DØ-note 4308
March 3, 2003
Characteristics of the Outer Layer Silicon Sensors
for the Run IIb Silicon Detector
M. Demarteau1, R. Demina2, S. Korjenevski2, F. Lehner3, R. Lipton1, H.S. Mao1, R.
McCarthy4, R. Smith1
1) Fermilab, Batavia, USA
2) Kansas State University, Manhattan, USA
3) University of Zurich, Switzerland
4) State University of New York at Stony Brook, USA
Abstract
The characteristics of the first received prototype Hamamatsu silicon sensors for the outer
layers for the Run IIb DØ silicon detector are described. The results of the electrical and
mechanical characterizations indicate that the overall sensor quality is excellent. The
sensors behave in all aspects very satisfactory and our specifications are well matched.
1
1. Introduction
Extended running of the Tevatron at a center of mass energy approaching 2 TeV provides
an enormous physics potential for the collider detectors. Our current theoretical prejudice
indicates that the ability to identify heavy quarks in the final state is absolutely necessary
to resolve these new physics channels. A silicon strip detector is currently the tool most
suited to identify heavy flavors in proton-antiproton collisions. It is anticipated that the
current silicon detector of the DØ experiment will start performing marginally at an
integrated luminosity of 4 fb-1 due to radiation damage and low signal to noise ratio. It is
for these reasons that the collaboration has designed a new silicon tracker [1]. The silicon
tracking system will be a six layer device, divided into two radial groups. The inner two
layers, covering a radius between ~18 mm and ~35 mm, will have axial readout only.
These layers have a significantly reduced radius with respect to the current tracker. The
outer group is comprised of layers 2 through 5. The basic building block of the outer
layers is a stave. A stave is a two layer structure of silicon sensors. Silicon sensors are
mounted on each side of a rohacell stave core, which has embedded cooling channels.
One side of the stave will have axial readout, the other side stereo readout. The stereo
angle is obtained by rotating the sensor. Each side of the stave is in turn populated with
two readout modules. Closest to the interaction point is a 10-10 module, with two
independent ~10 cm long readout sections. Next to it is mounted a 20-20 module, with
two independent ~20 cm long readout sections. Each side of the stave thus has six silicon
sensors for a total of twelve sensors per stave. Our design calls for 168 staves, for a total
of 2016 outer layer sensors. This note described the results of the tests mainly done at
Fermilab and KSU on a set of prototype outer layer sensors.
2. Sensor Specifications
All silicon detectors are p+n type single sided sensors, with AC coupling and biased
through poly-silicon resistors and need to withstand a dose of 2 x 1013 1 MeV equivalent
neutrons per cm2. Only one vendor is being considered for the outer layer sensors,
Hamamatsu Photonics. The silicon sensors will have a single-guard ring with peripheral
n-well as designed by Hamamatsu in order to improve the high voltage stability after
irradiation. The detailed specifications for the sensors can be found in [2]. Table 1
summarizes some of the main sensor characteristics. A detailed mechanical drawing of
the sensors, drawing number 3823.210-ME-399565, on which the location of strips,
fiducial marks, bonding and testing pads as well as the strip numbering definition and
other features are indicated, is available and was provided to Hamamatsu (see Fig. 1).
The sensor also has a 24-field scratch pad for unique identification.
2
Specifications:
Wafer thickness
Depletion voltage
Leakage current
Junction breakdown
Implant width
Al width
Al strip resistivity
Coupling capacitance
Coupling capacitor
breakdown
Interstrip capacitance
Polysilicon bias resistor
Not working strips
Active Length (mm)
Active Width (mm)
Cut Length (mm)
Cut Width (mm)
Strip Pitch (m)
Readout Pitch (m)
# of Readout strips
Not working strips
Layer 2-5
32020m, wafer warp less
than m
V<300V
<100nA/cm2 at RT and
FDV+10%V, total current <
16A at 350V
>350V
8m
2-3 m overhanging metal
< 20 /cm
>12pF/cm
>100V
<1.2pF/cm
0.8  0.3 M
<1%
98.33
38.34
100.00
40.34
30
60
639
<1%
Table 1: Specifications for outer layer sensors
3. Prototype Sensors and Test Structures
In July 2002 a set of 100 prototype sensors were ordered from Hamamatsu Photonics,
which were shipped to Fermilab on November 29, 2002. The specifications of the sensors
are detailed in reference 1. In Table 1 a summary of the specifications for the outer layer
sensors is given. The sensors are produced on 6” wafers. Figure 2 shows a layout of the
wafer. The sensors received have two different lot numbers, indicating that they were
produced in two different batches. If the serial numbers on the sensors are an indication
of the processing yield at the company, the yield is about 70%. In addition to the sensors,
test structures are an integral part of the wafer. There are four separate test structures on a
wafer of which we receive one of the two adjacent to the long side of the sensor. They are
indicated by the serial numbers of the two sensors on the wafer. The test structure
contains two ‘baby-sensors’. One is identical to the full-size sensor and is different only
in the fact that it has eight readout strips. The other baby-sensor has no poly-silicon
resistors. Adjacent to the baby-sensors are four implants with aluminization with separate
contacts for each. As per our specifications, there are also silicon diodes on the test
3
Figure 1: Detailed layout of outer layer sensor
4
Figure 2: Layout of 6" Hamamatsu wafer
structure, with and without guard ring structure. One area on the test structure contains a
field MOS structure for flatband voltage measurements, and monitors for the implant
resistance and coupling capacitance. The remaining features on the test structure are
arrays of mainly poly-silicon resistors.
