Apparatus for Retaining Wafers

This invention relates to an apparatus for accepting, retaining and releasing a workpiece, and in particular, to an apparatus for accepting, retaining and releasing a wafer in a semiconductor processing system.

Prior art devices for holding wafers during processing include a spring-loaded clamping ring as shown, for example, in Faretra, U.S. Pat. No. 4,282,924, which is assigned to the assignee of the present invention. This clamping apparatus has the disadvantage that the clamping ring presses against the surface of the semiconductor wafer being held in position by the clamping apparatus, thus wasting valuable silicon surface area which is not available for processing. Since such a clamping ring is adjacent to the wafer surface, sputter contamination of the surface of the wafer during ion implantation may result from energetic ions striking the clamping ring. Further, the sliding surfaces and rubbing springs of such devices tend to generate particles which may contaminate the wafer surface.

In commercially available batch processing ion implantation systems, a plurality of platens is arranged concentrically around the periphery of a disk, and wafers held on the platens are implanted as the disk rotates. In these systems, the devices used to hold wafers on the platens against the centrifugal force generated by the rotation of the disk include an arcuate bumper on the outer edge of each platen and a spring driven mechanism to slide the wafers against the bumper. The sliding of a wafer across the platen tends to generate particles on the lower surface of the wafer. Such particles impede thermal contact between the wafer and the platen and may be transported to contaminate the wafer surface.

The wafer holding apparatus of the present invention avoids these negative features of prior art holding devices.

A device for releaseably holding a workpiece is disclosed which includes a base, a resilient member attached to the base having a plurality of finger members, and means for moving the finger members to selectably engage or disengage the workpiece by elastically deforming the resilient member.

In one embodiment, the finger members are attached to the base by means of a resilient collet having the shape of a closed loop. The collet is elastically deformed to pivot the finger members from a closed position for engaging and thus holding the workpiece to an open position for releasing the workpiece. In a particular embodiment, the base is a platen for holding a flat workpiece. When the fingers are in the open position, the workpiece may be placed on or removed from the platen without contacting the finger members. When the fingers are in the closed position, they do not contact the top planar surface of the flat workpiece, so that the entire top surface of the workpiece is available for processing.

In one embodiment, the finger members are an integral part of the resilient collet, and the collet is attached to the platen by engaging an annular elastomeric ring fixed to the platen. In another embodiment, the collet is attached to the platen by elastomer members attached to the outer periphery of the collet between the finger members. In one preferred embodiment, the surface of the finger members proximate the central axis of the platen are inclined a few degrees with respect to the central axis and are dimensioned so that they do not extend above the surface of the workpiece.

The holding mechanism of the present invention is particularly useful in retaining the workpiece on the platen when the platen is part of a disk in a batch processing apparatus which rotates the disk at high speeds during processing of the workpiece. In such an embodiment, a counterweight is attached to the collet for the purpose of generating a moment equal and opposite to the moment generated by the workpiece pressing against the finger members.

The metallic surfaces of the moving parts of the device are separated by elastomers which practically eliminate generation of particulates.

The invention and its other advantages may be more fully understood by reference to the drawings and accompanying detailed description of the invention.

FIG. 1 shows a simplified schematic cross sectional diagram of an end station for a batch processing ion implantation system;

FIG. 2 shows a partially cut away perspective view of an implant chamber;

FIG. 3 shows a partially cut away view of a transfer chamber for the implant chamber of FIG. 2;

FIG. 4 shows a side view of the wafer loading station in the transfer chamber of FIG. 3;

FIG. 5 shows a top view of a platen P and the wafer retaining apparatus of the present invention;

FIG. 6 shows a top view of the collet connecting the fingers shown in FIG. 5;

FIG. 7 shows a cross sectional view along the line 7--7 of FIG. 5;

FIG. 8 shows an expanded scale cross sectional view of a region 8--8 of FIG. 7;

FIG. 9 shows another embodiment of apparatus for releaseably holding a workpiece;

FIG. 10 is a cross sectional view of a further embodiment of a wafer retaining apparatus;

