FABRICATING BINARY OPTICS - PROCESS VARIABLES CRITICAL TO OPTICAL-EFFICIENCY

While conventional photolithographic and reactive ion etching techniques are employed in the manufacture of binary optical elements, the nested character of these diffractive surface relief structures, coupled with submicron linewidths and additive etch depths, puts stringent demands on process tolerances. To quantitatively assess the effects of process variables, we have utilized a mask set that incorporates ten different lenslet designs as single elements and as 10 x 10 arrays. All lenslets are 200-mu-m x 200-mu-m with focal lengths ranging from 170-mu-m to 14 mm at 632.8 nm wavelength. Overlay registration accuracy is evaluated by optical microscopy of vernier-style alignment marks, etch depths are determined by stylus profilometry, and linewidths are measured by optical and scanning electron microscopy. The optical efficiency is evaluated as a function of the focal length and compared to theoretical predictions. The eight-phase-level F/4 microlenses, measured at 96% of the expected value, exhibit the highest optical efficiency reported to date for such a lens in the visible.