WO2013119313A2

WO2013119313A2 - Microplasma generator with array of tapered microstrips

Info

Publication number: WO2013119313A2
Application number: PCT/US2012/068427
Authority: WO
Inventors: Naoto Miura
Original assignee: Tufts University
Current assignee: Tufts University
Priority date: 2011-12-09
Filing date: 2012-12-07
Publication date: 2013-08-15
Anticipated expiration: 2014-06-09
Also published as: WO2013119313A4; WO2013119313A3; WO2013119313A9

Description

TITLE OF THE INVENTION

Microplasma Generator With Array of Tapered Microstrips

CROSS-REFERENCE TO RELATED APPLICATION

N/A

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR

DEVELOPMENT

The invention was made with support from Grant #DE-SC0001923 from the U.S. Department of Energy and Grant #CBET-0755761 from the National Science Foundation. The United States Government has certain rights in the invention.

BACKGROUND OF THE INVENTION

A plasma is created when electrons are heated by an electric field that then creates ionized gas atoms. At a low gas pressure, the hot electrons inside a plasma have relatively few collisions with the gas atoms, therefore, the gas remains cool, as one observes in a fluorescent light (p ~ 1 Torr). At, or near, atmospheric pressure (p ~ 760 Torr), however, the free electrons in the plasma collide more frequently with gas atoms and heat the gas to very high temperatures, e.g., 5,000-10,000° K. Examples of atmospheric plasmas include lightning and welding arcs. High temperature plasmas tend to be destructive and are unsuitable for many industrial processes, including photo-voltaic manufacturing.

Recently, plasma generators have been developed that produce plasma that is relatively low-temperature at or near atmospheric pressure. These low-temperature, atmospheric -pressure plasmas are known as "cold" plasmas, and are characterized by their lower gas temperatures, generally in the range of 300-1000° K and often less than 500°K. These cold plasma discharges are not constricted arcs but are typically quite small (< 1 mm) but do not cover relatively broad areas of up to 1 m as can be required for industrial processes. These cold plasmas, however, are advantageous for numerous industrial processing applications, and in particular for processing inexpensive commodity materials that are sensitive to heat, such as, for example, plastics, polymers and textiles.

At atmospheric pressure, generating uniform glow discharge is challenging, because the discharge tends to become so intense at a localized spot that it transits to an arc discharge. This problem is often called "glow to arc transition." Pulsed DC discharges are known to solve this issue by pulsing the power, i.e., each power pulse is shut off before the plasma changes into an arc. Dielectric Barrier Discharge (DBD) solves this issue by coating the electrode with an insulating material. The coating limits the current and suppresses the glow to arc transition.

The generation of plasma using microwave discharges has been developed by several groups. There is not, however, much available using microwave discharge as it is considerably more difficulty to implement, due to the complexity of the microwave circuitry, as compared to the pulsed DC and DBD modes of operation. While most microwave sources produce small (point type) plasmas, there has been work performed recently on a stripline array that generates long (line type) plasmas. One difficulty when using the microwave device is being able to control the electromagnetic waves as the wavelength is comparable to the length of plasma. As an example, if the wavelength is much longer than a plasma, the electric field is uniformly applied to plasma. This is the case for low frequency plasma sources, however, low frequency sources have the glow-to-arc problem. A non-uniformity in the electric field along the discharge gap will cause a non-uniform plasma.

What is needed, therefore, is a device for generating a microplasma that can be better controlled and tuned for specific applications and that can provide plasma over a larger area.

BRIEF SUMMARY OF THE INVENTION

According to one embodiment of the present invention, a plasma generator provides a uniform and linear plasma at near atmospheric pressure. The plasma generator is composed of two power supplies, a power divider, an array of transmission line tapers and an ignition resonator. One power supply, driven at a microwave frequency, drives the taper array and the other power supply drives the ignition resonator. The power divider splits the taper array power evenly and delivers it to each taper. A plasma is ignited by the ignition resonator and then the plasma propagates to the taper array. The taper array does not start a plasma by itself and the ignition resonator is provided to do so. By increasing the number of tapers, the length of the plasma can be increased, depending upon the needs of the specific application.

