U.S. patent application number 09/823922 was filed with the patent office on 2001-11-29 for laser system and method for single pass micromachining of multilayer workpieces.
Invention is credited to Barrett, Spencer, Dunsky, Corey M., Lo, Ho W., Sudhakar, Raman, Whiteman, Ken, Wilt, Donald R..
Application Number | 20010045419 09/823922 |
Document ID | / |
Family ID | 22714215 |
Filed Date | 2001-11-29 |
United States Patent
Application |
20010045419 |
Kind Code |
A1 |
Dunsky, Corey M. ; et
al. |
November 29, 2001 |
Laser system and method for single pass micromachining of
multilayer workpieces
Abstract
A single pass actuator (70, 200), such as a deformable mirror
(70), quickly changes, preferably in less than 1 ms, the focus and
hence the spot size of ultraviolet or visible wavelength laser
pulses to change the fluence of the laser output (66) at the
workpiece surface between at least two different fluence levels to
facilitate processing top metallic layers (264) at higher fluences
and underlying dielectric layers (266) at lower fluences to protect
bottom metallic layers (268). The focus change is accomplished
without requiring Z-axis movement of the laser positioning system
(62). In addition, the spot size can be changed advantageously
during trepanning operations to decrease via taper, reduce lip
formation, increase throughput, and/or minimize damage.
Inventors: |
Dunsky, Corey M.; (Portland,
OR) ; Lo, Ho W.; (Portland, OR) ; Whiteman,
Ken; (Aloha, OR) ; Wilt, Donald R.;
(Lexington, MA) ; Barrett, Spencer; (Beaverton,
OR) ; Sudhakar, Raman; (Tigard, OR) |
Correspondence
Address: |
STOEL RIVES LLP
900 SW FIFTH AVENUE
SUITE 2600
PORTLAND
OR
97204
US
|
Family ID: |
22714215 |
Appl. No.: |
09/823922 |
Filed: |
March 30, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60193581 |
Mar 30, 2000 |
|
|
|
Current U.S.
Class: |
219/121.76 ;
219/121.61; 219/121.78; 219/121.85 |
Current CPC
Class: |
H05K 3/0038 20130101;
B23K 26/08 20130101; B23K 26/02 20130101; H05K 2203/108 20130101;
H05K 3/384 20130101; B23K 26/082 20151001; B23K 26/042 20151001;
H01L 21/486 20130101; B23K 26/073 20130101; B23K 26/707 20151001;
B23K 26/389 20151001; B23K 26/3568 20180801; B23K 26/384 20151001;
B23K 26/702 20151001; B23K 26/046 20130101 |
Class at
Publication: |
219/121.76 ;
219/121.61; 219/121.78; 219/121.85 |
International
Class: |
B23K 026/00; B23K
026/02; B23K 026/08 |
Claims
1. A method for depthwise laser machining through multiple layers
at multiple target locations on a multilayered workpiece including
at least first and second layers of respective first layer and
second layer materials having respective first and second ablation
fluence thresholds, comprising: addressing a beam positioner toward
a first target location on the workpiece; generating a first laser
output having a wavelength shorter than 550 nm; propagating the
first laser output along an optical path including a single pass
actuation assembly that is selectively changeable to provide at
least two different focal effects including a first focal effect to
provide a first spot area and a second focal effect to provide a
second spot area; applying the first laser output to the first
target location to remove first layer material from the first
target location, the first laser output containing at least a first
laser pulse that acquires the first focal effect and has a first
fluence over the first spot area, and the first fluence being
greater than the first ablation fluence threshold; causing the
single pass actuation assembly to provide the second focal effect
that is different from the first focal effect; generating a second
laser output having a wavelength shorter than 550 nm; propagating
the second laser output along the optical path including the single
pass actuation assembly; applying the second laser output to the
first target location to remove second layer material from the
first target location, the second laser output containing at least
a second laser pulse that acquires the second focal effect and has
a second fluence over a second spot area, and the second fluence
being greater than the second ablation fluence threshold;
addressing the beam positioner toward a second target location,
different from the first target location, on the workpiece; causing
the single pass actuation assembly to provide a third focal effect;
generating a third laser output having a wavelength shorter than
550 nm; propagating the third laser output along an optical path
including the single pass actuation assembly; applying the third
laser output to the second target location to remove first layer
material from the second target location, the third laser output
containing at least a third laser pulse that acquires the third
focal effect and has a third fluence over a third spot area, and
the third fluence being greater than the first ablation fluence
threshold; causing the single pass actuation assembly to provide a
fourth focal effect that is different from the third focal effect;
generating a fourth laser output having a wavelength shorter than
550 nm; propagating the fourth laser output along the optical path
including the single pass actuation assembly; applying the fourth
laser output to the second target location to remove second layer
material from the second target location, the fourth laser output
containing at least a fourth laser pulse that acquires the fourth
focal effect and has a fourth fluence over a fourth spot area, and
the fourth fluence being greater than the second ablation fluence
threshold.
2. The method of claim 1 further comprising: causing the single
pass actuation assembly to switch between providing the first focal
effect and the second focal effect in less than 2 ms.
3. The method of claim 2 further comprising: causing the single
pass actuation assembly to switch between providing the first focal
effect and the second focal effect in less than 1 ms.
4. The method of claim 1 in which the first and second target
locations are separated by a distance that incurs a positioning
move time, further comprising: causing the single pass actuation
assembly to switch between providing the first focal effect and the
second focal effect in a focal time that is shorter than the
positioning move time.
5. The method of claim 1 wherein the single pass actuation assembly
comprises a deformable mirror having a mirror surface of a first
shape for providing the first focal effect, further comprising:
changing the mirror surface to have a second shape to provide the
second the second focal effect.
6. The method of claim 5 further comprising: applying a voltage to
an actuator supporting the mirror surface to switch between the
focal effects.
7. The method of claim 6 wherein the actuator comprises an
electrostrictive PMN device.
8. The method of claim 5 wherein the deformable mirror has a
response time of less than 0.5 ms.
9. The method of claim 5 in which the mirror is actuated at a
frequency of greater than 100 Hz.
10. The method of claim 9 in which the mirror is actuated at a
frequency of greater than 300 Hz.
11. The method of claim 1 wherein the single pass actuation
assembly comprises distinct first and second focal paths that
create the respective first and second focal effects.
12. The method of claim 11 in which a pair of galvanometer mirrors
effect switching between the first and second focal paths.
13. The method of claim 1 in which the first fluence is greater
than the second fluence.
14. The method of claim 1 wherein the first layer comprises a first
conductor material and the second layer comprises a dielectric
material; wherein the conductor material is positioned above the
dielectric material; wherein a third layer of a second conductor
material is positioned below the dielectric material and has a
second conductor ablation fluence threshold; and wherein the second
fluence is less than the second conductor ablation fluence
threshold such that the second conductor material is substantially
undamaged and a depthwise self-limiting blind via is formed.
15. The method of claim 14 wherein the first and second conductor
materials are substantially the same.
16. The method of claim 1 in which the laser pulses of the first
and second laser outputs have pulse energies that are substantially
the same.
17. The method of claim 1 in which the first and second laser
outputs are generated by a solid-state laser comprising Nd:YAG,
Nd:YLF, Nd:YAP, or Nd:YVO.sub.4.
18. The method of claim 1 in which the first spot area is smaller
than the second spot area.
19. The method of claim 1 in which the first spot area has a
1/e.sup.2 diameter that is less than about 25 .mu.m.
20. The method of claim 19 in which the first spot area has a
1/e.sup.2 diameter that is less than about 15 .mu.m.
21. The method of claim 1 in which the first and second laser
outputs comprise substantially the same wavelength.
22. The method of claim 1 in which the wavelengths of the first and
second outputs comprise about 355 nm or 266 nm.
23. The method of claim 1 in which the first and third fluences
comprise a fluence of greater than or equal to 10 J/cm.sup.2 in at
least some region of the first and third spot areas.
24. The method of claim 1 in which the second and fourth fluences
comprise a fluence of greater than or equal to 0.5 J/cm.sup.2 in at
least some region of the second and fourth spot areas.
25. The method of claim 1 in which the single pass actuation
assembly is selectively changeable to provide multiple focal
effects for respective multiple laser pulses with respective
multiple spot areas of sizes between those of the first and second
spot areas, the respective multiple spot areas thereby receiving
respectively different fluences from the multiple laser pulses.
26. The method of claim 25 wherein the first layer comprises a
first conductor material and the second layer comprises a
dielectric material; wherein the conductor material is positioned
above the dielectric material; wherein a third layer of a second
conductor material is positioned below the dielectric material and
has a second conductor ablation fluence threshold; wherein the
second fluence is less than the second conductor ablation fluence
threshold; and wherein the respectively different fluences
generally diminish as the multiple laser pulses approach the third
layer such that the second conductor material is substantially
undamaged and a depthwise self-limiting blind via is formed.
27. The method of claim 26 wherein the blind via has a bottom at
the third layer and diminished fluence pulses are employed to clean
out dielectric material from the bottom of the blind via.
28. The method of claim 1 in which the spot areas define respective
spot sizes that are smaller than and fit within first and second
spatial regions of the respective first and second target
locations, the first and second spatial regions being divisible
into multiple positions defining a contiguous set of spot areas
that cover the spatial regions, the method further comprising:
directing the first laser outputs to first multiple positions
associated with the first spatial region to remove multiple amounts
of target material corresponding to multiple respective first spot
areas; directing the second laser outputs to second multiple
positions associated with the first spatial region to remove
multiple amounts of target material corresponding to multiple
respective second spot areas; directing the third laser outputs to
multiple positions associated with the second spatial region to
remove multiple amounts of target material corresponding to
multiple respective third spot areas; and directing the fourth
laser outputs to multiple positions associated with the second
spatial region to remove multiple amounts of target material
corresponding to multiple respective fourth spot areas.
