Method of producing thin single-crystal sheets

A method of producing thin single-crystal sheets is disclosed. A thin single-crystal layer is formed on a substrate, with the material of the layer having a different absorption coefficient for laser radiation than does the material of the substrate at their interface. The laser radiation is focused into a region contiguous to the interface and extending the width of the interface, and is swept across the entire interface region. The energy that is absorbed from the laser radiation in the focus region liquifies material in this region. The layer is progressively separated from the substrate as the laser radiation is swept across the interface, until the entire layer is separated from the substrate. The method is applicable to the production of thin single-crystal sheets of semiconductor material which may be used, for example, in the manufacture of solar cells or integrated circuits.

This invention relates to a low-cost method of producing thin 
single-crystal sheets. It can be used, for example, to produce thin 
single-crystal sheets of semiconductor materials for use in the 
manufacture of solar cells and integrated circuits. 
This application is a continuation-in-part of my co-pending application 
entitled "METHOD OF PRODUCING THIN CRYSTALS," Ser. No. 112,938, filed on 
Jan. 17, 1980, now abandoned. 
The prior art includes a variety of methods for the production of thin 
single-crystals. Most commonly, the interest is in the production of thin 
single-crystal wafers of semiconductor material for use in the electronics 
industry. 
In producing such wafers, the starting point is the preparation of 
highly-purified materials, which are then used in growing large 
single-crystals called boules. Growth methods for this purpose include the 
Bridgeman, Czochralski, and floating-zone methods. The resulting boule is 
sawed into slices, and the slices are then lapped and polished to obtain a 
smooth and defect-free surface on which subsequent processing steps are 
performed. Approximately half of the boule material is lost in making the 
wafers. Moreover, for reasons of structural integrity in these operations, 
the wafers are generally far thicker than is necessary functionally for 
the final product made using the wafers, thereby adding to the cost of the 
wafers. 
Peeled film technology, involving separating a layer of crystal from a 
substrate, has recently been developed for use in the production of 
thin-film single-crystals for use in making solar cells, for example. Such 
techniques were reported in A. G. Milnes and D. L. Feucht, Proceedings of 
the 11th I. E. E. E. Photovoltaic Specialist Conference, pp. 338-341 
(1975), and in M. Konagi, M. Sugimoto, and K. Takahashi, Journal of 
Crystal Growth, v. 45, pp. 277-280 (1978). These techniques involve 
peeling films by melting or dissolving a layer formed intermediate the 
film and the substrate, the intermediate layer being different in its 
material composition from the film and the substrate. The techniques 
described suffer from disadvantages that significantly adversely affect 
film quality, cost, and obtainable size. 
If the technique that involves melting an intermediate layer lower in its 
melting point temperature than the film or substrate adjacent to it is 
utilized to peel a film, it is necessary that a liquified layer be 
generated between the entire film and the substrate from which it is to be 
removed. To accomplish such a result, the entire structure including film, 
intermediate layer, and substrate must be heated to a temperature at least 
equal to the melting point temperature of the intermediate layer, and held 
at such a temperature until the necessary liquified layer is developed 
over the entire interface between the film and the substrate, so that when 
removal of the film is attempted it will be successful and the film will 
not tear or otherwise be damaged. Particularly for the III-V compounds, 
the temperatures that must be attained are fairly high. Diffusion of 
material throughout a crystalline material takes place rapidly at such 
temperatures. When the liquified material derived from the intermediae 
layer is held in contact with the film, material from this liquid will 
enter the film and alloy with it or constitute impurities in it. As the 
material of the film differs in composition from that of the intermediate 
layer in order that their melting point temperatures may differ, such 
diffusing material may lower charge carrier lifetimes and mobilities. This 
can result in a lowering of the efficiency of a final device made from the 
film. In addition, it is desirable for certain applications to produce a 
film having graded impurity distribution. For example, it may be useful to 
produce a film with a p-n junction already formed in it. The film can be 
grown in such a form. However, if the film is held at elevated 
temperatures for a significant interval of time, diffusion processes will 
tend to even out impurity distributions in it, and thus the efficiency of 
the p-n junction will be lowered. In general, the larger the product film 
size, the longer the structure must be held at an elevated temperature in 
order to assure a liquified layer exists over the entire interface region 
between the film and the substrate so that successful peeling can be 
accomplished. Thus, the diffusion problem becomes more serious for the 
larger film product sizes than for the small sizes. Since it is more 
cost-effective to use the larger film size material in manufacturing solar 
cells, the cost of such a product using the prior art described above is 
negatively impacted by limitations on product film size. As it is 
precisely the cost of solar cells that limits their use, this is a serious 
disadvantage of the prior art in terms of this final product application. 