In another embodiment, a microplasma generator includes a substrate of dielectric material with a first surface and a second surface. A linear array of tapered conductive strips is provided on the first surface and each strip has a first end and a second end, and the second ends are aligned with one another. An ignition resonator is provided on the first surface of the substrate and adjacent the array and the ignition resonator has an input end and an output end where the output end is aligned with the second ends of the tapered strips. A ground electrode is placed on the first surface and opposite the second ends of the tapered strips and the output end of the ignition resonator to define a linear gap. A ground plane is disposed on the second surface of the substrate and each of the ignition resonator and the ground electrode is electrically coupled or connected to the ground plane.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

Features and advantages of the present invention will be apparent from the following description of embodiments thereof and from the claims, taken in conjunction with the accompanying drawings, in which:

Fig. 1 is a conceptual schematic of a plasma generator in accordance with an embodiment of the present invention;

Figs. 2A and 2B are side views of the plasma generator shown in Fig. 1;

Fig. 3 is a schematic view of a taper array in accordance with an embodiment of the present invention;

Fig. 4 is a schematic representation of a power divider in accordance with an embodiment of the present invention;

Fig. 5 is a schematic view of a taper array where each taper includes an extended end portion in accordance with an embodiment of the present invention;

Figs. 6 A - 6C represent different taper shapes in accordance with embodiments of the present invention;

Fig. 7 is a schematic view of a plasma limiter in accordance with an embodiment of the present invention;

Figs. 8 A, 8B and 8C are, respectively, a schematic view, a top view and a side view, of a plasma jet generator in accordance with an embodiment of the present invention;

Fig. 9 is an exploded view of the plasma jet generator of Figs. 8A-8C;

Figs. 10A and 10B are, respectively, a schematic view and a side view, of a plasma jet generator in accordance with an embodiment of the present invention;

Fig. 11 is a schematic view of a flow guide used in diverting a plasma jet in the embodiment shown in Figs. 10A and 10B; and

Figs. 12A and 12B are, respectively, a side view and a schematic view of a plasma generator in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the various embodiments of the present invention. It will be understood by those of ordinary skill in the art that these embodiments of the present invention may be practiced without some of these specific details. In other instances, well- known methods, procedures, components and structures may not have been described in detail so as not to obscure the embodiments of the present invention.

Prior to explaining at least one embodiment of the present invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments or of being practiced or carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein are for the purpose of description only and should not be regarded as limiting.

It is appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention, which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable subcombination.

As will be described with more detail below, embodiments of the present invention are directed to a plasma generator that is driven with an input at a microwave frequency that ignites and sustains a uniform and linear plasma near atmospheric pressure. Referring to the conceptual schematic of one embodiment in Fig. 1, a plasma generator 100 includes a taper array portion 104 and a power divider portion 108. A taper power input 112 receives taper power 116 which is divided into N outputs by the power divider portion 108. These N outputs are provided, respectively, to N taper electrodes 120-n, also referred to as microstrips, that make up a taper array 122 provided in the taper portion 104. The taper portion 104 includes a ground electrode 124 positioned opposite the taper array 122. An ignition resonator 128 is provided in the taper portion 104 and includes a ground via 132 coupling the ignition resonator 128 to a ground plane as will be described below. An ignition power input 136 is coupled to the ignition resonator 128 to received ignition power 140. The ground electrode 124 is spaced apart from the taper array 122 and the ignition resonator 128 to define a plasma gap 144.

Referring now to Fig. 2A, a side cross-section view of the taper array portion 104, one can see that the taper array portion 104 includes a body 204 made of dielectric material with the tapers 120-n and the ground electrode 124 provided on a first surface 205 thereof. A ground plane 208 is provided on a second surface 209 of the dielectric body 204. The ground electrode 124 is coupled, i.e., connected, to the ground plane 208 by a ground electrode via 212 running through the body 204. Alternately, the connection could be made around the edge of the dielectric body 204 by, for example, a metal trace or similar structure. Viewing the taper array portion 104 from the other side, as shown in Fig. 2B, the ignition resonator 128 is also provided on the upper surface 205 of the body 204. An ignition resonator via 216 connects the ignition resonator 128 to the ground plane 208. As will be described below in more detail, the location of the ignition resonator via 216 is chosen such that a predetermined impedance value, in one instance 50 ohms, is presented at the ignition power input 136. The microstrips 120-n and the ignition resonator 128 are made of metal disposed on the dielectric substrate 204. A dielectric material with a small loss tangent is advantageous and could be air (ε_Γ =1), Teflon (ε_Γ = 1.9), aluminum oxide (ε_Γ =9.3) or commercial high frequency laminates such as Rogers TMM series, Duroid series materials. Performance may be improved if a highly conductive material is chosen for the microstrips 120-n and the ignition resonator 128, e.g., gold, copper, aluminum, etc. It has been observed, however, that magnetic materials such as nickel should be avoided, since the microwave energy heats this material very well. The taper impedance can be changed by adjusting the dielectric constant (ε_Γ) and the thickness of dielectric material 204. This allows one to roughly match the taper impedance to the plasma impedance. The thickness of the dielectric material 204 should be much smaller than the wavelength of the input power.

