Time-dependent variation of the target mode in reactive sputtering of Al–O2 system

Reactive sputtering techniques have been widely used for forming compound thin films. In this study, the time-dependent variation of the target mode of an Al–O₂ system was investigated by measuring the target voltage for various reactive O₂ gas flow ratios and presputtering (i.e., sputtering in pure Ar gas to clean the target surface) times. The Al target remains in metallic mode for a certain period of time after being exposed to an Ar–O₂ plasma before it begins to be oxidized. This period increases with decreasing O₂ flow ratio and with increasing the presputtering time. It is considered that the period is caused by the gettering of O₂ gas by Al films deposited on the substrate and the chamber wall during presputtering.     

Formation process of Al2O3 thin film by reactive sputtering

Al and Al₂O₃ films were deposited by RF magnetron sputtering using a mixed gas of Ar and O₂. The surface of the Al target was changed from the metallic mode to the oxide mode at a critical O₂ flow ratio of 8%. The atomic ratio of sputtered Al atoms to supplied oxygen atoms was found to be approximately 2:3 at the critical O₂ flow ratio. The oxide layer thickness formed on the Al target was estimated to be 5-7 nm at an O₂ flow ratio of 100% by ellipsometry.     

Effects of total gas flow rate and sputtering power on the critical condition for target mode transition in Al-O2 reactive sputtering

Reactive sputtering is one of the most commonly used techniques for the fabrication of compound thin films, and the critical condition for target mode transition from metal mode to oxide mode is very important. We investigated the effects of total gas flow rate and sputtering power on the critical condition in Al-O₂ reactive sputtering. It was found that the ratio of the number of sputtered Al atoms (N_Al) to the number of supplied O atoms (N_O) at the critical condition was almost constant, and the ratio of N_Al to N_O was close to the stoichiometric ratio of Al₂O₃ (2 to 3). It is thought that the introduced oxygen is gettered by Al atoms almost completely and the target remains in the metal mode below the critical condition. By increasing the amount of supplied O atoms above the stoichiometric ratio of Al₂O₃, the oxygen supply overcomes the gettering effect. Then, oxygen concentration in the plasma increases abruptly and the target mode changes from metal mode to oxide mode.     

DC reactive sputtering of aluminium doped zinc oxide films for solar modules controlled by target voltage

Reactive sputtering is an option to further reduce costs associated with the deposition of the transparent front contact in chalcopyrite-based solar modules. In view of the difficulties reported in scaling-up reactive ZnO sputtering we have chosen a simple and robust approach. It is comprised of a dual magnetron operated in DC/DC mode, a constant oxygen flow and the process is controlled by target voltage. After process optimisation, the optical and electrical properties of the reactively sputtered films are comparable to those of reference films (RF-sputtered from ceramic targets). Likewise, the efficiency of monolithically integrated CuInS₂-based module test structures is not affected by the modified ZnO process.     

Structure and mechanical properties of ZrO2 films deposited by gas flow sputtering

ZrO₂ films were deposited by reactive gas flow sputtering (GFS) where voltage is applied to a cyindrical hollow-cathode target from a DC source, the discharge being produced at relatively high sputtering pressure. In this system, secondary electrons form a major component of the total current flow and lead to heating of the substrate which in turn has an effect on the properties of deposited films. The present experiments were carried out under the following conditions: Ar gas flow rate of 200 sccm, O₂ flow rate FO₂ in the range between 0.003 and 1 sccm, and sputtering power (P_S) in the range of 50-800 W. The reults showed that the crystal structure of the films deposited for P_S below 200 W was monoclinic but for P_S above 400 W, the films included tetragonal cystals of stable structure formed at high temperature by the electron bombardment. The films were formed with grains of 20-100 nm in diameter in a porous structure. The mechanical properties of the films were determined by a nanoindentation technique. Martens hardness (H_M) of the porous films was found to be in the range between 220 and 330 MPa which is substantially less than that of films typically deposited by rf magnetron sputtering.     

