WO2012099548A1

WO2012099548A1 - Device for high-frequency gas plasma excitation

Info

Publication number: WO2012099548A1
Application number: PCT/SI2012/000002
Authority: WO
Inventors: Rok ZAPLOTNIK; Alenka Vesel; Miran Mozetič
Original assignee: Institut ''jožef Stefan''
Priority date: 2011-01-20
Filing date: 2012-01-19
Publication date: 2012-07-26
Also published as: DE112012000015B4; SI23611A; DE112012000015T5

Abstract

The invention relates to a device for excitation of high-frequency gas plasma i. e. optimisation of transfer of electromagnetic power from a radio-frequency generator (8) into a gas plasma. The transfer of power is optimized by using two or more parallel overlapping and offset excitation coils (11, 12) that are serially connected into the assembly consisting of a generator (8), a high-frequency cable (9), a matching network (10) and an excitation coil (11, 12). The measurements on the connections of the excitation coil (11, 12) prove that for equal transfer of power a lower voltage is required on the double excitation coil (11 and 12) consisting of serially connected overlapping excitation coils (11, 12) than on the ordinary excitation coil (11). With the same voltage on the connections of the coil (11, 12), the plasma is also more intensive, if it is generated in two serially connected overlapping excitation coils (11 and 12).

Description

DEVICE FOR HIGH-FREQUENCY GAS PLASMA EXCITATION

The subject of the present invention is a device for excitation of high-frequency gas plasma i.e. optimisation of transfer of electromagnetic power from a radio-frequency generator into the gas plasma. The transfer of power is optimized by using two or more parallel overlapping and offset excitation coils that are serially connected into the assembly comprising a generator, high-frequency cable, matching network and coil. Such manner of connecting coils has several advantages over assemblies, described so far, among which are: power efficiency of the generator is increased, plasma in the discharge vessel is more homogenous, the voltage on the generator needed for generating plasma is lower, and, at the same time, side effects caused by capacitive coupling in the plasma system are reduced.

Technical Problem

Plasma has become the basis of numerous modern technologies in the last decades. Thermally and thermodynamically unbalanced plasmas are known. Thermal plasma, in which the particles of gas are in thermodynamic balance, is used for plasma cutting and welding, for synthesis of ceramics, for decomposition of dangerous chemical waste, for plasma spraying of thick protective coatings on tools and machines, etc. Demands for ecologically clean technologies have led to a development of new processes for treating materials, in which thermodynamically unbalanced plasmas are used. Some examples of use are: vacuum processes of applying thin layers, lighting industry, laser production, microelectronics, macroelectronics, e.g. plasma displays, micro-treatment of silicon, e.g. production of silicon pressure sensors, production of memory elements, etc. Numerous other examples of use can be found in automobile, optical and military industries as well as in biomedicine.

Different types of thermodynamically unbalanced plasmas are used for plasma treating of surfaces of materials and they can be excited with different types of discharge in gases. Transition of gas into plasma or discharge can be achieved by exposing the gas to an electric field. The gas through which the electrical current flows is partly ionised, which means that free electrons and ions are present besides neutral particles. Free electrons are accelerated in the electric field and through collisions with atoms or molecules of gas they cause the atom or molecule to change from the basic thermodinamically balanced state into different excitated states.

Types of discharges are categorised according to the frequencies of electrical field with which plasma is excited: direct current discharge, corona 50-450 kHz, radiofrequency discharge 5- 100 Mhz, microwave isotropic discharge without a magnet 2.45 GHz and ECR discharge with a magnet 2.45 GHz.

With radiofrequency (hereinafter: RF) plasma, mainly two industry prescribed frequencies of 13.56 MHz and 27.12 MHz are used. RF plasmas are divided into capacitively and inductively coupled plasmas according to the method used to create the electric field. Electrodes or a capacitor is used for capacitive coupling for the generation of the electric field and an excitation coil is used for inductive coupling.

For inductively coupled plasma two types of operation are known: E-type and H-type. Low emission of light, low density of electrons and a relatively high temperature of electrons are characteristic for the discharge in the inductively coupled plasma when using lower excitation powers. Since the capacititive component of RF power tranfer into plasma predominates here, this type is called E-type. When the critical value is achieved by increasing RF excitation power, the luminosity of plasma and the density of electrons are instantaneously increased and the temperature of electrons is somewhat decreased. In this type of operation the inductive method of RF power transfer into plasma predominates, therefore it is called H-type. In the H- type, which is relatively easy to achieve in smaller plasma reactors, the plasma is concentrated inside a small volume inside the excitation coil or in the vicinity of the excitation coil. However, the problem of generating equally distributed inductively coupled RF plasma in the H-type in bigger reactors, which are interesting especially for the industry, remains unsolved.

