Phosphorus Doped Polysilicon for Double Poly Structures I. Morphology and Microstructure

In double poly structures, the isolation between the two polysilicon films is achieved by oxidation of the first poly film. The breakdown characteristics and conductivity of the interpoly oxide depend on the surface morphology of the polysilicon film. The surface morphology and microstructure are investigated atvarious tages in the fabrication process for polysilicon films prepared by five different methods. A high surface quality in combination with a high doping level is only achieved for in situ doped films, deposited in the amorphous state. Polysilicon is widely used in integrated circuits for gate electrodes in MOS devices, as emitter contacts in bipolar transistors, as interconnect material, and as resistors. As a standard, the polysilicon is deposited in the crystalline state at a temperature of typically 620 ~ (1, 2). More re-cently, it was found that deposition f the film in the amor-phous state at temperatures below 580 ~ followed by an annealing results in a polysilicon film with strongly im-proved surface smoothness (3-5). The film is made conduc