WO2014167392A1

WO2014167392A1 - Monitoring device and method for wire web monitoring and wire saw

Info

Publication number: WO2014167392A1
Application number: PCT/IB2014/000429
Authority: WO
Inventors: Daniel ZINGG
Original assignee: Meyer Burger Ag
Priority date: 2013-04-09
Filing date: 2014-03-26
Publication date: 2014-10-16
Also published as: TW201505742A; CN105050759A; JP2016518996A

Abstract

An improved and simplified system and method for monitoring the wire web of a wire saw is described. The system contains a sensor that is moved over the wire web and thus determines the deflection of individual wires of the wire web or of wire groups thereof.

Description

Monitoring device and method for wire web monitoring and wire saw

FIELD OF THE INVENTION

[001] The invention concerns a wire saw, especially a multi-wire saw, and a method for monitoring the condition of a wire web in a wire saw especially for cutting hard, brittle materials such as multi and mono crystalline silicon as used in the semi-conductor and the photovoltaic industry and a sensor for monitoring the condition of a wire web. More specifically the invention is concerned with a wire saw for cutting a workpiece comprising (at least two) wire guide rollers for forming a wire web, the wire web containing adjacent wire segments from at least one cutting wire, for cutting the workpiece, a feed table for mounting the workpiece and moving it towards the wire web, a controller for controlling the movement of the wire guide rollers and the feed table and a wire web monitoring system for monitoring wire segments of the wire web, containing deflection measuring means, containing one or multiple sensors, for measuring the distance to or presence of at least one wire segment.

[002] Wire saws according to the state of the art further comprise pulleys for guiding cutting wire from a fist (new wire) spool to the wire web and then to a second (old wire) spool. The wire may be moved back and forth between the first and the second spool. Tensioning means may be provided to keep the wire under the desired tension.

TECHNICAL BACKGROUND

[003] The invention refers to a wire saw, particularly a multi-wire saw, comprising a cutting wire or multiple loops of cutting wire forming a wire web for cutting a workpiece and wire sensing means for detecting the presence or location of cutting wires. Such wire saws are used for cutting hard brittle materials such as sapphire, quartz, silicon (multi, mono-like and mono), boron, etc. The material is e.g. cut into slices referred to as wafers. The wafers may be very thin (as used for the solar cells, typically 100-200 pm), relatively thick (as used for electrical components in the semiconductor industry, typical 200 - 900 μηη) or even thicker as is for instance the case for optical components made of sapphire or quartz or elements made of rare earth metals.

[004] Wafers, e.g. for use in solar cells or semiconductor industry, are cut from a block (also called brick or ingot) in a wire saw employing a wire and abrasives. Usually abrasives are suspended in a slurry, in this application also called cutting fluid, that is supplied to the wire web, e.g. by means of a spray means. Nowadays, wafers are cut more and more using fixed abrasives, which are directly attached to the cutting wire. Such wire is referred to as diamond wire. Also in the latter case a cutting fluid is used, e.g. for high quality cuts and for cooling purposes. Both slurry and true cutting fluids will be referred to as cutting fluid. The invention relates to wire saws using both slurry and fixed abrasives.

[005] A wire saw forms a wire web or wire row by winding one or multiple wires on a plurality of grooved rollers (2, 3 or 4 are use commonly) and runs the wire at a high speed and presses a workpiece against the wire web to thereby slice the workpiece. Since the workpiece may not be of the same quality over its complete length (especially for poly-crystalline), not all "wires" or wire segments extending between the wire guide rollers of the wire web will advance through the workpiece at the same rate. Note that with wire segment actually the tube-like volume is meant between two grooves on consecutive wire guide rollers where the wire extends in and that extends in a single cut in the workpiece. In this document the wire extending (at a given moment) in this volume will be referred to as wire or wire segment of the wire web. With wire web or wire row the part of the wire is meant extending between the wire guide rollers, normally used for cutting. If multiple wire webs for cutting are present each of them may be equipped with an inventive monitoring system. Under workpiece an object to be cut is understood, typically the object being a hard brittle material such as silicon, sapphire, quartz, Boron or the like. The workpiece may in fact consist of multiple blocks of material, e.g. multiple silicon bricks. [006] Wire saws are well known and used widely in the semiconductor and solar industry.

[007] From US 6,178,961 B1 as closest prior art it is known to determine the cutting load (the load on the workpiece in a direction opposite to a workpiece feed direction) and control the movement of the feed table. The cutting load should not exceed a maximum value because this would lead to wire breakage. Having a too low cutting load will prolong the time it takes to cut the workpiece and also impair wafer quality.

[008] In order to determine the cutting load, it is proposed to measure the displacement or bow of some wires of the wire web. The cutting load on the wire is calculated according to the wire deflection and the initial tension of the wire. As a sensor an eddy-current displacement meter as a proximity sensor may be used. This sensor may be protected from the displaced wire and the slurry or cutting fluid by a cover, which is made of the nonconductive material, without affecting the measurement of the sensor. [009] One disadvantage of this system is that only some wires are used to calculate the cutting load on the complete wire web. Due to e.g. inclusions in the workpiece, however, the load on a single wire segment may become undesirable high, leading to wire breakage.

[0010] Another disadvantage is that the sensor is positioned in the very harsh environment of the processing or cutting chamber of the wire saw. Especially in the case of abrasive cutting fluids (slurries), the sensor will get contaminated and will be hard to maintain.

