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/nimaStudy of surface properties of ATLAS12 strip sensors and theirradiation resistance
M. Mikestikova s,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, M. Ullan d, I. Bloch e, I.M. Gregor 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,l, 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, J. Stastny 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, K. Sato x,y,
M. Hagihara x, S. Iwabuchi x, 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, USAc
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, Universität Freiburg, Hermann-Herder-Str. 3, D-79104 Freiburg, Germanyg
DPNC, University of Geneva, 24, Quai Ernest-Ansermet, CH-1211 Geneve 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 Kingdom
nJožef 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, The 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 Shefﬁeld, Hicks Building, Hounsﬁeld Road, S3 7RH Shefﬁeld, 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, SpainbnCorresponding author.
E-mail address: mikestik@fzu.cz (M. Mikestikova).

http://dx.doi.org/10.1016/j.nima.2016.03.056
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
3 March 2016
Accepted 16 March 2016

A radiation hard n þ -in-p micro-strip sensor for the use in the Upgrade of the strip tracker of the ATLAS
experiment at the High Luminosity Large Hadron Collider (HL-LHC) has been developed by the “ATLAS
ITk Strip Sensor collaboration” and produced by Hamamatsu Photonics.
Surface properties of different types of end-cap and barrel miniature sensors of the latest sensor
design ATLAS12 have been studied before and after irradiation. The tested barrel sensors vary in “punchthrough protection” (PTP) structure, and the end-cap sensors, whose stereo-strips differ in fan geometry,in strip pitch and in edge strip ganging options. Sensors have been irradiated with proton ﬂuences of up
to 1  1016 neq/cm2, by reactor neutron ﬂuence of 1  1015 neq/cm2 and by gamma rays from 60Co up todose of 1 MGy. The main goal of the present study is to characterize the leakage current for microdischarge breakdown voltage estimation, the inter-strip resistance and capacitance, the bias resistanceand the effectiveness of PTP structures as a function of bias voltage and ﬂuence. It has been veriﬁed that
the ATLAS12 sensors have high breakdown voltage well above the operational voltage which implies thatdifferent geometries of sensors do not inﬂuence their stability. The inter-strip isolation is a strong
function of irradiation ﬂuence, however the sensor performance is acceptable in the expected range for
HL-LHC. New gated PTP structure exhibits low PTP onset voltage and sharp cut-off of effective resistanceeven at the highest tested radiation ﬂuence. The inter-strip capacitance complies with the technicalspeciﬁcation required before irradiation and no radiation-induced degradation was observed. A summaryof ATLAS12 sensors tests is presented including a comparison of results from different irradiation sites.
The measured characteristics are compared with the previous prototype of the sensor design, ATLAS07.
& 2016 Elsevier B.V. All rights reserved.

Keywords:
HL-LHCATLAS ITk
Micro-strip sensor
Radiation resistance
Surface properties
Inter-strip capacitanceInter-strip resistancePunch-through protection

1. Introduction
The Phase-II upgrade of the Large Hadron Collider (LHC) to thehigher luminosity (5  1034 cm  2 s  1) machine, called the High
Luminosity-LHC (HL-LHC) [1], will require replacement of the entire
current ATLAS Inner Detector with a new all-silicon tracker containing
new type and design of silicon sensors. It will have pixel sensors at the
inner radii surrounded by micro-strips. As currently planned, themicro-strip detector in the upgraded Inner Tracker (ITk) [2,3] willconsist of 2 barrel layers of “short” strips (length 23.8 mm), 2 barrel
layers of “long” strips (length 47.8 mm) and seven end-cap discs oneach side. The end-cap strips length varies from 15 mm to 60 mm
depending on the radius. The strip length is chosen so as to maintainthe average hit occupancy to be less than 1% at the expected maximum instantaneous luminosity. The maximum hit occupancy islimited by the bandwidth of the read-out system.
Strip sensors in the ITk will be exposed to charged particles,
neutrons and gammas. The predicted maximum ﬂuence andionizing dose [4] for the integrated luminosity of 3000 fb  1 in the
strip barrels is 5.3  1014 1-MeV neutron equivalent (neq)/cm2 and216 kGy for the short strips in the layer 1, and in the strip end-caps
8.1  1014 neq/cm2 and 288 kGy in the inner regions of the outermost disk at a distance of 300 cm from interaction point. The
estimates stated above have no safety factors applied. Such a highﬂuence of particles and ionizing dose causes bulk and surface
damage to the sensors. In this study the miniature sensors were
irradiated by protons, neutrons and gammas from 60Co source.
Protons and neutrons displace silicon atoms via non-ionizing
energy losses, which results in point and cluster defects in thebulk. In addition protons ionize both the Si bulk and the insulating
layer (SiO2). The latter causes the accumulation of positive charges
and traps in the SiO2 and at the Si-SiO2 interface that leads todeterioration of the sensor surface. The gamma irradiation isusually used to study the oxide/interface damage separately fromthe bulk damage. However, gamma rays from 60Co source areabsorbed in the sensor mainly through Compton scattering andthe scattered Compton electrons have high enough energy (hundreds of keV) to produce point defects in the Si bulk. The surface

