Abstract:
A trench forming method for forming trenches without creating gouges at the boundary between a masking oxide film and a semiconductor layer and at the boundary between an oxide film insulating layer and the semiconductor layer, includes at least three etching steps each using, as the etching gas, one of at least two types of etching gases that respectively contain different components.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method of forming trenches in an SOI substrate. 
     2. Description of the Related Art 
     Methods of forming trenches are known in which trenches are formed by anisotropic dry etching in an SOI (Silicon On Insulator) substrate in which an oxide film insulating layer and a silicon semiconductor layer are laid one over another on a silicon support substrate. This etching is performed by etching the silicon semiconductor substrate from its surface through the plasma action of an etching gas. A mixed gas containing fluorine-containing gas and oxygen gas, for example, is used as the etching gas. For example, Japanese Patent Kokai No. 2006-93269 (Patent literature 1) discloses an etching method including a first etching step of forming a tapered opening in a silicon semiconductor layer using a first etching gas containing fluorine-containing gas and O 2  and a second etching step of performing etching through the tapered opening using a second etching gas containing fluorine-containing gas, O 2 , and HBr. 
     SUMMARY OF THE INVENTION 
     Using a mixed gas containing fluorine-containing gas (SF6) and oxygen gas (O 2 ) as the etching gas for etching the surface of the silicon semiconductor layer, however, causes the problem that so-called “gouges” are created by notching at the boundary between a masking oxide film, which serves as an etching mask, and the silicon semiconductor layer.  FIG. 1A  is a cross-sectional photographic image of a trench  70   b  formed using a mixed gas containing fluorine-containing gas (SF6) and oxygen gas (O 2 ) as the etching gas. The figure shows results of etching at a gas flow rate of 30 sccm (standard cc/min) for SF6 and 30 sccm for O 2  and a chamber internal pressure of 25 mT. The trench depth is about 10 μm and the etching time is about 260 seconds. 
       FIG. 1B  is a cross-sectional photographic image showing, in enlarged view, a portion  111   b  of a boundary  110   b  between a masking oxide film  40   b  and a silicon semiconductor layer  30   b . A gouge  112  due to etching occurs at the boundary  110   b .  FIG. 1C  is a cross-sectional photographic image showing, in enlarged view, a portion  121   b  of a boundary  120   b  between an oxide film insulating layer  20   b  and the silicon semiconductor layer  30   b . A gouge  122  due to etching occurs at the boundary  120   b.    
     In applications such as high power MOS, an insulating film is formed inside the trench  70   b . However, there is the problem that the presence of gouges  112 ,  122  decreases the breakdown voltage. Further, in cases where electrodes are formed on the insulating film formed on the inner wall of the trench  70   b , there is the problem that the gouge  112  or  122  is likely to cause a short. 
     The present invention was made in view of the above problems and an object thereof is to provide a trench forming method of forming trenches without creating gouges at the boundary between a masking oxide film and a semiconductor layer or at the boundary between an oxide film insulating layer and the semiconductor layer. 
     A trench forming method of the present invention includes a substrate preparation step of preparing an SOI substrate in which an insulating layer and a silicon semiconductor layer are laid one over another on a support substrate; an oxide film forming step of forming an oxide film on a surface of the silicon semiconductor layer; an oxide film removing step of removing a portion of the oxide film; and an etching step of etching, using an etching gas, the surface of the silicon semiconductor layer with the oxide film as a mask to form a trench that reaches the insulating layer. The above etching step includes at least three etching steps each using, as the etching gas, one of at least two types of etching gases that respectively contain different components. 
     The trench forming method of the present invention can form trenches without creating gouges at the boundary between a masking oxide film layer and a semiconductor layer or at the boundary between an oxide film insulating layer and the semiconductor layer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A-1C  are cross-sectional photographic images of a trench formed by a conventional trench forming method; 
         FIGS. 2A-2E  are cross-sectional views of steps in a trench forming method according to a first embodiment; 
         FIGS. 3A-3C  are cross-sectional photographic images of a trench formed by the trench forming method according to the first embodiment; 
         FIGS. 4A-4C  are cross-sectional views of steps in a trench forming method according to a second embodiment; and 
         FIGS. 5A-5C  are cross-sectional photographic images of a trench formed by the trench forming method according to the second embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of the present invention will be described in detail below with reference to the accompanying drawings. 
     First Embodiment 
       FIGS. 2A-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  FIGS. 2A-2E . 
     First, as illustrated in  FIG. 2A , 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  FIG. 2B , 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  FIG. 2C . 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  FIG. 2D . 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  FIG. 2E . 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. 
       FIG. 3A  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. 
       FIG. 3B  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 .  FIG. 3C  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. 
     Second Embodiment 
       FIGS. 4A-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  FIGS. 4A-4C . Note that the steps up to etching the masking oxide film  40  are the same as those in the first embodiment ( FIGS. 2A 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  FIG. 4A . 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  FIG. 4B . 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  FIG. 4C . 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. 
       FIG. 5A  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. 
       FIG. 5B  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 .  FIG. 5C  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.