FIELD OF THE INVENTION
Embodiments of the invention are related generally to carriers used in polishing machines for polishing thin, flat work pieces and more specifically to a wafer carrier for simultaneously polishing a batch of wafers.
BACKGROUND
Thin, flat wafers may be used in the manufacture of semiconductor devices, optical components, and other products and applications. A wafer polishing machine may be used to remove material from at least one large face of a wafer to produce a polished wafer. A wafer polishing machine may simultaneously polish a batch comprising more than one wafer until all wafers in the batch meet specifications for polishing quality. Polishing quality may be represented by one or more parameters, for example but not limited to variations in wafer thickness at different locations on a wafer, surface roughness, variations in thickness from one wafer to another in a batch, amount of material removed from each wafer in a batch, or other measured or calculated values.
During polishing, wafers may be held in a rotating carrier in a wafer polishing machine. A carrier may include a base plate having a front side made uniformly flat and smooth by lapping. The back side of a base plate may include mounting features for attaching the base plate to a polishing head in a polishing machine. Base plates may be made from stiff materials such as stainless steel, aluminum alloy, or ceramic. Because wafers may be quite thin relative to their diameter, variations from planarity of the base plate's front side may affect smoothness, flatness, or other parameters related to quality of polished wafers. Nearly the entire front side of a baseplate may be lapped smooth and flat, even when wafers being polished on the baseplate cover only a fraction of the baseplate's surface area. Lapping a large area takes more time and is more expensive than lapping a small area to the same conditions of planarity and surface roughness.
A wafer to be polished may be placed in an aperture formed in a template with the front face of the wafer protruding outward from the front surface of the template. A template adapted for simultaneous polishing of more than one wafer may have a separate aperture for each wafer. A template may include an upper layer made from fiberglass, an intermediate layer made from backing material, and a bottom layer comprising adhesive for attaching the template to the front face of the base plate on the carrier. The intermediate layer in a template may be referred to as the backing layer and the upper layer may be referred to as the front layer. During polishing, wafers are placed in apertures formed in the upper and backing layers. Examples of wafer templates attached to a lapped or polished front side of a base plate on a carrier are described in U.S. Pat. Nos. 3,449,870 to Jensen, 4,512,113 to Budinger, 5,193,316 to Olmstead, 5,267,418 to Currie et al., and 6,612,905 to Nguyen. An example of a base plate and a cast aluminum alloy carrier is described in U.S. Pat. No. 4,194,324 to Bonora et al.
During polishing, wafers press against the edges of the apertures in the front layer of a template, subjecting the template to wear along the edges of the apertures. Even a small amount of wear of the aperture edges may reduce the quality of polished wafers. Worn templates may be replaced periodically, for example after a selected number of wafer batches have been polished. Removing a template from a carrier may be difficult when adhesive has been used to bond the template to the carrier. After a template is removed from a baseplate, residual adhesive remaining on the baseplate may be removed by thorough cleaning to provide a smooth, flat surface for installation of a new template. The lapped front surface of a base plate may be damaged during removal of a template or cleaning of the base plate and may therefore be lapped again to restore the base plate to specifications for surface planarity and surface roughness. A worn template may be discarded after being removed from a baseplate. Template replacement, baseplate cleaning, and baseplate lapping represent significant contributors to wafer polishing cost.
The thicker a template's front layer, the better the template may resist deformation and wear during polishing and the better it may able to hold a wafer for achieving high quality polishing results. The front layer of a template is preferably thinner than the thickness of a wafer to be polished. The thinner the front layer, the higher the pressure that is applied to edges of the front layer, increasing deformation and wear of the template edges that come into contact with wafers. These contradictory requirements for template thickness may be difficult to resolve for thin wafers such as silicon wafers for semiconductor production, which may have a wafer thickness that is a small fraction of wafer diameter.
The depth by which an aperture for holding a wafer penetrates into a template limits a range of wafer thicknesses which may be polished with the template. The depth of an aperture may correspond to the thickness of a template's front layer, or the depth may be made less than the thickness of the front layer by placing inserts in the aperture or by other means. A range of wafer thicknesses that may be polished in any one template may be relatively narrow. A wafer that is too thick for the template in which it is placed may escape from the template during polishing, possibly damaging the wafer or the template. If a wafer is too thin for a particular template, the front surface of the wafer may not protrude sufficiently outside the template to achieve desired polishing quality.
Variations in diameter from one wafer to another may prevent some wafers from being held securely in a template. Some wafers may be too large to fit within an aperture in a template. Other wafers may have a diameter that is too small for a template to hold the wafers securely, allowing the wafer to move excessively within the aperture. Excessive movement of a wafer within a template aperture may reduce polishing quality. For templates having more than one backing layer behind the front layer, dimensional tolerances for each layer add uncertainty to the precise position of a wafer relative to the base plate and may increase uncertainty in the polishing results achievable for some combinations of wafer thickness and polishing machine operating parameters.
