Sensors for the CDF Run2b Silicon Detector
T. Akimoto, M. Aoki, P. Azzi, N. Bacchetta, S. Behari, D. Benjamin, D. Bisello, G. Bolla, P. Booth, D.
Bortoletto, A. Burghard, G. Busetto, S. Cabrera, A. Canepa, G. Cardoso, M. Chertok, C. I. Ciobanu, P.
Cooke, G. Derylo, I. Fang, E.J. Feng, J.P. Fernandez, B. Flaugher, J. Freeman, L. Galtieri, J. Galyardt, M.
Garcia-Sciveres, G. Giurgiu, I. Gorelov, C. Haber, D. Hale, K. Hara, R. Harr, C. Hill, M. Hoeferkamp, J.
Hoff, B. Holbrook, S.C. Hong, M. Hrycyk, T. H. Hsiung, J. Incandela, E.J. Jeon, K.K. Joo, T. Junk, H.
Kahkola, S. Karjalainen, S. Kim, K. Kobayashi, D.J. Kong, B. Krieger, M. Kruse, S. Kyre, R. Lander, T.
Landry, R. Lauhakangas, J. Lee, R.-S. Lu, P.J. Lujan, P. Lukens, E. Mandelli, C. Manea, P. Maksimovic, P.
Merkel, S.N. Min, S. Moccia, Y. Nakamura, I. Nakano, D. Naoumov, T. Nelson, B. Nord, J. Novak, T.
Okusawa, R. Orava, Y. Orlov, K. Osterberg, D. Pantano, V. Pavlicek, D. Pellett, J. Pursley, P. Riipinen, B.
Schuyler, S. Seidel, A. Shenai, A. Soha, D. Stuart, R. Tanaka, M. Tavi, H. Von der Lippe, J.-P. Walder, Z.
Wang, P. Watje, M. Weber, W. Wester, K. Yamamoto, Y.C.Yang, W. Yao, W. Yao, R. Yarema, H.
Yoshitama, J.C. Yun, F. Zetti, T. Zimmerman, S. Zimmermann, S. Zucchell

Abstract-- We describe the characteristics of silicon microstrip
sensors fabricated by Hamamatsu Photonics for the CDF Run 2b
Manuscript received November 15, 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, and operated by Universities Research Association
Inc. under Contract No.DE-AC02-76CH03000 with the United States
Department of Energy..
T. Akimoto, M. Aoki, K. Hara (contact e-mail: hara@px.tsukuba.ac.jp), S.
Kim, Y. Nakamura, and H. Yoshitama are with University of Tsukuba,
Tsukuba, Ibaraki 305-8571, Japan. P. Azzi, N. Bacchetta, D. Bisello, G.
Busetto, C. Manea, D. Pantano, and Z. Wang are with Universita' di Padova
and INFN-Padova, Italy. S. Behari, P. Maksimovic, B. Nord, J. Pursley, and B.
Schuyler are with Johns Hopkins University, Baltimore, MD 21218F. D.
Benjamin, S. Cabrera, and M. Kruse are with Duke University, Durham, NC
27708. G. Bolla, D. Bortoletto, A. Canepa, J.P. Fernandez, and J. Novak arewith Purdue University, West Lafayette, IN 47907. P. Booth and P. Cooke arewith University of Liverpool, Liverpool L69 7ZE England. A. Burghard, I.
Gorelov, M. Hoeferkamp, D. Naoumov, S. Seidel, and P. Watje are with
University of New Mexico, Albuquerque, NM 87131. G. Cardoso, G. Derylo,
I. Fang, B. Flaugher, J. Hoff, M. Hrycyk, P. Lukens, P. Merkel, S. Moccia, T.
Nelson, Y. Orlov, V. Pavlicek, A. Shenai, W. Wester, R. Yarema, J.C. Yun,and T. Zimmerman are with Fermilab, Batavia, IL 60510. M. Chertok, B.
Holbrook, R. Lander, T. Landry, D. Pellett, A. Soha, and W. Yao are with
University of California, Davis, CA 95616. C. I. Ciobanu, T. H. Hsiung, and T.
Junk are with University of Illinois at Urbana-Champaign, Urbana, IL 61801.
E.J. Feng, J. Freeman, L. Galtieri, M. Garcia-Sciveres, C. Haber, B. Krieger,
P.J. Lujan, E. Mandelli, H. Von der Lippe, J.-P. Walder, M. Weber, W. Yao, F.
Zetti, and S. Zimmermann are with Lawrence Berkeley Laboratory, Berkeley,
CA 94720. J. Galyardt and G. Giurgiu are with Carnegie Mellon University,
Pittsburgh, PA 15213. D. Hale, C. Hill, J. Incandela, S. Kyre, and D. Stuart
are with University of California, Santa Barbara, CA 93160. R. Harr is with
Wayne State University, Detroit, MI 48202. S.C. Hong, E.J. Jeon, K.K. Joo,
D.J. Kong, J. Lee, S.N. Min, and Y.C.Yang are with Kyungpook National
University, Daegu, 702-701 Korea. H. Kahkola, S. Karjalainen, and M. Tavi
are with Academia Sinica, Taipei, Taiwan 11529, Republic of China. K.
Kobayashi, I. Nakano, and R. Tanaka are with Okayama University, Okayama
700-8530, Japan. R. Lauhakangas, R. Orava, K. Osterberg, and P. Riipinen
are with Pohjois-Savo Polytechnic (PSPT), Kuopio, Finland. R.-S. Lu is with
University of Helsinki and Helsinki Institute of Physics, Finland. T. Okusawa
and K. Yamamoto are with Osaka City University, Osaka 558-8585, Japan. S.
Zucchell is with Universita' di Bologna and INFN-Bologna, Italy.

