MECHANICAL-STRESS IN MICROMACHINED COMPONENTS CAUSED BY HUMIDITY-INDUCED INPLANE EXPANSION OF THIN POLYMER-FILMS

Thin polymer films have found widespread application in silicon-based microelectronic devices and micromechanical systems due tc, their uniq ue material properties. When exposed to ambient humidity polymers tend to expand due to the uptake of moisture in the film. In most microele ctronic applications. this characteristic is rather undesirable due to associated reliability problems. We have investigated humidity-depend ent mass uptake and expansion for a range of polymers at room temperat ure and have found that the isotherms are generally nonlinear. Based o n the obtained relationships. the polymer process can be optimized in viers of reduced mass uptake and reduced mechanical stress in the poly mer. Results are given for polyimide PI2540. Both mass uptake and swel ling can be approximated by a linear humidity dependence with an error of about 10%. Based on these results, the empirical quantities swelli ng ratio and stress coupling factor are defined, and linearized sorpti on parameters are discussed. (C) 1998 Elsevier Science S.A.