M. Quaas

Formation of TiOx films produced by high-power pulsed magnetron sputtering

Formation of thin TiOx films produced by pulsed planar magnetron sputtering deposition is reported in this paper. The formation process and layer growth were controlled by (i) the ratio of reactive O2 in Ar/O2 working gas mixture and (ii) the pressure in the vacuum chamber. The magnetron, operated in a high-power pulse mode with a low repetition frequency of 250 Hz, reached maxima peak current Ip ~ 50 A and magnetron current density peaks at ip ~ 1 A cm−2. Particular spectral lines (Ar = 420.07 nm, Ar+ = 487.98 nm, Ti = 518.96 nm) emitted by the discharge were investigated using time-resolved photon counting measurements. The phases of deposited TiOx films were determined by grazing incidence x-ray diffractometry and thickness and density were calculated from x-ray reflectometry measurements; in addition composition and chemical bounds were revealed by x-ray photoelectron spectroscopy. The film diagnostics survey the existence of different crystalline phases in the Ti–O system and their formation. Discharge properties for example, deposition rate and time evolution of discharge current are also discussed.

Effect of nitrogen doping on TiOxNy thin film formation at reactive high-power pulsed magnetron sputtering

The paper is focused on a study of formation of TiOxNy thin films prepared by pulsed magnetron sputtering of metallic Ti target. Oxygen and nitrogen were delivered into the discharge in the form of reactive gases O2 and N2. The films were deposited by high-power impulse magnetron sputtering working with discharge repetition frequency f = 250 Hz at low (p = 0.75 Pa) and high (p = 10 Pa) pressure. The substrates were on floating potential and thermally stabilized at room temperature during the deposition process. Post-deposition thermal annealing was not employed. The chemical composition from x-ray photoelectron spectroscopy diagnostic reveals formation of TiOxNy structure at low flow rate of oxygen in the discharge gas mixture. This result is confirmed by XRD investigation of N elements incorporation into the Ti–O lattice. Decrease in band-gap to values Eg ~ 1.6 eV in TiOxNy thin film is attributed to formed Ti–N bonds. The discharge properties were investigated by time-resolved optical emission spectroscopy. Time evolution of particular spectral lines (Ar+, Ti+, Ti) is presented together with peak discharge current.

Physical properties of homogeneous TiO2 films prepared by high power impulse magnetron sputtering as a function of crystallographic phase and nanostructure

Optical, photo-electrochemical, crystallographic and morphological properties of TiO2 thin films prepared by high power impulse magnetron sputtering at low substrate temperatures (<65 °C) without post-deposition thermal annealing are studied. The film composition—anatase, rutile or amorphous TiO2—is adjusted by the pressure (p ~ 0.75–15 Pa) in the deposition chamber. The different crystallographic phases were determined with grazing incidence x-ray diffractometry. The surface morphology and size of TiO2 grains/clusters were imaged with atomic force microscopy. Basic plasma parameters were determined by means of the time-resolved Langmuir probe technique. The power density influx on the substrate was estimated from calorimetric probe measurement. The data from calorimetric probe measurements and time-resolved Langmuir probe served as input parameters for the calculation of influx contributions of particular species. The band-gap energy Eg depends on the film composition and crystallographic phase. Optical parameters (refractive index n + ik, transmittance T, reflectance R and absorbance A) are measured as functions of photon energy in the UV–Vis range by spectroscopic ellipsometry. For the rutile and anatase films agreement with the respective bulk phase is found. Incident photon-current conversion efficiency determined by photo-electrochemical measurements reached the highest values (0.312) for the anatase film.

Examples for application and diagnostics in plasma?powder interaction

Low-pressure plasmas offer a unique possibility of confinement, control and fine tailoring of particle properties. Hence, dusty plasmas have grown into a vast field and new applications of plasma-processed dust particles are emerging. There is demand for particles with special properties and for particle-seeded composite materials. For example, the stability of luminophore particles could be improved by coating with protective Al2O3 films which are deposited by a PECVD process using a metal-organic precursor gas. Alternatively, the interaction between plasma and injected micro-disperse powder particles can also be used as a diagnostic tool for the study of plasma surface processes. Two examples will be provided: the interaction of micro-sized (SiO2) grains confined in a radiofrequency plasma with an external ion beam as well as the effect of a dc-magnetron discharge on confined particles during deposition have been investigated.

