Title of Invention

"A METHOD OF DEPOSITING SILICON THIN FILM ON A SUBSTRATE"

Abstract

In a method of depositing a silicon thin film by using a vertical plasma CVD apparatus having steps of holding a substrate (21) having an area not smaller than 1,200 cm2 and having a conductive film (22) formed thereon with a substrate holder (1), disposing the substrate (22) to face an electrode, and depositing a silicon thin film under a power density of 100 mW/cm2 or more, the substrate holder (1) is electrically insulated from the conductive film (22) formed on the surface of the substrate (21) by forming a separation groove (24) in the conductive film (22).

Full Text

The present invention relates to method of depositing a silicon thin film on a substrate Background Art A thin film solar cell module is constructed such that string-like solar cells each consisting of a transparent electrode layer, a photovoltaic semiconductor layer, and a back electrode layer, which are stacked one upon the other on a transparent substrate, are connected in series. The photovoltaic semiconductor layer formed of amorphous silicon is low in cost, but is defective in that the photovoltaic efficiency is low. In order to improve the photovoltaic efficiency, it is advantageous to use a hybrid type photovoltaic semiconductor layer in which pin-type amorphous silicon and pin-type polycrystalline silicon (polysilicon) layer are stacked one upon the other or a polysilicon type photovoltaic semiconductor layer using pin-type polysilicon alone. Also, a substrate having a large area has come to be used for improving the manufacturing efficiency of the thin film solar cell module. The JP-5-265033 described a method for forming an amorphous silicon film by bringing gate electrode wires formed on a glass substrate and a jig for setting the glass into a non-conducting state, when a lower substrate of an active matrix liquid crystal display is produced. Further setting an insulating spacer between the gate electrode wires and the jig for setting the glass and forming the gate electrode wires in a region other than the periphery of the glass substrate are described as specific ways to bring the gate electrode wires and the jig for setting the glass into a non-conductive state. The JP-10-120491 discloses that the power density is set to 0.01-0.2 W/cm2, when an amorphous silicon film is formed on the substrate. The JP 2635950 discloses that the power density is set to 0.1 mW/cm2 or higher when the amorphous silicon film is formed on the substrate The JP -5-166733 discloses that the power density is set to 100 mW/cm2 or higher when an amorphous silicon thin film is formed on the substrate. In order to deposit a photovoltaic layer on a transparent electrode layer formed on a transparent substrate having a large area, it is efficient to use vertical-type in-line plasma CVD apparatus. The method of depositing a photovoltaic semiconductor layer by using vertical-type in-line plasma CVD apparatus will now be described with reference to FIGS. 1A and 1B. As shown in FIG. 1A, a frame-like substrate holder 1 is constructed to have a recess, slightly larger than a substrate 2, on the back surface. The substrate holder 1 is first placed horizontally, and the substrate 2 is fitted into the recess of the substrate holder 1 from the backside under the state that the transparent conductive film is positioned on the front side. As shown in FIG. 1B, a back plate 3 is put on the back surface of the substrate holder 1, and pins are slid between fixing tools la of the substrate holder 1 and fixing tools 3a of the back plate 3 so as to hold the substrate 2. The substrate holder 1 holding the substrate 2 under the particular state is held upright and moved within the vertical-type in-line plasma CVD apparatus to the position of an electrode 4. Under this condition, a photovoltaic semiconductor layer is deposited by plasma CVD. Incidentally, a conductive material such as SUS is used for the substrate holder 1 in view of the mechanical strength required for holding the substrate having a large area. FIG. 2 shows in a magnified fashion the contact portion between the substrate holder 1 and the substrate in the process of depositing a photovoltaic semiconductor layer by the conventional method. As shown in FIG. 2, a transparent conductive film 22 is formed on a transparent substrate.. 21, and the peripheral region of the transparent conductive film 22 is in contact with the inner edge portion of the substrate holder 1. No problem was generated in the case of depositing an amorphous silicon film by plasma CVD under the state shown in FIG. 2. However, in the case of depositing a polysilicon film, abnormal distribution or defects have been generated in the thin film. In the worst case, it has been found that the substrate is cracked. It has been clarified that the difficulty is caused as follows. Amorphous silicon has a relatively high absorption coefficient and, thus, the thickness of the amorphous silicon film can be decreased. In the case of a polysilicon film, however, it is necessary to increase the thickness of the film because polysilicon has a low absorption coefficient. In order to improve the productive efficiency by shortening the time required for depositing the polysilicon layer, it is necessary to