Abstract:
A first trench having a first width and a second trench having a width which is smaller than the first width are formed on a major surface of a semiconductor substrate. A first isolation insulator having an outer side wall is formed to fill up the first trench. A second isolation insulator having an outer side wall is formed to fill up the second trench. The first isolation insulator includes a side wall insulator film forming the outer side wall and an internal insulator film enclosed with the side wall insulator film for filling up the first trench. The second isolation insulator includes an internal insulator film forming the outer side wall for filling up the second trench. Thus obtained are a highly reliable semiconductor device comprising isolation insulators having different widths and a method of fabricating the same.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a semiconductor device and a method of fabricating the same, and more specifically, it relates to a semiconductor device comprising trench isolation insulator films and a method of fabricating the same. 
     2. Description of the Prior Art 
     Following refinement and high integration of a semiconductor device, trench isolation oxide films are generally employed for electrically isolating elements which are formed on a major surface of a semiconductor substrate from each other. FIG. 26 is a sectional view for illustrating conventional trench isolation oxide films. With reference to FIG. 26, the conventional trench isolation oxide films are now described. 
     Referring to FIG. 26, trenches  104   a  to  104   c  are formed on a major surface of a semiconductor substrate  101 . Thermal oxide films  105   a  to  105   c  are formed on surface portions of the semiconductor substrate  101  located in the trenches  104   a  to  104   c  respectively. Oxide films  113   a  to  113   c  serving as isolation insulator films are formed in the trenches  104   a  to  104   c  by chemical vapor deposition (hereinafter referred to as CVD) respectively. The widths W 1  of the oxide films  113   a  to  113   c  and the intervals therebetween (the widths of element forming regions on the major surface of the semiconductor substrate  101 ) are set at various values in a single semiconductor device. 
     FIGS. 27 to  30  are sectional views for illustrating steps of forming the conventional trench isolation oxide films  113   a  to  113   c  shown in FIG.  26 . With reference to FIGS. 27 to  30 , the steps of forming the conventional trench isolation oxide films  113   a  to  113   c  are now described. 
     First, a thermal oxide film (not shown) is formed on the major surface of the semiconductor substrate  101  (see FIG.  27 ). A silicon nitride film (not shown) is formed on this thermal oxide film. A resist pattern (not shown) is formed on this silicon nitride film and thereafter employed as a mask for partially removing the silicon nitride film and the thermal oxide film by anisotropic etching. Thereafter the resist pattern is removed. Thus, thermal oxide films  102   a  to  102   d  and silicon nitride films  103   a  to  103   d  are formed on the major surface of the semiconductor substrate  101 , as shown in FIG.  27 . 
     Then, the silicon nitride films  103   a  to  103   d  are employed as masks for partially removing the semiconductor substrate  101  by anisotropic etching, thereby forming the trenches  104   a  to  104   c  as shown in FIG.  28 . These trenches  104   a  to  104   c  are set at a depth capable of electrically isolating elements in the element forming regions from each other. For example, it is inferred that the depth of such trenches is not more than about 0.35 μm in a DRAM (dynamic random access memory) having a storage capacity of at least  1  gigabyte (G). 
     After formation of the trenches  104   a  to  104   c , the surface portions of the semiconductor substrate  101  located in the trenches  104   a  to  104   c  may have defects resulting from the aforementioned anisotropic etching. In order to remove such defects, the surface portions of the semiconductor substrate  101  located in the trenches  104   a  to  104   c  may be thermally oxidized and thereafter partially removed with an HF solution. Alternatively, the surface portions of the semiconductor substrate  101  located in the trenches  104   a  to  104   c  may be removed by isotropic etching, or the semiconductor substrate  101  may be heat-treated, in order to remove the aforementioned defects. 
     Then, the surface portions of the semiconductor substrate  101  located in the trenches  104   a  to  104   c  are thermally oxidized, thereby forming the thermal oxide films  105   a  to  105   c , as shown in FIG.  29 . Then, an oxide film  113  is deposited on the silicon nitride films  103   a  to  103   d  and in the trenches  104   a  to  104   c  by CVD. 
     Then, the oxide film  113  is partially removed by anisotropic etching, thereby obtaining a structure shown in FIG.  30 . 
     Then, the silicon nitride films  103   a  to  103   d  and the thermal oxide films  102   a  to  102   d  are removed from the major surface of the semiconductor substrate  101  by etching, thereby obtaining the structure shown in FIG.  26 . The conventional trench isolation oxide films  113   a  to  113   c  are formed in this method. 
     Higher integration and refinement are increasingly required to semiconductor devices in recent years, particularly in semiconductor memory devices such as DRAMs. Therefore, the widths W 1  of the trench isolation oxide films  113   a ,  113   b  and  113   c  shown in FIG. 26 must be further reduced. For example, it is predicted that the widths W 1  of such trench isolation oxide films  113   a ,  113   b  and  113   c  are about 0.1 to 0.2 μm in a 1-gigabyte DRAM. 
     When the widths W 1  of the trench isolation oxide films  113   a ,  113   b  and  113   c  are reduced, however, a part of the oxide film  113  formed in the trench  104   c  and on the silicon nitride films  103   c  and  103   d  by CVD may come into contact with an upper portion of the trench  104   c  to close its opening and define a void  114  in the trench  104   c  before filling up the same, as shown in FIG.  31 . Such a void  114  formed in the trench isolation oxide film  104   c  deteriorates the isolation property thereof, leading to insufficient electric isolation between the elements formed on the surface of the semiconductor substrate  101 . This results in a problem such as a malfunction of the semiconductor device. 
