Nuclear Instruments and Methods in Physics Research A 636 (2011) S118–S124

Contents lists available at ScienceDirect

Nuclear Instruments and Methods inPhysics Research A
journal homepage: www.elsevier.com/locate/nima

Optimization of surface structures in n-in-p silicon sensors using
TCAD simulation
Y. Unno a,n, Y. Ikegami a, T. Kohriki a, S. Terada a, K. Hara b, K. Yamamura c, S. Kamada caInstitute of Particle and Nuclear Study, High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305-0801, JapanThe Institute of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japanc
Solid State Division, Hamamatsu Photonics Co. Ltd., Hamamatsu, Shizuoka 435-0051, Japanba r t i c l e in f o

a b s t r a c t

Available online 28 April 2010

In order to develop a novel n-in-p radiation-tolerant silicon microstrip sensor with a p-stop structure,surface structures are systematically analyzed for variations in the width, position, and multiplicity of
p-stops. With the help of technology CAD (TCAD) simulation, three dynamics are quantitatively
investigated: the dependence of the electric potential of the p-stops on the width of the p-stops, thesimilarity between the electric potential of multiple p-stops, which therefore function more like a single
large p-stop, and the correlation between the electric potential of the p-stops and the electric ﬁeld
strength in various conﬁgurations. The understanding of the electric potential of p-stops and the
maximum electric ﬁeld strength provides a guideline for reducing the electric ﬁeld at the implant edges
and thus for designing a p-stop structure for very-high-voltage operation.
& 2010 Elsevier B.V. All rights reserved.

Keywords:
Silicon detectors
Microstrip
Semiconductor device simulation
Radiation tolerant
Strip isolation

1. Introduction
A silicon semiconductor position-sensitive device, the silicon
microstrip sensor, has been widely used in elementary-particlephysics experiments. To date, the foremost radiation environmenthas been the large hadron collider (LHC) [1]. In the future, the LHC
will be upgraded to the super LHC (SLHC), which will result in an
increase in instantaneous luminosity that is 10 times that of the
LHC, i.e. increase to 1035 cm 2 s 1, and an integrated luminosity 5
times that of the LHC, i.e. increase to 3000 fb 1. Radiation-tolerant
silicon microstrip sensors will then face the new challenge of
coping with a particle ﬂuence of about 1  1015 1-MeV neutron
equivalent/cm2 at a radius of 30 cm from the beamline.
With the radiation damage in silicon, full or over-depletion of
300 mm thick silicon is unlikely even if the applied bias voltage
is high, e.g. 500 V. The aim of the research and development ofradiation-tolerant silicon microstrip sensors for the SLHC is toutilize a non-inverting material, a p-type wafer, and to readsignals out from the p–n junction side, the n + -implant strips, theso-called ‘n-in-p’ sensor [2,3]. Because a high voltage is required
towards the end of life of the sensor in order to cope withradiation damage, the proof for the high voltage, e.g. up to 1000 V
implicit in design, is required not only to secure a safety margin
for any degradation of the breakdown voltage after irradiation,
but also to ensure the high quality of the fabrication of sensors

nCorresponding author. Tel: + 81 29 864 5791; fax: + 81 29 864 2580.
E-mail address: yoshinobu.unno@kek.jp (Y. Unno).

0168-9002/$ - see front matter & 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.nima.2010.04.095

and in verifying the detector system to meet the system
speciﬁcations upon completion.
In an n-in-p sensor, the ﬁxed and the trap charges in the
Si–SiO2 interface, being predominantly positive, create an inversion layer on the n-side. An isolation structure is therefore
required to overcome this layer. During high-voltage operation, asudden increase in leakage current, called microdischarge, occurs
at positions where the electric ﬁeld strength exceeds silicon’s
avalanche breakdown ﬁeld strength of 300 kV/cm. Such a hot
spot in an n-in-p sensor is shown in Fig. 1. Optimization of thesurface structures in order to reduce the associated electric ﬁeld
in silicon becomes more critical in the n-in-p sensor for the SLHC.
Studies on n-side isolation in n-bulk wafers have been
conducted in the past [4–8], and those in p-bulk wafers havebeen conducted recently, in order to compare the isolation
techniques of p-stop, p-spray, and moderated p-spray (orequivalently, of p-stop+p-spray) structures [9,10]. The study
described in this paper is a systematic evaluation of thegeometries of p-stop structures, conducted in order to gain insight
into their electric ﬁelds and to provide a guideline for achieving
very-high-voltage operation. Other factors such as strip isolationand signal-to-noise ratio were beyond the scope of this study.