supply high power to the substrate so as to increase the film deposition rate. To be more specific, for depositing a polysilicon layer, the power density on the substrate is set at a high level not lower than 100 mW/cm2. The power density noted above is at least 4 to 6 times as high as the power density for depositing an amorphous silicon layer. If plasma CVD is performed under a power density not lower than 100 mW/cm2 under the state that the transparent conductive film 22 formed on the surface of the substrate 21 is brought into contact with the substrate holder 1 as shown in FIG. 2, problems are generated such as blackish discoloring of the transparent conductive film, defects such as flaws and scrapes, and a substrate crack. These defects are rendered prominent with increase in the supplied power. It is considered reasonable to understand that a charge is accumulated in the transparent conductive film 22 in performing the plasma CVD so as to bring about abnormal discharge (a spark) between the tip of the substrate holder 1 and the transparent conductive film 22, leading to the defects referred to above. If the substrate holder 1 could be brought into a tight contact with the transparent conductive film 22, it would be theoretically possible to release the charge accumulated on the surface of the transparent conductive film 22 through the substrate holder 1 so as to overcome the difficulty noted above. However, it is practically impossible to bring the substrate holder 1 into a tight contact with the transparent conductive film 22 because of, for example, the warp of the substrate 21. An object of the present invention is to provide a method of depositing a silicon thin film on a substrate having a large area under a high power density by using vertical-type plasma CVD apparatus, which permits improving uniformity of the silicon thin film and also permits preventing a substrate crack so as to realize stable production. Disclosure of Invention The present invention provides a method of depositing a silicon thin film by using a plasma CVD apparatus, comprising: holding a substrate having an area not smaller than 1,200 cm2 and having a conductive film formed thereon with a substrate holder; disposing the substrate to face an electrode; and depositing a silicon thin film under a power density of 100 mW/cm2 or more, preferably 200 mW/cm2 or more in view of the production efficiency, characterized in that the substrate holder is electrically insulated from the conductive film formed on the substrate. The present invention also provides a silicon thin film solar cell, comprising a conductive film formed on a surface of a rectangular substrate, characterized in that at least one separation groove formed in the conductive film along each of the four sides of the substrate in a region within 3 mm to 40 mm from the outer periphery of the substrate. This invention therefore provides a method of depositing a silicon thin film on a substrate by using a plasma CVD apparatus, comprising holding the substrate having an area not smaller than 1,200 CM2 and having a conductive film formed thereon with a substrate holder disposing the substrate to face an electrode and depositing a silicon thin film on the substrate under a power density of 100 mW/cm or more, wherein the substrate holder is electrically insulated from the conductive film formed on the surface of the substrate, and wherein a separation groove is formed in the conductive film formed on the surface of the substrate such that the separation groove is positioned away from the inner edge of the substrate holder by 0.1 to 30 mm. Brief Description of Drawings FIGS. 1A and IB are a plan view and a cross sectional view, respectively, collectively showing the mounted state of a substrate to a substrate holder; FIG. 2 is a cross sectional view showing the contact state between the substrate holder and the substrate in the conventional method; FIG. 3 is a cross sectional view showing the contact state between the substrate holder and the substrate in a method according to one embodiment of the present invention; FIG. 4 is a plan view showing the separation groove shown in FIG. 3; FIG. 5 is a cross sectional view showing the contact state between the substrate holder and the substrate in a method according to another embodiment of the present invention; FIG. 6 is a cross sectional view showing the contact state between the substrate holder and the substrate in a method according to another embodiment of the present invention; FIG. 7 is a cross sectional view showing the contact state between the substrate holder and the substrate in a method according to another embodiment of the present invention; FIG. 8 is a cross sectional view showing the contact state between the substrate holder and the substrate in a method according to another embodiment of the present invention; FIG. 9 schematically shows the method of measuring insulating properties in the present invention; FIG. 10 is a cross sectional view showing the contact state between the substrate holder and the substrate in a method according to another embodiment of the present invention; and FIG. 11 is a cross sectional view showing the contact state between the substrate holder and the substrate in a method according to still another embodiment