3-D self assembled SOI MEMS: fabrication and numerical simulation

The advantages of thin film SOI (Silicon-on-Insulator) technology for MOS circuits compared with thick SOI and bulk silicon MOS techniques are nowadays well known. Among them, we can mention a lower power consumption, better performance on high frequency operation and resistance to irradiation as well as better high temperature characteristics. These advantages in addition to a direct MEMS-IC integration make thin film SOI an attractive process for the manufacture of high quality MEMS and MOS integrated circuits for space applications. In this work, we present tridimensional (3-D) self-assembled multilayered SOI MEMS structures that can be used (but not limited) as flow sensors, thermal actuators and high frequency inductors. These MEMS consist of layers of Si, Si3N4 and Al deposited at different temperatures, and the final shape of the structure can be very different depending on the number, the nature and the thickness of the layers that are deposited. If the system consists only of one layer of Si, the structure remains flat after the release, whereas in the bilayer case (Si and Si3N4) the structure bends up. This is due to the thermal expansion coefficient of the Si3N4, which is higher than the Si one, and therefore the upper part of the structure contracts more than the lower part resulting in a curved shape. The thermal expansion coefficient of the Al being even greater than the Si3N4 one, we would then expect a larger curvature of the structure in the trilayer case, but considering the Al deposition temperature and the geometry we can demonstrate that the shape is rather flat. To obtain a curly shape in the trilayer structure an additional thermal step is needed. It could be a two minutes RTA (Rapid Thermal Annealing) at 600°C or a lower temperature annealing (432°C) but for a longer time (30 min). In order to make quantitative comparisons with numerical simulations, the final shape of these structures was completely characterized using laser interferometry.