Angular distribution and sputtering yield of Al and Al2O3 during 40 key argon ion bombardment

The angular distribution and sputtering yield of Al and Al₂O₃ during 40 keV argon ion bombardment have been measured by collecting the sputtered particles on a semi-cylindrical collector and analysing them by Rutherford backscattering spectrometry (RBS). It was found that due to the relatively high residual pressures (1–4 · 1O^?4Pa) in the sputtering chamber not only aluminium but also oxygen atoms are deposited on the collector through the mechanism of reactive sputtering both when sputtering the oxide and the metal target. The angular distribution of the collected aluminium and oxygen atoms in the case of pure aluminium sputtering follows a cosine law while in the case of Al₂O₃ sputtering a considerable deviation from the cosine law is observed. This deviation is explained by a preferred orientation (texture) of the crystallites in the polycrystalline oxide targets. It was found that the very thin films deposited on the collector when sputtering both types of targets have a composition close to AlO₂. The sputtering yield of Al and Al₂O₃ by 40 keV argon ions has been determined. On the basis of the obtained values an estimation of the productivity of the reactive sputter deposition of Al₂O₃ films from oxidized and non-oxidized targets is made.     

Hysteresis-free reactive high power impulse magnetron sputtering

High power impulse magnetron sputtering (HIPIMS) of an Al target in Ar/O₂ mixtures has been studied. The use of HIPIMS is shown to drastically influence the process characteristics compared to conventional sputtering. Under suitable conditions, oxide formation on the target as the reactive gas flow is increased is suppressed, and the hysteresis effect commonly observed as the gas flow is varied during conventional sputtering can be reduced, or even completely eliminated, using HIPIMS. Consequently, stoichiometric alumina can be deposited under stable process conditions at high rates. Possible explanations for this behavior as well as a model qualitatively describing the process are presented.     

Direct current reactive sputtering of aluminium

The influence of the oxygen content in the gas flow on the discharge current and on the chemical composition of sputtered AlO_x was investigated. Unlike SiO₂, Al₂O₃ films are formed on the substrate only for oxygen contents sufficient for full oxidation of the target surface. The minimum oxygen content can be determined by the sharp change in discharge current at constant working voltage and total gas pressure. Infrared spectra confirming this statement are given. The kinetics of the processes of forming and sputtering off of an oxide layer on the aluminium target surface were also investigated. Some speculations about the interrelation between oxide formation on the substrate and on the target for direct current reactive sputtering are given.     

Control of aluminum doping of ZnO:Al thin films obtained by high-power impulse magnetron sputtering

This work reports a method used to control Al doping of ZnO thin films deposited by high-power impulse magnetron sputtering of a pure Zn target in low-pressure Ar/O₂ gas mixture. The method uses sputtering of an electrically negative biased aluminum electrode placed in the proximity of the negative glow of the magnetron discharge. Resonant laser absorption measurements of Al atom concentration in vapor phase and the X-ray Photoelectron Emission Spectroscopy measurements of Al concentration in the deposited ZnO:Al films confirm that the electrode biasing potential is the key parameter that controls the Al doping process. Optically transparent ZnO:Al films with resistivity as low as 3.6 × 10^− 3 Ω × cm have been obtained at an optimum value of Al concentration of 1.5 at.%. It has been found that the film electrical conductivity is limited by the effect of decreasing of crystalline grain size in the films with the increased Al doping concentration.     

Effect of sputtering pressure and post-annealing on hydrophilicity of TiO2 thin films deposited by reactive magnetron sputtering

A series of TiO₂ thin films was deposited onto glass substrates without intentional heating or biasing by magnetron sputtering of a titanium target using Ar/O₂ reactive mixtures over a broad range of total sputtering pressures from 0.12 Pa to 2.24 Pa. Each of the film types was deposited by the threshold poisoned mode at a specific given oxygen flow rate monitored in-situ by optical emission spectroscopy. Both the sputtering pressure and thermal annealing are the key factors for the TiO₂ films to yield fast-response superhydrophilicity with a water contact angle of 5°. The mechanism of superhydrophilicity for the TiO₂ films deposited by high-pressure sputtering will be discussed based on empirical studies of X-ray diffractometry, high-resolution scanning microscopy and atomic force spectroscopy.     

RF sputtered electrochromic V2O5 films

The properties of vanadium pentoxide (V₂O₅) films deposited by r.f. reactive sputtering from V₂O₅ target are investigated. In particular the composition and structure of these films are analysed by Rutherford backscattering spectrometry (RBS), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) together with their electrochromic properties. Information on electrochromic behaviour of these films, like the injected charge and the width of transition from bleached to coloured state when lithium was electrochemically injected, is obtained by voltammetric and spectrophotometric measurements. The dependence of all these properties on the oxygen flow used in the argon–oxygen gas mixture during deposition is analysed. The highest value of injected charge and the greatest differences in transmittance values due to electrochromic transition are shown by samples deposited at low O₂ flow. The films appear to be amorphous when the O₂ flow is lower than 20% and the stoichiometry is not greatly affected by the O₂ flow.     

