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Laser diode absorption spectroscopy is used to experimentally measure Ar(1s5) metastable density and translational gas temperature within a 94 GHz microplasma. A square two-dimensional photonic crystal (PhC) at this resonance frequency serves to ignite and sustain the plasma from 20 to 200 Torr (2.7 × 103–2.7 × 104 Pa) by using millimeter wave power from 300 to 1000 mW. Metastable density within the plasma is estimated from the absorption line shape of the laser traversing the PhC. The metastable density reaches an order of 1019 m−3 at lower pressure and decreases as pressure increases. From the Lorentzian line shape of the absorption profile at 811.53 nm, the gas temperature is extracted and found to increase from 500 K at 20 Torr to 1300 K at 200 Torr. These data are analyzed and compared with a zero-dimensional plasma model and with previous experimental plasma results at 43 GHz. [ABSTRACT FROM AUTHOR]

Short pulses of millimeter wave (MMW) radiation at 43 GHz create microplasma within a photonic crystal for pressures from 40 to 600 Torr (1.3 × 103–8.0 × 104 Pa). Gas breakdown occurs within a photonic crystal, which acts as an electromagnetic resonator to create a strong initial electric field. The time response of the argon metastable density is experimentally determined during the pulse and in the afterglow using laser absorption. The metastable density overshoots the steady-state condition at the beginning of the pulse and during the afterglow. Modeling is presented to understand these observations. The overproduction of argon 1s5 at the beginning of each pulse is due to a concurrent overshoot in the MMW electric field within the photonic crystal. This field overheats the plasma electrons and enhances the production of excited states. The burst of argon metastables observed in the afterglow is due to the pooled energy of the plasma stored in electrons, ions, and excited states of argon. Understanding metastable production is an important intermediate step to ionization and is also critical in the study of diode-pumped rare gas lasers. [ABSTRACT FROM AUTHOR]

The electron density (n e) and argon metastable density (1 s 5) of a 43 GHz microplasma are obtained using the zero-dimensional plasma kinetics solver (ZDPlasKin®) for an incident power of 160 and 1000 mW and argon pressures of 10 – 600 Torr (1.3 × 10 3 to 8.0 × 10 4 Pa). These simulations are compared with previously published experimental data. To determine the self-consistent electric field in the modeled plasma, the three-dimensional millimeter wave fields are computed as a function of electron density using ANSYS EM19.2, HFSS®. This electromagnetic field model is coupled to ZDPlasKin such that any increase in the simulated plasma density correctly attenuates the simulated electric field within the microplasma. The electron density is found to be sensitive to argon gas temperature, so a two-zone temperature model was needed to obtain agreement with experimental measurements. The temperature in the central core of the microplasma was determined by a previous experimental study. That temperature was used as an input to the model for the simulation of volume recombination losses. The outer regions of the microplasma are assumed to be in equilibrium with the walls (300 K). This second temperature was used in the model to determine diffusion losses. The modeled electron and metastable densities are of the order 10 20 and 10 18 m − 3 , respectively. This is in good agreement with those measured experimentally as long as the two-zone temperature model is used. In the absence of a hot gas core, the modeled three-body recombination rates are excessive and the simulation severely under-predicts the electron density and over-estimates the metastable density. We conclude that the millimeter wave microplasma has a hot core (2500 K at 600 Torr) that rarifies the argon gas and effectively reduces three-body recombination. This allows one to achieve high electron density on the order of 10 20 m − 3 with only 100 mW of wave power. [ABSTRACT FROM AUTHOR]

Argon (1s5) metastable density and translational gas temperature are experimentally measured using laser diode absorption spectroscopy within a 43 GHz microplasma. The plasma is initiated and sustained within a photonic crystal constructed from a rectangular array of alumina rods, each 1 mm in diameter. This configuration generates stable microplasma from 10 to 600 Torr using millimeter wave power from 100 to 1200 mW. The metastable density is in the order of 1018 m−3 at low pressure. However, Ar(1s5) density decreases to undetectable levels with increasing pressure and wave power. The gas temperature is extracted from the Lorentzian line shape of the absorption profile at 811.53 nm. The gas temperature increases from approximately 400 K at low pressure to 2000 K at 320 Torr (427 mbar, 4.27 × 104 Pa). These data are compared with previous results and suggest that the microplasma has a dense core of electrons that depletes the metastable density at high gas pressure and wave power. [ABSTRACT FROM AUTHOR]

