U.S. patent number RE38,760 [Application Number 08/903,077] was granted by the patent office on 2005-07-19 for controlled etching of oxides via gas phase reactions.
This patent grant is currently assigned to Penn State Research Foundation. Invention is credited to Robert W. Grant, Jerzy Ruzyllo, Kevin Torek.
United States Patent |
RE38,760 |
Grant , et al. |
July 19, 2005 |
Controlled etching of oxides via gas phase reactions
Abstract
Oxides are etched with a halide-containing species and a low
molecular weight organic molecule having a high vapor pressure at
standard conditions, where etching is performed at preset wafer
temperature in an enclosed chamber at a pressure such that all
species present in the chamber, including water, are in the gas
phase and condensation of species present on the etched surface is
controlled. Thus all species involved remain in the gas phase even
if trace water vapor appears in the process chamber. Preferably,
etching is performed in a cluster dry tool apparatus.
Inventors: |
Grant; Robert W. (Allentown,
PA), Ruzyllo; Jerzy (State College, PA), Torek; Kevin
(Boise, ID) |
Assignee: |
Penn State Research Foundation
(University Park, PA)
|
Family
ID: |
22821450 |
Appl.
No.: |
08/903,077 |
Filed: |
July 30, 1997 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
Reissue of: |
219961 |
Mar 30, 1994 |
05439553 |
Aug 8, 1995 |
|
|
Current U.S.
Class: |
438/723; 134/1.2;
134/1.3; 216/73; 216/79; 216/80; 438/906; 438/908 |
Current CPC
Class: |
H01L
21/31116 (20130101) |
Current International
Class: |
B08B
6/00 (20060101); H01L 21/302 (20060101); H01L
21/02 (20060101); H01L 021/302 (); B08B 006/00 ();
B44C 001/21 () |
Field of
Search: |
;438/723,906,908
;156/645.1,646.1,642.1 ;134/1.2,1.3 ;216/73,79,80 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Bersin, R.L., "The Dry Ox Process for Etching Silicon Dioxide",
Solid State Technology, Apr. 1977, pp. 78-80. .
Butterbaugh, J.W., Hiatt, C.F. and Gray, D.C.; "Gas-Phase Etching
of Silicon Oxide with Anhydrous HF and Isopropanol", Third Int'l
Symposium on Cleaning Technology in Semiconductor Development Mfg.,
Subcommittee of Electrochemical Society, Oct. 15-20, 1993. .
Miki, N., Kikuyama, H., Maeno, M., Murota, J., and Ohmi, T.;
"Selective Etching of Native Oxide By Dry Processing Using Ultra
Clean Anhydrous Hydrogen Fluoride", 1988 IEEE, 730-733. .
Jackman, R.B., Ebert, H. and Foord, J.S.; Reaction Mechanisms For
The Photon-Enhanced Etching Of Semiconductors: An Investigation Of
The UV-Stimulated Interaction of Chlorine With Si(100), Surface
Science 176 (1986) 183-192. .
Miki, N., Kikuyama, H., Kawanale, I. Miyashita M., and Ohmi T.;
"Gas-Phase Selective Etching Of Native Oxide", IEEE 37:1 (1190)
107-115. .
Philipossian, A.; "The Activity of HF/H.sub.2 O Treated Silicon
Surfaces in Ambient Air Before And After Gate Oxidation", J.
Electrochem. Soc., vol. 139, No. 10, Oct. 1992, 2956-2961. .
Ito, T., "Wafer Dry Cleaning With Photo-Excited Halogen Radicals",
Proceedings--Institute of Environmental Sciences, pp. 808-813,
1991. .
Ruzyllo, J., Torek, K., Daffron, C., Grant, R., and Novak, R.;
"Etching of thermal Oxides in Low Pressure Anhydrous HF/CH.sub.3 OH
Gas Mixture at Elevated Temperature", J. Electrochem. Soc., vol.
