Microstructural characterization of SiO_x surface contaminants on ashed aluminum thin films

Thin films of aluminum were exposed in short increments to a plasma environment using a semiconductor plasma asher. The process gas was a mixture of air and oxygen. The total exposed oxygen fluence was 1.2 × 10²¹ oxygen atoms cm^-2 accumulated over 34 h. Heavy contamination of film surfaces resulted from plasma etching of the chamber seals. The surface microstructure of this deposited contaminant was studied with atomic force microscopy (AFM). The contaminants appear to nucleate and grow at protruding surface features. These features appear in the AFM data as dome-shaped growths rising rapidly from the surface. The density of these observed growths is much higher on sputtered films than on electron-beam evaporated films. These differences are most likely due to the microstructure of the as-deposited films. The as-deposited sputtered films are rougher than their evaporated counterparts and provide more sites for nucleation of the observed contaminant growths. The surface roughness of the contaminated films increases quickly by a factor of 3-6 within 300 min of exposure and saturates thereafter. It is postulated that the surface roughness increases until a continuous, but rough, contaminant layer is formed. After the formation of a continuous layer, additional material deposits while maintaining the existing surface microstructure. The growth of rough contaminant features has a significant effect on the specular reflectance of the films. Sputtered films show larger losses in specular reflectance than evaporated films after asher exposure.