Analysis and modelling of the impact of plasma RF harmonics in semiconductor plasma processing

Reactive ion etching (RIE) has been used extensively in the last few decades in the microelectronics industry for integrated circuit fabrication. However, the monitoring and control of this process is quite challenging because the plasma process is complex and not fully understood The use of a newly developed Plasma Impedance Monitor (PIM) to monitor the RIE process is reported. The sensitivity and the usefulness of the PIM to detect the end pomt of a RIE are tested in a SF6 plasma m a capacitively coupled planar reactor with silicon (Si) and silicon dioxide (S i0 2) samples. The measurement o f harmonic components of the current-voltage (I-V) characteristics are tested as parameters to monitor the RIE processes and detect the end point when a S i 02 layer on a Si substrate undergoes SF6 RIE. The parameter used for the end pomt detection is empirically modelled as a polynomial equation of the mput factors (1 e , RF source power, chamber pressure and gas flow rate) usmg the Box-Behnken experimental design. The end point can be predicted from the modelled equation of the parameter used for the end point detection. An analytical solution for a non-symmetnc, capacitively coupled plasma driven by a non-sinusotdal radio frequency (RF) current is obtained under the assumptions of time-independent, collisionless ion motion, mertialess electrons and uniform current density. Modelling is developed considering that the RF current can be expressed as a summation of the Fourier components whose frequency is exactly an mteger multiple of the fundamental frequency. The different plasma parameters obtamed from the present model are compared with those of an established model developed by Lieberman [M A Lieberman, IEEE Trans Plasma Set, vol 16, p 638, 1988]. The present model always found the even harmonic components of IlF voltage and hence the overall RF impedance, whereas the Lieberman model found no even harmonic component of RF voltage. The sheath resistance and capacitance could always be estimated usmg the present model, whereas the Lieberman model fails to relate either of these parameters while considering the harmonic frequencies. The overall RF impedance and RF voltage obtamed from the present model are verified with experimental values. The relative magnitudes of RF voltage and impedance harmonics determined by the present model are shown to follow qualitatively the values measured m the experiment. The values of the normalized RF voltage and impedance harmonics assume lower values both for calculated and measured quantities as the asymmetry of the plasma chamber decreases.