U.S. patent application number 10/724791 was filed with the patent office on 2005-06-02 for removal of mems sacrificial layers using supercritical fluid/chemical formulations.
Invention is credited to Baum, Thomas H., Ghenciu, Eliodor G., Korzenski, Michael B..
Application Number | 20050118832 10/724791 |
Document ID | / |
Family ID | 34620140 |
Filed Date | 2005-06-02 |
United States Patent
Application |
20050118832 |
Kind Code |
A1 |
Korzenski, Michael B. ; et
al. |
June 2, 2005 |
Removal of MEMS sacrificial layers using supercritical
fluid/chemical formulations
Abstract
A method and composition for removing silicon-containing
sacrificial layers from Micro Electro Mechanical System (MEMS)
substrates having such sacrificial layers is described. The etching
compositions include a supercritical fluid, an etchant species, a
co-solvent, and optionally a surfactant. Such etching compositions
overcome the intrinsic deficiency of SCFs as cleaning reagents,
viz., the non-polar character of SCFs and their associated
inability to solubilize polar species that must be removed from the
semiconductor substrate. The resultant etched MEMS substrates
experience lower incidents of stiction relative to MEMS substrates
etched using conventional wet etching techniques.
Inventors: |
Korzenski, Michael B.;
(Danbury, CT) ; Baum, Thomas H.; (New Fairfied,
CT) ; Ghenciu, Eliodor G.; (King of Prussia,
PA) |
Correspondence
Address: |
ATMI, INC.
7 COMMERCE DRIVE
DANBURY
CT
06810
US
|
Family ID: |
34620140 |
Appl. No.: |
10/724791 |
Filed: |
December 1, 2003 |
Current U.S.
Class: |
438/745 ;
257/E21.228 |
Current CPC
Class: |
B81C 2201/117 20130101;
B81C 1/00936 20130101; B08B 7/0021 20130101; H01L 21/02063
20130101; B81C 2201/0108 20130101 |
Class at
Publication: |
438/745 |
International
Class: |
H01L 021/302; H01L
021/461 |
Claims
What is claimed is:
1. A sacrificial silicon-containing layer etching composition,
comprising a supercritical fluid (SCF), at least one co-solvent, at
least one etchant species, and optionally at least one
surfactant.
2. The composition of claim 1, wherein the SCF is selected from the
group consisting of carbon dioxide, oxygen, argon, krypton, xenon,
and ammonia.
3. The composition of claim 1, wherein the SCF is carbon
dioxide.
4. The composition of claim 1, wherein the co-solvent comprises at
least one C.sub.1-C.sub.6 alcohol.
5. The composition of claim 1, wherein the co-solvent comprises
methanol.
6. The composition of claim 1, wherein the co-solvent comprises
isopropanol.
7. The composition of claim 1, wherein the sacrificial
silicon-containing layer comprises silicon oxide.
8. The composition of claim 7, wherein the etchant species
comprises at least one bifluoride compound selected from the group
consisting of ammonium bifluoride and tetraalkylammonium bifluoride
((R).sub.4NHF.sub.2), wherein R is a C.sub.1-C.sub.4 alkyl
group.
9. The composition of claim 7, wherein the etchant species
comprises ammonium bifluoride.
10. The composition of claim 7, wherein the surfactant comprises at
least one nonionic surfactant.
11. The composition of claim 10, wherein the surfactant is selected
from the group consisting of fluoroalkyl surfactants, polyethylene
glycols, polypropylene glycols, polyethylene ethers, polypropylene
glycol ethers, carboxylic acid salts, dodecylbenzenesulfonic acid,
dodecylbenzenesulfonic salts, polyacrylate polymers, dinonylphenyl
polyoxyethylene, silicone polymers, modified silicone polymers,
acetylenic diols, modified acetylenic diols, alkylammonium salts,
modified alkylammonium salts, and combinations comprising at least
one of the foregoing.
12. The composition of claim 10, wherein the surfactant comprises a
modified acetylenic diol.