FIG. 11A is a top view of the collet used in the embodiment of FIG. 10;

FIG. 11B is a cross sectional view of the collet taken along the line 11B--11B of FIG. 11A;

FIG. 11C is a cross sectional view of the collet taken along the line 11C--11C of FIG. 11A;

FIG. 12 is an enlarged, cross sectional view of the area 12--12 of FIG. 10 shown with the fingers closed;

FIG. 12A is an enlarged, cross sectional view of the area 12--12 of FIG. 10 shown with the fingers open;

FIG. 13 is an enlarged, cross sectional view of the area 13--13 of FIG. 10; and

FIGS. 14A and 14B illustrate the counterweight ring retainer used in the apparatus of FIG. 10.

FIG. 1 shows a simplified schematic cross sectional diagram of an end station 1 for a batch processing ion implantation system which employs the wafer holding device (not shown in FIG. 1) of the present invention. End station 1 includes a rotatable disk 2 mounted at its center to drive shaft 5 of spin drive motor 6 which rotates drive shaft 5 about its spin axis D. Spin drive motor 6 and drive shaft 5 are contained in housing 4 which extends through housing 12 to the exterior of vacuum chamber 11. Power line C provides external electric power to spin drive motor 6. Disk 2 is shown in its generally vertical implant orientation in FIG. 1. Disk 2 includes a peripheral conical ring 2a having a selected small base angle B, for example 7.degree.. A plurality of semiconductor wafers W are retained on a corresponding plurality of platens P mounted on ring 2a by the wafer holding devices of the present invention. The surface of each platen P is flat, and each platen is partially recessed in ring 2a and attached thereto. As explained below, during operation of the system, each wafer W is forced against its corresponding platen on peripheral conical ring 2a by the component of centrifugal

force normal to the surface of the wafer generated by rotation of disk 2 on drive shaft 5.

If desired, drive shaft 5 may include a rotary vacuum feedthrough 8. Fluid lines J and K extend from outside vacuum chamber 11 via housing 4 to rotary feedthrough 8. A first fluid, typically water, for cooling the platens P flows from fluid line J through feedthrough 8 to channels (not shown) in disk 2 which extend from vacuum feedthrough 8 to the platens P. Fluid line K provides a second selected fluid, typically gas, via feedthrough 8 to other channels (not shown) in disk 2 and platens P which carry the selected gas to the back surface of wafers W which are held against platens P. The gas provided between a platen P and a wafer W enhances the cooling of wafer W when wafer W is subjected to the heat caused by the impact of ion beam 14 on the surface of wafer W. Ion beam 14 is formed in an ion source (not shown) and is passed through appropriate mass analysis and ion optical elements (not shown) before being applied to wafers W. Vacuum pump 7 is coupled through isolation valve 9 to vacuum implant chamber 11 defined by housing 12. Vacuum pump 7 serves to evacuate vacuum chamber 11 prior to implantation of wafers W.

FIG. 2 shows a partial cut away perspective view of implant chamber 11 with disk 2 in horizontal position. Disk 2 is rotated about axis A by flip drive motor 14 and associated linkage (not shown) from the position shown in FIG. 2 to the implant position shown in FIG. 1. Scan drive motor 16 and associated linkage (not shown) reciprocate disk 2 in a plane approximately perpendicular to ion beam 14 when disk 2 is orientated for implantation as shown in FIG. 1. Motors 14 and 16 are mounted exteriorly of chamber 11.

In operation, a wafer in loadlock 29 (FIG. 1) is conveyed to a selected platen p on disk 2a by means of transfer arm 22 located in transfer chamber 24 as is explained in more detail in connection with FIG. 3.

FIG. 3 shows a partially schematic plan view of transfer chamber 24 and elevator chamber 20 of load-lock 29. Cassettes 10 holding wafers W are placed in elevator chamber 20 of loadlock 29 via loading chambers not shown in FIG. 2 and which are not pertinent to the present invention. The details of loadlock 29 are provided in copending U.S. patent application Ser. No. 856,814, entitled "Wafer Transfer System" and assigned to the assignee of the present invention, which application is incorporated herein by reference. Wafers W are removed from a selected cassette holder 10 one at a time by wafer transfer arm 22.

