ICP-assisted Sputtering Research Articles

Properties of tungsten thin film deposited using inductively coupled plasma assisted sputtering for next-generation interconnect metal

Tungsten (W) is actively investigated as one of the future metallization materials which can replace Cu for the metal thickness lower than tens of nanometer due to its sfmall electron mean free path of 19 nm and high melting temperature of 3673 K. In this study, inductively coupled plasma (ICP) assisted DC magnetron sputtering of W has been investigated for the lower resistivity of nm scale W film (30 nm). When the characteristics of W thin films deposited with and without ICP assistance were investigated, the decrease of the W thin film resistivity with the increase of deposition rate was observed as the ICP power is increased regardless of substrate heating. The decrease of W thin film resistivity by the ICP assisted DC sputter deposition was related to the change of crystal structure from A-15 to BCC, decreased oxygen incorporation in the film, decreased surface roughness, and increased grain size due to the increased ion flux to the substrate.

Low-temperature growth of VO2 films on transparent ZnO/glass and Al-doped ZnO/glass and their optical transition properties

Vanadium dioxide (VO2) is a promising compound for thermochromic applications because of its insulator-metal transition (IMT) characteristic with concomitant transmittance change for infrared-light. For practical applications, we deposited VO2 thin films on transparent ZnO films on glass substrate. Growth of (010)-oriented VO2 film was obtained on c-axis oriented hexagonal ZnO layer, however, zinc vanadate (Zn2V2O7) was formed at the interface region in conventional radio-frequency magnetron sputtering at substrate temperature of 350 °C. On the other hand, (010)-oriented VO2 films were successfully grown on ZnO/glass substrates even at lower temperature of 250 °C by introducing inductively coupled plasma (ICP)-assisted sputtering, realizing steep interface between VO2 and ZnO without undesirable compounds. The VO2 films showed resistivity drop of more than two orders of magnitude together with transmittance change for infrared-light from 45 to 10% at around 70 °C. Simultaneous achievement of steep changes for both resistivity and transmittance is a milestone for practical application of VO2 films into thermochromic devices, such as smart windows and optoelectrical devices. Further, we succeeded the low temperature growth of VO2 films on conductive Al-doped ZnO (AZO) by using ICP-assisted sputtering technique. The transmittance change of VO2 on AZO/glass from 50 to 10% was observed, realizing a basis for electrically controllable intelligent optical windows.

Mechanisms of the structural modification of Ti films by pulsed direct current and inductively coupled plasma-assisted pulsed direct current sputtering

Titanium (Ti) thin films have been deposited by direct current (dc), pulsed dc, and inductively coupled plasma (ICP)-assisted pulsed dc sputtering to investigate the effects of current pulsing and ICP assistance on film structure and properties. Ti thin films were deposited from a Ti target using Ar as a discharge gas. Ion energy was measured by energy-resolved mass spectrometry. The discharge plasma parameters were measured using a single probe with a time-resolved data acquisition system. The crystal structure was investigated by X-ray diffraction. Surface and cross-sectional morphologies were investigated by atomic force microscopy and scanning electron microscopy, respectively. The cross-sectional images demonstrated that the film structure changed from a coarse columnar structure with voids to a fine columnar structure. Changes in the roughness and crystallite sizes of the deposited Ti thin films indicated that the thin films were amorphized by the ICP-assisted sputtering, resulting in the smoothing of the film surface. Energy and mass analyses indicated that the energy fluxes to the substrate via Ar+ and Ti+ ions increased by more than four and two orders of magnitude, respectively, when ICP assistance was employed. This large difference in degree of enhancement generated by ICP assistance for Ar+ and Ti+ ions suggests different ionization efficiencies between gas-phase Ar and sputter-emitted energetic Ti, particularly in the ICP-assisted pulsed dc discharge. As a result of the increase in energy flux to the substrate, the mean energy transferred by both Ar+ and Ti+ ions per Ti metal particle (atom or ion) arriving at the substrate increased from approximately 40eV for dc sputtering to >104eV for ICP-assisted pulsed dc sputtering, and the mean energy transferred by Ti+ ions per Ti metal particle (atom or ion) arriving at the substrate increased from a few eV for dc sputtering to approximately 29–30eV for ICP-assisted pulsed dc sputtering. These increases in transferred energy reflect the formation of intense and broad high-energy peaks, as shown in the ion-energy spectra. It is shown that the formation of an additional high-energy peak to the single peak of the dc discharge results from the current pulsing, and that the further high-energy shift and splitting of the bimodal peaks result from the combination of the increase in plasma sheath potential to the grounded substrate and the increases in the electron density and electron temperature due to the subsequent ICP assistance to the pulsed dc discharge.

