February, 2004 CMS Tracker Week 1

Sensor Stability Measurements

Objective: find simple method of observing sensor stability

-> measure fluctuations in overall leakage current

Outline: setup and experimental method description results so far for 4 sensors (3 ST, 1 Hamamatsu)

Mark Raymond, Imperial College

February, 2004 CMS Tracker Week 2

Experimental setup

transimpedance circuit chosen to give ~ low impedance to guard ring, but could justuse 100 k resistor to ground

all measurements @ room temp. (24 oC (+/- ~ 1)) (RH ~ 30 – 40 %)

100 k => 100 mV / A

February, 2004 CMS Tracker Week 3

Experimental Setup

sensor

op-ampcircuit

February, 2004 CMS Tracker Week 4

scope setup

Tektronix (TDS724D) operated in peak detect mode

internal sampling (Gs/s) much faster than intervals between displayed samples displayed samples alternate between highest in one interval and lowest in next

allows capture of fast transients over relatively long time period (few seconds)

only red samplesstored anddisplayed

February, 2004 CMS Tracker Week 5

4 sensors studied

1st ST sensortype W7B (ID 30211433739909)failed acceptance test - high leakage on one strip (and neighbours) (~ 10 A)

2nd ST sensortype W7B (ID 30211433961106)good sensor (I think), Ileak ~ 650 nA @ 550 V

3rd ST sensorTOB (ID 30210432016302)one leaky strip (~ 6 A)

Hamamatsutype W4 (ID 3022883101103)good sensor, Ileak ~200 nA @ 800 V

February, 2004 CMS Tracker Week 6

Measurements – 1st ST sensor

DC leakage measurement in close agreement with data provided (Vienna) take off at > 150V attributed to group of 3 strips only

scope measurements show instability appearing forbias > 150 V, spiky transients and step changes

6.7 A DC current measurement clean => no significant noise contribution from measuring system

DC leakage only

February, 2004 CMS Tracker Week 7

Measurements – DC + AC

shows relative magnitudes of DC leakage and transient behaviour for all 4 sensors studied (instability not correlated with overall leakage current magnitudes)

February, 2004 CMS Tracker Week 8

AC only

DC leakage subtracted – baselines offset

February, 2004 CMS Tracker Week 9

AC only

instability characteristics different to 1st

ST sensor (no step changes)some evidence of instability but muchbetter than first two

February, 2004 CMS Tracker Week 10

additional observations

vacuum effects

leakage current changes ~ randomly if vacuum to chuck turned on/off initially observed 50 Hz interference also traced to vacuum pressure variations these effects also observed on Hamamatsu sensor

… not clear what’s going on here – leakage sensitive to stress?

February, 2004 CMS Tracker Week 11

Summary

simple scope method used to monitor stability of overall sensor leakage current sensitivity at ~ few 10’s nA possible

results for four sensors only – 3 ST and 1 Hamamatsu instability observable in 2 ST sensors (1 already rejected on overall leakage measurement)

but instability characteristics not identical for these two

probably worth looking at larger sample