Development of a TiW plasma etch process using a mixture experiment and response surface optimization

The characteristics of SF₆/He plasmas used to etch TiW have been studied with statistically designed experiments using a Tegal 804 single wafer system. Two processes were developed using both positive and negative photoresist as the mask material for etching TiW. The goal was to consolidate both processes into one. A two-phase experimental approach was taken to generate the processes. In phase 1 a fractional factorial screening experiment was used to identify key factors, and in phase 2 a mixture experiment was used for process optimization. The fractional factorial experiment was initially used to study the effects of reactor pressure, RF power, SF₆/He gas ratio, overetch time, and hard bake. The results of this initial experiment were used to identify the appropriate levels for the main process parameters. Then, at these parameter levels, a mixture experiment was conducted using the partial pressures of SF₆, He, and the nitrogen ballast as the design variables. Since the total pressure in the system is fixed, these three variables are the components of a mixture, and thus form a constrained design space for the experiment. Quadratic and special cubic response surface models were generated for the following responses: TiW etch rate, photoresist etch rate, selectivity between the TiW and photoresist, uniformity of all etch rates and selectivities, and critical dimension control for the photoresist and TiW. Contour plots for all responses as a function of the partial pressure of SF₆, He, and nitrogen ballast were generated. The contours from these empirical models were analyzed jointly to optimize the processes.