The orientation of twinning and other effects in silicon crystal ribbon growth is controlled by use of a starting seed crystal having a specific (110) crystallographic plane and (112) crystallographic growth direction

Leipold, M. H.

English

NASA Technical Reports Server

REPLY TO
ATTN OF: GP
NATIONAL AERONAUTICS AND Si3ACE ADMINISTRATION
WASHINGTON, D.C. 20546
NOV 2 1 1978
NST-44
TO: X2CK/Scientific & Technical Information Division
Attn: Miss Winnie M. Morgan
FROM: GP/Office of Assistant General
Counsel for Patent Matters
SUBJECT: Announcement of NASA-Owned U.S. Patents in STAR
In accordance with the procedures agreed upon by Code GP and Code
KSI, the attached NASA-owned U.S. Patent is being forwarded for
abstracting and announcement in NASA STAR.
The following information is provided:
U.S. Patent No.
Government or*
Corporate Employee
<f
Supplementary|Corporate
Source (if applicable)
NASA Patent Case No. :
», '
?
NOTE - If this patent covers an invention made by a corporate employee
of a NASA Contractor, the following is applicable:
YES 6T7 N O / 7
ft
Pursuant to Section 305(a) of the National Aeronautics and. Space Act,
the name of the Administrator of NASA appears on the first page of the
patent; however, the name of the actual inventor (author) appears at
the heading of column No. 1 of the Specification, following the words
"...with respect t© an invention of
1- <*,
*" f
Bonnie L. Hendersori
Enclosure ?
(NASA-Case-NPO-13918~-1)~ ^ METHOD JOF
CONTROLLING DEFECT ORIENTATION IN SILICON
CRYSTAL RIBBON GROWTH Patent (NASA) U p
CSCL 20B
00/76
N79-11920
Onclas
37232
https://ntrs.nasa.gov/search.jsp?R=19790003749 2020-03-20T18:06:03+00:00Z
United States Patent
Leipold
[ii] 4,121,965
[45] Oct. 24,1978
[54] METHOD OF CONTROLLING DEFECT
ORIENTATION IN SILICON CRYSTAL
RIBBON GROWTH
[75] Inventor: Martin H. Leipold, La Canada, Calif.
[73] Assignee: The United States of America as
represented by the Administrator of
the National Aeronautics & Space
Administration, Washington, D.C.
[21] Appl. No.: 706,073
[22] Filed: Jul. 16,1976
[51] Int. C1.2 B01J 17/18; C01B 33/02
[52] U.S. Q 156/608; 156/617 SP;
156/DIG. 64; 156/DIG. 65; 156/DIG. 88
[58] Field of Search 156/DIG. 64, DIG. 88,
156/DIG. 65, 608, 617 SP; 23/273 SP
[56] References Cited
U.S. PATENT DOCUMENTS
3,759,671
3,977,934
9/1973
8/1976
Bleil 156/608
Lesk 156/608
B 584,997 3/1976
2,961,305 11/1960
3,130,040
3,341,361
3,591,348
4/1964
9/1967
7/1971
Cizzek 156/608
Dash 156/608
Faust 156/617 SP
Gorski 156/DIG. 88
La Belle, Jr 156/608
FOREIGN PATENT DOCUMENTS
913,674 12/1962 United Kingdom 156/617 SP
938,917 10/1963 United Kingdom 156/DIG. 65
OTHER PUBLICATIONS
Dash, Growth of Silicon Crystals Free From Disloca-
tions, J. of App. Phys., vol. 30, #4, Apr. 1959, pp.
459-474.
Barrett, "Growth of Wide, Flat Crystals of Silicon
Web, J. Electro. Chem. Soc., Solid State Sci., Jun. 1971,
vol. 118, #6, pp. 952-957.
Primary Examiner—Stephen J. Emery
Attorney, Agent, or Firm—Monte F. Mott; Wilfred
Grifka; Robert Kinberg
[57] ABSTRACT
The orientation of twinning and other effects in silicon
crystal ribbon growth is controlled by use of a starting
seed crystal having a specific {110} crystallographic
plane and <112> crystallographic growth direction.
1 Claim, 2 Drawing Figures
15
<II2> DIRECTION
PLANE
12
U.S. Patent Oct. 24, 1978 4,121,965
A ^IZ) DIRECTION
15 PLANE
12
LIQUID-SOLID
INTERFACE
GtTfGZ^93 Vp<i VNCI-.il 26
FIG. 2
4,121,965
METHOD OF CONTROLLING DEFECT
ORIENTATION IN SILICON CRYSTAL RIBBON
GROWTH
ORIGIN OF 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 10
(72 Stat. 435; U.S.C. 2457).
BACKGROUND OF THE INVENTION
This invention relates to thick film silicon growth
techniques. 15
An important photovoltaic device useful in the direct
conversion of solar energy into electrical power is the
silicon solar cell, but its use on a large commercial scale
has not been feasible due to the lack of an industrially
feasible process for the growth of single crystal silicon 20
directly and continuously from a melt. A method which
has been most successful is an edge-defined, film-fed
growth (EFG) process by which crystals may be grown
with their thickness and width defined and controlled
by the outside dimensions of a die. . 25
The EFG process employs two spaced apart sheets of
suitable material partially immersed in a silicon melt.
The silicon melt wets the sheets and rises to the top of
the sheets by capillary action. A single-crystal silicon
seed is brought into contact with the melt to initiate 30
crystallization. After proper adjustment of the melt
temperature, crystal withdrawal rate, and crystalliza-
