Vacuum variable capacitor

Abstract

This invention relates to a vacuum variable capacitor (1), comprising: a vacuum-sealed housing (2) for containing a vacuum dielectric, wherein the housing (2) includes a first plate (3) and a second plate (4), the first plate (3) and the second plate (4) being separated by an electrical insulating element (5); a fixed electrode (6) attached to the first plate (3) within the housing (2); and a movable electrode (8) attached to a movable plate (7), wherein the movable plate (7) is attached to the second plate (4) within the housing (2) via at least one vacuum bellows (9). The vacuum capacitor (1) includes a mechanical drive system (10) for moving (particularly translating) the movable plate (7) relative to the first plate (3) to change the capacitance of the vacuum capacitor (1). The mechanical drive system (10) includes a ball screw (11) configured to drive the movable plate (7). The mechanical drive system (10) includes a limiting element (14) outside the vacuum-sealed housing (2) to limit the maximum distance between the first plate (3) and the movable plate (7). The drive system includes a nut (13) attached to the ball screw (11). The nut (13) includes a first shoulder (13a) configured to abut against the limiting element (14) to limit the maximum distance between the first plate (3) and the movable plate (7).

Description

Technical Field

[0001] This invention relates to the field of vacuum capacitors, and more particularly to the field of vacuum variable capacitors. Prior Technology

[0002] Vacuum capacitors are capacitors suitable for high-power applications. Due to their excellent vacuum dielectric properties, they can typically withstand voltages of several kilovolts or even tens of thousands of volts. Because the conductive parts (i.e., the metal electrodes and rings used to connect the vacuum capacitor to other circuit elements) have very low losses, vacuum capacitors can carry alternating currents (especially radio frequency currents) of tens or even hundreds of amperes.

[0003] Vacuum variable capacitors constitute a subclass of vacuum capacitors and are also known for their excellent suitability for high-power applications. These vacuum variable capacitors allow for capacitance adjustment by changing the position of one electrode relative to another, both of which are located within the vacuum casing of the capacitor. In practice, it is not the electrode spacing that is changed, but rather the electrode surface overlap is altered by changing the distance between the plates carrying the electrodes. The range of capacitance adjustment is called adjustability and is defined by t = (C max - C min) / C max, where C max and C min are the maximum and minimum capacitance values ​​achievable by a given capacitor. Most vacuum variable capacitors are designed to provide at least 90% adjustability, and some offer 98% or even higher. Typically, a screw and nut system associated with a retractable connector (also known as a bellows) allows actuation to be brought from outside the vacuum to the adjustable electrode within the vacuum casing. For example, a stepper motor controlled by control software can be used to actuate the parts to precisely position the movable electrode, thereby achieving a specific, precise value of the capacitance of the vacuum variable capacitor. The aforementioned components for adjusting the capacitance of a vacuum variable capacitor define its mechanical drive system.

[0004] In addition to high voltage and high current capabilities, one of the key quality standards for this subclass of vacuum variable capacitors is the performance of their drive system. Valuable features include not only high adjustability but also a long-life drive system (defined by the number of cycles between Cmax and Cmin until failure), a drive system adjustable with relatively moderate torque, and fast and precise mechanical adjustments to meet the high demands of many applications.

[0005] Typical applications of vacuum capacitors include power delivery systems in the semiconductor manufacturing industry, where deposition and etching processes are performed using radio frequency (RF) (e.g., 13.56 MHz) powered plasma tools. The primary function of a vacuum variable capacitor is to adjust and match the impedance of the applied load to the impedance of the RF generator in the power delivery circuit. Because plasma loads have time-varying impedances (dependent on each processing step, ionized gas state, residual gas pressure, and other factors), it needs to be adapted (matched) to the impedance of the generator and the RF cable, which is optimized to operate at 50 ohms impedance according to industry standards. Any impedance mismatch can lead to unsuccessful semiconductor manufacturing processes and, in some cases, damage to the generator due to reflected power resulting from the impedance mismatch between the generator and the load.

