Negative-ion Bombardment Research Articles

Negative ion beam bombardment of a protic ionic liquid: Alleviating surface charging and damage and analyzing the surface of organic insulating materials

Positive ion beams are widely used in surface processing and analysis; however, serious surface charging can occur in the case of insulating materials. To address this issue, we investigate bombardment effects of ionic liquid negative ions emitted from the tip of a sharp needle wetted with the protic ionic liquid, diethylmethylammonium trifluoromethanesulfonate. Experimental results show that the potential of an electrically floating metal target bombarded with the ionic liquid negative ions is slightly higher (about 1 V) than that of a front electrode, indicating that the target potential can be controlled by adjusting the potential of a nearby electrode. We also investigate the application of the negative ion bombardment in secondary ion mass spectrometry. Two types of insulating materials, polytetrafluoroethylene and polyethylene glycol, are analyzed. Experimental results show that the negative ion bombardment allows one to analyze organic insulating materials by adjusting sample bias potential, without charge neutralization such as electron flooding. Results obtained show that the ionic liquid negative ion beam is a useful tool for alleviating sample charging and damage because tens of negatively charged low-energy constituent atoms hit a surface locally and simultaneously. The ionic liquid negative ion bombardment is shown to have the advantages of both negative and polyatomic ion bombardment.

Negative-ion bombardment increases during low-pressure sputtering deposition and their effects on the crystallinities and piezoelectric properties of scandium aluminum nitride films

Scandium aluminum nitride (ScAlN) films are being actively researched to explore their potential for use in bulk acoustic wave and surface acoustic wave resonators because of their good piezoelectric properties. Sputtering is commonly used in ScAlN film deposition. Unfortunately, it has been reported that film quality metrics such as the crystallinity and piezoelectric properties can deteriorate before the Sc concentration reaches 43% without an isostructural phase transition. One reason for this is bombardment with negative ions generated from carbon and oxygen impurities in the Sc ingots. Because the number of negative ions increases during low-pressure sputtering deposition, their effect on film quality may be considerable. In this study, we investigated negative-ion bombardment of the substrate during sputtering deposition and its effects on ScAlN crystallinity and piezoelectric properties. Negative-ion energy distribution measurements indicated that many more negative ions collide with the substrate during ScAlN film deposition than during AlN deposition. In addition, decreasing the sputtering pressure further increased the number of negative ions and their energies. It is well known that film quality improves at low pressures because increasing the mean free path reduces thermalization and scattering of sputtered particles. Although, AlN crystallinity and piezoelectric properties improved at low pressures, the properties of ScAlN films deteriorated dramatically. Therefore, the results indicated that ion bombardment increase at low pressure adversely effects ScAlN crystal growth, deteriorating crystallinity and piezoelectric properties. ScAlN films may be improved further by suppressing negative-ion bombardment of the substrate.

1J2-3 Effect of negative ion bombardment increased in low-pressure sputtering deposition on piezoelectric properties of ScAlN thin films

1J2-3 Effect of negative ion bombardment increased in low-pressure sputtering deposition on piezoelectric properties of ScAlN thin films

Sputter deposition of copper oxide films

Copper oxide thin films are grown by reactive magnetron sputter deposition. To define the parameter space to obtain CuO films, the influence of the oxygen partial pressure, the total pressure, and the discharge current was investigated on the phase formation. A clear change from pure copper, over cuprite (Cu2O), and paramelaconite (Cu4O3) to tenorite (CuO) thin films with increasing oxygen partial pressure was observed using X-ray diffraction and Fourier transform infrared spectroscopy. The main driving force defining the phase composition is the oxygen partial pressure, while the influence of the total pressure, and the discharge current is minimal. A clear condition to obtain phase pure CuO films could be defined based on the measured discharge voltage. Both the domain size, and the Bragg peak position for pure CuO thin films can be correlated to the negative ion bombardment during film growth.

