Robots for microelectronic workpiece handling

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation application of International PCT Patent Application No. PCT/US99/15567, designing the U.S., filed Jul. 9, 1999, entitled ROBOTS FOR MICROELECTRONIC WORKPIECE HANDLING, which claims priority from U.S. patent application Ser. No. 09/114,105, filed Jul. 11, 1998.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable

BACKGROUND OF THE INVENTION

There are a wide range of apparatus types for processing workpieces that ultimately become microelectronic devices. As the microelectronics industry advances toward efficient and economical mass production of the devices, the demands on the apparatus used in processing of the workpieces have increased. Increasingly, automation of the apparatus is being used to meet these ever-increasing demands. More particularly, many of the increased demands relate to automated devices for handling the microelectronic workpieces during processing.

An automated apparatus used for processing a microelectronic workpiece, such as a semiconductor workpiece, is disclosed in U.S. Ser. No. 08/991,062, filed Dec. 15, 1997, and titled “Semiconductor Processing Apparatus Having Lift and Tilt Mechanism”, which is hereby incorporated by reference. This apparatus utilizes a plurality of workpiece processing modules or stations for performing various processing steps. Workpiece transport units are used to access workpiece cassettes and transfer workpieces throughout the processing apparatus. A workpiece conveyor supports and guides the workpiece transport units for transferring individual workpieces between workpiece interface modules and the workpiece processing modules or stations. The workpiece conveyor also includes a transport unit guide, such as an elongated rail, which defines a path for one or more workpiece transport units within the apparatus. The workpiece transport units which move along the rail are configured to have a workpiece transfer arm assembly having an end with a vacuum effector for holding a workpiece. The transfer arm assembly can be adjusted in vertical elevation and can be rotated about the vertical axis for precise positioning of the effector and the workpiece.

Workpieces are typically handled and stored with the face to be processed (the “front” face) oriented facing upwardly. This orientation avoids contact on the front face by the supporting structure. Some processing modules, on the other hand, require the workpiece to be oriented with the face to be processed facing downwardly. To accommodate such requirements, some processing modules such as electroplating reactors, utilize a processing head which can be “flipped”, i.e., rotated, between a first position in which the processing head is positioned to receive the workpiece with a front side of the workpiece facing up and a second positioned in which the front side of the workpiece faces down for processing.

Making provision for each processing module or station to “flip” the workpiece for processing requires complicated head operator mechanisms for rotating the processing heads. Such operator mechanisms can require substantially heavy or large structures for rotating the processing heads, and can require significant overhead operating room for the rotational movement.

The present inventors have recognized that reducing or eliminating the requirement for processing modules to turn over or flip a workpiece for processing would simplify the overall workpiece apparatus. The present inventors have also recognized that cost savings and process simplicities would be enhanced by eliminating the requirement for flipping the workpiece. Still further, the inventors have recognized that a wider range of processing stations of different types may be integrated into a single processing tool. Such processing stations may have varying wafer orientation requirements, one station requiring a front-face up orientation for processing while another station requires a front-face down orientation for processing. An apparatus that addresses each of these recognized problems is set forth.

Additionally, the present inventors have recognized that it would be advantageous to provide a workpiece conveyor with transport unit slidable thereon which minimizes the required working space or “footprint” of the conveyor and transport units operating between laterally disposed process units. An apparatus which provides this advantage is set forth.

SUMMARY OF THE INVENTION

The present invention is directed to a workpiece conveyor system that is used for transporting individual workpieces between workpiece processing stations and the/or interface modules in a workpiece processing apparatus. The workpiece conveyor system includes an improved workpiece transport unit that carries the workpieces within the apparatus on, for example, a conveyor rail or the like. The transport unit includes a vertical member extending from a housing. An arm member extends from the vertical member at a base end of the arm member. A workpiece-holding end effector is disposed at a distal end of the arm member and is selectively driven in rotation about a horizontal axis to “flip” the workpiece between a face-up orientation and a face-down orientation. The effector is preferably configured to grip an edge of a workpiece, such as a semiconductor wafer, and can have a workpiece presence sensor for informing a control unit that a workpiece is present on the effector.

