Patent Publication Number: US-2002000242-A1

Title: Thin-film semiconductor device and its manufacturing method and apparatus and thin-film semiconductor solar cell module and its manufacturing method

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Field of the Invention  
       [0002] This invention relates to a thin-film semiconductor device such as solar cell made by first making a semiconductor device on a substrate and then transferring it onto another substrate. The invention also relates to a method and an apparatus for manufacturing such thin-film semiconductor devices, a thin-film semiconductor solar cell module, and its manufacturing method.  
       [0003] 2. Description of the Related Art  
       [0004] Solar cells have recently been brought into practice but still in limited uses. For full-scale use of solar cells, it is especially important to realize resource saving and cost reduction. Accounting the issues of energy conversion efficiency (photoelectric conversion) and energy pay-back period, thin-film solar cells are preferable to thick film solar cells. Since thin-film solar cells have a certain flexibility and can be mounted on curved portions of vehicle bodies or curved outer surfaces of portable electric appliances for electricity generation, they are available for wider uses.  
       [0005] To facilitate fabrication of such thin-film solar cells, The Applicant previously proposed a method for separating a device-making layer from a substrate (U.S. Ser. No. 595,382) and a method for manufacturing a thin-film semiconductor, solar cell and light emitting device (Japanese Patent Application No. 8-234480).  
       [0006] The method disclosed in U.S. Ser. No. 595,382 uses a crystal substrate (single-crystal silicon substrate) as the substrate, forms a porous layer as a separation layer, then grows on the porous layer a semiconductor layer forming a solar cell, bonds a plastic plate on the semiconductor layer by an adhesive, and then applies a tensile stress to separate the semiconductor layer together with the plastic plate from the crystal substrate. In this method, the crystal substrate can be used repeatedly, and therefore contributes to resource saving and cost reduction.  
       [0007] The method disclosed in Japanese Patent Application No. 8-234480 is an improvement of the former method, in which the porous layer as the separation layer varies in porosity in its thickness direction to weaken the tensile strength of the separation layer, and the quality of the semiconductor layer on the porous layer is improved. Especially, anodic oxidation current varying from 1 mA/cm 2  through 7 mA/cm 2  to 200 mA/cm 2  is applied to a silicon substrate to form a porous silicon layer used as the separation layer, and the semiconductor device layer is formed on the porous silicon layer by epitaxial growth. This method can make a separation layer having a weak tensile strength within the porous silicon layer.  
       [0008] This method, however, involves the following problems. When the tensile strength of the separation layer is too weak, the semiconductor device layer separates partly or entirely from the substrate during formation of an oxide film, for example, in a process for fabricating a solar cell or other semiconductor device in the semiconductor layer, due to a stress caused by a difference in expansion coefficient between the oxide film and silicon. Moreover, if the semiconductor device is exposed to a vacuum while a vapor deposition film, for example, is made, then the device layer may partly strips away from the substrate due to a stress caused by a difference in air pressure between pours in the porous silicon layer and the vacuum on the device surface. Especially when the substrate is a silicon substrate, which is liable to cleave and liable to break down with a weak stress, the problem often occurs during application of a tensile stress. In contrast, when the tensile strength of the separation layer is too large, the problem of separation does not occur in the process of forming the semiconductor device. However, the tensile strength in the separation layer increases when the tensile stress is applied. This results in separation between the plastic plate and the adhesive or between the semiconductor device and the adhesive, and makes it very difficult to separate the semiconductor device as a reliable product from the substrate.  
       [0009] As reviewed above, it is difficult for the formerly proposed methods of manufacturing a thin-film semiconductor device to make an separation layer having an appropriate tensile strength satisfying both the requirement that the semiconductor device never separates from the substrate during the manufacturing process and the requirement that the semiconductor device, maintaining a high quality, can be readily separated from the substrate when it is transferred to another substrate. Especially for manufacturing a semiconductor device, such as solar cell or LSI (Large Scale Integrated circuit), having a large area, the issue of the tensile force of the separation layer is one of most serious problems to be overcome.  
       [0010] As to the requirement of cost reduction of solar cells, cost reduction is necessary not only for individual solar cells themselves but also for a module containing a plurality of solar cells connected in series and in parallel. However, since single-crystal or polycrystalline silicon solar cells conventionally used for electricity generation use a silicon wafer having the thickness of approximately 300 μm, individual silicon wafers supporting solar cells must be connected electrically for incorporating them into a module, and the cost of the module remains high. Therefore, there is a demand for solar cells in form of a monolithic device.  
       [0011] For applications of solar cells to various portable appliances, such as wrist watches or portable electric calculators, miniaturization, cost reduction and high flexibility in design choice of appliances are required, and here again is a demand for a monolithic device of solar cells. Amorphous silicon solar cells can be made in form of a monolithic device on a substrate made of amorphous silicon. In this respect, amorphous silicon is an excellent material of solar cells. However, the photoelectric conversion efficiency of amorphous silicon is low, and the use of amorphous silicon solar cells is limited to applications to portable electric calculators, or the like.  
       [0012] If it is possible to realize a monolithic device of solar cells using single-crystal silicon having a higher photoelectric conversion efficiency than that of amorphous silicon, the cost of solar cells will be reduced, and they will be used in more applications. Japanese Patent Laid-Open Publication No. 54-6791 discloses a monolithic device of solar cells using a thick single-crystal silicon film. However, due to the thickness, it involves various problems, such as high cost, long energy pay-back period, less flexibility, and so on.  
       [0013] For mounting solar cells in portions thorough which light must pass through, such as house windows, vehicle windows, sun roofs, etc., for electricity generation, the solar cells must be see-through.  
       [0014] Amorphous silicon solar cells permit part of incident light to pass through. Therefore, see-through solar cells, in which a number of fine holes are made in a uniform distribution in an amorphous silicon film to form the solar cells, are being manufactured. However, amorphous silicon solar cells originally have a low photoelectric conversion efficiency, and making a number of fine holes results in an unacceptable decrease in generated output.  
       [0015] If solar cells using single-crystal or polycrystalline silicon higher in conversion efficiency than amorphous silicon can be readily made in a see-through form, then the use of solar cells in house windows, or the like, will be increased drastically. However, it is very difficult to make a number of fine holes in currently available single-crystal or polycrystalline silicon solar cells having the thickness of approximately 300 μm.  
       OBJECTS AND SUMMARY OF THE INVENTION  
       [0016] It is therefore an object of the invention to provide a method and an apparatus for manufacturing a thin-film semiconductor device, capable of readily separating and transferring a thin-film semiconductor device from a substrate to another substrate, free from the problem of separation during the semiconductor device being made, and suitable also for manufacturing a thin-film semiconductor device with a large area.  
       [0017] Another object of the invention is to provide a thin-film semiconductor device manufactured by the above-mentioned method, therefore improved in quality, and capable of being made as a large-scaled device.  
       [0018] Another object of the invention is to provide a monolithic thin--film single-crystal semiconductor solar cell and its manufacturing method, which enables cost reduction, flexibility, miniaturization, flexibility in design choice of appliances using solar cells, and extension over a wide area.  
       [0019] Another object of the invention is to provide a thin-film single-crystal semiconductor solar cell and its manufacturing method, which is see-through but exhibits a high conversion efficiency.  
       [0020] According to the invention there is provided a method for manufacturing a thin-film semiconductor device configured to form the thin-film semiconductor device on a first substrate and thereafter transfer the thin-film semiconductor device from the first substrate to a second substrate, comprising the steps of:  
       [0021] forming a porous layer containing a separation layer on the first substrate;  
       [0022] forming the thin-film semiconductor device on the porous layer; and  
       [0023] after bonding the second substrate different from the first substrate in contraction coefficient by cooling onto the thin-film semiconductor device, cooling the product by cooling means to produce a shear stress in the separation layer in the porous layer and to separate the thin-film semiconductor device from the first substrate along the separation layer.  
       [0024] According to another aspect of the invention, there is provided a method for manufacturing a thin-film semiconductor device configured to first form the thin-film semiconductor device on a first substrate and thereafter transfer the thin-film semiconductor device from the first substrate to a second substrate, comprising the steps of:  
       [0025] forming a porous layer containing a separation layer on the first substrate;  
       [0026] forming the thin-film semiconductor device on the porous layer; and  
       [0027] after bonding the second substrate onto the thin-film semiconductor device, irradiating an ultrasonic wave to separate the thin-film semiconductor device from the first substrate along the separation layer.  
       [0028] According to another aspect of the invention, there is provided a method for manufacturing a thin-film semiconductor device configured to first form the thin-film semiconductor device on a first substrate and thereafter transfer the thin-film semiconductor device from the first substrate to a second substrate, comprising the steps of:  
       [0029] forming a porous layer containing a separation layer on the first substrate;  
       [0030] forming the thin-film semiconductor device on the porous layer; and  
       [0031] after bonding the second substrate onto the thin-film semiconductor device, applying a centrifugal force to separate the thin-film semiconductor device from the first substrate along the separation layer.  
