A plasma-assisted atomic layer deposition (ALD) process has been developed that allows for low temperature (100 °C) synthesis of virtually 100% pure palladium thin films with low resistivity of 24 ± 3 μΩ cm on oxide substrates. This process is based on Pd(hfac) 2 (hfac = hexafluoroacetylacetonate) precursor dosing followed by sequential H 2 plasma and O 2 plasma steps in a so-called ABC-type ALD process. Gas-phase infrared spectroscopy studies revealed that the O 2 plasma pulse is required to remove carbon contaminants from the Pd surface that remain after the H 2 plasma reduction step. Omitting the O 2 plasma step, that is, Pd ALD from Pd(hfac) 2 and H 2 plasma in a typical AB-like ALD process, leads to a carbon contamination of >10% and significantly higher resistivity values. From transmission electron microscopy, it has also been observed that the ABC-type process leads to a faster nucleation of the Pd nanoparticles formed during the initial stage of film growth. As this novel process allows for the deposition of high-purity Pd at low temperatures, it opens prospects for various applications of Pd thin films and nanoparticles.
Two novel heteroleptic titanium precursors for the atomic layer deposition (ALD) of TiO 2 were investigated, namely, titanium (N,N′diisopropylacetamidinate)tris(isopropoxide) (Ti(O i Pr) 3 (N i Pr-Me-amd)) and titanium bis(dimethylamide)bis(isopropoxide) (Ti(NMe 2 ) 2 (O i Pr) 2 ). Water was used as the oxygen source. These two precursors are liquid at room temperature and present good volatility, thermal stability and reactivity. The self-limiting ALDgrowth mode was confirmed at 325 °C for both precursors. The titanium (N,N′diisopropylacetamidinate)tri(isopropoxide)/water process showed an ALD window at 300−350 °C, and titanium bis(dimethylamide)bis(isopropoxide) exhibited an interestingly high growth rate of 0.75 Å/cycle at 325 °C. The films were crystallized to the anatase phase in the as-deposited state. X-ray photoelectron spectroscopy analysis demonstrated that the films were pure and close to the stoichiometric composition. The refractive indexes and absorption coefficient of the films were measured by spectroscopic ellipsometry.
HfO2 thin films were prepared by plasma-enhanced atomic layer deposition using a cyclopentadienyl-alkylamido precursor [HfCp(NMe2)3, HyALD™] and an O2 plasma over a temperature range of 150–400 °C at a growth per cycle around 1.1 Å/cycle. The high purity of the films was demonstrated by x-ray photoelectron spectroscopy and elastic recoil detection analyses which revealed that by increasing the deposition temperature from 200 to 400 °C, the atomic concentrations of residual carbon and hydrogen reduced from 1.0 to <0.5 at. % and 3.4 to 0.8 at. %, respectively. Moreover, Rutherford backscattering spectroscopy studies showed an improvement in stoichiometry of HfO2 thin films with the increase in deposition temperature, resulting in Hf/O ratio close to ∼0.5 at 400 °C. Furthermore, grazing incidence x-ray diffraction measurements detected a transition from amorphous at the deposition temperature of 300 °C to fully polycrystalline films at 400 °C, consisting of a mixture of monoclinic, tetragonal, and cubic phases. Finally, the surface morphology and conformality of HfO2 thin films studied by atomic force microscopy and transmission electron microscopy are also reported.
Strontium titanate (SrTiO3, STO) films were deposited by plasma-assisted ALD using cyclopentadienyl-based Sr- and Ti-precursors with O2 plasma as the oxidizing agent. Spectroscopic ellipsometry (SE) was employed to determine the thickness and the optical properties of the layers. As determined from Rutherford backscattering spectrometry (RBS), [Sr]/([Sr]+[Ti]) ratios ranging from 0.42 to 0.68 were achieved for 30–40 nm thick films by tuning the [SrO]/[TiO2] ALD cycle ratio. Films deposited at 250°C were amorphous and required post-deposition annealing to crystallize into the ultrahigh-k perovskite structure. The crystallinity of the films after rapid thermal annealing strongly depended on the film composition as observed by X-ray diffraction measurements. Using RBS data for a set of as-deposited samples, an optical constant library was built to determine the film stoichiometry from SE directly for the amorphous as-deposited films. After rapid thermal annealing the crystalline phase could be determined from the position of critical points of the measured dielectric function and the estimation of the stoichiometry was also possible for crystallized layers. These results open up a new way to use SE as a real-time characterization method to monitor and tune the STO film composition and crystallinity.
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