Application Example 22: Oxidation of Sputtered Metal Surface in Air – The Main Cause of Surface Alternation

Abstract

After final sample surface cleaning by rf-GD sputtering, samples are transferred into the specimen chamber of FE-SEM. During this short period of time for sample transfer, normally less than 1min, clean sample surfaces are exposed to air. Hydrocarbons in air adsorb to form a thin contaminant layer, less than 1 nm thick. For metals, chemisorption of oxygen and subsequent formation of a monolayer of the oxide also takes place rapidly. At room temperature and at relatively low humidity (<40%), however, further growth of the oxide is insignificant or, more precisely, proceeds at negligibly slow rates for most metals. Oxide formation during this short period of sample transfer is described below for a stainless steel as an example. A commercial stainless sheet, 0.2mm thick, for architecture use was cut into a dimension of 20mm × 20mm. Then, the sheets were mechanically polished using suspensions containing successively finer alumina particles of sizes down to 0.05 μm, rinsed thoroughly in distilled water, and dried in a cold air stream. Through XPS, it has been shown that the surface of a mechanically polished stainless steel is covered with a thin air-formed oxide layer, ~2nm thick, which consists of two layers [29,30]. Next to the metal, there is an inner layer highly enriched with Cr₂O₃. Over this inner layer, there is an outer Fe₂O₃ layer. It is also suggested that the surface of the outer Fe₂O₃ layer is hydrated.