embodiments of the present invention will be described in detail below with reference to the accompanying drawings . fig2 a - 2e are cross - sectional views of steps in a trench forming method according to the present embodiment . the process steps of forming a trench in an soi substrate 1 will be described below with reference to fig2 a - 2e . first , as illustrated in fig2 a , an soi substrate 1 is prepared which has a support substrate 10 made of a semiconductor such as silicon , an insulating layer 20 made of an insulating film such as an oxide film , and a semiconductor layer 30 made of a semiconductor such as silicon laid one over another in that order . next , a masking oxide film 40 , which serves as an etching mask , is formed on the surface of the semiconductor layer 30 by the cvd ( chemical vapor deposition ) method . the masking oxide film 40 is a silicon oxide film and has a thickness of about 500 nm , for example . next , a resist 50 , which serves as an etching mask , is formed by photolithography on the masking oxide film 40 except on a trench forming region 60 where a trench is to be formed in a later step . the resist 50 is , for example , a positive photosensitive resist cured by exposure to light and has a thickness of 10 μm , for example . next , as illustrated in fig2 b , a portion of the masking oxide film 40 corresponding to the trench forming region 60 is etched by dry etching with the resist 50 as a mask . the resist 50 is then removed . then , the semiconductor layer 30 is etched by dry etching with the masking oxide film 40 as a mask . the dry etching includes three etching steps . first , the first etching step will be described with reference to fig2 c . the first etching step uses , as an etching gas , a first mixed gas containing hydrogen bromide gas ( hbr ). the soi substrate 1 having the masking oxide film 40 formed thereon is secured to a substrate - securing table inside a not - shown chamber . the chamber is first evacuated , and the first mixed gas is then introduced into the chamber at a gas flow rate of , for example , 30 sccm for hbr , so that the chamber internal pressure is at 25 mt , for example . the surface of the semiconductor layer 30 is then etched with the masking oxide film 40 as a mask , by gas plasma gp 1 of the first mixed gas created by a high - frequency electric field formed between the substrate - securing table and a not - shown electrode disposed opposite the table . at this time , the layer 30 is etched down by a thickness d 1 equal to , e . g ., about 10 % of the distance from the surface of the semiconductor layer 30 to the insulating layer 20 ( which distance is hereinafter referred to as a trench depth ). note that the first mixed gas may be a gas consisting only of hydrogen bromide gas ( hbr ). instead , the first mixed gas may be a mixed gas containing hydrogen bromide gas ( hbr ) as its main component and also containing a slight amount of other gas such as fluorine - containing gas . in this case , the flow rate of the other gas is , for example , within a range of 1 / 20 to 1 / 10 , preferably 1 / 50 or less , of the flow rate of hydrogen bromide gas ( hbr ). even in cases where the first mixed gas contains fluorine - containing gas and oxygen gas ( o 2 ) as the other gas , the flow rate thereof ( or the content in the mixed gas ; the same applies hereinafter ) is , as a matter of course , smaller than the flow rate of fluorine - containing gas and oxygen gas ( o 2 ) contained in a second mixed gas in the subsequent second etching step . in this case , the flow rate of fluorine - containing gas and oxygen gas ( o 2 ) in the first mixed gas is , for example , within a range of 1 / 20 to 1 / 10 , preferably 1 / 50 or less , of the flow rate of fluorine - containing gas and oxygen gas ( o 2 ) in the second mixed gas . next , the second etching step will be described with reference to fig2 d . the second etching step uses , as an etching gas , the second mixed gas containing fluorine - containing gas ( such as sf 6 ) and oxygen gas ( o 2 ). the first mixed gas is exhausted from the not - shown chamber , and the second mixed gas is then introduced into the chamber at a gas flow rate of , for example , 30 sccm for sf 6 and 30 sccm for o 2 , so that the chamber internal pressure is at 25 mt , for example . the semiconductor layer 30 is then etched with the masking oxide film 40 as a mask , by gas plasma gp 2 of the second mixed gas created in the same way as above . at this time , the layer 30 is etched down by a thickness d 2 equal to , e . g ., about 80 % of the trench depth . note that the second mixed gas may be a mixed gas consisting only of fluorine - containing gas and oxygen gas ( o 2 ) and containing no other gas whatsoever such as hydrogen bromide gas ( hbr ). instead , the second mixed gas may be a mixed gas containing fluorine - containing gas and oxygen gas ( o 2 ) as its main components and also containing a slight amount of other gas such as hydrogen bromide gas ( hbr ). in this case , the flow rate