A silicon dendrite is grown as a ribbon forming two silicon crystal layers which are separated by an interface layer which contains a large number of defects. Significant increase of minority carrier lifetime with homogeneous distribution at the outer surfaces of the two silicon crystal layers is achieved by processing the web in an atmosphere of a selected gas, e.g., oxygen, nitrogen or an inert gas, for about 30 minutes to several hours at a temperature preferably on the order of 900 to 1200 C

Koliwad, K. M.

Liu, J. K.

Schwuttke, G. H.

English

NASA Technical Reports Server

United States Patent 1191 
Liu et al. 
[111 4,401,505 
[45] Aug. 30, 1983 
[54] METHOD OF INCREASING MINORITY 
CARRIER LIFETIME IN SILICON WEB OR FOREIGN PATENT DOCUMENTS 
THE LIKE 1816083 6/1970 Fed. Rep. of Germany ... 156/DIG. 
66 
[75] Inventors: James K. Liu, Glendale, Calif.; OTHER PUBLICATIONS 
Biedermann; “Producing Silicon Semiconductor Wa- 
fers . . .  ”; IBM Tech. Bull.; vol. 19; No. 4; p. 1295; Sep. 
1976. 
Guenter H. Schwuttke, 
Poughkeepsie, N.Y.; Krishna M. 
Koliwad, La Canada, Calif. 
[73] Assignee: The United States of America as 
represented by the Administrator 
National Aeronautics and Space 
Administration, Washington, D.C. 
[21] Appl. NO.: 364,092 
[22] Filed: Mar. 31, 1982 
[51] 
[52] 
[58] 
Int. Cl.3 ...................... C30B 15/34; C30B 29/06; 
C30B 33/00 
U.S. C1. ........................... 156/608; 156/DIG. 66; 
156/DIG. 73 
Field of Search ............... 156/605, 608, DIG. 64, 
156/DIG. 66, DIG. 73, DIG. 88, DIG. 89 
[561 References Cited 
U.S. PATENT DOCUMENTS 
3,856,472 12/1974 Schweitzer et al. ....... 156/DIG. 66 
4,018,626 4/1977 Schwuttke et al. .................. 148/1.5 
4,165,249 8/1979 Heywang .................... 156/DIG. 66 
4,220,483 9/1980 Cazcarra ..................... 156/DIG. 66 
4,257,827 3/1981 Schwuttke et al. .................. 148A.5 
4,276,114 6/1981 Takano et al. .............. 156/DIG. 66 
“Lifetime Control in Silicon through Impact Sound 
Stressing” by Schwuttke et al.; Phys. Stat. Sol.; 42; p. 
553; Apr. 1977. 
“Minority Carrier Lifetime Improvement in Silicon 
through Laser Damage Gettering” by Yang et al.; Phys. 
Stat. Sol.; 58; p. 127; Oct. 1979. 
“Dislocation Reactions in Silicon Web-Dendrite Crys- 
tals” by S. O’Hara et al.; J. Appl. Phys.; vol. 36; No. 8; 
p. 2475; Aug. 1965. 
Primary Examiner- Bradle y Garris 
Attorney, Agent, or Firm-John R. Manning; Paul F. 
McCaul; Thomas H. Jones 
[571 ABSTRACT 
A silicon dendrite is grown as a ribbon forming two 
silicon crystal layers which are separated by an inter- 
face layer which contains a large number of defects. 
Significant increase of minority carrier lifetime with 
homogeneous distribution at the outer surfaces of the 
two silicon crystal layers are achieved by processing the 
web in an atmosphere of a selected gas, e.g. oxygen, 
nitrogen or an inert gas, for about 30 minutes to several 
hours at a temperature preferably on the order of 900” 
C.-1200” c. 
10 Claims, 2 Drawing Figures 
26 20 25 I 
https://ntrs.nasa.gov/search.jsp?R=19830027617 2020-03-22T01:12:15+00:00Z
U.S. Patent Aug. 30, 1983 4,401,505 
FIG. I 
PRJOR ART 
12 
10 t 
14 
-7- 
c 
FIG. 2 
21 
I 26 20 25 
4,401,505 
1 2 
viewed as consisting of the two single crystal silicon 
layers, separted by an interface layer, hereafter referred 
to as the twin plane. The twin plane contains a large 
number of defects as the result of the web’s growth 
5 process. It has been discovered that by annealing a 
Silicon dendrite web at a selected temperature range for 
a selected period in a selected environment, Very Signifi- 
cant increases in lifetime at Or near the outer surfaces of 
the web are realized. 
The novel features of the invention are set forth with 
METHOD OF INCREASING MINORITY CARRIER 
LIFETIME IN SILICON WEB OR THE LIKE 
ORIGIN O F  THE INVENTION 
The invention described herein was made in the per- 
formance of work under a NASA contract and is sub- 
ject to the provisions of Section 305 of the National 
Aeronautics and Space Act of 1958, Public Law 85-568 
(72 Stat. 435: 42 USC 2457). 10 
particularity in the appended claims. The invention will 
be best understood from the following description when 
read in conjunction with the accompanying drawings. 
BACKGROUND OF THE INVENTION 
1. Field of the Invention 
BRIEF DESCRIPTION OF THE DRAWINGS 
FIG. 1 is a cross sectional view of a silicon crystal 
FIG. 2 is a cross sectional view of a silicon dendrite 
The present invention is directed to a method of 
improving semiconductor matter and, more particu- * 
larly, to a method of engineering minority carrier life- 
time in silicon material as needed for in the fabrication 
of high density Very Large Scale Integration (VLSI) 
semiconductor devices and high performance solar 
cells. 20 
useful in explaining the prior art methods; and 
web, useful in explaining the present invention. 
2. Description of the Prior Art DETAILED DESCRIPTION OF THE 
Increasing the lifetime of minority carriers in Silicon DRAWINGS 