The characterization of the sensors is divided into three distinct parts. In the next section
the measurements on the test structures will be described. The section following it is the
main section, describing the measurements performed on the sensors themselves. All
measurements follow the procedures as outlined in reference [3]. The note concludes
with a summary of the mechanical properties of the sensors. Both the test structures and
sensors were also irradiated and characterized after irradiation. Those results are
described in an accompanying note [4].
4. Electrical Characterization of the Test Structures
Two test structures, 35/36 and 65/66, were measured at Fermilab and test structures
67/68, 69/70 and 73/74 were measured at KSU. Four parameters are determined from
measurements on the test structures:
i.
Coupling Capacitance
5
ii.
Implant resistance
iii.
Aluminum strip resistance
iv.
Poly-silicon resistance
v.
Breakdown voltage of the coupling capacitor
4.1. Coupling Capacitance and Coupling Capacitor Breakdown
On the test structure there is a series of four strips consisting of the p+ implant only, the
coupling capacitor and the aluminization. Pads are connected to the implant (DC-pad)
and the aluminum strip (AC-pad) at each end of the strip (see Figure 3). To facilitate the
measurement, the corresponding pads of two adjacent strips are wirebonded at the far end
of the strip. Using this configuration, the coupling capacitance of the strip, the implant
resistance and the aluminum strip resistance were measured.
Figure 3: Implant and Coupling Capacitor only structure on the Test Structure
The capacitance of the coupling capacitor for two sets of two strips was measured to be
93 pF at a frequency of 1 kHz. This gives a value of 9.5 pF/cm. This is still large enough
compared to the interstrip capacitance (see section 5.6), so that only a small capacitive
crosstalk is expected.
Several strips were selected on the baby sensor on the various test structures and a
voltage applied across the coupling capacitor by placing a probe on the DC-pad and the
AC-pad. Breakdown of the capacitor is defined as the voltage when the current reaches
100nA. Figure 4 shows the dependence of the current versus voltage for a few channels
6
on test structures 35/36 and 65/66. Test structure 65/66 breaks down at a slightly lower
voltage than test structure 35/36, but both are well above our specified breakdown value
of 100V.
Ccc Breakdown Voltage (Test Structures)
Current (nA)
4500
35/36, Ch. 8
4000
35/36, Ch. 5
3500
35/36, Ch. 3
35/36, Ch. 2
3000
65/66, Ch. 1
2500
65/66, Ch. 2
2000
65/66, Ch. 3
65/66, Ch. 6
1500
65/66, Ch. 7
1000
500
0
0
50
100
150
200
250
300
Voltage (V)
Figure 4: IV-Curve for measurement of Ccc breakdown
4.2. Implant Resistance
The implant resistance was measured by applying a voltage differential between two
neighboring DC pads and mapping the current versus voltage. Sets of two strips were
measured on test structure 35/36 and give an implant resistance of 104 k/cm. Figure 5
shows the measurement for the second set of strips. The implant resistance is not part of
our specifications, but the measured value agrees with our expectations of a typical p+
doping.
7
Implant Resistance Measurement
Voltage (V)
1.5
1
0.5
0
-0.5
-0.3
-0.1
0.1
0.3
0.5
-0.5
-1
-1.5
I ( A)
Figure 5: Implant resistance measurement
4.3. Aluminum Strip Resistance
The aluminum strip resistance was measured by applying a voltage differential between
two neighboring AC pads and mapping the current versus voltage. Sets of two strips were
measured on test structure 35/36 and give a resistance for the aluminization of 16 /cm,
well within our specification which requires a resistivity of less than 20 /cm. Figure 6
shows the measurement for the set consisting of strips 6 and 8. The measurement on all
other strips show identical results. The measured aluminum resistance corresponds to an
aluminum thickness of about 1.5m. The introduced ENC noise of a total series
resistance of around 160  for a strip length of 10cm in front of the preamplifier amounts
to a noise contribution less than 200e. It can therefore be neglected. If the strip length is
20cm, as is the case for the 20-20 modules, the ENC noise is less than 500e.