     In order to fill up narrow trenches with an oxide film while forming no void, proposed is a method of forming trench isolation oxide films through deposition such as HDP-CVD (high density plasma CVD) simultaneously progressing deposition and etching of an oxide film. FIG. 32 is a sectional view showing trench isolation oxide films formed by HDP-CVD. Referring to FIG. 32, trenches  124   a  to  124   c  are formed on a major surface of a semiconductor substrate  101 . Thermal oxide films  105   a  to  105   c  are formed on surface portions of the semiconductor substrate  101  located in the trenches  124   a  to  124   c  respectively. Silicon oxide films  115   a  to  115   c  are formed by HDP-CVD, to fill up the trenches  124   a  to  124   c  respectively. The width W 2  of the trenches  124   a  to  124   c  is set at 0.25 μm, and the interval W 3  between the trenches  124   a  to  124   c  is set at 0.55 μm. 
     FIG. 33 is a sectional view for illustrating a method of forming the trench isolation oxide films shown in FIG. 32 by HDP-CVD. With reference to FIG. 33, the method of forming the trench isolation oxide films by HDP-CVD is now described. 
     First, thermal oxide films  102   a  to  102   d  (see FIG. 33) and silicon nitride films  103   a  to  103   d  (see FIG. 33) are formed on the major surface of the semiconductor substrate  101  (see FIG.  33 ), and the trenches  124   a  to  124   c  are formed on the semiconductor substrate  101 , through steps similar to those shown in FIGS. 27 and 28. 
     Then, the thermal oxide films  105   a  to  105   c  (see FIG. 33) are formed on the surface portions of the semiconductor substrate  101  located in the trenches  124   a  to  124   c  respectively. Then, a silicon oxide film  115  is formed in the trenches  124   a  to  124   c  and on the silicon nitride films  103   a  to  103   d  by HDP-CVD, as shown in FIG.  33 . 
     At this time, the silicon oxide film  115  is deposited in the trenches  124   a  to  124   c  and simultaneously partially sputter-etched on upper portions of the trenches  124   a  to  124   c . Therefore, the parts of the silicon oxide film  115  do not come into contact with the upper portions of the trenches  124   a  to  124   c  to close the openings thereof, dissimilarly to the general step of forming the silicon oxide film by the CVD. Parts of the silicon oxide film  115  deposited on the silicon nitride films  103   a  to  103   d  are formed to have side surfaces inclined at an angle of about 45°, since corner portions of the silicon oxide film  115  are sputter-etched on the upper portions of the trenches  124   a  to  124   c.    
     Thereafter the parts of the silicon oxide film  115  located on the silicon nitride films  103   a  to  103   d  are removed and the silicon nitride films  103   a  to  103   d  and the thermal oxide films  102   a  to  102   d  are removed from the semiconductor substrate  101  through a step similar to that shown in FIG. 30, thereby obtaining the structure shown in FIG.  32 . 
     Thus, trench isolation oxide films having a narrower width can be formed by HDP-CVD. However, the inventors have further studied and carried out steps of forming a plurality of trench isolation oxide films having different widths and separating from each other at different intervals under conditions closer to those for actual steps of fabricating a semiconductor device, to find out the following subject: 
     FIGS. 34 to  36  are sectional views for illustrating the steps of forming trench isolation oxide films carried out by the inventors. The steps of forming trench isolation oxide films carried out by the inventors are now described with reference to FIGS. 34 to  36 . 
     First, thermal oxide films  102   a  to  102   d  and silicon nitride films  103   a  to  103   d  are formed on a major surface of a semiconductor substrate  101  as shown in FIG. 28, through steps similar to those for forming the conventional trench isolation oxide films shown in FIGS. 27 and 28. The silicon nitride films  103   a  to  103   d  are employed as masks for partially removing the semiconductor substrate  101  by anisotropic etching, thereby forming trenches  104   a  to  104   c . The widths W 6  and W 7  (see FIG. 34) of the trenches  104   b  and  104   a  are set at 0.2 μm and 2.5 μm respectively. 
     Then, surface portions of the semiconductor substrate  101  located in the trenches  104   a  to  104   c  are thermally oxidized, thereby forming thermal oxide films  105   a  to  105   c  (see FIG. 34) respectively. Then, a silicon oxide film  115  (see FIG. 34) is deposited in the trenches  104   a  to  104   c  and on the silicon nitride films  103   a  to  103   d  by HDP-CVD, thereby obtaining the structure shown in FIG.  34 . 
     As hereinabove described, HDP-CVD is adapted to simultaneously progress deposition and removal of the silicon oxide film  115 . In this HDP-CVD, the parts of the silicon oxide film  115  removed by sputter etching are deposited on other regions again, to form the silicon oxide film  115 . In this HDP-CVD, the balance between the rates for depositing the oxide film  115  and removing the same by sputter etching can be changed by adjusting the type of gas or the pressure of the atmosphere. 
     Referring to FIG. 34, consider the case of depositing the silicon oxide film  115  in the trenches  104   a  and  104   b  having the different widths W 7  and W 6  respectively. The portion of the silicon oxide film  115  deposited on an upper portion of a side wall of the trench  104   b  having the relatively narrow width W 6 , i.e., a corner portion  119  of the silicon nitride film  103   c , is partially removed by sputter etching in HDP-CVD. The part of the silicon oxide film  115  removed by sputter etching is deposited in the vicinity of a corner portion  120  of the opposite silicon nitride film  103   b  again. 
     On the other hand, a part of the silicon oxide film  115  similarly removed by sputter etching from an upper portion of a side wall of the trench  104   a  having the large width W 7  may be deposited on an upper portion of an opposite side wall of the trench  104   a , while the amount of this deposition is smaller than that in the trench  104   b , due to the large width W 7  of the trench  104   a . Therefore, the conditions for HDP-CVD for filling up the trench  104   b  having the narrow width W 6  must be adjusted to increase the rate for partially removing the silicon oxide film  115  by sputter etching as compared with that for filling up the trench  104   a  having the large width W 7 , in order to prevent the silicon oxide film  115  from closing the opening in the upper portion of the trench  104   b.    
