U.S. patent application number 09/833777 was filed with the patent office on 2001-09-13 for low defect density silicon and a process for producing low defect density silicon wherein v/g0 is controlled by controlling heat transfer at the melt/solid interface.
Invention is credited to Falster, Robert J., Holzer, Joseph C..
Application Number | 20010020437 09/833777 |
Document ID | / |
Family ID | 21918645 |
Filed Date | 2001-09-13 |
United States Patent
Application |
20010020437 |
Kind Code |
A1 |
Falster, Robert J. ; et
al. |
September 13, 2001 |
Low defect density silicon and a process for producing low defect
density silicon wherein V/G0 is controlled by controlling heat
transfer at the melt/solid interface
Abstract
The present invention relates to single crystal silicon, in
ingot or wafer form, which contains an axially symmetric region
which is free of agglomerated intrinsic point defects, and a
process for the preparation thereof. The process comprises
controlling growth conditions, such as growth velocity, v,
instantaneous axial temperature gradient, G.sub.0, and the cooling
rate, within a range of temperatures at which silicon
self-interstitials are mobile, in order to prevent the formation of
these agglomerated defects. In ingot form, the axially symmetric
region has a width, as measured from the circumferential edge of
the ingot radially toward the central axis, which is at least about
30% the length of the radius of the ingot. The axially symmetric
region additionally has a length, as measured along the central
axis, which is at least about 20% the length of the constant
diameter portion of the ingot.
Inventors: |
Falster, Robert J.; (London,
GB) ; Holzer, Joseph C.; (St. Charles, MO) |
Correspondence
Address: |
SENNIGER POWERS LEAVITT AND ROEDEL
ONE METROPOLITAN SQUARE
16TH FLOOR
ST LOUIS
MO
63102
US
|
Family ID: |
21918645 |
Appl. No.: |
09/833777 |
Filed: |
April 12, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09833777 |
Apr 12, 2001 |
|
|
|
09057907 |
Apr 9, 1998 |
|
|
|
60041845 |
Apr 9, 1997 |
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Current U.S.
Class: |
117/13 ;
257/E21.321 |
Current CPC
Class: |
C30B 15/00 20130101;
C30B 29/06 20130101; H01L 21/3225 20130101; C30B 33/00 20130101;
C30B 15/206 20130101; C30B 15/203 20130101; C30B 15/14 20130101;
Y10T 428/21 20150115; Y10T 117/1004 20150115 |
Class at
Publication: |
117/13 |
International
Class: |
C30B 015/00; C30B
021/06; C30B 027/02; C30B 028/10; C30B 030/04 |
Claims
What is claimed is:
1. A single crystal silicon wafer having a central axis, a front
side and a back side which are generally perpendicular to the axis,
a circumferential edge, and a radius extending from the central
axis to the circumferential edge of the wafer, the wafer comprising
an axially symmetric region which is substantially free of
agglomerated intrinsic point defects, the axially symmetric region
extending radially inwardly from the circumferential edge of the
wafer and having a width, as measured from the circumferential edge
radially toward the center axis, which is at least about 40% the
length of the radius of the wafer.
2. The wafer as set forth in claim 1 wherein the axially symmetric
region is generally annular in shape and the wafer additionally
comprises a generally cylindrical region consisting of vacancy
dominated material which is radially inward of the annular
region.
3. The wafer as set forth in claim 1 wherein the wafer has as
oxygen content which is less than about 13 PPMA.
4. The wafer as set forth in claim 1 wherein the wafer has as
oxygen content which is less than about 11 PPMA.
5. The wafer as set forth in claim 1 wherein the wafer has an
absence of oxygen precipitate nucleation centers.
6. A single crystal silicon ingot having a central axis, a
seed-cone, an end-cone, and a constant diameter portion between the
seed-cone and the end-cone having a circumferential edge and a
radius extending from the central axis to the circumferential edge,
the single crystal silicon ingot being characterized in that after
the ingot is grown and cooled from the solidification temperature,
the constant diameter portion contains an axially symmetric region
which is substantially free of agglomerated intrinsic point defects
wherein the axially symmetric region extends radially inwardly from
a circumferential edge of the ingot, has a width as measured from
the circumferential edge radially toward the central axis of the
ingot which is at least about 30% the length of the radius of the
constant diameter portion, and has a length as measured along the
central axis of at least about 20% the length of the constant
diameter portion of the ingot.
7. The single crystal silicon ingot as set forth in claim 6 wherein
the length of the axially symmetric region is at least 40% the
length of the constant diameter portion of the ingot.
8. The single crystal silicon ingot as set forth in claim 7 wherein
the length of the axially symmetric region is at least 60% the
length of the constant diameter portion of the ingot.
9. The single crystal silicon ingot as set forth in claim 6 wherein
the axially symmetric region has a width which is at least about
60% the length of the radius of the constant diameter portion.
10. The single crystal silicon ingot as set forth in claim 9
wherein the axially symmetric region has a width which is at least
about 80% the length of the radius of the constant diameter
portion.
11. A process for growing a single crystal silicon ingot in which
the ingot comprises a central axis, a seed-cone, an end-cone and a
constant diameter portion between the seed-cone and the end-cone
having a circumferential edge and a radius extending from the
central axis to the circumferential edge, the ingot being grown
from a silicon melt and then cooled from the solidification
temperature in accordance with the Czochralski method, the process
comprising controlling a growth velocity, v, and an instantaneous
axial temperature gradient, G.sub.0, of the crystal during the
growth of the constant diameter portion of the ingot to cause the
formation of an axially symmetrical segment which, upon cooling of
the ingot from the solidification temperature, is substantially
free of agglomerated intrinsic point defects wherein the axially
symmetric region extends inwardly from the circumferential edge of
the ingot, has a width as measured from the circumferential edge
radially toward the central axis of the ingot which is at least
about 30% the length of the radius of the ingot, and has a length
as measured along the central axis of at least about 20% the length
of the constant diameter portion of the ingot.
12. The process as set forth in claim 11 wherein the length of the
axially symmetric region is at least 40% the length of the constant
diameter portion of the ingot.
13. The process as set forth in claim 12 wherein the length of the
axially symmetric region is at least 60% the length of the constant
diameter portion of the ingot.
14. The process as set forth in claim 11 wherein the axially
symmetric region has a width which is at least about 60% the length
of the radius of the constant diameter portion.
15. The process as set forth in claim 14 wherein the axially
symmetric region has a width which is at least about 80% the length
of the radius of the constant diameter portion.
16. A process for growing a single crystal silicon ingot, the
single crystal silicon ingot being characterized in that, after the
ingot is grown from a silicon melt and cooled from the
solidification temperature in accordance with the Czochralski
method, a constant diameter portion of the ingot contains an
axially symmetric region which is substantially free of
agglomerated intrinsic point defects, the process comprising
controlling a growth velocity, v, and an instantaneous axial
temperature gradient, G.sub.0, such that a ratio v/G.sub.0 ranges
in value from about 0.6 to about 1.5 times the critical value of
v/G.sub.0.
