Plasma processing equipment

Abstract

To provide a plasma processing apparatus that can process materials continuously and uniformly. [Solution] The plasma processing apparatus 10 comprises a pair of electrodes 30, 31 to which an AC voltage is applied to generate plasma, a discharge gas supply unit 34 that supplies discharge gas between the electrodes 30, 31, and a dielectric 33 placed between the electrodes 30, 31. A breathable belt 20 for transporting the workpiece 11 is placed between the electrodes 30, 31.

Description

Technical Field

[0001] The present invention relates to a plasma processing apparatus for processing a workpiece.

Background Art

[0002] Conventionally, plasma processing apparatuses have been used for surface treatments such as ashing, etching, or film formation, surface modifications such as improvement of adhesiveness or wettability or surface hardening, and treatments such as sterilization of medical instruments and foods. For example, Patent Document 1 below discloses a plasma processing apparatus in which a belt configured as a dielectric is disposed between a pair of electrodes that generate plasma, and the workpiece is processed while being conveyed by the belt.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In the above-described plasma processing apparatus, the workpiece is processed while being conveyed using a belt that constitutes a dielectric, but in the case of workpieces such as foodstuffs, continuous and uniform processing is desired.

[0005] The problem to be solved by the present invention is to provide a plasma processing apparatus capable of continuously and uniformly processing a workpiece.

Means for Solving the Problems

[0006] The plasma processing apparatus of the present invention includes a pair of electrodes to which an alternating voltage is applied to generate plasma, a discharge gas supply unit that supplies a discharge gas between the electrodes, a dielectric disposed between the electrodes, and a belt having air permeability that is disposed between the electrodes and conveys a workpiece.

Effects of the Invention

[0007] According to the present invention, a plasma processing apparatus capable of continuously and uniformly processing materials can be provided. [Brief explanation of the drawing]

[0008] [Figure 1] This is a schematic side view of a plasma processing apparatus according to one embodiment. [Figure 2] This is a schematic front view of the plasma processing apparatus shown above. [Figure 3] This is a perspective view showing a schematic of the plasma processing apparatus. [Figure 4] A side view of the plasma processing apparatus shown above, with a cover added to illustrate its general structure. [Figure 5] This is a schematic side view of a plasma processing apparatus according to another embodiment. [Figure 6] This is a perspective view showing a schematic of a plasma processing apparatus according to another embodiment. [Figure 7] This graph shows the characteristics of the discharge gas used in the plasma processing apparatus of each embodiment. [Figure 8] This graph shows the characteristics of the discharge gas used in the plasma processing apparatus of each embodiment. [Modes for carrying out the invention]

[0009] Hereinafter, one embodiment of the present invention will be described with reference to the drawings.

[0010] Figures 1 to 3 show the plasma processing apparatus 10. The plasma processing apparatus 10 of this embodiment is used as a plasma sterilization apparatus to sterilize food ingredients as the processed material 11 under atmospheric pressure and at room temperature. Food ingredients include powdered or granular materials such as beans, wheat, sesame seeds, pepper, or tea leaves (including tencha and matcha).

[0011] The plasma processing apparatus 10 includes a transport device 12 for transporting the workpiece 11, and a plasma generating device 13 for sterilizing the workpiece 11 transported by the transport device 12 by generating plasma using dielectric barrier discharge.

[0012] The conveying device 12 uses a belt conveyor that includes an endless belt 20, rollers 21 and 22 that are mounted on the belt 20 in a substantially horizontal manner, and a drive unit (not shown) such as a motor that rotates the rollers 21 to rotate the belt 20 in the conveying direction F. The rotation speed of the belt 20 is, for example, about 1 to 20 mm / s, and can be determined experimentally in advance according to the required processing of the material to be processed 11.

[0013] The belt 20 is a flat belt, and its upper surface, which is wrapped between the rollers 21 and 22, is configured as a conveying surface 23 for carrying and transporting the workpiece 11. The belt 20 is formed in sheet form from a glass cloth material that has heat resistance, flexibility, and breathability, allowing discharge gas to pass through. The material of the belt 20 is not limited to glass cloth material; it may also be formed in sheet form from insulating materials such as fluororesin or polyamide resin, as long as it is non-conductive and has heat resistance, flexibility, and breathability.

