Remote Plasma Sputtering Research Articles

Reactive remote plasma sputtering of TiOx thin films and controlled growth of textured single-phase rutile using rf substrate biasing

Titanium oxide (TiOx) thin films were deposited by remote plasma sputtering (RPS) with the use of rf substrate bias onto unheated and water-cooled glass, Si, and Kapton substrates. The rf remote plasma power was kept constant (2.0 kW) and the properties of the films were studied as a function of deposition rate and substrate bias voltage. Four different reactive processes at four different deposition rates were developed under zero bias conditions by altering the target bias voltage and the flow of oxygen to give highly transparent TiOx films. These processes were studied with varying substrate bias voltage and the film structural, optical, and surface properties were investigated by GI-XRD, SEM, transmission, and sessile drop measurements. The changes in film structure and properties observed were related to the plasma conditions of the RPS system, investigated by dc electrical probe and OES measurements, and compared with other observations for films deposited under energetic conditions in the literature. All films deposited under zero bias conditions exhibited amorphous structures in GI-XRD and SEM images. Sufficient application of substrate bias was seen to promote columnar growth in cross-sectional SEM images and to crystallise phase pure rutile with no detection of the anatase phase in GI-XRD. Rutile bearing films were noted for their change in optical properties. Further increase of the substrate bias voltage caused a change in the texture of the rutile films from a low-energy [110] preferred orientation to a high-energy [101] and [002] preferred orientation. Contact angles of water with deposited films were found to increase with increasing substrate bias voltage and decreasing deposition rate. Film stress was also found to be influenced mainly by the substrate bias voltage process parameter.

Role of Materials Chemistry on Transparent Conductivity of Amorphous Nb-Doped SnO2 Thin Films Prepared by Remote Plasma Deposition

In this study, remote plasma sputtering deposition of niobium-doped SnO2 transparent conductive oxides on glass substrates was carried out at ambient temperature with no post-deposition annealing. The microstructure, optical, electrical, and surface morphology of the thin films were characterized using a combination of advanced techniques, such as X-ray diffraction (XRD), UV-Vis spectrophotometer, Hall-effect measurements, as well as field emission scanning electron microscope (FESEM), high-resolution transmission electron microscopy, and high-resolution X-ray photoelectron spectroscopy. It was determined that the oxygen defects of the films have a substantial impact on their transparent conductivity. The crystalline films, which were crystallized by annealing at 450 °C, had higher resistivities due to a decreased concentration of oxygen vacancies, which restricted conduction. In comparison, the amorphous films exhibited remarkable conductivity. The best amorphous films (Nb:SnO2) exhibited a resistivity of less than 4.6 × 10−3 Ω·cm, with a 3 × 1020 cm−3 carrier concentration and a 4.4 cm2/(V·S) of Hall mobility. X-ray amorphous Nb:SnO2 films can be used to make conductive and transparent protective layers that can be used to shield semiconducting photoelectrodes used in solar water splitting. These layers can also be used with more conductive TCO films (ITO or AZO) when needed.

The effect of RF plasma power on remote plasma sputtered AZO thin films

Aluminium-doped ZnO (AZO) thin films were deposited by remote plasma sputtering of a ZnO:Al2O3 98:2 wt% ceramic target in a pulsed DC configuration. The target power was kept constant at 445 W and the RF plasma power was varied between 0.5 and 2.5 kW. The as-deposited AZO thin films exhibited an optimum resistivity of 6.35 × 10−4 Ω·cm and optical transmittance of 92% at a RF plasma power of 1.5 kW. The thin film microstructure, chemical composition, and residual stress were investigated using SEM, RBS, XPS and XRD. Accurate determination of the chemical composition and correct interpretation of GIXRD data for AZO thin films are a particular focus of this work. The AZO layer thickness was 500–700 nm and Al content in the range of 2.3–3.0 at.%, determined by RBS. The AZO thin films exhibited a strong (002) preferential orientation and grain sizes between 70 and 110 nm. The (103) peak intensity enhancement in GIXRD is proven to be a result of the strong (002) preferential orientation and GIXRD geometrical configuration rather than a change in the crystallite orientation at the surface. XPS depth profiles show preferential sputtering of O and Al using a 500 eV Ar+ beam, which can be reduced, but not eradicated using an 8 keV Ar150+ beam. The preferential sputtering can be successfully modelled using the simulation software TRIDYN. A plasma power of 1.5 kW corresponds to a highly ionised plasma and various microstructural and compositional factors have all contributed to the optimum low resistivity occurring at this plasma power. The grain size exhibits a maximum in the 1.25–1.5 kW range and there is improved (002) orientation, minimising grain boundary scattering. The highest carrier concentration and mobility was observed at the plasma power of 1.5 kW which may be associated with the maximum in the aluminium doping concentration (3.0 at.%). The lowest residual stress is also observed at 1.5 kW.

