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INSTITUTE OF PHYSICS PUBLISHING

JOURNAL OF PHYSICS D: APPLIED PHYSICS

J. Phys. D: Appl. Phys. 38 (2005) 1698–1703

doi:10.1088/0022-3727/38/11/009

A microfabricated atmospheric-pressure microplasma source operating in air J Hopwood1 , F Iza1 , S Coy2 and D B Fenner3 1 2 3

Northeastern University, Boston, MA 02115, USA Sionex Corporation, Waltham, MA 02451, USA Verionix, Inc., North Andover, MA 01845, USA

Received 30 November 2004, in final form 18 January 2005 Published 20 May 2005 Online at stacks.iop.org/JPhysD/38/1698 Abstract An atmospheric-pressure air microplasma is ignited and sustained in a 25 µm wide discharge gap formed between two co-planar gold electrodes. These electrodes are the two ends of a microstrip transmission line that is microfabricated on an Al2 O3 substrate in the shape of a split-ring resonator operating with a resonant frequency of 895 MHz. At resonance, the device creates a peak gap voltage of ∼390 V with an input power of 3 W, which is sufficient to initiate a plasma in atmospheric pressure air. Optical emission from the discharge is primarily in the ultraviolet region. In spite of an arc-like appearance, the discharge is not in thermal equilibrium as the N2 rotational temperature is 500–700 K. The intrinsic heating of the Al2 O3 substrate (to 100˚C) causes a downward shift in the resonant frequency of the device due to thermal expansion. The temperature rise also results in a slight decrease in the quality factor (142 > Q > 134) of the resonator. By decreasing the power supply frequency or using a heat sink, the microplasma is sustained in air. Microscopic inspection of the discharge gap shows no plasma-induced erosion after 50 h of use.

1. Introduction One of the goals of microplasma research is to develop a discharge that operates in atmospheric-pressure air using minimal power. This would allow for the creation of a lightweight and inexpensive source of plasma by eliminating the need for vacuum pumps, compressed gases and bulky power supplies. Although scaling-down to a microplasma inherently reduces the power consumption, the microstructures of the plasma generator are vulnerable to plasma-induced erosion, such as sputtering and evaporation. Therefore, long-term operational reliability requires that ion-induced sputtering of the microplasma reactor be eliminated and gas temperatures be low. In the following sections, we describe a microplasma device that ignites and sustains a discharge in atmosphericpressure air using 3 W of power at 894 MHz. This power range is compatible with the amplifier chip found in many cellular telephones. The high frequency of operation and a symmetrical excitation potential eliminate sputter erosion. This results in a device that has operated for more than 50 h without degradation. A promising application for microplasmas is analytical chemistry instrumentation. The realization of portability 0022-3727/05/111698+06$30.00

© 2005 IOP Publishing Ltd

would allow bulky analytical equipment to emerge from the laboratory and be used in the field. A review of microplasma developments in microanalytical instrumentation was recently published by Karanassios [1]. The variety of microplasma sources spans the frequency spectrum: dc [2, 3], dc microhollow cathodes [4–7], ac dielectric barriers [8], RF coronas [9, 10], RF capacitively [11, 12] and inductively [13] coupled plasmas, and microwave induced [14–16] microplasma sources have been reported. However, long-term operation in air at 1 atm with portable power has been elusive. Sputter erosion can be reduced by operating a plasma source at microwave frequency. If the power supply frequency exceeds the ion plasma frequency, the ions are effectively immobile in the rf field. However, ions may also be accelerated by dc potential gradients in the sheath region. The plasma sheath behaves similar to a capacitor, but at high frequency the impedance of the sheath capacitance becomes negligible compared to the plasma resistance (1/ωCs < Rp ) and the potential drop across the sheath decreases. These factors decrease the ion energy near the electrodes and reduce the sputter erosion rate. We have recently reported an argon microplasma at 1 atm that is sustained between two copper electrodes separated by 45 µm at a frequency of 900 MHz [17].

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An atmospheric-pressure air microplasma

The electron density is approximately 1014 cm−3 . However, using 3 W of power it was not possible to sustain an air discharge in the 45 µm gap above 200 Torr. In addition, the air plasma aggressively etched the Teflon components of that microplasma source. The difficulty encountered in air plasmas is that most of the electron energy is partitioned into non-ionizing collisions. In particular, more than 90% of the discharge power may be coupled into molecular vibrational states if the electron temperature is low (∼1–2 eV) [18]. Non-ionizing collisions increase the energy cost of creating a single electron–ion pair above several keV/electron [19]. If the electron temperature, or more accurately the average electron energy, can be increased by increasing the ratio of the electric field to the gas density (E/N), then the ionization efficiency can reach the Stoletov constant of 66 eV/electron in air. Macheret et al [18] have demonstrated this principle using high-field nanosecond pulses. In the present work, the required electric field intensity is created within a 25 µm gap excited at 895 MHz (T /2 = 0.56 ns). Although a somewhat smaller discharge gap would provide a more intense field, the further reduction of gap dimensions would result in ballistic electron transport across the gap because the electron mean free path is of the order of 5 µm. Here, with an average of five collisions per electron transit, the electrons are accelerated in a semi-ballistic manner and gain energy equal to ∼20% of the gap voltage between collisions.

