U.S. patent application number 16/669478 was filed with the patent office on 2020-07-16 for laser texturing of ceramic-containing articles.
The applicant listed for this patent is M Cubed Technologies, Inc.. Invention is credited to Michael K. Aghajanian, Edward J. Gratrix, Daniel Mastrobattisto, Austin Scott McDannald.
Application Number | 20200223013 16/669478 |
Document ID | / |
Family ID | 62567726 |
Filed Date | 2020-07-16 |
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United States Patent
Application |
20200223013 |
Kind Code |
A1 |
Gratrix; Edward J. ; et
al. |
July 16, 2020 |
LASER TEXTURING OF CERAMIC-CONTAINING ARTICLES
Abstract
A laser texturing process modifies the surface of a
semiconductor wafer-handling device so that flatness is maintained,
but controlled roughness is imparted to prevent unwanted wafer
sticking. The laser texturing may be from a thermal laser, a cold
ablation laser, or either laser modified with an inert cover gas.
The laser etches or burns away a portion or fraction of a flat
surface, thereby reducing the area of contact to the semiconductor
wafer and thereby reducing friction and sticking. The etched or
burned-away portion is thus at a reduced, relieved or lower
elevation than the unaffected portion. The laser texturing may take
the form of a plurality of channels cut into the surface, or a
plurality of holes. Laser machining can yield a semiconductor wafer
handling device having finer detail than can be produced by other
shaping techniques, with feature sizes on the order of 50 microns
being achievable.
Inventors: |
Gratrix; Edward J.; (Monroe,
CT) ; Aghajanian; Michael K.; (Newark, DE) ;
Mastrobattisto; Daniel; (Milford, CT) ; McDannald;
Austin Scott; (Ansonia, CT) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
M Cubed Technologies, Inc. |
Newtown |
CT |
US |
|
|
Family ID: |
62567726 |
Appl. No.: |
16/669478 |
Filed: |
October 30, 2019 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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PCT/US2018/030748 |
May 2, 2018 |
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16669478 |
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62500482 |
May 2, 2017 |
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62500491 |
May 2, 2017 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C04B 41/91 20130101;
B23K 26/032 20130101; B23K 26/123 20130101; B23K 2103/02 20180801;
B23K 2103/50 20180801; B23K 26/364 20151001; C04B 41/0036 20130101;
B23K 26/402 20130101; B23K 2103/08 20180801; B23K 26/355 20180801;
C04B 41/5346 20130101; B23K 26/10 20130101; B23K 26/14 20130101;
H01L 21/68735 20130101; B23K 26/0876 20130101; B23K 26/36 20130101;
B23K 2103/52 20180801; B23K 26/362 20130101; B23K 26/40 20130101;
B23K 26/1464 20130101; B23K 26/0624 20151001 |
International
Class: |
B23K 26/352 20060101
B23K026/352; B23K 26/03 20060101 B23K026/03; B23K 26/364 20060101
B23K026/364 |
Claims
1-31. (canceled)
32. An extremely flat, machined article of controlled roughness
configured as a component for handling semiconductor wafers,
comprising: (a) a chuck having a support surface; (b) wherein said
support surface features a ceramic-containing material having (i) a
first portion at a first elevation, said first portion being
optically flat, and (ii) a second portion at a second elevation
recessed or relieved with respect to said first elevation, thereby
reducing optical contact bonding; (c) wherein said second portion
includes at least two sets of parallel grooves or channels that are
angled with respect to one another to form a cross-hatched pattern
on said support surface; and (d) said second portion exhibiting no
visible surface oxidation.
33. The article of claim 32, wherein said parallel grooves or
channels are no more than 500 microns in width.
34. The article of claim 32, wherein said parallel grooves or
channels are spaced no more than 500 microns apart.
35. The article of claim 32, wherein said second portion of said
support surface is made by laser machining.
36. The article of claim 32, wherein said component comprises at
least one member selected from the group consisting of vacuum wafer
chuck, electrostatic chuck, vacuum wafer table, wafer arm, end
effector, and susceptor.
37. The article of claim 34, wherein said parallel grooves or
channels are spaced no more than 100 microns apart.
38. The article of claim 32, wherein the no visible oxidation is
determined using optical or scanning electron microscopy.
39. The article of claim 32, wherein said component includes a
wafer chuck populated with a plurality of pins.
40. The article of claim 39, wherein each of said plurality of pins
features a top surface, wherein said top surfaces collectively
define said first portion, and further wherein said second portion
is present on at least one top surface of at least one pin of said
plurality of pins.
41. The article of claim 32, wherein said second portion of said
support surface has a roughness of at least about 0.75 micron
R.sub.A.
42. The article of claim 32, wherein said second portion of said
support surface exhibits no melted texture.
43. The article of claim 32, wherein said second portion of said
support surface exhibits no evidence of microstructural
modification.
44. The article of claim 32, wherein said ceramic-containing
material has an electrical resistivity greater than about
1.times.10.sup.6 (10E6) ohm-cm.
45. The article of claim 32, wherein said ceramic-containing
material is exclusive of ceramic oxides.
46. The article of claim 32, wherein said ceramic-containing
material comprises silicon carbide.