17. A process for growing a single crystal silicon ingot, the
single crystal silicon ingot being characterized in that, after the
ingot is grown from a silicon melt and cooled from the
solidification temperature in accordance with the Czochralski
method, a constant diameter portion of the ingot contains an
axially symmetric segment which is substantially free of
agglomerated intrinsic point defects, the process comprising
controlling a growth velocity, v, and an instantaneous axial
temperature gradient, G.sub.0, such that a ratio v/G.sub.0 ranges
in value from about 0.6 to about 1.5 times the critical value of
v/G.sub.0; and controlling a cooling rate within a temperature
range of about 1400.degree. C. to about 800.degree. C., such that
the rate ranges from about 0.2.degree. C./minute to about
1.5.degree. C./minute.
18. The process of claim 17 wherein the growth velocity, v, and the
instantaneous axial temperature gradient, G.sub.0, are controlled
such that the ratio v/G.sub.0 ranges in value from about 0.75 to
about 1 times the critical value of v/G.sub.0.
19. The process of claim 17 wherein the cooling rate is controlled
within a temperature range of about 1400.degree. C. to about
1000.degree. C.
20. The process of claim 19 wherein the cooling rate is controlled
such that the rate ranges from about 0.2.degree. C./minute to about
1.degree. C./minute.
21. The process of claim 17 wherein oxygen precipitate nucleation
centers formed during the growth of the single crystal are
dissolved by annealing the single crystal silicon.
Description
REFERENCE TO RELATED APPLICATIONS
[0001] This application is a non-provisional application claiming
priority from provisional application serial no. 60/041,845 filed
Apr. 9, 1997.
BACKGROUND OF THE INVENTION
[0002] The present invention generally relates to the preparation
of semiconductor grade single crystal silicon which is used in the
manufacture of electronic components. More particularly, the
present invention relates to single crystal silicon ingots and
wafers having an axially symmetric region which is devoid of
agglomerated intrinsic point defects, and a process for the
preparation thereof.
[0003] Single crystal silicon, which is the starting material for
most processes for the fabrication of semiconductor electronic
components, is commonly prepared by the so-called Czochralski
("Cz") method. In this method, polycrystalline silicon
("polysilicon") is charged to a crucible and melted, a seed crystal
is brought into contact with the molten silicon and a single
crystal is grown by slow extraction. After formation of a neck is
complete, the diameter of the crystal is enlarged by decreasing the
pulling rate and/or the melt temperature until the desired or
target diameter is reached. The cylindrical main body of the
crystal which has an approximately constant diameter is then grown
by controlling the pull rate and the melt temperature while
compensating for the decreasing melt level. Near the end of the
growth process but before the crucible is emptied of molten
silicon, the crystal diameter must be reduced gradually to form an
end-cone. Typically, the end-cone is formed by increasing the
crystal pull rate and heat supplied to the crucible. When the
diameter becomes small enough, the crystal is then separated from
the melt.
[0004] In recent years, it has been recognized that a number of
defects in single crystal silicon form in the crystal growth
chamber as the crystal cools after solidification. Such defects
arise, in part, due to the presence of an excess (i.e. a
concentration above the solubility limit) of intrinsic point
defects, which are known as vacancies and self-interstitials.
Silicon crystals grown from a melt are typically grown with an
excess of one or the other type of intrinsic point defect, either
crystal lattice vacancies ("V") or silicon self-interstitials
("I"). It is understood that the type and initial concentration of
these point defects in the silicon, which become fixed at the time
of solidification, are controlled by the ratio v/G.sub.0, where v
is the growth velocity and G.sub.0 is the instantaneous axial
temperature gradient in the crystal at the time of solidification.
Referring to FIG. 1, for increasing values of the ratio v/G.sub.0,
a transition from decreasingly self-interstitial dominated growth
to increasingly vacancy dominated growth occurs near a critical
value of v/G,.sub.0, which based upon currently available
information appears to be about 2.1.times.10.sup.-5 cm.sup.2/sK. At
the critical value, the concentrations of these intrinsic point
defects are at equilibrium.
[0005] As the value of v/G.sub.0 exceeds the critical value, the
concentration of vacancies increases. Likewise, as the value of
v/G.sub.0 falls below the critical value, the concentration of
self-interstitials increases. If these concentrations reach a level
of critical supersaturation in the system, and if the mobility of
the point defects is sufficiently high, a reaction, or an
agglomeration event, will likely occur. Agglomerated intrinsic
point defects in silicon can severely impact the yield potential of
the material in the production of complex and highly integrated
circuits.
[0006] Vacancy-type defects are recognized to be the origin of such
observable crystal defects as D-defects, Flow Pattern Defects
(FPDs), Gate Oxide Integrity (GOI) Defects, Crystal Originated
Particle (COP) Defects, crystal originated Light Point Defects
(LPDs), as well as certain classes of bulk defects observed by
infrared light scattering techniques such as Scanning Infrared
Microscopy and Laser Scanning Tomography. Also present in regions
of excess vacancies are defects which act as the nuclei for ring
oxidation induced stacking faults (OISF). It is speculated that
this particular defect is a high temperature nucleated oxygen
agglomerate catalyzed by the presence of excess vacancies.
[0007] Defects relating to self-interstitials are less well
studied. They are generally regarded as being low densities of
interstitial-type dislocation loops or networks. Such defects are
not responsible for gate oxide integrity failures, an important
wafer performance criterion, but they are widely recognized to be
the cause of other types of device failures usually associated with
current leakage problems.
[0008] The density of such vacancy and self-interstitial
agglomerated defects in Czochralski silicon is conventionally
within the range of about 1*10.sup.3/cm.sup.3 to about
1*10.sup.7/cm.sup.3. While these values are relatively low,
agglomerated intrinsic point defects are of rapidly increasing
importance to device manufacturers and, in fact, are now seen as
yield-limiting factors in device fabrication processes.
[0009] To date, there generally exists three main approaches to
dealing with the problem of agglomerated intrinsic point defects.
The first approach includes methods which focus on crystal pulling
techniques in order to reduce the number density of agglomerated
intrinsic point defects in the ingot. This approach can be further
subdivided into those methods having crystal pulling conditions
which result in the formation of vacancy dominated material, and
those methods having crystal pulling conditions which result in the
formation of self-interstitial dominated material. For example, it
has been suggested that the number density of agglomerated defects
can be reduced by (i) controlling v/G.sub.0 to grow a crystal in
which crystal lattice vacancies are the dominant intrinsic point
defect, and (ii) influencing the nucleation rate of the
agglomerated defects by altering (generally, by slowing down) the
cooling rate of the silicon ingot from about 1100.degree. C. to
about 1050.degree. C. during the crystal pulling process. While
this approach reduces the number density of agglomerated defects,
it does not prevent their formation. As the requirements imposed by
device manufacturers become more and more stringent, the presence
of these defects will continue to become more of a problem.
[0010] Others have suggested reducing the pull rate, during the
growth of the body of the crystal, to a value less than about 0.4
mm/minute. This suggestion, however, is also not satisfactory
because such a slow pull rate leads to reduced throughput for each
crystal puller. More importantly, such pull rates lead to the
formation of single crystal silicon having a high concentration of
self-interstitials. This high concentration, in turn, leads to the
formation of agglomerated self-interstitial defects and all the
resulting problems associated with such defects.
