Control Of Ion Energy Research Articles

Optimizing HiPIMS pressure for deposition of high-k (k = 18.3) amorphous HfO2

Stoichiometric amorphous HfO2 films have been deposited by reactive High Power Impulse Magnetron Sputtering (HiPIMS) from a Hf target in a 1:1 Ar:O2 atmosphere at pressures 2–4.5mTorr. An optimum pressure was found for depositing smooth, high refractive index and amorphous films. Stress and refractive index reached a maximum as deposition pressure was increased to 3.5mTorr. At 3.5mTorr, HfO2 films were deposited with a refractive index of 2.15 at 500nm, low leakage currents, moderate fixed charge density and a high dielectric constant of ∼18.3. The intensification of energetic ion bombardment upon the film with increase in HiPIMS pressure plays a dominant role in film properties. Increase in pressure above the optimum relieved the stress in the films and degraded the optical and electrical properties. HiPIMS pressure enables to gain indirect control of ion flux and energy in the plasma and can be used to modify the properties of depositing films.

Evolution of electron energy distribution function in capacitively coupled argon plasma driven by very high frequency

Capacitively coupled plasma driven by a very high frequency power has attracted much attention due to its rather independent control of ion flux and energy. In this paper, Langmuir probe diagnostic technique is used to observe the evolution of plasma properties such as electron energy distribution function, electron temperature and density, etc. Our experiment is performed in capacitively coupled argon plasma driven by a 40.68 MHz frequency. Experimental results show that the electron energy probability function changes from bi-Maxwellian type to single-Maxwellian type and then to Druyvesteyn type with the increase of the discharge pressure. At a low gas pressure, the electron collisionless heating in bulk plasma leads to bi-Maxwellian type in electron energy possibility function (EEPF), which has a double temperatures structure in EEPF. As the gas pressure increases, the electrons with low energy are able to collide with the neutral species more frequently, thus they gain energies through collisional heating. Therefore, these electrons can overcome the dc ambipolar potential and the collisional heating becomes a main electron heating mechanism. Increasing the input power enhances the electron population with low energy. From the discharge center to the edge, electron population with low energy decreases clearly due to the dc ambipolar potential, and they are unable to reach an oscillating sheath where collisionless heating occurs. However, electron population with high energy is slightly increased. The result indicates that more uniform plasma can be achieved at a high gas pressure. Additionally, EEPFs are measured for different discharge gaps between electrodes. The change of electrode gap for the plasma leads to a transition of electron heating mode along the axial direction. In order to characterize the electron behavior further, we introduce the ratio of the cold electron density to hot electron density (α) and the ratio of cold electron temperature to hot electron temperature (β). The ratios also show the proportional distributions of the cold and hot electron populations. The electrode gap has a great influences on α while little influence on β. When the discharge gap between electrodes varies from 20 to 40 mm, α changes from 0.2 to 0.5 while β has the same trend. Spatial distributions of electron density and temperature with low and high energy are also discussed.

Dry development for a directed self-assembly lithography hole-shrink process using CO / H 2 plasma

A dry development, directed self-assembly lithography (DSAL) hole-shrink process was studied, with a focus on the selectivity of the etching of poly(methyl methacrylate) (PMMA) over polystyrene (PS), and the suppression of etch stop. The highly selective etching of PMMA over PS was achieved using CO gas chemistry. However, it was found that the PMMA etching did not proceed beyond a certain depth. Scanning transmission electron microscopy and x-ray photoelectron spectroscopy analysis indicated that a deposition layer formed not only on the PS but also on the PMMA. The addition of H2 to the CO plasma was effective in controlling the thickness of the deposition layer and in suppressing the etch stop. The CO/H2 plasma process was combined with ion energy control to achieve a dry development for hole shrinkage. The dry development DSAL hole-shrink process was successfully realized by tailoring the etching gas chemistry and controlling the ion energy.

