# Basics of Chemical Vapor Deposition

CVD is widely used to fabricate semiconductor devises. It depends on availability of a volatile gaseous chemical that can be converted to solid film through some thermally activated chemical reaction. Chemical Vapor Deposition can be used to produce a large variety of thin films with different precursors. It is very crucial that the chemical reaction takes place on the substrate surface only, so that a thin film can be deposited onto the substrate. If undesired chemical reactions occur in the gas phase, the solid particles can be formed which may fall onto the substrate or coat the chamber walls. To avoid the undesired chemical reaction, the substrate surface temperature, deposition time, pressure, and surface specificity should be carefully selected. The chemical reaction during a CVD process is usually accomplished in several steps. The path of chemical reactions can be altered by changing the substrate temperature. For example, when titanium tetrabromide (TiBr4) is used as a precursor to deposit titanium film, the chemical reaction is accomplished in the following steps (Mazumder and Kar, 1995):

\begin{align} & \text{TiB}{{\text{r}}_{\text{4}}}\text{(g)}\to \text{TiB}{{\text{r}}_{\text{2}}}\text{(s)+B}{{\text{r}}_{\text{2}}}\text{(g)} \\ & \text{3TiB}{{\text{r}}_{\text{2}}}\text{(s)}\to \text{2TiBr(s)+TiB}{{\text{r}}_{\text{4}}}\text{(g)} \\ & \text{4TiBr(g)}\to \text{3Ti(s)+TiB}{{\text{r}}_{\text{4}}}\text{(g)} \\ \end{align}

The mechanisms of chemical reactions for many CVD processes are not clear, so the chemical reactions occurring in a CVD process are often represented by a single overall chemical reaction equation. Table 1 summarizes some examples of the overall chemical reactions occurring in CVD processes (including LCVD).

Table 1 Overall chemical reaction of CVD processes (including LCVD)

 Thin films Overall reaction Temperature of reaction References Al2O3 Al(l) + H2O(g) = AlO(g) + H2(g) AlO(g) + H2O(g) = Al2O3(s) + H2(g) 1230-1255°C Powell et al. (1966) C CxHy(g)=xC(s)+(y/2)H2(g) 700-1450°C Taylor et al. (2004) GaAs GaCI(g) + (1/4)As4(g) = GaAs(s) + HCI(g) Sivaram (1995) GaAs GaAs(g) + HCI(g) =GaCI(g) + 1/4(As4(g)) + 1/2(H2(g)) 700-850°C Sivaram (1995) Ga(CH3)3+AsH3 = GaAs+3CH4 Al(CH3)3+AsH3 = AlAs+3CH 500-800°C Ueda (1996) GaN Ga(g) + NH3 = GaN(s) + (3/2)H2 (g) 650°C Elyukhin et al. (2002) Ge(s) GeH4 =Ge(s) + 2H2 Herring (1990) Si SiH4(g)=Si(s)+2H2(g) >600oC (polysilicon) >850-900oC (single crystal) Herring (1990) SiC Si(CH3)4(g)=SiC(s)+3CH4(g) 700-1450 °C Sun et al. (1998) SiO2 SiH4+ O2 = SiO2 + 2H2 Sivaram (1995) SiH4 + 2N2O = SiO2 + 2H2O + 2N2 800°C Sivaram (1995) SiH2Cl2 + 2N2O = SiO2 + 2HCI + 2N2 > 900°C Sivaram (1995) TiO2 TiCl4(g)+O2(g)=TiO2(S)+2Cl2(g) Jakubenas et al. (1997) TiN TiCl4(g)+2H2(g)+(1/2)N2(g)=TiN(s)+4HCl(g) 900°C Mazumder and Kar (1995)

Figure 1: Common CVD reactors

CVD reactors may operate at atmospheric reduced pressure (APCVD) – which varies from 0.1 to 1 atm – or at low pressure (LPCVD). The typical pressure for LPCVD is 10-3 atm. A wide variety of CVD reactors have been developed for its various applications; some of them are illustrated in Fig. 1 (Jensen et al., 1991; Mahajan, 1996). The horizontal reactor shown in Fig. 1(a) is one of the most established configurations: a rectangular duct. The wafers to be coated are placed on a heated susceptor that is tilted by about 3° in order to ensure uniformity of deposition (Mahajan, 1996). The horizontal reactor is primarily used in CVD research and epitaxial growth of silicon semiconductors (Jensen et al., 1991). In the vertical reactor shown in Fig. 1(b), the precursors are injected into a slowly-rotating susceptor on which CVD takes place (Evans and Greif, 1987). The barrel reactor shown in Fig. 1(c) is frequently used for large volume production of silicon epitaxial wafers. The wafers sit in shallow pockets on a slightly tapered, slowly rotating heated susceptor. In the CVD reactors shown in Figs. 1 (a), (b) and (c), the activation energy for chemical reaction is supplied directly to the susceptors, and the walls are either unheated or cooled. The CVD reactor shown in Fig. 1(d), however, is a hot wall tubular reactor that is heated from outside; it is commonly used to deposit polycrystalline

