Process for preparing a semiconductor body

Abstract: A process for preparing a semiconductor member comprises the steps of: forming a member having a non-porous monocrystalline semiconductor region on a porous monocrystalline semiconductor region, bonding the surface of a member of which the surface is constituted of an insulating substance onto the surface of the non-porous monocrystalline semiconductor region, and then removing the porous monocrystalline semiconductor region by etching.

Description: BACKGROUND OF THE INVENTION Field of the Invention This invention relates to a process for preparing a semiconductor member, more particularly to a semiconductor which is suitable for separation of dielectric materials or electronic devices, integrated circuits prepared on a monocrystalline semiconductor layer on an insulating material. Related Background Art Formation of a monocrystalline Si semiconductor layer on an insulating material has been widely known as the silicon on insulator (SOI) technology, and since a large number of advantages which can not be reached by bulk Si substrates for preparation of conventional Si integrated circuits are possessed by the device utilizing the SOI structure, so many researches have been done. More specifically, by utilizing the SOI structure, the following advantages can be obtained: 1. Dielectric isolation can be easily done to enable high degree of integration; 2. Radiation hardness is excellent; 3. Stray capacity is reduced to attain high speed: 4. Well formation step can be omitted; 5. Latch-up can be prevented; 6. Fully depleted field effect transistor can be made by thin film formation. In order to realize the many advantages in device characteristics as mentioned above, studies have been made about the method for forming the SOI structure for these some 10 years. The contents are summarized in, for example, the literature as mentioned below: Special Issue; "Single-crystal silicon on non-single-crystal insulators"; edited by G.W. Cullen, Journal of Crystal Growth, volume 63, No.3, pp. 429 - 590 (1983). Also, it has been known for a long time to form the SOS (silicon on sapphire) structure by heteroepitaxy of Si on a monocrystalline sapphire substrate by CVD (chemical vapor deposition) method. This was successful to some extent as the most mature SOI technique, but for such reasons as a large amount of crystal defects because of lattice mismatching at the interface between the Si layer and the sapphire substrate, introduction of aluminum from the sapphire substrate into the Si layer, and above all the high cost of the substrate and delay in enlargement of the substrate wafer size, it is obstructed from being widely applied. Relatively in recent years, attempts to realize the SOI structure without use of a sapphire substrate have been done. Such attempts may be broadly classified into the three shown below. (1) After surface oxidation of a Si monocrystalline substrate, a window is formed to have the Si substrate partially exposed, and epitaxial growth is proceeded in the lateral direction with that exposed portion as the seed to form an Si monocrystalline layer on SiO 2 . (In this case, deposition of Si layer on SiO 2 is accompanied). (2) By use of a Si monocrystalline substrate itself as active layer, SiO 2 is formed therebeneath. (This method is accompanied with no deposition of Si layer). (3) After epitaxial growth of Si on a Si monocrystalline substrate, isolation separation is effected. (This method is accompanied with deposition of Si layer). As the means for realizing the above (1), there have been known the method in which a monocrystalline Si layer is formed directly to lateral epitaxial growth by CVD, the method in which amorphous Si is deposited and subjected to solid phase lateral epitaxial growth by heat treatment, the method in which amorphous or polycrystalline Si layer is irradiated convergently with an energy beam such as electron beam, laser beam, etc. and a monocrystalline layer is grown on SiO 2 by melting and recrystallization, and the method in which a melting region is scanned in a zone fashion by a rod-shaped heater (Zone melting recrystallization). These methods have both advantages and disadvantages, the still have many problems with respect to controllability, productivity, uniformity and quality, and none of them have been industrially applied yet up to date. For example, the CVD method requires sacrifice-oxidation in flat thin film formation, while the crystallinity is poor in the solid phase growth method. On the other hand, in the beam annealing method, problems are involved in controllability such as treatment time by converged beam scanning, the manner of overlapping of beams, focus adjustment, etc. Among these, the Zone Melting Recrystallization method is the most mature, and a relatively larger scale integrated circuit has been trially made, but still a large number of crystal defects such as point defect, line defect, plane defect (sub-boundary), etc. remain, and no device driven by minority carriers has been prepared. Concerning the method using no Si substrate as the seed for epitaxial growth which is the above method (2), for example, the following methods may be included. 1. An oxide film is formed on an Si monocrystalline substrate with V-grooves as anisotropically etched on the surface, a polycrystalline Si layer is deposited on the oxide film thick to the extent as the Si substrate, and thereafter by polishing from the back surface of the Si substrate, Si monocrystalline regions dielectrically separated by surrounding with the V-grooves on the thick polycrystalline Si layer are formed. In this method, although crystallinity is good, there are problems with respect to controllability and productivity in the step of depositing the polycrystalline Si thick as some hundred microns and the step in which the monocrystalline Si substrate is polished from the back surface to leave only the Si active layer as separated. 2. This is the method called SIMOX (Separation by ion-implanted oxygen) in which an SiO 2 layer is formed by ion implantation of oxygen into an Si monocrystalline substrate, which is one of the most mature methods because of good matching with the Si-IC (Integrated Circuit) process. However, for formation of the SiO 2 layer, 10 18 ions/cm 2 or more of oxygen ions are required to be implanted, and the implantation time is very long to be not high in productivity, and also the wafer cost is high. Further, many crystal defects remin, and from an industrial point of view, no sufficient level of quality capable of preparing a device driven by minority carriers has been attained. 3. This is the method to form an SOI structure by dielectric isolation according to oxidation of porous Si. This is a method in which an N-type Si layer is formed on the surface of a P-type Si monocrystalline substrate in shape of islands by way of proton ion implantation (Imai et al., J. Crystal Growth, Vol. 63,547 (1983)), or by epitaxial growth and patterning: only the p-type Si substrate is made porous by anodization in HF solution so as to surround the Si islands from the surface; and then the N-type Si islands are dielectrically isolated by accelerated oxidation. In this method, the separated Si region is determined before the device steps, whereby there is the problem that the degree of freedom in device and circuit design may be limited in some cases. As the method (3) as described above, the method described in Japanese Laid-open Patent Application No. 56-16464 has the steps of forming an N-type monocrystalline S1 layer on a P-type Si wafer, providing a glass layer containing an oxide of the N-type impurity thereon and the step of bonding the glass layer to a glass layer containing the oxide of an N-purity impurity provided on another silicon wafer by heat treatment. And, subsequent to the bonding step, the P-type Si wafer is made porous, and then the porous layer is oxidized, followed by etching to remove the porous layer, thereby forming an SOI structure. Also, Japanese Patent Publication No. 53-45675 discloses a method in which a silicon monocrystalline wafer is made porous, then oxidized to make the porous layer higher in resistance; a monocrystalline Si layer is formed on the porous silicon layer; and a part of the monocrystalline Si layer is made porous and higher in resistance so as to surround the monocrystalline Si region, thereby separating the monocrystalline Si region. The methods described in these publications all include the step of oxidizing a porous layer, and because the volume of porous layer is increased by oxidation, sometimes an influence of distortion may be exerted on the monocrystalline Si layer, and therefore in these methods, a monocrystalline Si layer with constantly good quality could not necessarily be formed on the insulator. US-A-3 997 381 describes a process for preparing a semiconductor member comprising the steps of starting with a silicon monocrystalline, parent substrate; forming an epitaxial silicon monocrystalline layer upon said parent substrate, said epitaxial layer having substantially different doping concentrations than said parent substrate; forming an insulating layer on said parent substrate with epitaxial layer and on a silicon, supportive substrate; bonding said parent and supportive substrates whereby said insulating layers on said parent and supportive substrates are in physical contact and whereby said epitaxial layer is positioned between said parent and supportive substrates; lapping a portion of said parent substrate; etching said parent substrate by a preferential chemical etchant; measuring an electromotive force between an electrode pair immersed in said chemical etchant; and adding an oxidizing agent to said etchant in response to said measuring of electromotive force to control the etching characteristics of said etchant and to terminate said etching when removal is substantially complete. In Materials Letters Vol. 7, No. 3 (1988), pp. 94-98, a low-pressure vapor-phase epitaxy formation process of silicon on porous silicon is described. In this process, a much lower temperature than in conventional CVD epitaxy is applied. After pre-oxidation to stabilize the porous structure, the surface of pre-oxidized porous silicon was chemically cleaned and finally thermally cleaned at temperatures below 860 °C just before the epitaxial step. Full oxidation of the buried porous silicon layer was subsequently possible. In Journal of Applied Physics, Vol. 64, No. 10 (1988), pp. 4943-4950, the bonding of oxidized silicon wafers for preparing SOI is described. The bonding was achieved by heating in a inert atmosphere a pair of wafers with hydrophilic surfaces contacted face-to-face. In Applied Physics Letters, Vol. 43, No. 3 (1983), pp. 263-265, another process for preparing single crystal Si film on an insulating substrate is described. A first substrate was prepared by epitaxially growing Si on Si substrate, then oxidizing the epitaxial Si film and then depositing silicon nitride and silicon dioxide films on the oxidized epitaxial Si film surface by CVD. A second substrate was thermally oxidized, and then a glass of specific type was used for bonding the two substrates. The Si wafer of the first substrate was removed by sequential polishing and etching steps to obtain the epitaxial Si film on an insulating substrate. SUMMARY OF THE INVENTION An object of the present invention is to provide a semiconductor member capable of overcoming the problems as described above and meeting the demands as described above and a process for preparing the member. Another object of the present invention is to provide a semiconductor having a monocrystalline layer excellent in crystallinity equally as monocrystalline SOI wafer, and a process for preparing the member excellent also in aspects of productivity, uniformity, controllability, economy. Still another object of the present invention is to provide a semiconductor member having excellent characteristics which can be sufficiently alternative for expensive SOS or SIMOX even in preparing a large scale integrated circuit by the SOI structure and a process for preparing the member economically. The present invention provides a process for preparing a semiconductor member according to claim 1. A preferred embodiment for preparing a semiconductor member according to the present invention is specified in claim 25. BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1 through 13 are schematic illustrations of the examples of the steps of the process for preparing the semiconductor member of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Semiconductors obtained by the process according to the present invention are as described below. A semiconductor member of the present invention comprises: a first member having a non-porous monocrystalline semiconductor layer provided on a porous monocrystalline semiconductor layer, and a second member having the surface constituted of an insulating substance bonded onto the surface of the non-porous monocrystalline semiconductor layer. Another semiconductor member comprises: a first member having a non-porous monocrystalline semiconductor layer and a layer constituted of an insulating substance arranged in this order on a porous monocrystalline semiconductor layer, and a second member bonded through a layer constituted of an insulating substance onto the surface of the layer constituted of the insulating substance. Still another semiconductor member has a non-porous silicon monocrystalline semiconductor layer arranged on a layer constituted of an insulating substance, wherein the dislocation defect density in the non-porous silicon monocrystalline semiconductor layer is 2.0 x 10 4 /cm 2 or less, and the life time of carriers is 5.0 x 10 -4 sec or longer. Still another semiconductor member has a non-porous silicon monocrystalline semiconductor layer arranged on a layer constituted of an insulating substance, wherein the dislocation defect density in the non-porous silicon monocrystalline semiconductor layer is 2.0 x 10 4 /cm 2 or less, and the life time of carriers is 5.0 x 10 -4 sec or longer, and also the difference between the maximum value and the minimum value of the thickness of the silicon monocrystalline semiconductor layer is 10% or less, with respect to the maximum value. Processes for preparing the semiconductor member of the present invention are as described below. A process for preparing a semiconductor member of the present invention comprises the steps of: preparing a first member having a non-porous monocrystalline semiconductor layer on a porous monocrystalline semiconductor layer, bonding said first member to the insulating layer of a second member in a manner so as to place said non-porous monocrystalline semiconductor layer inside the multilayer structure to be obtained, and then selectively etching said porous monocrystalline semiconductor layer. Furthermore, in a preferred embodiment for preparing a semiconductor member according to the present invention, the first member is prepared by making a non-porous monocrystalline semiconductor substrate partially porous to form a porous monocrystalline semiconductor layer on a first non-porous monocrystalline semiconductor layer, and by forming a second non-porous monocrystalline semiconductor layer on said porous monocrystalline semiconductor layer, the bonding step is carried out by bonding the surface of the second non-porous monocrystalline semiconductor layer onto an insulating layer of the second member, and before the porous monocrystalline semiconductor layer is removed by etching, the first non-porous monocrystalline semiconductor layer is removed. The semiconductor member obtained by the process of the present invention has a monocrystalline semiconductor layer where carrier life time is long and the defect is extremely little on an insulating material with excellent uniformity of thickness, and is applicable to various semiconductor devices. Also, the semiconductor member obtained by the process of the present invention is capable of high speed response, and is applicable to semiconductor device rich in reliability. The semiconductor member obtained can be also an alternative for expensive SOS or SIMOX. The process for preparing a semiconductor member of the present invention provides a process excellent in aspects of productivity, uniformity, controllability, economy in obtaining an Si crystal layer having crystallinity equal to monocrystalline wafer on an insulating material. Further, according to the process for preparing a semiconductor member of the present invention, it is possible to provide semiconductor members wherein the advantages of the SOI device of the prior art can be realized and applied. Also, according to the present invention, there can be provided a process for preparing a semiconductor member which can be an alternative for expensive SOS or SIMOX also in preparing a large scale integrated circuit of SOI structure. The process for preparing a semiconductor member of the present invention, as is described in detail in Examples, makes it possible to perform the treatment efficiently within a short time, and is excellent in its productivity and economy. Referring now to silicon as an example of semiconductor materials, the present invention is described in detail, but the semiconductor material in the present invention is not limited to silicon at all. In porous Si layer, according to observation by transmission electron microscopy, pores with diameters of about 600 angstroms on an average are formed, and despite the fact that their density is half or lower as compared with monocrystalline silicon, momocrystallinity is maintained. Monocrystal refers to a crystalline solid such that, when calling attention on an optional crystal axis, its direction is the same at any part of the sample, and the porous layer used in the present invention, although having pores therethrough, the crystal axis in the crystalline region is the same in the direction at any part, thus being monocrystalline. And, epitaxial growth of a monocrystalline Si layer onto the porous layer is possible. However, at a temperature of 1000 °C or higher, rearrangement of the atoms positioned around the inner pores takes place, whereby the characteristics of accelerated chemical etching may be sometimes impaired. For this reason, in the present invention, for epitaxial growth of Si layer, there may be suitably used the crystal growth method