US3662708A

US3662708A - Apparatus for supporting a substrate holder

Info

Publication number: US3662708A
Application number: US21837A
Authority: US
Inventors: Robert Leland Shrader
Original assignee: Airco Inc
Current assignee: Airco Inc
Priority date: 1970-03-23
Filing date: 1970-03-23
Publication date: 1972-05-16
Anticipated expiration: 1989-05-16

Abstract

Apparatus is described for supporting a substrate holder in a vacuum coating system. The substrate holder is supported and, if desired, rotated within the vacuum system by an electromagnetically energized armature.

Description

United States Patent Shrader [54] APPARATUS FOR SUPPORTING A SUBSTRATE HOLDER [72] lnventor: Robert Leland Shrader, Castro Valley,

Calif.

[73] Assignee: Airco, Inc., New York, NY.

[22] Filed: Mar. 23, 1970 21 App1.No.: 21,837

[52] 11.8. CI ..118/49, 118/500, 269/8 [51] Int. Cl ..C23c 13/08 [58] Field of Search ..1 18/48-50. 1, 500,

118/503, 620; 294/655; 279/1 M; 269/8; 204/297 M; 214/1 NQ; 198/1 NO; 134/] NO; 317/1 NQ;

[56] References Cited UNITED STATES PATENTS 2,463,906 3/1949 Pride ..118/49 X [451 May 16, 1972 2,746,420 5/1956 Steigerwald ..1 18/49.1 X 2,819,905 1/1958 Stead .279/l M 3,206,322 9/1965 Morgan. 1 18/49 X 3,228,794 1/1966 Ames..... 118/49 X 3,381,947 5/1968 Beggs 118/50 X 3,396,696 8/1968 Becker ..1 18/49 3,469,560 9/1969 Bukkila et a1. ...1 18/49 X 3,568,632 3/1971 Cawthon ..118/49 Primary Examiner-Morris Kaplan Atlarney-Fitch, Even, Tabin & Luedeka ABSTRACT Apparatus is described for supporting a substrate holder in a vacuum coating system. The substrate holder is supported and, if desired, rotated within the vacuum system by an electromagnetically energized armature.

6 Claims, 3 Drawing Figures e I fi 67 89 PATENTEDMAY 16 m2 SHEET 2 OF 2 APPARATUS FOR SUPPORTING A SUBSTRATE HOLDER This invention relates to vacuum coating systems and, more particularly, to apparatus for supporting a substrate holder in a vacuum coating system.

The coating of substrates within a high vacuum has been recognized for some time as being a highly advantageous one in many applications. For example, vacuum coating may be used to provide non-metallic coatings for the control of optical properties of lenses, zinc or aluminum coated papers and plastics for electrical capacitors and decorative applications, refined high pressure metals, the production of metallic foils, and various other applications where high purity or unusual alloy compositions are desirable.

A typical vacuum coating system includes a vacuum tight enclosure in which a vapor source is positioned. The vapor source may comprise a crucible or boat in which molten metal is contained and wherein the molten metal is heated by a resistance heated filament. Where longer production runs, high evaporation rates, or higher purity is desired, electron beam evaporation may be employed. Typically, in electron beam evaporation, the evaporant is contained within a cooled crucible so that the material to be evaporated is insulated from the crucible by a solid skull of its own composition. The material in the crucible is heated by directing an electron'beam or a plurality of electron beams at the surface of the material.

ln addition to the vapor source, the substrate to be coated is also positioned within the vacuum enclosure. Frequently it is desirable to rotate or otherwise move the substrate with respect to the vapor in order to ensure uniformity of deposition over the entire surface of the substrate, or with respect to other adjacent substrates. ln many cases, the substrate or several substrates may be suitably mounted to a device for supporting and moving the substrate or substrates in the vapor flow prior to the time the vacuum enclosure is evacuated. Such arrangements, most often comprising the well known bell jar type of vacuum enclosure, are usually unsatisfactory for high production rates. This is because the vacuum enclosure must be pumped down for each new substrate or batch of substrates which are to be coated.

