Characteristics of Irradiated Silicon
Microstrip Detectors with h100i and h111i
Substrates
T. Akimoto
Y. Iwataca

S. Araia

T. Kohriki

M. Shimojima
N. Ujiieba

K. Harab

T. Nakayama

T. Kondo

S. Shinma

Y. Unnoabab

I. Nakano

R. Takashima

K. YamamotofeadY. Ikegami
T. Ohsugi

S. Terada

K. Yamamurabcbf

a University of Tsukuba, Institute of Physics, Tsukuba, Ibaraki 305-8571, Japanb IPNS, High Energy Accelerator Organization (KEK), Tsukuba, Ibaraki 305-0801,

Japanc Hiroshima University, Dep. of Physics, Higashi-Hiroshima, Hiroshima 739-8526,

Japan
d Okayama University, Dep. of Physics, Okayama, Okayama 700-8530, Japane Kyoto University of Education, Dep. of Education Fushimi, Kyoto 612-0863,

Japanf Solid State Division, Hamamatsu Photonics K.K., Hamamatsu, Shizuoka

435-8558, JapanAbstract
In a program of developing radiation-hard silicon sensors for the LHC-ATLAS experiment, we have irradiated various types of silicon sensors with 12-GeV protons atKEK, and made a comparative study, among the other properties, of characteristicsof the sensors with two wafer planes h111i and h100i. The studied sensors are p-onn type, which satisfy the ATLAS-SCT speci cations. Possible dependence on thesubstrate orientation could result from di erent dangling-bond con gurations. The
compared characteristics are the charge collection eciency, interstrip capacitance,and noise levels measured with a real ATLAS-SCT electronics system. A substantial
di erence is observed in the interstrip capacitance at 10 kHz, while the di erenceis small at 1 MHz. Also the di erences in the charge collection eciency and in
the noise levels appear to be small.
>silicon microstrip detector; radiation damage; wafer direction; h111i;
h100i; charge collection; interstrip capacitance; ATLAS SCT;

Key words:

Preprint submitted to Elsevier Preprint

1 August 2000

1 Introduction

Silicon microstrip detectors will be extensively used for tracking in the ATLAS
experiment at the LHC. The ATLAS SCT (Semiconductor Tracker) system is
described in detail in [1]. The sensors are AC-coupled and single sided withdimensions of 63.56 (width) 2 63.96 (length) 2 0.28 (thickness) mm3 . The
electrode strips of 12 m width are made on the p side at a pitch of 80 mwith the number of readout strips being 768. Two fo such sensors are wirebonded together, constructing a readout strip length of 12 cm. The radiation
tolerance is one of the key issues to operate silicon microstrip detectors in ATLAS, where the radiation environment is estimated to correspond to 3 21014
multi-GeV proton equivalent per cm2 in 10 years of operation.
In a program of developing radiation-hard silicon sensors, we have fabricatedvarious types of silicon sensors and irradiated them with 12-GeV protons atKEK. The irradiations were performed for two separate sets of sensors to
1:721014 cm02 and 4:221014 cm02. The present measurements were made 6{12months after the initial annealing is completed. The details of the irradiation
conditions and performance tests which include the leakage current increase,change in the full depletion voltage and drop in the charge collection arereported elsewhere[2].
In this report, we examine the possible di erence between the sensors with thetwo wafer planes, h100i and h111i. The compared sensors are summarized inTable 1. All the sensors are the p-on-n type, satisfying the ALTAS-SCT speci cations. Comparative studies are made on the charge collection eciency,
interstrip capacitance, and noise performance. As pointed out in [3], di erent behaviors between these wafer orientations could result from the di erent
amount of the dangling bonds at the Si-SiO2 interface.
Table 1
Tested silicon microstrip sensors. * is either 0, 17 or 42, denoting the proton uencein unit of 1013 cm02.
sensor namemakerresistivity wafer plane uence (cm02)
Hp4k-*
Hamamatsu 4 k cm
h111i
0, 17 21013
Hp1k-*
Hamamatsu 1 k cm
h100i
17, 42 21013
Sp4k-*
SINTEF4 k cmh100i
17, 42 21013