Transfer vacuum chamber 24 opens off implant chamber 11 and is defined by housing 51. Chamber 24 is connected to a vacuum pumping system (not shown) for evacuation thereof. Transfer arm 22 is located in chamber 24 and is supported and operated by a transfer drive system which includes a drive assembly 54 that supports and drives transfer arm 22. Drive assembly 54 and arm 22 attached thereto are laterally moveable along the Y axis shown in FIG. 1. Drive assembly 54 is moveable along the Y axis under the control of drive motor 56 positioned outside chamber 24 and a lead screw 58 so that drive assembly 54 and arm 22 mounted thereon can be positioned opposite a selected cassette 10. Additional details concerning the operation of transfer arm 22 are provided in the above referenced copending U.S. Patent Application. Once positioned opposite a selected cassette, transfer arm 22 is extended through opening 48 into cassette 10 beneath a selected wafer, and then cassette 10 is lowered by an elevator mechanism (not shown) a selected distance so that the selected wafer is transferred to transfer arm 22. Transfer arm 22 is then withdrawn from cassette 10.

In order to transfer the wafer W on arm 22 to selected platen P on conical ring 2a, disk 2 is rotated about axis A and about axis D so that the selected platen P is horizontal and located above wafer loading station 28. Arm 22 with the wafer W thereon is then extended over the selected platen.

FIG. 4 shows a side view of wafer loaded station 28 with a selected platen P having been rotated on ring 2a above station 28. Arm 22 is shown extended above the selected platen P with wafer W resting thereon. During wafer loading and unloading, disk 2 is incrementally rotated about axis D so that each wafer platen located on annular ring 2a is presented to wafer transfer location 28.

Lift pins 30, typically 3 in number, are selectably moved upwardly and downwardly by actuating air cylinder 50 through holes 31 in wafer support platen P on disk 2. Pins 30 lift a wafer from a platen surface for removal by arm 22 or lift a wafer from transfer arm 22 for placement on platen surface P. When a wafer is being loaded onto a platen P positioned above station 28, pins 30 are extended through cylindrical holes 31 which extend through conical ring 2a and platen P until they contact wafer W carried by arm 22 and then are extended a small distance further so that wafer W is lifted from transfer arm 22 which is then retracted by drive mechanism 54. When a wafer is being removed from the disk, it is first lifted by pins 30. Next, transfer arm 22 is extended between wafer W and platen P, and lift pins 30 are then lowered through cylindrical holes 31, thereby lowering wafer W onto transfer arm 22.

Wafer W is retained on platen P by the mechanism described in connection with FIG. 5 through FIG. 8.

FIG. 5 shows a top view of platen P with retaining finger f.sub.1 through f.sub.14, which when attached thereto as explained below, extend slightly above the flat surface P.sub.s of platen P. In one embodiment, the fingers f.sub.1 through f.sub.14 are dimensioned to extend above surface P.sub.s by only the thickness of the wafer to be retained by the fingers, so that they do not extend above the surface of a wafer being retained. The inside faces of fingers f.sub.1 through f.sub.14 are machined to conform to the shape of the edge of the workpiece. In particular, in FIG. 5, they form portions of a circle to match the circumference of the circular wafer. Platen P includes holes 31 for pins 30 (FIG. 4) which lower a wafer to platen P (and which lift a wafer from platen P). Grooves g.sub.1 through g.sub.14 are machined in aluminum platen P and dimensioned to accommodate the corresponding fingers f.sub.1 through f.sub.14. The number of fingers need not be 14, but it is preferable to employ at least two fingers and preferable to employ more than two fingers to accommodate variations in the geometry of the workpiece. For example, if a wafer has one or more flats, typically a finger will not make contact with the flat portion of the wafer W held on platen P.

