Sensors for the CDF Run2b Silicon Detector
Kazuhiko Hara
For the CDF Collaboration

Abstract-- We describe the characteristics of silicon microstrip
sensors fabricated by Hamamatsu Photonics for the CDF Run 2b
silicon detector. A total of 953 sensors, including 117 sensors fromprototyping, have been produced and tested for electrical andmechanical properties. Five sensors were irradiated withneutrons up to 1.4 × 1014 n/cm2, as a part of the sensor quality
assurance program. The electrical and mechanical characteristics
are found to be superior in all aspects and fulfill our
specifications.
I.

radiation hard to 1.4 × 1014 1-MeV equivalent neutrons per
square centimeters, the fluence estimated to correspond to 30
fb-1 of Tevatron luminosity at the innermost layer. The two
requirements in concern are the high-voltage operation of
exceeding 500 V, and relatively large initial full depletion
voltage with the minimum of 100 V.

INTRODUCTION

CDF collaboration has designed a new siliconTHE
tracker system [1] for Tevatron Collider Run 2b. The
system is a six-layer device located between 2.1 cm and 16 cm
radius, consisting of about 2300 single sided silicon microstrip
sensors. Among these, 144 inner axial sensors (or L0 sensors)
are glued with epoxy to a CFRP structure, which is supportedby the beam-pipe [2]. The rest is the outer sensors (or L1-L5
sensors), which are glued with epoxy to both sides of selfsupported structures of about 4 cm width and 66 cm length,
called staves [3]. Each stave has an embedded cooling channel
in order to efficiently sink the heat generated in the sensors
after receiving radiation and in ASIC chips on hybrids. The
hybrids are glued on the silicon sensors. Such staves cover the
tracking volume as illustrated in Fig. 1. Two of these sets arealigned along the beam direction, covering the pseudorapidity
region of |η|<2. Details of the stave design are given in [1, 3].
Each stave consists of twelve silicon sensors: L1 and L5
have double axial sensors on both sides, while L2 to L4 haveaxial and 1.2o stereo sensors on each side. The silicon sensors
reported here are axial and stereo types for L1 to L5. The
sensors are p+ microstrips on n-bulk with 512 AC-coupled
readout strips. An un-read strip, intermediate strip, is locatedbetween readout strips in order to enhance the position
resolution. The dimensions of axial (stereo) sensors are 40.6
(41.1) mm wide and 96.4 mm long so that two sensors can betaken from 6” wafer. The sensors are fabricated by Hamamatsu
Photonics (HPK). Table I summarizes the main specifications.
We employed HPK standard 6” wafers of 320-µm thickness
and in <100> direction. We require that the sensors are
Manuscript received November 6, 2003. This work was supported in partby the Japanese Ministry of Education, Culture, Sports, Science andTechnology under Grant in Aid for Scientific Research (KAKENHI) on
Priority Areas, 13047101.

Fig. 1. Cross sectional view of the new silicon system, consisting of 90
staves in five layers, and a beam-pipe L0 layer (16 cm outer radius). Two sets
are aligned in the beam direction covering 120 cm long region.
Table I. Main specifications of axial and stereo sensors

The designed system requires 1512 axial and 648 stereo
sensors in total. Although the plan was to replace the current
CDF L00 and SVX-II detectors, and the sensor production wasin progress, a total of 953 sensors having been delivered, thewhole project is cancelled. The main reason is that the
University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan (e-mail:hara@px.tsukuba.ac.jp).

integrated Tevatron luminosity will likely be a 4.3 to 8.4 fb-1
range through 2009, which is roughly a factor of two smallerwhen the project started.
First, we describe the electrical properties of the sensors.
Five axial sensors were irradiated with neutrons up to1.4 × 1014
n/cm2. The characteristics of these sensors are given. Finally,
we summarize the results.
II. ELECTRICAL PROPERTIES
A. General Quality Assurance (QA) Procedure
Hamamatsu Photonics performs intensive QA tests: I-V, CV, AC-pad scan, and DC-pad scan. The AC-pad scan classifies
defective strips to “AC Open” (the aluminum strip has a
break), and “AC Short” (neighboring aluminum strips areshorted). The DC-pad scan reports “Open” (the implant strip
has a break), “Short” (neighboring implant strips are shorted),“Leaky” (the individual strip current is large), and “BadIsolation” (the interstrip resistance is small).
We have performed additional tests in order to verify the
HPK test results and to evaluate the electrical characteristics
further to detail. The tests are:
1. I-V curve: Total leakage current was measured up to
1000V bias at a step of 10V.
2. I-V curve stability: I-V curves were measured every 30
min at least for 10 times, typically 30 times. The bias
voltage between the I-V measurements was set at 500
V.
3. C-V curve: Total capacitance was measured as afunction of bias voltage. The curve was used to extract
the full depletion voltage.
4. AC scan: Oxide coupling capacitance, equivalent series
resistance (implant and bias resistance), and oxide
punch-through at 100V were measured for eachreadout strip with an LCR meter.
5. DC scan: The leak current of individual strip was
measured for sensors with large total leakage. The bias
was set above the micro-discharge onset voltage.
6. Interstrip isolation: The interstrip resistance wasmeasured with applying voltages (−1 to +1V) to theneighboring intermediate DC pads and measuring thecurrent emerging from the readout DC pad.
7. Interstrip capacitance: The capacitance between theneighboring AC pads was measured with other strips
floating to the ground.
We measured I-V and C-V characteristics for all the sensors.
The AC scan and IV stability tests were performed on asampling basis in order to verify the HPK probing results andto monitor the surface quality of the sensors. The rest of thetests were made less frequently to understand the generalelectrical properties in concern and to identify the problem of
the sensor found to be extraordinary.

