A design for a linear array PIN photodiode for use in a Computed mammo-Tomography (CmT) system

A p–i–n (PIN) photodiode has been used in a solid-state detector for X-ray detection as a photosensor of visible light from the scintillator. The most sensitive material used as low-energy X-ray detector in the mammography system is a Gd2O2S (GOS). As the light from GOS having a short wavelength in the range of 450–700 nm (peak at 510 nm) is absorbed within a very shallow layer near the surface of photodiode before arriving at depletion region and does not contribute to the signal.For designing the PIN photodiode, it is important to make p-layer as shallow as possible. In order to achieve shallow junction, the optimum conditions of ion implantation such as thickness of SiO2 oxide barrier, tilting angle of the wafer with respect to incident ion beam, and annealing conditions, have been determined using simulation results. The penetration depths are about 2 μm for 510 nm, and 7 μm for 700 nm. It is necessary for adequate depletion depth (about 10 μm) to acquire the entire incident light. So far, wafers of ≥1000 and ≥150 Ω cm resistivity were chosen, which offer about 15 and 6 μm depletion depth, respectively. The pixel pitch of photodiode is 0.4 mm×3.0 mm and one module has 64 channels in linear array. Depth of the active p-layer is under 0.3 μm in zero bias. Measured leakage currents under 10 pA/mm2 for both diodes and junction capacitances are 16 and 29 pF/mm2 in zero bias for the diodes of ≥150 and ≥1000 Ω cm resistivity, respectively.The breast phantom, which was scanned by the Computed mammo-Tomography (CmT) system with two different detector modules and the data acquisition system, was developed. Little differences for distinct light absorption were shown in the three-dimensional images acquired in this study.