Influence of Radiation and AC Coupling on Time Performance of Analog Pixels Test Structures in 65 nm CMOS technology

This study demonstrates that Analog Pixel Test Structures fabricated in 65 nm CMOS technology, utilizing both DC- and AC-coupled designs, maintain high detection efficiency and sub-70 ps time resolution even after exposure to radiation levels up to 10^15 NIEL, confirming their suitability for future high-energy physics tracking systems.

The Gist

Imagine you are trying to catch tiny, invisible messengers (particles) flying through the air at nearly the speed of light. To do this, you need a super-fast, super-sensitive camera made of silicon. This paper is about testing a new, high-tech version of that camera to see if it can survive the harsh environment of a giant particle collider (like the Large Hadron Collider) and still take perfect "photos" of these messengers.

Here is the story of their experiment, broken down into simple concepts.

1. The Goal: Building a Better "Eye" for Physics

Scientists are building a new generation of particle detectors. They need these detectors to be:

• Tiny: To see very small details.
• Fast: To catch particles moving incredibly quickly.
• Tough: To survive being bombarded by radiation (like a constant hailstorm of invisible bullets) without breaking.

The team tested a prototype sensor made using 65nm CMOS technology. Think of this as the "smartphone chip" technology of the future, but adapted for catching particles. They made two versions of this sensor to see which one works better.

2. The Two Designs: The "Direct Line" vs. The "Transformer"

The researchers built two types of sensors to connect the particle catcher to the computer brain:

• The DC-Coupled Sensor (The Direct Line):
  Imagine a telephone where the microphone is wired directly to the speaker. There is no middleman. This design is very simple and efficient. It has a very low "capacitance" (which we can think of as electrical "weight" or "inertia"). Because it's light, it can react very quickly to a signal. However, it has a limit: it can't handle very high voltage, like a car engine that can only go up to 60 mph before the engine blows.

• The AC-Coupled Sensor (The Transformer):
  Imagine putting a special transformer box between the microphone and the speaker. This box allows you to crank up the voltage (the "pressure" pushing the signal) much higher than the direct line can handle. This is great because higher pressure usually means a clearer signal. However, the transformer adds a little bit of "static" or "weight" to the connection, which can make the signal slightly noisier.

3. The Stress Test: The Radiation Hailstorm

To see if these sensors are tough enough for the real world, the team took them to a particle beam at CERN. They shot particles at the sensors to simulate years of radiation damage all at once.

• Level 1: A light drizzle of radiation ($10^{14}$ particles).
• Level 2: A massive hurricane of radiation ($10^{15}$ particles).

The Results:

• The Direct Line (DC) Sensor: It was incredibly tough. Even after the "hurricane" of radiation, it kept working perfectly. It could still detect particles with 99% efficiency and measure their arrival time with a precision of about 63 picoseconds.
  • What's a picosecond? It's one-trillionth of a second. To visualize this: If a picosecond were a second, a second would be about 31,700 years. That is incredibly fast!
• The Transformer (AC) Sensor: It also survived well. It could detect particles just as well as the other one. However, because of the "static" from the transformer, it was slightly noisier. But, because it could handle higher voltage (like turning up the volume on a radio), it eventually caught up to the Direct Line sensor's speed when they pushed the voltage to the limit.

4. The "Time Walk" Problem

When you measure time, sometimes a loud sound seems to arrive slightly earlier than a quiet sound, even if they happened at the same time. This is called "time walk."

• In their sensors, if a particle hit the edge of a pixel, the signal was a bit slower than if it hit the center.
• The researchers developed a clever math trick (a "correction") to fix this. It's like telling the computer: "If the sound is quiet, wait a tiny bit before you record the time." Once they applied this fix, both sensors became incredibly accurate.

5. The Big Takeaway

The paper concludes that both designs work, but they have different superpowers:

• The Direct Line (DC) is naturally faster and cleaner because it's lighter, but it can't handle high voltage.
• The Transformer (AC) is a bit heavier and noisier, but it can handle high voltage, which helps it catch signals better in tough conditions.

