May, 'The Energies of Ions, Electrons and
Neutrals in Reactive Ion Etching Plasmas',
PhD Thesis, 1991.
Radio frequency plasmas are used extensively in the microelectronics industry for
delineation of submicron features in dry etching processes. We have investigated the
energies that species within RF plasmas possess, both from an experimental and
theoretical viewpoint. This work is divided into 3 main sections.
- The kinetic energy of excited Cl* and Ga* atoms in the bulk of an RF discharge was measured
for Cl2, CFCCl3 and CF2Cl2 plasmas etching GaAs. This was achieved using Fabry Perot
Interferometry to study the Doppler linewidths of optical emission lines with a resolution of better
than 0.01Å. Results indicate that Cl* atoms have kinetic energies of 0.2-0.6eV, suggesting the
primary mechanism for Cl formation is electron impact dissociation of a parent molecule. Ga*
exhibited translational energies of up to 2eV, probably resulting from electron impact dissociation of
the primary etch product, GaCl.
- Theoretical calculations were made for the trajectories of positive ions as they are accelerated
through the oscillating sheath potential to strike the cathode and anode in a reactive ion etcher.
Recently derived expressions for the DC bias and plasma potential in RF systems were incorporated
into a Monte Carlo computer program. This program simulates the energies at which ions strike
either electrode in plasmas such as Ar, O2, H2 and CF4 under a variety of process conditions.
Calculated ion energy distributions (IEDs) invariably show two peaks (e.g. 180eV
and 235eV for a set of Ar plasma conditions we adopted as standard). Calculations
also produced an average ion energy (eg. 207eV for the standard conditions). Our
predicted IEDs closely reproduce available experimental data both qualitatively and
quantitatively. Ion angular distributions (IADs) are also calculated for the same
The calculations have been extended for Ar plasmas to include collisional effects
(such as scattering and charge exchange) in the sheath. This allows high pressure (up
to 500 mtorr) IEDs and IADs and neutral particle distributions (created by ion-neutral
collisions) to be calculated.
Electron energy distributions were also calculated, both for primary electrons
striking the electrodes, and for secondary electrons ejected from the electrodes (due
to energetic particle bombardment) and being accelerated into the plasma region.
- Sputter sidewall profiles have been calculated using a computer program that incorporates our
IEDs and IADs and a model of the interaction of Ar+ ions with an amorphous Si surface. The
calculations have been made for a variety of process conditions and accurately predict observed
etching phenomena. The phenomena include mask undercut at high pressures, anisotropic etching
and black silicon effects at low pressure, and faster etch rates at high RF powers. The calculations
were extended to include resist erosion and chemical (isotropic) etching.