of the other gas is , for example , within a range of 1 / 20 to 1 / 10 , preferably 1 / 50 or less , of the total flow rate of fluorine - containing gas and oxygen gas ( o 2 ). even in cases where the second mixed gas contains hydrogen bromide gas ( hbr ) as the other gas , the flow rate thereof is , as a matter of course , smaller than the flow rate of hydrogen bromide gas ( hbr ) contained in the first mixed gas . in this case , the flow rate of hydrogen bromide gas ( hbr ) in the second mixed gas is , for example , within a range of 1 / 20 to 1 / 10 , preferably 1 / 50 or less , of the flow rate of hydrogen bromide gas ( hbr ) in the first mixed gas . further , the flow rate of hydrogen bromide gas ( hbr ) in the second mixed gas is smaller than the flow rate of hydrogen bromide gas ( hbr ) contained in a third mixed gas in the subsequent third etching step . in this case , the flow rate of hydrogen bromide gas ( hbr ) in the second mixed gas is , for example , within a range of 1 / 20 to 1 / 10 , preferably 1 / 50 or less , of the flow rate of hydrogen bromide gas ( hbr ) in the third mixed gas . next , the third etching step will be described with reference to fig2 e . the third etching step uses , as an etching gas , the third mixed gas containing hydrogen bromide gas ( hbr ). the second mixed gas is exhausted from the not - shown chamber , and the third mixed gas is then introduced into the chamber at a gas flow rate of , for example , 30 sccm for hbr , so that the chamber internal pressure is at 25 mt , for example . the semiconductor layer 30 is then etched with the masking oxide film 40 as a mask , by gas plasma gp 3 of the third mixed gas created in the same way as above . at this time , the layer 30 is etched down by a thickness d 3 equal to , e . g ., about 10 % of the trench depth . note that the third mixed gas may be a gas consisting only of hydrogen bromide gas ( hbr ). instead , the third mixed gas may be a mixed gas containing hydrogen bromide gas ( hbr ) as its main component and also containing a slight amount of other gas such as fluorine - containing gas . in this case , the flow rate of the other gas is , for example , within a range of 1 / 20 to 1 / 10 , preferably 1 / 50 or less , of the flow rate of hydrogen bromide gas ( hbr ). even in cases where the third mixed gas contains fluorine - containing gas and oxygen gas ( o 2 ) as the other gas , the flow rate thereof is , as a matter of course , smaller than the flow rate of fluorine - containing gas and oxygen gas ( o 2 ) contained in the second mixed gas . in this case , the flow rate of fluorine - containing gas and oxygen gas ( o 2 ) in the third mixed gas is , for example , within a range of 1 / 20 to 1 / 10 , preferably 1 / 50 or less , of the flow rate of fluorine - containing gas and oxygen gas ( o 2 ) in the second mixed gas . further , it is desirable to over - etch the surface of the insulating layer 20 down by a thickness equal to , e . g ., about 10 % of the trench depth , to assure the insulating layer 20 being exposed . by undergoing the above process steps , the trench 70 is formed . fig3 a is a cross - sectional photographic image of the trench 70 formed by the trench forming method of the present embodiment . the trench depth is about 10 μm , and the trench width is about 1 . 8 μm . the trench 70 that reaches the insulating layer 20 is formed by etching the semiconductor layer 30 down by about 1 μm from the surface in the first etching step , then etching the layer 30 down by about 8 μm in the second etching step , and finally etching the layer 30 down by about 1 μm in the third etching step . fig3 b is a cross - sectional photographic image showing , in enlarged view , a portion 111 of a boundary 110 between the masking oxide film 40 and the semiconductor layer 30 . no gouge due to etching occurs at the boundary 110 . fig3 c is a cross - sectional photographic image showing , in enlarged view , a portion 121 of a boundary 120 between the insulating layer 20 and the semiconductor layer 30 . no gouge due to etching occurs at the boundary 120 either . according to the trench forming method of the present embodiment as described above , in initial etching of the surface of the semiconductor layer , the etching is conducted using a mixed gas containing hydrogen bromide gas ( hbr ) ( the first etching step ). it is said that etching conducted using a mixed gas containing hydrogen bromide gas ( hbr ) is more likely to create so - called deposits ( sediments ) as compared to etching conducted using a mixed gas containing fluorine - containing gas ( such as sf 6 ) and oxygen gas ( o 2 ) but not hydrogen bromide gas ( hbr ). thus , etching conducted using a mixed gas containing hydrogen bromide gas ( hbr ) does not give rise to gouges near the trench opening because the deposits adhering to the inner wall of the trench produce effects such as preventing the boundary between the masking oxide film and the semiconductor layer from being etched sideways . further , according to the trench forming