Abstract:
A method of forming a trench isolation structure is provided, which prevents generation of defects such as voids, cracks, and depressions of an isolation dielectric formed in an isolation trench without problems such as isolation region expansion, isolation capability degradation, and current leakage increase. In a first step, an isolation trench is formed in a semiconductor substrate to expose a top of the trench from a main surface of the substrate. In a second step, the whole main surface of the substrate is covered with a solution of a silazane perhydride polymer by spin coating, thereby forming a film of the solution covering the whole main surface of the substrate. The trench is entirely filled with the film of the solution. The film of the solution may be formed directly on the main surface of the substrate or formed indirectly over the main surface of the substrate via any intervening film or films. In a third step, the film of the solution covering the main surface of the substrate is converted to an oxide film of silicon covering the main surface of the substrate due to chemical reaction. In a fourth step, the oxide film of silicon covering the main surface of the substrate is selectively removed, thereby leaving a part of the oxide film that is used as an isolation dielectric of a trench isolation structure.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to semiconductor device fabrication and more particularly, to a method of forming a trench isolation structure provided in a semiconductor device, which uses spin coating. 
     2. Description of the Prior Art 
     The isolation structure of a semiconductor device is, in general, provided to electrically isolate a semiconductor element or elements such as transistors, resistors, and capacitors in an active region from other semiconductor element or elements in a neighboring active region on a same semiconductor substrate. 
     In recent years, the need to narrow the isolation regions has been becoming stronger with the increasing level of integration of Large-Scale Integrated circuits (LSIs). Thus, with the well-known LOcal Oxidation of Silicon (LOCOS) method where the isolation regions are formed by producing a patterned isolation dielectric on a main surface of a semiconductor substrate of silicon (Si) due to selective oxidation, the isolation regions corresponding to a desired integration level have been being unable to be realized. 
     To respond to the need, the “trench isolation structures” has been often used, in which neighboring active regions are electrically isolated from one another by an isolation dielectric filled in trenches formed vertically into a semiconductor substrate. The isolation dielectric is typically made of silicon dioxide (SiO 2 ). The trenches are formed in the substrate according to a desired pattern of isolation regions and then, the isolation dielectric is selectively formed so as to fill the trenches. 
     The trench isolation structure makes it possible to decrease the width of the isolation trenches (i.e., the isolation regions) compared with the isolation regions realized by the conventional LOCOS method. Thus, the trench isolation structure can produce narrower isolation regions corresponding to a recent, high integration level of LSIs. 
     A conventional method of forming the trench isolation structure is explained below with reference to FIGS. 1A to  1 E. In this method, it is needless to say that a lot of isolation trenches are formed in a semiconductor substrate to electrically isolate adjoining active regions from one another. However, only one of the trenches is illustrated to isolate two adjoining ones of the active regions and explained below for the sake of simplification of description. 
     It is known that Chemical Vapor Deposition (CVD) is effective to form selectively an isolation dielectric of SiO 2  to fill fine isolation trenches (e.g., approximately 0.1 μm in width), because CVD produces SiO 2  with a good filling property of the trenches, in other words, SiO 2  produced by CVD (i.e., CVD-SiO 2 ) has a good trench-filling property. In the conventional method explained below with reference to FIGS. 1A to  1 E, high-density plasma CVD, which produces SiO 2  with a better trench-filling property, is used. 
     First, a SiO 2  film  105  with a thickness of approximately 20 nm, which serves as a pad oxide, is formed on a main surface of a single-crystal Si substrate  101  by thermal oxidation of the substrate  101 . Then, a silicon nitride (Si 3 N 4 ) film  106  with a thickness of approximately 200 nm is formed on the SiO 2  film  105  by reduced-pressure CVD. The Si 3 N 4  film  106  is used as a mask for an isolation trench. The state at this stage is shown in FIG.  1 A. 