47. The article of claim 32, wherein said channel has a depth of at
least 1 micron.
48. An extremely flat, machined article of controlled roughness,
comprising: (a) a chuck having a support surface; (b) wherein said
support surface features a ceramic-containing material having (i) a
first portion at a first elevation, said first portion being
optically flat, and (ii) a second portion at a second elevation
recessed or relieved with respect to said first elevation, thereby
reducing optical contact bonding; (c) wherein said second portion
includes at least two sets of parallel grooves or channels that are
angled with respect to one another to form a cross-hatched pattern
on said support surface; and (d) said second portion exhibiting no
visible surface oxidation.
49. The article of claim 48, configured as a component for
conditioning a chemical-mechanical planarization pad.
50. The article of claim 48, wherein said support surface comprises
at least one material selected from the group consisting of diamond
and silicon carbide.
51. The article of claim 48, wherein at least said support surface
comprises titanium, silicon, and silicon carbide.
Description
CROSS-REFERENCE TO RELATED APPLICATION(S)
[0001] This patent document claims the benefit of U.S. Provisional
Application Ser. Nos. 62/500,482 and 62/500,491, each filed on May
2, 2017. Where permitted by law, the entire contents of each of
these commonly owned patent applications are expressly incorporated
by reference herein.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
[0002] None.
TECHNICAL FIELD
[0003] In one aspect, the present invention relates to machining
techniques for use on ceramic-containing materials, including
composites and glasses. The present invention particularly relates
to laser machining techniques.
[0004] In another aspect, the present invention relates to
machining methods for imparting a controlled roughness to an
otherwise flat, smooth surface, particularly in articles or
components for handling semiconductor wafers (e.g., silicon) wafers
for processing, which could be, but is not limited to,
lithography.
BACKGROUND ART
[0005] As Moore's Law pushes semiconductor feature sizes smaller
and smaller, the need for highly precise wafer handling components
(vacuum chucks, electrostatic chucks, wafer arms, end effectors,
etc.) grows. Desired features for wafer handling components include
high mechanical stability (high stiffness and low density), high
thermal stability (high thermal conductivity and low coefficient of
thermal expansion), low metallic contamination, machinability to
high tolerance, low wear (to maintain precision), low friction (to
prevent wafer sticking), and the ability to be fabricated to sizes
of up to 450 mm. Furthermore, these chucks should have low friction
with respect to the semiconductor wafers that they support, and,
critically, must be free of particle contamination on the support
surfaces.
[0006] These needs have pushed the manufacturing of wafer handling
components to precision materials (e.g., SiC-based ceramics) and
precision finishes, i.e., wafer contact surfaces with extreme
flatness and low roughness.
[0007] However, it is well known in the technical literature that
when two very flat and smooth surfaces are contacted, they will
stick together (a phenomenon typically referred to as "optical
contacting" or "contact bonding" or "stiction").
[0008] When stickiness is present between a wafer and a wafer
support surface, it is difficult to quickly chuck and de-chuck a
wafer, and it is difficult to hold a wafer in a precision
fashion.
[0009] The Applicant manufactures several ceramic composites (i.e.
Si/SiC, SiC/Diamond, etc.) that meet the mechanical and thermal
stability requirements. Many of these ceramic composites are based
on, or formed by, a reaction bonding process involving infiltration
of molten silicon metal or silicon alloy into a porous mass
containing an inert (or rendered inert) ceramic reinforcement. An
example is infiltrating molten Si into a porous mass featuring SiC
to form "reaction bonded silicon carbide" (or "RBSC" or "Si/SiC").
In order to reduce the friction between the chuck and the wafer, a
pattern of pins is machined into the top surface of the chuck. By
virtue of machining these pins, the overall contact area with the
wafer--and thus the friction--is greatly reduced. A further benefit
of low contact area is reduced wafer contamination.
[0010] Currently available methods to machine these pin patterns,
such as Electric Discharge Machining (EDM), struggle to meet the
demand for smaller and more precisely controlled pins. EDM
preferentially machines the metallic component of these composites,
leading to sub-surface damage (i.e. cracks and voids) and particle
formation. Moreover, EDM leaves a surface oxide layer (also known
as "re-cast") that can lead to particle contamination in wafer
handling operations. Still further, EDM suffers from poor
dimensional control. See, for example, the scanning electron
microscope photograph of FIG. 14, which shows a pin in a wafer
chuck machined by EDM. Of particular interest in this photograph
are the loss of circularity 11 (e.g., a rough perimeter around the
pin), crack initiation 13, and void formation 15.
[0011] EDM also requires that the material being machined be
electrically conductive.
[0012] Laser machining provides a potential alternative to EDM. The
laser can cut through all phases in a multi-phase material,
allowing uniform geometries with very minimal sub-surface damage.
However, conventional laser machining can locally heat the
material, which can cause surface modifications and oxide
formation. Such surface modifications and oxide formation are
highly undesirable due to an increased propensity for particle
contamination.
[0013] Another machining technique involves the use of a laser
beam, most notably a laser machining technique based on "cold
ablation". Cold-ablation is a relatively new laser machining
technique that uses short, high energy laser pulses to quickly
ablate the material while minimizing the local heating experienced
by the material (part being machined). Cold-ablation does not
completely avoid local heating, however, and some oxide formation
can occur.