The "Aha!" Moment:
The scientists realized that the perfect sensor might be a hybrid. Imagine taking the lightweight, fast body of the Direct Line sensor and giving it the high-voltage engine of the Transformer. If they could combine the "low weight" of the first design with the "high pressure" capability of the second, they could create a sensor that is even faster and more precise than anything they have today.

Why Does This Matter?

This technology is crucial for the future of particle physics. As we build bigger colliders to find new particles (like the Higgs boson or dark matter), we need sensors that can survive the radiation and tell us exactly when and where a particle passed through. These 65nm sensors are a major step forward, proving that we can build "smart" silicon eyes that are tough enough for the most extreme environments in the universe.

Technical Summary

Here is a detailed technical summary of the paper "Influence of Radiation and AC Coupling on Time Performance of Analog Pixels Test Structures in 65 nm CMOS technology."

1. Problem Statement

Next-generation particle physics experiments, such as the ALICE Inner Tracking System (ITS3) upgrade at the Large Hadron Collider (LHC), require tracking detectors with extreme radiation hardness, high spatial resolution, and precise timing capabilities. Monolithic Active Pixel Sensors (MAPS) in advanced CMOS technologies are promising candidates for these applications. However, two critical challenges remain:

1. Radiation Hardness: Sensors must maintain performance (timing resolution and detection efficiency) after exposure to high fluences of non-ionizing energy loss (NIEL), up to $10^{15}$$1 \text{ MeV } n_{eq}/\text{cm}^2$.
2. Coupling Scheme Trade-offs: While Direct Current (DC) coupling offers low input capacitance (beneficial for timing), it limits the reverse bias voltage due to breakdown risks in the shallow junctions. Alternating Current (AC) coupling allows for higher reverse bias (enhancing charge collection) but introduces coupling capacitance, which increases input noise and potentially degrades timing jitter.

The paper aims to evaluate the Analog Pixel Test Structures (APTS) fabricated in the TPSCo 65 nm CMOS process to determine if they meet these requirements and to compare the performance of DC-coupled versus AC-coupled designs under irradiation.

2. Methodology

Device Architecture

• Technology: TPSCo 65 nm CMOS imaging process.
• Sensor Design: A $4 \times 4$matrix of$10 , \mu\text{m}$ pitch pixels.
• Process Variant: "Modified with gap" design, featuring a small n-well collection electrode ($\sim 1 \, \mu\text{m}$) and a deep n-type implant with a gap to create a lateral electric field for faster charge collection.
• Readout: Each pixel includes a fast operational amplifier (OA)-based buffering stage for direct analog readout.
• Variants Tested:
  • DC-Coupled: Sensor directly connected to the front-end. Limited to $\sim 4.8 \text{ V}$ substrate bias to avoid junction breakdown.
  • AC-Coupled: Sensor separated from the front-end by a metal-to-metal coupling capacitor ($C_{coupling} \approx 7.9 \text{ fF}$). Allows higher reverse bias (tested up to $18 \text{ V}$, designed for$50 \text{ V}$).

Irradiation and Test Setup

• Irradiation: DC-coupled sensors were irradiated at JSI Ljubljana with neutrons to fluences of $10^{14}$and$10^{15}$$1 \text{ MeV } n_{eq}/\text{cm}^2$.
• Beam Test: Conducted at CERN-SPS H6 facility using $120 \text{ GeV}/c$ positive hadrons.
• Telescope Setup:
  • Reference Planes: ALPIDE sensors (Planes 0, 1, 4) for track reconstruction.
  • Trigger Plane: APTS-SF (Plane 3).
  • Time Reference: Low Gain Avalanche Detector (LGAD) (Plane 5) with $\sim 23 \text{ ps}$ resolution.
  • Device Under Test (DUT): APTS-OA (Plane 2).
• Readout: High-speed oscilloscope ($40 \text{ GS/s}$,$13 \text{ GHz}$) for central pixels; ADCs for outer pixels.

