Ms. Sarah P. Formica Profile

Sarah P. Formica

at Univ of Albany

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Publications (3)

Proceedings Article | 31 August 2005 Paper

Polycapillary optic-enhanced x-ray diffraction and fluorescence for rapid materials analysis

Joerg Schmidt, Sarah Formica, Susanne Lee

Proceedings Volume 5918, 591809 (2005) https://doi.org/10.1117/12.625191

KEYWORDS: Tin, X-rays, Germanium, X-ray optics, Sensors, Luminescence, Thin films, Crystals, Signal detection, Diffraction

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Proceedings Article | 7 April 2005 Paper

An in situ XRF system for composition mapping of thin film IR sensors

Sarah Formica, Susanne Lee

Proceedings Volume 5726, (2005) https://doi.org/10.1117/12.588419

KEYWORDS: Tin, Germanium, X-rays, X-ray optics, Thin films, Chemical species, Geometrical optics, Sensors, X-ray sources, Optics manufacturing

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Ge_1-xSn_x thin films are capable of detecting mid- to long IR wavelengths. However, due to the size difference between Ge and Sn atoms, homogeneous alloys are difficult to manufacture in thicknesses greater than a couple hundred nanometers. To address the difficulty in manufacturing these IR detecting alloys, we developed an in situ-capable x-ray fluorescence (XRF) system to characterize and monitor thin film composition and concentration information as a function of position during thin film growth. The inclusion of a focusing polycapillary optic enabled the x-ray source and detector distances from the sample to be large enough for placement outside the deposition chamber. RF sputtered, metastable Ge_1-xSn_x thin films (~ 4 μm, ~ 8 μm) were compositionally mapped with the XRF system. Since Sn phase separation destroys the IR capabilities of the Ge_1-xSn_x sensor, it is imperative to prevent this during deposition. Additionally, to produce an IR sensing array, more gradual changes in composition are desired. With our system, both Sn phase separation and more gradual changes in alloyed Sn concentrations were observed in 1000 s. However, the signal-to-noise ratio was such that 100 s would have been sufficient, meaning this system could be used in situ to characterize the alloy composition during deposition. A spatial resolution of ± 25 μm was obtained by oversampling with the focusing optic (100 μm spot size). From these measurements, the minimum detectable limits were on the order of nanograms using the Sn-L_a signal, which corresponds to picograms from a Sn-K_a signal. Such low levels are usually only possible with a rotating anode source 10³ times more powerful than the low power sealed tube source used in this experiment. Additional ultra thin samples (< 100 nm) made by ion implantation were also analyzed with this XRF system. Sn was ion implanted into single crystal Ge, resulting in a sample representative of early stage thin film growth. In 300 s, a detectable signal was obtained, indicating the viability of this system for in situ, thin film, composition monitoring and characterization of uniformly alloyed metastable semiconductors for IR sensors.

Proceedings Article | 23 October 2002 Paper

Physical analysis of high-intensity focusing x-ray optics: doubly curved crystals vs. highly focusing polycapillaries

Sarah Formica, Susanne Lee

Proceedings Volume 4781, (2002) https://doi.org/10.1117/12.453743

KEYWORDS: X-ray optics, Optical filters, X-rays, Crystals, Optics manufacturing, Sensors, Nickel, X-ray fluorescence spectroscopy, Geometrical optics, Solids

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Choosing the best performing x-ray optic for a specific x-ray fluorescence application is tricky since x-ray fluorescence requirements can vary from extremely small x-ray beams for spatially resolved measurements to intense, highly monochromatic beams for locating trace amounts of impurities in samples. Additionally, due to the wide variety of commercially available x-ray optics to choose from, each providing different outputs, one has to decide which optic suits one's application requirements best. Two such optics, a doubly curved crystal (DCC) optic and a monolithic polycapillary focusing optic, were examined for use in micro-beam x-ray fluorescence and low level impurity detection. In these two measurements, intense output beams are needed. With the two optics examined, the average CuKa x-ray intensities were 6 x 10⁷ photons/sec-μm² for the polycapillary optic and 8 x 10⁵ photons/sec-μm² for the DCC optic from a 20W sealed-tube, 120μm diameter Cu source. Thus, with an extremely low power source, the polycapillary output intensity was almost 100 times more than from the DCC optic. Because the spot sizes from the two optics were different, a better intensity comparison is insertion gain, which showed the polycapillary optic had an 8 times higher insertion gain than the DCC optic. In addition to intensity, lowering the minimum detectable limit (MDL) in x-ray fluorescence measurements requires highly monochromatic x-ray beams. Of the two optics examined, the DCC optic (with a 0.37mm Ni filter) produced an x-ray beam that would detect about a 20% lower impurity concentration than the polycapillary optic (with a 0.44mm Ni filter). In addition, the DCC optic's MDL can be improved, since this optic produces a highly monochromatic beam by diffraction, eliminating the need for a filter. On the other hand, the polycapillary optic transmits polychromatic x rays, requiring a filter to create a monochromatic beam from the polycapillary output, thereby reducing the characteristic line intensity. Thus we found the

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