Chemical mechanical polishing drive assembly

Abstract

The application provides a chemical mechanical polishing driving assembly, comprising a driving unit, a driving frame, a rotary joint, a bearing head and a damping shaft system; the damping shaft system comprises a first damping module and a second damping module to absorb vibration in horizontal and / or vertical directions respectively. The damping shaft system provided by the application absorbs vibration energy in different degrees of freedom without increasing the self weight of the driving frame, blocks the transmission path of the vibration energy, prevents the loosening and even failure of parts due to vibration, and improves the stability of wafer grinding quality to some extent.

Description

Technical Field

[0001] This invention belongs to the field of wafer manufacturing technology, and more specifically, relates to a chemical mechanical polishing drive component. Background Technology

[0002] The integrated circuit industry is the core of the information technology industry, playing a crucial role in promoting the digital and intelligent transformation and upgrading of the manufacturing industry. Chips are the carriers of integrated circuits, and chip manufacturing involves processes such as chip design, wafer fabrication, wafer processing, electrical measurement, dicing, packaging, and testing.

[0003] Chemical Mechanical Polishing (CMP) is one of the five core processes in wafer manufacturing. It's an ultra-precision surface finishing technique that achieves global planarization. The entire process involves alternating chemical and mechanical actions to polish the wafer surface. Compared to commonly used mechanical polishing, CMP produces a flatter wafer surface and offers advantages such as lower processing costs and simpler methods, making it the most prevalent surface planarization technology for semiconductor materials.

[0004] In the chemical mechanical polishing process of wafers, both the bearing head and the polishing disk rotate in the same direction. At the same time, the bearing head also needs to make linear movements within a short stroke. The complex motion process causes the bearing head to generate strong vibrations in different degrees of freedom during polishing. Especially for some high-pressure polishing processes, the vibration may be aggravated.

[0005] Currently, the main solution to reduce the vibration amplitude and frequency of the bearing head is to increase the rigidity of the frame and shaft system. However, this increases the weight of the frame or shaft system itself and does not allow for adjustment of the system rigidity. Therefore, it is urgent to improve the existing equipment structure to reduce equipment vibration. Summary of the Invention

[0006] This invention aims to at least partially solve one of the technical problems in related technologies. To this end, this invention proposes a chemical mechanical polishing drive assembly.

[0007] This invention provides a chemical mechanical polishing drive assembly, comprising:

[0008] Drive unit, drive frame, rotary joint, bearing head, and vibration damping shaft system;

[0009] The vibration damping shaft system includes a first vibration damping module and a second vibration damping module to absorb vibrations in the horizontal and / or vertical directions, respectively.

[0010] In some embodiments, the drive frame is formed as a rigid support frame structure, the damping shaft system is fixedly mounted on the drive frame and moves with it, the upper part of the drive unit is connected to the damping shaft system, the lower part is configured for a support head, and the rotary joint is used to supply multiple gas to the support head during its rotational operation.

[0011] In some embodiments, the rotary joint is disposed on the upper part of the damping shaft system and has an air passage for conveying gas, at least a portion of which is disposed inside the main shaft of the damping shaft system.

[0012] In some embodiments, the first damping module includes a wing plate formed in a plate-like structure, disposed on the upper circumference of the main shaft of the damping shaft system.

[0013] In some embodiments, the wing has a hollow structure to improve its overall elastic modulus.

[0014] In some embodiments, the outer periphery of the wing plate is fixedly connected to the drive frame, and the main shaft of the vibration damping shaft system is suspended at its inner periphery by a plurality of vibration damping structures.

[0015] In some embodiments, the second damping module is disposed on the outer periphery of the upper part of the damping shaft system to absorb vibrations in the horizontal direction.

[0016] In some embodiments, the second vibration damping module includes a vibration isolation sleeve and a plurality of damping columns located within the vibration isolation sleeve, the plurality of damping columns being uniformly arranged around the outer periphery of the bearing at the upper part of the main shaft of the vibration damping shaft system;

[0017] The vibration isolation sleeve is equipped with multiple adjusting components for adjusting the preload between the bearing and the damping column.

[0018] In some embodiments, the chemical mechanical polishing drive assembly is further configured with a third vibration damping module, and a cavity is formed inside the main shaft of the vibration damping shaft system to accommodate the third vibration damping module.

