Positive Ion Bombardment Research Articles

Insights on film growth conditions on a floating substrate during reactive Ar/O2 bipolar high power impulse magnetron sputter deposition

We report several effects found in reactive bipolar high-power impulse magnetron sputtering (abbreviated as bipolar HiPIMS or BPH) discharges at the substrate level. The role of the negative peak current and the positive pulse voltage, U, on the deposition flux at the electrically insulating substrate is discussed and the results are compared to conventional DC discharges. In particular, the normalized energy flux, ϕ n , delivered to the substrate is found higher under BPH conditions at U = +300 V ( eV·nm−3) as compared to the HiPIMS ( eV·nm−3) and DCMS ( eV·nm−3) regimes which is assumed to be provoked by an intensification of the positive ion bombardment. Accordingly, the mean energy and the count of positively charged metal and gaseous ions increase in line with U as measured by energy-resolved mass spectrometry. Contrariwise, our analysis has shown a significant decrease (≈20 %) of negative oxygen ion count under BPH conditions.

Bipolar HiPIMS: The role of capacitive coupling in achieving ion bombardment during growth of dielectric thin films

Abstract Bipolar high-power impulse magnetron sputtering (HiPIMS) is used to achieve ion acceleration for ion bombardment of dielectric thin films. This is realized by increasing the plasma potential (Up), during the interval in-between the HiPIMS-pulses, using a positive reversed voltage (Urev). As long as the film surface potential (Us) is maintained low, close to ground potential, this increase in Up results in ion-acceleration as ions approach the film surface. The effect of Urev on the ion bombardment is demonstrated by the growth of dielectric (Al,Cr)2O3 films on two sets of substrates, Si (001) and sapphire (0001) utilizing a Urev ranging from 0 to 300 V. A clear ion bombardment effect is detected in films grown on the conductive Si substrate, while no, or a very small, effect is observed in films grown on the dielectric sapphire substrate. This is ascribed to the changes in Us when the substrate is subjected to the bombardment of positive ions. For a film surface that has a high capacitance to ground, Us remains close to ground potential for an extended time in-between the HiPIMS pulses, while if the capacitance is low, Us quickly attains floating potential (Ufloat) close to Up. The simulated temporal evolutions of Us for the films by using capacitors show that for a 1 μm thick (Al,Cr)2O3 film on a conductive substrate, Us is maintained close to ground potential during the entire 20 μs that Urev is applied after the HiPIMS pulse. On the other hand, when a capacitance corresponding to the 0.5 mm thick sapphire substrate is used, Us rapidly attains a potential close to Urev.

Pulsed DC bias for the study of negative-ion production on surfaces of insulating materials in low pressure hydrogen plasmas

In this work, negative-ion production on the surface of a sample negatively DC biased in a hydrogen plasma is studied. The negative ions created under positive ion bombardment are accelerated toward the plasma, self-extracted, and detected according to their energy and mass, by a mass spectrometer placed in front of the sample. The use of a pulsed bias allows applying a quasi-DC bias on insulating material during a short period of time and offers the possibility to extend the measurement method to nonconductive samples. The pulsed-bias tests were performed first with highly oriented pyrolytic graphite (HOPG), a conductive material, to demonstrate the feasibility of the method. By changing the pulsed-bias frequency, it was possible to obtain HOPG material with different hydrogen surface coverages and hence different surface states leading to an increase of negative-ion production by up to 30%–50% as compared to the continuous bias case. To establish a protocol for insulating materials, charge accumulation on the surface during the bias pulse and influence of the bias duration and frequency were explored using microcrystalline diamond (MCD) thin layers. By using a pulse short enough (10 μs) at 1 kHz frequency, it has been possible to measure negative-ions on the MCD sample at a quasi-constant surface bias of 130 V, with only 1 V variation during the measurement. Negative-ion surface production on MCD has been studied in pulsed mode with surface temperature from room temperature to 800 °C. It is shown that pulsing the bias and increasing the temperature allows limiting defect creation on MCD which is favorable for negative-ion production. Consequently, at 400 °C, the yield on MCD in pulsed mode is one order of magnitude higher than the yield on HOPG in continuous mode at room temperature.

