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Microdosimetry provides measurements of stochastic lineal energy deposition in a micrometric sensitive volume (SV) that are comparable to human cell dimensions, typically in the order of 10–15 $\mu \text{m}$ in diameter. Silicon-based microdosimeters have been fabricated using “3-D technology,” providing true cell-like SVs that are fully encapsulated by a 3-D through substrate electrode. The unique geometry has been made feasible using modern microelectromechanical systems (MEMS). The combination of MEMS techniques and radiation detector fabrication brings forth challenges in achieving reliability, consistency, and high yield. This article reviews the technological challenges encountered in several prototype runs completed at SINTEF. The technological review and prototype iterations aim to deliver a future technology plan at the manufacture level with a high yield throughput.

Funding Information: Funding: This project (18SIB10 chipS.CALe) received funding from the EMPIR program co-financed by the Participating States and from the European Union’s Horizon 2020 research and innovation program. This research was funded, in part, by The Research Council of Norway (Tildeling av grunnbevilgning for 2021—prosjektnr. 194068/F40). A CC BY or equivalent license is applied to any author accepted manuscript (AAM) version arising from this submission, in accordance with the grant’s open access conditions. Authors of Aalto University acknowledge the funding by the Academy of Finland Flagship Programme, Photonics Research and Innovation (PREIN), decision number: 320167. Publisher Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. We performed a systematic study involving simulation and experimental techniques to develop induced-junction silicon photodetectors passivated with thermally grown SiO2 and plasma-enhanced chemical vapor deposited (PECVD) SiNx thin films that show a record high quantum efficiency. We investigated PECVD SiNx passivation and optimized the film deposition conditions to minimize the recombination losses at the silicon-dielectric interface as well as optical losses. Depositions with varied process parameters were carried out on test samples, followed by measurements of minority carrier lifetime, fixed charge density, and optical absorbance and reflectance. Subsequently, the surface recombination velocity, which is the limiting factor for internal quantum deficiency (IQD), was obtained for different film depositions via 2D simulations where the measured effective lifetime, fixed charge density, and substrate parameters were used as input. The quantum deficiency of induced-junction photodiodes that would be fabricated with a surface passivation of given characteristics was then estimated using improved 3D simulation models. A batch of induced-junction photodiodes was fabricated based on the passivation optimizations performed on test samples and predictions of simulations. Photodiodes passivated with PECVD SiNx film as well as with a stack of thermally grown SiO2 and PECVD SiNx films were fabricated. The photodiodes were assembled as light-trap detector with 7-reflections and their efficiency was tested with respect to a reference Predictable Quantum Efficient Detector (PQED) of known external quantum deficiency. The preliminary measurement results show that PQEDs based on our improved photodiodes passivated with stack of SiO2/SiNx have negligible quantum deficiencies with IQDs down to 1 ppm within 30 ppm measurement uncertainty.

Introduction An accurate description of the radiation quality of proton beams is a precondition to increase our understanding of radiobiological mechanisms and to develop accurate biological response models for radiotherapy. However, there are few detectors capable of measuring microdosimetric quantities with high spatial resolution along the entire Bragg curve due to the rapid increase in stopping power at the Bragg peak (BP) and distal dose fall-off (DDF). The aim of this work was to measure the microdosimetric spectra along the Bragg curve in a low energy proton beamline used for radiobiological experiments with a novel 3D silicon-on-insulator (SOI) “mushroom” microdosimeter. Method A silicon microdosimeter with an array of 3D structured diodes, creating well-defined sensitive volumes (SV) with excellent spatial resolution was used for microdosimetry. The microdosimeter was used to measure microdosimetric spectra and the relative dose throughout the Bragg curve of a 15 MeV proton beam by sequential insertion of 16 μm thick polyamide absorption films in front of the microdosimeter. The results were tissue corrected with a novel correction function and compared to Monte Carlo (MC) simulations performed in GATE. Results The measured dose-mean lineal energy ( y D ‾ ) increased from 8 keV/μm at the entrance to 24 keV/μm at the BP, rising to a maximum of 35 keV/μm at the DDF. The measured y D ‾ showed an overall good agreement with the MC simulated values, with deviation of less than 2% at the BP and DDF, while the largest deviation (12%) was found at the entrance. Clear changes in microdosimetric spectra were seen for each 16 μm step at the BP and DDF. Conclusion The SOI microdosimeter with its well-defined 3D sensitive volumes is an excellent tool for characterizing low energy beamlines that demands very high spatial resolution. The good overall agreement between experimental and simulated results indicated that the detector is capable of accurate microdosimetric measurements.