silicon detector. A total of 953 sensors, including 117 sensors fromprototyping, have been produced and tested for electrical andmechanical properties. Five sensors were irradiated with neutrons
up to 1.4 × 1014 n/cm2, as a part of the sensor quality assurance
program. The electrical and mechanical characteristics are foundto be superior in all aspects and fulfill our specifications. We
comment on charge-up susceptibility of the sensors that employ<100> wafers.
I.

INTRODUCTION

CDF collaboration has designed a new silicon trackerTHEsystem [1] for Tevatron Collider Run 2b. The system is a
six double-layer device located between 2.1 cm and 17 cm
radius, consisting of about 2300 single sided silicon microstrip
sensors. The whole detector is divided into an inner layer (L0)with 144 axial sensors and five outer layers (L1 to L5) with
1512 axial and 648 1.2-degree stereo sensors. In the following
we report on the outer sensors results, as the small fraction of
the inner sensors are similar although smaller in size (78.5mm × 15 mm) and with a finer pitch (25 µm with alternate
readout strips). Details of the L0 design can be found in [2].
The outer layers are built out of compact stave structures [3].
Each stave consists of twelve silicon sensors. Layers 1 and 5
have axial sensors on both sides, while Layers 2 to 4 have axial
sensors on one side and stereo on the other side. The double
axial layers, L1 and L5, provide efficiently unambiguous
anchor hit points that can be used for L0 hit search and for
track extrapolation to the outer tracking system. The siliconsensors are glued to a rohacell structure sandwiched by carbonfiber (CF) sheets and internal bus cables. The readout hybrids
are glued directly on the silicon sensors. The cooling tube,
embedded in the rohacell, sinks efficiently the heat generated
in the sensors after receiving radiation and in ASIC chips on
the hybrids: The innermost sensors are designed to operate at –5oC. Such staves construct a 66 cm long cylindrical barrelstructure with each end supported by a CF bulkhead. Fig. 1
shows an endview of the barrel. Two barrels, each containing
90 staves, are arranged along the beamline (in z) covering theinteraction region to |η|<2 with a small gap between the barrels
at z=0. Details of the stave design are given in [1, 3].
The sensors are fabricated by Hamamatsu Photonics (HPK),
see Fig. 2. The sensors are p+ microstrips on n-bulk with 512
AC-coupled readout strips. An un-read strip, called anintermediate strip, is located between readout strips, which
enhances the position resolution by charge division. The
dimensions of axial (stereo) sensors are 40.6 (41.1) mm wideand 96.4 mm long so that two sensors can be taken from a 6”
wafer. Table I summarizes the main specifications. We
employed the HPK standard 6” wafer of 320-µm thickness andwith <100> crystal orientation.