of the present invention. Best Mode for Carrying Out of the Invention In the method of the present invention, the substrate holder is electrically insulated from the conductive film formed on the surface of the substrate. Therefore, it is possible to prevent abnormal discharge between the substrate holder and the conductive film formed on the surface of the substrate in depositing a silicon thin film on the substrate having a large area not smaller than 1,200 cm2 under a power density set at a high value not lower than 100 mW/cm2. As a result, uniformity of the silicon thin film can be improved and a substrate crack can be prevented. In the present invention, the substrate holder is insulated from the substrate so as to suppress the generation of abnormal discharge in the contact portion between the two members. The abnormal discharge is considered to take place in the case where a considerably large amount of electric charge is accumulated when the charge accumulated on the conductive film escapes to the substrate holder. Since the accumulated charge tends to escape through the contact portion between the substrate and the substrate holder, the amount of the electric charge that escapes at once to the substrate holder is dependent on the ratio of the substrate area over the peripheral length of the substrate, taking into account of the construction of the substrate holder used in the present invention. Since the particular ratio is proportional to the square of the substrate size, the abnormal discharge tends to take place easily with increase in the substrate area. Such being the situation, the method of the present invention is rendered indispensable in the case where a silicon thin film is formed on a substrate having a large area under high power. The specific methods for electrically insulating the substrate holder from the conductive film formed on the surface of the substrate in the present invention will now be described with reference to the accompanying drawings. For example, a substrate holder 1 is electrically insulated from a transparent conductive film 22 by forming a separation groove 24 in the transparent conductive film 22 formed on the surface of a substrate 21, as shown in FIG. 3. The separation groove 24 is formed away from the inner edge of the substrate holder 21 by a distance d of 0.1 to 30 mm. It is more desirable for the distance d between the inner edge of the substrate holder 21 and the separation groove 24 to fall within a range of between 1 mm and 30 mm. Where the distance d is smaller than 0.1 mm, it is difficult to prevent the abnormal discharge. In addition, it is difficult to ensure a desired distance d because of the positional deviation of the substrate. On the other hand, if the distance d exceeds 30 mm, the utilization ratio of the solar cell on the substrate is lowered. Also, in order to improve insulating reliability or in order to supply higher power, it is desirable to form two or three separation grooves, which are 0.5 mm to 2 mm away from each other, in the region where the distance d falls within a range of between 1 mm and 30 mm. If the number of separation grooves is three or less, the tact time for performing laser scribing to the transparent conductive film is relatively short, which is practical in terms of productivity. Also, it is desirable for the width of the overlapping portion between the substrate holder and the substrate to be at least 3 mm for supporting the substrate without fail. On the other hand, it is desirable for the width noted above to be not larger than 10 mm because, if the width in question is excessively large, the effective area of the semiconductor layer is decreased. It follows that, in actually forming the separation groove in the transparent electrode formed on the substrate, it is desirable to form the separation groove in a region that is 3 mm to 40 mm away from the outer periphery of the substrate. It is also desirable to form at least one separation groove along each of the four outer sides of the rectangular substrate. The separation groove 24 will now be described with reference to FIG. 4. As shown in FIG. 4, in order to form string-like solar cells, a scribing line 23 is formed zigzag on the transparent conductive film 22 on the surface of the substrate 21 by using a laser scriber before deposition of a photovoltaic semiconductor layer. Further, two separation grooves 24 are formed by laser scribing in the vicinity of the two sides parallel to the integration direction of the solar cells denoted by an arrow in the drawing. These two separation grooves are formed inside the portion where the scribing lines 23 are connected with each other so as to separate the transparent conductive film 22 into the peripheral region and the cell-integrated region. If the separation grooves 24 are formed in this fashion, the transparent conductive film 22 is separated into the peripheral region and the cell-integrated region naturally by the scribing lines 23 in the front and rear of the integration direction of the solar cells. A photovoltaic semiconductor layer is deposited under this condition. Incidentally, Jpn. Pat. Appln. KOKAI Publication / No. 11-186573 teaches the idea that a photovoltaic semiconductor layer is deposited after formation of a peripheral separation