Radiation Damage Studies of Silicon Microstrip Sensors
T. Nakayama1 , S. Arai1 , K. Hara1 , M. Shimojima1 , Y. Ikegami2 , Y. Iwata3 , L. G. Johansen4 ,H. Kobayashi1 , T. Kohriki2 , T. Kondo2 , I. Nakano5 , T. Ohsugi3 , P. Riedler6 , S. Roe6 , S. Stapnes7 ,B. Stugu4 , R. Takashima8 , K. Tanizaki1 , S. Terada2 , Y. Unno2 , K. Yamamoto9 , and K. Yamamura9
1

Institute of Physics, University of Tsukuba, Tsukuba, Ibaraki 305-8751, Japan
IPNS, High Energy Accelerator Organization (KEK), Tsukuba, Ibaraki 305-0801, Japan3
Department of Physics, Hiroshima University, Higashi-Hiroshima, Hiroshima 739-8526, Japan
4
Department of Physics, University of Bergen, N-5007 Bergen, Norway
5
Department of Physics, Okayama University, Okayama, Okayama 700-8530, Japan
6
PPE, CERN, CH-1211 Geneva 23, Switzerland
7
Department of Physics, University of Oslo, N-0316 Oslo 3, Norway
8
Department of Education, Kyoto University of Education, Fushimi, Kyoto 612-0863, Japan
9
Solid State Division, Hamamatsu Photonics K.K., Hamamatsu, Shizuoka 435-8558, Japan
2

Abstract

respects. The detection efficiency is not much degraded at bias
Various types of large area silicon microstrip detectors were voltages lower than the full depletion voltage, since thefabricated for development of radiation-tolerant detectors readout is the junction side after type inversion. New types ofoperational in the LHC ATLAS SCT. The detectors were p-stop structures are configured in the n-on-n samples, which
irradiated with 12-GeV protons at KEK to fluences of 1.7×1014 are to be compared for radiation hardness.
The SCT group has chosen p-on-n detectors as the baseline
and 4.2×1014 protons/cm2. Irradiated samples include n-on-ndetectors with 4 kΩ cm bulk resistivity and p-on-n detectors despite the above benefit. Through the development of edge
with 1 kΩ cm and 4 kΩ cm bulk resistivities. Four patterns of structures which can sustain high bias voltages, p-on-np-stop structures are configured in the n-on-n detectors. detectors are shown to work at voltages exceeding ~350 V. An
Although Hamamatsu fabricated most of the detectors, included efficient cooling system is, though, required to transmit theare p-on-n detectors by SINTEF, and those fabricated in heat generated by leakage current in the detectors operated at
modified process by Hamamatsu. The performances after such a high bias voltage. Since temperature has also a very
irradiation are compared through the probability of creation of strong effect on the reverse annealing of the full depletion
faulty coupling capacitors, C-V characteristics, charge voltage [4,5,6], operation at lower temperatures makes thecollection curves, and total leakage current. The p-on-n detector lifetime longer. Other important issues in choosing pdetectors are similar to n-on-n detectors in these performances, on-n detectors are that the electric field around the strips
reduces with irradiation because the junction side moves to theand are operational in the ATLAS radiation environment.
backplane and that they require only one type implantation andare fabricated in simpler true single-sided process. The latter
I. INTRODUCTION
fact can increase the production yield and reduce the production
The silicon microstrip detectors are extensively used in cost, which is crucial for a large-scale production as for the
recent high-energy physics experiments for compactness and ATLAS SCT.
capability of high precision tracking. Intensive studies are
Irradiation was performed in two periods on separate
being carried out in order to make it possible to operate the samples to fluences of 1.7×1014 protons/cm2 andῌ 4.2×1014
detectors in future hadron collider experiments such as at LHC. protons/cm2. The leakage current characteristics and body
The radiation environment of e.g. ATLAS barrel SCT (semi- capacitances were measured as a function of bias voltage. The
conductor tracker), which will be located in the region 30−52 charge collection was evaluated from the pulse height
cm from the beamline, will receive approximately 3×1014 cm−2 distribution for penetrating β rays. For the samples irradiatedmulti-GeV protons equivalent in 10 years of operation [1]. to 1.7×1014 protons/cm2, we probed the coupling capacitors to
Such a heavy radiation produces defects within the silicon, measure the number of faulty capacitors created by radiation.
which manifest themselves through an increase in leakage We also measured the resistivity at the detector edge in order tocurrent, in a change to the effective resistivity and in a drop in study the possibility of biasing from the front side.
the collectable charge. A notable change occurs in the bulk
One of the subjects in this study is to compare thetype transition from n to p, as radiation-induced defects tend to performances between n-on-n and p-on-n detectors, and between
act as acceptors.
new p-stop structures employed for the n-on-n detectors. The
In a program of developing radiation-tolerant detectors for p-strip detectors included two initial bulk resistivities, 1
the ATLAS SCT, we have fabricated various types of detectors kΩ cm and 4 kΩ cm. The second subject is to compare theseand irradiated them with 12-GeV protons at the KEK PS. All two at the two proton fluences. If the effective acceptors
the detectors are full-sized prototypes of single side readout, created by radiation are cancelled partially by the initial donors,
including p-strip readout (p-on-n) and n-strip readout (n-on-n). we expect the detectors made using low resistive bulks become
Previous studies [2,3] have shown that irradiated n-strip radiation harder. On the other hand, there are studies that thereadout detectors perform better than p-strip readout in some

