ICMCTF2012 Session FP: Symposium F Poster Session

Cadmium oxide (CdO) with a cubic structure is an n-type semiconductor and shows a wide direct band gap of ~2.6 eV and a narrow indirect band gap of ~0.5 eV. Recently, CdO has attracted much attention because of its technological applications such as photodetectors, solar cells, gas sensors and nonlinear optics.

Chemical vapor deposition (CVD) methods utilizing the vapor-liquid-solid (VLS) mechanism are favorable for position- and size-controlled growth of nanostructures. In this paper, shape controllability and optical properties of CdO nanostructures grown by atmospheric-pressure CVD methods using Cd and H₂O as source materials and Au naocolloids as a catalyst will be discussed in terms of substrate temperature, source supply ratio and growth time.

The catalytic solution of Au nanocolloids diluted with ethanol was coated on the c-plane Al₂O₃ substrates by the spin-coating technique (1500 rpm, 5 sec). Substrate temperature (T_S) was varied in the range from 825 to 975 °C. Source temperature of Cd (T_Cd) was kept at 500 °C. The vaporizer containing H₂O (T_H2O) was kept at 54 °C. The nitrogen carrier gas flow rates for Cd and H₂O (F_Cd and F_H2O) were changed in the range from 10 to 60 sccm.

Various shapes of nanostructures, such as nanorods (NRs), nanotrees (NTs) and nanobelts (NBs), were successfully grown. Especially, it was found that the shapes of the NRs change from the cylinders to the cones with increasing T_S, so-called “tapering”. In general, two different types of growth mechanisms are in progress simultaneously during the CVD process; one is the axial growth due to the VLS mechanism through the catalyst and the other is the radial growth due to the film growth mechanism on the NR’s side wall (VS growth). The tapering behavior is probably due to the rapid increase in radial growth rate with increasing T_S. Moreover, the appearances of Y- and T-shape NTs and their 3D network structures suggest that the catalytic particles split and migrate during the growth process. We believe that the simultaneous work of the VS and VLS mechanisms together with the splitting and migration of catalytic particles is the driving force of the morphological variety of nanostructures

LiCoO₂ in thin film form is identified as one of the best cathode materials by its high energy density, long cycle life and high capacity retention. Investigations are aimed to enhance the electrochemical performance of the as-grown films using textured Si as a base substrates and compared the results with the films grown on polished Si substrate. Thin films of LiCoO₂ were prepared by rf magnetron sputtering technique on Au/Ti/SiO₂/(polished)Si and Au/Ti/SiO₂/(textured)Si maintained at a substrate temperature 300^oC. Comprehensive investigation was performed on the growth, microstructure and electrochemical properties of thick LiCoO₂ films deposited at various reactive gas composition and rf powers. The as-grown LiCoO₂ films on polished Si metalized substrate at low O₂ to Ar gas composition ratio of 1:9 and rf power 150 W exhibited predominant (0 0 3) orientation representing partially ordered HT-LiCoO₂ structure with R3m crystalline space group. The average surface area fraction observed from SEM analysis is observed to be 4.1 %. The films exhibited first ordered phase transition during intercalation/de-intercalation reaction and demonstrated a discharge capacity of 53 µAhcm^-2 µm^-1 with a higher capacity fading rate of 0.26 %. Where as, the films deposited on Au/Ti/SiO₂/(textured)Si substrates represented predominant (0 0 3) orientation with relatively enhanced surface area fraction of 16.7 %. The surface of the film observed from SEM contains pyramidal shaped clusters composed of grains with an average size of 210 nm. These films exhibits improved electrochemical performance in terms of discharge capacity (57.5 µAhcm^-2 µm^-1), capacity retention (95.4% per 50 cycles) and lower capacity fade rate (0.07 % per cycle) which is demonstrated by the substrate texturing influence.