[0014] The material to be processed 11 is supplied to the belt 20 continuously or intermittently from a material supply device or the like.

[0015] Next, the plasma generator 13 includes a pair of opposing main electrodes 30 and 31, a pre-ionization electrode 32 disposed on at least one side of electrode 30, a main dielectric 33 disposed between electrodes 30 and 31, a discharge gas supply unit 34 that supplies discharge gas between electrodes 30 and 31, and a power supply unit (not shown) that applies an AC voltage between electrode 30 and the pre-ionization electrode 32 and between electrodes 30 and 31.

[0016] One electrode 30 is disposed above the conveying surface 23 of the belt 20, and the other electrode 31 is disposed below the lower surface opposite to the conveying surface 23 of the belt 30. The opposing surfaces 30a and 31a of these electrodes 30 and 31 are disposed parallel to each other with the belt 20 and the dielectric 33 interposed therebetween. The opposing interval between the electrodes 30 and 31 is about 5 mm to 20 mm and can be adjusted according to the workpiece 11.

[0017] The electrodes 30 and 31 are made of a conductive material such as aluminum, and are formed in a flat plate shape with a length of 400 mm, a width of 100 mm, and a smooth curved surface with a radius of curvature of 8 mm at the ends of the electrodes. The electrodes 30 and 31 are disposed in the width direction W that intersects the conveying direction F in which the workpiece 11 is conveyed by the belt 20. Note that if the electrodes 30 and 31 have conductivity, materials other than aluminum, such as copper, brass, tungsten, or graphite, may be used.

[0018] A plurality of accommodating portions 37 for disposing the pre-ionization electrodes 32 are formed on the opposing surface 30a of the electrode 30. The accommodating portions 37 are concave and open to the opposing surface 30a of the electrode 30, are arranged at a predetermined interval in the short side direction of the electrode 30 (corresponding to the conveying direction F of the belt 20), and extend along the long side direction of the electrode 30 (corresponding to the width direction W of the belt 20). A flow path 38 through which the discharge gas supplied between the electrodes 30 and 31 by the discharge gas supply portion 34 flows is formed in the electrode 30.

[0019] The electrodes 30 and 31 are supported by a support 40. The support 40 includes a first support 41 that supports the upper electrode 30, a second support 42 that supports the lower electrode 31, and a connecting portion 43 that connects these supports 41 and 42. The interval between the electrodes 30 and 31 can be arbitrarily adjusted according to the dimensions of the connecting portion 43 that connects the supports 41 and 42.

[0020] Furthermore, the pre-ionization electrodes 32 are rod-shaped, and multiple electrodes are used. Each electrode is placed in a housing portion 37 formed on the opposing surface 30a of the electrode 30, generating pre-ionization plasma Ps between it and the electrode 30. The pre-ionization electrodes 32 housed in the housing portion 37 are arranged at predetermined intervals in the short-side direction of the electrode 30 (corresponding to the conveying direction F of the belt 20), and their long-side direction (axial direction) is aligned with the long-side direction of the electrode 30 (corresponding to the width direction W of the belt 20). The pre-ionization electrodes 32 are supported together with the electrode 30 by the support 40.

[0021] The pre-ionization electrode 32 comprises a conductor 45 and a pre-ionization discharge dielectric 46. The conductor 45 is formed by stretching a conductive material into a long length. The conductor 45 is made of, for example, copper wire, but any conductive material may be used, such as copper, brass, tungsten, or aluminum. The pre-ionization discharge dielectric 46 covers the conductor 45 and insulates the conductor 45 from the electrode 30 and the like. The pre-ionization discharge dielectric 46 is made of a transparent quartz tube formed in a bottomed cylindrical shape that houses the conductor 45. The pre-ionization discharge dielectric 46 is not limited to a quartz tube; any heat-resistant dielectric with insulating properties that covers the conductor 45 may be used, such as heat-resistant glass, ceramic materials other than glass, resin materials, or rubber materials.

