Nuclear Instruments and Methods in Physics Research A 435 (1999) 437}445

Prototype Si microstrip sensors for the CDF-II ISL detector
K. Hara *, K. Hata , K. Kanao , S. Kim , M. Ogasawara , T. Ohsugi,M. Shimojima , K. Takikawa
University of Tsukuba, Institute of Physics, Tsukuba, Ibaraki 305-8571, JapanDepartment of Physics, Hiroshima University, Higashi-Hiroshima, Hiroshima 739-8526, Japan
Received 17 May 1999

Abstract
Prototype Si microstrip sensors for the CDF-II ISL were fabricated by Hamamatsu Photonics and SEIKO Instruments using 4 technology. The sensor is AC coupled and double-sided forming a stereo angle of 1.2073. The strip pitch is
112 lm on both sides. The main di!erences between the two manufacturers lie on the technologies of passivation and thestructure of coupling capacitors. We describe the design of the sensor and evaluation results of the performance. The
evaluations include the total and individual strip currents and interstrip capacitance measured before and after Coc irradiation.  1999 Elsevier Science B.V. All rights reserved.

1. Introduction
The CDF detector at Fermilab Tevatron is currently being upgraded to CDF-II [1] in order totake full advantage of 10-fold increase in luminosity. One of the major upgrades is the replacementof the entire central tracking system; the new system will consist of "ve layers of double-sided Si
microstrip detectors, SVX-II (2.5(r(10.6 cm),the intermediate radius Si layers, ISL (20(r(30cm), and the central open cell tracker, COT(46(r(131 cm). Combining the measurementswith SVX-II and ISL allows precision standalone
tracking with up to seven axial and seven stereo
measurements.

* Corresponding author. Tel.: #81-298-53-4270; fax: #81298-53-4491.
E-mail address: hara@hep.px.tsukuba.ac.jp (K. Hara)

The ISL is constructed from "ve barrels in total;
one central barrel, inner and outer barrels at forward and backward regions. The Si sensors are
AC-coupled and double-sided with the electrodestrips forming a small stereo angle of &1.23. Eachbarrel has single layer of such Si sensors arranged
into ladders, each ladder consisting of six Si microstrip sensors lined along the beam direction. The
strips of three Si sensors in a ladder are wirebonded so that the signals can be read out fromeither end of the ladder. Each barrel is composed of
28, 24 and 36 such ladders for the central, inner andouter forward/backward barrels. The total numbers thus count to 148 ladders and 888 Si sensors.
About a half of the sensors are fabricated using 4
technology. We describe the prototype of thesesensors.
Four-inch ISL prototype sensors have been fabricated by Hamamatsu Photonics Co. (HPK) and
SEIKO Instruments Inc. (SII). The sensitive area is0168-9002/99/$ - see front matter  1999 Elsevier Science B.V. All rights reserved.
PII: S 0 1 6 8 - 9 0 0 2 ( 9 9 ) 0 0 6 5 3 - 1

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K. Hara et al. / Nuclear Instruments and Methods in Physics Research A 435 (1999) 437}445

57.23 mm wide and 67.5 mm long with a thicknessof 300 lm. The p-side strips have small stereo angleof 1.2073 with respect to the n-side strips which runparallel to the longer side. To compromise between
the number of readout channels and the required
spatial resolution, the strip pitch was determined tobe 112 lm with the number of readout strips being
512 per side.

2. Prototype design
2.1. Strip width considerations
The prototype sensor design is based on the
HPK Si microstrip sensor developed for the SDC[2]. Given the strip pitch of 112 lm, the implant
strip width and Al electrode width are chosen to be
22 and 16 lm; the Al electrode is recessed by 3 lmfrom the implant strip edges. This design reduces
the electric "eld strength in the bulk in the vicinityof the implant strip edges, which helps to reduce themicro-discharge [3]. The implant strip width isdetermined from a compromise of e!ects on the full
depletion voltage, the detector capacitance, and Al
electrode resistivity, the latter two contributing to
the ampli"er noise. With increasing the strip width,
the detector capacitance increases while the full
depletion voltage and the Al electrode resistivity
decreases. Because of fringing "elds near the strip
structures, excess bias voltages are necessary for
strip detectors compared to simple diodes in order
to be fully depleted, which is discussed in Ref. [4].
There are several measurements available [5}7] ofthe dependence of the full depletion voltage on the
strip width for double-sided Si detectors. Although
the measured excess voltages are slightly smaller
than the calculation [4], they tend to become substantially high for strip widths narrower than
&20% of the strip pitch.
The interstrip capacitance on the p-side is wellstudied and can be expressed approximately by(0.75#1.8=/P) pF/cm [5] for strip width of= and pitch P, which gives 1.0 pF/cm for our
sensors. We note that the measured value is1.2> pF/cm (0.15 being the estimate of the stray\ 
capacitance) for both HPK and SII sensors, and isconsistent with this empirical formula.

