CDF/DOC/TRACKING/PUBLIC/6286
Draft v1
Jan.28, 2003

Characteristics of Prototype Silicon Sensors for Run2b (I)K. Hara and Y. Takei
University of Tsukuba, Tsukuba, Ibaraki, Japan

Abstract
We describe the characteristics of Hamamatsu silicon sensors prototyped for Run2b silicondetector. In the prototype program, 60 axial and 53 stereo sensors have been fabricated. The
electrical and mechanical characteristics are superior in all the aspects and fulfill our
specifications.

1. Introduction
The CDF collaboration is building a new silicon tracker system for Tevatron Collider Run2b,
replacing the present L00 and SVXII detectors. The new silicon system will be a six-layer device
located between 2.1 cm and 16 cm radius, consisting of ~2300 single sided silicon sensors. The
new silicon detector is expected to become ready in 2005.
The specifications of silicon sensors are described in detail in CDF 6283. Table 1 summarizes
the main specifications:ooooSensors should be operational up to 500V
Full depletion voltage should be in the range from 120 to 250V
Dead channel fraction should be less than 1%
Sensors should be uniform in coupling capacitance, bias resistance, interstrip resistance,and other electrical properties

Table 1: Main sensor specifications
Specifications:
Wafer
Full depletion voltage
Leakage current
Junction breakdown
Implant width
Al width
Coupling capacitance
Coupling capacitor breakdown
Interstrip capacitance
Polysilicon bias resistor
Defective strips
Readout strip pitch
Intermediate strip
Interstrip resistance

Inner Axial

Outer Axial/Outer Stereo

Orientation: <100> Thickness: 320±15µm Warp: less than 130µm
120<Vdep<250V<50nA/cm2 at RT and at 500V
>500V
7 µm
8 µm
1-3 µm overhanging metal
2-3 µm overhanging metal>12pF/cm>100V<1.2pF/cm
1.5±0.5 MΩ
<1%
50 µm
75µm /80µm
yes>1GΩ (after irradiation)
12. Production of Prototype Sensors
The prototype sensors, outer axial and outer stereo, were fabricated by Hamamatsu Photonics(HPK). One of the drawings prepared by HPK is attached below for the stereo sensor. The
number of readout strips is 512. Single un-read strip, intermediate strip, helps to enhance theposition resolution with use of charge division. The readout pitch is 75 µm (80 µm) for the axial(stereo) sensors. The outer dimensions are 40.55 mm (41.146 mm) in width and 96.392 mm inlength: two sensors are made on 6” wafer. We received 60 axial and 53 stereo sensors. While theaxial sensor production completed as scheduled, the stereo sensor production was delayed by 2-3
weeks due to trouble with a process line device. During the repair, the stereo wafers were
removed manually from the line, which created unusual stains on the sensor surface. The stereo
sensor yield was thus not as high as the axial sensor yield.

23. Electrical Characteristics
We have evaluated the following electrical characteristics. The measurement procedures aredescribed in detail in the accompanying note (CDF-6283). This note summarizes the results on
the following items:(1) I-V curve: Total leakage current is measured as a function of bias voltage up to 1000V at a
step of 10V.
(2) Stability of I-V curve: I-V curves are measured every 30 min at least for 10 times. The
bias voltage between the I-V measurements is kept at 200 V.
(3) C-V curve: Total capacitance is measured as a function of bias voltage. The curve is used
to extract the full depletion voltage.
(4) AC scan: Coupling capacitance, implant + bias resistance, and oxide punch-through at
100V are measured for each readout strip.
(5) DC scan: The leak current of individual strip is measured for sensors with large total
leakage. The bias is set above the micro-discharge onset voltage.
(6) Interstrip isolation: The interstrip resistance is measured with applying voltages (−1 to+1V) to the neighboring intermediate DC pads and measuring the current emerging fromthe readout DC pad.
(7) Interstrip capacitance: The capacitance between the neighboring AC pads is measured
with other strips floating to the ground.
(8) Long-term stability: The leakage current is read out periodically while keeping the bias
voltage at 500V.
In addition to the above, diagnostic measurements were carried out to identify the problems.
Examples of such diagnostic measurements are:(1) DC scan on intermediate strips: When micro-discharge was identified from I-V measurement,
we measured the individual strip current for the strips reported “Leaky” by HPK, or
performed the full DC scan. This includes the DC scan on intermediate strips. Note,otherwise intermediate strips are not tested in usual scans. We can safely ignore somewhat
leaky intermediate strips as far as the total leakage current is manageable.
(2) Visual inspection: When the implant strip is reported “Open” by HPK, we first look at AC
scan data (the coupling capacitance and series resistance are smaller if implant is open), thenthe strip is carefully inspected under microscope. If there is no clear open found, we measure
the resistance between the DC pad and the bias-ring (very large if the intermediate implant isopen), and other characteristics. We always inspect carefully under microscope when any
irregularities are recognized.

