X-ray-to-current signal conversion characteristics of trench-structured photodiodes for direct-conversion-type silicon X-ray sensor

Abstract

To reduce the radiation dose required in medical X-ray diagnoses, we propose a high-sensitivity direct-conversion-type silicon X-ray sensor that uses trench-structured photodiodes. This sensor is advantageous in terms of its long device lifetime, noise immunity, and low power consumption because of its low bias voltage. With this sensor, it is possible to detect X-rays with almost 100% efficiency; sensitivity can therefore be improved by approximately 10 times when compared with conventional indirect-conversion-type sensors. In this study, a test chip was fabricated using a single-poly single-metal 0.35 µm process. The formed trench photodiodes for the X-ray sensor were approximately 170 and 300 µm deep. At a bias voltage of 25 V, the absorbed X-ray-to-current signal conversion efficiencies were 89.3% (theoretical limit; 96.7%) at a trench depth of 170 µm and 91.1% (theoretical limit; 94.3%) at a trench depth of 300 µm.

Silicon trench photodiodes on a wafer for efficient X-ray-to-current signal conversion using side-X-ray-irradiation mode

“…24) The photodiode characteristics and their basic operation as a direct-conversion-type X-ray sensor have already been confirmed. 25) X-rays can be efficiently detected by directly irradiating the beam along a depletion layer formed around the trench photodiode. By this method, the X-ray detection capability of Si can be improved to the level of CdTe.…”