Microdosimetry is a particularly powerful method to estimate the relative biological effectiveness (RBE) of any mixed radiation field. This is particularly convenient for therapeutic heavy ion therapy (HIT) beams, referring to ions larger than protons, where the RBE of the beam can vary significantly along the Bragg curve. Additionally, due to the sharp dose gradients at the end of the Bragg peak (BP), or spread out BP, to make accurate measurements and estimations of the biological properties of a beam a high spatial resolution is required, less than a millimetre. This requirement makes silicon microdosimetry particularly attractive due to the thicknesses of the sensitive volumes commonly being ∼10 [Formula: see text]m or less. Monte Carlo (MC) codes are widely used to study the complex mixed HIT radiation field as well as to model the response of novel microdosimeter detectors when irradiated with HIT beams. Therefore it is essential to validate MC codes against experimental measurements. This work compares measurements performed with a silicon microdosimeter in mono-energetic [Formula: see text], [Formula: see text] and [Formula: see text] ion beams of therapeutic energies, against simulation results calculated with the Geant4 toolkit. Experimental and simulation results were compared in terms of microdosimetric spectra (dose lineal energy, [Formula: see text]), the dose mean lineal energy, y D and the RBE10, as estimated by the microdosimetric kinetic model (MKM). Overall Geant4 showed reasonable agreement with experimental measurements. Before the distal edge of the BP, simulation and experiment agreed within ∼10% for y D and ∼2% for RBE10. Downstream of the BP less agreement was observed between simulation and experiment, particularly for the [Formula: see text] and [Formula: see text] beams. Simulation results downstream of the BP had lower values of y D and RBE10 compared to the experiment due to a higher contribution from lighter fragments compared to heavier fragments.

Silicon radiation detectors with minimum dead area in the sensor periphery are desirable and sometimes necessary for many applications in nuclear medicine, high energy physics, and X-ray​ science. The dead area typically includes a guard ring structure that is required for facilitating a uniform electric field distribution around the active area of the sensor, and consequently assuring high breakdown voltages, as well as for limiting the active area leakage current. The dead sensor periphery can be drastically reduced or even completely eliminated by replacing the conventional guard ring structure with the so-called ”active-edge”. The active edge is fabricated by etching through-substrate trenches surrounding the active area of the sensor using a micromachining technique known as deep reactive ion etching, followed by passivation of trench walls with doping. The active edge thus provides an excellent isolation of the active area from defects resulting from sensor separation while occupying only negligible physical space. Several laboratories worldwide have investigated the fabrication of active-edge sensors using a support wafer, which is required to provide mechanical integrity once the trenches are etched. However, the post-processing removal of the support wafer is a cumbersome and unreliable step, making this fabrication approach unsuitable for high yield manufacturing. We have recently developed a new fabrication method that eliminates the challenges associated with processing on a support wafer and thus facilitates mass-manufacturing. We have successfully demonstrated the fabrication of edgeless sensors with edge insensitivity of

This paper presents recent development of Silicon on Insulator (SOI) detectors for microdosimetry at the Centre for Medical Radiation Physics (CMRP) at the University of Wollongong. A new CMRP SOI microdosimeter design, the 3D mushroom microdosimeter is presented. Modification of SOI design and changes to the fabrication processes have led to improved definition of the microscopic sensitive volumes (SV), and thus to better modelling of the deposition of ionizing energy in a biological cell. The electrical and charge collection properties of the devices have been presented in previous works. In this study, the response of the microdosimeters in monoenergetic and spread out Bragg peak therapeutic 12C ion beam at Heavy Ion Medical Accelerator in Chiba (HIMAC, Japan) are presented. Derived relative biological effectiveness (RBE) in 12C ion radiation therapy matches the tissue equivalent proportional counter (TEPC) well, along with outstanding spatial resolution. The use of SOI technology in experimental microdosimetry offers simplicity (no gas system or HV supply), high spatial resolution, low cost, high count rates capabilities for beam characterisation and quality assurance (QA) in charged particle therapy.

New 10- $\mu \text{m}$ -thick silicon microdosimeters utilizing 3-D technology have been developed and investigated in this paper. The TCAD simulations were carried out to understand the electrical properties of the microdosimeters’ design. A charge collection study of the devices was performed using 5.5-MeV He2+ ions which were raster scanned over the surface of the detectors and the charge collection median energy maps were obtained and the detection yield was also evaluated. The devices were tested in a 290 MeV/u carbon ion beam at the Heavy Ion Medical Accelerator in Chiba (HIMAC) in Japan. Based on the microdosimetric measurements, the quality factor and dose equivalent out of field were obtained in a mixed radiation field mimicking the radiation environment for spacecraft in deep space.

