# Patent

## Title
Reduced RC delay between adjacent substrate wiring lines

## Abstract

A process for producing a semiconductor substrate comprises the steps of forming a porous layer in a first substrate comprising monocrystalline silicon; forming a protective film on a side wall of the pores of the porous layer; forming a nonporous monocrystalline silicon layer on the porous layer; bonding the surface of the nonporous monocrystalline silicon layer onto a second substrate with interposition of an insulating layer; and etching off selectively the porous layer by use of a chemical etching solution.

## Background
1. Field of the Invention

The present invention relates to a semiconductor substrate, and a process for producing the semiconductor substrate. More particularly, the present invention relates to a semiconductor substrate suitable for dielectric isolation, and electronic devices and integrated circuits formed in a monocrystalline semiconductor layer on an insulator, and to a process for producing such a semiconductor substrate.

2. Related Background Art

Formation of monocrystalline Si semiconductor layer on an insulator is well known as silicon-on-insulator technique (SOI). Many investigations have been made thereon since the devices made by the SOI technique have many advantages which are not achievable with a bulk Si substrate for preparing usual Si integrated circuits. The advantages brought about by the SOI technique are as below: effect transistor,

and so forth.

In order to realize the aforementioned advantages in device characteristics, investigations have been made on the methods of forming the SOI structure for the last few decades. The results of the investigations are collected in the literature, for example: Special Issue: "Single-crystal silicon on non-single-crystal insulators"; edited by G. W. Cullen, Journal of Crystal Growth, Vol. 63, No. 3, pp. 429-590 (1983).

Formerly, the SOS technique (silicon-on-sapphire) is known which forms heteroepitaxial Si on monocrystalline sapphire substrate by CVD (chemical vapor deposition). This technique, although successful as completed SOI technique, is not widely applied because of many crystal defects caused by insufficient matching of the lattice at the interface between the Si layer and the underlying sapphire substrate, migration of aluminum from the sapphire substrate to the Si layer, and, above all, the high cost of the substrate and difficulty in enlarging the area thereof.

In recent years, the SOI structure without use of the sapphire substrate is going to be realized. This attempt is made in two methods: Si substrate, forming an aperture in the oxidized layer to uncover partially the Si substrate, growing Si epitaxially in a lateral direction using the uncovered Si as the seed to form an monocrystalline Si layer on the SiO.sub.2 (deposition of Si on SiO2), and use of the monocrystalline Si substrate itself as the active layer (no deposition of Si layer).

The means for practicing the methods (1) above include direct epitaxial growth of monocrystalline Si layer in a lateral direction by CVD; deposition of amorphous Si and subsequent epitaxial growth in solid in a lateral direction by heat treatment; growth of amorphous or polycrystalline Si into monocrystalline layer by melting-recrystallization by focusing thereon an energy beam such as electron beam and laser light; and zone melting recrystallization by a long heater. These methods have both advantages and disadvantages, still involving problems in controllability, productivity, uniformity, and quality of the products. Therefore, none of these methods has been practiced industrially.

For example, the CVD process requires sacrificial oxidation. The solid growth results in low crystallinity. The beam annealing process involves problems in treating time, control of superposition of the beam and focus adjustment. Among the above methods, zone melting recrystallization is most highly developed, and has been employed for experimental production of relatively large integrated circuits. This method, however, still causes crystal defects such as subgrain boundary, etc., and does not give a minority carrier device.

The methods (2) above, which does not use the Si substrate as the seeds for epitaxial growth, are practiced in the three ways below: form V-shaped grooves on the surface. An oxide film is formed thereon. On the oxide film, a polycrystalline Si layer is deposited in a thickness that is nearly the same as the Si substrate. Then the Si substrate is abraded at the backside to form dielectrically separated monocrystalline Si regions surrounded by the V-shaped grooves on the thick polycrystalline Si layer. Although this technique gives satisfactory crystallinity, it involves problems in control and productivity in the process of depositing polycrystalline Si in a thickness of as much as several hundred .mu.m and in the process of abrading the monocrystalline Si substrate from the backside to leave only the separated active Si layer. implantation. This method is called SIMOX (separation by ion implanted oxygen). This process exhibits excellent coherency with the Si process, and is the most highly developed technique at the moment. However, the process requires implantation of oxygen ions as much as 10.sup.18 ions/cm.sup.2 to form the SiO.sub.2 layer, and the implantation takes long time, so that the productivity is not high, and the wafer cost is high. Furthermore, the product has many remaining crystal defects, and does not have satisfactory quality for industrial production of minority carrier devices. porous Si. In this method, on a surface of a P-type monocrystalline Si substrate, an N-type Si layer is formed in an island shape by proton ion implantation (Imai, et al.: J. Crystal Growth, vol. 63, 547, (1983)), or by epitaxial growth and patterning, then the P-type Si substrate only is made porous by anodization in an HF solution so as to surround the island-shaped Si regions, and the N-type Si islands are separated dielectrically by accelerated oxidation. This method has disadvantage that the freedom in device design is frequently restricted since the Si region to be separated has to be decided prior to the device production process.

For example, Japanese Patent Publication No. 53-45675 discloses a process for forming SOI in which a monocrystalline silicon layer is grown on a porous layer or a porous insulating layer. In this process, a porous layer is heat-treated in an oxidizing atmosphere to give the layer of high resistance, thereby obtaining an SOI structure. This process, although proposed more than 10 years ago, has not been practically employed. This is because the porous material is thermally instable and nonuniform in structure. In the porous material, extremely fine pores are formed in monocrystalline silicon. The thickness of monocrystalline silicon layer remaining between the pores can be made to be in the range of as small as from several nm to several tens of nm. Such porous silicon is oxidizable in a much larger reaction velocity than normal monocrystalline silicon. By utilizing this phenomenon, the SOI can be prepared by selectively oxidizing the porous lower portion of the monocrystal layer. This method, however, involves the disadvantage that satisfactory insulating layer cannot readily be prepared owing to residual silicon in the insulating layer and variation of the thickness of the oxide layer which are caused by nonuniformity in size of the silicon regions remaining in the porous layer. This method involves a further disadvantage that the porous material comes to have coarser structure during the heat treatment at the high temperature required for oxidation, whereby the rate of oxidation falls. Furthermore, if the porous silicon or a high-resistance porous material remains on the substrate, it induces denaturation such as structural change or coarsening of the porous structure in heat treatment for formation of an element on the substrate, which tends to cause warpage or distortion of the substrate.

