AVS 60th International Symposium and Exhibition
    Plasma Science and Technology Tuesday Sessions
       Session PS1-TuA

Paper PS1-TuA1
Real Time Feedback Control of Photoresist ashing in an Electron Cyclotron Resonance Plasma Chamber

Tuesday, October 29, 2013, 2:00 pm, Room 102 B

Session: Plasma Diagnostics, Sensors and Control
Presenter: B.J. Keville, Dublin City University, Ireland
Authors: B.J. Keville, Dublin City University, Ireland
C. Gaman, Dublin City University, Ireland
Y. Zhang, Dublin City University, Ireland
A.M. Holohan, Dublin City University, Ireland
S. Daniels, Dublin City University, Ireland
M.M. Turner, Dublin City University, Ireland
Correspondent: Click to Email
Present practice in plasma-assisted semiconductor manufacturing specifies recipes in terms of inputs such as gas flow rates, 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 process metrics, is required to ensure acceptable results. Once matched, chambers 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 may reduce sensitivity to disturbances of the plasma, would be to specify a recipe in terms of quantities such as active species 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 Cyclotron 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, real time control of an O2/Ar plasma used for photoresist ashing has been implemented. More specifically, the oxygen 844 and argon 750 line intensities measured by optical emission spectrometry have been regulated by adjusting microwave power and oxygen flow rate in real time by means of a control algorithm. In order to test the efficacy of the control algorithm in reducing the sensitivity of the ashing rate to wall conditions, a wall disturbance was simulated by fluourinating the chamber walls using an SF6 plasma prior to O2/Ar ashing. In the open loop case, wall fluorination results in a large increase in the 844 line intensity, which is indicative of an increase in the density of atomic oxygen, and a concomitant increase in the ash rate is observed. However, under closed loop control, the average ash rate is unaffected by wall fluorination, thus demonstrating effective attenuation of the disturbance. Although this may not necessarily be of intrinsic interest – one generally ashes to end point without being overly concerned about tight control of the ash rate – it indicates that real time control of a plasma application – ashing, in this case - may be achieved indirectly by control of active species in the plasma. This has important implications for applications where tight control of dimensions is critical – etch profile, for example.