Al2O3 films with precisely controlled thicknesses and excellent conformality were grown on Si(100) at low temperatures of 350-650 K using sequential surface chemical reactions. This controlled deposition was achieved by separating a binary reaction for Al2O3 chemical vapor deposition (2Al(CH3)(3) + 3H(2)O --> Al2O3 + 6CH(4)) into two half-reactions : (A) AlOH* + Al(CH3)(3) --> AlOAl(CH3)(2)* + CH4 (B) AlCH3* + H2O --> AlOH* + CH4 In the above reactions, the trimethylaluminum [Al(CH3)(3)] (TMA) and H2O reactants were employed alternately in an ABAB... binary reaction sequence where the asterisks designate the surface species. At the optimal reaction conditions, a growth rate of 1.1 Angstrom per AB cycle was measured on the Si (100) substrate using ellipsometry. These Al2O3 films had an index of refraction of n = 1.65 and a corresponding density of rho = 3.50 g cm(-3). Additional ellipsometric measurements revealed that the Al2O3 deposition rate per AB cycle decreased at substrate temperatures >450 K. The decrease in the growth rate closely matched the thermal stability of the AlOH* and AlCH3* surface species previously measured with FTIR spectroscopy. This correlation supports a reaction mechanism based on self-limiting surface chemistry. Atomic force microscope images revealed that the deposited Al2O3 films were exceptionally flat with a surface roughness of only +/-3 Angstrom (rms) after 500 AB cycles and the deposition of a film thickness of similar to 560 Angstrom. The power spectra of the surface topography measured by AFM also demonstrated that the surface roughness was nearly identical for the initial Si(100) substrate and the deposited Al2O3 films after 20-500 AB reaction cycles.