WO2004032176A1

WO2004032176A1 - Nanoporous dielectrics for plasma generator

Info

Publication number: WO2004032176A1
Application number: PCT/KR2003/002023
Authority: WO
Inventors: Kyu-Wang Lee; Sung-Jin Park; Jin-Hoon Cho
Original assignee: Kyu-Wang Lee; Sung-Jin Park; Jin-Hoon Cho
Priority date: 2002-10-04
Filing date: 2003-10-01
Publication date: 2004-04-15
Also published as: KR20040031146A; KR100530765B1; AU2003265112A1

Abstract

The present invention provides a plasma electrode with nanoporous dielectrics of their pore size of few nm to few um. According to an embodiment of the present invention, regular or irregular nanoporous dielectrics are formed on the electrodes and arranged symmetrically or asymmetrically to produce a glow discharge with various modes of volume, surface and coplanar discharge. Simple modification of plasma parameters and consequently the improvement of stability and efficiency of plasma can be achieved by changing the thickness, homogeneity, pore diameter and porosity of the nanoporous dielectrics and by impregnation or forming the plasma reforming materials such as conductor, semiconductor or secondary electron emissive materials inside of the pore or surface of the dielectrics.

Description

NANOPOROUS DIELECTRICS FOR PLASMA GENERATOR

Background of the Invention

Technical Field The present invention relates to a method for generating a highly efficient and stable plasma and a device thereof using nanoporous dielectrics. The present invention is suitable for wide range of applications of plasma induced physical and chemical processes such as lamp, surface treatment, thin film deposition, etching, cleaning and sterilization etc.

Background Art

Plasma is a collection of electrons, ions and excited neutral species.

Especially, unlike an arc discharge plasma, a glow discharge plasma is non-thermal and highly reactive while gas remains ambient temperature and finds its many applications in lamp, surface treatment, thin film deposition, etching, cleaning and sterilization etc.

A number of techniques have been developed to obtain efficient, stable, high pressure and large volume glow discharge mainly by suppressing arc transition.

Notably, pin or brush type electrodes (Akishev et al: J. Phys. D., 1630, 1993), insertion of dielectric barrier between electrodes (Kogoma et al: J. Phys. D., 1125,

1990), capillary electrode (Kunhardt, IEEE Trans. Plasma Sc, 189, 2000), triode structure (Penetrante et al: J. Appl. Phys., 1332, 1986), micro structured electrode

( Gerike et al; Vacuum 291, 2002), and the use of highly resistive electrode (Alexeff,

US patent 6232723) have been suggested. But, they accomplished a limited success in efficiency and stability wise. It is, therefore, an object ofthe present invention to provide a highly efficient and stable glow discharge plasma electrode with modified dielectrics that substantially obviate one or more problems ofthe related art.

Another object of the present invention is to provide a plasma electrode device that produces a stable glow discharge in various gases up to atmospheric pressure. The distribution and dimension of pores and the thickness of barrier layer etc. affect the density and energy of electrons as well as current density and this results in the prevention of arc and further controls the plasma density. Further, cathode fall is minimized and settled during glow discharge and this prevents transition to arc. Avalanche like propagation of electrons in nano channel increases the electron density significantly and the sharp edge of the nano structures generates very high electric fields. In addition, inclusion of conductor, semiconductor and secondary electron emitter inside of pores or pore peripherals enhance the above effects and lowering the breakdown voltage and increasing the plasma efficiency. Further, low work function materials enhance the electron emission efficiency.

Still another object ofthe present invention is to provide diverse devices for various applications by simple modification of electrodes. Wider range of dielectric property and capacitance can be varied via modification of dielectric structure, compared to the conventional bulk dielectrics. This enables to generate the low to high density, the low to high pressure and the small and large volume plasma.

Disclosure ofthe Invention

Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. The dielectrics of the present invention has regular or irregular array of pores with diameter (D) of few nm to um. Hereinafter, this structure will be referred to as a 'nanoporous structure', and a dielectric including such structure will be referred to as a 'nanoporous dielectric'.

