A method for growing a high purity, low defect layer of semiconductor is described. This method involves depositing a patterned mask of a material impervious to impurities of the semiconductor on a surface of a blank. When a layer of semiconductor is grown on the mask, the semiconductor will first grow from the surface portions exposed by the openings in the mask and will bridge the connecting portions of the mask to form a continuous layer having improved purity, since only the portions overlying the openings are exposed to defects and impurities. The process can be iterated and the mask translated to further improve the quality of grown layers

Daud, Taher

Morrison, Andrew D.

English

NASA Technical Reports Server

United States Patent [191 p i ]  Patent Number: 4,612,072 
Morrison et al. [45] Date of Patent: Sep. 16, 1986 
[54] METHOD FOR GROWING LOW DEFECT, 
HIGH PURITY CRYSTALLINE LAYERS 
UTILIZING LATERAL OVERGROWTH OF A 
PATTERNED MASK 
[75] Inventors: 
[73] Assignee: 
[21] Appl. No.: 
[22] Filed: 
Andrew D. Morrison, Altadena; 
Taher Daud, LaCrescenta, both of 
Calif. 
The United States of America as 
represented by the Administratpr of 
the National Aeronautics and Space 
Administration, Washington, D.C. 
706,564 
Feb. 28, 1985 
Related U.S. Application Data 
[62] Division of Ser. No. 507,624, Jun. 24, 1983, Pat. No. 
4,522,66 1. 
[51] Int. (3.4 ...................... HOlL 21/20; HOlL 21/76 
[52] US. (3. .................................. 148/175; 29/576 E, 
29/576 J; 29/576 W; 29/575; 29/578; 148/174; 
148/DIG. 26; 148/DIG. 85; 148/DIG. 150; 
156/612; 156/DIG. 65; 156/DIG. 88; 357/4; 
357/50 
[58] Field of Search ............. 29/576 E, 576 W, 576 J, 
29/578, 575; 148/174, 175, DIG. 26, DIG. 85, 
DIG. 150; 156/612, DIG. 65, DIG. 88; 357/4, 
50 
[561 References Cited 
U.S. PATENT DOCUMENTS 
3,564,358 2/1971 Hahnlein ................... 148/DIG. 150 
3,634,150 1/1972 Horn ................................... 148/175 
4,378,629 4/1983 Bozler et al. ..................... 29/578 X 
4,482,422 11/1984 McGinn .............................. 156/612 
OTHER PUBLICATIONS 
Tausch et al., “Novel Crystal Growth. . .  Overgrowth 
Onto Silicon Dioxide”, J. Electrochem. SOC., vol. 112, 
Primary Examiner-William G. Saba 
Attorney, Agent, or Firm-Paul F. McCaul; Thomas H. 
Jones; John R. Manning 
[571 ABSTRACT 
The purity and perfection of a semiconductor is im- 
proved by depositing a patterned mask (12) of a material 
impervious to impurities of the semiconductor on a 
surface (14) of a blank (10). When a layer (40) of semi- 
conductor is grown on the mask, the semiconductor 
will first grow from the surface portions exposed by the 
openings (16) in the mask (12) and will bridge the con- 
necting portions of the mask to form a continuous layer 
(40) having improved purity, since only the portions 
(42) overlying the openings (16) are exposed to defects 
and impurities. The process can be reiterated and the 
mask translated to further improve the quality of grown 
layers. 
NO. 7, Jul. 1965, pp. 706-709. 
7 Claims, 6 Drawing Figures 
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substrates in order to reduce the cost of good devices 
have been made, but without substantial success. 
Growing of films on solid state substrates has been 
practiced as a means of forming junctions, or for salvag- 
5 ing the substrate. Baliga et al. (U.S. Pat. No. 4,251,299) 
in FIG. 5 shows non-planar growth of silicon extending 
beyond etched areas to form a continuous film. How- 
ever, Baliga et al. use liquid phase epitaxial growth and 
only want growth to occur in the etched grooves. 
There are several references that teach the salvaging 
of imperfect semiconductor or doped semiconductor 
grow a film extending into and bridging bevels on 
mesas. However, he only a mask to etch the bevels. 
15 There is no mask present during liquid or vapor deposi- 
tion. Mayberry et al. (U.S. Pat. No. 3,559,281) reclaims 
wafers by stripping the front surface, passivating it and 
then stripping and polishing the opposite surface and 
conducting crystal growth 
pat. N ~ .  3,671,338), and Revesz et al. (uaS. pat. No. 