     Consequently, the silicon oxide film  115  can fill up the trench  104   b  having the relatively narrow width W 6  while forming no void. However, the oxide film  115  is excessively removed from the upper portion of the side wall of the trench  104   a  having the large width W 6  by sputter etching. Thus, not only parts of the silicon nitride films  103   a  and  103   b  and the thermal oxide films  102   a  and  102   b  employed as masks but a part of the semiconductor substrate  101  is finally removed from the upper portion of the side wall of the trench  104   a  by sputter etching, as shown in FIG.  34 . 
     Then, portions of the silicon oxide film  115  located on the silicon nitride films  103   a  to  103   d  are removed by etching or chemical mechanical polishing (CMP), as shown in FIG.  35 . 
     Then, the silicon nitride films  103   a  to  103   d  and the thermal oxide films  102   a  to  102   d  are removed from the semiconductor substrate  101  by etching, thereby obtaining a structure shown in FIG.  36 . In the step of depositing the silicon oxide film  115  by HDP-CVD shown in FIG. 34, the silicon nitride films  103   a  and  103   b  and the thermal oxide films  102   a  and  102   b  employed as masks and the semiconductor substrate  101  are partially removed from the trench  104   a  having the large width W 7 . Therefore, a trench isolation oxide film  115   a  has overhang portions  116   a  and  116   b , as shown in FIG.  36 . Consequently, the interval W 4  between this isolation oxide film  115   a  and an isolation oxide film  115   b  on the major surface of the semiconductor substrate  101  is smaller than the interval W 5  therebetween under the major surface of the semiconductor substrate  101 . 
     Due to such overhang portions  116   a  and  116   b  of the isolation oxide film  115   a , the thickness of a polysilicon film deposited for forming a wire  117  of polysilicon on the isolation oxide film  115   a  may be irregular in a portion  118  close to the overhang portion  116   b , as shown in FIG.  37 . In a photolithographic step for forming the wire  117 , further, the wire  117  cannot attain a designed shape but may be disconnected or shorted due to the presence of the overhang portions  116   a  and  116   b . Thus, the semiconductor device is disadvantageously reduced in reliability. Further, such a defect results in reduction of the yield of the semiconductor device. 
     When the silicon oxide film  115  is deposited by HD-CVD, the semiconductor substrate  101  is also partially removed from the upper portion of the side wall of the trench  104   a  by sputter etching, as shown in FIG.  34 . Thus, the semiconductor substrate  101  causes a defect due to such partial removal by sputter etching, to disadvantageously increase leakage currents from element forming regions. Such increase of the leakage currents causes deterioration of the electric characteristics of the semiconductor device, to result in reduction of the reliability of the semiconductor device. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a highly reliable semiconductor device comprising a plurality of isolation insulators having different widths and a method of fabricating the same. 
     In a semiconductor device according to a first aspect of the present invention, a first trench having a first width and a second trench having a second width which is narrower than the first width are formed on a major surface of a semiconductor substrate. A first isolation insulator having an outer side wall for isolating element forming regions from each other is formed to fill up the first trench. A second isolation insulator having an outer side wall for isolating element forming regions from each other is formed to fill up the second trench. The first isolation insulator includes a side wall insulator film forming the outer side wall and an internal insulator film, enclosed with the side wall insulator film, filling up the first trench. The second isolation insulator includes an internal insulator film forming the outer side wall and filling up the second trench. 
     The first isolation insulator comprises the side wall insulator film forming the outer side wall. When an upper portion of a side surface of the first trench is partially removed in case of forming the internal insulator film in a step of fabricating the semiconductor device, therefore, the side wall insulator film can be formed to have a thickness exceeding that of the removed part. Thus, the semiconductor substrate or the like can be prevented from being partially removed from the upper portion of the side wall of the first trench. Therefore, it is possible to prevent generation of a leakage current from any element forming region of the semiconductor substrate resulting from partial removal of the semiconductor substrate from the upper portion of the side wall of the first trench. Consequently, a semiconductor device having high reliability can be obtained. 
     Further, it is possible to prevent partial removal of the upper portion of the side wall of the first trench, thereby preventing formation of an overhang portion on an upper portion of the first isolation insulator. In case of forming a wire on the first isolation insulator, therefore, it is possible to prevent such a problem that the wire cannot attain a designed pattern but is disconnected or shorted in photolithography therefor. Consequently, a semiconductor device having high reliability can be obtained. 
     In the semiconductor device according to the first aspect of the present invention, the side wall insulator film may be made of a material having a thermal expansion coefficient which is substantially identical to that of the semiconductor substrate. 
     In case of performing heat treatment or the like in a step of fabricating the semiconductor device, therefore, it is possible to prevent generation of stress resulting from difference in thermal expansion between the semiconductor substrate and the side wall insulator film in a contact region therebetween. Thus, the semiconductor substrate can be prevented from a defect caused by such stress. Therefore, generation of a leakage current resulting from such a defect can be prevented in the semiconductor substrate. Consequently, a semiconductor device having high reliability can be obtained. 
     In the semiconductor device according to the first aspect of the present invention, the side wall insulator film may be a silicon nitride film, and the internal insulator films may be silicon oxide films. 
     In the semiconductor device according to the first aspect of the present invention, a covering film may be formed on the side wall insulator film. 
     Thus, a surface portion of the side wall insulator film can be prevented from damage caused by etching or the like in a step of fabricating the semiconductor device and can be prevented from fluctuation of its thickness. 
     In a method of fabricating a semiconductor device according to a second aspect of the present invention, a covering film is first formed on a major surface of a semiconductor substrate. This covering film is selectively removed, thereby exposing the major surface of the semiconductor substrate in element isolation regions. The semiconductor substrate is partially removed through the covering film serving as a mask, thereby forming a first trench and a second trench having a narrower width than that of the first trench. A side wall insulator film is formed on at least a side surface of the first trench. An insulator film for defining internal insulator films is formed on the side wall insulator film, in the first and second trenches and on the covering film. A part of the insulator film located on the covering film is removed. The covering film is removed. 
