U.S. patent application number 12/856374 was filed with the patent office on 2011-02-03 for program controlled dicing of a substrate using a pulsed laser beam.
This patent application is currently assigned to Electro Scientific Industries, Inc.. Invention is credited to Adrian Boyle, Oonagh Meighan.
Application Number | 20110029124 12/856374 |
Document ID | / |
Family ID | 29252466 |
Filed Date | 2011-02-03 |
United States Patent
Application |
20110029124 |
Kind Code |
A1 |
Boyle; Adrian ; et
al. |
February 3, 2011 |
PROGRAM CONTROLLED DICING OF A SUBSTRATE USING A PULSED LASER
BEAM
Abstract
A substrate is diced using a program-controlled pulsed laser
beam apparatus having an associated memory for storing a laser
cutting strategy file. The file contains selected combinations of
pulse rate .DELTA.t, pulse energy density E and pulse spatial
overlap to machine a single layer or different types of material in
different layers of the substrate while restricting damage to the
layers and maximising machining rate to produce die having
predetermined die strength and yield. The file also contains data
relating to the number of scans necessary using a selected
combination to cut through a corresponding layer. The substrate is
diced using the selected combinations. Gas handling equipment for
inert or active gas may be provided for preventing or inducing
chemical reactions at the substrate prior to, during or after
dicing.
Inventors: |
Boyle; Adrian;
(Monasterevin, IE) ; Meighan; Oonagh; (Dublin,
IE) |
Correspondence
Address: |
ELECTRO SCIENTIFIC INDUSTRIES/STOEL RIVES, LLP
900 SW FIFTH AVE., SUITE 2600
PORTLAND
OR
97204-1268
US
|
Assignee: |
Electro Scientific Industries,
Inc.
Portland
OR
|
Family ID: |
29252466 |
Appl. No.: |
12/856374 |
Filed: |
August 13, 2010 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10511743 |
Apr 18, 2005 |
7776720 |
|
|
PCT/EP03/04069 |
Apr 17, 2003 |
|
|
|
12856374 |
|
|
|
|
Current U.S.
Class: |
700/121 |
Current CPC
Class: |
B23K 2101/40 20180801;
B23K 26/40 20130101; B23K 2103/50 20180801; H01L 21/78 20130101;
B23K 26/123 20130101; B23K 26/126 20130101; B23K 26/0648
20130101 |
Class at
Publication: |
700/121 |
International
Class: |
G05B 19/02 20060101
G05B019/02 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 19, 2002 |
IE |
S2002/0289 |
Oct 28, 2002 |
GB |
0225033.0 |
Claims
1. A method of using a pulsed laser for program-controlled dicing
of a substrate comprising at least one layer, the method comprising
the steps of: a. providing program control means and associated
data storage means for controlling the pulsed laser; b. providing
in the associated data storage means a laser cutting strategy file
of at least one selected combination of pulse rate, pulse energy
and pulse spatial overlap of pulses produced by the laser at the
substrate to restrict damage to the respective at least one layer
while maximising machining rate for the at least one layer; c.
providing in the laser cutting strategy file data representative of
at least one selected plurality of scans of the respective at least
one layer by the pulsed laser necessary to cut through the
respective at least one layer when the pulsed laser is operating
according to the respective at least one combination stored in the
laser cutting strategy file; and d. using the laser under control
of the program control means driven by the laser cutting strategy
file to scan the at least one layer with the respective at least
one selected plurality of scans at least to facilitate dicing of
the substrate such that a resultant die has at least a
predetermined die strength and a yield of operational die equals at
least a predetermined minimum yield.
2. A method as claimed in claim 1, wherein the steps b and c of
providing a laser cutting strategy file comprise, for each of the
at least one layer, the steps of: b1. varying at least one of a
combination of pulse rate, pulse energy, pulse spatial overlap to
provide a respective combination; b2. measuring a cutting rate of
the respective layer using the respective combination; b3.
examining the layer to determine whether damage is restricted to a
predetermined extent; b4. dicing the substrate and measuring yield
of the resultant die; b5. measuring die strength of the resultant
die; b6. creating a laser cutting strategy file of a selected
combination which maximises cutting rate while resulting in a yield
of operational die which have at least the predetermined minimum
yield and for which the die have at least the predetermined die
strength; c1. scanning the at least one layer using the selected
combination to determine a plurality of scans necessary to cut
through the layer; and c2. storing the selected plurality of scans
in the laser cutting strategy file.
3. A method as claimed in claim 1, wherein the selected combination
is used for less than the selected plurality of scans, which
corresponds to the selected combination, to machine a layer to be
cut and the layer is scanned for further scans up to the selected
plurality using a combination which will not significantly machine
an underlying layer such that substantially no machining occurs of
the underlying layer should the laser continue to scan the
substrate after the layer to be cut has been cut through.
4. A method as claimed in claim 1, wherein the substrate includes
an active layer, wherein the step of providing a selected
combination to restrict damage to the at least one layer comprises
providing a selected combination which does not significantly
affect the subsequent operation of active devices in the active
layer.
5. A method as claimed in claim 1, wherein the step of providing a
selected combination comprises the steps of: b7. providing an
initial combination at which the laser machines the substrate at an
initial rate which does not cause significant crack propagation due
to thermal shock at an ambient temperature, and such that a
temperature of the substrate is raised by the machining after a
predetermined plurality of scans of the substrate by the laser to a
raised temperature above ambient temperature; b8. and providing a
working combination at which the laser machines the substrate at a
working rate, higher than the initial rate, which does not cause
significant crack propagation due to thermal shock at the raised
temperature; and step d of machining the substrate includes: d4.
machining an initial depth of the substrate using the initial
combination for at least the predetermined plurality of scans; and
d5. machining at least part of a remaining depth of the substrate
using the working combination.
