Abstract:
A method for producing a Schottky diode formed of a doped guard ring in an edge area of the Schottky contact is described. The guard ring is produced by depositing a high barrier material, especially made of platinum, on the surface of the semiconductor layer. The surface is provided with a structured masking layer beforehand, and which is subsequently etch-backing.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This is a continuation of copending International Application PCT/DE99/01428, filed May 11, 1999, which designated the United States. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates to a method for producing Schottky diodes having a protective ring in an edge region of a Schottky contact. The invention furthermore relates to a Schottky diode. 
     During the production of very small-area Schottky diodes, that is to say of Schottky diodes having a diameter of the order of magnitude of 5 to 10 μm, eliminating the edge effects poses considerable difficulties. It is known to reduce the edge effects by use of a doped protective ring being introduced into the semiconductor substrate and/or into the topmost epitaxial layer using implantation technology. In the case of very small-area Schottky diodes, however, a doped protective ring produced by photomasks constitutes such a considerable additional capacitance that Schottky diodes produced in such a way cannot be used in relatively high frequency ranges. applications in the 77 GHz frequency range demand that the Schottky diode have a total capacitance of approximately 40 fF, which permits a parasitic capacitance of at most 10 fF. 
     SUMMARY OF THE INVENTION 
     It is accordingly an object of the invention to provide a method for producing Schottky diodes which overcomes the above-mentioned disadvantages of the prior art method of this general type, which have typical diameters of 5 to 10 μm and can be used in high-frequency ranges typically of 77 GHz and above. 
     With the foregoing and other objects in view there is provided, in accordance with the invention, a method for producing a Schottky diode. The method includes the step of providing a semiconductor layer having a surface. A mask layer is produced on the semiconductor layer. The mask layer is formed so as to leave a region free of the mask layer on the surface of the semiconductor layer, the region provided for forming a Schottky contact. The mask layer and the semiconductor layer are covered by a barrier metal layer having edges for forming the Schottky contact. A ring shaped area remains free on the surface of the semiconductor layer between the mask layer and the edges of the barrier metal layer. The ring shaped area is covered by a high barrier metal and the high barrier metal forms a protective ring. 
     The invention provides for the protective ring to be produced after the production of the Schottky contact by depositing an electrically conductive protective ring material onto the surface of the semiconductor layer, which surface is provided with a patterned masking layer. 
     Following the principle of the invention, the protective ring material constitutes a metal, in particular a high barrier metal, which has, in particular, platinum. 
     The protective ring material is expediently applied by vapor deposition or by being sputtered on. 
     A further preferred embodiment of the invention may provide for the patterned masking layer to be etched back by undercutting prior to the deposition of the Schottky contact material using a patterned covering layer. 
     In accordance with an added feature of the invention, there is the step of applying the high barrier metal to a thickness of approximately 0.1 μm. 
     In accordance with an additional feature of the invention, there is the step of forming the mask layer as a two-part layer formed of a silicon oxide layer and, on top of that, a silicon nitride layer is applied and the mask layer is patterned. The silicon nitride layer is undercut for forming overhanging rims. 
     In accordance with another feature of the invention, there is the step of applying the silicon oxide layer to a thickness of approximately 0.2 μm and applying the silicon nitride layer to a thickness of approximately 0.1 μm. 
     In accordance with a further feature of the invention, there is the step of forming the barrier metal layer by depositing titanium to a thickness of at most 0.1 μm. 
     In accordance with a concomitant feature of the invention, there is the step of forming the semiconductor layer from silicon and siliconizing the high barrier metal after its applied. 
     Other features which are considered as characteristic for the invention are set forth in the appended claims. 
     Although the invention is illustrated and described herein as embodied in a method for producing Schottky diodes, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. 
