WO2011081532A1

WO2011081532A1 - Sawing blocks into wafers using initially bowed wires

Info

Publication number: WO2011081532A1
Application number: PCT/NO2010/000490
Authority: WO
Inventors: Kristian Leonard Aas; Erik Sauar; Øivind GJERSTAD; Stian Sannes; Torgrim Andersen
Original assignee: Rec Wafer Norway As
Priority date: 2009-12-30
Filing date: 2010-12-29
Publication date: 2011-07-07
Also published as: GB0922708D0; GB2476658A; TW201134634A

Abstract

Process for cutting a block of material 12 into a multiplicity of wafers by movement of a planar array of parallel fast moving wires 14 relative to the block (or vice versa) in a direction perpendicular to the plane of the wires and applying a slurry or fluid to the wires before the wires pass through the block 12, in which, before application of the slurry or fluid, the array of wires 14 is brought into contact with the block in a dry condition, thus to introduce a bow in the array of wires caused by a contact pressure between the wires and the block and thereby keeping the array of wires against the block in their intended positions, introducing slurry onto the wires, whereby the wires cut through the block as the relative movement in a perpendicular direction takes place.

Description

SAWING BLOCKS INTO WAFERS USING INITIALLY BOWED WIRES

Field of the Invention

The invention relates to the sawing of blocks into a multiplicity of thin wafers. In particular, the invention relates to the sawing of silicon blocks into wafers for use in the electronics, semiconductor and photovoltaic cell industries, and in similar applications.

Background to the invention

The technique of using a multi wire array of rapidly moving wires for sawing blocks of material into thin wafers is known. An example of apparatus for abrasive wire sawing is disclosed in UK Patent Specification 2,414,204. The present invention is not limited to use in that apparatus, and is applicable to any process for wire sawing blocks of material into thin wafers.

Wafers of various materials such as Si, SiC, GaAs and sapphire can be sawn from blocks of such materials for electronics, semiconductor and photovoltaics applications. For photovoltaic applications, multi or monocrystaline Si wafers are produced by sawing large Si blocks. Currently, multi-wire sawing is used for high volume cutting of Si wafers from Si blocks, and is capable of rapidly producing high quality thin wafers (<200 μιη). The slurry may carry abrasive particles, or alternatively the wire may be embedded with abrasive particles. The slurry can be a solvent such a Polyethylene glycol carrying abrasive particles such as silicon carbide particles.

The solvent is carried into the block by an array of parallel wires moving into the block at right angles while running lengthwise at high speeds (5-20 m/s). The aim is to cut with high throughput and minimum loss of solvent, resulting in high quality wafers at a low cost. Wafers can be sawn within a process window defined by properties such as viscosity, size and shape of the abrasive particles, solvent properties and saw parameters.

One of the biggest loss categories is in Local Area Thickness Fluctuations (LATF). These fluctuations occur during the beginning of a cut, and can result in thickness variations and, as a consequence, unacceptably thick and thin wafers.

The reason for this is the pairing of wires as the wires are attracted to each other. One of the main reasons for this to happen is the surface tension in the solvent used for cutting. This pulls two adjacent wires together thus pairing them and resulting in thick and thin wafers on the 'cutting in' edge or in the worst case wafers twice as thick as the pitch. This problem escalates as wafer thickness and wire thickness decrease. The force with which the wires are pulled together is given by

F = 2yL sm^' (a + θ) + 2γ∑— sin a where γ is the surface tension of solvent, L is the

R

solvent film length between wires, r is the wire radius, R is the curvature of solvent surface between wire, d is the center distance between wire=(pitch-2r), 9=wetting angle between solvent and wire, and a is the angle determined by contact point between solvent and wire, see Fig. 5.

The present invention depends on the recognition that the surface tension of the solvent is a key parameter controlling the force between the wires, and should be reduced to prevent local area thickness variations (LATF). Summary of the invention

The invention provides a process for cutting a block of material into a multiplicity of wafers, where

- a planar array of parallel running wires is moved relative to the block in a direction perpendicular to the plane of the running wires,

characterised in that

- the array of wires is, in a dry condition, brought into contact with the block and pressed to form a contact pressure between the wires and the block in the range from 0.2 to 1.4 N of each wire, and

- the wires are, in a dry condition, made to run at an speed in the range from

0.5 to 2.0 m/s and allowed to cut into the block for a period in the range from

0.075 to 60.0 seconds before the sawing slurry is applied.

