Methyl silsesquioxane

High-resoIution proton and silicon NMR has been used to study structure formation in solution mixtures of ethylene oxide/propylene oxide triblock copolymers and methyl silsesquioxane. These mixtures are precursors to ultra low dielectric constant filim used in the fabrication of integrated circuits. The solution NMR results show that micelle formation is suppressed during solvent casting and curing of the films, and that miscibility is enhanced by the interactions of both the ethylene oxide and propylene oxide blocks of the triblock copolymer with the methyl silsesquioxane matrix. 

Proton NMR in solution has been used to study composite formation in mixtures of ethylene oxide/propylene oxide triblock copolymers and methyl silsesquioxane. The porous films for low constant applications are prepared (Figure 1) by mixing the polymers and methyl silsesquioxane in butanol followed by spin casting (5). The film is then heated to 120°C to condense the methyl silsesquioxane into a relatively rigid network. After the matrix has been cured, a high temperature treatment (> 400 °C) is used to remove the polymer and obtain the final porous film. 

Figure 2. The solution (left) and solid-state (right) silicon NMR spectra of methyl silsesquioxane before and after heating to 12(fCfor 1 hour.

The low-k films are prepared by solution casting the methyl silsesquioxane and the triblock copolymers mixtures from butanol. The size and distribution of polymer domains in the film will depend on the degree to which the polymer self-associates. Figure 4 shows the solution proton NMR spectra of the P103 triblock copolymer (EOirPOgo-EOn) mixed with methyl silsesquioxane in butanol-dio. The important feature to note is the narrow line widths for the polymer methyl and main chain resonances. Both sets of peaks are narrower than the broad peaks shown in Figure 3 for the triblock... 

Figure 4. The solution proton NMR spectrum of the 50 50 methyl silsesquioxane P013 mixture in butanol-djo-...

The results for the butanol solution show that micelles are unlikely to form in the initial solvent mixture. As the film is cast and heated, the environment for the polymer changes substantially as solvent is driven off and the hydroxyl groups on the methyl silsesquioxane are condensed. To study the effect of the solvent loss we prepared a neat sample of the methyl silsesquioxane and the LlOl triblock copolymer (EO4-PO59-EO4). We were able to study the solution structure of the neat mixture because the LlOl has a low ethylene oxide content and is liquid-like at ambient temperature. Figure 5 compares the solution spectra of the neat LlOl, the neat methyl silsesquioxane and the 50 50 mixture. Again we note the narrow lines for the polymer peaks, suggesting that micelle formation is also suppressed in the neat mixture with methyl silsesquioxane. 

Figure 5. The 500 MHz proton NMR spectra of (a) neat methyl silsesquioxane, (b) the 50 50 methyl silsesquioxane L101 mixture, and (c) neat LlOl, Residual solvent lines are marked (s).

Figure 6. The 2D exchange spectrum of the 50 50 mixture of methyl silsesquioxane and LI01 obtained with a 0.25 s mixing time. The solvent lines are marked (s).

Ethylene oxide/propylene oxide triblock copolymers have been successfully used to template pore formation in ultra low-k films, and dielectric constants as low as 1.5 have been observed with polymer loading levels of 50 wt% (5). These films have good mechanical properties, a high breakdown voltage and a low moisture uptake. We have characterized the films with high-resolution solid-state proton NMR and found that the triblock copolymers form nm-sized core-shell structures with the propylene oxide block at the interface between the ethylene oxide block and the methyl silsesquioxane matrix (14). 

In these studies we have used solution NMR to study structure formation and the intermolecular interactions between the polymer and the matrix during the cure that affect the miscibility. The triblock copolymers form micelles in aqueous solution with the propylene oxide block at the center and the ethylene oxide block at the exterior (6), and micelle formation can be monitored via the proton linewidths. The NMR studies show that the triblock copolymers do not form micelles in the butanol solutions used for solution casting films of the low-k dielectrics, or in neat mixtures of the triblock copolymers with the methyl silsesquioxane. The methyl silsesquioxane starting material contains a... 

Kohl, A.T., et al., 1999. Low k, porous methyl silsesquioxane and spin-on-glass. Electrochem. Solid State Lett. 2 (2), 77-79. 

Low molecular weight versions usually have abundant chain end functionality, although reaction procedures have been developed to promote intramolecular condensation leading to soluble low molecular wei t SSQ derivatives with relatively little SiOH fimctionality." Here we haA% studied three different low molecular weight resins, two were pure methyl silsesquioxane (MSSQ) while die third was a copolymer containing 25-35% of SiOa linkages which inqirove the mechanical properties of the resin. 

A number of different types of nanoporous materials for use in low dielectric constant iq lications have been developed in recent years, including nanoporous silica, polyimides, poly(arylefhers), and poly(methyl silsesquioxanes). Recently, much research has been done in die field of siqiercritical carbon dioxide (SCCO2) and its use in the synthesis of polymers for microelectronic applications. A variety of different methods using supercritical CO2 to form micro- and nanoporous materials towards applications in the microelectronic industry are described. 

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