140, No. 4, Apr. 1993. .
Kao, D.B., Cairns, B.R., and Deal, B.E., "Vapor-Phase pre-Cleans
for Furnace-Grown and Rapid-Thermal Thin Oxides." in: Ruzyllo, J.,
Novak, R.E., Proceedings of the Second International Symposium on
Cleaning Technology in Semiconductor Device Manufactoring, the
Electrochemical Society Softbound Proceedings Series (Pennington,
NJ 1992), pp. 251-259. .
Izumi, A., Matsuka, T., Takeuchi, T. and Yamano, A., "A New
Cleaning Method By Using Anhyrous HF/CH.sub.3 OH Vapor System." in:
Ruzyllo, J., Novak, R.E., Proceedings of the Second International
Symposium on Cleaning Technology in Semiconductro Device
Manufacturing, the Electrochemical Society Softbound Proceedings
Series (Pennington, NJ 1992), pp. 260-267. .
Deal, B.E. and Helms, R., "Vapor Phase Wafer Cleaning Technology",
Handbook of Semiconductor Wafer Cleaning Technology, pp. 274-297.
.
Ruzyllo, J., "Overview of Dry Wafer Cleaning Processes", Chapter 5
in: Handbook of Silicon Wafer Cleaning Technology, pp. 274-297,
Noyes Publications, 1993. .
Abstract, Toke, 1987 Derwent Publications Ltd. .
Ruzyllo, J., "Dry Cleaning Processes", Intensive Course on Feb.
23-24, 1993 in Austin, Texas. .
Donovan, R., et al., "Semiconductor Wafer Cleaning Technology",
Intensive Course, Feb. 23-24, 1993..
|
Primary Examiner: Nguyen; Ha Tran
Attorney, Agent or Firm: Jaeger; Hugh D.
Claims
What is claimed is:
1. A method of etching oxides on a surface of a substrate, said
substrate being at a temperature .Iadd.from 30.degree. C. to
70.degree. C., .Iaddend.said method comprising etching with a gas
phase mixture of .[.a halide-containing species, an organic
material having a higher vapor pressure than water at standard
conditions of room temperature and pressure of 1 atmosphere and
water, in an enclosed chamber at a pressure such that.]. .Iadd.HF
and acetone in an enclosed chamber at a pressure of 500 Torr such
that a reaction product .Iaddend.water is substantially maintained
in the gas phase and condensation of species present on the etched
surface is minimized..[.
2. A method of etching as claimed in claim 1 where the etching is
performed in an enclosed chamber of a cluster dry tool
apparatus..]..[.
3. A method of etching as claimed in claim 1 where the organic
material having a higher vapor pressure than water at standard
conditions of room temperature and pressure of 1 atmosphere is
selected from the group consisting of alcohols, organic acids,
ketones and alkanes..]..[.
4. A method of etching as claimed in claim 3 where the organic
material having a higher vapor pressure than water at standard
conditions of room temperature and pressure of 1 atmosphere in
selected from the group consisting of methanol, isopropanol,
acetone, acetic acid and methane..]..[.
5. A method of etching as claimed in claim 1 where the
halide-containing species is selected from the group consisting of
HF, NF.sub.3, ClF.sub.3 and F.sub.2..]..[.
6. A method of etching as claimed in claim 1 where the
halide-containing species is HF, the organic material having a
higher vapor pressure than water at standard conditions of room
temperature and pressure of 1 atmosphere is methanol, the pressure
is 100 Torr and the preset temperature is from 20.degree. C. to
95.degree. C..]..[.
7. A method of etching as claimed in claim 1 where the
halide-containing species is HF, the organic material having a
higher vapor pressure than water at standard conditions of room
temperature and pressure of 1 atmosphere is methanol, the pressure
is 300 Torr and the temperature is from 80.degree. C. to 95.degree.
C..]..[.