13. The composition of claim 7, wherein the etching composition
comprises about 75.0 wt % to about 99.5 wt % SCF, about 0.3 wt % to
about 22.5 wt % co-solvent, about 0.01 wt % to about 5.0 wt %
etchant species, and about 0.01 wt % to about 5.0 wt % surfactant,
based on the total weight of the composition.
14. The composition of claim 1, wherein the sacrificial
silicon-containing layer consists essentially of silicon.
15. The composition of claim 14, wherein the etchant species is
XeF.sub.2.
16. The composition of claim 14, wherein the etching composition
comprises about 75.0 wt % to about 99.5 wt % SCF, about 0.3 wt % to
about 22.5 wt % co-solvent, about 0.01 wt % to about 5.0 wt %
etchant species, based on the total weight of the composition.
17. A method of removing silicon-containing substances from a
substrate having same thereon, said method comprising contacting
the substrate with a SCF-based composition comprising a SCF, at
least one co-solvent, at least one etchant species, and optionally
at least one surfactant, for sufficient time and under sufficient
contacting conditions to remove the silicon-containing substances
from the substrate.
18. The method of claim 17, wherein the SCF is selected from the
group consisting of carbon dioxide, oxygen, argon, krypton, xenon,
and ammonia.
19. The method of claim 17, wherein the SCF is carbon dioxide.
20. The method of claim 17, wherein the contacting conditions
comprise pressures in a range of from about 1400 to about 4400
psi.
21. The method of claim 17, wherein said contacting time is in a
range of from about 30 seconds to about 30 minutes.
22. The method of claim 17, wherein the co-solvent comprises at
least one C.sub.1-C.sub.6 alcohol.
23. The method of claim 17, wherein the co-solvent comprises
methanol.
24. The method of claim 17, wherein the co-solvent comprises
isopropanol (IPA).
25. The method of claim 17, wherein the silicon-containing
substance comprises a sacrificial silicon oxide layer.
26. The method of claim 25, wherein the etchant species comprises
at least one bifluoride compound selected from the group consisting
of ammonium bifluoride and tetraalkylammonium, bifluoride
((R).sub.4NHF.sub.2), wherein R is a C.sub.1-C.sub.4 alkyl
group.
27. The method of claim 25, wherein the etchant species comprises
ammonium bifluoride.
28. The method of claim 25, wherein the surfactant comprises at
least one nonionic surfactant.
29. The method of claim 28, wherein the surfactant is selected from
the group consisting of fluoroalkyl surfactants, polyethylene
glycols, polypropylene glycols, polyethylene ethers, polypropylene
glycol ethers, carboxylic acid salts, dodecylbenzenesulfonic acid,
dodecylbenzenesulfonic salts, polyacrylate polymers, dinonylphenyl
polyoxyethylene, silicone polymers, modified silicone polymers,
acetylenic diols, modified acetylenic diols, alkylammonium salts,
modified alkylammonium salts, and combinations comprising at least
one of the foregoing.
30. The method of claim 25, wherein the etching composition
comprises about 75.0 wt % to about 99.5 wt % SCF, about 0.3 wt % to
about 22.5 wt % co-solvent, about 0.01 wt % to about 5.0 wt %
etchant species, and about 0.01 wt % to about 5.0 wt % surfactant,
based on the total weight of the composition.
31. The method of claim 17, wherein the silicon-containing
substance is selected from the group consisting of silicon,
post-ash residue and post-etch residue.
32. The method of claim 31, wherein the etchant species is
XeF.sub.2.
33. The method of claim 31, further comprising dehydrating the
substrate prior to contacting the substrate with the SCF-based
etching composition.
34. The method of claim 31, wherein the etching composition
comprises about 75.0 wt % to about 99.5 wt % SCF, about 0.3 wt % to
about 22.5 wt % co-solvent, about 0.01 wt % to about 5.0 wt %
etchant species, based on the total weight of the composition.
35. The method of claim 17, wherein the contacting step comprises a
etching cycle including (i) dynamic flow contacting of the etching
composition with the silicon-containing substance, and (ii) static
soaking contacting of the etching composition with the
silicon-containing substance.