       [0032] The above, and other, objects, features and advantage of the present invention will become readily apparent from the following detailed description thereof which is to be read in connection with the accompanying drawings. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0033]FIGS. 1A through 1D are cross-sectional views of a thin-film semiconductor device in different steps of a manufacturing process according to the first embodiment of the invention;  
     [0034]FIGS. 2A and 2B are cross-sectional views of the thin-film semiconductor device in steps subsequent to those of FIGS. 1A through 1D;  
     [0035]FIGS. 3A and 3B are cross-sectional views of the thin-film semiconductor device in steps subsequent to those of FIGS. 2A and 2B;  
     [0036]FIG. 4 is a cross-sectional view illustrating a construction of a cooling device used in steps shown in FIGS. 2A through 3B;  
     [0037]FIG. 5 is a cross-sectional view illustrating a construction of another cooling-device;  
     [0038]FIGS. 6A and 6B are cross-sectional views of a thin-film semiconductor device in different steps of a manufacturing process according to the second embodiment of the invention;  
     [0039]FIG. 7 is a diagram of a construction for explaining ultrasonic irradiation;  
     [0040]FIGS. 8A and 8B are cross-sectional views of a thin-film semiconductor device in different steps of a manufacturing process according to the third embodiment of the invention;  
     [0041]FIG. 9A is a plan view of a centrifugal separator used in the step shown in FIG. 8B, and FIG. 9B is a cross-sectional view taken along the A-A line of FIG. 9A;  
     [0042]FIG. 10 is a cross-sectional view illustrating a construction of a transfer holder in the centrifugal separator shown in FIGS. 9A and 9B;  
     [0043]FIG. 11 is a cross-sectional view illustrating another construction of the transfer holder;  
     [0044]FIG. 12 is a cross-sectional view for explaining a method for manufacturing thin-film single-crystal silicon solar cells according to the fourth embodiment of the invention;  
     [0045]FIG. 13 is a cross-sectional view for explaining the method for manufacturing the thin-film single-crystal silicon solar cells according to the fourth embodiment of the invention;  
     [0046]FIG. 14 is a cross-sectional view for explaining the method for manufacturing the thin-film single-crystal silicon solar cells according to the fourth embodiment of the invention;  
     [0047]FIG. 15 is a cross-sectional view for explaining the method for manufacturing the thin-film single-crystal silicon solar cells according to the fourth embodiment of the invention;  
     [0048]FIG. 16 is a cross-sectional view for explaining the method for manufacturing the thin-film single-crystal silicon solar cells according to the fourth embodiment of the invention;  
     [0049]FIG. 17 is a cross-sectional view for explaining the method for manufacturing the thin-film single-crystal silicon solar cells according to the fourth embodiment of the invention;  
     [0050]FIG. 18 is a cross-sectional view for explaining the method for manufacturing the thin-film single-crystal silicon solar cells according to the fourth embodiment of the invention;  
     [0051]FIG. 19 is a cross-sectional view for explaining the method for manufacturing the thin-film single-crystal silicon solar cells according to the fourth embodiment of the invention;  
     [0052]FIG. 20 is a cross-sectional view for explaining the method for manufacturing the thin-film single-crystal silicon solar cells according to the fourth embodiment of the invention;  
     [0053]FIG. 21 is a cross-sectional view for explaining the method for manufacturing the thin-film single-crystal silicon solar cells according to the fourth embodiment of the invention;  
     [0054]FIG. 22 is a cross-sectional view for explaining the method for manufacturing the thin-film single-crystal silicon solar cells according to the fourth embodiment of the invention;  
     [0055]FIG. 23 is a cross-sectional view for explaining the method for manufacturing the thin-film single-crystal silicon solar cells according to the fourth embodiment of the invention;  
     [0056]FIG. 24 is a cross-sectional view for explaining the method for manufacturing the thin-film single-crystal silicon solar cells according to the fourth embodiment of the invention;  
     [0057]FIG. 25 is a cross-sectional view for explaining the method for manufacturing the thin-film single-crystal silicon solar cells according to the fourth embodiment of the invention;  
     [0058]FIG. 26 is a plan view showing a plan-view configuration of the thin-film single-crystal silicon solar cells according to the fourth embodiment of the invention;  
     [0059]FIG. 27 is a plan view showing another plan-view configuration of the thin-film single-crystal silicon solar cells according to the fourth embodiment of the invention;  
     [0060]FIG. 28 is a cross-sectional view for explaining a method for manufacturing a thin-film single-crystal silicon solar cell according to a fifth embodiment of the invention;  
     [0061]FIG. 29 is a cross-sectional view for explaining a method for manufacturing a thin-film single-crystal silicon solar cell according to a seventh embodiment of the invention;  
     [0062]FIG. 30 is a cross-sectional view for explaining a method for manufacturing a thin-film single-crystal silicon solar cell according to an eighth embodiment of the invention;  
     [0063]FIG. 31 is a cross-sectional view for explaining the method for manufacturing the thin-film single-crystal silicon solar cell according to the eighth embodiment of the invention;  
     [0064]FIG. 32 is a cross-sectional view for explaining the method for manufacturing the thin-film single-crystal silicon solar cell according to the eighth embodiment of the invention;  
     [0065]FIG. 33 is a cross-sectional view for explaining the method for manufacturing the thin-film single-crystal silicon solar cell according to the eighth embodiment of the invention;  
     [0066]FIG. 34 is a cross-sectional view for explaining the method for manufacturing the thin-film single-crystal silicon solar cell according to the eighth embodiment of the invention;  
     [0067]FIG. 35 is a cross-sectional view for explaining the method for manufacturing the thin-film single-crystal silicon solar cell according to the eighth embodiment of the invention;  
     [0068]FIG. 36 is a cross-sectional view for explaining the method for manufacturing the thin-film single-crystal silicon solar cell according to the eighth embodiment of the invention;  
     [0069]FIG. 37 is a cross-sectional view for explaining the method for manufacturing the thin-film single-crystal silicon solar cell according to the eighth embodiment of the invention;  
     [0070]FIG. 38 is a cross-sectional view for explaining the method for manufacturing the thin-film single-crystal silicon solar cell according to the eighth embodiment of the invention;  
     [0071]FIG. 39 is a cross-sectional view for explaining the method for manufacturing the thin-film single-crystal silicon solar cell according to the eighth embodiment of the invention;  
     [0072]FIG. 40 is a cross-sectional view for explaining the method for manufacturing the thin-film single-crystal silicon solar cell according to the eighth embodiment of the invention;  
     [0073]FIG. 41 is a plan view showing a plan-view configuration of the thin-film single-crystal silicon solar cell according to the eighth embodiment of the invention; and  
     [0074]FIG. 42 is a cross-sectional view for explaining a method for manufacturing a thin-film single-crystal silicon solar cell according to a ninth embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0075] Embodiments of the invention are explained below in detail with reference to the drawings.  
     [0076] (First Embodiment)  
     [0077]FIGS. 1A through 3B are cross-sectional views of a thin-film semiconductor device in different steps of a manufacturing process according to an embodiment of the invention. The thin-film semiconductor device taken here is a thin-film single-crystal silicon solar cell.  
     [0078] First prepared as a first substrate is a single-crystal silicon substrate (p-type, 0.01 to 0.02 Ω.cm)  100  (hereinafter referred to as silicon substrate  100 ), for example, as shown in FIG. 1A. Formed on the silicon substrate  100  is a porous silicon layer  110  by anodic oxidation, for example, as shown in FIG. 1B. Anodic oxidation pertains to a method relying on electric conduction in a water solution of hydrofluoric acid, using the silicon substrate  100  as the anode, and it can be conducted by a double-cell method proposed by Ito, et al. in “Anodic Oxidation of Porous Silicon” (Surface Technologies, Vol. 46, No. 5, pp 8-13, 1995). In this method, a silicon substrate to form a porous layer thereon is disposed between two electrolytic solution tanks, and platinum electrodes connected to a d.c. source are set in both electrolytic solution tanks. Then, electrolytic solutions are poured into both electrolytic solution tanks, and a d.c. voltage is applied to the platinum electrodes, to use the silicon substrate as the anode and the platinum electrode as the cathode. Thus, one of opposite surfaces of the silicon substrate is corroded to become porous.  
     [0079] In this embodiment, using anodic oxidation, a porous silicon layer having a separation layer whose tensile strength is weak is made on the single-crystal silicon substrate, and a solar cell is formed on the porous silicon layer. Then, the solar cell is separated from the separation layer in the porous silicon layer, using a cooling means explained later, and transferred onto another substrate.  
     [0080] More specifically, using an electrolytic solution containing HF (hydrogen fluoride): C 2 H 5 OH (ethanol) =1:1, for example, as an electrolytic solution (anodic oxidation solution), first-step anodic oxidation is executed for 8 minutes under the current density of approximately 0.5 to 3 mA/cm 2 , for example, to form a first porous silicon layer with a small porosity. After that, second-step anodic oxidation is effected for 8 minutes under the current density of approximately 3 to 30 mA/cm 2 , for example, to form a second porous silicon layer with an intermediate porosity. Further executed third-step anodic oxidation for several second under the current density of approximately 40 to 300 mA/cm 2 , for example, to form a third porous silicon layer with a large porosity. When the third porous silicon layer (porous silicon layer  110 ) is being formed, a layer with a very large porosity as the origin of the separation layer  111  (FIG. 1D), explained later, is also formed in the porous silicon layer  110 . The silicon substrate  100  is preferably a p-type single crystal from the viewpoint of forming the porous silicon layer  110  thereon by anodic oxidation. However, n-type single crystal or polycrystalline silicon are usable under appropriate conditions.  
