Modeling the etching rate and uniformity of plasma-aided manufacturing using statistical experimental design

The response characteristics of an O-2/CF4-based plasma process used to desmear and etch back multi-layer rigid-flex printed circuit board were examined using a two-level fractional factorial experimental design. The effects of variation in RF power, temperature, gas proportion and gas flow (CF4 and O-2) on several output variables, including etch rate, process uniformity and selectivity were investigated. The screening factorial experiment was designed to isolate the most significant input parameters. In the experiments conducted, increases in the etching rate generally corresponded to decreases in uniformity. Etch uniformity was strongly dependent on temperature and gas proportion. The relative significance of polymer deposition and ion bombardment was separated. Using this information as a platform from which to proceed, the subsequent phase of the experiment developed empirical models of etch behavior using response surface 3-D plots. The models were subsequently used to optimize the etching process.