29. The method of claim 28, further comprising: after applying the
second laser output and prior to addressing the beam positioner
toward the second target location, causing the single pass
actuation assembly to provide a fifth focal effect; generating a
fifth laser output having a wavelength shorter than 550 nm;
propagating the fifth laser output along the optical path including
the single pass actuation assembly; applying the fifth laser output
to the first target location to remove second layer material from
the first target location, the fifth laser output containing at
least a fifth laser pulse that acquires the fifth focal effect and
has a fifth fluence over a fifth spot area, and the fifth fluence
being greater than the second ablation fluence threshold but
different from the second fluence; after applying the fourth laser
output, causing the single pass actuation assembly to provide a
sixth focal effect; generating a sixth laser output having a
wavelength shorter than 550 nm; propagating the sixth laser output
along the optical path including the single pass actuation
assembly; and applying the sixth laser output to the second target
location to remove second layer material from the second target
location, the sixth laser output containing at least a sixth laser
pulse that acquires the sixth focal effect and has a sixth fluence
over a sixth spot area, and the sixth fluence being greater than
the second ablation fluence threshold but different from the fourth
fluence.
30. The method of claim 29 in which the spatial regions having a
periphery and a central portion; in which the fifth and sixth
fluences are respectively greater than the second and fourth
fluences; and in which second and fourth spot areas are applied to
the central portion and the fifth and sixth spot areas are applied
to the periphery.
31. The method of claim 1, further comprising: after applying the
second laser output and prior to addressing the beam positioner
toward the second target location, causing the single pass
actuation assembly to provide a fifth focal effect; generating a
fifth laser output having a wavelength shorter than 550 nm;
propagating the fifth laser output along the optical path including
the single pass actuation assembly; applying the fifth laser output
to the first target location to remove second layer material from
the first target location, the fifth laser output containing at
least a fifth laser pulse that acquires the fifth focal effect and
has a fifth fluence over a fifth spot area, and the fifth fluence
being greater than the second ablation fluence threshold but
different from the second fluence; after applying the fourth laser
output, causing the single pass actuation assembly to provide a
sixth focal effect; generating a sixth laser output having a
wavelength shorter than 550 nm; propagating the sixth laser output
along the optical path including the single pass actuation
assembly; and applying the sixth laser output to the second target
location to remove second layer material from the second target
location, the sixth laser output containing at least a sixth laser
pulse that acquires the sixth focal effect and has a sixth fluence
over a sixth spot area, and the sixth fluence being greater than
the second ablation fluence threshold but different from the fourth
fluence.
32. The method of claim 31 in which the fifth and sixth fluences
are respectively smaller than the second an fourth fluences.
33. The method of claim 31 in which the fifth and sixth fluences
are respectively greater than the second an fourth fluences.
34. The method of claim 1 in which the first and third fluences are
substantially the same and in which the second and fourth fluences
are the same.
35. The method of claim 1 in which the first and third focal
effects are substantially the same.
36. The method of claim 1 in which the first and third laser
outputs have different pulse repetition rates so the first and
third fluences are different, and in which the second and fourth
laser outputs have different pulse repetition rates so the second
and fourth fluences are different.
37. The method of claim 1 in which the first and third spot areas
are different so the first and third fluences are different.
38. The method of claim 1 in which the first and second laser
outputs have different pulse repetition rates, and in which the
third and fourth laser outputs have different pulse repetition
rates.
39. The method of claim 18 in which the second spot area has a
1/e.sup.2 diameter that is greater than about 40 .mu.m.
40. The method of claim 19 in which the second spot area has a
1/e.sup.2 diameter that is greater than about 60 .mu.m.
41. The method of claim 5 in which the spot areas define respective
spot sizes that are smaller than and fit within first and second
spatial regions of the respective first and second target
locations, the first and second spatial regions being divisible
into multiple positions defining a contiguous set of spot areas
that cover the spatial regions, the method further comprising:
directing the first laser outputs to first multiple positions
associated with the first spatial region to remove multiple amounts
of target material corresponding to multiple respective first spot
areas; directing the second laser outputs to second multiple
positions associated with the first spatial region to remove
multiple amounts of target material corresponding to multiple
respective second spot areas; directing the third laser outputs to
multiple positions associated with the second spatial region to
remove multiple amounts of target material corresponding to
multiple respective third spot areas; and directing the fourth
laser outputs to multiple positions associated with the second
spatial region to remove multiple amounts of target material
corresponding to multiple respective fourth spot areas.
42. The method of claim 41, further comprising: after applying the
second laser output and prior to addressing the beam positioner
toward the second target location, causing the single pass
actuation assembly to provide a fifth focal effect; generating a
fifth laser output having a wavelength shorter than 550 nm;
propagating the fifth laser output along the optical path including
the single pass actuation assembly; applying the fifth laser output
to the first target location to remove second layer material from
the first target location, the fifth laser output containing at
least a fifth laser pulse that acquires the fifth focal effect and
has a fifth fluence over a fifth spot area, and the fifth fluence
being greater than the second ablation fluence threshold but
different from the second fluence; after applying the fourth laser
output, causing the single pass actuation assembly to provide a
sixth focal effect; generating a sixth laser output having a
wavelength shorter than 550 nm; propagating the sixth laser output
along the optical path including the single pass actuation
assembly; and applying the sixth laser output to the second target
location to remove second layer material from the second target
location, the sixth laser output containing at least a sixth laser
pulse that acquires the sixth focal effect and has a sixth fluence
over a sixth spot area, and the sixth fluence being greater than
the second ablation fluence threshold but different from the fourth
fluence.
43. The method of claim 42 in which the spatial regions having a
periphery and a central portion; in which the fifth and sixth
fluences are respectively greater than the second and fourth
fluences; and in which second and fourth spot areas are applied to
the central portion and the fifth and sixth spot areas are applied
to the periphery.
44. The method of claim 5, further comprising: after applying the
second laser output and prior to addressing the beam positioner
toward the second target location, causing the single pass
actuation assembly to provide a fifth focal effect; generating a
fifth laser output having a wavelength shorter than 550 nm;
propagating the fifth laser output along the optical path including
the single pass actuation assembly; applying the fifth laser output
to the first target location to remove second layer material from
the first target location, the fifth laser output containing at
least a fifth laser pulse that acquires the fifth focal effect and
has a fifth fluence over a fifth spot area, and the fifth fluence
being greater than the second ablation fluence threshold but
different from the second fluence; after applying the fourth laser
output, causing the single pass actuation assembly to provide a
sixth focal effect; generating a sixth laser output having a
wavelength shorter than 550 nm; propagating the sixth laser output
along the optical path including the single pass actuation
assembly; and applying the sixth laser output to the second target
location to remove second layer material from the second target
location, the sixth laser output containing at least a sixth laser
pulse that acquires the sixth focal effect and has a sixth fluence
over a sixth spot area, and the sixth fluence being greater than
the second ablation fluence threshold but different from the fourth
fluence.
45. The method of claim 44 in which the fifth and sixth fluences
are respectively smaller than the second an fourth fluences.
46. The method of claim 44 in which the fifth and sixth fluences
are respectively greater than the second an fourth fluences.
47. A method for depthwise laser machining through layers of a
multilayered workpiece including at least first and second layers
of respective first layer and second layer materials having
respective first and second ablation fluence thresholds,
comprising: generating a first laser output having a wavelength
shorter than 356 nm; propagating the first laser output along an
optical path including a deformable mirror having a mirror surface
of a first shape to provide a first focal effect; applying the
first laser output to a target location on the workpiece to remove
first layer material within the target location, the first laser
output containing at least a first laser pulse having a first
fluence over a first spot area, and the first fluence being greater
than the first ablation fluence threshold; changing the mirror
surface to have a second shape in less than two milliseconds to
provide a second focal effect that is different from the first
focal effect; generating a second laser output having a wavelength
shorter than 356 nm; propagating the second laser output along the
optical path including the deformable mirror having the mirror
surface of the second shape; applying the second laser output to
the target location on the workpiece to remove second layer
material within the target location, the second laser output
containing at least a second laser pulse having a second fluence
over a second spot area that is greater than the first spot area,
and the second fluence being greater than the second ablation
fluence threshold and less than the first ablation fluence
threshold.
48. A laser system for processing multiple layers at multiple
target locations on a multilayered workpiece including at least
first and second layers of respective first layer and second layer
materials having respective first and second ablation fluence
thresholds, comprising: a beam positioner including an optical path
for directing a laser beam toward at least first and second target
locations on the workpiece and having an operational move time
between the first and second target locations; a laser for
generating first and second laser outputs at a wavelength shorter
than or equal to 550 nm; a single pass actuation assembly that is
positioned along an optical path and selectively changeable to
provide at least two different focal effects including a first
focal effect to provide a first spot area for the first laser
output and a second focal effect to provide a second spot area for
the second laser output such that the second spot area is different
from the first spot area; and an actuation controller capable of
changing the first focal effect to the second focal effect during
processing of the first target location and in a focal change time
that is shorter than the operational move time.
49. The laser system of claim 48 in which the laser comprises a
diode-pumped, Q-switched Nd:YAG, Nd:YLF, Nd:YAP, or
Nd:YVO.sub.4.
50. The laser system of claim 48 further comprising: causing the
single pass actuation assembly to switch between providing the
first focal effect and the second focal effect in less than 2
ms.
51. The laser system of claim 50 further comprising: causing the
single pass actuation assembly to switch between providing the
first focal effect and the second focal effect in less than 1
ms.
52. The laser system of claim 48 in which the first and second
laser outputs comprise first and second time intervals, the first
target location is associated with a target processing time that
includes the first and second time intervals, and the focal change
time is shorter than half of the target processing time.
53. The laser system of claim 48 in which laser pulses of the first
and second laser outputs have pulse energies that are substantially
the same.
54. The laser system of claim 48 in which the first spot area has a
1/e.sup.2 diameter that is smaller than about 25 .mu.m and the
second spot area has a 1/e.sup.2 diameter that is greater than
about 40 .mu..
55. The laser system of claim 48 wherein the single pass actuation
assembly comprises a deformable mirror having a mirror surface of a
first shape for providing the first focal effect, further
comprising: changing the mirror surface to have a second shape to
provide the second the second focal effect.
56. The laser system of claim 55 further comprising: applying a
voltage to an actuator supporting the mirror surface to switch
between the focal effects.
57. The laser system of claim 56 wherein the actuator comprises an
electrostrictive PMN device.
58. The laser system of claim 55 wherein the deformable mirror has
a response time of less than 0.5 ms.