     [0081] Next formed is a solar cell on the porous silicon layer  110 . That is, first executed is hydrogen annealing for 30 minutes at the temperature of 1100° C., for example, to cover holes opening to the surface of the porous silicon layer  110 . Thereafter, as shown in FIG. 1C, an epitaxial layer  120  is made as a semiconductor device film on the porous silicon layer  110  by epitaxial growth using gas of SiH 4  or other appropriate material at the temperature of 1070° C., for example. The thickness of the epitaxial layer  120  is 1 to 50 μm, for example, when a single-crystal silicon solar cell is to be made. In order to increase the efficiency of the solar cell, first grown as the epitaxial layer  120  is a p + -type layer  121  with the high concentration of 10 19 /cm 3 , for example, up to the thickness of approximately 1 μm, and a p-type layer  122  with a concentration from 10 15  to 10 18 /cm 3  is grown on the p + -type layer  121  up to the thickness of 1 to 49 μm. In this structure, electrons generated by light in the p-type layer are reflected by the p + -type layer  121 , which results in less recombination in the p + -type layer  121 , and the solar cell is made highly efficient.  
     [0082] After that, as shown in FIG. 1D, an oxide film  130  is formed on the epitaxial layer  120  by thermal oxidation, for example, and then patterned. Using the patterned oxide film  130  as a mask, an -type impurity is doped into the p-type layer  122  to form a high-concentrated n + -type layer  140  which will behave as a the cathode of the solar cell. Further formed on the n + -type layer  140  is a non-reflective film  150 , and an electrode aperture is formed in the non-reflective film  150 . Then, a metal electrode  160  made of aluminum (Al), for example, is selectively formed in the aperture. During the above-mentioned hydrogen annealing and epitaxial growth, silicon atoms in the porous silicon layer  110  move and recombine. As a result, a portion of the porous silicon layer  110  heretofore having a large porosity again changes largely and becomes a layer with the smallest tensile force, namely, the separation layer  111 . However, the separation layer  111  has a tensile strength large enough to prevent partial or entire separation of the epitaxial layer  120  from the silicon substrate  100  during formation of the solar cell in the epitaxial layer  120 .  
     [0083] After the metal electrode  160  is formed, as shown in FIG. 2A, a plastic plate  170  made of PET (polyethylene terephthalate), for example, as the second substrate, is bonded to the surface of the silicon substrate  100  (oxide film  130 ), using an adhesive  171  (for example, ultraviolet setting adhesive)  171  strong against a tensile force.  
     [0084] In the next step of the embodiment, the solar cell formed in the epitaxial layer  120  and the plastic plate  170  thereon are separated from the silicon substrate  100 , using a cooling means. That is, as shown in FIG. 2B, the silicon substrate  100  and the plastic plate  170  are cooled by vapor  180 A of liquid nitrogen, for example. The silicon substrate  100  and the plastic plate  170  exhibit different contraction coefficients when cooled. In general, the contraction coefficient of the plastic plate  170  is much larger than that of the silicon substrate  100 . Therefore, although the silicon substrate  100  contracts when cooled by the liquid nitrogen vapor  180 A, the plastic plate  170  contracts with a larger rate than the silicon substrate  100 . Due to the difference in contraction coefficient, a large shear stress occurs in the separation layer  111 . As a result, the semi-product solar cell (epitaxial layer  120  and plastic plate  170 ) separates from the silicon substrate  100  along the separation layer  111  as shown in FIG. 3A. That is, the solar cell is transferred from the silicon substrate  100  to the plastic plate  170 .  
     [0085] After that, the porous silicon layer  110 A remaining on the epitaxial layer  120  is removed by etching. Then, as shown in FIG. 3B, a bottom electrode  161  is formed on the bottom surface of the epitaxial layer  120  by printing, for example. After that, another plastic plate  173  made of PET or PC (polycarbonate), for example, is bonded to the bottom electrode  161  using the adhesive  172 . As a result, a thin-film single-crystal silicon solar cell sandwiched by two plastic plates  170  and  173  is completed. The silicon substrate  100  after separation of the solar cell can be re-used-by removing the remainder porous silicon layer  110 B on the surface by etching.  
     [0086] As explained above, according to the embodiment, by using a shear stress in the separation layer  111  due to a difference in contraction coefficient between the silicon substrate  100  and the plastic plate  170  when cooled, the solar cell can be readily separated from the silicon substrate  100  along the separation layer  111 , and an improvement in throughput is realized. Additionally, the method makes it possible to manufacture high-quality solar cells without the problem that the solar cell unintentionally strips away while it is being made.  
     [0087] Next explained with reference to FIG. 4 is a cooling apparatus for realizing the cooling method employed in the foregoing embodiment.  
     [0088] The cooling apparatus  200  includes a container  201  which contains liquid nitrogen  202 . A heater  203  is mounted in the container  201  to heat the liquid nitrogen  202 . A support plate  204  having a central aperture  204   a  is fixed to the inner wall of the container  201  above the liquid nitrogen  202  to support a semi-product (the silicon substrate  100  before separation of the solar cell  120 A) to be cooled with a support portion  204   b  formed along the aperture  204   a . A top plate of the container  201  has an aperture  201   a  for discharging vapor after cooling.  
     [0089] After the solar cell  120 A is formed in the epitaxial layer on the silicon substrate  100 , and the plastic plate  170  is bonded to the solar cell  120 A by the adhesive  171 , in steps explained with reference to FIGS. 1A through 2A, the silicon substrate  100  in this status is put on the support plate  204  in the cooling apparatus  200 . After that, the liquid nitrogen  202  is heated by a heater  203  and evaporated into vapor  180 A. Thus, the vapor  180 A is blown onto the silicon substrate  100 , adhesive  171  and plastic plate  170 , and the silicon substrate  100 , adhesive  171  and plastic plate  170  are gradually cooled. As a result, a shear stress is produced and increased in the separation layer  111  between the silicon substrate  100  and plastic plate  170 , and finally starts separation of the plastic plate  170  and the solar cell  120 A from the silicon substrate  100  along the separation layer  111  as shown in FIG. 3A. Note here that the density of vapor  180 A changes with temperature of the heater  203  and that the cooling speed can be controlled by adjusting the temperature of the heater  203 . It is important to prevent sudden decrease of the temperature to prevent damages to the solar cell to be separated.  
     [0090] In the cooling apparatus of this type, in general, the temperature of the silicon substrate  100  and the plastic plate  170  can be decreased to approximately 100K. If separation is not attained even at 100K, the silicon substrate  100  and the plastic plate  170  may be immersed into the liquid nitrogen  202 , or liquid helium vapor or dry ice may be blown, in order to further decrease the cooling temperature. When liquid helium is used, a helium recovery apparatus is preferably provided for cost reduction.  
     [0091] In a specific example where the porous silicon layer  110  was formed by anodic oxidation by electric conduction first for 8 minutes under the current density of approximately 1 mA/cm 2 , next for 8 minutes under the current density of 8 mA/cm 2 , and finally for 2.6 seconds under the current density of 200 mA/cm 2 , and the plastic plate  170  was bonded to the solar cell by an ultraviolet setting adhesive as the adhesive  171 , the solar cell could be separated from the silicon substrate  100  at the temperature of approximately 180K.  
     [0092]FIG. 5 shows the construction of an alternative cooling apparatus. In the cooling apparatus  300 , liquid nitrogen  304  is supplied to the container  301  through a liquid nitrogen inlet  302  and a pipe  303 , and the liquid nitrogen  304  is heated by a heater  305  and evaporated into vapor  180 A. A fixation plate  306  supported by support legs  306   a  is mounted above the liquid nitrogen  304 . The fixation plate  306  is made of a metal having a high heat conductivity, such as copper (Cu), for example. A hold plate  307  here again made of copper, for example, is fixed on the fixation plate  306 . The solar cell  120 A, formed on the silicon substrate  100  and having a transparent plastic plate  308  made of polycarbonate, for example, bonded thereto, is put on the hold plate  307 . In this status, along the periphery of the transparent plastic plate  308 , the transparent plastic plate  308  and the hold plate  307  are bound integrally by rivets  309   a ,  309   b  used as an anti-warpage means. The transparent plastic plate  308  has a pair of L-shaped engage portions  310   a ,  310   b  on its upper surface for engagement with a handle  311  when the transparent plastic plate  308  and the hold plate  307 , after cooled, are removed together from the container  301 .  
     [0093] After the solar cell  120 A is formed in the epitaxial layer on the silicon substrate  100  in steps explained with reference to FIGS. 1A through 2A, and the transparent plastic plate  308  is bonded to the solar cell  120 A, the silicon substrate  100  in this status is put on the hold plate  307  in the cooling apparatus  300 . After that, the liquid nitrogen  302  is heated by the heater  305  and evaporated into vapor  180 A. Thus, the vapor  180 A directly touches the fixation plate  306  and cools the fixation plate  306  and the hold plate  307  thereon to decrease their temperature. In this case, since the hold plate  307  is in contact over its entire area with the transparent plastic plate  308  and the silicon plate  100 , the temperature of the transparent plastic plate  308  and the silicon plate  100  are cooled uniformly, and their temperature is decreased uniformly. As a result, similarly to the case using the cooling apparatus  200 , the solar cell  120 A is separated together with the transparent plastic plate  308  from the silicon substrate  100  due to a difference in contraction coefficient by cooling. The aspect of the separation can be visually observed from above the transparent plastic plate  308  through the transparent plastic plate  308 . Although the transparent plastic plate  308  tends to warp away while it contracts due to cooling by liquid nitrogen vapor  180 A, the transparent plastic plate  308  is bound integrally with the hold plate  307  by rivets  309   a ,  309   b  as anti-warpage means, and such warpage is prevented. The anti-warpage means may be screws or other binding means instead of rivets  309   a ,  309 b, or a weight of a metal may be put on the upper surface of the transparent plastic plate  308 . It is also possible to combine two or more of these means.  
