Vacuum Variable Capacitor

Abstract

The present invention relates to a vacuum variable capacitor 1 comprising a vacuum sealed enclosure 2 containing a vacuum dielectric medium, the enclosure 2 comprising a first plate 3 and a second plate 4, said first plate 3 and said second plate 4 being separated by an electrical insulating element 5, the enclosure 2 further comprising a fixed electrode 6 attached to the first plate 3 inside the enclosure 2 and a movable electrode 8 attached to a movable plate 7, the movable plate 7 being attached to the second plate 4 inside the enclosure 2 by at least one vacuum bellows 9, the vacuum capacitor 1 being adapted to vary the capacitance of the vacuum capacitor 1. The vacuum sealed enclosure 2 has a mechanical drive system 10 for displacing, in particular translating, the movable plate 7 relative to the plate 3, the mechanical drive system 10 including a ball screw 11 arranged to drive the movable plate 7, the mechanical drive system 10 including a limiting element 14 outside the vacuum sealed enclosure 2 for limiting a maximum distance between the first plate 3 and the movable plate 7, the drive system including a nut 13 attached to the ball screw 11, the nut 13 including a first shoulder 13a configured to abut against the limiting element 14 for limiting the maximum distance between the first plate 3 and the movable plate 7.

Description

[Technical field]

[0001] The present invention relates to the field of vacuum capacitors, and in particular to the field of vacuum variable capacitors. [Background technology]

[0002] Vacuum capacitors are electrical capacitors suitable for high power applications, typically suitable for voltages of several thousand volts, or even tens of thousands of volts thanks to their excellent vacuum dielectric properties. Vacuum capacitors can pass alternating currents (especially radio frequency currents) of tens of amperes, or even hundreds of amperes thanks to the very low losses of the conductive parts, i.e. the metal electrodes and the metal collars used to connect the vacuum capacitor to other circuit elements.

[0003] Vacuum variable capacitors constitute a subgroup of vacuum capacitors and are furthermore known for their excellent suitability for high power applications. These vacuum variable capacitors have both electrodes located inside the vacuum enclosure of the vacuum capacitor and the capacitance value can be adjusted by adjusting the position of one electrode relative to the other. In fact, it is not the electrode separation that is changed but the overlap of the electrode surfaces by changing the distance between the plates carrying the electrodes. The adjustment range of the capacitance value is called tunability and is given by t=(C max -C min ) / C max where C max and C minare the maximum and minimum capacitance values ​​that can be achieved by a given capacitor. Most vacuum variable capacitors are designed to achieve at least 90% tunability, and some achieve 98% or even higher. Typically, a system of screws and nuts associated with an expandable connecting joint, also called a bellows, can provide actuation from outside the vacuum to the adjustable electrode inside the vacuum enclosure. For example, a stepper motor controlled by control software can be used for actuation to precisely position the movable electrode and thus achieve a specific and precise value of the vacuum variable capacitor's capacitance. The aforementioned elements used to adjust the capacitance value of the vacuum variable capacitor define its mechanical drive system.

[0004] In addition to the high voltage and current capabilities, a particular quality criterion for the subgroup of vacuum variable capacitors is their drive system performance. Vacuum variable capacitors are not only highly tunable, but also advantageously offer a long life of the drive system, which is achieved by the relatively moderate torque adjustable drive system for fast and precise mechanical adjustment to meet the high demands of many applications, and the C until failure. max and C. min It can be defined as the number of cycles between

[0005] A typical application of vacuum capacitors is the power delivery systems used in semiconductor manufacturing using radio frequency (e.g. at 13.56 MHz) powered plasma tools for deposition and etching processes. The original function of the vacuum variable capacitor is to adjust and match the impedance of the application load to the impedance of the radio frequency generator in the power delivery circuit. Since the plasma load has a time-varying impedance (depending on each processing step, ionized gas conditions, residual gas pressure, and other factors), it needs to be matched ("matched") to the impedance of the generator and RF cable, which are optimized by industry standards to function against a 50 ohm impedance. Any impedance mismatch can result in the failure of the semiconductor manufacturing process and in some cases even the destruction of the generator due to reflected power due to the impedance mismatch between the generator and the load.

