Description Technical Field

[0001 ] The invention relates to a fixed abrasive sawing wire as is used during sawing of hard and brittle materials.

Background Art

[0002] Currently the predominant technology for sawing hard and brittle materials like silicon, quartz, gallium arsenide, silicon carbide, sapphire, magnetic materials or any other expensive material with a cutting length below one to two meter is by means of a multi-loop single wire saw. In such a saw slurry comprising abrasive particles (mostly silicon carbide) suspended in a carrier liquid (usually poly ethylene glycol) is dragged by a round, thin, high tensile steel wire into the cut of the work piece. The wire is guided by grooved capstans in loops arranged parallel to one another thus forming a wire web. The work piece is cut due to the roll-stick action of the abrasive particles rolling between the wire and the work piece. The process is delicate as the slurry composition changes during use, expensive as both the wire and abrasive gets worn and must be replenished constantly, dirty as the slurry is spilled around by the capstans and has a high

environmental cost as the used slurry (with work piece material debris and steel dust in it) and wire must be discarded in a controlled way.

[0003] Therefore the industry is looking for solutions to dispense with the slurry preparation, slurry control and discarding. One of the solutions proposed is to eliminate the use of a carrier liquid for the abrasive particles and to fix the abrasive particles directly on the wire. Only a coolant is then still needed to cool the work piece and wash away the debris that may collect on the wire. This results

- in a better sawing action as the complete impulse of the abrasive

particle is transferred to cut in the work piece (no rolling of particles anymore), - in a purer residual products as the coolant mainly contains work piece debris and

- in a better use of consumables as the abrasives are better used and the wire wear is much less (as the abrasive is immobile relative to the wire).

Such a type of wire is called a 'fixed abrasive sawing wire': the abrasive particles are firmly attached to a carrier wire. Usually fine diamond particles (typically 10 to 50 μιτι size) are used as abrasive particles. Such a wire retains its cutting action much longer as the wire does not get worn by the abrasive - the abrasive is fixed to the wire - contrary to the loose abrasive sawing process wherein the wire also wears like the piece that is being sawn.

[0004] Wire sawing inherently results in a loss of - sometimes precious - work piece material. There is therefore also a constant strive to keep the 'kerf loss' (the amount of work piece material lost as sawing debris) as low as possible. Currently used fixed abrasive sawing wires have a smallest circumscribed outer diameter (i.e. the smallest diameter of an imaginary circle surrounding the abrasive particles) range between 310 μιτι for cropping and ingot shaping down to 145 μιτι for wafering.

[0005] Furthermore in wire sawing the wire must be tensioned. The longitudinally applied tension is transferred to a transversal force in the plane of the cut as the wire is pushed against the work piece. This transversal force - in combination with the wire longitudinal movement - makes the wire cut the work piece. The wire therefore takes the shape of a bow during sawing. The higher the tension, the smaller the bow, the faster the cut. In practice this tension force is about 25 newton in a wafering web but is much larger (> 70 N) in case of cropping and ingot shaping.

[0006] In loose abrasive cutting of workpieces with low geometrical requirements

- such as solar cell wafers - the wire can be driven unidirectional i.e. the wire always moves in the same direction during cutting. Alternatively, for more demanding wafer parametric uses - such as for the semicon industry

- the process can be driven bidirectional. In that case fresh wire is drawn from the spool over a forward length FL in the forward direction, then the sawing direction is reversed over a backward length BL. FL is larger than BL. 'Bidirectional sawing' is sometimes called 'back- and forward sawing', 'reciprocal sawing' or 'pilgrim-mode sawing'. Fixed abrasive sawing wires are nowadays uniquely used in pilgrim mode where the backward length BL is only marginally less than the forward length FL. Hence the same wire length is used multiple times i.e. has a high use rate.

[0007] This backward and forward sawing brings some particular problems with it.

During the backward direction sawing used wire - possibly soiled with swarf (i.e. debris from the work piece) - is wound back on clean wire over a distance BL. During this winding back, the tension is at cutting head tension i.e. 25 N or larger while typically the winding tension of the spool during production of the wire is only 2 to 5 newton. This may lead to problems such as 'cutting' of used wire into the fresh wire pack, followed by wire fracture at the moment the used wire end is snapped out of the fresh wire. As the wire is coated with abrasive, some abrasive grain may act as transversal cutting instrument for the wire and cut or damage the wire at the tensions and related forces used.