     [0094] In the first embodiment, if the separation layer  111  formed in the porous silicon layer  110  has a relatively large strength, an additional method of irradiating an ultrasonic wave or applying a tensile stress may be used in addition to cooling by the cooling means. In this case, the cooling temperature need not be so low as compared with the method relying on cooling means alone, and the energy of the ultrasonic wave and the tensile stress need not be large. Therefore, even a solar cell with a large area can be separated without damages.  
     [0095] (Second Embodiment)  
     [0096] Next explained is a second embodiment of the invention. Parts or elements identical or equivalent to those of the first embodiment are labeled with common reference numerals, and their explanation is omitted. This embodiment is the same as the first embodiment from the step of forming the metal electrode  160  until bonding the plastic plate  170  as the second substrate onto the surface of the silicon substrate  100  by the adhesive (for example, ultraviolet setting adhesive )  171  strong against a tensile force as shown in FIG. 6A. Therefore, subsequent steps alone are explained below.  
     [0097] In this embodiment, as shown in FIG. 6B, an ultrasonic wave  180 B is irradiated to the silicon substrate  100 , solar cell  120 A and plastic plate  170 . More specifically, as shown in FIG. 7, the silicon substrate  100  having formed the solar cell  120 A is immersed in a liquid  291 , such as water or ethanol, contained in a container  290 , and an ultrasonic wave  180 B of 250 kHz and 600 W, for example, is irradiated from an ultrasonic wave generator  292 . In this arrangement, the energy of the ultrasonic wave is effectively transmitted to the silicon substrate  100 , solar cell  120 A and plastic plate  170  to cut silicon atoms of the porous silicon layer  110  and to greatly weaken the tensile strength of the separation layer  111 . As a result, similarly to the aspect shown in FIG. 3A of the first embodiment, the solar cell  120 A and the plastic plate  170  on the epitaxial layer  120  are separated from the silicon substrate  100 . That is, the solar cell  120 A is transferred from the silicon substrate  100  to the plastic plate  170 . Subsequent steps are the same as those of the first embodiment, and their explanation is omitted here.  
     [0098] According to the embodiment explained here, since an ultrasonic wave is used to weaken the tensile strength of the separation layer  111  in the porous silicon layer  110 , the solar cell can be separated without decreasing the room temperature as required in the first embodiment. Additionally, this method makes it possible to manufacture solar cells with a large area and a high quality without the problem of unintentional separation during the solar cell being made. Moreover, since this embodiment need not apply a tensile stress, the solar cell does not receive a bending stress, and the problem of cleavage of the silicon substrate is eliminated.  
     [0099] In this embodiment, the tensile strength of the separation layer  111  becomes smaller as the energy of the ultrasonic wave becomes higher, or the frequency becomes lower, and the solar cell can be separated from the silicon substrate  100  and transferred to the plastic plate  170  only by irradiation of the ultrasonic wave. However, if the energy of the ultrasonic wave is too high, undesirable phenomenon such as cracking of the silicon substrate  100  may occur. Therefore, the energy of the ultrasonic wave should be determined to a value not cracking the silicon substrate  100 . An ultrasonic wave in the range of 50 kHz to 100 kHz, for example, is unlikely to cause damages to the silicon substrate  100 , and can be recommended for use in the embodiment.  
     [0100] In the above-explained embodiment, separation is done solely by irradiation of an ultrasonic wave. If irradiation of the ultrasonic wave is not sufficient for complete separation, then a tensile stress may be applied between the silicon substrate  100  and the plastic plate  170  in addition to irradiation of the ultrasonic wave. However, the tensile stress must be regulated within an appropriate range not to damages the epitaxial layer  120  having formed the solar cell.  
     [0101] If the use of the tensile stress in addition to irradiation of the ultrasonic wave is not sufficient for complete separation, the ultrasonic wave may be once again irradiated to further decrease the tensile strength of the separation layer  111  and to separate the solar cell from the silicon substrate  100 . In this case, the energy of the ultrasonic wave irradiated to the silicon substrate is desirably increased by increasing the output of the ultrasonic wave than the former irradiation or by decreasing the frequency of the ultrasonic wave.  
     [0102] If the separation is not completed even by these means, a tensile force is once again applied between the silicon substrate  100  and the plastic plate  170 . By repeating irradiation of the ultrasonic wave and application of a tensile stress, the tensile strength of the separation layer  111  gradually decreases to finally permit the separation.  
     [0103] If the separation is still difficult even after repeating irradiation of the ultrasonic wave and application of the tensile stress, then the separation layer  111  must be treated to exhibit a smaller tensile strength. That is, by elongating the time of anodic oxidation by several seconds or by increasing the current for anodic oxidation to absolutely decrease the tensile strength of the separation layer  111 , for example, the solar cell can be readily separated form the silicon substrate  100 . However, excessive decrease in tensile strength of the separation layer  111  will invite destruction of the solar cell while it is formed, and such situations must be avoided.  
     [0104] (Third Embodiment)  
     [0105] Next explained is a third embodiment of the invention. Parts or elements identical or equivalent to those of the first embodiment are labeled with common reference numerals, and their explanation is omitted. This embodiment is the same as the first and second embodiments from the step of forming the metal electrode  160  until bonding the plastic plate  170  as the second substrate onto the surface of the silicon substrate  100  by the adhesive (for example, ultraviolet setting adhesive )  171  strong against a tensile force as shown in FIG. 8A. Therefore, subsequent steps alone are explained below.  
     [0106] In this embodiment, as shown in FIG. 8B, a centrifugal force  180 B is applied to the solar cell and the plastic plate  170 . The centrifugal force  180 C cuts silicon atoms in the porous silicon layer  110  of the silicon substrate  100 , and greatly weakens the tensile strength of the separation layer  111 . As a result, similarly to the aspect shown in FIG. 3A of the first embodiment, the solar cell and the plastic plate  170  on the epitaxial layer  120  are separated from the silicon substrate  100 . That is, the solar cell is transferred from the silicon substrate  100  to the plastic plate  170 . Subsequent steps are the same as those of the first embodiment, and their explanation is omitted here.  
     [0107] According to the embodiment explained here, since a centrifugal force is applied to decrease the tensile strength of the separation layer  111  in the porous silicon layer  110  after the plastic plate as the second substrate is bonded to the solar cell, the solar cell can be readily separated together with the plastic plate  17 . This method enables fabrication of the solar cell without the problem of unintentional separation of the solar cell during formation thereof. Moreover, the method explained here can be conducted at the room temperature like the second embodiment, and requires less time for separation of the solar cell than the first and second embodiments. Therefore, a further improvement in through-put is realized.  
     [0108] Additionally, the embodiment explained here enables separation of elements of the same material, such as silicon from silicon, which is required for fabrication of a three-dimensional LSI (Large Scale Integrated circuit). That is, when two bonded silicon layers are to be separated, since they are identical in material and linear expansion coefficient, no stress occurs in the polycrystalline silicon layer even by changing the temperature (cooling) like the first embodiment. Theoretically, therefore, the method according to the first embodiment is not applicable to fabrication of three-dimensional LSI. In contrast, the embodiment shown here uses a centrifugal force instead of temperature variation. Since the centrifugal force applied to the unit area concentrates to the column portion of the separation layer  111 , the layers of the same material, such as silicon-silicon, can be separated along the separation layer  111 .  
     [0109] If the centrifugal force alone is not sufficient for complete separation, a tensile stress may be additionally applied between the silicon substrate  100  and the plastic plate  170  to promote separation. Moreover, the cooling method used in the first embodiment and irradiation of an ultrasonic wave used in the second embodiment may be combined.  
     [0110] A centrifugal separator for realizing centrifugal separation according to the embodiment is explained below in detail with reference to FIGS. 9 and 10.  
     [0111] The centrifugal separator  400  includes a ring-shaped rotary body  403  within a casing  402  supported by two support legs  401   a ,  401   b , for example. The rotary body  403  is made of a light material, such as duralmin, and configured to set a plurality of, i.e. three, transfer holders  404  inside it through an aperture formed in its upper plate. These transfer holders  404  are preferably located in equal intervals. Balancers  405  are also set in the rotary body  403  between the transfer holders  404  to smooth rotation of the rotary body  403 .  
     [0112] The centrifugal separator  400  further includes a driver  406  as means for applying a centrifugal force for rotating the rotary body  403 . The driver  406  is located at a central position of the rotary body  403 , and includes a rotary shaft  406   a  coupled to the rotary body  403  through three arms  407 , for example, a drive motor  406   b  located under the casing  403 , and gear mechanism  406   c  transmitting the driving power of the drive motor  406   b  to the rotary shaft  406   a . The entirety of the driver  406  is fixed to the bottom of the protective casing  403  via a support  408 .  