36. The method of claim 35, wherein said etching cycle comprises
alternatingly and repetitively carrying out dynamic flow contacting
(i) and static soaking contacting (ii) of the silicon-containing
substance.
37. The method of claim 17, further comprising the step of washing
the substrate, at a region at which the silicon-containing
substance has been removed, with a SCF/methanol/deionized water
wash solution in a first washing step, and with a SCF in a second
washing step, to remove residual precipitated chemical additives in
said first washing step, and to remove residual precipitated
chemical additives and/or residual alcohol in said second washing
step.
38. The method of claim 37, wherein the SCF is SCCO.sub.2.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to supercritical fluid-based
compositions useful in semiconductor manufacturing for the removal
of sacrificial layers, e.g., silicon or silicon oxide, from Micro
Electro Mechanical System (MEMS) substrates having such sacrificial
layers. The compositions also have utility for removing post-ash
and post-etch residue.
DESCRIPTION OF THE RELATED ART
[0002] Micro Electro Mechanical Systems (MEMS) are devices that
integrate mechanical and electrical components on a single silicon
wafer. The electrical and mechanical components are fabricated
using traditional integrated circuit (IC) techniques and
"micromachining" processes, respectively. Micromachining is used to
produce a number of mechanical devices on the wafer that are able
to sense and control the environment, including cantilever beams,
hinges, accelerometers, microsensors, microactuators and
micromirrors.
[0003] The mechanical components on a MEMS wafer are created by
depositing sacrificial and structural layers onto a substrate
followed by selective etching of the sacrificial layer relative to
the structural layer, leaving behind a suspended or freestanding
micromechanical structure, such as a beam or a lever. A major
problem with fabricating MEMS structures is that as aqueous based
etching of the sacrificial layer proceeds, stiction may occur,
wherein the surface adhesion forces are higher than the mechanical
restoring force of the microstructure. In effect, the
microstructure bends down toward the substrate and sticks to it,
generally permanently. Proposed causes of stiction include; van der
Waals forces, hydrogen bridging and/or electrostatic attractions
between the microstructure and the substrate, surface tension
forces generated from diminishing liquid menisci trapped in the
etched space, and etch by-products precipitating out of solution
during drying steps.
[0004] Several methods of minimizing stiction have been proposed,
including wet etching with HF, increasing surface roughness to
minimize the surface tension energy, and eliminating water by
drying the structures with a liquid that has no or little surface
tension, e.g., isopropanol (IPA). Proposed alternative water-free
etching compositions include anhydrous HF gas, which does not leave
residues. However, etching with neat anhydrous HF can require up to
ten hours to form complex microstructures and as such, the presence
of some water is necessary to initiate the etch reaction thereby
eliminating the advantages of using a water-free etchant.
[0005] Alternatively, supercritical fluids (SCF) can be used to
etch MEMS devices. Because of low viscosity and near zero surface
tension, SCFs avoid many of the problems associated with typical
wet processes. For example, because SCFs exhibit a gas-like
density, surface tension forces are low and thus the microstructure
does not stick to the substrate. Because of high diffusion rates,
SCFs can generally penetrate a solid sample faster than liquid
solvents. Further, SCFs can rapidly transport dissolved solutes
because of their low viscosity. However, SCFs are highly non-polar
and as such, many contaminant species are not adequately
solubilized therein.
[0006] There is therefore a continuing need in the field for
improved etching compositions, since the etching of sacrificial
layers from semiconductor substrates is critical to ensure proper
production and operation of MEMS devices and emerging integrated
circuits.
SUMMARY OF THE INVENTION
[0007] The present invention relates to supercritical fluid-based
compositions useful in semiconductor manufacturing for the etching
of sacrificial silicon-containing layers from semiconductor
substrates, and methods of using such compositions for removal of
same.
[0008] Further, the present invention relates to supercritical
fluid-based compositions useful in semiconductor manufacturing for
the removal of post-ash and post-etch residue from semiconductor
surfaces, and methods of using such compositions for removal of
same.
