N-side Readout Research Articles

Development of Si-CMOS hybrid detectors towards electron tracking based Compton imaging in semiconductor detectors

Electron tracking based Compton imaging is a key technique to improve the sensitivity of Compton cameras by measuring the initial direction of recoiled electrons. To realize this technique in semiconductor Compton cameras, we propose a new detector concept, Si-CMOS hybrid detector. It is a Si detector bump-bonded to a CMOS readout integrated circuit to obtain electron trajectory images. To acquire the energy and the event timing, signals from N-side are also read out in this concept. By using an ASIC for the N-side readout, the timing resolution of few μs is achieved. In this paper, we present the results of two prototypes with 20 μm pitch pixels. The images of the recoiled electron trajectories are obtained with them successfully. The energy resolutions (FWHM) are 4.1keV (CMOS) and 1.4keV (N-side) at 59.5 keV. In addition, we confirmed that the initial direction of the electron is determined using the reconstruction algorithm based on the graph theory approach. These results show that Si-CMOS hybrid detectors can be used for electron tracking based Compton imaging.

Silicon sensors for HL-LHC tracking detectors

It is foreseen to significantly increase the luminosity of the LHC by upgrading towards the HL-LHC (High Luminosity LHC) in 2021 in order to harvest the maximum physics potential. After the upgrade, unprecedented levels of radiation will require the experiments to upgrade their tracking detectors to withstand hadron fluences equivalent to over 1016 1MeV neutrons per cm2. Within the RD50 Collaboration, a massive R&D program is underway to develop silicon sensors with sufficient radiation tolerance. Recent defect characterization and Edge-TCT measurement results improved the understanding of irradiated detector performance. RD50 results show that sensors with n-side readout, easiest made with p-type silicon, have a superior radiation hardness due to the high overlap of electric and weighting field after irradiation, larger contribution of electrons to the total signal and finally due to charge multiplication which may enhance the collected charge at high bias voltages in this type of detector. A further area of activity is the development of advanced sensor types like 3D silicon and thin pixel detectors designed for the extreme radiation levels expected for the inner layers.

Results with p-type pixel sensors with different geometries for the HL-LHC

Pixel detectors will be extensively used for the four innermost layers of the upgraded ATLAS experiment at the future High Luminosity LHC (HL-LHC) at CERN. The total area of pixel sensors will be over 5m2. The silicon sensors that will instrument the pixel volume will have to face several technology challenges. They will have to withstand doses up to 2×1016neqcm−2, to have a reduced inactive area at the edge of the sensors still being able to hold 1000V bias voltage and to be relatively low cost considering the large area to be covered. N-side readout on p-type bulk is the most promising technology for satisfying the various requirements. Several sensor types have been produced in the UK, conceived for various readout systems, for studying the properties of n-in-p and n-in-n sensors before and after irradiation with test beam and laboratory measurements. The status of these studies is presented here.

Beam Test Measurements With Planar and 3D Silicon Strip Detectors Irradiated to sLHC Fluences

The planned luminosity upgrade of the CERN LHC to the super LHC (sLHC) requires investigation of new radiation hard tracking detectors. Compared to the LHC, tracking detectors must withstand a 5–10 times higher radiation fluence. Promising radiation hard options are planar silicon detectors with n-side readout and silicon detectors in 3D technology, where columnar electrodes are etched into the silicon substrate. This article presents beam test measurements performed with planar and 3D n-in-p silicon strip detectors. The detectors were irradiated to different fluences, where the maximum fluence was <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$3\times 10^{15}$</tex></formula> 1 MeV neutron equivalent particles per square centimeter <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">$({\rm n}_{\rm eq}/{\rm cm}^{2})$</tex></formula> for the planar detectors and <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${2\times 10^{15}~{\rm n}_{\rm eq}/{\rm cm}^{2}}$</tex></formula> for the 3D detectors. In addition to signal measurements, charge sharing and resolution of both detector technologies are compared. An increased signal from the irradiated 3D detectors at high bias voltages compared to the signal from the unirradiated detector indicates that charge multiplication effects occur in the 3D detectors. At a bias voltage of 260 V, the 3D detector irradiated to <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">${2\times 10^{15}~{\rm n}_{\rm eq}/{\rm cm}^{2}}$</tex> </formula> yields a signal almost twice as high as the signal of the unirradiated detector. Only 30% of the signal of an unirradiated detector could be measured with the planar detector irradiated to <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${3\times 10^{15}~{\rm n}_{\rm eq}/{\rm cm}^{2}}$</tex></formula> at a bias voltage of 600 V, which was the highest bias voltage applied to this sensor.

