DE3100818A1 - METHOD FOR PRODUCING THIN CRYSTAL PLATES - Google Patents

Description

Patent attorney Dipl.-Ing. Harro ^

Gfalfs Palentanwalt Am Burgetpark 8 D 3300 Bfaunschw «e Germany

At the Bürgeipark 8

D 3300 Braunschweig, Gen ι um ν

Telephone 0531-74798

Cable patmarks braunsdiwou .;

G / WS - C li » ^r ji {

Melvin S. Cook

kj> Westerly Road

Saddle River, New Jersey 07-58

UNITED STATES.

Process for the production of thin crystal plates

The invention relates to a method for the production of thin crystal plates, in particular for the production of thin single crystal plates of semiconductor materials for use in the manufacture of solar cells and integrated Circuits.

One of the most well-known methods for the production of thin single crystal plates or disks consists of rod-shaped Cutting off single crystals from semiconductor materials by sawing thin plates or wafers. The slices will then lapped and polished to a smooth and defect free surface for the following procedural steps. When cutting of the disks, also known as "wafers", almost half of the crystal material goes through the Chipping lost while sawing. In addition, for mechanical reasons, the wafers generally have to be very much thicker be produced as it is functionally necessary for the intended use.

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It is also known to produce thin film single crystals by a peeling technology. Such thin-film single crystals are used, for example, for the production of solar cells. Such techniques are reported by AQ Milnes and DL Feucht in "Proceedings of the 11th IEEE Photovoltaic Specialist Conference ¹ , ¹ pp. 338-3 ^ 1 (1975) and by M. Konagi, M. Sugimoto and K. Takahashi in" Journal of Crystal Growth ", Edition J" 5, pp. 277-280 (1978). In these techniques, films are peeled off by melting or dissolving a layer between the film and the substrate. The intermediate layer is different in its material composition from the crystal film and The known techniques have tile parts that have a significant negative impact on the film quality, the costs and the achievable area.

Bti the technique in which an intermediate layer is melted, whose Sehmel point temperature is lower than the melting point of the film or substrate, it is necessary that a liquefied layer is formed between the entire film and the substrate from which the film will be removed poll. In order to To achieve this result, the entire structure with the crystal film, intermediate layer and substrate must be heated to a certain temperature which at least corresponds to the melting point of the intermediate layer and are kept at such a temperature, until the liquefied layer has formed over the entire contact surface swisohen film and substrate, so that the The film never tears or is damaged in any way when it is peeled off the substrate. For III-V connections in particular, the Temperatures that have to be reached »relatively high. at At such temperatures, material diffusion quickly occurs through the crystalline material. If that liquefied If material of the intermediate layer remains in contact with the film, this liquid material enters the film and alloys with it this or forms impurities in the film. Because the material

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of the film is or must be different in its composition from the material of the intermediate layer so that If their melting points are different, such a diffused material can reduce the charge carrier transit time and mobility. This in turn can result in a reduction in the effectiveness of the final product, that is made on the film. In addition, for certain applications it is desirable to produce a film that which has a precisely predetermined distribution of foreign atoms. For example, it is desirable to produce a film that is, for example, pn-conductive through appropriate dopants. The film itself can be in this form be bred. However, if the film is held at higher temperatures for a significant period of time, it will occur by diffusion to a leveling of the distribution of the impurities, which then reduces the effectiveness of the pn line is reduced. In general, the larger the area of the film, the longer the structure has to be on the raised surface Temperature must be maintained to ensure that the entire contact area between the film and the substrate a liquefied layer exists so that the film can be peeled off successfully. The problem of diffusion is thus all the more larger, the larger the area of the crystal film is. For example, in the production of solar cells, the costs decrease with the film size, it is not possible to achieve significant cost savings with this known method, because for the reasons mentioned, the surface size of the film is limited in the known processes. But it is just that Cost of solar cells that limit their use.

