Wafer-Level Bonding using Anisotropic Conductive Adhesive for 3D MEMS Applications

Abstract

As opposed to planar ICs, micro-electro mechanical systems (MEMS) are three dimensional structures with movable parts, membranes and cavities. Fragile movable structures are generally encapsulated inside a cavity by bonding a cap wafer to a MEMS device wafer. The use of through silicon vias (TSVs) in MEMS allows a significant miniaturization of the devices. TSVs through the cap wafer require an electrical connection between the TSVs and bond pads on the device wafer. This paper presents a novel bonding method that allows to do this in a simple and cost-effective way. We demonstrate the use of Benzocyclobutene (BCB) containing metal coated monodisperse polymer spheres (MPS) as an anisotropic conductive adhesive (ACA) for wafer-to-wafer bonding. The technology is demonstrated by bonding glass cap wafers with cavities and a patterned metallization to silicon wafers with a patterned metallization. The ACA is spray-coated onto the cap wafer and the bonding is performed in vacuum at 250 °C for 60 minutes with 300 mbar pressure applied. The electrical performance is evaluated using daisy-chain and kelvin structures, and the bond strength is verified by shear testing. Variations in bond frame size, contact area and minimum distance between conductor lines are investigated. Four wafers with a total of 880 dies are electrically tested. An average resistance of34 ±14mΩ is obtained for a contact area of 300 × 300 μm2 and 78 ±55 mΩ for 200 x 200 μm2. Daisy chains with 16 contact areas have a yield of 98 % for a 300 × 300 μm2 contact area and 95 % for 200 x 200 μm2. The technology is expected to be scalable to smaller contact areas by increasing the amount of MPS in the BCB. An average bond shear strength of 36 ± 9MPa is measured.