METHOD OF MONITORING ATMOSPHERIC PRESSURE GLOW DISCHARGE PLASMA

Abstract: The generation of glow plasma due to discharge at an atmospheric pressure and its stabilization are monitored by counting the number of current pulses. Alternatively, they are monitored by the change of a retention voltage and/or the change of the full current value besides the number of the current pulses, or by Lissajous figures, to detect the plasma state and to stabilize and control the plasma. These procedures make it possible to stabilize and monitor the atmospheric pressure glow discharge plasma accurately, rapidly and readily, and to control the plasma.

Description: Technical Field The present invention relates to a method to monitor atmospheric pressure glow discharge plasma. More particularly, the present invention relates to a novel method to monitor atmospheric pressure glow discharge plasma that allows accurate monitoring and control of the generation and stabilization of atmospheric pressure glow discharge plasma which is used in surface treatment of solids, formation of thin films, decomposition of vapor-phase waste gases and reaction of vapor-phase synthesis, and the like. Background Technology It has been actively studied to utilize plasma, which are generated by discharge, in various areas such as the surface treatment of semiconductors, electronics devices and other electronic materials, bio-materials, new functional polymers and other materials, the formation of thin film for developing new materials, the decomposition of waste gas, and the reaction of vaporphase synthesis. As a result, the technology has been established as a practical basic technology in several areas. Of these plasma techniques, the method to utilize glow discharge plasma generated under a so-called high vacuum condition, is an important part of the plasma CVD, ion plating, plasma etching, plasma surface treatment and other techniques. These low pressure glow discharge plasma techniques are recognized as effective in producing surfaces and thin films of relatively good quality. However, because it is necessary to maintain a low pressure condition, these techniques have the drawbacks that a vacuum system must be provided and maintained: it is difficult to sustain discharge; and a huge amount of cost is necessary to keep the system air-tight. This is because, according to the conventional technological knowledge, generation of glow discharge plasma generally requires a low pressure condition of several Torr or below. This is because, with rising pressure, the discharge concentrates on one point with the pressure at about 100 Torr, until the discharge shifts to sparks and arc discharge at about atmospheric pressure, making it impossible to uniformly process the subject to be processed. Now therefore the inventor of the present invention has established a glow discharge plasma technique which can be used under atmospheric pressure to replace conventional low pressure glow discharge plasma techniques, and has actively studied its applications as well. As a result, the inventor of the present invention has found that the present atmospheric pressure glow discharge plasma is capable of performing highly efficient and high quality surface treatment, thin film formation, etc., without using a conventional troublesome vacuum system, and has been studying commercialization of the technique in various fields. Since then, a new development has been made for the technique on the understanding that the technique of the present invention is useful also in the so-called waste gas processing and vapor-phase synthesis reaction. However, to further develop this atmospheric pressure glow discharge plasma technique, a plasma reaction method that is capable of generating and utilizing plasma more accurately and capable of controlling the plasma in more diverse shapes must be developed. To generate and control plasma in this way, the state of the plasma must be monitored in the first place. This monitoring method is also indispensable in the conventional low pressure method, yet in the case of the atmospheric pressure glow discharge plasma, conventionally known silent discharge and arc discharge must be accurately avoided since the so-called atmospheric pressure condition is used. For this reason, the establishment of this monitoring method has been an important task. The present invention was made in the above circumstances and provides a novel method to monitor atmospheric pressure glow discharge plasma accurately, quickly and simply. To solve the above problems, the present invention provides a method of monitoring atmospheric pressure glow discharge plasma contained within a discharge device to which an a.c. voltage is applied, characterised by the steps of detecting the number of electric current pulses in the current flow of the discharge device per one half-cycle of the a.c. voltage applied to the device, and determining that the plasma is in a glow discharge state when the number of pulses in the current flow per one half-cycle is less than or equal to a predetermined maximum number, and determining that the plasma is in a filament discharge state when the number of pulses in the current flow per one half-cycle is greater than the predetermined maximum number. Preferably, the monitoring of the plasma is also based on the change in retained voltage and/or total current, in addition to the number of electric current pulses, and the plasma is stably controlled by detecting the state of the plasma using the said above monitoring methods. Further preferably, the plasma is monitored using a Lissajous