A backing layer in a template may provide a planar surface against which wafers are held during polishing. All wafers in a batch being polished simultaneously on one template may be placed against a same planar surface defined by a backing layer in a template. Parts of a template that are behind a wafer, that is, between the back surface of a wafer and the front surface of the base plate, may be compressed more than other parts of the template by pressure applied to the wafer and template during wafer polishing. A gap between the edges of a wafer being polished and the side walls of the aperture in which the wafer is held may permit excessive motion of the wafer relative to the template and may lead to wear of the backing layer. One or more template layers may wear or compress unevenly or at a different rate than other layers. One or more of these effects may lead to a template that becomes progressively thinner or less flat, possibly causing polishing quality to degrade over time and limiting the service lifetime of a template. Wafers polished on worn templates may not meet specifications for surface planarity, surface roughness, or other specifications related to polishing quality. Template wear and compression may lead to a reduction in the yield of wafers that meet specifications for polishing quality.
Variations in backing layer tolerances, wear, compression, and other factors may cause uneven distribution of polishing slurry across the face of a wafer. Uniform distribution of polishing slurry across the faces of wafers being polished may contribute to high quality polishing results. Slurry deficiency may be especially pronounced near the center of a wafer during polishing and may lead to inefficient or uneven removal of material, possibly reducing the quality of polished wafers.
With some polishing machines, there may be an increasing deviation in surface flatness from the center of a polished wafer toward the periphery. Wafer thickness near the edges of a wafer, sometimes referred to as “edge exclusion”, may decrease from the center of the wafer toward the periphery of the wafer and may be referred to as “roll off”. Although less common, in some wafer polishing machines wafer thickness increases from the center of the wafer toward the wafer's periphery. Variations in wafer thickness may reduce the yield of wafers having acceptable polishing quality.
SUMMARY
A carrier assembly for wafer polishing includes a base plate with a base plate front face and at least three carrier protrusions extending outward from the base plate front face. Each carrier protrusion has a protrusion front face separated from the base plate front face and from any other of the at least three carrier protrusions. Each carrier protrusion has a peripheral wall extending outward from the base plate to a perimeter of the protrusion front face. In some embodiments of the invention, a frame is removably attachable to the base plate front face, for example with a plurality of threaded fasteners. The frame is formed with a plurality of separate carrier protrusion apertures, one carrier protrusion aperture for each carrier protrusion. A carrier assembly embodiment of the invention may optionally further include at least three detachable rings. Each detachable ring is formed with a flange. The frame engages the flange on each detachable ring to hold the rings against the base plate front face, with each detachable ring held within a separate carrier protrusion aperture around a different one of the at least three carrier protrusions.
An alternative embodiment of a carrier assembly includes at least three carrier protrusions, each having a carrier protrusion front face. The carrier assembly further includes at least three detachable rings, each surrounding a separate one of the at least three carrier protrusions with a close fit, at least three retaining rings, each surrounding a separate one of the at least three detachable rings with a close fit, and a base plate including a base plate front face and at least three ring grooves formed in the base plate front face. Each of the at least three ring grooves includes a flat bottom surface parallel to the front face on each of the at least three carrier protrusions. Each of the at least three ring grooves surrounds a separate one of the at least three carrier protrusions, thereby separating the baseplate front face from the carrier protrusion front face on each of the at least three carrier protrusions.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an exploded pictorial view of an example of a carrier assembly adapted for simultaneously polishing a batch of five wafers.
FIG. 2 is a view toward the front side of the example of a carrier assembly of FIG. 1.
FIG. 3 is a view toward a side of the example of a carrier assembly of FIGS. 1-2.
FIG. 4 is a pictorial view of an example of a base plate with five carrier protrusions extending outward from the front side of the base plate.
FIG. 5 is a view toward the front side of an example of a frame adapted for holding a batch of five wafers.
FIG. 6 is a pictorial view of an example of a detachable ring having a peripheral flange.
FIG. 7A is a cross-sectional view A-A of examples of a carrier protrusion, a detachable ring, and a frame. The location of section A-A is marked by section line A-A in FIG. 2.
FIG. 7B is an alternate view of section A-A, showing examples of a wafer and an optional insert in a cavity formed by the detachable ring and front face of the carrier protrusion of FIG. 7A.
FIG. 8 is a view toward the front side of an example of an alternative embodiment of a carrier assembly having a frame with a separate aperture for each carrier protrusion without a detachable ring between the frame and each carrier protrusion.
FIG. 9 is a cross-sectional view E-E of the examples of a carrier protrusion and a frame from the carrier assembly of FIG. 8, showing that the frame does not contact the carrier protrusion in the illustrated example. The location of section E-E is marked by section line E-E in FIG. 8.
FIG. 10 is a view toward the front surface of an example of a frame on a carrier assembly adapted for polishing a batch of three wafers simultaneously.
FIG. 11 is an exploded pictorial view of the example of a carrier assembly of FIG. 10, showing a front side of a base plate with three carrier protrusions, three detachable rings, and a frame with three carrier protrusion apertures.
FIG. 12 is a view toward the front side of an example of alternative embodiment of the invention comprising a base plate with three carrier protrusions, a detachable ring surrounding each carrier protrusion, and three retaining rings for holding the detachable rings against the base plate.
FIG. 13 is an exploded pictorial view of the example of a carrier assembly of FIG. 12, showing examples of grooves formed in the front surface of the base plate and examples of retaining rings for holding a detachable ring in each groove.
FIG. 14A is a cross-sectional view B-B of examples of a carrier protrusion, a detachable ring surrounding the carrier protrusion, and a retaining ring surrounding the detachable ring and holding the detachable ring to the base plate. The location of section B-B is marked by section line B-B in FIG. 12.
