A method to simulate the observed surface properties of proton irradiated silicon strip sensors

T Peltola,A Messineo,K Ranjan,R Eber,K Lalwani,M Printz,T Eichhorn,A Bhardwaj,R Dalal

doi:10.1088/1748-0221/10/04/c04025

Abstract

During the scheduled high luminosity upgrade of the LHC, the world's largest particle physics accelerator at CERN, the position sensitive silicon detectors installed in the vertex and tracking part of the CMS experiment will face a more intense radiation environment than the present system was designed for. To upgrade the tracker to the required performance level, extensive measurements and simulation studies have already been carried out. A defect model for the bulk properties of proton irradiated silicon has been created with the Synopsys Sentaurus TCAD simulation package which produces simulated results that closely match the measured properties of silicon strip detectors. However, the expected behaviour due to increased surface damage is not so well predicted by the model. The solution requires an approach that does not affect the accurate bulk properties produced by the proton model, but only adds to it the required radiation induced properties close to the surface. These include the observed position dependence of the strip detector's charge collection efficiency (CCE). In this paper a procedure to find a defect model that reproduces the correct CCE loss, along with other surface properties of a strip detector up to a fluence 1.5×1015 1 MeV neq cm−2 (Φeq), will be presented. When applied to CCE loss measurements at different fluences, this method may provide a means for the parametrization of the accumulation of oxide charge at the SiO2/Si interface as a function of dose.

Highlights

The average cluster collection efficiency (CCE) loss between the strips measured with the Silicon Beam Telescope (SiBT) [9] was determined to be 30 ± 2% [10] in 200 μm active thickness float zone and magnetic-Czochralski sensors with p-stop and p-spray isolations (FZ200P/Y and MCz200P/Y, respectively) at Φeq = (1.4 ± 0.1) × 1015 cm−2
The measured position dependency of CCE was succesfully reproduced at a fixed fluence of Φeq = 1.5 × 1015 cm−2
The CCE loss results showed here are strictly tuned for the aforementioned fluence

Summary

Surface damage in silicon strip sensors

At the high radiation environment of the LHC, defects are introduced both in the silicon substrate (bulk damage) and in the SiO2 passivation layer, that affect the sensor performance through the interface with the silicon bulk (surface damage). Surface damage consists of a positively charged layer accumulated inside the oxide and interface traps may be created close to the interface with silicon bulk [3]. These are approximated in the simulation by placing a fixed charge at the interface. High oxide charge densities Qf are detrimental to the detector performance since the electron layer generated under the SiO2/Si interface can cause very high electric fields near the p+ strips in p-on-n sensors and loss of position resolution in n-on-p sensors by providing a conduction channel between the strips. Important strip sensor surface characteristics include the interstrip capacitance Cint, interstrip resistance Rint, position dependency of the charge collection efficiency CCE(x) and electric field distribution between strips E(x). The following work has been done to overcome this shortcoming

Simulated device structures

Non-uniform three level defect model

From uniform to non-uniform defect concentration

Conclusions