Internal RF Research Articles

Feedback Analysis and Design of RF Power Links for Low-Power Bionic Systems

This paper presents a feedback-loop technique for analyzing and designing RF power links for transcutaneous bionic systems, i.e., between an external RF coil and an internal RF coil implanted inside the body. The feedback techniques shed geometric insight into link design and minimize algebraic manipulations. We demonstrate that when the loop transmission of the link's feedback loop is -1, the link is critically coupled, i.e., the magnitude of the voltage transfer function across the link is maximal. We also derive an optimal loading condition that maximizes the energy efficiency of the link and use it as a basis for our link design. We present an example of a bionic implant system designed for load power consumptions in the 1-10-mW range, a low-power regime not significantly explored in prior designs. Such low power levels add to the challenge of link efficiency, because the overhead associated with switching losses in power amplifiers at the link input and with rectifiers at the link output significantly degrade link efficiency. We describe a novel integrated Class-E power amplifier design that uses a simple control strategy to minimize such losses. At 10-mW load power consumption, we measure overall link efficiencies of 74% and 54% at 1- and 10-mm coil separations, respectively, in good agreement with our theoretical predictions of the link's efficiency. At 1-mW load power consumption, we measure link efficiencies of 67% and 51% at 1- and 10-mm coil separations, respectively, also in good accord with our theoretical predictions. In both cases, the link's rectified output dc voltage varied by less than 16% over link distances that ranged from 2 to 10 mm.

Generation of uniform plasmas by crossed internal oscillating current sheets: Key concepts and experimental verification

The results of comprehensive experimental studies of the operation, stability, and plasma parameters of the low-frequency (0.46MHz) inductively coupled plasmas sustained by the internal oscillating rf current are reported. The rf plasma is generated by using a custom-designed configuration of the internal rf coil that comprises two perpendicular sets of eight currents in each direction. Various diagnostic tools, such as magnetic probes, optical emission spectroscopy, and an rf-compensated Langmuir probe were used to investigate the electromagnetic, optical, and global properties of the argon plasma in wide ranges of the applied rf power and gas feedstock pressure. It is found that the uniformity of the electromagnetic field inside the plasma reactor is improved as compared to the conventional sources of inductively coupled plasmas with the external flat coil configuration. A reasonable agreement between the experimental data and computed electromagnetic field topography inside the chamber is reported. The Langmuir probe measurements reveal that the spatial profiles of the electron density, the effective electron temperature, plasma potential, and electron energy distribution/probability functions feature a high degree of the radial and axial uniformity and a weak azimuthal dependence, which is consistent with the earlier theoretical predictions. As the input rf power increases, the azimuthal dependence of the global plasma parameters vanishes. The obtained results demonstrate that by introducing the internal oscillated rf currents one can noticeably improve the uniformity of electromagnetic field topography, rf power deposition, and the plasma density in the reactor.

Chemically enhanced physical vapor deposition of tantalum nitride-based films for ultra-large-scale integrated devices

Physical vapor deposition (PVD) using ionized metal plasmas (ionized PVD or IPVD) is widely used to deposit conducting diffusion barriers and liners such as Ta and TaN for use in ultra-large-scale integrated (ULSI) interconnect stacks. Ionized PVD films exhibit the low resistivity, high density, and good adhesion to underlying dielectric desired for this application. On the other hand, extending PVD beyond the 45 nm technology node is problematic since IPVD may not provide sufficient step coverage to reliably coat features having high aspect ratio and sub-100 nm dimensions. Alternatively, chemical vapor deposition (CVD) and atomic layer deposition (ALD) can be used to deposit highly conformal metal films, but the electrical performance and interfacial quality may not equal that of PVD. To address future ULSI barrier∕liner deposition needs, a method providing PVD-like film quality and CVD-like step coverage would be highly attractive. We have recently reported a hybrid approach to film deposition, referred to as chemically enhanced physical vapor deposition (CEPVD), in which a chemical precursor is introduced at the substrate during IPVD to provide a CVD component to the overall deposition process. The isotropic precursor flux is intended to provide film deposition on surfaces that are not impacted by the directional ions, such as the lower sidewall of a narrow via or trench. Conversely, the kinetic energy delivered to the surface by the flux of ionized metal may serve to enhance the desorption of CVD byproducts, reduce incorporation of impurities, and increase film density. In order to investigate the potential of CEPVD to deposit barrier∕liner films, we have focused on the Ta-N material system since Ta∕TaN is widely used as a diffusion barrier in Cu damascene processing. IPVD TaN films were deposited by reactive sputtering of a Ta target in Ar∕N2 using a planar magnetron and internal rf coils to provide a secondary ionization plasma for the sputtered neutrals. CEPVD was carried out by introducing a Ta-containing, organometallic precursor [tert-butylimino tris(diethylamino) tantalum] in the vicinity of the substrate surface during IPVD. Film thickness and step coverage were determined by cross-sectional scanning electron microscopy (SEM). Film composition, chemical state, and crystal structure were characterized using Auger electron spectroscopy, x-ray photoelectron spectroscopy, and x-ray diffraction, respectively. Resistivity was measured by four-point probe. Cross-sectional SEM showed improved step coverage over IPVD TaN. CEPVD film properties were highly process dependent; however, unlike IPVD TaNx films that vary in stoichiometry but not purity, CEPVD “TaN” films contained relatively large amounts of carbon (∼30%–60%) and could best be described as TaCxNy. Resistivity as low as ∼370μΩcm was obtained for planar films of approximately 90 nm in thickness.

