Reactive Ion Beam Deposition Research Articles

Switchable optics based on guided mode resonance in lithographically patterned vanadium dioxide with integrated heating layer

Vanadium dioxide (VO2) has promising applications in smart windows and active micro-optical devices due to its thermochromic properties. However, the successful fabrication and patterning of VO2 thin films with the correct stoichiometry and phase are challenging. In this study, we investigated lithographically patterned and non-patterned VO2 thin films fabricated by reactive ion beam deposition, using variable angle spectroscopic ellipsometry, Raman spectroscopy, and transmission and reflection measurements. The results show that the refractive index and extinction coefficient exhibit significant changes for near-infrared wavelengths when heated above 68 °C, confirming its thermochromic properties. The Raman spectroscopy results indicate the formation of the monoclinic phase VO2(M) after annealing, which was not changed by reactive ion etching. Lithographically structured VO2-layers were successfully realized demonstrating the potential of VO2 as a material for active micro-optical devices, such as guided mode resonance filters with switchable reflectance. The results suggest that VO2 has great potential as a promising material for actively switched optical elements and micro-optical devices.

Исследование возможности получения структур с нанометровыми толщинами слоев и резкими границами раздела между ними с помощью процессов ионно-лучевого и реактивного ионно-лучевого осаждения

В настоящее время для получения структур с нанометровыми толщинами слоев и резкими границами раздела между ними в производстве интегральных микросхем применяются процессы атомно-слоевого осаждения и магнетронного распыления. Однако существуют процессы ионно-лучевого и реактивного ионно-лучевого осаждения, которые в основном используются для получения многослойных оптических покрытий. Исследована возможность получения структур с нанометровыми толщинами слоев и резкими границами раздела между ними в процессах ионно-лучевого и реактивного ионно-лучевого осаждения. Исследования проведены методами времяпролетной вторичной ионной масс-спектрометрии и спектральной эллипcометрии. На примере структуры Ta (3 нм)/Nb (3 нм)/Ta (3 нм) впервые показано, что процесс ионно-лучевого осаждения позволяет формировать структуры с нанометровыми толщинами слоев и резкими границами раздела между ними. Тогда как в процессе реактивного ионно-лучевого осаждения структуры Nb (3 нм)/Ta2O5(3 нм)/Nb (3 нм) происходит окисление на всю толщину металлического слоя, следующего за слоем оксида металла, за счет ионов, атомов и молекул кислорода, содержащихся в ионном пучке.

NbTiN/AlN/NbTiN SIS Junctions Realized by Reactive Bias Target Ion Beam Deposition

The current state-of-the-art approach for superconductor–insulator–superconductor (SIS) junction fabric-ation is based on magnetron sputtering and the Gurvitch Al overlayer trilayer process, where an Al overlayer is deposited onto the Nb base electrode in order to subsequently grow a critical $\sim$ 1-nm AlO $_{x}$ or AlN tunnel barrier. While the switch from AlO $_x$ to AlN barriers has significantly increased the achievable critical current density, and significant research has been performed to understand and optimize inductively coupled plasma AlN growth, the use of Nb electrodes provides an upper limit to the frequency range for low-noise operation when employed in terahertz (THz) mixer applications and has led to the study and use of alternative materials with higher transition temperatures ( $T_{C}$ ) such as NbN and NbTiN. The Nb electrodes also impose stringent cooling requirements; replacement of both electrodes with higher $T_{C}$ materials can increase the required device operating temperature to 10 K, thus reducing the power consumption of the refrigeration system, a building block for the realization of more energy efficient superconducting computing systems, such as those based on single-flux-quantum logic. In this work, we have departed from conventional SIS material growth techniques through the use of an alternative material deposition technology—reactive bias target ion beam deposition (RBTIBD)—that offers unique capabilities to tailor materials and interfaces. Using RBTIBD technology, we have realized the first ever NbTiN/AlN/NbTiN SIS junctions with highest yet reported sum-gap voltages exceeding 5.0 mV, potentially extending the theoretical limit of low-loss SIS mixing applications beyond 1.2 THz and relaxing the cooling requirements for superconducting device applications.

