Nuclear Instruments and Methods in Physics Research A ∎ (∎∎∎∎) ∎∎∎–∎∎∎

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

Embedded pitch adapters: A high-yield interconnection solution forstrip sensors
M. Ullán d,n, P.P. Allport a, M. Baca a, J. Broughton a, A. Chisholm a, K. Nikolopoulos a,S. Pyatt a, J.P. Thomas a, J.A. Wilson a, J. Kierstead b, P. Kuczewski b, D. Lynn b,
L.B.A. Hommels c, C. Fleta d, J. Fernandez-Tejero d, D. Quirion d, I. Bloch e, S. Díez e,
I.M. Gregor e, K. Lohwasser e, L. Poley e, K. Tackmann e, M. Hauser f, K. Jakobs f, S. Kuehn f,
K. Mahboubi f, R. Mori f, U. Parzefall f, A. Clark g, D. Ferrere g, S. Gonzalez Sevilla g, J. Ashby h,A. Blue h, R. Bates h, C. Buttar h, F. Doherty h, T. McMullen h, F. McEwan h, V. O'Shea h,S. Kamada i, K. Yamamura i, Y. Ikegami j, K. Nakamura j, Y. Takubo j, Y. Unno j, R. Takashima k,
A. Chilingarov l, H. Fox l, A.A. Affolder m, G. Casse m, P. Dervan m, D. Forshaw m,1,
A. Greenall m, S. Wonsak m, M. Wormald m, V. Cindro n, G. Kramberger n, I. Mandić n,
M. Mikuž n, I. Gorelov o, M. Hoeferkamp o, P. Palni o, S. Seidel o, A. Taylor o, K. Toms o,
R. Wang o, N.P. Hessey p, N. Valencic p, K. Hanagaki q,j, Z. Dolezal r, P. Kodys r, J. Bohm s,
M. Mikestikova s, A. Bevan t, G. Beck t, C. Milke u, M. Domingo u, V. Fadeyev u, Z. Galloway u,
D. Hibbard-Lubow u, Z. Liang u, H.F.-W. Sadrozinski u, A. Seiden u, K. To u, R. French v,
P. Hodgson v, H. Marin-Reyes v, K. Parker v, O. Jinnouchi w, K. Hara x,y, J. Bernabeu z,
J.V. Civera z, C. Garcia z, C. Lacasta z, S. Marti i Garcia z, D. Rodriguez z, D. Santoyo z, C. Solaz z,
U. Soldevila zaSchool of Physics and Astronomy, University of Birmingham, Birmingham B15 2TT, United Kingdom
Brookhaven National Laboratory, Physics Department and Instrumentation Division, Upton, NY 11973-5000, USA
Cavendish Laboratory, University of Cambridge, JJ Thomson Avenue, Cambridge CB3 0HE, United KingdomdCentro Nacional de Microelectronica (IMB-CNM, CSIC), Campus UAB-Bellaterra, 08193 Barcelona, Spaine
DESY, Notkestrasse 85, 22607 Hamburg, GermanyfPhysikalisches Institut, Universitä t Freiburg, Hermann-Herder-Str. 3, d-79104 Freiburg, Germanyg
DPNC, University of Geneva, 24, Quai Ernest-Ansermet, CH-1211 Genve 4, Switzerlandh
SUPA- School of Physics and Astronomy, University of Glasgow, Glasgow G12 8QQ, United Kingdomi
Solid State Div., Hamamatsu Photonics K.K., 1126-1, Ichino-cho, Higashi-ku, Hamamatsu-shi, Shizuoka 435-8558, Japanj
Institute of Particle and Nuclear Study, KEK, Oho 1-1, Tsukuba, Ibaraki 305-0801, Japank
Department of Science Education, Kyoto University of Education, Kyoto 612-8522, Japanl
Physics Department, Lancaster University, Lancaster LA1 4YB, United KingdommOliver Lodge Laboratory, Department of Physics, University of Liverpool, Oxford St., Liverpool L69 7ZE, United KingdomnJožef Stefan Institute and Department of Physics, University of Ljubljana, Ljubljana, SloveniaoDepartment of Physics and Astronomy, University of New Mexico, MSC07 4220, 1919 Lomas Blvd. NE, Albuquerque NM 87131, USAp
Nikhef, Science Park 105, 1098 XG Amsterdam, Netherlandsq
Department of Physics, Osaka University, Machikaneyama-cho 1-1, Toyonaka-shi, Osaka 560-0043, Japanr
Charles University in Prague, Faculty of Mathematics and Physics, V Holesovickach 2, Prague 8, Czech Republics
Academy of Sciences of the Czech Republic, Institute of Physics, Na Slovance 2, 18221 Prague 8, Czech RepublictSchool of Physics and Astronomy, Queen Mary University of London, London E1 4NS, United KingdomuSanta Cruz Institute for Particle Physics (SCIPP), University of California, Santa Cruz, CA 95064, USAv
Department of Physics and Astronomy, The University of Sheeld, Hicks Building, Hounsﬁeld Road, S3 7RH Sheeld, United Kingdomw
Institute of Science and Engineering, Tokyo Institute of Technology, Ookayama 2-12-1, Meguro-ku, Tokyo 152-8551, Japanx
Institute of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8751, JapanyCenter for Integrated Research in Fundamental Science and Engineering, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japanz
IFIC/CSIC-UVEG, Ed. Inst. Investigacion, PO Box 22085, 46071 Valencia, SpainbcnCorresponding author.
E-mail address: miguel.ullan@imb-cnm.csic.es (M. Ullán).
1
Now at Syracuse University.

