Initial Stage Of Atomic Layer Deposition Research Articles

Nanotribological Properties of ALD-Made Ultrathin MoS2 Influenced by Film Thickness and Scanning Velocity.

Solid lubricating films are usually applied to reduce friction and adhesion of micro-/nano-electromechanical systems (MEMS/NEMS). Due to the special structure, MoS2 is endowed with excellent lubricity. Layer-controlled ultrathin MoS2 films with strong interactions to the underlying substrates were prepared by atomic layer deposition (ALD) using MoCl5 and H2S and characterized by various measures. Nanotribological properties of the MoS2 films with different thicknesses were observed by an atomic force microscope tip under various loads. In the initial stage of ALD, bigger roughness induced by discontinuous nanoparticles can increase friction. However, when the number of ALD cycles is more than 3, the friction force can be effectively reduced by about 40-55% because of interlayer sliding and lower adhesion. Finally, friction properties against sliding velocities of the MoS2 films were also observed. The ultrathin MoS2 film obtained by ALD exhibits a huge application value as reducing friction surface in MEMS/NEMS.

Role of interfacial layer on complementary resistive switching in the TiN/HfOx/TiN resistive memory device

The role of the bottom interfacial layer (IL) in enabling stable complementary resistive switching (CRS) in the TiN/HfOx/IL/TiN resistive memory device is revealed. Stable CRS is obtained for the TiN/HfOx/IL/TiN device, where a bottom IL comprising Hf and Ti sub-oxides resulted from the oxidation of TiN during the initial stages of atomic-layer deposition of HfOx layer. In the TiN/HfOx/Pt device, where formation of the bottom IL is suppressed by the inert Pt metal, no CRS is observed. Oxygen-ion exchange between IL and the conductive path in HfOx layer is proposed to have caused the complementary bipolar switching behavior observed in the TiN/HfOx/IL/TiN device.

Atomic Layer Deposition of Hafnium Oxide from Tetrakis(ethylmethylamino)hafnium and Water Precursors

We have calculated the atomistic mechanism for the HfO2 atomic layer deposition (ALD) using Hf(NEtMe)(4) and H2O precursors using density functional theory. On hydroxylated Si surface, our results show overall Hf(NEtMe)(4) half-reaction is exothermic by 1.65 eV with a small activation barrier of 0.10 eV. The activation barriers for water half-reaction are 0.24 and 0.20 eV. This indicates HfO2 ALD with Hf(NEtMe)(4) and H2O has a faster deposition rate than that with HfCl4 and H2O and can be run at relatively low temperature. However, on H-terminated Si surface, the Hf(NEtMe)(4) half-reaction is endothermic by 2.39 eV with high activation barriers. The H2O half-reaction is similar to that on hydroxylated surface which is kinetically favorable. The results suggest that long Hf(NEtMe)(4) pulse time and high deposition temperature is required during the initial stage of ALD on H-terminated surface. Moreover, our calculations indicate the reaction byproduct HNEtMe is likely to be bound to the surface after each half-reaction because of high desorption energy of 0.7-0.9 eV. Thus, long precursor purge time should be used if HNEtMe is needed to be removed completely.

Micro-Raman spectroscopy and X-ray diffraction studies of atomic-layer-deposited ZrO2 and HfO2 thin films

Raman spectroscopy and X-ray diffraction (XRD) methods were applied to characterize ZrO2 and HfO2 films grown by atomic layer deposition (ALD) on silicon substrates in chloride- based processes. A dramatic enhancement in spectral quality of Raman data resulted from the use of the film’s freestanding edges for experimental runs between 80 and 800 cm−1. Both techniques detected a preferential formation of a metastable phase in ZrO2 and HfO2 films at 500 and 600∘C, respectively, during the initial stages of ALD. In the case of ZrO2 films this phase was identified as the tetragonal polymorph of ZrO2 (t-ZrO2). XRD and Raman spectroscopy data showed that, in contrast to the monoclinic phase (m-ZrO2), the absolute amount of t-ZrO2 remained approximately constant while its relative amount decreased with the increase of the film thickness from 56 to 660 nm. Neither XRD nor Raman spectroscopy allowed unambiguous identification of the metastable phase formed in otherwise monoclinic HfO2 films.

Analysis of a transient region during the initial stage of atomic layer deposition

In atomic layer deposition (ALD), it is well known that a linear relationship exists between the deposited film thickness and the number of deposition cycles, which is due to its inherent characteristics of self-limited surface reaction between reactants. However, during the initial stage of ALD, the outermost surface is gradually converted from pre-deposited substrates into an actual film as ALD of the film proceeds. Therefore, a transient region should exist, which causes a nonlinear dependence of film thickness on the number of deposition cycles, because the characteristics of the surface adsorption of reactants is dependent on the exposed film surface. To estimate the accurate film thickness, especially for film thickness less than 10 nm, we propose a simple analytical kinetic model in the transient region. The experimental results of TiN–ALD performed on the SiO2 substrate are consistent with the existence of the transient region. Furthermore, it has been found that the probability of adsorption of a reactant to predeposited SiO2 is considerably lower than that to TiN films.