[0019] In some embodiments, the third vibration damping module includes a vibrating core coaxially disposed with the cavity, and a gap is left between the vibrating core and the inner wall of the cavity.

[0020] The two ends of the vibrating core are fitted with elastic elements, which abut against the top or bottom surface of the cavity;

[0021] A damping ring is also disposed between the end of the vibrating core and the elastic element. The outer periphery of the damping ring is in close contact with the inner wall of the cavity, and the gap is 0.5 to 2 times the radius of the damping ring.

[0022] Compared with the prior art, the beneficial effects of the present invention include:

[0023] This invention provides a vibration damping shaft system for a bearing head, which absorbs vibration energy in different degrees of freedom directions without increasing the self-weight of the drive frame, blocks the transmission path of vibration energy, prevents components from vibrating and loosening or even failing, and at the same time improves the stability of wafer grinding quality to a certain extent. Attached Figure Description

[0024] The advantages of the present invention will become clearer and easier to understand through the following detailed description in conjunction with the accompanying drawings, which are merely illustrative and do not limit the scope of protection of the present invention, wherein:

[0025] Figure 1 This is a schematic diagram of the structure of a chemical mechanical polishing drive assembly provided in an embodiment of the present invention;

[0026] Figure 2 This is a schematic diagram of the structure of a vibration damping shaft system provided in an embodiment of the present invention;

[0027] Figure 3 This is a cross-sectional view of a vibration-damping shaft system provided in an embodiment of the present invention;

[0028] Figure 4 This is a schematic diagram of the structure of the third vibration reduction module provided in an embodiment of the present invention. Detailed Implementation

[0029] The technical solutions of the present invention will be described in detail below with reference to specific embodiments and accompanying drawings. The embodiments described herein are specific implementations of the present invention, used to illustrate the concept of the present invention; these descriptions are explanatory and exemplary, and should not be construed as limiting the implementation methods or the scope of protection of the present invention. In addition to the embodiments described herein, those skilled in the art can employ other obvious technical solutions based on the content disclosed in the claims and specification of this application. These technical solutions include those that make any obvious substitutions and modifications to the embodiments described herein.

[0030] The accompanying drawings in this specification are schematic diagrams to aid in illustrating the concept of the invention, and schematically show the shapes of the various parts and their interrelationships. It should be understood that, in order to clearly demonstrate the structure of the components in the embodiments of the invention, the drawings are not drawn to the same scale, and the same reference numerals are used to indicate the same parts in the drawings. The technical solutions of the invention will be further described below through specific embodiments.

[0031] In this invention, "Chemical Mechanical Polishing (CMP)" is also called "Chemical Mechanical Planarization (CMP)," and the wafer (W) is also called the substrate (Substrate), with the same meaning and actual function.

[0032] Figure 1 A chemical mechanical polishing drive assembly according to an embodiment of the present invention includes:

[0033] Drive unit 100, drive frame 200, rotary joint 300, bearing head 400 and vibration damping shaft system 500;

[0034] The vibration damping shaft system 500 includes a first vibration damping module 510 and a second vibration damping module 520.

[0035] The chemical mechanical polishing (CMP) components, such as the bearing head 400 and the spindle 540, exhibit motion in different degrees of freedom during the CMP process, including circumferential rotation and linear reciprocating movement. Due to these complex motion states and paths, the CMP equipment experiences strong vibrations during operation. To address this, this embodiment provides a vibration-damping shaft system 500 for the bearing head 400. Without increasing the weight of the drive frame 200, this system absorbs vibration energy in different degrees of freedom, blocks the transmission path of vibration energy, and prevents components from loosening or even failing due to vibration. Simultaneously, it improves the stability of wafer polishing quality to a certain extent.

[0036] Figure 1 In the illustrated embodiment, the drive frame 200 is formed as a rigid support frame structure. The drive frame 200 includes a base and a fixing frame fixed to the base. The fixing frame is a portal frame structure, including symmetrically arranged two side columns and a crossbeam located between the two side columns. The top of the vibration damping shaft system 500 is detachably suspended on the drive frame 200 and moves with it. The upper part of the drive unit 100 is connected to the vibration damping shaft system 500, and the lower part of the drive unit 100 is equipped with a bearing head 400. The rotary joint 300 is used to supply multiple gas channels to the bearing head 400 during its rotational operation.

