Impact of nitrogen (N/sub 2//sup +/) implantation into polysilicon gate on thermal stability of cobalt silicide formed on polysilicon gate

A novel process which uses N/sub 2//sup +/ implantation into polysilicon gates to suppress the agglomeration of CoSi/sub 2/ in polycide gated MOS devices is presented. The thermal stability of CoSi/sub 2//polysilicon stacked layers can be dramatically improved by using N/sub 2//sup +/ implantation into polysilicon. The sheet resistance of the samples without N/sub 2//sup +/ implantation starts to increase after 875/spl deg/C RTA for 30 s, while the sheet resistance of CoSi/sub 2/ film is not increased at all after 950 and 1000/spl deg/C RTA for 30 s if the dose of nitrogen is increased up to 2/spl times/10/sup 15/ cm/sup -2/ and 6/spl times/10/sup 15/ cm/sup 2/, respectively, and TEM photographs show that the agglomeration of CoSi/sub 2/ film is completely suppressed. It is found that the transformation to CoSi/sub 2/ from CoSi is impeded by N/sub 2//sup +/ implantation such that the grain size of CoSi/sub 2/ with N/sub 2//sup +/ implantation is much smaller than that without N/sub 2//sup +/ implantation. As a result, the thermal stability of CoSi/sub 2/ is significantly improved by N/sub 2//sup +/ implantation into polysilicon.