Cure rate and dry etch patterning of thermoset polymers

Abstract

High quality thermoset polymer solutions are available from several commercial suppliers. These are suitable for forming thin films for optical waveguides because of their high transmission, suitable refractive indices and thickness uniformity of films obtained by spinning solutions on a substrate such as silicon. The solution's viscosity and the spinning speed determine the thickness of resulting films. The plasma etch rate was examined for trenches (of the order of 1 μm depth, suitable for photonic waveguide fabrication) formed in such films from a relatively high viscose polymer solution (UV15 from Master Bond). The cross-link density of the polymer is dependent on its curing process, that is, the exposure to ultra-violet light radiation and heat. The curing process can have a profound affect on the etch rate. Different paths were taken in the curing and etching process of the spun polymer film in order to examine the relation between the cross-link density and the polymer etching process. We examine the polymer films using FTIR to qualitatively measure the cross-link density and DSC for changes in the glass transition temperature (Tg). FTIR is used to show if chemical changes occur for different levels of curing. Determining Tg is important because this will show how it changes with the baking step. We have observed that for films with low Tg, the plasma etch process can cause the polymer surface to flow and hence wrinkle. Post-curing at higher temperatures increases Tg. By choosing appropriate curing steps, Tg and the etch rate can be optimized to obtain an optimum etch rate and preserve the smooth polymer surface during etching.