常压化学气相沉积APCVD

1、Atmospheric Pressure CVD （APCVD） Luomin(罗敏) History of CVDWhat 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

2、 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 .CVD i

3、s not a new processits 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 depositio

4、n.”1960: Introduction of CVD in semiconductor fabrication.1960: CVD TiC coating on cemented carbide tools introduced and development of CVD tungsten.1963: Introduction of plasma CVD in electronics.1968: Start of industrial use of CVD coated cemented carbides.1980s: Introduction of CVD diamond coatin

5、gs.1990s: Rapid expansion of metallo-organic CVD (MOCVD) for ceramic and metal deposition.Thermodynamic of CVD 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 transp

6、ort 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 thermodyn

7、amics. 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.G Calculations and Reaction FeasibilityThe first step of a theoretical analysis is to ensure that the desired CVD reaction will take

8、 place. This will happen if the thermodynamics is favorable, that is if the transfer of energythe free-energy change of the reaction known as Gris negative. To calculate Gr, it is necessary to know the thermodynamic properties of each component, specifically their free energies of formation (also kn

9、own as Gibbs free energy), Gf. The relationship is expressed by the following equation:Eq. (1)ofoforGproductsGGreactantsThe free energy of formation is not a fixed value but varies asa function of several parameters which include the type of reactants,the molar ratio of these reactants, the process

10、temperature, and theprocess pressure. This relationship is represented by the followingequation:QRTGGofrlnoifijorGZG.Eq. (2)where: =stoichiometric coefficient of species “i” in the CVD reaction (negative for reactants, positive for products) = standard free energy of formation of species“i” at tempe

11、rature T and 1 atm.R = gas constantT = absolute temperatureQ = = activity of species “i” which is = 1 for pure solids and = = for gases =partial pressure of species “i” = mole fraction of species “i” = total pressureiZoifG.ziiiaiaipTiPxipixTP(K is the equilibrium constant)KRTGln It is the equilibriu

12、m conditions of composition and activities (partial pressure for gases) that are calculated to assess the yield of a desired reaction.By definition, the free energy change for a reaction at equilibrium is zero, hence:Eq. (3)Deposition Sequence Reactant gases enter the reactor by forced flow. Gases d

13、iffuse 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. The sequence of events taking place during a CVD reaction c

14、an be summarized as follows: Fundamentals of APCVD Illustration of a horizontal APCVD reactor Gas Velocity Reactant-Gas Concentration TemperatureMain factors influence the depositionIllustration of structures of APCVD system Equipment structuresDeposition of SiO2 on the substrate APCVD is often used

15、 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 + O2SiO2 + 2H2 (T = 430

16、 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.ExampleCharacteristics (advantages)simple reaction systemDepositional speed is fast Low temperatureCharacteristics (