Prior Art

Large plasma reactors for inductively coupled RF plasma are used in the industry during different processes for the treatment of surfaces, described in the following patents: WO2004098259A3, SI1828434T1, EP1828434A1, US200914621A1, US2010024845A1, etc. Most of these processes require plasma with the largest density of ions or neutral atoms possible. If we want to achieve a large denstiy of neutral atoms or ions at a given pressure, the plasma should be excitated with the highest possible power and, as disclosed in patent No. SI21903A, the plasma reactor should be built from a material with low recombination coefficients that ensure a high dissociation of gas.

For the maximum transfer of power from the RP generator into plasma, matching networks are used. Matching networks consist of passive elements, connected serially or parallely: capacitors and coils. These can be found in several patents, e.g. for capacitively coupled RF plasma reactors: EP1812949A2, US5815047A, and for inductively coupled reactors: US2002130110A1 , US5689215 A, etc.

Besides the matching network, the form of excitation coil is important for the inductively coupled RP plasma for the transfer of power to plasma and even more for the homogeneity of the plasma. There exist several patents relating to the form of excitation plasma. The patents US5578165A and US2002096999A1 e.g. present planar excitation coils and methods for achieving more uniform plasma density on the planar axis.

LILAC excitation coil disclosed in patent US6184488B1 is a planar coil for generating big surfaces of plasma. The coil has a low inductivity, which reduces the problems with matching impedance and maximum transfer of power.

The capacitive component of transferring RP power into the plasma is also often a problem with the inductively coupled plasma. Although the plasma is generated in the electric field generated with the coil, also a capacititive component of coupling appears besides the inductive component, yet is mostly undesired. There are two possible solutions to this problem: using a Faraday shield that is described for a planar coil in patent US2002023899A1 and for a regular coil in patent WO0049638A1, or using a so-called floating coil. The floating coil mentioned in patent US5683539A reduces the capacitive component of coupling, because it is separated from the high-frequency generator and matching network by a transformer and is therefore on floating potential.

Much research and development has been performed so far on planar inductively coupled reactors, since they are similar in their shape to capacitively coupled reactors, which were developed first. However, many unresearched and unsolved problems like the transfer of maximum power, homogeneity of plasma and decrease in capacitive coupling in large plasma systems, where the excitation coil is coiled around the tube of reactor, remain. Due to a large diameter of the coil the inductivity can soon become very high, which presents a problem for the matching network or for the efficiency of the generator.

Solution to the Technical Problem and Embodiment

The subject of the invention is a device for the optimisation of transfer of electromagnetic power form a radio-frequency generator into a gas plasma. The method is based on a specially designed excitation coil. It consists of two or more serially connected coils. The coils are offset so that the loops of individual coils do not overlap.

The device for excitation of high-frequency gas plasma is described by way of figures representing in:

Fig. 1 Vacuum diagram of the plasma system

Fig. 2 Excitation part of the embodiment

Fig. 3 Double exciataion coil of the embodiment

Fig. 4 Measurements of voltage on the ordinary exciatation coil and the double excitation coil, to which this invention relates, as a function of the power of generator

Fig. 5 Measurements of the intensity of radiated light in the centre of the ordinary excitation coil and the double excitation coil, to which this invention relates, as a function of the voltage on the excitation coil at a pressure of 10 Pa

Fig. 6 Measurements of the intensity of radiated light in the centre of the ordinary excitation coil and the double excitation coil, to which this invention relates, as a function of the voltage on the excitation coil at a pressure of 40 Pa.

The diagram of the vacuum part of the plasma system used in our embodiment is displayed in Figure 1. The vacuum system consists of a vacuum pump 1, a valve 2, an air intake valve 3, a discharge quartz tube 4, an absolute pressure gauge 5, an accurate dosing valve 6 and a gas bottle 7. The vacuum pump 1 and the gas supply system with the valve 6 and the gas bottle 7 ensure a pressure in the discharge tube 4 between 1 Pa and 10⁴ Pa, preferably between 10 Pa and 1000 Pa. The discharge tube 4 is significantly larger than conventional laboratory plasma systems. Its diameter in the embodiment is 200 mm and its length is 2000 mm. The tube is made of quatz, therefore it can withstand higher temperatures caused by recombinations of atoms and neutralisations of charged particles on the walls of the reactor.

The electric or excitation part of the device, shown in Figure 2, consists of a radio-frequency generator 8, a co-axial cable 9, a matching network 10 and an excitation coil 1 1, 12. The radio- frequency generator 8 can operate in the frequency range between 100 kHz and 310 MHz. In the embodiment a 8 kW radio-frequency generator 8 is used and operates at a frequency 27.12 MHz. The matching network 10 is connected via co-axial cable 9 to the radio-frequency generator 8. The matching network 10 consists of two high-frequency, high- voltage, variable vacuum capacitors, the capacity of which is changed by means of servos, controlled by the radio-frequency generator 8. The connection of capacitors can be changed by moving the connection plate.

