Patent Grant
9460884
As is known in the art, plasma is an ionized gas, in which electrons heated by an electric field are responsible for ionizing 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 frequently collide 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, often less than 500° K. and generally in the range of 300-1000° K. These cold plasma discharges are not constricted arcs but are typically quite small (<1 mm) and do not cover relatively broad areas of up to 1 m2 as can be required for industrial processes. These low-temperature atmospheric-pressure plasmas, however, are advantageous for numerous industrial processing applications, and in particular for processing inexpensive commodity materials that are sensitive to heat, such as plastics.
An example of a microplasma generator for generating cold plasma at atmospheric pressure is a split ring resonator (SRR). In this device, the microplasma is generated in a discharge gap, e.g., 25 μm, formed in a ring-shaped microstrip transmission line. The cold atmospheric plasma is generated by coupling microwave energy (0.4-2.4 GHz) to plasma electrons using a resonating circuit. The circuit generates high electric fields (E˜10 MV/m) that heat the plasma electrons without strong coupling to the rotational and vibrational modes of the gas molecule, i.e., without generating significant heat. The gas temperature within the plasma can be measured using the rotational spectra of nitrogen molecules and is typically in the range of 100-400° C. Exemplary embodiments of SRR plasma generators are described in U.S. Pat. No. 6,917,165 to Hopwood et al., the entire contents of which are incorporated herein by reference for all purposes.
Known microplasma generators employ a microwave resonating circuit to generate a low-temperature atmospheric-pressure plasma. Of the known cold plasma technologies, the microwave resonator approach offers the most intense electron density while maintaining the lowest gas temperature and the longest electrode life.
One drawback to the existing cold plasma generators is that their geometries are not optimized for some industrial processing, particularly processes for altering the surface of a substrate. The SRR device, for example, is limited to a single “point” geometry, that severely limits its effectiveness for processing a wide-area substrate. Quarter-wave microstrip resonators have been demonstrated to generate microplasmas and can be assembled into linear arrays. These arrays do not scale well to sizes of industrial interest, however, as at larger linear array sizes plasma might not be generated by the resonators near either edge of the array.
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.
According to one embodiment of the present invention, a microplasma generator comprises a substrate made from dielectric material with first and second conductive strips disposed on a first surface of the dielectric substrate. The first and second conductive strips are arranged in line with one another with a gap defined between a first end of each strip. A ground plane is disposed on a second surface of the dielectric substrate. A second end of each of the first and second strips is coupled to the ground plane. A power input connector is coupled to the first strip at a first predetermined distance from the second end wherein the first predetermined distance is chosen as a function of the impedance of the first conductive strip.
In another embodiment, a microplasma generator includes first and second pluralities of conductive strips disposed on a first surface of a substrate of dielectric material. Each strip of the first plurality is arranged with respect to a corresponding strip of the second plurality to define a gap between a first end of each corresponding strip. A second end of each strip in the first and second pluralities of strips is electrically coupled to a ground plane disposed on a second surface of the substrate. A power input connector is coupled to at least one strip in the first plurality of strips at a first predetermined distance from the second end of the at least one strip and the first predetermined distance is chosen as a function of an impedance of the at least one strip.
In yet another embodiment, a microplasma generator array has a block of dielectric material with a ground plane disposed on a first surface of the block. A plurality of spaced apart resonators are disposed in the block where the resonators are substantially parallel to one another. A first end of each resonator is electrically coupled to the ground plane and a second end of each resonator is exposed in a second surface of the dielectric block. A power input connector is coupled to at least one of the resonators a first predetermined distance from the first end that is chosen as a function of the impedance of the at least one resonator.
In one embodiment of the microplasma generator array, a ground electrode is disposed on the second surface of the dielectric block where the ground electrode has a plurality of openings corresponding to each resonator and the second end of each resonator is exposed in the corresponding opening.
Other 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:
This application claims priority from U.S. Provisional Patent Application, Serial No. 61/512,739, filed Jul. 28, 2011 and entitled “Microplasma Generating Array,” the entire contents of which is hereby incorporated by reference in its entirety for all purposes.
Referring now to
A direct electrical connection to the ground plane is required. As is known, a via is a connection through the dielectric substrate. Alternately, the connection could be made around the edge of the dielectric by, for example, a metal trace or similar structure.
