Collimated beam taper

A number of advantages exist for ion milling compared to plasma etching or RIE. Because of the collimated beam of ions, essentially vertical profiles are possible. Also, profile tapering can be achieved by tilting the substrate relative to the ion beam. In addition, ion milling is performed at pressures at least 100 times lower than those used in plasma etching or RIE. Therefore, redeposition of sputtered material is reduced. 

The parameters for the linear taper of Fig. 5-6(a) are identical with those of the previous example and the taper angle is SI We can determine very simply the minimum taper length for total power transmission when a meridional ray enters the taper at z = —L and makes angle 0q with the axis. This result apphes to both direct on-axis illumination by a collimated beam, i.e. 0q = 0, and the inclusion of a lens, as discussed in Section 4-11, which transforms the on-axis beam into acone of meridional rays at angle d(, > 0. 

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