LOW RESISTANCE SCHOTTKY DIODE ON POLYSILICON/METAL-SILICIDE

Abstract: A Schottky diode is fabricated according to the following steps: forming a layer of metal-silicide on an underlying dielectric layer, forming a polysilicon layer on the upper surface of the metal-silicide layer, forming a second dielec­ tric layer on the upper surface of the polysilicon layer and patterning the second dielectric layer to create a contact window through the second dielectric layer to an exposed surface region of the polysilicon layer, and forming a metal contact to the exposed surface region.

Description: LOW RESISTANCE SCHO't'TRY DIODE ON POLySILICON/METAL-SILICIDE BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is directed to Schottky diodes and more particularly to a polysilicon Schottky diode which is fabricated by forming a sandwich of metal-silicide, polysilicon and metal on silicon dioxide. 2. Brief Description of Prior Art Schottky diodes are commonly fabricated in single crystal silicon together with bipolar transistors. In such devices, an N+ buried layer is used as the diode cathode. This causes both diode series resistance and buried layer to substrate capacitance to be very high. Schottky diodes have also been fabricated on polysilicon. However, two problems associated with this practice have been observed. First, Schottky diodes on polysilicon produce excessive leakage and low breakdown voltages. Second, such structures typically have high diode lateral series resistance due to the low mobility of electrons in polysilicon. SUMMERY OF THE INVENTION A general object of the invention is to provide an improved Schottky diode and a method for manufacturing such a diode. This and other objects are attained, in accordance with one aspect of the invention by a method for fabricating a Schottky diode, comprising: (a) forming a layer of metal-silicide on an underlying layer of dielectric; (b) forming a layer of polysilicon on the upper surface of said metal-silicide layer; (c) forming a second dielectric layer on the upper surface of said polysilicon layer and patterning said second dielectric layer to create a contact window through said second dielectric layer to an exposed surface region of said polysilicon layer; and (d) forming a metal contact to said exposed surface region. Another aspect of the invention includes a Schottky diode structure, comprising: (a) a layer of metal-silicide formed on an underlying layer of dielectric; (b) a layer of polysilicon formed on the upper surface of metal-silicide layer; (c) a second dielectric layer formed on the upper surface of said polysilicon and patterned such that a contact window extends through said second dielectric layer to a surface region of said polysilicon; and (d) a metal contact to said polysilicon surface region, said contact forming the anode of said Schottky diode structure and said metal-silicide forming the cathode of said structure. In fabricating an integrated Schottky diode structure according to a preferred method of the present invention, a metal-silicide layer is deposited on an underlying layer of silicon dioxide. A polysilicon layer is then deposited on the upper surface of the metal silicide layer. Phosphorous is then implanted into the polysilicon layer to provide a lower N+ polysilicon layer which forms an ohmic interface with the metal-silicide layer and a surface layer comprising N- polysilicon. The structure is then laser annealed such that about the top 3000 Angstroms of the polysilicon layer become single crystal. The polysilicon layer is then oxidized to form an overlying layer of silicon dioxide. The silicon dioxide is then masked and etched to create contact windows to exposed selected underlying surface regions of the polysilicon layer. N+ impurities are then implanted near the surface of the polysilicon at one selected contact window to form an N+ region which serves as a metal-silicide pick-up. Finally, metal contacts are formed at the contact windows to create Schottky barrier anodes and a contact to the metal silicide pickup. As stated above, the Schottky barrier diode of the present invention provides several advantages over prior art structures. First, it substantially eliminates capacitance problems. Second, by laser annealing the polysilicon surface, the polysilicon is converted to single crystal which inhibits leakage and improves breakdown voltages. Third, by utilizing the metal-silicide as a common cathode, lateral series resistance of the diode is reduced significantly. DESCRIPTION OF A PREFERRED EMBODIMENT Fig. 1 shows a semiconductor wafer 10 which comprises a silicon dioxide layer 12 formed on the upper surface of an underlying silicon substrate 11. A layer of metal-silicide 13, preferably tantalum-silicide, perfereably about 1500 Angstroms thick, is deposited upon the upper surface of the silicon dioxide layer 12 by either chemical vapor deposition or evaporation. A polysilicon layer 14, preferably about 5000 Angstroms thick, is deposited on the upper surface of the metal-silicide layer 13 by chemical vapor deposition. As shown in Fig. 2, N-type impurities, typically phosphorous, are implanted in the polysilicon layer 14 to provide a lower N+ polysilicon layer 14b which forms an ohmic inter race with the metal-silicide layer 13 and a surface layer 14a comprising N- polysilicon. A doping profile the polysilicon layer 14 is shown in Figure 2a. The upper surface of the wafer 10 is then laser annealed such that about the top 3000 Angstroms of the polysilicon layer 14 become single crystal. This is done to inhibit leakage between the metal anodes described below and the polysilicon layer. Then, using a photoresist mask (not shown), isolated islands of polysilicon/silicide are defined and oxidized to protect the sidewalls and the tops of the islands. As shown in Fig. 3, the silicon dioxide layer which remains overlying the island is then masked and etched according to conventional photolithographic techniques to form regions 15a, 15b, 15c, lSd and 15e which define contact windows to exposed selected surface regions 16a, 16b, 16c and 16d of the polysilicon layer 14. N+ impurities are then implanted near the surface of the polysilicon layer 14 at one selected contact window to form an N+ region 17 which serves as a pick-up for the metal-silicide cathode. Metal, typically aluminum, is then deposited on the surface of the polysilicon layer 14 and defined by conventional techniques to form contacts at each contact window to create Schottky barrier anodes 18a, 18b, 18c and a contact 18d to the metal-silicide cathode. Fig. 4 shows a topgraphical view of a completed Schottky diode island.