Experimental visualization of the cathode layer in AC surface dielectric barrier discharge

A narrow etched polyimide line at the bottom edge of a biased electrode (BE) and a non-etched dielectric surface near the biased electrode were observed in an atmospheric AC flexible surface dielectric barrier discharge of polyimide dielectric. These findings are attributed to the bombardment of positive oxygen ions on the bottom edge of the BE and the electron breakdown trajectory not contacting the polyimide surface following the electric field lines formed between the BE edge and the surface charge layer on the dielectric. The length of the non-etched dielectric surface during the first micro-discharge was observed as 22 μm. This occurred, regardless of three different operating durations, which is in good agreement with the length of the cathode layer according to Paschen's law.

Carboxyl-Functionalized Nanoparticles Produced by Pulsed Plasma Polymerization of Acrylic Acid.

Carboxyl-enriched and size-selected polymer nanoparticles (NPs) may prove to be very useful in biomedical applications for linker-free binding of biomolecules and their transport to cells. In this study, we report about the synthesis of such NPs by low-pressure low-temperature pulsed plasma polymerization of acrylic acid. Gas aggregation cluster source was adapted to operate plasma with a constant pulse period of 50 μs and with varying duty cycle. The NPs were produced with the size ranging from 31 ± 5 to 93 ± 14 nm and with retention of the carboxyl groups ranging from 4.0 to 12.0 atom %. Two regimes of the NP formation were identified. In the large duty cycle regime, the NP growth was interfered with by positive ion bombardment which resulted in the ion-driven detachment of the carboxyl species and in the formation of carboxyl-deficient NPs. In the small duty cycle regime, the NP growth was accompanied by the radical-driven chain propagation with the attachment of intact monomer molecules. Improved efficacy of the monomer retention resulted in increased concentration of the carboxyl groups.

Negative-ion surface production in hydrogen plasmas: Determination of the negative-ion energy and angle distribution function using mass spectrometry

This work focuses on the understanding of the production mechanism of negative-ions on surface in low pressure plasmas of H2/D2. The negative ions are produced on a Highly Oriented Pyrolytic Graphite sample negatively biased with respect to plasma potential. The negative ions created under the positive ion bombardment are accelerated towards the plasma, self-extracted, and detected according to their energy and mass by a mass spectrometer placed in front of the sample. The shape of the measured Negative-Ion Energy Distribution Function (NIEDF) strongly differs from the NIEDF of the ions emitted by the sample because of the limited acceptance angle of the mass spectrometer. To get information on the production mechanisms, we propose a method to obtain the distribution functions in energy and angle (NIEADFs) of the negative-ions emitted by the sample. It is based on an a priori determination of the NIEADF and on an a posteriori validation of the choice by comparison of the modelled and experimental NIEDFs.

Optimum plasma grid bias for a negative hydrogen ion source operation with Cs.

The functions of a biased plasma grid of a negative hydrogen (H(-)) ion source for both pure volume and Cs seeded operations are reexamined. Proper control of the plasma grid bias in pure volume sources yields: enhancement of the extracted negative ion current, reduction of the co-extracted electron current, flattening of the spatial distribution of plasma potential across the filter magnetic field, change in recycling from hydrogen atomic/molecular ions to atomic/molecular neutrals, and enhanced concentration of H(-) ions near the plasma grid. These functions are maintained in the sources seeded with Cs with additional direct emission of negative ions under positive ion and neutral hydrogen bombardment onto the plasma electrode.

The behaviour of negative oxygen ions in the afterglow of a reactive HiPIMS discharge

Using a single Langmuir probe, the temporal evolution of the oxygen negative ion, n−, and electron, ne, densities in the afterglow of a reactive HiPIMS discharge operating in argon–oxygen gas mixtures have been determined. The magnetron was equipped with a titanium target and operated in ‘poisoned’ mode at a frequency of 100 Hz with a pulse width of 100 µs for a range of oxygen partial pressures,.In the initial afterglow, the density of the principle negative ion in the discharge (O−) was of the order of 1016 m−3 for all conditions. The O− concentration was found to decay slowly with characteristic decay times between 585 µs and 1.2 ms over the oxygen partial pressure range. Electron densities were observed to fall more rapidly, resulting in long-lived highly electronegative afterglow plasmas where the ratio, α = n−/ne, was found to reach values up to 672 (±100) for the highest O2 partial pressure. By comparing results to a simple plasma-chemical model, we speculate that with increased ratio, more O− ions are formed in the afterglow via dissociative electron attachment to highly excited metastable oxygen molecules, with the latter being formed during the active phase of the discharge. After approximately 2.5 ms into the off-time, the afterglow degenerates into an ion–ion plasma and negative ions are free to impinge upon the chamber walls and grounded substrates with flux densities of the order of 1018 m−2 s−1, which is around 10% of the positive ion flux measured during the on-time. This illustrates the potential importance of the long afterglow in reactive HiPIMS, which can act as a steady source of low energy O− ions to a growing thin film at the substrate during periods of reduced positive ion bombardment.