Effect of Post Deposition Annealing on the Structural and Electrical Properties of NbTiN Thin Films Deposited by Reactive Bias Target Ion Beam Deposition Technique

Nb-based superconducting alloys, such as NbN and NbTiN, can have a superconducting energy gap near twice that of elemental Nb, thus, being ideal candidates for low-loss superconducting circuits operating above the gap frequency of Nb. We are particularly interested in these materials for THz-frequency superconducting-insulating-superconducting (SIS) mixers, kinetic inductance detectors, and traveling wave kinetic inductance parametric amplifier devices. We recently reported the use of an alternative synthesis technique, reactive bias target ion beam deposition (RBTIBD), used to realize high-quality NbTiN films and high-energy gap NbTiN/AlN/NbTiN SIS heterostructures. In this paper, the effect of post-deposition annealing on the electrical and structural properties of room temperature deposited RBTIBD NbTiN was investigated. Our room temperature RBTIBD NbTiN films were annealed using rapid thermal annealing from 200 to 1000 °C. The change in structural and electrical properties was characterized by X-ray diffraction, atomic force microscopy, and four-point probe measurements. The annealed films demonstrated an enhancement in transition temperature (T c ), increased crystallinity, larger grain size, and significantly reduced intrinsic stress.

Growth and Characterization of NbTiN Films Synthesized by Reactive Bias Target Ion Beam Deposition (RBTIBD)

High energy gap Nb-based superconducting alloys with low normal state resistivity are fundamentally important for realization of low-loss high frequency circuits operating above the typical ∼670 GHz gap frequency of elemental Nb. NbTiN has been shown to have a superconducting energy gap nearly twice that of elemental Nb, and is emerging as an important material for various detector and superconducting computing technologies. In this paper, we report on the growth of high-quality face-centered cubic (FCC) NbTiN films through the use of an alternative materials deposition technology—reactive bias target ion beam deposition. This novel technique offers more degrees of freedom compared to conventional sputtering, notably through decoupling the plasma generation and target bias, through use of a hollow-cathode ion source and independently pulse biased sputtering targets. The reported films are of high quality, with increased transition temperatures, reduced normal state resistivity, and surface roughness, compared to our previously reported reactive magnetron sputtered NbTiN films. In particular, at UVA, we are investigating these NbTiN films for wide-bandgap all-NbTiN superconducting-insulating-superconducting mixers, kinetic inductance detectors, and traveling-wave kinetic inductance parametric amplifier devices.

Research of the Possibility to Obtain Structures with Nanometer Layer Thicknesses and Sharp-Cut Interfaces between Them Using Ion-Beam and Reactive Ion-Beam Deposition Processes

Abstract—At present, atomic layer deposition and magnetron sputtering processes are used in the production of integrated circuits (IC) to obtain structures with nanometer layer thicknesses and sharp interfaces between them. However, there are also ion-beam and reactive ion-beam deposition processes, which are mainly used to produce multilayer optical coatings. The aim of this work is to study the possibility of obtaining structures with nanometer layer thicknesses and sharp interfaces between them in the processes of ion-beam and reactive ion-beam deposition. The studies were carried out by time-of-flight secondary ion mass spectrometry (SIMS) and spectral ellipsometry methods. Study of the structure Ta (3 nm)/Nb (3 nm)/Ta (3 nm) reveals that ion-beam deposition can form structures with nanometer layer thicknesses and sharp boundaries between them. On the other hand, in reactive ion-beam deposition of the structure Nb (3 nm)/Ta2O5 (3 nm)/Nb (3 nm), oxidation occurs on the entire thickness of the metal layer following the metal oxide layer due to ions, atoms, and molecules of oxygen contained in the ion beam.

