Stimulated CVD

The reaction rate limitation presents a practical lower limit to CVD process temperatures in many cases. This is a problem because high temperatures allow reactions among layers in the substrate and permit bulk diffusion. Many substrate materials, especially polymers, are also unable to withstand high process temperatures. Therefore it is of interest to reduce the temperature. Furthermore, some reactants are too stable to be used in conventional CVD, and wdl never decompose under normal CVD conditions. The classic example is N2. These restrictions have driven the development of methods to stimulate CVD reaetions. A stimulated reaction may not require as much heat from the substrate to proceed, therefore 

Light enters through vacuum system windows 

optical absorption in the gas phase can be used to probe the reactant concentrations. Unfortunately, this stimulation method is difficult to implement and hard to control. Windows through which the stimulating light passes become coated with a film over time, which reduces intensity. Finding exactly which wavelength to use for selective stimulation requires more experimentation than most process developers care to apply. As with normal incidence photo-assisted CVD, the parallel incidence method has become relatively rare. 

For PECVD to work the reactant molecules must retain their energy long enough on the surface to reach a potential reaction site and to react. Furthermore, to obtain adequate film properties the surface temperature cannot be reduced below that necessary for atoms to diffuse to acceptable growth sites after reaction and to eliminate ciystalline defects. This explains why very large reductions in substrate 

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Photosensitization is used for large-area photochemically stimulated CVD, because the generation of a sufficient photon flux over a large area to drive the chemistry directly is difficult. Usually, Hg excited by an external Hg lamp is used as a sensitizer. The energy in the excited Hg is then transferred to other gas-phase species that decompose and react to form a thin film. The process is used in horizontal reactors for the deposition of SiOj and SiN Hs from SiH4, NzO, and NH3 (40-42) and to assist the deposition of CdHgTe, in which Hg is a natural gas-phase constituent (43). 

Stimulated CVD adds energy to the reactants such that the substrate need not be heated as much as in a conventional CVD process. The most common method is plasma-enhanced CVD. The reaction is generally transport limited as a result of the added energy. 

In chemical vapor deposition (CVD) complex shaped surfaces can be coated with homogeneous layers, especially when carried out at low pressure (LPCVD, low pressure chemical vapor deposition) (review Ref. [410]). A gas reacts with the heated substrate surface to give a solid coating and gaseous by-products which have to be removed continously. Layer thicknesses created by chemical vapor deposition are usually in the order 5-10 pm.. In cases where it is necessary to keep the temperature low, a plasma can stimulate the surface reaction in plasma enhanced chemical vapor deposition (PECVD). 

Renewed interest in the optical properties of silicon films prepared by various new methods, like chemical vapor deposition (CVD), glow-discharge deposition, and sputter deposition, also stimulated investigations of the characteristics of these films for optical recording. 

Commercial n-type 6H-SiC wafers with a concentration of uncompensated donors Nd - Na = 3x lO cm were used as substrates. These substrates were covered with a thin epitaxial n-GaN buffer layer grown by hybrid vapor phase epitaxy (HVPE). This buffer layer had a thickness of 0.2 pm and a donor concentration of Nd-Na = (2-3)xl0 cm . On top. Mg doped / -type AlGaN epitaxial layers with thickness of 0.8 pm and an Al content of 12 at.% were also grown by HVPE with an acceptor concentration of Na-Nd = (5-8)xl0 cm. Then, Ga doped n-ZnO epitaxial layers were deposited using chemical vapor deposition (CVD) with a thickness of 0.8 pm and a donor concentration ofNd - Na = 7xl0 cm. The growth process was stimulated by a discharge plasma, that allowed to reduce the substrate temperature by up to 400 °C and thus improve the structural quality of the ZnO layers. ... 

In the Chemical Vapor Deposition (CVD) methods, the starting material undergoes specific chemical reactions at the hot surface of the substrate to form thin layers of the desired material. The reaction can be stimulated by various energy sources, e.g. plasma, giving plasma enhanced CVD (PECVD), or a laser, giving laser CVD. 

