WAFER CARRIER FOR BATCH WAFER POLISHING IN WAFER POLISHING MACHINES
A carrier assembly includes carrier protrusions extending outward from a base plate front face, one protrusion for each wafer to be polished. Each carrier protrusion has a front face separated from the base plate front face. An optional detachable ring may surround each protrusion front face to form a cavity for holding a wafer. In some examples of a carrier assembly, a frame having a separate aperture for each carrier protrusion holds all the detachable rings against the base plate front face. In other examples of a carrier assembly, the detachable rings are omitted, or a separate retaining ring for each carrier protrusion replaces the frame. A carrier protrusion may optionally be formed with edge relief to allow part of the wafer to deflect away from a polishing pad during polishing. A compressible elastic material may optionally be placed between a wafer and a carrier protrusion.
This application claims priority to U.S. Provisional Patent Application No. 61/646,761, filed May 14, 2012, incorporated herein by reference in its entirety.
FIELD OF THE INVENTIONEmbodiments 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.
BACKGROUNDThin, 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.
SUMMARYA 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.
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
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
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
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
A view toward the front surface 64 of an example of a frame 30 is shown in
A pictorial view toward the front side of an example of a circular detachable ring 40 is shown in
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
Alternative Section A-A in
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
An example of an alternative embodiment of the invention adapted for simultaneous polishing of a batch of three wafers is shown in
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
In the examples of a carrier assembly from
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
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
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
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.
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.
Claims
1. A carrier assembly for wafer polishing, comprising:
- a base plate comprising a base plate front face;
- at least three carrier protrusions extending outward from said base plate front face, wherein each of said carrier protrusions comprises: a protrusion front face separated from said base plate front face and from any other of said at least three carrier protrusions; and a peripheral wall extending from said base plate to a perimeter of said protrusion front face; and
- a frame comprising a front surface, wherein said frame is removably attachable to said base plate front face with a plurality of threaded fasteners, said frame is formed with a separate carrier protrusion aperture for each of said at least three carrier protrusions, and each of said carrier protrusion extends into a separate one of said carrier protrusion aperture with said front surface of said frame farther from said base plate front face than each of said protrusion front face.
2. The carrier assembly of claim 1, further comprising at least three detachable rings, wherein each of said at least three detachable rings fits around a different one of said at least three carrier protrusions.
3. The carrier assembly of claim 2, wherein each of said at least three detachable rings comprises a flange and said frame holds said flange against said base plate front face.
4. The carrier assembly of claim 2, wherein each of said at least three detachable rings further comprises:
- an inner wall;
- an outer wall; and
- a plurality of transverse grooves from said inner wall to said outer wall.
5. The carrier assembly of claim 1, wherein said base plate consists of five of said carrier protrusions.
6. The carrier assembly of claim 1, wherein said protrusion front face on each of said at least three carrier protrusions has a peripheral shape corresponding to a peripheral shape of a wafer to be polished.
7. The carrier assembly of claim 1, wherein said frame further comprises a front face, said front face is formed with a separate carrier protrusion aperture for each of said at least three carrier protrusions, and said front face is formed with at least three grooves, one of said at least three grooves between each adjacent pair of said carrier protrusion apertures.
8. The carrier assembly of claim 1, wherein each of said at least three carrier protrusions is formed with a chamfer from said peripheral wall to said protrusion front face.
9. The carrier assembly of claim 1, wherein each of said at least three carrier protrusions is formed with a step from said peripheral wall to said protrusion front face.
10. The carrier assembly of claim 9, further comprising an elastic ring on said step formed on each of said at least three carrier protrusions.
11. A carrier assembly, comprising:
- at least three carrier protrusions, each of said at least three carrier protrusions comprising a carrier protrusion front face;
- at least three detachable rings, each of said at least three detachable rings surrounding a separate one of said at least three carrier protrusions;
- at least three retaining rings, each of said at least three retaining rings surrounding a separate one of said at least three detachable rings; and
- a base plate comprising: a base plate front face; and at least three ring grooves formed in said base plate front face, each of said at least three ring grooves having a flat bottom surface parallel to said carrier protrusion front face on each of said at least three carrier protrusions, and each of said at least three ring grooves surrounding a separate one of said at least three carrier protrusions, thereby separating said base plate front face from each of said carrier protrusion front face.
12. The carrier assembly of claim 11, wherein each of said at least three detachable rings comprises a flange and said at least three retaining rings hold said flange for each of said at least three detachable rings against said flat bottom surface in each of said at least three ring grooves.
13. The carrier assembly of claim 11, wherein said carrier protrusion face on each of said at least three carrier protrusions has a perimeter shape corresponding to a perimeter shape of a wafer to be polished.
14. The carrier assembly of claim 11, consisting of:
- five of said at least three carrier protrusions;
- five of said at least three detachable rings; and
- five of said at least three retaining rings.
15. The carrier assembly of claim 11, wherein each of said at least three rings further comprises:
- an inner wall;
- an outer wall; and
- a plurality of transverse grooves from said inner wall to said outer wall.
16. The carrier assembly of claim 11, wherein said carrier protrusion front face on each of said at least three carrier protrusions is formed with a chamfer all the way around said carrier protrusion front face.
17. The carrier assembly of claim 16, wherein an acute angle between said chamfer and said carrier protrusion front face on each of said at least three carrier protrusions is in a range from 0.02 degrees to 5 degrees.
18. The carrier assembly of claim 11, wherein each of said at least three carrier protrusions is formed with a step.
19. The carrier assembly of claim 18, wherein each of said at least three carrier protrusions further comprises an elastic ring positioned on said step.
20. The carrier assembly of claim 18, wherein a height dimension of said step formed on each of said at least three carrier protrusions is selected to cause a selected magnitude of deflection of a part of a wafer surface toward said base plate during wafer polishing.
Type: Application
Filed: May 2, 2013
Publication Date: Nov 28, 2013
Applicant: Edmond Arzuman Abrahamians (Fremont, CA)
Inventor: Edmond Arzuman Abrahamians
Application Number: 13/886,063
International Classification: B24B 37/30 (20060101); B24B 37/32 (20060101);