Pran Mukherjee

Diffraction efficiency of 200-nm-period critical-angle transmission gratings in the soft x-ray and extreme ultraviolet wavelength bands

We report on measurements of the diffraction efficiency of 200-nm-period freestanding blazed transmission gratings for wavelengths in the 0.96 to 19.4 nm range. These critical-angle transmission (CAT) gratings achieve highly efficient blazing over a broad band via total external reflection off the sidewalls of smooth, tens of nanometer thin ultrahigh aspect-ratio silicon grating bars and thus combine the advantages of blazed x-ray reflection gratings with those of more conventional x-ray transmission gratings. Prototype gratings with maximum depths of 3.2 and 6 μm were investigated at two different blaze angles. In these initial CAT gratings the grating bars are monolithically connected to a cross support mesh that only leaves less than half of the grating area unobstructed. Because of our initial fabrication approach, the support mesh bars feature a strongly trapezoidal cross section that leads to varying CAT grating depths and partial absorption of diffracted orders. While theory predicts broadband absolute diffraction efficiencies as high as 60% for ideal CAT gratings without a support mesh, experimental results show efficiencies in the range of ∼50-100% of theoretical predictions when taking the effects of the support mesh into account. Future minimization of the support mesh therefore promises broadband CAT grating absolute diffraction efficiencies of 50% or higher.

Plasma etch fabrication of 60:1 aspect ratio silicon nanogratings with 200 nm pitch

The authors present a breakthrough multistage dry-etch process to create 100 nm half-pitch gratings in silicon with depths up to 6 μm. Interference lithography was used to pattern gratings in an optically matched stack of materials to form a 400-nm-thick silicon oxide hard-mask. The oxide was then used to mask the subsequent deep reactive-ion etching of silicon. In this article, the authors describe their grating patterning, pattern transfer, and deep etch processes, and present progress toward combining this technique with coarser scale lithography steps designed to form an integrated mechanical support structure to produce freestanding x-ray diffraction gratings.

Fabrication of nanoscale, high throughput, high aspect ratio freestanding gratings

A nanofabrication process has been developed for a novel critical-angle transmission grating for astronomical x-ray spectroscopy. The pitch of the gratings is 200 nm and the depth is 4 μm, which exceeds the state-of-the-art in aspect ratio by over a factor of 2 for ultrahigh aspect ratio freestanding nanoscale gratings with open areas on the order of 50% and spanning several square centimeters. They have a broad array of other applications, including neutral mass spectroscopy, ultraviolet filtration, and phase contrast imaging x-ray spectroscopy. The gratings are fabricated as a monolithic structure in silicon via two lithographic and pattern transfer processes, integrated together on a silicon-on-insulator wafer. The grating is patterned via interference lithography and transferred into the 4 μm device layer via a Bosch deep reactive-ion etch (DRIE). The grating channels are then filled without voids by spinning photoresist on them, which wicks into the channels. The sample is then bonded under vacuum vi...

Fabrication and testing of freestanding Si nanogratings for UV filtration on space-based particle sensors

We demonstrate complete fabrication process integration and device performance of sturdy, self-supported transmission gratings in silicon. Gratings are patterned with nanoimprint lithography and aluminum liftoff on silicon-on-insulator wafers. Double-sided deep reactive ion etching (DRIE) creates freestanding 120 nm half-pitch gratings with 2000 nm depth and built-in 1 mm pitch bulk silicon support structures. Optical characterization demonstrates 10(-4) transmission of UV in the 190-250 nm band while a 25-30% geometric transparency allows particles to pass unimpeded for space plasma measurements.

Potassium hydroxide polishing of nanoscale deep reactive-ion etched ultrahigh aspect ratio gratings

A fabrication process has been developed to chemically polish the sidewalls of 200 nm-pitch gratings via potassium hydroxide (KOH) etching following the Bosch deep reactive-ion etching (DRIE) process. Previous KOH polishing experiments focused on micron scale features. This work is the first reported combined DRIE-KOH etching process on the nanoscale for ultrahigh aspect ratio structures with feature sizes 30 times smaller than previously published work. The primary application of the gratings is x-ray spectroscopy and requires polished sidewalls for efficient x-ray reflection. Polishing is also critical for increasing the open area by narrowing the grating bars, which increases the throughput of x-rays. The increased open area is also valuable for other applications such as ultraviolet filtration, neutron spectroscopy and biofiltration. Advanced Bosch processes leave approximately 4 nm, root mean square (RMS), of roughness on the sidewalls. This roughness needs to be reduced to below 1 nm to efficiently ...

