Optical coupling between scintillators and standard CMOS detectors

In the digital X-ray imaging systems, one of the main problems to be solved is the efficiency of the optical interfaces between scintillators and photodetectors. This article presents the theoretical analysis, simulations, and experimental results on two different optical interfaces (SiO2SiO2 and Si3N4Si3N4), in order to maximize the X-ray detection efficiency. The working principle is the following: the X-ray photons reach the scintillator, which produces visible light. The visible light is then absorbed by the CMOS photodetector. Due to the fact that the refractive indexes of the scintillator and the photodetector are different, the visible light produced by the scintillator is partially reflected by the surface of the photodetector. In order to minimize this problem it is necessary to cover the photodetector with an anti-reflective filter. The anti-reflective filters were fabricated using two different dielectric layers available in a standard CMOS process, with no increase of the production time and costs of the devices.