In one device, shown in Fig. 3, and described for explanatory purposes only, the tapers 120-n and the resonator 128 were made by machining a high frequency copper laminate (Rogers, TMM3, 2.5 mm thick, ε_Γ=3.27). In this example, triangular tapers are shown but the tapers could be shaped differently as will be described below.

A taper input end width, Wl, is set to match the power line and provide an appropriate impedance. In the exemplary case, 50 ohm transmission lines were used for power delivery. A taper input end length, LI, is the length of the transmission line from the input to the tapered portion of the taper electrode 120-n. The taper input end length LI may improve the field uniformity at the taper, if the field at the input is not uniform. The taper input end length LI may range from zero up to ¼ of the wavelength λ of the input power, i.e., 0 < LI < ¼ λ . A taper length, L2, is at least longer than half the wavelength of the input power in order to reduce power reflection, but the optimal length depends on the taper shape. There is a tradeoff in performance, however, because although the longer the taper length L2 is, the smaller the reflection coefficient, there is more wave propagation loss. Generally, the width of the taper 120-n should be less than the wavelength to suppress the standing wave for better plasma uniformity.

Each taper 120-n has a taper end portion 304 and its longitudinal length, L3, can be zero or near zero. A taper end portion width, W2, is generally set to be less than a quarter wavelength of the input power to ensure electric field uniformity along the discharge gap 144. In order to reduce coupling to the adjacent taper, the taper end portion length L3 should be minimized. It is noted, however, that a plasma needs a small contact area with the electrodes, around a few hundred microns at atmosphere, and a smaller taper end portion length L3 may improve the spatial continuity of the plasma around the edges of the tapers 120-n. Generally, a taper-to-taper distance TS1 is directly related to the taper input width Wl and the taper end portion width W2, so TS1=W2+S1-W1. Generally, the taper-to-taper distance TS1 is close to the taper end portion width W2, i.e., about ¼λ with respect to the input power.

The ignition resonator 128 has a length LR of about 47 mm and a width of about 1 mm. A resonator input portion 308 has a length and width chosen to present a predetermined impedance, in this case, 50 ohms, to a power source with an input signal of 0.9 GHz. It should be noted that, while the resonator input portion 308 is shown as being adjacent the resonator 128 and the ground via 132, the resonator input portion 308 may be located anywhere along the resonator 128 in order to provide the predetermined impedance to the power source.

A ground length, L4, as shown in Fig. 2A, is the distance from the discharge gap 144, across the ground electrode 124, to the ground electrode via 212. The ground length L4 should be much smaller than the wavelength λ of the input power.

A width, G, of the discharge gap 144, for operation at atmospheric pressure, is in the range of 50 to 500 microns with a nominal value chosen to be around 100 microns.

The taper array 122 is formed by linearly arranging the tapers 120-n side by side as shown in Figs. 1 and 3. A taper separation distance SI between the taper end portions 304 of adjacent tapers 120-n, electrically isolates the tapers 120-n from one another. In the exemplary structure, the taper separation distance SI is about 100 microns. An ignition separation distance S2 between the taper array 122 and the ignition resonator 128 is about 100 microns. If the ignition separation distance S2 is too large, the plasma does not propagate from the ignition resonator 128 to the tapers 120-n. In general, the ignition separation distance S2 is 50 - 200 microns.