[0011] A second approach to dealing with the problem of
agglomerated intrinsic point defects includes methods which focus
on the dissolution or annihilation of agglomerated intrinsic point
defects subsequent to their formation. Generally, this is achieved
by using high temperature heat treatments of the silicon in wafer
form. For example, Fusegawa et al. propose, in European Patent
Application 503,816 A1, growing the silicon ingot at a growth rate
in excess of 0.8 mm/minute, and heat treating the wafers which are
sliced from the ingot at a temperature in the range of 1150.degree.
C. to 1280.degree. C. to annihilate the defects which form during
the crystal growth process. Such heat treatments have been shown to
reduce the defect density in a thin region near the wafer surface.
The specific treatment needed will vary depending upon the
concentration and location of agglomerated intrinsic point defects
in the wafer. Different wafers cut from a crystal which does not
have a uniform axial concentration of such defects may require
different post-growth processing conditions. Furthermore, such
wafer heat treatments are relatively costly, have the potential for
introducing metallic impurities into the silicon wafers, and are
not universally effective for all types of crystal-related
defects.
[0012] A third approach to dealing with the problem of agglomerated
intrinsic point defects is the epitaxial deposition of a thin
crystalline layer of silicon on the surface of a single crystal
silicon wafer. This process provides a single crystal silicon wafer
having a surface which is substantially free of agglomerated
intrinsic point defects. Epitaxial deposition, however,
substantially increases the cost of the wafer.
[0013] In view of these developments, a need continues to exist for
a method of single crystal silicon preparation which acts to
prevent the formation of agglomerated intrinsic point defects by
suppressing the agglomeration reactions which produce them. Rather
than simply limiting the rate at which such defects form, or
attempting to annihilate some of the defects after they have
formed, a method which acts to suppress agglomeration reactions
would yield a silicon substrate that is substantially free of
agglomerated intrinsic point defects. Such a method would also
afford single crystal silicon wafers having epi-like yield
potential, in terms of the number of integrated circuits obtained
per wafer, without having the high costs associated with an
epitaxial process.
SUMMARY OF THE INVENTION
[0014] Among the objects of the present invention, therefore, is
the provision of single crystal silicon in ingot or wafer form
having an axially symmetric region of substantial radial width
which is substantially free of defects resulting from an
agglomeration of crystal lattice vacancies or silicon
self-interstitials; and the provision of a process for preparing a
single crystal silicon ingot in which the concentration of
vacancies and self-interstitials is controlled in order to prevent
an agglomeration of intrinsic point defects in an axially symmetric
segment of a constant diameter portion of the ingot, as the ingot
cools from the solidification temperature.
[0015] Briefly, therefore, the present invention is directed to a
single crystal silicon wafer having a central axis, a front side
and a back side which are generally perpendicular to the axis, a
circumferential edge, and a radius extending from the central axis
to the circumferential edge. The wafer comprises an axially
symmetric region which is substantially free of agglomerated
intrinsic point defects. The axially symmetric region extends
radially inwardly from the circumferential edge of the wafer and
has a width, as measured from the circumferential edge radially
toward the central axis, which is at least about 40% of the length
of the radius of the wafer.
[0016] The present invention is further directed to a single
crystal silicon ingot having a central axis, a seed-cone, an
end-cone, and a constant diameter portion between the seed-cone and
the end-cone which has a circumferential edge and a radius
extending from the central axis to the circumferential edge. The
single crystal silicon ingot is characterized in that, after ingot
growth is complete and the ingot has cooled from the solidification
temperature, the constant diameter portion contains an axially
symmetric region which is substantially free of agglomerated
intrinsic point defects. The axially symmetric region extends
radially inwardly from the circumferential edge and has a width, as
measured from the circumferential edge radially toward the central
axis, which is at least about 30% of the length of the radius of
the constant diameter portion. The axially symmetric region also
has a length, as measured along the central axis, of at least about
20% of the length of the constant diameter portion of the
ingot.
[0017] The present invention is still further directed to a process
for growing a single crystal silicon ingot in which an ingot,
comprising a central axis, a seed-cone, an end-cone and a constant
diameter portion between the seed-cone and the end-cone which has a
circumferential edge and a radius extending from the central axis
to the circumferential edge, is grown from a silicon melt and then
cooled from the solidification temperature in accordance with the
Czochralski method. The process comprises controlling a growth
velocity, v, and an instantaneous axial temperature gradient,
G.sub.0, of the crystal during growth of the constant diameter
portion to cause the formation of an axially symmetric region
which, upon cooling the ingot from the solidification temperature,
is substantially free of agglomerated intrinsic point defects. The
axially symmetric region extends radially inwardly from the
circumferential edge, has a width as measured from the
circumferential edge radially toward the central axis which is at
least about 30% of the length of the radius of the constant
diameter portion, and a length as measured along the central axis
of at least about 20% of the length of the constant diameter
portion.
[0018] Other objects and features of this invention will be in part
apparent and in part pointed out hereinafter.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 is a graph which shows an example of how the initial
concentration of self-interstitials, [I], and vacancies, [V],
changes with an increase in the value of the ratio v/G.sub.0, where
v is the growth rate and G.sub.0 is the instantaneous axial
temperature gradient.
[0020] FIG. 2 is a graph which shows an example of how
.DELTA.G.sub.I, the change in free energy required for the
formation of agglomerated interstitial defects, increases as the
temperature, T, decreases, for a given initial concentration of
self-interstitials, [I].
[0021] FIG. 3 is a graph which shows an example of how
.DELTA.G.sub.I, the change in free energy required for the
formation of agglomerated interstitial defects, decreases (as the
temperature, T, decreases) as a result of the suppression of the
concentration of self-interstitials, [I], through the means of
radial diffusion. The solid line depicts the case for no radial
diffusion whereas the dotted line includes the effect of
diffusion.
[0022] FIG. 4 is a graph which shows an example of how
.DELTA.G.sub.I, the change in free energy required for the
formation of agglomerated interstitial defects, is sufficiently
decreased (as the temperature, T, decreases), as a result of the
suppression of the concentration of self-interstitials, [I],
through the means of radial diffusion, such that an agglomeration
reaction is prevented. The solid line depicts the case for no
radial diffusion whereas the dotted line includes the effect of
diffusion.
[0023] FIG. 5 is a graph which shows an example of how the initial
concentration of self-interstitials, [I], and vacancies, [V], can
change along the radius of an ingot or wafer, as the value of the
ratio v/G.sub.0 decreases, due to an increase in the value of
G.sub.0. Note that at the V/I boundary a transition occurs from
vacancy dominated material to self-interstitial dominated
material.
[0024] FIG. 6 is a top plan view of a single crystal silicon ingot
or wafer showing regions of vacancy, V, and self-interstitial, I,
dominated materials respectively, as well as the V/I boundary that
exists between them.
[0025] FIG. 7a is a graph which shows an example of how the initial
concentration of vacancies or self-interstitials changes as a
function of radial position due to radial diffusion of
self-interstitials. Also shown is how such diffusion causes the
location of the V/I boundary to move closer to the center of the
ingot (as a result of the recombination of vacancies and
self-interstitials), as well as the concentration of
self-interstitials, [I], to be suppressed.
