Fast signal transfer in a large-area X-ray CMOS image sensor

Abstract

For 2-d X-ray imaging, such as mammography and non-destructive test, a sensor should have a large-area because the sensor for typical X-ray beams cannot use optical lens system. To make a large-area 2-d X-ray image sensor using crystal Si, a technique of tiling unit CMOS image sensors into 2 x 2 or 2 x 3 array can be used. In a unit CMOS image sensor made of most common 8-inch Si wafers, the signal line can be up to similar to 180 mm long. Then its parasitic capacitance is up to similar to 25 pF and its resistance is up to similar to 51 k Omega (0.18 mu m, 1P3M process). This long signal line may enlarge the row time up to similar to 50 mu sec in case of the signal from the top row pixels to the readout amplifiers located at the bottom of the sensor chip. The output signal pulse is typically characterized by three components in sequence; a charging time (a rising part), a reading time and a discharging time (a falling part). Among these, the discharging time is the longest, and it limits the speed or the frame rate of the X-ray imager. We proposed a forced discharging method which uses a bypass transistor in parallel with the current source of the column signal line. A chip for testing the idea was fabricated by a 0.18 mu m process. A active pixel sensor with three transistors and a 3-pi RC model of the long line were simulated together. The test results showed that the turning on-and-off of the proposed bypass transistor only during the discharging time could dramatically reduce the discharging time from similar to 50 mu sec to similar to 2 mu sec, which is the physically minimum time determined by the long metal line capacitance.