U.S. patent application number 11/801706 was filed with the patent office on 2008-03-27 for laser-based method and system for processing targeted surface material and article produced thereby.
This patent application is currently assigned to GSI Group Corporation. Invention is credited to Steven P. Cahill, Jonathan S. Ehrmann, Bo Gu, Kevin E. Sullivan, Donald J. Svetkoff.
Application Number | 20080073438 11/801706 |
Document ID | / |
Family ID | 35512828 |
Filed Date | 2008-03-27 |
United States Patent
Application |
20080073438 |
Kind Code |
A1 |
Gu; Bo ; et al. |
March 27, 2008 |
Laser-based method and system for processing targeted surface
material and article produced thereby
Abstract
A laser-based method and system for processing targeted surface
material and article produced thereby are provided. The system
processes the targeted surface material within a region of a
workpiece while avoiding undesirable changes to adjacent
non-targeted material. The system includes a primary laser
subsystem including a primary laser source for generating a pulsed
laser output including at least one pulse having a wavelength and a
pulse width less than 1 ns. A delivery subsystem irradiates the
targeted surface material of the workpiece with the pulsed laser
output including the at least one pulse to texture the targeted
surface material. The pulsed laser output has sufficient total
fluence to initiate ablation within at least a portion of the
targeted surface material and the pulse width is short enough such
that the region and the non-targeted material surrounding the
material are substantially free of slag.
Inventors: |
Gu; Bo; (North Andover,
MA) ; Ehrmann; Jonathan S.; (Sudbury, MA) ;
Svetkoff; Donald J.; (Ann Arbor, MI) ; Cahill; Steven
P.; (Newton, MA) ; Sullivan; Kevin E.;
(Everett, MA) |
Correspondence
Address: |
BROOKS KUSHMAN P.C.
1000 TOWN CENTER
TWENTY-SECOND FLOOR
SOUTHFIELD
MI
48075
US
|
Assignee: |
GSI Group Corporation
Billerica
MA
|
Family ID: |
35512828 |
Appl. No.: |
11/801706 |
Filed: |
May 10, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11514660 |
Oct 27, 2006 |
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11801706 |
May 10, 2007 |
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11152509 |
Jun 14, 2005 |
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11514660 |
Oct 27, 2006 |
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60584268 |
Jun 30, 2004 |
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Current U.S.
Class: |
235/494 |
Current CPC
Class: |
B23K 26/0676 20130101;
B23K 26/0624 20151001; B41M 5/24 20130101; H01L 2223/54406
20130101; B23K 26/355 20180801; B23K 26/361 20151001; H01L
2223/54426 20130101; B23K 26/40 20130101; H01L 2223/54413 20130101;
H05K 3/0026 20130101; G06K 1/126 20130101; B33Y 80/00 20141201;
B23K 2103/50 20180801; H01L 23/544 20130101; H01L 2223/54433
20130101; H01L 2223/54453 20130101; H05K 1/0266 20130101 |
Class at
Publication: |
235/494 |
International
Class: |
G06K 19/06 20060101
G06K019/06 |
Claims
1-57. (canceled)
58. An article of manufacture comprising: at least one surface
material having discernible indicia formed thereon during at least
one step of manufacturing the article, the indicia being formed by
a method of selectively irradiating targeted surface material
within a region of a workpiece with a pulsed laser output, the
indicia being at least semi-permanent and useable during a
subsequent step of manufacturing the article; the region and
non-targeted material surrounding the region are substantially
slag-free; and surface roughness is increased within at least a
portion of the region during the at least one step of
manufacturing, thereby reducing reflection of energy used for
reading the indicia.
59. The article as claimed in claim 58, wherein high reflectance
contrast is obtained between the region and a background of the
region over a wide range of viewing angles.
60. The article as claimed in claim 58, wherein a surface of the
background of the region has a strong specular reflection
component.
61. The article as claimed in claim 58, wherein reflectance
contrast between the discernible indicia and a background of the
region exceeds 30:1 over an angular viewing range of at least 20
degrees.
62. The article as claimed in claim 58, wherein the indicia include
an alphanumeric indicium having a font dimension 0.3 mm or
finer.
63. The article as claimed in claim 58, wherein the indicia include
a two-dimensional matrix code.
64. The article as claimed in claim 58, wherein the indicia are
useable for one or more steps of manufacturing the article in
addition to identification.
65. The article as claimed in claim 58, wherein the indicia are
distinguishable from a background of the region with a roughness
measurement obtained by at least one of SEM (scanning electron
microscope) data, and AFM (atomic force microscope) data.
66. The article as claimed in claim 58, wherein DIN 4768 roughness
measurement standards may be utilized to compare roughness of a
portion of the indicia with a background of the region.
67. The article as claimed in claim 58, wherein the indicia are
distinguishable from a background of the region with a measurement
of image contrast.
68. The article as claimed in claim 58, wherein the indicia are
machine readable.
69. The article as claimed in claim 58, wherein the indicia appear
as a sequence of non-overlapping dots that form a dot matrix
code.
70. The article as claimed in claim 58, wherein the indicia are
usable in at least one of traceability, component identification,
and sorting.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims the benefit of U.S. provisional
application Ser. No. 60/584,268, filed Jun. 30, 2004.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to laser marking and
texturing, particularly forming at least semi-permanent or erasable
indicia on one or more materials of a microelectronic device. The
materials may include semiconductor substrates, thin films,
metallization, and dielectric layers. One or more embodiments may
also be applied for forming indicia on MEMs, optoelectronic
devices, or biomedical microchips. Various embodiments are useable
for various micromachining or microfabrication applications.
[0004] 2. Background Art
[0005] Prior to 1999, Silicon wafer marking was used for
identification at wafer level. Initially driven by the Known Good
Die, and more recently by traceability and component
identification, laser marking on the backside of the wafer at the
die level has become the trend, and applied to various packaging
technologies, including MCM, Flip Chip, DCA, and CSP. For the past
years, research and development effort occurred to develop such a
marking tool for production.
[0006] One of the emerging challenges for die marking is the recent
introduction of very thin wafers. Previously, wafer thickness
specifications of 300 to 700 microns (.mu.m) were typical. Present
requirements for smaller die, both in terms of area and thickness,
are resulting in wafers as thin as 150 .mu.m. Long-term projections
are for wafer thickness to be reduced to a feasible limit.
[0007] Another challenge is the continuing shrinking of die sizes.
For example, die used in DCA (Direct Chip Attach) applications are
in the 3 mm to 8 mm dimension. However, products like the RFID tags
can be as small as 0.3 mm yet require much of the same information
that is included in large die marking. This trend creates the need
for further development in die marking to shrink the actual
alphanumeric character sizes.
[0008] Traditional wafer marking systems are not well suited to
present and emerging requirements.
[0009] Valuable advancements have been demonstrated, for instance
as disclosed in published US Patent Application Number 2003/0060910
entitled "High Speed, Laser-Based Marking Method And System For
Producing Machine Readable Marks On Workpieces And Semiconductor
Devices With Reduced Subsurface Damage Produced Thereby," assigned
to the assignee of the present invention, published 1 Apr. 2004.
However, there is a need to produce high contrast indicia while
providing for decreased feature sizes--and to form indicia on
microelectronic materials, known to have widely varying optical
properties.
[0010] Desirable advancements for precision laser marking systems
includes: increasing mark density (e.g., smaller effective dot size
or line width), control over the marking depth, and improved mark
repeatability with control of or substantial elimination of a heat
affected zone. There is a need for improvement of readability
(e.g., mark contrast with the background), preferably, angle
independent contrast.
[0011] The ideal mark will be formed with little or no material
removed, and will provide contrast that will survive through one or
more subsequent fabrication steps. Further, shrinking sizes are
expected to mandate increasing density requirements, for instance,
font sizes less than 0.3 mm and decreased font spacing.
SUMMARY OF THE INVENTION
[0012] An object of at least one embodiment of the invention is to
provide a method of forming indicia/texture on at least one
material of a microelectronic article. The method includes the step
of applying a pulsed laser output to a localized region of the
material, the output having sufficient total fluence to initiate
ablation within at least a portion of the region and a pulse width
short enough such that the region and proximate material
surrounding the region are substantially slag-free.
[0013] Another object of at least one embodiment of the invention
is to provide an article of manufacture produced by the above
method.
