Abstract:
In fabrication of a semiconductor device, firstly an isolation trench is formed on a substrate to isolate a plurality of semiconductor elements, and then a thermal oxide film is formed on a sidewall of the trench, whereupon a silicon oxide film is formed on the substrate by chemical vapor deposition. Finally the entire substrate is annealed in a high-pressure ambient.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method of manufacturing a semiconductor device and more particularly to a method of manufacturing a semiconductor device with circuit-element-isolation trenches on a substrate. 
     2. Description of the Related Art 
     With the recent increase of integration and operating speed of semiconductor devices, various technologies have been advanced remarkably to miniature individual circuit elements to be loaded on each semiconductor device, and application of the LOCOS (Local Oxidation of Silicon) isolation using a silicon nitride film for electrically isolating individual circuit elements has increasingly been limited. 
     Consequently new element isolation technologies utilizing trenches have been developed. For example, Japanese Patent Laid-Open Publication No. Sho60-124840 proposed a technology of forming trenches, filling an insulation film in the trenches and then annealing at a temperature equal to or below a melting point of a substrate. 
     Another solution was proposed by a paper “An Optimized Densification of the Filled Oxide for Quarter Micron Shallow Trench Isolation” presented in Symposium on VLSI technology Digest Technical Papers 1996 by Han Sim Lee, et al. This technology will now be described with reference to FIG. 4 of the accompanying drawings of the present specification. 
     As shown in FIG. 4, a number of predetermined trenches  104  are formed on a silicon substrate  101 , and then a thermal oxide film  102  is formed on the inside walls of the trenches  104 , whereupon a silicon oxide film  105  is filled in the trenches  104  by low pressure chemical vapor deposition (hereinafter also called LP-CVD) and is leveled by chemical and mechanical polishing (hereinafter called the CMP process). 
     In their paper, Lee, et al. disclosed that densification of the silicon oxide film, namely, resistance to wet etching was achieved by annealing at a lower temperature in a water-vapor ambient. 
     Lee, et al. also pointed out that since the inside walls of the trenches were oxidized, stresses occurred in the trenches to cause crystalline faults. 
     However, in this conventional technology, since the silicon oxide film used to fill the trenches by CVD was inadequately densified, the wet etching rate was great. Consequently, as shown in FIG. 4, after leveling by the CMP method, pits  106  occurred in the film filled in the trenches or a non-illustrated slit occurred centrally in the individual trench. 
     In the meantime, in order to improve the foregoing inconvenience, as an example, it is necessary to densify the silicon oxide film as by annealing at a high temperature equal to or higher than 1200° C. But if this technique was adopted, large thermal stresses would have occurred in the substrate to cause slippage and/or crystal faults. 
     Yet in this conventional technology, since densification by annealing is carried out at a low temperature, the inside walls of the trenches are oxidized so that large stresses occur in the trenches, thus causing faults in the substrate. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a method of manufacturing a semiconductor device which method adequately densifies a trench-forming film at a low temperature using a high-pressure ambient can reduce occurrences of possible faults and, at the same time, can effectively isolate individual circuit elements from one another. 
     According to a first aspect of the invention, the above object is accomplished by a method of manufacturing a semiconductor device, comprising the steps of: forming on a substrate an isolation trench for isolating a plurality of semiconductor circuit elements; forming a thermal oxide film on a sidewall of the isolation trench formed in the trench-forming step; forming a silicon oxide film on the substrate by chemical vapor deposition (CVD); and annealing the silicon oxide film along with the substrate in a high-pressure ambient. 
     Preferably, the annealing of the CVD silicon oxide film and the substrate is carried out in a high-pressure ambient so that the silicon oxide film is densified at a lower temperature compared to normal-pressure ambient. 
     According to a second aspect of the invention, the above object is accomplished alternatively by a method of manufacturing a semiconductor device, comprising the steps of: forming on a substrate an isolation trench for isolating a plurality of semiconductor circuit elements; forming a thermal oxide film on a sidewall of the isolation trench formed in the trench-forming step; forming a silicon nitride film on the substrate; forming a silicon oxide film on the substrate by chemical vapor deposition (CVD); and annealing said silicon oxide film along with the substrate in a high-pressure ambient. 
     In the second aspect of the invention, like the first aspect of the invention, the annealing of the silicon oxide film and the substrate is carried out preferably in a high-pressure ambient so that the silicon oxide film is densified at a lower temperature compared to normal-pressure ambient. Also preferably, the high-pressure ambient contains water vapor. 