The n strips in n-bulk have a larger capacitancethan p strips since the charge accumulated betweenn strips makes the n electrode e!ectively wider.
Implantation of p> between n strips, or p-stop, isfor the purpose of isolating n strips. We employed
the individual p-stop con"guration [6] where eachn strips is surrounded by #oating p>. The region
between the neighboring n strips is covered asmuch as possible by p-stop implants, leaving a gapof 8 lm between the neighboring p-stop implants
and 10 lm between the n> and p-stop. The n-side
capacitance is studied by the RD20 group [8,9]both in the individual con"guration and in thecommon con"guration where all the p-stop implants are connected together beyond the strip
ends. According to their study, the interstrip
capacitance is slightly smaller in the individual con"guration. Although RD20 quotes an n-side
capacitance of &2 pF/cm, it is known to decrease
further with the bias voltage. Our capacitancemeasurement is described in Section 3.
The signal is read out with SVX3 chips whose
integrator is essentially a double cascode con"guration with a PMOS input transistor [10]. Amongthe various noise contributions (see, for example,[11]), we compared the FET Johnson noise andthe thermal noise from the series input resistancein order to evaluate the in#uence of the Al electrode resistance and detector capacitance. The
two factors contribute to the noise charge
through

2 1(Noise charge)&4k¹C R #"  3 g

(1)where k is the Boltzmann constant, C the detector"
capacitance, g the transconductance, and R the
e!ective resistance input to the preampli"er.
The signal picked up by the Al electrode is transmitted through an RC chain whose e!ective resistance is  of the actual Al electrode resistance inthe frequency region concerned. The measured Al
electrode resistivity is 12 )/cm for HPK and 13)/cm for SII. Under these conditions and otherparameters taken as inputs, a calculation resulted
in that the noise at a strip width of 22 lm islocated at minimum in the wide parameter range
considered.

K. Hara et al. / Nuclear Instruments and Methods in Physics Research A 435 (1999) 437}445

2.2. Other designs common to the manufacturers
A single guard ring structure is employed. Inorder to reduce the insensitive area, the p> guard
ring on the p-side is located underneath the bias
ring and its potential is held to the bias voltage.
Similarly, the n> guard ring on the n-side is locatedunderneath and DC connected to the bias ring.
A pair of #oating p-stop rings are located insideand outside the n> guard ring for isolation.
The bias voltage to individual implant strips isprovided by polycrystalline silicon (poly-Si) resistors with a mean resistance of 4 M). The poly-Si
with a width of 5 lm and a unit resistance of &1
M)/mm (HPK) or &1.5 M)/mm (SII) is placed
inside the guard ring making several turns. While
the Al electrodes are terminated not to overlap withthe poly-Si resistors, the implant strips are extended underneath the poly-Si towards the guard
ring. Charge accumulated by the strips in this region can still be picked up by the Al electrodes. This
design therefore allows the region inside the bias
ring to be fully e$cient; The distance between the
scribed edge and the bias ring is 700 }750 lm.
A single row of bonding pads, 200;60 lm, arelocated 1.7 mm from the detector end on the polySi resistor side. The pads at the other side arelocated at 1.0 mm. On the stereo side, 12 short
strips which are not read out are biased from theend opposite to the readout strips.
2.3. Designs specixc to the manufacturers
Although the geometrical design is similar, thecoupling capacitors, passivation, and edge treatments are speci"c to the manufacturers.

439

The coupling capacitor of HPK sensors has
a double-layer structure of SiO and Si N de veloped by chemical vapor deposition. The
double-layer structure helps to reduce creation of
pinholes. The thicknesses are 0.2 and 0.1 lm, respectively, which achieved a coupling capacitanceof 19 pF/cm (measured value). The passivation is
SiO .