capable of low temperature growth such as molecular beam epitaxial growth, plasma CVD, low pressure CVD method, photo CVD, bias-oputtering method, liquid phase growth method, etc. Since the porous layer has a large amount of voids formed internally thereof, its density can be reduced to half or lower. As the result, the surface area per unit volume (specific surface area) can be dramatically increased, and therefore its chemical etching rate is remarkably accelerated as compared with that of the etching rate of conventional non-porous monocrystalline layer. The present invention utilizes the two above-mentioned characteristics of the semiconductor as made porous, namely the fact that single crystallinity is maintained and a non-porous semiconductor monocrystal can therefore be epitaxially grown on the semiconductor substrate as made porous, and the fact that etching rate is remarkably more rapid as compared with non-porous monocrystal, whereby a non-porous semiconductor monocrystalline layer of high quality can be formed on a substrate having an insulating material surface within a short time. The porous layer can be more readily formed on a P-type Si layer than an N-type Si layer for the following reason. First, a porous Si was discovered by Uhlir et al. in the research process of electrolytic polishing of semiconductor in 1956 (A. Uhlir, Bell Syst. Tech. J., vol 35, p.333 (1956)). Unagami et al. studied about the dissolving reaction of Si in anodization, and has reported that positive holes are necessary for the anodic reaction of Si in an HF solution, and the reaction is as follows (T. Unagami, J. Electrochem. Soc., vol. 127, p. 476 (1980)). Si + 2HF + (2-n)e + → SiF 2 + 2H + + ne - SiF 2 + 2HF → SiF 4 + H 2 SiF 4 + 2HF → H 2 SiF 6 or Si + 4HF + (4 - λ)e + → SiF 4 + 4H + + λe - SiF 4 + 2HF → H 2 SiF 6 . Here, e + and e - represent a positive hole and an electron, respectively. n and λ are respectively numbers of positive holes necessary for dissolving one atom of silicon, and it has been postulated that porous silicon is formed when satisfying the condition of n > 2 or λ > 4. For the reasons mentioned above, the P-type silicon in which positive holes exist will be more readily made porous than the N-type silicon of the opposite characteristic. Selectivity in such pore structure formation has been verified by Nagano et al. (Nagano, Nakajima, Yasuno, Ohnaka, Kajiwara; Denshi Tsushin Gakkai Gijutsu Kenkyu Hokoku, vol. 79, SSD 79-9549(1979)) and Imai, (K. Imai; Solid-State Electronics vol. 24, 159 (1981)). However, depending on the conditions, the N-type silicon can be also made porous. Referring now to the drawings, the present invention is described in more detail. Embodiment 1 Description is made about the process for obtaining a semiconductor substrate by making porous all of a P-type substrate and permitting a monocrystalline layer to be epitaxially grown. As shown in Fig. 1A, first a P-type Si monocrystalline substrate is provided, and all of it is made porous. According to the crystal growth method capable of low temperature growth as mentioned above, epitaxial growth is effected on the substrate surface as made porous to form a thin film monocrystalline layer 22. The above P-type Si substrate is made porous by the anodization method by use of an HF solution. The porous Si layer 21 can be varied in its density to a range of 1.1 to 0.6 g/cm 3 by varying the HF solution concentration to 50 to 20% as compared with the density 2.33 g/cm 3 of the monocrystalline Si. Subsequently, as shown in Fig. 1B, by providing another Si substrate 23 and after forming an oxidized layer 24 on its surface, the Si substrate 23 having the oxidized layer 24 on the surface is bonded onto the surface of the monocrystalline layer 22 on the porous Si substrate 21. Then, as shown in Fig. 1C, the porous Si substrate 21 is all etched away to form a thin film-like monocrystalline silicon layer 22 remaining on the SiO 2 layer 24. In the present invention, since the porous semiconductor layer is etched away without application of the oxidation treatment on the porous semiconductor layer, the oxidation swelling of the porous semiconductor layer can be prevented, whereby the influence of distortion on the monocrystalline layer as epitaxially grown can be prevented. According to this method, the monocrystalline Si layer 22 equal to silicon wafer in crystallinity is formed flat and yet uniformly thinly over the whole wafer region with a large area on the oxidized Si layer 24 which is an insulating material. The semiconductor substrate thus obtained can be suitably used also with respect to preparation of an insulation separated electronic device. Here, the thickness of the non-porous semiconductor crystalline layer to be formed on the porous semiconductor substrate may be desirably made preferably 50 µm or less, more preferably 20 µm or less, for forming a thin film semiconductor device on the above semiconductor monocrystalline layer. Bonding between the above non porous semiconductor monocrystal and the substrate having the insulating material surface should be preferably performed in an atmosphere of nitrogen, an inert gas or a gas mixture of these, or in an atmosphere containing an inert gas or nitrogen, and further desirably under heated state. As the etchant for selectively etching the above semiconductor substrate as made porous with the above non-porous semiconductor monocrystalline layer as bonded onto the substrate having the insulating material surface being left to remain, for example, there may be employed etchants such as aqueous sodium hydroxide solution, aqueous potassium hydroxide solution, hydrofluoric acid-nitric acid-acetic acid mixed solution, etc. The substrate having an insulating material which can be used in the present invention may be one with at least its surface being constituted of an insulating material, or one with all of the substrate being constituted of an insulating material. Examples of the substrate with the surface being constituted of an insulating material may include monocrystalline or polycrystalline silicon substrates having the surface oxidized, electroconductive or semiconductive substrates having a layer of an insulating material such as oxide, nitride, boride, etc. formed on the surface, etc. Specific examples of the substrates wholly constituted of an insulating material may include substrates comprising insulating materials such as fused silica glass, sintered alumina, etc. Meanwhile, in the present Embodiment 1, an example of forming a non-porous semiconductor monocrystalline layer on a porous semiconductor substrate has been shown, but the present invention is not limited only to the mode of the Embodiment 1 as described above, but a porous semiconductor substrate having a non-porous semiconductor monocrystalline layer may be also formed by applying a pore forming treatment on a substrate having a monocrystalline layer comprising a material which can be made porous with difficulty (e.g. N-type silicon) and a layer comprising a material which can be readily made porous (e.g. P-type silicon). Also, in the step of removing the porous semiconductor substrate by etching, the substrate may be also coated with an etching preventive material except for the porous semiconductor substrate during etching, so that the non-porous semiconductor monocrystalline layer and the substrate having the insulating material surface may not be deleteriously influenced by the etchant. The non-porous monocrystalline layer on the insulating material thus formed can be one having 5.0 x 10 -4 sec. or longer in terms of the life time of carriers, having remarkably less crystal defect such as through dislocation, etc. and also being extremely small in distribution of the layer thickness of the semiconductor monocrystalline layer. Specifically, the dislocation defect density becomes 2 x 10 4 /cm 2 or less, and concerning the layer thickness of the semiconductor monocrystalline layer, within the range of the area of the semiconductor monocrystalline layer from 20 cm 2 to 500 cm 2 (5 cm (2 inches) wafer to 25 cm (10 inches) wafer), the difference between the maximum value and the minimum value of the thickness of the semiconductor monocrystalline layer can be suppressed below 10% or less based on the maximum value. In the following, other embodiments are shown. Embodiment 2 Referring now to Figs. 2A to 2D, Embodiment 2 is described in detail. First, as shown in Fig. 2A, a low impurity concentration layer 122 is formed by epitaxial growth according to various thin film growth methods. Alternatively, protons may be ion implanted into the surface of the P-type Si monocrystalline substrate 121 to form an N-type monocrystalline layer 122. Next, as shown in Fig. 2B, the back surface of the P-type monocrystalline substrate 121 is denatured into a porous Si substrate 123 according to the anodization method by use of an HF solution. The porous Si layer 123 can be varied in its density to a range of 1.1 to 0.6 g/cm 3 by varying the HF solution concentration within the range of 50 to 20% as compared with the density of the monocrystalline Si of 2.33 g/cm 3 . The porous layer, as described above, is formed on the P-type substrate. As shown in Fig. 2C, by providing another Si substrate 124 and after forming an oxidized layer 125 on its surface, the Si substrate 124 having the oxidized layer 125 on its surface is bonded to the surface of the monocrystalline Si layer 122 on the porous Si substrate 123. Then, the porous Si substrate 123 is all etched to leave a monocrystalline silicon layer 122 as thin film on the SiO 2 layer 125, thereby forming a semiconductor substrate. According to this process, the monocrystalline Si layer 122 equal in crystallinity to silicon wafer is formed on the oxidized layer 125 which is an insulating material to be flat and to be yet made uniformly into thin layer, over the whole wafer region with a large area. The semiconductor substrate thus obtained can be used suitably also with respect to preparation or an insulation separated electronic device. The above Embodiment 2 is an example of the process of forming an N-type layer on a P-type substrate before pore formation, and then making selectively only the P-type substrate porous by anodization. Also in the present Embodiment, a semiconductor substrate having a semiconductor monocrystalline layer with the same performences as Embodiment 1 can be obtained. Embodiment 3 As shown in Fig. 3A, first a P-type Si monocrystalline substrate is prepared and all of it is made porous. According to various growth methods, epitaxial growth is effected on the substrate surface made porous to form a thin film monocrystalline layer 12. As shown in Fig. 3B, by preparing another Si substrate 13 and after forming an oxidized layer 14 on its surface, the Si substrate having the oxidized layer 14 on the surface is bonded to the surface of the monocrystalline Si layer 12 on the porous Si substrate 11. Next, as shown in Fig. 3B, an Si 3 N 4 layer 15 is deposited to cover over the whole of the bonded two silicon wafers as the anti-etching film. Subsequently, as shown in Fig. 3C, the Si 3 N 4 layer on the surface of the porous silicon substrate is removed. As another anti-etching film material, Apiezon Wax may be also employed in place of Si 3 N 4 . Then, the porous Si substrate 11 is all etched and a monocrystalline silicon layer 12 made into a thin film remains on the SiO 2 layer 14 to form a semiconductor substrate. Fig. 3C shows a semiconductor substrate obtained in the present invention. More specifically, by removing the Si 3 N 4 layer 16 as the anti-etching film in Fig. 3B, a monocrystalline Si layer 12 of which crystallinity is equal to that of bulk wafer is formed through the SiO 2 layer 14 which is an insulating substance on the Si substrate 13, flat and yet uniformly into a thin layer, over the whole wafer region with a large area. The semiconductor substrate thus obtained can be used suitably also with respect to preparation of a discrete insulated electronic device. Also in the present Embodiment, a semiconductor substrate having a semiconductor monocrystalline layer with the same performances as Embodiment 1 can be obtained. Embodiment 4 Referring now to Figs. 4A to 4B, the Embodiment 4 of the present invention is described below in detail. First, as shown in Fig. 4A, a low impurity concentration layer 112 is formed by epitaxial growth according to various thin film growth methods. Alternatively, protons may be ion-implanted into the surface of the P-type Si monocrystalline substrate 111 to form an N-type monocrystalline layer 112. Next, as shown in Fig. 4B, the back surface of the P-type monocrystalline substrate 111 is denatured into a porous Si substrate 113 according to the anodization method by use of an HF solution. The porous Si layer 113 can be varied in its density to a range of 1.1 to 0.6 g/cm 3 by varying the HF solution concentration to 50 to 20% as compared with the density of the monocrystalline Si of 2.33 g/cm 3 . The porous layer 113, as described above, is formed from the P-type substrate. As shown in Fig. 4C, by preparing another Si substrate 114 and after forming an oxidized layer 115 on its surface, the Si substrate 114 having the oxidized layer 115 on the surface is bonded on the surface of the monocrystalline Si layer 112 on the porous Si substrate 113. Here, as shown in Fig. 4C, as the anti-etching film 116, an Si 3 N 4 layer 116 is deposited to cover over the whole of the bonded two silicon wafers as the anti-etching film. Subsequently, as shown in Fig. 4C, the Si 3 N 4 layer on the surface of the porous silicon substrate is removed. As another anti-etching film 116, a material excellent in etching resistance such as Apiezon Wax may be also employed in place of Si 3 N 4 . Then, the porous Si substrate 113 is all etched to form a semiconductor substrate by retaining a monocrystalline silicon layer 112 made into a thin film on the SiO 2 layer 115. Fig. 4D shows a substrate having the semiconductor layer obtained in the present invention. That is, by removing the Si 3 N 4 layer 116 as the anti-etching film 116 shown in Fig. 4C, a monocrystalline Si layer 112 is formed on the SiO 2 layer 115 which is the insulating material, flat and yet uniformly into a thin layer, over the whole wafer region with a large area. The semiconductor thus obtained will not be deleteriously affected by the etchant, and can be used suitably also with respect to preparation of a discrete insulated electronic device. Also, the semiconductor substrate obtained in the present Embodiment has the same performances as that obtained in Embodiment 1. Embodiment 6 As shown in Fig. 5A, first, a P-type Si monocrystalline substrate is prepared and all of it is made porous. A thin film monocrystalline layer 32 is formed by effecting epitaxial growth on the substrate surface made porous according to various growth methods. As shown in Fig. 5B, by preparing another Si substrate 33 and after forming an oxidized layer 34 on its surface, the Si substrate 33 having the oxidized layer 34 on the surface is bonded to the surface of the oxidized layer 36 formed on the monocrystalline Si layer 32 on the porous substrate 31. The bonding step is practiced by closely contacting the cleaned surfaces with each other, followed by heating in an inert gas atmosphere or nitrogen atmosphere. The oxidized layer 34 is formed in order to reduce the interface level of the non-porous monocrystalline layer 32 which is the final active layer. As shown in Fig. 5B, an Si 3 N 4 layer 35 is deposited as the anti-etching film to be coated on the bonded two silicon wafers as a whole. Subsequently, as shown in Fig. 5C, the Si 3 N 4 layer 35 on the surface of the porous silicon substrate 31 is removed. As another anti-etching film material, Apiezon Wax, etc. may be also employed in place of Si 3 N 4 . Then, all of the porous Si substrate 31 is etched to have the monocrystalline silicon layer 32 made into a thin film remain on the SiO 2 layer, thereby forming a semiconductor substrate. Fig. 5C shows the substrate having the semiconductor layer obtained in the present invention. More specifically, by removing the Si 3 N 4 layer 35 as the anti-etching film shown in Fig 5B, the monocrystalline Si layer 32 equal in crystallinity to silicon wafer is formed through the SiO 2 layers 34 and 36 on the Si substrate 33, flat and yet uniformly into a thin layer, over the whole wafer region with a large area. The semiconductor substrate thus obtained can be used suitably also with respect to preparation of a discrete insulated electronic device. Also, the semiconductor substrate obtained in the present Embodiment has the same performances as that obtained in Embodiment 1. Embodiment 6 Referring now to Figs. 6A to 6D, the Embodiment 6 of the present invention is described below in detail. First, as shown in Fig. 6A, a low impurity concentration layer 132 is formed by epitaxial growth according to various thin film growth methods. Alternatively, the surface of the P-type Si monocrystalline substrate 131 is subjected to ion implantation of protons to form an N-type monocrystalline layer 132. Next, as shown in Fig. 6B, the P-type Si monocrystalline substrate 131 is denatured from the back surface to a porous Si substrate 133 by the anodization method with an HF solution. The porous Si layer 133 can be varied in its density in the range of 1.1 to 0.6 g/cm 3 by varying the HF solution concentration to 50 to 20% as compared with the density of the monocrystalline Si of 2.33 g/cm 3 . The porous layer, as described above, is formed from the P-type substrate. As shown in Fig. 6C, by preparing another Si substrate 134 and after forming an oxidized layer 135 on its surface, the Si substrate 134 having the oxidized layer 135 is bonded onto the surface of the oxidized layer 137 formed on the monocrystalline Si layer 132 on the porous Si substrate 133. Subsequently, as the anti-etching film 136, an Si 3 N 4 layer 136 is deposited to be coated on the bonded two silicon wafers as a whole. Then, as shown in Fig. 6D, the Si 3 N 4 layer 136 on the surface of the porous silicon substrate 133 is removed. Then the porous Si substrate 131 is all chemically etched to have a monocrystalline silicon layer made into a thin film remain on the SiO 2 layers 135 and 137, thereby forming a semiconductor substrate. The semiconductor substrate thus obtained is excellent in adhesion between the respective layers, and can be also used suitable