ARTICLE IN PRESS

Nuclear Instruments and Methods in Physics Research A 541 (2005) 15–20
www.elsevier.com/locate/nima

Beam splash effects on ATLAS silicon microstrip detectors
evaluated using 1-w Nd : YAG laser
K. Haraa,, T. Kuwanoa, G. Moorheadb, Y. Ikegamic, T. Kohrikic,
S. Teradac, Y. Unnoca

Graduate School of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Ten’nodai, Tsukuba, Ibaraki 305-8571, Japanb
School of Physics, University of Melbourne, Parkville Victoria 3010, AustraliacHigh Energy Accelerator Research Organization (KEK), Oho 1-1, Tsukuba, Ibaraki 305-0801, Japan
Available online 5 February 2005

Abstract
On an incident of accelerator beam loss, the tracking detector located close to the beam line is subjected to receive
intensive radiation in a short period. We used a 1-W focused Nd : YAG laser and simulated the effects on the ATLAS
microstrip detector. The laser corresponds to intensity of up to 1  109 mips=pulse with a pulse width of about 10 ns.
We observed breaks on Al strips on extreme conditions, depending on the laser intensity and bias voltage applied to thesilicon sensor. The break can be interpreted as the oxide breakdown due to a large voltage locally created across theoxide by the intensive signal charges. The robustness of the Semiconductor Tracker (SCT) module including readout
ASICs is also evaluated.
r 2005 Published by Elsevier B.V.
PACS: 29.40.Gx; 29.40.Wk
Keywords: Silicon microstrip detector; ATLAS SCT; YAG laser; Beam loss; Beam splash; Breakdown

1. Introduction
The LHC will circulate approximately 1011
protons per bunch and collide them at a frequencyof 40 MHz. The detector, especially tracking
detector located close to the beampipe, must be
robust against possible beam loss where theCorresponding author.

E-mail address: hara@hep.px.tsukuba.ac.jp (K. Hara).
0168-9002/$ - see front matter r 2005 Published by Elsevier B.V.
doi:10.1016/j.nima.2005.01.033

detector may receive intensive particles in a shortperiod. According to a scenario and simulation,
the CMS group predicted that a loss of
109 proton=cm2 within 260 ns should occur at least
once a year and conducted a testbeam measurement [1]. They have shown that the CMS Si
tracker survives a beam of 7  1010 protons spreadover 10  3 cm2 : The survival is explained statingthat the large number of free carriers created by
the intensive radiation effectively reduces theARTICLE IN PRESS
16

K. Hara et al. / Nuclear Instruments and Methods in Physics Research A 541 (2005) 15–20

electric ﬁeld across the bulk and the bias voltage is
not sustained. This mechanism of electric ﬁeld
breakdown is pointed out in Ref. [2]. The external
RC bias network supplies quickly the charge torecover from the breakdown. Therefore the featureof the electric ﬁeld breakdown should depend on
the values of R and C [2].
In the studies reported in Refs. [1,2], the silicon
microstrip detectors are found to be safe in theirtest conditions. We used a Nd : YAG laser of1064 nm, as in Ref. [2], to simulate the passage ofcharged particles. The laser power we used was
1 W with the intensity continuously reducible byorders of magnitude. The laser pulse width is short(6 ns FWHM) and is focused to 8 mm in diameter,
which are to be compared with 1 ms and 10 mm indiameter in Ref. [2].
ATLAS Semiconductor Tracker (SCT) barrel
modules [3] are used in this study. The sensors are
p-on-n single-sided microstrips, fabricated byHamamatsu Photonics (HPK). The dimensionsof the sensor are 63:56  63:96  0:285 mm3 : The
sensor has 768 readout AC-coupled Al electrodesof 22 mm width at a pitch of 80 mm: The implantelectrodes of 16 mm width are grounded via
1:25 MO poly-silicon bias resistors, with the backplane being biased. Each module has four sensors
glued back-to-back on to a baseboard. The two
sensors on the same side are wirebonded together,having effectively 124 mm long strips. The signals
from the both sides, 1536 channels in total, areread out with 12 binary readout ASICs, ABCD3T
chips [4], which are mounted on a Cu-polyimide
hybrid [5]. The sensors will be biased up to
between 350 to 500 V, depending on irradiation,
thus on the distance from the collision point. The
hybrid is equipped with 50 nF capacitance between
the backplane and ground, and 5:1 kO series
resistance to the backplane.