Claims: We Claim: 1. A method for fabricating a Schottky diode, characterized by: (a) forming a layer of metal-silicide on an underlying layer of dielectric; (b) forming a layer of polysilicon on the upper surface of said metal-silicide layer; (c) forming a second dielectric layer on the upper surface of said polysilicon layer and patterning said second dielectric layer to create a contact window through said second dielectric layer to an exposed surface region of said polysilicon layer; and (d) forming a metal contact to said exposed surface region. 2. A method according to claim 1 characterized in that said metal-silicide is tantalum-silicide. 3. A method according to claim 1 or 2 characterized in that said dielectric is silicon dioxide. 4. A method according to claim 1, 2 or 3 characterized in that said polvsilicon is N-type polysilicon. 5. A method according to claim 1, 2, or 3 for fabricating an integrated Schottky diode structure, characterized by: forming a plurality of contact windows through said second dielectric layer to a corresponding plurality of exposed surface regions of said polysilicon layer; forming a relatively heavily doped region of the same conductivity type as said polysilicon layer in said polysilicon layer beneath one of said exposed surface regions; and in that said step of forming metal contacts produce metal contact to said relatively heavily doped region of said polysilicon layer to form a pick-up for said metal-silicide layer, said metal contacts to said remaining surface regions of said polysilicon layer form anodes of said integrated Schottky diodes and said metal-silicide layer forms a common cathode of said Schottky diodes. 6. A method according to claim 5 characterized in that said polysilicon layer is doped to provide a lower N+ polysilicon layer which forms an ohmic interface with said metal-silicide layer and a surface layer comprising N- polysilicon. 7. A method according to claim 6 characterized in that said polysilicon layer is laser annealed such that about the top 3000 Angstroms of said polysilicon layer become single crystal. 8. A Schottky diode structure, comprising: (a) a layer of metal-silicide formed on an underlying layer of dielectric; (b) a layer of polysilicon formed on the upper surface of metal-silicide layer; (c) a second dielectric layer formed on the upper surface of said polysilicon and patterned such that a contact window extends through said second dielectric layer to a surface region of said polysilicon; and (d) a metal contact to said polysilicon surface region, said contact forming the anode of said Schottky diode structure and said metal-silicide forming the cathode of said structure. 9. A structure according to claim 8 characterized in that said metal-silicide is tantalum-silicide. 10. A structure according to claim 8 or 9 characterized in that said dielectric is silicon dioxide. 11. A structure according to claim 8, 9, or 10 characterized in that said polysilicon is N-type polysilicon. 12. A structure according to claim 8, 9 or 10 for an integrated Schottky diode characterized in that a plurality of said contact windows being formed through said second dielectric layer to a corresponding plurality of surface regions of said polysilicon layer; a relatively heavily doped region of the same conductivity type as said polysilicon layer being formed in said polysilicon layer beneath one of said surface regions; and a plurality of said metal contacts to said exposed surface regions, said metal contact to said relatively heavily doped region of said polysilicon layer forming a pick-up for said metal-silicide layer, said metal contacts to said remaining surface regions of said polysilicon layer forming anodes of said integrated diode structure and said metal-silicide layer forming a common cathode of said structure. 13. A structure acccording to claim 12 characterized in that said polysilicon layer is doped to provide a lower N+ polysilicon layer which forms an ohmic interface with said metal-silicide layer and a surface layer comprising N- polysilicon. 14. A structure according to claim 12 or 13 characterized in that said polysilicon layer has been laser annealed such that about the top 3000 Angstroms of said polysilicon layer is single crystal.

Inventors: HINGARH, HEMRAJ K., VORA, MADHUKAR B.
Applicants: FAIRCHILD CAMERA & INSTRUMENT CORPORATION
Application year: 1982
Main classification: H01L27
IPC codes: H01L29, H01L21