8
Aluminum Strip Resistance
Voltage (V)
1.5
Test Structure 35/36, Ch. 6-8
1
0.5
0
-0.004
-0.002
0
0.002
0.004
-0.5
-1
-1.5
I (A)
Figure 6: Measurement of aluminum strip resistivity
4.4. Polysilicon Resistance
4.4.1. Strip Poly-silicon Resistance
The resistance of the poly-silicon resistor was measured on the baby sensor. The baby
sensor has exactly the same layout as the full strip detector. Also on the baby sensor there
are intermediate and readout strips. Simply because of space constraints, the poly-silicon
resistors cannot all reside on one end. The resistors alternate between strips.
Consequently, the poly-silicon resistors for the intermediate strips are all at one end, and
the resistors for the readout strip are all at the other end. The poly-silicon resistor value
has been measured on the intermediate strips, by placing the probe on the DC-pad and the
bias line and applying a negative voltage at the DC pad. Figure 7 shows the I-V curve as
measured for the first channel on test structure 35/36. It yields a poly-silicon resistor
value of 0.8 M. This measurement has been repeated for more channels and test
structures and they all yield the same result within an error of 0.1 M.
Performing the same measurement on the adjacent strip, the readout strip, still provides
valuable information. Placing the probe on the DC pad measures the implant and polysilicon resistance in series. The average value of the resistance was then determined to be
2.4Mand hence rather consistent with our previous findings of polysilicon and implant
resistors in series.
9
Rpoly Measurement on Test Structure
0
-3.0
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
Voltage (v)
-0.5
-1
-1.5
-2
-2.5
Current ( A)
Figure 7: Measurement of the poly-silicon resistance for the first intermediate strip on
test structure 35/36
4.4.2. Monitor Polysilicon Resistors
The test structure also contains various arrays of poly-silicon resistors for monitoring
purposes. These arrays are labeled ‘PSxx’, with xx a numeric identifier. The resistors
labeled ‘ps20’ look identical to the strip poly-silicon resistors and were measured. Figure
8 shows one of the measurements. The measured resistance is 0.88 M.
Resistance Measurement 'PS20'
0.6
Voltage (V)
0.4
0.2
0
-0.7
-0.5
-0.3
-0.1
0.1
0.3
0.5
0.7
-0.2
-0.4
-0.6
Current ( A)
Figure 8: Measurement of resistance of monitoring resistor 'PS20' on test structure
10
5. Electrical Characterization of the Sensors
Hamamatsu delivered 100 prototype sensors to Fermilab early in December 2002. The
sensors were probed both at Fermilab and KSU. The following sets of measurements are
performed on the sensors:
i.
I-V curve, to measure total detector leakage current as function of bias voltage up
to 700V.
ii.
Long-term stability of detector total leakage current
iii.
C-V curve, to measure the total detector capacitance as function of bias voltage.
The depletion voltage is extracted from this measurement
iv.
AC-scan, to measure the coupling capacitance and the current of individual strips.
Sensor defects, most commonly shorted strips and pinholes, are identified with
this measurement.
v.
DC-scan, to measure the leakage current of individual strips. Sensor defects, most
commonly leaky strips, are identified with this measurement.
vi.
Interstrip isolation by measuring the interstrip resistance and measurement of
resistance of bias resistor
vii.
Interstrip capacitance and total load capacitance
Not all sensors undergo the full series of tests. Our quality assurance program calls for
every sensor to undergo an IV-scan and a CV-scan. Due to scheduling pressures, at the
time of writing this note, not all sensors have been submitted to an IV- and CV test yet.
The emphasis was placed on mapping out a few sensors in detail. Table 3 shows a
summary of which sensors were measured to date (see also section 5.7). Some of the
sensors tested were irradiated and the results are described in reference [4]. Some other
sensors were used to build detector modules. The results of the module readout noise
tests are described in an accompanying note as well [5].
5.1. Characterization at the Manufacturer
The contract with the vendor calls for an electrical characterization of the sensors at the
company. Each sensor should undergo:
o I-V curve to 500 V at a temperature of 25  3C and a relative humidity of <50%
o Optical inspection for defects, opens, shorts and defects, and verification of mask
alignment to better than 2.5 m
o Depletion voltage as determined from the C-V method
o
AC capacitance value measurement and pinhole determination
11
In addition, the manufacturer is asked to verify the poly-silicon resistor value, implant
resistivity, coupling capacitor value and its breakdown as well as the aluminium
resistance on the test structures. The vendor provides these results as average value per
delivered batch (the 100 sensors came in two batches: SWA61737 and SWA61738). Our
measured results on the teststructures as they are described in section 4 of this document,
are in good agreement with Hamamatsu’s findings on the aforementioned quantities.
