WO1999033725A1

WO1999033725A1 - Contactless wafer pick-up chuck

Info

Publication number: WO1999033725A1
Application number: PCT/US1998/027111
Authority: WO
Inventors: Peter H. Rose; Haruo Tokunaga
Original assignee: Krytek Corporation; Kuraitekku Corporation
Priority date: 1997-12-30
Filing date: 1998-12-18
Publication date: 1999-07-08

Abstract

A semiconductor wafer pick-up chuck (300) including a support structure and a plurality of air bearings, each having a bearing surface associated with the support structure which supports a wafer (101) without contacting the bearing surface. The air bearings may be vacuum stabilized to hold the wafer (101) in position. Alternatively, mechanical devices can be used to clamp the wafer between upper and lower air bearings. The air bearings include at least one first port (203, 204) for providing a high pressure gas between the bearing surface and the wafer (101) and at least one second port (312, 314) for providing vacuum suction between the bearing surface and the wafer (101).

Description

CONTACTLESS WAFER PICK-UP CHUCK

BACKGROUND OF THE INVENTION

The invention relates to a system and method for contactless wafer pick-up chuck.

It is well understood to those engaged in semiconductor manufacture that the wafer on which semiconductor devices are formed must be moved from one process to another, and usually inside a particular processing tool itself. FIG. 1 is a top plan view of a generic example of a wafer handling system 100. Wafers 101 are provided to a process tool in a cassette 102 holding a number of wafers, typically 25, as is well understood by those engaged in the art of semiconductor manufacture. An air robot 106 with a vacuum pick-up 104 or a gravity pick-up can transfer a wafer into a second cassette 108 in a vacuum load lock, which can be isolated by an entrance valve 1 10 and an exit valve 112. After going through an appropriate vacuum cycle, the wafer is ready to be transferred by a pick-up 114 associated with a vacuum robot 116, and transferred from the load lock 108 to the processing position in one of the process chambers 118. Since the robot is in vacuum, a vacuum pick-up cannot be used and the pick-up must rely on gravity for adherence of the wafer to the pick-up chuck which if necessary can be assisted by an electrostatic chuck.

In all the above steps, which are only representative of actual processing activities, it will be appreciated that the back side of the wafer is in contact with the pick-up tool. Dirt and contamination are well known to be transferred to the back side of the wafer by the pick-up because the pick-ups become dirty through use. This dirt can interfere with the production process. For example, in lithography dirt between the back of the wafers and the process chuck will distort the wafer surface and impair the performance of the lithographic tool which has a very small depth of focus. This is just one example of the many ways contamination can interfere with processing.

The concept that air could be used to lubricate a bearing or support a load dates back more than a hundred years. Today, air bearings have found a wide range of applications, for example, on high speed grinding spindles, sliding ways on machine tools and on linear and rotating metrology devices. Applications extend to the minute read write heads which fly over magnetic memory discs with air gaps as small as 0.02 microns. For a detailed analysis of air bearings, see for example the work of D.D. Fuller "Theory and Practice of Lubrication for Engineers", John Wiley and Sons, 1984.

FIG. 2A is a side cross sectional view of the simplest form of a planar air bearing assembly 200 designed to move over or support a flat surface. As illustrated the figure shows a wafer 101 floating on a cushion of air between the wafer and an air bearing chuck 202. The air cushion, which must have a pressure above that of the surrounding atmosphere, is generated by high pressure air or a clean dry gas such as nitrogen provided to a tubular connection 205 connected to the air bearing, and thereafter leaked through a restrictive orifice 203 into the region between the wafer and the air bearing chuck.

As illustrated in the top plan view of FIG. 2B, the gas is fed to the center of the air bearing chuck via the centrally positioned restrictive orifice 203 commonly between 25 and 100 microns in diameter. A filter 204 located just before the orifice is commonly provided to remove particulates from the gas flowing through the orifice to prevent the small diameter orifice from becoming clogged by contaminants.

