Analysis of a CCD Register Driven through the Barrier for Full-Frame Mode Image Sensor.

The full-frame CCD image sensor is most suitable for digital still cameras. However, its disadvantages include low light sensitivity due to light absorption in the driving electrodes, a high dark current generated at the depleted Si-SiO2 interface of the CCD register, and a small charge-handling capability due to the surface pinning mode of operation used to reduce dark current. In this paper, we propose a new CCD register that solves these problems. The cell of the register is an inverted version of the conventional photo-diode with overflow drains. Therefore, the register is suitable for back illumination. The overflow drains, which are isolated from each other with an SiO2 layer, are used as driving electrodes. We analyze the performance of the register by using a three-dimensional numerical simulator and discuss its applicability to full-frame CCD image sensors. The leakage current between electrodes and between electrodes and channels was reduced to a negligible level by adjusting the barrier height. Also, the dark current generated at the Si-SiO2 interface successfully flowed into the electrodes and decreased drastically. The transfer inefficiency for 6 × 104 signal electrons, which is determined by the anti-blooming function of the cell, was as low as 10-11 within a 8 ns transfer time for pushing a pulse with a 4 ns fall time. The new CCD register, which is driven by four-phase drive pulses, handled a charge amount that was several times larger than that of a two-phase CCD using surface pinning mode. We also discuss the relation between the register's design and its performance.