22 Charge collection

The charge collection could degrade due to trapping and recombination centers created by intense radiation. The surface charge accumulation could also
modify the electric eld around the strips and hence the charge collection efciency. If the in uence associated with the wafer orientation emerges itself
only at the Si-SiO2 interface and not in the bulk, we expect small di erence
at least in the former e ect.
The charge collection curves were measured using penetrating rays from aSr source[2]. The sample sensor was sandwiched between a pair of A` slits
with a scintillation counter set underneath, which created gate signals to an
ADC. Three adjacent strips were read out separately. The peak ADC countof the summed signals is plotted as a function of the bias voltage. Figure 1 isan excerption from [2], which shows the charge collection curves of the threetypes of the sensors irradiated to 1:7 2 1014 cm02 . The ordinate is normalized
by the charge collection plateau of a non-irradiated Hp4k sample.
Collected charge

90

1.0
0.9
0.8
0.7
0.6
0.5
0.40.3

Hp4k-17<111>
Sp4k-17<100>
Hp1k-17<100>0.2100500
Bias voltage (V)Fig. 1. Charge collection curves of the three sensors irradiated to 1 7 2 1014 protons
cm02 . The vertical axis is normalized by the charge collection plateau of an Hp4k-0sensor.
:

We note that the shapes of Hp4k-17 and Sp4k-17 sensors are similar whereas
the Hp1k-17 curve remains high at lower voltages, which is due to the di erencein the initial resistivity.
3Collected charge

In Fig. 2 we plot the charge collection values at plateau for various samples
we tested. Typically there are measurements for two samples at the same
condition (only one for SINTEF), which are plotted by the same marks in
the gure. Although the set of the samples is not complete, there seems small
di erence between h111i and h100i wafer orientations. As is pointed out in [2],though, n-on-n sensors are superior to p-on-n sensors in the charge collection,
since the junction side of irradiated n-on-n sensors is on the strip side and theelectric eld becomes preferable for charge collection.
1.00.90.8
Hp4k <111>
Sp4k <100>
Hp1k <100>0.70.6012345142
Fluence (x10 /cm )Fig. 2. The charge collection of three types of the sensors as a function of proton
uence.
3 Interstrip capacitanceThe characterization of the interstrip capacitance was based on an HP4192A
LCR meter. Several A` strips were wire-bonded to a printed circuit board,which provided the connection to the LCR meter through Lemo cables. Hereafter we de ne the interstrip capacitance as that between two neighboring
strips with the adjacent strips oating.
Figure 3 shows the frequency dependence of the interstrip capacitance. Thebias voltages were set on the plateau in the charge collection curves. Nonirradiated sample exhibits a cuto at a few kHz, which is due to existenceof bias resistors parallel to the interstrip capacitance. The irradiated Hp1k
h100i sensors show a small deviation at a frequency lower than 10 kHz. The
curves of Sp4k-17 and Sp4k-42 (not shown in the plot, which behaves similarto Sp4k-17) show a larger deviation than Hp1k sensors. However, these curves
are essentially similar in the shape, considering the cuto of SINTEF sensors
being at a smaller frequency. On the contrary, the irradiated h111i sensors show
an enhancement of 30% in a wider frequency range below 100 kHz. They
all, however, tend to become identical above 5 MHz. It can be concluded
that the frequency dependence of the interstrip capacitance is not dependent4Interstrip C (pF)

on the proton uence in the range we investigated. Since a major di erence inthe curves is observed between h100i and h111i sensors, the wafer orientation
should be attributed to the changes.
543
2

Hp4k-17 <111>
Sp4k-17 <100>
Hp1k-17 <100>
Hp1k-42 <100>
Hp4k-0 <111>

10
1

2

10

10

3

10

410Frequency (kHz)Fig. 3. Frequency dependence of the capacitance between neighboring strips. Thebias voltage is set on the plateau in the charge collection curves.

We evaluated the bias voltage dependence of the interstrip capacitance at10 kHz, where the di erence appears to be most distinctive. The results areshown in Fig. 4. While the interstrip capacitance at lower bias voltages seems
di erent between Hp1k-17 and Sp4k-17 sensors, it converges into a similarvalue at the full depletion voltage of 300 V. The di erent behavior at lower
voltages should be related to the di erence in the wafer resistivity. On theother hand, the interstrip capacitance of Hp4k-17 is high at lower voltages
and continues to decrease across the full depletion voltage, resulting that theinterstrip capacitance is higher than the others even at 300 V.
It is reported in [3] that the interstrip capacitance is in uenced more by irradiation if the frequency is decreased (from 1 MHz to 100 kHz) and the bias
voltage is lowered (from 500 V to 300 V) for h111i substrate with resistivity of 6
k cm. It also reports that there is essentially small change for h100i substratewith resistivity of 1 k cm. The present results are qualitatively consistent
with their measurements.
4 Contribution to the electronics noise

The interstrip capacitance contributes dominantly to the detector capacitance.
The actual contribution to the electronics noise may not be substantial since5Interstrip C (pF)8
Hp4k-17<111>
Sp4k-17<100>
Hp1k-17<100>
Hp4k-0 <111>765432
f= 10 kHz

100

100

200

300

400

500

Bias Voltage (V)Fig. 4. Interstrip capacitance at 10 kHz as a function of bias voltage.

the increase is small in the range above 1 MHz which is the responsible frequency range for the electronics.

