To measure space plasmas and neutral particles one must filter out high-energy ultraviolet photons that would increase background count or damage sensors. To enable sensitive neutral particle measurements, a photon-to-particle rejection rate of 10(14) is desired, far exceeding the requirements of prior filters. The authors propose a high-aspect ratio Si grating with densely packed, sub-100 nm slits. In this article, the authors report the development of a new technique for fabricating sturdy, self-supported transmission gratings in silicon using nanoimprint lithography and deep reactive ion etching, resulting in grating slits with scalloping under 7 nm and high (8.5:1) aspect ratios.