JP5966212B2 - Inductively coupled microplasma source in which part of floating electrode faces inside gas flow path and apparatus using the same - Google Patents

Description

The present invention relates to downsizing, microdeviceization, ignition promotion, and power saving of an inductively coupled microplasma source.

Many types of microplasma sources have been reported where some or all of the plasma region size is below cm. Since the pressure is high, even a small size is advantageous for plasma generation. Above all, atmospheric pressure plasma is suitable for energy saving and low-cost manufacturing because a vacuum chamber is unnecessary. Compared to capacitive coupling type, inductively coupled plasma is promising because the plasma density can be higher.
However, there is a problem that plasma ignition and lighting are difficult. Adding an igniter mechanism requires a new dedicated power supply, electrode fabrication, and connection between the electrode and the power supply. It also hinders downsizing of plasma sources and micro devices.
For example, Patent Documents 1 to 5 and Non-Patent Document 1 disclose techniques for adding electrodes other than the inductively coupled plasma generating coil electrode and an accompanying mechanism to the plasma source for inductively coupled microplasma ignition. Yes.

In Patent Documents 1 and 2, a metal wire (tungsten and iron are exemplified) is introduced into a capillary for a material supply source of minute size metal dots. After high-frequency induction heating of this metal wire, the connected igniter is actuated for a moment to apply a high voltage between the wire tip and the coil electrode for inductive coupling type plasma generation to discharge the inductive coupling type micro It is disclosed to turn on plasma.
Patent Document 3 discloses that an electrode for generating a dielectric barrier discharge is additionally provided inside and outside the insulator tube for inductively coupled microplasma lighting.
Patent Document 4 mentions that the apparatus control unit includes a gas supply unit, a high-frequency power source, and a plasma ignition unit, although not shown. In at least some of the devices handled by the applicant, a mechanism equivalent to a gas stove ignition device using a piezoelectric element is combined.
Patent Document 5 discloses a plasma generation method that uses an electrode (not a wire shape but a cylindrical shape) that applies a high voltage and small current to start plasma in addition to an electrode that applies a low voltage and large current for maintaining plasma. Yes.
One of the authors of Non-Patent Document 1 is included in the inventors of Patent Documents 1 and 2. Exactly the same, but explained that the tungsten wire is usually connected to ground potential, promoting the generation of thermoelectrons, and applying a DC voltage of about 15 kV for 0.5 seconds for plasma ignition. Yes. In any case, the added electrode structure requires connection and a high voltage power source for ignition.

Non-Patent Document 2 shows an electrode structure for microplasma in which two tungsten needles face each other with a gap of 2.5 mm and one side is a floating electrode. A voltage of about 4 kV is applied at 48 kHz.
Non-Patent Document 3 shows microplasma using a silicon device having a size of about several hundreds of micrometers. Neon gas is prepared by preparing a floating electrode of 100 μm square or 50 μm square at the center of a square-shaped nickel electrode to which a positive / negative bipolar pulse voltage of about 260 V is applied at 7.5 kHz on the substrate plane. It is mentioned that the discharge is promoted. Neither of the above two is an inductively coupled microplasma source.
Patent Document 6 is filed by the inventor. It is assumed that a floating electrode is placed inside the gas flow path. For this reason, the design of the floating electrode depends on the design of the gas flow path. Non-Patent Document 4 is an academic paper by the inventors and is an inductively coupled microplasma source that is excited by a U-shaped flat coil in which a floating electrode is disposed in a gas flow path. A metal wire such as aluminum or tungsten is disposed in a groove having a width of 2 mm, a length of 40 mm, and a depth of 1.2 mm. In order to arrange in the groove which is a gas flow path, there are restrictions on the shape, length and direction.

