Available online at www.sciencedirect.com

Physics Procedia 37 (2012) 838 – 843

TIPP 2011 – Technology and Instrumentation in Particle Physics 2011

Design of Punch-Through Protection of Silicon Microstrip
Detector against Accelerator Beam Splash
K. Haraa*, N. Hamasakia, Y. Takahashia, S. Mitsuib Y. Ikegamic, Y. Takuboc,
S. Teradac, Y. Unnoc
aIPAS, University of Tsukuba, Tennodai 1-1-1, Tsukuba, 305-8571 Ibaraki, Japan
Graduate University for Advanced Studies (Soken-Dai), Oho 1-1, Tsukuba, 305-0801 Ibaraki, JapancINPS, High Energy Accelerator Research Org.(KEK), Oho 1-1, Tsukuba, 305-0801 Ibaraki, Japanb

Abstract
In experiments at high intensity accelerators such as the LHC and its planned upgradeHL-LHC, the silicon
microstrip detector should be designed to survive against possible beam loss that may create large amount of signal
charges in short period. We are designing a built-in protection structure based on a punch through mechanism for thep-bulk n+-readout sensors under development for the HL-LHC. The performance is evaluated by applying DCvoltages and also by using a pulsed infrared laser to generate a large amount of current. The latter test provides thetime information of the protection system.
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by Elsevier
B.V.
Selection
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under responsibilityof thecommitteeorganizingforcommitteefor© 20122011 PublishedElsevier BV.
Selection
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responsibilityof the organizing
TIPP 2011.
TIPP 11.
Keywords: microstrip sensor; punch-through; strip protection; beam loss;

1. Introduction
The Large-Hadron Collider (LHC) under operation at CERN will collide the proton bunches (1011protons/bunch) at 40 MHz at its design. Such high intensity beams are the key to explorer the nature ofparticle physics to depth. At an incident of beam loss happening in the detector area, a beam aborting
system will be triggered but the activation takes typically 1 ms. Therefore several times 1012 protons could
be splashed over the detector area in the worst case.
The ATLAS SCT employs the n-bulk and AC-coupled p-readout microstrip sensors (p+-in-n sensors).
A protection mechanism against such beam splashes is implemented on this senor by extending the p+-*Email: hara@hep.px.tsukuba.ac.jp

1875-3892 © 2012 Published by Elsevier B.V. Selection and/or peer review under responsibility of the organizing committee for TIPP 11.
doi:10.1016/j.phpro.2012.02.427

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implant ends close as much as 5 Pm [1][2] to the bias-ring to activate the punch-through (PT). A study [3]points out that even under such protection the strip could be damaged by an extremely intense beamsplash due mainly to the finite implant resistance. Fig. 1(a) shows a hole was formed on an SCT sensor
operated at 500 V when a charge equivalent to 108 minimum ionizing particles (mips) was injected in anarea 10 Pm square aside from the electrode. This sensor for example went un-operational for failure ofsustaining the bias voltage. Other effects were also observed such as several strips at neighbours or wholechip started to show substantial increase in noise. The studied case should be regarded as quite rare at theLHC since the protons are expected to splash in wider area, but the study suggests that an efficient
protection design is mandatory to minimize the risk of damage.
A new type of microstrip sensors is designed utilizing p-bulk wafers [4] for the planned High
Luminosity LHC. As schematically explained in Fig. 1(b), protection of the insulator from breaking isachieved by low impedance between the implant and the bias-ring, which is realized by a punch-throughmechanism, or PTP [5]. One difficulty in this design is the existence of the p-stop. The p-stop, which isrequired for p-bulk sensors for preventing mobile electrons accumulated at the surface to degrade the strip
isolation, acts as a punch-through blocker at the same time. We examined the PTP for various
configurations of the strip ends to investigate the most suitable design.
(a)(b)

Fig. 1. (a) A hole was created on a present SCT sensor (the strip width is 22 Pm) by a laser pulse equivalent to 108 mips. The laser
was spotted in the area shown in the square. The sensor bias was 500 V; (b) Schematic drawing of the strip end for the p-bulk sensorin design. The circuit including Vtest and an ampere-meter is used for the DC test (see text). The current generated from the beamloss is to be efficiently drained towards the bias-ring such that the implant voltage should not exceed the rating of the insulator.