http://dx.doi.org/10.1016/j.nima.2016.04.019
0168-9002/& 2016 Elsevier B.V. All rights reserved.

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2

art ic l e i nf o

a b s t r a c t

Article history:
Received 20 November 2015
Received in revised form
11 March 2016
Accepted 7 April 2016

A proposal to fabricate large area strip sensors with integrated, or embedded, pitch adapters is presented
for the End-cap part of the Inner Tracker in the ATLAS experiment. To implement the embedded pitch
adapters, a second metal layer is used in the sensor fabrication, for signal routing to the ASICs. Sensors
with different embedded pitch adapters have been fabricated in order to optimize the design andtechnology. Inter-strip capacitance, noise, pick-up, cross-talk, signal efﬁciency, and fabrication yield havebeen taken into account in their design and fabrication. Inter-strip capacitance tests taking into accountall channel neighbors reveal the important differences between the various designs considered. These
tests have been correlated with noise ﬁgures obtained in full assembled modules, showing that the tests
performed on the bare sensors are a valid tool to estimate the ﬁnal noise in the full module. The fullmodules have been subjected to test beam experiments in order to evaluate the incidence of cross-talk,pick-up, and signal loss. The detailed analysis shows no indication of cross-talk or pick-up as no additional hits can be observed in any channel not being hit by the beam above 170 mV threshold, and thesignal in those channels is always below 1% of the signal recorded in the channel being hit, above 100 mV
threshold. First results on irradiated mini-sensors with embedded pitch adapters do not show anychange in the interstrip capacitance measurements with only the ﬁrst neighbors connected.
& 2016 Elsevier B.V. All rights reserved.

Keywords:
Silicon radiation detectors
Strip sensors
HL-LHCATLAS upgrade
ITk
Pitch adapters