3.1 I-V curves
Figures 1a and 1b show I-V curves of 60 axial and 53 stereo sensors. Most of the sensors do notshow significant micro-discharge up to 1000V we measured. Exceptions are #010 and #048
stereo sensors, which showed micro-discharge at voltages above 300V-450V. As described in 3.5,
single leaky strip contributes to the micro-discharge for both sensors. The I-V curve of thesesensors became moderate by keeping the bias, as described in 3.2. We set our leakage current
requirement to be less than 2 µA at 500V. This limit ensures that no single readout strip has asignificant leakage current exceeding ~1µA. Actually, ISL sensors, which were double sided, had
an average leakage current roughly one order more than the present sensors, and substantial

3fraction had strip leakage current close to even a relaxed specification. The limit was set so thatthe noise performance is not degraded.
All the sensors fulfill our specification (S010 stereo is just on the boundary). Since the fractionof such micro-discharge sensors is small, two out of 113, we should avoid using such sensors as
far as the production delivery goes as scheduled.

Leak Current (uA)

Axial Prototypes (60)

T=28-29degC

10.00

1.00

0.10

0.01
0

200

400

600

Stereo Prototype (53)

800
1000
Bias Voltage (V)T=25-27degC

10.00

Leak Current (uA)

#10

#48

1.00

0.10

0.01
0

200

400

600

800

1000

Bias Voltage (V)Fig. 1a (top) I-V curves of 60 axial sensors.
Fig. 1b (above) I-V curves of 53 stereo sensors.

4

3.2 Stability of I-V curves
SVX2b Stereo Prototype 045

Leak Current (µA）

1:17:02

1.E-01

1:51:28
2:26:00
3:00:32
3:35:04
4:09:35
4:44:07
5:18:39
5:53:11
6:27:43

1.E-02
0

200

400

600

800

1000

Ｂｉ
ａｓVoltage (V)Fig. 2a Typical I-V stability of sensor S045
(stereo).
16:55
17:30

SV X2b A xial Prototype 032

18:05
18:40

10.00

19:15

Vbias=200V while not ramping

19:50
20:24

Leak Current (µA)

The I-V measurement was repeated 20-30
times (10 times for some sensors) for
sampled sensors. All the sensors whichshowed micro-discharges were tested.
Between the I-V measurements, the sensors
are biased to 200V and kept for 30 minutes.
Figure 2a overlays I-V curves measured for atypical sensor, stereo #045. The curves are
consistent and stable although the initial I-V
tends to be somewhat different. This may be
attributed to that the sensor surface condition
has stabilized in time between.
Among the 7 axial sensors we measured, 6sensors showed similar to the figure. This
statistics includes one sensor measured by
Purdue Group.
One sensor (axial A032: see Fig. 2b)
showed initially good I-V. The I-V curves
became in chaos after 7hr and the sensor
drew a large current at 800V after 10 hr. We
investigated the cause with an IR camera andfound a deep trace of discharge on the bias ring. We suspect that a discharge occurred
between the bias-ring (which is DC
connected to a p+ implant underneath) andthe substrate probably via defect in the wafer.
The I-V of this sensor has not recovered
indicating creation of permanent junction
breakdown.
For stereo sensors, we measured I-Vstability for 10 sensors. Sensor S010 (see Fig.
2c), which showed micro-discharge in the