[0011] A further disadvantage of the shown sensor is that in the position of the sensor, the distribution of the cutting fluid will be disturbed. This has a negative influence on the quality of the cut in that region. [0012] Also this sensor is only suited for carrying out a load determination and does not provide a solution for numerous other tasks as the inventive sensor does.

[0013] From DE 10232768 a sensor system is known that can monitor all wires of the wire web. A video or thermo camera is used for making images of the wire web. It is recognized that the bow may locally vary due to inclusions (e.g. SiC) and therefore a camera with a high enough resolution is used. Also disclosed is the use of inductive or capacitive sensors for the detection of the bow. From the disclosure it is unclear how such sensor could detect inclusions since each wire segment would require its own sensor. With typically 1000 wires segments in a wire web cutting the workpiece, this would lead to a very costly system.

[0014] Clearly such camera based systems will have great difficulty making accurate images of the wire web that is covered in slurry or cutting fluid. In both cases the fluid will be thrown around the working chamber obscuring the optical system of the camera or at least blocking the view.

SUMMARY OF THE INVENTION

[0015] An object of the present invention is to overcome some of the disadvantages of systems according to the state of the art. [0016] The invention is set forth and characterized in the independent claims, while the dependent claims describe other characteristics of the invention.

[0017] An improved, simplified more cost-effective system for monitoring the condition of wire groups containing no more than two or three wires, ideally containing only one wire of a wire web is described which can detect small wire groups/wires more accurately. By monitoring any kind of inspection of the wires is meant as will become apparent later. The inventive sensor monitors the wires by determining their relative (to each other or to the sensor) or absolute position (relative to a fix reference) or presence (e.g. difference in bow of wires or pairing of neighbouring wires). [0018] The present invention is based on the realisation that it is paramount to detect small wire groups or even single wire segments, but that the rate at which the groups are looked at does not have to be high. Inspecting each range of the wire web once per minute or a couple of times per minute will suffice for controlling the process. The cutting process being slow, no short reaction times are needed. [0019] In one aspect of the present invention, a wire saw, particularly a multi-wire saw, is provided having moving means provided for moving the deflection measuring means along the wire web, preferably over the wire web in a direction mainly parallel to the wire web.

[0020] In a preferred embodiment of the invention, the deflection measuring means is movable along a direction which is essentially parallel to the plane of the wire web and transverse, preferably perpendicularly, to the longitudinal extensions of the wire segments in the wire web.

[0021] As can be seen from the drawings, the wire web (which comes in contact with the workpiece during cutting) is formed by the wire segments monitored by the inventive wire web monitoring system.

[0022] By monitoring the wire web in this degree, the complete recipe may be controlled, the recipe including parameters such as starting point of the cut, relative speed of the feed table to the wire web, (rotational) speed of the wire web, cutting fluid/slurry supply and/or temperature, rocking of the work piece if a rocking unit is provided (moves the work piece in an oscillating manner while it is pushed thought the wire web). The movement of the wire guide rollers and the feed table are driven by drives, normally separate motors. The moving means or drives are controlled by a sensor controller unit that may be electrically connected to the controller for controlling the movement of the wire guide rollers and the feed table or maybe an integral part thereof. The controller unit may also trigger the measurements of the deflection measuring means and may contain a memory for storing measurement data.

[0023] The deflection measuring means may be any suitable means for determining the presence of (like a binary distance sensor: no wire visible means far away, wire visible means close to the sensor) or distance to wire segments. If a binary distance sensor is used, its distance to the wire segments may be changed, for example once per run (one complete round-trip over all relevant wire segments) or controlled on basis of the last known distance to the wire segments, as to determine the exact distance. [0024] The advantages of the system are many. Firstly, the whole wire web can be monitored with only one or only a few sensors that is at close range. Sensors are quite costly if one considers that a wire web can consists of up to 4000 wires. It is also very cumbersome to mount so many sensors and connected them to a controller for controlling the cutting process.

[0025] Secondly, by moving the sensor the effective resolution of the sensor can be augmented by processing the data of consecutive measurements. In this way a sensor that can only detect multiple wires, now can be used to detect single wires, thus avoiding the need for expensive, accurate sensors.

[0026] By mounting the deflection measuring means to a linear bearing, the former can be moved in an easy, precise and cost-effective way over the wire web. The sensor may be moved by a drive or motor that drives a (toothed) belt or the like that is directly or indirectly connected to the sensor controller unit.

[0027] To make the relation between the position of the deflection measuring means, also called sensor here, and the wire segments as easy as possible, it is beneficial if the movement of the sensor extends mainly parallel to the axes of the wire guide rollers. Moreover, in the direction the wire guide roller extend in. Since the wires do not extend perpendicularly between the wire guide rollers (they spiral around them), the movement of the sensor may be corrected for this, making the movement perpendicular to the direction the wire segments extend in and parallel to the plane of the wire web when no load of a workpiece or the like is bearing on it. This correction was found not to be essential for good measurements though. [0028] Important for the invention is that the sensor moves over all relevant wires segments.

Moreover, the sensor should move in the direction from one wire to the next, adjacent wire.

