# Upgrade of the Belle II Vertex Detector with Depleted Monolithic Active Pixel Sensors

D. Auguste, M. Babeluk, M. Barbero, P. Barrillon, J. Baudot, T. Bergauer, F.

Bernlochner, G. Bertolone, C. Bespin, S. Bettarini, et al.

► To cite this version:

D. Auguste, M. Babeluk, M. Barbero, P. Barrillon, J. Baudot, et al., Upgrade of the Belle II Vertex Detector with Depleted Monolithic Active Pixel Sensors. 11th International Workshop on Semiconductor Pixel Detectors for Particles and Imaging, Nov 2024, Strasbourg, France. pp.C10013, 10.1088/1748-0221/20/10/C10013. [hal-05324475](https://hal.science/hal-05324475v1)

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Distributed under a Creative Commons Attribution 4.0 International License
11TH INTERNATIONAL WORKSHOP
ON SEMICONDUCTOR PIXEL DETECTORS FOR PARTICLES AND IMAGING
STRASBOURG, FRANCE
18–22 NOVEMBER 2024

Upgrade of the Belle II Vertex Detector with Depleted Monolithic Active Pixel Sensors

D. Auguste,⁸ M. Babeluk,ª M. Barbero,ᵇ P. Barrillon,ᵇ J. Baudot,ᵇ T. Bergauer,ª

F. Bernlochner,ᵢ G. Bertolone,ᵇ C. Bespin,ᵢ S. Bettarini,ᵈ,ᵉ A. Bevan,q M. Bona,q J. Bonis,⁸

F. Bosi,ᵉ R. Boudagga,ᵇ P. Breugnon,ᵇ Y. Buch,ᵏ G. Casarosa,ᵈ,ᵉ L. Corona,ᵉ J. Dingfelder,ᵢ

A. Dorokhov,ᵇ G. Dujany,ᵇ L. Federici,ᵇ A. Fernandez Prieto,ₛ C. Finck,ᵇ F. Forti,ᵈ,ᵉ

D. Fougeron,ᵇ A. Frey,ᵏ A. Gabrielli,ᵇ,ᵉ,ᵉ L. Gaioni,ᵇ A. Gallas Torreira,ₛ X. Gao,ᵈ

S. Giroletti,ᶜ J. Gong,ᵈ K. Hara,¹,>m T. Higuchi,ⁿ A. Himmi,ᵇ D. Howgill,q C. Hu-Guo,ʰ

C. Irmler,ª D. Jeans,¹,>m A.B. Kaliyar,ª M. Karagounis,ⱼ T. Kishishita,ⱼ H. Krüger,ᵢ A. Kumar,ʰ

C. Lacasta,ₚ C. Marinas,ₚ M. Massa,ᵉ L. Massaccesi,ᵈ,ᵉ M. Maushart,ʰ J. Mazorra de Cos,ₚ

M. Minuti,ᵉ A. Moggi,ᵉ L. Molina-Bueno,ₚ S. Mondal,ᵈ,ᵉ F. Morel,ʰ K.R. Nakamura,¹,>m

Y. Okazaki,¹,>m Y. Onuki,'o P. Pangaud,ᵇ Y. Peinaud,⁸ H. Pham,ʰ B. Pilsl,ª L. Ratti,ᶜ V. Re,ᵇ

E. Riceputi,ᵇ I. Ripp-Baudot,ʰ G. Rizzo,ᵈ,ᵉ L. Schall,ᵢ C. Schwanda,ª B. Schwenker,ᵏ

M. Schwickardi,ᵏ R. Sefri,ʰ J. Serrano,ᵇ W. Song,ᵈ P. Stavroulakis,ʰ G. Traversi,ᵇ I. Valin,ʰ

V. Vobbilisetti,ₚ M. Vogt,ᵢ S. Wang,'o M. Winterg and D. Xu²

ªInstitute of High Energy Physics, Austrian Academy of Sciences, Nikolsdorfer Gasse 18, 1050 Vienna, Austria

ᵇDepartment of Engineering and Applied Sciences, University of Bergamo,
Viale Marconi 5, I-24044 Dalmine (BG), Italy

ᶜDepartment of Electrical, Computer and Biomedical Engineering, University of Pavia,
Via Ferrata 5, I-27100 Pavia, Italy