The main feature of the SII process is the ONO(oxide}nitride}oxide) coupling capacitor structurewith the LOCOS (local oxidation silicon) process[12}14] (see Fig. 1). A layer of LOCOS SiO isdeveloped "rst keeping its thickness reduced at the
strip positions, where n> and p> strips are implanted underneath by ion bombardment. The
coupling capacitor is an ONO strip consistingof 0.15 lm thick Si N sandwiched by 0.04
 
and &0.005 lm thick SiO layers. On top of
the ONO strip located is a strip of poly-Si which is
DC coupled to the Al electrode. The ONO coupling structure can provide a large coupling
capacitance, which was measured to be 25 pF/cm.
The surface is "nally passivated by P-SiN (SiN
passivation deposited by plasma enhanced chemical vapor deposition). Although the structure
employed by SII looks complicated in comparison
with the HPK structure, the processes arecommonly available in conventional CMOS
technology.
HPK employed an n> scribe line implant and Al
covering the oxide layer on top, which eliminatesvariation in the charge on the oxide surface
and reduces the e!ects from the scribed edges. In
the SII design, no such structures were instrumented and the region outside the guard ring is
kept clear.

Fig. 1. Passivation and coupling capacitor of (a) SII and (b) HPK sensors.

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K. Hara et al. / Nuclear Instruments and Methods in Physics Research A 435 (1999) 437}445

3. Performance
3.1. Leakage current
Fig. 2 compares the total leakage current of someof HPK and SII sensors. The full depletion voltage
de"ned from C}< curves is about 90 V for HPK
and 55 V for SII sensors. We specify the leakage
current be less than 4 lA at the operation voltages,de"ned as 10 V above the full depletion voltages.
The HPK sensors showed a leakage current wellunder the speci"cation. The total leakage current of
SII sensors was about one order higher than that of
HPK sensors at the operation voltages, which stillsatis"es the speci"cation. We note that the I}<
characteristics vary among the production batches.
Though the leakage current of SII sensors shown inthe "gure increases gradually above the full depletion voltage, many other SII sensors showeda steep increase above 100}120 V. On the other
hand, some other HPK sensors showed no steep
increase up to 200 V. Along with the speci"cationon the total leakage current, we require that thecurrent increase above the full depletion voltage besmall in order to assure no such steep increase
exists in the voltage range below 100 (SII) or 160

Fig. 2. Total leakage current as a function of bias voltage. HPKand SII sensors are compared.

Fig. 3. Strip leakage current of HPK sensor C18 at 160 V.

V (HPK). The leakage current of the delivered 10
HPK sensors ranged from 0.2 to 1.8 lA (0.8 lA on
average) at 160 V, and that of 8 delivered SII
sensors ranged from 2.5 to 26 lA (10 lA on average) at 100 V.
In order to investigate the reasons for the increase of the leakage current, individual strip leakage currents were measured by probing the DC
pads instrumented on the implant strips andmeasuring the induced current across the probe
and the bias ring on the side under evaluation. The
strip leakage currents measured on the p-strip side
are plotted in Figs. 3 and 4 against the strip number. The data shown were taken at 160 V for HPK,and at 100 V for SII. There are some leaky strips
evident especially for the SII sensor. The SII sensor
in the plot showed a steep increase in the leakage
current above 100 V, and the leaky strips are apparently responsible for this increase. Similar tendency
was observed for other sensors and also for HPK
sensors measured at bias voltage where the leakage
current starts to increase steeply. We also con"rmed with an infrared CCD camera that IR lights
are emitted locally near strip edges. From theseobservations we attribute the steep current increase
to a number of strips which draw larger currents
due to micro-discharge.
The location of leaky strips is not identicalamong the di!erent sensors, which eliminates thepossibility of errors in the masks. We have experienced that HPK single-sided p-strip-type sensors
showed such a steep current increase much less

K. Hara et al. / Nuclear Instruments and Methods in Physics Research A 435 (1999) 437}445

Fig. 4. Strip leakage current of SII sensor C22 at 100 V. The
strip currents of more than 200 nA are truncated and plotted at
200 nA.

frequently while single-sided n-strip-type sensors
showed micro-discharges. As far as the HPK process is concerned, leaky strips probably originate in
the n-side process such as p-stop processing. De"nitive conclusion is, however, di$cult since quality
monitoring is not possible until the "nal passivation process is completed.
The strip current of quiet channels is &1 nA for
HPK and 5}8 nA for SII, which explains the total
leakage current di!erence at operation voltage. It is
pointed out [3,15] that the additional layer of silicon nitride increases the total leakage current, and[16] that etching o! Si N in the region between 
the strips prevents an increase of the leakage current. The present observation implies that etchingo! Si N does not reduce the leakage current to the level a single SiO layer can achieve, although we
cannot conclude de"nitively since the di!erence
might lie on the process quality of the two manufacturers.
3.2. Interstrip capacitance
The n-side interstrip capacitance was evaluatedby probing the AC bonding pads and using an
HP4275A LCR meter. The interstrip capacitance isde"ned as the capacitance between one central strip
to the next neighboring Al electrodes on both sides
while others are #oating. The data are taken as
a function of bias voltage at various LCR meter

