WO1995005060A1 - A radiofrequency gas discharge - Google Patents
A radiofrequency gas discharge Download PDFInfo
- Publication number
- WO1995005060A1 WO1995005060A1 PCT/GB1994/001722 GB9401722W WO9505060A1 WO 1995005060 A1 WO1995005060 A1 WO 1995005060A1 GB 9401722 W GB9401722 W GB 9401722W WO 9505060 A1 WO9505060 A1 WO 9505060A1
- Authority
- WO
- WIPO (PCT)
- Prior art keywords
- radiofrequency
- electrodes
- discharge
- gas
- power
- Prior art date
Links
Classifications
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
- H01J37/00—Discharge tubes with provision for introducing objects or material to be exposed to the discharge, e.g. for the purpose of examination or processing thereof
- H01J37/32—Gas-filled discharge tubes
- H01J37/32009—Arrangements for generation of plasma specially adapted for examination or treatment of objects, e.g. plasma sources
- H01J37/32082—Radio frequency generated discharge
- H01J37/32174—Circuits specially adapted for controlling the RF discharge
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
- H01J37/00—Discharge tubes with provision for introducing objects or material to be exposed to the discharge, e.g. for the purpose of examination or processing thereof
- H01J37/32—Gas-filled discharge tubes
- H01J37/32009—Arrangements for generation of plasma specially adapted for examination or treatment of objects, e.g. plasma sources
- H01J37/32082—Radio frequency generated discharge
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/02—Constructional details
- H01S3/03—Constructional details of gas laser discharge tubes
- H01S3/0315—Waveguide lasers
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/09—Processes or apparatus for excitation, e.g. pumping
- H01S3/097—Processes or apparatus for excitation, e.g. pumping by gas discharge of a gas laser
- H01S3/0975—Processes or apparatus for excitation, e.g. pumping by gas discharge of a gas laser using inductive or capacitive excitation
-
- H—ELECTRICITY
- H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
- H05H—PLASMA TECHNIQUE; PRODUCTION OF ACCELERATED ELECTRICALLY-CHARGED PARTICLES OR OF NEUTRONS; PRODUCTION OR ACCELERATION OF NEUTRAL MOLECULAR OR ATOMIC BEAMS
- H05H1/00—Generating plasma; Handling plasma
- H05H1/24—Generating plasma
- H05H1/46—Generating plasma using applied electromagnetic fields, e.g. high frequency or microwave energy
Definitions
- the present invention relates to transverse radio- frequency gas discharges, and particularly those involving parallel or concentric electrodes of large area, such discharges may be used, for example, for the excitation of high power gas lasers, or for applications in plasma processing of materials, where the spatial uniformity of a large area high power density discharge is important.
- the present invention relates to a power distribution network which facilitates the uniform excitation of a radiofrequency discharge of large electrode area.
- the invention may be applied to a number of applications of large area RF discharges
- the background and need for the invention is described in terms of the requirements for uniform excitation of large area slab lasers, and particularly slab waveguide lasers employing one of the usual gas mixtures appropriate for operation of the carbon dioxide laser.
- gas lasers such as the carbon dioxide laser
- transverse discharges provide the excitation of the laser gain medium.
- a transverse electromagnetic field in the radiofrequency regime (approximately lMHz to 1 GHz) is applied between parallel or concentric metal, or metal- clad electrodes, whose separation may be such as to permit either optical waveguiding between the said electrodes, or alternatively, the propagation along the optical axis of a free space Gaussian laser beam which exhibits negligible interaction with the said electrodes.
- the said longitudinal voltage variations give ruse to spatial non-uniformities in the discharge which are often una ⁇ ceptably high and may even cause severe discharge instabilities.
- the said patent asserts the necessity of multiple RF discharge feed points or of inductive termination to reduce the scale of the deleterious discharge non-uniformities along the length of the electrode structure.
- the avoidance of the alpha-to-gamma transition is a matter of the highest importance in the design of transverse RF excited gas lasers.
- the issues involved in the choice of the frequency of the RF generator If it were the case that very low values of drive frequency, say a few tens of MHz, could be chosen without prejudice to the efficient operation of the discharge and the laser, then the deleterious transmission line effects which cause longitudinal variations in the transverse RF discharge voltage, as discussed above, could be significantly reduced.
