Sub-micrometer solid-state adhesive bonding with aromatic thermosetting copolyesters for the assembly of polyimide membranes in silicon-based devices

Abstract

An adhesive bonding process is presented that utilizes sub-micrometer thick bondlines of all-aromatic thermosetting copolyesters (ATSP) for the assembly of polyimide membranes in silicon-based sensors and actuators. Due to the unique ability of ATSP to form void-free self-adhesive bonds through solid-state interchain transesterification reactions, sub-micrometer adhesive bonding technology offers new avenues for the precision assembly of high-performance, three-dimensional microscopic and mesoscopic devices. As a model process, PMDA-ODA polyimide membranes, 4-6 µm thick, are fabricated on glass carrier substrates, selectively bonded, transferred, and assembled on bulk-micromachined silicon substrates in the fabrication of mesoscopic circular diaphragm structures, 2-8 mm in diameter. Experimental load-deflection behavior of adhesively bonded polyimide diaphragms demonstrate that assembled membranes exhibit a tensile residual stress of 19 MPa, a value roughly equal to that measured for a PMDA-ODA polyimide film (derived from a thermally imidized polyamic acid precursor) deposited directly on silicon. Using a standard blister-type peel test, the debond energy range of an assembled polyimide membrane is shown to be 15-23 J m-2, approximately 15-25% of the debond energy measured for a PMDA-ODA polyimide film deposited directly on a silicon substrate with a native oxide surface.