2. Technology CAD simulation
The semiconductor industry has developed sophisticatedprograms called ‘technology CAD (or TCAD)’ to simulate various
processes and devices, a ﬁeld that was pioneered by Dutton et al.
[11]. Though the program is capable of 3D simulations, we used

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the TCAD program ENEXSS/TiSSiEN [12] to simulate a device intwo dimensions (2D), which resulted in simplicity that was
sufﬁcient to elucidate the relative differences we explored.
A simpliﬁed surface geometry of p-stop structures is shown in
Fig. 2 for (a) a common p-stop, (b) an individual p-stop, and(c) a combined p-stop structure, that were abstracted from
Refs. [3,6–8]. Simulations were performed for the cross-section
between the two n + -strips. These three geometries describe thecases in which there are one, two, and three p-stop sections
between the n + -strips. The cross-sectional geometry model isshown in Fig. 3. An example of electric ﬁeld strength E (V/cm) is
shown in Fig. 4. In next sections, electric ﬁeld distributions near
the surface are measured at a depth of
1.3 mm where theelectric ﬁeld is the strongest at the corner of n-strips and p-stops,in order to show the relative differences.
The default values for the input simulation parameters are
summarized in Table 1. These values are used throughout thesimulations unless otherwise mentioned. An interface trap chargeof 1  1011 cm 2 is the default, except in selected simulations inwhich a charge of 1  1012 cm 2 is considered. The former charge
value is a typical of the /1 0 0S orientation [13], and the latter istypical in the case of ionizing radiation [14]. The concentration of

the p-stop of 4  1012 cm 2 is taken as a representative value andis a factor that is twice as dense as that used in the case of the
p-spray in Ref. [3]. A backplane bias of 200 V that was chosen
was considered sufﬁcient to deplete the n-side and to evaluate therelative differences among p-stop structures. The surface of the
SiO2 passivation is ﬂoating.

Fig. 1. Hot spot of microdischarge observed in an n-in-p sensor by an infrared
camera. The spot is at the edge of the n + electrode.

Fig. 3. 2D model of p-stop structures in TCAD simulation. The number andposition of p-stops are shown for the analyzed p-stop structures.

2.1. Common p-stop
The electric ﬁeld strength E (V/cm) distribution near the surface
is shown in Fig. 5 for a common p-stop structure in which thep-stop is 6 mm wide with an interface trap charge of 1  1011 cm 2(the legend 1e11) and 1  1012 cm 2 (1e12). Although position of
the peak moves from the edge of the n + -strip to the p-stop with

Fig. 2. Simpliﬁed schematics of p-stop structures: (a) common p-stop, (b) individual p-stop, and (c) combined p-stop. One, two, and three p-stop sections exist between the
n + strips.

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Fig. 4. 2D plot of electric ﬁeld strength E in the common p-stop structure. The x
coordinate is between the n + -strip and the p-stop. The bias voltage condition is
0 V in the n + -strips and 200 V in the backplane. High electric ﬁelds are at theedges of the n + -strip and p-stop.

Fig. 5. Electric ﬁeld strength E in a common p-stop structure with a 6 mm wide
p-stop, with interface trap charges of 1  1011 cm 2 (1e11) and 1  1012 cm 2
(1e12).

Table 1
Simulation input parameters.
SymbolItem

Default value

Pbulk

Bulk material

p-type: 4.7  1012 cm 3
(Resistivity: 3 kO cm)
320 mm (Full depletion: 365 V)
n-type: 1  1014 cm 2
8 mm
0.9+ 0.1 mm

tSi
Nsub
wNsub
tNsub

Bulk thickness
n + -strip materialn + -strip width (half)n + -strip thickness(depth+ smear)wAl
Al electrode width (half)tAl
Al electrode thicknesstCp
AC coupling material andthicknesstOx
Passivation material andthicknessPsub
p-stop material
wPsub
p-stop width
tPsub
p-stop thickness(depth+ smear)
Pback
Backplane materialtPback
Backplane thickness
Qit
Si–SiO2 interface trap
charges+V(n -strip)
Voltage of n + -strip
V(Al)Voltage of Al metalV(Backplane) Voltage of backplane