A source 121 of high frequency power is connected to the first metal strip 101, nominally at a location on the strip 101 where the input impedance matches that of the power supply. In this embodiment, the operating frequency of the power is selected such that the length of the first strip 101 is an odd integer multiple of ¼ of the wavelength (λ), i.e., a “quarter-wave resonator,” of the signal traveling on the strip. As will be discussed below, other lengths or arrangements may be chosen in order for a resonant frequency of the device to match the frequency of the voltage signal. When power is applied, a microplasma 122 forms in the gap 115 between the strips 101, 102 due to the electric fields in that region.
The determination of impedance will now be discussed with reference to
Here, the microwave power is connected directly to the resonator without a matching network because the physical position of the input port has been chosen to match the power supply impedance (50 Ω). The input impedance of the quarter-wave resonator is based on the equivalent transmission line circuit of
In one embodiment of a plasma generator, as shown in
In the embodiment of
In yet another embodiment, as shown in
In some instances of operation, the microplasma generator 400 may result in microplasma that migrates outwardly from the center along the resonators resulting in uneven power distribution. To reduce this occurrence, as shown in
It is generally impractical to drive each resonator with an individual power source as phase coherency would be lost. Accordingly, in one embodiment, the linear array of resonators is driven from a single power source through the connector 307 with the aid of strong resonant coupling. As known, coupled mode theory provides an accurate model for energy-exchange among resonators. Thus, considering an array of n linear resonators, if one defines the energy stored in the ith resonator as |ai|2, then the coupling among the n resonators can be expressed to lowest order as:
dai/dt=−j(ωi−jΓi)ai+jΣm≠iκmam+Fi, (i=1, 2, . . . , n) (Eq. 1)
where ωi is the resonance frequency of the ith resonator in isolation, Γi is the damping factor, Fi is the external input function, and κm is the coupling coefficient to the ith resonator from the mth resonator.
If the external inputs Fi are assumed to be small, the solution of the n differential equations in Eq. 1 for an n-resonator system results in n eigenfrequencies for the system of coupled resonators. Power is applied only to any one resonator and the remaining resonators operate through resonant coupling. In order to generate various arrangements of plasma along the array, the resonators can be operated by a superposition of several eigenmodes. The discharge produced by the addition of two or more modes may also demonstrate improved uniformity. This superposition of modes is implemented by combining two or more frequencies from two or more RF signal generators using an RF power combiner and applying this amplified waveform to the first resonator only, as described above.
With reference to
In one approach, the lower frequency F1 excites the resonators located toward the center of the array and the higher frequency F2 excites the resonators towards the ends or edges of the array. It should be noted, however, that the input location is not critical, except some locations will not be 50 ohms. The deliberate superposition of the two or more modes provides a nearly uniform line of microplasmas. In this embodiment, two frequencies are added at the input, where each frequency excites a mode of the array. It will be understood that more than two frequencies can be added at the input. In one embodiment, up to N different frequencies, each corresponding to an excitation frequency of a mode of operation of the array, can be added at the input. In one aspect, the superimposition of mode excitation frequencies improves plasma uniformity.
Microwave resonators have been shown to be an efficient method of generating stable microplasmas in micron-scale electrode gaps. Another embodiment of the present invention is a two-dimensional array of such sources, generating a dense array of microplasmas on a surface. Advantageously, in one embodiment, energy coupling among resonators allows an entire array to be powered by a single microwave power supply. This energy coupling causes a variety of possible operating modes, generating patterned sheets of microplasma.
As shown in
Further, two of the 2D generators 600 could be positioned opposite one another to create a plasma between them This arrangement is analogous to that shown in
It should be further noted that the reference to a resonant wire is not intended to limit the structure to a wire shape and that other structures or shapes that provide the same functionality may be used. While the cross-section of the resonator is not critical to operation of the device, it appears that symmetric cross-sections may be advantageous, for example, circles, squares, hexagons, etc. over a flat strip.
In an alternate embodiment, as shown in
The energy coupling between resonators causes the system resonant frequency to split. The lowest-frequency mode results in a relatively uniform distribution of energy, while non-uniform modes generate plasma only at distinct locations. Several examples of predicted mode patterns are shown in
At a first resonant frequency, as shown in
At the fourth resonant frequency, as shown in
At the eleventh resonant frequency, referring to
As described above, in either a planar array of resonators or a 2D array, power is applied to one resonator and the other resonators acquire energy due to resonant coupling. While this resonant coupling is often sufficient, it can be enhanced by providing electrical connections between adjacent resonators.