In accordance with one embodiment of the present invention, the workpiece transport unit provides five “axes” of movement. To this end, the transport unit can be driven linearly on the rail along a horizontal axis (Y). The vertical member can be raised or lowered vertically along a vertical axis (Z

1

). The arm member can be rotated about the vertical axis (Z

1

) and a distal portion of the arm member can be rotated about the vertical axis (Z

2

). The end effector can rotate or “flip” about a horizontal axis (R), for example, to orient the workpiece in either the front-face up or front face down orientation. To execute such rotation, the arm member preferably includes a rotary actuator mounted within the arm member to turn the end effector about the horizontal axis.

By providing a workpiece transport unit with increased flexibility of movement, including a rotation about a horizontal axis, more expensive, heavy and complicated mechanisms for flipping workpieces at a plurality of process modules is avoided. Additionally, it becomes possible to integrate processing stations having different workpiece orientation requirements into a single processing apparatus.

In a further aspect of the invention, a workpiece transport unit is provided having a vacuum gripping mechanism for holding a workpiece to the end effector. The vacuum gripping mechanism includes a plurality of raised pads for pressing against an edge region of the workpiece, and vacuum ports through the pads for urging the workpiece onto the pads.

In a still further aspect of the invention, two workpiece transport units are slidable on opposite lateral sides of a guide rail structure. At least one of the transport units includes a first end effector which is elevated above an adjacent section of its respective first robot arm, providing a vertical space therebetween. The vertical space is sufficiently projected in a horizontal direction for the respective other end effector of the other transport unit, operating at a lower elevation, to pass under the first end effector and over the first robot arm. Thus, wafers held by the two end effectors can be overlapped in plan, and the two transport units can be moved longitudinally along the conveyor rail, together, or individually with respect to each other, without interference between end effectors or wafers held thereby. This arrangement minimizes the lateral footprint needed between opposing process units of the tool.

Numerous other advantages and features of the present invention will become readily apparent from the following detailed description of the invention and the embodiments thereof, from the claims and from the accompanying drawings in which details of the invention are fully and completely disclosed as part of this specification.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1

is an exploded perspective view of a workpiece processing tool incorporating an improved workpiece conveyor system constructed in accordance with one embodiment of the present invention;

FIG. 2

is a perspective view of the improved workpiece conveyor system shown in

FIG. 1

;

FIG. 3

is a sectional view taken generally along line

3

—

3

of

FIG. 2

;

FIG. 4

is a perspective view of a workpiece transport unit constructed in accordance with one embodiment of the present invention;

FIG. 5

is an exploded perspective view of the workpiece transport unit shown in

FIG. 4

;

FIG. 6A

is a partial exploded perspective view of the robot arm components of the transport units of

FIG. 5

;

FIG. 6B

is a partial exploded perspective view of the robot arm components of

FIG. 6A

,

FIG. 6B

being a continuation of

FIG. 6A

;

FIG. 7

is a side view of the robot arm components of

FIGS. 6A

,

6

B, as assembled;

FIG. 8

is a sectional view taken generally along line

8

—

8

of

FIG. 7

;

FIG. 9

is a sectional view taken generally along line

9

—

9

of

FIG. 8

;

FIG. 10

is an enlarged fragmentary sectional view from

FIG. 8

;

FIG. 11

is an enlarged fragmentary right side view taken from

FIG. 7

;

FIG. 12

is an enlarged fragmentary sectional view taken from

FIG. 8

;

FIG. 13

is an enlarged perspective view of one embodiment of an end effector suitable for use in the workpiece transport unit shown in

FIG. 4

;

FIG. 14

is a rear perspective view of the workpiece transport unit of

FIG. 4

in which the arm is in a different rotary position and in which the end effector is holding a workpiece;

FIG. 15

is a plan view of the end effector of

FIG. 13

;

FIG. 16

is a sectional view taken generally along line

16

—

16

of

FIG. 15

;

FIG. 17

is an enlarged fragmentary sectional view taken from

FIG. 16

, shown holding a workpiece;

FIG. 18

is an enlarged fragmentary sectional view taken generally along line

18

—

18

of

FIG. 15

;

FIG. 19

is an enlarged fragmentary sectional view taken from

FIG. 16

;

FIG. 20

is an enlarged fragmentary sectional view of an alternative embodiment robot arm;

FIG. 21

is an enlarged view taken from

FIG. 20

;

FIG. 22

is an end view of an alternative workpiece processing tool having a workpiece conveyor system using alternative transport units which incorporate the robot arms of

