Solid-State Detector Module Design for Computed mammo-Tomography (CmT) System

PIN photodiode has been used in solid-state detector for X-ray detection as a photosensor of visible light from the scintillator. The most sensitive material for low energy X-ray detector as used in the mammography system is a Gd202S (GOS). As the light from the GOS is a short wavelength having a range of 450-700 nm(peak at 510 nm), the light is absorbed within a very shallow layer near the surface of the photodiode before arriving at the depletion region and does not contribute to the signal. In designing the PIN photodiode, it is important to make the p-layer as shallow as possible. In order to achieve a shallow junction, the optimum conditions of ion implantation such as the thickness of the SiO₂ oxide barrier, the tilting angle of the wafer with respect to the incident ion beam, and the annealing conditions, have been determined by using the results of simulation. The Penetration depth is about 7 jm in wavelength of 700 nm. It is necessary for adequate depletion depth (about 15 mum) to acquire all of incident light. So far, we chose the wafer of 1500 Ohm-cm resistivity offers about 17 muw depletion depth. The pixel pitch of photodiode is 0.4 mm times 3.0 mm and one module has 64-channels in linear array. Depth of P-layer, depletion region, and intrinsic layer are 0.3, 17, and 250 mum in zero bias. Measured leakage current is under 1 nA/cm², capacitance is 200 pF in zero bias.