The ignition resonator 128 is provided as the taper array 122 is generally not able to ignite a plasma by itself, although it can sustain a plasma. The ignition resonator 128 is a quarter-wavelength resonator and amplifies the electric field due to resonance and the high electric field makes it possible to ignite a plasma near atmospheric pressure with 2-3 watts of input power. Thus, the length LR is chosen to be an odd integer multiple of 1/4 the wavelength of an operating frequency of the ignition power 140. The resonant frequency of the ignition power 140, 0.9 GHz (2.7 GHz, third resonance) was chosen so that it would not overlap with the 2.45 GHz frequency of the taper power 116. A first end of the resonator strip is connected to the ground plane by the via 132 and the second end is aligned with the ends of the tapered strips.

The power divider portion 108 combines a plurality of hybrid ring couplers 404-n, as shown in Fig. 4. Each hybrid ring coupler 404-n has four ports: a first (input) port 408-n, a second port 412-n, a third port 416-n and a fourth port 420-n. The ring coupler 404-n functions as a power divider with the ports 408-n, 412-n, 416-n and 420-n being separated from one another by ¼ wavelength of the input signal 116 about the circumference of the ring and the ring's circumference is 1.5 wavelengths.

Referring now to Fig. 4, a first ring coupler 404-1 has its second port 412-1 coupled to the first port 408-3 of a third ring coupler 404-3 and a third port 404-1 coupled to a first port 408-2 of a second ring coupler 404-2. A fourth port 420-1 of the first coupler 404-1 is terminated in a 50 ohm resistor 424-1.

The second ring coupler 404-2 has a second port 412-2 coupled to an output 428-3 and a third port 416-2 coupled to an output 428-4 to supply power to a respective taper electrode 120- n. A fourth port 420-2 of the second coupler 404-2 is terminated in a 50 ohm resistor 424-2.

The third ring coupler 404-3 has a second port 412-3 coupled to an output 428-1 and a third port 416-3 coupled to an output 428-2 to supply power to a respective taper electrode 120- n. A fourth port 420-3 of the third coupler 404-3 is terminated in a 50 ohm resistor 424-3.

Characteristics of a hybrid ring coupler will be described, generally, with respect to the first coupler 404-1, as an example. A signal that is input into the first port 408-1 will be evenly split between the second port 412-1 and the third port 416-1 as respective outputs. Two signals with the same amplitude and phase (even mode) presented, respectively, to the second port 412- 1 and the third port 416-1, will be combined into a signal out of the first port 408-1. If, however, two signals with the same amplitude but 180° out of phase from each other (odd mode) are presented, respectively, to the second port 412-1 and the third port 416-1, then the coupler combines the signals as an output on the fourth port 420-1.

Therefore, when a hybrid ring coupler is implemented, any in-phase reflected signals will go back to the power supply and the out of phase reflected signals are emitted from the fourth port 420 and can be dissipated in a 50 ohm terminal resistor 424. The advantages of the coupler 404-n include high isolation between output ports and any out-of -phase reflected power can be dissipated in the external 50 ohm resistor. In embodiments of the present invention, large out-of -phase signals can be generated due to reflected signals from the taper array when plasmas are not ignited at all of the tapers. The explanatory circuit shown in Fig. 4 represents a four- way power divider design using three hybrid couplers. Conceptually, a T- way divider can be created with an n level design (cascading n couplers). A power divider as shown in Fig. 4 can be made, for example, by machining a high frequency copper laminate such as is available from Rogers, Duroid 6010.2, 2.5 mm thick, ε_Γ = 10.2.

In another embodiment of the present invention, as shown in Fig. 5, an extended taper end portion 504 may be positioned as the taper end portion. This is, essentially, a modified version of the taper end portion 304 shown in Fig. 3. The additional width EW and length ED are chosen to be much smaller than the wavelength of the input power, generally 1/10 the wavelength λ. The extended end portion 504 allows for a larger distance TS2 between adjacent tapers 120-n as compared to the same size tapers 120-n with taper end portion 304. Advantageously, this shape provides for slightly more electrical isolation.

The shape of the taper 120-n may be modified as shown in Figs. 6A-6C. A triangular taper 604 includes straight edges, while an exponential taper 608 has edges that follow an exponential path. A Klopfenstein taper 612 can also be implemented. Each taper has a slightly different frequency response. For example, the Klopfenstein taper 612 is optimum in the sense that it has a minimum reflection coefficient magnitude in the passband for a specified length of taper. In addition, the tapers in an array could be a mix of shapes where, for example, some are triangular, i.e., straight-edged, and some are exponentially shaped.

