Chemically resistant encapsulant to enable a novel MEMS fabrication process

We have proposed and demonstrated a novel sequence in MEMS fabrication process flow. The novel MEMS fabrication process flow can be shortly described as a “packaging first, MEMS release second”, whereas a standard process starts form MEMS release and ends up with packaging. The process is explored on a 3D capacitive MEMS sensor (3 × 3 mm²). Unreleased wafer is singulated by sawing on individual dies, then the individual sensor is mounted to the package, wire bonded and encapsulated. Because the sensors are still unreleased there is no damage occurred during the assembly. However the choice for the encapsulant material is not evident. The encapsulant must survive the chemical attack during the MEMS release process (mixture of 73%HF and IPA (isopropanol)), followed by a triple rinse in IPA. We pre-selected 6 different encapsulants: a silicone-, an epoxy- and an urethane-based. At least one encapsulant passed the acceptance criteria: there is no delamination, there is no texture change and the encapsulant maintains a sufficient mechanical adhesion. Additionally to that we measured micro-hardness of the encapsulant before and after the HF release test. We also performed an electrical characterization of the flow meter sensor before and after the HF release and we detected no changes in the sensor´s performance caused by HF exposure. We have proposed and demonstrated a novel sequence for MEMS fabrication. We packaged the sensor first, and performed the release after that. The key enabler for the novel process is the encapsulant which can withstand an exposure to the release solution (73%HF:IPA).