U.S. patent application number 16/839808 was filed with the patent office on 2022-07-14 for process for preparing ingot having reduced distortion at late body length.
The applicant listed for this patent is GlobalWafers Co., Ltd.. Invention is credited to Sumeet S. Bhagavat, Hong-Huei Huang, Tapas Jain, Zheng Lu, Feng-Chien Tsai.
Application Number | 20220220631 16/839808 |
Document ID | / |
Family ID | |
Filed Date | 2022-07-14 |
United States Patent
Application |
20220220631 |
Kind Code |
A9 |
Jain; Tapas ; et
al. |
July 14, 2022 |
PROCESS FOR PREPARING INGOT HAVING REDUCED DISTORTION AT LATE BODY
LENGTH
Abstract
A method for growing a single crystal silicon ingot by the
Czochralski method having reduced deviation in diameter is
disclosed.
Inventors: |
Jain; Tapas; (Hsinchu City,
TW) ; Bhagavat; Sumeet S.; (St. Charles, MO) ;
Lu; Zheng; (O'Fallon, MO) ; Tsai; Feng-Chien;
(Taipei, TW) ; Huang; Hong-Huei; (Shuishang
Township, TW) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
GlobalWafers Co., Ltd. |
Hsinchu |
|
TW |
|
|
Prior
Publication: |
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Document Identifier |
Publication Date |
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US 20200325594 A1 |
|
|
US 20210269936 A9 |
September 2, 2021 |
|
|
Appl. No.: |
16/839808 |
Filed: |
April 3, 2020 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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62832561 |
Apr 11, 2019 |
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International
Class: |
C30B 15/22 20060101
C30B015/22; C30B 15/14 20060101 C30B015/14; C30B 30/04 20060101
C30B030/04; C30B 29/06 20060101 C30B029/06; C30B 15/20 20060101
C30B015/20 |
Claims
1. A method of preparing a single crystal silicon ingot by the
Czochralski method, the method comprising: adding an initial charge
of polycrystalline silicon to a crucible contained within a growth
chamber, wherein the crucible comprises a bottom wall and a
sidewall and further wherein the growth chamber comprises a bottom
heater located next to the bottom wall of the crucible, a side
heater located next to the sidewall, and a reflector; supplying
power to the bottom heater, the side heater, or both the bottom
heater and side heater to thereby heat the crucible comprising the
initial charge of polycrystalline silicon to cause a silicon melt
to form in the crucible, wherein the power supplied to the side
heater is greater than the power supplied to the bottom heater and
further wherein the silicon melt has a free melt elevation level;
contacting a silicon seed crystal with the silicon melt contained
within the crucible; withdrawing the silicon seed crystal from the
silicon melt in a direction perpendicular to the melt elevation
level at an initial pull rate to thereby form a solid neck portion
of the single crystal silicon ingot; withdrawing a solid outwardly
flaring seed-cone adjacent the neck portion of the single crystal
silicon ingot from the silicon melt by modifying the initial pull
rate to thereby achieve an outwardly flaring seed-cone pull rate;
and withdrawing a solid main body of the single crystal silicon
ingot adjacent the outwardly flaring seed-cone from the silicon
melt by modifying the outwardly flaring seed-cone pull rate to
thereby achieve a main body pull rate, wherein the solid main body
of the single crystal silicon ingot has a radial diameter and an
axial length and surface tension arising as the solid main body of
the single crystal silicon ingot is withdrawn from the molten
silicon results in a melt-solid interface located above the free
melt elevation level and further wherein a meniscus comprising
molten silicon is between the melt-solid interface and the free
melt elevation level; wherein a cusp magnetic field is applied to
the silicon melt during growth of the main body of the single
crystal silicon ingot; and wherein a heat flux in an axial
direction between the melt-solid interface and the free melt
elevation level during growth of at least 40% of a total axial
length of the solid main body of the single crystal silicon ingot
has an absolute value of at least about 20,000 W/m.sup.2 over at
least about 85% of the radial length of the solid main body of the
single crystal silicon ingot.
2. The method of claim 1 wherein the bottom wall of the crucible is
insulated.
3. The method of claim 1 wherein the total axial length of the
solid main body of the single crystal silicon ingot is at least
about 1100 mm.
4. The method of claim 1 wherein the total axial length of the
solid main body of the single crystal silicon ingot is between
about 1200 mm and about 1300 mm.
5. The method of claim 1 wherein the radial length of the solid
main body of the single crystal silicon ingot is about 75 mm, at
least about 75 millimeters, about 100 mm, or at least about 100
millimeters.
6. The method of claim 1 wherein the radial length of the solid
main body of the single crystal silicon ingot is about 150 mm or at
least about 150 mm.
7. The method of claim 1 wherein the heat flux in the axial
direction between the melt-solid interface and the free melt
elevation level during growth of at least 60% of the axial length
of the solid main body of the single crystal silicon ingot has an
absolute value of at least about 20,000 W/m.sup.2 over at least
about 85% of the radial length of the solid main body of the single
crystal silicon ingot.
8. The method of claim 1 wherein the heat flux in the axial
direction between the melt-solid interface and the free melt
elevation level during growth of at least 80% of the axial length
of the solid main body of the single crystal silicon ingot has an
absolute value of at least about 20,000 W/m.sup.2 over at least
about 80% of the diameter of the solid main body of the single
crystal silicon ingot.
9. The method of claim 1 wherein the heat flux in the axial
direction between the melt-solid interface and the free melt
elevation level during growth of at least 90% of a total length of
the solid main body of the single crystal silicon ingot has an
absolute value of at least about 20,000 W/m.sup.2 over at least
about 85% of the radial length of the solid main body of the single
crystal silicon ingot.
10. The method of claim 1 wherein a temperature gradient along a
meniscus curve between the melt-solid interface and the free melt
elevation level has an average value of at least about
0.16.degree./mm during growth of at least 40% of a total length of
the solid main body of the single crystal silicon ingot.
11. The method of claim 1 wherein a temperature gradient along a
meniscus curve between the melt-solid interface and the free melt
elevation level has an average value of at least about
0.18.degree./mm during growth of at least 40% of a total length of
the solid main body of the single crystal silicon ingot.
12. The method of claim 1 wherein a temperature gradient along a
meniscus curve between the melt-solid interface and the free melt
elevation level has an average value of at least about
0.16.degree./mm during growth of at least 60% of a total length of
the solid main body of the single crystal silicon ingot.
13. The method of claim 1 wherein a temperature gradient along a
meniscus curve between the melt-solid interface and the free melt
elevation level has an average value of at least about
0.18.degree./mm during growth of at least 60% of a total length of
the solid main body of the single crystal silicon ingot.
14. The method of claim 1 wherein a temperature gradient along a
meniscus curve between the melt-solid interface and the free melt
elevation level has an average value of at least about
0.16.degree./mm during growth of at least 80% of a total length of
the solid main body of the single crystal silicon ingot.
15. The method of claim 1 wherein a temperature gradient along a
meniscus curve between the melt-solid interface and the free melt
elevation level has an average value of at least about
0.18.degree./mm during growth of at least 80% of a total length of
the solid main body of the single crystal silicon ingot.
16. The method of claim 1 wherein a temperature of the molten
silicon in the meniscus is at least 1691K as measured at the free
melt elevation level during growth of at least 40% of a total
length of the solid main body of the single crystal silicon
ingot.
17. The method of claim 1 wherein a temperature of the molten
silicon in the meniscus is at least 1692K as measured at the free
melt elevation level during growth of at least 40% of a total
length of the solid main body of the single crystal silicon
ingot.
18. The method of claim 1 wherein a temperature of the molten
silicon in the meniscus is at least 1691K as measured at the free
melt elevation level during growth of at least 60% of a total
length of the solid main body of the single crystal silicon
ingot.
19. The method of claim 1 wherein a. temperature of the molten
silicon in the meniscus is at least 1692K as measured at the free
melt elevation level during growth of at least 60% of a total
length of the solid main body of the single crystal silicon
ingot.
20. The method of claim 1 wherein a temperature of the molten
silicon in the meniscus is at least 1691K as measured at the free
melt elevation level during growth of at least 85% of a total
length of the solid main body of the single crystal silicon
ingot.
21. The method of claim 1 wherein a temperature of the molten
silicon in the meniscus is at least 1692K as measured at the free
melt elevation level during growth of at least 85% of the axial
length of the solid main body of the single crystal silicon
ingot.