Typically, the high pressure gas is at 3 to 5 atmospheres, and the gap between the wafer and the chuck is between 2 and 20 microns. For example, a chuck having a diameter of 3.0 cm, and an air supply at 4 atmospheres pressure has a load bearing capability of 25 newtons and the thin air cushion has a stiffness of 11 newtons/micron at a gap of 2 microns. The small spacing requires that the chuck and the surface on which it moves must be smooth and precisely flat, which makes a semiconductor wafer an ideal candidate workpiece. At these small spacings, the impedance to gas flow between the orifice 203 and the outside of the chuck 202 is large and a relationship is established between the input pressure Pi, the pressure at the center of the chuck Pc and atmospheric pressure Pa, such that Pi » Pc > Pa.

The load that the vacuum chuck can bear will be

W = f PcA Newtons, where A is the area of the chuck and f is a factor which takes into account the pressure drop between the center and edge of the chuck. Due to the large pressure drop across the restrictive nozzle, the flow at the entrance to the nozzle reaches sonic velocities, and consequently the flow is choked and the flow becomes substantially independent of the outlet pressure Pc. If the flow is not choked, the flow becomes lamina and isothermal, and the volume of gas flowing through the bearing can be calculated using Euler's equation which takes into account compressibility. The calculated volume of gas flowing through the bearing is

K h³ rPi cc -Pa j 7_{cu w} ^■ _cf_jes/ / _sec

6μ\n(d_/dι) 2p_c

given by at a pressure Pc, and where μ is the viscosity, di and d₂ are the inner and outer diameters of the bearing as defined by FIG. 2B, and h is the spacing between the bearing and the wafer.

FIG. 2C is a top plan view of a more stable and preferred embodiment of an air bearing chuck 207. Multiple nozzles 208 are provided near the circumference of the air bearing chuck, which give greater stability when the loads on the chuck, as may often be the case, are asymmetric. The pressure inside the circle of nozzles is approximately constant which gives improved load bearing capability.

The distribution of pressure is shown graphically by FIG. 2D which shows the drop in pressure between the central pressure region and the outer circumference is almost linear for isothermal laminar flow. Preferably an air bearing should be very stiff.

A change in the load ΔW should produce a very small change in the gap h, a fact well understood by those skilled in the art of designing air bearings.

SUMMARY OF THE INVENTION The invention involves the application of air bearing technology to provide a means for picking up and moving a workpiece such as a silicon wafer with no or minimum contact. The workpiece, which can be any flat object, is supported only by the air cushion of one or more air bearings that prevent the workpiece from touching the bearings themselves. It is preferable that the air bearing be vacuum stabilized if the workpiece has a small mass like a wafer or it will not be held firmly in position and may even become unstable. It is also clear that for the pick-up to work, the surface of the workpiece and the plane in which the air bearings of the pick-up lie, must be flat to great precision, to approximately ± 1 micron over the area encompassed by the air bearings. A workpiece such as a silicon wafer has a flatness specification and sufficient conformabilty because of its thinness to lie within this range. A group of air bearings can also be made mechanically flat with this precision, provided mechanical stable materials and proper manufacturing techniques are employed as is implemented by those skilled in the art. Accordingly, in one exemplary embodiment of the invention a wafer can be picked up and floated on one or more air bearings and that the gap between the workpiece and the air bearing is small and can be controlled with good accuracy by vacuum loading.

In contrast, motion in plane of the workpiece and bearing is unrestricted because there is virtually no friction between the workpiece and the pick-up. Consequently, to have a practical pick-up device, the workpiece must be restrained in the plane of the bearing by some means. Most simply, the workpiece can be prevented from moving by mechanical means such as a series of pins around its periphery or by friction sufficient to prevent motion of the workpiece when the pick-up is moved. Mechanical restraints of these types can be restricted to the edge of the workpiece or to areas very close to the edge of the workpiece reducing contamination from these restraining means to a negligible amount. The invention also provides for all mechanical contact to be eliminated by replacing the mechanical fence or friction forces by using the aerodynamic drag of gas flowing over the workpiece surface to hold the edge of the workpiece against a series of air bearing cushions. By this means, the workpiece can be held firmly on the pick-up when it is moved. It should also be noted that when vacuum stabilized air bearings are employed, it is possible to hold the workpiece at any angle because the vacuum force prevents the workpiece from falling off the pick-up.