[0014] Yet another object of at least one embodiment of the
invention is to provide a laser marking/texturing system for
carrying out the above method.
[0015] In carrying out the above objects and other objects of the
present invention, a method for processing targeted surface
material within a region of a workpiece while avoiding undesirable
changes to adjacent non-targeted material is provided. The method
includes generating a pulsed laser output including at least one
pulse having a wavelength and a pulse width. The method further
includes irradiating the targeted surface material of the workpiece
with the pulsed laser output including the at least one pulse to
texture the targeted surface material. The pulsed laser output has
sufficient total fluence to initiate ablation within at least a
portion of the targeted surface material and the pulse width is
short enough such that the region and non-targeted material
surrounding the region are substantially free of slag.
[0016] The textured surface material may include indicia.
[0017] The indicia may be at least semi-permanent or erasable.
[0018] The workpiece may be a microelectronic device and the
textured surface material may be a microelectronic material.
[0019] The targeted surface material may be at least one of a
semiconductor substrate, a thin film, a metal layer and a
dielectric layer.
[0020] The workpiece may be one of a MEMs device, an optoelectronic
device and a biomedical chip.
[0021] The non-targeted surface material may include indicia.
[0022] The indicia may be machine-readable.
[0023] The indicia may have a font size less than 0.3 mm.
[0024] The textured surface material may include a microtextured
pattern formed on the workpiece.
[0025] The workpiece may be a semiconductor wafer and wherein the
microtextured pattern forms indicia on the wafer.
[0026] The method may further include generating a secondary laser
output and irradiating the textured surface material with the
secondary laser output to process the textured surface
material.
[0027] The textured surface material may include indicia, and the
indicia may be erased during the step of irradiating with the
secondary laser output.
[0028] The textured surface material may be formed on at least one
side of the workpiece.
[0029] The workpiece may be a semiconductor wafer.
[0030] The step of generating may be at least partially performed
with a femtosecond or picosecond laser.
[0031] The pattern may be a bar pattern, an alphanumeric character
string, or a logotype.
[0032] The pulse width of the at least one pulse may be below about
1 ns.
[0033] The pulse width may be about 100 ps or less, or may be less
than about 10 ps.
[0034] The textured surface material may include microtextured
surface material.
[0035] The microtextured surface material may include nanotextured
surface material.
[0036] The total fluence may be measurable over a spatial dimension
of a spot of the output.
[0037] The textured surface material may include indicia, and the
step of irradiating may include the step of directing the laser
output in response to at least one control signal that represents a
first location of at least a part of the indicia to impinge the
region at the first location.
[0038] The region may be within the spatial dimension of the
spot.
[0039] The step of irradiating may substantially increase surface
roughness of the targeted surface material within at least a
portion of the region.
[0040] The non-targeted surface material surrounding the region may
have a surface with a strong specular reflection component.
[0041] Diffuse reflectance of the indicia may be in a range of 0.5%
to 5%.
[0042] The total fluence may exceed about 0.1 J/cm.sup.2.
[0043] The wavelength may be less than an absorption edge of the
targeted surface material.
[0044] The wavelength may be ultraviolet.
[0045] The pulse width of the at least one pulse may be in a range
of about 15 fs to 500 ps.
[0046] The pulse width of the at least one pulse may be in a range
of about 100 fs to 50 ps, or may be in a range of about 300 fs to
15 ps.
[0047] The targeted surface material may be silicon, or may be a
metal or a dielectric.
[0048] The targeted surface material may be a portion of a
dielectric passivation layer. The dielectric of the layer may be an
inorganic, organic, or a low-k dielectric.
[0049] The targeted surface material may be part of a MEM
device.
[0050] A portion of the indicia may have surface variations in a
range of about 0.25 microns to about 1 micron.
[0051] A feature dimension of the indicia may be in a range of
several microns to tens of microns, or may be a few wavelengths of
the at least one pulse.
[0052] The step of irradiating may include the step of controlling
polarization of the pulsed laser output to enhance or control a
characterization of the textured surface material.
[0053] The pulsed laser output may include a focused laser
processing beam, and the step of irradiating may include the step
of relatively moving the workpiece and the focused laser processing
beam.
[0054] The textured surface material may include a microtextured
pattern, and the step of relatively moving may create the
microtextured pattern on the workpiece.
[0055] The step of irradiating may include the step of shaping the
spot to obtain a shaped spot.
[0056] The shaped spot may have a top-hat irradiance profile.
[0057] The shaped spot may have a depressed center with energy
concentrated in a perimeter of the shaped spot.
[0058] The step of irradiating may include the step of controlling
an aspect of the spot.
[0059] The wavelength may be below an absorption edge of a material
of the workpiece.
[0060] The pulsed laser output may finely texture the targeted
surface material and the secondary laser output may coarsely
process the textured surface material.
[0061] The pulsed laser output may coarsely texture the targeted
surface material and the secondary laser output may finely process
the textured surface material.
[0062] The textured surface material may include indicia, and a
negative window mark may be created during the step of irradiating
with the secondary laser output.
[0063] The textured surface material may include a pattern, and the
step of irradiating with the secondary laser output may
micromachine the pattern.
[0064] The step of irradiating with the secondary laser output may
trim an electrical or mechanical parameter of the textured surface
material.
[0065] The secondary laser output may include at least one pulse
having a wavelength which is absorbed into the textured surface
material.
[0066] The wavelength of the at least one pulse of the secondary
beam may or may not be absorbed into the non-targeted material
surrounding the region.
[0067] Further in carrying out the above objects and other objects
of the present invention, a system for processing targeted surface
material within a region of a workpiece while avoiding undesirable
changes to adjacent non-targeted material is provided. The system
includes a primary laser subsystem which includes a primary laser
source for generating a pulsed laser output including at least one
pulse having a wavelength and a pulse width. The system further
includes a delivery subsystem for irradiating the targeted surface
material of the workpiece with the pulsed laser output including
the at least one pulse to texture the targeted surface material.
The pulsed laser output has sufficient total fluence to initiate
ablation within at least a portion of the targeted surface
material. The pulse width is short enough such that the region and
the non-targeted material surrounding the material are
substantially free of slag.
[0068] The primary laser source may include an ultrafast laser.
[0069] The ultrafast laser may be a picosecond laser, or may be a
femtosecond laser.
[0070] The delivery subsystem may include a controller that accepts
data that represents a location of the targeted surface material to
be textured and produces at least one position control signal.
[0071] The delivery subsystem may include a positioning subsystem
for directing the laser output to the location of the targeted
surface material so as to texture the targeted surface material in
response to the at least one position control signal.
[0072] The system may further include a secondary laser subsystem
which includes a secondary laser source for generating a secondary
laser output which irradiates the textured surface material.
[0073] The secondary laser output may at least erase, micromachine,
weld or actuate the region of the textured surface material.
[0074] The secondary laser source may include one of a pulsed,
modulated or CW source.
[0075] Irradiation with the secondary laser output may be below the
fluence breakdown threshold of the targeted surface material to
heat the region.
[0076] Irradiation with the secondary laser output may be above the
fluence breakdown threshold of the targeted surface material to
effect at least one property change of the targeted surface
material.
[0077] The secondary laser output may include at least one pulse
having a wavelength near or exceeding the absorption edge of the
material of the workpiece.
[0078] The primary laser source may include the secondary laser
source or may be separate from the secondary laser source.
[0079] The delivery subsystem may include a polarization controller
for controlling polarization of the laser output.
[0080] The primary laser source may include a diode-pumped,
solid-state UV laser, and the pulse width may be less than about 20
ns.
[0081] The pulse width may be less than 1 ns.
[0082] The positioning subsystem may include at least one
translation stage to move the workpiece relative to the laser
output.
[0083] The positioning subsystem may include fine and coarse
positioners.
[0084] The positioning subsystem may include translation and
rotation stages to move the workpiece relative to the laser
output.
[0085] The positioning subsystem may include an optical scanner to
move the laser output relative to the workpiece.
[0086] The positioning subsystem may include two or more stages and
scanners to move the laser output relative to the workpiece.
[0087] The laser output may be a laser beam having a beam waist.
The positioning subsystem may include at least one component for
moving the beam waist relative to the workpiece.
[0088] The delivery subsystem may include a focusing subsystem.
[0089] The focusing subsystem may be a refractive optical
subsystem.
[0090] The system may further include an inspection subsystem for
inspecting the textured surface material.