     Next, after a photoresist film (not shown) is formed on the Si 3 N 4  film  106  by coating, the photoresist film is patterned by popular exposure and development processes. The patterned photoresist film has a pattern corresponding to the plan shape of a desired isolation trench. In other words, the photoresist film has a window corresponding to the isolation trench to be formed. 
     Using the patterned photoresist film as a mask, the Si 3 N 4  film  106  and the SiO 2  film  105  are successively patterned by dry etching. Thus, a hole  118  is formed to penetrate through the Si 3 N 4  and SiO 2  films  106  and  105 . The hole  118 , which has a plan shape corresponding to the window of the photoresist film, is reached the main surface of the substrate  101 , as shown in FIG.  1 B. 
     After removing the photoresist film, the main surface of the substrate  101  is selectively and vertically removed by dry etching using the Si 3 N 4  film  106  as a mask, thereby forming an isolation trench  103  in the substrate  101 , as shown in FIG.  1 C. The isolation trench  103  has a plan shape corresponding to the window of the photoresist film. For example, the trench  103  has a width of 0.1 μm and a depth of 0.5 μm, resulting in an aspect ratio of 5 (=0.5/0.1). 
     Subsequently, as shown in FIG. 1D, a SiO 2  film  113  is formed on the Si 3 N 4  film  106  to cover the whole main surface of the substrate  101 . The formation process of the SiO 2  film  113  is carried out by high-density plasma CVD that produces SiO 2  with a better trench-filling property. As a result, the SiO 2  film  113  is deposited on the Si 3 N 4  film  106  and at the same time, it is deposited in the trench  103  and the penetrating hole  118 . The state at this stage is shown in FIG.  1 D. 
     The SiO 2  film  113  is then polished by Chemical Mechanical Polishing (CMP) until the surface of the underlying Si 3 N 4  film  106  is exposed. Thus, the SiO 2  film  113  is removed while the part of the SiO 2  film  113  located under the surface of the Si 3 N 4  film  106  is left and at the same time, the surface of the Si 3 N 4  film  106  is planarized. 
     Finally, the remaining Si 3 N 4  film  106  and the underlying SiO 2  film  105  are successively removed by wet etching. As a result, as shown in FIG. 1E, only the part of the SiO 2  film  113  located under the surface of the Si 3 N 4  film  106  is left. The remaining part of the SiO 2  film  113 , almost all of which is located in the trench  103  and a top of which is protruded from the main surface of the substrate  101  by a height corresponding to the total thickness of the films  106  and  105 , serves as an isolation dielectric. The trench  103  and the remaining SiO 2  film  113  constitute a trench isolation structure  102  that isolates electrically two adjoining active regions A 101  and A 102 . 
     With the conventional method of forming a trench isolation structure shown in FIGS. 1A to  1 E, a void (i.e., unfilled part)  114  tends to be formed in the remaining SiO 2  film  113  (i.e., the isolation dielectric) during the process of forming the SiO 2  film  113  by high-density plasma CVD, as shown in FIG.  1 D. This is caused by the fact that the isolation trench  103  is narrow in width and high in aspect ratio. In this case, even if high-density plasma CVD, which produces SiO 2  with a better trench-filling property, is used for forming the SiO 2  film  113 , the whole trench  103  is difficult to be filled with the SiO 2  film  113 . 
     If the void  114  exists in the isolation dielectric  113 , not only the mechanical strength of the trench isolation structure  102  but also the electrical isolation capability thereof will degrade. Also, there is a possibility that the void  114  appears on the main surface of the substrate  101  after the CMP process of the SiO 2  film  113 , as shown in FIG. 1E in this case, the exposed void  114  will cause a problem that overlying wiring layers or lines (which will be formed in subsequent processes) are broken or cut. 
     As an improvement of the above-described conventional method shown in FIGS. 1A to  1 E, a method using a different condition of the high-density plasma CVD has been developed. In this method, the void  114  is prevented from being generated due to the enhanced plasma-etching action. 
     With the improved method using the different CVD condition, although the void  114  can be prevented, the neighborhood of the hole  118  of the films  106  and  105  and the top of the isolation trench  103  tend to be etched by the enhanced plasma-etching action. As a result, as shown in FIG. 2, the sidewalls of the hole  118  and the trench  103  become oblique. The oblique sidewalls  115  of the trench  103  and the hole  118  lead to substantial expansion of the trench  103  or isolation regions with respect to that of the conventional method of FIGS. 1A to  1 E, which is contrary to the need to shrink the isolation regions. Moreover, the expanded trench  103  will cause a problem that current leakage is increased due to degraded isolation capability. 
