常压化学气相沉积(APCVD)

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•By definition, the free energy change for a reaction at equilibrium is zero, hence: Eq. (3)
∆G = − RT ln K
(K is the equilibrium constant)
• It is the equilibrium conditions of composition and activities (partial pressure for gases) that are calculated to assess the yield of a desired reaction.
• 1963: Introduction of plasma CVD in electronics. • 1968: Start of industrial use of CVD coated cemented carbides. • 1980s: Introduction of CVD diamond coatings. • 1990s: Rapid expansion of metallo-organic CVD (MOCVD) for ceramic and metal deposition.
Equipment structures
Illustration of structures of APCVD system
Example
Deposition of SiO2 on the substrate
• APCVD is often used for deposition of doped and undoped oxides. The deposited oxide has a low density and the coverage is moderate due to a relatively low temperature. Because of improved tools, the APCVD undergoes a renaissance. The high wafer throughput is a big advantage of this process SiH4 + O2→SiO2 + 2H2 (T = 430 °C, p = 105Pa) As process gases silane SiH4 (highly deluted with nitrogen N2) and oxygen O2 are used. The gases are decomposed thermal at about 400 °C and react with each other to form the desired film.
Thermodynamic of CVD
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A CVD reaction is governed by thermodynamics, that is the driving force which indicates the direction the reaction is going to proceed (if at all), and by kinetics, which defines the transport process and determines the rate-control mechanism, in other words, how fast it is going. Chemical thermodynamics is concerned with the interrelation of various forms of energy and the transfer of energy from one chemical system to another in accordance with the first and second laws of thermodynamics. In the case of CVD, this transfer occurs when the gaseous compounds, introduced in the deposition chamber, react to form the solid deposit and by-products gases.
Eq. (2)
∆Gr = ∆G o + RT ln Q f
∆Gro = ∑ Z ij ∆G o .i f
where:
Z i =stoichiometric coefficient of species “i” in the CVD reaction (negative for reactants, positive for products) ∆G o .i = standard free energy of formation of species f • “i” at temperature T and 1 atm. • R = gas constant • T = absolute temperature zi • Q = Π i ai • ai = activity of species “i” which is = 1 for pure solids and = pi = xi P for T gases • pi =partial pressure of species “i” • x = mole fraction of species “i” • i = total pressure
Atmospheric Pressure CVD （APCVD） ）
Luomin(罗敏)
History of CVD
• What is APCVD? • Chemical vapor deposition may be defined as the deposition of a solid on a heated surface from a chemical reaction in the vapor phase. It belongs to the class of vapor-transfer processes which is atomistic in nature, that is the deposition species are atoms or molecules or a combination of these. • APCVD is a CVD method at normal pressure (atmospheric pressure) which is used for deposition of doped and undoped oxides. The deposited oxide has a low density and the coverage is moderate due to a relatively low temperature .
Deposition Sequence
The sequence of events taking place during a CVD reaction can be summarized as follows: 1. 2. 3. 4. 5. Reactant gases enter the reactor by forced flow. Gases diffuse through the boundary layer. Gases come in contact with surface of substrate. Deposition reaction takes place on surface of substrate. Gaseous by-products of the reaction are diffused away from the surface, through the boundary layer.
Characteristics (advantages)
I. II. simple reaction system Depositional speed is fast
III. Low temperature
Characteristics (limitations)
•
Eq. (1)
o ∆Gr = ∑ ∆G o products − ∑ ∆G o reactants f f
The free energy of formation is not a fixed value but varies as a function of several parameters which include the type of reactants, the molar ratio of these reactants, the process temperature, and the process pressure. This relationship is represented by the following equation:
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∆G Calculations and Reaction Feasibility
• The first step of a theoretical analysis is to ensure that the desired CVD reaction will take place. This will happen if the thermodynamics is favorable, that is if the transfer of energy—the free-energy change of the reaction known as ∆Gr—is negative. To calculate ∆Gr, it is necessary to know the thermodynamic properties of each component, specifically their free energies of formation (also known as Gibbs free energy), ∆Gf. The relationship is expressed by the following equation:
Hale Waihona Puke Baidu
• CVD is not a new process • its first practical use was developed in the1880s in the production of incandescent lamps to improve the strength of filaments by coating them with carbon or metal. • 1960: Introduction of the terms CVD and PVD to distinguish “chemical vapor deposition” from “physical vapor deposition.” • 1960: Introduction of CVD in semiconductor fabrication. • 1960: CVD TiC coating on cemented carbide tools introduced and development of CVD tungsten.
Fundamentals of APCVD
Main factors influence the deposition
a) Gas Velocity b) Reactant-Gas Concentration c) Temperature
Illustration of a horizontal APCVD reactor