1. Introduction and motivation
The interconnection of sensors and readout electronics is asubject of critical impact in the module design for High Energy
Physics experiments, such as ATLAS. The sensors are made progressively larger, and the readout ASICs smaller, and both contain
increasingly more channels. On the other hand, the pitch between
bonding pads is usually very different between sensors and ASICs,
which leads to unrealizable wire-bonding angles between them.
With all this, the actual design of the electrical connection between sensors and ASICs is not trivial and it becomes a yield issue
when the experiment contains tens or hundreds of millions ofstrip channels. The question is not anymore if the wire-bonding
can be done, but if it can be done reliably enough to build a whole
tracker in the required production time.
In the case of the current ATLAS experiment the interconnection was solved by adding external pitch adapters which facilitatedthe bonding at both sides, sensor and ASIC, and made the pitch
adaptation by routing with metal tracks on a high-density metalon-glass technology [1]. The downside of this solution is that, although the bonding is very much facilitated, the number of bonds
is doubled. On top of that, an extra piece is added to the module,
increasing the total mass, assembly complexity, and costs. The
groups involved in the developments of the ATLAS Upgrade Inner
Tracker (ITk) want to avoid this solution. Several studies have been
made in order to identify the maximum angle that can be achievedby direct wire-bonding between detectors' and ASICs' bonding
pads [2]. Angles below 20° are considered safe, which results inthat direct wire-bonding can be used for the barrel design, but for
the End-cap a solution might be needed due to the larger pitch
variations in the 6 different sensor layouts [3].
In this paper, a proposal to fabricate large-area strip sensors
with integrated, or embedded, pitch adapters is presented for the
End-cap part of the Inner Tracker in the ATLAS experiment. To
implement the embedded pitch adapters (EPA), a second metal
layer is used at the end of the sensor fabrication in order to form
additional metal tracks which contact the top metal of the strip's
coupling capacitor (where the standard AC pad is) with an “embedded pad” that is placed on the sensor right in front of thecorresponding ASIC pad. In this way, the routing for the pitch
adaptation from sensor to ASIC is done on the sensor itself, anddirect bonding from sensor to ASIC is facilitated without the needto double the number of wire-bonds, making the wire-bonding
faster and more reliable.
Nevertheless, there are some possible effects that have to betaken into account in the evaluation and use of the EPA:Inter-strip capacitance: An increase of the inter-strip capacitance can be expected from the increase of the total metal lengthand from the additional coupling of the second-metal tracks witheach other and with the ﬁrst-metal tracks, especially taking into
account that the second-metal tracks have to cross over several
ﬁrst-metal tracks. This increase in the inter-strip capacitance implies an increase in the overall noise, thus the sensor channels with
EPA will show more noise than the standard one-metal channels.
Also, as the inter-strip capacitance in the EPA will depend strongly
on the particular design, the noise can also vary from channel tochannel, resulting in larger noise variability through the sensor.
Cross-talk: Signal transmitted between ﬁrst-metal tracks(standard detector strip metal) and second-metal tracks (EPA
metal) due to the coupling between them. This can result inspurious signals in not hit channels and in loss of signal (efﬁciency)
for the hit channel.
Pick-up: Charge that can be induced in the second-metal tracks
directly from the bulk when a particle crosses the sensor andcharge is created in the bulk. This can also induce spurious signals
in channels that are away from the actual hit channel and in loss of
signal (efﬁciency) for the hit channel.
Yield: From a technological point of view, as there is an increase of the number and length of tracks plus the addition of an
extra photolithographic step with the EPA, this could increase theprobability of a short-circuit between channels, or an open circuit
in a channel during the processing. This means a possible reduction in the number of good channels per sensor, and consequently
also a possible reduction of the number of good sensors in a whole
production.
The EPA have been implemented on the sensors fabricated forthe prototype of the End-cap part of the ATLAS Upgrade Inner
Tracker (ITk), the “petalet” [4,5]. Initial results on these sensors,which had ﬁrst-generation layout designs, have shown no indication of cross-talk or pick-up from laser tests [6]. Neverthelessan increase in noise and noise variability has been observed in theﬁrst modules fabricated with those sensors as can be seen in Fig. 1,
where the noise results for petalet modules made with andwithout the ﬁrst designs of EPA are shown. These tests were takenusing threshold scans on a digital readout system to determine thenoise level in electron noise equivalent. Fig. 1 shows the noise perchannel comparing typical sensors of “One metal” and “Embedded”
designs. The electrical noise stemming from the readout system
alone (without the sensors) is 380 þ/  10 ENC and has not been
subtracted in the ﬁgure. For a typical one-metal sensor, the ENC

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Fig. 1. Noise results on module for sensors with and without EPA with the ﬁrst
layouts implemented.

noise is 550 with a deviation of at most 50 ENC. Six sensors with
EPA have been built into modules and showed consistent behavior
as seen in Fig. 1.