Microplasma generated within a millimeter wave (MMW) photonic crystal (PhC) is analyzed by direct measurement of the electron density and the wave transmission spectra (S21) of the crystal. A continuous wave (cw) drive frequency of 43.66 GHz maintains plasma, while a low power frequency sweep (43.5–44.1 GHz) simultaneously probes the wave transmission in the presence of microplasma. Rotational gas temperature and electron density are measured from the CH emission spectrum and the Stark broadening of the Hβ atomic transition. The permittivity of the plasma depends on the electron density. As higher cw power and argon gas pressure increase the electron density in the PhC vacancy, the resonant frequency of the PhC shifts upward in accordance with the measured electron density and plasma permittivity. As the PhC obscures the central core of the microplasma, we present a simple diffusion model that approximates the electron density distribution within the microplasma. The diffuse electron density is then used in a numerical model for S21(ω). The measured and modeled transmission spectra can only be reconciled using the diffusion density gradient of the microplasma, thus resolving previous discrepancies observed when using simplified, abrupt-boundary plasma models. [ABSTRACT FROM AUTHOR]

We present the transmission characteristics of pulsed 44 GHz millimeter waves (MMWs) through a plasma formed within a photonic crystal. Initially, a low density plasma (n e = 0.3 × 10 18 m − 3 ) is sustained in a crystal vacancy defect using a continuous wave (cw) MMW power of 0.06 W. This weak plasma does not much attenuate wave transmission, but it does provide seed electrons such that dense plasma forms in response to incoming MMW pulses. The dense plasma formation limits the transmission of high power pulses through the photonic crystal. A high power pulse increases the plasma electron density and shifts the resonant frequency of the crystal vacancy in accordance with the plasma permittivity. A rapid decrease in the transmitted power is observed by following the time-evolution of the plasma. A numerical electromagnetic model estimates electron density and traces the time-evolution of the plasma during MMW pulsing. The plasma-filled photonic crystal is evaluated as a power limiter. As plasma properties are readily tunable, we observe an improved operational bandwidth for incoming pulse frequencies by controlling the cw power and gas pressure of the initial weak plasma. [ABSTRACT FROM AUTHOR]

Plasma formation between coupled dielectric resonators (DRs) is frequency scaled from 1 GHz to 7 GHz. Previous research found that applying low-power microwaves to a pair of calcium titanate (CaTiO3) resonators could ignite plasma between them using the HEM111 resonance mode. Those large, 1 GHz DRs are scaled down in size, and thus scaled up in frequency. The optimum relative permittivity of DRs for plasma formation is reported to be 200, confirming that CaTiO3 (ɛr ∼ 170 ± 5) is a good material to use for plasma-forming DRs. This paper presents data collected for a 2D array of resonators operating in the 6–7 GHz range for separation distances of 0.25 mm and 0.5 mm in a 1–10 Torr argon gas environment. Gas breakdown data for two DR configurations are compared to the Raizer theory at 1.1 GHz and 6.5 GHz. Plasma modulation of the reflection and transmission of DR arrays with plasma on and off indicates possible future applications as a frequency selective filter. [ABSTRACT FROM AUTHOR]

Optically pumped rare gas laser systems require a lasing medium of high density metastable atoms. A scalable two-dimensional array of quarter-wave resonators produces high microwave electric fields suitable for generating a dense gas of argon 1 s 5 states. This resonator array is immersed within an evacuated waveguide in order to create a volume of plasma in a relatively wide discharge gap (13 mm) at argon-helium pressures up to 100 Torr using 30 W of continuous microwave power at 2.3 GHz. Spatial density profiles of argon 1 s 5 states are measured using diode laser absorption spectroscopy with 0.5 mm resolution. The region of argon 1 s 5 states is mapped in the discharge and shown to have an estimated density in excess of 1 × 10 18 m − 3 within a volume of 11.6 × 3.2 × 0.8 m m 3. [ABSTRACT FROM AUTHOR]

