Laser Testing of Silicon Detectors Rhorry Gauld University of Saint Andrews IPM program – PPD Mentor: Ronald Lipton 30/07/08 1.

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


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


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)


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

)


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




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


-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


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


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)


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.


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


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 University of Saint Andrews IPM program – PPD Mentor: Ronald Lipton 30/07/08 1.

Documents

laser picoprobe

laser pulsing

strip detector

silicon detector work

laser pulse duration

detector image

error slide

bulk silicon