RF Excitation Frequency Research Articles

Helicon wave plasma generated by a resonant birdcage antenna: magnetic field measurements and analysis in the RAID linear device

This paper describes a helicon-wave-sustained plasma generated by a novel birdcage antenna with cosine azimuthal current distribution. The resonant birdcage source maintains a stable intense plasma column from low (300 W) to high (10 kW) steady-state RF power in hydrogen or argon, enabling plasma diagnostic measurements over the whole volume. The plasma density, 1018 to 1019 m−3, and uniform static magnetic field, 200–800 G, are typical of high density, low B-field helicon sources where the RF excitation frequency, 13.56 MHz here, is far above the ion cyclotron frequency and much lower than the electron cyclotron and plasma frequencies. Magnetic field measurements of the unbounded plasma column show a helicon wave propagating from the birdcage source to the target 1.5 m away. The axial wavelength, measured for a range of plasma density and magnetic field, is described by the whistler dispersion relation; it is not determined by the antenna length but varies smoothly as a function of electron density irrespective of the gas type, and shows no discontinuous transitions. This wavelength was compared with semi-analytical and purely numerical models, showing a good fit with the m = +1 helicon eigenvalue of shortest axial wavelength. The birdcage resonant antenna is a suitable source for continuous helicon-sustained plasma experiments, from fundamental wave studies up to high power applications in hydrogen or deuterium.

A 5-to-8-GHz Wideband Miniaturized Dielectric Spectroscopy Chip With <inline-formula> <tex-math notation="LaTeX">$I/Q$ </tex-math> </inline-formula> Mismatch Calibration in 65-nm CMOS

This paper presents a fully integrated broadband biosensor chip for miniaturized dielectric spectroscopy (DS). The proposed biosensor chip consists of wideband exciting signal source and high sensitivity receiver. The exciting signal source consists of a wideband phase-locked loop (PLL) and output drivers. The wideband receiver is composed of low noise amplifier, mixer, and output buffers. The proposed receiver with in-phase ( ${I}$ ) and quadrature-phase ( ${Q}$ ) signals mismatch calibration measures the frequency-dependent S21 magnitude and phase of a microfluidic dielectric sensor. The DS system implements a modified frequency response analysis by converting the biosensor response signal from the RF excitation frequency to low frequency using a passive mixer. Dielectric measurements results of ethanol with the proposed biosensor chip in the range of 5–8 GHz are in excellent agreement (error <1%) with theoretical values. The chip is fabricated in 65-nm CMOS process. The total power consumption is 18 mW with 1.2-V supply.

The electromagnetic environment of Magnetic Resonance Imaging systems. Occupational exposure assessment reveals RF harmonics

Magnetic Resonance Imaging (MRI) systems played a crucial role in the postponement of the former occupational electromagnetic fields (EMF) European Directive (2004/40/EC) and in the formation of the latest exposure limits adopted in the new one (2013/35/EU). Moreover, the complex MRI environment will be finally excluded from the implementation of the new occupational limits, leading to an increased demand for Occupational Health and Safety (OHS) surveillance. The gradient function of MRI systems and the application of the RF excitation frequency result in low and high frequency exposures, respectively. This electromagnetic field exposure, in combination with the increased static magnetic field exposure, makes the MRI environment a unique case of combined EMF exposure. The electromagnetic field levels in close proximity of different MRI systems have been assessed at various frequencies. Quality Assurance (QA) & safety issues were also faced. Preliminary results show initial compliance with the forthcoming limits in each different frequency band, but also revealed peculiar RF harmonic components, of no safety concern, to the whole range detected (20-1000MHz). Further work is needed in order to clarify their origin and characteristics.

Occupational Electromagnetic Fields exposure in Magnetic Resonance Imaging systems – Preliminary results for the RF harmonic content

PurposeEuropean legislation concerning the protection of workers from exposure to Electromagnetic Fields (EMF) was recently (26.6.2013) completed by Directive 2013/35/ΕU. This Directive is a specific one of the framework Directive 89/391/EEC and part of the overall legislation for Occupational Health and Safety (OHS). Magnetic Resonance Imaging (MRI) systems have played a key role, both in the postponement of the former 2004/40 EMF Directive and in the formation of the latest limits adopted by the new Directive. On the other hand, MRI systems are associated with the exposure of personnel to EMF of various frequencies and modulations, arousing peculiar safety issues. Therefore, we will try to acquire the highly important knowledge of the exact occupational exposure levels, in all working scenarios and practices. MethodsDifferent MRI systems (1.5 and 3 T) have been chosen for a variety of measurements in order to assess occupational exposure compared to the limits (ALs) of the Directive and to the main OHS principles. Gradient function of MRI systems results in low frequency exposure, while high frequency exposure comes from the application of the RF excitation frequency. ResultsIn most of the cases the RMS and peak value measurements do not exceed the corresponding ALs, apart from a few specific hot spots, manageable through OHS principles. DiscussionComplete occupational exposure results can form the basis for dealing with multiple exposures present in MRI systems. Peculiar RF harmonic components, of no safety concern, were detected. Their origin is under examination.

