F. Corsi

Electrical Characterization of Silicon Photo-Multiplier Detectors for Optimal Front-End Design

Silicon Photo-Multiplier (SiPM) detectors represent an attractive solution for the detection of low energy photons in several fields of both high energy physics and medical imaging. We present here an accurate electrical model for this kind of detectors, which can be conveniently used to perform reliable simulations at circuit level. A suitable extraction procedure for the parameters involved in the model is also described, based on both static and dynamic measurements. The proposed model allows to reproduce accurately the waveform of the signal generated by the SiPM when coupled to the front-end electronics, as shown by excellent fittings obtained between simulations and measurements taken on real devices. This is particularly useful in order to choose the most suitable front-end architecture for SiPM detectors, since the performance of the whole detection system, especially in terms of dynamic range and timing resolution, can be correctly predicted as a function of the detector parameters and of the main characteristics of the coupled electronics.

Defect level evaluation in an IC design environment

The purpose of this paper is to present a methodology for the evaluation of the Defect Level in an IC design environment. The methodology is based on the extension of Williams-Brown formula to nonequiprobable faults, which are collected from the IC layout, using the information on a typical IC process line defect statistics. The concept of weighted fault coverage is introduced, and the Defect Level (DL) evaluated for the Poisson and the negative binomial yield models. It is shown that DL depends on the critical areas associated with undetected faults, and their correspondent defect densities. Simulation results are presented, which highlight that the classic single Line Stuck-At (LSA) fault coverage is a unreliable metric of test quality. Moreover, results show that the efficiency of a given set of test patterns strongly depends on the physical design and defect statistics.

Multistate Markov Models and Structural Properties of the Transition-Rate Matrix

This paper explains how to use Markov models for evaluating the availability and reliability of a system when the transition rates of each component depend on the state of the system. To this purpose, the structural properties of the system transition-rate matrix, obtained by the Kronecker sum of transition rate matrices of the components, are analyzed. As a result, an efficient use of the Markov model is possible by mapping the transition rates of the single components into the elements of the system transition-rate matrix. The potential of parallel computers can facilitate this approach, giving renewed interest to the Markov method for complex systems.

Novel Silicon Photomultipliers for PET Applications

Silicon photomultipliers (SiPMs) with quantum efficiency maximized for a wavelength between 420 and 470 nm have been developed at ITC-irst Trento (Italy), and are being tested for their application in the construction of a ultra high resolution small animal PET tomograph. The devices have an area of 1 mm times 1 mm and 625 microcells. The breakdown voltage is around 30 V, and the gain of the order of 106. The intrinsic timing resolution is 70 ps rms at the single photoelectron level. The first tests as readout for scintillators show an energy resolution of 21% FWHM with Na-22 employing LSO crystals. The first matrices of SiPMs have been produced and are being tested.

BASIC: An 8-channel front-end ASIC for Silicon Photomultiplier detectors

Multi-channel, integrated front-end electronics suitable for Silicon Photomultiplier detectors and mainly intended for medical imaging applications has been developed in a CMOS standard technology, according to a current-mode approach. Full exploitation of the good performance of the detector in terms of fast response and gain has been made possible by this design approach. An 8-channel, self-triggered prototype with an on-chip ADC has been designed and realized, also featuring a good degree of programmability and sparse read-out capabilities. Characterization measurements, carried out by coupling the circuit to both an injection capacitance and a SiPM manufactured from FBK-irst, confirm the expected results in terms of overall charge to voltage gain, dynamic range (more than 70pC at 1% non-linearity error), equivalent input noise charge (about 50fC) and timing accuracy.

ASIC development for SiPM readout

The design of CMOS front-end electronics suitable for Silicon Photo-Multipliers (SiPM) is described in this paper, starting with the specification of an accurate electrical model of the detector and its experimental validation. A novel current-mode solution is proposed for the preamplifier and the discriminator, to cope with the large dynamic range and the extremely fast rise time of the detector signal. Experimental results achieved from front-end prototypes designed according to this current-mode approach demonstrate its effectiveness: dynamic range of the order of 50 pC and timing accuracy of the electronics alone of about 30 ps have been measured.

Defect level as a function of fault coverage and yield

An extension of the well known formula relating yield, fault coverage and defect level has been derived by removing the hypothesis of equally likely faults and exploiting the concept of critical area to evaluate the probabilities of individual faults, under the hypothesis of Poissons yield model. A generalized weighted fault coverage figure has been introduced with reference to realistic faults and implications on the test strategy to adopt for a given product have been put into evidence.<<ETX>>

Preliminary results from a current mode CMOS front-end circuit for silicon photomultiplier detectors

We propose a CMOS front-end circuit suitable for Silicon Photomultiplier detectors (SiPM) based on a current buffer, as input stage, which features small input impedance and large bandwidth, thanks to the application of current feedback techniques. The current mode approach enhances the dynamic range of the front-end and does not suffer from possible voltage limitations due to deep-submicron CMOS implementation. We report the first measurement results obtained by coupling the circuit prototype to a SiPM detector excited by a blue LED light source. The measurements confirm the effectiveness of the proposed front-end approach and demonstrate its capability of managing large current signals with good linearity.

Infrared microscopy study of anomalous latchup characteristics due to current redistribution in different parasitic paths

Anomalous effects such as abrupt variations of the latchup current in steady-state conditions and window effects, i.e. the existence of a well-defined interval of I/O injected currents for latchup to occur, can occur during pulsed latchup tests. Infrared microscopy allows the correlation of electrical characteristics with latchup current distribution and reveals that anomalous effects are due to the dynamic competition between different latchup paths. This is confirmed by a SPICE simulation of the lumped equivalent circuit of a CMOS output comprising two coupled p-n-p-n parasitic structures. >

Mixed-signal circuit classification in a pseudorandom testing scheme

Pseudo-random testing techniques for mixed-signal circuits offer several advantages compared to explicit time-domain and frequency-domain test methods, especially in a BIST structure. To fully exploit these advantages a suitable choice of the pseudo-random input parameters should be done and an investigation on the accuracy of the circuit response samples needed to reduce the risk of misclassification should be carried out. Here these issues have been addressed for a testing scheme based on the estimation of the impulse response of the device under test (DUT) by means of input-output cross-correlation. Moreover, new acceptance criteria for the DUT are suggested which solve some ambiguity problems arising if the classification of the DUT as good or bad is based on a few samples of the cross-correlation function. Examples of application of the proposed techniques to real cases are also shown in order to assess the impact of the measurement system inaccuracies on the reliability of the test.