[0009] In one aspect, the invention relates to a sacrificial
silicon-containing layer etching composition, comprising a
supercritical fluid, at least one co-solvent, at least one etchant
species, and optionally at least one surfactant.
[0010] In another aspect, the invention relates to a method of
removing silicon-containing substances from a substrate having same
thereon, said method comprising contacting the substrate with a
SCF-based composition comprising a SCF, at least one co-solvent, at
least one etchant species, and optionally at least one surfactant,
for sufficient time and under sufficient contacting conditions to
remove the silicon-containing substances from the substrate.
[0011] Other aspects, features and embodiments of the invention
will be more fully apparent from the ensuing disclosure and
appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1 is a control sample before SCF-based etching
composition processing including a silicon substrate, a 100 nm
thick silicon oxide film on the substrate and a 100 nm thick
polysilicon film on the oxide.
[0013] FIG. 2 is the control sample in FIG. 1 after the sacrificial
silicon oxide layer was etched with a SCF-based etching composition
of the present invention, illustrating a free standing
microstructure.
[0014] FIG. 3 is a control sample before SCF-based etching
composition processing including a silicon substrate, a 100 nm
thick silicon oxide film on the substrate and a 100 nm thick
polysilicon film on the oxide.
[0015] FIG. 4 is the control sample in FIG. 3 after the sacrificial
silicon oxide layer was etched with a SCF-based etching composition
of the present invention, illustrating a free standing
microstructure.
[0016] FIG. 5 is a sample etched with a SCF-based etching
composition of the present invention, illustrating a free standing
microstructure.
DETAILED DESCRIPTION OF THE INVENTION, AND PREFERRED EMBODIMENTS
THEREOF
[0017] The present invention is based on the discovery of
supercritical fluid (SCF)-based etching compositions that are
highly efficacious for the etching of sacrificial
silicon-containing layers from semiconductor substrates. The
compositions and methods of the invention are effective for etching
sacrificial layers, including silicon and silicon oxide layers, and
related post-etch residue removal from patterned wafers.
[0018] Because of its readily manufactured character and its lack
of toxicity and negligible environmental effects, supercritical
carbon dioxide (SCCO.sub.2) is a preferred SCF in the broad
practice of the present invention, although the invention may be
practiced with any suitable SCF species, with the choice of a
particular SCF depending on the specific application involved.
Other preferred SCF species useful in the practice of the invention
include oxygen, argon, krypton, xenon, and ammonia. Specific
reference to SCCO.sub.2 hereinafter in the broad description of the
invention is meant to provide an illustrative example of the
present invention and is not meant to limit the same in any
way.
[0019] SCCO.sub.2 might at first glance be regarded as an
attractive reagent for removal of oxides and residue contaminants,
since SCCO.sub.2 has the characteristics of both a liquid and a
gas. Like a gas, it diffuses rapidly, has low viscosity, near-zero
surface tension, and penetrates easily into deep trenches and vias.
Like a liquid, it has bulk flow capability as a "wash" medium.
[0020] However, despite these ostensible advantages, SCCO.sub.2 is
non-polar. Accordingly, it will not solubilize many polar species,
including ionic etchant species comprising fluoride or inorganic
salts and polar organic compounds that are present in many
post-etch and post-ash residues. The non-polar character of
SCCO.sub.2 thus poses an impediment to its use for etching
sacrificial layers and the subsequent cleaning of wafer surfaces of
contaminant residues.
[0021] The present invention, however, is based on the discovery
that disadvantages associated with the non-polarity of SCCO.sub.2
and other SCFs can be overcome by appropriate formulation of
SCF-based etching compositions with additives as hereinafter more
fully described, and the accompanying discovery that etching a
sacrificial silicon-containing layer with a SCF-based medium is
highly effective and achieves damage-free, residue-free etching of
the substrate having such sacrificial silicon-containing layer
thereon.