59. The laser system of claim 55 wherein the mirror surface is
oriented at about a 45 degree angle with respect to the laser
outputs propagating along the optical path.
60. The method of claim 48 wherein the single pass actuation
assembly comprises distinct first and second focal paths that
create the respective first and second focal effects.
61. The method of claim 60 in which a pair of galvanometer mirrors
effect switching between the first and second focal paths.
62. The method of claim 48 in which the mirror is actuated at a
frequency of greater than 100 Hz.
63. The method of claim 62 in which the mirror is actuated at a
frequency of greater than 300 Hz.
64. A method for laser processing a layer of material within a
spatial region on a workpiece, the material having an ablation
fluence threshold and the spatial region having a peripheral region
and a central region, comprising: addressing a beam positioner
toward a first region selected from the central region or the
peripheral region of the spatial region on the workpiece;
generating a first laser output; propagating the first laser output
along an optical path including a single pass actuation assembly
that is selectively changeable to provide at least two different
focal effects including a first focal effect to provide a first
spot area and a second focal effect to provide a second spot area
wherein the first and second spot areas are smaller than and fit
within the spatial region; applying the first laser output to the
first region to remove material from the spatial region, the first
laser output containing at least a first laser pulse that acquires
the first focal effect and has a first fluence over the first spot
area, and the first fluence being greater than the ablation fluence
threshold of the material; addressing a beam positioner toward a
second region selected from the central region or the peripheral
region of the spatial region on the workpiece; causing the single
pass actuation assembly to provide the second focal effect that is
different from the first focal effect; generating a second laser
output; propagating the second laser output along the optical path
including the single pass actuation assembly; applying the second
laser output to the second region to remove material from the
spatial region, the second laser output containing at least a
second laser pulse that acquires the second focal effect and has a
second fluence over a second spot area, and the second fluence
being greater than the ablation fluence threshold of the material
and different from the first fluence.
65. The method of claim 64 in which the first region is the central
region and the second region is the peripheral region, or in which
the first region is the peripheral region and the second region is
the central region.
66. The method of claim 65 further comprising: applying the second
output prior to addressing the beam positioner toward a first
region of a second spatial region on the workpiece wherein the
second spatial region is noncontiguous with the first spatial
region.
67. The method of claim 65 wherein the fluence applied to the
peripheral region is greater than the fluence applied to the
central region.
68. The method of claim 67 wherein the material is metal.
69. The method of claim 67 wherein the material is dielectric.
70. The method of claim 65 wherein the material comprises first
layer material, wherein the workpiece comprises multiple layers
including at least first and second layers of respective first
layer and second layer materials having respective first and second
ablation fluence thresholds, and wherein the ablation threshold is
the first ablation threshold, further comprising: addressing the
beam positioner toward a first addressed region selected from the
central region or the peripheral region of the spatial region on
the workpiece; causing the single pass actuation assembly to
provide a third focal effect; generating a third laser output;
propagating the third laser output along the optical path including
the single pass actuation assembly to provide the third focal
effect to provide a third spot area that is smaller than and fits
within the spatial region; applying the third laser output to the
first addressed region to remove second layer material from the
spatial region, the third laser output containing at least a third
laser pulse that acquires the third focal effect and has a third
fluence over the third spot area, and the third fluence being
greater than the second ablation fluence threshold and less than
first ablation threshold; addressing a beam positioner toward a
second addressed region selected from the central region or the
peripheral region of the spatial region on the workpiece; causing
the single pass actuation assembly to provide a fourth focal effect
that is different from the third focal effect; generating a fourth
laser output; propagating the fourth laser output along the optical
path including the single pass actuation assembly; applying the
fourth laser output to the second addressed region to remove second
layer material from the spatial region, the fourth laser output
containing at least a fourth laser pulse that acquires the fourth
focal effect and has a fourth fluence over a fourth spot area, and
the fourth fluence being greater than the second ablation fluence
threshold and less than first ablation threshold and different from
the third fluence.
71. The method of claim 70 in which the first addressed region is
the central region and the second addressed region is the
peripheral region, or in which the first addressed region is the
peripheral region and the second addressed region is the central
region.
72. The method of claim 71 further comprising: causing the single
pass actuation assembly to switch between providing the first focal
effect and the second focal effect in less than 2 ms.
73. The method of claim 72 further comprising: causing the single
pass actuation assembly to switch between providing the first focal
effect and the second focal effect in less than 1 ms.
74. The method of claim 71 wherein the single pass actuation
assembly comprises a deformable mirror having a mirror surface of a
first shape for providing the first focal effect, further
comprising: changing the mirror surface to have a second shape to
provide the second the second focal effect.
75. The method of claim 74 further comprising: applying a voltage
to an actuator supporting the mirror surface to switch between the
focal effects.
76. The method of claim 75 wherein the actuator comprises an
electrostrictive PMN device.
77. The method of claim 74 wherein the deformable mirror has a
response time of less than 0.5 ms.
78. The method of claim 71 wherein the single pass actuation
assembly comprises distinct first and second focal paths that
create the respective first and second focal effects.
79. The method of claim 78 in which a pair of galvanometer mirrors
effect switching between the first and second focal paths.
Description
[0001] This patent application derives priority from U.S.
Provisional Application No. 60/193,581, filed Mar. 30, 2000.
TECHNICAL FIELD
[0002] The present invention relates to laser micromachining and,
in particular, to a method and apparatus employing a single pass
actuation (SPA) assembly to vary the power density of ultraviolet
laser output applied to a target surface during processing of
multilayer workpieces having at least two layers with different
absorption characteristics in response to ultraviolet light.
BACKGROUND OF THE INVENTION
[0003] The background is presented herein only by way of example to
multilayer electronic workpieces, such as integrated-circuit chip
packages, multichip modules (MCMs) and high-density interconnect
circuit boards, that have become the most preferred components of
the electronics packaging industry.
[0004] Devices for packaging single chips such as ball grid arrays,
pin grid arrays, circuit boards, and hybrid microcircuits typically
include separate component layers of metal and an organic
dielectric and/or reinforcement materials, as well as other new
materials. A standard metal component layer typically has a depth
or thickness of greater than 5 .mu.m, a standard organic dielectric
layer typically has a thickness of greater than 30 .mu.m, and a
standard reinforcement component "layer" typically has a thickness
of greater than 5 .mu.m disbursed throughout the dielectric layer.
Stacks having several layers of metal, dielectric, and
reinforcement material are often thicker than 2 mm.
[0005] Much recent work has been directed toward developing
laser-based micromachining techniques to form vias in, or otherwise
process, these types of electronic materials. Vias are discussed
herein only by way of example to micromachining and may take the
form of complete through-holes or incomplete holes called blind
vias. Unfortunately, laser micromachining encompasses numerous
variables including laser types, operating costs, and laser- and
target material-specific operating parameters such as beam
wavelength, power, and spot size, such that the resulting machining
throughputs and hole quality vary widely.
[0006] In U.S. Pat. No. 5,593,606, Owen et al. describe advantages
of employing UV laser systems to generate laser output pulses
within advantageous parameters to form vias through at least two
layers of 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.
[0007] In U.S. Pat. No. 5,841,099, Owen et al. vary laser output
within similar parameters to those described above to have
different power densities while machining different materials. They
change the intensity by changing the laser repetition rate and/or
the spot size. In one embodiment, they employ a first laser output
of a high intensity to ablate a metallic layer and a second laser
output of lower intensity to ablate an underlying dielectric layer
so a lower metal layer can act as a laser etch stop in blind via
operations.
[0008] In one implementation, Owen et al. change spot size by
raising and lowering the objective lens to change the energy
density of the laser spot impinging upon the workpiece. In most
conventional laser systems, changing the height of the objective
lens is a slow process because moving the vertical (Z) stage
requires at least several hundred milliseconds (ms).
[0009] In another implementation, Owen et al. change the repetition
rate of the laser to change the energy density of the laser spot
impinging the workpiece. However, for a given laser power, if the
energy per pulse is decreased, for example, by increasing the
repetition rate, then more pulses and consequently more time is
needed to apply the total energy that must be delivered to the
workpiece to drill the via. Thus, this implementation also
generally impacts throughput.
[0010] Even within the parameters established by Owen et al.,
skilled persons would need to further tailor the repetition rate
changes and other process parameters to suit particular workpieces
to produce vias meeting all the criteria for quality, including the
via wall taper, the degree of melting of the copper layer at the
bottom of the via, and the height of the "rim" around the periphery
of the via caused by the splash of molten copper during drilling.
These parameters are difficult to optimize for throughput as well
as for all the criteria for quality.
[0011] Because these energy density changing methods are time
consuming or complex, the conventional process for machining
through multilayer devices is typically at least a two pass
operation. Such two pass operations involve sequentially removing a
first layer of a first material at all of the desired target
locations at a first energy density. Once all of the holes are made
through the first layer, the spot size and/or repetition rate is
changed to achieve a second energy density, which is then used to
remove a second layer of a second material at all of the desired
target locations.
[0012] The major disadvantage of such two pass operations is that
the typical hole-to-hole move time of 2-10 ms is relatively slow
and each hole must be addressed twice, resulting in a total via
formation time of 4-20 ms plus actual drilling time. FIG. 1 shows a
best-case conventional time line for a double-pass, two-step via
drilling process, assuming a hole drilling time of 2 ms and a 2 ms
move time.
[0013] A faster and more reliable way of changing the energy
density of laser output between first and second layer machining
operations is therefore desirable.
SUMMARY OF THE INVENTION
[0014] An object of the present invention is, therefore, to provide
a method or apparatus for quickly changing the energy density of
laser output to facilitate machining of workpieces.
[0015] Another object of the invention is to improve the throughput
of workpieces in such laser machining operations.
[0016] A further object of the invention is to facilitate one-pass
processing of workpieces.
[0017] Changing the laser spot size is more practical than changing
the repetition rate to alter the energy density because if a laser
system decreases the power density by maintaining the energy per
pulse but spreading it out over a larger laser spot area, the laser
system can apply the same total energy with fewer pulses. Hence,
the system can process the workpieces faster. The present invention
preferably, therefore, conserves the total energy per pulse and
employs a system or method that rapidly changes the area of the
laser spot impinging upon the workpiece. By changing the laser spot
size in a period of less than a few milliseconds, the present
invention can eliminate the conventional second pass of
hole-to-hole moves, and the throughput of the overall process can
be substantially increased.