initial donors are removed at low fluences and therefore detector
performance after heavy irradiation becomes similar [7,3].
Most of the irradiated samples were fabricated byHamamatsu Photonics (HPK). Some p-on-n samples by
SINTEF [10] were also irradiated to see possible difference
between the manufacturers. In the tested samples included are
Hamamatsu p-on-n detectors of which n-side processing was
modified. Since the leakage current after irradiation is almost
proportional to the detector volume and thinner detectors of~285 µm are thus preferred for the ATLAS SCT, Hamamatsu
fabricated such detectors starting from 325 µm thick wafers.
Performance comparison of these modified p-on-n detectors
with the conventional ones is another subject in this study.
Details of the modified process are described below.

All the detectors have the same dimensions of 64.0 mm(along strips) × 63.6 mm, processed with a 4” technology
using n-type wafers. The detector thickness ranged from 285 to
328 µm. The detectors are AC coupled and single sided with areadout strip width of 16 µm and a strip pitch of 80 µm. There
are 768 readout strips in a detector. The detector bias isprovided through 1.5 MΩ poly-silicon resistors.
Table 1 summarizes the proton fluences and types ofdetectors used in this study. Typically two detectors were
irradiated at each condition.
Figure 1 shows the four p-stop configurations of theHamamatsu n-on-n detectors irradiated to 1.7×1014 protons/cm2.
In the full common configuration (Hn4kFC), the p-stop is
implanted between the strips keeping a gap of 10 µm between
the p-stop and n + implants. The p-stop implants are floating
and connected together beyond the ends of the n-strips. In theother three, Hn4kID, Hn4kIA and Hn4kIB, the p-stop
configuration is identical where each n-strip is surrounded by
the p-stop individually.
Table 1
Tested silicon microstrip detectors. Hn4k* denotes four variants
in p-stop configurations: Hn4kID (individual), Hn4kFC (fullcommon), Hn4kIA (individual A) and Hn4kIB (individual B). See
text for details of the structure.
Fluence[p/cm2 ]

Maker

Bulk
resistivity

p-on-nn-on-n0HPK

4 kΩcm

Hp4k-0Hn4k*-0

Hp4k-17

Hn4k*-17

HPK

SINTEF

4.2×1 014

HPK
SINTEFAl
p-stopn+

Individual

A. Detector Samples

1.7×1 014

Aln+II. DETECTOR IRRADIATION

4 kΩcmIn Hn4kIA a poly-silicon plate is in addition developed on
the surfaceῌcovering each p-stop, and in Hn4kIB single polysilicon strip is made covering the two p-stop implants atneighbor. The poly-silicon plates, which are extended in width
over the p-stop, are DC connected to the p-stop implants
through the SiO2 layer. With this configuration the potential
underneath the extended region is stabilized and the charge
accumulation isῌtherefore suppressed. Without such a plate,
negative ῌ charge accumulated by irradiation constructs astrong electric field in the vicinity of the edges of p-stop
implants, which tends to cause micro-discharge [8].

