Nuclear Instruments and Methods in Physics Research A 699 (2013) 36–40

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

Evaluation of slim-edge, multi-guard, and punch-through-protection
structures before and after proton irradiation
S. Mitsui a,n, Y. Unno b, Y. Ikegami b, Y. Takubo b, S. Terada b, K. Hara c, Y. Takahashi c, O. Jinnouchi d,
R. Nagai d, T. Kishida d, K. Yorita e, K Hanagaki f, R. Takashima g, S. Kamada h, K. Yamamura ha

The Graduate University for Advanced Studies (SOKENDAI), 1-1 Oho, Tsukuba-shi, Ibaraki-ken 305-0801, Japan
Institute of Particle and Nuclear Study, High Energy Accelerator Research Organization (KEK), 1-1 Oho, Tsukuba-shi, Ibaraki-ken 305-0801, Japanc
Institute of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennoudai, Tsukuba-shi, Ibaraki 305-8571, Japand
Department of Physics, Tokyo Institute of Technology, 2-12-1 Okayama, Meguro-ku, Tokyo 152-8550, Japane
Research Institute for Science and Engineering, Waseda University, 1-6-1 Nishi-Waseda, Shinjuku-ku, Tokyo 169-8050, Japanf
Department of Physics, Osaka University, Machikaneyama-cho, Toyonaka-shi, Osaka 560-0043, Japang
Department of Education, Kyoto University of Education, 1 Fukakusa-Fujimori-cho, Fushimi-ku, Kyoto 612-8522, Japanh
Solid-state Division, Hamamatsu Photonics K.K., 1126-1 Ichino-cho, Higashi-ku, Hamamatsu-shi, Shizuoka 435-8558, Japanba r t i c l e i n f o

a b s t r a c t

Available online 2 June 2012

Planar geometry silicon pixel and strip sensors for the high luminosity upgrade of the LHC (HL-LHC)
require a high bias voltage of 1000 V in order to withstand a radiation damage caused by particle
ﬂuences of 1  1016 1 MeV neq/cm2 and 1  1015 1 MeV neq/cm2 for pixel and strip detectors, respectively. In order to minimize the inactive edge space that can withstand a bias voltage of 1000 V, edge
regions susceptible to microdischarge (MD) should be carefully optimized. We fabricated diodes withvarious edge distances (slim-edge diodes) and with 1–3 multiple guard rings (multi-guard diodes). AC
coupling insulators of strip sensors are vulnerable to sudden heavy charge deposition, such as anaccidental beam splash, which may destroy the readout AC capacitors. Thus various types of punchthrough-protection (PTP) structures were implemented in order to ﬁnd the most effective structure toprotect against heavy charge deposition. These samples were irradiated with 70 MeV protons at
ﬂuences of 5  1012 1 MeV neq/cm2–1  1016 1 MeV neq/cm2. Their performances were evaluated before
and after irradiation in terms of an onset voltage of the MD, a turn-on voltage of the PTP, and PTP
saturation resistance.
& 2012 Published by Elsevier B.V.

Keywords:
HL-LHCn-in-p sensor
Slim-edge
Multi-guard ring
Punch-through-protection

1. n-in-p pixel and strip sensor for ATLAS upgrade
The Large Hadron Collider (LHC) has been running at CERN in
Geneva since 2009 [1]. The LHC is planned to achieve an
integrated luminosity of 350 fb  1. From 2022, the upgraded
LHC (HL-LHC) will deliver an instantaneous luminosity 5 times
as high as that of the existing LHC. A target number of theintegrated luminosity is 3000 fb  1 at the HL-LHC. The ATLAS
inner detectors will be replaced with new silicon detectors for the
HL-LHC during the 2020 shutdown. In the upgraded ATLAS
detector at the HL-LHC, pixel sensors placed 3.3 cm and microstrip sensors placed 30 cm from the beam pipe will be exposed toa radiation ﬂuence of  2  1016– 1  1015 1 MeV neq/cm2
respectively. Therefore, we have been developing highly radiation
tolerant n-in-p silicon microstrip and pixel sensors for the HLLHC [2].

nCorresponding author.
E-mail address: smitsui@post.kek.jp (S. Mitsui).

