Trilayer Stacks Research Articles

Considerations for extracting moiré-level strain from dark field intensities in transmission electron microscopy

Bragg interferometry (BI) is an imaging technique based on four-dimensional scanning transmission electron microscopy (4D-STEM) wherein the intensities of select overlapping Bragg disks are fit or more qualitatively analyzed in the context of simple trigonometric equations to determine local stacking order. In 4D-STEM based approaches, the collection of full diffraction patterns at each real-space position of the scanning probe allows the use of precise virtual apertures much smaller and more variable in shape than those used in conventional dark field imaging such that even buried interfaces marginally twisted from other layers can be targeted. With a coarse-grained form of dark field ptychography, BI uses simple physically derived fitting functions to extract the average structure within the illumination region and is, therefore, viable over large fields of view. BI has shown a particular advantage for selectively investigating the interlayer stacking and associated moiré reconstruction of bilayer interfaces within complex multi-layered structures. This has enabled investigation of reconstruction and substrate effects in bilayers through encapsulating hexagonal boron nitride and of select bilayer interfaces within trilayer stacks. However, the technique can be improved to provide a greater spatial resolution and probe a wider range of twisted structures, for which current limitations on acquisition parameters can lead to large illumination regions and the computationally involved post-processing can fail. Here, we analyze these limitations and the computational processing in greater depth, presenting a few methods for improvement over previous works, discussing potential areas for further expansion, and illustrating the current capabilities of this approach for extracting moiré-scale strain.

A nonvolatile memory element for integration with superconducting electronics

We demonstrate a nonvolatile cryogenic magnetic memory element needed to support emerging superconducting- and quantum-computing technologies. The central element is a switchable tri-layer thin film magnetic dot comprising two semiconducting ferromagnetic GdxSm1−xN layers separated by an exchange-blocking Al layer. The materials are explored for their tunable magnetic responses, the potential to engineer compensating magnetic moments in the anti-parallel tri-layers. The stability of the parallel and anti-parallel states and the reproducibility over repeated cycles are also demonstrated. We show that the tri-layer stacks can be formed into dots as small as 4 μm diameter, without affecting their magnetic behavior.

Crystallization and stability of rare earth iron garnet/Pt/gadolinium gallium garnet heterostructures on Si

The crystallization of rare earth iron garnet films such as dysprosium iron garnet on Si substrates provides a path for integration of these complex oxides into magnetic devices. We report the growth of 50–75 nm thick dysprosium yttrium iron garnet (YDyIG) films, deposited without a seed layer then crystallized by a 750 °C rapid thermal anneal, forming a polycrystalline film with grains of several µm diameter containing radiating low-angle boundaries. The Y:Dy ratio affects the magnetization and anisotropy of YDyIG. Crystallization becomes more challenging as the garnet film thickness decreases. To crystallize thinner rare earth garnet films, tri-layer stacks consisting of a 50 nm thick gadolinium gallium garnet (GGG) seed layer, a 1.5 nm thick Pt diffusion barrier, and a 10 nm thick YDyIG film were prepared. The YDyIG/Pt/GGG tri-layers showed agglomeration of the Pt leading to a morphology consisting of a polycrystalline garnet film enriched with Dy and Fe and containing a layer of Pt nanoparticles embedded near its surface.

Plasma-based area selective deposition for extreme ultraviolet resist defectivity reduction and process window improvement

Extreme ultraviolet (EUV) lithography has overcome significant challenges to become an essential enabler to the logic scaling roadmap. However, it remains limited by stochastically driven defects, such as line breaks and line bridges for aggressive pitches. This is especially relevant for the back end of line, which requires the most aggressive scaling. Stochastic defects reduce device yield and may push device manufacturers to move to EUV multipatterning beyond 36 nm pitch single exposure, which is a costly option. While the lithography and patterning stack can be optimized to provide the largest process window with the lowest number of defects, process margins decrease as smaller pitches are required. Currently, for some lithography stacks, especially spin-on glass based trilayer stacks, the defect-free process window beyond 36 nm pitch is limited by line collapse. Reduction in resist thickness may mitigate pattern collapse, but it may also increase the number of line breaks—trading one killer defect for another. In this paper, we expand on an area selective deposition (ASD) process in situ of an etch chamber to selectively deposit material on the EUV photoresist prior to transferring the pattern downstream. We demonstrate mitigation of resist line notching and breaks while maintaining deposition-free open areas and clear alignment marks. Due to the inherent chemical selectivity of the deposition process as opposed to a purely aspect ratio driven deposition process, thinner resists that, with a normal etch condition would result in line breaks, can now be considered. This drives down flopover defect issues seen with thicker EUV resists and enables several underlayer systems that could otherwise not be considered. Finally, we demonstrate that defectivity levels measured by e-beam inspection post lithography and post pattern transfer and yield are both improved at 30 nm pitch when this ASD process is used.

