Atomic Shadowing Research Articles

Comparative study on the effects of suspension way of sample holder on the large-aperture uniformity of refractive index of HfO2 films

For large-aperture dielectric film optics, the uniformity of film refractive index across the aperture is crucial to the uniformity of reflectance and transmittance. Herein, a comparative research has been performed on the refractive index uniformity of HfO2 films across the Φ560 mm-aperture sample holder for inclined suspension and horizontal suspension. It’s revealed that the refractive index of HfO2 film at edge position of sample holder increased about 1.52% compared to that of HfO2 film at center position for inclined suspension, while the refractive index varied only about 0.25% for horizontal suspension. The further analysis performed on film crystallization, morphology and microstructure show that the large variation of refractive index between the films at center and edge positions of inclined suspension sample holder was attributed to the different growth properties. The film at the edge crystallized more slightly and was denser, thus being of higher refractive index. And the different growth properties of the two films at center and edge positions can be explained in the light of the co-effects of atomic shadowing, reemission and diffusion of ad-atoms during film growth.

Study of ZrO2 thin films deposited at glancing angle by radio frequency magnetron sputtering under varying substrate rotation

In present work, structural, morphological and optical properties of glancing angle deposited (GLAD~82°) ZrO2 thin films by RF magnetron sputtering at different substrate rotation (ω) have been investigated and compared with normally deposited (ND~0°) ZrO2 films. Both GLAD and ND ZrO2 films exhibit preferential structural growth of monoclinic phase oriented in different directions. GLAD films also depict a tetragonal peak which has been attributed to the presence of fine nano-crystallite size (~13nm). Surface correlation length of such films decreases with the increase in ω. The decrease has been attributed to the change in columnar microstructure with ω. RMS roughness values for GLAD and ND ZrO2 films are 4.6–5.1nm and 1nm, respectively. Dominant atomic shadowing is responsible for high roughness of GLAD films. Refractive index derived from ellipsometric measurements manifests a decreasing trend with parameter, ω. The trend has also been explained in terms of varying columnar micro-structure. Refractive index for GLAD ZrO2 films varies from 1.901 and 2.011 on varying ω from 0.50 to 4rpm. ND ZrO2 film exhibits refractive index value of 2.178 which is substantially greater than that of GLAD films. Lowering in refractive index of GLAD films has been attributed to dominant atomic shadowing at glancing angles. Residual stress switches from large compressive to small tensile as the deposition angle changes from 0° to 82°. For GLAD films, tensile stress increases with the increase in ω except for the film deposited at the highest substrate rotation. Lowering in stress for GLAD films compared to ND films and trend of stress with ω have been explained in terms of varying inter-molecular forces with inter-columnar distance.

Reduced atomic shadowing in HiPIMS: Role of the thermalized metal ions

In magnetron sputtering, the ability to tailor film properties depends primarily on the control of the flux of particles impinging on the growing film. Among deposition mechanisms, the shadowing effect leads to the formation of a rough surface and a porous, columnar microstructure. Re-sputtered species may be re-deposited in the valleys of the films surface and thereby contribute to a reduction of roughness and to fill the underdense regions. Both effects are non-local and they directly compete to shape the final properties of the deposited films. Additional control of the bombarding flux can be obtained by ionizing the sputtered flux, because ions can be controlled with respect to their energy and impinging direction, such as in High-Power Impulse Magnetron Sputtering (HiPIMS). In this work, the relation between ionization of the sputtered species and thin film properties is investigated in order to identify the mechanisms which effectively influence the shadowing effect in Deep Oscillation Magnetron Sputtering (DOMS), a variant of HiPIMS. The properties of two Cr films deposited using the same averaged target power by d.c. magnetron sputtering and DOMS have been compared. Additionally, the angle distribution of the Cr species impinging on the substrate was simulated using Monte Carlo-based programs while the energy distribution of the energetic particles bombarding the substrate was evaluated by energy-resolved mass analysis. It was found that the acceleration of the thermalized chromium ions at the substrate sheath in DOMS significantly reduces the high angle component of their impinging angle distribution and, thus, efficiently reduces atomic shadowing. Therefore, a high degree of ionization in HiPIMS results in almost shadowing effect-free film deposition and allows us to deposit dense and compact films without the need of high energy particle bombardment during growth.