Abstract:

Lithium transition metal oxides have received a considerable attention in recent years as high voltage positive electrode materials in the fabrication of all solid state microbatteries. Among various lithium based cathode materials, LiMn₂O₄ is one of the most promising cathode material as it offers high energy density, high cell voltage, low cost, and low toxicity over the other electrode materials. In the present investigation, thin films of LiMn₂O₄ were prepared by radio frequency magnetron sputtering on gold coated silicon substrates in an Oxygen to Argon ratio of 1:6 and an oxygen partial pressure of 2×10^-2 mbar. The films were deposited from different substrate temperatures ranging from 400 K – 700 K and RF power was varied from 50-125 W. The influence of substrate temperatures and RF power on growth, microstructure and electrochemical properties was studied. The XRD and SEM results revealed that with the increase of substrate temperature the film structure changed from amorphous to polycrystalline. The films deposited at T_s = 673 K with RF power 80 W exhibited predominantly (111) orientation representing cubic spinel structure of Fd3m symmetry with an average grain size and lattice parameter of 275 nm and 8.23 Å. Further increase in crystallanity and electrochemical performance was observed by annealing these films at 700⁰C. The films annealed at 973 K exhibited better electrochemical performance with initial discharge capacity of 53.5 μAh cm^-2 μm^-1 in the aqueous media suggesting that the film can be used as binder free cathode in Li-ion battery application.

In the last ten years, Bismuth-film electrodes (BiFEs), prepared by coating a suitable substrate with a metallic Bi thin film, have proved valuable tools for electroanalysis in the reductive potential regime, especially for anodic and adsorptive stripping analysis. They have been proposed as a green alternative to substitute Hg film electrodes.

In order to evaluate the physicochemical properties and the electrochemical behavior of Bi films produced by DC magnetron sputtering and to assess their feasibility to be used as BiFEs for detection of heavy metal traces in water, in the present work Bi films were deposited by DC magnetron sputtering onto glass substrates using a high purity Bi target (99.9 at. %, 4” in diameter), a working pressure of 4 Pa (Ar atmosphere), a current density of 0.2 A and deposition times of 5, 10 and 15 minutes.

The structural properties of the films were characterized by X-ray Diffraction, X-ray Photoelectron Spectroscopy and Raman Spectroscopy. The thickness, roughness and dielectric constant of the films were also characterized. The structure of the deposited films was basically the Bi-rhombohedral phase with a preferred orientation (012) & (104). The films were polycrystalline and have large deposition rates.

For the electrochemical tests of the films, a 1 cm² area was exposed to buffer solutions. First, the potential window of the films at which the electrode might be useful and their stability upon potentiostatic extreme conditions were characterized in buffer solutions of pHs 4.6, 7 and 10, in an electrochemical cell with a saturated calomel reference electrode and a Pt wire as the counter electrode. No strong variations in the potential windows with deposition time (film thickness) were observed. The oxidation potential became more negative as the pH increased, from -0.3 to -0.5 V. The potential windows were about 1 V for all the films in the 3 different pHs.

After determination of the potential window an electrochemical impedance spectroscopy test was performed; the spectra were very similar for the 3 pH values and the different film thickness, probably indicating similar interfacial effects. Then, the samples were subjected to a cyclic potential from -1.5 to 1.5 V; the films could support these extreme conditions without undergoing delamination. Finally, the performance of the films as BIFEs was evaluated by anodic adsorptive stripping analysis in water solutions containing either, Cu or Sn traces.

The current trend towards downsized integrated electronic and optical devices has strongly motivated the intensive study of various one-dimensional (1D) nanostructures. In particular, one attempt has been made to create coaxial 1D structures with a core/sheath geometry, which may assist in the realization of various tailor-made functions by assembling the different features of both nanowires (as cores) and nanotubes (as sheaths) with different chemical compositions in the radial direction. Indeed, the great potential has recently been demonstrated in nanodevice applications such as coaxial-gated transistors and laser diodes. SnO₂ is a well-studied functional material that has been extensively used in dye-based solar cells, transparent conducting electrodes, and gas sensors.