The prior art methods have another disadvantage. In order to grow a single crystal layer on a single crystal surface, the lattice parameters must be very precise must be coordinated and the two materials

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be isomorphic. The materials must also be precisely matched to one another in terms of their coefficients of thermal expansion, since both materials have to be in contact with one another over a wide temperature range. The vote of the Temperature coefficient is hardly possible. In the case of a large temperature range, there are tensions, displacements and Defects that appear in the finished film. The need to structure including film, interlayer and to bring the substrate to the melting temperature of the intermediate layer and to keep it at this temperature for some time, therefore have the negative effect of causing defects of various kinds to appear in the finished film.

In another known method, the intermediate layer between the substrate and the film is dissolved. It's clear, that considerable problems in the production of large-area films also occur with this method. This includes, in particular, the diffusion of materials into the intermediate layer and out of the intermediate layer through the very narrow channels which are dissolved away between the film and the substrate, wherein it has to be taken into account that this process is naturally a very slow process. These diffusion hazards apply Because of the slow process, even more so for large-area films. The slowness of the process also makes this one costly · In addition, the film cannot be expected to be completely inert to the chemicals through which the intermediate layer is dissolved. The solution chemicals can thus keep unwanted contaminants in bring in the film.

Finally, it should be pointed out again that the known methods discussed above, the formation of a Require intermediate layer between the film and the substrate. The formation of intermediate layers makes the system complex and increases the cost of the finished film.

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The object of the invention is to create a method with the ea it is possible to produce thin crystal films from semiconductor materials and others at low cost Materials. In particular, it is an object of the invention to provide a method by which thin single crystal films can be produced from semiconductor materials or others Materials. In addition, the method should be particularly suitable for producing large-area thin-film single crystals To manufacture semiconductor materials or other materials.

The starting point of the process is the growth of a single crystal layer on the surface of a single crystal substrate. the Techniques for such growth are known, both epitaxial and heteroepitaxial Education. For example, methods for growing epitaxial layers from the vapor phase or from the are known Plush phase, growing from a melt and precipitation by molecular beams.

The object is achieved according to the invention by a The method as set out in the main claim and, in an appropriate embodiment, is the subject of the subclaims.

For example, the substrate can be grown in the form of a large cylindrical body. This is then first shaped into an accurate geometric cylinder. A single crystal splint is then grown on the cylinder surface of the substrate with the exception of a linear area, which lies parallel to the cylinder axis and in which peeling begins later. It then emits laser radiation into one Focused on the area of the substrate that is close to the crystal layer. The focus area extends over the full Width of the layer parallel to the cylinder axis. The optical system used for this creates a uniform one

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Radiation distribution in the focal area and thus a uniform effect in this area. The laser radiation becomes «next to one of the axially parallel edges of the layer directed on the substrate where peeling of the film can then begin.

Since the substrate material absorbs laser radiation, while the material of which the single crystal film is made for Laser radiation is permeable, the substrate is heated directly by the absorbed radiation, while the Crystal layer is only heated by the heat transfer from the substrate. However, only part of the heat generated passes into the crystal layer. So that is Heat effect due to the radiation in the crystal film significantly less than in the substrate. For this reason, substrate material can be liquefied in the focus area, while the adjoining material of the single crystal film layer resting on it remains firm, even if that The material of this film layer has a lower melting point than the material from which the substrate is made.

While the substrate material liquefies in the focus area the cylinder rotates under the fixed optical system and the separation of the film layer from the material is carried out mechanically while the substrate material is still liquid. Since the actual amount of heat is small, regardless of the local Aufohmelzungen, the layer must be localized by the Substrate are separated because as soon as the laser radiation emerges from this area, the melted material quickly resolidifies to the extent that heat flows into neighboring material that is colder. In this way, the The film is progressively peeled off as the laser radiation is moved across the substrate and layer. the rapid resolidification of the liquefied material

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indicates that hot melt, which differs in its material composition, in no local area for one remains in contact with the film layer for a significant period of time. This reduces the appearance of material from the Substrate in the finished single crystal film. The time during the significant heating of the film layer occurs, is very brief in every point. But with this there are diffusion effects reduced to a minimum. The impurity distribution formed in the layer before separation remains exist. This is beneficial in lowering the cost of various types of solar cells because it is less costly is to make the layer pn-conductive as such a line at a later point in time in the manufacturing process, after the film is separated from the substrate is.