[0022] In one aspect, the invention relates to SCF-based etching
compositions useful in removing sacrificial silicon-containing
layers from a semiconductor substrate. The formulation of the
present invention comprises a SCF, at least one co-solvent, at
least one etchant, and optionally at least one surfactant, present
in the following ranges, based on the total weight of the
composition:
1 component of % by weight SCF about 75.0% to about 99.5%
co-solvent about 0.3% to about 22.5% etchant about 0.01% to about
5.0% surfactant about 0.01% to about 5.0%
[0023] In the broad practice of the invention, the SCF-based
etching formulations may comprise, consist of, or consist
essentially of a SCF, at least one co-solvent, at least one etchant
and optionally at least one surfactant.
[0024] The inclusion of the co-solvent with the SCF serves to
increase the solubility of the composition for sacrificial
silicon-containing species. In general, the specific proportions
and amounts of SCF, co-solvent, etchant, and optionally surfactant,
in relation to each other may be suitably varied to provide the
desired etching action of the SCF-based etching composition for the
silicon oxide species and/or processing equipment, as readily
determinable within the skill of the art without undue effort.
[0025] The co-solvent used in the SCF-based etching composition is
preferably an alcohol. In one embodiment of the invention, such
alcohol includes a straight-chain or branched C.sub.1-C.sub.6
alcohol (i.e., methanol, ethanol, isopropanol, etc.), or a mixture
of two or more of such alcohol species. In a preferred embodiment,
the alcohol is methanol or isopropanol (IPA).
[0026] With regards to conventional silicon oxide etching
solutions, the etchant of choice is HF, which dissociates in water
to form the etchant species F.sup.-, H.sub.2F.sup.- and
H.sub.2F.sub.2. However, in a CO.sub.2 rich environment, the
ionization of HF to form etchant species does not readily occur
because the water reacts with the CO.sub.2 (to form carbonic acid
(H.sub.2CO.sub.3)) or is removed by the alcohol co-solvent.
[0027] As such, the silicon oxide etchant used in the SCF-based
etching composition of the present invention includes a pre-ionized
fluoride source, such as a bifluoride species, including ammonium
difluoride and tetraalkylammonium difluorides, such as those
produced by the following reaction:
(R).sub.4NOH+2HF.fwdarw.(R).sub.4NHF.sub.2+H.sub.2O
[0028] where R is methyl, ethyl, butyl, phenyl or fluorinated
C.sub.1-C.sub.4 alkyl groups.
[0029] With regards to conventional silicon etching solutions,
XeF.sub.2 is particularly well suited to MEMS applications.
XeF.sub.2 etchants exhibit nearly infinite selectivity of silicon
to photoresist, silicon oxides, silicon nitrides and aluminum.
Being a vapor phase etchant, XeF.sub.2 avoids many of the problems
typically associated with wet processes. For example, XeF.sub.2
surface tension forces are negligible and thus stiction between the
microstructure and the substrate is less likely. In addition,
etching rates using XeF.sub.2 are much faster.
[0030] It has been proposed that XeF.sub.2 etching of silicon
involves the physisorption of XeF.sub.2 onto the silicon surface.
Because the bond energies of both the F atoms to the Xe atoms and
the Si atoms to other Si atoms are sufficiently weak, and the
attraction forces between Si and F are relatively strong, F will
dissociate from Xe and bond to Si to form various silicon fluoride
products, as illustrated in the following reactions:
XeF.sub.2(g)+Si(s).fwdarw.Xe(g)+SiF.sub.2(s)
XeF.sub.2(g)+SiF.sub.2(s).fwdarw.Xe(g)+SiF.sub.4(s)
[0031] An etching reaction occurs when volatile SiF.sub.4 is
formed, which leaves the surface spontaneously, thus removing
sacrificial silicon material.
[0032] Notably, the XeF.sub.2 etch rate is highly dependent on the
dryness of the silicon surface. If water is present on the surface
of the silicon, a thin silicon fluoride polymer layer forms.
