Cryogenic cyclical etching of Si using CF4 plasma passivation steps: The role of CF radicals

Ultraviolet (UV) absorption spectroscopy is used to monitor the CF radical density in CF4 inductively coupled plasma (ICP) plasmas as a function of the substrate temperature. The CF density decreases dramatically when the wafer temperature is reduced from 20 to −130 °C by applying identical plasma conditions, demonstrating that the CF surface sticking coefficient increases as the surface temperature is reduced. This suggests that CF4 plasma could be used to form sidewall passivation layers and perform anisotropic etching at cryogenic temperature, which is impossible at room temperature. Subsequently, a cyclical Bosch type etching process of silicon was evaluated at −100 °C using CF4 plasma to passivate the trench sidewalls. Anisotropic etch profiles were obtained with an etch rate of 4.4 μm/min. Compared to a typical Bosch process using highly polymerizing c-C4F8 plasma, chamber wall contamination could be significantly reduced, alleviating a major issue of this cyclic process. Furthermore, CF4 has a 28% lower global warming potential than c-C4F8.