As shown in FIG. 6, circular collet 41 comprises a thin, resilient generally annular ring portion 41a connecting fingers f.sub.1 through f.sub.14. In this embodiment, finger members f.sub.1 through f.sub.14 are an integral part of collet 41 which is machined from a single piece of aluminum or other suitable material. The pair of notches n.sub.a, n.sub.b in collet 41 on each side of each finger are not essential to the operation of the collet, but serve to reduce stress in the region where a finger joins the resilient portion 41a of collet 41. Resilient portion 41a is actually conical in shape in the inner edge H is lower than outer edge E (see angle .alpha. in FIG. 8).

FIG. 7 shows a cross section of platen P and the attached collet 41 of FIG. 5 along the line 7--7. FIG. 8 shows an enlarged view of the region 8--8 of the cross-section in FIG. 7. As shown in FIG. 7, annular groove 49 is machined into the base of platen P near the periphery of platen P so that arc like skirt sections 45 of the platen are formed between the grooves g.sub.1 through g.sub.14 shown in FIG. 5. An annular elastomer 43 is molded to the top surface of annular groove 49 and to the inside surface of arc like skirt sections 45 of platen P. Annular groove sections 47 are molded into elastomer 43 near the top portion of elastomer 43 on the inside skirt sections 45. Collet 41 is attached to platen P (FIG. 7) by snapping the edge portions E (only a few of which are labeled in FIG. 6 for the sake of clarity) between fingers f.sub.1 through f.sub.14 into grooves 47. Thus, there is no metal on metal contact of collet 41 and platen P.

When collet 41 has been snapped into place, the fingers f.sub.1 through f.sub.14 are located in grooves g.sub.1 through g.sub.14 as shown in FIG. 5. FIG. 8 shows the mechanism for moving a typical finger f from a closed position (shown in FIG. 8) for contacting and holding wafer W on platen P to an open position (not shown) wherein finger f does not contact wafer W.

Annular counterweight ring 53 is attached to the underside of collet 41 by means of elastomeric annular ring 55 which is molded on collet 41. Ring 55 has a generally semicircular cross section, as shown in FIG. 8, except that at selected locations around ring 55 molded finger like projections 57 of the elastomer of ring 55 extend downward through circular holes 65 through counterweight ring 53. Circular hole 65 has a top portion 67 of larger diameter than the bottom portion 69 with a narrow neck portion 71 in between top portion 67 and bottom portion 69. Counterweight ring 53 is attached to collet 41 by pulling the elastomeric projections 52 through the narrow neck portions 71 and then trimming the projections to be flush with the bottom of counterweight ring 53 as shown in FIG. 8. The thin resilient connecting ring portion 41a of collet 41, which is angled (by .alpha. degrees) slightly downward from the horizontal, acts as a spring element biasing the fingers f.sub.1 through f.sub.14 inward to the closed position for engaging a wafer shown in FIG. 8. In one embodiment, .alpha. is 5.degree..

In operation, the finger f is pivoted counter clockwise, as indicated by arrows C, about a virtual pivot arc P.sub.a to an open position for allowing the wafer W to be lowered onto platen P by prongs 30 (FIG. 4) or to be removed from platen P by prongs 30 without contacting finger f.

The pivoting of finger f to the open position is accomplished by air cylinder 50 (FIGS. 4 and 8), or other means, driving prongs 32 through openings (not shown) in ring 2a and against the bottom side of counterweight ring 53, which causes counterweight ring 53 to move upward, elastically deforming thin resilient portion 41a of collet 41 until it is stopped by pressing against elastomer 43. Deforming the spring portion 41a causes a type of "oil can" action in the spring portion similar to the action of a Bellville spring washer which yields almost constant spring force over the full stroke range. Rate characteristics maybe selected by selecting the geometry of the collet. This elastic deformation causes finger f to pivot about virtual pivot arc P.sub.a. The edge E of collet 41 pressed into groove 47 in elastomer 43 acts as a pivot in a sensitive gravimetric scale where finger f is the balance beam. The pivot arc P.sub.a is a virtual extension of groove 47.