It is preferred that the block is formed with chamfers on its corners first contacted by the array of wires, and initial contact between the wires and the block is made on an edge of a chamfer closest to the array of wires.

It is further preferred that the wires within the array are kept in their intended positions by friction and by reduction of attractive surface tension forces between wires and by regular equally spaced guide indentations formed on the corner of the block.

In one form the bow in the wires is introduced by pressing the block against the wires before the start up of the wires.

In an alternative form initial contact with the block is made with the wires running at an initial speed, and as the slurry is added, the wires are accelerated to a higher running speed to cut through the block.

It is preferred that the initial speed of the cutting wires is in the range 0.5 - 2.0 m/s. Experiments performed by the inventors indicates that a minimum distance of 15 cm wire may be allowed to contact the block during the initial dry sawing before wire with sawing sludge is entered. This corresponds to a minimum time of dry sawing of 15/200 or 0.075 seconds. The wire will tolerate dry cutting up to one minute before the cooling effect of the slurry is requested.

It is further preferred that the bow in the cutting wires is between 1 mm and 7 mm of a 660 mm wire span, corresponding to a force F on each wire from 0.2 to 1.4 N. More specifically, it is preferred that the bow in the cutting wires is between approximately 2.5 mm and 5 mm of a 660 mm wire span, corresponding to a force F on each wire from 0.5 to 1.0 N.

It is preferred that the higher running speed of the cutting wires is in the range 14.0 - 18.0 m/s.

In one form the wire is plain wire, and the slurry contains abrasive particles for cutting the block. The abrasive particles may be of silicon carbide or diamond. In this form it is preferred that the solvent is polyethylene glycol (PEG), or is

5 - 100 % by weight of water.

In an alternative form the wire has embedded abrasive particles for cutting the block, and the slurry is added for the purposes of cooling and removal of material from the block. The abrasive particles may be of diamond. In this form it is preferred that the solvent is polyethylene glycol (PEG), or is 5 - 100 % by weight of water.

The invention also provides apparatus for use in the process of the invention as described above.

The invention also provides wafers when made in the process of the invention as described above or in the apparatus for use with that process.

Brief description of the drawings

Specific embodiments of the invention will now be described by way of examples with reference to the accompanying drawings, in which:

Fig 1 is a diagrammatic end view of a silicon block being sawn into wafers, and showing how an initial force is applied by introducing a bow into the cutting wires where they contact the lower face of the block;

Fig 2 is a diagram showing the directions of tension in the cutting wires;

Fig 3 is a further diagram showing the resolution into vertical components of that tension for a very small displacement of the block; and

Fig 4 is an illustration showing how the silicon block is shaped at its lower corners, and how initial indentations are made in that block.

Fig 5 is an illustration showing the force between parallel wires. Detailed description of the drawing(s)

As shown diagrammatically in Fig 1 , apparatus for cutting a block of silicon into thin wafers (for photovoltaic or other uses) comprises two parallel guide rollers 10 and 1 1 arranged below a block of silicon 12. A tensioned cutting wire 14 is run repeatedly round the rollers with narrow spaces between successive wires, so to form a planar array (or 'web') of parallel wires. The wires are drawn round the guide rollers 10 and 1 1 , so that the wires can run past the block 12 at high speed (e.g. 14 to 18 m/s) to effect cutting of the block into wafers. The block is moved downwardly (D) through the array of wires to cut the block into wafers. When the wires are running at high speed, slurry must be introduced onto the wires to cool the block and to remove waste material. The slurry may carry abrasive particles, or alternatively the wire may be embedded with abrasive particles.

As shown in Fig 1 , the block is moved down through the array of wires. However, the block could be moved in any direction provided that its movement is

perpendicular to the parallel array of wires.

