Deiters, K

Ingram, Q

Musienko, Yu V

Nicol, S

Patel, B

Renker, D

Reucroft, S

Rusack, R W

Sakhelashvili, T M

Singovsky, A V

Swain, J D

Vikas, P

CERN Document Server

CMS ECAL APD quality assurance facilityDeiters,K.  Ingram,Q.  Renker,D.  Sakhelashvili,T.Paul Scherrer Institute, Villigen, SwitzerlandPatel,B. Rusack,R.  Singovski,Aa).  Vikas,P.University of Minnesota, Minneapolis, USAMusienko,Y.  Nicol,S. Reucroft,S. Swain,J.Northeastern University, Boston, USAa) E-mail:Alexander.Singovski@cern.chAbstractThe S8148 Avalanche Photo Diode (APD), developed byHamamatsu Photonics for CMS electromagnetic calorimeterfits well to the detector specifications. The dedicated qualityassurance/control facility is installed at CERN by CMSMINNEUPSI (Minnesota University + NortheasternUniversity + Paul Scherre Institute) collaboration, for APDinput control during the mass production.I. INTRODUCTIONA high discovery potential of the CMS experiment shouldbe based on an excellent performance of all sub-detectors, inparticular the electromagnetic calorimeter (ECAL) [1]. Thisdetector will be made of two innovative materials never usedin calorimetry before: heavy and fast PbWO4 scintillatingcrystal read out by the large area Avalanche Photo Diode. Thelatest one was developed by Hamamatsu Photonics for CMSECAL following a very demanding specification: the bestpossible parameters essential for the calorimeter performance,like high quantum efficiency, low dark current etc., and verygood stability and reliability.The R&D on APD optimisation is practically finished andthe current device parameters (see below) fit well to the CMSECAL specification. The reliability issue is becoming crucialnow, in particular the way of APD testing during the massproduction to ensure 10-4 fault rate required by specification.To address this problem, the CMS collaboration has createdthe APD Laboratory at CERN and has equipped it by severalcomputer controlled test benches dedicated to the APDquality assurance and quality control during the massproduction.II. APD STATUSThe electromagnetic calorimeter performance is usuallyexpressed in terms of the energy resolution, which isparameterised as: EcbEaEE ⊕⊕=σ , where thestochastic term a is due to the intrinsic shower fluctuationcombined with the photo statistics, the constant term b isrelated to the detector stability, uniformity and precision ofthe calibration and the term c is the noise contribution due toelectronics, pile-up etc. The CMS ECAL design goal is :a~3%, b~0.5% and c~200 MeV [1]. APD contributes to allthree terms: to the stochastic term a by photo statistics, inparticular by APD area and quantum efficiency and by theexcess noise factor F [2], which reflects the avalanche natureof the photo multiplication by APD; to the constant term b bythe gain variation with bias voltage and temperature, ageingand radiation damage effects; to the noise term c by thecapacitance as a series noise and the dark current as a parallelnoise. The goal of R&D was to maximise the ‘useful’parameters like APD area, quantum efficiency and minimisethe ‘bad’ parameters like excess noise factor, dark current,capacitance and the slopes of the bias voltage and temperaturedependencies.A. APD parametersThe final values of the essential APD parameters aresummarised in Table. 1Table 1: Hamamatsu S8148 APD parametersActive area 5x5mm2Operating voltage(Vr) ~380VCapacitance 70pFSerial resistance 3ΩDark Current <10nAQuantum Efficiency ~72%@420nm1/Ma)×dM/dV(M=50) 3.3%1/M×dM/dT(M=50) -2.3%Excess Noise Factor (M=50) 2Distance to breakdown (Vb-Vr) (30-40)VEffective thickness ~5µmGain range Up tp 1000a)M is a gain value, T-temperatureThe typical APD gain curve is shown in Figure 1. It isclear that the gains up to several hundreds can be used. Thegain dependence on the bias voltage and the temperature areshown in Figure 2. The example of the capacitance, quantumefficiency and excess noise factor curves are shown in figures3, 4 and 5 respectively.Figure 1: The typical APD gain vs. bias voltage curve.Figure 2: The gain dependence on the bias voltage and temperature.Figure 3: Capacitance vs. bias voltage.Figure 4: APD quantum efficiency.Figure 5: Excess niose factor vs. gain. Statistical fluctuation of theavalanche multiplication F=kM+(2-1/M)(1-k), where k is a ratio ofthe ionisation coefficients for holes and electrons and M is an APDgain, contributes to the constant term as sqrt(F/Np.e.)B. StabilityThe critical parameters of the photo detector, scheduled torun in the LHC experiment are the radiation hardness and thelong-term stability. First, because both neutron and photonradiation levels will be rather high, second because thedetectors will have to