2. Experiment Figure 1 shows a sketch of a split-ring resonator (SRR) microplasma source. The microplasma is generated in a 25 µm-wide discharge gap formed in a ring-shaped microstrip transmission line. The transmission line is formed from a 20 mm diameter metal trace that was photolithographically defined on a 29 mm-diameter Al2 O3 substrate. Aluminium oxide was chosen for its chemical resistance to plasma radicals, its high dielectric constant, and good thermal conductivity. The back of the substrate was coated with TiW/Cu/Au and forms the ground plane of the microstrip transmission line. The high dielectric constant of the substrate (εr = 9.8) reduces the wavelength of the 895 MHz excitation potential such that the circumference of the ring is exactly one half of a wavelength. Therefore, when the potential at one side of the gap is maximum, the potential at the opposite side of the gap is minimum. When the SRR is excited at its resonant frequency, microstrip line SMA rf connector

discharge gap

ground plane

Al2O3 substrate

Figure 1. Schematic of the SRR microfabricated on a 29 mm diameter Al2 O3 wafer. The microstrip line and the ground plane are a 10 µm thick multilayer of TiW, copper and gold. The microplasma forms in open air on the top surface of the substrate.

the voltage across the gap prior to plasma ignition is [20]  Z0 QPabs , (1) Vgap = 4 π where Z0 is the characteristic impedance of the transmission line (70 ), Q is the quality factor of the resonator (Q = 142) and Pabs is the power absorbed in watts. The minimum power required to ignite a plasma in air is 3 W, which corresponds to 390 V across the gap. Once the plasma is lit, however, the plasma resistance decreases Q and the gap voltage becomes ∼45 V. Power is introduced to the SRR through a subminiature type-A (SMA) rf connector placed at an appropriate angle relative to the discharge gap (168˚) such that the input impedance of the device is 50 . Power is supplied by an ENI, Inc. linear amplifier (model 603L, 4 W max.) and a HP8656A signal generator. Forward and reflected powers are monitored by a dual directional coupler (Pasternack PE2217-20) and an HP435A power meter with an HP8482H power sensor. Power input to the plasma generator is always reported as forward power minus reflected power, but the reflected power is typically less than a few tenths of a watt. The plasma was ignited and operated in open laboratory air with no external flow. No compressed gases were used in this work, although the microplasma has also been operated with flowing argon, O2 and N2 . Figure 2(a) shows a photograph of the SRR, rf connector and the miniature coaxial cable leading to the power amplifier. The microplasma is visible in figure 2(b), and actually has a blue colour. Note that the exposure time has been changed to accommodate the brightness of the discharge. A close-up of the microplasma is shown in figure 2(c). An afterglow extends approximately 500 µm outside the central 25 µm discharge gap. The narrowest segment of the gap is obscured by the bright central portion of the microplasma, but resides on the outer radius of the ring as shown in figure 1. Because the outermost layer of the microstrip line is made of gold, it is particularly at risk for sputter erosion due to a high sputter yield. The SRR design, however, minimizes the dc sheath potential between the plasma and the goldcoated electrode because the entire ring structure is at the same dc potential. In addition, the use of microwave frequency excitation prevents ions from transiting the discharge gap in a single cycle and reduces the sheath impedance, as described earlier. To test the durability of the SRR design, an air plasma was operated for 50 h at 3 W of power. Figure 3 shows a micrograph of the region near the discharge gap after this experiment. Although there is some deposition (presumably carbon from CO2 in the air), the gold electrodes remain intact with no evidence of rounding or facetting from ion bombardment. Some of the performance characteristics of the SRR were measured in an attempt to better understand the physics of the device and the microplasma. A survey scan of the optical emission spectrum from the discharge was measured by an Ocean Optics spectrometer (model USB2000), while more detailed scans of the emission spectrum were captured using an optical multichannel analyzer (EG&G 1453A) and a JarrelAsh diffraction grating monochromator. The latter instrument was used for rotational temperature measurements and was 1699

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Figure 2. Photographs of the SRR show: (a) the device with no plasma, (b) the device with a 1 atm air plasma and (c) a close-up of the discharge gap and air microplasma.