In operation, air cylinder 50 drives all four prongs 32 (FIG. 4) against ring 53 simultaneously. Since counterweight ring 53 is substantially more rigid than the resilient portion 41a of collet 41, the four actuating prongs 32, which are spaced 90.degree. apart beneath counterweight ring 53, simultaneously pivot all of the fingers f.sub.1 through f.sub.14 when prongs 32 are driven against counterweight ring 53. When actuating prongs 32 are lowered by air cylinder 50, the elastically deformed resilient portion 41a elastically restores itself. Air cylinder 50 simultaneously drives actuating prongs 32 and lifting prongs 30. These prongs are dimensioned relative to platen P so that prongs 32 engage counterweight ring 53 to drive collet portion 41a against elastomeric stop 43 and thus pivot fingers f.sub.1 through f.sub.14 to the open position before lifting prongs 30 emerge above the top surface of platen P. Air cylinder 50 then continues to drive lifting prongs 30 above the top surface of platen P while actuating prongs 32 continue to press on counterweight ring 53 by means of spring compliance. The operation of air cylinder 50 is controlled by a controller (not shown).

In operation, when the implant system is in the implant orientation (FIG. 2), spin drive motor 6 typically spins disk 2 at a rate on the order of 1000 rev/min, which generates a centrifugal force having a component normal to the planar surface of wafer W and a component parallel to the planar surface of wafer W. In the operating environment described above, forces of approximately 500 g's are generated near the periphery of portion 2a of the spinning disk 2, and the fingers f must remain in the closed position even in this extreme environment.

The component normal to the surface of wafer W presses wafer W against platen P. The component parallel to the surface of wafer W causes wafer W to press outwardly against those fingers near the outer edge of portion 2a of disk 2. When wafer W presses outwardly against a selected finger f (FIG. 8), the force generated by the wafer has a lever arm of length L.sub.1.

The counterweight ring 53 is subject to the same centrifugal forces as wafer W. As wafer W presses outward against finger f, the elastomeric interface 61 on counterweight ring 53 also presses against finger f. Molded elastomeric ring 61 recessed in the outer edge of counterweight ring 53 provides a soft contact surface between counterweight ring 53 and finger f. The force exerted on finger f by elastomeric interface 61 has a lever arm of length L.sub.2. The weight of counterweight ring 53 is selected so that the rotational moment about pivot arc P.sub.a generated by wafer W is approximately balanced by the opposing rotational moment generated by counterweight ring 53. The slender profile of aluminum counterweight 53 allows ring 53 to elastically deform under high g loads to an oval shape. Thus, the ring 53 conforms to the radius change due to wafer size tolerance variations.

Finger f shown in FIG. 8 extends slightly above the surface of the wafer. In another embodiment, the finger f is dimensioned so that it extends only to the top surface of wafer W. The phantom line 53 in FIG. 8 indicates the contour of a finger f which does not extend above the surface of wafer W. This contour is advantageous for reducing the sputter of the wafer surface caused by energetic ions striking fingers f.

It should also be noted that inside face, f.sub.a of finger f is angled at a few degrees, typically 3.degree.-4.degree., from the normal to the flat surface of platen P. This provides a good contact with the semicircular edge of a typical wafer and provides a small component of the force of finger f pressing against wafer W normal to platen P, which also tends to hold wafer W on platen P. In the embodiment shown in FIG. 8, the inside face f.sub.a is also curved (in a plane perpendicular to the edge of the wafer, thus spreading the load and reducing stress on the wafer. Finger f is also slightly tapered to reduce mass. In one embodiment (not shown), the inside surface f.sub.a is coated with Teflon or other suitable material to prevent wafer W from contacting the metal of finger f, thus reducing particle generation. The grip of finger f on wafer W can be improved by addition of an elastomeric compound on surface f.sub.a.

FIG. 9 shows another embodiment of the invention suitable for use in a device which is not part of a rotating system. In the embodiment of FIG. 9, collet 41 is attached to base B in the same way that collet 41 is attached to platen P in FIG. 8. Wafer W in FIG. 9 is supported by notches f.sub.g in fingers f and does not contact base B. Fingers f are moved to an open position by air cylinder 72 (or other suitable driving means) which drives a plurality of prongs 75 against annular ring 71. Elastomeric ring 73 is molded on ring 71 and elastically deforms resilient portion 41a of collet 41 when air cylinder 72 drives prongs 75 and ring 71 upward.