22. The method of claim 1 wherein the cusp magnetic field applied
to the silicon melt during growth of the main body of the single
crystal silicon ingot is derived from an upper magnetic coil and a
lower magnetic coil, and further wherein an upper magnetic field
strength derived from the upper magnetic coil is greater than a
lower magnetic field strength derived from the lower magnetic
coil.
23. The method of claim 1 wherein the cusp magnetic field applied
to the silicon melt during growth of the main body of the single
crystal silicon ingot is derived from an upper magnetic coil and a
lower magnetic coil, and further wherein an upper magnetic field
strength derived from the upper magnetic coil exceeds a lower
magnetic field strength derived from the lower magnetic coil by at
least 10%, or at least 15%.
Description
CROSS REFERENCE TO RELATED APPLICATION(S)
[0001] This application claims the benefit of priority to U.S.
provisional application Ser. No. 62/832,561, filed Apr. 11, 2019,
the disclosure of which is incorporated by reference as if set
forth in its entirety.
FIELD OF THE DISCLOSURE
[0002] The field of the disclosure relates to a method to grow a
single crystal silicon ingot using the Czochralski method.
BACKGROUND
[0003] Single crystal silicon, which is the starting material for
most processes for the fabrication of semiconductor electronic
components, is commonly prepared by the 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, for example, 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 is typically reduced gradually to form a tail end in the
form of an end-cone. The end-cone usually 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] Czochralski growing techniques include the batch Czochralski
method and the continuous Czochralski method. In batch CZ, a single
polycrystalline charge is loaded into the crucible, the single
charge being sufficient to grow a single crystal silicon ingot,
after which the crucible is essentially depleted of silicon melt.
In continuous Czochralski (CCZ) growth, polycrystalline silicon may
be continually or periodically added to the molten silicon to
replenish the melt during the growth process and, as a result,
multiple ingots can be pulled from a single crucible during a
growth process.
[0005] To carry out the CCZ process, the traditional batch
Czochralski growth chamber and apparatus are modified to include a
means for feeding additional polycrystalline silicon to the melt in
a continuous or semi-continuous fashion without adversely affecting
the properties of the growing ingot. As the seed crystal is
continuously grown from the melt, solid polycrystalline silicon
such as granular polycrystalline silicon is added to the melt to
replenish the melt. The feed rate of the additional solid
polycrystalline silicon added to the melt is typically controlled
to maintain process parameters. In order to reduce the adverse
effects of this replenishing activity on simultaneous crystal
growth, the traditional quartz crucible is often modified to
provide an outer or annular melt zone into which the added material
is delivered along with an inner growth zone from which the silicon
ingot is pulled. These zones are in fluid flow communication with
one another.
[0006] The continuously shrinking size of the modern
microelectronic device imposes challenging restrictions on the
quality of the silicon substrate, which is essentially determined
by the size and the distribution of the grown-in microdefects. Most
of the microdefects formed in silicon crystals grown by the
Czochralski (CZ) process and the Float Zone (FZ) process are the
agglomerates of intrinsic point defects of silicon--vacancies and
self-interstitials (or, simply, interstitials).
[0007] A series of studies have established that the interstitial
agglomerates exist in two forms--globular interstitial clusters,
termed B swirl defect (or B-defects), and the dislocation loops,
termed A swirl defect (or A-defects). Later discovered vacancy
agglomerates, known as D-defects, have been identified as
octahedral voids. Voronkov provided the well-accepted explanation
for the microdefect distributions observed in silicon crystals on
the basis of the crystal growth conditions. According to Voronkov's
model, or theory, the temperature field in the vicinity of the
melt/crystal interface drives the recombination of the point
defects providing driving forces for their diffusion from the
melt/crystal interface--where they exist at their respective
equilibrium concentrations--into the crystal bulk. The interplay
between the transport of the point defects, both by the diffusion
and the convection, and their recombination establishes the point
defect concentration beyond a short distance away from the
interface, termed the recombination length. Typically, the
difference between the vacancy concentration and the interstitial
concentration beyond the recombination length, termed the excess
point defect concentration, remains essentially fixed away from the
lateral surface of the crystal. In a rapidly pulled crystal, the
spatial redistribution of the point defects by their diffusion
beyond the recombination length is generally not important--with
the exception of a region close to the lateral surface of the
crystal that acts as a sink or a source of the point defects.
Therefore, if the excess point defect concentration beyond the
recombination length is positive, vacancies remain in excess, and
agglomerate to form D-defects at lower temperatures. If the excess
point defect concentration is negative, interstitials remain the
dominant point defects, and agglomerate to form A-defects and
B-defects. If the excess point defect concentration is below some
detection threshold, no detectable microdefects are formed. Thus,
typically, the type of grown-in microdefects is determined simply
by the excess point defect concentration established beyond the
recombination length. The process of establishing the excess point
defect concentration is termed the initial incorporation and the
dominant point defect species is termed the incorporated dominant
point defect. The type of the incorporated point defects is
determined by the ratio of the crystal pull-rate (v) to the
magnitude of the axial temperature gradient in the vicinity of the
interface (G). At a higher v/G, the convection of the point defects
dominates their diffusion, and vacancies remain the incorporated
dominant point defects, as the vacancy concentration at the
interface is higher than the interstitial concentration. At a lower
v/G, the diffusion dominates the convection, allowing the
incorporation of the fast diffusing interstitials as the dominant
point points. At a v/G close to its critical value, both the point
defects are incorporated in very low and comparable concentrations,
mutually annihilating each other and thus suppressing the potential
formation of any microdefects at lower temperatures. The observed
spatial microdefect distribution can be typically explained by the
variation of v/G, caused by a radial non-uniformity of G and by an
axial variation of v. A striking feature of the radial microdefect
distribution is the oxide particles formed through the interaction
of oxygen with vacancies in the regions of relatively lower
incorporated vacancy concentration--at a small range of v/G
marginally above the critical v/G. These particles form a narrow
spatial band that can be revealed by thermal oxidation as the OSF
(oxidation-induced stacking faults) ring. Quite often, the OSF ring
marks the boundary between adjacent crystal regions that are
vacancy-dominated and interstitial-dominated, known as the V/I
boundary.
[0008] The microdefect distributions in CZ crystals grown at lower
rates in many modern processes, however, are influenced by the
diffusion of the point defects in the crystal bulk, including the
diffusion induced by the lateral surfaces of the crystals.
Therefore, an accurate quantification of the microdefect
distributions in CZ crystals preferably incorporates the
2-dimensional point defect diffusion, both axially and radially.
Quantifying only the point defect concentration field can
qualitatively capture the microdefect distribution in a CZ crystal,
as the type of the microdefects formed is directly determined by
it. For a more accurate quantification of the microdefect
distribution, however, capturing the agglomeration of the point
defects is necessary. Traditionally, the microdefect distribution
is quantified by decoupling the initial incorporation of the point
defects and the subsequent formation of the microdefects. This
approach ignores the diffusion of the dominant point defects in the
vicinity of the nucleation region, from the regions at higher
temperatures (where the microdefect density is negligible) to the
regions at lower temperatures (where the microdefects exist in
higher densities and consume the point defects). Alternatively, a
rigorous numerical simulation based on predicting the size
distributions of the microdefect populations at every location in
the crystal is numerically expensive.
[0009] The transition between vacancy and interstitial dominated
material occurs at a critical value of v/G, which currently appears
to be about 2.5.times.10.sup.-5 cm.sup.2/sK. If the value of v/G
exceeds the critical value, vacancies are the predominant intrinsic
point defect, with their concentration increasing with increasing
v/G. If the value of v/G is less than the critical value, silicon
self-interstitials are the predominant intrinsic point defect, with
their concentration increasing with decreasing v/G. Accordingly,
process conditions, such as growth rate (which affect v), as well
as hot zone configurations (which affect G), can be controlled to
determine whether the intrinsic point defects within the single
crystal silicon will be predominantly vacancies (where v/G is
generally greater than the critical value) or self-interstitials
(where v/G is generally less than the critical value).
[0010] Agglomerated defect formation generally occurs in two steps.
First, defect "nucleation" occurs, which is the result of the
intrinsic point defects being supersaturated at a given
temperature; above this "nucleation threshold" temperature,
intrinsic point defects remain soluble in the silicon lattice. The
nucleation temperature for agglomerated intrinsic point defects is
greater than about 1000.degree. C.
[0011] Once this "nucleation threshold" temperature is reached,
intrinsic point defects agglomerate; that is, precipitation of
these point defects out of the "solid solution" of the silicon
lattice occurs. The intrinsic point defects will continue to
diffuse through the silicon lattice as long as the temperature of
the portion of the ingot in which they are present remains above a
second threshold temperature (i.e., a "diffusivity threshold").
