BACKGROUND OF THE INVENTION

[0001] This invention relates to an apparatus that allows direct on-line transfer of slurry from downstream of a multi-wire wafer saw back to the wires upstream. More specific, the invention relates to an apparatus in the wafer sawing operation for on-line removing and transferring slurry and abrasive through the downstream guide roller back through a pipe to go through the upstream guide rollers to the wires.

[0002] Also runoff and drippings from the workpiece may be transferred on-line back through the upstream guide roller.

[0003] The three main objectives with this invention are:

1. Provide an efficient, clean, reproducible, known and controlled manner for on-line adding and reusing cutting liquid with abrasive grains in the multi-wire wafer sawing system.

2. Minimize loss of expensive SiC abrasive particles and cutting liquid by direct recycling from downstream to upstream in a closed system

3. Reduce the amount of slurry and large SiC particles losses to the collection tank, thus facilitating off-line treatment with recovery of SiC and Si in mind and allowing "bleeding-off of undersize SiC particle and iron, copper and other contaminants.

[0004] In recent years the solar cell industry has had a very strong growth rate which reached 40 % from 2003 until today. Due to this growth rate, apart from the lack of solar grade silicon, there is a shortage of size fraction SiC in the range of 10 micrometers. Nearly half of the silicon in the work piece is lost as micrometer size silicon kerf during the sawing operation. Consumption of SiC is of the same order of magnitude. Only a maximum of 30% of the SiC lost to the slurry collection tank is recovered. [0005] Multi-crystalline solar cell wafers are typically produced by melting and casting silicon of sufficient purity in a directional solidification furnace. This produces a rectangular ingot of multicrystalline solar grade silicon metal which is sliced into 16-25 blocks with a quadratic cross- section. As illustrated in Fig 1 each block is then sliced into wafers of thickness about 0.2 mm. After slicing, the wafers are surface treated, textured, doped, given electric leads etc. to form solar cells that may be mounted together to form solar panels.

[0006] Wire-sawing, free abrasive machining- allows the production of very thin wafers from the large diameter ingots. A thin steel wire of about 160-180 micrometer diameter and several hundreds of km long is wrapped around three or four rollers with grooves to hold and guide the wires, thus producing a wire web.

[0007] When the silicon work piece is pressed against the rows of moving wire stretching between rollers upstream and downstream of the work-piece, parallel mechanical cutting processes take place with the interaction of wire, SiC, carrier liquid and silicon. The wires move at a speed in the order of 10 m/s in the cutting process and are wetted with a slurry composed of ethylene glycol or mineral oil carrying SiC particles in the size range of 5-15 micrometers.

[0008] In wire-sawing abrasives roll between the continuously moving wire and the material which is to be cut. The hard abrasives, mostly SiC, indent the comparatively soft material, silicon. Forced along by the wire moving at high speed, the abrasives roll and remove material in the form of chips. Chipping is a function of force on the abrasive and of the hardness difference between the abrasive and the material. The total material removal rate depends on the concentration of abrasives, viscosity of the slurry and the speed of the wire. Viscosity depends on the temperature.

[0009] It is difficult to prevent runoff and dripping from the workpiece. These effects depend on the dimensions of the workpiece, wire tension and force on the workpiece, vibrations, wetting, viscosity and particle size.

[0010] To achieve a good dimensional accuracy it is important that the same amounts and composition of cutting solution are fed to each and every one of the neighboring wires. [0011] The fine silicon particles may be covered by a layer of oxide due to the somewhat oxidizing conditions during cutting and in the collection tank. Also the silicon kerf particles may adhere to SiC grains.

DESCRIPTION OF THE PRIOR ART

[0012] A problem in adding cutting solution to the wires is that of hitting and wetting a rapidly moving thin wire target. In practice only a small fraction of the slurry seems to reach the wires. One patented method, WO 0191982, is to position two nozzles on opposite sides of the ingot holder so that slurry is dispensed onto the wire web forming a curtain or sheet of slurry. The slurry curtain extends across a full width of the wire web so that slurry is to be delivered to every reach of wire and every slice of ingot. Obviously, the openings between the wires cause losses. The velocity difference between the slurry curtains and the moving wires is also a difficulty. As much as possible of the cutting solution that actually reaches the wire should enter the channel between the work piece and the cutting wire. Another approach to reduce losses is Japanese Patent 2000000750, adding slurry from a perforated pipe where each hole sprays slurry onto the corresponding wire in the web, ideally without scattering the slurry. Again the goal is to control feeding so that no excess of cutting liquid is lost in flight or drips or flows off the wires before they cut into the workpiece. [0013] It is important to control the temperature and thus also the viscosity of the cutting liquid. One method, European Patent 0827798, for controlling the temperature in the upstream wire guide roller is by circulating a cooling liquid such as water through a hole along the axial direction of the center axis. In a related patent, United States Patent 793641 , cutting liquid is fed to the operative abrasive surface of a porous grinding wheel. The cutting liquid fed in must be free of particles. Centrifugal forces or a high-pressure pump drive the cutting liquid through the pores of the wheel to its operative surface.

