Machine For Grinding An Edge Contour On A Semiconductor Wafer

Abstract

A grinding machine for producing a rounded chamfer on the edge of a workpiece such as a semiconductor wafer. The workpiece is attached to a revolving chuck and a grinding head having oblique resilient grinding members is located opposite the chuck. The grinding members lie along generators of a cone and extend beyond the edges of the workpiece and press against the edge. Semiconductor wafers are ground in this machine by pressing their edges against the resilient grinding devices while rotating the wafer and moving the grinding devices in a lateral circular path concentric with the axis of rotation of the wafer.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a machine for grinding semiconductor wafers, and more specifically it relates to machine adapted to chamfer the edge of such wafers.

2. Description of the Prior Art

In the production of semiconductor devices, such as transistors, integrated circuits, and solid-state targets for image pick-up devices, one of the important processes is vapor-deposition or epitaxial-growth on the semiconductor wafer substrate. The wafers are usually sliced from an ingot of single crystal semiconductor material. If the edges of the wafers are not chamfered, the epitaxial material tends to build up a ridge at the perimeter of the wafers and this ridge interferes with proper usage of the coated wafers. For one thing, it makes unusable the area that is excessively heavily coated and for another thing it prevents the usual optical mask from being placed directly in conact with the major part of the surface of the coated wafer, thereby interfering with the precise optical focusing considered necessary for production of semiconductor devices. Hence, an attempt has previously been made to chamfer the edge of such wafers, but the machines used to carry out the chamfering have created addiional problems. For one thing, semiconductor wafers are usually round with a straight edge portion along one side related to the crystallographical structure of the semiconductor material. The chamfering devices used heretofore have not been able to achieve equal chamfering of both the round and straight portions of the total perimeter of such wafers, and this has lead to unequal growth of the epitaxial material.

Accordingly, it is one of the objects of the present invention to provide an improved machine for grinding the edge portion of semiconductor wafers.

A further object is to provide an improved machine capable of grinding specific contours on the edge portions of semiconductor wafers having perimeters which are primarily round but have straight portions at one specific location.

Still further objects will be apparent from the following specification together with the drawings.

BRIEF DESCRIPTION OF THE INVENTION

In accordance with the present invention, semiconductor wafers which are to be ground, or chamfered, along the edge are mounted on rotatable chucks beneath grinding heads which comprise separately rotatable devices so arranged they they follow eccentric curves with respect to the axis of the heads on which the wafers themselves are mounted. The grinding devices, which extend downwardly from the heads, are short rods or strips bent or formed at appropriate angles to achieve the desired ground contour of the edge of the wafers. Using these grinding strips in a machine with the combined relative eccentric and rotary motions of the grinding heads and the chucks produces uniformly contoured and lapped edges of semiconductor wafers of different sizes without having to change to grinding strips of different configuration. The contour of the edges of the semiconductors can further be controlled by raising or lowering the grinding heads.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are enlarged cross-sectional views illustrating epitaxial-grown layers on semiconductor wafers.

FIG. 3 is an enlarged perspective view illustrating the shape of semiconductor wafers.

FIG. 4 is a drawing illustrating one example of a conventional grinding machine.

FIG. 5 is an enlarged partial view of a wafer illustrating the specific profile ground according to the present invention.

FIG. 6 is a schematic front view of the grinding machine of the present invention.

FIG. 7 is a cross-sectional view of the grinding machine along the line A--A in FIG. 6.

FIG. 8 is a cross-sectional view of the machine along the line B--B in FIG. 6.

FIGS. 9 and 10 illustrate the grinding of wafers by resilient grinding tools.

FIG. 11 illustrates one example of the eccentric movement mechanism used in the machine shown in FIG. 6.

FIG. 12 is a cross-sectional view, partially broken away, illustrating one example of a vacuum chuck of the type shown in FIG. 6.

FIGS. 13 and 14 are enlarged side-elevational views illustrating one example of a holder of the resilient grinding devices used in the machine in FIG. 6.

FIG. 15 is an enlarged view illustrating the resilient grinding tool in FIG. 14.

FIG. 16 is a cross-sectional view of a grinding device along the line C--C in FIG. 15.

FIG. 17 is a cross-sectional view illustrating a resilience adjusting ring along the line D--D in FIG. 13.

