Note: Descriptions are shown in the official language in which they were submitted.
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ULTRAVIOLET LASER ABLATIVE PATTERNING
OF MICROSTRUCTURES IN SEMICONDUCTORS
RELATED APPLICATIONS
[0001] This patent application derives priority from U.S. Provisional
Application
No. 60/265,556, filed January 31, 2001, and from U.S. Patent Application No.
09/803,382, filed March 9, 2001.
TECHNICAL FIELD
[0002] This invention relates to a method and/or apparatus for high-speed
formation of
micron-scale features by ablation of semiconductors, and particularly silicon,
using pulsed
output of an ultraviolet (UV) laser.
BACKGROUND OF THE INVENTION
[0003] The semiconductor industry utilizes numerous techniques to separate
distinct
electronic devices, often referred to as die, from the semiconductor wafer
upon which
devices are fabricated. A common method for such separation is the use of a
diamond saw.
Methods for reducing the area on the semiconductor wafer required to be
allocated for saw
streets are much desired to enable greater area utilization of the wafer for
useful die,
thereby increasing the yield of die per wafer. Laser technology offers such an
opportunity
to reduce the street dimensions for dicing of semiconductor wafers.
[0004] The use of infrared lasers, such as Q-switched 1064 nm Nd:YAG lasers,
for
laser processing of silicon is well known to 'those skilled in the art.
However, since silicon
is a weak absorber at 1064 nm, significant problems have been encountered in
laser dicing
processes operating at or near this wavelength. The cut quality is typically
observed to be
marred by redeposition of silicon along the wafer surface and along the walls
of the cut.
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[0005] U.S. Pat. No. 4,541,035 of Carlson et al. and U.S. Pat. No. 4,589,190
of
Anthony describe fabrication of features in silicon devices using 1064 nm
pulsed output
such as from an acousto-optic Q-switched, infrared (IR) Nd:YAG laser
integrated into an
ESI Model 25 Laser Scribing System. (See also "Diodes FoYmed by Laser Drilling
and
Diffusio~a," T.R. Anthony, JouftZal of Applied Plzysics, vol. 53, Dec. 1982,
pp. 9154-
9164). U.S. Pat. No. 4,618,380 of Alcorn et al. also describes a method of
fabricating an
imaging spectrometer by processing a silicon device with a laser.
[0006] In U.S. Patent No. 5,543,365, Wills et al. describe a laser scribing
apparatus for
the purpose of forming polysilicon streaks in silicon wafers using 1064 nm
pulsed output
such as from a Nd:YAG Laser with a pulsewidth exceeding 4 ns. Alternatively,
they teach
that the frequency-doubled wavelength of 532 nm may be employed.
[0007] In "Excimer VS Nd:YAG Laser Creation of Silicon Vias for 3D
Interconnections" (1992 IEEE/CHMT Int'1 Electronics Manufacturing Technology
Symposium), Lee et, al. (Lee) report use of Nd:YAG laser wavelengths at 1064
nm and
532 nm to create vias throughout the surface of a silicon wafer for the
purpose of enabling
production of multichip modules. Lee reports that when laser drilling through
holes in
silicon wafers at 1064 mn, molten material frequently condensed onto the walls
of the holes
once an appreciable depth was reached. This apparent redeposition of silicon
made the
holes unsuitable for further processing. Lee reports employment of a double
drilling
process at 1064 nm to improve 'hole quality. Lee describes employing 532 nm
frequency-
doubled pulsed laser output from a lamp-pumped, Q-switched Nd:YAG in a
trepanning
process using rotating lenses that are offset with respect to the incoming
laser beam to cut 4
mil (approximately 100 micron (~,m)) diameter holes in silicon. He reports the
processing
parameters used as 833 pJ per pulse at a pulse repetition frequency of 3 kHz
with a
pulsewidth of~'70 ns. Redeposition of silicon around the perimeter and along
the walls of
the laser drilled via was still observed and a chemical etching process was
used to clean the
holes .
[0008] Lee further reports on using an excimer laser at a wavelength of 248 nm
to drill
holes in silicon. Holes with very smooth sidewalk were reported due to the
very high pulse
energies employed. He reports using an energy per pulse of 290 mJ at a pulse
repetition
frequency of 250 Hz and a focused spot size of 5 mils (approximately 125 ~,m)
to drill a
hole through a silicon wafer in 30 seconds. He compared drilling time to the 3
seconds
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required for the holes drilled using his 532 nm Nd:YAG trepanning technique.
Lee
suggests a method for reducing the drilling time required for silicon holes by
a 248 nm
excimer laser through use of a projection technique. As those skilled in the
art will
recognize, such a technique is reliant upon an appropriate aperture mask for
each pattern of
holes to be formed using such a technique.
[0009] In U.S. Pat. No. 5,870,421, Dahm discusses the problem of use of near
infrared
lasers for the purpose of dicing silicon wafers. He teaches that the primary
cause for poor
cut quality resulting from redeposition when employing near infrared lasers is
use of laser
pulsewidths exceeding about 1 ns. Dahm teaches the use of near infrared lasers
with short
pulsewidths of less than about 1 ns to solve the deep absorption depth of near
infrared
wavelengths in silicon, stating that such short pulsewidths may produce
surface plasmas
which can act as highly absorbing layers. Dahm also mentions that near
infrared lasers,
such as 1064 nm Nd:YAG lasers, are used for high speed applications because of
their
ability to produce greater power than UV lasers, arguing that UV lasers cannot
develop
sufficient power to process silicon at high speeds.
[0010] 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 ,um, and average intensities or
irradiances of greater
than 100 mW over the spot areas at repetition rates of greater than 200 Hz.
