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The RD50-MPW4: A Radiation Hard HV CMOS Sensor for Future Colliders

The RD50-MPW4, a 150 nm HV CMOS sensor prototype developed by the CERN RD50 collaboration, demonstrates exceptional radiation hardness with over 99% efficiency after irradiation to $1 \times 10^{15}$ 1 MeV n $_{\mathrm{eq}}/\mathrm{cm}^2$ , alongside high spatial (16 $\mu$ m) and timing (10 ns) resolutions, making it a promising candidate for future collider detectors.

Original authors: Jory Sonneveld, Thomas Bergauer, Raimon Casanova, Harald Handerkas, Christian Irmler, Jorge Jiménez-Sánchez, Uwe Krämer, Ricardo Marco-Hernandez, José Mazorra de Cose, Fernando Muñoz-Chavero, Rogelio

Published 2026-07-22

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CC BY 4.0

Original authors: Jory Sonneveld, Thomas Bergauer, Raimon Casanova, Harald Handerkas, Christian Irmler, Jorge Jiménez-Sánchez, Uwe Krämer, Ricardo Marco-Hernandez, José Mazorra de Cose, Fernando Muñoz-Chavero, Rogelio Palomo, Bernhard Pilsl, Sebastian Portschy, Samuel Powell, Patrick Sieberer, Helmut Steininger, Eva Vilella, Benjamin Wade, Chenfan Zhang, Sinuo Zhang

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the universe as a giant, chaotic ballroom where invisible particles are dancing at speeds close to light. To understand the music of the cosmos, scientists need to build cameras that can snap pictures of these dancers. But here's the catch: the ballroom is filled with "radiation," a kind of cosmic static that usually fries delicate electronics like a microwave overheating a smartphone. For decades, scientists have been trying to build a camera sensor that is tough enough to survive this radiation storm while still being sharp enough to see the tiniest details. They are looking for a sensor that is thin, fast, and incredibly resilient, capable of surviving in the heart of a particle collider where the environment is harsher than the surface of the sun. The key to this survival is a special type of chip called a "High Voltage CMOS" sensor. Think of it like a superhero suit: it's a standard electronic chip, but it's been supercharged with high voltage to create a deep, protective "depletion zone" that sweeps away damage and keeps the signal clear, even when the radiation is pounding on the door.

This paper introduces the latest version of that superhero suit, a sensor called the RD50-MPW4. Built by a team of scientists from across Europe, this chip is designed to be the ultimate tracker for future particle colliders. The team created a prototype using a 150 nm manufacturing process, resulting in a tiny grid of 64 by 64 pixels. Imagine a microscopic chessboard where each square is a sensor pixel, spaced just 62 micrometers apart (about the width of a human hair). The big innovation here is how they power it: instead of a weak battery, they apply a massive voltage of over 600 volts. This is like turning up the pressure in a garden hose so high that it can blast through a wall of mud; in this case, the "mud" is radiation damage. By pushing the voltage this high, the sensor can stay "depleted" (ready to work) even after being bombarded by an enormous amount of radiation, specifically up to a fluence of 1×10¹⁵ 1 MeV neq/cm².

The researchers tested these chips in a lab, first with fresh samples and then with samples that had been blasted with radiation to simulate years of collider use. The results were impressive. The fresh chips worked with more than 99.9% efficiency, meaning they caught almost every particle that hit them, and they could pinpoint the location of a particle within 16 micrometers. Even after being irradiated to the extreme levels mentioned above, the chips still managed to catch over 99% of the particles. They also measured how deep the "depletion zone" went inside the chip using a special laser technique called Two-Photon Absorption. This laser acts like a super-precise flashlight, allowing them to see inside the chip and map out exactly how deep the electric field reaches. They found that at a bias voltage of -90 V, the depletion depth was 226 µm, and they could clearly see the electronics at the top and the edge of the active region at the bottom.

One of the cleverest features of the RD50-MPW4 is how it handles its data. Instead of a messy tangle of wires, it uses a "column-drain" readout, which is like a high-speed assembly line where information flows down columns and gets sorted out efficiently. This design helps prevent the signals from different pixels from getting mixed up (a problem called "cross-talk"). The team also found that by applying the high voltage from the back of the chip (backside biasing), they created a stronger, more uniform electric field, which made the sensor even more resistant to radiation damage. While the chips did show some noise when hit with the very highest levels of radiation (5×10¹⁵ neq/cm²), they performed remarkably well up to 3×10¹⁵ neq/cm², maintaining a low rate of "fake hits" (mistaken signals).

