Patent Application
20110042008
The present invention relates to a plasma generator and, more particularly, to a so-called atmospheric plasma generator.
The present inventors previously developed atmospheric plasma generators and filed patent applications therefor (see Patent Documents 1 and 2). In the plasma generators, electrode surfaces facing each other are provided with micro-scale recesses, to thereby induce hollow cathode electric discharge, through which a plasma is generated. When a plasma-generating gas is caused to pass through the plasma-generating zone, a gas containing at least a plasma can be jetted. Through employment of such a plasma generator, high-density plasma can be readily generated under atmospheric pressure with a high-frequency voltage of about some kilovolts obtained from a single-phase commercial power source (100 V) by means of a voltage booster.
Patent Document 1: Japanese Patent Application Laid-Open (kokai) No. 2006-196210
Patent Document 2: Japanese Patent Application Laid-Open (kokai) No. 2006-272039
In generating plasma under atmospheric pressure, when a short inter-electrode distance is employed, electric discharge is unstable, whereas when the distance is 1 cm or longer, electric discharge cannot be maintained. Thus, according to Patent Documents 1 and 2, in the case where the plasma-generating zone is, for example, a longitudinally elongated zone, electrodes having a certain length and extending in the longitudinal direction are employed. However, when such long electrodes are provided to face each other, uniformity in density of the generated plasma is problematically impaired. Specifically, electric discharge starts at spots on the electrode surfaces, and electric discharge cannot be uniformly generated in the relevant electrode surfaces. Due to unstable electric discharge, the plasma generators cannot be effectively employed for the plasma treatment of a considerable wide area of an object such as a surface portion of a liquid-crystal panel or the like. Furthermore, difficulty is encountered in increasing the volume of the plasma-generating zone. Therefore, limitations are imposed on the effective uses of the aforementioned plasma generators, although the plasma generators can generate high-density plasma under atmospheric pressure.
The present invention has been conceived in order to solve the aforementioned problem, and an object of the invention is to provide an atmospheric plasma generator having an elongated plasma-generating zone.
In a first aspect of the present invention, there is provided an atmospheric plasma generator comprising:
a longitudinally extending casing which is made of an insulator and which defines a columnar plasma-generating zone;
a gas intake section for supplying a plasma-generating gas to the plasma-generating zone in a direction normal to the longitudinal direction of the plasma-generating zone so as to attain a uniform gas distribution, in the longitudinal direction, over the plasma-generating zone;
a pair of electrodes disposed in the plasma-generating zone with a spacing in the longitudinal direction of the plasma-generating zone; and
a gas discharge section for discharging the plasma generated in the plasma-generating zone, which section is connected to the plasma-generating zone, which section is disposed along the longitudinal direction of the plasma-generating zone, and which section comprises a number of elongate holes that extend in the direction of the flow of the generated gas-form plasma.
The present invention is directed to an apparatus for generating linear plasma under atmospheric pressure and for irradiating an object to be treated (hereinafter may be referred to as treatment object) with the linearly extended plasma. The holes of the gas discharge section may be provided in a line or a plurality of lines along the longitudinal direction. The diameter of each hole at the base (i.e., the end connecting to the plasma-generating zone) may be different from that at the tip, and the diameter at the tip may be smaller than that at the base. Alternatively, the diameter at the tip may be larger than that at the base. The hole may be tapered, or the diameter of the hole may be varied stepwise. When the apparatus of the invention is employed with the tip of each hole being apart from the treatment object, no electric discharge occurs between the gas discharge section and the treatment object, even though the length of the hole is shortened. In this case, the treatment effect is weak. However, through increasing the amount of gas flow, a satisfactory treatment effect can be attained, even when the tips are apart from the treatment object.
In a second aspect of the present invention, which is directed to a specific embodiment of the first aspect, the gas intake section has a diffusion section for supplying the plasma-generating gas over the plasma-generating zone uniformly in the longitudinal direction, the diffusion section having a number of guide portions for guiding the plasma-generating gas in a direction normal to the longitudinal direction of the plasma-generating zone. Through this configuration, a plasma-generating gas can be supplied over the plasma-generating zone uniformly in the longitudinal direction. These guide portions may be formed as a grid-like pattern or formed of a plurality of walls of the holes.