Limits to the area that can be achieved for the single crystal film are determined by the requirements placed on the laser radiation and by the availability of substrates of the appropriate size.

It is possible to use laser radiation to heat the entire substrate surface at the same time. this however, it is less expedient, since the reduction in product quality in such a process can be significantly greater because the total heat input and the time in which the film is in contact with the hot melt, each is very large would be bigger.

The absorption of radiation in a semiconductor material can, for example are described in the form of parameters that allow easy comparison of different materials,

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2
PalIf FCx) is the photon flux in cm / s is

P (x) »P (O) exp (-oC _A x)

Here oGi is the absorption coefficient for the radiation of wavelength Λ and F (O) is the value of P (x) at reference point χ »0. The jump coefficient can be graphed as a puncture of A and the photon energy for the different materials are plotted.

A laser radiation with a wavelength = 1.06 μ can be generated by an Nd: YAG continuous beam laser. The photon energy "flti" is characteristic of such radiation is 1.17 eV. This radiation can be absorbed by a semiconductor material with a band gap energy of 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 is InP is equal to 1.33 eV at 300 ⁰ K. The absorption coefficient thus essentially zero for a 1.06 / um radiation. Quaternary ^Qa - ^ln _x i _x ^ ^Aa y ^ i ^ _y alloys can be Qitterübereinstimmung with InP, and can be produced with band gaps energies that span the range between 0.75 β V to 1.3 eV. It can be used to produce an alloy that is an effective absorber of 1.06 / laser radiation. If an InP crystal layer is formed on the surface of such an alloy with, for example, a band gap energy equal to 0.8 eV, a laser can be used to separate the alloy from the substrate in the manner described above. If the absorption coefficient of the alloy is 10 * em " ¹ 71 of the photons striking the surface of the alloy are absorbed at a depth of 0.125 yu /.

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Further details of the invention are described below with reference to the drawing. In this one is:

Fig. 1 is a graphic representation of the relationship between the absorption coefficient, the wavelength of the Radiation and photon energy for various semiconductor materials;

Pig. 2 is a schematic representation of the separation of a

Using single crystal layer from a substrate from laser radiation passing through the single crystal layer;

Fig. 3 is a schematic representation of the system of FIG. 2, but in which an intermediate layer of a radiation absorbing material is used;

Figure 4 is a schematic illustration of a thin single crystal layer being separated from a substrate below Using laser radiation that passes through the substrate;

Fig. 5 is a schematic representation of a substrate with a single crystal layer formed thereon and with a metal foil which is in contact with the under pressure Layer stands and

6 shows a schematic representation of the separation of the crystal layer adhering to the metal foil from the substrate.

In Fig. 1 is the ratio of the absorption coefficient to the radiation wavelength and the photon energy graphically for various different semiconductor materials

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shown. It can be seen from this that the materials GaP and Si with indirect gap have a ratio of increasing the Have absorption coefficients with respect to an increase in photon energy that is less than that for the Materials QaAs and CdS, the materials with a direct gap. For effective absorption of radiation passing through these materials must pass through with an indirect gap the wavelength of the radiation is therefore further from the basic absorption edge (longest wavelength) in the direction of shorter wavelengths (greater photon energy) than this is the pail for materials with a direct gap.

