Deposition processes surface reaction control

In silicon based MEMS processing, common CVD films include polysilicon, silicon oxide, and silicon nitride. For polysilicon films (usually the structural layer), an LPCVD pyrolysis method is generally used with silane (SiH4) as the source gas [see Eq. (1)]. To obtain a imiform film across the wafer, the process is carried out at low pressure to ensure that the deposition is surface reaction controlled and not diffusion limited. Typical process temperatures are in the range of 580-650°C, and pressures between 0.1 and 0.4Torr. 

Thin films formed by atomistic deposition techniques are unique materials that seldom have handbook properties. Properties of these thin films depend on several factors (4), including substrate surface condition, the deposition process used, details of the deposition process and system geometry, details of film growth on the substrate surface, and post-deposition processing and reactions. For some applications, such as wear resistance, the mechanical properties of the substrate is important to the functionality of the thin film. In order to have reproducible film properties, each of these factors must be controlled. 

The relationship of the stirring rate in these experiments to the rates of hydrolysis reactions of basalt phases is indicative of surface-reaction controlled dissolution (21). First order kinetics are not inconsistent with certain rate-determining surface processes (22). Approximate first order kinetics with respect to dissolved oxygen concentration have been reported for the oxidation of aqueous ferrous iron (23) and sulfide (24), and in oxygen consumption studies with roll-type uranium deposits(25). 

FIGURE 5.8 Summary of the key kinetic concepts associated with CVD under the surface reaction, diffusion, and mixed-control regimes, (a) Schematic illustration and deposition rate equation for CVD under surface reaction control, (b) Schematic illustration and deposition rate equation for CVD under reactant diffusion control, (c) Schematic iUusIration and deposition rate equation for CVD under mixed control, (d) Illustration of the crossover from surface-reaction-controlled behavior to diffusion-controlled behavior with increasing temperature. The surface reaction rate constant (k ) is exponentially temperature activated, and hence the surface reaction rate tends to increase rapidly with temperature. On the other hand, the diffusion rate increases only weakly with temperature. For CVD processes where the reactions become less thermodynamically favorable with increasing temperature (common), the rate will eventually fall at higher temperatures as the CVD process becomes unfavorable thermodynamically. The slowest process determines the overall rate. 

In the A sector (lower right), the deposition is controlled by surface-reaction kinetics as the rate-limiting step. In the B sector (upper left), the deposition is controlled by the mass-transport process and the growth rate is related linearly to the partial pressure of the silicon reactant in the carrier gas. Transition from one rate-control regime to the other is not sharp, but involves a transition zone where both are significant. The presence of a maximum in the curves in Area B would indicate the onset of gas-phase precipitation, where the substrate has become starved and the deposition rate decreased. 

This CVD procedure is somewhat different from that used to deposit semiconductor layers. In the latter process, the primary reaction occurs on the substrate surface, following gas-phase decomposition (if necessary), transport, and adsorption. In the fiber optic process, the reaction takes place in the gas phase. As a result, the process is termed modified chemical vapor deposition (MCVD). The need for gas-phase particle synthesis is necessitated by the slow deposition rates of surface reactions. Early attempts to increase deposition rates of surface-controlled reactions resulted in gas-phase silica particles that acted as scattering centers in the deposited layers, leading to attenuation loss. With the MCVD process, the precursor gas flow rates are increased to nearly 10 times those used in traditional CVD processes, in order to produce Ge02-Si02 particles that collect on the tube wall and are vitrified (densified) by the torch flame. 

Heat transfer is an extremely important factor in CVD reactor operation, particularly for LPCVD reactors. These reactors are operated in a regime in which the deposition is primarily controlled by surface reaction processes. Because of the exponential dependence of reaction rates on temperature, even a few degrees of variation in surface temperature can produce unacceptable variations in deposition rates. On the other hand, with atmospheric CVD processes, which are often limited by mass transfer, small susceptor temperature variations have little effect on the growth rate because of the slow variation of the diffusion with temperature. Heat transfer is also a factor in controlling the gas-phase temperature to avoid homogeneous nucleation through premature reactions. At the high temperatures (700-1400 K) of most... 

Chemical vapor deposition is a key process for the growth of electronic materials for a large variety of devices essential to modern technology. Its flexibility and relatively low deposition temperatures make CVD attractive for future device applications in Si and compound-semiconductor technologies. The process involves gas-phase and surface reactions that must be controlled to achieve desired material and electronic properties. 

In the analysis of CVD reactions, it is important to recognize the rates of the various processes. The slowest rate will be controlling, and which one is the slowest or fastest can depend on gas as well as surface conditions. For example, surface reactions may be fast at high surface temperatures. In this case, the CVD process will tend to be limited by the rate at which reactants can get to the surface or products leave it. For this situation, the fluid dynamic boundary layer phenomena will govern the deposition rate. On the other hand, at low pressures diffusion is very rapid and the rate at which surface reactions proceed will tend to govern the deposition rate. Alternatively, low surface temperatures will have low reaction rates, and this will govern no matter how much material diffuses to the surface. 

Microbiological action in starch dispersions results in a drop in pH, loss of viscosity and the development of odor. Retrogradation may be accelerated by the drop in pH or especially if butanol, which complexes with amylose, is generated via starch fermentation. Sulfate-reducing bacteria will cause black deposits due to reaction with iron in the process water. For quality control, preservatives are added to starch slurry, cooked starch, surface size and coating color. 

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