A design tool fully adapted to the development of the thin film packaging process used for MEMS devices
Authors: Frederic Souchon , Anne-Charlotte Gervais , Laurent Thouy , Damien Saint-Patrice , louis Pornin Jean 
Published: October 2015

MEMS Wafer Level Packaging is required for mass production of MEMS devices: wafer to wafer bonding is usually the current solution, however thin film encapsulation becomes a promising alternative method [1]. Nevertheless, major challenges should be overcome to develop thin film encapsulation, namely the development of a thin cap strong enough to withstand high mold pressures. Consequently, design tools are required to develop successfully thin film encapsulation [2–4]. For that, finite element models (FEM) are commonly used, and this article proposes a generic methodology based on an efficient convergence loop to fit FEM results with experimental data. Our convergence loop guarantees reliable predictive FEM results because our results are double checked with experimental characterizations: we use not only the cap geometry evolution during the process flow, but also the mechanical properties of the cap and especially its stiffness.

A study case which shows how to manage the cap deflection during the cap release operation is used to illustrate the relevance of our methodology. To recall [5], the thin film encapsulation requires closed cavities formed above the MEMS devices with surface micromachining techniques: the cavity is formed with a sacrificial layer recovered by a cap. The cap is then perforated by holes to remove the sacrificial layer. Finally, a film is deposited on the cap to seal the cap holes. In practice, the release of the sacrificial layer is one of the most critical operations because the cap can damage the MEMS device due to a buckling effect. Indeed, the residual stresses within the capping layer (compressive residual stresses are usually mandatory) and the geometry of the sacrificial layer have to be tuned in order to control the final shape of the cap.

The study case is focused on a test structure with a silicon oxide quadratic plate of 800 μm side length and 3 μm thickness. In practice, the cap geometry has been characterized with a mechanical profilometer; and, a force/displacement curve obtained by nano-indentation technique has been used to extract accurately the mechanical properties of the cap. Then, these experimental data have been used to build our FEM model.

The correlation between experimental data and FEM results allows verifying our model because we show that the simulated profile and the simulated stiffness fit successfully with experimental data. The best result has been obtained with a 60MPa compressive residual stress; and, this value is in agreement with experimental measurements. We have used our FEM model to detail the effect of several parameters like the silicon oxide thickness, the residual stresses, the height of the cap edge rolls, or the added value of reinforcement solutions as corrugated membrane or metallic layer.

Finally, we conclude that our model is an efficient design tool to optimize the thin film encapsulation. For example, it becomes possible to monitor the buckling effect of the cap by the cavity geometry or the cap material residual stresses.