Vapor precursors

In PECVD, the plasma generation region may be in the deposition chamber or precede the deposition chamber in the gas flow system. The latter configuration is called remote plasma-enhanced CVD (RPECVD). In either case, the purpose of the plasma is to give activation and partial reaction/reduction of the chemical precursor vapors so that the substrate temperature can be lowered and still obtain deposit of the same quaUty. 

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. 

Precursor Vapor pressure (torr) Comments and temperature References... 

Sb precursor Vapor pressure (torr) Minimum achievable growth temperature (alloy grown ) (°C) Comment References... 

A schematic illustration of the gas-to-particle conversion route is shown in Figure 7.37. Precursor vapors react at high temperatures to form molecules of intermediate... 

Frequently, bursts of new particles are detected within the boundary layer over land or sea. Such phenomena have been investigated using simultaneous measurements of ultrafme aerosols and their precursor vapors—namely HjSO,—which strongly constrain the mechanisms of particle formation. Observations obtained at a clean continental site at Idaho Hill, Colorado on September 21, 1993 have been analyzed with the APM [30]. Since the variations in H2S04 vapor were carefully characterized in this instance, its production rate was constrained in the simulations to match the observed... 

Figure 4.S7. SEM images of an electroluminescent phosphor particle, ZnS (used in backhght displays for cell phones, watches, etc.), before (a) and after (b) the deposition of an aluminum oxide thin film. This film is a transparent coating that prevents the phosphor particle from undergoing humidity-accelerated decay. A technique known asfluidized-bed CVD was used, where a carrier gas both delivered the precursors to a vertically aligned CVD chamber, and dispersed the powdery sample in order to expose all surface regions to the precursor vapors.

VDP of polyimides is usually performed in vacuum (pressures <10 Pa). Just as in any CVD process, the deposition parameters greatly influence the properties of the polyimide thin films. The effect of a few notable ones viz., substrate temperature and the relative fluxes of the precursor vapors is the focus of the next section. 

As the VDP of polyimides involves two precursors, diamine and dianhydride, the relative amounts of fluxes of the precursor vapors need to be controlled carefully to obtain high quality polyimide thin films. The effects of both excess dianhydride as well as the diamine components have been studied and reported in literature, It was observed that, excess dianhydride (PMDA) undergoes desorption when cured after deposition leading to poor thermal stability. 

Microwave-assisted fabrication of ceramic coatings on fibers and powders can be done at intermediate temperatures. For example, at temperatures of about 800-900° C, carbon fibers have been coated with thin TiN layers using a microwave plasma-assisted fluidized bed with the precursor vapor (TiCU in this case) being introduced into the reactor by the fluidizing gas. ... 

Thin film dielectrics are usually deposited using chemical vapor deposition (CVD). A variation of CVD utilizing a plasma discharge is called plasma-enhanced CVD (PECVD) and is the standard in IC fabrication for the deposition of dielectric films. Plasma-enhanced CVD involves the formation of a solid film in a substrate surface from volatile precursors (vapor or gas) in a plasma discharge. The precursors are chosen to contain the constituent elements of the final film and chemical reactions in the gas phase are encouraged. They are condensed in a substrate that is heated or cooled. It will be shown later that porosity can be introduced in the PECVD films. Spin coating is another preparation technique and a popular choice... 

One approach to avoiding intermolecular agglomeration is the additional coordination of the alkaline earth metal ions with neutral ligands [110]. Addition of Lewis bases such as free ligand, tetrahydrofuran, ammonia, or amines to the carrier gas has afforded some improvement in vapor pressure characteristics of the most commonly used strontium and barium MOCVD precursors, Sr(dpm)2 and Ba(dpm)2 [111-119]. These effects may be due to saturation of the Lewis acidic metal centers with the gaseous bases, thereby increasing precursor vapor pressure and stability in a transient fashion. 

Chemical vapor deposition (CVD) of polymers allows the formation of polymer thin films directly on substrates from a precursor vapor stream of monomers or oligomers. Two of the more common are briefly described. 

YSZ films were prepared a vertical cold-wall type CVD apparatus [II]. Source precursors of Zr(dpm)4 and Y(dpm)3 were vaporized at 483 to 593 and 393 to 473 K, respectively. The source vapors were carried with Ar gas into the CVD reactor. O2 gas was separately introduced by using a double tube nozzle, and mixed with precursor vapors in a mixing chamber placed above a... 

Molecular beam epitaxy (MBE). This technique is another class of PVD processes. The main difference from the traditional PVD process resides in the control of the vapor flow, which, for MBE allows the growth of monocrystalline films (epi above and taxis in ordered manner ). To achieve such a highly crystalline film, a crystalline substrate is required, the substrate must be heated to several hundred degree Celsius during deposition, and the precursor vapor must travel through a very high vacuum (10 Pa) at relatively low flow. 

Figure 12. A schematic diagram of the flow-type ALE reactor. The reaction chamber (1) consists of a quartz cup which can hold up to 20 g of porous material. The precursor vessels are for solid (2) and liquid or gaseous precursors (3). The reaction chamber, the precursor vessels, and the tubes leading the precursor vapors into the chamber are resistively heated. The heating and the flow of the precursor and carrier gas (4) are computer controlled. The reaction chamber is kept at 6-10 kPa, and the pumping takes place from the bottom of the fixed bed of the porous material (5).

In flame C, the dilution of the TiCU-laden Ar stream with the CH4 stream is not that significant since both streams have comparable flowrates 0.25 and 0.4 1/min, respectively. Furthermore, the mixing of precursor vapor and methane takes place much earlier than that in flame A and B, the newly formed titania particles stay longer in the flame and sinter, resulting in rather large primary particles (Figure 5 d - e). 

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