WO2000047366A2

WO2000047366A2 - Apparatus and method for grinding, lapping and polishing semiconductor wafers

Info

Publication number: WO2000047366A2
Application number: PCT/GB2000/000376
Authority: WO
Inventors: Kobayashi Masayuki
Original assignee: Shin-Etsu Handotai Europe Limited
Priority date: 1999-02-10
Filing date: 2000-02-09
Publication date: 2000-08-17
Also published as: WO2000047366A3; AU2311000A; GB9902818D0

Abstract

Apparatus and methods for grinding, lapping and polishing semiconductor wafers (W), particularly the peripheral edges thereof, and most particularly the orientation notch (40). A lapping/polishing tool comprises a flexible, elongate member (10) which is substantially circular in transverse cross section, which is substantially inelastic in its longitudinal direction and which is elastic and deformable (compressible) in its radial direction. The tool has an inner core (12) which is substantially inelastic at least in its longitudinal direction surrounded by a cylindrical outer shell (14) which is elastic and deformable in its radial direction. Apparatus employing the tool comprises a first spool (30) upon which a length of said tool is wound, guide means for guiding said tool along a predetermined path from said first spool (30) to a second spool (32) may be wound, and drive means for driving said spools so as to drive the tool along said path between the spools, a portion of said path comprising a work station (33) at which a workpiece may be positioned to be operated on by said tool. The workstation (33) permits the wafer (W) to be translated and rotated relative to the tool to lap and/or polish the periphery of the wafer, including an orientation notch (40) formed therein. Methods of operation are also disclosed.

Description

"Apparatus and Method for Grinding, Lapping and Polishing Semiconductor Wafers"

The present invention relates to the lapping and polishing of semiconductor wafers. The invention provides improved apparatus and methods for effecting such lapping and polishing and is concerned particularly, but not exclusively, with the lapping and polishing of the surfaces of orientation notches formed in the peripheral edges of semiconductor wafers. The invention may also be applicable to the grinding of wafers .

The manufacture of semiconductor wafers comprises a series of steps, beginning with the pulling of a monocrystalline ingot, typically of single crystal silicon. The raw ingot is machined to a cylindrical shape and a groove is then formed along the surface of the cylindrical ingot, parallel to its longitudinal axis. The ingot is then sliced into a plurality of disc-shaped wafers. The groove formed in the ingot prior to slicing results in each wafer having a notch in its peripheral edge. This notch provides a reference or index mark to assist in correctly orienting the wafer during subsequent processing of the wafer and is commonly referred to as an "orientation notch" . The notch is normally V-shaped, with an internal angle of 90°.

The slicing of the ingot provides roughly finished wafers which then undergo a series of processes, typically including mechanical, thermal and chemical processes to provide finished wafers. The precise details of such processes vary according to the required characteristics of the finished wafer, but generally include a variety of grinding, lapping and polishing steps. Normally, the peripheral edges of the wafers, including the edges of the notch, are chamfered. Typically, at least one of the major surfaces of the wafer and the chamfered edges are polished to a mirror finish.

A variety of lapping and polishing devices and techniques are known for performing different lapping and polishing processes. These include various types of lapping/polishing pads, discs, tapes and the like, with associated mechanisms for positioning the workpiece relative to the lapping/polishing tool. The lapping and polishing of orientation notches presents particular difficulties due to the shape and small size of the notches. Conventional techniques employing pads, discs and tapes as lapping/polishing tools suffer from a variety of disadvantages, including the need to profile and dress the tools prior to and during use, limited working life, and difficulties in controlling operation in order to obtain high quality and high efficiency.

Accordingly, it is an object of the present invention to provide improved apparatus and methods for grinding, lapping and polishing semiconductor wafers, particularly the peripheral edges thereof, and most particularly the orientation notch.

In accordance with a first aspect of the invention, there is provided a lapping/polishing tool comprising a flexible, elongate member which is substantially circular in transverse cross section, which is substantially inelastic in its longitudinal direction and which is elastic and deformable (compressible) in its radial direction.

Preferably, the tool comprises an inner core which is substantially inelastic at least in its longitudinal direction surrounded by a cylindrical outer shell which is elastic and deformable in its radial direction.