hardness of these sensors are the high voltage operation inexcess of 500 V (this might be necessary after a large radiationdose) and the relatively high starting depletion voltage with aminimum value set at 100 V.
The system requires 1512 axial and 648 stereo sensors. We
report on characteristics of 953 delivered sensors. 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 also given. We
observed some charge-up susceptibility and studied tounderstand its characteristics. Mechanical properties are alsostudied. Finally, we summarize the results.

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.

II. ELECTRICAL PROPERTIES

Fig. 2. Photograph of the sensor corner area: (1) bias ring, (2) AC pads, (3)DC pads for readout and intermediate strips, (4) bias resistors, (5) guard ring,(6) edge ring, and (7) Nsub pad, which is connected to the bulk. The distanceof the bias ring to the scribed edge is 1 mm.

We require that the sensors are radiation hard to 1.4 × 1014 1MeV equivalent neutrons per square centimeters, the fluence
being estimated to correspond to 30 fb-1 of Tevatron luminosity
at the innermost layer. Specific requirements for radiation

Table I. Main specifications of axial and stereo sensors

A. General Quality Assurance (QA) Procedure
Hamamatsu Photonics performed intensive QA tests: I-V,
C-V, AC-pad scan, and DC-pad scan. The AC-pad scanclassifies defective strips as “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 strip current is large), and “Bad Isolation” (theinterstrip resistance is too small).
We have performed additional tests in order to verify the
HPK test results and to evaluate the electrical characteristics infurther detail. The tests are:
1. I-V curve: Total leakage current was measured up to
1000 V bias at a step of 10 V.
2. I-V curve stability: I-V curves were measured every 30
min at least for 10 times, typically for 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, and equivalent
series resistance (implant and bias resistance) were
measured for each readout strip with an LCR meter.
Also, the leakage current was measured when 100 V
was applied across the oxide.
5. DC scan: The leak current of individual strip was
measured for the sensor which showed a rapid increasein leakage current, or micro-discharge [4]. 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 under test
7. Interstrip capacitance: The capacitance between theneighboring AC pads was measured. Other strips were
floating to the ground.
We measured I-V and C-V characteristics for all the sensors(except for some prototype sensors). The AC scan and IV
stability tests were performed on a sampling basis. The rest of
the tests were made less frequently: In particular they were
performed to diagnose the sensors that showed deviated resultsfrom the expectation.
B. I-V characteristics
The I-V curve is relatively easy to measure, yet providing
overall view on the sensor quality. In Fig. 3 we show typicalI-V curves of a 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 isquite small and most of the sensors do not show any significant
micro-discharge up to 1000 V we measured 1. There are two
sensors in this sample of 116 sensors, which show microdischarge with the onset voltages of around 800 and 900 V.
The leakage currents at 500 and 950 V are shown in Fig. 4 forall 953 sensors.

Fig. 3. Typical I-V curves (curves for 116 sensors are overlaid).

The leakage current is typically 0.1 µA. We set our leakage
current requirement to be less than 2 µA at 500 V in order to1

Although our specification is 500 V, we investigated the possibility ofoperating the same sensor at higher voltages for future applications. Since ourinitial results were promising up to 1000 V, we chose 1000 V as the maximum
to apply.

insure that no single readout strip has a leakage current
exceeding ~1µA as this would degrade the noise performance.
All the sensors fulfilled this specification. In addition, 97% of
the sensors showed leakage current below 0.5 µA even at 950
V. HPK has realized this superior performance by employing asingle guard-ring structure with 1 mm distance between the
scribed edge and the sensitive area (see Fig. 2).

Fig. 4. 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). It would bealso possible to use these sensors if such strips are
disconnected from the readout.