[0018] The present invention employs a single pass actuation
assembly to change the energy density of laser output pulses
between at least two different values to facilitate processing
different layers at different energy densities. In a preferred
embodiment of the present invention, a deformable mirror permits a
quick, preferably less than one millisecond, change of focus of UV
laser output to change the spot size of the focused beam waist and
hence its energy density without requiring Z-axis movement of the
laser positioning system.
[0019] Deformable mirrors have been employed as adaptive optics for
IR- and visible-wavelength lasers in astronomy and climatology
applications to compensate for atmospheric turbulence in order to
keep the fluence constant.
[0020] In U.S. Pat. No. 5,667,707, Klingel et al. employ a laser
system with a deformable mirror to cut or weld metal of huge panels
having surfaces that are not particularly flat. Their laser
operation requires a high-energy tightly-focused laser spot to
efficiently process the metal target. They employ the deformable
mirror to change the focus height of the laser spot to maintain the
size of the laser spot at the target surface regardless of its
flatness and hence maintain the laser spot's high fluence
throughout the metal processing operation. The deformable mirror
has a soft surface whose curvature is manipulated by varying fluid
pressure. The mirror response time is relatively slow.
[0021] A preferred deformable mirror employs a flexible face sheet
made from an optically flat and coated machined piece of glass or
other common optical substrate that is rigidly attached to two
concentric circles of an electrostrictive actuator, preferably made
of PMN (lead magnesium niobate). The outer circle of the actuator
is active and increases in length with applied voltage. The inner
circle is not connected to power and is therefore inactive.
Whenever a voltage is applied to the actuator, the outer PMN
material expands, pushing on the outer rim of the face sheet while
the inner PMN material holds the center of the face sheet firmly in
position. The resulting surface contour of the face sheet is
concave. The back of the face sheet is machined such that the
active concave surface contour is smooth and continuous and has the
correct optical figure over its clear aperture so the reflected
beam wavefront is precisely spherical. The use of the PMN
electrostrictive actuator allows focus changes to be accomplished
in less than about 0.5 ms.
[0022] Other embodiments may employ galvanometer-driven mirrors to
divert the laser beam to an alternative focal path to change the
size of the laser spot area.
[0023] 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
[0024] FIG. 1 is a conventional time line for a double-pass,
two-step via drilling process.
[0025] FIG. 2 is an isometric view of a simplified laser system
incorporating a deformable mirror in accordance with present
invention.
[0026] FIG. 3 is an isometric sectional view of a deformable mirror
mechanism employed in the laser system of FIG. 2 and depicting a
mirror face sheet in an inactive shape.
[0027] FIG. 4 is a frontal view of the deformable mirror mechanism
that depicts the flexible sheet in an active shape.
[0028] FIG. 5 is an exploded view of actuator parts forming part of
the deformable mirror mechanism of FIG. 3.
[0029] FIG. 6 is an exploded view of an actuator housing.
[0030] FIG. 7 is a partly exploded isometric view of a mounting
assembly employed to align the deformable mirror mechanism within a
beam path.
[0031] FIG. 8 is an exemplary time line for a single-pass, two-step
via drilling process of the present invention.
[0032] FIG. 9 is a simplified schematic view of an alternative
single pass actuator assembly, which employs a galvanometer mirror
assembly, that can be substituted for the deformable mirror
mechanism in the laser system of FIG. 2.
[0033] FIG. 10 is a detailed side view of the galvanometer mirror
assembly shown in FIG. 9.
[0034] FIG. 11 is a top view of the galvanometer mirror assembly
shown in FIG. 10.
[0035] FIG. 12 is an enlarged sectional side elevation view of a
multilayered workpiece having a through-hole and a blind via.
[0036] FIG. 13 is an irradiance versus spot diameter graph showing
a profile of a first laser output pulse having irradiance
sufficient to ablate metal and a second laser output pulse having
irradiance sufficient to ablate dielectric but not ablate
metal.
[0037] FIG. 14 is a diagram that shows qualitatively the
differences in spot size that correspond to different distances
between the workpiece and the laser beam focal plane.
[0038] FIG. 15 is fluence versus repetition rate graph showing
metal and dielectric ablation thresholds for a fixed spot size.
[0039] FIG. 16 is a graph showing high and low focus level profiles
for fluence versus pulse repetition frequency.
[0040] FIGS. 17A and 17B are fragmentary cross-sectional views
showing the sequential steps carried out to form a depthwise
self-limiting blind via of a workpiece composed of a layer of
dielectric material positioned between a top conductor layer and a
bottom conductor layer.
[0041] FIG. 18 is a fragmentary cross-sectional view of an
incomplete top layer opening for a via that can be machined in
accordance with the present invention.
[0042] FIG. 19 is a fragmentary cross-sectional view of a workpiece
that is similar to the workpiece of FIGS. 17A and 17B but has a
second dielectric layer positioned between conductor layers.
[0043] FIG. 20 is a fragmentary cross-sectional view of the
workpiece of FIG. 19 but with a blind via characterized by a
depthwise-stepped width of increasing diameter from a top conductor
layer to a bottom conductor layer.
[0044] FIGS. 21 and 22 show cutting profiles for forming a
through-hole and a blind via, respectively.
[0045] FIG. 23 is an alternative trepanning profile for forming a
blind via.
[0046] FIG. 24 is a conventional line-cutting profile.
[0047] FIG. 25 is an exemplary fluence versus time profile.
[0048] FIG. 26 is an exemplary focus degree versus time profile for
a laser, two-step blind via-drilling process.
[0049] FIG. 27 is an alternative exemplary focus level versus time
profile for a laser blind via-drilling process.
[0050] FIG. 28 is an alternative exemplary focus level versus time
profile for a laser blind via-drilling process.
[0051] FIG. 29 is an alternative exemplary focus level versus time
profile for a laser blind via-drilling process.
[0052] FIGS. 30 and 31 demonstrate how via taper is a function of
beam profile.
[0053] FIG. 32 shows an exemplary trepanning profile for reducing
via taper.
[0054] FIG. 33 is an exemplary focus level versus time profile for
a blind via trepanning process employing a second step of a single
subpass.
[0055] FIG. 34 is an ideal focus level versus time profile for a
blind via trepanning process employing a second step of at least
two subpasses.
[0056] FIG. 35A and 35B are respective steps in an alternative
exemplary trepanning profile.
[0057] FIG. 36 is an exemplary line-cutting profile that can be
implemented with the present invention.
[0058] FIG. 37 is an enlarged sectional side elevation view of a
multilayered workpiece showing a lip formed around the surface
perimeter of a via.
[0059] FIG. 38 is a depiction of how low beam energy affects lip
formation.
[0060] FIG. 39 is a depiction of how high beam energy affects lip
formation.
[0061] FIGS. 40-42 show comparisons between single pass and double
pass via drilling operations having a variety of via drilling times
and hole-to-hole move times.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENT
[0062] With reference to FIG. 2, a preferred embodiment of a laser
system 10 of the present invention includes Q-switched,
diode-pumped (DP), solid-state (SS) laser 12 that preferably
includes a solid-state lasant such as Nd:YAG, Nd:YLF, Nd:YAP, or
Nd:YVO.sub.4, or a YAG crystal doped with holmium or erbium. Laser
12 preferably provides harmonically generated laser output 38 of
one or more laser pulses at a wavelength shorter than 550 nm such
as about 532 nm, and preferably shorter than 400 nm 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. Lasers 12 and harmonic generation
techniques are well known to skilled practitioners. Details of one
exemplary laser 12 are described in detail in U.S. Pat. No.
5,593,606 of Owen et al. Skilled persons will also appreciate that
other pumping sources, such as a krypton are lamp, or other
wavelengths are available from the other listed lasants. The
pumping diodes, arc lamp, or other conventional pumping means
receive power from a power supply 14.
[0063] With reference to FIG. 2, laser output 38 may be manipulated
by a variety of well-known optics including beam expander lens
components 44 and 46 that are positioned along beam path 48 before
being directed by a series of beam-directing reflectors 50, 52, and
54 (such as Z, Y, and X positioning mirrors), flexible mirror face
sheet 56, turn mirror 58, and fast positioner 60 (such as a pair of
galvanometer mirrors) of beam positioning system 62. Finally, laser
output 38 is passed through a focusing lens 64 before being applied
as processing output beam 66 with laser spot 67 at workpiece
40.
[0064] A preferred beam positioning system 62 is described in
detail in U.S. Pat. No. 5,751,585 of Cutler et al. and may include
ABBE error correction means described in U.S. patent application
Ser. No. 09/755,950, filed Jan. 5, 2001, of Cutler. Beam
positioning system 62 preferably employs a translation stage
positioner that preferably controls at least two platforms or
stages 63 and 65 and supports positioning components 56, 58, and 60
to target and focus processing output beam 66 to a desired laser
target position 68. In a preferred embodiment, the translation
stage positioner is a split-axis system where a Y stage 63,
typically moved by linear motors, supports and moves workpiece 40,
an X stage 65 supports and moves fast positioner 60 and objective
lens 64, the Z dimension between the X and Y stages is adjustable,
and beam-directing reflectors 50, 52, and 54 align the beam path 64
through any turns between laser 12 and flexible sheet 56. Beam
positioning system 62 permits quick movement between target
positions 68 on the same or different circuit boards or chip
packages to effect unique or duplicative processing operations
based on provided test or design data.
[0065] A laser system controller 16 preferably synchronizes the
firing of laser 12 to the motion of stage 63 and 65 and fast
positioner 60 in a manner well known to skilled practitioners. One
example of such coordination is described in U.S. Pat. No.
5,453,594 of Koneeny for Radiation Beam Position and Emission
Coordination System. Laser system controller 16 is shown
generically to control fast positioner 60, stages 63 and 65, power
supply 14, laser 12, and DMM controller 20. Skilled persons will
appreciate that laser system controller 16 may include integrated
or independent control subsystems to control and/or provide power
to any or all of these laser components and that such subsystems
may be remotely located with respect to laser system controller
16.