Figure 9 shows, for completeness, the IV-curves for all 100 sensors as measured by
Hamamatsu. There is one point that is off-scale. Sensor 103 has a leakage current of
731nA at 500V bias voltage. Although not visible in this graph, there is a slight increase
in the leakage current for a fraction of sensors around 200 Volts. We will come back to
this later.
Total Leakage Current, Hamamatsu
500
I (nA)
400
300
200
100
0
0
100
200
300
400
500
Bias Voltage (V)
Figure 9: Total detector leakage current as function of bias voltage as measured by
Hamamatsu for all hundred prototype sensors
All sensors have a depletion voltage between 110-130V, with a granularity of 10V. HPK
measures the C-V dependence in steps of 10 Volts and defines the full depletion voltage
as the lowest voltage where the increase in 1/C2 is found to be less than 2% [6]. This
method tends to overestimate the depletion voltage.
Table 2 lists all the sensor defects as noted by Hamamatsu. Only three sensors out of 100
(!) are flagged as having a defect. Sensor 51 and 65 have one and two occurrences of
shorted aluminum strips, respectively. Sensor 108 has two shorted coupling capacitors.
All three sensors were visually inspected and the shorts confirmed.
12
Lot No.
Serial No.
SWA61737
51
SWA61737
65
SWA61738
108
Ch. No.
Type
AC-AL short 20-21
AC-AL short
52-53
AC-AL short
39-40
Coupling short
94
Coupling short
95
Table 2: The defects on the three sensors as noted by Hamamatsu
5.2. IV-Scan
All sensors that were tested have excellent detector leakage current. No sensor exceeded
the specification of a total detector current in excess of 16A at 350V. In retrospect, it
seems that this criterion was too loose and should be significantly tightened for the
production order. Figure 10 shows the difference in total detector leakage current as
measured at Hamamatsu and Fermilab for a subset of sensors. The difference is nearly
always less than 50 nA, and independent of bias voltage. The offset in current can easily
be attributed to the different environmental conditions for the measurements at the two
different locations. The slight increase in current at 200V and 400V for some sensors is
only present in the Hamamatsu data. Its origin is not clear. The measurements at KSU are
done at a lower temperature, typically 1oC lower.
HPK - FNAL Measurements
100
19
20
21
47
48
51
88
131
75
D Idet (nA)
50
25
0
-25
-50
-75
-100
0V
100V
200V
300V
400V
500V
Vbias (V)
Figure 10: Difference in leakage current for a subset of sensors as measured at
Hamamatsu and Fermilab
13
The breakdown voltage is well above the specified 350V for all sensors. Figure 11 shows
the detector currents up to a bias voltage of 700V. Most detectors show a gradual increase
incurrent in the vicinity of 600V.
Leakage Current
30000
19
20
25000
21
I (nA)
20000
47
48
15000
51
88
10000
131
5000
0
0
100
200
300
400
500
600
700
800
Vbias (V)
Figure 11: Detector currents for a subset of sensors
5.3. Long Term Stability
A long term test facility is available at Fermilab to monitor sensor behavior under bias for
extended periods. The facility consists of a light and gas tight box which can
accommodate up to six sensors in aluminum holders and a PC/Labview based readout
system shown in figure 12. Data for sensor currents, box temperature and humidity are
recorded at set intervals, typically every five minutes, during the burn-in period.
Figure 13 shows the results of a burn-in of 5 layer 2 prototype sensors over a period
spanning approximately six days. Detectors were biased to 150 V. After the first ten
points the sensors are in dry air with a dew point below –40C. Room temperatures
varied by 3.5C during the test and the currents are corrected to 20C. Some residual time
dependence is visible. These variations are most prominent during periods when the
temperatures change most rapidly. This residual variation appears to be due to a time lag
between the air temperature recorded by the PC and the actual sensor temperature.
However, no long-term drifts of the leakage currents have been observed and the sensors
turned out to exhibit a rather stable bias behavior.
14
Temp,
humidity
HP 3456
DVM
GPIB
Keithley 706 switch box
Keithley
485 pa
meter
Keithley
2410 ps
Test Box with detector diodes
PC
Labview
Figure 12: Setup for long term sensor current stability
Sensor Burn-In 2/23/2003 corrected to 20 deg C
9.00E-08
8.00E-08
7.00E-08
Current
6.00E-08
5.00E-08
L2-47
4.00E-08
L2-48
3.00E-08
lL2-51
2.00E-08
L2-20
L2-21
1.00E-08
0.00E+00
0.E+00
5.E+04
1.E+05
2.E+05
2.E+05
3.E+05
3.E+05
4.E+05
4.E+05
5.E+05
5.E+05
Elapsed Tim e (sec)
Figure 13: Six day burn-in results of five outer layer Hamamatsu sensors.