damage is expected to inﬂuence the breakdown voltage, the interelectrode isolation and capacitance.
The current ATLAS Semi-Conductor Tracker is based on singlesided type of micro-strip sensors made with p-strips implanted onn-type silicon bulk (p-in-n). n þ -in-p type sensors, which are much
more radiation hard [5,6], are being considered for use in the HLLHC trackers. This type of sensor has much faster response as itcollects electrons instead of holes. It also has no radiation induced
type inversion and therefore it always depletes from the segmented side that allows sensor operation in partially depletedmode. This is particularly useful after high radiation ﬂuencies
when the full depletion voltage becomes higher. However, n þ -in-p
strip sensors need additional strip isolation.
A large area n þ -in-p silicon sensor for the use in ITk has beendeveloped by the ATLAS ITk Strip Sensor Collaboration [7,8] andproduced by Hamamatsu Photonics (HPK) [9] in a 6-inch (150 mm)
wafer. Besides the large sensor (9.75 cm  9.75 cm), the wafer also
includes sets of miniature sensors, which are used for irradiation
studies. The sensors were irradiated by different particles, energies
and doses and were distributed for extensive test to many institutions of the collaboration for studies of sensor properties that havereceived up to double maximal ﬂuence predicted in ITk.
The samples for the surface irradiation study presented in thispaper are the barrel and end-cap (EC) miniature strip sensors of
the latest sensor design, called ATLAS12 [8] which was developed
from ATLAS07 design [7]. The surface properties before and after
ionizing radiation up to the proton ﬂuence of 1  1016 neq/cm2,
neutron ﬂuence 1  1015 neq/cm2 and gamma dose of up to 1 MGy
were evaluated. The tests include measurements of the inter-strip
capacitance and resistance as a function of ﬂuence, and the interstrip resistance as a function of bias, temperature and annealing
time. The effectiveness of different types of punch-through protection structures was tested with DC voltage scan method.
Some of the surface studies with previous design ATLAS07
were reported in Ref. [10] to maximal ﬂuence of 1.5  1013 neq/cm2.
The bulk damage of latest design ATLAS12 is presented in Ref. [11]for barrel and in Ref. [12] for end-cap sensors.