2. Tested PTP Structures
In order to protect the sensor effectively, it is preferable that the punch-through occurs at low enoughvoltages and the impedance of the channel be low. Negative voltages are concerned on the implant strip
ends against the bias-ring, since the signal current is negative in the n+-in-p sensors. Various design issues
exist for efficient PTP such as (1) the distance between the electrodes, (2) the shapes of the electrode ends,(3) the p-stop density, and (4) the gate structure above the PT region.
In the previous studies [4,5] we have discussed (2) and (3). A clear dependence on the p-stop density isobserved, as explained by the p-stop functioning as a PT blocker. On the other hand, no obvious results
were obtained concerning (2) for various end shapes. In fact the investigated samples were not wellcontrolled in view of (4) the gate structure. Therefore, in this paper we systematically changed (1) thedistance and (4) the gate coverage of the PT region by extending the aluminum of the bias-ring, as shownin Fig. 2. The structure Z4D3 is identical to Z4D previously studied [4,5]. The samples Z4D3, Z4D4 andZ4D5 are to be compared for the gate coverage. Z4D4 and Z4D2 are for the distance between theelectrodes. For investigation of the p-stop as PT blocker, Z4D1 was added where the p-stop was excluded.

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Fig. 2. Examined PTP structures (Z4D2 through D5). The “common” p-stop is 5 Pm wide and floating. The overhang of thealuminum, which is DC connected to the bias-ring, is varied from 3 Pm (D3), 12 Pm (D4) to 30 Pm (D5) to control the coverage ofthe PT region. Z4D2 has a distance of 5 Pm between the implant end to the p-stop and between the p-stop to the bias-ring to becompared to 7 Pm of Z4D4. Z4D1 is a comparison structure eliminating the p-stop.

3. Evaluation Procedures and Results
3.1. DC method
Fig. 1(b) shows the schematic diagram of the DC evaluation method of the PTP. The resistance acrossthe DC pad and the bias-ring is measured by applying a test voltage Vtest on the DC pad. The DC pad islocated 0.5 mm from the implant strip end.
Typical resistance curves are plotted in Fig. 3(a). Since the channel is composed of n+(Vtest)-p-n+(0V),
the PT occurs at the implant strip end for positive and at the bias-ring for negative Vtest. We define the PTvoltage VPT as Vtest when the effective resistance is one half of the bias resistance of about 1.5 M:. Since
the curves are not identical in shape, we also define 90% and 10% points of the bias resistance, V-PT andV+PT, respectively.
The results obtained for the test structures Z4D1 through Z4D5 are summarized in Fig. 3 where thesamples are both non- and proton irradiated up to 1015 neq/cm2[6]. Among these Z4D3 is the worst,
showing larger VPT and V+PT, while Z4D5 is the best, showing notably small V+PT. This observation(a)Fig. 3. (a) Examples of measured resistances as a function of Vtest; (b) Summary of the DC results for the samples non- and proton
irradiated up to 1015 neq/cm2. The vertical bars represent the range of V-PT and V+PT defined in (a).

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concludes the effectiveness of the gate structure that covers fully up to the strip ends where the PT occurs.
The similarity between Z4D1 and Z4D5 especially at larger doses implies the effect of p-stop as a PT
blocker is weakened by the gate. The sample Z4D2 having a 5 Pm electrode distance is almost inagreement with Z4D4 having a 7Pm.
3.2. AC method
The DC method provides us estimates on the resistance values of the channel, while not on the timestructure. We injected infrared laser pulses to simulate passage of charged particles and triggered the PTto extract the timing information.
We employed a 1064 nm Nd:YAG laser pulses with a 10 ns base width [7]. The laser intensity wastuned by adjusting the angle of a polarizer. The laser intensity expressed in terms of number of mips wascalculated from the average current when the laser was pulsed at 1 kHz, assuming that a 0.32 mm thicksilicon sensor produces 3.6 fC per mip. The intensity calibration is shown in Fig. 4(a). A total current of40 PA corresponds to 1.1 u 106 mips/pulse.
The layout of the strips is sketched in Fig. 4(b). The PTP structures were implemented on one side atright. The laser focused to 10 Pm square was spotted at the pre-defined locations, NEAR or FAR. Thestrip signals were read out from the DC pads at either ends with an oscilloscope via passive probes
TEK6139B. While injecting the laser nearly at the middle of the strips, the two strip signals nearest to theinjection point and one at a neighbor were read out in order to investigate the effects to neighboring strips.
Characteristic signal shapes were obtained depending on the laser injection point and readout side
configurations. The two extreme cases taken for 1.1 u 106 mips/pulse are shown in Figs. 5(a) and (b) forNEAR(injection)-NEAR(readout) and FAR-FAR configurations. The quick rises (<20ns) are immediately
dumped by the PTP for NEAR-NEAR, Fig. 5(a), while it takes a while for FAR-FAR, Fig. 5(b), due tothe finite implant strip resistance of about 20 k:. In both cases, the signals rise again when the impedancechanges after losing PT.
The voltages while the PT is on are plotted in Fig. 6(a). All except in FAR-FAR configuration, thestable voltage after the first peak representing the finite resistance across the PT times the current being
swept away. For FAR-FAR, where the voltage gradually decreases, the maximum values are plotted.
Note that below 10 PA there was no constant part representing the PT being on. Fig. 6 (b) shows the time
duration while the PT is on. The duration increases linearly with the laser intensity, providing a time scaleof about 2 Ps for 1.1 u 106 mips/pulse.
The observed signal shapes are dependent on the impedances among the sensor electrodes and theFig. 4 (a) Total detector current as a function of the polarizer angle. The curves are shown for five different samples; (b) Diagram showing
the dimensions (in Pm) of test structures. The PTP structures 4D1 to Z4D5 are implemented at the right side end of the implant strip. Thelaser was spotted either at pre-defined NEAR or FAR positions.