[0026] FIG. 7b is a graph of .DELTA.G.sub.I as a function of radial
position which shows an example of how the suppression of
self-interstitial concentration, [I], (as depicted in FIG. 7a) is
sufficient to maintain .DELTA.G.sub.I everywhere to a value which
is less than the critical value at which the silicon
self-interstitial reaction occurs.
[0027] FIG. 7c is a graph which shows another example of how the
initial concentration of vacancies or self-interstitials changes as
a function of radial position due to radial diffusion of
self-interstitials. Note that, in comparison to FIG. 7a, such
diffusion caused the location of the V/I boundary to be closer to
the center of the ingot (as a result of the recombination of
vacancies and self-interstitials), resulting in an increase in the
concentration of interstitials in the region outside of the V/I
boundary.
[0028] FIG. 7d is a graph of .DELTA.G.sub.I as a function of radial
position which shows an example of how the suppression of
self-interstitial concentration, [I], (as depicted in FIG. 7c) is
not sufficient to maintain .DELTA.G.sub.I everywhere to a value
which is less than the critical value at which the silicon
self-interstitial reaction occurs.
[0029] FIG. 7e is a graph which shows another example of how the
initial concentration of vacancies or self-interstitials changes as
a function of radial position due to radial diffusion of
self-interstitials. Note that, in comparison to FIG. 7a, increased
diffusion resulted in greater suppression the self-interstitial
concentration.
[0030] FIG. 7f is a graph of .DELTA.G.sub.I as a function of radial
position which shows an example of how greater suppression of the
self-interstitial concentration, [I], (as depicted in FIG. 7e)
results in a greater degree of suppression in .DELTA.G.sub.I, as
compared to FIG. 7b.
[0031] FIG. 7g is a graph which shows another example of how the
initial concentration of vacancies or self-interstitials changes as
a function of radial position due to radial diffusion of
self-interstitials. Note that, in comparison to FIG. 7c, increased
diffusion resulted in greater suppression the self-interstitial
concentration.
[0032] FIG. 7h is a graph of .DELTA.G.sub.I as a function of radial
position which shows an example of how greater suppression of the
self-interstitial concentration, [I], (as depicted in FIG. 7g)
results in a greater degree of suppression in .DELTA.G.sub.I, as
compared to FIG. 7d.
[0033] FIG. 7i is a graph which shows another example of how the
initial concentration of vacancies or self-interstitials changes as
a function of radial position due to radial diffusion of
self-interstitials. Note that in this example a sufficient quantity
of self-interstitials recombine with vacancies, such that there is
no longer a vacancy-dominated region.
[0034] FIG. 7j is a graph of .DELTA.G.sub.I as a function of radial
position which shows an example of how radial diffusion of
self-interstitials (as depicted in FIG. 7i) is sufficient to
maintain a suppression of agglomerated interstitial defects
everywhere along the crystal radius.
[0035] FIG. 8 is a longitudinal, cross-sectional view of a single
crystal silicon ingot showing, in detail, an axially symmetric
region of a constant diameter portion of the ingot.
[0036] FIG. 9 is a longitudinal, cross-sectional view of a segment
of a constant diameter portion of a single crystal silicon ingot,
showing in detail axial variations in the width of an axially
symmetric region.
[0037] FIG. 10 is a longitudinal, cross-sectional view of a segment
of a constant diameter portion of a single crystal silicon ingot
having axially symmetric region of a width which is less than the
radius of the ingot, showing in detail that this region further
contains a generally cylindrical region of vacancy dominated
material.
[0038] FIG. 11 is a latitudinal, cross-sectional view of the
axially symmetric region depicted in FIG. 10.
[0039] FIG. 12 is a longitudinal, cross-sectional view of a segment
of a constant diameter portion of a single crystal silicon ingot
having an axially symmetric region of a width which is equal to the
radius of the ingot, showing in detail that this region is a
generally cylindrical region of self-interstitial dominated
material which is substantially free of agglomerated intrinsic
point defects.
[0040] FIG. 13 is an image produced by a scan of the minority
carrier lifetime of an axial cut of the ingot following a series of
oxygen precipitation heat treatments, showing in detail a generally
cylindrical region of vacancy dominated material, a generally
annular shaped axially symmetric region of self-interstitial
dominated material, the V/I boundary present between them, and a
region of agglomerated interstitial defects.
[0041] FIG. 14 is a graph of pull rate (i.e. seed lift) as a
function of crystal length, showing how the pull rate is decreased
linearly over a portion of the length of the crystal.
[0042] FIG. 15 is an image produced by a scan of the minority
carrier lifetime of an axial cut of the ingot following a series of
oxygen precipitation heat treatments, as described in Example
1.
[0043] FIG. 16 is a graph of pull rate as a function of crystal
length for each of four single crystal silicon ingots, labeled 1-4
respectively, which are used to yield a curve, labeled v*(Z), as
described in Example 1.
[0044] FIG. 17 is a graph of the axial temperature gradient at the
crystal/melt interface, G.sub.0, as a function of radial position,
for two different cases as described in Example 2.
[0045] FIG. 18 is a graph of the initial concentration of
vacancies, [V], or self-interstitials, [I], as a function of radial
position, for two different cases as described Example 2.
[0046] FIG. 19 is a graph of temperature as a function of axial
position, showing the axial temperature profile in ingots for two
different cases as described in Example 3.
[0047] FIG. 20 is a graph of the self-interstitial concentrations
resulting from the two cooling conditions illustrated in FIG. 19
and as more fully described in Example 3.
[0048] FIG. 21 is an image produced by a scan of the minority
carrier lifetime of an axial cut of an entire ingot following a
series of oxygen precipitation heat treatments, as described in
Example 4.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0049] As used herein, the following phrases or terms shall have
the given meanings: "agglomerated intrinsic point defects" mean
defects caused (i) by the reaction in which vacancies agglomerate
to produce D-defects, flow pattern defects, gate oxide integrity
defects, crystal originated particle defects, crystal originated
light point defects, and other such vacancy related defects, or
(ii) by the reaction in which self-interstitials agglomerate to
produce dislocation loops and networks, and other such
self-interstitial related defects; "agglomerated interstitial
defects" shall mean agglomerated intrinsic point defects caused by
the reaction in which silicon self-interstitial atoms agglomerate;
"agglomerated vacancy defects" shall mean agglomerated vacancy
point defects caused by the reaction in which crystal lattice
vacancies agglomerate; "radius" means the distance measured from a
central axis to a circumferential edge of a wafer or ingot;
"substantially free of agglomerated intrinsic point defects" shall
mean a concentration of agglomerated defects which is less than the
detection limit of these defects, which is currently about 10.sup.4
defects/cm.sup.3; "V/I boundary" means the position along the
radius of an ingot or wafer at which the material changes from
vacancy dominated to self-interstitial dominated; and "vacancy
dominated" and "self-interstitial dominated" mean material in which
the intrinsic point defects are predominantly vacancies or
self-interstitials, respectively.
[0050] In accordance with the present invention, it has been
discovered that the reaction in which silicon self-interstitial
atoms react to produce agglomerated interstitial defects can be
suppressed during the growth of single crystal silicon ingots.