As shown in Fig 1 , a single block is being cut. However, more than one block can be cut. For instance, there might be four blocks in one row on one table, or eight blocks in two rows split between upper and lower tables. Other arrangements of blocks are possible. Accurate alignment of the blocks is important when more than one bloc is cut simultaneously.

The present invention is concerned with avoiding LATF, which may occur due to the surface tension between adjacent wires, causing them to pair up and to result in successive wafers being thick, thin, thick, thin, rather than of uniform thickness.

The problems resulting from surface tension in the slurry can be reduced or avoided by bringing the block 12 into contact with the array of wires to form a bow 15 in the wires 14 before the slurry is introduced. This is illustrated in the accompanying diagrammatic figures, in which the vertical dimension of the initial bow is greatly exaggerated. Initial contact is made with the wires not moving, or with the wires moving slowly (e.g from 0.5 to 2.0 m/s).

As shown in Fig 2, downward movement D of the block 12 onto the tensioned wires 14 moves the tension T into a slight angle to the plane of the wires. By introducing the bow 15, a vertical component of this force will act on the silicon block. (The angle is greatly exaggerated in the figures.)

Fig 3 shows the resolution of the force relative to the block into a vertical component Ty. For relatively small displacements, the increase in tension in the wire will be negligible, and so the resulting force F from each wire on the block is F = 2Ty = 2T · sin(a). Displacements for practical use during pre-tensioning and cutting will be small. Examples of workable dimensions for use in the cutting processes are given below. Fig 4 shows the particular configuration of the lower edges of the block 12. In Fig 4, the left hand side 16 of the block 12 is designated as the 'wire in' surface, the bottom 17 of the block is designated as the 'cutting in' surface, and the right hand side 18 of the block is designated as the 'wire out' surface. The lower corners of the block 12 have small chamfers 21 and 22 between the surfaces 16 and 17, and surfaces 17 and 18 respectively. These chamfers may be at 45 degrees to the adjacent surfaces, and may be from 0.5 mm to 1.25 mm long. (With mono-silicon blocks the chamfers could be much bigger, and might even have some curvature.) When the block 12 is lowered to form the bow 15, the wires 14 engage with the lower edges of the chamfers 21 and 22. Initial contact with the wires and movement of the wires at low speeds without slurry will create evenly spaced indentations in the lower edges of the chamfers at 23 and 24. These indentations guide the cutting wires 14 as the wires are speeded up and the slurry is added.

As mentioned above LATF is avoided or reduced by the introduction of a bow in the array of wires and by starting the sawing in a dry condition. The bow in the array of wires is caused by an increase in the contact pressure between the wires and the block. This gives an increased friction between the wires and the block. The initial sawing without slurry assures that the high friction is maintained. Further, the tendency for unwanted wire pairing due to surface-tension effects from the wet slurry is reduced by starting the sawing in the dry condition. This wire pairing effect will be less when the wires are affected by the contact pressure of the block. In addition, regular equally spaced indentations are formed by the wires, which also contribute to preventing unwanted lateral movement of the wires.

Initial contact with the wires can be done without the wires moving. After contact between the block and the wires is made and the bow established, the saw is turned on so that the wires start moving at low speed (typically 0.5 m/s to 2.0 m/s).

After a time period, slurry is added. The time period can typically be 3 seconds up to 40 seconds, more preferably 5 seconds to 30 seconds, and most preferably 5 to 15 seconds. It also possible to run the saw without slurry in a longer time period, but this will likely not improve the results and may cause increased chance for wire breakage. It is also possible to add slurry at the initiation of the sawing. Then there will be sawing with about 5 cm of dry wires before the wet wires enter the block. The time period with dry sawing is in this case less than 1 second.

Initial contact with the wires can also be done with the dry wires moving (typically at a speed of 0.5 m/s to 2.0 m/s). Then the slurry is added, typically after 3 seconds to 40 seconds, more preferably after 5 seconds to 30 seconds, and most preferably after 5 to 15 seconds. It is also possible to run the saw in a longer time period, but this will likely not improve the results and may cause increased chance for wire breakage.

After slurry has been introduced onto the wires, the wires can be speeded up to a cutting speed (e.g. from 14 to 18 m/s).