Reduction and uniformization of the resistivity of Ga-doped ZnO by combining short-gap magnetron sputtering and buffer layer

To obtain low-resistance metal-doped ZnO film with a uniform resistivity distribution, a combination of short-gap magnetron sputtering and a buffer layer is proposed. The influence of the distance between the target and the substrate (T–S distance) on properties such as the thickness, resistivity, carrier density, carrier mobility, and crystallinity of Ga-doped ZnO thin films deposited by conventional RF planar magnetron sputtering was investigated. As the T–S distance decreased, the film resistivity remarkably decreased. Although spontaneous annealing greatly reduced the film’s resistivity, the opposing part of the target erosion area showed high resistivity, possibly due to the bombardment of oxygen negative ions. The spatial uniformity in resistivity was greatly improved using a glass substrate on which a solid-phase-crystallized thin buffer layer was formed.

Impact of negative oxygen ions on the deposition processes of RF-magnetron sputtered SrTiO3 thin films

This work reports on the deposition of strontium titanate (SrTiO3) thin films by on-axisradio-frequency magnetron sputtering. The on-axis sputter deposited SrTiO3 thin film showed drastic substrate etching due to the production of accelerated energetic negative oxygen ions, when applied power density on target is above a threshold value. The negative ion bombardment leads to the formation of surface micro-effects on the as-deposited SrTiO3 film or even complete removal of the substrate metallization. In order to understand the impact of negative ion bombardment and to limit these re-sputtering effects, the chemical composition and the thin film growth process were systematically studied as a function of deposition parameters. At high RF-power (~8 W/cm2), we observe surface etching at the center of the silicon wafer (~5.08 cm2). However, deposition of the thin film was possible in regions outside this etching zone. The key to achieve stoichiometric SrTiO3 thin film deposition at the axis-center was by controlling both deposition pressure and oxygen concentration, it minimized re-sputtering of the negative oxygen ions on the substrate surface. Polycrystalline growth of SrTiO3 thin films on platinized silicon substrates was optimized systematically and further extended to the deposition of polycrystalline SrTiO3 on MgO substrates.

Ga-doped ZnO films by magnetron sputtering at ultralow discharge voltages: Effects of defect annihilation

Preparation of high quality transparent conductive oxide (TCO) films by sputter deposition involves an intricate balance of defect generation by the highly energetic negative oxygen ions (depending on the discharge voltage) and the concomitant annihilation of these defects during film growth. Ga-doped ZnO films with a low Ga content (1.7at.%) were deposited to investigate the effects of defect annihilation on the microstructure evolutions as well as the optical and electrical properties. To achieve this aim, we prepared the GZO films by magnetron sputtering at ultralow discharge voltages (~70V) to minimize the defect generation, and varied the substrate temperature (from 130°C to 380°C) to adjust the annihilation rates. The microstructure was systematically characterized by X-ray Diffraction (XRD), Transmission Electron Microscopy (TEM), and Raman Spectroscopy. The electrical and optical properties were obtained by a Hall-effect measurement system and Spectroscopic Ellipsometry (SE), respectively. It was found that (i) even under the condition of highly controlled defect generation from the bombardment of negative oxygen ions, a sufficient annihilation of the defects cannot be realized without externally heating the substrate; and (ii) there existed a threshold temperature, above which both the structural quality and the electrical properties were improved with the increased temperature over the temperature range we examined. These results reveal that the growth temperature during the GZO film deposition has played an important role in effective annihilation of the irradiation-induced structural defects, which may be due to the higher diffusion barrier of Ga atoms in our GZO films.

Room temperature deposition of high figure of merit Al-doped zinc oxide by pulsed-direct current magnetron sputtering: Influence of energetic negative ion bombardment on film's optoelectronic properties

Aluminum-doped zinc oxide is regarded as a promising indium-free transparent conductive oxide for photovoltaic and transparent electronics. In this study high transmittance (up to 90,6%) and low resistivity (down to 8,4°1−4Ωcm) AZO films were fabricated at room temperature on thermoplastic and soda-lime glass substrates by means of pulsed-DC magnetron sputtering in argon gas. Morphological, optical and electrical film properties were characterized using scanning electron microscopy, UV–vis–nIR photo-spectrometer, X-ray spectroscopy and four probes method. Optimal deposition conditions were found to be strongly related to substrate position. The dependence of functional properties on substrate off-axis position was investigated and correlated to the angular distributions of negative ions fluxes emerging from the plasma discharge. Figure of merit as high as 2,15±0,14Ω−1 were obtained outside the negative oxygen ions confinement region. Combination of high quality AZO films deposited on flexible polymers substrates by means of a solid and scalable fabrication technique is of interest for application in cost-effective optoelectrical devices, organic photovoltaics and polymer based electronics.