8. A method of etching as claimed in claim 1 where the
halide-containing species is HF, the low molecular weight organic
molecule having a higher vapor pressure than water at standard
conditions of room temperature and pressure of 1 atmosphere is
methanol, the pressure is 500 Torr and the preset temperature is
from 95.degree. C. to 105.degree. C..]..[.
9. A method of etching as claimed in claim 1 where the etching is
performed in an enclosed chamber which is a reactor body of an
apparatus further comprising: a gas delivery system; a UV source;
an IR source; a rotating mechanism to rotate the wafer; and a
vacuum control system..]. .[.
10. A method of etching as claimed in claim 1 where the organic
material having a higher vapor pressure than water at standard
conditions of room temperature and pressure of 1 atmosphere is
anhydrous..]..[.
11. A method of etching as claimed in claim 1 where the
halide-containing species is anhydrous..]..[.
12. A method of etching oxides on a surface of a substrate, said
substrate being at a temperature of from 30.degree. C. to
70.degree. C., said method comprising etching with a gas phase
mixture of HF and acetone, in an enclosed chamber at a pressure of
500 Torr such that water is substantially maintained in the gas
phase and condensation of species present on the etched surface is
minimized..].
Description
BACKGROUND OF THE INVENTION
The present invention relates to a method of etching oxides on a
substrate to minimize or prevent deposition of contamination on the
substrate. In particular, the present invention relates to the
etching of silicon oxides.
In integrated circuit manufacturing, the etching of oxides is
typically carried out using a halide-containing water solution, for
instance a HF/water solution, or reactive ion etching (RIE). The
former method is not compatible with integrated processing of
integrated circuits while the latter damages the surface of the
substrate and contaminates the near-surface region of the
substrate. As a result, neither method permits integration of oxide
etching with other steps performed in a cluster processor.
The use of gaseous HF and water vapor, for example, is an
alternative to RIE for gas-phase oxide etching. In this process,
water vapor is needed to uniformly initiate etching. Gas phase
etching can produce the same level of hydrogen termination as the
traditional HF/water rinse process. The process is well developed
and is supported by commercial reactors. It works on the premise
that the HF-water vapor mixture delivered to the oxide condenses
there, and hence, the etching process is eventually taking place in
the liquid phase. One of the problems encountered with this etching
mode is that some of the reaction products may precipitate on the
surface where they are difficult to remove without an additional
deionize water rinse. Also, difficulties with precise control of
the condensed liquid film composition and its uniform coverage of
the oxide may, under certain conditions, result in inadequate etch
uniformity and run-to-run reproducibility. Moreover, since existing
HF vapor etch chemistry generally involves the addition of water
vapor, the process is difficult to integrate with reduced pressure
reactors and stainless steel components.
U.S. Pat. No. 5,022,961 to Izumi et al., incorporated herein by
reference, implements an approach in which etching is performed
using HF and alcohol where water added to the system is minimized.
Etching is performed at room temperature and atmospheric pressure
and liquid as well as vapor state etching are contemplated. Ionic
contaminant, however, remains on the surface after etching,
contained within the alcohol layer. After-treatment is again
required to remove the contamination.
Prior art etching techniques suggest that use of particular solvent
molecules for etching in certain circumstances. Izumi et al., for
instance, discusses etching with HF or other halide-containing
species only in combination with alcohol. Wet etching has
prevalently been disclosed with HF, water and acetic acid alone or
a mixture of organic acids. U.S. Pat. No. 4,746,397 to Maeda et al.
does disclose etching with the use of a fluorine-containing species
and an alcohol ketone or carboxylic acid. However, liquid or vapor
phase etching is contemplated under reduced pressure where the
addition of water is specified.
It is disclosed in U.S. Pat. No. 4,857,142 to Syverson and U.S.
Pat. No. 4,749,440 to Blackwood that etching may be accomplished
with a minimum of added water where the oxide itself contains
sufficient water. It is thus considered in the art that at least
some water is required for etching.
