US20090083953A1

US20090083953A1 - Clamp mechanism for a backing plate disposed in a pecvd chamber

Info

Publication number: US20090083953A1
Application number: US12/199,456
Authority: US
Inventors: Sam H. Kim; Beom Soo Park; William N. Sterling
Original assignee: Individual
Current assignee: Applied Materials Inc
Priority date: 2007-09-27
Filing date: 2008-08-27
Publication date: 2009-04-02
Also published as: CN201217693Y; CN201442977U

Abstract

Embodiments described herein relate to a clamp mechanism for a backing plate disposed in a PECVD chamber, comprising an upper clamp portion fixed to an interior portion of a chamber lid, and a lower clamp portion in sliding contact with the upper clamp portion and the backing plate, wherein the lower clamp portion comprises at least one fastener disposed through the lower clamp portion and the chamber lid, the at least one fastener comprising an adjustment member, and wherein rotation of the adjustment member causes lateral movement and compression to the backing plate.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit of U.S. Provisional Patent Application Ser. No. 60/975,691 (Attorney Docket No. 11816L), filed Sep. 27, 2007, which is herein incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention
Embodiments of the present invention generally relate to an apparatus used in the fabrication of electronic devices on large area substrates and related chamber hardware, and more specifically, to a clamp mechanism for a backing plate disposed in a plasma-enhanced chemical vapor deposition (PECVD) chamber used for deposition of material on large area substrates.
2. Description of the Related Art
Plasma Enhanced Chemical Vapor Deposition (PECVD) is a method for depositing a material onto a substrate by igniting process gases into a plasma state. Process gas may be provided to a gas distribution plate or showerhead disposed in an opposing relationship to a temperature controlled substrate support assembly or susceptor disposed within a sealable processing volume in a processing chamber. The susceptor supports a large area substrate and the process gases are disassociated above the substrate to deposit material forming electronic devices, such as thin film transistors (TFT's), organic light emitting diodes (OLED's), and photovoltaic cells used in solar cell fabrication. Negative pressure may be provided to the processing volume and the process gases are disassociated by thermal energy, vacuum, radio frequency (RF) power, and combinations thereof, to form a plasma and facilitate deposition of materials on the large area substrate.
An upper portion of the processing chamber includes the showerhead and associated hardware, such as a backing plate, that may be coupled to a lid assembly. The lid assembly generally includes a cover and process gas inlets, and the cover may support a remote plasma unit. The backing plate is typically disposed above the showerhead and may be configured to facilitate support of the showerhead. Generally, a perimeter of the backing plate is adapted to compress a seal between the perimeter of the backing plate and the lid assembly to form an upper boundary of the processing volume. The seal typically includes a polymer or an elastomeric material that provides a vacuum-tight interface and may additionally facilitate electrical insulation. Thermal forces and/or pressure cycling may cause the interface to loosen, which may cause the interface to be compromised.
Therefore, there is a need in the art for an improved assembly for ensuring vacuum sealing of the chamber.