The apparatus described above for releaseably retaining a wafer on a platen has several other advantages over prior art clamping devices.

No surface area of the wafer W is contacted by the wafer retaining apparatus, so that all of the surface area of the wafer is available for the production of semiconductor devices.

Since the wafer is lowered vertically onto the platen and then held there by the retaining fingers, wafer sliding against the platen, which causes contamination by generating particulates, is virtually eliminated. None of the moving parts of the apparatus slide or rub against the wafer.

All metal parts of the wafer retaining apparatus snap together by means of elastomeric interfaces, eliminating metal to metal contact, which generates particles, and also eliminating metal fasteners such as screws. In general, assuming at least 3 fingers contacting the wafer along an arc greater than 180.degree., the sum of the forces on the wafer exerted by the fingers (and vice versa) is independent of the number of fingers contacting the edge of the wafer. Thus, by increasing the number of fingers, the force exerted by each finger on the wafer is reduced, which reduces wafer breakage. If a finger is positioned opposite a wafer flat, the holding device remains in equilibrium.

Finally, the design is intrinsically safe. Wafer W will be retained on the rotating platens for the duration of the implant process even if the spring portion of the collet breaks or if the counterweight ring 53 is no longer retained by elastomeric projections 57.

The above embodiments are intended to be exemplary and not limiting, and in view of the above disclosure, many modifications and substitutions will be obvious to one of average skill without departing from the scope of the invention. For example, while the resilient member 41a in FIGS. 8 and 9 is in the shape of a continuous, generally annular ring angled slightly downward from the outer edge, in other embodiments, the resilient member may be given other shapes to better accommodate the shape of the workpiece. For example, the resilient member 41a may be an oval loop or square loop. In the embodiment shown, the resilient member 41 forms a simple closed loop; but if desired, the resilient member may comprise more than one disjoint resilient section, each section connecting a plurality of fingers, with a separate means for selectively elastically deforming each resilient section to move the fingers attached thereto into and out of engagement with a workpiece. It should also be clear that the invention may be used for releaseably retaining any workpiece and that the workpiece need not be shaped like a disk. The invention may also be employed to retain wafers in conjunction with many different semiconductor processes in addition to ion implantation.

An alternate embodiment of the wafer retaining apparatus is shown in FIGS. 10-14. The top view of the apparatus is represented by FIG. 5. A platen 102 is provided with a flat, generally circular surface 104. A resilient, thermally conductive layer 106 is molded to surface 104, and a workpiece such as a semiconductor wafer W is retained on resilient layer 106 by a plurality of fingers f.sub.1 through f.sub.14. The resilient layer 106 provides highly efficient thermal transfer in vacuum between the semiconductor wafer W and the platen 102. The details of the resilient layer 106 are provided in pending application Ser. No. 141,709, filed Jan. 7, 1988 now U.S. Pat. No. 4,832,781, which application is hereby incorporated by reference.

Grooves g.sub.1 through g.sub.14 are spaced apart around the periphery of platen 102 for receiving fingers f.sub.1 through f.sub.14, as best seen in FIG. 5. An annular recess 116 defined by surfaces 118 and 120 (FIG. 12) is machined near the periphery of platen 102. The platen 102 is provided with arc shaped skirt portions 122 between grooves g.sub.1 through g.sub.14. A collet 110 includes an annular ring 112 interconnecting fingers f.sub.1 through f.sub.14 (FIG. 11A). The fingers f.sub.1 through f.sub.14 are preferably an integral part of collet 110. The annular ring 112 of collet 110 is mounted in recess 116 such that fingers f.sub.1 through f.sub.14 extend slightly above resilient layer 106 for retaining wafer W. A counterweight ring 130 is mounted below annular ring 112 and is positioned to deform annular ring 112 when counterweight ring 130 is moved upwardly. Deformation of annular ring 112 causes fingers f.sub.1 through f.sub.14 to be pivoted between a closed position in which the fingers f.sub.1 through f.sub.14 bear against the edge of wafer W and retain it on platen 102, and an open position in which the wafer W is released.