Infrared laser protection of multi-wavelength with high optical switching efficiency VO2 film

Vanadium dioxide (VO2) film with nanoparticles is fabricated by reactive ion beam deposition (RIBD) technology and post-annealing method on a quartz glass substrate. RIBD can enhance the damage threshold of VO2 film and reduce its scattering at insulator-state. And post-annealing can eliminate the structure defects and residual stress. VO2 film exhibits first-order and reversible metal-to-insulator (MIT) phase transition at a temperature of 68 ℃. It also exhibits photo-induced MIT, in which process a metal-like phase of monoclinic VO2 appears. With many surprising features in heat-induced and photo-induced MIT processes, VO2 film turn to satisfy all the characteristics needed for a laser protection system. The thickness of VO2 film used in these experiments and simulations is about 100 nm. The double-frequency He-Ne laser at a wavelength of 3 m is used to perform the experiment of heat-induced MIT, with a temperature controlling system. The exact optimal annealing temperature is demonstrated to be 465 ℃, as the sample annealing at this temperature shows the sharpest transition properties and unmixed VO2 phase peaks in X-ray diffraction pattern. Drude and Drude-Lorentz dispersion models are taken to analyze the dielectric constant of VO2. Then, the complex refractive index is calculated for simulation. Simulations with the TFCale software show that the transmissions at high temperature and low temperature have high contrasts in the infrared range. MIT experiments at multi-wavelength, which cover heat-induced and photo-induced MIT phase transition, are performed to investigate the applicability of VO2 film in multi-wavelength laser protection for both continuous wave and pulsed lasers Thus the excellent performance of VO2 film for laser protection is roundly verified. The laser protection experiments on silicon photocell exhibit that the VO2 film enhances the anti-jamming capability of photocell system by about 2.6 times, demonstrating the applicability of VO2 film to laser protection system. The power density of MIT transition threshold of VO2 film with a thickness of 100 nm is 4.35 W/cm2 at room temperature, which is investigated with a continuous wave laser at a wavelength of 1.08 m with a continuous tunable system. In addition, atomic force microscope is used to observe the film surfaces, which are irradiated by lasers with different power densities for different times The experimental results demonstrate that the power density damage threshold of VO2 film becomes very high (404 W/cm2). The low MIT transition threshold and high damage threshold of VO2 film further demonstrate its applicability as a key role for a laser protection system. With the high switching efficiency ratio and high damage threshold, VO2 thin film can be used in optical switch, smart windows and photoelectric device.

High temperature coefficient of resistance molybdenum oxide and nickel oxide thin films for microbolometer applications

Molybdenum oxide (MoOx) and nickel oxide (NiOx) thin films were deposited by reactive biased target ion beam deposition. MoOx thin film resistivity varied from 3 to 2000 Ω·cm with a temperature coefficient of resistance (TCR) from −1.7% to −3.2%/K, and NiOx thin film resistivity varied from 1 to 300 Ω·cm with a TCR from −2.2% to −3.3%/K, both easily controlled by varying the oxygen partial pressure. Biased target ion beam deposited high TCR MoOx and NiOx thin films are polycrystalline semiconductors and have good stability in air. Compared with commonly used vanadium oxide thin films, MoOx or NiOx thin films offer improved process control for resistive temperature sensors.

Epitaxial niobium dioxide thin films by reactive-biased target ion beam deposition

Epitaxial NbO2 thin films were synthesized on Al2O3 (0001) substrates via reactive bias target ion beam deposition. X-ray diffraction and Raman spectra were used to confirm the tetragonal phase of pure NbO2. Through XPS, it was found that there was a ∼1.3 nm thick Nb2O5 layer on the surface and the bulk of the thin film was NbO2. The epitaxial relationship between the NbO2 film and the substrate was determined. Electrical transport measurement was measured up to 400 K, and the conduction mechanism was discussed.