SUMMARY OF THE INVENTION

Embodiments described herein relate to a clamp mechanism for a backing plate disposed in a PECVD chamber. Embodiments of the clamp mechanism provide and maintain a compressive force to the backing plate to facilitate vacuum sealing of the chamber. The clamp mechanism as described herein provides adjustment of the clamp mechanism from a location outside of the processing chamber without disassembly of the chamber.
In one embodiment, a clamp mechanism for a backing plate disposed in a plasma chamber is described. The clamp mechanism includes a first clamp portion fixed to an interior portion of a chamber lid assembly, the first clamp portion having at least one first slanted contact surface, and a second clamp portion having at least one second slanted contact surface in sliding contact with the first slanted contact surface and the backing plate. The second clamp portion includes at least one fastener disposed through the second clamp portion and the interior portion of the chamber lid assembly, the at least one fastener comprising an adjustment member exposed to an exterior of the plasma chamber and providing lateral movement of the second clamp portion relative to the backing plate and the first clamp portion by rotation of the adjustment member.
In another embodiment, a plasma chamber having a lid assembly including a backing plate, a cover spaced-apart from the backing plate by a wall, and a plurality of clamp mechanisms disposed in the area between the lid and the backing plate is described. Each of the plurality of clamp mechanisms include a first clamp portion fixed to an interior portion of the wall, the first clamp portion having at least one first slanted contact surface, and a second clamp portion having at least one second slanted contact surface in sliding contact with the first slanted contact surface and the backing plate. The second clamp portion includes at least one fastener disposed through the second clamp portion and the wall, the at least one fastener comprising an adjustment member exposed to an exterior of the plasma chamber and providing lateral movement of the second clamp portion relative to the backing plate and the first clamp portion by rotation of the adjustment member.
In another embodiment, a plasma chamber having a lid assembly including a backing plate, a cover spaced-apart from the backing plate by a wall, and a plurality of clamp mechanisms disposed in the area between the lid and the backing plate is described. Each of the plurality of clamp mechanisms include a first clamp portion fixed to an interior portion of the wall by a screw, the first clamp portion having at least one first slanted contact surface, and a second clamp portion having at least one second slanted contact surface in sliding contact with the first slanted contact surface and the backing plate, the second clamp portion comprising at least one fastener disposed through the second clamp portion and the wall, the at least one fastener comprising a nut exposed to an exterior of the plasma chamber and providing lateral movement of the second clamp portion relative to the backing plate and the first clamp portion by rotation of the nut.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.

FIG. 1 is a schematic cross-sectional view of one embodiment of a PECVD chamber.

FIG. 2 is an isometric view of on embodiment of a clamp device.

FIG. 3 is a cross-sectional view of the clamp device of FIG. 2.

FIG. 4 is a cross-sectional view of one embodiment of a nut.

To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements disclosed in one embodiment may be beneficially utilized on other embodiments without specific recitation.