Effects of energetic substrate-incident ions on the growth of crystalline vanadium dioxide films in inductively coupled plasma-assisted sputtering

We report on the effects of energetic ions incident to a substrate on the growth of vanadium dioxide (VO2) films on conductive layers by inductively coupled plasma (ICP)-assisted sputtering (ICPS). Ion energy distributions (IEDs) of Ar+ were measured using an electrostatic energy analyzer consisting of three meshed plates. Ions with kinetic energies up to 150 eV with peak positions corresponding to the plasma space potential were observed in ICP-assisted sputtering, in contrast with lower energies in conventional sputtering. Crystalline VO2 films with an insulator-to-metal transition (IMT) exhibiting resistance change over 2 orders of magnitudes at around 70 °C were successfully grown on conductive layers of titanium (Ti) and indium tin oxide (ITO) even at a low substrate temperature of 250 °C. Characteristic IMTs with multistep abrupt changes and hysteresis width of around 30 °C were first realized for polycrystalline VO2 film with the M2 phase grown on ITO layers on glass substrates. Through the analyses of surface morphology and stress states, we elucidated that high-energy ion irradiation strongly assists the low-temperature (<250 °C) crystalline growth of VO2, while it is, at the same time, accompanied by high growth stress.

Tribological properties and thermal stability of TiAlCN coatings deposited by ICP-assisted sputtering

In this study, the tribological and thermal properties of TiAlCN coatings were investigated to evaluate their feasibility in automobile applications. TiAlCN coatings with carbon compositions between 25 and 65at.% were prepared by inductively coupled plasma (ICP) assisted sputtering and were annealed at 400, 500, and 600°C in air. The structures and compositions of the coatings were studied by X-ray diffraction, Raman spectroscopy, transmission electron microscopy, and Auger electron spectroscopy. The hardness and tribological performance were evaluated by micro-indentation and ball-on disk tests. The wear rates were calculated using a non-contact 3D surface profiler. At a carbon composition>50at.%, the coatings featured a mixture of amorphous and crystalline phases and exhibited superior tribological performance compared to that of commercialized carbon or nitride coatings. Raman spectroscopy revealed that even when the amorphous carbon phase has a relatively high sp2 to sp3 ratio, there are sufficient amounts of TiAlN grains to support the hardness in the coatings. The coating hardness decreased slightly after high-temperature annealing due to graphitization in the amorphous carbon phase, however, the friction coefficients and wear rates were reduced with the annealing temperature.

Phase selective growth and characterization of vanadium dioxide films on silicon substrates

We report on selective growth of VO2 films with M1, M2, and intermediate T phases on silicon (Si) substrates by using inductively coupled plasma (ICP)-assisted sputtering (ICPS) under particular conditions. The film composed of M2 phase was proved to be under strong in-plane compressive stress, which is consistent with stress-induced M2 phase. Crystalline structural phase transition (SPT) properties of these films were demonstrated together with infrared light transmittance as a measure of insulator-metal transition (IMT) against temperature. Characteristic correlations between SPT and IMT for films with M2 and intermediate-T phases were reported. Ultraviolet photoelectron spectroscopy measurements probed an energy gap of the film in the M2 phase at around 0.4 eV from the Fermi level indicating the presence of a Mott gap.

Improved performance of dye-sensitized solar cells with compact TiO2 blocking layer prepared using low-temperature reactive ICP-assisted DC magnetron sputtering

This study discusses about carrier blocking effects of a compact TiO2 deposited onto fluorine-doped tin oxide (FTO) to improve the performance of dye-sensitized solar cells (DSSCs). The compact TiO2 blocking layer is prepared by reactive inductively coupled plasma (ICP)-assisted DC magnetron sputtering without substrate heating. The resulting layer yields a 47% improvement in the energy conversion efficiency, indicating that the blocking layer prepared by low temperature ICP-assisted sputtering effectively prevents physical contact with the FTO. This process may suggest a new method for fabricating photoelectrodes which consist of TiO2 blocking layer for use in flexible DSSCs.

Superhard Nanocrystalline Titanium Nitride Films Formed by Inductively Coupled Plasma-Assisted Sputtering

Nano fabrication technology of superhard TiN films with sub-nanometered crystallites was developed using an Inductively coupled plasma (ICP) during deposition. Nanocrystalline TiN coatings were fabricated by ICP assisted sputtering and the properies of the coatings were investigated. The ICP assisted TiN coatings showed a much higher nano-hardness (>43 GPa) compared to coatings produced by the conventional DC sputtering process. The ICP assisited TiN coatings also showed superior properties in dense microstructure and surface roughness compared to the DC sputtered TiN coatings. The superior mechanical properties of ICP assistted TiN coatings were attributed to the fine and dense microstructure and high compressive residual stress.