[0011] In U.S. Pat. No. 5,841,099, Owen et al. vary UV 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 repetition rate
of the laser to
change the energy density of the laser spot impinging the workpiece and/or
they change the
spot size.
[0012] In U.S. Pat. No. 5,751,585, Cutler et al. describe a high speed, high
accuracy
mufti-stage positioning system for accurately and rapidly positioning a wide
variety of
tools, such as a laser beam relative to targets on a workpiece. They employ a
mufti-rate
positioner system which processes workpiece target positioning commands and
converts
them to commands to slow and fast positioners. These positioners move without
necessarily stopping in response to a stream of positioning data. In one
embodiment, this
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technique enables the laser micromachining of a pattern of small features
across a large
workpiece, thereby allowing increased throughput of laser micromachined parts.
SUMMARY OF THE INVENTION
[0013] An object of the invention is to provide an improved method for using a
laser to
create microfeatures in semiconductors, including silicon, gallium arsenide
(GaAs), silcon
carbide (SiC), silicon nitride (SiN), and/or Ge:Si, and/or also including such
semiconductors subsequently treated in semiconductor processes, including but
not limited
to photolithography and etching, well known to those skilled in the art to
contain additional
layers for the purpose of creating useful electronic and optoelectronic
circuits on
semiconductor substrates, including semiconductor wafers.
[0014] Another object of the invention is to provide such a method that
employs a
highly reliable nonexcimer UV laser capable of operating with high pulse
energy output at
high pulse repetition frequencies.
[0015] The present invention provides a method for rapidly and directly
forming
patterns with feature sizes of less than 50 ~,m in semiconductor workpieces
using ultraviolet
laser ablation. A compound beam positioner is used to rapidly position at the
workpiece
the focused output of a nonexcimer UV laser capable of emitting high energy
per pulse
output at high pulse repetition frequencies. These patterns may include:
formation of very
high-aspect cylindrical openings, such as through-holes or blind vial, for
integrated circuit
connections; curvilinear or rectilinear singulation of processed dies
contained on silicon
wafers; microtab cutting to separate microcircuits formed in semiconductor
workpieces
from parent wafer; formation of curvilinear or rectilinear features in optical
waveguides,
such as arrayed waveguide gratings (AWGs) or microelectronic machine systems
(MEMS);
and scribing alignment, identification, or other markings into the wafer
surface.
[0016] The present invention utilizes laser wavelengths shorter than 390 nm
for which
the optical absorption coefficient of silicon is more than 1000 times greater
than at the
wavelength of 1064 nm used in U.S. Pat. Nos. 4,541,035, 4,589,190, and
5,543,365. A
Q-switched frequency-tripled Nd:YAG, Nd:YVOa, or Nd:YLF diode-pumped laser
provides the preferred source of the ablative ultraviolet output. The laser's
optical system
produces a Gaussian spot size of about 10 Vim. Alternatively, an optical
system producing a
top hat beam profile may be used. Exemplary pulse energy for high-speed
ablative
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processing of silicon using this focused spot size is greater than 200 ~cJ per
pulse at pulse
repetition frequencies of greater than 5 kHz and preferably above 15 kHz. The
laser
pulsewidth measured at the full width half maximum points is preferably less
than 80 ns.
[0017] An advantage of using UV wavelengths is the capability of producing
spot sizes
significantly smaller than those achievable with longer wavelength sources.
This small spot
size capability enables the production of micron-scale feature sizes in
silicon. Also, for a
fixed spot size achievable with conventional Gaussian focusing techniques, a
shorter
wavelength allows for formation of features with improved aspect ratios due to
the greater
depth of focus afforded at ultraviolet wavelengths.
[0018] The present invention also provides methods for reducing damage or
marring of
processed semiconductor workpieces resulting from stray reflections of the
ultraviolet
processing beam from workpiece supporting structures, such as wafer chucks, by
use of
substantially nonreflecting materials and novel fabrication techniques for
workpiece
supporting structures.
[0019] 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
[0020] FIG. 1 is a graph displaying the optical absorption coefficient of
silicon as a
function of wavelength.
[0021] FIG. 2 is a graph displaying the optical absorption coefficient of
gallium
arsenide (GaAs) as a function of wavelength.
[0022] FIG. 3 is a simplified pictorial diagram of a preferred laser system
for
ultraviolet laser ablative patterning of microstructures in semiconductors in
accordance with
the present invention.
[0023] FIG. 4 is a simplified pictorial diagram of an alternative preferred
laser system
for ultraviolet laser ablative patterning of microstructures in
semiconductors.
[0024] FIG. 5 is a simplified pictorial diagram of an optional imaged optics
module
that may be used in a laser system for ultraviolet laser ablative patterning
of
microstructures in semiconductors.
[0025] FIG. 6 is a graph displaying the characteristic relationship between
pulse energy
and pulse repetition frequency of the laser employed during practice of the
invention.
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[0026] FIG. 7 is a representative illustration of ultraviolet ablative
patterning of a
cylindrical opening in silicon.
[0027] FIG. 8 is a representative illustration of ultraviolet ablative
patterning of a trench
pattern in silicon.
[0028] FIG. 9 is a simplified representation of an exemplary segmented cutting
profile
for making long cuts in semiconductor materials.
[0029] FIG. I0 is a simplified representation of an alternative segmented
cutting profile
for making long cuts in semiconductor materials.
[0030] FIG. 11 is a representative illustration of ultraviolet ablative
patterning of a
MEMS device on a semiconductor wafer.
[0031] FIG. 12 is a representative illustration of ultraviolet ablative
patterning of an
AWG device fabricated on a semiconductor wafer.