Studies on the effect of finite geometrical asymmetry in dual capacitively coupled radio frequency plasma

In recent years, dual capacitively coupled radio frequency (CCRF) glow discharge plasma has been widely studied in the laboratory because of its simpler design and high efficiency for different material processing applications such as thin-film deposition, plasma etching, sputtering of insulating materials etc. The main objective of studies on dual frequency CCRF plasma has been the independent control of ion energy and ion flux using an electrical asymmetry effect (EAE). Most studies have been reported in electrode configurations that are either geometrically symmetric (both electrodes are equal) or completely asymmetric (one electrode is infinitely bigger than the other). However, it seems that most of the laboratory CCRF plasmas have finite electrode geometry. In addition, plasma series resonance (PSR) and electron bounce resonance (EBR) heating also come into play as a result of geometrical asymmetry as well as EAE. In this study, a dual frequency CCRF plasma has been studied in which the dual frequency CCRF has been coupled to the lumped circuit model of the plasma and the time-independent fluid model of the plasma sheath, in order to study the effect of finite geometrical asymmetry on the generation of dc-self bias and plasma heating. The dc self-bias is found to strongly depend on the ratio of the area between the electrodes. The dc self-bias is found to depend on the phase angle between the two applied voltage waveforms. The EAE and geometrical asymmetry are found to work differently in controlling the dc self-bias. It can be concluded that the phase angle between the two voltage waveforms in dual CCRF plasmas has an important role in determining the dc self-bias and may be used for controlling the plasma properties in the dual frequency CCRF plasma.

Calculation of Effective Molecules Collision Frequency and Cross-Sectional Ratio for Electromagnetic Discharge in Various Gases Mixtures

Estimation of gases quantities in gases mixture in plasma were accomplished at 300K. Each mixture has an effective value of the molecular collision frequency, electro-ions collision frequency and collision cross-sectional ratio. The computational and experimental results are correlated. In this work, the concept of separate control ion energy and ion flux by the two frequencies works generally well for the investigated discharge setup. The high frequency mainly determines ion flux and radical formation in the mixture discharges while ion energies are strongly influenced by the low frequency. Deviations from an ideally separated control of ion flux and energy are caused by influences on discharge parameters and interaction between the two frequencies since the radio frequency (RF) power sources are coupled to each other in the current setup.

Control of ion energy and angular distributions in dual-frequency capacitively coupled plasmas through power ratios and phase: Consequences on etch profiles

Anisotropic etching, enabled by energetic ion bombardment, is one of the primary roles of plasma–assisted materials processing for microelectronics fabrication. One challenge in plasma etching is being able to control the ion energy-angular distributions (IEADs) from the presheath to the surface of the wafer which is necessary for maintaining the critical dimension of features. Dual frequency capacitive coupled plasmas (DF-CCPs) potentially provide flexible control of IEADs, providing high selectivity while etching different materials and improved uniformity across the wafer. In this paper, the authors present a computational investigation of customizing and controlling IEADs in a DF-CCP resembling those industrially employed with both biases applied to the substrate holding the wafer. The authors found that the ratio of the low-frequency to high-frequency power can be used to control the plasma density, provide extra control for the angular width and energy of the IEADs, and to optimize etch profiles. If the phases between the low frequency and its higher harmonics are changed, the sheath dynamics are modulated, which in turn produces modulation in the ion energy distribution. With these trends, continuously varying the phases between the dual-frequencies can smooth the high frequency modulation in the time averaged IEADs. For validation, results from the simulation are compared with Langmuir probe measurements of ion saturation current densities in a DF-CCP.

