Ageing studies of discontinuous copper and silver thin films

The results of experiments carried out on the post-deposition resistance changes in discontinuous films of copper and silver with and without overlayers of SiO and Al₂O₃ are presented. The changes in the sheet resistance of the films with time and pressure were studied for the above combinations. Mobility coalescence is assumed to be responsible for the resistance increase of an uncovered copper film of initial resistance 1.9 MΩ/□. On exposure to the atmosphere, it was found that an Ag/SiO combination of initial resistance of 0.1 MΩ/□ achieved stability in the sheet resistance much quicker than a Cu/Al₂O₃ combination of initial resistance 20 MΩ/□. The fall in resistance of the Cu/Al₂O₃ composite is attributed to the formation of Al₂(OH)₆ due to the interaction of Al₂O₃ with the water vapour in atmosphere. Copper films with and without overlayers of Al₂O₃ show an abrupt increase in the sheet resistance as a function of pressure at a pressure of about 5 × 10^-2 torr with the maximum rate of change of resistance occurring at higher pressure for the higher resistance film. This indicates that the overlayer of Al₂O₃ is very porous in nature. Field effect studies were carried out on an uncovered copper film of initial resistance 10 MΩ/□ and the behaviour was found to be ohmic up to a field of 800 V cm^-1.