Pre-grown SnO₂ 1D nanostructures are coated via a DC sputtering technique using a metal target. By carrying out the subsequent thermal annealing, we have generated the metal nanoparticles on the core nanowires. M etallic catalysts are known to functionalize the surface of nanomaterials. For example, n oble metals anchored on semiconducting oxides facilitate the dissociation of oxygen molecules into oxygen species, including atomic oxygen, thereby enhancing the oxygen sensitivity. Accordingly, some research groups have attempted to functionalize the surface of oxide nanowires with nanosized noble metals using various methods. In the present work, we have investigated the effect of metal nanoparticles on the gas sensor properties of nanowires. .

Recently, carbon nanotubes (CNTs) have been researched to develop a high resolution x-ray image system for medical applications such as early diagnosis of cancers, using the superior properties of CNTs such as chemical stability, thermal conductivity, mechanical strength, and structural aspect ratio. For use of CNT-emitters as electron source for x-ray generation, a large number of investigations have been focused on how to enhance the emission current level and to reduce the turn-on field for electron-emission. Especially, to obtain high-resolution x-ray images, the diameter of the beam incident area must be tens of micrometers or less when the electron beam emitted from the CNT cold cathode collides against the x-ray generation target. For this purpose, some researchers have recently developed CNT emitters by growing them on very sharp tip-type substrate. In these tip-type CNT emitters, however, the long-term emission stability should be ensured due to a relatively low bonding force between the CNT and the substrate. In order to achieve desirable performances in emission current and stability, coating of CNTs with various materials, such as metal carbides like titanium carbide (TiC), metal nitrides like boron nitrides (BN), and metal oxides including magnesium oxide (MgO), have been studied.

In this work, the effects of coating of lithium (Li) thin layers with various thicknesses have been investigated for the purpose of enhancing the electron-emission current and the long-term stability of CNT-emitters. The CNTs were grown on metal-tip (tungsten, approximately 500 nm in diameter at the summit part) substrates via electrophoretic deposition (EPD). The Li layers were coated on CNTs using an electroplating method. The morphologies, microstructures, and chemical compositions of the Li-coated CNTs were analyzed as a function of the thickness of the Li layer. For all the fabricated Li-coated CNT-emitters, the electron-emission characteristic and the long-term (up to 20 h) stability of the emission current were measured, which were also compared with those of the conventional non-coated CNT-emitter. The experimental results showed that the electron-emission capacity was noticeably enhanced by coating Li layers on the surface of CNTs. This was attributed to the fact that the effective work function of CNTs was reduced by Li coating. It was also observed that the Li-coated CNT-emitters exhibited a more stable electron-emission characteristic than that of the non-coated one.

Carbon nanotubes (CNTs) have much attraction for high-current density applications because of their superior properties, such as chemical stability, thermal conductivity, mechanical strength, and structural aspect ratio. Among the many promising applications of CNTs, the electron emitted source of cold cathode for miniature x-ray system is that which can be most immediately realized because CNT-based cold cathodes could overcome many problems with conventional thermionic cathodes, such as limited temporal resolution, a short lifetime, a high operating cost, and restricted miniaturization. Currently, in order to use CNTs as electron sources, many investigations have been focused on how to enhance the emission current level and to reduce the turn-on field for electron-emission. The CNT-based field emitters have been fabricated either by direct growth methods like chemical vapour deposition (CVD) or by indirect printing methods, with various types of substrates such as pin-type or flat type. In general, the pin-type substrates have some advantages for obtaining high resolution of emitted electron beams, but they are hard to meet the current level required for x-ray generation. On the other hand, the flat-type substrates may produce sufficient currents, but they have poor adhesion between the CNTs and the substrates due to the weak van der Waals force.

In this study, we present a novel method for fabricating CNT-based field emitters with high emission current level by using flat-type polymer substrates (such as cellulose and polyester) with high absorbency. The CNTs were grown using a dip-coating method by dipping the substrates in the CNT suspension. The CNTs attached on the substrates formed covalent and hydrogen bonds with the polymer surface that has hydrophilic groups (i.e., OH^-) after acid purification of CNTs. The morphologies and microstructures of polymer substrates and CNTs were monitored via field-emission scanning electron microscopy (FESEM) and high-resolution transmission microscopy (HRTEM). Fourier transform infrared spectroscopy (FTIR) was used to identify the covalent and hydrogen bonds between CNTs and substrates. These analyses indicated that the polymer substrate was chemically combined with a large number of CNTs with a strong adhesion. The electron emission properties of the fabricated CNT emitters were also measured at a pressure of below 10^-5 Pa, with a distance of 1mm between the cathode (CNTs) and the anode. The results showed that the CNT emitters fabricated with the polymer substrates produced more than 1 mA at 1 V/µm of applied field.