Vacuum coating systems designed for higher production rates typically employ one or more vacuum locks through which ingress or egress to the main coating chamber may be obtained without losing the vacuum conditions therein. In such systems, the substrate or substrates are secured by clamps or other suitable means in a substrate holder, which is a frame or similar type of fixture. The substrate holder with the substrate or substrates attached thereto is then moved by suitable mechanical means through the vacuum lock and into the coating chamber. After the coating operation is completed, the substrate holder with the substrate or substrates attached thereto is then removed from the coating chamber through the same lock or through another separate vacuum lock.

In order to support the substrate holder within the coating chamber in the proper position for coating the substrate, vacuum coating systems of the general type described employ various types of devices. Prior art devices for supporting a substrate holder in a vacuum coating system include mechanical mechanisms such as toggles, latches, sprockets, and gears for the purpose of supporting the substrate holder and, if necessary, rotating the substrate holder. Where a large number of moving parts are utilized within the vacuum enclosure itself, complications arise in providing the necessary motive power for such parts and in providing lubrication for their proper functioning. If the sources of power are to be outside of the vacuum enclosure, relatively complicated sealing arrangements may be necessary in relation to the elements which transmit the power into the vacuum enclosure. If the power sources are to be contained within the vacuum enclosure itself, a larger volume is required necessitating correspondingly greater pumping capacity and therefore greater operating costs, and more expensive hermetically sealed power sources are required. 1

It is an object of the present invention to provide improved apparatus for supporting a substrate in a vacuum coating system.

Another object of the invention is to provide apparatus for supporting a substrate holder in a vacuum coating system which employs a minimum number of moving parts within the vacuum. i

It is another object of the present invention to provide apparatus for supporting a substrate holder in a vacuum coating system which occupies a minimum volume within the vacuum enclosure.

Other objects of the invention will become apparent to those skilled in the art from the following description, taken in connection with the accompanying drawings wherein:

FIG. 1 is a schematic sectional view of a vacuum coating system employing apparatus constructed in accordance with the invention;

FIG. 2 is an enlarged top plan view of the apparatus of the invention; and

FIG. 3 is a further enlarged sectional viewtaken along the line 3-3 of FIG. 2, with some parts removed for clarity.

Very generally, the apparatus of theinvention includes an electromagnetic coil 11 and means 12 for supporting the electromagnetic coil within the vacuum coating system. An armature l3 is provided for engaging a ferromagnetic part of the substrate holder and the armature is supported adjacent the coil so that the armature is magnetized upon energization of the coil for holding the ferromagnetic part of the substrate holder and therebysupporting the substrate holder. Where the substrate holder is to be rotated,.the armature 13 is supported by a shaft 14 which is rotated by means 16. Where the nature of the substrate holder requires, the coil supporting means 12 include means 17 for moving the coiland the armature axially to engage and disengage the ferromagnetic part of the substrate holder.

Referring now more particularly to FIG. 1, the vacuum coating system illustrated schematically therein includes a vacuum tight enclosure 21 defining a coatingchamber 23 in which the vacuum coating operation takes place. The chamber23 is evacuated through a duct 25 in-the lower wall of the enclosure21 by a suitable vacuumpump An opening 29 is provided in the top wall. of theenclosure 2 l.for accommodating the apparatus of the invention, explained more fully below. I

The vapor source in the illustrated embodiment comprises a water cooled copper crucible 31 having a plurality of coolant passages 32 therein through which coolant is circulated by suitablemeans, not shown. As a result of the cooling, a skull 35 of solidified evaporant material forms between the cooled crucible 31 and a molten pool 37 from which the vapor for coating is produced. The molten pool is heated by an electron beam 39 produced by an electron beam gun4l. The electron beam gun may be of the type shown in US. Pat. No. 3,177,535 assigned to the assignee of the-present invention-Means, not shown, are provided for establishing transverse magnetic fields in order to deflect the beam 39 through the curving path illustrated to impinge upon the top of the molten pool 37. By positioning the electron beam gun 41 beneath the crucible, the gun is less susceptible to impingement of vapor particles thereon which would deleteriously affect its operation.

Ingress and egress to and from the chamber ;23 are provided by a pair of

vacuum valves

43 and 45. The vacuum valve 43 includes a pair of

walls

47 and 49 forming a valve chamber 51.

Openings

53 and 55 are provided in the

walls

47 and ,49, respectively, thereby forming a port through which objects may pass into the chamber 23. A valve plate 57 is movable in the chamber 51 in a sealed relationship with the

walls

47 and 49 by means of a pneumatic actuator 59. having an extensible actuator rod 61 attached to the valve plate 57. The .valve plate 57 is shown in its retracted position thereby maintaining the valve 43 in the open condition. The valve 45 is identical to the valve 43 and is illustrated in the closed condition.