Without being bound to any particular theory, it is believed that
the concentration of self-interstitials is controlled during the
growth and cooling of the crystal ingot in the process of the
present invention, such that the change in free energy of the
system never exceeds a critical value at which the agglomeration
reaction spontaneously occurs to produce agglomerated interstitial
defects.
[0051] In general, the change in system free energy available to
drive the reaction in which agglomerated interstitial defects are
formed from silicon self-interstitials in single crystal silicon is
governed by Equation (I): 1 G I = kT ln ( [ I ] [ I ] eq ) ( I
)
[0052] wherein
[0053] .DELTA.G.sub.I is the change in free energy,
[0054] k is the Boltzmann constant,
[0055] T is the temperature in K,
[0056] [I] is the concentration of self-interstitials at a point in
space and time in the single crystal silicon, and
[0057] [I].sup.eq is the equilibrium concentration of
self-interstitials at the same point in space and time at which [I]
occurs and at the temperature, T.
[0058] According to this equation, for a given concentration of
self-interstitials, [I], a decrease in the temperature, T,
generally results in an increase in .DELTA.G.sub.I due to a sharp
decrease in [I].sup.eq with temperature.
[0059] FIG. 2 schematically illustrates the change in
.DELTA.G.sub.I and the concentration of silicon self-interstitials
for an ingot which is cooled from the temperature of solidification
without simultaneously employing some means for suppression of the
concentration of silicon self-interstitials. As the ingot cools,
.DELTA.G.sub.I increases according to Equation (I), due to the
increasing supersaturation of [I], and the energy barrier for the
formation of agglomerated interstitial defects is approached. As
cooling continues, this energy barrier is eventually exceeded, at
which point a reaction occurs.
[0060] This reaction results in the formation of agglomerated
interstitial defects and the concomitant decrease in .DELTA.G.sub.I
as the supersaturated system is relaxed.
[0061] The agglomeration of self-interstitials can be avoided as
the ingot cools from the temperature of solidification by
maintaining the free energy of the silicon self-interstitial system
at a value which is less than that at which an agglomeration
reaction will occur. In other words, the system can be controlled
so as to never become critically supersaturated. This can be
achieved by establishing an initial concentration of
self-interstitials which is sufficiently low such that is critical
supersaturation is never achieved. However, in practice such
concentrations are difficult to achieve across an entire crystal
radius and, in general, therefore, critical supersaturation may be
avoided by suppressing the initial silicon self-interstitial
concentration subsequent to crystal solidification.
[0062] FIGS. 3 and 4 schematically illustrate two possible effects
of suppressing [I] upon the increase in .DELTA.G.sub.I as the ingot
of FIG. 2 is cooled from the temperature of solidification. In FIG.
3, the suppression of [I] results in a decrease in the rate of
increase of .DELTA.G.sub.I, but in this case, the suppression is
insufficient to maintain .DELTA.G.sub.I everywhere at a value which
is less than the critical value at which the reaction occurs; as a
result, the suppression merely serves to reduce the temperature at
which the reaction occurs. In FIG. 4, an increased suppression of
[I] is sufficient to maintain .DELTA.G.sub.I everywhere to a value
which is less than the critical value at which the reaction occurs;
the suppression, therefore, inhibits the formation of defects.
[0063] Surprisingly, it has been found that due to the relatively
large mobility of self-interstitials, it is possible to effect the
suppression over relatively large distances by the radial diffusion
of self-interstitials to sinks located at the crystal surface or to
vacancy dominated regions. Radial diffusion can be effectively used
to suppress the concentration of self-interstitials, provided
sufficient time is allowed for the radial diffusion of the initial
concentration of self-interstitials. In general, the diffusion time
will depend upon the radial variation in the initial concentration
of self-interstitials, with lesser radial variations requiring
shorter diffusion times.
[0064] Typically, the axial temperature gradient, G.sub.0,
increases as a function of increasing radius for single crystal
silicon which is grown according to the Czochralski method. This
means that the value of v/G.sub.0 is typically not singular across
the radius of an ingot. As a result of this variation, the type and
initial concentration of intrinsic point defects is not constant.
If the critical value of v/G.sub.0, denoted in FIGS. 5 and 6 as the
V/I boundary 2, is reached at some point along the radius 4, the
material will switch from being vacancy dominated to
self-interstitial dominated. In addition, the ingot will contain an
axially symmetric region of self-interstitial dominated material 6
(in which the initial concentration of silicon self-interstitial
atoms increases as a function of increasing radius), surrounding a
generally cylindrical region of vacancy dominated material 8 (in
which the initial concentration of vacancies decreases as a
function of increasing radius).
[0065] FIGS. 7a and 7b schematically illustrate the effect of
suppressing [I] upon the increase in .DELTA.G.sub.I as an ingot is
cooled from the temperature of solidification in accordance with
one embodiment of the present invention. When the ingot is pulled
in accordance with the Czochralski method, the ingot contains an
axially symmetric region of interstitial dominated material
extending from the edge of the ingot to the position along the
radius at which the V/I boundary occurs and a generally cylindrical
region of vacancy dominated material extending from the center of
the ingot to the position along the radius at which the V/I
boundary occurs. As the ingot is cooled from the temperature of
solidification, radial diffusion of interstitial atoms causes a
radially inward shift in the V/I boundary due to a recombination of
self-interstitials with vacancies and a significant suppression of
the self-interstitial concentration outside the V/I boundary.
Furthermore, the suppression of [I] is sufficient to maintain
.DELTA.G.sub.I everywhere to a value which is less than the
critical value at which the silicon self-interstitial reaction
occurs.
[0066] Referring now to FIGS. 8 and 9, in the process of the
present invention a single crystal silicon ingot 10 is grown in
accordance with the Czochralski method. The silicon ingot comprises
a central axis 12, a seed-cone 14, an end-cone 16 and a constant
diameter portion 18 between the seed-cone and the end-cone. The
constant diameter portion has a circumferential edge 20 and a
radius 4 extending from the central axis to the circumferential
edge. The process comprises controlling the growth velocity, v, and
the instantaneous axial temperature gradient, G.sub.0, of the
crystal during the growth of the constant diameter portion of the
ingot to cause the formation of an axially symmetric region 6
which, upon cooling the ingot from the solidification temperature,
is substantially free of agglomerated intrinsic point defects.
[0067] The growth conditions are preferably controlled to maintain
the V/I boundary 2 at a position which maximizes the volume of the
axially symmetric region 6 relative to the volume of the constant
diameter portion 18 of the ingot 10. In general, therefore, it is
preferred that the axially symmetric region have a width 22 (as
measured from the circumferential edge radially toward the central
axis of the ingot) and a length 24 (as measured along the central
axis of the ingot) which equals the radius 4 and length 26,
respectively of the constant diameter portion of the ingot. As a
practical matter, however, operating conditions and crystal puller
hardware constraints may dictate that the axially symmetric region
occupy a lesser proportion of the constant diameter portion of the
ingot. In general, therefore, the axially symmetric region
preferably has a width of at least about 30%, more preferably at
least about 40%, still more preferably at least about 60%, and most
preferably at least about 80% of the radius of the constant
diameter portion of the ingot. In addition, the axially symmetric
region extends over a length of at least about 20%, preferably at
least about 40%, more preferably at least about 60%, and still more
preferably at least about 80% of the length of the constant
diameter portion of the ingot.