Preferably, said outer shell is formed of woven textile material .

Preferably also, the outer shell comprises at least two layers of said woven, textile material.

Suitably, the outer shell may be formed from a material such as nylon, aramid fibre, cotton, leather or the like. Preferably also, the outer shell is woven from multi- filament twine. Most preferably, each filament of said twine is coated with a bonding agent by means of which the filaments are bonded to one another. Preferably, the thickness of the coating of bonding agent is equal to or greater than the radius of the filaments. The bonding agent might suitably comprise a polyurethane or wax material .

Preferably, the inner core comprises a single filament of a material such as nylon, aramid fibre, cotton, leather, steel or the like.

Preferably, the tool has a diameter in the range 2 mm to 3 mm. Preferably also, the thickness of the outer shell is approximately equal to or greater than the radius of the inner core.

The tool may be adapted for use in grinding operations by bonding abrasive materials to its outer surface.

In accordance with a second aspect of the invention, there is provided a lapping/polishing apparatus for use with the lapping/polishing tool of the first aspect, comprising a first spool upon which a length of said tool is wound, guide means for guiding said tool along a predetermined path from said first spool to a second spool upon which a length of said tool delivered from said first spool may be wound, and drive means for driving said spools so as to drive the tool along said path between the spools, a portion of said path comprising a work station at which a workpiece may be positioned to be operated on by said tool .

Preferably, said guide means comprises a plurality of pulleys.

Preferably also, the apparatus further includes tensioning means for applying tension to said tool . Preferably, the tensioning means comprises a free weight tensioning mechanism.

Preferably, the drive means is adapted to drive said tool backwards and forwards between said first and second spools.

Preferably, the apparatus further includes workpiece handling means for positioning a wafer relative to said tool at said work station.

Preferably also, said work station includes a length of said tool extending in a first direction and said wafer handling means is adapted to hold a wafer and: to translate said wafer along first, second and third mutually orthogonal axes, said first axis (Z- axis) extending substantially parallel to said first direction, said second axis (Y-axis) extending substantially at right angles to said first direction across said tool and said third axis (X-axis) extending substantially at right angles to said first direction towards and away from said tool; and to rotate said wafer about said first and second axes. In accordance with a third aspect of the invention, there is provided a method of lapping or polishing a semiconductor wafer, using apparatus in accordance with the second aspect of the invention, in which a semiconductor wafer is positioned in contact with said tool so as to create a bow in said tool which, in combination with a predetermined tension applied to the tool, applies a desired force to the wafer, and the tool is driven lengthwise relative to the wafer.

Preferably, the tool is driven backwards and forwards relative to the wafer. Preferably also, the wafer is oscillated relative to the tool.

The orientation and position of the wafer relative to the tool is adjusted so that a desired force is applied to a desired portion of the surface of the wafer.

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Fig. 1A is a side view, on an enlarged scale, of a length of a lapping/polishing tool in accordance with the first aspect of the invention;

Fig. IB is a transverse cross-sectional view of the tool of Fig. 1;

Fig. 2A and 2B are transverse cross-sectional views of twines forming part of the tool of Fig. 1; Fig. 3 is a schematic diagram illustrating a lapping/polishing apparatus employing the tool of Fig. 1, in accordance with the second aspect of the invention;

Figs. 4A, 4B and 4C are diagrams illustrating the positioning and movement of a wafer relative to the apparatus of Fig. 3;

Fig. 5A is a perspective view of an orientation notch of a semiconductor wafer and Fig. 5B is a fragmentary cross sectional view illustrating the general cross sectional profile of the wafer edge including chamfer surfaces;

Fig. 6 is a plan view of an orientation notch of a semiconductor wafer illustrating the manner in which the tool of Fig. 1 interacts therewith in use;

Fig. 7 illustrates the manner in which forces transmitted from a tool to a wafer may be varied.

Figs. 8A and 8B, 9A and 9B and 10A and 10B illustrate the use of the tool in accordance with the invention for lapping/polishing wafer chamfer surfaces, Figs, 9A and 9B comprising enlarged details of Figs. 8A and 8B respectively.