Capacitively coupled microplasma generation becomes more efficient at microwave frequencies. Helium and argon microplasmas are characterized using excitation frequencies of 450 MHz, 900 MHz, and 1.8 GHz. These microplasmas are tested at both atmospheric pressure and 0.4 torr. We have experimentally determined the microplasma's electrical impedance, which consists of bulk plasma resistance and capacitive sheath reactance. These two parameters were measured by fitting theoretical power reflection coefficients to experimental forward and reflected microwave power as a function of frequency. Microplasma resistance decreases with increasing frequency, showing that the generation of free electrons depends on the driving frequency. In addition, the reactive sheath impedance and the microwave electrode voltage also decrease with an increase of frequency. A 3-D microplasma simulation shows that a narrower sheath width exists for higher frequency microplasma, and this is responsible for reducing the reactive impedance and the peak-to-peak electrode voltage. At higher microwave frequency, the decreased electrode voltage reduces both the plasma potential and the ion kinetic energy losses, thus increasing the electron density. Index Terms--Microplasma, microwave plasma, plasma impedance, plasma simulation.

The localized potential gradients created by a microplasma are capable of trapping and concentrating micro- and nanoparticles. In this paper, argon microplasma is generated within a 350-[micro]m discharge gap formed within a microstrip transmission line. Melamine formaldehyde particles (1 [micro]m) are released approximately 2 cm away from the microplasma. The microparticles are then negatively charged by stray electrons, electrostatically drawn toward the potential well of the microplasma, and trapped within the microplasma. The particles are observed to form Coulomb crystals. Time-of-flight experiments show that the particles are trapped in the microplasma by balancing the electrostatic force of the potential well against the molecular drag force. Pulsed plasma data show that the particles retain a net negative charge after the plasma has been extinguished, allowing detection and sorting by electrostatic methods. Index Terms--Dusty plasma, microparticle, microplasma.

Microwave-driven plasmas operating near atmospheric pressure have been shown to be a promising technique for producing the high density of argon metastable atoms required for optically pumped rare gas laser systems. Stable microwave-driven plasmas can be generated at high pressures using microstrip-based resonator circuits. We present results from computational modeling and laser absorption measurements of argon metastable densities in such plasmas operating in argon-helium gas mixtures at pressures up to 300 Torr. The model and measurements resolve the plasma characteristics both perpendicular to the substrate surface and along the resonator length. The measurements qualitatively and in many aspects quantitatively confirm the accuracy of the model. The plasmas exhibit distinct behaviors depending on whether the operating gas is mostly argon or mostly helium. In high-argon plasmas, the metastable density has a large peak value but is confined very closely to the electrode surfaces as well as being reduced near the discharge gap itself. In contrast, metastable densities in high helium-fraction mixtures extend through most of the plasma. In all systems, increasing the power extends the region of metastable along the resonator length, while the extent away from the substrate surface remains approximately constant. [ABSTRACT FROM AUTHOR]

The physical mechanisms for the interaction between reactive plasma and a cesium iodide surface are investigated. Atomic fluorine offers only a limited increase in etch rate and acts primarily as an inhibitor.

Integration of microplasma sources in portable systems sets constraints in the amount of power and vacuum levels employed in these plasma sources. Moreover, in order to achieve good power efficiency and prevent physical deterioration of the source, it is desirable to keep the discharge temperature low. In this paper, the thermal characteristics of an atmospheric argon discharge generated with a low-power microwave plasma source are investigated to determine its possible integration in portable systems. The source is based on a microstrip split-ring resonator and is similar to the one reported by Iza and Hopwood, 2003. Rotational, vibrational, and excitation temperatures are measured by means of optical emission spectroscopy. It is found that the discharge at atmospheric pressure presents a rotational temperature of ~300 K, while the excitation temperature is ~0.3 eV (~3500 K). Therefore, the discharge is clearly not in thermal equilibrium. The low rotational temperature allows for efficient air-cooled operation and makes this device suitable for portable applications including those with fight thermal specifications such as treatment of biological materials. Index Terms--Excitation temperature, microplasma, microwave plasma, rotational temperature, split-ring resonator, vibrational temperature.