[0008] The high use rate of the wire implies that only very little wire is taken into the web at each cycle and used wire is wound back on fresh wire many times before entering the web. For example, before a fresh length (FL-BL) of wire enters the web completely - and does not return anymore to payoff spool - it has been rewound BL/(FL-BL) times on the pay-off spool. This easily amounts to over 100, even 200 back windings on the pay-off spool, depending on the pilgrim mode used. Note that for the purpose of this application the spool from which the wire is consumed will be called the 'pay-off spool' (even though it temporarily acts as a spool on which wire is wound up i.e. receives wire) while the spool receiving the used wire will be called the 'take-up spool' (although it intermittently used as a spool delivering wire).

[0009] Moreover - in most of the current machine designs - the tension used for sawing of about 25 newton or higher is without reduction transferred to the windings on the spool. To make matters worse, the abrasive particles protrude out of the wire surface (as otherwise they would not cut) and during the many backward windings wires that are on the spool and that arrive on the spool reciprocally damage each other which will be called the 'self-damage problem'. The problem is most severe on the pay-off side as there the wire is wound back on fresh wire that still has to be used. Fresh wire therefore enters the web in an already damaged state, thereby greatly increasing the risk for wire fracture or decreased sawing performance during the sawing process.

[0010] The 'self-damage' problem can be prevented by a machine design such as exemplified in EP0261695, US3942508 or US5052366 that collects used wire in a pulley array system and thereby also reduces the wire tension from the cutting head to the spool, but such designs are more complicated and expensive.

[001 1 ] On the take-up side the self-damage problem is less severe as the wire is worn out and the abrasive has become dull: wire entering the take-up spool lands on already worn out wire. Hence the wire on the take-up spool can not harm much the wire landing on the take-up spool. Vice versa: the wire re-entering the cutting head on winding back, can not be harmed much by the wire it landed on.

[0012] Attempts to resolve this problem have been searched in carefully winding the wires in layers on the spools in hexagonal or square relationship (as described in EP 1698433 A1 ) in order to prevent point contacts and spread the contact pressure. However, given the fineness of the fixed abrasive sawing wire - typically about 150 μιτι - this is not easy.

[0013] Another solution that has been suggested is to use an adhesive layer on which several abrasive sawing wires are adhered parallel to one another (US 6178962).

[0014] Currently various techniques are explored in order to fix the abrasive

particles - that are usually diamond dust particles - to a carrier wire:

- Fixation can be done through a mechanical bond: by pressing diamond particles in a soft sheath high tensile wire such as e.g. described in application with application number EP2010/055678 of the current applicant. In such a wire part of the abrasive particles are in the soft sheath, thereby diminishing the protrusion of the abrasive particles, which is beneficial for self-damage.

- A metallurgical bond has also been considered e.g. by brazing or

soldering the particles to the surface of the wire like e.g. described in WO 99/46077. About the same amount of protrusion is expected for this kind of wire as for mechanically fixed abrasive particles.

- A resin bond such as e.g. described in US 6070570 has also been studied extensively. However, it turns out to be difficult to hold the particles in the resin during sawing as the fixation is rather weak.

- By electrolytic or electroless fixation of the diamond particles. This route has emerged from the superseded saw blade technology for cutting silicon ingots and is regularly used for other diamond cutting tools as well. The abrasive particles protrude very much out of the surface of the wire making it vulnerable to the self-damage problem.

In general a distinction can be made between organic bound abrasive particles (resin bond) and inorganically bound abrasive particles (indented in a metallic layer, brazed particles, electrolytic or electroless plated particles).

[0015] The inventors found a solution to this problem that will be described in the next sections.

Disclosure of Invention

[0016] The object of the invention is primarily to eliminate the self-damage

problem. The object is more specifically to eliminate this problem with the least possible adaptations of the process or the machine. The basic principle of the invention is to protect the fixed abrasive sawing wire from self-damage by putting a layer of an easy removable and disposable organic coating around it.

[0017] According a first aspect of the invention, a fixed abrasive sawing wire is presented. Fixed abrasive sawing wires are generally built around a high tensile element which is mostly steel such as plain carbon steel or stainless steel. Most preferred is a steel core that is made of plain carbon steel (with a minimum carbon content of 0.70 wt% carbon) or a stainless steel. In the striving to reduce the overall diameter of the fixed abrasive sawing wire the tensile strength of the wire must increase accordingly. Preferred diameters and tensile strength of the core of the fixed abrasive sawing wire are: if diameter is smaller than... then tensile strength is larger than...

