Plasma atomic layer deposition

Plasma atomic layer deposition (ALD) is optimized through modulation of the gas residence time during an excited species phase, wherein activated reactant is supplied such as from a plasma. Reduced residence time increases the quality of the deposited layer, such as reducing wet etch rates, increasing index of refraction and/or reducing impurities in the layer. For example, dielectric layers, particularly silicon nitride films, formed from such optimized plasma ALD processes have low levels of impurities remaining from the silicon precursor

Remote plasma atomic layer deposition of thin films of electrochemically active LiCoO2

One of the remaining challenges in the field of portable electronics is the miniaturization of lithium-ion batteries without decreasing their storage capacity. To tackle this challenge and to effectively integrate battery technology in even a wider variety of applications, it is essential to produce high quality thin films for all-solid-state batteries. A remote plasma ALD process for the positive electrode material LiCoO2 was developed using the combination of CoCp2 as the cobalt precursor, LiOtBu as the lithium precursor and O2 plasma as the oxidant source. The thin films were deposited at a temperature of 325 °C with a virtually linear growth rate of 0.06 nm/cycle. After annealing the samples at 700 °C for 6 minutes the high temperature phase LiCoO2 was obtained, as demonstrated by XRD and Raman spectroscopy measurements. Electrochemical charge/discharge cycling showed good electrochemical activity with a promising storage capacity

Atomic layer deposition of aluminum fluoride using Al(CH₃)₃ and SF₆ plasma

Metal fluorides typically have a low refractive index and a very high transparency and find many applications in optical and optoelectronic devices. Nearly stoichiometric, high-purity AlF3 films were deposited by atomic layer deposition (ALD) using trimethylaluminum [Al(CH3)3] and SF6 plasma. Self-limiting growth was confirmed and the growth per cycle was determined to range from 1.50 Å to 0.55 Å for deposition temperatures between 50 °C and 300 °C. In addition, the film density of ∼2.8 g cm-3 was found to be relatively close to the bulk value of 3.1 g cm-3. Vacuum ultraviolet spectroscopic ellipsometry measurements over the wavelength range of 140-2275 nm showed a refractive index n of 1.35 at 633 nm, and an extinction coefficient k of &lt;10-4 above 300 nm, for all deposition temperatures. Optical emission spectroscopy during the SF6 plasma exposure step of the ALD cycle revealed the formation of C2H2 and CF2 species, resulting from the interaction of the plasma with the surface after Al(CH3)3 exposure. On the basis of these results, a reaction mechanism is proposed in which F radicals from the SF6 plasma participate in the surface reactions. Overall, this work demonstrates that SF6 plasma is a promising co-reactant for ALD of metal fluorides, providing an alternative to co-reactants such as metal fluorides, HF, or HF-pyridine.</p

3D negative electrode stacks for integrated all-solid-state lithium-ion microbatteries

The deposition feasibility and electrochemical evaluation of highly structured negative electrode stacks for 3D-integrated batteries is demonstrated. The stacks comprise a TiN thin film, serving as both current collector and Li-barrier layer, covered by a polycrystalline Si (poly-Si) thin film as electrode material. In comparison with planar films, these poly-Si films present a storage capacity increase of about 5x for the highest pore aspect ratio electrodes. The step coverage of poly-Si can be considerably improved by growing TiN and poly-Si into wide trenches. This results in much smoother poly-Si films and significantly improved step coverage. Further optimization of the trench dimensions should result in poly-Si films with a Li-storage capacity increase of more than one order of magnitude with respect to planar films

Reaction mechanisms of atomic layer deposition of TaNx from Ta(NMe2)5 precursor and H2-based plasmas

The reaction mechanisms of plasma-assisted atomic layer deposition (ALD) of TaNx using Ta(NMe2)5 were studied using quadrupole mass spectrometry (QMS). The fact that molecule dissociation and formation in the plasma have to be considered for such ALD processes was illustrated by the observation of 4% NH3 in a H2-N2 (1:1) plasma. Using QMS measurements the reaction products during growth of conductive TaNx using a H2 plasma were determined. During the Ta(NMe2)5 exposure the reaction product HNMe2 was detected. The amount of adsorbed Ta(NMe2)5 and the amount of HNMe2 released were found to depend on the number of surface groups generated during the plasma step. At the beginning of the plasma exposure step the molecules HNMe2, CH4, HCN, and C2H2 were measured. After an extended period of plasma exposure, the reaction products CH4 and C2H2 were still present in the plasma. This change in the composition of the reaction products can be explained by an interplay of aspects including the plasma-surface interaction, the ALD surface reactions, and the reactions of products within the plasma. The species formed in the plasma (e.g., CHx radicals) can re-deposit on the surface and influence to a large extent the TaNx material composition and propertie

