Fabrication of thin periodic structures in photoresist: a model

We present a technique for designing holo-graphically fabricated structures in photoresist. A method is given for obtaining a suitable initial resist thickness. The etch depth (Deltat) vs exposure (E) characteristic is determined experimentally for a fixed development time (T). The characteristics for other values of T are found using the linearity of Deltat with T. Whole families of grating profiles can then be generated by using a polynomial fit to the exposure characteristics. For example, using a single holographic exposure plus an optional uniform preexposure, a family of grating profiles is obtained by varying the parameter T. A desired profile can thus be realized by a suitable combination of initial resist thickness, exposure, and development time. Residual resist layers may be eliminated by this method thereby minimizing scattering and other losses. Experimental verification of the various features of the model is given.