[0029] The deflection detection means or sensor may be any type of suitable sensor capable of measuring distance to the wire segments or detecting the presence of wire segments. A distance sensor may determine the absolute correct distance or give an output depending on factors that are not known such as for example the cutting fluid or slurry used. In the latter case the relative position of the wires (to each other or to the sensor) may be monitored. The sensor may be any sensor from the group consisting of optical sensors (reflex light barrier or curtain, sensors using time of flight or triangulation/stereo "imaging", etc.), inductive sensors, capacitive sensors and ultrasound sensors, mechanical sensors or any combination thereof. Preferably, sensors are used that can "see" through the used cutting fluid, such as capacitive or inductive sensors. If combinations of sensor types are used, the measurement may be more robust or accurate. The distance to the wire segments may be defined from a reference point in/near the sensor.

[0030] Preferably, the deflection measuring means are located close to the wire web, preferably if the wire web extends freely between the wire guide rollers, not further than 5mm away from the wire web in a direction of measurement, even more preferred not further than 4mm or even less than 3mm away. This distance being defined from the front side (facing the wire web) of the sensor or housing it is in to the wire segment being measured. In principle the sensor should be as close the wire segments as possible in order to achieve the highest accuracy that normally deteriorates with distance. However, in order not to disturb the flow of the cutting fluid and not to come in contact with the wires wire retracting the wafers thought the web, a certain distance should be maintained. The direction of measurement being the direction the sensor "looks" in such as its optical axis or the centre line of a measurement field or cone. Since normally the measurement range of the deflection detection means is convergent, this ensures that the deflection detection means 'see' only a small number of wire segments. The sensor may be placed underneath or above the wire web it is monitoring. Since retracting the cut workpiece from the wires and pushing the workpiece through the wires may lead to different requirements, two inventive sensors arrangements may be used: one above and one underneath the wire web. Since sensors with smaller measurement range are more cost-effective, it is beneficial to keep the distance to the web small. The distance between (a reference, such as a wire segment, on) the wire web and the at least one sensor may be controlled. Moreover, the sensor may be moved towards the wire web as the wire segments move away from the sensor under the influence of the pressure of the work piece. Alternatively, for example when the wire segments have a bow towards the sensor, the sensor may have a constant distance to the (virtual) relaxed wire web. In the latter case, the wire segments will be in detection range of the sensor all the time (and most visible when having a large bow) and it only has to be prevented (by changing the recipe) that the sensor runs into the wire segments as they bend.

[0031] Since a wire segment may extend under different angles on both sides of the workpiece (an inclusion may be offset to one side of the brick), two inventive wire web monitoring systems may also be used, one on each side of the workpiece.

[0032] It is preferred that the deflection measuring means moves along a line parallel to the crest of a wire guide roller it is associated with, the distance between the crest and the centre of the measurement range of the deflection measuring means being smaller than 6 cm, preferably smaller than 4 cm or even more preferred between 30 and 40 mm, this distance being measured parallel to the wire segments when it extends freely between the wire guide rollers. The measurement range of the deflection detection means is limited. In order to not stretch the need for more range and maybe even allow more accuracy, it is beneficial to keep the range small. This can be assured by doing measurements in a range where the wire segments do not deflect too much: close to the (crest of the) wire guide rollers. On the other hand, pressing down the wires will make them follow the contour of the rollers. In this case, all wire segments pressed down more than a given amount (what is allowed by the rollers) will have the same deflection, rendering the measurement worthless. The crest of the wire guide roller is defined as the highest point of the wire web, when the freely extending wire web (no load) extends in horizontal direction.

[0033] Preferably, the deflection measuring means and the moving means for moving the deflection measuring means together are inside a single housing, preferably a waterproof housing. By placing all moving parts inside one housing, the wear is reduced to a minimum. Also the deflection measuring means and controller are not exposed to the slurry or cutting fluid. The housing also is a protection, especially for the deflection measuring means, for (broken) wires. Wires, that may be moving at high speed, may come close and even touch the deflection measuring means thus damaging them. The housing may be releasably mounted in the wire saw for easy maintenance or replacement. The deflection measuring means and the means for moving the deflection measuring means may be releasably mounted in the housing for easy access. The housing having means for holding the latter. The housing preferably has one connector for electrical power and signals, the housing thus forming a self-contained unit or sensor.

[0034] In order not to impair the detection capabilities of the deflection measuring means, a window may be provided in the housing that is transparent for the sensor such as a polymer or glass window for sensors using electrical fields to detect the wires. Sensors using electromagnetic radiation may use a glass or transparent polymer window. Preferably the portion of the housing not facing the wires/not impairing the view of the sensor is made of a robust material such as steel. The housing may even have an inexpensive replaceable member that can be replaced easily. [0035] Since the housing is located close to the wire web, it may influence the flow of the cutting fluid or slurry. It is preferred that the housing has a constant shape or cross-section over the whole width of the (used) wire web, at least on the side facing the wires. In this way influence on the distribution of the cutting fluid or slurry is constant over the whole cutting range. The housing may even be adapted to influence the fluid flow in a desired manner. The housing further may be formed so that it bends away from the wire, thus preventing wire from cutting the housing, e.g. when retracting the wafers from the wire web. The housing thus having a convex part facing the wire web.