     On the other hand, it has been known that so-called “spin coating” is effective to produce SiO 2  with a good trench-filling property. The spin coating process may be termed the “Spin-On-Glass (SOG)” process. When this spin coating or SOG process is used, a solution of a Si-containing material is dropped onto a main surface of a Si substrate (or, a layer located on the substrate) while rotating the substrate in a horizontal plane, thereby forming a uniformly-coated film of the solution on the entire main surface of the substrate (or, the layer located on the substrate) due to the effect of centrifugal force. Subsequently, the substrate  101  is heated to vaporize or volatilize the solvent of the solution from the coated film and to cause a chemical reaction of the Si-containing material with oxygen existing in the atmosphere, thereby forming a SiO 2  film on the whole main surface of the substrate  101  (or, the layer located on the substrate). 
     With the method using the spin coating process, since the solution of the Si-containing material is dropped onto the surface of the substrate (or, the layer located on the substrate) and coated thereon by the effect of centrifugal force, there arises an advantage of excellent trench-filling property. As the solution of the Si-containing material, a solution containing silicon hydroxide (i.e., silanol, SiOH 4 ) dispersed in an organic solvent such as alcohol is typically used. 
     A conventional method of forming a trench isolation structure using the above-described spin coating or SOG process is shown in FIGS. 3A and 3B. 
     First, in the same way as shown in the above-described conventional method of FIGS. 1A to  1 E, an isolation trench  103  is formed in a Si substrate  101  with a SiO 2  film  105  and a Si 3 N 4  film  106 , as shown in FIG.  3 A. Then, through a spin coating process using a solution containing SiOH 4  dispersed in an organic solvent, a film  107  of the SiOH 4  solution is formed on the Si 3 N 4  film  106  to cover the whole main surface of the substrate  101 . At this stage, the film  107  fills the entire trench  103  and the entire hole  118 , as shown in FIG.  3 A. 
     Subsequently, the film  107  of the SiOH 4  solution is subjected to a specified heat treatment, thereby converting the film  107  to a SiO 2  film  104  due to hydrolysis and dehydrating condensation reactions. 
     Finally, in the same way as shown in the above-described conventional method of FIGS. 1A to  1 E, the SiO 2  film  104  located over the surface of the Si 3 N 4  film  106  is selectively removed by CMP and then, the Si 3 N 4  film  106  and the SiO 2  film  105  are successively removed by wet etching. As a result, the part of the SiO 2  film  104  located below the surface of the Si 3 N 4  film  106  is left in the trench  103  and the hole  118 , thereby forming a trench isolation structure  102  that isolates electrically two adjoining active regions A 101  and A 102 , as shown in FIG.  33 . 
     The conventional method using the spin coating shown in FIGS. 3A and 3B, however, has the following problem. 
     During the heat-treatment process for converting the Si-containing material film  107  to the SiO 2  film  104 , a large-volume shrinkage occurs in the film  107  due to the dehydrating condensation reaction. As a result, the part of the SiO 2  film  104  located in the trench  103  and the hole  118  (i.e., the isolation dielectric), which has a comparatively larger thickness or height than the remaining part, is unable to resist the shrinkage action, resulting in cracks  117  in the remaining SiO 2  film  104 , as shown in FIG.  3 B. 
     Furthermore, it has been known that the SiO 2  film  104  generated by the dehydrating condensation reaction of SiOH 4  contains a lot of SiOH 4  groups as well as the film  104  is porous due to insufficient densification. These properties of she SiO 2  film  104  affect badly the CMP process for removing selectively the SiO 2  film  104  and/or the wet etching processes for removing the Si 3 N 4  film  106  and the SiO 2  film  105 . As a result, the top of the remaining SiO 2  film  104  tends to be lowered to form a depression or hollow  116  thereon in the trench  103 , as shown in FIG.  3 B. 
     In particular, when the pad SiO 2  film  105  is formed by thermal oxidation, the depression or hollow  116  is likely to be formed. This is due to the fact that the SiO 2  film  104  generated by the chemical reaction of the film  107  of the SiOH 4  solution is much larger in etch rate than the SiO 2  film  105  formed by thermal oxidation in the wet etching process for removing the SiO 2  film  105 . 
