Influence of Highly Ionising Events on the CMS APV25 Readout Chip

The Compact Muon Solenoid (CMS) experiment is one of four large high energy physics experiments presently being constructed for operation at the Large Hadron Collider (LHC) facility at CERN, Geneva. The motivation for the LHC and its experiments is the large range of new physics expected at the TeV energy scale. The CMS Silicon Strip Tracker (SST) will play a major role in all physics searches, providing precision tracking within a hostile radiation environment. The SST readout system is based on the APV25 front-end chip, of which 73000 are needed to fully instrument the sub-detector. It is essential for the SST readout system to be of the highest quality in order to maximise the physics performance of the sub-detector. Therefore, a production test station has been developed to screen wafers, each containing several hundred APV25 chips, prior to the integration of individual die into the final readout system. Several hundred wafers have already been screened and the number of chips identified to be fully functional and exhibit excellent performance corresponds to 70% of the SST requirement. Analysis results are presented. Inelastic nuclear collisions of hadrons incident on silicon sensors frequently generate highly ionising particles that can deposit as much energy within the sensor bulk as several hundred minimum ionising particles. These large signals can saturate the APV25 front-end chip, resulting in deadtime and introducing inefficiencies into the readout system. Analyses of beam test data that quantify the effect are presented. A change of a front-end component is shown to significantly reduce the induced inefficiencies, which are predicted to be at the sub-percent level for the final SST readout system. Subsequent Monte-Carlo studies have shown that the induced inefficiencies will have negligible effect on the tracking performance and b-tagging efficiency of the SST sub-detector.