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In-situ FT-IR curing studies for low-temperature cure thin film polymer dielectrics in solid state
Keywords: Low temperature cure , reaction kinetics, In-situ FT-IR spectroscopy
In this study Rapid Scan in-situ Fourier Transform Infrared Spectroscopy (FT-IR) was used to characterize the cure process in thin films of two common low-temperature cure polymer materials in the temperature range between 160�C � 400�C with 4 seconds time interval. This methodology has the power to characterize the cure process in real time by following directly the chemical conversion. It delivers detailed data to describe the kinetic process by mathematical models quantitatively. In contrast to usually published results based on ex-situ methods, which only measures the final conversion, or thermoanalytical techniques, which senses not directly specific chemistry, much more details of the polymer cure process can be disclosed. First, a photosensitive low-temperature cure ester-type polyimide pre-cursor was investigated. The imidization reaction is characterized by a chemically-controlled and a diffusion controlled region with significant different reaction rates. During cure the imide ring is formed in the pre-cursor polymer film and the ester-type crosslinker is cleaved out of the material. The in-situ FT-IR measurements show that in the temperature range between 190 � 220�C the diffusion controlled reaction part has a significant influence on the final degree of imidization. At temperatures above 230�C the diffusion limited reaction part is negligible for unexposed films. As this material is a negative working photosensitive polyimide pre-cursor, the impact of the photo-crosslinking on the imidization rate was studied. It will be shown that at certain temperature / time conditions with increasing exposure dose the imidization rate and final imidization degree will be significantly affected in a negative manner compared to unexposed films. Second, the thermosetting process of the 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. The network formation has a high impact on the molecular mobility of the reacting groups, leading to a significant change in the reaction rate. Especially above 80% degree of cure at temperatures below 210�C a significant reduction of the reaction rate was measured. This is caused by a diffusion limited process in the thin DVS bis-BCB polymer film. Above 230�C the reaction is more chemically controlled, which yields rather fast a nearly full converted polymer film. Based on the aforementioned results a quantitative kinetic cure model for both polymer materials has been developed, which takes into account the conversion dependent diffusion contribution. The model is suitable to characterize the cure process of other low-temperature cure polymer films. It will help to compare different polymer formulations and to optimize the process conditions to meet the requirements in the wafer-level packaging field.
Frank Windrich, Scientist
Fraunhofer IZM
Moritzburg, Saxony

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