Three topics are presented: (1) Brief explanation of evolution of high-speed video cameras, (2) our recent work toward 1 Gfps, and (3) the temporal resolution limit of silicon image sensors.

   In 1991, Etoh developed a commercially available digital high-speed video camera for visualization of motion of water. Since then, we have been updating the frame rate: 4,500 fps in 1991 (KODAK HS4540), 1 Mfps in 2002 (Shimadzu HPV and HPV-2), and 16 Mfps in 2011.

Currently, we are developing a high-speed video camera operating at 100 Mfps and a multi-framing camera operating at 1 Gfps. The image sensors are backside-illuminated with multiple collection gates on the front side, arranged like a flower pedals at the center of each pixel (BSI-MCG image sensors). By applying a high voltage to the collection gates in turn, signal charges are successively collected by the collection gates at an interval of 1 ns. A special driver circuit is also invented to generate a train of pulses with the width of 1 ns to drive the multiple collection gates. The circuit is a ring oscillator consisting of invertor circuits each equipped with an XNOR circuit (ROXNOR circuit). The performance of the ROXNOR driver has been confirmed for the test chip. Furthermore, simulation study showed that a frame interval of 200 ps can be achieved by 3D-stacking of the BSI-MCG image sensor chip and the ROXNOR driver chip manufactured with a 130 nm-process technology.

The frame rate of the image sensors seems approaching the upper bound. It’s time to search for the ultimate high-speed that a silicon image sensor can reach, as Rayleigh criterion was proposed when resolution of lenses approached the limit. Fortunately, the temporal resolution limit was theoretically derived. The expression is extremely simple, including only four parameters, the drift velocity, the diffusion coefficient, the penetration depth and the width of the photo-electric conversion layer. The limit is calculated by substituting the parameter values specific to the layer and incident photons. For example, for green light of 550 nm to a silicon layer, the temporal resolution limit is 11.1 ps. The inverse, the ultimate frame rate, is 90.9 Gfps