Post on 18-Dec-2015
Laser Testing of Silicon Detectors
Rhorry GauldUniversity of Saint Andrews
IPM program – PPD Mentor: Ronald Lipton
30/07/08
1
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, 30/07/082
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
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
Image 1: strip detector showingionization as a charged particlepasses through
Laser Testing of Silicon Detectors, Rhorry Gauld, 30/07/083
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, 30/07/084
First Attempts & ProblemsSetup 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, 30/07/085
Experiment Set-up
Vertical Height Fix
0
5
10
15
20
0 20 40 60 80 100 120 140 160
Vertical Height (no units)
Vo
ltag
e P
eak (
mV
)
Setup 2-Vertical laser height fix-Laser attenuation fix-Pulse rate at which base line fell beforenext pulse-Measurement of detector spacing-Laser pulse duration
Problems come from picoprobe tip & connections in circuit
Laser Attenuation vs. Voltage peak
0
1
2
3
4
5
0 2 4 6 8 10 12
Laser Attenution
Vo
ltag
e P
eak
(mV
)
Laser Testing of Silicon Detectors, Rhorry Gauld, 30/07/086
Co-ordinate systemMeasured 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 Scan
05
1015202530
-720 -360 0 360 720 1080
1440
Horizontal Distance Between strip and laser
(uM)
Vo
ltag
e P
eak
Hei
gh
t (m
V)
Voltage Peak(mV) 50Vsupply
Voltage Peak(mV) 100Vsupply
Voltage Peak(mV) 150Vsupply
Laser Testing of Silicon Detectors, Rhorry Gauld, 30/07/087
Strip Scan vs Laser Scan
0
2
4
6
8
10
12
14
-600 -400 -200 0 200 400 600 800
Horiztontal Spacing between Laser and strip (um)
Peak
Hei
ght (
mV)
8
Errors
Laser Testing of Silicon Detectors, Rhorry Gauld, 30/07/08
-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
Oscilloscope Readout
Time( T = 40us)
Vo
ltag
e O
ut
(mV
)
Set-up CheckGaussian for 50V
-5
0
5
10
15
20
25
-500 0 500 1000 1500
Laser - Strip displacement (um)
Vo
ltag
e P
eak
(mV
)
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 C
Calculate 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 C
Pulse 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, 30/07/089
Results
d = 300 um and V = 175V
d =
1/2
e ND
2 ε ε0 V d = depletion width
V = voltage bias at full depletion
ε 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
=> ND = 2.561 x 10^12 cm^-3
Laser Testing of Silicon Detectors, Rhorry Gauld, 30/07/0810
Levelling Off
0
5
10
15
20
25
30
0 30 60 90 120 150 180 210
Voltage Supply (V)
Hei
gh
t o
f V
olt
age
Pea
k (m
V)
More ResultsFall time ~1/ (V^1/2) leveling off at175V
Agrees with C ~ 1/ (V^1/2)
Agrees with RC time constant
suggests a C 1pF/cm
90%-10% Peak fall time vs Bias
02468
1012141618
0 50 100 150 200 250
Voltage Bias (V)
Tim
e (u
s)
Laser Testing of Silicon Detectors, Rhorry Gauld, 30/07/0811
Full calculations still to come
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.
Laser Testing of Silicon Detectors, Rhorry Gauld, 30/07/0812
Rise Time
Time to peak ~420ns
Vo
ltag
e o
ut
pea
k ~
23
0mV
Currently Testing rise/fall times atdifferent values of Voltage Bias
Will calculate depletion width thendoping concentration next
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
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, 30/07/0814