2. Design of the embedded pitch adapters
In view of the results just mentioned about noise variability
across the detector due to the different inter-strip capacitance ineach channel for the initial design, 4 new layouts were designed inorder to attempt reducing this variability and the rest of effects
mentioned above. Fig. 2 shows the 5 layouts that have been tried.
(a) Basic: This is the ﬁrst layout made for the initial batches andtests. The second-metal tracks keep the same angle (in eachquadrant), i.e. they are parallel to each other, and this angle isthe maximum that can be used for a minimum separation
between tracks of 20 mm. This distance is chosen as a small but
technologically safe distance between tracks in order not tocompromise the yield.
(b) Equalized: In this layout the second-metal tracks are parallel asin the Basic, but they are enlarged in order for them all to havethe same length. This is done to equalize the inter-strip capacitance between all the channels and to avoid the variability
in the noise.
(c) Varying: In this layout the second-metal tracks have a constantangle with respect to each other. This angle is calculated sothat they occupy all the 360° (180°/63 ¼2.86°). In this way, thechannels will be separated from each other, reducing theirinter-strip capacitance. The channels are also enlarged in thecenter to make them have roughly the same length.
(d) Rectangular-A: In this case, the second-metal channels go
parallel to the ﬁrst-metal channels from the pads until they
can cross perpendicularly to them to reach their corresponding contact. The second-metal channels go in between theﬁrst-metal ones, and on top of the p-stop. In this way, theinter-strip capacitance to the ﬁrst-metal channels is
minimized.
(e) Rectangular-B: In this case, the second-metal channels go
parallel to the ﬁrst-metal channels, but on top of them. Thislayout is more like a test structure (a worst-case) because inthis case the inter-strip capacitance will be increased, althoughmaybe the pick-up is minimized.
Additionally, there are two options of each of these layouts: onein which the second-metal tracks have 20 mm width (as in the ﬁrst

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designs), and another in which there tracks have 10 mm width. Thissecond case is still far from the resolution limits of the technology
but will allow a lower coupling between metal tracks.
All these layouts of EPA were added in a new full wafer mask
which includes the two TOP sensors of the petalet prototype [4].
Each of these sensors has 2 strip rows and each strip row has 384
channels. The strip pitch in these sensors is variable, because thechannels are oriented toward the beam in the End-cap part of the
ITK, plus the strips are designed with built-in stereo angle. The
strip pitch ranges then roughly from 93 mm to 104 mm across thesesensors. The chips used for the readout in the petalet prototypes
are the so-called ABC250 [7] which have 128 readout channels andpads staggered in two rows with constant pitch of 50 mm. Therefore there is room for 12 EPA in the whole wafer. One sensor (TOPLeft) includes the 5 EPA layouts with 20 mm track width, plus one
basic design with 10 mm track width; and the other sensor (TOPright) includes the 5 EPA layouts with 10 mm track width, plus one
basic design with 20 mm track width. The full wafer design can beseen in Fig. 3.

3. Technology and fabrication
One full batch of TOP petalet sensors have been fabricated in
the clean room of the Centro Nacional de Microelectronica (IMBCNM, CSIC), Barcelona, Spain. High-resistivity, p-type, 300 mm
thick wafers with 4 in. diameter have been used as substrates. The“standard” ﬁrst-metal tracks, which are the top plate of the coupling capacitors, are made with a 0.5 mm thick sputtered Aluminum layer. The inter-metal oxide layer is deposited by a lowtemperature Plasma Enhanced Chemical Vapor Deposition(PECVD) processing step. Four different inter-metal oxide thicknesses have been implemented in order to see the inﬂuence of thisparameter and to study the technological difﬁculties. Finally, thesecond-metal tracks are made with a sputtered Aluminum layer
which is 1.5 mm thick. A picture of one of the fabricated wafers isshown in Fig. 3, where the different EPA designs can be clearlyseen.
Due to the different thermal expansion coefﬁcients of siliconoxide and silicon, the inter-metal oxide layer creates stress. Many
yield issues in multilevel technologies are related to stress [8]:cracking of dielectric layers, hillocks or stress voiding in aluminumlines, etc. These have not been observed during fabrication. We
will therefore focus on the resulting wafer bowing which can affect the subsequent photolithographic steps, because it interfereswith the alignment marks that are deﬁned before and after theinter-metal oxide. The bowing of the sensors can also affect theassembly of the whole module, plus position resolution can becompromised. All this limits the inter-metal oxide thickness thatcan be used for the EPA fabrication. Consequently, the bowing of
the different wafers was monitored during the fabrication of thesensors. Fig. 4 shows the results of wafer bowing tests for thedifferent inter-metal oxide thicknesses. The bow is deﬁned here,according to standards [9], as the distance between a point at
wafer center, and a reference plane deﬁned by three points at thewafer edges. Note that the bow can be positive or negative, which
results respectively from compressive or tensile stress. In our case,the bow is positive because the stress from the inter-metal oxide iscompressive. As it can be seen, the bowing is already very signiﬁcant (roughly 300 mm) with 3 mm thick oxide, and although thephotolithography could be realized without problems, this bowing
would be too large for the proper assembly of the modules, and forparticle position resolution for the tracking. In the case of 4 mm
oxide thickness, the bowing of the wafers already generated difﬁculties in the processing, so this option is discarded already fortechnological reasons.

