Bias Voltage Source Research Articles

An Experimental Complex for Studying the Pulse-Periodic Diffuse Discharge Used for the Sterilization of Medical Instruments

An experimental complex for studying the processes that govern the initiation of a diffuse discharge was developed. The complex contains gas discharge chambers with various electrode systems and a high-voltage pulse-periodic generator. A specific feature of this complex's power supply system is that the dc bias-voltage source and the pulse generator can be used simultaneously. The pulse-periodic generator is based on a pulse transformer and a high-voltage shaper of nanosecond pulses with an amplitude of up to 120 kV and a rise time of <10 ns.

Deposition of hard carbon coatings using combined inductively and capacitively coupled plasma sources

A combined inductively coupled plasma (ICP) and capacitively coupled plasma (CCP) source has been presented. Diamond-like carbon (DLC) films can be prepared on BK7 glass substrates using only the CCP source. An efficient cooling of the substrates is very important to obtain DLC films when the ICP source is also supplied. The refractive index n and the thickness of the DLC films on BK7 glass substrates were determined from transmission spectra by a computer program. Without CCP power the deposits on scratched Si samples at the pressures of 0.1–1 mbar in a CH 4/H 2/Ar gas mixture are microcrystalline or nanocrystalline diamonds. The morphology of the diamond films changes to pyramidal-like on scratched and ball-like crystals on unscratched Si samples when a CCP power is applied to use as a bias voltage source. The effect of the ratio of CCP to ICP power on the structure and morphology of diamond films has been investigated. Ball- and pyramidal-like crystals size and sp 3 content increase with increasing ratio of CCP to ICP power.

Angular dependence of etch rates in the etching of poly-Si and fluorocarbon polymer using SF6, C4F8, and O2 plasmas

The dependences of etch rates on the angle of ions incident on the substrate surface in four plasma/substrate systems that constitute the advanced Bosch process were investigated using a Faraday cage designed for the accurate control of the ion-incident angle. The four systems, established by combining discharge gases and substrates, were a SF6/poly-Si, a SF6/fluorocarbon polymer, an O2/fluorocarbon polymer, and a C4F8/Si. In the case of SF6/poly-Si, the normalized etch rates (NERs), defined as the etch rates normalized by the rate on the horizontal surface, were higher at all angles than values predicted from the cosine of the ion-incident angle. This characteristic curve shape was independent of changes in process variables including the source power and bias voltage. Contrary to the earlier case, the NERs for the O2/polymer decreased and eventually reached much lower values than the cosine values at angles between 30° and 70° when the source power was increased and the bias voltage was decreased. On the other hand, the NERs for the SF6/polymer showed a weak dependence on the process variables. In the case of C4F8/Si, which is used in the Bosch process for depositing a fluorocarbon layer on the substrate surface, the deposition rate varied with the ion incident angle, showing an S-shaped curve. These characteristic deposition rate curves, which were highly dependent on the process conditions, could be divided into four distinct regions: a Si sputtering region, an ion-suppressed polymer deposition region, an ion-enhanced polymer deposition region, and an ion-free polymer deposition region. Based on the earlier characteristic angular dependences of the etch (or deposition) rates in the individual systems, ideal process conditions for obtaining an anisotropic etch profile in the advanced Bosch process are proposed.

Simulation of a two-dimensional sheath over a flat insulator–conductor interface on a radio-frequency biased electrode in a high-density plasma

A combined fluid/Monte Carlo (MC) simulation was developed to study the two-dimensional (2D) sheath over a flat insulator/conductor interface on a radio-frequency (rf) biased electrode in a high-density plasma. The insulator capacitance increased the local impedance between the plasma and the bias voltage source. Thus, for uniform ion density and electron temperature far away from the wall, the sheath potential over the insulator was only a fraction of that over the conductor, resulting in a thinner sheath over the insulator. The fluid model provided the spatiotemporal profiles of the 2D sheath electric field. These were used as input to the MC simulation to compute the ion energy distribution (IED) and ion angular distribution (IAD) at different locations on the surface. The ion flux, IED, and IAD changed drastically across the insulator/conductor interface due to the diverging rf electric field in the distorted sheath. The ion flux was larger on the conductor at the expense of that on the insulator. Both the ion impact angle and angular spread increased progressively as the material interface was approached. The ion impact energy and energy spread were smaller on the insulator as compared to the conductor. For given plasma parameters, as the insulator thickness was increased, the sheath potential and thickness over the insulator decreased, and sheath distortion became more pronounced.