groove in a transparent electrode layer. However, the method proposed in this prior art is intended to ensure sufficient insulation between the cell-integrated region and the peripheral region in the final product. This prior art does not teach the technical idea of the present invention that abnormal discharge is prevented in depositing a silicon thin film on a substrate having a large area under a high power density by using vertical-type plasma CVD apparatus. It is also possible to employ the method shown in FIG. 5. Specifically, the transparent conductive film 22 is removed in the peripheral region of the substrate 21, and the substrate holder 1 is electrically insulated from the transparent conductive film 22 by bringing the substrate holder 1 into contact with the peripheral region of the substrate 21 having the transparent conductive film 22 removed therefrom so as to permit the substrate 21 to be held by the substrate holder 1. Incidentally, Japanese Patent Disclosure No. 2000-225547 discloses a method of mechanically removing a~transparent conductive film by a prescribed width from the outer peripheral region of the substrate. This method is intended to perform sufficient processing of an insulating separation between the cell-integrated region and the peripheral region in a short time. However, this prior art does not teach the object of the present invention that abnormal discharge is prevented in depositing a silicon thin film on a substrate having a large area under a high power density by using vertical-type plasma CVD apparatus. It is also possible to electrically insulate the substrate holder 1 from the transparent conductive film 22 by arranging an insulator between the transparent conductive film 22 formed on the surface of the substrate 21 and the substrate holder 1, as shown in FIG. 6. It is possible to use, as the insulator, an insulating tape 25 such as a polyimide tape low in degassing. It is also possible to use, as the insulator, an insulating coating prepared by, for example, thermally spraying anodized aluminum to the surface of the substrate holder 1 in a thickness of, for example, about 100 /m. Incidentally, Japanese Patent Disclosure No. 56-40282 discloses a method of depositing an amorphous silicon film by plasma CVD, with an insulating spacer interposed between an oxide transparent electrode formed on the surface of the substrate and the substrate holder for holding the substrate. However, this prior art is intended to prevent the oxide transparent electrode from being brought into contact with the substrate holder. If the oxide transparent electrode is brought into contact with the substrate holder, it is grounded and, thus, is reduced into a metal under a reducing atmosphere, thereby losing the transparency. This prior art also does not teach the object of the present invention that abnormal discharge is prevented in depositing a silicon thin film on a substrate having a large area under a high power density by using vertical-type plasma CVD apparatus. Further, it is possible in the present invention to employ the method of electrically insulating the substrate holder 1 from the transparent conductive film 22 as shown in FIG. 7. To be more specific, the separation groove 24 is formed on the transparent conductive film 22 formed on the surface of the substrate 21 in a position which is 0.1 to 30 mm away from the inner edge of the substrate holder 21, and the insulating tape 25 is arranged between the transparent conductive film 22 formed on the surface of the substrate 21 and the substrate holder 1, thereby electrically insulating the substrate holder 1 from the transparent conductive film 22. Likewise, it is also possible to employ the method shown in FIG. 8. Specifically, the separation groove 24 is formed on the transparent conductive film 22 formed on the surface of the substrate 21 in a position which is 0.1 to 30 mm away from the inner edge of the substrate holder 21, and the insulating coating 26 is arranged on the contact portion of the substrate holder 1 with the substrate 21, thereby electrically insulating the substrate holder 1 from the transparent conductive film 22. The methods shown in FIGS. 7 and 8 are most effective for electrically insulating the substrate holder 1 from the transparent conductive film 22. In these methods, it is possible to prevent effectively abnormal discharge between the tip of the substrate holder 1 and the transparent conductive film 22 even in the case where the power density on the surface of the substrate 21 is very high. Example 1 A glass substrate sized at 910 mm x 910 mm and having a transparent conductive film formed on the surface thereof was prepared. As shown in FIGS. 3 and 4, a separation groove 24 was formed in a width of about 100 Aim by laser scribing in the transparent conductive film 22 formed on the surface of the glass substrate 21 such that the separation groove 24 was positioned 3 mm away from the inner edge of the substrate holder 1 when the glass substrate 21 was mounted to the substrate holder 1. As shown in FIG. 9, the probes 27 of Megatester were brought into contact with the transparent conductive film 22 such that the probes 27 were positioned away from each other by a distance of about 8 mm with locating the separation groove 24 between