[0014] When a major part of the slurry used in the cutting operation, splashes off as waste to a collection tank for the wafer sawing unit, an excessive amount of SiC particles are lost. In the collection tank SiC particles and silicon kerf particles and iron and other contaminants accumulate. Furthermore, also the relative amounts of kerf and SiC and the distribution of SiC particle sizes varies with time. Such a slurry is difficult to process both for recovery of SiC and silicon from the kerf.

[0015] Slurry that can be reused may contain as much as 30 % kerf by mass in photovoltaic slurry, and up to 10 % in electronic slurry. However, a high content of kerf gives an unwanted increase in the "effective viscosity" of the slurry. Also the SiC particles may be covered with a layer of Si and traces of Fe that reduces the efficiency of the cutting operation. As slurry becomes contaminated by reuse, the thickness changes of the wafers produced. Specifically, wafer thickness, total thickness variation (TTV) and standard deviation (SD) of TTV change. The general trend is that the thickness of the wafers increase with reuse, while TTV typically declines initially, and then increases considerably with the amount of reuse. [0016] It may be a problem to clean the wires before they can be reused. A method, European Patent 1020271 , is to remove slurry from the wires in a separate chamber using a cleaning liquid and shutting off the cleaning nozzles when the wires return to the cutting chamber.

[0017] A solution employed in the industry in the cutting operation is to add only fresh PEG and new SiC. However, this approach consumes large amounts of SiC and PEG.

SUMMARY OF THE INVENTION

[0018] To increase the efficiency of slurry containing expensive abrasive it is important in a controlled and reproducible manner, on-line to recycle back to the upstream wires the slurry and abrasives. Since excessive recycling could lead to unwanted accumulation of kerf and undersize SiC on the wires, a compromise must be struck where only an optimal part should be recycled directly from downstream.

[0019] The invention concerns an apparatus, sketched in Fig2, for upstream of the work-piece adding to the cutting wires slurry with abrasives directly recycled from the wires downstream and some fresh or reconstituted slurry with abrasives. More specific, the invention relates to an apparatus in the wafer sawing operation for reliably and uniformly adding cutting liquid with abrasive particles through the roller to the cutting wires upstream of the workpiece and downstream from the workpiece directly recycling the abrasive slurry from the wire through the downstream roller back through a transfer pipe. [0020] Runoff and drippings from the workpiece can also be recycled directly through the upstream roller as indicated in Fig.3.

[0021] From the outlet of the downstream roller also a fraction of the slurry is removed in order to offline treat spent slurry to recover SiC and the fine silicon kerf particles and to bleed-off contaminants such as iron and copper.

[0022] Silicon and SiC and PEG are mentioned explicitly. However, the patent also applies to similar systems for production of other wafers than Si, the use of abrasives other than SiC and the use of slurry compositions not based on PEG. An example of the use of other abrasives than SiC is the use of diamonds fixed to the wires.

[0023] In the following only one roller is mentioned for upstream and one roller for downstream. However, the patent also includes the case that several rollers have the same function.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 shows the rollers upstream and downstream of the work-piece and indicates the origin of the workpiece.

[0025] Figure 2 schematically shows transfer of slurry and abrasives through the guide rollers upstream and downstream of the workpiece, and on-line recycling back through a pipe from the downstream to the upstream roller.

[0026] Figure 3 schematically shows on-line recycling of runoff and drippings from the workpiece back through the upstream roller.

DESCRIPTION OF THE INVENTION

[0027] The invention relates to an apparatus in the wafer sawing operation for adding and removing cutting liquid with abrasive particles from the wires. This is to be carried out in an efficient, clean, known and controlled manner with a minimum of losses. Spent slurry containing silicon kerf and waste abrasive is to be removed.

