CVD precursors

The choice of a precursor is governed by certain general characteristics which can be summarized as follows  

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Compounds like ds-[PdMe2(PR3)2] (R = Me, Et) have been suggested as chemical vapour deposition (CVD) precursors for palladium [108b]. 

PF3A11CI, prepared from Au2C16 and PF3 in SOCl2 has a vapour pressure of 10 4mbar at room temperature and has been suggested as a laser CVD precursor [80],... 

Carbonyl Nitric Oxides. Another group of metal-carbonyl complexes, worthy of investigation as CVD precursors, consists of the carbonyl nitric oxides. In these complexes, one (or more) CO group is replaced by NO. An example is cobalt nitrosyl tricarbonyl, CoNO(CO)3, which is a preferred precursor for the CVD of cobalt. It is a liquid with a boiling point of 78.6°C which decomposes at 66°C. It is prepared by passing NO through an aqueous solution of cobalt nitrate and potassium cyanide and potassium hydroxide. ... 

Many elements form hydrides but only few hydrides are presently used as CVD precursors. These are the hydrides of elements of groups nib, IVb, Vb, and VIb, which are all covalent. They are listed in Table 3.7. 

To be useful as CVD precursors, a metallo-organic compound should be stable at room temperature so that its storage and transfer are not a problem. It should also decompose readily at low temperature, i.e., below 500°C. The compounds listed in Table 4.1 meet these conditions with the exception of the alkyls of arsenic and phosphorus, which decompose at higher temperatures. For that reason, the hydrides of arsenic and phosphorus are often preferred as CVD precursors (see Ch. 3). These hydrides however are extremely toxic and environmental considerations may restrict their use. 

Also increasingly common, as CVD precursors, are many halogen-acetylacetonate complexes, such as trifluoro-acetylacetonate thorium, Th(C5H4F302)4, used in the deposition of thoriated tungsten for thermionic emitters, the trifluoro-acetylacetonates of hafnium and zirconium and the hexafluoro-acetylacetonates of calcium, copper, magnesium, palladium, strontium, and yttrium. 

If the temperature and supersaturation are sufficiently high in a CVD reaction, the product is primarily powder precipitated from the gas phase (see Ch. 2). Such powders have few impurities provided that the CVD precursors are carefully purified. Their small diameter and great uniformity are important factors in the production of high quality hot-pressed or sintered ceramic bodies with good mechanical and electrical properties. In addition, the sintering temperatures required for CVD powders are lower than those for conventional powders. 

The relatively high volatility of Tg[CH = CH2]8 has enabled it to be used as a CVD precursor for the preparation of thin films that can be converted by either argon or nitrogen plasma into amorphous siloxane polymer films having useful dielectric propertiesThe high volatility also allows deposition of Tg[CH = CH2]g onto surfaces for use as an electron resist and the thin solid films formed by evaporation may also be converted into amorphous siloxane dielectric films via plasma treatment. ... 

The CVD precursors are generally corrosive, hygroscopic and air sensitive or toxic. Thus CVD processing is usually carried out in closed systems. However, in many cases, deposition can be accomplished at atmospheric pressure in relatively simple systems. Schematic drawings of two experimental CVD reactors are shown in Figure 4.33. 

Initially, the main disadvantage of using single-source precursors was their lack of volatility. However, the advent of ultralow-pressure and vacuum CVD techniques, along with the use of alternative delivery methods which do not rely primarily on precursor volatility, means that many compounds that are essentially nonvolatile can now be considered as potential CVD precursors. A major problem that remains, however, is that of stoichiometry control a molecular precursor containing two (or more) elements in a specific ratio, as desired within the thin film to be grown, will not necessarily deposit those elements in the same proportions. 

In a search for CVD precursors for beryllium, the air sensitive, volatile solid CpBe(SiMe3) 17 was prepared from the reaction of Li[SiMe3] with CpBeCl in pentane.62 Characterized with single crystal X-ray diffraction, multinuclear NMR, and mass spectra, the compound displays a Be-Si bond length of 2.185(2) A that is somewhat longer than the sum of covalent radii (2.01 A). The lengthening is similar to that observed in the related Be(Si(/-Bu)3)2 (Be-Si = 2.193(1) A),63 so there is not a pronounced effect from the Cp ligand on the Be-Si interaction. 

The reaction of 49 with alkylamines has been examined in the context of Cp2Mg-MR3-NH3 (M = group 13 metal) mixtures as CVD precursors. Addition of primary and secondary amines to 49 at ambient temperature in toluene affords stable amine adducts in good yield (Table 3). Most adducts can be sublimed at under 100 °C/0.05 torr in... 

The conductors, semiconductors, and superconductors that have been discussed are materials that can be prepared via some type of CVD process. In order to prepare each material, a precursor is required. The precursor chemistry of these materials is based heavily on organometallic and inorganic chemistry. Numerous ligand platforms have been investigated for use in the preparation of suitable CVD precursors. 

Fig. 15. Fluorinated ligands that are used in the preparation of volatile group 2 CVD precursors.

If a single metal film is deposited on an oxide, the sheet resistance measurement results can by easily interpreted and converted to the thickness. In practice, however, this is not usually the case. For example, in W CVD, the tungsten is not directly deposited on oxide due to high residual stress and unreliable adhesion. A titanium (Ti) layer must be first deposited as a glue layer. In addition, to prevent the fluorine in the CVD-precursor WFg from directly reacting with Ti (a strong catalytic reaction will occur), a barrier layer of titanium nitride (TiN) must be deposited on top of the Ti. As a result, we have a trilayer film of W on TiN on Ti on oxide, as shown schematically in Fig. 21. This poses some problems in accuracy in the four-point probe measurements. Based on the resistivities in Table VI, the... 

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