Ray Conley

Focusing of hard x-rays to 16 nanometers with a multilayer Laue lens

We report improved results for hard x-ray focusing using a multilayer Laue lens MLL. We have measured a line focus of 16 nm width with an efficiency of 31% at a wavelength =0.064 nm 19.5 keV using a partial MLL structure with an outermost zone width of 5 nm. The results are in good agreement with the theoretically predicted performance.

Hard x-ray nanofocusing by multilayer Laue lenses

Multilayer Laue lens (MLL) is a new class of x-ray optics that offer great promise for achieving nanometre-level spatial resolution by focusing hard x-rays. Fabricating an MLL via thin-film deposition provides the means to achieve a linear Fresnel-zone plate structure with zone widths below 1?nm, while retaining a virtually limitless aspect ratio. Despite its similarity to the Fresnel-zone plate, MLL exhibits categorically distinctive focusing properties and their fabrication comes with a wide array of challenges. This article provides a comprehensive review of advances in MLLs, and includes extensive theoretical modelling on focusing performance, discussion on fabrication challenges, their current capabilities and notable results from x-ray focusing experiments.

Two dimensional hard x-ray nanofocusing with crossed multilayer Laue lenses

Hard x-ray microscopy with nanometer resolution will open frontiers in the study of materials and devices, environmental sciences, and life sciences by utilizing the unique characterization capabilities of x-rays. Here we report two-dimensional nanofocusing by multilayer Laue lenses (MLLs), a type of diffractive optics that is in principle capable of focusing x-rays to 1 nm. We demonstrate focusing to a 25 × 27 nm(2) FWHM spot with an efficiency of 2% at a photon energy of 12 keV, and to a 25 × 40 nm(2) FWHM spot with an efficiency of 17% at a photon energy of 19.5 keV.

Wedged multilayer Laue lens

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. The sequence of layer thicknesses is controlled to satisfy the Fresnel zone plate law and the multilayer is sectioned to form the optic. An improved MLL geometry can be created by growing each layer with an in-plane thickness gradient to form a wedge, so that every interface makes the correct angle with the incident beam for symmetric Bragg diffraction. The ultimate hard x-ray focusing performance of a wedged MLL has been predicted to be significantly better than that of a nonwedged MLL, giving subnanometer resolution with high efficiency. Here, we describe a method to deposit the multilayer structure needed for an ideal wedged MLL and report our initial deposition results to produce these structures.

Sectioning of multilayers to make a multilayer Laue lens

We report a process to fabricate multilayer Laue lenses (MLLs) by sectioning and thinning multilayer films. This method can produce a linear zone plate structure with a very large ratio of zone depth to width (e.g., >1000), orders of magnitude larger than can be attained with photolithography. Consequently, MLLs are advantageous for efficient nanofocusing of hard x rays. MLL structures prepared by the technique reported here have been tested at an x-ray energy of 19.5 keV, and a diffraction-limited performance was observed. The present article reports the fabrication techniques that were used to make the MLLs.

Multilayer Laue lenses as high-resolution x-ray optics

Using Fresnel zone plates, a spatial resolution between 20 nm for soft x-rays and 70 nm for hard x-rays has been achieved. Improvement of the spatial resolution without loss of efficiency is difficult and incremental due to the fabrication challenges posed by the combination of small outermost zone width and high aspect ratios. We describe a novel approach for high-resolution x-ray focusing, a multilayer Laue lens (MLL). The MLL concept is a system of two crossed linear zone plates, manufactured by deposition techniques. The approach involves deposition of a multilayer with a graded period, sectioning it to the appropriate thickness, assembling the sections at the optimum angle, and using it in Laue geometry for focusing. The approach is particularly well suited for high-resolution focusing optics for use at high photon energy. We present a theory of the MLL using dynamic diffraction theory and Fourier optics.

Profile coating and its application for Kirkpatrick-Baez mirrors

For microfocusing x-ray mirrors, an elliptical shape is essen- tial for aberration-free optics. However, it is difficult to polish elliptical mirrors to x-ray quality smoothness. A differential coating method to con- vert a cylindrical mirror to an elliptical one has been previously reported. The coating was obtained by varying the sputter source power over a moving substrate. Here we report a new method of profile coating: the sputter source power is kept constant, while the substrate is passed over a contoured mask at a constant speed to obtain a desired profile along the direction perpendicular to the direction of substrate motion. The shape of the contour depends on the desired profile and the thickness distribution directly above the gun at the substrate level. The thickness distribution was measured on films coated on Si wafers using a spectro- scopic ellipsometer with computer-controlled X-Y translation stages. A model was developed to fit the measured thickness distribution, which determines the relative thickness weightings. When the substrate moves during a deposition, the film thickness is proportional to the length of the opening on the shield can along the direction of motion. By equating the sum of relative weightings to the required relative thickness at the same position, the length of the opening at that position can be determined. By repeating the same process for the whole length of the required profile, a contour can be obtained for a desired thickness profile. The number of passes and the speed of the substrate are determined according to the required thickness and the growth-rate calibration of a test run. The mir- ror coating profile is determined from the difference between the ideal surface figure of a focus ellipse and the surface figure obtained from a long-trace profiler measurement on the substrate. A Kirkpatrick-Baez (KB) mirror pair was made using Au as a coating material and cylindri- cally polished mirrors as substrates. Synchrotron x-ray results using this KB mirror pair showed a focused spot size of 0.430.4 mm 2 .

Multimodality hard-x-ray imaging of a chromosome with nanoscale spatial resolution.

We developed a scanning hard x-ray microscope using a new class of x-ray nano-focusing optic called a multilayer Laue lens and imaged a chromosome with nanoscale spatial resolution. The combination of the hard x-ray’s superior penetration power, high sensitivity to elemental composition, high spatial-resolution and quantitative analysis creates a unique tool with capabilities that other microscopy techniques cannot provide. Using this microscope, we simultaneously obtained absorption-, phase-, and fluorescence-contrast images of Pt-stained human chromosome samples. The high spatial-resolution of the microscope and its multi-modality imaging capabilities enabled us to observe the internal ultra-structures of a thick chromosome without sectioning it.

Multilayer growth in the APS rotary deposition system.

We report our progress in the growth of periodic and depth-graded multilayers in the APS rotary deposition system, a machine designed for fabrication of films tens of microns thick with thousands of layers. A computational method was employed to design depth-graded multilayers for use as wide-angular bandpass reflective optics. We present experimental results for a 154-layer WSi2/Si multilayer system with bilayer thickness ranging from 2.2 nm to 5.5 nm that closely match theoretical flat-top reflectivity predictions of 9.8% from 15.6 mrad to 23.3 mrad at 8 keV.

Multilayer Laue Lens: A Path Toward One Nanometer X-Ray Focusing

The multilayer Laue lens (MLL) is a novel diffractive optic for hard X-ray nanofocusing, which is fabricated by thin film deposition techniques and takes advantage of the dynamical diffraction effect to achieve a high numerical aperture and efficiency. It overcomes two difficulties encountered in diffractive optics fabrication for focusing hard X-rays: (1) small outmost zone width and (2) high aspect ratio. Here, we will give a review on types, modeling approaches, properties, fabrication, and characterization methods of MLL optics. We show that a full-wave dynamical diffraction theory has been developed to describe the dynamical diffraction property of the MLL and has been employed to design the optimal shapes for nanofocusing. We also show a 16 nm line focus obtained by a partial MLL and several characterization methods. Experimental results show a good agreement with the theoretical calculations. With the continuing development of MLL optics, we believe that an MLL-based hard x-ray microscope with true nanometer resolution is on the horizon.