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Fig. 2. Images of the 5 different EPA layouts studied in this work.

Fig. 4. Wafer bowing as a function of the inter-metal oxide thickness for all thewafers fabricated.

Fig. 3. Picture of one of the wafers fabricated in the batch of TOP petalet sensors
with the ﬁve EPA layouts, including also different mini-sensors and test structures.

The quality of the via contact between the second metal andthe ﬁrst metal was also checked in order to ﬁnd out if this affects
the yield of good channels in the sensors. Daisy-chain test structures including more than 200 vias were implemented in thewafer design for this purpose. The daisy chains are just series ofvias connected in series such that the ﬁrst via goes from metal 2 tometal 1, then a short line of metal 1 connects to the next via whichgoes from metal 1 to metal 2, then a short line of metal 2 connects
to the next via with goes from metal 2 to metal 1, and so forth.
Fig. 5 shows the results obtained for the via resistance when different number of vias are measured in series. As it can be seen, thevalue stabilizes very nicely at about 0.18 Ω/via for high number ofvias series, when the inﬂuence of parasitic resistances from the

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Fig. 5. Values of the via resistance obtained from measurements of resistance ofdaisy-chains of vias in series, and for different number of vias. As it can be seen,
when the number of vias in series is too small the error in the result is large because the parasitic resistances of the measurement become relevant. Measurements shown for different daisy-chains and for different inter-metal oxide
thicknesses.

setup becomes negligible (high number of vias measured in series). The resistance value is independent of the inter-metal oxide
thickness, indicating a very good ﬁll of the via contact with themetal. Regarding yield, not a single daisy-chain failure was foundin measurements of 200 vias in series, measuring at least 2 daisy
chains per wafer in all the wafers. The daisy chain test structures
could not be distributed across the wafer, as it would have been
optimal, due to layout limitations. The structures were placed only
on one side, not very close to the center of the wafer. It has to bementioned that a technological problem was found in the thickoxide wafers, where an incomplete etching of the vias produced
defective contact from second to ﬁrst metal in the center of thewafers. This is due to the combined inhomogeneity of the oxide
deposition and etching process, and can be easily corrected by acontrolled via over etch. Still, this is not considered a via yieldproblem, as the vias that are correctly etched make a correct lowresistance contact with no-measurable statistical failure.

4. Inter-strip capacitance Tests
As explained above, the use of an additional second-metal trackon every channel in order to make the connectivity and pitch
adaptation to the ASIC, can produce an increase in the inter-strip
capacitance of the channel. This will result in an increase of thenoise per channel. Additionally, the inter-strip capacitance willvary from channel to channel as the second-metal track is inprinciple different for every channel, and so will do the noise,increasing the noise variability. In order to control that and to
identify the differences between the ﬁve EPA layouts with respect
to noise, inter-strip capacitance has to be measured for all thechannels.
On the other hand, one speciﬁc feature of the EPA is that thesecond-metal channels cross on top of many of the ﬁrst-metalchannels, increasing the coupling between channels that are far
from each other in the sensor, the “far neighbors”. This implies thatthe usual way of testing inter-strip capacitance by measuring it
only with respect of the 2 or 4 neighbors, is not a good approximation in this case. A test of the inter-strip capacitance of every
channel has to take into account all the rest of the channels, “allneighbors”, in order to have a proper measurement. Only this way
the channel noise variability could be deduced from the inter-strip