Reactive biased target ion beam deposited W–DLC nanocomposite thin films — Microstructure and its mechanical properties

Tungsten incorporated diamond like carbon (W–DLC) nanocomposite thin films with variable fractions of tungsten were deposited by using reactive biased target ion beam deposition technique. The influence of tungsten incorporation on the microstructure, surface topography, mechanical and tribological properties of the DLC were studied using X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, Atomic force microscope (AFM), transmission electron microscopy (TEM), nano-indentation and nano-scratch tests. The amount of W in films gets increases with increasing target bias voltage and most of the incorporated W reacts with carbon to form WC nanoclusters. Using TEM and FFT pattern, it was found that spherical shaped WC nanoclusters were uniformly dispersed in the DLC matrix and attains hexagonal (W2C) crystalline structure at higher W concentration. On the other hand, the incorporation of tungsten led to increase the formation of C-sp2 hybridized bonding in DLC network and which is reflected in the hardness and elastic modulus of W–DLC films. Moreover, W–DLC films show very low friction coefficient and increased adhesion to the substrate than the DLC film, which could be closely related to its unique nanostructure of the W incorporated thin films.

Bioactivity and mechanical properties of nickel‐incorporated hydrogenated carbon nanocomposite thin films

In this paper, the influence of nickel incorporation on the mechanical properties and the in vitro bioactivity of hydrogenated carbon thin films were investigated in detail. Amorphous hydrogenated carbon (a‐C:H) and nickel‐incorporated hydrogenated carbon (Ni/a‐C:H) thin films were deposited onto the Si substrates by using reactive biased target ion beam deposition technique. The films' chemical composition, surface roughness, microstructure and mechanical properties were investigated by using XPS, AFM, TEM, nanoindentation and nanoscratch test, respectively. XPS results have shown that the film surface is mainly composed of nickel, nickel oxide and nickel hydroxide, whereas at the core is nickel carbide (Ni3C) only. The presence of Ni3C has increased the sp2 carbon content and as a result, the mechanical hardness of the film was decreased. However, Ni/a‐C:H films shows very low friction coefficient with higher scratch‐resistance behavior than that of pure a‐C:H film. In addition, in vitro bioactivity study has confirmed that it is possible to grow dense bone‐like apatite layer on Ni/a‐C:H films. Thus, the results have indicated the suitability of the films for bone‐related implant coating applications. Copyright © 2011 John Wiley & Sons, Ltd.

Effect of nickel incorporation on structural, nanomechanical and biocompatible properties of amorphous hydrogenated carbon thin films prepared by low energy biased target ion beam deposition

Nickel incorporated amorphous hydrogenated carbon (Ni/a:C–H) thin films were deposited onto the Si substrates via biased target ion beam sputtering of nickel combined with reactive ion beam deposition of a:C–H using CH 4/Ar gas mixture. The effects of Ni doping and target bias voltage on the microstructure and mechanical properties of the as-deposited films were investigated by means of X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, transmission electron microscopy and nanoindentation. It was found that the Ni content in the films gets increased with increasing target bias voltage, and most of the Ni atoms react with C atoms to form NiC x phases in the Ni/a:C–H films. Moreover, the nickel carbide nanoparticles attain crystallinity even at low deposition temperature and get embedded in the cross-linked carbon matrix. It was found that the presence of Ni 3C nanoparticles tends to increase the content of sp 2 carbon, thus decreasing the hardness of Ni/a:C–H films as compared with that of a:C–H films. Additionally it was found that the nickel incorporated films do not show any adverse effect on the osteoblast cellular adhesion. Overall, these carbidic nanocrystals initiate direct graphitization and intend to change diamond-like to graphite-like carbon structure in Ni/a:C–H films with promising biocompatibility.

