Tiny Blinds Bend Beams

A new way of bending x-ray beams could lead to greatly improved space telescopes, as well as new tools for biology and semiconductor chip manufacturing.

X-rays from space provide astronomers with important information about the most exotic events and objects in our universe, such as dark energy, black holes and neutron stars. But x-rays are notoriously difficult to collect and many interesting cosmic sources are faint, which makes collecting these high-energy rays difficult and time-consuming, even with telescopes on satellites far above our x-ray-absorbing atmosphere.

Now a group of researchers from the Massachusetts Institute of Technology has fabricated a new, highly efficient nanoscale Venetian-blind-like device that contains thousands of ultrasmooth mirror slats per millimeter for use in future improved space-based x-ray telescopes. The so-called critical-angle transmission (CAT) gratings feature dense arrays of tens-of-nanometer-thin, freely suspended silicon structures that serve as efficient mirrors for the reflection and diffraction of nanometer-wavelength light -- otherwise known as x-rays.

Gratings used to manipulate x-rays for future space telescopes and other applications, like tiny miniaturized venetian blinds, were created using this interference lithography patterning tool, called the nanoruler, developed at MIT's Space Nanotechnology Laboratory. The colorful, diffracting wafer at center has a diameter of 12 in. (Photo copyright ©Ralf Heilmann)

New instrument designs based on these gratings could also lead to advances in fields beyond astrophysics, from plasma physics to the life and environmental sciences, as well as in extreme ultraviolet lithography, a technology of interest to the semiconductor industry because it could lead to faster circuits with smaller features. The concept behind CAT gratings might also open new avenues for devices in neutron optics and for the diffraction of electrons, atoms and molecules.

Based on an invention by Ralf Heilmann and Mark Schattenburg of the Space Nanotechnology Laboratory (SNL) at the MIT Kavli Institute of Astrophysics and Space Research, the fabrication challenges were overcome by graduate student Minseung Ahn of the Department of Mechanical Engineering at MIT in a yearlong effort, with a Samsung fellowship and financial support from NASA.

Motivated by technology goals for NASA's next-generation x-ray telescope, called Constellation-X, the new devices promise to improve more than fivefold upon the efficiency of the transmission gratings on board NASA's Chandra X-Ray Observatory (launched in 1999), which were also built at SNL. The improvement comes from the new design, in which x-rays are reflected very efficiently at very shallow angles -- akin to skipping stones on water -- from the subnanometer-smooth sidewalls of the silicon slats, through the spaces between the slats. Also, in the earlier version the x-rays had to pass through a supporting substrate of polyimide, which absorbed many of the rays and reduced the grating's efficiency.

The silicon slats -- as thin as 35 nm, which is comparable to the smallest feature sizes still under development in commercial computer chip manufacturing -- are parallel to each other and separated by as little as about 150 nm. The slats have to extend many micrometers in the remaining two dimensions.

"Imagine a thin, 40-foot-long, 8-foot-tall mirror, with surface roughness below a tenth of a millimeter," said Heilmann. "Then put tens of thousands of these mirrors next to each other, each spaced precisely an inch from the next. Now shrink the whole assembly -- including the roughness -- down by a factor of a million, and you have a good CAT grating."

Recent x-ray test results from a prototype device, obtained with the help of Eric Gullikson of Lawrence Berkeley National Laboratory, confirmed that it met theoretical expectations.

The results of the research were published in the June 9 edition of Optics Express. They were also presented May 28 at the 52nd International Conference on Electron, Ion and Photon Beam Technology and Nanofabrication in Portland, Ore., and will be presented again at the SPIE Conference on Astronomical Telescopes and Instrumentation in Marseille, France, June 23.

For more information, visit: www.mit.edu