Investigation of diffusion and crystallization processes in thin ITO films by temperature and time resolved grazing incidence X-ray diffractometry

Oxygen diffusion into metallic In/Sn films and crystallite growth of thin indium tin oxide (ITO) films were investigated by in situ high temperature grazing incidence X-ray diffractometry (HT-GIXRD) at temperatures ranging from 100 to 300 °C. The investigated films were deposited by dc magnetron sputtering from a metallic target at different oxygen flows and bias voltages. The deposition process influences not only the film properties but also the film reactions during the post-deposition annealing process. The ITO formation is determined by two processes: the diffusion of oxygen into the metallic grains and a fast crystallization process. Kinetic parameters for both processes were derived. A model was developed which allows the determination of the diffusion coefficient D from the time dependence of the integral intensity of the ITO X-ray reflection. Diffusion coefficients as well as the activation energies are influenced by the bias voltage but not by the oxygen flow. According to the Johnson–Mehl–Avrami theory, the crystallization can be described as a two-dimensional process.

Complex (dusty) plasmas: Examples for applications and observation of magnetron- induced phenomena*

Low-pressure plasmas offer a unique possibility of confinement, control, and fine tailoring of particle properties. Hence, dusty plasmas have grown into a vast field, and new applications of plasma-processed dust particles are emerging. During the deposition of thin amorphous films onto melamine formaldehyde (MF) microparticles in a C2H2 plasma, the generation of nanosized carbon particles was also studied. The size distribution of those particles is quite uniform. In another experiment, the stability of luminophore grains could be improved by coating with protective Al2O3 films that are deposited by a plasma-enhanced chemical vapor deposition (PECVD) process using a metal-organic precursor gas. Coating of SiO2 microparticles with thin metal layers by magnetron sputtering is also described. Especially the interaction of the microsized grains confined in a radio frequency (rf) plasma with the dc magnetron discharge during deposition was investigated. The observations emphasize that the interaction between magnetron plasma and injected microdisperse powder particles can also be used as a diagnostic tool for the characterization of magnetron sputter sources.

In situ studies of diffusion and crystal growth in plasma deposited thin ITO films

Abstract Tin-doped indium oxide (ITO) films were deposited on Si(100) substrates without external heating by means of DC-planar magnetron sputtering. A metallic In/Sn (90/10) target and an argon/oxygen gas mixture were used. The flow of the reactive gas oxygen was varied between 0 and 2 sccm. DC bias voltages between 0 and −100 V were used. With increasing oxygen flow, the film structure and composition changed from crystalline metallic In/Sn to amorphous ITO. The diffusion of oxygen into metallic In/Sn films and the amorphous-to-crystalline transformation of ITO were studied using in situ grazing incidence X-ray diffractometry (GIXRD), grazing incidence reflectometry (GIXR), and AFM. In situ measurements of GIXRD were made during annealing in vacuum (10 −6 mbar) at temperatures between 100 and 300°C. Two processes determine the ITO crystal growth: the diffusion of oxygen into the metallic film, and a fast crystallization of amorphous ITO. From the X-ray integral intensities the following kinetic parameters were extracted: diffusion constant, activation energy of the diffusion, reaction order and activation energy of the crystal growth process. The diffusion constants depend on the bias voltage in following manner: D (0 V) D (−100 V) D (−50 V). The oxygen flow during deposition does not influence the diffusion constant. The activation energy of the diffusion was determined in films deposited without oxygen flow. Two activation energies were found: E A =25 kJ/mol for processes below 150°C and E A =12.4 kJ/mol for diffusion higher than 150°C. The bias voltage does not influence the activation energy. Crystallization of amorphous ITO occurs via classical nucleation and growth mode parameters, as described by the Johnson–Mehl–Avrami equation. Reaction orders between 2.4 and 3.0 were estimated. This is consistent with a two-dimensional transformation geometry. The activation energy is E A =74 kJ/mol for films deposited without bias voltage.