- the freedom to choose low RF frequencies is reduced, particularly for lasers without dielectric ballast for two important reasons.
- This deficiency may be particularly acute and damaging when applied to discharges whose geometry is planar in cross section, forming for example a rectangular slab of discharge, which may be used for example as a laser gain medium.
- This situation is in contrast to the case of square or circular cross section discharges (of smaller electrode area) which are characteristic of the devices whose transverse RF discharge voltage spatial uniformity has been the subject of the prior art inventions described above.
- the resultant voltage non-uniformity may lead to the onset of alpha-to-gamma transitions, or to the non-uniform deposition of electrical energy in the discharge, so producing, at the very lease, inefficient laser excitation and for severe non-uniformities, localised gas heating and the generation of RF arcs.
- the said transverse voltage uniformity is such as to prevent the onset of any of the above-mentioned discharge instabilities, and more- over may be translated into a corresponding uniformity of the electrical power deposited in the discharge.
- Figure 1(a) shows a large area discharge structure in the form of a rectangular slab formed by a pair of parallel plate electrodes separated by a pair of dielectric spacers;
- Figure 1(b) shows a large area discharge structure in the form of a rectangular slab formed by a pair of parallel plate electrodes with no dielectric sidewall spacers;
- Figure 2 shows a large area high frequency discharge structure connected to a single source of RF power through a power distribution network in accordance with the present invention and a reactive impedance matching network;
- FIG. 3 illustrates an aspect of the operation of the circuit shown in Figure 2;
- Figure 4 shows the normalised deviation of the transverse inter-electrode voltage from the mean value as a function of the distance from the end of the electrode structure, when the distributed parallel resonance technique of the prior art is employed to provide power distribution;
- Figure 5 shows the normalised deviation of the transverse inter-electrode voltage from the mean value as a function of the distance from the end of the electrode structure when a power distribution network in accordance with the present invention is employed;
- Figure 6 shows an alternative representation of the network shown in Figure 2;
- Figure 7(a) shows a power destination network embodying the present invention in which the power distribution network is supplied by a plurality of power amplifiers, each with an individual input;
- Figure 7(b) shows a power distribution network according to a second embodiment of the present invention in which the power distribution network is supplied by a plurality of power amplifiers which are in turn supplied by a second power distribution network similar to that shown in Figure 2 from a single low power source;
- Figure 7(c) shows a power distribution network according to a third embodiment of the present invention in which positive feedback is provided by a reactive network to produce self oscillation at the resonant frequency of the power distribution network;
- Figure 7(c) shows a power distribution network according to a fourth embodiment of the present invention in which self-oscillation is achieved by distributed positive feedback from multiple points along the power distribution network via multiple reactive networks to multiple distributed power sources;
- Figure 8 shows a particular mechanical embodiment of the invention
- Figure 9 shows an alternative particular mechanical embodiment of the invention.
- FIG 1 two arrangements of a pair of electrodes 1.2 for the creation of a transverse radiofrequency discharge gas discharge 3 are shown.
- the electrodes may be provided with a means of temperature control, such as for example as water cooling, and will also normally be inside a gas-sealed enclosure. They may have surfaces prepared for efficient optical wave- guiding.
- the present invention is particularly appropriate for this discharge geometry in which the electrode spacing is small, and the electrode area is large, resulting in a very low electrical impedance.
- planar electrodes as shown in Figure 1 may be commonly used, other electrode surface shapes of matched curvature are equally applicable to the invention.
- Figure 1(a) shows an arrangement where dielectric material pieces 4 are used to set the spacing of the electrodes and define the area of electrode where the discharge 3 may exist.
- the electrodes are supported at the desired spacing by an external means not shown, and the area of the discharge 3 is that of the whole area of the opposing faces of the electrodes 1,2.
- the electrode and dis ⁇ charge arrangement may be of any of the types just described.
- a pair of electrodes 1,2 is used to define a region of high frequency gas discharge 3 in the form of a large area sheet of small thickness.
- the properties of the discharge are such as, by way of example, to excite efficiently a gas laser gain medium, or in a second type of example to allow plasma processing of suitable surfaces.
- a second pair of metal plates 5,6 is separated by a sheet of insulating or dielectric material 7 so as to form a low impedance electrical transmission line which is situated in close proximity to the said discharge electrodes 1,2.
