X-ray reflectivity (XRR) is an effective non-destructive characterization method that has recently gained interest in the semiconductor industry for routine quality control. XRR is capable of measuring thin film properties such as density, thickness and interfacial characteristics. In particular, this method is being studied to determine its usefulness in characterizing porous SiO<sub>2</sub>, one possible replacement for standard SiO<sub>2</sub> as a low-k dielectric for device miniaturization. A necessary component to evaluating these porous materials is to understand the level of porosity as determined by the overall density of the material. The density information can be obtained from the critical angle observed in XRR data, hence the necessity of accurate measurements. In this work, the authors explore the limitations of XRR for determining the overall density of a layer using a simulation and fitting program, SimulReflec, designed for x-ray and neutron reflectivity studies. This fitting program is applied to both experimental and simulated data. Various versions of noise have been added to simulated data and then re-fit to determine the sensitivity of the technique.
Polycapillary fibers and a prototype collector for high energy x rays with a 2 m focal length have been fabricated and characterized. Measurements of a prototype collector, performed in collimating mode, show that the optic has high transmission, good uniformity, and small exit divergence. The transmission as a function of energy was analyzed using an extended single fiber geometrical optic simulation and the result shows that the simulation fits the data fairly well. Scatter transmission and contrast enhancement were measured in focusing mode using a parallel beam input.
A prototype capillary optical system has been developed to further test the possible use of polycapillary optics for a hard x-ray spectrometer for astrophysical applications. It has been evaluated both as a concentrator and a collimator of x-rays with energies between 10 and 60 keV. Transmission efficiency, angular acceptance and focal spot size have been measured. Both experiment and simulation results for the prototype optic have demonstrated the potential of x-ray polycapillary optics for astrophysical applications. Further design and fabrication improvements indicated by prototype studies are discussed.
Polycapillary X-ray optics, which is comprised of bundles of tens of thousands to millions of hollow glass capillary tubes, can be used as concentrators of astronomical X-rays for spectroscopic studies. Measurements have been performed of transmission efficiency of straight polycapillary fibers in the range of 10-80 keV. A geometrical optics simulation has been developed which accurately models experimental results and includes the effects of surface roughness and profile error. An optic designed for 8-keV photons has been tested as a concentrator for parallel beam synchrotron radiation. The results, a factor of 65 in intensity gain, are in good agreement with optics simulation. A prototype optic designed for 10-50 keV is currently under construction with a predicted gain of more than 100. Design requirements for higher energy photons are considered. By using a small or position sensitive detector, improvements of two orders of magnitude at 80 keV are expected from the use of this type of collector.
Measurements have been performed on a prototype CdZnTe linear array designed for direct digital mammography. Direct detection of x-ray photons without conversion to visible light avoids the trade-off between resolution and efficiency with phosphor thickness inherent in the conversion process. Polycapillary x-ray optics can be used to shape the x-ray image in a manner similar to the use of fused fiber optic tapers with visible light. The polycapillary optics also provide significant scatter rejection and resultant contrast enhancement. The theoretical detector quantum efficiency of CdZnTe at mammographic energies (20 keV) is quite high. Measurements were performed of DQE values and uniformity from 13 - 256 keV in large single pixel detectors. Uniformity and imaging measurements were also performed on a prototype 1 cm long linear detector array with 50 micrometer pixels attached to read-out electronics using indium bump bonding technology.