[0091] The inspection subsystem may include a machine vision
subsystem.
[0092] The primary laser source may include a mode-locked
oscillator and a diode-pumped, solid-state laser amplifier.
[0093] The optical scanner may be a two-dimensional,
galvanometer-based scanner.
[0094] The at least one position control signal may be produced
during at least one step of manufacturing an article from the
workpiece.
[0095] The primary laser subsystem may include a seed laser and a
fiberoptic amplifier.
[0096] The primary laser subsystem may further include a
frequency-doubled, diode-pumped, solid-state laser.
[0097] The primary laser subsystem may still further include a
mode-locked oscillator, a diode-pumped, solid-state laser
amplifier, and a wavelength shifter.
[0098] The primary laser subsystem may yet further include one of a
frequency doubler, a frequency tripler and a frequency
quadrupler.
[0099] The laser output may have a repetition rate greater than 10
KHz.
[0100] The laser output may have an average laser output power in
the range of 0.01 W-2 W.
[0101] The textured surface material may include indicia. The
system may further include a viewing subsystem for reading the
indicia. The viewing subsystem may include an illuminator and an
electronic imaging subsystem.
[0102] The illuminator may be one of a bright-field, a dark-field,
and a combination of both bright- and dark-field.
[0103] Yet still further in carrying out the above objects and
other objects of the present invention, an article of manufacture
is provided. At least one surface material has discernible indicia
formed thereon during at least one step of manufacturing the
article. The indicia are formed by a method of selectively
irradiating targeted surface material within a region of a
workpiece with a pulsed laser output. The indicia are at least
semi-permanent and useable during a subsequent step of
manufacturing the article. The region and non-targeted material
surrounding the region are substantially slag-free. Surface
roughness is increased within at least a portion of the region
during the at least one step of manufacturing, thereby reducing
reflection of energy used for reading the indicia.
[0104] High reflectance contrast may be obtained between the region
and a background of the region over a wide range of viewing
angles.
[0105] A surface of the background of the region may have a strong
specular reflection component.
[0106] Reflectance contrast between the discernible indicia and a
background of the region may exceed 30:1 over an angular viewing
range of at least 20 degrees.
[0107] The indicia may include an alphanumeric indicium having a
font dimension 0.3 mm or finer.
[0108] The indicia may include a two-dimensional matrix code.
[0109] The indicia may be useable for one or more steps of
manufacturing the article in addition to identification.
[0110] The indicia may be distinguishable from a background of the
region with a roughness measurement obtained by at least one of SEM
(scanning electron microscope) data, and AFM (atomic force
microscope) data.
[0111] DIN 4768 roughness measurement standards may be utilized to
compare roughness of a portion of the indicia with a background of
the region.
[0112] The indicia may be distinguishable from a background of the
region with a measurement of image contrast.
[0113] The indicia may be machine readable, and may appear as a
sequence of non-overlapping dots that form a dot matrix code.
[0114] The indicia may be usable in at least one of traceability,
component identification, and sorting.
[0115] The above objects and other objects, features, and
advantages of the present invention are readily apparent from the
following detailed description of the best mode for carrying out
the invention when taken in connection with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0116] These and other features, aspects, and advantages of the
invention will become better understood with regard to the
following description, accompanying drawings and attachments
where:
[0117] FIG. 1 is a schematic block diagram showing some elements of
a laser processing system corresponding to one embodiment of the
present invention wherein a microtextured pattern is formed on a
workpiece, for instance, to form indicia on a portion of a
semiconductor wafer;
[0118] FIG. 2 is a schematic block diagram showing some elements of
a laser processing system corresponding to one embodiment of the
present invention wherein a microtextured pattern, which may be
formed with the system of FIG. 1a, is further processed with a
secondary beam, for instance to erase a mark;
[0119] FIGS. 3 and 4 illustrate some details of first and second
sides of semiconductor wafer, an example of a workpiece which may
be processed with various embodiments of the present invention;
[0120] FIG. 5 is a schematic illustrating an exemplary
microtextured region which may be formed with a femtosecond laser
system;
[0121] FIGS. 6 and 7 are schematics comparing prior art marks with
marks formed in accordance with the present invention, and
illustrate improved density of a dot matrix pattern;
[0122] FIGS. 8 and 9 are schematics comparing prior art marks with
marks formed in accordance with the present invention, and
illustrate improved density of a bar pattern;
[0123] FIGS. 10 and 11 are schematics illustrating, by way of
example, a prior art laser mark formed on a specular surface, for
instance bare silicon, and a corresponding surface profile showing
exemplary defects associated with a deep ("hard") mark--for
instance slag and melted zones, debris, and microcracking;
[0124] FIGS. 12 and 13 are schematics illustrating, by way of
example, a mark formed on the specular surface of FIG. 10 using a
system of the present invention and an exemplary profile for
comparison with FIGS. 10 and 11;
[0125] FIGS. 14 and 15 are schematic block diagrams showing some
elements of a semiconductor wafer processing system;
[0126] FIGS. 16 and 17 illustrate subsystems of a semiconductor
wafer processing system corresponding to FIGS. 14 and 15 in further
detail;
[0127] FIGS. 18 and 19 show some components of an exemplary laser
beam positioning system which may be included in FIG. 1 or FIG. 2
for practicing various embodiments of the present invention;
[0128] FIG. 20 is a schematic block diagram, similar to FIG. 16,
showing some elements of a semiconductor wafer processing system,
specifically, elements related to secondary processing of FIG.
2;
[0129] FIGS. 21 and 22 illustrate the structure of various laser
marks produced with conventional and more recent laser marking
systems for further comparison with a marks formed in accordance
with the present invention;
[0130] FIG. 23 illustrates removing material, for instance, erasing
a mark using a secondary processing beam corresponding, for
example, to FIG. 2;
[0131] FIG. 24 illustrates removing material, for instance,
modifying a microtextured pattern, using a secondary processing
beam corresponding, for example, to FIG. 2; and
[0132] FIGS. 24-37d illustrate various examples and results wherein
silicon wafers having ground, polished, smooth, or rough surfaces
are marked in accordance with the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0133] For the purpose of the following description of various
embodiments of the invention, the following non-limiting guidelines
are used:
[0134] "Ultrafast laser" or "ultrashort laser" generally refers to
a pulsed laser providing one or more pulses, each pulse having a
duration below 1 ns, for instance 100 ps or less, or typically less
than 10 ps;
[0135] "Microtexture" generally refers to micron sized surface
variations, but may also include surface variations of a finer
scale, for instance 0.5 microns or 0.1 microns; and
[0136] "Nanotexture" generally refers to surface variations below
one micron in size.
Overview
[0137] New laser marking technology has been developed to overcome
limitations of present laser marking systems. Permanent and high
contrast shallow marks (less than 1 micron) on the backside of
wafers are achieved with little or no material removed by using
this new laser technology. Viewing of these marks is strongly
independent of the viewing angle, a significant advancement. The
so-called micro marking technology allows the mark font size to be
much less than 0.3 mm.
[0138] As will become apparent in sections that follow, various
embodiments may be also applied to mark erasing, microjoining,
laser welding, and microactuation.
[0139] At least one embodiment of the invention may be applied to
micro machining of fine, laser-marked patterns.
[0140] One or more embodiments of the invention may be applicable
to laser micro assembly techniques for silicon articles including
laser welding and laser actuation of micro device members.
[0141] Laser Marking/Texturing Method
[0142] One aspect of the invention features a method of marking a
material of a microelectronic device with a pulsed laser output to
form high-density, discernible indicia on the material. The method
includes: generating a pulsed laser output having at least one
pulse with a pulse duration less than about 1 ns and having total
fluence sufficient to initiate ablation of a portion of the
material, the fluence being measurable over a spatial spot
dimension of the output. The method further includes directing the
laser output in response to at least one control signal that
defines a first location of at least a portion of indicia to be
formed on the material, to impinge a localized region of the
material at the first location, the localized region being within
the spatial spot dimension. The laser output initiates ablation of
at least a portion of the material and substantially increases
surface roughness within at least the portion of the region. The
region and background material that proximately surrounds the
region are both substantially slag-free.
[0143] A background surface may have a strong specular reflection
component.
[0144] The diffuse reflectance of the indicia may be in a range of
0.5% to 5%.
[0145] The total fluence may exceed about 0.1 J/cm.sup.2.
[0146] The laser output may have a wavelength less than an
absorption edge of the material.