1.E+00

20:59

1.00

21:34
22:09
22:44
23:19
23:54

0.10

0:29
1:03
1:38
2:13
2:48

0.01
0

200

400

600

800

1000

Bias V oltage (V )Fig. 2b Faulty I-V stability of sensor A032 (axial).
23:59
0:33

0:34
1:38
2:43
3:48
4:53
5:58
7:02
8:07
9:12:34

Leak Current (μA）

100.00

10.00

1.00
0.10

1:08
1:43

100.00

2:16
2:50
3:24

Leak Current (µA）

S V X 2b S tereo P rototype 010

SVX2b Stereo Prototype 048

10.00

3:58
4:32
5:06
5:40

1.00

6:14
6:49
7:23

0.10

7:57
8:30
9:04

0.01

0.01
0

200

400

600

800

1000

Ｂｉ
ａｓV oltage (V )Fig. 2c I-V stability of sensor S010 (stereo).

9:38

0

200

400

600

800

1000

Ｂｉ
ａ
ｓVoltage (V)Fig. 2d I-V stability of sensor S048 (stereo).
5

10:12
10:47

initial I-V, improved the I-V curve gradually. The micro-discharge onset voltage of Sensor #048
(see Fig. 2d) was fluctuating around 800V. The I-V became flat to 1000V in the last threemeasurements. Note that the initial I-V is already better than the one shown in Fig. 1b because thesensor was biased for other test before this stability test. The rest of eight sensors showed stable

I-V curves up to 1000V.
3.3 C-V curves
The C-V curves are measured for 19
axial and 53 stereo sensors at an LCR
frequency of 400 Hz. We extract the1/sqrt(150)full depletion voltage as the interceptof two straight lines in a C-V-1/2 plot
2000
(see Fig. 3a). HPK measures C-V at
a voltage step of 10V. The data isused to evaluate the “full depletion
voltage”, which is defined to be the1500
lowest voltage where the increase of
1/C2 is found to be less than 2%.
1/sqrt(132)stereo 001
There is a tendency that HPK gives
1000
somewhat larger voltage, as can be
0.05
0.07
0.09
0.11
0.13
0.15
0.17
seen in Fig. 3b. The distributions of
Tsukuba data (axial and stereo
1/sqrt(Voltage)
separately) and of HPK are given in
Fig. 3a C-V curve and evaluation of “the full depletion
Fig. 3c. The spread of thevoltage”. HPK determines (see text) it to be 150V, whereas
distributions are similar, indicating
we obtain 132V for this sensor, stereo #001.
that the full depletion voltages are
extracted consistently apart from theabsolute value. As a matter of fact, the value 2% is an arbitral choice so that no fitting isnecessarily performed. For a choice of 4%, the agreement between the two should be improved.
Three sensors have failed in the full depletion voltage specification, but all of them are more than
115V.
Purdue Group has performed C-V measurement for four other axial sensors. The fitted full
depletion voltages are larger than HPK values by 18-27 V, consistent to the present result.

Total Capacitance (pF)

2500

50
Number of Sensors

Full Depletion Voltage (HPK

180

160

140

a x ia l

120

72 sensors
40
stereo

30

axial
HPK

20

10

s te r e o

0

100
100

120

140

160

Fu ll De p e tio n l V o lta g e ( Ts u ku b a )Fig. 3b Correlation of full depletion voltages
defined by HPK and Tsukuba.

