Precursor Delivery System

Even though all three reactors share the same precursor delivery system, each tool offers specific advantages. For example, a cold-wall reactor (reactor B) helps prevent decomposition of the precursor before it reaches the substrate. A pulsed aerosol injection system at low pressure (reactor C) allows the film to grow under better-defined conditions than in a continuous process (reactor A) because of the minimization of undesirable transient effects caused by the high volatility of the solvents used.46 A more detailed description of each of the conditions for film growth, including reactor type, precursor type, delivery method, deposition temperature, growth time, and other parameters are summarized in Table 6.2. Depositions were done on bare and Mo-coated... 

Similar to chemical vapor deposition, reactants or precursors for chemical vapor synthesis are volatile metal-organics, carbonyls, hydrides, chlorides, etc. delivered to the hot-wall reactor as a vapor. A typical laboratory reactor consists of a precursor delivery system, a reaction zone, a particle collector, and a pumping system. Modification of the precursor delivery system and the reaction zone allows synthesis of pure oxide, doped oxide, or multi-component nanoparticles. For example, copper nanoparticles can be prepared from copper acetylacetone complexes [70], while europium doped yttiria can be obtained from their organometallic precursors [71]. 

Standard CVD reactors employing the aforementioned precursors have been employed for the formation of Nb Sn films, whiskers/single-crystals, and powders. In addition, two novel reactor designs have been employed for the fabrication of continuous tapes and small diameter particles. Figure 2-12 shows apparatus for the continuous CVD growth of Nb.iSn films on metallic ribbons and wires developed by researchers at RCA [40]. The system consists of niobium and tin chlorinators, a precursor delivery system for the introduction of the metal chlorides plus reactive HCI and H2 gases, and a deposition chamber. The ends of the reaction chamber are fitted with carbon elec-... 

Surfx Technologies has developed a device for PECVD [54]. A volatile precursor containing the coating elements is fed separately to the source. The precursor and plasma gas mix together, react, and deposit the desired thin film. The A-PD precursor delivery system includes a temperature-controlled bath, gas manifold system with vent/run, and dilution mass-flow controllers, which provide accurate delivery of precursors and act as moisture barriers. 

The plasma source and the precursor delivery system are shown in Fig. 20.34. 

Figure 20.34 (A) Device for PECVD. (B) The A-PD precursor delivery system. (Reproduced with permission from Surfx Technologies).

The vapor delivery system for sublimeable soM precursors is similar to liquid precursor delivery systems, specifically in its use of a carrier gas. Heating the solid precursor increases the sublimation rate. Sublimeable solid precursors are the most difficult to handle for the following reasons. The sublimation rate of a solid is a function of temperature, the quantity of the solid precursor in the vessel and the carrier gas flow rate. Consequently, the delivery rate of precursor to the chamber can vary during the course of consecutive experiments unless the quantity and form of the starting material is the same at the start of each experiment However, if care is taken this is typically not a problem. 

Adjusting the residence time of the precursor molecules by changing the gas flow rate, the pressure difference between the main chamber and precursor delivery system occurs. Then the temperature of the hot wall reactor results in the fertile production of nanosized particles of ceramics and metals instead of thin films as in CVD processing. Composite structures or more complex oxides such as BaTiOs can be formed by this technique. 

A number of new approaches to the way in which precursors are delivered to a substrate have been developed in which the precursor is dispersed into the gas phase without having to be volatile. These new systems each have their own advantages for a particular precursor depending on its physical state. Most of the delivery systems in use can be classified as one of the following a liquid injection system (LIS), where a precursor is vaporized directly from a neat liquid or solution containing the precursors a solid delivery system (SDS), where the precursor is vaporized... 

Noncovalent interactions play a special role in synthetic procedures used to assemble various types of supramolecular species. These syntheses rely on the stabilization provided by non-covalent interactions between recognition sites incorporated within precursors. Various types of non-covalent interactions can be used as a recognition motif utilized to guide the synthesis.Targeted synthesis of macro- and supramolecular structures of various sizes, shapes, and functionality has now become possible. Supramolecular chemistry offers incredible applications in various fields such as medicinal chemistry (drug delivery systems),host-guest chemistry,catalysis,and molecular electronics. ... 

O Brien P, Pickett NL, et al (2002) Developments in CVD delivery systems, A chemist s perspective on the chemical and physical interactions between precursors. Chemical Vapor Deposition 8(6), 237-249... 

India Declared in June 1997 that it possessed a chemical weapons stockpile. Has begun to destroy its stockpile under the CWC. Its industry retains the ability to produce agent precursors - chemicals that can be used in chemical weapons production. Delivery systems include short-range anti-ship cruise missiles, air-launched tactical missiles, fighter aircraft, artillery and rockets. 

The role of this system is to generate precursor vapour, and then deliver it to the CVD reactor. Gaseous precursors are preferred because they can be readily metered to control the gas flow parameters. Liquid and solid reactants can usually be vaporised at suitable temperatures and transported by a carrier gas to the reaction chamber. A typical gas delivery system with four delivery lines is shown in Figure 3.3. In lines 1, 2 and 3 gas flows and their associated parameters (e g. flow rate, pressure) are regulated by three mass flow controllers and the relevant valves. H2, MTS-H2 and Ar gases are conveyed to the gas mixer, then to the CVD chamber. Line 4 is employed to purge the reactor before and after the CVD process. For a delivery system, three types of precursors should be considered. 

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