Arthur Sherman - Chemical Vapor Deposition for Microelectronics (779637), страница 8
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J.Phys. Chern. 68: 3797 (1964).9. Wan, C.F., and Spear, K.E., in: Proc. Sixth International Conference onChemical Vapor Deposition (L.F. Donaghey, P. Rai-Choudhury, R.N.Tauber, eds.), p. 47, Electrochemical Society, Princeton, NJ (1977).These figures were originally presented at the Fall 1977 Meeting ofthe Electrochemical Society, Inc. held in Atlanta, Georgia.10. Besmann, T.M., and Spear, K.E., Analysis of the chemical vapor deposition of titanium diboride.
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Sherman, A., Numerical modeling of CVD reactors including gas phasekinetics. To be published.Fundamentals of Thermal CVO3914. Eversteijn, F.C., Severin, P.J.W., v.d. Brekel, C.H.J., and Peek, H.l., Astagnant layer model for the epitaxial growth of silicon from silane ina horizontal reactor. JECS 117 :925 (1970).15. Kee, R.J., Warnatz, J., and Miller, J.A., A FORTRAN computer codepackage for the evaluation of gas-phase viscosities, conductivities, anddiffusion coefficients. Sandia Report SAN 083-8209 (March 1983).16. Zucros, M.J., and Hoffman, J.D., Gas Dynamics, Vol.
II, New York, J.Wiley & Sons (1977).17. Newman, J.S., Electrochemical Systems, New Jersey: Prentice-Hall (1983).18. Claassen, W.A.P., Bloem, J., Valkenburg, W.G.J., and Van den Brehel,C.H.J., The deposition of silicon from silane in a low-pressure hot-wallsystem.J. Crys. Growth 57:259 (1982).19. Murarka, S.P., Interactions in metallization systems for integrated circuits,J.
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Solid State Techn. 20(4) :63 (1977).2Fundamentals of Plasma-Assisted CVD2.1 INTRODUCTIONAs discussed in the previous chapter, CVD of thin films depends on aheterogeneous su rface reaction between a reacti ng gas and a hot surface. Inmany applications, the temperatures required to obtain commercially realisticdeposition rates are prohibitive. For example, depositing titanium nitride ontool steel has to be done at temperatures above the softening temperature ofthe tool steel.
This necessitates hardening the tool steel after the CVD deposition, making it difficult to maintain critical dimensions. A similar difficultyoccurs if si Iicon nitride is to be deposited on integrated circuits as a final passivation layer. Since the final metallization is aluminum. which melts at about600°C, and the temperature necessary to deposit silicon nitride thermally is800° to 900°C, this clearly cannot work, although such a film would be an excellent barrier film.One solution to such difficulties is to use a glow discharge in the reactinggas to create a higher than equilibrium quantity of free radicals.! The flux ofsuch free radicals to a sol id surface at low temperatures is then sufficient topermit acceptable deposition rates.
For example, plasma-enhanced CVD ofsilicon nitride has been a commercial process in the integrated circuit industry2for some years. Good quality films are prepared at 350°C. Other films havealso been deposited by the same technique. These include amorphous silicon,3iron oXide,4 epitaxial silicon,s as well as refractory metal and metal silicides,6among others.
Silicon dioxide films have also been produced by this technique 7 in spite of the fact that sil icon diox ide can be deposited thermal! y atabout 400°C. This points out the fact that there are often reasons other thanlow temperature deposition for using plasma-enhanced CVD. For silicon dioxide,the interest is in varying the film stoichiometry, which cannot be done thermally. For amorphous sil icon used for solar cells, a substantial quantity of hydrogen is incorporated in the film when a plasma is used.
The hydrogen incorporated enables the amorphous sil icon to behave as a semiconductor. 340Fundamentals of Plasma-Assisted CVD41Commercial plasma-enhanced CVD reactors all operate with RF glowdischarges, creating a low pressure plasma. The reacting gases are ionized anddissociated by electron impact, and in the bulk of the plasma, there are equalnumbers of electrons and ions. The electron temperature can be on the orderof 20,OOOoK or higher, while the gas temperature remains near room temperature, depending on the pressure at which the discharge is operated. Alongelectrode surfaces, a thin sheath forms where electrical charge neutrality is nolonger maintained.