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readout probe, while the time information in the real case where the readout amplifier [8] is connected isconcerned. The impedance of the probe is expressed as Z[M:@~20/f with frequency f [kHz]. Taking theamplifier impedance [8] into account, the impedance of the measurement system is about twice as that of
the real case if the PT is not set. This should slow the current flow, providing almost similar voltage peaksin the two cases. The voltage is present entirely across the insulator for slower pulses (f<10MHz) orentirely across the amplifier inputs for f>100MHz. Once the PT is set, however, the impedances coincidewith each other, hence the voltage and time information is presumably reproduced in this measurement.
The fast rise that appears right at laser injection is responsible for the amplifier break while theconstant or slowly decaying voltages, which are summarized in Fig. 6(a), and the bump that appears after
the PT is deactivated are responsible for the insulator break. As far as for the sensor, the voltages at FARFAR configuration are largest and are to be concerned. The probe response is linear up to 300 Vrms forf<3MHz and up to 50 Vrms for f>20MHz. This limitation for faster signals underestimates the fast signals
right after the laser injection, responsible for the amplifier break.

 Fig. 5 Two sets of oscilloscope traces for (a) NEAR-NEAR and (b) FAR-FAR configurations for laser injection and readout
positions. CH1 and CH2 are of the nearest strips to the laser injection point, CH3 is of the next neighbor. CH4 is for the laser trigger
signal. Horizontal: 1 Ps/div, Vertical: 20 V/div for CH1-CH3. The laser intensity was 106 mips/pulse with the detector bias of 200 V.
The low and constant part in (a) and slowly decaying part in (b) are due to PT, the difference being described by the impedance dueto the implant length. The latest bumps appear when the PT is deactivated, changing the impedance accordingly.

Fig. 6 (a) The voltage and (b) time while the PT is on, as a function of the detector current. The curves are for four laser injection
and readout position configurations. 40 PA corresponds to 106 mips/pulse. The PTP structure is Z4A described in [1][2].

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The voltages measured in FAR-FAR configuration are plotted in Fig. 7 for Z4D1 to Z4D5 PTP
configurations in comparison with another configuration Z3 [1] where no PTP design is applied keeping awider distance of 30 Pm between the implants. The similarity of Z3 and Z4D3 (smallest gate coverage)
curves suggests that the gate is more important than the separation of the electrodes for the PT to occur.
The effectiveness of p-stop as the PT blocker is seen from the Z4D1 data when the gate coverage isinsufficient. The Z4D5 (PT region covered entirely) is the best among the designs.
We measured also for the samples irradiated to 1015 n/cm2. Since the collected charge is almost halved
for the irradiated samples due to degraded charge collection, the apparent effects are smaller with respect
to the laser intensity. If the curves are compared in terms of the induced current, small radiation-induced
degradation was observed.

Fig. 7 Maximum voltages in FAR-FAR configuration, (a) compared among various PTP structures, (b) compared among irradiatedand non irradiated Z4D5 samples. The detector was biased at 200 V.

Conclusions
The performance of various PTP designs was investigated both by DC and AC methods. We showedthe effectiveness of the gate structure that suppresses the p-stop to function as a PT blocker. The finite
implant resistance is most concerned, which creates a voltage difference across the strip insulator
exceeding 100 V with a duration of a few Ps when 1 u 106 mips/pulse is injected 8 mm from the PTPstructure. The durability of the insulator against the pulses with described time structures needs to beinvestigated.
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