A simple model is used to study the dependence of the sputtering yield on the incident atomic number (Z1). Scatterings between incident and target atoms are classified into two types, that is, large and small angle scatterings. Large angle scatterings are represented by random walks with a certain energy loss and small angle scatterings by another certain energy loss. The range distribution derived from this model is compared with other rigorous calculations. Sigmund's theory considerably overestimates the sputtering yield in the region where M2≫M1, M1 and M2 the mass of incident and target atoms respectively. Our simple model explains this as a surface boundary effect, i.e., as an effect due to escaping of energetic atoms from the surface.
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