Rapid Scan In-Situ FT-IR Curing Studies of Low-Temperature Cure Thin Film Polymer Dielectrics in Solid State

Rapid scan in-situ Fourier Transform Infrared Spectroscopy (FT-IR) was used to characterize the cure process of two common low-temperature cure thin film polymer materials for wafer-level packaging applications. Beside a discussion of the spectral changes during the cure reaction, aspects of quantification the degree of cure will be shown. First, a photosensitive low-temperature cure ester-type polyimide precursor was investigated. As this material is a negative working photosensitive polyimide precursor, the impact of the photo-crosslinking on the imidization rate will be discussed in comparison to unexposed films. It will be shown, that at certain temperature / time conditions the exposure dose should be carefully adjusted to yield fully imidized films with minimized cure temperatures. Second, the thermosetting process of a low-k polymer dielectric based on divinyl siloxane bis-benzocyclobutene (DVS bis-BCB) was studied. Due to the chemical nature of the DVS bis-BCB resin a highly crosslinked network is formed during the cure process. Especially above 80% degree of cure at temperatures below 210°C a significant reduction of the reaction rate was measured. A two step cure process was developed, which can minimize the process time at elevated temperatures and yield a rather high degree of conversion in a reasonable process time. Both polymer cure reactions are characterized by a chemically-controlled and a diffusion controlled region with significant different reaction rates. Based on the aforementioned results a time, temperature and conversion dependent kinetic / diffusion model was used to describe the experimental data quantitatively. This model allows calculating very precisely the conversion in dependence on both, temperature and time, which will help to optimize the cure process for the two thin film polymers with respect to thermal budget and / or process time. Therefore this paper shows a method, which will help comparing different thin film polymer formulations regarding cure kinetics. Modeling of the data allows optimizing the process conditions to meet the temperature requirements in the area of wafer-level packaging and 3D integration.