6. A method as claimed in claim 1, including the further steps of:
e. providing gas handling means to provide a gaseous environment
for the substrate; f. using the gaseous environment to control a
chemical reaction with the substrate at least one of prior to and
during dicing the substrate to enhance a strength of the resultant
die.
7. A method as claimed in claim 6, wherein the step of providing a
gaseous environment comprises providing a passive inert gas
environment for substantially preventing oxidation of walls of a
die during machining.
8. A method as claimed in claim 6, wherein the step of providing a
gaseous environment comprises providing an active gas
environment.
9. A method as claimed in claim 8 wherein the step of providing an
active gas environment comprises etching walls of a die with the
active gas to reduce surface roughness of the walls and thereby
improve the die strength.
10. A method as claimed in claim 8, wherein the step of providing
an active gas environment comprises etching walls of a die with the
active gas substantially to remove a heat affected zone produced
during machining, and thereby improve the die strength.
11. A method as claimed in claim 8, wherein the step of providing
an active gas environment comprises reducing debris, produced
during machining, adhering to surfaces of machined die.
12. A method as claimed in claim 1, for producing die with rounded
corners by scanning the laser beam along a curved trajectory at
corners of the die using a galvanometer based scanner, wherein the
selected combination is adapted to maintain the selected pulse
spatial overlap between consecutive laser pulses around an entire
circumference of the die.
13. A method as claimed in claim 1, wherein the selected
combination is adapted to deliver pulses at an arcuate portion or
corner of the die such that substantially no over-cutting or
undercutting generating a defect at the arcuate die edge or corner
occurs.
14. A method as claimed in claim 1, to form a tapered dice lane
having arcuate walls tapering inwards in a direction away from the
laser beam by varying a width of the dice lane as the laser scans
through the substrate wherein the selected combination is modified
to give a finely controlled taper and smooth die sidewalls, and
thereby increase die strength of the resultant die.
15. A method as claimed in claim 1, wherein the substrate is
mounted on a tape and energy of final scans of the laser is
controlled substantially to prevent damage to the tape.
16. A program-controlled substrate dicing apparatus for dicing a
substrate comprising at least one layer, the apparatus comprising:
a pulsed laser; program control means and associated data storage
means for controlling the pulsed laser using a laser cutting
strategy file, stored in the data storage means, of at least one
respective selected combination of pulse rate, pulse energy and
pulse spatial overlap of pulses produced by the laser at the
substrate and data representative of at least one respective
selected plurality of scans of the respective at least one layer by
the pulsed laser necessary to cut through the respective at least
one layer; telecentric scan lens means for scanning a laser beam
from the pulsed laser across the substrate; and laser power
measuring means for mapping a laser energy density received in a
focal plane of the telecentric scan lens to produce a laser energy
density map of a field of view of the telecentric lens using the
selected combination of pulse rate, pulse energy and pulse spatial
overlap of pulses for storing the laser energy density map as an
array in the data storage means for modifying the at least one
respective selected combination to compensate for irregularities,
introduced by the telecentric lens, of laser energy density at the
substrate, such that in use a resultant die has at least a
predetermined die strength and a yield of operational die equals at
least a predetermined minimum yield.
17. An apparatus as claimed in claim 16, wherein the program
control means includes control means for varying at least one of
pulse rate, pulse energy and pulse spatial overlap for controlling
the laser subject to the at least one respective selected
combination.
18. An apparatus as claimed in claim 16, further comprising gas
handling means for providing a gaseous environment for the
substrate for controlling a chemical reaction with the substrate at
least one of prior to and during dicing the substrate to enhance
strength of the resultant die.
19. An apparatus as claimed in claim 16, further comprising a
galvanometer-based scanner for producing die with rounded corners
by scanning a laser beam along a curved trajectory at corners of
the die, wherein the selected combination is arranged to maintain
the selected pulse spatial overlap between consecutive laser pulses
around an entire circumference of the die.
20. An apparatus as claimed in claim 16, arranged for forming a
tapered dice lane having arcuate walls tapering inwards in a
direction away from the laser beam by varying a width of the dice
lane as the laser scans through the substrate wherein the selected
combination is modified to give a finely controlled taper with
smooth die walls, and thereby increase die strength of the
resultant die.
Description
RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 10/511,743, which is a national stage
application of International Application No. PCT/EP03/04069, filed
Apr. 17, 2003, which claims priority to Irish Patent Application
No. S2002/0289, filed Apr. 19, 2002 and Great Britain Patent
Application No. 0225033.0, filed Oct. 28, 2002.
TECHNICAL FIELD
[0002] The invention relates to program-controlled dicing of a
substrate, particularly, but not limited to, a semiconductor
substrate, using a pulsed laser.
BACKGROUND INFORMATION
[0003] Wafer dicing is a critical aspect of package assembly that
facilitates all subsequent operations in an assembly process. Wafer
dicing is conventionally achieved by dicing a wafer with a
mechanical saw. Use of a mechanical saw has disadvantages such as
low yield, chipping and cracking. Thin wafers cannot be machined
due to the stresses induced in the wafer by the saw resulting in
low die strength. The strength of the dies produced when a
semiconductor substrate is diced is an important factor as low die
strengths reduce reliability. Improving die strength minimizes
breakages and the onset of micro-cracking and improves device
reliability.
