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Results from SINTEF's 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.

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 design's 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 × 1014 protons cm−2.

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.

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.

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.

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.

Low energy electrons, alpha particles and X-rays has a short penetration depth in silicon. For detection of these types of radiation it is important that the detector has a shallow inactive region at the side which is irradiated. A series of shallow dead layer test detectors produced by n-side implantation of phosphorus, arsenic or antimony has been characterized. Leakage current levels in the range 2-5 nA/cm/sup 2/ were achieved with dead layers in the range 0.1-0.2 /spl mu/m. A silicon pixel detector which will be used in a hybrid photon detector has been produced with thin dead layer technology. The lower detection limit for electrons with this detector is 2.5 keV.

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 silicon microstrip detector has been developed with 50- mu m-pitch strips on both the p- and n-side, using the principle of capacitive coupling between p/sup +/ diode strips (respectively, n/sup +/ strips) and the metallization strips which connect to the front-end preamplifiers. The detector is biased on both sides via polysilicon resistors connecting each p/sup +/ or n/sup +/ line to a common bias bus. To allow ohmic separation at the n-side, the accumulation layer of electrons has to be disrupted between the n/sup +/ strips. This has been achieved in three different ways: by separate polysilicon lines on thick oxide between two adjacent n/sup +/ lines to break the conducting accumulation layer by externally induced field depletion or by using the metal lines of the n/sup +/ strips on thick oxide or on thin oxide. Results on 20*20-mm/sup 2/ test devices are presented. A preliminary analysis of the spatial resolution gives sigma =16 mu m on both sides. These results demonstrate that double-sided readout Si strip detectors can be used for experiments where spatial resolution in the 10 mu m range is needed. >

A low-capacitance photodiode based on the principle of the solid-state drift chamber has been constructed and tested. The device is based on a cellular design with an anode at the center of each of five cells, allowing electrons liberated by ionization to drift up to 1 mm to the readout strip. Two designs were produced with p/sup +/ implant widths of 75 mu m and 85 mu m, respectively, using n-type material of 3.5-k Omega -cm resistivity. The opposite surface is a uniform p-type implant, features matching those of the other surface, and is unmetallized to ensure good photosensitivity. Results on the performance of the detector, including leakage current, capacitance, and drift properties, are presented and compared with simulation results. >

A silicon microstrip detector has been developed with capacitive coupling of the diode strips to the metallization and with polysilicon bias resistors to each diode. It allows the decoupling of the leakage current from the input to the charge-sensitive amplifier. Results are given on the coupling capacity and the breakdown voltage as well as on the polysilicon line resistance. It is found that the coupling capacitance, varying between 25 pF and 80 pF as a function of the oxide thickness, is large enough to avoid capacitance signal losses to the backplane. The 200-nm-thick silicon oxide withstands a potential difference of 100 V or more, thus allowing the operation of the detector with the metal strips and backplane at a ground potential and the bias voltage applied to the diodes. >

Punch-through currents flowing between adjacent p/sup +/ strips on the surface of silicon microstrip drift detectors have been observed. Measurements of the floating strip potential have shown that a p/sup +/ strip acquires a voltage such that the punch-through current and the leakage current are equal and opposite. The factors influencing the threshold for the punch-through effect have been compared with a simple computer model, and the predicted variation with the interstrip gap is found to be in reasonable agreement with measured values for a variety of detectors. >

A silicon microstrip detector with capacitive coupling of the diode strips to the metallization and with individual polysilicon resistors to each diode has been developed. The detector was tested in a minimum ionizing particle beam showing a performance similar to conventional strip detectors and a spatial resolution of 3.5 μm. Capacitive coupling allows the decoupling of the leakage current from the input to the charge sensitive preamplifier especially in the case of LSI electronics.

Si strip detectors with integrated coupling capacitors between diode and metallization and with separate bias resistors for each strip have been exposed to ionising radiation. Results from measurements of detector response before and after irradiation are presented. >