A Broadband Sensor Interface IC for Miniaturized Dielectric Spectroscopy From MHz to GHz

This paper describes a broadband sensor interface IC as part of a miniaturized measurement platform for MHz-to-GHz dielectric spectroscopy. Developed in 0.35 μm 2P/4M RF CMOS, the IC measures frequency-dependent S 21 magnitude and phase of a microfluidic dielectric sensor fabricated in a thick gold-on-glass microfabrication process and loaded with a material-under-test (MUT). The IC architecture implements a broadband frequency response analysis (bFRA) method by first down-converting the sensor response signal from the RF excitation frequency to an intermediate frequency (IF) of 1 MHz using a low-noise amplifier (LNA) and active mixer, followed by down-converting the IF signal to dc using a coherent detector employing IF amplification stages with programmable gain, a passive mixer driven by in-phase (I) and quadrature-phase (Q) signals and an active-RC low-pass filter (LPF). The sensor interfaced with the IC is fully capable of differentiating among deionized (DI) water, phosphate buffered saline (PBS), ethanol and methanol in tests conducted at four different excitation frequencies of 50 MHz, 500 MHz, 1 GHz and 3 GHz. Further, dielectric readings of ethanol from the sensor interfaced with the IC at five excitation frequencies in the range of 50 MHz to 2 GHz are in excellent agreement (error <;1%) with those from using a vector network analyzer (VNA) as the sensor readout. A bulk-solution reference measurement by an Agilent 85070E dielectric probe kit interfaced with a VNA is also performed to verify proof-of-concept feasibility in conducting MHz-to-GHz dielectric spectroscopy with a miniaturized measurement platform using μL-sample volumes.

Effects of RF power and pressure on performance of HF-PECVD silicon thin-film solar cells

High-frequency plasma-enhanced chemical vapor deposition (HF-PECVD) is a widely applicable method of deposition over a large area at a high rate for fabricating silicon thin-film solar cells. This investigation presents the properties of hydrogenated amorphous silicon (a-Si:H) films and the preparation of highly-efficient p–i–n solar cells using an RF (27.1 MHz) excitation frequency. The influence of the power (10–40 W) and pressure (20–50 Pa) used during the deposition of absorber layers in p–i–n solar cells on the properties and mechanism of growth of the a-Si:H thin films and the solar cells is studied. The a-Si:H thin films prepared under various deposition conditions have widely varying deposition rates, optical-electronic properties and microstructures. When the deposition parameters were optimized, amorphous silicon-based thin-film silicon solar cells with efficiency of 7.6% were fabricated by HF-PECVD. These results are very encouraging for the future fabrication of highly-efficient thin-film solar cells by HF-PECVD.

The bounce resonance heating of low-energy electrons in capacitively coupled discharges

Low-energy electrons are confined in the bulk plasma by the ambipolar potential and are typically heated inefficiently due to their low collisionality and the weak rf electric field present in the bulk when their mean-free path is much larger than the system length in capacitively coupled discharges. It is shown in this study, however, that electrons in the bulk that bounce inside the electrostatic potential well with a frequency equal to the rf excitation frequency are efficiently heated by coherent interaction with the rf field. This resonant heating manifests itself as a plateau in the electron energy probability function and is observed in a wide range of pressures from 25 mTorr to 1 Torr while decreasing the gap distance. The weak transverse magnetic field significantly influences the bounce frequency of the low-energy electrons. As a result, the electrons not in the resonant condition in the absence of a magnetic field can be led to satisfy the resonant condition, becoming effectively heated in the presence of a weak transverse magnetic field.

Abnormal Heating of Low-Energy Electrons in Low-Pressure Capacitively Coupled Discharges

In low-pressure capacitively coupled plasmas, high-energy electrons are collisionlessly heated by large rf fields in the sheaths while low-energy electrons are confined in the bulk plasma by the ambipolar potential. Low-energy electrons are typically inefficiently heated due to their low collisionality and the weak rf electric field present in the bulk. It is shown, however, that as a result of the nonlinear interaction between the electron motion and the weak rf field present in the bulk, low-energy electrons can be efficiently heated. Electrons in the bulk that bounce inside the electrostatic potential well with a frequency equal to the rf excitation frequency are efficiently heated by the coherent interaction with the rf field. This resonant collisionless heating can be very efficient and manifest itself as a plateau in the electron energy probability function.

