Influence of Surface Damage and Bulk Defects on the Interstrip Isolation of p-type Silicon Strip Sensors

Silicon strip sensors of upcoming tracking detectors in high luminosity colliders usually consist of a p-doped bulk with n-type strip implants. The general consensus is that such a design requires an additional interstrip isolation structure such as a p-stop implant. If there is no additional implant between the strips, it is expected that the strip isolation will be insufficient. Before irradiation, impurities and defects in the material lead to positive charge in the oxide and Si/SiO$_2$ interface which attracts electrons from the bulk. Those electrons accumulate just beneath the surface and between the n$^+$ strip or pixel implants, which decreases the interstrip resistance significantly. Ionising radiation introduces even more charge inside the silicon dioxide, which further decreases the interstrip resistance. Contrary to that expectation of a decreasing interstrip resistance due to irradiation, a high interstrip resistance was sometimes observed after proton irradiation. Hence, bulk defects induced by proton irradiation seem to have a non-negligible impact on the strip isolation. Therefore, an irradiation campaign with sensors without any interstrip isolation implant has been performed. The use of sensors without isolation structures provides deeper insight into the dominant isolation effects. The sensors' performance is first evaluated before irradiation. Afterwards, a set of sensors is irradiated with different particles in order to systematically introduce a mixture of surface and bulk defects. This enables the distinction and leads to a better understanding of the different effects. Finally, a combined TCAD model is derived which is able to describe the beneficial effect of bulk defects and the complicated interplay between surface and bulk damage.