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Silsesquioxanes

Background

Silsesquioxane in cube form

Silsesquioxanes have a cage like structure which is most commonly cubes, hexagonal prisms and octagonal prisms.

In recent years, silsesquioxanes have become commonly known as polyhedral oligosilsesquioxanes (POSS), which is a trademarked name by Hybrid Plastics. Hybrid Plastics was formed by Dr. Joseph Lichtenhan and his colleagues in 1998 and since has been a large catalyst in expanding the field. This is largely due to Hybrid Plastics making many silsesquioxane feedstocks commercially available. (http://www.hybridplastics.com) Addidtionally, a competitor Mayaterials was founded in 2003. This companies objectives are “to commercialize coating technologies based on octasilsesquioxanes of the type [ROSiO_1.5]₈ and its polymeric derivatives, and [RphenylSiO_1.5]₈ and its polymeric derivatives.” (http://www.mayaterials.com/).

Since their initial discovery, silsesquioxanes have been a frequent and productive topic of research that has become interwoven into many fields of science. This has been proof of their potential for many applications, with an amazing amount of variation possible. The extent of this variation is largely due to the molecules themselves, which have been found to form a number of different structure types. Though the basic formula for all silsesquioxanes has been found to be RSiO_3/2 with the identity of R typically being alkyl or organo-functional groups, the combined structure of these RSiO_3/2 units vary depending on synthesis methods, starting materials and the catalyst used. The four most common silsesquioxane structures are cage structures in which the units form a cage of n units in a designated Tn cage, partial cage structures in which the aforementioned cages are formed but lack complete connection of all units in the cage, ladder structures in which two long chains composed of RSiO_3/2 units are connected at regular intervals by Si-O-Si bonds, and finally random structures which include RSiO_3/2 unit connections without any organized structure formation.

Chemical Structure and Synthesis

The structure of silsesquioxanes depends on the preparation method. To simplify, a silicon with a hydrolytically stable organic substituent and three easily hydrolyzed groups such as chlorine or alkoxy groups, which are reacted with water and an acid or base catalyst. The final structure depends on the function of the concentration of initial monomer, concentration of water, temperature, type of catalyst, and the nature of the non-hydrolyzing substituent. This can be seen in the following equations. Solvent hydrogen bonding can have a large effect on rates and types of molecular condensations.

The basic silsesquioxane synthesis methods introduced by those who pioneered its formation in the silicone industry typically involved producing trichlorosilane precursors. These precursors were often formed by the reaction of methylene chloride or hydrogen chloride with silicon metal in the presence of a metal catalyst. The subsequent reaction used to form the silsesquioxanes were typically metal-catalyzed hydrosilation reactions with chloro- or alkylsilanes or organometallic coupling reactions with chlorosilanes. One key factor in selecting the R substituents is the catalyst, which is can change depending on the type of R group to be attached (for instance, with the addition of alkyl groups larger than methyl or organo-functional groups, platinum is used as a catalyst). Some work has been done for limited alterations of substituent addition, but in general there is to date no known method to direct or control the formation of particular substitutional isomers when introducing two or more different substituents in a silsesquioxane cage synthesis.

When characterizing silsesquioxanes the typical methods involved are x-ray diffraction, nuclear magnetic resonance spectroscopy (proton, carbon and silicon), and infrared radiation spectroscopy, though SEM and TEM have been used for visualization in crystal growth studies. Features of general importance when describing silsesquioxane compounds are the number of RSiO3/2 units in the cage compounds and the degree to which the silsesquioxane is condensed. In a fully condensed silsesquioxane compound, the general formula is RaSiaO(1.5a–0.5b)(OH)b with b=0, indicating that all oxygen atoms in the compound are bridging silicon atoms. A less condensed silsesquioxane compound (b>0) is indicative of the compound having less coordinated Si-O-Si connections, thus in general how condensed the compound is gives the nature of the macroscopic molecule (i.e. polymeric forms are typically highly condensed- almost fully connected networks). It is worth noting that in when the cage formations of silsesquioxanes are produced, though many different sizes are possible (i.e. T8, T10, T12), the most preferred formation are the cubic T8 compounds due to the high stability of the Si4O4 rings in the cage.