15
5.3. CV-Scan
Depletion Voltage HPK (V)
The CV-scan is used to determine the depletion voltage. The depletion voltage at our
testing centers is extracted as the intercept of two straight lines in a 1/C2 - Vbias plot. Both
Fermilab and KSU measure depletion voltage, which are consistently lower than
Hamamatsu, which is attributed to Hamamatsu’s different method. Figure 14a shows a
typical CV-curve for sensor 56. The depletion voltages as measured at the test centers
and by Hamamatsu are summarized and compared in figure 14b.
700
Sensor 56
1/C2 (1/pF)2
600
500
400
Vdep = 96V
300
200
100
140
130
120
110
100
90
80
80
90
100
110
0
0
50
100
150
200
250
300
350
400
450
Depletion Voltage CV Scan (V)
Vbias (V)
Figure 14: a) CV-curve for sensor 56, and determination of depletion voltage; b)
comparison of depletion voltage measured with the CV method and Hamamatsu
measurements; the line indicates one-to-one correspondence.
5.4. AC-Scan
The integrity of each strip is verified with the AC-scan. The measurement is performed
by probing the AC-pad and the bias ring. A backside voltage of 20V is applied. The scan
consists of two steps. First, a voltage of 80V is applied to the AC pad and the current
drawn by the strip is measured. In a second step the voltage is lowered to 0V and the
capacitance is measured at a low frequency either at 1kHz (KSU) or 10kHz (FNAL). The
capacitance represents the coupling capacitance of the silicon oxide layer. Figure 15
shows a typical AC scan taken at 10kHz. The coupling capacitors are lower due to the
frequency response of a low-pass filter consisting of coupling capacitor and implant
resistor. At 1kHz the measured capacitances approach their zero-frequency limit and the
so obtained values are then consistent with the teststructure measurements in section 4.
Figure 16 shows the spread of the measured coupling capacitors values for 10 sensors.
These data have been obtained at 1kHz and the mean value of the coupling capacitance is
in agreement with our specifications. The spread (RMS-value) over the strips and sensors
is less than 5%. The AC defects observed on three sensors by Hamamatsu could all be
reproduced.
16
50
5.0
45
4.5
40
4.0
35
3.5
30
3.0
25
2.5
20
2.0
15
1.5
10
1.0
5
0.5
0
0.0
0
200
400
I (nA)
C (pF)
AC-Scan Sensor 21 (10 kHz)
600
Channel #
Figure 15: AC-scan for sensor 21; the upper data set corresponds to the capacitance
measurement.
Figure 16: AC-scan for 10 sensors. The scans have been performed with the LCR meter
at 1kHz.
17
5.5. DC-Scan
A full DC-scan was also performed on most tested sensors, with the backside at full bias.
No anomalies were observed. Sometimes a measurement gave a very low current reading,
which normally could be attributed to a poor contact between the probe and the DC-pad.
A repeat of the measurement usually confirmed that it was just a poor contact. Figure 17
gives an example of a DC-scan.
DC-Scan, Sensor 28
ILeak (pA)
200
180
160
140
120
100
80
60
40
20
0
0
100
200
300
400
500
600
Channel #
Figure 17: DC-scan results on sensor 28.
5.6. Polysilicon bias and interstrip resistors
The DC-scans performed at KSU were also used to determine the polysilicon resistor
values on all readout strips by applying a small voltage of 1V across the DC-pad and the
bias rail while the backside was kept at a bias potential. Such a scan is sensitive to strip
regions on the sensor with low interstrip resistances or to bias resistor inhomogeneities.
Figure 18 shows the result of one scan performed on sensor 63. Since the polysilicon
resistors are at the opposite end of the DC-pads, an effective series resistance of
polysilicon and implant resistance is probed. The obtained values of 2.2-2.5 MOhm are in
good agreement to the teststructure observations of section 4. The obtained values of 2.22.4 MOhm are in good agreement to the teststructure observations of section 4.
Moreover, from the measurement of the strip resistances in Figure 18 we can conclude
that the interstrip resistance is high enough (in the order of O(Gohm)) so that the strips
are well isolated.
18
Figure 18: Measurement of the polysilicon resistors for sensor 63
5.7. Interstrip Capacitance
The interstrip capacitance was measured extensively on sensors 47 and 88. The procedure
as outlined in the Quality Assurance document [2] was followed. The setup with the
equivalent circuit diagram is shown in figure 18. Two measurements are made. The first
measurement involves a three probe setup. Probe 1 is placed on the AC pad of strip N;
Probe 2 and 3 are placed on the AC pads of the adjacent strips, strip N-1 and N+1,
respectively. The probes on strip N-1 and N+1 are tied together at the input of the LCR
meter. The capacitance measured this way is, to first order, the total load capacitance the
readout chip sees and is denoted by CL or C1. In a second measurement one of the probe
tips adjacent to the center probe tip, is lifted. The capacitance measured in the second
measurement is denoted C2. From the equivalent circuit diagram one finds that
CL = C1 = 2 Ci + C b
C2 = Ci + Cb .
