CdTe photon-counting X-ray image sensor with charge-counting readout IC was developed for high quality X-ray imaging. CdTe is one of the good material for X-ray imaging, because it has i) high sensitivity for X-rays by high atomic number (Average 50), ii) room temperature operation with low thermal noise by large band-gap (1.44eV) , and iii) direct conversion from X-ray photon to electron-hole pair (charge). CdTe also has electron mobility (1,100cm²/V/s) and large ut. It is very good properties for X-ray sensor, and it can be processed by using laser processes and some semiconductor post processes.

It is very difficult to make integrated circuit on CdTe wafer, however, we shold stack Si-readout IC and CdTe multi-pixel sensor chip. In this paper, we use sliver paste based bumping for this stack connection. We developed new readout IC, and report in this paper.

A direct charge conversion LSI (Si readout IC) for photon counting X-ray imaging sensor has been fabricated by TSMC 0.18um CMOS process. This LSI is designed to prove a behavior of the direct charge conversion architecture based on charge injection [1] for photon counting with energy information. The LSI has charge-to-digital convertors (QDC) used to read out a charge generated by an X-ray detector such as CdTe and digitize energy information of the source X-ray photon. The QDC realizes 15 energy windows within 200 keV and the LSI provides two 12-bit memories to output two images with differential energy information simultaneously. The LSI has 1600 QDCs and its pixel pitch is 80um. Analogue behaviors of the QDC, characteristics of energy discrimination and a capability to take X-ray/gamma-ray images have been tested. As the results, a desired gain and good linearity are equipped on the QDC, spectra of ²⁴¹Am and ⁵⁷Co can be discriminated, and differences between with and without a material have been shown as image. We demonstrate X-ray penetrate image and report in this paper.

References: [1] K. Takagi, et al., Sensors and Materials, Vol. 30, No. 7 (2018) 1611–1616

Fig.1. Direct charge conversion LSI with pixilated CdTe detector.