The dielectrics of the present invention contains regular or irregular array of pores with diameter (D) of few nm to um and their pore distribution, pore length (L), pore diameter (D) and inter pore distance (d) can be modified with ease, depending on the dielectric material and processing method. To avoid confusion in wording, thickness and height will be expressed by 'length'.

Since a wet process is typically used for manufacturing an electrode device with a nanoporous structure, every specific process can be performed successively.

In addition, using an electrochemical method, it is possible to make a linear adjustment on variables, and to easily modify the shape, size or pattern of the nanoporous dielectric without much restriction.

Brief Description of the Drawings

FIG. 1 is a schematic view of a basic structure of an electrode with regular shape nanoporous dielectrics.

FIGS. 2A to 2C are cross-sectional views of the structural variation of an electrode with regular shape nanoporous dielectrics. FIG. 3 is a cross-sectional view of a basic structure of an electrode with irregular shape nanoporous dielectrics.

FIGS. 4A and 4B are the cross-sectional views of the structural variation of an electrode with irregular shape nanoporous dielectrics.

FIGS. 5A to 5D are schematic views of a diode type plasma device with one or more electrodes with nanoporous dielectrics. FIG. 6 is a schematic view of a triode type plasma device with one or more electrode with nanoporous dielectrics.

FIG. 7 is a graph showing electrical properties for the FIG 5A type plasma device with porous anodic alumina dielectric (device A) and with porous alumina dielectric (device B).

FIG. 8 is a photograph illustrating a homogeneous glow discharge plasma in the FIG 5A type plasma device.

FIGS. 9 A and 9B are photographs illustrating a homogeneous glow discharge plasma in the FIG 6 type plasma device with different power supplying scheme.

****** DESCRIPTION OF SYMBOLS IN THE DRAWING******

10: conducting electrode 20, 20, 50: dielectrics

22: barrier layer 24, 24, 54: pore peripherals 26, 26, 66, 66, 56: nano pore 27,28: plasma reforming materials

60: nano porous dielectric 67:insulating layer

58: metal layer 70, 90: electrode 80, 82, 84: power supply

Best Mode for Carrying Out the Invention

FIG. 1 is a schematic view of a basic structure of the electrode with regular shape nanoporous dielectrics, according to a first embodiment of the present invention. As shown in FIG.l nanoporous dielectric 20 (hereinafter, it is referred to simply as 'dielectric') is formed on the conducting electrode 10. The dielectric 20 is composed of the porous area with pore 26 (hereinafter, it is referred to simply as 'pore') and barrier layer 22 without pore having a designated thickness (t2). Meanwhile, nano peripheral pores 24 indicate a peripheral portion of pores.

The cross sectional shape of the pore 26 can be circular or other shape. The ratio of the pore diameter (D) to the length (L) as well as the ratio of the pore length (L) to the thickness of dielectric (tl) affect the effects from nanoporous dielectric and plasma characteristics significantly and can be optimized with respect to the specific plasma application. In addition, the variation ofthe thickness (t2) ofthe barrier layer 22 controls the electron flow from conducting electrode 10 and the capacitance of dielectric 20 and consequently controls the plasma characteristics. The shape of the pore also can be modified to vary the electrical property of dielectric 20. For example, whole or certain area ofthe pore mouth can be closed to make a nano chamber.

More specifically, a diameter of the pore (D) can be in the range of few nm to few um and the length (L) is in the range of few tens of nm to few hundreds of um. A inter pore distance (d) is in the range of few nm and few tens of um. A porous anodic alumina is an example of electrode described in FIG 1. Its electrochemical preparation method, forming mechanism and modification of the structure are fairly well known (Yakoleva et al, Inorg. Mat., 34,711, 1998, Jessensky et al. Appl. Phys. Lett., 72, 1773, 1998).