tion spread across the top of the sheets through their full
width, a ribbon of crystal can be grown of indefinite
length. This spreading across the top of the sheets is 35
halted by the sharp change in the effective contact
angle at the outer perimeter of the ends of the sheets.
The result is a thick film silicon crystal ribbon the thick-
ness and width of which are defined by the perimeter of
the ends of the sheets. 40
To better control the thickness of the silicon crystal
ribbon, it has been suggested that the sheets be beveled
to provide two sharp ends, i.e., to provide only sharp
edges wetted by the silicon melt. The perimeter of the
ends of the sheets is thus defined by the two parallel 45
edges at the upper ends of the beveled sheets. In other
words, by reducing the outer surfaces wetted by the
silicon melt to sharp edges, the silicon crystal formed is
virtually restricted in thickness to the space between the
sheets. 50
A problem with the EFG process is the random oc-
currence of twinning and other effects as the silicon
ribbon is grown. When twinning occurs in a direction
parallel to the length of the ribbon, the ribbon is better
for the manufacture of solar cells, but when twinning 55
occurs randomly, and in an uncontrolled manner so that
there are different orientations of the twinning defects
along the length of the ribbon, that portion of the ribbon
is poorer and is discarded.
The defect called twinning is evidenced by replica- 60
tion of crystal planes in mirror image formations. As
much as 5 feet of ribbon may be crystallized before
twinning stabilizes in the preferred direction parallel to
the length of the ribbon and normal to the crystallo-
graphic plane. Since ribbon growth is a slow process, 65
such loss of ribbon is costly because of the substantial
loss of process time and the unnecessary wear on the
walls of the die surrounding the opening imposed by the
passage of the hot, corrosive liquid silicon. The latter
affects both configuration and dimension of the opening
in time so that the die must be replaced. A ribbon grow-
ing technique which would reduce or eliminate the
uncontrolled orientation of the twinning effect would
be of considerable advantage.
SUMMARY OF THE INVENTION
In accordance with the present invention, anomalous
crystallographic developments, particularly twinning,
are obtained in the preferred direction by seeding with
{110} <112< crystals. Any other orientation results in
a complete breakdown on the ribbon structure before
the anomalies stabilize in the preferred direction. The
.{110} <112> crystal seed may be a single crystal, or
may be a crystal with anomalies in the preferred direc-
tion. In the case of the latter, the initial structure of the
ribbon will generally be a direct continuation of the
seed structure. In the case of twinning in the seed, the
same spacing between the twinning planes is maintained
in the crystal ribbon grown. This allows control of
spacing between the twinning planes in all crystal rib-
bon grown once a seed with the desired spacing is ob-
tained.
The novel features that are considered characteristic
of this invention are set forth with particularity in the
appended claims. The invention will best be understood
from the following description-when read in connection
with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic sectional drawing of a crucible
and die arrangement for edge-defined, film-fed growth
of silicon ribbon.
FIG. 2 is a schematic sectional drawing of an alterna-
tive crucible and die arrangement for growth of silicon
ribbon in which the die is not wetted by the silicon melt.
DESCRIPTION OF PREFERRED
EMBODIMENTS
Referring now to FIG. 1, the EFG process by which
silicon crystals may be grown is generally practiced
with a crucible 10 made of refractory material such as
quartz (silicon dioxide) or silicon nitride heated, as by
an RF coil 11, to produce a melt (silicon solution). Two
sheets of silicon carbide 12 and 13 are semi-immersed in
the melt. The sheets are spaced sufficiently close to each
other to make the liquid silicon rise by capillary force
and wet a slot 14 between their upper edges. A silicon
seed crystal is then brought into contact with the liquid
silicon in the slot. After the mutual effects of melt tem-
perature, withdrawal rate, and crystallization spread
across the top of the sheets have been properly ad-
justed, the silicon crystal can be grown in a ribbon of
indefinite length. The growth of the silicon crystal rib-
bon is established in thickness and width by the upper
horizontal surfaces of the sheets. It is for that reason
that the sheets are commonly referred to as a "die." The