figure showing the correlation between voltage and electric quantity, and the invention preferably provides a method to generate and stabilize atmospheric pressure glow discharge plasmas more strictly by monitoring said Lissajous figure and the number of electric current pulses simultaneously. The attached drawings show the following: Figure 1 is a cross-sectional view of an atmospheric pressure glow discharge plasma reactor of an embodiment of the present invention. Figure 2 is a pattern diagram showing changes in voltage and current in silent discharge. Figure 3 is an electric current pulse diagram as observed in the atmospheric pressure glow discharge. Figure 4 is an equivalent circuit diagram. Figure 5 is a circuit diagram for the Lissajous figure. Figure 6 is a Lissajous figure of filament discharge. Figure 7 is a Lissajous figure of the atmospheric pressure glow discharge. Figure 8 is a pattern diagram showing electric current pulses in silent discharge and glow discharge. Figure 9 is a diagram showing the change in retained voltage of silent discharge and glow discharge. Figure 10 is a correlational diagram showing the change in retained voltage in glow discharge. Figure 11 is a correlational diagram showing the change in retained voltage in filament discharge. Figure 12 is a correlational diagram for the current and voltage in the glow and arc discharge. Figure 13 shows the change in the total current over time. The following symbols are used in the drawings: 1, 2 electrode 3, 4 dielectric 5 substrate A silent discharge B glow discharge C arc discharge As described above, the present invention enables, among others, by using the above methods, monitoring of the atmospheric pressure glow discharge plasma which is important in the commercialization of atmospheric pressure glow discharge plasma, constant monitoring of its stability, detection and identification of the presence or absence of a shift to arc or filament discharge (silent discharge) without operator attendance, and the automatic recovery of stable glow. The present invention, in order to achieve the above, is so designed as to fully detect the features of stable glow discharge, arc discharge and filament discharge, respectively, thereby detecting the shift to and/or change between these different discharge types. As the inventor of the present invention has already proposed, atmospheric pressure glow discharge plasma can be generated by a reaction device comprising various structures and is actually used for many purposes. For example, it is possible to process the surface of a substrate 5 or form a thin film thereupon under atmospheric pressure, or trigger vapor-phase reaction such as gas decomposition without using the substrate 5, by using 90% or above rare gas or reactive gas diluted by inert gas, by opposingly arranging, for example, dielectrics 3 and 4 on both or either of upper and lower electrodes 1 and 2, respectively, as shown in Figure 1. In this case, the electrode 1 may be a needle-like metallic electrode, a needle-like electrode covered by a dielectric, or a hollow cylindrical electrode. In the case of atmospheric pressure glow discharge plasma with this type of device and under these conditions, shift to filament (silent) and arc discharge must be monitored. Number of Electric Current Pulses For this reason, the present invention detects and identifies the most probable shift, or a shift to filament (silent) discharge by an number of pulses of electric current. This is because, in the case of glow discharge plasma, the number of electric current pulses is generally unity except when the voltage is extremely high, whereas in the case of filament (silent) discharge, the number of pulses is two or more. For example, Figure 2 shows typical current waveforms in filament discharge (50 Hz) in an oxygen atmosphere, and Figure 3 those of He glow discharge (3 kHz). It is thus possible to avoid filament (silent) discharge and keep stable atmospheric pressure glow discharge by clearly classifying and recognizing these typical examples. Figure 4 shows an example of a circuit for monitoring the number of electric current pulses. The discharge tube is shown by an equivalent circuit in the figure. The following symbols are used in the figure: r: Discharge impedance Cd: Equivalent capacity R : Electric current measuring resistance In the figure, capacity current Ic is the capacity component of discharge current at the electric power frequency which is applied. Ir is the true discharge current. Id is the sum of both components, thus it is necessary to separate them in the measurement. Now therefore, it is possible to separate the discharge current pulses for measurement from the capacity component (low frequency component) by passing said discharge current pulses through a high frequency filter. In addition, the pulse wave height (electric current) of the atmospheric pressure glow discharge is proportional to the discharge current if the number of pulses is 1/half cycle, whereas the number of pulses increases to a maximum of several (2 to 3) pulses/half-cycle as the impressed voltage increases. However, in the case of filament discharge, the number of pulses shows an unlimited number of fine discharge states, so that it is easy to identify filament discharge. It is possible to observe with an oscilloscope using the wave height of electric current pulses and the number of pulses. It is possible also to monitor the discharge current in the equation below, where H is pulse length, and n the number of pulses/half-cycle: (where "a" is a constant determined by resistance R). Ic, Ir