C. Stability of I-V curves
The I-V stability was measured for 42 sampled sensors. This
test is motivated to monitor the surface quality of the sensor.
Between the I-V measurements, the sensors were biased at
500-V for 30 min. The results can be categorized into fourtypes. For most of the sensors (35 sensors), the I-V curves wereconsistent and stable, as in Fig. 5(a). Two sensors showedstability as in Fig. 5(b), where small micro-discharge
developed above 900 V. We found four sensors that showedmuch significant micro-discharge as shown in Fig. 5(c).
Among these, two sensors that showed significant microdischarge were selected from the initial I-V measurement: theonset 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 withtime, reaching 800 V or higher at the last measurement.
An exceptional behavior was observed for one sensor. The
I-V stability is shown in Fig. 5(d). The I-V, initially good to1000 V, started to become chaotic after 8 hr and finally went
into breakdown at 800V after 10 hr. We investigated the causewith an IR camera, and found a discharge hole on the bias-ring.
Since we recognized no hint of discharge bridging to nearby
structures, the discharge must have occurred towards the bulk
substrate probably via some defect in the wafer. The sensor
did not recover, indicating the creation of a permanent junction
breakdown.

In summary, most sensors are good to 1000 V, but somecreated micro-discharge after a time scale of 1 hr. In any case,
operation below 500 V should be safe.

Fig. 5. Classification of I-V stability of 42 sensors. (a) Typical (35 sensors),(b) 2 sensors with small micro-discharge, (c) 4 sensors with significant microdischarge, and (d) one sensor which showed breakdown at 800 V at 18th time.

D. Full Depletion Voltage
We measured the C-V curves of 908 sensors using an LCR
meter with test pulses at 400 Hz and 1 V. 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 shown in Fig. 6.
Our depletion voltage specification of 100 V to 250 V isbased on the resistivity range from 1.25 to 3.25 kΩcm, a wafer
class employed by HPK. Some fraction, 7.7%, of the sensors
are found not to maintain the specification: It is excused by
HPK that the wafer resistivity is not uniform and applied
sampling, three points per wafer, was not enough. Since therequirement, minimum of 100 V, is to prolong the sensor
lifetime before inversion, such sensors are safely usable if weplace them in the outer layers.

Fig. 6. Distribution of the full depletion voltages.

E. AC scan and Number of Defect Strips
The strip integrity was intensively evaluated by AC scan
where the measurement was performed in two steps. Firstly, thecapacitance and series resistance were measured by probing
between the AC pad and bias-ring with setting the LCR meterat Cs-Rs mode and at 400 Hz. Secondly, a 100 V pulse wasapplied across the oxide and the leakage current through wasmeasured 1 second after. The capacitance represents the oxide
coupling capacitance, and the resistance the sum of the bias
resistance and implant electrode resistance. The implant
resistance contributes approximately 0.3-0.5 MΩ.

Fig. 7. AC scan result for a stereo sensor. The coupling capacitor Ccp
increases with strip length and reaches about 120 pF. The series resistance R,
starting from poly-silicon resistance of 1.5 MΩ, reaches about 1.8 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: An example plot isgiven in Fig. 7 for a stereo sensor, where first 25 strips startingfrom –256 are shorter due to angle. The leakage current in thisplot, about 10 nA, is due to the leakage through the CMOS
relays that we used in the apparatus to isolate the LCR inputs
from 100 V pulses. This is larger than the leakage throughgenuine coupling capacitors, but is much smaller than theleakage when the oxide has a punch-through, 10 µA, which isdefined by protection resistors in the system.
With HPK’s AC scan, they evaluate various defects byinjecting step pulses into the bulk from the backside contact
and measuring the signal emerging out from the AC pad, thus
eliminating use of an LCR meter. No bias is applied in thismeasurement. One of the purposes of our AC scan is to verify
the HPK 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 later 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 because the both breaks
are mostly created by dusts in the process. This apparent
discrepancy is explained by that HPK probing is barely
sensitive to detect the readout implant opens because of thesensor layout we employed. In our sensor layout, the polysilicon resistors for readout strips are arranged at the probingside while those for intermediate strips are at opposite side.
The opposite side where the hybrid is glued on top iscompletely passivated, and does not allow probing. Because ofthis limitation, the readout implant breaks are not detected
efficiently by just probing DC pads and bias-ring at the sameside, which is not the case for intermediate implants. In Table
II we added six readout breaks with the fraction calculated for
49 sensors we measured. The fraction of the readout implant
breaks is now comparable with that of intermediate implant
breaks. Adding the defects listed in the table, we estimate thefraction of the readout strip defects to 0.04%. The fraction of
implant breaks, which would degrade the resolution locally, is
0.05%.
Table II. Numbers and fractions of defect strips measured by HPK. The
numbers in brackets are estimations from our measurements (see text).

