Silicon films, dielectric properties

A recent competitor to CVD in the planarization of silicon dioxide is the sol-gel process, where tetraethylorthosilicate is used to form spin-on-glass (SOG) films (see Appendix). This technique produces films with good dielectric properties and resistance to cracking. Gas-phase precipitation, which sometimes is a problem with CVD, is eliminated. 

Adhesion of polyimides to inorganic substrates is of great importance to the microelectronics industry [1, 2]. The polyimide films are deposited most often by spin coating the polyamic acid (PAA) usually from a TV-methylpyrrolidone (NMP) solution onto the substrate surface followed by thermal imidization at temperatures up to 400<>C. The most studied polyimide is the pyromellitic dianhydride-oxydianiline (PMDA-ODA), which exhibits excellent mechanical and dielectric properties, but not so good adhesion characteristics. The latter has been generally overcome by application of an adhesion promoter, such as y-aminopropyltriethoxysilane [3-7]. The reactions of APS (coated from water solution) with the silicon dioxide surface as well as with polyamic acid have been well characterized by Linde and Gleason [4] however, we do not have such detailed information available on APS interaction with other ceramic surfaces. 

Silicon oxides (SiOx) are the most widely used thin films in silicon microelectronic and micromechanical devices. Similar to silicon nitride (Section 5.5.4), these amorphous films exhibit dielectric properties. Silicon oxide is often utilized as part of a dielectric membrane, as a passivation or insulating layer, or as a sacrificial layer, which can be etched with hydrofluoric acid (HF)-containing etchants. Two different approaches to forming a silicon oxide thin film are... 

Dimethylformamide stands out from the other solvents in the group. The AOS films synthesized in an electrolyte based on it demonstrate a relatively uniform dielectric properties over the anode surface. The molecules of this solvent have the largest donor number suggesting their enhanced capabilities to take part in the electron transfer to the groups with uncompensated bonds which form on the silicon anode surface. The adsorption of such molecules on the solid surface is likely to be assigned to chemisorption. 

The next two chapters deal with investigations concerning solid silicon monoxide. The application of thin films of this material is based on its unique mechanical, chemical, and dielectric properties. It is related to Si-Si systems in so far as solid SiO consists of small particles of Si and Si02. Depending on the conditions for synthesis, the material has different local structures. In the contribution of U. Schubert and T. Wieder (Chapter 18), the structure and reactivity of a special SiO modification (Patinal ) is described. This material consists of Si and Si02 regions of 0.25 - 0.5 nm in diameter, which are connected by a thin interface. Most of the SiO reactions are also observed for elemental silicon. H. Hofineister and U. Kahler (Chapter 19) show that thermal processing of solid SiO (from BALZERS) up to 1300°C leads to phase separation into Si nanocrystallites embedded in an SiOx matrix. Their internal structure is determined by solid-phase crystallization processes. 

Niinisto, J., M Putkonen, L. Niinisto, K Kukli, M. Ritala, M Leskela (2004) Structural and dielectric properties of thin Zr02 films on silicon grown by atomic layer depesition from cyclopentadienyl precursor. Journal of Applied Physics, 95,84-92... 

Polyimides exhibit outstanding dielectric and mechanical properties at elevated temperatures. Nevertheless, relatively high values of water sorption (up to 3-4 wt%) and coefficients of thermal expansion (5x10 K l) impede (micro)electronic applications, e.g., forming stress-free films on silicon substrates. From this point of view silica (SiCh), that exhibits very low values of water sorption and coefficients of thermal expansion (5x10- K- ), would be more suited for (micro)electronic applications but dielectric properties and planarizability are inferior to PI [17].Combined materials exhibiting favourable properties of both polyimides and silica are therefore in great demand. 

The measurement of properties such as the resistivity or dielectric constant of PS requires some kind of contact with the PS layer. Evaporation of a metal onto the PS film-covered silicon sample produces a metal/PS/Si sandwich, which behaves like an MIS structure with an imperfect insulator. Such sandwich structures usually exhibit a rectifying behavior, which has to be taken into account when determining the resistivity [Si3, Bel4]. This can be circumvented by four-terminal measurements of free-standing PS films, but for such contacts the applied electric field has to be limited to rather small values to avoid undesirable heating effects. An electrolytic contact can also be used to probe PS films, but the interpretation of the results is more complicated, because it is difficult to distinguish between ionic and electronic contributions to the measured conductivity. The electrolyte in the porous matrix may short-circuit the silicon filaments, and wetting of PS in-... 

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