[0036] It is preferred that the deflection measuring means has a measuring range wider than the distance between two adjacent wire segments, preferably wider than 10 times this distance or even more preferred larger than 20 times this distance. Moreover, in order to reduce costs, preferably a sensor is used that cannot detect single wire segments in a wire web. The movement (which is given anyhow) of the wire sensor together with smart data processing will enable the detection of single wires. The sensor data is processed by a data processing unit that for example uses any of a de-convolution filter, a Wiener Filter, motion estimation filter, differential filter or any combination thereof. The sensor may have a measuring range (in the location the wire is expected or the range where it may move under different loads) wider than 1 mm, preferably wider than 5 mm, even more preferred wider than 10 mm. The measuring range as used here means the area or range the wires are detected in. With a wire pitch between 0.2 and 0.4 mm, the sensor thus "sees" 2 to 50 wires. [0037] Preferably the wire saw or the wire web monitoring system further comprises a memory for storing measurement data from the deflection measuring means. The representation may be seen as an image (position of "pixels" next to each other is known), also called bow profile and/or may be deduced using filtering of the raw data as described above. The memory may be used to store (partial) time series or images that can then be processed. Multiple such images may also be stored to evaluate the change of the wire web over time. Also a look-up table for calibrating the sensor may be stored here.

[0038] The condition of the wire web (bow, wire breakage, etc.) may be deduced from (shape) features of the representation, raw data or images, selected from the group consisting of maximal distance (wire segment with the largest distance to a reference plane), minimal distance (wire segment with the smallest distance to a wire reference plane, average distance (average distance of a number of wire segments to a reference plane), median, variance or standard deviation (calculated over multiple wire segments relative to a reference plane), deviation in axial direction (distance between two neighbouring wire segments or pitch), missing wire, pairing of wires or any combination thereof . It was found that when sawing, looking at the slowest wire segment (the one cutting the slowest) can prevent wire breakage due to applying too much load to the wire web.

[0039] Preferably the deflection measuring means make a reciprocating movement or run, preferably with a scan frequency smaller than 5/min, even more preferred smaller than 2/min or even smaller than 1/min. By reciprocally moving the deflection measuring means over the wire web, the whole web or at least the relevant parts thereof can be monitored in a high enough rates. The cutting of e.g. silicon ingots progresses at about 1-5 mm/min. Moving the sensor over the wire web (half a run) 1-3 times per minute results in 1-3 measurements of each wire every minute. In this minute, the cutting has progressed 1-5 mm. Typically a wire has to change its relative position

(measured where entering the sawing gap) to the other wires by 3 mm in order to pose a problem, meaning that the wire segment or group of wire segments will be "seen" at least 2 times before this unacceptable deviation occurs. The rate of reciprocally moving may be adapted to the needs of a given operation or the state or shape of the web. When determining when the work piece first touches the wire web, the feed table may move faster when approaching the wire web than during cutting. In the former case the reciprocal movement may need to be faster.

[0040] With scan frequency the number of times per time unit the sensor moves over the complete wire web (= all relevant wire segments) and back to its original position is meant. Moreover, a run is one period of its movement. The run only needs to extend over the relevant wire segments such as over those that actually take part in the cutting.

[0041] The deflection measuring means may be held still while measuring. Alternatively the deflection measuring means may be moved in continuous fashion over the web. After moving over the web, the sensor may wait or move back immediately. In the latter case the movement of the sensor is such as to achieve the desired scan rate. In the former case, the movement is quicker and the scan rate follows from the speed of the sensor, the waiting time and the width of the wire web that is monitored.

[0042] Preferably the deflection measuring means makes a measurement at least twice while moving from one wire segment to the next, neighbouring wire segment (also called pitch of the wire web), preferably at least 3 times and even more preferred at least 4 times. This ensures that wires can be detected individually. Typically the pitch of the wires (distance from moving from one wire to the next) is 80 to 400 pm, meaning that one measurement may be made every 40 to 200 pm. Preferably every measurement is made multiple times to increase the accuracy of the measurement. The sensor preferably "sees" at least 3 wires at once, even more preferably at least 10 wires at once. This allows using cost-effective sensors. A fast scanning rate (multiple measurements per pitch) and data processing enables detecting wires individually, as long as the bows of neighbouring wire segments are not too different. An inductive sensor may be used, depending on the pitch and material of the wires.

[0043] If a higher detection speed is required, the wire web monitoring system may comprise multiple deflection measuring means that preferably are associated with one moving means. If the same moving means and controller are used for moving multiple deflection measuring means, only an additional sensor is needed. Also the sensors may have different measuring ranges so that wire segments can be detected over a larger range. In addition the accuracy may be augmented by using multiple sensors.

[0044] Preferably the wire saw comprises a controller unit for controlling the movement of the wire guide rollers and the feed table, the control unit preferably in addition being capable of any of setting an alarm, controlling the movement of the feed table and/or wire guide rollers and any combination thereof. The controller may control the movement of the deflection measuring means and the measurements as well. In that way the measurements are made on a known location so that the data can be seen as an image. Based on the measured data the controller can directly or indirectly (through other controllers in the wire saw) influence the movement of the feed table, the speed of the wire or any other parameter of the cutting process. Also an alarm may be set of so that an operator can influence the cutting process.