Study of various high voltage protection structures for reduction of the insensitive region of silicon sensors designed for extreme radiation tolerance

We studied several devices with different distances of the sensitive region to the cut edge after severe hadron irradiation. They have been characterized in term of the bias voltage drop on the various guard ring (GR) structures and their charge collection performance.Current pixel detectors are made with n-type silicon with n-side readout. This geometry requires backplane processing for implanting GR structures on the backplane of the sensor. This increases the cost and the complexity of the device, but on the other hand offers GR structures on both sides, with possibly a more efficient reduction of the edge potential and beneficial effects on reducing the insensitive area. We present the comparison of n-in-p and n-in-n detectors with similar geometry to show the effect of the double GR structure before and after hadron irradiation.

Radiation hardness of p-type silicon detectors

Finely segmented silicon detectors made with n-side readout on p-type substrate have emerged as the most promising choice for the replacement of the tracker systems for the CERN LHC upgrade to higher luminosity. They have practically assumed the status of baseline devices for the silicon microstrip layers of the upgrades, and are now also being considered as possible candidates even for the innermost pixel layers. A review of the reasons for the success of the p-type bulk devices is presented here.

Annealing studies of silicon microstrip detectors irradiated at high neutron fluences

Silicon p-type detectors are being investigated for the development of radiation-tolerant detectors for the luminosity upgrade of the CERN large hadron collider (Super-LHC (sLHC)). Microstrip detectors have been fabricated by CNM-IMB with an n-side read-out on p-type high-resistivity float zone substrates. They have been irradiated with neutrons at the TRIGA Mark II nuclear reactor in Ljubljana. The irradiation fluxes match with the expected doses for the inner tracker at the sLHC (up to 10 16 equivalent 1 MeV neutrons cm −2). The macroscopic properties of the irradiated prototypes after irradiation were characterized at the IFIC-Valencia laboratory. The charge collection studies were carried out by means of a radioactive source setup as well as by an infrared laser illumination. The annealing behavior was studied in detail on a microstrip detector irradiated with a flux of 10 15 equivalent 1 MeV neutrons cm −2. Collected charge measurements were made after accelerated annealing times at 80 °C up to an equivalent annealing time of several years at room temperature. This note reports on the recorded results from the annealing of the irradiated p-type microstrip sensor.

LHCb–VELO module production with n-side read-out on n- and p-type silicon substrates

The modules for the Vertex Locator detector of the LHCb experiment represent a technical challenge for their complexity. The design of the sensors uses a complex double metal routing of the connection to the read-out strips and a high density of metal lines has to be accommodated in the module. The detectors are n-side read-out to be able to survive the highest radiation damage of any micro-strip sensor used in LHC experiments. The present choice is n-strips on n-type substrates (n-in-n geometry). Double-sided lithography is required, which impact on the cost of the devices and on the module construction. Moreover, the compact size of the hybrid imposes sophisticated technical solutions for cooling the electronics and the detector. The module construction and the possible benefits offered by the choice of p-type substrate detectors compared to the present n-in-n devices are here discussed in details.

Effects of accelerated annealing on p-type silicon micro-strip detectors after very high doses of proton irradiation

Miniature (1 cm length) strip detectors have been produced with n-side read-out on p-type substrates to exploit the advantages, after exposure to large fluxes of hadrons, of reading out segmented devices from the n-side. These p-type prototypes have been irradiated with 24 GeV/ c protons to three fluences compatible with the expected doses (at different radii) for the inner tracker at the upgraded LHC (sLHC), namely 1.1, 3.5 and 7.5×10 15 cm −2. We have previously shown that those devices exhibit a signal to noise greater than 8:1 with LHC speed electronics even after the highest dose. The measurements were performed at the minimum of the curve of the annealing of the full depletion voltage after irradiation before the reverse annealing sets in. We have now studied the charge collection performance after accelerated annealing at 80 °C. At this temperature, the effects of the annealing are accelerated by a factor of about 7400 compared to room temperature, and our study spans an equivalent annealing time of several years at room temperature. The results, in terms of charge collection efficiency, are in sharp contrast with the evolution of the full depletion voltage as measured with the capacitance-voltage technique and suggest that these heavily irradiated detectors can be kept at room temperature for long periods without degrading their performance (e.g., during maintenance time). It is also apparent that the description of these heavily irradiated devices in terms of their full depletion voltage is no longer appropriate for the prediction of their charge collection properties.