     The construction and method of operation of the invention, however, 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 diagrammatic, sectional view of a semiconductor wafer with a patterned oxide and nitride layer mask and a photoresist mask according to the invention; 
     FIG. 2 is a sectional view of the semiconductor wafer according to FIG. 1 after a Schottky contact material has been applied; 
     FIG. 3 is a sectional view of the semiconductor wafer after the deposition of a protective ring material and after the deposition and patterning of contact metallization layers; 
     FIG. 4 is a sectional view of the semiconductor wafer according to FIG. 1 after the vapor deposition of the Schottky contact material in accordance with a second exemplary embodiment of the invention has been performed; 
     FIG. 5 is a sectional view of the semiconductor wafer according to FIG. 4 after the deposition of the protective ring material; and 
     FIG. 6 is a sectional view of the semiconductor wafer according to FIG. 5 after the deposition and patterning of the contact metallization layers. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In all the figures of the drawing, sub-features and integral parts that correspond to one another bear the same reference symbol in each case. Referring now to the figures of the drawing in detail and first, particularly, to FIG. 1 thereof, there is shown a substrate  1  as the starting point of the method of the invention. The substrate  1  is composed of a semiconductor material, and has a semiconductor layer  2 , which is deposited epitaxially thereon. On the epitaxially deposited semiconductor layer  2  there is a masking layer formed of a silicon oxide layer  3 A having a thickness of approximately 0.2 μm, and a low-pressure silicon nitride layer  3 B having a thickness of approximately 0.1 μm, which is deposited on the silicon oxide layer  3 A under low pressure. For the definition of an active area of the Schottky diode and for the etching of the silicon nitride layer  3 B, a photoresist layer  6  is patterned using lithography technology known per se. This is followed by targeted undercutting of the silicon oxide layer  3 A likewise using known methods, the depth of the undercutting typically being at most 0.2 μm. Reference numeral  5  designates an axis of symmetry of the Schottky diode, which is configured to be rotationally symmetrical. The substrate  1  may be composed for example of n + -doped silicon, while the semiconductor layer  2  may be composed of n-doped silicon. This choice of material is not mandatory, however. The substrate  1  and semiconductor layer  2  may also have the mirror-reversed p-type doping or be composed of different semiconductor material such as, for example, germanium, gallium arsenide or indium phosphide. The masking layer  3 A,  3 B may be composed of any desired insulator. 
     FIG. 2 shows a semiconductor wafer according to FIG. 1 in which, after the removal of the photoresist layer  6  by resist stripping, a barrier metal  10  has been deposited over the whole area for the purpose of forming the Schottky contact, in particular has been vapor-deposited perpendicularly to a surface  9  of the semiconductor layer  2 . The thickness of the metal layer  10  is approximately 0.1 μm at most. A suitable metal for the layer  10  is titanium, for example. 
     Afterwards, a high barrier metal  4  such as platinum, for example, is sputtered on isotropically, the metal  4  indiffusing through a gap  12  between a nitride edge  7  and a titanium edge  8  and covering an open silicon ring area  13  between the titanium layer  10  and the oxide layer  3 A (FIG.  3 ). 
     This is followed by vapor deposition with a gold layer  14  according to FIG. 3 in order, if appropriate, to fill the gap still remaining between the nitride edge  7  and the titanium edge  8  and to produce a bondable surface. The thus completed contact with gold, illustrated in the sectional view in FIG. 3, is also known by the designation Bell contact. An electrode layer  11  deposited on the rear side of the substrate  1  forms a second terminal of the Schottky contact. 
     FIGS. 4 to  6  show a second exemplary embodiment of the invention. Proceeding from the configuration according to FIG. 1, according to FIG. 4 the barrier metal  10  such as titanium, for example, is applied, to a thickness typically of at most 0.1 μm, to the surface  9  of the epitaxial layer  2  and the masking layer  3 A,  3 B covered with photoresist  6 , the application preferably being effected by perpendicular vapor deposition. This is then followed by a lift-off strip in order to remove the photoresist  6  and the titanium material  10  situated thereon. Overhanging rims  15  of the nitride layer  3 B are removed by etching in hot phosphoric acid selectively with respect to the titanium material  10 . 
     Afterwards, in accordance with FIG. 5, the high barrier metal layer  4  made of platinum is sputtered on isotropically. Platinum regions  16  situated between the titanium layer  10  and the oxide layer  3 A are siliconized in order to form the the protective ring. The platinum is sputtered back as far as the nitride layer  3 B, the titanium and the platinum silicide  16  in the protective ring being subjected to only superficial sputtering. 
     The Schottky contact is completed after the vapor deposition and pattering of a Bell contact with a layer  17  containing, for example, titanium-platinum-gold, and the deposition of the electrode layer  11  on the rear side of the substrate  1  for the second terminal. 
     In the two exemplary embodiments according to FIG.  3  and FIG. 6, the high barrier metal  4  made of platinum or its silicide  16  fulfills the requirements of a protective ring because it compensates the barrier lowering at the junction between the Schottky metal  10  and the oxide  3 A, which is caused by reduction in the interface state densities toward the oxide. For Schottky diodes with a typical diameter of 5 μm, the parasitic capacitance is approximately 5 fF (measured for a voltage U=0 V).