     [0113]FIG. 10 shows a specific construction of each transfer holder  404  set in the rotary body  403  of the centrifugal separator  400 . The transfer holder  404  is a box-shaped element large enough to contain a semi-product, and may be made of a light metal, such as duralmin. The transfer holder includes a main body  404   a  open to its front end, for example, and a cover  404   b  covering the opening of the main body  404   a . The cover  404   b  is attached to the main body  404   a  by screws  409   a ,  409   b , for example. An inner vertical wall surface of the main body  404   a  behaves as a hold portion  404   c  to which the back surface of a semi-product silicon substrate  100  inserted through the opening is bonded by an adhesive  500 . A metal plate  404   d  of copper (Cu), for example, used as a weight is bonded to the plastic plate  170  as the second substrate by an adhesive  501 . On the other hand, a damping element  404   e  made of sponge, for example, is bonded to an inner wall surface of the cover  404   b . A gap  404   f  is provided between the damping element  404   e  and the metal plate  404   d  bonded to the plastic plate  170 .  
     [0114] In the centrifugal separator  400 , when the drive motor  406   b  rotates, its driving power is transmitted to the rotary shaft  406   a  through the gear mechanism  406   c . As a result, the rotary body  403  rotates, and a centrifugal force  180 C is applied to the metal plate  404  and the plastic plate  170  in the transfer holder  404  as shown by the arrow in FIG. 10. Due to the centrifugal force  180 C, the solar cell  120 A and the plastic plate  170 , together with the metal plate  404   d , are separated from the silicon substrate  100  as shown by the dots-and-dash line. The solar cell  120 A and the plastic plate  170  separated from the silicon substrate  100  fly toward the cover  404   b . However, since the damping element  404   d  is bonded to the cover  404   b , the metal plate  404   d  hits the damping element  404   e  and stops there. Therefore, the solar cell  120 A is never damaged. The metal plate  404   d  is removed from the plastic plate  170  after separation of the solar cell  120 A.  
     [0115] Since the centrifugal force F generated by the centrifugal separator  400  is determined, in general, by mrω 2  (m: mass, r: rotational radius, and ω: angular velocity) the embodiment may rely on increasing the length (r) of the arm  407 , revolution (ω) or mass (m) to increase the centrifugal force. Considering that an increase of the length (r) of the arm  407  results in increasing the size of the apparatus, the length of the arm  407  used in the embodiment is 30 cm, for example. Additionally, considering that an increase of the revolution (ω) results in applying a large load to the drive motor, a motor of 1000 to 5000 rpm, for example, is used. That is, the embodiment mainly rely on an increase of the mass (m) by bonding the metal plate  404  to the plastic plate  170  to increase the centrifugal force. For example, when the length (r) of the arm  407  is 30 cm, and a motor whose revolution (ω) is 300 rpm is used, a metal plate with the thickness of 2 mm per 1 cm 2  may be used to apply a centrifugal force as large as five times the gravity when the metal plate is made of copper (Cu).  
     [0116] If a spindle motor rotatable at a high speed around decade thousands rpm, for example, is used as the drive motor, then a sufficient centrifugal force can be obtained without using the metal plate. For example, if the revolution of the motor is 30,000 rpm, the centrifugal force is equivalent to 500 times the gravity. Therefore, it is sufficient to bond a PC plate with the thickness of approximately 0.2 mm in order to obtain a centrifugal force equivalent to five times the gravity. Although the centrifugal force is referred to as being five times the gravity for convenience in calculation, the same is applicable also to a sufficiently small sample in which the strength of the porous silicon layer is 1 G or less.  
     [0117] The foregoing embodiment has been explained as bonding the silicon substrate  100  to a wall surface (hold portion  404   c ) in the transfer holder  404  by an adhesive. However, the arrangement shown in FIG. 11, for example, may be used alternatively. In FIG. 11, the plastic plate  170  has a smaller diameter than that of the silicon substrate  100 , and a wall surface of the transfer holder  404  confronting to the periphery of the silicon substrate  100  form a hold portion  410 . In this arrangement, when a centrifugal force is applied similarly to the above case, the silicon substrate  100  is engaged by the hold portion  410  along its peripheral edge, and the solar cell  120 A and the plastic plate  170  are separated in the same manner. Since this arrangement requires no adhesive, the silicon substrate after separation of the solar cell  120 A can be readily removed from the transfer holder  404  to ensure more reliable re-use of the silicon substrate.  
     [0118] Moreover, although the foregoing embodiment has been explained as the hold portion  404   c  grasping the silicon substrate  100  as the first substrate in the transfer holder  404 , the holder  404   c  may be configured to grasp the plastic plate  170  as the second substrate. However, the structure of the embodiment configured to grasp the silicon substrate  100  at the hold portion  404   c  is preferable because the plastic plate  170  is more suitable for attaching the metal plate, and the plastic plate  170  having attached the metal plate is heavier and more likely to separate than the silicon substrate.  
     [0119] Location the opening of the transfer holder  404  may be selected as desired, for example, on the top end, although it is on the front end in the foregoing embodiment.  
     [0120] Although the invention has been described by way of various embodiments, the invention is not limited to these specific examples, but rather involves various changes and modifications within the range of equivalents. For example, although the first to third embodiments have been explained as making a solar cell as the thin-film semiconductor device, any appropriate thin-film device, such as photo detector, light emitting device, liquid crystal display device, integrated circuit device, or the like, may be made. Also these devices may be made of a polycrystalline or amorphous layer, or their compound film, instead of a single-crystal layer.  
     [0121] Although the embodiments have been explained as using the plastic plate  170  as the second substrate, also usable are a glass plate, SUS (stainless steel) or other metal plate, and a silicon or other semiconductor substrate, case by case.  
     [0122] Next explained are a solar cell module and its manufacturing method according to the invention.  
     [0123]FIGS. 12 through 25 show a process for manufacturing a thin-film single-crystal silicon solar cell, taken as the fourth embodiment of the invention. The thin-film single-crystal silicon solar cell is made in form of a monolithic device containing an appropriate number of such thin-film single-crystal silicon solar cells required for a specific use. In FIGS. 12 through 25, only two thin-film single-crystal silicon solar cells are shown.  
     [0124] In the fourth embodiment, first prepared is a single-crystal silicon substrate  601  as shown in FIG. 12. The single-crystal silicon substrate  601  is preferably of a p-type from the viewpoint that the a porous silicon layer is to be made thereon by anodic oxidation explained later. However, even if an n-type substrate is used, the porous silicon layer can be made under appropriately determined conditions. The single-crystal silicon substrate  601  has a specific resistance in the range of 0.01 to 0.02, for example.  
     [0125] Next formed on the surface of the single-crystal silicon substrate  601  is a porous silicon layer  602  by anodic oxidation as shown in FIG. 13. The porous silicon layer  602  is made in three different steps. In the first step, in order that an epitaxial layer having a good crytallographic property be made on the porous silicon layer  602 , anodic oxidation is done for 8 minuets, for example, under the current density of approximately 0.5 to 3 mA/cm 2 , for example, to form a porous silicon layer with a small porosity. In the next second step, anodic oxidation is executed for 8 minutes, for example, under the current density of approximately 3 to 20 mA/cm 2 , for example, to make a porous silicon layer with an intermediate porosity. In the next third step, anodic oxidation is done for several seconds, for example, under the current density of approximately 40 to 300 mA/cm , for example, to make a porous silicon layer with a large porosity. By anodic oxidation in the third step, a thin porous silicon layer  602   a  having a very large porosity as the origin of the separation layer is formed in the porous silicon layer  602 . For anodic oxidation in respective steps, an anodic oxidation solution containing HF:C 2 H 2 OH=1:1, for example. Considering that the single-crystal silicon substrate  601  is used repeatedly, the thickness of the porous silicon layer  602  is preferably as thin as possible, preferably in the range of 5 to 15 μm, more preferably around 8 μm, for example, to alleviate the decrease in thickness of the single-crystal silicon substrate  601  and to maximize the re-usable time.  
     [0126] Next conducted is hydrogen annealing for 30 minutes at the temperature of 1100° C., for example, to cover holes opening to the surface of the porous silicon layer  602 . Thereafter, as shown in FIG. 14, a p -type single-crystal silicon layer  603  and a p-type single-crystal silicon layer  604  are epitaxially grown in sequence on the porous silicon layer  602  at 1070° C., for example, by CVD using SiH 4 , for example, as the material gas. The total thickness of the p + -type single-crystal silicon layer  603  and the p-type single-crystal silicon layer  604  is preferably 1 to 50 μm. The p + -type single-crystal silicon layer  603  has an impurity concentration around 10 19 /cm 3 , for example, and a thickness around 1 μm. The p-type single-crystal silicon layer  604  has an impurity concentration around 10 15  to 10 18 /cm 3 , for example, and a thickness in the range of 1 to 49 μm, for example.  
     [0127] During the hydrogen annealing and the epitaxial growth, silicon atoms in the porous silicon layer  602  move and recombine. As a result, the thin porous silicon layer  602   a  having a large porosity in the porous silicon layer  602  becomes a layer with a very low tensile strength, namely, the separation layer.  
     [0128] After that, a silicon oxide layer  605  is formed on the entire surface of the p-type single-crystal silicon layer  604  by thermal oxidation or CVD, as shown in FIG. 15.  
     [0129] Next, as shown in FIG. 16, a resist pattern (not shown) of a configuration corresponding to the solar cell to be made is formed on the silicon oxide film  605  by lithography, and the silicon oxide film  605  is etched using the resist pattern as a mask. The resist pattern is removed thereafter. Then, the patterned silicon oxide film  605  is used as a mask to sequentially wet-etch the p-type single-crystal silicon layer  604  and the p + -type single-crystal silicon layer  603  sequentially, using an alkali etchant, such as KOH. As a result, separate solar cell layers  606 ,  706  each made of the p + -type single-crystal silicon layer  603  and the p-type single-crystal layer  604  are obtained. To fully separate the solar cell layers  606 ,  607 , the wet etching may be continued until an upper part of the porous silicon layer  602  is etched. However, as explained later, the wet etching is preferably stopped before reaching the porous silicon layer  602   a  as the separation layer, in order to facilitate separation of the solar cell layers  606 ,  607  from the single-crystal silicon substrate  601 .  
