Monolithic Active Pixel Sensor Technologies in High-Energy Physics

Monolithic Active Pixel Sensors (MAPS) have emerged as a transformative class of silicon detectors in high-energy physics, uniting sensing and readout electronics on a single substrate. By eliminating the need for bump bonding, these devices drastically reduce material budget and production complexity while enhancing spatial resolution. Recent technological advances focus on full depletion of the sensitive layer to accelerate charge collection and strengthen radiation tolerance, enabling operation in the demanding environment of modern colliders. Innovations in deep p-well implantation and reverse substrate bias have yielded Depleted Monolithic Active Pixel Sensors (DMAPS) with fast signal response on the order of nanoseconds and radiation hardness exceeding 10^15 neq cm^−2. Integrated sparsification and low-power front-ends permit pixel-by-pixel data reduction, crucial for higher interaction rates. Such sensors underpin upgrades of inner tracking systems at the Large Hadron Collider and inform applications in beam monitoring, medical imaging and space radiation detection.

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A landmark development is the ALPIDE sensor architecture for the ALICE Inner Tracking System upgrade. Fabricated in a 180 nm CMOS imaging process, ALPIDE exploits a deep p-well to accommodate full CMOS circuitry within each pixel. This design achieves a spatial resolution of around 5 µm, a power consumption below 40 mW cm^−2 and timing peaking times near 2 µs, while enduring radiation fluences up to 10^13 neq cm^−2. Its in-matrix sparsification sends only hit addresses to the periphery, significantly reducing data volumes.

A recent evaluation of the MALTA telescope illustrates the integration of DMAPS planes into a compact beam test system. Using six DMAPS layers realised in 180 nm CMOS, the telescope delivers sub-5 µm residuals and sub-10 ns time stamping. On-chip readout and a configurable trigger logic unit enable real-time monitoring of hit multiplicities and timing distributions, facilitating rapid track reconstruction during test-beam campaigns at CERN.

For extreme radiation environments, the Mini-MALTA prototype implements a small-electrode DMAPS design on high-resistivity epitaxial silicon. Pixels measuring 36.4 × 36.4 µm^2 achieve full detection efficiency after irradiation up to 1 × 10^15 neq cm^−2, demonstrating suitability for the outer layers of the ATLAS Inner Tracker. The combination of low capacitance, fast charge collection and simplified monolithic integration marks a significant step towards large-scale deployment in high-luminosity experiments.

Monolithic Active Pixel Sensor Technologies in High-Energy Physics publication trend

The graph below shows the total number of articles in monolithic active pixel sensor technologies in high-energy physics across all publications each year (not limited to Nature Index journals).

Technical terms

Monolithic Active Pixel Sensor (MAPS): A silicon detector in which both the sensing diode and the readout electronics are fabricated on a single chip, eliminating interconnects between sensor and readout ASIC.

Depleted Monolithic Active Pixel Sensor (DMAPS): A MAPS variant in which the epitaxial layer is fully depleted, enabling fast charge collection by drift and enhanced radiation tolerance.

Charge collection efficiency: The fraction of generated charge carriers successfully collected by the sensing electrode, critical for signal fidelity and noise performance.

Deep p-well: An implanted p-type region that shields the sensing volume from n-well circuitry, allowing full CMOS logic within the pixel without degrading charge collection.

Pixel pitch: The centre-to-centre distance between adjacent pixels, governing spatial resolution and influencing capacitance and power consumption.

References

1. ALPIDE, the Monolithic Active Pixel Sensor for the ALICE ITS upgrade. Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment (2016).
2. Performance of the MALTA telescope. European Physical Journal C (2023).
3. Mini-MALTA: radiation hard pixel designs for small-electrode monolithic CMOS sensors for the High Luminosity LHC. Journal of Instrumentation (2020).
4. A process modification for CMOS monolithic active pixel sensors for enhanced depletion, timing performance and radiation tolerance. Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment (2017).
5. First tests of a novel radiation hard CMOS sensor process for Depleted Monolithic Active Pixel Sensors. Journal of Instrumentation (2017).