ARTICLE IN PRESS

Nuclear Instruments and Methods in Physics Research A 541 (2005) 122–129
www.elsevier.com/locate/nima

Test of ATLAS SCT barrel modules with Nd:YAG laser
K. Haraa,, M. Minagawaa, T. Kuwanoa, K. Nakamuraa, Y. Nakamuraa,
H. Sengokua, Y. Ikegamib, T. Kohrikib, T. Kondob, S. Teradab, Y. Unnob,
R. Takashimac, I. Nakanod, R. Tanakad, Y. Iwatae, T. OhsugieaGraduate School of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Ten’nodai, Tsukuba, Ibaraki 305-8571, Japanb
High Energy Accelerator Research Organization (KEK), Oho 1-1, Tsukuba, Ibaraki 305-0801, Japanc
Kyoto University of Education, Fukakusa-Fujimori, Kyoto 612-8522, Japand
Department of Physical Science, Okayama University, 3-1-1 Tsushima-naka, Okayama 700-8530, Japane
Department of Physical Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima 739-8526, Japan
Available online 4 February 2005

Abstract
As a part of the quality assurance procedure of the ATLAS SCT (Semiconductor Tracker) barrel modules, theresponse of the microstrip detector is measured by injecting focused Nd:YAG laser at each strip. The test is sensitive tosensor originated problems and a cross check to the results obtained from the electrical tests performed with the test
pulse system implemented in the readout ASICs. Combining these results with the probing results obtained by thesilicon sensor manufacturer, we veriﬁed the reliabilities of these tests and classiﬁed the type of defects for overall
performance evaluation of the modules.
r 2005 Published by Elsevier B.V.
PACS: 29.40.Gx; 29.40.Wk
Keywords: Silicon microstrip detector; ATLAS SCT; Laser test

1. Introduction
The ATLAS SCT (Semiconductor Tracker)
group is constructing silicon microstrip detector
modules [1] for the LHC Collider Experiment. The
SCT modules are arranged into four concentric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.2005.01.048

barrels in the central region and nine discs each in
the forward/backward regions. All the barrel
modules, in total 2112, are identical in design.
Each module has four microstrip sensors,
63:56 mm  63:96 mm; glued back-to-back on toa baseboard. The strips of the two sensors on thesame side are wirebonded together. This effectively
makes the strips 124 mm long with a pitch of
80 mm: A stereo angle of 40 mrad is subtended
between the top and bottom strips.

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A ﬂexible Cu-polyimide base hybrid [2], where
12 ABCD3T [3] binary readout ASICs aremounted, bridges over the module at the middle.
With carbon–carbon backing, the hybrid is glued
on extrusions of the baseboard without touching
the sensor surface. The carbon–carbon backing
and TPG (thermal pyrolytic graphite) baseboard
efﬁciently draws the heat generated in ABCD3Ts
and sensors (after radiation damage) toward oneof the extrusions where a cooling pipe is coupled.
The barrel modules are constructed at four
qualiﬁed construction sites. Although the module
assembling systems are not identical among thesites [4], a thorough quality assurance is being
carried out based on an agreed program. Together
with sensor IV scans and thermal cycling tests, acommon electrical test procedure is carried out
using the test pulse system incorporated in the
ABCD3T chips. This fully characterizes thefunctionality of the chip. The ABCD3T chippossesses 4-bit trim DACs to adjust channel-bychannel offsets. The chip gain, noise and offset aremeasured with and without the trim DAC adjustment. Channels with abnormal gain or offset may
fail this adjustment. These channels can be masked
in subsequent tests and during data taking.
Another aspect of the measurements is determining the status of the wirebonding to the sensor
strips. There are two categories which are unbonded and partially bonded that are determinedfrom noise measurements. Small noise of the
ABCD3T indicates either both strips, ‘‘unbonded’’, or one strip, ‘‘partbonded’’, are not‘‘seen’’ at the input.
All the sensors for the barrel modules were
fabricated by Hamamatsu Photonics (HPK),
where extensive sensor probing was performed.
The defects are categorized into A‘ bridge, A‘break, implant bridge, implant break, and pinhole.
The reliability of HPK probing had been checkedat the prototyping and early production stages andwas found to be acceptable.
The laser test described here has been performedon every module exclusively at the Japanesemodule production site. The purpose is to crosscheck the results obtained by the electrical test andsensor probing, and to provide an overall defectevaluation of the modules.