Analysis Techniques

• Signal Processing: Constant fraction discrimination (10% amplitude for DUT, 40% for LGAD) to minimize time walk.
• Correction: A time-walk correction based on cluster size and signal amplitude was applied to account for signal shape variations.
• Metrics: Time resolution ($\sigma_t$), detection efficiency, charge collection (Most Probable Value), and noise RMS.

3. Key Contributions

1. Radiation Hardness Validation: Demonstrated that 65 nm MAPS with OA buffering maintain high performance up to $10^{15}$$1 \text{ MeV } n_{eq}/\text{cm}^2$, a fluence relevant for future high-luminosity collider upgrades.
2. AC vs. DC Coupling Comparison: Provided a comprehensive experimental comparison of AC and DC coupling schemes, quantifying the trade-off between higher bias capability (AC) and lower capacitance/noise (DC).
3. Timing Performance Optimization: Showed that while AC coupling initially suffers from lower signal-to-noise ratio (SNR), increasing the reverse bias can compensate for this, achieving timing resolutions comparable to DC-coupled sensors.
4. Mechanism Insight: Identified that radiation-induced trapping centers suppress slow-drifting carriers (peripheral charge), which paradoxically reduces the "long tail" in time residual distributions, improving the overall RMS in some irradiated cases.

4. Key Results

Radiation Hardness (DC-Coupled)

• Timing Resolution:
  • Non-irradiated: $62.7 \pm 1.6 \text{ ps}$.
  • Irradiated ($10^{14}$):$62.4 \pm 1.0 \text{ ps}$ (No degradation).
  • Irradiated ($10^{15}$):$68.6 \pm 1.0 \text{ ps}$($\sim 10%$ degradation).
• Detection Efficiency: Maintained $>99\%$ efficiency up to a threshold of $100 , e^-$even at$10^{15}$ fluence.
• Noise Behavior: Unlike previous studies with Source-Follower (SF) readout, the OA-based readout showed a decrease in noise RMS after irradiation. This is attributed to the reduction in sensor capacitance (due to trapping) outweighing the increase in leakage current noise.

AC-Coupled Performance

• Bias Dependence: Time resolution improved with increasing reverse bias ($4.8 \text{ V}$to$18 \text{ V}$).
• Peak Performance: At $18 \text{ V}$, the AC-coupled sensor achieved a time resolution of **$67.2 \pm 3.6 \text{ ps}$**, which is comparable to the DC-coupled sensor (within$2\sigma$ uncertainty).
• Trade-off: The AC-coupled sensor exhibited higher noise RMS and lower signal amplitude due to the coupling capacitor, leading to a larger jitter contribution. However, the higher bias capability allowed for a stronger electric field, improving charge collection speed and compensating for the SNR loss.

Charge Collection and Efficiency

• Cluster Size: Decreased with higher bias and irradiation (due to charge trapping), reducing charge sharing.
• Efficiency: Both DC and AC versions achieved $>99\%$ efficiency for thresholds below $150 , e^-$.
• Charge Loss: Irradiated DC sensors showed a reduction in collected charge (MPV) due to trapping, but the efficiency remained high due to the low threshold.

5. Significance and Conclusion

This work confirms that 65 nm CMOS MAPS are highly suitable for future high-energy physics vertex and timing detectors.

• Viability: The technology demonstrates exceptional radiation tolerance, maintaining sub-70 ps timing resolution and high efficiency after extreme irradiation.
• Design Flexibility: The study validates that AC coupling is a viable alternative to DC coupling. While AC coupling introduces higher capacitance and noise, the ability to apply higher reverse bias ($>18 \text{ V}$) compensates for these drawbacks, yielding timing performance on par with DC-coupled designs.
• Future Outlook: The authors suggest that a hybrid approach—combining the low capacitance of the DC-coupled design with the high-bias capability of the AC-coupled design—could further push time resolution limits. This makes the 65 nm TPSCo process a strong candidate for the ALICE ITS3 upgrade and other high-luminosity collider experiments.