On the other hand, a light-transmissive substrate typified by glass allows Si to grow only into an amorphous or polycrystalline layer under the influence of the disorderness of the crystal structure thereof, and is unsuitable for production of devices of high performance. Simple deposition of Si on such a substrate will not give excellent monocrystal layer because of the amorphous structure of the substrate. The light-transmissive substrate is important in constructing a contact sensor as a light-receiving element, a projection type liquid crystal image displaying apparatus, and the like. In order to provide a sensor or a display apparatus with image elements (picture elements) in higher density, higher resolution, and higher fineness, extremely high performance of the driving element is required. Therefore, the element on a light-transmissive substrate have to be made from monocrystal layer having excellent crystallinity.

In other words, amorphous Si or polycrystalline Si will not generally give a driving element which exhibits the satisfactory performance required or to be required in the future because of many defects in the crystal structure.

On a light-transmissive substrate, however, none of the above methods for a monocrystalline Si substrate is suitable for forming an excellent monocrystal layer.

It is, therefore, an object of the present invention to provides a semiconductor substrate which is free from the aforementioned problems and satisfies the above requirements, and to provide a process for production thereof.

Another object of the present invention is to provide a process for producing a semiconductor substrate of SOI structure which prevents strongly coarsening of the porous structure at the high temperature in the epitaxial growth and the substrate bonding operation.

Another object of the present invention is to provide a semiconductor substrate which has the advantage of the conventional SOI structure and is applicable to various high-performance electronic devices, and to provide a process for production thereof.

Still another object of the present invention is to provide a semiconductor substrate which is useful in place of expensive SOS and SIMOX in production of large scale integrated circuits, and to a process for production thereof.

Yet another object of the present invention is to provide a process for producing a semiconductor substrate having, on an insulating layer, an Si layer of crystallinity as high as monocrystalline wafer with prominence in productivity, uniformity, controllability, and cost.

Again, another object of the present invention is to provide a process for producing a semiconductor substrate having, on a transparent (light-transmissive) base plate, an Si layer of crystallinity as high as monocrystalline wafer with prominence in productivity, uniformity, controllability, and cost.

FIGS. 1A to 1E are schematic views for explaining an example of the process of the present invention.

FIG. 2 is a graph showing the etching characteristics of porous silicon and nonporous silicon.

FIGS. 3A to 3E are schematic views for explaining another example of the process of the present invention.

FIGS. 4A to 4E are schematic views for explaining another example of the process of the present invention.

A process for producing a semiconductor substrate of the present invention comprises the steps of forming a porous layer in a first substrate comprising monocrystalline silicon; forming a protective film on a side wall of the pores of the porous layer; forming a nonporous monocrystalline silicon layer on the porous layer, bonding the surface of the nonporous monocrystalline silicon layer onto a second substrate with interposition of an insulating layer; and etching off selectively the porous layer by use of a chemical etching solution.

In the present invention, forming a protective film on the side wall of the pores of the porous silicon layer prevents coarse structure formation in the porous silicon layer, and maintains the selectivity of the etching. Therefore, heat treatment at a high temperature is practicable in order to form nonporous monocrystalline silicon with high crystallinity or to form strong bonding. Thereby, a monocrystalline Si layer with extremely few defects or weak bonding points is obtainable on a base member which has an insulating layer on the surface or on a light-transmissive base member.

The present invention is described in more detail by reference to examples of the embodiment.

A method is explained in which an Si substrate is made porous and thereon a monocrystal layer is allowed to grow epitaxially.

A monocrystalline Si substrate 11 is provided and made porous entirely as shown in FIG. 1A or partially as shown in FIG. 4A, thereby forming a porous layer 15.

The Si substrate is porousified by anodization in an HF solution. While the density of monocrystalline Si is 2.33 g/cm.sup.3, the density of porous monocrystalline Si layer 15 can be varied from 0.6 to 1.1 g/cm.sup.3 by varying the concentration of the HF solution in the range of from 20 to 50%. The porous monocrystalline Si layer 15 tends to be formed readily in a P-type substrate by the reasons below. The pores formed have an average diameter of from about 50 .ANG. to about 600 .ANG. according to observation by transmission electron microscopy.

The porous monocrystalline Si was found in the course of study on electropolishing of semiconductors in the year 1956 by A. Uhlir et al. (A. Uhlir, et al.: Bell Syst. Tech. J., vol. 35, p. 333 (1956)). The dissolving reaction of Si in anodization was studied by Unagami, who reported that positive holes are required for anodic reaction of Si in an HF solution and the reaction proceeds according to the formulas below (T. Unagami: J. Electrochem. Soc., vol. 127, p. 476 (1980):

or otherwise,

where e.sup.+ and e.sup.- represent respectively a positive hole and an electron; n and .lambda. represent respectively the number of the positive holes required for dissolving one atom of monocrystalline Si, and it was also reported that porous silicon is formed if n>2, or .lambda.>4.

Accordingly, a P-type Si having positive holes is more readily made porous. This selectivity in porous structure formation has already been actually proved by Nagano et al. (Nagano, Nakajima, Yasuno, Ohnaka, and Kajihara: Densi Tsushin Gakkai Gijutsu Kenkyu Hokoku (Technical Bulletin of Electronic Communication Society) vol. 79, SSD 79-9549 (1979)) and K. Imai: Solid State Electronics, vol. 24, p. 159 (1981)). Thus, P-type silicon having positive holes can be selectively porousified.

On the other hand, it was also reported that high-concentration N-type silicon can also be made porous (R. P. Holmstorm, I. J. Y. Chi: Appl. Phys. Lett. vol. 42, p. 386 (1983)). Therefore it is important to select the substrate capable of being made porous, regardless of whether the structure is of P-type or of N-type.

After the monocrystalline silicon substrate is porousified, a protective film is formed on the side wall of the pores of the porous layer as shown in FIG. 1B or FIG. 4B.

The protective film is made of a material including silicon oxide, silicon nitride, etc. The protection film is formed by deposition or adhesion of another film, or by denaturation of the silicon itself by chemical reaction. Specifically the protection film is formed, for example, by (1) oxidation treatment such as thermal oxidation, oxidation by oxygen plasma, and immersion in an oxidizing solution; (2) nitridation treatment such as heat treatment in a nitrogen atmosphere or an ammonia atmosphere, and nitridation by ammonia plasma; and the like methods. However, the protective film is not limited thereto, but any protective film may be used which allows the epitaxial growth and the bonding in the subsquent steps.

