Fundamentals of Sputtering

From a user’s point of view, one would wish that a secondary-ion-mass-spectroscopic (SIMS) signal [1,2] S be given by S=YP, where Y is a sputter yield and P an ionization probability. As we all know, things are more complex. As a next step, one could write schematically (1) $$S\left( {x,y} \right) = \smallint dz Y\left( {x;y,z} \right) P\left( {y,z} \right) ,$$ where x stands for a set of bombardment parameters (ion type, energy, angle of incidence etc.), y for the detected signal (mass, charge, energy etc. of the emitted ion), and z for all those parameters that may influence the ionization probability without being specified in the measured signal. The existence of the latter class of parameter (the original site of the emitted ion, the physical and chemical state of the surroundings of the point of emission, etc.) causes S to be a convolution rather than a product and makes it questionable to draw conclusions from a SIMS signal on the sputter process [3], despite increasing knowledge of the ionization mechanism [4]. This is true at least when P is small (≪1). Therefore, studies of the sputter process have to rely predominantly on other than SIMS measurements.