180

100

120

140

160

180

200

Full Depletion Voltage (V)Fig. 3c Full depletion voltage distributions for axial
and stereo sensors (Tsukuba) and for all (HPK).
6

3.4 AC scanfrequency = 400 Hz

S068
250

250

A007
Ccp (pF) / R (×10kΩ）/Icp (×nA)

Ccp (pF) /R (×10kΩ）/Icp (×0.1nA)

Ccp (pF) /R (×10kΩ）/Icp (×0.1nA)

The strip integrity is intensively
evaluated by AC scan where the200
measurement is performed in two
steps. Firstly, capacitance and series
resistance are measured by probing
150
between the AC pad and bias-ring
setting the LCR at Cs-Rs mode.
100
Secondly, 100V is applied across for
1 second and then the leakage current
through is measured. The capacitance50
represents the oxide coupling
Ccp
capacitance, and the resistance theRsum of the bias resistance and0-256 -192 -128 -64
064
128 192 256 Icp
implant electrode resistance. The
latter contributes approximately 0.3strip
0.5 MΩ.
Fig. 4a Example of AC scan result (Stereo S068).
An example is shown in Fig. 4a,
where the AC scan results are shownfor stereo S068. Nominal values are about 120 pF (Ccp: coupling capacitance), 1.85 MΩ (R:series resistance), and less than 10 nA (Icp: leakage) for the strips with full length. As shown in
the figure, coupling capacitance and R are smaller for shorter strips. Note that the 1st data point of
R is small due to the frequency setting (1 kHz) is not at the optimum. Most part of the measured
leakage Icp is actually through relays used in the measurement system. The leakage throughgenuine oxide layer is much smaller. If the oxide is completely punch-through, protection
resistors in the system determine the leakage to ~9µA.
Some example results are shown in Fig.4b, where we found some defective strips. In total, ACscan is made for 18 axial sensors and 18 stereo sensors. The comparison of dead channels
reported by HPK is described later.
Among the defective strips, there are a couple of cases where Icp is normal but R and Ccp are
irregular. An example of such AC scans is given in Fig. 4c, where RD24 (-24 in the plot) has

200
150
100
50
0
-256 -192 -128 -64

0

64

128

192

256

250

A054

200
150
100
50
0
-256 -192 -128 -64

strip

0

64

128

192

256

strip

Fig. 4b AC scan result for (left) axial A007 and (right) axial A054

7

Fig. 4d AC scan for stereo S053. One strip(-24) shows irregular R and Ccp values. A
clear break was found in this implant strip,
while this defect was not recognized by
HPK.

Ccp (pF) /R (×10kΩ）/Icp (×0.1nA)

smaller R and Ccp values but Icp isnormal. In this case, we found a clear
break in the implant. HPK reports no
defect with this strip. Later, we
describe this discrepancy further indetail.

250

S053

200
150
100
50
0
-256 -192 -128

-64

0

64

128

192

256

strip

3.5 DC scan

Leakagae Current (uA)

Stereo Prototype 010 (Vb=400V )DC scan was made only for
1.E+00
leaky sensors. An example result
for stereo sensor S010 is given in
Fig. 5, where the total leakage
1.E-01current evolution during themeasurement is also plotted. The
1.E-02bias is set to 400 V, above the
Totalmicro-discharge onset voltage.
Individual
1.E-03
The total leakage decreases as
the measurement proceeds (scansfrom –256 to 256), as we have
1.E-04seen in I-V stability test. Since
the total leakage current is about
1.E-05
0.1-0.2 µA for good sensors, the-256 -192 -128
-64
0
64
128
192
256
excess of this sensor is initially
Readout strip #about 1 µA and finally 0.1 µA.
The excess is totally explained
Fig. 5 DC scan (stereo S010) measured at 400V
by a single leaky strip, U225.
This strip is classified as dead (Leaky) by HPK. Another leaky sensor (stereo S048) has also
single leaky intermediate strip, which is identified by HPK.

3.6 Interstrip Resistance
The interstrip resistance is evaluated by measuring the increase/decrease in the strip leakage
current when DC voltages ( ± 1,±0.5 V) are applied to the intermediate strips at neighbor. The four
resistance values so obtained are averaged to represent the interstrip resistance. Three axial
sensors have been measured. Two distributions are shown in Fig. 6. Although the characteristics
look different, the averages are 50-200 GΩ and no strip is smaller than 1GΩ, our specification.
Another sample (Axial A016) showed a distribution very similar to A026 Sensor. A001 wasmeasured right after the delivery (i.e. production) and other two were measured about one month
later. The difference could be instrumental, but we suspect that A001 is exceptional“remembering” the process conditions and usual sensors should look like S026 after conditioned.
Unfortunately this hypothesis can not be checked, since A001 is irradiated with neutrons.