Referring now to
The coupling strip 354 may be placed anywhere along the length of the resonator 352 and may be co-located with the power input 307. Generally, however, the coupling strip 354 is not located at either end of the resonator 354 as one end, at least with a quarter-wave resonator, is coupled to ground and the plasma is being generated at the other end.
The coupling strip can also be applied to the “hub-and-spoke” embodiment of
As shown in
The coupling sheet 650 comprises a conductive material electrically connecting adjacent resonators. Different locations may be desirable in different situations. Typically, placing the electrical connections closer to the first end of the resonators, i.e., near the surface where plasma is generated, results in a more uniform distribution of energy in the lowest-frequency operating mode, while placing the connections closer to the second end of the resonators, i.e., closer to the ground plane, results in resonant modes that have more closely-spaced resonant frequencies. The presence of the coupling strip or coupling sheet alters the coupling coefficients Kim among the resonators, as used in Eq. 1. The increased coupling improves uniformity and may allow for single-frequency operation. Further, a plurality of coupling strips may be implemented although the locations should be chosen carefully as there is a possibility that placement of an additional coupling strip could eliminate or distort some of the higher modes of operation.
In another embodiment of the present invention, referring now to
The logic plane 1202 comprises a plurality of power field-effect transistors (FETs) 1302 where each FET 1302 couples (or decouples) a respective resonator 602 to (from) ground 606, as shown in
Changing the connection on the end of a resonator 602 will affect its resonant frequency. Thus, opening a FET 1302 between a resonator 602 and ground 606, as depicted in
A microprocessor 1306 may be provided to control the logic switch controller 1304 for setting a state of each FET 1302. The microprocessor 1306 may run a program stored in a memory 1308. When a FET 1302 is configured to couple a respective resonator 602 to ground 606, the resonator is “active” but when disconnected from ground 606, it will not be capable of producing a microplasma. Thus, individual resonators 602 can be controlled and a desired microplasma pattern obtained. Of course, one of ordinary skill in the art will understand that there are other mechanisms for controlling the FETs in the logic plane. These include, but are not limited to, circuits made of analog and/or digital components, programmable devices such as ASICs and PALs and other approaches.
Advantageously, as the resonators 602 can be individually turned off and on, well-defined spatial and temporal patterns of microplasmas can be used to serve as an array of high quality millimeter (mm) wave switches with a high ION/IOFF ratio (IOFF is virtually zero) and excellent isolation performance. These switches can be used to reconfigure and tune the mm-wave circuits in a signal-processing plane 1402 as shown in
The signal processing plane consists of multiplicities of any of the following circuit elements: filter, resonator, phase shifter, attenuator, coupler, mixer, etc., which are components used for processing millimeter wave signals.
The ability to tune the conductivity (and reactivity) of the microplasmas also offers the opportunity to use microplasma not just as a switch but also as an adjustable capacitive circuit element. This allows for implementing tunable filters that can change from low-pass to bandpass to high-pass behavior with minimal “reconfiguration overhead,” as will be discussed below in more detail.
In one embodiment, a 2D array of microplasma switches utilizes microplasma as a virtual switch that can be arbitrarily positioned between any two terminals of interconnection with high ION/IOFF ratio (IOFF is almost zero) and excellent isolation. The S-parameters of these switches, their insertion loss and isolation behavior, as a function of microplasma properties, can be characterized and defined. Switches such as these can be used for reconfigurable millimeter wave front ends, tunable capacitor banks for filters and oscillators as shown in
Metamaterials are artificially-designed bulk materials typically consisting of sub-wavelength metallic inclusions in dielectric media. Metamaterial absorbers and reflectors have been recently shown to be the thinnest and highest performance absorbing (or scattering) materials that depend only on the geometrical design of their unit cells and not on their material properties.