[0147] The laser wavelength may be ultraviolet.
[0148] The pulse duration may be in the range of about 15 fs to 500
ps.
[0149] The pulse duration may be in the range of about 100 fs to 50
ps.
[0150] The pulse duration may be in the range of about 300 fs to 15
ps.
[0151] The material may be silicon.
[0152] The material may be a metal or dielectric.
[0153] The material may be a portion of a dielectric passivation
layer, and the dielectric may be inorganic, organic, or a Low-k
dielectric.
[0154] The material may be part of a MEM device.
[0155] A portion of the indicia may have surface height variations
in the range of about 0.25 microns to about 1 micron.
[0156] A feature dimension of the indicia may be in the range of
several microns to tens of microns.
[0157] A feature dimension of the indicia may be a few optical
wavelengths.
[0158] Laser Marking/Texturing System
[0159] Another aspect of the invention features a laser marking
system. The marking system includes: means for generating a pulsed
laser output having at least one pulse with a pulse duration less
than about 1 ns and having total fluence over a spatial region of
the output sufficient to initiate ablation of a portion of a
material to be marked; a controller that accepts data that define
indicia to be formed on a material of a microelectronic article and
produces at least one position control signal to direct a laser
output to mark the material and thereby form the indicia; and means
for directing the laser output to a surface location of the
material to be marked so as to form discernible, high contrast,
high-density indicia on the material surface.
[0160] The means for directing may include an optical scanner.
[0161] The means for directing may include a positioning subsystem
for positioning the material relative to the laser beam in three
dimensions.
[0162] The positioning subsystem may have three or more degrees of
freedom.
[0163] The optical scanner may be a two-dimensional,
galvanometer-based scanner.
[0164] The means for directing may include an X-Y stage and a beam
deflector coupled to the controller.
[0165] The position control signal may be produced during at least
one step of manufacturing the article.
[0166] The means for generating may includes a seed laser and a
fiber optic amplifier.
[0167] The means for generating may include a frequency doubled,
diode-pumped, solid-state laser.
[0168] The means for generating may include a mode-locked
oscillator, a diode-pumped, solid-state laser amplifier, and a
wavelength shifter.
[0169] The means for generating may include a frequency doubler,
frequency tripler or frequency quadrupler.
[0170] The laser output may have a repetition rate greater than 10
KHz.
[0171] The average laser output power may be in the range of 0.01
W-2 W.
[0172] The system may further include a viewing system for reading
the indicia, including an illuminator and an electronic imaging
system.
[0173] The illuminator may be bright-field.
[0174] The illuminator may be dark-field.
[0175] The illuminator may be a combination of bright and dark
field.
[0176] Article of Manufacture
[0177] One aspect of the invention features an electronic article.
The article includes at least one material having discernible
indicia formed on the material during at least one step of
manufacturing the article. The indicia are formed by a method of
selectively irradiating at least one localized material region with
a pulsed laser output. The indicia are at least semi-permanent and
useable during a subsequent step of manufacturing the article. A
marked region and background material that proximately surrounds
the region are both substantially slag-free. The surface roughness
is increased within at least a portion of the material region, and
thereby reduces reflection of energy used for reading the
indicia.
[0178] Preferably, the method of selectively irradiating at least
one localized material region with a pulsed laser output is the
above section entitled "Laser Marking/Texturing Method."
[0179] High reflectance contrast may be obtained between the region
and the background over a wide range of viewing angles.
[0180] A background surface may have a strong specular reflection
component.
[0181] Reflectance contrast between the discernible indicia and the
background may exceed 30:1 over an angular viewing range of at
least 20 degrees.
[0182] The indicia may include an alphanumeric indicium having font
dimension 0.3 mm or finer.
[0183] The indicia may include a two-dimensional matrix code.
[0184] The indicia may be useable for one or more manufacturing
steps in addition to identification.
[0185] The indicia may be distinguished from the background with a
roughness measurement obtained by at least one of SEM (scanning
electron microscope) data, and AFM (atomic force microscope)
data.
[0186] DIN 4768 roughness measurement standards may be utilized to
compare the roughness of a portion of the indicia with the
background.
[0187] The indicia may be distinguished from the background with a
measurement of image contrast.
[0188] The indicia may be machine readable.
[0189] The indicia may appear as a sequence of non-overlapping dots
that form a dot matrix code.
[0190] The indicia may be used in at least one of traceability,
component identification, and sorting.
Laser Processing
[0191] Embodiments of the present invention may be used to form
indicia on ground, polished, or smooth material surfaces, based on
specific application requirements. A surface may be coated. By way
of example, the surface may be either side of a semiconductor
wafer, or another material used in a step of fabricating a
microelectronic device.
[0192] With reference to FIG. 1, an ultrafast laser source 102 of a
laser processing system, generally indicated at 100, generates a
laser output 104 which includes one or more pulses. The laser
output 104 propagates through delivery optics 106 that focus the
output and deliver the beam into a process chamber 110, the chamber
110 being used if the laser processing is to be carried out in a
gaseous environment. The process chamber 110 may contain a gaseous
processing environment at a pressure, partial vacuum, or
temperature as is known in the field of laser material interaction
to produce microtextured silicon. For processing in ambient,
atmospheric conditions the chamber is not required. The focused
beam 108 produces a working spot 112 that is incident on the target
material, which may be a portion of a silicon semiconductor wafer
114, and produces marked material 116 (not to scale).
[0193] With reference to FIGS. 3 and 4, the wafer 114 may have a
bare (unpatterned) backside 117, which may be coated, polished, or
rough. Fiducials 118 are used for alignment. The topside shown in
FIG. 3 may have a large number of die 119 and corresponding dense
circuit patterns.
[0194] An optional secondary laser processing system may be used
for further processing. With reference to FIG. 2, to erase,
micromachine, weld, or actuate the region of the mark, a second
laser source 120 generates a secondary beam 122. High absorption
within the marked region can support such further processing. For
example, the actuation may occur as a result of differential
thermal expansion of the material and background subsequent to
heating the material with the secondary processing beam. The beam
122 propagates through secondary delivery optics 124 that focus the
beam. The secondary laser source 120 may be a pulsed , modulated,
or CW source, depending on the application. The focused beam 126
produces a secondary working spot 130 that is incident on the wafer
114 and irradiates the marked material 116. The irradiation may be
below the ablation threshold of the marked material to heat the
region for actuation or above the threshold to effect material
property changes.
[0195] In various embodiments, the two laser systems 100, 129
having beam paths 104, 122 may be included in separate systems, or
the optical systems may be combined by various well-known methods
into a single laser system. There may be two optical axes, one for
each beam path, or the beam paths may be combined for a coaxial
beam path. A single laser head may produce the beams of the laser
sources 102 and 120, or there may be two laser sources. By way of
example, it is preferred that the secondary laser 120 has a
wavelength near or exceeding the absorption edge of silicon. The
choice of integrating into a single system may be based on specific
design considerations, for example, workpiece dimensions, laser
wavelengths and power, optical design considerations, component
costs, available factory floor space, X-Y positioning requirements,
etc.
[0196] The delivery systems 106 and 124, corresponding to the
texturing system 100 and the secondary processing system 129,
respectively, will often include some elements operated under
computer control, as indicated by lines 107 and 127, respectively.
For example, focusing, spot size adjustment, polarization control,
and energy control functions may be controlled. Suitable
combinations of electro-optic devices, modulators, and
opto-mechanical devices for positioning may be utilized. For
instance, either delivery system 106 or 124 may include a
polarization controller to enhance or control a mark
characteristic.
[0197] The ultrafast laser 102 may produce one or more femtosecond
pulses. However, picosecond lasers may provide many of the benefits
of femtosecond lasers but at reduced cost and complexity.
Controlled laser-material interaction at the ultrafast scale can be
used to control the contrast of the marks over a wide range to meet
specific application requirements. Of further significance is the
reduction or elimination of debris, slag, cracking, and other
undesirable effects generally associated with traditional marking
lasers.
[0198] Discrete areas of laser-patterned, microtextured material
may be used to create high contrast marking on silicon, especially
silicon wafers, and on other materials such as titanium or steel.
By way of example, FIG. 5 illustrates an exemplary "spiky" textured
region 135 with the surface height 136 illustrated by a typical
spike. Such nearly periodic and sharp variation in roughness may be
produced with femtosecond laser pulses. The spikes may have a
height ranging from a fraction of one micron to tens of microns.