Referring now to the drawings, Figs. 1A and IB illustrate an embodiment of a lapping/polishing tool in accordance with the present invention. The lapping/polishing tool comprises a flexible, elongate member 10 which is substantially circular in transverse cross section, typically having a diameter of 2 mm to 3 mm. The tool is of indefinite length, normally of the order of hundreds or thousands of metres, being fed between first and second spools in use, as shall be described below. The tool 10 is substantially inelastic in its longitudinal direction but is elastic and deformable (compressible) in its radial direction.

In the preferred embodiments, the tool 10 comprises an inner core 12 which is substantially inelastic at least in its longitudinal direction, surrounded by a cylindrical outer shell 14 which is elastic and deformable in its radial direction.

The outer shell 14 is formed of woven textile material, preferably comprising at least two layers 14A, 14B, of any suitable material such as nylon, aramid fibre, cotton, leather or the like. As indicated in Fig. 1A, the textile material of the outer shell 14 may be formed as a square-woven material 15A or a cross-woven material 15B, for example.

The outer shell 14 is preferably woven from multi- filament twine 16, as illustrated in Figs. 2A and 2B. Most preferably, each filament 18 of said twine 16 is pre-coated with a bonding agent 20 by means of which the filaments 18 are bonded to one another. The thickness of the coating of bonding agent 20 is preferably equal to or greater than the radius of the filaments 18, so that the overall diameter of the coated filament is at least twice that of the basic filament 18. The bonding agent suitably comprises a polyurethane or wax material. Preferably, the bonding agent has a Shore hardness in the range 30A to 200A, depending upon the intended application of the tool 10.

After the shell material is woven, it is subjected to a bonding process using heat and/or solvents to bond the filaments together. Fig. 2A shows a cross-sectional view of the pre-coated filaments 18 of the twine 16 prior to bonding. Fig. 2B shows a similar cross- sectional view after bonding, showing how the bonding agent 20 provides a homogeneous matrix encapsulating the filaments 18. The use of pre-coated filaments eliminates unreliable impregnation of the twine with the bonding agent which might result in weak bonding. The thickness of the coating 20 of bonding agent is selected so as to provide an elastic layer of appropriate thickness between the filaments 18 and twines 16 after dissolution/fusion and solidification of the bonding agent 20 during the bonding process.

The inner core 12 of the tool 10 preferably comprises a single filament of suitable tensile strength, typically formed from a material such as nylon, aramid fibre, cotton, leather, steel or the like. The outer shell 14 will typically have a thickness approximately equal to or greater than the radius of the inner core 12. Referring now to Fig. 3 of the drawings, lapping/polishing apparatus for use with the tool 10 of Figs. 1 and 2 comprises a first spool 30 upon which a length of the tool 10 is wound, and guide means, suitably comprising a series of pulleys 31, for guiding the tool 10 along a predetermined path from the first spool 30 to a second, take-up spool 32. The spools 30 and 32 are driven by any suitable drive means (not shown) so as to drive the tool lengthwise along its path between the spools 30and 32. The drive means may comprise servo motors, for example, and is adapted to provide smooth acceleration and deceleration of the spools and to provide a high and consistent driving speed (typically of the order of up to 10 m/sec) .

A portion of the path, in this example being that portion between pulleys 31A and 31B, provides a work station 33 at which a wafer W is positioned to be operated on by the tool 10.

The apparatus further includes tensioning means for applying tension to the tool 10 as it travels along the path between the spools 30 and 32. Preferably, the tensioning means comprises a free weight tensioning mechanism 34, having a weight 36 suspended by the tool 10 between a pair of said guide pulleys 31. The tensioning mechanism 34 may include sensors 38 to monitor the tensioning of the tool 10.

The work station 33 includes wafer handling apparatus for translating and rotating the wafer relative to the tool 10, as shall now be described with reference to Figs. 4A to 4C of the drawings. Wafer handling apparatus of this general type is well known in the art and will not be described in detail herein. Typically, such apparatus includes wafer holding means such as a vacuum chuck in order to hold the wafer securely without causing damage thereto. The wafer holding means is in turn mounted on handling apparatus for translating and rotating the holding chuck as required.