In order to pass objects through the vacuum valve 43 into the chamber 23 without having to bring the chamber 23 up to atmospheric pressure, a vacuum lock 63 is provided. Similarly, in order to remove objects from the chamber 23 without having to bring the chamber 23 up to atmospheric pressure, a vacuum lock 65 is provided. The vacuum lock 63 communicates with the chamber 23 through the valve 43 and the vacuum lock 65 communicates with the chamber 23 through the valve 45.

The vacuum lock 63 includes an evacuated enclosure 67 which is evacuated through a port 69 in the lower wall of the chamber 67 by a suitable vacuum pump 71. The vacuum lock 65 includes a vacuum tight enclosure 73 which is evacuated through a port 75 in the lower wall thereof by a vacuum pump 77. A vacuum valve 79 identical with the vacuum valve 43 is disposed at the opposite end of the vacuum lock 63 from the valve 43. 'A similar vacuum valve 81 closes the end of the vacuum lock 65 opposite the vacuum valve 45. Thus, the

locks

63 and 65 may be opened to the atmosphere without loss of vacuum in the chamber 23 provided that the

valve

43 and 45 are closed at appropriate times. Flow of materials into and out of the chamber 23 may be on a straight line basis for production efficiency and the use of the vacuum locks enables loading and unloading of substrates while at the same time other substrates are being coated in the chamber 23.

In order to support a substrate or substrates for coating, one or more substrate holders 83 are provided. One of the substrate holders 83 is illustrated and includes a frame 85 which is supported on a cart 86 having a plurality of wheels 87. The wheels 87 are guided on a track 89 which extends through the vacuum lock 63, the chamber 23, and the vacuum lock 65 to facilitate the straight line throughput arrangement for moving the substrates through the vacuum system. The substrates 91 are supported underneath the frame 85 by suitable clamps 93. The substrate holder 83 is at least partially ferromagnetic for reasons which will be explained below. In the particular design of the substrate holder illustrated in FIG. 1, a ferromagnetic plate 95 is secured to the top of the frame 85 and constitutes the ferromagnetic part of the substrate holder 83,

When the cart 86 is moved into the chamber 23 with a substrate holder 83 supported on the cart, the apparatus of the invention is employed to support the substrate holder 83 within thechamber 23 independently of the cart 86. In this manner, the cart may be removed from the chamber 23 prior to beginning the vacuum coating operation.

Referring now additionally to FIGS. 2 and 3, the apparatus of the invention is mounted to the top of the vacuum enclosure 21 by means of a base flange 101. The base flange is secured to the top wall of the enclosure 21 by a plurality of cap screws 103. An annular seal 105 is disposed in a suitable recess in the lower surface of the base flange 101 to provide a vacuum tight seal between the base flange and the top of the vacuum enclosure 21. The base flange 101 is provided with a central opening 107 and an annular rim 109 projects upwardly from the top surface of the base flange 101 about one-third of the way from the opening 107 to the periphery of the base flange. The base flange 101 and other elements described below constitute the means 12 which support the coil 11 within the vacuum coating system and, more particularly, within the vacuum chamber 23.

The coil supporting means 12 additionally include a tube or hollow cylinder 1 11 which comprises a cam body as explained below. The cylinder 111 is supported for rotation by an annular bearing 113 which is seated in the comer between the projection 109 and the top surface of the base flange 101. Three roller support brackets 115 extend vertically upward from the top surface of the base flange 101 near the periphery thereof. I

disposed, respectively, in three cam slots 131 in the cylinder or cam body 111. The cam slots 131 thus provide cam surfaces which the rollers 127 may follow upon rotational displacement of the cylinder 111, as will be explained below. In order to maintain spacing between the roller mounting plate and the cylinder 111, three rollers 133 are provided at 120 intervals around the outside of theplate 125. The roller 133 is shown, in FIG. 3, outof position. The rollers 133 are mounted to the plate by axles 135 which depend vertically downward from the plate 125 so that the rollers revolve around a vertical axis. l