[0006] Now let's return to the drive system of the vacuum variable capacitor. As mentioned above, it is desirable to provide a system capable of actuation with moderate or low torque. Currently, most vacuum variable capacitors typically require a stepper motor or other actuator to provide a torque greater than 0.4 N. The torque m is adjusted accordingly. One objective of this invention is to disclose a vacuum variable capacitor that can be actuated with reduced torque without compromising the accuracy and repeatability of the drive system. To achieve this, the vacuum variable capacitor requires a mechanical end stop at the movable electrode position corresponding to the minimum capacitance Cmin. Conventional vacuum variable capacitors typically do not provide a means for determining the precise value of Cmin. The only available means is essentially to fully compress the bellows. The bellows is not a high-precision component, so that even using the same component (nominally), this end stop corresponds to, for example, 32pF in some cases, and 35pF or 27pF in others. Due to this problem, current datasheets for mass-produced vacuum variable capacitors must show higher Cmin values ​​resulting in a smaller capacitance range, thus causing worse adjustability simply because of this uncertainty. In the example just mentioned, the manufacturer would publish a "safer" Cmin value (e.g., 40pF) because the manufacturer knows from experience that regardless of the bellows characteristics within its component tolerance range, a "safe Cmin" will be achievable. In other words, for all types of capacitors published, customers can indeed achieve the values ​​published in the datasheet. This reliability is important, so manufacturers agree to publish adjustable characteristics that are worse than necessary.

[0007] Vacuum variable capacitors including limiting elements for limiting the maximum distance between a movable plate and a fixed plate are known from US3040220A, JP2964248B1, US2002163398A1 and US2010254066A1, but the proposed limiting elements do not provide a satisfactory solution to the above problems.

[0008] Therefore, the object of the present invention is to provide a vacuum variable capacitor whose capacitance can be adjusted with low torque, while allowing for precise definition of its minimum capacitance value. Summary of the Invention

[0009] Therefore, the object of the present invention is to propose a novel vacuum variable capacitor that can completely overcome or at least greatly reduce the above-mentioned disadvantages of known systems.

[0010] The present invention specifically aims to provide a vacuum variable capacitor whose capacitance can be adjusted with low torque while ensuring high accuracy of the capacitance value.

[0011] According to the present invention, these objectives are achieved in particular through elements of the independent claim.

[0012] Specifically, the object of the present invention is achieved by a vacuum variable capacitor comprising a vacuum-sealed housing for containing a vacuum dielectric, wherein the housing includes a first plate and a second plate separated by an electrically insulating element; a fixed electrode attached to the first plate within the housing; and a movable electrode attached to a movable plate, wherein the movable plate is attached to the second plate within the housing via at least one vacuum bellows, wherein the vacuum capacitor includes a mechanical drive system for moving (particularly translating) the movable plate relative to the first plate to change the capacitance of the vacuum capacitor, wherein the mechanical drive system includes a ball screw configured to drive the movable plate, and wherein the mechanical drive system includes a limiting element outside the vacuum-sealed housing that limits the maximum distance between the first plate and the movable plate, wherein the drive system includes a nut attached to the ball screw, and wherein the nut includes a first shoulder configured to abut against the limiting element to limit the maximum distance between the first plate and the movable plate.

