SOS island edge passivation structure
REFERENCE TO RELATED APPLICATIONS 
     This is a division of application Ser. No. 085,148, filed Oct. 15, 1979, now U.S. Pat. No. 4,242,156, issued December 30, 1980. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to silicon-on-sapphire (SOS) semiconductor devices, and in particular to structures for SOS devices which operate at high operating voltages. 
     2. Description of the Prior Art 
     The typical prior art SOS semiconductor device structure consists of a silicon island on a sapphire substrate. A gate oxide is grown on the surface of the silicon island, and gate metal is deposited thereover. The thickness of the oxide at the island edge is typically only 400 A. 
     Although for many device applications such prior art structures are acceptable for SOS circuits with high operating voltages, such structures are disadvantageous. The relatively thin oxide thickness is a frequent source of circuit failure due to oxide breakdown. 
     Prior to the present invention there has not been a simple and easy-to-fabricate SOS semiconductor device structure which is relatively immune to oxide breakdown. 
     The novel features which are considered as characteristic for the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a highly simplified cross-section of an SOS semiconductor device structure according to the prior art; 
     FIG. 2 is a highly simplified cross-section of an SOS semiconductor device structure according to the present invention; 
     FIG. 3(a) shows the first step in the fabrication process for producing the semiconductor device according to the present invention, in which a silicon layer on a sapphire substrate is etched to form an island; 
     FIG. 3(b) shows the second step in the fabrication process for producing the semiconductor device according to the present invention, in which an oxide layer is grown over the silicon island; 
     FIG. 3(c) shows the third step in the fabrication process for producing the semiconductor device according to the present invention, in which a silicon nitride layer is deposited over the oxide layer and the sapphire substrate; 
     FIG. 3(d) shows the fourth step in the fabrication process for producing the semiconductor device according to the present invention, in which a photoresist layer is applied over the silicon nitride layer and exposed from the backside; 
     FIG. 3(e) shows the fifth step in the fabrication process for producing the semiconductor device according to the present invention, in which the silicon nitride and oxide layer on the top major surface of the silicon island is etched to bare silicon; 
     FIG. 3(f) shows the sixth step in the fabrication process for producing the semiconductor device according to the present invention, in which gate oxide is grown over the silicon island; and 
     FIG. 3(g) shows the final step in which a metal or polysilicon electrode is deposited over the gate oxide. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Turning now to FIG. 1 there is shown a highly simplified cross-sectional view of an SOS semiconductor structure according to the prior art. The FIGURE indicates a silicon island disposed on the major surface of a sapphire substrate with a gate oxide layer grown over the top surface of the silicon island. Over the gate oxide, as well as over the sapphire substrate is placed a gate metal for forming a gate electrode of a field effect transistor to be formed therein. As pointed out above, the relatively thin gate oxide layer is particularly susceptible to failure due to oxide breakdown. 
     FIG. 2 is a highly simplified cross-sectional view of an SOS semiconductor device structure according to the present invention. There is shown a similar silicon island disposed on the major surface of a sapphire substrate with a gate oxide layer disposed over the exposed major surface of the silicon island, i.e. a top portion and a side portion. A chemical vapor deposited (CVD) nitride layer is disposed over the surface of the sapphire substrate as well as the gate oxide located over the side or end portion and corner edge of the silicon island. Finally, over the surface of the gate oxide on the top surface of the silicon island, as well as over the CVD nitride layer, a layer of gate metal is disposed. 
     In a typical example the device according to the present invention is used in an MNOS memory circuit in which the silicon nitride layer, shown in the diagram, may be the same silicon nitride layer deposited for the memory gate insulator. 
     In a preferred embodiment, the silicon nitride layer in FIG. 2 is approximately 300-800 A in thickness, so that on top of the thermal oxide layer which is from 100-800 A in thickness, a relatively thick layer is provided over the edge of the silicon island which prevents oxide breakdown and circuit failure. 
     A controlled test at 260° C. using 830 A oxides and nitride thickness layers were done which compared devices with edge protection and without edge protection, at applied voltages ranging from 25-35 volts. Devices without edge protection after a period between 1-10 hours began to show rapidly increasing failures with a number of surviving devices approaching zero after two hours for the applied voltage of 35 volts and approaching zero after 12 hours with an applied voltage of -35 volts. A comparative situation with edge protected devices showed no failures. 
     Turning now to FIG. 3 there is shown a sequence of steps for fabricating the device according to the present invention. 
     FIG. 3(a) shows the first step in the fabrication process for producing the semiconductor device according to the present invention, in which a silicon layer on a sapphire substrate is etched to form an island. 
     FIG. 3(b) shows the second step in the fabrication process for producing the semiconductor device according to the present invention, in which a first oxide layer is grown over the silicon island. 
     FIG. 3(c) shows the third step in the fabrication process for producing the semiconductor device according to the present invention, in which a silicon nitride layer is deposited entirely over the oxide layer over the silicon island, as well as over the sapphire substrate. The preferred method of deposition is by chemical vapor deposition (CVD). 
     FIG. 3(d) shows the fourth step in the fabrication process for producing the semiconductor device according to the present invention, in which a photoresist layer is applied over the silicon nitride layer and exposed from the backside. 
     FIG. 3(e) shows the fifth step in the fabrication process for producing the semiconductor device according to the present invention, in which the silicon nitride and oxide layer on the top major surface of the silicon island is etched to bare silicon. 
     FIG. 3(f) shows the sixth step in the fabrication process for producing the semiconductor device according to the present invention, in which a second or gate oxide layer is grown over the exposed top surface of the silicon island. 
     FIG. 3(g) shows the final step in which a conductive layer (e.g., metal or polysilicon) is deposited over the second oxide layer. 
     The use of such semiconductor structures in the formation of active devices would be known to those skilled in the art, and need not be elaborated here. 
     While the invention has been illustrated and desc ribed as embodied in an SOS island edge passivation structure and method of fabrication, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention. 
     Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can, by applying current knowledge, readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitutes essential characteristics of the generic or specific aspects of this invention, and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims.