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

• Published in: Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment Vol. 610; no. 1; pp. 210 - 214
• Main Authors: Park, Shin-Woong, Yuk, Sunwoo, Park, Jung-Byung, Yi, Yun
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 21.10.2009
• Subjects: Breast imaging; Computed mammo-Tomography(CmT); Mammo-CT; PIN photodiode; Mammo-CT; Breast imaging; Computed mammo-Tomography(CmT); PIN photodiode
• ISSN: 0168-9002; 1872-9576
• DOI: 10.1016/j.nima.2009.05.170

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 Gd 2O 2S (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 SiO 2 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/mm 2 for both diodes and junction capacitances are 16 and 29 pF/mm 2 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.