Role of the modifications induced by ion beam irradiation in the optical and conducting properties of polyimide

Ion beam irradiation produces drastic changes in the polymer properties, which result from the important rearrangement of the chemical bonds and the loss of the more volatile elements. The increase of the optical absorption in the visible and the near-infrared and the appearance of an electrical conductivity in polyimide films have been studied. A strong increase of the absorption is observed for doses between 1014 and 1015 Xe+ implanted at 700 keV, without induced conductivity changes, which may be explained by the formation of carbonized islands along the tracks of the incident particles. The optical absorption is calculated using the Maxwell-Garnett theory and the effective medium theory, which give good agreement for a filling factor of 40% of graphite inclusions embedded in the polymeric matrix. A strong conductivity increase (18 orders of magnitude) takes place for higher ion fluences. The critical dose for this insulator-metal transition is a function of the incident energy (2x1016 Xe+ at 700 keV) and the conductivity level a function of the beam flux. Technological implications for interconnections in microelectronics are noted. (orig.)