B. I-V characteristics
The I-V curve is relatively easy to measure, yet providing
overall view on the sensor quality. In Fig. 2 we show I-V
curves of a typical subset of the sensors. The measured leakage
current values were normalized at 20oC while the test benchtemperature was in the range 26 to 30oC. The leakage current
is quite small and most of the sensors do not show any
significant micro-discharge up to 1000 V we measured. There
are two sensors in this sample of 116 sensors showing microdischarge with the onset voltages of 800 and 900 V. For all
953 sensors the leakage currents at 500 and 950 V are shownin Fig. 3.
We set our leakage current requirement to be less than 2 µA
at 500 V in order to insure that no single readout strip has asignificant leakage current exceeding ~1µA, thus not todegrade the noise performance. All the sensors fulfilled thisspecification. In addition, 97% of the sensors showed leakage
current below 0.5 µA even at 950 V. HPK has realized thissuperior performance by employing a single guard-ring
structure with 1 mm distance between the scribed edge and thesensitive area.

Fig. 2. Typical I-V curves (curves for 116 sensors are overlaid).

Fig. 3. Leakage currents at 500 and 950 V for all 954 sensors. The
numbers exceeding 0.5 µA are given in extremely right bin

For the sensors that show micro-discharge, we have verified
that the micro-discharge is caused at a single or a few strips bymeasuring the individual strip leakage (DC scan). Therefore it
should be able to use such sensors also just by disconnecting
those strips from readout.

The leakage current of typical sensors increases to a 0.1 µA
level in 30 V, this level being kept up to 1000 V. Such an I-V
curve is not expected for sensors with full depletion voltagesof about 100 V. We understand that the bulk current is quite
small and the surface current is dominating in the low voltage
region.

C. Stability of I-V curves
The I-V stability was measured for 42 sampled sensors.
Between the I-V measurements, the sensors were biased at
500V for 30 min. The results can be categorized into fourtypes, as shown in Fig. 4. For most of the sensors (35 sensors),
the I-V curves were consistent and stable, as in Fig. 4(a). Two
sensors showed stability as shown in Fig. 4(b), where small
micro-discharge developed above 900 V. We found four
sensors that showed much significant micro-discharge asshown in Fig. 3(c). Two of them showed micro-discharge
from the first measurement: the onset voltage increased
gradually with time. The other two were good to 1000 V
initially but the onset voltage of 600 V showed up within 1 hr.
The onset voltage then increased with time, reaching 800 V or
higher at the last measurement.

D. Full Depletion Voltage
The C-V curves were measured for 908 sensors at an LCR
frequency of 400 Hz. We extracted the full depletion voltage
as the intercept of two straight lines in a C−2 –V plot. The full
depletion voltage distributed from 87 V and 165 V, as shownin Fig. 5. Our depletion voltage specification of 100 V to 250
V is based on the resistivity range from 1.25 to 3.25 kΩcm, awafer class employed by HPK. The minimum voltage of 100 V
was not maintained: 7.7% of the sensors are out ofspecifications. This discrepancy is excused by HPK that thewafer resistivity is not uniform and applied resistivity
sampling was not enough.

Fig. 5. Distribution of the full depletion voltages.

E. AC scan and Number of Defect Strips

Fig. 4. Classification of I-V stability of 42 sensors. (a) Typical (35 sensors),(b) 2 sensors, (c) 4 sensors with significant micro-discharge, and (d) one
sensor which showed breakdown at 800 V at 18th time.

An exceptional behavior was observed for one sensor. The
I-V stability is shown in Fig. 4(d). The I-V, initially good to1000 V, started to become chaotic after 8 hr and finally went tobreakdown at 800V after 10 hr. We investigated the cause withan IR camera, and found a discharge hole on the bias-ring.
Since we recognized no hint of discharge bridging to nearby
structure, the discharge must have occurred towards the bulk
substrate probably via some defect in the wafer. The sensor
did not recover, indicating creation of permanent junction
breakdown.
In summary, most sensors are good to 1000 V but some cancreate micro-discharge after a time scale of 1 hr. In either case,
operation below 500 V should be safe.