ᵈDipartimento di Fisica “E. Fermi”, Università di Pisa, L.go B. Pontecorvo 3, I-56127 Pisa, Italy

ᵉINFN Sezione di Pisa, L.go B. Pontecorvo 3, I-56127 Pisa, Italy

fAix Marseille Univ, CNRS/IN2P3, CPPM, Marseille, France

gLaboratoire de Physique des 2 infinis Irène Joliot-Curie - IJCLab, Université Paris-Saclay, CNRS/IN2P3,
IJCLab, 91405 Orsay, France

hUniversité de Strasbourg, CNRS, IPHC UMR 7178, F-67000 Strasbourg, France

iPhysikalisches Institut, Rheinische Friedrich-Wilhelms-Universität Bonn,
Nussallee 12, 53115 Bonn, Germany

jUniversity of Applied Sciences and Arts Dortmund, Sonnenstraße 96-100, 44139 Dortmund, Germany

2025 JINST 20 C10013

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<footnote>*Corresponding author.</footnote>
kII. Physikalisches Institut, Georg-August-Universität Göttingen,
Friedrich-Hund-Platz 1, 37077 Göttingen, Germany

¹High Energy Accelerator Research Organization (KEK), Tsukuba 305-0801, Japan

mThe Graduate University for Advanced Studies (SOKENDAI), Hayama 240-0193, Japan

nKavli Institute for the Physics and Mathematics of the Universe (WPI), University of Tokyo,
Kashiwa-no-ha 5-1-5, Kashiwa 277-8583, Japan

oDepartment of Physics, University of Tokyo, Hongo 7-3-1, Tokyo 113-0033, Japan

pInstituto de Física Corpuscular (IFIC), CSIC-UV, Catedrático José Beltran, 2. E-46980 Paterna, Spain

qSchool of Physical and Chemical Sciences, Department of Physics and Astronomy,
Queen Mary University of London, 327 Mile End Road, London, E1 4NS, U.K.

rCollege of Physics, Jilin University, 2699 Qianjin Street, Changchun, Jilin, China

sUniversidade de Santiago de Compostela, 2010 Instituto Galego de Física de Altas Enerxías (IGFAE),
Colexio de San Xerome, PZ Obradoiro, S/N. E-15782 Santiago de Compostela, Spain

E-mail: alice.gabrielli@pi.infn.it

ABSTRACT: The Belle II experiment, operating at the SuperKEKB $e^+e^-$ collider, is developing a new pixelated vertex detector (VTX) to handle an higher luminosity, up to $6 \times 10^{35} \text{ cm}^{-2} \text{ s}^{-1}$. The VTX will feature 5–6 layers, with a material budget under 3% of $X_0$, and will utilize depleted monolithic active pixel sensors. The chip developed for the upgrade of the vertex detector, OBELIX, is based on the TJ-Monopix2 sensor which uses Tower Semiconductor 180 nm CMOS process. OBELIX sensor offers a 33 µm pitch, 50 ns time-stamping, and a digital trigger logic that matches Belle II’s 30 kHz trigger rate. The sensor is designed to withstand radiation levels up to $5 \times 10^{14} \text{ neq/cm}^2$ and 1 MGy while maintaining low power dissipation (200 mW/cm$^2$). This paper outlines the motivation for the upgrade and the VTX concept, and presents characterisation results and beam test campaigns on the TJ-Monopix2 sensor, including both non-irradiated and irradiated samples, which are crucial for validating key performance characteristics for the OBELIX design.

KEYWORDS: Particle tracking detectors (Solid-state detectors); Radiation-hard detectors; Solid state detectors
# Contents

<table><tr><td>1 Introduction</td><td>1</td></tr><tr><td>2 Upgrade motivations and VTX requirements</td><td>2</td></tr><tr><td>3 VTX proposal</td><td>2</td></tr><tr><td>4 TJ-Monopix2 as a forerunner for OBELIX</td><td>3</td></tr><tr><td>5 The OBELIX chip for Belle II</td><td>4</td></tr><tr><td>6 Conclusions</td><td>6</td></tr></table>

## 1 Introduction

The Belle II experiment [1] is a B-factory dedicated to the search for physics beyond the Standard Model at the intensity frontier, aiming at high-precision measurements of the properties of hadrons containing heavy quarks and the tau leptons. It operates at the SuperKEKB collider [2] in Japan, which provides asymmetric collisions with 7 GeV electrons and 4 GeV positrons at the $\Upsilon(4S)$ resonance. SuperKEKB employs a nano-beam scheme and very high beam currents to achieve very high luminosity, aiming to collect an integrated luminosity of $50 \text{ ab}^{-1}$ with a target peak luminosity of $6 \times 10^{35} \text{ cm}^{-2} \text{ s}^{-1}$. These operational conditions impose significant challenges on the detector system, particularly on the innermost vertex detector (VXD).

