Nucleation gas-phase

Fine-grained polycrystals Amorphous deposits Gas-phase nucleated snow... 

Various appHcations such as lubricant additives, dyes, pigments, and catalysts are under investigation. Tungsten can be deposited from tungsten hexacarbonyl, but carbide formation and gas-phase nucleation present serious problems (1,2). As a result, tungsten halides are the preferred starting material. 

Other CVD Processes. CVD also finds extensive use in the production of protective coatings (44,45) and in the manufacture of optical fibers (46-48). Whereas the important question in the deposition of protective coatings is analogous to that in microelectronics (i.e., the deposition of a coherent, uniform film), the fabrication of optical fibers by CVD is fundamentally different. This process involves gas-phase nucleation and transport of the aerosol particles to the fiber surface by thermophoresis (49, 50). Heating the deposited particle layer consolidates it into the fiber structure. Often, a thermal plasma is used to enhance the thermophoretic transport of the particles to the fiber walls (48, 51). The gas-phase nucleation is detrimental to other CVD processes in which thin, uniform solid films are desired. 

Chemical Reaction Mechanisms and Kinetics. CVD chemistry is complex, involving both gas-phase and surface reactions. The role of gas-phase reactions expands with increasing temperature and partial pressure of the reactants. At high reactant concentrations, gas-phase reactions may eventually lead to gas-phase nucleation that is detrimental to thin-film growth. The initial steps of gas-phase nucleation are not understood for CVD systems, not even for the nucleation of Si from silane, which has a potential application in bulk Si production (97). In addition to producing film precursors, gas-phase reactions can have adverse effects by forming species that are potential impurity sources. 

Films grown by chemical vapor deposition are similar to the films described above, with one exception. The CVD process allows for the possibility of gas phase particle nucleation, and the incorporation of such particles in a growing surface can contribute to a surface roughness. This is one reason that atmospheric CVD is being used less and less, as compared to low-pressure CVD where such gas phase nucleation is less likely. 

The problem of assuring uniform depositions on many wafers closely spaced in a long uniform tube was solved when operation of the reactor at low pressure was considered.22 Normally, in an atmospheric pressure cold wall CVD system, the reactant gas is heavily diluted in N2 in order to reduce gas phase nucleation. At the pressures used for low pressure CVD (0.5-1.0 Torr), this is less of a problem so less diluent is needed. The net effect then is that deposition rates only fall by a factor of five. However, as many as 100 wafers can be processed in such a reactor at one time (see Figure 26), and this more than compensates for the lower deposition rate. In addition, due to the low pressure, diffusion occurs at high rates and the deposition tends to be controlled by the surface temperature. Given the very uniform temperatures available in a diffusion furnace, the deposition uniformity tends to be excellent in such a system. 

If the SiH4/02 mixture is not sufficiently diluted with an inert gas, then gas phase nucleation typically occurs and Si02 particulates are formed. Generally, N2 is used as the diluent, but some work has been done with Ar, C02, and He. Depending on the reactor configuration, an inert gas effects the deposition rates in different ways. 

The low-temperature depositions described in the present section can be used for either interlayer dielectrics or final passivation films. Their primary disadvantage is one of film quality, because the process is susceptible to gas-phase nucleation and incorporation of particles into the film. 

Another feature of this approach is a claimed lower particulate count that the wafers are exposed to. If the longer flow path in an LPCVD furnace promotes gas phase nucleation and stirs up particles, then this system should minimize such effects. 

Gas-phase nucleation Flame synthesis of particles (e.g., carbon black, silica) cluster formation in chemical vapor deposition manufacture of high-purity silicon cluster structure and energetics plasma synthesis of refractory materials and coatings. 

Gas Phase Nucleation Amorphous Deposits Fine Grained Deposits Poiycrystais Dendrites Whiskers Piateiets Epitaxiai Growth... 

In addition to process time and uniformity, other factors must be considered in selecting optimal conditions. As in any CVD process, a specific temperature range will usually be required to obtain a desired morphology of the deposited material. Specific process conditions may also be required either to induce or to inhibit certain gas-phase reactions, such as those involved in the production of necessary deposition precursors or those leading to undesirable gas-phase nucleation of particles. Here, we will present an optimization approach that can be used to maximize deposition rates subject to any of these constraints. 

This is an irreversible system, and the reaction does not depend on transport from a hotter to a colder zone. Gas phase nucleation is reduced, and the reaction temperature is low (300-500°C). The hydrogen passivation occurs, but still this effect may be partially alleviated by substitution of an alkyl chalcogenide to avoid the hydrogen selenide. Alkylselenium compounds originally were used to avoid the very toxic H2Se. The criteria (e.g., toxicity, decomposition into stable products) for selection of these alternative precursors have been reviewed . 

As polynuclear aromatic hydrocarbons show a high tendency towards undesired gas-phase nucleation and soot formation, they should be avoided by choosing a suitable critical residence time and other appropriate processing parameters. 

