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

An adhesive bonding process is presented that utilizes sub-micrometer thick bondlines of all-aromatic thermosetting copolyesters (ATSP) for the assemb ly of polyimide membranes in silicon-based sensors and actuators. Due to th e unique ability of ATSP to form void-free self-adhesive bonds through soli d-state interchain transesterification reactions, sub-micrometer adhesive b onding technology offers new avenues for the precision assembly of high-per formance, three-dimensional microscopic and mesoscopic devices. As a model process, PMDA-ODA polyimide membranes, 4-6 It in thick, are fabricated on g lass carrier substrates, selectively bonded, transferred, and assembled on bulk-micromachined silicon substrates in the fabrication of mesoscopic circ ular diaphragm structures, 2-8 mm. in diameter. Experimental load-deflectio n behavior of adhesively bonded polyimide diaphragms demonstrate that assem bled 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 therm ally 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 t he debond energy measured for a PMDA-ODA polyimide film deposited directly on a silicon substrate with a native oxide surface.