[0006] Returning now to the original vacuum variable capacitor drive system, as mentioned above, it is desirable to provide a system that can be operated with moderate or low torque. Today, most vacuum variable capacitors would typically require a stepper motor or other actuator providing a torque greater than 0.4 N·m to adjust. The goal of the present invention is to provide a vacuum variable capacitor that can be operated with a reduced torque without compromising on the accuracy and repeatability of the drive system. To achieve this goal, the vacuum variable capacitor must have a minimum capacitance C min Prior art vacuum variable capacitors typically require a mechanical stop at the location of the movable electrode corresponding to C minThe end stops do not provide a means to determine an exact value for C. The only available means is essentially for the bellows to be fully compressed. Because the bellows is not a high precision component, in some cases this end stop may correspond to, say, 32pF, and in other cases 35pF or 27pF, even though the (nominal) same components are used. Because of this problem, data sheets for serially produced vacuum variable capacitors today often specify a smaller capacitance range for higher C min Therefore, it is only because of this uncertainty that tunability is shown to be less. In the example just given, the manufacturer could choose to use the "safe C" characteristic of the bellows as a reference for whatever characteristic is within the tolerances of the component. min " will be achievable, so we choose a "safer" C of, say, 40 pF. min In other words, the value published in the data sheet is actually achievable by all customers for all offered capacitors of that type. This reliability is important, and manufacturers agree to publish tunability characteristics that are inferior to what is required.

[0007] Vacuum variable capacitors including limiting elements for limiting the maximum distance between the movable and fixed plates of the capacitor are known from US Pat. No. 3,040,220A, US Pat. No. 2,964,248B1, US Pat. No. 2002,163,398A1 and US Pat. No. US2010254066A1, however the proposed limiting elements do not provide a satisfactory solution to the above mentioned problems.

[0008] It is therefore an object of the present invention to provide a vacuum variable capacitor whose capacitance can be adjusted by low torque while allowing an accurate definition of its minimum capacitance value. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] U.S. Patent No. 3040220A [Patent Document 2] Patent No. 2964248B1 [Patent Document 3] U.S. Patent No. 2002163398A1 [Patent Document 4] U.S. Patent No. US2010254066A1 Summary of the Invention [Problem to be solved by the invention]

[0010] SUMMARY OF THE DISCLOSURE The object of the present invention is therefore to propose a novel vacuum variable capacitor in which the above-mentioned drawbacks of known systems are completely overcome or at least significantly reduced.

[0011] The object of the invention is in particular to propose a vacuum variable capacitor whose capacitance can be adjusted with low torque while ensuring a high precision of the capacitance value. [Means for solving the problem]

[0012] According to the present invention, these objects are achieved particularly through the elements of the independent claims.

[0013] In particular, the object of the invention is achieved by a vacuum variable capacitor comprising a vacuum sealed enclosure containing a vacuum dielectric medium, the enclosure comprising a first plate and a second plate, said first plate and second plate being separated by an electrical insulating element, the enclosure further comprising a fixed electrode attached to the first plate inside the enclosure and a movable electrode attached to the movable plate, the movable plate being attached to the second plate inside the enclosure by at least one vacuum bellows, the vacuum capacitor having a mechanical drive system for displacing, in particular translating, the movable plate relative to the first plate so as to vary the capacitance of the vacuum capacitor, the mechanical drive system comprising a ball screw arranged to drive the movable plate, the mechanical drive system comprising a limiting element outside the vacuum sealed enclosure for limiting a maximum distance between the first plate and the movable plate, the drive system comprising a nut attached to the ball screw, the nut comprising a first shoulder configured to abut against the limiting element for limiting the maximum distance between the first plate and the movable plate.