[0068] Referring to FIG. 9, the width 22 of the axially symmetric
region 6 may have some variation along the length of the central
axis 12. For an axially symmetric region of a given length,
therefore, the width is determined by measuring the distance from
the circumferential edge 20 of the ingot 10 radially toward a point
which is farthest from the central axis. In other words, the width
22 is measured such that the minimum distance within the given
length 24 of the axially symmetric region 6 is determined.
[0069] Referring now to FIGS. 10 and 11, when the axially symmetric
region 6 of the constant diameter portion 18 of the ingot 10 has a
width 22 which is less than the radius 4 of the constant diameter
portion, the region is generally annular in shape. A generally
cylindrical region of vacancy dominated material 8, which is
centered about the central axis 12, is located radially inward of
the generally annular shaped segment. Referring to FIG. 12, it is
to be understood that when the width 22 of the axially symmetric
region 6 is equal to the radius 4 of the constant diameter portion
18, the region does not contain this vacancy dominated region;
rather, the axially symmetric region itself is generally
cylindrical and contains self-interstitial dominated material which
is substantially free of agglomerated intrinsic point defects.
[0070] While it is generally preferred that the crystal growth
conditions be controlled to maximize the width of the interstitial
dominated region, there may be limits for a given crystal puller
hot zone design. As the V/I boundary is moved closer to the central
crystal axis, provided the cooling conditions and G.sub.0(r) do not
change, where G.sub.0(r) is the radial variation of G.sub.0, the
minimum amount of radial diffusion required increases. In these
circumstances, there may be a minimum radius of the vacancy
dominated region which is required to suppress the formation of
agglomerated interstitial defects by radial diffusion.
[0071] FIGS. 7c and 7d schematically illustrate an example in which
the minimum radius of the vacancy dominated region is exceeded. In
this example, the cooling conditions and G.sub.0(r) are the same as
those employed for the crystal of FIGS. 7a and 7b in which there
was sufficient outdiffusion to avoid agglomerated interstitial
defects for the position of the V/I boundary illustrated. In FIGS.
7c and 7d, the position of the V/I boundary is moved closer to the
central axis (relative to FIGS. 7a and 7b) resulting in an increase
in the concentration of interstitials in the region outside of the
V/I boundary. As a result, more radial diffusion is required to
sufficiently suppress the interstitial concentration. If sufficient
outdiffusion is not achieved, the system .DELTA.G.sub.I will
increase beyond the critical value and the reaction which produces
agglomerated interstitial defects will occur, producing a region of
these defects in an annular region between the V/I boundary and the
edge of the crystal. The radius of the V/I boundary at which this
occurs is the minimum radius for the given hot zone. This minimum
radius is decreased if more radial diffusion of interstitials is
allowed.
[0072] FIGS. 7e, 7f, 7g and 7h illustrate the effect of an
increased radial outdiffusion on interstitial concentration
profiles and the rise of system .DELTA.G.sub.I for a crystal grown
with the same initial vacancy and interstitial concentration
profiles as the crystal exemplified in FIGS. 7a, 7b, 7c and 7d.
Increased radial diffusion of interstitials results in a greater
suppression of interstitial concentration, thus suppressing the
rise in the system .DELTA.G.sub.I to a greater degree than in FIGS.
7a, 7b, 7c and 7d. In this case the system .DELTA.G.sub.I is not
exceeded for the smaller radius of the V/I boundary.
[0073] FIGS. 7i and 7j illustrate an example in which sufficient
radial diffusion is allowed such that the minimum radius is reduced
to zero by insuring sufficient radial diffusion to achieve a
suppression of agglomerated interstitial defects everywhere along
the crystal radius.
[0074] In a preferred embodiment of the process of the present
invention, the initial concentration of silicon self-interstitial
atoms is controlled in the axially symmetric, self-interstitial
dominated region of the ingot. Referring again to FIG. 1, in
general, the initial concentration of silicon self-interstitial
atoms is controlled by controlling the crystal growth velocity, v,
and the instantaneous axial temperature gradient, G.sub.0, such
that the value of the ratio v/G.sub.0 is relatively near the
critical value of this ratio, at which the V/I boundary occurs. In
addition, the instantaneous axial temperature gradient, G.sub.0,
can be established such that the variation of G.sub.0 (and thus,
v/G.sub.0) as a function of the ingot radius is also
controlled.
[0075] The growth velocity, v, and the instantaneous axial
temperature gradient, G.sub.0, are typically controlled such that
the ratio v/G.sub.0 ranges in value from about 0.5 to about 2.5
times the critical value of v/G.sub.0 (i.e., about
1.times.10.sup.-5 cm.sup.2/sK to about 5.times.10.sup.-5
cm.sup.2/sK based upon currently available information for the
critical value of v/G.sub.0) Preferably, the ratio v/G.sub.0 will
range in value from about 0.6 to about 1.5 times the critical value
of v/G.sub.0 (i.e., about 1.3.times.10.sup.-5 cm.sup.2/sK to about
3.times.10.sup.-5 cm.sup.2/sK based upon currently available
information for the critical value of v/G.sub.0) . Most preferably,
the ratio v/G.sub.0 will range in value from about 0.75 to about 1
times the critical value of v/G.sub.0 (i.e., about
1.6.times.10.sup.-5 cm.sup.2/sK to about 2.1.times.10.sup.-5
cm.sup.2/sK based upon currently available information for the
critical value of v/G.sub.0) . These ratios are achieved by
independent control of the growth velocity, v, and the
instantaneous axial temperature gradient, G.sub.0.
[0076] In general, control of the instantaneous axial temperature
gradient, G.sub.0, may be achieved primarily through the design of
the "hot zone" of the crystal puller, i.e. the graphite (or other
materials) that makes up the heater, insulation, and heat shields,
among other things. Although the design particulars may vary
depending upon the make and model of the crystal puller, in
general, G.sub.0 may be controlled using any of the means currently
known in the art for minimizing axial variations in heat transfer
at the melt/solid interface, including reflectors, radiation
shields, purge tubes, light pipes, and heaters. In general, radial
variations in G.sub.0 are minimized by positioning such an
apparatus within about one crystal diameter above the melt/solid
interface. G.sub.0 can be controlled further by adjusting the
position of the apparatus relative to the melt and crystal. This is
accomplished either by adjusting the position of the apparatus in
the hot zone, or by adjusting the position of the melt surface in
the hot zone. Either, or both, of these methods can be used during
a batch Czochralski process in which melt volume is depleted during
the process.
[0077] It is generally preferred for some embodiments of the
present invention that the instantaneous axial temperature
gradient, G.sub.0 , be relatively constant as a function of
diameter of the ingot. However, it should be noted that as
improvements in hot zone design allow for variations in G.sub.0 to
be minimized, mechanical issues associated with maintaining a
constant growth rate become an increasingly important factor. This
is because the growth process becomes much more sensitive to any
variation in the pull rate, which in turn directly effects the
growth rate, v. In terms of process control, this means that it is
favorable to have values for G.sub.0 which differ over the radius
of the ingot. Significant differences in the value of G.sub.0,
however, can result in a large concentration of self-interstitials
near the wafer edge and, thereby, increase the difficultly in
avoiding the formation of agglomerated intrinsic point defects.