Combination of Metal Oxide and Polytriarylamine: A Design Principle to Improve the Stability of Perovskite Solar Cells

In the last decade, perovskite photovoltaics gained popularity as a potential rival for crystalline silicon solar cells, which provide comparable efficiency for lower fabrication costs. However, insufficient stability is still a bottleneck for technology commercialization. One of the key aspects for improving the stability of perovskite solar cells (PSCs) is encapsulating the photoactive material with the hole-transport layer (HTL) with low gas permeability. Recently, it was shown that the double HTL comprising organic and inorganic parts can perform the protective function. Herein, a systematic investigation and comparison of four double HTLs incorporating polytriarylamine and thermally evaporated transition metal oxides in the highest oxidation state are presented. In particular, it was shown that MoOx, WOx, and VOx-based double HTLs provided stable performance of PSCs for 1250 h, while devices with NbOx lost 30% of their initial efficiency after 1000 h. Additionally, the encapsulating properties of all four double HTLs were studied in trilayer stacks with HTL covering perovskite, and insignificant changes in the absorber composition were registered after 1000 h under illumination. Finally, it was demonstrated using ToF-SIMS that the double HTL prevented the migration of perovskite volatile components within the structure. Our findings pave the way towards improved PSC design that ensures their long-term operational stability.

Modulations of interfacial magnetic properties using structurally inverted magnetic tri-layer systems: Toward skyrmion formation

Various magnetic properties such as the interfacial Dzyaloshinskii–Moriya interaction (DMI) and the perpendicular magnetic anisotropy (PMA) in structurally inverted Pd/Co/Pt and Pt/Co/Pd tri-layer stacks are systematically investigated by performing an inelastic light scattering method. Two examined individual multilayer structures are identical, but the stacking order is inverted for each other. Here, we demonstrate that the interfacial DMI energy densities and the saturation magnetization values for both systems are almost similar. However, the surface magnetic anisotropy energy densities of Pd/Co/Pt systems are dramatically decreased up to 34% compared with Pt/Co/Pd system, while the interfacial DMI is kept. Since a large interfacial DMI and a small PMA (or domain wall energy) are required to form magnetic skyrmion phases, the Pd/Co/Pt system can be one of the promising candidates. According to the thickness dependent measurements, we show a possibility of the existence of skyrmion phases in the Pd/Co/Pt system within a certain range of Co layer thickness at room temperature without external magnetic fields.

Fabrication and room temperature characterization of trilayer junctions for the development of superconducting qubits on 300 mm wafers

We present the development of Nb/Al–AlO x /Nb trilayer stacks and the implementation of a full 300 mm process flow for the fabrication of trilayer-based superconducting qubits. Room temperature electrical characterization of tens of thousands of Josephson junctions showed good agreement between blanket resistance-area (RA) product and RA product of processed wafers. Cross-bridge Kelvin resistor structures with dimensions ranging from 200 nm to 1.2 μm were tested and exhibited excellent yield and exceptionally low resistance variability down to <1%. This result is expected to translate to reduced qubit device variability and improved predictability of qubit transition frequencies at cryogenic temperatures.

Stability phase diagrams and tuning of magnetic skyrmionium and other states

Topological magnetic states such as skyrmions have widely been investigated as a suitable candidate for memory bits in next generation data storage device applications including random access memory (RAM) or race-track memory as well as spin-torque nano-oscillators etc. Energy efficient multistate switching of these states based on electric control of magnetization involves the electric field tuning of perpendicular magnetic anisotropy (PMA). By using micromagnetic simulation study, we demonstrate the strain-mediated electric manipulation of various topological magnetic states in nanodisc with [Pt/Co/Ta]n trilayer stacks residing on a piezoelectric substrate. A stability phase diagram is derived for the representation of regions of the lowest energy states against the applied strain as a function of disc dimensions. Utilizing this derived phase diagram, various electric field-induced strain-mediated switching scenarios including non-volatile reversible switching of skyrmion into skyrmionium, 3π state and helical stripe domain state, the creation of skyrmionium from uniform single-domain magnetic state and its annihilation and volatile reversible switching between skyrmionium and vortex are achieved. Moreover, an extended phase diagram based on the estimated critical values of the applied strain for different domain structures within the nanodisc is also built and further implemented in designing a multistate sequential switching between various magnetic states triggered by a continuously varying strain pulse. These findings hold a potential for contributing in the development of low-power high-density multistate magneto-electric or magneto-elastic memory and logic device applications involving simple architectural concept applicable to future microelectronic integrated device strategies.