3,904,453) do grow crystals on a perforated, masked 
substrate but they restrict growth to the window open- 
ings. Lawrence (U.S .  Pat. No. 3,923,567) reclaims crys- 
METHOD FOR GROWING LOW DEFECT, HIGH 
PURITY CRYSTALLINE LAYERS UTILIZING 
LATERAL OVERGROWTH OF A PATTERNED 
MASK 
ORIGIN OF THE INVENTION 
The invention described herein was made in the per- 
formance Of work under a NASA 'Ontract and 's 
ject to the provisions of Section 305 of the National 
(72 Stat 435; 42 USC 2457). 
June 24, 1983, now U.S. Pat. No. 4,522,661. 
10 
Aeronautics and 'pace Act Of 1958, Law 83-568 blanks. Endler et al. (U.S. pat. No. 4,255,206) does 
This is a division of application Ser. No. 507,624, filed 
TECHNICAL FIELD 
The present invention relates to techniques for grow- 
ing low defect, high purity layers of semiconductors 
high purity silicon on high defect, impure silicon sub- 
strates. 
BACKGROUNDART 
and, more particularly, to the growth Of low defect* 20 Barnett et a]. (U.S. Pat. No. 3,647,578), Fujii (U.S.  
Self-sufficiency in energy is a stated goal' 25 tals by gettering to concentrate defects at the surface. 
Rode (U.S. pat. No. 4,050,964) improves smoothness of Many of the proposed to achieve this goal, espe- 
cially those dependent on fossil fuels, are either environ- growth by misorienting the substrate. Lawrence et al. 
mentally unacceptable or not feasible. (US. Pat. No. 4,062,102) processes reject wafers by 
Of the available alternatives, Solar energy is the most stripping and etching to form a pattern for electrodes. 
abundant, inexhaustible single resource available. How- 30 MacDonald, J ~ .  et al. (u.s. Pat. N ~ .  4,131,472) tracks 
ever, capturing and utilizing Solar energy is not simple. the defects to dies of a mask so as to correct the die 
Methods are being sought to convert solar energy to a openings. Baliga (U.S. Pat, No. 4,128,440) lowers the 
concentrated, storable form of energy. temperature to control diffusion. Esseluhn (U.S. Pat. 
No. 4,160,682) shows an improved slider apparatus. 
form being prominently considered is the deployment 35 Kotval el at. (U.S. Pat. No. 4,124,410) directionally 
of large arrays of photovoltaic solar cells, especially in pulls relatively impure silicon to form low cost, single 
the sunbelt areas such as the southwestern and Western crystal silicon and Mizrah (u.s. Pat. NO. 4,199,379) 
regions of the United States. The most promising candi- selectively forms a metal pattern on a crystal by deposi- 
date for the solar cell is a doped silicon material. tion, masking and etching. 
earth's crust. However, solar cells are presently fabri- SUMMARY OF THE INVENTION 
cated from semiconductor-grade silicon, which has a Because a growing film typically contains all the 
market Price of about $65.00 Per kilogram. A number of defects of the substrate from which it grows, the obvi- 
current projects are directed to developing the national ous solution is to have the film grow from a substrate 
capability to produce low-cost, long-life photovoltaic 45 with limited defects. However, low defect substrates 
modules at a rate greater than 500 MW per year and at are expensive. The process of the invention produces 
a price of less than $500 per peak kilowatt by the year high quality, low-defect films or layers on low-quality, 
1986. A drastic reduction in price of material is neces- inexpensive substrates using well established vapor de- 
sary to meet these important national objectives. The position, epitaxial growth techniques. 
presence of transition metal impurities has been identi- 50 This is accomplished in accordance with the inven- 
fied as one of the major factors causing degradation of tion by masking most of the surface of the substrate with 
silicon photovoltaic cells. These impurities have a nega- a mask having windows exposing a portion of the sur- 
tive effect on the carrier lifetime and also on the eff- face, generally 50% to 95% thereof. The mask is imper- 
ciency of silicon cells. Both of these factors have been vious to diffusion of impurities in the substrate. Epitax- 
considered as inter-related critical limitations. The mini- 55 ial vapor deposition onto the masked substrate results in 
mization of all transition-metal impurities is a major film nucleation growing only in the windows bridging 
concern in the production of silicon for wide-spread use the land area of the mask and forming a continuous film 
in solar arrays. which completely covers the surface of the substrate. 
