Technology steps to fabricate a thin Silicon Carbide membrane based particle detector

(1) Physics Department, University of Lund, Box 118, SE-221 00 Lund, Sweden
(2) Linköping University, Department of Physics, Chemistry and Biology, SE-58183 Linköping, Sweden
(3)  also Graphensic AB, Teknikringen 7, SE- 58330 Linköping, Sweden
(4) Sensor System, ACREO Swedish ICT AB, Box 1070, SE-164 25 Kista, Sweden
(5)  also Graphensic AB, Teknikringen 7, SE- 58330 Linköping, Sweden

Thin ion transmission detectors are of interest in several application areas that span over a wide range, for example as a part of a telescope detector for mass and energy identification but also as a pre-cell detector in a microbeam system for studies of biological effects from single ion hits on individual living cells. Such single ion thin detectors were previously fabricated by etching deep cavities in silicon that leaves a membrane thickness down to 5 µm [1]. The membranes are the active part of the detector where passing MeV ions deposit a small part of their kinetic energy. However, silicon detectors show noise and single protons are not easy to measure. In addition, beam damage may limit the detector lifetime. We propose a structure of graphene on silicon carbide (SiC) as a concept for a transmission proton detector with low noise. Generally, noise is related to the leakage current, which for 4H-SiC is several decades lower than for silicon due to the difference in bandgap (3.3 vs 1.1 eV). In our proposed structure, we will use graphene as a contact material that collects carriers generated by protons at the inner cavity of the membrane and graphene covers the inner cavity all the way to the backside of the SiC substrate. This can be used to evaluate the number of protons by a change in electrical conductivity of the graphene. A great experimental advantage in many scenarios is the physical strength of SiC which allows it to be used as a combined vacuum window and detector. The hardness of SiC is, however, a challenge in the fabrication process to create a thin membrane inside of a deep etched cavity. We have used ICP technique to etch circular cavities with depths down to 370 µm in prototype samples. At the next step graphene is created by a special high temperature process converting the outermost layers of SiC into graphene (a layer of carbon atoms only one or a few atomic layers thick with extremely high electric conductivity). The smoothness of the etched membrane is critical for the graphene forming process, and in turn depends on the etching procedure, cavity depth and the quality of the SiC material. Evaluation of the membrane quality was done through surface profiler, SEM, AFM as well as by energy loss measurements using the focused micro-beam at Lund Ion Beam Analysis Facility. The graphene fabrication is challenged by that the graphene formation differs between the on-axis surfaces of substrate backside, the wall of cavity, and the bottom of cavity. This paper describes the technological steps in selection of SiC material, the etching process, graphene formation and evaluation of the prototype devices.  

[1]  N. S. Abdel, J. Pallon, M. Graczyk and I. Maximov. Journal of Instrumentation, JINST 9 T06002, 2014.