In principal, it is also possible to use an air bearing in a vacuum provided the flow of air leaking from the bearing is reduced by differential pumping. Stabilizing a bearing by suction is of course impossible in a vacuum but the bearing can stiffened by having two opposing bearings one on each side of the wafer. A force pressing the wafer between the two bearings will stiffen the bearings sufficiently to hold the wafer firmly.

Accordingly, the invention provides a semiconductor wafer pick-up chuck including a support structure and a plurality of vacuum stabilized air bearings each having a bearing surface associated with the support structure which supports a wafer without contacting the bearing surface. The air bearing may include at least one first port for providing a high pressure gas between the bearing surface and the wafer and at least one second port for providing vacuum suction between the bearing surface and the wafer

BRIEF DESCRIPTION OF THE DRAWINGS

FIG 1 is a top plan view of a generic example of a wafer handling system, FIGs 2 A-2C are top plan views of conventional air bearing chucks,

FIGs 3A and 3B are cross sectional views of alternative embodiments of air bearing assemblies with vacuum preloading,

FIG 4 is an example of a system showing that a gas bearing can be used in combination with a vacuum region provided the gas bearing region is separated from the vacuum region by a series of differentially pumped concentric channels,

FIGs 5 A and 5B are plan and a side views, respectively, of an exemplary pick-up chuck, FIGs 5C and 5D are schematic diagrams of the pick up chuck of FIGs 5 A and

5B showing the high pressure and vacuum connections to the bearings, FIGs 5E-5G illustrate an operational sequence showing how the pick-up chuck of FIGs 5 A and 5B can be used to pick up and remove a wafer from a cassette,

FIGs 6A-6C are a top plan partially cut away view and cross sectional views, respectively, of a pneumatic air bearing fence,

FIG 7A is a top plan partially cut away view of a pick-up chuck in accordance with the invention, FIGs 7B-7D are side view operational sequence diagrams of the method of using the chuck described with reference to FIG 7 A,

FIGs 8A and 8B are a top plan view and a side view, respectively, of an embodiment of a pick-up chuck in accordance with the invention, FIGs 8C and 8D are a top plan view and a cross sectional view, respectively, of yet another embodiment of a moveable clamp for use with a pick-up such as the pick-up chuck of FIG 8 A, FIGs 8E, 8F and 8G are top plan and cross sectional views, respectively, of an alternative embodiment of a moveable clamp in accordance with the invention,

FIGs 9A and 9B are top plan and cross sectional views, respectively, of yet another embodiment of a pick-up chuck in accordance with the invention, FIGs 9C-9F illustrate an operational sequence showing how the pick-up chuck of FIGs 9A and 9B can be used to pick up and remove a wafer from a cassette,

FIG 10 is a schematic diagram of an air bearing system, which shows how most of the gas from an air bearing can be prevented from entering a vacuum system by a series of concentric differentially pumped channels surrounding the air bearing region, and

FIGs 11 A and 1 IB are a top plan view and cross sectional view, respectively, of a pick-up chuck utilizing opposing differentially pumped air bearings which are used to support a wafer in a vacuum environment

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS Applying the air bearing concept to wafer pick-up and handling which will be used as an exemplary example of a workpiece is assisted by the wafers inherent smoothness and flatness The SEMI recommendation for a wafer includes the requirement for a mirror like surface, a flatness such that for a 300mm diameter wafer, the bow over the whole wafer is less than 65 microns and the warp is less than 75 microns The wafer is smooth enough and flat enough over a 10-50mm distances to be useable as one of the surfaces in an air bearing Additionally, a wafer has a degree of flexibility because it is thin, which gives it compliance to conform to the plane of the air bearings These fortunate circumstances make the application of air bearings for wafer pick-up and handling practical

The small mass of a wafer is also advantageous, the pick-up chuck can be made thin and light, but on the other hand the small wafer mass makes it difficult to provide stiffness in the air bearing unless the force on the bearing is increased by some means For air bearings of the type shown in FIG 2A and 2C, a mechanical means must be provided as illustrated to press the air bearings against the workpiece in the embodiments shown in FIGs 8 A and 9A In the embodiment of FIG 8 A, the wafer is clamped between an upper air bearing 814 pressing down on the top surface of the wafer and a lower air bearing 804 and pins 806a-c supporting the lower side of the wafer In this manner, the air or gas in the bearing gap is reduced to between 2 and 40 microns, depending on the application, giving the bearings adequate stiffness and force to support the wafer firmly and precisely