F. Other Electrical Properties
1) Interstrip Resistance
We evaluated the interstrip resistance measuring theincrease/decrease in the readout strip leakage current when DC
voltages ( ± 1,±0.5 V) were applied to the two intermediate strips
at neighbor. The sensor was biased to 200 V. The four
resistance values so obtained were averaged to represent theinterstrip resistance. Three axial sensors were measured. One
sensor showed resistance of in the 50-200 GΩ range, and othertwo sensors showed 100-300 GΩ.
2) Interstrip Capacitance
We evaluated the interstrip capacitance probing neighboring(readout) AC pads with other Al electrodes at floating and thedetector bias at 200 V. The LCR frequency was set to 1 MHz.
We have measured the interstrip capacitance of in total 9 axial

and 2 stereo sensors. Among 5600 pairs of strips, only twopairs showed irregular values of 5% off the nominal value.
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.
Although radiation in CDF is composed of various hadrons, we
irradiated 5 sensors with neutrons at MNRC Irradiation Facility
at UC Davis in order to verify that our sensors are radiation
hard to the similar level as previously reported [5]. Three ofthe sensors received 1.4 × 1014 cm−2 1-MeV neutron equivalent
fluence and the other two 0.7 × 1014 cm−2 1-MeV equivalent[6]. The actual doses were calibrated by sulfur activation to
1.40 × 1014 and to 0.67 × 1014 n/cm2 with typical uncertainty of
10%. The irradiation time was 69 min in total. No sensor bias
was applied and no temperature control was attempted during
the irradiation.
Right after the irradiation the sensors were stored at −7oC.
The sensor temperature was then increased for certain periods
so that the beneficial annealing is almost complete hence thefull depletion voltage at the minimum.
B. Total Leakage Current and Full Depletion Voltage
Fig. 8 shows the I-V curves of the irradiated sensors. The
current is normalized at 20oC. The sensors do not cause any
micro-discharge up to 1 kV and show clear shoulders
indicating that dominating is the leakage current through thebulk.
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 (an example plot is given in
the inset in Fig. 11) 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 withprevious measurements [5], which predict that the full
depletion voltage be around 120 V and 40 V, respectively.
The damage constant α is expressed by α = ∆I /(Vφ), where
φ is the neutron fluence, ∆I the increase in the total leakage
current measured at the full depletion voltage, and V thevolume of the sensor. Previous studies show that the leakage
current of irradiated sensors decreases (anneals) with time.
Because of this feature and temperature dependence of theleakage current, α is calculated after the annealing is almost
completed and at 20oC. Systematic study of the annealing
phenomena is available such as from Rose Collaboration [5].
At room temperatures, the current decreases substantiallywithin a month, with α reaching 3 × 10-17 A/cm.
Instead of evaluating the damage constant, we use our resultsto evaluate the neutron fluence. The evaluated neutron fluenceis 1.1 × 1014 n/cm2 for the nominal of 1.4 × 1014 n/cm2, and is(0.33-0.64) × 1014 n/cm2 for 0.7 × 1014 n/cm2. The present

results prefer somewhat smaller neutron fluence but are in
reasonable agreement.

for the two sensors irradiated to 1.4 × 1014 n/cm2. The LCR test
signals were at 1 MHz frequency and 5 V amplitude. Although
the curves at small bias voltages behave differently due toindividual difference, the shoulders around 130 V areconsistent with the full depletion voltage. The interstrip
capacitance decreases gradually with bias voltage, approachingto asymptotic values consistent with those of non-irradiatedsensors. We notice, though, that the bias should besubstantially larger than the full depletion voltage to reach theasymptotic value.

