Microplasma Devices Research Articles

Pressure-independent point in current-voltage characteristics of coplanar electrode microplasma devices operated in neon

We introduce the idea of a pressure-independent point (PIP) in a group of current-voltage curves for the coplanar electrode microplasma device (CEMPD) at neon pressures ranging from 15 to 95 kPa. We studied four samples of CEMPDs with different sizes of the microcavity and observed the PIP phenomenon for each sample. The PIP voltage depends on the area of the microcavity and is independent of the height of the microcavity. The PIP discharge current, IPIP, is proportional to the volume (Vol) of the microcavity and can be expressed by the formula IPIP=IPIP0+D×Vol. For our samples, IPIP0 (the discharge current when Vol is zero) is about zero and D (discharge current density) is about 3.95 mA mm−3. The error in D is 0.411 mA mm−3 (less than 11% of D). When the CEMPD operates at VPIP, the discharge current is quite stable under different neon pressures.

Microplasma Discharge Ionization Source for Ambient Mass Spectrometry

In this paper, we demonstrate the first use of a microplasma ionization source for ambient mass spectrometry. This device is a robust, easy-to-operate microhollow discharge that enables ambient direct analysis of gaseous, liquid, and solid-phase samples with minimum requirements in terms of operating power and high purity gas consumption. The initial performance of the microplasma device has been evaluated by ionizing samples containing dimethyl sulfoxide (DMSO), dimethylformamide (DMF), methyl salicylate, caffeine, l-leucine, l-histidine, loratadine, ibuprofen, acetaminophen, acetylsalicylic acid, and cocaine in various forms. These molecules are diverse in nature, but almost all have relatively high proton affinities. Thus, the major species observed in all obtained mass spectra corresponded to protonated molecules. Though these microplasmas are known to produce significant densities of metastable species and electrons with mean energies greater than several electronvolt, minimal fragmentation was observed. Background spectra showed prominent signals corresponding to H(+)(H(2)O)(2) ions and a distinct lack of H(3)O(+). Small water cluster ions are likely the dominant proton transfer agents, giving rise to mass spectral data very similar to that obtained using other plasma-based ambient ionization techniques. The simplicity, low cost, low power, low rate of gas consumption, and possibility of being batch-fabricated, makes these microplasma devices attractive candidates as ion sources for miniaturized mass spectrometry and other field detection applications.

AC Microplasma Device With a Cylindrical Hollow Electrode for Improving Luminous Efficacy

In this paper, a device utilizing the hollow cathode discharge is demonstrated to diagnose the possibility of improving luminous efficacy of a microplasma device. A series of experiments is conducted by measuring luminous efficacy, which is an important factor in display devices, in accordance with specifications of the cylindrical hollow and gas pressure. The proposed structure has two electrodes, and one of them contained a cylindrical hole and is called a hollow electrode. The hollow electrode works as either a cathode or an anode alternately due to the ac operating conditions. When the hollow electrode operated as a cathode, the luminous efficacy is higher than that of the anode case, and this tendency is observed consistently throughout all experiments. Moreover, from the measurement results on the tendency of luminous efficacy in terms of hole specifications, different aspects of luminous efficacy are obtained, depending on whether the variation is in the diameter or in the depth. In case of the diameter, the luminous efficacy is improved when p middotD (i.e., the product of the gas pressure and the diameter of the cylindrical hole) is less than one Torr ldr cm. In the case of the depth, the maximum value of luminous efficacy exists at the certain p middotd (i.e., the product of the gas pressure and the depth of the cylindrical hole).