Effects of the Bombardment of Negative Oxygen Ions on Gate Bias Stability of InGaZnO Thin Film Transistors

During the sputtering process, the bombardment effect of negative oxygen ions (NOIs) on the bias stability of amorphous indium gallium zinc oxide based thin film transistors (a-IGZO TFTs) was investigated. To control the bombardment energy of NOIs, a-IGZO active layer was prepared by using a superimposed rf/dc magnetron sputter. The electrical characteristics of a-IGZO TFTs under a negative bias illumination stress (NBIS) shows the different trend with that in the dark. We propose that the heavy defects such as neutral oxygen vacancy defects (Vo) as bulk defects are closely related with the bombardment energy of the negative oxygen ions.

Effects of the Bombardment of Negative Oxygen Ions on Gate Bias Stability of InGaZnO Thin Film Transistors

Amorphous InGaZnO (α-IGZO) thin-film transistors (TFTs) are very promising due to their potential use in thin film electronics and display drivers. However, the stability of AOS-TFTs under the various stresses has been issued for the practical AOSs applications. Up to now, many researchers have studied to understand the sub-gap density of states (DOS) as the root cause of instability. Nomura et al. reported that these deep defects are located in the surface layer of the α-IGZO channel. Also, Kim et al. reported that the interfacial traps can be affected by different RF-power during RF magnetron sputtering process. It is well known that these defect states can influence on the performances and stabilities of α-IGZO TFTs. Nevertheless, it has not been reported how these defects are created during conventional RF magnetron sputtering. In general, during conventional RF magnetron sputtering process, negative oxygen ions (NOI) can be generated by electron attachment in oxygen atom near target surface and accelerated up to few hundreds eV by a self-bias of RF magnetron sputter; the high energy bombardment of NOIs generates bulk defects in oxide thin films and can create the defects of α-IGZO thin films. The a-IGZO TFTs have been fabricated with different dc biases (200 V, 300V, and 400V). Figure 1 shows that the transfer characteristics of the a-IGZO TFTs with a positive bias stress (PBS in (a)), a negative bias stress (NBS in (b)), and a negative bias illumination stress (NBIS in (c)). It is observed that the changes of the transfer characteristics with PBS, and NBS are not depended on different dc biases. However, a huge shift of a threshold voltage on a-IGZO TFTs is observed under NIBS condition. In this study, we have investigated that the NOIs accelerated by dc self-bias were one of the dominant causes of instability in α-IGZO TFTs when the channel layer was deposited by conventional RF magnetron sputtering system. Finally, we will introduce our novel technology named as Magnetic Field Shielded Sputtering (MFSS) process to prevent the NOIs bombardment effects and present how much to be improved the instability of α-IGZO TFTs by this new deposition method.

Effects of bombardment of negative oxygen ions on amorphous indium gallium zinc oxide thin films by superimposed rf/dc magnetron sputtering

During the sputtering process, the bombardment effect of negative oxygen ions (NOIs) on the thin-film properties of amorphous indium gallium zinc oxide (a-IGZO) was investigated. The energies of NOIs determined by in situ ion energy analyses are in good agreement with the values obtained using the Meyer equation. Furthermore, we find that the energies of NOIs are sufficiently high to affect the lattice distortion and/or displacement in a-IGZO films. The overall analysis revealed that the bombardment of NOIs has a considerable effect on a-IGZO film properties. Our work demonstrates that the bombardment of NOIs is a crucial issue for achieving high-quality a-IGZO films.

The correlation between the radial distribution of high-energetic ions and the structural as well as electrical properties of magnetron sputtered ZnO:Al films

The origin of the pronounced radial distributions of structural and electrical properties of magnetron sputtered ZnO:Al films has been investigated. The film properties were correlated with the radially resolved ion-distribution functions. While the positive ions exhibit low energies and a radial distribution with a maximum intensity opposite the center of the target, the negative ions can have energies up to several hundred eV, depending on the target potential, with a radial distribution with two maxima opposite the erosion tracks. The most prominent positive ion is that of the working gas (Ar+), while the highest flux of the negative ions is measured for negative oxygen O−. The radial distribution of the flux of the high-energetic negative ions can clearly be related to the radial variations of the structural (c-axis lattice parameter, crystallite size) and electronic (resistivity) properties for sputtering from the planar target, which points to the decisive role of the high-energetic negative oxygen ions for the film quality. The relation between the negative ion bombardment and the structural as well as electronic properties can be explained by a qualitative model recently developed by us. The same target has also been investigated in the eroded state. In this case, the limited acceptance angle of the mass spectrometer leads to a misinterpretation of the radial distribution of the flux of the high-energetic negative ions. This effect can be explained by a simulation, based on the assumption that the high-energetic negative ions are mainly accelerated in the cathode (target) sheath perpendicular to the uneven substrate surface.