Effect of inductively-coupled plasma assist on the crystal orientation of magnesium oxide thin films produced by reactive sputtering

High-performance reactive sputter-deposition of magnesium oxide (MgO) thin films is investigated. In this research, planar magnetron (PM) sputtering of Mg target in argon/oxygen mixture is assisted by an inductively coupled plasma (ICP) that is located between the target and the substrate and is driven by an internal RF coil antenna at 13.56 MHz. The changes in deposition rates and X-ray diffraction patterns due to the independent power control of PM and ICP are investigated. As a result, we found that deposition rate of MgO films was predominantly controlled by the PM discharge power and that the crystallinity of deposited MgO films was controlled by the ICP-RF power.

Inductively coupled plasmas sustained by an internal oscillating current

A global electromagnetic model of an inductively coupled plasma sustained by an internal oscillating current sheet in a cylindrical metal vessel is developed. The electromagnetic field structure, profiles of the rf power transferred to the plasma electrons, electron/ion number density, and working points of the discharge are studied, by invoking particle and power balance. It is revealed that the internal rf current with spatially invariable phase significantly improves the radial uniformity of the electromagnetic fields and the power density in the chamber as compared with conventional plasma sources with external flat spiral inductive coils. This configuration offers the possibility of controlling the rf power deposition in the azimuthal direction.

Recent developments of long pulse RF ion sources for NBI systems

For the new radial injector of the Wendelstein 7-AS stellerator, two medium size 40 kW RF sources (‘W7-AS RF source’) have been built and tested. A positive hydrogen ion beam of 28 A at 50 kV with a proton fraction of 60% has been extracted. For future injection systems, in particular those based on negative ions, large area beam extraction and long pulse capability up to cw operation are envisaged. At IPP two different design concepts of RF sources are being investigated in order to meet these requirements: in the first one, the plasma is generated in one or two small circular RF sources (‘drivers’), which are mounted onto the back plate of a 30 l expansion volume (‘bucket’) with magnetic multi-cusp confinement of the plasma (‘Type 6-1’ and ‘Type 6-2’ respectively). In the second concept the plasma is generated inside the bucket by an internal RF coil, which is insulated by a quartz tube (‘Type 5’). In both cases the applicability for negative as well as for positive ion production is being tested. The next application will be in the positive ion based injectors in the third injector of JET and in the W7X stellerator.

A Green's function approach to local rf heating in interventional MRI.

Current safety regulations for local radiofrequency (rf) heating, developed for externally positioned rf coils, may not be suitable for internal rf coils that are being increasingly used in interventional MRI. This work presents a two-step model for rf heating in an interventional MRI setting: (1) the spatial distribution of power in the sample from the rf pulse (Maxwell's equations); and (2) the transformation of that power to temperature change according to thermal conduction and tissue perfusion (tissue bioheat equation). The tissue bioheat equation is approximated as a linear, shift-invariant system in the case of local rf heating and is fully characterized by its Green's function. Expected temperature distributions are calculated by convolving (averaging) transmit coil specific absorption rate (SAR) distributions with the Green's function. When the input SAR distribution is relatively slowly varying in space, as is the case with excitation by external rf coils, the choice of averaging methods makes virtually no difference on the expected heating as measured by temperature change (deltaT). However, for highly localized SAR distributions, such as those encountered with internal coils in interventional MRI, the Green's function method predicts heating that is significantly different from the averaging method in current regulations. In our opinion, the Green's function method is a better predictor since it is based on a physiological model. The Green's function also elicits a time constant and scaling factor between SAR and deltaT that are both functions of the tissue perfusion rate. This emphasizes the critical importance of perfusion in the heating model. The assumptions made in this model are only valid for local rf heating and should not be applied to whole body heating.