FIG. 14B is an alternate view of section B-B, showing examples of a wafer and an optional insert in a cavity formed by the detachable ring and the front face of the carrier protrusion from FIG. 14A.
FIG. 15 is a cross-sectional view C-C of an example of a carrier protrusion with edge relief in the form of a chamfer all the way around the periphery of the front face of a carrier protrusion, and further showing examples of a wafer and an optional insert in their undeflected positions before polishing begins. The location of section C-C is marked by section line C-C in FIG. 12.
FIG. 16 is a cross-sectional view C-C of an alternative example of a carrier protrusion with edge relief formed as a step all the way around the front peripheral edge of a carrier protrusion.
FIG. 17 is an alternative cross-sectional view C-C of the example of a carrier protrusion of FIG. 16, showing an example of an elastic material placed between the back side of the wafer and base plate in the peripheral step around a carrier protrusion.
FIG. 18 is a view toward the front side of a carrier protrusion with edge relief as shown in the example of FIG. 17, and an example of an elastic ring positioned on the peripheral step.
FIG. 19 is another alternative cross-sectional view C-C of the example of a carrier protrusion of FIG. 16, showing an example of a carrier protrusion formed with a front face having a curved profile.
FIG. 20 is a cross-sectional view of an example of a carrier assembly with a frame and a carrier protrusion with edge relief in the form of a peripheral step. The location of section D-D is marked by section line D-D in FIG. 10.
DESCRIPTION
Embodiments of the invention comprise a wafer carrier for simultaneously polishing a batch of wafers on a wafer polishing machine. As used herein, a batch refers to all the wafers being polished simultaneously on one wafer carrier embodiment of the invention. An embodiment of the invention includes a base plate having at least three carrier protrusions on the front side of the base plate. Each carrier protrusion has a front face for supporting one wafer. The carrier protrusion front face against which a wafer is held during polishing is separated from the carrier protrusion front face against which any other wafer is held. In some embodiments of the invention, a carrier protrusion front face is formed as a flat, smooth surface. In alternative embodiments of the invention, the front face of a carrier protrusion may be formed with a curved, chamfered, or stepped profile so that the height of a carrier protrusion is less near the perimeter of the carrier protrusion than near the center. A carrier protrusion with a curved, chamfered, or stepped profile may have advantages for controlling edge exclusion or roll off effects compared to carriers with flat support surfaces for wafers.
In contrast to embodiments of the invention, carriers previously known in the art have a base plate with a front face that is made flat everywhere across the front face. For carriers previously known in the art, all the wafers in one batch are supported by the single, approximately planar surface of the backing layer of a template attached to the base plate's front face, rather than resting on separated surfaces as in an embodiment of the invention.
The examples of a wafer carrier described herein have a front face with a circular perimeter shape, although other shapes may be used. The front face of the wafer carrier is the face immediately adjacent to a polishing pad during wafer polishing in a wafer polishing machine. In general, the front side or face of components described herein will be the side facing the polishing pad during wafer polishing. For example, the front face of a wafer refers to the face being polished by contact with the polishing pad. The back face of a wafer faces toward the front face of a carrier protrusion during wafer polishing. One or more inserts or spacers may optionally be interposed between the back face of a wafer and the front face of a carrier protrusion. A carrier, also referred to herein as a carrier assembly or wafer carrier, may be driven in rotation during wafer polishing by a drive shaft or other components coupled to the back side of the carrier. Components for driving a carrier in rotation may optionally be omitted from an embodiment of the invention.
A reference to a thickness dimension of an element, for example a wafer thickness, a thickness of a carrier protrusion, or a thickness of a frame, refers to a direction normal to a front face on a base plate when the element is in position for wafer polishing. A dimension or direction referred to as thickness may alternately be referred to as height.
A workpiece that may be processed by an embodiment of the invention is referred to herein as a wafer. A wafer suitable for use with an embodiment of the invention comprises at least one relatively large face to be polished to specified conditions of planarity and smoothness, possibly expressed by values related to surface roughness, dimensions of polishing marks, deviation from an ideal surface, or other values related to polishing quality. A wafer may alternatively have, for example but not limited to, a circular perimeter, a polygonal perimeter, a curved perimeter with at least one straight edge, or an irregular perimeter shape. A silicon wafer used in the manufacture of integrated circuits is an example of a workpiece referred to herein as a wafer, but other relatively thin, relatively flat workpieces may also be suitable for processing by embodiments of the invention. Embodiments of the invention are especially well suited for polishing wafers that are very thin compared to their largest linear dimension. For some applications, polished wafers preferably have uniform thickness and very low surface roughness on at least one polished face.
Referring now to the figures, an exploded view of an example of a wafer carrier embodiment of the invention is shown in FIG. 1. The example of a wafer carrier 10, also referred to as a carrier assembly 10, includes a base plate 12 adapted for attachment to a drive shaft in a wafer polishing machine (not illustrated) for driving the carrier assembly 10 in rotation during wafer polishing. The example of a carrier assembly 10 in FIG. 1 shows an example of a base plate 12 having five carrier protrusions 14 integrally formed onto the front side of the base plate. In some embodiments of a base plate 12, each carrier protrusion 14 extends outward from a base plate front face 18. Each carrier protrusion 14 includes a protrusion front face 16 that is spatially separated from the base plate front face 18 and from the front face 16 of any other carrier protrusion 14. For example, in some embodiments of the invention a protrusion front face 16 may be positioned outward from the base plate front face 18 by a thickness dimension of a carrier protrusion 14. A carrier protrusion 14 may be attached to or alternately formed as an integral part of the base plate front face 18.