Here, Cb refers to the capacitance of the strip to the backplane and Ci is the interstrip
capacitance. CL is to first order the total load capacitance on the readout chip. The second
measurement can of course be performed for both neighboring strips and the results
should be identical. In the results presented here, unless mentioned explicitly, the
capacitance C2 is the average capacitance measured for the pair (N, N+1) and the pair (N,
N-1):
C2 = ( C2 (N, N-1) + C2 (N, N+1) ) / 2 .
A few words about the limitations of the current setup are appropriate. The blocking
capacitors used in the setup were 0.1 F. The results obtained for frequencies below 1
kHz were therefore not very stable. If measurements below 1 kHz are presented it is
19
implicit that they are accompanied with rather large error bars. The capacitance of the
probes and leads was measured to be on average about 1 pF. In case ‘corrected’ results
are shown, the measured capacitance of the leads, dependent on frequency of course, was
subtracted from the measurement. Note that in the extraction of Ci no correction is
needed.
probe 2 on
AC pad
(strip N+1)
probe 1
Ext. LCRprobe 3 on adapter meter
AC pad (strip
N-1)
Cintrstr
Cs
Cs
Cs
Test chuck
Figure 19: Setup for interstrip capacitance measurements
Figure 20 shows the measurement of C1 as function of frequency for the triplet of strips
(109, 110, 111) on sensor 88. At a frequency of 1 MHz, the frequency of relevance for
operation with the SVX4 readout chip, the total capacitance is about 1 pF/cm.
20
Total Load Capacitance, Sensor #88
12.00
Strips 109-110-111
CL (pF)
10.00
8.00
6.00
4.00
2.00
0.00
1.0E+02
1.0E+03
1.0E+04
1.0E+05
1.0E+06
Frequency (Hz)
Figure 20: Frequency dependence of C1 (see text for details)
In figure 21 the frequency dependence of the capacitance on the backside bias voltage is
shown. An initial backside voltage of 5 Volts was applied; the voltage was then increased
to 10 Volts and then incremented in steps of 10 to 100 Volts. Figure 10 was taken at a
backside bias voltage of 110 Volts, which corresponds to full depletion voltage plus 10%.
Total Load Capacitance, Sensor #88
14.00
5V
10 V
12.00
20 V
30 V
C L (pF)
40 V
10.00
50 V
60 V
8.00
70 V
80 V
90 V
6.00
100 V
4.00
2.00
0.00
1.0E+02
1.0E+03
1.0E+04
1.0E+05
1.0E+06
Frequency (Hz)
21
Figure 21: Frequency dependence of C1 for various backside bias voltage settings
Total Load Capacitance, Sensor #88
12.00
CL (pF)
10.00
8.00
6.00
4.00
1 kHz
2.00
1 MHz
0.00
0
20
40
60
80
100
120
Backside Bias (V)
Figure 22: Dependence of C1 on backside bias voltage for 1 kHz and 1 Mhz
Figure 22 shows the dependence on the backside bias voltage for two frequencies. The
dependence on the backside bias voltage decreases for increasing frequency. Moreover,
there is a change in slope of the dependence on backside voltage at a frequency of about
5 kHz.
The measurement of C2 for adjacent strips are in very good agreement, justifying the
averaging of the two sets of strips to determine Ci and Cb. Figure 23 shows the relations
of C2 corresponding to figures 21 and 22. The dependence of the capacitance C2 on the
frequency and backside voltage is less than for the total capacitance.
C2, Sensor #88
C2 (pF)
10
8
5V
10 V
20 V
30 V
40 V
50 V
60 V
70 V
80 V
90 V
C2, Sensor #88
8
7
C2 (pF)
12
100 V
6
4
6
5
4
3
2
1 kHz
2
1
0
1.0E+02
1 MHz
0
1.0E+03
1.0E+04
1.0E+05
1.0E+06
Frequency (Hz)
0
20
40
60
80
100
120
Backside Bias (V)
Figure 23: Dependence of C2 on frequency and backside voltage
The measurements were repeated for three sets of triples, (109, 110, 111), (509, 510, 511)
and (519, 520, 521), on sensor 88 and three sets of triplets on sensor 47. The results are
all very consistent. Using the two equations given at the beginning of this section, the
interstrip capacitance and backplane capacitance are extracted. Figure 24 shows the
22
results for the three sets of strips measured on sensor 88. The points labeled ‘background’
in the lower graph of figure 24 indicates the capacitance of the leads in the setup and
ought to be subtracted from the plotted values to obtain the intrinsic backside
capacitance.