[0014] Thanks to the ball screw, the distance between the first plate and the movable plate and therefore the capacitance of the capacitor can be changed with low torque. This is particularly advantageous in applications where the capacitance has to be changed repeatedly and quickly. A low torque allows lower requirements on the motor that is usually used to drive the movable plate, so that smaller and cheaper motors can be used. Due to the limiting element, an exact value for the maximum distance between the plates and therefore for the minimum value for the capacitance can be defined. This value can therefore be used as a reference value for all capacitance values ​​corresponding to other distances between the plates. Using the minimum capacitance as a reference value is advantageous, since the capacitance varies inversely with the distance between the plates. Absolute errors on the distance between the plates have less effect on the capacitance at the minimum capacitance value than at the maximum capacitance value. Therefore, an accurate reference value at the minimum capacitance is advantageous. In prior art vacuum variable capacitors, the minimum capacitance value is defined by elements inside the vacuum enclosure, such as a bellows. This means that this value must be defined before the enclosure is assembled and evacuated, and cannot be defined afterwards. It would therefore be advantageous to be able to define a reference value for the minimum capacitance by a limiting factor external to the vacuum enclosure. In fact, the capacitance can be measured and this value defined as a function of this measurement. Furthermore, some prior art vacuum variable capacitors define the minimum capacitance by the maximum compression rate of the bellows. This is problematic, as it may not lead to an accurate reference value for the minimum capacitance.

[0015] As explained above, typical applications of vacuum capacitors include power delivery systems used in semiconductor manufacturing using radio frequency (e.g., at 13.56 MHz) powered plasma tools for deposition and etching processes. The primary function of a vacuum variable capacitor is to tune and match the impedance of the application load to the impedance of the radio frequency generator in the power delivery circuit. Because the plasma load has a time-varying impedance (depending on each processing step, ionized gas conditions, residual gas pressure, and other factors), it needs to be matched ("matched") to the impedance of the generator and RF cable, which are optimized by industry standards to function against a 50 ohm impedance. Any impedance mismatch can result in the failure of the semiconductor manufacturing process and, in some cases, even the destruction of the generator due to reflected power due to the impedance mismatch of the load. It is therefore of utmost importance to provide a vacuum variable capacitor that includes an accurate reference value at minimum capacitance.

[0016] Finally, providing a drive system having a nut (including a first shoulder configured to abut a limiting element to limit a maximum distance between the first plate and the movable plate) attached to a ball screw represents a simple and cost-effective manner for defining a reference value with a minimum capacitance value.

[0017] In a first preferred embodiment of the invention, the maximum distance between the first plate and the movable plate is adjustable by means of a limiting element. This is particularly advantageous since the properties of the capacitor may change over time or due to external conditions such as temperature. The possibility of being able to adjust the maximum distance and therefore the reference value at the minimum capacitance therefore represents an advantage over vacuum variable capacitors known in the prior art.

[0018] In a further preferred embodiment of the invention, the nut is at least partially disposed in a guide element of the enclosure, the nut including a second shoulder configured to abut an edge of the guide element to limit a minimum distance between the first plate and the movable plate, thereby defining a maximum capacitance value of the capacitor.

[0019] In yet another preferred embodiment of the invention, the vacuum variable capacitor comprises a cap configured to cover the ball screw. This is advantageous to protect the drive system, and in particular the ball screw, in order to have a capacitor whose performance is guaranteed over a long period of time. The cap can advantageously be configured to completely cover the ball screw in order to protect it against dust and mechanical shocks.

[0020] In a further preferred embodiment of the invention, the limiting element is at least partially arranged inside the cap, whereby the limiting element is additionally protected by the cap and it is ensured that the reference value at the minimum capacitance value remains constant over time.

[0021] In yet a further preferred embodiment of the invention, the cap comprises a thread onto which the restrictive element is at least partially screwed. This represents a particularly simple and cost-effective way of implementing the restrictive element.

[0022] In another preferred embodiment of the invention, the limiting element is a threaded pin. A threaded pin can be easily provided and the length of the pin can be used to define the minimum capacitance value.

