Constant Rf Voltage Research Articles

Similarity of capacitive radio-frequency discharges in nonlocal regimes

Similarity transformations are essential for correlating discharges at different scales, which are mostly utilized with local field or local energy approximations. In this work, we report the fully kinetic results from particle-in-cell/Monte Carlo collision simulations that unambiguously demonstrate the similarity of radio frequency (rf) discharges in nonlocal regimes where the electron energy relaxation length is much larger than the gap dimension. It is found that at a constant rf voltage amplitude, discharges will be similar if the gas pressure, inverse of gap distance, and rf driving frequency are all changed by the same scaling factor. The scaling relations of fundamental parameters are illustrated for rf discharges in the alpha-mode with secondary electron emission ignored, and the temporal electron kinetics are shown to have invariance in similar discharges. The results explicitly validate the scaling laws in nonlocal kinetic regimes, indicating promising application potentials of the similarity transformations across a wide range of kinetic regimes.

Two-Dimensional Tandem Mass Spectrometry in a Single Scan on a Linear Quadrupole Ion Trap.

A two-dimensional tandem mass spectrometry (2D MS/MS) scan has been developed for the linear quadrupole ion trap. Precursor ions are mass-selectively excited using a nonlinear ac frequency sweep at constant rf voltage, while simultaneously, all product ions of the excited precursor ions are ejected from the ion trap using a broad-band waveform. The fragmentation time of the precursor ions correlates with the precursor m/z value (the first mass dimension) and also with the ejection time of the product ions, allowing the correlation between precursor and product ions. Additionally, the second mass dimension (product ions' m/z values) is recovered through fast Fourier transform of each mass spectral peak, revealing either intentionally introduced "frequency tags" or the product ion micropacket frequencies, both of which can be converted to product ion m/z through the classical Mathieu parameters, thereby revealing a product ion mass spectrum for every precursor ion without prior isolation. We demonstrate the utility of this method for analyzing a broad range of structurally related precursor ions, including chemical warfare agent simulants, fentanyls and other opioids, amphetamines, cathinones, antihistamines, and tetracyclic antidepressants.

Inducing locally structured ion energy distributions in intermediate-pressure plasmas

Ion energy distribution functions (IEDFs) incident upon material surfaces in radio frequency (rf) capacitively coupled plasmas are coupled to spatial and temporal sheath dynamics. Tailoring the ion energy distribution function within intermediate-pressure plasmas (≈133 Pa, 1 Torr), which find application in surface modification and aerospace industries, is challenging due to the collisional conditions. In this work, experimentally benchmarked 2D fluid/Monte-Carlo simulations are employed to demonstrate the production of structured IEDFs in a collisional (200 Pa 1.5 Torr argon) rf hollow cathode discharge. The formation of structures within the IEDFs is explained by an increase in the Ar+ ion-neutral mean-free-path and a simultaneous decrease in the phase-averaged sheath extension as the rf voltage frequency increases over 13.56–108.48 MHz for a constant rf voltage amplitude (increasing plasma power) and gas flow rate. Two distinct transitions in the shape of the IEDF are observed at 450 V, corresponding to the formation of “mid-energy” (60–180 eV) structures between 40.68 and 54.24 MHz and additional “high energy” (≳180 eV) structures between 81.36 and 94.92 MHz, with the structures within each region displaying a distinct sensitivity to the applied voltage amplitude. Transitions between these energy ranges occurred at lower applied voltages for increased applied voltage frequencies, providing increased control of the mean and modal ion energy over a wider voltage range. The capabitlity to extend the range of access to an operational regime, where the structured IEDFs are observed, is desirable for applications that require control of the ion-bombardment energy under collisional plasma conditions.

Implementation of Precursor and Neutral Loss Scans on a Miniature Ion Trap Mass Spectrometer and Performance Comparison to a Benchtop Linear Ion Trap.

Implementation of orthogonal double resonance precursor and neutral loss scans on the Mini 12 miniature rectilinear ion trap mass spectrometer is described, and performance is compared to that of a commercial Thermo linear trap quadropole (LTQ) linear ion trap. The ac frequency scan version of the technique at constant rf voltage is used here because it is operationally much simpler to implement. Remarkably, the Mini 12 shows up to two orders of magnitude higher sensitivity compared to that of the LTQ. Resolution on the LTQ is better than unit at scan speeds of ~ 400Th/s, whereas peak widths on the Mini 12, on average, range from 0.5 to 2.0Th full width at half maximum and depend heavily on the precursor ion Mathieu q parameter as well as the pump down time that precedes the mass scan. Both sensitivity and resolution are maximized under higher pressure conditions (short pump down time) on the Mini 12. The effective mass range of the product ion ejection waveform was found to be 5.8Th on the Mini 12 in the precursor ion scan mode vs. that of 3.9Th on the LTQ. In the neutral loss scan mode, the product ion selectivity was between 8 and 11Th on the Mini 12 and between 7 and 8Th on the LTQ. The effects of nonlinear resonance lines on the Mini 12 were also explored. Graphical Abstract ᅟ.