[0014] The instant invention addresses these problems, and provides
a solution.
DISCLOSURE OF THE INVENTION
[0015] Next generation ceramic-containing wafer chuck machining
processes require:
[0016] High dimensional accuracy
[0017] Low sub-surface damage
[0018] Low local heating
[0019] Low surface modification
[0020] Low oxide formation
[0021] Low particle contamination.
[0022] EDM struggles to meet ever-tightening requirements, as
described above. Conventional laser machining meets dimensional
requirements but allows for undesirable surface modification. Cold
ablation laser machining does not completely solve the surface
modification problem.
[0023] In accordance with one aspect of the present invention, what
is provided is an assist to the cold ablation laser machining
technique, the assist being provided by an inert gas
atmosphere.
[0024] In another aspect of the invention, and per the embodiments
of the instant invention, in a device for handling semiconductor
wafers, a portion of the highly flat surface that supports the
wafer ("the wafer support surface"), is removed by machining,
thereby reducing the area of contact between the support surface
and the wafer. This action reduces the friction between these two,
and thus the propensity for "optical sticking". The machining may
take the form of a groove or channel, or a plurality of such
grooves or channels, or a plurality of holes such as "blind" holes.
In this way, a "texture" or controlled roughness is imparted to the
support surface. Embodiments of the present invention may use a
laser for this texturing, which laser may be a thermal laser, a
cold ablation laser, or a laser (thermal or cold ablation) modified
with an inert "cover" gas to reduce oxidation of non-oxide
materials.
BRIEF DESCRIPTION OF THE FIGURES
[0025] FIGS. 1A and 1B are SEM photographs at different
magnifications of pins in a Si/SiC wafer chuck that were laser
machined according to the instant invention.
[0026] FIG. 2 is a side view drawing of an inert gas-assisted cold
ablation laser machining apparatus.
[0027] FIG. 3 shows a hole drilled in a Si/SiC composite material
by means of a laser, the process being conducted in air.
[0028] FIG. 4 shows a hole drilled in a Si/SiC composite material
by means of a laser, the process being conducted in a flowing argon
cover gas.
[0029] FIGS. 5A, 56 and 5C are SEM photographs at the same
magnification of pocket cuts made into samples of Si/SiC composite
material by EDM, the prior art cold ablation laser in air, and the
instant cold ablation laser machining under protective argon gas
flow, respectively.
[0030] FIGS. 6A-6C are SEM photographs showing pins that were laser
machined according to the instant invention into a surface of a
Si/SiC composite material, a TiSi/SiC composite, and a Si/Diamond
composite material.
[0031] FIGS. 7A-7C are SEM photographs further illustrating the
capabilities of the instant embodiments of the invention.
[0032] FIG. 8A (inset) is a photomicrograph of one pin laser
machined in CVD SiC. FIG. 8B is a photograph of this CVD SiC
showing the pattern of a plurality of pins machined into the
surface.
[0033] FIGS. 9A and 9B are high magnification and low magnification
SEM photos, respectively, of a pin machined in a CVD diamond body.
FIG. 9B shows the pattern of a plurality of pins machined into the
surface.
[0034] FIGS. 10A and 10B are photomicrographs taken at two
different magnifications of a cross-hatched pattern of channels
laser etched on the semiconductor wafer support surface of a Si/SiC
body.
[0035] FIGS. 11A-11C are photomicrographs of an untextured wafer
support pin, a wafer support pin textured with a cross-hatch
pattern with 80 micron spacing, and a wafer support pin textured
with a cross-hatch pattern with 50 micron spacing,
respectively.
[0036] FIGS. 12A-12C are SEM photographs further illustrating the
capabilities of the instant embodiments of the invention.
[0037] FIGS. 13A and 13B are photographs at two different
magnifications of a Si/SiC wafer handling device to support a
semiconductor wafer, the device featuring pins on the support
surface.
[0038] FIG. 14 is a scanning electron microscope (SEM) photograph
highlighting the performance limitations of EDM. It is
representative of the prior art.
MODES FOR CARRYING OUT THE INVENTION
[0039] Next generation wafer handling components (e.g., wafer
chucks) require:
[0040] High mechanical stability (high stiffness, low density)
[0041] High thermal stability (high thermal conductivity, low
CTE)
[0042] High wear resistance
[0043] Low sticking
[0044] Precision flatness and low roughness
[0045] Applicant supplies SiC-based ceramic components, typically
with surface pins) for the application--see, for example, FIGS. 13A
and 13B. An issue is that as flatness and roughness improve,
unwanted stickiness becomes a problem due to the optical contact
bonding effect. When stickiness is present, it is difficult to
quickly chuck and de-chuck a wafer, and it is difficult to hold a
wafer in a precision fashion.
[0046] Mechanical grinding or Electrical Discharge Machining (EDM)
can potentially be used to impart a texture to the optically flat
surface, with certain limitations. As will be discussed in more
detail below, both mechanical grinding and EDM are limited to
machining feature sizes of about 1000 microns, and with precision
(or reproducibility) of about 200 microns. Where the semiconductor
wafer support surface is a collection of "pins" all having a
defined and very precise elevation, mechanical grinding will be
insufficiently precise. Similarly, EDM is of questionable
precision, as the size (diameter) of a support pin may be on the
order of 200 microns. EDM machining of such a pin may obliterate
the entire pin top surface instead of only a portion of the
surface.