SUMMARY OF THE DISCLOSURE
[0004] According to a first aspect of the invention, there is
provided a method of using a pulsed laser for program-controlled
dicing of a substrate comprising at least one layer, the method
comprising the steps of: providing program control means and
associated data storage means for controlling the pulsed laser,
providing in the associated data storage means a laser cutting
strategy file of at least one selected combination of pulse rate,
pulse energy and pulse spatial overlap of pulses produced by the
laser at the substrate to restrict damage to the respective at
least one layer while maximizing machining rate for the at least
one layer, providing in the laser cutting strategy file data
representative of at least one selected plurality of scans of the
respective at least one layer by the pulsed laser necessary to cut
through the respective at least one layer when the pulsed laser is
operating according to the respective at least one combination
stored in the laser cutting strategy file; and using the laser
under control of the program control means driven by the laser
cutting strategy file to scan the at least one layer with the
respective at least one selected plurality of scans at least to
facilitate dicing of the substrate such that a resultant die has at
least a predetermined die strength and a yield of operational die
equals at least a predetermined minimum yield.
[0005] Preferably, the steps of providing a laser cutting strategy
file comprise, for each of the at least one layer, the steps of
varying at least one of a combination of pulse rut; pulse energy,
pulse spatial overlap to provide a respective combination;
measuring a cutting rate of the respective layer using the
respective combination; examining the layer to determine whether
damage is restricted to a predetermined extent; dicing the
substrate and measuring yield of the resultant die; measuring die
strength of the resultant die; creating a laser cutting strategy
file of a selected combination which maximizes cutting rate while
resulting in a yield of operational die which have at least the
predetermined minimum yield and for which the die have at least the
predetermined die strength scanning the at least one layer using
the selected combination to determine a plurality of scans
necessary to cut though the layer; and storing the selected
plurality of scans in the laser cutting strategy file.
[0006] Conveniently, the die strength is measured using a Weibull
die strength test.
[0007] Advantageously, the step of using the laser to scan the at
least one layer includes providing a galvanometer-based
scanner.
[0008] Conveniently, the step of using the laser to scan the at
least one layer includes providing a telecentric scan lens for
scanning a laser beam from the laser across the substrate and the
step of providing a laser cutting strategy file comprises the steps
of: mapping a laser energy density received in a focal plane of the
telecentric scan lens to produce a laser energy density map of a
field of view of the telecentric lens using the selected
combination of pulse rate, pulse energy and pulse spatial overlap
of pulses; storing the laser energy density map as an array in the
storage means; and using the laser energy density map to modify,
with the control means, at least one of the pulse repetition rate
and the pulse energy of the selected combination to produce a
constant laser energy density at scanned points in the field of
view at the substrate.
[0009] Conveniently, the step of mapping a laser energy density
comprises using a laser power meter to measure laser energy density
at representative locations within the field of view of the
telecentric lens.
[0010] Advantageously, the step of providing a selected combination
comprises providing a selected combination which restrict thermal
loading of the material of the respective layer to restrict
mechanical stress to a predetermined maximum.
[0011] Conveniently, the selected combination is used for less than
the selected plurality of scans, which corresponds to the selected
combination, to machine a layer to be cut and the layer is scanned
for further scans up to the selected plurality using a combination
which will not significantly machine an underlying layer such that
substantially no machining occurs of the underlying layer should
the laser continue to scan the substrate after the layer to be out
has been cut through.
[0012] Advantageously, the method comprises scribing a substrate
through the layer to be cut for subsequent mechanical dicing of the
substrate.
[0013] Conveniently, where the substrate includes an active layer,
the step of providing a selected combination to restrict damage to
the at least one layer comprises providing a selected combination
which does not significantly affect the subsequent operation of
active devices in the active layer.
[0014] Advantageously, the step of providing a selected combination
which does not significantly affect the subsequent operation of
active devices in the active layer comprises providing a
combination which does not cause significant cracks to propagate
through the active layer.
[0015] Conveniently, the step of providing a selected combination
comprises the steps of: providing an initial combination at which
the laser machines the substrate at an initial rate which does not
cause significant crack propagation due to thermal shock at an
ambient temperature, and such that a temperature of the substrate
is raised by the machining after a predetermined plurality of scans
of the substrate by the laser to a raised temperature above ambient
temperature; and providing a working combination at which the laser
machines the substrate at a working rate, higher than the initial
rate, which does not cause significant crack propagation due to
thermal shock at the raised temperature; and the step of machining
the substrate includes: machining an initial depth of the substrate
using the initial combination for at least the predetermined
plurality of scans; and machining at least part of a remaining
depth of the substrate using the working combination.
[0016] Preferably, an energy of at least a first of the plurality
of scans is lower than an energy of succeeding scans of the
plurality of scans such that a generation of surface micro-cracks
is less than would otherwise be produced.
[0017] Advantageously, an energy of at least a final of the
plurality of scans is lower than an energy of preceding scans of
the plurality of scans such that backside chipping of the substrate
is less than would otherwise be produced.
[0018] Advantageously, energy of the plurality of scans is varied
between scans to facilitate removal of debris generated during
dicing of the substrate, conveniently by increasing laser energy
with increasing machining depth to remove debris from a dice
lane.
[0019] Advantageously, the method includes the further steps of:
providing gas handling means to provide a gaseous environment for
the substrate; using the gaseous environment to control a chemical
reaction with the substrate at least one of prior to, during and
after dicing the substrate to enhance a strength of the resultant
die.
[0020] Conveniently, the step of providing gas handling means
includes providing gas delivery head means for delivering gas
substantially uniformly to a cutting region of the substrate to
facilitate substantially uniform cutting of the substrate.
