[Paper Review] Radiation Hardness of MALTA2 Monolithic CMOS Sensors on Czochralski Substrates

M. Van Rijnbach, Dumitru Vlad Berlea+27 more|arXiv (Cornell University)|Aug 25, 2023

Particle Detector Development and PerformancePhysics and Astronomy3 citations

TL;DR

This paper presents MALTA2, a next-generation monolithic CMOS pixel sensor fabricated on high-resistivity Czochralski (Cz) silicon substrates with backside metallisation, demonstrating radiation hardness up to 3 × 10¹⁵ 1 MeV nₑq/cm². It achieves 98% detection efficiency and 6.3 ns timing resolution RMS at 110 e⁻ threshold, with superior performance and uniformity in samples featuring very high doping of the n⁻ layer, confirming enhanced radiation tolerance for future high-energy physics experiments.

ABSTRACT

MALTA2 is the latest full-scale prototype of the MALTA family of Depleted Monolithic Active Pixel Sensors (DMAPS) produced in Tower Semiconductor 180 nm CMOS technology. In order to comply with the requirements of High Energy Physics (HEP) experiments, various process modifications and front-end changes have been implemented to achieve low power consumption, reduce Random Telegraph Signal (RTS) noise, and optimise the charge collection geometry. Compared to its predecessors, MALTA2 targets the use of a high-resistivity, thick Czochralski (Cz) substrates in order to demonstrate radiation hardness in terms of detection efficiency and timing resolution up to 3E15 1 MeV neq/cm2 with backside metallisation to achieve good propagation of the bias voltage. This manuscript shows the results that were obtained with non-irradiated and irradiated MALTA2 samples on Cz substrates from the CERN SPS test beam campaign from 2021-2023 using the MALTA telescope.

Motivation & Objective

• To develop a radiation-hard monolithic CMOS pixel sensor suitable for high-luminosity LHC experiments requiring fluence tolerance beyond 10¹⁵ 1 MeV nₑq/cm².
• To evaluate the performance of MALTA2 sensors fabricated on high-resistivity Czochralski (Cz) silicon substrates, which offer larger depletion volumes and improved charge collection.
• To assess the impact of backside metallisation on bias voltage propagation and sensor performance in Cz substrates.
• To investigate the influence of n⁻ layer doping levels on timing resolution and detection uniformity after irradiation.
• To validate the radiation hardness of MALTA2 through test beam measurements at CERN's SPS facility using the MALTA telescope.

Proposed method

• Fabrication of MALTA2 sensors using Tower Semiconductor’s 180 nm CMOS process with three pixel flavours: standard (STD), n⁻ layer with gap (NGAP), and additional deep p-well (XDPW).
• Use of high-resistivity p-type Czochralski substrates (100 µm thick) to increase depletion volume and improve charge collection efficiency.
• Application of backside metallisation to ensure uniform bias voltage distribution across the sensor, critical for performance in thick Cz substrates.
• Irradiation of MALTA2 samples to fluences up to 3 × 10¹⁵ 1 MeV nₑq/cm² using the CERN SPS test beam to simulate HL-LHC conditions.
• Characterisation using the MALTA telescope setup with scintillator references for precise timing and position reconstruction of particle tracks.
• Analysis of key performance metrics: detection efficiency, cluster size, timing resolution (RMS of time difference distribution), and time shift uniformity across the pixel.

Experimental results

Research questions

• RQ1 How does the use of high-resistivity Czochralski substrates affect the radiation hardness and charge collection performance of MALTA2 sensors?
• RQ2 What is the impact of backside metallisation on the uniformity of bias voltage distribution and sensor response in thick Cz substrates?
• RQ3 How does the doping level of the n⁻ layer influence timing resolution and detection uniformity after irradiation to 3 × 10¹⁵ 1 MeV nₑq/cm²?
• RQ4 Can MALTA2 achieve sufficient detection efficiency and timing resolution at the highest fluence levels required for HL-LHC applications?
• RQ5 What is the relative performance of the XDPW pixel design with very high n⁻ doping compared to high doping under equivalent irradiation conditions?

Key findings

• Non-irradiated MALTA2 sensors on Cz substrates achieve 99% detection efficiency and 1.7 ns timing resolution (RMS) at a 150 e⁻ threshold, with 98% of hits collected within 25 ns.
• At 3 × 10¹⁵ 1 MeV nₑq/cm², the MALTA2 sample with very high n⁻ layer doping achieves 98% detection efficiency and 1.7 pixel average cluster size at 110 e⁻ threshold.
• The same irradiated sample with very high n⁻ doping exhibits a timing resolution of 6.3 ns RMS, with 95% of clusters collected within 25 ns.
• The very high n⁻ doping sample shows superior timing uniformity across the pixel diagonal compared to the high doping sample, indicating better radiation tolerance.
• The very high n⁻ doping sample performs comparably to a high doping sample irradiated to only 2 × 10¹⁵ 1 MeV nₑq/cm², suggesting enhanced radiation hardness.
• Backside metallisation enables effective bias voltage propagation in thick Cz substrates, which is critical for achieving consistent performance across the sensor.