Comparison of Measurements at Kansas State and Stony Brook
Introduction
Kansas State University (KSU) and the State University of New York at Stony
Brook (SB) have obtained similar systems for testing silicon microstrip sensors. Each
system contains an Alessi (Cascade Microtech) 6100 automatic probe station, a Keithley
237 Source Measure Unit (SMU) picoammeter and voltage source (does both at once), a
Keithley 487 picoammeter and voltage source (separate), and an LCR meter. For an
LCR meter KSU uses the HP 4284, but SB uses the Agilent (HP) 4263B. Both also use
the HP34904A 4  8 Matrix Switch to perform multiple electrical tests with the needle
probes only touching the sensor pads once. KSU runs 3 coaxial probes with an
additional probe riding on the chuck. SB runs 4 coaxial probes plus a fifth riding on the
chuck.
We will discuss the comparison of results from KSU and SB for two sensors
which both labs have scanned. L2_014 is a perfect outer sensor from HPK. Both labs
agree this sensor has no problems. L1_010 is an Elma sensor which is more interesting
because it has problems, so we will discuss measurements on this sensor in detail.
L1_010 IV Curves
450
400
350
Current (nA)
300
250
SB IVCV
200
SB
KSU
150
100
50
0
0
100
200
300
400
500
600
700
800
900
-50
Bias Voltage (Volts)
Figure 1. Total leakage current versus bias voltage for L1_010.
IV Scans
Figure 1 shows IV scans of L1_010. The important point is that the sensor does
not break down with voltages as high as 800 V. Two SB measurements are shown with
one (IVCV) only going up to 200 V. Temperature differences could account for the
different current levels observed at SB on different days. (Fine temperature control at SB
is being installed now.) Differences in shape of the IV curves taken at KSU and SB
could be due to different settling times before the SMU measurement. (SB uses 1
second.)
CV curve for L1_010
3
2.5
1/(C(nF)*C(nF))
2
1.5
SB
KSU
1
0.5
0
0
50
100
150
200
250
-0.5
Bias Voltage
Figure 2. 1/Capacitance(nF)2 versus bias voltage for L1_010.
CV Scans
Figure 2 shows 1/C(nF)2 versus bias voltage for L1_010. SB obtained a full
depletion voltage of 23 V for this sensor whereas KSU obtained 25 V.
Idiel
Figure 3 shows the current through the dielectric of the coupling capacitor for
each strip. The purpose of this measurement is to detect pinholes, current greater than 10
nA. L1_010 does have some pinholes. A summary of bad strips is presented below for
both sensors and both institutions.
It should be noted that SB follows our QA document (“Silicon Sensor Quality
Assurance for the D0 Run IIB Silicon Detector: Procedures and Equipment”, A. Bean et
al., D0 note 4120, draft 4.0) more closely than KSU in performing many measurements.
To measure Idiel, SB places a probe on a strip’s AC pad and measures the current out of
the SMU at 80 V with the bias line grounded. KSU places the voltage of 20 V on a DC
pad and measures the current to the corresponding AC pad at ground. Hence the KSU
Idiel for L1_010
0.2
0.19
0.18
0.17
0.16
0.15
0.14
0.13
Idiel (nA)
0.12
0.11
SB
0.1
KSU
0.09
0.08
0.07
0.06
0.05
0.04
0.03
0.02
0.01
0
0
50
100
150
200
250
300
350
400
450
strip num ber
Figure 3. Current through the coupling capacitor dielectric for each strip of L1_010.
measurement is made by the K487 picoammeter, whereas the SB measurement is made
by the K237 SMU picoammeter. SB observes a background current (current with the
probe lifted) of typically 40 pA with the K237 SMU but less than 1 pA with the K487.
So approximately 40 pA should be subtracted from the SB measurements on both plots
for the sake of comparison. We have not bothered to do this because 40 pA is much less
than the currents we care about (10 nA). The fact that SB uses a higher testing voltage
than KSU could be significant.
The differences in measurement technique do lead to one physical effect. On the
L1_010 SB measures both of the end strips to be “pinholes”. Pinholes usually have
abnormal values of the coupling capacitance, but not these two end strips. So it is
suggested that these end strip “pinholes” actually result from surface conduction to the
bias line. KSU probably should not see this because their AC pad is at ground, close to
the same potential as their bias line. KSU does see an increase in Idiel by a factor of 7
on one end strip of L1_010, but this current does not exceed 10 nA.
Coupling Capacitance for L1_010
100
95
90
85
80
75
70
Capacitance (pF)
65
60
55
SB
50
KSU
45
40
35
30
25
20
15
10
5
0
0
50
100
150
200
250
300
350
400
450
strip num ber
Figure 4. Coupling capacitance versus strip number for L1_010.