DETAILED DESCRIPTION OF THE INVENTION

If flat wafers 1 of the type shown in FIG. 1 are subjected to vapor-depositing to produce an epitaxial layer 2, a ridge 3 is built up at the edge 1a of the wafer because growth of the vapor-deposited material is not uniform over the entire surface of the wafer. Such wafers have the disadvantage that the ridge portion 3 is not suitable for further manufacturing processes and the further important disadvantage that a photo-mask cannot be applied in contact with the surface of the wafer in photographically processing the semiconductor devices. As a result of the latter disadvantage, resolution will be reduced or the photo-mask or the wafer itself or a photoresist layer on the wafer will be damaged. Thus, the quality of the complete devices is deteriorated and productivity is also reduced.

To avoid such disadvantages, it has been proposed to make wafers having a profile of the type illustrated in FIG. 2. A beveled portion 1b is produced on the edge of the wafer 1. However, it is difficult to bevel the wafers uniformly. This is due to the fact that semiconductor wafers usually have a flat portion 1c, as shown in FIG. 3, which is used to align the wafers according to their cyrstallographical orientations.

FIG. 4 shows one example of a conventional technique for beveling the wafer 1. The wafer is fixed on a support 4 and is ground, or lapped, by a cylindrical grindstone 5 rotating on its own axis. Simultaneously, either the grindstone 5 is rotated around the periphery of the wafer 1 so that the axis of the grindstone generates a generally conical surface of revolution, or the wafer 1 is rotated about a vertical axis while the grindstone is rotated in a single location. The flat edge 1a' and the round edge 1a cannot be lapped to have the same profile because the axis of the grindstond tilts more nearly perpendicular when it reaches the flat portion 1a'. The grindstone causes rather deep mechanical distortion in the wafer, and as a result crystal defects occur in the epitaxial-grown layer 2 due to such distortion.

Both the round edge portion 1a and the flat edge 1a' should have the same specific cross-sectional profile. A particular form of this profile is illustrated in FIG. 5 in greater detail. The angle .theta. and the width w of the beveled portion 1b have specific values with respect to the thickness t of the wafer 1. The surface of the beveled portion 1b is rounded with a curvature R. For example, if t = 250.mu..about.300.mu., it is preferable that w .div. 0.5mm. and .theta. = 10.degree..about.15.degree.. These data have been selected on the basis of extensive experiments by the present inventors.

FIGS. 6, 7, and 8 show one embodiment of a grinding machine incorporating the present invention. A frame 6 forms the body of the grinding machine and has a rectangular oil pan 7 with an outlet 7' at one side of it. The oil pan is set into the top of the frame. A panel 8 is located above the oil pan 7 and columns 9 extend up from the oil pan to support the panel. The columns 9 are joined to the panel 8 and to the oil pan 7 by individual connectors 9'. A swinging plate 10 between the oil pan 7 and the panel 8 makes an eccentric rotational movement. The mechanism of the eccentric rotation will be described hereinafter in connection with FIG. 11.

The swinging plate 10 is provided with two rows of holding means 11. Each of these holding means comprises a cylinder 12 operated by air or hydraulic fluid through suitable air or hydraulic lines 12a and 12b. In addition, there is a holder 13 located below the swinging plate 10 and a sliding rod 14 mounted in the holder 13. Each rod 14 is prevented from rotating around its own axis but can be moved up and down by a piston in the cylinder 12. Attached to the lower end of each sliding rod 14 is a grinding device 15 that comprises a plurality of tools in the form of resilient strips.

Vacuum chucks 16, each having a head 17, are provided on the oil pan 7 at locations corresponding to each of the grinding devices 15. One of the wafers 1 is held and rotated by each of the heads 17 and is contacted by the lower surfaces of the strips attached to the grinding device 15. Each grinding device 15 rotates eccentrically with respect to the wafer below it, and each wafer is rotated by the respective head 17. Thus, the edge of each of the wafers 1 is ground to have a specific cross-section. During the grinding process, suitable amounts of grinding liquid, such as water, light oil, etc., are supplied to the wafers 1.

The grinding machine also includes a common seat plate 18 and an elevator 19 for a wafer magazine. These items will be described in greater detail hereinafter.