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2. Laser test system and measurements
Since the energy of Nd:YAG laser 1.165 eV isjust above the Si band gap energy of 1.12 eV, mostof the laser light penetrates the Si sensor. Thus thepassage of charged particles can be simulated if thelaser is collimated and its intensity properly
adjusted.
The laser test system consists of a set [5] of
the laser head (Spectra-Physics Model S12-1060)
with intensity controls and focusing lens, XY
and Z stages, and an SCT-DAQ system based on
VME [6].
The module under test is placed inside a module
box for protection. It has slit windows for injectionof the laser light. The module temperature isstabilized by placing the box on a water cooledð5  CÞ Cu block which is ﬁxed to the XY stage. The
module is shifted horizontally by the XY-stage
controlled through RS-232C with a feedback
precision of 1 mm: The Z-stage is a GPIB
controlled stepping motor (1:5 mm step) that
regulates the vertical movement of the laser head
and focusing microscope.
The whole system is triggered by a master
pulser. On receiving a TTL-level signal, the laser
power supply stops sending RF pulses to the Q
switch in the laser head. The laser lights accumulated in the YAG rod are then liberated in a time
duration of about 15 ns (6 ns FWHM). The laser
intensity is adjusted through a polarizer and a setof neutral-density ﬁlters. The laser is focused on
the sensor surface with a spot 2–3 mm square.
The master pulse is branched and fed to level
one accept (L1A) of the SCT-DAQ system toinitiate data acquisition. The response of a channel
is evaluated by performing a threshold scanbecause of the binary readout scheme employedby the ABCD3T. Once the threshold scan iscompleted the module is shifted to the next strip
so its evaluation can continue. Since two sensor
strips are wirebonded to one ABCD3T channel,
the scan is performed along two lines per side inorder to be sensitive to sensor related problems
such as A‘ electrode breaks. The two scan lines(named Line-1 and Line-2) trace a path acrossthe A‘ strips at the far ends with respect to thereadout ends.

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124

Fig. 1 shows the response variation as a functionof the laser injection point measured from the strip
center. Also shown are dependences on Z. The XY
position of the module is calibrated by locating
two ﬁducial marks of one of the sensors. The
precision of this measurement is typically 3 mm:
This precision can be translated to 5 mm at theposition along the scan lines. The thermal deformation could further degrade the precision. In
order to avoid this the XY calibration is performed
after the system has achieved thermal stability.
This takes 5–10 min after the module is attached tothe system. The nominal injection point is set at
25 mm from the strip center with the laser focusedon the sensor surface. Concerning the Z control,
although Z dependence is moderate, the module Z

Mean response (mV)

350

25+5 m

300
250
200

focus (ch-)
focus (ch+)+15um (ch-)+15um (ch+)-15um( ch-)-15um (ch+)

150
100
50
0
10

15

20

25 30 35 40
Distance (µm) from ch-

45

50

55

Fig. 1. Mean outputs (mV) of two neighboring strips as afunction of the distance from the left strip. The curves areshown for the three cases where the laser spot is focused on thesensor surface (crosses), 15 mm upper (squares), and 15 mm
lower. In the laser test, the spot is located nominally 25  5 mmfrom the left when the response of the left strip is examined. The
strip pitch is 80 mm:

position is measured at three points along each of
the two scan lines and parabola functions are used
to interpolate Z at other strips. This is necessary
because the module is not ﬂat by about 50 mm due
to warps of the sensors and the baseboard [4]. The
Z calibration can be achieved better than 15 mm:
From Fig. 1, the Z and XY precisions could
introduce a module-to-module response variationof 7% at most.
The most important system performance isthe time required to complete the scan whileachieving precise measurements of the chip’s gain
and noise. The peak production rate of four
modules a day introduced the constraint that oneach module the test must complete in 2 h. This
includes the time required to achieve thermal
stability, the XY and Z calibrations and pedestal
measurements. One parameter that can have a
direct impact on the time taken to perform a scan
is the master clock frequency. In Fig. 2 the output
response and time required to complete a set ofthreshold scan are shown as a function of themaster clock frequency. The laser output starts todecrease above 1 kHz, and SCT-DAQ systemhangs above 2 kHz. From these curves, the masterclock is set to 1 kHz.
The laser intensity is adjusted to about 300 mVat the ABCD3T threshold. Taking into accountthe pedestal of 20 mV on the average, thiscorresponds to the input charge of approximately
5 fC. The scan is composed of 10 threshold points
and for each point 50 events are acquired. Under
these conditions the measurement per strip takes
1.6 s with the threshold scan requiring 1.3 s and thestage movement and data validation taking 0.3 s.
Hence a total of 40 min is required to complete one