- One of the electrodes 2 is connected to one of the transmission line plates 5 by a plurality of metal rods or a continuous metal plate or other means, so as to form a low inductance electrical path, as illustrated by lines 101, between the electrode 2 and the transmission line plate 5.
- the opposing electrode 1 and transmission line plate 6 are connected electrically by a plurality of coils 8.
- the inductances of the said coils 8 are chosen in conjunction with the capacitance of the electrodes 1,2 in the presence of a gas discharge, the capacitance of the transmission line and stray inductances so as to create an electrical resonance at or near the desired operating frequency.
- the network enclosed as 11 in Figure 2 will be referred to as a power distribution network. Electrical power is introduced into this network 11 from a high frequency power generator 9 through a reactive impedance matching network 10 connected electrically to the centre of the transmission line plate 6 and to the centre of the electrode 1.
- the reactive impedance matching network 10 is well known in the prior art.
- the high frequency power generator 9 may be of the oscillator-amplifier type or of the power self-oscillator type, both well known in the prior art, or of any other general type.
- the purpose of the structure depicted in Figure 2 is to ensure the uniform distribution of electrical power to all points of the discharge electrode by creating a highly uniform value for the magnitude of the high frequency voltage between all matching regions of the electrodes 1,2 referred to hereafter as the inter-electrode voltage.
- the operation of the network 11 in Figure 2 involves two aspects associated with the metal plates 5,6 and dielectric 7.
- the first aspect is illustrated in
- FIG. 3 which shows one section of the full network of Figure 2.
- This circuit consists of one of the coils 8, the capacitance and resistance of a section of the discharge region 3 between electrodes 1,2, the capaci- tance of a section of the transmission line formed by the metal plates 5,6 and dielectric material 7, and the stray inductance of the connections between the capaci ⁇ tances of these sections are a fraction 1/n of the total capacitance of the inter-electrode region 3 and the transmission line 5,6,7 where n is the number of coils 8 used in the network.
- the circuit in Figure 3 operates as a series resonant circuit with resonant frequency close to the frequency of the electrical power generator 9.
- the presence of the capacitance introduced by the transmission line acts to increase the Q or quality factor of this section of the full network when the impedance present between the electrodes 1,2 is the very low value encountered in discharges between large areas of electrode such as used for the excitation of high power slab or coaxial gas lasers.
- the full network 11 in Figure 2 is made up of sections as illustrated in Figure 3 essentially in parallel, except that the high operating frequency in combination with the large physical size requires the use of an electrical transmission line viewpoint for the description of current flow between the sections.
- the trans ⁇ mission line formed by the electrodes 1,2 and their discharge region 3 is effectively connected in parallel with the second transmission line formed by metal plates 5,6 and dielectric material 7, for the transmission of high frequency current between the resonant sections of the type in Figure 3.
- the two transmission lines form a composite line of very low characteristic impedance which assists the attainment of a very uniform inter-electrode voltage as power is distributed from the electrical input at the centre of the network 11 in Figure 2 to the large area of dis ⁇ charge 3.
- the dielectric constant of the dielectric material 7 should be a low value amongst the range available for high frequency insulating materials.
- FIG. 4 shows the voltage distribution along the electrode expressed as a percentage of the average inter-electrode voltage for the aforesaid planar electrodes when the prior art method of parallel resonance is used.
- the voltage variations are large and cannot be reduced further from the values of ⁇ 12% to -8% shown in Figure 4 by an variations of the number of coils or their inductance value. Because of the highly non-linear properties of gas discharges, such voltage variations would be sufficient to give rise both to regions where the discharge is extinguished and other regions where it operates at too high a power density, with possible severely deleterious consequences.
- the electrically resonant circuit depicted in Figure 2 involves the input of power at or near the central point through connections to the transmission line 6 and the electrode 1.
- This type of configuration is particularly suited to the use of a remote power generator 9 connected to the network in Figure 2 by the usual 50 ⁇ coaxial cable, in that it presents an impedance to the matching network 10 which is much larger than the discharge impedance and which is of similar magnitude to that of the connecting coaxial l cable.
- This provides a simplification in the design of the matching network 10.
- Connection of the power source may also be at other than a central point, for example at one or other end of the power distributing network 11, with some small reduction in the degree of inter-electrode voltage uniformity.
