Chemical vapor deposition metal oxide film precursors

The most intensive development of the nanoparticle area concerns the synthesis of metal particles for applications in physics or in micro/nano-electronics generally. Besides the use of physical techniques such as atom evaporation, synthetic techniques based on salt reduction or compound precipitation (oxides, sulfides, selenides, etc.) have been developed, and associated, in general, to a kinetic control of the reaction using high temperatures, slow addition of reactants, or use of micelles as nanoreactors [15-20]. Organometallic compounds have also previously been used as material precursors in high temperature decomposition processes, for example in chemical vapor deposition [21]. Metal carbonyls have been widely used as precursors of metals either in the gas phase (OMCVD for the deposition of films or nanoparticles) or in solution for the synthesis after thermal treatment [22], UV irradiation or sonolysis [23,24] of fine powders or metal nanoparticles. 

For the chemical vapor deposition of ZnO, the ratio of the various precursors that participate in the chemical reaction leading to ZnO formation is an important parameter it influences the stoichiometry of the deposited films and therefore, also, their properties. DEZ and DMZ, which are the metal-organic precursors mostly used for ZnO formation, react in the presence of oxidizing agents like O2 or H2O. The equation for the complete oxidation reaction of DEZ as well as the equation for the complete reaction of DEZ with H2O are given here as examples ((6.5) and (6.6)) ... 

Aerosol-assisted chemical vapor deposition is illustrated by work published by Siadati et al. (2004). An aerosol is a vapor suspension of finely divided particles. The particles can be solid, as in smoke, or liquid, as in fog. A toluene solution of Zr(tfac)4, Y(hfac)3, and Ce(tmhd)4, converted to an aerosol, was used to deliver the metal in a carrier gas of O2. The film deposited was CeC>2-doped Y2O3-stabilized zirconia. The ligands used in this work to produce volatile metal compounds are frequently used in chemical vapor deposition. Trifluoroacety-lacetonate (tfac), hexafluoroacetylacetonate (hfac), and tetramethylheptanedionate (tmhd) are all 3-diketonates, and their structures are shown in Figure 3.22. Like the diethyl dithiocarbamates mentioned above, these precursors could potentially be the source of both the metal and the oxygen in the chemically deposited film. However, to ensure that the metal remains at its highest oxidation state and to avoid a film with mixed valencies on the metal, oxygen is used as the carrier gas. 

High-vacuum dry-processes, such as chemical vapor deposition (CVD) and molecular beam epitaxy (MBE), have made it feasible to control precisely the thickness of metal oxide thin films. In these techniques, the preparative conditions like pressure and substrate temperature can be widely varied, and the elemental composition in individual atomic layers is controllable by sequential supply of precursor gases [1]. The dense, defect-less oxide films thus prepared are frequently used as underlayers of microelectronics devices. 

This chapter is intended to cover major aspects of the deposition of metals and metal oxides and the growth of nanosized materials from metal enolate precursors. Included are most types of materials which have been deposited by gas-phase processes, such as chemical vapor deposition (CVD) and atomic layer deposition(ALD), or liquid-phase processes, such as spin-coating, electrochemical deposition and sol-gel techniques. Mononuclear main group, transition metal and rare earth metal complexes with diverse /3-diketonate or /3-ketoiminate ligands were used mainly as metal enolate precursors. The controlled decomposition of these compounds lead to a high variety of metal and metal oxide materials such as dense or porous thin films and nanoparticles. Based on special properties (reactivity, transparency, conductivity, magnetism etc.) a large number of applications are mentioned and discussed. Where appropriate, similarities and difference in file decomposition mechanism that are common for certain precursors will be pointed out. 

Numerous ceramics are deposited via chemical vapor deposition. Oxide, carbide, nitride, and boride films can all be produced from gas phase precursors. This section gives details on the production-scale reactions for materials that are widely produced. In addition, a survey of the latest research including novel precursors and chemical reactions is provided. The discussion begins with the mature technologies of silicon dioxide, aluminum oxide, and silicon nitride CVD. Then the focus turns to the deposition of thin films having characteristics that are attractive for future applications in microelectronics, micromachinery, and hard coatings for tools and parts. These materials include aluminum nitride, boron nitride, titanium nitride, titanium dioxide, silicon carbide, and mixed-metal oxides such as those of the perovskite structure and those used as high To superconductors. 

The chemical features of tin(IV) alkoxides, such as pre-existing metal-oxygen bonds in molecular units, high volatility and low decomposition temperatures make them attractive precursors for deposihon of Sn02. The heterometallic complex [Sn(dmae)2Cd(acac)2],Figures.1.2, (acac = 2,4-pentanedionato dmae = N,N -dimethylamino-ethanoate) has been decomposed in aerosol-assisted chemical vapor deposition conditions, producing amorphous tin(IV) oxide films with no detectable cadmium. ... 

Other techniques have been used for the fabrication of thin-film metal-oxide gas sensors. At NIST in the USA, Cavicchi et al. (1995) and Semancik et al. (2001) produced gas sensors by chemical vapor deposition (CVD). By applying a current and thus heating the hotplate, sensing films could be deposited locally (i.e. only on heated active areas) using an adequate organ-ometaUic precursor. SnOj and ZnO films were obtained with tetramethyltin and diethylzinc in an oxygen atmosphere. They were deposited onto different seed layers, which played a significant role in terms of gas selectivity. 

Electropolymerization is a convenient method for the preparation of insoluble conjugated polymers such as PPV because it yields thin films directly on an electrode surface. Such a film may then be directly utilized in an application requiring a conducting contact, such as electroluminescence. Several methods have been reported for the electrochemical preparation of PPV films. Electrochemical reduction of 4,5,6 has been shown to yield PPV films on metal and indium tin oxide (ITO) electrodes (19-21). Another direct route to thin-films is chemical vapor deposition (CVD) from precursors such as 7 or 8 (22,23). 

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