In a third aspect of the present invention, which is directed to a specific embodiment of the first or second aspect, said holes of the gas discharge section have such a length that no electric discharge occurs with respect to an object to be irradiated with plasma. Through this configuration, the object is not damaged. In consideration of deactivation of radicals, the length of the hole is most preferably the shortest length, so long as no such discharge occurs.
In a fourth aspect of the present invention, which is directed to a specific embodiment of any of the first to third aspects, said holes of the gas discharge section have a tip having a diameter of 0.1 mm to 1 mm. Through this configuration, a treatment object can be irradiated selectively with only radicals among the plasma particles. That is, electrons are absorbed by the walls of the holes.
In a fifth aspect of the present invention, which is directed to a specific embodiment of any of the first to fourth aspects, said holes of the gas discharge section have a length which is ½ times or more the distance between said pair of electrodes. In this case, electric discharge with respect to a treatment object can be prevented.
In a sixth aspect of the present invention, which is directed to a specific embodiment of any of the first to fifth aspects, said holes of the gas discharge section have an oblique portion which is oblique with respect to a direction normal to the longitudinal direction. Through this configuration, electrons can be efficiently absorbed by the inner walls of the oblique holes, whereby a treatment object can be irradiated with radicals of higher purity. In addition, irradiation of the treatment object with UV rays, vacuum UV rays, visible light, etc. can be prevented. UV rays emitted in the plasma-generating zone are intercepted by the inner walls of the oblique holes. Thus, irradiation of the treatment object with UV rays, which would otherwise be emitted through the tips of the holes, can be prevented. Therefore, damage of the treatment object, which would otherwise be caused by the light (e.g., UV light) emitted in the plasma-generating zone, can be prevented.
In a seventh aspect of the present invention, which is directed to a specific embodiment of any of the first to sixth aspects, said pair of electrodes are disposed with a spacing of 1 cm to 50 cm.
In an eighth aspect of the present invention, which is directed to a specific embodiment of any of the first to seventh aspects, at least one electrode of said pair of electrodes is provided with hollow portions on a surface thereof which face the other electrode.
In a ninth aspect of the present invention, which is directed to a specific embodiment of any of the first to eighth aspects, the longitudinal length L (cm) of the columnar plasma-generating zone and the cross-sectional area σ (mm2) normal to the longitudinal direction satisfy the following relationships:
2≦Lσ≦200 and 3≦σ≦25. When the conditions are satisfied, plasma can be effectively generated. More preferred conditions are 2≦Lσ≦100 and 3≦σ≦25.
In a tenth aspect of the present invention, said holes of the gas discharge section have a cross-section normal to a direction of gas flow, the cross-section being at least one selected from a group consisting of a circle, an oval, and a rectangle or a slit-like pattern having a longer side normal to the line of arrangement of the holes.
In the present invention, an atmospheric plasma is generated in the space defined (or surrounded) by the casing made of an insulator. In the columnar space defined (or surrounded) by the insulator casing, an elongated plasma zone is realized. In the present invention, a conceivable role of the insulator is such that the inner surfaces of the casing are electrically charged, to thereby stabilize plasma generation occurring in the entire plasma-generating zone extending in the longitudinal direction and having a large volume.