Cac
Figure 4 shows the coupling capacitance for each strip. At this writing the
absolute SB values are in question because differences from the hand measurements at
SB have been observed. Nevertheless measurements at SB and KSU usually agree in the
identification of bad channels (Cac greater than 1.2 times the typical value (short) or less
than 0.8 times the typical value (open)). A summary of bad channels is provided below.
The pinholes usually also qualify as opens or shorts. KSU and SB usually agree on
whether the coupling capacitance is good or bad but for the bad channels, they often
disagree on whether the bad value is an open or a short. This could be due to the
differences in method. SB measures capacitance from the AC pad to the bias line,
whereas KSU measures the capacitance between the DC pad and AC pad. The regular
structure seen in the SB plots is real. It is due to the fact that every 10th AC pad is larger
than the others.
Strip Leakage Current for L1_010
1.2
1
Istrip (nA)
0.8
SB
0.6
KSU
0.4
0.2
0
0
50
100
150
200
250
300
350
400
450
strip num ber
Figure 5. Leakage current to each strip for L1_010.
Istrip
Figure 5 shows the leakage current from each strip. Both KSU and SB use the
K487 to measure this current and the agreement between the two measurements is quite
good. There is a quantization in the SB data (about 5 pA for L1_010) which KSU does
not see. For L1_010 the SB data are more spikey than the KSU data. The KSU data
follow some spikes, but not all. All of the spikes are much less than the bad channel limit
of 10 nA.
Polysilicon Resistance of Each Strip on L1_010
1400
Rpoly (kOhm)
1350
SB
1300
KSU
1250
1200
0
100
200
300
400
500
strip num ber
Figure 6. Polysilicon resistance on each strip on L1_010.
Rpoly
Figure 6 shows measurements of the polysilicon resistor on each strip. On
L1_010 both labs find that ALL strips have resistors which are out of spec. Since all the
channels are bad, we do not mention this in the summary. We arbitrarily call a channel
bad if its resistance is greater than the average value (1272 kOhm) plus 300 kOhm or less
than the average value minus 300 kOhm. SB sees one bad channel which is not
confirmed by KSU.
Interstrip Capacitance at 100 kHz for L1_010
4.5
4
3.5
Cint(pF)
3
2.5
SB
2
1.5
1
0.5
0
0
50
100
150
200
250
300
350
400
450
strip num ber
Figure 7. Interstrip capacitance between adjacent strips starting with an even number.
Cint
Figure 7 shows the SB measurements of the interstrip capacitance of the even
pairs of strips. KSU does these measurements on test structures. SB performs this
measurement at 100 kHZ. Other labs see that the frequency dependence of Cint is rather
flat between 100 kHz and 1 Mhz, at which Cint should be less than 1/2  1.2 pF/cm or 4.6
pF for a L1 sensor. There are no bad channels (among half the pairs).
Interstrip Conductance of L1_010
70
60
50
Interstrip Conductance (pA/V)
40
30
20
SB
10
0
0
50
100
150
200
250
300
350
400
450
-10
-20
-30
strip num ber
Figure 8. Interstrip conductance between adjacent strips starting with an even number.
Rint
Figure 8 shows SB measurements of the interstrip conductance for even pairs of
strips on L1_010. KSU does these measurements on test structures. Between any pair of
strips the interstrip resistance must be greater than 2 GOhm. This limit corresponds to an
interstrip conductance of 500 pA/V. All even pairs of strips pass. The quantization of
the SB data is not currently understood and reflects a systematic error. Some negative
values of the conductance are observed, but all are much less than 500 pA/V in absolute
value. The average interstrip conductance of L1_010 is 14.8 pA/V. This average
corresponds to interstrip resistance 68 GOhm.
Summary of Bad Channels
Sensor
strip
L2_014
L1_010
0
80
239
240
241
242
243
247
316
376
380
383
Kansas State
Stony Brook
none
none
OK
open
pinhole, short
pinhole, short
pinhole, short
pinhole, short
pinhole, short
pinhole, short
OK
open
OK
OK
“pinhole”
OK
pinhole, open
pinhole, open
pinhole, open
pinhole, open
pinhole, open
pinhole, short
Rpoly high
open
short
“pinhole”
SB and KSU agree that L2_014 is perfect.
For L1_010:
SB saw 6 pinholes which KSU confirmed.
SB saw one open (without pinhole) which KSU confirmed.
SB saw 2 “pinholes” which KSU did not confirm.
SB saw one short which KSU did not confirm.
SB saw one Rpoly high which KSU did not confirm.
KSU saw one open which SB did not confirm.
In the future both labs plan to remeasure all bad channels, to confirm the measurements.