P.W. 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.

  1. 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 0.2-0.6 eV, 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.
  2. 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 process conditions.
    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.
  3. 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.