[0045] In another aspect of the invention the inventive wire saw or sensor is used for determining the zero point: the point close to where the cutting recipe starts. This may be done by forming the wire web, preferably mounting a workpiece to the feed table, moving the feed table towards the wire web, pushing the feed table or the workpiece attached thereto against the wire web and monitoring the wire web while the feed table or workpiece touches it. The moment this happens is the zero point. These steps may be carried out in any convenient order. For example the wire web may be formed prior to mounting the workpiece. Measured data and position information may be stored in a sensor control unit with a memory. [0046] By looking at what wires are depressed, it can be recognized how long the workpiece is or workpieces are and where they are located. This information may be used for both the cutting recipe and for controlling the inventive wire web sensor.

[0047] If the wire web is rotated, normal cutting may commence while the cutting process is monitored. Now the workpiece is pushed through the wire web, normally until the all wafer have been cut completely. [0048] In a further aspect of the invention, the wire saw or inventive sensor is used to monitor the cutting by mounting a workpiece to the feed table, forming and rotating the wire web, moving the workpiece though the wire web, monitoring the wire web while the workpiece is moved.

[0049] Also after cutting, the wires may be monitored by monitoring the wire web while the workpiece is retracted. Once a cut is complete, the cut workpiece is normally retracted through the wire web. When doing so wires may get stuck. This may be monitored by the inventive wire web monitoring system as well, stopping the movement if one or more wires stay behind too far.

[0050] In order to calibrate the sensor, the feed table or the workpiece may be moved against the non-rotating wire web and then moved over known distances, storing the position of the feed table and the sensor readings. For example if the web is pressed downward for the first time, this position of the feed table is stored: zero point, no bow. The feed table or workpiece is then moved further over a predetermined distance (resulting in a known bow). The sensor now measures a deflection of the wire segments that goes with this predetermined distance or bow and stores these values as well and so on. Thus the reading of the sensor at known positions of the feed table and thus known bows (provided the width of the workpiece or feed table is known) are recorded thus calibrating the system. In this way a look-up table can be made that later is used to measure the bow e.g. during cutting.

[0051] It is further possible to detect when the cut is completed. This can be done by looking at the relative position of the wires. When cutting in the beam, all wires will quickly move to the same plane since the beam is not as hard as the workpiece and does normally have no inclusions. The bow of the wires is reduced at a high rate once in the beam. This fact can be monitored as well thus recognizing what wire segments are in the beam. Further, the absolute position of the wire may be determined based on the position of the feed table and the known geometry of the wire saw. The benefit of quickly recognizing the end of the cut is that wires quickly make large holes in the beam. This causes wafers to be cut from the beam (cutting the beam above a wafer) or to detach themselves when wires get stuck as the wires are retracted from the beam. [0052] The following steps may be part of operating the inventive sensor:

- Determining the position of the feed table in which it or workpiece attached to it first touches the wire web. From this e.g. the size of the work piece may be deduced or the position where the actual cutting starts.

- Moving the feed table over a predetermined distance and determining the amount the web is pressed down may e.g. be used for making a look-up table as was described above.

- Determining the difference between the position of the feed table in which the first wire is

touched and the last wire is touched gives e.g. an indication of how parallel the wire web is to the lower surface of the workpiece to be cut.

- Determining what wire segments are pressed down and what wire segments are not may e.g. be used to determine the number of, location of and length of the workpiece(s). This gives an idea of what wire segments should be monitored but also may be used to predict the amount of material that has to be cut: number of wire segments actually cutting multiplied by the cross- section of the workpiece multiplied by the wire thickness gives an estimate of the amount of material that has to be removed. This can be used to determine the cutting recipe or predict when new wire has to be mounted on the wire saw.

- Determining the difference between the angles of at least two wires gives e.g. insight of the relative cutting speed of the respective wires.

- Determining the value the deflection sensor is measuring as the web is first pressed down may e.g. be used for calibrating the sensor.

- Monitoring the speed of a wire segment in the direction perpendicular to the wire web may e.g. be used to determine if the cut is complete. The same holds for recognizing when all relevant wire segments have returned to one plane and determining the absolute position wires relative to the wire saw.

All these steps may be combined with each other and with other methods described.

[0053] A further aspect of the invention is a wire web monitoring system or sensor to be used in any of the above wire saws or methods.

[0054] The inventive system may further be used for detecting wire breakage or wire pairing. If one or multiple wires are not detected in a range where they can be expected, the wire saw may be stopped.

[0055] These together with other objects of the invention, along with the various features of novelty that characterize the invention, are described in more detail below. For a better understanding of the invention, its operating advantages and the objects and advantages of the present invention, reference is made to the accompanying drawings and description of the embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS [0056] For a better understanding of the nature of the present invention, reference should be made in the detailed description taken in conjunction with the accompanying drawings in which:

FIG. 1 schematically illustrates a state of the art multi-wire saw cutting a workpiece;

FIG. 2 illustrates how the wire segments of the wire web are depressed and bent during cutting forming a negative bow; FIG. 3 is a schematic view of an embodiment of the inventive wire web monitoring system;

FIG. 4A-4C show the relative position of wire segments in a wire web before and during cutting and while retracting the wafers;

FIG. 5 shows the members of an embodiment of the inventive system inside a housing;

FIG. 6 shows the location of an embodiment of the inventive wire web monitoring system in relation to the wire web during cutting;

FIG. 7 shows another view of an embodiment of the inventive sensor mounted above a wire web;

FIG. 8 shows the steps for determining the zero point;

FIG. 9 shows the steps for cutting;

FIG. 10 shows the steps for creating a reference table; FIG. 11 shows the steps for determining the length of the workpiece; FIG. 12 shows an

embodiment of the inventive system with two sensors; and

FIG. 13 shows the steps for monitoring the retrieval of wafers.