The plasma-generating gas is supplied through the gas intake section to the plasma-generating zone in a direction normal to the longitudinal direction of the zone, and discharged through a number of holes included in the gas discharge section. In this manner of gas flow, radicals are generated at a uniform density over the zone along the longitudinal direction thereof, and a treatment object is irradiated with the thus-generated radicals which are jetted straight through the tips of the holes. Through employment of hollow cathode electric discharge provided by means of electrodes having a recessed surface disclosed in Patent Document 1 or 2, plasma can be readily generated under atmospheric pressure. When the holes of the gas discharge section have a length which is ½ times or more the distance between said pair of electrodes, electric discharge with respect to a plasma treatment object is prevented. The hole length is most preferably ½ times the distance between said pair of electrodes. When the length is longer than the ½ length; that is, when the length from the plasma-generating zone to the treatment object is long, the generated radicals are deactivated, which is not preferred. In the case where the inter-electrode distance is 40 mm, the most preferred hole length has been found to be 20 mm. The columnar plasma-generating zone of the invention has a longitudinal length L (cm) of 1 to 50, and a cross-sectional area σ (mm2) normal to the longitudinal direction is 3 to 25. In the columnar plasma-generating zone, the smaller the cross-sectional area σ, the longer the length L. The length L can be increased, when the casing is substantially tubular. Some experiments have revealed that stable plasma can be formed under the relationship: 2≦Lσ≦200. For more effective generation of plasma, the following relationships: 2≦Lσ≦100 and 3≦σ≦25, are satisfied.
The casing must be made from a material which has high resistance to plasma generated in the inside thereof. For example, a ceramic material such as sintered boron nitride (PBN) is preferred. The electrodes may be formed from stainless steel, molybdenum, tantalum, nickel, copper, tungsten, platinum, or an alloy thereof. The electrode surface in which hollow portions for providing hollow cathode electric discharge are provided preferably has a thickness (in the Y-axis direction in
Examples of the gas which may be used at atmospheric pressure for generating plasma include air, oxygen, rare gas (He, Ne, Ar, etc.), nitrogen, and hydrogen. By use of air or oxygen, active oxygen radicals are formed, and organic contaminants can be effectively removed. Use of air is advantageous in view of economy. In the case where Ar gas (rare gas) is used, oxygen radicals are generated from oxygen molecules existing in an atmosphere around a treatment object by the Ar plasma during irradiation of a treatment object with the Ar plasma. By the mediation of oxygen radicals, organic contaminants present on the surface of the treatment object can be effectively removed. This process is economical because a sole gas of Ar is used. For the aforementioned reasons, a mixture of air and Ar may also be used. The flow rate and supply amount of the gas and the degree of vacuum may be selected as desired. In the present invention, plasma is not generated by high-frequency voltage. The power source connected to each electrode may be a DC power source, an AC power source, a pulse power source, or another power source, and no particular limitation is imposed on the frequency.
When a plasma gas is jetted through the outlets (i.e., a number of holes included in the gas discharge section) to a treatment object, the distance between each jetting outlet and the treatment object, which depends on the gas flow rate, is preferably, for example, 2 mm to 20 mm, more preferably 3 mm to 12 mm, most preferably 4 mm to 8 mm. When oxygen radicals are generated, the distance is preferably adjusted so that, on the surface of the treatment object, the oxygen radical density is maximized and the electron density is minimized. Through employment of the above conditions, charge-up damage of the treatment object can be prevented, and the treatment object can be cleaned in the most effective manner. Alternatively, the treatment object may be irradiated with plasma in a direction slanted from the gas flow direction. Through plasma irradiation in this manner, impairment of products caused by irradiation of plasma to a polarized film, a liquid crystal-sealant, etc. can be prevented. Onto an area where plasma irradiation should be avoided, a gas such as plasma-free air may be sprayed, so as to prevent diffusion of plasma.
Oxidation of the electrodes is preferably prevented through lowering the oxygen concentration by use of a gas containing nitrogen, Ar, or hydrogen (reducing gas). Alternatively, a plurality of plasmas may be generated so as to remove organic contaminants and prevent reaction with the remaining area. Also, a gas generated from the portion of the treatment object irradiated with plasma after reaction is preferably removed via suction. Through removal of the gas, deposition of the molecules which have been reacted with organic contaminants on remaining areas of the treatment object can be prevented. The temperature and density of the plasma, which are measured through, for example, laser beam absorption spectrometry, are preferably adjusted to predetermined levels by feed-back-controlling of applied voltage, duty ratio (in the case of pulse voltage application), irradiation time, gas flow rate, etc.