[0078] In view of the foregoing, the control of G.sub.0 involves a
balance between minimizing radial variations in G.sub.0 and
maintaining favorable process control conditions. Typically,
therefore, the pull rate after about one diameter of the crystal
length will range from about 0.2 mm/minute to about 0.8 mm/minute.
Preferably, the pull rate will range from about 0.25 mm/minute to
about 0.6 mm/minute and, most preferably, from about 0.3 mm/minute
to about 0.5 mm/minute. It is to be noted the stated ranges are
typical for 200 mm diameter crystals. However, the pull rate is
dependent upon both the crystal diameter and crystal puller design.
In general, the pull rate will decrease as the crystal diameter
increases.
[0079] The amount of self-interstitial diffusion may be controlled
by controlling the cooling rate as the ingot is cooled from the
solidification temperature (about 1410.degree. C.) to the
temperature at which silicon self-interstitials become immobile,
for commercially practical purposes. Silicon self-interstitials
appear to be extremely mobile at temperatures near the
solidification temperature of silicon, i.e. about 1410.degree. C.
This mobility, however, decreases as the temperature of the single
crystal silicon ingot decreases. Experimental evidence obtained
to-date suggests that the diffusion rate of self-interstitials
slows such a considerable degree that they are essentially immobile
for commercially practical time periods at temperatures less than
about 700.degree. C., and perhaps at temperatures as great as
800.degree. C., 900.degree. C., or even 1,000.degree. C.
[0080] Within the range of temperatures at which self-interstitials
appear to be mobile, and depending upon the temperature in the hot
zone, the cooling rate will typically range from about 0.2.degree.
C./minute to about 2.degree. C./minute. Preferably, the cooling
rate will range from about 0.2.degree. C./minute to about
1.5.degree. C./minute and, more preferably, from about 0.2.degree.
C./minute to about 1.degree. C./minute. Control of the cooling rate
can be achieved by using any means currently known in the art for
minimizing heat transfer, including the use of insulators, heaters,
and radiation shields.
[0081] As previously noted, a minimum radius of the vacancy
dominated region exists for which the suppression of agglomerated
interstitial defects may be achieved. The value of the minimum
radius depends on v/G.sub.0(r) and the cooling rate. As crystal
puller and hot zone designs will vary, the ranges presented above
for v/G.sub.0(r) , pull rate, and cooling rate will also vary.
Likewise these conditions may vary along the length of a growing
crystal. Also as noted above, the width of the interstitial
dominated region free of agglomerated interstitial defects is
preferably maximized. Thus, it is desirable to maintain the width
of this region to a value which is as close as possible to, without
exceeding, the difference between the crystal radius and the
minimum radius of the vacancy dominated region along the length of
the growing crystal in a given crystal puller.
[0082] The optimum width of the axially symmetric region and the
required optimal crystal pulling rate profile for a given crystal
puller hot zone design may be determined empirically. Generally
speaking, this empirical approach involves first obtaining readily
available data on the axial temperature profile for an ingot grown
in a particular crystal puller, as well as the radial variations in
the instantaneous axial temperature gradient for an ingot grown in
the same puller. Collectively, this data is used to pull one or
more single crystal silicon ingots, which are then analyzed for the
presence of agglomerated interstitial defects. In this way, an
optimum pull rate profile can be determined.
[0083] FIG. 13 is an image produced by a scan of the minority
carrier lifetime of an axial cut of a section of a 200 mm diameter
ingot following a series of oxygen precipitation heat-treatments
which reveal defect distribution patterns. It depicts an example in
which a near-optimum pull rate profile is employed for a given
crystal puller hot zone design. In this example, a transition
occurs from the optimum v/G.sub.0(r) at which the axially symmetric
region has the maximum width, to a v/G.sub.0(r) in which the
maximum width of the interstitial dominated region is exceeded,
resulting in the generation of regions of agglomerated interstitial
defects 28.
[0084] In addition to the radial variations in v/G.sub.0 resulting
from an increase in G.sub.0 over the radius of the ingot, v/G.sub.0
may also vary axially as a result of a change in v, or as a result
of natural variations in G.sub.0 due to the Czochralski process.
For a standard Czochralski process, v is altered as the pull rate
is adjusted throughout the growth cycle, in order to maintain the
ingot at a constant diameter. These adjustments, or changes, in the
pull rate in turn cause v/G.sub.0 to vary over the length of the
constant diameter portion of the ingot. In accordance with the
process of the present invention, the pull rate is therefore
controlled in order to maximize the width of the axially symmetric
region of the ingot. As a result, however, variations in the radius
of the ingot may occur. In order to ensure that the resulting ingot
has a constant diameter, the ingot is therefore preferably grown to
a diameter larger than that which is desired. The ingot is then
subjected to processes standard in the art to remove excess
material from the surface, thus ensuring that an ingot having a
constant diameter portion is obtained.
[0085] For an ingot prepared in accordance with the process of the
present invention and having a V/I boundary, experience has shown
that low oxygen content material, i.e., less than about 13 PPMA
(parts per million atomic, ASTM standard F-121-83), is preferred.
More preferably, the single crystal silicon contains less than
about 12 PPMA oxygen, still more preferably less than about 11 PPMA
oxygen, and most preferably less than about 10 PPMA oxygen. This is
because, in medium to high oxygen contents wafers, i.e., 14 PPMA to
18 PPMA, the formation of oxygen-induced stacking faults and bands
of enhanced oxygen clustering just inside the V/I boundary becomes
more pronounced. Each of these are a potential source for problems
in a given integrated circuit fabrication process.
[0086] The effects of enhanced oxygen clustering may be further
reduced by two methods, used singularly or in combination. Oxygen
precipitate nucleation centers typically form in silicon which is
annealed at a temperature in the range of about 350.degree. to
about 750 .degree. C. For some applications, therefore, it may be
preferred that the crystal be a "short" crystal, that is, a crystal
which has been grown in a Czochralski process until the seed end
has cooled from the melting point of silicon (1410.degree. C.) to
about 750.degree. C. after which the ingot is rapidly cooled. In
this way, the time spent in the temperature range critical for
nucleation center formation is kept to a minimum and the oxygen
precipitate nucleation centers have inadequate time to form in the
crystal puller.
[0087] Alternatively, and more preferably, oxygen precipitate
nucleation centers formed during the growth of the single crystal
are dissolved by annealing the single crystal silicon. Provided
they have not been subjected to a stabilizing heat-treatment,
oxygen precipitate nucleation centers can be annealed out of
silicon by rapidly heating the silicon to a temperature of at least
about 875.degree. C. and preferably continuing to increase the
temperature to at least 1000.degree. C. By the time the silicon
reaches 1000.degree. C., substantially all (e.g., >99%) of such
defects have annealed out. It is important that the wafers be
rapidly heated to these temperatures, i.e., that the rate of
temperature increase be at least about 10.degree. C. per minute and
more preferably at least about 50.degree. C. per minute. Otherwise,
some or all of the oxygen precipitate nucleation centers may be
stabilized by the heat-treatment. Equilibrium appears to be reached
in relatively short periods of time, i.e., on the order of 1
minute. Accordingly, oxygen precipitate nucleation centers in the
single crystal silicon may be dissolved by annealing it at a
temperature of at least about 875.degree. C. for a period of at
least about 30 seconds, preferably at least about 10 minutes. The
dissolution may be carried out in a conventional furnace or in a
rapid thermal annealing (RTA) system. In addition, the dissolution
may carried out on crystal ingots or on wafers, preferably
wafers.
