From single molecules to micelles – An in situ study of porogen aggregation and nanopore formation mechanisms in porous thin films

The generation and formation mechanisms of nanopore structure using a degradable porogen, 5,11,17,23,29,35-hexa-tert-butyl-37,38,39,40,41,42-hexa-acetoxyl calix[6] arene (CA[6]), in a silsesquioxane (SSQ) matrix have been studied by means of ellipsometric porosimetry (EP) and in situ positron annihilation lifetime spectroscopy (PALS). A clear transition of the final nanopore structure resulting from a transition from molecularly dispersed porogen to micelle-like assembled porogen domains was observed in the range of 13–16% CA[6] porogen loading. By utilizing PALS measurements during in situ heating of hybrid nanocomposite samples, the curing temperature for decomposition and removal of the porogen to produce nanopores is completely different for samples where the porogen is dispersed as single molecules as compared to those where assembled porogen domains are present. It is determined that this is largely due to inherently different paths for diffusion of the degraded porogen fragments. This methodology for investigating pore generation mechanisms in porous low-k films can be used as a characterization tool for a wide range of thermally degradable porogen systems.

In this paper, the generation and formation mechanisms of nanopore structure using a calixarene (CA[6]) based porogen in a silsesquioxane (SSQ) matrix have been studied in two complementary ways: (i) mesopore characterization by EP and PALS of relatively low porosity (<20%) films with a fine porogen loading gradient and (ii) PALS analysis during in situ vacuum thermal curing of hybrid samples with a molecularly dispersed state (porogen loading; 10%) and micelle-like assembled state (porogen loading; 20%). The requisite curing temperature for nanopore generation is completely different between molecular dispersed porogen (420 °C, see Fig. (a)) and assembled porogen domains (300–373 °C, see Fig. (b)). It is suggested that the highly interconnected micelle-produced pores provide their own readily accessible diffusion paths to remove the degraded porogen fragments whereas molecularly dispersed and isolated porogen domains retain the fragments until the matrix is cured to a high enough degree to allow for diffusion of porogen fragments.Figure optionsDownload as PowerPoint slideHighlights
► The generation and formation mechanisms of nanopore structure are studied.
► Novel methodology for investigating pore generation mechanisms is suggested.
► EP and in situ PALS are adapted in this methodology.