GB2468874A

GB2468874A - Apparatus for cutting wafers using wires and abrasive slurry

Info

Publication number: GB2468874A
Application number: GB0905059A
Authority: GB
Inventors: Erik Sauar; Stian Sannes
Original assignee: REC Wafer Norway AS
Current assignee: REC Wafer Norway AS
Priority date: 2009-03-24
Filing date: 2009-03-24
Publication date: 2010-09-29
Also published as: WO2010110673A1; GB0905059D0; TW201107101A

Abstract

Apparatus for cutting a multiplicity of wafers from a block comprises means to form a flat closely spaced parallel array of wires 11, and means to move the block 14 through the array, or vice versa. The apparatus also includes means 15 to apply abrasive slurry to the array before it reaches the block, in which there is provision to limit the length of wire along which there is a full supply of abrasive slurry during the start up of the cutting process into the block. The provision may comprise a cylinder shaped piece of softer material 21, which is rotatable between subsequent sawing runs. Alternatively, a separate tube or strip of softer material (23, fig 3) may be attached to the block by glue, so that the array of wires initially cuts into the softer material with little or no slurry; or a brush or scraper (26, fig 6) may be arranged to remove slurry from the array of wires. The supply of abrasive slurry to the block prior to starting the main cutting process may be reduced or eliminated. The apparatus is aimed at preventing the wires of the array from combining and so giving rise to local area thickness fluctuations.

Description

WIRE SAWING TO FORM THIN WAFERS

Technical Field of the Invention

The invention relates to wire sawing of a solid block to form a multiplicity of thin wafers.

Background of the Invention

An apparatus for producing a multiplicity of thin wafers from a block of solid material was described in UK Patent Specification 2,414,204. That apparatus was a development of machinery which used cutting wire to slice thin plates or wafers from bars or blocks of expensive and fragile semi conductor or electro-optic material. A current application of this apparatus has been the production of silicon wafers e.g. for PV panels.

However, the present invention is not limited to that particular apparatus for cutting pairs of blocks with a diversion of the cutting wire between them, and may be applied to the cutting of single blocks, as shown in Figure 3 of UK 2,414,204..

In such machinery, a high tensile steel wire is passed under tension from a supply reel, round wire guide rollers, and is taken up on a collection reel. The surface of each of the wire guide rollers has a series of closely spaced V' grooves at a pitch separation equal to the thickness of the wire plus the thickness of the required wafer. The current configuration of the rollers is such that the wire can pass round them up to 3000 times producing a flat loom or web of wires, with the wire passing around the rollers at a speed of up to 20 meters per second.

Heretofore the web has been flooded with a suspension or slurry of finely divided abrasive powder such as 10 micron silicon carbide powder in a lubricating or cooling medium such as paraffin oil or polyethylene glycol. The material to be sliced is pressed against and slowly moved through the web of wires where the fast moving wires (together with the abrasive slurry) cut a regular series of thin slots through the material. This produces a multiplicity of plates or wafers of accurate thickness with a fine surface finish. In future the wire itself may also be developed into containing the abrasive particles.

It has been found that in the struggle to make thinner wafers in order to save manufacturing cost per wafer, some new challenges have arisen. The abrasive slurry used to perform and cool the sawing process also stimulates cohesion forces (surface tension and capillary forces) between the sawing wires -and in particular before they enter the silicon block. This effect has been sufficient to draw adjacent wires together, and so jam the performance of the apparatus.

The problem has been seen to lie in the characteristics of the abrasive slurry; and the distance between the wires and the tension in the wires, as the wires move into the silicon block.

