Real-time monitor for thin film etching in atomic oxygen environments

Abstract

Various films have been deposited on gold-coated quartz crystals as part of a quartz crystal microbalance. In situ measurements of the effects of atomic oxygen on the net mass change in these materials have been determined by observing the frequency shift of the oscillator in conjunction with a mass-frequency calibration. Films were exposed to a neutral atomic oxygen beam produced by supersonic expansion of a lightly seeded helium plasma. Atomic oxygen fluxes as large as 10 17 cm -2 s -1 and incident kinetic energies up to 2.5 eV were typically used. Based on an assumed reaction path between amorphous carbon (a-C) and atomic oxygen, the reaction probability is 0.01. Hydrogenated amorphous carbon (a-C:H) films show a three-fold increase in etch rate. Layered coatings of amorphous carbon films can be identified by their differing etch rates. Amorphous carbon films were used to show how reaction rates vary with changes in reaction parameters. A 25% reduction in reactivity is found for a 25% reduction in incident kinetic energy. Synergistic effects were identified by comparing mass loss rates during exposure of amorphous and hydrogenated amorphous carbon films to a neutral atomic oxygen beam and to an oxygen plasma. Plasma-deposited SiO x , SiN x , a-Si:H and plasma polymer hexamethyldisiloxane are shown to be quite inert and are useful as protective coatings. The protective nature of a coating may be ascertained by overcoating a film of reactive material as shown for plasma-deposited silicon oxide and silicon nitride over amorphous carbon films. Films of aluminium have a uniform and adherent oxide layer demonstrated to be stable in atomic oxygen environments. This is in contrast to silver films, which indicate that a protective oxide layer is not formed.