DETAILED DESCRIPTION

FIG. 1 is a schematic cross-sectional view of one embodiment of a plasma enhanced chemical vapor deposition (PECVD) system 100. The system 100 may be a PECVD system available from AKT®, a division of Applied Materials, Inc., of Santa Clara, Calif. The system 100 includes a processing chamber 102 having walls 106, a backing plate 101, and a bottom 108 that partially define a process volume 112. The process volume 112 is typically accessed through a selectively sealable port (not shown) in the walls 106 that facilitates transfer of a large area substrate 140 into and out of the processing chamber 102. The substrate 140 may be made of glass, polymers, or other material capable of having electronic devices formed thereon and typically includes a surface area of 2.2 meters², or greater. The walls 106 and bottom 108 are typically fabricated from aluminum or other material compatible with processing.
A temperature controlled susceptor or substrate support assembly 138 is centrally disposed within the processing chamber 102. The substrate support assembly 138 supports the large area substrate 140 during processing. In one embodiment, the substrate support assembly 138 comprises an aluminum body 124 that includes an embedded heater 132. The heater 132, such as a resistive element, disposed in the support assembly 138, is coupled to a power source 174 that controllably heats the support assembly 138 and the large area substrate 140 positioned thereon to a predetermined temperature. Typically, in a CVD process, the heater 132 maintains the large area substrate 140 at a uniform temperature between about 150° to at least about 460° C., depending on the deposition processing parameters for the material being deposited.
The substrate support assembly 138 has a lower side 126 and a substrate receiving surface 134 and the substrate receiving surface 134 supports the large area substrate 140. The lower side 126 has a stem 142 which couples the substrate support assembly 138 to a lift system (not shown) that moves the substrate support assembly 138 between an elevated processing position (as shown) and a lowered position that facilitates substrate transfer to and from the processing chamber 102. The stem 142 additionally provides a conduit for electrical and thermocouple leads between the substrate support assembly 138 and other components of the chamber 102. A bellows 146 may be coupled between the substrate support assembly 138 (or the stem 142) and the bottom 108 of the processing chamber 102. The bellows 146 facilitates vacuum sealing around the stem 142 while facilitating vertical movement of the substrate support assembly 138.
The substrate support assembly 138 has a plurality of holes 128 disposed therethrough that accept a plurality of lift pins 150. The lift pins 150 are comprised of ceramic or anodized aluminum. When the substrate support assembly 138 is lowered, a lower portion of the lift pins 150 also lower to make contact with the chamber bottom 108. As the substrate support assembly 138 is lowered further, the upper portion of the lift pins 150 are elevated above the substrate receiving surface 134 to space the large area substrate 140 apart from the substrate receiving surface 134 to facilitate transfer of the large area substrate by allowing access to a robot blade or end effector.
In one embodiment, the substrate support assembly 138 is grounded such that radio frequency (RF) power, supplied by a power source 122, is provided to a showerhead 118 positioned between the lid assembly 110 and substrate support assembly 138, or other electrode positioned within or near the lid assembly of the chamber. The RF power excites gases present in the process volume 112 between the substrate support assembly 138 and the showerhead 118 to facilitate deposition on the substrate 140. The RF power from the power source 122 is generally selected commensurate with the size of the substrate to drive the chemical vapor deposition process.
The walls 106 support a lid assembly 110 that includes a lid cover 105 and a backing plate support member 111 facilitating support of the backing plate 101. The backing plate 101 is a substantially rigid plate and may support the showerhead 118. The backing plate 101 also includes an inlet 180 coupled to a gas source 104 by a conduit 121 through which process gases, provided by the gas source 104, are introduced into the processing chamber 102. The inlet 180 is also coupled to a cleaning source 182, which may comprise a remote plasma unit disposed on and coupled to the lid cover 105 by brackets 181. The cleaning source 182 typically provides a cleaning agent, such as a molecular or disassociated fluorine containing gas that is introduced into the processing chamber 102 to remove deposition by-products and films from processing chamber hardware including the showerhead 118. The process gases and/or cleaning agent may flow from the inlet 180 to the process volume 112 through a plurality of gas passages 162 formed in the showerhead 118. A vacuum pump 119 is coupled to the chamber 102 to facilitate negative pressure in the process volume 112 and exhaust of process gases from the process volume 112.
The showerhead 118 is typically fabricated from stainless steel, aluminum (Al), anodized aluminum, nickel (Ni) or other conductive material. The showerhead 118 may be supported in a center region by a plurality of support members 123. Each of the plurality of support members 123 may be adjustable fasteners coupled to the backing plate 101 or through the backing plate 101 and coupled with a support assembly outside of the chamber 102 (not shown).
The showerhead 118 typically may also include a flexible support 160 adapted to allow for thermal expansion and contraction of the perimeter of the showerhead 118. The plurality of gas passages 162 are formed through the showerhead 118 to allow a predetermined distribution of gas passing through the showerhead 118 and into the process volume 112. The flexible support 160 also maintains the showerhead 118 in a spaced-apart relation relative to the substrate support assembly 138.