In another embodiment, metamaterials are implemented or controlled by a microplasma generator array according to an embodiment of the present invention. The metamaterial can be tailored to achieve a relatively exotic function such as, for example, operating as a so-called “perfect” absorber or reflector, an electrically small antenna, a so-called “perfect” lens, etc. Embedded microplasma generator arrays, in accordance with embodiments of the present invention, may arbitrarily adjust the absorption and reflection profile of the absorber and, therefore, be tuned over a wide frequency range. A two-dimensional microplasma generator with spatial and temporal control provides a mechanism for a widely tunable, widely programmable metamaterial with minimal reconfiguration overhead. As shown in
In one embodiment, a tunable absorber operates at 110 GHz and 230 GHz with more controlled absorption/reflection and at least 20% frequency tunability. Absorbers may be implemented in ultra-thin SOI substrates and are positioned physically over the microplasma array.
Operation of the proposed device design is robust to the influence of radiation and temperature. The array 1502 itself is similar to the 2D structures described above. The array 1502 may be driven by a single power supply, not shown, which can be shielded and cooled as appropriate. A control logic plane 1506, similar to the control logic plane 1202 described above, is placed below the surface of the discharge, shielding the transistors from the external environment.
Advantageously, the microplasma generating arrays of the present invention are capable of steady-state operation with relatively simple control circuitry. In contrast, the “flashFET,” a three-electrode discharge device described by Mitra et al., is excited by a pulsed applied voltage that requires electrode charging via leakage current that severely limits its duty cycle. A resonator array in accordance with one or more embodiments of the present invention, however, is capable of running up to steady-state duty cycles. Known commercial dielectric barrier discharge-based plasma display panels require complex control circuitry to control pixels, while in the current array, plasma control is achieved by either selection of the drive frequency or the on/off signals provided to an array of transistors. The current resonator array also will, advantageously, allow generation of microplasmas on an exterior surface, as opposed to inside a cell, easing integration into signal processing circuitry.
In the foregoing embodiments, the resonators were configured as quarter-wave resonators. Alternatively, the resonators could be configured as half-wave resonators where the operating frequency is selected such that the length of the resonator is an integer multiple of half the wavelength of the signal. If operating at a same frequency, the half-wave resonator will be twice as long as the quarter-wave resonator. A half-wave resonator configuration differs from the quarter-wave configuration in that, in a half-wave implementation, the end of each resonator that, in a quarter-wave implementation is coupled to ground, floats. The logic control described in
As the length of each resonator is fixed, the generator 370 can be operated in either half-wave or quarter-wave by coupling or decoupling the second ends to ground, through the switching elements 376, 378 and setting the power voltage to the appropriate frequency. Further, once operating, specific resonators may be turned on and off with the switching elements, as grounding the second end of a half-wave resonator will turn it off and, conversely, disconnecting the second end of a quarter-wave resonator will disable it. The lengths L1 and L2 may be equal to one another, and operated by resonant coupling or of different lengths with different supplies to provide power.
The PCT Publication No. WO2010/129277, which claims priority to U.S. Provisional Application Ser. No. 61/173,334, each of which is incorporated herein by reference in its entirety for all purposes, describes a microplasma generator that includes a plurality of strips provided opposite a respective ground electrode where power is provided directly to one of the strips. As a result of the arrangement and length of each strips, power resonates through them and a microplasma is created.
An improvement to this structure is presented in
Still further, as shown in
The microplasma generator devices of the present invention can be fabricated using a substrate of aluminum oxide (Al2O3), glass, or Duroid® material. In one embodiment, aluminum oxide is used due to its resistance to chemical reactions. Any dielectric that exhibits low electromagnetic loss (i.e., has a low loss tangent) is appropriate. The dielectric thickness may be between 0.1 mm and several mm. The surfaces of the dielectric layer are coated with adhesion promoting layers to ensure structural integrity to the high conductivity metals used as resonators in the embodiments using microstrips, as shown at least in, for example,
It may be useful to coat the metal layers with a thin protective layer of dielectric (such as glass) or a refractory metal, such as tungsten, on top of the ¼ λ microstrip. In certain of the described embodiments, a second dielectric layer can be provided over the metal strips and the ground electrodes such that the metal structures of the microplasma generator are protected from the plasma. The microplasma forms on the upper surface of this protective dielectric layer. The layer can be comprised of any dielectric, though glass and aluminum oxide have properties that make their use advantageous. The thickness of this protective dielectric layer can be between, for example, 1 micrometer and 500 micrometers. Thicker protective layers will provide more protection, though the intensity of the microplasma is reduced with thicker protective layers.