Fig. 4A provides a schematic illustration of the work station 33, showing the manner in which the wafer handling means is adapted to rotate and translate the wafer W relative to the tool 10. The wafer W is shown with its orientation notch 40 facing the tool 10.

As seen in Fig. 4A, the work station 33 includes a length of the tool 10 extending in a first direction between pulleys 31A and 3IB. The wafer handling means is adapted to hold the wafer W for translation and rotation along and about various axes as follow: to translate the wafer along first, second and third mutually orthogonal axes, Z, Y and X respectively, the first axis (Z-axis) extending substantially parallel to said first direction defined by the working length of the tool 10, the second axis (Y-axis) extending substantially at right angles to said first direction across the tool and the third axis (X-axis) extending substantially at right angles to said first direction towards and away from said tool; and to rotate the wafer W about the first (Z) and second (Y) axes as indicated by arrows B and A respectively.

Taking the illustrated position of the wafer W with the notch 40 facing the tool 10 as a reference, the wafer handling means preferably permits the following movements wafer W relative to the tool 10: The wafer can pivot about a diameter thereof in the direction A through +/- 90 degrees, so that the angle at which the peripheral edge of the wafer W is presented to the tool 10 may be varied in order to provide positive contact between the tool 10 and the chamfered surfaces of the wafer. The wafer can swivel about its own centre in the direction B through at least +/- 60 degrees so that different points on the circumference of the wafer may be presented to the tool 10 and, in particular, so that the tool 10 may be brought into positive contact with all of the surfaces of the notch 40. Preferably, the wafer can be oscillated along the direction B with an adjustable speed in the range 0 to 200 cpm. The wafer can oscillate along the Y-axis from the illustrated position by +/- 100 mm to ensure positive contact between the tool 10 and the surfaces of the notch 40, this oscillation being synchronised with the oscillation along arrow B with a similarly adjustable oscillation speed of 0 to 200 cpm. The wafer may be translated along the X-axis towards and away from the tool 10 so as to create a desired bow in the tool 10 (see Fig. 4B) . The wafer may be translated along the Z-axis so as to vary the point on the length of tool 10 between pulleys 31A and 3 IB at which the wafer makes contact, in order to allow different parts of the wafer edge profile to be brought into contact with the tool. Fig. 4C shows the wafer displaced upwardly along the Z-axis from the reference position and inwardly along the X- axis so as to create an asymmetric bow in the tool 10 (as compared with the symmetric bow of Fig. 4B) .

Figs. 5A and 5B illustrate the geometry of the notch 40 and the edge profile of the wafer W. The notch 40 typically has an internal angle of 90 degrees. The edge of the wafer W, including the notch 40, has a top chamfer surface XI, a bottom chamfer surface X2 and a peripheral edge surface X3. The tool 10 is required to operate on all of these surfaces along the length of the V-shaped edge of the notch 40.

Fig. 6 illustrates the manner in which the tool 10 deforms upon contact with the wafer notch 40, under pressure generated by the tension on the tool 10. As can be seen, the outer shell 14 deforms against the wafer edge so as to provide good, uniform contact therewith, conforming with the edge profile even at the innermost angle of the notch 40. The inelastic inner core 12 prevents the outer shell 14 from stretching under tension and thereby losing its radial elasticity. Thus, the outer shell 14 remains elastic and flexible in the radial direction, allowing it to follow the notch profile while tension is applied to the tool 10. Accordingly, no profiling or dressing of the tool 10 is required in advance of the tool being used.

The properties of the tool 10 may be varied to suit particular applications by varying the materials from which the core 12 and outer shell 14 are formed. In particular, the radial elasticity of the outer shell 14 may be adjusted by employing bonding materials 20 of different hardness. The properties of the outer shell 14 may also be adjusted by varying the diameters of the filaments 18 and the weave of the material, including the weave density and pattern. By varying the properties of the tool and of the slurries used with the tool it may be adapted for a variety of lapping and polishing operations.

Typically, in use, the tool 10 will be driven backwards and forwards between the spools 30 and 32. The tool will typically be driven in a forward direction by a predetermined amount and driven backwards by a lesser amount. The distances by which the tool is driven backwards and forwards may be adjusted to obtain an optimal balance between lapping/polishing efficiency and cost .