     As described above, the conventional method shown in FIGS. 1A to  1 E has a problem that the void  106  tends to be formed in the remaining SiO 2  film  113  in the isolation trench  103 . The conventional method shown in FIG. 2 solves this problem relating to the void  106 . However, it has a problem that the isolation region is expanded, the isolation capability is degraded, and the current leakage is increased. The conventional method shown in FIGS. 3A and 3B has a problem that the cracks  117  and/or the depression  116  tend to be formed in the remaining SiO 2  film  104  in the isolation trench  103 . 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of the present invention to provide a method of forming a trench isolation structure that prevents generation of defects such as voids, cracks, and depressions of an isolation dielectric formed in an isolation trench without problems such as isolation region expansion, isolation capability degradation, and current leakage increase. 
     Another object of the present invention to provide a method of forming a trench isolation structure that makes it possible to fill a fine isolation trench having a width of approximately 0.1 μm with a dielectric. 
     The above objects together with others not specifically mentioned will become clear to those skilled in the art from the following description. 
     A method of forming a trench isolation structure according to the present invention is comprised of the following first to fourth steps. 
     In the first step, an isolation trench is formed in a semiconductor substrate to expose a top of the trench from a main surface of the substrate. 
     In the second step, the whole main surface of the substrate is covered with a solution of a silazane perhydride polymer by spin coating, thereby forming a film of the solution covering the whole main surface of the substrate. The trench is entirely filled with the film of the solution. 
     The film of the solution may be formed directly on the main surface of the substrate or formed indirectly over the main surface of the substrate via any intervening film or films. 
     In the third step, the film of the solution covering the main surface of the substrate is converted to an oxide film of silicon covering the main surface of the substrate due to chemical reaction. 
     In the fourth step, the oxide film of silicon covering the main surface of the substrate is selectively removed, thereby leaving a part of the oxide film that is used as an isolation dielectric of a trench isolation structure in the trench. 
     With the method of forming a trench isolation structure according to the present invention, the film of the solution of the silazane perhydride polymer is formed to cover the whole main surface of the semiconductor substrate in the second step by using spin coating having an excellent trench-filling property. Then, the film of the solution covering the main surface of the substrate is converted to the oxide film of silicon due to chemical reaction in the third step. Thus, even if the isolation trench has a small width of approximately 0.1 μm, the oxide film of silicon can be well formed to fill the entire trench without any problems such as isolation region expansion, isolation capability degradation, and current leakage increase. 
     Moreover, since the film of the solution of the silazane perhydride polymer is formed by spin coating, no void is formed in the remaining part of the oxide film of silicon in the trench, i.e., in the isolation dielectric. The film of the solution of the silazane perhydride polymer scarcely shrinks during the transformation to the oxide film of silicon in the third step and therefore, no crack is formed in the isolation dielectric. 
     Furthermore, the oxide film of silicon generated from the film of the solution of the silazane perhydride polymer due to chemical reaction is dense and high in etch resistance. Thus, the oxide film of silicon is scarcely affected in a process (e.g., a CMP process) for removing the unused part of the oxide film of silicon in the fourth step and a process (e.g., a wet etching process) for removing other film or films (e.g, silicon nitride or silicon dioxide film) formed on the main surface of the substrate. As a result, no depression nor hollow is formed at the isolation dielectric. 
     As described above, with the method of forming a trench isolation structure according to the present invention, even if the isolation trench is fine to have a small width of approximately 0.1 μm, the defects of the isolation trench such as voids, cracks and depressions can be prevented from being generated without any problem such as isolation region expansion, isolation capability degradation, and current leakage increase. In other words, even if the isolation trench has a small width of approximately 0.1 μm, the isolation dielectric is well formed in the isolation trench. 