More specifically, during the start of cutting the wafers in one particular configuration, there is a continuous slurry film on the sawing wires necessarily exceeding the length of the block edge, thereby establishing a large area for cohesion forces between the wires. As soon as sawing starts, this distance is automatically reduced to less than half as the silicon block being cut interrupts the continuous liquid film between the wires. The cohesion forces in the initial phase lead to adjacent wires combining in pairs, thereby creating the problem sometimes referred to as LATF [Local Area Thickness Fluctuation] or "thick-thin" in production. (The result is that every second wafer becomes too thick and the intermediate wafers become too thin on the edges.) The forces stimulating the wires to combine are approximately proportional to the length of the continuous liquid film along the wires. In the situation prior to the potential combining into wire pairs, the wires (except at the very edges) are balanced between forces on each side, while as soon as a pair is formed, both the neighbouring wires to the pair experience a greater distance to one neighbour than the other, and the phenomenon spreads to other neighbouring wires. As a consequence, this problem frequently impacts either all wafers in a block or none.

Disclosure of the inventici

As explained above, the liquid film formed by the slurry (necessary to bring the cutting particles in contact with silicon, to transport the cut material away from the cut slit, and to lubricate and cool the cutting process) creates the problem of capillary and surface tension forces drawing the sawing wires together even before they touch the silicon block. By interrupting this liquid film before the wires cut into the silicon block, this length will be greatly reduced, and the problem mitigated. The same problem could also be reduced by using smaller wire guides just next to the silicon block, but this increases the complexity of the wafer saw itself.

The invention provides apparatus for cutting a multiplicity of wafers from a block, the apparatus comprising -means (a supply reel) to supply one or more wires, -means (a collection reel) to collect the used wire/wires, -means (roller guides) to align the wire or wires to form a flat closely spaced parallel array of wires, and -means to move the block through the wires or vice versa in a direction generally perpendicular to the plane of the array of wires, in which there is means (a manifold) to apply abrasive slurry to the wires before they reach the block, so to effect the main cutting of the block by dragging the abrasive slurry surrounding the wires into the block, and in which there is a provision to limit the length of wire along which there is a full supply of abrasive slurry during the start up of the cutting process into the block.

In one form it is preferred that there is provision to attach a separate tube or strip of softer material to the block, and the arrangement is such that the wires initially cut into this softer material with little or no slurry applied to them.

In this form it is further preferred that there is provision to remove the cut pieces of tube or strip from the block after it has passed the wires.

In another form it is preferred that at least a part of the surface of the block nearer to the array of wires is covered with a soft material, so that initial contact between the block and the wires is made through this soft material with little or no slurry on the wires, before the cutting of the block with the normal amount of abrasive slurry is started.

The soft material may be glued to the block, and the soft material may be glue.

The soft material may be planar and may cover the entire block evenly.

Alternatively, the soft material may be unevenly distributed across the block, and may be distributed in stripes.

In another form it is preferred that the soft material comprises a separate sheet of material disposed close to the surface of the block.

In yet another form, the soft material is present on the opposite side of the wire web compared to the block so that initial sawing is made into the soft material on one side with little or no slurry on the wires, before the cutting direction is reversed, while keeping the soft material and silicon block in direct contact and the application of a normal amount of abrasive slurry is started.

In this last mentioned form it is preferred that the soft material has a cylindrical shape and is rotatable to allow rotation between subsequent sawing runs or sets of sawing runs. Alternatively, the soft material may be of a regular shape such as a square, rectangle or hexagon, and is movable through a corresponding angle between subsequent sawing runs or sets of sawing runs.

The wire may initially cut into soft material completely without slurry.

Alternatively, the supply of slurry is restricted to half its normal quantity subsequent to the cutting through the soft material.

In another form, the provision limiting the length of the continuous slurry film on the wire web may be a brush or scraper arranged to remove slurry from the wires.

The invention also provides a method of cutting a multiplicity of wafers from a block, comprising the steps of moving the block through an array of fast moving closely spaced parallel wires in a direction generally perpendicular to the plane of the array, and providing a supply of abrasive slurry to the wires for the main cutting process, in which, prior to commencement of the main cutting process, the supply of abrasive slurry to the block is reduced or eliminated, so that the start up of cutting into the block is effected with the wires having only a little or no slurry on those wires.