work several years practically withoutany maintenance and reparation.The radiation hardness of APDs is tested in the 70 MeVproton beam of PSI. The beam rate is 9x1012 protons/cm2,which is equivalent to 2x1013 of 1MeV neutrons/cm2, a 10years fluence expected in CMS barrel [3]. The dark currentversus neutron flux for several APDs is shown in figure 6.Figure 6: Total current for APD under Vr vs. neutron flux.The rise of the dark current to 10-15 µA by 10 years willnot degrade very much the overall ECAL performance. Thebreakdown voltage is decreased after irradiation by severalvolts, which is not dangerous because the operating tobreakdown voltage distance is typically more than 30 volts.Several percents of APD during irradiation go quickly to thebreakdown. This effect is under investigation now and issupposed to be removed before the mass production start. Anew facility with a 252Cf  source is currently setting up inMinnesota University. Irradiation by “pure” neutrons at lowerrates will give a complimentary information, very importantfor understanding of the nature of APD damage effects.The long-term stability is tested during the acceleratedageing: keeping APD at 80°C under the bias corresponding tothe gain=50 at this temperature. No significant degradation ofAPD#3030.11101001000100000 100 200 300 400 500Bias [V]Gain01020304050600 500 1000 1500 2000Gain1/M*dM/dV [%]-30-25-20-15-10-500 500 1000 1500 2000Gain1/M*dM/dT [%]101001000100000 50 100 150 200 250 300 350 400Voltage [V]Capacitance [pF]0102030405060708090100300 400 500 600 700 800 900 1000Wavelength [nm]Quantum Efficiency [%]135791113150 500 1000 1500 2000GainExcess Noise FactorTotal Current (dark current + inonisation current)024681012141618200.00E+00 5.00E+12 1.00E+13 1.50E+13 2.00E+13total Neutron Flux / cm2Current [ µµA]the essential APD parameters was observed after 2 monthsageing, corresponding to 10 years of operation under CMSECAL running temperature of 18°C.III. QUALITY ASSURANCE/CONTROL PROCEDUREThe scheme of the APD path from the Producer to thefinal capsule mounting is presented in Figure 7.Figure 7: APD / capsule testA small fraction of APD, 5 samples from each wafer, willbe first packaged and sent to CERN where all parameters willbe measured and after to PSI for the radiation hardnessacceptance test. In case of negative result the correspondingwafer will be rejected. In case of positive – all APDs will bepacked and tested at the Hamamatsu test bench, similar toones installed at CERN and PSI. All APDs which meet thespecification will be delivered to CERN where a certainfraction of them will be tested at the CERN APDlab testbenches. In case of the significant deviation from the desiredparameters, full lot or some wafers could be returned toHamamatsu. Some fraction of APD will be placed to the ovenfor the accelerated ageing test.A quality assurance facility installed at CERN APDlab isequipped with six computer-controlled set-ups to measure:- gain as function of the bias voltage and temperature,- capacitance versus bias voltage- quantum efficiency and gain versus wavelength- noise and excess noise factor- timing response- accelerated ageing.All set-ups can run in parallel. The gain and quantumefficiency set-ups are equipped with the temperaturestabilisation systems.C. Gain set-upThe sketch of the gain set-up is shown in figure 8. The set-up includes the APD housing box, Keithley487picoammeter/voltage source, Keithley7001 bipolar switch andKeithley485 picoammeter. The APD housing box consists ofa water cooled aluminium plate to which a printed circuitboard with 20 APDs mounted in ZIF sockets is attached. Theplastic fibres of the light distribution system bring the lightfrom the blue LED (Nichia NSPB 500S) to the APDs frontwindow (figure 9).Figure 8: Sketch of the Gain measurement set-upA PIN diode is mounted on one of fibres and is used tomonitor the light intensity. A water from the Neslab RTE-211chillier is pumped through the Al plate allowing the APDtemperature to be held between 8 ant 80°C with 0.2°Cprecision. The APDs temperature is measured by the AD590sensor.Figure 9: APD housing box of the Gain set-upThe APD bias and the current readout is provided byKeithley487 picoammeter/voltage supply coupled to twentychannel bipolar switch Keithley2001. The reference PINdiode is read out by the Keithley485. A PC running LabViewcontrols all devices via GPIB interface. The gain curvesmeasurement for 20 APDs is fully automatic and takes about2 hours.D. Capacitance set-upThe bench where capacitance and series resistance aremeasured consists of a HP LCZmeter 428A, a Keithley2410picoammeter/voltage source and a Keithley2000 switch. Bothparameters are measured as a