[0032] FIG. 13 is a representative illustration of an ultraviolet transparent
chuck on
which semiconductor workpieces are placed for through processing using the
ultraviolet
ablative patterning method.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0033] FIG. 1 displays the optical absorption coefficient of silicon as a
function of
wavelength. With reference to FIG. l, silicon exhibits a very sharp rise in
the optical
absorption at wavelengths in the ultraviolet. The present invention
advantageously utilizes
laser wavelengths shorter than 390 nm and takes advantage of the increased
absorption of
silicon in the ultraviolet to efficiently ablate silicon and thereby form a
variety of useful
patterns or features directly in silicon. The absorption behavior facilitates
strongly ablative
removal of silicon in the ultraviolet with a greatly reduced thermally
affected zone in
comparison with features formed using either 532 nm or 1064 nrn pulsed output
as taught
by the prior art.
[0034] FIG. 2 displays the optical absorption coefficient of GaAs as a
function of
wavelength. With reference to FIG. 2, GaAs exhibits a very sharp rise in the
optical
absorption at wavelengths in the ultraviolet. The absorption coefficient at
355 nm of GaAs
and silicon are quite close. GaAs is a key material in optoelectronic devices,
such as diode
lasers and detectors.
[0035] FIGS ~ 3 and 4 illustrate alternative preferred embodiments of
respective laser
processing systems 10a and 10b (generically 10) utilizing a compound beam
positioning
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system 40 equipped with a wafer chuck assembly 140 that can be employed for
ultraviolet
laser ablative patterning of microstructures in semiconductor workpieces 12 in
accordance
with the present invention. With reference to FIGS. 3 and 4, a preferred
embodiment of a
laser system 10 includes a Q-switched, diode-pumped (DP), solid-state (SS) UV
laser 14
that preferably includes a solid-state lasant such as Nd:YAG, Nd:YLF, or
Nd:YVOa.
Laser 14 preferably pxovides harmonically generated UV laser output 16 of one
or more
laser pulses at a wavelength such as 355 nm (frequency tripled Nd:YAG), 266 nm
(frequency quadrupled Nd:YAG), or 213 nm (frequency quintupled Nd:YAG) with
primarily a TEMoo spatial mode profile.
[0036] Although Gaussian may be used to describe the irradiance profile of
laser output
16, skilled persons will appreciate that most lasers 14 do not emit perfect
Gaussian output
16 having a value of MZ=1. For convenience, the term Gaussian is used herein
to include
profiles where MZ is less than or equal to about 1.5, even though MZ values of
less than 1.3
or 1.2 are preferred.
[003.7] In a preferred embodiment, laser 14 includes a Model 210-V06 Q-
switched,
frequency-tripled Nd:YAG laser, operating at about 355 nm, and commercially
available
from Lightwave Electronics. This laser has been employed in the ESI Model 2700
microvia drilling system available from Electro Scientific Industries, Inc. of
Portland,
Oregon. In an alternative embodiment,, a Lightwave Model 210-V09 Q-switched,
frequency-tripled Nd:YAG laser, operating at about 355 nm may be employed in
order to
employ high energy per pulse at a high pulse repetition frequency (PRF).
Skilled persons
will appreciate that other lasers could be employed and that other wavelengths
are available
from the other listed lasants. Although laser cavity arrangements, harmonic
generation,
and Q-switch operation, and positioning systems are all well known to persons
skilled in the
art, certain details of some of these components will be presented within the
discussions of
the preferred embodiments.
[0038] LTV laser output 16 is optionally passed through a variety of well-
known
expansion and/or collimation optics l~, propagated a long an optical path 20,
and directed
by a beam positioning system 30 to impinge laser system output pulses) 32 on a
desired
laser target position 34 on workpiece 12. Beam positioning system 30
preferably includes a
translation stage positioner that preferably employs at least two transverse
stages 36 and 38
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that support, for example, X, Y, and/or Z positioning mirrors 42 and 44 and
permit quick
movement between target positions 34, on the same or different workpieces 12.
[0039] In a preferred embodiment, the translation stage positioner is a split-
axis system
where a Y stage 36, typically moved by linear motors along rails 46, supports
and moves
workpiece 12, and an X stage 38, typically moved by linear motors along rails
48, supports
and moves a fast positioner 50 and associated focusing lens(es). The Z
dimension between
X stage 38 and Y stage 36 may also be is adjustable. The positioning mirrors
42 and 44
align the optical path 20 through any turns between laser 14 and fast
positioner 50, which is
positioned along the optical path 20. The fast positioner 50 may for example
employ high
resolution linear motors or a pair of galvanometer mirrors that can effect
unique or
repetitive processing open ations based on provided test or design data. The
stages 36 and
38 and positioner 50 can be controlled and moved independently or coordinated
to move
together in response to panelized or unpanelized data.
[0040] Fast positioner 50 preferably also includes a vision system that can be
aligned to
one or more fiducials on the surface of the workpiece 12. Beam positioning
system 30 can
employ conventional vision or beam to work alignment systems that work through
objective
lens 36 or off axis with a separate camera and that are well known to skilled
practitioners.
In one embodiment, an HRVX vision box employing Freedom Library software in a
positioning system 30 sold by Electro Scientific Industries, Inc. is employed
to perform
alignment between the laser system 10 and the target locations 34 on the
workpiece 12.
Other suitable alignment systems are commercially available. The alignment
systems
preferably employ bright-field, on-axis illumination, particularly for
specularly reflecting
workpieces like lapped or polished wafers.
[0041] In addition, beam positioning system 30 also preferably employs non-
contact, small-displacement sensors to determine Abbe errors due to the pitch,
yaw, or roll
of stages 36 and 38 that are not indicated by an on-axis position indicator,
such as a linear
scale encoder or laser interferometer. The Abbe error correction system can be
calibrated
against a precise reference standard so the corrections depend only on sensing
small
changes in the sensor readings and not on absolute accuracy of the sensor
readings. Such
an Abbe error' correction system is described in detail in International
Publication No. WO
01/52004 A1 published on July 19, 2001 and U.S. Publication No. 2001-0029674
A1
published on October 18, 2001. The relevant portions of the disclosure of the
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corresponding U.S. Pat. Appl. No. 09/755,950 of Cutler are herein incorporated
by
reference.
