Analysis of Successive Focused Ion Beam Slices by Scanning Electron Imaging and 3D Reconstruction
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Analysis of Successive Focused Ion Beam Slices by Scanning Electron Imaging and 3D Reconstruction T. S. Yeoh, N. A. Ives, N. Presser, G. W. Stupian, and M. S. Leung The Aerospace Corporation El Segundo, CA 90245-4691, U.S.A. J. L. McCollum, F. W. Hawley Actel Corporation Mountain View, CA 94043, U.S.A.
ABSTRACT An antifuse structure was analyzed using scanning electron microscope imaging and focused ion beam image slicing to generate a form of three-dimensional microscopy. This method reveals nanometer scale features that could not be easily imaged using a single focused ion beam cross-section. A novel end-point detection technique has been developed to control the thickness of the slice to about 2 nm. Voxel imaging and interpretive three-dimensional reconstruction was used to resolve volumes as small as 2 nm3. It was determined that the fusing region for an antifuse is a complex mixture of material phases with an elliptical volume approximately 75 nm in diameter. INTRODUCTION Visualization of device structures at the micron and submicron scale is crucial for improving microelectronic and optoelectronic device performance and for investigating the fundamental causes of device failure. Imaging techniques based on scanning electron microscopy (SEM) and transmission electron microscopy (TEM) have become standard tools for failure analysis extending to the nanoscale (< 100 nm). Focused ion beam (FIB) nanotomography has been a topic of considerable interest because it offers the ability to view a reconstruction of a structure in its entirety in three dimensions (3D) with nanometer scale resolution. This capability is becoming increasingly useful as critical microelectronic device feature dimensions continue to shrink[1]. At these dimensions, nanometer-scale chemistry as well as quantum effects are significant, and the orientation and relative positions of the constituent materials are important to the understanding of the device. The principle of FIB tomography is based on FIB cross-sectioning and consecutive scanning electron micrograph (SEM) images collected after each slice. For state-of-the-art FIB systems, FIB tomographic resolution is limited by how thin a slice of material can be removed reproducibly throughout the deconstruction process. Recent advances in automatic FIB registration[2] have brought this volumetric resolution down to 17 nm per slice. Although FIB tomography has previously been demonstrated on a variety of structures with nanoscale features[3-7], previous work was limited in volumetric resolution whereas much higher volumetric resolution of less than 2 nm3 has been achieved by the method described below.
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EXPERIMENTAL DETAILS The sample was an Actel field programmable gate array. The region of interest is comprised an antifuse structure. This structure includes a tungsten interconnect, titanium nitride contacts and a dielectric stack consisting of amorphous silicon. The antifuse is created by increasing the contact voltage potentials across the insulating stack unt
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