A separate carrier protrusion 14 is provided for each wafer 60 to be polished on the carrier assembly 10. Each of the five wafers 60 shown in broken lines in FIG. 1 is an example of a work piece having a front surface 62 to be polished by contact with polishing slurry and a rotating polishing pad (not illustrated). The front surface 62 of a wafer 60 may be referred to as the front face of the wafer. An optional insert 50, also shown in broken lines, may be placed between a back surface of each wafer 60 and the front face 16 of each carrier protrusion 14. When an insert is omitted, the back surface of a wafer 60 may contact the front face 16 of a carrier protrusion 14 during polishing of the front surface 62 of the wafer 60.
A detachable ring 40 may optionally be provided for each carrier protrusion 14. A detachable ring 40 may optionally be made from a plastic material and may optionally fit around a carrier protrusion with a close fit. In the carrier assembly example of FIG. 1, detachable rings 40 are interposed between the sides of the carrier protrusions 14 and the edges of apertures 32 in a frame 30 and are retained against the base plate front face 18 by the frame 30. The frame 30, also referred to as a mounting plate, is formed with one carrier protrusion aperture 32 for each carrier protrusion 14 on the base plate 12. The frame may be formed with countersunk through-holes 22 through which threaded fasteners (not shown in FIG. 1) pass to removably assemble the frame 30 to the base plate 12. When the frame 30 is assembled to the base plate 12, the detachable ring 40 surrounding each carrier protrusion 14 fits within a corresponding carrier protrusion aperture 32. The detachable rings 40 prevent contact between wafers 60 and the frame 30. During wafer polishing, the front surface 62 of each wafer 60 extends outward from the front surface 64 of the frame 30 so that the front surface of the wafer may come into contact with a polishing pad and polishing slurry in a wafer polishing machine. Each carrier protrusion, protrusion front face, detachable ring, and protrusion aperture in the frame may optionally have a perimeter shape corresponding to the perimeter shape of a wafer to be polished. Alternatively, a carrier protrusion, detachable ring, aperture in a frame, or other parts of an embodiment of the invention may have a perimeter shape that matches part of the perimeter of a wafer to be polished but does not match other parts of the wafer's perimeter. A wafer may be held firmly enough by the detachable ring 40 and frame 30 to prevent the wafer 60 from escaping from the detachable ring during wafer polishing.
A wafer 60 may slide laterally relative to the front face 16 of the carrier protrusion 14 behind the wafer, possibly resulting in wear of the detachable ring 40. For frames adapted for use with detachable rings, the detachable rings 40 reduce wear of the frame 30 and may be replaced more frequently and at lower cost than the frame 30, extending the service lifetime of the frame and base plate. A frame 30 adapted for use with detachable rings 40 may have a substantially longer service life than a template used with polishing machines previously known in the art. A carrier assembly 10 having a base plate 12 with carrier protrusions 14, a frame 30, and detachable rings 40 may have a substantially longer service lifetime before re-lapping is required than carrier assemblies previously known in the art.
The example of a carrier assembly 10 from FIG. 1 is shown in a view toward the front surface 64 of the frame 30 in FIG. 2 and in a side view in FIG. 3. The example of a frame 30 in FIG. 2 is formed with five (5) carrier protrusion apertures 32. A detachable ring 40 surrounds each carrier protrusion 14 and fits within the corresponding carrier protrusion aperture 32. The frame 30 may be made of a stiffer material than the rings 40 to reduce flexure of the rings and to reduce wafer motion relative to the carrier assembly 10 during wafer polishing. A flange 42 having an outer flange radius 86 from the center of the detachable ring extends around the back side of each detachable ring 40 and fits in a channel formed on the back side of the frame 30 around each carrier protrusion aperture 32, holding the detachable rings 40 between the frame 30 and the front surface of the base plate 12 in the carrier assembly 10. In the example of FIG. 2, the cylindrical carrier protrusions 14 have a circular perimeter shape with a radius 82 corresponding to an inner radius of a detachable ring 40. The carrier protrusion apertures 32 are formed with a radius 84, corresponding to an outer radius of a detachable ring 40. The centers of the cylindrical carrier protrusions 14 are located on a circle having a radius 88. The center of each carrier protrusion 14 is separated from the centers of adjacent carrier protrusions on either side by equal angular intervals 90. A plurality of threaded fasteners 70 pass through countersunk apertures in the frame 30 and engage corresponding threaded apertures in the base plate 12 to enable removable attachment of the frame 30 to the base plate 12. The front face 16 of a carrier protrusion is visible in FIG. 2 within the area surrounded by a detachable ring 40.
FIG. 4 shows a pictorial view of an example of a base plate 12 adapted for simultaneous polishing of a batch of five wafers. Five carrier protrusions 14 extend outward from the base plate front face 18. The front face 16 of each carrier protrusion 14 is joined to the base plate front face 18 by a peripheral wall 20 forming the side of each carrier protrusion. The height of the peripheral wall 20 corresponds to a thickness of a carrier protrusion at the periphery of the carrier protrusion. In the example of FIG. 4, the peripheral wall 20 spatially separates a carrier protrusion front face 16 from the base plate front face 18.