Through performing control in the aforementioned manner, high-quality cleaning and shortened cleaning time can be realized. In the case where a plurality of outlets of the holes are disposed in a line, irradiation with plasma can be limited only to a required portion of the treatment object by appropriately adjusting the diameters and lengths of the holes. Also preferred is cooling of the plasma-generating gas supplied to the plasma generator of the invention. Through cooling, elevation of the plasma temperature to a level higher than the required level can be prevented, and impairment of products such as liquid crystal displays; e.g., damage to polarized film, can be prevented. According to the present invention, a very small-scale plasma generator can be realized. Thus, in one possible mode, a plurality of plasma jetting holes are provided in the gas discharge section, and irradiation with high-density plasma can be confined to an anisotropic conductive film (ACF)-attached substrate portion. Also, the present cleaning device can be effectively installed in a very narrow space of a liquid crystal display fabrication apparatus. A hole of the gas discharge section may have an oblique angle with respect to the imaginary line which is normal to the longitudinal direction of the plasma-generating zone, and the oblique direction may be any direction within 360° along the imaginary line.
Throughout the above-described aspects of the invention, plasma generation is preferably performed under atmospheric pressure. However, reduced or increased pressure may be employed. The term “atmospheric pressure” herein encompasses about 0.5 to about 2 atm. The holes of the gas discharge section may have a tip diameter of 0.1 mm to 1 mm. When the hole diameter is shorter, the gas flow rate increases. In this case, the treatment object is irradiated at high possibility with plasma without deactivation of radicals, which is preferred, and suitable radical irradiation can be realized. When the holes of the gas discharge section have a length which is ½ times or more the distance between said pair of electrodes, electric discharge with respect to a treatment object can be effectively prevented.
The plasma generator 100 shown in
The plasma-generating zone P has a rectangular cross-section (short side: 2 mm, long side 5 mm) in a direction normal to the x-axis direction. The long side is along the y-axis direction. The x-axis length of the zone P is 4 cm. As shown in
The plasma generator 100 can enhance adhesion between the glass substrate of a liquid-crystal display and anisotropic conductive film (ACF), in the case where an area of the glass substrate to which ACF is attached is cleaned by the plasma generator before attachment of ACF to the substrate.
Then, through varying the length of the side of the cross-section normal to the x-axis direction of the plasma-generating zone P while the distance (along the x-axis) between the electrodes 2a and 2b was fixed to 4 cm, stable electric discharge was generated when the side length was 5 mm or less. When the distance between the electrodes 2a and 2b was varied while the length of the side of the cross-section of the plasma-generating zone P was fixed to 5 mm, stable linear electric discharge was generated when the distance was 4 cm or shorter.
Subsequently, through employment of the above apparatus, the surface of a glass substrate of a liquid-crystal display was hydrophilicized with argon gas plasma. The contact angle of the surface before the treatment was 50°, but was reduced to 7° after the treatment. When the same apparatus as shown in
With reference to
In the aforementioned Embodiments, a plurality of said plasma generators may be provided along the longitudinal direction (x-axis direction) or in parallel to the x-axis, whereby an object having a wide area can be treated. When n units of plasma generators are provided along the x-axis direction, an object can be treated in a width of 4n cm (in the above Embodiments). In this case, through conveying the treatment object along the axis normal to the X-axis and Y-axis, a wider area of the object can be treated. When a plurality of plasma generators are provided in parallel to the x-axis, through conveying the treatment object along the axis normal to the X-axis and Y-axis, thorough plasma irradiation can be performed. The holes 23, 24, and 26 have a cross-section normal to the gas flow direction, which cross-section is a circle or an oval, or a rectangle or slit-like pattern having a longitudinal axis normal to the x-axis and y-axis (an axis normal to the sheet of each drawing) in
In the above Embodiments, when the cross-section of the plasma-generating zone P normal to the x-axis is a rectangle, a side thereof preferably has a length of 2 mm to 5 mm, and the cross-section preferably has an area of 3 mm2 to 25 mm2.
The apparatus of the present invention can allow an object to be irradiated with linear plasma for the surface treatment of the object.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2008-042056 | Feb 2008 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2009/052956 | 2/20/2009 | WO | 00 | 11/1/2010 |
