Charge collection studies in irradiated HV-CMOS particle detectors

Abstract

Charge collection properties of particle detectors made in HV-CMOS technology were investigated before and after irradiation with reactor neutrons. Two different sensor types were designed and processed in 180 and 350 nm technology by AMS. Edge-TCT and charge collection measurements with electrons from 90 Sr source were employed. Diffusion of generated carriers from undepleted substrate contributes significantly to the charge collection before irradiation, while after irradiation the drift contribution prevails as shown by charge measurements at different shaping times. The depleted region at a given bias voltage was found to grow with irradiation in the fluence range of interest for strip detectors at the HL-LHC. This leads to large gains in the measured charge with respect to the one before irradiation. The increase of the depleted region was attributed to removal of effective acceptors removal. The evolution of depleted region with fluence was investigated and modeled. Initial studies show a small effect of short term annealing on charge collection.

Charge collection characterisation with the Transient Current Technique of the ams H35DEMO CMOS detector after proton irradiation

“…For all samples, the depletion depth follows the same evolution with the increasing fluence: a strong growth is observed at low fluences, reaching values between 140 µm and 200 µm, depending on the resistivity, and then it shrinks again with the increase of the fluence. This effect has been observed before in sensors with p-doped substrates [18][19][20] and can be explained using a combination of two different effects: proton irradiation causes the introduction of deep acceptors, which lead to a decreased depletion region, and, at the same time, shallow acceptors are removed. In this context the term "shallow" is used to indicate that the levels are close enough to the valence band to be fully ionised at room temperature.…”

Irradiation study of a fully monolithic HV-CMOS pixel sensor design in AMS 180 nm

“…The influence of the depletion voltage on the efficiency and noise as a function of the threshold reference is shown in figure 10 for a neutron irradiated sensor with 5 × 10 15 n eq /cm 2 which was operated at a sensor temperature of about 8 • C. The sensor efficiency increases with the applied depletion voltage, consistent with the expectation that the active depletion zone grows proportional to √ U HV , leading to higher signals. At very high negative voltages additional avalanche effects contribute to charge amplification, which sets in at about −80 V for nonirradiated sensors and shifts to slightly higher negative voltages for irradiated sensors [17]. For a depletion voltage of −85 V an efficiency of about 90 % is measured at a threshold reference of 715 mV and a noise rate of about 100 Hz per pixel.…”

“…For the proton irradiated sensors (figure 8c) a non-uniform distribution of the TDACs is obtained which we attribute to a non-uniform irradiation beam profile. The observed non-uniform radiation damage causes position dependent depletion and varying noise levels [17]. The distribution of the TDAC values (figure 8c) has a shoulder towards lower values which originate from the bottom right part of the TDAC map where the irradiation beam center was probably located 8 40 735 38 N00 40 700 19 60 725 21 60 718 20 70 740 21 70 725 19 85 734 20 85 725 19 P1515 60 700 19 N514 60 740 18 70 680 19 N115 40 711 23 85 568 20 60 711 22 P7815 40 770 26 70 740 23 60 755 26 85 730 24 70 760 28 N515 40 675 22 75 740 27 60 675 21 70 690 25 85 690 25 Table 2: HV and configuration parameters used for chip characterization.…”

Investigation of HV/HR-CMOS technology for the ATLAS Phase-II Strip Tracker Upgrade

“…The details of the study will be reported elsewhere [8]. The results show a non-trivial 178 dependence for the default value of 20 cm, due to several phenomena.…”

“…The issue of signal increase is not trivial. The charge collection study [8] indicated that 346 the acceptor removal rate is a function of bulk doping. Empirical calculations based on 347 the information known so far show an increase of minimal signal with initial resistivity 348 ( Figure 12).…”