[0047] EDM also requires that the material being machined be
electrically conductive. Where the material being machined is a
metal-ceramic composite, EDM preferentially machines the metallic
component of these composites, leading to sub-surface damage (i.e.
cracks and voids) and particle formation. Moreover, EDM leaves a
surface oxide layer (also known as "re-cast") that can lead to
particle contamination in wafer handling operations.
[0048] One attractive solution is a laser textured, or laser
machined, surface, which prevents stickiness by inducing a
pseudo-roughness to the surface. According to this technique, the
surface of the wafer-handling article that is intended to support
the semiconductor wafer is first configured, e.g., machined,
polished and/or lapped to the desired flatness, which may be
optically flat, that is, "flat" to within a tolerance measured on
the nanometer scale. Typically, such a precise degree of flatness
also has associated with it a similar degree of smoothness. Some of
the wafer support surface ("a second portion") is then further
processed, e.g., textured or machined, thereby removing some
material of that portion of the support surface. This leaves a
first portion of the wafer support surface untextured, thereby
leaving it at its existing level of flatness and smoothness. The
second portion of the surface thus is relieved or recessed (e.g.,
at a lower elevation) relative to the elevation of the first
portion. So, one portion of the support surface is left intact, and
another portion is machined or etched away. This is what is meant
by "pseudo roughness". This procedure has the effect of further
reducing the contact area between the support surface and the
article being supported (e.g., semiconductor wafer), which reduces
friction and particularly the optical sticking phenomenon.
[0049] This texturing or machining of the second portion of the
support surface can be performed by a thermal laser. The laser beam
can be manipulated to provide the relief or recesses ("texturing")
in a random, or in an organized way. The laser beam can be moved
relative to the support surface to remove at least a surface layer
of material, thereby creating a groove or channel in the material
of the support surface. The texturing may be in the form of a
plurality of channels, some of which may be parallel to one
another, for example. Two sets of such parallel channels may be
angled with respect to one another to create a cross-hatch pattern.
Alternatively, the laser beam can be held in a fixed position
relative to the support surface to create a hole, such as a blind
hole. The depth of the groove, channel or hole should be at least 1
micron.
[0050] There is a "characteristic width" associated with the laser
machining or texturing operation. One aspect of the characteristic
width is the width of the groove, channel or hole that is produced
by the laser beam. If a plurality of grooves, channels, or holes
are produced, another aspect of the characteristic width is the
spacing between adjacent holes, or parallel grooves or channels. In
other words, this aspect measures the width of a region of material
(e.g., unmachined material) between adjacent textured surfaces
(grooves, channels or holes). A regular, periodic, repeating form
of such spacing is sometimes referred to as "pitch". Both aspects
of the characteristic width of the laser machining operation are
finer (e.g., smaller width) than can be achieved by mechanical
machining such as grinding, or by EDM, as will be discussed in
greater detail in Example 3 below.
Exemplary Laser Processing Conditions (Thermal Laser)
[0051] 1.064 micron Nd:YAG
[0052] 100 Watt max average power
[0053] 50-300 microsecond pulse rate
[0054] Direct beam
[0055] 300 mm.times.300 mm stage
[0056] 1 micron repeat stage precision
Exemplary Modified Cold Ablation Laser Machining Technique
[0057] Laser machining provides a potential alternative to EDM. The
laser can cut through all phases in a multi-phase material,
allowing uniform geometries with very minimal sub-surface damage.
However, conventional laser machining can locally heat the
material, which can cause surface modifications and oxide
formation, particularly where the material includes a metal and/or
non-oxide ceramic. Such surface modifications and oxide formation
are highly undesirable due to an increased propensity for particle
contamination.
[0058] "Cold-ablation" is a relatively new laser machining
technique that uses short, high energy laser pulses to quickly
ablate the material while minimizing the local heating experienced
by the material (part being machined). Cold-ablation does not
completely avoid local heating, however, and some oxide formation
can occur if the machining is performed in air.
[0059] The Applicant has developed a laser machining technique
based on cold-ablation with an inert assist gas, which minimizes
the surface modifications and mitigates the oxide formation. In
particular, Applicant has modified the cold-ablation laser
machining equipment to include a gas nozzle that envelopes the
local area being machined in an inert atmosphere (e.g., Argon or
other such as Helium, Neon, Krypton, Xenon). The atmosphere, being
inert, prevents oxide formation. The overall result is a machining
technique that can be used on ceramic-containing materials such as
monolithic ceramics and composites based on ceramic and/or metal.
The instant machining technique can be used to machine a pin
pattern into a supporting platform or device such as a wafer chuck,
achieving high dimensional control (tight tolerances) and with
little to no surface modification (including oxide formation).