Embodiments of the present invention are designed to generate plasma essentially only near the discharge gap 144. In some situation, the plasma extends over the electrodes 120-n and the ground electrode 124 and may affect the uniformity of the plasma. A plasma limiter 702 comprising two pieces 704, 708, as shown in Fig. 7, restricts plasma to remain near the discharge gap 144. In addition, the plasma limiter 702 can transfer heat from the plasma because of its relatively larger contact area with the plasma and, advantageously, keep the gas temperature low. The limiter 702 is typically made of a dielectric material and a highly thermally conductive material will enhance the heat transfer. Examples of materials that could be used include, but are not limited to, macor, aluminum oxide and sapphire (crystalline aluminum oxide).

In another embodiment of the present invention a plasma jet is produced by generating a plasma in flowing gas surrounded by air. In one non-limiting example, argon is used as a source gas. A plasma is selectively generated in argon, but not generated in the surrounding air. Thus, the air creates "virtual walls" that confine a plasma. As a result of the need for the air to confine the plasma, it is advantageous if a uniform sheet of the gas is provided. As an overview, by flowing the gas through the discharge gap, a sheet of gas is produced. In the sheet of gas, a plasma is ignited and a plasma jet is produced. Referring now to Fig. 8A, a device 800 for generating a plasma jet is based on the embodiment described above. A dielectric material body 804 has an upper surface 808 on which a plurality of taper electrodes 812 and an ignition resonator 816 are provided. The taper electrodes 812 and ignition resonator 816 may be arranged and powered as described above. A ground plane 820 is provided and a ground electrode 824 is connected to the ground plane 820. A gas flow channel 828 is disposed adjacent the ground electrode 824. The gas flow channel 828 receives the source gas and is in fluid connection with a discharge gap 830 provided between the taper electrodes 812 and the ground electrode 824. In operation, a plasma jet 832 is directed away from, and perpendicular to, the upper surface 808.

Referring now to Figs. 8B and 8C, a top-view and side-view, respectively, of the plasma jet generator 800, a plurality of gas holes 836 are shown that fluidly couple the gas flow channel 828 to the discharge gap 830. The source gas is supplied to the gas flow channel 828 and then distributed through the gas holes 836 and guided to the linear discharge gap 830 between the ground electrode 824 and the dielectric body 804 of the taper array. An electric field created between the end of the taper electrodes 812 and the ground electrode 824 sustains the plasma jet 832 igniting from the gas input.

As shown in Fig. 9, the gas flow channel 828, the ground electrode 824 and the gas holes 836 may be incorporated into a single structure that is attached to an end of the dielectric body 804. When attached, it is then connected to the ground plane 820.

In yet another embodiment of the present invention, the plasma jet generator 800 is modified to direct the plasma jet 832 in a direction that is parallel to the upper surface 808. As shown in Fig. 10A, and from a side view in Fig. 10B, a plasma jet generator 900 includes a gas flow guide 904 disposed over the discharge gap 830. In operation, therefore, the plasma jet 832 is diverted by the flow guide 904. The operation of the device 900 is generally the same as that described above with respect to the other embodiments.

The flow guide 904 may include a recess 908 to direct the plasma jet, as shown in Fig.

11.

Referring now to Figs. 12A and 12B, a plasma generator in accordance with another embodiment of the present invention does not include a separate ground electrode as is found in the embodiments described above. A dielectric body 950 includes a side face 954. A ground plane 958 is provided on a bottom surface of the dielectric body 950 and has a ground portion 960 that extends around to the side face 954. A plurality of tapers 962 are provided on an upper surface of the dielectric body 950, in a manner similar to those described above, however, each taper 962 has a taper portion 964 that extends around to the side face 954. Accordingly, a discharge gap 966 is defined between the taper portions 964 and the ground portion 960.

It should be noted that, simply for convenience, the tapers 962 could be shaped as those described above although not shown as such in Figs. 12A and 12B. In addition, a plasma ignition device is not shown but could be included and/or provided, also as is described above.