"Low melting point substrate with minute dots or lines, deposition method and apparatus using microplasma" Publication number: Patent publication 2005-262111 Publication date: September 29, 2005 Applicant: National Institute of Advanced Industrial Science and Technology Publication number: Patent Publication 2008-306209 Publication date: December 18, 2008 Applicant: National Institute of Advanced Industrial Science and Technology "Plasma Generator" Publication Number: Patent Publication 2008-198583 Publication Date: August 28, 2008 Applicant: Terashima Kazuo "Microplasma Jet Control Method and Apparatus" Publication Number: Patent Publication 2007-213821 Publication Date: August 23, 2007 Applicant: Matsushita Electric Industrial Co., Ltd. "Plasma Generator and Plasma Generation Method" Publication Number: Patent Publication 2009-289432, Publication Date: December 10, 2009 Applicant: Tokyo Institute of Technology "Inductively coupled microplasma source with floating electrode" Publication number: JP 2011-249289 Publication date: December 8, 2011 Applicant: Toyota Gakuen Toyota Technological Institute "Thermoelectron-enhancedmicrometer-scale plasma generation", T. Ito, K. Terashima, Applied Physics Letters, Vol. 80, No. 15 (2002) 2648-2650. “A simple cold Ar plasma jetgenerated with a floating electrode at atmospheric pressure”, Q.-Y.Nie, C.-S.Ren, D.-Z.Wang, J.-L.Zhang, Applied Physics Letters, Vol. 93 (2008) 011503. "Mode Change Observed on Spatial Distribution of Microplasma Emission in a Microdischarge Cell with aFloating Electrode", D.-S. Lee, O. Sakai, K. Tachibana, Japanese Journalof Applied Physics, Vol. 48 (2009) 106002. "Novel Atmospheric Pressure Inductively Coupled Micro Plasma Source Using Floating Wire Electrode", S. Kumagai, H. Matsuyama, Y. Yokoyama, M. Hori, M. Sasaki, Japanese Journal of Applied Physics, Vol. 50 (2011) 08JA02.

Inductively coupled microplasma can generate high density plasma. However, there was a problem that plasma ignition was difficult. Although a method of adding an igniter mechanism is known, it requires preparation of a dedicated power supply, production of an additional electrode, and connection between the electrode and the power supply. These hinder the miniaturization and microdevices of the plasma source.
In order to solve this problem, the inventor previously applied for a patent for a method of facilitating ignition without adding a power source for ignition by disposing a floating electrode inside the gas flow path.
However, disposing the electrode inside the gas flow path means that the design of the floating electrode depends on the design of the gas flow path. Since the flow path and electrode design appropriate for the plasma source are different, there is a problem that they are not necessarily optimal for each other.

The present invention solves the problem by realizing an inductively coupled microplasma source in which a part of the floating electrode faces the inside of the gas flow path.
According to the present invention, although the floating electrode is used, an inductively coupled microplasma source can be obtained by facing only part of the floating electrode inside the gas flow path.
Since inductive coupling is used for microplasma excitation, energy can be supplied to the floating electrode and ignition can be promoted without special connection. Since only a part of the floating electrode faces a gas that is easily ionized, the ignition point is limited, but the ignition position is determined depending on the electrode arrangement, so that it can be incorporated into the design of the plasma source.
Thereby, each of the gas flow path and the floating electrode can be optimally designed. For example, the length and direction of the floating electrode can be designed independently of the gas flow path. The longer the electrode, the lower the ignition power, and the orientation is related to the design of outgoing light extraction in the case of light source applications. Since it is plasma emission, there is an appropriate direction to avoid self-absorption.

According to the present invention, an inductive coupling is characterized in that a part of the floating electrode faces the inside of the gas channel and the other part is close to the material having a high dielectric constant outside the gas channel. Type microplasma source is obtained.
Since a high-frequency power source of about 100 MHz is used for microplasma applications, the wavelength is about 3 m. Since the structure of microplasma is usually less than cm, it is sufficiently smaller than the wavelength. As the floating electrode becomes larger, the effect of receiving an electromagnetic field is obtained as in the case of the antenna, which is advantageous for plasma ignition. The proximity of the floating electrode to the material having a high dielectric constant outside the gas channel has an effect of making the electromagnetic field longer than the actual size. That is, the effect of facilitating ignition while maintaining the size of the plasma source small can be obtained.