Arrays of Novel Coplanar-Electrode Microplasma Devices

Arrays of coplanar-electrode microplasma devices have been fabricated and operated under neon pressures. The microcavity of the device is composed of a couple of electrodes instead of a dielectric or a single cathode, which can enhance the density of the microcavities. When the size of the microcavity is 150 mum, the density of the microcavities could exceed 103 cm-2. The devices are fabricated by only one wet lithography process, which can simplify the fabrication processes and develop the fabrication uniformity. The current-voltage characteristics of the devices under different operation pressures and with different frequencies are investigated. The experimental results show that the current-voltage characteristics under different neon pressures exhibit a positive differential resistance of about 78 kOmega. The discharge current will be enhanced with the increase in operation pressure and driven frequency. With the increase in driven voltage and operation pressure, the discharge in the gaps will be enhanced, and the degree of the enhancement in the gaps will exceed that in the microcavities, particularly when the driven voltage is above 350 V and the driven frequency is above 20 kHz.

Helium–hydrogen microplasma device (MPD) on postage-stamp-size plastic–quartz chips

A new design of a miniaturized, atmospheric-pressure, low-power (e.g., battery-operated), self-igniting, planar-geometry microplasma device (MPD) for use with liquid microsamples is described. The inexpensive MPD was a hybrid, three-substrate quartz-plastic-plastic structure and it was formed on chips with area the size of a small postage stamp. The substrates were chosen for rapid prototyping and for speedy device-geometry testing and evaluation. The approximately 700-microm (diameter) and 7-mm (long) He-H(2) (3% H(2)) microplasma was formed by applying high-voltage ac between two needle electrodes. Operating conditions were found to be critical in sustaining stable microplasma on plastic substrates. Spectral interference from the electrode materials was not observed. A small-size, electrothermal vaporization system was used for introduction of microliter volumes of liquids into the MPD. The microplasma was operated from an inexpensive power supply. And, operation from a 14.4-V battery has been demonstrated. Microplasma background emission in the spectral range between 200 and 850 nm obtained using a portable, fiber-optic spectrometer is reported. Analyte emission from microliter volumes of dilute single-element standard solutions of Cd, Cu, K, Li, Mg, Mn, Na, Pb, and Zn is documented. Element-dependent precision was between 10-25% (the average was 15%) and detection limits ranged between 1.5 and 350 ng. The system was used for the determination of Na in diluted bottled-water samples.

Mixed LTCC and LTTT Technology for Microplasma Generator Fabrication

An issue of paramount importance for the fabrication of microplasma devices is internal structure flatness. To overcome this problem we devised a fabrication method using LTCC (Low Temperature Cofired Ceramic) and LTTT (Low Temperature Transfer Tape) technologies. The flexibility resulting from mixing these technologies could enable fabrication of certain microfluidic applications. In the present work, the fabrication of a microplasma generator using a mixed LTCC and LTTT technology is presented. Silver-palladium electrodes were screen printed on the green tapes and were cofired after machining of microchannels and other structures. Two electrode plates were obtained separately in this way using a setter powder sheet to ensure flatness. In the postfire process, transfer tapes were used to bind the two electrode plates and the electrode spacer together to make the final device structure. Uniform and stable glow discharges were obtained in argon, nitrogen, oxygen, and sulfur hexafluoride gases. Paschen's curves, V-I characteristics, and optical emission spectra were obtained in DC discharges. The fabrication method presented has been demonstrated to be very reproducible and produced very flat electrodes.

Electrical properties of nanotip-assisted microplasma devices

Nanotip-assisted microplasma devices have been fabricated and characterized in Ne, Ar, or Ne/2% Ar. The electrical properties of the glow discharge in three different environments, Ne, Ar, and Ne/2% Ar, have been examined with direct current (dc) bipolar-pulsed waveforms with frequencies from 2to20kHz at pressures from 300to800torr. The operating voltage of the nanotip-assisted microplasma devices was decreased around 10–20V compared with devices without a nanotip electrode. The 6×1cm panel-type microplasma device with nanotip electrodes revealed low discharge voltages around 300V.