The successful commercialization of lithium ion batteries for electronics, automobiles, and technology has led to many research groups to invest considerable amount of money in this battery technology that utilizes LiCoO₂, LiNiO₂, and LiMnO₂ cathodes. However, lower cost cathode materials are required for various applications. In addition, these materials limit the applications to small batteries due to the high cost, toxicity, and environmental harmful of the materials. LiFePO4 has received significant attention and commercialized as a cathode for Li-ion batteries. The exceptional stability of LiFePO₄ at elevated temperatures enables safe, large lithium ion batteries for large scale applications such as electric vehicles or space applications. The present work was performed to understand the effect of temperature, an important thermodynamic variable, on the microstructure and electronic properties of LiFePO₄ films fabricated by radio-frequency (RF) magnetron sputtering. LiFePO₄ films were grown under varying deposition temperatures in the range of 25 to 400 °C. In addition, LiFePO₄ films were annealed in temperature ranges of 400 to 800 °C for 1 and 2 hours. The effect of growth temperature on the crystal structure, surface morphology, chemical quality and electronic properties is investigated in detail. Characterizations of the films were performed using X-ray diffraction (XRD), high resolution scanning electron microscopy (HRSEM), energy dispersive X-ray spectrometry (EDS), optical spectrophotometer, and electrical resistivity measurements. The grain size increased as the annealing temperature increased from 400 to 800 °C. The optical properties of the LiFePO₄ films indicate that, as the growth temperature is increased, the transmittance of the films increases. The band gap increases from 2.75 eV to 3.28 eV with increasing temperature from RT- 400 °C. When the films were annealing at 1 hour form 400-800 °C, the band gap increased from 3.12 eV to 3.7 eV. Annealing for 2 hours at temperatures from 400 to 800 °C showed an increase in band gap 3.12 eV to 3.75 eV showing the maximum value at 600 °C. The electrical condcutivity indicates that with an increase in substrate temperature, the resistivity of the films also increases. The results will be presented and discussed.

In the last years there has been an increasing interest in electrical energy storage. The requirements of the industry are clear and unambiguous: the storage solutions should be powerful, compact and save. All these properties can be achieved with thin film lithium ion technology. The research on cathode materials plays a key role because the performance of a lithium ion cell is mostly limited by its cathode. Today the most commercially available lithium ion batteries are still based on the toxic and expensive LiCoO₂ as standard cathode material. Cheaper and environmentally friendlier are lithium manganese based cathode materials.

In this work LiMn₂O₄ spinel and orthorhombic LiMnO₂ thin films have been prepared by non-reactive r.f. magnetron sputtering from commercial ceramic LiMn₂O₄- and LiMnO₂-targets in a pure argon discharge. The deposition parameters target power and working gas pressure were optimized in combination with a post deposition furnace annealing with respect to microstructure and electrochemical behavior. The chemical composition was determined using inductive coupled plasma optical emission spectroscopy (ICP-OES) and inert gas fusion analysis (IGFA) and the results were compared with laser ablation mass spectroscopy (LA-MS) and high frequency glow discharge optical emission spectroscopy (HF-GDOES). The films crystal structure, phase evolution and microstructure were investigated by X-ray techniques, micro Raman spectroscopy and scanning electron microscope (SEM). Due to the fact that these thin films consist of the pure active material without impurities like binders or conductive additives like carbon black, they are particularly well suited for measurements of the pure intrinsic physical properties.

The electrochemical behavior of these films was investigated by galvanostatic methods in lithium half cells with a standard EC:DMC (1:1) liquid electrolyte containing 1 mol LiPF₆. Both the influence of the charging and discharging currents and of the voltage window was investigated. To get a deeper insight into the electrochemical reactions cyclic voltammetry was carried out. Finally both materials will be compared and some ideas for structural improvements will be given.