250 pm 2900 N/mm ²

150 μηη 3600 N/mm ²

140 m 3700 N/mm ²

120 μηη 3900 N/mm ²

The tensile strength is the breaking load of the wire (in newton) divided by the cross sectional surface area (in mm ² ).

[0018] On the core metal wire abrasive particles are fixed. The abrasive particles can be superabrasive particles such as diamond (natural or artificial, the latter being somewhat more preferred because of their lower cost and their grain friability), cubic boron nitride or mixtures thereof. For less demanding applications particles such as tungsten carbide (WC), silicon carbide (SiC), aluminium oxide (AI2O3) or silicon nitride (Si3N ) can be used: although they are softer, they are considerably cheaper than diamond. However, most preferred remains diamond.

[0019] The abrasive particles are firmly attached or fixed to the core wire by

means of a fixation layer that is inorganic. With 'inorganic' is meant any type of fixation layer that does not rely on a carbon or silicon chemistry to hold the particles. In a fixed abrasive sawing wire using a resin to hold the abrasive particles to the core wire, the fixation layer is organic.

[0020] Examples of inorganic fixation layers are:

^■ Fixation by means of indentation of abrasive particles into a soft layer surrounding the core wire followed by covering the abrasive particles by means of a metallic coating such as described in WO 2010/125083 and WO 2010/092151 . By preference the first soft layer is a copper layer and the covering layer is nickel.

^■ Fixation by means of soldering such as described in WO

2010/071 198. By preference the used solder is a low temperature melting solder such as a tin-silver or, tin-zinc solder to limit the thermal loading of the core wire. Alternatively a high temperature resistance wire can be used such as tungsten wire as described in US 6102024. Clearly such a metal layer is an inorganic layer.

^■ Fixation by means of an electroless or electrolytic deposition process.

The inorganic coating is then a metal coating, most preferred being a nickel metal coating. An example of a preferred fixed abrasive sawing wire is described in US 7704127 or EP2277660.

■ Fixation by other inorganic means such as for example embedding in a glass coating.

From the above it will be clear that by preference the inorganic fixation layer is metallic in nature wherein different layers are used for different functionalities. Metals that are preferred for indentation layers are soft such as zinc, copper, brass, low carbon, metals used for fixing the abrasive particles are mostly nickel or nickel alloys (nickel phosphate, nickel cobalt etc....).

[0021 ] Most preferred is an electrolytic deposited fixed abrasive layer as there the protrusion of the diamonds is substantial leading to a high cutting efficiency. However, this high level of protrusion leads to a large risk for self-damage which can be solved as described below.

[0022] The fixed abrasive sawing wire with the abrasive particles attached to it in an inorganic coating layer is further covered with an organic coating layer. The organic coating layer is not intended to fix the abrasive particles in place. The organic coating layer is intended to protect the fixed abrasive sawing wire from self-damage. As the organic layer is on the wire prior to use but has disappeared after use, used wire that is wound back on fresh wire will not be damaged by the fresh wire as this used wire lands in a 'cushion' formed by the organic coating layer of the fresh wire. Conversely the fresh wire that is on the spool is protected by the organic coating layer from the used wire - substantially without organic coating layer - landing on it at high tension.

[0023] As the function of the organic coating layer is merely for protection of the wire during transport it is of no use for the sawing function of the wire and should not interfere with this function. For the purpose of this application 'transport of the wire' should be considered broadly in that not only the physical transport of a coil of wire in its entirety is meant, but also if only a part of the wire is transported on or off the carrier i.e. winding on and winding off wire are also considered as 'transport' of the wire. Hence the primary function of the organic coating layer is 'to protect the wire during transport from self-damage'. An additional advantage of the organic coating layer - as organic coating layers generally have high self-friction coefficients - is that the different windings in the spool are held immovable to one another. In this way entanglement of the wires during transport or during threading of the wire web is prevented. Therefore tacky, sticky types of organic coating layers are preferred. Such coatings have the advantage that transverse movement of wires relative to one another is prevented: it is precisely this scratching by the abrasive particles, oblique to the axis of the wire, that are most harmful to the wire and makes it break prematurely.