The strip integrity was intensively evaluated by AC scan
where the measurement was performed in two steps. Firstly,
capacitance and series resistance were measured by probing
between the AC pad and bias-ring setting the LCR meter at CsRs mode and at 400 Hz. Secondly, 100 V pulse was applied
across the oxide for 1 second and the leakage current throughwas measured. The capacitance represents the oxide coupling
capacitance, and the resistance the sum of the bias resistanceand implant electrode resistance. The implant resistancecontributes approximately 0.3-0.5 MΩ.
AC scan was performed for 49 sensors. The coupling
capacitors and resistors were found to be very uniform anddefective strips can be easily identified. For the oxide leakcurrent, the leakage through the CMOS relays that we used toisolate the LCR inputs from 100 V pulses contributes by a 10
nA level. This is larger than the leakage through genuine
coupling capacitors, but is much smaller than the leakage
through oxide punch-through of 10 µA, which is defined byprotection resistors in the system.
With HPK’s AC scan, they evaluate various defects byinjecting a step pulse into the bulk and measuring the signal
emerging out from the AC pad, thus eliminating use of an LCR
meter. One of the purposes of our AC scan is to compare theboth results.
The 49 sensors we selected have defects reported by HPK.
Except for some leaky strips and bad isolation strips, we
succeeded to identify all the other defects. Those unidentifieddefects can be ignored from the following reasons. Largeleakage current tends to decrease with time, as described in
Section II.C. Also, isolation degradation can be understood as
non-permanent charge up, as is described in Section IV.
We created a couple of new damages associated with the
probing system manipulation. Such damages are obvious by
visual inspection. Apart from them, we have observed six new
defects which were identified with a microscope as readout
implant breaks.
Table II summarizes the number of defect strips measuredby HPK. The percentages are given with respect to the total
number of (readout or intermediate) strips of 488 thousands.
Among these, the fraction of intermediate strip breaks isdominating. The readout implant breaks are substantially fewer,
although we expect similar fractions if the breaks are createdby dusts in the process. This apparent discrepancy is explainedby the sensor layout and that HPK probing is barely sensitive
to detect the readout implant opens. In our sensor layout, thepoly-silicon resistors for readout strips are arranged at the
probing side and those for intermediate strips at opposite side,
the opposite side being completely passivated because thehybrid is glued on top. Because of this layout, the intermediate
strip beaks are efficiently detected by probing DC pads andbias-ring, which is not the case for readout strips. In Table II
we added six readout breaks with the fraction calculated for 49
sensors we measured. The fraction of readout implant breaks iscomparable with that of intermediate implant breaks. We
estimate the fraction of the readout strip defects to 0.04%
adding the defects listed in the table.
Table II. Numbers and fractions of defect strips measured by HPK. The
numbers in brackets are from our measurements.

sulfur activation to 0.67 × 1014 and to 1.40 × 1014 n/cm2 withtypical uncertainty of 10%.
Right after the irradiation the sensors were stored at −7oC.
The sensor temperature was then increased for certain periods
so that the sensors should almost complete the initial annealing
and to have the full depletion voltage almost at the minimum.
B. Total Leakage Current and Full Depletion Voltage
The I-V curves of irradiated sensors show clear shoulders
corresponding to the full depletion, which indicates that
dominating is the leakage current through the bulk.
The measurement of full depletion voltage is strongly
affected by high leakage current and therefore was performedin the environmental chamber at –25oC. The full depletion
voltages derived from C-2-V plots ranged from 128 to 130 V
for the sensors irradiated to 1.4 × 1014 n/cm2, and about 50 V
for the sensors irradiated to 0.7 × 1014 n/cm2. These values are
consistent with previous measurements [4], which predict thatthe full depletion voltage be around 120 V and 40 V,
respectively.
Instead of evaluating the damage constant, we use ourresults to evaluate the neutron fluence. The evaluated neutron
fluence is 1.1 × 1014 n/cm2 for nominal of 1.4 × 1014 n/cm2, andis (0.33-0.64) × 1014 n/cm2 for nominal of 0.7 × 1014 n/cm2.
The present results prefer somewhat smaller neutron fluence
but are in reasonable agreement.
C. Poly-silicon Resistance and Interstrip Resistance
The poly-silicon resistance was measured and compared
before and after irradiation. Fig. 6a shows the bias dependence
measured for a particular strip. Although the resistancesaturates below about 100V before irradiation, much larger
bias voltage, about 350V, is required to reach the asymptotic
value.
Fig. 6b shows the bias dependence of the interstrip
resistance. The resistance is substantially degraded: it is greaterthan 50 GΩ before irradiation. In order to keep the resistanceto be more than 1GΩ, a bias of 250V is required at –10oC.
Although the data are shown for a particular strip (pair), we
found that the degradation is very uniform among the strips
both for the poly-silicon resistance and interstrip resistance.

