Fabrication of high-throughput critical-angle X-ray transmission gratings for wavelength-dispersive spectroscopy

Abstract

The development of the critical-angle transmission (CAT) grating seeks both an order of magnitude improvement in the effective area, and a factor of three increase in the resolving power of future space-based, soft x-ray spectrometers. This will enhance further studies of the universe's make-up, such as the composition of the intergalactic medium, black holes, neutron stars and other high energy sources. Conceptually, x-rays are reflected in the device off nanoscale silicon grating bars at shallow angles, such that the diffraction orders are at the specular reflection angle, which is designed to be less than the critical-angle for total external reflection. This blazing effect boosts the efficiency of the device; however, the grating bars are required to form very deep channels to reflect all the incoming x-rays at shallow angles. Previous attempts to fabricate the grating were done with wet potassium hydroxide (KOH) etching of silicon. This process successfully fabricated small areas of grating and enabled a successful demonstration of the soft x-ray diffraction efficiency. However, the open-area fraction was limited to below 20 percent for four micron-tall CAT grating bars due to diagonal etch stops in the silicon crystal lattice. This limitation prevents the past fabrication technique from achieving the desired open-area fraction for a future x-ray observatory. New nanofabrication techniques are presented that can lead to CAT gratings with an open-area fraction in excess of 50 percent. Specifically, three major nanofabrication processes were developed and are described in detail; a two-dimensional, thermal, silicon dioxide mask, an integrated plasma-etch process to create free-standing, ultra-high aspect ratio gratings, and a polishing process to smooth the grating sidewalls. The two-dimensional mask was used to develop a record-performance deep reactive-ion etch (DRIE) for ultra-high aspect ratio gratings. The mask is the integration of a 5 micron and 200 nanometer-pitch grating into a single layer of 300 nanometer-thick thermal silicon dioxide. It spans 5 centimeters on a side, with vertical sidewalls, and is cleanable which enables consistent high quality etches. Experiments with chrome and polymer masking materials for DRIE are also presented. The DRIE was critical for the integrated process, which combined two plasma-etch processes on the front and back side of a silicon-on-insulator wafer. DRIE is not significantly affected by the silicon crystal orientation and therefore avoids the open-area restrictions of wet etching. The result of the process was a free-standing grating with a period of 200 nanometers, a depth of four microns, and a span of three centimeters. These free-standing gratings exceed the state-of-the-art by more than a factor of two in aspect ratio at the nanoscale. The sidewall roughness is one shortcoming of DRIE, which is often greater than 4 nanometers RMS, and it needs to be approximately one nanometer to efficiently reflect soft x-rays. To address this, the world's first reported nanoscale polishing process has been developed to smooth the sidewalls of DRIE'd, ultra-high aspect ratio silicon. This process utilizes potassium hydroxide etching, an anisotropic etch of single crystal silicon. Specifically, the [111] planes etch approximately 100 times slower than the non-[111] planes. A novel alignment technique is presented to align the CAT grating pattern to the [111] silicon planes to within 0.2 degrees. This precise alignment enables KOH to etch away sidewall roughness and slowly widen the channels without fully destroying the structure. The result of polishing was a reduction in sidewall roughness to approximately 1 nm RMS, while decreasing the widths of the grating bars. In addition to the nanofabrication developments, this work provides a preliminary analysis of launching and deploying CAT gratings in space. The nanofabrication developments are focused towards the CAT grating; however, they have other applications as well. High quality masks have applications in MEMS structures and photonic devices. The free-standing structure as a stand-alone device has applications such as neutral mass spectroscopy, ultraviolet filtration, and x-ray phase contrast imaging. The polishing process is valuable to numerous optical applications where smooth sidewalls are critical, as well as filtration techniques which seek to maximize open-area.