441

frequencies, ranging from 100 kHz to 10 MHz. Theinterstrip capacitance is characterized better athigher LCR frequencies since the bias resistors exist
in parallel in the circuit. We measured that theinterstrip capacitance is not a!ected by the frequency choice in this frequency range in the bias
voltage region above the full depletion. Typical
data are shown in Section 3.3, where we discuss theirradiation e!ects. The n-side interstrip capacitancewas 9 pF/strip at the operation voltages. The
contribution from other strips was measured to be&1 pF. We therefore can achieve an n-side
capacitance of 30 pF when three sensors are wire
bonded.
3.3. Performance to Co c's irradiation
The ISL sensors will not receive more than 0.1
Mrad of radiation for an integrated Tevatron luminosity of 10 fb\. Taking these conditions into
account, we measured the electrical properties one
week after the irradiation of 0.16 Mrad Co c-rays.
We require that the total leakage current be lessthan 120 lA and no major changes be present inelectrical properties. Although the substantial partof the radiation at the Tevatron is due to hadrons,
we conducted a Co irradiation test to investigate
the di!erence of surface conditions between thetwo manufacturers; e!ects to the bulk are studied
elsewhere [17].
The irradiation was performed at the Co c-ray
irradiation facility of the Japan Atomic Energy
Research Institute (JAERI). The sensor was held ina test frame with &60 electrodes and bias-rings
wire-bonded to electrodes printed on the frame.
Four sensors, two of each manufacturer, were irradiated. The irradiation was interrupted at accumulated doses of 7, 35 and 160 krad while the I}<
characteristics were measured without the Co
source. The temperature of the irradiation cave was
173C. The interstrip and bulk capacitances were
measured before and one week after the irradiationof 0.16 Mrad.
Fig. 5 shows the leakage currents versus theelapsed time of two SII sensors and the average of
two HPK sensors. The data points correspond to
the last two irradiation periods and the intermission in between. The current readings of three of the442

K. Hara et al. / Nuclear Instruments and Methods in Physics Research A 435 (1999) 437}445

Fig. 5. Total leakage current as a function of elapsed time. The
source was on during the time indicated by the arrows. The bias
voltage of SII sensors was changed in the last irradiation period
as shown in the plot.

four sensors were noted as frequently as shown in
the plot with that of the fourth sensor (SII C12)
being recorded automatically. Right after thesource was set, all the sensors showed roughly thesame leakage current of &70 lA which is due tothe ionization by the source. The sensors of thesame manufacturer behaved similarly, but the SII
sensors showed a steep increase during the irradiation while the HPK sensors showed a constant but
small increase over the irradiation period. The bias
voltage of the SII sensors was lowered from 55 to
45 V midpoint in the last irradiation period whilethat of HPK sensors was kept at 120 V. The excellent behavior of HPK sensors is due to the superior
design of the edge n> implants and Al metallization
on top.
The I}< curve was measured typically 10}30 min
after the source was removed. Fig. 6 shows the I}<
curves of HPK sensor C12. The onset voltage of
micro-discharge shifted from 180 to 130 V by irradiation. The charge accumulation in the oxide
layers is attributed to this change, which enhances
the electric "eld near the p> implants. Another
HPK sensor showed essentially the same e!ects but
the shift of the onset voltage was about 20 V. The
shift in the SII sensors was not visible because of
the large leakage through the edges.
After the irradiation, the sensors were brought to
the University of Tsukuba where the sensors were
kept biased at a temperature of 203C. The total

Fig. 6. Leakage current of HPKC12 sensor, measured rightafter the Co source was removed. The number attached toeach curve is the accumulated dose.