In addition, according to the present invention, an inductively coupled microplasma source is obtained in which a part of the floating electrode faces the inside of the gas flow path and there is one or more other floating electrodes. In the case of one floating electrode, plasma ignition is promoted by obtaining a high electric field between the electrode end and the surrounding potential. When two or more floating electrodes are prepared, positive and negative potentials with different signs can be formed at the ends of the electrodes depending on the size and layout. Since the distance between the electrodes can be designed to be short, the effect of forming a high electric field can be enhanced.
Further, according to the present invention, a light source, a spectroscopic system, surface treatment, doping, film deposition using an inductively coupled microplasma source, wherein a part of the floating electrode faces the inside of the gas flow path An apparatus such as etching can be realized.

According to the present invention, an inductively coupled microplasma source that is small and easy to ignite can be realized.
When the plasma is ignited at a lower power, even when the power is further increased, the plasma grows to form a brighter plasma at a lower power. Promotion of higher density and improved energy efficiency are expected. The degree to which the design of the floating electrode and the design of the gas flow path depend on each other can be reduced, making them suitable for plasma ignition and lighting. It becomes a stable plasma source. This is an important performance when applying a plasma source as a component in a larger device. The effects of downsizing, microdevices, high functionality, improved reliability, and energy saving of an apparatus using an inductively coupled microplasma source can be obtained.

Hereinafter, embodiments of the present invention will be described with reference to the drawings. The drawing is merely an example, and the material, shape, arrangement, and the like of the electrodes and gas flow paths that constitute the plasma source are not limited.
FIG. 1 is a schematic view of an embodiment of an inductively coupled microplasma source characterized in that a part of a floating electrode faces the inside of a gas flow path according to an embodiment of the present invention.
In FIG. 1, 1 is a plasma region. 2 is a floating electrode. The floating electrode 2 basically has a triangular shape, and a line slightly extends from a sharp corner and faces the gas flow path. Ignition occurs around the end of the electrode facing the gas flow path. 3 is another floating electrode (sub-electrode). 4 is a material having a high dielectric constant close to the floating electrode. As the dielectric constant increases, the electromagnetic action of the floating electrode 2 becomes longer than the actual size.

Reference numeral 5 denotes an inductively coupled plasma generating coil electrode made of a spiral pipe.
In the example of FIG. 1, three other floating electrodes are arranged in the gas flow path, and are smaller than the two floating electrodes and are orthogonal to each other.
At the time of plasma ignition, energy is supplied to the floating electrode 2 or another floating electrode 3 through electromagnetic induction.
A gas flow path 6 forms a gas flow 7. In this example, a cylindrical glass capillary is used.
Reference numeral 8 denotes a high frequency power source. In atmospheric pressure plasma, a frequency in the VHF band typified by 100 MHz is often used so that energy loss due to gas collision with the flow path wall is reduced.

In the embodiment of FIG. 1, the inductively coupled plasma generating spiral coil 5 is formed by a spiral pipe, and the gas flow path 6 is formed by a cylindrical glass thin tube.
The floating electrode 2 is configured such that the floating electrode 2 is sandwiched in a gap connecting cylindrical glass capillaries of the gas flow path 6 and a part thereof faces the inside of the gas flow path.
A magnetic field generated by the spiral coil 5 electromagnetically acts on the floating electrode 2. The shape of the floating electrode 2 is such that a line slightly extends beyond the tip of an acute triangle. The floating electrode is covered with a material having a high dielectric constant (for example, polyimide) outside the gas flow path. For this reason, it has the function of receiving an electromagnetic field that is the same as the electrode size larger by the dielectric constant than the actual dimension.
In addition, a floating electrode (sub-electrode) 3 having a smaller size is disposed in the flow path 6.

