Precursor gases

The ability of a given volatile organic compound (VOC) to produce during its atmospheric oxidation secondary organic aerosol depends on three factors  

As a result of the above constraints, VOCs that for all practical purposes do not produce organic aerosol in the atmosphere include all alkanes with up to six carbon atoms (from methane to hexane isomers), all alkenes with up to six carbon atoms (from ethene to hexene isomers), benzene and many low-molecular-weight carbonyls, chlorinated compounds, and oxygenated solvents. 

Organic aerosols formed by gas-phase photochemical reactions of hydrocarbons, ozone, and nitrogen oxides have been identified in both urban and rural atmospheres (Grosjean, 1977). Most of these species are di- or poly-functionally substituted alkane derivatives. These compounds include aliphatic organic nitrates (Grosjean and Friendlander, 1975), di-carboxylic acids (adipic and glutaric acids) (O Brien et al., 1975), carboxylic acids derived from aromatic hydrocarbons (benzoic and phenylacetic acids), polysubstituted phenols, and nitroaromatics from aromatic hydrocarbons (Kawamuraet al., 1985 Satsumakayashi et al., 1989, 1990). Some species that have been identified in ambient aerosol and are be- 

TABLE 13.11 Measured and Estimated Total Yields of Condensable Compounds During the Oxidation of Selected Precursors  

TABLE 13.12 Some Secondary Organic Compounds Identified in Ambient Particles in Urban Air 

---

CVD processing can be accompanied by volatile hot-reaction by-products such as HCl or HF, which, along with unused precursor gases, must be removed from the exhaust gas stream. This is done by scmbbing the chemicals from the gas using water to dissolve soluble products or by burning the precursor gases to form oxides. 

The activation energies for the decomposition of these precursor gases still are not accurately known and show a large scatter. Some reported values are as follows ... 

In ion beam deposition, hydrocarbon gas such as methane or ethyene is ionized into plasma by an ion source such as the Kaufman source [3]. The hydrocarbon ions are then extracted from the ion source and accelerated to form an ion beam. The ions and the unionized molecules condense on the substrate surface to form DEC coating. However, in this method, ionized ratio of precursor gases could hardly exceed 10 %. In order to obtain a better quality of DEC coatings. 

CVD of SiC normally uses silane and a hydrocarbon as the precursor gases and a hydrogen carrier gas. The gases pass over a heated graphite susceptor that is coated by SiC or tantalum carbide (TaC). 

The concentration of precursor gases will decrease with respect to the flow direction over the susceptor due to the consumption of growth species, which results in a tapered layer thickness. This effect is known as depletion. To compensate for the depletion it is common to taper the susceptor such that the velocity of the gases increases along the flow direction over the susceptor and thus the boundary layer will be pushed downward, resulting in a shorter diffusion for the active species to the substrate. 

Consider an atmospheric-pressure process to deposit a silicon film from a silane (SifLj) precursor. The showerhead-to-wafer distance is 3 cm. In this process a helium carrier gas makes up the bulk of the flow, with the active silane accounting for only 0.17% of the inlet mixture. The precursor gases enter the reactor at 300 K, but the wafer temperature and inlet velocity are varied to observe different process characteristics. 

FIGURE 5 Schematic diagram of a typical large-scale horizontal gas foil Planetary MOCVD reactor chamber. The precursor gases are injected in the center of the rotating wafer carrier and the gas flows horizontally over the individually rotating wafers. 

The deposition process is illustrated in the left part of Fig. 5. The precursor gases are sprayed on the surface by the nozzle , where they are adsorbed. In a second step, the incoming ion beam decomposes the adsorbed precursor gases. Then the volatile reaction products desorb from the surface and are removed through the vacuum system, while the desired reaction products remain fixed on the surface as a thin film. The deposited material is not fully pure however, because organic contaminants as well as Ga ions (from the ion beam) are inevitably included in the deposited film [23],... 

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. 

Another consideration concerns the profiles obtained for the different precursor gases. We expect the same radicals to be present in the plasmas of different gases. For example, C2H5 will certainly be produced in an ethane plasma by simple dissociation, but it will also be produced in a methane discharge due to various gas phase reactions. Therefore, it seems reasonable to model the profiles from all cavity positions and for all precursor gases with superpositions of one common set of single-/ profiles. The question that arises... 

Fig. 11.6. Film thickness profiles of cavities exposed to plasmas of different precursor gases in three different positions in the vacuum vessel. The dots represent the measured profiles, the solid lines the calculated three-component fits. The profiles in each column are from the same position, the ones in a row are from the same precursor gas...

---