In one embodiment, the backing plate support member 111 is adapted to couple to the walls 106 by one or more fasteners 113. The backing plate support member 111 is compressed against a seal 148 by compression provided by one or both of the fasteners 113 and the sheer weight of the lid assembly 110. The lid assembly 110, which includes the backing plate 101, provides an upper boundary to the process volume 112 by sealing against the walls 106. In particular, an interior surface 120 of the backing plate 101, at least between a seal 114, provides the upper boundary to the process volume 112.
The lid assembly 110 also includes a plurality of clamp devices 117 disposed between the lid cover 105 and the backing plate 101. Specifically, the clamp devices 117 provide compression to the upper surface of the backing plate 101, which compresses the backing plate 101 against the seal 114 to provide an upper boundary for negative pressure within the chamber 102. While the seal 114 is described as a pressure sealing member, the seal 114 may also be adapted as an electrical insulator to insulate the backing plate 101 from the backing plate support member 111 and/or other portions of the chamber 102. In one embodiment, the seal 114 is a polymer or elastomer that provides a vacuum seal and additionally facilitates electrical insulation.
FIG. 2 is an isometric view of on embodiment of a clamp device 117. In this embodiment, the clamp device 117 comprises an upper clamp portion 204 and a lower clamp portion 208. The upper clamp portion 204 includes an opening 206 which allows a first fastener 212, such as a screw, to couple the upper clamp portion 204 to the interior portion of a lid support 103 which is a portion of the lid assembly 110. Alternatively, the upper clamp portion 204 may be attached to the lid support by adhesives or welding. In another alternative, the upper clamp portion 204 may be an integral part of the lid support 103, such as an extension or protrusion disposed on the lid support 103. The upper clamp portion 204 is adapted to be stationary or immovable relative to the lid support 103 while the lower clamp portion 208 is movable relative to the lid support 103. In one embodiment, one or both of the lower clamp portion 208 and the upper clamp portion 204 may be made of polyetheretherketone (PEEK) material.
In one embodiment, the lower clamp portion 208 and the upper clamp portion 204 are configured as wedges to provide a compressive force to the backing plate 101. For example, the lower clamp portion 208 and the upper clamp portion 204 include at least one slanted or inclined surface adapted to provide a movable interface therebetween. As the slanted surfaces move relative to each other in at least one direction of travel, compression is applied to the backing plate 101. In one embodiment, the lower clamp portion 208 includes a first slanted surface 218A and the upper clamp portion 204 includes a second slanted surface 218B. The slanted surfaces 218A, 218B of both the upper clamp 204 and lower clamp 208 are adapted to be in sliding contact to vary compressive force to the backing plate 101 as the clamp device 117 is adjusted.
FIG. 3 is a cross-sectional view of the clamp device 117 of FIG. 2. The upper clamp portion 204 is configured to be immovable relative to the lid support 103. The upper clamp portion 204 is statically coupled to the lid support 103 by a first fastener 212, such as a screw, inserted through a hole in the upper clamp portion 204 and tightened against a threaded portion formed in the lid support 103. While only one fastener is used, it is understood that more than one fastener may be used to couple the upper clamp portion 204 with the lid support 103. A second fastener 214, such as a bolt, may be inserted through aligned openings in the both of the lower clamp portion 208 and the lid support 103. A rotatable coupling, such as a nut 216 may be rotated onto an exposed threaded portion of the second fastener 214. The nut 216 is exposed on the exterior side of the lid support 103 and can be accessed by personnel from the exterior of the chamber 102 without having to disassemble the entire lid assembly 110, or at the minimum, remove the lid cover 105 and any hardware disposed therethrough or thereon.
In one embodiment, the second fastener 214 may be made of stainless steel having a head 320 and a threaded end 330 opposite the head 320. In another embodiment, the second fastener 214 may be an Allen head bolt having a rotation prevention feature 325 in or on the head 320 to minimize rotation of the second fastener 214 during adjustment of the nut 216. The rotation prevention feature 325 may be a flattened portion, a stop or a pin or protrusion adjacent the head 320. In other embodiments, the rotation prevention feature may be a slot adapted to receive a screw driving device, a flattened portion disposed on an outer surface of the head 320 adapted to mate with a corresponding flat or stop disposed in the lower clamp portion 208, an orifice or depression adapted to receive a tool, or other feature adapted to minimize rotation of the Allen head bolt during adjustment of the nut 216. In yet another embodiment, the second fastener 214 may include a hex head bolt, a square head bolt, or other fastener. In one embodiment, the nut 216 may be made of stainless steel with anti-bind/gall coating. In yet another embodiment, the anti-bind/gall coating may be made of tungsten disulfide (WS₂) or dicronite.
Continued rotation or tightening of the nut 216 causes lateral movement of the lower clamp portion 208 relative to the upper clamp portion 204 and along a periphery of the backing plate 101. The lower clamp portion 208 includes a sliding contact surface 305 adapted to contact an upper surface 310 of the backing plate 101. As the lateral movement of the lower clamp portion 208 continues, contact between the first slanted surface 218A and the second slanted surface 218B is established. When the first slanted surface 218A and the second slanted surface 218B contact each other, the lower clamp portion 208 provides a compressive force to the upper surface 310 of the backing plate 101. Thus, the compressive pressure provided to the upper surface 310 of the backing plate 101 translates to compress a sealing surface 315 of the backing plate 101 against the seal 114. In one embodiment, all sliding contact surfaces may include a radius or bevel to minimize one surface gouging or cutting into another as the surfaces move relative to each other.
