Lars Evensen

Guard ring design for high voltage operation of silicon detectors

Abstract The termination of the depletion zone towards the non-depleted part of silicon affects the total device leakage current, the long term stability, the noise level and the radiation hardness of silicon detectors. This paper describes computer simulations and experiments to develop guard ring structures for use in silicon detectors requiring thick depletion layers, high operating voltages and biasing beyond depletion without increase in the leakage current and the noise. Computer simulation of a simplified structure is used to understand the influence from the oxide charges and the substrate doping concentration for a segmented guard structure with several floating diffusion strips. Results from the simulations are compared with measurements on devices. The numerical results are found to be in agreement with experimental data. It is found that segmented guard structures with floating diffusion strips have high breakdown voltages and low leakage currents. The effects of floating metal field plates over the oxide between the floating diffusion strips are studied on two different guard structures by measuring the potential on the diffusion strips and the leakage currents in the guard and active diode. The results show that floating intermediate field plates reduces the influence from oxide charges and stabilises the device against environmental influence.

Radiation tolerance of single-sided silicon microstrips

Abstract The RD20 collaboration is investigating the design and operation of an LHC inner tracking detector based on silicon microstrips. Measurements have been made on prototype detectors after irradiation with electrons, neutrons, photons, and protons for doses up to 5 Mrad and fluences up to 10 15 particles/cm 2 . The annealing of effective doping changes caused by high neutron fluences, one of the major limits to detector lifetime at the LHC, is shown to be strongly inhibited by cooling below room temperature. Detailed results are presented on the critical issue of microstrip capacitance. We have also investigated bulk damage caused by high-energy protons, interstrip isolation after neutron irradiation, and MOS capacitors irradiated with electrons and photons.

A fast low noise silicon detector for electron spectroscopy up to 1 MeV

Abstract A 1 mm thick silicon pad detector has been developed for the Atlas Positron Experiment (APEX) [1,2]. This experiment requires efficient detection of positrons and electrons with energies up to 1 MeV with an energy resolution of better than 10 keV and time resolution of less than 2 ns. To satisfy these requirements, the detectors should be 1 mm thick with low leakage current and be capable of operation at voltages greater than needed for full depletion. To enhance the charge collection time, the detectors should also withstand cooling to liquid nitrogen temperature. These requirements have been satisfied using diodes manufactured on high resistivity silicon in combination with a segmented guard ring structure (multiguard) and a process optimized to reduce the bulk generated leakage current. A room temperature leakage current of 1 nA/cm2 was achieved and an energy resolution of 3.6 keV FWHM was measured with 30 pF input capacitance on a charge sensitive preamplifier. At −130°C with 90 pF input capacitance an energy resolution of 3.4 keV FWHM and a time resolution of 1.2 ns was measured for 500 keV electrons in coincidence with gamma rays detected in a BaF2 scintillator detector. Results from the evaluation of different designs and process approaches are discussed.

THE EFFECT OF METAL FIELD PLATES ON MULTIGUARD STRUCTURES WITH FLOATING P+GUARD RINGS

Abstract The performance and stability of silicon diode detectors can be improved by implementing guard ring structures around the active detector area. The purpose of this work is to study design parameters influencing the performance of multiguard structures, especially the effect of metal field plates. An important feature is the potential distribution in the multiguard ring structure which depends on the bulk doping concentration, the oxide charge, the size of the gap between guard rings and the field-plate design. We have made a systematic investigation of the effect of distance between floating p+ guard rings with two different metal field plate designs. We have also varied the width of the field plates and studied the effect of gamma irradiation. Numerical simulations have been done to compare with results from the experimental potential distributions between guard rings.

Radiation damage studies of field plate and p-stop n-side silicon microstrip detectors

Abstract We present results from studies of the properties of dedicated n-side microstrip structures before and after irradiation, with photons to 7 Mrad and fast neutrons to 8 × 10 13 ncm −2 . Both p-stop and field plate devices were investigated, each having a range of strip geometries in order to determine optimal configurations for long-term viability and performance.

Silicon drift photodiodes

Abstract Low capacitance photodiodes based on the principle of the solid state drift chamber have been constructred and tested. The devices are based on a cellular design with an anode at the centre of each of five cells allowing electrons liberated by ionisation to drift up to 1 mm to the readout strip. Results on the performance of the detectors, including leakage current, capacitance and drift properties, are presented and compared with simulations. A second series drift photodiodes have been processed which incorporate improvements based on results acquired from the first design. They have leakage currents of a few nA and shorter drift times at the periphery of the drift cells. We present measurements showing the performance of the new devices.

Temperature dependence of the behaviour of silicon drift detectors

Abstract Measurements have been carried out on the temperature dependence of the behaviour of two types of drift device: photodiodes and position sensitive drift chambers with segmented anode and cathode structures. Leakage currents and electron mobility have been investigated at low temperature for the drift photodiodes. Self-triggering has been achieved for the position sensitive drift chambers using 60 keV photons, and differences in arrival time between the prompt trigger signal from the cathode and the delayed anode signal have been studied as a function of drift distance and temperature.

Single-sided microstrip detector for high radiation doses

Abstract A “single-sided” microstrip detector with n-type strip implants in n-type silicon substrate materials has been developed for use in high luminosity vertex tracker experiments. This technology has been chosen to make a detector which will operate satisfactorily well beyond the radiation dose which converts the substrate to p-type material. This full-size detector of dimensions 60 mm × 60 mm has 1025 strips (with 512 read-out strips) biased with polysilicon resistors. Separate “p-stop” implants between the strips isolate the strips from each other electrically. These individual p-stops are implemented in order to reduce the detector noise. A combination of silicon oxide and silicon nitride gives a very reliable dielectric layer for the coupling capacitors, which yields a low fraction of defective read-out strips. The backside p-n-junction area is surrounded by a high-stability guard which makes it possible to operate the detector at bias voltages corresponding to several times the depletion voltage. Preliminary irradiation tests carried out in order to determine the detector designs suitability for the experiments at the CERN-Large Hadron Collider have shown that the detectors are not fatally damaged after a radiation dose of 2 × 10 14 protons cm −2 .

Recent detector developments at SINTEF (industrial presentation)

Abstract Results from SINTEFs research on radiation hardness of silicon detectors, thin silicon detectors, silicon drift devices, reach-through avalanche photodiodes, and detectors with thin dead layers are presented.

RADIATION HARDNESS OF PUNCH-THROUGH AND FET BIASED SILICON MICROSTRIP DETECTORS

Silicon microstrip detectors can be biased with polysilicon resistors or Field Effect Transistor (FET) biasing structures. Polysilicon resistors are radiation hard, but using the FET biasing principle reduces processing costs and can give better noise performance. A set of microstrip detectors has been manufactured with a standard radiation sensor process in order to assess the radiation hardness of punch-through and FET biasing. Eight different bias geometry designs were used in order to study the effects of bias gap lengths and strip end geometries on the detector characteristics. The test detectors were irradiated at several dose levels up to 75 kGy with a 60Co source. Initially the devices had very low oxide charge (3 × 1010 cm−2) and leakage current levels (60 pA per strip). The dynamic resistance was in the 1 GΩ range, which is higher than the values which can be achieved by conventional polysilicon resistors. Radiation exposure gave significant increases in the leakage current of the devices. This causes large reductions in the dynamic resistance, and detector performance will degrade. The degradation due to increased leakage current was present for all strip end geometries, and it could not be compensated by changing the gate voltage.