[0060] What is shown in FIG. 2 is a side-view drawing of the
modified cold ablation laser apparatus. In particular, the set-up
shows: a laser objective lens 21 for short duration high energy
laser pulses, a nozzle 23 for directing inert gas onto the surface
to be machined, a mechanical measurement probe 25, and an optical
measurement camera 27. Not shown is a movable galvanometer mirror
to direct and focus the laser light onto and ablate the material to
be machined. The modified pressurized gas nozzle achieves two
goals: (i) the flow of gas, removes machining debris from the
immediate area, and (ii) the flow of gas envelops the machining
area in an inert argon atmosphere, preventing oxidation.
[0061] Acceptable ranges for the modified cold ablation laser
processing conditions are:
[0062] Wavelengths used between 1080 nm and 150 nm
[0063] Pulse widths between 300 ns and 1 fs
[0064] Repetition rate between 100000 Hz and 1 Hz
[0065] Scan speed up to 1000 mm/s
[0066] Power up to 120 W
[0067] Argon as assist (cover) gas
[0068] The present modified cold ablation laser machining methods
(featuring an inert gas "assist") will work with most metals and
ceramics, including composites, and including metal-ceramic
composites. Other materials that can be machined with this
technique and are used for semiconductor wafer handling are AlN,
sintered SiC, CVD SiC, Al.sub.2O.sub.3, glass, and glass ceramics.
It is especially important when the metal in question has a
tendency to quickly form an oxide when heated (by, for example,
laser exposure) in air (i.e. Si, Ti, Al, etc.). Furthermore, it is
important when the ceramic phase thermally decomposes under laser
exposure into one of the aforementioned metals (i.e. SiC, TiC, AlN,
etc.).
[0069] Silicon carbide (SiC) has desirable properties for use as a
wafer chuck: low density, low thermal expansion coefficient, and
high thermal conductivity, to name three.
[0070] Silicon carbide-based bodies can be made to near net shape
by reactive infiltration techniques, and such has been done for
decades. In general, such a reactive infiltration process entails
contacting molten silicon (Si) with a porous mass containing
silicon carbide plus carbon in a vacuum or an inert atmosphere
environment. A wetting condition is created, with the result that
the molten silicon is pulled by capillary action into the mass,
where it reacts with the carbon to form additional silicon carbide.
This in-situ silicon carbide typically is interconnected. A dense
body usually is desired, so the process typically occurs in the
presence of excess silicon. The resulting composite body thus
contains primarily silicon carbide (for example, 40-80 volume
percent), but also some unreacted silicon (which also is
interconnected), and may be referred to in shorthand notation as
Si/SiC. The process used to produce such composite bodies is
interchangeably referred to as "reaction forming", "reaction
bonding", "reactive infiltration" or "self bonding".
[0071] For added flexibility, one or more materials other than SiC
can be substituted for some or all of the SiC in the porous mass.
For example, replacing some of this SiC with diamond particulate
can result in a Si/diamond/SiC composite. The volume fraction of
diamond can be engineered to range from 10 percent to 70 percent.
Further, the silicon metal may be alloyed, or the porous mass may
contain a metal other than silicon, to yield a reaction formed
composite containing the alloying element. For example, the silicon
constituent in a Si/SiC composite may be modified with titanium to
yield a SiC-containing composite body featuring both silicon and
titanium, which may be denoted as "TiSi/SiC". Successful
infiltrations to form such titanium-containing reaction-formed SiC
composites have been carried out using infiltrant metals containing
15 wt % and 40 wt % titanium, respectively, balance silicon. The
instant embodiments of the invention are used on all of these
materials.
Characterization of a Surface Machined by Various Techniques
[0072] The following describes how to characterize or differentiate
a machined surface prepared by traditional
grinding/lapping/polishing from one machined by EDM, and from one
machined with a laser beam. Note: a metal-ceramic composite
material, namely, Si/SiC was used for this characterization.
[0073] The surfaces resulting from the different operations can be
distinguished by the surface roughness. In grinding operations
(i.e. surface grinding, spin grinding, jig grinding or lapping)
with course abrasive diamond tool, will leave scratches in the
surface (parallel scratches in surface grinding, concentric in spin
grinding, randomized for jig grinding and lapping). In grinding
operations with randomized motion and fine diamond abrasive (i.e.
jig grinding with fine tools or lapping) the surface becomes
polished. The polished surface will have will have flat tops, all
at the same height, of the ceramic grains with the metal between
the ceramic slightly relieved.
[0074] In contrast, a laser machined surface will have a controlled
roughness. The laser machining removes material in a series of
circular areas that is scanned over the surface. The roughness is
controlled by the degree of overlap of the circular areas. Within
each circular exposure area, the ceramic grains will have some
roughness and stand slightly proud of the inter-granular metal.
[0075] While grinding and laser machining have a small difference
between the material removal rate of the ceramic and the metal, EDM
operations almost exclusively machine the metal. Because of this,
the EDM surface is very different with a melted/oxidized textures
(a "recast layer"). An electrical discharge machined surface will
have a random roughness. The metal is removed deeply between the
ceramic grains. There will also be micro-cracks of removed metal
that will extend deep below the surface. In some cases, these
micro-cracks can through the feature and connect to cracks from the
opposing surface several hundred microns away.
[0076] Table 1 quantifies the roughness discussed above. The table
shows that laser machining of a finely ground (polished) Si/SiC
composite material surface increases the roughness by about an
order of magnitude at relatively low laser power, but roughly
triples this roughness at higher power levels. Nevertheless, the
roughness obtained through electrical discharge machining was still
more than double the roughness produced by the 85 watt laser.
