Johannes Meersschaut

BEOL compatible high tunnel magneto resistance perpendicular magnetic tunnel junctions using a sacrificial Mg layer as CoFeB free layer cap

Perpendicularly magnetized MgO-based tunnel junctions are envisaged for future generation spin-torque transfer magnetoresistive random access memory devices. Achieving a high tunnel magneto resistance and preserving it together with the perpendicular magnetic anisotropy during BEOL CMOS processing are key challenges to overcome. The industry standard technique to deposit the CoFeB/MgO/CoFeB tunnel junctions is physical vapor deposition. In this letter, we report on the use of an ultrathin Mg layer as free layer cap to protect the CoFeB free layer from sputtering induced damage during the Ta electrode deposition. When Ta is deposited directly on CoFeB, a fraction of the surface of the CoFeB is sputtered even when Ta is deposited with very low deposition rates. When depositing a thin Mg layer prior to Ta deposition, the sputtering of CoFeB is prevented. The ultra-thin Mg layer is sputtered completely after Ta deposition. Therefore, the Mg acts as a sacrificial layer that protects the CoFeB from sputter-indu...

Interplay between structural and magnetic properties of L10-FePt(001) thin films directly grown on MgO(001)

We present a detailed study of the magnetic and structural properties of L10-FePt thin films. The films are prepared via molecular beam epitaxy directly onto MgO(001) substrates, i.e., without buffer layer. Despite the large lattice misfit between the in-plane lattice parameters of L10 FePt and MgO, highly ordered thin films are obtained with the easy magnetization c axis perpendicular to the film plane. Via high resolution transmission electron microscopy and Rutherford backscattering measurements we focus on the FePt/MgO interface to study the misfit relaxation and the defect density. Further, the influence of elevated substrate temperatures and of postgrowth high temperature annealing on the structural and magnetic properties is discussed.

Multilayer MoS2 growth by metal and metal oxide sulfurization

We investigated the deposition of MoS2 multilayers on large area substrates. The pre-deposition of metal or metal oxide with subsequent sulfurization is a promising technique to achieve layered films. We distinguish a different reaction behavior in metal oxide and metallic films and investigate the effect of the temperature, the H2S/H2 gas mixture composition, and the role of the underlying substrate on the material quality. The results of the experiments suggest a MoS2 growth mechanism consisting of two subsequent process steps. At first, the reaction of the sulfur precursor with the metal or metal oxide occurs, requiring higher temperatures in the case of metallic film compared to metal oxide. At this stage, the basal planes assemble towards the diffusion direction of the reaction educts and products. After the sulfurization reaction, the material recrystallizes and the basal planes rearrange parallel to the substrate to minimize the surface energy. Therefore, substrates with low roughness show basal plane assembly parallel to the substrate. These results indicate that the substrate character has a significant impact on the assembly of low dimensional MoS2 films.

Exchange bias by implantation of O ions into Co thin films

An original approach for the formation of an exchange bias system is presented. Alternative to surface oxidation or deposition for the formation of Co/CoO bilayer exchange bias systems, implantation of oxygen ions into Co films is applied. The implantation results in the formation of CoxOy embedded in a Co matrix. Comparison with noble gas implantation unambiguously demonstrates that the observed exchange bias effect is induced by the implanted oxygen. Opposed to bilayers formed by surface oxidation, the implantation results in a different morphology of the interface between Co and CoxOy and also gives rise to a radically different magnetization reversal mechanism.

Alternative high-k dielectrics for semiconductor applications

Although the next generation high-k gate dielectrics has been defined for the 45nm complementary metal oxide semiconductor technology node, threshold voltage control and equivalent oxide thickness (EOT) scaling remain concerns for future devices. Therefore, the authors explored the effect of incorporating dysprosium in the gate stack. Results suggest that improved EOT-leakage scaling is possible by adding Dy to the interfacial SiO2 layer in a 1:1 ratio or by adding 10% Dy to bulk HfO2. The deposition of a 1nm Dy2O3 cap layer lowered the threshold voltage by ∼250mV. In addition, for future dynamic random access memory capacitor applications, dielectrics with e of 50–130 are projected by the International Technology Roadmap for Semiconductors, unachievable with standard high-k dielectrics. Theoretical modeling can help direct the experimental work needed for extensive screening of alternative dielectrics. Moreover, materials such as perovskites only exhibit a sufficiently high-k value when properly crystall...

