Investigation of silicon sensors quality as a function of the ohmic side processing technology

Silicon sensors designed for the CMS Preshower detector must have a high breakdown voltage in order to be fully efficient after a strong irradiation. Studies made by several groups left bracket 1,2,3 right bracket have underlined the importance of the p**+ side geometrical parameters, such as the metal width and the number and spacing of guard rings. We have in addition investigated the effects related to the ohmic side processing and found that the breakdown voltage depends strongly on the depth of the effective "dead" n**+ layer. By increasing this thickness from mum to 2.5mum, the fraction of sensors with breakdown voltage higher than 500V increased from 22% to more than 80%. On the other hand, it was noticed that the starting surface quality of the wafer (double side polished or single side polished) does not affect the detectors parameters for a given production technology. The thick n**+-layer protects against initial wafer surface and defects caused by the technological treatment during the detector production. 10 Refs

Does radiation improve silicon detectors

Sensors designed for the CMS Preshower detector were irradiated with protons and neutrons to fluences equivalent up to 2x10^14 n/cm^2. The leakage current and the capacitance as well as the charge collection efficiency and the noise were measured, before and after the irradiation, for most of the detectors. We noticed, that for some detectors of a lower quality, the breakdown voltage increases after type inversion and that their leakage current, charge collection efficiency and noise are comparable to good detectors. We explain this phenomenon by two effects: a change of the distribution of the electric field and a decrease of the carriers lifetime. Defects on the p-side do much less harm after type inversion, because the maximum of the E-field is now on the n-side. Defects on the n-side still generate charge carriers, but their lifetime is much shorter and most of them recombine beforereaching by diffusion the space charge volume. The article presents the measurements of the breakdown voltage, the charge collection efficiency and the noise before and after irradiation of such sensors compared with detectors of a high initial quality

Performance of Si sensors irradiated 5x1014 n/cm2

The expected particle fluence in the inner part of the CMS Preshower is calculated to be 1.6x1014 cm^-2 for neutrons and 0.4x1014 cm2 for charged hadrons. Since the error of the calculation is high and/or unexpected accidental beam miss-alignments might happen we have irradiated Preshower silicon sensors to fluences up to 5x1014 n/cm2 to verify that they hold voltages necessary to reach the full charge collection efficiency. All irradiated sensors showed no signs of breakdown up to 1000 V, the maximum voltage applied. For all but one the operating voltage was lower than 900 V and a charge collection efficiency plateau was at least 200 V long. No noisy channels were observed at the extreme voltages. The measured charge was 71+-12% and 59+-12% for sensors irradiated to 3x1014 n/cm2 and 5x1014 n/cm2, respectively. These values are consistent with the extrapolation from previous measurements made on sensors irradiated to 2.3x1014 n/cm2. The work presents the results of static and dynamic measurements and shows that our design and the technology are very robust

High-voltage performance of silicon detectors irradiated under bias

The CMS preshower detector contains 16 m/sup 2/ of silicon. The silicon sensors' design is being finalized by taking into account their performance after five years of operation at high luminosity. Three detectors from different manufacturers were irradiated by neutrons and photons under bias and at low temperature. Their electrical parameters and their response to alpha and beta particles were measured. The charge collection efficiency attains a plateau at around 300 V. The irradiation set-up and the results of the measurements are presented in this paper. (17 refs)

High Voltage Performance of Silicon Detectors Irradiated under Bias

The CMS Preshower detector contains 16 sq. m of silicon. The silicon sensors' design is being finalized by taking into account their performance after five years of operation at high luminosity. Three detectors from different manufacturers were irradiated by neutrons and photons under bias and at low temperature. Their electrical parameters and their response to alpha and beta particles were measured. The charge collection efficiency attains a plateau at around 300 V. The irradiation set-up and the results of the measurements are presented in this paper

Silicon sensors for the CMS preshower detector

This paper is a summary of a research and development programme, conducted during the past 3 years on the CMS Preshower silicon sensors to define the specifications. The main purpose was to study the radiation hardness of these devices resulting from the specific design (metal lines wider than the p/sup +/ implants) and the production technology, a deep n/sup +/ layer on the ohmic side. An acceptable noise and a uniform charge collection were guaranteed by an appropriate choice of the interstrip region width. About 65 sensors, of different designs and produced by six manufacturers, were irradiated with neutrons and protons and thoroughly tested before and after irradiation. The results of the tests and the final specifications are presented. (18 refs)