TABLE-US-00001 TABLE 1 Surface Measured roughness (R.sub.A in
microns) Ground 0.098 85 W laser machined 2.4 29 W laser machined
0.89 15 W laser machined 0.75 EDM 5.1
[0077] Another difference among grinding, EDM and laser machining
is in terms of potential modifications to the chemistry and
crystallinity of the machined surface. The predominant feature of a
thermal laser machined surface is a melted or heat-affected zone.
In contrast, when a Si/SiC material is laser machined with an
Ar-assisted cold ablation laser, this instant process provides:
[0078] For more precise control of machined features [0079] Much
less sub-surface damage [0080] Smoother surface finishes [0081]
Much less roughness on feature edges [0082] No evidence of crack or
void formation [0083] Avoidance of oxidation and reduced propensity
for particle formation.
[0084] By "avoidance of oxidation", the Applicant means that no
oxidation was observed visually, in optical microscopes, or even in
the SEM. This does not preclude the possibility, however, of atomic
scale oxidation being present on metal or non-oxide ceramic
surfaces, for example one, several, perhaps up to half a dozen
atomic layers of oxide present on the machined surface. This result
is in distinct contrast with thermal laser and EDM, where there is
an observed layer of oxidation.
[0085] The invention will now be further described with reference
to the following examples.
Example 1: Laser Machining Pins in a Si/SiC Wafer Chuck
[0086] Refer now to FIGS. 1A and 1B, which are SEM photographs at
different magnifications of pins in a Si/SiC wafer chuck that were
laser machined according to the instant invention. The laser
machining was performed by means of a cold ablation laser apparatus
modified or supplemented with a means (e.g., a nozzle) to direct an
inert gas (here, argon gas) onto the surface of the Si/SiC material
to be machined in the vicinity, zone or region of the laser
beam.
[0087] In addition to little-to-no surface modification of the
machined surface of the Si/SiC material, the Ar-assisted
cold-ablation laser machining provides; [0088] For more precise
control of machined features [0089] Much less sub-surface damage
[0090] Smoother surface finishes [0091] Much less roughness on
feature edges [0092] No evidence of crack or void formation [0093]
Avoidance of oxidation and reduced propensity for particle
formation.
[0094] By "avoidance of oxidation", the Applicant means that no
oxidation was observed visually, in optical microscopes, or even in
the SEM. This does not preclude the possibility, however, of atomic
scale oxidation being present on metal or non-oxide ceramic
surfaces, for example one, several, perhaps up to half a dozen
atomic layers of oxide present on the machined surface. This result
is in distinct contrast with thermal laser and EDM, where there is
an observed layer of oxidation.
Example 2: Comparison of Laser Drilled Holes in Si/SiC
[0095] This example shows the effect of adding an inert gas
"assist" to a cold ablation laser machining process.
[0096] Here, the process is in drilling a hole in a Si/SiC
composite material formed by a reaction-forming process. FIG. 3
shows the process being conducted in air. The left side of the
figure is a SEM photo of the hole. The right side of the figure
shows the elemental analysis of the edge of the hole according to
energy dispersive analysis by x-ray (EDAX).
[0097] Similarly, FIG. 4 shows the process being conducted in
flowing argon cover gas. The left side of the figure is a SEM photo
of the hole. The right side of the figure shows the elemental
analysis of the edge of the hole according to energy dispersive
analysis by x-ray (EDAX). Comparing the ratio or relative sizes of
the oxygen peak to the silicon peak in FIG. 3 versus FIG. 4, one
can see that the oxygen peak is greatly reduced where the argon
cover gas was used, indicating much less oxide formation during the
laser drilling process. In fact, the amount of oxide formed using
the argon cover gas was less than half as much as the amount formed
when laser drilling was conducted in air. Here, the argon gas
contained some oxygen and/or water vapor impurity; a purer source
gas would have reduced the amount of oxide formed still
further.
[0098] The formation of an oxide layer can cause a number of
problems, particularly in the context of the fabrication of
components for handling semiconductor wafers, such as wafer chucks.
These problems include: reduced tolerance control, reduced surface
hardness, potential particle contamination (e.g., from spalling of
oxide), non-uniform surface properties, and stress and strain
(e.g., warping) problems caused by joined materials having
different thermal expansion coefficients (bi-metallic strip
effect).
Example 3: Comparison of Pocket Cuts in Si/SiC
[0099] This example compares the quality of a "pocket cut" among
the modified cold ablation laser machining technique of the instant
invention, a prior art cold ablation laser machining process, and a
prior art electrical discharge machining process. In each instance,
the pocket cut was prepared on a sample of Si/SiC composite
material produced by reaction bonding. A pocket cut may be prepared
by providing an orthogonal prism of material such as a cube, and
proceeding to shave off material on one side to a desired depth but
leaving a region near the top surface intact, and then doing the
same on an adjacent side surface, again leaving a region near the
top surface undisturbed.