Below this "diffusivity threshold" temperature, intrinsic point
defects are no longer mobile within commercially practical periods
of time.
[0012] While the ingot remains above the "diffusivity threshold"
temperature, vacancy or interstitial intrinsic point defects
diffuse through the silicon lattice to sites where agglomerated
vacancy defects or interstitial defects, respectively, are already
present, causing a given agglomerated defect to grow in size.
Growth occurs because these agglomerated defect sites essentially
act as "sinks," attracting and collecting intrinsic point defects
because of the more favorable energy state of the
agglomeration.
[0013] 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 clusters of oxygen or silicon dioxide. Some
of these clusters remain small and relatively strain-free, causing
essentially no harm to a majority of devices prepared from such
silicon. Some of these clusters are large enough to act as the
nuclei for ring oxidation induced stacking faults (OISF). It is
speculated that this particular defect is facilitated by previously
nucleated oxygen agglomerates catalyzed by the presence of excess
vacancies. The oxide clusters are primarily formed in CZ growth
below 1000.degree. C. in the presence of moderate vacancy
concentration.
[0014] 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.
[0015] In this regard it is to be noted that, generally speaking,
oxygen in interstitial form in the silicon lattice is typically
considered to be a point defect of silicon, but not an intrinsic
point defect, whereas silicon lattice vacancies and silicon
self-interstitials (or, simply, interstitials) are typically
considered to be intrinsic point defects. Accordingly, essentially
all microdefects may be generally described as agglomerated point
defects, while D-defects (or voids), as well as A-defects and
B-defects (i.e., interstitial defects) may be more specifically
described as agglomerated intrinsic point defects. Oxygen clusters
are formed by absorbing vacancies; hence, oxygen clusters can be
regarded as agglomerates of both vacancies and oxygen.
[0016] It is to be further noted that the density of such vacancy
and self-interstitial agglomerated point defects in Czochralski
silicon historically has been within the range of about
1.times.10.sup.3/cm.sup.3 to about 1.times.10.sup.7/cm.sup.3,
whereas the density of oxygen clusters varies between around
1.times.10.sup.8/cm.sup.3 to 1.times.10.sup.10/cm.sup.3.
Agglomerated intrinsic point defects are therefore of rapidly
increasing importance to device manufacturers, because such defects
can severely impact the yield potential of the single crystal
silicon material in the production of complex and highly integrated
circuits.
[0017] In view of the foregoing, in many applications it is
preferred that a portion or all of the silicon crystal, which is
subsequently sliced into silicon wafers, be substantially free of
these agglomerated intrinsic point defects. To-date, several
approaches for growing substantially defect-free silicon crystals
have been reported. Generally speaking, all these approaches
involve controlling the ratio v/G, in order to determine the
initial type and concentration of intrinsic point defects present
in the growing CZ single crystal silicon crystal. Additionally,
however, such approaches may involve controlling the subsequent
thermal history of the crystal to allow for prolonged diffusion
time to suppress the concentration of intrinsic point defects
therein, and thus substantially limit or avoid the formation of
agglomerated intrinsic point defects in a portion or all of the
crystal. (See, for example, U.S. Pat. Nos.: 6,287,380; 6,254,672;
5,919,302; 6,312,516 and 6,328,795; the entire contents of which
are hereby incorporated herein by reference.) Alternatively,
however, such approaches may involve a rapidly cooled silicon (RCS)
growth process, wherein the subsequent thermal history of the
crystal is then controlled to rapidly cool at least a portion of
the crystal through a target nucleation temperature, in order to
control the formation of agglomerated intrinsic point defects in
that portion. One or both of these approaches may also include
allowing at least a portion of the grown crystal to remain above
the nucleation temperature for a prolonged period of time, to
reduce the concentration of intrinsic point defects prior to
rapidly cooling this portion of the crystal through the target
nucleation temperature, thus substantially limiting or avoiding the
formation of agglomerated intrinsic point defects therein. (See,
e.g., U.S. Patent Application Publication No. 2003/0196587, the
entire disclosure of which is incorporated herein by reference.)
Still further, methods have been developed to reduce or eliminate
agglomerated point defects from the center of the ingot to the edge
by the simultaneous control of the cooling rate of the solidified
ingot and the radial variation of the axial temperature gradient in
the vicinity of the interface (G). (See, e.g., U.S. Pat. No.
8,673,248, the entire disclosure of which is incorporated herein by
reference.)
[0018] This section is intended to introduce the reader to various
aspects of art that may be related to various aspects of the
disclosure, which are described and/or claimed below. This
discussion is believed to be helpful in providing the reader with
background information to facilitate a better understanding of the
various aspects of the present disclosure. Accordingly, it should
be understood that these statements are to be read in this light,
and not as admissions of prior art.
SUMMARY
[0019] One aspect of the present disclosure is directed to a method
of preparing a single crystal silicon ingot by the Czochralski
method. The method comprises adding an initial charge of
polycrystalline silicon to a crucible contained within a growth
chamber, wherein the crucible comprises a bottom wall and a
sidewall and further wherein the growth chamber comprises a bottom
heater located next to the bottom wall of the crucible, a side
heater located next to the sidewall, and a reflector; supplying
power to the bottom heater, the side heater, or both the bottom
heater and side heater to thereby heat the crucible comprising the
initial charge of polycrystalline silicon to cause a silicon melt
to form in the crucible, wherein the power supplied to the side
heater is greater than the power supplied to the bottom heater and
further wherein the silicon melt has a free melt elevation level;
contacting a silicon seed crystal with the silicon melt contained
within the crucible; withdrawing the silicon seed crystal from the
silicon melt in a direction perpendicular to the melt elevation
level at an initial pull rate to thereby form a solid neck portion
of the single crystal silicon ingot; withdrawing a solid outwardly
flaring seed-cone adjacent the neck portion of the single crystal
silicon ingot from the silicon melt by modifying the initial pull
rate to thereby achieve an outwardly flaring seed-cone pull rate;
and withdrawing a solid main body of the single crystal silicon
ingot adjacent the outwardly flaring seed-cone from the silicon
melt by modifying the outwardly flaring seed-cone pull rate to
thereby achieve a main body pull rate, wherein the solid main body
of the single crystal silicon ingot has a radial diameter and an
axial length and surface tension arising as the solid main body of
the single crystal silicon ingot is withdrawn from the molten
silicon results in a melt-solid interface located above the free
melt elevation level and further wherein a meniscus comprising
molten silicon is between the melt-solid interface and the free
melt elevation level; wherein a cusp magnetic field is applied to
the silicon melt during growth of the main body of the single
crystal silicon ingot; and wherein a heat flux in an axial
direction between the melt-solid interface and the free melt
elevation level during growth of at least 40% of a total axial
length of the solid main body of the single crystal silicon ingot
has an absolute value of at least about 20,000 W/m.sup.2 over at
least about 85% of the radial length of the solid main body of the
single crystal silicon ingot.
[0020] Various refinements exist of the features noted in relation
to the above-mentioned aspects of the present disclosure. Further
features may also be incorporated in the above-mentioned aspects of
the present disclosure as well. These refinements and additional
features may exist individually or in any combination. For
instance, various features discussed below in relation to any of
the illustrated embodiments of the present disclosure may be
incorporated into any of the above-described aspects of the present
disclosure, alone or in any combination.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIGS. 1A and 1B illustrate melt temperature profile near the
crystal/melt interface for a regular hot zone (FIG. 1A) and a low
power hot zone (FIG. 1B).
[0022] FIGS. 2A and 2B illustrate melt temperature profile near the
meniscus for a regular hot zone (FIG. 2A) and a low power hot zone
(FIG. 2B). Temperatures were obtained after 400 mm growth and after
800 mm growth.
[0023] FIG. 3 is a depiction of a hot zone suitable for carrying
out the method of the present invention.
[0024] FIG. 4 is a series of illustrations depicting movement of
the crucible during growth of an ingot during a batch Czochralski
method.
[0025] FIG. 5 is a simplified depiction of the hot zone
configuration suitable for use in the method of the present
invention.
[0026] FIG. 6 is an illustration depicting the meniscus curve.
[0027] FIGS. 7A and 7B are graphs depicting relative crucible
relative height (HR) protocols during growth of an ingot according
to some embodiments of the method of the present invention.
[0028] FIGS. 8A and 8B are graphs depicting seed rotation rate
protocols during growth of an ingot according to some embodiments
of the method of the present invention.