In their pure state, these semiconductors are largely transparent for wavelengths longer than their fundamental absorption edge. In the impure state they are impermeable and therefore absorbing for wavelengths from ultraviolet to radio wavelengths. In areas where the pure materials absorb strongly, they have absorption coefficients in the

■ 5-1
Of the order of ICr cm

In Fig. 2 is an arrangement 3 with a cylindrical substrate on which a single crystal is formed in the form of a thin film layer. Techniques for breeding a such crystal layers are known. You can, for example, from the liquid or gaseous phase in epitaxy or Heteroepitaxy can be deposited, grown from a melt, or by molecular beam growth.

The laser 11 emits a laser radiation 9 which is directed by an optical system 13 into the focus area 15, the extends over the entire length of the layer on the substrate. The amount of radiation in the focus area is sufficient, material to melt in the focus area. The substrate rotates in the direction of arrow 17. First, the edge 19 is the

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Schioht detached from the substrate. This can be done by mechanically inserting a wedge when the substrate is in the Focus area is liquefied. This wedge is not shown in the drawing. However, conventional wedges can be used useful for this purpose.

The first separated edge of the layer 19 can be held mechanically and used to peel off the film, so that while the laser radiation travels over the surface of the substrate, the liquefied areas of their overlying crystal layer can be released by pulling on the holding device. This mechanical holding device is also not shown. But it can conventional holding devices are used for this. The process is continued until the entire crystal layer is removed Substrate is separated.

In the Pig. 2 the crystal layer is transparent to the laser radiation which is absorbed by the substrate. If the layer is an InP crystal, the substrate can be composed of a Ga In, As P ₁ alloy, and a Nd: YAG laser which processes with a wavelength of 1.06 can be used. In general, the composition of the alloy used should be selected so that the laser radiation used is effectively absorbed.

In FIG. 3, essentially the same arrangement as in FIG. 1 is used. In contrast, an intermediate layer is made here Radiation-absorbing material 23 is provided between the single crystal layer 7 and the cylindrical substrate 5 lies. The liquefaction of this intermediate layer with the help of the absorption of laser radiation enables the crystal layer 7 to be separated, which then represents the desired film product.

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In this case it is advantageous to use a material for the substrate 5 which is transparent to the laser radiation is because the latter is and should be absorbed in the intermediate layer. This helps reduce local warming to a minimum, which could otherwise deteriorate the quality of the product. In this arrangement the intermediate layer 23 is used as the substrate on which the crystal layer 7 is grown or deposited.

The embodiment of Figure 3 is particularly useful when it is difficult to grow large single crystals on a suitable radiation absorbing material which is used as a substrate in conjunction with the material from which the thin film is formed. For example, it is much easier to grow large single crystals from a silicon than large single crystals from silicon-germanium alloys with a mole percent silicon in the range of 15 % to 95 % an arrangement according to FIG. 3 is used, an intermediate layer of a silicon-germanium alloy being used in connection with a silicon substrate. The intermediate layer can be provided with a thickness of 0.5 yu /, while the silicon crystal layer has a thickness of 25 yu ^. A silicon layer of this thickness is sufficiently flexible to be easily handled and, on the other hand, thick enough for the production of efficient solar cells with pn conduction.

In Fig. ¹ I a crystal layer 27 is formed on a substrate 25. In this embodiment, the substrate is transparent to the laser (or other) radiation 29, which is guided through the substrate via the optical system 31 into the focus area 33. Local liquefaction then occurs in the focus area due to the absorption of radiation, so that a

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Wedge 35 can be advanced by mechanical means in the direction of arrow 37 so as to remove the crystal layer from the Separate the substrate in the area of the liquefied material. Feed means which can be used for this purpose are not shown and known in the art. At the same time, the optical system and the laser radiation are directed in the direction of arrow 39 moved further so that the entire crystal layer can be progressively separated from the substrate. The liquefied material tends to solidify with a crystal structure that portrays the crystal structure of the underlying crystal layer is. As long as the material is liquid, it can be removed mechanically or chemically. After solidification the crystal structure, if it is unsatisfactory, can be recrystallized, for example by the laser radiation is passed over the surface so that melting occurs at the surface. But this layer can also can be removed chemically or mechanically.