[0022] Furthermore, the dielectric 33 is formed in a flat plate shape from a dielectric material such as heat-resistant glass, quartz, or alumina, and is arranged parallel to the opposing surfaces 30a and 31a of the electrodes 30 and 31. The dielectric 33 is positioned between the opposing surfaces 30a and 31a of the electrodes 30 and 31, between the lower surface of the belt 20 opposite to the conveying surface 23 and the opposing surface 31a of the lower electrode 31. The dielectric 33 is supported by the support 40 together with the electrodes 30 and 31.

[0023] Furthermore, the discharge gas supply unit 34 supplies discharge gas, which is drawn from a discharge gas supply source including a pump and a tank, between the electrodes 30 and 31. The discharge gas supply unit 34 supplies the discharge gas between the electrodes 30 and 31 through a flow passage 38 provided in the upper electrode 30. The flow passage 38 has an inlet into which the discharge gas flows in on the upper surface of the electrode 30, and multiple outlets are provided adjacent to each housing portion 37 on the opposing surface 30a to blow out the discharge gas that has flowed in from the inlet. The outlets have, for example, a hole diameter of 0.5 mm to 1.0 mm and are formed at intervals of 2 to 5 mm along the longitudinal direction of the housing portion 38 of the electrode 30.

[0024] The discharge gas is a gas that facilitates the generation of a pre-ionization plasma Ps by the pre-ionization electrode 32 and guides the generated pre-ionization plasma Ps to the dielectric 33 side to generate and maintain a plasma (hereinafter referred to as the main discharge plasma Pm) between electrodes 30 and 31. The discharge gas is composed of gases with a lower ionization voltage than air, such as nitrogen, argon, and helium, either individually or in mixtures thereof, and further, depending on the application, by adding small amounts of gases such as oxygen, water vapor, or ammonia to these.

[0025] The discharge gas supply unit 34 optimizes the flow rate of discharge gas introduced between electrodes 30 and 31 according to the distance between electrodes 30 and 31, increasing the flow rate of discharge gas introduced between electrodes 30 and 31 when the distance between electrodes 30 and 31 is wide, and decreasing the flow rate of discharge gas introduced between electrodes 30 and 31 when the distance is narrow. It also stably generates plasma (pre-ionization plasma Ps, main discharge plasma Pm) by changing the type of discharge gas or, in the case of a mixed gas, the mixing ratio.

[0026] The plasma generator 13 is covered with a cover (not shown) to prevent the outflow of ozone generated during atmospheric pressure discharge, as well as to prevent the diffusion of discharge gas supplied from the discharge gas supply unit 34, thereby maintaining the discharge gas atmosphere between the electrodes 30 and 31. The cover allows the passage of the workpiece 11 on the belt 20 and the conveying surface 23.

[0027] The power supply unit also includes a pre-ionization power supply unit that applies an AC voltage between electrode 30 and pre-ionization electrode 32, and a main discharge power supply unit that applies an AC voltage between electrodes 30 and 31. The pre-ionization power supply unit outputs an AC voltage with, for example, a pre-ionization voltage of ±5kV and a frequency of 10kHz in order to generate pre-ionization plasma Ps between electrode 30 and pre-ionization electrode 32. The main discharge power supply unit outputs an AC voltage with, for example, a main discharge voltage of ±15kV and a frequency of 10kHz, which is in opposite phase to the pre-ionization electrode voltage, in order to generate main discharge plasma Pm between electrodes 30 and 31. These are just examples, and the AC voltage, current, and frequency output by the power supply unit can be experimentally determined in advance according to the electrode spacing or the processing content required for the workpiece 11.

[0028] Next, the operation of the plasma processing device 10 will be explained.

[0029] In this embodiment, the plasma processing apparatus 10 is described in a case where it is incorporated into a manufacturing and processing line for powdered or granular food ingredients such as beans, wheat, sesame seeds, pepper, or tea leaves (including tencha and matcha), and these are treated as processed materials 11 for sterilization.