[0021] Advantageously, the step of providing gas handling means
comprises providing means to control at least one of flow rate,
concentration, temperature, type of gas and a mixture of types of
gases.
[0022] Conveniently, the step of providing a gaseous environment
comprises providing a passive inert gas environment for
substantially preventing oxidation of walls of a die during
machining.
[0023] Alternatively, tie step of providing a gaseous environment
comprises providing an active gas environment.
[0024] Conveniently, the step of providing an active gas
environment comprises etching walls of a die with the active gas to
reduce surface toughness of the walls and thereby improve the die
strength.
[0025] Advantageously, the step of providing an active gas
environment comprises etching walls of a die with the active gas
substantially to remove a heat affected zone produced during
machining, and thereby improve the die strength.
[0026] Advantageously, the step of providing an active gas
environment comprises reducing debris, produced during machining,
adhering to surfaces of machined die.
[0027] Conveniently, the method comprises the further step after
dicing of scanning an edge of the resultant die with the laser with
sufficient energy to heat sidewalls of the resultant die to reduce
surface roughness thereof and thereby increase die strength of the
resultant die.
[0028] Conveniently, the method is adapted for producing die with
rounded corners by scanning the laser beam along a curved
trajectory at corners of the die using a galvanometer based
scanner, wherein the selected combination is adapted to maintain
the selected pulse spatial overlap between consecutive laser pulses
around an entire circumference of the die.
[0029] Conveniently, the selected combination is adapted to deliver
pulses at an arcuate portion or corner of the die such that
substantially no over-cutting or undercutting generating a defect
at the arcuate die edge or corner occurs.
[0030] Advantageously, the method is adapted for forming a tapered
dice lane having arcuate walls tapering inwards in a direction away
from the laser beam by varying a width of the dice lane as the
laser scans through the substrate wherein the selected combination
is modified to give a finely controlled taper and smooth die
sidewalls, and thereby increase die strength of the resultant
die.
[0031] Conveniently, the laser is a Q-switched laser device.
[0032] Preferably, a laser beam from the laser is directed by
rotatable mirrors.
[0033] Conveniently, the substrate is mounted on a tape and energy
of final scans of the laser is controlled substantially to prevent
damage to the tape.
[0034] Preferably, the tape is substantially transparent to
ultraviolet radiation.
[0035] Advantageously, the tape is polyolefin-based.
[0036] According to a second aspect of the invention, there is
provided an apparatus for program-controlled dicing of a substrate
comprising at least one layer, the apparatus comprising: a pulsed
laser; and program control means and associated data storage means
for controlling the pulsed laser using a laser cutting strategy
file, stored in the data storage means, of at least one respective
selected combination of pulse rate, pulse energy and pulse spatial
overlap of pulses produced by the laser at the substrate and data
representative of at least one respective selected plurality of
scans of the respective at least one layer by the pulsed laser
necessary to cut through the respective at least one layer; such
that in use a resultant die has at least a predetermined die
strength and a yield of operational die equals at least a
predetermined minimum yield.
[0037] Preferably, the program control means includes control means
for varying at least one of pulse rate, pulse energy and pulse
spatial overlap for controlling the laser subject to the at least
one respective selected combination.
[0038] Conveniently, the apparatus includes telecentric scan lens
means for scanning a laser beam from the laser across the
substrate.
[0039] Advantageously, the apparatus includes laser power measuring
means for mapping a laser energy density received in a focal plane
of the telecentric scan lens to produce a laser energy density map
of a field of view of the telecentric lens using the selected
combination of pulse rate, pulse energy and pulse spatial overlap
of pulse for stowing the laser energy density map as an array in
the data storage mean for modifying the at least one respective
selected combination to compensate for irregularities, introduced
by the telecentric lens, of laser energy density at the
substrate.
[0040] Preferably, the apparatus flier comprises gas handling means
for providing a gaseous environment for the substrate for
controlling a chemical reaction with the substrate at least one of
prior to, during and after dicing the substrate to enhance strength
of the resultant die.
[0041] Advantageously, the gas handling means includes gas delivery
head means for uniformly delivering gas to a cutting region of the
substrate.
[0042] Preferably, the gas handling means comprises control means
for controlling at least one of flow rate, concentration,
temperature, type of gas and a mixture of types of gases.
[0043] Conveniently, the gas handling means is arranged to provide
an inert gas environment for substantially preventing oxidation of
walls of a die during machining.
[0044] Alternatively, the gas handling means is arranged to provide
an active gas environment.
[0045] Advantageously, the gas handling means is arranged to etch
walls of a die with the active gas to reduce surface roughness of
the walls, and thereby increase die strength.
[0046] Advantageously, the gas handling means is arranged to etch
walls of a die with the active gas substantially to remove a heat
affected zone produced during machining, and thereby increase die
strength.
[0047] Advantageously, the gas handling means is arranged to etch
walls of a die with the active gas to reduce debris, produced
during machining, adhering to surfaces of machined die.
[0048] Conveniently, the apparatus further comprises a
galvanometer-based scanner for producing die with rounded corners
by a laser beam along a curved trajectory at corners of the die,
wherein the selected combination is arranged to maintain the
selected pulse spatial overlap between consecutive laser pulses
around an entire circumference of the die.
[0049] Advantageously, the selected combination is arranged to
control laser pulse delivery at an arcuate portion or corner of a
die edge such that substantially no over-cutting or undercutting
occurs which would generate a defect at the die edge.