     [0130] Next, as shown in FIG. 17, the silicon oxide film  605  is partly removed by etching until exposing the p-type single-crystal silicon layer  604  overlying end portions of the solar cell layers  606 ,  607 . After that, by diffusing a p-type impurity, such as boron, into the exposed portions and side wall portions of the solar cell layers  606 ,  607  to form p + -type single-crystal silicon layers  608 .  
     [0131] Next, as shown in FIG. 18, a silicon oxide film  609  is formed by thermal oxidation or CVD to cover exposed surfaces of the p + -type single-crystal silicon layers  608 .  
     [0132] Next, as shown in FIG. 19, the silicon oxide film  906  is selectively removed by etching to make apertures  609   a , and an n-type impurity, such as phosphorus, is diffused into the p-type single-crystal silicon layer  604  through the apertures  609   a  to form n + -type single-crystal silicon layers  610 .  
     [0133] Each n + -type single-crystal silicon layer  610 , p-type single-crystal silicon layer  604  and p + -type single-crystal silicon layer  603  construct a thin-film single-crystal silicon solar cell  611  or  612  having an n + -p-p +  structure. The n + -type single-crystal silicon layer  610  and the p + -type single-crystal silicon layer  608  behave as the cathode and the anode of each thin-film single-crystal silicon solar cell  611  or  612 . The p + -type single-crystal silicon layers  603  have the role of increasing the conversion efficiency of the thin-film single-crystal silicon solar cells  611 ,  612 . That is, since electrons generated by incident light into the p-type single-crystal silicon layer  604  are reflected by- the p +-type single-crystal silicon layer  603 , recombination of electron-hole pair decreases in the p + -type single-crystal silicon layer  603 , and a high conversion efficiency is realized.  
     [0134] Next, as shown in FIG. 20, an anti-reflection film  613  in form of a silicon nitride film, for example, is formed on the entire surface by CVD, for example, and the anti-reflection film  613  and silicon oxide film  609  are selectively removed by etching to form apertures  614  and  615  where the p + -type single-crystal silicon layer  608  and the n + -type single-crystal silicon layer  610  are exposed.  
     [0135] After that, a metal film such as aluminum film, for example, is formed on the entire surface by vacuum evaporation or sputtering, for example, and then patterned into a predetermined configuration by etching. As a result, a metal electrode  616  is formed as shown in FIG. 21. In this status, the n + -type single-crystal silicon layer  610  behaving as the cathode of the thin-film single-crystal silicon solar cell  611  and the p + -type single-crystal silicon layer  608  behaving as the anode of the thin-film single-crystal silicon solar cell  612  are connected by the metal electrode  616 .  
     [0136] Next, as shown in FIG. 22, a transparent substrate  618  made of a transparent plastic film, for example, is bonded to surfaces of the thin-film single-crystal silicon solar cells  611 ,  612  by using an adhesive  617  preferably having a high tensile strength.  
     [0137] After that, while the single-crystal silicon substrate  601  is immersed in water or ethanol solution, for example, an ultrasonic wave with the frequency of 25 kHz and the power of 600 W, for example, is irradiated to decrease the separation strength of the porous silicon layer  602   a  as the separation layer due to the energy of the ultrasonic wave and to separate the single-crystal silicon substrate  601  along the porous silicon layer  602   a  as shown in FIG. 23. Alternatively, tensile forces in opposite directions may be applied to the transparent substrate  618  and the single-crystal silicon substrate  601  to separate the single-crystal silicon substrate  601  along the porous silicon layer  602   a  as the separation layer. Alternatively, the single-crystal silicon substrate  601  and the transparent substrate  618  may be cooled by blowing cold nitrogen gas evaporated from liquid nitrogen, for example, to produce a shear stress caused by a difference in thermal contraction between the single-crystal silicon substrate  601  and the transparent substrate  618  and to thereby separate the single-crystal silicon substrate  601  along the porous silicon layer  602   a  as the separation layer. Alternatively, two or more of the above-mentioned processes may be combined to separate the single-crystal silicon substrate  601  along the porous silicon layer  602   a  as the separation layer.  
     [0138] In this status, the thin-film single-crystal silicon solar cells  611 ,  612  are short-circuited by the porous silicon layer  602  remaining on their back surfaces. Therefore, wet-etching is done using an alkali etchant, for example, to remove the porous silicon layer  602  from the back surface of the thin-film single-crystal silicon solar cells  611  and to fully isolate these thin-film single-crystal silicon solar cells  611 ,  612  from each other as shown in FIG. 24.  
     [0139] After that, a metal film, such as aluminum film, is formed on the entire bottom surface of the thin-film single-crystal silicon solar cells  611 ,  612  by vacuum evaporation or sputtering, for example, and the metal film is patterned into a predetermined configuration by etching to form metal electrodes  619  on bottom surfaces of the thin-film single-crystal silicon solar cells  611 ,  612 . By lining the bottom surfaces of the thin-film single-crystal silicon solar cells  611 ,  612  by the metal electrodes  19 , the serial resistances of the thin-film single-crystal silicon solar cells  611 ,  612  can be decreased. This is especially effective when the thin-film single-crystal silicon solar cells  611 ,  612  are wide and their serial resistances are large. These metal electrodes  619  also function as reflective mirrors for reflecting light passing through the thin-film single-crystal silicon solar cells  611 ,  612 , and hence increase the conversion efficiency of the thin-film single-crystal silicon solar cells  611 ,  612 .  
     [0140] After that, as shown in FIG. 25, a substrate  621  made of a plastic film, for example is bonded to the bottom surfaces of the thin-film single-crystal silicon solar cells  611 ,  612  by an adhesive  620 .  
     [0141] As a result, monolithic thin-film single-crystal silicon solar cells separated from each other and connected in series are completed on the transparent substrate  618 .  
     [0142] A plan-view configuration of the thin-film single-crystal silicon solar cells is shown in FIG. 26, and another in FIG. 27. In the example of FIG. 26, a plurality of strip-shaped thin-film single-crystal silicon solar cells are formed and isolated by separation regions. In the example of FIG. 27, a plurality of rectangular thin-film single-crystal silicon solar cells are formed and isolated by longitudinal and transverse separation regions.  
     [0143] According to the forth embodiment, the following advantages are obtained. That is, since a plurality of thin-film single-crystal silicon solar cells are made in form of a monolithic device in an isolated relationship on the transparent substrate  618 , the cost of solar cell modules can be reduced remarkably, and hence the cost of solar cells can be decreased. Moreover, the monolithic design contributes to miniaturization of solar cells, and permits a variety of designs of portable appliances in which solar cells are mounted. Further, since the thin-film single-crystal silicon solar cells comprises a thin-film solar cell layer and flexible transparent substrate  618  and substrate  612 , flexible solar cells having a high conversion efficiency can be realized, and applications of solar cells are increased remarkably. In particular, with regard of the flexibility, since a plurality of thin-film single-crystal solar cells are held in an isolated relationship on the transparent substrate  618  and the substrate  621 , and the adhesive  617  fills portions between adjacent thin-film single-crystal silicon solar cells, the product has a sufficient strength against a certain degree of bending. Additionally, by using a rectangular single-crystal silicon substrate obtained by cutting a single-crystal silicon ingot obtained by crystal growth along its lengthwise direction, for example, solar cells extending over a large area as large as square meters can be realized.  
     [0144] Moreover, after the porous silicon layer  602  formed on the single-crystal silicon substrate  601  is removed, the silicon substrate  601  restores the original status shown in FIG. 12, and can be re-used to execute the step shown in FIG. 13. That is, the single-crystal silicon substrate  610  can be re-used, the cost of thin-film single-crystal silicon solar cells can be decreased. More specifically, if the thickness of the porous silicon layer  602  is 8 μm, and a thickness around 3 μm of the single-crystal silicon substrate  601  is lost by polishing for its re-use, then the single-crystal silicon substrate  601  loses the thickness 11 μm in one cycle of the manufacturing process of thin-film single-crystal silicon solar cells. Therefore, even after the single-crystal silicon substrate  601  is used ten times, the single-crystal silicon substrate  601  loses the thickness of only 110 μm. Thus, the single-crystal silicon substrate  601  can be used at least ten times.  
     [0145] Etching or electrolytic polishing may be used for removal of the porous silicon layer  602  formed along the surface of the single-crystal silicon substrate  601 . An example of conditions for removing the porous silicon layer  602  by electrolytic polishing is: the electrolytic polishing solution being a solution with a low HF concentration, namely, HF:C 2 H 5 OH=1:1, for example, and the current density being 400 mA/cm 2 .  
