U.S. patent number RE43,400 [Application Number 11/332,815] was granted by the patent office on 2012-05-22 for laser segmented cutting, multi-step cutting, or both.
This patent grant is currently assigned to Electro Scientific Industries, Inc.. Invention is credited to Brian W. Baird, Kevin P. Fahey, Richard S. Harris, James N. O'Brien, Yunlong Sun, Michael J. Wolfe, Lian-Cheng Zou.
United States Patent |
RE43,400 |
O'Brien , et al. |
May 22, 2012 |
Laser segmented cutting, multi-step cutting, or both
Abstract
UV laser cutting throughput through silicon and like materials
is improved by dividing a long cut path (112) into short segments
(122), from about 10 .mu.m to 1 mm. The laser output (32) is
scanned within a first short segment (122) for a predetermined
number of passes before being moved to and scanned within a second
short segment (122) for a predetermined number of passes. The bite
size, segment size (126), and segment overlap (136) can be
manipulated to minimize the amount and type of trench backfill.
Real-time monitoring is employed to reduce rescanning portions of
the cut path .[.112.]. .Iadd.(112) .Iaddend.where the cut is
already completed. Polarization direction of the laser output (32)
is also correlated with the cutting direction to further enhance
throughput. This technique can be employed to cut a variety of
materials with a variety of different lasers and wavelengths.
.Iadd.A multi-step process can optimize the laser processes for
each individual layer..Iaddend.
Inventors: |
O'Brien; James N. (Bend,
OR), Zou; Lian-Cheng (Portland, OR), Sun; Yunlong
(Beaverton, OR), Fahey; Kevin P. (Portland, OR), Wolfe;
Michael J. (Portland, OR), Baird; Brian W. (Oregon City,
OR), Harris; Richard S. (Portland, OR) |
Assignee: |
Electro Scientific Industries,
Inc. (Portland, OR)
|
Family
ID: |
27360805 |
Appl.
No.: |
11/332,815 |
Filed: |
January 13, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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10017497 |
Dec 14, 2001 |
7157038 |
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09803382 |
Mar 9, 2001 |
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60297218 |
Jun 8, 2001 |
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60265556 |
Jan 31, 2001 |
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60233913 |
Sep 20, 2000 |
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Reissue of: |
10165428 |
Jun 6, 2002 |
6676878 |
Jan 13, 2004 |
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Current U.S.
Class: |
264/400;
219/121.67; 219/121.62; 264/482; 219/121.8; 219/121.81;
219/121.69 |
Current CPC
Class: |
B23K
26/083 (20130101); B23K 26/0622 (20151001); B23K
26/40 (20130101); B23K 26/073 (20130101); B23K
26/066 (20151001); B23K 26/38 (20130101); B23K
26/364 (20151001); B23K 26/032 (20130101); B23K
26/0869 (20130101); B23K 2103/50 (20180801); B23K
2101/40 (20180801) |
Current International
Class: |
B23K
26/04 (20060101); C04B 41/91 (20060101) |
Field of
Search: |
;264/400,482
;219/121.67,121.69,121.85,121.71,121.75,121.81 |
References Cited
[Referenced By]
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|
Primary Examiner: Wollschlager; Jeffrey
Attorney, Agent or Firm: Stoel Rives LLP
Parent Case Text
.Iadd.More than one application for reissue of U.S. Pat. No.
6,676,878 has been filed. The reissue applications are U.S. patent
application Ser. Nos. 12/351,562, 12/350,767, 12/256,156, and
11/332,815 (the present application), all of which are divisional
applications for reissue of U.S. Pat. No. 6,676,878..Iaddend.
RELATED APPLICATIONS
.Iadd.This is an application for reissue of U.S. Pat. No.
6,676,878, which issued on U.S. patent application Ser. No.
10/165,428, filed Jun. 6, 2002..Iaddend.
This patent application derives priority from U.S. Provisional
Application No. 60/297,218, filed Jun. 8, 2001, .[.and.]. is a CIP
of U.S. patent application Ser. No. 10/017,497, filed Dec. 14,
2001, .Iadd.(now U.S. Pat. No. 7,157,038), .Iaddend.which claims
priority from U.S. Provisional Application No. 60/265,556, filed
Jan. 31, 2001.Iadd., and is a CIP of U.S. patent application Ser.
No. 09/803,382, filed Mar. 9, 2001, (now abandoned), which derives
priority from U.S. Provisional Patent Application No. 60/233,913,
filed Sep. 20, 2000..Iaddend.
Claims
What is claimed is:
1. A method of increasing throughput in a laser cutting process,
comprising: directing a first pass of first laser pulses to impinge
along a first segment of a cutting path .[.having.]. .Iadd.that is
continuous and has .Iaddend.a cutting path length greater than 100
.mu.m, each first laser pulse having a first spot area on a
workpiece, the first .Iadd.spot area having a first major axis and
the first .Iaddend.segment having a first segment length that is
longer than the first .[.spot area.]. .Iadd.major axis .Iaddend.and
shorter than the cutting path length; directing a second pass of
second laser pulses to impinge along a second segment of the
cutting path, each second laser pulse having a second spot area on
the workpiece, the second .Iadd.spot area having a second major
axis and the second .Iaddend.segment having a second segment length
that is longer than the second .[.spot area.]. .Iadd.major axis
.Iaddend.and shorter than the cutting path length, the second
segment overlapping the first segment by an overlap length greater
than .[.at least.]. the first or second .[.spot areas.].
.Iadd.major axis.Iaddend.; and after directing at least the first
and second passes of laser pulses, directing a third pass of third
laser pulses to impinge along a third segment of the cutting path,
each third laser pulse having a third spot area on the workpiece,
the third .Iadd.spot area having a third major axis and the third
.Iaddend.segment having a third segment length that is longer than
the third .[.spot area.]. .Iadd.major axis .Iaddend.and shorter
than the cutting path length, the third segment including a
subsequent portion of the cutting path other than the first or
second .[.segments.]. .Iadd.segment.Iaddend., wherein the
subsequent portion of the cutting path has a nonoverlap length
greater than the first, second, or third .[.spot areas.].
.Iadd.major axis.Iaddend..
2. The method of claim 1 in which major portions of the first and
second segments overlap.
3. The method of claim 1 in which the second segment includes the
first segment.
4. The method of claim 3 in which the first and second segments are
processed in a same direction.
5. The method of claim 3 in which the first and second segments are
processed in opposite directions.
6. The method of claim 1 in which the first and second segments are
processed in a same direction.
7. The method of claim 1 in which the first and second segments are
processed in opposite directions.
8. The method of claim 1 in which additional sets of first and/or
second laser pulses are applied to the first and/or second segments
to form a through trench within the first and/or second segments
prior to applying the third laser pulses.
9. The method of claim 1 further comprising: forming a through
trench in the first and/or second segments prior to applying the
third laser pulses.
10. The method of claim 1 further comprising: forming a through
trench in the first and/or second segments with multiple passes of
laser pulses prior to applying the third laser pulses; and forming
a through trench within the third segment.
11. The method of claim 10 further comprising: forming a through
trench along the entire cutting path length.
12. The method of claim 11 in which the cutting path length is
greater than 1 mm and the first, second, and third segment lengths
are between about 10 .mu.m and about 500 .mu.m.
13. The method of claim 1 in which the cutting path length is
greater than 1 mm and the first, second, and third segment lengths
are between about 10 .mu.m and about 500 .mu.m.
14. The method of claim 13 in which the cutting path length is
greater than 10 mm and the first, second, and third segment lengths
are between about 200 .mu.m and about 500 .mu.m.
15. The method of claim 13 in which the first, second, and third
laser pulses are characterized by a UV wavelength, a pulse
repetition frequency of greater than 5 kHz, pulse energies of
greater than 200 .mu.J, and a bite size of about 0.5 to about 50
.mu.m.
16. The method of claim 1 in which the first, second, and third
laser pulses are characterized by a UV wavelength, a pulse
repetition frequency of greater than 5 kHz, pulse energies of
greater than 200 .mu.J, and a bite size of about 0.5 to about 50
.mu.m.
17. The method of claim 16 in which the workpiece has a thickness
greater than 50 .mu.m.
18. The method of claim 17 in which the workpiece has a thickness
greater than 500 .mu.m.
19. The method of claim 12 in which the workpiece has a thickness
greater than 50 .mu.m.
20. The method of claim 12 in which the workpiece has a thickness
greater than 500 .mu.m, the cutting path length is greater than 100
mm, and the throughcut along the entire length of the cutting path
is made with fewer than 25 passes of laser pulses over any position
along the cutting path.
21. The method of claim 13 in which the workpiece has a thickness
greater than 200 .mu.m, further comprising: cutting through the
entire thickness along the cutting path at a cutting speed of
greater than 10 mm per minute.
22. The method of claim 21 in which a major portion of the
thickness of the workpiece comprises a semiconductor material, a
glass material, a ceramic material, or a metallic material.
23. The method of claim 21 in which a major portion of the
thickness of the workpiece comprises Si, GaAs, SiC, SiN, indium
phosphide, or AlTiC.
24. The method of claim 22 in which the laser pulses are generated
from a solid-state laser or a CO.sub.2 laser.
25. The method of claim 1 in which the laser pulses are generated
from a solid-state laser or a CO.sub.2 laser.
26. The method of claim 2 in which the overlap length .[.of the
first and second portions.]. or the first or second segment
.[.lengths are sufficiently short such that.]. .Iadd.length is
selected to enable .Iaddend.the second laser pulses .Iadd.to
.Iaddend.impinge along the overlap length before a major portion of
any debris generated by the first laser pulses cools along the
overlap length to ambient temperature.
27. The method of claim 1 in which the third segment excludes the
first or second .[.segments.]. .Iadd.segment.Iaddend..
28. The method of claim 1 in which the first laser pulses impinge
along the cutting path in a first cutting direction and the first
laser pulses have a first polarization orientation that is parallel
to the first cutting direction, in which the third laser pulses
impinge along the cutting path in a third cutting direction and the
third laser pulses have a third polarization orientation that is
parallel to the third cutting direction, and in which the first and
third cutting directions are transverse.
29. The method of claim 28 further comprising: employing a
polarization control device to change from the first polarization
orientation to the third polarization orientation.
30. The method of claim 10 further comprising: monitoring
throughcut status with a throughcut monitor to determine throughcut
positions where throughcuts have been affected along the cutting
path; and reducing impingement of the throughcut positions during
the passes of first, second, third, or subsequent laser pulses in
response to information provided by the throughcut monitor.
.[.31. The method of claim 1 in which the laser pulses within the
first pass have generally similar parameters..].
.[.32. The method of claim 1 in which the laser pulses of the
first, second, and third passes have generally similar
parameters..].
.[.33. The method of claim 1 in which the laser pulses of at least
two of the first, second, and third passes have at least one
generally different parameter..].
.[.34. The method of claim 1 in which at least two of the laser
pulses in at least one of the first, second, or third passes have
at least one generally different parameter..].
35. The method of claim 1 in which multiple passes of laser pulses
are applied to the first segment to form a throughcut within the
first segment.
36. The method of claim 35 in which the throughcut is formed in the
first segment before the pass of second laser pulses is applied to
the second segment.
37. The method of claim 36 in which multiple passes of laser pulses
are applied to the second segment to form a throughcut within the
second segment.
38. The method of claim 37 in which the throughcut is formed in the
second segment before the pass of third laser pulses is applied to
the third segment.
39. The method of claim 38 in which multiple passes of laser pulses
are applied to subsequent segments to sequentially form throughcuts
within the respective subsequent segments to form a full length
throughcut along the cutting path length.
40. The method of claim 1 in which only minor portions of the first
and second segments overlap.
41. The method of claim 1 in which the first laser pulses impinge
along the cutting path in a first cutting direction and the first
laser pulses have a first polarization orientation that is oriented
to the first cutting direction to enhance throughput or cut
quality, in which the third laser pulses impinge along the cutting
path in a third cutting direction and the third laser pulses have a
third polarization orientation that is oriented to the third
cutting direction to enhance throughput or cut quality, and in
which the first and third cutting directions are transverse and the
first and third polarization orientations are transverse.
42. The method of claim 1 in which at least one of the segments is
an arc.
43. The method of claim 1 in which a purge gas is employed to
facilitate blowing potential backfill debris through throughcuts
along the cutting path.
44. The method of claim 1 in which an elongated laser pass that
includes at least three first, second, and third segments is
applied to the cutting path.
45. The method of claim 1 in which each spot area along a segment
is in proximity to or partly overlaps the spot area of a preceding
laser pulse.
46. A method of increasing throughput for forming a cut along a
cutting path .[.having.]. .Iadd.that is continuous and has
.Iaddend.a cutting path length on a workpiece, comprising:
selecting a segment length that is shorter than the cutting path
length; directing a first pass of first laser pulses having first
spot areas to impinge the workpiece along a first segment of about
the segment length along the cutting path.Iadd., each of the first
spot areas having a first major axis.Iaddend.; directing a second
pass of second laser pulses having second spot areas to impinge the
workpiece along a second segment of about the segment length along
the cutting path, .Iadd.each of the second spot areas having a
second major axis, and .Iaddend.the second segment overlapping the
first segment by an overlap length greater than at least the first
or second .[.spot areas.]. .Iadd.major axis.Iaddend.; and after
directing at least the first and second passes of laser pulses,
directing a third pass of third laser pulses having third spot
areas to impinge along a third segment of about the segment length
along the cutting path, .Iadd.each of the third spot areas having a
third major axis, and .Iaddend.the third segment including a
portion of the cutting path that extends beyond the first or second
.[.segments.]. .Iadd.segment.Iaddend., wherein the portion of the
cutting path has a portion length greater than the first, second,
or third .[.spot areas.]. .Iadd.major axis.Iaddend..
47. The method of claim 46 in which impingement of laser pulses
along the cutting path generates debris and in which the overlap
length or the segment length is .[.sufficiently short such that.].
.Iadd.selected to enable .Iaddend.the .Iadd.second laser pulses of
the .Iaddend.second pass .[.of second laser pulses.]. .Iadd.to
.Iaddend.impinge along the overlap length before a major portion of
any debris generated by the first laser pulses cools to ambient
temperature along the overlap length.
48. A method of increasing throughput in a laser cutting process,
comprising: directing a first pass of first laser pulses to impinge
along a first segment of a cutting path .[.having.]. .Iadd.that is
continuous and has .Iaddend.a cutting path length, each first laser
pulse having a first spot area on a workpiece, the first .Iadd.spot
area having a first major axis and the first .Iaddend.segment
having a first segment length that is longer than the first .[.spot
area.]. .Iadd.major axis .Iaddend.and shorter than the cutting path
length; directing second passes of second laser pulses to impinge
along a second segment of the cutting path, the second segment
including an overlap length that overlaps at least a portion of the
first segment until a throughcut is made within the overlap length,
each second laser pulse having a second spot area on a workpiece,
the second .Iadd.spot area having a second major axis and the
second .Iaddend.segment having a second segment length that is
longer than the second .[.spot area.]. .Iadd.major axis
.Iaddend.and shorter than the cutting path length, the overlap
length being greater than .[.at least.]. the first or second
.[.spot areas.]. .Iadd.major axis.Iaddend.; and after directing at
least the first and second passes of laser pulses, directing third
passes of third laser pulses to impinge along a third segment of
the cutting path until a throughcut is made within the third
segment, each third laser pulse having a third spot area on a
workpiece, the third .Iadd.spot area having a third major axis and
the third .Iaddend.segment having a third segment length that is
longer than the third .[.spot area.]. .Iadd.major axis .Iaddend.and
shorter than the cutting path length, the third segment including a
portion of the cutting path that extends beyond the first or second
.[.segments.]. .Iadd.segment.Iaddend., wherein the portion of the
cutting path has a portion length greater than the first, second,
or third .[.spot areas.]. .Iadd.major axis.Iaddend..
49. The method of claim 1 in which the overlap length of the first
and second portions or the first or second segment .[.lengths
are.]. .Iadd.length is .Iaddend.in a range appropriate so as to
exploit with second laser pulses persistence of a selected
transient effect arising from the interaction of first pulses with
the workpiece along the overlap length.
50. The method of claim 46 in which the overlap length of the first
and second portions or the first or second segment .[.lengths
are.]. .Iadd.length is .Iaddend.in a range appropriate so as to
exploit with second laser pulses persistence of a selected
transient effect arising from the interaction of first pulses with
the workpiece along the overlap length.
51. The method of claim 48 in which the overlap length of the first
and second portions or the first or second segment .[.lengths
are.]. .Iadd.length is .Iaddend.in a range appropriate so as to
exploit with second laser pulses persistence of a selected
transient effect arising from the interaction of first pulses with
the workpiece along the overlap length.
.Iadd.52. The method of claim 1 in which a cutting blade is
employed to sever the workpiece along the cutting
path..Iaddend.
.Iadd.53. The method of claim 1 in which the laser pulses of at
least two of the first, second, and third passes have different
wavelengths, including first and second wavelengths..Iaddend.
.Iadd.54. The method of claim 53 in which the first wavelength is a
UV wavelength or a visible wavelength and the second wavelength is
an IR wavelength or visible wavelength..Iaddend.
