Controlling agglomerated point defect and oxygen cluster formation induced by the lateral surface of a silicon single crystal during CZ growth
View Patent ↗Processes for preparing a single crystal silicon ingot are disclosed. In certain embodiments, the processes involve controlling (1) a growth velocity, v, of the ingot as well as (2) an average axial temperature gradient, G, a corrected average axial temperature gradient, G corrected , or an effective average axial temperature gradient, G effective , during the growth of at least a segment of the constant diameter portion of the ingot.
1. A process for growing a single crystal silicon ingot from a silicon melt wherein the melt/solid interface between the ingot and the silicon melt is not flat, the ingot comprising a central axis, a seed-cone, an end opposite the seed-cone, and a constant diameter portion between the seed-cone and the opposite end, said constant diameter portion having a lateral surface, a radius (R) extending from the central axis to the lateral surface, and a nominal diameter of at least about 150 mm, the ingot being grown in accordance with the Czochralski method, the process comprising: controlling (i) a growth velocity, v, and (ii) an effective average axial temperature gradient, G effective , during the growth of at least a segment of the constant diameter portion of the ingot over the temperature range from solidification to a temperature of about 1200° C., wherein G effective is defined by the following equation:
G
effectve
=
[
G
corrected
v
]
x
,
flat
×
v
x
,
iface
and G corrected is defined by the following equation:
1
T
corrected
=
(
1
/
T
m
)
+
(
1
/
T
m
2
)
zG
corrected
,
such
that
∑
f
(
T
-
T
corrected
)
=
0
wherein: G effective represents a revision to G corrected , revised to account for the deviation in the interface shape from a flat profile; T is the temperature of at any fixed radial location, r, within the segment; T m is the temperature at the melt/solid interface; z is the axial distance from the interface at the given radial location; the function f denotes an acceptable statistical agreement between T and T corrected ; the subscript x denotes the critical value at which the predominate intrinsic point defect transitions between vacancy dominated and interstitial dominated material, such that [G corrected /v] x,flat is the ratio of G corrected /v at which the material transitions from interstitial dominated to vacancy dominated for a flat melt-solid interface and v x,iface is the critical value of v wherein G effective /v x,iface is the ratio at which the material transitions from interstitial dominated to vacancy dominated for the non-flat melt-solid interface; flat denotes a flat interface; iface denotes any non-flat interface; such that the ratio of v/G effective , at a given axial position within said segment, varies radially by less than about ±30%, relative to the critical value of v/G effective ; and, cooling said segment from the solidification temperature to about 750° C.
2. The process of claim 1 , wherein during said cooling of the segment from the solidification temperature to at least about 750° C., the cooling rate of said segment is controlled (i) from the solidification temperature to a temperature of at least about 1250° C., such that said segment cools at an average rate of at least 2.5° C./minute, and (ii) between a temperature of less than about 1250° C. and about 1000° C., such that said segment cools at an average rate of between about 0.3 and about 0.025° C./minute.
3. The process of claim 1 , wherein said constant diameter portion of said ingot has a nominal diameter of about 200 mm.
4. The process of claim 2 wherein, upon cooling, said segment comprises an axially symmetric region which has (i) a measurable radial width that is less than or equal to about 1 R, and (ii) a length, as measured along the central axis, of at least about 10% of the length of the constant diameter portion of the ingot.
5. The process of claim 4 , wherein the axially symmetric region is substantially free of agglomerated intrinsic point defects.
6. The process of claim 4 , wherein silicon lattice vacancies are the predominant intrinsic point defect in the axially symmetric region.
7. The process of claim 6 , wherein the axially symmetric region additionally contains detectable agglomerated vacancy defects and/or oxygen clusters therein, the agglomerated vacancy defects having an average radius of less than about 30 nm and the oxygen clusters having an average radius of less than about 10 nm.
8. The process of claim 4 , wherein silicon self-interstitials are the predominant intrinsic point defect in the axially symmetric region.
9. The process of claim 8 , wherein the segment additionally contains B-defects.
10. The process of claim 4 , wherein the axially symmetric region extends from the central axis of the ingot radially outward toward the lateral surface for a radial width that is less than about 0.95 R.
11. The process of claim 4 , wherein the axially symmetric region extends from the central axis of the ingot radially outward toward the lateral surface.
12. The process of claim 4 , wherein the axially symmetric region forms an annular ring located between the central axis and the lateral surface.
13. The process of claim 4 , wherein the segment further comprises a first annular ring extending radially inward from about the lateral surface toward the axially symmetric region, wherein said first annular ring comprises silicon lattice vacancies as the predominant intrinsic point defect.
14. The process of claim 13 , wherein the first annular ring further comprises agglomerated vacancy defects having an average radius of less than about 30 nm and/or oxygen clusters having an average radius of less than about 10 nm.
15. The process of claim 13 , wherein the axially symmetric region forms a second annular ring that surrounds an axially symmetric core that contains silicon lattice vacancies as the predominant intrinsic point defect and, optionally, voids having an average radius of less than about 30 nm and/or oxygen clusters having an average radius of less than about 10 nm.
16. The process of claim 13 , wherein the axially symmetric region forms a second annular ring that surrounds an axially symmetric core that contains silicon self-interstitials as the predominant intrinsic point defect, and optionally B-defects.
17. The process of claim 13 , wherein the first annular ring has a radial width, as measured from about the lateral surface radially toward the central axis, of less than about 0.25 R.
18. The process of claim 1 , wherein said control of the cooling rate from the solidification temperature to a temperature of at least about 1250° C. is such that said segment cools at a rate between about 2.5° C./minute and about 3.5° C./minute.
19. The process of claim 2 , wherein said process further comprises controlling the cooling rate of said segment between about 1000° C. and about 750° C., such that said segment cools at a rate of at least about 0.25° C./minute.
20. The process of claim 2 , wherein the cooling rate of said segment is controlled (i) from the solidification temperature to a temperature of at least about 1250° C., such that said segment cools at a rate of at least 2.5° C./minute, and (ii) between a temperature of less than about 1250° C. and about 1000° C., such that said segment cools at a rate of between about 0.3 and about 0.025° C./minute.