[0029] FIG. 9 illustrates the location of the magnetic coils
suitable for generating a cusp magnetic field.
[0030] FIGS. 10A, 10B, and 10C illustrate the magnetic field
strength and cusp position during Czochralski crystal growth during
a conventional method (FIG. 10A) and during methods according to
the present invention (FIGS. 10B and 10C).
[0031] FIGS. 11A and 11B depict meniscus temperature profile (FIG.
11A) and heat flux on the melt side (FIG. 11B) according to an
embodiment of the present invention. Temperatures and heat flux
data were obtained after 400 mm growth and after 800 mm growth.
[0032] FIGS. 12A and 12B depict meniscus temperature profile (FIG.
12A) and heat flux on the melt side (FIG. 12B) according to an
embodiment of the present invention. Temperatures and heat flux
data were obtained after 400 mm growth and after 800 mm growth.
[0033] FIGS. 13A and 13B depict meniscus temperature profile (FIG.
13A) and heat flux on the melt side (FIG. 13B) according to an
embodiment of the present invention.
[0034] FIGS. 14A and 14B depict diameter of a single crystal
silicon ingot during main body growth during a conventional low
power hot zone process (FIG. 14A) and during a process according to
an embodiment of the present invention (FIG. 14B).
[0035] Corresponding reference characters indicate corresponding
parts throughout the drawings.
DETAILED DESCRIPTION
[0036] According to the method of the present invention, a single
crystal silicon ingot is grown by the Czochralski (CZ) method under
process conditions suitable to inhibit or prevent ingot distortion
during late body length (after 600 mm) growth in a low power hot
zone type. It has been observed during ingot growth in a low power
hot zone growth chamber, the cross-section of the crystal may alter
from the initial desired circular shape to a star shape during
later growth of the main body of the single crystal silicon ingot.
Disadvantageously, the distortion may cause the camera which
monitors ingot growth conditions to lose track of the growing ingot
and shape of the meniscus during the growth process. According to
some embodiments, suitable growth conditions are selected during
growth of the ingot to suppress the drop in the melt temperatures
near the crystal/melt interface and meniscus, which in turn reduces
the potential of supercooling and thus distortion. That is,
conditions are selected to minimize temperature changes at the free
meh surface level as well as the melt under the melt/crystal
interface during growth of the entire length of the ingot, which in
turn minimizes or eliminates distortion so that cross sections of
the ingot retain the desired circular shape along the entire length
of the ingot.
[0037] The method of the present invention may be applied to
Czochralski pullers with an applied magnetic field, e.g., cusp
magnetic field, and added insulation in the bottom portion of the
hot zone, which is referred to as low power hot zone (LPHZ). The
regular hot zone pullers usually have a requirement of high bottom
heater power during the growth process. The low power hot zone is a
modification of the regular hot zones where the bottom heater power
requirement was reduced by installing additional insulation at the
bottom to prevent escaping of heat from the bottom, which helped in
reducing the power consumption and cost of production.
[0038] Crystal growth in low power hot zones are more prone to have
ingot distortion at late body length (after 600 mm), and this
problem is not commonly observed in regular hot zone pullers in
which the bottom heater is set to high power. According to current
understanding, the distortion at late body length results at least
in part from the melt temperature near the melt/crystal interface.
FIGS. 1A and 1B compare the thermal field in the melt at an early
body length (400 mm) and late body length (800 mm) in an ingot
grown in a regular hot zone (FIG. 1A) and a low power hot zone
(FIG. 1B). The figures illustrate the temperatures in the melt near
the crystal/melt interface. As shown in FIG. 1B, these temperatures
had dropped significantly in the low power hot zones at late body
length, whereas the temperature difference was quite less in the
regular hot zones, as shown in FIG. 1A. For illustration a contour
at T=1690 K is highlighted in each of the thermal field for
comparison in FIGS. 1A and 1B.
[0039] FIGS. 2A (regular hot zone) and 2B (low power hot zone) are
graphs depicting the temperature profiles along the meniscus curve
at early and late body length for both hot zone types. As shown in
FIG. 2A, in regular hot zone, the meniscus end temperature, i.e.,
at the melt free interface, at late body length, i.e. 800 mm, is
around 1694 K (where no distortion was seen at this stage) which is
very close to the early body, i.e., 400 mm meniscus temperature at
the melt free interface of 1694.4K. See FIG. 6 for a depiction of
the meniscus curve. The distance 0 mm in FIG. 2A corresponds to the
solid-melt interface, while the distance 35 mm corresponds to the
melt free interface. These distances may vary depending upon hot
zone design and pull conditions. As depicted in FIG. 2B, during
crystal growth in a low power hot zone, the meniscus temperature at
the melt free interface for 800 mm has dropped to a value of 1689.9
K at 800 mm growth, and has a significant difference from the
temperature of 1691.4 K at 400 mm body length. It can also be
observed from FIGS. 2A and 2B that the gradient of temperature has
also decreased from 400 mm to 800 mm body length at the interface
as well as at the meniscus for low power hot zone. It is possible
that these changes are responsible for increased likelihood of
distortion at 800mm body length in a low power hot zone
configuration.
[0040] According to the method of the present invention, hot zone
conditions are selected to increase the melt side heat flux and
gradient along the meniscus during growth of the full body length
of a single crystal silicon ingot. Suitable process conditions that
are selected according to some embodiments of the present invention
include bottom heater power, relative crucible height, seed
rotation rate, crucible rotation rate, and magnetic field strength
a suitable condition was found. Certain variables, including
relative crucible height (RH), seed rotation rate, bottom heater
power, and magnetic field position were discovered to increase melt
side heat flux and thereby increase the temperatures near the melt
and the temperature gradients along the meniscus. In turn, the
temperature profiles achieved according to the method of the
present invention resulted in ingots grown to have reduced or
eliminated distortion throughout the body length of the ingot.
[0041] FIG. 3 is a depiction of a hot zone suitable for carrying
out the method of the present invention. The hot zone configuration
includes a quartz crucible 10 of suitable diameter for holding a
silicon melt and for pulling an ingot having a diameter of 450 mm
or more. A graphite crucible 20 envelopes and supports the quartz
crucible 10. Other configurations for holding the quartz crucible
10 are within the scope of the method of the present invention,
e.g., a configuration that lacks the graphite crucible 20. The hot
zone configuration includes a side heater 30 located near the
sidewalls of the crucible 10 and optionally a bottom heater 40
located below the crucible 10. The hot zone configuration includes
insulation 50 configured to retain heat within the hot zone. The
low power hot zone according to the present invention includes
additional insulation 50 near the bottom of the crucible 10. The
growing crystal diameter and the shape and height of the meniscus
is monitored by a camera 60 (e.g., a CCD camera) located at the top
window 70. Data obtained from the camera enables feedback to the
side heater 30 and bottom heater 40. During crystal growth, the
power distribution may be adjusted between the heaters to enable
uniformity of the melt/solid interface, i.e., maintain the desired
shape and height of the meniscus. The heat shield or reflector 80
reflects heat flux from the hot part of the furnace including the
heater 30, 40 and crucible 10 to the melt 90. The reflector 80
reduces heat transfer from the hot part to the cold part of the
furnace and thereby maintains a separation between these two
regions of the furnace. The reflector 80 helps control the axial
and radial temperature gradients, which drive the solidification
and crystallization of the molten silicon 90 into the growing ingot
100.
[0042] The Czochralski method begins by loading polycrystalline
silicon into a quartz crucible 10, with reference to FIG. 3. The
solid polysilicon added to the crucible 10 is typically granular
polysilicon, although chunk poly silicon may be used, and it is fed
into the crucible using a polysilicon feeder that is optimized for
use with granular polysilicon. Chunk polysilicon typically has a
size of between 3 and 45 millimeters (e.g., the largest dimension),
and granular polysilicon typically has a size between 400 and 1400
microns. Granular polysilicon has several advantages including
providing for easy and precise control of the feed rate due to the
smaller size. However, the cost of granular polysilicon is
typically higher than that of chunk polysilicon due to the chemical
vapor deposition process or other manufacturing methods used in its
production. Chunk polysilicon has the advantage of being cheaper
and being capable of a higher feed rate given its larger size.