An analysis of flicker noise rejection in low-power and low-voltage CMOS mixers

The sensitivity of RF CMOS receivers using a direct conversion or a low-IF architecture is strongly affected by flicker noise. This paper gives theoretical guidelines to predict the flicker noise in Gilbert-cell mixers. The conversion gain, the equivalent input and output noise, and the effect of the pole at the single internal RF node are discussed. For the first time, results which are valid in all modes of operation are given. Such complete results are required for some ultra low-power and low-voltage applications, since the transistors might be operated in moderate or even weak inversion region. The theoretical gains are found to remain within a 2-dB margin with respect to the measurements of a UHF downconverter built in a 0.5 /spl mu/m process, for a large range of bias conditions and local oscillator swing.

Bipolar cathode transferred-electron device for millimeter-wave generation

The potential of a BIpolar Cathode Transferred-Electron Device (BICTED) for mm-wave RF-power generation is theoretically investigated by means of a time-domain circuit simulator based on a semiconductor device macroscopic physical model. In this N +P +NN + bipolar transistor-like structure, the emitter-base junction is used as an electron injection mechanism. The collector region operates under accumulation and transit-time mode classically observed in mm-wave GUNN diodes. In a first step, the device internal RF operation is analysed by means of CW pure sine simulations. Next, more realistic BICTED oscillator modeling demonstrates significant RF-performance at 62.5 GHz.

An rf sustained argon and copper plasma for ionized physical vapor deposition of copper

Langmuir probe, optical emission spectroscopy, and biased quartz crystal microbalance measurements were used to investigate an argon and copper plasma used for ionized physical vapor deposition of copper. Copper vapor generated by a magnetron sputter discharge is ionized upon passing through an argon discharge excited by an internal rf induction antenna. Argon plasma characteristics such as electron temperatures Te, plasma densities ne, and plasma and floating potentials Vp and Vf, were studied as a function of argon pressure and rf power. An increase of plasma density versus rf discharge power and argon pressure was observed. The radial profile of plasma density measured by a Langmuir probe reveals a peak ion density at the center of the rf antenna and an increase in the radial ion concentration gradient with argon pressure. The ratios of optical emission intensities from Cu+ ion and Cu neutral lines increase with rf discharge power and argon pressure. The biased quartz crystal microbalance measurements show an increase of both Cu+ ion flux and the ratio of Cu+ ion to Cu neutral fluxes with rf power and argon pressure; however, they also show a decrease of total Cu flux with increasing argon pressure.

Communications receiver design using digital processing

A digital processing receiver performs most receiver functions using digital processing techniques. Examples of such functions are frequency translation, filtering, demodulation, automatic gain control, mode of operation, equalization of group delay distortion, and passband ripple. An initial step in defining the receiver bandwidth and signal level is performed in the analog domain. The information-bearing signal is then sampled and quantized at a suitable IF frequency. The remainder of the processing is performed digitally using digital signal processing (DSP) techniques. The digital approach takes advantage of the everincreasing capability of digital signal processing and the potential of miniaturization of all digital receivers using VHSIC technology. Other advantages of using DSP are that it increases the reliability and repeatability of demodulating and monitoring the received signals. The signal output from an analog receiver can vary from unit to unit, and even the same unit can vary with time, temperature, and calibration. Conventional analog receivers use RF stages, frequency synthesizers, IF amplifiers, and analog demodulators which not only pose internal RF interference problems but also do not lend themselves to low-cost production or reduction in size, weight, and cost. The objectives of the digital receiver design are to provide overall improved performance, reliability, maintainability, and reduced cost and weight and to be cost-effective. The architecture and design lend themselves to flexibility in providing enhanced functional capability through the sharing of software and hardware and can be expanded to integrate other functions in the receiver for future growth. The research and development necessary for the advanced digital receiver technology were performed at Cubic Defense Systems under various IRD (2) preliminary analysis and performance assessment; (3) comparison and selection of the preferred approach; (4) distortion analysis of each receiver stage for the various modulations; and (5) analysis of the overall receiver performance. The application of DSP technology to the design of the digital receiver can be illustrated by following the design of the VHF/UHF communication receiver developed at Cubic Defense Systems. The specifications of this receiver are tabulated in Table 1. Minimization of signal distortion is one of the objectives of such a design, and Section 2 discusses the sources of distortion in analog and digital receivers. Section 3 describes four different receiver architectures and compares their performance, advantages, and limitations. Section 4 discusses various digital demodulation schemes, and finally, Section 5 summarizes these design steps.