The chemical factor ( chem) shows the efficiency of the principal active discharge species in the chemical reaction of interest. In the process of NF3 dissociation and F-atom generation for CVD chamber cleaning, the chemical factor ( chem) shows the efficiency of the atoms in the gasification of impurities. In plasma-chemical reactions stimulated by vibrational excitation, chem can be presented as... 

The investigations of Schafer et al. were stimulated by observations made during HF-CVD experiments using arrays of four parallel uncoiled Ta wires, 11.5 mm from each other. Under conditions of free convection of the gas phase a strong decrease of the diamond growth rate from the edges of a wafer towards its center was noted. The authors concluded that the reason for this observation was a virtually complete conversion of carbon species to acetylene, which does not act as a growth species. 

The chemical inertness of diamond, even in hot acidic or caustic oxidizing media [81], and the possibility of making conductive CVD diamond films with resistivities down to less than 0.01 Hem [82] on a wide variety of substrates and over large areas, has in the recent years stimulated an increased interest in applications for analytical and preparative electrochemistry. In a comparative study, Swain... 

Chemical methods of material processing were known for years, existing in parallel with physical and other methods of film deposition. Recent advances in electron microscopy and scanning nanoprobe microscopy (STM, ATM) have revealed that some of the materials produced by the chemical methods have distinctive nanocrystalline structure. Furthermore, due to the achievements of colloid chemistry in the last 20 years, a large variety of colloid nanoparticles have become available for film deposition. This has stimulated great interest in further development of chemical methods as cost-effective alternatives to such physical methods as thermal evaporation magnetron sputtering chemical and physical vapor deposition (CVD, PVD) and molecular beam epitaxy (MBE). 

PECVD is a technique commonly used in microfabrication to deposit layers of insulating materials and amorphous or polycrystalline silicon. The plasma is used to help stimulate a reaction on the substrate surface of two or more species from the gas phase. The plasma helps break down the parent molecules and allows the reaction to occur at a lower temperature than cOTiventional CVD. The major advantage of PECVD, in fact, is its capability of working at a lower temperature with respect to other conventional CVD systems. For example, while deposition temperatures of 700-900 °C are required for silicon deposition in CVD, a temperature range of 250-350 °C is sufficient in the PECVD systems. 

The mechanisms by which ascorbate supplementation prevents the exacerbation of hypercholesterolemia and related CVD include an increased catabolism of cholesterol. In particular, ascorbate is known to stimulate 7-a-hydroxylase, a key enzyme in the conversion of cholesterol to bile acids and to increase the expression of LDL receptors on the cell surface. Moreover, ascorbate is known to inhibit endogenous cholesterol synthesis as well as oxidative modification of LDL (for review see 1). 

Ascorbate supplementation prevents the exacerbation of CVD associated with hypertriglyceridemia. Type III hyperlipidemia, and related disorders by stimulating lipoprotein lipases and thereby enabling a normal catabolism of triglyceride-rich lipoproteins." Ascorbate prevents the oxidative modification of these lipoproteins, their uptake by scavenger cells and foam cell formation. Moreover, we propose here that, analogous to the LDL receptor, ascorbate also increases the expression of the receptors involved in the metabolic clearance of triglyceride-rich lipoproteins, such as the chylomicron remnant receptor. 

Finally, ascorbic acid may inhibit atherosclerosis and clinical CVD by its effects on (smooth muscle) cell proliferation and its antihistamine action. Histamine stimulates platelet aggregation and leukocyte recruitment, thus potentially contributing to atherosclerotic vascular disease. The interested reader is referred to Chapters 5 and 10 in this volume for detailed information. 

CVDs are associated with higher amounts of cholesterol in the blood. The major causes of heart diseases include stress, high intake of refined sugars or oils, and constant consumption of animal fats [89]. In coronary disease, initially atherosclerotic plaques originate in the arteries, which become narrower towards the lumen, resulting in a decrease in blood flow. Initially, low density lipoproteins (LDL) deposit at scratched sites in the arteries and become oxidized after the depletion of protective substances (tocopherols). With the oxidation of LDL, the lipoproteins are oxidized and stimulate inflammatory reactions. Consequently, this... 

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