Figure 3. This micrograph shows the discharge gap after 50 h of operation in open air using 3 W of power. Carbon deposition is observed, but the gold microstrip electrodes are not eroded.

calibrated for spectral response intensity using a variable monochromatic light source that was characterized by a NISTtraceable optical power meter. The plasma’s gas temperature was estimated from the rotational temperature (Trot ) of the second positive system of N2 . The R, P and Q branches of the ν = 0, 1, 2 bands were modelled, convolved with the instrument’s spectral resolution (λ = 1 nm), and fitted to the measured UV emission spectrum to determine Trot [17]. The electrical properties of the SRR were measured by a HP8714ET network analyzer which determined the resonant frequency (fc ) and the quality factor (Q) of the SRR as a function of substrate temperature. The substrate temperature was measured by placing a thermocouple in the centre of the Al2 O3 wafer with electronic heat sink compound to ensure good thermal contact. 1700

3. Results Dissipating ∼3 W of power in the 29 mm diameter SRR causes the substrate temperature to rise to approximately 100˚C as shown in figure 4. By attaching the ground plane of the SRR to an aluminium heat sink (area = 1000 cm2 ), the temperature rise was reduced to 37˚C in non-flowing air. Careful observation of figure 4 reveals that the rise time and the fall time for the thermally isolated SRR are not equal. This is because the power absorbed by the SRR decreases as the temperature rises. Evidence of the decreased plasma power is seen in the top curves of figure 4 where the integrated optical emission intensity (300 < λ < 800 nm) decreases as the temperature increases. The absorbed power decreases from 3.15 W at t = 0 to 2.60 W at t = 480 s if the heat sink is not

An atmospheric-pressure air microplasma

Figure 4. The temperature of the Al2 O3 substrate versus time is shown for operation at 3 W of power (f = 896 MHz) both with and without a heat sink. The integrated optical emission intensity (300 < λ < 800 nm) from the microplasma is also plotted as a function of time. The plasma is extinguished at t = 600 s.

Figure 5. The centre of the resonance frequency (fc ) and the quality factor (Q) of the resonator are plotted as a function of substrate temperature (no plasma was present).

used. When the heat sink is used, however, the absorbed power decreases far less, from 3.15 to 3.02 W. Figure 5 shows the electrical characterization of the SRR as measured by a network analyser with no plasma in the gap. The temperature increase of the SRR was simulated using a heat gun, and the reflection coefficient (as a function of frequency) was measured to determine the centre frequency and quality factor of the resonator. At room temperature the centre frequency of the device is fc = 895 MHz and the quality factor is Q = 142. This tells us that the bandwidth of the SRR is B = fc /Q = 6.3 MHz. As the SRR temperature rises, the resonant frequency shifts downwards to 890.6 MHz due to thermal expansion of the substrate. At the same time, Q decreases to 135 as a result of increased conductor losses at higher temperature (the dielectric loss is negligible). Fortunately, the decrease

in Q is minimal and, therefore, the power transfer from the SRR to the microplasma is not significantly altered. The decrease in resonant frequency, however, is nearly equal to the bandwidth (B). The input signal source must be slightly adjusted to track the shifted centre frequency, or a heat sink must be used to minimize the temperature rise and accompanying frequency shift. Fortunately, the plasma resistance broadens the bandwidth of the resonator when a discharge is present in the gap, and decreasing the operating frequency to 894 MHz is sufficient to sustain the plasma. The emission spectrum of the air microplasma shows excited N2 as well as NO and atomic oxygen (figure 6). Most of the emission is in the ultraviolet band, so the visible and infrared spectra are multiplied by 25 in figure 6. Although not evident in the emission spectra, the microplasma also produces noticeable amounts of ozone odour. The second positive system of N2 was used to measure the rotational temperature of the gas as shown in figure 7. The temperature increases slightly with increasing power from 610 K at 2.5 W to 670 K at 3.5 W. This is significantly higher than the rotational temperature measured in Ar + N2 (0.1%), which was 400 K at 1 W [17] and may indicate the inefficient partitioning of power into excited molecular states. In spite of the higher temperature, however, the air microplasma is far from thermal equilibrium and is clearly not an arc. Below 2.5 W, the emission intensity of the plasma quickly diminishes and the rotational temperature decreases to ∼500 K. The plasma cannot be sustained in air for an input power of less than 2.3 W.

4. Conclusion An atmospheric pressure air microplasma can be sustained between two co-planar electrodes separated by 25 µm using 3 W of power at ∼900 MHz. The narrow spacing and high peak voltage across the discharge gap give rise to an electric field of 1701

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Figure 6. The emission spectrum for an air microplasma operated at 3 W of power (f = 894 MHz). Emission above 500 nm is multiplied by 25.

Acknowledgment This work is supported in part by NSF Grant No DMI-0078406.

References

Figure 7. The rotational temperature of the second positive system of N2 in air is calculated from the emission spectra of the ν = 0, 1, 2 vibrational bands. The substrate was held at 330 K by attaching a heat sink.

∼15 MV m−1 , which is sufficient to initiate air breakdown. Ion erosion of the electrodes is avoided by symmetrical excitation of a resonant ring and the reduction of dc sheath potential at the two electrodes. The temperature of the thermally isolated SRR rises to approximately 100˚C for 3 W of applied power. The temperature increase shifts the resonant frequency due to thermal expansion of the SRR, but the conductor losses do not significantly increase. In spite of a very high power density, the gas temperature (∼Trot ) remains relatively low (500–700 K) and the microplasma does not reach thermal equilibrium. The peak gap voltage of 45 V allows electrons to be accelerated to an energy of ∼10 eV between collisions. This relatively high energy electron component improves the ionization efficiency of the air plasma and allows for low-power operation. 1702

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