CONTROL OF AN UNSTABLE ELECTRON-CYCLOTRON RESONANCE PLASMA

Although plasmas are used throughout the microelectronics industry for etching, deposition, and cleaning of thin films, control of plasma pr ocesses has been a long-standing problem. Because of the nonlinear pro perties of plasmas, such as the coupling between wave propagation, den sity profile, and power absorption, plasma reactors are prone to unsta ble operation, multiple steady states, and hysteresis. We report obser vation and suppression of an abrupt transition in the operating mode o f an electron cyclotron resonance reactor that alters the ion flux to device wafers by more than twofold. While the origin of this mode chan ge is not well understood, we show here that it is strongly correlated with the neutral gas density, which slowly decreases as the reactor t emperature increases during a process or from run to run. By measuring the quartz liner temperature and adjusting the pressure to maintain a n approximately constant neutral gas density, the mode change can be a voided indefinitely. In a simulated manufacturing process, where the p lasma is pulsed on and off, a mode change occurs after several cycles unless the neutral density, instead of the pressure, is controlled.