Controlled deposition of sulphur-containing semiconductor and dielectric nano-structured films on metals in SF6 ion-ion plasma

In the present paper, the deposition processes and formation of films in SF6 ion-ion plasma, with positive and negative ion flows accelerated to the surface, are investigated. The PEGASES (acronym for Plasma Propulsion with Electronegative GASES) source is used as an ion-ion plasma source capable of generating almost ideal ion-ion plasma with negative ion to electron density ratio more than 2500. It is shown that film deposition in SF6 ion-ion plasma is very sensitive to the polarity of the incoming ions. The effect is observed for Cu, W, and Pt materials. The films formed on Cu electrodes during negative and positive ion assisted deposition were analyzed. Scanning electron microscope analysis has shown that both positive and negative ion fluxes influence the copper surface and leads to film formation, but with different structures of the surface: the low-energy positive ion bombardment causes the formation of a nano-pored film transparent for ions, while the negative ion bombardment leads to a continuous smooth insulating film. The transversal size of the pores in the porous film varies in the range 50–500 nm, and further analysis of the film has shown that the film forms a diode together with the substrate preventing positive charge drain, and positive ions are neutralized by passing through the nano-pores. The film obtained with the negative ion bombardment has an insulating surface, but probably with a multi-layer structure: destroying the top surface layer allows to measure similar “diode” IV-characteristics as for the nano-pored film case. Basing on results, practical conclusions for the probes and electrodes cleaning in ion-ion SF6 plasmas have been made. Different applications are proposed for the discovered features of the controlled deposition from ion-ion plasmas, from Li-sulphur rechargeable batteries manufacturing and nanofluidics issues to the applications for microelectronics, including low-k materials formation.

Comparison of ion energies and fluxes at the substrate during magnetron sputtering of ZnO : Al for dc and rf discharges

Energy spectra of positive and negative ions have been measured in magnetron sputtering of a ZnO : Al target. The discharges have been operated in dc or rf, the latter with a frequency of 13.56 or 27.12 MHz, at the same geometry, argon pressure, and discharge power. While the average energy of the positive ions against a grounded surface increases with increasing frequency due to increased plasma potential, the average energy of the negative ions strongly decreases due to a target voltage decrease. The flux of negative ions simultaneously decreases, whereas that of the positive ions slightly increases. Combining these quantities to the energy flux onto floating substrates shows a drastic decrease of the energy flux with increasing frequency for high-energetic negative ion bombardment of the films while the low-energetic positive ion impingement is weakly increased. From the point of the energetic bombardment rf discharges are hence favoured to obtain high-quality films. The energy distributions of negative ions have a sharp peak in the dc mode but broad distributions in the rf discharges. Additionally, the latter are characterized by an overlaid peak structure that is explained by the oscillation of target and plasma potential.

The impact of negative oxygen ion bombardment on electronic and structural properties of magnetron sputtered ZnO:Al films

In order to study the impact of negative oxygen ion bombardment on the electronic transport properties of ZnO:Al films, a systematic magnetron sputtering study from ceramic targets with excitation frequencies from DC to 27 MHz, accompanied by strongly varying discharge voltages, has been performed. Higher plasma excitation frequencies significantly improve the transport properties of ZnO:Al films. The effect of the bombardment of the films by energetic particles (negative oxygen ions) can be explained by the dynamic equilibrium between the formation of acceptor-like oxygen interstitials compensating the extrinsic donors and the self-annealing of the interstitial defects at higher deposition temperatures.