die accurately controls the crystal ribbon cross section.
Growth rate is limited only by the rate at which heat
can be removed from the solid-liquid interface.
Orientation of anomalies, such as twinning, can be
stabilized immediately at the beginning of the growth
process by starting ribbon growth with a seed crystal 15
having the anomalies oriented with respect to one spe-
cific plane and direction, namely {110 } and <112<.
This seed crystal orientation has been proven successful
for controlling twinning direction parallel to the ribbon
4,121,965
edges and perpendicular to the crystal plane by actual
fabrication of ribbon.
FIG. 1 illustrates a length of grown crystal 16 with-
drawn from the die after the seed crystal. The vertical
lines on the {110} plane of the seed crystal and the
grown crystal represent the interfaces of crystal planes
in mirror image formations, the evidence of the so-
called twinning anomaly. This orientation is determin-
able by conventional X-ray diffraction techniques. The
fate of growth is maintained by controlling the rate of.,
cooling and the rate of moving the belt to maintain the
liquid-solid interface at the tip of the nozzle even as the
melt is used, until there is not sufficient melt to keep the
nozzle full.
The silicon crystal ribbon grown by this process, an
inverted Stepanov process, differs from the more con-
ventional Stepanov process only in that the slabs are
placed at the bottom of the crucible instead of the top in
crystallographic direction < 112 < is in the direction of 10 order that capillary forces that feed the nozzle be aided
crystal growth. by the pressure head of the liquid silicon in the crucible.
As shown in FIG. 1, the twinning anomaly present in
the preferred orientation in the seed crystal is generally
continued in the grown crystal. Other anomalies, such
as crystal dislocations, will be similarly controlled in
orientation. That reduces or eliminates uncontrolled
orientation of the anomalies, and significantly reduces
loss of grown crystal. The substantial reduction in any
loss of process time reduces wear on the die. With such
In every one of these techniques, the orientation of
anomalies is controlled by using a starting seed crystal
having a specific {110} crystallographic plane and
15 < 112 < crystallographic direction. If such a seed crys-
tal is not used, anomalies can occur randomly and in an
uncontrolled manner so that there are different orienta-
tions of the anomalies along the length of ribbon. As
much as 5 feet of ribbon crystal may be grown before
significant reduction in the wear on the die, the die can 20 the anomalies stabilize in the preferred orientation. That
be sharply beveled as in the alternative arrangement of portion of the ribbon cannot be used for manufacture of
FIG. 2 to better define the thickness of the grown crys-
tal ribbon. The use of a beveled edge in a conventional
capillary EFG for better definition of the crystal thick-
solar cells and is discarded,
what is claimed is:
1. In a film-fed process of growing crystalline silicon
ness has been reported in the literature by others. 25 in a ribbon form by drawing molten silicon through a
Referring now to FIG. 2, which illustrates an in- die, starting crystallization of silicon thus fed through
verted Stepanov technique, a quartz crucible 20 is
heated, as by an RF coil 21, and liquid silicon is fed
through a "nozzle" 22 formed by two horizontal and
said die as it cools with a seed crystal, and continuing to
pull the crystal ribbon thus formed from said die as
crystallization continues, the improvement consisting of
beveled slabs 23 and 24 spaced apart the desired thick- 30 using as said seed crystal a rectangular crystalline sili-
ness of the crystal ribbon. The fine edges of the beveled con of appropriate dimensions having a {110} crystallo-
slabs define the limits of the liquid silicon. A seed crys-
tal 25 is brought in contact with the melt. After adjust-
ment of the crucible temperature for the cooling rate of
the nozzle, the seed crystal is drawn away from the 35
nozzle, as by a moving belt 26. The growth of a silicon
crystal ribbon 27 is then established. The established
graphic plane parallel to the broad surfaces of the seed
and the ribbon grown, a < 112 < crystallographic direc-
tion parallel to the direction of growth of said crystal
ribbon, and {111} twinning planes parallel to said
growth direction and normal to said broad surfaces.
40
45
50
55
60
65


Method of controlling defect orientation in silicon crystal ribbon growth

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server