The oxidation or the nitridation proceeds from the wall of the pores to the interior thereof in the porous material. It is important that the oxidation is stopped before all of the porous layer is oxidized and silicon region is lost, and that the monocrystallinity of silicon is retained. The monocrystallinity of silicon is confirmed by X-ray diffraction, Rutherford backscattering spectroscopy, etc. The thickness of the protective film is decided depending on the porous matter, and preferably in the range of from 1 nm to 40 nm, more preferably 2 nm to 30 nm. The formation of the protective film is desirably conducted at a low temperature where structural change or coarsening of the porous matter does not occur. The temperature is preferably in the range of from 100.degree. C. to 900.degree. C., more preferably from 200.degree. C. to 700.degree. C. In particular, in the thermal oxidation, the temperature is preferably not lower than 400.degree. C.

In the aforementioned oxidation in an oxidizing solution, the oxidizing solution includes aqueous hydrogen peroxide solution, sulfuric acid solution, and nitric acid solution, and mixtures of aqueous hydrogen peroxide with an acid such as sulfuric acid, nitric acid, and hydrochloric acid, but is not limited thereto.

Subsequently, on the porous surface of the substrate, a thin monocrystalline layer 12 is formed as shown in FIG. 1C or FIG. 4C by any epitaxial growth method. If necessary, prior to the epitaxial growth, the oxide layer on the porousified substrate surface of the oxide layers formed by the oxidation treatment is removed by etching with aqueous hydrofluoric acid or other procedure.

In the porous Si layer, pores of about 600 .ANG. in average diameter are formed according to observation with transmission electron microscopy. Although the density thereof is half or less than the density of monocrystalline Si, the monocrystallinity is retained, and epitaxial growth of monocrystalline Si is practicable on the surface. However, rearrangement of the internal pores occurs in epitaxial growth at a temperature above 1000.degree. C., which impairs the characteristics of enhanced etching. Therefore, the epitaxial growth of an Si layer is preferably conducted by a low temperature growth process such as molecular beam epitaxial growth, plasma CVD, thermal CVD, photo-assisted CVD, bias sputtering, liquid phase growth, etc.

In the epitaxial growth on the porous Si, the strain caused in the heteroepitaxial growth can be relaxed, and development of defects can be prevented by the structural properties of the porous Si.

The porous layer has a density of half or less than the density of nonporous layer because of the large amount of void in the interior thereof, and has greater surface area for the volume. Consequently, the rate of the chemical etching is significantly increased than that of a normal monocrystalline layer.

The known methods for the etching of porous Si include: K. Barla and J. C. Pfister: J. Electrochem. Soc., vol. 130, no. 7, p. 1611 (1983)), and monocrystalline Si.

In the above method (2), a hydrofluoric nitric acid type etching solution is used normally. With this etching solution, the etching proceeds with oxidation of Si by nitric acid to form SiO.sub.2 and subsequent etching of the resulting SiO.sub.2 by hydrofluoric acid as shown below:

The known methods for etching crystalline Si include etching with an etching solution of ethylene diamine type, KOH type, hydrazine type or the like as well as the above hydrofluoric nitric acid type etching solution.

The selective etching of porous Si, which is particularly effective and important in the present invention, employs hydrofluoric acid or a buffered hydrofluoric acid which does not have etching action for crystalline Si. In this etching, an aqueous hydrogen peroxide solution may be used additionally as an oxidizing agent. The reaction rate can be controlled by changing the ratio of addition of the hydrogen peroxide. An alcohol may be added which serves as a surface active agent to remove instantaneously the bubbles of the gaseous reaction product from the etching surface, and enables selective etching of porous Si uniformly and efficiently.

Preliminary oxidation of the porous silicon accelerates the etching of the porous silicon layer by hydrofluoric acid. Further, since the silicon wall of the porous layer is made thin, the etching rate is increased practically by 20%, thereby raising the selectivity of etching of the porous monocrystalline silicon layer relative to the nonporous monocrystalline silicon layer. FIG. 2 shows the etching characteristics of the porous silicon and the nonporous silicon. In FIG. 2, the solid line shows the progress of the etching of oxidized porous silicon, and the broken line shows that of unoxidized porous silicon.

The porous silicon, when thermally treated at a high temperature, is subjected to rearrangement and especially coarsening of the pores, resulting in decrease of the etching rate. However, the oxidation of the porous silicon suppresses the coarsening of the pores in the porous layer and prevents the decrease of the etching rate of the porous layer even when high temperature treatment is conducted later for high temperature growth to form a nonporous monocrystalline silicon layer with high crystallinity and for strengthening the bonding to the substrate. In particular, when a hydrofluoric type etching solution is employed, the oxidation of the porous silicon accelerates the etching of the porous layer by 20% in comparison with unoxidized porous silicon even after the heat treatment.

Subsequently, as shown in FIG. 1D and FIG. 4D, the surface of the monocrystalline Si layer 12 on the porous Si substrate 14 is bonded to a surface of a second substrate 13 composed of a base material such as a silicon base and an insulating layer formed thereon, or a light-transmissive substrate 13.

Prior to the bonding, an oxide layer may be formed on the surface of the monocrystalline layer 12 on the porous Si to form beforehand an interface between the monocrystalline silicon layer and the insulating layer. The oxide layer plays an important role in preparation of a device. That is, as to the interfacial level caused by an interface with an underlying base of an Si active layer, the level of the interface with an underlying base formed by oxidation of a monocrystalline silicon layer can be made smaller than that of a bonding interface especially with glass. In other words, by keeping a bonding interface apart from an active layer, the energy level which may be caused in the bonded interface is kept apart, whereby the performance of the electronic device is remarkably improved. The monocrystalline Si layer on porous Si, on the surface of which an oxide layer is formed, may be bonded to a desired substrate such as an Si substrate.

Thereafter, the porous Si substrate 14 is entirely removed by chemical etching to obtain a substrate which is constituted of a base plate having an insulating layer on the surface thereof or a light-transmissive base plate and a thin-layered monocrystalline silicon layer remaining thereon. Prior to the etching, an etching prevention film may be formed on the portion other than the porous silicon portion. For example, the two substrates bonded together is entirely coated by deposition of Si.sub.3 N.sub.4, and the portion of the Si.sub.3 N.sub.4 on the porous silicon surface is removed before the etching. As the etching prevention film, Apiezon Wax may be used in place of the Si.sub.3 N.sub.4.