The surface profile may be strongly dependent on laser parameters
including pulse duration (i.e., width), peak energy, spot diameter,
and spot irradiance profile. Researchers have postulated that such
spike formation involves both laser ablation and laser-induced
chemical etching.
[0199] Embodiments of the present invention can be used to create
microtextured regions having lower amplitude (e.g., sub-micron)
surface height variations than exemplified in the above paragraph,
but with sufficient variations to create high contrast, slag-free
marks. Further, enhanced contrast and absence of debris provides
for improved mark density when compared to traditional marking
approaches. FIGS. 10 and 11 are schematics illustrating, by way of
example, a prior art laser mark formed on a specular surface, for
instance bare silicon, and a corresponding surface profile showing
debris and cracking associated with a deep, "hard" mark. FIGS. 12
and 13 are schematics illustrating, by way of example, a mark
formed on the specular surface of FIG. 10 using a system of the
present invention. The pulsed laser may be a picosecond laser
producing a pulsed output with total energy density (in one or more
pulses) sufficient to initiate ablation within a portion of a spot
area on the substrate surface. The surface height variations may be
tens to hundreds of nanometers, the marked region generally showing
significant roughness and eliminating at least strong reflection
components.
[0200] FIGS. 6 and 7 are schematics comparing prior art marks with
marks formed in accordance with the present invention,
respectively, and illustrate improved density of a dot matrix
pattern. FIGS. 8 and 9 are schematics comparing prior art marks
with marks formed in accordance with the present invention,
respectively, and illustrate improved density of a bar pattern.
[0201] FIGS. 21 and 22 illustrate the structure of various laser
marks produced with traditional and more recent laser marking
systems for further comparison with a marked substrate produced in
accordance with the present invention. FIGS. 21 and 22,
respectively, are side and top schematic views of a mark 250 formed
by the assignee of the present invention using a NdYVO4 laser with
a pulse width of about 15 ns at 532 nm. The laser system produced
shallow marks having a depth of about 1.5-4 microns without
substrate cracking. FIG. 22 is a top view of a mark 250
illustrating the presence of ejected material 252 adjacent to the
mark 250. This recent example corresponds to results disclosed in
published US Patent Application Number 2004/0060910 entitled "High
Speed, Laser-based Marking Method and System for Producing Machine
Readable Marks on Workpieces and Semiconductor Devices with Reduced
Subsurface Damage Produced Thereby," assigned to the assignee of
the present invention, and published 1 Apr. 2004. FIG. 21 is a side
schematic view of a relatively deep traditional "hard" mark 254,
about 10 microns in depth, wherein cracking of silicon is observed
with relatively deep, laser penetration 256.
[0202] In certain applications it may also be of interest to remove
or erase a previously-formed, laser mark. The formation of
highly-absorbing, microtextured regions provides for such
capability because the region may be controllably modified using
the secondary laser system configured with appropriate laser
parameters.
[0203] With bright field visible at near infrared illumination,
these textured (marked) areas formed in accordance with the present
invention provide high contrast relative to the reflective wafer
background surface 115. For instance, the wafer surface 115 may be
smooth relative to a visible wavelength, thereby resulting in a
strong specular reflection component with negligible diffuse
reflection. A textured region may appear opaque, and will
preferably have diffuse reflectance corresponding to the darkest
shades of "grey scale charts" used for calibrating imaging
systems.
[0204] By way of example, the diffuse reflectance may be in a range
of about 0.5% to 5%, corresponding to about 6 shades of grey. The
contrast provides detection for improved-detection,
machine-readable marks such as alphanumeric strings, bar codes,
matrix codes, etc. Such indicia may be viewed with a mark
inspection system 201, which may be one component of a complete
laser processing system as shown in FIG. 14 which, by way of
example, includes both the marking laser system 100, and the
secondary laser processing system 129 of FIGS. 1 and 2,
respectively. The system 129 may be used to erase a mark.
Alternatively, a system may include only laser marking and an
optional mark inspection system 201 without secondary processing.
With an ultrafast (or ultra-violet) source, shallow marks with
highly controllable depths are possible that would be advantageous
for marking or coding very thin wafers, for instance, with mark
dimensions substantially finer than present commercially available
systems.
[0205] A following section entitled "Marking Examples" and
referenced drawing figures show exemplary results using picosecond
lasers to produce laser marks on silicon substrates having rough,
ground, or smooth surfaces. The readability of the marks is
improved over conventional laser marks. The marks appear
substantially opaque and the contrast with the background remains
high as a function of illumination and viewing angles, for
instance, as the relative angle between an illumination source and
receiver is varied over 30 degrees. This invariance increase the
reliability of machine vision algorithms.
Laser Marking Systems
[0206] Various embodiments of the invention provide for high
contrast marks on wafers and marks on other microelectronic
articles or devices made from silicon. Further application is
expected to MEMS and MOEMS devices and for providing marks or other
patterns on materials such as Ti and steel.
[0207] The marking may be used primarily for identification, or it
may be used to change the material's optical properties at discrete
sites for functional or subsequent process reasons.
[0208] In a complete marking system, wafers to be processed are
removed from a wafer carrier by a robotic wafer handling system
205, as illustrated schematically in FIGS. 14 and 15. Orientation
of the wafer is determined by optical alignment, including
operation of a pre-aligner 206 and any other required steps, for
instance identifying the wafer type with a reader 207.
[0209] The laser 102 generates a processing beam. With reference to
FIG. 16, the beam positioning system 106 delivers and focuses the
ultrafast processing beam onto a workpiece, which may be a silicon
substrate. Interaction of the focused processing beam with the
material of the workpiece in an atmosphere or ambient environment
creates microtexture on the surface of the workpiece.
[0210] Laser pulses 104 (i.e., FIG. 1) generated by the ultra-short
pulse laser 102 propagate along the optical path 104, and are
deflected or otherwise positioned with a beam positioner 220 (FIG.
16). The beam positioning will typically be carried out using two
galvanometer scan mirrors which generally provide for deflection
over a wide angle with a scan lens used to focus the output onto
the wafer 114 which is typically mounted on an X-Y stage 208. Each
laser pulse (or pulse sequence) forms a microtextured spot of
material on the wafer 114. The positioning of the beam is
determined by a controller so that a sequence of laser pulses forms
characters or other indicia on the wafer 114. When the mark is
complete, the wafer 114 is removed from the processing area and
reloaded into its wafer carrier. Alternatively, a secondary
processing step may occur with the system 129 before the wafer 114
is transferred.
[0211] Relative controlled movement of the workpiece and the
focused processing beam selectively creates microtexture that forms
discernible patterns on the workpiece.
[0212] Various spot-shaping and aligning methods such as shaping by
dithering taught by Fillion in U.S. Pat. Nos. 6,341,029 and
6,496,292, and aspect and orientation modification taught by
Ehrmann in U.S. Pat. No. 6,639,177 and others can be applied to
laser microtexturing. For example, top-hat irradiance profile spots
can provide indicia with more uniform microtexturing and reduced
melting at the ablation threshold transition zone. With a top hat
profile, the area of the spot at or above the ablation threshold
can be increased without an increase in the pulse energy, thereby
efficient microtexturing is achieved. It may be desirable to use a
spot with a depressed center and energy concentrated in the
perimeter to further limit melting and increase sharpness of the
microtexture substrate transition at the kerf edge.
[0213] Round spots provide consistent exposure for line elements in
any orientation. However, square and rectangular spots can further
increase efficiency and uniformity by delivering a uniform dose
across an aligned kerf width and to larger areas requiring filling.
Control of the aspect of a spot can be used to precisely control
the irradiance and spot overlap. For example, maximum pulse energy
may be selected and spot length along a trajectory may be expanded
or contracted to modify the shape of the area exposed at or above
the ablation threshold. The spot may be expanded to expose a larger
area at or above the ablation threshold without increasing the kerf
width or can allow the kerf width to be reduced without increasing
irradiance. The spot may be expanded to allow expose of at least a
portion of the spot to an increased number of laser pulses.
Conversely, compression of the spot along the kerf may be used to
increase the exposure to or above the ablation threshold without
decreasing the kerf width or can allow the kerf width to be
increased without decreasing irradiance. Compression of the spot
may also be used to allow exposure of at least a portion of the
spot to a decreased number of pulses.
