In a multilevel interconnection technology, propagation delay due to parasitic capacitance is one of the important issues to be solved. The delay becomes more serious in advanced integrated circuits (ICs) as interconnect dimensions are scaled down. To reduce the delay, various materials with low dielectric constant have been investigated.
SiOF thin film is fluorine doped silicon oxide. SiOF thin film is one of attractive materials for a low dielectric constant inter-metal dielectric layer. It has some strong points that the fabrication process is similar to that of $SiO_2$, which is currently used for the inter layer dielectric materials although its dielectric constant is slightly higher than those of other low-dielectric materials.
However, SiOF thin film easily absorbs water and its properties degrade when it is exposed to air. It is well known that the high reactivity of Si-F or $Si-F_2$ bond causes chemical reaction and results in the formation of Si-OH bond, which has high polarizability and makes the dielectric constant increase. There have been many researches on the chemical reaction of Si-F bond with water.
In this study, we measured the variation of residual stress to expect the water absorption behavior. The variation of residual stress is known to be very sensitive to the water absorption. SiOF film with high fluorine concentration shows instability of residual stress while it has lower dielectric constant than that of film with low fluorine concentration. It is also confirmed that there is no or little change of chemical bonding structure, Si-F and Si-OH, during the water absorption. We expected that the water absorption occur by physical adsorption. To clarify the reason of the water absorption, the film was dried after the water absorption. The residual stress returned to its initial value. We found that the water absorption occurs considerably although no chemical reaction occurs between Si-F and $H_2O$. It is suggested the water absorption be ...