In this study, the bipolar resistive switching characteristics of the resistive random access memory (RRAM) device based on sputter–deposited TaON thin film was investigated. The resistive switching behavior of the Pt/TaON/TiN structure can be traced by dc voltage and pulse voltage. The proposed memory device exhibits excellent resistance switching with a high resistance state/ low resistance state ration of 2.5 order, write/erase endurance of about 2 order, and long retention time of 10⁴ s at 85 ℃. In addition, the device was investigated to achieve multilevel operation, which could increase storage density for next generation memory application. It was also found that the polarity of the forming process would not influence the resistive switching characteristic but influence the first reset process behavior. The switching behavior could be regarded as the oxygen redox near the TiN interface. However, the first reset behavior of negative forming process was related with the oxygen concentration gradients near the Pt electrode and the Joule heating enhanced oxidation.

ZnO/Ru/ZnO trilayer films sandwiched between Ru electrodes were prepared for nonvolatile resistive memory applications. These structures show resistance switching under electrical bias both before and after a rapid thermal annealing treatment, while it is found that the resistive switching effects in the two cases exhibit distinct characteristics. The ZnO devices after RTA treatment demonstrates remarkable device parameter improvements including lower threshold voltages and lower write current. Furthermore, the RTA treatment has triggered a switching mechanism transition from a carrier trapping/de-trapping type to an electrochemical-redox-reaction-controlled conductive filament formation/rupture process, as indicated by different features in current-voltage characteristics.

Bismuth oxide has five known crystalline phases: alpha, beta, gamma, delta and omega, each with its distinct properties and stability domains. The delta phase exhibits ionic conductivity 1-2 orders of magnitude higher than yttria-stabilized zirconia, the compound widely used in solid oxide fuel cells. In bulk, the delta-Bi₂O₃ phase is not stable at room temperature but from 750 C to 825 C, the melting point of bismuth oxide. Outside this temperature range, other phases are stable but their conductivities are up to three orders of magnitude lower than that of the delta phase. The stability of the delta phase stability can be extended to lower temperatures by doping, but this process yields a severe reduction in ionic conductivity.

In the present work, we have synthesized Bi₂O₃ films by RF reactive magnetron sputtering using a Bismuth target in an Ar/O₂ gas discharge. We investigate how the structure of the films is influenced by substrate temperature, source power, and oxygen flow rate/total gas flow rate ratio. As expected, the substrate temperature influences the initial nucleation process and growth kinetics. Deposition at ambient temperature yields mainly amorphous films while higher temperature yields crystalline films. Different stoichiometric compositions can be obtained by varying the source power and the oxygen flow ratio. By tuning all these parameters we obtain Bi₂O₃ with cubic structure, with thickness of hundreds of nanometers. XRD shows crystalline films with peaks around 27 and 55 two theta degrees corresponding to (111) and (222) planes for cubic structure. Pole figures XRD analysis was also performed for those peaks and diffractions rings were observed near phi angle of 70 degree which corresponds to the angle between (1 1 1) and (1 1 -1) planes in a cubic structure, consistent with the delta phase. SEM showed a columnar structure of our films with very good uniformity of the films. Electrical and optical measurements have also been performed; ellipsometry yields a value of the band gap around 2.5eV while for optical constants values of n≈2.5 and k≈0.5 are obtained for UV-Vis wavelength interval.

Three films of varying ITO thicknesses were examined: sheet resistance and optical transmission were monitored and tensile testing performed before and after exposure to acrylic acid. Also the tribological properties of the coated films were examined under a specially-modified fretting rig.

It appears that even low concentrations of acrylic acid that would often go unnoticed, may cause failure to occur more readily. In both tensile testing and the monotonic bending test where the film is subjected to tension, high concentrations of acrylic acid would normally be a cause of failure. Nevertheless, even low concentrations such as 0.1M can also cause the critical onset strain to occur more readily due to stress-corrosion cracking. However when the films are subjected to stresses in compression, the exposure to acrylic acid shows little or no effect.

