High-Efficiency Silicon Photodiode Detector for Sub-keV Electron Microscopy

A silicon photodiode detector is presented for use in scanning electron microscopy (SEM). Enhanced imaging capabilities are achieved for sub-keV electron energy values by employing a pure boron (PureB) layer photodiode technology to deposit nanometer-thin photosensitive anodes. As a result, imaging using backscattered electrons is demonstrated for 50-eV electron landing energy values. The detector is built up of several closely packed photodiodes, and to obtain high scanning speed, each photodiode is engineered with low series resistance and low capacitance values. The low capacitance (<; 3 pF/mm²) is facilitated by thick, almost intrinsically-doped epitaxial layers grown to achieve the necessarily wide depletion regions. For the low series resistance, diode metallization has been patterned into a conductive grid directly on top of the nanometer-thin PureB-layer front-entrance window. Finally, a through-wafer aperture in the middle of the detector is micromachined for flexible positioning in the SEM system.