An effect of rapid post-annealing temperature on the properties of cupric oxide thin films deposited by a remote plasma sputtering technique

Traditional annealing has limitations of long-treatment time, excessive grain size and energy-intensive, inhibiting the quality and applications of the semiconductor thin films. Herein, an effect of rapid post-annealing (RPA) on the properties of cupric oxide (CuO) thin films deposited by a remote plasma sputtering technique was studied systemically. The chemical composition, microstructure, and the optoelectronic properties, including transmission, absorption, band gap, Urbach energy, carrier concentrations, and hall mobility, etc., were characterized by XRD, Raman, XPS, AFM, SEM TEM, optical transmission/absorption spectra and Hall measurements, correspondingly. The results reveal that the as-deposited and annealed (250, 350 and 450oC for 5 min) films all possess a single monoclinic CuO phase. All films possess compact nanostructures and the surface root-mean-square (RMS) roughness is around 1.5 nm because tiny and plain grains distribute uniformly within the films. The thermal annealing under higher temperature partially removes (or suppresses) the defects and this yields a broader bandgap for the CuO thin film, while the Urbach energy monotonous decreases with the increasing annealing temperature, signifying the minimizing disorder of the thin films. Meanwhile, the annealed CuO thin films possess decreased carrier concentration and enhanced hall mobility with increasing annealing temperature. For the 450oC-annealed CuO thin film, the carrier concentration is of 5.3 × 1015 cm−3 and the Hall mobility is of 2.3 cm2 V−1 s−1. This offers an essential material basis for photonic devices.

Blue-blocking antireflective coatings based on a selective absorber in Cr2O3 media

Antireflection (AR) coatings allow increased substrate transmission and reduce surface reflection over a specific wavelength range. We proposed a blue-blocking (BB) AR coating that allows maximal visible-light transmittance while minimizing the transmittance of blue light, owing to selective absorption. Admittance diagrams of BB Cr2O3 / SiO2 coatings and traditional AR Ta2O5 / SiO2 coatings were shown to demonstrate that small absorption affects the reflectance much less than it affects the transmittance. Therefore, by controlling the degree of absorption, the lower residual reflection can be obtained by optimizing the thickness of different layers of the overall stacked structure. Coatings were fabricated using remote plasma sputtering to meet the requirements of low residual reflectance (<1.16 % ), high visible-light transmittance (exceeding 90%), and short-wavelength transmittance attenuation by ∼20 % . These promising results indicate that this type of BB coatings is a good candidate for vision protection applications, such as eyeglasses, desktop and mobile digital screens, and solar control films used in architectural glass.

Design and fabrication of metameric interference thin films based on metal-dielectric structure for optical security devices

Interference thin film based on metamerism is the latest means in the field of optical anti-counterfeiting. In this paper, we propose a pair of metal-dielectric metameric interference thin films that could create a hidden image for optical security devices. To improve color match tolerance of the metameric pair, the angle insensitive metal-dielectric Fabry-Perot structure was designed as the metameric pair background film B; The logo film A adopts the five-layer angle sensitive structure to enhance the contrast. The film B is designed to have a weak color-shift effect as the observation angle is increased. When the observation angle increases from 0° to 15°, the reflectance spectrum of film B changes smoothly, and the color difference indexes between film A (0°) and film B (0°, 5°, 10°, 15°) are 0.968, 0.988, 1.798 and 3.778, respectively. This design is less sensitive to manufacturing errors and has a good color match effect. Color target optimization optimac method is utilized in the design process and remote plasma sputtering is used in the deposition process. The color difference index of the experiment is 2.753, which shows good hidden image effect for the optical security devices.