Redeposition of etch products on sidewalls during SiO2 etching in a fluorocarbon plasma. II. Effects of source power and bias voltage in a CF4 plasma

Variations in the properties of sidewalls due to the redeposition of etch products emitted from the bottom during SiO2 etching in a CF4 plasma were studied under different conditions of source power and bias voltage, in the range of 100 and 500 V, and 0 and 400 W, respectively. A Faraday cage and a step-shaped SiO2 pattern located in a transformer coupled plasma etcher permitted the control of the incident angle of ions, thus permitting the redeposition phenomenon to be observed on a macroscopic scale. Under all process conditions, the deposition rate on sidewall (A), affected by the redeposition of particles emitted from the bottom, was larger than that on sidewall (B), which was unaffected by the redeposition, because particle redeposition induced the formation of a sidewall passivation layer on sidewall (A). It was indirectly confirmed that the amount and the kinetic energy of particles sputtered from the bottom were closely related to the formation of the sidewall passivation layer. The redop-effect, which indicates the extent to which the redeposition contributes to the deposition rate on the sidewall, increased with bias voltage and source power. The extent of etching of the SiO2 underlayer of sidewall (B) increased with both the source power and bias voltage, in contrast to the case of sidewall (A). On sidewall (A), the extent of SiO2 etching increased continuously with source power, but only increased with bias voltage when the latter was under 300 V. The etching decreased at higher bias voltages.

Resist trimming in high-density CF4/O2 plasmas for sub-0.1 μm device fabrication

Resist trimming using a CF4/O2 plasma generated by an inductively coupled high-density plasma source is investigated. Results show that the resist trimming process is applicable for the fabrication of a sub-0.1 μm polysilicon gate electrode with conventional 248 nm lithography. The trim rate depends strongly on gas composition, temperature, rf source power, bias voltage, reactor pressure and total gas flow. The trim rate increases with increasing temperature, rf source power and reactor pressure but decreases with higher bias voltage. For a typical trimming condition at gas composition of CF4 and O2 with a ratio of 3.3:1, the resist trim rates are 1.0 and 1.2 nm/s for dense and isolated lines, respectively. By examining the effect of various process parameters, it is found that the trimming process is dominated by the neutral reactant species. Angle-resolved x-ray photoelectron spectroscopy shows that a fluorinated carbon oxyfluoride polymer is deposited on the sidewall of photoresist. The degree of carbon fluorination of the film is influenced by the process parameters. A more fluorinated film, obtained at lower bias voltage and higher source power, gives more trimming.

Low-frequency noise characteristics of metamorphic In 0.52Al 0.48As/In 0.60Ga 0.40As double-heterostructure pseudomorphic high electron mobility transistors grown on a GaAs substrate

Low-frequency noise characteristics of metamorphic In 0.52Al 0.48As/In 0.60Ga 0.40As double-heterostructure pseudomorphic high electron mobility transistors (p-HEMTs) grown on a GaAs substrate are investigated. Low-frequency noise spectral density of the In 0.52Al 0.48As/In 0.60Ga 0.40As metamorphic HEMT (MM-HEMT) having two 0.5×50 μm 2 gates was measured for temperatures between 200 and 400 K and frequencies between 1 Hz and 53 kHz. Dependence of input noise spectral density on gate–source and drain–source bias voltages and transconductance frequency dispersion characteristics were also characterized. The low-frequency input noise spectra of the MM-HEMT showed pure 1/ f noise characteristics for the temperature and the frequency ranges investigated, indicating that there exists insignificant level of deep traps associated with the metamorphic epitaxial layer growth. The MM-HEMT showed a very low-noise spectral density (0.8×10 −13 V 2/Hz at 1 kHz) and the Hooge parameter (3.7×10 −5) that are comparable to those of the state-of-the-art In 0.52Al 0.48As/In 0.53Ga 0.47As HEMT grown on an InP substrate and an extremely small transconductance frequency dispersion ( Δg m/ g m0<1%). The results indicate a great potential of the In 0.52Al 0.48As/In 0.60Ga 0.40As MM-HEMT grown on a GaAs substrate for millimeter-wave circuit applications requiring low phase noise characteristics.