also with respect to preparation of an isolation separated electronic device. Also, the semiconductor substrate obtained in the present Embodiment has the same performances as that obtained in Embodiment 1. Embodiment 7 An shown in Fig. 7A, first, a P-type Si monocrystalline substrate is prepared and all of it is made porous. According to various growth methods, epitaxial growth is effected on the substrate surface made porous to form a thin film monocrystalline layer 42. As shown in Fig. 7B, by preparing another Si substrate 43 and after forming an oxidized layer 44 on its surface, the Si substrate 43 having the above oxidized layer 44 on the surface is bonded onto the surface of the oxidized layer 45 formed on the monocrystalline Si layer 42 on the porous Si substrate 41. This bonding step is practiced by closely contacting cleaned surfaces with each other, followed by heating in an inert gas atmosphere or nitrogen atmosphere. The oxidized layer 44 is formed in order to reduce the interface level of the monocrystalline layer 42 which is the active layer as the final semiconductor. As shown in Fig. 7C, the porous Si substrate 41 is all etched to have the monocrystalline silicon layer made into a thin film on the SiO 2 layers 44 and 45, thereby forming a semiconductor substrate. Fig. 7C shows the semiconductor substrate obtained in the present invention. A monocrystalline Si layer 42 equal in crystallinity to silicon wafer is formed through the SiO 2 layers 44 and 45 on the Si substrate 43, flat and yet uniformly into a thin layer, over the whole wafer region with a large area. The semiconductor substrate thus obtained can be also used suitably also as seen from the standpoint of preparation of an isolation separated electron device. Also, the semiconductor substrate obtained in the present embodiment has the same performances as that obtained in Embodiment 1. Embodiment 8 Referring now to Figs. 8A to 8D, the Embodiment 8 of the present invention is described below in detail. First, as shown in Fig. 8A, a low impurity concentration layer 142 is formed by epitaxial growth according to various thin film growth methods. Alternatively, the surface of the P-type Si monocrystalline substrate 141 is subjected to ion implantation of protons to form an N-type monocrystalline layer 142. Next, as shown in Fig. 8B, the P-type Si monocrystalline substrate 141 is denatured from the back surface to a porous Si substrate 143 by the anodization method with an HF solution. The porous Si layer 143 can be varied in its density in the range of 1.1 to 0.6 g/cm 3 by varying the HP solution concentration to 50 to 20% as compared with the density of the monocrystalline Si of 2.33 g/cm 3 . The porous layer, as described above, is formed on the P-type substrate 141. AS shown in Fig. 8C. by preparing another Si substrate 144 and after forming an oxidized layer 145 on its surface, the Si substrate 144 having the oxidized layer 145 is bonded onto the surface of the oxidized layer 146 formed on the monocrystalline Si layer 142 on the porous Si substrate 143. Then the porous Si substrate is all chemically etched to have a monocrystalline silicon layer made into a thin film remain on the SiO 2 layers 145 and 146, thereby forming a semiconductor substrate. Fig. 8D shows the semiconductor substrate obtained in the present invention. A monocrystalline Si layer 142 equal in crystallinity to silicon wafer is formed through the SiO 2 layers 145 and 146 on the Si substrate 144, flat and yet uniformly into a thin layer, over the whole wafer region with a large area. The semiconductor substrate thus obtained can be also used suitably also with respect to preparation of an isolation separated electronic device. Also, the semiconductor substrate obtained in the present Embodiment has the same performances as that obtained in Embodiment 1. Embodiment 9 As shown in Fig. 9A, first, a P-type Si monocrystalline substrate is prepared, and all of it is made porous. According to various growth methods, epitaxial growth is effected on the surface of the substrate 51 made porous to form a thin film monocrystalline layer 52. As shown in Fig. 9B, a light-transmissive substrate 53 represented by glass is prepared and the light-transmissive substrate 53 is bonded onto the surface of the monocrystalline Si layer 52 on the porous Si substrate 51. Here, as shown in Fig. 9B, as the anti-etching film 54, an Si 3 N 4 layer 54 is deposited to be coasted on the bonded two substrates as a whole. Subsequently, as shown in Fig. 9C, the Si 3 N 4 layer 54 on the surface of the porous silicon substrate is removed. Then, the porous Si substrate 51 is all etched away to have a monocrystalline silicon layer 52 made into a thin film remain on the light-transmissive substrate 53, thereby forming a semiconducor substrate. Fig. 9C shows the semiconductor substrate obtained in the present invention. The semiconductor substrate thus obtained can be also used suitable also as seen from the standpoint of preparation of an electronic device isolation separated with a light-transmissive insulating material. Also, the semiconductor obtained in the present Embodiment has the same performances as that obtained in Embodiment 1. Embodiment 10 Referring now to Figs. 10A to 10D, the Embodiment 10 of the present invention is described below in detail. First, as shown in Fig. 10A, a low impurity concentration layer 152 is formed by epitaxial growth according to various thin film growth methods. Alternatively, the surface of the P-type Si monocrystalline substrate 151 is subjected to ion implantation of protons to form an N-type monocrystalline layer 152. Next, as shown in Fig. 10B, the P-type Si monocrystalline substrate 151 is denatured from the back surface to a porous Si substrate 153 by the anodization method with an HF solution. The porous Si layer 163 can be varied in its density in the range of 1.1 to 0.6 g/cm 3 by varying the HF solution concentration to 50 to 20% as compared with the density of the monocrystalline Si of 2.33 g/cm 3 . The porous layer 153, as described above, is formed on the P-type substrate 151. As shown in Fig. 10C, after preparing a light-transmissive substrate 154, onto the surface of the monocrystalline Si layer 152 on the porous Si substrate 153 is bonded the light-transmissive substrate 154. Then, as shown in Fig. 10C, as an anti-etching film 155, an Si 3 N 4 layer, etc. is deposited to be coated on the bonded two substrates as a whole. Subsequently, as shown in Fig. 10D, the Si 3 N 4 layer 155 on the surface of the porous silicon substrate 153 is removed. Then, the porous Si substrate 153 is all etched away to have the monocrystalline silicon layer 152 made into a thin film remain on the light-transmissive substrate 154, thereby forming a semiconductor substrate. Fig. 10D shows the semiconductor substrate obtained in the present invention. It is the monocrystalline Si layer 152 equal in crystallinity to silicon wafer formed on the light-transmissive substrate 154, flat and yet uniformly into a thin layer, over the whole wafer region with a large area. The semiconductor substrate thus obtained can be also used suitably also with respect to preparation of an isolation separated electronic device with a light-transmissive insulating material. Also, the semiconductor substrate obtained in the present Embodiment has the same performances as that obtained in Embodiment 1. Embodiment 11 As shown in Fig. 11A, first, a P-type Si monocrystalline substrate is prepared, and all of it is made porous. According to various growth methods, epitaxial growth is effected on the surface of the substrate 61 made porous to form a thin film monocrystalline layer 62. As shown in Fig. 11B, a light-transmissive substrate 63 represented by glass is prepared and the light-transmissive substrate 63 is bonded to the surface of the monocrystalline Si layer 62 on the porous Si substrate 61. Then, the porous Si substrate 61 is all etched to have the monocrystalline silicon layer 62 made into a thin film remain on the light-transmissive substrate 63, thereby forming a semiconductor substrate. Fig. 11C shows the semiconductor substrate obtained in the present invention. It is the monocrystalline Si layer 62 equal in crystallinity to silicon wafer formed on the light-transmissive substrate 63, flat and yet uniformly into a thin layer, over the whole wafer region with a large area. The semiconductor thus obtained can be also suitably used also with respect to preparation of an electronic device isolation separated with a light-transmissive insulating material. Embodiment 12 Referring now to Figs. 12A to 12D, the Embodiment 12 of the present invention is described below in detail. First, as shown in Fig. 12A, a low impurity concentration layer 162 is formed by epitaxial growth according to various thin film growth methods. Alternatively, the surface of the P-type Si monocrystalline substrate 161 is subjected to ion implantation of protons to form an N-type monocrystalline layer 162. Next, as shown in Fig. 12B, the P-type Si monocrystalline substrate 161 is denatured from the back surface to a porous Si substrate 163 by the anodization method with an HF solution. The porous Si layer 163 can be varied in its density in the range of 1.1 to 0.6 g/cm 3 by varying the HF solution concentration to 50 to 20% as compared with the density of the monocrystalline Si of 2.33 g/cm 3 . The porous layer 163, as described above, is formed on the P-type substrate 163. As shown in Fig. 12C, after preparing a light-transmissive substrate 164, onto the surface of the monocrystalline Si layer 162 on the porous Si substrate 163 is bonded the light-transmissive substrate 164. As shown in Fig. 12C, the porous Si substrate 163 is all etched away to have the monocrystalline silicon layer 162 made into a thin film remain on the light-transmissive substrate 164, thereby forming a semiconductor substrate. Fig. 12D shows the semiconductor substrate obtained in the present invention. It is the monocrystalline Si layer 162 equal in crystallinity to silicon wafer formed on the light-transmissive substrate 164, flat and yet uniformly into a thin layer, over the whole wafer region with a large area. The semiconductor substrate thus obtained can be also used suitably also with respect to preparation of an isolation separated electronic device with a light-transmissive insulating material. Also, the semiconductor substrate obtained in the present Embodiment has the same performances as that obtained in Embodiment 1. Embodiment 13 This Embodiment is described by referring to Figs. 13A to 13F. As shown in Fig. 13A, first a porous region 1301 is formed on a part of an Si monocrystalline substrate 1300. Subsequently, a thin film Si monocrystalline layer 1302 is formed according to various crystal growth methods on the porous region 1301 (Fig. 13B). On the thin film Si monocrystalline layer 1302 is formed an oxidized film 1303 (Fig. 13C). The oxidized film 1305 formed on the surface of another Si substrate 1304 is bonded to the above oxidized film 1303 (Fig. 13D). Subsequently, the Si monocrystalline substrate 1300 remaining without being made porous is removed by mechanical polishing or etching to have the porous region 1301 exposed (Fig. 13E). The porous region 1301 is etched away to form a semiconductor substrate having a thin film Si monocrystalline layer on an insulating material (Fig. 13F). When such step is employed, the time required for making the substrate porous can be shortened and also the time for etching away the porous Si substrate can be shortened, whereby substrate formation can be effected highly efficiently. It is also possible to bond the thin film Si monocrystalline layer 1302 directly to the oxidized film 1305 without forming the oxidized film 1303 shown in Fig. 13, and it is also possible to bond an insulating substrate such as glass, etc. in place of the oxidized film 1305 formed on the Si substrate 1304. Also, the respective steps in the Embodiments 1 to 12 can be also incorporated in the present Embodiment. The semiconductor substrate thus obtained has the same excellent performances as those of the semiconductor substrate obtained in Embodiments 1 to 12. Example 1 A P-type (100) monocrystalline Si substrate (Si wafer) having a diameter of 7.6 cm (3 inches) and a thickness of 200 microns was anodized in a 50% HF solution. The current density at this time was 100 mA/cm 2 . The porous structure formation rate at this time was 8.4 µm/min. and the P-type (100) Si substrate having a thickness of 200 microns as a whole was made porous in 24 minutes. On the P-type (100) porous Si substrate 21 was grown an Si epitaxial layer to a thickness of 0.5 microns by the MBE (Molecular Beam Epitaxy) method. The deposition conditions are as follows: Temperature: 700 °C Pressure: 1 x 10 -9 Torr ( 1 Torr = 133.5 Pa) Growth rate: 0.1 nm/sec. Next, on the surface of the epitaxial layer 21 was superposed another Si substrate 23 with an oxidized layer of 5000 angstroms (1 angstrom = 0.1 nm) formed on the surface, and by heating in a nitrogen atmosphere at 800 °C for 0.5 hour, the two Si substrates were bonded firmly together. Then, the porous Si substrate 21 was chemically etched away by use or hydrofluoric acid-nitric acid-acetic acid solution (1:3:8). As described above, chemical etching rate of ordinary Si monocrystal relative to hydrofluoric acid-nitric acid-acetic acid solution is about a little lower than 1 micron per minute (hydrofluoric acid-nitric acid-acetic acid solution, 1:3:8), but the chemical etching rate of a porous layer is accelerated by about 100-fold thereof. That is, the Si substrate 21 made porous having a thickness of 200 microns was removed in 2 minutes. Thus, a monocrystalline Si layer 22 having a thickness of 0.5 µm could be formed on the SiO 2 layer 24. Also the thickness of the monocrystalline Si layer obtained was examined by use of Scanning Ellipsometer. Specifically, the whole surface of the 3 inches wafer was scanned for the measurement. As the result, within the plane of the 3 inches wafer, the difference between the maximum value and the minimum value of the thickness of the monocrystalline Si layer was found to be suppressed 5% or less relative to the maximum value of the thickness. As the result of plan view observation of the monocrystalline Si layer by transmission electron microscopy, the dislocation defect density was found to be suppressed 1 x 10 3 /cm 2 or less, whereby it could be confirmed that no new crystal defect had been introduced in the monocrystalline Si layer formation step to maintain good crystallinity. When the life time of minority carriers was measured for the monocrystalline Si layer by use of the MOS C-t method, a high value of 2.0 x 10 -3 sec. was exhibited. Example 2 A P-type (100) monocrystalline Si substrate having a diameter of 4 inches and a thickness of 500 microns was anodized in a 50% HF solution. The current density at this time was 100 mA/cm 2 . The porous structure formation rate at this time was 8.4 µm/min, and the P-type (100) Si substrate having a thickness of 500 microns as a whole was made porous in 60 minutes. On the P-type (100) porous Si substrate 21 was grown at low temperature an Si epitaxial layer 22 to a thickness of 0.5 microns by the plasma CVD method. The deposition conditions are as follows: Gas: SiH4 Radio frequency power: 100 W Temperature: 800 °C Pressure: 1 x 10 -2 Torr Growth rate: 2.5 nm/sec. Next, on the surface of the epitaxial layer 22 was superposed another Si substrate 23 with an oxidized layer 24 of 5000 angstroms formed on the surface, and by heating in a nitrogen atmosphere at 700 °C for 0.5 hour, the two Si substrates were bonded firmly together. Then, the porous Si substrate 21 was chemically etched away by use of hydrofluoric acid-nitric acid-acetic acid solution (1:3:8). As described above, chemical etching rate of ordinary Si monocrystal relative to hydrofluoric acid-nitric acid-acetic acid solution is about a little lower than 1 micron per minute (hydrofluoric acid-nitric acid-acetic acid solution, 1:3:8), but the chemical etching rate of a porous layer is accelerated by about 100-fold thereof. That is, the Si substrate 21 made porous having a thickness of 500 microns was removed in 5 minutes. Thus, a monocrystalline Si layer having a thickness of 0.5 µm could be formed on the SiO 2 layer 24. Also the thickness of the monocrystalline Si layer obtained was examined by use of Scanning Ellipsometer. Specifically, the whole surface of the 4 inches wafer was scanned for the measurement. As the result, within the plane of the 4 inches wafer, the difference between the maximum value and the minimum value of the thickness of the monocrystalline Si layer was found to be suppressed 7% or less relative to the maximum value of the thickness. As the result of plan view observation of the monocrystalline Si layer by transmission electron microscopy, the dislocation defect density was found to be suppressed 1 x 10 3 /cm 2 or less, whereby it could be confirmed that no new crystal defect had been introduced in the monocrystalline Si layer formation step to maintain good crystallinity. When the life time of minority carriers was messured for the monocrystalline Si layer by use of the MOS C-t method, a high value of 2.0 x 10 -3 sec. was exhibited. Example 3 A P-type (100) monocrystalline Si substrate (Si water) having a diameter of 3 inches and a thickness of 200 microns was anodized in a 50% HF solution. The current density at this time was 100 mA/cm 2 . The porous structure formation rate at this time was 8.4 µm/min. and the P-type (100) Si substrate having a thickness of 200 microns as a whole was made porous in 24 minutes. On the P-type (100) porous Si substrate 21 was grown an Si epitaxial layer 22 to a thickness of 0.5 microns by the bias-sputtering method. The deposition conditions are as follows: RF frequency: 100 MHz Radio frequency power: 600 W Temperature: 300 °C Ar gas pressure: 8 x 10 -3 Torr Growth time: 60 minutes Target direct current bias: - 200 V Substrate direct current bias: 5 V. Next, on the surface of the epitaxial layer 22 was superposed another Si substrate 23 with an oxidized layer 24 of 5000 angstroms formed on the surface, and by heating in a nitrogen atmosphere at 800 °C for 0.5 hour, the two Si substrates were bonded firmly together. Then, the porous Si substrate 21 was chemically etched away by use of hydrofluoric acid-nitric acid-acetic acid solution (1:3:8). As described above, chemical etching