11 mm
1.8 mm
SiO2: 0.2 mm
SiO2: 0.8 mm
p-type: 4  1012 cm
6 mm
0.5+ 0.5 mm

2p-type: 1  1014 cm 2
5 mm
Positive: 1  1011 cm 2
0V
0V
200 V

increasing interface trap charge, the dependence of the two peakvalues on other parameters, e.g. the p-stop width, is similar, as canbe seen in Section 2.6 (Fig. 13).
The electric potential Psi (V) is shown in Fig. 6 for p-stop
widths of 6, 10, 15, 30, and 45 mm (legends P6, P10, P15, P30, andP45, respectively), together with no p-stop and a nil (NP Nil) or
1  1011 cm 2 interface trap charge (NP). With no p-stop and aninterface trap charge of 1  1011, the Psi is nearly ﬂat, as expected,
due to the conduction in the inversion layer. With no p-stop andnil interface trap charge, the Psi drops to50 V at the centre
between the n + -strips. This is the potential distribution in theideal case, i.e. the reference. As the width of the p-stop increases,

Fig. 6. Electric potential Psi near the silicon surface between n + -strips in common
p-stop structures with p-stop widths of 6–45 mm (P6–P45), together withreferences without p-stop and with interface trap charges of 1  1011 cm 2 (NP)and nil (NP Nil).

Psi drops more steeply, to about 100 V at 45 mm. The Psi value
vertically across depth (y) is shown in Fig. 7. The interface trap
charges of 1  1011 and 1  1012 cause no noticeable difference inthe potential of the p-stops. The electric ﬁeld strength E isbasically governed by this potential Psi and its activity in different
situations is summarized in Section 2.6 (Fig. 13).

2.2. Individual p-stop
The idea behind making the potential of the p-stop shallower
is to split the p-stop, thus creating a single ring of p-stops aroundY. Unno et al. / Nuclear Instruments and Methods in Physics Research A 636 (2011) S118–S124

the n + -strips. Two p-stops, each with a width of 6 mm, are placed
between the n + -strips. The potentials for four cases are shown in
Fig. 8: oP184, in which the two p-stops are separated by 6 mm,making the outer-edge-to-outer-edge distance between the two
p-stops equivalent to that of a common p-stop with a width of
18 mm; o P304 , in which they are separated by 24 mm and awidth of 30 mm; oP45 4, in which they are separated by 33 mm
and a width of 45 mm; and oP53 4, in which they are separated
by 41 mm and a width of 53 mm. The potential of the two p-stops
becomes deeper as the separation widens and approaches an

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equivalent width of 45 mm; the potential does not follow
the potential distribution of the reference. The potential of the
p-stops is shallower than that of common p-stops of equivalent
width. The potential, however, is deeper than that of common
p-stops of the width narrower than 15 mm.
2.3. Combined p-stop
A mixture of the common and individual p-stop yields the
combined p-stop. In this case, three p-stops exist between the n + strips. The electric potential Psi is shown in Fig. 9 for these threecases: for the legend oP304, the outer-edge-to-outer-edge
distance between the three p-stops is equivalent to a common
p-stop with a width of 30 mm, for oP45 4, it is equivalent to
45 mm, and for oP53 4, it is equivalent to 54 mm. The potentialof the outer two p-stops is dragged to the one at the centre,
although it is slightly shallower as the separation from the centre
widens. The potential is deeper than that of the equivalent width
in the case of an individual p-stop.
2.4. P-stop in an asymmetric position
The position of the p-stop between the n + -strips is varied tosimulate the asymmetric placement of the p-stop. The case of acommon p-stop that is 6 mm wide is considered. The electric
potential Psi is shown in Fig. 10 for the following cases: P6 for thecase of symmetric placement; oP184, in which the nearest edgeof the p-stop to the n + -strip is equivalent to a common p-stopwith a width of 18 mm; oP304, in which it is equivalent to
30 mm; and oP45 4, in which it is equivalent to 45 mm. The
potential of the p-stop has only weak dependence on the location.
2.5. Strip pitch

Fig. 7. Electric potential Psi charted vertically through silicon in common p-stop
structures with p-stop widths of 6–45 mm at the centre between the n + -strips(P6–P45), and at the n + -strip (P6 N-strip).

