Paper PS1-TuM10
Multivariable Control of a Capacitively Coupled Plasma

Tuesday, October 30, 2012, 11:00 am, Room 24

Present practice in reactive ion etching specifies etch recipes in terms of inputs such as gas flow rates, RF power and pressure. However, ostensibly identical chambers running identical recipes may produce very different results. Extensive ‘chamber matching’, i.e. initial iterative, empirical tuning of the process recipe, which entails time-consuming, ex situ statistical analysis of etch metrics, is required to ensure that an etch chamber produces acceptable results. Once matched, etchers are run ‘open loop’ and are thus sensitive to disturbances such as actuator drift, wall seasoning and substrate loading, which may have deleterious effects on process metrics such etch depth, uniformity, anisotropy and selectivity,. An alternative approach, which would reduce sensitivity to disturbances of the plasma generating process, would be to specify a recipe in terms of plasma quantities such as radical densities, and to regulate these in real time by adjusting the inputs with a suitable multivariable control algorithm.

Multivariable closed loop control of an SF₆/O₂/Ar plasma in an Electron Cyclon Resonance (ECR) etcher is the focus of a major research program in the National Centre for Plasma Science and Technology (NCPST) in Dublin City University (DCU). As an intermediate step, a multivariable LQG/LTR control algorithm has been implemented on a capacitively coupled plasma using the same gas mixture. This presentation describes the design of the algorithm, and its efficacy is demonstrated via simulation with a variety of disturbances. The performance of the multivariable algorithm is compared to that of a suite of decentralised single-input-single-output (SISO) controllers.