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2. Samples and irradiation
The tested sensors of ATLAS12A, ATLAS12M and ATLAS07
layouts1 are fabricated in p-type, ﬂoat zone wafers with o1 0 0 4
crystal orientation and thickness of 320 mm. For each layout,slightly different wafer resistivity was used, and therefore the full
depletion voltage (VFD) differs. The VFD of non-irradiated sensors,
estimated from capacitance-voltage measurements of strip miniature sensors, was typically 350–360 V for 12 A, 225 V for 12 M,
and 200 V for 07.
The readout aluminum strips are capacitively coupled with nstrips biased through polysilicon resistors. The strip isolation isdone with common p-implant (p-stop) with surface ion concentration of approximately 4  1012 cm  2. The tested sensors are1  1 cm2. The barrel sensors have 74.5 μm strip pitch withvarious “punch-through protection” (PTP) structure geometries
examined. The end-cap sensors evaluated here have stereo-strips
in fan geometry that differ in strip pitch (“large” and “small” pitches: 103.4 and 64.3 mm on average, respectively) and in edge strip
biasing. Details of the design including strip isolation structure andthe edge strips ganging are described in Ref. [8].
The proton irradiations were performed at Karlsruhe Inst. Tech.,Germany (KIT) [13] with 23 MeV, at Birmingham, UK [14] with
27 MeV and at CYRIC (Tohoku University, Japan) with 70 MeV
protons. Samples were kept in a cooled box during irradiations
and the box was scanned to cover the whole target area of sensors.
The stage scan speed at the CYRIC irradiation was 20 mm/s, at KIT
115 mm/s and at the Birmingham 1 mm/s (2013 irradiation run)and 4 mm/s (2015 run). The scan time depends on the beam current and the area of the samples to be irradiated at the same run.
The 1015 neq/cm2 ﬂuence took 15 min at KIT at a beam current
1.7 μA,  80 s in the Birmingham cyclotron at 1000 nA (2013 run),and  100 min at CYRIC at 400 nA (2015 run) and at 700 nA (2014run). The measurement of the equivalent proton ﬂuence has anerror of approximately 20%, taking the precision of the dosimetry
and of the estimation of hardness factor into account. The irradiated samples were immediately stored in a refrigerator to prevent any post-irradiated annealing. The neutron irradiations wereachieved at Ljubljana JSI TRIGA Reactor [15]. The accuracy ofneutron ﬂuence is 10%. The source of uncertainty is the precisionof the dosimetry and errors in timing of irradiation. 60Co gamma
irradiation was carried out at the Solid State Gamma-ray irradiation facility of the Brookhaven National Laboratory (BNL) [16] andat the Institute of Nuclear Fuels, Prague (UJP) [17]. The sensors
were maintained at room temperature during gamma irradiation.
The dose rate was 220 Gy/hour and 13.2 kGy/hour in BNL and UJP,
respectively. The dose rate uncertainty is  10% in both cases.

3. Punch-through protection structures
AC coupled strip sensors can be subjected to large voltages
between the readout metal strips and the implant strips. Theselarge voltages can be caused by charge accumulation in the bulk
due to beam splash accidents. A dedicated punch-through protection (PTP) structure, at the ends of the implant strips is used tokeep the implant voltage low. The PTP structure creates a shortcircuit in current path in parallel to the bias resistor at the voltages
higher than the PTP onset voltage, thus avoiding the capacitor
breakdown. The ATLAS12 miniature sensors have different types of
PTP structures (A–F type) (Fig. 1) [8]. The E and F types are
1
In ATLAS12A wafer layout the large-area main sensor is made of all “axial”
strips where the strips are parallel to the sensor edge. In ATLAS12M two segmentsof strips are “axial” and two are “stereo” strips where the strips are inclined to thesensor edge. More details about 12 A and 12 M wafer design are in [8].

3standard PTP structure with 20 and 70 mm gap between the bias
ring and the end of the implant n þ -strip. The B and C types arenew, improved PTP structure with an extended polysilicon electrode of the bias ring over the PTP gap; so called the full-gatestructures.