III. PROPERTIES OF IRRADIATED SENSORS
A. Neutron Irradiation
Radiation damage of silicon sensors has been studied
intensively for the application to e.g. LHC experiments [4]. In
order to verify that our sensors are radiation hard to the similarlevel, we irradiated 5 sensors with neutrons at MNRC
Irradiation Facility at UC Davis. Three of the sensors received
1.4 × 1014 cm−2 1-MeV neutron equivalent fluence and theother two 1.4 × 1014 cm−2 1-MeV equivalent. The irradiation
time was 69 min in total. The actual doses were evaluated by

Fig. 6. (a) Poly-silicon resistance vs bias voltage and (b) interstrip
resistance vs. bias voltage measured at various temperatures.

D. Interstrip CapacitanceFig. 7 shows the bias dependence of the interstrip
capacitance measured for a particular set of neighboring
strips for two modules irradiated to 1.4 × 1014 n/cm2. The two
sensors were annealed similarly but the temperature wasslightly different at measurement. The LCR frequency is 1
MHz. The shoulders around 130 V are consistent with thefull depletion voltage. The interstrip capacitance decreases
gradually with bias voltage, approaching to asymptotic
values consistent with those of non-irradiated sensors. We
notice, though, that the bias is substantially larger than thefull depletion voltage to reach the asymptotic value.

Fig. 7. Interstrip capacitance of irradiated sensors measured as a function ofbias. The two sensors are measured at 13oC (filled circles) and at 14oC (opentriangles).

E. Charge collection
The two sensors irradiated to 1.4 × 1014 n/cm2 were used toconstruct a module where a readout hybrid was attached. This
module was illuminated with Nd:YAG laser to investigate thecharge collection. The laser wavelength 1064 nm corresponds
to the energy slightly above the energy gap of silicon, hence
the laser is absorbed almost uniformly along its trajectory,
which simulates well the passage of charged particles.

obtained somewhat smaller charges at saturation for theirradiated module, but the shapes are similar.
The voltage dependence of the charge collection is as
expected from the C-V curve of the same sensor, which isshown in the inset in the figure.
IV. CONCLUSIONS
We evaluated the electrical and mechanical properties of
microstrip sensors designed for the CDF Run2b silicondetector.
The leakage current is small (0.1µA typically) and mostsensors show no breakdown up to 1000V. The fraction of
defective readout channels is estimated to 0.04%. The coupling
capacitance, bias resistance and other electrical values arefound to be uniform.
We have characterized various electrical parameters of thesensors irradiated with neutrons up to 1.4 × 1014 cm-2. The
evolution of the full depletion voltage and increase of theleakage current are consistent with previously known values.
Many parameters such as interstrip capacitance, interstrip
resistance, and bias resistance are apparently degraded due tocharges accumulated in oxide layer and type inversion. We
observed that it requires a bias of 250-300V to reach theasymptotic values while the full depletion voltage is evaluatedto be around 130 V.
V. REFERENCES
[1]

[2]
[3]

[4]

Fig. 8. Charge collection curve of irradiated sensors (filled squares) and of
non-irradiated sensors (open diamonds). The inset is C-2 –V curve of theirradiated sensor.

Fig. 8 shows the collected charge as a function of the bias
voltage. Since the laser spot spreads over a few strips, ADC
sum of neighboring strips is plotted. The same curve measured
for a non-irradiated module is superimposed in the figure. We

M. Aoki et al. (CDF Collaboration), “CDF Run IIb Silicon Detector”,
submitted for Proceedings of 9th Pisa Meeting on Advanced Detectors,
La Biodola, Isola d'Elba May 25, 2003; CDF Collaboration, “The CDF
IIb Detector: Technical Design Report”, FERMILAB-TM-2198 (Feb
2003).
P. Merkel (CDF Collaboration), “CDF Run IIb Silicon: The New
Innermost Layer”, submitted for IEEE NSS/MIC Conference, Portland,
USA, October 19, 2003.
M.M. Weber (CDF Collaboration), “Electrical Performance and Deadtimeless Operation of “Staves” for the New CDF Silicon Detector”;
submitted for IEEE NSS/MIC Conference, Portland, USA, October 19,
2003; R.-S. Lu (CDF Collaboration), “Stave Design and Testing of
SVXIIb of CDF at Fermilab”, ibid; C. Cardoso (CDF Collaboration),“Polyimide and BeO Mini Portcard Performance”, ibid.
ROSE Collaboration, see, for example, http://cern.ch/rd48.