Automatic Coupling Control of a Loop–Gap Resonator by a Variable Capacitor Attached Coupling Coil for EPR Measurements at 650 MHz

A coupling coil was fabricated that can electrically change the magnetic coupling with a loop–gap resonator (LGR) for EPR studies at 650 MHz. It is composed of a single-turn coil and a coupling control circuit that includes a varactor diode. The coarse control of the magnetic coupling is made by mechanically changing the distance between the LGR and single-turn coil. The fine control is obtained by changing the capacitance of the varactor diode that is connected in parallel with the single-turn coil. This capacitance is controlled by changing reverse voltage from a variable bias voltage source. Because this can be located far from the resonator, remote control of coupling of the LGR is possible. Automatic coupling control (ACC) was accomplished by negative feedback of the DC component in the radiowaves reflected from the LGR to the coupling control circuit when the LGR was irradiated precisely at its resonant frequency. To accomplish this, automatic frequency control (AFC) is used. In EPR measurements of a phantom that included a physiological saline solution containing a nitroxide radical, it was confirmed that the drifts in the coupling and resonant frequency caused by the perturbation of the resonant nature could be sufficiently compensated by the ACC and AFC systems. In the in vivo EPR studies, it was found that the deviation of coupling at the chest of a mouse is greater than that at the head of a rat, but the ACC system could compensate for the respiratory motions of a living animal.

Effects on the linearity in Ka band of single or double-recessed PHEMT's

The effects of the gate recess topologies on the linearity performance of PHEMT's have been investigated in the Ka band. The authors highlight the reasons why, at a given output power level, the double recessed device exhibits a very large improvement of its intermodulation performance as its drain source bias voltage is increased whereas its linearity is inferior to that of the single recessed device at low drain source bias voltage. The effect of the load impedance on the linearity behavior has also been investigated. At a given output power the load impedance contour of the double recess structure is shown to exhibit a much larger variation of the intermodulation ratio than that of the single recess structure.

Analysis of Schottky gate electron tunneling in polarization induced AlGaN/GaN high electron mobility transistors

( gate=nickel)/(barrier=GaN/Al (y) Ga (1−y) N)/(buffer=GaN)/(substrate=SiC ) polarizationinduced high electron mobility transistors (PI-HEMTs) show promise for ultrahigh power microwave amplification. The polarization fields in these Ga-face, wurtzite structures are used to engineer the conduction band profile in the absence of any doping. One important attribute of these PI-HEMTs is their ability to withstand electric fields up to 3 MV/cm without avalanche breakdown. The combination of high electric field under the gate necessary to pinch off the channel, and the high electric field fringing to the gate from the large drain–source bias voltage, makes it possible for electron tunneling from the gate into and through the GaN/Al(y)Ga(1−y)N barrier into the GaN channel. A one-dimensional theoretical analysis of the tunneling current at pinchoff as a function of the aluminum fraction (0.05&amp;lt;y&amp;lt;0.45) in the barrier, the operating temperature of the gate metal at 300 and 573 K, and a uniform additional electric field of −1 and −2 MV/cm from drain bias reveals the potential for these GaN based devices to impose negligible current loading by the gate on a microwave driver. Unlike GaAs, avalanche breakdown induced by gate leakage is unimportant in these GaN based power devices.

Progress in the use of 4H-SiC semi-insulating wafers for microwave power MESFETs

The use of Silicon Carbide semi-insulating wafers has to be mastered in order to reach output power densities up to 3–4 W mm −1 in the 1–2 GHz frequency range (values that are currently targeted). This study shows that the buffer layer doping level and thickness have a great influence on the MESFET behavior. We have studied three epilayer configuration on SI substrates that differ only by the buffer layer. On two of them, a slow drain current decrease in d.c. mode was observed. On the third one, no d.c. current drift was observed without rf input power, but drain current decreases instantaneously when rf input power is switched on. Traps, located either in the buffer layer or in the substrate are supposed to be responsible for these drift phenomena. Load–pull measurements were performed at 2 GHz on transistors fabricated on the three different structures. One of them, with a 2 mm gate periphery, has been measured under 72 V drain–source bias voltage and 2.1 W mm −1 power density was obtained at 2 GHz. We believe these results are the first to be published on a SiC MESFET with d.c. bias voltage over 70 V.