.Iadd.55. The method of claim 53 in which the first and second
wavelengths are different UV wavelengths..Iaddend.
.Iadd.56. The method of claim 1 in which the laser pulses of at
least two of the first, second, and third passes have different
irradiances..Iaddend.
.Iadd.57. The method of claim 1 in which the laser pulses of at
least two of the first, second, and third passes have different
repetition rates..Iaddend.
.Iadd.58. The method of claim 1 in which the laser pulses of at
least two of the first, second, and third passes have different
bite sizes..Iaddend.
.Iadd.59. The method of claim 1 in which the laser pulses of at
least two of the first, second, and third passes have different
scan speeds..Iaddend.
.Iadd.60. The method of claim 1 in which at least two of the first,
second, and third passes have different lengths..Iaddend.
.Iadd.61. The method of claim 1 in which the laser pulses of at
least two of the first, second, and third passes have different
pulse widths..Iaddend.
.Iadd.62. The method of claim 1 in which the laser pulses of at
least two of the first, second, and third passes have different
fluences..Iaddend.
.Iadd.63. The method of claim 1 in which the laser pulses of at
least two of the first, second, and third passes have different
spot areas..Iaddend.
.Iadd.64. The method of claim 1 in which the laser pulses of at
least two of the first, second, and third passes employ a different
laser, wavelength, pulse width, fluence, and/or bite
size..Iaddend.
.Iadd.65. The method of claim 1 in which the laser pulses are
generated by different lasers..Iaddend.
.Iadd.66. The method of claim 1 in which the work piece comprises
first and second layers of different materials and in which the
laser pulses applied to the first and second layers are generated
by different lasers, including first and second
lasers..Iaddend.
.Iadd.67. The method of claim 66 in which the first laser is a UV
or visible laser and the second laser is an IR or visible
laser..Iaddend.
.Iadd.68. The method of claim 66 in which the first and second
lasers are both UV lasers that generate output at different
wavelengths..Iaddend.
.Iadd.69. The method of claim 1 in which the laser pulses of at
least one of the first, second, and third passes have: spot areas
that successively overlap and impinge nonoverlapping areas, each
having a spatial major axis of about 0.01 to 9.5 microns, and a
wavelength shorter than or equal to about 355 nm..Iaddend.
.Iadd.70. The method of claim 1 in which the laser pulses of at
least one of the first, second, and third passes have a
substantially Gaussian irradiance profile at a wavelength shorter
than or equal to about 532 nm..Iaddend.
.Iadd.71. The method of claim 1 in which the workpiece has a
substrate supporting a layer, the substrate having a wafer material
and the layer having a material different from that of the
substrate and prone to propagating cracks that initiate during a
cutting technique, and in which the cutting path is a second
cutting path that is addressed after a first cutting path is
addressed, the method further comprising: applying a first
technique to form a first kerf through the layer along the first
cutting path, the first technique including directing a first laser
output having a first set of first parameters along the first
cutting path across the layer to form the first kerf through the
layer, and the first parameters adapted to minimize initiation of
cracks; and applying along the second cutting path a second
technique to form in the substrate a second kerf parallel to the
first kerf, the second cutting path being parallel to the first
cutting path, the second technique including directing the first,
second and third passes, the second technique being different from
the first technique, and the second technique initiating cracks in
the layer that begin at the second kerf, propagate in a direction
toward the first kerf, and terminate at or prior to the first
kerf..Iaddend.
.Iadd.72. The method of claim 1 in which the workpiece has a
substrate supporting a layer, the substrate having a wafer material
and the layer having a material different from that of the
substrate and having a tendency to delaminate from the substrate at
or near a layer-substrate interface during a cutting technique, and
in which the cutting path is a second cutting path that is
addressed after a first cutting path is addressed, the method
comprising: applying a first technique along the first cutting path
to form a first kerf through the layer and into the substrate, the
first technique including directing a first laser output having a
first set of first parameters along the first cutting path across
the layer to form the first kerf through the layer, and the first
parameters adapted to minimize initiation of delamination of the
layer from the substrate; and applying along the second cutting
path a second technique to form in the substrate a second kerf
parallel to the first kerf, the second cutting path being parallel
to the first cutting path, the second technique including directing
the first, second and third passes, the second technique being
different from the first technique, and the second technique
initiating delamination of the layer from the substrate that begins
at the second kerf, propagates in a direction toward the first
kerf, and terminates at or prior to the first kerf..Iaddend.
.Iadd.73. The method of claim 1 in which the workpiece comprises an
electronic device having an edge formed from the cutting path, the
edge having first and second transverse surfaces, the method
further comprising: generating first laser output having a
wavelength shorter than or equal to about 355 nm; directing the
first laser output toward a first target location on the first
surface in proximity to the edge of the electronic device such that
a first output spot area of first laser output impinges the first
surface; generating second laser output having a second output spot
area and a wavelength shorter than or equal to about 355 nm; and
directing the second laser output toward a second target location
on the first surface in proximity to the edge of the electronic
device, such that the second output spot area impinges the first
surface and such that the second output spot area partly overlaps
the first output spot area and impinges a nonoverlapping area
having a spatial major axis of 0.5-9 .mu.m, thereby converting the
edge to a rounded edge in proximity to the first and second target
locations..Iaddend.
.Iadd.74. The method of claim 1 in which the workpiece comprises a
brittle, high melting temperature ceramic or glass material, having
a surface; and in which the laser pulses of at least one of the
first, second, or third pass have a wavelength shorter than or
equal to about 355 nm and spot areas that successively overlap and
impinge nonoverlapping areas each having a spatial major axis of
0.01 to 9.5 .mu.m and generate redeposited debris that primarily
comprises nonmolten materials that contact the surface and are
nonpermanent and removable from the surface by conventional
cleaning techniques..Iaddend.
.Iadd.75. The method of claim 1 in which the workpiece comprises a
wafer supporting multiple electronic devices; in which the laser
pulses of at least one of the first, second, or third pass have a
wavelength shorter than or equal to about 355 nm and spot areas
that successively overlap and impinge nonoverlapping areas each
having a spatial major axis of 0.01 to 9.5 .mu.m; and in which the
cutting path is employed to separate groups of electronic devices
or separate individual electronic components..Iaddend.
.Iadd.76. The method of claim 1 in which the laser pulses of at
least one of the first, second, or third pass have a substantially
Gaussian irradiance profile as generated and are subsequently
propagated through an aperture to provide apertured output for the
laser pulses..Iaddend.
.Iadd.77. The method of claim 76 in which the laser pulses of the
apertured output are shaped by at least one beam shaping element
before the laser pulses are propagated through the aperture to
provide shaped apertured output for the laser pulses..Iaddend.
.Iadd.78. The method of claim 77 in which the laser pulses of the
shaped apertured output have a wavelength shorter than or equal to
about 532 nm..Iaddend.
.Iadd.79. The method of claim 1 in which the workpiece comprises a
wafer supporting rows of electronic devices, the method further
comprising: identifying a first feature on a first surface of a
first row of electronic devices; aligning with respect to the first
feature on the first surface, a first target position of a laser
system such that the target position is in proximity to a first
intended edge of a first electronic device having surface features
in a first orientation; directing at least one of first, second,
and third passes of laser pulses to impinge the first surface at
the first target position and linearly therewith to form a first
kerf that traverses between a first set of rows of electronic
devices, one of the first set of rows including the first
electronic device; identifying a second feature on a second surface
of a second row of electronic devices; aligning with respect to the
second feature on the second surface a second target position of
the laser system such that the second target position is in
proximity to a second intended edge of a second electronic device
having surface features in a second orientation that is different
from the first orientation; and directing at least one of first,
second, and third passes of laser pulses to impinge the second
surface at the second target position and linearly therewith to
form a second kerf that traverses between a second set of rows of
electronic devices, one of the second set of rows including the
second electronic device..Iaddend.
.Iadd.80. The method of claim 1 in which the laser pulses of at
least one of the first, second, or third pass are generated by a
holmium or erbium-doped laser..Iaddend.
Description
FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not Applicable
COPYRIGHT NOTICE
.COPYRGT. 2001 Electro Scientific Industries, Inc. A portion of the
disclosure of this patent document contains material which is
subject to copyright protection. The copyright owner has no
objection to the facsimile reproduction by anyone of the patent
document or the patent disclosure, as it appears in the Patent and
Trademark Office patent file or records, but otherwise reserves all
copyright rights whatsoever. 37 CFR .sctn.1.71(d).
TECHNICAL FIELD
This invention relates to a laser cutting and, in particular, to a
method and/or system for advantageous beam positioning and scanning
to improve the throughput of laser .Iadd.material processing and/or
.Iaddend.cutting in silicon or other materials .Iadd.and/or to
employing laser output to dice, notch, or drill vias in
semiconductor wafers which have a film layer or multilayer which
resides on top of the wafer surface.Iaddend..
BACKGROUND OF THE INVENTION
.Iadd.Most semiconductor and related products, for example
transistors, diodes, light emitting diodes, MEMS devices, planar
waveguide structures and integrated circuits, are fabricated in the
form of a large number of elements manufactured simultaneously on a
large wafer. This wafer is typically composed of Si, GaAs, GaP,
InP, Sapphire, or other material. The creation of devices is most
often performed using conventional fabrication techniques such as
photolithography, oxidation, implantation, deposition, etching,
epitaxial growth, and/or spin coating. Upon completion of these
device wafers, the individual devices must be singulated, a process
which is typically referred to as "dicing." The individual devices
are referred to as "die" or "dice." The area on the wafer in
between active parts of adjacent die is referred to as the "street"
or "dice lane." The streets are limited to a minimum width because
of the wafer material which is removed or destroyed during the
dicing process. The wafer area which is completely removed by the
dicing process is called the "kerf," while the rest of the street
must accommodate any damage zone around the cut and any
misalignment or deviation from straightness of the
cut..Iaddend.
.Iadd.Conventionally, dicing is performed by the use of a wafer saw
or by the technique of "scribe and break," where the wafer is
notched, often by a diamond point, and is then cleaved along this
scribe line. Due to issues with scribe and break such as low yield,
dicing saws have taken over in recent years as the predominant
technique for dicing wafers. Conventional slicing blades typically
have a narrow dimension of about 50 to 200 .mu.m along their
cutting axes and produce cuts that are wider than the blades. The
slicing blades currently need to be this wide to withstand stresses
of making straight cuts through the strength and thickness of
conventional wafers, for example. The wide cuts made by the
mechanical cutting blades often significantly reduce the number of
rows and columns of die that can be fit onto each
wafer..Iaddend.
.Iadd.Skilled persons will also note that dicing blades tend to
wear relatively quickly such that the widths of their cuts may vary
over time. In some cases, the blades can be inadvertently bent and
then such blades produce curved or slanted cuts or increased
chipping. The dicing process creates small chips as it creates
sharp edges and sharp corners along singulation paths..Iaddend.
FIG. 1 is a simplified representation of a traditional continuous
cutting profile 8. Traditional laser cutting employs sequentially
overlapping spots from consecutive laser pulses to continuously
scan through an entire cut path. Numerous complete passes are
performed until the target is severed along the entire cut path.
When the target material is thick, many passes (in some cases over
100 passes) may be necessary to complete the cutting process,
particularly with limited laser power.
A method for .Iadd.dicing wafers and .Iaddend.increasing laser
cutting throughput for thick materials is, therefore,
desirable.
SUMMARY OF THE INVENTION
.Iadd.As a result, laser sawing is becoming an attractive
alternative to these conventional techniques for dicing. Some
reasons for the consideration of laser dicing would be that lasers
can cut curved die such as Arrayed Waveguide Gratings (AWGs) from a
wafer, unlike either of the two conventional techniques. In
addition, lasers can often cut without the use of water, which is
of great importance for the manufacture of devices which are water
sensitive, such as MEMS. Dicing saws, which are today the
predominant technique in use, typically require the use of water as
a lubricant and/or coolant. Lasers also offer the potential of the
smallest street width available, due to a potentially very small
kerf width and the possibility of very accurate alignment of the
laser to the workpiece (wafer)..Iaddend.
An object of the present invention is, therefore, to provide a
method and/or system for improving the throughput for laser cutting
silicon or other materials.
For convenience, the term cutting may be used generically to
include trenching (cutting that does not penetrate the full depth
of a target workpiece) and throughcutting, which includes slicing
(often associated with wafer row separation) or dicing (often
associated with part singulation from wafer rows). Slicing and
dicing may be used interchangeably in the context of this
invention.
.Iadd.Lasers also can offer the ability to pattern wafers, creating
features such as trenches or notches which are made by scanning the
laser across the surface but only cutting partially through the
wafer (unlike dicing). This technique can be used to make features
on die, or can also be used to perform laser scribing for a
scribe-and-break process, for example..Iaddend.
.Iadd.Lasers also offer great potential for the drilling of vias
through or into the substrate material. This is of interest for
reasons that may include but are not limited to: allowing ground to
be contacted through the backside of the die; allowing die to be
stacked atop each other inside one package (so-called
"three-dimensional packaging"); or the ability to mount devices in
a "flip-chip" BGA fashion, but with the active devices facing up
(important for MEMS or front-side cooling of integrated circuits or
laser diodes). These vias can range in diameter up from several
microns to several hundred microns, and the die thicknesses of
interest vary from tens of microns to almost 1000 microns. Few
production-worthy solutions exist for the drilling of such
high-aspect ratio vias, and those such as plasma etching tend to be
cumbersome, expensive, and often slow..Iaddend.
.Iadd.While laser processing capability has advanced greatly in the
last few years due to advances both in available lasers and in
process understanding, there are still some significant issues with
the use of lasers for dicing, drilling, or patterning processes.
Attempts may have been made to use infrared (IR) lasers to machine
alumina, alumina/titanium carbide (Al.sub.2O.sub.3/TiC) mixtures
(also commonly referred to as AlTiC), silicon, or silicon oxides.
IR wavelengths to a limited extent have been shown to machine these
materials, and have been used successfully as laser scribing tools
for marking die or scribe and break. These lasers, however, tend to
damage pure alumina or Si such as by unpredictably cracking the
alumina, Si, or oxide layers and by throwing permanent redeposited
material (redep), such as melted slag, onto the top surface of the
wafer and by creating a "melt lip" where the edge of the cut pulls
backward and up..Iaddend.
.Iadd.One embodiment of the invention provides such a method or
system that facilitates the manufacturing of sliders..Iaddend.
.Iadd.Another embodiment of the invention provides such a method or
system that eliminates the cutting-formed sharp edges and chips on
either the front or back sides of ceramic, glass, or silicon
sliders or dies during the manufacturing process..Iaddend.
.Iadd.Another embodiment of the invention provides such a method or
system that decreases the widths of the cutting lanes or paths
between the rows and sliders..Iaddend.
.Iadd.U.S. Pat. Nos. 5,593,606 and 5,841,099 of Owen et al.
describe techniques and advantages for employing UV laser systems
to generate laser output pulses within advantageous parameters to
form through-hole or blind vias through at least two different
types of layers in multilayer devices. These parameters generally
include nonexcimer output pulses having temporal pulse widths of
shorter than 100 ns, spot areas with spot diameters of less than
100 .mu.m, and average intensities or irradiances of greater than
100 mW over the spot areas at repetition rates of greater than 200
Hz..Iaddend.
.Iadd.Despite the foregoing, lasers have not been employed
successfully to dice Si wafers or to pattern oxide or other layers
on top of Si wafers. The same is true for other types of
semiconductor wafers, and for sapphire or other insulator wafers.
In particular, solid-state UV lasers have not been employed
successfully to machine sliders and particularly have not been
employed successfully to machine brittle, high melting temperature
ceramic, glass, or glass-like materials such as alumina or
alumina/titanium carbide (Al.sub.2O.sub.3/TiC, also known as AlTiC)
in the context of sliders. One of the major issues results from the
fact that most devices are made of several different materials,
usually deposited or grown on top of the wafer in a build-up
process. These materials include but are not limited to metals,
oxide dielectrics, nitrides, silicides, polymer dielectrics, and
other semiconductor layers. Of late, several attempts have been
made to dice or scribe Si using IR lasers, and although some
success has been achieved for cutting Si, these lasers are unable
to cut through SiO.sub.2 or other oxide layers on top of the Si
wafer..Iaddend.
FIG. 2A is a graph showing that for conventional long continuous
throughcuts, the effective dicing speed decreases very quickly as
silicon wafer thickness increases. Thus, as thickness increases,
the number of laser passes increases almost exponentially and
consequently exponentially decreases the dicing speed. The cutting
width may be on the order of only a few tens of microns (.mu.m),
and the wafer thickness is typically much greater than the cutting
width.
Traditional laser cutting profiles may suffer from trench backfill
of laser ejected material. When the wafer thickness is increased,
this backfill becomes much more severe and may be largely
responsible for the dramatic decrease in dicing speed. Moreover,
for some materials under many process conditions, the ejected
backfill material may be more difficult to remove on subsequent
passes than the original target material. Because trench backfill
with laser ejected material has a somewhat random nature, the
degree of backfill along any portion of a traditional cutting
profile may be large or small such that some portions of the
cutting path may be cut through (opened) in fewer passes than other
portions of the cutting path. Traditional laser cutting techniques
ignore these phenomena and continuously scan an entire cut path,
including areas that may already be opened, with complete passes of
laser output until the target material is severed along the entire
cut path.