DETAILED DESCRIPTION OF THE INVENTION [0057] The present invention will be described with reference to exemplary embodiments and the present invention is not limited to a particular wire saws, methods or sensor, except as defined in the appended claims. Embodiments of the present invention may be used with a variety of methods and systems. It will be apparent to one skilled in the art that the present invention may be practiced in a variety of ways within the scope of the claims. All features shown in relation to the figures may be applied mutatis mutandis to the invention as described in the claims.

[0058] As used herein, the indefinite article ("a", "an") denotes the presence of at least one of the referenced item, and the term 'a plurality' denotes the presence of more than one.

[0059] The present invention relates to systems, methods and to production equipment, more precise to a wire saw, particularly a multi-wire saw, for cutting hard, brittle materials such as silicon, sapphire and quartz and methods for operating the same as well as its inventive components.

[0060] In the figures only parts essential for the current invention are shown schematically, for better understanding of the invention. Wire saws and their components parts as such are widely known in the industry. [0061] As is shown in figure 1 , wafers are normally cut in a wire saw 1 by pushing (arrow M) a workpiece such as a brick, ingot or core 2 through a field of moving cutting wires 3, also called the wire web or simply the web. The wire of the wire web may have abrasive material attached to it, so called diamond wire, or an abrasive may be suspended in the cutting fluid used, the so called slurry. The workpiece 2 is attached to a sacrificial plate 5 that is attached to a fixture attachment 6. The fixture attachment 6 is used for attaching the workpiece 2 to the movable part or feed table 7 of the wire saw 1. The fixture attachment 6 is normally detachably (by tensioning rods) attached to the feed table 7. The beam 5 is normally glued to both the workpiece to be cut 2 and the fixture attachment 6. The purpose of the (disposable) beam 5 is to prevent the wire 3 from cutting into the fixture attachment 6. The wire web in this figure extends freely i.e. without load". [0062] As can be seen from figure 2, during cutting the workpiece 2 pushes the wire web down giving it the so called (negative) bow. After the wafers have been cut, the workpiece is normally moved upward, pulling the wires with it forming a (positive) bow in the opposite direction. The wire portions that at a given moment are inside the brick (in the sawing gap or cut) follow a curved path. Moreover, the left and right edges 2a, 2b of the brick 2 are cut before the centre part 2c. To prevent the wire from cutting into the fixture attachment 6, the beam 5 is used to offset the brick 2 to the fixture attachment 6.

[0063] The wire web is normally formed by one wire that is wound multiple times (e.g. 3000-5000 times) around two or more wire guide rollers 4. It is however also known to use multiple wire loops that run around the wire guide rollers 4. The invention can be used for both systems and is not limited to any of the two. The wire web 3 extends beyond the longitudinal extension of the brick, so that the complete brick is cut into wafers even if the position of the brick in that direction is not exactly controlled.

[0064] With wire segment or simply wire, the wire extending between the wire guide rollers at any given moment is meant. It is the path the wire follows from one wire guide roller to the next, possibly through a cut in the workpiece. Since the wire during cutting moves at high speed, any portion of it only is part of a wire segment for a short time.

[0065] On the outer surface of a wire guide roller normally grooves are provided for holding the cutting wire. Since the grooves on consecutive wire guide rollers are normally not aligned, the wire segments in fact do not extend perpendicularly to the direction the axis of the wire guide rollers. Since the angle does not deviate greatly from 90 degrees, for all purposes in this document the angle is considered to be perpendicular unless stated otherwise.

[0066] Figure 3 shows a more detailed and more realistic view of the wire web 3 during cutting. As can be seen, the wire segments 3', 3", 3"' from the wire web 3 are not pressed downward by an equal amount. For clarity only three wires 3', 3", 3"' are shown, but normally each wire segment lies in its own plane. The wires 3', 3", 3"' may also be considered as wire groups: groups of possibly adjacent wire segments that lay in roughly the same plane.

[0067] As the workpieces advances, the wires 3', 3", 3"' touch the workpiece and are pushed downward, the wires deflect form the zero point onward. The cutting rate of each wire segment depends on the local quality of the workpiece. If a brick has hard inclusions a cut will not advance as fast as in other places. This leads to the effect that in practice the wire web will have an irregular shape, having wire segments that advance faster than others.

[0068] Shown in figure 3 are three representative wire segments 3', 3", 3"'. As the brick 2 advances in downward direction, some gaps advance relatively fast; here wire segment 3'. Other segments may not advance as fast as e.g. the segments 3" and especially segment 3'". [0069] It is shown that the centre of the housing 10 of the wire web monitoring system 3 is located at a distance d from the crest or top 16 of the wire guide roller 4. It is assumed that in this embodiment the centre of the detection range of the sensor is also located at this distance d from the crest 16 of the wire guide roller 4. [0070] If the wire segments are retracted from the gaps after cutting, the wires may stick by different amounts, leading to the same effect in opposite direction: some wires advance faster than other out of the cut.

