Process for preparing an SOI-member

Abstract: A process for preparing a semiconductor member comprising the steps of: preparing a first member having a porous monocrystalline semiconductor layer and a non-porous monocrystalline semiconductor layer, bonding said first member to a light-transmissive second member in a manner so as to place said non-porous monocrystalline semiconductor layer inside the multi-layer structure to be obtained, and then selectively etching said porous monocrystalline semeconductor layer.

Description: BACKGROUND OF THE INVENTION Field of the Invention This invention relates to a process for preparing a semiconductor member, more particularly for preparing 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. 55-16464 has the steps of forming an N-type monocrystalline Si 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. SUMMARY OF THE INVENTION An object of the present invention is to provide a process for preparing 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 process for preparing 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 process for preparing 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. BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1 through 6 are schematic illustrations of the examples of the steps of the process for preparing the semiconductor member of the present invention or the reference embodiments. 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 light-transmissive second member being 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 light-transmissive second member being an insulating substance and bonded onto the surface of the layer constituted of the insulating substance. In the semiconductor member obtained 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. Also the difference between the maximum value and the minimum value of the thickness of the silicon monocrystalline semiconductor region 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 comprising the steps of: preparing a first member having a porous monocrystalline semiconductor layer and a non-porous monocrystalline semiconductor layer, bonding said first member to a light-transmissive second member being an insulating material in a manner so as to place said non-porous monocrystalline semiconductor layer inside the multi-layer structure to be obtained, and then selectively etching said porous monocrystalline semiconductor layer. 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 devices 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 (10Å=1nm) 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-sputtering 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. Reference 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. 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 Reference 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 Reference 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 (2 inches wafer to 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. Other reference embodiments and reference examples are illustrated by the description of embodiments 2 to 8 and examples 1 to 32 and 48 in connection with Figures 1 to 8 of the published application, EP-A-747 935. Embodiment 1 As shown in Fig. 2A, 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. 2B, 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. 2B, 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. 2C, 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. 2C 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 Reference Embodiment 1. Embodiment 2 Referring now to Figs. 3A to 3D, the Embodiment 2 of the present invention is described below in detail. First, as shown in Fig. 3A, 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. 3B, 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 153 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. 3C, 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. 3C, 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. 3D, 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. 3D 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 Reference Embodiment 1. Embodiment 3 As shown in Fig. 4A, 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. 4B, 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. 4C 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 4 Referring now to Figs. 5A to 5D, the Embodiment 4 of the present invention is described below in detail. First, as shown in Fig. 5A, 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. 5B, 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. 5C, 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. 5C, 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. 5D 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 Reference Embodiment 1. Reference Embodiment 2 This Reference Embodiment is described by referring to Figs. 6A to 6F. As shown in Fig. 6A, 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. 6B). On the thin film Si monocrystalline layer 1302 is formed an oxidized film 1303 (Fig. 6C). The oxidized film 1305 formed on the surface of another Si substrate 1304 is bonded to the above oxidized film 1303 (Fig. 6D). 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. 6E). The porous region 1301 is etched away to form a semiconductor substrate having a thin film Si monocrystalline layer on an insulating material (Fig. 6F). 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. 6. For the design of an inventive embodiment, 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 above Embodiments 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 above Embodiments. Example 1 A P-type (100) monocrystalline Si substrate having a diameter of 3 inches (1 inch=25.4 mm) 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 pm/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 (1 Torr = 133 Pa) 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 2 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 substrate 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 3 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 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 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 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 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 Si 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 6 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 7 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 8 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 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 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 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 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 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 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 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 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 the 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 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) 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 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 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 14 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 15 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 16 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 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 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 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 (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.

Claims: A process for preparing a semiconductor member comprising the steps of: preparing a first member having a porous monocrystalline semiconductor layer (51, 153, 61, 163, 1301) and a non-porous monocrystalline semiconductor layer (52, 152, 62, 162, 1300), bonding said first member to a light-transmissive second member (53, 154, 63, 164, 1304) being an insulating material in a manner so as to place said non-porous monocrystalline semiconductor layer (52, 152, 62, 162, 1300) inside the multi-layer structure to be obtained, and then selectively etching said porous monocrystalline semiconductor layer (51, 153, 61, 163, 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 (51, 153, 61, 163, 1301) is P-type. A process for preparing a semiconductor member according to claim 1, wherein said non-porous monocrystalline semiconductor layer (52, 152, 62, 162, 1300) 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 (52, 152, 62, 162, 1300) is formed by epitaxial growth. A process for preparing a semiconductor member according to claim 1, wherein said non-porous monocrystalline semiconductor layer (52, 152, 62, 162, 1300) 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 (51, 153, 61, 163, 1301) 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 1, wherein said non-porous monocrystalline semiconductor layer (52, 152, 62, 162, 1300) 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 light-transmissive second member comprises a glass. A process for preparing a semiconductor member according to claim 13, wherein said glass is a quartz glass. A process for preparing a semiconductor member according to claim 1, wherein selectively etching of said porous monocrystalline semiconductor layer (51, 153, 61, 163, 1301) is carried out under the state where said first and second members bonded to each other are covered with an anti-etching material (54, 155). A process for preparing a semiconductor member according to claim 15, wherein said anti-etching material (54, 155) is silicon nitride. A process for preparing a semiconductor member according to claim 15. wherein said anti-etching material (54, 155) is Apiezon Wax. A process for preparing a semiconductor member according to any of the claims 1 to 17, wherein said first member having a porous monocrystalline semiconductor layer (51, 61) and a non-porous monocrystalline semiconductor layer (52, 62) is formed by making a non-porous monocrystalline semiconductor layer porous to form the porous monocrystalline semiconductor layer (51, 61) and forming said non-porous monocrystalline semiconductor layer (52, 62) on said porous monocrystalline semiconductor layer (51, 61). A process for preparing a semiconductor member according to any of the claims 1 to 17, wherein said first member having a porous monocrystalline semiconductor layer (153, 163) and a non-porous monocrystalline semiconductor layer (152, 162) is formed by the steps of forming a second monocrystalline semiconductor layer (152, 162) of a second electroconduction type on a first monocrystalline semiconductor layer (151, 161) of a first electroconduction type, and making said first monocrystalline semiconductor layer (151, 161) porous to form said porous monocrystalline semiconductor layer (153, 163). A process for preparing a semiconductor member according to any one of claims 1 to 17, 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
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1991
Main classification: H01L21
IPC codes: H01L21