Control of the Deposition Rate

In many systems that are conventionally used for CVD, the chemical equilibrium is on the solid side, but high temperatures are required in order to achieve sufficient reaction rates. Several examples of such systems in use for fabrication of optical fibers and electronic devices 

Very limited mechanistic studies of these reactions indicate that electron impact induced processes, such as dissociation and ionization, are the primary steps This conclusion is likely to apply to most systems since the contribution of uncharged radicals to the overall reaction rates is small in weak discharges at low temperatures However, caution is advisable in considering any mechanistic interpretations of such complicated processes. For example, more recent results show that ion- and electron-impact induced process taking place on the surface of the deposit are those actually controlling the deposition rate of microcrystalline silicon 55.i59.i60) 

Vepfek et al. have shown that, whereas increasing ion bombardment under a strongly negative bias increases the deposition rate only slightly, a dramatic decrease of the deposition rate is observed when the substrate potential is driven above the wall potential and it reaches zero value at a substrate potential which is still somewlmt negative with respect to the surrounding plasma 5 .i60) 

As mentioned above, weak discharge operating at a low current can catalyze reactions which cannot be promoted thermally because of the instability of the reaction products at high temperatures. This applies to systems involving stable, strongly bonded educt molecules, such as and CO2 and products which are unstable at high temperatures (e.g. MoN, WN). Example of such a system which has been studied in some detail is the Boudouard s reaction 

Below cs 300 °C the equilibrium is far on the right-hand-side but the decomposition of carbon monoxide with formation of CO2 and deposition of solid carbon proceeds only at an extremely slow rate because of the high dissociation energy of CO A weak discharge operating at a current density of a few mA/cm efficiently enhances the reaction rate the probable reaction mechanism being the dissociative attachment on CO followed by associative detachment of 0 and CO 

---

Some of the most significant developments in the CVD of Si02 include experiments in plasma CVD at 350°C via electron cyclotron resonance (ECR) to gain improved control of the deposition rate and obtain a quality equivalent to that of the thermally grown oxide (see Ch. 5). Deposition from diacetoxyditertiarybutoxy silane at 450°C has also been shown to significantly improve the Si02 film properties. " ]... 

Fig. 9.7. Precise control of the deposition rates for Alq3 in two separate runs with seven sequential depositions.

Several methods are used to deposit the films onto molybdenum-coated soda-lime glass substrates. Coevaporation of Cu, In, Se, and Ga from elemental sources onto a heated substrate, with careful control of the deposition rate of each, has achieved the highest efficiency. Another common method is selenization. A metal precursor layer of Cu, In, and Ga is deposited by, for example, sputtering and reacted in H2Se or Se vapor. For layers without Ga, the final reaction in a series is... 

Automatic control of Film Deposition Rate with the crystal oscillator for preparation of alloy films. 

Although the concentration of fluorine is the most important quantity in the control of the reaction rate and must be maintained within certain limits, in practice the stoichiometry, the molecular fluorine to substrate H-atom molar ratio, is used to determine the reaction parameters leading to a successful and efficient perfluorination. AF is most successful when sublimable solids are introduced into the hydrocarbon evaporator unit of the aerosol fluorinator as solutions by a syringe pump. This now common procedure emphasizes the individual molecule s isolation as it is fluorinated using AF. No intermolecular reactions between solute and solvent have been observed Choice of the solvent is important as it must not boil at a temperature below the melting point of the solute in order to prevent solid deposition in the tubes feeding the evaporator. It must also fluorinate to a material easily separable from the solid reactant after perfluorination. In most cases it has been found that aliphatic hydrochlorocarbons are excellent choices, but that carbon tetrachloride and chloroform and other radical-scavenging solvents are not (sec ref 6). 

An effective control of the deposition process operating temperature is vital for the consistent performance of any deposition bath. Deviations of more than 5°C from optimum temperature are sufficient to harm plate quality, deposition rates, and other properties. Baths can usually be formulated, however, to operate satisfactorily at any given temperature within a relatively wide range (typically up to 60°C). 

Thickness controllability (Table 9.1, no. 6) and reproducibility in OVPD is achieved by accurate adjustment of the flow of carrier gas by means of mass-flow controllers whereas in VTE quartz crystal monitors are used to control the rate of deposition by adjustment of the evaporation temperature. In VTE small deviations of the evaporation temperature are known to affect the stability of the deposition rate and consequently the layer thickness, which may also affect the roughness and morphology of the VTE-deposited layer. 

Thermodynamic control confirms that the deposition rate is equal to the mass input rate into the reactor, corrected for reaction yield. This often is observed to occur at deposition conditions of low flow rate, and high substrate temperatures. The temperature dependence of the deposition rate may be obtained from standard thermodynamic calculations. 

This reaction mechanism which proceeds without any intermediate reactions involving other species generally favors a compact deposit and closely obeys Faraday s law, see Eq.(5.1), making a precise control over the deposition rate and thickness possible. Furthermore, the metal deposit is much more stable than the corresponding metal oxides that might be hydrated and readily redissolve. Since the metal electroplating is not considered to be a precipitation process, as in the case of various metal oxide depositions, the inclusion of impurities from the electrolyte are less likely. 

The -TCNQ thin films were prepared by the ICB method in high vacuum Pa). A time-of-flight mass spectrometer was used to monitor in situ the size distribution of the deposited clusters. Pure and TCNQ clusters were deposited alternately onto an amorphous carbon substrate, forming TCNQ-C o-TCNQ sandwiched layers. The thickness of each individual layer was controlled by the deposition rate and time and was adjusted to obtain the desired structure. The total thickness of the multilayered thin films was about 70 nm. 

In Chapter 1, R.C. De Mattel and R.S. Feigelson review electrochemical methods for the deposition and doping of semiconductors. Potential advantages of these methods over thermally driven processes include electrical control over the deposition rate, relatively low deposition temperatures, and applicability to a wide range of materials. Despite these advantages, electrochemical methods have been overlooked as a route to electronic semiconductors. The incentive for research described in this chapter has come largely from photovoltaic applications. 

Fluorinated copper phthaocyanine (FieCuPc) is one of the few molecules that exhibit air-stable n-channel semiconducting behavior. The mobility of the OFET is 0.03 cm /Vs when the thin film is fabricated on Si02 by vapor deposition [ 122]. The structure of the FieCuPc film on Si02 depends on the thickness of the film, which can be controlled by the deposition rate and... 

The actual CVD reaction process can be significantly more complex than the simplified view presented above. In general, to maintain a constant pressure in the reactor at various reactant flow rates, a significant part of the reactor gas is H2. Thus, the H2 along with the temperature allows independent control of the reaction rate, gas density, and reactor pressure. The H2 also enhances the quality of the deposited material by reducing the importance of atomic vacancies in the resulting semiconductor. 

---