     According to a third aspect of the invention, the above object is accomplished yet alternatively by a method of manufacturing a semiconductor device, comprising the steps of: forming on a substrate an isolation trench for isolating a plurality of semiconductor circuit elements; forming a thermal oxide film on a sidewall of the isolation trench formed in the trench-forming step; forming a silicon nitride film on the thermal oxide film formed on the sidewall of the trench; forming a silicon oxide film on the substrate by chemical vapor deposition (CVD); and annealing the entire substrate in a high-pressure ambient. 
     In the third aspect of the invention, the second aspect of the invention, the annealing of the silicon oxide film and the substrate is carried out preferably in a high-pressure ambient so that the silicon oxide film is densified at a lower temperature compared to normal-pressure ambient. Yet preferably, the high-pressure ambient contains water vapor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, advantages and features of the present invention will be more apparent from the following description taken in conjunction with the accompanying drawings in which: 
     FIGS.  1 ( a ) to  1 ( d ) are schematic cross-sectional views showing sequential process steps of a semiconductor device fabrication method according to a first embodiment of the present invention; 
     FIGS.  2 ( a ) to  2 ( f ) are schematic cross-sectional views showing sequential process steps of another semiconductor device fabrication method according to a second embodiment of the invention; 
     FIG. 3 is a characteristic graph showing dependency of the viscosity of a silicon oxide film on the water content in the film; and 
     FIG. 4 shows the manner in which circuit elements are isolated from one another by trenches formed according to the conventional technology. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The principles of the present invention are particularly useful when applied to a method of manufacturing a semiconductor device, a couple of preferred embodiments of which will now be described with reference to the accompanying drawings. 
     In the preferred embodiments of the invention, with silicon oxide and nitride films, which are formed and patterned on a substrate, as a mask, trenches are formed between circuit elements, and then over the substrate a silicon oxide film is formed by CVD. 
     Then the silicon oxide film is densified in structure by annealing in a high-pressure ambient, and the resultant silicon oxide film is then flattened by the CMP process to leave the last-named silicon oxide film only in the trenches. 
     In the absence of the silicon nitride film on the trench sidewalls, the ambient to be used is preferably any of water vapor, oxygen, nitrogen, hydrogen and N 2 O or any combination of them. 
     First Embodiment 
     FIGS.  1 ( a ) to  1 ( d ) are schematic cross-sectional views showing sequential process steps of the semiconductor device fabrication method according to a first embodiment. 
     In this fabrication method, a silicon nitride film  3  is formed over a substrate  1  via a thermal oxide film  2 . The silicon nitride film  3  and thermal oxide film  2  are patterned in a desired pattern of trenches by photolithography and dry etching. 
     Then with the silicon nitride film  3  as a mask, the silicon of the substrate  1  is dry-etched to form trenches  4  in a desired shape. After these trenches  4  have been formed, a thermal oxide film  1 A for protection of the trenches  4  is formed on inside walls of the trenches  4  (FIG.  1 ( a )). 
     A silicon oxide film  5  is then formed over the substrate  1  by low pressure CVD to fill the trenches  4  (FIG.  1 ( b )). 
     Subsequently, in order to make it tight in structure, the silicon oxide film  5  is annealed in a high-pressure ambient devoid of either water vapor or oxygen. 
     During the annealing in such ambient, the silicon oxide film is in general a much viscous liquid and assumes a solid-like state when it is low in temperature like room temperature. As temperature rises, the viscosity is lowered and, about its melting point, the silicon oxide film takes a flowable characteristic liquid. 
     This lowering of viscosity occurs also in a relatively low temperature; for example, it is less than approximately 10 12  poises at 1000° C. At that time, if pressure is exerted on the surface of the silicon oxide film, its structure will be rearranged. 
     Whereas under normal pressure, no force would exert from outside so that the structure will not be rearranged until it becomes higher in temperature. Accordingly, it is possible to make a smaller quantity of silicon oxide film tight in structure by annealing the silicon oxide film in a high-pressure ambient. 
     And since high-pressure annealing is carried out in an ambient free of either water vapor or oxygen, the silicon in the trenches would not be oxidized so that stresses inside the trenches due to the growth of the silicon oxide film on the trench sidewalls can be prevented from increasing. 
     After tightening of structure is done by annealing in a high-pressure ambient, flattening is done by the CMP process. At that time, the silicon nitride film  3  serves as a barrier layer against chemical and mechanical polishing so that optimum polishing can be secured (FIG.  1 ( c )). 
     This leveling by the CMP process takes place in a cleaning step using a solution containing fluoric acid. Consequently the adequately densified silicon oxide film  5  would lose its mass less causing no pits. 
     Then the silicon nitride film  3  used as a barrier layer is removed by a wet method or by dry etching using plasma, and the thermal oxide layer  2  also is removed (FIG.  1 ( d )). 
     Subsequently, also during washing with a solution containing fluoric acid in formation of a gate oxide film, this adequately densified silicon oxide film  5  would lose its mass less to become level or flat. 
     Embodiment 2 
     FIGS.  2 ( a ) to  2 ( f ) are schematic cross-sectional views showing sequential process steps of the semiconductor device fabrication method according to a second embodiment. 
     In this fabrication method, like the method of the first embodiment of FIG. 1, a first silicon nitride film  13  is formed over a sheet-like substrate  11  via a first thermal oxide film  12 , whose thickness is 100 angstroms. 
     Then the first silicon nitride film  13  and the first thermal oxide film  12  are patterned in a desired pattern by photolithography and dry etching. Then with the first silicon nitride film  13  as a mask, the silicon of the substrate  11  is dry-etched to form trenches  14  in a desired shape. After these trenches  14  have been formed, a second thermal oxide film  11 A for protection of the trenches  14  is formed over inside walls of the trenches  14  (FIG.  2 ( a )). 
     A second silicon nitride film  16  is then formed over the substrate  11  by low pressure CVD to cover the first silicon nitride film  13  and the second thermal oxide film  11 A (FIG.  2 ( b )). At that time, if it is too thick, the second silicon nitride film  16  would be overetched to cause pits after chemical and mechanical polishing and, if it is too thin, the second silicon nitride film  16  would become less resistant to oxidation during annealing. Consequently the thickness of the second silicon nitride film  16  is preferably in the range of approximately 50 to 100 angstroms. 
     As shown in FIG.  2 ( c ), the second silicon nitride film  16  is etched in such a manner that its portion left unetched covers only the inside walls of the trenches  14 . This enables to reduce stresses to be exerted on the second silicon nitride film  16 . Then a CVD silicon oxide film  17  is formed over the substrate  11  by low pressure CVD, filling the trenches  14  (FIG.  2 ( d )). 
     Subsequently, in order to densify it in structure, the silicon oxide film  17  is annealed in a high-pressure ambient containing water vapor. 
     During the annealing in the water-vapor-containing ambient, a large quantity of water penetrates into the silicon oxide film  17 . The relation between the water content of the silicon oxide film  17  and the viscosity of the same film is shown in a graph of FIG.  3 . It turns out from the graph of FIG. 3 that with the increase of the water content, the silicon oxide film  17  becomes sharply less viscous. 
     Thus in the water-vapor-containing ambient, the silicon oxide film  17  can be softened at a low temperature. By adding a high pressure with the temperature restricted to a relatively low range, it is possible to make the silicon oxide film  17  adequately densified. 
     At that time the second silicon nitride film  16  on the inside walls of the trenches  14  serves to prevent oxidation of the silicon of the substrate  11  so that the internal stresses of the trenches due to the oxidation of silicon of their inside walls would not increase. 
     Subsequently, after being densified by annealing, the silicon oxide film  17  is leveled or flattened by the CMP process. At that time, the first silicon nitride  13  serves as a barrier layer during the chemical and mechanical polishing (CMP) process and, as a result, optimum polishing can be achieved (FIG.  2 ( e )). 
     In this case, during washing with a solution containing fluoric acid after chemical and mechanical polishing, the silicon oxide film  17  has already been adequately densified due to annealing and loses its mass less causing no pits. 
     Then the first silicon nitride film  13  used as a barrier layer is removed by a wet method or by dry etching using plasma, and the first thermal oxide film  12  is removed (FIG.  2 ( f )). 
     The adequately densified silicon oxide film  17  loses its mass less also while washing with a fluoric-acid-containing solution during a subsequent process of forming a gate oxide film, thus resulting in a smooth surface. 
     According to the semiconductor device of the present invention, since the silicon oxide film to be used in forming trenches by CVD is adequately densified by annealing in a high-pressure ambient, it is possible to completely eliminate pits, which would have occurred at opposite ends of the trench-forming film after leveling according to the conventional art. It is therefore possible to reduce occurrences of faults in the substrate and, at the same time, to surely isolate the individual circuit elements on the substrate. 
     In addition, given that the silicon nitride film is formed on the sidewalls of trenches, it is possible to prevent oxidation of silicon of the trench sidewalls much more effectively. It is therefore possible to avoid any possible stresses, which would have tended to occur in the trenches with the lapse of time, thus improving the durability of the whole device. 
     It is thus apparent that the present invention should by no means be limited to the illustrated embodiment and various modifications and changes may be suggested without departing from the scope and spirit of the invention. 
     Finally, the present application claims the priority of Japanese Patent Application No. Hei9-156256 filed on Jun. 13, 1997, which is herein incorporated by reference.