Paper PS-ThA8
Real Time, Multivariable Control of an SF₆/O₂/Ar Plasma

Thursday, November 3, 2011, 4:20 pm, Room 201

Plasmas of sulphur hexafluoride, SF6, mixed with oxygen and argon have been used for silicon etching in microelectronics manufacturing. Fluorine atoms produced by dissociation of SF6 etch Si with very high rates. Lateral etching, which reduces feature anisotropy, may be inhibited by the formation of a silicon oxide passivating layer on feature sidewalls. It has been demonstrated experimentally that feature profile shape is determined to a large extent by the balance between O and F radical densities at the surface of the substrate. In general, etch recipes are specified in terms of inputs such as gas flow rates, RF power and pressure and processes are run ‘open loop’. ‘Chamber matching’, which entails ex situ statistical analysis of metrics such as etch depth, uniformity, anisotropy and selectivity, is required to ensure that each chamber produces acceptable results. However, process reproducibility may be degraded due to real-time disturbances such as MFC and match network drift, wall seasoning and substrate loading. An alternative approach which would reduce the need for chamber matching and reduce process sensitivity to disturbances would be to specify a recipe in terms of plasma parameters such as O and F radical densities, and the fluxes and energies of ions at the wafer surface and to regulate these in real time by adjusting the inputs with a suitable real time control algorithm. This presentation describes how a real time, multivariable control algorithm for an SF₆/O₂/Ar plasma plasma may be designed with the aid of a control-oriented process model. The stability and efficacy of the control algorithm is demonstrated using a model of the process and a variety of simulated disturbances. Experimental implementation of the control algorithm on a laboratory capacitively coupled plasma is described.