A view toward the front surface 64 of an example of a frame 30 is shown in FIG. 5. The frame 30 is formed with five carrier protrusion apertures spaced at equal angular intervals, one aperture for each carrier protrusion on the base plate. The frame 30 may also be formed with countersunk through-holes 22 through which threaded fasteners pass to assemble the frame to a base plate. Each carrier protrusion aperture 32 has a peripheral edge 38 which may contact and support an optional detachable ring around each carrier protrusion. As shown in the example of FIG. 5, an “I”-shaped aperture 36 may optionally be formed in the front surface 64 of the carrier 30 in each of the places where adjacent carrier protrusion apertures 32 are closest to one another. The length, width, and depth dimensions of an “I”-shaped aperture may be selected to provide a desired flow rate of polishing compound through the gap between adjacent wafers during wafer polishing. The “I”-shaped apertures are effective for maintaining a uniform distribution of polishing compound across the face of each wafer. The apertures 36 may alternatively be formed as straight channels or other shapes. In a frame 30 lacking “I”-shaped apertures 36, polishing compound may not be distributed evenly to all areas on the front surfaces of wafers being polished. “I”-shaped apertures 36 may optionally penetrate the front layer of the frame 30 without penetrating any of the frame's backing layers, or may alternatively penetrate the front layer and a selected number of other layers.
A pictorial view toward the front side of an example of a circular detachable ring 40 is shown in FIG. 6. An inner wall 43 of the detachable ring example of FIG. 6 is optionally sized for a close fit around the peripheral wall of a carrier protrusion. The thickness or height of the detachable ring may optionally be somewhat greater than the thickness of a frame into which the detachable ring fits. The peripheral shape of a detachable ring, represented by the shape of an inner edge 38, is approximately the same as the peripheral shape of a carrier protrusion and of a wafer to be polished. The outer wall 45 of the detachable ring 40 may optionally be sized for a close fit into a carrier protrusion aperture in a frame. A flange 42 extends around the periphery of the back side of the outer wall 45. The flange 42 fits into a corresponding channel formed into the back side of the frame around each carrier protrusion aperture. The back side of the detachable ring 40 (back side not visible in FIG. 6) contacts a front surface of the base plate in a carrier assembly. A detachable ring 40 may optionally be formed with a split 44 through the outer wall 45 and flange 42, and may optionally include at least one transverse groove 46 through which polishing compound may flow during wafer polishing, thereby providing uniform distribution of polishing compound across the front face of a wafer being polished and improving the quality of polished wafers.
The front face of a carrier protrusion and the inner wall of a detachable ring form a cavity into which a wafer fits for wafer polishing, with the front surface of the wafer extending outward from the front surface of the frame. For embodiments of the invention having a frame, the front surface of the frame is farther from the base plate front face than the protrusion front face. The front face of a carrier protrusion may be formed by conventional metalworking processes, for example by lapping, and may be referred to as the lapped face of a carrier protrusion. An example of a cavity for holding a wafer is shown in FIGS. 7A and 7B. FIG. 7A represents a view of cross section A-A from the example of FIG. 2. FIG. 7B is an alternative view of cross section A-A having a wafer 60 and an optional insert in the cavity formed by the detachable ring and carrier protrusion, with the wafer front surface 62 extending above the detachable ring 40 and front surface 64 of the frame 30.
FIG. 7A shows an example of a cross-section A-A through the base plate 12, detachable ring 40, and frame 30 of FIG. 2. Parts of a carrier assembly behind the base plate 12 (ref. FIG. 3) are omitted in FIGS. 7A-7B and other cross-sectional views. The carrier protrusion 14 in the example of FIG. 7A may optionally be formed as an integral part of the base plate 12. In alternative embodiments of the invention, carrier protrusions may be formed separately from the base plate and then attached to the base plate. The protrusion front face 16 is separated from the base plate front face 18 by a peripheral wall 20 around the perimeter of the carrier protrusion 14. Unlike carriers previously known in the art, the front face 18 of the base plate 12 may not be lapped. The front face 18 of the base plate 12 may therefore be less smooth than the front face 16 of a carrier protrusion. The detachable ring 40 fits against the peripheral wall 20 of the carrier protrusion 14, extending outward from the protrusion front face 16 and outward from the front of the frame 30. The frame 30 traps the flange 42 on the detachable ring 40 against the front face 18 of the base plate 12. Threaded fasteners (not shown in the examples of FIGS. 7A-7B) may be used to attach the frame 30 to the base plate 12.
Alternative Section A-A in FIG. 7B repeats the example of FIG. 7A and adds an example of a wafer 60 to be polished and an example of an optional insert 50. The front face 62 of the wafer 60 protrudes beyond the front side of the detachable ring 40 and the frame 30 so that material may be removed efficiently and evenly from the front face during wafer polishing. As suggested in the example of FIG. 7B, the largest dimension of the wafer is smaller than the inner diameter of the detachable ring 40. Lateral motion of the wafer 60 relative to the front surface 16 of the carrier protrusion 14 may cause frictional wear of the detachable ring 40, but the detachable ring prevents frictional wear of the frame 30 by the wafer 60.