Many polymeric forms of silsesquioxanes have been developed with varying molecular weights and synthesis methods. The first high molecular weight tractable polymeric silsesquioxane was a ladder type repeating unit, polyphenylsilsequioxane, reported by Brown et al. in 1960 [1]. Brown et als. findings were used as a basis for further research and synthesis variations by plethora of additional research groups investigating polyphenylsilsequioxanes. Though many alterations were made depending on the researchers, the origin synthesis proposed by Brown involved a three step process outlined as follows: (1) the hydrolysis of phenyltrichlorosilane in a solvent with excess water to give a hydrolyzate, (2) equilibration of the hydrolyzate with potassium hydroxide at a low concentration and temperature to give the prepolymer, and (3) equilibration of the prepolymer at a high concentration and temperature to give the final polymeric form. In the many attempts to vary synthesis in hopes of increasing the polymer molecular weight, it was found that the critical factors were high concentration and temperature during the equilibration of the prepolymer. Another noteworthy milestone in the silsesquioxane polymeric materials is the development of soluble and stable polymethylsilsesquioxane with high molecular weights by Japan Synthetic Rubber [2]. This polymer which, unlike its phenyl derivative, gels easily during the course of its synthesis, has found a wide range of alternative applications including cosmetics [3], resins [4], and chemical amplification resist for electron beam lithography [5].

Another scientific development in the field of silsequioxanes was the first synthesis of hydridosilsesquioxanes by Frye and Collins [6]. Hydridosilsesquioxanes are a silsesquioxane type with only hydrogen substituents on the silicon, and are thus purely inorganic compounds. Initial synthesis methods involved the adding benzene solutions of trichlorosilane to a mixture of benzene, concentrated sulfuric acid, and fuming sulfuric acid to yield the T10-16 oligomers. The T8 oligomer was also synthesized, but by the reaction of trimethylsilane with a mixture of acetic acid, cyclohexane, and hydrochloric acid. It has been found that these compounds can be converted to silica coatings for application in environmental protection, and for application as an interlayer dielectric for integrated circuits. [7,8]

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. Some examples of starting materials are bis-1,4’-(triethoxysilyl)benzene and bis-4,4’-(triethoxysilyl)biphenyl. 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 [R-SiO1.5]x[H-SiO1.5]y 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 -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 - 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.

Applications

Electronics

There are many companies and universities that have done extensive research on silsesquioxanes as semiconductors, insulators and OLED’s. Some of these companies include Dow Corning, IBM, Honeywell, Japan Rubber Co, Hitachi, Mayaterials, and Hybrid Plastics just to name a few. These materials can be used in semiconductor devices as both semiconducting materials, with tuned functionality resulting from the attached organic groups, or as insulators in there native forms to form spacing in layers in semiconducting devices. These materials tend to have low dielectric constants, which makes them good thin film insulators. As organic OLED’s, polyhedral oligomeric silsesquioxanes make up an inorganic core with peripheral organic emitters making up the peripheral of a complex. This incorporation allows for an improved stability and an enhancement in electroluminescence properties.

The first example of a type of silsesquioxane is poly(hydridosilsesquioxane), which is being used for interlayer dielectric applications, which represents a linked-cage structure, which is sold under the name Fox Flowable Oxide.(SCP) These hydrogen silsesquioxanes are readily used for ceramic coatings in devices such as semiconductors. These structures can be found not only in the linked-cage, but also in the ladder form as well. (see Images) These compounds are often applied to an electronic device with organic solvent through evaporative techniques for thin-film coating. These devices can be difficult to prepare due to the fact that silsesquioxanes can be unstable in solvents and thickness modification, since striae are formed. Recently, methods have been developed such as plasma curing, which can give a more uniform coating with an elastic modulus(elasticity) of 3.82 GPa, Material Hardness of 0.62, and dielectric constant of 2.32. Methylsilsesquioxane materials are useful as spin-on-glass (SOG) dielectrics.(SCP) Bridged silsesquioxanes have been used for quantum confined nano-size semiconductors. Silsesquioxane resins have also been used for these applications because they have high dielectric strengths, low dielectric constants, high volume resistivities, and low dissipation factors, making them very suitable for electronics applications. These resins have heat and fire resistant properties, which can be used to make fiber-reinforced composites for electrical laminates.

With electronics getting smaller and smaller, the need for materials that keep these devices from short-circuiting is growing in demand. Single microchips contain thousands of interconnecting transistors that when overlapped, can cause interference problems, power dissipation and voltage issues. Silsesquioxane properties have the ability to prevent short-circuiting by acting as a rigid, insulating spacer; preventing corrosion or oxidation of metal conductors. They can level uneven topography, and fill gaps between closely spaced conductors. These films are easily applied through solvent evaporation with hydrogen silsesquioxane resins, and turned ceramic by heating the substrate in air. These films are known as interlevel dielectric (ILD) and protective overcoat films (PO). Industrial applications of OLED’s have a limited application. Traditional OLED’s do not typically contain inorganic materials, however due to the instability of (OLED’s) on their own, research is being conducted to look at hybrid materials that increase the stability of these compounds. Polyhedral oligomeric silsesquioxanes have been looked at in order to form an inorganic core. These compounds give better mechanical properties and stability, with an organic matrix for good optical and electrical properties. The mechanisms of degradation in these devices is not well understood, but it is believed that material defect understanding is important for understanding the optical and electronic properties.