Electrical properties of RF magnetron-sputtered insulating silicon nitride thin films.
Si3N4 thin films were prepared by RF magnetron sputtering using N2 or Ar as the
sputtering gas. The films were amorphous, with the deposition rate for Ar-sputtered films
increasing with RF power and Ar pressure.
Sandwich samples having both Al and Au electrodes were prepared. Capacitancevoltage
measurements indicated that the contacts for Nj-sputtcred samples were ohmic, while
Ar-sputtered samples with Al electrodes exhibited depletion regions. Values of the relative
permittivity of 6.3 (AI electrodes) and 6.8 (Au electrodes) were determined from geometric
capacitance variations in Ny-sputrered films. Current density-voltage characteristics normally
showed ohmic and space charge limited conductivity with trap levels distributed exponentially
within the insulator band gap, but exceptionally in N2-sputtered films with Au electrodes
electroforming behaviour was observed, with Poole-Frenkel conductivity in the preformed
region. Hopping was dominant at low temperatures.
AC conductivity was higher for Ar-sputtering, and with Au electrodes. These effects
were related to the possible structure of the films, and the diffusion of Au. AC conductivity
increased with increasing frequency and temperature, appearing to be via a free band process
at high temperatures and hopping at low temperatures. Plausible values of the density of
localised states were deri ved using Elliott's model, but this could not be considered uni versally
applicable. Loss tangent was also frequency and temperature dependent in Ny-sputtered films,
showing a minimum value which shifted towards higher frequencies with increasing
temperature. In Ar-sputtered samples minima were not observed in the frequency range
covered. The model of Goswami and Goswami appears consistent with these results,
particularly in the former case. Variations in the loss tangent values with the sputtering gas and electrode species were consistent with the observed conducti vity variations.
Optical properties were also investigated. In Ar-sputtered films, the optical band-gap
appeared narrower and the optical absorption higher than for Ny-sputtered films, and a direct
transition was also identified.
Values of the electrical properties determined for such sputtered films are comparable
to those prepared using more sophisticated methods, particularly in the case of Nj-sputtered
films. Sputtering may therefore prove useful in semiconductor processing, where a relatively
inexpensive method of deposition is required.