Positron annihilation lifetime (PAL) spectroscopy was used to study the effect of water uptake on the free volume in 6FDA-ODA polyimide (PI2566, DuPont). Orthopositronium (o-Ps) lifetime (τ 3 ) and intensity (I 3) data were analyzed assuming a Gaussian size distribution of preexisting (excess free volume) holes and further assuming that holes occupied by water molecules would not be detected in the measurement. When exposed to humidity, both the mean hole volume and the number of holes not occupied by water molecules decrease with increasing relative humidity, as indicated by decreasing τ 3 and I 3. Our PAL results correlated with humidity-induced mass uptake and volume expansion support a model according to which water absorption in polyimides occurs in two stages. At relative humidities smaller than about 30%, water is absorbed mostly in large, preexisting holes, with each hole typically occupied by a single water molecule. At larger humidities, an increasing fraction of the sorbed water molecules will occupy sites other than preexisting empty holes. A multiple occupation of larger holes by water molecules is discussed as a possible mechanism.

Polymers are currently considered as a possible alternative to silicon dioxide in the fabrication of interlevel dielectrics. To penetrate mainstream semiconductor device fabrication polymers have to meet a number of requirements regarding their long-term stability. One aspect is the mechanical stability of integrated polymer films under changing climatic conditions. In the present work, the impact of ambient moisture on the mechanical properties of thin polymer films (PI, BCB, and PFCB) was investigated. The sorption of water molecules in these materials typically causes an anisotropic volume expansion, resulting in increased mechanical film stress if the film is physically constrained by adjacent inorganic structures. Especially polyimides show both considerable moisture uptake and large changes in the mechanical film stress, while BCB and PFCB are virtually insensitive to ambient moisture. In the paper, experimental data (water uptake, in-plane swelling, out-of-plane swelling) are presented and discussed in detail.

Polyimide films are now finding application in microelectronics and microelectromechanical systems technology. One problem with polyimide films is their tendency to adsorb water from ambient humid air, which can cause significant changes in the mechanical and electrical behavior of these films and even lead to their failure. To find ways to minimize water adsorption in polyimide films, we have studied different types of polyimides by both analytical and phenomenological means. It appears that the process of water uptake is by sorption of water molecules in micropores inside the polymer. The sorption of water in thin polymer films typically results in volume expansion. Past a certain saturation level, each sorbed water molecule causes an expansion of about 15 A 3 , which is about half of the effective volume of the water molecule. If the expansion of the polymer film is restrained by adjacent inorganic layers, the result is humidity-induced compressive stress in the polymer film. We have found that the amount of water uptake and the resulting anisotropic swelling depend strongly on the particular type of polyimide. In this paper, new experimental results are presented and discussed in detail.