As an example, a UV laser, having laser output power of only about
4 W at 10 kHz, requires about 150 passes to make a complete cut
through a 750 .mu.m-thick silicon wafer using a conventional laser
cutting profile. The conventional cutting profiles typically
traverse the entire lengths of wafers, which typically have
diameters of about 200-305 mm. The resulting cutting rate is too
slow for commercial dicing applications of silicon this thick.
Although the segmented cutting technique can be employed to cut any
laser-receptive material and employed at any laser wavelength, the
segmented cutting technique is particularly useful for laser
processing at wavelengths where laser power is limited, such as
solid-state-generated .[.V.]. .Iadd.UV.Iaddend., and particularly
where such wavelengths provide the best cutting quality for a given
material. For example, even though IR lasers tend to provide much
more available output power, IR wavelengths tend to crack or
otherwise damage silicon, alumina, AlTiC and other ceramic or
semiconductor materials. UV is most preferred for cutting a silicon
wafer for example.
U.S. patent application Ser. No. 09/803,382 ('382 application) of
Fahey et al., describes a UV laser system and a method for
separating rows or singulating sliders or other components. These
methods include various combinations of laser and saw cutting
directed at one or both sides of a wafer and various techniques for
edge modification.
.[.U.S. patent application derives priority from U.S. Provisional
Application No. 60/297,218, filed Jun. 8, 2001, and is a CIP of
U.S. patent application Ser. No. 10/017,497, filed Dec. 14, 2001,
which claims priority from U.S. Provisional Application No.
60/265,556, filed Jan. 31, 2001..]. .Iadd.U.S. patent application
Ser. No. 10/017,497 ('497 Application) of Baird et al. further
describes using ultraviolet laser ablation to directly and rapidly
form patterns with feature sizes of less than 50 .mu.m in
hard-to-cut materials, such as silicon. These patterns include:
formation of very high-aspect cylindrical openings or blind vias
for integrated circuit connections; singulation of processed dies
contained on silicon wafers; and microtab cutting to separate
microcircuits formed in silicon from a parent wafer..Iaddend.
FIG. 2B is a graph showing the results of a recent experiment
comparing the number of passes to complete a dicing cut versus the
cutting length of the cutting profile in 750 .mu.m-thick silicon. A
wedge or "pie slice" was taken from a 750 .mu.m-thick silicon
wafer, and cutting profiles of different lengths were executed from
edge to edge. The experiment revealed that shorter cutting profiles
could be diced with fewer passes.
The present invention, therefore, separates long cuts into a
cutting profile containing small segments that minimize the amount
and type of trench backfill. For through cutting or trench cutting
in thick silicon, for example, these segments are preferably from
about 10 .mu.m to 1 mm, more preferably from about 100 .mu.m to 800
.mu.m, and most preferably from about 200 .mu.m to 500 .mu.m.
Generally, the laser beam is scanned within a first short segment
for a predetermined number of passes before being moved to and
scanned within a second short segment for a predetermined number of
passes. The beam spot size, bite size, segment size, and segment
overlap can be manipulated to minimize the amount and type of
trench backfill. A few scans across the entire cut path can be
optionally employed in the process, particularly before and/or
after the segment cutting steps, to maximize the throughput and/or
improve the cut quality.
The present invention also improves throughput and quality by
optionally employing real-time monitoring and selective segment
scanning to reduce backfill and overprocessing. The monitoring can
eliminate rescanning portions of the cut path where the cut is
already completed. In addition, polarization of the laser beam can
be correlated with the cutting direction to further enhance
throughput. These techniques generate less debris, decrease the
heat affected zone (HAZ) surrounding the cutting area or kerf, and
produce a better cut quality.
Although the present invention is presented herein only by way of
example to silicon wafer cutting, skilled persons will appreciate
that the segmented cutting techniques described herein may be
employed for cutting a variety of target materials with the same or
different types of lasers having similar or different
wavelengths.
.Iadd.As discussed in detail in U.S. Patent Applications 60/265,556
(Baird et al.) and Ser. No. 09/803,382 (Fahey et al.), there are
several laser and optic processing parameters which must be
optimized to cleanly cut a given material using a laser. These
include but are not limited to the wavelength, the repetition rate,
the distance of new target material impinged by each sequential
laser pulse (the bite size d.sub.bite), the energy of each laser
pulse, the temporal pulse width, the spot size and the spatial
energy distribution within the spot. The parameters of choice for
cutting a particular material can vary considerably, and the
"process windows," the area of parameter space in which a given
material can be cleanly ablated, differs for different materials.
Even materials which appear to be the same (like various types of
SiO.sub.2 or SiON) can have very different optical and mechanical
and thermal/ablative properties due to factors which include but
are not limited to: different dopant, different stoichiometry,
different deposition technique, different microstructure (due to
the above or due to different underlayer, processing temperature
profile, etc.), or different macrostructure (porosity, geometry,
thickness). This means that such closely related materials may
still have non-matching process parameters and process
windows..Iaddend.
.Iadd.When cutting through a wafer, the majority of the wafer
thickness is usually taken up by the substrate material. The laser,
however, must cut through the overlying device layers first before
reaching the substrate material. Since in general two or more
different materials do not respond in the same fashion to a
particular set of laser parameters, it is common that the layers
atop a wafer substrate are compromised during the laser cutting of
the wafer. This can result in problems ranging from decreased
cutting rate (if the laser is not efficient in cutting the
overlayers) to the creation of a large damage region in the layers
if the laser interacts in a destructive fashion with
them..Iaddend.
.Iadd.The same is true for the saws to dice wafers where it is well
known that the saw can cause cracking, chipping and/or delamination
in layers, especially ones that are brittle and/or have low
adhesion. As layer stacks get more complicated, and with the
introduction of more fragile materials such as oxide-based low K
dielectrics, this problem is expected to become worse, and
certainly not less of an issue..Iaddend.
.Iadd.As such, it is of interest to find a technique by which one
could use a laser to dice, pattern, or drill a wafer with layers on
it while effectively cutting through the layers and the
substrate..Iaddend.
.Iadd.An object of the present invention is, therefore, to provide
a better method and/or system for dicing, cutting, or drilling of
wafers which include layers of various materials on one or both
sides of the wafer substrate..Iaddend.
.Iadd.One embodiment of the invention provides such a method or
system that allows for removal of the layer or layers with one
laser process or several laser processes and is then followed by
one subsequent laser process or several subsequent laser processes
which complete the cutting or drilling by only having to remove or
cut through the wafer substrate material. Accordingly, one example
of the present invention employs a UV laser to cut ceramic, glass,
polymer or metal films which may comprise the layers on the top or
bottom surfaces of the wafer substrate, while a different laser,
such as a 532 nm or IR laser, or the same laser/optic system run
with different process parameters (for example using laser
segmented dicing) is used to cut through the substrate material
after the layers have been cleared away. A preferred process
entails covering the surfaces of the wafer with a sacrificial layer
such as photoresist; optionally removing a portion of the
sacrificial layer to create uncovered zones over intended cutting
areas; laser cutting the layers atop the wafer substrate to a width
equal to or greater than that which will occur in the subsequent
substrate dicing or drilling step; then dicing or drilling the
wafer with a separate processing step or steps using a different
laser, wavelength, pulse width, fluence, bite size, or other laser
processing parameters. In cutting through the layers, it may be
preferable to use overlapping adjacent passes to widen the notch
through the layer. This may allow for subsequent passes to be
contained completely within the notch opened in the upper layer. It
may be useful to use several different parameters for various
passes in one notching step to tailor the notch geometry, including
sidewall angle..Iaddend.
.Iadd.Another embodiment of the invention provides such a method or
system that allows for removal of the layer or layers with one
laser process or several laser processes and is then followed by
one subsequent process or several subsequent processes which
complete the cutting or drilling with a non-laser technique which
then only has to remove the wafer substrate material. One example
of this technique is the removal of all metal, polymer or other
soft material from the dice lane using the laser, such that during
subsequent dicing with a saw blade, the blade only makes contact
with the substrate material. In this way, there will be no blade
degradation due to the presence of a softer material on the more
brittle substrate material. The benefits of this may include but
are not limited to improved lifetime of saw blades, or the
reduction of damage to the edges of the cut in the substrate due to
a contaminated blade. This technique will be of particular use when
dicing wafers with metallization in the dice lanes, such as that
due to the presence of test devices, or wafers which have a polymer
dielectric material such as some of the low-K materials which are
presently on the market..Iaddend.
.Iadd.Another embodiment of the invention includes laser processing
which is done after the substrate dicing step in order to correct
any damage which may have been created during the substrate dicing
step. For example, the laser can be used to melt the layers in
order to seal the edges and eliminate any cracks or crack
initiation sites which may have originated. The laser may also be
used to round the corner of the diced edge, as described U.S.
patent application Ser. No. 09/803,382 (Fahey et al), to eliminate
any sharp edges or chips which may have occurred during
dicing..Iaddend.
.Iadd.Another embodiment of the invention includes laser processing
which is performed after the notching or trenching or removal of
the surface layers in order to correct any damage which may have
been created during the laser processing steps which cut through
the layers. This may be done before or after the dicing of the
substrate..Iaddend.
.Iadd.Another embodiment of the invention is the use of imaged,
shaped output to notch or cut through the layers. This may be
beneficial for several reasons, including but not limited to the
ability to stop more precisely upon a lower layer or on the
substrate material without causing damage; the ability to more
precisely control the sidewall angle of the cut through the layer;
or the ability to use a larger spot size while achieving uniform
irradiance across the spot area..Iaddend.
.Iadd.Another embodiment of the invention is making laser cuts from
both sides of the wafer in order to cut entirely through the wafer.
This technique is useful for reasons which include but are not
limited to: the ability to use a more pristine cutting technique
from the device side of the wafer, while using a more aggressive
technique to cut at higher speed from the backside without
compromising the devices; also, it allows the use of two lasers at
the same time to cut the same dice lane for increased throughput.
In addition, it is well known that laser cutting rate decreases
with increasing depth into the wafer. As such, cutting from both
sides would allow faster cutting of the wafer, since two half
thickness cuts are faster than one cut of the full thickness.
Furthermore, this technique would allow for cuts to be made through
thicker materials, where half the thickness is below the saturation
depth, but the full thickness is too deep to cut due to saturation.
The cutting from both sides can be accomplished either by the use
of a laser system which has laser beams impinging on the wafer from
both sides, or by flipping the wafer in order to expose both sides
to the laser irradiance in succession. Another embodiment of the
invention is the use of a laser to drill holes or other alignment
marks through the wafer in order to align the backside cuts to the
frontside devices or cuts. These marks can be cut in from the edge
of the wafer or drilled through somewhere in the center area of the
wafer. They can also be made in a wafer carrier like a tape frame
on which the wafer is mounted..Iaddend.
.Iadd.In the creation of semiconductor and related devices, there
exists a variety of wafer types and layer types which are used.
These wafer types include, but are not limited to Si, GaAs, GaP,
InP, Sapphire, SiGe, Silicon on Sapphire, Silicon on Insulator, and
various types of ceramic material such as alumina/titanium carbide.
Layer types include but are not limited to various metals, oxides,
nitrides, polymers, epitaxial or amorphous or polycrystalline
semiconductor materials. As such, there is a wide range of
combinations of lasers and laser/optic process parameters which
must be used for the multi-step laser dicing or drilling of all
such devices. While much of this invention description uses Si and
SiO.sub.2 as the substrate and layer, respectively, in the various
examples, those skilled in the art will recognize that there are a
number of types of laser which may be useful for specific
combinations of layers and substrates. These laser types include
but are not limited to: Excimer, CO.sub.2, Nd:YAG, Nd:YLF, Vanadate
(or harmonic generated versions of the previous three), Ar-Ion, Cu
vapor, and many others. The wavelengths available from these
various lasers range from the UV, through the visible spectrum and
into IR..Iaddend.
.Iadd.Although a preferred laser for patterning or trenching
surface layers such as SiO.sub.2 is a UV Q-switched, solid-state
laser providing imaged, shaped output at a bite size of between
about 1 to 7 .mu.m, other UV lasers including excimers can be
employed for those materials which require UV wavelengths. In the
case of the cutting of materials which require visible or infrared
wavelengths, although the preferred laser may be a Q-switched,
solid-state laser providing imaged, shaped output, many other
lasers such as non-Q-switched or CO.sub.2 may be
employed..Iaddend.
Additional objects and advantages of this invention will be
apparent from the following detailed description of preferred
embodiments thereof, which proceeds with reference to the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a simplified representation of a traditional continuous
cutting profile.
FIG. 2A is a graph showing effective dicing speed versus silicon
wafer thickness for traditional continuous cuts.
FIG. 2B is a graph showing the number of passes to complete a cut
versus the cutting length in silicon.
FIG. 3 is a simplified partly pictorial and partly schematic
diagram of an exemplary laser system for performing segmented
cutting in accordance with the present invention.
FIG. 4 is a simplified pictorial diagram of an alternative
preferred laser system for performing segmented cutting in
accordance with the present invention.
FIG. 5 is a simplified pictorial diagram of an optional imaged
optics module that may be used in a laser system for performing
segmented cutting in accordance with the present invention.
.Iadd.FIGS. 5A-5F are simplified and partly schematic views of
several possible embodiments of a laser processing system suitable
for multi-step laser dicing or drilling..Iaddend.
FIG. 6 is a graph displaying the characteristic relationship
between pulse energy and pulse repetition frequency of the laser
employed during practice of the invention.
FIG. 7 is a simplified representation of a real time cut status
monitor optionally employed by an exemplary laser system for
performing segmented cutting in accordance with the present
invention.
FIG. 8 depicts a cut path having respective first and second
transverse directions through which cutting speed is enhanced by an
optional polarization tracking system.
FIG. 9 is a representative illustration of an ultraviolet
transparent chuck on which semiconductor workpieces are placed for
.[.throughout.]. .Iadd.throughcut .Iaddend.processing using
ultraviolet ablative segmented cutting in accordance with the
present invention.
FIG. 10 is a simplified representation of a segmented cutting
profile produced in accordance with the present invention.
FIG. 11 is a simplified plan view of an enlarged cutting segment
sequentially impinged by overlapping laser spots.
FIG. 12 is a simplified representation of an alternative segmented
cutting profile produced in accordance with the present
invention.
FIG. 13 is a simplified representation of an alternative segmented
cutting profile produced in accordance with the present
invention.
FIG. 14 is a simplified representation of an alternative segmented
cutting profile produced in accordance with the present
invention.
FIG. 15 is a simplified representation of an alternative segmented
cutting profile produced in accordance with the present
invention.
FIG. 16 is a simplified representation of an alternative segmented
cutting profile produced in accordance with the present
invention.
FIG. 17 is a simplified representation of an alternative segmented
cutting profile produced in accordance with the present
invention.
FIG. 18 is a representative illustration of a trench pattern formed
by segmented cutting processing of silicon.
FIG. 19 is a representative illustration of patterning of a MEMS
device by a segmented cutting process on a semiconductor wafer.
FIG. 20 is a representative illustration of an AWG device
fabricated by a segmented cutting process on a semiconductor
wafer.
.Iadd.FIG. 21 is a deposited end perspective view of a prior art
slider including a magnetic recording head..Iaddend.
.Iadd.FIG. 22 is an enlarged cross-sectional view of a trailing end
of a slider with its head oriented toward a magnetic recording
disk..Iaddend.
.Iadd.FIG. 23 is a plan view of a wafer having a plurality of
thin-film magnetic heads, such as the magnetic head shown in FIG.
22, deposited thereon..Iaddend.
.Iadd.FIG. 24 is a plan view of a carrier supporting diced into
rows of sliders, the air-bearing surface of the sliders being
patterned with a photoresist mask..Iaddend.
.Iadd.FIG. 25 is a simplified plan view of a carrier supporting a
number of slider rows, some of which exhibit row defects including
misalignment, prior to dicing into individual sliders..Iaddend.
.Iadd.FIG. 26 is a deposited end perspective view of a slider
processed in accordance with one embodiment of the
invention..Iaddend.
.Iadd.FIG. 27 is a deposited end perspective view of a slider
processed in accordance with another embodiment of the
invention..Iaddend.
.Iadd.FIG. 27A is a deposited and perspective view of a slider
processed in accordance with yet another embodiment of the
invention..Iaddend.
.Iadd.FIGS. 28a-28h are simplified side sectional views of a
generic workpiece as it undergoes process steps of an exemplary
laser rounding process..Iaddend.
.Iadd.FIGS. 29a-29f are simplified side sectional views of a
generic workpiece as it undergoes process steps of an exemplary
laser cutting process..Iaddend.
.Iadd.FIG. 30 is a simplified side section view of a generic
workpiece undergoing a number of lines or rows of laser passes
whose positions vary with distance from an edge..Iaddend.
.Iadd.FIG. 31 is a plan view of a portion of a row carrier
supporting bowed and angled slider rows that can be diced by laser
row defect compensation..Iaddend.