Resilient annular ring 112 has a truncated conical shape in its relaxed position. By way of example, the collet 110 can be fabricated of aluminum with annular ring 112 having a width of about 0.28-inch, a thickness of about 0.025-inch and an outside diameter approximately equal to the diameter of the wafer being held. In its relaxed state, the surface of ring 112 forms an angle 132 of about 8.degree. with the plane of ring 112 (FIG. 11B). Outer peripheral sections 112b of ring 112 are separated by fingers f.sub.1 through f.sub.14. Fingers f.sub.1 through f.sub.14 of collet 110 have generally the same size, shape and function as the fingers f.sub.1 through f.sub.14 of collet 41 shown and described hereinabove.

Elastomer members 134 are attached to each of the outer peripheral sections 112b, preferably by molding. The elastomer members 134 provide a resilient, pivoting connection between the annular ring 112 and the platen 102. The annular ring 112, with elastomer members 134 attached thereto, is mounted in recess 116 with an outer surface 134a of each elastomer member abutting against an inner surface 122a of the respective skirt portion 122. An upper surface 134b of each elastomer member abuts against a surface 124 of platen 102. In a preferred embodiment, the inner surface 122a of each skirt portion 122 is tapered inwardly by an angle 126 (FIG. 12) of about 3.degree. to 4.degree.. Similarly, the surface 134a of each elastomer member 134 is tapered outwardly by an angle 128 (FIG. 11C) of about 3.degree. to 4.degree.. The angled surfaces 122a and 134a provide a snap fit of the collet 110 into position between skirt portions 122 and surface 124 as shown in FIG. 12.

The molding of elastomer members 134 to the outer peripheral sections 112b of annular ring 112 provides several advantages over the configuration wherein an elastomer member having a groove is molded to the platen. The requirement for molding a very small groove (about 0.025-inch in width) is eliminated. Furthermore, any difficulty associated with mounting of the collet in the groove is eliminated.

The outer peripheral sections 112b of the collet between fingers f.sub.1 through f.sub.14 are preferably fabricated with a numerically controlled, thin and flexible end mill causing errors up to 0.020-inch. Any errors in the outer periphery of the collet can produce errors in the positioning of the collet relative to the platen, since the outer periphery is used to locate the collet relative to the platen. By using an accurate mold, the outer diameter of the elastomer members 134 can be held to a tolerance of 0.002-inch, and the surface 122a which locates the elastomer members 134 can be turned with an accuracy of 0.0002-inch. As a result, the fingers f.sub.1 through f.sub.14 are located with extreme accuracy relative to platen 102.

Finally, the resilient layer 106 which is molded to surface 104 of platen 102 may require elevated temperatures for processing. It is desirable to avoid a requirement for platen 102 to have two molded elements, each having different processing parameters.

An elastomer bumper 140 is molded to the inner periphery 112a of annular ring 112. The elastomer bumper 140 typically extends in a closed loop around collet 110. A cross section of the bumper 140 is generally circular except where inner periphery 112a is molded into bumper 140. When the collet 110 is deformed to the open position of fingers f.sub.1 through f.sub.14, bumper 140 prevents direct, metal to metal contact between annular ring 112 and surface 118 of platen 102.

The elastomer bumper 140 includes a plurality of downwardly extending elastomer pins 142 as best shown in FIGS. 11C and 12. In a preferred embodiment, four elastomer pins 142 are equally spaced around collet 110. Each of the elastomer pins 142 includes a tapered portion 142a near its lower end. The counterweight 130 is provided with recesses 144 for receiving pins 142. Each recess 144 is tapered so that its wall engages and retains tapered portion 142a of elastomer pin 142. The molding of bumper 140 to collet 110 eliminates the requirement for molding of a bumper to surface 118 of platen 102. Elastomer members 134 and elastomer bumper 140 are interconnected by elastomer strips 145. The strips 145 result from interconnecting passages in the mold used to fabricate elastomer elements 134 and 140 and help to retain those elements in position on annular ring 112. Molding of elastomer members 134, elastomer bumper 140 and elastomer pins 142 to annular ring 112 can be accomplished by casting, compression or transfer molding.

