U.S. patent application number 09/929585 was filed with the patent office on 2002-01-03 for non-oxygen precipitating czochralski silicon wafers.
Invention is credited to Falster, Robert J..
Application Number | 20020000185 09/929585 |
Document ID | / |
Family ID | 26795148 |
Filed Date | 2002-01-03 |
United States Patent
Application |
20020000185 |
Kind Code |
A1 |
Falster, Robert J. |
January 3, 2002 |
Non-oxygen precipitating Czochralski silicon wafers
Abstract
The present invention relates to a process for the treatment of
Czochralski single crystal silicon wafers to dissolve existing
oxygen clusters and precipitates, while preventing their formation
upon a subsequent oxygen precipitation heat treatment. The process
comprises (i) heat-treating the wafer in a rapid thermal annealer
at a temperature of at least 1150.degree. C. in an atmosphere
having an oxygen concentration of at least 1000 ppma, or
alternatively (ii) heat-treating the wafer in a rapid thermal
annealer at a temperature of at least about 1150.degree. C. and
then controlling the rate of cooling from the maximum temperature
achieved during the heat-treatment through a temperature range in
which vacancies are relatively mobile in order to reduce the number
density of vacancies in the single crystal silicon to a value such
that oxygen precipitates will not form if the wafer is subsequently
subjected to an oxygen precipitation heat-treatment.
Inventors: |
Falster, Robert J.; (London,
GB) |
Correspondence
Address: |
SENNIGER POWERS LEAVITT AND ROEDEL
ONE METROPOLITAN SQUARE
16TH FLOOR
ST LOUIS
MO
63102
US
|
Family ID: |
26795148 |
Appl. No.: |
09/929585 |
Filed: |
August 14, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09929585 |
Aug 14, 2001 |
|
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|
09379383 |
Aug 23, 1999 |
|
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60098822 |
Sep 2, 1998 |
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Current U.S.
Class: |
117/2 ;
257/E21.321 |
Current CPC
Class: |
H01L 21/3225 20130101;
C30B 29/06 20130101; C30B 15/206 20130101 |
Class at
Publication: |
117/2 |
International
Class: |
C30B 001/00 |
Claims
We claim:
1. A process for heat-treating a Czochralski single crystal silicon
wafer to dissolve oxygen precipitates, the process comprising
heat-treating the wafer in a rapid thermal annealer at a
temperature of at least about 1150.degree. C. in an atmosphere
having an oxygen concentration of at least about 1000 ppma.
2. A process for heat-treating a Czochralski single crystal silicon
wafer to dissolve oxygen precipitates, the process comprising
heat-treating the wafer in a rapid thermal annealer at a
temperature of at least about 1150.degree. C. and controlling the
rate of cooling from the maximum temperature achieved during the
heat-treatment through a temperature range in which vacancies are
relatively mobile to reduce the number density of vacancies in the
single crystal silicon to a value such that oxygen precipitates
will not form in the heat-treated wafer upon subjecting the wafer
to an oxygen precipitation heat-treatment.
3. A process for heat-treating a Czochralski single crystal silicon
wafer to dissolve oxygen clusters and to prevent future precipitate
formation resulting from an oxygen precipitation heat treatment,
the process comprising: heat-treating the wafer at a temperature of
at least about 1150.degree. C. in a rapid thermal annealer to
dissolve pre-existing oxygen clusters; cooling the heat-treated
wafer to a temperature between about 950.degree. and 1150.degree.
C. at a rate in excess of about 20.degree. C.; thermally annealing
the cooled wafer at a temperature between about 950 and
1150.degree. C. to produce a wafer which is incapable of forming
oxygen precipitates upon being subjected to an oxygen precipitation
heat treatment.
Description
REFERENCE TO RELATED APPLICATION
[0001] This application claims priority from U.S. provisional
application Ser. No. 60/098,822, filed on Sep. 2, 1998.