10
Time (sec) per scan

Mean response (mV)

400

(a)

1000
Trigger Freq.(Hz)

200
100
0
100

1
100

300

10000
(b)

1000
Trigger Freq. (Hz)10000

Fig. 2. (a) Time (s) to complete a threshold scan and (b) mean response (mV) as a function of the trigger frequency.

ARTICLE IN PRESSK. Hara et al. / Nuclear Instruments and Methods in Physics Research A 541 (2005) 122–129

3. Results
The module production is currently under way.
The results to be shown are gathered from a totalof 656 modules.
3.1. Response mean and spread
The laser response is 271 mV on the averageafter pedestals are subtracted. This is about 5.12 fC
at the input charge by taking account of the mean
ABCD3T gain of 52.9 mV/fC. The RMS spread ofthe mean is 7.7%, representing the sum ofcontributions of the ABCD3T gain variation(4.6%), response variation due to incomplete XY
and Z calibrations ðo7%Þ; and laser intensity
variation. Among these, the contribution of thelaser intensity variation is largest. The same system
was used for other study where the laser intensity
and spot size were varied and no effort was madeto set precisely the conditions back. Instead therelative spread, deﬁned as the RMS spread divided
by the mean, was evaluated for each side of themodule. Fig. 3 plots the correlation between therelative spread calculated from the raw response
and from the response which is corrected for
different ABCD3T gain per channel basis. The

0.12
Side UP
0.10
RMS/mean, laser raw.

side of the module. The mean and sigma of theerror function which is ﬁtted to the threshold curve
are the mean strip response and its spread. Whenpicking up the 10 different threshold points there
are two considerations. The ﬁrst one is that at least
two points are low enough but above the pedestal
to study cross-talk between strips. The points 100
and 130 mV are chosen for this purpose. The
second is that at least two points should be locatedin the transition region of the threshold curve inorder to extract the mean and its spread with areasonable precision. The nominal region is from
220 to 360 mV at a step of 20 mV. The region isshifted accordingly and the threshold curve is remeasured if the ﬁtted mean is outside the nominal
region. Also, in order to deal with the cases wheresome dust absorbs the laser, the same thresholdscan is repeated at a shifted illumination point
when the response is low.

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Side Down

0.08
0.06
0.04
0.02
0.00
0.00

0.02
0.04
0.06
0.08
0.10
RMS/mean, Chip gain normalized

0.12

Fig. 3. Correlation between the relative RMS spread for laser
raw response and that for the response normalized by the chipgain (see text).

average of the relative spread is 3.07% (RMS
variation is 0.75%) for raw response and 3.00%(RMS variation is 0.84%) for the gain corrected
response.
In the plot, the data points located upper left arecharacteristic where the large raw response variation is explained by the large ABCD3T gain
spread. On the other hand, those located lower
right are worse if corrected by the ABCD3T gains.
This could be explained partially by wrong
evaluation of the calibration capacitors of thechip. This is one of the purposes of the laser test
which can provide direct and independent evaluations of the chip gains. In fact it turned out that
one chip of the module at the extreme point istaken from a wrong chip production batch where
the calibration capacitors are quite different.
Many of the points, though, are for irregularmodules [6] and larger variation of gain corrected
response can be explained by the temperature
difference between the laser and electrical tests.
The ABCD3T chips of such modules are found to
be sensitive to the operation conditions such as the
strobe timing and shaper current, both being
dependent on the temperature. Nevertheless, it
can be noted that 98.4% of the module sides havethe gain spread smaller than 5% without the chipgain correction.