FIG. 4 is a cross-sectional view of one embodiment of a nut 216. The nut 216 includes a body 400 having threads 405 formed therethrough. In one embodiment, the body is made of stainless steel. In another embodiment, the body 400 may be made of stainless steel and the threads 405 may include a coating 410 configured to minimize binding with the second fastener 214. In one embodiment, the coating 410 may be made of tungsten disulfide (WS₂) or dicronite.
While the weight of the backing plate 101 facilitates contact with the seal 114 and may facilitate a vacuum-tight interface by compression on the seal 114, the clamp devices 117 provide additional compression of the sealing surfaces of the backing plate 101 against the seal 114. In one embodiment, the clamp devices 117 are adapted to form an initial vacuum seal between the backing plate 101 and the sealing surfaces. For example, the weight of the backing plate 101 may provide insufficient contact between the sealing surfaces of the backing plate 101 and the seal 114. In this example, the clamp devices 117 may provide compression and facilitate vacuum sealing with assistance from the vacuum pump 119, which pulls the interior surface of the backing plate 101 toward the process volume 112 providing additional compressive force to the backing plate 101 against the seal 114. Once the initial vacuum seal has been established, the clamp devices 117 may remain in compressive contact with the backing plate 101 or even loosen to not be in compressive contact with the backing plate 101, but the initial vacuum seal may remain intact to withstand many vacuum applications or processing cycles without readjustment of the clamp devices 117.
However, over time, the initial seal may be breached and the clamp devices 117 may need to be readjusted to restore the vacuum seal. For example, cyclic vacuum application and/or expansive/contractive movement of the backing plate 101 relative to the seal 114 may cause failure of the vacuum seal. Additionally, a user may choose to periodically adjust the clamp devices 117 to ensure compressive contact between the clamp devices 117 and the backing plate 101 thereby maintaining the compressive contact and the vacuum seal.
In another scenario, the lid assembly 110 may be periodically separated from the walls 106 to access and/or service interior portions of the processing chamber 102. The lid assembly 110 may be removed from the walls 106 by lifting the lid assembly 110 with a crane or other lifting device adapted to separate the lid assembly 110 from the walls 106. As the lid assembly 110 is removed or separated from the walls 106, components such as the showerhead 118 may be accessed by personnel for cleaning, inspection or other maintenance procedure. Additionally, in this separated state, the lid cover 105 may be removed to access the backing plate 101 for maintenance, inspection or other servicing procedure.
After maintenance procedures have been performed, the lid assembly 110 may be reassembled or otherwise readied for re-attachment to the walls 106. As the lid assembly 110 is reassembled, the upper clamp portions 204 of each of the clamp devices 117 are statically coupled to an interior portion of the lid assembly 110 by the first fastener 212 and the lower clamp 208 of each of the clamp devices 117 are positioned adjacent the upper clamp 204 and the upper surface of the backing plate 101. The second fastener 214 is inserted through the lower clamp portion 208 and lid support 103 and is tightened by the nut 216 to be snug. The lid assembly 110 may be lifted by a crane and is positioned against the walls 106. The lid assembly 110 is then secured to the walls 106 by fasteners 113. After the lid assembly 110 is suitably coupled to the walls 106, personnel may tighten each of the nuts 216 of each of the clamp devices 117 to compress the backing plate 101 against the seal 114. The nuts 216 are exposed on the exterior side of the lid support 103 and can be accessed from the exterior without having to disassemble the entire lid assembly 110, or at the minimum, remove the lid cover 105 (and any hardware disposed therethrough or thereon). Thereafter, negative pressure may be applied to the process volume 112 to perform a deposition process.
As mentioned above, during multiple deposition processes, the backing plate 101 may move relative to the seal 114, which may breach the vacuum seal. Other vacuum breaches may occur due to loosening of the clamp devices 117 and/or backing plate 101 by causes or forces such as vibration, disassembly of portions of the chamber 102, among other causes or forces. In one scenario, one or more of the previously tight clamp devices 117 may be loosened due to vacuum application to the backing plate 101, which may further bind the backing plate against the seal 114 to a point where one or more of the clamp devices 117 may not be in compressive contact with the backing plate 101. The structure and location of the clamp devices 117 as described herein provide a suitable remedy to tighten the backing plate 101 against the seal 114.
In any of the situations discussed above, and other situations requiring tightening of or providing additional compression to the backing plate 101, the clamp devices 117 allow adjustment by personnel from a location outside of the chamber 102. Additionally, disassembly of portions of the chamber 102 is not required to access the clamp devices 117 for adjustment. Thus, downtime may be minimized and throughput of the chamber 102 may be enhanced.
While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.