4. Experimental methods and results
About 200 miniature sensors of different types have been tested for surface studies before and after irradiations at siliconlaboratories at the Academy of Sciences of the Czech Republic in
Prague, University of California Santa Cruz, University of Tsukuba,
University of Freiburg, Lancaster University and IFIC Valencia. The
sensors were either bonded to PCB board and measured in temperature control box down to  50 °C or in probe station with acooled chuck where the contact was provided by micromanipulators (down to  30 °C). Nitrogen environment ensured
humidity below 5% at 20 °C.
4.1. Leakage current
The leakage current as a function of the bias voltage (IV) was
measured for all types of the barrel and end-cap sensors in order todetermine whether sensor stability is inﬂuenced or not by sensor
geometry, wafer used and surface radiation damage. For all the tested
non-irradiated sensors, the value of the leakage current at 600 V is lessthan 4.8 nA/cm2 at 20 °C, which is well below the technical speciﬁcation's limit (o2 mA/cm2) [18]. There were no observed onsets of
micro-discharges in any of the sensors up to 1000 V.
The upper plot of Fig. 2 shows the current of the proton irradiated ATLAS12A, 12 M and 07 barrel sensors. The lower plot ofFig. 2 shows the current of the end-cap sensors irradiated by protons and gammas. Neutron irradiated barrel sensor is included forcomparison. All sensors were measured after annealing them for
80 min at 60 °C. Most of the proton irradiated sensors did not show
micro-discharge breakdown below 1000 V. The wiggle at 250–
350 V in low ﬂuence samples (0.52 and 1  1013 neq/cm2) is probably due to surface generated current in the backside electrode –when the depletion had reached the backside [8]. The average
current values at bias voltage 600 V and temperature  10 °C are 30,
51 and 103 μA/cm2 for the proton ﬂuence 0.5, 1 and 2  1015 neq/
cm2, respectively. The leakage current increase with ﬂuence isconsistent with bulk current increase. The values correspond towhat one would expect from the estimate I(Φ,t,T)¼α(t,T)VΦ, where
α is the leakage current damage factor,2 V is the active sensor
volume and Φ is ﬂuence in neq/cm2. Measured current value above
ﬂuence of 1  1015 neq/cm2 is lower than what is estimated using
the sensor volume because sensors are not fully depleted at 600 V.
The full depletion voltage increases with increasing proton ﬂuence
except for a little variation at low ﬂuence – the VFD decreases up to1  1014 neq/cm2 due to initial acceptor removal process [21]. The
VFD of ATLAS12 sensors is 350–450 V and 500–700 V for 3  1014
and 1  1015 neq/cm2, respectively. At ﬂuences greater than
3  1015 neq/cm2 the VFD is above 1000 V.
The total leakage current increases  100 times after gamma
irradiation, which is the result of the surface current increase due
to surface damage. Unfortunately, the guard ring in the tested
sensors does not have a probing pad and thus it is not possible toseparate the leakage current of the active area inside of the bias
ring from the current at the periphery. The micro-discharge
breakdown at 900 V of sensors irradiated by 10 kGy gamma dose2
α¼ 4  10  17 A/cm2 at 20 °C after annealing for t ¼80 min at 60 °C [19]. Bulk
current temperature normalization with activation energy of 1.2 eV was used.

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Fig. 1. PTP structures in ATLAS12 miniature sensors. New PTP structure with full gate (C-type) is implemented in main ATLAS12 sensor in addition to the miniature test
sensors.

is attributed to surface damage [22]. It is well above the maximum
operating voltage 600 V and it disappears after annealing andadditional irradiation. Observation that the current is lower for thehigher gamma dose is a subject of further studies.
4.2. Inter-strip capacitanceImportant strip sensor surface characteristics which give us
information about the effect of surface radiation damage are theinter-strip capacitance (Cint) and inter-strip resistance (Rint). Theinter-strip capacitance contributes to the input capacitance of thereadout electronics and is the main contributor to the detector
noise. Therefore, it should be as low as possible. The capacitance ismeasured by LCR meter between a central metal strip and its
nearest neighbors with other strips ﬂoating (Fig. 3). This way themeasurement also includes additional  10% contribution from thenext neighbors [23,10].
The inter-strip capacitance before irradiation was measured on
25 ATLAS12 sensors at 100 kHz test frequency as required by
speciﬁcations. The average values of Cint are equal to 0.767 0.02,

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Fig. 2. The leakage current vs. bias voltage after annealing 80 min at 60 °C for (upper) ATLAS12A, 12 M and 07 barrel sensors irradiated by protons (CYRIC) to ﬂuences inrange 0.52  1013–8.8  1015 neq/cm2 taken at  20 °C and for (lower) various types of ATLAS12 end-cap sensors after proton (Birmingham and Karlsruhe), gamma andneutron irradiation compared with non-irradiated sensor. Proton and neutron sensors were measured at  10 °C, gamma and non-irradiated ones at room temperatureþ 20 °C [20]. SP and LP in the legend denote for small and large pitch sensors, respectively; E and C are referring to different PTP structure types.

Fig. 3. Measuring method for the inter-strip capacitance: AC pads of metal strips are bonded to PCB board. The capacitance between the central strip and its neighbors ismeasured by an LCR meter in parallel mode.

0.79 70.01 and 0.557 0.01 pF/cm for barrel, small and large pitch
end cap sensors, respectively. The value for the barrel miniature
sensors is in very good agreement with value of large area sensor
having the same strip pitch [24]. The bias voltage dependence ofCint was measured at different proton ﬂuences and gamma doses
at test frequency 1 MHz.3 Fig. 4a shows the results for gamma
irradiation. The higher the radiation dose the higher the value ofthe bias voltage needed for Cint to saturate to the pre-radiation
value. This is caused by the surface effect when the increasing
charge trapped in the oxide upon irradiation induces an increasein carrier concentration in thin accumulation layer between strips.
3
The inter-strip capacitance measurements are dependent on the frequency of
the AC signal. There is a radiation induced resonance at 10 kHz that increases withincreasing proton ﬂuence and also affects measurements of Cint at 100 kHz [20].
Therefore, for irradiated sensors the higher test frequency of 1 MHz was used. The
resonance effect is not present at this value.

