Optimization of overshoot in the pulsed radio frequency inductively coupled argon plasma by step waveform modulation

Abstract

The pulsed inductively coupled plasma (ICP) has considerable potential to satisfy multiple stringent scaling requirements for use in the semiconductor industry. However, overshoot of plasma parameters during the rising period of the pulse affects the stability and uniformity of the plasma and can lead to a breakdown of the wafer and over-sputtering of the film. In this study, a step waveform modulation method is used to reduce the overshoot at the initial stage of the pulse. The behavior of the discharge is monitored by measuring (i) the modulated step waveform signal on the function generator, (ii) the input power (by a time-resolved VI-probe), and (iii) the amplitudes of the coil voltage and current (by voltage and current probes, respectively), as well as (iv) the plasma parameters including the electron density, the effective electron temperature, and the electron energy probability distribution function (by a time-resolved Langmuir probe). It was found that the state of the plasma can be controlled by changing the waveform, such as varying the time of the rising edge, varying the initial amplitude, and varying the duration of the low-high amplitude. The results indicated that the overshoot value of the electron density can be reduced by using a low-high step waveform. When the amplitude of the waveform was 500/550 mV and the duration was 200/300 μs, the overshoot value observed was 1/4 of that of the conventional ICP pulse discharge. In addition, increasing the duty cycle of the pulse could also reduce the overshoot value due to the high electron density that occurs during the afterglow period. Moreover, the plasma can reach a steady state more quickly at high pressure by using a step waveform of high amplitude.