[0066] An example of a preferred laser system 10 that contains many
of the above-described system components employs a Model 210
UY-3500 laser sold by Lightwave Electronics of Mountain View,
Calif. in a Model 5320 laser system or others in its series
manufactured by Electro Scientific Industries, Inc. (ESI) in
Portland, Oreg. Skilled persons will also appreciate that a system
with a single X-Y stage for workpiece positioning and a fixed beam
position and/or stationary galvanometer for beam positioning may
alternatively be employed.
[0067] Laser system output beam 66 preferably produces a spot area
67 of diameter, d.sub.spot, at target position 68 on workpiece 40.
Although spot area 67 and d.sub.spot generally refer to
1/e.sup.2dimensions, especially with respect to the description of
laser system 10, 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.
[0068] FIG. 3 is an isometric sectional view of a deformable mirror
mechanism (DMM) 70 that employs an actuator 72 that supports and
creates a shape change in flexible sheet 56, which is preferably
made from an optically flat piece of glass or similar material. In
a preferred embodiment, flexible sheet 56 is rigidly attached by a
UV resistant adhesive 74, such as epoxy, to inner and outer
concentric zones 76 and 78 of a ferroelectric ceramic actuator
material, such as PMN. The electrostrictive PMN actuator material
has high electromechanical conversion efficiency, exhibits wide
operating and manufacturing temperature ranges, does not require
permanent polarization, and provides useful mechanical activity
with small electrical drive voltages.
[0069] Although a piezoelectric-type (PZT) actuator 72 could be
employed, PMN material is preferred because it avoids silver
migration, which is a function of field effect and humidity, that
is common with PZT actuators 72. Thus, the PMN material does not
creep with time and requires no re-calibration, so once flexible
sheet 56 has been assembled and polished, it will remain flat, for
example, without an offset voltage. Furthermore, it is likely that
a suitable PZT actuator 72 would last only 25% (about one year) as
long as a suitable PMN actuator. In addition to PZT actuators,
skilled persons will appreciate that any precision high-bandwidth
actuators 72 such as voice coils could be employed for DMM 70.
Skilled persons will appreciate that flexible face sheet 56 could
be actuated by a small array of DMMs 70, such as 6-9 DMMs 70, to
provide greater control. Such arrays would, however, typically
employ closed loop feedback and would be more expensive to
implement for the model 53xx and 54xx laser systems of ESI.
[0070] FIG. 4 is a frontal view of DMM 70 that depicts flexible
sheet 56 in an activated shape. With reference to FIGS. 3 and 4, in
a preferred embodiment, outer zone 78 of actuator 72 is active and
increases in length with applied voltage, and inner zone 76 is not
connected to power and is therefore always inactive. Whenever a
voltage is applied to outer zone 78, its PMN material expands,
pushing on the outer rim 80 of flexible sheet 56 while the inner
PMN material 82 holds center 83 of flexible sheet 56 firmly in
position. The resulting active surface contour of flexible sheet 56
is concave. Backside 84 of flexible sheet 56 is machined such that
the active concave surface contour is smooth and continuous and has
the correct optical figure over its clear aperture so the wave
front of reflected beam 86 is precisely spherical. Actuator 72
allows focus changes to be accomplished in less than about 2 ms,
preferably less than 1 ms, and most preferably less than 0.5
ms.
[0071] FIG. 5 is an exploded view of actuator parts forming part of
DMM 70. With reference to FIGS. 3-5, single crystal silicon (Si),
fused silica, or fused silica/ULE.TM. (ultra low expansion-UV
grade) are preferred options for the material of flexible sheet 56
and collar 85. Fused silica transmits 355 nm light that might
damage the adhesive or actuator 72 if coatings on flexible sheet 56
are not 100% reflective. Silicon crystal absorbs 355 nm light.
Fused silica/ULB.TM. is a good choice for applications exposed to
large temperature changes. A highly transmissive flexible sheet 56
minimizes the chances of absorbing any radiation that passes
through the reflective coatings and incurring heat absorption
damage that could affect the beam size (and hence its intensity),
beam shape, or beam position. Skilled persons will appreciate that
other suitable materials could be employed, or they can be
particularly selected for use at a different wavelengths of
interest, such as 266 nm.
[0072] In a preferred embodiment, flexible sheet 56 is coated to
provide a reflectivity of at least 99% at 355 nm and protect
actuator 72 from laser energy damage. Because bare glass has only a
4% reflectivity at 355 nm, the reflectivity is brought up from 4%
to 99% with many layers of dielectric coating. However, if aluminum
(Al), which is 85% reflective to 355 nm, is used as a base coating
layer, then fewer dielectric layers can be employed to increase the
reflectance from 85% to 99%. A preferred dielectric coating
includes 5-20 layers of SiO.sub.2 and 5-20 layers of
Ta.sub.2O.sub.5, has a total a thickness of about 1-4 .mu.m, and is
applied at about 140 to 170.degree. C. by ion assisted deposition.
The coatings provide good durability against mechanical damage from
deformation as well as low absorption so adhesive 74 and actuator
72 are not significantly exposed to the UV light. Other coating
preparations are well known to skilled practitioners and can be
selected to facilitate the use of different wavelengths such as 266
nm.
[0073] In a preferred embodiment, deformable mirror mechanism 70
has an operational temperature range of about 40.degree. C. This
range includes a wide range around typical room temperature, but
can be made wider if temperature feedback is used. PMN actuator
material is sufficiently stable that DMM 70 can be operated in an
open-loop control mode. However, due to possible deformation caused
by temperature changes (such as a 2% change in stroke per 1.degree.
C. at operating temperatures), DMM 70 may include an internal
temperature sensor that permits compensation for stroke changes due
to temperature effects. If an Si flexible sheet 56 absorbs 1% of 3
watts, the temperature of DMM 70 could increase by 0.04.degree.
C.
[0074] The preferred DMM 70 is also adapted to withstand 30
milliwatts (mW) or greater of laser energy. If, for example, the
dielectric coating reflects 99% of the beam and flexible sheet 56
passes all of the unreflected energy, then DMM 70 will be exposed
to a fraction of the 30 mW proportional to its surface area.
Coating the backside of flexible sheet 56 with aluminum is one
preferred method for absorbing the laser light and protecting
actuator 72 and adhesive 74. At 99% reflectivity, the 30 mW of heat
from a 3-W laser beam and/or air temperature variation could cause
thermal expansion which could affect flatness. The actuator
position can thus be adjusted to compensate for any such
temperature related effects to flatness.
[0075] Deformable mirror mechanism 70 has dimensions that are
preferably less than about 50 mm.times.50 mm.times.25 mm, and most
preferably less than about 25 mm.times.25 mm.times.6 mm. In a
preferred laser system 10, DMM 70 is mounted above fast positioner
60 on the X positioning stage and replaces the 90 degree turn
mirror of some conventional beam positioning systems.
[0076] Precise alignment of DMM 70 to beam path 48 is desirable as
it is for conventional mirrors in beam path 48. In particular, it
is preferable to align DMM 70 so that center 83 of flexible face
sheet 56 is in the center of beam path 48 than to align the center
of beam path 48 to hit center 83 of flexible face sheet 56.
[0077] A variety of factors affect the ability of beam path 48 to
strike center 83 of flexible sheet 56. For example, if beam path 48
is not parallel to the motion of X-stage 65, a change in X will
cause a lateral displacement in the beam path 48 relative to
flexible sheet 56. In addition, misalignment of beam path 48 from
center 83 of flexible sheet 56 can cause the position of output
beam 66 on the surface of workpiece 40 to shift when flexible sheet
56 is actuated.
[0078] FIG. 6 is an exploded view of a preferred actuator housing
88, and FIG. 7 is a partly exploded view of a preferred mounting
assembly 100 that is employed to support and align DMM 70. With
reference to FIGS. 5-7, actuator 72 is preferably mounted with
epoxy to an invar plate 90, and bolts 92 are bolted through plate
holes 93 to holes 94 in actuator frame 95. Power control wires 96,
which are connected to actuator 72 at terminals 97, are fed through
plate holes 98. Power supplied through control wires 96 is
responsive to commands from DMM controller 20.
[0079] Actuator housing 99, including plate 90 and frame 95, is
then mounted within mounting assembly 100 having a mounting base
102. Although a conventional turn mirror has only a pitch and yaw
adjustment, mounting assembly 100 is preferably also provided with
an X/Y translation adjustment to facilitate alignment of flexible
sheet center 83 with beam path 48. Mounting assembly 100 thus
preferably has four degrees of freedom, two for laser alignment and
two for centering flexible sheet 56 within beam path 48. The first
two degrees of freedom are translation in the X and Y axes of
positioning system 62. Along both the X and Y axes, mounting
assembly 100 has an adequate range of adjustment, such as .+-.6 mm,
to accommodate maximum positional tolerances of the beam position
as directed by the other components of positioning system 62. Both
X plate 104 and Y plate 106 move along ball bearings in grooves
between them. The X and Y plates 104 and 106 have retaining screws
110 and translation adjustment screws 112 and 114 that are loaded
by extension springs 116 and 118 for easy adjustment. Retaining
screws 110 and adjustment screws 112 and 114 also act to prevent
mounting assembly 100 from separating under extreme loads. The
total weight of assembly 100 is preferably low enough so that its
mass contribution to X stages 65 does not adversely affect desired
acceleration and deceleration of the total mass of X stage 65. An
exemplary weight for assembly 100 is less than about 200 grams.
Assembly 100 is held securely in the event that X stage 65 hits a
limit and experiences high decelerations.
[0080] Rotation plate 120 has grooves for mounting over ball
bearings 122 to provide flexible sheet center 83 with the two
degrees of freedom of angular rotation with respect to the X and Y
axes. Adjustment screws 124 and preload screws 126 and springs 128
lock and pre-load rotation plate 120. Other kinematic mounting
assemblies could be employed and are well within the knowledge of
skilled practitioners.