and Id are mentioned as electric current in the above description, but it is reasonably understood that they are in fact a quantity of electricity. This is because a total of quantity of electricity flowing in one second is treated as electric current. However, in the description of the present invention, the term electric current is used because a term such as "quantity of electricity of pulses" is not generally used. Retained Voltage/Total Current A shift from a stable glow discharge or a filament discharge to an arc discharge can also be detected by the retained voltage for the purpose of discharge. For example, a shift from a stable glow to an arc discharge and from a filament discharge to an arc discharge can be detected by the change in the level of the retained voltage. This is because the voltage drops sharply when a shift to an arc discharge takes place. This is also reported by A. Bauer, P. Schulz (Z. Phys. vol. 39, p. 1302, 1971). These shifts can also be detected by the change in total electric current. This is because the electric current increases considerably when an arc discharge takes place, and the change due to these shifts have already been observed. For this reason, more comprehensive and accurate monitoring is made possible by combining the retained voltage and electric current with the above detection and identification of the number of pulses. Lissajous Figure As mentioned above, the discharge retained voltage serves as an index in the monitoring of the discharge state. Actually, however, the measurement is not so simple, strictly speaking. This is because in the atmospheric pressure glow discharge system, as shown in Figure 1, there are solid dielectrics between metallic electrodes and it is difficult, strictly speaking, to measure retained voltage of the discharge space and the electric current flowing through the space simply by determining electric current/voltage externally. Now therefore, in order to use the discharge retained voltage as a more accurate index in the monitoring of the atmospheric pressure glow discharge, the present invention provides a method to monitor by looking at a Lissajous figure which displays the relation between voltage and quantity of electricity. Figure 5 shows an example of a measuring circuit for the purpose of Lissajous figures. More particularly, a capacity C is charged with electric current flowing from a high voltage H.V. power supply, and the resultant voltage Y is depicted against the longitudinal axis on an oscilloscope. The high voltage H.V. is divided by a high R 1 and a low R 2 resistance and the voltage X is depicted against the horizontal axis on the oscilloscope. In this way, a Lissajous figure comprising the correlation between the above-mentioned X and Y is derived as an oscillogram. It goes without saying that the values for R 1 , R 2 and C can be appropriately selected according to the size of the atmospheric pressure glow discharge and the level of impressed voltage H.V.. Figure 6 shows a typical Lissajous figure for filament discharge, and Figure 7 that for glow discharge. In these Lissajous figures, for example, capacity current equivalent Ya, space discharge quantity of electricity/half cycle Yb and discharge retained voltage Xa/2 are expressed as shown in Figure 6. More particularly, in these Lissajous figures, discharge retained voltage Xa/2 is accurately determined. Inclination of the Lissajous figure is due to the fact that the discharge tube has a capacity (solid dielectric) component. In the case of filament (silent) discharge, as is clear from the comparison of Figures 6 and 7, many pulses are generated for each 1/2 cycle and due to these multiple pulses, space discharge takes place. For this reason, a continuous or closed parallelogram is formed as shown in Figure 6. In the case of the atmospheric pressure glow discharge, on the other hand, there are only very few pulses, and thus a single pulse electric current is used to draw a figure, with the result that there is a jump between two voltage lines. Although a small difference exists depending on the type of plasma gas, only two apparently parallel lines are thus observed as shown in Figure 7. More particularly, the number (small or large) of the pulses governs the clarity of the lines of the quantity of electricity (or how clearly they appear) in the Lissajous figure. Difference between filament and glow discharge is thus quite clear. The advantage of using a Lissajous figure is that, in addition to clear indication of the difference between the above retained voltage and the state of discharge, it can determine the state of mixture of a filament and a glow discharge (it is difficult, as a matter of fact, to identify simply by monitoring the electric current pulses). More particularly, the pure glow discharge can also be confirmed with this Lissajous figure simultaneously with the measurement of retained voltage. Working Examples (a) Discharge was conducted with a device of the same basic construction as shown in Figure 1 in an argon (Ar) gas atmosphere (atmospheric pressure), under the following conditions: Impression of 3 kHz high voltage Argon (Ar) flow rate 2000 cm 3 /min Filament-like pulse discharge, or so-called silent discharge, occurred. Discharge was conducted again by mixing a trace of acetone (Ac) with argon (Ar). Glow discharge occurred, which was also visually confirmed. The state of electric current pulses of the above silent and glow discharge was compared to find that, as shown in Figure 8, silent discharge (A) has several pulses while the stable glow discharge (B) has only one. The unity