Calculation of the electrode shape for suppression of the standing wave effect in large area rectangular capacitively coupled reactors

The electromagnetic standing wave becomes one of the main sources of plasma nonuniformity in large area capacitively coupled rf reactors. In cylindrical reactors with a central rf connection or one-dimensional linear reactors with rf connections at both extremities, Gaussian shaped electrodes can be used to suppress this standing wave. In this work, we present a two-dimensional quasiplanar circuit model and a numerical method for calculation of the electrode shape that can suppress the standing wave effect in large area rectangular reactors. It is shown that the calculated shapes are not Gaussian, and are not only a function of the reactor dimensions and excitation frequency, but are also strongly influenced by the position and number of rf connections, as will also be the case for a cylindrical reactor with a noncentral rf connection. However, when a shape has been determined for a given reactor geometry and rf excitation frequency, then it is shown that it remains independent of the plasma provided that the electromagnetic skin depth in the plasma remains large enough such that skin effects remain negligible.

Amorphous and microcrystalline silicon solar cells prepared at high deposition rates using RF (13.56 MHz) plasma excitation frequencies

This paper presents a-Si:H and μc-Si:H p–i–n solar cells prepared at high deposition rates using RF (13.56 MHz) excitation frequency. A high deposition pressure was found as the key parameter to achieve high efficiencies at high growth rates for both cell types. Initial efficiencies of 7.1% and 11.1% were achieved for a μc-Si:H cell and an a-Si:H/μc-Si:H tandem cell, respectively, at a deposition rate of 6 Å/s for the μc-Si i-layers. A μc-Si:H cell prepared at 9 Å/s exhibited an efficiency of 6.2%.

A multi-variable analysis of the influence of process parameters on VHF PECVD deposition process for amorphous silicon

It is reported about the influence of the parameters gas flow, gas pressure and RF power on a PECVD (plasma enhanced chemical vapor deposition) process of amorphous silicon (a-Si:H) in the excitation frequency range from 40 to 80 MHz. In contrast to the usual method, the process parameters are varied by a special experimental design in order to study the principal effect and contribution ratio of each process parameter separately with respect to the a-Si:H film deposition rate and the characteristic dark conductivity and photo conductivity. The analyzed principal influence of the process parameters RF power and excitation frequency on the PECVD process is explained by the distribution of the electric field on the surfaces of planar plasma electrodes. As a result it is shown that the geometry of the plasma electrodes strongly influences the PECVD process and a new definition of the commonly applied value for power density in the discharge volume should be introduced, since this parameter depends on the excitation frequency as well as on the given plasma electrode system.

Frequency effects in capacitively coupled radio-frequency glow discharges: A comparison between experiments and a two-dimensional fluid model

The results of a two-dimensional fluid model for argon rf discharges in a closed cylindrical vacuum chamber are compared with experimental data from an amorphous silicon deposition reactor. Good agreement is obtained for the relation between the dc autobias voltage and the dissipated power in the frequency range 50–100 MHz at pressures between 12 and 90 Pa. A scaling law is presented for the relation between the power, the dc bias voltage, the rf excitation frequency, and the background pressure. The model yields a linear relation between the applied rf voltage and the dc bias voltage. This relation depends only on the geometry of the discharge chamber and shows an offset.

Role of photoacoustics in optogalvanics

Time-resolved laser optogalvanic (LOG) signals have been induced by pulsed laser excitation (lsj→2pk, Paschen notation) of a ∼30 MHz radio-frequency (rf) discharge in neon at ∼5 torr. Dramatic changes of the shape/polarity of certain parts of the LOG signals occur when the rf excitation frequency is scanned over the electrical resonance peak of the plasma and the associated driving/detecting circuits. These effects are attributed to ionization rate changes (i.e., laser-induced alterations of the plasma conductivity), with concomitant variations in the plasma resonance characteristics. In addition to ionization rate changes, it is shown that photoacoustic (PA) effects also play a significant role in the generation of the LOG signal. Those parts of the LOG signal that are invariant with respect to the rf frequency are attributed to a PA effect. The similarity of LOG signal shapes from both rf and dc discharges suggests that photoacoustics play a similar role in the LOG effect in dc discharges. Contrary to common belief, most reported LOG signal profiles, ones produced by excitation to levels that do not lie close to the ionization threshold, appear to be totally mediated by the PA effect.

Superconducting accelerometer using niobium‐on‐sapphire rf resonator

An accelerometer is described which uses a rf niobium-on-sapphire resonator as its sensor element. The accelerometer uses a magnetically levitated spool as a test mass and the spool modulates the inductance of the resonator; its position is servo controlled to maintain the resonator at the external rf excitation frequency. The accelerometer has high sensitivity over the full audio frequency range, but is optimized for frequencies between 100 Hz and 1 kHz, where the calculated displacement sensitivity approaches 10(-15) cm for a 1 Hz measurement bandwidth. The system noise sources are analyzed and possible improvements are discussed.