Factors determining energy values of ion beams for ion-beam deposition

IBD (ion-beam deposition) is performed with a very low-energy ion beam, whose energy value is not simply determined. For the accurate evaluation an excess energy due to the source-plasma potential should be added to the energy value obtained simply from the source-bias voltage. However, the plasma potential is changing dependently on the secondary electrons from chamber wall as well as the plasma parameters. We studied theoretically how the plasma potential is affected by the chamber material as well as by the plasma parameters. Then, we measured floating potentials of several materials immersed in a microwave plasma for studying the influence of secondary electrons from the wall. The result suggests that the potential of a plasma surrounded by material of a high secondary-electron coefficient becomes low and consequently the excess energy decreases.

Light emission from tunnelling mode GaAs static induction transistor

Vertical type GaAs static induction transistors (SIT) with 160 Å channels were fabricated with selective regrowth, using molecular layer epitaxy. The structure with a sidewall channel is suitable for electron transport investigation with electroluminescence spectra. The room temperature spectra (below bandgap, in the range from 0.6 eV to 1.4 eV) indicate tunnelling as the room temperature electron transport mechanism for this device. At sufficiently high drain–source or gate–source bias voltages, most of the electrons are injected directly from the source to the drain. The electron transit time was roughly estimated as 2 × 10–14 s.

Measurement of excess energies of ion beams extracted from a microwave ion source

When an ion beam is used as a tool for analysis based on its energy value, the precision of the measurement depends on how accurately the ion energy is evaluated. However, as far as a plasma type ion source is concerned, the energy value of a very low-energy ion beam, lower than 100 eV specifically, should be carefully treated. Strictly speaking, the ion energy is equal to the ion charge multiplied by the sum of the source bias potential supplied to the chamber and the plasma potential with respect to the chamber. Of course, when the bias potential is sufficiently high, the plasma potential can be neglected. Theoretically, the plasma potential is determined by the electron temperature which is changeable depending on the source operating condition. Furthermore, the fact, that most plasmas of industrially used ion sources have plural electron temperatures and not a single one, makes the problem difficult to resolve. In this paper, we used the magnetic field of a mass separator to directly evaluate the plasma potential in a microwave ion source by utilizing the linear relationship between the squared magnetic flux of the mass separator and the ion acceleration voltage, i.e. the sum of the source bias potential and the plasma potential. The measurement showed that the excess energy of the ion beam due to the plasma potential rises above 100 eV at a microwave power of more than 500 W. Thus, when we handle a low energy ion beam of the order of 100 eV, the real ion energy may be more than twice the value given only by the source bias voltage at the chamber. The result is compared with the theoretical calculation for a plasma model of double electron temperatures.

Microwave ion sources for material processing (invited)

Microwave ion sources were developed and used for material processing in industrial manufacture. Major applications of them were concentrated on semiconductor device fabrication, especially on modifying electrical properties of semiconductor material by ion implantation, as well as on processing submicron structures of semiconductor devices by so-called plasma etching. Most ion implanters for manufacturing silicon on insulator wafers exclusively adopt microwave ion sources because the sources work very stably for oxygen ions. Recently, the application of ion beams is extending to nonsemiconductor fields. Thus, ions are applied to surface modification of nonsemiconductor materials and to thin-film formation by ion beam deposition which is carried out with low energy of several tens of eV. The characteristics of films are very dependent on the beam energy values which are strictly determined by sum of the source bias voltage and the plasma potential with respect to the chamber. Microwave ion sources are very suitable to such applications of low-energy ion beams because the ions extracted from them have a very small energy dispersion. In this article, the features of microwave ion sources are specified and a variety of the sources are reviewed with respect to their applications.