FIG. 20

;

FIG. 23

is an enlarged view taken from

FIG. 22

;

FIG. 24

is a plan view of the workpiece processing tool of

FIG. 22

;

FIG. 25

is an exploded perspective view of an end effector of the robot arm shown in

FIG. 20

, and a workpiece;

FIG. 26

is a plan view of the end effector of

FIG. 25

;

FIG. 27

is a bottom view of the end effector of

FIG. 26

;

FIG. 28

is an enlarged view taken from

FIG. 26

;

FIG. 29

is an enlarged view taken from

FIG. 26

;

FIG. 30

is a sectional view taken along line

30

—

30

in

FIG. 26

;

FIG. 31

is a plan view of the end effector of

FIG. 25

, holding a workpiece;

FIG. 32

is a sectional view taken along line

32

—

32

in

FIG. 31

;

FIG. 33

is a sectional view taken along line

33

—

33

in

FIG. 31

; and

FIG. 34

is a sectional view taken along line

34

—

34

in FIG.

31

.

DETAILED DESCRIPTION OF THE INVENTION

While this invention is susceptible of embodiment in many different forms, there are shown in the drawings and will be described herein in detail specific embodiments thereof with the understanding that the present disclosure is to be considered as an exemplification of the principles of the invention and is not intended to limit the invention to the specific embodiments illustrated.

FIG. 1

illustrates an exemplary modular workpiece processing apparatus

10

that may use the improved conveyor system of the present invention. As illustrated, apparatus

10

includes an input/output assembly

12

, and left and right processing modules

14

,

16

. The apparatus

10

also includes the improved workpiece conveyor system

20

, a top exhaust assembly

24

, and an end panel

25

. As illustrated, left and right processing modules

14

,

16

, which each include a plurality of workpiece processing stations, may be secured to one another about the workpiece conveying system

20

to form a processing chamber having a longitudinally disposed inlet and outlet. Preferably, workpiece conveyor

20

is disposed in the processing chamber so that it can access each of a plurality of workpiece cassette interface modules within the input/output assembly

12

and, further, can access each workpiece processing station within the left and right processing modules

14

,

16

.

A plurality of the processing modules

14

,

16

may be secured in an end-to-end configuration to thereby provide an extended processing chamber capable of performing a substantial number of processes on each workpiece or, in the alternative, process a larger number of workpieces concurrently. In such instances, the workpiece conveying system

20

of one apparatus

10

is programmed to cooperate with the workpiece conveying system

20

of one or more prior or subsequent conveying systems

20

.

FIG. 2

illustrates further details of the workpiece conveyor

20

for transporting workpieces throughout the processing apparatus

10

of FIG.

1

. As shown, the workpiece conveyor

20

generally includes one or more workpiece transport units

30

,

32

that are coupled for movement along workpiece transport unit guide

26

. The transport unit guide

26

preferably comprises an elongate spine

26

a

mounted on a frame

28

. Alternatively, transport unit guide

26

may be formed as a track or other elongate configuration for guiding workpiece transport units

30

,

32

thereon. The length and shape of workpiece conveyor

20

and transport unit guide

26

may be varied, and configured to permit the workpiece transport units

30

,

32

to access each processing station within the apparatus

10

.

In the illustrated embodiment, the workpiece transport unit guide

26

includes a spine that supports a pair of upper guide rails

36

,

38

mounted on opposite sides of the upper portion of spine

26

a

and a pair of lower guide rails

40

,

42

mounted on opposite sides of the lower portion of spine

26

a

. Each workpiece transport unit

30

,

32

preferably engages a respective pair of the upper and lower guide rails

36

,

40

and

38

,

42

. Each pair of guide rails can mount one or more transport units along the spine

26

a.

Each workpiece transport unit

30

,

32

is powered along the respective path by a suitable driver. More specifically, drive operators

61

,

64

are mounted to respective sides of transport unit guide

26

to provide controllable axial movement of workpiece transport units

30

,

32

along the transport unit guide

26

. The drive operator

61

,

64

may be linear magnetic motors for providing precise positioning of workpiece transport units

30

,

32

along the guide

26

. In particular, drive operators

61

,

64

are preferably linear brushless direct current motors. Such preferred drive operators

61

,

64

utilize a series of magnetic segments which magnetically interact with a respective electromagnet

69

mounted on each of the workpiece transport units

30

,

32

to propel the units along the transport unit guide

26

.

