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Atomic Layer Deposition (ALD)
Atomic Layer Deposition (ALD) is a coating technique based on Chemical Vapor Deposition (CVD). The main difference is that in ALD a binary reaction is split into two half-reactions in order to achieve a precise control over the film thickness. Atomic layer deposition (ALD) provides a unique method for depositing ultrathin films on surfaces [1]. This technique uses sequential surface reactions to coat substrates with high conformality and precise thickness control at the atomic scale.
Figure 1. ALD based on sequential, self-limiting surface reaction
Coating schemes have been developed for several materials such as alumina (Al2O3), silica (SiO2), titania (TiO2), boron nitride (BN), silicon nitride (Si3N4), zinc oxide (ZnO), tungsten (W) among many others [2-7]. For the case of alumina ALD, alternate doses of trimethylaluminium (TMA) and water are performed in an ABAB. . . sequence. The growth of ultrathin and conformal Al2O3 nanolayers using sequential surface reactions is based on the binary reaction [2]: 2Al(CH3)3 + 3H2O →Al2O3 + 6CH4 Al2O3 ALD is achieved by splitting this binary reaction into two separate half-reactions [2-3]: A)
AlOH* + Al(CH3)3 --> AlOAl(CH3)2*
+ CH4 where the asterisks denote the surface species. During each dose, the reactants completely saturate the active sites on the substrate, making this a self-limiting and self-controlling process. The thickness of the film deposited after sequential exposures of Al(CH3)3 and H2O. Much work has been done on the deposition of ultrathin films using ALD on flat surfaces. However, the interest of modifying the surface chemistry of fine particles has continuously grown over the past years. The growth of ultrathin films on nanoparticles has been successfully demonstrated at small scale [ 8-10 ] for several ALD systems. Metal and ceramic micron-sized particles have been coated with alumina using ALD in a fluidized bed (ALD-FBR). Bulk processing of ultrafine particles allowed testing the improved properties of coated particles. Enhanced oxidation resistance and increased surface reactivity are some examples of these superior characteristics [11-13] . As the interest in the use of nanoparticles continuously grows the need for bulk processing also rises. The conformal nanocoating of nanoparticles at a large scale using a fluidized bed reactor has been demonstrated [14-16]. Figure 2 shows TEM micrographs of alumina-coated silica and zirconia nanoparticles by ALD-FBR.
Figure 2. Alumina-coated silica and zirconia nanoparticles
The ability to control the surface chemistry of nanoparticles at the nanoscale may open up a wide variety of new applications. The inherent characteristics of ALD make it the coating technique of choice to build the materials of the new century.
References 1. S.M. George, A.W. Ott and J.W. Klaus, "Surface Chemistry for Atomic Layer Growth", J. Phys. Chem. 100, 13121-13131 (1996). 2. A.C. Dillon, A.W. Ott, S.M. George, and J.D. Way, "Surface Chemistry of Al2O3 Deposition Using Al(CH3)3 and H2O in a Binary Reaction Sequence", Surf. Sci. 322, 230-242 (1995). 3. A.W. Ott, J.W. Klaus, J.M. Johnson and S.M. George, "Al2O3 Thin Film Growth on Si(100) Using Binary Reaction Sequence Chemistry", Thin Solid Films 292, 135-144 (1997). 4. J.W. Klaus, A.W. Ott, J.M. Johnson and S.M. George, "Atomic Layer Controlled Growth of SiO2 Films Using Binary Reaction Sequence Chemistry", Appl. Phys. Lett. 70, 1092-1094 (1997). 5. J.W. Klaus, A.W. Ott, A.C. Dillon and S.M. George, "Atomic Layer Controlled Growth of Si3N4 Films Using Sequential Surface Reactions", Surf. Sci. 418, L14-L19 (1998). 6. J.W. Klaus, S.J. Ferro and S.M. George, "Atomic Layer Deposition of Tungsten Using Sequential Surface Chemistry with a Sacrificial Stripping Reaction", Thin Solid Films 360, 145-153 (2000). 7. J.W. Elam, Z.A. Sechrist and S.M. George, "ZnO/Al2O3 Nanolaminates Fabricated by Atomic Layer Deposition: Growth and Surface Roughness Measurements", Thin Solid Films 414, 43-55 (2002). 8. J.D. Ferguson, A.W. Weimer and S.M. George, "Atomic Layer Deposition of Ultrathin and Conformal Al2O3 Films on BN Particles", Thin Solid Films 371, 95-104 (2000). 9. J.D. Ferguson, A.W. Weimer and S.M. George, "Atomic Layer Deposition of SiO2 Films on BN Particles Using Sequential Surface Reactions", Chem. Mater. 12, 3472-3480 (2000). 10. J.D. Ferguson, A.W. Weimer and S.M. George, "Atomic Layer Deposition of Boron Nitride Using Sequential Exposures of BCl3 and NH3", Thin Solid Films 413, 16-24 (2002). 11.
Wank, J.R., George, S.M., and A.W. Weimer, “Nanocoating Individual Cohesive
Boron Nitride Particles in a Fluidized Bed by ALD,” Powder Technology, 142
(1), 59-69 (2004). 12.
Wank, J.R., K.J. Buechler, L. Hakim, S.M. George, and A.W. Weimer, “Coating
Fine Iron Particles with an Oxidation-Resistance g-Alumina
Nanolayer Using ALD in a Fluidized Bed Reactor,” in Fluidization XI - Present
and Future of Fluidization Engineering,
ECI International (Brooklyn, NY) (U. Arena, R. Chirone, Ml. Miccio, and P.
Salatino, editors), 603-610 (2004). 13.
Wank, J.R., S.M. George, and A.W. Weimer, “Coating Fine Nickel Particles with
Al2O3 Utilizing an Atomic Layer
Deposition Fluidized Bed Reactor (ALD-FBR),” J. Amer. Ceram. Soc., 87
(4), 762-765 (2004). 14. Hakim, L.F., S.M. George, and A.W. Weimer, ”Conformal Nanocoating of Zirconia Nanoparticles by ALD in a Fluidized Bed Reactor,” Nanotechnology 16 (2005) S375-S381. To see this article click here 15. Hakim, L. F., Blackson, J., George, S. M., Weimer, A. W., “Nanocoating Individual Silica Nanoparticles by Atomic Layer Deposition in a Fluidized Bed Reactor” Chemical Vapor Deposition 11(10) 425-425 (2005). To see this article click here 16. Hakim, L. F., McCormick, J. A., Zhan, G. D., Li, P., George, S. M., Weimer, A. W., “Surface Modification of Titania Nanoparticles Using Ultrathin Ceramic Films” accepted for publication in Journal of the American Ceramic Society (2006)
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