1. Influence of humidity on the IV characteristics A.Chilingarov Lancaster University ATLAS SCT Week CERN, 22-26.09.03

2. A.Chilingarov, Influence of humidity on the IV characteristics 2 Examples of the IV curves measured in the QA. Each plot contains the data for 4 detectors with consecutive numbers. All currents are corrected to 20 o C. Note the plateau developing with humidity.

3. A.Chilingarov, Influence of humidity on the IV characteristics 3 Recollect our earlier reports that in a typical detector the current increases and the inter-strip capacitance decreases after bias application. (The plots are for U=150V.) The stabilisation time decreases rapidly with the ambient humidity. According to Rainer Richter, MPI, these effects can be explained by a charging of the insulation layer covering the detector surface, which in turn depletes the electron accu- mulation layer at the Si-SiO 2 interface.

4. A.Chilingarov, Influence of humidity on the IV characteristics 4 The same effect can explain the plateau development in the IV curves. It can be attributed to the surface charging having been already completed during the measurement. In our case the IV from 5 to 500V takes ~13min. Eight w21 detectors tested during the QA at low and medium humidity were recently re-measured at high humidity. The results clearly confirm the above model.

5. A.Chilingarov, Influence of humidity on the IV characteristics 5 This effect explains a systematic difference between the module IV, measured in a box flushed with dry nitrogen (RH<15%), and the sum of the four detector currents, measured during the QA in a clean room at ~50% humidity. The results on this page (by courtesy of Graham Beck) are for Barrel modules measured at QMUL up to August 03. All currents are corrected to 20 o C. On top is a typical example of the IV curves. The histogram below shows the ratio of the module current to the sum of the detector currents at 350V for 48 modules. Similar distributions were observed also at 150 and 500V. The average values of this ratio are: 0.8, 0.75, 0.75 at 150, 350 and 500 volts respectively.

6. A.Chilingarov, Influence of humidity on the IV characteristics 6 Simulations by Rainer Richter and Graham Beck show that the surface charging leads also to a decrease of the electric field near the strip edge. Thus a faster depletion at higher humidity should be accompanied by a suppression of breakdown if it develops near the strip edge. This is indeed observed in some cases. By the same reason keeping detector at 150V for ~45 min helps to suppress breakdowns at higher voltages as was reported by Rainer Richter on 16.04.03.

7. A.Chilingarov, Influence of humidity on the IV characteristics 7 However the humidity increase does not always suppress the breakdowns as shown here. Note a non-standard evolution of the current with humidity below the breakdown in all of these cases. This indicates that here in addition to the usual bulk and surface currents there is a significant contribution of another current, possibly produced at a local defect or at the detector edge. If the breakdown is related to this current the effect of humidity can’t be predicted by the model described above.

8. A.Chilingarov, Influence of humidity on the IV characteristics 8 Some statistics on the breakdown behaviour Of ~1200 Hamamatsu detectors tested at Lancaster only 10 showed a breakdown behaviour during the QA. These breakdowns were observed in the humidity range from 26% to 44% at about +20 o C. All 10 detectors were re-tested recently at humidity from 49% to 58%. For 4 detectors the breakdown disappeared or started at higher voltages at higher humidity. For 3 detectors the breakdown threshold went down with humidity. For another 3 detectors the change in the breakdown behaviour was insignificant. Rainer Richter reported at CERN Detector QA meeting on April 16 this year that keeping detector for a prolonged time (~45min) at 150V suppresses the breakdown, which otherwise would happen at higher voltages. Of 46 CiS detectors subjected to this Long-Term-Treatment procedure the breakdown disappeared completely for 29 detectors. For 12 detectors a significant improvement in the breakdown behaviour was observed and 5 detectors showed little or no improvement.

9. A.Chilingarov, Influence of humidity on the IV characteristics 9 Conclusions 1.The IV curves at low humidity typically do not saturate up to 500V. At higher humidity a plateau appears and its onset voltage decreases with humidity. This behaviour agrees with the model of the long-term stabilisation effects reported earlier. 2.When the current evolution with humidity follows this pattern the breakdown is usually suppressed by the humidity. This can be explained by the decrease of the electric field at the strip edges predicted by the same model. 3.All the above effects should be taken into account when comparing the IV measurements made at different stages of the module production.