Overvoltage protection for battery powered equipment

An overvoltage protection circuit for a CMOS electronic multimeter circuit uses a pair of complementary field-effect protection transistors connected to each input line of the meter circuit. The gate electrodes of the protection transistors are connected to bias voltage sources which provide bias voltages with magnitudes slightly less than the meter power supply voltages. The protection transistors go into conduction when an overvoltage condition causes the voltage on the input to exceed the bias voltages and the conducting protection transistors clamp the input voltage to substantially the bias voltage. Current caused by an overvoltage condition is shunted to ground thereby avoiding a charging condition in the power supplies.

Tunable acoustic resonator for clinical ultrasonic transducers

A tunable ultrasonic probe includes a body of a first piezoelectric material acoustically coupled in series with a body of a second piezoelectric material. The second piezoelectric material has a Curie temperature that is substantially different than that of the first piezoelectric material. Preferably, the first piezoelectric material is a conventional piezoelectric ceramic, such as lead zirconate titanate, while the second piezoelectric material is a relaxor ferroelectric ceramic, such as lead magnesium niobate. At an operating temperature of the probe, the first piezoelectric material has a fixed polarization. In contrast, the second piezoelectric material has a polarization that is variable relative to the fixed polarization of the first piezoelectric material. A preferred novel arrangement of electrodes electrically couples the bodies in parallel with one another. An oscillating voltage for exciting the acoustic signals in the probe is coupled with the electrodes. The polarization of the second piezoelectric material is variably controlled by a bias voltage coupled with the electrodes. In a preferred embodiment, the bias voltage has a reversible electrical polarity for selecting one resonant frequency from a plurality of resonant frequencies of the probe. In another preferred embodiment, the bias voltage source has a variable voltage level for selecting at least one of a plurality of resonant frequencies of the probe.

Ensemble Monte Carlo study of interface-state generation in low-voltage scaled silicon MOS devices

An ensemble Monte Carlo (MC) model coupled with an interface-state generation model was employed to predict the quantity and lateral distribution of hot-electron-induced interface states in scaled silicon MOSFETs. Constant field and more generalized scaling methods were used as the basis to simulate devices with 0.33-, 0.20-, and 0.12-/spl mu/m channel lengths. The dependencies of interface-state generation on applied bias and electric field profiles were investigated. Hot-electron injection and interface-state density profiles were simulated at biases as low as 1.44 V (i.e., lower than the 3.1 V potential barrier at the Si/SiO/sub 2/ interface). These simulations demonstrate that "lucky electron" and/or electron temperature models are no longer accurate for predicting hot-electron effects in such regimes. Electron-electron scattering is shown to be a critical consideration for simulation of hot-electron injection at low drain to source bias voltages, where local interfacial barrier heights are greater than the energy gained by an electron from the applied electric field. Simulations indicate that a scaled decrease in the channel length of a device may be accompanied by an increase in the lateral electric field without incurring a penalty for higher hot-electron degradation. It is also shown that conventional hot-electron stressing using accelerated stress bias conditions may continue to be valuable for predicting the reliability of device designs scaled to 0.1-/spl mu/m channel lengths.

Profiling interface traps in MOS transistors by the DC current-voltage method

Position profiling the interface trap density along the channel length of metal-oxide-silicon transistors by the Direct-Current Current-Voltage method is illustrated for five density variations: zero, peaked in drain junction space-charge layer, constant in channel, nonconstant in channel, and peaked in drain junction space-charge layer and nonconstant in channel. The interface trap densities were monitored by MOS transistor's d.c. body current and the density profiles were obtained from the body-drain and body-source differential conductance versus drain or source bias voltage. An experimental demonstration is given for a 1.6 μm n-channel Si MOS transistor with about 10/sup 11/ traps/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> generated by channel hot electron stress.

An analytical model for the photodetection mechanisms in high-electron mobility transistors

The use of microwave high-electron mobility transistors (HEMTs) as photodetectors or optically controlled circuit elements have attracted interest. A model of the optical characteristics of HEMTs, which takes into account carrier transport as well as the quantum mechanical nature of the two-dimensional (2-D) electron gas channel, is presented. It is shown that the effect of illumination is equivalent to a shift in the gate to source bias voltage, referred to as the internal photovoltaic effect. The theoretical model is supported by experimental results that demonstrate that the HEMT photoresponse is a nonlinear function of light intensity with very high responsivity at low optical power levels.