Microplasma sources can be integrated into portable devices for applications such as bio-microelectromechanical system sterilization, small-scale materials processing, and microchemical analysis systems. Portable operation, however, limits the amount of power and vacuum levels that can be employed in the plasma source. This paper describes the design and initial characterization of a low-power microwave plasma source based on a microstrip split-ring resonator that is capable of operating at pressures from 0.05 torr (6.7 Pa) up to one atmosphere. The plasma source's microstrip resonator operates at 900 MHz and presents a quality factor of Q = 335. Argon and air discharges can be self-started with less than 3 W in a relatively wide pressure range. An ion density of 1.3 x [10.sup.11] [cm.sup.-3] in argon at 400 mtorr (53.3 Pa) can be created using only 0.5 W. Atmospheric discharges can be sustained with 0.5 W in argon. This low power allows for portable air-cooled operation. Continuous operation at atmospheric pressure for 24 h in argon at 1 W shows no measurable damage to the source. Index Terms--Low-power microplasma, microwave plasma, split-ring resonator.

Microwave resonator-driven microplasmas are a promising technology for generating the high density of rare-gas metastable states required for optically pumped rare gas laser systems. We measure the density of argon 1s5 states (Paschen notation) in argon-helium plasmas between 100 Torr and atmospheric pressure using diode laser absorption. The metastable state density is observed to rise with helium mole fraction at lower pressures but to instead fall slightly when tested near atmospheric pressure. A 0-D model of the discharge suggests that these distinct behaviors result from the discharge being diffusion-controlled at lower pressures, but with losses occurring primarily through dissociative recombination at high pressures. In all cases, the argon metastable density falls sharply when the neutral argon gas fraction is reduced below approximately 2%. [ABSTRACT FROM AUTHOR]

We systematically measure and model the behavior of argon microplasmas sustained by a broad range of microwave frequencies. The plasma behavior exhibits two distinct regimes. Up to a transition frequency of approximately 4 GHz, the electron density, directly measured by Stark broadening, increases rapidly with rising frequency. Above the transition frequency, the density remains approximately constant near 5×1020m-3. The electrode voltage falls with rising frequency across both regimes, reaching approximately 5V at the highest tested frequency. A fluid model of the plasma indicates that the falling electrode voltage reduces the electron temperature and significantly improves particle confinement, which in turn increases the plasma density. Particles are primarily lost to the electrodes at lower frequencies, but dissociative recombination becomes dominant as particle confinement improves. Recombination events produce excited argon atoms which are efficiently re-ionized, resulting in relatively constant ionization rates despite the falling electron temperature. The fast rates of recombination are the result of high densities of electrons and molecular ions in argon microplasmas. [ABSTRACT FROM AUTHOR]

Argon microplasmas generated at 0.9 GHz by a split-ring resonator form a single filament-shaped nonequilibrium glow discharge. The internal structure of these microplasmas is probed using laser diode absorption and imaging emission spectrometry. These two-dimensional diagnostics are then Abel inverted to reveal that the core of the filament has a strongly depleted central metastable argon population. The microdischarges are approximately 0.2 mm in width at 1 atm and expand with increasing input power between 0.05-1.5 W. The relative electron density and the Ar(4p) density are estimated from the emission detected by a CCD camera through various bandpass filters. Absolute Ar(4s) densities, on the other hand, are determined by Ar 801.4 nm absorption. The Ar(4s) profile transitions from center-peaked at low power (0.05 W) to center-depleted above 0.25 W, saturating at 1019 m-3. The electron density profile within the microplasma, however, remains center-peaked regardless of the power. The spatially-resolved gas temperature was estimated from the broadening of the Ar 801.4 nm absorption profile. The error in this gas temperature measurement due to the depletion of the metastable atoms is corrected using numerical heat transfer models and shown to be 1650 K if the plasma power is 1.2 W. Differences between the peak and average temperatures based on the nitrogen rotational spectrum are also explained using nitrogen emission imaging. [ABSTRACT FROM AUTHOR]