The roller mounting plate 125 includes a central opening 136 therein for reasons explained below. An adapter flange 138 is mounted to the top of the roller mounting plate 125 by suitable means, not shown. An annular seal 144 is provided between the two elements. A transition flange 140 is mounted to the underside of the adapter flange 138, in the opening 136 of the plate 125, by a plurality of cap screws 142. I

The coil 11 is supported from the transition flange 140 by three support rods 137 which depend vertically from the flange 140. At their upper ends, the support rods 137, positioned at 120 intervals around the axis of the flange, are secured to the flange by cap screws 139 which pass through the flange. At their lower ends the rods 137 are suitably secured to the top of an annular coil block 141. The coil block 141 is weldedto the support surface of a coil housing 149, which is a ferromagnetic material, such as a suitable stainless steel. The coil housing 149 contains an annular recess 151 therein in which the electromagnetic coil 11 is disposed. Suitable electrical connection is made to the coil 11 through a conduit extending through the wall of the coil housing 149. A separator ring 150 is disposed immediately adjacent the coil 11 and is secured to the coil housing 149 to close the recess 151 containing the coil and to hermetically seal the coil in the recess. A tube 153 communicates with the recess 151 to enable testing the seal, and is pinched off once the integrity of the seal is established. I

The coil housing 149 is provided with a recess 157 therein and the armature 13 is disposed in the recess. The armature 13 consists of a center disc 158 and a surrounding sleeve 160 radially spaced therefrom. A ring 161, of non-magnetic material maintains the spacing between the disc 158 and the sleeve 160. The disc 158 is secured to the lower end of the shaft 14 by means of a cap screw 159. The armature 13 is spaced from the coil housing 149 and is able to rotate with respect thereto as will be explained. The sleeve 160 and the ring 161 rotate with disc 158. The end of the sleeve 160 opposite the ring 161 is spaced from both the inner wall of the coil housing 149 and the separator ring 150. When the coil 11 is energized, the sleeve 160, positioned adjacent the outer perimeter of the coil, becomes one polarity while the disc 158, positioned adjacent the inner perimeter of the coil becomes the other polarity. The non-magnetic ring 161 maintains a gap between the poles of the armature, i.e. the disc 158 and the sleeve 160. The armature may be assembled first and machined subsequently to provide exactness in dimensions.

-In order that the shaft 14 and the armature 13 attached thereto may rotate with respect to the'coil 11 and the coil housing 149 with constant clearance, the lower end of the shaft 14 is provided with a section 167 of reduced diameter which is journalled in a bearing 169 supported in an opening 171 centrally of the coil housing. A retaining ring 173 secures the bearing 169 against a shoulder 175 in the opening 171 of the coil housing 149. A bushing 177 maintains spacing between the armature 13 and the housing 149 in the axial direction. A disc 179 surrounds the shaft 14 just above the housing 149 to prevent foreign objects from falling into the v bearing. The bearing 169 maintains the armature 13 with respect to the coil housing so that very small air gaps may be used for maximum magnetic forces.

Since the assembly of the coil 11 and the armature 13 with their associated elements which are part of the support assembly 123 are disposed within the coating chamber 23, excessive heating of these elements may occur due to the high heat within the chamber. To prevent damage to the coil, the coil housing is provided with a coolant channel 181 of annular configuration in the top surface of the housing surrounding the opening 171. The channel is closed by the annular block which is brazed over the channel, and a suitable coolant, such as water, is circulated in the channel to conduct heat out of the housing 149 and thereby cool the coil and associated elements.

In order to supply water to the channel 181 and conduct water therefrom, coolant conduits are provided. Only one conduit is illustrated and may be either the inlet or outlet conduit for the coolant. The conduit is comprised of a passage 183 in the annular block 141 which communicates through a suitable coupling 185 with a tube 187. The upper end of the tube 187 passes through a slot 189 in the transition flange 140 and communicates with a passage system 191 in the adapter flange 138. The passage system 191 passes out of the top of the flange 140 and a suitable fluid connection 197 is provided for connecting the passage 191 to a coolant supply system, not

shown.

Returning for a moment to FIG. 1, it may be possible to locate the level of the armature 13 such that, when the cart 86 is moved into the chamber 23, the contact plate 95 is positioned sufficiently close to the armature that energization of the coil will cause the plate to be securely held to the armature. In the apparatus of the invention, however, provision is made for moving the armature and the coil axially to provide additional clearance. This permits the substrate holder to be easily moved into position, or allows for raising the substrate holder off of the cart 86 in order that the cart may be easily removed.