Negative-ion production on carbon materials in hydrogen plasma: influence of the carbon hybridization state and the hydrogen content on H− yield

Highly oriented polycrystalline graphite (HOPG), boron-doped diamond (BDD), nanocrystalline diamond, ultra-nanocrystalline diamond and diamond-like carbon surfaces are exposed to low-pressure hydrogen plasma in a 13.56 MHz plasma reactor. Relative yields of surface-produced H− ions due to bombardment of positive ions from the plasma are measured by an energy analyser cum quadrupole mass spectrometer. Irrespective of plasma conditions (0.2 and 2 Pa), HOPG surfaces show the highest yield at room temperature (RT), while at high temperature (HT), the highest yield (∼3–5 times compared to HOPG surface at RT) is observed on BDD surfaces. The shapes of ion distribution functions are compared at RT and HT to demonstrate the mechanism of ion generation at the surface. Raman spectroscopy analyses of the plasma-exposed samples reveal surface modifications influencing H− production yields, while further analyses strongly suggest that the hydrogen content of the material and the sp3/sp2 ratio are the key parameters in driving the surface ionization efficiency of carbon materials under the chosen plasma conditions.

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.

Emission of Negative Ions From Surfaces With High Emissivity

Spectra of the negative ion fraction in the low field emission from MPG-8 graphite with rough surface and tungsten surface with nano fuzz as a function of the residual gas composition and pressure are measured and analyzed. The intensity of the emitted negative ions increases linearly with the pressure of the working gas in the range of <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">$10^{-5}$</tex></formula> to <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$10^{-7}~{\rm Torr}$</tex></formula> and depends on the atomic mass of the residual gas. A simple model based on the secondary negative ion emission of adsorbed particles under positive ion bombardment of the cathode is proposed.

Negative-ion surface production in hydrogen plasmas: modeling of negative-ion energy distribution functions and comparison with experiments

A negatively biased graphite sample (highly oriented pyrolitic graphite) was placed in a H2 low-pressure plasma. The negative ions were produced on the graphite surface upon positive-ion bombardment. Surface-produced H− negative-ion distribution functions (NIDFs) were measured by means of an energy-resolved mass spectrometer. The shapes of the recorded NIDFs depend not only on the surface production mechanisms but also on the negative-ion trajectories in the plasma and their collection probability by the mass spectrometer. In order to gain an insight into the surface production mechanisms, NIDFs were computed using Stopping and Range of Ions in Matter simulations and calculations of the ion transmission function between the sample and the mass spectrometer detector. The calculations were based on 3D modeling of the sheath potential in the extraction region and 3D modeling of ion transport inside the mass spectrometer. The excellent agreement between experiments and computations led to a better understanding of the experimental NIDFs. The method developed in this work to study H-surface production on graphite can be generalized to any other negative ions and/or surface material.

High-performance gas sensors with temperature measurement

There are a number of gas ionization sensors using carbon nanotubes as cathode or anode. Unfortunately, their applications are greatly limited by their multi-valued sensitivity, one output value corresponding to several measured concentration values. Here we describe a triple-electrode structure featuring two electric fields with opposite directions, which enable us to overcome the multi-valued sensitivity problem at 1 atm in a wide range of gas concentrations. We used a carbon nanotube array as the first electrode, and the two electric fields between the upper and the lower interelectrode gaps were designed to extract positive ions generated in the upper gap, hence significantly reduced positive ion bombardment on the nanotube electrode, which allowed us to maintain a high electric field near the nanotube tips, leading to a single-valued sensitivity and a long nanotube life. We have demonstrated detection of various gases and simultaneously monitoring temperature, and a potential for applications.