[0042] Many variations of positioning systems 30 are well known to skilled
practitioners and some embodiments of positioning system 30 are described in
detail in
U.S. Pat. No. 5,751,585 of Cutler et al. The ESI Model 5320 microvia drilling
system
available from Electro Scientific Industries, Inc. of Portland, Oregon is a
preferred
implementation of positioning system 30 and has been used for laser drilling
of resin coated
copper packages for the electronics industry. Other preferred positioning
systems such as a
Model series numbers 27xx, 43xx, 44xx, or 53xx, manufactured by Electro
Scientific
Industries; Inc. in Portland, Oregon, can also be employed. Some of these
systems which
use an X-Y linear motor for moving the workpiece 12 and an X-Y stage for
moving the
scan lens are cost effective positioning systems for making long straight
cuts. Skilled
persons will also appreciate that a system with a single X-Y stage for
workpiece positioning
with a fixed beam position and/or stationary galvanometer for beam positioning
may
alternatively be employed. Those skilled in the art will recognize that such a
system can be
programmed to utilize toolpath files that will dynamically position at high
speeds the
focused UV laser system output pulses 32 to produce a wide variety of useful
patterns,
which may be either periodic or non-periodic. Those skilled in the art will
also recognize
that this capability has many advantages over the suggestion made by Lee to
produce vias in
silicon through use of a projection imaging arrangement.'
[0043] An optional laser power controller 52, such as a half wave plate
polarizer, rnay
be positioned along optical path 20. In addition, one or more beam detection
devices 54,
such as photodiodes, may be downstream of laser power controller 52, such as
aligned with
a positioning mirror 44 that is adapted to be partly transmissive to the
wavelength of laser
output 16. Beam detection devices 54 are preferably in communication with beam
diagnostic electronics that convey signals to modify the effects of laser
power controller 54.
[0044] With reference to FIG. 4, laser system 10b preferably employs at least
two
lasers 14a and 14b that emit respective laser outputs 16a and 16b that are
linearly polarized
in transverse directions and propagate along respective optical paths 20a and
20b toward
respective reflecting devices 42a and 42b. An optional waveplate 56 may be
positioned
along optical path 20b. Reflecting device 42a is preferably a polarization
sensitive beam
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combiner and is positioned along both optical paths 20a and 20b to combine
laser outputs
16a and 16b to propagate along the common optical path 20.
[0045] Lasers 14a and 14b may be the same or different types of lasers and may
produce laser outputs 16a and 16b that have the same or different wavelengths.
For
example, laser output 16a may have a wavelength of about 266 nm, and laser
output 16b
may have a wavelength of about 355nm. Skilled persons will appreciate that
lasers 14a and
14b may be mounted side by side or one on top of the othex and both attached
to one of the
translation stages 36 or 38. Laser system lOb is capable of producing very
high energy
laser output pulses 32b. A particular advantage of the arrangement shown in
FIG. 4 is to
produce a combined laser output 32 impinging on the work surface having an
increased
energy per pulse which could be difficult to produce from a conventional
single laser head.
Such an increased energy per pulse can be particularly advantageous for
ablating deep
trenches or deep cylindrical openings in thick silicon wafers.
[0046] Despite the substantially round profile of laser system output pulse
32, improved
beam shape quality can be achieved with an optional imaged optics module 62
whereby
unwanted beam artifacts, such as residual astigmatism or elliptical or other
shape
characteristics, are filtered spatially. With reference to FIG. 5, image
optics module 62
preferably includes an optical element 64, a lens 66, and an aperture mask 68
placed at or
near the beam waist created by the optical element 64 to block any undesirable
side lobes
and peripheral portions of the beam so that a precisely shaped spot profile is
subsequently
imaged onto the work surface. In a preferred embodiment, optical element 64 is
a focusing
lens, and lens 66 is a collimating lens to add flexibility to the
configuration of laser system
48.
[0047] Varying the size of the aperture can control the edge sharpness of the
spot
profile to produce a smaller, sharper-edged intensity profile that should
enhance the
alignment accuracy. In addition, with this arrangement, the shape of the
aperture can be
precisely circular or also be changed to rectangular, elliptical, or other
noncircular shapes
that can be aligned parallel or perpendicular to a cutting direction. The
aperture of mask
68 may optionally be flared outwardly at its light exiting side. In imaged
optics module 62,
mask 68 may comprise a UV reflective or UV absorptive material, but is
preferably made
from a dielectric material such as UV grade fused silica or sapphire coated
with a
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multilayer highly UV reflective coating other UV resistant coating. Skilled
persons will
appreciate that aperture mask 68 can be used without optical elements 64 and
66.
[0048] In an alternative preferred embodiment, optical element 64 includes one
or more
beam shaping components that convert laser pulses having a raw Gaussian
irradiance profile
into shaped (and focused) pulses that have a near-uniform "top hat" profile,
or particularly
a super-Gaussian irradiance profile, in proximity to an aperture mask 68
downstream of
optical element 64. Such beam shaping components may include aspheric optics
or
diffractive optics. In a preferred embodiment, lens 66 comprises imaging
optics useful for
controlling beam size and divergence. Skilled persons will appreciate that a
single imaging
lens component or multiple lens components could be employed. Skilled persons
will also
appreciate that shaped laser output can be employed without using an aperture
mask 68.
