arXiv subjects

J. Schwandt

Publications and source records attributed to J. Schwandt.

28 records · Page 2Linked to original sources

arXiv2017

Characterisation of highly radiation-damaged SiPMs using current measurements

The characterisation of radiation-damaged SiPMs is a major challenge, when the average time between dark counts approaches, or even exceeds, the signal decay time. In this note a collection of formulae is presented, which have been developed and used for the analysis of current measurements for SiPMs in the dark and illuminated by an LED, before and after hadron irradiation. It is shown, how parameters like the breakdown voltage, the quenching resistance, the dark-count rate, the reduction of the photo-detection efficiency due to dark counts and the Geiger discharge probability can be estimated from current-voltage measurements. The only additional SiPM parameters needed are the pixel capacitance, the number of pixels and the correlated noise. Central to the method is the concept of the pixel occupancy, the probability of a Geiger discharge in a single pixel during a given time interval, for which the decay time of the SiPM signal has been assumed. As an illustration the formulae are used to characterise a KETEK SiPM before and after irradiation by a fluence of 5E13 cm$^{-2}$ of reactor neutrons for temperatures of -30°C and +20°C, where dark-count rates exceeding 1E11 Hz are observed.

physics.ins-det↗

arXiv2017

Neutron irradiation effect on SiPMs up to $Φ_{neq}$ = 5 $\times$ 10$^{14}$ cm$^{-2}$

Silicon Photo-Multipliers (SiPM) are becoming the photo-detector of choice for increasingly more particle detection applications, from fundamental physics to medical and societal applications. One major consideration for their use at high-luminosity colliders is the radiation damage induced by hadrons, which leads to a dramatic increase of the dark count rate. KETEK SiPMs have been exposed to various fluences of reactor neutrons up to $Φ_{neq}$ = 5$\times$10$^{14}$ cm$^{-2}$ (1 MeV equivalent neutrons). Results from the I-V, and C-V measurements for temperatures between $-$30$^\circ$C and $+$30$^\circ$C are presented. We propose a new method to quantify the effect of radiation damage on the SiPM performance. Using the measured dark current the single pixel occupation probability as a function of temperature and excess voltage is determined. From the pixel occupation probability the operating conditions for given requirements can be optimized. The method is qualitatively verified using current measurements with the SiPM illuminated by blue LED light.

physics.ins-det↗

arXiv2017

Test Beam Performance Measurements for the Phase I Upgrade of the CMS Pixel Detector

A new pixel detector for the CMS experiment was built in order to cope with the instantaneous luminosities anticipated for the Phase~I Upgrade of the LHC. The new CMS pixel detector provides four-hit tracking with a reduced material budget as well as new cooling and powering schemes. A new front-end readout chip mitigates buffering and bandwidth limitations, and allows operation at low comparator thresholds. In this paper, comprehensive test beam studies are presented, which have been conducted to verify the design and to quantify the performance of the new detector assemblies in terms of tracking efficiency and spatial resolution. Under optimal conditions, the tracking efficiency is $99.95\pm0.05\,\%$, while the intrinsic spatial resolutions are $4.80\pm0.25\,μ\mathrm{m}$ and $7.99\pm0.21\,μ\mathrm{m}$ along the $100\,μ\mathrm{m}$ and $150\,μ\mathrm{m}$ pixel pitch, respectively. The findings are compared to a detailed Monte Carlo simulation of the pixel detector and good agreement is found.

physics.ins-det↗

arXiv2017

Determination of the p-spray profile for n+p silicon sensors using a MOSFET

The standard technique to electrically isolate the $n^+$ implants of segmented silicon sensors fabricated on high-ohmic $p$-type silicon are $p^+$-implants. Although the knowledge of the $p^+$-implant dose and of the doping profile is highly relevant for the understanding and optimisation of sensors, this information is usually not available from the vendors, and methods to obtain it are highly welcome. The paper presents methods to obtain this information from circular MOSFETs fabricated as test structures on the same wafer as the sensors. Two circular MOSFETs, one with and one without a $p^+$-implant under the gate, are used for this study. They were produced on Magnetic Czochralski silicon doped with $\approx 3.5 \times 10^{12}$ cm$^{-2}$ of boron and $\langle 1 0 0 \, \rangle$ crystal orientation. The drain-source current as function of gate voltage for different back-side voltages is measured at a drain-source voltage of 50 mV in the linear MOSFET region, and the values of threshold voltage and mobility extracted using the standard MOSFET formulae. To determine the bulk doping, the implantation dose and profile from the data, two methods are used, which give compatible results. The doping profile, which varies between $3.5 \times 10^{12}$ cm$^{-3}$ and $2 \times 10^{15}$ cm$^{-3}$ for the MOSFET with $p^+$-implant, is determined down to a distance of a fraction of a $μ$m from the Si-SiO$_2$ interface. The method of extracting the doping profiles is verified using data from a TCAD simulation of the two MOSFETs. The details of the methods and of the problems encountered are discussed.

physics.ins-det↗

arXiv2017

On the characterisation of SiPMs from pulse-height spectra

Methods are developed, which use the pulse-height spectra of SiPMs measured in the dark and illuminated by pulsed light, to determine the pulse shape, the dark-count rate, the gain, the average number of photons initiating a Geiger discharge, the probabilities for prompt cross-talk and after-pulses, as well as the electronics noise and the gain fluctuations between and in pixels. The entire pulse-height spectra, including the background regions in-between the peaks corresponding to different number of Geiger discharges, are described by single functions. As a demonstration, the model is used to characterise a KETEK SiPM with 4382 pixels of 15 μm x 15 μm area for voltages between 2.5 and 8 V above the breakdown voltage at 20°C. The results are compared to other methods of characterising SiPMs. Finally, examples are given, how the complete description of the pulse-eight spectra can be used to optimise the operating voltage of SiPMs, and a method for an in-situ calibration and monitoring of SiPMs, suited for large-scale applications, is proposed.