The structures that comprise the metal layers of the device can be formed by, for example, (1) milling the unwanted surface layers using a circuit board prototyping tool (e.g., an LPKF circuit board milling tool can be used to pattern Duriod/copper laminates), or (2) by photolithographically defining the desired structures (according to procedures known in the electronics industry) and then etching the metal layers using acids or plasmas with the photoresist mask protecting the structures that are desired to be preserved. A further fabrication method includes defining the metal structures by photolithography directly on the dielectric substrate followed by deposition of metal on the photoresist layer. Removal of the photoresist layer leaves a metal pattern on the dielectric; this process is known as lift-off. All of these procedures are commonly practiced by the electronics industry, and in particular the microwave integrated circuit industry.
Typical feature sizes for the device are, according to some embodiments:
Gap: 1 micrometer to 1000 micrometers with a gap width in the range of 25-250 micrometers depending on the gas used (air=20 microns; argon=200 microns)
Microstrip width: 1 mm
Microstrip length: λ/4 (approximately 60 mm at 450 MHz using Al2O3; the length depends on the relative dielectric constant)
Microstrip thickness: 50 microns
Dielectric thickness: 2.5 mm
Power Frequency: 100 MHz to 10 GHz (in one embodiment, in the range of 1-3 GHz)
Power: 0.1-1.0 watts per resonator (though this parameter is gas and process dependent).
It should be appreciated that certain features, which were, 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 were, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination.
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.
This application claims priority from U.S. Provisional Patent Application, Ser. No. 61/512,739, filed Jul. 28, 2011 and entitled “Microplasma Generating Array.”
The invention was made with support from Grant DE-SC0001923 from the U.S. Department of Energy. The United States Government has certain rights in the invention.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/US2012/048268 | 7/26/2012 | WO | 00 | 1/28/2014 |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO2013/016497 | 1/31/2013 | WO | A |
| Number | Name | Date | Kind |
|---|---|---|---|
| 5017404 | Paquet et al. | May 1991 | A |
| 5406238 | Suzuki | Apr 1995 | A |
| 5733511 | De Francesco | Mar 1998 | A |
| 5938854 | Roth | Aug 1999 | A |
| 6124592 | Spangler | Sep 2000 | A |
| 6211754 | Nishida | Apr 2001 | B1 |
| 6225879 | Schallner | May 2001 | B1 |
| 6653914 | Gapski | Nov 2003 | B2 |
| 6917165 | Hopwood | Jul 2005 | B2 |
| 7159536 | Grosse et al. | Jan 2007 | B1 |
| 7309842 | Doughty | Dec 2007 | B1 |
| 7915979 | Chang | Mar 2011 | B2 |
| 20040164682 | Hopwood | Aug 2004 | A1 |
| 20070170995 | Dutton | Jul 2007 | A1 |
| 20070170996 | Dutton | Jul 2007 | A1 |
| 20080073202 | Lee et al. | Mar 2008 | A1 |
| 20090065177 | Ouyang | Mar 2009 | A1 |
| 20100194278 | Ouyang | Aug 2010 | A1 |
| 20100327155 | Lopez-Avila | Dec 2010 | A1 |
| 20110175531 | Urdahl | Jul 2011 | A1 |
| 20120045863 | Hopwood | Feb 2012 | A1 |
| 20140049259 | Poon | Feb 2014 | A1 |
| 20140159571 | Hopwood | Jun 2014 | A1 |
| Number | Date | Country |
|---|---|---|
| 2004-128159 | Apr 2004 | JP |
| 2004-220935 | Aug 2004 | JP |
| 2008-034735 | Feb 2008 | JP |
| 10-0345543 | Jul 2002 | KR |
| 10-2008-0043597 | May 2008 | KR |
| 10-2009-0037438 | Apr 2009 | KR |
| 10-2009-0055515 | Jun 2009 | KR |
| WO 2010-129277 | Nov 2010 | WO |
| Entry |
|---|
| Jaeho Kim, Makoto Katsurai, Dongmin Kim, and Hyroyuki Ohsaki; Microwave-excited atmospheric-pressure plasma jets using a microstrip line; Applied Physics Letters 93, 191505 (2008); American Institute of Physic's; Nov. 13, 2008. |
| Number | Date | Country | |
|---|---|---|---|
| 20140159571 A1 | Jun 2014 | US |
| Number | Date | Country | |
|---|---|---|---|
| 61512739 | Jul 2011 | US |