.Iadd.FIG. 32 shows a flow diagram of notching, rounding, and
separating process with simplified side sectional views of a
generic workpiece as it undergoes process steps..Iaddend.
.Iadd.FIG. 33 shows a flow diagram of a rounding and separating
process..Iaddend.
.Iadd.FIG. 34 shows a flow diagram of an alternative rounding and
separating process..Iaddend.
.Iadd.FIG. 35 shows examples of excimer mask lines used for resist
removal, edge rounding, slicing, or dicing..Iaddend.
.Iadd.FIGS. 36a-36f are simplified side sectional views of a wafer
with multiple layers as it undergoes process steps of a generic
multi-step dicing or drilling process..Iaddend.
.Iadd.FIG. 37 is a simplified side sectional view of a wafer with
layers as it undergoes laser geometry modification prior to
subsequent cutting or drilling processes..Iaddend.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
FIGS. 3 and 4 illustrate alternative embodiments of respective
exemplary laser processing systems 10a and 10b (generically 10)
utilizing a compound beam positioning system 30 equipped with a
wafer chuck assembly 100 that can be employed for performing
segmented cutting, such as trenching, slicing, or dicing
semiconductor workpieces 12, in accordance with the present
invention. With reference to FIGS. 3 and 4, an exemplary embodiment
of a laser system 10 includes a Q-switched, diode-pumped (DP),
solid-state (SS) UV laser 14 that preferably includes a solid-state
lasant such as Nd:YAG, Nd:YLF, .Iadd.Nd:YAP, .Iaddend.or
Nd:YVO.sub.4.Iadd., or a YAG crystal doped with holmium or
erbium.Iaddend.. Laser 14 preferably provides harmonically
generated UV laser output 16 of one or more laser pulses at a
wavelength such as 355 nm (frequency tripled Nd:YAG), 266 nm
(frequency quadrupled Nd:YAG), or 213 nm (frequency quintupled
Nd:YAG) with primarily a TEM.sub.00 spatial mode profile.
In a preferred embodiment, laser 14 includes a Model 210-V06 (or
Model Q301) Q-switched, frequency-tripled Nd:YAG laser, operating
at about 355 nm with 5 W at the work surface, and commercially
available from Lightwave Electronics of Mountain View, Calif. This
laser has been employed in the ESI Model 2700 micromachining system
available from Electro Scientific Industries, Inc. of Portland,
Oreg. In an alternative embodiment, a Lightwave Electronics Model
210-V09 (or Model Q302) Q-switched, frequency-tripled Nd:YAG laser,
operating at about 355 nm may be employed in order to employ high
energy per pulse at a high pulse repetition frequency (PRF).
Details of another exemplary laser 22 are described in detail in
U.S. Pat. No. 5,593,606 of Owen et al. Skilled persons will
appreciate that other lasers could be employed and that other
wavelengths are available from the other listed lasants. Although
laser cavity arrangements, harmonic generation, .[.and.]. Q-switch
operation, and positioning systems 30 are all well known to persons
skilled in the art, certain details of some of these components
will be presented within the discussions of the exemplary
embodiments.
Although Gaussian may be used to describe the irradiance profile of
laser output 16, skilled persons will appreciate that most lasers
14 do not emit perfect Gaussian output 16 having a value of
M.sup.2=1. For convenience, the term Gaussian is used herein to
include profiles where M.sup.2 is less than or equal to about 1.5,
even though M.sup.2 values of less than 1.3 or 1.2 are preferred. A
typical optical system produces a Gaussian spot size of about 10
.mu.m, but this may easily be modified to be from about 2-100
.mu.m. Alternatively, an optical system producing a top hat beam
profile and or employing a mask, such as described later herein,
may be used to create a predetermined spot size. The pulse energy
used for cutting silicon using this focused spot size is greater
than 200 .mu.J, and preferably greater than 800 .mu.J, per pulse at
pulse repetition frequencies greater than 5 kHz and preferably
above 10 kHz. An exemplary setting provides 9.1 W at 13 kHz. An
exemplary laser pulsewidth measured at the full width half-maximum
points is less than 80 ns. Alternative and/or complementary
exemplary process windows include, but are not limited to, about
3.5-4.5 W UV at the work surface at about 10 kHz through about
20-30 W UV at 20-30 kHz, such as 15 W at 15 kHz.
UV laser output 16 is optionally passed through a variety of
well-known expansion and/or collimation optics 18, propagated along
an optical path 20, and directed by a beam positioning system 30 to
impinge laser system output 32 of one or more pulses on a desired
laser target position 34 on workpiece 12 such as a silicon wafer.
An exemplary beam positioning system 30 may include a translation
stage positioner that may employ at least two transverse stages 36
and 38 that support, for example, X, Y, and/or Z positioning
mirrors 42 and 44 and permit quick movement between target
positions 34 on the same or different workpieces 12.
In an exemplary embodiment, the translation stage positioner is a
split-axis system where a Y stage 36, typically moved by linear
motors along rails 46, supports and moves workpiece 12, and an X
stage 38, typically moved by linear motors along rails 48, supports
and moves a fast positioner 50 and associated focusing lens(es) or
other optics 58 (FIG. 7). The Z dimension between X stage 38 and Y
stage 36 may also be adjustable. The positioning mirrors 42 and 44
align the optical path 20 through any turns between laser 14 and
fast positioner 50, which is positioned along the optical path 20.
The fast positioner 50 may for example employ high resolution
linear motors or a pair of galvanometer mirrors 60 (FIG. 7) that
can effect unique or repetitive processing operations based on
provided test or design data. The stages 36 and 38 and positioner
50 can be controlled and moved independently or coordinated to move
together in response to panelized or unpanelized data. A split axis
positioning system 30 is preferred for use in large area of travel
applications, such as cutting 8'' and especially 12'' wafers.
Fast positioner 50 may also include a vision system that can be
aligned to one or more fiducials on the surface of the workpiece
12. Beam positioning system 30 can employ conventional vision or
beam to work alignment systems that work through objective lens 58
or off axis with a separate camera and that are well known to
skilled practitioners. In one embodiment, an HRVX vision box
employing Freedom Library software in a positioning system 30 sold
by Electro Scientific Industries, Inc. is employed to perform
alignment between the laser system 10 and the target positions 34
on the workpiece 12. Other suitable alignment systems are
commercially available. The alignment systems preferably employ
bright-field, on-axis illumination, particularly for specularly
reflecting workpieces like lapped or polished wafers.
For laser cutting, the beam positioning system 30 is preferably
aligned to conventional typical saw cutting or other fiducials or a
pattern on wafer surface. .Iadd.These may include existing features
on the devices, pole tips or rails of sliders, dedicated alignment
targets, vias or alignment marks which have been previously drilled
through the wafer. .Iaddend.If the workpieces 12 are already
mechanically notched, alignment to the cut edges is preferred to
overcome the saw tolerance and alignment errors. Beam positioning
system 30 preferably has alignment accuracy of better than about
3-5 .mu.m, such that the center of the laser spot is within about
3-5 .mu.m of a preferred cutting path, particularly for laser beam
spot sizes such as 10-15 .mu.m. For smaller spot sizes, the
alignment accuracy may preferably be even better. For larger spot
sizes, the accuracy can be less precise.
In addition, beam positioning system 30 may also employ
non-contact, small-displacement sensors to determine Abbe errors
due to the pitch, yaw, or roll of stages 36 and 38 that are not
indicated by an on-axis position indicator, such as a linear scale
encoder or laser interferometer. The Abbe error correction system
can be calibrated against a precise reference standard so the
corrections depend only on sensing small changes in the sensor
readings and not on absolute accuracy of the sensor readings. Such
an Abbe error correction system is described in detail in
International Publication No. WO 01/52004 A1 published on Jul. 19,
2001 and U.S. Publication No. 2001-0029674 A1 published on Oct. 18,
2001. The relevant portions of the disclosure of the corresponding
U.S. patent application Ser. No. 09/755,950 of Cutler are herein
incorporated by reference.
Many variations of positioning systems 30 are well known to skilled
practitioners and some embodiments of positioning system 30 are
described in detail in U.S. Pat. No. 5,751,585 .Iadd.and/or U.S.
Pat. No. 5,847,960 .Iaddend.of Cutler et al. The ESI Model 2700 or
5320 micromachining systems available from Electro Scientific
Industries, Inc. of Portland, Oreg. are exemplary implementations
of positioning system 30. Other exemplary positioning systems such
as a Model series numbers 27xx, 43xx, 44xx, or 53xx, manufactured
by Electro Scientific Industries, Inc. in Portland, Oreg., can also
be employed. Some of these systems which use an X-Y linear motor
for moving the workpiece 12 and an X-Y stage for moving the scan
lens are cost effective positioning systems for making long
straight cuts. Skilled persons will also appreciate that a system
with a single X-Y stage for workpiece positioning with a fixed beam
position and/or stationary galvanometer for beam positioning may
alternatively be employed. Those skilled in the art will recognize
that such a system can be programmed to utilize toolpath files that
will dynamically position at high speeds the focused UV laser
system output pulses 32 to produce a wide variety of useful
patterns, which may be either periodic or non-periodic.
An optional laser power controller 52, such as a half wave plate
polarizer, may be positioned along optical path 20. In addition,
one or more beam detection devices 54, such as photodiodes, may be
downstream of laser power controller 52, such as aligned with a
positioning mirror 44 that is adapted to be partly transmissive to
the wavelength of laser output.[...]. 16. Beam detection devices 54
are preferably in communication with beam diagnostic electronics
that convey signals to modify the effects of laser power controller
52.
Laser 14 and/or its Q-switch, beam positioning system 30 and/or its
stages 36 and 38, fast positioner 50, the vision system, any error
correction system, the beam detection devices 54, and/or the laser
power controller 52 may be directly or indirectly coordinated and
controlled by laser controller 70.
With reference to FIG. 4, laser system 10b employs at least two
lasers 14a and 14b that emit respective laser outputs 16a and 16b
that are linearly polarized in transverse directions and propagate
along respective optical paths 20a and 20b toward respective
reflecting devices 42a and 42b. An optional waveplate 56 may be
positioned along optical path 20b. Reflecting device 42a is
preferably a polarization sensitive beam combiner and is positioned
along both optical paths 20a and 20b to combine laser outputs 16a
and 16b to propagate along the common optical path 20.
Lasers 14a and 14b may be the same or different types of lasers and
may produce laser outputs 16a and 16b that have the same or
different wavelengths. For example, laser output 16a may have a
wavelength of about 266 nm, and laser output 16b may have a
wavelength of about 355 nm. Skilled persons will appreciate that
lasers 14a and 14b may be mounted side by side or one on top of the
other and both attached to one of the translation stages 36 or 38,
or lasers 14a and 14b can also be mounted on separate independently
mobile heads. The firing of lasers 14a and 14b is preferably
coordinated by laser controller 70. Laser system 10b is capable of
producing very high energy laser output pulses 32b. A particular
advantage of the arrangement shown in FIG. 4 is to produce a
combined laser output 32 impinging on the work surface having an
increased energy per pulse which could be difficult to produce from
a conventional single laser head. Such an increased energy per
pulse can be particularly advantageous for ablating deep trenches,
or slicing or dicing through thick silicon wafers or other
workpieces 12.
Despite the substantially round profile of laser system output
pulse 32, improved beam shape quality may be achieved with an
optional imaged optics module 62 whereby unwanted beam artifacts,
such as residual astigmatism or elliptical or other shape
characteristics, are filtered spatially. With reference to FIG. 5,
imaged optics module 62 may include an optical element 64, a lens
66, and an aperture mask 68 placed at or near the beam waist
created by the optical element 64 to block any undesirable side
lobes and peripheral portions of the beam so that a precisely
shaped spot profile is subsequently imaged onto the work surface.
In an exemplary embodiment, optical element 64 is a diffractive
device or focusing lens, and lens 66 is a collimating lens to add
flexibility to the configuration of laser system 10.
Varying the size of the aperture can control the edge sharpness of
the spot profile to produce a smaller, sharper-edged intensity
profile that should enhance the alignment accuracy. In addition,
with this arrangement, the shape of the aperture can be precisely
circular or also be changed to rectangular, elliptical, or other
noncircular shapes that can be aligned parallel or perpendicular to
a cutting direction. The aperture of mask 68 may optionally be
flared outwardly at its light exiting side. For UV laser
applications, mask 68 in imaged optics module 62 preferably
comprises sapphire. Skilled persons will appreciate that aperture
mask 68 can be used without optical elements 64 and 66.
In an alternative embodiment, optical element 64 includes one or
more beam shaping components that convert laser pulses having a raw
Gaussian irradiance profile into shaped (and focused) pulses that
have a near-uniform "top hat" profile, or particularly a
super-Gaussian irradiance profile, in proximity to an aperture mask
68 downstream of optical element 64. Such beam shaping components
may include aspheric optics or diffractive optics. In one
embodiment, lens 66 comprises imaging optics useful for controlling
beam size and divergence. Skilled persons will appreciate that a
single imaging lens component or multiple lens components could be
employed. Skilled persons will also appreciate, and it is currently
preferred, that shaped laser output can be employed without using
an aperture mask 68.
In one embodiment, the beam shaping components include a
diffractive optic element (DOE) that can perform complex beam
shaping with high efficiency and accuracy. The beam shaping
components not only transform the Gaussian irradiance profile to a
near-uniform irradiance profile, but they also focus the shaped
output to a determinable or specified spot size. Although a single
element DOE is preferred, skilled persons will appreciate that the
DOE may include multiple separate elements such as the phase plate
and transform elements disclosed in U.S. Pat. No. 5,864,430 of
Dickey et al., which also discloses techniques for designing DOEs
for the purpose of beam shaping. The shaping and imaging techniques
discussed above are described in detail in International
Publication No. WO 00/73013 published on Dec. 7, 2000. The relevant
portions of the disclosure of corresponding U.S. patent application
Ser. No. 09/580,396 of Dunsky et al., filed May 26, 2000 are herein
incorporated by reference. .Iadd.Alternatively, the shaped laser
output can be employed without using an aperture..Iaddend.
.Iadd.Employing a clipped or imaged shaped Gaussian beam may
facilitate better singulation in a multi-step process. In addition
to facilitating greater spot shape control and consistency and
depth control (particularly for imaged shaped), beam spots with
minimized tails generate redep debris that are more easily cleaned
by nonaggressive cleaning techniques than redep debris generated by
unmodified Gaussian beam spots. Furthermore, the uniform irradiance
profile facilitates the selectivity of a notch cut of a film over
an underlying substrate or another underlying film since there is
little or no change in the illuminated intensity across the spot,
allowing better selectivity between different
materials..Iaddend.
.Iadd.FIGS. 5A-5F are simplified and partly schematic views of
several possible embodiments of a laser processing systems 10a-10f,
including respective lasers 14a-14f, lenses 58a-58f, and suitable
for multi-step laser dicing or drilling of respective workpieces
12a-12f. With reference to FIGS. 5A-5F, a preferred embodiment of a
laser processing system of the present invention includes two
lasers with two separate beam delivery paths impinging upon the
same wafer. Variants of this embodiment include having the laser
beams impinge from opposite sides of the wafer, having the laser
beams impinge at adjacent positions with the ability to move the
wafer between the two positions, or having the system handle two
wafers at the same time, each of which is cut by one laser beam.
The system in the last case may be required to have the ability to
flip the wafer as it passes from one laser head position to the
other. Other variants which may work in some cases include the use
of a system which has two lasers which are selectable and create
beams which impinge on the wafer through the shared scan head and
scan lens, a system with one laser cavity but which has an
insertable harmonic generation device 15, or the use of only one
laser which has a range of process parameters suitable for cutting
the layers and wafer with their own individually optimized
process..Iaddend.
For the purpose of providing increased flexibility in the dynamic
range of energy per pulse, a fast response amplitude control
mechanism, such as an acousto-optic modulator or electro-optic
modulator may be employed to modulate the pulse energy of
successive pulses. Alternatively, or in combination with the fast
response amplitude control mechanism, the pulse repetition
frequency may be increased or decreased to effect a change in the
pulse energy of successive pulses. FIG. 6 displays the
characteristic relationship between pulse energy and pulse
repetition frequency (PRF) of a laser 14 employed during practice
of the invention. As FIG. 6 indicates, pulse energies of greater
than 200 .mu.J can be obtained from the Model 210-V06. In addition,
the characteristic relationship between pulse energy and PRF for
alternative lasers, Lightwave Electronics 210-V09L and Lightwave
Electronics 210-V09H, are also shown. Those skilled in the art will
appreciate that FIG. 6 is illustrative of the principal described
and alternate embodiments of laser system 10 will produce different
characteristic relationships between pulse energy and pulse
repetition frequency.