The current VXD consists of six layers of silicon sensors for precise track reconstruction near the interaction point. It has two inner layers of DEPFET pixel sensors (PXD) [3], which provide high spatial resolution, with 50–70 µm fine pitch, but long integration time of 20 µs, and four outer layers of double-sided silicon strip sensors (SVD) [4], offering excellent time resolution of 3 ns and extended spatial coverage. This configuration achieves a spatial resolution of 10–25 µm and provides polar angle coverage from 17° to 150°, with a material budget per layer ranging from 0.25% to 0.75% of a radiation length, and a total material budget of 3.5% $X_0$. While the VXD has delivered excellent performance under low background conditions, its ability to operate “under the high background conditions” projected for the target luminosity remains limited. The expected increase in machine-induced backgrounds, coupled with uncertainties in their extrapolation due to possible changes in the interaction region (IR), could lead to performance degradation due to high occupancy. To address these challenges and improve tracking and vertexing performance, a new pixel vertex detector (VTX) [5] is proposed. Based on CMOS monolithic active pixel sensors (MAPS), the VTX will provide improved robustness to harsh background conditions, enhanced tracking efficiency and higher spatial resolution (cf. section 2). In addition, it will be designed to adapt to potential IR redesign, ensuring that Belle II will continue to provide high precision measurements for years to come.
## 2 Upgrade motivations and VTX requirements

The upgrade of the vertex detector is driven by uncertainties in background extrapolation at the target luminosity, in particular with a potential upgrade of the interaction region [5]. The current detector operates with limited safety margins, with projected occupancies of up to 2% in PXD layer 1 and 9% in SVD layer 3 at higher background scenario, approaching operational limits. To maintain performance, the new VTX will require increased granularity and improved time resolution.

The key requirements for the detector are:

* **Spatial Resolution:** better than 15 µm, with pitches of 30–40 µm.

* **Low Material Budget:** 0.2%–0.8% $X/X_0$ per layer for the inner and outer layers respectively.

* **Hit Rate Capability:** up to 120 MHz/cm² with triggered read-out.

* **Fast Timestamping:** 50–100 ns.

* **Radiation Tolerance:** TID up to 1 MGy and NIEL fluence of $5 \times 10^{14} n_{eq}/cm^2$ in the innermost layer.

* **Power Dissipation:** at or below 200 mW/cm² to minimize the material budget and cooling services.

The technology that meets these specifications is the Depleted Monolithic Active Pixel Sensors (DMAPS), specifically the TJ-Monopix2 sensor [6], manufactured using the Tower Semiconductor 180 nm CMOS process and originally developed for the outer layers of the ATLAS ITk [7].

## 3 VTX proposal

The proposed VTX consists of five straight detection layers, all based on the Optimised BELle II pIXel (OBELIX) sensor, derived from the TJ-Monopix2 sensor. The VTX design prioritises higher space and time granularity compared to the current VXD while minimising the material budget. The integration of these advanced DMAPS layers is expected to significantly improve tracking efficiency, particularly for low-momentum particles and in high hit-rate scenarios, as described in [5]. A 5-layers layout for VTX is shown in figure 1.

It comprises an inner section (iVTX) and an outer section (oVTX). The iVTX includes two layers positioned at radii of 14 mm and 22 mm. These layers feature an “all-silicon ladder” design with a material budget of 0.2% $X/X_0$ per layer. The design uses a block of four OBELIX chips from the same wafer, interconnected through a post-process redistribution layer (RDL) and selectively thinned to approximately 50 µm, leaving a 400 µm border for stiffness. Air cooling is being evaluated as a method to dissipate an average power between 200 and 300 mW/cm², depending on the hit rate. This may be further enhanced by the addition of thin cooling pipes at the edges of the ladder. The oVTX extends to radii of up to 140 mm and comprises three to four additional layers. These layers are built on a carbon fiber support structure, inspired by ALICE ITS2 [8] design, featuring an omega-shaped design that sandwiches Rohacell foam between two carbon fiber sheets. The structure incorporates a cold plate with liquid cooling pipes, sensors glued directly to the plate, and flex circuits for interconnections. This configuration achieves a material budget of up to 0.8% $X/X_0$ per layer, while the omega-shaped design enhances mechanical stability and supports the integration of up to four layers in the oVTX region.
<figure><img src="image_3.png" /><figcaption>**Figure 1.** Schematic view of the VTX baseline layout with five detection layers.</figcaption></figure>