- FIG. 2 The basic network of Figure 2 can be used with other power input configurations which can be understood with reference to Figure 6.
- This is a simplified electrical circuit diagram which shows all the coils 8 in parallel as a single coil 17, the total capacitance of the electrodes 1,2 and discharge region 3 as single capaci- tance 16 and the total capacitance of the transmission line made up of 5,6 and 7 as a single capacitor 15.
- Figure 6 shows that there are 3 possible configurations for the supply of high frequency power to the network. Connection of the power source between circuit nodes 12 and 14 corresponds to that in Figure 2 and its advant ⁇ ages have been discussed above. Connection between nodes 12 and 13 produces a configuration of intermedi ⁇ ate impedance which can make use of high values of stray inductance associated with vacuum feedthroughs as a significant fraction or all of the inductance 17.
- circuit nodes 13 and 14 Connection between circuit nodes 13 and 14 produces a low impedance which is suited to the use of low voltage power sources based on semiconductors.
- inventions use multiple sources of high frequency power.
- sources may be connected in one of the three configurations just described in association with Figure 6, and with the multiple power sources connected at a range of points evenly spaced along the power distributing network 11 as in Figure 7(a), where the power sources are configured as amplifiers with individual inputs 19.
- Each power source 18 consists of an active device or devices in association with reactive components and DC power supply so as to form an effective high frequency power amplifier, in accordance with prior art.
- the distributed power sources 18 may be connected between nodes 12 and 14 or between nodes 12 and 18 for best effect.
- the distributed power sources may be connected between nodes 14 and 18 for best effect.
- an external means may be provided to adjust the phase of each signal applied to the inputs 19 to match the requirement of the power distribution network 11.
- a specific embodiment of the invention occurs when the multiple power sources 18 are assembled in close association with the power distribution network 11, and no coaxial cable or other transmission line method is used between the power sources 18 and the distribution network.
- This integrated power source is most appropriate when a large number of transistors are the active devices but many also be appropriate for a smaller number of moderate power rating vacuum tube devices.
- This embodiment limits high current signals to the immediate surroundings of the gas laser device and avoids the need for high power, high frequency coaxial cables. Advantages are gained from the reduced risk of electromagnetic interference with other equip- ment and improved electrical reliability and safety.
- Such integrated power sources may be operated either in an amplifier mode or as a self-excited power oscillator.
- a second power distribution network 20 similar to that in Figure 2 can be configured to drive the inputs of the power sources 18 in parallel from a single source connected at 21, as in Figure 7(b).
- the input impedances of the power sources 18 act as a distributed load on the power distribution network 20 in the same manner as the gas discharge 3 loads the network 11 in Figure 2.
- the network 20 ensures uniform power delivery to all active devices, and can be arranged to provide the correct relative phase of input signal to all active devices.
- the electrodes 1,2 and associated power distribution network components 5,6,7,8 are assembled within a vacuum envelope which is arranged to contain the low pressure gas required by the discharge and separate it from the atmosphere.
- This embodiment has the advantage of requiring only one electrical feedthrough in the vacuum envelope, but requires great care to avoid unwanted gas discharges within or between components. It is appropriate for the simplest configurations of the invention such as that in Figure 2.
- FIGs 8 and 9 show cross-sections of particular mechanical embodiments of the invention for the excitation of a planar discharge region of the type in Figure 1(a). These embodiments place the power distri ⁇ bution components outside the vacuum envelope and are appropriate for all the electrical configurations previously described.
- the electrodes 1,2 are mounted within a metallic vacuum enclosure 26 which acts to separate the low pressure gas fill from the atmosphere. Electrical connections to the electrode are made by a row of feedthroughs composed of metal rods 24 surrounded by insulating material 27. Electrical connections to electrode 1 are made by the internal connecting plates 25 to the vacuum enclosure 26.
- the electrical transmission line made up of plates 5,6 and dielectric 7 is mounted on the rods 24 with the upper plate 6 connected to the outer surface of the metallic vacuum enclosure 26 by the multiplicity of coils 8.
- an alternative arrangement which achieves the same electrical configuration, has two rows of electrical feedthroughs made up of rods 24 and insulators 27.
- a large area radiofrequency discharge or a plurality of such discharges, excited in a uniform fashion according to the present invention is used as a technique for energising a high power carbon dioxide laser.
