Feedforward power distortion correction in RF power delivery systems for plasma processing systems

Abstract

Many critical technologies rely and will continue to rely on processes that utilize plasma based material processing. Plasma processing is a cornerstone technology for the semiconductor industry, and plays a critical role in the continued adherence of technology advances to the now famous Moore's Law. Paramount to the continuum of plasma processing advances for thin-film manufacturing are reliable and repeatable RF power delivery systems used for RF plasma discharges. In conventional RF power delivery systems, the RF power supply performs local control to regulate power for the required power level. An impedance tuning network resides between the RF power supply and the RF discharge. In a manner analogous to the RF power supply, this impedance tuning network, with dual actuators for load and tune compensation, adjusts variable impedance devices (i.e. variable capacitors) with motor-controlled actuators to adapt the network for maximum power transfer from the RF source to the plasma discharge. When power transfer is not at a maximum, some portion of the power is reflected by the load back to the RF source. We consider this power loss a distortion that requires a correction to achieve an optimal power transfer condition. Our centralized control approach is economical and achieves the vexing performance objectives necessary for advancing thin-film manufacturing. In our scheme, power regulation is also conducted with autonomous feedback control in the power supply. An RF sensor in the power supply serves a dual purpose of (1) coupling feedback to the power controller, and (2) providing a quantitative power distortion measurement of the RF power delivery system. We centralize control and remove systematic redundancy with a feedforward controller, also in the RF power supply, to correct the power distortion by adjusting elements in the impedance tuning network. We demonstrate the primary instantiation of our feedforward controller with a frequency-agile RF power supply. By adjusting frequency, power distortion in the RF power delivery system is corrected. Our feedforward framework is a significant and substantial departure from the industry practice of using heuristic methods for impedance tuning to overcome RF power distortion. We demonstrate fast frequency tuning operation that is required by short-cycle thin-film processes and RF pulsing applications.