consists of a laser head (Spectra-Physics Model
S12-1060), intensity controls and a focusing lens.
We have two independent intensity control systems, Glan polarizer and neutral-density (ND)
ﬁlters. Since the laser is 100% polarized, theintensity can be varied by rotating the angle ofthe polarizer. There are two pairs of ‘‘thick’’ and‘‘thin’’ ND ﬁlters located downstream the Glanpolarizer. The laser aperture is then deﬁned by XY
windows and the laser is focused with a microscope (50 magniﬁcation). The laser with no ND
ﬁlter is intensive enough to evaporate Al electrodes.
The laser intensity was calibrated using a real
ATLAS SCT module. The signal with all ND
ﬁlters inserted was read out via an SCT–DAQ
system, and is calibrated with ABCD3T chip gain
(55 mV/fC) [7]. The transparency curve of the Glanpolarizer in terms of the rotation potentiometer
was then obtained by choosing the laser spot size
so that the intensity is within the measurement
range of the ABCD3T (less than 680 mV). After
this procedure, setting the Glan polarizer at a
reduced intensity, one of thin or thick ND ﬁlters istaken out to calibrate their transparencies. The
intensity at the different setting was calculated
from the Glan polarizer transparency curve andtransparencies of the ND ﬁlters.
The laser was operated at 1 kHz: the intensity isconstant up to 1 kHz then decreases gradually [7].
Fig. 1 plots the calculated laser intensity in termsof number of mips per pulse and the expectedsignal current. Here we assumed that 2  104 holes
are collected per mip as the signal. The detector
currents in the ﬁgure were measured with anamperemeter. Although there exists a discrepancy
up to a factor of two at lower intensities, the two
curves are in reasonable agreement. The maximum
laser intensity is 1  109 mips=pulse:

2. Laser system and intensity calibration

3. Robustness of the HPK microstrip sensor

Because the energy of Nd : YAG laser is justabove the Si band-gap energy of 1.12 eV, most of
the laser light penetrates the Si sensor and theenergy is liberated, if absorbed, it will be almost
uniformly along the path. The laser system [6]3.1. Creation of break in Al electrodes
We observed that the Al electrode of the
microstrip sensor breaks above certain bias
voltage and laser intensity. Fig. 2 is a typical traceARTICLE IN PRESSK. Hara et al. / Nuclear Instruments and Methods in Physics Research A 541 (2005) 15–20

1000
measured
Detector current (µA)

expected
100

10

1
106

107

108

109

1010

Laser intensity (mips/pulse)
Fig. 1. Calibration of the laser intensity. The expected detector
current (see text) and the measured current are in reasonableagreement, verifying the reliability of the laser intensity
calibration in terms of number of mips per laser pulse.

Fig. 2. Hole is created on the Al electrode at 500 V bias voltage
and 1  108 mips: The laser is injected about 30 mm off the Al
electrode. The Al electrode is 22 mm wide.

of the break. Note that the laser, conﬁned in adiameter of 8 mm; is injected just aside the Al not
to hit it directly. Since the laser at highest intensity
would evaporate the Al if it hits the Al directly,
this procedure is carefully completed so that thepresent study is not sensitive to the damage speciﬁcto the laser. Note also that the damage trace
shown in the ﬁgure is apparently different from the17