Critical-angle transmission grating spectrometer for high-resolution soft x-ray spectroscopy on the International X-ray Observatory

High-resolution spectroscopy at energies below 1 keV covers the lines of C, N, O, Ne and Fe ions, and is central to studies of the Interstellar Medium, the Warm Hot Intergalactic Medium, warm absorption and outflows in Active Galactic Nuclei, coronal emission from stars, etc. The large collecting area, long focal length, and 5 arcsecond half power diameter telescope point-spread function of the International X-ray Observatory will present unprecedented opportunity for a grating spectrometer to address these areas at the forefront of astronomy and astrophysics. We present the current status of a transmission grating spectrometer based on recently developed high-efficiency critical-angle transmission (CAT) gratings that combine the traditional advantages of blazed reflection and transmission gratings. The optical design places light-weight grating arrays close to the telescope mirrors, which maximizes dispersion distance and thus spectral resolution and minimizes demands on mirror performance. It merges features from the Chandra High Energy Transmission Grating Spectrometer and the XMM-Newton Reflection Grating Spectrometer, and provides resolving power R = E/ΔE = 3000 - 5000 (full width half max) and effective area >1000 cm2 in the soft x-ray band. We discuss recent results on ray-tracing and optimization of the optical design, instrument configuration studies, and grating fabrication.

Fabrication update on critical-angle transmission gratings for soft x-ray grating spectrometers

Diffraction grating-based, wavelength dispersive high-resolution soft x-ray spectroscopy of celestial sources promises to reveal crucial data for the study of the Warm-Hot Intergalactic Medium, the Interstellar Medium, warm absorption and outflows in Active Galactic Nuclei, coronal emission from stars, and other areas of interest to the astrophysics community. Our recently developed critical-angle transmission (CAT) gratings combine the advantages of the Chandra high and medium energy transmission gratings (low mass, high tolerance of misalignments and figure errors, polarization insensitivity) with those of blazed reflection gratings (high broad band diffraction efficiency, high resolution through use of higher diffraction orders) such as the ones on XMM-Newton. Extensive instrument and system configuration studies have shown that a CAT grating-based spectrometer is an outstanding instrument capable of delivering resolving power on the order of 5,000 and high effective area, even with a telescope point-spread function on the order of many arc-seconds. We have fabricated freestanding, ultra-high aspect-ratio CAT grating bars from silicon-on-insulator wafers using both wet and dry etch processes. The 200 nm-period grating bars are supported by an integrated Level 1 support mesh, and a coarser external Level 2 support mesh. The resulting grating membrane is mounted to a frame, resulting in a grating facet. Many such facets comprise a grating array that provides light-weight coverage of large-area telescope apertures. Here we present fabrication results on the integration of CAT gratings and the different high-throughput support mesh levels and on membrane-frame bonding. We also summarize recent x-ray data analysis of 3 and 6 micron deep wet-etched CAT grating prototypes.

Progress in the development of critical-angle transmission gratings

Recently developed Critical-Angle Transmission (CAT) grating technology - in combination with x-ray CCD cameras and large collecting-area focusing optics - will enable a new generation of soft x-ray spectrometers with unprecedented resolving power and effective area and with at least an order of magnitude improvement in figures-of-merit for emission and absorption line detection. This technology will be essential to address a number of high-priority questions identified in the Astro2010 Decadal Survey “New Worlds New Horizons” and open the door to a new discovery space. CAT gratings combine the advantages of soft x-ray transmission gratings (low mass, relaxed figure and alignment tolerances, transparent at high energies) and blazed reflection gratings (high broad band diffraction efficiency, utilization of higher diffraction orders to increase resolving power). We report on progress in the fabrication of large-area (31× 31 mm2) free-standing gratings with two levels of low-blockage support structures using highly anisotropic deep reactive-ion etching.