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capacitance tests. For this purpose, a probe card with 128 probes
has been used for inter-strip capacitance tests in this work. Only
this way, the correct capacitance measurement can be done between the strip under tests and all the rest of the channels that are
purposely grounded. In practice, our switching matrix has only 24
input channels, therefore the strips are measured in groups of 20,
by moving the probe card across the EPA pads. But with the 128
probe card we can be assured that all the relevant neighborchannels (the ones that can cross the strips under test) are
grounded during the measurement. The measurements have been
performed using and Agilent 4284A LCR meter. A correction has
been applied to all the measurements individually. Before probing
the pads, a capacitance measurement is performed in the 20 active
probes without connecting the probe card to anything (“OPEN”
conﬁguration). This value is recorded for each probe and subtracted for every individual capacitance measurement that is takenwith this particular probe.
The results of the inter-strip capacitance tests for the 5 EPA
structures fabricated according to the 5 layouts described above
using 20 mm track width can be seen in Fig. 6. In the ﬁrst plot(a) one can conﬁrm that the inter-strip capacitance reproduces
qualitatively the noise shape shown in Fig. 1 for modules assembled with the ﬁrst EPA layout implemented. The second plot(b) reveals that although the number of crossings between second
metal and ﬁrst metal and the total track length is made the samein this layout for all the channels, the coupling of the second-metal
tracks with far neighbors is dominant because of the short separation between tracks, therefore, the channels with moreneighbors show increased inter-strip capacitance, actually enhancing the undesired shape of the Basic layout. The third plot(c) shows that with the Varying layout, the inter-strip capacitanceis “equalized” in the center of the structure with respect to the
Basic structure, but the inter-strip capacitance reduction at thestructure edges keeps its variability. The forth plot (d) shows thebest equalization on inter-strip capacitance achieved with all
structures, corresponding to the Rectangular-A layout. The fact thatthe second-metal tracts are quite separated from each other andfrom ﬁrst-metal tracks, seems to be the dominant inﬂuence in thisresult. Finally, the ﬁfth plot (e) shows the expected behavior of the
Rectangular-B layout where the fact that the central tracks are on
top of the ﬁrst-metal tracks for long distances, increases the interstrip capacitance for them.
Fig. 7 represents the differences in inter-strip capacitance for3 selected structures for second-metal track width of 20 or 10 mm.
As can be seen in the ﬁrst plot (a) the variability of the Basic layout
is quite improved when the track width is reduced due to theimportant reduction in second-metal tracks coupling with eachother when the track width is halved while maintaining the track
pitch, which has effectively increased the track separation by a
50%. The same effect is observed, although enhanced, in the
Equalize structure (b), where the inter-strip capacitance variability
has been quite reduced. Finally, Rectangular-A structure shows
only a small effect of the track width reduction on inter-strip capacitance, as the separation between tracks is already large with
20 mm track width.
Finally, Fig. 8 shows the effect of inter-metal oxide thickness on
the inter-strip capacitance in Basic and Rectangular-A layouts. In
plot (a) one can see that the increase in inter-metal oxide does nothave an effect in the inter-strip capacitance variability. This agrees
with the fact that the inter-strip capacitance variability in the Basiclayout comes mainly from the second-metal tracks coupling toeach-other, and not with the coupling with the ﬁrst-metal tracks.
In plot (b) it can be seen that the inter-strip capacitance in Rectangular-A is ﬂattened even more, resulting in quite a ﬂat plot,
reducing very much the variability. This is consistent with the factthat the inter-strip capacitance in Rectangular-A layout is mostly

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Fig. 6. Inter-strip capacitance tests results for the 5 EPA layouts with 20 mm track width. (a) Basic, (b) Equalize, (c) Varying, (d) Rectangular-A, and (d) Rectangular-B.