Alternatively, the bearings stiffness can be provided to pre-load the bearing with a suction force as illustrated in FIG 3A FIG 3A is a cross sectional view of a vacuum stabilized air bearing assembly 300 supporting a wafer 101 in accordance with the invention The assembly 300 has a cavity 302 evacuated by a connecting a vacuum pump to the tube 304 which evacuates the cavity 302 Pre-loaded bearings of this type are commercially available, for example, from New Way Machine Components, Inc The stiffhess and gap of the bearing can be controlled by varying the suction pressure and the area of the suction cavity in order to provide contactless pick-up of a semiconductor wafer 101. FIG. 3B is a cross sectional view of an air bearing assembly 310 in accordance with the invention, which shows that the suction force could also be exerted by a pumped annulus 312 surrounding the bearing area.

For example, a vacuum compensated air bearing 300 with the suction region 302 having a diameter of 2.5cm and a bearing gap h=10 micrometers will have a gas flow into the suction region of about 3 Torr cmVsec. A tube connected to the suction region with an inner diameter of 10mm and a 1.5 meters long, and connected to a diaphragm vacuum pump would have no difficulty reducing the pressure in the suction region to one half an atmosphere or below. At half an atmosphere, the suction force on the bearing would be 2.5kg more than enough to compress the bearing to a reasonable working gap of 10 micrometers.

The flexibility of the wafer 101 must be considered carefully when designing a pick-up. For example, consider the situation where a wafer is supported by a large flat air bearing and in the center of which there is a circular aperture of diameter di where the air pressure is reduced to below atmospheric pressure as is the case with the vacuum stabilized air bearing 300. A pressure difference of one third of an atmosphere over this region, i.e., a 1cm diameter hole will bow a 0.8mm thick, 200mm diameter wafer by 50 microns at the center. However, it will be appreciated by those skilled in the art that the same holding force can be exerted by a plurality of smaller vacuum regions of diameter d₂ with distortion reduced by

but adding up to the same holding force distributed over the area of the bearing. The air bearing 310 has the advantage that the wafer deflection over a narrow suction annulus will be very small. Air or gas bearings are not only employed when they are surrounded by atmospheric pressure air, but also when they are surrounded by corrosive gases or a vacuum. For example, an air bearing has been described by Fox in U.S. Pat. No. 4,191,385 where an air bearing surrounds a high vacuum region. In order to prevent gas from the bearing from flowing into the vacuum system, the embodiment of a air bearing assembly 400 shown in FIG. 4 includes three concentric channels, 402, 404 and 406, which isolate gas flow from the air bearing from a vacuum system 408. Channel 402 allows gas from the bearing to vent to air, channels 404 and 406 are connected to vacuum pumps 410, 412, 414 which reduce the pressure in stages so the pressure in the vacuum system can be maintained at the desired level.

A wafer pick-up is often used to transfer a wafer from one cassette to another. The pick-up must be at least 1mm thinner than the gaps between wafers in a cassette to 5 avoid interference at the time of pick-up or replacement. Using this as a guide and referring to the SEMI cassette spacing specification, the maximum thickness of the pickup for wafers of different size is shown in the table below. Note that all dimensions are in millimeters.

0 Wafer Wafer Cassette Pick-up

Diameter Thickness Pitch Thickness

150 .67 4.8 3.1

200 .73 6.3 4.6

300 .80 10.0 8.2 5

The table shows that as the wafers become larger, the pick-up becomes considerably thicker. It is, therefore, much easier to provide for the high pressure gas and vacuum connections to the air bearings as the wafer size increases. This is particularly 0 advantageous for the vacuum connection, which should have as large a cross-section as possible.

