Thin films of tungsten (W) and tungsten nitride (W2N) were grown with atomic layer control using sequential surface reactions. Tungsten atomic layer deposition was accomplished by separating the reaction WF6 + Si2H6 --> W + 2SiHF(3) + 2H(2) into two half-reactions. The tungsten nitride atomic layer growth was performed by dividing the reaction 2WF(6) + NH3 --> W2N + 3HF + 9/2F(2) into two half-reactions. Successive exposure to WF6 and Si2H6 (NH,) in an ABAB... reaction sequence produced W (W2N) deposition at substrate temperatures between 425-600 K (600-800 K). The W deposition rate was 2.5 Angstrom/AB cycle for WF6 and Si2H6 reactant exposures > 800 and 3000 L, respectively. The W2N deposition rate was also 2.5 Angstrom/AB cycle for WF6 and NH3 reactant exposures > 3000 and 10,000 L, respectively. Atomic force micrographs of the deposited films on Si(100) were remarkably flat indicating smooth deposition. X-ray diffraction investigations revealed that the deposited tungsten and tungsten nitride films were either amorphous or composed of very small crystalline grains. X-ray photoelectron spectroscopy demonstrated that the films contained very low impurity concentrations. The results for tungsten represent the first demonstration of atomic layer deposition (ALD) of smooth single-element films using sequential surface reactions.