method of the present embodiment , the surface of the semiconductor layer is first etched down by a thickness equal to about 10 % of the entire trench depth , and then the semiconductor layer is etched down by a thickness equal to about 80 % of the trench depth using a mixed gas containing fluorine - containing gas ( such as sf 6 ) and oxygen gas ( o 2 ) but not hydrogen bromide gas ( hbr ) ( in the second etching step ). etching conducted using a mixed gas containing hydrogen bromide gas ( hbr ) tends to create deposits . therefore , if etching were conducted using this mixed gas down to the insulating layer , a so - called tapered trench that gradually narrows would be formed . in contrast , by etching the semiconductor layer down by a thickness equal to about 80 % of the entire trench depth using a mixed gas containing fluorine - containing gas ( such as sf 6 ) and oxygen gas ( o 2 ) but not hydrogen bromide gas ( hbr ) as in the present embodiment , a trench having vertical side walls can be formed in a relatively short time . further , according to the trench forming method of the present embodiment , after etching the semiconductor layer down to about 90 % of the entire trench depth from the surface in the first and second etching steps , the layer is etched down by the remaining 10 % of the depth using a mixed gas containing hydrogen bromide gas ( hbr ) ( the third etching step ). etching conducted using a mixed gas containing hydrogen bromide gas ( hbr ) does not give rise to gouges at the bottom of the trench because the deposits adhering to the inner wall of the trench produce effects such as preventing the boundary between the insulating layer and the semiconductor layer from being etched sideways . as described above , according to the trench forming method of the present embodiment , a mixed gas containing hydrogen bromide gas ( hbr ) is used as the etching gas when etching parts of the semiconductor layer near its boundary with the masking oxide film and near its boundary with the insulating layer , whereas a mixed gas containing fluorine - containing gas ( such as sf 6 ) and oxygen gas ( o 2 ) is used as the etching gas when etching the intermediate part of the semiconductor layer . by this means , without gouges being created at the boundary between the masking oxide film and the semiconductor layer and at the boundary between the insulating layer and the semiconductor layer , a trench having vertical side walls can be formed . fig4 a - 4c are cross - sectional views of steps in a trench forming method according to the present embodiment . the steps of forming a trench in an soi substrate 1 will be described below with reference to fig4 a - 4c . note that the steps up to etching the masking oxide film 40 are the same as those in the first embodiment ( fig2 a and 2b ). after etching the masking oxide film 40 , the semiconductor layer 30 is etched by dry etching with the masking oxide film 40 as a mask . the dry etching includes three etching steps . first , the first etching step will be described with reference to fig4 a . the first etching step uses , as an etching gas , a first mixed gas containing chlorine gas ( cl 2 ). note that the first mixed gas contains no hydrogen bromide gas ( hbr ). the soi substrate 1 having the masking oxide film 40 formed thereon is secured to a substrate - securing table inside a not - shown chamber . the chamber is first evacuated , and the first mixed gas is then introduced into the chamber at a gas flow rate of , for example , 30 sccm for cl 2 , so that the chamber internal pressure is at 25 mt , for example . the surface of the semiconductor layer 30 is then etched with the masking oxide film 40 as a mask , by gas plasma gp 1 of the first mixed gas created by a high - frequency electric field formed between the substrate - securing table and a not - shown electrode disposed opposite the table . at this time , the layer 30 is etched down by a thickness d 1 equal to , e . g ., about 10 % of the distance from the surface of the semiconductor layer 30 to the insulating layer 20 ( which distance is hereinafter referred to as a trench depth ). note that the first mixed gas may be a gas consisting only of chlorine gas ( cl 2 ). instead , the first mixed gas may be a mixed gas containing chlorine gas ( cl 2 ) as its main component and also containing a slight amount of other gas such as fluorine - containing gas . in this case , the flow rate of the other gas is , for example , within a range of 1 / 20 to 1 / 10 , preferably 1 / 50 or less , of the flow rate of chlorine gas ( cl 2 ). even in cases where the first mixed gas contains fluorine - containing gas and oxygen gas ( o 2 ) as the other gas , the flow rate thereof is , as a matter of course , smaller than the flow rate of fluorine - containing gas and oxygen gas ( o 2 ) contained in a second mixed gas in the subsequent second etching step . in this case , the flow rate of fluorine - containing gas and oxygen gas ( o 2 ) in the first mixed gas is , for example , within a range