FIGs. 5A and 5B are a top plan view and a side view, respectively, of an exemplary pick-up chuck 500 which includes pins 502a-502c surrounding a wafer 101, and vacuum assisted air bearing assemblies 504a-504c. The pin 502a is designed so that it

25 can be extended immediately after pick-up and retracted after placement, allowing the pick-up to be withdrawn.

FIGs. 5C and 5D are respective top plan and cross sectional views of schematic diagrams of the pick-up chuck 500 showing the gas line connections for the air bearing assemblies. An air source 506 and vacuum source 508 are connected via lines 510 and

30 512, respectively, to each of the air bearings 504. Quick acting valves 514,516 are required to activate the gas and vacuum connections. Connections to the air bearings can be made in a variety of ways, and FIGs. 5C and 5D are meant to be merely exemplary of the variety of arrangements possible.

FIGs. 5E, 5F and 5G are side view operational sequence diagrams showing how the pick-up 500 is used. The pick-up 500 is inserted between two wafers 101 a, 101c in a cassette 520 of wafers lOla-lOld as illustrated in FIG. 5E. The gas and vacuum is supplied to the air bearings by opening the quick acting valves 514 and 516, the pick-up is raised as shown in FIG. 5F so the wafer 101a is no longer resting on the cassette, but held on the air bearings 504a-c, and the pin 502a is raised to restrain the wafer 101a. The pick-up 500 can now be withdrawn from the cassette 520 as shown in FIG. 5G. The wafer is now resting on the air bearings and is prevented from sideways motion by the pins 502a, 502b, and 502c. The vacuum suction forces of the enhanced air bearing 500 will allow the pick-up to be rotated and even prevent the wafer from falling off if the wafer is held upside down.

In accordance with another embodiment of the invention, restraint in the plane of the pick-up without mechanical contact can be provided by using the pneumatic properties of an air bearing at the edge of the wafer, thus minimizing or reducing the edge contact as illustrated by the pneumatic fence of FIGs. 6A-6C. FIGs. 6A-6C are a top plan view and partial cross sectional views, of an air bearing fence assembly 600. The assembly includes a housing 602 which envelopes the edge of a wafer 101 within an annular chamber 604. Selected regions of the chamber 604 are alternately air bearings 606 or vacuum suction regions 610 as illustrated in FIGs. 6B and 6C, which are taken along section lines A-A and B-B of the housing 602. The air flowing across the wafer surface from the suction regions 610 drags the wafer towards the air bearings 606. The wafer's edge is prevented from contacting the edge of the fence bv the high pressure air Pi emerging from the air bearings 606 forming a small air cushion or air bearing. The fence assembly 600 can be incorporated as part of a pick-up as shown for example in FIG. 7A. FIG. 7A is a top plan partially cut away view of a pick-up chuck 700 in accordance with the invention. The surface 703 of the pick-up 700 is illustrated with one vacuum assisted air bearing 704 located midway between the center and the edge of wafer 101. However, as is understood by those of skill in the art, a plurality of air bearings can be employed when that is desirable. The chuck also includes a high pressure air source 706 and a vacuum source 708.

The method of using the chuck 700 is described with reference to the side view operational sequence diagrams of FIGs 7B-7D The pick-up 700 is inserted into a cassette 710 of wafers lOla-lOld such that the pick-up does not touch either the upper or lower side the selected wafer 101a The degree of insertion is such that the pneumatic fence 600 does not touch the edge of the wafer and is a small distance from it The air

5 bearing 704 is activated by opening the quick acting valves 706 and 708, and the pick-up is raised so the air bearing 704 holds the wafer firmly and the wafer is no longer in contact with the cassette Gas and vacuum suction is immediately supplied to the pneumatic fence 600 by opening valves 710 and 712 so that the wafer is sucked against the air bearing cushions in the pneumatic fence, thus holding the wafer firmly in position The wafer

10 101a can then be withdrawn from the cassette and moved to some other position

In some instances a small area of contact with the backside of the wafer may be allowable and even desirable and an embodiment using a combination of pins and air bearings is illustrated in FIGs 8A and 8B, which are a top plan view and a side view, respectively, of an embodiment of a pick-up chuck 800 in accordance with the invention