Fig. 8. I-V curves of irradiated sensors. The leakage current isnormalized at 20oC. Sensor (1) was measured at 14oC and Sensor (2) at −15oC.
Sensor (1) is not completely annealed while Sensor (2) should have completed
the initial annealing.

C. Bias Resistance and Interstrip Resistance
The resistances of various sensor components are key
properties to evaluate the radiation effect on silicon sensors.
The oxide layer accumulates positive charges created byradiation, which degrades among others such as effective bias
resistance and interstrip resistance. In the following, we
evaluated the degradation using real sensors instead of using
test pieces, thus the results represent the effective changes.
Fig. 9a shows the bias dependence of the bias resistance,compared for a particular strip before and after irradiation.
Although the resistance saturates below about 100V before
irradiation, much larger bias voltage, about 350 V, is required
to reach the asymptotic value.

Fig. 10. Interstrip capacitance of irradiated sensors measured as afunction of bias. The data are shown for two sensors irradiated to 1.4 × 1014
n/cm2.

E. Charge collection
The two sensors irradiated to 1.4 × 1014 n/cm2 were used toconstruct a module, where a real readout hybrid [3] wasattached. This module was illuminated with Nd:YAG laser toinvestigate the charge collection. The laser wavelength 1064
nm corresponds to the energy slightly above the energy gap ofsilicon, hence the laser is absorbed almost uniformly along its
trajectory. This simulates well the passage of charged particles.

Fig. 9. (a) Bias resistance vs. bias voltage and (b) interstrip resistance vs.
bias voltage measured at various temperatures.

Fig. 9b shows the bias dependence of the interstrip
resistance. The resistance is substantially degraded: it was
greater than 50 GΩ before irradiation. In order to keep theresistance to above 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 bias resistance and interstrip resistance.
D. Interstrip CapacitanceFig. 10 shows the bias dependence of the interstrip
capacitance measured for a particular set of neighboring strips

Fig. 11. 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. 11 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.
The voltage dependence of the charge collection is almost as
expected from the C-V curve of the same sensor, which isshown in the inset in the figure. We though notice that the
charge collection of the irradiated module increases even above
the full depletion voltage. The deficit in the collected charge
can be explained by followings. After irradiation there are
active trapping and detrapping centers, which spread the signal
arrival time. A tail was actually observed in the collectedcharge vs. time distribution, indicating that the integration timeof the SVX4 chips, 112 ns, is not long enough to collect all the
charges. This effect can be mitigated by raising the bias voltage,
as we see in Fig. 11.
IV.

SENSOR SUSCEPTIBILITY OF CHARGE UP

In the early stage of testing, we often observed the sensor toshow degraded performance in a region spreading over about
30 to 50 strips. The symptom is that the coupling capacitances
are larger, the series (implant and poly-silicon) resistances are
smaller, and interstrip resistances are smaller, which implies
that the strip isolation is degraded in the region. Afterinvestigation it turned out that the vacuum tweezers used topick up the sensor caused the charge-up phenomenon. The
phenomenon disappears if the sensor is placed back to theenvelope provided by HPK.
The wafer orientation we employed is <100>. Because thepositive charges trapped at the Si-SiO2 interface and in theoxide layer are fewer in <100> than in <111>, they attract
fewer electrons underneath the interface. Thus, externalpotentials can create easier a p+ layer, inversion layer, between
the p+ strips, degrading the strip isolation. MOSFET devices
usually use <100> wafers for this reason.
We observed that the charge-up phenomenon could remain
at least for a week if the sensor surface is isolated. In ourpractical sensor application, though, the wire-bonds to theamplifiers, for example, stabilize the Al electrode potential,
which should eliminate the inversion layer. In order to verify
this hypothesis, we performed the following study.
We created a charge-up sample and wire-bonded three AC
pads in the charge-up region to the ground, simulating theconnection to the amplifiers. We repeated the AC scan tomonitor the recovery. The results are given in Fig. 12. The
initial charge-up, as visible in Fig. 12(d), almost recovered 20
h after wire-bonding (Fig. 12(e)). The recovery seems to be
somewhat slower at the mid-point between the wire-bonded
strips; see the distribution 1 h after (Fig. 12(b)). Therefore, we
can conclude that charge-up phenomena, if exist, are todisappear once the sensors are assembled to modules.
From our experience, we are able to diminish the creation of
charge-up if we eliminate use of vacuum tweezers. Also wefind that the charge-up susceptibility degrades with time: we
tried to charge-up the sensors intentionally, but it was hard forthe samples 1 month after the delivery. Our hypothesis is thatthe sensor accumulates positive charges in the oxide layer inthis period and became resistive against creation of inversion
layer.