A nano structure array was formed on a cleaned and electropolished aluminum substrate by anodization in acidic solution or by specifically shaped nano patterning such as photolithography followed by the removal of patterning layer. This seeded substrate was further anodized to a desirable thickness of dielectric. Pore diameter, pore density and barrier layer thickness were controlled by the applied voltage and the type of electrolyte. Further pore widening can also be achieved by dissolution of anodic alumina. Since anodic alumina grows on the aluminum conducting electrode, the adhesion between the conducting electrode and the dielectric is very strong and results in a very durable and efficient electrode.

Nano porous silica is another example which can be prepared by lithography and patterning. (Tonucci et al. Science, 258, 783, 1992) The conducting electrode can be formed on a nano porous silica dielectric.

FIGS. 2 A to 2C illustrate cross-sectional views of the structural variation of the electrode with regular shape nanoporous dielectrics, according to the second embodiment of this invention. As shown in FIG. 2A, barrier layer was selectively removed. Referring to FIG 2B, conductors or semiconductors, such as Ni, Au, Ag, graphite, MnO₂, TiO₂, conducting polymers, or secondary electron emissive materials, such as MgO, BaO, KCl, NaCl, diamond, SnO₂ etc. are embedded in the nano pore. The length (hi) of the embedding layer 27 that is related to the current density through pore can be adjusted in the range of few nm to few tens of um. FIG 2C, conductors or semiconductors, such as Ni, W, Mo, Au, Ag, Pt, Al:Li graphite, MnO₂, TiO₂, or secondary electron emissive materials, such as MgO, BaO, LiF, KCl, NaCl, diamond, SnO₂ etc. are coated on the nano pore peripherals 24. The length (h2) of this coating layer 28 can be adjusted in the range of few nm to few tens of um. Further, the nanoporous dielectrics can be prepared directly from the dielectric composite of conductors, semiconductors and secondary electron emitting materials. An appropriate amount ofthe above materials can be mixed with dielectric materials in preparation of dielectric layer or their alloy can be used in electrochemical preparation of dielectric layer. For example, the anodic anodization of Al-Mg alloy produces a nanoporous composite of alumina and magnesia. The device with this dielectric layer shows excellent plasma characteristics without adhesion problem that is often observed in the devices of the examples of FIGS. 2B and 2C.

The combination of structural variation as illustrated in FIG. 2A, FIG. 2B and FIG. 2C is also possible.

FIG. 3 is a cross-sectional view of a basic structure of the electrode with irregular shape nanoporous dielectrics, according to the third embodiment of this invention. As seen in FIG. 3, the nm to um size dielectric particles such as alumina, silica and polystyrene are deposit on the conducting electrode 10 to form the dielectric layer 50 with pores 54. This dielectric layer can be deposit by various methods of precipitation, electrochemical, plasma jet, sol-gel and electrophoretic deposition etc. The dielectric materials must have suitable dielectric constant, durability and preferably, good adhesion to conducting electrode.

Good secondary electron emitting materials such as Csl and KCl film show a columnar structure with pores, those have been applied to a channel plate (Shikhaliev et al. Nucl. Inst. Met. Phys. Res., 487, 676, 2002), can be used as a dielectric. Since their durability and low or even negative electron affinity, nanoporous diamond and diamond like carbon can be utilized too.

FIG. 4A shows a nanoporous conducting layer 60 while FIG. 4B shows a nanoporous dielectric layer 67 as well as nanoporous conducting electrode layer 60, according to the fourth embodiment of this invention. The diameters (D) of pores 66, 66' are in the range of few nm to few hundreds of nm. The thickness and the ration to the diameter are similar to those illustrated in above embodiments and their effects are also similar. An example of FIG. 4B structure can be obtained by electro photochemical etching of silicon. On top ofthe nano porous conducting electrode 60, a silicon oxide for dielectric layer was formed via thermal or electrochemical oxidation and followed by forming a conducting electrode. This device is similar to the ballistic electron surface-emitting device (BSD) for field emission display (FED). (Ichihara et al. J. Cryst. Growth, 1915, 2002)

In addition, nano fiber bundle of conductors, semiconductors and insulators such as Ag, Au, carbon nanotube, CdS, MgO, TiO₂ etc. can be used as conducting electrodes or dielectrics. In so doing, plasma efficiency can be improved by secondary electron emission or field emission effect from the nanoporous structure or the nano bundle. In this case the adhesion is the most concerning factor. Plasma and heat resistant nano porous polymer, for example, polycarbonate, polystyrene, or polyester, can also be used as a dielectric for cold plasma device.