     Thus, the insulator film for defining the internal insulator films is formed on the side wall insulator film, in the first and second trenches and on the covering film. In case of simultaneously removing upper portions of side walls of the trenches in a step of forming the insulator film, therefore, it is possible to prevent partial removal of the covering film and the semiconductor substrate from the upper portions of the side walls of the trenches by forming the side wall insulator film in a thickness exceeding that of the removed parts. In an upper portion of the first trench, therefore, the width of an isolation insulator consisting of the side wall insulator film and the internal insulator film can be prevented from exceeding a designed value to form an overhang portion. In case of forming a wire or the like on the isolation insulator, therefore, it is possible to prevent such a problem that the wire cannot attain a designed pattern but is disconnected or shorted in photolithography therefor. Consequently, a semiconductor device having high reliability can be obtained. 
     Further, the semiconductor substrate can be prevented from partial removal as described above, whereby no leakage current is generated from any element forming region thereof. Consequently, a semiconductor device having high reliability can be obtained. 
     Further, the vertical positions of parts of the internal insulator films and the side wall insulator film exposed on the major surface of the semiconductor substrate can be adjusted by adjusting the thickness of the covering film. 
     In the method of fabricating a semiconductor device according to the second aspect of the present invention, the step of forming the insulator film for defining the internal insulator films on the side wall insulator film, in the first and second trenches and on the covering film may be carried out with a deposition method simultaneously progressing deposition and removal of the insulator film. 
     Therefore, it is possible to deposit the insulating film in a trench having a narrow width, which is hard to fill up in the conventional CVD, while removing a part of the insulator film deposited on the upper portion of this trench. Therefore, the insulator film can be prevented from closing the upper portion of the trench before filling up the same. Thus, the insulator film can fill up the trench having a narrow width while forming no void. Consequently, it is possible to form an isolation insulator having a narrow width, which is hard to form in the conventional CVD. 
     In the method of fabricating a semiconductor device according to the second aspect of the present invention, an insulator film for defining the side wall insulator film may be formed in the first and second trenches and on the covering film, and a protective film may be formed on the insulator film in a region located on the first trench, in the step of forming the side wall insulator film on at least the side surface of the first trench. The protective film may be employed as a mask for removing the insulator film from the second trench. Then, the protective film may be removed. 
     Thus, a surface portion of the insulator film located in the first trench is covered with the protective film, whereby the surface portion of the insulator film located in the first trench can be prevented from damage caused by etching or the like and can be prevented from fluctuation of its thickness when the insulator film is removed from the second trench through the protective film serving as a mask. 
     The surface portion of the insulator film is not damaged in the first trench, whereby the thickness of the insulator film may not be increased beyond a value necessary for preventing removal of the covering film or the semiconductor substrate on the upper portion of the side wall of the first trench. 
     In the method of fabricating a semiconductor device according to the second aspect of the present invention, the protective film may be prepared from a resist. 
     Thus, the fabrication step can be simplified as compared with the case of forming the protective film by a silicon nitride film or the like. In case of employing a silicon nitride film as the protective film, a resist pattern must be formed on the silicon nitride film in a region located on the first trench after forming the silicon nitride film on the insulator film, for removing a part of the silicon nitride film located on the second trench or the like by etching through the resist pattern serving as a mask. When the protective film is prepared from a resist, however, no step is required for partially removing the aforementioned silicon nitride film by etching. 
     In the method of fabricating a semiconductor device according to the second aspect of the present invention, an insulator film for defining the side wall insulator film may be formed in the first and second trenches and on the covering film so that the thickness of the portion located in the first trench is larger than half the width of the second trench and a first part of the insulator film may be removed by isotropic etching to leave a second part of the insulator film on at least the side surface of the first trench, thereby forming the side wall insulator film in the step of forming the side wall insulator film on at least the side surface of the first trench. 
     Thus, the side wall insulator film can be formed on at least the side surface of the first trench without forming a protective film on the insulator film in the region located on the first trench. Consequently, the fabrication steps can be simplified. 
     In the method of fabricating a semiconductor device according to the second aspect of the present invention, the insulator film may be left on the covering film in the step of forming the side wall insulator film by removing the first part of the insulator film by isotropic etching to leave the second part on at least the side surface of the first trench. 
     Thus, it is possible to increase precisional allowance for a position for stopping the step of removing the insulator film located on the covering film. 
     In the method of fabricating a semiconductor device according to the second aspect of the present invention, the side wall insulator film may be made of a material having a thermal expansion coefficient which is substantially identical to that of the semiconductor substrate. 
     Thus, it is possible to prevent generation of stress resulting from difference in thermal expansion between the semiconductor substrate and the side wall insulator film in a contact region therebetween in case of performing heat treatment or the like in any fabrication step for the semiconductor device. Further, the semiconductor substrate can be prevented from a defect caused by such stress. Thus, it is possible to prevent generation of a leakage current resulting from such a defect in the semiconductor device. Consequently, a semiconductor device having high reliability can be obtained. 