the probes. When a voltage of 250 V was applied, it was possible to obtain insulation not lower than 0.5 MΩ . As shown in FIG. 1, a single glass substrate 21 of the size referred to above was held with the substrate holder 1 of vertical-type in-line plasma CVD apparatus. In this case, the distance between the inner edge of the substrate holder 1 and the separation groove 24 falls within a range of 3±2 mm in view of the positional deviation of the glass substrate 21. The substrate holder 1 holding the glass substrate 21 was moved to the position where the electrode 4 sized at 115 cm x 118 cm was arranged, and a hydrogen gas and a silane gas were introduced as reactant gases. Under this condition, a polysilicon film was deposited by supplying electric power of 3 kW. Under these conditions, the power density on the surface of the substrate is about 221 mW/cm2. As a result, no defect of the film caused by abnormal discharge was observed in the polysilicon film thus deposited. No substrate crack was generated either. Then, another polysilicon film was deposited under the conditions exactly equal to those described above, except that electric power of 5 kW (power density of about 368 mW/cm2) or 8 kW (power density of about 590 mW/cm2) was supplied in depositing the polysilicon film. No defect of the film caused by abnormal discharge was observed in the polysilicon film thus deposited in each of these cases. No substrate crack was generated either. Abnormal discharge was not observed either in the case where the separation groove 24 formed by laser scribing had a width of about 40 µm or about 200 µm. Comparative Example 1: A polysilicon film was deposited under the conditions as described in Example 1, except that the separation groove 24 was not formed in the transparent conductive film 22 formed on the surface of the glass substrate 21. In this case, defects of the film caused by abnormal discharge were observed in the polysilicon film deposited under the power supply of any of 3 kW and 5 kW. Also, a substrate crack was generated in some of the samples. Example 2: A glass substrate sized at 910 mm x 455 mm and having a transparent conductive film formed on the surface thereof was prepared. Then, a separation groove 24 was formed in a width of about 100 µm. by laser scribing in the transparent conductive film 22 formed on the surface of the glass substrate 21 such that the separation groove 24 was positioned 3 mm away from the inner edge of the substrate holder 1 when the glass substrate 21 was mounted to the substrate holder 1, as in Example 1. Two glass substrates 21 of the size described above were mounted to the substrate holder 1 of vertical-type in-line plasma CVD apparatus. In this case, another substrate holder 1 was also arranged intermediate between the two glass substrates 21. Then, a polysilicon film was deposited by supplying electric power of 3 kW or 5 kW as in Example 1. As a result, no defect derived from abnormal discharge was observed in the polysilicon film deposited under any condition. Also, no substrate crack was generated. Example 3: A glass substrate sized at 400 mm x 300 mm and having a transparent conductive film formed on the surface thereof was prepared. Then, a separation groove 24 was formed in a width of about 100 µm by laser scribing in the transparent conductive film 22 formed on the surface of the glass substrate 21 such that the separation groove 24 was positioned 3 mm away from the inner edge of the substrate holder 1 when the glass substrate 21 was mounted to the substrate holder 1, as in Example 1. Further, a polysilicon film was deposited by supplying electric power of 3 kW or 5 kW as in Example 1. As a result, no defect derived from abnormal discharge was observed in the polysilicon film deposited under any condition. Also, no substrate crack was generated. Example 4: A glass substrate sized at 910 mm X 910 mm and having a transparent conductive film formed on the surface thereof was prepared. As shown in FIG. 5, the transparent conductive film 22 was removed by polishing in the region of at least 5 mm from the edge of the substrate 21, in place of forming the separation groove 24. Then, a polysilicon film was deposited by supplying electric power of 3 kW or 5 kW as in Example 1. As a result, no defect derived from abnormal discharge was observed in the polysilicon film deposited under any condition. Also, no substrate crack was generated. Example 5: A glass substrate sized at 910 mm x 910 mm and having a transparent conductive film formed on the surface thereof was prepared. As shown in FIG. 6, an insulating tape 25 made of polyimide was arranged between the transparent conductive film 22 formed on the surface of the substrate 21 and the substrate holder 1, in place of forming the separation groove 24. Then, a polysilicon film was deposited by supplying electric power of 3 kW or 5 kW as in Example 1. As a result, no defect derived from abnormal discharge was observed in the polysilicon film deposited under any condition. Also, no substrate crack was generated. Example 6: A glass substrate sized at 910 mm x 910 mm and having a transparent conductive film formed on the surface thereof was prepared. As shown in FIG. 8, a separation groove 24 was formed in the transparent conductive film 22, and an insulating