Returning to the other figures of the drawings, axial movement of the armature 13 and the coil 11 is effected by an air or pneumatic cylinder 201. The cylinder is suitably mounted by a lug 203 and cap screw 205 to the upper surface of the vacuum chamber 21. The piston rod 207 of the cylinder 201 is pivotally attached by a suitable connection 209 to a bracket 211 which projects radially outward from the cylinder or cam body 111. When actuation of the cylinder 201 causes the piston 207 to retract from the position shown in solid lines in FIG. 2 to the position shown in dotted lines, the cylinder or cam body 111 is rotated counterclockwise a corresponding distance. This causes the rollers 127 to follow the cam surface of the slot 131 and thereby cause the roller plate 125 to move vertically upward. This causes the coil 11 and the armature 13 to move vertically upward with the support assembly 123 a distance according to the displacement of the rollers 127. When the coil and armature are to be returned to the position illustrated in the drawings, the pressure in the cylinder 201 is relieved to allow the vacuum in the chamber to pull the mechanism down, returning the cylinder or cam body 111 to its original position.

In order to maintain a vacuum seal between the roller plate 125 and the base flange 101 for all the vertically displaced positions of the support assembly, a cylindrical bellows 213 is provided joined at its upper end to the plate 125 at the opening 136 therein and joined at its lower end to the base flange 101 at the opening 107 therein. Thus, the plate 125 is able to move vertically with respect to the base flange 101 while maintaining integrity of the vacuum sealing between the two elements. The bellows also keeps the flange 125 from rotating when the cam body is rotated.

During many vacuum coating operations, it is desirable to rotate or otherwise move the substrates being coated within the vapor flow in order to assure uniformity of deposit. This is because the amount of vapor and hence the amount of condensation of the vapor on the substrates in differentregions'of the chamber may vary. Pseudo random exposure of the substrates to all regions of the vapor will aid in achieving uniform coating thicknesses and quality. In order to provide for rotation of the substrate holder attached and held by the armature 13, the shaft 14 which depends from and is supported by the support assembly 123 is rotated. The shaft 14 passes upwardly through a central opening 215 in the transition flange 140 and is secured to the lower end of a drive pin 217 by a spring pin 219. The drive pin 217 is joumalled by a pair of

bearings

221 and 223 mounted in a bearing support collar 225. The bearing support collar is itself supported in a top sealing plate 227 which is mounted over a central opening 229 in the adapter flange 138 by cap screws 231. A vacuum seal is provided between the plate 227 and flange 138 by an annular seal 233. The opening 229 accommodates the lower end of the collar 225 and accommodates the bearing 223.

By observing the structure thus far described, it may be seen that the system of

seals

105, 213, 144 and 233 effectively seal off the evacuated interior of the chamber 23 except for the region at the

bearings

221 and 223. To complete the seal, a sealed rotary drive is provided for the drive 217. The sealed rotary drive includes a housing 235 which is secured over and welded to the top of the plate 227, surrounding a slightly raised boss 237 thereon. The sealed rotary drive may be of any convenient design known in the art and includes a pair of flanges 239 at the top thereof. The drive pin 217 is rotated by a suitable bellows arrangement, not shown, in the interior of the housing 235 and a shaft 240 projects upwardly above the flanges 239. A pulley 241 is suitably secured to the upper end of the shaft 240. The pulley 241, which provides rotational drive for the drive pin 217, is coupled through a drive belt 247 to a drive pulley 249. The drive pulley 249 is mounted to a vertical shaft 255 driven, through a suitable gear drive 253, by a motor 257. The motor 257 has a mounting plate 259 which is bolted to a mounting bracket 263. The mounting bracket 263 is itself mounted to the upper one of the flanges 239. Thus, torque for rotating the shaft 14 is supplied through the drive pin 217 and the

pulleys

241 and 249 from the motor 257. Once the substrate holder is attached or held by the armature 13, it may be rotated by energizing the motor 257 to thereby rotate the armature with respect to the coil 11.

