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Planar waveguide gratings are finding applications in wide band signal processing for imaging and location radars. Advanced forms may take the form of a `blased' topology, in which height as well as line and space dimensioning are engineered. This allows more complicated beam steering and wave interaction along the grating, promising better control over efficiency and more diverse engineering application. Planar lenses are being investigated as a method of coupling optical signals to the substrate. Realizing these devices also requires modification of the host substrate in three dimensions and is a difficult technological hurdle. Inherently low contrast resists can be shaped with the aid of clever processing techniques and have been classically used to obtain smaller line widths than the lithography technique would have normally allowed. In this work we utilize an experimental negative tone resist formulation to realize three dimensional features on GaAs substrates. The negative tone resist of interest, P(SI-CMS)-20, is under development (AT&T Bell Labs, Murray Hill, NJ) as a high performance single component system to be used in the fabrication of x-ray masks. Its properties include high resolution and the more unusual ability to faithfully retain a post processed film thickness that is primarily dependent upon e-beam dose, while using a fixed post exposure processing methodology. A curve of film thickness retention versus dose is then selected to define a required post exposure processed film thickness. A nominal 200 nm thick film is first spun onto the GaAs host wafer and softbaked. A Leica EBMF-10.5 vector scan electron beam lithography tool working at 25 KeV beam energy is used for patterning. A saw tooth or step ramping in processed resist height may now be achieved with a series of single pass lines or small areal features of successively higher dose density. The minimum dose corresponds to the minimum incipient gel of the resist and clears the foot of the saw tooth ramp. Successive features of increasing dose will build an increasingly thicker ramp of resist. Images are developed in ethanol, a first rinse in methanol and a final rinse in IPA/H2O. Planar lenses may also be attempted in this way by again doing a piece-wise construction of the shape, using a varying dose. The processed 3-D resist pattern is then transferred to the wafer by a magnetron RIE dry etch in an argon and boron trichloride atmosphere. Etching of the resist pattern and the wafer is performed so that protected areas of the wafer receive the least etch and the smallest relief. Etching selectivity may in part be set by choosing an appropriate mix in the etching atmosphere.
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