Microplasmas were diagnosed by spatially resolved diode laser absorption using the Ar 801.4 nm transition (1s5-2p8). A 900 MHz microstrip split ring resonator was used to excite the microplasma which was operated between 100-760 Torr (13-101 kPa). The gas temperatures and the Ar 1s5 line-integrated densities were obtained from the atomic absorption lineshape. Spatially resolved data were obtained by focusing the laser to a 30 μm spot and translating the laser path through the plasma with an xyz microdrive. At 1 atm, the microplasma has a warm core (850 K) that spans 0.2 mm and a steep gradient to room temperature at the edge of the discharge. At lower pressure, the gas temperature decreases and the spatial profiles become more diffuse. [ABSTRACT FROM AUTHOR]

The experimental values for electron temperature and plasma density are measured in a microfabricated, inductively coupled plasma on a chip (ICP). The miniature ICP system consists of a planar plasma source that is microfabricated on a glass wafer and a miniature aluminum vacuum chamber ([lowercase_phi_synonym]=6 mm). A reliable Langmuir probe diagnostic for microplasma reactors was developed because classic probe measurements would be hindered by the small plasma volume, the small surface area of grounded reactor walls, collisional ion sheathes, and the radio-frequency (rf) interference from the high-frequency power supply. Once the ion density and electron temperature are determined, the neutral gas temperature was estimated to be nearly equal to room temperature. At low pressures, one finds that the plasma sheath width limits the dimensional scaling of the reactor, since the sheath is, typically, ∼0.6 mm. The plasma operates from 0.1 to 10 Torr, and requires less than 3 W of transmitted power at a frequency of 493 MHz. Direct measurement of the power absorbed by the gas discharge, however, shows that between 2% and 18% of the power supplied to the microstructure is absorbed by the discharge. The transfer of power to the discharge is most inefficient at low pressures where the electron neutral collision frequency is less than that of the rf power supply. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The design, fabrication, and characterization of a surface micromachined plasma generator is described for the first time in this paper. The plasma is sustained without electrodes by inductively coupling a 450-MHz current into a region of low-pressure gas. Both argon and air plasmas have been generated over a range of gas pressures from 0.1 to 10 torr (13.3-1333 Pa). Typically, the power used to sustain the plasma is 350 mW, although ~1.5 W is required to initiate the discharge. Network analysis of the plasma generator circuit shows that over 99% of the applied RF power can be absorbed by the device. Of this, ~50% is absorbed by the plasma and the remainder of the power is dissipated as ohmic heating. An argon ion current of up to 4.5 mA/[cm.sup.2] has been extracted from the plasma and the electron temperature is 52 000 K at 0.1 torr. This plasma source is intended for electronic excitation of gas samples so that the presence of impurities and toxins may be detected using optical emission spectroscopy. [540] Index Terms--Electromagnetic induction, microelectromechanical devices, micromachining, microplasma, plasma generation.

The scaling laws associated with the miniaturization of planar inductively coupled plasmas (ICP's) are investigated. The applications for miniature ICP's include microelectromechanical systems (MEMS) for chemical analysis and micro ion propulsion systems. Langmuir probe and microwave interferometry measurements of three ICP's with spiral-shaped coil diameters of 5, 10, and 15 mm show that electron density typically falls in the range of [10.sup.16]-[10.sup.17] [m.sup.-3]. The electron density is about an order of magnitude lower than large-scale ICP's as a result of the large surface-to-volume ratio of small discharges. The measured electron temperature is higher than predicted by a simple 'global model' unless the plasma dimensions are determined more precisely by subtracting the sheath width from the chamber dimensions. Since the sheath width does not scale with the plasma size, the sheath width may ultimately limit the minimum size of ICP's. Plasma initiation power is determined to have a minimum at a gas pressure for which the electron collision frequency equals the frequency of the RF power supply. Small scale ICP's operating at 460 MHz can therefore be started most easily at ~1 tort. The design of the coil is critical to miniature ICP performance. Unlike large-scale ICP's that operate efficiently using a broad range of coil shapes, the miniature coil must be carefully designed and constructed to minimize parasitic resistance. Index Terms--Inductively coupled plasma (ICP), microelectromechanical systems (MEMS).