Silsesquioxane - Silsesquioxanes For Electronics

Silsesquioxanes For Electronics

The synthesis of silsesquioxane materials for electronics applications can be quite detailed, with many variations occurring through the desired shapes of these structures as well as the different organic groups attached to these structures.

The bridged polysilsesquioxanes were developed originally to produce controlled porosity in structures. Bridging refers to structures where two or more –SiO_(3/2) units are attached by the same organic fragment to form molecular composites. These are most readily prepared from molecular building blocks that contain two or more trifunctional silyl groups attached to non-hydrolysable silicon-carbon bonds, with typical sol-gel processing. Monomers are usually dissolved in miscible solvent of water, with hydrolysis and condensation reactions catalyzed by acid, base or fluoride. The catalyst changes the physical properties of the silsesquioxane structures. Acid catalysts give clear, brittle solids, and base catalysts give opaque solids. It was found that mesopore size is proportional with the length of the bridge.

Synthesis of organosilsesquioxane films for semiconducting devices can be summarized as follows. A trichlorosilane is added drop-wise to distilled water and some non-polar solvent such as hexanes at 0 °C. Reaction left to stir for some time to allow precipitate to form, which is then filtered. Hexane is then added to the aqueous reaction medium to extract the product. This general reaction gives a basic synthesis for hydrogen silsesquioxanes. These reactions often use platinum catalysts such as chloroplatinic acid to get desired properties. Commercially available silsesquioxanes can then be modified to alkylated silsesquioxanes by the cross-metathesis of alkenes with readily available vinyl-substituted silsesquioxanes. In order to form a low k dielectric film, copolymers of alkylsilanes are copolymerized with trichlorosilane, with properties being controlled by the ratios of each. These polymers are then separated by molecular weight, since only low molecular weight polymers can be applied by Chemical Vapor Deposition (CVD) to a device. This is usually obtained by heating above the vapor pressure in a vacuum. There are also many other methods of applying these thin films for semiconductor devices such as spin coating, dip-coating, and spraying. The resulting material would have a molecular formula of _xy with x+y being an integer between 5 and 30. The methods described for forming thin films are useful in filling in empty space in electric materials as well as giving them an even surface.

There has also been interest in applying caged silsesquioxanes to these materials. Poly(methylsilsesquioxane), as mentioned above is an example of such a species. These materials give cage structures of varying sizes that are controlled by the synthetic processing. In general hydrolyzing hydrido- or organo- trichlorosilanes forms cages. Temperatures are below room temperature, and the system is kept dilute to favor intramolecular condensations. Condensation rates have also been found to slow by hydrogen bonding solvents. In general, caged structures are formed by kinetic not thermodynamic control.

In the application of light emitting diodes, there have been many more recent advances in synthetic techniques and functionalization of cubic silsesquioxanes. One of the first precursors used in light emitting application was octadimethylsiloxysilsesquioxane, which can be prepared in yields of >90% by treating tetraethoxysilane or rice hull ash with tetramethylammonium hydroxide followed by dimethylchlorosilane. The general method of hydrolyzing organotrichlorosilanes is still effective here. Recent research is looking at the effects of phenyl(silsesquioxane) structures, which can be functionalized to have a light emitting component from the organic end, through aromatic substitution reactions. When brominated or aminated, these structures can be coupled with epoxies, aldehydes, and bromoaromatics. The main goal is to attach these silsesquioxanes to π-conjugated polymer systems. Which can be done through the same functionalization methods mentioned above. These methods can use copolymerization techniques, Grignard reagents, and different coupling strategies. There has also been research on the ability of conjugated dendrimer silsesquioxanes to behave as light emitting materials. Though, highly branched substituents tend to have π-π interactions, which hinder high luminescent quantum yield.

It has been demonstrated by many research groups that chemically incorporating silsesquioxanes, can improve materials properties such as solubility, amorphousness, thermal and oxidative stability. This in turn leads to improved OLED device efficiencies and lifetimes. Whether the strategy involves linking the silsesquioxane cage to a polymer backbone to minimize aggregation, or linking active moieties to the rigid silsesquioxane core to form amorphous materials, it is clear that improved properties can be achieved.