[0214] The laser may be an ultrafast laser. Generally, the laser
parameters will depend of the linewidth to be marked, the material
optical properties, and numerous system considerations and features
(e.g., overall optical efficiency of the laser processing system).
The ultrafast laser may have a wavelength below the absorption edge
of the substrate, for example 532 nm for silicon processing. The
average power, which relates to the required threshold fluence and
repetition rate may be in the approximate range of 0.01 W-2 W for
marking silicon wafers using various embodiments of the present
invention. The pulse width (duration) may be less than 1 ns.
Preferably, the laser pulse width will be in the range of about 100
fs to about 50 ps, and most preferably in the range of about 300 fs
to 10 ps.
[0215] Certain embodiments may use a diode-pumped, solid-state UV
laser with a pulse width less than about 20 ns, and preferably less
than 1 ns. Formation of the desired microtexture may utilize
gas-assist or a gaseous environment in the chamber 110, as shown in
FIG. 1, for best results with UV processing.
[0216] FIGS. 16-19 show additional details of an exemplary laser
processing system which may be used for laser marking of
substrates, for instance marking of silicon wafers (front and/or
backside). A positioning subsystem may include one or more
translation stages 208 for moving the workpiece relative to the
processing beam 108 along at least two axes. Numerous combinations
of relative substrate and beam positioning devices are known to
those skilled in the art of laser material processing, for
instance, semiconductor processing, stereolithography,
semiconductor laser repair, laser drilling, or semiconductor wafer
trimming.
[0217] The features of FIGS. 7 and 9 (dots and bars, respectively)
are typically tens of microns in commercially available marking
systems. A laser marking system of the present invention may be
used to produce significantly smaller feature sizes, for instance
5-10 micron dots. The features in FIGS. 7 and 9 may be
characterized by a minimum resolvable distance between features of
the pattern. The minimum resolvable distance may be on the order of
a wavelength of the pulsed laser output, for instance 0.25
microns.
[0218] By way of example, it may be desirable to form indicia on a
portion of a MEM, in a restricted area. By controlling the fraction
of the energy above threshold in a merely diffraction-limited spot,
sub-micron features may be formed, for instance 0.25 micron
features, or 0.5-2 micron features if margin is to be provided. As
such, the positioning system may include fine and coarse
positioners to match or exceed the pattern resolution and provide
for ultra-fine patterning. The fine stage may have a travel range
on the order of millimeters positioning accuracy substantially
finer than 1 micron, for instance 0.05 microns. The system may also
include other auxiliary fine positioners, for instance precision,
small-angle, beam deflectors (e.g., an acousto-optic deflector),
together with closed loop control. Such positioning systems may be
used in embodiments of the present invention for laser marking,
patterning, joining, or other applications.
[0219] With reference to FIG. 17, embodiments of the invention for
practicing laser marking and other applications with laser systems
100,129 may include irradiating first and second sides of the
workpiece. By way of example, wafer chuck 249 may be mounted to an
X-Y stage in a configuration suitable for laser systems 100,129 to
irradiate opposite sides of the workpiece surface. The wafer chuck
249 may include a Z-axis translator and capability for tilting the
workpiece (rotation about the x-y axes). Such precision translation
and rotation stages have been applied in lithography, laser
trimming, and similar applications.
[0220] With reference to FIGS. 16, 18, 19 and 20, either or both
beam positioning systems 220, 220' may include a two-axis,
galvanometer-based, beam scanner 240,242 to move the beam relative
to the workpiece. Either beam positioning system 220, 220' may
include a combination of two or more stages and scanners to move
the processing beam relative to the workpiece in at least two axes.
Further, components to position the beam waist relative to the
workpiece through movement 246 of one or more optical elements
within the secondary laser system may be included. Similarly,
components may be included to move the beam waist relative to the
workpiece in at least 3 axes using various combinations of movement
246 of optical system components with z-axis translation 246'
using, for example, a motorized wafer chuck assembly.
[0221] Precision galvanometric scanning heads 240,242 are available
from GSI Lumonics Corporation (assignee of the present invention),
Cambridge Technologies and Scan Labs LTD. Exemplary options for an
associated optical system are illustrated in FIGS. 18 and 19
include: (1) a telecentric lens or f-theta corrected lens 221 with
programmable spot size adjustment 222; and (2) a widefield
post-objective system 223 and motorized dynamic focus adjustment
224 (not shown in FIG. 16).
[0222] The focusing subsystem 106 in the ultrafast system is
illustrated as a refractive optical system. In embodiments where a
femtosecond laser system produces a laser output, an all reflective
system may provide for improved performance as a result of
dispersion compensation. For example, an ultrashort pulse may have
a wavelength spread of about 8 nm or more about the central
wavelength. Femtooptics, Inc. is a supplier of femtosecond optical
components.
[0223] Extremely high absorption microtextures may be produced in a
non-ambient atmosphere, with processing carried out in a processing
chamber. The processing atmosphere may containing gases or may be a
vacuum to assist the formation of microstructures. However, it is
preferred that microstructures are formed in a open atmosphere of
gases, and it is most preferred that the workpiece is placed in
ambient air.
[0224] The microtexture created reduces reflected energy off the
surface of the workpiece. Generally, the microtexture created is
spike or cone-like structures, on the order of or smaller than the
wavelength of light, the surface profiles of which are illustrated
in FIG. 5 as an exemplary regular arrangement and FIGS. 12 and 13
as a microtextured region with reduced height variation (peak
amplitude) and regularity. Very highly absorbing structures have
been made in process chambers. However, moderately absorbing
structures (e.g., FIGS. 12 and 13) are considered for various
embodiments of the present invention, thereby reducing the system
requirements needed to create the microtextured regions.
[0225] In application to marking, the patterns formed may be
alpha-numeric characters. The patterns formed may be logotypes. The
patterns formed may be machine-readable. The patterns may be also
be human-readable. A font size of finer than 0.3 mm is achievable
using one or more embodiments of the present invention.
[0226] The system may include an integral mark inspection system
201 (i.e., FIG. 14) to identify the patterns, or the inspection may
be included in a separate system or station of a manufacturing
process. Commercially available machine vision technology, for
instance pattern recognition systems supplied by Cognex, Inc. may
be used for mark identification. When compared with conventional
wafer marking technologies, laser-based microtexturing has produced
high contrast marks, relatively insensitive to illumination
conditions, including camera angle and lighting angle over a wide
range. Similarly, for certain applications, if the marks result in
imagery that is at least weakly dependent upon illumination or
viewing angles, the inspection vision system 201 may be implemented
(or replaced) with an optional "though the lens" vision system
configuration integrated within a galvanometer system, for instance
designs corresponding to FIGS. 18 and 19.
[0227] The substrate material may be a metal, a silicon wafer (bare
or with various coatings). Other exemplary materials include
inorganic or organic dielectrics (including low-k materials,
metallization, refractory metals, and plastics).
[0228] The material to be marked may be part of a multimaterial
device, which may include a layer of silicon dioxide on silicon,
for instance. The material may be an inorganic or organic
dielectric, for instance a passivation layer. The marked areas may
be permanent, semi-permanent, or erasable so as to be utilized in
the process of making the multimaterial device, for instance, to
control or select various fabrication steps. Exemplary devices
include a multimaterial semiconductor memory, damascene structure,
processors, peripheral chips, etc., RFID tags, MCMs, and the
like.
[0229] In one arrangement, allowing for efficient integration into
existing systems supplied to the semiconductor industry, an
ultra-short pulse laser source is to be included within a
commercially soft-mark type of wafer marking system such as the GSI
Lumonics WaferMark SigmaClean. Details regarding various laser
system alternatives are included in a later section. The soft marks
normally created in the wafer marking system are replaced with
shallow, low-reflectance, angle-insensitive marks. Integration of
the ultra-short pulse laser source requires mechanical and optical
changes to couple the laser energy into an optical path of the
machine, propagate the beam along the optical path and focus the
beam onto the substrate. The integration may be carried out by
design practices well-known to those skilled in the art of
designing laser beam processing systems.