Beyond VFD the Cint becomes constant for both types of irradiationand for all tested doses. Cint is not inﬂuenced by the PTP structure
type. In the case of gamma irradiation, the VFD does not changewith irradiation dose, unlike of the proton irradiation. At highproton ﬂuences the depletion depth does not reach the fullthickness of the sensor and small increase in values of measured
Cint at 400 V is probably due to the contribution of the strip-tobackplane capacitance (Cbulk) as seen in Fig. 4b. Fig. 5 shows the
Cint as a function of proton ﬂuence of ATLAS12A, 12 M and 07
measured at various institutions. The inter-strip capacitance iswafer independent and does not change with proton ﬂuence athigh bias.
4.3. Inter-strip resistance
A well performing strip sensor has to have a good isolation
between the strips in order to maintain the high spatial resolution

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Fig. 6. Measurement set-up for inter-strip resistance measurement. The applied
voltage was in range (  5V, þ 5 V) for non-irradiated sensors with low inter-strip
currents (  100's pA) and (  1 V,þ 1 V) for irradiated sensors. Bias voltage wasapplied to the sensor up to  1000 V.

Fig. 4. (a) Inter-strip capacitance as a function of bias voltage: ATLAS12A end-cap
sensors of small pitch (C and E PTP type) irradiated by gamma. Unirradiated sensor
value shown for comparison. Beyond VFD the Cint saturates to pre-radiation value.
(b) Inter-strip capacitance vs. proton ﬂuence measured at bias voltage  400 V and 1000 V: ATLAS12A barrel sensors. The measurements were done at 1 MHz.

Fig. 5. Proton ﬂuence dependence of inter-strip capacitance. ATLAS12A, 12 M and07 barrel sensors. Vbias ¼  1000 V.

and adequate signal level of the recorded hits. The isolation
between neighboring strips was veriﬁed by measurements of theinter-strip resistance (Rint). The measuring method is illustrated on
Fig. 6. Three adjacent DC pads were contacted. Voltage Vapp was
applied by the Source-Measurement Unit to the central strip andthe current Iint was measured on the outer strips. The measurements were performed at þ20 °C for non-irradiated sensors andat cold temperatures (from  10 °C to  50 °C) for irradiated ones.
The inter-strip resistance was calculated by
Rint ¼ 2 = ðdI int = dV app Þ:

The inter-strip resistance values of non irradiated sensors aretens of GΩ's which signiﬁcantly exceeds the minimum limit
10  Rbias  15 MΩ required in the technical speciﬁcation [18]. Theinter-strip resistance as a function of bias voltage for ATLAS12A
sensors irradiated by protons at different ﬂuences from 1  1014 to1  1016 neq/cm2 is shown in Fig. 7. A non-irradiated sample is also
shown for comparison. The sensors with PTP structure of type C
and E are compared. Before irradiation the inter-strip resistance isindependent of bias voltage. After proton irradiation the inter-strip
resistance is reduced and is bias voltage dependent. The bias ﬁeld
decreases conductivity between strips. The results of various
institutions are consistent. No dependence on PTP structure typewas observed.
The inter-strip current as a function of applied voltage forsensors irradiated to different proton ﬂuences is shown in Fig. 8a.
The inter-strip current is linear function of applied voltage up to
the highest proton ﬂuence 1  1016 neq/cm2. The total leakage
current is in this case 124 μA. The Fig. 8b presents a comparison of
proton and neutron irradiated sensors with the same ﬂuence of1  1015 neq/cm2. Both sensors have a high total leakage current of
26 μA (at Vbias ¼  300V), but the neutron irradiated sensor has ahigh Rint ¼ 5.287 0.14 GΩ, whereas proton irradiated sensor has
lower Rint ¼0.92 70.01 GΩ.
The inter-strip resistance of irradiated sensors is temperaturedependent as illustrated in Fig. 9. The inter-strip current has thesame temperature dependence as bulk generation current. Using
the activation energy of 1.2 eV, the values of inter-strip resistance,
normalized to  20 °C, are very consistent in the range  10 °C to 25 °C at which sensors were measured in various laboratories(right plot of Fig. 9). In a larger range from 0 °C to  50°C the interstrip resistance values are also very close.
The ATLAS12A, 12 M and 07 barrel sensors were irradiated in
the same irradiation campaign for comparison. Inter-strip resistance as a function of proton ﬂuence measured at various
laboratories at the bias voltage 400 V is shown in Fig. 10. All sensor
types have the same inter-strip resistance degradation with proton
ﬂuence. Inter-strip resistance values exceed the minimum specs
limit up to 3  1015 neq/cm2. Up to 1  1016 neq/cm2 the inter-strip
resistance is still larger than the pre-ampliﬁer input impedance ofo1 kΩ for the ABC130 chip [25].
The correlations between the total ionizing dose (TID) andinter-strip resistance has been studied by comparison of proton,
neutron and gamma irradiated sensors. Due to the different
damage mechanism caused by proton and gamma in silicon sensor, the proton ﬂuence was converted to TID by: 
Φ½neq =cm2  dE
;
TID ¼
κ
dx Ep
where Φ is the proton ﬂuence given by irradiation facilities inis the stopping power of protons
1 MeV neutron equivalent, dE
dx Ep
of energy Ep in silicon taken from tables in [26] and κ is the