Hp4km-17

1 kΩcm#Hp1k-174 kΩcm#Sp4k-17
Hn4kID-42

4 kΩcm1 kΩcm#Hp1k-424 kΩcm#Sp4k-42

The wafer plane direction is <111> except for those denoted by #,which are <100>.

n+n+Full common
poly-Sin+p-stopp-stopIndividual A

poly-Sin+n+p-stopn+

Individual B

Figure 1: Four p-stop configurations of HPK n-on-n detectors.

In the conventional Hamamatsu p-on-n detector process, n+layer on the backplane is deposited first and then p-strip sidestructures are processed. More careful handling is required forprocessing on thinner wafers. The sample Hp4km is a p-on-ndetector modified in the n side processing. These detectors were
processed first on p-strip side using 325 µm thick wafers.
After the p-strip side processing is completed, the backplane
was etched down to 285 µm. Then the n+ layer was developedon the backplane by ion implantation where lower
temperatures can minimize the damage to the pre-processed pstrip side structures.

B. Proton Irradiation
The detectors were irradiated with 12-GeV protons at the
KEK PS. The irradiation setup is described elsewhere [11]. The
irradiation to 4.2×1014 protons/cm2 was performed in
January1998 for two p-on-n and one n-on-n detectors (seeTable 1). Reference [9] describes the conditions of theirradiation to 4.2×1014 protons/cm2, and [3] describes theresults including the detection efficiency in the interstrip
region studied in a π − testbeam. The irradiation to 1.7×1014
protons/cm2 was performed in June 1998 for 16 detectors. The
detectors were placed in a thermo-box. The temperature in thethermo-box was −5oC (−7oC) during the January (June) proton
irradiation period. The detector temperature at storage was 0oC.
The proton beam spread was monitored with PIN
photodiodes attached in front of the detectors. With thisinformation we moved the thermo-box periodically in order toirradiate the detectors uniformly. The actual fluence anduniformity was measured through the process p+Al→ 7Be+X
by activating 1×1 cm2 aluminum foils arranged covering theentire detector surface. The neutron contamination in thebeamline was evaluated by comparing the reactions
n+Al!7Be+X and n+Al!22Na+X. The contamination
amounted to ~20%, and was added to the proton fluence. The
fluence over the detector plane was uniform to 14% and 5% in
the first and second irradiations, respectively.

III. PERFORMANCE RESULTS

We note that large rms spreads of n-on-n detectors are due tosome of the faulty capacitors and the spreads of genuine
capacitors are quite similar among the Hamamatsu detectors.
The coupling capacitances of the two SINTEF detectors turned
to be smaller near the end strips by at maximum 40%. The
faulty capacitances for SINTEF were defined by 25% deviationfrom the local average taking into account of the structure overthe strips.

B. I-V and C-V characteristics

The coupling capacitors could potentially be damaged byintense radiation during the ATLAS experiment. Although thereadout strips are held at ground in the ATLAS SCT design,large leakage current after radiation damage will be routed tothe preamplifier if the damaged capacitor turns to be shorted.
The coupling capacitors of the detectors irradiated to
1.7×1014 protons/cm2 were measured at 1 kHz with applying 5
V across the aluminum strip and the implant strip underneath.
No reverse bias voltage was applied to the bulk. We defined
faulty capacitors if the capacitance is off by more than 25%
from the average. Table 2 lists the numbers of faulty
capacitors and the average capacitances before and after
irradiation. Most of the faulty capacitors after irradiation werefaulty at pre-irradiation measurement, and quite a few faulty
capacitors were created by irradiation (see numbers inparentheses in the table). Except for some capacitors of Sp4k17 and Hn4kID-17, which became nearly shorted, most of thedamaged capacitors are still usable.
The average capacitances are found to increase for p-strip
and decrease for n-strip detectors. Negative charge induced
nearby the implant electrodes effectively widens the electrode,so it can influence the capacitance measurement, which
explains the increase observed for p-strip detectors. For n-strip
detectors on the other hand, negative charge around the implant
strips is swept away after the ohmic n+-n contact side changed
to n+-p junction side.