In the case where the Si substrate is partially made porous as shown in FIG. 4D, the nonporous part of the substrate is removed to uncover the porous layer by grinding or abrasion, or etching with a mixture of hydrofluoric acid, nitric acid and acetic acid or other etching solution as usually employed in Si wafer production.

FIG. 1E and FIG. 4E illustrate respectively a semiconductor substrate obtained according to the present invention, in which a monocrystalline Si layer 12 having crystallinity similar to a silicon wafer is formed in a shape of uniform flat thin layer in a large area on the substrate 13 having an insulating layer on the surface or a light-transmissive substrate 13 over the entire wafer.

The resulting semiconductor substrate is can be used suitably for production of dielectrically isolated electronic elements.

Another example of the embodiment of the process for producing a semiconductor substrate of the present invention is described in detail in reference to the drawings.

FIGS. 3A to 3E illustrate schematically the steps of producing a semiconductor substrate of the present invention by cross-sectional views.

Firstly, as shown in FIG. 3A, a low-impurity-density layer 32 is formed epitaxially by a thin film growth method, or otherwise a N-type monocrystalline layer 32 is formed by proton ion implantation, on the surface of a P-type monocrystalline Si substrate 31.

Then as shown in FIG. 3B, the P-type monocrystalline Si substrate 31 is made porous from the backside by anodization by use of an HF solution to form a porous Si layer 33. While the density of monocrystalline Si is 2.33 g/cm.sup.3, the density of porous monocrystalline Si layer 33 can be varied from 1.1 to 0.6 g/cm.sup.3 by varying the concentration of the HF solution in the range of from 50 to 20%. The porous layer is formed in a P-type substrate as described before. After the porous structure formation in the monocrystalline silicon substrate, a protective film is formed on the side wall of the pores of the porous layer as shown in FIG. 3C. In forming the protective layer, the procedure as described in Embodiment 1 is also employed.

Then as shown in FIG. 3D, a substrate 34 having an insulating layer on the surface is bonded to the surface of the monocrystalline Si layer 32 on the porous Si substrate 35 or to the oxidized surface of the monocrystalline Si layer 32. An oxide layer may be formed on the monocrystalline Si layer on the porous Si layer, and may be bonded to a desired substrate such as an Si substrate.

The porous layer 35 of the porous Si substrate is entirely removed by etching to obtain a substrate having an insulting layer thereon and a thin monocrystalline silicon layer remaining thereon as shown in FIG. 3E.

FIG. 3E illustrates a semiconductor substrate obtained according to the present invention, in which a monocrystalline Si layer 32 having crystallinity similar to a silicon wafer is formed in a shape of uniform flat thin layer in a large area on a substrate 34 having an insulating layer on the surface or a light-transmissive substrate 34 over the entire wafer.

The resulting semiconductor substrate can be used suitably for production of dielectrically isolated electronic elements.

In the above embodiment, an N-type layer is formed before the porous structure formation, and subsequently the P-type portion of the substrate only is selectively made porous.

The present invention is described in more detail by reference to specific examples.

A P-type (100) monocrystalline Si substrate of 200 .mu.m thick was anodized in a 50% HF solution at a current density of 100 mA/cm.sup.2. The porous structure formation rate was 8.4 .mu.m/min. The entire of the P-type (100) Si substrate of 200 .mu.m thick was made porous in 24 minutes.

The porous substrate was immersed for 10 minutes in a 5% aqueous hydrogen peroxide solution heated to 80.degree. C. On the resulting P-type (100) porous Si substrate, an Si epitaxial layer was made to grow at a low temperature by an MBE (molecular beam epitaxy) method to a thickness of 0.52 .mu.m. The deposition conditions were as follows:

The surface of this epitaxial layer was thermally oxidized in a thickness of 50 nm. Onto the thermally oxidized layer, was superposed a monocrystalline silicon substrate having a silicon oxide layer of 1 .mu.m thick formed on the surface. The combined substrates were heated at 950.degree. C. for 2 hours in an oxygen atmosphere to bond the two substrates tightly.

The bonded substrates were subjected to selective etching by use of a mixture of a buffered hydrofluoric acid, alcohol, and hydrogen peroxide solution (10:6:50) as the etching solution without stirring. In 175 minutes, the porous Si substrate was selectively etched and removed completely with the monocrystalline Si layer remaining unetched as the etch-stop material.

The rate of etching of the nonporous monocrystalline Si by the etching solution is as low as 40 .ANG. or less for 175 minutes. The selectivity of the etching of the porous layer to the nonporous layer was 10.sup.5 or more, so that the amount of etching of the nonporous layer (several tens of .ANG.) is practically negligible. After removal of 200 .mu.m thick porous Si substrate and the Si.sub.3 N.sub.4 layer, there was obtained a monocrystalline Si layer formed in a thickness of 0.5 .mu.m on the surface of the silicon oxide surface on the silicon substrate.

The Si layer was confirmed to have sufficient crystallinity without introduction of new crystal defect according to observation by transmission electron microscopy.

A P-type (100) monocrystalline Si substrate of 200 .mu.m thick was anodized in a 50% HF solution at a current density of 100 mA/cm.sup.2. The porous structure formation rate was 8.4 .mu.m/min. The entire of the P-type (100) Si substrate of 200 .mu.m thick was made porous in 24 minutes.

The porous substrate was subjected to heat treatment at 300.degree. C. for 2 hours in an oxygen atmosphere. On the resulting P-type (100) porous Si substrate, an Si epitaxial layer was made to grow at a low temperature by a plasma CVD method in a thickness of 5 .mu.m. The deposition conditions were as follows:

The surface of this epitaxial layer was thermally oxidized to a thickness of 50 nm. Onto the thermally oxidized layer, was superposed an optically polished fused quarts substrate. The combined substrates were heated at 1000.degree. C. for 2 hours in an oxygen atmosphere to bond the two substrates tightly.

The bonded two substrates were coated with 0.1 .mu.m thick Si.sub.3 N.sub.4 by plasma CVD. Then the nitride film on the porous substrate only was removed by reactive ion etching.

The bonded substrates were subjected to selective etching by use of a mixture of a buffered hydrofluoric acid, alcohol, and hydrogen peroxide solution (10:6;50) as the etching solution without stirring. In 175 minutes, the porous Si substrate was selectively etched and removed completely with the monocrystalline Si layer remaining unetched as the etch-stop material.