Atomic layer deposition of aluminum fluoride using Al(CH₃)₃ and SF₆ plasma

Metal fluorides typically have a low refractive index and a very high transparency and find many applications in optical and optoelectronic devices. Nearly stoichiometric, high-purity AlF3 films were deposited by atomic layer deposition (ALD) using trimethylaluminum [Al(CH3)3] and SF6 plasma. Self-limiting growth was confirmed and the growth per cycle was determined to range from 1.50 Å to 0.55 Å for deposition temperatures between 50 °C and 300 °C. In addition, the film density of ∼2.8 g cm-3 was found to be relatively close to the bulk value of 3.1 g cm-3. Vacuum ultraviolet spectroscopic ellipsometry measurements over the wavelength range of 140-2275 nm showed a refractive index n of 1.35 at 633 nm, and an extinction coefficient k of &lt;10-4 above 300 nm, for all deposition temperatures. Optical emission spectroscopy during the SF6 plasma exposure step of the ALD cycle revealed the formation of C2H2 and CF2 species, resulting from the interaction of the plasma with the surface after Al(CH3)3 exposure. On the basis of these results, a reaction mechanism is proposed in which F radicals from the SF6 plasma participate in the surface reactions. Overall, this work demonstrates that SF6 plasma is a promising co-reactant for ALD of metal fluorides, providing an alternative to co-reactants such as metal fluorides, HF, or HF-pyridine.</p

Plasma-enhanced atomic layer deposition of tungsten oxide thin films using (tBuN)2(Me2N)2W and O2 plasma

The growth of tungsten oxide (WO3) thin films by atomic layer deposition (ALD) offers numerous merits including atomic-scale thickness control at low deposition temperatures. In this work, we have developed and characterized a new plasma-enhanced ALD process for WO3 thin films using the metalorganic precursor (tBuN)2(Me2N)2W and O2 plasma as co-reactant over a wide temperature range of 100 °C-400 °C. The influence of deposition temperature on the growth behaviour and film properties is investigated in detail. The WO3 ALD process developed in this work yields a relatively high growth per cycle (GPC) which varies from ~0.7 Å at 100 °C to ~0.45 Å at 400 °C, as-determined by in-situ spectroscopic ellipsometry (SE). Rutherford backscattering spectrometry (RBS) measurements revealed a mass density of 5.9 g/cm3 and near stoichiometric film composition (O/W = 2.9). Both RBS and X-ray photoelectron spectroscopy (XPS) measurements confirmed no detectable C as well as N impurity incorporation. Grazing incidence X-ray diffraction (GI-XRD) measurements indicated that the films deposited at 400 °C were polycrystalline in nature

In situ spectroscopic ellipsometry for atomic layer deposition

The application of in situ spectroscopic ellipsometry during thin film synthesis by atomic layer deposition (ALD) is examined for results obtained on Al2O3, TaN2, and TiN films with thicknesses ranging from 0.1 to 100 nm. By analyzing the film thickness and the energy dispersion of the optical constants of the films, the layer-by-layer growth and material properties of the ALD films can be studied in detail. The growth rate per cycle and the nucleation behavior of the films can be addressed by monitoring the thickness as a function of the number of cycles. It is shown that from the energy dispersion relation, insight into the conductive properties of metallic films can be derived. Moreover, the shape of the dispersion relation can be used to discriminate between different material compositions

Optical modeling of plasma-deposited ZnO films : electron scattering at different length scales

In this work, an optical modeling study on electron scattering mechanisms in plasma-deposited ZnO layers is presented. Because various applications of ZnO films pose a limit on the electron carrier density due to its effect on the film transmittance, higher electron mobility values are generally preferred instead. Hence, insights into the electron scattering contributions affecting the carrier mobility are required. In optical models, the Drude oscillator is adopted to represent the free-electron contribution and the obtained optical mobility can be then correlated with the macroscopic material properties. However, the influence of scattering phenomena on the optical mobility depends on the considered range of photon energy. For example, the grain-boundary scattering is generally not probed by means of optical measurements and the ionized-impurity scattering contribution decreases toward higher photon energies. To understand this frequency dependence and quantify contributions from different scattering phenomena to the mobility, several case studies were analyzed in this work by means of spectroscopic ellipsometry and Fourier transform infrared (IR) spectroscopy. The obtained electrical parameters were compared to the results inferred by Hall measurements. For intrinsic ZnO (i-ZnO), the in-grain mobility was obtained by fitting reflection data with a normal Drude model in the IR range. For Al-doped ZnO (Al:ZnO), besides a normal Drude fit in the IR range, an Extended Drude fit in the UV-vis range could be used to obtain the in-grain mobility. Scattering mechanisms for a thickness series of Al:ZnO films were discerned using the more intuitive parameter "scattering frequency" instead of the parameter "mobility". The interaction distance concept was introduced to give a physical interpretation to the frequency dependence of the scattering frequency. This physical interpretation furthermore allows the prediction of which Drude models can be used in a specific frequency range