Preparation and Nanomechanical Characterisation of Metal Containing Amorphous Hydrogenated Carbon Nanocomposite Films

The relationship between metal-induced chemical bonding and the mechanical properties of Me/a:C-H (Me- Ti and Ni) films are discussed. Nanocomposite films were deposited onto the Si substrates via biased target ion beam sputtering of metal combined with reactive ion beam deposition of a:C-H using CH4/Ar gas mixture. The chemical composition, microstructure and mechanical properties were characterized using X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and nanoindentation. XPS analysis revealed that both Ti and Ni atoms were preferentially bonded to carbon to form its metal carbide phase. Irrespective of its kind, both the metal carbide clusters induces more graphite like bondings in a:C-H matrix. From the nanoindentation analysis it was found that there is no crack propagation and damage around the indent region even at very high indentation loads. Additionally, it was found that a:C-H film shows higher hardness compared to that of Ti/a:C-H and Ni/a:C-H films.

The metal-insulator transition in vanadium dioxide: A view at bulk and surface contributions for thin films and the effect of annealing

Vanadium dioxide is investigated as potential oxide barrier in spin switches, and in order to incorporate VO2 layers in complex multilayer devices, it is necessary to understand the relation between bulk and surface/interface properties. Highly oriented VO2 thin films were grown on (0001) sapphire single crystal substrates with reactive bias target ion beam deposition. In the analysis of the VO2 films, bulk-sensitive methods [x-ray diffraction (XRD) and transport measurements] and surface sensitive techniques [photoelectron spectroscopy (PES) and scanning tunneling microscopy and spectroscopy] were employed. The samples were subjected to heating cycles with annealing temperatures of up to 425 and 525K. Prior to annealing the VO2 films exhibit the transition from the monoclinic to the tetragonal phase with the concurrent change in conductivity by more than a factor of 103 and their phase purity is confirmed by XRD. Annealing to 425K and thus cycling across the metal-insulator transition (MIT) temperature has no impact on the bulk properties of the VO2 film but the surface undergoes irreversible electronic changes. The observation of the valence band with PES during the annealing illustrates that the surface adopts a partially metallic character, which is retained after cooling. Annealing to a higher temperature (525K) triggers a modification of the bulk, which is evidenced by a considerable reduction in the MIT characteristics, and a degradation in crystallite morphology. The local measurement of the conductivity with scanning tunneling spectroscopy shows the transition of the surface from predominantly semiconducting surface prior to annealing to a surface with an overwhelming contribution from metallic sections afterward. The spatial distribution of metallic regions cannot be linked in a unique manner to the crystallite size or location within the crystallites. The onset of oxygen depletion at the surface is held responsible for this behavior. The onset of bulk modification at higher temperatures is most likely linked to oxygen loss and effusion along the grain boundaries and concurrent onset of sintering. Our study focuses on the comparison of the MIT in the bulk and at the surface of thin VO2 layers and establishes an irreversible modification of the crystallite structure and surface for temperatures exceeding the MIT. The surface modification impacts on the strategies which will be employed to build the metallic contacts to VO2 layers.

Comparative study of self-assembling of multilayers using reactive sputter deposition and mass selective ion beam deposition

While experimenting with the growth of metal-containing amorphous carbon (a-c:Me) thin films using two different growth processes, self-assembled multilayered structures were observed. One of the processes is a reactive magnetron sputter deposition process. The other process is a mass selective ion beam deposition process. Despite of the differences in the growth method and the growth condition, self-assembled multilayered thin films, consisting of alternating dark layer and bright layer, were obtained in both processes. Based on the consideration of energy for atomic diffusion in the thin films, the growth mechanism is discussed.

Fundamental reliability of 1.5‐nm-thick silicon oxide gate films grown at 150°C by modified reactive ion beam deposition

The reliability of 1.5‐nm-thick silicon oxide gate films grown at 150°C by modified reactive ion beam deposition (RIBD) with in situ pyrolytic-gas passivation (PGP) using N2O and NF3 was investigated. RIBD uses low-energy-controlled reactive, ionized species and potentializes low-temperature film growth. Although the oxide films were grown at a low temperature of 150°C, their fundamental indices of reliability, such as the time-dependent dielectric breakdown lifetime and interface state density, were almost equivalent to those of oxide films grown at 850°C using a furnace. This is probably due to localized interfacial N and F atoms. The number density of interfacial N atoms was about seven times larger than that for the furnace-grown oxide films, and this is a key factor for improving the reliability through the compensation of residual inconsistent-state bonding sites.