FIG. 7 depicts a simplified monitoring system 80 that employs one
or more sensors 82 optically in communication with the target
position 34 on the workpiece 12. In one embodiment, a mirror 84 is
positioned along the optical path 20, upstream or downstream of
fast positioner 50, and is transmissive to the outgoing beam but
reflects any incoming radiation to the sensors 82. Skilled persons
will appreciate, however, that mirrors and other optics associated
with monitoring system 80 may be aligned completely independently
from optical path 20 and a variety of detection techniques can be
employed. The sensors 82 of monitoring system 80 may be sensitive
to the intensity, albedo, wavelength and/or other properties of
light emitted, scattered, or reflected from the target material or
support material positioned beneath it. Sensors 82 may, for
example, be photodiodes and may include or form part of beam
detection devices 54. Typically, sensors 82 detect less feedback
when the cut path 112 (FIG. 10) is open. Sensors 82 may, for
example, communicate with laser controller 70 and/or beam
positioning system 30 to provide the cut-status information
continuously or for one or more discrete points along a given
segment 122 (FIG. 10). By employing real-time monitoring of the
completed and uncompleted portions or areas of the cut path 112,
the laser system 10 through a beam positioning system 30 can direct
the laser system output 32 only to portions of the cut path 112
that need additional cutting. This monitoring and selective segment
processing reduce the amount of time spent along a traditional cut
path 112 impinging already-completed portions along the entire
path. Thus, cutting throughput is improved.
FIG. 8 depicts a cut path 112 having respective first and second
transverse directions 92 and 94. Laser system 10 optionally employs
a polarization tracking system 90 (FIG. 3) that includes a
polarization control device, such as a rotatable half wave-plate or
a Pockel's cell, to change the polarization direction or
orientation of laser system output 32 to track changes in the
cutting path direction. The polarization control device may be
positioned upstream or downstream of fast positioner. When laser
system output 32 is in a trench and moving relative to the target
material, the laser system output 32 impinges the target material
at a nonnormal angle, resulting in a polarization effect that is
not present when impingement is nonmoving and normal to the target
material. Applicants have noted that coupling efficiency and
therefore throughput are increased when the polarization direction
is in a particular orientation with respect to the cutting
direction. Therefore, the polarization tracking system 90 may be
employed to keep the polarization orientation in an orientation
that maximizes throughput. In one embodiment, polarization tracking
system 90 is implemented to keep the polarization orientation
parallel with the cutting direction or orientation to increase the
coupling energy of the laser system output into the target
material. In one example, when cutting directions 92 and 94 differ
by an angle theta, the half waveplate is rotated by theta/2 to
change a first polarization orientation 96 to a second polarization
orientation 98 to match the cutting direction change of theta.
The polarization control device may also be implemented as a
variable optical retarder, such as a Pockel's cell. A drive circuit
conditions a polarization state control signal, which the drive
circuit receives from a processor associated with beam positioning
system 30 and/or laser controller 70. In this example, there is a
one-to-one correspondence between the magnitude of the polarization
state control signal and a beam positioning signal such that the
polarization direction of the light beam is maintained generally
parallel to its cutting path. U.S. Pat. No. 5,057,664 of Johnson et
al. describes a method for correlating the direction of beam
polarization with trimming direction. Skilled persons will
appreciate that the optimized polarization orientation versus
cutting direction may vary with laser systems and materials, such
that the preferred polarization orientation may be parallel,
vertical, orthogonal, elliptical (with the long axis in any given
orientation), or any other orientation with respect to the laser
pass or cutting direction.
FIG. 9 is a representative illustration of a chuck assembly 100 on
which silicon workpieces 12 are preferably placed for
.[.throughout.]. .Iadd.throughcut .Iaddend.processing using an
ultraviolet segment cutting method. Chuck assembly 100 preferably
includes a vacuum chuck base 102, a chuck top 104, and an optional
retaining carrier 106 placed over chuck top 104 for the purpose of
supporting a silicon workpiece 12 and retaining it after a
.[.throughout.]. .Iadd.throughcut .Iaddend.application. Base 102 is
preferably made from traditional metal material and is preferably
bolted to an additional plate 108 (FIG. 3). Plate 108 is adapted to
be easily connected to and disengaged from at least one of the
stages 36 or 38. The engagement mechanism is preferably mechanical
and may include opposing grooves and ridges and may include a
locking mechanism. Skilled .[.person.]. .Iadd.persons .Iaddend.will
appreciate that numerous exact alignment and lock and key
mechanisms are possible. Skilled persons will also appreciate that
the base 102 may alternatively be adapted to be secured directly to
the stages 36 or 38.
Chuck top 104 and optional retaining carrier 106 may be fabricated
from a material that has low reflectivity (is relatively absorbent
or relatively transparent) at the ultraviolet wavelength selected
for the particular patterning application to minimize backside
damage to silicon workpieces 12 around through trenches from
reflective energy coming off the metal chuck top after through
processing has been completed. In one embodiment, chuck top 104 or
retaining carrier 106 may be fabricated from an ultraviolet
absorbing material, such as Al or Cu, in order that laser system 10
may use a tool path file of the pattern of shallow cavities to be
drilled into the workpiece 12 to cut the corresponding pattern into
the material of chuck top 104 and/or retaining carrier 106. The
cavities may, for example, correspond to intended throughcuts and
prevent backside damage to the workpiece 12 during .[.throughout.].
.Iadd.throughcut .Iaddend.operations. In addition, any debris from
the process may settle into the cavities away from the backside of
workpiece 12. In one preferred embodiment, the pattern of the
shallow cavities is processed to have dimensions slightly larger
than those of the corresponding workpieces 12 after processing,
thereby enabling processed workpieces 12 to settle into the
cavities of the retaining carrier 106. A retaining carrier 106 with
cavities or through holes may be very thick to increase the
distance between chuck top 104 and the focal plane. Retaining
carrier 106 may also be machined to contain shallow cavities into
which the processed silicon workpieces 12 settle after through
processing operations. In an alternative embodiment, where 355 nm
output is employed, a UV-transparent chuck top 104 may be
fabricated from ultraviolet-grade or excimer grade fused silica,
MgF.sub.2, or CaF.sub.2. In another embodiment, UV-transparent
chuck top 104 may alternatively or additionally be liquid-cooled to
assist in maintaining the temperature stability of the silicon
workpieces 12. More details concerning exemplary chuck assemblies
100 can be found in the '497 application of Baird et al.
The above-described performance characteristics of UV laser system
10 can be used for high-speed cutting of semiconductors, and
particularly silicon. Such cutting operations may include, but are
not limited to, formation or trepanning of large diameter vias
through or partially through silicon wafers or other silicon
workpieces 12; formation of through or partly through trenches of
complex geometry for the purpose of singulation of processed die on
silicon wafers or silicon workpieces 12; formation of microtab
features to separate microcircuits formed in silicon from parent
wafers; formation of features on and/or singulation of AWGs and
sliders; and formation of features in MEMS. In addition, the
present invention facilitates feature formation without significant
melt lip formation, without significant slag formation, and without
significant peel back of the feature edge.
Applicants have discovered that laser cut rates for silicon, and
other like materials, can be significantly improved by segment
scanning or cutting instead of traditional methods of full path
cutting. The processing throughput can be enhanced by appropriate
selection of segment length, segment overlap, and/or overlap of
subsequent passes within each segment, as well as by selection of
other processing parameters.
By segment cutting, the consequences of material backfill in the
cut trench may be avoided or minimized. FIG. 2B suggests that
trench backfill may be a significant limitation to dicing speed. It
is proposed that by making quick short open segments or
subsegments, the laser system 10 can provide an avenue for much of
the laser ejected material to escape rather than refill the
trenches as they are being cut. Hence, reduced trench backfill will
decrease the number of passes necessary to cut through a given
portion of the cut path 112. FIGS. 10-17 present exemplary
segmented cutting profiles 110a-110f (generically profiles 110)
employed in the present invention. The techniques presented below
generally permit a 750 .mu.m-thick silicon wafer to be cut with
only about 4 W UV laser power at 10 kHz in about 26 or fewer passes
compared to the 150 passes needed using a conventional laser
cutting profile.
FIG. 10 depicts a simplified representation of an exemplary
segmented cutting profile 110a of the present invention. With
reference to FIG. 10, cutting profile 110a is shown, for
convenience, having a path cutting direction (indicated by the
direction of the arrow) from left to right along cut path 112 and
having generally distinct cutting segments 122a, 122b, and 122c
(generally, cutting segments 122) formed in a segment cutting
direction (or laser pass direction) that is the same as the path
cutting direction by respective groups of passes 132a, 132b, and
132c (generically, laser passes 132) of laser system output 32. In
this example, the lengths of the laser passes 132 substantially
equal the lengths 126 of the segments 122. Skilled persons will
appreciate that cutting profile 110a, and subsequent exemplary
cutting profiles 110, may preferably include from two to an
infinite number of cutting segments 122, depending on total
respective lengths 124 of cutting profiles 110.
FIG. 11 is a simplified plan view of an enlarged cutting segment
122 sequentially impinged by slightly overlapping spots having a
spot area of diameter, d.sub.spot, on workpiece 12. With reference
to FIG. 11, although the spot area and d.sub.spot generally refer
to the area within the outside edge of the laser spot when the
laser power falls to 1/e.sup.2 of the laser peak power, these terms
are occasionally used to refer to the spot area or diameter of the
hole created by a single pulse or the width of a kerf created in a
single pass of pulses. The difference between the 1/e.sup.2
dimension and the kerf diameter will vary with the laser, the
material, and other parameters.
The distance of new target material impinged by each sequential
laser pulse is called the bite size d.sub.bite. A preferred bite
size d.sub.bite for laser cutting of materials of interest, such as
silicon, includes an advantageous bite size range of about 0.5
.mu.m to about d.sub.spot, and more preferably a range of about
1-50 .mu.m, with a typical range of about 1-5.5 .mu.m, and most
typically a bite size of about 1 .mu.m. For some materials,
adjusting the bite size results in a condition where the redep
debris generated may be easier to remove. The bite size can be
adjusted by controlling the speed(s) of the laser beam positioning
system 30 and coordinating the movement speed (s) with the
repetition rate of the firing of the laser 14.
With reference again to FIGS. 10 and 11, generally a preferred
length 126 for cutting segments 122 may be dependent on the
characteristics of the material being processed, its thickness, and
the response time of the positioning system 30, including its
acceleration/deceleration limits, degree of ringing of the
mechanical components, and return movement time. For example, if
segments are too short, the number of segments for a given cut will
be very large, and the amount of time lost to change of direction
between passes will be very large. Thus, positioning system
characteristics may impact determination of the minimum segment
length. Segment length 126 may be a function of bite size,
repetition rate, and positioning system performance as well as
other possible factors, and each or all of these factors may be
optimized based on laser pulse intensity. Skilled persons will
appreciate that segments 122a-122c need not have the same lengths
126.
Generally each segment 122 is scanned substantially collinearly
with consecutive passes 132 of laser output 32 (skipping over
completely processed portions) until it is completely processed,
e.g. a throughcut is made along the entire length 126 of the
segment 122 or until the target material is trenched to a desired
depth before a subsequent segment 122 is processed. If snapstrates
are desired, a series of discontinuous throughcuts may be
desirable, or no through hole cutting may be desirable and nearly
throughcut trenches may be desirable. One to several scans across
the entire cut path length can be optionally employed in the
process, particularly before and/or after the segment cutting
steps, to maximize the throughput and/or improve the cut
quality.Iadd.. .Iaddend.Typically, a through hole can be made in
each segment in from 5-10 laser passes such that some of the debris
can escape through the through holes. However, if desired, each
segment 122 can be processed with multiple passes to an
intermediate depth, and the cutting profile can be reapplied,
perhaps even in the opposite direction if desirable. If segments
are initially processed only to a status where they each have a
through hole in one portion, then it may also be advantageous in
some circumstances to implement a traditional cutting profile as
soon as all the segments 122 include significant through holes. To
distinguish from laser punching, skilled persons will appreciate
that the segment length 126 is greater than d.sub.spot.
Furthermore, laser punching each spot to create a through hole
before moving along the cut path 112 would take longer, possibly
damage the target material, and cause other less favorable
results.
In an exemplary embodiment, for cutting thick silicon, each segment
122 has a segment length 126 of about 10 .mu.m to 1 mm, typically
from about 100 .mu.m to 800 .mu.m, and most preferably from about
200 .mu.m to 800 .mu.m. With respect to cutting profile 110a,
segments 122 are preferably slightly overlapped by an overlap
distance 136 that may be as small as the bite size or larger than
several spot sizes. However, skilled persons will appreciate that
the final pass processing segment 122a and the first pass
processing segment 122b may be combined into a double length
segment 122 (without overlap). Although it is preferred to maintain
the same laser parameters during any given pass 132 along a segment
122, skilled persons will appreciate that it is possible to change
laser parameters during any given pass 132 to accommodate specific
applications.
FIG. 12 depicts a simplified representation of an exemplary
segmented cutting profile 110b. With reference to FIG. 12, cutting
profile 110b is shown, for convenience, having a path cutting
direction from left to right and having distinct cutting segments
122d, 122e, and 122f (generally, cutting segments 122) formed from
respective laser passes 132d, 132e, and 132f in a segment cutting
direction that is opposite the path cutting direction. Thus,
segment 122d is processed from right to left and then segment 122e
is processed from right to left, etc.
An advantage of cutting profile 110b over cutting profile 110a is
that the debris generated while cutting segment 122d is generally
scattered in the direction of segment 122e (backwards with respect
to the laser pass direction) where there is no preexisting trench
to be backfilled by the debris. Any such debris that does land
along the subsequent segment 122 to be cut will be immediately
processed. In addition, since the path cutting direction is
opposite the segment cutting direction, the debris generated will
generally not occlude the trench of the previously cut segment 122.
Skilled persons will appreciate that other than the difference
between path cutting direction and segment cutting direction, most
of the discussion concerning FIGS. 10 and 11 is germane to FIG.
12.
FIG. 13 depicts a simplified representation of an exemplary
segmented cutting profile 110c. With reference to FIG. 13, cutting
profile 110c is shown, for convenience, having a path cutting
direction from left to right and having distinct cutting segments
122g, 122h, and 122i (generally, cutting segments 122) formed from
respective laser passes 132g, 132h, and 132i that each proceed from
left to right and from right to left in a back and forth
overlapping scanning fashion. In particular, segment 122h is first
processed from left to right and then from right to left, etc.
until it is completely processed, for example, and then segment
122i is similarly processed. Because the segments 122 are being
processed in both directions, the nonprocessing movement returns of
the positioning system 30 is eliminated, resulting in a higher
usage of the system capability. Because a laser pass 132 may take
longer than nonprocessing movement returns of the positioning
system 30, segments 122 in FIG. 13 may be shorter than those used
in FIGS. 10 and 12 in applications where it is desirable to impinge
debris or exposed portions of a trench within a prescribed period
of time from the previous impingement. Other than some of the
details specified above, most of the discussion concerning FIGS.
10-12 is germane to the example in FIG. 13.
FIG. 14 depicts a simplified representation of an exemplary
segmented cutting profile 110d. With reference to FIG. 14, cutting
profile 110d is shown, for convenience, having a path cutting
direction from left to right along cut path 112 and having distinct
cutting segments 122j, 122k, and 122m (generally, cutting segments
122) formed from right to left. FIG. 14 also depicts multiple,
substantially collinear laser pass sets 140.sub.1, 140.sub.2, and
140.sub.3 (generically laser pass sets 140), each comprising an
initial pass .[.132.sub.k.]. .Iadd.132k .Iaddend.and multiple
gradually lengthening overlapping and substantially collinear
passes .[.132.sub.m-132.sub.r.]. .Iadd.132m-132r.Iaddend.,
preferably processed in alphabetical order. Although cutting passes
132k.sub.1-132r.sub.3 are depicted as parallel in FIG. 14 for
convenience, cutting passes 132k.sub.1-132r.sub.3 are preferably
substantially collinear and collinear with the respective segments
122.
Unlike the slight optional overlaps between adjacent segments 122
associated with the examples in FIGS. 10, 12, and 13, the overlap
lengths associated with adjacent segments 122 or passes 132 in this
and the following examples are typically greater than about 10%,
more typically greater than about 25%, and most typically greater
than about 50%, and occasionally exceeding 67% or 85%. In one
particular example where a 300 .mu.m segment is employed, an
overlap length of 200 .mu.m is employed; and in another example
where a 500 .mu.m segment length is employed, a 250 .mu.m overlap
length is employed.
One reason to employ laser passes 132 that have different end
points within a segment 122 is to prevent a "scan end" effect where
more material is stacked at the end of segment 122 whenever it is
processed by identical overlapping passes 132. Thus, an advantage
of lengthening of consecutive passes 132 or consecutive small
groups of passes is to spread the scan effect over longer cut
lengths so that the cutting speed across an entire segment 122 or
the entire cut path 112 becomes more uniform, thereby enhancing
throughput and cut quality. The scan effect on quality can also be
mitigated by employing full cutting path length scans or passes 132
after the segment cutting process is finished.