Inactivation of bacteria by a mixed argon and oxygen micro-plasma as a function of exposure time

Purpose: A radio-frequency dielectric barrier discharge (DBD) was applied as a micro-plasma device for the inactivation of bacteria, e.g., Escherichia coli.Materials and methods: The cultured bacteria were placed on a polydimethyl siloxane (PDMS) film and placed inside the DBD cavity. The bacteria were exposed to micro-plasmas of varying oxygen/argon ratios for different exposure times. The survival of the bacteria was measured by determining bacterial growth using optical methods.Results: The excited oxygen species increased with the increase in the oxygen to argon ratio as measured by optical emission spectroscopy (OES), but the increase of excited oxygen species in argon micro-plasma did not enhance the inactivation of bacteria. In contrast, increases in the time the bacteria were exposed to the micro-plasma were of importance. The results show that a continuous plasma flow containing energetic and reactive species may result in electro-physical interactions with bacteria exposed to the plasma leading to their inactivation.Conclusion: For currently-employed DBD device, addition of 0.5% oxygen to the argon micro-plasma for an exposure time of 30 sec was optimum for the inactivation of E. coli.

Kinetics of microplasma atmospheric ion generation correlated with discharge current

The conditions necessary for achieving a stable bipolar ion generation (in the order of 10 6 ion/cm 3) and lower ozone concentration (less than 50 ppb) using a surface discharge microplasma device (SMD) by adjusting the applied voltage and frequency were experimentally determined and investigated. Measurements of the discharge current characteristics of the SMD revealed saturation against the frequency (1.5–2.5 kHz, depending on the applied voltage). The ion and ozone concentrations both increased in step with the discharge current in the lower frequency region. The ion concentration reached equilibrium in the frequency range of 200–500 Hz, and the point of equilibrium within that range depended on the applied voltage. The ozone concentration did not reach equilibrium under our experimental conditions (ozone concentration < 100 ppb). The kinetics of the ion/ozone generation rate with a focus on the plasma reaction and recombination of bipolar ions is discussed.

A Line Array of Microplasma Devices With Coplanar Electrodes Operating in Argon

A line array of microplasma devices with coplanar electrodes has been fabricated, and the discharge characteristics of the microplasma devices under different argon pressures were investigated in experiments which were driven by sinusoidal voltage and alternating pulse voltage, respectively. The microcavities of the proposed device are composed of two coplanar electrodes instead of dielectric or single cathode. The voltage-current characteristics of the device driven by sinusoidal voltage show that the power loading of each microcavity could reach several kilowatts per cubic centimeter. Driven by alternating pulse voltage, the device performed a trend that the discharge current grows up first and then goes down gradually with the increase of operation pressure. Moreover, with the increase of driven frequency, the device operated with high discharge stability on both time and intensity. The proposed device possesses the advantages of simple structure and simple fabrication processes.

Microplasma Device Utilizing SU-8 Photoresist as a Barrier Rib

A microplasma device utilizing SU-8 photoresist as a barrier rib has been fabricated and characterized operating in the abnormal mode for neon pressures from 300 to 800 torr and having a hexagonal structure, 5 times 5 arrays of microplasma. The microplasma device, which has a simple fabrication process, offers advantages such as low firing voltage, low cost, and stable glow discharge. The electrical properties have been examined by bipolar voltage waveforms with different frequencies. The geometric patterns of the barrier rib can be simply changed by lithographic techniques.

Nanotip-Enhanced Microplasma Devices

Nanotip-enhanced microplasma devices have been successfully fabricated and operated with neon gas at room temperature. The panel-type microplasma device included nanotip and metal electrodes on the rear panel, which provided low discharge voltage, strong emission, and stable discharge phenomena. The nanotip with a radius below 10 nm enhances the electrical field distribution density. The electrical properties of neon discharges have been examined with pulsed waveforms from 2 to 20 kHz in neon with pressures from 4 times 104 to 105 Pa.

