Fabrication of high-numerical-aperture phase zone plates with a single lithography exposure and no etching

We present a process for fabricating phase zone plates and other diffractive-optical elements that is capable of achieving high resolution, requires only a single lithography step, and no etching. For this process we use the negative resist hydrogen silsesquioxane (HSQ) (Dow Corning), which is sensitive to both electrons and x rays. HSQ's extraordinarily high resolution (similar to10 nm) and its SiO2-like properties make it an optimal choice for fabricating diffractive-optical elements that operate in the ultraviolet and deep ultraviolet. HSQ has an index of refraction very close to that of fused silica, and a negligible absorption down to 157 nm. As a result, if it is spun to the thickness corresponding to the desired phase step for the optic, patterning and development are the only required process steps. It is often desirable to place an absorbing layer on the areas surrounding the diffractive elements to prevent unwanted radiation from reaching the substrate when the optics are used for lithography and microscopy applications. We have developed a method that achieves this by means of a metal evaporation followed by a selectively controlled chemical etching (Fulton/ Dolan process). We present e-beam and x-ray lithographic exposures in HSQ of phase zone plate arrays, and provide a full description of the selectively controlled chemical etching process as applied to the fabrication of diffractive optics. The focusing properties of the fabricated arrays are also presented. (C) 2003 American Vacuum Society.