Material Removal/Erasing a Mark
[0230] A highly absorbing marked area may be irradiated for
subsequent processing according to various methods of this
invention. The secondary irradiation may use lasers that are more
weakly absorbed in the substrate, for instance lasers that operate
near or beyond the absorption edge of silicon. Various studies have
shown increased absorptance within the microtextured regions both
in the visible and the NIR. Near and beyond the absorption edge of
silicon, high power processing lasers can be used with minimal
damage to the normally transmitting substrate. The first ultrafast
step may produce fine patterning that is processed with a coarse
secondary beam, and conversely, the first step may produce a coarse
mark that is finely patterned with the secondary step. The
absorbing material may be ablated in a micromachining step with
controlled depth.
[0231] With reference to FIG. 23, this ablation may be for the
purpose of removing or erasing 410 the mark, creating a negative
"window" mark on an absorbing field, or it may be to micromachine a
fine pattern, or trim an electrical or mechanical parameter.
[0232] FIG. 24 shows a microtextured area that is modified at 420
with secondary irradiation 126. The operations to produce the
microtextured region may be carried out within a system as
illustrated in FIG. 14, or at a different location in the
manufacturing process.
[0233] FIGS. 2 and 20 illustrate several components which may be
used in a secondary processing system and these system components
may be similar or identical to those used in the ultrafast system.
The laser 120 generates the secondary processing beam. A beam
positioning system, which may correspond to any suitable
arrangement of optical or mechanical beam positioners, delivers and
focuses the secondary processing beam 126 onto a microtextured area
of a workpiece. The secondary laser energy is absorbed by the
microtexture. The microtexture is heated and the increased
temperature is sufficient to cause a phase change in the
material.
[0234] The secondary processing laser 120 may be a YAG or CO2
laser. Preferably, the laser 120 is selected to minimize damage to
non-microtextured areas. Most preferably, the laser beam 126 is
transmitted through the non-microtextured material. For example, if
the material is silicon the preferred wavelength may be about 1.2
microns (e.g., using a Raman laser) where the silicon transmission
is generally maximized. Alternatively, a commercially available
1.32 micron laser may also be nearly optimum, and such lasers are
widely available.
[0235] The beam positioning system may include one or more stages
208 for moving the workpiece relative to the processing beam, in an
arrangement similar or identical to FIG. 20. The beam positioning
system may include a beam scanner 240,242 to move the beam relative
to the workpiece and a focusing lens. The specific choice of
components and other features (e.g., lens coating) will generally
depend on the laser wavelength, spot size requirements, damage
threshold considerations, etc.
[0236] The beam positioning system may include a combination of two
or more stages and scanners to move the processing beam relative to
the workpiece in at least two axes. The beam positioner may
position the beam waist relative to the workpiece through movement
246 of one or more optical elements within the secondary laser
system. The beam positioning system may move the beam waist
relative to the workpiece in at least three axes. The beam
positioning system may move the beam waist relative to the
workpiece in at least three axes using various combinations of
movement of optical system components with z-axis translation 246'
using, for example, a motorized wafer chuck assembly.
[0237] The microtextured area may be formed according to an
embodiment of the current invention, but is not restricted to
ultrashort laser processing. Generally, the microtextured area is
to decrease the reflection coefficient of the workpiece
material.
[0238] The irradiated material may be ablated from the surface to
remove the absorbing structure. The irradiated material may be
melted and recast to form an area with modified properties.
[0239] For microtexture formed on materials with a transmission
band, it is preferred to select a laser wavelength in the
transmission band which is absorbed into the microtexture. In this
way, the properties of the microtextured material can be modified
and the properties of adjacent material can be left unchanged. The
secondary beam may have a wavelength which corresponds to maximum
transmission through the non-textured material. For example, if the
non-textured region is also silicon, the secondary beam may have a
wavelength beyond the absorption edge of silicon, for instance
greater than 1.2 microns. If the non-textured material is glass,
the wavelength may be in the visible or near IR region. In some
applications it may be desirable to use a laser that is absorbed in
the microtexture and the adjacent material
MARKING EXAMPLES
[0240] Each of FIGS. 25 to 36 relates to actual results obtained
using a commercially available picosecond laser to form marks on
silicon substrates having ground, polished, or smooth surfaces.
Laser and system parameters for some marked regions are as
follows:
[0241] FIG. 25 [0242] Grinded silicon wafer (with good mark);
[0243] Marking condition: [0244] Wavelength 532 nm; [0245]
Repetition rate 30 KHz; [0246] Average power 460 mw; [0247] 15
.mu.j pulse energy; [0248] Linear mark speed 100 mm/sec; [0249]
Line width 115 .mu.m; [0250] Energy density 0.15 J/cm.sup.2; [0251]
Peak power density 10.sup.10 W/cm.sup.2; [0252] Overlap: 34.
[0253] FIG. 26 [0254] Grinded silicon wafer; [0255] Marking
condition: [0256] Wavelength 532 nm; [0257] Repetition rate 30 KHz;
[0258] Average power 500 mw; [0259] 16.3 .mu.j pulse energy; [0260]
Linear mark speed 100 mm/sec; [0261] Line width 140 .mu.m; [0262]
Energy density 0.16 J/cm.sup.2.
[0263] FIG. 27 [0264] Grinded silicon wafer; [0265] Marking
condition: [0266] Wavelength 532 nm; [0267] Repetition rate 30 KHz;
[0268] Average power 300 mw; [0269] 9.8 .mu.j pulse energy; [0270]
Linear mark speed 16.6 mm/sec; [0271] Energy density 0.1
J/cm.sup.2.
[0272] FIG. 28 [0273] Grinded silicon wafer (with good mark);
[0274] Marking condition: [0275] Wavelength 532 nm; [0276]
Repetition rate 30 KHz; [0277] Average power 100 mw; [0278] 3.3
.mu.j pulse energy; [0279] Linear mark speed 100 mm/sec; [0280]
Line width 40 .mu.m; [0281] Energy density 0.26 J/cm.sup.2; [0282]
Peak power density 1.7.times.10.sup.10 W/cm.sup.2.
[0283] FIG. 29 [0284] Polished silicon wafer (with good mark);
[0285] Marking condition: [0286] Wavelength 532 nm; [0287]
Repetition rate 30 KHz; [0288] Average power 100 mw; [0289] 3.3
.mu.j pulse energy; [0290] Linear mark speed 100 mm/sec; [0291]
Line width 40 .mu.m; [0292] Energy density 0.26 J/cm.sup.2; [0293]
Peak power density 1.7.times.10.sup.10 W/cm.sup.2.
[0294] FIGS. 30a and 30b [0295] Silicon wafer (with mark on
specular surface); [0296] Marking condition: [0297] Wavelength 532
nm; [0298] Repetition rate 30 KHz; [0299] Average power 7.2 mw;
[0300] 0.24 .mu.j pulse energy; [0301] Linear mark speed 100
mm/sec; [0302] Line width 8 .mu.m; [0303] Energy density 0.12
J/cm.sup.2; [0304] Peak power density 0.8.times.10.sup.10
W/cm.sup.2.
[0305] FIGS. 31a and 31b [0306] Silicon wafer (with mark on rough
surface); [0307] Marking condition: [0308] Wavelength 532 nm;
[0309] Repetition rate 30 KHz; [0310] Average power 7.2 mw; [0311]
0.24 .mu.j pulse energy; [0312] Linear mark speed 100 mm/sec;
[0313] Line width 8 .mu.m; [0314] Energy density 0.12 J/cm.sup.2;
[0315] Peak power density 0.8.times.10.sup.10 W/cm.sup.2.
[0316] FIGS. 32a, 32b and 32c [0317] Silicon wafer (with 15 line
mark within 0.28 mm); [0318] Marking condition: [0319] Wavelength
532 nm; [0320] Repetition rate 30 KHz; [0321] Average power 7.2 mw;
[0322] 0.24 .mu.j pulse energy; [0323] Linear mark speed 100
mm/sec; [0324] Line width 8 .mu.m; [0325] Energy density 0.12
J/cm.sup.2; [0326] Peak power density 0.8.times.10.sup.10
W/cm.sup.2.
[0327] The samples were mounted on X-Y stage, and the mark
linewidth was varied by adjustment of a combination of the optical
system and incident laser energy, the adjustments controlling the
energy density incident on the material.
[0328] The images of the marked material samples were taken using a
"through the lens" (brightfield) microscope system and CCD camera.
Various regions were profiled using either a SEM (scanning electron
microscope) or AFM (atomic force microscope). Certain marks were
also compared with marks formed using a nanosecond laser system.