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Fig. 7. The inter-strip resistance as a function of bias voltage for non- irradiated and proton irradiated (CYRIC) sensors ATLAS12A. The inter-strip resistance values arenormalized to  20 °C. C and F in the legend denote for PTP structure type, (*) for not annealed sensors.

Fig. 10. Inter-strip resistance vs. proton ﬂuence for ATLAS12A, 12 M and 07 barrel
samples irradiated in the same irradiation campaign at CYRIC and measured byvarious laboratories at bias voltage of  400 V. The results are normalized totemperature of  20 °C.

Fig. 8. The inter-strip current as a function of applied voltage: (a) comparison ofdifferent proton ﬂuences and (b) comparison of proton and neutron irradiatedsensors to the same ﬂuence of 1  1015 neq/cm2. The inter-strip current is linear
function of applied voltage.

Fig. 11. Inter-strip resistance vs. total ionizing dose for ATLAS12 sensors irradiatedby protons, neutrons and gammas at different irradiation sites. Results at bias
voltage  400 V are presented. The values are normalized to the temperature of 20 °C.

Fig. 9. Temperature dependence of total leakage current (left) and inter-strip resistance (middle) measured in temperature interval from  50 °C to 0 °C. Inter-strip
resistance is normalized to  20 °C using Ea ¼ 1.2 eV (right). The measured sample is ATLAS12M barrel sensor irradiated to ﬂuence of 1  1015 neq/cm2.