Preferably, each pass 132 is employed only once and each laser set
140 is employed only once to process the respective segment 122 to
a desired intermediate depth or to a complete through cut before
the next segment 122 is processed. Alternatively, laser set 140, of
cutting passes 132k.sub.1-132r.sub.1 can be repeated until a
throughcut is made along some or all of segment 122j, then
subsequent laser sets 140 can be repeated segment by segment until
the entire cut path 112 is throughcut. Although only five
overlapping passes 132 are shown for each laser pass set 140,
skilled persons will appreciate that a substantially greater number
of overlapping passes 132 could be employed, particularly with
smaller incremental length increases as needed to accommodate the
thickness of the target material. Skilled persons will also
appreciate that any or all of the passes 132 employed in cutting
profile 110d could be processed in both directions instead of a
single direction as shown in FIG. 14. Skilled persons will also
appreciate that multiple applications of each laser pass set 140
could be employed, that multiple applications of one or more passes
132 in a laser pass set 140 could be employed, that the numbers of
each distinct pass 132 within a pass set 140 may differ, and that
the number of applications of laser pass sets 140 and laser passes
132 may differ during the processing of a single cut path 112. Any
of these variables may be adjusted in real time in response to
monitoring information. Other than the details specified above,
much of the discussion concerning FIGS. 10-13 is germane to the
example in FIG. 14.
FIG. 15 depicts a simplified representation of an exemplary
segmented cutting profile 110e that is somewhat similar to profile
110d, the cutting segments 122n, 122p, and 122q overlap to a
greater degree and the subsequent laser pass sets 140.sub.2a and
140.sub.2b omit laser passes 132k. With reference to FIG. 15,
profile 110e begins with the same laser pass set 140.sub.1 that
begins profile 110d. However, laser pass sets 140.sub.2a and
140.sub.2b omit laser passes 132k and their laser passes 132
increasingly overlap (about 86% in the following example) the
previously laser pass set 140. In one example of this embodiment,
laser pass 132k.sub.1, which has a length of 200 .mu.m, is applied
30 times. Then, laser pass 132m.sub.1, which has a length of 240
.mu.m (200 .mu.m plus 1/5 of the length of pass 132k.sub.1), is
applied 6 times (1/5 of 30 passes). Then, laser pass 132n.sub.1,
which has a length of 280 .mu.m (200 .mu.m plus of the length of
pass 132k.sub.1), is applied 6 times. This sequence is continued
until laser pass set 140.sub.1 is completed and then performed in
connection with laser pass sets 140.sub.2a and 140.sub.2b with
laser passes 132k omitted. In this example, the later portions of
each segment 122 may not be throughcut until some of the
subsequence segment 122 is processed. An advantage of overlapping
the segments 122 to include portions of cut path 112 that are
already throughcut is that any debris created by the shorter laser
passes 132 that is deposited on the sides of throughcut portions is
removed by the subsequent longer laser passes 132. The pass sets
140 in this example can exhibit dicing speeds of greater than or
equal to 8.5 mm/minute with a 3.5 W UV laser, operated at 10 kHz,
on a 750 .mu.m-thick silicon wafer.
FIG. 16 depicts a simplified representation of an exemplary
segmented cutting profile 110f.Iadd.. .Iaddend.With reference to
FIG. 16, cutting profile 110f is shown, for convenience, having a
path cutting direction from left to right and having distinct laser
passes 132s.sub.1-132t.sub.5 formed from right to left. Although
laser passes 132s.sub.1-132t.sub.5 are depicted as parallel in FIG.
16 for convenience, they are preferably substantially collinear.
FIG. 16 depicts an initial laser pass 132s and multiple gradually
lengthening overlapping passes 132s.sub.1-132t.sub.5, preferably
processed in numerical subscript order. In an exemplary embodiment,
the length of laser pass 132s is about 200 .mu.m or 300 .mu.m and
the length of each subsequent laser pass 132t is about 500 .mu.m.
This exemplary profile can yield dicing speeds of greater than or
equal to 10.4 mm/minute with a 3.5 W UV laser, operated at 10 kHz,
on a 750 .mu.m-thick silicon wafer. For shallow trenches, each pass
132 may be applied only once, and for throughcuts in thick target
materials, each pass 132 may be applied multiple times before the
next sequential pass 132 is undertaken. Preferably, each laser pass
132 is applied multiple times to reach a selected intermediate
depth before the next laser pass 132 is processed. In one
embodiment, each consecutive laser pass 132 receives a single pass
of laser output 32 and then the entire profile 110f is repeated or
the laser passes 132 are processed in reverse order.
Although only five overlapping laser passes 132t are shown, skilled
persons will appreciate that a substantially greater number of
overlapping laser passes 132 could be employed, particularly with
smaller incremental length increases as needed to accommodate the
thickness of the target material. Skilled persons will also
appreciate that any or all of the laser passes 132 employed in
cutting profile 110f could be sequentially processed in both
directions instead of a single direction as shown in FIG. 16. Other
than the details specified above, much of the discussion concerning
FIGS. 10-15 is germane to the example in FIG. 16.
FIG. 17 depicts a simplified representation of an exemplary
segmented cutting profile 110g that is somewhat similar to profile
110f. With reference to FIG. 17, odd subscripted laser passes
132.sub.1, 132.sub.3, 132.sub.5, 132.sub.7, and 132.sub.9, have an
exemplary pass length of 200 .mu.m and even subscripted laser
passes 132.sub.2, 132.sub.4, 132.sub.6, and 132.sub.8 have an
exemplary pass length of 270 .mu.m. A group of one of these laser
passes 132 is delivered before the next sequential group is
delivered. In one example the odd subscripted laser passes 132 are
applied more times or to a greater relative depth (60% of cut depth
versus 40% of cut depth, for example) than the even subscripted
passes. This cutting profile with the exemplary pass lengths avoids
an overlap junction until 5.4 mm along the cut path 112. Skilled
persons will appreciate that a variety of cutting profiles and pass
lengths can be employed to reduce scan effects and backfill and
thereby facilitate enhanced throughput.
FIG. 18 is a representative illustration of ultraviolet ablative
patterning of a trench or .[.throughout.]. .Iadd.throughcut
.Iaddend.150 in a workpiece 12 such as a wafer having an intrinsic
silicon substrate 148 of a height or thickness 152 of 750 .mu.m
overlaid with a 0.5 .mu.m-thick passivation layer of SiO.sub.2 (not
shown). Those skilled in the art will recognize that the thickness
of the silicon workpieces and the thickness of the passivation
layers will vary.
The trench 150 is preferably patterned by positioning the silicon
workpiece 12 at the focal plane of the laser system 10 and
directing a string of successively overlapping laser system output
pulses 32 at the silicon workpiece 12 as the laser positioning
system 30 moves workpiece 12 along the X- and/or Y-axes of the
workpiece 12. The Z-height of the laser focus position can be
simultaneously moved coincident with each succeeding laser pass 132
to place the laser focus at a sequentially deeper position in the
silicon workpiece 12, thereby maintaining the focused spot at a
position more coincident with the remaining silicon surface.
For forming a trench or throughout 150 in silicon, an exemplary
energy per pulse range is about 100 .mu.J to 1500 .mu.J, with a
typical .[.a.]. energy per pulse range of about 200 .mu.J to 1000
.mu.J and a more typical energy per pulse range of about 400 .mu.J
to 800 .mu.J, and most preferably an energy per pulse over about
800 .mu.J is employed. An exemplary PRF range is about 5 kHz to 100
kHz, with a typical PRF range from about 7 kHz to 50 kHz and a more
typical PRF range from about 10 kHz to 30 kHz. Those skilled in the
art will recognize that the laser performance as shown in FIG. 6
can achieve energy per pulse output at PRFs within the typical
ranges described above. An exemplary focused spot size range is
about 1 .mu.m to 25 .mu.m, with a typical focused spot size range
from about 3 .mu.m to 20 .mu.m and a more typical focused spot size
range from about 8 .mu.m to 15 .mu.m. An exemplary bite size range
is about 0.1 .mu.m to 10 .mu.m, with a typical .[.a.]. bite size
range from about 0.3 .mu.m to 5 .mu.m and a more typical bite size
range from about 0.5 .mu.m to 3 .mu.m. The bite size can be
adjusted by controlling the speed of either or both of the stages
of the laser beam positioning system 30 and coordinating the
movement speed(s) with the repetition rate and firing of the laser.
An exemplary segment size is about 200 .mu.m to 800 .mu.m. An
exemplary combination employing a V06 laser on a 2700
micromachining system used a segment length of 300 .mu.m and a
segment overlap of 200 .mu.m provided a very fast dicing speed.
Skilled persons will appreciate that for different applications
with different lasers for processing different materials, the
preferred laser, segment, pass, and other parameters can be
extremely different.
In one example, a trench or .[.throughout.]. .Iadd.throughcut
.Iaddend.150 can be made through 750 .mu.m-thick intrinsic silicon
overlaid with a 2.0 .mu.m passivation layer of SiO.sub.2 using an
output pulse energy from the laser 14 of about 360 .mu.J and using
a bite size of 1 .mu.m with a stage velocity of 10 mm/s in fewer
than 25 passes over the length of a cut path 112 over an
8''-diameter workpiece 12 with laser pulses having a focused spot
size (1/e.sup.2) diameter of 12 .mu.m at the work surface. A trench
150 produced employing parameters described above may, for example,
have a top surface opening width (diameter) (d.sub.t) 154 of about
20 .mu.m and an exit width (diameter) (d.sub.b) 156 of about 13
.mu.m, thereby producing an aspect ratio for this trench of about
30:1 and an opening taper angle of 0.4.degree.. In some
applications, it may be desirable to create an initial though hole
before scanning a segment.
Persons skilled in the art will further appreciate that the
selected segmented profile and segment length and the values of
energy per pulse, focused spot size, and number of pulses employed
to efficiently produce high quality trenches or throughcuts 150 in
silicon may vary according to the material and thickness 152 of the
silicon workpiece 12, relative thickness and composition of
overlayers, of which Sio.sub.2 is only one example, and the
wavelength employed. For example, for production of throughcuts 150
in silicon only 50 .mu.m thick, fewer than ten passes may be
employed to produce the desired throughout.
Those skilled in the art will recognize that various patterns of
varying geometry, including, but not limited to, squares,
rectangles, ellipses, spirals, and/or combinations thereof, may be
produced through programming of a tool path file used by laser
system 10 and positioning system 30 to position silicon workpiece
12 along X and Y-axes during processing. For laser cutting, the
beam positioning system 30 is preferably aligned to conventional
typical saw cutting or other fiducials or a pattern on the wafer
surface. If the wafers are already mechanically notched, alignment
to the cut edges is preferred to overcome the saw tolerance and
alignment errors. The various segmented cutting profiles may be
preprogrammed into the tool path file or other positioning system
command files.
Laser system 10 can be employed to produce one or more groups of
small through holes, such as by laser punching using the laser
parameters set forth above. These through holes can be positioned
on the top side near the periphery of workpieces 12, circuits or
dies, or within scribing, slicing, or dicing streets or their
intersections such that the back or bottom side of workpiece 12 can
be precisely aligned to with respect to features on the top side.
Such alignment facilitates backside processing such as laser
scribing or sawing to enhance processing speed or quality.
Techniques for front and/or backside wafer slicing or dicing are
discussed in more detail in U.S. patent application Ser. No.
09/803,382 ('382 .[.application.]. .Iadd.Application.Iaddend.) of
Fahey et al., entitled "UV Laser Cutting or Shape Modification of
Brittle, High Melting Temperature Target Materials such as Ceramics
or Glasses,.Iadd." .Iaddend.which is incorporated herein by
reference. This information was published on Mar. 21, 2002 under
U.S. Patent Publication No. US-2001-0033558 and published on Mar.
28, 2002 under International Patent Publication No. WO 02/24396,
which correspond to the '382 .[.application.].
.Iadd.Application.Iaddend..
.Iadd.Sliders move from track to track of direct access storage
devices (DASD), such as disk drives including rotatable magnetic
recording disks, to read or record desired information on the
tracks. FIG. 21 is a deposited end perspective view of a trailing
edge 212 of a prior art slider 210, and FIG. 22 is a
cross-sectional view of trailing edge 212 of slider 210 with its
magnetic head 214 oriented toward a magnetic recording disk 220.
The figures accompanying this description are generally not drawn
to scale or in proportion. For example, in FIGS. 21 and 22, the
components of slider 210 are not drawn to scale or in proportion. A
conventional "pica" slider 210 may have a slider height, h.sub.s,
of about 300 microns (.mu.m), a slider width, w.sub.s, of about
1000 .mu.m, and a slider depth or length, l.sub.s, of about 1250
.mu.m..Iaddend.
.Iadd.With reference to FIGS. 21 and 22, a typical slider 210
includes a non-magnetic substrate 22 typically made of a ceramic
material. Substrate 222 typically has a substrate depth, d.sub.s,
of about 300 .mu.m deep and forms a majority of the body of slider
210. Substrate 222 generally, therefore, defines an air-bearing
surface (ABS) 224 having an aerodynamic configuration suitable for
lifting slider 210 a desired distance above the surface of disk 220
as it rotates. Transducer or magnetic head 214 has first and second
spaced-apart magnetic pole pieces 228 and 230 which are located in
proximity to trailing edge 212 of slider 210. Magnetic pole pieces
228 and 230 include first and second pole tips 232 and 234 that are
aligned with the air-bearing surface 224. A non-magnetic gap layer
236 is located between the first and second pole pieces 228 and
230. Additionally, an insulating layer 238 is positioned between
the non-magnetic layer 236 and the second magnetic pole piece 230.
The insulating layer 238 is typically made of a polymeric material
such as hard-baked photoresist, and a coil 240 is located within
insulating layer 238. Finally, an overcoat layer 242, typically
comprising 20-50 microns of a vacuum-deposited alumina
(Al.sub.2O.sub.3), covers magnetic head 214 and forms trailing edge
212 of slider 210..Iaddend.
.Iadd.FIGS. 23-25 illustrate various steps or stages of a method
for manufacturing typical sliders 210. FIG. 23 shows a deposited
end view of a ceramic wafer 250 supporting a plurality of sliders
210. The various layers of each slider 210 are built up layer by
layer upon the wafer 250 to form the previously described slider
features by deposition processes known to the semiconductor
industry. An exemplary technique for generating the layers of a
slider having a thin-film magnetic head is described in U.S. Pat.
No. 4,652,954..Iaddend.
.Iadd.Wafer 250 is then typically cut into sections and then sliced
into rows 260 along straight slicing lanes 262 by a mechanical
cutting blade to form coarse air-bearing surfaces 224 and generally
parallel nonair-bearing surfaces 264. The mechanical cutting
process creates sharp edges 266 and 268 (FIGS. 21 and 22) with
small chips along slicing lanes 262. Conventional slicing blades
typically have a narrow dimension of about 200-300 .mu.m along
their cutting axis and produce cuts that are wider than the blades.
The slicing blades currently need to be this wide to withstand
stresses of making straight cuts through the strength and thickness
of conventional slider wafers 250, for example. Thus, the lane
width, w.sub.1, between rows 260 of sliders 210 is greater than cut
width to accommodate cut width variations due to blade wear and
misalignments. Hence, the row pitch equals w.sub.1 plus h.sub.s,
and the maximum number of rows equals the usable wafer diameter,
d.sub.w, divided by the row pitch. A conventional row pitch is, for
example, 600 .mu.m..Iaddend.
.Iadd.Course air-bearing surfaces 224 formed in the wafer slicing
process are polished using advanced but cumbersome and
time-consuming lapping techniques and slurries. Rows 260 are
mounted on a fixture or carrier 270 after ABS polishing so that
multiple rows 260 can simultaneously be processed through
subsequent steps. The mounting procedure must employ an adhesive
between nonair-bearing surfaces 264 and carrier 270 that is
selected for sufficient mechanical strength to withstand the
stresses of a later step of mechanically dicing the rows 260 into
individual sliders 210. Unfortunately, these adhesives make it
difficult to debond sliders 210 from carrier 270 at a later
time..Iaddend.
.Iadd.FIG. 24 illustrates rows 260 of sliders 210 mounted on
carrier 270 and oriented so that the air-bearing surfaces 224 of
magnetic heads 214 are facing upwards. With reference to FIG. 24,
polished air-bearing surfaces 224 are covered by photoresist
pattern masks 272 that correspond with a desired air-bearing
surface configuration having aerodynamic characteristics suitable
for causing heads 214 to fly a desired level above disks 220.
Photoresist masks are formed by first coating the entire surface
with photoresist. Then, a masking tool having a predetermined
pattern is aligned relative to the pole tips 232 and 234 or other
fiducials, and light is directed through the masking tool so that
selected portions of the photoresist on the polished ABSs 224 are
exposed. Alignment of the masking tool is achieved by using a
stepper with row-bar alignment or a well-aligned contact/projected
aligner. After exposure, the photoresist is developed such that the
desired air-bearing surface configurations are left covered with
the photoresist masks 272, while the remainder of the photoresist
is removed..Iaddend.
.Iadd.Once rows 260 of sliders 210 have been masked with the
desired pattern of photoresist, the polished ABSs are etched by
etching techniques such as ion milling or reactive ion etching
which are expensive and slow. Such etching techniques etch away the
exposed regions 274 of surfaces 224 to a desired depth to form
raised covered regions or rails 276 underlying masks 272. The
photoresist mask 272 is finally stripped away to reveal the desired
patterns on the air-bearing sides of sliders 210..Iaddend.