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126

3.2. Defective channels

Table 1
Summary of the defective channels for 656 modules

The number of defective channels per module ishistogrammed in Fig. 4, where the data are given
with and without including marginal noisy channels (see below). Modules are classiﬁed as good if
it possesses less than 15 (1%) defective channels.
Only one out of the 656 tested modules failed.
The defective channels are categorized by
combining all the information, HPK probing
result, electrical test result, laser test result andvisual inspection. A summary is given in Table 1.
For 656 modules, the number of defective channels
judged by the electrical test is 774 including noisy
channels. Although the laser test is not precise
enough to perform detailed comparison of thenoisy channels because of limited threshold step
size and number of events, it can detect noisy
channels if the noise exceeds typically 50% larger
than the normal. The total number 477 in the table
excludes most of the noisy channels but includes
28 very high noise channels identiﬁed also by thelaser test. Of 477 defects, about a half are due toincomplete wirebonding. We did not try to rewirebond, when wirebonding failed, for a certain
period of the production, since it was thought toincrease the detector leakage current. After thedecision that we re-wirebond but only once, thedefect fraction decreased. About a quarter are dueto the sensor related problems, and 15 are the
ABCD3T chip related problems. The detailed
breakdown of the chip problem can be foundNumber of modules tested

656

Number of channels tested

1,007,616

1000
Number of Modules.

noisy excluded
noisy included

100

10

1

0

5
10
Numberof Irregular Strips/Module>15

Fig. 4. Number of defective channels per module for a total of
656 modules. The open (ﬁlled) histogram is with noisy channels
excluded (included).

Defective channels (excl. noisy)(A) Sensor defects
A‘ bridge
A‘ breakImplant break(B) ABCD3T chip defects
dead/digital problem
gain or offset problem(C) Pitch adaptor defects(D) Incomplete wirebonding(E) Very high noise(F) Reason not identiﬁed

477
116
(87)
(22)
(7)
93
(30)
(63)
5
232
28
3

100%
0.047%(0.012%)(0.009%)(0.001%)(0.023%)(0.003%)(0.000%)The number of defective channels (excluding 318 marginal
noisy channels) was 477. Their defects are arranged into
categories (A)–(F). The numbers listed below (A) and (B) are
their breakdowns.

elsewhere [6]. The defects of the pitch adaptors are
three breaks and one short. This only one short iscreated by a droplet of silver epoxy glue. Thereexisted in addition six pairs of bridges, which are
not shown in the table. These bridges were
trimmed off by the same laser system but set at
higher intensity. There are three channels identiﬁed to be defective by the laser but judged normal
by the electrical test. Repeated measurements
provided consistent disagreement. Although a partof the sensor surface is covered by the hybrid,visual inspections could not ﬁnd any trace of
defects.
3.3. Consistency with HPK probing
The comparison between the HPK probing
and laser results is summarized in Table 2. The
HPK probing judged 620 sensor strips to be
defective.
Among the 85 A‘ bridges, 81 are identiﬁed bylaser. For the remaining two pairs, a remnant ofbridge is recognized by visual inspection. It seems
that the pairs were in contact originally but thebridge should have brown off when the neighboring strip was probed. We found six new bridges,

ARTICLE IN PRESSK. Hara et al. / Nuclear Instruments and Methods in Physics Research A 541 (2005) 122–129

two 3-strip bridges. These are known by HPK
probing but have been judged as pinholes. There isa tendency that HPK judges bridges as pinholes if
more than two strips are connected.
Among the 31 A‘ breaks, we identiﬁed 19 andthe rest turned out to be normal. In addition, we
identiﬁed three new breaks which were not seen by
HPK. Photographs of the three defects andanother one (described later) are shown in Fig. 5.
Since a trace of discharge is visible in (a), the strip
may have broken by discharge initiated at theneighboring strip which was probed after the‘‘later broken’’ strip. The passivation in (b) isdamaged, which may have disturbed the HPKTable 2Comparison with HPK probing results
Number of strips

A‘ bridge
A‘ breakImplant bridge
Implant break
Pinhole2,015,232
HPK

ID’ed

New

85
31
0
8
496

81+4
19
0
(7)
—

+6
+3
+0
—
—

The numbers in Column ‘‘ID’ed’’ are the defects identiﬁed byour tests out of the numbers in Column HPK. The column‘‘new’’ lists the numbers which are not detected by HPK. The
numbers in parentheses are from visual inspections, and those
with—are not detectable by our tests.

