Radiation and Temperature Effects on the APV25 Readout Chip for the CMS Tracker

The Compact Muon Solenoid (CMS) is one of four particle detectors designed for use at the Large Hadron Collider (LHC) currently under construction at CERN, the European Laboratory for Particle Physics in Geneva. The LHC will accelerate two counterrotating beams of protons to energies of 7 TeV and produce 109 proton-proton collisions per second at a bunch-crossing frequency of 40 MHz. These collisions occuring at the centre of CMS will generate a very hostile radiation environment. The CMS sub-detector system closest to the collision point is the highly segmented Tracker, consisting of a silicon pixel detector with 45 million channels and a silicon microstrip detector with 10 million channels. The microstrip detector will be read out by the APV25, a custom-made chip manufactured in a commercial 0.25 µm CMOS microelectronics process. Radiation and temperature studies are required to ensure that the APV25 can operate reliably in the CMS environment. The radiation effects to which the APV25 could be susceptible are total dose effects and single event effects (SEE), such as single event upsets (SEU), single event gate ruptures (SEGR) and single event latchups (SEL). Approximately 75 000 chips will be used in CMS and confidence in total dose radiation tolerance will come from irradiating a subset of these chips to the radiation levels expected at CMS. For this purpose, a total dose radiation testing procedure was set up and the results obtained on a set of chips are presented, along with a detailed analysis of effects on discrete transistors. The need to periodically reset the chip during operation at CMS was determined from SEU tests. Along with transistor measurements, these also demonstrate immunity to SEGR and SEL. The APV25 will be operated at –10 o C in CMS. Until recently, all testing of the chip was carried out at room temperature. An environmental chamber with a temperature range of 130o C to –40o C was used to investigate temperature effects on the APV25. The measurements performed provide strong evidence that the APV25 will be fully functional throughout the lifetime of the CMS experiment. This is largely due to a combination of special design rules and intrinsic total dose radiation tolerance attributed to the thin gate oxide of the 0.25 µm CMOS process.