In the embodiment of FIG. 1, the floating electrode (sub-electrode) 3 is arranged on the upstream side where the gas flows. Since the gap between the floating electrode 2 and the floating electrode (sub-electrode) 3 is small, a high electric field can be formed. Thereby, the effect of plasma ignition can be improved more. The glass tube 6 forming the gas flow path 7 is arranged perpendicular to the spiral coil 5, but is convenient for the configuration of a normal straight tube, and is optional except in the region where plasma is formed. The effect of plasma ignition is the same even with this tube shape.
Since inductive coupling is used for microplasma excitation, energy can be supplied to the floating electrode and ignition can be promoted without special connection.
Since only a part of the floating electrode faces a gas that is easily ionized, the ignition point is limited, but the ignition position is determined depending on the electrode arrangement, so that it can be incorporated into the design of the plasma source. Thereby, each of the gas flow path and the floating electrode can be optimally designed. The plasma source can be reduced in size, made into a microdevice, and easily ignited, while at the same time, further promoting the effect of generating plasma with lower power consumption.

FIG. 2 is a schematic view of another embodiment of an inductively coupled microplasma source characterized in that a part of the floating electrode faces the inside of the gas flow path according to an embodiment of the present invention.
In FIG. 2, 1 is a plasma region. 2 is a floating electrode. 3 is another floating electrode (sub-electrode). Whether to add this electrode depends on the purpose. In this example, the floating electrode 3 is smaller than the floating electrode 2 and the directions are orthogonal.
The tips where 2 and 3 face each other are pointed to obtain electric field strength. Ignition occurs around the part of the electrodes 2, 3 facing the gas flow path. 4 is a material having a high dielectric constant close to the floating electrode. For example, silicon, which is a substrate material for forming a micro device, may be used. Reference numeral 5 denotes an inductively coupled plasma generating coil electrode made of a spiral pipe. During plasma ignition, energy is supplied to the 2 and 3 floating electrodes via electromagnetic induction. Reference numeral 6 denotes a flow path, which forms a gas flow 7. In this example, a trench structure manufactured by microfabrication is used. Actually, the trench is covered with a flow path, which is not shown for the sake of simplicity. Reference numeral 8 denotes a high frequency power source.

The inductively coupled plasma generating spiral coil is an example in which a spiral pipe is formed, and the gas channel is formed by a channel manufactured by microgrooving. For example, a meandering flow path can be formed by fine silicon processing. The shape of the floating electrode is an acute triangle. Since this floating electrode is on a material having a high dielectric constant (for example, silicon) outside the gas flow path, it has the function of receiving an electromagnetic field that is the same as the electrode size larger by the dielectric constant than the actual dimension. Further, another floating electrode (sub-electrode having a smaller size) is disposed in the flow path. In this figure, it is the example arrange | positioned in the upstream from which gas flows. Since the gap between the two floating electrodes is small, a high electric field can be formed. Thereby, the effect of plasma ignition can be improved more.

FIG. 3 shows the function of the floating electrode of the embodiment of FIG. 1 by an equivalent circuit model. An electromotive force is generated in the floating electrode 2 by receiving a high-frequency magnetic field generated from the inductively coupled plasma generating spiral coil 5. This action can be expressed by a mutual inductance M existing between the coil 5 and the floating electrode 2. Various conductive materials can be used for the floating electrode. It can be considered that both ends a and b of the linear floating electrode are respectively coupled to the surrounding ground potential. This can be represented by capacitances Ca and Cb. Accordingly, the floating electrode 2 forms a series circuit composed of the ground potential-capacitance Ca-inductance L2 of the floating electrode itself-internal resistance R2-capacitance Cb-ground potential of the floating electrode itself. An electromotive force acts on this, and a current flows in the circuit. A current generates a voltage corresponding to the impedance of each element while passing through the series circuit.