     The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional view for illustrating trench isolation oxide films in a semiconductor device according to an embodiment 1 of the present invention; 
     FIGS. 2 to  9  are sectional views for illustrating first to eighth steps of forming the trench isolation oxide films in a method of fabricating the semiconductor device according to the embodiment 1 of the present invention shown in FIG. 1; 
     FIG. 10 is a sectional view for illustrating trench isolation oxide films in a semiconductor device according to an embodiment 2 of the present invention; 
     FIG. 11 is a sectional view for illustrating a step of forming trench isolation oxide films in a method of fabricating a semiconductor device according to an embodiment 3 of the present invention; 
     FIGS. 12 to  15  are sectional views for illustrating first to fourth steps of forming trench isolation oxide films in a method of fabricating a semiconductor device according to an embodiment 4 of the present invention; 
     FIG. 16 is a sectional view for illustrating trench isolation oxide films in a semiconductor device according to an embodiment 5 of the present invention; 
     FIGS. 17 to  20  are sectional views for illustrating first to fourth steps of forming the trench isolation oxide films in a method of fabricating the semiconductor device according to the embodiment 5 of the present invention shown in FIG. 16; 
     FIG. 21 is a sectional view for illustrating trench isolation oxide films in a semiconductor device according to an embodiment 6 of the present invention; 
     FIGS. 22 to  25  are sectional views for illustrating first to fourth steps of forming the trench isolation oxide films in a method of fabricating the semiconductor device according to the embodiment 6 of the present invention shown in FIG. 21; 
     FIG. 26 is a sectional view for illustrating conventional trench isolation oxide films; 
     FIGS. 27 to  30  are sectional views for illustrating first to fourth steps of forming the conventional trench isolation oxide films shown in FIG. 26; 
     FIG. 31 is a sectional view for illustrating a void formed in a trench in the step shown in FIG. 29; 
     FIG. 32 is a sectional view for illustrating trench isolation oxide films formed by HDP-CVD; 
     FIG. 33 is a sectional view for illustrating a method of forming the trench isolation oxide films shown in FIG. 32 by HDP-CVD; 
     FIGS. 34 to  36  are sectional views for illustrating first to third steps of forming trench isolation oxide films carried out by the inventors; and 
     FIG. 37 is a model diagram for illustrating a wire formed on a trench isolation oxide film. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the present invention are now described with reference to the drawings. 
     (Embodiment 1) 
     Referring to FIG. 1, trenches  4   a  to  4   c  are formed on a major surface of a semiconductor substrate  1  in a semiconductor device according to an embodiment 1 of the present invention. Thermal oxide films  5   a  to  5   c  are formed on surface portions of the semiconductor substrate  1  located in the trenches  4   a  to  4   c  respectively. The trenches  4   a  and  4   b  have widths W 7  and W 6  of 2.5 μm and 0.2 μm respectively. A silicon oxide film  8   a  is formed in the trench  4   a  by CVD. A silicon oxide film  6   a  is formed on the silicon oxide film  8   a  by HDP-CVD. Silicon oxide films  6   b  and  6   c  are formed in the trenches  4   b  and  4   c  by HDP-CVD respectively. The interval W 4  between the silicon oxide films  6   b  and  8   a  on the major surface of the semiconductor substrate  1  is substantially identical to the interval W 5  between the silicon oxide films  6   b  and  8   a  under the major surface of the semiconductor substrate  1 , and the trench isolation oxide films  6   a  to  6   c  and  8   a  have no overhang portions. 
     Thus, the silicon oxide film  8   a  for serving as a side wall insulator film is formed in the semiconductor device according to the embodiment 1 of the present invention. Even if an upper portion of a side wall of the trench  4   a  is partially removed when the insulator films  6   a  to  6   c  are formed in a step described later, therefore, the semiconductor substrate  1  or the like can be prevented from partial removal from the upper portion of the side wall of the trench  4   a  if the silicon oxide film  8   a  is formed to have a thickness exceeding that of the removed part. Thus, it is possible to prevent generation of leakage currents from element forming regions resulting from partial removal of the semiconductor substrate  1 . 
     Further, the trench isolation oxide films  6   a  to  6   c  and  8   a  have no overhang portions. In case of forming wires on the silicon oxide films  6   a  to  6   c  and  8   a  serving as isolation insulators, therefore, it is possible to prevent such a problem that the wires cannot have designed patterns but are disconnected or shorted in photolithography therefor. 
     With reference to FIGS. 2 to  9 , steps of forming the trench isolation oxide films  6   a  to  6   c  and  8   a  in a method of fabricating the semiconductor device according to the embodiment 1 of the present invention are now described. 
     First, a thermal oxide film (not shown) is formed on the major surface of the semiconductor substrate  1  (see FIG.  2 ). A silicon nitride film (not shown) is formed on this thermal oxide film. A resist pattern (not shown) is formed on this silicon nitride film. This resist pattern is employed as a mask for partially removing the silicon nitride film and the thermal oxide film, thereby forming silicon nitride films  3   a  to  3   d (see FIG. 2) and thermal oxide films  2   a  to  2 d (see FIG.  2 ). Thereafter the resist pattern is removed. Thus, the structure shown in FIG. 2 is obtained. 
     Then, the silicon nitride films  3   a  to  3   d  are employed as masks for partially removing the semiconductor substrate  1  by anisotropic etching, thereby forming the trenches  4   a  to  4   c  as shown in FIG.  3 . 
     Then, the portions of the major surface of the semiconductor substrate  1  located in the trenches  4   a  to  4   c  are thermally oxidized, thereby forming the thermal oxide films  5   a  to  5   c  respectively, as shown in FIG.  4 . 
     Then, a silicon oxide film  8  (see FIG. 5) is formed in the trenches  4   a  to  4   c  and on the silicon nitride films  3   a  to  3   d  by CVD. A silicon nitride film  12  (see FIG. 5) is formed on the silicon oxide film  8 . A resist pattern  11  (see FIG. 5) is formed on the silicon nitride film  12  in a region located on the trench  4   a . Thus, the structure shown in FIG. 5 is obtained. 
     The silicon nitride film  12  may be replaced with another material such as a polysilicon film, for example, which can be selectively etched with the silicon oxide film  8 , to attain a similar effect. 
     Further, the silicon nitride film  12  can be replaced with a silicon oxide film or a polysilicon film when the silicon oxide film  8  is replaced with a silicon nitride film, while the silicon nitride film  12  can be replaced with a polysilicon film, a silicon oxide film or a silicon nitride film when the silicon oxide film  8  is replaced with an oxi-nitride film. An effect similar to the above can be attained also in this case. 
     Then, the resist pattern  11  is employed as a mask for removing portions of the silicon nitride film  12  located on the trenches  4   b  and  4   c . Thereafter the resist pattern  11  is removed, thereby obtaining a structure shown in FIG.  6 . 
     Then, the silicon nitride film  12  is employed as a mask for removing portions of the silicon oxide film  8  located in the trenches  4   b  and  4   c  and on the silicon nitride films  3   c  and  3   d . Thereafter the silicon nitride film  12  is removed. Thus, a structure shown in FIG. 7 is obtained. 