15 The pick-up chuck 800 includes a lower support 802 having an air bearing assembly 804 and pins 806a-806c pressing against the bottom outer side of a wafer 101 A moveable upper clamp 808 has a hinge bearing 810 and an operating mechanism 812 to control the lowering and raising of the clamp The end of the clamp over the top surface of the wafer incorporates an air bearing assembly 814

20 In order to pick-up a wafer, the upper clamp 808 of the pick-up is raised slightly by a spring 817 in the bellows 819 of the mechanism 812 so the pick-up can be inserted under the wafer 101 High pressure gas is supplied to the air bearings 804 and 814 by opening the valve 805 The pick-up is raised until the wafer rests on the air bearing 804 and the two or more small pins 806a-c located on the pick-up at the outer bottom surface

25 of the wafer, and the wafer no longer touches the cassette The upper clamp 808 is then closed by opening valve 816 applying a vacuum to the operating mechanism 812 which contracts the bellows, thus compressing the spring and causing the movable arm to press the air bearing 814 against the top surface of the wafer

The wafer is now clamped in position by the balance of the forces between the air

30 bearing 814 on the movable clamp 808, the pins 806a-c and the air bearing 804 supporting the underside of the wafer The pins press against the lower wafer surface and the wafer is prevented from moving by the frictional forces between these pins and the wafer. The pins may be made of any material which does not scratch or contaminate the back side of the wafer. A preferred material would be Delrin. The pick-up chuck 800 of FIG. 8A is a very practical embodiment, because wafers are usually stored in cassettes where some edge contact is unavoidable, and the small contact area of the pins of this

5 pick-up would add a negligible amount of contamination compared with the conventional gravity or vacuum pick-up.

FIGs. 8C and 8D are a top plan view and a cross sectional view, respectively, of yet another embodiment of a moveable clamp 820 for use with a pick-up such as the pickup chuck 800 of FIG. 8A. The clamp 820 utilizes an air bearing assembly 822, a first

10 control mechanism 824 consisting of a bellows 819, and second control mechanism 826 consisting of a spring 817 disposed on the other side of a hinge bearing 810. An inlet 830 provides high pressure air via valve 832 to line 834 for feed to the air bearing assembly 822. The high pressure gas the bellows 819 of the control mechanism 824, thus lowering the air bearing 822 to the top wafer surface against the force of the spring 817 which

15 normally holds the movable clamp open.

FIGs. 8E, 8F and 8G are top plan and cross sectional views, respectively, of an alternative embodiment of a moveable clamp 840 in accordance with the invention. The clamp 840 utilizes a vacuum enhanced air bearing assembly 842, a raise/lower control mechanism 844 comprising a bellows 819 and an internal spring 817, and a hinge bearing

20 810. An inlet 850 provides a vacuum suction via valve 852 to line 854 for feed to the air bearing assembly 842 and the bellows 819, thus opening valve 852 simultaneously compresses the spring and brings the air bearing in contact with the wafer. An inlet 860 provides high pressure air via valve 862 to line 864 for feed to the air bearing assembly 842. The configuration provides increased holding forces and precision by adding the

25 vacuum suction in the pick-up chuck.

The motion of the clamps 808, 820 and 840 can also be provided by a number of means such solenoids or motors, but it is convenient to use the pressurized gas or the vacuum needed for the air bearing to control their motion, thereby avoiding the need for a separate activating mechanism. For example, referring again to FIG. 8C, when the valve

30 832 is opened, high pressure gas inflates the bellows of control mechanism 824 so that the clamp 820 closes, bringing the activated air bearing 822 down to the wafer. When the valve 832 is closed, the pressure bleeds off and the spring 826 raises the clamp as it pivots on the hinge bearing 810.

The vacuum channels 854 can occupy most of the cross section of the clamp 840, giving the channel a high conductance as shown by the cross-section of FIG. 8F. Since the clamp has thin walls, it is prevented from collapsing under vacuum by support members 872, shown in the FIG. 8F taken along section line A-A of FIG. 8E, which divides the interior of the thumb into separate vacuum channels 854. An internal spring within the bellows 819 elongates the bellows when the vacuum is released, thus raising the clamp.