.Iadd.With reference again to FIG. 24, rows 220 are diced by
mechanical dicing blade along straight dicing singulation or paths
278 to create edges 282. The dicing blades for this cutting
operation have a narrow dimension of about 75-150 .mu.m along their
cutting axis and produce cuts of about 150 .mu.m wide. Thus, the
path width, w.sub.p, between rows 260 of sliders 210 is slightly
greater. Hence, the slider pitch equals w.sub.p plus w.sub.s, and
the maximum number of sliders 210 per row 260 equals the row length
(or usable wafer diameter) divided by the slider pitch. A
conventional slider pitch is, for example, 1150 .mu.m for a 100
.mu.m wide dicing path. The dicing process creates small chips as
it creates sharp edges 282, 284, and 286 and sharp corners 285 and
287 (FIG. 21) along singulation paths 278..Iaddend.
.Iadd.FIG. 25 also shows carrier 270 supporting a number of rows
260a, 260b, 260c, and 260d (generically rows 260) prior to dicing
into individual sliders 210 with sides 280. Although row 260a
depicts a typical row 260, rows 260b, 260c, and 260d demonstrate
common slider manufacturing problems. Row 260b is relatively
straight but is fixed to carrier 270 such that it is askew to row
260a. Row 260c is also relatively straight and relatively parallel
to row 260a, but the pole tips 232 and 234 and/or the rails 276 of
row 260c are offset with respect to those in row 260a. Row 260d
exhibits row bow that may be primarily caused by stresses resulting
from the mechanical slicing of wafer 250 into rows
260..Iaddend.
.Iadd.Because the dicing blade must cut along straight singulation
paths 278, the sides 280 of sliders 210 in any column must be
aligned within about one-half of the remainder of the path width
minus the cut width. In view of the foregoing, rows 260b, 260c, and
260d can create a problem for the mechanical dicing operation and
may reduce yield of sliders 210 with acceptable magnetic or
aerodynamic properties. If the slant of row 260b is significant,
the edges 282 of sliders in row 260b are askew with respect to
rails 276, and the sliders 210 in row 260b will be defective.
Similarly, many of sliders 210 in bowed row 260d, especially those
at the ends for the case depicted, will be defective depending on
the significance and position of the curves. With respect to row
260c, if the ABS features are sufficiently offset with respect to
the other rows 260, then all sliders in row 260c will be defective
since the edges of the sliders will be in improper positions or the
dice paths will cut into ABS features..Iaddend.
.Iadd.The above-described process for manufacturing sliders 10 has
several other drawbacks. In particular, sharp edges 266, 268, 282,
284, and 286, sharp corners 285 and 287, and chips formed during
the dicing process make sliders 210 more susceptible to damage. For
example, external shocks, such as by dropping a disk drive on the
floor, can cause the sharp corners of the slider 210 to cut into
the disk media, can cause cracks to propagate, or can cause
particles to break loose at chipped locations which can then
interfere with the ability of head 214 to make proper contact with
disk 220. Polishing steps, which are time-consuming and employ
expensive reagents, do not generally eliminate these chips or sharp
edges..Iaddend.
.Iadd.In addition, the wide cuts made by the mechanical cutting
blades significantly reduce the number of rows 260 and sliders 210
that can be fit onto each wafer 250. Skilled persons will also note
that dicing blades tend to wear relatively quickly such that the
width of their cuts may vary over time. In some cases, the blades
can be inadvertently bent and then they produce curved or slanted
cuts or increased chipping..Iaddend.
.Iadd.U.S. Pat. Nos. 5,872,684 of Hadfield et al. ('684 Patent)
describes a method for etching a portion 288 of overcoat layer 242
wherein the etched portion 288 extends between the second pole tip
234 and trailing end 212 of slider 210. Etched portion 288 is
sloped with respect to air-bearing surface 224 of slider 210 and is
arranged and configured for preventing the overcoat layer from
protruding past the air-bearing surface upon expansion of overcoat
layer 242 during operation of magnetic head 214. Otherwise,
overcoat layer 214 could form a protruding portion 290 due to
localized heating when coil 240 is subjected to write currents and
could interfere with slider/disk contact. Photolithography masking
and etching techniques, like those described above, are used to
etch away the potential protrusion regions of alumina overcoat
layer 242. The '684 Patent does not address the dicing-generated
chips or other dicing-related reliability problems..Iaddend.
.Iadd.Accordingly, one embodiment of the present invention employs
a UV laser to cut ceramics, glasses, or silicon which may comprise
the body of sliders 210, and particularly separate rows 260 or
sliders 210 or round edges. A preferred process entails covering
the surfaces of wafers 250, rows 260, or sliders 210 with a
sacrificial layer such as photoresist; removing a portion of the
sacrificial layer to create uncovered zones along existing edges or
over intended edges; laser cutting wafers 250 into rows 260 or rows
260 into sliders 250; laser rounding edges 266, 268, 282, 284,
and/or 286, and/or corners 285 and/or 287; cleaning debris form the
uncovered zones such as by ion milling; and removing the
sacrificial layer. Another process sequence includes an initial
notching of the air-bearing surface 224 to form kerfs between rows
260 or sliders 210; laser processing to round the edges of the
corners formed during the notching; and a final cutting to separate
the rows or singulate the sliders..Iaddend.
.Iadd.FIGS. 26 and 27 are exemplary deposited end perspective views
of alternative slider embodiments after processing in accordance
with the invention as described herein. With reference to FIGS. 26
and 27, processed slider 350 exhibits rounded edges 352 where edges
282 have been processed by laser system output 330, and processed
slider 360 exhibits rounded edges 362, 364, and 366 where edges
282, 266, and 286 have been processed by laser system output.
Processed slider 360 also exhibits rounded corners 368 even when
corners 285 have not been separately and intentionally processed by
laser system output. Separately and intentionally processing
corners 285 provides, however, a greater radius of curvature.
Skilled persons will appreciate that upper edges 268 and/or 284
and/or upper corners 287 can also be rounded by laser system output
if desirable. Sliders 350 and 360 are less susceptible to external
shocks or chip generation than sliders 210, and sliders 350 and 360
can also ride closer to and make proper contact with disk
220..Iaddend.
.Iadd.FIG. 27A shows a variation of FIG. 27. With reference to FIG.
27A, a selected portion of edge 266 in proximity tip 369 is not
rounded. In general, selected portions of any edge can be left
unrounded whenever it is beneficial to do so. The positioning
system 314 can simply be instructed to pass over such
portions..Iaddend.
.Iadd.FIGS. 28a-28h (collectively FIG. 28) show simplified side
sectional views of a generic workpiece as it undergoes process
steps of an exemplary laser rounding process. In one embodiment, a
mechanical cutting blade separates rows 60 or sliders 10 along
lanes 62 or paths 78 to form surfaces 24 or sides 80, respectively.
The respective edges 66 and/or 82 can then be rounded with laser
system output. An advantage of this technique is that it suits the
established infrastructure in the industry. Another advantage of
mechanically cutting lanes 262 or paths 278 first is that there is
no debris surrounding the cut so mechanical cutting provides the
laser rounding operation with a flat surface that facilitates
rounding the edges to a preferred radius of curvature..Iaddend.
.Iadd.With reference to FIG. 28a, an optional sacrificial
protection layer 370 may be applied to patterned ABS 224 or all of
the workpiece surfaces prior to laser rounding to protect ABS
surface 224 and important ABS features 372, including rails 276 and
pole tips 232 and 234, from redep and/or to facilitate cleaning of
nonpermanent redep. A preferred sacrificial layer 370 comprises a
conventional lithographic photoresist or a laser ablatable resist.
Unfortunately, conventional materials used for sacrificial layer
370 have a tendency to burn when impinged by laser output suitable
for laser rounding..Iaddend.
.Iadd.With reference to FIGS. 28b and 28c, it is preferable,
therefore, to remove about a 10-25 .mu.m wide area of sacrificial
layer 370 from covering the ABS 224 in proximity to edges 266 or
282 to create a small uncovered zone 374. Uncovered zone 374 is
preferably wider than the spot area of output but narrow enough so
that all ABS features 372 remain covered. These strips of
sacrificial layer 370 can be removed by conventional lithographic
techniques, or by direct ablation or expose and etch solid-state UV
laser techniques disclosed in U.S. Pat. No. 6,025,256 of Swenson et
al. An example of parameters for resist-processing laser output 376
includes a beam positioning offset 378 of 10-20 .mu.m from edge 266
or 282, a 7 .mu.m bite size, at 14 kHz at 30 .mu.J at 266 nm. If
direct laser ablation is performed, the laser output parameters,
particularly the power density, are adapted to be insufficient to
adversely affect ABS 24. In a preferred embodiment, the same laser
system that is used to round edges 266 or 282 is used to remove the
strip of sacrificial layer 370, but the laser output is generated
at a higher repetition rate or the laser spot may be defocused to
reduce the power density. FIG. 28c shows uncovered zone 374 after a
strip of sacrificial layer 370 has been removed..Iaddend.
.Iadd.With reference to FIG. 28d, laser output is applied to ABS
224 in uncovered zone 374. Laser output is preferably positioned
perpendicular to the ABS 224, with the spot centered at edges 266
or 282 (or corners 287), as shown; however, skilled persons will
appreciate that other impingement angles and offsets from edges 266
or 282 can be employed. Although a single laser pass is preferable,
multiple passes of laser output can be employed. FIG. 28e shows
redep 380a on the surface of sacrificial layer 370 and redep 380b
on the surface of rounded edge 362 or 164, collectively redep 380,
that may result from application of laser output..Iaddend.
.Iadd.After the laser rounding operation shown in FIG. 28d, a
cleaning operation shown in FIG. 128f can be used to remove any
laser-generated debris 380 that may have accumulated in the
uncovered zone 374. A major advantage of employing a sacrificial
layer is that it permits the use of more aggressive cleaning
techniques, such as ion milling or reactive ion etching (RIE), to
remove redep 380b without risk of damage to ABS features 372. These
aggressive cleaning techniques may also remove a surface portion of
sacrificial layer 370 and any redep 380a thereon. Without
sacrificial layer 370, less aggressive cleaning techniques, such as
solvent or surfactant applications with or without ultrasound or
mechanical scrubbing, are preferred. FIG. 28g shows slider 210
after cleaning. Finally, sacrificial layer 370 is stripped off the
entire ABS 224, removing any remaining laser-generated debris 380a
with it. FIG. 28h shows an uncovered slider 350 or 360 with its
sharp edge removed..Iaddend.
.Iadd.FIGS. 29a-29f (collectively FIG. 29) show simplified side
sectional views of a generic workpiece as it undergoes process
steps of an exemplary laser cutting process (row slicing or slider
dicing). With reference to FIG. 29a, an optional sacrificial
protection layer 370 may be applied to patterned ABS 224 or all of
the workpiece surfaces, as previously described, prior to laser
cutting. With respect to the overall process of manufacturing
sliders 210, in one example, sacrificial layer 370 is applied
directly after ABS 224 has been patterned and before the
photoresist mask 272 has been removed. Alternatively, the rounding
and/or severing processes can be performed using mask 272 before or
after patterning. It can also alternatively be applied after mask
272 has been removed or after sliders 210 have been singulated.
Instead of, or in addition to, covering the surface with
sacrificial layer 370, laser cutting may be performed from the back
side of wafer 250 so that laser-generated debris 380 becomes
irrelevant. Back side alignment can be accomplished with laser or
other markings or through holes made from ABS 224 side of wafer
250, and/or edge alignment and/or calibration with a camera view of
ABS features 372 or deposited face of trailing end
212..Iaddend.
.Iadd.With reference to FIGS. 29b and 29c, preferably a 10-50 .mu.m
wide area of sacrificial layer 370 covering ABS 224 in proximity to
intended edges 266 and 268 or 282 is removed to create an uncovered
zone 374. These strips of sacrificial layer 370 can be removed as
previously described. If appropriate for a specific layout of rows
260 or sliders 210, a larger spot size 376a or multiple adjacent or
overlapping trim lines 340 of laser output 376 can be employed for
ablative removal of a strip of sacrificial layer 370. FIG. 29c
shows uncovered zone 374 after the strip of sacrificial layer 370
has been removed..Iaddend.
.Iadd.With reference to FIG. 29d, laser output 390 is applied to
ABS 224 in uncovered zone 374. Laser output 390 is preferably
positioned perpendicular to the ABS 224, with the spot centered
between intended edges 266 and 268 or 282 (or on corners 285), as
shown; however, skilled persons will appreciate that other
impingement angles and offsets from intended edges 266 and 268 or
282 can be employed. Multiple passes of laser output 390 are
typically employed for both row slicing and slider dicing; however,
slider dicing can be achieved in a single pass. Laser output 390
used for laser cutting may employ a higher peak power density than
laser output used for laser rounding..Iaddend.
.Iadd.Although using common parameters for slicing through both the
alumina and the AlTiC is advantageous for simplification, it may be
desirable for throughput, for example, to employ different
parameters for alumina slicing output 390a to slice through the
alumina than for AlTiC slicing output 390b to slice through AlTiC.
In particular, it may be desirable to use 266 nm or 355 nm to cut
the alumina and 355 nm or 532 nm to cut the AlTiC. In one
embodiment, row slicing through the alumina on multiple rows is
performed with output 390a and then slicing through the AlTiC is
performed in the notches with output 390b to finish the cuts.
Alternatively, a row 260 may be sliced completely through with
outputs 390a and 390b before a second row 260 is sliced. Each of
the two different laser outputs 390 maybe applied in a single or in
multiple passes. Switching the parameters of output 390 can be
achieved with a single laser employing a switchable wavelength,
repetition rate, or focus depth, or can be achieved through a multi
laser head system, with different laser heads responsible for the
different laser outputs 390. With respect to slider dicing, each
traverse cut 396 (FIG. 31) traverses regions of slider 210 that are
completely alumina and regions that are completely AlTiC.
Accordingly, output 390a can be applied in one or more passes along
the alumina portions of cuts 396 and then output 390b can be
applied in one or more passes along the AlTiC portions of cuts 396.
Alternatively, each cut 396 can be made completely one at a time,
switching between alumina processing output 390a and AlTiC
processing output 390b for each pass..Iaddend.
.Iadd.FIG. 29e shows separated edges 266, 268, or 282 with redep
380a on the surface of sacrificial layers 370 and redep 380b on the
surface of edges 266, 268 or 282. FIG. 29f shows the beginning of
the laser rounding process, described in connection with FIG. 28,
that is applied to both edges 266 or 282. The debris 380 can
optionally be cleaned off before the laser rounding process is
performed to provide a flatter surface to facilitate rounding the
edges to a preferred radius of curvature of about 20-25 .mu.m.
Although laser cutting without the additional laser rounding step
will provide benefits over mechanical cutting, performing a laser
rounding step in addition to laser cutting is
preferred..Iaddend.
.Iadd.Applying one or more additional laser processing passes along
the newly formed edges can change the radius of curvature along the
edges. Furthermore, a more gradual slope can be obtained by
employing one or a small number of passes slightly interior of an
edge and gradually increasing the number of passes as the beam is
positioned more closely to the edge. FIG. 30 shows a symbolic
representation of forming such a gradually sloped edge 400 with the
number of arrows in each column representing the number of passes.
It is noted that an increased radius of curvature can also be
achieved by performing one or multiple passes directly centered at
the edge. Generally, the slope or angle of the edge or sidewall can
be controlled by controlling the spacing of the lines of laser
spots as well as the distances from the edge and number of passes.
More passes at or near the edge results in a steeper angle, and
passes further from the edge can be used to produce a shallower
slope..Iaddend.
.Iadd.Although laser sacrificial layer strip removal, laser
cutting, and laser rounding may entail multiple laser process steps
at different parameters, an all laser process has many advantages
and employs repositioning along only a single axis for each linear
operation..Iaddend.
Laser cutting destroys significantly less material (kerfs of less
than 50 .mu.m wide and preferably less than 25 .mu.m wide and
typically about 10 .mu.m wide) than does mechanical cutting
(slicing lanes of about 300 .mu.m wide and dicing paths of about
150 .mu.m wide) so that devices on wafers can be manufactured much
closer together, allowing many more devices to be produced on each
wafer. Thus, the laser cutting process minimizes the pitch between
rows and the pitch between devices. .Iadd.In an example, the pitch
between rows 260 can be 350 .mu.m and the pitch between slider can
be 1025 .mu.m, realizing about a 33% increase in the number of rows
260 and a gain of about one slider 210 for every thirteen sliders
210 per row 260..Iaddend.
Elimination of the mechanical cutting can also simplify manufacture
of devices on workpieces 12. In particular, mechanical cutting can
impart significant mechanical stress to devices such that they come
off their carriers. To avoid losing rows, device manufacturers may
employ strong adhesives or epoxies between the rows and the
carrier. An all laser process significantly reduces the mechanical
strength requirements of the adhesive used for fixturing the rows
onto a carrier. Laser .Iadd.rounding and .Iaddend.cutting,
therefore, permits the elimination of strong adhesives or epoxies
used to affix the rows to the carrier and the harsh chemicals
needed to remove them. Instead, the adhesives can be selected for
ease of debonding, such as the reduction of debond time and less
exposure to potentially corrosive chemicals, and for amenability to
UV laser processing, greatly reducing risk of damage to the
devices, and thereby enhancing yield.