[0013] Because of the ball screw, the distance between the first plate and the movable plate, and thus the capacitance of the capacitor, can be changed with low torque. This is particularly advantageous in applications where capacitance must be changed repeatedly and rapidly. With low torque, the requirements for the motor typically used to drive the movable plate can be reduced, allowing for the use of smaller, cheaper motors. The limiting element can define a precise value for the maximum distance between the plates, thereby defining a minimum capacitance. This value can then be used as a reference for all capacitance values ​​corresponding to other distances between the plates. Because capacitance is inversely proportional to the distance between the plates, it is advantageous to use the minimum capacitance as the reference value. The absolute error in the distance between the plates has less effect on the capacitance at the minimum capacitance value than on the capacitance at the maximum value. Therefore, an accurate reference value at the minimum capacitance is advantageous. In conventional vacuum variable capacitors, the minimum capacitance value is defined by an element (e.g., a bellows) within the vacuum housing. This means that the value must be defined before the housing is assembled and evacuated, and cannot be defined afterward. Therefore, it is advantageous to limit the reference value of the minimum capacitance by a limiting element outside the vacuum enclosure. In fact, the capacitance can be measured and this value defined as a function of that measurement. Furthermore, some conventional vacuum variable capacitors limit the minimum capacitance by the maximum compressibility of the bellows. This is problematic because it cannot produce an accurate reference value at the minimum capacitance.

[0014] As mentioned above, typical applications of vacuum capacitors include power delivery systems in the semiconductor manufacturing industry, where plasma tools are driven by radio frequency (RF) (e.g., 13.56 MHz) for deposition and etching processes. The primary function of a vacuum variable capacitor is to adjust and match the impedance of the applied load to the impedance of the RF generator in the power delivery circuit. Because the plasma load has time-varying impedance (depending on each processing step, ionized gas state, residual gas pressure, and other factors), it needs to be adapted ("matched") to the impedance of the generator and the RF cable, which is optimized to operate at 50 ohms impedance according to industry standards. Any impedance mismatch can lead to unsuccessful semiconductor manufacturing processes and, in some cases, damage to the generator due to reflected power resulting from the load impedance mismatch. Therefore, it is crucial to provide a vacuum variable capacitor with an accurate reference value at minimum capacitance.

[0015] Finally, a drive system including a nut attached to the ball screw represents a simple and cost-effective way to limit the reference value to the minimum capacitance value, wherein the nut includes a first shoulder configured to abut against the limiting element to limit the maximum distance between the first plate and the movable plate.

[0016] In a first preferred embodiment of the invention, the maximum distance between the first plate and the movable plate can be adjusted by the limiting element. This is particularly advantageous because the characteristics of the capacitor change over time or due to external conditions (such as temperature). Therefore, the ability to adjust the maximum distance and thus the possibility of the reference value at minimum capacitance represents an advantage over vacuum variable capacitors known in the art.

[0017] In another preferred embodiment of the invention, the nut is at least partially disposed in a guide element of the housing, and the nut includes a second shoulder configured to abut against an edge of the guide element to limit the minimum distance between the first plate and the movable plate. This limits the maximum capacitance value of the capacitor.

[0018] In yet another preferred embodiment of the invention, the vacuum variable capacitor includes a cover configured to cover the ball screw. This is beneficial for protecting the drive system, and in particular the ball screw, so as to ensure the long-term performance of the capacitor. The cover can advantageously be configured to completely cover the ball screw to protect it from dust and mechanical shock.

[0019] In another preferred embodiment of the invention, the limiting element is at least partially disposed within the cover. Thus, the limiting element is also protected by the cover, thereby ensuring that the reference value at the minimum capacitance remains constant over time.

[0020] In yet another preferred embodiment of the invention, the cover includes a thread in which the limiting element is at least partially screwed. This represents a particularly simple and cost-effective way of implementing the limiting element.

[0021] In another preferred embodiment of the invention, the limiting element is a threaded pin. The threaded pin can be readily provided, and its length can be used to define the minimum capacitance value.

[0022] In yet another preferred embodiment of the invention, the maximum distance between the first plate and the movable plate can be adjusted from outside the cover. Thus, the minimum capacitance value can be defined and adjusted when the capacitor is fully assembled or even in operation.

[0023] In another preferred embodiment of the invention, the maximum distance between the first plate and the movable plate can be adjusted by screwing the limiting element into and out of the cover. This represents a particularly simple and cost-effective method of providing a limiting element that allows adjustment of the minimum capacitance value.

