A detailed study is performed to understand and show the potential of high-speed, deep reactive ion etching (DRIE) of silicon using oxygen inhibitor pulses as a replacement for hydro-fluorocarbons (HFCs). This process might be considered the 'holy grail' in DRIE as the environmental restrictions for the use of HFCs are becoming increasingly stronger. When compared to the usual cryogenic mixed oxygen DRIE and with respect to profile control, the proposed cryogenic pulsed oxygen DRIE is virtually independent of silicon loading, mask material and trench width, and it is less prone to the formation of black silicon (BS). Some indication is found that one of the major causes for the formation of BS is the existence of dust inside the plasma. Dust is created when oxygen and silicon tetra fluoride (SiF4 being the reaction product of silicon etching) coincide inside the plasma glow. This occurs in mixed oxygen plasma; the silica dust falls onto the wafer where it starts to form BS when directional etching is requested. Dust particles can also form when strong polymerizing gases are fed into the plasma. This is the case for the Bosch process using HFC pulses forming carbonic dust. The particles, and consequently the BS, are observed to be limited when the SiF4 and O2 gases are time separated, which forms the basis of the proposed pulsed oxygen DRIE. Another advantage of pulsed oxygen DRIE with respect to Bosch processing is that the protective skin of the sidewall during etching—a kind of native oxide—is believed to be self-terminating. This makes the process insensitive to profile variations caused by parameter fluctuations. It is found that inhibiting oxygen pulses give excellent results with respect to profile control at cryogenic temperatures (between −120 and −80 °C) and can still compete with HFC pulses up to intermediate low temperatures (between −80 and −40 °C). When selecting the proper DRIE conditions, the oxygen pulsed mode performs with excellent profile verticality and selectivity while keeping the silicon clean. It also shows a high etch rate up to 25 µm min−1 (<1% Si load of a 100 mm wafer). When the temperature is raised further (between −40 and 0 °C), the strength of the oxygen sidewall protection progressively fails and more oxygen with more bias is needed to keep sufficient profile directionality. The use of stronger oxide-forming agents is suggested in order to enable good performance at the more convenient, higher temperature, low bias conditions.
Jansen, H. V., de Boer, M. J., Ma, K., Girones nogue, M., Unnikrishnan, S., Louwerse, M. C., & Elwenspoek, M. C. (2010). Black silicon method XI: oxygen pulses in SF6 plasma. Journal of micromechanics and microengineering, 20(7), . https://doi.org/10.1088/0960-1317/20/7/075027