Fig. 12. AC scan results for charge-up study. (d) Initial distribution where
strips 105 to 135 show irregular coupling capacitance (100 pF nominal) andseries resistance (1.4 MΩ nominal). (a) Expanded plot of the charge-up region.
(b) Enlarged plot 1 h after strips 109, 110 and 120 are wire-bonded. (c)Expanded and (e) entire distributions 20 h after. LCR frequency is 1 kHz.

V. MECHANICAL PRECISIONS
The following mechanical precisions were measured forsampled sensors with a measuring microscope, which has a
reproducibility of about 2 µm horizontally and 3 µm vertically.
A. Wafer thickness
The wafer thickness was measured for 78 sensors at thecenter of shorter sides. The sensors were placed vertically on astage under the microscope. The central value was 320.4 µm,
with the distribution shown in Fig. 13. The spread is muchsmaller than the precision quoted by HPK, 15 µm. The
thickness difference at the two shorter sides of the same sensor
was at maximum 5 µm.

Fig. 13. Distribution of the detector thickness.

B. Edge Cut Precision
The distance between the fiducial marks to the edge wasmeasured at the four corners of 44 sampled sensors. The
nominal distance is 330 µm. The distance distributed from 323
to 335 µm along the longer side and from 326 to 335 µm
along the shorter side. The edge cut precision was better than 7µm, while the precision quoted by HPK is 20 µm.

C. Sensor Planarity
The sensors tend to be bowed due to different thermal
expansions between SiO2 and Si: They are flat at hightemperature when processed. The bow is more significant for
single-sided sensors than for double-sided sensors.
The planarity was measured on 1 cm grids. Typical profile isgiven in Fig. 14, where the three corner data are used to define
the reference plane and the deviations to it are plotted. The
positive deviations mean that the sensor is bowed with thestrips on the 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 the microscope with respect to theheights at the fiducials at the four corners. The deviations
distributed 80 to 100 µm for 11 axial sensors and 55 to 90 µm
for 11 stereo sensors. The difference could be qualitatively
explained by that the stereo sensors are wider then axial
sensors. The twists, deviations of the 4th fiducial height which
was not used to define the reference plane, distributed from –6
to 21 µm for the 22 samples.

The mechanical precisions, wafer thickness uniformity, edge
cut precision and sensor bowing, were measured. All of them
are found to be within our specifications.
VII. ACKNOWLEDGMENT
The authors would like to thank K. Yamamura ofHamamatsu Photonics for useful discussions. They also
acknowledge MNRC for neutron exposure, D. Newell and H.
Ben Liu for providing fluence calibration information.
VIII. REFERENCES
[1]

[2]

[3]

[4]

[5]

[6]

Fig. 14. Height profile of a stereo sensor measured on 1 cm grids.

VI. CONCLUSION
We evaluated the electrical and mechanical properties of
microstrip sensors designed for the CDF Run2b silicondetector.
The leakage current is small (0.1 µA typically) and mostsensors show no breakdown up to 1000 V. The fraction of
defective readout channels is estimated to 0.04%. The coupling
capacitance, bias resistance and other electrical values arefound to be quite 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.

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The 1-MeV equivalent fluence was calculated based on ASTM E722
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