[0024] In yet another preferred embodiment of the invention, the ball screw is configured to operate with a torque of less than or equal to 0.5 N. m (advantageously less than or equal to 0.3N) m, especially less than or equal to 0.25N The torque (m) is used to increase the distance between the first plate and the movable plate. This allows for a reduction in the requirements of the motor used to drive the movable plate and thus adjust the capacitor value. Consequently, a smaller and cheaper motor can be used.

[0025] In yet another preferred embodiment of the invention, the mechanical drive system includes a coupling element for attaching a threaded shaft of the ball screw to a drive motor. This allows the ball screw to be driven by a motor. Simple Explanation of the Diagram

[0026] Figure 1 shows a schematic cross-sectional view of a vacuum variable capacitor according to a specific example of the present invention. Implementation

[0027] Figure 1 shows a schematic cross-sectional view of a vacuum variable capacitor 1 according to a specific example of the present invention.

[0028] The vacuum variable capacitor 1 includes a vacuum-sealed housing 2 having a first plate 3 and a second plate 4 electrically isolated by insulating elements 5. Inside the housing 2, fixed electrodes 6, advantageously helical or cylindrical electrodes (or a set of cylindrical surfaces forming electrodes), are attached to the first plate 3. A movable plate 7 having movable electrodes 8 (advantageously helical or cylindrical electrodes (or a set of cylindrical surfaces forming electrodes)) is attached to the second plate 4 inside the housing 2 by means of a vacuum bellows 9. As is well known to those skilled in the art, these capacitance-generating electrodes can be configured as wound helices or concentrically spaced cylinders to optimize the voltage and power performance of the capacitor for a given volume of available vacuum dielectric, and thus for a given capacitor size or coverage area.

[0029] A mechanical drive system 10 is provided to translate a movable plate 7 relative to a first plate 3 and a fixed electrode 6, and consequently, a movable electrode 8, to change the capacitance of a capacitor. The mechanical drive system 10 includes a ball screw 11 with a threaded shaft 12; the ball screw 11 is configured to adjust the distance between the movable plate 7 and the first plate 3 by rotating the shaft 12. Due to the ball screw, the distance between the movable electrode 7 and the first plate 3 can be changed by applying a small torque to the threaded shaft 12. Advantageously, the ball screw 11 is configured to be able to change the distance by applying a torque less than or equal to 0.5 N. m (or even more advantageously less than or equal to 0.3 N) m, especially 0.25N The capacitance of the capacitor is adjusted by using a torque of m.

[0030] As shown in Figure 1, the nut 13 is attached to the ball screw 11 and has a first shoulder 13a, which is configured to abut against a limiting element 14. The limiting element 14 is configured to limit the maximum distance between the first plate 3 and the movable plate 7, thereby limiting 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 movable plate is reached, thus minimizing the capacitance of the capacitor. In other words, the minimum capacitance is not limited by the maximum compressibility of the vacuum bellows 9, as is the case with conventional capacitors, but by the distance between the end of the limiting element facing the movable plate and the first shoulder of the nut.

[0031] In the specific example of Figure 1, the limiting element 14 is in the form of a threaded pin, which is partially screwed into the cover 15, which covers the mechanical drive system 10, particularly the ball screw 11. It can be easily understood from Figure 1 that the threaded pin 14 can be screwed in and out of the cover 15, for example, using a screwdriver. By screwing the threaded pin 14 in and out, the maximum distance between the first plate 3 and the movable plate 7 can be precisely adjusted, 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.

[0032] Nut 13 is partially disposed in guide element 16 and presents a second shoulder 13b, wherein the second shoulder 13b is configured to abut against edge 16a of guide element in order to limit the minimum distance between first plate 3 and movable plate 7, thereby limiting the maximum capacitance of capacitor.

[0033] Advantageously, a coupling element 17 is provided so that the threaded shaft 12 can be attached to the drive motor.