     In a preferred embodiment of the method according to the present invention, the third step is a heat treatment carried out in an atmosphere containing at least one selected from the group consisting of oxygen, ozone, and water vapor. This is to ensure the transformation of the silazane perhydride polymer to an oxide of silicon through the chemical reaction in the third step. 
     In another preferred embodiment of the method according to the present invention, the third step is carried out at a temperature of 350° C. or higher. This is to ensure the transformation of the silazane perhydride polymer to an oxide of silicon through the chemical reaction in the third step. 
     In still another preferred embodiment of the method according to the present invention, there is provided with a step of densifying the oxide film of silicon by heat treatment at a temperature in a range from 700° C. to 1100° C. between the third and fourth steps. If the temperature is lower than 700° C., the oxide film of silicon tends to emit some gas and/or to shrink in a subsequent process step or steps, thereby affecting badly the subsequent process step or steps. If the temperature is higher than 1100° C., there arises a possibility that the semiconductor substrate is degraded due to heat. 
     In a further preferred embodiment of the method according to the present invention, there is provided with a step of volatilizing a solvent contained in the film of the silazane perhydride polymer by heat treatment in an inert atmosphere between the second and third steps. In this embodiment, there is an additional advantage that the solvent contained in the film of the silazane perhydride polymer can be removed without changing its film quality. 
     In a still further preferred embodiment of the method according to the present invention, there is provided with a step of rounding sidewalls at the top of the isolation trench by oxidizing the main surface of the substrate between the first and second steps. In this embodiment, since the sidewalls are rounded at the top of the isolation trench, the electric field occurring in the substrate is prevented from concentrating on the corners at the top of the isolation trench. Thus, there is an additional advantage that current leakage is further suppressed to raise an obtainable withstand voltage. 
     In the method of forming a trench isolation structure according to the present invention, the first step may be performed by any process or processes. Any spin coating process may be applied to realize the spin coating in the second step. The solution of the silazane perhydride polymer may be readily produced by, for example, dispersing a silazane perhydride polymer in a suitable solvent (preferably, organic solvent). As the organic solvent, for example, xylene or dibutyl ether may be preferably used. Any process may be used to perform the third step if it is able to form an oxide film of silicon through chemical reaction of the film of the solution of the silazane perhydride polymer. Although a heat treatment process is typically used for this purpose, any other process may be used therefor. Any process such as a CMP process and a dry or wet etching process may be used for the fourth step. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In order that the present invention may be readily carried into effect, it will now be described with reference to the accompanying drawings. 
     FIGS. 1A to  1 E are partial cross-sectional views showing a conventional method of forming a trench isolation structure, respectively. 
     FIG. 2 is a partial cross-sectional view showing another conventional method of forming a trench isolation structure. 
     FIGS. 3A and 3B are partial cross-sectional views showing still another conventional method of forming a trench isolation structure, respectively. 
     FIGS. 4A to  4 G are partial cross-sectional views showing a method of forming a trench isolation structure according to a first embodiment of the present invention, respectively. 
     FIG. 5 is a flowchart showing the heat treatment process of the film of the silazane perhydride polymer in the method according to the first embodiment of the present invention. 
     FIGS. 6A to  6 E are partial cross-sectional views showing a method of forming a trench isolation structure according to a second embodiment of the present invention, respectively. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will be described in detail below while referring to the drawings attached. 
     First Embodiment 
     A method of forming a trench isolation structure according to a first embodiment of the present invention is explained below with reference to FIGS. 4A to  4 G. 