Simultaneous Growth of Nano and Submicrometer Sized ZnO Islands on Si and the Surface Electrical Properties

Simultaneous growth of two types of ZnO islands with different sizes has been observed on silicon substrate using radio frequency (rf) magnetron sputtering under an rf power of at substrate bias. Depositions were carried out using an (80:20) and gas mixture environment. The islands of the first type are well faceted, truncated pyramidal shaped ZnO islands on the Si surface with average base diameter and height . The other type is cone shaped ZnO islands with hemispherical base (upside down) which are embedded into the Si substrate. The size and height of the embedded ZnO cone shaped islands have been found to be in a range of and , respectively. These islands result from the bombardment of positive Ar ions into a negatively biased Si substrate. Morphology, shape, size, and microstructure have been investigated by transmission electron microscopy. Surface electrical properties of the bigger faceted ZnO islands have been measured by conductive atomic force microscopy. Enhanced current conduction has been found at the edges of the truncated pyramidal islands. This indicates that the nonpolar facets of and (10-11) are more electrically active than the polar facet of (0002).

Modeling of Silicon Carbide ECR Etching by Feed-Forward Neural Network and Its Physical Interpretations

The behavior of electron cyclotron resonance etching of 4H-SiC based on SF6 + O2 plasma is studied using a three-layered feed-forward neural network model trained by the Broyden, Fletcher, Goldfarb, and Shanno optimization algorithm. The etch rate of 4H-SiC is modeled as a function of microwave power, dc bias, process pressure, and O2/(SF6 + O2) flow ratio referred to as the O2 fraction. The results clearly reveal the interaction of the various process parameters and their resultant effects on the etch rate. It is found that, by varying the O2 fraction and process pressure, optimized etch rate peaks can be achieved. Increasing dc bias and microwave power is found to result in the optimized etch rate peaks occurring at higher O2 fraction and process pressure, respectively. In addition, increasing dc bias or microwave power will increase the etch rate. However, there is saturation in the etch rate at higher dc bias, which does not occur with microwave power. Based on these modeling results, detailed physical interpretations of the etching process are given in terms of the chemical reaction of 4H-SiC with F, O, positive ion bombardment, etc.

N 2 effect on GaAs etching at 150 mTorr capacitively-coupled Cl 2/N 2 plasma

The role of N 2 on GaAs etching at 150 mTorr capacitively-coupled Cl 2/N 2 plasma is reported. A catalytic effect of N 2 was found at 20–25% N 2 composition in the Cl 2/N 2 discharges. The peak intensities of the Cl 2/N 2 plasma were monitored with optical emission spectroscopy (OES). Both atomic Cl (725.66 nm) and atomic N (367.05 nm) were detected during the Cl 2/N 2 plasma etching. With the etch rate and OES results, we developed a simple model in order to explain the etch mechanism of GaAs in the high pressure capacitively-coupled Cl 2/N 2 plasma as a function of N 2 ratio. If the plasma chemistry condition became positive ion-deficient at low % N 2 or reactive chlorine-deficient at high % N 2 in the Cl 2/N 2 plasma, the GaAs etch rate is reduced. However, if the plasma had a more balanced ratio of Cl 2/N 2 (i.e. 20–25% N 2) in the plasma, much higher etch rates (up to 150 nm/min) than that in pure Cl 2 (50 nm/min) were produced due to synergetic effect of neutral chlorine adsorption and reaction, and positive ion bombardment. Pure Cl 2 etching produced 14 nm of RMS surface roughness of GaAs. Introduction of ⩾20% N 2 gas in Cl 2/N 2 discharges significantly reduced the surface roughness to 2–4 nm. SEM photos showed that the morphology of photoresist mask was strongly degraded. Etch rate of GaAs slightly increased from 10 to 40 nm/min when RIE chuck power changed from 10 to 150 W at 12 sccm Cl 2/8 sccm N 2 plasma condition. The surface roughness of GaAs etched at 12 sccm Cl 2/8 sccm N 2 plasma was 2–3 nm.

H− production on a graphite surface in a hydrogen plasma

We measure H− negative ions by means of a mass spectrometer in a helicon plasma reactor. The H2 plasma operates at a low injected RF power (50–300 W), in a capacitive regime, under low pressure conditions (between 0.4 and 1 Pa). A highly oriented pyrolytic graphite (HOPG) graphite sample centred in the expanding chamber and facing the mass spectrometer nozzle placed 40 mm away is negatively biased. Negative ions formed on the graphite surface upon positive ion bombardment are detected according to their energy by the mass spectrometer. We obtain the H− ion distribution function (IDF) showing two main features: first, a high energy tail attributed to negative ions created via two-electron capture following and impact on the HOPG sample and, second, a main peak which can be attributed to negative ions created on the surface by the sputtering of adsorbed hydrogen and/or two-electron capture. Finally, we show that negative ion production is proportional to the positive ion flux and strongly depends on the positive ion energy.

