Thermal CVD

The choice of heating methods largely depends on such factors as the type of deposition process and the shape, size, and composition of the substrate material, as well as the process economics. 

A simple form of heating is most applicable when the substrate has some conductivity, since passing an electric current through the substrate gives direct resistance heating. This method particularly is suitable for continuous coating of long objects such as wires, filaments, rods, tubes, and hollow fiber membranes. 

Direct resistance heating most simply is effected in a cold wall reactor. For such a reactor, the typically employed form of input energy is radio frequency (Rf) induction heating of a conducting substrate support, known as a susceptor. Typical susceptor 

More recently, heating by photo-radiation with high intensity photons from quartz iodine or tungsten filament lamps has become popular. One advantage of photoradiation is that often only the substrate is heated significantly. 

For some applications, the high temperature required for rigorous mass transport limited control, or, occasionally, to a greatly reduced extent, even the lower temperature required for kinetic limited control, can induce thermal substrate damage. One potential solution to these problems is to employ alternate forms of energy input, permitting deposition at lower substrate temperatures (see Sect. 1.3.1.3). 

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Plasmas can be used in CVD reactors to activate and partially decompose the precursor species and perhaps form new chemical species. This allows deposition at a temperature lower than thermal CVD. The process is called plasma-enhanced CVD (PECVD) (12). The plasmas are generated by direct-current, radio-frequency (r-f), or electron-cyclotron-resonance (ECR) techniques. Eigure 15 shows a parallel-plate CVD reactor that uses r-f power to generate the plasma. This type of PECVD reactor is in common use in the semiconductor industry to deposit siUcon nitride, Si N and glass (PSG) encapsulating layers a few micrometers-thick at deposition rates of 5—100 nm /min. 

Many materials have been deposited by PECVD. Typically, the use of a plasma allows equivalent-quaUty films to be deposited at temperatures several hundred degrees centigrade lower than those needed for thermal CVD techniques. Often, the plasma-enhanced techniques give amorphous films and films containing incompletely decomposed precursor species such as amorphous siUcon (i -Si H) and amorphous boron (i -B H). 

Processing variables that affect the properties of the thermal CVD material include the precursor vapors being used, substrate temperature, precursor vapor temperature gradient above substrate, gas flow pattern and velocity, gas composition and pressure, vapor saturation above substrate, diffusion rate through the boundary layer, substrate material, and impurities in the gases. Eor PECVD, plasma uniformity, plasma properties such as ion and electron temperature and densities, and concurrent energetic particle bombardment during deposition are also important. 

Describe the various processes and equipment used in R D and production such as thermal CVD, plasma CVD, photo CVD, MOCVD, and others. 

Metallo-organic CVD (MOCVD) is a specialized area of CVD, which is a relatively newcomer, as its first reported use was in the 1960s for the deposition of indium phosphide and indium anti-monide. These early experiments demonstrated that deposition of critical semiconductor materials could be obtained at lower temperature than conventional thermal CVD and that epitaxial growth could be successfully achieved. The quality and complexity of the equipment and the diversity and purity of the precursor chemicals have steadily improved since then and MOCVD is now used on a large scale, particularly in semiconductor and opto-electronic applications.91P1... 

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. 

As stated in the introduction to this chapter, CVD can be classified by the method used to apply the energy necessary to activate the CVD reaction, i.e., temperature, photon, or plasma. This section is a review of temperature-activation process commonly known as thermal CVD. 

Thermal CVD requires high temperature, generally from 800 to 2000°C, which can be generated by resistance heating, high-frequency induction, radiant heating, hot plate heating, or any combination of these. Thermal CVD can be divided into two basic systems known as hot-wall reactor and cold-wall reactor (these can be either horizontal or vertical). 

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]... 

Thermal CVD, reviewed above, relies on thermal energy to activate the reaction, and deposition temperatures are usually high. In plasma CVD, also known as plasma-enhanced CVD (PECV) or plasma-assisted CVD (PACVD), the reaction is activated by a plasma and the deposition temperature is substantially lower. Plasma CVD combines a chemical and a physical process and may be said to bridge the gap between CVD andPVD. In this respect, itis similar to PVD processes operating in a chemical environment, such as reactive sputtering (see Appendix). 

Plasma CVD was first developed in the 1960s for semiconductor applications, notably for the deposition of silicon nitride. The number and variety of applications have expanded greatly ever since and it is now a major process on par with thermal CVD. 

Advantages of Plasma CVD. As shown in Table 5.4, with plasma CVD, a deposit is obtained at temperatures where no reaction whatsoever would take place in thermal CVD. This is its major advantage since it permits the coating of low-temperature sub-... 

In addition to the thermal CVD systems mentioned above, molybdenum is deposited by plasma CVD using Reaction (3) in hydrogen.Annealing is required to remove incorporated carbon and oxygen. 

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... 

In a plasma-activated reaction, the substrate temperature can be considerably lower than in thermal CVD (see Ch. 5, Sec. 9). This allows the coating of thermally sensitive materials. The... 

By increasing the electrical energy in a fixed amount of gas, the temperature is raised and may reach 5000°C or higher.P i Such high temperatures produce an almost complete dissociation of the hydrogen molecules, the CH radicals, and other active carbon species. From this standpoint, arc-plasma deposition has an advantage over microwave-plasma or thermal CVD since these produce much less atomic hydrogen. 

In addition to the thermal CVD reactions mentioned above, plasma CVD is used for the low temperature deposition of boron.i l... 

Common deposition reactions are based on the combination of silane with various oxidizers, either as thermal CVD or plasma CVD as follows 1... 

In addition to the thermal CVD reactions described above, a glow discharge plasma at 480-650°C has been used to deposit HB2 from the mixed chlorides,... 

Two variations of the general process are used. The first is a classical thermal CVD method. The reactants are usually the halides, i.e. 

A similar deposition system uses a plasma which is produced by a traveling microwave cavity. No other source of heat is required. The deposition system is shown schematically in Fig. 16.12. The reactants are the same as in the thermal CVD process. Pressure is maintained at approximately 1 Torr. In this case, the deposition occurs at lower temperature, no soot is formed and a compact glass is produced directly. A main advantage of this method is the more accurate grading of the refractive index of the cladding material. 

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). 

It has been suggested that the difference between the lower temperatures required for PECVD (300-400 °C) and those required for thermal CVD from Si114 (500-700°C) is... 

An attempt has been reported to detect reactive intermediates in the gas-phase chemistry in thermal CVD from Si2Hg, from 300 to 1000 K and 1 to 10 torr402. The reaction... 

Using thermal CVD methods with B2H6-NH3-H2 [98, 99] or BC13-NH3-H2 [100] gas mixtures, different BN-layers can be deposited e.g., h-BN, t-BN, or a-BN. BN with higher boron contents can be deposited at enhanced deposition temperatures. To deposit crystalline h-BN from the gas phase, temperatures above 1100 °C and a N/B ratio of 10 1 are necessary. 

Using conventional thermal CVD various BN modifications but no c-BN or nano-cBN are formed. Therefore, to synthesize c-BN a plasma is applied to... 

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