[0088] Although the temperature at which a self-interstitial
agglomeration reaction occurs may in theory vary over a wide range
of temperatures, as a practical matter this range appears to be
relatively narrow for conventional, Czochralski grown silicon. This
is a consequence of the relatively narrow range of initial
self-interstitial concentrations which are typically obtained in
silicon grown according to the Czochralski method. In general,
therefore, a self-interstitial agglomeration reaction will
typically occur, if at all, at temperatures within the range of
about 1100.degree. C. to about 800.degree. C.
[0089] As the Examples given below illustrate, the present
invention affords a process for preparing a single crystal silicon
ingot in which, as the ingot cools from the solidification
temperature in accordance with the Czochralski method, the
agglomeration of intrinsic point defects is prevented within an
axially symmetric region of the constant diameter portion of the
ingot, from which wafers may be sliced.
[0090] The following Examples set forth one set of conditions that
may be used to achieve the desired result. Alternative approaches
exist for determining an optimum pull rate profile for a given
crystal puller. For example, rather than growing a series of ingots
at various pull rates, a single crystal could be grown at pull
rates which increase and decrease along the length of the crystal;
in this approach, agglomerated self- interstitial defects would be
caused to appear and disappear multiple times during growth of a
single crystal. Optimal pull rates could then be determined for a
number of different crystal positions. Accordingly, the following
Examples should not be interpreted in a limiting sense.
EXAMPLE 1
Optimization Procedure For A Crystal Puller Having A Pre-existing
Hot Zone Design
[0091] A first 200 mm single crystal silicon ingot was grown under
conditions in which the pull rate was ramped linearly from 0.75
mm/min. to 0.35 mm/min. over the length of the crystal. FIG. 14
shows the pull rate as a function of crystal length. Taking into
account the pre-established axial temperature profile of a growing
200 mm ingot in the crystal puller and the pre-established radial
variations in the instantaneous axial temperature gradient,
G.sub.0, i.e., the axial temperature gradient at the melt/solid
interface, these pull rates were selected to insure that ingot
would be vacancy dominated material from the center to the edge at
one end of the ingot and interstitial dominated material from the
center to the edge of the other end of the ingot. The grown ingot
was sliced longitudinally and analyzed to determine where the
formation of agglomerated interstitial defects begins.
[0092] FIG. 15 is an image produced by a scan of the minority
carrier lifetime of an axial cut of the ingot over a section
ranging from about 635 mm to about 760 mm from the shoulder of the
ingot following a series of oxygen precipitation heat-treatments
which reveal defect distribution patterns. At a crystal position of
about 680 mm, a band of agglomerated interstitial defects 28 can be
seen. This position corresponds to a critical pull rate of v*(680
mm)=0.33 mm/min. At this point, the width of the axially symmetric
region 6 (a region which is interstitial dominated material but
which lacks agglomerated interstitial defects) is at its maximum;
the width of the vacancy dominated region 8, R.sub.v*(680) is about
35 mm and the width of the axially symmetric region, R.sub.I*(680)
is about 65 mm.
[0093] A series of four single crystal silicon ingots were then
grown at steady state pull rates which were somewhat greater than
and somewhat less than the pull rate at which the maximum width of
the axially symmetric region of the first 200 mm ingot was
obtained. FIG. 16 shows the pull rate as a function of crystal
length for each of the four crystals, labeled, respectively, as
1-4. These four crystals were then analyzed to determine the axial
position (and corresponding pull rate) at which agglomerated
interstitial defects first appear or disappear. These four
empirically determined points (marked "*") are shown in FIG. 16.
Interpolation between and extrapolation from these points yielded a
curve, labeled v*(Z) in FIG. 16. This curve represents, to a first
approximation, the pull rate for 200 mm crystals as a function of
length in the crystal puller at which the axially symmetric region
is at its maximum width.
[0094] Growth of additional crystals at other pull rates and
further analysis of these crystals would further refine the
empirical definition of v*(Z).
EXAMPLE 2
Reduction of Radial Variation in G.sub.0(r)
[0095] FIGS. 17 and 18 illustrate the improvement in quality that
can be achieved by reduction of the radial variation in the axial
temperature gradient at the crystal/melt interface, G.sub.0(r) .
The initial concentration (about 1 cm from the crystal/melt
interface) of vacancies and interstitials are calculated for two
cases with different G.sub.0(r) : (1)
G.sub.0(r)=2.65+5.times.10.sup.-4r.sup.2 (K/mm) and (2)
G.sub.0(r)=2.65+5.times.10.sup.-5r.sup.2 (K/mm). For each case the
pull rate was adjusted such that the boundary between vacancy-rich
silicon and interstitial-rich silicon is at a radius of 3 cm. The
pull rate used for case 1 and 2 were 0.4 and 0.35 mm/min,
respectively. From FIG. 18 it is clear that the initial
concentration of interstitials in the interstitial-rich portion of
the crystal is dramatically reduced as the radial variation in the
initial axial temperature gradient is reduced. This leads to an
improvement in the quality of the material since it becomes easier
to avoid the formation of interstitial defect clusters due to
supersaturation of interstitials.
EXAMPLE 3
Increased Out-diffusion Time for Interstitials
[0096] FIGS. 19 and 20 illustrate the improvement in quality that
can be achieved by increasing the time for out-diffusion of
interstitials. The concentration of interstitials is calculated for
two cases with differing axial temperature profiles in the crystal,
dT/dz. The axial temperature gradient at the crystal/melt interface
is the same for both cases, so that the initial concentration
(about 1 cm from the crystal/melt interface) of interstitials is
the same for both cases. In this example, the pull rate was
adjusted such that the entire crystal is interstitial-rich. The
pull rate was the same for both cases, 0.32 mm/min. The longer time
for interstitial out-diffusion in case 2 results in an overall
reduction of the interstitial concentration. This leads to an
improvement in the quality of the material since it becomes easier
to avoid the formation of interstitial defect clusters due to
supersaturation of interstitials.
EXAMPLE 4
[0097] A 700 mm long, 150 mm diameter crystal was grown with a
varying pull rate. The pull rate varied nearly linearly from 1.2
mm/min at the shoulder to 0.4 mm/min at 430 mm from the shoulder,
and then nearly linearly back to 0.65 mm/min at 700 mm from the
shoulder. Under these conditions in this particular crystal puller,
the entire radius is grown under interstitial-rich conditions over
the length of crystal about 320 mm to about 525 mm from the
shoulder of the crystal. At an axial position of about 525 mm and a
pull rate of about 0.47 mm/min, the crystal is free of agglomerated
intrinsic point defects clusters across the entire diameter. Stated
another way, there is one small section of the crystal in which the
width of the axially symmetric region, i.e., the region which is
substantially free of agglomerated defects, is equal to the radius
of the ingot.
[0098] In view of the above, it will be seen that the several
objects of the invention are achieved.
[0099] 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.
* * * * *