of 1 / 20 to 1 / 10 , preferably 1 / 50 or less , of the flow rate of fluorine - containing gas and oxygen gas ( o 2 ) in the second mixed gas . next , the second etching step will be described with reference to fig4 b . the second etching step uses , as an etching gas , a second mixed gas containing fluorine - containing gas ( such as sf 6 ) and oxygen gas ( o 2 ). note that the second mixed gas contains no hydrogen bromide gas ( hbr ). the first mixed gas is evacuated from the not - shown chamber , and the second mixed gas is then introduced into the chamber at a gas flow rate of , for example , 30 sccm for sf 6 and 30 sccm for o 2 , so that the chamber internal pressure is at 25 mt , for example . the semiconductor layer 30 is then etched with the masking oxide film 40 as a mask , by gas plasma gp 2 of the second mixed gas created in the same way as above . at this time , the layer 30 is etched down by a thickness d 2 equal to , e . g ., about 80 % of the trench depth . note that the second mixed gas may be a mixed gas consisting only of fluorine - containing gas ( such as sf 6 ) and oxygen gas ( o 2 ) and containing no other gas whatsoever such as chlorine gas ( cl 2 ). instead , the second mixed gas may be a mixed gas containing fluorine - containing gas ( such as sf 6 ) and oxygen gas ( o 2 ) as its main components and also containing a slight amount of other gas such as chlorine gas ( cl 2 ). in this case , the flow rate of the other gas is , for example , within a range of 1 / 20 to 1 / 10 , preferably 1 / 50 or less , of the total flow rate of fluorine - containing gas ( such as sf 6 ) and oxygen gas ( o 2 ). even in cases where the second mixed gas contains chlorine gas ( cl 2 ) as the other gas , the flow rate thereof is , as a matter of course , smaller than the flow rate of chlorine gas ( cl 2 ) contained in the first mixed gas . in this case , the flow rate of chlorine gas ( cl 2 ) in the second mixed gas is , for example , within a range of 1 / 20 to 1 / 10 , preferably 1 / 50 or less , of the flow rate of chlorine gas ( cl 2 ) in the first mixed gas . further , the flow rate of chlorine gas ( cl 2 ) in the second mixed gas is smaller than the flow rate of chlorine gas ( cl 2 ) contained in a third mixed gas in the subsequent third etching step . in this case , the flow rate of chlorine gas ( cl 2 ) in the second mixed gas is , for example , within a range of 1 / 20 to 1 / 10 , preferably 1 / 50 or less , of the flow rate of chlorine gas ( cl 2 ) in the third mixed gas . next , the third etching step will be described with reference to fig4 c . the third etching step uses , as an etching gas , a third mixed gas containing chlorine gas ( cl 2 ). note that the third mixed gas contains no hydrogen bromide gas ( hbr ). the second mixed gas is evacuated from the not - shown chamber , and the third mixed gas is then introduced into the chamber at a gas flow rate of , for example , 30 sccm for cl 2 , so that the chamber internal pressure is at 25 mt , for example . the semiconductor layer 30 is then etched with the masking oxide film 40 as a mask , by gas plasma gp 3 of the third mixed gas created in the same way as above . at this time , the layer 30 is etched down by a thickness d 3 equal to , e . g ., about 10 % of the trench depth . note that the third mixed gas may be a gas consisting only of chlorine gas ( cl 2 ). instead , the third mixed gas may be a mixed gas containing chlorine gas ( cl 2 ) as its main component and also containing a slight amount of other gas such as fluorine - containing gas . in this case , the flow rate of the other gas is , for example , within a range of 1 / 20 to 1 / 10 , preferably 1 / 50 or less , of the flow rate of chlorine gas ( cl 2 ). even in cases where the third mixed gas contains fluorine - containing gas and oxygen gas ( o 2 ) as the other gas , the flow rate thereof is , as a matter of course , smaller than the flow rate of fluorine - containing gas and oxygen gas ( o 2 ) contained in the second mixed gas . in this case , the flow , rate of fluorine - containing gas and oxygen gas ( o 2 ) in the third mixed gas is , for example , within a range of 1 / 20 to 1 / 10 , preferably 1 / 50 or less , of the flow rate of fluorine - containing gas and oxygen gas ( o 2 ) in the second mixed gas . further , it is desirable to over - etch the surface of the insulating layer 20 down by a thickness equal to , e . g ., about 10 % of the trench depth , to assure the insulating layer 20 being exposed . by undergoing the above process steps , the trench 70 is formed . fig5 a is a cross - sectional photographic image of the trench 70 formed by the trench forming method of the present embodiment . the trench depth is about 10 μm , and the trench width is about 1 . 