2017 May 17 - June 29
2017 October 11 - December 21
2017 Proposal deadline: 08/01/17
2017 BTR deadline: 09/10/17
K.D. Finkelstein (CHESS) and Paul Cottle (Cornell
CHESS, always at the cutting edge of innovation in x-ray optics, has begun exploiting resources at the Cornell Nanofabrication Laboratory to develop a new generation of focusing elements. The work adapts micro-electro-mechanical system (MEMS) fabrication techniques to reduce the size of structures in silicon used to collect, diffract, and focus x-ray beams.
As a first application, we are improving our sagittal focusing monochromator crystals that gather and focus a large horizontal swath of synchrotron radiation. These crystals are thin (~1mm) triangles with deep rectangular groves (gums) cut perpendicular to the altitude. The "teeth" adjacent to the gums counteract the anticlastic bending that occurs due to the sagittal bend. This type of x-ray optic produces a focal spot limited in size by the synchrotron source (particle beam size in the storage ring), the sagittal tooth size (individual diffracting regions on the crystal), and the focal distances. For the typical 1 to 3 focal ratio, a small focal spot requires the tooth width be ¼ of the source size. If one also wants to maximize flux in the focal spot, it is important to maximize the number of teeth cut into these crystals.
We are currently making full sized crystals from 4 inch diameter wafers, with equal gum and tooth widths of 250 microns, containing 125 teeth approximately 1 mm. tall. These features are smaller than can typically be achieved with traditional diamond-blade cutting, and with much less breakage of delicate crystals during processing. This technique produces virtually no surface damage and so there is no need to use strong acids to etch away damage.
A triangular crystal is etched out of a 4 inch silicon wafer. The inset shows the 1mm deep, 250 micron wide grooves that give the crystal enough flexibility to bend without breaking or becoming distorted. The light horizontal lines are groove edges.