High quality surfaces of crystalline materials absent The number of defects visible to the observer of this 
defects such as dislocations are required for growing 60 masked surface and available to the growing film is 
layers which are to be fabricated into electronic devices reduced to the number of defects in the windows, all 
having optimum performance. Device performance other defects are masked. Growth of a crystalline layer 
usually depends upon the degree of perfection of the on this new surface, if nucleated in the windows, will 
substrate material on which the device is fabricated, contain only the number of defects which are present in 
because a growing film typically contains all the defects 65 the windows and defects occurring at the boundaries of 
of the substrate from which it grows. This has required the growing film as the film which originated at each 
the use of expensive substrates of high perfection. Many window begins to coalesce. Every reiteration of the 
attempts to produce highly perfect films on imperfect masking and growth from the vapor will result in a 
One method of converting solar energy to a 
. 
Silicon is one of the most plentiful elements in the 40  
4.612,072 
3 
further decrease in the number of defects in the surface 
film. Additionally, the masking will inhibit the diffusion 
of impurities in the imperfect substrate into the growing 
films. Each reiteration will result in a more perfect film 
and thicker, more perfect films can be grown with each 
reiteration. 
The substrate can be single crystal or polycrystalline 
and can be a semiconductor material other than silicon, 
such as germanium or the wider band gap materials, 
such as 111-IV compounds, suitably gallium arsenide or 
phosphide. A suitable masking material for silicon is 
silicon oxide which also can be deposited from the 
vapor phase. The silicon oxide film is then etched to 
form an array of windows exposing the underlying 
substrate surface. Silicon films are then deposited on the 
mask, nucleate only in the windows, and grow to form 
a layer which completely covers the low-quality, low- 
cost substrate. 
The mask has the added advantage of acting as a 
diffusion barrier to limit the diffusion of impurities from 
a less pure substrate into the purer, grown layer. Reiter- 
ations of the masking and the growth process will insure 
increasingly more perfect layers. This can be an auto- 
mated and an inexpensive process. The equipment nec- 
essary to produce the masks and do the epitaxial growth 
is available in the market place from a variety of 
sources. The process itself is extremely simple and rela- 
tively low-cost. 
These and many other features and attendant advan- 
tages of the invention will become apparent as the in- 
vention becomes better understood by reference to the 
following detailed description when considered in con- 
junction with the accompanying drawings. 
BRIEF DESCRIPTION O F  THE DRAWINGS 
FIG. 1 is a schematic view of a deposition apparatus; 
FIG. 2 is a top view of a mask applied to a semicon- 
FIG. 3 is a top view of another type of mask applied 
FIG. 4 is a side view of the coated blank; 
FIG. 5 is a side view of a further mask applied to the 
FIG. 6 is a side view of the blank showing two grown 
ductor blank; 
to a seimconductor blank; 
grown layer; and 
layers of semiconductor according to the invention. 
DESCRIPTION O F  THE PREFERRED 
EMBODIMENTS 
Referring now to FIGS. 1 and 2, the process of the 
invention utilizes a low cost, relatively impure blank 10 
of semiconductor. The mask 12 of an impurity-impervi- 
ous material is applied to a surface 14 of the blank. The 
mask has openings 16 separated by bridges 18 of the 
mask material. The openings 16 can be in the form of 
lines 20 in the gradient-like mask shown in FIG. 2, or 
the openings can be in the form of polygons or circles 
22, as shown in FIG. 3. 
The pattern can be applied by applying a continuous 
layer across the surface 14 and then selectively remov- 
ing material for the openings by photolithography and 
etching. The pattern of the mask could also be applied 
by deposition through a stencil onto the surface, such as 
a sheet of metal containing a pattern of openings. A 
layer of semiconductor is then deposited on the surface 
of the blank by any suitable means, such as by vapor or 
liquid epitaxial deposition techniques according to pro- 
cedures well known in the art. Vapor deposition is 
preferred with silicon. 