[0043] Generally, the melt 90 from which the ingot 100 is drawn is
formed by loading polycrystalline silicon into a crucible 10 to
form an initial silicon charge. In general, an initial charge is
between about 100 kilograms and about 1000 kilograms, or between
about 100 kilograms and about 800 kilograms, or between about 100
kilograms and about 500 kilograms, of polycrystalline silicon,
which may be granular, chunk, or a combination of granular and
chunk. The mass of the initial charges depends on the desired
crystal diameter and HZ design. In some embodiments, the initial
polycrystalline silicon charge is sufficient to grow one single
crystal silicon ingot, i.e., in a batch method. In general, the
total axial length of the solid main body of the single crystal
silicon ingot is at least about 1100 mm, such as between about 1200
nun and about 1300 mm, such as between about 1200 mm and about 1250
mm. In a continuous Czochralski method, the initial charge does not
reflect the length of crystal, because polycrystalline silicon is
continuously fed during crystal growth. Accordingly, the initial
charge may be smaller, such as between about 100 kg and about 200
kg. If polycrystalline silicon is fed continuously and the chamber
height is tall enough, crystal length can be extended to 2000 mm,
3000 mm, or even 4000mm in length. A variety of sources of
polycrystalline silicon may be used including, for example,
granular polycrystalline silicon produced by thermal decomposition
of silane or a halosilane in a fluidized bed reactor or
polycrystalline silicon produced in a Siemens reactor. Once
polycrystalline silicon is added to the crucible to form a charge,
the charge is heated to a temperature above about the melting
temperature of silicon (e.g., about 1412.degree. C.) to melt the
charge, and thereby form a silicon melt comprising molten silicon.
The silicon melt has an initial volume of molten silicon and has an
initial melt elevation level, and these parameters are determined
by the size of the initial charge. In some embodiments, the
crucible comprising the silicon melt is heated to a temperature of
at least about 1425.degree. C., at least about 1450.degree. C. or
even at least about 1500.degree. C. The initial polycrystalline
silicon charge is heated by supplying power to the bottom heater
40, the side heater 30, or both the bottom heater 40 and side
heater 30. According to some embodiments, the power supplied to the
side heater 30 is greater than the power supplied to the bottom
heater 40. In some embodiments, the bottom heater power 40 is 2 kW
or less, such as 1 kW or less. In some embodiments, the bottom
heater 40 has no power supplied thereto, i.e., the power supplied
is 0 kW, such that the hot zone configuration is a low power hot
zone configuration. The low power hot zone according to the present
invention includes additional insulation 50 near the bottom of the
crucible 10.
[0044] With reference to FIG. 4, once the solid polycrystalline
silicon charge 150 is liquefied to form a silicon melt 120
comprising molten silicon, the silicon seed crystal 160 is lowered
to contact the melt. The silicon seed crystal 160 is then withdrawn
from the melt with silicon being attached thereto to thereby
forming a melt-solid interface near or at the surface of the melt.
Generally, the initial pull speed to form the neck portion is high.
In some embodiments, the silicon seed crystal and neck portion is
withdrawn at a neck portion pull rate of at least about 1.0
mm/minute, such as between about 1.5 mm/minute and about 6
mm/minute, such as between about 3 mm/minute and about 5 mm/minute.
In some embodiments, the silicon seed crystal and the crucible are
rotated in opposite directions, i.e., counter-rotation.
Counter-rotation achieves convection in the silicon melt. Rotation
of crystal is mainly used to provide a symmetric temperature
profile, suppress angular variation of impurities and also to
control crystal melt interface shape. In some embodiments, the
silicon seed crystal is rotated at a rate of between about 5 rpm
and about 30 rpm, or between about 5 rpm and about 20 rpm, or
between about 5 rpm and about 15 rpm, such as about 8 rpm, 9 rpm,
or 10 rpm. In some embodiments, the seed crystal rotation rate may
change during growth of the main body of the single crystal silicon
ingot. In some embodiments, the crucible is rotated at a rate
between about 0.5 rpm and about 10 rpm, or between about 1 rpm and
about 10 rpm, or between about 4 rpm and about 10 rpm, or between
about 5 rpm and about 10 rpm. In some embodiments, the seed crystal
is rotated at a faster rate than the crucible. In some embodiments,
the seed crystal is rotated at a rate that is at least 1 rpm higher
than the rotation rate of the crucible, such as at least about 3
rpm higher, or at least about 5 rpm higher. In general, the neck
portion has a length between about 300 millimeters and about 700
millimeters, such as between about 450 millimeters and about 550
millimeters. However, the length of the neck portion may vary
outside these ranges.
[0045] After formation of the neck, the outwardly flaring seed-cone
portion 170 adjacent the neck is grown, with reference to FIG. 4.
In general, the pull rate is decreased from the neck portion pull
rate to a rate suitable for growing the outwardly flaring seed-cone
portion. For example, the seed-cone pull rate during growth of the
outwardly flaring seed-cone 170 is between about 0.5 mm/min and
about 2.0 mm/min, such as about 1.0 mm/min. In some embodiments,
the outwardly flaring seed-cone 170 has a length between about 100
millimeters and about 400 millimeters, such as between about 150
millimeters and about 250 millimeters. The length of the outwardly
flaring seed-cone 170 may vary outside these ranges. In some
embodiments, the outwardly flaring seed-cone 170 is grown to a
terminal diameter of about 150 mm, at least about 150 millimeters,
about 200 mm, at least about 200 millimeters, about 300 mm, at
least about 300 mm, about 450 mm, or even at least about 450 mm.
The terminal diameter of the outwardly flaring seed-cone 170 is
generally equivalent to the diameter of the constant diameter of
the main ingot body 180 of the single crystal silicon ingot.
[0046] After formation of the neck and the outwardly flaring
seed-cone 170 adjacent the neck portion, the main ingot body 180
having a constant diameter adjacent the outwardly flaring seed-cone
170 is then grown. The constant diameter portion of the main ingot
body 180 has a circumferential edge, a central axis that is
parallel to the circumferential edge, and a radius that extends
from the central axis to the circumferential edge. The central axis
also passes through the cone portion and neck. The diameter of the
main ingot body 180 may vary and, in some embodiments, the diameter
may be about 150 mm, at least about 150 millimeters, about 200 mm,
at least about 200 millimeters, about 300 mm, at least about 300
mm, about 450 mm, or even at least about 450 mm. Stated another
way, the radial length of the solid main ingot body 180 of the
single crystal silicon ingot is about 75 mm, at least about 75
millimeters, about 100 mm, at least about 100 millimeters, about
150 mm, at least about 150 mm, about 225 mm, or even at least about
225 mm. The main ingot body 180 of the single crystal silicon ingot
is eventually grown to be at least about 1000 millimeters long,
such as at least 1200 millimeters long, such as at least 1250
millimeters long, such as at least 1400 millimeters long, such as
at least 1500 millimeters long, or at least 2000 millimeters long,
or at least 2200 millimeters, such as 2200 millimeters, or at least
about 3000 millimeters long, or at least about 4000 millimeters
long. In some preferred embodiments, the total axial length of the
solid main ingot body 180 of the single crystal silicon ingot is at
least about 1100 mm, such as between about 1200 mm and about 1300
mm, such as between about 1200 min and about 1250 mm.
[0047] In some embodiments, the main ingot body 180 may be pulled
according to a pull rate protocol. The pull rate declines from a
relatively high pull rate, to a minimum pull rate, and then rising
to a constant pull rate for a significant portion of growth of the
main body of the single crystal silicon ingot. The initial high
pull rate may be between about 0.5 mm/min and about 2.0 mm/min,
such as about 1.0 mm/min, then decreasing to a pull rate that may
be as low as about 0.4 mm/min or even as low as about 0.3 mm/min,
before increasing to the constant pull rate between about 0.4
mm/min and about 0.8 mm/min, between about 0.4 mm/min and about 0.7
mm/min, or between about 0.4 mm/min and about 0.65 mm/min.
[0048] In a continuous Czochralski method, during growth of the
main ingot body 180 of the single crystal silicon ingot,
polycrystalline silicon, i.e., granular, chunk, or a combination of
granular and chunk, is added to the molten silicon to thereby
achieve a constant volume of molten silicon and constant melt
elevation level. According to the method of the present invention,
maintenance of a substantially constant melt volume during growth
of a substantial portion of the axial length of the main body of
the single crystal silicon ingot enables the achievement of high
ingot quality over a substantial portion of the axial length of the
main body of the single crystal silicon ingot at a constant pull
rate. The constant melt volume regardless of the crystal length
enables maintaining a constant crystal/melt interface and thus
uniform crystal quality over a substantial portion of the main body
of the ingot. Accordingly, in some embodiments, the volume of
molten silicon varies by no more than about 1.0 volume % during
growth of at least about 90% the main body of the single crystal
silicon ingot, or by no more than about 0.5 volume % during growth
of at least about 90% the main body of the single crystal silicon
ingot, or even by no more than about 0.1 volume % during growth of
at least about 90% the main body of the single crystal silicon
ingot. Stated another way, in some embodiments, the melt elevation
level varies by less than about +/-0.5 millimeter during growth of
at least about 90% the main body of the single crystal silicon
ingot.
