Mechanical dynamical analysis of ultra thin resist films for microlithography applications

Abstract

In this paper we investigate an in situ measurement of polymer and lithographic resist film mechanical properties on a silicon substrate, with a Dynamical Mechanical Analysis tool (DMA). This technique allows the measure of the glass transition temperature (Tg) of the resist film (thickness range: several μm to few nm) and its elastic and viscous modulus variations with a high precision and reproducibility. Indeed, DMA appears to be more sensitive than other thermal analysis methods like Differential Scanning Calorimetry (DSC), to monitor Tg variations induced by film thickness reduction. First we will discuss the performance of the tool and present the variations of the glass transition temperature of a PMMA (PolyMethylMethAcrylate) layer as a function of its thickness: we observe a shift towards higher temperatures up to 30°C when the film thickness decreases from one micrometer to 10nm. This behavior highlights the importance of surface properties versus bulk. We will also discuss the interest of the DMA technique applied to more complex chemistries, as it is the case for lithographic resist formulations, i.e. a blend of polymer with grafted functionalities, photoactive compounds and various additives. We successfully applied this technique to characterize different resist film thicknesses and we observed the effect of the thickness on the thermal events. Such kind of change is important to take into account in the optimization of material performance for thin film applications. This material understanding allows to better define the process conditions and can be applied to different microelectronic topics such as: thermal flow treatment of positive tone photo resists, hot-embossing nanoimprint, cross linking reactions with negative tone resists or so called hardening processes.