Liquid Encapsulation by Bonding-in-Liquid Technique

Abstract

AbstractWe propose and demonstrate Bonding-in-Solution Technique (BiST) for encapsulation of liquid in MEMS devices. Liquid encapsulation enables innovative MEMS devices with various functions, such as hydraulic displacement amplification and scanning mirrors. Interfusion of air bubbles and leakage of the encapsulated liquid must be averted not to deteriorate device performances. Several liquid encapsulation processes have been proposed, such as parylene deposition and polymer thermal bonding. However, they involve vacuum and/or thermal processes and cannot be applied to volatile liquids. In BiST, two structural layers are passively aligned and brought into contact in solution, where the encapsulation cavities are uniformly filled up with the liquid without air bubbles. A UV-curable resin is used as an adhesive that does not require heat or vacuum environment but UV to bond the two layers. The detail processes of BiST are (a)UV-curable adhesive resin is coated onto the bonding surface of a structural layer. The layer contains not only encapsulating cavities but also concave-shaped structures for the following passive alignment. (b) The other layer with convex-shaped structures is brought into contact in solution, when the two layers are passively aligned by matching concave and convex structures. (c) UV light is irradiated and the two layers are permanently bonded while the contact is maintained by the jig. We successfully achieved encapsulation of DI water and glycerin in PDMS and silicon structural layers. No liquid remains in the bonding interface. Since conventional aligners are not applicable to BiST, we experimentally evaluated the accuracy of the passive alignment process in solution that makes use of matching concave and convex structures. We used a PDMS layer with cylinders (concave) and a silicon layer with cavities (convex) to evaluate the alignment in BiST. The height and depth of the cylinders and cavities are designed such that the PDMS cylinders elastically deform when the two layers are brought into contact. The elastic averaging enables the passive alignment of the two layers. We investigated the bonding accuracy with respect to the number of pairs of concave/convex structures and the height of PDMS cylinders while in contact. The bonding accuracy improved as the number of pairs increased while the height of PDMS cylinders did not show a correlation. The alignment accuracy of 5μm in BiST was achieved with 12 pairs of the concave/convex structures. The ultimate goal of our research is to develop innovative MEMS devices with encapsulated liquid, such as a hydraulic displacement amplification mechanism applicable to a tactile display. Glycerin was encapsulated by largely-deformable-PDMS thin membranes and silicon cavities by using BiST. The displacement at the input was successfully amplified at the output associated with the ratio of the cross-sectional areas.