Advanced Pressure Control in Time Division Multiplexed (TDM) Plasma Etch Processes

摘要

Time division multiplexed (TDM) plasma etch processes have been widely applied to MEMS device manufacturing due to the capability of defining high aspect ratio features at high etch rates and mask selectivity. To etch anisotropic features using F-based chemistry, a TDM process cyclically alternates between etch and passivation steps, which are normally carried out with different gases introduced into a reaction chamber at different flow rates, and during which chamber pressures are maintained at different levels. Conventional process control methods often result in chamber pressure overshoot and/or undershoot, slow pressure response times, and long-term pressure drifts. These are undesirable effects in manufacturing MEMS devices due to the requirements on process stability, reliability and repeatability. At Unaxis USA Inc., a proprietary control technique has been developed for the TDM etch processes to better control chamber pressures and improve process stability. Controls over the movement of a throttle valve are realized through a combination of pre-positioning the valve and regulating it with the proportional, integral and derivative (PID) function mechanisms. Using this technique, we have demonstrated in fast TDM processes that pressure overshoot and undershoot are significantly suppressed, pressure response times are improved, and long-term pressure drifts are eliminated. To this end, this new control technique has been successfully tested in processes where the etch/passivation process steps are alternating at frequencies up to 1 Hz. Applications of this advanced technique in deep silicon etching have demonstrated improved etch performance. As a result, this advanced pressure control technique enables the TDM dry etching technologies for MEMS devices manufacturing to become markedly more reliable and stable.