Effect of angle of deposition on micro-roughness parameters and optical properties of HfO2 thin films deposited by reactive electron beam evaporation

Oblique angle deposited oxide thin films, in which refractive index profiles can be tailored across depth by tuning their microstructure using varying angle of deposition, have opened up new dimensions in fabrication of optical interference devices. Since surface morphology plays an important role for the qualification of these thin film devices for optical or other applications, it is important to investigate morphological properties of obliquely deposited thin films. In the present work, a set of HfO2 thin films have been deposited at several oblique angles. Morphological parameters of such thin films viz., correlation length, intrinsic roughness, fractal spectral strength, etc., have been determined through suitable modelling of extended power spectral density measured by atomic force microscopy. It has been found that intrinsic roughness and fractal spectral strength show an interesting behaviour with deposition angle and the same has been discussed in the light of atomic shadowing and re-emission and diffusion of ad-atoms. Further refractive index and thickness of such thin films have been estimated from optical transmission spectra through suitable modelling. Refractive index of such thin film varies from 1.93 to 1.37 as the deposition angle varies from normal to glancing angle (80°). Further, refractive index and grain size depict an opposite trend with deposition angle. This variation has been explained in terms of varying film porosity and column slanting with angle of deposition.

Structure zone model for extreme shadowing conditions

Previously reported data on the microstructure of glancing angle deposited (GLAD) metal layers is used to extend the qualitative arguments of the structure zone model for physical vapor deposition to growth conditions with exacerbated atomic shadowing. At low growth temperatures Ts relative to the melting point Tm, the microstructural development is governed by atomic shadowing for both normal deposition and GLAD, resulting in fibrous grains with voided boundaries (Zone I). As the homologous growth temperature θ=Ts/Tm is raised above approximately 0.3, GLAD layers continue to exhibit well separated columns while conventional thin films show dense columnar microstructures (Zone II). θ>0.5 leads to equiaxed grains independent of deposition angle (Zone III). Therefore, strong shadowing during GLAD suppresses Zone II microstructures, causing a direct transition from Zone I to Zone III. GLAD microstructures can be divided into four distinct zones: rods, columns, protrusions, and equiaxed grains: separated self-affine rods form for θ<θc=0.24±0.2, while considerably broader columns develop at θ>θc, due to exacerbated self-shadowing associated with an increased growth front roughness, causing larger growth exponents. Above θ≈0.35, protrusions develop on top of some columns as they capture an overproportionate amount of deposition flux and grow much higher than the surrounding layer. At θ>0.5, diffusion processes dominate over atomic shadowing, leading to faceted rough layers with equiaxed grains. In addition, the large mass transport facilitates the formation of whiskers that form for many metal GLAD layers at θ>0.4.

Multi-component nanostructure design by atomic shadowing

This paper presents a method for multi-component three dimensional nanostructuring during physical vapor deposition, termed simultaneous opposite glancing angle deposition (SO-GLAD), which exploits atomic shadowing effects to control both the shape and the composition of nanorods parallel (in-plane) and perpendicular to the rod-axis. Periodic arrays of ~ 200-nm-wide Si–Ta two-component rods are grown by SO-GLAD onto monolayers of self-assembled close-packed 260-nm-diameter silica spheres. Simultaneous deposition of Si and Ta from opposite sides on a stationary substrate yields nanorods consisting of laterally separated Si and Ta components, since atomic shadowing causes deposition only onto the sphere-sides that are exposed to the respective deposition fluxes. Sequential deposition with a stationary and rotating substrate results in zigzag two-component nanosprings and vertical multilayer nanorods, respectively. A checkerboard arrangement is achieved by simultaneous deposition with intermittent 180° substrate rotations.