## 4 TJ-Monopix2 as a forerunner for OBELIX

Manufactured using Tower Semiconductor 180 nm CMOS technology with modifications to improve radiation hardness [6], the TJ-Monopix2 sensor serves as forerunner to the OBELIX chip. Figure 2 shows the cross-section of the DMAPS in pixel of both sensors, with the small collecting electrode DMAPS pixel structure.

<figure><img src="image_4.png" /><figcaption>**Figure 2.** Cross section of the DMAPS pixel in the TowerJazz 180 nm with modified process for radiation hardness.</figcaption></figure>

TJ-Monopix2 features a large 512 × 512 pixel matrix with a 33 × 33 µm pitch, 25 ns timestamping, 7-bit Time-over-Threshold (ToT) analog information. Four distinct front-end versions are implemented, including both DC- and AC-coupled designs, with two of them employing a cascode transistor in the charge-sensitive amplifier stage. The chip has a column-drain readout architecture which can handle hit rates higher than 100 MHz/cm² [6]. However, while the TJ-Monopix2 operates with a triggerless readout and lacks memory in the periphery, OBELIX integrates a new digital periphery specifically designed to meet the requirements of Belle II. Extensive laboratory tests have been conducted on TJ-Monopix2 to assess the key parameters crucial for OBELIX design validation. These tests demonstrated stable operation of the TJ-Monopix2 at thresholds around 250 e⁻, with a threshold dispersion of 17 e⁻ and a noise level of approximately 8 e⁻ [9, 10]. Accurate comparisons between measurements and simulations are essential for refining the OBELIX design. To validate the
performance of TJ-Monopix2 under realistic conditions, several test beam campaigns with 3–5 GeV electrons were performed at DESY. A first beam test with non-irradiated sensors showed a hit efficiency of over 99% and a position resolution of 9 µm at normal incidence. Subsequent campaigns included irradiated samples, which also showed good performance. During the 2023 campaign, sensors irradiated with 24 MeV protons up to $5 \times 10^{14} \text{ n}_{\text{eq}}/\text{cm}^2$ showed a position resolution comparable to that of non-irradiated sensors. High efficiencies of 98–99.9% were achieved at all front-ends by increasing the sensor bias to mitigate radiation damage. A small decrease in efficiency was observed at the pixel corners away from the collector, as expected from the cell layout, as shown in figure 3.

Additional studies conducted in July 2024 on irradiated samples (TID up to 1 MGy and NIEL up to $5 \times 10^{14} \text{ n}_{\text{eq}}/\text{cm}^2$) showed that both DC and AC coupled front-ends achieved efficiencies of 99.9% at TID levels. However, at high NIEL fluences, performance degraded slightly as a result of increased temperature and leakage current. The DC-coupled cascode FE maintained efficiency above 99.5% with approximately 3% of pixels masked, while the AC-coupled cascode FE reached 98.5%. After irradiation, when chip temperatures exceeded 40°C, threshold and noise levels increased with an increase in leakage current, leading to a drop in efficiency. Temperature dependence is therefore a critical factor in sensor performance, particularly in high background conditions. Various cooling solutions for the iVTX are being evaluated, as described in section 3, to manage OBELIX’s power consumption mitigate temperature-induced degradation and ensure optimal performance at high hit rates.

<figure><img src="image_5.png" /><figcaption>**Figure 3.** Layout of the 2 × 2 superpixel cell of TJ-Monopix2 (left). Reproduced with permission from [6]. In pixel efficiency on irradiated sample with 24 MeV protons up to $5 \times 10^{14} \text{ n}_{\text{eq}}/\text{cm}^2$ (right), for the DC front-end, with a reverse bias of 20 V. Reproduced from [9]. CC BY 4.0.</figcaption></figure>

An additional beam test campaign is scheduled for 2025 to evaluate the sensor's performance across different operating temperatures and NIEL fluences, providing further insight for the optimisation of OBELIX.