Some embodiments of a frame have carrier protrusion apertures into which carrier protrusions are positioned without an intervening detachable ring. An example of a frame 30 adapted for polishing wafers without the use of detachable rings is shown in FIG. 8 and FIG. 9. As suggested in FIG. 8, the frame 30 is formed with carrier protrusion apertures 32 into which carrier protrusions 14 fit so that the front face 16 of each carrier protrusion 14 is located within an aperture 32. The inner wall 39 of each aperture 32 is preferably separated from the peripheral wall 20 of each carrier protrusion 14 so that the inner wall 39 does not contact the peripheral wall 20. The frame 30 along the front surface 64 is thicker than the height of a carrier protrusion 14 so that a cavity for holding a wafer to be polished is formed by the inner wall 39 of the carrier protrusion aperture 32 in the frame 30 and the front face 16 of the carrier protrusion 14. In an embodiment of the invention which operates without detachable rings, for example as shown in FIGS. 8-9, the edges of a wafer being polished may come into direct contact with the inner wall 39 of a carrier protrusion aperture 32 because there is no detachable ring intervening between the wafer and frame 30. Although the example of a frame 30 in FIG. 8 is adapted for polishing five wafers, in alternative embodiments of the invention the frame is adapted for polishing three wafers or other numbers of wafers.
An example of an alternative embodiment of the invention adapted for simultaneous polishing of a batch of three wafers is shown in FIG. 10. The example of a carrier assembly 10 of FIG. 10 includes a frame 30 having a front surface 64. The frame 30 may be removably attached to a base plate 12 by threaded fasteners 70. Three carrier protrusion apertures 32 formed in the frame 30 are spaced at equal angular intervals 90 around the frame. A detachable ring 40 fits within each carrier protrusion aperture 32 and surrounds a front face 16 on each of three carrier protrusions 14. In the example of FIG. 10, each carrier protrusion front face 16 has a circular perimeter shape with a radius 82 corresponding to an inner radius of a detachable ring 40. Each detachable ring 40 fits around and against the sides of a separate carrier protrusion 14. A peripheral flange 42 having an outer radius 86 on each detachable ring 40 contacts the back side of the frame 30, enabling the frame 30 to hold the detachable rings 40 against the base plate 12. The center of each carrier protrusion 14 is positioned on a circle having a radius 88 originating at a rotational center of the base plate. An outer diameter 84 of a detachable ring 40 corresponds to a diameter of a carrier protrusion aperture 32.
The example of a carrier assembly 10 with a frame 30, three flanged detachable rings 40, and a baseplate 12 with three raised carrier protrusions 14 is shown again in the pictorial view of FIG. 11. FIG. 11 further illustrates an example of a threaded fastener 70 which passes through a countersunk through-hole 22 in the frame 30 and engages a corresponding threaded hole 34 formed in the front face 18 of the base plate 12 for retaining the frame and rings against the base plate. A carrier assembly 10 may optionally use a different number and location of threaded fasteners 70, countersunk holes 22, and threaded holes 34 than are shown in the example of FIG. 11. Dashed lines in FIG. 11 show the intervening position of a detachable ring 40 between a carrier protrusion 14 and the frame 30 and the alignment of the front face 16 of a carrier protrusion and detachable ring 40 with a carrier protrusion aperture 32 in the frame.
In the examples of a carrier assembly from FIGS. 1 and 9, carrier protrusions extend outward from the front face of a base plate, a detachable ring fits around each carrier protrusion, and each detachable ring and carrier protrusion fit into a corresponding aperture in a frame. In an alternative embodiment of a carrier assembly, the front face of each carrier protrusion may be isolated from the front face of the base plate by a groove around the perimeter of each carrier protrusion. However, instead of a frame for holding more than one detachable ring to the base plate as in the examples of FIG. 1 and FIG. 11, a separate retaining ring may be provided for individually holding each detachable ring to the baseplate. A retaining ring functions like the frame in other embodiments of the invention for supporting a detachable ring against a wafer protrusion. However, each retaining ring may be attached to or removed from a base plate independently from other retaining rings on the same baseplate.
FIG. 12 shows a view toward the front face 18 of an example of a base plate 12 adapted for holding a detachable ring 40 in place around a carrier protrusion front face 16 with a retaining ring 48. The carrier assembly 10 of FIG. 12 is configured for simultaneously polishing a batch of three wafers. In the example of FIG. 12, the carrier protrusions 14 have a circular perimeter shape with a radius 82 corresponding to an inner radius of a detachable ring 40. The detachable ring 40 has an outer radius 84 corresponding to an inner radius of the retaining ring 48. The detachable ring 40 has a circumferential flange with an outside radius 86 that fits in a corresponding step formed in the back side of the retaining ring 48. The centers of the circular perimeters of the carrier protrusions 14 are positioned on a circle of radius 88 from the rotational center of the base plate 12. An outer radius 92 of the retaining ring 48 is chosen so that the three retaining rings, carrier protrusions 14, and detachable rings 40 may be spaced at equal angular intervals 90. In FIG. 12, the front face 16 of each carrier protrusion 14 is visible within the area surrounded by each detachable ring 40.