     In FIGS. 4A to  4 G, although a lot of isolation trenches are practically formed in a semiconductor substrate to electrically isolate adjoining active regions from one another, only one of the trenches to isolate two adjoining active regions is illustrated and explained below for the sake of simplification. 
     First, as shown in FIG. 4A, a SiO 2  film  5  with a thickness of approximately 20 nm, which serves as a pad oxide, is formed on a main surface of a single-crystal Si substrate  1  by thermal oxidation of the substrate  1 . Then, a Si 3 N 4  film  6  with a thickness of approximately 200 nm is formed on the SiO 2  film  5  by reduced-pressure CVD. The Si 3 N 4  film  6  is used as a mask for an isolation trench. The state at this stage is shown in FIG.  4 A. 
     At this stage, same SiO 2  and Si 3 N 4  films as the films  5  and  6  are formed on the back surface of the substrate  1 . However, they are omitted here because they have no relationship to the present invention and they are removed in subsequent processes. 
     Next, after a photoresist film (not shown) is formed on the Si 3 N 4  film  6  by coating, the photoresist film is patterned by using a popular photolithography technique. The patterned photoresist film has a pattern corresponding to the plan shape of a desired isolation trench, in other words, has a window corresponding to the isolation trench to be formed. 
     Using the patterned photoresist film as a mask, the Si 3 N 4  film  6  and the SiO 2  film  5  are successively patterned by dry etching Thus, a hole  18  is formed to penetrate through the Si 3 N 4  and SiO 2  films  6  and  5 . The hole  18 , which has a plan shape corresponding to the window of the photoresist film, is reached the main surface of the substrate  1 , as shown in FIG.  4 B. 
     After removing the photoresist film, the substrate  1  is selectively and vertically removed through its main surface by dry etching using the Si 3 N 4  film  6  as a mask, thereby forming vertically an isolation trench  3  in the substrate  1  to expose the top of the trench  3  from the main surface of the substrate  1 , as shown in FIG.  4 C. The isolation trench  3  has a plan shape corresponding to the window of the photoresist film. For example, the trench  3  has a width of 0.1 μm and a depth of 0.5 μm, resulting in an aspect ratio of 5. 
     The above-described process steps are the same as those in the conventional method shown in FIGS. 1A to  1 E. 
     Subsequently, as shown in FIG. 4D, the whole main surface of the substrate  1  is covered with a film  7  of a solution of a silazane perhydride polymer [(SiH 2 NH) n ]. The film  7 , which is located on the Si 3 N 4  film  6 , has a thickness of approximately 400 nm. The film  7  of the solution of [(SiH 2 NH) n ] is formed using a spin coating process in the following way. 
     First, the solution of [(SiH 2 NH) n ] is generated by dispersing a liquid [(SiH 2 NH) n ] in a liquid xylene. Then, the solution of [(SiH 2 NH) n ] thus generated is dropped onto the surface of the Si 3 N 4  film  6  while rotating the substrate  1  in a horizontal plane. Thus, the uniform film  7  of the solution of [(SiH 2 NH) n ] is formed to cover the whole main surface of the substrate  1 . As a result, as shown in FIG. 4D, the trench  3  can be entirely filled with the film  7  even if the trench  3  has a narrow width of 0.1 μm and a high aspect ratio of 5. Unlike the conventional method shown in FIGS. 1A to  1 E, no void is formed in the part of the film  7  located in the trench  3  and the hole  18 . 
     A typical condition of the spin coating process is that the rotation speed of the substrate  1  is set as 4000 rpm and the rotation time is set as 20 seconds. 
     Thereafter, the film  7  of the solution of [(SiH 2 NH) n ] on the Si 3 N 4  film  6  is subjected to a heat treatment process, thereby converting the film  7  to a SiO 2  film  4 . This heat treatment process contains three steps S 1 , S 2 , and S 3  shown in FIG.  5 . 
     In the step S 1 , the substrate  1  having the film  7  of the solution of [(SiH 2 NH) n ] is placed on a hot plate (not shown) in an inert atmosphere held at 200° C. for three minutes, thereby volatilizing the organic solvent (i.e., xylene) from the film  7 . 
     In the step  52 , the substrate  1  having the film  7  of the solution of [(SiH 2 NH) n ] is placed in a water-vapor (H 2 O) atmosphere of an electric furnace (not shown) held at 400° C. for 60 minutes, thereby transforming the film  7  of the solution of [(SiH 2 NH) n ] to the SiO 2  film  4 . The step S 2  may be carried out in an oxygen (O 2 ) or ozone (O 3 ) atmosphere instead of the water vapor (H 2 O) atmosphere. 
     In the step S 2 , the [(SiH 2 NH) n ] film  7  is transformed to the SiO 2  film  4  according to the following reaction formula (1). 
     