An elastomer ring 146 is molded to the outer periphery of counterweight ring 130. When the disk 2 (FIG. 1) is spinning, centrifugal force displaces counterweight ring 130 radially outward and causes elastomer ring 146 to bear against those of fingers f.sub.1 through f.sub.14 which are located on the outer periphery of the wafer site. The force exerted by counterweight ring 130 offsets the centrifugal force exerted by wafer W on the fingers, thereby maintaining the wafer firmly clamped in position.

The counterweight ring 130 is retained in the assembly by a plurality of retainers 150 spaced at intervals around the platen 102. As shown in FIGS. 14A and 14B, each of the counterweight retainers 150 is preferably formed as a semicircular plate having a raised dimple 152. The retainer 150 is inserted in a slot 154 in surface 120 of platen 102. The raised dimple 152 holds the retainer in slot 154 by friction. In a preferred embodiment, four retainers 150 are equally spaced around the periphery of counterweight ring 130.

The fingers f.sub.1 through f.sub.14 are normally in a closed position as shown in FIG. 12, and the annular ring 112 is in a relaxed position. In the relaxed position, the inner periphery 112a of the annular ring is slightly lower than the outer peripheral sections 112b. The resilience of annular ring 112 maintains fingers f.sub.1 through f.sub.14 firmly but gently engaged with the edge of wafer W. In order to release the wafer W, an upward force is applied to the counterweight ring 130. Actuating prongs 32 (FIG. 4) engage counterweight ring 130 between retainers 150 and push it upwardly. The counterweight ring, acting through elastomer pins 142, pushes the inner periphery 112a of annular ring 112 upwardly until elastomer bumper 140 contacts surface 118 of platen 102.

Since the outer peripheral portion 112b of annular ring 112 are coupled to platen 102 by elastomer members 134, the outer peripheral portions 112b and fingers f.sub.1 through f.sub.14 are prevented from moving upwardly. Instead, annular ring 112 is deformed and simultaneously pivots about an imaginary pivot ring 114. As used herein, "pivot ring" refers to an imaginary circle coincident or nearly coincident with outer peripheral sections 112b. Pivoting of annular ring 112 occurs by elastic deformation of elastomer members 134. The manner in which annular ring 112 is coupled by elastomer members 134 to platen 102 permits annular ring 112 to pivot about pivot ring 114 without being linearly displaced. While the annular ring 112 as a whole is deformed, a cross section taken through annular ring 112 remains substantially fixed in shape as the ring is deformed. As annular ring 112 pivots, fingers f.sub.1 through f.sub.14 pivot away from platen 102 to an open position, as shown in FIG. 12A, in which wafer W is released.

It is important to note that the pivot ring 114 is coaxial with wafer W and has approximately the same diameter as wafer W. As a result, when fingers f.sub.1 through f.sub.14, pivot between their open and closed positions, each moves in a nearly radial direction relative to the wafer W, and scraping and abrasion of the wafer edge are avoided.

When the actuating prongs 32 are lowered, the annular ring 112 elastically returns to its relaxed position, and fingers f.sub.1 through f.sub.14 rotate to the closed position in contact with the edge of wafer W. The resilience of annular ring 112 ensures that wafer W will be retained on platen 102, even if operating power is lost.

The construction illustrated in FIGS. 10-14 avoids metal to metal contact between moving parts. The collet 110 pivots about pivot ring 114 by elastic deformation of elastomer members 134. The elastomer bumper 140 prevents contact between the inner periphery 112a and the platen 102. The counterweight ring 130 is connected to annular ring 112 by pins 142 and elastomer bumper 140. Thus, all motion is effected by deformation of elastomer elements 134, 140 and 142, and particulate generating metal to metal contact is avoided.

While there has been shown and described what is at present considered the preferred embodiments of the present invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention as defined by the appended claims.