[0081] For alignment, first the two X and Y rotational adjustments
are made to align the laser, then center 83 of flexible sheet 56 is
aligned to the center of beam path 48. Flexible sheet 56 can be
actuated and relocated to focus the beam spot. Mounting assembly
100 in conjunction with a beam path alignment procedure allows beam
path 48 to be aligned so that less than .+-.10 .mu.m of shift is
realized over the full travel length of X stage 65. Such a full
travel length shift may result in a typical displacement that is
less than 1 .mu.m and is comparable to or less than high-frequency
random beam positioning variations of pulse to pulse from a
Q-switched laser. Because the spot diameter in many applications is
about 100 .mu.m at maximum defocus, this maximum positioning error
is inconsequential. When desirable, however, skilled practitioners
can select application parameters to compensate for this error.
[0082] In a preferred embodiment, mounting assembly 100 is adapted
for easy upgrade of existing lasers and positioning systems 62,
such as employed in models 5200 or 5320 manufactured by Electro
Scientific Industries, Inc. of Portland, Oreg., and can be easily
exchanged for the 90 degree turn mirror on X stage 65 of
conventional laser systems.
[0083] In a preferred embodiment, beam path 48 preferably strikes
flexible sheet 56 at a 45 degree angle and then travels through the
galvanometer mirror scanners of fast positioner 60 and the
objective lens 64. For laser drilling operations, a preferred
objective lens focal length is about 50-100 mm, and a preferred
distance from the flexible mirror face sheet 56 to objective lens
64 is as small as practical within design constraints and
preferably less than about 300 mm when the Z-stage (not shown) is
at its normal focus height.
[0084] When the flexible sheet 56 is flat, it has a clear aperture
that is large enough to accommodate the desired size of the laser
spot and is preferably about 5-10 mm at a 45 degree angle of
incidence, and the reflected beam 86 remains collimated. When the
flexible sheet 56 is actuated, the mirror surface becomes concave
in an elliptical fashion (to compensate for 45 degree beam
incidence) and causes the beam to converge proportionately to the
radius of the mirrored surface. From an applications standpoint, it
is more desirable to have the beam undistorted when focused than
when unfocused. Therefore, the flexible sheet 56 is preferably flat
when an in focus output beam 66 is applied to workpiece 40 and is
preferably concave when an out of focus output beam 66 is applied
to workpiece 40. When flat, deformable mirror mechanism 70
preferably produces a diffraction-limited focused spot. DMM 70
preferably compensates for astigmatism and keeps the spot round
when in the defocused configuration. The oval shape of the concave
mirror corrects for astigmatism caused by the 45 degree incidence.
The un-actuated/actuated mirror preferably maintains its flatness
specification under the thermal stress of reflecting a 3 watt or
higher wattage beam. The curvature of the deformed mirror should
minimize the distortion of irradiance at the beam center. A convex
actuator could alternatively be employed.
[0085] With reference again to FIGS. 2-5, DMM controller 20 is in
communication with actuator 72 and controls the focal effect of
flexible sheet 56. DMM controller 20 is also preferably in
communication with laser system controller 16 such that focal
effect changes to flexible sheet 56 can be coordinated with the
firing of laser output 38 and/or the movements directed by
positioning system 62. Actuator 72 is preferably driven in the
range of about 0 to 100 volts. However, 120 V or higher voltage can
be employed if more stroke is desired but may be limited by the
amount of stress applied to DMM 70. Actuator 72 preferably has a
response time to focus of less than about 2 ms, and more preferably
less than about 1 ms, including settling time. The settling
criteria is preferably less than about .+-.1% focal length. A
preferred design actuation frequency is about 300 Hz.
[0086] If a temperature sensor is employed to compensate for
changes in the actuator stroke due to temperature, the drive
electronics of DMM controller 20 can receive a temperature sensor
signal from DMM 70 and laser system controller 16 is adapted to
support a calibration procedure and provide closed loop control of
the mirror temperature compensation. DMM controller 20 preferably
interfaces with actuator 72 through a DAC and one or more FETs, and
temperature feedback can be applied through an ADC. Actuator 72
preferably exhibits a hysteresis of less than 10%.
[0087] A graphical user interface and/or host software common to
laser system controllers 16 is preferably modified to add mirror
focal length to the properties of the drilling tool/operations, and
diagnostics can be updated to allow an operator direct control of
the mirror focal length for testing and alignment.
[0088] FIG. 8 is an exemplary time line for a single-pass, two-step
via drilling process of the present invention. The throughput
analysis assumes a hole drilling time of 2 ms and a 2 ms move time.
Actuator 72 decreases the total time to process a hole to 4.5 ms.
The time for re-establishing focus is absorbed in the move time to
the next hole. With reference to FIGS. 27-29, it is also possible
to keep the laser on during most or all of the profiled defocusing
to save an additional 0.5 ms per hole.
[0089] FIG. 9 is a simplified schematic view of an alternative
single pass actuator assembly 200, which employs alternative
galvanometer mirror pathways 202 and 204, that can be substituted
for the deformable mirror mechanism 70 in the laser system of FIG.
2. FIG. 10 is a detailed side view of the galvanometer mirror
pathways 202 and 204 shown in FIG. 9. FIG. 11 is a top view of the
galvanometer mirror pathways 202 and 204 shown in FIG. 10. With
reference to FIGS. 2 and 9-11, beam path 48 is directed toward
galvanometer mirror 206 that either permits laser output 38 to
propagate along pathway 202 through collimating lens components 210
and past galvanometer mirror 208 or reflects laser output 38 off
mirror 212, along pathway 204, through collimating lens components
214, off mirror 216, off galvanometer mirror 208, and toward
workpiece 40.
[0090] One of collimating lens components 210 or 214 creates focus
while the other creates defocus. Skilled persons will appreciate
that either or both of collimating lens components 210 or 214 can
be variable to modify the spatial spot size to suit different
applications. Skilled persons will also appreciate that collimating
lens components 210 in pathway 202 may be omitted as shown in FIGS.
10 and 11. For example, pathway 202 can implement defocus and
pathway 204 can implement focus. Alternatively, for example,
pathway 204 can implement defocus and collimating components 210
can be positioned after galvanometer mirror 208 or before
galvanometer mirror 206.
[0091] FIG. 12 is a cross-sectional side view of an enlarged
portion of a generic laser workpiece 40 that may, for example, be
an IC chip package, MCM, capacitor, circuit board, resistor, or
hybrid or semiconductor microcircuit. For convenience, workpiece 40
is depicted as having only four layers 264, 266, 268, and 270.
[0092] Layers 264 and 268 may contain, for example, standard metals
such as, aluminum, copper, gold, molybdenum, nickel, palladium,
platinum, silver, titanium, tungsten, metal nitrides, or
combinations thereof. Conventional metal layers 264 and 268 vary in
thickness, typically between 9-36 .mu.m (where 7.8.times.10.sup.-3
kg of metal equals a thickness of about 9 .mu.m), but may be
thinner or as thick as 72 .mu.m. Conductive layers 264 and 268 are
typically made of the same material.
[0093] Dielectric matrix or layer 266 may, for example, contain a
standard organic dielectric material such as benzocyclobutane
(BCB), bismaleimide triazine (BT), cardboard, cyanate esters,
epoxies, phenolics, polyimides, polytetrafluorethylene (PTFE),
various polymer alloys, or combinations thereof. Conventional
organic dielectric layers 266 vary considerably in thickness, but
are typically much thicker than metal layers 264 and 268. An
exemplary thickness range for organic dielectric layers 266 is
about 30-400 .mu.m.
[0094] Layer 266 may also contain a standard reinforcement
component 270 depicted as a woven line in FIG. 12 for convenience.
Components 270 may be fiber matte or dispersed particles of, for
example, aramid fibers, ceramics, or glass woven or dispersed
throughout organic dielectric layer 266 and may comprise much of
its thickness. Conventional reinforcement components 270 are
typically individual filaments or particles of about 1-10 .mu.m in
size and/or woven bundles of 10 .mu.m to several hundreds of
microns. Skilled persons will appreciate that reinforcement
components 270 may be introduced as powders into the organic
dielectrics and can be noncontiguous and nonuniform. Such composite
or reinforced dielectric layers 266 typically require laser
processing at a higher fluence than is needed to abate unreinforced
layers 266. Skilled persons will also appreciate that layers 264,
266, and 268 may also be internally noncontiguous, nonuniform, and
nonlevel. Stacks, having several layers of metal, dielectric, and
reinforcement material, may be larger than 2 mm.
[0095] Workpiece 40 in FIG. 12 also depicts a through-hole via 272a
and a blind via 272b (generically via 272) produced by laser system
10. Through-hole 272a cleanly and evenly penetrates all layers and
materials of workpiece 40 and exhibits negligible taper from its
top 276 to its bottom 278. Taper angle .phi. is preferably less
than 45.degree., more preferably less than 30.degree., and most
preferably 0-10.degree. with respect to normal axis 277.
[0096] Blind via 272b does not penetrate all layers and/or
materials. In FIG. 12, blind via 272b stops at and does not
penetrate layer 268. Thus, proper selection of the laser parameters
permits layer 268 to remain unaffected even if it comprises the
same metal component(s) as layer 264.
[0097] Via diameters preferably range from 25-300 .mu.m, but laser
system 10 may produce vias 272 that have diameters as small as
about 5-25 .mu.m or greater than 1 mm. Because the preferred
ablated spot size of output beam 66 is preferably about 25-75 .mu.m
in diameter, vias larger than 25 .mu.m may be produced by
trepanning, concentric circle processing, or spiral processing.
[0098] FIG. 13 shows a graph demonstrating the inverse relationship
between irradiance and spot size for a given laser output power,
taking into account the natural Gaussian spatial irradiance profile
of laser beam 66. In FIG. 13, an exemplary profile of a pulse of
first laser system output 222 has a spot diameter and sufficient
energy (above metal ablation threshold 224) to ablate metal layer
264 and an exemplary profile of a pulse of second laser system
output 226 has a spot diameter that is substantially larger and
sufficient energy (above dielectric ablation threshold 228) to
ablate dielectric layer 266 but not to ablate metal layer 264.