[0023] In yet another preferred embodiment of the invention, the maximum distance between the first plate and the movable plate is adjustable from the exterior of the cap, allowing the minimum capacitance value to be defined and adjusted even when the capacitor is fully assembled and in operation.

[0024] In a further preferred embodiment of the invention, the maximum distance between the first plate and the mobile plate is adjustable by screwing and unscrewing the limiting element into the cap. This represents a particularly simple and cost-effective way to provide a limiting element making it possible to adjust the minimum capacitance value.

[0025] In yet a further preferred embodiment of the invention, the ball screw is configured such that the distance between the first plate and the movable plate can be increased with a torque of 0.5 N·m or less, advantageously 0.3 N·m or less, in particular 0.25 N·m or less. This allows lowering the requirements of the motor used to drive the movable plate and therefore the capacitance value to be adjusted. This allows a smaller and cheaper motor to be used.

[0026] In yet another preferred embodiment of the present invention, the mechanical drive system includes a coupling element for attaching the threaded shaft of the ball screw to a drive motor, thereby driving the ball screw together with the motor. [Brief description of the drawings]

[0027] [Figure 1] FIG. 1 shows a schematic cross-sectional view of a vacuum variable capacitor according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0028] FIG. 1 shows a schematic cross-sectional view of a vacuum variable capacitor 1 according to an embodiment of the present invention.

[0029] The vacuum variable capacitor 1 comprises a vacuum sealed enclosure 2 including a first plate 3 and a second plate 4 electrically separated by an insulating element 5. Inside the enclosure 2, a fixed electrode 6, advantageously a helical or cylindrical electrode (or a set of cylindrical surfaces forming an electrode), is attached to the first plate 3. Inside the enclosure 2, a movable plate 7 including a movable electrode 8, advantageously a helical or cylindrical electrode (or as a set of cylindrical surfaces forming an electrode), is attached to the first plate 4 by a vacuum bellows 9. As is well known to those skilled in the art, these capacitance generating electrodes can be arranged as intertwined spirals or concentric, evenly spaced cylinders in order to optimize the voltage and power capabilities of the capacitor for a given available volume of vacuum dielectric and thus for a given size or footprint of the capacitor.

[0030] A mechanical drive system 10 is provided for translating the mobile plate 7, and therewith the mobile electrode 8, relative to the first plate 3 and the fixed electrode 6 in order to vary the capacitance of the capacitor. The mechanical drive 10 has a ball screw 11 comprising a threaded shaft 12 arranged such that by rotating the shaft 12 the distance between the mobile plate 7 and the first plate 3 can be adjusted. Thanks to the ball screw the distance between the mobile plate 7 and the first plate 3 can be modified by applying a small torque to the threaded shaft 12. Advantageously the ball screw 11 is configured such that the capacitance of the capacitor can be adjusted by applying a torque of less than or equal to 0.5 N·m, even more advantageously less than or equal to 0.3 N·m, in particular less than or equal to 0.25 N·m.

[0031] As can be seen in Fig. 1, the nut 13 is attached to the ball screw 11 and shows a first shoulder 13a arranged to abut against a limiting element 14, which is arranged to limit the maximum distance between the first plate 3 and the mobile plate 7 and thus to define the minimum capacitance of the capacitor 1. It is important to understand that the limiting element 14 is configured such that when the first shoulder 13a of the nut 13 abuts against the limiting element 14, the maximum distance between the first plate and the mobile plate is reached, and thus the minimum capacitance of the capacitor. In other words, the minimum capacitance is not defined by the maximum compression of the vacuum bellows 9, as in prior art capacitors, but by the distance between the end of the limiting element facing the mobile plate and the first shoulder of the nut.

[0032] In the embodiment of FIG. 1, the limiting element 14 has the form of a threaded pin which is partially screwed into a cap 15 which covers the mechanical drive system 10, in particular the ball screw 11. As can be easily seen from FIG. 1, the threaded pin 14 can be screwed and unscrewed from the outside of the cap 15, for example by means of a screwdriver. By screwing and unscrewing the threaded pin 14, the maximum distance between the first plate 3 and the movable plate 7, and thus the minimum capacitance of the capacitor 1, can be precisely adjusted. Of course, the minimum capacitance of the capacitor 1 can also be adjusted by selecting limiting elements of different lengths.