evaporation trace that would be created by thelaser hitting directly.
Fig. 3 shows the probability to create suchbreaks as a function of the bias voltage. The
histograms are shown for three different
laser intensities, 0:6  107 ; 2  107 and 8 
107 mips=pulse: Most of the samples, except only
one, are good to 500 V for 0:6  107 and 2 
107 mips=pulse: The laser was pulsed at 1 kHz andfor a duration up to 10 s. For 8  107 mips=pulse;
however, some strips broke at 350 V and theprobability reached about 50% at 450 V. Most of
the breaks were created within 1 s, or by the ﬁrst
shot when the laser was operated at 1 Hz, but somerequired a few seconds.
At lower bias voltages of 150 and 200 V, no
breaks were created up to the highest laser
intensity, 1  109 mips=pulse; and up to the orderof 104 pulses we injected.
We note that Al breaks of the HPK sensors werealways initiated at the point closest to the laser
injection point. Although the study was not
systematic, we also tested a CiS sensor of thesame ATLAS design. The CiS sensor is found to
break typically at 350 V for 0:6  107 mips: Distinctive difference is that the break was initiated on
the nearby Al electrode but at a point somewhat
away (30 mm and 0.5 mm, two samples) from thelaser injection point. We interpret the above asthat the HPK sensors were broken at the point
where the electric ﬁeld is maximum while the CiS
sensors break where the oxide layer has some weak
point.
3.2. SPICE simulation
In order to understand the reason for the
breaks, we modeled the microstrip detector andexternal bias RC network in a SPICE simulation.
The model used in the simulation is illustrated inFig. 4, where the sensor parameters are given in
the ﬁgure caption. A triangular shaped current
source with 10 ns base width was generated at apoint located 5 cm apart from the readout end.
The detector bias is 500 V. Fig. 5 shows a time
evolution of voltages on the implant and Al
electrodes at the point where the current was
generated, and at 1.5 mm apart from the current

ARTICLE IN PRESSK. Hara et al. / Nuclear Instruments and Methods in Physics Research A 541 (2005) 15–20

Vbias (V)

18

500

500

500

450

450

450

400

400

400

350

350

350

300

300

0.6x107
good

200

20

dead

200

dead

150

150
10

good

good

200

0

8x107 mips/strip

250

250

250

150

300

2x107

0

#samples5#samples

10

0

20

40

60

80

#samplesFig. 3. Probability of hole creation as a function of the bias voltage. Filled (open) histogram represents the number of passed (failed)samples. The data are shown for three laser intensities.

Fig. 4. SPICE model of the microstrip detector: Al resistivityðRAl Þ 15 O=cm; implant resistivity ðRi Þ 20 kO=cm; equivalent
ASIC resistance ðRic Þ 1 kO; bias resistor 1:25 MO; coupling
capacitance ðC cp Þ 25 pF/cm, bulk capacitance ðC bk Þ 2.5 pF/cm,
interstrip capacitance 5 pF/cm, ﬁnite backplane resistance ðRbk Þ
10 O (overestimated to see effects), bias voltage network R ¼
5:1 kO; C ¼ 50 nF:

SPICE3
400

(a)

350

Volts (V)

300
250
200
150

(c)

100

(b)

50
0

0

10

20

30
40
time (ns)

50

60

70

Fig. 5. Evolution of (a) implant and (b) Al electrode voltages atthe signal source, and (c) the implant voltage 1.5 mm away
(SPICE simulation).

source. Although effects such as space charge must
be taken into account, the generated charge
corresponds to 5  106 mips: The simulation results in different peak voltages, creating a large
voltage difference across the oxide layer at the
injection point. Also the peak voltage at the
implant decreases with the distance from the signal
source.
Contrary to the implant voltage increase, thebias voltage stays almost constant. The bias
voltage drop can occur only through Rbk and issmall, since the generated charge, 16 nC, is far
below the charge 25 mC stored in the external
capacitor. In the electric ﬁeld breakdown model[2], the large signal charge pulls down the bias
voltage and pulls up the implant voltage. Such achange in the bias voltage should be seen only for4109 mips in our external circuit. In our simulation, only the implant voltage is pulled up due tothe smaller bulk capacitance and larger implant
resistance compared with the coupling capacitanceand Al resistance. The implant resistance induces
also a local voltage difference along the electrode,as shown in the ﬁgure, peaking the oxide voltage at
the point where charges are created.
The HPK coupling capacitors are speciﬁed to be
durable up to 100 V. We tested the coupling
capacitors of the HPK sensor applying DC
voltages across. The break voltages were foundto be about 190 V and were very uniform. The
present simulation describes qualitatively well thatthe coupling capacitors of the HPK sensors canARTICLE IN PRESSK. Hara et al. / Nuclear Instruments and Methods in Physics Research A 541 (2005) 15–20