The rate of etching of the nonporous monocrystalline Si by the etching solution is as low as 40 .ANG. or less for 175 minutes. The selectivity of the etching of the porous layer to the nonporous layer was 10.sup.5 or more, so that the amount of etching of the nonporous layer (several tens of .ANG.) is practically negligible. After removal of the 200 .mu.m thick porous Si substrate and the Si.sub.3 N.sub.4 layer, there was obtained a monocrystalline Si layer of 5 .mu.m thick on the fused quartz glass substrate.

A P-type (100) monocrystalline Si substrate of 200 .mu.m thick was anodized in a 50% HF solution at a current density of 100 mA/cm.sup.2. The porous structure formation rate was 8.4 .mu.m/min. The entire of the P-type (100) Si plate of 200 .mu.m thick was made porous in 24 minutes.

The porous substrate was subjected to heat treatment at 400.degree. C. for one hour in an oxygen atmosphere. Then on the resulting P-type (100) porous Si substrate, an Si epitaxial layer was made to grow by a CVD method in a thickness of 1 .mu.m. The deposition conditions were as follows: Gas:                   SiH.sub.2 Cl.sub.2 (0.6 l/min) H.sub.2 (100 l/min) Temperature:           950.degree. C. Pressure:              80 Torr Growth rate:           0.3 .mu.m/min

The surface of this epitaxial layer was thermally oxidized to a thickness of 50 nm. Onto the thermally oxidized layer, was superposed an optically polished glass substrate having a softening point of about 500.degree. C. The combined substrates were heated at 450.degree. C. for 0.5 hour in an oxygen atmosphere to bond the two substrates tightly.

The bonded two substrates were coated with 0.1 .mu.m thick Si.sub.3 N.sub.4 by plasma CVD. Then the nitride film on the porous substrate only was removed by reactive ion etching.

The bonded substrates were subjected to selective etching by use of a mixture of a buffered hydrofluoric acid, alcohol, and hydrogen peroxide solution (10:6:50) as the etching solution without stirring. In 175 minutes, the porous Si substrate was selectively etched and removed completely with the monocrystalline Si layer remaining unetched as the etch-stop material.

The rate of etching of the nonporous monocrystalline Si by the etching solution is as low as 40 .ANG. or less for 175 minutes. The selectivity of the etching of the porous layer to the nonporous layer was 10.sup.5 or more, so that the amount of etching of the nonporous layer (several tens of .ANG.) is practically negligible. After removal of the 200 .mu.m thick porous Si substrate and the Si.sub.3 N.sub.4 layer, there was obtained a monocrystalline Si layer of 1 .mu.m thick on the glass substrate having a low softening point.

In the case where the Si.sub.3 N.sub.4 layer was replaced by Apiezon Wax coating or Electron Wax coating, the effect was the same, and the porous Si substrate only was removed completely.

A P-type (100) monocrystalline Si substrate of 200 .mu.m thick was anodized in a 50% HF solution at a current density of 100 mA/cm.sup.2. The porous structure formation rate was 8.4 .mu.m/min. The entire of the P-type (100) Si plate of 200 .mu.m thick was made porous in 24 minutes.

The porous substrate was subjected to heat treatment at 500.degree. C. for one hour in an atmosphere containing ammonia. On the resulting P-type (100) porous Si substrate, an Si epitaxial layer was made to grow by a bias sputtering method in a thickness of 1.0 .mu.m. The deposition conditions were as follows:

On the surface of this epitaxial layer, 500 nm thick silicon oxide layer was formed by thermal oxidation. Onto the thermally oxidized layer, was superposed an Si substrate. The combined substrates were heated at 1000.degree. C. for 2 hours in a nitrogen atmosphere to bond the two substrates tightly.

The bonded substrates were subjected to selective etching by use of a mixture of a buffered hydrofluoric acid, alcohol, and hydrogen peroxide solution (10:6:50) as the etching solution without stirring. In 175 minutes, the porous Si substrate was selectively etched and removed completely with the monocrystalline Si layer remaining unetched as the etch-stop material.

The rate of etching of the nonporous monocrystalline Si by the etching solution is as low as 40 .ANG. or less for 175 minutes. The selectivity of the etching of the porous layer to the nonporous layer was 10.sup.5 or more, so that the amount of etching of the nonporous layer (several tens of .ANG.) is practically negligible.

By the etching, the porous Si substrate of 200 .mu.m thick was removed, and a monocrystalline Si layer of 0.75 .mu.m thick was formed on the Si substrate with interposition of the oxide layer of 500 nm thick.

A P-type (100) monocrystalline Si substrate of 600 .mu.m thick was anodized in a 50% HF solution at a current density of 5 mA/cm.sup.2. The porous structure formation rate was 1 .mu.m/min. On the surface of the P-type (100) Si substrate of 600 .mu.m thick was formed a porous layer in a thickness of 20 .mu.m.

The porousified substrate was subjected to heat treatment at 300.degree. C. for 2 hours in an oxygen atmosphere. The heat-treated substrate was found to be monocrystalline by an RHEED method. On the P-type (100) porous Si substrate, an epitaxial Si layer was made to grow at a low temperature by a liquid phase growth method in a thickness of 10 .mu.m. The growth conditions were as follows:

Onto the surface of this epitaxial Si layer, a monocrystalline silicon substrate was superposed which had a silicon oxide layer of 1 .mu.m thick formed on the surface. The combined substrates were heated at 900.degree. C. for 5 hours in an oxygen atmosphere to bond the two substrates tightly.

Then the nonporous portion of the substrate having the porous layer of the bonded substrates was removed by grinding to uncover the porous layer.

The bonded substrates were subjected to selective etching by use of a mixture of a buffered hydrofluoric acid, alcohol, and hydrogen peroxide solution (10:6:50) as the etching solution without stirring. In 20 minutes, the porous Si substrate was selectively etched off completely with the monocrystalline Si layer remaining unetched as the etch-stop material.

The rate of etching of the nonporous monocrystalline Si by the etching solution is as low as 10 .ANG. or less for 20 minutes. The selectivity of the etching of the porous layer to the nonporous layer was 10.sup.5 or more, so that the amount of etching of the nonporous layer (several tens of .ANG.) is practically negligible.

By the etching, the 600 .mu.m thick Si substrate having the porous layer was removed, and the monocrystalline Si layer of 10 .mu.m thick was formed on the silicon substrate having an oxide surface layer.

