25-299 Design improvement and characterization of a Floating Gate Dosimeter

25-299 Design Improvement and Characterization of a Floating Gate Dosimeter

• Ph.D., 36 months
• Full-time

Mission

Both in space and particle accelerators, if for different radiation environments, TID (Total Ionising Dose) effects are responsible for electronic components degradation. Therefore, it is necessary to monitor dose levels and perform dosimetry in a mixed radiation field. At particle accelerators at CERN (the European Organization for Nuclear Research), multiple dosimetry systems are already in place. The general aim is to ascertain the life expectancy of electronics already deployed in facilities and to assess TID levels in future deployment spots. Similar needs are witnessed within the space industry. Radiation Hardness Assurance has been part of the Product Assurance procedures since the Eighties and is bound to evolve with the industry’s shift towards COTS components. This evolution requires increasing monitoring of the space radiation environment for modeling and real-time purposes, leading to a growing need for integration of dosimeters in space systems.

In mixed-field environments, the dosimetry technologies used vary depending on the aim of the measurement. For instance, at CERN, fiber-based dosimetry is widely used along with solid-state dosimetry. The Floating Gate Dosimeter (FGDOS), designed and produced by Sealicon, is a notable example. This device is based on a gate stack with features significantly larger than those of a floating gate-based memory but functions similarly. The stack comprises two silicon dioxide layers with a floating polysilicon gate in between. The floating gate is positively pre-charged by the tunnel effect, generating an electric field in the two SiO2 volumes above and below it, which are the active volumes. Radiation interacts with the oxide, generating electron-hole pairs, whose carriers are then drifted by the electric field in the gate. The collected negative charge discharges the polysilicon gate, which is read out by a front-end readout transistor. Accurate characterization allows for precise measurement of the charge generated in the silicon dioxide volume, thus determining the energy deposited by radiation in the detector, which is TID. This sensor is already characterized and integrated into an IC form, enabling easy integration into satellites, as demonstrated by its use on the ISS. However, it has drawbacks and margins for improvement, which could be the subject of further research.

The main issues and improvement areas are in charge collection efficiency. Charge collection efficiency, defined as the percentage of collected charge relative to the generated one, is crucial in optical detectors but is comparatively low in SiO2. This means a significant amount of the charge generated in the gate stack recombines before detection. The aim of this research proposal is to investigate improvements in the material, geometry of the gate stack, and the overall architecture of the FGDOS. New oxides can now be included in the fabrication of electronic components, and some may exhibit better characteristics for dosimetry than silicon dioxide. Additionally, the device’s geometry still has room for improvement.

The work associated with this thesis will be divided into three main blocks. The first will involve literature research on the topic and development of a Geant4 simulation model of the detector. Simulations will be performed on particle-matter interactions for the device, and different geometries and materials will be numerically tested.

The second phase will include preparing test setups for the prototypes, which may differ from the optimal solutions found in simulations due to time and industrial constraints. The prototypes will be tested at the Co-60 facility at CERN and during heavy ions and proton test campaigns in other facilities. In this phase, the possibility of inclusion in payloads may be discussed with CNES.

Finally, the third phase will encompass data analysis of the collected data during the previous test campaigns, further Geant4 simulations, and interpretation of the results. Ideally, the work will conclude with a summary of optimized design solutions and use recommendations.

The research associated with this thesis will be performed partially at CERN and partially in CNES laboratories, with some test campaigns for data collection conducted in particle accelerators across Europe.

For more information about the topics and the co-financial partner, contact Directeur de thèse:

Then, prepare a resume, a recent transcript, and a reference letter from your M2 supervisor or engineering school director, and you will be ready to apply online before March 14th, 2025, Midnight Paris time!

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