Characterization of a thick layer a-Si:H pixel detector with TFA technology using a scanning electron microscope

AbstractThe electron beam induced current (EBIC) technique was used to characterize a 32 lm thick hydrogenated amorphous silicon n–i–pdiode deposited on top of an ASIC, containing several channels of active feedback pre-amplifiers (AFP) with peaking time of 5 ns. Thehomogeneity of the sample together with the edge effects induced by the unevenness of the ASIC substrate were studied with low doses of10–30 keV electron beam. The degradation of a-Si:H pixel detectors was measured with intense electron beam. Their charge collectionand transient time were characterized with a 660 nm pulsed laser before and after the thermal annealing. All the diodes show approx-imately a full recovery of charge collection after thermal annealing. 2006 Elsevier B.V. All rights reserved. PACS: 73.61.Jc; 61.43.Dq; 29.40.Wk; 61.82.FkKeywords: Amorphous semiconductors; Detectors; Radiation; SEM; EBIC; ASIC 1. IntroductionElectron beam induced current (EBIC) is a powerfulsemiconductor analysis technique [1,2], which employs a10–30 keV electron beam from a scanning electron micro-scope (SEM) to scan the detector active area, and inducinga signal on the pixels’ electrodes. This signal can be directlyreadout or can be feedback to the SEM system in such away that any change in the generation, drift or recombina-tion of the generated carriers in the detector is displayed asvariations of contrast in EBIC images. EBIC imaging isvery sensitive to electron–hole recombination, so the EBICanalysis is very useful in finding defects such as voids,crystallographic imperfections, dislocations and grainboundaries.The EBIC method has a high potential for the character-ization of n–i–p hydrogenated amorphous silicon (a-Si:H)diodes on application specified integrated circuit (ASIC)[3], as it can also be used to study the electric field in thedepletion region and the degradation of a-Si:H with highdoses of electrons [4]. The EBIC signal depends primarilyon the strength of the electric field, so for a given potentialacross a junction, high dopant conditions result in narrowdepletion region and higher electric field, which can be usedto study the doping homogeneity. It must be noted that inaddition to electric field strength, the relative dimension ofdepletion region compared to generation volume is also animportant factor in EBIC collection efficiency. This is thereason why it is valuable to understand the basic devicestructure in order to interpret the results correctly.In this work, we tried to study the homogeneity of theactive area and the effects of the unevenness of the ASICsubstrate, which may induce regions with high electric field.This can also explain the high leakage currents that we