There exists another difficulty with this prior art. In order to grow a 
single-crystal layer over a single-crystal surface, the lattice parameters 
must be very-well matched and the two materials must be isomorphic. 
However, if the two materials are taken over a wide excursion of 
temperatures, then their coefficients of thermal expansion must also be 
very-well matched. It is rarely possible to achieve this latter matching. 
Therefore, if the temperature excursion is wide, strains, dislocations, 
and defects will tend to appear in the product film as a result. The 
necessity of bringing the structure (including film, intermediate layer, 
and substrate) to the melting point temperature of the intermediate layer 
and hold it there for some time has the negative effect of tending to 
cause defects of various types in the product film. 
The other prior art technique referred to above involves dissolving away an 
intermediate layer formed between the substrate and the film. It is clear 
that significant problems in producing larger size film product exist with 
this art. Diffusion of materials to and from the intermediate layer 
through the very narrow channels dissolved away between the film and the 
substrate during the process is inherently a very slow process--much more 
so for the larger film sizes. The slowness of the process makes it costly. 
In addition, the film cannot be expected to be completely inert to the 
chemicals eating away the intermediate layer, and such materials may also 
introduce undesirable impurities into the film. 
In addition, both of the prior art processes in question require the 
formation of an intermediate layer between the film and the substrate. 
Forming such an intermediate layer adds to system complexity and to the 
cost of the product film. 
It is an object of my invention to provide a low-cost method of producing 
thin sheets of semiconductor and other materials. 
It is a further object of my invention to provide a low-cost method of 
producing thin sheets of single-crystal semiconductor and other materials. 
It is a further object of my invention to provide a low-cost method of 
producing large sheets of thin-film single-crystal semiconductor and other 
materials. 
Briefly, in accordance with the principles of my invention and in the 
preferred embodiment thereof, a single-crystal layer is grown on a 
single-crystal substrate surface. Techniques for such growth are 
well-known in the art, both epitaxial and heteroepitaxial, and include, 
for example, vapor epitaxial methods, liquid epitaxial methods, growth 
from a melt, and deposition by molecular beams. 
The substrate material is highly absorptive of laser radiation to which the 
film material is transmissive in this embodiment of my invention. In the 
case of semiconductor materials, this means that the band-gap energy of 
the film is greater than the photon energy characterizing the laser 
radiation, while the band-gap energy of the substrate is less than this 
photon energy. 
If the substrate has been grown in the form of a large cylindrical boule, 
it is first shaped into a geometrically true cylinder. A single-crystal 
layer is then grown onto the cylindrical surface of this substrate, except 
for a bare linear region running parallel to the cylinder axis which will 
later be the region where peeling is commenced. 
Laser radiation is then focused into a substrate region adjacent the 
crystal layer. The focus region extends the full length of the layer 
parallel to the cylinder axis. The optical system employed to achieve this 
also produces a uniform radiation distribution in the focus region, and 
thus a uniform effect in this region. The laser radiation is initially 
directed adjacent an edge of the layer on the substrate where film peeling 
can commence. 
Since the substrate material absorbs laser radiation while the layer 
material is transparent to it, the substrate will be directly heated by 
the absorbed radiation whereas the layer will be heated only by transfer 
of heat from the substrate (as a result of such radiation). However, only 
a portion of the heat developed will transfer to the layer. Thus, the 
heating effect due to the radiation will be significantly less in the 
layer than in the substrate. For this reason, substrate material in the 
focus region can be liquified while adjacent layer material remains 
solid--even if the layer material has a lower melting point temperature 
than that of the substrate material. 
In the present embodiment of the invention, liquification of substrate 
material in the focus region occurs as the cylinder rotates under the 
(fixed) optical system, and separation of the layer from the substrate is 
mechanically performed while this material is still liquid. Since the 
actual amount of heat absorbed is small despite the local melting it 
engenders, the layer must be locally separated from the substrate as the 
laser radiation leaves the region since the melt material will quickly 
solidify as heat flows to adjacent material which is lower in temperature. 
Thus, the film is progressively peeled as the laser radiation sweeps 
across the substrate under the layer. 
The rapid solidification of the liquified material means that hot melt 
differing in material composition does not remain in contact with the 
layer in any given local region for a significant time interval. This 
minimizes the appearance of material derived from the substrate in the 
product film. The time during which significant heating of the layer 
occurs is very brief in any local region. Thus, diffusion effects are 
minimized. Impurity gradations established in the layer prior to the 
separation process can be maintained. This is advantageous in minimizing 
the cost of certain varieties of solar cells, for example, in that it is 
less costly to form the layer with a p-n junction, for example, than to 
form such a junction later in the manufacturing process after the film is 
separated from the substrate. 
Limitations on product size are set by laser radiation requirements and the 
availability of substrates of the appropriate sizes. 