FIG. 11 shows an eccentric rotation mechanism 20. A holder 21 is bolted to the upper panel 8 and a gear 22 is rotatably supported below the panel 8 by means of a short hollow shaft 23 held in two bearings 24. The upper end of the shaft 23 is threaded and a nut 25 is fitted thereon to hold the shaft 23 in place. While the shaft 23 is hollow, its inner bore 23' is not concentric with the outer surface of the shaft but is offset with respect thereto by a distance a. Below the gear 22 is a support structure 26 which comprises a main shaft 27 having an upper flange 28a and a lower flange 28b. An integral extension 29 of the main shaft 27 extends into the bore 23'. The axis of the main shaft 27 is offset with respect to the axis of the integral extension 29 by a distance b. Below the lower flange 28b, at the lower end of the main shaft 27, is another integral extension 30 which is also an integral extension of the main shaft 27, and both the integral extensions 29 and 30 are threaded at their outermost ends.

The extension 29 is held by means of a nut 31 and is provided with an adjusting knob 32. This extension is capable of rotating within the bore 23' but has a number of preferred stopping points determined by a detent arrangement comprising a spring-biased ball 33 and a number of recesses 34. By selecting the appropriate one of these recesses, the eccentricity of the main shaft 27 with respect to the axis of the gear 22 and its shaft 23 may be adjusted to a number of values between a - b and a + b. If b=a, a - b = 0, which means that the axis of the main shaft 27 may be brought into alignment with the axis of the shaft 23. In the position indicated in FIG. 11, the main shaft 27 is offset to the maximum eccentricity, which is a + b. The integral shaft must be rotated 180.degree. within the bore 23' to bring the axis of the main shaft 27 into alignment with the axis of the shaft 23. The recesses 34 in the upper flange 28a may be equally spaced around the flange 28a and identification of the eccentricity can be determined by noting the angular alignment of the knob 32 and by listening to the clicks as the ball 33 moves from one recess 34 to the next. The ball can retract into a hole 73 in the gear 22 to move from one recess 34 to the next but is pressed against the flange 28a by a spring 74. When the knob 32 has been set to the position that causes the axis of the shaft 27 to have the correct eccentricity, the nut 31 must be tightened to secure the shaft 27 in place.

The other integral extension 30 of the main shaft 27 is supported in a hanger 35 by a pair of bearings 36 separated by a spacer 40. The hanger 35 is bolted directly to the swinging plate 10. A nut 37 is threaded on the end of the extension 30 to hold it in place, and a ring 41 is threaded into the end of the hanger 35 to hold the bearings 35.

Referring back to FIG. 6, it will be seen that there are two of the swinging mechanisms 20 in the grinding machine. Each of the mechanisms 20 acts like an eccentric having a small throw, which is determined by the eccentricity setting of the knob 32. Both of the mechanisms 20 must have the same eccentricity. The gears 22 of both of these mechanisms are coupled to a common pinion 42, which is connected to the low-speed shaft of a gear motor 43. By this mechanism, the swinging plate 10 can be moved laterally around a circular path; but since there are two mechanisms 20, the plate 10 is constrained to move with a translatory motion and cannot rotate about the axis of either of the mechanisms 20.

FIG. 12 shows the in greater detail the vacuum chuck mechanism 16 and its rotatable head 17. As stated, the vacuum chuck 16 extends through the oil pan 7 and through the common seat plate 18, which is bolted to the oil pan 7. A bearing holder 44 is bolted to the common seat plate 18 and extends through the oil pan 7. This bearing holder plate has two bearings 45 in it and separated by a spacer 46. A rotatable hollow shaft 47 is located in the bearings 45 and extends both below and above the bearing holder 44. The upper end of the hollow shaft 47 is an enlarged head 48 that has a re-entrant lower grooved surface that fits over a raised nipple on the common seat plate 18. A gear 49 is rigidly attached to the shaft 47 to rotate the shaft, and a nut 50 is threaded onto the lowermost end of the shaft to hold the entire shaft assembly together. The gear 49 extends through an open section of the cover 51 and meshes with a main gear 53 that rotates the gear 49 and the shaft 47.

The vacuum system for the chuck 16 includes a pipe fitting 52 that has a pipe column 52' which extends up into the shaft 47. A vacuum line 54 indicated as a single dotted line is connected by an elbow 55 to the pipe fitting 52. Both the head 17, which extends down into the enlarged head 48, and the pipe column 52' have packing material 56 to achieve the necessary hermetic seal to prevent leakage of air in the vacuum system.

