Synthesis of MFI Zeolite Mesoporous Low-k Films Using Poly (ethylene oxide)-based Surfactants as Template

The synthesis of coating solutions with pure-silica-zeolite (PSZ) of MFI structure for low-k (dielectric constant) films applications was reported a few years ago. Although the k values can be around 2, the morphology of the films was poor. In order to solve this problem and to increase the mesopore volume in the films, the poly (ethylene oxide)-based non-ionic surfactants (such as Tween80, Tween60, Tween40 and Tween20) were added in the coating solution (with PSZ and MFI structure) in this research. The weight ratio of surfactant to tetraethylorthosilicate (TEOS, a silicon source) is 0.41. Two types of coating solutions with small crystalline particles or with large crystalline particles prepared through hydrothermal process were studied. The physical properties (dielectric constant, elastic modulus and harness) of the low-k films were characterized to make a comparison. Moreover, by burning away Tween in the films, the mesopores formed and the pore volume in the films should be increased, which may lower the k values of the films. Therefore, the nitrogen adsorption/ desorption measurements on different films were carried out at 77 K to characterize the effect of different Tweens on the pore volume and pore sizes in the low-k films.
It was found in this research that the films synthesized with the addition of Tween80 had the lower k value and the higher mechanical strength (Elastic modulus and hardness), as compared with those prepared with the addition of the other types of Tween. Nitrogen adsorption/ desorption results indicate that the total pore volume is consistent with the molecular size of Tween; in other words, a larger molecular size of Tween possesses a higher total pore volume. However, the dielectric constants are not consistent with the porosity (total pore volume) variation of the low-k films. In order to address the phenomena, TGA/ DTA results were provided to study the interaction between hydroxyl groups and surfactant molecules or surfactant micelles in the coating solutions. The results suggest that the interaction result in that the hydroxyl groups would be located in the position where is hard to be modified to become hydrophobic after the surfactant were removed. Therefore, the k values would be influenced.
The mechanical strength was also affected by the interactions between hydroxyl groups and the surfactant. From the results of 29Si solid state NMR spectra, a higher interaction would limit the polycondension reaction between hydroxyl groups during the stirring step. As a result, the low-k films would have a more completed polycondensation reaction among the hydroxyl groups after the calcinations, and the low-k film possessed a higher mechanical strength.
All of the low-k films synthesized with the addition of poly (ethylene oxide)-based surfactant met the requirements of IC industry; the low-k value were lower than 2 or around 2, elastic modulus were of > 10 GPa and hardness were of > 1 Gpa, and the values of leakage current density were under the order of 10-7 (A/cm2).