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conversion damage factor (κ is 1.5, 2.2 and 2 for CYRIC, Birmingham and Karlsruhe irradiation, respectively [27]). In proton irradiated samples the ﬂuence of 1  1015 neq/cm2 then corresponds to
0.80, 1.13 and 1.44 MGy for CYRIC, Birmingham and Karlsruhe
irradiation, respectively. The TID in neutron irradiated sensors due
to secondary particles has been assessed to be 10 kGy for1  1015 neq/cm2 [28]. The dependence of inter-strip resistance on
TID is presented in Fig. 11. Gamma and proton irradiated sensors
have a similar dependence of inter-strip resistance on the TID,
underscoring the inﬂuence of the surface damage on the strip
isolation. Exceptions are samples from Birmingham 2013 irradiation campaign. This discrepancy is a subject of further studies andtests at the Birmingham facility.
We further sought to check the inﬂuence of the bulk damageon the strip isolation by studying neutron irradiated sensors. They
have one order of magnitude lower inter-strip resistance than
gamma irradiated ones at the same TID. Neutrons in silicon, unlike
gammas, cause mainly bulk damage resulting in increased leakage
current. The leakage current of gamma irradiated sensors is morethan 100 times lower than in neutron ones. Assuming that the TID
dose determination for the neutron samples is sufﬁciently accurate, the difference indicates that the inter-strip resistance may beaffected by the bulk damage by its current dependence in addition
to the lead effect of the surface damage by TID.
To study the annealing effect on inter-strip resistance, a proton
irradiated sensor (2  1015 neq/cm2) was submitted to successive
annealing steps at 60 °C up to total time of 44,000 min. The resultsof this study are presented in Fig. 12. The annealing of the leakage
current is shown for comparison. The leakage current shows
typical annealing behavior – decrease at low bias voltage withannealing time and increase at high voltage as charge multiplication (CM) appears at longer annealing. With annealing time
the space charge concentration rises and higher electric ﬁeld nearstrips causes CM [29]. Inter-strip resistance shows opposite
behavior related to leakage current – signiﬁcant increase at low
bias voltage with annealing time and decrease at higher voltage.
Annealing for 5000 min causes inter-strip resistance increase to
pre-rad value, but the leakage current is still 4 orders of magnitude
higher than before irradiation. Charge multiplication near the
strips inﬂuences the inter-strip resistance at 700 V after annealingfor 5000 min and at 400 V at more than 22,000 min.
4.4. Coupling capacitance and bias resistanceThe coupling capacitance (Ccoupl) was measured between strip
metal and strip implant at 1 kHz test frequency. The values ofCcoupl/cm achieved are greater than 24 pF for both non-irradiatedand proton irradiated sensors, which meets the speciﬁcations ofCcoupl/cm 420 pF [18].
The direct method was used for an estimate of the bias resistance. A current–voltage scan was performed applying voltages(Vapp) from  1 to þ1 V in 0.4-V steps to the implant DC pad, andmeasuring the implant strip current (Iimpl). Iimpl is a linear functionof applied voltage and resistivity (Rbias) was estimated as
Rbias ¼dVapp/dIimpl. Gamma and proton irradiated sensors havemuch lower inter-strip resistance, and inter-strip currents inﬂuence the bias resistance measurements. To eliminate these interstrip effects the voltage Vtest was applied also on two neighbors,and on next neighbors at the same time. IV was performed on thecentral strip.
The polysilicon bias resistance of non-irradiated sensors is
1.48 70.07 MΩ which is well within the speciﬁcation's range(Rbias ¼ 1.57 0.5 MΩ). It slightly increases with proton ﬂuence andgamma dose. However it agrees with the speciﬁcation up to proton ﬂuence of 3  1015 neq/cm2 and gamma dose 1 MGy.
4.5. Stability measurements
Possible time evolution of leakage current and surface parameters is an important operational aspect for strip sensor. Therefore the stability of the leakage current, as well as the inter-strip
capacitance and resistance was studied by applying the bias voltage to the sensors for a few hours. Measurements performed at
low temperature (from  10 °C to  25 °C) and low humidity
veriﬁed that the inter-strip capacitance and resistance does notchange in time. The leakage current showed relatively small
decrease up to 3% during 20 h in some sensors. The results are inagreement with previous studies for the current Semiconductor
tracker of ATLAS experiment [30].
4.6. Punch-through protection
The effectiveness of the punch-through protection structure
was tested by using the static DC method [10,31,32]. The test
voltage Vtest (0–100 V) was applied between the implant (DC pad)and the grounded bias rail and resulting current Itest was recorded.
The Itest consists of the punch-through current IPTP and the current
ﬂowing through the polysilicon bias resistor (the bias resistance
Rbias E1.5 MΩ). The punch-through onset voltage VPTP onset is thendeﬁned to be the test voltage at which IPTP ¼ 10 μA. A well functioning PTP structure demands low VPTP onset, a steep current IPTP
increase above VPTP onset along with maximal current ﬂow below

Fig. 12. The leakage current (left) and the inter-strip resistance (middle) measured at different annealing time at 60 °C, inter-strip resistance vs. annealing time (right)
measured at 400 V and 700 V. ATLAS12A sensor irradiated by proton ﬂuence 2  1015 neq/cm2 (Karlsruhe) was studied. The arrows in the left and middle plot indicate thedata trends with the annealing time.

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5. Summary and conclusions

Fig. 13. Current Itest as a function of applied test voltage Vtest for different types of
PTP structures (A–F) implemented in ATLAS12M sensors. Sensors were irradiatedby protons to ﬂuence of 1  1015 neq/cm2and measured at bias voltage 1000 V andtemperature  20 °C.