[0050] Conveniently, the apparatus is arranged for forming a
tapered dice lane having arcuate walls tapering inwards in a
direction away from the laser beam by varying a width of the dice
lane as the laser scans through the substrate wherein the selected
combination is modified to give a finely controlled taper with
smooth die walls, and thereby increase die strength of the
resultant die.
[0051] Preferably, the laser is a Q-switched laser device.
[0052] Conveniently, the apparatus includes rotatable mirrors
directing a laser beam from the laser on the substrate.
[0053] Preferably, the apparatus is arranged for a substrate
mounted on a tape, wherein the laser is controlled in final scans
of the substrate not substantially to damage the tape.
[0054] Conveniently, the tape is substantially transparent to
ultraviolet light.
[0055] Advantageously, the tape is polyolefin-based.
BRIEF DESCRIPTION OF THE DRAWINGS
[0056] The invention will be more clearly understood from the
following description of some embodiments thereof, given by way of
example only with reference to the accompanying drawings in
which:
[0057] FIG. 1 is a plan view of a diced silicon wafer;
[0058] FIG. 2 shows transmitted laser intensity as a percentage of
incident laser intensity over a field of view (40 mm.times.40 mm)
of a telecentric scan lens objective for use with the invention and
also the variation in laser pulse energy for machining a trench of
uniform depth according to the invention;
[0059] FIG. 3(i) is a vertical cross-section of a multilayered
substrate suitable for dicing according to the invention;
[0060] FIG. 3(ii) represents a four-step laser process used to dice
the multilayered substrate in FIG. 3(i) according to the
invention;
[0061] FIGS. 4(i) to 4(iii) are vertical cross-sections of a
multilayered structure in which a pre-scribe trench is machined in
a top layer according to the invention;
[0062] FIGS. 5(i)a and 5(i)b are vertical cross-sections of a
semiconductor substrate diced from an active device side according
to the invention;
[0063] FIGS. 5(ii)a to 5(ii)c are vertical cross-sections of a
semiconductor substrate diced from a side opposed to the active
device side according to the invention;
[0064] FIG. 6 is a graph, helpful in understanding the invention,
of relative die strength as abscissa and proportion of die
surviving as ordinates for different spatial overlaps of successive
laser pulses;
[0065] FIG. 7(i) shows a plurality of die with rounded corners,
produced according to the invention;
[0066] FIG. 7(ii) shows a plurality of conventionally diced die
according to the prior art;
[0067] FIG. 8 is a schematic vertical cross-section of tapered dice
lane side walls produced according to the invention;
[0068] FIGS. 9(i) to 9(iii) are vertical cross-sections of a single
layer structure machined, according to the invention; and
[0069] FIG. 10 is a vertical cross-section of a substrate machined
according to the invention mounted on a carrier tape.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0070] A laser beam may be used to dice a semiconductor wafer 10
and thereby singulate devices 11 from the wafer by scanning a
Q-switched laser beam over the wafer surface using rotating mirrors
in a galvanometer type system to form a pattern such as that shown
in FIG. 1. Focusing of the laser beam may be achieved using a
telecentric type scan lens.
[0071] In embodiments of this invention, the temporal separation of
consecutive laser pulses (.DELTA.t) and the laser pulse energy (E)
is varied during machining of a single or multilayered substrate in
order to reduce thermal loading in different portions of the single
layer or in each of the materials in the substrate and the
subsequent mechanical stress or damage that results.
[0072] By way of example, a multilayered material workpiece 30
consisting of four layers 31, 32, 33 and 34 of three different
material types is shown in FIG. 3(i). These materials could be, for
example, a polymer material first layer 31 on a metal second layer
32 on a polymer third layer 33 on a semiconductor substrate 34.
FIG. 3(ii), which is a plot of time (corresponding to distance
machined though the multilayer s) as abscissa and pulse energy as
ordinates, illustrates a four step approach to dicing the
substrate. In order to machine the first layer 31 in such a way as
to reduce thermal loading and consequent mechanical damage in the
polymer material laser pulse energy E.sub.1 is low and an
inter-pulse separation .DELTA.t.sub.1 is high. Polymer materials
will melt and be damaged at high laser energies of, for example
several hundred microJoules per pulse, but they will be cut cleanly
at lower laser pulse energies, for example 10 microJoules per
pulse. Also if the repetition rate is too high (i.e. .DELTA.t is
too low) too much heat will enter the polymer material over too
short a time and the polymer will melt and be damaged, so for
polymers the repetition rate is kept low (i.e. .DELTA.t is high).
In general, values of .DELTA.t and E are chosen based on known
optical and thermal properties of the material or are determined
experimentally. The number of laser pulses delivered at
.DELTA.t.sub.1 and E.sub.1 is determined by the thickness of first
layer 31.
[0073] After machining through the first layer 31 with laser beam
35, the laser parameters are changed to .DELTA.t.sub.2 and E.sub.3,
where chosen values of .DELTA.t.sub.2 and E.sub.3 (like .DELTA.t
and E for all layers in the substrate) are determined by the
thermal properties and also the optical absorption properties of
the material at the laser wavelength used. After the machining of
the second layer 32, the laser properties are returned to
.DELTA.t.sub.1 and E.sub.1 to machine the third layer 33 which is
of similar material to the first layer 31. After machining of the
third layer 33, the laser properties are changed to .DELTA.t.sub.3
and E.sub.2 to machine the fourth layer 34. During machining of
each layer in the multilayer substrate the pulse energies E.sub.1,
E.sub.2 and E.sub.3 may be varied in a manner to be described
across the field of view of the focusing objective in order to
compensate for irregularities in the laser energy transmitted by
the telecentric lens, to ensure uniformity of machining through
each layer of the substrate.
