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X-Ray Nanotomography of Integrated CircuitsIntegrated circuit interconnects consist of three-dimensional metal lines with submicrometer cross sections surrounded by insulating material. Traditionally, the metals chosen are aluminum for horizontal connections and tungsten for vertical connections, but recently copper lines have been introduced into production. An integrated circuit interconnect might consist of several one-micrometer-thick layers.The 2IDB beamline of the Advanced Photon Source of the Argonne National Laboratory produces a high flux of soft x rays with a spot size of 100 nm. By turning the sample about an axis perpendicular to the beam direction, and detecting the x-ray flux passing through the sample, it is possible to use tomographic reconstruction techniques to reconstruct a region of an integrated circuit interconnect. Such experiments are underway, in conjunction with the SRC Center for Advanced Interconnect Science and Technology (CAIST) at Rensselaer Polytechnic Institute. Additional detail on current projects can be found at the Nanotomography Research home page. Work in our group seeks to prepare and measure samples, develop advanced tomographic reconstruction techniques, and predict the interaction of x rays and materials found in integrated circuit interconnects. A graphical example of three-dimensional x-ray reconstruction is given below. The first image is a three-dimensional representation of two parallel aluminum lines connected by a tungsten via. The second image is a three-dimensional reconstruction of the same object based on simulated x-ray measurements made from ten different angles.
Figure 1. Simulated interconnect.
Figure 2. Simulated reconstruction. We have recently applied this approach to image an integrated circuit interconnect at the 400 nm length scale. A 50 µm diameter portion of an integrated circuit interconnect was thinned to several micrometers. The sample was brought to the 2IDB beamline; x rays of 1573 eV were focused to a 170 nm spot size, with the sample in the focus of the beam. Using a two-dimensional scanning stage and one rotational degree of freedom, thirteen microradiographs of the sample were taken over 138° with at least 301x301 pixels in each with a 77 nm x 57 nm step size. A three-dimensional image was reconstructed from this data, showing two levels of metalization, connecting vias, and focused ion beam (FIB) markers (Fig. 3).
For more information contact: Zachary H. Levine
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