Improving AMS Detection of the Biomedical Radiotracer 41Ca with Segmented Radio-Frequency Quadrupoles

41Ca is an important biomedical radiotracer finding many applications in biological, nutritional and medical studies. The detection of 41Ca by AMS is however limited by an important background signal of 41K originating from biological samples and from contaminated cesium in the source. An approach consisting of using PbF2-assisted in-source fluorination in combination with an Isobar Separator for Anions (ISA), a device incorporating a low energy radio frequency quadrupole (RFQ) gas cell, promises to push down the limit of detection of 41Ca attainable on small (<3 MV) accelerator mass spectrometry (AMS) systems by several orders of magnitude. Such on-line reduction of 41K should also result in a simplification of biological sample preparation and less concern about variable 41K contamination of the cesium beam. The selective collision-induced fragmentation of KF3− versus CaF3-, occurring in the gas cell of an ISA equipped with a double segment RFQ, have been reported earlier1), leading to K being suppressed by a factor of 1e4 over Ca. We present here the future configuration of the ISA, redesigned using multi-segmented RFQ to enhance further this effect and improve transmission through the gas cell. A segmented RFQ is an appropriate tool to finely control ion energy down to the few eV's separating the fragmentation energies of the two fluoride species. This pre-commercial ISA destined to be used at the newly established A. E. Lalonde AMS laboratory at University of Ottawa (Canada) will be presented. Some practicalities of integrating a low energy RFQ-based device in a high energy AMS system will also be discussed.

Ion Energy Distribution Skew Control Using Phase-Locked Harmonic RF Bias Drive

The energy distribution of ions accelerated through a radio frequency sheath and incident on a plasma-facing material significantly influences material interaction with the plasma and can impact manufacturing at the nanoscale. Ion energy distributions are controlled through appropriate mixing of drive frequencies, which has been shown to control distribution width. This paper presents a modification to multifrequency drive for ion energy control by exploiting a digital frequency and phase controller that enables modification of the higher order moments of the distribution, specifically, controlling the skew of the distribution. By modulating the sheath with two frequencies where one frequency is the harmonic of the other and controlling the relative phase of these two waveforms incident on the plasma-facing surface, skew control is achieved. A simple empirical model is presented to describe this method, as well as experimental validation of the model and demonstration of skew control in a parallel plate capacitively coupled reactor.

Phase modulation in pulsed dual-frequency capacitively coupled plasmas

Particle-in-cell/Monte Carlo collision simulations, coupled with an external circuit, are used to investigate the behavior of pulsed dual-frequency (DF) capacitively coupled plasmas (CCPs). It is found that the phase shift θ between the high (or low) frequency source and the pulse modulation has a great influence on the ion density and the ionization rate. By pulsing the high frequency source, the time-averaged ion density shows a maximum when θ = 90∘. The time-averaged ion energy distribution functions (IEDFs) at the driven electrode, however, keep almost unchanged, illustrating the potential of pulsed DF-CCP for independent control of ion density (and flux) and ion energy. A detailed investigation of the temporal evolution of the plasma characteristics indicates that several high frequency harmonics can be excited at the initial stage of a pulse period by tuning the phase shift θ, and this gives rise to strong sheath oscillations, and therefore high ionization rates. For comparison, the pulsing of the low frequency source is also studied. In this case, the ion density changes slightly as a function of time, and the time-averaged ion density shows the same trend as in the HF modulation for different phase shifts θ. Moreover, the time-averaged IEDFs at the driven electrode can be modulated, showing the potential to reduce the maximum ion bombardment energy.

Selective etch of poly(methyl methacrylate) in block copolymer based on control of ion energy and design of gas chemistry for directed self assembly lithography

The selective etching of poly(methyl methacrylate) (PMMA) in a block copolymer was studied with a focus on the material structures of polystyrene (PS) and PMMA. Based on our predictions, we investigated the effect of ion bombardment and designed a carbon-containing gas plasma to improve selectivity. The etching characteristics of the carbon-containing gas plasma on the polymers were examined. Highly selective etching of PMMA to PS was achieved using the carbon-containing gas plasma. The carbon species in the plasma increased with increasing carbon-containing gas ratio and suppressed the PS etch rate drastically. The CO plasma process was successfully applied to a dry development process for directed-self assembly lithography.