[0049] In one preferred embodiment, the beam shaping components include a
diffractive optic element (DOE) that can perform complex beam shaping with
high
efficiency and accuracy. The beam shaping components not only transform the
Gaussian
irradiance profile to a near-uniform irradiance profile, but they also focus
the shaped output
to a determinable or specified spot size. Although a single element DOE is
preferred,
skilled persons will appreciate that the DOE may include multiple separate
elements such as
the phase plate and transform elements disclosed in U.S. Pat. No. 5,864,430 of
Dickey et
al., which also discloses techniques for designing DOEs for the purpose of
beam shaping.
The shaping and imaging techniques discussed above are described in detail in
International
Publication No. WO 00/73013 published on December 7, 2000. The relevant
portions of
the disclosure of corresponding U.S. Patent Application No. 09/580,396 of
Dunsky et al.,
filed 'May 26, 2000 are herein incorporated by reference.
[0050] For the purpose of providing increased flexibility in the dynamic range
of energy
per pulse, a fast response amplitude control mechanism, such as an acousto-
optic modulator
or electro-optic modulator may be employed to modulate the pulse energy of
successive
pulses. Alternatively, or in combination with the fast response amplitude
control
mechanism, the pulse repetition frequency may be increased or decreased to
effect a change
in the pulse energy of successive pulses. FIG. 6 displays the characteristic
relationship
between pulse energy and pulse repetition frequency (PRF) of a laser 14
employed during
practice of the invention. As FIG. 6 indicates, pulse energies of greater than
200 N.J can be
obtained from the Model 210-V06. In addition, the characteristic relationship
between
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pulse energy and PRF for alternative lasers, Lightwave 210-V09L and Lightwave
210-
V09H, are also shown. Those skilled in the art will appreciate that FIG. 6 is
illustrative of
the principal described and alternate embodiments of laser system 10 will
produce different
characteristic relationships between pulse energy and pulse repetition
frequency.
[0051] The above-described performance characteristics of UV laser system 10
can be
used for high-speed formation of micron-scale features by ablation of
semiconductors, and
particularly silicon. These features may include, but are not limited to,
formation of very
high aspect cylindrical openings 100 through or partially through silicon
wafers or other
silicon workpieces 12; formation of through or partially through trenches of
complex
geometry for the purpose of singulation of processed die on silicon wafers or
silicon
workpieces 12; formation of microtab features to separate microcircuits formed
in silicon
from parent wafers; formation of features on and/or singulation of AWGs; and
formation of
features in MEMS. In addition, the present invention facilitates feature
formation without
significant melt lip formation, without significant slag formation, and
without significant
peel back of the feature edge.
[0052] FIG. 7 presents a representative illustration of a cylindrical opening
100 formed
by ultraviolet ablative patterning in a silicon workpiece 12 such as a wafer
having 500 ~,m-
thick intrinsic silicon substrate 70 overlaid with a 0.5 ~,m-thick passivation
layer of Si02
(not shown). Those skilled in the art will recognize that the thickness of the
silicon
workpieces and the thickness of the passivation layers will vary.
[0053] The cylindrical opening 100 is preferably patterned by positioning the
target
position 34 of the silicon workpiece 12 at the focal plane of laser system 10
and directing a
series of laser system output pulses 32 at target position 34 on silicon
workpiece T2. In this
embodiment, laser system 10 is directed to move the silicon workpiece 12 in X
and Y axes
to a computer programmed centroid target position 34 of the desired location
for cylindrical
opening 100. The sequential laser system output pulses 32 are each incident on
the
programmed centroid target position 34. ,
[0054] For ablative patterning by sequential overlapping pulses, referred
herein as
punching, of cylindrical openings 100 in silicon workpieces, a preferred range
of combined
processing parameters, including energy per pulse, pulse repetition frequency
(PRF), and
focused spot size are particularly advantageous for rapid punching of useful
cylindrical
openings 100.
12
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[0055] In a punching process, sequential laser system output pulses 32 are
each incident on
the programmed centroid target position 34 while workpiece 12 is fixed in X
and Y axes
positions. For this exemplary ablative patterning of silicon process, the
preferred energy
per pulse range is about 100 p,J to 1500 p,J, more preferably, a energy per
pulse range of
about 200 ~J to 1000 ~,J, even more preferably from about 300 ~,J to 800 ~J,
and most
preferably over about 360 ~,J. The preferred PRF range is about 5 kHz to 100
kHz, and
more preferably; a PRF range from about 7 kHz to 50 kHz, and most preferably,
a PRF
range from about 10 kHz to 30 kHz. The preferred focused spot size range is
about 1 ~.m
to 25 ~,m, and more preferably, a focused spot size range from about 3 ~.m to
20 ~,m, and
most preferably, a focused spot size from about 8 ~,m to 15 ~,m. Those skilled
in the art
will recognize that the laser performance as shown in FIG. 6 can achieve
energy per pulse
output at PRFs within the most preferred ranges described above. In reduction
to practice,
programming of an ESI Model 2700 to operate with the most preferred process
parameters
resulted in a throughput of one hundred cylindrical openings per second, each
cylindrical
opening having a 35 ~,m diameter punched through on a 750 pm thick silicon
wafer
workpiece 12.
[0056] In another embodiment, the Z-height of the laser focus position is
simultaneously moved coincident with each succeeding laser system output
pulse.32 to
place the laser focus at a sequentially deeper position in the silicon
workpiece 12, thereby
maintaining the focused spot at a position more coincident with the remaining
silicon
surface.
[0057] In a preferred embodiment, cylindrical opening 100 completely
penetrates the
entire thickness 102 of the workpiece 12 using an output pulse energy from the
laser 14 of
greater than 300 microjoules (~J) using about 100 sequential laser system
output pulses 32.