coating 26 having a thickness of about 100 µm was formed by thermal spraying of anodized aluminum in the contact portion between the substrate holder I and the glass substrate. Then, a polysilicon film was deposited by supplying electric power of 3 kW, 5 kW or 8 kW (power density of about 590 mW/cm2) as in Example 1. As a result, no defect derived from abnormal discharge was observed in the polysilicon film deposited under any condition. Also, no substrate crack was generated. Example 7: A glass substrate sized at 910 mm x 910 mm and having a transparent conductive film formed on the surface thereof was prepared. As shown in FIG. 7, a separation groove 24 was formed in the transparent conductive film 22, and an insulating tape made of polyimide was arranged between the transparent conductive film 22 formed on the surface of the substrate 21 and the substrate holder 1. Then, a polysilicon film was deposited by supplying electric power of 8 kW (power density of about 590 mW/cm2) as in Example 1. As a result, no defect derived from abnormal discharge was observed in the polysilicon film. Also, no substrate crack was generated. Example 8: A glass substrate sized at 910 mm x 910 mm and having a transparent conductive film formed on the surface thereof was prepared. As shown in FIG. 10, a first separation groove 24 was formed in a width of about 100 µm by laser scribing in the transparent conductive film 22 formed on the surface of the glass substrate 21 such that the first separation groove 24 was positioned about 1 mm away from the inner edge of the substrate holder 1 when the glass substrate 21 was mounted to the substrate holder 1. Also, a second separation groove 28 was formed in a width of about 100 yum by laser scribing such that the second separation groove 28 was positioned on the inner region of the glass substrate than the first separation groove 24 and away from the first separation groove 24 by about 0.7 mm. As in Example 1, the probes of Megatester were brought into contact with Lhe transparent conductive film 22 such that the probes were positioned away from each other by a distance of about 8 mm with locating the first and second separation grooves 24 and 28 between the probes. When a voltage of 250 V was applied, it was possible to obtain insulation not lower than 0.5 MΩ . A polysilicon film was deposited by supplying electric power of 3 kW or 5 kW as in Example 1. No defect derived from abnormal discharge was observed in the polysilicon film deposited under any condition. Also, no substrate crack was generated. Further, no defect derived from abnormal discharge was observed in the polysilicon film deposited by supplying electric power of 8 kW. Next, the distance between the first separation groove 24 and the second separation groove 28 was set at about 0.5 mm or about 2 mm. When a voltage of 250 V was applied by using Megatester, insulation not lower than 0.5 MQ was obtained in each of these cases. Further, no defect derived from abnormal discharge was observed in the polysilicon film deposited by supplying electric power of 3 kW, 5 kW or 8 kW. Still further, abnormal discharge was not generated also in the case where the width of each of the separation grooves 24, 28 formed by laser scribing was set at about 40 µm or about 200 µm. Example 9: A glass substrate sized at 910 mm X 910 mm and having a transparent conductive film formed on the surface thereof was prepared. As shown in FIG. 11, a first separation groove 24 was formed in a width of about 100 µm by laser scribing in the transparent conductive film 22 formed on the surface of the glass substrate 21 such that the first separation groove 24 was positioned about 1 mm away from the inner edge of the substrate holder 1 when the glass substrate 21 was mounted to the substrate holder 1. Also, a second separation groove 28 was formed in a width of about 100 µm by laser scribing such that the second separation groove 28 was positioned on the inner region of the glass substrate than the first separation groove 24 and away from the first separation groove 24 by about 0.7 mm. Further, a third separation groove 29 was formed in a width of about 100 µm. by laser scribing such that the third separation groove 29 was positioned on the inner region of the glass substrate than the second separation groove 28 and away from the second separation groove 28 by about 0.7 mm. As in Example 1, the probes of Megatester were brought into contact with the transparent conductive film 22 such that the probes were positioned away from each other by a distance of about 8 mm with locating the first to third separation grooves 24, 28 and 29 between the probes. When a voltage of 250 V was applied, it was possible to obtain insulation not lower than 0.5 MΩ . A polysilicon film was deposited by supplying electric power of 3 kW, 5 kW or 8 kW as in Example 1. No defect derived from abnormal discharge was observed in the polysilicon film deposited under any condition. Next, each of the distance between the first separation groove 24 and the second separation groove 28 and the distance between the second separation groove 28 and the third separation groove 29 was set at about 0.5 mm or about 2 mm. When a voltage of 250 V was applied by using Megatester, insulation not lower than 0.5 MQ was obtained in each