of the electric current pulse in glow discharge is also confirmed in the glow discharge conducted in the 3 kHz helium (He) atmosphere (Figure 3), as mentioned above. For this reason, it was possible, for the purpose of stabilization of atmospheric pressure glow discharge plasma, to automatically detect the number of electric current pulses, and control the current, voltage and distance between electrodes by the resultant signals, thereby assuring the continuation of a stable glow discharge. (b) The inventor of the present invention further monitored the difference between silent and glow discharge in the above argon and argon plus acetone atmosphere from the viewpoint of discharge retained voltage. Figure 9 shows the relation between electrode gap (mm) and retained voltage (V). The figure shows the difference between glow discharge plasma B and filament (silent) discharge A. Both are different from an arc in that the voltage is above a few hundred volts. As described above, a more accurate monitor with a better precision was made possible by monitoring the voltage impressed on a specified electrode gap using a specified gas and by identifying the number of electric current pulses. Figure 10 shows the relation between the gap (mm) and the retained voltage for an atmospheric pressure glow discharge plasma in the 2 kHz, He atmosphere. Figure 11 shows the relation between the gap and the retained voltage for a filament discharge using oxygen. In both cases, a high voltage, different from an arc discharge, is shown. Further, as described above, and as shown in Figure 12, the difference between, on the one hand, filament A and glow B discharge and, on the other, arc discharge C, or the features of a shift from filament A and glow B discharge to arc discharge C, are confirmed by the correlation between current A and voltage V. The voltage drops sharply with the shift to the arc discharge. (c) It is also confirmed, by the observation of the total current at discharge, that the change is the same as that shown in Figure 13 which is already reported (Kekez et al, 1970), that is, the current level increases significantly with the shift from a glow B to a filament A and then to an arc C discharge. (d) The pulses indicating generation of a glow discharge were observed in the measurement of the number of electric current pulses. In this measurement, a great number of fine wave heights were observed between pulses. We evaluated them by the Lissajous figure and confirmed that this state represents the mixture of Figures 6 and 7. We then changed the conditions such as discharge voltage, gap between electrodes, rare gas concentration, and so on, with the result that we could confirm generation of a pure glow discharge shown in Figure 7. A great number of fine wave heights were not observed in the electric current pulses. (e) The above indicates that the detection of the number of electric current pulses, retained voltage, total electric current, and Lissajous figure enables accurate identification of a glow, silent and arc discharge. By combining these indices, stabilization control of the atmospheric pressure glow discharge plasma has been further enhanced. Industrial Field of Utilization As described above in detail, the present invention allows accurate, quick and simple monitoring of stabilization of atmospheric pressure glow discharge plasma and the control therefor. This monitoring method is effectively used in the surface treatment of various solids, formation of thin film on the surface of a solid, vapor-phase decomposition reaction, vapor-phase synthesis method, and the like, which use an atmospheric pressure plasma.

Claims: A method of monitoring atmospheric pressure glow discharge plasma contained within a discharge device to which an a.c. voltage is applied, characterised by the steps of detecting the number of electric current pulses in the current flow of the discharge device per one half-cycle of the a.c. voltage applied to the device, and determining that the plasma is in a glow discharge state when the number of pulses in the current flow per one half-cycle is less than or equal to a predetermined maximum number, and determining that the plasma is in a filament discharge state when the number of pulses in the current flow per one half-cycle is greater than the predetermined maximum number. The method of claim 1, wherein said monitoring is also based on a change in the retained voltage and/or the total electric current across the plasma. The method of claim 1 or 2, wherein said monitoring is also based on Lissajous figures indicating the correlation between the voltage and the current flow of the device. The method of claim 3, wherein the plasma is determined to be in a glow discharge state when the Lissajous figure consists of a jump between two voltage lines, and wherein the plasma is determined to be in a filament discharge state when the Lissajous figure is in the form of a closed parallelogram. The method of claim 3, wherein the plasma is determined to be in a glow discharge state when the Lissajous figure consists of a pair of substantially parallel lines, and wherein the plasma is determined to be in a filament dischage state when the Lissajous figure is in the form of a closed parallelogram. The method of any preceding claim, wherein the predetermined maximum number is between one and three inclusive.

Inventors: OGOMA MASUHIRO, OKAZAKI SATIKO, OGOMA, MASUHIRO, OKAZAKI, SATIKO
Applicants: OGOMA MASUHIRO, OKAZAKI SATIKO, OGOMA, MASUHIRO, OKAZAKI, SATIKO
Application year: 1992
Main classification: H05H1
IPC codes: H05H1, C23C16, C23C14