Claims

1. A clamp mechanism for a backing plate disposed in a plasma chamber, comprising:

a first clamp portion fixed to an interior portion of a chamber lid assembly, the first clamp portion having at least one first slanted contact surface; and

a second clamp portion having at least one second slanted contact surface in sliding contact with the first slanted contact surface and the backing plate, the second clamp portion comprising:

at least one fastener disposed through the second clamp portion and the interior portion of the chamber lid assembly, the at least one fastener comprising an adjustment member exposed to an exterior of the plasma chamber and providing lateral movement of the second clamp portion relative to the backing plate and the first clamp portion by rotation of the adjustment member.

2. The clamp mechanism of claim 1, wherein the first clamp portion and the second clamp portion are made of a polyetheretherketone (PEEK) material.

3. The clamp mechanism of claim 1, wherein the first clamp portion is fixed to the interior portion of the chamber lid assembly by a screw.

4. The clamp mechanism of claim 1, wherein the at least one fastener is a bolt.

5. The clamp mechanism of claim 4, wherein the bolt is an Allen head bolt.

6. The clamp mechanism of claim 4, wherein the bolt is a hex head bolt.

7. The clamp mechanism of claim 4, wherein the bolt includes a rotation prevention feature.

8. The clamp mechanism of claim 1, wherein the adjustment member is a nut.

9. The clamp mechanism of claim 8, wherein the nut is made of stainless steel and includes an anti-bind or anti-gall coating.

10. The clamp mechanism of claim 9, wherein the coating is made of tungsten disulfide or dicronite.

11. A plasma chamber having a lid assembly including a backing plate, a cover spaced-apart from the backing plate by a wall, and a plurality of clamp mechanisms disposed in the area between the lid and the backing plate, each of the plurality of clamp mechanisms comprising:

a first clamp portion fixed to an interior portion of the wall, the first clamp portion having at least one first slanted contact surface; and

at least one fastener disposed through the second clamp portion and the wall, the at least one fastener comprising an adjustment member exposed to an exterior of the plasma chamber and providing lateral movement of the second clamp portion relative to the backing plate and the first clamp portion by rotation of the adjustment member.

12. The clamp mechanism of claim 11, wherein the first clamp portion is fixed to the wall by a screw.

13. The clamp mechanism of claim 11, wherein the at least one fastener is a bolt.

14. The clamp mechanism of claim 13, wherein the bolt is an Allen head bolt.

15. The clamp mechanism of claim 13, wherein the bolt is a hex head bolt.

16. The clamp mechanism of claim 13, wherein the bolt includes a rotation prevention feature.

17. The clamp mechanism of claim 11, wherein the adjustment member is a nut.

18. The clamp mechanism of claim 17, wherein the nut is made of stainless steel and includes an anti-bind or anti-gall coating.

19. The clamp mechanism of claim 18, wherein the coating is made of tungsten disulfide or dicronite.

20. A plasma chamber having a lid assembly including a backing plate, a cover spaced-apart from the backing plate by a wall, and a plurality of clamp mechanisms disposed in the area between the lid and the backing plate, each of the plurality of clamp mechanisms comprising:

a first clamp portion fixed to an interior portion of the wall by a screw, the first clamp portion having at least one first slanted contact surface; and

at least one fastener disposed through the second clamp portion and the wall, the at least one fastener comprising a nut exposed to an exterior of the plasma chamber and providing lateral movement of the second clamp portion relative to the backing plate and the first clamp portion by rotation of the nut.

21. The clamp mechanism of claim 20, wherein the at least one fastener is a bolt.

22. The clamp mechanism of claim 21, wherein the bolt is an Allen head bolt.

23. The clamp mechanism of claim 21, wherein the bolt is a hex head bolt.

24. The clamp mechanism of claim 21, wherein the bolt includes a rotation prevention feature.

25. The clamp mechanism of claim 20, wherein the nut is made of stainless steel and includes an anti-bind or anti-gall coating.

26. The clamp mechanism of claim 25, wherein the coating is made of tungsten disulfide or dicronite.