Fabrication of suspended membranes for thermal sensors using high-density plasma etching A. Tserepi, C. Tsamis, E.Gogolides and A. G. Nassiopoulou Institute of Microelectronics, NCSR “Demokritos”, 15310 Aghia Paraskevi, Athens, Greece Abstract In this work, we describe a front-side Si micromachining process for the fabrication of suspended membranes for thermal sensors. Membrane release is achieved by means of lateral isotropic etching of the bulk silicon substrate, the etching being optimised for high rates and high selectivity with respect to the membrane material and the photoresist that is used to protect the device. Lateral Si etch rates of the order of 6-7 µm/min have been achieved in a high-density F- based plasma, which permit a reasonable etching time for the release of the membrane and the simultaneous formation of the cavity underneath that ensure thermal isolation of the final device. Keywords: isotropic Si etching; high-density plasma; micromachining; suspended membranes; thermal sensors; 1. Introduction During the last years, a considerable number of thermal transducers has been developed and produced. One major requirement for low power applications is the reduction of thermal losses, which is achieved by the fabrication of the active elements of the sensors on suspended structures (micro-hotplates). Several strategies have been developed for the fabrication of the micro-hotplates. The two different types of micro-hotplates utilized are [1]: The closed-type membrane, where the membrane overlaps the silicon substrate along its periphery and the suspended-type membrane, where the membrane element is supported to the Si substrate by means of supporting beams, its central part being suspended over a cavity etched in the substrate. In the latter case, the thermal losses to the substrate take place only through the supporting beams, and they are thus minimized compared to the closed type membrane. For the fabrication of these suspended membranes, various front-side micromachining technologies have been applied. One approach is to remove the silicon substrate underneath the membrane, by wet etching with KOH or EDP solutions [2]. Alternatively, surface micromachining techniques can be used. In that case, the release of the membrane is achieved by the sacrificial etching of oxide or PSG layers [3]. Sacrificial etching of porous silicon is another possibility to obtain suspended membranes. High etch rates of the porous silicon can be achieved during wet etching in various solutions, due to the porosity of the material. Mono- and poly-crystalline silicon [4-6] as well as nitride membranes [7] have been successfully fabricated with this technique. Another method that used for the fabrication of silicon suspended structures is dry etching. A variety of processes such as SCREAM [8] and SIMPLE [9] have been developed. A combination of high aspect ratio anisotropic and isotropic etching in F-based plasmas has been employed for the release of freestanding microcantilevers and bridges from multi-layer substrates (Si-SiO 2 -polySi-SiO 2 -Si sandwiched wafers) [10]. In this case, the released