Accordingly, the broad practice of the invention includes wafer
surface drying prior to exposure to XeF.sub.2. SCCO.sub.2 provides
an efficient and environmentally safe way to dehydrate the wafer
surface, thus eliminating the formation of the unwanted silicon
fluoride polymer layer. Further, pre-drying the silicon surface
with SCCO.sub.2 is also a necessary safety measure since most
XeF.sub.2 contains small amounts of XeF.sub.4, which upon reaction
with water forms the contact explosive XeO.sub.3.
[0033] Species such as XeF.sub.2 are largely insoluble in the
non-polar SCF solvents. Accordingly, co-solvents are added to the
composition to increase the solubility of XeF.sub.2 in the silicon
SCF-based etching composition of the present invention.
[0034] Surfactants are optionally added when the sacrificial
silicon-containing layer includes silicon oxide. The surfactant
used in the SCF-based etching composition may include nonionic
surfactants, such as fluoroalkyl surfactants, polyethylene glycols,
polypropylene glycols, polyethylene or polypropylene glycol ethers,
carboxylic acid salts, dodecylbenzenesulfonic acid or salts
thereof, polyacrylate polymers, dinonylphenyl polyoxyethylene,
silicone or modified silicone polymers, acetylenic diols or
modified acetylenic diols, and alkylammonium or modified
alkylammonium salts, as well as combinations comprising at least
one of the foregoing surfactants. In a preferred embodiment, the
surfactant is a modified acetylenic diol.
[0035] In one embodiment, the silicon dioxide etching composition
of the invention includes SCCO.sub.2, methanol, ammonium
bifluoride, and a modified acetylenic diol.
[0036] In another embodiment, the silicon etching composition of
the invention includes SCCO.sub.2, methanol and XeF.sub.2.
[0037] In another aspect, the invention relates to methods of
removal of sacrificial silicon-containing layers including, but not
limited to, silicon, silicon oxide and post-ash and post-etch
residues, from a semiconductor substrate using the appropriate
SCF-based etching composition.
[0038] The sacrificial silicon-containing layers and/or post-ash
and post-etch residues may be removed using a SCF-based etching
composition including a SCF, at least one co-solvent, at least one
etchant, and optionally at least one surfactant, as described
herein. Another possible application is removal of SiO.sub.2
particles via reaction or dissolution.
[0039] At present, the favored technique to remove developed
photoresist is plasma ashing. Plasma ashing involves exposing the
photoresist-covered wafer to oxygen plasma in order to oxidatively
decompose the unexposed photoresist film from the substrate
surface. However, plasma etching usually results in the formation
of plasma-ash and plasma-etch residue, and this residue must
subsequently be removed.
[0040] The removal of post-ash and post-etch residue is a well
known problem in light of the continuing and rapid decrease in
critical dimensions of microelectronic device structures, since any
residue remaining on the substrate can render the final device
deficient or even useless for its intended purpose.
[0041] Conventional post-ash and post-etch residue cleaning by wet
chemical treatment has not proven wholly satisfactory in effecting
complete removal of residues from the substrate, especially from
trenches, vias and microstructures in low k dielectrics. Further,
these conventional cleaning approaches are time-consuming, costly,
require substantial amounts of chemical reagents for the cleaning
operation and produce substantial quantities of chemical waste.
[0042] The SCF-based compositions of the present invention overcome
the disadvantages of the prior art post-ash and post-etch residue
removal treatments for Si and SiO.sub.2 based residues.
[0043] The appropriate SCF-based etching composition can be
employed to contact a substrate having a sacrificial layer, e.g.,
silicon oxide or silicon, and/or post-ash and post-etch residue, at
a pressure in a range of from about 1400 to about 4400 psi for
sufficient time to effect the desired etching of the sacrificial
layer and/or residue, e.g., for a contacting time in a range of
from about 30 seconds to about 30 minutes and a temperature of from
about 40 to about 70.degree. C., although greater or lesser
contacting durations and temperatures may be advantageously
employed in the broad practice of the present invention, where
warranted.
[0044] The removal process in a particularly preferred embodiment
includes sequential processing steps including dynamic flow of the
SCF-based etching composition over the substrate having the
sacrificial layer and/or residue, followed by a static soak of the
substrate in the SCF-based etching composition, with the respective
dynamic flow and static soak steps being carried out alternatingly
and repetitively, in a cycle of such alternating steps.
