8-6	Nondestructive Evaluation of Spatial Distribution of Interlayer Structure in Multilayer X-Ray Mirrors Using X-Ray Standing-Wave

Fig. 8-13 X-ray standing-wave spectra of the Mo/Si multilayer, which were measured at the center position (denoted by A), the middle positions (Bx, By), and the edge positions (Cx, Cy) on the 4-inch sample plane The energy position of the X-ray standing-wave peak depends on the periodic-length of interlayers. In the Mo/Si multilayer, the X-ray standing-wave peak is observed at 97.68 eV in the center position (A). However, toward the periphery the peak shifts to the higher energy region.

Fig. 8-14 Mapping spectrum of the X-ray standing-wave signals in a quarter of the 4-inch-wafer-size Mo/Si multilayer measured with normal incidence. The photon energy of incident X-rays is fixed at 97.68 eV The photocurrent intensity on the X-ray standing-wave gradually decreases from the center toward the periphery, which means the periodic-lengths of the interlayers gradually become shorter from the center toward the periphery. The contour line of the photocurrent intensity also is shown on the upper side of the mapping spectrum.

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Multilayer X-ray mirrors have been employed in high-flux X-ray optical systems for highly brilliant synchrotrons and laser-plasma X-rays. X-ray mirrors have two kinds of nano-meter-thickness thin films deposited alternately on a substrate several inches in diameter. In the development of such multilayer X-ray mirrors, evaluations of interlayer structure are necessary to improve their optical quality. The interlayer structure usually is evaluated by transmission electron microscopy (TEM). This is a destructive method because TEM evaluations require the multilayer X-ray mirrors be cut. To avoid this problem, we have demonstrated nondestructive evaluation of the spatial distribution of the interlayer structure in multilayer X-ray mirrors utilizing an X-ray standing-wave. In this method, the monochromatized synchrotron radiation beam is irradiated perpendicularly on multilayer samples. The resulting photocurrent generated in the samples is measured. When scanning the photon energy of the incident beam, the measured photocurrent drastically changes when the photon energy satisfies the specific optical condition (Bragg condition), which is dependent on the interlayer thickness (Fig. 8-13). Mapping measurements in the sample plane (x, y-axes) of the photocurrent (z-axis) around the Bragg condition can therefore nondestructively provide spatial information of the interlayer structure in the sample plane. The mapping spectrum of the quarter of the 4-inch-diameter Mo/Si multilayer X-ray mirror visually illustrates that the periodic-lengths of the Mo/Si layers gradually become shorter from the center toward the periphery (Fig. 8-14). This mapping measurement can be performed within one hour, which indicates that this is a less complicated evaluation of the interlayer distribution than conventional methods. This X-ray standing-wave method is expected to be utilized in the development of multilayer X-ray mirrors.

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Reference Y. Muramatsu et al., Total-Electron-Yield X-Ray Standing-Wave Measurements of Multilayer X-Ray Mirrors for Interface Structure Evaluation, Jpn. J. Appl. Phys., 41, 4250 (2002)

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