8

A001

Interstrip Resistance (GΩ)

1000

100

100

10

10

1
-256 -192 -128 -64

A026

1000

0

64

128 192 256

1-256 -192 -128 -64

0

64

128

192 256

strip umber

strip number

Fig. 6 Interstrip resistance of (left) A001 and (right) A026 Sensors.

3.7 Interstrip Capacitance

006 Stereo

frequency = 1 MHz

Cint (pF)

The interstrip capacitance is measured
5by probing neighboring (readout) AC
4
pads with other strip at floating. The bias
is set to 200 V and the LCR frequency at
3
1 MHz. The interstrip capacitance isquite uniform representing the strip
2
length (See Fig 7a). Note that theabsolute values are different by 0.3 pF,
1
which should be attributed to thecalibration accuracy of our system.
0
We have measured the interstrip-256 -192 -128 -64
064 128 192 256
capacitance of in total 9 axial and 2
strip
stereo sensors. Although most of theFig. 7b Interstrip capacitance: Sensor S006
sensors showed distributions quite(stereo) with an irregular value.
similar to those in Fig. 7a, 1 axial and 1
stereo sensors showed irregular values,
one pairs for each. An example is given in Fig. 7b. In fact the irregularity for this sensor did not

A010

frequency = 1MHz

5

4

4

3

3

Cint (pF)

Cint (pF)

5

2

1

0
-256 -192 -128 -640-256 -192 -128

strip

64

128 192256

frequency = 1 MHz

2

1

0

S053

-64

0

64

128

192

256

strip

Fig. 7a Interstrip capacitance for (left) axial 010 Sensor and (right) Stereo 053 Sensor.
9present at the first measurement, but it appeared. In the time between the probe scratched out theAC pad by miss-operation. Similar thing may have happened to other sensor, and we suspectthese irregularities are created during the testing at Tsukuba. There is no other problem found forthese strips by visual inspection under microscope.

3.8 Long-term stability test
The stability of leakage current is tested as a part of the electrical characterization. The
measurement is still on going, and the results are presented in a separate note.

3.9 Visual Inspections
The delivered sensors have beenvisually inspected under microscope.
Also, faulty strips reported by HPK andfound by our scan are inspected more
carefully.
The axial sensors are in general clean
and we did not recognize major flaws. As
described in Section 2, though, the stereo
sensors showed visible problems. Figure
8a is a typical picture found for about 1/3
of delivered stereo sensors. They areclassified as Class B, since visualinspection does not pass the criteria.
Enlarged view (Fig.8a right) shows thatthe black stains are on the very surface of Fig. 8a Example of stains observed for stereo sensors.
Al traces. As explained before, these (Right) Enlarged view.
were created when the wafers were takenout manually from the process line,
which is not expected to happen in the
production. We checked whether there
are any correlations between faulty strips
and the stains, and between magnitude ofstains and shape of I-V curves, but there
are no apparent correlations.
A yellow spot shown in Fig. 8b isfound only for S010 sensor. Stereo S010
sensor showed micro-discharge, but thecontributing strip is not the one in thisyellow spot.
Fig. 8b Yellow spot found for Stereo 010 Sensor.
From these observations, we conclude
that these sensors are good in electricalperformance. We classify them as ClassB (do not use them, unless delivery becomes a major problem for completion of the project).
Figure 8c shows examples of implant opens both for intermediate strips. The open for A011
sensor is exceptional, and all the other opens look similar to the one found for Axial 010 Sensor(intermediate strip IU125).