While the foregoing embodiments are implemented with an ignition resonator, alternate plasma starters could be used. For example, a microwave frequency resonator, another microplasma source, a piezoelectric spark generator or a high power UV light source, such as a laser, could be used. An input would be provided to couple any of these alternate plasma ignition devices to the discharge gap.

Advantageously, embodiments of the present invention provide a plasma source that can generate indefinitely long, continuous and uniform plasma by placing any desired number of tapers, as long as the power divider is designed to handle the high power. Such a plasma is very attractive to many applications including an atmospheric roll-to-roll plasma processing device and as a light source , e.g., a UV lamp, an excimer lamp, or a laser source such as an excimer laser.

When implemented in an atmospheric roll-to-roll plasma processing device, a long, uniform and high density plasma, as provided by the present invention, is a very attractive feature. This makes it possible to treat material uniformly and the throughput can be improved due to its higher plasma density than pulsed DC and DBD sources. For example, the device can coat (deposit) a thin film on a surface to change a surface property or make a device using such a film. Also, the plasma can be used to sanitize a surface which may be important for biomedical applications.

While a light source (UV lamp, excimer lamp) or laser source (excimer laser) based on

DBD are known, a DBD plasma is made of many small plasma pulses and the discharge is not temporally uniform. On the other hand, a device according to the present invention produces a uniform plasma, and this may be able to generate a temporally uniform light exposure to some target material.

The power divider is a combination of the classic hybrid couplers and this absorbs the reflected power in odd mode at the external 50 ohm load resistors 424-n. For the taper array plasma generator, reflected power in odd mode can become large, and the power needs to be dissipated. A large heat sink can be attached to the external 50 ohm resistor 424-n and the heat handling is not an issue for the power divider described here. The reflected power in the odd mode needs to be dissipated at internal resisters in a commercial Wilkinson power divider and the heat handling is harder, although it can be done.

Advantageously, the plasma generator with microstrip tapers 120-n is a wideband device and, as a result, the driving frequency can be changed. Further, as the taper is a highpass device if, for example, the cutoff frequency is 1 GHz, then any frequency above 1 GHz can be used. In addition, the driving frequency does not have to be well regulated and the device can tolerate some amount of instability in the input regulation. As a result, a power supply with a nominal output frequency of 2.45 GHz + 0.02 GHz is acceptable. The driving frequency changes the plasma properties, for example, the electron density, and, therefore, adjusting the driving frequency provides another type of control for plasma processing. As a result of the device's tolerance of poor regulation of its input power, the device can be driven by a commercially available regulated 2.45 GHz magnetron power supply that may have relatively poor frequency regulation. These known types of magnetron power supplies, often found in microwave ovens, have a high power output but are more cost effective than other microwave power supplies, e.g., bench-top laboratory microwave supplies.

Embodiments of the present invention provide advantages over pulsed DC/Corona discharges. Specifically, the pulsed DC/Corona discharges are non uniform discharges, i.e., they use a needle array electrode. The discharge is non-uniform at a micro scale whereas the plasma generated in this work is more uniform.

Embodiments of the present invention also provide advantages over DBD. Specifically, the plasma is pulsed for DBD, similarly to the pulsed DC discharge. The peak electron density of DBD plasma is 10 13 -101¹5^J cm^" 3 , but the average electron density is expected to be 1010 cm^" 3 to

10 13 cm -^"3. On the other hand, the plasma generated in this work produces a higher density - on the order of 10 1¹3^J cm^"J 3 to 101¹5^J cm^" 3. In addition, the DBD plasma is formed by many small filamentary pulsed discharges. On average, the whole discharge looks uniform, but it is not uniform temporally. On the other hand, the plasma generated in this work is driven by a continuous wave and it is uniform at any given moment. This difference may be important for uniformly treating a material.

Having thus described several features of at least one embodiment of the present invention, it is to be appreciated that various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be part of this disclosure and are intended to be within the scope of the invention.

Accordingly, the foregoing description and drawings are by way of example only, and the scope of the invention should be determined from proper construction of the appended claims, and their equivalents.