On a P-type (100) Si substrate of 200 .mu.m thick, an Si epitaxial layer was made to grow to a thickness of 0.5 .mu.m by a CVD method. The deposition conditions were as follows: Reactive gas flow rate:  SiH.sub.2 Cl.sub.2, 1000 SCCM H.sub.2, 230 l/min Temperature:             1080.degree. C. Pressure:                80 Torr Time:                    1 min

This substrate was anodized in a 50% HF solution at a current density of 100 MA/cm.sup.2. The porous structure formation rate was 8.4 .mu.m/min. The entire of the P-type (100) Si substrate of 200 .mu.m thick was made porous in 24 minutes. In the anodization, the P-type (100) Si substrate only was made porous, and the Si epitaxial layer remained unchanged as described above.

The substrate which had been made porous was immersed in a mixture of 36% hydrochloric acid, 30% aqueous hydrogen peroxide solution and water in the ratio of 1:1:3 at 80.degree. C. for 10 minutes and washed with water.

The surface of the epitaxial layer was thermally oxidized to a thickness of 50 nm. Onto the thermally oxidized layer, was superposed an optically polished fused quarts substrate. The superposed two substrates were heated at 1000.degree. C. for 0.5 hour in an oxygen atmosphere to bond the two substrates tightly.

The bonded two substrates were coated with Si.sub.3 N.sub.4 in a thickness of 0.1 .mu.m by low pressure CVD. Then the nitride film on the porous substrate only was removed by reactive ion etching.

The bonded substrates were subjected to selective etching by use of a mixture of a buffered hydrofluoric acid, alcohol, and hydrogen peroxide solution (10:6:50) as the etching solution without stirring. In 175 minutes, the porous Si substrate was selectively etched off completely with the monocrystalline Si layer remaining unetched as the etch-stop material.

The rate of etching of the nonporous monocrystalline Si by the etching solution is as low as 40 .ANG. or less for 175 minutes. The selectivity of the etching rate of the porous layer to that of the nonporous layer was 10.sup.5 or more, so that the amount of etching of the nonporous layer (several tens of .ANG.) is practically negligible. After removal of the 200 .mu.m thick porous Si substrate and the Si.sub.3 N.sub.4 layer, there was obtained the monocrystalline Si layer of 0.5 .mu.m thick on the fused quartz glass substrate.

In the case where the Si.sub.3 N.sub.4 layer was replaced by Apiezon Wax coating or Electron Wax coating, the effect was the same, and the porous Si substrate only was removed completely.

The Si layer was confirmed to have sufficient crystallinity without introduction of new crystal defect according to sectional observation by transmission electron microscopy.

On the surface of a 200 .mu.m thick P-type (100) Si substrate, an N-type Si layer was formed in a thickness of 1 .mu.m by proton ion implantation. The amount of implanted H.sup.+ was 5.times.10.sup.15 (ions/cm.sup.2).

This substrate was anodized in a 50% HF solution at a current density of 100 mA/cm.sup.2. The porous structure formation rate was 8.4 .mu.m/min. The entire of the P-type (100) Si substrate of 200 .mu.m thick was made porous in 24 minutes. In the anodization, the P-type (100) Si substrate only was made porous, and the N-type Si layer remained unchanged, as described above.

The substrate made porous was heat-treated at 300.degree. C. for 2 hours in an oxygen atmosphere.

The surface of the N-type monocrystalline layer was thermally oxidized to a thickness of 50 nm. Onto the thermally oxidized layer, was superposed an optically polished fused quarts substrate. The combined substrates were heated at 800.degree. C. for 0.5 hour in an oxygen atmosphere to bond the two substrates tightly.

The bonded two substrates were coated with Si.sub.3 N.sub.4 in a thickness of 0.1 .mu.m by low pressure CVD. Then the nitride film on the porous substrate only was removed by reactive ion etching.

The bonded substrates were subjected to selective etching by use of a mixture of a buffered hydrofluoric acid, alcohol, and hydrogen peroxide solution (10:6:50) as the etching solution without stirring. In 175 minutes, the porous Si substrate was selectively etched off completely with the monocrystalline Si layer remaining unetched as the etch-stop material.

The rate of etching of the nonporous monocrystalline Si by the etching solution is as low as 40 .ANG. or less for 175 minutes. The selectivity ratio of the etching rate of the porous layer to that of the nonporous layer was 10.sup.5 or more, so that the amount of etching of the nonporous layer (several tens of .ANG.) is practically negligible. After removal of the 200 .mu.m thick porous Si substrate and the Si.sub.3 N.sub.4 layer, there was obtained a monocrystalline Si layer of 1.0 .mu.m thick on the glass substrate.

In the case where the Si.sub.3 N.sub.4 layer was replaced by Apiezon Wax coating or Electron Wax coating, the effect was the same, and the porous Si substrate only was removed completely.

The Si layer was confirmed to have sufficient crystallinity without introduction of new crystal defect according to sectional observation by transmission electron microscopy.

As described above, the present invention provides a semiconductor substrate and a process for producing the semiconductor substrate which are free from the disadvantages of prior art and satisfy the requirements for the prior art.

The present invention also provides a process for producing a semiconductor substrate having an Si crystalline layer of crystallinity as high as that of a monocrystalline wafer, on a substrate having an insulating layer on the surface or a light-transmissive insulating substrate typified by glass, with prominence in productivity, uniformity, controllability, and cost.

The present invention further provides to provide a semiconductor substrate which has the advantage of the conventional SOI structure and is applicable to various high-performance electronic devices, and to provide a process for production thereof.

The present invention further provides a semiconductor substrate which is useful in place of expensive SOS and SIMOX in production of large scale integrated circuits of SOI structure, and to a process for production thereof.

In the present invention, by using a monocrystalline silicon substrate having originally high quality as a starting material, a surface monocrystalline layer is transferred onto a light-transmissive insulating substrate, and rest of the substrate is chemically removed, whereby a number of treatments are practicable in a short time and brings about great improvements in productivity and cost as described above.

## Claims

1. A process for producing a semiconductor substrate, comprising the steps of: having a porous monocrystalline semiconductor layer, the porous monocrystalline semiconductor layer having pores with inside walls; porous monocrystalline semiconductor layer and on a surface of the porous monocrystalline semiconductor layer; monocrystalline semiconductor layer while retaining the protective film formed on the inside walls of the pores of the porous monocrystalline semiconductor layer, monocrystalline semiconductor layer by epitaxial growth; layer therebetween to obtain a multi-layer structure member such that the non-porous monocrystalline semiconductor layer is positioned inside of the multi-layer structure member; and multi-layer structure member by chemical etching.