The I-V and C-V characteristics of the samples irradiated to
4.2 (1.7) ×1014 protons/cm2 were measured 16 (10) months
after the irradiation was completed. The detectors were kept at
0oC in a refrigerator but taken out for typically a few hours in
preparation at room temperature for charge collection
measurement described in the next section. The detectors
irradiated to 4.2×1014 protons/cm2 were in addition kept for 5days at room temperature and 7 days at 28oC [3].
The full depletion voltage of irradiated samples show
annealing at first and then anti-annealing because of different
time scales involved in these processes [5]. A calculation
results that the both sets of samples have experienced the firstannealing and the samples irradiated to 4.2×1014 protons/cm2
have increased the full depletion voltage by about 25%.

Table 2
Summary of numbers of faulty coupling capacitors measured before
and after irradiation. The numbers of faulty capacitors created byirradiation are shown in parentheses. The errors of pre-irradiateddetectors are rms spreads.
#bad strips
Capacitance (pF)
Detector
BeforeAfter
BeforeAfter
Hp4k-17
Hp1k-17
Sp4k-17
Hn4kFC-17
Hn4kID-17
Hn4kIA-17
Hn4kIB-17

0

4 (4)

131±3

+10

0

0 (0)

132±2

+10

0

0 (0)

117±4

+17

2

2 (0)

121±6

+26

5

4 (2)

151±14

+18

0

8 (8)

148±16

+19

6

6 (0)

144±21

−6

5

5 (0)

129±20−5

18

21 (6)

148±26−8

11

10 (1)

145±20

−6

1

1 (0)

143±5

−5

1

1 (0)

133±7

−3

2

2 (0)

142±6

−4

1

1 (0)

137±5

−6

Body capacitance (nF)

A. Damage to coupling capacitors

5

14

Hp1k-42
Hn4k-421

20

Hp4k-17
Hp1k-17
Hn4k-17

20

100

20

100

14

1.7x10500
Hp4k-0
Hp1k-0Hn4k-0

5

1

500

100

5

1

4.2x10pre-irrad

500

Bias voltage (V)Figure 2: C-V characteristics of Hamamatsu Hp4k, Hn4kID and
Hp1k detectors, measured at pre-irradiation and after 1.7×10 14
and 4.2×1 014 protons/cm2 .

The body capacitance was measured at a frequency of 1 kHz.
The post-irradiation measurements were made at −30oC inorder to keep the total leakage current small while the preirradiation measurements were at room temperature. Figures 2
and 3 show the C-V and I-V curves of samples Hp4k, H4nk
and Hp1k. For ideal planer diodes the body capacitance is
expected to decrease with bias voltage as V−0.5 until the bulk isfully depleted, as can be seen in the data before irradiation. The
C-V curves of irradiated detectors, however, show different
characteristics: C decreases as V−0.5 at low voltages and thensteeper before the capacitance becomes constant. Similartendency was observed for other detectors. The C-V10of Hp4k-0 detector. Among the above samples, Hn4kID-42
showed large noise and the peak was not reliably determined atabove 400 V. Sp4k-42 showed the peak increasing up to 500
V, which was the maximum our equipment was able to supply.

3a) ch-1b) ch-1a) ch-2b) ch-2a) ch-3b) ch-3

post-irrad.
Hp4k-17
10

2

0

100

200

Hp1k-17

Hp1k-42

Hn4k-17

Hn4k-42

300

400

500

Bias voltage (V)Figure 3: I-V characteristics of Hamamatsu Hp4k, Hn4kID and
Hp1k detectors (post-irradiation measurement). The leakage
current is expressed at −10 o C.

The leakage current at −10oC is typically 300-400 µA at
200 V after 1.7×1014 protons/cm2 and 800-900 µA at 300 V
after 4.2×1014 protons/cm2. It is not clear at this stage at
which voltages the leakage currents should be compared: The
above voltages are those determined from the charge collection
curves, as discussed in the next chapter. The leakage current is
proportional to the proton fluence after taking into account theanti-annealing effects in the depletion voltage for the samples
irradiated to 4.2×1014 protons/cm2.

