Radiation effects and noise evolution in NewAthena WFI flight-production sensors

Technical Summary: Radiation Effects and Noise Evolution in NewAthena WFI Flight-Production Sensors

Problem Statement
The Wide Field Imager (WFI) for ESA's NewAthena X-ray observatory relies on back-illuminated DEPFET (Depleted p-channel field-effect transistor) sensors to achieve high-energy resolution (near Fano-limited) and high count-rate capability in the 0.2–15 keV range. A critical performance requirement is an exceptionally low readout noise of approximately 3 electrons Equivalent Noise Charge (ENC) at the Beginning of Life (BOL). However, these sensors are highly sensitive to radiation-induced degradation. In the space environment, two primary mechanisms threaten performance: Total Non-Ionizing Dose (TNID), which causes lattice displacement damage leading to increased dark current, and Total Ionizing Dose (TID), which affects the gate oxide and threshold voltage. While previous studies characterized pre-flight sensors, there was a need to validate these findings using flight-production hardware and to disentangle the specific contributions of TNID and TID to the total noise budget, particularly regarding their temperature dependence and temporal evolution.

Methodology
The study utilized two distinct irradiation campaigns on detector modules containing 64×64-pixel DEPFET sensors from the first NewAthena flight-production wafer.

1. Proton Irradiation (TNID and Short-Term Annealing):

   • Facility: MedAustron accelerator (Wiener Neustadt, Austria).
   • Conditions: Two modules were irradiated with 62.4 MeV protons.
     • Module 1: Exposed to a high-flux, short-duration burst (19 s) to study transient effects and short-term annealing at the nominal operating temperature of 213 K.
     • Module 2: Exposed to a low-flux, long-duration irradiation (5.5 hours) to accumulate a total displacement damage dose equivalent to $2.6 \cdot 10^9$ 10-MeV protons/cm². This module was maintained at 213 K for 20 days post-irradiation to monitor dark current annealing.
   • Measurement: Detectors were fully biased and operated during irradiation. Noise components (dark current, DEPFET intrinsic noise, and ASIC noise) were separated by varying frame times and source-drain voltages.

2. X-ray Irradiation (TID Isolation):

   • Facility: Standard analytical X-ray tube (Mo-K$\alpha$ source).
   • Conditions: A separate flight-production module was irradiated to a total TID of 15 Gy. The readout ASICs were shielded to ensure only the DEPFET sensor received ionizing radiation.
   • Measurement: The study monitored the mean threshold voltage shift and the evolution of noise components over time, including annealing steps at 242 K and 273 K.

Key Contributions and Results

• Displacement Damage (TNID) and Dark Current:

  • Proton irradiation confirmed that radiation-induced dark current increases linearly with displacement damage dose.
  • At the expected End-of-Life (EOL) dose of $1.3 \cdot 10^9$ 10-MeV protons/cm², the radiation-induced dark current is predicted to be $3.0 \pm 0.6$ e⁻/(pix·ms). This result aligns closely with pre-flight production data ($2.5 \pm 0.4$ e⁻/(pix·ms)), confirming the radiation tolerance of the flight sensors.
  • Annealing Behavior: Significant annealing of lattice defects occurs even at the nominal operating temperature of 213 K. The dark current decreases over time with multiple time constants spanning from seconds to millions of seconds.
  • Temperature Constraints: To maintain the dark current noise within acceptable limits at a 500 Hz frame rate, the sensor temperature must remain below $(208.9 \pm 1.6)$ K at EOL.

• Ionizing Damage (TID) and Intrinsic Noise:

  • A 15 Gy TID exposure resulted in a mean threshold voltage shift of 50 mV.
  • Crucially, the study identified that TID induces an additional noise component in the DEPFET itself, distinct from dark current. This noise manifests as an increase in intrinsic DEPFET noise (both white and 1/f components) rather than an immediate spike in dark current.
  • Thermal Activation: The growth of this TID-induced noise is thermally activated. It remains minimal at 213 K but increases significantly when the sensor is annealed at higher temperatures (e.g., 242 K or 273 K). The authors attribute this to the diffusion of trapped holes/ions in the gate oxide forming interface traps at the Si/SiO₂ boundary.

• Noise Budget Evolution:

  • Below ~200 K, the reduction in dark current noise (due to lower temperature) outweighs the increase in intrinsic DEPFET noise.
  • However, if the sensor is warmed to room temperature for annealing, the intrinsic DEPFET noise increases measurably and does not fully recover, potentially degrading the total noise budget.

Significance and Operational Implications
The paper concludes that the flight-production sensors perform consistently with pre-flight generations regarding radiation tolerance. The primary operational finding is the necessity of a specific temperature profile to balance contamination risks and radiation damage:

1. Early Mission (Contamination Phase): The sensor should operate at 213 K to minimize contamination. At this temperature, TID-induced noise growth is suppressed.
2. Late Mission (Radiation Accumulation Phase): As radiation damage accumulates, the temperature should be gradually reduced to 193 K. This lower temperature is sufficient to suppress the radiation-induced dark current to a negligible level relative to the total noise budget.
3. Annealing Strategy: The authors advise against dedicated high-temperature annealing cycles during the mission. While higher temperatures would further reduce dark current, they would simultaneously activate the diffusion of radiation-induced defects in the gate oxide, leading to a permanent increase in intrinsic DEPFET noise that outweighs the benefits.

The study provides a validated noise budget and a recommended temperature profile (213 K transitioning to 193 K) to ensure the WFI meets its performance requirements throughout the NewAthena mission lifetime.