Several concentric circular grooves 17a and radial grooves are formed on the upper surface of the head 17 as shown in both FIG. 8 and FIG. 12. A small hole 17c connects these grooves and an air chamber 17b. The pressure within this chamber can be reduced by means of the exhaust system 54 so as to hold a wafer 1 tightly in place on the upper surface of the head 17.

The actual grinding mechanism will be described with reference to FIGS. 13-17. FIG. 13 shows an enlarged view of the holder 13 with the sliding rod 14 extending below it. A head 57 is screwed into the lower end of the rod 14 and a hollow central tube 58 is inserted into a hole at the center of the head 56. This tube is held in place by frictional engagement with the head and is joined by means of a connector 71 to a line 72 through which grinding fluid is supplied to the workpiece during the grinding operation.

The grinding elements themselves are bent strips 60 of a suitable material, such as stainless steel. Eight of these strips are attached to the lower side of the head 57 and are equally spaced around the head. Each of the strips comprises an upper or shank portion 61 that extends generally parallel to the axis of the holder 13 and a lower end portion 62 bent with respect to the upper end so that the included angle between the upper and lower ends is greater than 90.degree.. The strips 60, which are wider than they are thick, are bent so that the direction in which the thickness of the upper or shank portion and lower end portion is measured is in a common plane. The lower surface of the lower end portion 62 of each of the strips 60 is coated with a layer 62' of abrasive material, such as powdered diamonds, carborundum (silicon carbide) or the like to form a grinding surface. The abrasive material may be applied to the end 62 by electro-deposition, and the lower ends are curved in cross-section, as shown in FIG. 16, in the shape of sections of circular cylinders with the abrasive material 62 on the convex side. The cylindrical curvature makes the lower end more rigid, although the strip can still be considered resilient.

In a grinding device shown in FIG. 13 there are eight of the strips 60 equally spaced around the head 57 and attached to a ring 63 by a plurality of set screws 64 corresponding in number to the number of strips 60. As a further means of controlling the resilience of the strips 60 a resilience control device 65 may be used. This comprises a ring 66 having a set screw in it at the proper location to engage each of the upper or shank portions 61 of the strips 60 and an annular inner portion 68 that surrounds the central tube 58. The resilience control device can be moved up or down on the upper or shank portions 61 to any desired extent to control the bending freedom of the strips 60. Each of the lower end portions 62 lies along a direction that would constitute a generator of a cone. As shown particularly in FIG. 14 the included angle .theta. is only a little less than 180.degree. but is enough to allow the grinding material 62' to be brought down into contact with the outer edge of the semi-conductor wafer to be ground on this machine. The included angle .theta. is chosen to make the ground semiconductor wafer achieve the rounded profile shown in FIG. 5. This angle is also dependent on the material of which the strips 60 are made and is experimentally determined. Even if the angle .theta. is selected to have the most ideal value, there is inevitably some amount of variation in the ground profiles. In order to reduce such variation the resilience adjusting device 65 is provided.

FIGS. 6 and 8 show the grinding machine in operation with one of the wafers 1 on each of the heads 17 of the vacuum chucks 16. The rods 14 have been lowered by the cylinders 12 so that the grinding strips below each of the grinding devices 15 are in contact with the perimeters of the respective wafers. As may be noted, the axis of each of the rods 14 is offset with respect to the axis of the respective vacuum chuck 16.

The wafers 1 are loaded onto the heads 17 from a wafer magazine 73. As is shown particularly in FIG. 8, the wafer magazine is rectangular and has a plurality of holes through it corresponding in number and in location to the heads 17 of the vacuum chucks. The perimeter of each of the holes in the magazine 73 has a shelf that supports the edge of one of the wafers. This shelf allows each of the wafers to drop down below the top of the magazine 73, but more importantly, it permits the larger part of the perimeter of the hole above the shelf to serve as guide means to locate the wafer precisely so that when it is placed on the head 17 it will be in exactly the correct coaxial position with respect to the head.