C. Charge Collection
The bias voltage relevant at operation should be evaluatedfrom the charge collection efficiency. We have carried out themeasurement using a 90Sr β source. Signals out of threeneighboring strips were fed separately into CAMAC ADCs
through low-noise preamplifiers HIC-1576 [12] and shapers
with 200 ns shaping time. A pair of 300 µm wide and 10 mmlong slits were machined in 5 mm thick aluminum plates andwere aligned along the center strip with placing the detector in
between. The ADC gate was generated by a scintillating
counter set underneath the bottom slit. Several aluminumelectrodes besides the three readout strips were wire-bonded to
ground.
The detector and preamplifier were placed in a thermostat
chamber and cooled to −30oC. Figure 4 shows typical pulse
height distributions of the three channels. The distributions
before selection are evident for pedestals (around 100 counts)
and signals (around 450). In order to ensure that β rays passed
through the center strip, we required that the signal out of thecenter strip was maximal in an event. With this selection,
only the center strip shows a peak while the strip at neighborshows a pedestal and tail which corresponds to the case where
the track was shared between the two. The three ADC counts
were then summed with the selection applied, and the peak wasderived by fitting a Gaussian function with asymmetricvariances around the peak. The peak positions as a function ofbias voltage are plotted in Figure 5 for some detectors. The
voltage and charge values are corrected for the detector
thickness and expressed for 300 µm thickness. The ordinate isthen normalized by the plateau in the charge collection curve

Number of events

Total leakage current (µA)

characteristics at 1.7×1014 cm−2 clearly shows that detectors
with 1 kΩ cm reach the plateau at voltage lower than Hp4k andHn4k. However, since the full depletion voltage can not be
extracted reliably from such curves, other standard is setup in
the next chapter to evaluate the relevant voltage to compare.

ADC counts

F igure 4: Charge distributions out of three neighboring strips(a) without any selection and (b) with selection described in the
text. In b), pedestals are subtracted.
We evaluated the absolute charge scale by comparing thepulse height to β rays with the 60 keV photoelectric peak of241
Am γ 's. We assumed the energy of 3.6 eV to create anelectron-hole pair in silicon. The calibrated pulse height for βrays was 3.2 fC per 300 µm after correcting for detector
thickness and the β ray angle.
After 1.7×1014 cm−2, there is small difference between
initial p-on-n (Hp4k-17) and n-on-n (Hn4k-17) detectors in the
shapes of the charge collection curves though the maximum
collected charge is smaller by ~10% for initial p-on-n detectors.
The pre-irradiation detectors show that the collected charge
drops quickly for n-on-n detectors at low voltage, which is
explained by the fact that the readout strips are not wellisolated electrically. Despite the fact that reading out is on theohmic side for irradiated p-strip detectors, the charge collection
at low voltages is not much degraded. Negative charge
accumulated between the p-strips functions as to isolate themand the regions around the strips may be preferentially depleted
at lower voltage. At higher voltages the region between strips
is gradually depleted. As we have seen, C-V curves of irradiatedp-strip detectors show two slopes to be fully depleted. The two
structures in the C-V curves may correspond to the two
depletion processes mentioned here.
The difference between Hp1k-17 and Hp4k-17 is evident,
while the difference in shape becomes small between Hp1k-42
and Hn4k-42. Although we have no sample of 4 kΩ cm p-on-ndetectors irradiated to 4.2×1014 protons/cm2, the difference due
to bulk resistivity is probably small at this fluence for thesimilarity in shape between p-on-n and n-on-n detectors at
1.7×1014 protons/cm2. In the fluence range below ~2×1014
protons/cm2, the higher initial donor impurity can compensate
the radiation created acceptors and use of low resistivity wafer

1
0.9
0.8
0.7
0.6

1
0.9
0.8
0.7
0.6

0.5

0.5

0.4

0.4

0.3

Hp4k-0Hn4k-0

0.2
10

2

0.5

0.4

0.4
Hp4km-17
Sp4k-170.2
102

Hp4k-17
Hn4k-17
Hp1k-1710

0.5

V R = V
≥ 0
I0
I

I
V

n+

R

Biasring

A

Io

0.2
1
0.9
0.8
0.7
0.6

0.3

R=

0.3

1
0.9
0.8
0.7
0.6

Figure 6 shows R 0 as a function of V s for irradiated p-strip
detectors. The resistance increases with the fluence and reaches~100 kΩ cm. The resistance is even larger for the detectors
with 4 kΩ cm resistivity.
The detector current will increase to ~0.5 mA after 10 yearsof operation. The voltage drop across the above resistance isthen ~50 V, which has to be over biased. Comparing thedesigned maximum operation voltage of 350 V, ~50 V is at amarginal level, provided that R is not much different from R 0.
If we intend to operate the detectors further longer, the ultimate
detector lifetime is determined by the thermal runaway whichhappens at leakage current of ~2 mA [13]. The voltage drop
reaches 200 V in this extreme case, which certainly requires redesigning of the overall biasing system.