       
         SiH 2 NH+2O→SiO 2 +NH 3   (1) 
       
     
     As seen from the formula (1), the silazane perhydride (SiH 2 NH) reacts with activated oxygen (O) generated by decomposition of water vapor and as a result, silicon dioxide (SiO 2 ) and ammonia (NH 3 ) are generated. This NH 3  serves as a catalyst in the reaction expressed by the formula (1) and therefore, the SiH 2 NH contained in the film  7  are entirely converted to SiO 2  and the film  7  does not contain SiH 2 NH at all. Accordingly, the SiO 2  film  4  has a high density. Also, since the film  7  scarcely shrinks through the transformation or conversion to the SiO 2  film  4 , no crack is formed in the film  4 . Thus, the SiO 2  film  4  has an excellent etch resistance. 
     In the step S 3 , the substrate  1  having the SiO 2  film  4  thus obtained is placed in an inert atmosphere of the electric furnace held at 900° C. for 60 minutes, thereby removing impurities such as excessive ammonia and water from the film  7 . Through this step S 3 , the SiO 2  film  4  is further densified to have a higher density. 
     The state at this stage is shown in FIG.  4 E. 
     Subsequently, the SiO 2  film  4 , which has been formed by the heat treatment of the film  7 , is polished by a CMP process until the surface of the Si 3 N 4  film  6  is exposed. Thus, as shown in FIG. 4F, only the part of the SiO 2  film  4  located in the trench  3  and the penetrating hole  18  is left. The top of the remaining part of the SiO 2  film  4  is approximately flat. 
     The Si 3 N 4  film  6  is then removed by a wet etching process using a hot phosphoric acid heated to, for example, 160° C. The SiO 2  film  5  is then removed by a wet etching process using a buffered hydrofluoric acid. Thus, as shown in FIG. 4G, a trench isolation structure  2   a  is formed on the substrate  1 , where the remaining SiO 2  film  4  in the trench  3  serves as an isolation dielectric of the structure  2   a.    
     The remaining SiO 2  film  4  in the trench  3  is not etched during the etching process of the Si 3 N 4  film  6 . However, it is etched during the etching process of the SiO 2  film  5 . Therefore, the top of the remaining SiO 2  film  4  (i.e., the isolation dielectric) protrudes slightly from the main surface of the substrate  1 , as shown in FIG.  4 G. The trench isolation structure  2   a  formed through the above-described method according to the first embodiment is practically used in this state. 
     With the method according to the first embodiment, as described above, the film  7  of the solution of the silazane perhydride polymer is formed on the Si 3 N 4  film  6  to cover the whole main surface of the semiconductor substrate  1  by using a spin coating process having an excellent trench-filling property. Then, the film  7  of the solution covering the main surface of the substrate  1  is converted to the SiO 2  film  4  due to chemical reaction. Thus, even if the isolation trench  3  has a small width of approximately 0.1 μm, the SiO 2  film  4  can be well formed to fill the entire trench  3  without any problems such as isolation region expansion, isolation capability degradation, and current leakage increase. 
     Moreover, since the film  7  is formed by the spin coating process, no void is formed in the remaining part of the SiO 2  film  4  in the trench  3 , i.e., in the isolation dielectric. The film  7  scarcely shrinks during the transformation to the SiO 2  film  4  and therefore, no crack is formed in the isolation dielectric  4 . 
     Furthermore, the SiO 2  film  4  generated from the film  7  of the solution of the silazane perhydride polymer due to chemical reaction is dense and high in etch resistance. Thus, the SiO 2  film  4  is scarcely affected in the CMP process for removing the unused part of the SiO 2  film  4  and the wet etching processes for removing the Si 3 N 4  and SiO 2  films  6  and  5 . As a result, no depression or hollow is formed at the isolation dielectric  4 . 
     Thus, even if the isolation trench  3  is fine to have a small width of approximately 0.1 μm, the isolation dielectric  4  is well formed in the trench  3 . 
     Confirmation Test 
     To confirm the advantages of the present invention, the inventor performed a test under the different conditions (i), (ii), and (iii) listed below. 
     (i) A trench isolation structure was formed using a silazane perhydride polymer according to the above-described first embodiment. 
     (ii) A trench isolation structure was formed using silanol according to the previously-described conventional method shown in FIGS. 3A and 3B. 
     (iii) A trench isolation structure was formed using a high-density plasma CVD according to the previously-described conventional method shown in FIGS. 1A to  1 E. 
     Then, the shrinkage rate and the wet etch rate of the SiO 2  films formed by the methods under the conditions (i), (ii), and (iii) were measured and compared. The result of the test is shown in Table 1. 
     The shrinkage rate was calculated based on the measurement result of thickness change (i.e., a ratio of the difference between the resultant value and the initial value to the initial value) of the SiO 2  films through a heat treatment carried out in a nitrogen (N 2 ) atmosphere at 900° C. for 60 minutes. 
     The wet etch rate was calculated in the following way. Specifically, the SiO 2  films formed by the methods under the conditions (i), (ii), and (iii) and a SiO 2  film formed by thermal oxidation in a water vapor (H 2 O) atmosphere at 950° C. were etched using a buffered hydrofluoric acid, and etch rates of these SiO 2  films were measured. The buffered hydrofluoric acid was made by mixing hydrofluoric acid (HF) with ammonium fluoride (NH 4 F) at a ratio of 1:30. Then, the ratio of the etch rates of the SiO 2  films formed by the methods under the conditions (i), (ii), and (iii) with respect to that of the SiO 2  film formed by thermal oxidation was calculated. 
     