[0033] The nut 13 is partially arranged in the guide element 16 and presents a second shoulder 13b arranged to abut against an edge 16a of the guide element in order to limit the minimum distance between the first plate 3 and the mobile plate 7 and therefore to define the maximum capacitance of the capacitor.

[0034] Advantageously, a coupling element 17 is provided for attaching the threaded shaft 12 to a drive motor.

[0035] Finally, it should be noted that the foregoing has outlined one relevant non-limiting embodiment. It will be apparent to those skilled in the art that modifications to the disclosed non-limiting embodiment may be made without departing from the spirit and scope thereof. As such, the described non-limiting embodiment should be considered merely illustrative of some of the more prominent features and applications. Other beneficial results may be realized by applying the non-limiting embodiment in a different manner or by modifying them in manners known to those skilled in the art.

Claims

1. A variable vacuum capacitor (1) comprising a vacuum-sealed container (2) containing a vacuum dielectric medium, said container (2) including a first plate (3) and a second plate (4), said first plate (3) and said second plate (4) being separated by an electrical insulating element (5), said container (2) further including a fixed electrode (6) attached to said first plate (3) inside said container (2) and a movable electrode (8) attached to a movable plate (7), said movable plate (7) being attached to said second plate (4) inside said container (2) by at least one vacuum bellows (9), said vacuum capacitor (1) having a mechanical drive system (10) for displacing, in particular translating, said movable plate (7) relative to said first plate (3) so as to vary the capacitance of said vacuum capacitor (1), said mechanical drive system (10) including a ball screw (11) arranged to drive said movable plate (7), said mechanical drive system (10) including a limiting element (14) outside said vacuum-sealed container (2) for limiting the maximum distance between said first plate (3) and said movable plate (7), in said variable vacuum capacitor (1), said drive system including a nut (13) attached to said ball screw (11), said nut (13) including a first shoulder (13a) configured to abut said limiting element (14) for limiting the maximum distance between said first plate (3) and said movable plate (7), characterized in that it is a variable vacuum capacitor (1).

2. The vacuum capacitor (1) according to claim 1, wherein the maximum distance between said first plate (3) and said movable plate (7) is adjustable by said limiting element (14).

3. The vacuum capacitor (1) according to claim 1 or 2, wherein said nut (13) is at least partially disposed within a guiding element (16) of said container (2), said nut (13) including a second shoulder (13b) configured to abut an edge (16a) of said guiding element (16) for limiting the minimum distance between said first plate (3) and said movable plate (7).

4. The vacuum capacitor according to claim 1, comprising a cap (15) configured to cover the ball screw (11).

5. The vacuum capacitor according to claim 4, wherein the limiting element (14) is at least partially disposed inside the cap (15).

6. The vacuum capacitor according to claim 5, wherein the cap (15) has a thread (15a) into which the limiting element (14) is at least partially screwed.

7. The vacuum capacitor according to claim 6, wherein the limiting element (14) is a threaded pin.

8. The vacuum capacitor according to claim 4, wherein the maximum distance between the first plate (3) and the movable plate (7) is adjustable from outside the cap (15).

9. The vacuum capacitor according to any one of claims 6 or 8, wherein the maximum distance between the first plate (3) and the movable plate (7) is adjustable by screwing the limiting element (14) into the cap (15) and loosening the screw of the limiting element (14) in the opposite direction.

10. The ball screw (11) is configured such that the torque is 0.5 N·m or less, preferably 0.3 N·m or less, particularly 0.25 N·m or less, so as to be able to increase the distance between the first plate (3) and the movable plate (7). The vacuum capacitor according to claim 1.

11. The vacuum capacitor according to claim 1, wherein the mechanical drive system (10) has a coupling element (17) for attaching a drive motor to the threaded shaft (12) of the ball screw (11).

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