The magazine 73 is raised and lowered by the elevators 19, each of which has an L-shaped support 19a to lift the magazine loaded with its wafers. In raising and lowering the elevators 19, hydraulic pressure is applied to a pair of support rods 19b. The magazine 73 with the proper number of wafers loaded on it, in this case eight wafers, is placed on the support 19a at the location shown by the broken line in FIG. 6. In order to place the magazine at that location, each of the rods 14 must be raised by its respective cylinder 12 and the elevators 19 must be in their upper position. After the magazine has been loaded on the elevators, the latter are lowered enough to bring the magazine below the heads 17 of the vacuum chucks 16. This causes the heads 17 to lift the wafers 1 clear of the magazine and these heads can then be evacuated to hold the wafers firmly in place. The vacuum is maintained as long as grinding continues but when the grinding is finished, the vacuum is released and the elevators 19 once again raise the magazine 73 and lift the wafers 1 away from the heads 17. It is preferable to prepare a number of such magazines to facilitate inserting new sets of unground wafers into the machine as soon as a previous set has been ground to the proper contour.

The grinding machine described hereinabove can be automatically operated. When the predetermined grinding period is over the elevators 19 are not only lifted to support the wafers 1 again but the grinding devices 15 are also lifted automatically and the swinging plate 10 is brought to a stop. Successive operations of the machine may be automatically controlled according to a programmed cycle which takes approximately 30 seconds to complete.

The actual grinding of the edge of the wafer 1 is shown in somewhat greater detail in FIGS. 9a and 9b and FIG. 10. During the grinding operation the head 17 of the vacuum chuck rotates in the direction indicated by the arrow in FIG. 9a and carries the wafer 1 with it. The axis of the grinding device 15 is simultaneously moving continuously or orbiting in a circle having a center that coincides with the axis of the head 17. As a result, the grinding material 62' on the lower ends of the strips 60 moves in the direction of the arrows e. The grinding surfaces thus touch the edge of each of the wafers 1 while changing the angle of contact as shown in FIG. 9b. Over a cycle of operation this causes the edge of the semiconductor wafer 1 to be ground to the desired round contour.

Claims

What is claimed is:

1. A grinding machine for grinding a predetermined edge contour on a semiconductor wafer, said machine comprising: a chuck for holding the wafer; means for rotating said chuck; a grinding head having a plurality of resilient grinding elements extending therefrom, each of said resilient grinding elements having a grinding surface facing the wafer held by said chuck, means for moving said grinding head laterally in a circular path that is substantially coaxial with the axis of rotation of said chuck, said chuck and said head being longitudinally spaced to cause the grinding surfaces of said grinding elements to be pressed resiliently against the edge of said wafer at all relative positions of said head and said chuck, and the angle between said grinding surface of each of said grinding elements and the axis of said chuck being less than 90.degree. at each point of contact between said grinding surface and the edge of said wafer so as to form a rounded contour on said edge.

2. The grinding machine of claim 1 comprising, in addition, means to adjust the position of said grinding head laterally with respect to said wafer.

3. The grinding machine of claim 1 in which said edge of said semiconductor wafer comprises a circular portion and a straight portion, and said chuck is a vacuum chuck to hold one of said wafers at a time.

4. The grinding machine of claim 1 comprising, in addition, means to move said grinding head toward and away from said chuck to control the pressure of said grinding surface against said wafer.

5. A grinding machine for grinding a predetermined edge contour on a semiconductor wafer, said machine comprising: a chuck for holding the wafer; means for rotating the chuck; and a grinding device including a head spaced axially from said chuck, and a plurality of grinding elements spaced apart about said head, each of said grinding elements being constituted by a flexibly resilient rod having a shank portion extending from said head toward said chuck generally parallel to the axis of rotation of the chuck and an end portion extending from said shank portion generally laterally outward with respect to said axis of rotation of the chuck to provide a grinding surface facing toward the wafer held by said chuck, the angle included between said shank portion and end portion of said flexibly resilient rod being greater than 90.degree., and the axial spacing of said head and said chuck being selected to cause said grinding surface of each of said grinding elements to be pressed resiliently against the edge of said wafer held by the chuck in all rotary positions of the chuck.

