X-ray Phase Measurements Research Articles

Electrochemical Endpoint Lithography Techniques Applied to PMMA LIGA Fabrication of Xpci Gold Gratings

X-ray Phase contract imaging (XPCI) is a promising technique for medical and non-destructive testing[1]. XPCI requires High Aspect Ratio gratings made of high Z materials, e.g. gold. The preferred method of fabrication to realize the very high aspect ratio tall structures is X-ray based lithography, molding, and electroforming process (LIGA) capable of creating high density 1.2 micron features with straight sidewalls that are 120 micron in height [2]. One critical step to pattern such structures is to accurately develop the photoresist mold. This is so because the window is narrow between development and over development. We have proposed and demonstrated the use of a novel electrochemical technique to accurately develop LIGA molds with high repeatability prior to subsequent electroplating. The number and period of development cycles often can be a critical and subjective step where operator error and human judgement can play a significant role drastically affecting the outcome. The analysis must be performed when the sample is still immersed in DI water to avoid capillary action destroying the photoresist molds at high aspect ratios. Some experts have implemented drying techniques to avoid the collapse of high aspect ratio polymer molds when they are dried. Some scientist show a direct phase change from liquid to gas using cyclohexane to mitigate this phenomenon [3]. Sodium chloride (NaCl) salts have been added at ~100 mM to a GG developer LIGA based chemistry. After x-ray exposure of the PMMA the GG developer removes regions where x-ray have severed the PMMA backbone of the polymer mold resulting in a patterned mold that can then be filled with gold to create a high aspect ratio grating. LIGA substrates were prepared using high molecular weight PMMA, x-ray exposed, and actively monitored utilizing electrochemical technique during the development process to search for an endpoint where subsequent gold plating can then occur. As the high aspect ratio mold is developed, the electrochemical interactions are measured between the conducting seed layer and NaCl. We will discuss conductivity of the chemistry, optimizing the process for repeatability, the benefits, and the challenges. Results show an increase in impedance as the exposed PMMA is chemically removed and the conductive NaCl salts get closer to the conductive seed layer at the base of the LIGA PMMA mold. This process uses first of its kind real time electrochemical data capable of detecting more accurately the endpoint of the development process. Previous endpoints were determined by visual inspection, destructive inspection after the fact, large lot burn ins, and single use chemical processes. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA-0003525. The views expressed in the article do not necessarily represent the views of the U.S. Department of Energy or the United States Government. Momose, A. and W. Yashiro, X-ray Phase Measurements with Talbot Interferometry and Its Applications. International Conference on Advanced Phase Measurement Methods in Optics an Imaging, 2010. 1236: p. 195-199. Mohr, J., et al., High Aspect Ratio Gratings for X-Ray Phase Contrast Imaging. International Workshop on X-Ray and Neutron Phase Imaging with Gratings, 2012. 1466: p. 41-50. Koch, F., et al., Increasing the aperture of x-ray mosaic lenses by freeze drying. Journal of Micromechanics and Microengineering, 2015. 25(7).

Honeycomb Support Structures to Stabilize PMMA Mold during LIGA Processing of High Aspect Ratio Gratings for XPCI Systems

X-ray phase contrast imaging (XPCI) allows for medical and non-destructive testing of soft tissue [1]. High aspect ratio (HAR) gratings (100:1) are difficult to achieve and are a key element of XPCI gold gratings PMMA lithography, molding, and electroforming (LIGA) process [2]. Development of uniform, continuous HAR gold gratings over a large area is difficult as they easily tilt and delaminate. One technique for stabilizing the mold during development is to incorporate relief structures into mold, providing more surface area at the bottom, which results in segmented gratings [2]. In the current work, honeycomb supports are used to stabilize the PMMA mold during development and plating to realize continuous gratings centimeters in lateral length. Our goal is to fabricate micron size HAR gratings ~10 cm in lateral length that are ideal for XPCI applications enabling larger field of view for the XPCI imaging. Honeycomb structures were patterned on a 100 mm substrate. A sheet of PMMA is then bonded and fly cut to desired height. An x-ray exposure then patterns the PMMA mold prior to the fabrication of the top honeycomb structure. The gratings are then developed and electroplated having structural support on both sides. The use of bottom and top honeycomb arrays provide unique mechanical support to the fragile PMMA mold during development and electroplating of the HAR grating. Momose, A. and W. Yashiro, X-ray Phase Measurements with Talbot Interferometry and Its Applications. International Conference on Advanced Phase Measurement Methods in Optics an Imaging, 2010. 1236: p. 195-199. Mohr, J., et al., High Aspect Ratio Gratings for X-Ray Phase Contrast Imaging. International Workshop on X-Ray and Neutron Phase Imaging with Gratings, 2012. 1466: p. 41-50. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA-0003525. The views expressed in the article do not necessarily represent the views of the U.S. Department of Energy or the United States Government. Figure 1: Schematic of mold stabilization design (left), honeycomb pattern (right) Figure 1

X-ray dynamical diffraction in amino acid crystals: a step towards improving structural resolution of biological molecules via physical phase measurements.

In this work, experimental and data analysis procedures were developed and applied for studying amino acid crystals by means of X-ray phase measurements. The results clearly demonstrated the sensitivity of invariant triplet phases to electronic charge distribution in d-alanine crystals, providing useful information for molecular dynamics studies of intermolecular forces. The feasibility of using phase measurements to investigate radiation damage mechanisms is also discussed on experimental and theoretical grounds.

Absolute refinement of crystal structures by X-ray phase measurements.

A pair of enantiomer crystals is used to demonstrate how X-ray phase measurements provide reliable information for absolute identification and improvement of atomic model structures. Reliable phase measurements are possible thanks to the existence of intervals of phase values that are clearly distinguishable beyond instrumental effects. Because of the high susceptibility of phase values to structural details, accurate model structures were necessary for succeeding with this demonstration. It shows a route for exploiting physical phase measurements in the crystallography of more complex crystals.

X-ray phase measurements as a probe of small structural changes in doped nonlinear optical crystals

X-ray multiple diffraction experiments with synchrotron radiation were carried out on pure and doped nonlinear optical crystals: NH4H2PO4and KH2PO4doped with Ni and Mn, respectively. Variations in the intensity profiles were observed from pure to doped samples, and these variations correlated with shifts in the structure factor phases, also known as triplet phases. This result demonstrates the potential of X-ray phase measurements to study doping in this type of single crystal. Different methodologies for probing structural changes were developed. Dynamical diffraction simulations and curve fitting procedures were also necessary for accurate phase determination. Structural changes causing the observed phase shifts are discussed.

SEM and X-ray analysis of surface microstructure of a natural leather processed in a low temperature plasma

The plasma processing of natural leather by exposure to a non-equilibrium low temperature plasma was done for different plasma regimes. An SEM investigation of the structure and X-ray phase measurements of surface layers on the processed leather are reported. It was established that the changes of morphology and phase composition of the leather surface layers arose from crystallization of an organic phase that depended on the plasma processing times. The present results represent the practical importance for perfection of technology of manufacturing and processing of leather under plasma conditions.