Influence of ion-beam induced chemical and structural modification in polymers on moisture uptake

Polyimide thin films are a promising material for microelectronics and aerospace applications. In particular, they are sensitive to moisture and gas uptake. This leads to film swelling which may be monitored by a corresponding change in piezoresistance caused by plate bending of a polymer–silicon double-layer or bimorphic sensor, respectively. However, the expansion of the polymer, induced by moisture or gas uptake, can usually be influenced by surface ion-beam modification. By this, the selectivity to a partial gas may be enhanced or decreased. In this work, the influence of ion-beam induced surface modification on both polymer structure and moisture uptake of polyimide and polyethersulfone is investigated. To modify the polymer layer surface boron ions were implanted with energies from 50 to 180 keV and irradiation doses between 1013 and 1016 B+/cm2. It could be shown that increase of irradiation dose leads partly to a destruction of the imide and aromatic groups. The aromatic structure is degraded by hydrogen abstraction. This corresponds to the creation of a new amorphous and graphite-like structure, which increases the modified surface film conductivity by several orders of magnitude, and which decreases the Freundlich's coefficient of the moisture-uptake behaviour.