This paper focuses on deep reactive ion etching (DRIE) of sub-micrometer features. Very high aspect ratios up to 160:1 on trenches of 250nm have been achieved using the Bosch process and up to 120:1 on trenches of 35nm using a cryogenic process. The proposed etch recipes are specifically optimized for sub-micrometer features, and are not compatible with feature sizes in the tens of micrometer range. Based on analyzing data from our experiments and from literature, we show that a previously reported two-parameter empirical logarithmic law accurately describes the dependency of aspect ratio on trench width over a wide range of widths and etch parameters, including the sub-micrometer regime. We also propose a new figure of merit (FOM) that describes the ultimate aspect ratio achievable for any given etching process. This FOM also allows comparison of different aspect ratio performances, while taking into account in the same time, the dimension of the trench for which this performance is attained.

Three-dimensional (3D) integration of MEMS and ICs enables improvements in device performance, and often requires through-silicon vias (TSVs). TSV technologies presently available for micro electromechanical systems (MEMS) either have completely filled vias or hollow vias. Hollow vias imply perforated wafers, and limit further processing options. We have investigated dry film resist which enables photolithography on perforated wafers. The investigated resist was found to have adequate adhesion to silicon, SiO 2 , and aluminum surfaces. Resist patterns with square openings of side length 15 μm and resist squares of side length 15 μm were reliably realized on all three investigated surfaces. On silicon surfaces, resist patterns with square openings of side length 10 μm and resist squares of side length 7 μm could be realized. The resist could withstand wet etching of 1 μm aluminum, reactive ion etching (RIE) of 7500 A SiO 2 and deep silicon etching (DRIE) of 30 μm Si. Individual process steps for the future fabrication of DRIE etched TSVs with SiO 2 isolation, polysilicon conductor material and aluminum top contacts have been developed and verified.

A study of charge collection in SINTEF 3D active edge silicon detectors was carried out at ANSTO using Ion Beam Induced Charge (IBIC) technique. An IBIC study has shown that several different geometries of 3D detectors have full depletion under low applied bias. The effect of fast neutron and gamma radiation on their charge collection efficiency was also investigated. A 3D active edge silicon detector technology has demonstrated extremely promising performance for application of the 3D Sensitive Volumes (SVs) fabrication methods to SOI microdosimetry.

3D active edge sensors have advantages such as radiation hardness and edgeless capability. With the use of deep reactive ion etching and wafer bonding, 18.5 by 20.5 mm2 3D detectors with active edges have been successfully fabricated at SINTEF MiNaLab. These sensors are compatible with the ATLAS FE-I4 readout electronics. Fabrication process and difficulties are presented and the preliminary electrical measurements are also discussed.

Fragile micromachined MEMS structures are usually protected by bonding a capping wafer to the device wafer itself. As opposed to using lateral interconnects at the interface between the cap wafer and the device wafer, the use of vertical through silicon vias (TSVs) significantly simplifies the mounting of the components and it also results in the smallest footprint. This paper presents the concept chosen for fabricating a miniaturized MEMS acceleration switch with TSVs through the SOI (silicon on insulator) device wafer, as well as the experimental results of the TSV process development that was done for this particular application. Especially challenging was the development of an etching process that can etch the thick buried oxide of the SOI wafer through high aspect ratio trenches.

The Super-LHC upgrade puts strong demands on the radiation hardness of the innermost tracking detectors of the CMS, which cannot be fulfilled with any conventional planar detector design. The so-called 3D detector architectures, which feature columnar electrodes passing through the substrate thickness, are under investigation as a potential solution for the closest operation points to the beams, where the radiation fluence is estimated to reach 10(16) n(eq)/Tcm(2). Two different 3D detector designs with CMS pixel readout electronics are being developed and evaluated for their advantages and drawbacks. The fabrication of full-3D active edge CMS pixel devices with p-type substrate has been successfully completed at SINTEF. In this paper, we study the expected post-irradiation behaviors of these devices with simulations and, after a brief description of their fabrication, we report the first leakage current measurement results as performed on wafer.

Capacitive micromachined ultrasound transducers (CMUTs) can be used to realize miniature ultrasound probes. Through-silicon vias (TSVs) allow for close integration of the CMUT and read-out electronics. A fabrication process enabling the realization of a CMUT array with TSVs is being developed. The integrated process requires the formation of highly doped polysilicon electrodes with low surface roughness. A process for polysilicon film deposition, doping, CMP, RIE and thermal annealing that resulted in a film with sheet resistance of 4.0 Ω/□ and a surface roughness of 1 nm rms has been developed. The surface roughness of the polysilicon film was found to increase with higher phosphorus concentrations. The surface roughness also increased when oxygen was present in the thermal annealing ambient. The RIE process for etching CMUT cavities in the doped polysilicon gave a mean etch depth of 59.2 ± 3.9 nm and a uniformity across the wafer ranging from 1.0 to 4.7%. The two presented processes are key processes that enable the fabrication of CMUT arrays suitable for applications in for instance intravascular cardiology and gastrointestinal imaging.