Using the X-axis positioning mechanism, the wafer W is moved forward to contact the tool 10 at a position which creates a desired degree of bow in the tool 10. As the bow increases, the force in the X-Y plane increases, as illustrated in Fig. 7. The force applied to the wafer by the tool depends firstly on the tension applied to the tool 10. It can be seen that the component of the total force which is applied to the wafer increases as the bow increases . The amount of bow is selected to provide a balance between lapping/polishing efficiency and the contact angle between the tool 10 and the XI and X2 chamfer surfaces, in combination with the degree by which the wafer is pivoted about the Y-axis (arrow A) . The amount of bow might differ for operations performed on the peripheral X3 surface and the XI and X2 surfaces .

By way of example, lapping/polishing operations on the various surfaces XI, X2 and X3 may be performed as follows.

X3 Lapping/Polishing (Fig. 4B, Fig. 7)

The tool 10 is run back and forth between the spools 31A and 3 IB. The wafer is translated along the X-axis to create the desired bow. The wafer is oscillated along the Y-axis and about the Z-axis (arrow B) . These oscillations are synchronised so that the wafer, as seen in Fig. 4A, is rotated clockwise through 45 degrees about the Z-axis while being translated to the right along the Y axis and is rotated counter clockwise through 45 degrees about the Z-axis while being translated to the left along the Y-axis. These oscillations affect the bow of the tool, the operational parameters being selected to provide a desired force on the wafer.

XI and X2 Lapping/Polishing (Figs. 8A/9A and 8B/9B) The tool 10 is run back and forth. The wafer is translated along the X-axis and pivoted about the Y- axis so as to bring either surface XI or X2 into contact with the tool 10 at a desired angle. The further the wafer is translated in the X direction (i.e. the greater the bow on the tool 10) the less the wafer needs to be pivoted in the A direction, as can be seen by comparison of Figs. 8A/9A (lesser X translation, greater A pivot) and Figs. 8B/9B (greater X translation, lesser A pivot) . The same angle between the tool 10 and the main wafer surface can be obtained by different combinations of X translation and A pivot. The wafer is also translated along the Z-axis as required in order to fine tune the pressure specifically towards the XI and X2 surfaces as required. Figs. 10A and 10B illustrate how the applied force component is varied by varying the position of the wafer along the Z-axis. At the same time, the wafer is oscillated in the Y and B directions in the same synchronised manner as for the X3 surface. This ensures a positive pressure against the XI and X2 surfaces on the V-edge of the notch 40.

The use of the flexible tool 10 in combination with the independent positioning and orientation of the wafer relative thereto along and about the various axes allows the lapping/polishing forces to be controlled in any direction relative to the various surfaces of the wafer edge profile and the V-edge of the notch. This permits lapping/polishing operations to be carried out with greater efficiency and consistency. This is particularly true of the X3 surface. The tool may be as long as the driving system can accommodate. The shoulders of the notch would be lapped and polished as part of the main circumference of the wafer, in a separate operation from the lapping/polishing of the sides and inner angle of the notch.

The tool 10 may be employed in lapping processes with micro-grit abrasives, in mechanical and chemical polishing processes, and in secondary edge grinding processes, prior to lapping/polishing, using tools with micro-grit and/or diamond type abrasives bonded on the tool surface.

The tool is thus extremely flexible in its range of possible applications, providing higher productivity and better quality of finish as compared with conventional methods. No profiling, tooling or dressing is required for lapping/polishing operations either before or during production. A longer interval between changing tools can be expected and higher efficiency and quality of lapping/polishing can be obtained due to the multi -directional pressure control provided by the system.

Improvements and modifications may be incorporated without departing from the scope of the invention.

Claims

1. A lapping/polishing tool comprising a flexible, elongate member which is substantially circular in transverse cross section, which is substantially inelastic in its longitudinal direction and which is elastic and deformable (compressible) in its radial direction.

2. A lapping/polishing tool as claimed in Claim 1, wherein the tool comprises an inner core which is substantially inelastic at least in its longitudinal direction surrounded by a cylindrical outer shell which is elastic and deformable in its radial direction.

