Modelization of structural changes in ultra low k materials during ultraviolet cure

摘要

As of 45 nm node, ultralow k (ULK) materials (ε_r ＜ 2.55) are widely used in microelectronic interconnects to reduce signal propagation delay. In order to get such low dielectric constant, most of these ULK are obtained using subtractive method: after co-deposition of matrix and porogen a UV cure process is used to remove labile species (porogen). During this process, porosity is created and films are densified as shrinkage increases. This paper presents an in-depth study of the UV cure process. Two kinetic models are presented to describe shrinkage rate: the nth order and the autocatalytic model. These models also used to describe photo-polymerization shrinkage-strain of dental composite, gives predictions that are in good agreement with experimental data. We find that nth-order model is best suited to describe ULK transformations during UV cure process. CH_x bonds loss and carbon content can also be modeled using same kinetic model. In order to be able to build the model, initial SiOCH-C_xH_y film has to be over-cured to determine the maximum conversion that film can reach when submitted to extra-long cure times. Results show that such SiOCH materials have to be cured more than 250 times their nominal cure time to reach maximum shrinkage (～50%) and a complete loss of carbon, making them looking like SiO_2. When structural changes are observed at different temperature, activation energy of reactions can be determined. In this way, shrinkage reaction has an activation energy close to 1000-1100kJ/mol and CH_x bonds or carbon loss reactions have activation energy in the range of 250-400 kJ/mol, which is pretty close to C-H binding energy.