Characterization and Preparation of Microporous Non-crystalline Silica Low-k Films

In our previous study, coating solution containing non-crystalline silica for preparing mesoporous low k film with k value below 2 and high mechanical strength has been made. However, microporous low k films is necessary for future needs. Therefore, the goal of this research is to make microporous low k film with high mechanical strength. In this research, pore size distribution (PSD) and pore volume of powders made from coating solution by evaporation method or scraping method were measured by argon adsorption/desorption and analyzed by non local density functional theory (NLDFT) simulation. Coating solution consist of synthesized silica, tetrapropylammonium hydroxide (TPAOH) and surfactant. TPAOH was used as a structure directing agent to synthesis MFIlike microporous silica during hydrothermal process. Surfactant added in the solution can form pores between silica particles after the calcination. As the porosity increases, dielectric constant of the film should decrease. In order to increase the micropore volume, the molar ratio of TPAOH to TEOS (tetraethyl orthosilicate) was increased from 0.15 to 0.36. Nevertheless, the micropore volume was not successfully increased. Instead, the surfactant interacted with silica strongly, resulting in destroying micropores of MFI structure. On the other hand, because increasing hydrophilicities of silica and surfactant (or porogen) can increase the interaction between these two, decrease the formation of micelles and mesopores, and resulting more micropores, shortening hydrothermal period (less than 24 hours) was adopted to manufacture more hydrophilic silica. It was found that micropore volume was increased and mesopore volume was decreased successfully. However, that from too short hydrothermal period (i.e., 6 hours) was not, may be because synthesized silica would maintain amorphous structure from incomplete reaction between TEOS and water. In this research, the low k film with the largest micropore size of 13.6Å has been produced from the coating solution containing silica (synthesized from 12 hours hydrothermal process) and the surfactant with a reduced amount (i.e., ratio of surfactant to TEOS decreases from 0.41 to 0.1). In order to increase the micropore volume, poly(ethylene glycol) with molecular weight 200 (PEG200) acting as hydrophilic porogen was added to coating solution. It was found that the largest micropore increased to 17.3Å and the porosity increased to 0.31 or 0.32, which is close for making low-k value less than 2. Nevertheless, the measured k values are all well above 2, which are not suit for the future needs of IC industry. One of the reasons may be because of the non-uniform coating. Although it can be solved by waiting 5 seconds after dispersing coating solution to wafer before the spinning, the k values are still unexpectedly large.