Growth and characterization of vanadium dioxide thin films prepared by reactive-biased target ion beam deposition

Using a novel growth technique called reactive bias target ion beam deposition, the authors have prepared highly oriented VO2 thin films on Al2O3 (0001) substrates at various growth temperatures ranging from 250to550°C. The influence of the growth parameters on the microstructure and transport properties of VO2 thin films was systematically investigated. A change in electrical conductivity of 103 was measured at 341K associated with the well known metal-insulator transition (MIT). It was observed that the MIT temperature can be tuned to higher temperatures by mixing VO2 and other vanadium oxide phases. In addition, a current/electric-field induced MIT was observed at room temperature with a drop in electrical conductivity by a factor of 8. The current densities required to induce the MIT in VO2 are about 3×104A∕cm2. The switching time of the MIT, as measured by voltage pulsed measurements, was determined to be no more than 10ns.

1.5 - nm -thick silicon oxide gate films grown at 150°C using modified reactive ion beam deposition with pyrolytic-gas passivation

Low-temperature ultrathin silicon oxide gate film growth using modified reactive ion beam deposition (RIBD) with an in situ pyrolytic-gas passivation (PGP) method is described. RIBD uses low-energy-controlled reactive and ionized species and potentializes low-temperature film growth. By combining RIBD with PGP using N2O and NF3, 1.5-nm-thick silicon oxide gate films with high-potential barrier height energy, 3.51eV, and low-leakage current, less than about 10−5A∕cm2 at 2MV∕cm, can be obtained at a growth temperature of 150°C. From an evaluation of number densities of N, F, and O atoms near the 1.5–5.0-nm-thick RIBD-with-PGP silicon oxide films/Si(100) interfaces, it is believed that interfacial N and F atoms contribute to improve the electrical characteristics and F effectively compensates the residual inconsistent-state bonding sites after the N passivation.

Room-temperature growth of epitaxial Fe3O4 films by ion beam deposition

Epitaxial and polycrystalline Fe3O4 films were grown on MgO (100) and Si (100) substrates, respectively, at room temperature by using reactive ion beam deposition. The MS value of epitaxial Fe3O4 films was around 310 emu/cm3, and was almost independent of thickness from 45 to 195 nm. The MS value of polycrystalline films showed significant thickness dependence, which might be attributed to the formation of the initial layer. The Verwey transition at 110 K was observed on 195 nm epitaxial films, and decreased significantly with decreasing thickness. The reduction of the Verwey temperature may be related to the residual strain in the film.

Design and performance of a versatile, cost-effective microwave electron cyclotron resonance plasma source for surface and thin film processing

A plasma source has been developed, which makes use of electron cyclotron microwave resonance conditions without cost-intensive tuning or other active elements. A magnetron typical of domestic microwave ovens is coupled to a cylindrical resonator of fixed dimensions. A high density plasma is maintained with permanent magnets. Sources have been designed for both high vacuum and ultrahigh vacuum environments. In the latter, microwaves are conducted by a coaxial feedthrough from the resonator to a ceramic plasma cup inside the vacuum chamber. This permits shorter working distances, resulting in higher beam densities at the sample. Ions are extracted from the plasma by grid assemblies. Beam energies are in the range from 30 to 2000 eV, with densities from a few μA/cm2 up to 10 mA/cm2, respectively. Predominantly neutral species are emitted using a specially designed, ceramic grid. Operating with O2 gas, a flux of neutral monomers of 2×1016/cm2 s has been measured by the oxidation of Ag films. The performance of the source is demonstrated in various surface and thin film processes, like ion beam sputtering, reactive ion beam deposition, as well as sputter cleaning and reactive etching of semiconductor materials.