Downstream Electric Field Effects during Film Deposition with a Radio Frequency Plasma and Observations of Carbon Reduction

Strong electric fields are generated by radio frequency (RF) plasma sources, and though the RF portion is too high a frequency for ions to react, the direct current (DC) portion of these fields has been shown to cause the atomic migration of metals, which can influence film morphology even downstream of the plasma where ionized plasma species are absent. In particular, we have observed the growth of nanopillars due to metal atoms migrating toward the positive field of the remote plasma. A biased grid placed between the plasma and the substrate can shield the substrate from these fields so that, when grounded, smooth films can be grown to a root mean square roughness of less than 1 nm. Positively biasing the grid returns the growth of nanocolumns. Interestingly, negatively biasing the grid significantly reduced the carbon and hydrocarbon content of gallium nitride films grown at a low temperature (-660 degrees C) using a nitrogen plasma, as observed using secondary ion mass spectroscopy (SIMS) and optical absorption measurements. The films also showed a notable improvement in conductivity and visible appearance.