Precursor and Neutral Loss Scans in an RF Scanning Linear Quadrupole Ion Trap.

Methodology for performing precursor and neutral loss scans in an RF scanning linear quadrupole ion trap is described and compared to the unconventional ac frequency scan technique. In the RF scanning variant, precursor ions are mass selectively excited by a fixed frequency resonance excitation signal at low Mathieu q while the RF amplitude is ramped linearly to pass ions through the point of excitation such that the excited ion's m/z varies linearly with time. Ironically, a nonlinear ac frequency scan is still required for ejection of the product ions since their frequencies vary nonlinearly with the linearly varying RF amplitude. In the case of the precursor scan, the ejection frequency must be scanned so that it is fixed on a product ion m/z throughout the RF scan, whereas in the neutral loss scan, it must be scanned to maintain a constant mass offset from the excited precursor ions. Both simultaneous and sequential permutation scans are possible; only the former are demonstrated here. The scans described are performed on a variety of samples using different ionization sources: protonated amphetamine ions generated by nanoelectrospray ionization (nESI), explosives ionized by low-temperature plasma (LTP), and chemical warfare agent simulants sampled from a surface and analyzed with swab touch spray (TS). We lastly conclude that the ac frequency scan variant of these MS/MS scans is preferred due to electronic simplicity. In an accompanying manuscript, we thus describe the implementation of orthogonal double resonance precursor and neutral loss scans on the Mini 12 using constant RF voltage. Graphical Abstract ᅟ.

Plasma Dynamics in Low-Pressure Capacitively Coupled Oxygen Plasma Using PIC–MCC/Fluid Hybrid Model

Low-pressure ( <; 20 mT) capacitively coupled plasmas are now widely used for plasma processing in the semiconductor industry. In particular, O2 plasmas are being used for etching, photoresist ashing, and chamber cleaning. At low pressure, the electron mean free path increases, making kinetic effects more important. To consider kinetic effects, a hybrid plasma simulation tool has been developed that couples the particle-in-cell/Monte Carlo collision model for charged species with a fluid model for neutral species. Oxygen plasma has been modeled using this hybrid model for a range of pressures. It is observed that the electrons primarily absorb power at the sheath edge during sheath expansion. These energetic beam electrons are responsible for plasma production and sustenance through collisions. The beam electrons are stronger at lower pressure and retain their beam characters for a longer distance. For a constant RF voltage, the electron density increases with pressure.

Considerations for Thermal Injury Analysis for RF Ablation Devices~!2009-09-09~!2009-12-19~!2010-02-04~!

Background:The estimation of lesion size is an integral part of treatment planning for the clinical applications of radiofrequency ablation. However, to date, studies have not directly evaluated the impact of different computational estimation techniques for predicting lesion size. In this study, we focus on three common methods used for predicting tissue injury: (1) iso-temperature contours, (2) Cumulative equivalent minutes, (3) Arrhenius based thermal injury.Methods:We created a geometric model of a multi-tyne ablation electrode and simulated thermal and tissue injury profiles that result from three calculation methods after 15 minutes exposure to a constant RF voltage source. A hybrid finite element technique was used to calculate temperature and tissue injury. Time-temperature curves were used in the assessment of iso-temperature thresholds and the method of cumulative equivalent minutes. An Arrhenius-based formulation was used to calculate sequential and recursive thermal injury to tissues.Results:The data demonstrate that while iso-temperature and cumulative equivalent minute contours are similar in shape, these two methodologies grossly over-estimate the amount of tissue injury when compared to recursive thermal injury calculations, which have previously been shown to correlate closely with in vitro pathologic lesion volume measurement. In addition, Arrhenius calculations that do not use a recursive algorithm result in a significant underestimation of lesion volume. The data also demonstrate that lesion width and depth are inadequate means of characterizing treatment volume for multi-tine ablation devices.Conclusions:Recursive thermal injury remains the most physiologically relevant means of computationally estimating lesion size for hepatic tumor applications. Iso-thermal and cumulative equivalent minute approaches may produce significant errors in the estimation of lesion size.