[0028] The invention deals with an apparatus in the wafer sawing operation for reliably and uniformly adding cutting liquid with abrasive particles through the roller to the cutting wires upstream of the workpiece, and downstream directly removing slurry and abrasives from the wires, through the roller and then recycling the abrasive slurry back through a pipe to the upstream roller. [0029] It is difficult to prevent runoff and dripping from the workpiece. Thus an option is to collect this flow and to recycle it on-line back through the upstream roller as sketched in Fig.3. [0030] The cutting liquid, PEG, "wets" the SiC, the surface of the rollers and the wires. The slurry and abrasives spread out as a film over the surface and along the grooves of the guide rollers mainly due to gravity and shear between the rotating rollers and surrounding air. Wetting, surface tension effects and texture of the surface prevent the thin slurry film from running or dripping off the rollers and wires. Dripping from the wires and rollers are prevented by controlling the system so that the film on the wires is only about 20 micrometers thick and on the roller the thickness is less than the diameter of the wires, 0.2 mm.

[0031] The feed system of slurry and abrasives on the roller upstream is designed to give a film on the roller surface that wets and nearly immerses the wire, thus allowing transport of slurry and SiC to the workpiece. With proper design of the feed system, stable control is attained. For instance, if more slurry goes to the workpiece than enters the roller surface, the thickness of the film decreases and transfer by the wire to the workpiece is reduced. Similarly, if less slurry leaves the downstream roller than enters, the thickness of the film increases and transfer away from the roller to the wire and through the roller increases.

[0032] From the outlet of the downstream roller a fraction of the slurry is removed in order to off-line treat spent slurry to recover SiC and the fine silicon kerf particles and to bleed-off contaminants such as iron and copper.

[0033] To attain a uniform addition of slurry in the upstream roller and uniform removal of slurry downstream, shape and pressure drop should be identical in all the openings, respectively. Also, the major part of the pressure drop should be in the openings, slits or tubes. [0034] The rollers may also be composite, supported by a cylindrical cage. The material in the rollers walls may be perforated, composed of fibers, woven in a number of layers or tubes may pass the slurry and abrasives from the inlet/outlet to the surface. In the downstream roller abrasive slurry can move from the roller surface through the roller to the axial pipe and via a lead-through to the transfer pipe. For the upstream roller movement is in the opposite direction from the transfer pipe to the surface. In short the rollers must allow slurry including the SiC particles to pass through, while still sustaining the required pressure gradients.

[0035] The pressure drop for transfer of slurry and abrasives inside the rollers depends on the cross- section of the openings, viscosity, on the diameter of the SiC particles and on the specified feed rate for the slurry. Upstream the recycled PEG-water slurry is pressed from the inside to the surface, partly due to the centrifugal forces. The pressure at the lead-through inlet to the upstream roller may be higher than, equal to, or less than the ambient (atmospheric) pressure.

[0036] Downstream removal from the roller surface is due to suction that overcomes the effect of centrifugal forces.

[0037] New abrasive grains must be added to the slurry to compensate for degradation and loss of the optimal large size fraction abrasives, usually SiC.

PREFERRED EMBODIMENT OF THE INVENTION

[0038] In a first advantageous embodiment of the invention the wire-guiding rollers 1, 2 are perforated with openings- holes, slits or tubes- leading from the inside inlet of the rollers to the surface. In the preferred embodiment shown in Fig 2 tubes 3 lead from an inner axial pipe to the surface of both the upstream and downstream rollers. The openings in the tubes connect to an axial pipe going out of the roller to a transfer pipe via a lead-through 4 sealed by means of seals.

[0039] The openings-holes, slits or tubes- through the roller walls have a smallest dimension in the range from 20 micrometers to 40 micrometers.

[0040] The axial pipe in the downstream roller 2 is part of a vacuum system 5 pumping out via an axial lead-through 4 in the roller sealed by means of seals. The pressure at the lead-through may be down to 0.001 bar. The aim is to pump a given fraction to the upstream roller 1 through a transfer pipe

6. The remaining part is removed 8 and treated off-line to recover the large SiC grains and ideally to recover silicon kerf and to bleed-off unwanted contaminants such as iron and copper. Treatment may be by standard settling, filtration, froth flotation, centrifugation, hydro-cyclone, distillation, drying, electrostatic separation, crushing and screening methods.