[0037] Figure 1 In the illustrated embodiment, the rotary joint 300 is disposed on the upper part of the vibration damping shaft system 500, and the rotary joint 300 has multiple air passages 310 for conveying gas inside and / or outside, and at least a portion of the air passages 310 is disposed inside the main shaft 540 of the vibration damping shaft system 500.

[0038] Figure 2In the illustrated embodiment, the first damping module 510 includes a wing 511 formed as a plate structure, which is disposed on the circumference of the upper part of the main shaft 540 of the damping shaft system 500. Figure 2 As shown, the wing plates 511 on both sides of the main shaft 540 are symmetrically arranged, and are shaped like butterflies. They have an inner short arc edge and an outer long arc edge. The inner short arc edge of the wing plate 511 is detachably connected to the outer periphery of the vibration isolation sleeve 523 on the main shaft 540 of the hoisting vibration damping shaft system 500 through multiple vibration damping structures. The outer long arc edge of the wing plate 511 is fixed to the surface of the base through multiple fasteners 515.

[0039] The inner short arc edge of the wing plate 511 is connected and fixed to the vibration isolation sleeve 523 via a vibration damping structure. The vibration damping structure can optionally be as follows: Figure 2 As shown, it includes a damping column 513 and a disc spring 514. The disc spring 514, the wing plate 511, the damping column 513, and the vibration isolation sleeve 523 are detachably connected and fixed by connecting screws. The connecting screws pass through the vibration isolation sleeve 523, the damping column 513, the wing plate 511, and the disc spring 514 from bottom to top and are then locked in place. The disc spring 514 can provide a large elastic force in a small space, so that the vibration reaching the top of the main shaft 540 is first transmitted to the damping structure. The vibration energy is partially absorbed and weakened by the damping structure and then transmitted to the wing plate 511, where the vibration energy is absorbed and weakened again.

[0040] The wing plate 511 can be made of spring steel and has a hollow structure 512 to improve its overall modulus of elasticity. The hollow structure 512 on the wing plate 511 includes a through slot formed thereon. Specifically, the shape of the through slot can be as follows: Figure 2 The arc-shaped through slot is shown. The function of the wing plate 511 includes, but is not limited to, reducing the vertical vibration of the damping shaft system 500.

[0041] In the first vibration damping module 510, when vibration is transmitted through the main shaft 540 to the bearing 521 on the top outer periphery of the main shaft 540, the vibration energy is further consumed and absorbed by the vibration damping structure, preventing the vibration energy from being transmitted to the drive frame 200 through the wing plate 511. The elastic combination of the vibration damping structure and the wing plate 511 can also prevent the wing plate 511 from being excessively deformed.

[0042] Figure 3 In the illustrated embodiment, the second damping module 520 is disposed on the outer periphery of the upper part of the damping shaft system 500 to absorb vibrations in the horizontal direction. Specifically, as shown... Figure 3 As shown, the second vibration damping module 520 includes a vibration isolation sleeve 523 and a plurality of damping columns 522 located within the vibration isolation sleeve 523. The plurality of damping columns 522 are evenly arranged around the outer periphery of the bearing 521 on the upper part of the main shaft 540 of the vibration damping shaft system 500.

[0043] Multiple fixing grooves are formed inside the vibration isolation sleeve 523 for fixing the damping column 522. A retaining ring 526 is provided on the top of the vibration isolation sleeve 523, and a gap is left between the top of the damping column 522 and the retaining ring 526. A gap is left between the bearing 521 and the vibration isolation sleeve 523. When the main shaft 540 is vibrated and the bearing 521 wobbles, the bearing 521 can freely squeeze the damping column 522, converting the vibration energy into frictional damping inside the damping column 522 for consumption.

[0044] like Figure 2 As shown, the outer wall of the vibration isolation sleeve 523 is provided with multiple adjusting components 525 for adjusting the preload between the damping column 522 and the bearing 521, thereby changing their rigidity and tuning the damping effect. The adjusting component 525 optionally includes a bolt with a threaded sleeve fitted onto the stud bolt, forming a threaded engagement with the stud. A lock nut is also fitted onto the stud, and tightening the lock nut restricts the rotation of the threaded sleeve. After the bolt passes through the outer wall of the vibration isolation sleeve 523, it abuts against the damping column 522. By tightening the threaded sleeve, the amount of bolt protrusion inside the vibration isolation sleeve 523 can be adjusted, thereby adjusting the preload between the damping column 522 and the bearing 521.