Hardness of sputter deposited nanocrystalline Ni3Al thin films

Nanocrystalline thin films of nominal composition Ni-25 at.% Al have been sputter deposited from a target of the intermetallic compound Ni₃Al using different sputtering conditions. Increase in the pressure of sputtering gas resulted in a substantial reduction in the grain size of these nanocrystalline films and a consequent enhancement in their hardness. While films deposited onto heated substrates exhibited larger grain sizes as compared to those deposited on unheated substrates at the same sputtering pressure, the hardness of the former films was substantially higher. The reason for this enhanced hardness is the long-range chemical ordering in films deposited on heated substrates and the formation of L1₂-Ni₃Al, the thermodynamically stable phase for this composition.     

Reactive sputtering of titanium in Ar/CH4 gas mixture: Target poisoning and film characteristics

Reactive sputtering of titanium target in the presence of Ar/CH₄ gas mixture has been investigated. With the addition of methane gas to above 1.5% of the process gas a transition from the metallic sputtering mode to the poison mode was observed as indicated by the change in cathode current. As the methane gas flow concentration increased up to 10%, the target was gradually poisoned. The hysteresis in the cathode current could be plotted by first increasing and then subsequently decreasing the methane concentration. X-ray diffraction and X-ray photoelectron spectroscopy analyses of the deposited films confirmed the formation of carbide phases and the transition of the process from the metallic to compound sputtering mode as the methane concentration in the sputtering gas is increased. The paper discusses a sputtering model that gives a rational explanation of the target poisoning phenomenon and shows an agreement between the experimental observations and calculated results.     

Fabrication of highly c-axis textured ZnO thin films piezoelectric transducers by RF sputtering

The influence of fabrication parameters on ZnO film properties has been analyzed through conducting several experiment processes to develop an appropriate deposition condition for obtaining highly c-axis textured films. A transducer with the structure of Al/ZnO/Al/Si was fabricated at low deposition rate and under a temperature of 380 °C in a mixture of gases Ar:O₂ = 1:3, and RF power of 178 W. Pt/Ti was employed as the bottom electrode of the transducer fabricated in a suitable substrate temperature, which starts increasing at 380 °C with an increment of 20 °C for each 2 h stage of the deposition. Highly c-axis textured ZnO films have been successfully deposited on Pt/Ti/SiO₂/Si substrate under feasible conditions, including RF power of 178 W, substrate temperature of 380 °C, deposition pressure of 1.3 Pa and Ar:O₂ gas flow ratio of 50%. These conditions have been proposed and confirmed through investigating the influences of the sputtering parameters, such as substrate temperature, RF power and Ar:O₂ gas flow ratio, on the properties of ZnO films.     

Effect of O<Subscript>2</Subscript> partial pressure and thickness on the gasochromic properties of sputtered V<Subscript>2</Subscript>O<Subscript>5</Subscript> films

V₂O₅ thin films were deposited by reactive DC-diode sputtering technique in a mixed atmosphere of O₂/Ar gas at room temperature from a high purity target of 99.99% vanadium. For the investigation, the thickness of the films and the O₂/Ar ratio during the sputtering process were the parameters. The sputtering rate of the V₂O₅ films dramatically decreases with increasing the O₂/Ar ratio. By X-ray diffraction it was found that films sputtered with 1% O₂/Ar ratio grow preferentially in two orientations: the 200 and the 001 orientation. The increase of the O₂/Ar ratio enhances the growth preferentially in the c-axis (001) and strongly decreases the growth in the a-axis (200) direction. The scanning electron microscope pictures confirm these results. In the visible region the optical transmittance is increased with increasing the O₂/Ar ratio in the sputter gas. Additionally, the optical band gap is slightly larger for the films sputtered with an O₂/Ar ratio higher than 5%. Beyond a thickness of about 220 nm and an O₂/Ar ratio of 10% the electrical sheet resistance of the films increases dramatically. During the insertion/extraction of hydrogen ions, the change in the optical transmission was investigated. The gasochromism of the V₂O₅ films was explained by use of the Infra Red (IR) measurements during the insertion/extraction of hydrogen ions.     