rate or ordinary Si monocrystal to hydrofluoric acid-nitric acid-acetic acid solution is about a little lower than 1 micron per minute (hydrofluoric acid-nitric acid-acetic acid solution, 1:3:8), the chemical etching rate or a porous layer is accelerated by about 100-fold thereof. That is, the Si substrate 21 made porous having a thickness of 200 microns was removed in 2 minutes. Thus, a monocrystalline Si layer having a thickness of 0.5 µm could be formed on the SiO 2 layer 24. Example 4 A P-type (100) monocrystalline Si substrate having a diameter of 3 inches and a thickness of 200 microns was anodized in a 50% HF solution. The current density at this time was 100 mA/cm 2 . The porous structure formation rate at this time was 8.4 µm/min, and the P-type (100) Si substrate having a thickness of 200 microns as a whole was made porous in 24 minutes. On the P-type (100) porous Si substrate 21 was grown an Si epitaxial layer 22 to a thickness of 0.5 microns by the liquid phase growth method. The growth conditions are as follows: Solvent: Sn Growth temperature: 900 °C Growth atmosphere: H 2 Growth time: 10 minutes. Next, on the surface of the epitaxial layer 22 was superposed another Si substrate 23 with an oxidized layer 24 of 5000 angstroms formed on the surface, and by heating in a nitrogen atmosphere at 800 °C for 0.5 hour, the two Si substrates were bonded firmly together. Then, the porous Si substrate 21 was chemically etched away by use of hydrofluoric acid-nitric acid-acetic acid solution (1:3:8). As the result, the Si substrate 21 made porous having a thickness of 200 microns was removed in 2 minutes. Thus, a monocrystalline Si layer 22 having a thickness of 0.5 µm could be formed on the SiO 2 layer 24. Example 5 A P-type (100) monocrystalline Si substrate having a diameter of 3 inches and a thickness of 200 microns was anodized in a 50% HF solution. The current density at this time was 100 mA/cm 2 . The porous structure formation rate at this time was 8.4 µm/min, and the P-type (100) Si substrate having a thickness of 200 microns as a whole was made porous in 24 minutes. On the P-type (100) porous Si substrate 21 was grown an si epitaxial layer 21 to a thickness of 0.5 microns by the low pressure CVD method. The deposition conditions are as follows: Source gas: SiH 4 Carrier gas: H 2 Temperature: 850 °C Pressure: 1 x 10 -2 Torr Growth rate: 3.3 nm/sec. Next, on the surface of the epitaxial layer 22 was superposed another Si substrate with an oxidized layer 24 of 5000 angstroms formed on the surface, and by heating in a nitrogen atmosphere at 800 °C for 0.5 hour, the two Si substrates were bonded firmly together. Then, the porous Si substrate 21 was chemically etched away by use of hydrofluoric acid-nitric acid-acetic acid solution (1:3:8). As the result, the Si substrate 21 made porous having a thickness of 200 microns was removed in 2 minutes. Thus, a monocrystalline Si layer having a thickness of 0.5 µm could be formed on the SiO 2 layer 24. When SiH 2 Cl 2 was employed as the source gas, although it was necessary to elevate the growth temperture by some ten degrees, the accelerted etching characteristics inherent in the porous substrate were maintained. Example 6 On a P-type (100) Si substrate 121 having a diameter of 3 inches and a thickness of 200 microns was grown an Si epitaxial layer 122 with a thickness of 1 micron by the CVD method. The deposition conditions are as follows: Reactive gas flow rate: SiH 2 Cl 2 1000 SCCM H 2 230 l/min. Temperature: 1080 °C Pressure: 80 Torr Time: 2 min. The substrate 121 was anodized in a 50% HF solution. The current density at this time was 100 mA/cm 2 . The porous structure formation rate at this time was 8.4 µm/min. and the P-type (100) Si substrate 121 having a thickness of 200 microns as a whole was made porous in 24 minutes. In this anodization, only the P-type (100) Si substrate 121 was made porous, and there was no change in the Si epitaxial layer 122. Next, on the surface of the epitaxial layer 122 was superposed another Si substrate 124 with an oxidized layer 125 of 5000 angstroms formed on the surface, and by heating in a nitrogen atmosphere at 800 °C for 0.5 hour, the two Si substrates were bonded firmly together. Then, the porous Si substrate 123 was chemically etched away by use of hydrofluoric acid-nitric acid-acetic acid solution (1:3:8). As the result, the Si substrate 123 made porous having a thickness of 200 microns was removed in 2 minutes. Also the thickness of the monocrystalline Si layer obtained was examined by use of Scanning Ellipsometer. As the result, within the plane of the 3 inches wafer, the difference between the maximum value and the minimum value of the thickness of the monocrystalline Si layer was found to be suppressed 5% or less relative to the maximum value of the thickness. As the result of plan view observation of the monocrystalline Si layer by a transmission electron microscopy, the dislocation defect density was found to be suppressed 1 x 10 3 /cm 2 or less, whereby it could be confirmed that no new crystal defect had been introduced in the monocrystalline Si layer formation step to maintain good crystallinity. When the life time of minority carriers was measured for the monocrystalline Si layer by use of the microwave reflection method, a high value of 2.0 x 10 -3 sec. was exhibited. Example 7 On a P-type (100) Si substrate having a diameter of 3 inches and a thickness of 200 microns was grown an Si epitaxial layer 122 to a thickness of 0.5 micron by the CVD method. The deposition conditions are as follows: Reactive gas flow rate: SiH 2 Cl 2 1000 SCCM H 2 230 l/min. Temperature : 1080 °C Pressure: 80 Torr Time: 1 min. The substrate was anodized in a 50% HF solution. The current density at this time was 100 mA/cm 2 . The porous structure formation rate at this time was 8.4 µm/min. and the P-type (100) Si substrate 121 having a thickness of 200 microns as a whole was made porous in 24 minutes. In this anodization, only the P-type (100) Si substrate was made porous, and there was no change in the Si epitaxial layer 122. Next, on the surface of the epitaxial layer 122 was superposed another Si substrate 124 with an oxidized layer 125 of 5000 angstroms formed on the surface, and by heating in a nitrogen atmosphere at 800 °C for 0.5 hour, the two Si substrates were bonded firmly together. Then, the porous Si substrate 123 was chemically etched away by use of hydrofluoric acid-nitric acid-acetic acid solution (1:3:8). As the result, the Si substrate 123 made porous having a thickness of 200 microns was removed in 2 minutes. As the result of cross-sectional observation by transmission electron microscopy, it could be confirmed that no new crystal defect had been introduced in the Si layer 122 to maintain good crystallinity. Example 8 On a surface of a P-type (100) Si substrate 121 having a diameter of 3 inches and a thickness of 200 microns was formed an N-type Si layer 122 to a thickness of 1 micron by ion implantation of protons into the surface. The amount of H + implanted was found to be 5 x 10 15 (ions/cm 2 ). The substrate 121 was anodized in a 50% HF solution. The current density at this time was 100 mA/cm 2 . The porous structure formation rate at this time was 8.4 µm/min, and the P-type (100) Si substrate 121 having a thickness of 200 microns as a whole was made porous in 24 minutes. As described above, in this anodization, only the P-type (100) Si substrate 121 was made porous, and there was no change in the N-type Si layer 122. Next, on the surface of the N-type Si layer 122 was superposed another Si substrate 124 with an oxidized layer 125 of 5000 angstroms formed on the surface, and by heating in a nitrogen atmosphere at 800 °C for 0.5 hour, the two Si substrates were bonded firmly together. Then, the porous Si substrate 123 was chemically etched away by use of hydrofluoric acid-nitric acid-acetic acid solution (1:3:8). As the result, the Si substrate 123 made porous having a thickness of 200 microns was removed in 2 minutes. As the result of cross-sectional observation by transmission electron microscopy, it could be confirmed that no new crystal defect had been introduced in the Si layer 122 to maintain good crystallinity. Example 9 A P-type (100) monocrystalline Si substrate having a diameter of 3 inches and a thickness of 200 microns was anodized in a 50% HF solution. The current density at this time was 100 mA/cm 2 . The porous structure formation rate at this time was 8.4 µm/min, and the P-type (100) Si substrate having a thickness of 200 microns as a whole was made porous in 24 minutes. On the P-type (100) porous Si substrate 11 was grown an Si epitaxial layer 12 to a thickness of 0.5 microns by the MBE (Molecular Beam Epitaxy) method. The deposition conditions are as follows: Temperature: 700 °C Pressure: 1 x 10 -9 Torr Growth rate: 0.1 nm/sec. Next, on the surface of the epitaxial layer 12 was superposed another Si substrate 13 with an oxidized layer 14 of 5000 angstroms formed on the surface, and by heating in a nitrogen atmosphere at 800 °C for 0.5 hour, the two Si substrates were bonded firmly together. Subsequently Si 3 N 4 was coated on the bonded substrate by the low pressure CVD method to a thickness of 0.1 µm. Thereafter, only the silicon nitride film on the porous substrate was removed by reactive ion etching. Then, the porous Si substrate 11 was chemically etched away by use of hydrofluoric acid-nitric acid-acetic acid solution (1:3:8). As the result, the Si substrate 11 made porous having a thickness of 200 microns was removed in 2 minutes. After removal of the Si 3 N 4 layer 15, a substrate having a monocrystalline Si layer having a thickness of 0.5 µm on the SiO 2 layer 14 could be formed. As the result of cross-sectional observation by transmission electron microscopy, it could be confirmed that no new crystal defect had been introduced in the Si layer to maintain good crystallinity. Example 10 A P-type (100) monocrystalline Si substrate having a diameter of 3 inches and a thickness of 200 microns was anodized in a 50% HF solution. The current density at this time was 100 mA/cm 2 . The porous structure formation rate at this time was 8.4 µm/min, and the P-type (100) Si substrate having a thickness of 200 microns as a whole was made porous in 24 minutes. On the P-type (100) porous Si substrate 11 was grown an Si epitaxial layer 12 to a thickness of 0.5 microns by the plasma CVD method. The deposition conditions are as follows: Gas: SiH 4 Radio frequency power: 100 W Temperature: 800 °C Pressure: 1 x 10 -2 Torr Growth rate: 2.5 nm/sec. Next, on the surface of the epitaxial layer 12 was superposed another Si substrate with an oxidized layer 14 of 5000 angstroms formed on the surface, and by heating in a nitrogen atmosphere at 800 °C for 0.5 hour, the two Si substrates were bonded firmly together. Subsequently, Si 3 N 4 was coated on the bonded substrate by the low pressure CVD method to a thickness of 0.1 µm. Thereafter, only the silicon nitride film on the porous substrate 11 was removed by reactive ion etching. Then, the porous Si substrate 11 was chemically etched away by use of hydrofluoric acid-nitric acid acetic acid solution (1:3:8). As the result, the Si substrate 11 made porous having a thickness of 200 microns was removed in 2 minutes. After removal of the Si 3 N 4 layer 15, a substrate having a monocrystalline Si layer 12 having a thickness of 0.5 µm on the SiO 2 layer could be formed. Also the thickness of the monocrystalline Si layer obtained was examined by use of Scanning Ellipsometer. As the result, within the plane of the 3 inches wafer, the difference between the maximum value and the minimum value of the thickness of the monocrystalline Si layer was found to be suppressed 5% or less relative to the maximum value of the thickness. As the result of observation by defect delineation etching by use of Sirtle etching, the dislocation defect density was found to be suppressed 1 x 10 3 /cm 2 or less, whereby it could be confirmed that no new crystal defect had been introduced in the monocrystalline Si layer formation step to maintain good crystallinity. When the life time of minority carriers was measured for the monocrystalline Si layer by use of the MOS C-t method, a high value of 2.0 x 10 -3 sec. was exhibited. Example 11 A P-type (100) monocrystalline Si substrate having a diameter of 3 inches and a thickness of 200 microns was anodized in a 50% HF solution. The current density at this time was 100 mA/cm 2 . The porous structure formation rate at this time was 8.4 µm/min. and the P-type (100) Si substrate having a thickness of 200 microns as a whole was made porous in 24 minutes. On the P-type (100) porous Si substrate 11 was grown an Si epitaxial layer 12 to a thickness of 0.5 microns by the bias sputtering method. The deposition conditions are as follows: RF frequency: 100 MHz Radio frequency power: 600 W Temperature: 300 °C Ar gas pressure: 8 x 10 -3 Torr Growth time: 60 min. Target direct current bias: -200 V Substrate direct current bias: + 5 V Next, on the surface of the epitaxial layer 12 was superposed another Si substrate with an oxidized layer 14 of 5000 angstroms formed on the surface, and by heating in a nitrogen atmosphere at 800 °C for 0.5 hour, the two Si substrates were bonded firmly together. Subsequently, Si 3 N 4 was coated on the bonded substrate by the low pressure CVD method to a thickness of 0.1 µm. Thereafter, only the silicon nitride film on the porous substrate was removed by reactive ion etching. Then, the porous Si substrate 11 was chemically etched away by use of hydrofluoric acid-nitric acid-acetic acid solution (1:3:8). As the result, the Si substrate 11 made porous having a thickness of 200 microns was removed in 2 minutes. After removal of the Si 3 N 4 layer 15, a substrate having a monocrystalline Si layer 12 having a thickness of 0.5 µm on the SiO 2 layer 14 could be formed. Also, the same effect was obtained when Apiezon Wax was coated in place of the Si 3 N 4 layer, and only the Si substrate made porous could be completely removed. Example 12 A P-type (100) monocrystalline Si substrate having a diameter of 3 inches and a thickness of 200 microns was anodized in a 50% HF solution. The current density at this time was 100 mA/cm 2 . The porous structure formation rate at this time was 8.4 µm/min, and the P-type (100) 51 substrate having a thickness of 200 microns as a whole was made porous in 24 minutes. On the P-type (100) porous Si substrate 11 was grown an Si epitaxial layer 12 to a thickness of 0.5 microns by the liquid phase growth method. The growth conditions are as follows: Solvent: Sn Growth temperature: 900 °C Growth atmosphere: H 2 Growth time: 10 min. Next, on the surface of the epitaxial layer 12 was superposed another Si substrate 13 with an oxidized layer 14 of 5000 angstroms formed on the surface, and by heating in a nitrogen atmosphere at 800 °C for 0.5 hour, the two Si substrates were bonded firmly together. Subsequently, Si 3 N 4 was coated on the bonded substrate by the low pressure CVD method to a thickness of 0.1 µm. Thereafter, only the silicon nitride film on the porous substrate was removed by reactive ion etching. Then, the porous Si substrate 11 was chemically etched away by use of hydrofluoric acid-nitric acid-acetic acid solution (1:3:8). As the result, the Si substrate 11 made porous having a thickness of 200 microns was removed in 2 minutes. After removal of the Si 3 N 4 layer 15, a substrate having a monocrystalline Si layer 12 having a thickness of 0.5 µm on the SiO 2 layer 14 could be formed. Also, the same effect was obtained when Apiezon Wax was coated in place of the Si 3 N 4 layer, and only the Si substrate made porous could be completely removed. Example 13 A P-type (100) monocrystalline Si substrate having a diameter of 3 inches and a thickness of 200 microns was anodized in a 50% HF solution. The current density at this time was 100 mA/cm 2 . The porous structure formation rate at this time was 8.4 µm/min. and the P-type (100) Si substrate having a thickness of 200 microns as a whole was made porous in 24 minutes. On the P-type (100) porous Si substrate 11 was grown an Si epitaxial layer 12 to a thickness of 0.5 microns by the low pressure CVD method. The deposition conditions are as follows: Source gas: SiH 4 Carrier gas: H 2 Temperature: 850 °C Pressure: 1 x 10 -2 Torr Growth rate: 3.3 nm/sec. Next, on the surface of the epitaxial layer 12 was superposed another Si substrate 13 with an oxidized layer 14 of 5000 angstroms formed on the surface, and by heating in a nitrogen atmosphere at 800 °C for 0.5 hour, the two Si substrates were bonded flrmly together. Subsequently, Si 3 N 4 was coated on the bonded substrate by the low pressure CVD method to a thickness of 0.1 µm. Thereafter, only the silicon nitride film 15 on the porous substrate 11 was removed by reactive ion etching. Then, the porous Si substrate 11 was chemically etched away by use of hydrofluoric acid-nitric acid-acetic acid solution (1:3:8). As the result, the Si substrate 11 made porous having a thickness of 200 microns was removed in 2 minutes. After removal of the Si 3 N 4 layer 15, a substrate having a monocrystalline Si layer having a thickness of 0.5 µm on the SiO 2 layer 14 could be formed. When SiH 2 Cl 2 was employed as the source gas, although it was required to elevate the growth temperature by some ten degrees, the accelerated chemical etching characteristics inherent in the porous substrate could be maintained. Example 14 On a P-type (100) Si substrate 111 having a diameter of 3 inches and a thickness of 200 microns was grown an Si epitaxial layer 112 with a thickness of 1 micron by the CVD method. The deposition conditions are as follows: Reactive gas flow rate: SiH 2 Cl 2 1000 SCCM H 2 230 l/min. Temperature: 1080 °C Pressure: 80 Torr Time: 2 min. The substrate was anodized in a 50% HF solution. The current density at this time was 100 mA/cm 2 . The porous structure formation rate at this time was 8.4 µm/min, and the P-type (100) Si substrate 111 having a thickness of 200 microns as a whole was made porous in 24 minutes. In this anodization, only the P-type (100) Si substrate 111 was made porous, and there was no change in the Si epitaxial layer 112. Next, on the surface of the epitaxial layer 112 was superposed another Si substrate 114 with an oxidized layer of 5000 angstroms formed on the surface, and by heating in a nitrogen atmosphere at 800 °C for 0.5 hour, the two Si substrates were bonded firmly