Laser system output pulses 32 are incident at the work surface with a focused
spot size
(1/e2) diameter of about 12 ,um. The cylindrical opening 100 produced in this
embodiment
will typically have a top surface opening diameter (d~) 104 of about 20 ,um
and an exit
diameter (db) 106 of about 13 ~,m, thereby producing an aspect ratio for this
through hole
cylindrical opening of about 30:1 and an opening taper angle of 0.4°.
[0058] Persons skilled in the art will further appreciate that the precise
values of energy
per pulse, focused spot size, and number of pulses required to efficiently
produce high
quality cylindrical openings 100 through silicon may vary according to the
thickness 102 of
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WO 02/060636 PCT/US02/00867
the silicon workpiece 12, relative thickness and composition of overlayers, of
which SiOz is
only one example, and the precise ultraviolet wavelength employed. For
example, for
production of through-hole cylindrical openings 100 in silicon for use as
sites for direct
conductive interconnection of an integrated circuit patterned on a silicon die
to a printed
circuit, the silicon may be only 50 ~cm thick, for example. In this example,
as few as, about
ten pulses may be employed to produce the desired through-hole cylindrical
opening 100.
Those skilled in the art will realize that cylindrical openings, which do not
completely
penetrate through the entire thickness 102 of the silicon, (blind vias) can be
produced
through the correct selection of the parameters described.
[0059] Persons skilled in the art will appreciate that such cylindrical
openings 100
through silicon with high aspect ratio and very low taper angle are very
advantageous for
electronic packaging and interconnect applications. In addition, one or more
groups of
these small through-hole cylindrical openings 100 can be positioned on the top
side near the
periphery of workpieces 12, circuits or dies, or within scribing, slicing, or
dicing streets or
their intersections such that the back or bottom side of workpiece 12 can be
precisely
aligned to with respect to features on the top side. Such alignment
facilitates backside
processing such as laser scribing or sawing to enhance processing speed or
quality.
Techniques for front and/or backside wafer slicing or dicing are discussed in
more detail in
U.S. Pat Appl. No. 09/803,382, filed March 9, 2001, entitled UV Laser Cutting
or Shape
Modification of Brittle, High Melting Temperature Target Materials such as
Ceramics or
Glasses and in U.S. Prov. Pat. Appl. No. 60/301,701, filed June 28, 2001,
entitled Multi-
Step Laser Processing for the Cutting or Drilling of Wafers with Surface
Device Layers.
[0060] FIG. 8 is a representative illustration of ultraviolet ablative
patterning of a trench
110 in a silicon workpiece 12. The trench 110 is preferably patterned by
positioning the
'silicon workpiece 12 at the focal plane of the laser system 10 and directing
a string of
successively overlapping laser system output pulses 32 at the silicon
workpiece 12 as the
laser positioning system 30 moves workpiece 12 along the X-and/or Y-axes of
the
workpiece 12.
[0061] For the ablative patterning process for forming a trench in silicon,
the preferred
energy per pulse range is about 100 ~,J to 1500 ~J, and more preferably, a
energy per pulse
range of about 200 ~,T to 1000 ~,J, even more preferably from about 300 ~,J to
800 ~,J, and
most preferably over about 360 ~J. The preferred PRF range is about 5 kHz to
100.kHz,
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WO 02/060636 PCT/US02/00867
and more preferably, a PRF range from about 7 kHz to 50 kHz, and most
preferably, a
PRF range from about 10 kHz to 30 kHz. The preferred focused spot size range
is about 1
~,m to 25 ~,m, and more preferably, a focused spot size range from about 3 ~,m
to 20 ~,m,
and most preferably, a focused spot size from about 8 ~.m to 15 ~,m. The
preferred bite
size range is about 0.1 ~.m to 10 p,m, and more preferably, a bite size range
from about 0.3
~m to 5 pm, and most preferably, a bite size from about 0.5 pm to 3 ~.m. The
bite size can
be adjusted by controlling the speed of either or both of the stages of the
laser beam
positioning system and coordinating the movement speeds) with the repetition
rate and
firing of the laser.
[0062] Tn a preferred embodiment, a linear trench 110 is cut completely
through 750
,um-thick intrinsic silicon overlaid with a 2.0 ~,m passivation layer of SiOa
using an output
pulse energy from the laser 14 of about 360 ,uJ and using a bite size of 1 ~,m
with a stage
velocity of 10 mmls in 180 passes over workpiece 12. These laser pulses are
incident at
the work surface with a focused spot size (1/e2) diameter of 12 ~,m. Those
skilled in the art
will recognize that various patterns of varying geometry, including, but not
limited to,
squares, rectangles, ellipses, spirals, and/or combinations thereof, may be
produced
through progranzxning of a tool path file used by laser system 10 and
positioning system 30
to position silicon workpiece 12 along X and Y-axes during processing. For
laser cutting,
the beam positioning system 30 is preferably aligned to conventional typical
saw cutting or
other fiducials or a pattern on the wafer surface. If the wafers are already
mechanically
notched, alignment to the cut edges is preferred to overcome the saw tolerance
and
alignment errors.
[0063] Laser cutting destroys significantly less material (kerfs of less than
50 ~,m wide
and preferably less than 25 ,um wide) than does mechanical cutting (slicing
lanes of about
300 ,um and dicing paths of about 150 ,um) so that devices on wafers can be
manufactured
much closer together, allowing many more devices to be produced on each wafer.
Thus,
the laser cutting process minimizes the pitch between rows and the pitch
between devices.
[0064] Elimination of the mechanical cutting can also simplify manufacture of
devices
on workpieces 10. In particular, mechanical cutting can impart significant
mechanical
stress to devices such that they come off their carriers. To avoid losing
rows, device
manufacturers may employ strong adhesives or epoxies between the rows and the
carrier.