Diamond-Shaped Microplasma Device Using Thick Photoresist as a Barrier Rib in Ne–Ar Gas Mixtures

The discharge characteristics of the diamond-shaped microplasma devices with pure neon, neon-1% argon, and neon-5% argon gas mixtures are investigated. The 5 times 5 arrays of diamond-shaped microplasma devices operating at pressures from 300 to 800 torr have been characterized. The diamond-shaped microplasma devices, offer advantages such as having a high yield process, low ignition voltage, stable glow discharge, and low cost. The electrical properties of neon discharges have been examined with different argon fractions. The diamond-shaped microplasma device operates at voltages as low as 190 V, at 800 torr.

Spatial dynamics of the light emission from a microplasma array

The spatial dynamics of the optical emission from an array of 50×50 individual microplasma devices is reported. The array is operated in noble gas at atmospheric pressure with an ac voltage. The optical emission is analyzed with phase and space resolution. It has been found that the emission is not continuous over the entire ac period, it occurs only twice in each cycle. Each of the observed emission phases shows a self-pulsing of the discharge, with several bursts of emission of a fixed width and repetition rate. Cross-talk between the individual devices can be observed through spatially resolved measurements.

Characteristic responses of an atmospheric pressure DC micro-plasma detector for gas chromatography to organic functional groups

A gas chromatographic detector using direct current micro-plasma discharged a distance of less than 50 μm under helium at atmospheric pressure is described. The micro-plasma device was assembled on glass substrates; the volume of the plasma region was only 0.58 nL. Very low power consumption (15 mW) and only carrier helium flow were needed to maintain micro-plasma stability. Organic compounds with various functional groups were chromatographically separated and fed into the micro-plasma. Fluctuations in plasma current were detected as the sample passed through the micro-plasma. All non-halogen organics showed positive peaks, while simple alkyl halides showed negative peaks. Compounds with both aromatic and halogen groups exhibited hybrid peak shapes, consisting of both positive and negative peaks. The detection limits (3 σ) ranged from 24.5 pg for m-xylene to 756 pg for 1,2-dichloroethane, depending on differences in ionization and electron-capture efficiency.

Micromachined, planar-geometry, atmospheric-pressure, battery-operated microplasma devices (MPDs) on chips for analysis of microsamples of liquids, solids, or gases by optical-emission spectrometry

Because of their desirable characteristics, for example small size, lightness, low power and gas consumption, and potential for portability, miniaturized plasma sources are receiving significant attention in the scientific literature. To take advantage of these characteristics we micromachined and fabricated new, planar-geometry, self-igniting, atmospheric-pressure microplasma devices (MPDs) on chips. These microplasmas required such low power for their operation they could be operated from a re-chargeable battery (of the type used in cordless power-tools). Despite their advantages, most miniaturized plasma sources reported in the literature have not performed well with liquid samples; analysis of powders or solids that can be converted to a powder (and processed and used as slurries) is even more difficult. To address these shortcomings we coupled an electrothermal, mini-in-torch vaporization (mini-ITV) "dry" sample-introduction system to the low-power planar microplasma devices we developed. In this preliminary investigation, absolute detection limits obtained from microsamples of single-element liquid standards and optical emission spectrometry with photomultiplier-tube detection and a spectral bandpass similar to that of portable, commercially available fiber-optic spectrometers were in the low-pg to ng range, for example 2 pg (for K) to 25 ng (for Pb). Mini-ITV also enabled (as far as we are aware, for the first time) measurement of analyte emission from microsamples of powdered solids (as slurries). In addition to the 3% H2 in Ar mixtures, the ac-operated microplasmas were sustained by use of a variety of electrode materials and different plasma-support gases (e.g. Ar, He and 3% H2 in He) thus indicating fabrication versatility and operational flexibility. Such flexibility has the potential to enable microplasmas to be tailored to analytical problems, and this is demonstrated by using a He MPD and chlorine emission measurements (837.594 nm) from gaseous microsamples as an example.