[0049] In a batch Czochralski method, the initial charge of
polycrystalline silicon is sufficient to grow the entire length of
the ingot. Rather than maintain a constant melt elevation level,
the silicon melt volume declines as the ingot grows. Accordingly,
with Reference to FIG. 4 (inset), the crucible 110 holds molten
silicon 120, from which the crystal ingot 140 is drawn. As
illustrated from FIG. 4 sections a) through j), the initial charge
of solid polycrystalline silicon 150 is melted to form the melt 120
by application of heat from the heater 130. A seed crystal 160 is
brought into contact with the molten silicon 120, and a single
crystal ingot 140 is grown by slow extraction. As can be seen in
the illustration, as the length of the single crystal silicon ingot
140 increases, the volume of molten silicon 120 is depleted,
necessitating the vertical movement of the crucible 110 in the same
direction in which the ingot is pulled.
[0050] Regardless of whether the method is batch or continuous,
growth conditions are selected to achieve an optimized melt side
heat flux, temperature near the melt, and temperature gradients
near the meniscus in order to grow an ingot having reduced or
eliminated distortion along the entire length of the main body of
the ingot. Among these growth conditions is the monitoring of the
relative height of the bottom of the reflector 200 above the
surface of the melt 210, with reference to FIG. 5. The relative
height "HR" can be measured directly as a distance between the
bottom of the reflector 200 and the melt level 210, as shown in
FIG. 3, the vector labeled "HR." If the configuration of the hot
zone does not enable direct measurement, i.e., the camera 220
cannot locate the bottom of the reflector 200, the relative height
"HR" can be measured indirectly with a reference point located, for
example, below the crucible. The distance between the melt level
and the reference point 230 is measured, and the distance between
the bottom of the reflector 200 and the reference point 230, which
is labeled "RZ" is measured. The relative height, "HR", is
calculated by subtracting the distance between the melt level 210
and the reference point 230 from "RZ." In a batch process, the melt
level changes as the ingot is grown. Accordingly, the crucible must
be moved, i.e., raised, in order to maintain a desired "HR."
[0051] According to some embodiments, the "HR" begins with a
relatively high value, such as between about 60 mm to 120 mm, or
between about 70 mm to 100 mm. In some embodiments, the relatively
high value of HR occurs during growth of the neck and crown, i.e.,
outwardly flaring seed cone, of the single crystal silicon ingot.
The HR distance is large initially to allow the camera to capture
the meniscus in the necking and crowning stage. The HR value is
large initially to keep the meniscus in the view window of the
camera tracking it. In some embodiments, the relatively high value
of HR may continue during growth of the initial part of the ingot
body, for example, during growth of the first 200 mm to 400 mm of
the ingot body. In some embodiments, after growth of the neck and
crown, the distance between the bottom of the reflector and the
melt level is rapidly decreased by bringing the crucible close to
the bottom of the reflector. In some embodiments, the distance
between the bottom of the reflector and the melt level may be
closed at a rate of at least -0.05 millimeter per millimeter of
ingot growth, or at least about -0.06 millimeter per millimeter of
ingot growth, such as about -0.065 millimeter per millimeter of
ingot growth, preferably less than about -0.1 millimeter per
millimeter of ingot growth, or less than about -0.08 millimeter per
millimeter of ingot growth. The values are stated as negative since
the relative height is decreasing from a higher value to a lower
value. In some embodiments, the HR is brought to a distance between
40 mm and 50 mm during growth of a significant portion of the main
body of the ingot, such as between 45 mm and 50 mm, or between 45
mm and 48 mm. In some embodiments, the HR is brought to a distance
of 45 mm during growth of a significant portion of the main body of
the ingot. In some embodiments, the HR is brought to a distance of
47 mm during growth of a significant portion of the main body of
the ingot. These HR distances apply to growth of at least about
50%, at least about 60%, at least about 70% of the length of the
main body of the ingot. "HR" profiles according to exemplary
embodiments of the present invention are shown in FIGS. 7A and 7B.
The crystal edge gradient is altered by the HR values according to
the present invention, and the defects profile in the crystal is
controlled by this crystal edge gradient. So basically, the defects
profile decides to what value the HR is ramped to and when should
the ramp be started. For instance, in the early body, the crystal
gets heated a lot from the side and thus is pulled at a low rate
initially to get the desired diameter and then the seed lift is
ramped. Since this seed lift changes with the body length there is
a need to change the temperature gradient also to control the
defects, which in turn is controlled by changing the HR during the
crystal growth. As shown therein, the "HR" is maintained at a
constant low value for most of the growth of the main body of the
ingot, which is rapidly increased as the ingot is completed and
pulled from the remaining melt.
[0052] In some embodiments, the lower HR is combined with a lower
seed rotation rate during growth of a significant length of the
single crystal silicon ingot main body. In some embodiments, the
seed rotation rate may start at an initial high rate. The high seed
rotation rate is suitable for growing the initial portion of the
ingot since the melt temperature and heat flux are generally high
during growth of the first 200 to 600 mm of the length of the main
body. In some embodiments, the initial seed rotation rate may be at
least about 10 rpm, such as at least about 11 rpm, or even at least
about 12 rpm. In some preferred embodiments, the initial seed
rotation rate may be at least about 11 rpm. After growth of an
initial portion of the ingot, the seed rotation rate is decreased.
In some embodiments, the seed rotation rate may be decreased at a
rate from about -0.005 rpm per millimeter of ingot growth to about
-0.020 rpm per millimeter of ingot growth, such as from about
-0.005 rpm per millimeter of ingot growth to about -0.014 rpm per
millimeter of ingot growth. The values are stated as negative since
the seed rotation rate is decreasing from a higher value to a lower
value.
[0053] Decreasing the seed rotation rates within this range may
bring the seed rotation rate to between about 5 rpm and about 10
rpm, such as about 8 rpm, about 9 rpm, or about 10 rpm between
about 600 mm to about 900 mm of ingot body length. In some
preferred embodiments, the seed rotation rate may be about 9 rpm
between about 600 mm to about 900 mm of ingot body length. In some
embodiments, the rotation rates may be decreased further, such as
between about 5 rpm and about 8 rpm, such as about 7 rpm or about 8
rpm, at late body growth, such as after about 1200 mm of growth. In
some preferred embodiments, the rotation rates may be about 7 rpm,
at late body growth, such as after about 1200 mm of growth. In some
preferred embodiments, the rotation rates may be about 8 rpm, at
late body growth, such as after about 1200 mm of growth. Selecting
these seed rotation values results in a higher temperature gradient
in the melt region near the solid-liquid interface and the meniscus
temperatures could also be increased. Very low seed rotation values
can result in degrading the oxygen radial gradient. Seed rotation
rate profiles according to exemplary embodiments of the present
invention are shown in FIGS. 8A and 8B.
[0054] Additionally, according to the process of the present
invention, a magnetic field may be applied to the crucible
comprising the silicon melt. Either cusp or horizontal magnet field
can be applied to set the appropriate crystal/melt interface, i.e.,
the shape and height of the meniscus. The magnetic field is used to
fix a desire crystal/melt interface shape and height primarily, and
control of the oxygen content, Oi, is a subordinate purpose.
[0055] Control of the melt flow and the shape of the melt/solid
interface and therefore the quality of the ingot may be enhanced by
the application of a magnetic field to the silicon melt during
growth of the main body of the single crystal silicon ingot. In
some embodiments, the applied magnetic field maintains a
substantially constant melt/solid interface profile during at least
about 70% of the growth of the main body of the single crystal
silicon ingot, or between about 70% and about 90% of the growth of
the main body of the single crystal silicon ingot. The magnetic
field applies electromagnetic force, which affects the silicon melt
flow, so the heat transfer in the melt is affected. It changes the
profile of crystal/melt interface and the temperature of growing
crystal.
[0056] The magnetic field impacts the oxygen content and uniformity
in the ingot. The source of oxygen is the ingot is from dissolution
of the quartz crucible wall, evaporation SiOx (g) at the melt free
surface (controlled by melt flow kinetics) and incorporation into
growing crystal front. The magnet field impacts the convective melt
flow during growth which can impact Oxygen evaporation and
incorporation. The variation of oxygen incorporation into the
single crystal silicon ingot by time increment is controlled by the
diffusion and convection of oxygen in the melt according to the
following equation:
.differential. C .differential. t = .gradient. C - v .times. .rho.