In-situ analyses on the reactive sputtering process to deposit Al-doped ZnO films using an Al-Zn alloy target

The kinetic energies of generated ions were investigated during the reactive sputtering process to deposit Al-doped ZnO (AZO) films using an Al-Zn alloy target. The sputtering system was equipped with specially designed double feedback system to stabilise the reactive sputtering processes and analysis was performed with a quadrupole mass spectrometer combined with an energy analyser. Negative ions O⁻, O₂⁻, AlO⁻ and AlO₂⁻ with high kinetic energies corresponding to cathode voltage are generated at the partially oxidised target surface, after which some of the ions undergo subsequent charge exchange and/or dissociation. Positive ions O⁺, Ar⁺, Zn⁺ and Al⁺ with lower kinetic energies (around 10 eV) are generated by charge exchange of sputtered neutral O, Ar, Zn and Al atoms, respectively. As the target surface oxidises, cathode voltage decrease, the flux of high-energy negative ions increases and the electrical properties of the AZO degrade by ion bombardment as well as the AZO films that are deposited by conventional magnetron sputtering using an AZO target.     

RF sputtering deposition of MgMn2O4 spinel thin films

In this paper we describe about the synthesis of MgMn₂O₄ thin films by means of off-axis RF magnetron sputtering and their structural and morphological characterisation realised by means of X-ray diffraction (XRD), micro-Raman spectroscopy and Scanning Electron Microscopy (SEM). We explored different synthesis conditions, namely the substrate temperature, T _s, and the sputtering gas composition and we could observe that well-crystallised films are obtained at T _s = 500°C while the effect of the sputtering gas composition is to affect the structural properties of the films deposited. The thin films deposited always present a crystal structure less distorted with respect to the one of the target material. The possibility of synthesising films of low distorted spinels or even cubic ones may be an interesting way of preparing buffering layers for the growth of isostructural materials such as other spinels or pseudocubic perovskites.     

Evaluation of ion conductivity of ZrO2 thin films prepared by reactive sputtering in O2, H2O, and H2O + H2O2 mixed gas

Hydrated ZrO₂ thin films were prepared by reactive sputtering in O₂, H₂O, and H₂O + H₂O₂ mixed gas, and the effect of the sputtering atmosphere on ion conductivity of the films was investigated. The results showed that the films deposited in O₂ gas exhibited poor ion conductivity; however, the ion conductivities of the films deposited in the other two kinds of atmosphere were similar and 300-500 times higher than that of the films deposited in O₂ gas. It was indicated that the higher ion conductivity of the films was caused by lower film density and higher water content.     

Dielectric properties of DC reactive magnetron sputtered Al2O3 thin films

Alumina (Al₂O₃) thin films were sputter deposited over well-cleaned glass and Si < 100 > substrates by DC reactive magnetron sputtering under various oxygen gas pressures and sputtering powers. The composition of the films was analyzed by X-ray photoelectron spectroscopy and an optimal O/Al atomic ratio of 1.59 was obtained at a reactive gas pressure of 0.03 Pa and sputtering power of 70 W. X-ray diffraction results revealed that the films were amorphous until 550 °C. The surface morphology of the films was studied using scanning electron microscopy and the as-deposited films were found to be smooth. The topography of the as-deposited and annealed films was analyzed by atomic force microscopy and a progressive increase in the rms roughness of the films from 3.2 nm to 4.53 nm was also observed with increase in the annealing temperature. Al-Al₂O₃-Al thin film capacitors were then fabricated on glass substrates to study the effect of temperature and frequency on the dielectric property of the films. Temperature coefficient of capacitance, AC conductivity and activation energy were determined and the results are discussed.     

Ion conducting properties of hydrogen-containing Ta2O5 thin films prepared by reactive sputtering

Hydrogen-containing Ta₂O₅ (Ta₂O₅:H) thin films are considered to be a candidate for a proton-conducting solid-oxide electrolyte. In this study, Ta₂O₅:H thin films were prepared by reactively sputtering a Ta metal target in an O₂ + H₂O mixed gas. The effects of sputtering power and post-deposition heat treatment on the ion conducting properties of the Ta₂O₅:H thin films were studied. The ionic conductivity of the films was improved by decreasing the RF power and a maximum conductivity of 2 × 10⁻⁹ S/cm was obtained at an RF power of 20 W. The ionic conductivity decreased by heat-treatment in air, and no ion-conduction was observed after treatment at 300 °C due to the decrease in hydrogen content in the films.