Cable guards

72

,

73

may be connected to respective workpiece transport units

30

,

32

and frame

28

for protecting communication or power cables therein. Cable guard

72

,

73

may comprise a plurality of interconnected segments to permit a full range of motion of workpiece transport units

30

,

32

along transport unit guide

26

.

As shown in

FIG. 3

, the workpiece transport unit

30

is coupled with a first side of the spine

26

a

of guide

26

, and the workpiece transport unit

32

is coupled to a second side of the spine

26

a

. Each workpiece transport unit

30

,

32

can include four linear bearings

136

,

140

,

138

,

142

for engagement with linear guide rails

36

,

40

,

38

,

42

respectively.

FIG. 4

illustrates a workpiece transport unit

30

which is substantially identical to the workpiece transport unit

32

. For simplicity, only the transport unit

30

will be described in detail. The transport unit

30

includes a robot arm or arm member

100

extending horizontally from a transport unit housing

106

at a base end of the arm member, to an edge-grip end effector

108

at a distal end of the arm member. The arm member

100

includes a first arm section

110

rotatably connected to a second arm section

114

. The first arm section

110

is rotatable about a vertical axis Z

1

with respect to the housing

106

. The second rotatable arm section

114

is rotatable about a vertical axis Z

2

with respect to the first arm section

110

. The end effector

108

is rotatable about a horizontal axis (or “flip” axis) R, perpendicular to the vertical axes Z

1

and Z

2

.

The housing

106

includes a vertically arranged base plate

120

, a first top cover plate

122

, a second top cover plate

124

, a bottom cover plate

126

and a U-shaped shroud

128

. The U-shaped shroud

128

comprises side walls

129

,

130

and a back wall

132

.

Mounted to the base plate are the four linear bearings

136

,

138

,

140

,

142

which receive the guide rails as shown in FIG.

3

. Arranged between the upper linear bearings

136

,

138

and the lower linear bearings

140

,

142

is a brushless motor

69

, which acts on the drive operator

61

of the guide

26

(shown in FIGS.

2

and

3

). A head reader linear encoder

149

provides a position signal corresponding to the position of the transport unit

30

on the guide

26

, to a control unit used to control the transport unit.

FIG. 5

illustrates the various components that are disposed inside of the housing

106

. As illustrated, a lift assembly

154

and cooperating components of arm assembly

100

are disposed within the housing

106

.

The lift assembly

154

includes the various components used drive the arm assembly

100

along vertical axis Z

1

. To this end, the lift assembly

154

includes a lead screw motor

156

which turns a threaded lead screw

158

that, and turn, is disposed for rotation within a lift bracket

160

. A lead screw nut

162

is threaded onto the lead screw

158

and fastened to a lift nut adaptor

164

. Vertical movement within the lift assembly

154

is guided by a linear rail

170

. Thus, rotation of the lead screw

158

about its axis will advance the nut

162

and the adaptor

164

upwardly, axially along the lead screw

158

. Reverse rotation of the lead screw motor

156

will lower the nut

162

and adaptor

164

along the lead screw

158

. A signal corresponding to the vertical position of the arm assembly

100

along the vertical axis Z

2

is provided by an absolute position sensor

165

.

The arm member

100

is connected to vertical rail

176

for movement along the vertical axis Z

2

. A vertical linear bearing assembly

170

having a track

172

and a sliding element

174

is arranged adjacent to the lift assembly

154

. The vertical member includes at a base end thereof a carrier plate

180

which is connected to the moving element

174

and the adaptor

164

such that the vertical rail

176

and the arm member

100

can be vertically raised and lowered by the adaptor

164

through actuation of the lead screw motor

156

. The linear bearing assembly

170

ensures a precise and stable vertical lifting of the vertical member. A lift encoder

177

is connected to the driven shaft of the lead screw motor

156

to send a precise lift position signal to a control for the transport unit.