A full-band waveguide SIS receiver with integrated tuning for 75-110 GHz

The performance of a broadband SIS (superconductor-insulator-superconductor) receiver with no mechanical tuning elements was modeled and tested. The mixer mount had a broadband-waveguide-to-microstripline transition consisting of a four-step Chebyshev single-ridge transformer. The last step of the ridge connected the waveguide to a microstripline circuit. The on-chip circuit consisted of a microstripline that transmitted the RF and the local oscillator signals to the SIS mixer. Both the SIS element and the inductor had RF grounds provided by 90 degrees radial stubs. The inductor tuned out the junction capacitance to allow operation over the full frequency band. The SIS element can be a single junction or series array using Nb-AlO/sub x/-Nb trilayer tunnel junctions with areas as small as 0.5 mu m/sup 2/ and V/sub m/(2 mV)=39 mV. Preliminary results indicated a DSB receiver noise temperature of 65-80 K across the band measured at 4.4 K with an internal cryogenic RF hot/cold source and with T/sub if/=21 K. With another device, a mixer noise temperature of 35 K at 100 GHz was achieved, increasing to 45 K at 79.5 and 110 GHz. Coupled mixer gain of up to +3 dB and negative dynamic resistance on the first photon step were observed. The lowest noise temperature was obtained for an untuned single-junction mixer at 80 GHz: T/sub R/=41 K and T/sub M/=20 K were measured.

Electromagnetic deposition in an anatomically based model of man for leakage fields of a parallel-plate dielectric heater

An anatomically based 5628-cell model of a human being was used to calculate local, layer-averaged, and whole-body-averaged specific absorption rates and internal RF currents at 27.12 and 40.68 MHz for spatially variable electromagnetic fields of a parallel-plate applicator representative of RF dielectric heaters used in industry. The conditions of exposure of the man model considered are: isolated from ground, shoe-wearing condition, feet in contact with ground, and an additional grounded top plate 13.1 cm above the head to simulate screen rooms that are occasionally used for RF heaters. Since peak E fields as high as 1000-2700 V/m have been measured at locations typically occupied by the operator, significant internal RF currents on the order of 0.5-2.3 A are projected for the operators. Measurements of the foot currents at 27.12 and 40.68 MHz for a human subject are in reasonable agreement with the calculated values for the various conditions of exposure.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Species and divergence measurements on the TFTR diagnostic neutral beam

The ion source of the TFTR diagnostic neutral beam utilizes a magnetic cusp bucket from a PLT source modified with an internal rf antenna in place of filaments. The species mix of the beam has been measured after the neutralizer as a function of several parameters in two ways: spectroscopically and by an electrostatic charge exchange analyzer. Source and neutralizer gas feeds (both hydrogen) were varied. Some measurements were made with the bending magnet alternately on and off to obtain neutralizer efficiency for each component. Accelerator current was varied, at fixed acceleration voltage and gas feed rate, by varying rf power to the antenna. Relative beam divergence during this scan was monitored by both diagnostics, and an absolute divergence was obtained from a spectroscopic profile scan.

Development of radio frequency induction plasma generators for neutral beams

The techniques, operational aspects, and experimental results of a radio frequency induction plasma generator, with an internal rf power coupler, intended for intense neutral beam applications are described. One of the development sources suitable for 10×10-cm2 extraction optics was operated to a deuterium ion current density of 250 mA/cm2, uniform to 5%, over a circular extraction area 15 cm in diameter with a coupled rf power of 20 kW. Temporal fluctuation levels in the extracted ion current were measured to be typically 1% of the dc level. A second developmental source suitable for 10×40-cm2 grid sets was operated to 200 mA/cm2, uniform to ±8% over a 10×40 cm2 area, with 40-kW coupled rf power.