127

probing judgement. The break of (c) is clear andneeds to be explained why this was not identiﬁedby HPK.
The silicon sensors with implant breaks areassembled into modules with the bias resistors
placed far side to the readout. Under thisarrangement, the implant electrode where the laser
is injected would be biased normally and detecting
the implant break is inefﬁcient because the signal
would be picked up normally to the A‘ electrode.
Instead, we investigated the implant breaks undera microscope. Out of 8 implant breaks, 7 are
identiﬁed and 1 is normal.
The pinholes cannot be detected both by theelectrical and laser tests. Summarizing the HPK
probing reliability, only one A‘ break out of 9 new
defects should be attributed as a thoroughmistake, while the HPK probing provides somewhat over-estimated evaluation of defective channels especially for A‘ breaks.
3.4. Consistency with electrical testing
Table 3 summarizes the comparison between theelectrical and laser tests.
There are 23 defective channels which are
identiﬁed by laser but not by electrical test. Amongthese, 16 are the A‘ breaks which have been
identiﬁed also by HPK. Of 4 defects which aredetected by the laser test only, 3 are new A‘ breaks

Fig. 5. (a)–(b) A‘ breaks and (d) a bridge between DC pad and bias-ring. These were not identiﬁed by HPK probing.

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K. Hara et al. / Nuclear Instruments and Methods in Physics Research A 541 (2005) 122–129

Table 3Comparison with electrical test results. The numbers inparentheses are breakdowns
Number of channels

1,007,616

Defects not detected by electrical testA‘ breaks identiﬁed by laser and HPK
New sensor defects identiﬁed by laser only
New defects, but reason not identiﬁed
Judgement error by electrical test
Trim error
Wire-bonding judge mistake23

Irregular ð415% off the average) but
usable

94

(16)
(4)
(3)
10
(7)
(3)

listed in Table 2, and 1 is a bridge found between
the DC pad and bias-ring (see Fig. 5d). The three‘‘new defects, but reason not identiﬁed’’ are thosechannels listed as (F) in Table 1. Of 116 sensor
defects, listed as (A) in Table 1, the electrical test
missed to detect 20 channels.
There are 10 channels which are to be attributed
to the errors of the electrical test judgement. Seven
are the errors in the trim DAC adjustment of
ABCD3T chips probably due to pick-up noise
during the calibration measurements: these werere-tested and found to be normal. The three ‘‘wirebonding judge mistakes’’ are due to that themeasured noise levels are just around the judgement boundary.
The laser test found 94 channels which have a
response off the local average by more than 15%
but are qualiﬁed to be usable by electrical test.
Note that the laser test is performed without trim
DAC adjustment. These channels turned out tohave negative pedestals but are within the range
ð4  100 mVÞ recoverable by the trim DAC
adjustment.
Summarizing the above, 7 trim errors could
be attributed to the real mistakes of the electrical test out of 2 million channels examined,
since the electrical test is expected not to besensitive to detect sensor originated problems (20and probably 3 more) and wire-bonding judge
mistakes (3) could be recovered by algorithm
tuning.

4. Summary
A total of 656 ATLAS SCT barrel modules
are tested by injecting focused Nd:YAG laser at
each strip. The test is sensitive to the sensor
originated problems and a cross check to the HPK
probing results and calibration results obtainedby the electrical test. All the above information and visual inspections are gathered toevaluate the reliability of the HPK probing andelectrical tests.
The overall defective channel fraction is 0.047%
on the average without including marginal noisy
channels. Incomplete wirebonding, sensor defects,
and ASIC chip defects are the main causes. The
HPK probing provided in general reliable but
somewhat over-estimated evaluation of the A‘strip breaks. Only one A‘ break was missed todetect out of over 2 million strips probed. The
electrical test missed to detect 20 sensor defects out
of in total of 116 defects, and resulted 7 wrong
trim DAC calibrations out of over 1 million ASIC
channels.

Acknowledgements
We would like to acknowledge the many people
involved in the SCT module design and production, and also who have developed the SCT-DAQ
system. Especially we would like to thank G.
Moorhead for providing us the software for the
SCT data acquisition. S. Inaba and M. Yokoshima
are also to be acknowledged for their continuous
help in the module measurement.
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