Design of a Segmented LGAD Sensor for theDevelopment of a 4-D Tracking Detector

University of Tsukuba
Tsukuba, Ibaraki 3058571, Japan
E-mail: wada@hep.px.tsukuba.ac.jp

Kyoji Onaru
University of Tsukuba
Tsukuba, Ibaraki 3058571, Japan

Kazuhiko Hara
University of Tsukuba
Tsukuba, Ibaraki 3058571, Japan

Yoshinobu Unno
High Energy Accelerator Research (KEK)
Tsukuba, Ibaraki 3050801, Japan

Koji Nakamura
High Energy Accelerator Research (KEK)
Tsukuba, Ibaraki 3050801, Japan
Tracking detectors capable of recording the hit-time information at a precision of O(10 ps) willbe breakthrough devices in the future of hadron collider experiments to resolve signal tracks in aharsh environment with significant background tracks. The low-gain avalanche detector (LGAD)
is an example of such a device, which is expected to provide real-time fine spatial resolution for
a 4-D tracking detector. In this study, we have fabricated LGAD diodes and segmented striptype detectors at Hamamatsu Photonics. We tested pad type detectors using a proton beam andobtained a time resolution better than 30 ps for the devices with an active thickness of 50 µm. The
non-uniformity of the gain across the strip was modeled using TCAD simulation, which was thenused to design segmented LGAD sensors with a uniform gain. The study suggests that uniform
LGAD sensors can be made with a broader operation bias voltage range. In this paper, we present
the test beam and the TCAD simulation results.

The 28th International Workshop on Vertex Detectors - Vertex2019
13-18 October, 2019
Lopud, Croatia∗ Speaker.

c Copyright owned by the author(s) under the terms of the Creative Commons
Attribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND 4.0).

https://pos.sissa.it/PoS(Vertex2019)057

Sayaka Wada∗

Sayaka Wada

Design of a Segmented LGAD Sensor

1. Introduction

Low-gain avalanche detectors (LGADs), [3, 4, 5] are a new type of sensors where a p+ implantlayer added underneath the readout n+ electrode provides an internal signal gain due to avalanche
multiplication in the region of a high electric field created across the p+ -n+ junction. The additional
gain can compensate for reduction in the charge collection. Furthermore, LGADs with a moderate
gain have shown excellent timing of around 30 ps or faster because a large signal is created during
the hole movements in the vicinity of the thin multiplication region. These results encourage us todevelop semiconductor detectors with fine spatial and time resolutions, namely 4D detectors.
We examined the LGAD samples fabricated by Hamamatsu Photonics and evaluated theirelectrical properties [6]. The samples are pad-type detectors with a sensitive area of 1 mm φand strip-type, as shown in Fig. 1. Both types have an active thickness of 50 and 80 µm, and four
different p+ doping profiles (A-D), where A-D refers to the doping concentrations in the increasingorder.
The timing characteristics were evaluated for the pad-type LGADs using a high-energy proton
beam at FNAL, FTBF in February 2018. The gain uniformity of the strip-type was measured [6]by injecting an IR laser, and the data were used to tune the parameters of a TCAD simulation [7].
The simulation was used to design a uniform gain LGAD sensor with segmentation by trenching[8].

Figure 1: Two types of the evaluated HPK LGAD sensors: pad type (chip size 2.5×2.5 mm2 , active area
1 mmφ ) and strip type (chip size 6×12 mm2 , strip pitch 80 µm)

1PoS(Vertex2019)057

Silicon semiconductor detectors have become essential in many high-energy experiments whereprecise spatial resolution and fast response are required. The n+ -in-p-type sensors, or p-bulk sensors, where fast electrons produce the main part of the signal, are widely used in experiments at
the LHC hadron collider as well as the forthcoming HL-LHC. Intensive studies have been carriedout to investigate the radiation hardness, and it was found [1, 2] that the charge collection is approximately reduced by half at a bias of 500 V for typical strip sensors irradiated at a fluence of
1×1015 1-MeV neq /cm2 . Such a degradation is tolerable for the HL-LHC; however, innovation isrequired for future experiments where the radiation level will become higher. This implies that theparticle density will also become high, which can be partially overcome by reducing the pixel size.

