Effect of atmospheric oxygen on evaporated chromium films

Abstract

The composition of thin Cr films deposited under ultrahigh vacuum conditions onto plain and surface-treated Cu foils has been investigated by SIMS. It was found that the oxygen content in the Cr–Cu transition region is determined by the oxygen concentration pressure at the Cu substrated surface. Chromium deposited in ultrahigh vacuum on heat-treated but not surface-treated Cu foils shows that the high Cu-surface oxygen concentration extends about 100 monolayers deep into the chromium film. However, for ion sputtered Cu surfaces, the region of oxygen influence is reduced to about 50 monolayers. Exposing the chromium film sample on the plain copper foil to air reveals a constant value of the Cr+ and O2− signal throughout the entire chromium film, not counting the outermost top monolayers. From the Cr–Cu transition region extending into the Cu substrate, an O2− intensity increase is recorded which is due to the high oxygen concentration at the copper foil surface. Exposure to air of the Cu surface-treated sample leads to an O2− intensity which is not constant through the entire Cr film region, but decreases monotonically to a depth of about 70 Cr monolayers. This result is interpreted to mean that oxygen moves from the oxygen-rich top Cr layer through the oxygen-depleted transition region into the Cu substrate.