Heated substrate

CVD gaseous reactants (precursors) delivered to a heated substrate in a flow reactor undergo tliennal reaction to deposit solid films at atmospheric or reduced pressure, and volatile side products are pumped away. CVD is used for conductors, insulators and dielectrics, elemental semiconductors and compound semiconductors and is a workliorse in tire silicon microelectronics industry. 

MetallorganicMBE (MOMBE). tire solid source Knudsen cells in conventional MBE are replaced witli gaseous beams of organometallic precursors, directed toward a heated substrate in UHV. Compared to MOCVD, MOMBE eliminates gas phase reactions tliat may complicate tire deposition surface reactions, and provides lower growtli temperatures. 

Easily decomposed, volatile metal carbonyls have been used in metal deposition reactions where heating forms the metal and carbon monoxide. Other products such as metal carbides and carbon may also form, depending on the conditions. The commercially important Mond process depends on the thermal decomposition of Ni(CO)4 to form high purity nickel. In a typical vapor deposition process, a purified inert carrier gas is passed over a metal carbonyl containing the metal to be deposited. The carbonyl is volatilized, with or without heat, and carried over a heated substrate. The carbonyl is decomposed and the metal deposited on the substrate. A number of papers have appeared concerning vapor deposition techniques and uses (170—179). 

On the other hand, organic materials of relatively low molecular weight such as acetylene, benzene, ethylene and methane, can produce vapour-grown carbon materials by imperfect combustion or by exposing their vapour to a heated substrate in an electric furnace in the presence of a metal catalyst. The latter process generates VGCFs. Other precursors to VGCF include polyacrylonitrile (PAN), isotropic or mesophase pitch, rayon or nylon [8]. 

In spray pyrolysis, very fine droplets are sprayed onto a heated substrate. The limitations of this process are the same as for spin-on coating. The same is often the case for preparing solid electrolytes by chemical vapor deposition (CVD) processes, which in addition are more expensive, and the precursors are often very toxic. 

Chemical vapor deposition is a synthesis process in which the chemical constituents react in the vapor phase near or on a heated substrate to form a solid deposit. The CVD technology combines several scientific and engineering disciplines including thermodynamics, plasma physics, kinetics, fluid dynamics, and of course chemistry. In this chapter, the fundamental aspects of these disciplines and their relationship will be examined as they relate to CVD. 

Figure 5.2. Two of the more common types of low pressure CVD reactor, (a) Hot Filament Reactor - these utilise a continually pumped vacuum chamber, while process gases are metered in at carefully controlled rates (typically a total flow rate of a few hundred cubic centimetres per minute). Throttle valves maintain the pressure in the chamber at typically 20-30 torr, while a heater is used to bring the substrate up to a temperature of 700-900°C. The substrate to be coated - e.g. a piece of silicon or molybdenum - sits on the heater, a few millimetres beneath a tungsten filament, which is electrically heated to temperatures in excess of 2200 °C. (b) Microwave Plasma Reactor - in these systems, microwave power is coupled into the process gases via an antenna pointing into the chamber. The size of the chamber is altered by a sliding barrier to achieve maximum microwave power transfer, which results in a ball of hot, ionised gas (a plasma ball) sitting on top of the heated substrate, onto which the diamond film is deposited.

Method (2) can be further subdivided depending on whether the mixed metal vapor is quench-cooled or if a heated substrate is used to encourage atom mobility after condensation. 

Hence, the decision to use a heated substrate with simultaneous evaporation of the component metals as an aid to homogenization requires consideration of whether or not it might have an adverse effect, i.e., causing preferential nucleation of one component, which interdiffusion may not be able to remedy. It was believed (60) that in preparing Pd-Rh alloys by simultaneous deposition on a substrate at 400°C, rhodium nucleated preferentially and that crystallites grew by the addition of palladium (and rhodium) atoms. The diffusion of palladium atoms into this kernel formed a phase with 88 =t 5% Rh (phase II). The outer shell of the crystallite, phase I, was in effect a solid solution deficient in rhodium compared with the overall film composition, and the Rh content of phase I therefore increased as the Rh flux was increased. 

MBE (molecular beam epitaxy), which involves epitaxial growth of thin films on either the same material as substrate (homoepitaxial) or a lattice-matched substrate (heteroepitaxial) the heated substrate reacts with a molecular beam of compounds containing the constituent elements of the semiconductor as well as any dopants the resultant film is essentially a single crystal slow growth rates produce films from a few nanometers thick to at most several hundred nanometers that have very high purity and controlled levels of dopants. 

In the absence of suitable volatile precursors, aerosol-assisted CVD (AACVD) has been used, whereby a precursor solution is atomized and the micron-sized particles deposited onto a heated substrate to grow into thin films [98], LSM thin layers (4 to 5 4m) have been deposited in this way [99-100],... 

A hot solution (0.1—1 %) is added drop-wise onto a pre-heated substrate such as a glass slide or a piece of mica sheet (the substrate may have been coated with vapor-deposited C). After being dried and solidified, the resulting thin film specimen is mounted onto a specimen grid with the aid of water surface if the specimen is insoluble in water. When a glass slide is used as a substrate, a dilute aqueous solution (<0.02M) of hydrofluoric acid (HF) might be used for easy detachment. The resulting thin film can be... 

Alternate Sample Introduction — Obviously, elimination of the sample dissolution stage would greatly reduce analytical time, as it is the slowest step in the analytical scheme. Pulsed-laser vaporization using a CO2—TEA laser seems promising(63, 64). Another possibility is the introduction of a suitable prepared slurry of the sample into the nebullzer(65). Thermal vaporization studies using heated substrates such as tanta-lum(66), carbon filaments(67), or carbon rods(39) have been reported. Silvester(39) de fined the problems of vapor transport, carrier gas expansion, and solid phase chemistry associated with electrothermal sample introduction to an ICP. 

Since the envisionaged application of a CD process in thin-fihn solar cells is a large-scale one, efforts have been made to optimize the deposition process used, particularly in minimizing the waste Cd-containing solutions. Dilute Cd solutions (ca. 1 mM), a flow system with filtration, and a heated substrate have been employed to this end. The heated substrate means that deposition occurs preferentially on the substrate rather than on the cooler walls of the deposition vessel. Also, ethylenediamine has been used as a complexant rather than the much more volatile ammonia. 

Fig. 4.2 Flow system for continuous deposition using a locally heated substrate. (After Ref. 74 with permission from Elsevier Science.)...

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