[0024] In order that the organic coating layer does not interfere with the sawing process some options remain open:

First the organic coating layer can be removed just prior to the use of the fixed abrasive sawing wire. In that case a removal method step must be provided on the pay-off spool or between the pay-off spool and the wire web that removes the organic coating on its first entry into the web. Hence at each cycle only a length (FL-BL) must liberated from organic coating as the remaining length BL has already been cleared in the previous cycle.

[0025] Of course the choice of the organic material for the organic coating layer is very important. The coating should be inexpensive, easy to apply, soft, non-adherent or at the most slightly adherent to metals and most important easy to remove and this by preference in an environmentally friendly way.

[0026] Most preferred are therefore mechanically weak, non-metal adhering

coatings that can be dissolved in a polar medium. Examples of polar media are: water, polyethyleneglycol, alcohol, di-ethyleenglycol, or mixtures thereof. These are also the liquids that are used as a basis in the coolants that are used for fixed abrasive sawing wire.

[0027] Particularly preferred polymers that can be used for the organic coating layer are therefore poly vinylpyrrolidone, polyvinyl acetate, poly (acrylic acid), poly (methyl acrylate), methylcellulose, polyvinyl alcohol, or ethylene/vinyl alcohol copolymers, polyethyloxazolines or mixtures thereof, as they easily dissolve in polar media such as the ones cited. Most preferred are polyvinyl alcohol and methylcellulose. Polyvinyl alcohol readily dissolves in water, has excellent film forming capabilities, and is non-toxic. Methylcellulose precipitates in a polar medium when the medium is heated, which provides a convenient way of removing it.

[0028] Biopolymers are also a good choice as they are biodegradable and are generally mechanically not very strong and some of them easily dissolve in water. Noteworthy examples are: caserne, starch derivates, hydrogels, poly saccharide or protein based polymers.

[0029] Alternative polymers are polymers that are not dissolvable in polar media such as thermoplastic polymers, hot melt polymers, or thermosetting polymers. These are somewhat less preferred - although not impossible per se - for the intended use.

[0030] The amount of organic coating layer present on the fixed abrasive sawing wire must not be much. It must be sufficient to cover the diamonds and to prevent the wires from slipping over one another. It suffices that the diameter of the smallest circumscribed circle of said fixed abrasive sawing wire inclusive said organic coating layer is larger than the diameter of the smallest circumscribed circle that only encloses said abrasive particles. Too much coating will result in too much waste (either in the removal apparatus or in the coolant). Not enough coating will not bring the advantageous effect of protection of the fixed abrasive sawing wire.

[0031 ] An alternative way of determining the amount of polymer is by

determination of the organic carbon residue content on the surface of the wire by means of carbon pyrolysis. In this test only a limited sample (1 to 2 grams) is needed. The sample is heated to 480°C till the organic residues (but not the carbon in the steel) on the sample decompose into carbon monoxide and carbon dioxide. In a catalyser at 850°C all carbon monoxide is converted to carbon dioxide. The total amount of carbon is calculated from the infra-red absorption of the carbon dioxide. The total amount of carbon remaining must at least be larger than 400 g carbon residue per gram of sawing wire. It should not be larger than about 5 mg carbon residue per gram of sawing wire. As the polymer of the organic coating layer is not only composed of carbon (but also hydrogen and oxygen) the numbers are less than those determined by other methods such as the double weighing method. [0032] According a second aspect of the invention a method to protect a fixed abrasive sawing wire during transport is claimed. The method comprises the steps of:

^■ Providing a fixed abrasive sawing wire whereon abrasive particles are fixed in an inorganic coating;

^■ Coating the fixed abrasive sawing wire with an organic coating layer that is removable prior to use or during use of the wire.

Optionally, the thus coated fixed abrasive wire is collected on a carrier such as a spool, packed and transferred to the sawing machine.

[0033] At the sawing machine a further method step is performed in that:

^■ the organic coating layer is removed prior to or during use of the fixed abrasive sawing wire.

[0034] The wire can be coated in a number of ways: coating can be done by

extrusion, powder coating, powder electrostatic coating, curtain coating, brushing, dipping or a combination thereof. These methods are known to the person skilled in the art. Extrusion can be through an extrusion head, but can also be very basic by guiding the fixed abrasive sawing wire through a pot containing the polymer followed by stripping off superfluous material in a rubber die (of diameter larger than the smallest circumscribed circle). 'Curtain coating' is the method whereby the wire is guided through laminar flowing curtain of the pure or dissolved polymer. Electrostatic coating is already complicated.