physics.ins-det↗

arXiv2016

Study of the breakdown voltage of SiPMs

The breakdown behaviour of SiPMs (Silicon PhotoMultiplier) with pixel sizes of 15$\times $15, 25$\times $25, 50$\times $50, and 100$\times $100 $μ$m$^2$, manufactured by KETEK, has been investigated. From the current-voltage characteristics measured with and without illumination by LED light of 470 nm wavelength, the current-breakdown voltage, $V_I$, and from linear fits of the voltage dependence of the SiPM gain, measured by recording pulse-area spectra, the gain-breakdown voltage, $V_G$, have been obtained. The voltage dependence of the Geiger-breakdown probability was determined from the fraction of zero photoelectron events with LED illumination. By comparing the results to a model calculation, the photodetection-breakdown voltage, $V_{PD}$, has been determined. Within experimental uncertainties, $V_I$ and $V_{PD}$ are equal and independent of pixel size. For $V_G$, a dependence on pixel size is observed. The difference $V_I - V_G$ is about 1 V for the SiPM with 15 $μ$m pixels, decreases with pixel size and is compatible with zero for the SiPM with 100 $μ$m pixels.

physics.ins-det↗

arXiv2016

The influence of edge effects on the determination of the doping profile of silicon pad diodes

Edge effects for square p+n pad diodes with guard rings, fabricated on high-ohmic silicon, are investigated. Using capacitance-voltage measurements of two pad diodes with different areas, the planar and the edge contributions to the diode capacitance are determined separately. It is shown that the doping concentration derived from the capacitance-voltage measurements with and without edge corrections differ significantly. After the edge correction, the bulk doping of the pad diodes is found to be uniform within +/- 1.5%. The voltage dependence of the edge capacitance is compared to the predictions of two simple models.

physics.ins-det↗

arXiv2013

Challenges for the Adaptive Gain Integrating Pixel Detector (AGIPD) design due to the high intensity photon radiation environment at the European XFEL

The European X-ray Free Electron Laser (XFEL) is a new research facility currently under construction in Hamburg, Germany. With a pulse length of less than 100 fs and an extremely high luminosity of 27000 flashes per second the European XFEL will have a unique time structure that demands the development of new detectors tailored to the requirements imposed by the experiments while complying with the machine specific operation parameters. The Adaptive Gain Integrating Pixel Detector (AGIPD) is one response to the need for large 2D detectors, able to cope with the 4.5 MHz frame rate, as well as with the high dynamic range needed by XFEL experiments ranging from single photons to more than 10$^4$ 12 keV photons per pixel per pulse. In addition it has to withstand doses of up to 1 GGy over three years.

physics.ins-det↗

arXiv2012

Design of the AGIPD Sensor for the European XFEL

For experiments at the European X-Ray Free-Electron Laser (XFEL) the Adaptive Gain Integrating Pixel Detector (AGIPD) is under development. The particular requirements for the detector are a high dynamic range of 0, 1 - to more than 10E4 12.4 keV photons per pixel within an XFEL pulse duration of < 100 fs, and a radiation tolerance of doses up to 1 GGy for 3 years of operation. The detector will have 1024 x 1024 p+ pixels with a pixel size of 200 um x 200 um and will be manufactured on 500 um thick n-type silicon. The design value for the operating voltage is 500 V, however, for special applications an operation up to ~ 1000 V should be possible. Experimental data on the dose dependence of the surface density of oxide charges at the Si-SiO2 interface and the surface-current density have been implemented in the SYNOPSYS TCAD simulation program in order to optimize the design of the pixel and guard-ring layout. The methodology of the sensor design, the optimization of the most relevant parameters and the layout are demonstrated. Finally the simulated performance, in particular the breakdown voltage, dark current and inter-pixel capacitance as function of the X-ray dose will be presented.

physics.ins-det↗

arXiv2011

Optimization of the Radiation Hardness of Silicon Pixel Sensors for High X-ray Doses using TCAD Simulations

The European X-ray Free Electron Laser (XFEL) will deliver 27000 fully coherent, high brilliance X-ray pulses per second each with a duration below 100 fs. This will allow the recording of diffraction patterns of single molecules and the study of ultra-fast processes. One of the detector systems under development for the XFEL is the Adaptive Gain Integrating Pixel Detector (AGIPD), which consists of a pixel array with readout ASICs bump-bonded to a silicon sensor with pixels of 200 μm \times 200 μm. The particular requirements for the detector are a high dynamic range (0, 1 up to 10E5 12 keV photons/XFEL-pulse), a fast read-out and radiation tolerance up to doses of 1 GGy of 12 keV X-rays for 3 years of operation. At this X-ray energy no bulk damage in silicon is expected. However fixed oxide charges in the SiO2 layer and interface traps at the Si-SiO2 interface will build up. As function of the 12 keV X-ray dose the microscopic defects in test structures and the macro- scopic electrical properties of segmented sensors have been investigated. From the test structures the oxide charge density, the density of interface traps and their properties as function of dose have been determined. It is found that both saturate (and even decrease) for doses above a few MGy. For segmented sensors surface damage introduced by the X-rays increases the full depletion voltage, the surface leakage current and the inter-pixel capacitance. In addition an electron accumulation layer forms at the Si-SiO2 interface which increases with dose and decreases with applied voltage. Using TCAD simulations with the dose dependent damage parameters obtained from the test struc- tures the results of the measurements can be reproduced. This allows the optimization of the sensor design for the XFEL requirements.

physics.ins-det↗