Physical properties and structure of thin ion-beam modified polymer films

In order to increase the sensitivity to moisture uptake of polyimide (PI) and polyethersulfone (PES) films applied in bimorphic humidity sensors, boron ions with an ion energy of 140 and 180 keV and with irradiation doses between 10(13) and 10(16) B+/cm(2) were implanted. A complex investigation of the following features has been carried out: chemical changes in the surface regions by X-ray photoelectron spectroscopy (XPS); density and Young's modulus by surface acoustic waves (SAW); conductivity of modified polymer films. It was shown, that the partial destruction of chemical bonds under ion bombardment leads to the creation of amorphous carbon islands, which increase the surface film conductivity by several orders of magnitude from an insulating to a semiconducting region and enhance the sensitivity of these polymer films to moisture uptake. The radiation-induced cross-linking results in an enhancement of the Young's modulus of the polymer layers up to a factor of 6-9. The density of the modified films increases from an as-prepared value of 1.37-1.44 to 1.8-2.2 g/cm(3). The changes in structure and physical properties of polymer films are discussed in dependence on irradiation conditions for the PI and PES films. Conditions for optimal sensor properties were evaluated. (C) 2003 Elsevier B.V. All rights reserved.