The absolute impedance value of each element is estimated assuming the case of the wire in the embodiment of FIG.
When estimated under the conditions of a frequency of 100 MHz and a total length of 22 mm, the internal resistance R2 is 0.04Ω, the impedance due to the inductance L2 is 6Ω, and the impedance due to the capacitances Ca and Cb is 300 kΩ. The inductance approximated that the interacting external electrode was at a position 0.5 mm away from the center at a distance equivalent to a vacuum, and the capacitance was at infinity. Since there is a gap length with the ground potential region, the capacitance becomes small. Therefore, even if the frequency is 100 MHz, the impedance absolute value is large. Since a series circuit is formed, the current flowing through each element is the same, and a potential difference and an electric field are increased between elements having a large impedance value. The accuracy of the impedance value is about the estimated value of the order, but the magnitude differs by five orders of magnitude depending on the impedance due to inductance and capacitance. The conditions under which the impedance due to capacitance becomes dominant are widely established. This means that the potential at point a or b is significantly different from the surrounding potential, which is consistent with ignition from the end of the floating electrode by the electric field. The values of the capacitances Ca and Cb can be adjusted depending on the shape and layout, and the electric field can be more concentrated on one of the ends of the linear floating electrode, which can be designed.
As a result, the electric field can be efficiently concentrated in the gas flow path by the floating electrode. This electric field promotes plasma ignition. Note that not only the ground potential but also the coil electrode 3 having another potential can be coupled to the floating electrode 1 via the capacitance. In this case, since the voltage takes an intermediate value with respect to the coupling portion, the electric field that is the voltage gradient in that region is suppressed, and this is not necessarily advantageous for ignition.

FIG. 4 is a graph showing ignition power when several floating electrodes are used using the inductively coupled microplasma source according to the embodiment of the present invention shown in FIG.
The helium gas flow rate was 0.5 L / min, the inner diameter of the glass capillary was 1 mm, the outer diameter was 1.5 mm, and the distance between the spiral coil and the center of the glass capillary was close to 2 mm or less. The spiral coil is connected to a 100 MHz power source via an impedance matching box. The horizontal axis of the figure is the floating electrode length L, and the vertical axis is the ignition power. The shape of the floating electrode is an acute triangle as shown in the inset, and the apex angle θ and how to take the length L are shown. The floating electrode is manufactured by manually cutting a copper foil (thickness 0.1 mm) and soldering the copper wire at the tip, so there is some variation in the structure. First, as an overall trend, when the electrode length L is shortened (that is, downsizing is advanced), the ignition power increases. The electrode of θ = 3.9 ° indicated by the x mark has a width of 3 mm or less within the range shown in the figure, but the ignition power is generally high. It is thought that the structure has a large power loss, including warpage during production.

Other electrodes having a value of θ are similar, including a single copper wire. However, the data of length L = 5 mm shows a significant difference, and a triangular shape of θ = 11.7 ° indicated by a circle is judged to be advantageous for ignition. -The wire indicated by a mark is a value when a copper wire having a diameter of 0.1 mm is arranged so that only a part thereof faces the inside of the gas flow path, like a triangular floating electrode. In addition, although it is only one point in length L = 22mm, after passing a glass thin tube through a spiral coil, the ignition power at the time of arrange | positioning the floating electrode which consists of a 0.1 mm diameter copper wire in a gas flow path Is indicated by + (corresponding to the configuration shown in Patent Document 6). Although it was slight, low power of ignition was obtained. It can be seen that the ability to optimally design the gas flow path and the floating electrode is important when the plasma source is downsized and made into a microdevice.

FIG. 5 shows ignition power when several floating electrodes and another floating electrode (sub-electrode) are combined using the inductively coupled microplasma source according to the embodiment of the present invention shown in FIG. It is a graph which shows.
The flow rate of helium gas is 0.5 L / min. The floating electrode portion outside the glass thin tube was covered with polyimide tape. In order to confirm the variation width of the experiment, the same experiment was performed three times. (A) is an example of a floating electrode having a triangular shape with a length L = 22 mm and a width w = 9 mm (θ = 23.1 °) as shown in the schematic diagram. The horizontal axis of the graph represents the difference between different floating electrodes (sub-electrodes). “None” indicates that there is no sub-electrode, “Upstream” indicates that a copper wire having a diameter of 0.1 mm and a length of 5 mm is placed in the glass capillary on the upstream side of the flow path, and “Downstream” indicates that the downstream side of the flow path When a copper wire having a diameter of 0.1 mm and a length of 5 mm was placed in a glass capillary, “upstream and downstream” were two copper wires having a diameter of 0.1 mm and a length of 5 mm placed on the glass capillary at the upstream and downstream of the flow path. Represents the case.
The floating electrodes were brought as close as possible to the extent that they did not contact each other.