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Figure 2: SALD system

silicon and other dielectric films. The reactor operates at a low pressure (0.1 to 10 Torr) and is nearly at isothermal condition, with temperatures ranging from 300 to 900 °C (Jensen et al., 1991). In addition to gaseous precursors discussed above, the precursor for CVD can also be liquid as reported by Versteeg et al. (1995).

Figure 2 shows a reaction chamber for the Selective Area Laser Deposition (SALD) process (Marcus et al., 1993). In contrast to conventional CVD, in which the entire susceptor is heated, only a very small spot on the substrate is heated by a directed laser beam. Scanning of the substrate surface is accomplished by a movable table. After the first layer of the solid is deposited, consecutive layers can be deposited to build the three-dimensional part based on the CAD design. The pressure inside the chamber is usually under 1 atm and the temperature of the spot under laser irradiation can range from 700 to 1500 °C. Successful deposition of various ceramic and metallic materials using various gaseous precursors has been reported.

## References

Elyukhin, V.A., Garcia-Salgado, G., and Pena-Sierra, R., 2002, “Thermodynamic Model for Low Temperature Metalorganic Chemical Vapor Deposition of GaN,” Journal of Applied Physics, Vol. 91, pp. 9091-9094.

Evans, G., and Greif, R., 1987, “A Numerical Model of the Flow and Heat Transfer in a Rotating Disk Chemical Vapor Deposition Reactor,” ASME Journal of Heat Transfer, Vol. 109, pp. 928-935.

Herring, R.B., 1990, “Silicon Epitaxy,” In Handbook of Semiconductor Silicon Technology, edited by O'Mara, W.C., Herring, R.B., and Hunt, L.P., Noyes Publications, Park Ridge, NJ, pp. 258-336.

Jakubenas, K.J., Birmingham, B., Harrison, S., Crocker, J., Shaarawi, M.S., Tompkins, J.V., Sanchez, J., and Marcus, H.L., 1997, “Recent Development in SALD and SALDVI,” Proceedings of 7th International Conference on Rapid Prototyping, San Francisco, CA.

Jensen, K.F., Einset, E.O., and Fotiadis, D.I., 1991, “Flow Phenomena in Chemical Vapor Deposition of Thin Films,” Annu. Rew. Fluid Mech. , Vol. 23, pp. 197-232.

Mahajan, R.L., 1996, “Transport Phenomena in Chemical Vapor-Deposition Systems,” Advances in Heat Transfer, Academic Press, San Diego, CA.

Marcus, H.L, Zong, G., and Subramanian, P.K., 1993, “Residual Stresses in Laser Processed Solid Freeform Fabrication, Residual Stresses in Composites,” Measurement, Modeling and Effect on Thermomechanical Properties, Barrera, E.V. and Dutta, I., eds., TMS, pp. 257-271.

Mazumder, J., and Kar, A., 1995, Theory and Application of Laser Chemical Vapor Deposition, Plenum Publishing Co., New York, NY.

Powell, C., Blocher, M., and Oxley, J., 1966, Vapor Deposition, John Wiley and Sons, New York.

Sivaram, S., 1995, Chemical Vapor Deposition: Thermal and Plasma Deposition of Electronic Materials, Kluwer Academic Publishers, Bordrecht, Netherlands.

Sun, L., Jakubenas, K.J., Crocker, J.E., Harrison, S., Shaw, L.L., and Marcus, H.L., 1998, “In Situ Thermocouples in Micro-Components Fabricated Using SALD/SALDVI Techniques: II Evaluation of Processing Parameters,” Materials and Manufacturing Processes, Vol. 13, pp. 883-907.

Taylor, C.A., Wayne, M.F., and Chiu, W.K.S., 2004, “Microstructural Characterization of Thin Carbon Films Deposited from Hydrocarbon Mixtures,” Surface and Coatings Technology, Vol. 182, pp. 131-137.

Ueda, O., 1996, Reliability and Degradation of III-V Optical Devices, Artech House, Inc., Boston.

Versteeg, V.A., Avedisian, C.T., and Raj, R., 1995, “Metalorganic Chemical Vapor Deposition by Pulsed Liquid Injection Using an Ultrasonic Nozzle: Titanium Dioxide on Sapphire from Titanium (IV) Isopropoxide,” Journal of the American Ceramic Society, Vol. 78, pp. 2763-2768. .