Unipolar resistive switching behavior in Al2O3/HfO2 multilayer dielectric stacks: fabrication, characterization and simulation

In this work, the impact of different HfO2/Al2O3-based multilayer dielectric stack (DS) configurations on the electrical characteristics and on the resistive switching (RS) performance of Ni/Insulator/Silicon devices has been systematically investigated. Significant differences are observed in the electrical characteristics of the fabricated bilayer, trilayer and pentalayer stacks compared to a single HfO2 layer of the same physical thickness. The RS analysis has shown similar low resistance state currents and set voltages for all the DS combinations whereas currents at the high resistance state and reset voltages depend on the DS. The shift of the reset voltage to lower values for HfO2 and Al2O3/HfO2/Al2O3 cases is explained by the results from thermal simulations that reveal that these differences could be associated to the different temperature distributions at the narrowest part of the conductive filament immediately before the thermally triggered reset process occurs.

Optical absorption in air/monolayer MoS2/S (S[dbnd]SiO2 or Si) trilayer stacks at oblique incidence

The monolayer MoS2 with strong light-matter interaction and optoelectronic properties is useful to design different optical devices such as photonic absorbers. In this paper, the MoS2 monolayer is sandwiched in the following stacks: SiO2/MoS2/air and Si/MoS2/air to study of increasing optical absorption. We study theoretically the absorption of these stacks as a function of polarization, incident angle and wavelength to reach the maximum value in the visible regime. The absorption intensity reaches to be as high as 65% for internal reflection and s polarization, around the blue wavelength for air/MoS2/SiO2 stack. Absorption behavior including maxima points as a function of wavelength is correlated with imaginary component of refractive index of the monolayer MoS2 except around critical angle. Our results recommend important role of internal and external reflection, cover and substrate layer, angle of incidence and polarization to design elements for optical energy harvesting applications.

Effect of heat treatment on the optical properties of sol–gel-derived, fully dielectric solar control coatings on glass

TiO2 and zeolite were used as high index and low index dielectric materials respectively to generate fully dielectric solar control coatings on glass. TiO2|Zeolite|TiO2 trilayer stacks as well as single layer TiO2 and zeolite coatings were generated on soda lime glass substrates using dip-coating technique. The coatings were densified at 450°C in air using a conventional muffle furnace and a conveyorized belt furnace with a total cycle time of 330 and 30 min, respectively, per layer. The effect of heat-treatment time on the optical properties of multilayered and single-layered coatings was studied. Spectroscopic ellipsometric analysis was carried out on individual layers of the stack to elucidate thickness, roughness, and refractive index data. The ellipsometry data were subsequently used as inputs to simulate the UV–Vis–NIR transmittance spectra of the trilayer stacks generated by TFCalc® software, for comparison with experimentally obtained data. The morphology and phase compositions of the coatings were studied using scanning electron microscopy and grazing angle incidence X-ray diffraction analysis. Different heat treatment schedules were found to result in different optical properties. The results from the present study could be used to recommend suitable heat treatment schedules for generating solar control coatings that could be used for either architectural or automotive applications.

Effects of post-growth annealing in a CoFeB/MgO/CoFeB trilayer structure

CoFeB/MgO/CoFeB tri-layer thin-film stacks have been widely used in the design of STT-RAM devices as functional magnetic-tunnel-junction (MTJ) structures. The materials properties of the CoFeB and MgO layers, including composition and lattice quality, have been extensively researched from the stand point of optimizing for the best MTJ performance. On the other hand, post-growth annealing is required for the MTJ structure to acquire its functional property, i.e. its TMR performance. In this work, we have studied the various possible effects resulting from the post-growth annealing process. Specifically, we show that the post-growth annealing causes boron in the top and bottom CoFeB layers to migrate into the adjacent Ta layers as well as deterioration in lattice quality of the MgO layer. Furthermore, we evaluate other effects that could be possibly induced during the annealing process, including Ta diffusion and layer intermixing in the CoFeB/MgO/CoFeB tri-layer structure. The post-growth annealing causes little change in the Ta diffusion and the layer intermixing. These annealing effects were also evaluated with respect to variations in the MgO growth process; more specifically, an additional natural oxidation treatment during the MgO layer deposition and the insertion of a Fe layer before the MgO layer. Our results indicate that the addition of a natural oxidation process during the MgO deposition process and the insertion of a thin-layer of Fe before the MgO layer both lead to a reduction in the layer intermixing between the MgO and the CoFeB layer and to an improvement in MgO lattice quality. We also show that the post-growth annealing does not alter the beneficial effect of these MgO growth process modifications.