Unusual growth of polycrystalline oxide film induced by negative ion bombardment in the capacitively coupled plasma deposition

A polycrystalline film usually grows in its most densely packed plane parallel to the substrate plane. We demonstrated that the unusual crystalline growth can occur by using energetic negative ions generated in the magnetron capacitively coupled plasma deposition without using separated ion source. Negative ion energy and flux entering the substrate were quantitatively measured and compared with the preferential crystalline growth of unusual (112¯0) orientation in ZnO films. Strong (112¯0) orientation was found at the cathode erosion area where large amount of high energy negative ion of 170–250 eV was observed in low gas pressure of 0.1 Pa.

The influences of high energetic oxygen negative ions and active oxygen on the microstructure and electrical properties of ZnO:Al films by MF magnetron sputtering

In this paper, ZnO:Al transparent conducting films were prepared on glass substrate by magnetron sputtering from Al doped ZnO ceramic targets. By measuring and analyzing the structure and electrical properties of films in front of targets at different target-to-substrate distance, it was concluded that the bombardment of energetic oxygen negative ions decreased with increasing target-to-substrate distance, dominating variation of resistivity and the microstructure in erosion area, while numbers of active oxygen decrease with increasing target-to-substrate distance, explaining variation of resistivity in non-erosion area. The influence of target-to-substrate distance on electrical and microstructure properties of ZnO:Al films on drum was also investigated in order to confirming our result. The result indicated that both energetic oxygen negative ions and numbers of active oxygen determined the properties of films on drum. While target-to-substrate distance is less than 95mm, the numbers of energetic oxygen ions are the key factor and vice versa. The optimum resistivity of post-annealed films on drum was 5.1×10−4Ωcm at target-to-substrate distance of 95mm.

Reactive magnetron sputtering of transparent conductive oxide thin films: Role of energetic particle (ion) bombardment

Abstract

The mechanisms of negative oxygen ion formation from Al-doped ZnO target and the improvements in electrical and optical properties of thin films using off-axis dc magnetron sputtering at low temperature

Transparent conducting aluminum-doped zinc oxide (AZO) films have been prepared on glass substrates by dc magnetron sputtering using ceramic ZnO with 2 wt% Al2O3 target. The mechanism of negative oxygen ion generation on an AZO target surface and its influence on the conductivity of films were discussed. The negative ion generation on an AZO target was contributed by the surface ionization leading to the spot emission from Al atoms adsorbed on the AZO target surface. The contribution of negative ions' current was mainly from the erosion area of the target due to its higher temperature. To reduce the damage caused by negative ion bombardment to film growth, an off-axis sputtering system was proposed, where the substrates were placed perpendicular to the target. The effects of distance (d) on the electrical properties of films were experimentally verified in detail. A low resistivity of 3.7 × 10−4 Ω cm, an average transmittance above 85% in the visible range (300–800 nm) and reflectance higher than 85% in the infrared range (2500–4000 nm) were obtained for the films deposited at d = 2.5 cm. The overall analysis revealed that the generation of negative ions on the AZO target has a great influence on film growth, especially in the ultra-low pressure deposition process. Our work demonstrates the feasibility of reducing the negative effects of ion bombardment on the quality of films, which would be of great merit for industrial applications.

Batch production of large-area double-sidedYBa2Cu3O7−δ thin films by DC magnetron sputtering

Using a novel DC magnetron sputtering system, high qualityYBa2Cu3O7−δ (YBCO) thin films were deposited on both sides of twelve pieces of 3-inchLaAlO3(00l) single crystal substrates in one batch. This sputtering system has a rotatable disc sample holder withthe LaAlO3 substrates being positioned in the middle of upper and lower YBCO targets. These twosputter guns are arranged to simultaneously deposit YBCO thin films on both sides of thesubstrate. The rotatable sample holder is an important approach to large scale depositionof high quality YBCO thin films. In this manner, the sputtered species can diffuse wellaround the substrates during one rotation cycle. Furthermore, the anode is installed in theinner of rotatable disc sample holder to avoid negative ion bombardment. Thus, a gooduniformity over the whole area for all the films has been realized. The full-width athalf-maximum (FWHM) values of the (005) rocking curves typically range from0.49° to0.56°. The superconducting critical temperatures fall in the range of89–90 K and the critical current densities are between 3.75 and4.0 MA cm−2 at 77 K. The microwavesurface resistance, RS (77 K, 10 GHz), of the YBCO thin films range typically from 0.56 to0.71 mΩ, which make these films suitable for microwave applications.