PhD Thesis abstract, I. Hedgecock, 1994

I. Hedgecock, 'The Methane/Hydrogen Reactive Ion Etching of InP', PhD Thesis, 1994.

The work presented here describes the extension and combination of two existing models, which were then used to simulate the sidewall profiles obtained when etching InP in a CH4/H2 plasma under a range of plasma conditions. The prediction of sidewall profiles as a function of Reactive Ion Etching (RIE) parameters has considerable commercial application.

The first model simulates the ion energy and angle distributions (IEDs and IADs) at the electrodes of a HE chamber and has been extended to model high pressure H2 plasmas. Eighteen ion-molecule reactions and their energy-dependent cross sections have been included to take account of collisions in the sheath region of the plasma which significantly affect the IED. This model which uses three fitting parameters has successfully reproduced IEDs measured by Dickenson at the anode of a reactive ion etcher. Using the fitting parameters obtained to reproduce experimental IEDs, cathode IEDs have been predicted. Cathode IEDs have also been predicted for CH4/H2 plasmas by including CH4 fragment ions using Langevin theory to calculate energy dependent cross sections for momentum transfer.

The second model which simulates etched sidewall profiles from calculated IEDs and IADs and calculated angle-dependent sputter yields has also been extended. This model now includes two sidewalls, variable resist height and angle and ion reflection from the resist. Options to model overetch, micromasking and layered substrate materials have also been included. The pure sputter profiles obtained from the model reproduce both common and unusual observed etch profiles. The profile simulations suggest that the trenching seen in some etch processes is due to ions being reflected into the developing etched feature from the resist.

InP has been etched in CH4/H2 and the trench profiles compared to the predictions derived from the combination of the two models described above. It was necessary to include an 'anisotropy factor' into the sidewall simulation program to model the experimental results. This work suggests that the mechanism behind the observed anisotropy in CH4/H2 etching of InP is predominantly sidewall passivation and not ion-enhanced reactivity.

The energies of In* CH4/H2 etching of InP have been studied using FabryPerot Interferometry (FPI). The envelope produced by the hyperfine In* emission lines has been successfully modelled as the superposition of Gaussian line profiles. The Doppler broadening shows a 'hot' In population with an average energy between 1 and 2 eV. This is significantly higher than the thermal energies in the plasma. A dissociative attachment mechanism involving In(CH3)x has been proposed to explain the observations.

FPI was also used to study the anomalous F* emission from the 3p 2P0 state in CF4/O2 etching of Si previously investigated in this laboratory. The results indicate that there are two F* production mechanisms at work, one which produces thermal F*, and one which yields hot F* with kinetic energies up to 5 eV. A dissociative recombination mechanism has been proposed to explain the production of hot F* and also to explain the anomalous intensity changes of emission lines originating from the 3p 2P0 state on addition of an Si wafer to the RIE chamber.