Real-time feedback control of ion energy flux in chlorine inductively coupled plasma for poly-silicon etch process

Summary form only given. Advanced semiconductor fabrication requires tighter process monitoring and control to improve production yield and reliability. Recently, advanced process control (APC), an in situ sensor based methodology, has been applied to achieve the desired process goals in operating individual process steps. For instance, in etching of polysilicon using chlorine discharges, in order to obtain a desired etch profile, the process often is operated at the ion-enhanced regime where the etch rate and etched profile are strongly dependent on the total ion energy flux incident on the wafer surface. Therefore, a better process control can be achieved if one can implement the real-time control of ion energy flux in etch processing. In this study, we have demonstrated experimentally the real-time multiple-input multiple-output (MIMO) control of both ion density and ion energy in etching of polysilicon using chlorine inductively coupled plasma. To measure relative positive ion density, the optical emission at 750.4 nm from trace amounts of Ar is used which is proportional to the total positive ion density. An RF voltage meter is adopted to measure the peak RF voltage on the electrostatic chuck which is linearly dependent on sheath voltage. One actuator is a 13.56 MHz RF generator having a maximum power of 5 kW to drive the inductive coil seated on a ceramic window, along with a L-type matching network to minimize the reflected power. The second actuator is also a 13.56 MHz RF generator to power the electrostatic chuck via a matching network. The two RF generator is locked in phase. The design of MIMO controller is applied by Quantitative Feedback Theory (QFT) to compensate process drift, process disturbance, and pilot wafer effect. The experiment results showed that the MIMO control system has a better reproducibility in etch rate as compared to current industrial practice.