2. The process according to claim 1, wherein the first substrate is formed of a monocrystalline silicon substrate having a porous monocrystalline silicon layer.

3. The process according to claim 2, wherein the step of providing the first substrate comprises a step of making porous at least a part of the monocrystalline silicon substrate by anodization.

4. The process according to claim 2, wherein the step of forming the protective film comprises an oxidizing step of partly oxidizing the inside walls of the pores of the porous monocrystalline silicon layer and the surface of the porous monocrystalline silicon layer.

5. The process according to claim 4, wherein the oxidizing step includes performing a thermal oxidation at a temperature of 400.degree. C. to 700.degree. C.

6. The process according to claim 4, wherein the oxidizing step includes dipping the porous monocrystalline silicon layer into an oxidative solution.

7. The process according to claim 6, wherein the oxidative solution comprises an aqueous hydrogen peroxide solution.

8. The process according to claim 4, wherein the step of removing the protective film formed on the surface of the porous monocrystalline silicon layer comprises a step of etching an oxide film formed on the surface of the porous monocrystalline silicon layer with hydrofluoric acid.

9. The process according to claim 2, wherein the step of forming the protective film comprises a step of partly nitriding the inside walls of the pores of the porous monocrystalline silicon layer and the surface of the porous monocrystalline silicon layer.

10. The process according to claim 1, wherein the second substrate comprises silicon.

11. The process according to claim 1, wherein the second substrate and the insulating layer are formed by oxidizing a surface of a silicon substrate.

12. The process according to claim 1, wherein the insulating layer is formed by oxidizing a surface of the non-porous semiconductor layer.

13. The process according to claim 1, wherein the insulating layer comprises a first insulating layer and a second insulating layer, the first insulating layer is formed by oxidizing a surface of the non-porous semiconductor layer, and the second insulating layer and the second substrate are formed by oxidizing a surface of a silicon substrate.

14. The process according to claim 2, wherein the chemical etching is carried out using a solution containing hydrofluoric acid.

15. The process according to claim 2, further comprising before the step of removing the porous monocrystalline semiconductor layer by chemical etching, a step of removing a remaining non-porous part of the monocrystalline silicon substrate from the multi-layer structure member to expose the porous monocrystalline semiconductor layer, and wherein the step of providing the first substrate comprises a step of making porous a part of the monocrystalline silicon substrate by anodization.

16. The process according to claim 15, wherein the step of removing the remaining non-porous part of the monocrystalline silicon substrate comprises a step of polishing or grinding the remaining non-porous part of the monocrystalline silicon substrate.

17. A process for producing a semiconductor substrate, comprising the steps of: having a porous monocrystalline semiconductor layer, the porous monocrystalline semiconductor layer having pores with inside walls; porous monocrystalline semiconductor layer and a surface of the porous monocrystalline semiconductor layer; monocrystalline semiconductor layer while retaining the protective film formed on the inside walls of the pores of the porous monocrystalline semiconductor layer; monocrystalline semiconductor layer by epitaxial growth; obtain a multi-layer structure member such that the non-porous monocrystalline semiconductor layer is positioned inside of the multi-layer structure member; and multi-layer structure member by chemical etching.

18. The process according to claim 17, wherein the first substrate is formed of a monocrystalline silicon substrate having a porous monocrystalline silicon layer.

19. The process according to claim 18, wherein the step of providing the first substrate comprises a step of making porous at least a part of the monocrystalline silicon substrate by anodization.

20. The process according to claim 18, wherein the step of forming the protective film comprises an oxidizing step of partly oxidizing the inside walls of the pores of the porous monocrystalline silicon layer and the surface of the porous monocrystalline silicon layer.

21. The process according to claim 20, wherein the oxidizing step includes performing a thermal oxidation at a temperature of 400.degree. C. to 700.degree. C.

22. The process according to claim 20, wherein the oxidizing step includes dipping the porous monocrystalline silicon layer into an oxidative solution.

23. The process according to claim 22, wherein the oxidative solution comprises an aqueous hydrogen peroxide solution.

24. The process according to claim 20, wherein the step of removing the protective file formed on the surface of the porous monocrystalline silicon layer comprises a step of etching an oxide film formed on the surface of the porous monocrystalline silicon layer with hydrofluoric acid.

25. The process according to claim 18, wherein the step of forming the protective film comprises a step of partly nitriding the inside walls of the pores of the porous monocrystalline silicon layer and the surface of the porous monocrystalline silicon layer.

26. The process according to claim 17, wherein the second substrate comprises glass.

27. The process according to claim 17, wherein the second substrate comprises quartz.

28. The process according to claim 17, further comprising before said bonding step, a step of oxidizing a surface of the non-porous monocrystalline semiconductor layer.

29. The process according to claim 18, wherein the chemical etching is carried out using a solution containing hydrofluoric acid.

30. The process according to claim 18, further comprising before the step of removing the porous monocrystalline semiconductor layer by chemical etching, a step of removing a remaining non-porous part of the monocrystalline silicon substrate from the multi-layer structure member to expose the porous monocrystalline semiconductor layer, and wherein the step of providing the first substrate comprises a step of making porous a part of the monocrystalline silicon substrate by anodization.

31. The process according to claim 30, wherein the step of removing the remaining non-porous part of the monocrystalline silicon substrate comprises a step of polishing or grinding the part remaining non-porous of the monocrystalline silicon substrate.

32. A process for producing a semiconductor substrate, comprising the steps of: having an impurity concentration different from the impurity concentration of the silicon substrate; layer, the porous monocrystalline silicon layer having pores with inside walls, the non-porous monocrystalline silicon layer provided on the porous monocrystalline silicon layer together forming a first substrate; monocrystalline silicon layer, the protective film preventing the porous monocrystalline silicon layer from forming a coarse structure during heat treatment; therebetween to form a multi-layer structure member such that the non-porous monocrystalline silicon layer is positioned at an inside of the formed multi-layer structure member; and structure member by chemical etching.

33. The process according to claim 32, wherein the step of forming the protective film comprises an oxidizing step of partly oxidizing the inside walls of the pores of the porous monocrystalline silicon layer.

34. The process according to claim 33, wherein the oxidizing step includes performing a thermal oxidation at a temperature of 400.degree. C. to 700.degree. C.

35. The process according to claim 33, wherein the oxidizing step includes dipping the porous monocrystalline silicon layer into an oxidative solution.

