An energy dispersive x-ray diffraction and fluorescence (EDXRD/XRF) system with no moving parts was developed to monitor <i>in situ</i> the initial stages of thin film growth. The EDXRD/XRF system utilized a low power 25 W microfocus x-ray source and collimating polycapillary optics manufactured by X-Ray Optical Systems (XOS). Metastable semiconductor thin film samples containing phase-separated inclusions of Sn were analyzed for simulation of early deposition stages. The time required to obtain sufficient diffraction data from these thin films was on the order of 60 seconds. Diffraction and fluorescence data were simultaneously obtained making it possible to simultaneously determine the crystal structure and composition of thin films while they are growing. This has the potential for revolutionizing how new materials are developed and commercialized, significantly cutting development and process control costs. An additional XRF system was developed that utilized a low power 20 W microfocus x-ray source and a focusing polycapillary optic. The Sn minimum detectable limit of this system (samples of interest were Ge<sub>1-x</sub>Sn<sub>x</sub>) was on the order of nanograms using the Sn-L<sub>α</sub> signal, which corresponds to picograms from a Sn-K<sub>α</sub> signal. Such low levels are usually only possible with a rotating anode source 10<sup>3</sup> times more powerful than the low power sealed tube source used in this experiment. An 100nm thin Ge<sub>1-x</sub>Sn<sub>x</sub> sample made by ion implantation was analyzed with this XRF system. In 300 s, a detectable signal was obtained, indicating the viability of this system for <i>in situ</i>, thin film, composition monitoring and characterization of the first several monolayers of thin film growth.
Ge<sub>1-x</sub>Sn<sub>x</sub> 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 <i>in situ</i>-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<sub>1-x</sub>Sn<sub>x</sub> thin films (~ 4 μm, ~ 8 μm) were compositionally mapped with the XRF system. Since Sn phase separation destroys the IR capabilities of the Ge<sub>1-x</sub>Sn<sub>x</sub> sensor, it is imperative to prevent this <i>during</i> 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 <i>in situ </i>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<sub>a</sub> signal, which corresponds to picograms from a Sn-K<sub>a</sub> signal. Such low levels are usually only possible with a rotating anode source 10<sup>3</sup> 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 <i>in situ</i>, thin film, composition monitoring and characterization of uniformly alloyed metastable semiconductors for IR sensors.
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 10<sup>7</sup> photons/sec-μm<sup>2</sup> for the polycapillary optic and 8 x 10<sup>5</sup> photons/sec-μm<sup>2</sup> 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