     The surface of the silicon oxide film  8   a  formed in the trench  4   a , which is covered with the silicon nitride film  12  (see FIG. 6) serving as a protective film, is not damaged by the etching step shown in FIG. 7, with no fluctuation of the thickness of the silicon oxide film  8   a.    
     Thus, the surface of the silicon oxide film  8   a  serving as a side wall insulator film is not damaged, with no fluctuation of the thickness. Therefore, the silicon oxide film  8   a  may be formed to have a thickness necessary for preventing partial removal of the silicon nitride films  3   a  and  3   b  and the semiconductor substrate  1  in deposition of a silicon oxide film  6  by HDP-CVD shown in FIG.  8 . Thus, the thickness of the silicon oxide film  8   a  may not be increased beyond necessity. 
     Then, the silicon oxide film  6  is formed in the trenches  4   b  and  4   c  and on the silicon nitride films  3   c  and  3   d  and the silicon oxide film  8   a  by HDP-CVD, as shown in FIG.  8 . At this time, the ratio of the deposition rate for the silicon oxide film  6  to that for sputter etching in HDP-CVD so that the silicon oxide film  6  can fill up the trench  4   b  having a narrow width while forming no void, whereby the rate for removing the silicon oxide film  6  or the like by sputter etching is higher than the deposition rate for the silicon oxide film  6  on an upper portion of the trench  4   a  having a large width. Therefore, the silicon oxide film  8   a  is partially removed from upper portions of side walls of the trench  4   a  by this sputter etching, to define inclined surfaces  7   a  and  7   b . However, the silicon nitride films  3   a  and  3   b  are hardly removed by this etching. 
     Thus, when a sufficient thickness is ensured for the silicon oxide film  8   a  serving as a side wall insulator film, the silicon nitride films  3   a  and  3   b  and the semiconductor substrate  1  can be prevented from partial removal from the upper portions of the side walls of the trench  4   a  in the HDP-CVD step for depositing the silicon oxide film  6 . In the trench  4   a , therefore, the width W 7  (see FIG. 1) of an isolation insulator consisting of the silicon oxide films  8   a  and  6   a  (see FIG. 9) can be prevented from being increased beyond a designed value to form an overhang portion. In case of forming a wire or the like on this isolation insulator, therefore, it is possible to prevent such a problem that the wire cannot attain a designed pattern but is disconnected or shorted in photolithography therefor. 
     Further, partial removal of the semiconductor substrate  1  can be prevented, thereby preventing generation of leakage currents from the element forming regions of the semiconductor substrate  1 . 
     In addition, it is possible to deposit the silicon oxide film  6 , deposited by HDP-CVD, in the trench  4   b  having a narrow width, which is hard to fill up in the conventional CVD, while removing a part of the insulator film  6  deposited on the upper portion of the trench  4   b . Thus, the insulator film  6  can be prevented from closing the upper portion of the trench  4   b  before filling up the same. Therefore, the insulator film  6  can fill up the trench  4   b  having a narrow width while forming no void. 
     Then, portions of the silicon oxide films  6  and  8   a  located on the silicon nitride films  3   a  to  3   d  are removed by etching or chemical mechanical polishing, as shown in FIG.  9 . 
     The vertical positions of portions of the silicon oxide films  6   a  to  6   c  and  8   a , serving as isolation oxide films, projecting from the major surface of the semiconductor substrate  1  can be adjusted by adjusting the thickness of the silicon nitride films  3   a  to  3   d.    
     Then, the silicon nitride films  3   a  to  3   d  and the thermal oxide films  2   a  to  2   d  are removed by etching, thereby obtaining the structure shown in FIG.  1 . 
     (Embodiment 2) 
     Referring to FIG. 10, trench isolation oxide films in a semiconductor device according to an embodiment 2 of the present invention are basically similar in structure to those of the semiconductor device according to the embodiment 1 shown in FIG.  1 . In a trench  4   a , however, a silicon nitride film  12  is formed on a silicon oxide film  8   a.    
     Thus, a step similar to that for removing the silicon nitride film  12  in the embodiment 1 of the present invention shown in FIG. 7 can be omitted, whereby steps of fabricating the semiconductor device according to the embodiment 2 can be simplified. 
     In order to form the trench isolation oxide films of the semiconductor device according to the embodiment 2 of the present invention shown in FIG. 10, steps similar to those of the embodiment 1 shown in FIGS. 2 to  5  are first carried out. Then, in steps similar to those shown in FIGS. 6 and 7, portions of a silicon oxide film  8  located in trenches  4   b  and  4   c  are removed and thereafter a silicon oxide film  6  is deposited by HDP-CVD as shown in FIG. 8, without removing the silicon nitride film  12  (see FIG. 6) employed as a protective film. Thereafter a step similar to that shown in FIG. 9 is carried out, thereby obtaining the structure shown in FIG.  10 . 
     (Embodiment 3) 
     With reference to FIG. 11, steps of forming trench isolation oxide films in a method of fabricating a semiconductor device according to an embodiment 3 of the present invention are now described. 
     First, steps similar to those for the semiconductor device according to the embodiment 1 of the present invention shown in FIGS. 2 to  4  are carried out, and thereafter a silicon oxide film  8  (see FIG. 11) is formed in trenches  4   a  to  4   c  (see FIG. 11) and on silicon nitride films  3   a  to  3   d  (see FIG. 11) by CVD. Then, a resist pattern  11  (see FIG. 11) is formed on the silicon oxide film  8  in a region located on the trench  4   a . Thus, the structure shown in FIG. 11 is obtained. 
     Then, the resist pattern  11  is employed as a mask for partially removing the silicon oxide film  8  from the trenches  4   b  and  4   c , as shown in FIG.  7 . Thereafter the resist pattern  11  is removed. Thus, a structure similar to that shown in FIG. 7 is obtained. Then, steps similar to those shown in FIGS. 8 and 9 are carried out, thereby obtaining the trench isolation oxide films similar to those shown in FIG.  1 . 