[0045] A "dynamic" contacting mode involves continuous flow of the
cleaning composition over the wafer surface, to maximize the mass
transfer gradient and effect complete removal of the sacrificial
layer and/or residue from the substrate. A "static soak" contacting
mode involves contacting the wafer surface with a static volume of
the etching composition, and maintaining contact therewith for a
continued (soaking) period of time.
[0046] For example, the dynamic flow/static soak steps may be
carried out for four successive cycles in the aforementioned
illustrative embodiment, as including a sequence of 30 sec-10 min
dynamic flow, 30 sec-5 min high pressure static soak, e.g., about
3000 psi to about 4400 psi, 30 sec-10 min dynamic flow, and 30
sec-10 min low pressure static soak, e.g., about 1400 psi to about
2800 psi.
[0047] With regards to the silicon layers to be etched, the wafer
surface should be dehydrated prior to the etching process. SCFs can
be used as drying media for patterned wafers in drying compositions
that include one or more water-reactive agents that chemically
react with water on the patterned wafer to form reaction product
species that are more soluble in the SCF than water.
[0048] As an illustrative example, hexafluoroacetone (HFA) is
usefully employed as a water-reactive agent in SCCO.sub.2 to
provide a highly effective SCF composition for drying of patterned
wafers. In such composition, HFA reacts instantly with water and
quantitatively forms a soluble and volatile diol as depicted in the
following reaction:
H.sub.2O+CF.sub.3COCF.sub.3.fwdarw.CH.sub.3C(OH).sub.2CF.sub.3
[0049] The product diol, CH.sub.3C(OH).sub.2CF.sub.3, is highly
soluble in SCCO.sub.2 and is readily dissolved by the SCF, thereby
effectively removing water from the patterned wafer substrate with
which the SCF composition, containing SCCO.sub.2 and HFA, is
contacted.
[0050] More generally, the water-reactive agent in the SCF-based
wafer drying composition can be of any suitable type, including for
example, other halogenated aldehydes and ketones; halogenated
diketones, e.g., 1,1,1,5,5,5-hexafluoro-2,4-pentanedione,
alternatively denoted as (hfac)H; halogenated esters; carboxylic
anhydrides, e.g., (CH.sub.3CO).sub.2O; siloxanes, halogenated
silanes; and any other compounds and materials that easily react
with water and form derivatives soluble in SCCO.sub.2 or other SCF
species.
[0051] Generally, the water-reactive agent can be formulated in the
SCF-based wafer drying composition at any suitable concentration
that is effective for water removal from the patterned wafer
substrate. In various embodiments, depending on the particular SCF
species employed, the concentration of the water-reactive agent can
be a concentration in a range of from about 0.01 to about 10.0% by
weight, based on the total weight of the supercritical fluid and
the water-reactive agent, with concentrations of from about 0.1 to
about 7.5% by weight, on the same total weight basis being more
preferred, and from about 0.1 to about 5.0% by weight, on the same
total weight basis being most preferred.
[0052] The contacting of the patterned substrate with the drying
composition is carried out for a suitable period of time, which in
a specific embodiment can for example be on the order of from about
20 to about 60 seconds, although other (longer or shorter) periods
of contacting may be usefully employed depending on the nature and
amount of the water to be removed from the patterned substrate, and
the process conditions employed for drying.
[0053] Following drying of the patterned substrate, the contacting
vessel in which the SCF-based wafer drying composition is contacted
with the patterned substrate can be rapidly decompressed to
separate the SCF composition from the patterned substrate and
exhaust the regasified SCF from the contacting vessel, so that the
non-supercritical component(s), such as the soluble water reaction
product(s), can be entrained in the regasified SCF and likewise be
removed from the drying locus. Thereafter, the contacting vessel
can be compressed and the SCF-based etching composition may be
introduced to the vessel to remove the sacrificial layer and/or
residue.
