Ion-Beam-Assisted Deposition and Synthesis
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tering, will often be very similar to the work described here. The parallels will be examined when appropriate, and we will correlate some general aspects of bombardment-related processes. The effects of energetic particle bombardment on a growing film can be classified as structural or chemical, but the net effect to the film is generally a combination of both phenomena. Ion beam bombardment-induced structural changes to the film include sputtering of the film surface, or preferential sputtering of one component in an alloy deposition, resulting in an altered alloy composition from the t a r g e t c o m p o s i t i o n — a "frozen-in" altered layer. The sputtering component can result in the formation, or elimination, of surface topographic features. A practical example of this phenomena is an increase in the film coherence or "step coverage" over underlying structures on the substrate surface with concurrent ion bombardment during deposition.? This effect is similar to biasing the substrate during rf deposition. The ion bombardment in this case causes sputtering of the depositing film and increases the surface mobility of the depositing atoms. The crystal structure of the film can be significantly altered by bombardment during deposition, resulting in changes in degree of crystallinity, orientation, and grain size. These changes can subsequently affect such parameters as the film density, electrical resistivity, optical d e n s i t y , film porosity, and surface topography. For example, altering these structural properties of the film can strongly influence a film's response to light. This is particularly important for dielectric thin films d u e to changes in the index of refraction, dielectric constant, and lifetime and stability of the film in air. The intrinsic stress of the film can be significantly altered by ion bombardment during film deposition. The stress in a film can affect how well the film adheres to the substrate, and can be particularly important in multilayer films. Several chemical processes at a film surface can be influenced by bombardment of the film during deposition. The adhesion of a thin film can be altered by concurrent ion b o m b a r d m e n t d u r i n g t h e earliest stages of film deposition. This could simply be due to cleaning of the interface, or could also be due to redistribution of the atoms at the film-substrate interface, leading to mixing and enhanced adhesion. Ion bombardment can dislodge or remove adsorbed residual gas contaminants from the sample surface during the deposition, resulting in a cleaner film and changes in the adhesion. Ion bombardment during deposition can
PAGE 40, MRS BULLETIN, FEBRUARY 16/MARCH 16, 1987
cause preferential sputtering of one component in a compound or alloy deposition, resulting in an altered stoichiometry from the nonbombarded case, the degree depending strongly on the relative arrival rates of the ions and on the condensing atoms and the ion energy. The bombarding ion species will be incorporated into the depositing film to an extent d
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