3. A lapping/polishing tool as claimed in Claim 1 or Claim 2, wherein said outer shell is formed of woven textile material.

4. A lapping/polishing tool as claimed in any preceding Claim, wherein the outer shell comprises at least two layers of said woven, textile material.

5. A lapping/polishing tool as claimed in Claim 3 or Claim 4, wherein the outer shell is formed from a material such as nylon, aramid fibre, cotton, leather or the like.

6. A lapping/polishing tool as claimed in any preceding Claim, wherein the outer shell is woven from multi-filament twine.

7. A lapping/polishing tool as claimed in Claim 6, wherein each filament of said twine is coated with a bonding agent by means of which the filaments are bonded to one another .

8. A lapping/polishing tool as claimed in Claim 7, wherein the thickness of the coating of bonding agent is equal to or greater than the radius of the filaments.

9. A lapping/polishing tool as claimed in Claim 7 or Claim 8, wherein the bonding agent comprise a polyurethane or wax material .

10. A lapping/polishing tool as claimed in any preceding Claim, wherein the inner core comprises a single filament of a material such as nylon, aramid fibre, cotton, leather, steel or the like.

11. A lapping/polishing tool as claimed in any preceding Claim, wherein the tool has a diameter in the range 2 mm to 3 mm.

12. A lapping/polishing tool as claimed in any preceding Claim, wherein, the thickness of the outer shell is approximately equal to or greater than the radius of the inner core.

13. A lapping/polishing tool as claimed in any preceding Claim, wherein the tool is adapted for use in grinding operations by bonding abrasive materials to its outer surface.

14. A lapping/polishing apparatus for use with the lapping/polishing tool as claimed in any one of Claims 1 to 13, the lapping/polishing apparatus comprising a first spool upon which a length of said tool is wound, guide means for guiding said tool along a predetermined path from said first spool to a second spool upon which a length of said tool delivered from said first spool may be wound, and drive means for driving said spools so as to drive the tool along said path between the spools, a portion of said path comprising a work station at which a workpiece may be positioned to be operated on by said tool .

15. A lapping/polishing apparatus as claimed in Claim 14, wherein said guide means comprises a plurality of pulleys.

16. A lapping/polishing apparatus as claimed in Claim 14 or Claim 15, wherein the apparatus further includes tensioning means for applying tension to said tool.

17. A lapping/polishing apparatus as claimed in Claim 16, wherein the tensioning means comprises a free weight tensioning mechanism.

18. A lapping/polishing apparatus as claimed in any one of Claims 14 to 17, wherein the drive means is adapted to drive said tool backwards and forwards between said first and second spools.

19. A lapping/polishing apparatus as claimed in any one of Claims 14 to 18, wherein the apparatus further includes workpiece handling means for positioning a wafer relative to said tool at said work station.

20. A lapping/polishing apparatus as claimed in Claim 19, wherein said work station includes a length of said tool extending in a first direction and said wafer handling means is adapted to hold a wafer and: to translate said wafer along first, second and third mutually orthogonal axes, said first axis (Z- axis) extending substantially parallel to said first direction, said second axis (Y-axis) extending substantially at right angles to said first direction across said tool and said third axis (X-axis) extending substantially at right angles to said first direction towards and away from said tool; and to rotate said wafer about said first and second axes.

21. A method of lapping or polishing a semiconductor wafer, using apparatus as claimed in any one of Claims 14 to 20, in which a semiconductor wafer is positioned in contact with said tool so as to create a bow in said tool which, in combination with a predetermined tension applied to the tool, applies a desired force to the wafer, and the tool is driven lengthwise relative to the wafer.

22. A method of lapping or polishing a semiconductor wafer as claimed in Claim 21, wherein the tool is driven backwards and forwards relative to the wafer.

23. A method of lapping or polishing a semiconductor wafer as claimed in Claim 21 or Claim 22, wherein the wafer is oscillated relative to the tool.

24. A method of lapping or polishing a semiconductor wafer as claimed in any one of Claims 21, 22 or 23, wherein the orientation and position of the wafer relative to the tool is adjusted so that a desired force is applied to a desired portion of the surface of the wafer.