Polysilane applications

Proceedings of the international conference on science and technology of synthetic metals (ICSM 88), Santa Fe, NM, USA, June 26-July 2, 1988, Aldissi M (ed) Polyheterocycles, Polysilanes, Applications and Processing, Volume C (reprinted from Synth Met 28 1-2 (1988), Elsevier, London (1989)... 

Investigations of silicon-metal systems are of fundamental interest, since stable coordination compounds with low valent silicon are still rare [64], and furthermore, silicon transition-metal complexes have a high potential for technical applications. For instance, coordination compounds of Ti, Zr, and Hf are effective catalysts for the polymerization of silanes to oligomeric chain-silanes. The mechanism of this polymerization reaction has not yet been fully elucidated, but silylene complexes as intermediates have been the subject of discussion. Polysilanes find wide use in important applications, e.g., as preceramics [65-67] or as photoresists [68-83],... 

There are, however, other classes of inorganic and organometallic polymers that deserve consideration due to their considerable scientific and applicative relevance, such as polysilanes [28-31], polycarbosilanes [32,33],polysilazanes [33],polyborazines [34,35],polythiazenes [36], and, as an example,polymetal-locenylsilanes [37]. 

In this chapter, an introduction and overview of polysilane chemistry will be presented, concentrating on the linear high polymers (polysilanes) and their technological applications. Polysilane polymers were reviewed in 1986,(1) and a more general review of polysilane chemistry appeared in 1982.(2)... 

Soluble polysilane polymers can also be used as precursors to silicon carbide. The first such application, using (PhMeSi)n-(Me2Si)m copolymers ("Polysilastyrene"), was to strengthen silicon nitride ceramics. The Si3N4 ceramic body was soaked in polysilane and refired, leading to the formation of silicon carbide whiskers in the pore spaces and a consequent increase in strength. (U)... 

Although polysilanes have been used mostly as photoinitiators for polymerization, they may also find application as initiators for other radical reactions. Experiments to test this possibility are now being carried out. 

Contrast enhancement lithography is a clever procedure which uses a bleachable contrast enhancing layer to restore the distorted aerial image of the mask which has been blurred by diffraction effects into a sharp image at the underlying photoresist surface. The process is too complicated to explain in detail here and the interested reader is referred to the cited literature (60,61). Suffice it to say that the large extinction coefficients of most polysilane derivatives coupled with their ready bleachability make them ideally suited for such purposes and we have demonstrated this application at 313 nm (16,18). 

In summary, the polysilanes comprise a new class of scientifically interesting, radiation sensitive materials for which many applications have become recently apparent. There is every reason to believe that future investigations will continue to be scientifically rewarding and result in new applications. 

A rapidly increasing number of publications on polysilanes documents current interest in these polymers (JJ. Polysilanes are potentially applicable in microlithography as high resolution UV-resists (2J, imageable etch barriers ), or contrast enhancement layers (4). They have been successfully used as precursors to Si-C fibers (5J and ceramic reinforcing agents ((L). Polysilanes have also initiated polymerization of vinyl monomers (J ). Doping of polysilanes have increased their conductivity to the level of semiconductors (8). Very recently polysilanes were used as photoconductors (9) and non-linear optical materials (10b... 

A great potential for new compounds is provided by structures with two carbon and two silicon atoms around the central silicon. These polysilanes with organic groups lead to silicon-carbide ceramics. A wide field of application would be opened up if one could make a polysilane as a plastic mass which could be extruded and modeled and if after pyrolysis silicon-carbide is formed without a strong contraction (this means a high ceramic yield). Polysilane fibers are only one product in a range of many... 

The application of chloromethyldisilanes, available on an industrial scale, yields polysilanes containing chlorine and a chlorine containing SiC is formed after pyrolysis. The properties of such SiC are not very useful. It is better to use methoxymethyldisilanes [17] or H-containing methyldisilanes [18]. 

The application of this method to dichlorosilanes results in the efficient formation of polysilanes (Scheme 41). Sonication results in a marked increase in the yields of polysilanes. The electrolysis of dichlorodisilajies also gives the corresponding polysilanes of higher molecular weight although the yields are low. 

The first CEM system described by Griffing and West (84) consisted of an organic dye dispersed in an inert polymer film that is spin coated onto the surface of a resist and subsequently removed following exposure but prior to resist development. The chemistry of this system is based on the photoisomerization of an aromatic dye to an oxaziridine (87) (Figure 10). Other workers have evaluated polysilanes (88) and diazonium salt chemistry (89,90) for CEM applications. 

Soluble polysilane derivatives (I) represent a new class of radiation sensitive materials for which a number of new applications have recently appeared. In this regard, they have been utilized as (i) thermal precursors to ceramic materials (2,3) ... 

Other polymers containing Si in the main chain are the polysilanes. The discovery that polysilane derivatives were radiation sensitive(20), coupled with their stability in an oxygen plasma, has led to a number of microlithographic applications(14, 21-24). For the... 

In another elegant application of the masked disilene method, the anionic polysilanyl chain ends were chemically bound to substrate surfaces to form end-graft polysilanes. These systems are discussed in Section 3.11.4.2.3. 

In a very interesting development, increasing the potential of polysilanes for application yet further, polysilanes have been end-grafted onto substrate surfaces using a variety of approaches. This surface-tethering of polysilanes requires the functionalization of one end (terminus) of the polymer main chain with a group which can be attached to a reactive surface or derivatized surface. 

The dynamics of polysilane dendrimer excited states were also studied by Watanabe as mentioned above in a comparison with polysilynes,360,364 and suggested that a configuration coordinate model is applicable to the photophysics of branched silicon chains. Calculations showed a distorted geometry of the excited state localized at a branching point. 

Went, M. J. Sakurai, H. Sanji, T. Modification and Functionalization of Polysilanes. In Silicon-based Polymers The Science and Technology of their Synthesis and Application-, Jones, R. G., Ando, W., Chojnowski, J., Eds. Kluwer Dordrecht, 2000 pp 419-437. 

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