6. A grinding machine for grinding a predetermined edge contour on a semiconductor wafer, said machine comprising: a chuck for holding the wafer; means for rotating the chuck; and a grinding device including a head spaced axially from said chuck and a plurality of grinding elements spaced apart about said head, each of said grinding elements being constituted by a flexibly resilient rod having a shank portion extending from said head toward said chuck generally parallel to the axis of rotation of said chuck and an end portion extending from said shank portion generally laterally outward with respect to said axis of rotation of the chuck, each of said rods being in the form of strip of resilient material having a width greater than its thickness and being bent so that the directions in which the thicknesses of said shank and end portions are measured lie in a common plane, said end portion having a curved cross-section comprising a segment of a circular cylinder with the convex side thereof facing said chuck to define a grinding surface, the angle included between said shank portion and end portion of each resilient rod being greater than 90.degree., and the axial spacing of said head and said chuck being selected to cause said grinding surface of each of the grinding elements to be pressed resiliently against the edge of the wafer held by said chuck in all rotary positions of the chuck.

7. The grinding machine of claim 6 in which the outer portion of said convex surface of each of said strips has grinding material electrodeposited thereon.

8. A grinding machine for grinding a predetermined edge contour on a semiconductor wafer, said machine cmprising: a chuck for holding the wafer; means for rotating said chuck; and a grinding device including a head spaced axially from said chuck a plurality of grinding elements spaced apart about said head, each of said grinding elements being a constituted by a flexibly resilient rod having a shank portion extending from said head toward said chuck generally parallel to the axis of rotation of said chuck and an end portion extending from said shank portion generally laterally outward with respect to said axis of rotation of the chuck to provide a grinding surface facing toward the wafer held by said chuck, the angle included between said shank portion and end portion of said flexibly resilient rod being greater than 90.degree., the axial spacing of said head and chuck being selected to cause said grinding surface of each of said grinding elements to be pressed resiliently against the edge of said wafer held by the chuck in all rotary positions of said chuck, and a resilience adjustment ring encirling and attached to said shank portion of each of said resilient rods and spaced from said head to control the flexing of said rods when said grinding surfaces of the rods are pressed against said wafer.

9. A grinding machine for grinding a predetermined edge contour simultaneously on a plurality of semiconductor wafers, said machine comprising:

A. a plurality of chucks to hold a plurality of said wafers in a substantially common plane;

B. means to rotate all of said chucks simultaneously, each on its own axis;

C. a plurality of grinding heads corresponding in number to the number of said chucks, each of said grinding heads comprising a plurality of resilient grinding devices attached thereto, each of said devices comprising an oblique grinding surface facing the respective wafer and extending beyond the edge thereof at an angle of less than 90.degree. with respect to the axis of the respective chuck;

D. common means to adjust the position of each of said grinding heads laterally with respect to the respective wafer; and

E. common means to swing all of said grinding heads simultaneously along a lateral circular path wherein each of said heads moves in a circle substantially coaxial with the axis of its respective chuck all of said chucks being longitudinally spaced from said heads to cause said grinding surfaces to be pressed against the edge of the respective one of said wafer at all rotary positions of the respective chuck.

10. The grinding machine of claim 9 in which said means to swing said grinding head comprises a plate having means therein for holding said grinding heads in position.

11. The grinding machine of claim 10 in which said means to adjust the position of said grinding heads comprises:

A. a pair of main shafts, each comprising a first extension at one end laterally offset with respect to the axis of said main shaft and a second extension at the other end;

B. means attaching said plate to each of said second extensions to allow rotation rotation of said second extension with respect to said plate;

C. a pair of driving gear means, each of said first extensions being attached to one of said driving gear means to be rotated thereby, each of said driving gear means comprising an eccentrically bored shaft to received a respective one of said first extensions; and

d. Means to adjust the angular orientation of each of said main shafts to adjust the diameter of the circle of motion of said plate.

12. The grinding machine of claim 11 comprising, in addition, detent adjustment means between each of said gear means and the respective one of said main shafts to set the diameter of the circle of motion of said plate to specific values.

13. The grinding machine of claim 9 comprising, in addition:

A. a plate having a plurality of apertures corresponding in number to the number of said chucks;

B. means to support said last-named plate to place each of said apertures in coaxial alignment with a respective one of said chucks;

C. guide means concentric with each of said apertures on said plate to locate one of said semiconductor wafers with respect to that aperture; and

D. means to lower said plate relative to said chucks to allow said chucks to lift said semiconductor wafers therefrom.