Abstract:
A matrix array of semiconductor diodes formed in an epitaxial layer of a semiconductor wafer and being dielectrically isolated from each other by two orthogonal sets of parallel insulating oxide regions, one set extending completely through the epitaxial layer and the other set extending only partially through the epitaxial layer. A preferred method of forming the matrix array is also disclosed.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to integrated circuits and more particularly to monolithic integrated circuits of the type having diodes (or other circuit components) formed on a common substrate and which are electrically isolated from each other throughout the substrate. 
     As used herein, the expression &#34;monolithic integrated circuits&#34; refers to a single substrate or wafer of semiconductive material, typically monocrystalline, on (or in) which individual active and/or passive circuit components are formed which, when appropriately interconnected, results in the desired circuit network. Accordingly, such circuit networks require electrical isolation between the various circuit components formed on or in the wafer. 
     In the past, various techniques have been developed to provide the electrical components with the required electrical isolation, one from another. For example, in one method, an epitaxial layer of semiconductive material is formed upon a substrate of opposite conductivity type and the discrete components are formed in the epitaxial layer. Thereafter, an isolation region is formed by doping the areas surrounding each device with an impurity having the same conductivity type as the substrate. This technique then requires that the device be heated to an elevated temperature in excess of 1,000°  C. for an extended period of time to create vacancies in the lattice structure of the epitaxial layer to enable the impurity to diffuse through the epitaxial layer and contact the substrate, thus forming the isolation barrier. 
     This prior art technique has a well known disadvantage in that, in diffusing through the epitaxial layer, the dopant tends to diffuse in all directions. The resultant lateral spread of the dopant extends through a larger area of the substrate than would be required if the dopant were made to diffuse in a vertical direction. Additionally, the high temperatures required to enable the dopant to diffuse through the epitaxial layer may, in certain instances, introduce contaminative impurities into the device and thus impair its characteristics. 
     SUMMARY OF THE INVENTION 
     Briefly, and in accordance with the present invention, a semiconductor wafer of one conductivity is provided, on one surface thereof, with an epitaxial layer of semiconductor material of an opposite conductivity and a nitride layer. A first set of substantially parallel grooves are etched through the nitride layer and partially through the epitaxial layer after which a second set of substantially parallel grooves, arranged to cross the first set of grooves, is etched through the nitride layer. The grooved wafer is then subjected to an oxidation step which converts the exposed grooved portions to a insulating oxide. The result is a structure that has a first set of grooves that extend completely through the epitaxial layer and a second set of crossing grooves that extend only partially through the epitaxial layer. The grooves, being filled with an insulator material forms discrete &#34;islands&#34; which are electrically isolated one from another on which circuit components may be fabricated. 
     It is, therefore, a principal object of the present invention to provide an improved method for fabricating integrated circuits on a semiconductor wafer wherein the individual circuits are dielectrically isolated one from another. 
     An additional principal object of the present invention is to provide an improved method of fabricating diodes on a semiconductor wafer wherein grooves etched therein provide dielectric isolation between adjacent diodes and the depth of the grooves facilitate the connection of the diodes to rows and columns. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A through 9A are a cross-sectional view of the various stages of processing taken through section &#34;A--A&#34; of FIG. 11; 
     FIGS. 1B through 9B are a cross-sectional view of the various stages of processing taken through section &#34;B--B&#34; of FIG. 11; 
     FIG. 10 is a schematic representation of a diode array formed by my novel process; and 
     FIG. 11 is a partial cross-sectional, isometric view of my novel device. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to FIGS. 1A and 1B, there is shown a cross-sectional view taken along lines &#34;A--A&#34;  and &#34;B--B&#34; respectively of FIG. 11 wherein a P-type semiconductor wafer 12 is shown provided with an N-type epitaxial layer 14 having a nitride layer 16 deposited thereon and photoresistive layer 18 having apertures 20 therein. While substrate 12 and epitaxial layer 14 have been shown as having P and N type conductivities, it will be obvious to those skilled in the semiconductor arts that these conductivities may be reversed without departing from the inventive concept. The reversal of the noted conductivities, together with appropriate changes in diffusion material, will also produce my novel diode array. It should also be obvious to those skilled in the art that semiconductor wafer 12 may be either purchased commercially with epitaxial layer 14 grown thereon or may be grown by the user prior to the deposition of silicon nitride layer 16. 
     Apertures 20 are utilized to define subsequent &#34;deep cut&#34;, parallel grooves (as distinguished from the &#34;shallow cut&#34;) as will be hereinafter defined. Having provided the structure of FIGS. 1A and 1B, a first nitride etch is performed on the structure so as to etch the portions of nitride layer 16 exposed by apertures 20. Thereafter, as shown in FIGS. 2A and 2B, photoresist layer 18 is removed from the structure and the now etched nitride layer 16 is used as a mask for a subsequent silicon etch of layer 14, thus providing the initial step in the subsequent formation of the parallel deep cut grooves. 
