Abstract:
Si regions separated from a Si-rich SiO 2  film are nitrided to provide a film mainly consisting of SiO 2  regions an Si 3  N 4  regions to be used for constituting a dielectric device or a capacitor.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a dielectric device having a high permittivity and good insulation characteristics. 
     Conventional capacitors are constituted, for instance, by first forming an N +  -type drain region on part of a Si substrate, then forming a SiO 2  layer thereon, and finally forming a polysilicon electrode on the SiO 2  layer. However, such capacitors cannot provide a sufficiently large permittivity. With a recent increase of the degree of integration of this type of devices and a resultant reduction of the electrode area, it has been desired to utilize a material having a high permittivity. 
     SUMMARY OF THE INVENTION 
     In view of the present need as described above, an object to the present invention is to provide a dielectric device having a high permittivity and sufficient dielectric characteristics. 
     According to the invention, a dielectric device comprises: 
     a Si substrate; 
     an oxide film formed on the Si substrate; and 
     a Si-rich SiO 2  nitride film formed on the oxide film and including SiO x  regions and SiN x  regions, the SiN x  regions being formed by nitriding Si regions separated from a Si-rich SiO 2  film. 
     Using more specific terms, the dielectric device of the invention includes the SiO 2  film and the Si-rich SiO 2  nitride layer that mainly consists of SiO 2  regions and Si 3  N 4  regions. The SiO 2  film is to provide sufficient insulation (i.e., dielectric breakdown) characteristics. On the other hand, in the Si-rich SiO 2  nitride film, while the SiO 2  regions also work to provide the sufficient insulation characteristics, the Si 3  N 4  regions serve to increase the permittivity. Since the Si 3  N 4  regions of the nitride film are a superior dielectric material though having insulation characteristics somewhat inferior to the SiO 2  film, the SiO 2  film used can be made thinner than the conventional dielectric device. 
     The capacitance of a capacitor is generally proportional to a relative permittivity of a dielectric material used therein. In the invention, the partial occupation by the Si 3  N 4  regions, which have a relative permittivity larger than SiO 2 , makes it possible to provide a larger capacitance compared with a same-sized dielectric device having only a SiO 2  film. When compared with a dielectric device having the same capacitance and having only a SiO 2  film, the area of the dielectric device can be made smaller by 10-20%. It is noted that relative permittivities of SiO 2  and Si 3  N 4  are 3.9 and 7.0, respectively. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional view of a DRAM cell according to an embodiment of the present invention; and 
     FIG. 2 is a sectional view of a capacitor according to another embodiment of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Embodiments of the present invention are described hereinafter with reference to the accompanying drawings. 
     FIG. 1 is a sectional view of a DRAM cell according to an embodiment of the invention. FIG. 2 is a sectional view of a capacitor according to another embodiment. 
     To explain the FIG. 2 embodiment first, reference numeral 10 represents a SiO 2  layer; 8, Si-rich SiO 2  nitride layer; 8a, SiO 2  region; 8b, Si 3  N 4  region; and 7, polysilicon electrode. The Si 3  N 4  region 8b is formed by nitriding Si regions that have separated from the Si-rich SiO 2  layer. 
     Turning now to the embodiment of FIG. 1, a transistor portion Tr is formed by parts 1, 2, 4, 5, 6, 9 and 12 that are located on the left side in the figure, and a capacitor portion C is constituted of parts 12, 6, 10, 8, 7 and 3 that are on the right side. Reference numeral 1 represents a bit line; 2, word line; 3, cell plate; 4, source region; 5, gate electrode; 6, drain region; 7, polysilicon electrode; 8, Si-rich SiO 2  nitride film; 9, gate oxide film, 11, LOCOS layer; and 12, Si substrate. 
     Referring to FIG. 1, the capacitor portion C is constituted according to the following procedure: formation of a SiO 2  thin film 10 (its thickness is less than about 10 nm), formation of a Si-rich SiO 2  film, nitriding of it, and formation of an electrode (i.e., cell plate 3 made of polysilicon, aluminum, etc.). The Si-rich SiO 2  nitride film formed in this manner mainly consists of SiO x  and SiN x  regions. Where high insulation performance is not required, the above SiO 2  thin film 10 can be omitted. Conversely, where it is required, another SiO 2  thin film may be added below the cell plate 3. 
     A dielectric device having the above configuration is produced according to the following procedure: formation of the oxide film 10 on the Si substrate 12, formation of the Si-rich SiO 2  film on part of the oxide film 10, and nitriding separated Si regions in the Si-rich SiO 2  film to form the Si-rich SiO 2  nitride film 8. 
     A capacitor is produced according to the following procedure: formation of the oxide film 10 on the Si substrate 12, formation of the Si-rich SiO 2  film on part of the oxide film 10, nitriding separated Si regions in the Si-rich SiO 2  film to form the Si-rich SiO 2  nitride film 8, and formation of the polysilicon electrode film 7 on the nitride film 8. 
     A DRAM is produced as a combination of the dielectric device and the capacitor which are produced in the above manner. 
     As described in the above embodiments, according to the invention, the capacitor is produced, for instance, by forming the Si-rich SiO 2  film after formation of the oxide film on the Si substrate, then nitriding the separated Si regions to form the Si-rich SiO 2  nitride film, and finally forming the polysilicon electrode. Therefore, by virtue of partial existence of the Si 3  N 4  regions, which have a relative permittivity larger than SiO 2 , a larger capacitance can be obtained compared with a same-sized capacitor having only a SiO 2  film (the capacitance is proportional to the relative permittivity). When compared with a capacitor having only a SiO 2  film and the same capacitance, the area of the capacitor of the invention can be made smaller by 10-20%. 
     The dielectric device of the invention makes it possible to utilize the material having a larger relative permittivity, to utilize the very thin SiO 2  film having a thickness less than 10 nm, and to reduce the area of the memory cell (capacitor) by increasing the whole permittivity.