5 
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40 
45 
50 
55 
60 
65 
4 
A blank 10 with patterned mask 12 is then placed in 
the holder 24 within a deposition chamber 26. The 
chamber is evacuated by vacuum pump 28. Deposition 
source 30 is then activated to feed vapors 32 into the 
chamber. The deposition chamber may be provided 
with appropriate heating means. As shown in FIG. 4 
the vapors react at or near the surface to deposit crystal- 
line semiconductor material which deposits at the sur- 
face portions 34 within the openings 16. The deposit 
grows to form a layer which fills the openings 16 and 
forms a connecting portion 36 to the next opening until 
a continuous layer 40 is formed. The layer 40 will have 
reduced defects throughout, but will have less impuri- 
ties and defects within the connecting portions 36, than 
the portions 42 overlying the openings 16. 
The coated blank is then removed and a device can be 
formed on the layers by doping to form a junction and 
applying electrodes. The front surface may be polished 
to remove non-planar portions and the grown layer can 
be sliced from the blank. 
A further enhancement of growth of more perfect 
and pure crystals can be achieved by reiteration steps in 
which the mask is displaced, so that solid portions over- 
lay the open areas of the previous mask. Referring now 
to FIGS. 5 and 6, the coated blank with grown layer 40 
is then covered with a mask 50 having openings 52 
overlying the portions 36 and having bridges 54 cover- 
ing the portions 42. Vapor deposition results in growth 
of a layer 56 which has fewer defects than if only mask 
12 had been utilized. This process can be repeated as 
many times as necessary to produce the desired quality 
of semiconductor. 
All the materials, apparatus and procedures necessary 
to practice the invention are available. A suitable proce- 
dure follows. A silicon blank, such as a material pre- 
pared by pulling ribbon or rod from a melt, is sliced, 
polished and placed in a deposition chamber. A layer of 
silicon dioxide having a thickness of at least 2000 Ang- 
stroms to 10,000 Angstroms, usually from 6000 to 
10,000 Angstroms, is grown on the surface by feeding 
oxygen into the chamber. A layer of appropriate photo- 
resist material is then applied and dried. The layer of 
photoresist is exposed through a mask, developed, 
rinsed and cured. The photoresist can be a positive or 
negative photoresist. The uncured areas are removed 
with solvent to expose portions of the silicon oxide 
layer. The layer is etched with a mixture of 10% hydro- 
fluoric acid, or other suitable solvent to expose portions 
of the silicon surface. The remaining portions of the 
photoresist are removed with suitable solvent, such as 
sulfuric acid-hydrogen peroxide. The silicon oxide 
masked blank is then placed in a deposition apparatus 
and a silicon layer is grown by epitaxial growth process 
and proceeds from the openings and across the bridging 
portions of the silicon dioxide mask to form a continu- 
ous layer with less defects and fewer impurities. 
It is to be understood that only preferred embodi- 
ments of the invention have been described and that 
numerous substitutions, modifications and alterations 
are permissible without departing from the spirit and 
scope of the invention as defined in the following 
claims. 
We claim: 
1. A method of forming an improved semiconductor 
applying to a surface of a semiconductor a layer of a 
first mask having a pattern of openings exposing 
comprising the steps of: 
5 
4,612,072 
6 
portions ofthe surface, said mask being impervious 2. A method according to claim 1 in which the first 
mask has openings 50% to 95% of the area of the semi- to diffusion of impurities from said surface; 
conductor surface. growing a first continuous layer of semiconductor on in which the first 
Said Surface extending from exposed Portions and 5 and/or second mask Dattern is deposited directly on the 
3. A method according to claim 
bridging the connecting portions of the mask to surface by shadow masking. 
form a continuous layer; 4. A method according to claim 2 in which the sur- 
applying to a surface of said first layer of semiconduc- face is covered with a continuous layer of the first and- /or second mask and portions of the mask layer are 
tor a second mask having a pattern Of Openings in 10 selectively removed to form said pattern using photoli- 
registration with the bridges between the openings thography. 
of the first mask and a pattern of bridges overlying 5. A method according to claim 4 in which the por- 
the openings in the first mask, and tions are removed by etching. 
growing a second continuous layer of semiconductor 6.  A method according to claim 1 in which the semi- 
on said first layer with the second layer extending 7. A method according to claim 6 in which the first 
from the openings in the second mask over the 
bridges of the second mask. 
15 conductor is silicon. 
and/or second mask is silcon dioxide. * * * * *  
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Method for growing low defect, high purity crystalline layers utilizing lateral overgrowth of a patterned mask

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