[0030] Discharge gas is discharged from the discharge gas supply unit 34 from the opposing surface 30a of the electrode 30 and supplied between the electrodes 30 and 31.

[0031] An AC voltage is applied between electrode 30 and pre-ionization electrode 32 by a power supply, generating pre-ionization plasma Ps between electrode 30 and pre-ionization electrode 32. Pre-ionization plasma Ps is generated when a portion of the discharge gas and atmosphere present between electrode 30 and pre-ionization electrode 32 is ionized and activated. In this way, pre-ionization plasma Ps is generated by dielectric barrier discharge under atmospheric pressure. In this case, pre-ionization plasma Ps is generated uniformly and stably in a rectangular planar shape in a planar view along the opposing surface 30a of electrode 30 and the longitudinal and width directions of pre-ionization electrode 32.

[0032] An AC voltage is applied between electrode 30 and electrode 31 by a power supply, generating a main discharge plasma Pm between electrode 30 and electrode 31 via a dielectric 33 or the like. The main discharge plasma Pm is generated when the discharge gas and a portion of the atmosphere present between electrode 30 and electrode 31 are ionized and activated by electrons and ions generated by the pre-ionization plasma Ps. In this way, the main discharge plasma Pm is generated by dielectric barrier discharge under atmospheric pressure. In this case, the main discharge plasma Pm is generated uniformly and stably in three dimensions, in a rectangular planar shape in a planar view along the longitudinal and width directions of the opposing surfaces 30a of electrode 30 and 31a of electrode 31, and also across the gap between electrodes 30 and 31.

[0033] Then, the belt 20 is rotated, and the material to be processed 11 is continuously supplied from the material supply device to the conveying surface 23 of the belt 20. The material to be processed 11, which is conveyed in the conveying direction F by the rotating belt 20, passes between the electrodes 30 and 31 of the plasma generator 13, and is continuously sterilized by being irradiated with plasma.

[0034] In the plasma processing apparatus 10 of this embodiment, since the belt 20 is permeable, the discharge gas supplied from the upper electrode 30 flows through the belt 20 and is uniformly distributed. As a result, plasma is generated uniformly and stably between electrodes 30 and 31, and the material 11 being transported on the belt 20 can be continuously and uniformly sterilized. Therefore, the plasma processing apparatus 10 can be incorporated into a food manufacturing and processing line to continuously and uniformly sterilize food.

[0035] By using a non-conductive glass cloth that is heat-resistant, flexible, and breathable for the belt 20, a plasma processing apparatus 10 can be realized that continuously and uniformly sterilizes the material 11 conveyed by the belt 20 as described above.

[0036] Furthermore, the plasma processing apparatus 10 may be equipped with stirring means for stirring the workpiece 11 on the belt 20 that is being sterilized by plasma. For example, a vibrator that vibrates the belt 20 may be used as the stirring means, and the workpiece 11 passing between the electrodes 30 and 31 may be stirred by the vibration of the belt 20, thereby uniformly irradiating the workpiece 11 with plasma and improving the uniformity of the sterilization effect on the workpiece 11. The stirring means may also be a screw, stirring plate, or stirring brush that comes into contact with and stirs the workpiece 11 on the belt 20.

[0037] Furthermore, as shown in Figure 4, the plasma processing apparatus 10 may also use a breathable cover 48 positioned between the conveying surface 23 of the belt 20 that conveys the workpiece 11 and the upper electrode 30 facing the conveying surface 23. The cover 48 is formed in sheet form from a glass cloth material that has heat resistance, flexibility, and breathability to allow discharge gas to pass through. The material of the cover 48 is not limited to glass cloth material, as long as it has heat resistance, flexibility, and breathability, it may also be formed in sheet form from an insulating material such as fluororesin or polyamide resin.

[0038] The cover 48 is placed over the workpiece 11 on the conveying surface 23 of the belt 20 so as to cover it between itself and the belt 20, and moves in the conveying direction F together with the conveying surface 23 of the belt 20 and the workpiece 11.