19

Table 1
Robustness of the SCT modules. The functionality is categorized as normal (‘‘OK’’), completely no response (‘‘dead’’), or some badchannels (‘‘?’’), as a function of the laser intensity in mips/pulse
Bais voltage (V)2  1074  1087  1081  109

comment

150
200
200
200
400

OK
OK
OK
OK
OK

OK
OK
OK
OK
OK

OK
?
?
OK
OK

OK
Dead
?
Dead
OK

laser spread over 3 strips

The laser (8 mm in diameter) is focused to one strip except for the one at the bottom where the laser was spreaded over three strips.

break at the point where a large current isgenerated. Quantitative description, however, requires a simulation which takes space-charge
effects into account and detailed data concerning
the coupling capacitor durability against voltages
applied in a short period of about 10 ns.

4. Robustness of the SCT modules
We observed that the chip connected to theelectrode lost the entire functionality whenever the
Al electrode was broken. It is found that the ASIC
input circuit trace connected to the broken
electrode got colored and the trace sparked toward
the power lines located underneath.
The study is extended to search the conditions
where the sensor electrodes are not broken but the
ASIC is damaged. Although statistically limited,
the results are summarized in Table 1. The
functionality of the module was evaluated using
the ASIC test pulse. The module with 150 V bias
functioned normally, giving proper gain and noise
values, up to 109 mips=pulse: The modules at 200 V
bias were completely dead at 109 mips=pulse: At
lower intensities with 200 V bias (marked ? in thetable), several channels around the laser injection
point became sick, providing larger noise values.
The above results are for the case where the laser
was focused at one strip. Since the local electricﬁeld must be dependent on the laser spot size, we
enlarged the spot to cover three strips. In thissituation, the module was found to function
properly at much higher bias of 400 V up to
109 mips=pulse: The Al electrodes broke at

109 mips=pulse at around 430 V, so the study athigher voltage was not possible.

5. Conclusions
Using a 1-W YAG laser, we have evaluated therobustness of the ATLAS microstrip sensor and of
module against possible incident of accelerator
beam loss. The Al electrodes of the microstrip
sensors can be broken at high biases and large
signal current. The break typically occurs at 350 V
for 108 mips=pulse if the laser is focused at one
strip. The reason can be interpreted as the oxide
breakdown. The ASIC chip can be dead at 109
mips/pulse at 200 V bias. If the laser spot is
expanded to three strips, the module at 400 V bias
functions properly up to 109 mips=pulse:
If we take the same robustness requirement of
the CMS Si tracker, 109 proton=cm2 in 260 ns, ourlast condition, 109 mips=ð0:2 mmÞ2 in 6 ns
(FWHM), is by far severe in view of the electricﬁeld strength. In order to assure that the ﬁeld
breakdown occurs within a few ns to quench thevoltage increase in the case of a beam loss of muchlarger number of particles, the ﬁnite backsideresistance Rbk must be kept below a few 10 mO; a
50 nF capacitor and Rbk determine the time
constant. Since such a small resistance is notrealized reasonably, we must foresee that the ﬁeld
breakdown follows after a substantial increase in
the local voltage across the coupling capacitors.
As a conclusion, the ATLAS SCT microstrip
detector can be damaged due to accelerator
beam loss, but only in very extreme conditions

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K. Hara et al. / Nuclear Instruments and Methods in Physics Research A 541 (2005) 15–20

where the detectors are fully biased exceedingabout 400 V and the beam of exceeding 109
particles is focused to a few strips, or in equivalentconditions where the generated charges create alarge enough voltage to break the oxide layer ofthe microstrips.

Acknowledgements
We would like to thank the members of the SCT
group, especially the module group, who inspired
us to conduct the present study.

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