In short, the RD50-MPW4 is a successful step forward in making particle detectors that can survive the harshest environments in physics. It proves that by using high voltage and smart design, we can build sensors that are both tiny and tough. The authors suggest that this technology will continue to evolve, with plans to use similar designs for future projects like the LHCb Mighty Tracker. While the paper doesn't claim to have solved every problem in particle detection, it shows that these high-voltage sensors are a very promising path toward seeing the universe's smallest secrets, even when the view is obscured by a storm of radiation.

Technical Summary: The RD50-MPW4 HV CMOS Sensor

Problem and Context
Future high-luminosity colliders require particle tracking and vertexing detectors capable of withstanding extreme radiation environments while maintaining excellent time resolution and high granularity. The former CERN RD50 collaboration addresses this need by developing radiation-hard semiconductor devices. Specifically, the High Voltage (HV) CMOS working group aims to create Depleted Monolithic Active Pixel Sensors (DMAPS) that integrate electronics directly into the sensor substrate. Previous prototypes (RD50-MPW1 through MPW3) utilized a 150 nm CMOS process from LFoundry, featuring 64x64 pixel matrices and LVDS readout. However, to further enhance radiation tolerance, the breakdown voltage limit needed to be pushed significantly higher, requiring advanced biasing schemes and guard ring structures.

Methodology and Design
The paper presents the RD50-MPW4, a prototype sensor produced by LFoundry in December 2023 using a 150 nm CMOS process. The design incorporates several key architectural advancements:

Sensor Structure: The sensor utilizes a deep n-well (DNWELL) in a p-substrate with embedded readout electronics. A deep p-well (PSUB) shields the collection electrode, allowing CMOS electronics to reside within the pixel. Two of the three sensor variants were backside-processed to enable backside biasing, which is known to create a stronger, more uniform electric field compared to topside biasing.
Biasing and Voltage: The substrate is thinned to 280 µm with a nominal resistivity of 3 kΩ⋅cm. A new biasing scheme with an advanced guard ring structure (based on the LF-Monopix2 prototype) allows for operating voltages exceeding 600 V. This high voltage is applied from the top side for topside-biased sensors and from the backside for backside-processed variants.
Readout Architecture: The sensor features a 64x64 pixel matrix with a 62 µm pitch. It employs a column-drain readout architecture similar to the ATLAS FEI3, utilizing a double-column layout where analog signal lines are separated from digital and shielding lines to reduce crosstalk. The in-pixel electronics include a charge-sensitive amplifier, a comparator with a 4-bit trim DAC for individual threshold adjustment, and a fully digital readout path.
Data Acquisition: The system uses a triggerless readout where row addresses are serialized at the end-of-column (EoC) and combined with column addresses to form an 8-bit pixel address. Time-over-threshold information is provided via 8-bit leading and trailing edges. Data is transmitted via a 640 Mbit/s LVDS link and acquired using the Caribou system with FPGA Mezzanine Cards.

Key Results
Extensive testing was conducted on unirradiated and irradiated samples to characterize performance:

Leakage Current and Biasing: Unirradiated samples exhibited leakage currents on the order of 10 nA at a depletion voltage of -190 V. The sensors were successfully operated safely at bias voltages beyond 600 V, with leakage current increasing as expected with temperature.
Efficiency and Resolution: Tests using a 4.2 GeV electron beam at DESY (with the Adenium telescope) showed that unirradiated samples achieved an efficiency of over 99.9% with a spatial resolution of 16 µm and a timing resolution of 10 ns. For samples irradiated to fluences of $1 \times 10^{15} \text{ neq/cm}^2$ , efficiencies remained above 99%.
Fluence Dependence: Efficiency degradation was observed at higher fluences, particularly in pixel corners due to charge sharing. Sensors irradiated up to $3 \times 10^{15} \text{ neq/cm}^2$ maintained acceptable fake hit rates, while those at $5 \times 10^{15} \text{ neq/cm}^2$ were significantly affected by noise. Backside-biased sensors generally performed better than topside-biased ones.
3D Characterization: Using the Two-Photon Absorption Transient Current Technique (TPA-TCT) with a 1550 nm femtosecond laser at CERN, a 3D map of charge collection was generated. This measurement clearly visualized the sensor's electronics at the top and the depletion region edge at the backside. At a bias of -90 V, a depletion depth of 226 µm was measured.

Significance and Outlook
The RD50-MPW4 demonstrates that monolithic HV CMOS sensors can achieve breakdown voltages exceeding 600 V, significantly pushing the limits of radiation tolerance for future collider applications. The successful implementation of separate analog and digital power domains, combined with the advanced guard ring structure, validates the design's capability to operate in high-radiation environments. The paper concludes that the RD50-MPW series represents a viable path for high-granularity, fast-timing pixel detectors. Future R&D within the DRD3 collaboration will continue with the LFoundry 150 nm process, specifically focusing on the RadPix sensor for the LHCb Mighty Tracker.

Technical Summary: The RD50-MPW4 HV CMOS Sensor

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