[0039] When the material to be processed 11 is a food ingredient in powder form, the irradiation of plasma onto the material 11 on the conveying surface 23 of the belt 20 can cause the material 11 to become charged and adhere to the electrode 30 and pre-ionization electrode 32, potentially changing and degrading the discharge characteristics over time. However, by using the cover 48, adhesion of the material 11 to the electrode 30 and pre-ionization electrode 32 can be prevented, and the change and degradation of the discharge characteristics over time can be suppressed. Moreover, since the cover 48 is breathable, the discharge gas blown out from the electrode 30 side passes through the cover 48 and is supplied to the material 11 and the belt 20, so that the plasma irradiated onto the material 11 is generated uniformly and stably, ensuring uniformity of the sterilization effect on the material 11.

[0040] The cover 48 only needs to be positioned between the conveying surface 23 of the belt 20 that conveys the processed material 11 and the upper electrode 30 facing the conveying surface 23. For example, it may be fixedly positioned on a support 40 or the like so as to cover the upper electrode 30.

[0041] Furthermore, as shown in Figure 5, the plasma processing apparatus 10 may be provided with a discharge gas supply unit 34 on the lower electrode 31, and discharge gas blown out from the opposing surface 31a of the lower electrode 31 may be supplied between the electrodes 30 and 31. In this case, the plate-shaped dielectric 33 may be attached to the opposing surface 30a of the upper electrode 30, or the dielectric material may be coated on at least one of the electrode surfaces of electrode 30 and electrode 31 to maintain insulation between the two electrodes.

[0042] The discharge gas supply unit 34 blows discharge gas from the opposing surface 31a of the lower electrode 31 to the lower side opposite to the conveying surface 23 of the belt 20, causing it to be discharged onto the conveying surface 23. As a result, the discharge gas discharged from the lower side of the belt 20 onto the conveying surface 23 causes the workpiece 11 on the conveying surface 23 to be lifted and moved, agitating it and uniformly irradiating the workpiece 11 with plasma, thereby improving the uniformity of the sterilization effect. Therefore, the discharge gas supply unit 34 provided on the lower electrode 31 is configured as an example of an agitation means for agitating the workpiece 11 on the conveying surface 23 of the belt 20.

[0043] Furthermore, as shown in Figure 5, the above-described cover 48 may also be used in the configuration of the plasma processing apparatus 10 in which the discharge gas supply unit 34 is provided on the lower electrode 31.

[0044] Furthermore, as shown in Figure 6, the plasma processing apparatus 10 may also include a moving mechanism 50 that reciprocates the electrodes 30 and 31 in a width direction W intersecting the transport direction F. In this case, the moving mechanism 50 may also reciprocate the pre-ionization electrode 32 and dielectric 33, which are supported by the support 40 along with the electrodes 30 and 31, in the width direction W via the support 40. The moving mechanism 50 includes a sliding mechanism that supports the support 40 so as to be slidable in the width direction W, and an actuator that reciprocates the support 40 in the width direction W. For example, the moving mechanism 50 reciprocates the electrodes 30 and 31 in the width direction W with a reciprocating amplitude of 20 to 30 mm and a moving speed of 50 mm / s.

[0045] The movement of the workpiece 11 by the belt 20 in the transport direction F and the reciprocating movement of the electrodes 30 and 31 by the moving mechanism 50 in the width direction W intersecting the transport direction F change the relative position of the plasma generated between the electrodes 30 and 31 and the workpiece 11. This allows the plasma to be uniformly irradiated onto the workpiece 11, improving the uniformity of the sterilization effect on the workpiece 11.

[0046] Figure 7 shows the experimental results regarding the types and proportions of discharge gases. In the experiment, plasma irradiation was performed using a biological indicator consisting of a strip of test paper embedded with spore-forming bacteria and wrapped in glassine paper, with argon, helium, and a mixed gas of argon and helium. The relationship between plasma treatment time and the number of surviving bacteria was then determined. In this case, the distance between electrodes 30 and 31 was 12 mm, the pre-ionization voltage was 5 kV, the main discharge voltage was 14 kV, and the frequency was 10 kHz.