0168-9002/$ - see front matter & 2012 Published by Elsevier B.V.
http://dx.doi.org/10.1016/j.nima.2012.05.071

We have chosen to focus on n-in-p type sensors, since radiation damage does not reverse the bulk type. Lattice defects
created by irradiation behave as p-type impurities in bulk silicon.
Therefore, radiation damage only increases the p-type carrier
concentration [3]. As a result, the n-in-p silicon sensor can always
read signals from the strip side. Furthermore, these sensors can be
operated under partial depletion conditions, because the depletion region always spreads from the readout strip side.

2. Hamamatsu sensors and proton irradiation tests
Various n-in-p test samples were fabricated by Hamamatsu
Photonics K.K [4] using ﬂoat-zone highly resistive silicon, FZ1 or
FZ3. FZ3 has a thinner depletion region than FZ1, since FZ3 is
processed with deeper backside Pþ implantation. Sample dimensions are 4  4 mm2 (slim-edge, multiguard) or 10  10 mm2(PTP) with a thickness of 150, 200, or 320 mm. PTP samples also
have a P-stop (Pþ is implanted between strips) or P-spray (Pþ issprayed on surface) strip isolation structure for preventing from

S. Mitsui et al. / Nuclear Instruments and Methods in Physics Research A 699 (2013) 36–40

37Table 1
Variety of slim-edge and multi-guard samples.
1st test, slim-edge (p-/n-edge) samples
200 mmN-bulkFZ1

320 mmFZ1

1st test, multi-guard (p-/n-edge) samples
200 mmP-bulk–N-bulkFZ1

320 mmFZ1
FZ1

2nd test, slim-edge (p-edge) samples
150 mmP-bulkFZ1, FZ3
P-bulk, P-spray
FZ1

320 mmFZ1
FZ1

2nd test, multi-guard (p-/n-edge) samples
150 mmP-bulkFZ1, FZ3
P-bulk, P-spray
FZ1

320 mmFZ1
FZ1

conduction between strips due to an electron accumulation layer
at the Si–SiO2 interface induced by positive charges in SiO2
insulation layer.
We performed two proton irradiation tests with the 70 MeV
beam at the Cyclotron and Radioisotope Center (CYRIC) at Tohoku
University [5] (Table 1). During the ﬁrst test, we irradiated the
PTP, slim-edge, and multi-guard ring samples of 200 and 320 mm
in thickness at 5.7  1012, 1.1  1013, 1.2  1014, 1.2  1015, and1.1  1016 1 MeV neq/cm2. All samples were annealed at 60 1C for
80 min after irradiation. During the second test, we irradiatedother slim-edge and multi-guard ring samples of 150 and 320 mm
in thickness at 1.1  1014, 1.2  1015, 5.7  1015, 1.2  1016
1 MeV neq/cm2. These samples were then annealed at 60 1C for
65 min equivalent after irradiation. The ﬂuences of 1 MeV neutrons is estimated from multiplying the ﬂuences of 70 MeV
protons by 1/0.7 by the NIEL hypothesis.
We evaluated the full depletion voltage (FDV), energy gap (Eg)and damage constant for the PTP samples in order to estimate theconsistency of the proton irradiation. FDV is estimated from body
capacitance of the sensorsﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ
sﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ
2eV NA þ N D
S
ee NA ND
d¼
, C bulk ¼ e ¼ S:
e NA NDd2V N A þ ND

Fig. 1. Comparison of the Full Depletion Voltage for present and previous
measurements.

Fig. 2. Temperature dependence of leakage current in BZ4D-5 irradiated at
1  1015; 1 MeV neq/cm2.

where NA is accepter density, ND is donor density, e is siliconpermittivity, V is bias voltage, e is elementary charge, d is thedepth of the depletion zone, and S is effective sensor area. The
inverse square of the body capacitance, 1/C2, is proportional todepletion thickness. After achieving the FDV, 1/C2 becomes aconstant, because the depth of the depletion zone, d becomes aconstant. In this way, we can estimate the FDV from the 1/C2
curve. The FDVs for BZ4D-1 through  5 are consistent withprevious measurements Fig. 1, [6].
The energy gap is obtained from the following formula:






 1 2
Eg 1
1

IðTÞ ¼ I T ref
exp
:
T ref
2kB T T refwhere Eg is the energy gap, kB is the Boltzmann constant, T istemperature, Tref is reference temperature, and I is leakage
current. Eg is calculated from the leakage currents at the FDV ata certain temperature [7]. We measured the PTP samples irradiated at 1.2  1015 1 MeV neq/cm2 at 60, 50,  40,  30, and20 1C. Tref was 20 1C. Eg was found to be 1.21 eV (Fig. 2).
The damage constant a is calculated fromDI ¼ a  V  F:
where DI is leakage current, V is effective sensor volume, F is the
ﬂuence. For a PTP sample irradiated at 1.2  1015 1 MeV neq/cm2

Fig. 3. Fluence dependence of leakage current in PTP samples.

at the FDV, the effective sensor volume is estimated to be
0.772 cm2  0.032 cm. The damage constant at the FDV is thenestimated as 4.6  10  17 A/m before annealing (Fig. 3).

38

S. Mitsui et al. / Nuclear Instruments and Methods in Physics Research A 699 (2013) 36–40

3. Slim-edge
We examined slim edge samples in order to ﬁnd the minimaledge width able to withstand a bias voltage of up to 1000 V for
minimizing the inactive area of the silicon sensor. MD caused byan avalanche of electrons occurs from the high electric ﬁeld
developed as the depletion region spreads from the implantelectrodes to the dicing edge.
At one side, edge width from the bias ring to the dicing edge
was varied from 80 to 964 mm. P þ or N þ regions are implanted
around the surface of the sensor edge. We examined p-bulk andn-bulk sensors with p-edge or n-edge investigating whether the
p–n junction in the edge region affects the bias voltage tolerance.
We measured the leakage current as a function of the bias voltage(IV curve) for 0 to 2000 V at  20 1C. We deﬁne the onset
voltage for MD as the point where the slope of the IV curve
becomes 5 times larger than the smallest slope in the lower
voltage region [8].
In Fig. 4 the abscissa shows the ﬁeld width (the width from thebias ring to the dicing edge without implantations) and theordinate shows the onset voltage for MD. The n-bulk n-edge
sensor breaks down less than 700 V, because a p–n junction exists
between the edge implantation and the bulk. We infer that then-bulk substrate changes to p-type through an irradiation ﬂuence
in excess of 1  1014 1 MeV neq/cm2, while the surface is still

Fig. 6. Field width dependence of onset voltage for MD for a p-bulk, p-edge
sample with 320 mm thickness.

Fig. 7. Structure of multi guard ring.

Fig. 4. Field width dependence of the onset voltage for MD.

n-type. The n-bulk p-edge samples have no p–n junction. Therefore, these samples show good bias voltage tolerance, although
the onset voltages of MD are about 700 V. We see that onset
voltage for MD increases with ﬁeld width until about 400 mm,
after which onset voltage becomes constant. The n-bulk or p-bulk(200 or 320 mm in thickness) may require Z450 mm ﬁeld width
to withstand 1000 V (Fig. 5). In Fig. 6, six samples have the sameﬁeld width of 230 mm with different edge widths. As each sample
breaks down at the same bias voltage, we conclude that the onset
voltage for MD is independent of the edge width.

4. Multiguard

Fig. 5. Fluence dependence of ﬁeld width hold up to 1000 V.

Guard rings are implanted around the bias ring in order toprevent high voltage break down around the edge by weakening
the electric ﬁeld. We investigated the various edge conﬁgurations,the number of guard rings, and the width of the guard ring inorder to ﬁnd the minimal overall edge width for withstanding
1000 V. We examined p-bulk and n-bulk sensors with 6 types ofmulti guard ring structures, 1GR-narrow, 1GR-middle, 1GR-wide,
2GR-middle, 2GR-wide and 3GR-wide (Fig. 7). Each sample has
the same ﬁeld width of 350 mm. We measured leakage current byapplying a bias voltage of 0 to  2000 V at  20 1C and estimatedthe onset voltage for MD in a manner as described previously [8].
Fig. 8 shows the MD onset voltage as a function of ﬂuence.
Before irradiation, the multi GR (2GR, 3GR) withstands a higher
bias voltage than the single GR. After irradiation, however, noS. Mitsui et al. / Nuclear Instruments and Methods in Physics Research A 699 (2013) 36–40

Fig. 8. Fluence dependence of onset voltage for MD for FZ1 p-bulk sample with150 mm thicknessc.

