Research On Silicon Pixel Detector Based On Heterojunction

Due to the continuous development of high-energy physics experiments,semiconductor detectors have attracted more and more attention,among which silicon-based photodetectors are the most widely researched and increasingly mature.The most common silicon-based photodetectors include silicon drift detector(SDD),silicon microstrip detector(SSD),silicon PIN detector and silicon pixel detector(SPD).With the rapid development of silicon-based photodetectors,their application fields have also been expanded from high-energy physics experiments to civil applications,such as X-ray fluorescence spectrometry(XRF)technology.In this work,the silicon pixel detector based on heterojunction is studied,and the device design,fabrication,optimization and characterization are carried out.Different from the homojunction formed by ion implatation,the heterojunction is formed by tunnel oxide passivated contact structure.The tunnel oxide passivated contact structure contains a tunnel oxide layer and a heavily doped polycrystalline silicon thin film.The tunnel oxide layer can passivates the dangling bonds at the surface of silicon substrate,and the heavily doped polycrystalline silicon thin film can adjusts the bend of energy band near the surface of silicon substrate,producing a surface electrical field.The combination of the two effects can improve the surface passivation of substrate.In this work,the tunnel oxide layer was prepared by wet chemical oxidation method and the polycrystalline silicon thin film was prepared by Low-Pressure Chemical Vapor Depositon(LPCVD)system.Several steps of photolithography were adoped to finish the patterning of pixel detector.The effective minority carrier lifetime was measured by WCT-120 instrument;the doping property was measured by electrochemical capacitance voltage equipment(ECV);the surface morphology and the cross-sectional diagram was measured by SEM;the IV and CV curves were measured by high and low temperature probe table,thus the device leakage current and depletion voltage can be evaluated.The effects of pixel structures and the substrate thickness on the device performance are studied.By bonding the pixel detector and the charge sensitive preamplifier(ASIC sensor)on a special PCB board,the energy resolution was obtained by a multi-channel analyzer(927 aspect model of Ortec company).Excellent performance can be obtained by the heterojunction pixel detector.The leakage current of 150?m-size pixels on 525?m-thick substrate was as low as 40 p A at bias voltage of 70 V.When the substrate was thinned to less than 300?m by KOH wet etching and chemical mechanical polishing(CMP),the leakage of 150?m-size pixels was reduced to 20 p A under the same bias voltage(70 V),and the depletion voltage of the detector was also reduced from 45 V to 15 V.Using ⁵⁵Fe as the radiation source,the energy resolution of the pixel detector on the 525?m-thick substrate was 230 e V(@5.9 ke V).For the thinned subtrate,an improved energy resolution of 200 e V was obtained.The test results of pixel detector indicate that the silicon pixel detector based on heterojunction can obtain low leakage current.Further more,by thinned the silicon substrate,the leakage current can be further lowered,and the depletion voltage can also be reduced,resulting in an improved energy resolution.Those results suggest that it is feasible to prepare high quality pixel detector by heterojunction technology.Because of the simple preparation process and the low cost of heterojunction pixel detector,it is very meaningful to realize the domestical production of silicon-based high-energy particle detector in the future.