Advantages of inductively coupled plasma-assisted sputtering for preparation of stoichiometric VO 2 films with metal–insulator transition

Inductively coupled plasma (ICP)-assisted sputtering with an internal coil enabled deposition of stoichiometric crystalline vanadium dioxide (VO 2) films on a sapphire (Al 2O 3) (001) substrate under widely various deposition conditions. The films showed a metal–insulator (M–I) transition around temperatures of 68 °C with several orders of change in resistivity. Particularly, low-temperature (250 °C) growth of VO 2 film with two orders transition decade was achieved in ICP-assisted sputtering, in contrast with conventional sputtering, which required 400 °C for VO 2 growth. Rutherford back scattering (RBS) measurements revealed that the VO 2 film prepared by ICP-assisted sputtering was exactly stoichiometric, containing no impurity atoms from the inserted coil material. The ICP-assisted sputtering was examined in comparison to conventional sputtering in view of plasma characteristics.

Electric Field Induced Metal-Insulator Transition of Vanadium Dioxide Films on Sapphire Substrate Prepared by Inductively Coupled Plasma-Assisted Sputtering

Single phase monoclinic vanadium dioxide (VO2) films were successfully grown on sapphire (001) substrate by means of an inductively coupled plasma (ICP)-assisted sputtering with an internal coil. In x-ray diffraction patterns, we observed reflections originating from monoclinic VO2 with a main peak at 2θ = 39.77° corresponding to VO2 (020) for films on sapphire (001) substrates. The VO2 films exhibited metal-insulater transition (MIT) at temperatures around 65°C with three orders of change in resistivity. A simple device of VO2 film on sapphire (001) substrate having aluminum electrodes as ohmic contacts with gap separation of 1.0 mm showed current jump at an applied voltage of 48 V. The current jumped from 7 mA to 200 mA which was a limited value of voltage source.

In-Plane Orientation and Annealing Behavior of Rutile TiO2 Films on MgO Substrate Prepared by Inductively Coupled Plasma-Assisted Sputtering

In-plane orientation and annealing behavior of Rutile TiO2 films grown on MgO (100) substrates by means of inductively coupled plasma-assisted sputtering were investigated. We deposited TiO2 films on relatively low temperature (≦400°C) MgO substrates in Ar–O2 mixture gases. Several films were annealed in atmospheric O2 at 800°C for 60 min. X-ray φ scan measurements and AFM images revealed that rutile TiO2 film grows on MgO (100) substrate with two orientations of TiO2 (110)[001]∥MgO (100)[011] and TiO2 (110)[001]∥MgO (100)[011]. Post annealing in atmospheric O2 at 800°C improved the crystalline preference of TiO2 film, however, formation of MgTiO3 interface layer was clearly shown by cross sectional TEM image and XPS depth analysis. Values of refractive index and band gap energy were evaluated from optical characteristics of the films. High refractive index of 2.65 at wavelength of 500 nm in as-deposited TiO2 film degraded to 2.0 in annealed film due to the formation of MgTiO3 interface layer. Through this study, we offer stable rutile (110) surface and a formation route of MgTiO3 dielectric layer.

Dynamic control of substrate bias for highly c-axis textured thin ferromagnetic CoCrTa film in inductively coupled plasma-assisted sputtering

This study shows highly c-axis textured thin ferromagnetic Co-based alloy (CoCrTa) film growth in inductively coupled plasma (ICP)-assisted sputtering with an internal coil with an insulated surface. Dynamic control of the substrate bias achieved highly c-axis textured CoCrTa film with a thickness of 70nm in 3min depositions on a Si substrate. The prepared film showed a smooth, dense surface consisting of small crystal grains. The film had a perpendicular magnetic coercivity of 1030Oe and coercive squareness of 0.36. ICP-assisted sputtering with an internal coil with an insulated surface enabled higher-density (⩾1.0×1011cm−3) plasma with lower space potential (⩽30V) compared to ICP-assisted sputtering with bare coil systems. Therefore, the proposed bias control is quite effective for textured growth of thinner Co layers via the effect of a high flux of ions with proper energies. This method can be a candidate for the deposition technique of c-axis textured films as perpendicular magnetic recording media.