10

A011-IU183

A010-IU125

Fig. 8c Examples of implant opens. (Left) Axial 011 Sensor and (Right) Axial 010 Sensor

Some opens are found in the readout
implant, one example being shown in Fig.
8d. This is a special case that the break islocated underneath the Al bonding pad.
Although AC scan at Tsukuba first detected
the irregularity of this strip, we failed tofind this open under microscope. Thissensor was then sent back to HPK forinvestigation, where this open was
identified. This lessons us that very carefulinspection is required to find the implant
open which is partially hidden by Al
electrode on top.
Fig. 8d Readout implant open found underneath theAC pad.

11

4. Summary of the QA Measurements and Comparison with Manufacturer’s Data
4.1

Axial Prototypes

Out of 60 delivered sensors, 18 are studied in detail the characteristics described so far. Sensors
with dead channels and worse I-V characteristics are preferentially selected. Table 2 summarizes
the dead channels compared between HPK and Tsukuba measurements.
Table 2. Dead channels of axial prototypes compared between HPK and Tsukuba measurements. For
disagreements, channels with clear defects are in red.

sensor
1
2
5
7
10

11
12
16

HPK





IU244 (Implant open)
IU241 (Implant open)
IU126 (Implant open)
IU183 (Implant open)





RU62
RD252
RD251
IU244
IU241
IU126
IU183
RU240




(Ccp=93pF, R=1.77MΩ,Implant open)(R=1.1MΩ)(R=1.1MΩ) trace of small discharge(R=large)(R=large)(Implant open)(Implant open)(Ccp=1.07nF, R=2.94MΩ, Icp=8.1uA )


RD231

(167pF, 1.45MΩ6uA)

RD217

RD242

RD241

RD240
RD200
RD194

RD116




ID237, ID240 (Leaky strip)
RD256
RD249, RD250 (Leaky strip) RD249
RD248

RD244






ID229, ID230, RD230 (Badisolation)
RU136(887pF, 2.63MΩ8uA)

20
21
23

26
53
54

56
62
64
67

Tsukuba
(252pF,3.7MΩ,8uA ),(149pF,2.9MΩ, 8uA )(4860pF, 1.7MΩ, 8.3uA)(197pF, 3.7MΩ, 8uA)
(155pF, 3.7MΩ, 8uA),(1920pF, 2.6MΩ, 8uA)
(129pF, 1.15MΩ, 8.1uA)(120pF, 2.0MΩ, 1.5uA)(3291pF, 1.6MΩ, 8.3uA)(292pF, 3.7MΩ, 8.0uA)
(Ccp=80pF, R=1.6MΩ, implant open)

12

The reason for the dead is given in the parentheses. In red, we denote that we found new
defects and the reason is understood. If we cannot identify the reason for new defects, they are inblue (There is no case for axial sensors).
The two strips (I-U244 and I-U241; “I” refers intermediate strip and “R” readout strip) of
Sensor 10 have no obvious opens found under microscope, but the resistance measured between
the DC pad and the bias-ring was large. The bias resistor is located other end for the intermediate
strips. Probably the joint between the DC pad and implant strip is not fine, which can not be
identified by microscope.
None of the leaky strips (Sensor 54) and bad isolation strips (Sensor 67) was identified at
Tsukuba. It is quite possible that they disappeared by keeping bias on for a while. Therefore,
these disagreements are ignored.
In total 12 punch-throughs are “created” by Tsukuba measurement. After discussion with HPK,
it turned out that HPK tested the punch-through by applying a voltage of 100 V for 1/6 sec. Since
we apply 100V for 1 sec (or multiple of 1 sec if the measurement is repeated), it is not strangethat some punch-throughs are created. HPK should have tested at 120 V, which is the
specification and was actually the case for the stereo prototypes. We can ignore thesedisagreements since we did not see any new punch-through for stereo sensors, as described later.
Sensors 5 and 67 have one implant open for each, which were detected at first by AC scan andthen identified by microscope, whereas HPK could not detect them. An explanation is given in
Section 4.3 after describing the summary for stereo sensors.
After all, two strips (Sensor 7, RD251 and RD 252) were new and not clearly understood. Thissensor (and some other sensors) was sent back to HPK to investigate the problem. According to
HPK investigation, small discharge should have happened. The photograph taken by HPK isshown in Fig. 8d. The DC pads of these strips are colored and black discharge traces are visible
bridging the two pads.