together. Subsequently, Si 3 N 4 was coated on the bonded substrate by the low pressure CVD method to a thickness of 0.1µm. Thereafter, only the nitride film on the porous substrate was removed by reactive ion etching. Then, the porous Si substrate 111 was chemically etched away by use of hydrofluoric acid-nitric acid-acetic acid solution (1:3:8). As the result, the Si substrate 113 made porous having a thickness of 200 microns was removed in 2 minutes. After removal of the Si 3 N 4 layer 116, a substrate having a monocrystalline Si layer 112 having a thickness of 1 µm could be formed on SiO 2 . Also the thickness of the monocrystalline Si layer obtained was examined by use of Scanning Ellipsometer. As the result, within the plane of the 3 inches wafer, the difference between the maximum value and the minimum value of the thickness of the monocrystalline Si layer was found to be suppressed 5% or less relative to the maximum value of the thickness. As the result of plan view observation of the monocrystalline Si layer by transmission electron microscopy, the dislocation defect density was found to be suppressed 1 x 10 3 /cm 2 or less, whereby it could be confirmed that no new crystal defect had been introduced in the monocrystalline Si layer formation process to maintain good crystallinity. When the life time of minority carriers was measured for the monocrystalline Si layer by use of the microwave reflection method, a high value of 2.0 x 10 -3 sec. was exhibited. Example 15 On a P-type (100) Si substrate 111 having a diameter of 3 inches and a thickness of 200 microns was grown an Si epitaxial layer 112 to a thickness of 0.5 micron by the CVD method. The deposition conditions are as follows: Reactive gas flow rate: SiH 2 Cl 2 1000 SCCM H 2 230 l/min. Temperature: 1080 °C Pressure: 80 Torr Time: 1 min. The substrate was anodized in a 50% HF solution. The current density at this time was 100 mA/cm 2 . The porous structure formation rate at this time was 8.4 µm/min. and the P-type (100) Si substrate 111 having a thickness of 200 microns as a whole was made porous in 24 minutes. In this anodization, only the P-type (100) Si substrate 111 was made porous, and there was no change in the Si epitaxial layer 112. Next, on the surface of the epitaxial layer 112 was superposed an Si substrate 114 with an oxidized layer of 5000 angstroms formed on the surface, and by heating in a nitrogen atmosphere at 800 °C for 0.5 hour, the two Si substrates were bonded firmly together. Subsequently, Si 3 N 4 was coated on the bonded substrate by the low pressure CVD method to a thickness of 0.1 µm. Thereafter, only the silicon nitride film 116 on the porous substrate 113 was removed by reactive ion etching. Then, the porous Si substrate 113 was chemically etched away by use of hydrofluoric acid-nitric acid-acetic acid solution (1:3:8). As the result, the Si substrate 113 made porous having a thickness of 200 microns was removed in 2 minutes. After removal of the Si 3 N 4 layer 116, a substrate having a monocrystalline Si layer 112 having a thickness of 0.5 µm could be formed on the SiO 2 layer 115. As the result of cross sectional observation by transmission electron microscopy, it could be confirmed that no new crystal defect had been introduced in the Si layer to maintain good crystallinity. Example 16 On a surface of a P-type (100) Si substrate 111 having a diameter of 3 inches and a thickness of 200 microns was formed an N-type Si layer 112 to a thickness of 1 micron by ion implantation of protons into the surface. The amount of H + implanted was found to be 5 x 10 15 (ions/cm 2 ). The substrate was anodized in a 50% HF solution. The current density at this time was 100 mA/cm 2 . The porous structure formation rate at this time was 8.4 µm/min. and the P-type (100) Si substrate 111 having a thickness of 200 microns as a whole was made porous in 24 minutes. In this anodization, only the P-type (100) Si substrate 111 was made porous, and there was no change in the N-type Si layer 112. Next, on the surface of the N-type Si layer 112 was superposed another Si substrate 114 with an oxidized layer 115 of 5000 angstroms formed on the surface, and by heating in a nitrogen atmosphere at 800 °C for 0.5 hour, the Si substrates were bonded firmly together. Subsequently, Si 3 N 4 was coated on the bonded substrate by the low pressure CVD method to a thickness of 0.1 µm. Thereafter, only the silicon nitride film on the porous substrate was removed by reactive ion etching. Then, the porous Si substrate 113 was chemically etched away by use of hydrofluoric acid-nitric acid-acetic acid solution (1:3:8). As the result, the Si substrate 113 made porous having a thickness of 200 microns was removed in 2 minutes. After removal of the Si 3 N 4 layer 116, a substrate having a monocrystalline Si layer 112 having a thickness of 1.0 µm could be formed on the SiO 2 . As the result of cross-sectional observation by transmission electron microscopy, it could be confirmed that no new crystal defect had been introduced in the Si layer to maintain good crystallinity. Example 17 A P-type (100) monocrystalline Si substrate having a diameter of 3 inches and a thickness of 200 microns was anodized in a 50% HF solution. The current density at this time was 100 mA/cm 2 . The porous structure formation rate at this time was 8.4 µm/min. and the P-type (100) Si substrate having a thickness of 200 microns as a whole was made porous in 24 minutes. On the P-type (100) porous Si substrate 31 was grown an Si epitaxial layer 32 to a thickness of 0.5 microns by the MBE (Molecular Beam Epitaxy) method. The deposition conditions are as follows: Temperature: 700 °C Pressure: 1 x 10 9 Torr Growth rate: 0.1 nm/sec. Next, on the surface of the epitaxial layer 32 was formed an oxide layer 36 with a thickness of 1000 angstroms. Then, on another Si substrate having an oxide layer 34 or 5000 angstroms formed on the surface was superposed the above oxide layer 36, and by heating in a nitrogen atmosphere at 800 °C for 0.5 hour, the both were bonded firmly together. Si 3 N 4 was coated on the bonded substrate by the low pressure CVD method to a thickness of 0.1 µm. Thereafter, only the silicon nitride film on the porous substrate was removed by reactive ion etching. Then, the porous Si substrate 31 was chemically etched away by use of hydrofluoric acid-nitric acid-acetic acid solution (1:3:8). As the result, the Si substrate 31 made porous having a thickness of 200 microns was removed in 2 minutes. After removal of the Si 3 N 4 layer 35, a substrate having a thin film monocrystalline Si layer 32 could be formed on the SiO 2 . As the result of cross-sectional observation by transmission electron microscopy, it could be confirmed that no new crystal defect had been introduced in the Si layer to maintain good crystallinity. Example 18 A P-type (100) monocrystalline Si substrate having a diameter of 3 inches and a thickness of 200 microns was anodized in a 50% HF solution. The current density at this time was 100 mA/cm 2 . The porous structure formation rate at this time was 8.4 µm/min, and the P-type (100) Si substrate having a thickness of 200 microns as a whole was made porous in 24 minutes. On the P-type (100) porous Si substrate 31 was grown an Si epitaxial layer 32 to a thickness of 5 microns according to the plasma CVD method. The deposition conditions are as follows: Gas: SiH 4 Radio frequency power: 100 W Temperature: 800 °C Pressure: 1 x 10 -2 Torr Growth rate: 2.5 nm/sec. Next, on the surface of the epitaxial layer 32 was formed an oxide layer 36 with a thickness of 1000 angstroms. Then, on another Si substrate 33 having an oxide layer 34 of 5000 angstroms formed on the surrace was superposed the above oxide layer 36, and by heating in a nitrogen atmosphere at 800 °C for 0.5 hour, the both were bonded firmly together. Si 3 N 4 was coated on the bonded substrate by the low pressure CVD method to a thickness of 0.1 µm. Thereafter, only the silicon nitride film on the porous substrate was removed by reactive ion etching. Then, the porous Si substrate 31 was chemically etched away by use of a KOH solution (6 M). As the result, the Si substrate 31 made porous having a thickness of 200 microns was removed in 2 minutes. After removal of the Si 3 N 4 layer, a substrate having a monocrystalline Si layer 32 with good crystallinity could be formed on the SiO 2 layer. Also the thickness of the monocrystalline Si layer obtained was examined by use of Scanning Ellipsometer. As the result, within the plane of the 3 inches wafer, the difference between the maximum value and the minimum value of the thickness of the monocrystalline Si layer was found to be suppressed 5% or less relative to the maximum value of the thickness. As the result of plan view observation of the monocrystalline Si layer by transmission electron microscopy, the dislocation defect density was found to be suppressed 1 x 10 3 /cm 2 or less, whereby it could be confirmed that no new crystal defect had been introduced in the monocrystalline Si layer formation process to maintain good crystallinity. When the life time of minority carriers was measured for the monocrystalline Si layer by use of the MOS C-t method, a high value of 2.0 x 10 -3 sec. was exhibited. Example 19 A P-type (100) monocrystalline Si substrate having a diameter of 3 inches and a thickness of 200 microns was anodized in a 50% HF solution. The current density at this time was 100 mA/cm 2 . The porous structure formation rate at this time was 8.4 µm/min. and the P-type (100) Si substrate having a thickness of 200 microns as a whole was made porous in 24 minutes. On the P-type (100) porous Si substrate 31 was grown an Si epitaxial layer 32 to a thickness of 1 micron according to the bias sputtering method. The deposition conditions are as follows: RF frequency: 100 MHz Radio frequency power: 600 W Temperature: 800 °C Ar gas pressure: 8 x 10 -3 Torr Growth time: 120 min. Target direct current bias: - 200 V Substrate direct curren bias: + 5V Next, on the surface of the epitaxial layer 32 was formed an oxide layer 36 with a thickness of 1000 angstroms. Then, on another Si substrate 33 having an oxide layer 34 of 5000 angstroms formed on the surface was superposed the above oxide layer 36, and by heating in a nitrogen atmosphere at 800 °C for 0.5 hour, the both were bonded firmly together. Si 3 N 4 was coated on the bonded substrate by the low pressure CVD method to a thickness of 0.1 µm. Thereafter, only the nitride film on the porous substrate 31 was removed by reactive ion etching. Then, the porous Si substrate 31 was chemically etched away by use of a hydrofluoric acid-nitric acid-acetic acid solution (1:3:8). As the result, the Si substrate 31 made porous having a thickness of 200 microns was removed in 2 minutes. After removal of the Si 3 N 4 layer, a substrate having a monocrystalline Si layer 32 with good crystallinity could be formed on the SiO 2 layer. The same effect was obtained also when Apiezon Wax was coated in place of the Si 3 N 4 layer 35, whereby only the Si substrate 35 made porous could be completely removed. Example 20 A P-type (100) monocrystalline Si substrate having a diameter of 3 inches and a thickness of 200 microns was anodized in a 50% HF solution. The current density at this time was 100 mA/cm 2 . The porous structure formation rate at this time was 8.4 µm/min. and the P-type (100) Si substrate having a thickness of 200 microns as a whole was made porous in 24 minutes. On the P-type (100) porous Si substrate 31 was grown an Si epitaxial layer 32 to a thickness of 5 micron by the liquid phase growth method. The growth conditions are as follows: Solvent: Sn Growth temperature: 900 °C Growth atmosphere: H 2 Growth time: 10 min. Next, on the surface of the epitaxial layer 32 was formed an oxide layer 36 with a thickness of 1000 angstroms. Then, on another Si substrate 33 having an oxide layer 34 of 5000 angstroms formed on the surface was superposed the above oxide layer 36, and by heating in a nitrogen atmosphere at 700 °C for 0.5 hour, the both were bonded firmly together. Si 3 N 4 was coated on the bonded substrate by the low pressure CVD method to a thickness of 0.1 µm. Thereafter, only the silicon nitride film on the porous substrate was removed by reactive ion etching. Then, the porous Si substrate 31 was chemically etched away by use of a hydrofluoric acid-nitric acid-acetic acid solution (1:3:8). As the result, the Si substrate 31 made porous having a thickness of 200 microns was removed in 2 minutes. After removal of the Si 3 N 4 layer 35, a substrate having a monocrystalline Si layer 32 could be formed on the SiO 2 . The same effect was obtained also when Apiezon Wax was coated in place of the Si 3 N 4 layer, whereby only the Si substrate made porous could be completely removed. Also the thickness of the monocrystalline Si layer obtained was examined by use of Scanning Ellipsometer. As the result, within the plane of the 3 inches wafer, the difference between the maximum value and the minimum value of the thickness of the monocrystalline Si layer was found to be suppressed 5% or less relative to the maximum value of the thickness. As the result of plan view observation of the monocrystalline Si layer by transmission electron microscopy, the dislocation defect density was found to be suppressed 1 x 10 3 /cm 2 or less, whereby it could be confirmed that no new crystal defect had been introduced in the monocrystalline Si layer formation process to maintain good crystallinity. When the life time of minority carriers was measured for the monocrystalline Si layer by use of the MOS C-t method, a high value of 2.0 x 10 -3 sec. was exhibited. Example 21 A P-type (100) monocrystalline Si substrate having a diameter of 3 inches and a thickness of 200 microns was anodized in a 50% HF solution. The current density at this time was 100 mA/cm 2 . The porous structure formation rate at this time was 8.4 µm/min. and the P-type (100) Si substrate having a thickness of 200 microns as a whole was made porous in 24 minutes. On the P-type (100) porous Si substrate 31 was grown an Si epitaxial layer 32 to a thickness of 1.0 micron at low temperature by the low pressure CVD method. The deposition conditions are as follows: Source gas: SiH 4 Carrier gas: H 2 Temperature: 850 °C Pressure: 1 x 10 -2 Torr Growth rate: 3.3 nm/sec. Next, on the surface of the epitaxial layer 32 was formed an oxidized layer 36 with a thickness of 1000 angstroms. Then, on another Si substrate 33 having an oxide layer 34 of 5000 angstroms formed on the surface was superposed the above oxide layer 36, and by heating in a nitrogen atmosphere at 700 °C for 0.5 hour, the both were bonded firmly together. Si 3 N 4 was coated on the bonded substrate by the low pressure CVD method to a thickness of 0.1 µm. Thereafter, only the silicon nitride film 35 on the porous substrate 31 was removed by reactive ion etching. Then, the porous Si substrate 31 was chemically etched away by use of a hydrofluoric acid-nitric acid-acetic acid solution (1:3:8). As the result, the Si substrate 31 made porous having a thickness of 200 microns was removed in 2 minutes. After removal of the Si 3 N 4 layer 35, a substrate having a monocrystalline Si layer 32 could be formed on the SiO 2 layer. When SiH 2 Cl 2 was employed as the source gas, although the growth temperature was required to be elevated by some ten degrees, the accelerated chemical etching characteristics inherent in the porous substrate could be maintained. Example 22 On a P-type (100) Si substrate 131 having a diameter of 3 inches and a thickness of 200 microns was grown an Si epitaxial layer 132 with a thickness of 1 micron by the CVD method. The deposition conditions are as follows: Reactive gas flow rate: SiH 2 Cl 2 1000 SCCM H 2 230 l/min. Temperature: 1080 °C Pressure: 80 Torr Time: 2 min. The substrate was anodized in a 50% HF solution. The current density at this time was 100 mA/cm 2 . The porous structure formation rate at this time was 8.4 µm/min. and the P-type (100) Si substrate 131 having a thickness of 200 microns as a whole was made porous in 24 minutes. As described above, only the P-type (100) Si substrate 131 was made porous in this anodization, and there was no change in the Si epitaxial layer 132. Next, on the surface of the epitaxial layer 132 was formed an oxidized layer 137, another Si substrate 134 having an oxidized layer 135 of 5000 angstroms formed on the surface was superposed on the above oxidized layer 137, and the two Si substrates were bonded together firmly by heating at 800 °C for 0.5 hour in a nitrogen atmosphere. By the low pressure CVD method, Si 3 N 4 was coated with a thickness of 0.1 µm on the bonded substrate. Then only the silicon nitride film on the porous substrate was removed by reactive ion etching. Subsequently, the porous Si substrate 133 was chemically etched away by use of hydrofluoric acid-nitric acid-acetic acid solution (1:3:8). As the result, the Si substrate 133 made porous having a thickness of 200 microns was removed in 2 minutes. After removal of the Si 3 N 4 layer 136, a substrate having the monocrystalline Si layer 132 having a thickness of 1 µm could be formed on SiO 2 . As the result of cross-sectional observation by transmission electron microscopy, it was confirmed that no new crystal defect had been introduced in the Si layer to maintain good crystallinity. Example 23 On a P-type (100) Si substrate 131 having a diameter of 4 inches and a thickness of 500 microns was grown an Si epitaxial layer 132 with a thickness of 0.5 micron by the CVD method. The deposition conditions are as follows: Reactive gas flow rate: SiH 2 Cl 2 1000 SCCM H 2 230 l/min. Temperature: 1080 °C Pressure: 80 Torr Time: 1 min. The substrate was anodized in a 50% HF solution. The current density at this time was 100 mA/cm 2 . The porous structure formation rate at this time was 8.4 µm/min. and the P-type (100) Si substrate 131 having a thickness of 500 microns as a whole was made porous. Only the P-type (100) Si substrate 131 was made porous in this anodization, and there was no change in the Si epitaxial layer 132. Next, on the surface of the epitaxial layer 132 was formed an oxidized layer 137 with a thickness of 1000 angstroms. Then, another Si substrate 134 having an oxidized layer 135 of 5000 angstroms formed on the surface was adhered on the above oxidized layer 137, and the both were bonded together firmly by heating at 700 °C for 0.5 hour in a nitrogen atmosphere. By the low