100 V. The measured current Itest as a function of the test voltage
for various PTP structure types for proton irradiated sensors at
1  1015 neq/cm2 is shown in Fig. 13 while the PTP onset voltage
and current at 100 V are shown in Fig. 14.
The full gate structures C and B exhibit the steepest increase incurrent while keeping VPTP onset low. In addition the C type havethe modest increase of PTP onset voltage up to 2  1015 neq/cm2
and large current IPTP (@VPTP ¼ 100 V)¼ 2.5 mA even at 1  1016 neq/
cm2 (Figs.13 and 14). The novel full gate PTP structure C doubles
the allowable current without increasing the onset voltage.

Surface properties of ATLAS12 n þ -in-p strip sensors intended
for upgraded ATLAS ITk fabricated by Hamamatsu were evaluatedby several participating institutes to verify if they can cope withpredicted high radiation environment. Different types of barrel
and end-cap miniature sensors were tested. Sensors have highmicro-discharge breakdown before and after irradiation which iswell above the maximum operational voltage (500–600 V). The
results show that sensor stability is not inﬂuenced by PTP structure type, strip pitch, type of edge strip ganging or the wafer used.
The inter-strip capacitance of all types of sensors is less than
0.9 pF/cm which complies with speciﬁcations and does not changewith irradiation. The inter-strip resistance decreases with proton,
neutron and gamma irradiations. The primary factor is TID dose.
However, the inter-strip resistance is also related to the bulk
leakage current, that changes with ﬂuence, temperature, andannealing. The values of inter-strip resistance above 100 MΩ for2  1015 neq/cm2 for proton-irradiated samples have proven thatthe strip isolation is sufﬁcient for HL-LHC ﬂuences. The most suitable PTP structure is the novel PTP structure with an extended
electrode of the bias ring over the PTP gap. This full gate structure
implemented in the ATLAS12 main sensor, shows a steep current
increase above the low onset voltage and low effective resistanceat large voltages even at the ﬂuence of 2  1015 neq/cm2. Compared
to a standard structure without a gate, the new structure doubles
the allowable current ﬂowing into bias rail without increasing theonset voltage. Results from the evaluation of various sensor
properties are summarized in Table 1.

Fig. 14. PTP onset voltage (upper left) and test current at 100 V (upper right) for different types of PTP structures in sensors irradiated to 1  1015 neq/cm2. Fluence
dependence of VPTP (bottom left) and Itest (bottom right) of PTP structure C.

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Table 1
Summarized measurement values for various sensor properties.

Leakage current [μA/cm2] (at Vbias ¼600 V)
Inter-strip capacitance [pF/cm] (at 1 MHz)
Coupling Capacitance [pF/cm] (at 1 kHz)Inter-strip resistance [GΩ/cm] (atVbias ¼ 400 V)
Bias resistance [MΩ]

Pre-radiation

Protons 1  1015 [neq/cm2] Protons 2  1015 [neq/cm2]Gamma 100 kGyo 0.0048 ( þ20 °C)
0.75-0.82
24
15–65 (þ 20 °C)

51 (  10 °C)
0.82
24
0.5–2.2 (  20 °C)

103 (  10 °C)
0.82
24
0.3-0.7 (  20 °C)

1.5 7 0.1 ( þ 23 °C) 1.7-1.8 (  10,
 20 °C)

2.0 (  20 °C)

2.1 (  20 °C)

0.22 ( þ 20 °C)
0.77–
0.0086 ( þ 20 °C) 0.6–1.1(  20 °C)
1.7 ( þ23 °C)

The present surface studies have demonstrated that developedn þ -in-p strip sensors of the latest sensor design ATLAS12 show
appropriate performance for operation in ATLAS Upgrade ITk.

Science and Technology Facilities Council (under Grant ST/
M006409/1), and the United States Department of Energy, grant
DE-FG02-13ER41983.

Acknowledgments

References

The irradiations were performed: with protons at the University of Birmingham MC40 cyclotron, supported by the H2020
project AIDA-2020, GA number 654168, and the UK's Science andTechnology Facilities Council, at Cyclotron and Radioisotope Center(CYRIC), Tohoku 45 University, with Y. Sakemi, M. Ito, and T.Wakui,at the Karlsruhe Institute of Technology (KIT) by A.Dierlamm,
supported by the Initiative and Networking Fund of the Helmholtz
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