Stereomicroscopy: 3D Imaging and the Third Dimension Measurement

Introduction Scanning electron microscopy (SEM) has been widely used for imaging objects with various dimensions ranging from millimeters to nanometers. Compared with other common microscopies, SEM offers a unique combination of imaging characteristics including high lateral resolution, broad magnification range, and large depth of field. As the working mechanism of SEM, a focused electron beam scans across the surface of a specimen rectilinearly, and the generated electrons (secondary or backscattered electrons) from the beam-specimen interaction are detected synchronically from pixel to pixel with various intensities resulting in the image contrast. Although SEM micrographs appear to be three dimensional, they are in fact purely two dimensional. The grey level of the pixel is not a function of the local height of the point, but rather of materials, morphologies, and certain properties. Additionally, the high depth of field of SEM could obscure the height difference of two objects particularly when both are in good focus. To overcome this limitation, many efforts have been made to recover the third dimension in SEM. Examples include shape-from-shading method [1], Monte Carlo electron transport modeling [2], and FIB/SEM dual beam techniques [3]. One technique, photogrammetry, based on stereo-pair images has been extensively studied and applied to reconstruct three dimensional features. The theoretical description of photogrammetry applied to SEM was first described by Piazzesi [4]. Building on the early work on photogrammetric analysis [5, 6], this technique has become more interesting in recent years partially due to the fast development of powerful software which enables good qualitative and quantitative 3D reconstructions of specimen surfaces. Currently quantitative measurements of specimen at microand nanoscales by a truly three dimensional characterization technique are highly demanded in a variety of applications such as high aspect ratio MEMS structures [7], surface roughness determination [8], nanomaterials and nanodevices, life sciences [3], fracture analysis [9], and many others.