36. The process according to claim 35, wherein the oxidative solution comprises an aqueous hydrogen peroxide solution.

37. The process according to claim 32, wherein the step of forming the protective film comprises a step of partly nitriding the inside walls of the pores of the porous monocrystalline silicon layer.

38. The process according to claim 32, wherein the second substrate is another silicon substrate.

39. The process according to claim 32, wherein the second substrate and the insulating layer are formed by oxidizing a surface of another silicon substrate.

40. The process according to claim 32, wherein the insulating layer is formed by oxidizing a surface of the non-porous monocrystalline silicon layer.

41. The process according to claim 32, wherein the chemical etching is carried out using a solution containing hydrofluoric acid.

42. The process according to claim 32, wherein the porous monocrystalline semiconductor layer having the protective film formed on the inside walls of the pores thereof is not wholly oxidized.

43. A process for producing a semiconductor substrate, comprising the steps of: having a porous monocrystalline semiconductor layer, the porous monocrystalline semiconductor layer having pores with inside walls: of the pores of the porous monocrystalline semiconductor layer; monocrystalline semiconductor layer; multi-layer structure member such that the non-porous monocrystalline semiconductor layer is positioned inside of the multi-layer structure member; and multi-layer structure member.

44. The process according to claim 43, wherein the first substrate is bonded to the second substrate with an insulating layer therebetween.

45. The process according to claim 32, wherein the non-porous monocrystalline silicon layer having the different impurity concentration is formed by epitaxially growing on the silicon substrate a thin film having a lower impurity concentration than the impurity concentration of the silicon substrate, or by implanting protons into a surface of a P-type silicon substrate.

46. A process for producing a semiconductor substrate, comprising the steps of: non-porous silicon substrate; porous silicon layer; silicon layer; structure member such that the non-porous monocrystalline semiconductor layer is positioned inside of the multi-layer structure member; etching.

47. The process according to claim 46, wherein the protective film forming step comprises oxidizing the porous silicon layer such that the porous silicon layer is not wholly oxidized.

48. The process according to claim 46, wherein the protective film forming step comprises thermally oxidizing the porous silicon layer at a temperature of 400.degree. to 700.degree. such that the porous silicon layer is not wholly oxidized.

49. The process according to claim 46, wherein the bonding step is carried out with an insulating layer between the first and the second substrates.

50. The process according to claim 46, further comprising, after the bonding step, a step of effecting heat treatment to increase the bonding strength.

51. The process according to claim 46, wherein the second substrate is a light-transmissive substrate or silicon substrate.

52. The process according to claim 46, further comprising, after the protective film forming step, a step of removing the protective film formed on a surface of the porous silicon layer.

53. The process according to claim 46, wherein the thickness of the protective film is 1-40 nm.

54. The process according to claim 46, wherein the step of exposing the porous silicon layer comprises grinding or polishing the non-porous silicon substrate.

55. The process according to claim 46, wherein the step of removing the porous silicon layer comprises etching with a solution containing hydrofluoric acid.

56. The process according to claim 46, wherein the step of removing the porous silicon layer comprises etching with a solution containing hydrofluoric acid and hydrogen peroxide.

57. A process for producing a semiconductor article, comprising the steps of: substrate; porous layer; via the step of exposing the porous layer.

58. A process for producing a semiconductor article, comprising the steps of: porous layer; substrate.

59. The process according to claim 58, wherein the first substrate comprises the porous layer on a non-porous substrate, and wherein the transferring step comprises exposing the porous layer.

60. The process according to claim 57, wherein the non-porous substrate comprises silicon.

61. The process according to claim 57, wherein the porous layer comprises porous silicon.

62. The process according to claim 58 or 59, wherein the porous layer comprises porous silicon.

63. The process according to any one of claims 58 to 61, wherein the protective film is a silicon oxide film.

64. The process according to any one of claims 58 to 59, wherein the protective film forming step comprises a step of oxidizing the porous layer.

65. The process according to claim 64, wherein the oxidizing step is carried out at a temperature of 400.degree. C. to 700.degree. C.

66. The process according to claim 64, further comprising, after the oxidizing step, a step of removing the oxide film from the surface of the porous layer.

67. The process according to any one of claims 57 to 59, wherein the non-porous semiconductor layer is a monocrystalline silicon layer.

68. The process according to any one of claims 57 to 59, wherein the second substrate is a silicon substrate.

69. The process according to claims 58 or 59, wherein the bonding step comprises first forming an insulating layer on the non-porous semiconductor layer and then bonding the insulating layer and the second substrate to one another.

70. The process according to claims 58 or 59, wherein the bonding step comprises first forming an insulating layer on the second substrate and then bonding the insulating layer and the non-porous semiconductor layer to one another.

71. The process according to claims 58 or 59, wherein the bonding step comprises first forming an insulating layer on each of the second substrate and the non-porous semiconductor layer, and then bonding the insulating layers to one another.

72. The process according to claim 57, wherein the step of exposing the porous layer comprises removing the non-porous substrate.

73. The process according to any one of claims 57 to 59, wherein the transferring step comprises a step of removing the porous layer by etching.

74. The process according to claim 73, wherein the etching is effected using a solution comprising hydrofluoric acid, or a solution comprising hydrofluoric acid and hydrogen peroxide.

75. The process according to claim 62, wherein the protective film is a silicon oxide film.

76. A process for transferring a semiconductor layer, comprising the steps of: porous layer;

77. The process according to claim 76, wherein the porous layer comprises porous silicon.

78. The process according to claim 76, wherein the protective film comprises silicon oxide.

79. The process according to claim 76, wherein the protective film forming step comprises a step of oxidizing the porous layer.

80. The process according to claim 79, wherein the oxidizing step is carried out at a temperature of 400.degree. C. to 700.degree. C.

81. The process according to claim 76, wherein the bonding step comprises first forming an insulating layer on the non-porous semiconductor layer and then bonding the insulating layer and the second substrate to one another.

82. The process according to claim 76, wherein the bonding step comprises first forming an insulating layer on the second substrate and then bonding the insulating layer and the non-porous semiconductor layer to one another.

83. The process according to claim 76, wherein the bonding step comprises first forming an insulating layer on each of the second substrate and the non-porous semiconductor layer, and then bonding the insulating layers to one another.

84. The process according to claim 45, wherein the removal of the porous silicon layer is carried out by chemical etching.

85. The process according to claim 45, wherein the step of removing the porous silicon layer comprises etching with a solution containing hydrofluoric said.