     Having accomplished the initial step for the deep cut parallel grooves, the structure is now provided with another photoresist layer 24, having apertures 26 therein (FIG. 3B), with layer 24 filling apertures 22 formed by the prior etching step (FIG. 3A). 
     Referring now to FIGS. 4A and 4B it will be seen that another nitride etching step is performed on the structure of FIGS. 3A and 3B to etch the parallel grooves in nitride layer 16 exposed by apertures 26. Photoresist layer 24 is then removed and the resulting structure is a plurality of grooves, shown in section as apertures 28 (FIG. 4B), in nitride layer 16 and represents the first step in the production of the pattern of parallel shallow grooves. Thus, with nitride layer 16 provided with apertures 22 (FIG 4A) and 28 (FIG. 4B) it may now be used as a mask for the formation or growth of oxide 30 in apertures 22 and 28 as shown in FIGS. 5A and 5B. This oxide may, for example, be grown using steam at approximately 1000° C.± 100° C. for a period of about 12 hours. 
     Thereafter, as shown in FIGS. 6A and B, the junction is diffused into epi layer 14 to form my novel device. This junction is formed, for example, by the diffusion of Boron Tribromide (BBr 3 ) into N-epi layer 14 at a furnace temperature of about 1050° C.± 10° C. Nitrogen is used as a primary carrier flowing at the rate of approximately 2950 cc per minute together with oxygen flowing at the rate of about 20 cc per minute is caused to flow through a bubbler containing 99.99% BBr 3  which is maintained at about 24° C. The boron doping is carried on until such time as layer 24 is doped for the required thickness, thus forming P-doped areas 32 (FIGS. 7A and 7B) which overlies N-epi areas 14 to form the required P-N junction and thus, a diode. 
     Referring to FIGS. 7A and 7B, my process requires that oxide layer 30.1 be grown over the exposed now doped areas 32 as well as oxide areas 30 for both the deep cut and the shallow cut. This oxide may be grown, for example, at a temperature of about 1000° C. utilizing dry oxygen for a sufficient period of time to form a thin, transparent layer about 1 micron thick, as is well known in the art. 
     Having now provided my novel device with a P-N junction (32-14) covered with a thin passivating oxide layer 30.1, it is now necessary to provide the required contacts to the various discrete elements, which, together with the appropriate lead lines, forms the array. This is accomplished by first providing the structure with a photoresistive mask 36 having apertures 38 therein wherein the apertures define the areas for the subsequent contact holes. In the embodiment shown, the contacts will be positioned at a corner of each P-N junction with the connecting leads running atop the oxide dividers 30 which act as insulators. To accomplish the formation of the contacts, the device is then subjected to an oxide etch which etches selected exposed portions of oxide layer 30.1 to form apertures 40 in oxide layer 30.1. 
     Thereafter, as shown in FIGS. 9A and 9B, metallic, conductive contacts 42 are deposited in apertures 40 to provide ohmic contacts to P-layer 32 while metallic, conductive lead line 42.1 provide ohmic connections to contacts 42. This may be done by either sputtering through another mask (not shown) or by an anodization technique (not shown) both methods being well known. 
     Referring now to FIG. 11, there is shown in partial section, an isometric view of my novel device consisting of P-type substrate 12 having an N-epitaxial layer 14 deposited thereon. However, by reason of the processing just described, N-epitaxial layer 14 is divided into rows 1, 2 and 3 due to deep cuts 22 and the deposition of oxide 30 therein (FIG 4A). The P-diffused areas 32, together with its associated areas of N-epitaxial layer 14, form the discrete P-N junction and thus, a diode. The diodes in each column (I, II, III and etc.) are separated from each other by reason of oxide 30 in deep cuts 22 (FIG. 4) while the diodes in each row (1, 2, 3 and etc.) are separated from each other by reason of oxide 30 in shallow cuts 22 (FIG. 4B). Further, contacts 42 provide the required ohmic connection to P-diffused areas 32 and each contact 42 is connected to corresponding P-type areas in the same column by reason of conductive leads 42.1. 
     Referring now to FIG. 10, it will thus be seen that a diode array is presented wherein the cathodes of each of the active devices in Column I are all connected together while all similar cathodes in Column II are connected to each other. To complete the array, the first anode of each diode of each row is connected together while similarly the anodes in rows 2, 3 and 4 are connected to a respective anode in the same row. 
     It should now become obvious to those skilled in the art that utilizing the proper driver circuitry as an input to the rows or columns of my array, one can readily determine the conductivity of the discrete P-N junction by appropriately interrogating the rows and columns, in accordance with well known techniques. 
     While the FIGS. 1-9 and 11 depict the grooves as being parallel and perpendicular with respect to each other, it will be obvious to those skilled in the art that while the parallelism is desired it is not necessary to have the crossing lines perpendicular to each other, as shown in FIG. 11. The important consideration being that the discrete islands be of the same general area so that they exhibit similar characteristics. Accordingly, it is not my intention to be limited to perpendicular crossings of deep and shallow grooves.