Precursor Volatility

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. 

To increase precursor volatility, interest focused on using ligands of considerable bulk to reduce the degree of molecular association. This was of limited success, with mercury proving to be an exception. Bradley and Kunchur218 reported that Hg(SBu )2 has only weak intermolecular interactions and occurs as discrete Hg(SR)2 units even in the solid state. Other attempts to prepare complexes of zinc or cadmium with limited degrees of polymerization have been undertaken. 

There are a number of chemical and physical properties that should be considered when designing or selecting metal-containing compounds as precursors for CVD of metal-containing materials. These properties can be broadly categorized into precursor volatility, reactivity, and solubility (for reasons that are described later). 

The other solution to the problem of low precursor volatility and instability that has been used extensively in the deposition of Group 2 metal-containing films is the incorporation of various donor ligands into the coordination sphere of the metal ion, which prevents oligomerization. 

Instruments and methods that can in near real time characterize more fully the speciated organic composition of secondary and combustion aerosols and that of the gas phase. In conjunction with laboratory studies, one may hope to use these techniques to elucidate the pathways and connect precursor volatile organic compounds to the nature of particulate matter. 

Experiments I to III were carried out under comparable conditions (250 g cucumbers, phosphatebuffer pH 6.8, 40 s homogenization, addition of 100 mg precursors, volatiles were enriched by extraction / or distillation-extraction and determined by GC-MS). It can be seen that linolenic acid is transformed into hexanal, ( )-3-nonenal, pentylfuran, and (E)-2-nonenal in the 40 s trial I and I (I = extraction I = distillation-extraction). During 120 s homogenization the labile intermediate (Z)-3-nonenal decreased from 0.23 mg to 0.02 mg and all carbonyls are reduced to some extent to the corresponding alcohols. 

The deposition process is governed by the precursor volatility, stability, and reactivity which mainly depend on the substrate/reactor temperature for a given reactor pressure, as illustrated in Fig. 8.2. The ideal process window, the so-called ALD window, in which growth is saturated and insensitive to overexposure with precursor. 

Fig. 8.2 Dependence of the ALD growth rate on the deposition temperature. The ALD window is characterized by a stable deposition rate reflecting an ideal precursor volatility, reactivity, and stability in this temperature range...

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