Interstrip Capacitance, Sensor 88
3.5
3.0
C i (pF)
2.5
2.0
1.5
1.0
Strips 109-110-111
Strips 509-510-511
Strips 519-520-521
0.5
0.0
1.0E+02
1.0E+03
1.0E+04
1.0E+05
1.0E+06
Frequency (Hz)
Backside Capacitance, sensor 88
6.00
5.00
C b (pF)
4.00
3.00
2.00
1.00
0.00
1.0E+02
Strips 109-110-111
Strips 509-510-511
Strips 519-520-521
Background
1.0E+03
1.0E+04
1.0E+05
1.0E+06
Frequency (Hz)
Figure 24: Interstrip and Backside capacitance as function of frequency
23
5.7. Summary
Table 3 gives a summary of the measurements. If a dash is shown in a column, it
indicates that the measurement was not performed for that particular sensor. The last
column indicates if that sensor has been used to build a module. Strip numbers refer to
strips that have been flagged as faulty by our testing procedures. The column labeled
HPK strip# indicates the strip number(s) identified by Hamamatsu as having a defect.
The type of defect was listed in Table 2. Two channels 290 and 299 in the AC-scan
column are labeled with an asterisk. This means that a potential defect on these strips has
to be confirmed, since the measured capacitance value is not conclusive and points to a
bad contact between probe tip and pad.
Sensor
number
Tested I-V scan I
C-V
Vdepl
DCscan
AC-scan
(nA) @ 350V
3
5
6
8
10
19
20
21
28
29
33
47
48
51
52
54
56
57
58
59
62
63
64
65
88
131
KSU
KSU
KSU
KSU
KSU
FNAL
FNAL
FNAL
FNAL
FNAL
FNAL
FNAL
FNAL
FNAL
KSU
KSU
KSU
KSU
KSU
KSU
KSU
KSU
KSU
KSU
FNAL
FNAL
31
34
34
38
88
174
132
128
---------134
117
136
102
71
78
43
35
38
56
56
78
116
204
120
105
106
106
105
106
100
100
87
98
100
87
87
95
95
101
104
96
96
95
85
96
100
100
98
98
90
0
0
0
3
--0
0
0
0
308
0
328,567
0
0
0
0
0
0
0
0
0
0
0
0
--0
0
1
290*
1
--335
0
0
214
285,286
0
0
0
20,21,52,53
0
0
299*
0
0
0
40,512
0
0
0
--0
Comment
HPK strip#
Module
20-20 stereo
20-20 axial
20-20 axial
20-20 axial
20,21,52,53
irradiated
irradiated
irradiated
irradiated
39,40
20-20 axial
10-10 stereo
10-10 stereo
10-10 axial
10-10 axial
20-20 stereo
20-20 stereo
20-20 stereo
Table 3: Summary of measurements.
24
Note also in table 3, that the KSU I-V measurements have been performed at a lower
temperature, so that in general the recorded leakage currents are somewhat smaller than
at FNAL.
All the four defects on sensor 51 which are claimed by HPK were found as well.
However, we did not detect two shorted strips flagged by HPK on sensor 65 for yet
unknown reasons. In addition we found out of 24 AC tested sensors nine additional bad
strips (the two channels with an asterisk are not included here) on seven sensors during
our AC-scans. These strips have not been discovered by HPK.
25
6. Mechanical Characterization of the Sensors
The layout of the sensor as specified in drawing number 3823.210-ME-399565, provides
a scratch pad field containing 6x4=24 pads. The vendor was asked to provide in three sets
of 4 scratch pads a unique serial number coding for the sensors, with the rightmost pad
representing the LSB. The remaining pads would be used for QC pass/fail marks. We
expected the vendor to provide a binary serial number in the scratch pads. Figure 25
shows the rightmost pads for sensor 37. Hamamatsu has opted to use a binary decimal
representation for the serial numbering, rather than a pure binary representation as we
expected. For the production order we expect the serial numbering to be a binary
representation and an additional footnote has been added to the drawing to that effect.
Figure 25: Hamamatsu decimal binary serial numbering for sensor 37.
A set of 10 sensors (serial numbers 66, 67, 69, 70, 71, 72, 73, 75, 76 and 77) was
measured on an Optical Gage Products (OGP) coordinate measuring machine to verify
the mechanical dimensions of the sensors. A reference system was established using the
same convention as in the drawing with a corner fiducial defining the origin. The x-axis
runs along the long side of the detector, parallel to the strip orientation; the y-axis is
along the short axis, perpendicular to the strip orientation. The flatness of the sensors was
measured by defining a grid of 11x11 points in x and y and measuring the z position of
the top surface of the sensor. The OGP has an intrinsic z-resolution of a few m. Figure
26 shows the measurements for sensor 75. The sensor is warped along both the short and
long axis. The difference between the minimum and maximum z-position on the sensor is
then determined (see figure 27). The average of the ‘highest’ and ‘lowest’ point on a
sensor is 67m, with a few sensors exceeding 70m. Our specifications call for a sensor
warp of less than 50m, agreed to by Hamamatsu only on a best effort basis. Although
the desired sensor warp is not obtained, the sensors are acceptable.