[0035] The methods can be applied on a single wire during producing the wire, but there are other possibilities to do this more efficiently. One can for example coat the fixed abrasive sawing wire as they are wound on the carrier e.g. the spool or bobbin: the coating can be applied - much like a paint e.g. by means of a brush - per layer formed on the bobbin. In this way all wires are well coated and there is sufficient protection between the windings. Alternatively, the carrier can be intruded with polymer coating from the outside inward by pressing the polymer through the wire pack and pumping air out of the core of the spool. In this way the fixed abrasive wire is well protected during transport. [0036] Possible methods for removing the organic coating layer are:

^■ Brushing off the organic coating e.g. by means of spiral brush through which the wire runs. The brush can be cleaned constantly with a stream of solvent in order to keep it free of debris;

^■ By means of pealing off e.g. by first rolling the wire against a roll so that the coating cuts slits itself through. Another preferred way of pealing is that the organic coating is also tacky to itself, but less to the sawing wire. During winding, the organic coating layers of the different windings will glue to each other. At unwinding, the organic layer will preferably remain attached to the other organic layers and the wire will be ripped out of the coating. The organic coating then remains on the spool, and must be removed at the spool.

^■ By means of buffing by a circular, rotating brush.

^■ By stripping the coating e.g. by leading it through a narrow matching slit. The slit can easily be made by letting the wire saw through a thin piece of material - preferably the material to be sawn - at the start-up of the machine. Once the slit has been formed, cutting of the work piece can start.

^■ By heating or burning of the coating.

^■ By dissolving the coating by guiding the wire through a solvent tank, the solvent removing the coating.

Any combination of the above may of course also be considered.

[0037] Secondly, the organic coating can be such that the coating dissolves in the coolant that is used in the sawing process. This has the additional advantages that no machine adaptations are necessary. Provisions will have to be made to remove the organic coating from the coolant as it accumulates in the coolant.

[0038] Thirdly: combinations of both 'prior to use' and 'during use' removal of the organic coating can be considered. As a 100 % prior cleaning will be practically difficult to implement, some remaining organic coating layer will be removed during sawing. Brief Description of Figures in the Drawings

[0039] FIGURE 1 shows a cross section of the inventive fixed abrasive sawing wire.

[0040] FIGURE 2 shows a cross section of the spool with the wires as they are used during the sawing.

Mode(s) for Carrying Out the Invention

[0041 ] Figure 1 shows a fixed abrasive sawing wire 100 with a core metal wire

102 that in this case is a high tensile (3995 N/mm ² ), high carbon (0.85%C) steel wire of diameter 120 μιτι. The wire has been electrolytically coated with nickel 106 and the abrasive particles 104 are microdiamonds with a median size of 9 μιτι with a range from 6 to 12 m (5% and 95% limits of the particle size distribution). Such wire is commercially available.

Protrusion of the abrasive particles is about 7 μιτι which is large compared to the overall diameter of the wire.

[0042] Circle 108 describes the circle with the smallest diameter D ₀ that

circumscribes all abrasive particles of the wire.

[0043] The wire has been coated with a layer 1 10 of poly vinylalcohol (PVA)

©Mowiflex TC obtainable from Kuraray by running it through a dip tank and winding it on a wire spool. The PVA forms a soft layer in between the wires thereby fixing the layers more in place, as shown in Figure 2, 206.

[0044] After coating, a circle 1 12 can be defined with the smallest diameter Di that circumscribes the fixed abrasive sawing wire inclusive the organic coating layer 1 10. Di is larger than D ₀ . Circle 1 12 is not necessarily concentric to circle 108. About 1 mg of residual carbon was measured per gram of wire in the carbon pyrolysis test.

[0045] During unwinding of the spool 202 it was observed that much of the PVA was held by the still remaining wire on the spool as the adhesive force between the PVA layers is larger than between the PVA and the wire. Only a small part remained on the wire as it went into the web which

subsequently dissolved in the water based coolant used during sawing. During backwinding, the now uncoated wires 208, 208', 208" landed on the still coated wires thereby reducing the self-damage of the fixed abrasive sawing wire.