Alan Lyon

Nanofabrication of high aspect ratio 24nm x-ray zone plates for x-ray imaging applications

Building high-performance zone plate is a critical step for achieving nanometer resolution in advanced x-ray imaging and microscopy. Zone plates with smaller outmost zone width and higher aspect ratio are increasingly in demand, simply because the resolution and efficiency of an x-ray microscope are ultimately determined by these two features. In this paper, we will present detailed discussion of achieving high aspect ratio and high resolution x-ray zone plate through electron beam lithography, trilevel resist process and gold plating, fabrication problems, and limitations. We will also present the technique to double the aspect ratio of the zone plate and measure the results of x-ray diffraction efficiency of single and aspect ratio doubled zone plates.

Ellipsoidal and parabolic glass capillaries as condensers for x-ray microscopes

Single-bounce ellipsoidal and paraboloidal glass capillary focusing optics have been fabricated for use as condenser lenses for both synchrotron and tabletop x-ray microscopes in the x-ray energy range of 2.5-18 keV. The condenser numerical apertures (NAs) of these devices are designed to match the NA of x-ray zone plate objectives, which gives them a great advantage over zone plate condensers in laboratory microscopes. The fabricated condensers have slope errors as low as 20 murad rms. These capillaries provide a uniform hollow-cone illumination with almost full focusing efficiency, which is much higher than what is available with zone plate condensers. Sub-50 nm resolution at 8 keV x-ray energy was achieved by utilizing this high-efficiency condenser in a laboratory microscope based on a rotating anode generator.

Near field stacking of zone plates for reduction of their effective zone period

Here we analyze the potential of a new fabrication method for high resolution zone plates with high aspect ratios based on near field stacking of frequency doubled atomic layer deposited (ALD) zone plates. The proposed method enables reduction of the effective zone period by a factor of four with two zone plate layers compared to the initial e-beam lithography exposed outermost zone period. It also overcomes the problem that very small zone widths with high aspect ratios have to be fabricated for high-resolution hard X-ray microscopy. Using rigorous coupled wave theory, we have analyzed the diffraction behavior of these near field stacked zone plates and investigated strategies to optimize fabrication parameters to compensate for separation of stacked zone plates. The calculations performed for 8 keV photon energy and effective outermost zone widths of 28 nm and 15 nm predict diffraction efficiencies ≥ 20% suggesting that such optics could find widespread practical applications.

Absolute efficiency measurement of high-performance zone plates

An absolute efficiency measurement technique for Fresnel zone plates using an electron impact micro-focus laboratory X-ray source (Lα line of Tungsten at 8.4 KeV) is demonstrated. A quasi-monochromatic x-ray image of a zone plate was obtained employing a pair of copper and cobalt filters. Applying this method to zone plates optimizes the zone plate fabrication process and provides the ability to explore zone geometry to achieve the best possible efficiency. Several zone plate parameters were tested with first order efficiency measuring from 1% to 29%.

Pathways to sub-10nm x-ray imaging using zone plate lens

The spatial resolution is a key optical parameter characterizing the performance of an imaging microscope. Zone plate based x-ray microscopy offers the highest spatial resolution over the whole electromagnet wave spectrum. Sub-20 nm resolution have been demonstrated with soft x-rays and sub-60 nm resolution have been obtained with multikeV x-rays using a laboratory source. There are two simple pathways to achieve sub-10 nm resolution x-ray imaging: (1) improving the fabrication technology to produce zone plates with an outermost zone width less than 10 nm and (2) using a higher diffraction order (such as the third diffraction order) of a currently available zone plate.

Dark field image of full-field transmission hard X-ray microscope in 8-11 keV

We have demonstrated dark-field imaging using a full-field hard x-ray microscope by using a custom capillary-based condenser. The condenser provides illumination with a numeric aperture about 3-mrad with high efficiency. This high illumination angle allows full-resolution imaging using a 50 nm hard x-ray zone plate. The zeroth order beam from the condenser is well out of the zoneplate range - which allows a high signal-to-noise ratio in the image plane. Small particles with high scattering power, such as colloidal gold markers used in biology are well-suited for dark-field imaging. Combining with high brightness source from NSRRC BL01B, the dark field image can be acquired within several minutes with high contrast ratio. In this paper, the dark field image of IC and the zoneplate defect will be demonstrated and studied in different energy under dark field mode.