Performance of inductively coupled plasma assisted sputtering with internal coil for ferromagnetic CoCrTa film deposition

The performance of inductively coupled plasma (ICP)-assisted sputtering with an internal coil was revealed for deposition of ferromagnetic Co-based alloy films. Three types of internal coil, a directly grounded bare coil, a capacitively coupled bare coil and an insulated coil, were investigated with regard to the crystalline structure of films and plasma characteristics. Highly c-axis textured hexagonal-close-packed (hcp) film growth was achieved in the bare coil system at a medium operating Ar pressure of 1.3 Pa. Pole figure measurements showed that the film grown on a low-temperature Si substrate contained small face-centered-cubic phase in the c-axis textured hcp crystal. Plasma diagnostics suggested that high plasma space potential beyond 80 V caused highly textured growth through the effect of ion bombardment with proper energies for crystallization. The superior capability of ICP-assisted sputtering with a capacitively coupled bare coil was revealed for textured growth in the thin layer on a substrate surface. A high deposition rate was achieved as a result of high electron density of the order of 1011 cm−3 in ICP-assisted sputtering with an insulated coil. However, the insulated coil system could not achieve textured growth.

Growth of Highly Oriented CoCrTa films by Inductively Coupled Plasma-Assisted Sputtering

Inductively coupled plasma (ICP)-assisted sputtering using an internal coil was applied to the deposition of ferromagnetic Co-based alloy (CoCrTa) films intended for magnetic recording media. The growth of highly oriented crystalline Co films on both low-temperature Si and glass substrates was achieved. Pole figure measurements indicated the growth of face-centered cubic Co with a (111) plane parallel to the substrate, while a hexagonal close-packed crystalline film with c-axis perpendicular to the glass substrate was obtained by adjusting the rf power applied to the inserted coil and the substrate temperature. In the ICP-assisted sputtering at a coil rf power of 120 W, the deposition rate at a pressure of 1.3 Pa was more than two times higher than that of conventional sputtering at the same target rf power of 150 W. The deposition performance responsible for highly oriented crystalline growth and higher deposition rate at low pressures was discussed in terms of target self-bias voltage and plasma parameters measured using a Langmuir probe. The film properties characterized by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and vibrating sample magnetometer (VSM) were also presented showing the superior capability of ICP-assisted sputtering for ferromagnetic targets.

557. Projection color television displays

Titanium (Ti) thin films have been deposited by direct current (dc), pulsed dc, and inductively coupled plasma (ICP)-assisted pulsed dc sputtering to investigate the effects of current pulsing and ICP assistance on film structure and properties. Ti thin films were deposited from a Ti target using Ar as a discharge gas. Ion energy was measured by energy-resolved mass spectrometry. The discharge plasma parameters were measured using a single probe with a time-resolved data acquisition system. The crystal structure was investigated by X-ray diffraction. Surface and cross-sectional morphologies were investigated by atomic force microscopy and scanning electron microscopy, respectively. The cross-sectional images demonstrated that the film structure changed from a coarse columnar structure with voids to a fine columnar structure. Changes in the roughness and crystallite sizes of the deposited Ti thin films indicated that the thin films were amorphized by the ICP-assisted sputtering, resulting in the smoothing of the film surface. Energy and mass analyses indicated that the energy fluxes to the substrate via Ar+ and Ti+ ions increased by more than four and two orders of magnitude, respectively, when ICP assistance was employed. This large difference in degree of enhancement generated by ICP assistance for Ar+ and Ti+ ions suggests different ionization efficiencies between gas-phase Ar and sputter-emitted energetic Ti, particularly in the ICP-assisted pulsed dc discharge. As a result of the increase in energy flux to the substrate, the mean energy transferred by both Ar+ and Ti+ ions per Ti metal particle (atom or ion) arriving at the substrate increased from approximately 40 eV for dc sputtering to > 104 eV for ICP-assisted pulsed dc sputtering, and the mean energy transferred by Ti+ ions per Ti metal particle (atom or ion) arriving at the substrate increased from a few eV for dc sputtering to approximately 29–30 eV for ICP-assisted pulsed dc sputtering. These increases in transferred energy reflect the formation of intense and broad high-energy peaks, as shown in the ion-energy spectra. It is shown that the formation of an additional high-energy peak to the single peak of the dc discharge results from the current pulsing, and that the further high-energy shift and splitting of the bimodal peaks result from the combination of the increase in plasma sheath potential to the grounded substrate and the increases in the electron density and electron temperature due to the subsequent ICP assistance to the pulsed dc discharge.