[0045] Figure 3 In the illustrated embodiment, the second vibration damping module 520 further includes a sleeve 524 fitted around the outer periphery of the shaft body for supporting the bearing 521 located above it. A locking element is provided on the bearing 521 to lock the relative movement of the inner ring of the bearing 521 and the sleeve 524, so that the inner ring of the bearing 521 and the sleeve 524 rotate synchronously.

[0046] During the chemical mechanical polishing process, the second vibration damping module 520 is fixedly connected to the top outer periphery of the main shaft 540 via the bearing 521. The bearing inner ring 5213, the sleeve 524 and the main shaft 540 are fixed, and the bearing outer ring 5211 is fixed to the vibration isolation sleeve 523. The relative rotation between the main shaft 540 and the vibration isolation sleeve 523 is achieved through the balls 5212 between the bearing inner ring 5213 and the bearing outer ring 5211. The bearing inner ring 5213, the sleeve 524 and the main shaft 540 rotate synchronously, while the bearing outer ring 5211, the vibration isolation sleeve 523 and a number of damping columns 522 located inside the vibration isolation sleeve 523 remain stationary.

[0047] In the second vibration damping module 520, multiple damping columns 522 surrounding the outer periphery of the main shaft 540 absorb vibrations in different directions within the horizontal plane. When lateral vibration is transmitted, all the damping columns 522 around the outer periphery of the bearing 521 deform accordingly. Since there are lateral vibrations in different directions within the same horizontal plane, the torque generated by the vibration energy in different directions on the damping columns 522 is also different. Therefore, the deformation of different damping columns 522 is also different, thereby better absorbing lateral vibrations in different directions within the horizontal plane and reducing the lateral vibration of the main shaft 540.

[0048] Figure 4 In the embodiment shown, the chemical mechanical polishing drive assembly is further configured with a third vibration damping module 530, and a cavity is formed inside the main shaft 540 of the vibration damping shaft system 500, with the third vibration damping module 530 located inside the cavity.

[0049] The bottom of the cavity is an open structure. The third vibration damping module 530 is inserted into the cavity from the bottom of the main shaft 540. The bottom opening of the cavity is provided with a cover plate 534. The cover plate 534 seals the bottom opening of the cavity, so that the cavity is sealed. The cover plate 534 at the bottom of the cavity of the main shaft 540 can be fixed to the main shaft 540 by a detachable connection method such as thread or snap, which can realize free disassembly and assembly, thereby reducing the maintenance cost and replacement cost of the vibrating core 531.

[0050] like Figure 4 As shown, the third vibration damping module 530 is located near the bottom of the main shaft 540. Since the bottom of the main shaft 540 is closer to the vibration source, the third vibration damping module 530 is placed at the bottom of the main shaft 540 to better utilize the damping and vibration reduction effect of the vibration core 531.

[0051] Specifically, the third vibration damping module 530 includes a vibrating core 531 coaxially arranged with the cavity. The outer diameter of the vibrating core 531 is smaller than the diameter of the cavity of the main shaft 540, so that an annular gap is formed between the internal cavity of the main shaft 540 and the vibrating core 531. The width of the annular gap is 0.5 to 2 times the radius of the vibration damping ring. Here, the "radius of the vibration damping ring" refers to the cross-sectional radius of the solid structure of the vibration damping ring, rather than the radius of the inner or outer circumference of the vibration damping ring. The vibrating core 531 provided in this embodiment is constructed of a high-density alloy material, which allows the vibrating core 531 to have a high weight in a limited space, thereby improving the static rigidity of the main shaft 540.

[0052] Optionally, the vibrating core 531 can be a columnar structure of equal diameter or unequal diameter. When using a vibrating core 531 of unequal diameter, the diameter of the area closer to the lower part of the spindle 540 is larger, and the mass is higher. During chemical mechanical polishing, the vibration amplitude of the spindle 540 is greater closer to the bearing head 400, requiring a vibrating core 531 with greater mass for dynamic vibration damping. By changing the diameter of different positions of the vibrating core 531, and thus changing the weight at different positions, the vibration intensity at different positions of the spindle 540 can be differentially damped. When the length of the spindle 540 increases, the length of the vibrating core 531 can also be extended to achieve a better vibration damping effect.