Capacitance measurement of Josephson tunnel junctions with microwave-induced dc quasiparticle tunneling currents

The microwave response of the dc quasiparticle tunneling current in Josephson tunnel junctions, where the Josephson current is suppressed by an external magnetic field, has been studied quantitatively in order to clarify its characteristics as a probe for the measurement of the junction capacitance. Extensive experiments for both small and long junctions are carried out for distinguishing between microwave behaviors of lumped and distributed constant junctions. It is shown that the observed voltage dependence of the dc quasiparticle tunneling current modified by an applied rf field is in good agreement with a theoretical result which takes into account the influence of the microwave circuit connected to the junction. The comparison between theory and experiment gives the magnitude of the internal rf field in the junction. Together with the applied rf field, this internal rf field leads to the junction rf impedance which is dominated by the junction capacitance in our experimental condition. In the case of lumped junctions, this experimental rf impedance is in reasonable agreement with the theoretical one with the junction capacitance estimated from the Fiske step of the distributed junction fabricated on the same substrate; the obtained ratio of the experimental impedance to the theoretical one is approximately 0.6–1.7. In the case of distributed junctions, however, experimental values of their characteristic impedances are approximately 0.2–0.3 of theoretical values calculated by assuming the one-dimensional junction model and taking account of the standing-wave effect in the junction.

Localized Fields and Density Perturbations Due to Self-Focusing of Nonlinear Lower-Hybrid Waves

Large-amplitude lower-hybrid wave bursts at ω0>ωLH are excited from a grid and propagate into a density gradient perpendicular to B0. The evolution of the internal rf fields and the plasma parameters is investigated. Above threshold localized field maxima appear (Eint>>Eappl) and, simultaneously, strong density perturbations are formed. The nonlinear phenomena are interpreted by ponderomotive-force and space-charge separation effects.

Mass-spectrometric sampling of the ionic and neutral species present in different regions of an RF discharge in methane

The ionic and neutral products of RF discharges in methane produced by capacitive and inductive coupling have been examined. The configurations studied were (1) axial sampling through an internal RF capacitor electrode; (2) axial sampling through an electrically grounded internal capacitor electrode; (3) radial sampling of a capacitively coupled discharge through an electrically floating electrode; and (4) axial sampling of an inductively coupled discharge through a grounded electrode. Major differences in the ions observed occur between configuratons (1) and (3), above. In the former, the dominant ions found are C +, CH +, CH 2 +, CH 3 + C 2H 2 +, and C 2H 3 +, with very little abundance of ions containing more than three carbon atoms. In the latter, the ions CH 3 +, CH 5 +, C 2H 3 +, and C 2H 5 + are dominant, with a significant fraction of the total comprising C 4 to C 6 ions. The dissimilarity observed between these two cases clearly can be attributed to the difference in energy of the electrons which cause the ionization and the difference in kinetic energy of the primary ions which undergo ion—molecule reactions to yield secondary ions. Ions containing two or more carbon atoms have been shown to result from ion—molecule reactions rather than from primary ionization of C 2 neutrals. The neutral gas composition is also function of position in the discharge and is clearly related to the energy available at any particular sampling point.

Retardation Effects Caused by Ferrite Sample Size on the Frequency Shift of a Resonant Cavity

Using perturbation theory, expressions are derived for the frequency shift of a circularly polarized resonant microwave cavity upon insertion of a ferrite sample small compared with the wavelength. Retardation effects are considered in deriving the rf field h2 internal to the sample. By integrating h1·h2dv (where h1 is the applied rf field) over the sample volume, a closed-form expression is obtained for the cavity frequency shift. The theory is applied to the magnetic resonance in ferrites. The calculated magnetic resonance line shape is found to remain symmetrical as the sample size is increased. Retardation effect, thus, does not explain the asymmetry of the resonance lines which have been observed for polycrystalline ferrite. Results of the theory are also compared with measurements on a single crystal of ferrite having the rather large maximum permeability loss term of 100. Theoretical and measured values are in close agreement. The theory is also compared with the somewhat different results of other recent theoretical treatments, in which an approximate expression for an effective magnetic dipole moment for the sample was computed, rather than integrating the internal rf fields over the volume of the sample.