[0054] Following the contacting of the etching composition with the
substrate bearing the sacrificial layer and/or residue, the
substrate thereafter preferably is washed with copious amounts of
SCF/methanol/deionized water solution in a first washing step, to
remove any residual precipitated chemical additives from the
substrate region in which etching and/or residue removal has been
effected, and finally with copious amounts of pure SCF, in a second
washing step, to remove any residual methanol co-solvent and/or
precipitated chemical additives from the substrate region.
Preferably, the SCF used for washing is SCCO.sub.2.
[0055] The SCF-based etching compositions of the present invention
are readily formulated by simple mixing of ingredients, e.g., in a
mixing vessel under gentle agitation.
[0056] Once formulated, such etching compositions are applied to
the substrate for contacting with the sacrificial layer and/or
residue thereon, at suitable elevated pressures, e.g., in a
pressurized contacting chamber to which the etching composition is
supplied at suitable volumetric rate and amount to effect the
desired contacting operation for removal of the sacrificial layer
and/or residue.
[0057] It will be appreciated that specific contacting conditions
for the etching compositions of the invention are readily
determinable within the skill of the art, based on the disclosure
herein, and that the specific proportions of ingredients and
concentrations of ingredients in the etching compositions of the
invention may be widely varied while achieving desired removal of
the sacrificial layer and/or residue from the substrate.
[0058] The features and advantages of the invention are more fully
shown by the illustrative example discussed below.
[0059] The sample wafers examined in this study included a
substrate, a 100 nm thick silicon oxide film on the substrate and a
100 nm polysilicon film on top of the oxide layer. The samples were
processed to etch the sacrificial silicon oxide layer using the
SCF-based etching composition of the following formulation:
2 Component Weight Percent ammonium bifluoride (32.3 wt %) 1.0%
surfynol-104 0.05% methanol 4.0% SCCO.sub.2 94.95%
[0060] Alternatively, the sample wafers may include a substrate, a
380 nm thick silicon film on the substrate, a 30 nm silicon oxide
film on the silicon film, and a 300 nm silicon nitride film on top
of the oxide layer. The samples may be processed to etch the
sacrificial silicon oxide layer using the SCF-based etching
composition of the following formulation:
3 Component Weight Percent ammonium bifluoride (32.3 wt %) 1.0%
surfynol-104 0.05% methanol 4.0% SCCO.sub.2 94.95%
[0061] The temperature was maintained at 50.degree. C. throughout
the cleaning/rinsing procedure. The optimal process conditions are
dynamic flow of the SCF-based etching composition for 45 sec at
4000 psi followed by a 1 min SCCO.sub.2 rinse. The samples were
then thoroughly rinsed with copious amounts of
SCCO.sub.2/methanol/deionized water and pure SCCO.sub.2 in order to
remove any residual co-solvent and/or precipitated chemical
additives.
[0062] The results are shown in FIGS. 1-5, as described
hereinbelow.
[0063] FIGS. 1 and 3 are optical microscope photographs of control
wafers prior to etching, showing unremoved sacrificial silicon
oxide layers.
[0064] FIGS. 2 and 4 show the optical image of the FIGS. 1 and 3
wafers after sacrificial silicon oxide layer removal, respectively,
using the composition and method described herein. Following
removal of the sacrificial silicon oxide layer, the free standing,
stiction-free microstructures can be clearly seen.
[0065] FIG. 5 is an optical image of a free-standing microstructure
produced using the composition and method of the present
invention.
[0066] The above-described photographs thus evidence the efficacy
of SCF-based etching compositions in accordance with the invention,
for removal of sacrificial layers from wafer substrates.
[0067] Accordingly, while the invention has been described herein
in reference to specific aspects, features and illustrative
embodiments of the invention, it will be appreciated that the
utility of the invention is not thus limited, but rather extends to
and encompasses numerous other aspects, features and embodiments.
Accordingly, the claims hereafter set forth are intended to be
correspondingly broadly construed, as including all such aspects,
features and embodiments, within their spirit and scope.
* * * * *