FIGs. 9A and 9B are top plan and cross sectional views, respectively, of yet another embodiment of a pick-up chuck 900 in accordance with the invention. The pickup chuck 900 includes a lower support member 902 and a moveable clamp 904. The movable clamp 904 utilizes air bearing assemblies 906, a raise/lower control mechanism 908, a spring 910 and a hinge bearing 912. An inlet 914 provides high pressure air via valve 916 for feed to the air bearing assemblies 906. The method of using the chuck 900 is described with reference to the side view operational sequence diagrams of FIGs. 9C-9F. In order to pick-up a wafer 101 from a cassette 920, the clamp 904 is raised slightly and the lower support 902 of the pick-up is inserted under the wafer. The lower support is raised so that the outside lower surface of the wafer is supported by three pins 918a-c. Upon opening the valve 916, high pressure gas flows to the air bearings and to the bellows of the control mechanism 908. The internal bellows of the control mechanism 908 forces the clamp down to the wafer surface until stopped and prevented from contact with the surface of the wafer by the gas pressure in the air bearing. The force exerted by the bearing can be controlled by selecting the bellows size and stiffness of the spring. The closure of the clamp, as shown in FIG. 9E, lifts the far edge of the wafer so that it is in the plane of the lower support 902, and the wafer can be removed from a cassette without touching the sides of the cassette. The friction of the pins on the back side of the wafer prevent the wafer from moving during transport. When the valve 916 is closed, the spring 910 raises the clamp to release the wafer. In the example chosen to illustrate the prior art of wafer handling and transport, as shown in FIG. 1, the air robot transfers a wafer or other workpiece from a cassette into a vacuum lock. It has been explained how this can be done without the pick-up touching the wafer. In a vacuum, the surrounding gas is effectively at zero pressure, suction therefore cannot be exerted to hold the wafer close to the bearing, and in any case the gas from a normal air bearing would overload the vacuum system.

FIG. 10 is a schematic diagram of a system 1000, which shows how most of the gas from an air bearing can be prevented from entering a vacuum system. The cross section shows two opposing pick-up chucks 1002, 1004 holding a wafer 101 between them. The gas escaping from the circumference of the air bearings provided by inlets 1005, 1007 is pumped away by two differentially pumped annular channels 1006 and 1008 having pumps 101 1 and 1012, substantially preventing the air leaking from the air bearing from entering the vacuum system. By this means, the gas load on the vacuum system can be reduced to a manageable level. The tolerance of the process chamber to a rise in gas pressure will depend upon whether wafer processing is continued during wafer transfer. A pressure rise to 10^" or even 10^" Torr can be tolerated in many vacuum systems if the process is not active at the time of the workpiece movement. The provision of opposing bearings holds the wafer firmly in position as illustrated by FIG 10.

FIGs. 1 1 A and 1 IB are a top plan view and a cross sectional view of an assembly 1100 that includes an upper support 1103 and a lower support 1105. The assembly includes one pair of opposing differentially pumped air bearings 1102, 1104, which hold a wafer 101 in a fixed plane. This exemplary embodiment of a pick-up, capable of operating in a vacuum, employs mechanical pins 1106a- 1106c to hold the wafer in position preventing motion when the wafer is moved. The pin 1106a is extended after the wafer has been picked up.

The foregoing descriptions have been set forth to illustrate the invention and is not intended to be limiting. Since modifications of the described embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art, the scope of the invention should be limited solely with reference to the appended claims and equivalents thereof.

What is claimed is:

Claims

CLAIMS 1. A workpiece pick-up chuck comprising: a support structure; and at least one vacuum stabilized air bearing having a bearing surface associated with said support structure which supports a workpiece in the plane of said bearing surface without contacting said bearing surface, said air bearing including at least one first port for providing a high pressure gas between said bearing surface and said workpiece and at least one second port for providing vacuum suction between said bearing surface and said workpiece.

2. The chuck of claim 1, wherein said at least one air bearing is positioned to support a surface region of said workpiece proximate to an edge of said workpiece.

3. The chuck of claim 2, wherein said another air bearing is positioned to support a central surface region of said workpiece.

4. The chuck of claim 1, wherein said support structure comprises a plurality of elements adjacent the edge of said workpiece which contain said workpiece in the proximity of said at least one air bearing.

