Electrical crosstalk in front-illuminated photodiode array with differentguard ring designs for medical CT applications

摘要

This paper presents electrical crosstalk studies on front-illuminated photodiode arrays for medicalcomputed tomography (CT) applications. Crosstalk is an important factor to the system noise and imagequality. The electrical crosstalk depends on silicon substrate properties and photodiode structures. Thephotodiode samples employed in this paper are planar processed on high-resistivity n-type siliconsubstrate, resulting in a p+/n—/n+ diode structure. Two types of guard ring structures are designed andapplied to the same geometry of two-dimensional photodiode arrays. One structure is an n guard ring inthe gap area between pixels, and the other structure is an additional p+ guard ring around each pixeltogether with the n guard ring. A 10 μm light spot with wavelength of 525 nm is used to scan across thesurface of the photodiode array in the electrical crosstalk measurements. The electrical currents of twoneighbor pixels are measured and the results are compared between two guard ring designs. The designwith the p+ guard ring structure gives better electrical crosstalk suppression. Moreover, themeasurement results show much smaller influence on surrounding pixels with the p+ guard ringstructure in the case of disconnected pixel. Besides the electrical crosstalk, the light sensitivity withinthe gap area is also discussed between two guard ring designs.