FIG. 13 shows an exploded pictorial view of the example of a carrier assembly 10 from FIG. 12. Dashed lines indicate how a flange 42 on each detachable ring 40 may be held by a retaining ring 48 in a ring groove 54 formed in the front face 18 of the base plate 12. The front face 16 of each carrier protrusion 14 is spatially separated from the front face 18 of the base plate 12 and from the front faces of other carrier protrusions by the ring groove 54. Retaining rings 48 may be removably attached to the base plate 12 by threaded fasteners (not illustrated) passing through apertures 22 in the retaining rings and engaging threaded holes 34 in the flat bottom surface 56 of each ring groove 54. Retaining rings 48 are preferably fabricated from a strong, chemically inert material, for example stainless steel.
FIG. 14A shows an example of a cross-section B-B through the base plate 12, detachable ring 40, and retaining ring 48 of FIG. 12. The carrier protrusion 14 in the example of FIG. 14A may be formed by a groove 54 in the base plate front face 18. The groove 54 defines the outer perimeter shape of the carrier protrusion, with an inner wall of the groove corresponding to the outer peripheral wall 20 of the carrier protrusion. The groove spatially isolates the front face of a carrier protrusion from other front-facing surfaces on a base plate. The perimeter shapes of carrier protrusions, retaining rings, and detachable rings may match the perimeter shapes of wafers to be polished.
The bottom surface 56 of the groove 54 may be parallel to the front face 16 of a carrier protrusion 14. The groove 54 may optionally be wide enough for the retaining ring 48 and detachable ring 40 to fit into the groove. Alternatively, the groove may be wider than the combined widths of the retaining ring and ring. The thickness of the base plate 12 in the region of the carrier protrusion 14 may be approximately the same as the thickness of the base plate just outside the area enclosed by a retaining ring 48 on the front face 18 of the base plate 12. In an alternative embodiment of the base plate 12, the thickness of the base plate in the region of the carrier protrusion is less than the thickness of the base plate outside the retaining ring, that is, the front surface 16 of the carrier protrusion 14 is below the front surface 18 of the base plate in the orientation shown in FIG. 14A. In another alternative embodiment of the invention, the thickness of the base plate in the region of the carrier protrusion is greater than the thickness of the base plate outside the retaining ring, that is, the front surface 16 of the carrier protrusion 14 is above the front surface 18 of the base plate in the example of FIG. 14A.
FIGS. 14A and 14B show an example of a detachable ring 40 held in the groove 54 by the retaining ring 48. An inner wall 43 of the detachable ring 40 surrounds the outer peripheral wall 20 of the carrier protrusion 14 with a close fit. The detachable ring 40 includes a peripheral flange 42 that engages a corresponding overhang or step 47 formed on the back side of the retaining ring 48. A back side of the flange 42 contacts the bottom surface 56 of the groove 54 in the base plate 12. The outer wall 45 of the detachable ring 40 fits against the corresponding inner wall 49 of the retaining ring 48 with a close sliding fit. At least two threaded fasteners (not illustrated) pass through countersunk holes 22 in the retaining ring 48 and engage threaded holes 34 in the base plate 12 to hold the retaining ring 48 and detachable ring 40 tightly against the bottom surface 56 of the groove 54. FIG. 14B further illustrates an example of a wafer 60 to be polished, with an optional insert 50 placed between a back side of the wafer and the front face 16 of the carrier protrusion 14. As suggested in FIG. 14B, a gap may exist between the edges of the wafer 60 and the inner wall 43 of the detachable ring 40. The front surface 62 of the wafer 60 preferably protrudes beyond the steel ring 48 and detachable ring 40 during wafer polishing. A wafer may be held firmly enough by the detachable ring 40 and retaining ring 48 to prevent the wafer 60 from escaping from the ring during wafer polishing.
In the examples of carrier protrusions shown in cross sectional views 7A, 7B, 12A, and 12B, the front surface 16 of a carrier protrusion has been drawn in profile as a straight line to represent a planar surface. In alternative embodiments of the invention, a carrier protrusion may have a surface with a curved profile, chamfered edges, stepped edges, or some other deliberate deviation from planarity. A deviation from planarity in the front surface of a carrier protrusion permits part of a wafer to deflect during wafer polishing, possibly causing different rates of removal of material at different locations on a wafer's surface. A nonplanar profile for a wafer carrier front surface may enable compensation during wafer polishing for non-uniform distribution of polishing compound, misalignment of various axes of rotation in a wafer polishing machine, or other factors that influence the quality of polished wafers. Shaping the surface of a carrier protrusion with a deliberate deviation from planarity may reduce uncertainties related to placing one or more layers of inserts between a wafer and carrier protrusion and may therefore improve the yield and reduce the cost of acceptably polished wafers.
An example of a carrier protrusion having a deliberate deviation from planarity, implemented as edge relief, is shown in the cross-sectional view of FIG. 15. Section C-C in FIG. 15 shows a partial cross section of the base plate 12, retaining ring 48, and detachable ring 40 in a groove 54 from the example of a carrier assembly 10 in FIG. 12. The detachable ring 40 fits close to the outer peripheral wall 20 of a carrier protrusion as previously explained with regard to FIGS. 12A-12B. A chamfer 94A is formed inward from the outer peripheral wall 20 along the front face 16 of the carrier protrusion. The chamfer extends inward a distance L 78 from the outer peripheral wall 20 of the carrier protrusion, all the way around the carrier protrusion. The thickness of the carrier protrusion at the outer peripheral wall 20 differs from the thickness inward of the chamfer 94A by a maximum dimension H 76. The height dimension H 76 represents a maximum amount of deflection of the wafer 60 toward the base plate 12 over a length D 74 from the side 66 of the wafer 60 during wafer polishing. Neither deflection of the wafer 60 during polishing nor quality of the polished wafer surface 62 is affected by planarity or smoothness of the front surface 18 of the base plate 12.