BACKGROUND OF THE INVENTION
[0002] The present invention generally relates to the preparation
of semiconductor material substrates, especially silicon wafers,
which are used in the manufacture of electronic components. More
particularly, the present invention relates to a process for the
treatment of Czochralski single crystal silicon wafers to dissolve
existing oxygen clusters and precipitates, while preventing their
formation upon a subsequent oxygen precipitation heat
treatment.
[0003] Single crystal silicon, which is the starting material for
most processes for the fabrication of semiconductor electronic
components, is commonly prepared with the so-called Czochralski
process wherein a single seed crystal is immersed into molten
silicon and then grown by slow extraction. As molten silicon is
contained in a quartz crucible, it is contaminated with various
impurities, among which is mainly oxygen. At the temperature of the
silicon molten mass, oxygen comes into the crystal lattice until it
reaches a concentration determined by the solubility of oxygen in
silicon at the temperature of the molten mass and by the actual
segregation coefficient of oxygen in solidified silicon. Such
concentrations are greater than the solubility of oxygen in solid
silicon at the temperatures typical for the processes for the
fabrication of electronic devices. As the crystal grows from the
molten mass and cools, therefore, the solubility of oxygen in it
decreases rapidly, whereby in the resulting slices or wafers oxygen
is present in supersaturated concentrations.
[0004] During the thermal treatment cycles typically employed in
the fabrication of electronic devices, oxygen precipitate
nucleation centers may form and ultimately grown into large oxygen
clusters or precipitates. The presence of such precipitates in the
active device region of the wafer can impair the operation of the
device. Historically, to address this problem electronic device
fabrication processes included a series of steps which were
designed to produce silicon having a zone or region near the
surface of the wafer which is free of oxygen precipitates (commonly
referred to as a "denuded zone" or a "precipitate free zone").
Denuded zones can be formed, for example, in a high-low-high
thermal sequence such as (a) oxygen out-diffusion heat treatment at
a high temperature (>1100.degree. C.) in an inert ambient for a
period of at least about 4 hours, (b) oxygen precipitate nuclei
formation at a low temperature (600-750.degree. C.), and (c) growth
of oxygen (SiO.sub.2) precipitates at a high temperature
(1000-1150.degree. C.). See, e.g., F. Shimura, Semiconductor
Silicon Crystal Technology, Academic Press, Inc., San Diego Calif.
(1989) at pages 361-367 and the references cited therein.
[0005] More recently, however, advanced electronic device
manufacturing processes such as DRAM manufacturing processes have
begun to minimize the use of high temperature process steps.
Although some of these processes retain enough of the high
temperature process steps to produce a denuded zone, the tolerances
on the material are too tight to render it a commercially viable
product. Other current, highly advanced electronic device
manufacturing processes contain no out-diffusion steps at all.
Because of the problems associated with oxygen precipitates in the
active device region, therefore, these electronic device
fabricators must use silicon wafers which are incapable of forming
oxygen precipitates anywhere in the wafer under their process
conditions.
[0006] Accordingly, a process is needed by which existing oxygen
clusters or precipitates in the silicon wafer may be dissolved,
prior to the device fabrication, in such a way that future
formation of oxygen precipitates within the wafer is prevented.
SUMMARY OF THE INVENTION
[0007] Among the objects of the invention, therefore, is the
provision of a Czochralski single crystal silicon wafer, as well as
the process for the preparation thereof, in which oxygen clusters
and precipitates have been dissolved; and, the provision of such a
wafer which will not form oxygen precipitates or clusters upon
being subjected to an oxygen precipitation heat treatment.
[0008] Briefly, therefore, the present invention is directed to a
process for heat-treating a Czochralski single crystal silicon
wafer in a rapid thermal annealer to dissolve oxygen clusters, and
to prevent future precipitate formation resulting from a subsequent
thermal processing step. The process comprises heat-treating the
wafer at a temperature of at least about 1150.degree. C. in an
atmosphere having an oxygen concentration of at least about 1000
ppma to dissolve existing oxygen clusters and yield a wafer which
is incapable of forming oxygen precipitates upon being subjected to
an oxygen precipitation heat treatment.