The invention also provides a block for use as a feed stock in the apparatus or method described above, and having a portion of soft easily cut material on the side on which the sawing is to be initiated.

Brief description of the drawings

Six specific embodiments of the invention will now be described by way of example with reference to the accompanying drawings, which are diagrammatic side elevations of machinery cutting silicon blocks into a multiplicity of thin wafers.

Description of a Specific Embodiment of the Invention As shown in the diagrammatic side elevation of Fig 1, a first guide roller 10 feeds an array of abrasive cutting wires 11 to a second guide roller 12. It will be understood that in the side elevation only one wire can be shown. The wire comes from a supply reel (not shown) and is drawn onto a collection reel (also not shown).In practice, an array of closely spaced abrasive cutting wires will be used (as shown in Figure 3 of ITK 2,414,204). The guide rollers position the wires in a flat evenly spaced cutting array. (It is known to have one or a few long wire(s) going more than 1000 times around the guide rollers.) There may be several thousand wires (or different sections of one or a few wires) spaced apart at a distance of 200 microns or less when measuring between adjacent wire surfaces. A silicon block 14, having its surface near to the array of wires, is slowly moved towards the array of wires. (The silicon block is shown with its contact face parallel to the array of wires. The block may be slightly inclined, with its edge nearer to the approaching wires slightly closer to the wires than its further edge.) The apparatus has provision (not shown, but already known from other machinery) for moving the block 14 downwardly through the array of wires 11. A manifold 15 is arranged above the array of wires for the supply of abrasive slurry.

The general arrangement shown in the diagrammatic side elevations and described above is common to all embodiments.

In the embodiment of Fig. 1, a polymer rod or tube 21 is moved upwardly into the array of wires from below, so that the wires 11 initially cut into the soft material of the polymer tube 21. The tube may be circular in cross section (as shown) but it could be square, rectangular, hexagonal or any other regular shape.

The tube need not be of polymer, but must be of a material soft enough to be cut by the wires with little or no slurry.

For the initial period of the Cut (possibly no more that 0.1mm) the wires cut partially into the tube 21. Then the block 14 is lowered until it comes into contact with the semi-cut tube 21. Abrasive slurry is supplied to the wires 11 from the manifold 15 at any time after the cutting into the tube 21 has started, and block 14 is thereafter lowered through the array of wires 11. Thus the wafers are formed. For the next block, the tube 21 may be rotated, so that a new part of the soft material is initially exposed to the cutting wires. In a variant of this embodiment (Fig 2), the tube 21 is replaced by a bar 22 In a third embodiment (Fig 3), the surface of the block 14 nearer to the wires 11 has a coating of soft material 23. Initially, as the block is lowered into contact with the wires, the wires will be able to saw through this soft material (which could be of a polymer/rubber composition) with little or no slurry. After the wires have -with certainty -started cutting into the soft material (taking machine and block size tolerances into account), abrasive slurry is introduced from the manifold 15, and sawing can continue as the block is lowered through the array of wires 11.

It is advantageous to use a soft material coating which will dissolve or fall off in the subsequent washing of the wafer. For instance, the soft material coating may be a thin layer of the same glue which is used to attach the semiconductor block to the sawing substrate (not shown).

In a forth embodiment (Fig 4), a separate web of soft material 24 is disposed between the silicon block 14 and the array of wires 11. This web of soft material will be engaged by the wires 11 before the abrasive slurry contacts the surface of the silicon block 14.

In a fifth embodiment (Fig 5) there is a strip of soft material 25 preferably attached close to the entry edge of the block 14 with respect to the direction of approach of the cutting wires 11. Initially, as the block is lowered into contact with the wires, the wires cut into this soft material with little or no abrasive slurry.

Thereafter, abrasive slurry can be introduced from the manifold 15 to effect the cutting of the block into wafers.

In a sixth embodiment, (Fig 6), a brush, shaver or scraper 26 is arranged to contact the array of wires 11 from below, and to remove most of the abrasive slurry from the wires before the initial contact of the wires with the silicon block 14.