Gas impurities. Limited gas solubility can provoke formation gas phase (nucleation), contenting not a vapour but this gas mainly. This effect will decrease Th-Tn as well. In our runs this effect is not important as concentrations of dissolved air and hydrogen jr oxygen are negligible. 

Figure 10. Fluorescence microscope images of PDA. The probe was NBD-hexadecylamine and the probe concentration was 1 mol%. (a) Isotherm at 20°C. The monolayer is prepared in the G-LE transition region (iii) bar = 100/tm. Expansion produces a foam structure (ii) that grows with time (i). With compression, (iii) is converted to an all-white phase and small dark islands appear at the LE-LC two-phase boundary (i v). The islands grow with further compression (v) and eventually deform and coalesce (viy The granular appearance of the LE phase in (vi) is the result of demixing of the dye at low dye concentrations this effect is not seen, (b) Measurements as a function of time after a quench from 20°C to 14°C at a fixed molecular area of 52 A. Circular bubbles of the gas phase nucleate around the LC domains and grow with time.

The succession of images observed in an experiment in which a PDA monolayer in the LE-LC region is cooled just below the triple point is shown in Fig. 10b. The gas phase nucleates on the LC phase and is easily distinguished from it because the gas bubbles are markedly noncircular and highly compressible. If the temperature is now held constant, the area of the LE phase diminishes and the LC and G phases grow. The transformation rate... 

In spite of these studies and results, the relative importance of the gas-phase nucleation compared to the surface nucleation is unclear as yet. In fact, the number of diamond particles collected from the gas phase is very small compared to the typical surface nucleation densities, thus the homogeneous nucleation mechanism cannot account for the large variety of nucleation densities observed on different substrate materials and from different surface pretreatments. It is speculated and also supported by a recent experimentl l that the nuclei formed in the gas phase may reach the growing surface and increase the surface nucleation density. However, how the diamond particles formed in the gas phase could serve as seeds on the substrate surface for the subsequent growth of a diamond film remains unknown. 

Thermochemical investigations have shown that temperature levels of about 900°C are required for the deposition of pure metal films at atmospheric pressure unless the carbonyl is fed as an extremely dilute gas mixture. At these high temperatures, however, gas-phase nucleation is a serious problem in obtaining dense and adherent films [184]. 

Another strategy to synthesize the particle from volatile organometallic precursors is achieved by promoting homogeneous gas-phase nucleation, the so-called chemical vapor synthesis (CVS). " In this case, the precursor is evaporated using a carrier gas and reacted with a co-substrate (e.g., O2) to produce the desired material, which can be collected as powder. The typical experimental setup is assembled according to Figure... 

The study of small and intermediate-sized clusters has become an important research field because of the role clusters play in the explanation of the chemical and physical properties of matter on the way from molecules to solids/ Depending on their size, clusters can show reactivity and optical properties very different from those of molecules or solids. The great interest in silicon clusters stems mainly from the importance of silicon in microelectronics, but is also due in part to the photoluminescence properties of silicon clusters, which show some resemblance to the bright photoluminescence of porous silicon. Silicon clusters are mainly produced in silicon-containing plasma as used in chemical vapor deposition processes. In these processes, gas-phase nucleation can lead to amorphous silicon films of poor quality and should be avoided.On the other hand, controlled production of silicon clusters seems very suitable for the fabrication of nanostructured materials with a fine control on their structure, morphological, and functional properties. ... 

There exists an extensive literature on nucleation theory. A great diversity of problems have been discussed. They range from homogeneous gas phase nucleation,to condensation of the primodial vapor in the solar system to form meteorites, and to formation of voids in nuclear reactor materials. Several collections of review articles " as well as a book have been published recently devoted solely to nucleation problems. In these works, each author has advocated his own particular approach to nucleation theory or dealt solely with his own pet nucleation problem. 

Initially we will examine homogeneous gas phase nucleation. This is the classic test problem of nucleation theory. It is believed that condensation of a gas is the simplest nucleation problem. This problem provides a test of the conceptual usefulness of nucleation theory. The lessons learned in discovering how to solve this as yet unsolved problem will hopefully provide a guide to more complex nucleation problems. 

In Section 2 we will examine very carefully the mathematical formalism for calculating nucleation rates. The expressions obtained in this section will be appropriate for homogeneous gas phase nucleation. Although the formal solutions appropriate to this problem are well known, they are worth examining closely as they must be thoroughly understood in order to treat complex nucleation phenomena. 

In Section 3 we will discuss actual homogeneous gas phase nucleation rate calculations. Extensive reliable experimental data exist and the predictions of various nucleation theories will be compared with experiment. The principle purpose of this section will be to undemtand fully what must be reliably known in order to predict nucleation behavior accurately. As we will show, the most serious problem in homogeneous nucleation theory is the... 

In the particular case of homogeneous gas phase nucleation with the rate equations expressed as in Eq. (16), it is actually straightforward to obtain an analytic expression for rig. 

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