The structure of Ta nanopillars grown by glancing angle deposition

Regular arrays of Ta nanopillars, 200 nm wide and 500 nm tall, were grown on SiO 2 nanosphere patterns by glancing angle sputter deposition (GLAD). Plan-view and cross-sectional scanning electron microscopy analyses show dramatic changes in the structure and morphology of individual nanopillars as a function of growth temperature T s ranging from 200 to 700 °C. At low temperatures, T s ≤ 300 °C, single nanopillars develop on each sphere and branch into subpillars near the pillar top. In contrast, T s ≥ 500 °C leads to branching during the nucleation stage at the pillar bottom. The top branching at low T s is associated with surface mounds on a growing pillar that, due to atomic shadowing, develop into separated subpillars. At high T s, the branching occurs during the nucleation stage where multiple nuclei on a single SiO 2 sphere develop into subpillars during a competitive growth mode which, in turn, leads to intercolumnar competition and the extinction of some nanopillars.

Surface morphological evolution of epitaxial CrN(001) layers

CrN layers, 57 and 230 nm thick, were grown on MgO(001) at Ts=600–800°C by ultrahigh-vacuum magnetron sputter deposition in pure N2 discharges from an oblique deposition angle α=80°. Layers grown at 600 °C nucleate as single crystals with a cube-on-cube epitaxial relationship with the substrate. However, rough surfaces with cauliflower-type morphologies cause the nucleation of misoriented CrN grains that develop into cone-shaped grains that protrude out of the epitaxial matrix to form triangular faceted surface mounds. The surface morphology of epitaxial CrN(001) grown at 700 °C is characterized by dendritic ridge patterns extending along the orthogonal ⟨110⟩ directions superposed by square-shaped super mounds with ⟨100⟩ edges. The ridge patterns are attributed to a Bales-Zangwill instability while the supermounds form due to atomic shadowing which leads to the formation of epitaxial inverted pyramids that are separated from the surrounding layer by tilted nanovoids. Growth at 800 °C yields complete single crystals with smooth surfaces. The root-mean-square surface roughness for 230-nm-thick layers decreases from 18.8 to 9.3 to 1.1 nm as Ts is raised from 600 to 700 to 800 °C. This steep decrease is due to a transition in the roughening mechanism from atomic shadowing to kinetic roughening. Atomic shadowing is dominant at 600 and 700 °C, where misoriented grains and supermounds, respectively, capture a larger fraction of the oblique deposition flux in comparison to the surrounding epitaxial matrix, resulting in a high roughening rate that is described by a power law with an exponent β&amp;gt;0.5. In contrast, kinetic roughening controls the surface morphology for Ts=800°C, as well as the epitaxial fraction of the layers grown at 600 and 700 °C, yielding relatively smooth surfaces and β⩽0.27.

Nanostaircases: An atomic shadowing instability during epitaxial CrN(001) layer growth

Epitaxial CrN(001) layers, 57 and 230 nm thick, were grown on MgO(001) at 700 °C by ultrahigh-vacuum magnetron sputter deposition in pure N2 discharges. An oblique deposition angle α=80° was utilized to purposely increase the effect of atomic shadowing on surface morphological and microstructural evolution. The layers are single crystals with a surface morphology that is characterized by dendritic ridge patterns extending along orthogonal ⟨110⟩ directions superposed by square-shaped supermounds with ⟨100⟩ edges. The ridge patterns are due to a two-dimensional growth instability related to a gradient in the adatom density while the supermounds form due to atomic shadowing. The supermounds protrude out of the surface and capture a larger deposition flux than the surrounding layer. This leads to both vertical and lateral growth and the formation of inverted pyramids that are epitaxially embedded in a single crystalline matrix. The inverted pyramids are terminated by 1–3-nm-wide tilted voids that form nanostaircases due to kinetic faceting along orthogonal {100} planes.

Epitaxial growth of metastable δ-TaN layers on MgO(001) using low-energy, high-flux ion irradiation during ultrahigh vacuum reactive magnetron sputtering