Nucleation and growth mechanisms of Al2O3 atomic layer deposition on synthetic polycrystalline MoS2

Two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs) are of great interest for applications in nano-electronic devices. Their incorporation requires the deposition of nm-thin and continuous high-k dielectric layers on the 2D TMDs. Atomic layer deposition (ALD) of high-k dielectric layers is well established on Si surfaces: the importance of a high nucleation density for rapid layer closure is well known and the nucleation mechanisms have been thoroughly investigated. In contrast, the nucleation of ALD on 2D TMD surfaces is less well understood and a quantitative analysis of the deposition process is lacking. Therefore, in this work, we investigate the growth of Al2O3 (using Al(CH3)3/H2O ALD) on MoS2 whereby we attempt to provide a complete insight into the use of several complementary characterization techniques, including X-ray photo-electron spectroscopy, elastic recoil detection analysis, scanning electron microscopy, and time-of-flight secondary ion mass spectrometry. To reveal the inherent reactivity of MoS2, we exclude the impact of surface contamination from a transfer process by direct Al2O3 deposition on synthetic MoS2 layers obtained by a high temperature sulfurization process. It is shown that Al2O3 ALD on the MoS2 surface is strongly inhibited at temperatures between 125°C and 300°C, with no growth occurring on MoS2 crystal basal planes and selective nucleation only at line defects or grain boundaries at MoS2 top surface. During further deposition, the as-formed Al2O3 nano-ribbons grow in both vertical and lateral directions. Eventually, a continuous Al2O3 film is obtained by lateral growth over the MoS2 crystal basal plane, with the point of layer closure determined by the grain size at the MoS2 top surface and the lateral growth rate. The created Al2O3/MoS2 interface consists mainly of van der Waals interactions. The nucleation is improved by contributions of reversible adsorption on the MoS2 basal planes by using low deposition temperature in combination with short purge times. While this results in a more two-dimensional growth, additional H and C impurities are incorporated in the Al2O3 layers. To conclude, our growth study reveals that the inherent reactivity of the MoS2 basal plane for ALD is extremely low, and this confirms the need for functionalization methods of the TMD surface to enable ALD nucleation.

Thermally stable high effective work function TaCN thin films for metal gate electrode applications

TaCN layers were deposited using metal-organic chemical-vapor deposition for applications as metal gate electrodes in p-type metal-oxide-semiconductor (pMOS) devices. The films were formed by thermal decomposition of tertiary-amylimido-tris(dimethylamido)tantalum (TAIMATA®) between 400 and 600 °C. The composition was dependent on the growth temperature with increasing C and decreasing N content at higher temperature. Films grown below 500 °C were nearly amorphous and became weakly polycrystalline with a cubic structure at higher growth temperature. The layer density was ∼8.1 g/cm3, about half of the TaCN bulk density. Grazing-incidence x-ray diffraction and transmission electron microscopy showed that the films consist of small polycrystalline grains in an amorphous matrix. The resistivity was found to decrease with increasing growth temperature. Lowest resistivity values were around 1 mΩ cm for films grown at 600 °C. The films formed an ∼4 nm thick insulating surface oxide, which leads to a thickness dep...

High-k Dielectrics and Metal Gates for Future Generation Memory Devices

The requirements and development of high-k dielectric films for application in storage cells of future generation flash and Dynamic Random Access Memory (DRAM) devices are reviewed. Dielectrics with k-value in the 9-30 range are studied as insulators between charge storage layers and control gates in flash devices. For this application, large band gaps (> 6 eV) and band offsets are required, as well as low trap densities. Materials studied include aluminates and scandates. For DRAM metal-insulator-metal (MIM) capacitors, aggressive scaling of the equivalent oxide thickness (with targets down to 0.3 nm) drives the research towards dielectrics with k-values > 50. Due to the high aspect ratio of MIMCap structures, highly conformal deposition techniques are needed, triggering a substantial effort to develop Atomic Layer Deposition (ALD) processes for the deposition of metal gates and high-k dielectrics. Materials studied include Sr and Ba-based perovskites, with SrTiO3 as one of the most promising candidates, as well as tantalates, titanates and niobates.

Artificial neural networks applied to the analysis of synchrotron nuclear resonant scattering data

The capabilities of artificial neural networks (ANNs) have been investigated for the analysis of nuclear resonant scattering (NRS) data obtained at a synchrotron source. The major advantage of ANNs over conventional analysis methods is that, after an initial training phase, the analysis is fully automatic and practically instantaneous, which allows for a direct intervention of the experimentalist on-site. This is particularly interesting for NRS experiments, where large amounts of data are obtained in very short time intervals and where the conventional analysis method may become quite time-consuming and complicated. To test the capability of ANNs for the automation of the NRS data analysis, a neural network was trained and applied to the specific case of an Fe/Cr multilayer. It was shown how the hyperfine field parameters of the system could be extracted from the experimental NRS spectra. The reliability and accuracy of the ANN was verified by comparing the output of the network with the results obtained by conventional data analysis.

The magnetism of UX3 compounds (X = In, Pb, Ga) from symmetry considerations and perturbed-angular-correlation spectroscopy

We show how symmetry considerations and ab initio calculations within the framework of the density-functional theory can be used in the analysis of the appearance and orientation of magnetic hyperfine fields. This approach, in which the incorporation of spin-orbit coupling is essential, provides information about hyperfine fields also for the sites where the Heisenberg exchange field is zero. This is of crucial importance in the interpretation of magnetic hyperfine-field measurements. We apply this theoretical scheme and perturbed-angular-correlation experiments to study the magnetic structure of UX3 compounds with X = Ga, In, Pb. For UPb3 we find a triaxial structure with U moments along the [111] axes. In UIn3 the U moments are arranged in a type-II antiferromagnetic structure with moments along [110].