pressure CVD method, Si 3 N 4 was coated with a thickness of 0.1 µm on the bonded substrate, and only the silicon nitride film 136 on the porous substrate 133 was removed by reactive ion etching. Then, the porous Si substrate was chemically etched away by use of hydrofluoric acid-nitric acid-acetic acid solution (1:3:8). As the result, the Si substrate made porous having a thickness of 500 microns was removed in 5 minutes. After removal of the Si 3 N 4 layer 136, a substrate having the monocrystalline Si layer 132 could be formed on SiO 2 . Also the thickness of the monocrystalline Si layer obtained was examined by use of Scanning Ellipsometer. As the result, within the plane of the 4 inches wafer, the difference between the maximum value and the minimum value of the thickness of the monocrystalline Si layer was found to be suppressed 8% or less relative to the maximum value of the thickness. As the result of plan view observation of the monocrystalline Si layer by transmission electron microscopy, the dislocation defect density was found to be suppressed 1 x 10 3 /cm 2 or less, whereby it could be confirmed that no new crystal defect had been introduced in the monocrystalline Si layer formation process to maintain good crystallinity. When the life time of minority carriers was measured for the monocrystalline Si layer by use of the MOS C-t method, a high value of 2.1 x 10 -3 sec. was exhibited. Example 24 On the surface of a P-type (100) Si substrate 131 having a diameter of 3 inches and a thickness of 200 microns was formed N-type Si layer 132 with a thickness of 1 micron by ion implantation of protons. The amount of H + implanted was 5 x 10 15 (ions/cm 2 ). The substrate was anodized in a 50% HF solution. The current density at this time was 100 mA/cm 2 . The porous structure formation rate at this time was 8.4 µm/min. and the P-type (100) Si substrate 131 having a thickness of 200 microns as a whole was made porous in 24 minutes. Only the P-type (100) Si substrate 131 was made porous in this anodization, and there was no change in the N-type Si layer 132. Next, on the surface of the epitaxial layer 132 was formed an oxidized layer 137 with a thickness of 1000 angstroms. Then, another Si substrate 134 having an oxidized layer 135 of 5000 angstroms formed on the surface was adhered on the above oxidized layer 137, and the two Si substrates were bonded together firmly by heating at 700 °C for 0.5 hour By the low pressure CVD method, Si 3 N 4 was coated with a thickness of 0.1 µm on the bonded substrate. Then, only the silicon nitride film on the porous substrate was removed by reactive ion etching. Subsequently, the porous Si substrate 133 was chemically etched away by use of hydrofluoric acid-nitric acid-acetic acid solution (1:3:8). As the result, the Si substrate made porous having a thickness of 200 microns was removed in 2 minutes. After removal of the Si 3 N 4 layer 136, a substrate having the monocrystalline Si layer 132 could be formed on SiO 2 . As the result of cross-sectional observation by transmission electron microscopy, it was confirmed that no new crystalline defect had been introduced in the Si layer to maintain good crystallinity. Example 25 A P-type (100) monocrystalline Si substrate having a diameter of 3 inches and a thickness of 200 microns was anodized in a 50% HF solution. The current density at this time was 100 mA/cm 2 . The porous structure formation rate at this time was 8.4 µm/min. and the P-type (100) Si substrate having a thickness of 200 microns as a whole was made porous in 24 minutes. On the P-type (100) porous Si substrate 41 was grown an Si epitaxial layer 42 to a thickness of 0.5 microns by the MBE (Molecular Beam Epitaxy) method. The deposition conditions are as follows: Temperature: 700 °C Pressure: 1 x 10 -9 Torr Growth rate: 0.1 nm/sec. Next, on the surface of the epitaxial layer 42 was formed an oxidized layer 45 with a thickness of 1000 angstroms. Then, another Si substrate 43 with an oxidized layer 44 of 5000 angstroms formed on the surface was superposed on the above oxidized layer 45, and by heating in a nitrogen atmosphere at 800 °C for 0.5 hour, the two Si substrates were bonded firmly together. Then, the porous Si substrate 41 was chemically etched away by use of hydrofluoric acid-nitric acid-acetic acid solution (1:3:8). As the result, the Si substrate 41 made porous having a thickness of 200 microns was removed in 2 minutes. Thus, a substrate having a thin film monocrystalline Si layer 42 could be formed on the SiO 2 . As the result of cross-sectional observation by transmission electron microscopy, it was confirmed that no new crystal defect had been introduced in the Si layer to maintain good crystallinity. Example 26 A P-type (100) monocrystalline Si substrate having a diameter of 3 inches and a thickness of 200 microns was anodized in a 50% HF solution. The current density at this time was 100 mA/cm 2 . The porous structure formation rate at this time was 8.4 µm/min. and the P-type (100) Si substrate having a thickness of 200 microns as a whole was made porous in 24 minutes. On the P-type (100) porous Si substrate 41 was grown an Si epitaxial layer 42 to a thickness of 5 microns by the plasma CVD method. The deposition conditions are as follows: Gas: SiH 4 Radio frequency power: 100 W Temperature: 800 °C Pressure: 1 x 10 -2 Torr Growth rate: 2.5 nm/sec. Next, on the surface of the epitaxial layer 42 was formed an oxidized layer 45 with a thickness of 1000 angstroms. Then, another Si substrate 43 with an oxidized layer 44 of 5000 angstroms formed on the surface was superposed on the above oxidized layer 45, and by heating in a nitrogen atmosphere at 800 °C for 0.5 hour, the two Si substrates were bonded firmly together. Then, the porous Si substrate 41 was chemically etched away by use of a KOH solution of 6 M. As described above, ordinary chemical etching rate of Si monocrystal relative to 6M KOH solution is about a little lower than 1 micron per minute, but the chemical etching rate of a porous layer is accelerated by about 100-fold thereof. Then, the Si substrate made porous having a thickness of 200 microns was removed in 2 minutes. Thus, a monocrystalline Si layer having good crystallinity could be formed on the SiO 2 . Example 27 A P-type (100) monocrystalline Si substrate having a diameter of 5 inches and a thickness of 600 microns was anodized in a 50% HF solution. The current density at this time was 100 mA/cm 2 . The porous structure formation rate at this time was 8.4 µm/min. and the P-type (100) Si substrate having a thickness of 600 microns as a whole was made porous in 70 minutes. On the P-type (100) porous Si substrate 41 was grown an Si epitaxial layer 42 to a thickness of 1 micron by the bias-sputtering method. The deposition conditions are as follows: RF frequency: 100 MHz Radio frequency power: 600 w Temperature: 300 °C Ar gas pressure: 8 x 10 -3 Torr Growth time: 120 minutes Target direct current bias: - 200 V Substrate direct current bias: + 5 V. Next, on the surface of the epitaxial layer 42 was formed an oxidized layer 45 with a thickness or 1000 angstroms. Then, another Si substrate 43 with an oxidized layer 44 of 5000 angstroms formed on the surface was superposed on the above oxidized layer 45, and by heating in a nitrogen atmosphere at 800 °C for 0.5 hour, the two Si substrates were bonded firmly together. Then, the porous Si substrate 41 was chemically etched away by use of hydrofluoric acid-nitric acid-acetic acid solution (1:3:8). As the result, the Si substrate 41 made porous having a thickness of 600 microns was removed in 7 minutes. Thus, a substrate having a monocrystalline Si layer 42 with good crystallinity could be formed on the SiO 2 . Also the thickness of the monocrystalline Si layer obtained was examined by use of Scanning Ellipsometer. As the result, within the plane of the 5 inches water, the difference between the maximum value and the minimum value of the thickness of the monocrystalline Si layer was found to be suppressed 8% or less based on the maximum value of the thickness. As the result of plan view observation of the monocrystalline Si layer by transmission electron microscopy, the dislocation defect density was found to be suppressed 1 x 10 3 /cm 2 or less, whereby it could be confirmed that no new crystal defect had been introduced in the monocrystalline Si layer formation step to maintain good crystallinity. When the life time of minority carriers was measured for the monocrystalline Si layer by use of the MOS C-t method, a high value of 2.1 x 10 -2 sec. was exhibited. Example 28 A P-type (100) monocrystalline Si substrate having a diameter of 3 inches and a thickness of 200 microns was anodized in a 50% HF solution. The current density at this time was 100 mA/cm 2 . The porous structure formation rate at this time was 8.4 µm/min. and the P-type (100) Si substrate having a thickness of 200 microns as a whole was made porous in 24 minutes. On the P-type (100) porous Si substrate 41 was grown an Si epitaxial layer 42 to a thickness of 5 microns by the liquid phase growth method. The growth conditions are as follows: Solvent: Sn Growth temperature: 900 °C Growth atmosphere: H 2 Growth time: 10 minutes. Next, on the surface of the epitaxial layer 42 was formed an oxidized layer 45 with a thickness of 1000 angstroms. Then, another Si substrate 43 having an oxidized layer 44 of 5000 angstroms formed on the surface was closely contacted, and by heating in a nitrogen atmosphere at 700 °C for 0.5 hour, the two Si substrates were bonded firmly together. Then, the porous Si substrate 41 was chemically etched away by use of hydrofluoric acid-nitric acid-acetic acid solution (1:3:8). As the result, the Si substrate 41 made porous having a thickness of 200 microns was removed in 2 minutes. Thus, a substrate having a monocrystalline Si layer could be formed on the SiO 2 . Example 29 A P-type (100) monocrystalline Si substrate having a diameter of 3 inches and a thickness of 200 microns was anodized in a 50% HF solution. The current density at this time was 100 mA/cm 2 . The porous structure formation rate at this time was 8.4 µm/min. and the P-type (100) Si substrate having a thickness of 200 microns as a whole was made porous in 24 minutes. On the P-type (100) porous Si substrate 41 was grown an Si epiraxial layer 42 to a thickness of 1.0 microns by to the low pressure CVD method. The deposition conditions are as follows: Source gas: SiH 4 Carrier gas: H 2 Temperature: 850°C Pressure: 1 x 10 -2 Torr Growth rate: 3.3 nm/sec. Next, on the surface of the epitaxial layer 42 was formed an oxidized layer 45 with a thickness of 1000 angstroms. Then, another Si substrate 43 having an oxidized layer 44 of 5000 angstroms on the surface was closely contacted, and by heating in a nitrogen atmosphere at 700 °C for 0.5 hour, the two Si substrates were bonded firmly together. Subsequently, the porous Si substrate 41 was chemically etched away by use of hydrofluoric acid-nitric acid-acetic acid solution (1:3:8). As the result, the Si substrate 41 made porous having a thickness of 200 microns was removed in 2 minutes. Thus, a substrate having a monocrystalline Si layer 42 on SiO 2 could be formed. When SiH 2 Cl 2 was employed, although it was necessary to elevate the growth temperature by some ten degrees, the accelerated chemical etching characteristics inherent in the porous substrate were maintained. Example 30 On a P-type (100) Si substrate 141 having a diameter of 3 inches and a thickness of 200 microns was grown an Si epitaxial layer 142 with a thickness of 1 micron by the CVD method. The deposition conditions are as follows: Reactive gas flow rate: SiH 2 Cl 2 1000 SCCM H 2 230 l/min. Temperature: 1080 °C Pressure: 80 Torr Time: 2 min. The substrate was anodized in a 50% HF solution. The current density at this time was 100 mA/cm 2 . The porous structure formation rate at this time was 8.4 µm/min, and the P-type (100) Si substrate having a thickness of 200 microns as a whole was made porous. Only the P-type (100) Si substrate 141 was made porous in this anodization, and there was no change in the Si epitaxial layer 142. Next, on the surface of the epitaxial layer 142 was superposed another Si substrate 144 having an oxidized layer 145 of 5000 angstroms formed on the surface, and the two Si substrates were bonded together firmly by heating at 800 °C for 0.5 hour in a nitrogen atmosphere. Subsequently, the porous Si substrate was chemically etched away by use of hydrofluoric acid-nitric acid-acetic acid solution (1:3:8). As the result, the Si substrate made porous having a thickness of 200 microns was removed in 2 minutes. Thus, a substrate having a monocrystalline layer 142 with a thickness of 1 µm on SiO 2 could be formed. As the result of cross-sectional observation by a transmission electron microscopy, it was confirmed that no new crystal defect had been introduced in the Si layer to maintain good crystallinity. Example 31 On a P-type (100) si substrate 141 having a diameter of 3 inches and a thickness of 200 microns was grown an Si epitaxial layer 142 with a thickness of 0.5 micron by the CVD method. The deposition conditions are as follows: Reactive gas flow rate: SiH 2 Cl 2 1000 SCCM H 2 230 l/min. Temperature: 1080°C Pressure: 80 Torr Time: 1 min. The substrate was anodized in a 50% HF solution. The current density at this time was 100 mA/cm 2 . The porous structure formation rate at this time was 8.4 µm/min. and the P-type (100) Si substrate 141 having a thickness of 200 microns as a whole was made porous in 24 minutes. Only the P-type (100) Si substrate 141 was made porous in this anodization, and there was no change in the Si epitaxial layer 142. Next, on the surface of the epitaxial layer 142 was formed an oxidized layer 146 with a thickness of 1000 angstroms. Then, another Si substrate having an oxidized layer 145 of 5000 angstroms formed on the surface was closely contacted, and the two Si substrates were bonded together firmly by heating at 700 °C for 0.5 hour. Then, the porous Si substrate was chemically etched away by use of hydrofluoric acid-nitric acid-acetic acid solution (1:3:8). As the result, the Si substrate made porous having a thickness of 200 microns was removed in 2 minutes. Thus, a substrate having a monocrystalline Si layer on SiO 2 could be formed. As the result of cross-sectional observation by transmission electron microscopy, it was confirmed that no new crystal defect had been introduced in the Si layer to maintain good crystallinity. Example 32 On the surface of a P-type (100) Si substrate 141 having a diameter of 3 inches and a thickness of 200 microns was formed an N-type Si layer 142 with a thickness of 1 micron by ion implantation of protons. The amount of H + implanted was 5 x 10 15 (ions/cm 2 ). The substrate was anodized in a 50% HF solution. The current density at this time was 100 mA/cm 2 . The porous structure formation rate at this tine was 8.4 µm/min. and the P-type (100) Si substrate having a thickness of 200 microns as a whole was made porous in 24 minutes. Only the P-type (100) Si substrate 141 was made porous in this anodization, and there was no change in the N-type Si layer 142. Next, on the surface of the epitaxial layer 142 was formed an oxidized layer 146 with a thickness of 1000 angstroms. Then, another Si substrate having an oxidized layer 145 of 5000 angstroms formed on the surface was closely contacted with the above oxidized layer 146, and the two Si substrates were bonded together firmly by heating at 700 °C for 0.5 hour. Subsequently, the porous Si substrate was chemically etched away by use of hydrofluoric acid-nitric acid-acetic acid solution (1:3:8). As the result, the Si substrate made porous having a thickness of 200 microns was removed in 2 minutes. A substrate having a monocrystalline Si layer on SiO 2 could be formed. Also the thickness of the monocrystalline Si layer obtained was examined by use of Scanning Ellipsometer. As the result, within the plane of the 3 inches wafer, the difference between the maximum value and the minimum value of the thickness of the monocrystalline Si layer was found to be suppressed 5% or less based on the maximum value of the thickness. As the result of plan-view observation of the monocrystalline Si layer by a transmission electron microscopy, the dislocation defect density was found to be suppressed 1 x 10 3 /cm 2 or less, whereby it could be confirmed that no new crystal defect had been introduced in the monocrystalline Si layer formation step to maintain good crystallinity. When the life time of minority carriers was measured for the monocrystalline layer by use of the MOS C-t method, a high value of 2.2 x 10 -3 sec. was exhibited. Example 33 A P-type (100) monocrystalline Si substrate having a diameter of 3 inches and a thickness of 200 microns was anodized in a 50% HF solution. The current density at this time was 100 mA/cm 2 . The porous structure formation rate at this time was 8.4 µm/min. and the P-type (100) Si substrate having a thickness of 200 microns as a whole was made porous in 24 minutes. On the P-type (100) porous Si substrate 51 was grown an Si epitaxial layer 52 to a thickness of 0.5 microns at a low temperature by the MBE method. The deposition conditions are as follows: Temperature: 700 °C Pressure: 1 x 10 -9 Torr Growth rate: 0.1 nm/sec. Next, on the surface of the epitaxial layer 52 was superposed a fused silica substrate applied with optical polishing, and by heating in a nitrogen atmosphere at 800 °C for 0.5 hour, the two substrates were bonded firmly together. By the low pressure CVD method, Si 3 N 4 was coated on the bonded substrate with a thickness of 0.1 µm. Subsequently, only the nitride film 54 on the porous substrate 51 was removed by reactive ion etching. Then, the porous Si substrate 51 was chemically etched away by use of hydrofluoric acid-nitric acid-acetic acid solution (1:3:8). As the result, the Si substrate 51 made porous having a thickness of 200 microns was removed in 2 minutes. After removal of the Si 3 N 4 layer 54, a substrate having the monocrystalline Si layer 52 with a thickness of 0.5 µm on the fused silica 53 could be obtained. As the result of cross-sectional observation by transmission electron microscopy, it was confirmed that no new crystal defect had been introduced in the Si layer to maintain good crystallinity. Example 34 A P-type (100) monocrystalline Si substrate having a diameter of 3 inches and a thickness of 200 microns was anodized in