5. The chuck of claim 1, wherein said at least one air bearing is positioned to support a first surface region of said workpiece.

6. The chuck of claim 2, wherein said another air bearing is positioned to support a second surface region which is on the opposite side of said workpiece.

7. The chuck of claim 6, wherein said another air bearing is associated with an arm which extends parallel to said support structure.

8. The chuck of claim 7, wherein said arm comprises an adjustable clamping arm.

9. The chuck of claim 8 further comprising adjustment means for raising and lowering said arm with respect to said support structure.

10. The chuck of claim 9, wherein said adjustment means comprises an assembly including a bellows and a spring.

11. The chuck of claim 1, wherein said chuck is associated with a robotic arm.

12. A semiconductor wafer pick-up chuck comprising: a support structure; and a plurality of vacuum stabilized air bearings each having a bearing surface associated with said support structure which supports a wafer without contacting said bearing surface, said air bearing including at least one first port for providing a high pressure gas between said bearing surface and said wafer and at least one second port for providing vacuum suction between said bearing surface and said wafer.

13. The chuck of claim 12, wherein at least one of said air bearings is positioned to support a surface region of said wafer proximate to an edge of said wafer

14. The chuck of claim 13, wherein said another of said air bearings is positioned to support a central surface region of said wafer.

15. The wafer of claim 12, wherein said support structure comprises a plurality of elements adjacent the edge of said wafer which contain said wafer in the proximity of said air bearings.

16. The wafer of claim 12, wherein at least one of said air bearings is positioned to support a first surface region of said wafer.

17. The chuck of claim 13, wherein another of said air bearings is positioned to support a second surface region which is on the opposite side of said wafer.

18. The chuck of claim 17, wherein said another of said air bearings is associated with an arm which extends parallel to said support structure.

19. The chuck of claim 17, wherein said arm comprises an adjustable clamping arm.

20. The chuck of claim 19 further comprising adjustment means for raising and lowering said arm with respect to said support structure.

21. The chuck of claim 20, wherein said adjustment means comprises an assembly including a bellows and a spring.

22. The chuck of claim 12, wherein said chuck is associated with a robotic arm.

23. The chuck of claim 12, wherein said plurality of vacuum stabilized air bearings are positioned orthogonal to the surface plane of said wafer so as to interact with one or the other of the planar surfaces of said wafer.

24. The chuck of claim 12, wherein said plurality of vacuum stabilized air bearings are positioned parallel to the surface plane of said wafer so as to interact with the circumferential edge surface of said wafer.

25. The chuck of claim 24, wherein said plurality of vacuum stabilized air bearings are positioned within a channel housing associated with said support structure which partially envelops the circumferential edge of said wafer.

26. The chuck of claim 25, wherein said support structure further comprises at least one vacuum stabilized air bearing which is positioned to interact with a planar surface of said wafer.

27. A semiconductor wafer pick-up chuck comprising: a support structure; at least one air bearing having a bearing surface associated with said support structure which interacts with a wafer without contacting said bearing surface, said air bearing including at least one first port for providing a high pressure gas between said bearing surface and a first surface of said wafer; and means for interacting with a second opposite surface of said wafer such that said wafer is supported between said interacting means and said at least one air bearing.

28. The chuck of claim 27, wherein said support structure comprises an arm which extends parallel to said support structure.

29. The chuck of claim 28, wherein said arm comprises an adjustable clamping arm.

30. The chuck of claim 29 further comprising adjustment means for raising and lowering said arm with respect to said support structure.

31. The chuck of claim 30, wherein said adjustment means comprises an assembly including a bellows and a spring.

32. The chuck of claim 28, wherein said at least one air bearing is positioned on said arm.

33. The chuck of claim 27, wherein said interacting means comprises at least one element which minimally contacts said second side of said wafer.

34. The chuck of claim 27, wherein said interacting means comprises at least one air bearing.

35. The chuck of claim 27, wherein said interacting means comprises at least one bearing and at least one element which minimally contacts said second side of said wafer.

36. The chuck of claim 27, wherein said chuck is associated with a robotic arm.