Drawbacks of prior practice are addressed by the present invention
in a method of etching oxides on a surface of a substrate
comprising etching at a preset wafer temperature with a gas phase
mixture of a halide-containing species and a low molecular weight
organic molecule having a high vapor pressure at standard
conditions, in an enclosed chamber at a pressure such that all
species present are substantially maintained in the gas phase and
condensation of species present on the etched surface is
controlled. Because trace water vapor may appear in the process
chamber, water vapor is taken into account as a species present in
the gas phase. Because water is less volatile than the constituent
reactants, the temperature and pressure parameters are chosen from
the area below the curve for water in the phase diagram. By
maintaining temperature and pressure parameters within this range,
all species present in the chamber, including water, are
substantially maintained in the gas phase. Low molecular weight
organic molecules having high vapor pressure at standard conditions
encompass low molecular weight alcohols, organic acids, ketones or
alkanes such as methanol, isopropanol, acetone, acetic acid and
methane. Preferably, etching is performed in a cluster dry tool
apparatus. The process achieves controlled, gas-phase etching of
oxides with etch rates up to two orders of magnitude lower than
typical rates obtained for vapor HF/water etching, without the
addition of water vapor to the input gases.
The present invention also addresses drawbacks of prior practice by
a method of etching oxides comprising etching with a
halide-containing species and water where the pressure and
temperature parameters are, as discussed above, selectively chosen
from parameters below the curve representing water in the phase
diagram.
It is an objective of the present invention to provide a method of
etching oxides in which the oxide etch rate is controlled.
It is an objective of the present invention to provide a method of
etching oxides in which condensation of species present on the
etched surfaces is controlled.
It is a further objective of the present invention to provide a
method which controls etch at a rate of up to two orders of
magnitude lower than typical rates obtained for vapor HF/water
etching.
SUMMARY OF THE INVENTION
The present invention provides a method of etching oxides on a
surface of a substrate comprising etching at a preset wafer
temperature with a gas phase mixture of a halide-containing species
and a low molecular weight organic molecule having a high vapor
pressure at standard conditions, in an enclosed chamber at a
pressure such that all species present are substantially maintained
in the gas phase and condensation of species present on the etched
surfaces is controlled. Because trace water vapor may appear in the
process chamber, water vapor is taken into account as a species
present in the gas phase. Because water is less volatile than the
constituent reactants, the temperature and pressure parameters are
chosen from the area below the curve for water in the phase
diagram. By maintaining temperature and pressure parameters within
this range, all species present in the chamber, including water,
are substantially maintained in the gas phase. Low molecular weight
organic molecules having high vapor pressure at standard conditions
encompass low molecular weight alcohols, organic acids, ketones or
alkanes such as methanol, isopropanol, acetone, acetic acid and
methane. Preferably, etching is performed in a cluster dry tool
apparatus. The process achieves controlled, gas-phase etching of
oxides with etch rates of up to two orders of magnitude lower than
typical rates obtained for vapor HF/water etching, without the
addition of water vapor to the input gases.
Alternatively, the present invention may be used with a
halide-containing species and water, without the addition of a low
molecule weight, high vapor pressure organic molecule where the
pressure and temperature parameters are again selectively chosen
from parameters below the curve representing water in the phase
diagram as discussed above.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a phase diagram representing vapor pressure of HF,
methanol and water as a function of temperature.
FIG. 2 is a graph of oxide etch rates in HF/methanol at three
different pressures as a function of wafer temperature.
FIG. 3 is a graph of oxide etch rate as a function of HF/methanol
flow ratio.
FIG. 4 is a graph of XPS spectra for HF/methanol treated substances
in accordance with the present invention.
FIG. 5 is a graph of oxide etched in HF/acetone as a function of
temperature.
FIG. 6 is a graph of oxide etched in HG/acetone as a function of HF
partial pressure.