Considerations for Thermal Injury Analysis for RF Ablation Devices

Background: The estimation of lesion size is an integral part of treatment planning for the clinical applications of radiofrequency ablation. However, to date, studies have not directly evaluated the impact of different computational estimation techniques for predicting lesion size. In this study, we focus on three common methods used for predicting tissue injury: (1) iso-temperature contours, (2) Cumulative equivalent minutes, (3) Arrhenius based thermal injury. Methods: We created a geometric model of a multi-tyne ablation electrode and simulated thermal and tissue injury profiles that result from three calculation methods after 15 minutes exposure to a constant RF voltage source. A hybrid finite element technique was used to calculate temperature and tissue injury. Time-temperature curves were used in the assessment of iso-temperature thresholds and the method of cumulative equivalent minutes. An Arrhenius-based formulation was used to calculate sequential and recursive thermal injury to tissues. Results: The data demonstrate that while iso-temperature and cumulative equivalent minute contours are similar in shape, these two methodologies grossly over-estimate the amount of tissue injury when compared to recursive thermal injury calculations, which have previously been shown to correlate closely with in vitro pathologic lesion volume measurement. In addition, Arrhenius calculations that do not use a recursive algorithm result in a significant underestimation of lesion volume. The data also demonstrate that lesion width and depth are inadequate means of characterizing treatment volume for multi-tine ablation devices. Conclusions: Recursive thermal injury remains the most physiologically relevant means of computationally estimating lesion size for hepatic tumor applications. Iso-thermal and cumulative equivalent minute approaches may produce significant errors in the estimation of lesion size.

Particle accumulation in a flowing silane discharge

Particle trapping in different areas of a parallel-plate, radio frequency silane discharge, and its effect on plasma optical emission of SiH and Hα, has been studied under high gas-flow and low power-density conditions, as used for “device-quality” hydrogenated amorphous silicon (a-Si:H) film deposition. The largest density of particles occurs between the electrodes, near the downstream corners of the rectangular electrodes. Particles are trapped in these positions by sheath fields, until reaching sufficient size to escape with the flow. The region of strong particle trapping has an increased intensity of optical emission, with Hα increased nearly fourfold. Slow oscillatory behavior of particle scattering and discharge emission was observed for pressures near 30 Pa. Power deposited in the discharge has also been measured; for a constant rf voltage and gas-flow speed it changes weakly with pressure, with the maximum at ∼40 Pa. Combined with film growth-rate measurements, this yields a discharge energy deposition of ∼17 eV per deposited Si atom.

Comparison of electron-density measurements made using a Langmuir probe and microwave interferometer in the Gaseous Electronics Conference reference reactor

A Langmuir probe and a microwave interferometer have been combined to measure the electron density of argon glow discharges in the Gaseous Electronics Conference reference reactor [Bull. Am. Phys. Soc. 36, 207 (1991)]. The two techniques indicate the same charge density at 100 mTorr to within 30%. This 30% difference is easily explained by the experimental peculiarities. While the predicted charge densities obtained from the two techniques track one another as the applied rf voltage is varied at constant pressure, they do not track one another as the pressure is varied at constant rf voltage. In fact, the charge densities predicted from the Langmuir probe using Laframboise’s theory (Tech. Rep. 100, Univ. Toronto Inst. Aerospace Study, 1966) are factors of 2 and 4 times lower than those from the interferometer at 250 and 500 mTorr, respectively. It appears that the probe alters the charge density in its vicinity when the probe radius becomes greater than the ion mean free path. The interferometer has also been used to investigate the macroscopic perturbation of the plasma electron density caused by the Langmuir probe. It was found that presence of the probe in a radio-frequency discharge does not appreciably alter the electron density measured by the interferometer at a set rf voltage irrespective of the potential placed upon it. This was not the case for dc discharges in the same apparatus where the volume-averaged electron density could be depleted by as much as 70% when the probe was biased even a few volts above plasma potential.

A computational investigation of the RF plasma structures and their production efficiency in the frequency range from HF to VHF

The influence of driving frequency on discharge structure and production efficiency has been investigated in Ar over the frequency range from 13.56 MHz to 100 MHz in terms of the relaxation continuum model. Under constant RF voltage, the plasma density increases in proportion to the square of the driving frequency. The net ionization and excitation rates, and the power density, show the same frequency dependence, although the collisional production efficiency per input power density is almost invariant with change of frequency. This has the great advantage for plasma processing and deposition from plasmas that parallel plate discharge in very high frequency (VHF) will lead to the system being rather free from ion damage or effects of confined plasma volume as compared with those in high frequency (HF) and microwave (MW).

Spatially resolved optical emission and electrical properties of SiH4 RF discharges at 13.56 MHz in a symmetric parallel-plate configuration

A symmetric capacitively coupled SiH4 radio-frequency discharge at 13.56 MHz is analysed by spatially resolved optical emission spectroscopy and by measurements of the RF voltage and RF current and of the ion conduction current and energy distribution through the sheath. A transition between two discharge regimes is observed, either by increasing the pressure at a constant RF voltage, or by increasing the RF voltage at a constant pressure. This transition between two discharge regimes is attributed to different mechanisms by which electrons gain energy from the sheaths and in the plasma. For a gas temperature of 200 degrees C, at a fixed pressure of 0.185 Torr, the transition occurs as the RF voltage exceeds 140 V. When comparing both discharge regimes at the same pressure and RF voltage, the authors observe a drastic change of the RF current waveform. The power dissipated in the discharge, as well as the total intensity of the optical emission from the discharge, increases by a factor 3. Moreover the spatial distribution of the emission is completely modified. These results are discussed in relation to recent experimental and modelling studies of RF discharges.