FIG. 15 illustrates an example of a reference position for the wafer 60 and optional spacer 50 corresponding to an undeflected position of the wafer, before the front surface 62 of the wafer is pressed into a polishing pad. During polishing, the back surface 80 of the wafer 60 bends closer to the chamfer 94A, thereby reducing an amount of material removed from the wafer over the distance D 74 compared to material removed from other parts of the wafer. If an optional insert 50 is present, the insert bends closer to the chamfer when the wafer bends. The height of the chamfer H 76 and the length of the chamfer L 78 may optionally be selected to cause a selected local variation in the rate of material removal across the front face 62 of the wafer 60. The chamfer 94A may optionally be formed with an acute angle between the chamfered face and a horizontal reference, for example the front surface 16 of the carrier protrusion, in a range from 0.02 degrees to 5 degrees.
FIG. 16 shows an alternative embodiment of a carrier protrusion with edge relief. Instead of a chamfer as in the example of FIG. 15, the carrier protrusion in the example of FIG. 16 is formed with a step 94B cut inward from the outer peripheral wall 20 toward the center of the carrier protrusion. As shown in alternative cross-sectional view C-C in FIG. 16, the step 94B is formed with width L 78 and depth H 76 all the way around the carrier protrusion. The back surface 80 of the wafer 60 deflects toward the base plate 12 over a distance D 74 from the side 66 of the wafer during wafer polishing. A magnitude of deflection may be controlled by parameters related to the operation of the wafer polishing machine, for example an amount of pressure applied to the carrier assembly to force the wafers being polished into a polishing pad, surface characteristics of an insert, surface characteristics of a polishing pad, properties of the polishing slurry, and possibly other parameters.
A compressible, elastic material may optionally be placed in the step 94B to affect an amount by which the wafer 60 deflects toward the base plate 12 during wafer polishing. An example of an elastic material in the form of an elastic ring 52 is shown in the alternate cross section C-C in FIG. 17. The elastic ring 52 is positioned on the step 94B on the carrier protrusion and contacts the optional insert 50. When the optional insert is omitted, the elastic ring contacts a back surface 80 of the wafer 60. The carrier protrusion and elastic ring from the example of FIG. 17 are shown in a view toward the front surface 16 of the carrier protrusion 14 in FIG. 18. The carrier protrusion 14 has a radius 82 from the center of the protrusion to the outer peripheral wall 20. Edge relief 94 extends inward a distance L 78 from the outer peripheral wall 20 of the carrier protrusion 14. Edge relief 94 may alternatively be implemented as a chamfer, a step, or a continuous curved surface, with a thickness of the carrier protrusion at the outer peripheral wall 20 less than or equal to a thickness of the carrier protrusion elsewhere on the front face 16. FIG. 19 shows a cross-sectional view of an example of a carrier protrusion having edge relief implemented as a smoothly curved surface 94C from the front surface 16 outward to the peripheral wall 20. In the example of FIG. 19, the carrier protrusion has a greater height near the left side of the figure than near the perimeter of the protrusion. FIG. 19 further illustrates an example of a wafer having a back surface 80 in direct contact with the front surface 16 of a carrier protrusion, without an intervening insert between the wafer and carrier protrusion.
In an embodiment of a carrier protrusion with edge relief, an optional elastic ring 52, when present, rests on the carrier protrusion 14 in the area of the edge relief 94. The elastic ring 52 may optionally be attached to the carrier protrusion 14 with adhesive. The elastic ring 52 may optionally be formed with a perimeter shape that matches the perimeter shape of wafers to be polished.
FIG. 18 may be viewed as an example of a carrier protrusion with edge relief in a carrier assembly having a frame. Section D-D in FIG. 20 shows more details of a carrier protrusion with edge relief on a base plate adapted for attachment of a frame. Section D-D shows a partial cross-sectional view of an alternative embodiment of the base plate 12 from the example of FIG. 10 in which the detachable ring 40 is held against the outer peripheral wall 20 of a carrier protrusion and against the front face 18 of the base plate 12 by a frame 30. As shown in FIG. 20, a carrier protrusion may be formed with edge relief on the front face 16 of the carrier protrusion. The edge relief may alternatively be implemented as a step 94B as shown in FIG. 20 or as a chamfer or curved surface as described above for carrier assemblies without frames. A ring of elastic material 52 may optionally be interposed between the wafer 60 and the base plate 12, or between the base plate 12 and an optional insert 50 if present. During wafer polishing, the wafer 60 and the optional insert 50 deflect toward the base plate 12 in the region of the edge relief.
Unless expressly stated otherwise herein, ordinary terms have their corresponding ordinary meanings within the respective contexts of their presentations, and ordinary terms of art have their corresponding regular meanings.
Patent Application
20130316627