Sayaka Wada

Design of a Segmented LGAD Sensor

2. Evaluation of timing precision in high-energy beam
2.1 Beam test setup

2.2 LGAD signal

Events /0.5mVThe maximum voltages (absolute values) in correlation with the beam are plotted on a histogram in Fig. 2. A cluster of events with large pulses with the |voltage| > 40 mV is eveident
corresponding to the tracks traversing the gain region. Over the pedestal (below 5 mV), the cluster
below 40 mV is for the tracks traversing the peripheral region with no LGAD gain. For those eventswith |voltage| > 40 mV, the signal shapes of the peak are also shown in Fig. 2.
104103102
101
10−1−0.15

−0.1

−0.05

0Voltage [V]Figure 2: Pad-type LGAD response measured with the FNAL test beam; (left) distribution of the maximum
voltages in correlation with the beam, (right) signal shapes for the events with a maximum voltage greaterthan 40 mV.

2.3 Time resolution
2.3.1 Evaluation methods
The time resolution achievable in typical practical electronics was evaluated from the pulse
shapes given in Fig. 2. As the time walk deteriorates the time resolution, the time exceedinga single threshold is not precise enough. Here, we investigated the time resolution based on aconstant fraction method where the time is given at a voltage exceeding a certain fraction of thepeak voltage. The measured pulse shape was fitted to a quintic function. The peak pulse height
Vpeak was then calculated and the time at a voltage exceeding f ×Vpeak was derived, where f stands
for the constant fraction.
2PoS(Vertex2019)057

Three pad-type LGADs with thicknesses 50 and 80 µm were stacked along the 120-GeV
proton beam at Fermilab. The sensors were enclosed in a Styrofoam box to control the temperatureinside; the temperature was lowered by placing blocks of dry ice on the box. The signals wereamplified by fast discrete amplifiers [9] of 75-GHz bandwidth and a gain of 100, then digitizedusing fast FADCs, CAEN V1742 based on DRS4 technology [10]. The pulse shape was obtainedin 12 bits (1 Vpp) at 5-GHz sampling for a total duration of 200 ns. The event trigger was provided
by two sets of scintillation counters and an ATLAS FE-I4 based pixel module [11] which restricts
the beam area by setting the region of interest.

Sayaka Wada

Design of a Segmented LGAD Sensor

The same method was applied to the three pad-type LGADs in the beam. As the two sensors
named 1 and 2 had a similar active thickness of 50 µm, the event-by-event time difference T1 − T2shows a Gaussian distribution with a standard deviation given byq√
σ (T1 − T2 ) = σ12 + σ22 ∼ 2σ ,2.3.2 Results of the time resolution

Time Resolution [ps]Figure 3 shows plots of the evaluated time resolutions as a function of the fraction f . Theresults are given for 50 and 80 µm active thicknesses and at different temperatures. Generally, thetime resolution reaches a minimum for the fraction f > 0.3. The resolution is better for the LGAD
with an active thickness of 50 µm than that with 80 µm. The resolution improved and the gainincreased by reducing the temperature.
6080µm:500V:22°C
50µm:340V:22°C

55
50
45
40
3530
25
20

00.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

1fraction fFigure 3: Evaluated intrinsic time resolution of the pad-type LGAD sensors: (left) 50 and 80 µm activethicknesses, (right) 50 µm active thickness measured at 22 and 8◦ C. The horizontal is the ratio of the threshold voltage to the peak voltage (see text).