DETAILED DESCRIPTION OF THE INVENTION
The gas-phase oxide etching method of the present invention
involves the use of an agent effective in stimulating etching with
gaseous HF as well as producing a hydrogen-transmitted surface.
Furthermore, the present invention involves the creation of
conditions under which the condensation of reactants and other
contaminants on the oxide surface is either prevented, or more
precisely controlled. The deposition of particulate contamination
on the substrate surface, which would be nonvolatalizable if
deposited, is thus prevented.
Etching is carried out at temperatures and pressures at which
condensation of reactants on the wafer surface can be effectively
controlled. To assure adequate process reproducibility and control
of the etch, temperature and pressure parameters which prevent
condensation of the reactants and water on the etched surface are
selected. Under the process conditions preventing condensation and
eliminating moisture from interacting with etched surfaces, BPSG:
thermal oxide etch selectivity in excess of 6000 is observed. Also,
there is indirect evidence suggesting hydrogen termination of
etched silicon surfaces.
Preferably, etching is performed with a gas phase mixture of a
halide-containing species and a low molecular weight organic
molecule having a high vapor pressure at standard conditions. It is
here recognized that organic molecules having low molecular weight
and high vapor pressure at standard conditions are generally
applicable in the method of the present invention. These molecules
include low molecular weight alcohols, organic acids, ketones or
alkanes such as methanol, isopropanol, acetone, acetic acid and
methane. In addition to their favorable solvent properties, these
molecules are volatalizable. It is most preferable that the low
molecular weight, high vapor pressure organic molecules be capable
of removing any water which may be present on the oxide surface,
particularly, for instance, when etching a BPSG oxide. Furthermore,
it is most preferable that the organic molecule be capable of
ionizing the halide-containing species which etches the oxide and
generating Si--OH precursor uniformly on the water surface. Such
molecules which are most preferable include low molecular weight
alcohols, organic acids and ketones such as methanol, isopropanol,
acetone and acetic acid. Moreover, it has been recognized that such
molecules actually initiate the etch when present with the
halide-containing species. It is also believed that the presence of
carbon in a low molecular weight, high vapor pressure molecule may
be sufficient to stimulate the initiation of etching. Therefore, in
a less preferred embodiment of the present invention, molecules
which do not contain oxygen-bearing nucleophilic moieties yet are
generally carbon containing organic molecules of low molecular
weight and high vapor pressure at standard conditions, e.g.,
methane, may be utilized.
Halide-containing species include, in particular, HF, NF.sub.3,
CIF.sub.3, F.sub.2 as well as other fluoride liberating species.
Generally, the halide-containing species are compounds that readily
crack with UV treatment in the presence of a stimulant. Other
examples of halide-containing species and low molecular weight,
high vapor pressure organic molecules will be readily apparent to
those skilled in the art.
The preferred aspect of the present invention focuses on the
minimization of water present during the oxide etch. Therefore, it
is preferred that either or both of the constituent reactant
species be present in anhydrous form. Accordingly, in the preferred
embodiment a low molecular weight organic molecule having a high
vapor pressure at standard conditions is utilized with a
halide-containing species, without the addition of water to the
input gases. In the most preferred embodiment, anhydrous forms or
both of these constituent reactant species are present, without the
addition of water.
The selection of process parameters is made on the basis of a phase
diagram showing the variation of vapor pressures vs. temperature.
Water is considered as it is the least volatile among the species
present including halide-containing species, low molecular weight
organic molecules having high vapor pressure is standard conditions
and any water present. FIG. 1 is a phase diagram representing
individual vapor pressure curves of HF, methanol and water as a
function of temperature. As long as the temperature and
corresponding pressure of the oxide etching process are selected
from the area below the curve for water as in FIG. 1, then all
reactants involved should remain in the gas phase even if trace
water vapor appears in the process chamber either as a product of
the etching reaction, or in the input gases. In particular, where a
surface of a substrate is at a preset temperature, etching is
performed at a pressure such that all species present are
substantially maintained in the gas phase and condensation of
species present on the etched surface is controlled.