In the embodiment, the double excitation coil 1 1 and 12, i.e. a central part of the invention, is connected to the matching network 10. The double excitation coil 1 1 and 12 consists of two parallely connected overlapping excitation coils 1 1 and 12. According to the invention at least two excitation coils 1 1 and 12 are used, but more can be used as well. The excitation coils 11 and 12 have all the same diameter and overlap so that they have the same axis and they are in contact only at the beginning and the end, where they are connected to the matching network. The excitation coils 11 and 12 have to be offset along the common axis by 1/N the distance between two consecutive loops, where N equals the number of excitation coils 11, 12. This means that in the case of two excitation coils 11 and 12 the loops of the second excitation coil 12 are coiled in the middle between the loops of the first excitation coil 11 , i. e. the second excitation coil 12 is offset along the common axis by a ½ of the distance between two consecutive loops. If three excitation coils 11, 12 are used, they are offset along the common axis by 1/3 of the distance between two consecutive loops etc. The excitation coils 1 1, 12 must be offset by at least the width of the band of each individual excitation coils 1 1, 12. The excitation coils 11, 12 are all coiled in the same direction, so that the loops of each excitation coil 11, 12 run paralell to the loops of every other excitation coil 11, 12. Consequently, despite the fact that the excitation coils 11, 12 overlap, the loops of each individual excitation coil 11, 12 do not overlap and are not in contact. Since all coils have the same diameter, they are in the same plane. The number of loops in individual coil 11, 12 shall be between 2 and 100. The number of loops on every excitation coil 1 1, 12 can be equal, but other or additional excitation coils 12 can also have one loop less than the first excitation coil 1 1. Even in this case it still holds that the loops of the excitation coils 1 1, 12 do not overlap. The only reason for having one loop less on other or additional excitation coils 12 is that the total length of the double excitation coil 11, 12 does not increase with the number N of excitation coils 11, 12, i. e. the length of the double excitation coil 1 1 and 12 remains the same as the length of the first excitation coil 11 , irrespective of number N.

The double excitation coil 11 and 12 is made of a band with the resistance of maximum 100 Ω for direct current, wherein the width of the band, from which each individual excitation coil 11 , 12 is made, is the same and the width of the band of each individual excitation coil 11, 12 is between 1 mm and 10 cm. The band is coiled around the discharge quartz tube 4 so that it touches the tube with the largest surface possible. The diameter of individual excitation coils 11, 12 that constitute the double excitation coil 1 1 and 12 is thus equal to the outer diameter of the tube, in the embodiment D = 200 mm. The length of the loops of each individual coil 11, 12 can be different from the multiple of a quarter of electromagnetic wavelength originating from the radio-frequency generator 8 by not more than 20 %.

In the embodiment shown in Figure 3, two parallel overlapping excitation coils 11, 12 are used that are made from 25 mm wide 0.4 mm thick copper strip. The copper strip is coiled around the discharge quartz tube 4 so that it touches the tube with the largest surface possible. The first excitation coil 11 has 5 loops and the second overlapping excitation coil 12 has 4 loops. The copper strip loops of the second excitation coil 12 do not overlap and do not touch the loops of the first excitation coil 11. The distance between the edges of copper strips of the first ecitation coil 1 1 and the second excitation coil 12 is approximately 60 mm in the embodiment. The total length of the double excitation coil 11 and 12 of the embodiment is 800 mm.

The measurements made in the oxygen plasma, generated in the ordinary excitation coil 11 with the length of 800 mm and with 5 coils, compared to the measurements made in the plasma, generated with the double excitation plasma 11 and 12, which is the subject of this invention, are shown in Figures 4-6.

Figure 4 shows the measurements of voltage on the connections of the ordinary excitation coil 11 and the double excitation coil 1 1 and 12 that is the subject of this invention in dependence of the power of the radio-frequency generator. The voltage on the double excitation coil 11 and 12 was measured with a high-voltage probe 13 and read by means of an oscilloscope 14.

The graphs show that a higher voltage is required for the transfer of the same power when only the ordinary excitation coil 11 is used. By using the double excitation coil 1 1 and 12 that is the subject of this invention, the voltage is reduced in comparison to the classic coil 11, which is very favourable from the technological viewpoint.