We report on the microstructural evolution and physical properties of epitaxial δ-TaN layers grown on MgO(001) at 600 °C by ultrahigh vacuum reactive magnetron sputtering of Ta in mixed Ar/N2 discharges as a function of the N2 fraction fN2 (0.100⩽fN2⩽0.275) and the incident ion energy Ei (8.4⩽Ei⩽65 eV). The ratio of the ion-to-Ta fluxes Ji/JTa incident at the growing film was maintained at 11±0.5 with 0.100⩽fN2⩽0.200 and 15±0.3 with 0.250⩽fN2⩽0.275. High-resolution x-ray diffraction, transmission electron microscopy, and Rutherford backscattering spectrometry results show that films grown with Ei≲40 eV are single-phase δ-TaNx(001), with x increasing from 0.94 with fN2=0.100 to 1.37 with fN2=0.275, exhibiting a cube-on-cube epitaxial relationship with the substrate: (001)δ-TaN‖(001)MgO and [100]δ-TaN‖[100]MgO. However, the use of Ei≲20 eV leads to δ- TaNx(001) layers which are underdense with a self-organized array of 1-nm-wide nanopipes oriented predominantly along orthogonal 〈100〉 directions. The nanopipes, which are first observed at film thicknesses of ≃5 nm and extend to the surface, form due to the combination of low adatom surface diffusivities, leading to kinetic roughening, and atomic shadowing. Fully dense stoichiometric epitaxial δ-TaN(001) films were obtained with Ei=30 eV and fN2=0.125. Increasing Ei to 40 eV results in a high density of {111} stacking faults. Films grown with even higher ion energies, 40&amp;lt;Ei⩽65 eV, contain large concentrations of residual extended defects as well as hexagonal γ-Ta2N second-phase inclusions. The room-temperature resistivity, hardness, elastic modulus, and relaxed lattice constant of fully dense stoichiometric δ-TaN(001) are 185±15 μΩ cm, 32.9±0.9 GPa, 435±15 GPa, and 0.4351±0.0002 nm, respectively.

Growth of single-crystal CrN on MgO(001): Effects of low-energy ion-irradiation on surface morphological evolution and physical properties

CrN layers, 0.5 μm thick, were grown on MgO(001) at Ts=570–775 °C by ultrahigh vacuum magnetically unbalanced magnetron sputter deposition in pure N2 discharges at 20 mTorr. Layers grown at Ts⩽700 °C are stoichiometric single crystals exhibiting cube-on-cube epitaxy: (001)CrN||(001)MgO with [100]CrN||[100]MgO. At higher temperatures, N2 desorption during deposition results in understoichiometric polycrystalline films with N fractions decreasing to 0.35, 0.28, and 0.07 with Ts=730, 760, and 775 °C, respectively. The surface morphologies of epitaxial CrN(001) layers were found to depend strongly on the incident ion-to-metal flux ratio JN2+/JCr which was varied between 1.7 and 14 with the ion energy maintained constant at 12 eV. The surfaces of layers grown with JN2+/JCr=1.7 consist of self-organized square-shaped mounds, due to kinetic roughening, with edges aligned along orthogonal 〈100〉 directions. The mounds have an average peak-to-valley height 〈h〉=5.1 nm and an in-plane correlation length of 〈d〉=0.21 μm. The combination of atomic shadowing by the mounds with low adatom mobility results in the formation of nanopipes extending along the growth direction. Increasing JN2+/JCr to 14 leads, due to increased adatom mobilities, to much smoother surfaces with 〈h〉=2.5 nm and 〈d〉=0.52 μm. Correspondingly, the nanopipe density decreases from 870 to 270 μm−2 to &amp;lt;20 μm−2 as JN2+/JCr is increased from 1.7 to 6 to 10. The hardness of dense CrN(001) is 28.5±1 GPa, but decreases to 22.5±1 GPa for layers containing significant nanopipe densities. The CrN(001) elastic modulus, 405±15 GPa, room-temperature resistivity, 7.7×10−2 Ω cm, and relaxed lattice constant, 0.4162±0.0008 nm, are independent of JN2+/JCr.

Interfacial reaction pathways and kinetics during annealing of 111-textured Al/TiN bilayers: A synchrotron x-ray diffraction and transmission electron microscopy study