More embodiments of a plasma discharge device with nanoporous dielectric electrode are discussed below. FIGS. 5 A to 5D illustrate the schematic view of the diode type plasma device with one or more electrode with nano porous dielectrics according to the fifth embodiment of this invention. Referring to FIG. 5 A, a conducting electrode 70 such as metal plate and an electrode with nano porous dielectrics are arranged to form a discharge space, such that the discharging electrodes face each other. In FIG. 5B, both of discharging electrodes are electrodes with nano porous dielectrics. A coplanar discharge and a surface discharge type device structures with electrode with nano porous dielectrics are illustrated in FIG. 5C and FIG. 5D, respectively.

Though the electrode with regular shape nanoporous dielectrics was illustrated in FIGS. 5 A to 5D, other types and their combinations of electrode with nano porous dielectrics suggested in this invention, such as illustrated in FIGS. 2 A to

4, can also be applied.

In these devices, there is no limitation in power supply 80 specification and

DC, pulsed DC, AC, RF and MW power supply can be used. Depending on kind of discharge gases, few tens of voltages to few hundreds of voltages, and hundreds of uA/cm² to tens of mA/cm² of current density can be used. In case of using a high energy plasma, few kV to tens of few kV can be applied.

With these devices, stable glow discharge can be generated at up to atmospheric pressure even in air ambient. The distance between discharge electrodes can be varied from few hundreds of um to few tens of cm, depending on the discharge gas composition and the pressure. There is no limitation in total thickness and area ofthe electrode. Further, dielectric properties of dielectric layer can be tailored precisely and no external ballast is necessary. In some cases, a gas flow system can be installed inside of the electrode to make gas flow in-between nanoporous structures.

FIG. 6 illustrates a schematic view ofthe triode type plasma device with one or more electrode with nano porous dielectrics according to the sixth embodiment of the present invention. A conducting or semi conducting layer 28, acting as a gate, is formed on top of nano porous dielectrics. The gate voltage is lower than that of discharge electrodes. If AC was applied between discharge electrodes, DC can be applied to the gate or the gate can be grounded, after shorting the switch 86.

The plasma device illustrated in FIG. 6 can be manufactured by using the electrode of FIG. 4B. In this case, a metallic thin film layer 68 functions as a gate layer. FIG. 7 is a graph showing electrical properties for the FIG. 5A type plasma device with porous anodic alumina dielectric (device A) and with porous alumina dielectric (device B). The 25 mm x 30 mm aluminum electrode and the ITO counter electrode was separated by 1 mm. Discharge gas was 50 Torr of Ne gas. In device A, 17 um thick porous anodic alumina with pore diameter of 50 nm was grew on the aluminum electrode, while 20 um thick sintered alumina paste was prepared on the aluminum electrode in device B. Both of the devices A and B show the stable glow discharge up to atmospheric pressure.

The most striking difference between devices A and B is current limiting ability. The regular shape nanoporous dielectric (device A) limits the cuπent more effectively. As seen in FIG. 7 power consumption of device B is 2-3 times higher than that of device A at a same voltage. Since the intensity of light outputs are nearly same for both devices and it indicates the efficiency of device A is 2-3 times higher. Further, dielectric breakdown voltage was much higher in device A and no sign of electrode degradation was observed, after prolonged operation. FIG. 8 is a photograph illustrating homogeneous glow discharge plasma in the FIG 5A type plasma device with porous anodic alumina dielectric. (Device A) The turn on voltage at 100 Torr neon gas was about 130V (AC, 2KHz). At atmospheric pressure turn on voltage rise about 160-190 V and its breakdown field is 800-900 V/cm, which is much lower than previously reported value of 2-3 KV/cm. FIGS. 9 A and 9B are photographs illustrating homogeneous glow discharge plasma in the FIG 6 type plasma device with different power supplying scheme. On top of the 20 um thick porous anodic alumina dielectric, 50 nm thick aluminum was evaporated as a gate. When gate was grounded, much stronger light intensity about