An important feature of all embodiments of the invention is to be able to initiate the sawing process with an array of dry or near dry cutting wires, which may for example (in Figs 1 to 5) be achieved by using a soft or easily cut material in the initial cutting phase.

While reference has been made to cutting wires, it will be understood that only a single (or few) cutting wire(s) isfare needed, and this/these is/are formed into a regular array (many wires across) by the guide rollers. 7.

Claims

CLAIMS1/ Apparatus for cutting a multiplicity of wafers from a block, the apparatus comprising: -means (a supply reel) to supply one or more wires, -means (a collection reel) to collect the used wire/wires, -means (roller guides) to align the wire or wires to form a flat closely spaced parallel array of wires, and -means to move the block through the wires or vice versa in a direction io generally perpendicular to the plane of the array of wires, in which there is means (a manifold) to apply abrasive slurry to the wires before they reach the block, so to effect the main cutting of the block by dragging the abrasive slurry surrounding the wires into the block, and in which there is a provision to limit the length of wire along which there is a full supply of abrasive slurry during the start up of the cutting process into the block.
2/ Apparatus as claimed in claim I in which there is provision to attach a separate tube or strip of softer material to the block, and the arrangement is such that the wires initially cut into this softer material with little or no slurry applied to them.
3/ Apparatus as claimed in claim 2, in which there is provision to remove the cut pieces of tube or strip from the block after it has passed the wires.
4/ Apparatus as claimed in claim 1, in which at least a part of the surface of the block nearer to the array of wires is covered with a soft material, so that initial contact between the block and the wires is made through this soft material with little or no slurry on the wires, before the cutting of the block with the normal amount of abrasive slurry is started.
5/ Apparatus as claimed in claim 4, in which the soft material may be glued to the block.
6/ Apparatus as claimed in claim 5, in which the soft material is the glue.
7/ Apparatus as claimed in any one of the preceding claims 4 to 6, in which the soft material is planar and covers the entire block evenly.
8/ Apparatus as claims in any one of claims 4 to 6, in which the soft material may be unevenly distributed across the block.
9/ Apparatus as claimed in claim 8, in which the soft material is distributed in stripes.
10/ Apparatus as claimed in claim 1 in which the soft material comprises a separate sheet of material disposed close to the surface of the block.
11/ Apparatus as claimed in claim 1, in which a soft material is present on the opposite side of the wire web compared to the block so that initial sawing is made intO the soft material on one side with little or no slurry on the wires, before the cutting direction is reversed, while keeping the soft material and silicon block in direct contact and the application of a normal amount of abrasive slurry is started.
12/ Apparatus as claimed in claim 11, in which the soft material has a cylindrical shape and is rotatable to allow rotation between subsequent sawing runs or sets of sawing runs.
13/ Apparatus as claimed in claim 11, in which the soft material is a regular shape such as a square, rectangle or hexagon, and is movable through a corresponding angle between subsequent sawing runs or sets of sawing runs.
14/ Apparatus as claimed in any one of the preceding claims, in which the wire is initially cutting into soft material completely without slurry.
15/ Apparatus as claims in any one of claims 4 to 13 in which the supply of slurry is restricted to half its normal quantity subsequent to the cutting through the soft material.
16 Apparatus as claimed in claim 1 in which the provision limiting the length of the continuous slurry film on the wire web is a brush or scraper arranged to remove slurry from the wires.

17/ Method of cutting a multiplicity of wafers from a block, comprising the steps of moving the block through an array of fast moving closely spaced parallel wires in a direction generally perpendicular to the plane of the array, and providing a supply of abrasive slurry to the wires for the main cutting process, in which, prior to commencement of the main cutting process, the supply of abrasive slurry to the block is reduced or eliminated, so that the start up of cutting into the block is effected with the wires having only a little or no slurry çse wires.

18/ Block for use as a feed stock in the apparatus or method as claimed in any one of the preceding claims and having a portion of soft easily cut material on the side on which the sawing is to be initiated.