[0047] As a result, the discharge gases showed better sterilization properties when mixed with argon or helium than when used individually. Furthermore, with the mixed gas, good sterilization properties were obtained, with the detection limit line L falling below the threshold in 50-60 seconds, when the gas flow rate ratio of Ar / (Ar+He) was approximately 70% (Ar: 7 sim, He: 3 sim). Therefore, when using a mixed gas of argon and helium as the discharge gas, it is acceptable to use a discharge gas with such a gas flow rate ratio.

[0048] Furthermore, in an experiment in which a polyethylene sheet coated with a spore-forming bacterial suspension and dried was covered with Cover Body 48 and irradiated with plasma, inactivation of the spore-forming bacteria in a short time was confirmed, making it possible to use it as a low-temperature sterilization device for medical instruments. One possible factor in the rapid inactivation of spore-forming bacteria is the inactivating effect of oxygen radical species and nitrogen radical species generated by the plasma discharge, but observations using an electron microscope suggest that the damage to the outer membrane of the spore-forming bacteria due to the direct impact of charged particles from the discharge itself is the main factor in the inactivation.

[0049] Figure 8 also shows the results of an experiment investigating the relationship between the flow rate of Ar / H2O added and the number of surviving spore-forming bacteria when a small amount of water vapor was added to argon as the discharge gas. In this case, the distance between electrodes 30 and 31 was 10 mm, the pre-ionization voltage was 4-5 kV, the main discharge voltage was 12.5 kV, and the plasma irradiation time was 30 seconds.

[0050] As a result, good sterilization characteristics were obtained when the Ar / H2O addition flow rate was approximately 150 sccm. Therefore, when using a mixed gas of argon and helium as the discharge gas, the use of helium can be a cost obstacle, but by adding water vapor at a specific addition flow rate to argon as the discharge gas, good sterilization characteristics can be obtained and costs can be reduced.

[0051] Furthermore, the plasma processing device 10 can be used not only for sterilizing food ingredients, but also for sterilizing seeds such as rice and wheat in addition to vegetables in the agricultural field, and can be applied to sterilizing various pathogenic microorganisms that harm the growth of seeds. In addition, the plasma processing device 10 can be applied to sterilizing medical devices. For example, when medical devices packaged in breathable Tyvek® packaging paper are subjected to plasma treatment, the plasma is generated by penetrating the space inside the Tyvek packaging paper, so the medical devices inside can be sterilized without damaging the Tyvek packaging paper. Moreover, the plasma processing device 10 can be applied to surface modification and surface treatment of conductive and resistant materials such as resin materials, and can be applied to a wide range of fields other than the product and medical fields.

[0052] Embodiments of the present invention and their variations have been described above, but various combinations of configurations, as well as partial omissions, substitutions, and modifications, are also possible. [Explanation of symbols]

[0053] 10 Plasma Processing Equipment 11. Processed materials 20 belts 23 Conveying surface 30,31 electrode 33 Dielectrics 34. Discharge gas supply unit 48 Covering body 50 Moving mechanism F Conveying direction

Claims

1. A pair of electrodes to which an alternating voltage is applied to generate plasma, A discharge gas supply unit that supplies discharge gas between the electrodes, A dielectric material is disposed between the electrodes, A breathable belt is placed between the electrodes and conveys the material to be processed. A plasma processing apparatus characterized by comprising the following features.

2. The belt is formed of glass cloth. The plasma processing apparatus according to feature 1.

3. The belt is equipped with a stirring means for stirring the material being processed on the belt. The plasma processing apparatus according to feature 1.

4. The discharge gas supply unit blows the discharge gas from the side opposite to the conveying surface of the belt that transports the processed material, and discharges it towards the conveying surface. The plasma processing apparatus according to feature 1.

5. The belt that transports the processed material is equipped with a breathable cover that is positioned between the transport surface of the belt and the electrode facing the transport surface. The plasma processing apparatus according to feature 1.

6. The system includes a moving mechanism that reciprocates the electrode in a direction intersecting the conveying direction in which the belt conveys the processed material. The plasma processing apparatus according to feature 1.

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