Simulations of electrical asymmetry effect on N2-H2 capacitively coupled plasma by particle-in-cell/Monte Carlo model

A N2-H2 capacitively coupled rf discharge has potential applications in etching of organic low dielectric constant (low-k) material for microelectronics technology. In this paper, we investigate the characteristic and electrical asymmetry effect (EAE) on the N2-H2 capacitively coupled plasma used for low-k material etching by particle-in-cell/Monte Carlo (PIC/MC) model, in which the two frequency sources of 13.56 MHz and 27.12 MHz are applied separately to the two electrodes in geometrically in symmetry. It is found that the plasma density profiles, the ion flux density profiles and the energy distribution of ion bombarding electrodes can be changed by adjusting the phase angle θ between the two harmonics. When the phase angle θ is 0°, the density of primary ion (H3+) near low frequencie electrode (LFE) (wafer) is smallest, whereas flux and average energy of ion (H+, H3+, H2+) bombarding LFE are biggest; if the phase angle θ is tuned from 0° to 90°, the dc self-bias increases almost linearly from -103 V to 106 V, ion flux bombarding the LFE decreases by ±18%, the maximum of the ion bombarding energy at the LFE decreases by a factor of 2.5. For the N2-H2 capacitively coupled rf discharge, for the case of two frequencies (13.56 MHz/27.12 MHz) applied separately to the two electrodes, can realize separate control of ion energy and flux via the EAE, and is generally in qualitative agreement with experimental and modeling investigation on the Ar and O2 plasma for a dual-frequency voltage source of 13.56 MHz and 27.12 MHz is applied to the powered electrode. This work supplies a references basis for experimental research and technology that the EAE on the H2-N2 plasmas is used for organic low-k material etching process.

Space and phase resolved ion energy and angular distributions in single- and dual-frequency capacitively coupled plasmas

The control of ion energy and angular distributions (IEADs) is critically important for anisotropic etching or deposition in microelectronic fabrication processes. With single frequency capacitively coupled plasmas (CCPs), the narrowing in angle and spread in energy of ions as they cross the sheath are definable functions of frequency, sheath width, and mean free path. With increases in wafer size, single frequency CCPs are finding difficulty in meeting the requirement of simultaneously controlling plasma densities, ion fluxes, and ion energies. Dual-frequency CCPs are being investigated to provide this flexible control. The high frequency (HF) is intended to control the plasma density and ion fluxes, while the ion energies are intended to be controlled by the low frequency (LF). However, recent research has shown that the LF can also influence the magnitude of ion fluxes and that IEADs are determined by both frequencies. Hence, separate control of fluxes and IEADs is complex. In this paper, results from a two-dimensional computational investigation of Ar/O2 plasma properties in an industrial reactor are discussed. The IEADs are tracked as a function of height above the substrate and phase within the rf cycles from the bulk plasma to the presheath and through the sheath with the goal of providing insights to this complexity. Comparison is made to laser-induced fluorescence experiments. The authors found that the ratios of HF/LF voltage and driving frequency are critical parameters in determining the shape of the IEADs, both during the transit of the ion through the sheath and when ions are incident onto the substrate. To the degree that contributions from the HF can modify plasma density, sheath potential, and sheath thickness, this may provide additional control for the IEADs.

Control of ion activation energy delivered to tissue and sensitive materials in atmospheric pressure plasmas using thin porous dielectric sheets

In atmospheric pressure discharges, such as dielectric barrier discharges (DBDs), it is common to have transient sheaths having electric fields of hundreds of kV cm−1. With these sheaths, even with short mean free paths, it may be possible to deliver short pulses of energetic ions (tens to hundreds of eV) to surfaces. The energies of these ions can be controlled to some extent by selecting the relative permittivity (ε/ε0) of the underlying surface. In using DBDs to treat sensitive materials or tissue in plasma medicine where control of ion energies may be desirable, one has a very limited ability, if any, to modify the dielectric properties of the tissue. In this paper, we use results from a computational study to propose a method to control the characteristics of the transient sheath formed at the surface of sensitive materials or biological tissue by using a thin porous film placed on top of the surface. By controlling the transient sheath, one can control the ion energies delivered to the surface. We show that ion energies delivered to the underlying surface through the pores can be controlled by the capacitance and thickness of the film and the width of the pore. Results are discussed for streamer penetration and ion energies delivered to the surface when the plasma filament directly strikes the pore and is offset from the pore.