Fig. 7. Inter-strip capacitance results for 20 and 10 mm track width for different structures: (a) Basic, (b) Equalize, and (c) Rectangular‐A.

dependent on ﬁrst metal to second metal coupling. On the other
hand, in both cases a general increase of the inter-strip capacitanceis observed for thicker oxides. We do not have an explanation forthis result yet.
The two TOP sensors from one of the wafers fabricated with
1 mm inter-metal oxide thickness were mounted in a module. Fig. 9shows the equivalent noise charge (ENC) measured for all thechannels in this module, indicating the correspondence with thedifferent EPA layouts. For the measurements, 2 fC charges are injected into all channels, then 200 triggers are sent into eachchannel and the ASICs are read out repeatedly with increasing
thresholds. From the resulting S-curve, all characteristics of eachchannel, such as noise, are determined in a ﬁt. The readout is donewith the full testing setup [10]: the module is connected to HSIO(“High Speed Input Output”) board developed by the ITk collaboration [11], and read out with a root program called SCTDAQ
developed at Rutherford Appleton Laboratory (RAL) [12]. One canconﬁrm the expected good correlation between the inter-strip

capacitance and the noise for the different structures [13,14],
which demonstrates that the inter-strip capacitance measurements, with all-neighbors considered, are a proper test to estimate
the noise behavior of the different EPA layouts [15].
Additionally, some miniature sensors with EPA have been irradiated with 70 MeV protons at the Cyclotron and Radioisotope
Center (CYRIC), Tohoku University, Japan, in order to see the inﬂuence of radiation on the inter-strip capacitance of sensors with
EPA. The ﬂuences achieved were 5.3  1014, and 1.2  1015 1 MeV
equivalent neutrons/cm2. The last ﬂuence is the current target
value for the strip sensors in the future ITk. Sensors irradiated tothose ﬂuences with the Basic EPA layout, 20 mm track width, and
1 mm inter-metal oxide thickness have been tested. At this point,
only inter-strip capacitance measurements considering ﬁrst
neighbor channels have been taken, because the irradiated sensors
need cooling in order to bias them at full depletion voltage with
controlled leakage current, and the test setup with the 128-probes
probe card does not have a cooling system available. Nevertheless,Please cite this article as: M. Ullán, et al., Nuclear Instruments & Methods in Physics Research A (2016), http://dx.doi.org/10.1016/j.
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Fig. 8. Inter-strip capacitance results for different inter-metal oxide thicknesses in two structures. (a) Basic and (b) Rectangular-A.

Fig. 10. First-neighbors interstrip capacitance tests for the irradiated miniaturedetectors.

Fig. 9. Noise results for the different EPA layouts measured on a module assembled
with one of the wafers fabricated.

as it can be seen in Fig. 10, the results of the tests performed on theirradiated miniature sensors do not show any inﬂuence of radiation in the inter-strip capacitance. This will have to be conﬁrmed
with all-neighbors inter-strip capacitance tests.

5. Beam test
The full module assembled with two TOP petalet sensors fromone of the wafers fabricated with 1 mm inter-metal oxide thickness
have been subjected to test beam experiments at the DiamondLight Source synchrotron in the UK. The 15 keV beam has a rate of
up to 2 kHz and it is microfocused to a spot size of
3.27 mm  1.65 mm (FWHM) which guarantees to hit only one strip

at a time. There is an xyz-stage with micron precision positioning
to move the beam across the target. As for the setup, the module
was held in place with vacuum, a chiller with a cooling liquid of atemperature of 15 °C was used to cool the module. The environmental conditions inside the box where the module was mounted
were monitored using temperature and humidity sensors placed
close to the module. A low ﬂow of nitrogen at room temperature
was used to prevent condensation inside the box. During thewhole duration of the measurement, inside the box temperatureand humidity were stable at 20 °C, and 50% humidity.
One quarter of each EPA structure was scanned with the beamhitting every other channel, meaning that 16 strips were scannedof each structure. Each structure was scanned twice: once on topof the second-metal tracks, and once away from them, in order todistinguish between cross-talk and pick-up. 3500 triggers wereallowed per strip and per threshold, but the photon conversion
efﬁciency inside a time window opened by a trigger is only
roughly 60%, which results in about 2000 events registered per
strip. Three threshold ranges were used: 80–200 mV in steps of
6.4 mV, 200–300 mV in steps of 39.4 mV, 300–450 mV in steps of
9.6 mV, resulting in 40 threshold steps per beam position on the
strips. For each photon undergoing interactions in the silicon, adeposited charge of about 0.6 fC is expected, corresponding to a

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8

produces a general increase of its value in all the channels of about
1 pF that is still to be fully understood. Measurements of miniature
sensors irradiated at different ﬂuences show no inﬂuence of radiation damage on the interstrip capacitance, for measurementswith ﬁrst neighbors. The analysis from the beam test shows no
indication of cross-talk or pick-up above 170 mV threshold. A
deeper analysis of the test beam results and more test beams withtracking capabilities are foreseen to conﬁrm this result in detail.