[0053] Figure 4In the illustrated embodiment, an elastic element is provided at both the upper and lower ends of the vibrating core 531, namely, a first elastic element 533a located at the upper end of the vibrating core 531 and a second elastic element 533b located at the lower end of the vibrating core 531. The elastic elements at both ends of the vibrating core 531 provide axial constraint and axial vibration reduction for the vibrating core 531.

[0054] Elastic elements are provided at both ends of the vibrating core 531 along the vibration reduction direction, which can effectively improve the lateral vibration isolation effect. The elastic elements can be columnar springs or rubber pads. The elastic elements are sleeved on the outer periphery of the limiting part. By setting the elastic elements, the stiffness of the vibrating core 531 in the vertical direction can be increased. At the same time, by replacing the elastic elements with different elastic moduli, the stiffness of the entire third vibration reduction module 530 in the vertical direction can be adjusted, thereby effectively avoiding the resonance phenomenon between the main shaft 540 and the third vibration reduction module 530.

[0055] Figure 4 In the illustrated embodiment, a damping ring is also provided at both the upper and lower ends of the vibrating core 531, namely, a first damping ring 532a located at the upper end of the vibrating core 531 and a second damping ring 532b located at the lower end of the vibrating core 531. The damping rings at both ends of the vibrating core 531 provide radial restriction and radial vibration reduction. The damping rings can also seal the cavity, preventing the cavity from being contaminated by the external environment. The damping effect can be tuned by adjusting the type / hardness and size design of the damping rubber material of the damping rings. When the vibration is transmitted to the main shaft 540, it will cause the vibrating core 531 to vibrate, which is converted into the kinetic energy of the oscillator and the heat energy of the damping rings under the action of the elastic element and the damping rings, so that the vibration is rapidly attenuated and absorbed.

[0056] Furthermore, such as Figure 4 As shown, both ends of the vibrating core 531 are provided with limiting portions. The limiting portions are formed by extending axially from both end faces of the vibrating core 531. The diameter of the limiting portion is smaller than the diameter of the vibrating core 531, thereby forming an annular stepped surface between the limiting portion and the vibrating core 531. The elastic elements at both ends of the vibrating core 531 are sleeved at the limiting portions. The top surface of the first elastic element 533a abuts against the top surface of the cavity of the main shaft 540, the bottom surface of the second elastic element 533b abuts against the bottom surface of the cavity of the main shaft 540, the bottom surface of the damping ring abuts against the annular stepped surface, and the outer peripheral surface of the damping ring abuts against the inner wall surface of the cavity.

[0057] In the third vibration damping module 530, the vibrating core 531 enhances vibration damping and anti-vibration effects, improves the structural stability of the spindle 540, and also improves the vertical vibration damping effect of the spindle 540. By fully utilizing the good compressive strength of the elastic element and the damping ring, the original working state under shear and tension is changed to a compressive working state, increasing the horizontal vibration damping capacity while also providing reset and limiting functions. The vibrating core 531, the damping ring, and the elastic element constitute the third vibration damping module 530 with a two-degree-of-freedom vibration damping effect, fully utilizing the vibration damping performance of the elastic element and the damping ring while also extending the service life of the polishing equipment.

[0058] The three vibration damping modules with different structures provided in this embodiment are arranged at different positions on the main shaft 540. The first vibration damping module 510 mainly includes two wing plates 511 and a damping structure, and is located on the top of the main shaft 540. The main shaft 540 is connected to the drive frame 200 through the first vibration damping module 510, which can absorb or even block the vibration energy between the main shaft 540 and the drive frame 200. The second vibration damping module 520 mainly includes several damping columns 522, which are arranged around the outer periphery of the main shaft 540 to absorb the lateral vibration of the main shaft 540. The third vibration damping module 530 mainly includes a vibrating core 531, an elastic element, and a damping ring, which is located inside the main shaft 540 and coaxially arranged with the main shaft 540 to absorb the longitudinal vibration of the main shaft 540.