[0071] Looking in the direction of movement of the wires (cross section XX' in figure 3), the wire web 3 looks as is seen in the figures 4A, 4B and 4C. In figure 4A the wire web is shown as it has not yet engaged by the brick. In all figures 4A, 4B and 4C, the horizontal dashed lines show the position of the non-deflected, i.e. freely extending wire web. In figure 4A all wires lie on this line.

[0072] The distance the wire segments deviate from the zero point may be measured. This is in fact a measure for the angle enclosed between the position a segment has when no load bares on it and the position it has e.g. during cutting. The angle therefore also is a measure for the displacement and may thus be used for the same purposes. Since the wire partially follows the curvature of the wire guide rollers, the relation between displacement and angle is not as straight forward. In order to make accurate measurements, a look-up table may be generated as described in relation to steps 301 to 307.

[0073] In figure 3 the wire web monitoring system 13 is placed slightly higher than the wire web 3 in its relaxed state and offset towards the workpiece 2. Since the detection range 14 (see figure 5) of the sensor 8 is limited, it is beneficial to mount the sensor not too far away from the wire web 3. On the other hand, the wire web monitoring system 13 may not disturb the cutting fluid (not shown) that is on the wire web 3 in an undesirable way and thus should be kept at a distance. Preferably the sensor housing 10 is not further than 1-5 mm away from the wire web 3 in its non-deflected state. [0074] Since this wire web monitoring system 13 always has some influence on the behaviour of the cutting fluid, the influence should be kept as constant as possible over the whole wire web 3 or at least that part of the wire web 3 that is used for cutting. Moreover, the outer surface of the housing 10 should at least near the wire web 3 be constant over the relevant part of the wire web 3.

[0075] Since the wire web is normally deflected, the deflection measuring means may be aligned to the deflected state of the web. Moreover, the measuring range of the deflection measuring means may not extend perpendicularly to the wire web as it is not loaded, but to its normal position during cutting. This may increase the accuracy of the measurements depending on what type of deflection measuring means are used.

[0076] Since the detection range 14 (see figure 5) of the sensor 8 is limited, it is further beneficial if the measurements are done close to the (crest of the) wire guide rollers 4. The wire segments 3', 3", 3"' clearly deflect not as much closer to the wire guide rollers 4 as compared to further towards the workpiece 2. Preferably the (horizontal) distance d from the axis (or crest) of the wire guide roller to the centre of the detection range 14 is about 20-60 mm, preferably around 30-40 mm, when measured in the place and direction of the non-deflected wire web 3. Put differently: distance d from the point the relaxed wire segments (on average) leave the wire guide roller 4 to the centre of the detection range 14 may be about 20-60 mm.

[0077] Figure 4B shows the web 3 after the cutting has started and all wire segments extend in a gap or cut. Clearly all wires are pushed down by the downward movement (M) of the brick (not shown). The gap wire segment 3"' extends in falls behind the most (the gap extend from the lower side of the brick up to the wire segment), while the gap of wire segment 3' advances the fastest. Also visible in figure 4B are two wires, the two left-most wires that have paired: they run in the same groove, greatly impairing wafer quality.

[0078] Figure 4C shows the retrieval of the wafers. After the wafers have been cut, they are still attached to the beam and have to be retracted though the wire web to leave the wire saw. While doing so, some wire segments get stuck between the wafers more than others and are pulled up more. This is the case for wire segment 3" in figure 4C.

[0079] Figure 5 schematically shows part of an embodiment of the present invention in the same perspective as figures 4A to 4C. Shown is the case where the brick (not shown) is cut while being moved in downward direction M. A deflection sensor or deflection measurement means 8 is mounted on a rail or linear guide 9 as to be movable above the wire web 3 in the direction of one wire to the next S. Moreover, the sensor 8 can be moved in the direction S mainly perpendicular to the direction the wire segments extend in, mainly parallel to the relaxed wire web. The important thing to note is that the direction S of movement of the sensor 8 enables the sensor 8 to measure the deflection of all relevant wires. The sensor 8 should therefore be movable over all relevant wire segments. Keeping the direction of movement parallel to the wire web 3 and parallel to the direction the wire guide rollers extend in, makes the measurements of the individual wire more comparable. [0080] The sensor 8 and the rail 9 are mounted inside housing 10 which is attached to the wire saw (not shown). The housing is completely closed. The wire web monitoring system is completely self- contained and only one outlet 11 for electrical power and signals is provided.

[0081] The deflection sensor or deflection measuring means 8 may be any kind of distance, presence or displacement sensor that may use electromagnetic radiation of any sort (visible light, radar, infra-red light etc.), electro-magnetic induction (inductive displacement sensors), capacitive coupling (capacitive displacement sensors), mechanical or (ultra-) sound. Preferably a non-contact sensor is used, but also sensors that touch the wires could be used.

[0082] As will be explained later in more detail, the deflection sensor 8 may be used to make an "image" of the wire web 3, the so-called bow profile. As the sensor 8 moves over the wire web 3, the data is recorded and stored according to the position the sensor has, when the measurement is made. The accuracy and/or depth resolution the sensor 8 needs to have depends on the analysis that is made on this "image". Therefore a sensor 8 only indicating the presence of a wire within a certain range may suffice. If a more sophisticated analysis is desired, the sensor 8 may need to determine the distance to the wires accurately (absolute or relative). With "distance" the distance between a reference plane and the observed wire segment(s) is meant. Typically the reference plane coincides with part of the sensor such as its front face or the sensitive part of the sensor.