10 - 20 times was observed, as seen in FIG. 9B, while current increase was about 50 - 200%. This may indicate that strong local electric field is induced by gate. It will be apparent to those skilled in the art that various modifications and variation can be made in the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope ofthe appended claims and their equivalents.

Industrial Applicability

As discussed above, nanoporous dielectrics for plasma generator of the present invention has the following advantages. According to the present invention, by introducing the nanoporous dielectrics, stable glow discharge can be obtained in various gases up to atmospheric pressure.

The distribution and dimension of pores and the thickness of barrier layer etc. affect the density and energy of electrons as well as current density and this results in a prevention of arc and further the controls the plasma density.

Further, cathode fall is minimized and settled during glow discharge and this prevents transition to arc.

Avalanche like propagation of electrons in nano channel increases the electron density significantly and the sharp edge of the nano structures generate very high electric fields. In addition, inclusion of conductor, semiconductor and secondary electron emitter inside of pores or pore peripherals enhance the above effects and lowering the breakdown voltage and increasing the plasma efficiency. Further, low work function materials enhance the electron emission efficiency.

Wider range of dielectric property and capacitance can be varied via modification of dielectric structure, compared to the conventional bulk dielectrics. The nano dimension of dielectrics surface is homogeneous and this suppresses the formation of filament or local micro plasma those tend to develop arc.

There is no limitation in power supply specification and one of the DC, pulsed DC, AC, RF and MW power supply can be used. Since plasma generation inside of nanopore is practically impossible (i.e.

Paschen's Law) in ordinary operating conditions, damage of nanoporous dielectrics is minimized and plasma operation lifetime is maximized.

Large surface area of nanoporous dielectrics promotes the heat dissipation ability that enhances the plasma efficiency and device lifetime. Since dielectrics are directly grown on the conducting electrode, adhesion, durability and plasma efficiency are maximized while it minimizes the resistive heat.

Also, the process of preparing dielectrics is very simple and large area device can be prepared very easily.

Claims

What Is Claimed Is:

1. A plasma generating device, comprising a conducting electrode with an appropriate size a nanoporous dielectric with the array of pores on the conducting electrode a power supply of one ofthe DC, pulsed DC, AC, RF or MW.

2. The device in accordance with claim 1, wherein the diameter of the pores is in the range of few nm to few um and the thickness ofthe dielectric is in the range of few tens of nm to few hundreds of um.

3. The device in accordance with claim 1, wherein the dielectric is solid.

4. The device in accordance with claim 1, wherein the barrier layer is formed in bottom ofthe nanopores in nanoporous dielectric.

5. The device in accordance with claim 1, wherein the pore mouth is closed.

6. The device in accordance with claim 1, wherein a conductor, a semiconductor and a secondary electron emitting material, or a combination thereof is inserted in the pore array with a designated thickness.

7. The device in accordance with claim 1, wherein a conductor, a semiconductor and a secondary electron emitting material layer, or a combination thereof is formed on in the pore peripherals with a designated thickness.

8. The device in accordance with claim 6 or claim 7, wherein a length of the layer is in the range of few nm to few tens of um.

9. The device in accordance with claim 1, wherein the dielectric is dielectric material composite that contains one or their combination of conductor, semiconductor and secondary electron emitting material.

10. The device in accordance with claim 1, wherein a plurality of pores with few nm to few um size is arrayed on the conducting electrode.

11. A plasma generating device comprising a conducting electrode with the nano structure of size in the range of few nm to few um; a power supply of one of DC, pulsed DC, AC, RF or MW.