Another factor in the design of a microstrip sensor is the pitchof the strips. For a p-stop structure, two main criteria can bethought: (1) to maintain the width of the p-stop constant or (2) tomaintain the ratio of the width of the p-stop to the strip pitch

Fig. 8. Electric potential Psi in individual p-stop structures with outer-edge-toouter-edge distances between the two p-stops being 18–53 mm (o P184 too P534) and the reference (NP Nil).

Fig. 9. Electric potential Psi in combined p-stop structures with the outer-edge-toouter-edge distance between the three p-stops being 30–53 mm (o P304 tooP53 4) and the reference (NP Nil).

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Fig. 10. Electric potential Psi of asymmetric positions of 6 mm wide p-stops incommon p-stop structures, with the minimum distance between the n + -strip andthe p-stop equivalent to a p-stop width of 18–45 mm (o P18 4 to o P45 4).

Fig. 11. Electric potential Psi, in common p-stop structures with a constant p-stop
width of 6 mm and strip pitches of 36–150 mm.

constant. The electric potential Psi in the common p-stop
structure for the case of a constant width of 6 mm is shown in
Fig. 11 with strip pitches of 36, 50, 75, 100, and 150 mm, and in
Fig. 12 for the case of a constant ratio of 1:6. The potential of the
p-stops deepens as the pitch widens, and it becomes even deeper
in the case of the constant ratio, because the p-stop width widens
with widening pitch.

2.6. Maximum electric ﬁeld strength
The electric potential of the p-stops plays a fundamental role.
The distribution of potential deﬁnes the electric ﬁeld and the
maximum electric ﬁeld strength Emax (V/cm) at the edges of theFig. 12. Electric potential Psi in common p-stop structures with a constant width/
pitch ratio (W/P) of 1/6 and strip pitches of 36–150 mm.

Fig. 13. Maximum electric ﬁeld strength Emax vs. minimum distance between the
n + -strip and the p-stop, N–P gap of common p-stop structures with interface trap
charges of 1  1011 cm 2 (CP 1e11) and 1  1012 cm 2 (CP 1e12), individual p-stop(IP 1e11), combined p-stop (BP 1e11), and asymmetric position of common p-stop(AP6 1e11).

n + -strips and p-stops, which are best known from the TCAD
simulations. The maximum electric ﬁeld strength Emax issummarized as a function of the gap between the n + -strip andthe nearest p-stop, which is denoted as N–P gap in Fig. 13 for thevarious p-stop structures and in Fig. 14 for the strip pitches. These
dependences provide a guideline for the design of an n-in-p
sensor with p-stop structures for very-high-voltage operation.
Emax can be reduced as the positioning of the p-stops becomes
more symmetric, the p-stop width narrows, and the N–P gap
widens, even with multiple p-stops. Although the absolute valueof Emax should be regarded as qualitative, a relative Emax value
provides a quantitative measure as a guideline.
The case of Fig. 2 corresponds to a combined p-stop structurewith an N–P gap of about 3 mm along the horizontal cross-section.

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insulator of about 100 V is provided by a punch-throughprotection (PTP) structure. This structure is a narrow gap between
the bias-ring (n + ) and the end of the n + -strips. The voltage of then + -strips drops when a large current passes through the bias
resistor from a splash of charge particles into the silicon sensor,
e.g. due to mishandling of the beam.
A PTP structure in an n-in-p microstrip sensor is analyzed forthe gap between the bias-ring (n + ) and the end of the n + -strips,
an N–N gap of 20 mm with a p-stop that is 6 mm wide at the centre(P6 20 mm). Variations in the structure are those with an extended
aluminium electrode of the bias-ring (n + ) over the entire area ofthe p-stop (P6 HX2) proposed in this paper and with an interface
trap charge of 1  1012 cm 2 (P6 HX2 1e12). For a comparison,
analyzed is the PTP with the p-spray of 2  1012 cm 2 with theFig. 14. Maximum electric ﬁeld strength Emax vs. minimum distance between the
n + -strip and the p-stop, N–P gap of common p-stop structures with a constantwidth p-stop (W 6 mm) and constant W/P of 1/6.

Fig. 16. Electric ﬁeld strength E in PTP structure of p-stop with an extended Al
electrode, with applied voltage of 50 V in the n + -strip, applied voltage of 0 V inthe bias-ring, and applied voltage of 200 V in the backplane.