An all laser process significantly reduces the mechanical strength
requirements of the
CA 02436736 2003-07-30
WO 02/060636 PCT/US02/00867
adhesive used for fixturing the rows onto a carrier. Laser cutting, therefore,
permits the
elimination of strong adhesives or epoxies used to affix the rows to the
carrier and the harsh
chemicals needed to remove them. Instead, the adhesives can be selected for
ease of
debonding, such as the reduction of debond time and less exposure to
potentially corrosive
chemicals, and for amenability to UV laser processing, greatly reducing risk
of damage to
the devices, and thereby enhancing yield.
[0065] Laser row slicing reduces row bow because laser slicing does not exert
as much
mechanical stress as mechanical slicing. However, if row bow or other of the
row defects
are apparent, the rows can be laser diced (and re-sliced) to compensate for
these defects
without concern for the critical device to device alignment needed between
rows for
mechanical dicing. For convenience, the term (through)cutting may be used
generically to
include slicing (often associated with wafer row separation) or dicing (often
associated with
part singulation from wafer rows), and slicing and dicing may be used
interchangeably in
the context of this invention.
[00G6] Because positioning system 30 can align to through holes 100 or
fiducials, laser
system 10 can process each row and/or each device independently. With respect
to slanted
rows, the laser spot can perform traverse cuts across the slanted rows at
appropriate
positions with respect to outer edges of the devices with stage and/or beam
translations
between each cut to effect a rectangular or curvilinear wave patterns as
desired. Thus,
laser dicing can compensate for row fixturing defects and perhaps save entire
rows of
devices that would be ruined by mechanical dicing.
[0067] UV laser cutting throughput through silicon and like materials can be
improved
by dividing a long cut path into short segments. For through cutting or trench
cutting in
thick silicon, for example, these segments are preferably from about 10 ~.m to
1 rnm, more
preferably from about 100 ,um to 1000 ~.m, and most preferably from about 200
~cm to 500
~cm. The laser beam is scanned within a first short segment for a
predetermined number of
passes before being moved to and scanned within a second short for a
predetermined
number of passes. The bite size, segment size, and segment overlap can be
manipulated to
minimize the amount and type of trench backfill.
[0068] FIG. 9 depicts a simplified representation of an exemplary segmented
cutting
profile 112a. With reference to FIG. 9, cutting. profile 112a is shown, for
convenience,
having an path cutting direction from left to right and having distinct
cutting segments
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116k~-116r3 (generally, cutting segments 116) formed from right to left.
Although cutting
segments 116k~-116rs are depicted as parallel in FIG. 9 for convenience,
cutting segments
116k~-116r3 are actually collinear. FIG. 9 depicts multiple segment sets 114a,
each
comprising an initial segment 116k and multiple gradually lengthening
overlapping
segments 116m-116r, preferably processed in alphabetical order. Preferably,
each set 114a
is processed to a selected intermediate depth or to a complete through cut
before the next
set is processed. Although only five overlapping segments are shown for each
set 114a,
skilled persons will appreciate that a substantially greater number of
overlapping segments
116 could be employed, particularly with smaller incremental length increases
as needed to
accommodate the thickness of the target material. Skilled persons will also
appreciate that
any or all of the segments 116 employed in cutting profile 112a could be
processed in both
directions instead of a single direction as shown in FIG. 9.
[0069] FIG. 10 depicts a simplified representation of an exemplary segmented
cutting
profile 112b that is somewhat similar to profile 112a. With reference to FIG.
10, profile
112b begins with the same segment set 114a that begins profile 112a. However,
segment
sets 114b omit segments 116k and increasingly overlap the previously processed
segment
sets as much as 60 % . In one example of this embodiment, segment 116k~ is cut
with 30
passes and has a length of 200 ~,m. Then, segment 116m~ is cut with 6 passes
(1/5 of 30
passes) and has a length of 240 ~m (200 ~,m plus 1l5 of the length of segment
116k~).
Then, segment 116m is cut with 6 passes and has a length of 280 ~,m (200 ,um
plus 2/5 of
the length of segment 116k~). This sequence is continued until segment set
114b is
completed. This example can exhibit dicing speeds of greater than or equal to
8.5
mm/minute.
[0070] Real-time monitoring can also be employed to reduce rescanning portions
of the
cut path where the cut is already completed. In addition, polarization
direction of the laser
beam can be correlated with the cutting direction to further enhance
throughput. These
segmented cutting techniques are discussed in detail in U.S. Provisional
Patent Application
No. 60/297,218, filed June 8, 2001, entitled Laser Segmented Slicing or
Dicing.
[0071] Another application of the present ultraviolet ablative patterning
method is to
produce MEMS (microelectronic machine system) devices 120. FIG. 11 is a
representative
illustration of ultraviolet ablative patterning of a MEMS device 120. In one
preferred
embodiment, the MEMS device 120 is patterned using the method described above
to create
17
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WO 02/060636 PCT/US02/00867
trenches 122a, 122b, 122c, 122d, and 122e (generically trenches 122) in
silicon. Skilled
persons will appreciate that through computer control of the X andlor Y axes
of the laser
positioning system 30, the directed laser system output pulses 32 can be
directed to the
work surface such that o~,rerlapped pulses create a pattern which expresses
any complex
curvilinear geometry. This capability combined with the ultraviolet ablative
patterning
method may be used to produce complex curvilinear geometric patterns in
silicon useful for
efficient production of a variety of MEMS devices 120.