[0059] This embodiment improves the stability of the bearing head 400 during the chemical mechanical polishing process by designing three different vibration damping structures. It adopts a dual-degree-of-freedom multi-dimensional dynamic vibration damping module. With the total mass of the vibration damping shaft system 500 remaining unchanged, its maximum response peak value is smaller than the maximum response amplitude of a conventional dynamic vibration damping system, and its amplitude-frequency response characteristics are more stable. This significantly improves the polishing quality of the wafer and also prevents other components from loosening or even failing due to the vibration of the bearing head 400, eliminating harmful vibrations and improving the machine's operating quality.

[0060] The applicant declares that the above are merely specific embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Those skilled in the art should understand that any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the scope of protection and disclosure of the present invention.

Claims

1. A chemical mechanical polishing drive assembly, characterized in that, include: Drive unit, drive frame, rotary joint, bearing head, and vibration damping shaft system; The vibration damping shaft system includes a first vibration damping module and a second vibration damping module. The first vibration damping module is used to absorb vibrations in the horizontal direction and / or vertical direction, and the second vibration damping module is used to absorb vibrations in the horizontal direction. A cavity is formed inside the main shaft of the vibration damping shaft system to accommodate a third vibration damping module. The first vibration damping module is arranged on the upper circumference of the main shaft, including a vibration damping structure and a wing plate, the wing plate having an elastic modulus; The second vibration damping module includes a bearing connected to the top outer periphery of the main shaft, multiple damping pillars surrounding the bearing, and a vibration isolation sleeve, with the multiple damping pillars located inside the vibration isolation sleeve; the wing plate is connected to the vibration isolation sleeve via a vibration damping structure. The third vibration damping module is placed at the bottom of the spindle to be close to the vibration source. It includes a vibration core, and an elastic element and a vibration damping ring are provided at both ends of the vibration core. The third vibration damping module can perform dual-degree-of-freedom vibration damping. The vibration core is used to improve the static rigidity of the spindle and damp the spindle. The elastic element is used to dampen the axial vibration of the vibration core, and the vibration damping ring is used to dampen the radial vibration of the vibration core. There is a gap between the bearing and the vibration isolation sleeve of the second vibration damping module. When the spindle vibrates or the bearing wobbles, the bearing freely squeezes the damping column, converting the vibration energy into frictional damping inside the damping column for consumption. The first vibration damping module first absorbs the vibration energy at the top of the spindle through the vibration damping structure, and then absorbs the vibration energy again through the wing plate.

2. The chemical mechanical polishing drive assembly according to claim 1, characterized in that, The drive frame is formed as a rigid load-bearing frame structure. The vibration damping shaft system is fixedly installed on the drive frame and moves with it. The upper part of the drive unit is connected to the vibration damping shaft system, and the lower part is configured for the load-bearing head. The rotary joint is used to supply multiple gas channels to the load-bearing head during its rotational operation.

3. The chemical mechanical polishing drive assembly according to claim 1, characterized in that, The rotary joint is disposed on the upper part of the vibration damping shaft system and has an air passage for conveying gas, at least a portion of which is disposed inside the main shaft of the vibration damping shaft system.

4. The chemical mechanical polishing drive assembly according to claim 1, characterized in that, The wing has a hollow structure to improve its overall elastic modulus.

5. The chemical mechanical polishing drive assembly according to claim 4, characterized in that, The outer periphery of the wing plate is fixedly connected to the drive frame, and the main shaft of the vibration damping shaft system is suspended at its inner periphery by multiple vibration damping structures.

6. The chemical mechanical polishing drive assembly according to claim 1, characterized in that, The second vibration damping module is disposed on the outer periphery of the upper part of the vibration damping shaft system to absorb vibrations in the horizontal direction.

7. The chemical mechanical polishing drive assembly according to claim 6, characterized in that, The vibration isolation sleeve is equipped with multiple adjusting components for adjusting the preload between the bearing and the damping column.

8. The chemical mechanical polishing drive assembly according to claim 1, characterized in that, A gap is left between the vibrating core and the inner wall of the cavity; The elastic element abuts against the top or bottom surface of the cavity; A damping ring is also disposed between the end of the vibrating core and the elastic element. The outer periphery of the damping ring is in close contact with the inner wall of the cavity, and the gap is 0.5 to 2 times the radius of the damping ring.

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