Thus, in a preferred embodiment, a first laser system output 222 of
localized high irradiance or power density is used to ablate
metallic layer 264, and a second laser system output 226 of equal
power and greater spot size (lower irradiance) is used to ablate an
underlying dielectric layer 266. This two step method is especially
useful for making blind vias having a metallic bottom layer 268,
because the second laser system output 226 has a lower irradiance
that is insufficient to ablate metal layer 268, so only dielectric
layer 266 is removed. Thus, the two-step machining method provides
a depthwise self-limiting blind via because the fluence of second
laser system output 226 is insufficient to vaporize metallic bottom
layer 268, even if the second laser system output 226 continues
after dielectric layer 266 is completely penetrated. Skilled
persons will appreciate that in accordance with single pass method
of the present invention, the first and second laser system outputs
222 and 226 are temporally contiguous rather than employing a
series of first laser outputs to multiple space-apart targets over
an extended surface area of workpiece 40 and then employing a
series of second laser outputs to the same multiple spaced-apart
targets over the same extended surface area.
[0099] FIG. 14 is a diagram showing exemplary differences in spot
area and d.sub.spot at the surface of workpiece 40 that correspond
to different focal effects of actuator 72. With reference to FIG.
14, spot area 240 represents a high degree of focus when flexible
sheet 56 is unactivated and flat. Spot area 242 represents an
intermediate focal degree imparted by actuator 72 to flexible sheet
56 as an intended partial activation of sheet 56 or during its
transition to a low degree of focus. Spot area 244 represents a
larger spot area that corresponds to a full activation of sheet 56
and a low degree of focus. Skilled persons will appreciate that an
alternative embodiment could employ a flat sheet 56 to impart a low
degree of focus and an activated sheet 56 to impart a high degree
of focus. In addition, another embodiment contemplates a flexible
sheet 56 that is activated when flat and in active when curved or
shaped.
[0100] In one embodiment, the focused spot has a minimum 1/e.sup.2
spot size of 8-20 .mu.m, and preferably 10-16 .mu.m; and the
defocused spot has a maximum 1/e.sup.2 spot size of 40-150 .mu.m,
preferably 45-130 .mu.m, and most preferably 60-90 .mu.m. These
1/e.sup.2 spot sizes can be smaller or larger than the ablated spot
size. In particular, the ablated spot size is typically smaller for
very large defocused 1/e.sup.2 spot sizes when much of the spot
area is below the ablation threshold for a particular material.
[0101] The parameters of processing output beam 66 are selected to
facilitate substantially clean, sequential drilling, i.e., via
formation, in a wide variety of metallic, dielectric, and other
material targets that may exhibit different optical absorption,
ablation threshold, or other characteristics in response to UV or
visible light. Preferred parameters of first laser system output
222 include average energy densities greater than about 120 .mu.J
measured over the beam spot area, preferably greater than 200
.mu.J; spot size diameters or spatial major axes of less than about
50 .mu.m, and preferably from about 1-50 .mu.m; and a repetition
rate of greater than about 1 kHz, preferably greater than about 5
kHz, and most preferably even higher than 20 kHz; and a wavelength
preferably between about 190-532 nm, and most preferably between
about 266 nm and 355 nm. The preferred parameters of processing
output beam 66 are selected in an attempt to circumvent certain
thermal damage effects by utilizing temporal pulse widths that are
shorter than about 100 ns, and preferably from about 40-90 ns or
lower. Skilled persons will also appreciate that the spot area of
output beam 66 is generally circular, but may be slightly
elliptical.
[0102] FIG. 15 shows a graph indicating that the fluence of laser
output varies inversely with the pulse repetition rate or frequency
(PRF). Thus, for a given spot size, the fluence (energy density) of
the first laser system output 222 is greater than metal ablation
threshold 224, then the PRF is increased so the fluence of the
second laser system output 226 is below metal ablation threshold
224 but above dielectric ablation threshold 228. Skilled persons
will appreciate that as lasers 10 achieve greater peak output pulse
energies, the fluence versus PRF curve will be displaced higher in
the graph.
[0103] FIG. 16 is a graph showing a high focus level profile 230
and a low focus level profile 232 of fluence versus PRF. With
reference to FIG. 16, when flexible sheet 56 is flat, it provides
high focus profile 230 for a given laser output power; and when
flexible sheet 56 is activated, it provides low focus profile 232
for the same output power. In a preferred embodiment shown by arrow
233, metal layer 264 is processed with first laser system output
222 above metal ablation threshold 224, then flexible sheet 56 is
activated so dielectric layer 266 is processed with second step
laser system output 226 above dielectric ablation threshold 228 but
below the metal ablation threshold 224. In an alternative
embodiment shown by arrow 237, the metal layer 264 is processed by
first step laser system output 222, then dielectric layer 266 is
processed with alternative second step laser system output 226a in
which flexible sheet 56 is activated and the PRF is increased such
that output 226a is below ablation threshold 228a for reinforced
dielectric layers, for example, but above ablation threshold 228b
for unreinforced dielectric layers 266, for example. Skilled
persons will appreciate that threshold 224 will vary with the
thickness of metal layer 264.
[0104] FIGS. 17A and 17B are fragmentary cross-sectional views
showing the sequential steps carried out to form a depthwise
self-limiting blind via in a workpiece 40. FIG. 17A represents the
first step of delivering laser beam pulses at a first irradiance
that is above ablation threshold 224 of conductor layer 264. The
first step removes conductor layer 264 and a portion of dielectric
layer 266. FIG. 17B represents the second step of delivering laser
pulses at a second irradiance that is below the ablation threshold
224 of conductor layer 266 but above the ablation threshold 228 of
dielectric layer 162. This two-step method provides a depthwise
self-limiting blind via because the laser beam power density is
insufficient to progress depthwise beyond dielectric layer 266
material to vaporize conductor layer 268.
[0105] FIG. 18 is a fragmentary cross-sectional view of an
incomplete top layer opening 269 for a hole or via (shown in
phantom lines) that can be machined in accordance with the
invention. The situation depicted in FIG. 18 typically arises in an
incompletely chemically pre-etched metal top layer 264 that does
not expose dielectric layer 266 in a workpiece 40 that was intended
to be machined only through dielectric layer 266. The method steps
described with reference to FIGS. 17A and 17B can be carried out to
machine a via 272 into this workpiece 40.
[0106] FIG. 19 is a fragmentary cross-sectional view of a workpiece
40a that is similar to workpiece 40 but has a second dielectric
layer 266a positioned between conductor layer 268 and a third
conductor layer 268a. Dielectric layer 266a and conductor layer
268a have ablation thresholds in the same relative proportion to
those of dielectric layer 266 and conductor layer 268,
respectively. Thus, conductor layers 268 and 268a become the
respective middle and bottom conductor layers of workpiece 40a. To
form a blind via in workpiece 40a, one repeats the first step by
increasing the laser beam pulses to the first irradiance to machine
through layer 268 and then repeats the second step by decreasing
the laser beam pulses to the second irradiance to machine through
dielectric layer 266a and stop at layer 268a. Skilled persons will
appreciate that the first step irradiance could be employed to
machine through layers 264, 266, and 268 before the focal effect is
changed to produce second step irradiance.
[0107] FIG. 20 is a fragmentary cross-sectional view of workpiece
40 with a blind via 280 characterized by a depthwise, stepped width
of decreasing diameter from top layer 264 to bottom layer 268a. The
changes in width are accomplished by selectively decreasing the
laser working area after each successive layer or pair of layers,
as shown, is penetrated.
[0108] Except for scale, FIGS. 12 and 17-20 would have these
general appearances if they were formed by two-pass or simple
single pass process or formed by trepanning or spiraling as
described below. FIGS. 21 and 22 show cutting profiles for forming
a through-hole 272c and a blind via 272d, respectively, that are
larger than the spot size of output beam 66. With reference to FIG.
21, through-hole 272c defines on the surface of workpiece 40 a
circular spatial region 290 having a periphery 292. Output beam 66
has a spot area 294 that is less than the area of region 290.
Through-hole 272c is formed by sequentially positioning beam 66
having spot area 294 at overlapping contiguous locations around
periphery 292. Beam 66 is preferably moved continuously through
each location at a speed sufficient for system 10 to deliver the
number of beam pulses necessary to achieve the depth of cut at the
location. After beam 66 completes the path around periphery 292,
the center target material 296 falls out to form through-hole 272c.
This process is called trepanning.
[0109] With reference to FIG. 22, blind via 272d also defines on
the surface of workpiece 40 a circular region 290 having a
periphery 292. Output beam 66 having spot 294 is initially
positioned at the center 298 of region 290. Blind via 272d is
formed by sequentially positioning beam 66 having spot area 294 at
overlapping contiguous locations along a spiral path 299 to
periphery 292. Beam 66 is preferably moved continuously through
each location at a speed sufficient for system 10 to deliver the
number of beam pulses necessary to achieve the depth of cut at the
location. As beam 66 proceeds along spiral path 299, the target
material is "nibbled" away to form a hole of increasing size each
time beam 66 is moved to a new cutting location. The final shape of
the hole is achieved when beam 66 moves along a circular path at
periphery 292.
[0110] An alternative beam cutting path to form blind via 272d
would be to start at center 298 and cut concentric circles of
incrementally increasing radii defined by spot area 294 of beam 66.
The overall diameter of via 272d would increase as the concentric
circles forming via 272d travel in a circular path at greater
distances from center 298 of region 290. Alternatively, this
process may begin by defining the desired circumference and
processing the edges toward the center.
[0111] Outward spiral processing tends to be a little more
continuous and quicker than concentric circle processing. Skilled
persons will appreciate that either workpiece 40 or processing
output beam 66 may be fixed or moved relative to the position of
the other. In a preferred embodiment, both workpiece 40 and
processing output beam 66 are moved simultaneously. Several
examples of through-hole vias and blind vias of various depths and
diameters produced on a number of different substrates are set
forth in U.S. Pat. No. 5,593,606.
[0112] FIG. 23 is an alternative conventional trepanning profile
300 that moves a laser spot of consistent diameter along path 302
to form blind via 272d. This trepanning profile 300 can be useful
for creating mid sized blind vias 272d that have diameters that are
greater than the largest useful laser spot diameter but small
enough to be less efficient for spiraling. Typically, the laser
spot starts in center of the via to be formed. Skilled persons will
appreciate that blind vias of larger size can be created by
trepanning by adjusting the size and number of inner loops.