Fig. 15. Dynamic change of resistance between n + -strip and bias-ring (n + ) in thepunch-through protection (PTP) structure with p-stop width of 6 mm and n–n +distance of 20 mm (P6 20 mm), PTP structure with the extended Al electrode(p6 HX2), PTP structure with the extended Al electrode and with the interface
trap charge of 1  1012 cm 2 (P6 HX2 1e12), and PTP structure with p-spray of then–n + distance of 12 mm (R 12 mm).

The onset of the microdischarge occurred at around 400 V. In
Fig. 13 Emax is  300 kV/cm, which is almost equal to theavalanche breakdown ﬁeld strength at a bias voltage of 200 V.
By enlarging the N–P gap to 15 mm, e.g., Emax is reduced too150 kV/cm. The distance of the N–P gap must be balanced withthe onset voltage of the punch-through that is larger for the larger
N–P gap. A new fabrication process that employs a modiﬁed maskwith the N–P gap of 7 mm has successfully pushed the onset of
microdischarge above 1000 V [3].
If a conductive layer on top of the passivation layer exits, itworks like a ﬁeld plate of the n-strip potential and reduces theelectric ﬁeld at the n-strip and enhances at the p-stop edges. The
dependence on the N–P gap of the electric ﬁelds at those edges
follows the same trend as in the above.
2.7. Punch-through protection structure+

Protection against a situation in which the voltage of the n strips drops below the breakdown voltage of the AC coupling

Fig. 17. Electric ﬁeld strength E in PTP structures of p-stop with an extended Al
electrode and an n + -strip and bias-ring (n + ) distance of 20 mm (P6 20 HX2) and of
p-spray with a distance of 12 mm (R 12), under normal conditions of 0 V in boththe n + -strip and the bias-ring.

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N–N gap of 12 mm. The resistance for the N–N gap is shown in
Fig. 15. An example of the electric ﬁeld strength E of the PTP
structure (P6 HX2) is shown in Fig. 16 for a voltage of 50 V for ann + -strip when the PTP is on. Addition of the extended electrodehelps to reduce the onset voltage of the p-stop with N–N gap of
20 mm nearly equivalent to the p-spray of 12 mm. The onset
voltage is rather sensitive to the interface trap charges, and should
therefore be regarded as only a qualitative measure. The trend,
however, persists.
The electric ﬁeld strength E is shown in Fig. 17 under thenormal condition of both the n + -strip and the bias-ring (n + ) at
0 V. The maximum electric ﬁeld strength Emax is in the range of
60–150 kV/cm, which would still be safe for high-voltage
operation.

3. Summary
A novel radiation-tolerant silicon microstrip sensor developed
for the SLHC uses a non-inverting silicon material, a p-type wafer
and to read signals out from the n + -strips. In n-in-p siliconsensors, a n + -strip isolation structure with a p-type impurity isrequired in the surface of silicon in order to cope with theinversion layer formed by the positive trap charges at the Si–SiO2
interface. Three cases of p-stop structures – common, individual,
and combined – are systematically analyzed in terms of width andposition of the p-stops.
Three dynamics are quantitatively investigated: the dependence of the electric potential of the p-stops on the width of the
p-stops, the similarity between the electric potential of multiple
p-stops which therefore function more like a single large p-stop,and the correlation between the electric potential of the p-stops
and the electric ﬁeld strength in various conﬁgurations. The
distribution of electric potential deﬁnes the maximum electricﬁeld strength Emax at the edge of the n + -strips and the p-stops,
which is best understood from the TCAD simulations.
The dependence of Emax on the gap between the n + -strip andthe p-stop, i.e. the N–P gap provides a guideline for the design ofan n-in-p sensor with p-stop structures for very-high-voltage
operation. Emax can be reduced as the positioning of the p-stops
becomes more symmetric, the p-stop width narrows, and theN–P gap widens, even with multiple p-stops. Although theabsolute value of Emax may yet be qualitative, the relative
Emax value provides a quantitative measure as a guideline. An
example of fabrication has demonstrated the usefulness of thisguideline.

Acknowledgments
We would like to express our thanks to the detector development program of KEK for use of the TCAD program. This research
was partly supported by the Japan Grant-in-Aid for ScientiﬁcResearch (A) (Grant no. 20244038), Research (C) (Grant no.
20540291) and Research on Priority Area (Grant no. 20025007).

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