Metal-dielectric pure red to gold special effect coatings for security and decorative applications

Because of high color saturation and fewer layers, metal-dielectric thin film has been used for years in the field of special effect coatings such as security and decorative applications. In the application of red color, purple red and orange red metal-dielectric thin films have already been realized, however, pure red thin film has not been achieved. In this paper, color target optimization method is adopted to eliminate the blue second order reflect peak (460 nm–470 nm) and compress the red first order reflect peak (675 nm), consequently, a seven-layer metal-dielectric pure red thin film is designed and fabricated. The chromaticity coordinate is (x,y) = (0.635,0.32) at normal incidence, which is located in pure red region, and the gold color target coordinate is (x,y) = (0.52,0.45) at 45°. The final base structure is Air/1.5Cr/230SiO2/6.3Cr/225SiO2/8.9Cr/213SiO2/100Al/Glass, where the numbers refer to layer thicknesses in nanometers. As the observation angle is increased from 0° to 45°, the thin film varies from pure red to gold. Remote plasma sputtering technique is used to fabricate the multilayer thin film. Finally, pure red thin films are demonstrated for the automobile coatings and optically variable ink applications, and the practical color effects are presented.

Analysis of grain size in FePt films fabricated using remote plasma deposition

Remote plasma sputtering (RPS) offers a high degree of control over the sputtering parameters used to deposit thin metallic films and has demonstrated a capability to control the media grain size distribution. Narrow grain size distributions remain a key requirement for future magnetic media. Here we report a comprehensive magnetometry, X-ray diffraction and transmission electron microscopy study of how RPS affects the grain size distribution of continuous, non-segregated L10 FePt thin films. These provide a model medium for heat-assisted magnetic recording and more generally for spintronic devices such as magnetoresistive random access memory and spin torque oscillators, where very high perpendicular magnetocrystalline anisotropy is required. Varying the target DC bias voltage, which in RPS can be tuned independently of the plasma generation, produces no meaningful, statistical change in average grain size, 6.5±0.1nm, for as-deposited, disordered FePt. Annealing at 800°C creates the well-ordered L10 phase but results in an increased average grain size of 8.3–13.6nm, and a significantly wider grain size distribution of 6.4–8.5nm. These results show that whilst RPS is capable of producing well-ordered L10 FePt thin films, it does not offer an advantage in controlling the grain size of FePt, as reported in other thin film systems.

Exploring the potential of remote plasma sputtering for the production of L10 ordered FePt thin films

Lowering the temperature at which the desirable L10 phase forms in FePt thin films is a key requirement in the development of next generation high-density data storage media and spintronic devices. Remote plasma sputtering offers a higher degree of control over the sputtering parameters, allowing the properties of films to be tailored, and potentially can affect the ordering kinetics of the L10 phase of FePt. Here, we report a comprehensive study of FePt thin films deposited under a range of temperatures and sputtering conditions. X-ray diffraction and magnetometry investigations show that whilst FePt thin films ordered in the L10 phase with high perpendicular anisotropy can be produced using this technique, there is no significant reduction in the required ordering temperature compared with films produced using conventional DC sputtering. Optimally ordered L10 FePt films were fabricated when the film was deposited at a substrate temperature of 200 °C, followed by post annealing at 750 °C.

Remote plasma sputtering deposited Nb-doped TiO2 with remarkable transparent conductivity

Nb-doped TiO2 (TNO) thin films with high electrical conductivity and excellent transparency were fabricated for the first time by reactive remote-plasma sputtering deposition. TNO films were deposited on non-alkali glass slides at room temperature, using a reactive remote plasma sputtering technology, followed by low-temperature annealing in ambient air. The as-deposited films were amorphous which were then crystallised into anatase nanocrystals after a short thermal exposure of 30min at a moderate temperature of 280°C. Such low temperature crystallisation induced remarkable enhancement of both conductivity and transparency, with the annealed samples demonstrating low resistivity of 6.4×10−4Ωcm at room temperature and up to 87% optical transmittance. Fundamentally, the excellent transparent conductivity from the current work is attributed to the interplay between doping Nb to the Ti sub-lattice sites of the anatase phase and energetic/electronic effects due to formation of native defects. The achieved optical transparency and electrical conductivity for TNO are comparable to those for tin-doped indium oxide (ITO), thus demonstrating great potential in low-carbon processing of TNO to substitute the ITO that is based on the depleting and expensive indium resource.