Laser row slicing reduces row bow because laser slicing does not
exert as much mechanical stress as mechanical slicing. However, if
row bow or other of the row defects are apparent, the rows can be
laser diced (and re-sliced) to compensate for these defects without
concern for the critical device to device alignment needed between
rows for mechanical dicing. For convenience, the term (through)
cutting may be used generically to include slicing (often
associated with wafer row separation) or dicing (often associated
with part singulation from wafer rows), and slicing and dicing may
be used interchangeably in the context of this invention.
Because positioning system 30 can align to through holes or
fiducials, laser system 10 can process each row and/or each device
independently. With respect to slanted rows, the laser spot can
perform traverse cuts across the slanted rows at appropriate
positions with respect to outer edges of the devices with stage
and/or beam translations between each cut to effect a rectangular
or curvilinear wave patterns as desired. Thus, laser dicing can
compensate for row fixturing defects and perhaps save entire rows
of devices that would be ruined by mechanical dicing.
.Iadd.FIG. 31 demonstrates an exemplary laser process for row
defect compensation using transverse cuts 396 and stage and/or beam
translations 398 to generally make cuts 396 at angles such that the
surfaces of sliders 210 are substantially perpendicular to each
other. Numerous other cutting patterns are possible such as making
all cuts in a first column before making all cuts in second column.
Sliders 210 in rows 260a, 260b, and 260c can be singulated in a
similar fashion regardless of angle or offset. With respect to row
260d, the rectangular wave cut and translate pattern can be curved
to align with the row bow..Iaddend.
.Iadd.FIG. 32 shows a flow diagram of a simplified cutting and
rounding process with simplified side sectional views of a generic
workpiece such as wafer 250 as it undergoes process steps. In this
alternative embodiment, a mechanical cutting blade or laser output
390 notches rows 260 or sliders 210 along lanes 262 or paths 278 to
a depth, preferably above an adhesive layer if a combination of
laser and mechanical notching or cutting is to be employed.
Alternatively, for preslice notching, laser output 390a may be
employed to notch all the way through the alumina material. FIG.
32b shows the result of laser notching with a solid line and shows
the result of mechanical notching with a broken line. Laser output
then rounds the desired edges and/or corners, and finally the
mechanical cutting blade or laser output 390 finishes the
separation of rows 260 or singulation of sliders 350 or 360. The
width of the kerf or diameter used for the cutting process can be
less than or equal to the width of the kerf or diameter used for
the notching process. A sacrificial layer 370 and the related steps
associated with it may be employed prior to a notching process.
Skilled persons will appreciate that edges on the bottom side can
optionally be done by this notching technique, preferably such that
top and bottom alignment is conserved. Such notching would greatly
facilitate subsequent laser separation of the rows 260 or sliders
210, 350, or 360. One advantage of this technique is that there are
fewer pieces to align since the parts are still referenced to each
other, i.e., the rounding is completed before the pieces are
separated. Another advantage is that the preliminary notch does not
expose the adhesive layer where mechanical cutting is to be
employed, since the adhesives needed to withstand mechanical
cutting are particularly volatile in response to laser
radiation..Iaddend.
.Iadd.FIG. 33 shows a flow diagram of an alternative cutting and
rounding process with simplified side sectional views of a generic
workpiece as it undergoes process steps. With reference to FIG. 33,
rounding laser output 330 is applied along two parallel trim lines.
The trim lines are spaced such that the edges 282 of the dice lane
278 align with the centers of the trenches 402 produced by the
laser outputs 330. In FIG. 33b, a dice blade or laser cuts the
workpiece surface between the trenches 402 to produced rounded
separate parts shown in FIG. 33c..Iaddend.
.Iadd.FIG. 34 shows an alternative rounding, notching, and
separating process. In FIG. 34a, multiple adjacent passes of laser
output 330 or 390 create an extra wide notch (FIG. 34b) with
rounded edges. Then output 390 or a cutting blade is applied to
separate the rows 260 or sliders 210. This process creates a
shelfed edge shown in FIG. 34c. The edges of the lower shelves can
be rounded with processes previously discussed. It may be useful to
use several different parameters for various passes in one notching
step to tailor the notch geometry, including sidewall angle (FIG.
34b.sub.2 or 34b.sub.3)..Iaddend.
.Iadd.With reference to FIGS. 29 and 31-34, it may be desirable to
notch through one side of the workpiece, preferably about one half
the thickness of the workpiece, and then finish the row or slider
separation from the opposite side, preferably by flipping the
workpiece and using alignment techniques previously discussed. This
embodiment may provide significant throughput advantages
particularly for high-aspect ratio kerfs. The rounding process can
be performed before or after notching or after row or slider
separation..Iaddend.
.Iadd.FIG. 35 demonstrates that an excimer laser at an appropriate
UV wavelength can be used with appropriate-sized line-making masks
410 or 412 (about the width of preferred Gaussian spot sizes) for
the above-described laser dicing or rounding operations without
employing the preferred bite size technique. The line-making masks
410 or 412 can have a length the size of an entire column or as
little as the desired edge. For example, the surfaces of wafers
250, rows 260, or sliders 210 can be covered with sacrificial layer
370; the portions of the sacrificial layer 370 can be removed to
create uncovered zones; wafers 250 and/or rows 260 can be diced and
edges 266, 268, 282, 284, and/or 286, and/or corners 285 and/or 287
can be rounded with a UV excimer through a line mask of an
appropriate shape and size; the entire surface can be aggressively
cleaned to remove debris from the uncovered zones; and the
sacrificial layer can be removed..Iaddend.
Another application of the segment cutting method is to produce
MEMS (microelectronic machine system) devices 160. FIG. 19 is a
representative illustration of ultraviolet laser cutting of a MEMS
device 160. In one preferred embodiment, the MEMS device 160 is cut
using the method described above to create trenches 162a, 162b,
162c, 162d, and 162e (generically trenches 162) in silicon and to
create a depression 164 by employing a pattern of adjacent trenches
162. Skilled persons will appreciate that through computer control
of the X and/or Y axes of the laser positioning system 30, the
directed laser system output pulses 32 can be directed to the work
surface such that overlapped pulses create a pattern which
expresses any complex curvilinear geometry. Skilled persons will
appreciate that the segmented cutting techniques and other
processing techniques disclosed herein can be used to cut arcs and
other curves for nonMEMS applications as well.
Another application of the segmented cutting method is to process
optical integrated circuits, such as an arrayed waveguide gratings
(AWG) device 170 produced on semiconductor wafer workpieces 12.
FIG. 20 is a representative illustration of ultraviolet ablative
patterning of an AWG device 170. In one preferred embodiment, the
AWG device 170 is patterned using the method described above to
create curvilinear trenches 172, with portions 172a, 172b, 172c,
172d, and 172e in silicon, for example. Although trench 172 is
shown to be symmetric, skilled persons will appreciate that through
computer control of the X and/or Y axes of the beam positioning
system 30, the laser system output pulses 32 can be directed to the
work surface such that overlapped pulses 32 create a pattern which
expresses any complex curvilinear profile or geometry. Skilled
persons will appreciate that segments 122 are not required to be
linear and can be arcs such that each portion 172 can be processed
with one or more nonlinear segments 122. This capability may be
used to produce complex curvilinear geometric patterns in silicon
useful for efficient production of a variety of AWG devices 170.
Skilled persons will also appreciate that the segmented cutting
techniques could be employed to produce large diameter through hole
or blind vias.
The '382 application of Fahey et al. describes techniques for
forming rounded edges along cuts, as well as for laser slicing and
dicing ceramic wafers. Many of these techniques, as well as the
alignment techniques disclosed therein, can be advantageously
incorporated into the present invention to cut silicon wafers and
further improve the quality of and processing speed for cutting
ceramic or other brittle, high melting temperature materials, such
as glasses. U.S. patent application Ser. No. 09/803,382 is herein
incorporated by reference.
It is contemplated that performing the cuts in a reactive gas
atmosphere, such as an oxygen-rich atmosphere, will generate debris
that is easier to cut. In an oxygen rich environment, for example,
it is proposed that the hot ejected silicon will more likely form
SiO.sub.2 in an exothermic reaction that may keep any resulting
SiO.sub.2 backfill redep at a higher temperature for a longer time
making it less likely to stick strongly on the silicon and/or
making it easier to clean from a trench with a quick subsequent
laser pass 132. To the extent that redep (or exposed trench
material) cooling or resolidification is a factor, this
recharacterization time interval may to some extent influence the
maximum preferred length 126 of segments 122 such that the laser
spot can process length 126 and return to impinge again any redep
(or warmed exposed trench material) at the initial laser pass 132a
and subsequent laser passes 132 before the redep (or exposed trench
material) cools or sticks strongly.
Skilled persons will also appreciate that purge gases, such as
nitrogen, argon, helium, and dry air, may be usefully employed to
assist in the removal of waste fumes from the workpiece 12 and more
preferably to blow potential backfill through any existing
throughcut portions along cut path 112. Such purge gases can be
delivered to the close vicinity of the work surface using delivery
nozzles attached to laser system 10.
If desirable, silicon workpieces 12 processed in accordance with
the present invention may be cleaned using ultrasonic baths in
liquids including but not limited to water, acetone, methanol, and
ethanol to improve the surface quality of affected areas. Those
skilled in the art will also recognize that cleaning of processed
silicon workpieces 12 in hydrofluoric acid can be beneficial in
removing unwanted oxide layers.
Although the present invention is presented herein only by way of
example to silicon wafer cutting, skilled persons will appreciate
that the segmented cutting techniques described herein may be
employed for cutting a variety of target materials including, but
not limited to, other semiconductors, GaAs, SiC, SiN, indium
phosphide, glasses, ceramics, AITiC, and metals with the same or
different types of lasers including, but not limited to,
solid-state lasers, such as YAG or YLF, and CO.sub.2 lasers, of
similar or different UV, visible, or IR wavelengths.
U.S. Prov. Pat. Appl. No. 60/301,701, filed Jun. 28, 2001, entitled
Multi-Step Laser Processing for the Cutting or Drilling of Wafers
with Surface Device Layers of Fahey et al., which is herein
incorporated by reference describes multi-step techniques for
cutting wafers and the device layers they support with different
severing processes, such as different laser parameters. This
multi-step process involves the optimization of laser processes for
each individual layer, such that the processing of any one layer or
the substrate material does not negatively affect the other layers.
A preferred process entails the use of UV lasers for cutting layers
that are transparent in the IR or visible range, allowing for a
different laser to be used for cutting the wafer than is used for
cutting the layers. This process permits significantly less damage
to the layer than would occur if only one laser, such as an IR
laser, were used to cut through the entire layer and wafer
structure. Furthermore, this laser processing of the layers allows
for the optimization of other cutting processes, such as the use of
a wafer saw, in order to reduce or eliminate the damage to the
layers on the wafer. One example employs a UV laser 10 to cut
layers that include ceramic, glass, polymer or metal films on the
top or bottom surfaces of the wafer substrate, while a different
laser, such as a 532 nm laser or IR laser, or the same laser or
optical system run with different process parameters is used to cut
through the substrate material after the surface layers have been
cleared away. Each of the laser processes may employ the same or
different segmented cutting techniques that cooperate with the
other laser parameters chosen to facilitate high quality and
throughput. Alternatively, surface layers may be processed by
conventional full scan processing while the thicker substrate layer
may be processed by a segmented technique.
One embodiment entails covering the surfaces of the wafer with a
sacrificial layer such as photoresist; optionally removing a
portion of the sacrificial layer to create uncovered zones over
intended cutting areas; laser cutting the layers atop the wafer
substrate to a width equal or greater than that which will occur in
the subsequent substrate cutting step; then cutting the wafer with
a separate processing step or steps using a different laser,
wavelength, pulse width, fluence, bite size, and/or other laser
processing parameters.
Another embodiment allows for removal of the surface layer or
layers with one laser process or several laser processes and then
employs a subsequent process or several subsequent processes that
complete the cutting with a non-laser technique that only has to
remove the wafer substrate material. One example of such technique
is the removal of all metal, polymer or other soft material from
the cutting lane using the laser, such that during subsequent
cutting with a saw blade, the blade only makes contact with the
substrate material. This technique will be of particular use when
cutting wafers with metallization in the dice lanes, such as that
due to the presence of test devices, or wafers which have a polymer
dielectric material such as some of the low-K materials that are
presently on the market.
.Iadd.FIGS. 36a-36f (collectively FIG. 36) show simplified side
sectional views of a generic workpiece as it undergoes process
steps of an exemplary laser dicing or drilling process. In one
embodiment, separate processing steps are used in succession to cut
through the layers 470, 472 in order from the top layer 470 down to
the substrate 522 (FIGS. 36a, 36b, 36c, and 36e). Depending upon
preference and upon the layer materials, it may be of interest to
choose the FIGS. 36c-36d process, in which lower layers 472 are
opened to successively smaller cut widths in order to not have the
successive processes affect the overlayer. The typical best case
would be the process of FIGS. 36e-36f, where all layers are cleared
open to the same width. Finally (FIGS. 36d, 36f), the substrate 522
is cut in the position where the layers have been cleared off by
the earlier processes. In this figure, the cuts shown are cross
sections of a cut which may either be a dicing cut or a drilled
via. If necessary, further process steps may occur where the laser
is used to clean up the edges of the cuts made in the earlier
steps. While the layer cuts and cleanup steps will be done with a
laser, the step which involves cutting through the substrate may be
done with a laser or with another technique such as mechanical
sawing..Iaddend.
.Iadd.With reference to FIG. 36a, the top layer 470 could either be
a device layer, or could be an optional sacrificial protection
layer to protect important features, such as solder bumps on die or
features on die (laser diodes, optical waveguides or MEMS
components, etc.) from redep and/or to facilitate cleaning of
nonpermanent redep. A preferred sacrificial layer comprises a
conventional lithographic photoresist or a laser ablatable resist.
Unfortunately, conventional materials used for sacrificial layer
have a tendency to burn when impinged by laser output suitable for
dicing or removal of many types of device layer. As shown in FIG.
36c, it is therefore preferable to remove about a 10-25 .mu.m wider
area of the sacrificial layer in proximity to the edges of the
notches to be made in the underlying layers to create a small
uncovered zone. These strips of sacrificial layer can be removed by
conventional lithographic techniques, or by direct ablation or
expose and etch solid-state UV laser techniques disclosed in U.S.
Pat. No. 6,025,256 of Swenson et al. An example of parameters for
resist-processing laser output includes a beam positioning offset
178 of 10-20 .mu.m from edge 266 or 282, a 7 .mu.m bite size, at 14
kHz at 30 .mu.J at 266 nm. If direct laser ablation is performed,
the laser output parameters, particularly the power density, are
adapted to be insufficient to adversely affect the underlying
device layers or substrate material. In a preferred embodiment, the
same laser system that is used to round edges 266 or 282 is used to
remove the strip of sacrificial layer, but the laser output 522 is
generated at a higher repetition rate or the laser spot may be
defocused to reduce the power density..Iaddend.
.Iadd.One skilled in the art will realize that an excimer laser at
an appropriate UV wavelength can be used with appropriate-sized
line-making masks (about the width of preferred Gaussian spot
sizes) for the above-described laser dicing operations for those
layers which require UV ablation. The line-making masks can have a
length the size of an entire column or as little as the desired
edge of each die. For example, in FIG. 36, a UV excimer through a
line mask of an appropriate shape and size could be used to perform
the ablation steps in FIG. 36b, 36c, 36e, 36d, or 36f if that laser
is appropriate for cutting the material of interest in that
particular process step. In addition, it could be used for the
removal of any sacrificial layer atop the wafer. Skilled persons
will appreciate that if the semiconductor industry moves toward
making die on different types of wafers, like InP, SOS, SOI, etc.,
the rounding and cutting processes disclosed herein can be applied
to devices manufactured with or on such wafers. Silicon carbide and
titanium carbide, or other insulating (non-semiconductor)
substrates, may also be similarly processed..Iaddend.
.Iadd.Another embodiment of the invention provides such a method or
system that modifies the geometry of the layer or layers by one or
several laser processes 530 such that the subsequent cutting or
drilling of the substrate does not cause damage in the active area
of the devices. For example (FIG. 8), a process sequence may
include an initial notching of layer or layers on either side of
the cut without removing all the material from the dice lane area
such that the outermost edges formed by the laser trenches are
unaffected by the subsequent substrate dicing process. As discussed
above, this laser notching would be performed using parameters
specifically optimized for cutting the layers cleanly without
inducing damage that would occur if substrate dicing parameters
were used. Use of this geometry modification would include, but is
not limited to, the formation of trenches or other shapes outside
the dicing kerf which would act as crack stops or mechanisms for
arresting delamination which may be induced by the wafer substrate
dicing step. These notches may extend only to the bottom of the
layers or may extend further into the substrate material depending
on the damage mode which is anticipated during the dicing process.
For example, if the layers are delaminating during the subsequent
cutting process, the notches need only go below the interface of
interest. If the substrate material is being damaged during the
subsequent cutting process, it may be of interest to make the
notches penetrate more deeply into the substrate
material..Iaddend.
It will be obvious to those having skill in the art that many
changes may be made to the details of the above-described
embodiment of this invention without departing from the underlying
principles thereof. The scope of the present invention should,
therefore, be determined only by the following claims.
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