39

Fig. 11. Effective resistance for BZ4D-2 irradiated at up to  1  1015 neq/cm2.

Fig. 9. Hot electron photograph at 1800 V and 1900 V.

Fig. 12. Effective resistance for BZ4D-3 irradiated at up to  1  1015 neq/cm2.

Fig. 10. PTP types of bias Al extension. Only one side is extended.

difference is apparent. Both the 150 and 320 mm thick samples(FZ1 and FZ3) behave the same way.
4.1. Hot electron photograph
Using hot electron photographs, we identiﬁed the hot spot
location caused by the MD. A back-thinned cooled CCD camera
having infrared sensitivity can image any hot spots caused by MD.
Fig. 9 shows the hot spots for multi guard samples with thebias voltage of 1800 V and 1900 V. Break-down at the bias ringoccurs ﬁrst. As bias voltage increase further the guard ring thenbreaks down. This phenomenon is common to all samples
regardless of the number and width of the guard rings.
5. Novel punch-through-protection (PTP)
PTP structures should protect the AC coupling capacitors, forwhich break down voltage is designed to be 120–150 V. This

Fig. 13. Effective resistance for BZ4D-4 irradiated at up to  1  1015 neq/cm2.

break voltage may be surpassed by a large induced charge frome.g. an accidental beam splash. The PTP structure is to mitigate
the situation by discharging directly to the bias ring. Test samples
are p-bulk sensors of 320 mm in thickness. As shown in Fig. 10, we40

S. Mitsui et al. / Nuclear Instruments and Methods in Physics Research A 699 (2013) 36–40

saturation resistance r20 kO and 5 mA per strip (equivalentto 10 kMIPS/25 ns/strip). BZ4D-5 can protect the AC coupling
capacitor against an accidental beam splash.

6. Conclusions

Fig. 14. Effective resistance for BZ4D-5 irradiated at up to  1  1015 neq/cm2.

considered 5 kinds of structures with various Al width extended
from the bias ring and with or without a p-stop:
BZ4D-1: no Al extension, no p-stop
BZ4D-2: Al extension up to p-stop
BZ4D-3: no Al extension, p-stop
BZ4D-4: Al extension over p-stop
BZ4D-5: full Al extension with p-stop
We measured the resistance between the bias ring and the DC
pad connected to the n-strip implant by applying a voltage, V test,
until the effective resistance became as low as 20 kO otherwise at
most  150 V [9]. When the PTP structure is inactive the effective
resistance is about 1.5 MO, which corresponds to the resistance ofthe bias resistor. After the PTP becomes active, the effective
resistance decreases (Figs. 11–14). The punch-though voltage,
Vpt, is deﬁned as the voltage where the bias resistance decreases
by half. As the ﬂuence increases, the onset voltage and Vpt
increases (Figs. 11–14). BZ4D-5 performs the best, showing alower Vpt, a sharper cutoff, and a lower saturation resistance. A
wider Al extension shows a lower Vpt and a lower saturation
resistance. With the full Al extension, Vpt o100 V with a

We have evaluated radiation tolerance of n-in-p sensors with aplanar geometry by irradiating various samples at ﬂuences of up
to 1  1016 1 MeV neq/cm2. We examined slim-edge samples witha narrow edge width for minimizing the ineffective area of thesilicon sensor. A large ﬁeld width leads to a higher bias voltage
tolerance. Both n-bulk and p-bulk require a ﬁeld widthZ450 mm
in order to withstand 1000 V. The MD onset voltage was found to
be independent of the edge width.
We investigated whether the number of guard rings and thewidth of the guard ring help reduce the overall edge width. With
increasing number of guard rings, bias voltage tolerance
improved before irradiation. With increasing bias voltage, bias
ring break-down occurs ﬁrst, followed by guard ring break-down.
We evaluated novel PTP structures and found that BZ4D-5(with full extension of the bias ring width) performed the best.
This structure should be able to protect the AC coupling capacitor
even after an irradiation of 1  10151 MeV neq/cm2.

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