+ .times. SOURCE . ##EQU00001##
[0057] C is the concentration of oxygen is the solidifying silicon,
t is time, v is the convection velocity (melt flow velocity), rho,
.rho., is the density of silicon melt, .gradient. is the gradient
(d/dx). The applied magnetic field affects the melt velocity (v)
and the gradient of oxygen concentration in the melt
(dC/dx=.gradient.C). Since magnetic field results in a steady state
melt flow, the incorporating of oxygen, Oi, into the ingot is time
constant, which enhances radial and axial oxygen concentration
uniformity. The SOURCE term is derived from two parameters, the
dissolution of quartz (SiO.sub.2) crucible which is the generation
of oxygen (Si (l)+SiO2(s).fwdarw.SiOx(g)), and the evaporation
which is the removal (disappearance) of oxygen (SiOx(g)) from melt.
In a batch Cz process, this SOURCE term is not constant. Instead,
it depends upon the crystal length since the melt mass decreases as
the crystal is grown. When the ingot has grown a substantial
portion of its body length, the remaining melt volume is low, so
that that amount of silicon melt in contact with the crucible is
decreased, which therefore leads to lower concentrations of oxygen
incorporated from the crucible into the melt. Therefore, the oxygen
incorporated into solidifying silicon crystal is decreased, if
other terms (diffusion, convection, evaporation) are constant. The
melt free surface (contact surface between melt and gas) area
affects the evaporation rate of SiOx(g). Less evaporation of
SiOx(g) means more oxygen incorporation into the solidifying
silicon crystal. According to the method of the present invention,
the melt mass is maintained as constant since polysilicon is added
as the crystal ingot grows. Accordingly, all source terms
(generation of Oxygen by SiO.sub.2 crucible dissolution into melt,
and evaporation of SiOx(g) gas through melt free surface) are
constant. Therefore, the diffusion and convection terms affect the
oxygen of solidifying silicon crystal. The applied magnetic field
makes melt flow more stable (i.e., melt flow is constant like as
time independent steady condition), so incorporating Oxygen is
uniform and stable in the axial and radial directions during growth
of the entire length of the ingot. In some embodiments,
interstitial oxygen may be incorporated into the ingot in a
concentration between about 4 PPMA and about 18 PPMA. In some
embodiments, interstitial oxygen may be incorporated into the ingot
in a concentration between about 10 PPMA and about 35 PPMA. In some
embodiments, the ingot comprises oxygen in a concentration of no
greater than about 15 PPMA, or no greater than about 10 PPMA.
Interstitial oxygen may be measured according to SEMI MF
1188-1105.
[0058] In some embodiments, a horizontal magnetic field is applied
to the silicon melt during growth of the main body of the single
crystal silicon ingot. Crystal growth in presence of a horizontal
magnetic field is achieved by placing the crucible holding the
silicon melt between the poles of a conventional electromagnet. In
some embodiments, the horizontal magnetic field may have a magnetic
flux density between about 0.2 Tesla and about 0.4 Tesla in the
melt area. Magnetic field variation in the melt is less than +/-
about 0.03 Tesla in a given strength. Application of a horizontal
magnetic field gives rise to Lorentz force along axial direction,
in a direction opposite of fluid motion, opposing forces driving
melt convection. The convection in the melt is thus suppressed, and
the axial temperature gradient in the crystal near the interface
increases. The melt-crystal interface then moves upward to the
crystal side to accommodate the increased axial temperature
gradient in the crystal near the interface and the contribution
from the melt convection in the crucible decreases.
[0059] In some embodiments, a cusp magnetic field is applied to the
silicon melt during growth of the main body of the single crystal
silicon ingot. Magnetic coil locations suitable for achieving a
cusp magnetic field is illustrated in FIG. 9. A cusp magnetic field
has two controlling parameters, namely the magnetic flux density
and magnetic field shape. A cusp magnetic field applies a
horizontal (radial) magnetic field component at the nearest surface
of the melt combined with a vertical (axial) magnetic field deeper
in the melt near the axis of the ingot. The cusp magnetic field is
generated using a pair of Helmholtz coils 300, 310 carrying current
in opposite direction. As a result, at the position halfway between
the two magnetic fields, vertically along the ingot axis, the
magnetic fields cancel each other out to make a vertical magnetic
field component at or near zero. For example, the cusp magnetic
flux density is typically about zero to about 0.2 Tesla in the
axial direction. The magnetic flux density in the radial direction
is generally higher than the magnetic flux density in the vertical
direction. For example, the cusp magnetic flux density is typically
between about 0 and about 0.6 T in the radial position, such as
between about 0.2 and about 0.5 T, dependent upon the radial
position. The radial cusp magnetic field restrains the flow of the
melt, to thereby stabilize the melt. In other words, application of
a radial cusp magnetic field induces convection at a portion
adjacent to the solid-liquid interface at which crystal growth
occurs, and suppresses convection at the remaining portions of the
melt, to thereby serve as an effective method for realizing uniform
oxygen distribution. Thermal melt convection can be locally and
independently controlled by the cusp magnetic field at the melt
free surface and at the melt crucible interface at the same time.
This enables to control the oxygen concentration in the growing
crystal by magnetic flux density only, irrespective of crystal
rotation speed. In presence of an axial or a radial magnetic field,
control of oxygen concentration is achieved via control of crystal
rotation speed. Application of the cusp magnetic field may enable
growth on an ingot comprising less oxygen content than an ingot
grown without an applied magnetic field, such as no greater than
about 15 PPMA, or no greater than about 10 PPMA. Interstitial
oxygen may be measured according to SEMI MF 1188-1105.
[0060] According to the method of the present invention, the cusp
magnetic field applied to the silicon melt during growth of the
main body of the single crystal silicon ingot is derived from an
upper magnetic coil 300 and a lower magnetic coil 310, wherein the
upper magnetic field strength derived from the upper magnetic coil
300 is greater than a lower magnetic field strength derived from
the lower magnetic coil 310. In some embodiments, the upper
magnetic field strength derived from the upper magnetic coil 300
exceeds a lower magnetic field strength derived from the lower
magnetic coil 310 by at least 10%, or at least 15%. By applying a
greater magnetic field strength to the upper magnetic coil compared
to the lower magnetic coil, the cusp position may be moved lower
into the melt, as illustrated in FIGS. 10B and 10C, as compared to
a conventional method as illustrated in FIG. 10A. Lowering the cusp
position has been found to increase the temperature in the meniscus
region.
[0061] According to the method of the present invention, the
conditions disclosed herein minimize the temperature changes at the
free melt surface level as well as the melt under the melt/crystal
interface in the late body as no distortion is seen in the early
body. The conditions according to the method of the present
invention help in preventing the melt temperatures near the
crystal/melt interface and meniscus to drop which in turn reduces
the potential of supercooling and thus distortion. Advantageous
factors that achieve the desired outcome are the melt side heat
flux, the temperature near the melt, and the temperature gradients
near the meniscus which could be increased by selecting, HR, seed
rotation, and magnetic cusp conditions.
[0062] Melt side heat flux is determined according to the following
equation:
Q melt = k m * ( dT d .times. x ) m ##EQU00002##
[0063] where k.sub.m is the thermal conductivity of the melt
and
( dT dx ) m ##EQU00003##
is the axial temperature gradient. The heat flux depends on the
axial temperature gradient in the melt. Thus, a greater heat flux
results in a larger axial temperature gradient which means higher
temperatures in the melt under the melt/crystal interface and
higher temperature gradients near the meniscus which results in
reduced ingot distortion along the axial length of the ingot.