[0009] The present invention is further directed to a process for
heat-treating a Czochralski single crystal silicon wafer to
dissolve oxygen precipitates or clusters, and to prevent future
precipitate formation resulting from a subsequent thermal
processing step. The process comprises heat-treating the wafer in a
rapid thermal annealer at a temperature of at least about
1150.degree. C. to dissolve existing oxygen clusters or
precipitates, and controlling the cooling rate of the heat-treated
wafer down to a temperature of less than about 950.degree. C. to
produce a wafer which is incapable of forming oxygen precipitates
upon being subjected to an oxygen precipitation heat treatment.
[0010] The present invention is still further directed to a process
for heat-treating a Czochralski single crystal silicon wafer to
dissolve oxygen precipitates or clusters, and to prevent future
precipitate formation resulting from a subsequent thermal
processing step. The process comprises heat-treating the wafer in a
rapid thermal annealer at a temperature of at least about
1150.degree. C. in an atmosphere to dissolve existing oxygen
clusters or precipitates. The heat-treated wafer is then cooled to
a temperature between about 950 and 1150.degree. C. at a rate in
excess of about 20.degree. C., and then thermally annealed at a
temperature between about 950 and 1150.degree. C. to produce a
wafer which is incapable of forming oxygen precipitates upon being
subjected to an oxygen precipitation heat treatment.
[0011] Other objects and features of this invention will be in part
apparent and in part pointed out hereinafter.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0012] The process of the present invention affords the means by
which to obtain a single crystal silicon wafer having a reduced
concentration of oxygen precipitates or clusters, as well as other
defects to which these precipitates are related. Additionally, the
present process yields a wafer which, during essentially any
subsequent oxygen precipitation heat treatment (e.g., annealing the
wafer at a temperature of 800.degree. C. for four hours and then at
a temperature of 1000.degree. C. for sixteen hours), will not form
oxygen precipitates. The process of the present invention therefore
acts to annihilate or dissolve a variety of pre-existing defects
such as large oxygen clusters and certain kinds of oxygen induced
stacking fault ("OISF") nuclei throughout the wafer. The dissolved
oxygen which remains in the wafer will not precipitate, even if the
wafer is subjected to an oxygen precipitation heat treatment.
[0013] The starting material for the process of the present
invention is a single crystal silicon wafer which has been sliced
from a single crystal ingot grown in accordance with conventional
Czochralski crystal growing methods. Such methods, as well as
standard silicon slicing, lapping, etching, and polishing
techniques are disclosed, for example, in F. Shimura, Semiconductor
Silicon Crystal Technology, Academic Press, 1989, and Silicon
Chemical Etching, (J. Grabmaier ed.) Springer-Verlag, New York,
1982 (incorporated herein by reference). The silicon wafer may be
polished or, alternatively, it may be lapped and etched but not
polished. In addition, the wafer may have vacancy or
self-interstitial point defects as the predominant intrinsic point
defect. For example, the wafer may be vacancy dominated from center
to edge, self-interstitial dominated from center to edge, or it may
contain a central core of vacancy of dominated material surrounded
by an axially symmetric ring of self-interstitial dominated
material.
[0014] Czochralski-grown silicon typically has an oxygen
concentration within the range of about 5.times.10.sup.17 to about
9.times.10.sup.17 atoms/cm.sup.3 (ASTM standard F-121-83). Because
the oxygen precipitation behavior of the wafer is essentially
erased by the present process (i.e., the wafer is essentially
rendered non-oxygen precipitating, even if subjected to an oxygen
precipitation heat treatment), the starting wafer may have an
oxygen concentration falling anywhere within or even outside the
range attainable by the Czochralski process. Depending upon the
cooling rate of the single crystal silicon ingot from the
temperature of the melting point of silicon (about 1410.degree. C.)
through the range of about 750.degree. C. to about 350.degree. C.,
oxygen precipitate nucleation centers may form in the single
crystal silicon ingot from which the wafer is sliced.
[0015] The presence or absence of these nucleation centers in the
starting material is not critical to the present invention.
Preferably, however, these centers are capable of being dissolved
by the rapid thermal anneal heat-treatment of the present
invention.