material used for fabricating semiconductor VLSI de- In order to highlight the advantages of the present 
vices such as high density memories Or high Perfor- invention attention is first directed to FIG. 1 wherein a 
mance solar cells is of paramount importance. As is also 25 single crystal 10, such as silicon, is shown in cross-sec- 
appreciated, increased minority carrier lifetime, hereaf- tional view. The crystal 10 of a thickness which is gen- 
ter also referred to as increased lifetime, is achievable erally on the order of6-10 mils has a top side or surface 
by gathering impurities in the semiconductor material 12, upon which devices are fabricated and a bottom side 
to defective points in such material. This process is 14. It is at or near surface 12 whereat increased lifetime called ‘gettering’. To  improve the material, it is neces- 30 of minority carriers is desired by removing impurities 
the material where the active circuits are fabricated. As at the bottom surface 14 as indicated by small x’s desig- used in the art and herein, impurities refer to foreign nated by numeral 15. atoms or bodies in a layer of material, while defects 
relate to distortion of the crystaline lattice symmetry of 35 The many disadvantages Of the proposed prior art 
methods have been discussed hereabove. These include: the material. 
time in a surface layer can be achieved by purposely to the surface 12, thus destroying the usefulness of the 
damaging the bottom side of a semiconductor material, crystal; (b) irreproducible results between batches of 
material. Although it is claimed that increased lifetime batch; (c) highly nonuniform increased lifetime at or 
is achieved, the need to purposely damage the crystal is near the top surface; and (d) most of the impurity getter- 
highly undesirable and the resulting increased lifetime is ing Occurs near the bottom side 14 where the damage is 
limited for the following reasons. produced. 
Since the Crystal iS relatively thick, e.g. 6-10 mils, in 
one wafer, cannot be duplicated in other wafers. Thus, order to effect the minority carrier lifetime at or near 
the increased lifetime is not reproducible. Also, due to the surface 12, relatively extensive damage has to be 
the thickness of the wafer, or crystal, relatively exten- Produced. A danger however is Present that the rela- 
sive damage must be induced. Quite often, the damage, tively large damage may itself extend to Or near surface 
which is intentionally produced at the wafer’s bottom 50 12, thereby destroying the usefulness of the crystal for 
side, extends to the top surface or close to it, thereby any Purpose. The extension ofthe damage to ~ r f a c e  12 
destroying the usefulness of the gettering process. Fur- may Occur when Producing the damage Or during sub- 
thermore, the damage in the crystal is typically not sequent processing. 
uniform and therefore, the increased lifetime at the Unlike the prior art, the method of the present inven- 
surface is highly irregular on a microscopic scale which 55 tion is directed to increasing the lifetime of minority 
is most undesirable. carriers in a dendrite web or ribbon, shown in cross 
sary to getter the impurities away from the surface Of therefrom. In the prior art, physical damage is induced 
Heretofore, it has been suggested that increased life- (a) physical damage to the crysta1 which Often extends 
such as silicon, and subsequently annealing the silicon 40 damaged crystals as well as beh‘een crystals in the Same 
The extent and distribution of damage, introduced in 45 
sectional view in FIG. 2 and designated by numeral 20. 
Basically, such a silicon ribbon consists of two opposite 
layers of silicon 24 and 26 with an interface layer 25. It 
OBJECTS AND SUMMARY OF THE 
INVENTION 
It is a primary object of the present invention to pro- 60 
vide a new improved method of producing homoge- 
nous minority carrier lifetime distributions in a silicon 
wafer. 
The above and other objects of the invention are 
achieved in connection with a silicon dendrite web. The 65 
process of forming a silicon dendrite web, also referred 
to as a ribbon is well known. Ignoring the raised edges, 
which characterize a silicon dendrite web, it can be 
is layer 25 also referred to as the defect plane, or twin 
plane which effectively separates the ribbon into the 
two layers 24 and 26. The width of the web or ribbon 20 
which is of uniform thickness t is designated by w. 
Layer 25 contains defects as a result of the web growth 
process. It is generally in the middle of the ribbon’s 
thickness. Thus, the thickness of each of layers 24 and 
26 is on the order of i t  or less, which is about 3-5 mils 
or less. It has been discovered that since the web 20, as 
3 
4,401,505 
a result of its growth in a furnace, contains the defect- 
containing layer 25 in non-equilibrium, such a web can 
be processed very effectively to produce dramatically 
enhanced lifetime of minority carriers distributed ho- 
mogeneously at or near the outer surface of both the 
layers 24 and 26. 