This letter studies the hot-carrier effect in indium–gallium–zinc oxide (IGZO) thin film transistors with symmetric and asymmetric source/drain structures. The different degradation behaviors after hot carrier stress in symmetric and asymmetric source/drain device indicate that different mechanisms dominate the degradation. Since the C-V measurement is highly sensitive to the trap state compared with the I-V characteristics, thus, the C-V curves are utilized to analyze the hot carrier stress induced trap state generation. Furthermore, the asymmetric C-V measurements (gate-to-drain capacitance, gate-to-source capacitance) are useful to analyze the trap state location. For asymmetric device structure, different source/drain structure under hot carrier stress will induce asymmetric electrical field and cause different degradation behaviors. In this work, the on-current and subthreshold swing (S.S) degrade under low electrical field, whereas the apparent V_t shift occurs under large electrical field. The different degradation behavior indicates that the trap state generates under low electrical field and channel-hot-electron (CHE) effect occurs under large electrical field.

Semiconductor quantum dots can confine the carriers such as electrons and holes in three-dimensional directions and exhibit unique electronic and optical properties. For highly efficient IR light-emitting devices using germanium (Ge) dots, the Ge dots with high density and small size less than 10nm in diameter is required. As regards the density of dots, it has been reported that a density of 2x10¹¹ cm^-2 was achieved by the pregrowth of submonolayer carbon.¹⁾ In this case, carbon atoms were incorporated into a Si(001) substrate and surface strain was induced, leading to the formation of a Si(001) c(4x4) reconstruction structure at the surface layer, which changed the Ge growth mode from the Stranski–Krastanov (SK) mode to the Volmer-Weber (VW) mode, leading to high-density dot formation. As a strong repulsive interaction can operate between Ge and C, Ge atoms deposited on the SiC layer do not interdiffuse. The c(4×4) surface was expected as a template substrate for the formation of high-density Ge nanodots. In our previous study, it was found that the c(4x4) structure was formed by the reaction of monomethylgermane (MMGe) on Si(001) 2x1 surface. The Ge nanodots embedded in SiC structure is expected to strongly confine the carriers due to the difference in the band-gap between Ge and SiC. As a result of the experiment of the Ge nanodot formation using MMGe, SiC and Ge nanodots were formed after the appearance of the c(4×4) reconstructed surface. Using a pulse-controlled nucleation method, the formation of high-density (1.3×10¹¹ cm^-2) and small size dots (6 nm) with small standard deviation (1 nm) was achieved.

In this study, surface structure of Ge and SiC nanodots fabricated using MMGe were investigated by reflection high-energy electron diffraction (RHEED) and scanning tunneling microscopy (STM). PL spectra of the Ge nanodots capped with SiC layer were also measured at a low temperature. The PL spectra obtained were deconvoluted into some components to consider the origin of each emission. The PL peaks around 1.01 eV and 1.07 eV, which are considered to be originated from the Ge nanodots, were observed. Eberl et al. have observed PL peak around 0.99 eV and 1.04 eV from Ge nanodots generated by a solid source molecular beam epitaxy (MBE) method. Blue shifts of the emission peaks observed in our sample were considered to be due to the difference in the chemical composition compared to that of the Ge dots formed by the solid source MBE or residual stress of nanodots.

Reference

1) M. Stoffel, L. Simon, J. L. Bischoff, D. Aubel, L. Kubler, and G. Castelein: Thin Solid Films 380 (2000) 32.

Thin film solar cells are currently demanded to be flexible with acceptable photovoltaic (PV) efficiency. Among a variety of PV materials, titanium-based oxide with low material cost, non-toxicity and most important, low crystal growth temperature, seemed to be a good candidate for PV material on flexible substrate. Strategies in this study is to produce oxygen-graded TiO_x(x=1.5-1.9) (Eg=1.8 - 2.7 eV) layer as the absorption layer followed by depositing stiochiometric TiO₂ (Eg=3.0 - 3.2 eV) as the window layer to form eventually a hetero-junction device on flexible polyimide (PI) substrate. This allows maximum light harvest of the device. To achieve low temperature deposition of titanium-based oxide on PI substrate, a high power impulse magnetron sputtering (HIPIMS) technique is employed, which is known to provide high density plasma to favor crystalline growth during deposition and hence a reduced substrate temperature.