Room temperature sputtering of inclined c-axis ZnO for shear mode solidly mounted resonators

ZnO films with a c-axis significantly inclined away from the surface normal were grown by a remote plasma sputtering technique at room temperature. The films were used to make solidly mounted resonators (SMRs) operating in shear mode at a resonant frequency of 1.35 GHz. Control of the ZnO microstructure was achieved using a polycrystalline AlN seed layer which can be added on top of a sputtered acoustic mirror to give a complete SMR device. The ZnO was reactively sputtered in an atmosphere of argon and oxygen from a zinc target. The c-axis of the ZnO was estimated to be at an angle of ∼45° to the surface normal. SMRs were measured to have quality factors (Q) of up to 140 and effective electromechanical coupling coefficients of up to 2.2% in air. Although an inclined c-axis can be achieved with direct growth onto the acoustic mirror, it is shown that the AlN seed layer provides higher coupling coefficients and narrower inclination angular distribution. The responses of the devices in liquids of different viscosities (acetone, water, and AZ5214E photoresist) were measured. The shear mode Q decreased by 45% in acetone, 72% in water, and 92% in AZ5214E.

Metal-dielectric metameric filters for optically variable devices

A pair of metal-dielectric metameric filters that could create a hidden image was presented for the first time. The structure of the filters is simple and only six layers for filter A and five layers for filter B. The prototype filters were designed by using the film color target optimization method and the designed results show that, at normal observation angle, the reflected colors of the pair of filters are both green and the color difference index between them is only 0.9017. At observation angle of 60°, the filter A is violet and the filter B is blue. The filters were fabricated by remote plasma sputtering process and the experimental results were in accordance with the designs.

The impact of substrate bias on a remote plasma sputter coating process for conformal coverage of trenches and 3D structures

With the progression towards higher aspect ratios and finer topographical dimensions in many micro- and nano-systems, it is of technological importance to be able to conformally deposit thin films onto such structures. Sputtering techniques have been developed to provide such conformal coverage through a combination of coating re-sputtering and ionised physical vapour deposition (IPVD), the latter by use of a secondary plasma source or a pulsed high target power (HiPIMS). This paper reports on the use of an alternate remote plasma sputtering technique in which a high density (>1013 cm−3) magnetised plasma is used for sputter deposition, and additionally is shown to provide IPVD and a re-sputtering capability. From the substrate I–V characteristics and optical emission spectroscopy (OES) data, it is shown that remote plasma sputtering is an inherently continuous IPVD process (without the need of a secondary discharge). Through the reactive deposition of Al2O3 onto complex structures, scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDX) results demonstrate that applying a negative substrate bias during film growth can result in re-sputtering of deposited material and film growth on surfaces obscured from the initial sputter flux. Using 5 : 1 (height : width) aspect ratio trenches, the substrate bias was set to 0,−245 and −334 V. At 0 V substrate bias, the alumina coating is predominantly deposited on the horizontal surfaces; at −344 V, it is predominantly deposited onto the side walls and at −245 V a more uniform layer thickness is obtained over the trench. The process was optimised further by alternating the substrate bias between −222 and −267 V, with a 50% residence time at each voltage, yielding a more uniform conformal coverage of the 5 : 1 aspect ratio structures over large areas.

Precise control of interface anisotropy during deposition of Co/Pd multilayers

We demonstrate the control of perpendicular magnetic anisotropy (PMA) in multilayer films without modification of either the microstructure or saturation magnetization by tuning the Ar+ ion energy using remote plasma sputtering. We show that for [Co/Pd]8 multilayer films, increasing the Ar+ ion energy results in a strong decrease in PMA through an increase in interfacial roughness determined by X-ray reflectivity measurements. X-ray diffraction and transmission electron microscope image data show that the microstructure is independent of Ar+ energy. This opens a different approach to the in-situ deposition of graded exchange springs and for control of the polarizing layer in hybrid spin transfer torque devices.

All-dielectric metameric filters for optically variable devices

In order to increase the anti-counterfeiting performance of optically variable devices, the innovative interference security image structures based on metamerism have been developed. In this letter, we show a pair of all-dielectric metameric filters offering a hidden image effect with the color shift at a specific angle of observation. These filters are designed by two materials TiO2/SiO2 based on the different angle color target optimization. The 6-layer- and 9-layer stacks are achieved and the performance of prototype filters prepared by remote plasma sputtering is shown. The color difference index of the experiment is up to 1.19, which shows good metameric matching effect.