Multiple frequency capacitively coupled plasmas as a new technology for sputter processes

A novel large area multiple frequency coupled plasma is introduced for sputter deposition purposes. The discharge is driven by three different excitation frequencies (13.56, 27.12 and 60 MHz) simultaneously for advanced control of Ar ion flux and energy at the target by applying the electrical asymmetry effect during sputter processes.Optical emission spectroscopy is performed to characterize the sputter plasma with respect to plasma parameters as well as the Al transport through the plasma. The spectroscopic data are compared with TRIDYN calculation in combination with a simulation of the transport of atoms through the plasma volume.

Fundamental investigations of capacitive radio frequency plasmas: simulations and experiments

Capacitive radio frequency (RF) discharge plasmas have been serving hi-tech industry (e.g. chip and solar cell manufacturing, realization of biocompatible surfaces) for several years. Nonetheless, their complex modes of operation are not fully understood and represent topics of high interest. The understanding of these phenomena is aided by modern diagnostic techniques and computer simulations. From the industrial point of view the control of ion properties is of particular interest; possibilities of independent control of the ion flux and the ion energy have been utilized via excitation of the discharges with multiple frequencies. ‘Classical’ dual-frequency (DF) discharges (where two significantly different driving frequencies are used), as well as discharges driven by a base frequency and its higher harmonic(s) have been analyzed thoroughly. It has been recognized that the second solution results in an electrically induced asymmetry (electrical asymmetry effect), which provides the basis for the control of the mean ion energy. This paper reviews recent advances on studies of the different electron heating mechanisms, on the possibilities of the separate control of ion energy and ion flux in DF discharges, on the effects of secondary electrons, as well as on the non-linear behavior (self-generated resonant current oscillations) of capacitive RF plasmas. The work is based on a synergistic approach of theoretical modeling, experiments and kinetic simulations based on the particle-in-cell approach.

Mass density control of carbon films deposited by H-assisted plasma CVD method

In order to obtain high density carbon films with keeping anisotropic deposition profile on trench substrates, we control mass density of carbon films deposited by a H-assisted plasma chemical vapor deposition (CVD) method by ion kinetic energy of ions irradiating on film surface during deposition. The highest mass density of 2.14g/cm3 is obtained for deposition under the ion energy of 75eV and it is 1.4 times as high as that for the ion energy of 32eV. We also have studied etching rate of these films using H2+N2 discharge plasmas. The lowest etch rate of 1.8nm/min is obtained for the ion energy of 75eV and it is 2.8 times as low as that for the ion energy of 32eV. Etching rate of carbon films decreases exponentially with increasing the mass density of carbon films. Control of ion energy is the key to obtain high mass density carbon films with keeping anisotropic deposition profile on trench substrates.

Coupling effects in inductive discharges with radio frequency substrate biasing

Low pressure inductively coupled plasmas (ICP) operated in neon at 27.12 MHz with capacitive substrate biasing (CCP) at 13.56 MHz are investigated by phase resolved optical emission spectroscopy, voltage, and current measurements. Three coupling mechanisms are found potentially limiting the separate control of ion energy and flux: (i) Sheath heating due to the substrate biasing affects the electron dynamics even at high ratios of ICP to CCP power. At fixed CCP power, (ii) the substrate sheath voltage and (iii) the amplitude as well as frequency of plasma series resonance oscillations of the RF current are affected by the ICP power.