In this manner FIG. 1 indicates, for instance, that at 100 Torr,
all species present during HF/methanol etching should be in the
vapor phase at 60.degree. C. and higher. At 300 Torr, all species
present should be in the vapor phase at 80.degree. C. and higher
and at 500 Torr, the vapor phase should be achieved at 95.degree.
C. and higher. Generally, FIG. 1 illustrates that as a higher
pressure is chosen, the minimum range of temperatures which may be
chosen is correspondingly higher. As a higher temperature is
chosen, the maximum range of pressure which may be chosen
increases, in order to maintain all species present in the gas
phase.
The same may not apply to the etching of heavily hydrated oxides,
such as borophosphosilicate glass (BPSG), where moisture contained
in the etched oxide may not be driven from the surface fast enough
to prevent fast etching. On the other hand, it is believed that
increased temperature and reduced pressure will enhance
vaporization of at least some reaction products, thereby reducing
formation of solid residues on the etched surfaces.
In certain experiments, moisture has been driven from the oxide
under vacuum conditions. Etching was then attempted with HF alone
and with further addition of a low molecular weight organic
molecule having a high vapor pressure at standard conditions. From
these experiments it has been determined that in etching with a
halide-containing species and a low molecular weight, high vapor
pressure organic molecule, the low molecular weight, high vapor
pressure organic molecule is responsible for initiation of the
etch. Moreover, reactions of halide-containing species with water
vapor and reactions of halide-containing species with low molecular
weight, high vapor pressure organic molecules appear to be driven
by different reaction mechanisms.
In a less preferred embodiment, the method of etching of the
present invention is applicable to etching with a halide-containing
species and water, without the use of a low molecular weight, high
vapor pressure organic molecule. Similar to the method described
above, temperature and pressure parameters are selectively chosen
such that all constituents remain in the gas phase.
One application for which the method of the present invention is
well suited is etching of sacrificial oxide in a cluster tool or
like environment immediately prior to gate oxidation. The cluster
tool apparatus has been disclosed in U.S. Pat. No. 5,228,206 to
Grant et al., which is incorporated herein by reference. The
cluster tool module permits dry process cleaning according to the
present invention on a commercial scale. It includes a 200 mm.
wafer compatible reactor equipped with IR heating lamps and a
vacuum control system such as a dry mechanical pump and turbo pump
allowing pressure reduction down to 10.sup.-6 Torr. To increase
process uniformity, rotation of the wafer using a frictionless
mechanism can be employed during etching. A high purity
halide-containing species is delivered from a nickel cylinder, and
a gas delivery system of stainless steel bubbler is used to contain
and supply liquid low molecular weight organic solvent having high
vapor pressure at standard conditions, with nitrogen as a carrier
gas. This gas delivery system permits the gas to pick up the vapor
without liquid transport. The system uses an all stainless steel
gas delivery system. Moreover, the UV lamp assists in driving off
molecules which may begin to condense, particularly after etching
is complete and the species present are replaced with nitrogen gas.
The use of the UV lamp markedly reduces processing time of the
wafers.
The following examples demonstrate the applicability of HF/methanol
and HF/acetone in the present invention. The present invention has
also been observed to achieve similar results using ethanol as the
low molecular weight, high vapor pressure organic molecule, as the
results set forth in Examples I and II using methanol and acetone.
A brief experiment using HF/acetic acid further indicated that
acetic acid is also applicable in the present invention. It has
also been documented that HF/isopropanol is applicable in the
present invention and achieves analogous results to those shown for
HF/methanol. See "Gas-Phase Etching of Silicon Oxide with Anhydrous
HF and Isopropanol", Third Int'l Symposium on Cleaning Technology
in Semiconductor Development Mfg., Subcommittee of Electro-chemical
Society, Oct. 15-20, 1993, citing Ruzyllo et al., J. Electrochem.