function of bias voltage. Thecapacitance measurements are made at a frequency  of500kHz and take about 15 minutes per APD. The C(V) curvesprovide an information on the APD doping profile, whichdepend on the stability of the production technology and is animportant parameter for the quality control and assurance.E. Quantum Efficiency set-upThe Quantum Efficiency set-up is similar to the Gain one,except the light source is the ORIEL Na Lamp with theCornerstone150 computer-controlled monochromator. It canprovide the light in the 300-1020nm range in several nano-meters bin (depend on the output collimator width) with0.5nm precision. The quantum efficiency is measured for thewavelengths from 340nm to 800nm with 10nm step. The APDbias is 10V(M=1). The measurement for 20 APD takes3hours.F. Excess Noise Factor set-upNoise, excess noise factor nuclear counter effect andresponse APD to gamma-rays are measured in a set-up whichconsist of the universal APD-housing box, the Ortec142preamplifier connected to the Ortec420 research amplifier.Signal and noise amplitudes are measured by the LeCroy2259 ADC and the LeCroy 2249W QADC or by theTektronix TDS 540 digital oscilloscope, which is used alsofor the noise spectra analysis. A Sr90 source is used for thenuclear counter effect  measurement and the system iscalibrated with a Hamamatsu 200µm thick PIN diode. Gainmeasurements are made with Fe55 and Am241 sources. Allmeasurement on this set-up are made for the single APD andtake, depending on the complexity, from 30 minutes to severalhoursG. Timing response set-upAPD timing response is measured using the Tektronix TDS540 digital oscilloscope. A 3ns pulse from VSL-337 UV laseris injected through a 200m long 100µm diameter quartz fibre.The APD response is measured directly at the 50Ω input ofthe oscilloscope.H. Accelerated ageing set-upAPD long term stability is tested at the accelerated ageingset-up. The sketch of set-up is shown in figure 10.The ageing set-up consists of two ovens, housing 5 printedcircuit boards (PCB) with 80 APDs in ZIF connectors each.All boards are connected to the Keithley7002 switchingsystem with ten 7011-C switching cards. Each PCB has twohigh voltage channels, connected to the CAEN SY-127 highvoltage system with A333 HV cards, hence 40 APDs arebiased by a single HV channel. APD current through the100kΩ resistor is read out by the Keithely2000 voltmeterthrough the Keithley7002 switch. Two APDs are connected toone readout channel. The bias voltage and temperature aremonitored using Keithley2000. PC running LabView controlsthe measurement procedure. APD currents are measuredperiodically, usually once per hour, and recorded to the localdisk.Figure 10: Sketch of the ageing set-upThe ageing slightly change the breakdown voltage,decreasing it for most of APDs by less than 1V. The darkcurrent increases by about 1na. No change of the quantumefficiency, capacitance and other parameters where found.The changes, although well visible, are not significant and theaccelerated ageing is considered as non-destructive.Some APDs produced at the beginning of this year showeda sharply rising dark current after several days of theaccelerated ageing at 80°C (figure 11). The problem,connected to the production technology, in particular etchingof the groove, was localised and removed by Hamamatsu.Figure 11: Current during accelerated ageing for good andbad APDsIV. CONCLUSIONSThe Hamamatsu S8148 APD, developed for CMS ECALdetector, is well suitable for use in this detector. The stabilityand reliability issues are crucially important at the massproduction stage. To coop with this challenge, CMScollaboration has defined an APD quality acceptance and testprocedure and set-up at CERN a dedicated qualityassurance/control facility. The facility consists of 6 computercontrolled test benches and is capable to test during the massAPDs delivery to CERN:APD aging studies (T=80 C)0100020003000400050006000700080009000100000 2 4 6 8 10Time (days)Dark Current (nA)Ch#76Ch#10Gain, dM//dV, dM/dT 100% of APDsCapacitance, serial resistance up to 20%Quantum efficiency up to 30%Noise, excess noise factor up to 5%The big fraction of the tested APDs will be very useful atthe beginning of the mass production and will be decreased tothe foreseen by TDR 2-5% at the stable APD productionphase.V. REFERENCES[1] The Electromagnetic Calorimeter Project, TechnicalDesign Report, CERN/LHC 97-31(1997).[2] A.Karar et al. Neucl. Instr. and Meth. 428A (1999), 413 [3] R. McIntyre, IEEE trans. Electron Dev. ED-19 (1972),703.

CMS ECAL APD quality assurance facility

CMS ECAL APD quality assurance facility

Abstract

Similar works

Full text

Available Versions

CERN Document Server