Control of plasma properties in capacitively coupled oxygen discharges via the electrical asymmetry effect

Using a combined experimental, numerical and analytical approach, we investigate the control of plasma properties via the electrical asymmetry effect (EAE) in a capacitively coupled oxygen discharge. In particular, we present the first experimental investigation of the EAE in electronegative discharges. A dual-frequency voltage source of 13.56 MHz and 27.12 MHz is applied to the powered electrode and the discharge symmetry is controlled by adjusting the phase angle θ between the two harmonics. It is found that the bulk position and density profiles of positive ions, negative ions, and electrons have a clear dependence on θ, while the peak densities and the electronegativity stay rather constant, largely due to the fact that the time-averaged power absorption by electrons is almost independent of θ. This indicates that the ion flux towards the powered electrode remains almost constant. Meanwhile, the dc self-bias and, consequently, the sheath widths and potential profile can be effectively tuned by varying θ. This enables a flexible control of the ion bombarding energy at the electrode. Therefore, our work proves the effectiveness of the EAE to realize separate control of ion flux and ion energy in electronegative discharges. At low pressure, the strength of resonance oscillations, which are found in the current of asymmetric discharges, can be controlled with θ.

Secondary electrons in dual-frequency capacitive radio frequency discharges

Two fundamentally different types of dual-frequency (DF) capacitively coupled radio frequency discharges can be used for plasma processing applications to realize separate control of the ion mean energy, ⟨Ei⟩, and the ion flux, Γi, at the substrate surface: (i) classical discharges operated at substantially different frequencies, where the low- and high-frequency voltage amplitudes, ϕlf and ϕhf, are used to control ⟨Ei⟩ and Γi, respectively; (ii) electrically asymmetric (EA) discharges operated at a fundamental frequency and its second harmonic with fixed, but adjustable phase shift between the driving frequencies, θ. In EA discharges the voltage amplitudes are used to control Γi and θ is used to control ⟨Ei⟩. Here, we report our systematic simulation studies of the effect of secondary electrons on the ionization dynamics and the quality of this separate control in both discharge types in argon at different gas pressures. We focus on the effect of the control parameter for ⟨Ei⟩ on Γi for different secondary yields, γ. We find a dramatic effect of tuning ϕlf in classical DF discharges, which is caused by a transition from α- to γ-mode induced by changing ϕlf. In EA discharges we find that no such mode transition is induced by changing θ within the parameter range studied here and, consequently, Γi remains nearly constant as a function of θ. Thus, despite some limitations at high values of γ the quality of the separate control of ion energy and flux is generally better in EA discharges compared with classical DF discharges.

The electrical asymmetry effect in capacitively coupled radio-frequency discharges

We present an analytical model to describe capacitively coupled radio-frequency (CCRF) discharges and the electrical asymmetry effect (EAE) based on the non-linearity of the boundary sheaths. The model describes various discharge types, e.g. single and multi-frequency as well as geometrically symmetric and asymmetric discharges. It yields simple analytical expressions for important plasma parameters such as the dc self-bias, the uncompensated charge in both sheaths, the discharge current and the power dissipated to electrons. Based on the model results the EAE is understood. This effect allows control of the symmetry of CCRF discharges driven by multiple consecutive harmonics of a fundamental frequency electrically by tuning the individual phase shifts between the driving frequencies. This novel class of capacitive radio-frequency (RF) discharges has various advantages: (i) A variable dc self-bias can be generated as a function of the phase shifts between the driving frequencies. In this way, the symmetry of the sheaths in geometrically symmetric discharges can be broken and controlled for the first time. (ii) Almost ideal separate control of ion energy and flux at the electrodes can be realized in contrast to classical dual-frequency discharges driven by two substantially different frequencies. (iii) Non-linear self-excited plasma series resonance oscillations of the RF current can be switched on and off electrically even in geometrically symmetric discharges. Here, the basics of the EAE are introduced and its main applications are discussed based on experimental, simulation, and modeling results.