a 50% HF solution. The current density at this time was 100 mA/cm 2 . The porous structure formation rate at this time was 8.4 µm/min, and the P-type (100) Si substrate having a thickness of 200 microns as a whole was made porous in 24 minutes. On the P-type (100) porous Si substrate 51 was grown an Si epitaxial layer 52 to a thickness of 5 microns by the plasma CVD method. The deposition conditions are as follows: Gas: SiH 4 Radio frequency power: 100 W Temperature: 800°C Pressure: 1 x 10 -2 Torr Growth rate: 2.5 nm/sec. Next, on the surface of the epitaxial layer 52 was superposed a glass substrate having a softening point around 500 °C applied with optical polishing, and by heating in a nitrogen atmosphere at 450 °C for 0.5 hour, the two substrates were bonded firmly together. By the low pressure CVD method, Si 3 N 4 was coated on the bonded substrate with a thickness of 0.1 µm. Subsequently, only the nitride film 54 on the porous subatrate 51 was removed by reactive ion etching. Then, the porous Si substrate was chemically etched away by use of 6M KOH solution. As the result, the Si substrate made porous having a thickness of 200 microns was removed in 2 minutes. After removal of the Si 3 N 4 layer, the monocrystalline layer 52 with a thickness of 5 µm on the low softening glass substrate 53 could be obtained. Example 35 A P-type (100) monocrystalline Si substrate having a diameter of 3 inches and a thickness of 200 microns was anodized in a 50% HF solution. The current density at this time was 100 mA/cm 2 . The porous structure formation rate at this time was 8.4 µm/min, and the P-type (100) Si substrate having a thickness of 200 microns as a whole was made porous in 24 minutes. On the P-type (100) porous Si substrate 51 was grown an Si epitaxial layer to a thickness of 1.0 micron by the bias-sputtering method. The deposition conditions are as follows: RF frequency: 100 MHz Radio frequency power: 600 W Temperature: 300 °C Ar gas pressure: 8 x 10 -3 Torr Growth time: 120 minutes Target direct current bias: - 200 V Substrate direct current bias: + 5 V. Next, on the surface of the epitaxial layer 52 was superposed a glass substrate 53 having a softening point around 500 °C applied with optical polishing, and by heating in a nitrogen atmosphere at 450 °C for 0.5 hour, the two substrate were bonded firmly together. By the low pressure CVD method, Si 3 N 4 was coated on the bonded substrate with a thickness of 0.1 µm. Subsequently, only the nitride film on the porous substrate was removed by reactive ion etching. Then, the porous Si substrate was chemically etched away by use of hydrofluoric acid-nitric acid-acetic acid solution (1:3:8). As the result, the Si substrate 51 made porous having a thickness of 200 microns was removed in 2 minutes. After removal of the Si 3 N 4 layer 54, a substrate having the monocrystalline layer 52 with a thickness of 1.0 µm on the low melting point glass could be obtained. The same effect could be also obtained when Apiezon Wax was coated in place of the Si 3 N 4 layer, and only the Si substrate 51 made porous could be removed. Also the thickness of the monocrystalline Si layer obtained was examined by use of Scanning Ellipsometer. As the result, within the plane of the 3 inches wafer, the difference between the maximum value and the minimum value of the thickness of the monocrystalline Si layer was found to be suppressed 5% or less based on the maximum value of the thickness. As the result of plan-view observation of the monocrystalline Si layer by a transmission electron microscopy, the dislocation defect density was found to be suppressed 1 x 10 3 /cm 2 or less, whereby it could be confirmed that no new crystal defect had been introduced in the monocrystalline Si layer formation step to maintain good crystallinity. When the life time of minority carriers was measured for the monocrystalline Si layer by use of the MOS C-t method, a high value of 2.0 x 10 -3 sec. was exhibited. Example 36 A P-type (100) monocrystalline Si substrate having a diameter of 3 inches and a thickness of 200 microns was anodized in a 50% HF solution. The current density at this time was 100 mA/cm 2 . The porous structure formation rate at this time was 8.4 µm/min, and the P-type (100) Si substrate having a thickness of 200 microns as a whole was made porous in 24 minutes. On the P-type (100) porous Si substrate 51 was grown an Si epitaxial layer to a thickness of 10 microns by the liquid phase growth method. The growth conditions are as follows: Solvent: Sn Growth temperature: 900 °C Growth atmosphere: H 2 Growth time: 20 minutes. Next, on the surface of the epitaxial layer 52 was superposed a glass substrate 53 having a softening point around 800 °C applied with optical polishing, and by heating in a nitrogen atmosphere at 750 °C for 0.5 hour, the two substrates were bonded firmly together. By the low pressure CVD method, Si 3 N 4 was coated on the bonded substrate with a thickness of 0.1 µm. Subsequently, only the nitride film on the porous substrate was removed by reactive ion etching. Then, the porous Si substrate was chemically etched away by use of hydrofluoric acid-nitric acid-acetic acid solution (1:3:8). As the result, the Si substrate 51 made porous having a thickness of 200 microns was removed in 2 minutes. After removal of the Si 3 N 4 layer 54, a substrate having the monocrystalline Si layer 52 with a thickness of 10 µm on the glass 53 could be obtained. The same effect could be also obtained when Apiezon Wax was coated in place of the Si 3 N 4 layer, and only the Si substrate made porous could be completely removed. Example 37 A P-type (100) monocrystalline Si substrate having a diameter of 3 inches and a thickness of 200 microns was anodized in a 50% HF solution. The current density at this time was 100 mA/cm 2 . The porous structure formation rate at this time was 8.4 µm/min. and the P-type (100) Si substrate having a thickness of 200 microns as a whole was made porous in 24 minutes. On the P-type (100) porous Si substrate 51 was grown an Si epitaxial layer 52 to a thickness of 1.0 micron by the low pressure CVD method. The deposition conditions are as follows: Source gas: SiH 4 800 SCCM Carrier gas: H 2 150 l/min. Temperature: 850 °C Pressure: 1 x 10 -2 Torr Growth rate: 3.3 nm/sec. Next, on the surface of the epitaxial layer 52 was superposed a fused silica substrate 53 applied with optical polishing, and by heating in a nitrogen atmosphere at 800 °C for 0.5 hour, the two 51 substrates were bonded firmly together. By the low pressure CVD method, Si 3 N 4 was coated on the bonded substrate with a thickness of 0.1 µm. Subsequently, only the nitride film on the porous substrate was removed by reactive ion etching. Then, the porous Si substrate 51 was chemically etched away by use of hydrofluoric acid-nitric acid-acetic acid solution. As the result, the Si substrate 51 made porous having a thickness of 200 microns was removed in 2 minutes. After removal of the Si 3 N 4 layer, a substrate having the monocrystalline Si layer 52 with a thickness of 1.0 µm on the fused silica substrate 53 could be formed. When SiH 2 Cl 2 as the source gas was employed, although it was necessary to elevate the growth temperature by some ten degrees, the accelerated chemical etching characteristics inherent in the porous substrate were maintained. Example 38 On a P-type (100) Si substrate 151 having a diameter of 4 inches and a thickness of 300 microns was grown an Si epitaxial layer 152 with a thickness of 1 micron by the CVD method. The deposition conditions are as follows: Reactive gas flow rate: SiH 2 Cl 2 1000 SCCM H 2 230 l/min. Temperature: 1080 °C Pressure: 80 Torr Time: 2 min. The substrate was anodized in a 50% HF solution. The current density at this time was 100 mA/cm 2 . The porous structure formation rate at this time was 8.4 µm/min, and the P-type (100) Si substrate 151 having a thickness of 300 microns as a whole was made porous in 36 minutes. As described above, only the P-type (100) Si substrate 151 was made porous in this anodization, and there was no change in the Si epitaxial layer 152. Next, on the surface of the epitaxial layer 152 was superposed a fused silica substrate 154 applied with optical polishing, and the two substrate were bonded together firmly by heating at 800 °C for 0.5 hour in a nitrogen atmosphere. By the low pressure CVD method, Si 3 N 4 was deposited with a thickness of 0.1 µm to be coated on the bonded substrate. Subsequently the nitride film 155 on the porous substrate 153 was removed by reactive ion etching. Then, the porous Si substrate was chemically etched away by use of hydrofluoric acid-nitric acid-acetic acid solution (1:3:8). As the result, the Si substrate 153 made porous having a thickness of 300 microns was removed in 4 minutes. After removal of the Si 3 N 4 layer 155, a substrate having the monocrystalline Si layer 152 having a thickness of 1 µm on the fused silica substrate 154 could be formed. As the result of cross-sectional observation by transmission electron microscopy, it was confirmed that no new crystal defect had been introduced in the Si layer to maintain good crystallinity. Example 39 On a P-type (100) Si substrate 151 having a diameter of 3 inches and a thickness of 200 microns was grown an Si epitaxial layer 152 with a thickness of 0.5 micron by the CVD method. The deposition conditions are as follows: Reactive gas flow rate: SiH 2 Cl 2 1000 SCCM H 2 230 l/min. Temperature: 1080 °C Pressure: 80 Torr Time: 1 min. The substrate was anodized in a 50% HF solution. The current density at this time was 100 mA/cm 2 . The porous structure formation rate at this time was 8.4 µm/min. and the P-type (100) Si substrate 151 having a thickness of 200 microns as a whole was made porous in 24 minutes. Only the P-type (100) Si substrate was made porous in this anodization, and there was no change in the Si epitaxial layer 152. Next, on the surface of the epitaxial layer 152 was superposed a fused silica substrate 154 applied with optical polishing, and the two substrates were bonded together firmly by heating at 800 °C for 0.5 hour. By the low pressure CVD method, Si 3 N 4 was deposited with a thickness of 0.1 µm to be coated on the bonded substrate. Subsequently the nitride film 155 on the porous substrate 153 was removed by reactive ion etching. Then, the porous Si substrate was chemically etched away by use of hydrofluoric acid-nitric acid-acetic acid solution (1:3:8). As the result, the Si substrate 153 made porous having a thickness of 200 microns was removed in 2 minutes. After removal of the Si 3 N 4 layer 155, a substrate having the monocrystalline Si layer 152 having a thickness of 0.5 µm on the fused silica 154 could be formed. As the result of cross-sectional observation by transmission electron microscopy, it was confirmed that no new crystal defect had been introduced in the Si layer to maintain good crystallinity. Example 40 On the surface of a P-type (100) Si substrate 151 having a diameter of 4 inches and a thickness of 300 microns was formed an N-type Si layer 152 with a thickness of 1 micron by ion implantation of protons. The amount of H+ implanted was 5 x 1015 (ions/cm 2 ). The substrate was anodized in a 50% HF solution. The current density at this time was 100 mA/cm 2 . The porous structure formation rate at this time was 8.4 µm/min. and the P-type (100) Si substrate 151 having a thickness of 300 microns as a whole was made porous in 37 minutes. Only the P-type (100) Si substrate was made porous in this anodization, and there was no change in the N-type Si layer 152. Next, on the surface of the N-type Si layer 152 was superposed a fused silica substrate 154 with optical polishing, and the two substrates were bonded together firmly by heating at 800 °C for 0.5 hour in a nitrogen atmosphere. By the low pressure CVD method, Si 3 N 4 was deposited to a thickness of 0.1 µm to be coated on the bonded substrate. Subsequently, only the nitride film 155 on the porous substate 153 was removed by reactive ion etching. Then, the porous Si substrate was chemically etched away by use of hydrofluoric acid-nitric acid-acetic acid solution. As the result, the Si substrate 151 made porous having a thickness of 300 microns was removed in 4 minutes. After removal of the Si 3 N 4 layer 155, a substrate having the monocrystalline Si layer 152 with a thickness of 1.0 µm on the fused silica 154 could be formed. Also the thicknees of the monocrystalline Si layer obtained was examined by use of Scanning Ellipsometer. As the result, within the plane of the 4 inches wafer, the difference between the maximum value and the minimum value of the thickness of the monocrystalline Si layer was found to be suppressed 6% or less based on the maximum value of the thickness. As the result of plan-view observation of the monocrystalline Si layer by a transmission electron microscopy, the dislocation defect density was found to be suppressed 1 x 10 3 /cm 2 or less, whereby it could be confirmed that no new crystal defect had been introduced in the monocrystalline Si layer formation step to maintain good crystallinity. When the life time of minority carriers was measured for the monocrystalline Si layer by use of the MOS C-t method, a high value of 2.2 x 10 -3 sec. was exhibited. Shortly speaking, according to the present invention, in obtaining an Si crystalline layer excellent in crystallinity equal to monocrystalline Si wafer also on a light-transparent insulating material substrate represented by glass, a process for forming a semiconductor substrate excellent in aspects of productivity, uniformity, controllability, economy can be provided. Further, according to the present invention, there can be provided a process for forming a semiconductor substrate broad in scope of application which can make avail of the advantages of the SOI device of the prior. Example 41 A P-type (100) monocrystalline Si substrate having a diameter of 3 inches and a thickness of 200 microns was anodized in a 50% HF solution. The current density at this time was 100 mA/cm 2 . The porous structure formation rate at this time was 8.4 µm/min. and the P-type (100) Si substrate having a thickness of 200 microns as a whole was made porous in 24 minutes. On the P-type (100) porous Si substrate 61 was grown an Si epitaxial layer 62 to a thickness of 0.5 microns by the MBE method. The deposition conditions are as follows: Temperature: 700 °C Pressure: 1 x 10 -9 Torr Growth rate: 0.1 nm/sec. Next, on the surface of the epitaxial layer 62 was superposed a fused silica substrate 63 applied with optical polishing, and by heating in a nitrogen atmosphere at 800 °C for 0.5 hour, the two substrates were bonded firmly together. Then, the porous Si substrate 61 was chemically etched away by use of hydrofluoric acid-nitric acid-acetic acid solution. As the result, the Si substrate 61 made porous having a thickness of 200 microns was removed in 2 minutes. A substrate having the monocrystalline Si layer 62 with a thickness of 0.5 µm on the fused silica 63 could be obtained. As the result of cross-sectional observation by transmission electron microscopy, it was confirmed that no new crystal defect had been introduced in the Si layer to maintain good crystallinity. Example 42 A P-type (100) monocrystalline Si substrate having a diameter of 3 inches and a thickness of 200 microns was anodized in a 50% HF solution. The current density at this time was 100 mA/cm 2 . The porous structure formation rate at this time was 8.4 µm/min. and the P-type (100) Si substrate having a thickness of 200 microns as a whole was made porous in 24 minutes. On the P-type (100) porous Si substrate 61 was grown an Si epitaxial layer 62 to a thickness of 5 microns by the plasma CVD method. The deposition conditions are as follows: Gas: SiH 4 Radio frequency power: 100 W Temperature: 800 °C Pressure: 1 x 10 -2 Torr Growth rate: 2.5 nm/sec. Next, on the surface of the epitaxial layer 62 was superposed a glass substrate having a softening point around 500 °C applied with optical polishing, and by heating in a nitrogen atmosphere at 450 °C for 0.5 hour, the two substrates were bonded firmly together. Then, the porous Si substrate 61 was chemically etched away by use of 6M KOH solution. As the result, the Si substrate 61 made porous having a thickness of 200 microns was removed in 2 minutes. A substrate having the monocrystalline Si layer 62 with a thickness of 5 µm on the low softening point glass 63 could be obtained. Example 43 A P-type (100) monocrystalline Si substrate having a diameter of 3 inches and a thickness of 200 microns was anodized in a 50% HF solution. The current density at this time was 100 mA/cm 2 . The porous structure formation rate at this time was 8.4 µm/min. and the P-type (100) Si substrate having a thickness of 200 microns as a whole was made porous in 24 minutes. On the P-type (100) porous Si substrate 61 was grown an Si epitaxial layer 62 to a thickness of 1.0 micron by the bias-sputtering method. The deposition conditions are as follows: RF frequency: 100 MHz Radio frequency power: 600 W Temperature: 300 °C Ar gas pressure: 8 x 10 -3 Torr Growth time: 120 minutes Target direct current bias: - 200 V Substrate direct current bias: + 5 V. Next, on the surface of the epitaxial layer 62 was superposed a glass substrate 63 having a softening point around 500 °C applied with optical polishing, and by heating in a nitrogen atmosphere at 450 °C for 0.5 hour, the two substrates were bonded firmly together. Then, the porous Si substrate 61 was chemically etched away by use of 7 M NaOH solution. As described above, the chemical etching rate of ordinary Si monocrystal relative to 7 M NaOH solution is about a little less than 1 micron per minute, but the chemical etching rate of the porous layer is accelerated by about 100-fold thereof. That is, the Si substrate 61 made porous having a thickness of 200 microns was removed in 2 minutes. A substrate having tho monocrystalline Si layer 62 with a thickness of 1.0 µm on the low melting point glass 63 could be formed. Also the thickness of the monocrystalline Si layer obtained was examined by use of Scanning Ellipsometer. As the result, within the plane of the 3 inches wafer, the difference between the maximum value and the minimum value of the thickness of the monocrystalline Si layer was found to be suppressed 5% or less based on the maximum value of the thickness. As the result of plan-view observation of the monocrystalline Si layer by a transmission electron