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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.

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sensor stability measurements
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

CMS Tracker Week

slide2

Experimental setup

100 kW => 100 mV / mA

transimpedance circuit chosen to give ~ low impedance to guard ring, but could just

use 100 k resistor to ground

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

CMS Tracker Week

slide3

Experimental Setup

op-amp

circuit

sensor

CMS Tracker Week

slide4

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 samples

stored and

displayed

CMS Tracker Week

slide5

4 sensors studied

1st ST sensor

type W7B (ID 30211433739909)

failed acceptance test - high leakage on one strip (and neighbours) (~ 10 mA)

2nd ST sensor

type W7B (ID 30211433961106)

good sensor (I think), Ileak ~ 650 nA @ 550 V

3rd ST sensor

TOB (ID 30210432016302)

one leaky strip (~ 6 mA)

Hamamatsu

type W4 (ID 3022883101103)

good sensor, Ileak ~200 nA @ 800 V

CMS Tracker Week

slide6

Measurements – 1st ST sensor

DC leakage only

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 for

bias > 150 V, spiky transients and step changes

6.7 mA DC current measurement clean

=> no significant noise contribution from

measuring system

CMS Tracker Week

slide7

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)

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slide8

AC only

DC leakage subtracted – baselines offset

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slide9

AC only

instability characteristics different to 1st

ST sensor (no step changes)

some evidence of instability but much

better than first two

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slide10

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?

CMS Tracker Week

slide11

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

CMS Tracker Week