According to some embodiments, a heat flux in an axial direction
between the melt-solid interface and the free melt elevation level
during growth of at least 40% of a total axial length of the solid
main body of the single crystal silicon ingot has an absolute value
of at least about 20,000 W/m.sup.2, at least about 21,000
W/m.sup.2, at least about 22,000 W/m.sup.2, at least about 23,000
W/m.sup.2, or at least about 24,000 W/m.sup.2 over at least about
85% of the radial length of the solid main body of the single
crystal silicon ingot. According to some embodiments, the heat flux
in the axial direction between the melt-solid interface and the
free melt elevation level during growth of at least 60% of the
axial length of the solid main body of the single crystal silicon
ingot has an absolute value of at least about 20,000 W/m.sup.2, at
least about 21,000 W/m.sup.2, at least about 22,000 W/m.sup.2, at
least about 23,000 W/m.sup.2, or at least about 24,000 W/m.sup.2
over at least about 85% of the radial length of the solid main body
of the single crystal silicon ingot. According to some embodiments,
the heat flux in the axial direction between the melt-solid
interface and the free melt elevation level during growth of at
least 80% of the axial length of the solid main body of the single
crystal silicon ingot has an absolute value of at least about
20,000 W/m.sup.2, at least about 21,000 W/m.sup.2, at least about
22,000 W/m.sup.2, at least about 23,000 W/m.sup.2, or at least
about 24,000 W/m.sup.2 over at least about 80% of the diameter of
the solid main body of the single crystal silicon ingot. According
to some embodiments, the heat flux in the axial direction between
the melt-solid interface and the free melt elevation level during
growth of at least 90% of a total length of the solid main. body of
the single crystal silicon ingot has an absolute value of at least
about 20,000 W/m.sup.2, at least about 21,000 W/m.sup.2, at least
about 22,000 W/m.sup.2, at least about 23,000 W/m.sup.2, or at
least about 24,000 W/m.sup.2 over at least about 85% of the radial
length of the solid main body of the single crystal silicon ingot.
According to some embodiments, the heat flux in the axial direction
between the melt-solid interface and the free melt elevation level
during growth of at least 95% of a total length of the solid main
body of the single crystal silicon ingot has an absolute value of
at least about 20,000 W/m.sup.2, at least about 21,000 W/m.sup.2,
at least about 22,000 W/m.sup.2, at least about 23,000 W/m.sup.2,
or at least about 24,000 W/m.sup.2 over at least about 85% of the
radial length of the solid main body of the single crystal silicon
ingot.
[0064] Still further, conditions are selected to achieve a
temperature gradient along the meniscus curve. See FIG. 6.
According to some embodiments, a temperature gradient along a
meniscus curve between the melt-solid interface and the free melt
elevation level has an average value of at least about
0.16.degree./mm or at least about 0.18.degree./mm during growth of
at least 40% of a total length of the solid main body of the single
crystal silicon ingot. According to some embodiments, a temperature
gradient along a meniscus curve between the melt-solid interface
and the free melt elevation level has an average value of at least
about 0.16.degree./mm or at least about 0.18.degree./mm during
growth of at least 60% of a total length of the solid main body of
the single crystal silicon ingot. According to some embodiments, a
temperature gradient along a meniscus curve between the melt-solid
interface and the free melt elevation level has an average value of
at least about 0.16.degree./mm or at least about 0.8.degree./mm
during growth of at least 80% of a total length of the solid main
body of the single crystal silicon ingot. According to some
embodiments, a temperature gradient along a meniscus curve between
the melt-solid interface and the free melt elevation level has an
average value of at least about 0.16.degree./mm or at least about
0.18.degree./mm during growth of at least 90% of a total length of
the solid main body of the single crystal silicon ingot.
[0065] By achieving these melt flux and temperature gradients, the
temperature of the molten silicon at the meniscus is higher than
can be conventionally achieved in a low power hot zone. According
to some embodiments, a temperature of the molten silicon in the
meniscus is at leak 1691K, or at least 1692K, as measured at the
free melt elevation level during growth of at least 40% of a total
length of the solid main body of the single crystal silicon ingot.
According to some embodiments, a temperature of the molten silicon
in the meniscus is at least 1691K, or at least 1692K, as measured
at the free melt elevation level during growth of at least 60% of a
total length of the solid main body of the single crystal silicon
ingot. According to some embodiments, a temperature of the molten
silicon in the meniscus is at leak 1691K, or at least 1692K,
measured at the free melt elevation level during growth of at least
80% of a total length of the solid main body of the single crystal
silicon ingot. According to some embodiments, a temperature of the
molten silicon in the meniscus is at least 1691K, or at least
1692K, as measured at the free melt elevation level during growth
of at least 85% of a total length of the solid main body of the
single crystal silicon ingot. According to some embodiments, a
temperature of the molten silicon in the meniscus is at least
1691K, or at least 1692K, as measured at the free melt elevation
level during growth of at least 90% of a total length of the solid
main body of the single crystal silicon ingot.
[0066] In some embodiments, process conditions suitable for
achieving the meniscus temperature profile and melt flux on the
melt side are depicted in FIGS. 11A and 11B include bottom heater
power (BH)=0 kW, seed rotation rate (SR)=9 rpm, and relative height
(HR)=47 mm (Test Condition 1). The heat flux (Qmelt) value is the
measurement of heat flux into the melt across the crystallization
front. The negative values. signify the heat movement from the melt
to the crystal across the melt/solid interface. As shown in FIG.
11A, the meniscus temperature is at least 1692K along the meniscus
curve at both 400 mm and 800 mm axial growth. Moreover, the heat
flux has an absolute value greater than 20,000 W/m.sup.2 over the
radial length of the interface at both 400 mm and 800 mm axial
growth, as shown in FIG. 11B.
[0067] In some embodiments, process conditions suitable for
achieving the meniscus temperature profile and melt flux on the
melt side are depicted in FIGS. 12A and 12B include bottom heater
power (BH)=0 kW, seed rotation rate (SR)=9 rpm, and relative height
(HR)=45 mm (Test Condition 1). The heat flux (Qmelt) value is the
measurement of heat flux into the melt across the crystallization
front. The negative values signify the heat movement from the melt
to the crystal across the melt/solid interface. As shown in FIG.
12A, the meniscus temperature is at least 1692K along the meniscus
curve at both 400 mm and 800 mm axial growth. Moreover, the heat
flux has an absolute value greater than 20,000 W/m.sup.2 over the
radial length of the interface at both 400 mm and 800 mm axial
growth, as shown in FIG. 12B.
[0068] In some embodiments, it was found that when the magnetic
cusp position is lowered, the temperature in the meniscus region
increases. To lower the magnetic cusp position, a gap of 15%
between the upper and the lower magnet, with the upper magnet at a
higher strength. A comparison between the temperature profile in
the meniscus region and the heat flux on the melt side was done as
shown below in FIGS. 13A and 13B. The lower temperatures and
gradient shown in FIG. 13A pertains to conventional low power hot
zone (conventional LPHZ) with a magnetic cusp position exemplified
in FIG. 10A. The higher temperatures and gradient shown in FIG. 13A
pertains to the hot zones according to embodiments of the present
invention (Test Condition 1 and Test Condition 2) with cusp
positions exemplified in FIGS. 10B and 10C. Accordingly, cusp
magnetic field position, along with seed rotation rate and relative
height, may be combined to increase the meniscus temperature
profile.
[0069] The process of the present invention enables growth of
single crystal silicon ingots in which the deviation from the
setpoint diameters is minimized. Minimizing/reducing distortion
results in an actual diameter profile for the grown crystal close
to the set point values. With reference to FIG. 14A, conventional
low power hot zone processes may result in significant deviation
from the desired crystal diameter. The set point diameter is
represented by the solid line, and FIG. 14A demonstrates that
actual diameters may deviate significantly from the set point
diameter. With reference to FIG. 14B, some vibration and deviation
may still occur, however, the process of the present invention
enables the growth of crystals in which the diameters values remain
close to the set point values. In view thereof, the process of the
present invention results in crystals having diameters that vary
little around the set point values, thereby reducing ingot
distortion over the entire length of the main body of the
ingot.
[0070] As used herein, the terms "about," "substantially,"
"essentially" and "approximately" when used in conjunction with
ranges of dimensions, concentrations, temperatures or other
physical or chemical properties or characteristics is meant to
cover variations that may exist in the upper and/or lower limits of
the ranges of the properties or characteristics, including, for
example, variations resulting from rounding, measurement
methodology or other statistical variation.
[0071] When introducing elements of the present disclosure or the
embodiment(s) thereof, the articles "a", "an", "the" and "said" are
intended to mean that there are one or more of the elements. The
terms "comprising," "including," "containing" and "having" are
intended to be inclusive and mean that there may be additional
elements other than the listed elements. The use of terms
indicating a particular orientation (e.g., "top", "bottom", "side",
etc.) is for convenience of description and does not require any
particular orientation of the item described.
[0072] As various changes could be made in the above constructions
and methods without departing from the scope of the disclosure, it
is intended that all matter contained in the above description and
shown in the accompanying drawing[s] shall be interpreted as
illustrative and not in a limiting sense.
* * * * *