Fig. 8d Photograph around DC pads of RD251 and RD252. Two
black traces are visible bridging the two DC pads. (Photo taken
by HPK)

13

4.2

Stereo Prototypes

Out of 53 delivered sensors, 18 sensors are studied in detail on the strip integrity. The
comparison is given in Table 3. All the defect strips reported by HPK are identified at Tsukuba.
In addition, we found three identified implant opens for readout strips (Sensor 10, RU75; Sensor
23, RU2; Sensor 53, RD24), and one un-identified defective strip (Sensor 13, RD83). There is nopunch-through created since HPK tested at 120V.
Table 3. Dead channels of stereo prototypes compared between HPK and Tsukuba measurements. For
disagreements, channels with understood defects are in red and others in blue.

sensorHPK
6 
7 
10 RU225 (Leaky)

23 
24 
47 IU158 (Implant open)
65 IU59 (Implant open)
66 IU238 (Implant open)
68 IU21 (Implant open)
69 
13 
46 
49 IU147 (Implant open)
50 
53 ID7 (Implant open)
57 ID62 (Implant open)
63 ID146 (Implant open)
25 ID230 (Implant open)

Tsukuba
RU225 (leaky)
RU75 (C=55,R=1.16; implant open)
RU2 (C=40,R=0.88; implant open)
IU158 (Implant open)
IU59 (R=20M)
IU238 (implant open)
IU21 (implant open)RD83 (C=111, R=1.77)

IU147(R=large)
ID7 (implant open)
RD24 (C=90,R=1.6, implant open)
ID62 (implant open)
ID146 (implant open)
ID230 (implant open)

14

4.3

Implant Opens and Some Technical Difficulties

We have detected five implant opens that were not detected by HPK. These are all readout strips.
A possible explanation is sketched in Fig. 9. HPK carries out two kinds of tests, DC and AC tests.
In the AC test, HPK injects a step pulse on the AC pad and measures the transient signal shape todetect oxide punch-through (evaluate coupling capacitance) and aluminum strip break/bridge.
This procedure is reliable and has been used for many years. In the DC test, HPK applies a DC
voltage between the DC pad and bias-ring, and measures the current. This is most sensitive forpoly-silicon resistance but also can provide information for the implant open and strip isolation.
Since the intermediate strip implant is connected to the bias resistor at far end, the measurement
is sensitive to any break in the implant. On the contrary, the system is not sensitive for thereadout strip open since the implant is connected to the ground at the probing end. Since we do
not request DC pads on other end to avoid openings where hybrids are glued, it is not possible toenhance the sensitivity for readout strip opens. HPK is expecting the AC scan has somesensitivity for the opens locating the near end. The five opens detected by Tsukuba locate about 2
cm, 3 cm, 7cm, 7cm and 8 cm from the probing end 1 . We have to conclude that the HPK
measurement is barely sensitive for detecting readout implant opens.
There is no reason for the fraction of implant opens being different between intermediate andreadout strips. HPK reports 13 opens for the intermediate while we identified 5 opens for thereadout. Therefore it is probable that we are still missing some more readout implant opens. Also
we should foresee to have some un-identified readout implant opens since our AC scan can bedone only for sampled sensors.
Similar difficulties exist for stereo sensors. In our design most of the probed short strips are notread out, but 12 (19 if transceivers are used) strips not starting from the end will be connected to
the ASICs. The DC pads for these strips are located at the other end but the nearby bias-ring,which is required for reliable probing, is passivated. Although we could change the mask andopen the passivation, probing these extra strips only is another time consuming task, which HPK
wants to avoid. It is a reasonable decision not to probe these short strips.
We therefore have to accept some un-identified defective short strips and readout implant
opens to exist before connecting to the hybrid. It should be safe to say that if the number ofidentified defects is small, the number of such un-identified defects should also be small. A goodthing is that most of HPK sensors have zero defect! .