     The resist pattern  11  is formed on the silicon oxide film  8  for serving as a side wall insulator film and employed as a mask for partially removing the silicon oxide film  8  from the trenches  4   b  and  4   c , whereby the number of the steps can be reduced as compared with the case of forming the silicon nitride film  12  (see FIG. 5) on the silicon oxide film  8  in the steps of the embodiment 1 shown in FIGS. 5 and 6. 
     (Embodiment 4) 
     With reference to FIGS. 12 to  15 , steps of forming trench isolation oxide films in a method of fabricating a semiconductor device according to an embodiment 4 of the present invention are now described. 
     First, steps similar to those for the trench isolation oxide films in the method of fabricating the semiconductor device according to the embodiment 1 of the present invention shown in FIGS. 2 to  4  are carried out, and thereafter a silicon oxide film  8  is deposited in trenches  4   a  to  4   c  and on silicon nitride films  3   a  to  3   d  by CVD, as shown in FIG.  12 . At this time, the silicon oxide film  8  is deposited in a thickness larger than half the width of the trench  4   b . Thus, growing surfaces of parts of the silicon oxide film  8  deposited on opposite side surfaces of the trench  4   b  come into contact with each other at a substantially central portion of the trench  4   b . The contact portion between such growing surfaces of the silicon oxide film  8  is referred to as a seam portion  13 . A similar seam portion  14  is defined also in the trench  4   c  which is substantially identical in width to the trench  4   b.    
     In the trench  4   a  having a large width, on the other hand, no growing surfaces of the silicon oxide film  8  collide with each other and hence the silicon oxide film  8  can be formed to have a thickness larger than half the width of the trench  4   b.    
     Then, the silicon oxide film  8  is partially removed by wet etching. At this time, an etching solution infiltrates into the trenches  4   b  and  4   c  along the seam portions  13  and  14  respectively. The etching rate on the seam portions  13  and  14  is higher than that on the remaining regions of the silicon oxide film  8 , due to influence by heterogeneity of crystals or the like. In the trenches  4   b  and  4   c , therefore, the etching progresses from the seam portions  13  and  14 . When the wet etching time is so adjusted that the thickness of the removed parts of the silicon oxide film  8  is at least substantially identical to half the width of the trench  4   b , it is possible to remove the silicon oxide film  8  (see FIG. 12) from the trenches  4   b  and  4   c  having narrow widths while leaving a silicon oxide film  8   a  in the trench  4   a  having a large width, as shown in FIG.  13 . Further, silicon oxide films  8   a  to  8   c  can be left on the silicon nitride films  3   a  to  3   d , since the thickness of the silicon oxide film  8  is larger than half the width of the trench  4   b.    
     When the silicon oxide film  8  is deposited by 0.4 μm, for example, it is possible to fill up trenches having widths of up to 0.8 μm. When the silicon oxide film  8  is partially removed by 0.3 μm by wet etching, a part filling up a trench having a width of up to 0.6 μm can be removed. In a trench having a width larger than 0.6 μm, on the other hand, the silicon oxide film  8  is left with a thickness of 0.1 μm. 
     Thus, it is possible to form the silicon oxide film  8   a  for serving as a side wall insulator film in the trench  4   a  without forming a mask such as the silicon nitride film  12  (see FIG. 6) in the embodiment 1 on the region located on the trench  4   a  by adjusting the thickness of the silicon oxide film  8  and employing wet etching. Consequently, the steps can be simplified. 
     Then, a silicon oxide film  6  is formed in the trenches  4   a  to  4   c  and on the silicon oxide films  8   a  to  8   c  by HDP-CVD, as shown in FIG.  14 . The silicon oxide film  8   a  serving as a side wall insulator film is formed in the trench  4   a  having a large width, whereby an effect similar to that of the embodiment 1 shown in FIG. 8 is attained. 
     Then, portions of the silicon oxide films  6  and  8   a  to  8   c  located on the silicon nitride films  3   a  to  3   d  are removed, thereby obtaining a structure shown in FIG.  15 . 
     Due to the formation of the silicon oxide films  8   a  to  8   c  (see FIG. 14) on the silicon nitride films  3   a  to  3   d , precisional allowance for a position for stopping etching or chemical mechanical polishing can be increased in the step of removing the portions of the silicon oxide film  6  located on the silicon nitride films  3   a  to  3   d.    
     Thereafter the silicon nitride films  3   a  to  3   d  and the thermal oxide films  2   a  to  2   d  are removed from a major surface of a semiconductor substrate  1 . Thus, a structure similar to that of the trench isolation oxide films in the semiconductor device according to the embodiment 1 of the present invention shown in FIG. 1 can be obtained. 
     (Embodiment 5) 
     Referring to FIG. 16, trench isolation oxide films in a semiconductor device according to an embodiment 5 of the present invention are basically similar in structure to those of the semiconductor device according to the embodiment 1 shown in FIG.  1 . In the semiconductor device shown in FIG. 16 according to the embodiment 5 of the present invention, however, a silicon nitride film  9   a  is formed in a trench  4   a , as shown in FIG.  16 . This silicon nitride film  9   a  serves as a side wall insulator film similarly to the silicon oxide film  8   a  shown in FIG. 1, whereby an effect similar to that of the embodiment 1 shown in FIG. 1 can be attained. 
     With reference to FIGS. 17 to  20 , steps of forming the trench isolation oxide films in a method of fabricating the semiconductor device according to the embodiment 5 of the present invention are now described. 
     First, steps similar to those for forming the trench isolation oxide films in the method of fabricating the semiconductor device according to the embodiment 1 shown in FIGS. 2 to  4  are carried out, and thereafter a silicon nitride film  9  is formed in trenches  4   a  to  4   c  and on silicon nitride films  3   a  to  3   d , as shown in FIG.  17 . This silicon nitride film  9  is deposited in a thickness larger than half the width of the narrow trench  4   b.    