Studies of long pulses of high power impulse magnetron sputtering (HiPIMS) have been reported, both by experiments and by modeling. The model used is based on IRM I, which referred to as IRM II here, is a time dependent plasma chemical discharge model developed for the ionization region in magnetron sputtering discharges. It is benchmarked against the experiments performed at Lawrence Berkeley National Laboratory, with square voltage pulses (330V to 1000V), 400μs long, applied to an Al target in Ar at 1.8Pa. The power is kept so low that the discharge does not go into the runaway self sputtering mode. The model uses two fitting parameters, the probability of ion back-attraction to the target, and the fraction of the electric discharge power, that goes to the electrons. Model calculations are found to give a close fit to an experimentally observed initial high current transient, as well as a later plateau value of constant lower current, but only within the limited parameters range. The following observations hold within all of this fitting parameter range. The peak in discharge current precedes a maximum in gas rarefaction. The time durations of the high current transient, and of the rarefaction maximum, are determined by the time it takes to establish a steady state diffusional refill of process gas from the surrounding volume. The dominating process for gas rarefaction is ionization losses, with only about a few percent due to the sputter wind, heating, and kick-out processes that dominate in dcMS. The plasma density peaks during the high current transient, and more than half of the sputtered metal ionized. During the whole plateau phase the electron density stays stable, while the degree of metal ionization is lower and almost constant. The electron temperature was steady throughout the pulse. The degree of self sputtering (here defined as the metal ion fraction of the total ion current to the target) varies during the pulse. It grows from zero at pulse start to a maximum some time later, coinciding in time with the maximum gas rarefaction. It then stabilizes during the plateau phase. This makes the degree of self-sputtering as an important parameter in the HiPIMS discharge. The part of the decrease in deposition rate that can be attributed to the back-attraction of the ionized sputtered species also varies during the pulse, in such a fashion that it is correlated to the momentary discharge current. It is low during the initial stage, then peaks during the current transient, and finally stabilizes during the plateau phase.

BIPOLAR sputtering is currently being implemented in progressing number of applications replacing older MF technology. Most important new feature of the technology is ability to react on changing parameters of the process and plasma very fast in order to enhance process and coating parameters.

New abilities of the BIPOLAR power supplies mainly result from ultra fast digital internal control system. Fast digital data processing, enables new BIPOLAR power supplies serve broad range of applications better than older MF technology. Faster and more precise digital control gives user enhanced control over the process, and enables power supply to react on process parameters in sophisticated way employing advanced algorithms.

Most importantly digital control platform enables new features supporting reactive sputtering with BIPOLAR technology. The solution enabled to deposit coatings with enhanced quality and at high deposition rate. Results of use of the solution will be presented and potential for industrial applications will be discussed.

The effect of hydrogen addition on the formation of cubic boron nitride thin film was investigated, especially focusing on the behavior of the residual stress of the film. The films were deposited by UBM (unbalanced magnetron sputtering) method. A boron nitride target of 3” diameter was used as a sputtering source, which was connected with 13.56 MHz RF (radio-frequency) electric power supply. The substrate holder was placed at 7.5 cm under the target and 200 KHz electric power supply was used as a substrate bias power supply. Either Si or Si wafer with nanocrystalline diamond thin film on it was used as a substrate. The chamber was evacuated down to 10^-6 torr and a mixed gas of Ar-10% nitrogen was flowed into the chamber during deposition. The hydrogen was added to the mixed gas up to 5 sccm while maintaining the total gas flow rate at 20 sccm. The deposition pressure was maintained at 2 or 4 mtorr. The electric power of the target was 500 W and the substrate self bias voltage was -60 V. The residual stress was calculated by Stoney equation by using the curvatures of thin Si strip of (3 x 40) mm² measured during deposition. FTIR, SEM, TEM as well as RBS were used to analyze the phase, structure and composition.

