Stress in thermally annealed parylene films

Abstract

Stress is a problem for thin film Multichip Modules (MCM) fabrication and reliability. Stress can cause adhesion failure, wire cracking and fracture of the polymer. Parylene can be deposited in a low compressive stress state, and here we explore its temperature-stress behavior. The stress generated by thermal coefficient of expansion (TCE) mis-match between thick parylene-n (PA-n) films (4-7 μm)and Si substrates has been studied as a function temperature. Measurements of stress were made on as-deposited films; after thermal annealing and also in-situ. The as-deposited films are under compressive stress of 20 MPa. As the films are annealed, the stress decreases and then becomes tensile, peaking at about 53 MPa. It was observed that stress was reduced in PA-n films during the crystal phase transformations at around 200 and 270°C. The stress has been compared to reported data from other sources and found to agree. Stress in PA-c (chlorinated), and co-polymerized PA-n ande (ethyl) films, and a fluorinated parylene (PA-f) are also evaluated. The effective glass transition temperatures and the linear coefficient of expansion have also been estimated.