     Then, the silicon nitride film  9  is partially removed by wet etching, thereby obtaining a structure shown in FIG.  18 . In this wet etching, the silicon nitride film  9  (see FIG. 17) is removed at least in a thickness substantially identical to half the width of the trench  4   b , whereby it is possible to leave a silicon nitride film  9   a  serving as a side wall insulator film in the trench  4   a  having a large width while simultaneously removing the silicon nitride film  9  (see FIG. 17) from the trenches  4   b  and  4   c  having narrow widths, similarly to the steps of forming the trench isolation oxide films in the method of fabricating the semiconductor device according to the embodiment 4 shown in FIGS. 12 and 13. Thus, an effect similar to that in the steps of forming the trench isolation oxide films in the method of fabricating the semiconductor device according to the embodiment 4 shown in FIGS. 12 and 13 can be attained. 
     Then, a silicon oxide film  6  is deposited in the trenches  4   a  to  4   c  and on the silicon nitride films  9   a  to  9   c  by HDP-CVD, as shown in FIG.  19 . Due to the silicon nitride film  9   a  formed in the trench  4   a  for serving as a side wall insulator film, an effect similar to that in the step of forming the trench isolation oxide films in the method of fabricating the semiconductor device according to the embodiment 1 shown in FIG. 8 can be attained. 
     Then, portions of the silicon oxide film  6  located on the silicon nitride films  3   a  to  3   d  are removed by etching or chemical mechanical polishing. Thus, a structure shown in FIG. 20 is obtained. 
     Due to the silicon nitride films  9   a  to  9   c  (see FIG. 19) formed on the silicon nitride films  3   a  to  3   d , an effect similar to that in the steps of forming the trench isolation oxide films in the method of fabricating the semiconductor device according to the embodiment 4 shown in FIGS. 14 and 15 can be attained. 
     Then, the silicon nitride films  3   a  to  3   d  and thermal oxide films  2   a  to  2   d  are removed from a major surface of a semiconductor substrate  1 , thereby obtaining the structure shown in FIG.  16 . 
     (Embodiment 6) 
     Referring to FIG. 21, trench isolation oxide films in a semiconductor device according to an embodiment 6 of the present invention are basically similar in structure to those of the semiconductor device according to the embodiment 1 shown in FIG.  1 . In the semiconductor device according to the embodiment 6 of the present invention, however, an oxi-nitride film  10   a  is formed in a trench  4   a  as a side wall insulator film. 
     In addition to an effect attained by the structure similar to that of the trench isolation oxide films in the semiconductor device according to the embodiment 1 shown in FIG. 1, therefore, it is possible to provide the oxi-nitride film  10   a  with a thermal expansion coefficient which is substantially identical to that of a semiconductor substrate  1  by adjusting the composition of the oxi-nitride film  10   a . In case of performing heat treatment or the like in any step of fabricating the semiconductor device, therefore, it is possible to prevent generation of stress resulting from difference in thermal expansion between the semiconductor substrate  1  and the oxi-nitride film  10   a . Thus, the semiconductor substrate  1  can be prevented from a defect caused by such stress. Thus, the semiconductor device can be prevented from generation of a leakage current resulting from such a defect. Consequently, a semiconductor device having high reliability can be obtained. 
     With reference to FIGS. 22 to  25 , steps of forming the trench isolation oxide films in a method of fabricating the semiconductor device according to the present invention are now described. 
     First, steps similar to those for forming the trench isolation oxide films in the method of fabricating the semiconductor device according to the embodiment 1 shown in FIGS. 2 to  4  are carried out, and thereafter an oxi-nitride film  10  is formed in trenches  4   a  to  4   c  and on silicon nitride films  3   a  to  3   c , as shown in FIG.  22 . The oxi-nitride film  10  is so deposited that its thickness is larger than half the width of the narrow trench  4   b.    
     Then, the oxi-nitride film  10  is partially removed by wet etching, thereby obtaining a structure shown in FIG.  23 . Similarly to the steps of forming the trench isolation oxide films in the method of fabricating the semiconductor device according to the embodiment 4 of the present invention shown in FIGS. 12 and 13, it is possible to leave the oxi-nitride film  10   a  for serving as a side wall insulator film in the trench  4   a  having a large width while simultaneously removing the oxi-nitride film  10  from the trenches  4   b  and  4   c  at least in a thickness substantially equal to half the width of the narrow trench  4   b.    
     Thus, an effect similar to that in the steps of forming the trench isolation oxide films in the method of fabricating the semiconductor device according to the embodiment 4 shown in FIGS. 12 and 13 can be attained. 
     Then, a silicon oxide film  6  is deposited in the trenches  4   a  to  4   c  and on oxi-nitride films  10   a  to  10   c  by HDP-CVD, as shown in FIG.  24 . 
     Due to the oxi-nitride film  10   a  for serving as a side wall insulator film formed in the trench  4   a  having a large width, an effect similar to that in the step of forming the trench isolation oxide films in the method of fabricating the semiconductor device according to the embodiment 1 shown in FIG. 8 can be attained. 
     Then, portions of the silicon oxide film  6  located on the silicon nitride films  3   a  to  3   d  are removed by etching or chemical mechanical polishing, thereby obtaining a structure shown in FIG.  25 . 
     Due to the formation of the oxi-nitride films  10   a  to  10   c  on the silicon nitride films  3   a  to  3   d , an effect similar to that in the steps of forming the trench isolation oxide films in the method of fabricating the semiconductor device according to the embodiment 4 shown in FIGS. 14 and 15 can be attained. 
     Then, the silicon nitride films  3   a  to  3   d  and thermal oxide films  2   a  to  2   d  are removed from a major surface of a semiconductor substrate  1 , whereby the structure shown in FIG. 21 can be obtained. 
     In each of the aforementioned embodiments 1 to 6, the semiconductor substrate  1  may be formed by a p-type silicon substrate or an n-type silicon substrate. 
     Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims.