Abstract
From the early times of the electron microscopy, film has been used for recording the electron images in TEM. A standard detector option commonly used instead of film is the charged coupled device (CCD), which was first used in electron microscopy in the late 1980s. The CCD systems have enabled the immediate access to images in real-time, and have significantly increased throughput in the electron microscopy field. The CCD camera system is an indirect imaging detection system because the CCD will suffer from radiation damage and charge saturation during direct exposure of electron beam in TEM. A scintillator is thus needed to convert the electron image to a photonic image, which is then relayed to the CCD cameras for image acquisition. Hence, indirect imaging detection system have some inherent restrictions, including spatial and temporal resolution, efficiency and frame rate. In order to improve performance of electron detection systems, the application of silicon strip detector in TEM as a direct imaging systems was proposed. The application of silicon strip detector in TEM can meet the requirements for two-dimensional position sensitivity, spatial resolution and frame rate with direct sensing. Compared to pixel detector like CCD, silicon strip detector has advantages of simple structure, less number of readout circuits and lower manufacturing complexity. But silicon strip detector has one major issue, ghost images, which is unavoidable. If more than one particle hits the silicon strip detector, the measured particles position is no longer unambiguous and ghost hits appear. In this work, we proposed a direct electron detector with unique readout metal strip arrangement, called silicon strip-pixel detector, to relieve multi-hits induced ghost image problems and greatly reduce the number of readout circuits compared with pixel detector. Owing to characteristics integration of silicon strip detector and pixel detector, it was called silicon strip-pixel detector. The silicon strip-pixel detector is fabricated by single-sided process on high resistivity n-doped 4 inch diameter silicon wafer with a thickness of 250 μm. The pitch of strips determines the spatial resolution of the detector. The performance of P-N junction can be obtained by fine-tuning of implantation energy, dose and annealing parameters. The guard rings are designed around the active area to reduce leakage current. The dark current of silicon strip-pixel detector is 0.3 pA, breakdown voltage is -120 V and leakage current is 2.3 μA. The detector with guard rings design can greatly reduce leakage current with 3.48 times and increase breakdown voltage of 75 V compared with detector without guard ring. The threshold energy of detector sensitivity is 5 keV, minimum signal current is 15.1 pA and signal to noise ratio (in full depletion) is 3.12. According to theoretical calculations, charge collection efficiency is 0.703, DQE is 0.016 and theoretical frame rate is 0.7 ms per frame. The strip-pixel detector can meet the generic requirements for direct electron imaging system and has great potential to be used as imaging sensor in TEM.