Al

Vs

p+

i
Guardringbulk: npn+

2

Ro (k Ω)

Collected charge

may have the possibility to enlarge the safety margin in thedetector operation.
The charge collection curve of Hp4km-17, manufactured in
the modified process, is shifted to lower voltage than that of
Hp4k-17. The maximum collected charges of these two
samples agree with each other within ~10%.
Comparing to Hamamatsu Hp4k detectors, SINTEF
detectors showed consistently larger voltages by ~20% to turnto the plateau. The maximum collected charges at 1.7×1014
cm−2 is somewhat smaller but are consistent at 4.2×1014 cm−2.

Hp1k-42
Hn4k-42Sp4k-4214

φ = 1.7x10

0.3

p/cm2

14

p/cm2

14

p/cm2

φ = 4.2x10

0.2
10

2

φ = 1.7x10

Bias voltage (V)Figure 5: Charge collection as a function of bias voltage. The
ordinate is normalized by maximum collected charge of Hp4k-0,
3.2 fC.

D. Front-biasing
In Hamamatsu detectors a wide n+ is implanted in the edge
region surrounding the entire sensitive area. Aluminumelectrodes are DC connected to the n+ implant through the
SiO2 layer. This design minimizes the effects on leakage
current increase from the scribed edges and from environment.
The same aluminum electrode can be biased to fully deplete thedetector through n+-n- n+ ohmic contact to the backplane.
If such an electrode is usable also for irradiated detectors,
the biasing scheme can be simplified: conductive epoxy or
wire-bonding to the backplane can be avoided. The resistance
between the front electrode and backplane is, though, expected
to increase after bulk inversion since reverse biasing is required
across n+-p junction. We have measured the bulk resistancenear the edge in a configuration shown in Figure 6. With
applying positive voltage (source voltage V s) on the edge
electrode with respect to the bias-ring, the voltage across theedge electrode to the backplane (V) was measured. The current
I0 generated by V s is collected through n+-(inverted p)-n+ andpartially through the surface. Therefore, the edge resistance R
is equal to or greater than R 0 defined as the ratio of V and I0.

Vs (V)

Figure 6: Edge resistance of irradiated detectors as a function ofsource voltage. The measurement configuration is shown above.

IV. DISCUSSIONS
We compare the performance of various detector samples
through the maximum collected charge Qmax and thecorrelation between C-V and charge collection curves. The
shape of the charge collection curve is characterized by thevoltage, V Q(95%), where 95% of Qmax is accumulated.
Similarly the C-V curve is characterized by the voltage,
V 1/C(95%), at 95% of the plateau in 1/C, which is supposed to
be proportional to the depletion thickness and hence the charge
generation. We corrected for the detector thickness and the data
are expressed corresponding to 300 µm thickness. The results
are summarized in Table 3. The leakage current in the table isat V Q(95%) and expressed at −10oC.
The voltages V 1/C(95%) are systematically larger than
V Q(95%). The correlation between the two is plotted in Figure
7. As can be seen, V 1/C(95%) is substantially lower than
V Q(95%) for p-strip detectors, whereas the deviations aremoderate for n-strip detectors. C-V curves are often used toTable 3
Summary of detector performance. The detector thickness iscorrected and the values are for 300 µm. The 95% voltages incharge collection of Sp4k-42 and Hn4k-42 were not reliably
determined as described in the text.
95%Voltage (V)
Ileak
Qmax
Detector
Charge1=3.2 fC
1/C[µA]
collection
Hp4k-0