Radiation tolerance of oxygenated n-strip read-out detectors

Following earlier work on ‘oxygenated’ detectors in terms of charge collection efficiencies after proton irradiation, full-size detectors for the LHC have been processed with n-side read-out on oxygen enhanced n-type silicon substrates. Two hundred-micron-thick detectors have been inhomogeneously irradiated up to doses of 7×10 14 p/ cm 2 using 24 GeV protons from the CERN PS. Results are presented on the charge collection efficiencies as a function of operating voltage for regions of the detectors irradiated to different doses, using LHC speed analogue read-out electronics. The measurements confirm the expectations which led to our original proposal of such detectors which are now being envisaged for the silicon-based detector systems at the LHC designed to withstand the greatest doses. The possibilities for survival at an upgraded luminosity LHC (Super-LHC) are also briefly discussed.

Charge collection and charge sharing in heavily irradiated n-side read-out silicon microstrip detectors

Hadron radiation damage of n-bulk silicon detectors leads to a change in the effective space charge from positive to negative. This effect is called type inversion. Type inversion occurs after exposure to a fluence of a few 10 13 proton cm −2 and is characterized by migration of the diode junction to the n + implanted side (n-side) of the detector. After inversion the charge collection efficiency (CCE) of segmented detectors at low voltage is higher when the n-side, rather than the p-side, is read out. A p-side read out (p–in–n) and an n-side read out (n–in–n) strip detector with identical strip geometry and a wafer thickness of 200 μm were simultaneously and inhomogenously irradiated to a maximum fluence of 7×10 14 proton cm −2 with 24 GeV c −1 protons. A comparison of the CCE at very high irradiation doses is shown with these two read out geometries. The inhomogeneous irradiation induces an inhomogenous distribution of the effective space charge with a transverse component of the electric field that could in principle affect the resolution properties of the microstrip detector. The inter-strip charge sharing properties, as a function of dose, for the n–in–n detector have been measured. No systematic distortion of the reconstructed cluster position was detected within the limits of the measurement accuracy. The detectors were manufactured using oxygen enriched silicon substrates to limit the degradation of the full depletion voltage under charged hadron irradiation. The measured CCE confirms that the use of oxygenated n–in–n detectors is viable up to fluences of 7×10 14 proton cm −2.

First results on the charge collection properties of segmented detectors made with p-type bulk silicon

Radiation damage of n-type bulk detectors introduces stable defects acting as effective p-type doping and leads to the change of the conductivity type of the silicon substrate (type inversion) after a fluence of a few times 10 13 protons cm −2. The diode junction after inversion migrates from the original side to the back plane of the detector. The migration of the junction can be prevented using silicon detectors with p-type substrates. Furthermore, the use of n-side readout gives higher charge collection efficiency for segmented devices operated below the full depletion voltage. Large area (≈ 6.4×6.4 cm 2) capacitively coupled 80 μm pitch detectors using polysilicon bias resistors have been fabricated on p-type substrates (n-in-p diode structure). These detectors have been irradiated with 24 GeV/ c protons to an integrated fluence of 3×10 14 cm −2 and kept for 7 days at 25°C to reach the broad minimum of the annealing curve. Results are presented on the comparison of their charge collection properties with detectors using p-strip read-out after corresponding dose and annealing.

Performance of a double-sided silicon microstrip detector with a wide-pitch n-side readout using a field-plate and a multi p-stop structure

The previous double-sided silicon microstrip detector (DSSDs) prototype with integrated coupling capacitors formed by oxide-nitride-oxide (ONO) dielectric film showed band-to-band tunneling (BET) current at the field-plate structure for the N/sup +/ strip, which represented a limitation of the biasing configuration. We report improved characteristics of the modified field-plate structure and the wide-pitch n-side readout using a multi p-stop structure combined thereon.