CONTROL OF SPHERICAL OPTICS STRESS BY X-RAY TOPOGRAPHY. Stanislaw Mikula 5,

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1 CONTROL OF SPHERICAL OPTICS STRESS BY X-RAY TOPOGRAPHY Richard Vitt 1, Jozef Maj 1, Szczesny Krasnicki 2, Lec Maj 3, Gary Navrotski 1, Paul Chow 4, 59 Stanislaw Mikula 5, 1 Argonne National Laboratory, Argonne, IL 60439; 2 Argonne National Laboratory-retiree 3 Yale University, New Haven, CT 06510; 4 HPCAT, Carnegie Institution of Washington, Washington, DC 20015; 5 Silesia Technical University, Gliwice, Poland ABSTRACT A novel method for spherical crystal bending is briefly described. The crystal bending is characterized by x-ray topography. Examples of recorded topographic images and the corresponding rocking curves are presented. Use of the new technology can lead to production of nearly uniformly concave single-crystal suitable for synchrotron beam line optics. INTRODUCTION Irregular stresses existing in concave crystal optics originating from the fabrication process have been a long-standing issue that has remained unresolved at the Advanced Photon Source (APS) and other synchrotron light sources. The lack of progress in reducing crystal stress was partially caused by insufficient testing of such devices during their fabrication phase. Checking quality of diffractive optics by x-ray topography is a well-known method (Krasnicki et al., 2003; Macrander et al., 2004; Zhong et al., 2005; Macrander et al., 2005); however, use of this technique to monitor the quality of the optics directly during the bending procedure is difficult. In the next sections of this paper, we introduce a novel method of thin single-crystal wafer bending, describe experiments that used the method, and demonstrate the method applicability to fabrication of high quality concave single-crystal optical devices. METHOD USED In order to use the new bending method, an assembly consisting of a small wafer bending device (prepared according to the specific dimensions of the wafer) and a vacuum pump, as well as access to an x-ray topography diffractometer are required. The wafer bending device (see Maj and Harmata, 2013) consists of an aluminum disc in which, a stepped cut-out, an inner cavity, and an orifice concentric with the center of the disc for connection to a vacuum pump were machined. The stepped cut-out is used to hold the wafer during the bending procedure. Therefore, its diameter and depth are to be adjusted to the

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3 diameter and thickness of the wafer. The diameter of the inner cavity should be a few millimeters smaller than the wafer diameter, while the depth of the cavity must supply enough space to accommodate the central part of the wafer after bending. Therefore, the cavity depth is dependent on the planned radius of the wafer curvature. 60 To start wafer bending, the device must be connected to a vacuum pump. The back side of the wafer is covered with a resin adhesive still in a semi-liquid state at the beginning of the operation. By placing the to-be-bent wafer inside of the cut-out, a small vacuum chamber under the wafer is created. Pumping the volume under the wafer causes uniform wafer bending. The low pressure required for a planned curvature radius is maintained for several hours until the resin adhesive is cured. After the bending procedure is completed, the device containing the concave wafer is placed on the x-ray diffractometer and the wafer curvature and the bending quality are measured and evaluated. EXPERIMENTS In order to demonstrate usefulness of the above described bending method, we used in our experiments the assembly shown in Fig.1. A highly polished silicon wafer of 100 mm diameter and 0.5mm thickness was utilized. Wafer was cut perpendicularly to the [111] crystallographic direction. Fig.1. Wafer bending assembly.

4 In order to measure and evaluate he final wafer radius with x-rays, the aluminum disc with the concave wafer was placed on the x-ray diffractometer called the Topo Test Unit (Krasnicki. 1996). The diffractometer was arranged to accept x-ray radiation originating from a Cu rotating anode. The monochromatic beam of approximately 60 mm x 60 mm cross section was obtained by using a Si (333) highly asymmetrically cut monochromator. After orienting the wafer to give the (333) reflection, the wafer full surface rocking curve was measured. Then, using a CCD camera or an X-ray film, a composite topographic image of the wafer was created by superimposing topographs taken at several equal-distance angular crystal positions covering the total rocking curve range. Based on such a composite image, both uniformity of bending and the radius of the concave wafer surface could be evaluated. 61 RESULTS Examples of the rocking curves measured for uniformly and non-uniformly concave bent Si wafers are shown in Figs. 2a and 2b, respectively. Fig.2 Rocking curve for a uniformly bent wafer A Rocking curve for a non-uniformly bent wafer Optic B.

5 Although the rocking curve half widths for bent wafers A and B were similar, i.e., about 1 0 and 1.4 0, respectively, the shapes of the rocking curves were very different. The shape of the wafer A rocking curve suggest a uniform wafer bending required for a good quality synchrotron radiation optical element. The final evaluation of the bending quality was based on taking multiple topographs. The resulting composite topographic images for wafers A and B are demonstrated in Figs.3a and 3b, respectively. 62 Fig. 3a. Composite topographic image for a uniformly bent wafer A obtained with a CCD camera and created by superimposing of topographs taken at many crystal positions that differed by

6 63 Fig. 3b Composite topographic image for a non-uniformly bent wafer B. DISCCUSION For many years, beamline scientists at several synchrotron facilities, in particular at the Advanced Photon Source, have tried to build concave optics with uniform stresses. Working closely with the APS onsite optics shop producing single-crystal flat optics with high quality stress- surface finishes and using the APS X-ray topography laboratory, we were recently able to produce excellent uniformly bent Si wafers with the concave surface of radii 1 m to 15 m. The composite topographic image shown in Fig. 3a corresponds to the radius 1m. This optics will be tested (Chow, 2014) at beam line 16BM of the Advanced Photon Source and used in an inelastic X-ray spectrometer. CONCLUSION We have demonstrated that the novel technique of bending single-crystal wafers combined with a special topographic testing technique can be successfully used to overcome problems connected with the difficult process of fabrication of high-quality concave optics for synchrotron beam lines.

7 64 ACKNOWLEDGMENTS The authors express their appreciation to: Neil Bartkowiak, Wieslaw Prucnal, Richard Ferry, John Pace, Xianrong Huang, Timothy Madden, Kurt Goetze, Josh Downey, John Attig, Michelle Givens, Xiaohuan Fu, Gregory Knott, John Hoyt and Dave Fallin for their contributions via valuable discussions. This work was supported by the U.S. Dept. of Energy, Office of Science, under Contract No. DE-AC02-06CH11357 REFERENCES Chow, P. (2014), Private communication. Krasnicki, S. (1996, Rev. Sci. Instrum., 67, Krasnicki, S., Zhong, Y., Macrander, A. (2003), Spatially Resolved Tilt and Strain Measurements in Diamond Crystals. Macrander, A., Krasnicki, S., Zhong, Y., Maj, J., Chu, Y. (2005) Strain mapping with partsper-million resolution in synthetic type-ib diamond plates, Appl. Phys. Lett. 87, Macrander, A., Zhong, Y., Krasnicki, S., Chu, Y.,.Maj, J. (2004), X-ray Topography and Tomography Studies of Surface Damage in Etched Diamond Crystal Plates, International Workshop on Diamond Single Crystals for 3 rd and 4 th Generation X-ray Sources, May 24-25, Grenoble, France. Maj, J., Harmata, C. (2013 System and method for implementing enhanced optics US Patent B2. Zhong, Y., Krasnicki, S., Macrander, A., Chu, Y. Maj, J. (2005) Bragg-case limited projection topography study of surface damage in diamond crystal plates, J. Phys. D: Appl. Phys. 38, A39.