Low cost Thin Film packaging for MEMS over molded

Abstract

Thin Film packaging (TFP) is now well known at CEA/LETI and mainly used as a protection for MEMS against degradation which can occur during back end processes: TFP is strong enough to endure thermo-mechanical constraints due to grinding, handling and protects the device from water during the sawing step. Our TFP process is also compatible with under bump metallisation, balling and flip chip processes. The main advantages of our TFP is a very low lost of silicon area, a low cost process with 3 mask levels, and is performed on equipments commonly used in IC fabs. In this paper we will speak about process improvement for a TFP compatible with over molding. The thermo-mechanical constraints due to the standard over molding step (100bar and up to 200°C) are much more challenging for TFP: the cavity is about 5 μm high, the cap layer 2μm thick and the polymer plugging layer 6Aμm thick. So TFP needs to be reinforced to withstand these high constraints. Two processes have been developed at wafer level with two materials. The aim of these processes is to keep compatibility with standard back-end manufacturing technologies with no increase of the die size. 200μm and 500μm wide cavities with TFP were reinforced with these processes and first tested under pneumatically pressure at room temperature: in case of contact between the cap and the substrate, a short circuit is measured between one electrode on the substrate and another electrode below the cap. Then, the same devices were over molded at 75bar and 100bar at 185°C. In the same run, BAW resonators with TFP and one type of reinforcement were overmolded at 100bar. The electrical performances of these resonators after over molding fit very well to the modeling of the test card and did not show degradation of RF characteristics. The main expected advantages of this technology are a reduction of final size and final thickness of the packaged devices. The compatibility between TFP and over molding constraints could make it a cost effective solution for MEMS packaging.