Acknowledgment

Fig. 11. Channel signal ratio to the hit channel signal for all the thresholds (background subtracted). The color code on the right corresponds to ratio between thesignal (number of hits) in the corresponding channel with respect to the signal in
the channel being hit, once the background has been subtracted. (For interpretationof the references to color in this ﬁgure legend, the reader is referred to the web
version of this article.)minimum signal of roughly 60 mV. Since multiple interactions canoccur, up to 3 fC can be deposited. A minimum ionizing particle isexpected to produce a signal of about 300 mV.
Fig. 11 shows one typical result for the beam pointing to a
speciﬁc strip (here strip 33) hit in a structure (here structure
Rectangular B as the worst-case structure). The ﬁgure represents
the ratio between the signal in every channel with respect to thesignal in the channel being hit, once the background has been
subtracted, for all the thresholds. The background has been determined by the spurious signal, that is, noise, measured in a
speciﬁc strip if the beam was pointing at a completely different
area of the sensor that is read out by a different readout chip, andaveraged over 16 times 10 measurements. As it can be seen, there
is no sign of spurious signal in any channel above 170 mV
threshold. On the other hand, the signal in the channels not being
hit is below 1% of the signal recorded in the channel being hit,
above 100 mV. The non-zero occupancy for the non-hit channel islikely due to the readout electronics performance at low thresholdlevels. All this is an indication that there is no sign of cross-talk or
pick-up, at least to the level of statistics achieved in this test beam.
A deeper analysis of the test beam results is being done to obtain
more detailed information on the possible spurious signals at
lower thresholds. More test beams are foreseen with tracking
capabilities and using irradiated sensors to conﬁrm these results.

6. Conclusion
A new proposal is presented to improve the throughput andreliability of the interconnection between sensors and electronics
in the Inner Tracker on the ATLAS Upgrade experiment for the HLLHC. A second metal layer is used to make the pitch adaptation
integrated in the sensor. New layouts for these Embedded Pitch
Adapters are tested, together with new design and technologicalimprovements such as the reduction of second-metal track width,
and the increase in the inter-metal oxide thickness. The effect ofthese factors on the inter-strip capacitance is measured by a new
method that takes into account the effect of all neighbors. The
results presented demonstrate the reduction in interstrip capacitance variability down to a few tenths of pF with some of thedesigns and with the reduction of the second-metal track width.
The increase in inter-metal oxide thickness helps in the ﬂatteningof the inter-strip capacitance for some of the structures, but

The irradiations were performed at Cyclotron and Radioisotope
Center (CYRIC), Tohoku University, with Y. Sakemi, M. Ito, and T.
Wakui. The test beam was performed in the Diamond Light Source
Synchrotron in the UK. The research was supported and ﬁnanced
in part by the Ministry of Education, Youth and Sports of the Czech
Republic (Grant no. LG13009), the German Federal Ministry of
Education and Research, and the Helmholtz Association, the European Social Fund and by the Ministry of Science, Research andArts, Baden-Wuerttemberg, Germany, the Japan Society for Promoting Science KAKENHI-A Grant number 20244038 and KAKENHI-C Grant number 20540291, the Ministry of Education,
Culture, Sports, Science and Technology-Japan, KAKENHI for Research on Priority Area Grant number 20025007 and for ScientiﬁcResearch on Innovative Areas Grant number 23104002, the Slovenian Research Agency, the Spanish Ministry of Economy andCompetitiveness through the Particle Physics National Program(Refs. FPA2012-39055-C02-01 and FPA2012-39055-C02-02) andco-ﬁnanced with FEDER funds, the ﬁnancial support of the State
Secretariat for Education, Research, and Innovation, the Swiss
National Science Foundation and the Canton of Geneva, Switzerland, the UK Science and Technology Facilities Council (underGrant ST/M006409/1), and the United States Department of Energy, Grant DE-FG02-13ER41983.

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nima.2016.04.019i