[0016] In accordance with the process of the present invention, a
single crystal silicon wafer is first subjected a heat treatment
step in which the wafer is heated to an elevated temperature.
Preferably, this heat treatment step is carried out in a rapid
thermal annealer in which the wafer is rapidly heated to a target
temperature and annealed at that temperature for a relatively short
period of time. In general, the wafer is subjected to a temperature
in excess of 1150.degree. C., preferably at least 1175.degree. C.,
more preferably at least about 1200.degree. C., and most preferably
between about 1200.degree. C. and 1275.degree. C. The wafer will
generally be maintained at this temperature for at least one
second, typically for at least several seconds (e.g., at least 3),
preferably for several tens of seconds (e.g., 20, 30, 40, or 50
seconds) and, depending upon the pre-existing defects, for a period
which may range up to about 60 seconds (which is near the limit for
commercially available rapid thermal annealers).
[0017] The rapid thermal anneal may be carried out in any of a
number of commercially available rapid thermal annealing ("RTA")
furnaces in which wafers are individually heated by banks of high
power lamps. RTA furnaces are capable of rapidly heating a silicon
wafer, e.g., they are capable of heating a wafer from room
temperature to 1200.degree. C. in a few seconds. One such
commercially available RTA furnace is the model 610 furnace
available from AG Associates (Mountain View, Calif.).
[0018] Heat-treating the wafer at a temperature in excess of
1150.degree. C. will cause the dissolution of a variety of
pre-existing oxygen clusters and OISF nuclei. In addition, it will
increase the number density of crystal lattice vacancies in the
wafer.
[0019] Information obtained to date suggests that certain
oxygen-related defects, such as ring oxidation induced stacking
faults (OISF), are high temperature nucleated oxygen agglomerates
catalyzed by the presence of a high concentration of vacancies.
Furthermore, in high vacancy regions, oxygen clustering is believed
to occur rapidly at elevated temperatures, as opposed to regions of
low vacancy concentration where behavior is more similar to regions
in which oxygen precipitate nucleation centers are lacking. Because
oxygen precipitation behavior is influenced by vacancy
concentration, therefore, the number of density of vacancies in the
heat-treated wafer is controlled in the process of the present
invention to avoid oxygen precipitation in a subsequent oxygen
precipitation heat treatment.
[0020] In a first embodiment of the process of the present
invention, the vacancy concentration in the heat-treated wafers is
controlled, at least in part, by controlling the atmosphere in
which the heat-treatment is carried out. Experimental evidence
obtained to date suggests that the presence of a significant amount
of oxygen suppresses the vacancy concentration in the heat-treated
wafer. Without being held to any particular theory, it is believed
that the rapid thermal annealing treatment in the presence of
oxygen results in the oxidation of the silicon surface and, as a
result, acts to create an inward flux of silicon
self-interstitials. This inward flux of self-interstitials has the
effect of gradually altering the vacancy concentration profile by
causing recombinations to occur, beginning at the surface and then
moving inward.
[0021] Regardless of the mechanism, the rapid thermal annealing
step is carried out in the presence of an oxygen-containing
atmosphere in the first embodiment of the process of the present
invention; that is, the anneal is carried out in an atmosphere
containing oxygen gas (O.sub.2), water vapor, or an
oxygen-containing compound gas which is capable of oxidizing an
exposed silicon surface. The atmosphere may thus consist entirely
of oxygen or oxygen compound gas, or it may additionally comprise a
non-oxidizing gas such as argon. It is to be noted, however, that
when the atmosphere is not entirely oxygen, preferably the
atmosphere will contain a partial pressure of oxygen of at least
about 0.001 atmospheres (atm.), or 1,000 parts per million atomic
(ppma). More preferably, the partial pressure of oxygen in the
atmosphere will be at least about 0.002 atm. (2,000 ppma), still
more preferably 0.005 atm. (5,000 ppma), and still more preferably
0.01 atm. (10,000 ppma).
