Highlights

Multilayer Laue Lenses for Nano-focusing of Hard X-rays

Scientific Achievement

A multilayer Laue lens (MLL) is an x-ray focusing optic fabricated from a multilayer structure consisting of thousands of layers of two different materials produced by thin-film deposition.  We have found that WSi₂/Si is a promising material system due to the low film stress between deposited layers and high atomic number difference of the system.  The sequence of layer thicknesses is controlled to satisfy the Fresnel zone plate law.  The latest MLL deposited consist of 5165 alternating layers of Si and WSi₂.  The thicknesses of layers on each side vary monotonically from 4nm to 160nm and the total thickness of the stack is about 40microns.

The challenges we face in this project are the deposition of thousands of graded layers with accurate thickness control; also sectioning and polishing such a delicate and small sample in order to be used for hard x-rays nano-focusing experiment is challenging.  By the help, training and support of experienced EMC staff, we are able to overcome the challenges.  High resolution images, at very high magnification, were obtained from Hitachi S-4700 SEM in the EMC facility over the course of last several years.  These images were essential as feedback for our deposition and sectioning process.  Sequences of overlapped images were used in accurate image processing in order to achieve accurate thickness and position data for each single layer.

We have measured a line focus of 16nm width with an efficiency of 31% at a wavelength ?=0.064 nm (19.5 KeX) using partial MLL structure with an outermost zone width of 5 nm.  This is the hard x-ray nano-focusing world’s record [Appl. Phys. Lett. 92, 2211114 (2008)].

Significance

Fresnel zone plates for x-ray focusing optics are typically made using lithography techniques.  However these lenses have low efficiency as well as limitations due to lithography limitations.  MLLs are designed and fabricated to overcome these limitations.  The linear zone plate structure produces very large aspect ratios of zone depth to width (e.g. >1000) which is an order of magnitude larger than can be attained with photolithography.  MLLs will significantly improve hard x-ray focusing and will assist the synchrotron community to reach higher resolutions for research and x-ray microscopy.   This work has been published in Phys. Rev. Lett. 96, 127401 (2006); Rev. Sci. Instrum. 78, 046103 (2007); Appl. Phys. Lett. 92, 2211114 (2008); and Rev. Sci. Instrum. 79, 053104 (2008).

Performers

N. Jahedi, C. Liu, J. Qian, B. Shi, A. Macrander (Argonne-APS)