3. TCAD simulation for segmented LGADs
3.1 Validation of TCAD parameters from comparison with measured characteristics
TCAD is a device simulator commonly used in semiconductor design, and we developed asimulation to represent the LGAD characteristics. The parameters, such as depth and lateral doping
profiles, are essential in the adoption of TCAD. After iterative trials by introducing best guesses,
the TCAD can reproduce the general trend as a function of the bias of the leakage current (I-V)
and the bulk capacitance (C-V) of a pad-type LGAD, as shown in Fig. 4 and Fig. 5. The precise
reproduction of the measurements is not complete because of the simplified lateral profiles, but
the breakdown voltages are well described. Moreover, the simulation can reproduce the response
profile in the strip-type detector, as shown in Fig. 6. Here the response across the strip was measured
by injecting a pulsed IR laser, which was collimated to 2×2 µm2 (some missing points are due to3PoS(Vertex2019)057

where σ is the intrinsic time resolution of the LGAD #1 and #2. The time resolution for LGAD
#3 with an active thickness of 80 µm was similarly derived from the T3 − T2 distribution and thepreviously measured σ2 value.

Sayaka Wada

Design of a Segmented LGAD Sensor

the Al electrode). From the overall results presented here, we were encouraged to extend the TCAD
to simulate the response of the LGAD that was segmented by trenches to evaluate the response
uniformity and the breakdown characteristics.

GainFigure 5: C-V curves of the pad-type LGAD sensors (80 µm active thickness and four different p+ implant
concentrations): (left) measurement, (right) TCAD simulation. The simulation includes the estimation forthe PIN diode (no amplification) in black.
109876543210−40

Strip Sensor

50D

250V

Non-irradiated.

200V
100V

−30−20−100102030

40Position [ µ m]

Figure 6: Gain distributions across the strip at different bias voltages; (left) measurement by injecting
collimated IR laser, (right) TCAD simulation.

4. TRENCH structure as a candidate for uniform gain
The readout n+ implant and the p+ implant underneath have a uniform lateral doping, and thereadout electrodes are separated physically by deep trenches [8]. Such an LGAD was simulated
4PoS(Vertex2019)057

Figure 4: I-V curves of the pad-type LGAD sensors (80 µm active thickness and four different p+ implant
concentrations): (left) measurement, (right) TCAD simulation. The simulation includes the estimation forthe PIN diode (no amplification) in black.

Sayaka Wada

Design of a Segmented LGAD Sensor

with the TCAD. Figure 7 illustrates the impact ionization profile, namely the multiplication profile,across the trenches located at 40 and 120 µm. The largest multiplication is located (shown in red)under the readout electrode (in blue) and it extends to the vicinity of the trenches. The trench widthis 3 µm and depth is 5 µm. The electric field is uniform with a vertical direction. The field aroundthe trench is significant, and this point initiates the breakdown.
The breakdown and response uniformity are shown in Fig. 7. The breakdown characteristics
are compared with the strip-type LGAD and the behavior of the trench is milder than that of thestrip type. The response is uniform to the vicinity of the trenches.

Figure 8: (left) I-V curves for the LGADs segmented by the trench (solid) and the strip (dotted) shownfor three p+ concentrations. (right) The gain uniformity at different bias voltages. The trench is located at±40 µm.

5. Conclusions
We are developing segmented LGAD sensors for a realization of a 4D tracker. Pad-type
LGADs with 50 µm active thickness showed a time resolution of 30 ps. A TCAD simulation wasdeveloped to reproduce the measured LGAD characteristics, which was used to design a segmented
LGAD with uniform gain. The trench separation was suitable to achieve effective segmentation.

Acknowledgements
This work was supported in part by JSPS KAKENHI Grant Number 19H04393 and 19H05193,
5

PoS(Vertex2019)057

Figure 7: Impact ionization distribution shown in a cross section of the trenched LGAD (TCAD simulation).
Enlarged is the detail around the trench.

Sayaka Wada

Design of a Segmented LGAD Sensorand U.S.-Japan Science and Technology Cooperation Program in High Energy Physics (FY20182019). The discussions with K. Yamamura and S. Kamata of Hamamatsu Photonics were very
inspiring. We appreciate the support provided by the FTBF operation group, led by M. Rominsky.
Cooperation with A. Apresyan of FNAL has been very fruitful, and his support was vital in carryingout the test beam measurements.

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