Noise (fC)

The contribution to the electronics noise was evaluated by using a non-irradiatedreal SCT readout system based on ABCD2T front-end chips. One ABCD2Tchip (128 readout channels) was mounted on a exible Kapton-based hybrid. Consecutive 6-cm long 16 strips were wire-bonded to the ABCD2T chip
through a pitch adapter. Another consecutive 16 strips were also wire-bonded
to the same chip inputs, thereby simulating 12 cm long strips in the real SCT
con guration. The sample was placed in a thermostat chamber with the temperature being kept at 010 C. The same input channels of the same ABCD2Tchip are used in order to compare the noise levels of the di erent sensor types.
0.26
0.24
0.22
0. 20
0.18
0.16
0.14
0.120.10

Hp4k-17 <111>
Sp4k-17 <100>
Hp1k-17 <100>

050

100

150200250

300350Bias voltage (V)Fig. 5. Noise as a function of the bias voltage for the three types of irradiatedsensors, two sensors for each.

6The ABCD2T is a binary readout system, therefore the noise was evaluatedfrom the sigma of the error function which was tted to the eciency vsinjected charge relation at a given comparator threshold voltage. The thresholdvoltage was adjusted to correspond to 2 fC of the charge injection. The noise
values of the connected 16 channels are consistent to each other. We plot inFig. 5 the noise average of the central channels only, though, in order to avoid
the possible in uence from the unconnected strips. Two sensors were measured
for each of the three types irradiated to 1:7 2 1014 cm02. Although the noiselevels at 300 V, where all the sensors are on the full depletion plateau, showthat Hp4k-17 (h111i) tends to be somewhat larger than Hp1k-17 and Sp4k-17(h100i) by 5{7%, the signi cance is weak due to the systematic uncertainty of
3%. Note that the noise level for a non-irradiated sensor is 0.20 fC.
In the gure we show the noise levels up to 350 V. Above this voltage thenoise level was found to increase. The sensors we used in this study are basedon \1997 design" where a substantial micro-discharge is observed around theguard rings. The current design (\1998 design") has modi ed the layout andsuch an increase is not observed [5].
5 SummaryWe have evaluated the characteristics of silicon microstrip sensors irradiatedwith protons and compared the results in view of h111i and h100i substrate orientations. The comparison was performed on the charge collection, interstrip
capacitance, and the contribution to the electronics noise. Although an increase of 30% was observed for h111i in the interstrip capacitance at 10 kHz,
there are small di erences in other characterstics. The noise levels measured
with a real SCT electronics system agreed to each other to a level of 5{7%.
References[1] ATLAS Technical Design Report 5, CERN/LHCC/97-17 (1997).
[2] T. Nakayama et al., \Radiation damage studies of silicon microstrip sensors",
Proceedings for 1999 IEEE Nuclear Science Symposium, 24-30 October 1999,Seattle, WA, USA.
[3] S. Albergo et al., \Comparative study of 111 and 100 crystals and capacitancemeasurements on Si strip detectors in CMS", Nuovo Cimento A, 112A (1999)
1261.
[4] W. Dabrowski et al., \The ABCD Binary Readout Chip for Silicon Strip
Detectors in the Atlas Silicon Tracker", Proc. of the Fourth Workshop

7

on Electronics for LHC Experiments, Rome, Spetember 21-25, 1998,
CERN/LHCC/98-36, p.175. Updates to the current version ABCD3T can befound in:http://scipp.ucsc.edu/groups/atlas/elect-doc/ABCD3T Spec v1.1.pdf.
[5] D. Morgan et al., \Chanracterization of p-in-n silicon microstrip detectors
fabricated by Hamamatsu Photonics and irradiated with 24-GeV/c protons to 321014 /cm02, Nuovo Cimento A, 112A (1999) 1245.
p8