leakage current decreased by about an order ofmagnitude in a day but nearly saturated thereafter;
the leakage current decreased by 21% (7%) for
HPK sensors and by 37% (28%) for SII sensors
when we compare the current one day (three days)
after the irradiation to that one week after.
The I}< characteristics measured before and one
week after the irradiation are shown in Fig. 7 forthe four samples. The HPK leakage current at theoperation voltage was about one order smaller
than our speci"cation of 120 lA while the SII leakage current was below but close to the speci"cation.
Both of the tested SII sensors had the onset voltageof micro-discharge at rather low voltages before
irradiation. We have conducted another irradiation
test of sample SII C01, one of the sensors with bestI}< characteristics (see Fig. 2), but we found it
behave similarly to the two SII sensors shown in
Fig. 7.
The strip current of one of the irradiated SII
sensors was measured six months after the irradiation. The results are plotted in Fig. 8. The leakage
current of quiet strips ranges from 50 to 60 nA at
100 V while it was about 10 nA before irradiation.
We note that the strip leakage current increases
gradually with bias voltage even above the fullK. Hara et al. / Nuclear Instruments and Methods in Physics Research A 435 (1999) 437}445

Fig. 7. The total leakage current as a function of the bias voltage(open marks) before and (solid marks) one week after the irradiation of 0.16 Mrad.

depletion voltage whereas a plateaux is visible
above the full depletion voltage for HPK sensors.
The mobility of ions in insulator materials is
small and radiation-induced ions could be accumulated in insulator layers, which could in#uences theinterstrip capacitance. On the n-side the accumulation layer e!ectively interconnects the strips andhence higher bias voltages are necessary to minimize the interstrip capacitance. The interstrip
capacitances on n-side and p-side were measured
before and one week after the irradiation, which are
shown in Figs. 9 and 10 for HPK and SII sensors.
The capacitance was measured at 1 MHz. After theirradiation, the bias voltage had to be raised by&5 V for HPK and &20 V for SII in order toaccomplish necessary n strip isolation. As described
before, additional silicon nitride layer degrades theleakage current characteristics. It can also be concluded that additional silicon nitride passivation
layer a!ects the interstrip capacitance change more
severely.
Silicon nitride such as in the MNOS
(metal}Si N }SiO }Si) devices is used for the pur pose of reducing the interface-trap density by

443

Fig. 8. Strip leakage current of irradiated SII sensor measured
six months after the irradiation of 0.16 Mrad. The bias voltages
are 50, 75 and 100 V.

Fig. 9. Interstrip capacitance of HPK sensor measured before
and after irradiation.

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K. Hara et al. / Nuclear Instruments and Methods in Physics Research A 435 (1999) 437}445

Fig. 10. Interstrip capacitance of SII sensor measured before
and after irradiation.

making a clean SiO }Si interface and of preventing(Na) ions to migrate by the outer Si N "lm [18].
 Also the conductivity of silicon nitride is orders ofmagnitude higher than SiO . From these argu
ments, we naively expect a better performance because of fewer oxide charges which include theinterface-trap charges, oxide-trap charges, mobile
ionic charges and "xed oxide charges [18]; change
in the sum of these charges should a!ect the interstrip capacitance. The function of the Si N }SiO 
interface possibly explains the observation. In
MNOS memory devices, although metal ions are
doped into SiO to form the interfacial chargestorage centers, the Si N }SiO interface can store charges [18]. At speci"c conditions such devices
can hold charges extending to 100 years.

4. Summary
Prototype double-sided Si microstrip sensors
were fabricated for the CDF-II ISL detector. The
sensors were manufactured by Hamamatsu Photonics and SEIKO Instruments using 4 technology.
The major di!erences in the design are the structureof the coupling capacitors and the passivation. SII
sensors have silicon nitride passivation while HPK
sensors have SiO .
Both HPK and SII sensors satis"ed the leakage
current requirements that the total current at theoperation voltage be less than 4 lA (100 nA/cm).
The leakage current of HPK sensors is, though,a factor of 10 smaller than that of SII sensors. Mostof the strips showed small leakage currents of 1}10
nA while some strips showed larger currents. These
strips are responsible for the steep increase in the
total leakage current, which is typically 80 V and
50 V above the full depletion voltage for HPK and
SII sensors, respectively.
The quality of the surface was compared using
Co c-rays. The leakage current at the operation
voltages increased by a factor of 40}100 one week
after 0.16 Mrad irradiation, which however are
within our speci"cation. The increase in the leakage
current, which is explained by charge accumulation
in insulation layers and/or at interfaces, remains atleast for months for SII sensors. The bias voltage
necessary to minimize the interstrip capacitance isshifted after the irradiation. SII sensors showeda larger shift of 20 V whereas HPK sensors showeda shift of 5 V.
Acknowledgements
We would like to appreciate K. Yamamuraof Hamamatsu Photonics, and Y. Saitoh andT. Akamine of SEIKO Instruments for discussions
on the sensor design. This work is supported in partby a Grant-in Aid from the Japanese Ministry of
Education, Science and Culture.
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