[0034] Finally, it should be noted that a suitable non-limiting example has been outlined above. Those skilled in the art will understand that modifications can be made to the disclosed non-limiting example without departing from its spirit and scope. Therefore, the described non-limiting example should be considered merely as an illustration of some more prominent features and applications. Other beneficial results can be achieved by applying the non-limiting examples in different ways or modifying them in ways known to those skilled in the art.

[0035] 1: Vacuum Variable Capacitor 2: Vacuum-sealed outer casing 3: First board 4: Second board 5: Insulating elements 6: Fixed electrode 7: Movable board 8: Movable electrode 9: Vacuum bellows 10: Mechanical drive system 11: Ball screw 12: Threaded shaft 13: Nut 13a: First shoulder 13b: Second shoulder 14: Limiting element 15: Lid 15a: Thread 16: Guiding element 16a: Edge 17: Coupling element

Claims

1. A vacuum variable capacitor (1), comprising: A vacuum-sealed housing (2) for containing a vacuum dielectric, wherein the housing (2) includes a first plate (3) and a second plate (4), the first plate (3) and the second plate (4) being separated by an electrical insulating element (5); A fixed electrode (6) is attached to the first plate (3) within the housing (2); and a movable electrode (8) is attached to a movable plate (7), wherein the movable plate (7) is attached to the second plate (4) within the housing (2) via at least one vacuum bellows (9), wherein the vacuum variable capacitor (1) includes a mechanical drive system (10) for moving, particularly translating, the movable plate (7) relative to the first plate (3) to change the capacitance of the vacuum variable capacitor (1), wherein the mechanical drive system (10) includes a ball screw (11) configured to drive the movable plate (7), and wherein the mechanical drive system (10) includes a limiting element (14) outside the vacuum-sealed housing (2) to limit the maximum distance between the first plate (3) and the movable plate (7), characterized in that: The drive system includes a nut (13) attached to the ball screw (11), wherein the nut (13) includes a first shoulder (13a) configured to abut against the limiting element (14) to limit the maximum distance between the first plate (3) and the movable plate (7).

2. The vacuum variable capacitor (1) as requested in item 1, wherein, The maximum distance between the first plate (3) and the movable plate (7) can be adjusted by the limiting element (14).

3. The vacuum variable capacitor (1) as requested in item 1 or 2, wherein, The nut (13) is at least partially disposed in a guide element (16) of the housing (2), and wherein the nut (13) includes a second shoulder (13b) configured to abut against an edge (16a) of the guide element (16) to limit the minimum distance between the first plate (3) and the movable plate (7).

4. The vacuum variable capacitor (1) of claim 1 further includes a cover (15) configured to cover the ball screw (11).

5. The vacuum variable capacitor (1) as requested in item 4, wherein, The limiting element (14) is at least partially disposed within the cover (15).

6. As in claim 5, the vacuum variable capacitor (1), wherein, The cover (15) includes a thread (15a) into which the limiting element (14) is at least partially screwed.

7. The vacuum variable capacitor (1) as requested in item 6, wherein, The limiting element (14) is a threaded pin.

8. The vacuum variable capacitor (1) of any one of claims 4 to 7, wherein, The maximum distance between the first plate (3) and the movable plate (7) can be adjusted from outside the cover (15).

9. The vacuum variable capacitor (1) as claimed in claim 6, wherein, The maximum distance between the first plate (3) and the movable plate (7) can be adjusted by screwing the limiting element (14) into and out of the cover (15).

10. The vacuum variable capacitor (1) as claimed in claim 8, wherein, The maximum distance between the first plate (3) and the movable plate (7) can be adjusted by screwing the limiting element (14) into and out of the cover (15).

11. The vacuum variable capacitor (1) as claimed in claim 1, wherein, The ball screw (11) is configured to increase the distance between the first plate (3) and the movable plate (7) with a torque of less than or equal to 0.5 Nm, advantageously less than or equal to 0.3 Nm, and especially less than or equal to 0.25 Nm.

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

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