As shown in the schematic diagram, (b) is an example of a floating electrode having an overall length L = 44 mm, half spread at an angle θ = 23.1 °, and the other half at a width w = 9 mm and a constant pentagonal shape. is there. (C) is an example of a floating electrode having a triangular shape with a length L = 44 mm and a width w = 9 mm (θ = 11.7 °), as shown in the schematic diagram.
There is variation in the data of any floating electrode shape, but there is a tendency that the ignition power decreases by combining another floating electrode (sub-electrode). When the ignition power is small, it is observed that the input power is increased and then brighter visually. In the case of (b), the power at which the polyimide tape connected to the glass tube burns and smoke starts to be emitted is 90 W in the case of “None” and 50 W in the case of “Upstream”. This suggests that the input power is efficiently transmitted to the plasma.

Many types of microplasma sources have been reported where some or all of the plasma region is below cm. When the pressure is increased, it is advantageous for plasma generation even in a small size. A high-density plasma can be formed according to the pressure. In particular, atmospheric pressure plasma eliminates the need for a vacuum chamber, which is essential for low-pressure plasma, so that the apparatus is simple and inexpensive, and the process time can be shortened because evacuation is not required. The light source application is often a point plasma, and can be used for a spectroscopic system. If the plasma source is arrayed, it can be formed in a linear or planar shape. Combination with a scanning mechanism of a plasma source can also be applied to surface treatment, doping, film deposition, etching, and the like.
Although there are many active species, a non-equilibrium plasma in which the gas temperature is not high can be obtained. Therefore, high-density plasma treatment can be performed on various materials such as plastic and film materials. Among various plasma generation methods, inductively coupled plasma is advantageous for high density. From the experimental results, if plasma ignition can be started at low power, the power until the plasma region spreads over a certain range or reaches a certain brightness can be reduced. Low cost, suitable for energy-saving manufacturing. A good plasma source will increase applicability.

1 is a schematic view of an example of an inductively coupled microplasma source characterized in that a part of a floating electrode faces the inside of a gas flow path according to an embodiment of the present invention. FIG. 6 is a schematic view of another example of an inductively coupled microplasma source characterized in that a part of the floating electrode faces the inside of the gas flow path according to the embodiment of the present invention. The function of the floating electrode is represented by an equivalent circuit model. 2 is a graph showing ignition power when several floating electrodes are used using the inductively coupled microplasma source according to the embodiment of the present invention shown in FIG. 1. FIG. 3 is a graph showing ignition power when combining several floating electrodes and another floating electrode (sub-electrode) using the inductively coupled microplasma source according to the embodiment of the present invention shown in FIG. 1. is there.

DESCRIPTION OF SYMBOLS 1 ... Plasma region 2 ... Floating electrode 3 ... Another floating electrode (sub electrode)
4 ... Material with high dielectric constant close to floating electrode 5 ... Spiral coil 6 for inductively coupled plasma generation ... Flow path (formed by glass tube, trench structure manufactured by microfabrication)
7 ... Gas flow 8 ... High frequency power supply

Claims (4)

In the microplasma source that is turned on when a high-frequency current flows through the coil,
An inductively coupled microplasma source characterized in that plasma is generated inside a gas channel by disposing a floating electrode so that a part thereof faces the inside of the gas channel. The inductively coupled microplasma source according to claim 1,
An inductively coupled microplasma characterized in that a part of the floating electrode faces the inside of the gas channel and the other part is close to a material having a dielectric constant higher than that of the discharge gas outside the gas channel. source. The inductively coupled microplasma source according to claim 1,
A part of the floating electrode faces the inside of the gas flow path, and another one or more floating electrodes face the gas with a gap in the gas flow path from a position facing the gas of the floating electrode. An inductively coupled microplasma source characterized by comprising: An apparatus for use in any one of a light source, surface treatment, doping, film deposition, or etching using the microplasma source according to any one of claims 1 to 3.

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