Fig. 9 Illustration of measurement configurations of readout and intermediate implant opens.

1

HPK re-visited their data if there is any hint for these three strips and also re-measured some of thesensors that have readout implant opens, but failed to identify the defects.
15

5. Mechanical Precisions
Mechanical precisions of the sensors are measured with Mitutoyo MF-UA measuring microscope.
The reproducibility is about 2 µm horizontally and 3 µm vertically. The vertical position is
obtained by focusing.

328

325

322

319

316

12
10
8
6
4
2
0
310

The wafer thickness was measured for 24
sensors at the center of shorter sides. The
sensors were placed vertically on the stage
under the microscope. The distribution of
48 measurement results is shown in Fig.
10a. The central value is 320 µm with aspread of about 5 µm. Since we usedwafers of the same lot, the spread may
become wider at production. The precision
quoted by HPK is 15 µm.
The thickness difference at the two sidesof the same sensor was at maximum 5 µm.

313

Number of sides

5.1 Wafer thickness

wafer thickness (um)Fig. 10a Wafer thickness distribution

5.2 Edge Cut Precision
The distance between the fiducial mark to the edge was measured at the four corners. The
nominal distance is 330 µm. Although no systematic difference between axial and stereo sensors
was seen, there are some systematic deviations among the different sides. The right side (shorter
hybrid side) tends to be wider than the nominal distance while the left side (shorter probing side)
tends to be narrower. Such a difference is smaller for the longer sides. The edge cutting for thepresent samples is precise to 5 µm, although the quoted precision is 20 µm.
18

14

16

lower
upper

12

14

10

12

left
right

10
8

8
6

6

4

4

2

2
0

0
320

325

330

335

340

X: fiducial-edge (um)

320

325

330

335

340

Y: fiducial-edge (um)Fig. 10b Distance between the fiducial to the edge.

5.3 Sensor Planarity
The sensors tend to be bowed due to the difference in CTEs between SiO2 and Si: They are flat at
high temperature when processed. The bow is more significant for single-sided sensors than for
double-sided sensors.

16

The general profile is measured on 1 cm grids with a laser system at first. Typical results are
given in Fig. 10c, where the three corner data are used to define the reference plane and thedeviations to it are plotted. The positive deviations mean that the sensor is bowed with the strip
on convex side. The profile is generally universal among the different sensors.
In order to increase the statistics, the height at the sensor center was measured with amicroscope together with the heights at the fiducials when the edge cut precision data were taken.
The height relative to the fiducial heights is histogrammed in Fig. 10d. The warp is typically 80 to
100 µm for axial sensors, while it is 55 to 90 µm for stereo sensors. The difference could be
qualitatively explained by that the stereo sensors are wider then axial sensors.
Planarity of Outer StereoSensor (S020)

Deviation(um

100

X=0

6

X=10mm
X=20mm
X=30mm

5

X=41mm
50
0
0

20

40

60

80

100

Number of sensors

150

axial
stereo

4
3
2
1

-50

0

-100
Y: Position along the sensor (mm)

0

20

40

60

80

100

120

sensor warp (um)Fig. 10c Typical Z profile of a stereo sensor

Fig. 10d The deviation in Z at the sensor center

8. Conclusions
We have evaluated the electrical performance of axial and stereo prototypes fabricated by HPK
for the SVX2b detector. The prototypes fulfill our specifications. The leakage current is small andmost sensors show no breakdown up to 1000V, our specification being 500V. The number ofdead channels is quite small. We fully understood the faulty strips reported by HPK. In addition,
we found some new faulty strips. We have recognized that HPK test system is not sensitive todetect readout implant opens: We found five new readout implant opens out of in total 36 tested
sensors (out of more than 18k strips).
The mechanical precisions, wafer width uniformity, edge cut precision and sensor bowing,
were measured. The wafer width and edge cutting are precise to 5 µm, within the specification
Characteristics of irradiated sensors and longer-term stability test results are covered in theseparate note (CDF 6287).

17