Additionally or alternatively, a partial defocus can be employed
for larger vias particularly with thinner metal layers 264 to
increase throughput.
[0113] Skilled persons will also appreciate that noncircular vias
may also be ablated through similar processes. Such vias may, for
example, have square, rectangular, oval, slot-like, or other
surface geometries. For example, FIG. 24 shows a conventional
line-cutting profile 304 including multiple parallel beam paths 306
impinged by a laser spot of consistent diameter to form a line
width 308.
[0114] FIGS. 25 and 26 are respective exemplary fluence versus time
and focus degree versus time profiles for a laser, two-step blind
via-drilling process. The focus degree is high for a sharply
focused beam 66 at the work surface, creating a small laser spot.
With respect to these profiles, all the pulses used in the first
step to process the first layer are focused at a first focus
degree, and all the pulses used in the second step to process the
second layer are focused at a second focus degree, regardless of
the via size (or trepanning or spiraling).
[0115] These profiles resemble, with the exception of move and
refocus time, the two step process of Owen et al., which sought to
minimize laser damage to the underlying layers. The damage can be
characterized in several ways: reflow of bottom metal layer 268,
ablation or material removal of the bottom metal layer 268, or
delamination of dielectric layer 266 from bottom metal layer 268.
With the standard two step process, the second step is performed
with beam characteristics such that the fluence level delivered to
bottom metal layer 268 is less than the damage threshold 224 of
that layer.
[0116] However, with respect to the present invention, the change
of focus degree is accomplished in an exclusively single pass
process employing a quick method of changing energy density at the
work surface. The focus profile of FIG. 26 can be achieved with
either DMM 70 or single pass actuation assembly 200, and the focus
"height" can be either decreased or increased to decrease the
degree of focus and hence decrease the energy density on the
surface at target location 68.
[0117] The single pass process of the present invention permits
numerous focus degree versus time profiles such as those shown in
FIGS. 27-29 that can be used advantageously for different
materials, thicknesses, and laser parameters. The laser spot size
changes can be performed while laser 12 is firing and beam
positioner 62 is moving. These latter profiles can be achieved with
DMM 70 but generally not with single pass actuation assembly 200
unless variable optics are employed. For the profiles of FIGS.
25-29, if the focus height is decreased (lowered) to bring the
laser beam out of focus, then energy density, focus degree, and
focus height could be used interchangeably to describe the Y-axis
with the realization that the relative scales would be
different.
[0118] With reference to FIGS. 25-27, as the second step begins to
remove the remaining dielectric layer 266, it would be more
efficient to use a higher fluence than used for the second step in
FIG. 25. However, as the second step clears away dielectric layer
266 and exposes the bottom metal layer 268 such that it begins to
absorb heat from laser beam 66, bottom metal layer 268 would be
better protected by using a lower fluence than used for the second
step in FIG. 25. Thus, a gradual defocus of the laser spot during
the second step as shown in FIG. 27 would be faster, more
efficient, and protect bottom metal layer 268 better than the
second step of FIG. 25. The curve can level off just above
dielectric ablation threshold 228 or just below the metal damage
level. Skilled persons will appreciate that the defocus curve of
FIG. 27 can be refined to suit particular types and heights of
dielectric layers 266 to increase the speed and efficiency of the
via drilling process.
[0119] Skilled persons will also appreciate that profiled focusing
can be implemented to form blind vias having diameters of the same
or larger size as the laser spot size that would require trepanning
or spiraling. For the larger sized vias, for example, profiled
focusing for machining the second layer as shown in FIGS. 27 or 28
could be coordinated with the positioner movement to control bite
size and repeated as desired to efficiently deliver the most energy
per time. The bite size is the area or distance of new target
material impinged by each sequential laser pulse. A focusing
profile that approximates such repetition might appear similar to
the focusing profile of FIG. 29.
[0120] With reference again to FIGS. 27-29, trepanning path changes
can also make more effective use of these and other focus profiles.
One net result of using such a focus profile method is that more
energy can be delivered per unit time than with a simple two-step
process without causing material damage to the inner layer. The
beam positioner can dynamically adjust the beam-path pitch to
ensure that the energy per unit area delivered to the hole or shape
remains more or less constant over the entire hole or shape. This
method can facilitate reduction of the heating effects caused by
the beam hitting the bottom layer 268.
[0121] FIGS. 30 and 31 demonstrate how sidewall taper .phi. is a
function of beam irradiance profile in the radial direction.
Sidewall taper .phi. results from the Gaussian nature of the
TEM.sub.00 beam profile of standard solid-state lasers. By
dynamically changing the pitch of a spiral in conjunction with the
profiled focus changes, it is possible to reduce the sidewall taper
.phi. of the cuts or holes created. A tightly focused beam at the
outermost spiral pitch also permits the sidewall to become
steeper.
[0122] FIG. 32 demonstrates a spiraling profile 299a for reducing
sidewall taper .phi.. When beam 66 is at the center 298 to begin
inner spiral section 314, beam 66 is further out of focus to ablate
a larger area at a lower fluence. When the beam positioner 62
reaches outer spiral section 316, beam 66 is focused more tightly
to remove or reduce sidewall taper .phi.. Spiraling profile 299a
can be employed for processing metal or dielectric at the
appropriate fluences. Because the bite size can be made larger at
the center 298 of spiraling profile 299a, corresponding to a larger
spot size of beam 66, this technique has the additional advantage
of decreasing the drilling time.
[0123] With reference to FIG. 33, for example, first layer 264 of
material is removed at one focus degree 310 and second layer 266 is
removed at two different lower focus degrees 312a and 312b. A large
inner pitch 318 ensures that bottom metal layer 268 is not damaged,
and a small outer pitch 320 facilitates reduction in taper .phi..
In an alternative embodiment not shown, the central region of first
layer 264 is processed at a focus degree 310 above metal ablation
threshold 224 and then the focus degree is increased to provide a
greater fluence around the peripheral region. Then, the focused
degree is dropped so that the fluence is between thresholds 224 and
228 for processing dielectric layer 266 in the peripheral region
and further dropped for processing dielectric layer 266 in the
central region.
[0124] Skilled persons will appreciate that FIG. 33 depicts a
profile where second layer 266 is removed in a single subpass.
Skilled persons will also appreciate that if the spiral is started
on the outside of the intended hole, a tighter focus would be
employed first for the periphery, and then inner spiral section 314
would be machined at greater defocus. This focus profile might
resemble that shown in FIG. 28.
[0125] FIG. 34 is an alternative focus degree versus time profile
for a via spiraling process where second layer 266 is removed in
two or more subpasses. The steepness of the "curve" is limited by
the speed of actuator 72 and a more representative profile may
resemble a sinusoidal focus level profile such as shown in FIG. 29.
Skilled persons will also appreciate that the spiraling profile of
FIG. 32 can be employed in the first step against the top layer 264
with both the outer tightly focused beam 66 and the less focused
inner beam 66 having greater power density than metal ablation
threshold 224.
[0126] FIGS. 35A and 35B are respective steps in an alternative
exemplary trepanning profile designed to reduce via sidewall taper
.phi. for machining layers 264 or 266. Trepan 322 immediately
precedes trepan 324 within a given hole and a given layer. Laser
spot 67a is larger than and has a lower fluence than spot 67b, and
trepan 320 has a smaller circumferential path than does trepan 322.
In addition, the laser is turn off during inner loop 324b of trepan
322.
[0127] Similarly, with reference to the line-cutting profiles
depicted in FIGS. 24 and 36, analogous trepanning-like profiles can
be used for arbitrary-shape cutting or drilling to minimize taper
.phi. or lip as desired. Hence, in FIG. 36, a larger laser spot 67a
is used to machine interior path(s) 306a, and a smaller laser spot
67b is used to machine exterior paths 306b.
[0128] FIG. 37 is an enlarged sectional side elevation view of a
multilayered 30 workpiece showing a lip 340 and redeposited debris
342 formed around the surface perimeter of a via. Lip size 344 is
related to both the beam profile (TEM.sub.00) and the low fluence
of the second step that can, under some processing conditions,
slowly melt top layer 264 around the perimeter of the via. A low
energy beam 66a increases areas of low fluence 346a, shown in FIG.
38, and increases the amount of lip formation. To minimize lip 340,
it would be desirable to minimize the exposure of top layer 264 to
fluences lower than that required to vaporize it. Thus, a high
energy beam 66b minimizes areas of low fluence 346b as shown in
FIG. 39 and decreases the amount of melted metal that causes lip
formation. FIGS. 38 and 39 also show the relative high fluence
areas 348a and 348b respectively. Fortunately, focus profiling can
also be employed to minimize the types of lips shown in FIG. 37
such as by employing the spiraling and focus level profiles shown
in FIGS. 32 and 34. To accomplish the change of focus for the outer
spiral with a conventional system would generally require an
additional pass or a slow Z height adjustment. By changing the beam
diameter at the outer most spiral pitch with a DMM 70 of the
present invention, the process developer can tailor lip size 340 to
meet specific application requirements without sacrificing
throughput.
[0129] Skilled persons will appreciate that an unconverted IR beam
from a solid-state laser can perform the focus profiling techniques
described herein. Skilled persons will also appreciate that output
beam 66 can be imaged or clipped of its wings or tails,
particularly for first step processing, if desired for specific
operations.
[0130] FIGS. 40-42 show throughput versus move time comparisons
between single pass and double pass via drilling operations having
a variety of via drilling times and hole-to-hole move times. In
particular, FIG. 40 shows exemplary conventional two-step, two-pass
(TSTP) throughput versus move time for 5.7-11.2 ms drill processing
times and 2-7 ms hole-to-hole move times. FIG. 41 shows exemplary
two-step, single-pass (TSSP) throughput versus move time for
5.7-11.2 ms drill processing times and 2-7 ms hole-to-hole move
times. FIG. 42 shows throughput versus move time comparisons for 5
ms drill processing times and 2-7 ms hole-to-hole move times of
TSTP, TSSP, and TSSP where the defocus time is buried in the
hole-to-hole move time. The throughput advantages facilitated by
the present invention are dramatic.
[0131] It will be obvious to those having skill in the art that
many changes may be made to the details of the above-described
embodiments 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.
* * * * *