In operating the apparatus of the invention, the substrate holder is first brought into the coating chamber on the cart 86. The assembly of the coil 11 and armature 13 is in the raised position by appropriately actuating the air cylinder 201 as previously explained. Once the substrate holder is centered in the vacuum chamber 23, with the ferromagnetic plate directly underneath the armature 13, the pressure in the air cylinder 201 is relieved to allow the cylinder or cam body 111 to rotate clockwise and the armature to be pulled down by the vacuum into position immediately adjacent the plate 95. The electromagnetic coil 11 is then energized and the substrate holder is picked up. In order to provide ample clearance, the assembly of the coil 11 and armature 13 with the substrate holder attached thereto is raised again by means of the air cylinder 201 and the cart 86 is removed from the vacuum chamber 23. The motor 257 is then energized and rotary motion is transmitted to the shaft 14 for rotating the armature 13 with respect to the coil 11.

By providing for axial movement of the coil 11 and annature 13 by the means previously described, the problem of providing adequate sealing of the vacuum chamber is simplified. The use of electromagnetic power for holding the substrate holder further reduces the required number of moving parts, since mechanical mechanisms such as toggles, latches, sprockets and gears are not required for lifting and rotating. By permitting the armature 13 to rotate with respect to the coil 11, electrical connection to the coil is simplified. The apparatus of the invention may be readily designed to handle substrate holders weighing as much as 60 pounds or more for rotational speeds exceeding 200 r.p.m.

The particular arrangement of the coil and armature also affords significant advantages. Because the coil is hermetically sealed, contamination of the vacuum by outgassing of the coil windings and insulation is prevented. Moreover, only the armature rotates, not the coil, obviating the need for sliding contacts or brushes for conducting electrical current to the coil.

It may therefore be seen that the apparatus of the invention provides an improvement in the supporting of substrate holders in vacuum coating systems. in particular, the apparatus of the invention utilizes a minimum of moving parts and greatly simplifies the problem of sealing to maintain the integrity of the vacuum system. Moreover, the apparatus of the invention is compact requiring little space within the coating chamber.

Various modifications of the invention in addition to those shown and described herein will become apparent to those skilled in the art from the foregoing description and accompanying drawings. Such modifications are intended to fall within the scope of the appended claims.

What is claimed is:

1. In a vacuum coating system apparatus for supporting a substrate holder wherein at least part of the substrate holder is ferromagnetic, comprising, an electromagnetic coil, means for supporting said electromagnetic coil within the vacuum coating system, an armature for engaging the ferromagnetic part of the substrate holder, a shaft supporting said armature adjacent said coil so that said armature is magnetized upon energization of said coil for holding the ferromagnetic part of the substrate holder and thereby supporting the substrate holder, and means for rotating said shaft to thereby rotate the substrate holder, said coil supporting means including means for moving said coil and said armature axially.

2. Apparatus according to claim 1 wherein said coil supporting means include a support assembly supporting said coil and saidshaft, said shaft being rotatable with respect to said support assembly, a cam body having at least one cam surface thereon, at least one cam follower secured to said support assembly and engaging said cam surface, and means for displacing said cam body to cause said cam surface to displace said support assembly and thereby displace said coil and said annature.

3. Apparatus according to claim 2 wherein said rotating means are mounted to said support assembly.

4. Apparatus according to claim 2 wherein said cam body comprises a hollow cylinder surrounding said support assembly, wherein said cam surfaces comprise a plurality of tracks in the wall of said cam body, and wherein said displacing means displace said cam body about its axis rotationally.

5. Apparatus according to claim 2 including means for mounting said apparatus over an opening in a vacuum enclosure of the vacuum coating system, and a generally cylindrical bellows attached to said support assembly and said mounting means to maintain a vacuum seal between said support assembly and said mounting means for all positions of said sup port assembly.

6. Apparatus according to claim 1 wherein said rotating means are mounted to said support assembly.

Claims

2. Apparatus according to claim 1 wherein said coil supporting means include a support assembly supporting said coil and said shaft, said shaft being rotatable with respect to said support assembly, a cam body having at least one cam surface thereon, at least one cam follower secured to said support assembly and engaging said cam surface, and means for displacing said cam body to cause said cam surface to displace said support assembly and thereby displace said coil and said armature.

5. Apparatus according to claim 2 including means for mounting said apparatus over an opening in a vacuum enclosure of the vacuum coating system, and a generally cylindrical bellows attached to said support assembly and said mounting means to maintain a vacuum seal between said support assembly and said mounting means for all positions of said support assembly.