FIGS. 6A and 12

illustrate a first rotational movement motor

200

which, by rotation of an output shaft

201

, effects rotation of the vertical member

176

and the first arm section

110

about the vertical axis Z

1

with respect to the housing

106

. The motor

200

is connected by a motor mount

202

to a lower housing

206

. The lower housing is connected by screws

210

to the carrier plate

180

. A coupling

214

connects the output shaft

201

of the motor

200

to an input shaft

218

of a tube assembly

220

. Between the tube assembly

220

and the lower housing

206

are arranged a bearing retainer

224

, a resolver sensor

226

, a roller bearing

230

(shown schematically), and a lower bearing retainer

232

. The resolver sensor

226

sends a precise rotary position signal of the tube assembly

220

with respect to the housing

106

to a control of the transport unit.

FIGS. 6B

,

8

and

9

illustrate the connection of tube

220

to a lower housing

242

of the first arm section

110

. Rotation of the tube

220

rotates the lower housing

242

and the first arm section

110

about the vertical axis Z

1

. A top cover

245

fits over the lower housing

242

to form a substantially closed volume

244

in which these components are held.

FIGS. 6B and 8

through

10

illustrate components for imparting rotation of the second arm section

114

about the vertical axis Z

2

. As shown, a second rotational motor

240

is housed within the tube

220

and the lower housing

242

. The motor

240

is vertically supported by a motor flange

248

which is fastened to a bottom wall

242

a

of the housing

242

and to the tube

220

. The flange

248

is also fastened to a top of the motor

240

as shown in

FIG. 8

, by fasteners (not shown). An output shaft

250

of the motor

240

receives a pulley flange

252

, a drive pulley

254

and a pulley clamp

256

which together constitute a driven pulley arrangement as shown assembled in FIG.

8

. The second rotation motor

240

includes a rotary position encoder (not shown) integrated therewith. The encoder sends a rotary position signal to a control unit for control of the transport unit operation.

As shown more clearly in

FIG. 10

, a wrist torque tube

260

is mounted for rotation within the lower housing

242

and is wrapped by an arm belt

290

. The arm belt

290

is driven by the drive pulley

254

. A bearing

264

(shown schematically) held by a bearing retainer

266

, and a torque tube retainer

272

support and guide the torque tube

260

. Upper and lower retaining rings

262

,

263

fit on the torque tube

260

and vertically retain the belt

290

circulating on the torque tube

260

. A read head mount

268

is mounted with a rotary absolute encoder

270

to the lower housing

242

. The rotary absolute encoder generates a rotational position signal of the second arm section

114

with respect to the first arm section

110

. The position signal is provided to a control for the transport unit. An absolute encoder cover

274

mates with the bottom of the lower housing

242

.

Located above the lower housing

242

is a robot wrist housing

280

fastened to the lower housing

242

, and a bottom cover

282

fastened to the torque tube

260

. Also held within the volume of the lower housing

242

is a flip axis amplifier

292

, and a spring loaded belt tensioner

294

.

Referring to

FIG. 9

, the tensioner

294

includes an idler pulley

295

for maintaining tension on the arm belt

290

. The idler pulley is carried by a plate

297

which is pivoted about a pin

296

with respect to the lower housing

242

. The plate is spring loaded by a spring (not shown) stretched between a fixed point on the lower housing

242

and a spring pin carried by the plate

297

. The force of the spring rotates the plate to press the idler pulley

295

against the belt

290

.

The second rotational motor

240

is selectively actuated to circulate the belt

290

which is wrapped around the wrist torque tube

260

. This actuation swings the second arm section

114

about the vertical axis Z

2

.

FIGS. 6B and 10

illustrate the flip axis components which allow rotation of the effector

108

about the horizontal axis R. Located beneath a flip axis cover

300

within the second arm section

114

is a flip axis motor

302

. The flip axis motor

302

is selectively actuated to rotate the end effector

108

about the horizontal axis R. The flip axis motor is connected to an actuator mount

304

. A bearing housing

306

is located within the cover

300

and holds a bearing

308

(shown schematically) together with a retainer

310

. A flip axis hub

312

is mounted to the end effector

108

.

The flip axis motor includes an output shaft

350

connected, at a back end of the motor

302

, to two rotary position encoders

351

. The redundant rotary position encoders provide a signal to a control unit of the transport unit that corresponds to the rotary position of the effector

108

about the horizontal axis R with respect to the second arm section

114

. The output shaft

350

is clamped to the flip axis hub

312

by the action of a clamp ring

352

and an interacting pressure flange clamp

354

which are squeezed between the flip axis hub

312

and a rear flange

356

of the effector

108

. The rear flange

356

is attached by fasteners to the flip axis hub

312

(registering fastener holes shown in FIG.