Soc., 140:4, April, 1993, p. L64-L66.
EXAMPLE I
A prototype cluster tool compatible commercial apparatus was used.
Special attention was given to the elimination of moisture and the
prevention of reactant condensation in any part of the hardware
upstream of the pressure control valve. Variation of process
parameters included changes of pressure from 100 to 500 Torr, wafer
temperature from 25.degree. to 110.degree. C., and CH.sub.3 OH
carrier gas/HF gas flow ratio from 0 to 6. X-ray photoelectron
spectroscopy (XPS) was used to evaluate the chemical condition of
the silicon surface following etching.
Thermal oxides were grown in dry oxygen to the thickness of 1100
.ANG.. For each etching run, initial and final oxide thicknesses
were determined using an automated ellipsometer with index of
refraction n=1.465. For the sake of preliminary evaluation of etch
selectivity, the rates of BPSG etching under the conditions defined
above were also determined. The BPSG films initially 6500 .ANG.
thick were formed using a conventional chemical vapor deposition
process.
The experimental portion was focused on: (i) establishing a
correlation between oxide etch rates and pressure of anhydrous
HF/CH.sub.3 OH/N.sub.2 mixture and wafer temperature, and (ii)
determining the effect of methanol on the etch rates and chemical
state of the Si surfaces.
Etching was performed with HF and methanol at various pressures and
wafer temperatures. An IR pyrometer was used to determine wafer
temperature. FIG. 2 presents the observed changes of thermal oxide
etch rates in HF/methanol at three different pressures as a
function of wafer temperature. As seen in FIG. 2, etch rates
decrease as wafer temperatures increase. Moreover, at higher
pressures these changes are significantly more pronounced. In
addition, the etch rates are much more reproducible at temperatures
which, at any given pressure, correspond to the etching region
assumed to be condensation-free. Thus the pressure and temperature
parameters discussed above in regard to the phase diagram of FIG. 1
are shown to have the most controlled etch rate in FIG. 2. These
effects are an indication that with increased wafer temperature,
less condensation is gradually taking place and gas-phase reactions
are becoming a dominant factor in the etching process. This
behavior is reflected by the oxide etch rates up to two orders of
magnitude lower than typical rates obtained for vapor HF/water
etching. In the condensation-controlled etching regime identified
in FIG. 2, variations of temperature and pressure can be used to
control etch rates of oxides from zero to about 200 .ANG./min.
Etching with HF/methanol was thus accomplished at a selectively
chosen pressures of between 100 to 500 Torr and corresponding wafer
temperatures of between 25.degree. to 120.degree. C. such that all
species present are in the gas phase and condensation of species
present on the etched surfaces is controlled. In particular, gas
phase etching with HF and methanol with a controlled etch was
achieved at a preset temperature and at a pressure such that water
is substantially maintained in the gas phase. As shown in FIG. 2,
such parameters were demonstrated in the HF/methanol system at 100
Torr from 20.degree. C. to 95.degree. C.; and 300 Torr from
80.degree. to 95.degree. C.; and at 500 Torr from 95.degree. to
105.degree. C.
In the case of BPSG however, under the same process conditions the
etch rates remain very high; typically etch rates in excess of 6000
.ANG./min were recorded which means that BPSG; thermal oxide etch
selectivity in the range of thousands can be accomplished. These
high etch rates are believed to be due to the heavy hydration of
BPSG resulting in significant enhancement of the etching reaction.
At the temperature and pressures applied in this experiment
moisture contained in BPSG is not driven away fast enough to
prevent fast oxide etching.
In order to test the effect of methanol on the oxide etching in the
condensation-controlled regime, moisture was driven from the oxide
under vacuum conditions and the etch rates for various CH.sub.3
OH/HF ratios were determined. Surprisingly, the results shown in
FIG. 3 clearly demonstrate that the methanol is responsible for
initiating the etching process. An important data point shown is
the apparent lack of any etching taking place at the CH.sub.3
OH/HF=0, i.e., during exposure of the oxide to gaseous HF only.