It is possible to use laser radiation to heat the entire substrate surface 
simultaneously. This is less preferable in that the degrading effects on 
product quality of such an approach would be far more substantial as the 
total heat input and the time spent in contact with hot melt would each be 
far greater. 
The process of absorption of radiation in a semiconductor material, for 
example, can be described in terms of parameters that permit comparison of 
various materials in a simple format. If F(x) is the photon flux per 
square centimeter per second, then the relationship: 
EQU F(x)=F(0) exp(-.alpha..sub..lambda. x) 
is obeyed, where .alpha..sub..lambda. is the absorption coefficient for 
radiation of wavelength .lambda., and F(0) is the value of F(x) at the 
reference point x=0. The absorption coefficient can be represented 
graphically as a function of .lambda. and of photon energy for different 
materials. 
Laser radiation of wavelength equal to 1.06 microns can be produced by a 
Nd:YAG continuous laser. The photon energy characteristic of such 
radiation is 1.17 eV. This radiation can be absorbed by semiconductor 
material with a band-gap energy less than 1.17 eV. A semiconductor 
material with a band-gap energy greater than 1.17 eV would be 
substantially transparent to such radiation. 
The band-gap energy of the semiconductor InP is equal to 1.33 eV at 
300.degree. K. Thus, its absorption coefficient is essentially zero for 
1.06 micron radiation. However, Ga.sub.x In.sub.1-x As.sub.y P.sub.1-y 
quaternary alloys can be lattice-matched to InP, and can be produced with 
band-gap energies spanning the range 0.75 ev to 1.3 eV. Such an alloy can 
be produced, therefore, which is an efficient absorber of 1.06 micron 
laser radiation. If an InP layer of crystal is formed on the surface of 
such an alloy with, e.g., a band-gap energy equal to 0.8 eV, then a laser 
can be used to separate the layer from the substrate as described 
previously. If the alloy absorption coefficient is 10.sup.5 cm.sup.-1, 
then 71% of the photons incident on the surface of the alloy will be 
absorbed in a depth of 0.125 microns.

In FIG. 1, the relationship of the absorption coefficient with respect to 
radiation wavelength and photon energy is shown graphically for several 
different semiconductors. It should be noted that the indirect gap 
materials GaP and Si have a rate of increase in absorption coefficient 
with respect to the increase in photon energy which is less than that for 
the direct gap materials GaAs and CdS. Thus, for efficient adsorption of 
radiation propagating through these indirect gap materials, the radiation 
wavelength must be further from the fundamental absorption edge (longest 
wavelength) in the direction of shorter wavelength (greater photon energy) 
than is the case for the direct gap materials. 
In pure state, these semiconductors are fairly transparent for wavelengths 
longer than that at their fundamental absorption edge. In impure state, 
they are opaque (they absorb) from the ultraviolet all the way up to radio 
wavelengths. In regions where pure materials absorb strongly, they have 
absorption coefficients of the order of 10.sup.5 cm.sup.-1. 
In FIG. 2, the numeral 3 denotes cylindrical substrate 5 on which a 
single-crystal thin-film layer 7 has been formed. Techniques for growing 
such a layer of crystal are well-known in the art and include, for 
example, vapor phase or liquid phase epitaxy or heteroepitaxy, growth from 
a melt, and molecular beam growth. 
Laser 11 emits laser radiation 9, which is focused by optical system 13 
into focus region 15 which extends the full length of the layer on the 
substrate. The amount of radiation in the focus region is sufficient to 
melt material in the focus region as the substrate rotates in direction 
17. Initially, edge 19 of the layer is separated from the substrate. This 
can be accomplished by mechanically inserting a wedge when the underlying 
substrate has been liquified. This wedge is not shown, but any of 
conventional wedges suitable for the purpose may be used. 
The initially-separated edge of the layer 19 may be mechanically held, and 
used to pull on the film in the direction 21 so that as the laser 
radiation sweeps along the substrate surface, the regions liquified may be 
freed of their overlying layer by pulling on the holding device. The 
mechanical holding device is not shown, but any conventional holding 
device may be employed for this purpose. This process is continued until 
the entire layer of crystal is separated from the substrate. 
In the embodiment of the invention shown in FIG. 2, the layer of crystal is 
transparent to the laser radiation absorbed by the substrate. If the layer 
is InP, then the substrate can be composed of a Ga.sub.x In.sub.1-x 
As.sub.y P.sub.1-y alloy, for example, and used in conjunction with a 
Nd:YAG laser operating at 1.06 microns wavelength. In general, the alloy 
utilized should be tailored in terms of its composition to efficiently 
absorb the particular laser radiation which is used. 