Claims

CLAIMS What is claimed is:

1. A microplasma generator, comprising:

a substrate of dielectric material having a first surface and a second surface;

an array of tapered conductive strips linearly disposed on the first surface of the substrate, each strip having a first end and a second end, and the second ends aligned with one another;

a plasma ignition device disposed on the first surface of the substrate and adjacent the array;

a ground electrode disposed on the first surface and opposite the second ends of the tapered strips and an output end of the plasma ignition device to define a discharge gap therebetween.

2. The microplasma generator of claim 1, further comprising:

a ground plane disposed on the second surface of the substrate,

wherein the ground electrode is connected to the ground plane.

3. The microplasma generator of claim 1, wherein the plasma ignition device comprises: an ignition resonator having an input end and an output end, the output end aligned with the second ends of the tapered strips, and

wherein the ignition resonator is connected to the ground plane.

4. The microplasma generator of claim 3, wherein each of the tapered strips, the ground electrode and the ignition resonator comprises a conductive metal.

5. The microplasma generator of claim 3, further comprising:

a first via running through the substrate and connecting the ignition resonator to the ground plane.

6. The microplasma generator of claim 5, wherein:

the first via is positioned with respect to the input end of the ignition resonator in order to present a first predetermined impedance value to a power source coupled to the ignition resonator.

7. The microplasma generator of claim 1, further comprising:

a power divider having an input and a same number of outputs as a number of strips in the array,

wherein the power divider provides microwave power to each taper in the array.

8. The microplasma generator of claim 7, wherein the power divider comprises:

a plurality of hybrid ring couplers arranged in a cascade.

9. The microplasma generator of claim 7, further comprising:

a power supply configured to provide microwave power to the input of the power divider.

10. The microplasma generator of claim 9, wherein:

the microwave power has a first frequency and a first wavelength; and

a length of each tapered strip in the array is chosen to be greater than one-half the wavelength of the microwave power.

11. The microplasma generator of claim 1, wherein each taper in the array comprises a portion having a shape chosen from:

a) a triangular shape;

b) an exponential shape; and

c) a Klopfenstein taper shape.

12. The microplasma generator of claim 1, wherein a respective input portion of each of the tapered strips in the array is configured to present a first predetermined impedance value to a power source coupled to the respective tapered strip.

13. A microplasma generator, comprising:

a substrate of dielectric material having a first surface and a second surface;

an array of elongated conductive strips arranged side-by-side with one another on the first surface of the substrate, each strip having a first end portion, a middle portion and a second end portion, and the second end portions aligned with one another; and

a ground electrode disposed with respect to the aligned second end portions to define a discharge gap between the ground electrode and the second end portions of the tapered strips, wherein each middle portion of a respective elongated conductive strip is of a tapered shape.

14. The microplasma generator of claim 13, further comprising:

a plasma ignition device disposed on the first surface of the substrate and adjacent the array, the plasma ignition device having an input end and an output end, the output end adjacent the second end portion of at least one of the elongated conductive strips.

15. The microplasma generator of claim 14, wherein the plasma ignition device comprises an ignition resonator.

16. The microplasma generator of claim 15, further comprising:

a ground plane disposed on the second surface of the substrate,

wherein each of the ignition resonator and the ground electrode is connected to the ground plane.

17. The microplasma generator of claim 16, further comprising:

18. The microplasma generator of claim 17, wherein:

the first via is positioned in order for the ignition resonator to present a first predetermined impedance value to a power source.

19. The microplasma generator of claim 13, further comprising:

a ground plane disposed on the second surface of the substrate,

wherein the ground electrode is positioned opposite an end portion of the substrate and is connected to the ground plane.

20. The microplasma generator of claim 19, further comprising:

a gas flow channel having a lumen therethrough in fluid connection with the discharge gap-

21. The microplasma generator of claim 20, further comprising:

a plurality of holes in the gas flow channel fluidly connecting to the discharge gap.

22. The microplasma generator of claim 13, wherein the ground electrode is disposed on the first surface.

23. The microplasma generator of claim 13, wherein each middle portion of each elongated conductive strip has a first end with a first width and a second end with a second with and wherein a width of the middle portion between the first end and the second end changes as one of:

a) a linear function;

b) an exponential function; and

c) a Klopfenstein function.

24. The microplasma generator of claim 13, further comprising:

a plasma limiter disposed over the discharge gap.

25. The microplasma generator of claim 13, further comprising:

an input connector coupled to the discharge gap and configured to receive plasma ignition energy.