[0072] Another application of the present ultraviolet ablative patterning
method is to
process optical integrated circuits, such as an arrayed waveguide gratings
(AWG) device
130 produced on semiconductor wafer workpieces 12. FIG. 12 is a representative
illustration of ultraviolet ablative patterning of an AWG device 130. In one
preferred
embodiment, the AWG 130 is patterned using the method described above to
create
curvilinear trenches 132, with portions 132a, 132b, 132c, 132d, and 132e in
silicon, for
example. Although trench 132 is shown to be symmetric, skilled persons will
appreciate
that through computer control of the X and/or Y axes of the laser positioning
system 30,
the directed laser system output pulses 32 can be directed to the work surface
such that
overlapped pulses 32 create a pattern which expresses any complex curvilinear
profile or
geometry. This capability combined with the ultraviolet ablative patterning
method may be
used to produce complex curvilinear geometric patterns in silicon useful for
efficient
production of a variety of AWG devices 120.
[0073] Employment of conventional metal chucks, such as fabricated from
ahuninum, is
not advantageous for through processing silicon workpieces 12 because the high
reflection
of these traditional metallic materials in the ultraviolet can cause backside
damage to silicon
workpieces 12. Experiments have shown significant evidence of backside damage
around
cylindrical through-hole openings 100 or through trenches 110 from'reflective
energy
coming off the metal chuck top after through processing has been completed.
However, no
backside damage was discovered in proximity to cylindrical through-hole
openings 100 or
through trenches 110 that were serendipitously drilled over tooling holes in
the chuck top.
[0074] FIG. 13 is a representative,illustration of a chuck assembly 140 on
which
silicon workpieces 12 are preferably placed for through processing using the
ultraviolet
ablative patterning method. Chuck assembly 140 preferably includes a vacuum
chuck base
142, -a chuck top 144, and an optional retaining carrier 146. Base 142 is
preferably made
18
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WO 02/060636 PCT/US02/00867
from traditional metal material and is preferably bolted to an additional
plate 148. Plate
148 is adapted to be easily connected to and disengaged from at least one of
the stages 36
or 38. The engagement mechanism is preferably mechanical and may include
opposing
grooves and ridges and may include a locking mechanism. Skilled person will
appreciate
that numerous exact alignment and lock and key mechanisms are possible.
Skilled persons
will also appreciate that the base 142 may alternatively be adapted to be
secured directly to
the stages 36 or 38.
[0075] Chuck top 144 is preferably fabricated from a dielectric material,
which has low
reflectivity at the ultraviolet wavelength selected for the particular
patterning application.
In one preferred embodiment, where 355 nm output from a frequency-tripled, Q-
switched
diode-pumped Nd:YAG laser is employed, UV-transparent chuck top 144 is
fabricated
from ultraviolet-grade or excimer grade fused silica, MgFz, or CaFa. In
another
embodiment, UV-transparent chuck top 144 may alternatively or additionally be
liquid-
cooled to assist in maintaining the temperature stability of the silicon
workpieces 12. Those
skilled in the art will recognize that fused silica is an ultraviolet
transparent material
composed of amorphous silicon dioxide and is formed by chemical combination of
silicon
and oxygen. ,
[0076] With reference again to FIG. 13, a retaining carrier 146 may be placed
over
chuck top 144 for the purpose of supporting a silicon workpiece 12 and
retaining it after
ultraviolet ablative patterning. Retaining carrier 146 is also preferably
fabricated from an
ultraviolet transparent material in order to prevent backside reflections from
damaging
workpieces 12 that are through processed. Retaining carrier 146 is preferably
machined to
contain shallow cavities into which the processed silicon workpieces 12 settle
after through
processing operations.
[0077] In an alternative embodiment, chuck top 144 or retaining carrier 146
may be
fabricated from an ultraviolet absorbing material, such as A1 or Cu, in order
that laser
system 10 may use a tool path file of the pattern of shallow cavities to be
drilled into the
workpiece 12 to cut the corresponding pattern into the material of chuck top
144 or
retaining carrier 146. The cavities may, for example, correspond to intended
drill holes or
edge patterns and prevent backside damage to the workpiece 12 during through
cut
operations. In addition, any debris from the process may settle into the
cavities away from
the backside of workpiece 12. In one preferred embodiment, the pattern of the
shallow
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WO 02/060636 PCT/US02/00867
cavities is processed to have dimensions slightly larger than those of the
corresponding
workpieces 12 after processing, thereby enabling processed workpieces 12 to
settle into the
cavities of the retaining carrier 146. In an alternative embodiment, retaining
carrier 146 is
fabricated from an ultraviolet transparent material by alternative means, such
as optical
fabrication or etching, and is subsequently aligned and affixed on the chuck
top 144. These
embodiments of chuck assembly 140 also have useful applications in UV via
drilling in
other materials such as polyamide.
[0078] Those skilled in the art will recognize that purge gases, such as
nitrogen, argon,
helium, and dry air, may be usefully employed to assist in the removal of
waste fumes from
the workpiece 12. Such purge gases can be delivered to the close vicinity of
the work
surface using delivery nozzles attached to laser system 10.
[0079] In order to improve the surface quality of the silicon workpieces 12
processed
using the ultraviolet ablative patterning method, processed workpieces 12 may
be cleaned
using ultrasonic baths in liquids including but not limited to water, acetone,
methanol, and
ethanol. Those skilled in the art will recognize that cleaning of processed
silicon
workpieces 12 in hydrofluoric acid can be beneficial in removing unwanted
oxide layers.
[0080] Although the description provided above has been largely directed
toward .
processing silicon and GaAs, the methods described are also generally
applicable to other
semiconductors that may be used as the substrate 70 for workpieces 12, such as
SiC, SiN,
or indium phosphide.
[0081] It will be obvious to those having skill in the art that many changes
may be made
to the details of the above-described embodiment of this invention without
departing from
the underlying principles thereof. The scope of the present invention should,
therefore, be
determined only by the following claims.
20.
