Thermal laser CVD

Plasma CVD and thermal laser CVD are also used particularly in the deposition of GaAs. The formation of epitaxial GaAs at 500°C and polycrystalline GaAs at 185°C has been reported,... 

If nitrogen is used, the ideal deposition temperature is 1000°C. The deposition temperature is lower for the ammonia reaction (575 700°C). Plasma processing can be used to reduce the processing temperature to 500°C . Thermal laser CVD has also been used to deposit TiN at reduced temperature . In an alternate approach, titanium tetraiodide is the precursor (with no plasma) at a deposition temperature under 450°CT... 

Thermal laser CVD involves the same chemical deposition processes that occur in thermal CVD. It, therefore, is applicable to depositing the same types of materials that the thermal CVD process does. Its major application is the direct writing of thin films in semiconductor processing. 

Laser CVD involves essentially the same deposition mechanism and chemistry as conventional thermal CVD and theoretically the same wide range of materials can be deposited. Some examples of materials deposited by laser CVD are listed in Table 5.2.h Hi8]... 

In addition to thermal MOCVD, laser CVD hasbeen successfully demonstrated. 0... 

Pyrolytic-laser-assisted CVD is analogous to thermally driven CVD, but instead of a diffuse heating source, a focused laser beam is used to define deposition areas spatially (32, 38, 39) or to heat the gas phase selectively (228). The use of laser has the added advantages of increased energy flux and rapid heating. To avoid photochemistry, the gas phase must be transparent to the radiation. 

In a pyrolytic or thermal LCVD experiment, the gas is transparent and the substrate absorbs the laser energy. This creates a so - called hot - spot on which a normal thermal CVD process occurs. Pyrolytic LCVD allows a very precise localization of the coating. In a sense, this technique may be compared to the cold - wall CVD technique in which the substrate may be heated by passing an electric current through it (resistance heating), or by induction, where the substrate itself acts as a susceptor. In these cases, the gas volume is not heated significantly (hence the name cold - wall CVD). The main difference between the cold-wall CVD and the pyrolytic laser CVD is that in the latter, the heated area can be localized and scanned very precisely. 

In general, several possible chemical reactions can occur in a CVD process, some of which are thermal decomposition (or pyrolysis), reduction, hydrolysis, oxidation, carburization, nitridization and polymerization. All of these can be activated by numerous methods such as thermal, plasma assisted, laser, photoassisted, rapid thermal processing assisted, and focussed ion or electron beams. Correspondingly, the CVD processes are termed, thermal CVD, plasma assisted CVD, laser CVD and so on. Among these, thermal and plasma assisted CVD techniques are widely used, although polymer CVD by other techniques has been reported. ... 

CVD can also be classified using its activation methods. Thermal activated CVD processes are initiated only with the thermal energy of resistance heating, RF heating or by infrared radiation. They are widely used to manufacture the materials for high-temperature and hard-to-wear applications. In some cases enhanced CVD methods are employed, which includes plasma-enhanced CVD (PECVD), laser-induced CVD (LCVD), photo CVD (PCVD), catalysis-assisted CVD and so on. In a plasma-enhanced CVD process the plasma is used to activate the precursor gas, which significantly decreases the deposition temperature. 

PVD processes typically use solid-state sources. The gas-phase species for thin-film deposition are generated from the source by thermal heating, electron beam evaporation, or sputtering. In CVD one or several gaseous precursors (gas mixtures) are activated to generate the reactive gas-phase species that forms the solid films. The precursor(s) are activated thermally ( standard CVD), within a plasma (plasma CVD), or by optical excitation (photo or laser CVD). 

As an alternative to thermal decomposition, CVD, and laser-assisted H elimination, the flash pyrolysis of soluble alkylpolysilyne in a vacuum is reported recently. Unless they are under vacuum conditions, polysilanes and polysilyne may convert to silicon carbide (SiC). The flash process, which is a rapid removal of volatile organic substances and H2 gas, enables the control of the dimension of Si structure from 2D to 3D. Flash pyrolysis of alkylpolysilyne in a vacuum above 500 °C leads to the formation of poly- Si with a minimal amount of SiH termini, although the size of the crystals is limited to the range between several pm and several nm. 

The various CVD processes comprise what is generally known as thermal CVD, which is the original process, laser and photo CVD, and more importantly plasma CVD, which has many advantages and has seen a rapid development in the last few years. The difference between these processes is the method of applying the energy required for the CVD reaction to take place. 

In addition to the thermal CVD reactions listed above, tungsten can be deposited by plasma CVD using Reaction(l)at350°C.[ ll P At this temperature, a metastable alpha structure (aW) is formed instead of the stable be.c. Tungsten is also deposited by an excimer laser by Reaction (1) at < 1 Torr to produce stripes on silicon substrate.P l... 

The preparation of CNTs is a prerequisite step for the further study and application of CNTs. Considerable efforts have been made to synthesize high quality CNTs since then-discovery in 1991. Numerous methods have been developed for the preparation of CNTs such as arc discharge, laser vaporization, pyrolysis, and plasma-enhanced or thermal chemical vapor deposition (CVD). Among these methods, arc discharge, laser vaporization, and chemical vapor deposition are the main techniques used to produce CNTs. 

The hybridizing component can also be formed directly on the surface of a pristine or modified nanocarbon using molecular precursors, such as organic monomers, metal salts or metal organic complexes. Depending on the desired compound, in situ deposition can be carried out either in solution, such as via direct network formation via in situ polymerization, chemical reduction, electro- or electroless deposition, and sol-gel processes, or from the gas phase using chemical deposition (i.e. CVD or ALD) or physical deposition (i.e. laser ablation, electron beam deposition, thermal evaporation, or sputtering). 

The reactions underlying CVD typically occur both in the gas phase and on the surface of the substrate. The energy required to drive the reactions is usually supplied thermally by heating the substrate or, in a few instances, by heating the gas. Alternatively, photons from an ultraviolet (UV) light source or from a laser, as well as energetic electrons in plasmas, are used to drive low-temperature deposition processes. 

In laser-assisted thermal CVD by gas-phase heating, the laser is used to vibrationally excite the gas (e.g., SiH4) and active film precursors (e.g., SiH2). The modeling of these processes revolves around the transport phenomena that control the access of the film precursors to the surface, as exemplified by the finite-element analysis by Patnaik and Brown of amorphous silicon deposition (228). 

Higher germanes have also been studied for the purpose of investigating routes for chemical vapor deposition (CVD) of germanium. For example, the results obtained from the sensitized thermal decomposition of Me4Ge promoted by multiphoton vibrational excitation of SF5 using a pulsed CO2 laser are consistent with the pyrolytic decomposition " that points out to a Ge—C bond cleavage as the primary process (equation 36). 

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