The behavior of the electrons, ions, and atoms as theytraverse the sheath on the way from the plasma to a surface, often deterrriinesthe nature of the resulting film.In addition to plasma-enhanced CVD reactors that we will study in thistext, there is another important commercial application for low-pressure plasmasin "dry" etching. s Here, a plasma converts an inert gas (i.e., from -CF 4 ) to amixture including free fluorine, which can then etch Si, Si0 2 , etc. There aretwo advantages. One, the starting gas is inert and, two, anisotropic etching ispossible due to ion bombardment.In the balance of this chapter, we will discuss plasmas in more detail toestablish a basis for a more complete understanding of the plasma-enhancedCVD process.
In addition, we will discuss reactor configurations and their influence on the films grown. In a later chapter, we will review in some detail themetallic and dielectric films that are being deposited by such techniques.2.2 PLASMASA plasma is characterized as a continuum gas, which is partially ionizedand wh ich has equal number densities of electrons and ions (charge neutral ity)at each point in the field. When a plasma is subjected to an applied electricfield, the electrons can achieve higher energies than heavy particles (atoms,molecules, ions) on average. They can then create many more free radicalsby electron impact than would be possible thermally.2.2.1 Elevated Electron Temperatures in PlasmasThe relationship of the applied electric field to the resulting elevated electron temperature can be seen by a simple analysis.
Consider a high velocityelectron colliding with an initially stationary heavy particle, as shown below.42Chemical Vapor Deposition for MicroelectronicsConservation of energy and axial momentum for this system gives.2 + 11(1 )"2 me ve.22" mi viand(2)Replacing vi in Equation (1) from Equation (2) gives12 me ve2 _ 12•2 _me ve-12mem;2[mi2ve2.~or(3)and the energy an electron loses in an elastic collision with a heavy particle inone collision is (2m e/md times the electrons' initial energy. Since the electronmass (me) is much smaller than the ion mass (miL we see that the electronloses very little energy in such collisions.
Now, if we think in terms of a continuum with thermal energy, the average particle energy will be 3/2 kT, wherek is the Soltzman constant. Then, the energy lost by an electron in such acoil ision is(4)/:;.t:__ 2m e [33]m"2 k Te -"2 k Tiwhere T e is the electron temperature and T is the heavy particle temperature.The energy gain of an electron in an electric field equals the force on ittimes the distance it moves.
In a time T, a particle acted on by a force F willmove x distance given byx=(~)T22"The force on an electron in an electric field isF=eE"So the distance it travels isx =2(~: ) 2 "TFundamentals of Plasma-Assisted CVD43Therefore, the energy the electron gains between collisions is(5)and we equate Equations (4) and (5) to obtain(6)i k [Te - T].For large values of E or T, we have T e ~ T. Large values of T correspond tolower pressure regimes, such as are found in glow discharges. The common example is the fluorescent lamp discharge which operates with a low pressureglow discharge in mercury vapor.2.2.2 Characteristic Parameters in PlasmasIn looking at plasmas in the most general way, we can categorize phenomena in terms of characteristic parameters.
For example, a simple gas canbe dealt with in terms of a mean-free path, mean collision frequency, or meanthermal energy. If the mean-free path is less than the dimensions of the vesselholding the gas, it can be treated as a continuum. Otherwise, we would haveto look at free molecular flow. In contrast, a plasma has many more characteristic parameters than a simple gas.It will be instructive to consider an electron density (n e ) versus temperature(T) plot 9 for a deuterium plasma, as shown in Figure 1. This plot was developedfor a deuterium plasma where the characteristic length is 1 cm. On this plotare displayed lines of constant magnetic field intensity (1, 104 and 108 gauss).Since we are dealing with a continuum plasma, these are the values of magneticfield necessary for the Lorentz force (J x B) to equal or exceed the pressuregradient. The main features of the plot are shown by the five regions (S, T, M,EM, and E).
They define the electron densities and temperatures where theplasma has scalar or tensor properties and where electric and/or magnetic fieldeffects predominate.The different regions are delineated by the Debye length (d), the meanfree path (A), and the Larmor radius rL). In a plasma, there can be many meanfree paths, since there are many different types of particles (different neutralspecies, electrons and ions). Of primary interest are the mean-free paths forcollisions between electrons and heavy particles (A e ) and ions and heavy partiel es (AD).If a magnetic field exists in a plasma, charged particles will tend to gyrateabout magnetic field Iines between coil isions. The radius of the circular pathit takes is called the Larmor radius and is expressed as44Chemical Vapor Deposition for MicroelectronicsCenter of sund ... Debye lengthrL =Larmor radiusA.
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