Jonathan Fulcher, Geoff Hall, Mark Raymond Imperial College, London, UK Marcus French Rutherford Appleton Lab, Chilton, Didcot, Oxon, UK D.Bisello, A.Paccagnella University of Padova, Italy !! During the research and design phases of the APV chip, much care has been taken to assure a high degree of total dose radiation tolerance. The chips have been fabricated in both the AVLSI-RA Bulk CMOS and DMILL processes. Extensive testing has been carried out on representative test structures from various processing runs, and the degree of radiation tolerance of these processes have been thoroughly investigated [1]. However, the susceptibility of the APV to SEU is not well known. A new version of the APV will be fabricated in the more SEU sensitive 0.25 micron technology, so a full understanding of the implications of these single event effects is imperative. An investigation of the behavior of the APV in a heavy ion beam would make it possible to extrapolate from the data to predict the SEU rate in the final system. SEU is a non-destructive phenomenon, which effects both the dynamic and static memory registers that temporarily store logic states. It manifests itself as a soft error appearing in a device and is caused by the deposition of charge by an ionizing particle. In the APV soft errors could cause a variety of undesirable effects, some of which would result in temporary malfunction and possible loss of data. In the event of such errors the APV can be reset and after a latency ( ~ 3 micro seconds ), normal operation would resume. It is clear that the acceptable upset rate is decreased by the existence of this latency dead time, and also depends on the total allowable number of dead channels. In order to calculate the predicted upset rate in the final system, an evaluation of the SEU sensitivity will be carried out by placing the APV in a beam of heavy ions, at the Legnaro accelerator in Italy, and measuring the SEU cross-section. Cross-section curves, in the case of heavy ion irradiation, typically represent the cross-section of the device as a function of Linear Energy Transfer (LET) of the bombarding ions. These curves generally have a threshold LET, where upsets begin to appear, and a saturating cross-section for high values of LET. These two defining features of the device behavior can then be used to extrapolate a prediction of the upset rate for other forms of bombarding radiation. In the case of the CMS tracker, the required calculations are complicated since the particles are typically of single charge and therefore only cause large enough ionization by virtue of nuclear interactions with silicon lattice sites. Evaluation of the APV will be carried out and later comparisons will be made between this and the previous evaluation of 0.25 micron test structures [2]. In this paper we present details of the experiment at Legnaro and results. [1] Radiation tolerance studies of Harris test structures. J.Fulcher, M.French, G.Hall, A.Holmes-Siedle, J.Matheson, M.Raymond, S.Watts. 4th Workshop for Electronics for LHC Experiments, in Rome 1998. [2] Estimate of the single event upset (SEU) rate in CMS. F.Faccio, C.Detcheverry, M.Huhtinen. 4th Workshop for Electronics for LHC Experiments, in Rome 1998. !! The microstrip tracker for the CMS experiment at the LHC will be read out using radiation hard APV chips. During high luminosity running of the LHC the tracker will be exposed to particle fluxes up to 10^6 cm^2 s^-1. This high rate of particles introduces a concern that the APV could occasionally suffer from Single Event Upset (SEU). In order to evaluate the expected upset rate under these circumstances the APV will be run under controlled conditions in a heavy ion beam. This should allow to exceed CMS SEE conditions by a large margin and allow to extrapolate to conditions in CMS. The upset cross-section for a range of beam energies will be measured and the referred threshold energy and saturated cross-section will be evaluated. These data can be used to predict the upset rate for the APV in the CMS tracker. !!