The encapsulation of a MEMS device constitutes one of the last but not least step of its fabrication process from the point of view of the numerous requirements that the package has to fulfill as well as the high costs it encounters. To sort out this issue, the thin film encapsulation technique has shown interesting results for many years now and constitutes a cheap and easy solution. An improvement of this method involves porous membranes obtained by anodization which allow a fast release and no sealing depositions in the cavity.
This master project intended to develop the on-chipencapsulation of a temperature-compensated resonator using a structural self-standing porous alumina shell obtained by an electrochemical process and to seal it with AlN (fig. 1). The aimed goals were to settle an easy and low-cost experimental setup, and obtain a membrane exhibiting through pores large enough to enable the quick release of underneath sacrificial layers but presenting at the same time a high aspect ratio to prevent sealing depositions.
Therefore, the anodization technique in an oxalic acid electrolyte at RT using a hand-made setup was applied to test samples of 15 mm by 15 mm composed from the bottom to the top of 1 μm silicon dioxide, 300 nm titanium, 10 nm gold and 1.5 μ m aluminum layers. The resulting porous alumina structure was then processed to get a membrane.
We successfully fabricated on a chip a 216 μ m long self-standing porous anodic alumina shell which was sealed with an approximate 1 μ m thick AlN coating. This confirmed the feasibility of the packaging technique. The shell had a thickness of 2 μ m and the diameter of the pores ranged from 20 nm to 50 nm thus giving a high aspect ratio (>40) which theoretically should avoid any AlN deposition in the cavity (fig. 2). The fabrication process developed lead to a total undercut of 16 μ m of which 14 μ m were due to the wet etching of the Au and Ti layers and 2 μ m to the SiO2 release. The pore diameter and thus the aspect ratio could be tuned thanks to an etching in a phosphoric acid solution and the measured etch rate of the SiO2 layer with HF vapor through the membrane ranged from 7.5 μ m/h to 12 μ m/h at 60°C. Unfortunately, the encapsulation of the temperature-compensated resonators was finally not undertaken due to technical problems but the exact fabrication process was however determined and only needs to be applied.