Arrays of Addressable Microcavity Plasma Devices

Addressable microcavity plasma devices with two- or three-electrode, dielectric barrier designs have been fabricated in 20 times 20 or 50 times 50 device arrays in Si(100) and characterized in the rare gases. Each device comprises a metal/Si electrode structure, a dielectric stack, and an inverted square pyramid microcavity having an emitting aperture of 50times50 mum <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> or 100times100 mum <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . Arrays with filling factors of 11% and 25% [for (100 mum) <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and (50 mum) <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> device arrays, respectively] and a crossed electrode (passive matrix) geometry exhibit operating voltages in Ne of ~220-300 V (RMS) when driven by a 20-kHz sinusoidal driving voltage. Displacement currents are ~50% of those for previous Si microplasma device arrays, and when exciting the array with 100-140-V pulses, the rise time of the wavelength-integrated fluorescence is observed to be < 600 ns for pure Ne or Ne/5%Xe gas mixtures at a pressure of 700 torr. A full address and sustain pulse sequence has also been demonstrated with a symmetrical three-electrode device structure

Development and characterization of thermoelectron enhanced microplasma devices for the generation of plasmas in micro-sized capillaries

Microplasmas offer the advantage of small size, low power consumption as well as higher rate of materials processing and are therefore promising for new applications in materials processing. While many different types of microplasma devices have been presented in recent years, the effects of the smaller scale on the properties of the plasma (gas and electron temperatures), distribution of electrons, radicals and ions, and the implications of these characteristics for materials processing have not been studied in detail yet. Here we present the generation and characterization by optical emission spectroscopy (OES) as well as the numerical simulation of atmospheric pressure argon microplasmas inside capillaries with inner diameters between 50 micro m and 1 mm using two different types of thennoelectron enhanced microplasma (TEMP) devices. The first type has a freestanding geometry whereas the second type is planar, fabricated by a surface mount technique. The latter allows for future size reduction of the microplasma setup and for an easier integration with other apparatuses and devices.

Flexible microdevices based on carbon nanotubes

This work reports the fabrication and testing of flexible carbon nanotube microdevices made using hot embossing material transfer. Both micro-plasma and photodetector devices were made using as-grown unpurified multi-wall carbon nanotubes printed on PMMA substrates. Optical detectors were fabricated by attaching metal wires and monitoring the resistance as a function of light exposure. The electrical resistance of the nanotubes showed a strong sensitivity to light exposure which was also enhanced by heating the devices. While such processes in MWCNTs are not fully understood, the addition of thermal energy is believed to generate additional free charge carriers in the nanotubes. The plasma-generating microdevices consisted of a thin layer of thermoplastic polymer having the CNT electrode on one side and a metal electrode on the reverse side. The devices were electrically tested under atmospheric conditions with 0.01–1 kV ac and at 2.5 kHz, with the plasma igniting near 0.7 kV. The fabrication of these flexible organic devices demonstrates the ability to pattern useful carbon nanotube microdevices in low-cost thermoplastic polymers.

Flexible Arrays of Ni/Polyimide/Cu Microplasma Devices with a Dielectric Barrier and Excimer Laser Ablated Microcavities

Microcavity plasma devices with circular, crescent or, for example, trapezoidal cross-section microcavities (characteristic dimension d=30–100 µm), produced by excimer laser ablation and overcoated with a silicon nitride barrier film, have been fabricated in Ni/30 µm polyimide/3 µm Cu layered substrates. 12×12 arrays of devices with cylindrical microcavities 100 µm in diameter exhibit turn-on voltages of 255–270 Vrms for a Ne pressure of 700 Torr and a sinusoidal excitation voltage having a frequency of 5–20 kHz. All of the device designs explored to date operate in the abnormal glow region, and an increase of 15–20% in the ignition voltage for these arrays is observed when pd is raised from 4 to 5 Torr cm. Tests in which the arrays were intentionally damaged or photoablation parameters were altered from the optimal values show the microplasma devices to be extraordinarily robust and insensitive to the cross-sectional shape of the microcavity.