The number of pulses corresponding to a particular linewidth can be
computed from the pulse repetition rate, linewidth, and table
speed. For instance, FIG. 25 corresponds to 34 applied pulses
during stage travel corresponding to the linewidth of 115
.mu.m.
[0329] The listed laser parameters and results are to be regarded
as exemplary rather than limiting. The laser parameters may be
optimized or adjusted based on various process conditions, surface
roughness figures, presence/absence of coatings, etc. Various
refinements and adjustments may provide for further improvements in
contrast and density.
[0330] Generally, the linewidth corresponds to an effective spot
size on the surface wherein the fluence is at or above an
approximate ablation threshold of silicon. Hence, if a greater
fraction of the spot is above the ablation threshold the linewidth
increases. For example, if the spot profile is diffraction limited
and Gaussian and the region above threshold corresponds to the
FWHM, then the nominal kerf width will be approximately the
FWHM.
[0331] Also, as a result of a nearly constant fluence threshold for
a specific material, larger linewidths, larger spots, generally
require more laser energy than small linewidths.
[0332] The laser used in the experiments and general laser system
specifications are as follows: [0333] Commercial pico-second laser
from Lumera Laser, Model Staccato; [0334] Primary specifications:
[0335] Pulse width 15 ps; [0336] Wavelength 532 nm; [0337]
Repetition rate 30 KHz; [0338] Average power (see attached
results); [0339] Linear polarization; [0340] M-squared less than
1.2. It is noted that whenever linear polarization is used the
direction of stage travel was aligned perpendicular to the
polarization direction.
[0341] FIGS. 25, 26 and 27 show marks formed on a ground silicon
substrate, exemplified by the grind direction (as opposed to
uniform, specular background). FIGS. 26 and 27 provide a rough test
of the "process energy window" which represent the energy range
over which acceptable processing is achieved. The larger linewidth
(140 microns) in FIG. 26 corresponds to a larger fraction of the
focused spot diameter above the ablation threshold. A slight
heat-affected zone (HAZ) is shown in the regions where melting
occurs, though insignificant. Undesirable slag, debris, or severely
melted zones are absent.
[0342] FIGS. 28 and 29 show parameters and results where a 40
micron linewidth was produced in the X and Y directions,
respectively. The polarization was perpendicular to the direction
of travel.
[0343] FIGS. 30a, 30b, 31a, 31b, 32a, 32b and 32c illustrate
improved mark density and a clearly resolved pattern within a 0.3
mm (0.28 mm) region, a result which demonstrates capability to form
finer indicia than available in current, commercially-available,
laser marking systems. FIGS. 30a and 30b correspond to a specular
wafer background (smooth finish), FIGS. 31a and 31b correspond to a
rough back-side wafer surface, and FIGS. 32a, 32b and 32c
correspond to a polished wafer. The enlarged photos provide some
local additional detail, the definition and sharpness reduction
believed to be caused by various limitations in the setup, for
instance the camera dynamic range, the high N.A. of the microscope
collecting additional scattered light, and other factors. The high
contrast images correspond to the approximate magnification to be
used by a typical mark reader/inspection system.
[0344] FIGS. 33a and 33b compare a high contrast mark obtained with
the picosecond system (i.e., FIG. 33a) with a mark formed with a
typical nanosecond laser-based marking system (i.e., FIG. 33b). The
nanosecond "dark" mark surface roughness is not significantly
altered, whereas the microtexture is formed only within the
picosecond mark.
[0345] FIGS. 34a, 34b and 34c show SEM images of the marked regions
further demonstrating the presence of the microtexture produced
with the picosecond system as follows:
[0346] FIG. 34a
[0347] SEM of mark by ps laser;
[0348] Submicron structures seen.
[0349] FIG. 34b
[0350] SEM of dark mark by ns laser;
[0351] No structure seen.
[0352] FIG. 34c
[0353] SEM of white mark by ns laser;
[0354] Larger ridge structure seen.
[0355] The nanosecond results show only insignificant roughness
("dark mark" case), the surface roughness variations evident with
the picosecond results were not detected in the nanosecond
data--neither the "dark" or "hard" marks show the microtexture. The
deep marks (traditional "hard marks") also show highly undesirable
ridge formations. Of significance is an observation that even for
"dark" nanosecond marks (which are relatively shallow compared to
"hard marks"), the microtexture is not detected in the images
corresponding to the nanosecond marking.
[0356] FIG. 35 shows SEM images obtained from a marked region of a
polished wafer. A surface region is shown at three SEM
magnifications: 15,000.times., 6,000.times., and 25,000.times.. The
texture boundary between the marked and unmarked region is evident,
even with the polished background variations. Further, the marked
region, its periphery, and the polished background are all
slag-free--ridges and kerf height negligible.
[0357] FIGS. 36, 37a, 37b, 37c and 37d show measurement of a marked
region of a polished wafer using an AFM to measure surface heights.
Sub-micron structures are evident, with peak heights in the range
of tens to hundreds of nanometers. The polished background regions
correspond to the semi-specular, strong directional reflectance
results shown in earlier figures. A very high contrast is
achievable for specular wafer backgrounds providing for clear
recognition of textured and non-marked specular wafer regions. In
such a case, the indicia may also be clearly distinguished from the
background using AFM or SEM measurements which will exemplify the
difference in texture.
[0358] The examples generally show marked regions which have
roughness greater than a reflective background. By way of example,
it is also possible to form positive contrast indicia by forming a
background which is microtextured, wherein the indicia are highly
reflective, and configuring the lighting so that the indicia have
positive contrast relative to a dark background. This example may
be of interest if, for example, the system throughput is not
degraded (or is improved) using the technique (e.g., wherein the
total area of the indicia is to be greater than the background
area). Further, certain applications may require such "reverse
contrast" as part of customer specifications. Other similar
variations and alternatives can be carried out without departing
from the scope and spirit of the present invention.
LASER EMBODIMENTS
[0359] The commercially available laser used to produce the high
contrast results of the above working example includes a mode
locked oscillator and a diode pumped solid-state laser amplifier.
Picosecond outputs are available with pulse parameters within a
desired range. The choice of a laser system is generally based upon
the requirements for pulse energy, repetition rate, average power,
pulse width required to irradiate the material to be marked with
total energy density sufficient to initiate ablation within the
spatial spot size on the material surface. Useful laser wavelengths
include near IR, visible (e.g., 532 nm), and ultraviolet. Other
factors include size, cost, reliability, and various practical
considerations for use in semiconductor production environment, for
example. An "off-the-shelf" solution is desirable when available.
Preferably, the laser system is compatible with available marking
equipment, for instance the commercially soft-mark type of wafer
marking system such as the GSI Lumonics WaferMark SigmaClean.
[0360] Published U.S. patent application No. 2004/0134894 entitled
"Laser-based System for Memory Link Processing with Picosecond
Lasers," is assigned to the assignee of the present invention and
is hereby incorporated in its entirety herein. Included therein are
various exemplary picosecond laser systems which may be used or
modified for use in one or more embodiments of the present
invention. Of particular interest are: the section entitled
"Picosecond Laser Embodiments", FIGS. 6a-8e, and the corresponding
portions of the published application.
[0361] Published U.S. patent application No. 2004/0226925 entitled
"Laser System and Method for Material Processing with Ultra Fast
Lasers," is assigned to the assignee of the present invention and
is hereby incorporated in its entirety herein. Included therein are
various exemplary femtosecond laser systems which may be used or
modified for use in one or more embodiments of the present
invention. Of particular interest are: the section entitled
"Ultrashort Laser Embodiments," FIGS. 1-8, and the corresponding
portions of the published application.
[0362] By way of example, the laser systems of the above-noted
incorporated patent applications may be modified for use at shifted
wavelengths (e.g., green and UV). The system output, for instance
average power and peak energy, may be adjusted to meet the energy
density requirements with reduction or increase or amplifier gain
as needed to process at a required energy density.
[0363] Ongoing developments are expected to lead to increased
commercial availability. For example, FCPA system (Fiber based
Chirped Pulse Amplification) reported by IMRA America includes 2
microjoules of pulse energy at a repetition rate of 500 KHz,
corresponding to 1 W average power operation at femtosecond pulse
widths.
[0364] While embodiments of the invention have been illustrated and
described, it is not intended that these embodiments illustrate and
describe all possible forms of the invention. Rather, the words
used in the specification are words of description rather than
limitation, and it is understood that various changes may be made
without departing from the spirit and scope of the invention.
* * * * *