Laser Testing of Silicon Detectors

1. Laser Testing of Silicon Detectors Rhorry GauldUniversity of Saint AndrewsIPM program – PPD Mentor: Ronald Lipton30/07/08

2. Introduction • Science involved • Aim of the project • First attempts • Development of technique • Application of technique • Future applications • Conclusion Laser Testing of Silicon Detectors, Rhorry Gauld, 30/07/08

3. How Does A Silicon Detector Work? Charged particle passes throughdetector and creates ionisation inthe bulk silicon This creates electron/hole pairs Holes drift in an applied electric-field towards the negatively charged p-type strips Image 1: strip detector showingionization as a charged particlepasses through Induces readable charge on the Aluminium strips Thus by recording which read-out channel is fired, it is possible to determine where the charged particle passed through the detector Laser Testing of Silicon Detectors, Rhorry Gauld, 30/07/08

4. Testing the Detectors • Why is it important?- ALICE experiment will use ~25000 detectors- Need to know each detector works efficiently- No defects ( make sure strips are working)- Properties of the detectors ( doping/electric field penetration depth etc.) • Benefits of technique?- compactness - ease of control of amount and location of deposited charge- allows detectors with no read-out system to be tested Laser Testing of Silicon Detectors, Rhorry Gauld, 30/07/08

5. First Attempts & Problems Setup 1 Problems;- Very difficult to position laser whilst probing detector- No scope reading for laser - Picoprobe not working Solutions;- Use a different type of detector which allows probing and laser pulsing at different points- Replace oscilloscope- New Detector also had built resistor and contacts ( so the ‘probe’ no longer required) Laser Testing of Silicon Detectors, Rhorry Gauld, 30/07/08

6. Experiment Set-up Setup 2 -Vertical laser height fix-Laser attenuation fix-Pulse rate at which base line fell beforenext pulse-Measurement of detector spacing-Laser pulse durationProblems come from picoprobe tip & connections in circuit Laser Testing of Silicon Detectors, Rhorry Gauld, 30/07/08

7. Co-ordinate system Measured detector spacing using Pixel DetectorMatched laser scan( number of turns) with strip scan ( um)Measured the effect that changing the voltage bias has onthe amplitude of our scan 1 Turn : 800 um Laser Testing of Silicon Detectors, Rhorry Gauld, 30/07/08

8. Errors • Trying to over lap laser scan vs. strip scan not completely accurateEstimated ~5% Error in overlap of graphs. 1 Turn : 800um +/- 40um- Easily fixed by installing coupled dial gauge • Error found when measuring output signal from oscilloscope- Fluctuations caused 1.2% read-out error Laser Testing of Silicon Detectors, Rhorry Gauld, 30/07/08

9. Set-up Check Charge deposited on 20um x 10cm strip( atx = 0) per pulse is Q = C VC = 1pF/cm V = 188mV=> Q = 1.88 x 10^-12 CCalculate expected current average producedby laserI = Qtotal / t Integrate curve fit for strip width / entire curveCharge deposited for strip at x = 0 : Q = 2.98%=> Qtotal = 6.31 x 10^-11 CPulse Duration = 5.6 nsPulse Generator Period = 40 us => I = 1.13 x 10^-2 A& Iave = 1.56 uAagrees with observed currentproduced by laser Laser Testing of Silicon Detectors, Rhorry Gauld, 30/07/08

10. Results d = depletion widthV = voltage bias at full depletionε ε0 = permittivity for silicon = 11.9 x 8.8541… × 10^−14 F/cm e = electric charge of electron = 1.602 x10^-19 C ND = doping level 2 ε ε0V e ND d = 300 um and V = 175V => ND = 2.561 x 10^12 cm^-3 Laser Testing of Silicon Detectors, Rhorry Gauld, 30/07/08

11. More Results Fall time ~1/ (V^1/2) leveling off at175VAgrees with C ~ 1/ (V^1/2)Agrees with RC time constantsuggests a C 1pF/cm Full calculations still to come Laser Testing of Silicon Detectors, Rhorry Gauld, 30/07/08

12. Application of technique • Using technique to detect efficiency, defects and properties of detectorsCurrently testing a small strip detector which we do not know the properties ofDifficult as I am scanning for an area which has < 0.1% area of a 2x2cm detector. Currently Testing rise/fall times atdifferent values of Voltage BiasWill calculate depletion width thendoping concentration next Laser Testing of Silicon Detectors, Rhorry Gauld, 30/07/08

13. Improvements • Set-up laser very accurately • Measure beam properties • Calculate electric field within detector at different BIAS • Relate the velocity of charge carriers to the rise time of out-put voltage peaks • Set-up coupled dial gauge • Use a very accurate current meter • Use Laser with different wavelength to see how penetration depth of the photons effects results Laser Testing of Silicon Detectors, Rhorry Gauld, 30/07/08

14. References • http://hepwww.rl.ac.uk/OpenDays98/Detectors/silicon.htm • http://www.physik.rwth-aachen.de/institut-ib-new-version/forschung/cms/ • Semiconductor Detector System – Helmuth Spieler – Oxford Science Publications – TK9180.S68 2005 C2 • http://ece-www.colorado.edu/~bart/book/book/chapter3/ch3_3.htm#fig3_3_2 • http://www.tf.uni-kiel.de/matwis/amat/semi_en/kap_2/backbone/r2_2_2.html • Prototyping of Silicon Strip Detectors for the Inner Tracker of the ALICEExperiment / Oleksiy Sokolov - [S.l.] : [s.n.], 2006 - Tekst. - ProefschriftUniversiteit Utrecht • Nuclear Instruments and Methods in Physics Research Section A: Accelerators,Spectrometers, Detectors and Associated EquipmentVolume 573, Issues 1-2, 1 April 2007, Pages 12-15 Laser Testing of Silicon Detectors, Rhorry Gauld, 30/07/08