[0074] In practice, prior to machining the layered substrate, a
laser cutting strategy file is generated to contain a series of
commands to the laser to change .DELTA.t and E for each layer and
to control a galvanometer scanner for positioning of the laser beam
on the workpiece surface. In addition, a respective number of laser
scans necessary to cut through each respective layer is
pre-programmed in the laser cutting strategy file from a prior
knowledge of thicknesses of each of the layers 31, 32, 33, 34.
[0075] Initially, this data may be collected experimentally, by
scanning layers of different materials using different pulse
energies and pulse repetition rates and observing any damage, for
example melting or crack propagation in the layer. The resultant
effect on die strength of different pulse energies and pulse
repetition rates may also be determined using, for example, a known
Weibull die strength test and a combination selected for each layer
which produces die with at least required die strength. In
addition, the yield of die may be determined to ensure that the
selected combination is not damaging devices on the substrate and
thereby adversely affecting the yield. Having selected a
combination of pulse energy and pulse repetition rate which causes
only acceptable damage and produces die of a required die strength
and acceptable yield, the number of scans required to cut though a
known thickness of material may then also be determined
experimentally. These values may then be used to write, the laser
cutting strategy file.
[0076] Dicing in this way leads to superior die strength compared
with conventional laser dicing methods.
[0077] In a further embodiment of this invention, the inter-pulse
temporal separation .DELTA.t and the laser pulse energy E are
changed during the machining of a single layer of a multilayered
material. Referring to FIGS. 4(i) to 4(iii), a first layer 41 to be
machined with a laser beam 44 overlies a second layer 42 on a
substrate 43. As the first layer 41 is machined, the pulse
properties .DELTA.t and E of the laser beam 44 are changed just
prior to completion of machining trough the first layer 41, as
illustrated by the changed broken line representing the changed
laser beam 441, during machining of the first layer 41 in order to
prevent damage to an underlying second layer 42. In general, damage
to the underlying layer 42 is prevented by reducing the pulse
energy E to below a melting threshold of the material constituting
the underlying layer. A trench 45 machined in layer 41 of FIG. 4
can be used as a pre-mechanical scribing trench. In this case die
strength is improved compared with the prior art as, by appropriate
choices of laser pulse energy and pulse repetition rate, there is
no cracking in the top layer 41 or in the underlying layer 42 that
could grow during a mechanical scribe and break process performed
after the laser prescribe step.
[0078] In a further embodiment of this invention, illustrated in
FIG. 5, low energy laser pulses of a laser beam 54 are used in a
first few passes along a dice lane 55 in order to prevent the
development of large cracks propagating through active devices 51
when machining from an active device side of the wafers 50, as
shown in FIG. 5(i)a. After the laser has cut through a depth of
material approximately equal to an active device layer thickness a
pulse energy E of the laser beam 54 can be increased to a higher
pulse energy of a laser beam 541 under control of a laser cutting
strategy file, as shown in FIG. 5(i)b, in order to machine more
quickly the bulk of a semiconductor substrate 53 of the wafer 50,
which is heated by the initial machining so that effects of thermal
shock in machining the substrate 53 are reduced. When machining
instead from a back side of a wafer substrate 53, as shown in FIG.
5(ii)a, opposed to a side caring the active devices 51, a similar
process can be adopted in order to prevent cracks propagating from
the initial laser cut down through the substrate material and so
the laser beam 54 with a low laser pulse energy is used initially.
In the bulk of the semiconductor substrate 53 the laser energy is
increased under control of a laser cutting strategy file using
higher energy laser beam 541 for faster machining, see FIG. 5(ii)b.
When the laser beam 541 machining from the backside of the wafer 50
reaches a region containing active devices 51, the laser pulse
energy of the laser beam 54 is reduced under control of the laser
cutting strategy file to prevent excessive damage in this region,
see FIG. 5(ii)c. In order to control laser machining in this
manner, the laser cutting strategy file also contains data
representative of a number of scans necessary to pass through the
active layer and through the remainder of the substrate
respectively, and the number of initial scans necessary to raise
the temperature of the substrate to a temperature at which the
effects of thermal shock are insignificant at the raised
temperature and raised pulse energy.
[0079] In a further embodiment of the invention, illustrated in
FIG. 9, when machining, for example, a trench or dice lane 92 in,
for example, a single layer substrate 93, by multi-pass cutting, a
laser beam 94 with lower pulse energy is used during an initial
pass or passes Man a laser beam 941 used when cutting a bulk of the
substrate in order to prevent, or at least to reduce to a lower
degree than would otherwise occur, generation of surface
micro-cracks in a first surface 91 from which the substrate 93 is
machined. Similarly, the energy of final passes of a laser beam 942
may be reduced below that used for cutting the bulk of the
substrate 93, to prevent, or at least to reduce below a degree than
would otherwise occur, chipping or cracking of a second surface 94
of the substrate opposed to the first surface 91, or, for example,
at a base of a trench. In the bulk of the substrate 93 higher
energy pulses are used for efficient material removal. The pulse
energy may be increased with increasing machining depth in order to
facilitate more efficient material removal.
[0080] Moreover, referring to FIG. 10, energy of a laser beam 104
may be varied throughout machining of a substrate 103 to facilitate
removal of debris 109 generated by the machining. That is, a higher
peak power of the laser beam 104 is used deep within the substrate
than close to surfaces of the substrate.