[0100] FIGS. 5A, 5B and 5C are SEM photographs at the same
magnification of pocket cuts made into the Si/SiC samples by EDM,
the prior art cold ablation laser in air, and the instant cold
ablation laser machining under protective argon gas flow. The
samples are oriented each the same way, showing a frontal view of
one of the machined side surfaces, and side views of the top
surface and the other side surface.
[0101] The prior art EDM pocket cut of FIG. 5A exhibits a
non-uniform edge, reflecting sub-surface damage as particles in the
Si/SiC composite pop out under the action of the electric
discharge. The cut surface also shows an oxide/recast layer. The
(prior art) cold ablation laser pocket cut of FIG. 5B exhibits a
more uniform edge (a straight cut), reflecting the laser cutting
through all phases of the Si/SiC composite. However, oxide build-up
at the cut surfaces is seen. The modified argon gas, cold ablation
laser pocket cut of the instant invention of FIG. 5C also exhibits
a uniform edge (a straight cut), but also shows that the cut
surfaces are "clean", with no formed oxide seen.
Example 4: Laser Machining of Si-Containing Composites of Different
Compositions
[0102] FIGS. 6A-6C are SEM photographs showing pins that were laser
machined according to the instant invention into a surface of a
Si/SiC composite material, a TiSi/SiC composite material, and a
Si/Diamond composite material. Thus, this example shows the
versatility of the instant techniques. Note that the
diamond-containing composite material cannot be machined using
mechanical techniques (e.g., grinding) because of the hardness of
the diamond particles.
Example 5: Dimensional Precision of the Instant Techniques
[0103] FIGS. 7A-7C are SEM photographs further illustrating the
capabilities of the instant embodiments of the invention. FIG. 7A
shows different geometric features that were laser machined and
being spaced apart from one another by less than 200 microns. FIG.
7B shows a laser machined pin in a wafer chuck, the pin having a
base measuring about 200 microns across. FIG. 7C shows a pin having
a base that is about 150 microns across, the pin also being laser
machined according to embodiments of the instant invention.
[0104] EDM and thermal laser have an accuracy issue due to
"over-burn". Essentially, there is a heat affected zone/melted zone
ahead of the cut. The modified cold laser ablation process of the
instant invention has a crisp cut because there is not a thermal
impact. EDM can machine features with sizes down to about 200
microns in diameter, and hold tolerances of .+-.(plus or minus) 12
microns in these materials. Mechanical machining such as by
grinding can machine features down to about 1000 microns in size,
and hold tolerances of .+-.200 microns. Net-shape molding similarly
has a tolerance capability of .+-.200 microns. The laser machining
can machine features that are 500 microns, 200, 100, even down to
50 microns in size, and hold tolerances of .+-.0.1 microns in these
materials. Feature sizes even smaller than 50 microns might be
possible.
[0105] In the context of the present invention embodiments, the
smallest feature size that can be machined can be expressed in
terms of the narrowest width of the passage, channel, or groove
that can be machined between two features, or the smallest diameter
hole that can be drilled. For example, in FIG. 7A, a pin and a
groove for a vacuum seal are shown to be about 200 microns apart.
Since ceramic material was removed (machined) to create these two
features, and the space (depressed area) between them, the instant
machining technique is shown to yield this narrow spacing. The
machined area has a lower elevation (is recessed) relative to the
elevation of the adjacent two features. Where a hole is concerned,
the machined surface defines the hole. For a "blind" hole, the
bottom of the hole is at a lower elevation than the elevation of
material outside and adjacent the hole.
[0106] Greater dimensional precision allows for more flexible pin
geometries and patterning. Because the material can be removed with
little surface modification, smaller features can be machined.
Inert gas-assisted cold ablation laser machining has greater
control over the profile of the pins and spacing to other features.
This will also benefit downstream processes.
Example 6: Laser Machining of Non-Conductive Materials
[0107] This example demonstrates the use of the modified cold
ablation laser machining technique of the invention to machine
materials that are insufficiently electrically conductive to be
machined by electrical discharge machining (EDM).
[0108] 1. CVD SiC:
[0109] FIG. 8A (inset) is a photomicrograph of a pin machined in
silicon carbide (SiC) produced by a chemical vapor deposition (CVD)
process. The machining was according to the instant invention,
e.g., cold ablation laser modified with protective inert gas. FIG.
8B is a photograph of the laser machined surface of this CVD SiC
showing the pattern of a plurality of pins machined into the
surface.
[0110] CVD SiC cannot be machined by EDM, as its electrical
resistivity is too high--on the order of 10E6 (one million)
ohm-cm.
[0111] 2. CVD Diamond:
[0112] FIG. 9A is a photomicrograph of a pin machined in diamond
produced by a chemical vapor deposition process. The machining was
according to the instant invention, e.g., cold ablation laser
modified with protective inert gas. FIG. 96 is a photograph of the
laser machined surface of this CVD diamond showing the pattern of a
plurality of pins machined into the surface.
[0113] CVD diamond cannot be machined by EDM, as its electrical
resistivity is too high--on the order of 1.times.10.sup.16 (10E16)
ohm-cm.
[0114] Inert gas-assisted cold ablation laser machining can meet
the requirements for machining of the next generation of
semiconductor wafer handling equipment.
