Generation of periodic surface corrugations.

The generation of periodic surface corrugations by ion-beam milling and chemical etching of grating relief patterns in photoresist is analyzed. A general treatment is developed for gratings of any desired period on substrates of arbitrary reflectivity, but particular emphasis is given to the generation of gratings with deep grooves and fine periods (Lambda < 3000 A) on GaAs. Analysis of the intensity distribution in photoresist for both p- and s-polarized incident beams reveals that the standing waves generated by reflection from the substrate are diminished for p-polarized beams, but the existence of a displaced grating for certain ranges of substrate reflectivity and angle of incidence severely limits groove depth in resist. The requirements are given for the establishment of an intensity maximum at the photoresist-substrate interface, a condition desired for subsequent chemical etching. It is shown further that the alternative use of a quarterwave intermediate oxide layer to achieve this condition on GaAs results in a lower limit being imposed on grating period. Constantintensity contours approximating the groove profiles in resist demonstrate that an imbalance in incident beam intensity may lead to severing of the resist stripes, and the dependence of this phenomenon on substrate reflectivity is determined. For beams of equal intensity, a similar phenomenon occurs with increasing reflectivity of the substrate. The transfer of a grating relief pattern to the substrate by ion-beam milling is treated by considering the erosion profiles produced by ion bombardment. This analysis is used to examine the influence of milling geometry on the depth and shape of the groove. Although the ion-beam milling rate of GaAs is several times greater than AZ-1350 photoresist, it is shown that the groove aspect ratio (depth/period) in GaAs can be no more than about 1.2, a figure that is obtained, surprisingly, by milling at the angle of maximum removal rate of photoresist. For a metal substrate, the groove aspect ratio decreases with increasing grating period. For gratings produced by chemical etch, the problem of the weakly exposed layer of resist adjacent to the substrate is solved by using a combination of ion-beam milling and chemical etching. Using a preferential chemical etch with a sufficiently slow etch rate, gratings with well-defined planar features, a period ~2500 A, and a groove aspect ratio >0.6 have been produced on GaAs.

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