A New In-line Laser-based Acoustic Technique for Pillar Bump Metrology

The drive to reduce the interconnect pitch and increase the number of connections for packaging in mobile devices has led to the development of copper pillar bumps. The key drivers for the adoption of copper pillars are improved performance, reduced form factor and lower cost. In this paper, we present a laser-based acoustic technique for the characterization of multi-layer pillars. This non-contact technique has a high sensitivity for materials characterization with micron-scale spatial resolution. Absorption of laser light causes excitation of elastic waves that propagate through the pillar and are reflected by the pillar walls, exciting vibrational modes in the structure. We have demonstrated that our approach is sensitive to the thicknesses of individual layers in bi-layer and tri-layer copper pillar stacks. Focused ion beam scanning electron microscopy (FIB SEM) has been used to optimize the model and to validate the accuracy of the technique.

X-ray fluorescence induced by standing waves in the grazing-incidence and grazing-exit modes: study of the Mg-Co-Zr system.

The characterization of Mg-Co-Zr tri-layer stacks using X-ray fluorescence induced by X-ray standing waves, in both the grazing-incidence (GI) and the grazing-exit (GE) modes, is presented. The introduction of a slit in the direction of the detector improves the angular resolution by a factor of two and significantly improves the sensitivity of the technique for the chemical characterization of the buried interfaces. By observing the intensity variations of the Mg Kα and Co Lα characteristic emissions as a function of the incident (GI mode) or detection (GE mode) angle, it is shown that the interfaces of the Si/[Mg/Co/Zr] × 30 multilayer are abrupt, whereas in the Si/[Mg/Zr/Co] × 30 multilayer a strong intermixing occurs at the Co-on-Zr interfaces. An explanation of this opposite behavior of the Co-on-Zr and Zr-on-Co interfaces is given by the calculation of the mixing enthalpies of the Co-Mg, Co-Zr and Mg-Zr systems, which shows that the Co-Zr system presents a negative value and the other two systems present positive values. Together with the difference of the surface free energies of Zr and Co, this leads to the Mg/Zr/Co system being considered as a Mg/CoxZry bi-layer stack, with x/y estimated around 3.5.

Optical spectroscopy of interlayer coupling in artificially stacked MoS2 layers

We perform an optical spectroscopy study to investigate the properties of different artificial MoS2 bi- and trilayer stacks created from individual monolayers by a deterministic transfer process. These twisted bi- and trilayers differ from the common 2H stacking in mineral MoS2 in the relative stacking angle of adjacent layers and the interlayer distance. The combination of Raman spectroscopy, second-harmonic-generation microscopy and photoluminescence measurements allows us to determine the degree of interlayer coupling in our samples. We find that even for electronically decoupled artificial structures, which show the same valley polarization degree as the constituent MoS2 monolayers at low temperatures, there is a resonant energy transfer between individual layers which acts as an effective luminescence quenching mechanism.

Energy level alignment in Au/pentacene/PTCDA trilayer stacks

Ultraviolet photoelectron spectroscopy is used to investigate the energy level alignment and molecular orientation at the interfaces in Au/pentacene/PTCDA trilayer stacks. We deduced a standing orientation for pentacene grown on Au while we conclude a flat lying geometry for PTCDA grown onto pentacene. We propose that the rough surface of polycrystalline Au induces the standing geometry in pentacene. It is further shown that in situ deposition of PTCDA on pentacene can influence the orientation of the surface pentacene layer, flipping part of the surface pentacene molecules into a flat lying geometry, maximizing the orbital interaction across the pentacene–PTCDA heterojunction.