Growth of TiN layers in most diffusion-barrier applications is limited to deposition temperatures Ts≲500 °C. We have grown polycrystalline TiN layers, 160 nm thick with a N/Ti ratio of 1.02±0.03 and a 111 texture, at Ts=450 °C on SiO2 by ultrahigh vacuum reactive magnetron sputter deposition in pure N2. Al overlayers, 160 nm thick with inherited 111 preferred orientation, were then deposited at Ts=100 °C without breaking vacuum. The as-deposited TiN layer is underdense due to the low deposition temperature (Ts/Tm≃0.23 in which Tm is the melting point) resulting in kinetically limited adatom mobilities leading to atomic shadowing which, in turn, results in a columnar microstructure with both inter- and intracolumnar voids. The Al overlayer is fully dense. Synchrotron x-ray diffraction was used to follow interfacial reaction kinetics during postdeposition annealing of the 111-textured Al/TiN bilayers as a function of time (ta=12–1200 s) and temperature (Ta=440–550 °C). Changes in bilayer microstructure and microchemistry were investigated using transmission electron microscopy (TEM) and scanning TEM to obtain compositional maps of plan-view and cross-sectional specimens. Interfacial reaction during annealing is initiated at the Al/TiN interface. Al diffuses rapidly into TiN voids during anneals at temperatures ⪝480 °C. In contrast, anneals at higher temperatures lead to the formation of a continuous nanocrystalline AlN layer which blocks Al penetration into TiN. At all annealing temperatures, Ti atoms released during AlN formation react with Al to form tetragonal Al3Ti at the interface. Al3Ti exhibits a relatively planar growth front extending toward the Al free surface. Analyses of time-dependent x-ray diffraction peak intensities during isothermal annealing as a function of temperature show that Al3Ti growth kinetics are, for the entire temperature range investigated, diffusion limited with an activation energy of 1.5±0.2 eV.

A molecular dynamics study of nickel vapor deposition: Temperature, incident angle, and adatom energy effects

The morphology and microstructure of metallic thin films synthesized by physical vapor deposition are known to be sensitive functions of the incident flux, its angular distribution and kinetic energy as well as the substrate temperature. Using an embedded atom method to represent the interatomic interactions, two-dimensional molecular dynamics simulations have been conducted to identify the role of these variables upon vacancy formation and surface morphology evolution during nickel deposition. The results reveal the existence of a critical incident angle above which mono-vacancies and vacancy clusters/voids are created by atomic shadowing. Increasing the substrate temperature and the adatom kinetic energy in the 0.1–2.0 eV range significantly reduces vacancy formation. Time lapse analyses have been used to identify the atomic mechanisms for this. They indicate that low defect concentration films can be synthesized at low temperatures by increasing the adatom kinetic energy to a few eV. A simple empirical model is proposed to relate the vacancy concentration to the controllable process conditions.

Application of ion beam sputtering for high‐resolution electron microscopy

AbstractIon beam sputtering for high‐resolution electron microscopy has basically provided miscellaneous operational features such as atomic shadowing, uncoated observation, and etching of biological specimens coupled with tungsten sputter coating and thinning of solid materials.Based on the power‐potential law of Lindhard for ionic impact phenomena on metal surfaces, the universal yield‐energy relationship has been derived. Thereby the sputtering deposition rate with reference to the sputtering removal rate was obtained as a function of sputtering yield, and the most important angular distribution of sputtering yield could be measured by using the hemicylindrical specimen stage.Evidence is presented to show that ion beam sputtering has become one of the most powerful tools for high‐resolution electron microscopy.

Ion bombardment of suitable targets for atomic shadowing for high resolution electron microscopy

Abstract The use of ion beam bombardment of suitable targets for the atomic shadowing of specimens for high resolution electron microscopy has been investigated. An apparatus is described consisting of a modified duoplasmatron source in which the ion beam is collimated and focused by an electrostatic lens and emerging atomic beam from the target is collimated by an aperture in a shield placed close to the specimen. The shadowing angle can be varied from about 10° to 75°. The thickness of the deposit or film and the rate of deposition have been studied in relation to the sputtering yield, flux of the primary ion beam and shadowing characteristics (contrast, grain size, nucleation and growth). Ion bombardment of tungsten yielded thin films with good contrast characteristics and with a barely resolvable (less than 0.4nm) grain size which, with other advantages of the technique as compared with evaporation (lower substrate temperatures and vaccum pressures) render this procedure valuable for high resolution studies. Films of tungsten prepared in this way, and up to 25nm thick, do not interfere with the phase changes of transmitted electrons at 60kV and thus have characteristics which also make them useful for serving as specimen supports. Investigations on the deposition of gold by ion bombardment revealed both the characteristics single crystall structure (1–3nm) and abnormal growth crystals up to about 0.2μm across.