# 5 The OBELIX chip for Belle II

Designed to meet the requirements of the upgraded VTX, the OBELIX chip [11] is a DMAPS based on the TJ-Monopix2 architecture. It features an extended matrix of 464 rows and 896 columns, including time stamping (47 ns), 7-bit time-over-threshold (ToT) resolution, and 3-bit in-pixel threshold tuning
to handle threshold variations across the matrix. The floorplan and dimensions of the OBELIX chip are shown in figure 4. To support the increased area and prevent voltage drops, the design includes low dropout regulators (LDOs) and power stabilisation features.

<figure><img src="image_6.png" /><figcaption>**Figure 4.** Floorplan and dimensions of the OBELIX chip. The values reported in the table are simulation results.</figcaption></figure>

With respect to TJ-Monopix2, OBELIX incorporates a new digital periphery specifically designed to meet the requirements of Belle II. A key innovation is its triggered readout architecture, realised through a Trigger Readout Unit (TRU), which effectively handles hit rates on average up to 120 MHz/cm², with peaks reaching 600 MHz/cm², and supports trigger rates up to 30 kHz with latencies of 10 µs. This architecture ensures minimal data loss (less than 0.02%, as indicated by simulations) while maintaining a data throughput of 320 Mbps. Two new features have been added in the digital periphery. The Track Trigger Transmission (TTT) module provides coarse but fast hit information to the Belle II trigger system, enabling VTX-based trigger decisions through a fast track recognition algorithm implemented in an FPGA that compares detector data with a predefined pattern table. Only the outer layers of the VTX can contribute to the trigger, as the higher hit rates in the inner layers could lead to pile-up due to the granularity available, coupled with the strict power dissipation constraints imposed on the iVTX layers. The Periphery Time-to-Digital (PTD) module is designed to provide precision timing in the external layers where a hit rate of ≤ 10MHz/cm² is expected. While the in-pixel clock provides a coarse timestamp with 47 ns granularity, the column-wise HitOr signal — an asynchronous OR of the discriminator outputs — can be sampled in the periphery with a ×16 faster clock, enabling finer timing. Measurements with TJ-Monopix2 show that, after applying corrections for time walk and position-dependent delays, a timing accuracy of 3 ns can be achieved [12]. This reconstruction is limited to low hit rates (≤ 10 MHz/cm²), as ambiguities arise when multiple hits occur in the same column within one clock cycle, and so it is only applicable in the outer layers of the VTX. The capability to append some hits to the tracks with a finer time resolution improves the rejection of off-time background tracks without increasing the main timestamp resolution and thus the power consumption. Simulations and preliminary tests confirm OBELIX’s performance, and the first full-scale prototype, OBELIX-1, is expected to be ready for submission in spring 2025.
# 6 Conclusions

The upgrade of the Belle II Vertex Detector with OBELIX DMAPS sensors represents a significant step forward for high luminosity operations at SuperKEKB. The tests on TJ-Monopix2 sensor, forerunner of OBELIX, showed high efficiency (> 99%), excellent spatial resolution (~ 9 µm) and good performance even after irradiation (NIEL up to $5 \times 10^{14} n_{eq}/cm^2$), which provided the basis for the development of OBELIX.

OBELIX integrates advanced features such as triggered readout, precise timing and advanced data handling to ensure optimal performance in high background conditions. With its capability to handle high hit rates and reduce off-time background, the new VTX will be crucial in achieving the scientific goals of Belle II and setting a benchmark for future high-energy physics experiments.

# Acknowledgments

This work has received the support from the European Union's Horizon 2020 Research and Innovation programme under Grant Agreements no 101004761 (AIDAinnova), Multilateral Scientific and Technological Cooperation in the Danube Region (MULT 03/23), Horizon 2020 ERC-Consolidator Grant No. 819127, TY-FJPPN (Toshiko Yuasa-France Japan Particle Physics Network), the MCIU with funding from the European Union NextGenerationEU (PRTR-C17.I01) and Generalitat Valenciana (GVANEXT), Project ASFAE/2022/016, the CYRCé member of the France Life Imaging network (ANR-11-INBS-0006).

The measurements leading to these results have been performed at the Test Beam Facility at DESY Hamburg (Germany), a member of the Helmholtz Association (HGF).

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