The statistical inception mechanism of the first corona in positive nonuniform air gaps ranging from 0.3 to 1.0 cm

AbstractThe aim of this paper is to understand the statistical inception mechanism of the firsYasujina in positive nonuniform air gaps ranging from 0.3 to 1.0 cm. We calculated first how the spatial distributions of negative ion density depend on the gap length, UV irradiation, and the concentration of water vapor using the steady‐state continuity equation for ion flow states. Next the cumulative probability of the first corona inception as a function of the statistical time lags was evaluated using the critical volume model in which initial electrons are provided by collisional detachment of negative ions. Furthermore, we also measured the cumulative probability of the corona inception under similar experimental conditions. From a comparison between the numerical and experimental results and their discussions, it was found that in the short range of the gap length, negative ions produced near the cathode associated with positive ion bombardment play an important role in the statistical inception mechanism of the first corona. Also, a much higher probability of corona inception in the case of UV irradiation was attributed to the number of negative ions produced near the cathode through the photo emission process. © 2006 Wiley Periodicals, Inc. Electr Eng Jpn, 158(2): 1–8, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20445

Carbon-13 Labeled Polymers: An Alternative Tracer for Depth Profiling of Polymer Films and Multilayers Using Secondary Ion Mass Spectrometry

13C labeling is introduced as a tracer for depth profiling of polymer films and multilayers using secondary ion mass spectrometry (SIMS). Deuterium substitution has traditionally been used in depth profiling of polymers but can affect the phase behavior of the polymer constituents with reported changes in both bulk-phase behavior and surface and interfacial interactions. SIMS can provide contrast by examining various functional groups, chemical moieties, or isotopic labels. 13C-Labeled PS (13C-PS) and unlabeled PS (12C-PS) and PMMA were synthesized using atom-transfer radical polymerization and assembled in several model thin-film systems. Depth profiles were recorded using a Cameca IMS-6f magnetic sector mass spectrometer using both 6.0-keV impact energy Cs+ and 5.5-keV impact energy O2+ primary ion bombardment with detection of negative and positive secondary ions, respectively. Although complete separation of 12C1H from 13C is achieved using both primary ion species, 6.0-keV Cs+ clearly shows improved detection sensitivity and signal-to-noise ratio for detection of 12C, 12C1H, and 13C secondary ions. The use of Cs+ primary ion bombardment results in somewhat anomalous, nonmonotonic changes in the 12C, 12C1H, and 13C secondary ion yields through the PS/PMMA interface; however, it is shown that this behavior is not due to sample charging. Through normalization of the 13C secondary ion yield to the total C (12C + 13C) ion yield, the observed effects through the PS/PMMA interface can be greatly minimized, thereby significantly improving analysis of polymer films and multilayers using SIMS. Mass spectra of 13C-PS and 12C-PS were also analyzed using a PHI TRIFT I time-of-flight mass spectrometer, with 15-keV Ga+ primary ion bombardment and detection of positive secondary ions. The (12)C7(1)H7 ion fragment and its 13C-enriched analogues have significant secondary ion yields with negligible mass interferences, providing an early indication of the potential for future use of this technique for cluster probe depth profiling of high molecular weight 13C-labeled fragments.

Relationship between X-ray Intensity and Electric Bias on Al&lt;SUB&gt;2&lt;/SUB&gt;O&lt;SUB&gt;3&lt;/SUB&gt; Surface during Low Energy Ga&lt;SUP&gt;+&lt;/SUP&gt; Irradiation

Low-energy characteristic X-ray emission is detected during bombardment of positive low energy ions onto insulator materials. The phenomenon is considered to be related to surface charge-up. To study further the mechanism, the characteristic X-rays was studied during 30keV Ga + ions bombardment onto Al 2 O 3 monocrystalline specimens applied with a direct current (DC) bias in the present work. The applied DC voltage builds an electric field parallel to the surface of the specimen. The results show that the characteristic X-rays of O-Kα and Al-Ka increased with the increasing of the applied DC voltages.