The intensity of radiated light in the middle of the excitation coil 11, 12 in comparison to the voltage on the excitation coil 11 , 12 i. e. the power of the radio-frequency generator tells us that the plasma generated in the double excitation coil 1 1 and 12, which is the subject of the invention, is much more intense than the plasma generated in the regular excitation coil 1 1. Figure 5 presents the results of the measurements of the intensity of oxygen emission lines of 777 nm and 845 nm plasma at a pressure of 10 Pa. The integration time of optical spectrometer was 200 ms. It can be noted that the intensity of the radiated light of plasma generated in the regular excitation coil 11 , is approximately three times lower than the intensity of the radiated light of the plasma generated in the double excitation coil 11 and 12 consisting of two parallel overlapping coils 11, 12.

Figure 6 shows the same results at a pressure of 40 Pa and the integration time of spectrometer of 100 ms. The difference is even more obvious than at a pressure of 10 Pa. The intensity of the light in the double excitation coil 1 1 and 12 consisting of two parallel overlapping coils 1 1, 12, can be up to 4-times higher than the intensity of light in the regular excitation coil 11 at the same voltage.

The device according to the invention for exciting high-frequency gas plasma, i. e. device for transferring electromagnetic power from the radio-frequency generator into the gas plasma with a high-frequency generator 8 connected into the system, consists of a discharge vessel 4, around which a plasma coil 11, 12 is wound, a vacuum pump 1, an accurate dosing valve 6, a gas bottle 7, where high-frequency generator 8, a matching network 10 and a plasma coil 1 1, 12 are serially connected. The plasma coil 1 1, 12 consists of two or more coils connected serially, so that the individual loops of each coil do not overlap and are wound around a common axis, so that the loops of the second coil are arranged between the loops of the first coil. All coils 1 1, 12 are made of a band with the resistance of maximum 100 Ω for direct current, where the width of the band, from which each individual coil 1 1, 12 is made, is the same and the width of the band of each individual coil 1 1 , 12 is between 1 mm and 10 cm. The coils 1 1, 12 are offset toward one another along the common axis by 1/N of the distance between two consecutive loops, where N equals the number of individual coils and at least by the width of the strip of each individual coils 1 1, 12. Each individual coil 1 1, 12 consists of at least 2 and at most 100 loops. The number of loops on all excitation coils 1 1 , 12 is either equal or other or additional excitation coils 12 have one loop less than the first excitation coil 11. The length of the loops of each individual coil 1 1, 12 is different from the multiple of a quarter of electromagnetic wavelength originating from the high-frequency generator 8 by not more than 20 %. The high-frequency generator 8 operates in the frequency range between 100 kHz and 310 MHz. Thevacuum pump 1 and the gas supply system with the valve 6 and the gas bottle 7 ensure a pressure in the discharge vessel between 1 Pa in 10⁴ Pa, preferably between 10 Pa and 1000 Pa.

Claims

1. A device for exciting high-frequency gas plasma, i. e. device for transferring electromagnetic power from a radio-frequency generator into a gas plasma with a high- frequency genrator (8) connected into the system, consists of a discharge vessel (4), around which a plasma coil (1 1, 12) is wound, a vacuum pump (1), an accurate dosing valve (6), a gas bottle (7), wherein the high-frequency generator (8), a matching network (10) and the plasma coil (11, 12) are serially connected, characterized in that the plasma coil (11, 12) consists of two or more coils connected serially, so that the individual loops of each coil do not overlap and are wound around a common axis, so that the loops of the second coil are arranged between the loops of the first coil.

2. The device of claim 1 , characterized in that all coils (11, 12) are made of a strip with the electric resistance of maximum 100 Ω for direct current, wherein the widths of the band, from which each individual coil (1 1, 12) is made, are the same and the width of the band of each individual coil (11 , 12) is between 1 mm and 10 cm.

3. The device of claims 1 and 2, characterized in that the coils (11, 12) are offset toward one another along the common axis by 1 N of the distance between two consecutive loops of each of individual coils (11, 12), where N equals the number of individual coils and at least by the width of the strip of each individual coils (1 1, 12).

4. The device of preceding claims, characterized in that each individual coil (11, 12) consist of at least 2 and at most 100 loops.

5. The device of claim 4, characterized in that the number of loops on all excitation coils (11, 12) is either equal or other or additional excitation coils (12) have one loop less than the first excitation coil (1 1).

6. The device of preceding claims, characterized in that the length of the windings of each individual coil (1 1, 12) is different from the multiple of a quarter of electromagnetic wavelength originating from the high-frequency generator (8) by not more than 20 %.

7. The device of claim 1 , characterized in that the high-frequency generator (8) operates in the frequency range between 100 kHz and 310 MHz.

8. The device of claim 1, characterized in that the vacuum pump (1) and the gas supply system with the valve (6) and the gas bottle (7) ensure a pressure in the discharge vessel (4) between 1 Pa in 10⁴ Pa, preferably between 10 Pa in 1000 Pa.