       
         
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                 WET 
               
               
                 SiO 2  DEPOSITION 
                 SHRINKAGE RATE 
                 ETCH RATE 
               
               
                 PROCESS 
                 (%) 
                 (times) 
               
               
                   
               
             
             
               
                 SILAZANE PERHYDRIDE 
                 3 
                  1.5 
               
               
                 POLYMER (SiH 2 NH) n   
               
               
                 SILANOL (SiOH 4 ) 
                 30  
                 10.0 
               
               
                 HIGH-DENSITY 
                 3 
                  1.5 
               
               
                 PLASMA CVD 
               
               
                   
               
             
          
         
       
     
     The following is clearly seen from Table 1. 
     In the case of the SiO 2  film under the condition (ii) using SiOH 4 , the shrinkage rate has a large value of 30%. On the other hand, in the cases of the SiO 2  films under the conditions (i) and (iii) using (SiH 2 NH) n  and high-density plasma CVD, the shrinkage rates have a very small value of 3%. As a result, it was confirmed that the shrinkage rate of the SiO 2  film formed by the method according to the first embodiment under the conditions (i) was as low as that formed using high-density plasma CVD under the conditions (iii). 
     In the case of the SiO 2  film under the condition (i) using (SiH 2 NH) n , the wet etch rate has a very small value of 1.5, which is much lower than the value of 10.0 in the case under the condition (ii) using SiOH 4 . As a result, it was confirmed that the wet etch rate of the SiO 2  film formed by the method according to the first embodiment under the condition (i) was as low as that formed using high-density plasma CVD. In other words, it was confirmed that the density of the SiO 2  film formed by the method according to the first embodiment was as high as that formed using high-density plasma CVD. 
     Second Embodiment 
     FIGS. 6A to  6 E show a method of a trench isolation structure according to a second embodiment of the present invention. 
     First, in the same way as that of the first embodiment, an isolation trench  3  is formed in a single-crystal Si substrate  1 . Thereafter, the substrate  1  is subjected to a thermal oxidation process, thereby forming a SiO 2  film B covering the main surface of the substrate  1  and the sidewalls and bottom wall of the trench  3 , as shown in FIG.  6 A. Through this thermal oxidation process, the top corners  9  of the trench  3  are rounded. 
     Subsequent processes are substantially the same as those in the first embodiment. 
     Specifically, a pad SiO 2  film  5  with a thickness of approximately 20 nm is formed on the SiO 2  film  8  by thermal oxidation of the substrate  1 . Then, a Si 3 N 4  film  6  with a thickness of approximately 200 nm is formed on the SiO 2  film  5  by reduced-pressure CVD. 
     Next, using a patterned photoresist film (not shown) as a mask, the Si 3 N 4  film  6  and the SiO 2  film  5  are successively patterned by dry etching, thereby forming a hole  18  penetrating through the Si 3 N 4  and SiO 2  films  6  and  5  to overlap with the trench  3 . The state at this stage is shown in FIG.  6 A. 
     After removing the photoresist film, as shown in FIG. 6B, a film  7  of a solution of a silazane perhydride polymer [(SiH 2 NH) n ] is formed on the Si 3 N 4  film  6  to cover the whole main surface of the substrate  1 . The film  7  has a thickness of approximately 400 nm. The film  7  of the solution of [(SiH 2 NH) n ] is formed using a spin coating process in the same way as described in the first embodiment. As a result, as shown in FIG. 6B, the trench  3  and the hole  18  can be entirely filled with the film  7  even if the trench  3  has a narrow width of 0.1 μm and a high aspect ratio of 5. Unlike the conventional method shown in FIGS. 1A to  1 E, no void is formed in the part of the film  7  located in the trench  3 . 
     Thereafter, the film  7  of the solution of [(SiH 2 NH) n ] on the Si 3 N 4  film  6  is subjected to a same heat treatment process as in the first embodiment, thereby converting the film  7  to a SiO 2  film  4 , as shown in FIG.  6 C. Subsequently, the SiO 2  film  4  is polished by a CMP process until the surface of the Si 3 N 4  film  6  is exposed. Thus, as shown in FIG. 6D, only the part of the SiO 2  film  4  located in the trench  3  and the penetrating hole  18  is left. The top of the remaining part of the SiO 2  film  4  is approximately flat. 
     The Si 3 N 4  film  6  is then removed by wet etching. The entire SiO 2  film  5  and the part of the SiO 2  film  8  located on the main surface of the substrate  1  are removed by wet etching. Thus, as shown in FIG. 6E, a trench isolation structure  2   b  is formed on the substrate  1 , where the remaining SiO 2  films  4  and  8  serve as an isolation dielectric. 
     The remaining SiO 2  film  4  in the trench  3  and the hole  18  is not etched during the etching process of the Si 3 N 4  film  6  and is etched during the etching process of the SiO 2  films  5  and  8 . Therefore, the top of the remaining SiO 2  film  4  (i.e., the isolation dielectric) protrudes slightly from the main surface of the substrate  1 , as shown in FIG.  6 E. The trench isolation structure  2   b  formed through the above-described processes is practically used in this state. 
     As described above, the method of forming the trench isolation structure  2   b  according to the second embodiment includes the same process steps as those in the first embodiment, it has the same advantages as those in the first embodiment. Moreover, the top corners  9  of the trench  3  are rounded and the isolation dielectric is formed by the remaining SiO 2  films  4  and  8 . Thus, the electric field occurring in the substrate  1  is prevented from concentrating on the corners  9 , which suppresses current leakage more effectively than the first embodiment. As a result, there is an additional advantage that an obtainable withstand voltage is higher than the first embodiment. 
     While the preferred forms of the present invention have been described, it is to be understood that modifications will be apparent to those skilled in the art without departing from the spirit of the invention. The scope of the invention, therefore, is to be determined solely by the following claims.