Transcript Document

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
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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 %)
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Experimental Setup
op-amp
circuit
sensor
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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
only red samples
stored and
displayed
allows capture of fast transients over relatively long time period (few seconds)
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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
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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
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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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AC only
DC leakage subtracted – baselines offset
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AC only
instability characteristics different to 1st
ST sensor (no step changes)
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some evidence of instability but much
better than first two
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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?
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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
February, 2004
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