[0081] The mechanical die strength of laser cut die is a function
of the spatial overlap between consecutive laser pulses. The
spatial overlap between consecutive laser pulses is therefore
preferably chosen so as to yield optimum mechanical die strength of
die obtained from a substrate to be machined. For example, the
dependence of mechanical die strength of a silicon substrate
machined using a 355 nm Q-switched laser is shown in FIG. 6, where
a probability of survival of a pressure test is plotted as
ordinates against a pressure applied to a die as abscisa for a
series of pulse overlaps from 30% to 76%. It is apparent in this
case, that the plot 61 having the highest die strengths is obtained
for a pulse overlap of 30%. It would appear that if the laser pulse
overlap is too high there is too much heating in a region and too
much cracking. If the laser pulse overlap is lower there is less
thermal damage in a region and less cracking. In practice, a
suitable overlap to give a required die strength and yield may be
determined experimentally and stored in the laser cutting strategy
file for use during machining. It will be understood that the
spatial overlap of laser pulses is in fact a function of the
scanning speed the pulse repetition rate and the diameter of the
incident laser beam, so that only these parameters need be stored
in the laser cutting strategy file.
[0082] When a telecentric lens is used to focus a laser beam the
received laser intensity varies across a field of view of the
telecentric lens. Laser parameters may be changed depending on the
location of a focal spot within a field of view of the focusing
scan lens objective in order to maintain a constant power density
at the workpiece surface across the entire field of view. The
variation in transmitted laser intensity as a percentage of
incident laser intensity over the field of view of a typical
telecentric scan lens is shown in a contour plot 20 in an upper
half of FIG. 2. Such a contour plot may be obtained by placing a
laser power meter beneath the telecentric lens in a plane in which
the substrate or workpiece is to be located. Laser power readings
are recorded at a number of positions across the field of view of
the lens (typically 40 mm.times.40 mm) and then plotted as a two
dimensional surface plot. The irregularities in the laser power
density map are mainly due to the quality of the antireflection
coating on the lenses. A telecentric lens consists of a number of
lenses and any irregularities in thickness or quality of the
coating on any of these lenses can cause the observed
irregularities in the laser power density map. Also, due to the
geometry of a telecentric lens, its inherent performance is not so
good at the edges of the field of view so the laser power density
is reduced because of distortion in the laser beam profile caused
by the telecentric lens itself.
[0083] Maintaining a constant power density across the entire scan
lens field of view necessitates changing at least one of laser
pulse energy and laser repetition frequency. In this embodiment of
the invention, laser parameters are changed depending on the
location of a focal spot within the field of view of the focusing
objective in order to maintain a constant power density at the
workpiece surface across the entire field of view. The variation in
transmitted laser intensity as a percentage of incident laser
intensity over the field of view of a typical telecentric scan lens
is shown in FIG. 2. Maintaining a constant power density across the
entire scan lens field of view necessitates changing at least one
of the laser pulse energy and the laser repetition rate and
conveniently changing the laser pulse energy at a fixed laser
repetition frequency or, alternatively, changing the laser
repetition frequency at a fixed laser pulse energy. Power density
(.phi.) is defined as the power (P in units of Watts) per unit area
(A in units of centimetres squared) at the focal spot of the laser
and is given by
.PHI. = P A ##EQU00001##
[0084] Where the power equals the pulse energy (E in units of
Joules) per second (s)
P = E s ##EQU00002##
[0085] By way of example, a lower half of FIG. 2, which is a plot
of pulse energy as ordinates versus distance along the line 21,
which is 10 mm from the lower edge of the field as seen in the
upper half of FIG. 2, demonstrates the modification of laser pulse
energy that is required to maintain a constant power density at the
substrate while scanning the laser across the field of view of the
scan lens to compensate for the variation in transmitted laser
intensity by the telecentric lens. In this example, the laser is
scanned along straight line 21 which is 40 mm in length, 10 mm from
the centre of the lens. In the upper half of FIG. 2 the field of
view of the lens is divided into regions wherein the intensity at
each point in a given region is within .+-0.5% of all points in
that region. For the 40 mm line 21 scanned by the laser in this
example, six different regions, corresponding to six portions 22,
23, 24, 25, 26, 27 of the scan line 21, are traversed and as a
result the laser energy is changed, under the control of the laser
cutting strategy file, five times. The laser pulse energy starts at
value 221 of E.sub.4 in region 1 for a first portion 22 of scan
line 21. The transmitted laser intensity at the workpiece in region
1 is 80 to 85% of the laser intensity incident on the scan lens and
as region 1 represents the region of lowest incident laser
intensity compared to all the regions 2 to 6, the energy per laser
pulse E.sub.4 in region 1 is consequently the highest. As the laser
is scanned from region 1 to region 2, corresponding to a second
portion 23 of scan line 21, the transmitted laser intensity
increases to 85 to 90% of the laser intensity incident on the scan
lens and in order to maintain a constant power density on the
surface of the workpiece the laser pulse energy is now reduced to a
value 231 of E.sub.3, where E.sub.3 is 5% lower in energy than
E.sub.4. As the laser beam traverses from one region to the next
the laser pulse energy is changed, under control of the laser
cutting strategy file, `on the fly` (on a pulse to pulse basis if
required) in order to maintain a constant value of power density
(.phi.) at the workpiece surface along the entire 40 mm length of
the dice lane 21.