- a large area radiofrequency discharge or a plurality of such discharges, excited according to the present invention is used to excite a carbon monoxide xenon or other gas laser type which is suitable for excitation by the well known radiofrequency discharge technique.
- a large area radiofrequency discharge, established in a suitable gaseous medium in a uniform fashion according to the present invention may be used to provide a uniform high power source for plasma processing of a suitably constituted material or surface.
Abstract
Description
Claims
Priority Applications (1)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
EP94923013A EP0664072A1 (en) | 1993-08-05 | 1994-08-05 | A radiofrequency gas discharge |
Applications Claiming Priority (2)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
GB9316282.4 | 1993-08-05 | ||
GB939316282A GB9316282D0 (en) | 1993-08-05 | 1993-08-05 | Uniformly excited radiofrequency gas discharge of large electrode area |
Publications (1)
Publication Number | Publication Date |
---|---|
WO1995005060A1 true WO1995005060A1 (en) | 1995-02-16 |
Family
ID=10740044
Family Applications (1)
Application Number | Title | Priority Date | Filing Date |
---|---|---|---|
PCT/GB1994/001722 WO1995005060A1 (en) | 1993-08-05 | 1994-08-05 | A radiofrequency gas discharge |
Country Status (3)
Country | Link |
---|---|
EP (1) | EP0664072A1 (en) |
GB (1) | GB9316282D0 (en) |
WO (1) | WO1995005060A1 (en) |
Cited By (3)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
WO1997018608A1 (en) * | 1995-11-14 | 1997-05-22 | Synrad, Inc. | Rf-excited gas laser system |
WO2000003415A1 (en) * | 1998-07-13 | 2000-01-20 | Applied Komatsu Technology, Inc. | Rf matching network with distributed outputs |
EP1217700A2 (en) * | 2000-12-07 | 2002-06-26 | John Tulip | Large area laser |
Citations (3)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US4443877A (en) * | 1982-02-01 | 1984-04-17 | United Technologies Corporation | Uniformly excited RF waveguide laser |
US4787090A (en) * | 1988-03-28 | 1988-11-22 | United Technologies Corporation | Compact distributed inductance RF-excited waveguide gas laser arrangement |
EP0486152A2 (en) * | 1990-10-12 | 1992-05-20 | Coherent, Inc. | Gas slab laser |
-
1993
- 1993-08-05 GB GB939316282A patent/GB9316282D0/en active Pending
-
1994
- 1994-08-05 EP EP94923013A patent/EP0664072A1/en not_active Ceased
- 1994-08-05 WO PCT/GB1994/001722 patent/WO1995005060A1/en not_active Application Discontinuation
Patent Citations (3)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US4443877A (en) * | 1982-02-01 | 1984-04-17 | United Technologies Corporation | Uniformly excited RF waveguide laser |
US4787090A (en) * | 1988-03-28 | 1988-11-22 | United Technologies Corporation | Compact distributed inductance RF-excited waveguide gas laser arrangement |
EP0486152A2 (en) * | 1990-10-12 | 1992-05-20 | Coherent, Inc. | Gas slab laser |
Cited By (7)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
WO1997018608A1 (en) * | 1995-11-14 | 1997-05-22 | Synrad, Inc. | Rf-excited gas laser system |
WO2000003415A1 (en) * | 1998-07-13 | 2000-01-20 | Applied Komatsu Technology, Inc. | Rf matching network with distributed outputs |
US6359250B1 (en) | 1998-07-13 | 2002-03-19 | Applied Komatsu Technology, Inc. | RF matching network with distributed outputs |
US6552297B2 (en) | 1998-07-13 | 2003-04-22 | Applied Komatsu Technology, Inc. | RF matching network with distributed outputs |
EP1217700A2 (en) * | 2000-12-07 | 2002-06-26 | John Tulip | Large area laser |
EP1217700A3 (en) * | 2000-12-07 | 2002-12-11 | John Tulip | Large area laser |
US6704333B2 (en) | 2000-12-07 | 2004-03-09 | John Tulip | Large area laser |
Also Published As
Publication number | Publication date |
---|---|
GB9316282D0 (en) | 1993-09-22 |
EP0664072A1 (en) | 1995-07-26 |
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