short communications J. Synchrotron Rad. (2012). 19, 425–427 doi:10.1107/S0909049512005249 425 Journal of Synchrotron Radiation ISSN 0909-0495 Received 21 October 2011 Accepted 6 February 2012 # 2012 International Union of Crystallography Printed in Singapore – all rights reserved A unique approach to accurately measure thickness in thick multilayers Bing Shi, a * Jon M. Hiller, b Yuzi Liu, c Chian Liu, a Jun Qian, a Lisa Gades, a Michael J. Wieczorek, a Albert T. Marander, a Jorg Maser a,c and Lahsen Assoufid a a X-ray Science Division, Argonne National Laboratory, Argonne, IL 60439, USA, b Materials Science Division, Argonne National Laboratory, Argonne, IL 60439, USA, and c Center for Nanoscale Materials, Argonne National Laboratory, Argonne, IL 60439, USA. E-mail: shi@anl.gov X-ray optics called multilayer Laue lenses (MLLs) provide a promising path to focusing hard X-rays with high focusing efficiency at a resolution between 5 nm and 20 nm. MLLs consist of thousands of depth-graded thin layers. The thickness of each layer obeys the linear zone plate law. X-ray beamline tests have been performed on magnetron sputter-deposited WSi 2 /Si MLLs at the Advanced Photon Source/Center for Nanoscale Materials 26-ID nanoprobe beamline. However, it is still very challenging to accurately grow each layer at the designed thickness during deposition; errors introduced during thickness measurements of thousands of layers lead to inaccurate MLL structures. Here, a new metrology approach that can accurately measure thickness by introducing regular marks on the cross section of thousands of layers using a focused ion beam is reported. This new measurement method is compared with a previous method. More accurate results are obtained using the new measurement approach. Keywords: multilayer Laue lenses; focused ion beam; scanning electron microscope; image processing; stitching. 1. Introduction Multilayer Laue lenses (MLLs) are conceptually capable of focusing X-rays to 1 nm (Kang et al., 2005). In addition, they provide a promising path to focusing hard X-rays with very high focusing effi- ciency at more moderate resolution between 5 nm and 20 nm. The MLLs developed at Argonne National Laboratory are based on WSi 2 /Si multilayers deposited by the DC magnetron sputtering technique (Liu et al., 2007). Other groups have also developed MLLs. WSi 2 /Si MLLs have been studied and reported by Wang’s group (Zhu et al., 2010); Koyama et al. fabricated and characterized MLLs using MoSi 2 /Si (Koyama et al., 2011); Liese et al. reported their work on the fabrication of Ti/ZrO 2 MLLs by a combination of pulsed laser deposition and a focused ion beam (Liese et al. , 2010). MLLs can be fabricated by a variety of methods, and is an area of active research (Liu et al., 2005; Jahedi et al., 2010). To date, a line focus of 16 nm with 31% efficiency (Kang et al., 2008) and a two- dimensional focus spot size of 25 nm  27 nm (FWHM) with an efficiency of 17% have been achieved with MLLs using 19.5 keV X-rays at the Advanced Photon Source/Center for Nanoscale Materials 26-ID nanoprobe beamline (Yan et al., 2011). 2. New method for layer thickness measurements MLLs have been successfully studied and characterized. However, key issues remain to develop MLLs that can focus X-rays to below 10 nm. One of these key issues is the lack of an accurate metrology tool for measuring the MLLs’ thin layer stack structure. To fabricate an MLL, thousands of depth-graded WSi 2 /Si thin layers need to be deposited. These layer thicknesses vary from a few nanometers to several hundred nanometers across the total width of the MLL. The deposited multilayers will be further processed into MLL structures. To protect the deposited multilayers, a sandwich structure is made by pasting another Si wafer on top of the multilayers before dicing and thinning the deposited multilayers into MLLs. M-bond is used as the glue. It is an adhesive that contains two-component epoxy-phenolic resins. The multilayers are then analyzed using a scanning electron microscope (SEM) to characterize the cross section to calibrate the thickness of each layer. More than a dozen images need to be taken to obtain a clear view of the thin layers on these thousands of depth- graded multilayers. The images are processed individually using image processing software and then stitched together. Once the thickness of each layer is measured, the corrective factor of the deposition can be determined. The multilayers with the designed layer thickness can then be deposited. In the aforementioned image processing method, SEM images were taken one after another such that each image overlapped the previous image. Overlapped areas are required for the stitching of images. They can be clearly distinguished because these areas are darker than the non-overlapping areas owing to electron-beam- enhanced deposition of an organic film. Overlapping cannot be controlled quantitatively. Images were stitched together by esti- mating the overlapped area of the adjacent images and by looking at the trend lines’ slope of 1/d (where d is the thickness of two adjacent layers). This is where errors can be introduced. To improve the absolute accuracy of our measurements, we have developed a new