     [0146] Moreover, according to the fourth embodiment, since the solar cell layers are separated into a plurality of regions by etching, the thin-film single-crystal silicon solar cells can be readily separated from the single-crystal silicon substrate  601 , and no trouble occurs there. That is, in order to facilitate separation of the thin-film single-crystal silicon solar cells from the single-crystal silicon substrate  601 , the tensile strength of the porous silicon layer  602   a  in the porous silicon layer  602 , i.e. the separation layer, may be made small. However, when it is excessively weak, a stress by heat increases and may results in unintentional separation of the solar cell layers from the single-crystal silicon substrate  601  while the product is put under a high-temperature condition in the manufacturing process of solar cells, namely, during diffusion of an impurity, for example. However, in the fourth embodiment, separation of solar cell layers  606 ,  607  is attained by wet etching using an alkali etchant, the stress applied to the solar cell layers  606 ,  607  are alleviated remarkably, and unintentional separation of the solar cell layers  606 ,  607  can be prevented effectively even when the product is exposed to a high temperature in subsequent steps. Especially when producing solar cells extending over a large area as large as 10 cm 2  or more, unintentional separation of solar cell layers in the manufacturing process of solar cells was a serious issue. Taking it into consideration, the method according to the fourth embodiment which can cut and separate the solar cell layer into parts of the size around 10 cm 2  prior to a high-temperature process is remarkably excellent, and can realize solar cells extending over an area as large as square meters as indicated above.  
     [0147] Next explained is a thin-film single-crystal silicon solar cell according to the fifth embodiment of the invention.  
     [0148] In the fifth embodiment, as shown in FIG. 28, the same steps as those of the fourth embodiment are executed until the p-type single-crystal silicon layer  604  is formed. After that, a single-layer film of silicon nitride, or a compound film combining a silicon nitride film and a chrome or metal film, is formed on the p-type single-crystal silicon layer  604  by CVD, vacuum evaporation or sputtering. Then, the film is patterned into a shape of solar cells by etching to form a mask  622 . By using the mask  622 , selective portions of the p-type single-crystal silicon layer  604  and the p + -type not covered by the mask  622  are changed to a porous layer by anodic oxidation. Then, the porous silicon layer, thus obtained, is removed by wet etching using Noah liquid to form isolated solar cell layers  606 ,  607 . The porous silicon layer can be removed easily by wet etching using Noah liquid.  
     [0149] After that, the process is progressed in the same manner as the fourth embodiment, and intended thin-film single-crystal silicon solar cells are completed.  
     [0150] Also the fifth embodiment gives the same advantages as those of the fourth embodiment.  
     [0151] Next explained is a thin-film single-crystal silicon solar cell according to the sixth embodimnt of the invention.  
     [0152] In the sixth embodiment, the same steps as those of the fourth embodiment are executed until forming the p-type single-crystal silicon layer  604  as shown in FIG. 28. After that, here again, a single-layer film of silicon nitride, or a compound film combining a silicon nitride film and a chrome or metal film, is formed on the p-type single-crystal silicon layer  604  by CVD, vacuum evaporation or sputtering. Then, the film is patterned into a shape of solar cells by etching to form the mask  622 . By using the mask  622 , selective portions of the p-type single-crystal silicon layer  604  and the p + -type single-crystal silicon layer  603  not covered by the mask  622  are changed to a porous layer by anodic oxidation. Additionally, the porous silicon layer is oxidized to form a silicon oxide layer (not shown) to form isolated solar cell layers  606 ,  607 . In this case, the silicon nitride film forming the mask  622  is used as the mask for oxidization.  
     [0153] After that, the process is progressed in the same manner as the fourth embodiment, and intended thin-film single-crystal silicon solar cells are completed.  
     [0154] Also the sixth embodiment gives the same advantages as those of the fourth embodiment.  
     [0155] Next explained is a method for manufacturing thin-film single-crystal silicon solar cells according to the seventh embodiment of the invention.  
     [0156] In the seventh embodiment, the same steps as those of the fourth embodiment are executed until the metal electrode  616  is formed as shown in FIG. 29. After that, spaces between thin-film single-crystal silicon solar cells  611 ,  612  are filled with a soft material  23 , such as adhesive or fibers, for example, having a strength against bending. Usable as the soft adhesive is an adhesive which does not set with ultraviolet rays, such as thermoplastic rubber adhesive or polyurethane adhesive. An appropriate fiber material is nylon or other transparent fibers.  
     [0157] Subsequently, the same steps as those of the fourth embodiment are executed, and intended thin-film single-crystal silicon solar cells are completed.  
     [0158] Also the seventh embodiment attains the same advantages as those of the fourth embodiment, and attains an additional advantage, namely, realization of thin-film single-crystal silicon solar cells flexibly resisting against bending.  
     [0159] Next explained is a method for manufacturing thin-film single-crystal silicon solar cells according to the eighth embodiment of the invention. FIG. 30 through  40  illustrate steps of the method according to the eighth embodiment.  
     [0160] In the eighth embodiment, as shown in FIGS. 30 and 32, the same steps as those of the fourth embodiment are executed to sequentially form on the single-crystal silicon substrate  601  the porous silicon layer  602 , p + -type single-crystal silicon layer  603  and p-type single-crystal silicon layer  604 .  
     [0161] After that, as shown in FIG. 33, a silicon oxide film  605  is formed on the entire surface of the p-type single-crystal silicon layer  604 , and then patterned by etching into a configuration corresponding to fine holes to be made. Using the patterned silicon oxide film  605  as a mask, the p-type single-crystal silicon layer  604  and the p + -type single-crystal silicon layer  603  are wet-etched sequentially, using an alkali etchant, such as KOH, to make a plurality of fine holes  624 . Since the fine holes  624  permit light to pass through, the thin-film single-crystal silicon solar cells become see-through. The diameter of each fine hole  624  is determined appropriately, depending on the intended use of the thin-film single-crystal silicon solar cells, for example, in the range of 1 μm to the order of cm. Although the wet-etching is continued until an upper part of the porous silicon layer  602  is removed, it is preferably stopped before reaching the porous silicon layer  602   a  as the separation layer in order to facilitate later separation of the solar cell layer  606  from the single-crystal silicon substrate  601 .  
     [0162] Next, as shown in FIG. 34, a p-type impurity, such as boron, is diffused into inner walls of the fine holes  624  not covered by the silicon oxide film  605  to form p + -type single-crystal silicon layers  625 . Then, silicon oxide films  609  are formed by thermal oxidation or CVD to cover exposed surfaces of the p + -type single-crystal silicon films  609 . The p + -type single-crystal silicon layers  625  have the same role as the p + -type single-crystal silicon layer  603 . That is, electrons generated by incident light into the p-type single-crystal silicon layer  604  are reflected by the p + -type single-crystal silicon layers  625 , recombination of electron-hole pairs decreases in the p + -type single-crystal silicon layers  625 , and the conversion efficiency of the thin-film single-crystal silicon solar cell  611  can be increased.  
     [0163] Next, as shown in FIG. 35, the silicon oxide film  605  is selective removed by etching to partly expose the p-type single-crystal silicon layer  604 . Then, an n-type impurity, such as phosphorus, is diffused into the exposed p-type single-crystal silicon layer  604  to form n + -type single-crystal silicon layers  610 .  
     [0164] Similarly to the fourth embodiment, each n + -type single-crystal silicon layer  610 , p-type single-crystal silicon layer  604  and p + -type single-crystal silicon layer  603  construct a thin-film single-crystal silicon solar cell  611  having an n + -p-p +  structure. In this case, the n + -type single-crystal silicon layer  610  behaves as the cathode of the thin-film single-crystal silicon solar cell  611 , and the p + -type single-crystal silicon layer  603  behave as the cathode of the thin-film single-crystal silicon solar cell  611 .  
     [0165] Next, as shown in FIG. 36, an anti-reflection film  613  in form of a silicon nitride film, for example, is formed on the entire surface by CVD, for example, and the anti-reflection film  613  is selectively removed by etching to expose the n + -type single-crystal silicon layer  610 . After that, a metal film, such aluminum film, is formed on the entire surface by vacuum evaporation or CVD, for example, and then patterned into a predetermined configuration by etching to form metal electrodes  616 .  
     [0166] Next, as shown in FIG. 37, a transparent substrate  618  made of a transparent plastic film, for example, is bonded to the surface of the thin-film single-crystal silicon solar cells  611 , using an adhesive  617  preferably having a high tensile strength.  
     [0167] Next, in the same manner as the fourth embodiment, the single-crystal silicon substrate  610  is separated from the thin-film single-crystal silicon solar cell  611 , as shown in FIG. 38. that is, an ultrasonic wave of the frequency of 25 kHz and the power of 600 W, for example, is irradiated to the single-crystal silicon substrate  601  immersed in water or ethanol solution, so that the energy of the ultrasonic wave decreases the tensile strength of the porous silicon layer  602   a  as the separation layer and promotes separation of the single-crystal silicon substrate  601  along the porous silicon layer  602   a . Alternatively, tensile forces in opposite directions may be applied to the transparent substrate  618  and the single-crystal silicon substrate  601  to separate the single-crystal silicon substrate  601  along the porous silicon layer  602   a  as the separation layer. Alternatively, the single-crystal silicon substrate  601  and the transparent substrate  618  may be cooled by blowing cold nitrogen gas evaporated from liquid nitrogen, for example, to produce a shear stress caused by a difference in thermal contraction between the single-crystal silicon substrate  601  and the transparent substrate  618  and to thereby separate the single-crystal silicon substrate  601  along the porous silicon layer  602   a  as the separation layer. Alternatively, two or more of the above-mentioned processes may be combined to separate the single-crystal silicon substrate  601  along the porous silicon layer  602   a  as the separation layer.  
     [0168] In this status, the porous silicon layer  602  remains on portions of the fine holes  624  and on back surfaces of the thin-film single-crystal silicon solar cells  611 . Therefore, wet-etching is done using an alkali etchant, for example, so that no porous silicon layer remain at portions of the fine holes  624  as shown in FIG. 39.  