26
3D Graph L2-75
ta
Z Da
0.08
0.06
0.04
0.02
0.00
50
40
30
20
10
0
0
20
120
100
80
4060
ata
D
X
-10
Y Data
Figure 26: Flatness measurements for outer layer sensor 75. Units along all axes are
mm.
Figure 27: Maximum variance in z for a set of ten Hamamatsu outer layer sensors.
27
The flatness data is analyzed further to see if there is a difference in sensor warp along
either the x-, and y-axis. The sensor is divided into eleven slices along the x (y-) axis and
the z-coordinate is plotted as function of the y- (x-) position along the sensor. The upper
graphs in figure 28 show the measurements for sensor 73. The set of data points for each
slice is fitted to a parabola, which fits the data points very well, and the coefficient of the
quadratic term extracted. The lower two graphs in figure 28 show the values of the
quadratic terms as determined by the fit for the ten sensors measured. The average values
for the coefficient for the quadratic terms are -0.22 10-4 (mm-1) for the curvature along the
x-axis, and -0.28 10-4 (mm-1) for the y-axis. The sensors have a slightly stronger warp
along the short axis, perpendicular to the strips, than along the long axis.
Figure 28: The upper two plots show the sensor z-position as function of x and y for
eleven slices in y and x, respectively. The lower two graphs show the coefficient of the
quadratic term, indicative of the sensor warp along the long and short axis.
28
In addition to the flatness measurements, a set of ten measurements each were taken
along the two short edges of the sensor and a set of twenty measurements were taken for
each of the long edges of the sensor. A straight line was then fitted to the data points.
With this data four characteristics were verified: the sensor cut width and cut length, the
accuracy of the cut edges, and the parallelism of the corresponding cut edges. In the
determination of the cut width and length, the center point of the fitted line was used.
The maximum difference between two measurements along each of the four sides for the
ten sensors measured is shown in figure 29. It is obvious that the cut edges are extremely
accurate. The absolute x or y positions of all edge measurements for the four edges for all
ten sensors measured, are plotted in figure 30. There are in total 200 measurements for
each of the two long edges, and 100 measurements for each of the short edges.
Combining the measurements in x and y, gives an average cut width of the sensors of
40.348mm and an average cut length of 100.010mm, to be compared to the nominal
values of 40.34mm and 100.00mm, respectively. The angle between the lines fitted to the
cut edges averages 90.00 ± 0.002 degrees. All in all, the sensors are superior in all
mechanical aspects, except for the sensor warp.
X2m ax-X2m in (m m )
Y1m ax-Y1m in (m m )
0.0035
0.0025
0.003
0.0025
0.002
0.002
0.0015
0.0015
0.001
0.001
0.0005
0.0005
0
0
1
2
3
4
5
6
7
8
9 10
1
2
3
4
5
6
7
8
9 10
8
9 10
X4m ax-X4m in (m m )
Y3m ax-Y3m in (m m )
0.0025
0.0035
0.003
0.002
0.0025
0.002
0.0015
0.0015
0.001
0.001
0.0005
0.0005
0
0
1
2
3
4
5
6
7
8
9 10
1
2
3
4
5
6
7
Figure 29: Maximum difference between two measurements (in mm) along
one of the four sensor cut edges. Each graph corresponds to a different cut
edge. The horizontal axis denotes a sensor identifier. Y3 and X4 refer to the
lines going through the origin of the coordinate system.
29
Figure 30: Absolute coordinates of x or y positions along the cut edges for ten sensors
combined. Each graph corresponds to a different cut edge.
30
References
[1]
[2]
[3]
[4]
[5]
[6]
DØ Run IIb Silicon Detector Technical Design Report, Chapter 3: Silicon Sensors
Silicon Sensor Specifications for Layers 2-5, Version 3.0, April ‘02
Silicon Sensor quality assurance for the D0 RunIIb silicon detector: procedures
and equipment, A. Bean et al., to be submitted as DØ note
Radiation Characteristics of Hamamatsu Layer 2 Sensors for the DØ Run IIb
Upgrade, T. Bolton et al., to be submitted as DØ note.
Readout of Run IIb Outer Layer Silicon Sensors for the Run IIb Silicon Detector,
A. Nomerotski and E. van Toerne
K. Hara, Tsukuba University, private communication
All documentations are available on:
http://www.physik.unizh.ch/~lehnerf/dzero/run2b/prr/prr_l2.html
31