microscopy, the dislocation defect density was found to be suppressed 1 x 10 3 /cm 2 or less, whereby it could be confirmed that no new crystal defect had been introduced in the monocrystalline Si layer formation step to maintain good crystallinity. When the life time of minority carriers was measured for the monocrystalline layer by use of the MOS C-t method, a high value of 2.1 x 10 -3 sec. was exhibited. Example 44 A P-type (100) monocrystalline Si substrate having a diameter of 3 inches and a thickness of 200 microns was anodized in a 50% HF solution. The current density at this time was 100 mA/cm 2 . The porous structure formation rate at this time was 8.4 µm/min. and the P-type (100) Si substrate having a thickness of 200 microns as a whole was made porous in 24 minutes. On the P-type (100) porous Si substrate 61 was grown an Si epitaxial layer 62 to a thickness of 10 microns by the liquid phase growth method. The growth conditions are as follows: Solvent: Sn Growth temperature: 900 °C Growth atmosphere: H 2 Growth time: 20 minutes. Next, on the surface of the epitaxial layer 62 was superposed a glass substrate 63 having a softening point around 800 °C applied with optical polishing, and by heating in a nitrogen atmosphere at 750 °C for 0.5 hour, the two substrates were bonded firmly together. Then, the porous Si substrate 61 was chemically etched away by use of hydrofluoric acid-nitric acid-acetic acid solution. As the result, the Si substrate 61 made porous having a thickness of 200 microns was removed in 2 minutes, A substrate having the monocrystalline Si layer 62 with a thickness of 10 µm on the glass 63 could be obtained. Example 45 A P-type (100) monocrystalline Si substrate having a diameter of 3 inches and a thickness of 200 microns was anodized in a 50% HF solution. The current density at this time was 100 mA/cm 2 . The porous structure formation rate at this time was 8.4 µm/min. and the P-type (100) Si substrate having a thickness of 200 microns as a whole was made porous in 24 minutes. On the P-type (100) porous Si substrate 61 was grown an Si epitaxial layer to a thickness of 1.0 micron by the low pressure CVD method. The deposition conditions are as follows: Source gas: SiH 4 800 SCCM Carrier gas: H 2 150 l/min. Temperature: 850 °C Pressure: 1 x 10 -2 Torr Growth rate: 3.3 nm/sec. Next, on the surface of the epitaxial layer 62 was superposed a fused silica substrate 63 applied with optical polishing, and by heating in a nitrogen atmosphere at 800 °C for 0.5 hour, the two substrates were bonded firmly together. Then, the porous Si substrate 61 was chemically etched away by use of hydrofluoric acid-nitric acid-acetic acid solution. As the result, the Si substrate 61 made porous having a thickness of 200 microns was removed in 2 minutes. A substrate having the monocrystalline Si layer 62 with a thickness of 1.0 µm on the fused silica 63 could be formed. When SiH 2 Cl 2 as the source gas was employed, although it was necessary to elevate the growth temperature by some ten degrees, the accelerated chemical etching characteristics inherent in the porous substrate were maintained. Example 46 On a P-type (100) Si substrate 161 having a diameter of 4 inches and a thickness of 300 microns was grown an Si epitaxial layer 162 with a thickness of 1 micron by the CVD method. The deposition conditions are as follows: Reactive gas flow rate: SiH 2 Cl 2 1000 SCCM H 2 230 l/min. Temperature: 1080 °C Pressure: 80 Torr Time: 2 min. The substrate was anodized in a 50% HF solution. The current density at this time was 100 mA/cm 2 . The porous structure formation rate at this time was 8.4 µm/min, and the P-type (100) Si substrate 161 having a thickness of 300 microns as a whole was made porous in 37 minutes. Only the P-type (100) Si substrate 161 was made porous in this anodization, and there was no change in the Si epitaxial layer 162. Next, on the surface of the epitaxial layer was superposed an optical polished fused silica glass substrate 164 and the two substrates were bonded together firmly by heating at 800 °C for 0.5 hour in a nitrogen atmosphere. Then, the porous Si substrate 163 was chemically etched away by use of hydrofluoric acid-nitric acid-acetic acid solution (1:3:8). As the result, the Si substrate 163 made porous having a thickness of 300 microns was removed in 4 minutes. A substrate having the monocrystalline Si layer 162 having a thickness of 1 µm on the fused silica glass substrate 164 could be formed. Also the thickness of the monocrystalline Si layer obtained was examined by use of Scanning Ellipsometer. As the result, within the plane of the 4 inches wafer, the difference between the maximum value and the minimum value of the thickness of the monocrystalline Si layer was found to be suppressed 7% or less based on the maximum value of the thickness. As the result of plan-view observation of the monocrystalline Si layer by a transmission electron microscopy, the dislocation defect density was found to be suppressed 1 x 10 3 /cm 2 or less, whereby it could be confirmed that no new crystal defect had been introduced in the monocrystalline Si layer formation process to maintain good crystallinity. When the life time of minority carriers was measured for the monocrystalline Si layer by use of the MOS C-t method, a high value of 2.0 x 10 -3 sec. was exhibited. Example 47 On a P-type (100) Si substrate 161 having a diameter of 3 inches and a thickness of 200 microns was grown an Si epitaxial layer 162 with a thickness of 0.5 micron by the CVD method. The deposition conditions are as follows: Reactive gas flow rate: SiH 2 Cl 2 1000 SCCM H 2 230 l/min. Temperature: 1080 °C Pressure: 80 Torr Time: 1 min. The substrate was anodized in a 50% HF solution. The current density at this time was 100 mA/cm 2 . The porous structure formation rate at this time was 8.4 µm/min. and the P-type (100) Si substrate 161 having a thickness of 200 microns as a whole was made porous in 24 minutes. Only the P-type (100) Si substrate 161 was made porous in this anodization, and there was no change in the Si epitaxial layer 162. Next, on the surface of the epitaxial layer 162 was superposed an optical polished fused silica glass substrate 164 and the two substrates were bonded together firmly by heating at 800 °C for 0.5 hour. Then, the porous Si substrate 163 was chemically etched away by use of hydrofluoric acid-nitric acid-acetic acid solution. As the result, the Si substrate made porous having a thickness of 200 microns was removed in 2 minutes. A monocrystalline Si layer having a thickness of 0.5 µm on the glass substrate could be formed. As the result of cross-sectional observation by transmission electron microscopy, it was confirmed that no new crystalline defect had been introduced in the Si layer to maintain good crystallinity. Example 48 On the surface of a P-type (100) Si substrate 161 having a diameter of 3 inches and a thickness of 200 microns was formed an N-type Si layer 162 with a thickness of 1 micron by ion implantation of protons. The amount or H + implanted was 5 x 10 15 (ions/cm 2 ). The substrate was anodized in a 50% HF solution. The current density at this time was 100 mA/cm 2 . The porous structure formation rate at this time was 8.4 µm/min. and the P-type (100) Si substrate 161 having a thickness of 200 microns as a whole was made porous in 24 minutes. Only the P-type (100) Si substrate 161 was made porous in this anodization, and there was no change in the N-type Si layer 162. Next, on the surface of the opitaxial layer 162 was superposed an optical polished fused silica glass substrate 164 and the two substrates were bonded together firmly by heating at 800 °C for 0.5 hour. Then, the porous Si substrate 163 was chemically etched away by use of hydrofluoric acid-nitric acid-acetic acid solution (1:3:8). As the result, the Si substrate 163 made porous having a thickness of 200 microns was removed in 4 minutes. A monocrystalline Si layer 162 with a thickness of 1.0 µm on the glass substrate 164 could be formed. As the result of cross-sectional observation by transmission electron microscopy, it was confirmed that no new crystal defect had been introduced in the Si layer to maintain good crystallinity. Example 49 A P-type (100) monocrystalline Si substrate having a diameter of 6 inches and a thickness of 600 microns was anodized in a 50 % HF solution. The current density at this time was 10 mA/cm 2 . In 10 minutes, a porous layer having a thickness of 20 microns was formed on the surface. On the P-type (100) porous Si substrate was grown an Si epitaxial layer to a thickness of 0.5 micron by the low pressure CVD method. The deposition conditions are as follows: Gas: SiH 2 Cl 2 (0.6 l/mm), H 2 (100 l/min.) Temperature: 850 °C Pressure: 50 Torr Growth rate: 0.1 µm/min. Next, the surface of the epitaxial layer was thermally oxidized to 50 nm. On the thus obtained thermally oxidized film was superposed another silicon substrate having an oxidized layer of 0.8 micron on the surface, and by heating in a nitrogen atmosphere at 900 °C for 1.5 hours, the two substrates were firmly bonded together. Thereafter, grinding of 585 microns was applied from the back side of the silicon substrate to have the porous layer exposed. By the plasma CVD method, Si 3 N 4 was deposited to a thickness of 0.1 µm to be coated on the bonded substrate. Subsequently, only the silicon nitride film on the porous substrate was removed by reactive ion etching. Then, the bonded substrate was subjected to chemically selective etching by use of a hydrofluoric acid-nitric acid-acetic acid solution. After 15 minutes, the porous Si layer was chemically selectively etched to be completely removed, and only the monocrystalline Si layer as the etch-stop material remained without being chemically etched. The chemical etching rate of non-porous Si monocrystalline relative to the etchant is very low as about 40 angstroms even after 15 minutes, whereby the selective ratio of the etching rate between the non-porous layer and the porous layer is very large and the etched amount in the non-porous Si layer was practically negligible. After removal of the Si 3 N 4 layer, a monocrystalline Si layer having a thickness of 0.5 µm on the silicon substrate having an insulating layer on the surface could be formed. The same effect was also obtained, when Apiezon Wax or Electron Wax was coated in place of the Si 3 N 4 , and only the Si layer made porous could be completely removed. Also the thickness of the monocrystalline Si layer obtained was examined by use of Scanning Ellipsometer. As the result, within the plane of the 6 inches wafer, the difference between the maximum value and the minimum value of the thickness of the monocrystalline Si layer was found to be suppressed 10% or less based on the maximum value of the thickness. As the result of plan view observation of the monocrystalline layer by a transmission electron microscopy, the dislocation defect density was found to be suppressed 1 x 10 3 /cm 2 or less, whereby it could be confirmed that no new crystal defect had been introduced in the monocrystalline Si layer formation process to maintain good crystallinity. When the life time of minority carriers was measured for the monocrystalline Si layer by use of the MOS C-t method, a high value of 2.0 x 10 -3 sec. was exhibited.

Claims: A process for preparing a semiconductor member comprising the steps of: preparing a first member having a non-porous monocrystalline semiconductor layer (22, 122, 12, 112, 32, 132, 42, 142, 1302) on a porous monocrystalline semiconductor layer (21, 123, 11, 113, 31, 133, 41, 143, 1301), bonding said first member to the insulating layer (24, 125, 14, 115, 34, 135, 44, 145, 53, 154, 63, 164, 1305) of a second member (23, 124, 13, 114, 33, 134, 43, 144, 1304) in a manner so as to place said non-porous monocrystalline semiconductor layer (22, 122, 12, 112, 32, 132, 42, 142, 1302) inside the multilayer structure to be obtained, and then selectively etching said porous monocrystalline semiconductor layer (21, 123, 11, 113, 31, 133, 41, 143, 1301). A process for preparing a semiconductor member according to claim 1, wherein said monocrystalline semiconductor comprises silicon. A process for preparing a semiconductor member according to claim 1, wherein said porous monocrystalline semiconductor layer (21, 123, 11, 113, 31, 133, 41, 143, 1301) is P-type. A process for preparing a semiconductor member according to claim 1, wherein said non-porous monocrystalline semiconductor layer (22, 122, 12, 112, 32, 132, 42, 142, 1302) has a thickness of 50 microns or less. A process for preparing a semiconductor member according to claim 1, wherein said bonding step is carried out in an atmosphere containing nitrogen. A process for preparing a semiconductor member according to claim 1, wherein said bonding step includes a heat treatment. A process for preparing a semiconductor member according to claim 1, wherein said non-porous monocrystalline semiconductor layer (22, 122, 12, 112, 32, 132, 42, 142, 1302) of said first member is formed by epitaxial growth. A process for preparing a semiconductor member according to claim 1, wherein said non-porous monocrystalline semiconductor layer (22, 122, 12, 112, 32, 132, 42, 142, 1302) of said first member is formed according to the method selected from the molecular beam epitaxial method, the plasma CVD method, the low pressure CVD method, the photo CVD method, the liquid phase growth method or the bias sputtering method. A process for preparing a semiconductor member according to claim 1, wherein said porous monocrystalline semiconductor layer (21, 123, 11, 113, 31, 133, 41, 143, 1301) of said first member is obtained by making a non-porous semiconductor monocrystalline layer porous by anodization. A process for preparing a semiconductor member according to claim 9, wherein said anodization is carried out in an HF solution. A process for preparing a semiconductor member according to claim 2, wherein said non-porous monocrystalline semiconductor layer (22, 122, 12, 112, 32, 132, 42, 142, 1302) of said first member is intrinsic or N-type. A process for preparing a semiconductor member according to claim 11, wherein said N-type silicon is formed by proton implantation or epitaxial growth. A process for preparing a semiconductor member according to claim 1, wherein said second member (53, 154, 63, 164, 1304) comprises a light-transmissive material. A process for preparing a semiconductor member according to claim 13, wherein said second member comprises a glass. A process for preparing a semiconductor member according to claim 14, wherein said glass is a quartz glass. A process for preparing a semiconductor member according to claim 1, wherein said second member (23, 124, 13, 114, 33, 134, 43, 144, 1304) is a silicon substrate. A process for preparing a semiconductor member according to claim 16, wherein a surface of said silicon substrate is oxidized for forming said insulating layer (24, 125, 14, 115, 34, 135, 44, 145, 1305). A process for preparing a semiconductor member according to claim 1, wherein selectively etching of said porous monocrystalline semiconductor layer (21, 123, 11, 113, 31, 133, 41, 143) of said first member is carried out under the state where said first and second members bonded to each other are covered with an anti-etching material (15, 116, 35, 136). A process for preparing a semiconductor member according to claim 18, wherein said anti-etching material (15, 116, 35, 136) is silicon nitride. A process for preparing a semiconductor member according to claim 18, wherein said anti-etching material (15, 116, 35, 136) is Apiezon Wax. A process for preparing a semiconductor member according to any of the claims 1 to 20 further comprising the step of forming a further insulating layer (36, 137, 45, 146, 1303) on the non-porous monocrystalline semiconductor layer (32, 132, 42, 142, 1302) of said first member before bonding said first member to the insulating layer (34, 135, 44, 145, 1305) of said second member (33, 134, 43, 144, 1304). A process for preparing a semiconductor member according to claim 21, wherein said further insulating layer (36, 137, 45, 146, 1303) is formed by thermal oxidation of the non-porous monocrystalline semiconductor layer (32, 132, 42, 142, 1302). A process for preparing a semiconductor member according to any of the claims 1 to 22, wherein said first member having a porous monocrystalline semiconductor layer (21, 11, 31, 41) and a non-porous monocrystalline semiconductor layer (22, 12, 32, 42) is formed by making a non-porous monocrystalline semiconductor layer porous to form the porous monocrystalline semiconductor layer (21, 11, 31, 41) and forming said non-porous monocrystalline semiconductor layer (22, 12, 32, 42) on said porous monocrystalline semiconductor layer (21, 11, 31, 41). A process for preparing a semiconductor member according to any of the claims 1 to 22, wherein said first member having a porous monocrystalline semiconductor layer (123, 113, 133, 143) and a non-porous monocrystalline semiconductor layer (122, 112, 132, 142) is formed by the steps of forming a second monocrystalline semiconductor layer (122, 112, 132, 142) of a second electroconduction type on a first monocrystalline semiconductor layer (121, 111, 131, 141) of a first electroconduction type, and making said first monocrystalline semiconductor layer (121, 111, 131, 141) porous to form said porous monocrystalline semiconductor layer (123, 113, 133, 143). A process for preparing a semiconductor member according to any one of claims 1 to 22, wherein said first member is prepared by making a non-porous monocrystalline semiconductor substrate partially porous to form a porous monocrystalline semiconductor layer (1301) on a first non-porous monocrystalline semiconductor layer (1300), and by forming a second non-porous monocrystalline semiconductor layer (1302) on said porous monocrystalline semiconductor layer (1301), the bonding step is carried out by bonding the surface of said second non-porous monocrystalline semiconductor layer (1302) onto an insulating layer (1305) of the second member (1304), and before said porous monocrystalline semiconductor layer (1301) is removed by etching, said first non-porous monocrystalline semiconductor layer (1300) is removed.

Inventors: YONEHARA TAKAO, YONEHARA, TAKAO, YONEHARA, TAKAO, C/O CANON KABUSHIKI KAISHA
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1991
Main classification: H01L21
IPC codes: H01L21