Pulsed CVD

Krumdieck, S. Sbaizero, O. Raj, R. 2001. Unique precursor delivery and control afforded by low-pressure pulsed-CVD process with ultrasonic atomization./. Phys. TV (Proceedings) 11 1161-1168. 

In terms of the process time control, CVD can be divided into three categories, namely continuous, discontinuous and pulsed CVD (P-CVD). Discontinuous CVD is used in most CVD processes, whereas continuous CVD is extensively used for fibre coating and high-volume production of semiconductor devices. In order to improve the coating thickness uniformity and its microstructure, P-CVD is commonly used. 

Based on pulsed CVD (P-CVD) by Bryant [55] in 1976 Sugiyama [56, 57] proposed the pulsed CVI method (P-CVI). The aim is to accelerate the in-diffusion of fresh precursor species and the out-diffusion of the by-products to and from the pores of the preform and thereby reduce the total infiltration time and density gradient along the thickness of the composites. The instantaneous introduction of precursor gases ensures the uniformity of the precursor gas composition along the depth of the pores within the preform and leads to uniform dense composites. 

Figure 6.24. Tungsten tapered tube produced by pulsed CVD [46] (a) a tube 1070 nun long and 0.75 mm thick (b) microstructure of W deposit...

DEC coating was first prepared by Aisenberg and Chabot using ion beam deposition in 1971 [2]. At present, PVD, such as ion beam deposition, sputtering deposition, cathodic vacuum arc deposition, pulsed laser deposition, and CVD, like plasma enhanced chemical vapor deposition are the most popular methods to be selected to fabricate DEC coatings. 

The approaches used for preparation of inorganic nanomaterials can be divided into two broad categories solution-phase colloidal synthesis and gas-phase synthesis. Metal and semiconductor nanoparticles are usually synthesized via solution-phase colloidal techniques,4,913 whereas high-temperature gas-phase processes like chemical vapor deposition (CVD), pulsed laser deposition (PLD), and vapor transfer are widely used for synthesis of high-quality semiconductor nanowires and carbon nanotubes.6,7 Such division reflects only the current research bias, as promising routes to metallic nanoparticles are also available based on vapor condensation14 and colloidal syntheses of high-quality semiconductor nanowires.15... 

Table 2.5 Catalytic performance of pulse reactions on the CVD-Re/ HZSM-5 (Si02/AI203 = 19) catalyst (Re 0.58 wt%) for direct phenol synthesis from benzene and 02 at 553 K in the absence of NH3a .

Fig. 1. Optical absorption spectra for various a-Si films ar.d laser treatments. E440 represents the absorption coefficient for electron-beam-deposited a-Si at 440°C substrate temperature, whereas LA and LC represent the laser-annealed and laser-crystallized spectra, respectively. The absorptivity curve of the CVD film is from Janai and Moser (1982) after pulse crystallization the curve PLA was measured.

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). 

The chemistry of ALD is similar to that of CVD, except that ALD makes nse of sequential precursor gas pulses to deposit a film layer-by-layer ALD film growth is selflimited and based on surface reactions, which makes achieving atomic-scale deposition control possible (monolayers can be obtained). In principle, the first ALD precnrsor is introduced into the process reactor producing only an adsorbed monolayer on the snbstrate surface because it cannot decompose completely without a second componnd. After the second precursor is brought into the reactor chamber it reacts with the first precnrsor to afford the desired monolayer, as illustrated in Figure 2. Since each of such cycles produces exactly one monolayer, the thickness of the resulting film may be precisely controlled by the number of deposition cycles. 

Pure hexagonal or turbostratic BN has poor sinterability, and so it is difficult to fabricate a bulky hBN compact by normal pressure sintering and even by hot pressing unless additives are used. Chemical vapor deposition allows simultaneous synthesis and consolidation of pure and dense films or shapes. CVD in the BCl3-NH3-H2-Ar reactant system was investigated in the low temperature regime 500-1300°C as well as by pulse chemical vapor infiltration in the B2H6-NH3-N2-Ar reactant system ... 

---