Investigation of plasma-deposited ITO films by GIXR and GIXRD

Abstract Thin indium tin oxide (ITO) films were deposited by reactive DC magnetron sputtering to study the influence of oxygen and low energy ion bombardment on film growth and film properties. Films were deposited at various oxygen gas flows (0 to 2 sccm) as well as negative substrate voltages (0 to −50 V). The film properties were investigated by grazing incidence X-ray reflectometry (GIXR), X-ray diffractometry (GIXRD), AFM and XPS. With increasing oxygen flow film structure and composition change from crystalline metallic In/Sn to X-ray-amorphous ITO. Simultaneously the deposition rates decrease from 0.6 to 0.25 nm/s and the film densities increase from 4.3 to 7.1 g/cm 3 . The metallic films consist of large grains forming a rough surface. The deposition with higher oxygen flows causes smooth surfaces, grain sizes are not clearly observable. In principle an increasing negative substrate voltage leads to the same film properties but it works like a diminished oxygen flow. Post deposition annealing causes the formation of crystalline ITO films. GIXRD measurements were carried out in situ at 200°C at 10 −5 mbar. Two processes determine the ITO crystallite growth, a fast crystallization of the deposited amorphous indium tin oxide and the diffusion of oxygen into the layer. The formation rate k of the diffusion limited reaction depends on the oxygen flows. With increasing oxygen flow k approximates zero.

The growth process of plasma-deposited ITO films investigated by grazing incidence X-ray techniques

Abstract Thin indium tin oxide (ITO) films were deposited on Si(100) substrates by reactive d.c. magnetron sputtering from a metallic In/Sn (9/1) target. The reactive gas flow as well as negative substrate voltage were varied. The films were investigated by in situ grazing incidence X-ray diffractometry (GIXRD), grazing incidence X-ray reflectometry (GIXR), AFM and XPS. Without oxygen crystalline metallic In/Sn layers were deposited. With increasing oxygen partial pressure the amount of amorphous ITO in the layers increases. A negative substrate voltage works like a diminished oxygen flow. Using high-temperature in situ GIXRD the formation of a crystalline ITO phase due to a post-deposition heat treatment can be obtained. This ITO crystallite growth is determined by two processes, a fast crystallization process and a diffusion limited process. Depending on the deposition conditions spontaneous crystallization or diffusion dominate the crystal growth and a different sample morphology occurs.

Influence of microstructure on oxygen diffusion in plasma-deposited In/Sn films

Abstract Thin tin-doped indium films were deposited by DC magnetron sputtering at different substrate voltages. The metallic films were annealed within a high-temperature chamber (10 −5 mbar) and studied by in situ grazing-incidence X-ray diffractometry (GIXRD). During the post-deposition annealing, crystalline indium tin oxide (ITO) forms in a diffusion-limited process. A mathematical model was applied to derive the effective diffusion coefficients D eff from the time dependence of the ITO(222) X-ray reflection integral intensity. A strong influence of the bias voltage on D eff was observed. From the temperature dependence of the diffusion coefficients, the activation energy values for oxygen diffusion into the metallic films were calculated. To understand the differences in D eff , the microstructure of the as-deposited films was investigated. Information on domain sizes, microstrains and dislocation densities was obtained from X-ray profile analysis using Warren–Averbach and Krivoglaz–Wilkens methods. A distinct change in grain size was found, depending on the negative substrate voltage applied. With respect to the film properties, the contribution of mass transport along grain boundaries, as well as through the lattice and dislocation cores, on the effective diffusion coefficient can be separated. It was shown that D eff is mainly influenced by grain boundary diffusion, whereas diffusion within the grains largely determines the activation energy of the process.