[0086] In summary, the laser power density .phi. at the workpiece
surface is directly proportional to the laser pulse energy E. The
value of the laser pulse energy at the workpiece surface will
differ from that emerging directly from the laser due to
attenuation in the scan lens. The contour map is stored as a two
dimensional array in a computer memory associated with a computer
control of the laser and depending on where the software directs
the galvanometer scanner to place the laser beam in the field of
view, a simultaneous command is sent to the laser to change the
pulse repetition rate and the laser pulse energy as indicated in
the laser cutting strategy file. Laser power may also be monitored
by an integral power meter in the laser head itself and any
variation in power in the laser can be compensated for. In
principle, rather than storing the contour map the laser power
could be monitored at the workpiece or substrate but there would be
a loss of laser power in doing so, and preferably the contour map
is stored in memory. In accordance with the invention, the
combination of pulse repetition rate and pulse energy are
controlled during scanning, the laser pulse energy E is varied in
indirect proportion to the transmission of the telecentric scan
lens, in order to maintain a constant power density at the
workpiece surface across the entire field of view of the scan lens.
This permits, for example, the machining of dice lanes and
pre-scribing trenches of uniform depth, where the depth of the dice
lane is directly proportional to the power density cp. In instances
where a substrate is laser machined so that the laser cuts down
through the entire thickness of the substrate, maintaining uniform
power density across the entire dice lane prevents partial cutting
of dice lanes. Partial cutting of dice lanes leaves material
between adjacent die and during the pick and place process, when
die are picked from a transport tape, such die which are stuck
together may break apart causing damage to the die, thus reducing
significantly their mechanical strength.
[0087] Laser dicing in accordance with the invention may be
performed in a non-ambient gas environment controlled by a gas
handling system. Gas parameters such as S flow rate, concentration,
temperature, gas type and gas mixes may be controlled at least one
of prior to, during and after the laser dicing process. A series of
different gases may be used at least one of prior to, during and
after the laser machining processes.
[0088] A gas delivery head may be used to ensure gas is uniformly
delivered to a cutting region of the substrate such that uniform
cutting is achieved.
[0089] Gases used may be passive or reactive with respect to the
semiconductor substrate and/or layers in the semiconductor wafer or
substrate. Inert gases (e.g. argon and helium) may be used to
prevent growth of an oxide layer on the die walls during laser
machining. Gases that react with silicon (e.g. chlorofluorocarbons
and halocarbons) may be used at least one of prior to, during and
after laser machining to reduce the surface roughness of die
sidewalls by etching the substrate material. Also, a heat affected
zone (HAZ) produced on the die sidewalls as a result of laser
machining can be etched away using a reactant gas. In this way the
quality of the die sidewalls is improved and therefore the die
strength is increased. Also reactive gases reduce the amount of
debris adhering on die sidewalls and top and bottom surfaces, thus
reducing potential stress points on laser machined die.
[0090] In a further embodiment of this invention, the laser pulse
energy is reduced to a value close to the melting threshold of the
wafer material (after die singulation) and the laser is scanned
along the die edge in order to heat (rather than ablate) the die
sidewalls. In doing so, the surface roughness of the die sidewalls
is reduced and the uniformity of the heat affected zone is
increased, thus resulting in increased die strength.
[0091] In a further embodiment of his invention, the laser is
scanned in such a way as to machine die 71 with rounded corners 72
as shown in FIG. 7(i). Die 75 diced with a conventional mechanical
saw according to the prior art are shown in FIG. 7(ii). Rounded
corner geometry is easier to achieve and is more accurate when
using a galvanometer based laser machining system rather than a
conventional mechanical saw based dicing system. However, the laser
pulse properties must be changed at the rounded corner sections if
as is typically the case, the galvanometer scanning mirrors used to
direct the laser beam have to slow down as they traverse the curved
features. Otherwise, when the scanning mirrors slow down the laser
pulse spatial overlap would increase, therefore the time between
pulses, .DELTA.t, needs to be increased in order to maintain an
overlap on the rounded corner sections that is the same as a
spatial overlap used on sight regions of the die. This data is
stored in the laser cutting strategy file for controlling the laser
beam during machining. Using a laser to produce die with rounded
corners improves die strength and enables dicing of thin wafers.
The rounded corners eliminate stresses that are induced by sharp
corners of rectangular die.
[0092] In addition, machining may be controlled by the laser
cutting strategy file and program control such that pulse delivery
on a corner or curved portion of a die edge is such that a "clear"
corner or curved section is obtained with no over-cutting or
undercutting which may otherwise generate a defect at the die
edge.
[0093] In a further embodiment of the invention, the taper of a
laser dice lane 85, cut with a laser beam 84 in a substrate 83, may
be varied in order to produce convex arcuate die sidewalls 82, as
shown in FIG. 8, to produce a cut which tapers in a direction of
the laser beam 84. As in the previously described embodiment, this
results in increased die strength by removing potential stress
points at sharp corners. Tapering of the dice lane sidewalls is
achieved by varying the width of the dice lane as the laser beam
scans down through the substrate. The tapered sidewalls shown in
FIG. 8 are achieved by reducing the number of adjacent laser scans
in the dice lane as the depth machined into the substrate
increases.
[0094] As illustrated in FIG. 10, a substrate 103 to be machined
may be mounted on a transport tape 110, for example to singulate
die 101 by machining dice lanes 102 in the substrate 103 in that
case, the laser beam energy may be controlled in final passes
through the substrate to ensure that damage to the tapes does not
occur, as described above in relation to FIG. 9(iii).
Alternatively, or in addition, a tape 110 may be used, such as a
polyolefin-based tape, which is substantially transparent to an
ultraviolet laser light beam 104 used to machine the substrate 103,
such that, with suitable choices of machining process parameters,
substantially no damage occurs to the tape.
[0095] The invention is not limited to the embodiments described
but may be varied in construction and detail.
* * * * *