It was observed that the residual stress was decreased with the increase of the hydrogen flow, while the cBN fraction was also decreased. The rate of the decrease, however, has shown different behavior. The former was faster than the latter at the initial stage of the added hydrogen amount, which implies that a little addition of the hydrogen could lessened the residual stress of the film to a much degree with only a trivial decrease of the cBN fraction. The reason of this behavior was discussed with the incorporation of Ar atoms into the film and its effect on the formation of the residual stress.

This research was supported by a grant from the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy, Republic of Korea.

The formation of cBN is known to be affected mainly by ion energy and flux. During sputtering process, both the ion energy and the ion flux are a function of substrate bias voltage and a mean free path of ions. In this study, the effect of deposition pressure on the formation was investigated to see the role of the mean free path length in the cBN formation. The films were deposited by UBM (unbalanced magnetron sputtering) method. A boron nitride target of 3” diameter was used as a sputtering source, which was connected with 13.56 MHz RF (radio-frequency) electric power supply. The substrate holder was placed at 7.5 cm under the target and 200 KHz electric power supply was used as a substrate bias power supply. Either Si or Si wafer with nanocrystalline diamond thin film on it was used as a substrate. The chamber was evacuated down to 1.2 x 10-6 Torr and a mixed gas of Ar-10% nitrogen was flowed at 20 sccm into the chamber during deposition. The deposition pressure was varied from 2 to 20 mTorr. For the system we used, the mean free path length was calculated to be around the range of the target to the substrate distance. The electric power of the target was 500 W and the substrate selfbias voltage was varied from -20 to -120 V. FTIR, SEM as well as TEM were used to analyze the phase and structure.

The formation of the cBN phase was affected both by the substrate bias voltage and the deposition pressure. There appeared a critical pressure above which only a hBN phase formed. With increasing the substrate bias voltage, the critical pressure increased. The cBN content in the FTIR spectrum was not varied in the range under the critical pressure. The critical substrate bias voltage was also increased with increasing deposition pressure. This behavior was discussed with a view point of ion energy and ion flux ratio relation curve for the cBN formation.

It was reported previously that minor addition of either the oxygen or the hydrogen had inhibited the formation of cBN during the PVD process. It is, thus, expected that the moisture adsorbed inside the deposition chamber could have a harmful effect on the deposition of the cBN film. Such possibility was investigated in this study. The chamber was open to the atmosphere before deposition for 1 h under the moisture content of about 30%. Then it was evacuated down to 3.5x 10-6 Torr. It took 30 min to arrive the pressure and then a mixed gas of Ar-10% nitrogen at 20 sccm was flown and the deposition process was started. The films were deposited by UBM (unbalanced magnetron sputtering) method. A boron nitride target of 3” diameter was used as a sputtering source, which was connected with 13.56 MHz RF (radio-frequency) electric power supply. The substrate holder was placed at 7.5 cm under the target and 200 KHz electric power supply was used as a substrate bias power supply. Either Si or Si wafer with nanocrystalline diamond thin film on it was used as a substrate. The deposition pressure was 4mTorr. We deposited the films for 30 min. Then, the sample was changed via load lock chamber and the chamber was evacuated again down to 1.2 x 10 -6 Torr with baking the chamber. The baking was done by heating the substrate heater whose temperature was adjusted between room temperature and 500 ℃. It took 90 min to arrive the base pressure. It is believed that little moisture was involved into the chamber during this stage. We compared the FTIR spectra of these samples to see the effect of moisture contamination.

The first deposited sample has shown that only a hBN phase formed in the film. The cBN phase began to appear as the deposition batch proceeded. The number of batch forming the cBN phase was smaller as the baking temperature increased. The baking time also affected the formation of the cBN content. We measured the OES (optical emission spectra) during deposition and compared them from batch to batch to find the possible indicator of cBN formation. The variation of the intensity of main emission peak with the number of the batch was shown and their relation with the cBN formation was discussed.

This research was supported by a grant from the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy, Republic of Korea