8 μm . the trench 70 that reaches the insulating layer 20 is formed by etching the semiconductor layer 30 down by about 1 μm from the surface in the first etching step , then etching the layer 30 down by about 8 μm in the second etching step , and finally etching the layer 30 down by about 1 μm in the third etching step . fig5 b is a cross - sectional photographic image showing , in enlarged view , a portion 111 of a boundary 110 between the masking oxide film 40 and the semiconductor layer 30 . no gouge due to etching occurs at the boundary 110 . fig5 c is a cross - sectional photographic image showing , in enlarged view , a portion 121 of a boundary 120 between the insulating layer 20 and the semiconductor layer 30 . no gouge due to etching occurs at the boundary 120 either . according to the trench forming method of the present embodiment as described above , in initial etching of the surface of the semiconductor layer , the etching is conducted using a mixed gas containing chlorine gas ( cl 2 ) ( the first etching step ). it is said that etching conducted using a mixed gas containing chlorine gas ( cl 2 ) is likely to create deposits ( sediments ). thus , etching conducted using a mixed gas containing chlorine gas ( cl 2 ) does not give rise to gouges near the trench opening because the deposits adhering to the inner wall of the trench produce effects such as preventing the boundary between the masking oxide film and the semiconductor layer from being etched sideways . further , according to the trench forming method of the present embodiment , the surface of the semiconductor layer is first etched down by a thickness equal to about 10 % of the entire trench depth , and then the semiconductor layer is etched down by a thickness equal to about 80 % of the trench depth using a mixed gas containing fluorine - containing gas ( such as sf 6 ) and oxygen gas ( o 2 ) but not chlorine gas ( cl 2 ) ( in the second etching step ). because the semiconductor layer is etched using this mixed gas which is considered less prone to form deposits , a trench having vertical side walls can be formed in a relatively short time . further , according to the trench forming method of the present embodiment , after etching the semiconductor layer down to about 90 % of the entire trench depth from the surface in the first and second etching steps , the layer is etched down by the remaining 10 % of the depth using a mixed gas containing chlorine gas ( cl 2 ) ( the third etching step ). etching conducted using a mixed gas containing chlorine gas ( cl 2 ) does not give rise to gouges at the bottom of the trench because the deposits adhering to the inner wall of the trench produce effects such as preventing the boundary between the insulating layer and the semiconductor layer from being etched sideways . note that in the first and third etching steps of the present embodiment , since containing chlorine gas ( cl 2 ), the mixed gas does not need to contain hydrogen bromide gas ( hbr ). further , in the second etching step of the present embodiment , the mixed gas usually contains no hydrogen bromide gas ( hbr ) in order to form a trench having vertical side walls . as described above , according to the trench forming method of the present embodiment , a mixed gas containing chlorine gas ( cl 2 ) is used as the etching gas when etching parts of the semiconductor layer near its boundary with the masking oxide film and near its the boundary with the insulating layer , whereas a mixed gas containing fluorine - containing gas ( such as sf 6 ) and oxygen gas ( o 2 ) is used as the etching gas when etching the intermediate part of the semiconductor layer . by this means , without gouges being created at the boundary between the masking oxide film and the semiconductor layer and at the boundary between the insulating layer and the semiconductor layer , a trench having vertical side walls can be formed . further , using chlorine gas ( cl 2 ) having a high etching rate as the etching gas also produces the effect of shortening the etching process time as compared to cases where hydrogen bromide gas ( hbr ) is used . the first and second embodiments employ three etching steps to form a trench . however , a trench may be formed using four or more etching steps . for example , in order to keep the side walls of the intermediate portion of the trench from expanding outward too much in cases where the trench depth is large , the trench may be formed by first etching down to 10 % of the trench depth from the surface using a mixed gas containing hydrogen bromide gas ( hbr ) in a first etching step , then etching down to 40 % of the trench depth from the surface using a mixed gas containing fluorine - containing gas ( such as sf 6 ) and oxygen gas ( o 2 ) in a second etching step , then etching down to 50 % of the trench depth using a mixed gas containing hydrogen bromide gas ( hbr ) in a third etching step , then etching down to 90 % of the trench depth using a mixed gas containing fluorine - containing gas ( such as sf 6 ) and oxygen gas ( o 2 ) in a fourth etching step , and finally etching down to 100 % of the trench depth using a mixed gas containing hydrogen bromide gas ( hbr ). this application is based on japanese patent application no . 2009 - 037807 which is herein incorporated by reference .