[0041] In a second advantageous embodiment of the invention shown in Fig.3 only the upstream roller 1 is perforated with openings- holes, slits or tubes 3- leading to the surface from the axial pipe, connected to the transfer pipe 6 via a lead-through 4 sealed by means of seals. The openings-holes, slits or tubes- through the roller walls have a smallest dimension in the range from 20 micrometers to

40 micrometers

[0042] New and/or reconstituted slurry and abrasives 9 is added to the pipe 6 to compensate for removal of slurry and abrasives.

[0043] The first and second advantageous embodiments can of course be combined.

Example 1 of preferred embodiment for low or negligible runoff or dripping (see Fig 2)

[0044] Slurry enters the upstream roller through an axial pipe going along the whole length of the roller. Tubes feed slurry and abrasives from inside this pipe radially to the surface of the roller. The design is the same for the downstream roller, except that slurry and abrasives now move from the surface of the roller to the inside axial pipe. The axial pipe is connected to the transfer pipe via lead- throughs sealed by means of seals. The grooves on the roller surface are semicircular with diameter equal to the wire diameter. [0045] Specifications are the following:

Wire speed: 10m/s

Wire diameter: 180 micrometer

Wafer thickness: 180 micrometer

Roller diameter: 0.2 m

Diameter of tubes 3 for feeding in or out of slurry with abrasives: 360 micrometers

Number of tubes in rollers 1, 2 per wire: 8

"Effective viscosity" of slurry: 0.15 Pa(scal) s(econd)

Mean size of SiC: 10 micrometer

Pressure at inlet 7 to upstream roller: ambient pressure, 1 bar

Pressure at outlet S of downstream roller: 0.01 bar (vacuum)

Pressure drop along upstream feeding tubes 3 (due to centrifugal forces) 0.5 bar

Pressure drop along downstream removal tubes 3 (suction-centrifugal forces): 0.5 bar

Fraction of slurry and abrasives recycled through rollers removed downstream 8: 0.1

Fraction added to transfer pipe 9: 0.1.

About 0.11 cubic cm per second of slurry and abrasives passes through the workpiece per wire

About 0.011 cubic cm per second of slurry and abrasives is fed through the upstream roller per wire

About 0.011 cubic cm per second of slurry and abrasives leaves through the downstream roller per wire

About 0.1 cubic cm per second of slurry and abrasives is recycled on the wire from the downstream to the upstream roller per wire. This means that 1/10 of the slurry and abrasives fed in on average are treated off- line. Then the slurry and abrasives has passed through the workpiece 100 times.

The thickness of the film on the rollers about 0.1mm.

Example 2 of preferred embodiment for major runoff or dripping (see Fig 3)

[0046] Slurry enters the upstream roller through an axial pipe going along the whole length of the roller. Tubes feed slurry and abrasives from inside this pipe radially to the surface of the roller. The axial pipe is connected to the transfer pipe via a lead-through sealed by means of seals The grooves on the roller surface are semicircular with diameter equal to the wire diameter. [0047] Specifications are the following:

• Wire speed: 10m/s

• Wire diameter: 180 micrometer

• Wafer thickness: 180 micrometer

• Roller diameter: 0.2 m

• Diameter of tubes 3 for feeding in slurry with abrasives: 540 micrometers

• Number of tubes in roller 1 per wire: 8

• The angle seen along the roller axis is 22.5 degrees between tubes for neighbouring wires.

• "Effective viscosity" of slurry: 0.15 Pa(scal) s(econd)

• Mean size of SiC: 10 micrometer

• Pressure at inlet 7 to upstream roller: ambient pressure, 1 bar

• Pressure drop along upstream feeding tubes 3 (due to centrifugal forces) 0.5 bar

• About 0.11 cubic cm per second of slurry and abrasives enters the workpiece per wire.

• About 0.055 cubic cm per second runoff the workpiece per wire into and out of tank 10.

• About 0.055 cubic cm per second of slurry and abrasives is fed through the upstream roller per wire.

• About 0.055 cubic cm per second of slurry and abrasives is recycled on the wire from the downstream to the upstream roller per wire

• Fraction of slurry and abrasives that runs off workpiece, that is removed to off-line treatment 8: 0.1

• Fraction of slurry and abrasives per second that is added 9 to pipe 6: 0.1. This means that 1/10 of the slurry and abrasives that run off the workpiece on average are treated off- line.

• The thickness of the film on the rollers: upstream about 0.1 mm and 0.05 mm downstream.