In accordance with the present invention, after the 
ribbon is grown, it is subjected to a preannealing step at 
a selected temperature range for a selected duration in a 
selected environment. The temperature range is prefer- 
ably on the order of 900"-1200" C .  and the duration is 
on the order of 30 minutes to 3 hours. As to the environ- 
ment, very satisfactory annealing results have been 
achieved with either oxygen, nitrogen or other inert 
gases. Similar satisfactory results have been obtained in 
oxygen, containing various amounts of HCI. 
There are major significant advantages from the 
practice of the present invention, as compared to the 
prior art. Herein, since the defect-containing layer 25 is 
inherently present in the ribbon, there is no need to 
purposely produce physical damage. Also, since the 
layer 24 extends uniformly across the web and along its 
length the defects are uniformly distributed, and there- 
fore, more uniform gettering of impurities is achieved, 
resulting in more uniform or homogeneous increased 
lifetime. 
This contrasts with the prior art, in which the damage 
does not produce uniformly distributed defects and 
therefore any increased lifetime is not uniform. In addi- 
tion in the dendrite web, the defect-containing layer 25 
is very close to the surface of either layer 24 or 26, e.g. 
about 3-5 mils. Thus, when practicing the present in- 
vention impurities are gathered by the defects in layer 
25 away from the surfaces of layers 24 and 26. 
Tests conducted on web dendrites processed in ac- 
cordance with the present invention indicate very uni- 
form increased lifetime. Tests at most incremental areas 
indicated lifetime of over 10 microseconds (ps), with 
many areas indicating a lifetime of several hundred ps, 
and up to and greater than 400 ps. On the other hand, 
tests on web dendrites which were not processed in 
accordance with the present invention indicated very 
low lifetimes with only few incremental areas exhibiting 
a lifetime of more than 10 ps.  Although particular em- 
bodiments of the invention have been described and 
illustrated herein, it is recognized that modifications and 
variations may readily occur to those skilled in that art 
and consequently, it is intended that the claims be inter- 
preted to cover such modifications and equivalents. 
What is claimed is: 
4 
1. A process of increasing the lifetime of minority 
simultaneously growing a multiple layer semiconduc- 
tor material to possess at least one layer containing 
impurities and a layer containing inherent defects 
resulting from the growing step: 
placing said material in a furnace containing at least 
one preselected gas; and 
heating said material in said furnace for a selected 
duration and temperature, whereby impurities in 
said one layer are gathered by defects in said layer 
containing defects. 
2. A process of increasing the lifetime of minority 
carriers as recited in claim 1 wherein said gas is selected 
3. A process of increasing the lifetime of minority 
carriers as recited in claim 1 wherein said semiconduc- 
tor material is silicon. 
4. A process of increasing the lifetime of minority 
20 carriers as recited in claim 3 wherein said gas is selected 
from the group consisting of oxygen and nitrogen. 
5. A process of increasing the lifetime of minority 
carriers in a silicon dendrite web, characterized by a 
pair of layers of silicon containing impurities which 
25 layers are spaced apart by an interfacial layer containing 
inherent defects resulting from growing the web, the 
steps comprising: 
placing said dendrite web in a furnace containing at 
annealing said dendrite web in said furnace by heat- 
ing the web to a temperature on the order of not less 
than 900" C. for a selected duration whereby impurities 
in at least one of said silicon layers are gathered by 
defects in said interfacial layer. 
6. A process of increasing the lifetime of minority 
carriers as recited in claim 5 wherein said selected dura- 
tion is on the order of not less than one half hour. 
7. A process of increasing the lifetime of minority 
carriers as recited in claim 6 wherein the web is heated 
8. A process of increasing the lifetime of minority 
carriers as recited in claim 5 wherein the gas is selected 
from the group consisting of oxygen and nitrogen. 
9. A process of increasing the lifetime of minority 
45 carriers as recited in claim 8 wherein said selected dura- 
tion is on the order of between one half to three hours. 
10. A process of increasing the lifetime of minority 
carriers as recited in claim 9 wherein the web is heated 
to a temperature in the range of 900" C.-12OO0 C. and 
carriers the steps comprising: 
5 
10 
15 from the group consisting of oxygen and nitrogen. 
least one preselected gas; and 
30 
35 
40 to a temperature in the range of 900" C.-1200" C. 
50 the gas is oxygen. * * * * *  
55 
60 
65 


Method of increasing minority carrier lifetime in silicon web or the like

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