TiN Deposition and Process Diagnostics using Remote Plasma Sputtering

The discharge voltage–current characteristics and the optical diagnostics of a remote plasma sputtering system called by high density plasma assisted sputtering source (HiPASS) were investigated. The remote plasma was generated by the hollow cathode discharge (HCD) gun and was transported to the target surface by external electromagnet coils. This showed a wide process window because the sputtering voltage and current could be individually controlled. The ion density and energy distribution could be also controlled unlike the conventional magnetron sputtering. Titanium nitride films were deposited under different sputtering voltage. The high voltage mode induced the high ionization ratio of the sputtered atoms and the high ion energy toward the substrate. That resulted in the enlarged grain size, and the preferred orientation toward (220). Eventually, this optimized condition of HiPASS obtained the best hardness of TiN films to be about 48 GPa at the sputtering voltage of -800 V.

Deposition of Low Stress Amorphous Zinc Tin Oxide at Ambient Temperature Using a Remote Plasma Sputtering Process Suitable for Delicate Substrates

ZnxSnyOz thin films (<100nm thickness), deposited by remote sputtering from a metal target using a confined argon plasma and oxygen gas jet near the sample, were investigated for their material properties. No visible deformation or curl was observed when deposited on plastic. Materials were confirmed to be amorphous and range between 5 and 10 at.% Sn concentration by x-ray diffraction, x-ray photoemission spectroscopy and energy-dispersive x-ray spectroscopy. Factors affecting the material composition over time are discussed. Depletion of the Sn as the target ages is suspected.

High‐density remote plasma sputtering of high‐dielectric‐constant amorphous hafnium oxide films

Abstractmagnified imageHafnium oxide (HfOx) is a high dielectric constant (k) oxide which has been identified as being suitable for use as the gate dielectric in thin film transistors (TFTs). Amorphous materials are preferred for a gate dielectric, but it has been an ongoing challenge to produce amorphous HfOx while maintaining a high dielectric constant. A technique called high target utilization sputtering (HiTUS) is demonstrated to be capable of depositing high‐k amorphous HfOx thin films at room temperature. The plasma is generated in a remote chamber, allowing higher rate deposition of films with minimal ion damage. Compared to a conventional sputtering system, the HiTUS technique allows finer control of the thin film microstructure. Using a conventional reactive rf magnetron sputtering technique, monoclinic nanocrystalline HfOx thin films have been deposited at a rate of ∼1.6 nm min−1 at room temperature, with a resistivity of 1013 Ω cm, a breakdown strength of 3.5 MV cm−1 and a dielectric constant of ∼18.2. By comparison, using the HiTUS process, amorphous HfOx (x = 2.1) thin films which appear to have a cubic‐like short‐range order have been deposited at a high deposition rate of ∼25 nm min−1 with a high resistivity of 1014 Ω cm, a breakdown strength of 3 MV cm−1 and a high dielectric constant of ∼30. Two key conditions must be satisfied in the HiTUS system for high‐k HfOx to be produced. Firstly, the correct oxygen flow rate is required for a given sputtering rate from the metallic target. Secondly, there must be an absence of energetic oxygen ion bombardment to maintain an amorphous microstructure and a high flux of medium energy species emitted from the metallic sputtering target to induce a cubic‐like short range order. This HfOx is very attractive as a dielectric material for large‐area electronic applications on flexible substrates.

Low loss Nb2O5 films deposited by novel remote plasma sputtering

We report the deposition of Nb2O5 films on unheated BK-7 glass substrates using remote plasma sputtering system. The remote plasma geometry allows pseudo separation of plasma and target bias parameters, which offers complete deposition rate control. Using appropriate oxygen flow rates, high-density and low-loss Nb2O5 films are deposited with rates up to 0.49 nm/s. Lower deposition rates (∼0.026 nm/s) can also be obtained by working at low target current and voltage and at low pressure. Nb2O5 films deposited at different rates have the refractive index of about 2.3 and the extinction coefficient as low as 6.9×10−5.

Room-temperature remote-plasma sputtering of c-axis oriented zinc oxide thin films

Highly c-axis oriented ZnO films have been deposited at room temperature with high rates (∼50 nm·min−1) using an innovative remote plasma sputtering configuration, which allows independent control of the plasma density and the sputtering ion energy. The ZnO films deposited possess excellent crystallographic orientation, high resistivity (&amp;gt;109 Ω·m), and exhibit very low surface roughness. The ability to increase the sputtering ion energy without causing unwanted Ar+ bombardment onto the substrate has been shown to be crucial for the growth of films with excellent c-axis orientation without the need of substrate heating. In addition, the elimination of the Ar+ bombardment has facilitated the growth of films with very low defect density and hence very low intrinsic stress (&amp;lt;100 MPa for 3 μm-thick films). This is over an order of magnitude lower than films grown with a standard magnetron sputtering system.