[0115] Among the benefits, both direct and indirect, are: [0116]
Because laser machining causes less surface modification and little
to no sub-surface damage, and has greater dimensional control, pin
pattern machining can be optimized to greatly benefit downstream
processes. [0117] Not limited to electrically conductive materials.
[0118] Greater flexibility of feature design can be utilized
(feature profile and footprint, surface texturing). [0119] Easily
scalable to 450 mm diameters and beyond. [0120] Large machining
centers are available, enabling many parts to be machined at
once.
[0121] What is illustrated next are several examples relating to
the laser texturing aspect of the invention.
Example 7: Laser Texturing a Wafer Handling Device
[0122] A Si/SiC wafer handling device with surface pins (also known
as "mesas" or "plateaus") to support a semiconductor wafer is
provided, and is similar to those shown in FIGS. 13A and 13B. The
pins are at a very uniform elevation, and greatly reduce the
contact area between device and wafer. Nevertheless, the pin tops
are very smooth, giving rise to the "optical sticking"
phenomenon.
[0123] Laser texturing was employed to roughen (provide a pseudo
roughness" to) the pin tops. Specifically, a thermal laser was used
to machine or etch a cross-hatch pattern into at least one pin top
of the wafer handling device.
[0124] The texturing (cross-hatching) can be conducted with a wide
range of laser parameters. For the present example, the Si/SiC was
textured with the following laser parameters:
[0125] 1.064 um Nd:YAG (thermal laser)
[0126] 100 W max average power
[0127] 50-300 um pulse rate
[0128] Direct beam
[0129] 300 mm.times.300 mm stage
[0130] 1 urn repeat stage precision
[0131] The results of this laser texturing are illustrated in the
two photomicrographs of FIGS. 10A and 10B. The photos are of the
same area on a pin top of the Si/SiC wafer handling device, just at
different magnifications. A right-angled cross-hatch pattern is
seen. The pattern corresponds to channels burned into the Si/SiC
material, with each channel being about 3 microns deep and about 10
microns wide. The pitch (periodicity) of the channels is about 80
to 90 microns.
Example 8
[0132] This Example demonstrates laser texturing a wafer handling
device using a cold ablation laser modified with an inert cover
gas. The material being laser machined was the same as in Example
1, namely, a silicon/silicon carbide composite material made by
reactive infiltration.
[0133] FIGS. 11A-11C are photomicrographs of an untextured wafer
support pin, a wafer support pin textured with a cross-hatch
pattern with 80 micron spacing, and a wafer support pin textured
with a cross-hatch pattern with 50 micron spacing, respectively.
The cross-hatch patterns cut into the Si/SiC ceramic with laser
machined using a 1064 nm cold ablation laser at 15W power with
Argon cover gas.
[0134] The cross-hatched pattern reduces surface contact with the
semiconductor wafer for reduced friction, reduced backside
contamination and improved flatness. The use of laser ablation to
form the cross-hatch pattern provides smaller feature sizes than
conventional machining, enhanced tolerance capability compared to
thermal laser cutting, and reduced thermal and mechanical damage
induced into the cut surface.
[0135] Reference herein to "one embodiment" or "an embodiment"
means that a particular feature, structure, or characteristic
described in connection with the embodiment can be included in at
least one embodiment of the invention. The appearances of the
phrase "in one embodiment" in various places in the specification
are not necessarily all referring to the same embodiment, nor are
separate or alternative embodiments necessarily mutually exclusive
of other embodiments. The same applies to the term
"implementation".
[0136] It should be understood that the steps of the exemplary
methods set forth herein are not necessarily required to be
performed in the order described, and the order of the steps of
such methods should be understood to be merely exemplary. Likewise,
additional steps might be included in such methods, and certain
steps might be omitted or combined, in methods consistent with
various embodiments of the present invention.
[0137] As used in this application, the word "exemplary" is used
herein to mean serving as an example, instance, or illustration.
Any aspect or design described herein as "exemplary" is not
necessarily to be construed as preferred or advantageous over other
aspects or designs. Rather, use of the word exemplary is intended
to present concepts in a concrete fashion.
FIELD OF USE/INDUSTRIAL APPLICABILITY
[0138] The Applicant's laser machining technique is versatile and
allows a wider variety (vs. EDM) of materials that can be
processed. Features can be machined regardless of ceramic particle
size in the composite. Many different compositions can be machined
(e.g., composite materials containing multiple phases).
[0139] The techniques, apparatus and articles of the present
invention should find utility in fabricating articles for the
semiconductor fabrication industry, and particularly in fabricating
the articles or components involved in handling semiconductor
wafers. Such articles or components include:
[0140] Vacuum Wafer Chucks
[0141] Vacuum Wafer Tables
[0142] Electrostatic Chucks
[0143] Wafer Arms
[0144] End Effectors
[0145] Other applications where heat-free or oxidation-free ceramic
material removal is desired include: [0146] Machining of next
generation materials (i.e. Diamond based) [0147] Anti-reflective
texturing [0148] Low friction texturing [0149] Chemical-mechanical
planarization (CMP) conditioner pad fabrication
[0150] An artisan or ordinary skill will appreciate that various
modifications may be made to the invention herein described without
departing from the scope or spirit of the invention as defined in
the appended claims.
* * * * *