Example 1

In a saw with a guide spacing of 660 mm and a 120 μπιΦ wire tensioned to 25 N, elongation of the wire is 1.053 % before any bow is introduced. A bow of 2.5 mm will increase the wire length between the guides to 660.025 mm. The effective additional elongation is to 1.057 % (assuming no slip on the guide rollers). The resulting wire tension is increased to 25.1 N in the wire section between the guide rollers. The resulting force from each wire towards the block is approximately 0.5 N. (The magnitude of the friction coefficient will then govern the force needed to move the wires out of their required equally spaced positions.) Example 2

In a similar saw, if the bow is 5 mm, the elongation is increased to 1.068 % giving a resulting wire tension of 25.4 N and a force on each wire towards the block of approximately 1.0 N.

The free length of wire is an important criterion in the process. In a currently available saw the free length between guide rollers ('a' in fig 1) is 660 mm, which is flushed with slurry and moves with a cutting speed of 5 m/s. With a pre-tensioned wire array, the distance from one guide roller to the block ('b' in Fig 1) is 252 mm (without slurry, or with only small traces of slurry) when the block is moved downwardly to introduce the bow 15.

Just before cutting, slurry is flushed on the 'wire in' side of the array of wires

(without turning on the guide rollers). During one test, the wetting of the wires did not reach all the way across the block. Thus the bottom of the block and the wires on the 'wire out' side of the array were dry. As soon as slow movement of the wire is started (e.g. 0.5 m/s) the array is wetted and the friction between the block and the wire is reduced, so the 'free length' of the wires becomes less evident.

Cutting action also takes place immediately, using the initial equally spaced parallel indentations, so that the wires become locked in their individual equally spaced positions, thus to avoid the problems arising from unequal spacing.

Advantages of the Invention

The benefit of using the method is that the occurrence of LATF is eliminated or reduced to a very low frequency. Thus the LATF phenomena would be 'turned off in a controlled manner.

Claims

1. Process for cutting a block of material into a multiplicity of wafers, where

characterised in that

- the wires are, in a dry condition, made to run at an speed in the range from

0.5 to 2.0 m/s and allowed to cut into the block for a period in the range from

0.075 to 60.0 seconds before the sawing slurry is applied.

2. Process as claimed in claim 1 , in which the block is formed with chamfers on its corners first contacted by the array of wires, and initial contact between the wires and the block is made on an edge of a chamfer closest to the array of wires.

3. Process as claimed in claim 1 or 2, wherein the wires within the array are kept in their intended positions by friction and by reduction of attractive surface tension forces between wires and by regular equally spaced guide indentations formed on the corner of the block.

4. Process as claimed in claim 1 or 2, wherein the block is pressed to form a contact pressure between the wires and the block in the range from 0.5 to 1.0 N of each wire.

5. Process as claimed in any one of the preceding claims, wherein the bow in the wires is introduced by pressing the block against the wires before the start up of the wires.

6. Process as claimed in claim 1, 2 or 3, wherein, as the slurry is added, the wires are accelerated to a higher running speed to cut through the block.

7. Process as claimed in claim 6, in which the higher running speed of the cutting wires is in the range from 14.0 to 18.0 m/s.

8. Process as claimed in any one of the preceding claims, in which the wire is a plain wire and the slurry contains abrasive particles for cutting the block.

9. Process as claimed in claim 8, in which the abrasive particles are of silicon carbide.

10. Process as claimed in claim 8, in which the particles are of diamond.

1 1. Process as claimed in any one of claims 8 to 10, in which the solvent is polyethylene glycol (PEG).

12. Process as claimed in any one of claims 8 to 10, in which the solvent is 5 - 100 % by weight of water.

13. Process as claimed in any one of claims 1 to 7 in which the wire has embedded abrasive particles for cutting the block, and the slurry is added for the purposes of cooling and removal of material from the block.

14. Process as claimed in claim 13, in which the abrasive particles are of diamond.

15. Process as claimed in claim 13 or 14, in which the solvent is polyethylene glycol (PEG)

16. Process as claimed in claim 13 or 14, in which the solvent is 5 - 100 % by weight of water.

17. Wafers when produced by the process as claimed in any one of claims 1 to 16.