PHASE HOLOGRAMS IN POLYMETHYL METHACRYLATE

Complex computer generated phase holograms (CGPHs) have been fabricated in polymethyl methacrylate (PMMA) by partial exposure and subsequent partial development. The CGPH was encoded as a sequence of phase delay pixels and written by e-beam (JEOL JBX-5DII), a different dose being assigned to each value of phase delay. Following carefully controlled, partial development, the pattern appears, rendered in relief, in the PMMA which then acts as the phase-delay medium. The exposure dose was in the range 20-200 muC/cm2, and very aggressive development in pure acetone led to low contrast. This enabled etch depth control to better than +/-20 nm corresponding to an optical phase shift in transmission, relative to air, of +/-lambda(vis)/60. That result was obtained by exposing isolated 50 mum square patches and measuring resist removal over the central area where the proximity effect dose was uniform and related only to the local exposure. For complex CGPHs with pixel size of the order of the proximity radius, the patterns must be corrected for proximity effects. In addition, the isotropic nature of the development process will produce sidewall etching effects. The devices fabricated were designed with 16 equal phase steps per retardation cycle, were up to 3 mm square, and consisted of up to 10 million 0.3-2.0 mum square pixels. Data files were up to 60 Mb long and exposure times ranged to several hours. No sidewall etch corrections were applied to the pattern and proximity effects were only treated approximately. A Fresnel phase lens was fabricated that had diffraction limited optical performance with 83% efficiency.