Recapture of ion-induced secondary electrons emitted from magnetron targets

The recapture of ion-induced secondary electrons emitted from magnetron targets is a significant but underappreciated part of our sputtering processes. Recaptured electrons are secondary electrons that return to and are captured by the target without interacting with the plasma. The fraction of electrons recaptured has been estimated to be as high as two-thirds to three-quarters of those that initially leave the target. We use the method described previously to determine ion-induced secondary electron emission (ISEE) coefficients under a variety of conditions with the goal of informing our thinking about the recapture process. The magnitude of the fraction of electrons recaptured has direct impacts on the ISEE coefficient. The latter is important in that it has a strong influence on cathode voltage. The cathode voltage often changes with reactive gas additions and is frequently used as a process control set point. This paper includes measurements of ISEE coefficients for Al as a function of pressure and magnetic field strength and for AlOx and AlNx both as their respective reactive gases flows are varied and also in the fully reacted mode as a function of pressure. This leads to a better appreciation of the significant role of electrons recaptured at the cathode surface.