1

47

62

0.004

Hn4kID-0

1.03

65

51

0.02

Hp4k-17

0 .8 7

195

119

460

Hp1k-17

0.85

168

77

570

Hp4km-17

0.80

222

77

500

The maximum collected charge Qmax is plotted in Figure 8
as a function of proton fluence. The charge decreases with thefluence at a rate of ~2% per 1014 protons/cm2 for Hn4k, and of~8% for Hp1k and Sp4k detectors. Although the Hp4k data at
4.2×1014 protons/cm2 is not available and the two data points
scatter at 1.7×1014, Hp4k is consistent with Hp1k and Sp4k.
There is small difference in the bulk resistivity and in the
manufacturers as far as the drop in the charge collection isconcerned.
Collected charge

estimate the operation voltage. It is, however, obvious fromthe plot that irradiated p-strip (n-strip) detectors require muchhigher voltage by roughly 70% (20%) to achieve sufficiently
high charge collection. At 1.7×1014 protons/cm2, there issmall difference in terms of V Q(95%) between the two
Hamamatsu p-on-n detectors with 1 kΩ cm and 4 kΩ cm bulk
(H4pk and H1pk), and p-on-n detector fabricated in modified
process (Hp4km). For SINTEF detectors (Sp4k-17 and Sp4k42), V Q(95%) is ~20% larger than that of Hamamatsu
detectors (Hp4k-17 and Hn4k-42).

1.0
0.9
0.8

Hn4k
Hp4k
Hp1k
Sp4k

0.7
0.6

0

1

2

3

4
14

Fluence (x10

5
2

/cm )

Figure 8: Maximum collected charge as a function of proton
fluence.

0.77

231

140

510

1.00

218

158

700

Hn4kFC-17

0.99

261

197

630

Hn4kIA-17

0.93

269

189

950

Hn4kIB-17

0.99

256

243

610

Hp1k-42

0.70

402

2 15

860

Hn4kID-42

0.93

~300

309

~820

IV. SUMMARY

Sp4k-42

0.71

~500

290

~1020

We have fabricated various types of silicon microstrip
detectors and irradiated them with 12-GeV protons to fluencesof 1.7×1014 or 4.2×1014 protons/cm2. We evaluated the detector
performances through I-V and C-V curves, and chargecollection, and by probing the coupling capacitors. The results
obtained in the present studies can be summarized as follows:

95% Voltage from C-V

Sp4k-17
Hn4kID-17

600
Hp4k
Hn4k
Hp4km
Hp1k
Sp4k

500

•
14

4.2x10400

300

We note that the leakage current at V Q(95%), Ileak, issystematically larger for n-strip detectors. This can be
attributed to the effects of micro-discharge. Among the two
new p-stop configurations, Hn4kIA draws a large current and
Hn4kIB shows the smallest current. In the design of Hn4kIA,
the region between the p-stop implants can accumulate thecharge because poly-silicon plates do not exist on top.

•

14

1.7x10200•

100

pre-irrad
0

0

100

200

300

400

500

600

95% Voltage from charge collection

Figure 7: Correlation between 95% voltage in charge collection
and that in 1 /C.

•

The coupling capacitors are barely broken by irradiation.
At 1.7×1014 protons/cm2, at most 1.5% of strips became
faulty whereas most of the detectors showed 0 or 1 strip tobe damaged out of 768 strips.
The leakage current of n-strip detectors is in general larger
than of p-strip detectors. A new p-stop configuration of
Hn4kIB is promising in reducing the leakage current.
There is small difference in initial p-on-n and n-on-ndetectors in view of the shape in the charge collection
curves. The charge collection of irradiated p-strip detectors
remains as high as that of n-strip detectors at voltages lower
than the full depletion voltage.
The difference in initial resistivity 1 kΩ cm or 4 kΩ cm issmall at the voltage where 95% of charge is collected.
However at lower voltages, more charge can be collected for1 kΩ cm detectors at 1.7×1014 protons/cm2. This difference
becomes smaller at 4.2×1014 protons/cm2.

••

The maximum collected charge decreases with the proton
fluence at a rate 2% per 1014 protons/cm2 for n-strip
detectors and ~8% for p-strip detectors. There is small
difference in bulk resistivity and in the manufacturers in the
maximum collected charge.
Hamamatsu detectors made with modified n-side process
showed performance equivalent to those made withconventional process.

V. ACKNOWLEDGMENTS
The authors would like to acknowledge the cooperation of
the beam channel and radiation safety groups of KEK. K.
Takikawa and S. Kim of University of Tsukuba are also
acknowledged for valuable suggestions. This work was
supported by Japan Ministry of Education, Science and Culture.

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