Fabrication challenges of silicon-based microdosimeter using 3D technology

Microdosimetry provides measurements of stochastic lineal energy deposition on a micrometric sensitive volume (SV), comparable to human cell dimensions. Conventional microdosimeter uses a tissue equivalent proportional counter (TEPC) that requires high voltage operation, are bulky and have poor spatial resolution. Silicon-on-insulator (SOI) microdosimeter fabricated using the so-called '3D technology', provides true cell-like SV that is encapsulated by a through substrate electrode. Furthermore, such detectors provide many attractive advantages such as micrometric spatial resolution, compact design, and easy coupling to readout electronics that provide real-time on-line monitoring. SINTEF in collaboration with Centre for Medical Radiation Physics (CMRP), have developed a full 3D SOI microdosimeter, a 5th generation SOI microdosimeter. Characterisation results of the first prototype run at several heavy ion therapy (HIT) centres demonstrated excellent results with derived radiobiological effectiveness (RBE), comparable with TEPC. Foreseen future manufacture of such dosimeters remains a challenge. One of the major difficulties is the integrity of metal connection joining individual sensitive volumes where the metal must trespass the the 3D circular electrodes encapsulating the entire SVs. The problem is further exasperated by the further enhancement in microdosimetry where all silicon surrounding the SVs is removed. This paper presents the first fabrication review of results and challenges over the several prototype runs carried out at SINTEF Minalab. The review and investigation aim to generate a fabrication technology that have a potential to provide a commercially viable manufacture process with a high yield throughput. Challenging processes such as removal of bulk silicon outside of the microscopic SVs and deposition of tissue equivalent material will also be discussed.