of these cases. Further, no defect derived from abnormal discharge was observed in the polysilicon film deposited by supplying electric power of 3 kW, 5 kW or 8 kW. Still further, insulation not lower than 0.5 MQ was obtained also in the case where the width of each of the separation grooves 24, 28 and 29 formed by laser scribing was set at about 40 µm or about 200 µm. Also, no defect derived from abnormal discharge was observed in the polysilicon film deposited by supplying electric power of 3 kW, 5 kW or 8 kW. Incidentally, in the examples shown in FIGS. 10 and 11, separation grooves were formed successively such that a separation groove was formed first in the outer region of the substrate and, then, another separation groove was formed in the inner region of the substrate. Alternatively, it is also possible to form separation grooves such that a separation groove is formed first in the inner region of the substrate and, then, another separation groove is formed in the outer region of the substrate. Industrial Applicability In the case of employing the method of the present invention, it is possible to improve uniformity of a silicon thin film in depositing the silicon thin film on a substrate having a large area under a high power density by using vertical-type plasma CVD apparatus. It is also possible to prevent a substrate crack so as to realize stable production. We claim: 1. A method of depositing a silicon thin film on a substrate (21) by using a plasma CVD apparatus, comprising holding the substrate (21) having an area not smaller than 1,200 CM2 and having a conductive film (22) formed thereon with a substrate holder (1) disposing the substrate(21) to face an electrode (4) characterized by depositing a silicon thin film on the substrate (21) under a power density of 100 mW/cm2or more, wherein the substrate holder (1) is electrically insulated from the conductive film (22) formed on the surface of the substrate (21), and wherein a separation groove (24) is formed in the conductive film (22) formed on the surface of the substrate (21) such that the separation groove (24) is positioned away from the inner edge of the substrate holder (1) by 0.1 to 30 mm. 2. A method of depositing silicon thin film on a substrate (21) by using a plasma CVD apparatus comprising: holding the substrate (21) having an area not smaller than 1,200 CM2 and having a conductive film (22) formed thereon with a substrate holder (1) disposing the substrate(21) to face an electrode (4) and depositing a silicon thin film on the substrate (21) under a power density of 100 mW/cm2 or more, wherein the substrate holder (1) is electrically insulated from the conductive film (22) formed on the surface of the substrate (21), and wherein a separation groove (24) in the conductive film (22) formed on the surface of the substrate (21) such that the separation groove (24) is positioned away from the inner edge of the substrate holder (1) by 0.1 to 30 mm, and an insulator is arranged between the conductive film (22) deposited on the surface of the substrate (21) and the substrate holder (1). 3. A method of depositing a silicon thin film on a substrate (21) by using a plasma CVD apparatus comprising: holding the substrate (21) having an area not smaller than 1,200 CM2 and having a conductive film (22) formed thereon with a substrate holder (1) disposing the substrate(21) to face an electrode (4) and depositing a silicon thin film on the substrate (21) under a power density of 100 mW/cm2 or more, wherein the substrate holder (1) is electrically insulated from the conductive film (22) formed on the surface of the substrate (21), and wherein a first separation groove (24) is formed in the conductive film (22) formed on the surface of the substrate (21) such that the separation groove (24) is positioned away from the inner edge of the substrate holder (1) by 0.1 to 30 mm, and a second separation groove (28) is formed in a region within 30 mm from the inner edge of the substrate holder (1) and away from the edge of the first separation groove (24) by 0.5 mm to 2 mm. 4. A method of depositing a silicon thin film on a substrate (21) by using a plasma CVD apparatus comprising: holding the substrate (21) having an area not smaller than 1,200 CM2 and having a conductive film (22) formed thereon with a substrate holder (1) disposing the substrate(21) to face an electrode (4) and depositing a silicon thin film on the substrate (21) under a power density of 100 mW/cm2 or more, wherein the substrate holder (1) is electrically insulated from the conductive film (22) formed on the surface of the substrate (21), and wherein a first separation groove (24) is formed in the conductive film (22) formed on the surface of the substrate (21) such that the separation groove (24) is positioned away from the inner edge of the substrate holder (1) by 0.1 to 30 mm, and a second separation groove (28) is formed in a region within 30 mm from the inner edge of the substrate holder (1) and away from the edge of the first separation groove (24) by 0.5 mm to 2 mm, and a third separation groove (29) is formed in a region within 30 mm from the inner edge of the substrate holder (1) and away from the edge of the second separation groove (28) by 0.5 to 2 mm.

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