     [0169] After that, a metal film, such as aluminum film, is formed on the entire bottom surface of the thin-film single-crystal silicon solar cells  611  by vacuum evaporation or sputtering, for example, and the metal film is patterned into a predetermined configuration by etching to form metal electrodes  619  on bottom surfaces of the thin-film single-crystal silicon solar cells  611 . Since the metal electrodes  619  behave also as reflective mirrors for reflecting light passing through the thin-film single-crystal silicon solar cells  611 , the conversion efficiency of the thin-film single-crystal silicon solar cells  611  can be increased.  
     [0170] After that, as shown in FIG. 40, a transparent substrate  626  made of a transparent plastic film, for example is bonded to bottom surfaces of the thin-film single-crystal silicon solar cells  611 , using an adhesive.  
     [0171] As a result, see-through thin-film single-crystal silicon solar cells having a plurality of fine holes  624  permitting light to pass through are completed on the transparent substrate  618 .  
     [0172] A plan-view configuration of the thin-film single-crystal silicon solar cells is shown in FIG. 41. In order to introduce external light in a natural form into a room of a house, for example, through the thin-film single-crystal silicon solar cells, the fine holes  624  must be made in a uniform distribution as shown in FIG. 41. However, if it is sufficient to obtain merely see-through thin-film single-crystal silicon solar cells, the fin holes  624  need not be distributed uniformly.  
     [0173] As described above, the eighth embodiment can realize see-through thin-film single-crystal silicon solar cells by making the fine holes  624  permitting beams of light to pass through. The see-through thin-film single-crystal silicon solar cells can generate a larger amount of electricity generation than conventional see-through amorphous silicon solar cells, and can greatly increase the cost performance. Additionally, the thin-film single-crystal silicon solar cells use a thin film as the solar cell layer and uses flexible substrates as the transparent substrate  618  and the substrate  621 , a high conversion efficiency and a high flexibility can be realized.  
     [0174] Moreover, the single-crystal silicon substrate  601  can be re-used repeatedly like that explained with the fourth embodiment, and the cost of the thin-film single-crystal silicon solar cells can be decreased.  
     [0175] Next explained is a thin-film single-crystal silicon solar cell according to the ninth embodiment of the invention.the following advantages are obtained.  
     [0176] In the ninth embodiment, the same steps as those of the fourth embodiment are conducted until the p-type single-crystal silicon layer  604  is formed as shown in FIG. 42. After that, a single-layer film of silicon nitride, or a compound film combining a silicon nitride film and a chrome or metal film, is formed on the p-type single-crystal silicon layer  604  by CVD, vacuum evaporation or sputtering. Then, the film is patterned into a shape defining fine holes by etching to form the mask  622 . By using the mask  622 , selective portions of the p-type single-crystal silicon layer  604  and the p + -type single-crystal silicon layer  603  not covered by the mask  622  are changed to a porous layer by anodic oxidation. Subsequently, the porous silicon layer is removed by wet etching using Noah solution, for example to form fine holes  624 . The porous silicon layer can be readily removed by wet etching using Noah solution.  
     [0177] After that, the process is progressed in the same manner as the fourth embodiment, and intended thin-film single-crystal silicon solar cells are completed.  
     [0178] Also the ninth embodiment attains the same advantages as those of the seventh embodiment.  
     [0179] Next explained is a thin-film single-crystal silicon solar cell according to the tenth embodiment of the invention.  
     [0180] In the tenth embodiment, after the same steps as those of the fourth embodiment are executed until the p-type single-crystal silicon layer  604  is formed as shown in FIG. 42, a single-layer film of silicon nitride, or a compound film combining a silicon nitride film and a chrome or metal film, is formed on the p-type single-crystal silicon layer  604  by CVD, vacuum evaporation or sputtering. Then, the film is patterned into a shape defining fine holes by etching to form the mask  622 . By using the mask  622 , selective portions of the p-type single--crystal silicon layer  604  and the p + -type single-crystal silicon layer  603  not covered by the mask  622  are changed to a porous layer by anodic oxidation. Additionally, the porous silicon layer is oxidized to form a silicon oxide layer (not shown) and thereby to form fine holes  624 . In this case, the silicon nitride film forming the mask  622  is used as the mask for oxidization.  
     [0181] After that, the process is progressed in the same manner as the fourth embodiment, and intended thin-film single-crystal silicon solar cells are completed.  
     [0182] Also the tenth embodiment attains the same advantages as those of the eighth embodiment.  
     [0183] Although the invention has been described by way of various embodiments, the invention is not limited to these specific examples, but involves various changes and modifications based on the spirit of the invention.  
     [0184] For example, glass substrates, for example, may be used as the transparent substrates  618 ,  626  and substrate  21  used in the fourth to tenth embodiments, if appropriate. Similarly, transparent electrodes made of ITO, for example, may be used instead of metal electrodes  616 ,  619 , if appropriate.  
     [0185] Although the fourth embodiment uses an alkali etchant, such as KOH, in wet etching for isolating the solar cell layers  606 ,  607  made of the p + -type single-crystal silicon layer  603  and the p-type single-crystal silicon layer  604 , an acid may be used for the wet etching, if appropriate. Also in the eighth embodiment, an acid may be used in lieu of an alkali etchant in wet etching for making fine holes  624  in the solar cell layer  606  comprising the p + -type single-crystal silicon layer  603  and the p-type single-crystal silicon layer  604 .  
     [0186] Still in the eighth embodiment, although the p + -type single-crystal silicon layer  625  is formed in the inner wall portion of each fine hole  624 , the p + -type single-crystal silicon layer  625  may be omitted when the silicon oxide layer  609 , for example, formed in the inner wall portion of the fine hole  624  can reduce the speed of recombination along the surface.  
     [0187] Further, the thin-film single-crystal silicon solar cells according to the fourth embodiment can be made as a see-through solar cell module by making fine holes in individual thin-film single-crystal silicon solar cells in the same manner as the eighth embodiment.  
     [0188] As explained above, the method for manufacturing thin-film semiconductor device according to the invention makes it easy to separate the thin-film semiconductor device from a first substrate and to manufacture a high-quality thin-film semiconductor device without the problem of unintentional separation during formation of the semiconductor device, because a shear stress is produced in a separation layer in the porous layer by cooling the product after bonding a second substrate onto the thin-film semiconductor device, so that the thin-film semiconductor device is separated from the first substrate together with the second substrate along the separation layer and results in being transferred to the second substrate. Especially, the method is effective when fabricating a thin-film semiconductor device having a large area. Additionally, by combining irradiation of an ultrasonic wave and/or application of a tensile stress with the cooling process, the method can separate thin-film semiconductor devices a second substrate is bonded to the thin-film semiconductor devices with a high yield without damages to the products.  
     [0189] Also another method according to the invention, configured to irradiate an ultrasonic wave to decrease the tensile strength of the separation layer in the porous layer, can readily separate thin-film semiconductor devices from the first substrate, and can fabricate high-quality thin-film semiconductor devices without the problem of unintentional separation during formation of the semiconductor devices. Here again, the method is especially effective when fabricating a thin-film semiconductor device extending over a wide area.  
     [0190] Also another method for manufacturing a thin-film semiconductor device according to the invention, configured to apply a centrifugal force to decrease the tensile strength of the separation layer in the porous layer, can separate thin-film semiconductor devices from the first substrate, easily and quickly, and can therefore fabricate high-quality thin-film semiconductor devices in a short time. Here again, the method is especially effective when manufacturing a thin-film semiconductor device extending over a wide area.  
     [0191] The apparatus for manufacturing thin-film semiconductor devices according to the invention, using anti-warpage means to prevent warpage of substrates upon cooling the first and second substrates, can reliably prevent unintentional separation of thin-film semiconductor devices, and can fabricate high-quality thin-film semiconductor devices.  
     [0192] Another apparatus for manufacturing thin-film semiconductor devices according to the invention, including a hold portion for holding thin-film semiconductor devices having bonded the second substrate and a damping portion confronting the hold portion in the transfer holder, can apply a centrifugal force without damages to thin-film semiconductor devices separated by the centrifugal force, and can fabricate high-quality thin-film semiconductor devices.  
     [0193] The thin-film semiconductor device according to the invention has a high quality and can be made as a device extending over a wide area, because it is transferred from the first substrate to the second substrate by utilizing a shear stress produced in the porous layer formed in the first substrate due to a difference in contraction coefficient between these substrates when cooled by cooling means.  
     [0194] Also another thin-film semiconductor device according to the invention has a high quality and can be made as a device extending over a wide area because it is transferred from the first substrate to the second substrate, utilizing a stress produced in the porous layer caused by an ultrasonic wave after it is formed on the porous layer on the first substrate.  
     [0195] Also another thin-film semiconductor device according to the invention has a high quality and can be made as a device extending over a wide area because it is transferred to the second substrate, utilizing a stress produced in the porous layer due to a centrifugal force after it is made on the porous layer on the first substrate.  
     [0196] Thus, the invention can provide a monolithic thin-film single-crystal semiconductor solar cell and its manufacturing method, which realize cost reduction, flexibility, miniaturization, flexibility in design choice of appliances having mounted solar cells, and extension over a wide area.  
     [0197] Additionally, the invention can provide a see-through thin film single-crystal semiconductor solar cell having a high conversion efficiency, and its manufacturing method.