A simulation study was performed to confirm the technical feasibility of a magnetic lens consisting of a multi-hole permanent magnet to focus a two-dimensional electron beam array. The permanent magnetic lens has a simple structure, and its interference with the deflection electric field is low. In contrast to expectations, the magnetic field intensity is substantially flat in the central region of the multi-hole permanent magnet with a diameter of 6 mm. Only a slight nonuniformity is observed in the radial direction in this region. Although the outer edge of the magnet strongly distorts the intensity distribution in the peripheral region, the area of the influence is only 1 mm wide. The beam convergence rate in the uniform area is approximately 1/10 for a simple model, and 1/40 for an improved model. This technology is applicable to ultra-high-speed imaging with a temporal resolution of the order of picoseconds. The number of recorded signals is expected to be M2 with 1/M convergence with the deflection in two directions. Therefore, a speed-up of 1600 times is achievable.
Two ultrahigh-speed CCD image sensors with different characteristics were fabricated for applications to advanced scientific measurement apparatuses. The sensors are BSI MCG (Backside-illuminated Multi-Collection-Gate) image sensors with multiple collection gates around the center of the front side of each pixel, placed like petals of a flower. One has five collection gates and one drain gate at the center, which can capture consecutive five frames at 100 Mfps with the pixel count of about 600 kpixels (512 x 576 x 2 pixels). In-pixel signal accumulation is possible for repetitive image capture of reproducible events. The target application is FLIM. The other is equipped with four collection gates each connected to an in-situ CCD memory with 305 elements, which enables capture of 1,220 (4 x 305) consecutive images at 50 Mfps. The CCD memory is folded and looped with the first element connected to the last element, which also makes possible the in-pixel signal accumulation. The sensor is a small test sensor with 32 x 32 pixels. The target applications are imaging TOF MS, pulse neutron tomography and dynamic PSP. The paper also briefly explains an expression of the temporal resolution of silicon image sensors theoretically derived by the authors in 2017. It is shown that the image sensor designed based on the theoretical analysis achieves imaging of consecutive frames at the frame interval of 50 ps.
Crosstalk in the backside-illuminated multi-collection-gate (BSI-MCG) image sensor was analyzed by means of Monte Carlo simulation. The BSI-MCG image sensor was proposed to achieve the temporal resolution of 1 ns. In this sensor, signal electrons generated by incident light near the back side travel to the central area of the pixel on the front side. Most of the signal electrons are collected by a collecting gate, to which a higher voltage is applied than that of other collection gates. However, due to spatial and temporal diffusion, some of the signal electrons migrate to other collection gates than the collecting gate, resulting in spatiotemporal crosstalk, i.e., mixture of signal electrons at neighboring collection gates and/or pixels. To reduce the crosstalk, the BSI-MCG structure is modified and the performance is preliminarily evaluated by Monte Carlo simulation. An additional donut-shaped N type implantation at the collection-gate area improves the potential gradient to the collecting gate, which reduces the crosstalk caused by the spatial diffusion. A multi-framing camera based on the BSI-MCG image sensor can be applied to Fluorescence Lifetime Imaging Microscopy (FLIM). In this case, crosstalk reduces accuracy in estimation of the lifetimes of fluorophore samples. The inaccuracy is compensated in a post image processing based on a proposed impulse response method.
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