In FIG. 3, the arrangement shown in FIG. 2 is used, except that an 
intermediate layer of radiation-absorbant material 23 is formed between 
the crystal layer 7 and the cylindrical substrate 5. Liquification of this 
intermediate layer by means of absorption of the laser radiation allows 
the crystal layer 7 to be separated to become the thin-film product. In 
this case, it is preferable that the substrate 5 be transmissive of the 
laser radiation (since it is the intermediate layer which absorbs the 
radiation); this helps minimize local heating which would otherwise act to 
degrade the product. In this arrangement, the "intermediate" layer 23 
actually functions as a substrate on which the crystal layer 7 is grown. 
The embodiment shown in FIG. 3 is particularly useful when it is difficult 
to grow large single-crystals of a suitable radiation-absorbant material 
for use as a substrate in conjunction with the desired thin-film product 
material. For example, it is much easier to grow large single-crystals of 
pure silicon than it is to grow large single-crystals of silicon-germanium 
alloys with a mol percent silicon in the range 15% to 95%. When a silicon 
thin-film product is required, therefore, the arrangement of FIG. 3 is 
preferably employed, and an intermediate layer of silicon-germanium alloy 
is used in conjunction with a silicon substrate. The layer can be grown in 
a thickness of 0.5 microns, and the silicon crystal layer can be grown to 
a thickness of 25 microns. Such a thickness of silicon is sufficiently 
flexible to be easily handled yet is sufficiently thick that it can be 
used to manufacture an efficient p-n junction solar cell. 
In FIG. 4, substrate 25 has a layer of crystal 27 formed on it. In this 
embodiment of the invention, the substrate is transparent to the laser (or 
other) radiation 29 which is transmitted through it by optical system 31 
into focus region 33. As local liquification in the focus region develops 
due to absorption of sufficient laser radiation, wedge 35 is advanced by 
mechanical means (these means are not shown, but conventional means may be 
used for this purpose) in direction 37 so as to separate the layer of 
crystal from the substrate at the liquified material. Simultaneously, the 
optical system and the laser radiation are swept across the layer in 
direction 39 so that the entire layer can be progressively separated from 
the substrate to become a thin-film product. The liquified material will 
tend to solidify with a crystal structure that is a continuation of the 
crystal structure of the underlying layer of crystal. While liquid, it can 
be mechanically or chemically removed, however. After solidification, if 
the crystal structure is unsatisfactory, it may be recrystallized for 
example, by sweeping laser radiation over it to produce superficial 
melting, or it can be removed by chemical or mechanical means. 
In FIG. 5, substrate 43 is shown with a thin single-crystal layer 45 formed 
on it. A metal foil 47 is pressed into contact with the layer by support 
49. This assembly is brought to a temperature and held there for a time 
such that a eutectic alloy of the material of the layer and the foil forms 
in the contact region of the two, in order to establish good mechanical 
adherence and electrical contact between them. The art of forming a 
eutectic alloy between a semiconductor material and a metal, for example, 
is well known in the art of semiconductor electronics manufacture. 
Aluminum and silicon form such a eutectic alloy at 577.degree. centigrade, 
for example. 
In FIG. 6, separation of the foil 47 with the adhering layer of crystal 45 
is accomplished by focusing laser radiation 55 through optical system 53 
into focus region 51, where local melting of material of the layer of 
crystal results from absorption of laser radiation. As such melting 
occurs, the foil with the adhering crystal is separated from the substrate 
43. Any conventional means may be employed for pulling on the foil to 
achieve such separation. To separate the entire foil with its adhering 
crystal, the laser radiation is swept across the entire layer of crystal 
by moving substrate 43 at a suitable speed in direction 57. Because only a 
very small amount of heat is developed to effect separation, metal atoms 
from the foil will not dissolve into the crystal layer and degrade its 
characteristics, unlike the situation which would result were gross 
heating employed to effect separation as in the prior art. 
The liquified material will tend to solidify with a crystal structure that 
is a continuation of that of the underlying solid. If that is not 
satisfactorily accomplished, it can be removed or treated to produce a 
more satisfactory crystal structure, as previously described. 
Although the invention has been described with reference to particular 
embodiments, it is to be understood that these embodiments are merely 
illustrative of the application of the principles of my invention. The 
basic concept of the invention is to provide a crystal layer which will 
become the product, grown on a material of different composition (called 
the substrate), with the two materials having different absorptive 
characteristics to the radiation employed to effect separation. The 
direction of the impinging radiation, as well as which of the two 
materials absorbs it, are not critical as long as the radiation passes 
through one of the materials and is absorbed by the other at their 
interface. The techniques of the invention can be used to obtain both 
single-crystal and polycrystalline products. Thus it is to be understood 
than numerous modifications may be made in the illustrative embodiments of 
the invention and other arrangements may be devised without departing from 
the spirit and scope of the invention.