Study of tri-layer antireflection coatings prepared by sol–gel method

Antireflective coatings (ARCs) on tri-layer thin film stacks were studied in this paper. Silica sols have been prepared by acid-catalyzed or base-catalyzed hydrolysis and condensation reactions of tetraethyl orthosilicate. Antireflective nanometric SiO2/TiO2 films are formed on both sides of the glass substrates by combining the sol–gel method and the dip-coating technique. Seen from the transmittance spectra of different films, a maximum light transmittance of 99.9% was obtained at the band of 300–800 nm. Scanning electron microscope (SEM) and atomic force microscopy (AFM) confirm the well-covered surface morphology. By the SEM observations we can see that the films are full of coverage on glass surface and containing no voids or cracks. The image root mean square roughness of the two types of ARCs provided by the AFM is 1.21 and 3.04 nm, respectively. Furthermore, a surface profiler was used to determine the thickness of each layer in the obtained multi-layer coating system.

CZTS thin films on transparent conducting electrodes by electrochemical technique

We have fabricated single phase Cu 2ZnSnS 4 (CZTS) films using a specially designed 3-stage electrochemical system. Sequential electrodepositon of constituent metallic layers was carried out on SnO 2/F coated glass substrates using a platinum counter electrode and a saturated calomel reference electrode. Unique bath compositions were formulated for each of these constituents. Sequentially deposited tri-layer stacks were annealed in sulfur environment to get CZTS phase. Detailed structural, morphological and optical characterization experiments were performed using several techniques including x-ray diffraction, Raman and UV–visible spectroscopy, scanning electron microscopy and atomic force microscopy. All characterization experiments indicated that the films are single phase with a measured direct band gap of 1.5 eV.

ELECTROMAGNETIC TUNNELING IN LOSSLESS TRILAYER STACKS CONTAINING SINGLE-NEGATIVE METAMATERIALS

We analyze the transverse-electric wave propagation through lossless trilayer stacks containing single-negative (SNG) materials in which only one of the two material constants, permittivity (epsilon) or permeability (mu), is negative. We consider the following combinations: ENG/MNG/ENG, ENG/DPS/MNG, DPS/ENG/DPS, and ENG/DPS/ENG, where ENG refers to epsilon-negative, MNG to mu-negative, and DPS to double-positive media. The transfer matrix formalism is applied. Although the waves are evanescent in the SNG media, combining the SNG layers or the SNG and DPS layers, leads to some unusual features, such as the complete tunneling. Since the symmetrical trilayer is equivalent to a single homogeneous layer, the complete tunneling conditions are easily predicted analytically for the trilayer stacks, and we show that in most of cases, they are rather well applicable to the respective bilayer stacks. The fleld and the Poynting vector distributions are studied in difierent trilayers and, in some cases, in the respective bilayers. In particular, we show that the complete tunneling is facilitated theoretically in the electrically thin stacks. Similar results could be obtained for the transverse-magnetic waves and the respective dual combinations by using the duality principle.

Graded Spin-on Organic Bottom Antireflective Coating for High NA Immersion Lithography

A spin-on organic Graded Bottom AntiReflective Coating (GBARC) was developed to enable appropriate reflectivity control during advanced photoresist patterning using 193nm immersion lithography up to 1.35 NA (limits of hyper-NA lithography using water as immersion fluid). This novel technology constitutes a simplified reflectivity control approach over more conventional schemes such as dual-layer BARCs, trilayer stacks or single layer BARCs. The GBARC layer is formed upon vertical phase separation of optically tuned polymeric components present in a GBARC casting solution during the spin-coating and post-applied baking steps. The substrate surface energy is found to dictate the direction of the spatial assembly for any given phase-separating polymer blend combination. The index of refraction (n193) of the GBARC polymeric components is approximately matched to that of the photoresist and substrate layers for optimum reflectivity control, while the extinction coefficient (k193) increases gradually towards the substrate. Analytical characterization of the varying optical constants throughout the GBARC film is achieved by Variable Angle Spectroscopic Ellipsometry (VASE) which allows for a quantitative description of the optical gradient present within the GBARC layer. The optical gradient sharpness is shown to be impervious to total GBARC thickness variations at constant relative polymer mass loading as well as changes in the relative mass loading of polymeric components at fixed total GBARC thickness. Also, the link between image profile afforded by various levels of reflectivity control and the mechanical stability of photoresist nanostructures is investigated at 1.2NA by designing GBARC formulations with variable residual reflectivity. Substrate reflectivity in excess of 1% is shown to limit the focus-exposure window due to pattern collapse. Last, adhesion of 193 nm photoresist patterns to a GBARC film is compared to a commercially available single layer BARC. Enhanced adhesion is demonstrated in the case of the GBARC surface, despite the low surface energy that typifies GBARC layers