Thin Film Solid State Proton NMR Measurements Using A Synthetic Mica Substrate: Polymer Blends
David L. VanderHart, Vivek M. Prabhu, Kristopher Lavery, Cindi L. Dennis, Ashwin Rao, Eric K. Lin
We demonstrate that a synthetic fluorophlogopite mica can be used as a proton-free, diamagnetic substrate for examining solid thin-film samples using conventional solid-state proton nuclear magnetic resonance (NMR) experiments. The context of this work extends our previous NMR studies from bulk samples to thin films. We have previously used NMR to characterize the intimacy of mixing in bulk glassy photoresist formulations that are relevant to the microelectronics industry. In application, however, photoresists are exclusively used as spin-coated thin films. Since observations for bulk samples do not necessarily carry over to thin films, we sought an NMR-friendly substrate for performing similar proton NMR experiments on glassy thin films. Proton detection and multiple pulse methods were chosen in order to maintain sensitivity. Two natural micas as well as the synthetic mica were each evaluated as possible substrates for spin coated films where the mica occupied over 98 % of the sample volume. A particularly attractive feature of mica is its layered character which permits the preservation of some level of sensitivity via the use of very thin mica sheets. However, the strong paramagnetism of the natural mica thwarts attempts to characterize the polymer thin films due to the strong associated spinning sidebands and linebroadening under magic angle spinning conditions. While we were able to demonstrate some reduction in these unwanted characteristics using oriented samples, the residual level of these effects still caused too much ambiguity in the results. Use of the synthetic mica, on the other hand, proved very successful, showing no deterioration in resolution and only a very slight increase in spinning sideband intensities. Investigations of the intimacy of mixing in a commercial polymer blend and in a more difficult photoresist blend as 250 nm-thick films are reported. It also emerges from these examples that the mica is truly proton free; yet, for a successful experiment designed to investigate the intimacy of mixing, one must pay scrupulous attention to excluding impurities during sample preparation and handling. In addition, one must also be careful to correct for other proton background signals associated with the empty probe. This synthetic mica should also be a very useful, NMR-friendly substrate for many other types of solid-state thin-film investigations. Finally, in order to provide a firmer basis for interpreting our observations, independent magnetic susceptibility measurements, including in-plane and out-of-plane anisotropies, were performed on all micas used.