In Fig. 5, a substrate 43 is shown on which a thin Single crystal layer 45 is formed. Through a carrier a metal foil 47 is under pressure in contact with the Brought crystal layer. This arrangement is then heated and at such a temperature for a sufficient time held that a eutectic alloy of the material of the crystal layer and the foil in the contact area of the two is designed to bring about a good mechanical connection and a good electrical contact between them. the Formation of such a eutectic alloy between a semiconductor material and a metal is for example known in the manufacture of electronic semiconductor components. Aluminum and silicon form such an alloy for example at 577 ° C.

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Like in Pig. 6 shows a separation of the film 47 carried out with the adhering crystal layer 45, by directing laser radiation 55 through an optical system 53 into the focal area 51, in which a local Melting of the metal of the metal layer occurs due to absorption of the laser radiation. When the melting temperature is reached, the film with the crystal adhering to it is progressively separated from the substrate 43. It Conventional means can be used to peel the film off to achieve this separation. Around the entire slide to separate with the attached crystal, the laser radiation is guided over the entire length of the crystal, by moving the substrate in the direction of arrow 57 at a corresponding speed. There is only one very If a small amount of heat is required to carry out the separation, there is no solution of metal atoms The foil in the crystal shows its properties would worsen. This is in contrast to the effect that occurs when the entire assembly is heated up Separation of the crystal layers is carried out as is the case in the prior art.

The liquefied material tends to solidify with crystal structure, which is a continuation of the crystal structure of the the unmelted crystal underneath. If no satisfactory recrystallization occurs, the melted layer can be removed or treated so that a more satisfactory crystal structure is obtained as described above.

The invention has been described with reference to a few specific embodiments. However, these embodiments are merely as an example of the application of the invention

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Principle has been chosen. The basic concept of the invention consists in placing a crystal layer, which is later the desired product, on a material of different types The so-called substrate, where the two materials have different absorption characteristics for the radiation that is used, to cause the separation. The direction from which the radiation hits, as well as the choice of which of the two Materials absorbing radiation is not critical as long as the radiation passes through one of the materials and is absorbed by the other at the contact surface will. The technology according to the invention can be used both for the production of single crystal products as well as polycrystalline products.

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Claims (9)

Expectations Iy process for the production of thin, film-like crystals, characterized in , that a crystal layer is grown on a substrate, either the substrate or the crystal layer being permeable are for radiation with at least one specific wavelength emitted by the substrate or the crystal layer is absorbable in the area of the contact surface between the crystal layer and the substrate, that a radiation with at least a certain wavelength in at least a part of the absorbing material is directed in the area of the contact surface, so that the absorbent material is liquefied there, and that the crystal layer is separated from the substrate in the region of the liquefied material. 2. The method according to claim 1, characterized in that the radiation is directed through the permeable material into the area of the contact surface. 3. The method according to claim 1 or 2, characterized in that the radiation is continuously moved over the contact surface and the crystal layer is separated from the substrate in the region of the radiation exposure. H. Method according to one of the preceding claims, characterized in that laser radiation is used as radiation. 5. The method according to any one of the preceding claims, characterized in that the crystal layer is grown as a single crystal. 130048/0559 6. The method according to any one of the preceding claims, characterized in that the crystal layer is connected to a layer of a different material prior to separation from the substrate. 7. The method according to claim 6, characterized in that the separation of the crystal layer from the substrate is carried out by pulling the layer made of a different material. 8. The method according to claim 6 or 7, characterized in that the layer made of a different material is a metal layer. 9. The method according to claim 1, characterized in that a first crystal layer is grown on the substrate, which absorbs the radiation and then a second, transparent to the radiation crystal layer is grown, and that the separation is carried out in that the first crystal layer is liquefied by the radiation. 130048/0559