Novel, High Brightness X-ray Source and High Efficiency X-ray Optic for Development of X-ray Instrumentation

In the past two decades, laboratory x-ray analysis equipment has made significant strides, particularly toward higher resolution capabilities [1-2]. The major bottleneck to continued advances to achieving simultaneously achieving higher sensitivity, resolution, and throughput is the relatively low flux of x-rays at the sample for micro-characterization techniques such as SAXS, high resolution XRD, microXRF, and x-ray microscopy [3]. We present two major innovations of a microfocus x-ray source and a high resolution, high efficiency x-ray optic to enable delivery of flux comparable to second generation bending magnet synchrotrons.

A New Approach to Microns-Resolution Trace Element and Mineralogy Mapping at PPM Sensitivity for Digital Rock and Geological Research

Analysis of geological materials has advanced substantially in the past decade, with the introduction of automated scanning electron microscope (SEM) based solutions such as QEMSCAN and MLA for mineralogy mapping and the development of the use of microCT systems for Digital Rock and mining applications. However, critical gaps in geological microanalysis still exist. For one, trace elements or minor elements within a mineral of similar chemistries (e.g. below 3% weight of gold in pyrite) are challenging to detect in the SEM-based mineralogy approaches [1], which is problematic for finding rare earth elements and for exploration of “invisible gold”. For microCT approaches, resolution limitations can prevent accurate determination of composition – as grayscale levels of regions of interest are influenced by both mineralogy and the presence of porosity [2].

Advances toward submicron resolution optics for x-ray instrumentation and applications

Sigray’s axially symmetric x-ray optics enable advanced microanalytical capabilities for focusing x-rays to microns-scale to submicron spot sizes, which can potentially unlock many avenues for laboratory micro-analysis. The design of these optics allows submicron spot sizes even at low x-ray energies, enabling research into low atomic number elements and allows increased sensitivity of grazing incidence measurements and surface analysis. We will discuss advances made in the fabrication of these double paraboloidal mirror lenses designed for use in laboratory x-ray applications. We will additionally present results from as-built paraboloids, including surface figure error and focal spot size achieved to-date.

New developments in ultra-high brightness microstructured x-ray sources for applications in Talbot-Lau imaging and dual-energy microanalytical/microXRF (Conference Presentation)

The limitations to achievable x-ray brightness within the laboratory1 for x-ray spectra is a well-known problem for improving the throughput, sensitivity, and resolution of a wide variety of x-ray techniques. Specific examples of such challenges include: throughput in Talbot-Lau interferometry for medical applications, limits to sensitivity in micro x-ray fluorescence (microXRF), and resolution in x-ray microscopy. We will present our patented x-ray source technology and recent developments. The major innovations in our x-ray source are the x-ray anodes, which are comprised of arrays of microstructured metal x-ray emitters embedded within a diamond substrate. The diamond substrate enables highly localized large thermal gradients that passively and rapidly cool the metal microstructures as heat is generated under the bombardment of electrons. Electron power densities, 4X higher than conventional solid metal targets can be achieved on the target even greater for metals of lower thermal conductivity. The thermal advantages of the anode design enables the use of many elements that were previously unsuitable as x-ray source materials, and will enable access to new x-ray characteristic lines to optimize performance in monochromatic x-ray analysis. In addition, we will review practical benefits of our patented FAASTTM (fine array anode source technology) x-ray source over both conventional x-ray sources and newer schemes such as liquid metal anodes2. Advantages include the ability to produce a patterned microbeam optimized for Talbot-Lau interferometry (phase contrast imaging) and the ability to produce various characteristic lines through the incorporation of novel materials (e.g. Au, Pt, Cr) for dual energy capabilities.