6

B).

The flip axis hub

312

includes an annular bearing surface

360

which is journaled for rotation by the bearing

308

. The bearing

308

is held in place by the bearing retainer

310

which is attached by fasteners to the bearing housing

306

(registering fastener holes shown in FIG.

6

B). The bearing housing

306

includes a base portion

362

which is fastened to the wrist torque tube

260

and to the bottom cover

282

by fasteners

364

. The actuator mount

304

is attached by fasteners

305

to a rear side of the bearing housing

306

. The actuator mount

304

is attached by fasteners to a front side of the motor

302

(registering fastener holes are shown in FIG.

6

B).

As illustrated in

FIG. 10

, a pneumatic cylinder

414

includes a spring

470

which exerts a thrusting force on a piston

472

which is connected to the plunger

434

via a threaded socket

473

. Pressurized air introduced into the port

422

acts on the piston

472

in opposition to the force of expansion of the spring and retracts the plunger

434

(to the left as shown in FIG.

10

).

As can be seen in

FIG. 10

, an annular space

600

is provided around the pneumatic cylinder

414

and beneath the flip axis cover

300

for the purpose of containing pneumatic tubing and signal and power conductors wound in a loose fashion to allow for rotation of the end effector

108

. This pneumatic tubing as well as the conductors can be routed from the space

600

backwardly, partly through the second arm section

114

, and downwardly through a central passage

260

a

of the torque tube

260

. Other conductors, such as from the motor

302

and the encoders

351

are routed via printed circuit cables disposed in cavities

260

b

. This arrangement winds up or unwinds these cables about torque tube

260

to thereby allow rotation of arm section

114

about axis Z

2

. The tubing and conductors can then be routed through the encoder housing

224

, upwardly into the volume

244

provided by the lower housing cover

245

, and down through the vertical member

176

, to exit the tube

220

at the opening

604

as shown in FIG.

6

A. To allow sufficient flexibility for the relative rotation between the first and second arm sections

110

,

114

, the conductors and tubing can be loosely coiled within the torque tube

260

before exiting.

FIGS. 13 through 16

illustrate one embodiment of the edge-gripping end effector

108

. As illustrated, the end effector

108

includes a paddle

400

extending from a base portion

400

a

(shown in

FIG. 19

) located over a bracket

402

. The paddle

400

is substantially Y-shaped with two substantially parallel prongs, a first prong

401

and a second prong

403

. A gripper body

404

is connected by fasteners

408

to the bracket

402

and acts to clamp the base portion

400

a

of the paddle

400

between the gripper body

404

and the bracket

402

. The pneumatic actuator

414

is connected to an upstanding leg

410

of the bracket

402

, connected by a plurality of fasteners

416

. The pneumatic actuator

414

is connected to the rear flange

356

of the effector

108

, by fasteners (not shown). The pneumatic actuator

414

includes the pressurized air inlet port

422

which can be a threaded opening for receiving a tube fitting of an air supply line (not shown).

The gripper body

404

includes a guide tab

428

at a front end thereof, overlying the paddle

400

. The guide tab includes, on a top surface thereof, a semicylindrical groove

430

. A plunger

434

is fit within a longitudinal bore through the gripper body

404

, in registry with the groove

430

. The tab

428

includes a ramp surface

440

on a front end thereof, declined downwardly in a forward direction toward a surface of the paddle

400

.

On a front surface of the gripper body

404

is a workpiece sensor

442

. The workpiece sensor is a light emitting and receiving sensor which emits a light beam and, if a workpiece is present on the paddle

400

, receives a light reflection from the workpiece. If no workpiece is present the reflection is not received, and a “no workpiece” signal or condition is transmitted. Preferably, the sensor

442

emits an infrared light beam.

At a front end of the paddle

400

are located two identical workpiece edge-gripping pins

450

,

452

. The pins are preferrably formed from plastic material. For simplicity, only the pin

452

will be described. As shown in

FIG. 17

, the pin

452

has a cylindrical body

456

with a radially extending top flange

458

and an intermediate base

460

. The base

460

fits onto a stepped region

462

of the prong

403

of the paddle

400

. A lower portion of the cylinder