This result indicates that methanol is the initiator of the etching
reactions, and hence, that the controlled, effective etching of dry
thermal oxide can be accomplished without the addition of water
vapor to the input gases. The participation of moisture likely
generated as a product of chemical reactions involved in etching
does not change the fact that the mechanism of HF/CH.sub.3 OH
etching appears to be different from vapor HF/H.sub.2 O etching.
This notion is supported by the comparison between predicted
SIO.sub.2 etch rates for the HF/H.sub.2 O process at 25.degree. C.
and 100 Torr with etch rates observed in this experiment at the
same temperature and pressure. In this experiment etch rates are
six to eight times lower than predicted rates for vapor HF/H.sub.2
O etching, which indicates a different process etching mechanism
for each.
The same HF.sub.2 -- species are likely responsible for oxide
etching in the case of both vapor HF/water and HF/CH.sub.3 OH
etching. Since the former is known to yield a mostly
hydrogen-terminated silicon surface, one can expect the same in the
case of HF/CH.sub.3 OH etching. Two observations support this
speculation. One is that silicon surfaces, from which the oxide was
removed entirely using the HF/CH.sub.3 OH chemistry, display
hydrophobic features as revealed through visual inspection of the
behavior of water droplets on the etched surfaces. The second comes
from the x-ray photoelectron spectroscopy (XPS) characterization of
HF/CH.sub.3 OH treated surfaces.
FIG. 4 is a graph of XPS spectra for HF/methanol treated substances
in accordance with the present invention. As XPS cannot detect
hydrogen, weak F.sub.15 and O.sub.15 peaks may be an indication of
a mostly hydrogen-terminated surface. In fact, the XPS spectrum in
this case is strikingly similar to the one reported for a vapor
HF/water treated silicon surface.
EXAMPLE II
The above experiments were performed under the same operating
conditions, although utilizing HF/acetone rather than
HF/methanol.
FIG. 5 is a graph of oxide etched in HF/acetone as a function of
temperature at a pressure of 500 Torr. Flow rates were as follows:
acetone at 300 standard cubic centimeters per minute (sccm);
nitrogen at 425 sccm; and HF at 100 sccm. While not depicting rate
of etch v. temperature as in FIG. 2 regarding the use of
HF/methanol, the results shown for HF/acetone are similar to those
of HF/methanol. At 500 Torr a sharp increase in etch rate occurs
with HF/acetone at lower temperatures, similar to that observed for
HF/methanol. At higher temperatures, above 30.degree. C., a slow,
controlled etch rate is observed, similar to the rate observed at
higher temperatures for HF/methanol.
As in Example I for HF/methanol, the gas phase oxide etching
results with HF/acetone indicate that etching was achieved at a
preset temperature and pressure such that all species present,
including water, are substantially maintained in the gas phase and
condensation of species present on the etched surfaces is
controlled. Thus it appears from FIG. 5, that a controlled etch is
obtained for HF/methanol at 500 Torr from about 30.degree. C. to
70.degree. C.
FIG. 6 is a graph of oxide etched in HF/acetone as a function of HF
partial pressure at 500 Torr and 20.degree. C., with a total flow
of 1000 sccm and an acetone flow of 364 sccm. The amount of oxide
etched increases exponentially with HF partial pressure. These
results are also similar to those shown in FIG. 3 for each rate v.
HF/methanol flow ratio.
It will be understood that changes may be made in the
above-described method without departing from the scope of the
invention. It is accordingly intended that all matter contained in
the above description or shown in the accompanying drawings be
interpreted as illustrative rather than in a limiting sense.
It is also to be understood that the following claims are intended
to cover all of the generic and specific features of the invention
as described herein, and all statements of the scope of the
invention which, as a matter of language, might be said to fall
therebetween.
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