[0022] Intrinsic point defects (vacancies and silicon
self-interstitials) are capable of diffusing through single crystal
silicon with the rate of diffusion being temperature dependant. The
concentration profile of intrinsic point defects, therefore, is a
function of the diffusivity of the intrinsic point defects and the
recombination rate as a function of temperature. For example, the
intrinsic point defects are relatively mobile at temperatures in
the vicinity of the temperature at which the wafer is annealed in
the rapid thermal annealing step, whereas they are essentially
immobile for any commercially practical time period at temperatures
of as much as 700.degree. C. Experimental evidence obtained to-date
suggests that the effective diffusion rate of vacancies slows
considerably at temperatures less than about 700.degree. C. and
perhaps as great as 800.degree. C., 900.degree. C., or even
1,000.degree. C., the vacancies can be considered to be immobile
for any commercially practical time period.
[0023] In a second embodiment of the present invention, therefore,
the concentration of vacancies in the heat-treated wafer is
controlled, at least in part, by controlling the cooling rate of
the wafer through the temperature range at which vacancies are
relatively mobile. As the temperature of the wafer is decreased
through this range of temperatures, the vacancies diffuse to the
wafer surface and are annihilated, thus leading to a change in the
vacancy concentration profile with the extent of change depending
upon the length of time the wafer is maintained at a temperature
within this range and the magnitude of the temperature; in general,
greater temperatures and longer diffusion times lead to increased
diffusion. In general, the average cooling rate from the annealing
temperature to the temperature at which vacancies are practically
immobile (e.g., about 950.degree. C.) is preferably no more than
20.degree. C. per second, more preferably no more than about
10.degree. C., and still more preferably no more than about
5.degree. C. per second.
[0024] Alternatively, the temperature of the wafer following the
high temperature anneal may be reduced quickly (e.g., at a rate
greater than about 20.degree. C./second) to a temperature of less
than about 1150.degree. C. but greater than about 950.degree. C.
and held for a time which is dependent upon the holding
temperature. For example, for temperatures near 1150.degree. C.,
several seconds (e.g., at least about 2, 3, 4, 6 or more) may be
sufficient whereas at temperatures near 950.degree. C. several
minutes (e.g., at least about 2, 3, 4, 6 or more) maybe required to
sufficiently reduce the vacancy concentration.
[0025] Once the wafer is cooled to a temperature outside the range
of temperatures at which crystal lattice vacancies are relatively
mobile in the single crystal silicon, the cooling rate does not
appear to significantly influence the precipitating characteristics
of the wafer and thus, does not appear to be narrowly critical.
[0026] Conveniently, the cooling step may be carried out in the
same atmosphere in which the heating step is carried out. Suitable
atmospheres include, for example, nitriding atmospheres (that is,
atmospheres containing nitrogen gas (N.sub.2) or a
nitrogen-containing compound gas, such as ammonia, which is capable
of nitriding an exposed silicon surface); oxidizing
(oxygen-containing) atmospheres; non-oxidizing, non-nitriding
atmospheres (such as argon, helium, neon, carbon dioxide), and
combinations thereof.
[0027] While the rapid thermal treatments employed in this process
may result in the out-diffusion of a small amount of oxygen from
the surface of the front and back surfaces of the wafer, the
resulting heat-treated wafer has a substantially uniform
interstitial oxygen concentration as a function of distance from
the silicon surface. For example, a heat-treated wafer will have a
substantially uniform concentration of interstitial oxygen from the
center of the wafer to regions of the wafer which are within about
15 microns of the silicon surface, more preferably from the center
of the silicon to regions of the wafer which are within about 10
microns of the silicon surface, even more preferably from the
center of the silicon to regions of the wafer which are within
about 5 microns of the silicon surface, and most preferably from
the center of the silicon to regions of the wafer which are within
about 3 microns of the silicon surface. In this context, a
substantially uniform oxygen concentration shall mean a variance in
the oxygen concentration of no more than about 50%, preferably no
more than about 20%, and most preferably no more than about
10%.
[0028] In view of the above, it will be seen that the several
objects of the invention are achieved. As various changes could be
made in the above compositions and processes without departing from
the scope of the invention, it is intended that all matter
contained in the above description be interpreted as illustrative
and not in a limiting sense.
* * * * *