[0083] The detection range 14 of the deflection sensor 8 may be wider than the vertical distance (parallel to the direction the wire guide rollers extend in) between the centre of two adjacent wires 3a and 3b. In this case normally more than one wire 3a, 3b will be "seen" by the sensor 8. A controller unit 12 connected to the sensor through connection 15 may be used to take multiple such measurements and from them, probably using the position of the sensor 8 at the time the measurement was made, deduce the position of single wire 3a. For this, digital filters such as a difference, a deconvolution, differential or any other filter may be used. The depth of view (detection range in the direction the sensor looks) extends beyond the maximum allowable position of the wires. In this way, the wires are always "visible".

[0084] The sensor 8 may be connected to a controller unit 12. The controller unit may control the position of the sensor, its speed and the time a measurement is made or stored. The controller unit 12 may have a memory 17 associated with it for storing the measurement data, preferably together with the position the measurement was made. In this way an "image" (actually a 1 D array) of the wire web can be made. The controller may slow down the speed of the sensor in areas where more or more accurate measurements are required. This may be the case if only a single wire falls behind and is (partially) obscured by its neighbours. In order to get a more accurate measurement, the sensor may slow down in this range and even hold. The connection 15 to sensor controller 12 may be part of or supported by the linear guide 9. [0085] Sensor 8 may also advantageously be used on the wire web 3 when it is not rotating.

Lowering the workpiece 2 onto the wire web 3 can be used to find out in what position of the feed table 7 and the workpiece 2 first touches the wire web 3. In this way the cutting speed and speed of the feed table 7 may be controlled during the staring of the cutting. Moreover, since now it is known in what position the cutting starts, the speed or force of the feed table, the speed of the wire web, length of wire fed in one direction before changing direction and maybe even the supply of cutting fluid (all part of the cutting recipe) may be controlled in any desired manner.

[0086] It also becomes possible to determine the exact location the workpiece(s) are in. This information may be used for controlling the reciprocating movement of the sensor 12 and or the location where measurement are made. Also the complete length of the workpiece(s) has an influence on the best cutting recipe.

[0087] Figure 12 shows the same situation as shown in figure 5 but with two deflection measuring means 8, 8'. The sensor 8' is also connected to the controller unit 12 by a connection 15'. The sensors 8 and 8' may have independent drives (not shown), may be mounted on independent rails (not shown) or may have a common drive (not shown) and/or be mounted on a common rail 9. [0088] Figures 6 and 7 show the position the inventive sensor 13 can have above the wire web 3 and the direction of movement of the senor 8 inside the housing 10. The invention is not limited to any position though. The system 3 may be located above and/or under the wire web 3 (between wire guide rollers 4) that is used for cutting and on one or both side of the workpiece.

[0089] The deflection of the wire segments may influence the effectively measured distance since the measuring range now spreads over a portions of the wire segment that have different distances to the sensor 8. The direction the sensor 8 points in and thus measures may be corrected for this by inclining it. Preferably the sensor is inclined thus that it measures perpendicularly to the wire segments when they are in their cutting operation. Alternatively the sensor is inclined (or not inclined) so that it measures the most accurate when the most accurate measurements are needed. This may e.g. be when the zero point has to be detected. The sensor 8 may also be rotatable to be adjusted to the average (over all wires at a given moment or over time) deflection of the wire or to the purpose it is serving at any given time.

[0090] Figure 8 shows a method for determining the zero point: the position of the feed table 7, where the workpiece 2 first touches the wire web. Ideally the cutting process is commenced very close to this position. The method 100 comprising the steps of:

- 101 : forming a wire web on the wire saw

- 102: attaching the workpiece 2 to the feed table 7, normally using a fixture attachment 6 and sacrificial plate 5;

- 103: moving the feed table 7 with workpiece 2 towards the wire web 3;

- 104: Monitoring the wire web by moving the sensor over the wire web and measuring the

deflection of the wires; and

- 105: storing the position of the feed table as soon as it is detected that a wire segment 3', 3", 3"' are first depressed.

[0091] This method may be carried out while rotating the wire web 3 (making the wire guide rollers rotate as to achieve the cutting action of the wire web) and with the wire web 3 standing still.

Especially steps 101 , 102, 103 and 104 may be done or commenced in any particular order. If the wire web is rotating, the speed of the feed table may be adjusted as soon as the wires are depressed.

[0092] The cutting may be carried out by the following steps of method 200:

- 201 : moving the workpiece to a predetermined distance above the wire web, ideally near the zero point;

- 202: giving the wire web its initial speed for initiating the cutting;

- 203: pressing the workpiece through the wire web;

- 204: optionally changing the wire web speed to the normal cutting speed and/or controlling the cutting recipe based on the measured features/shape of the wire web.

[0093] The features controlling the cutting process or cutting recipe maybe any statistical quantity or be selected or calculated based on elements from the group consisting of:

maximal deflection of a wire or wire group, minimal deflection of a wire or wire group, average deflection of a wire or wire group, variance or standard deviation of the deflection of a wire or wire group, deviation of a wire in axial direction (direction the wire guide rollers extend in), missing wire segments or any combination thereof. [0094] The features being determined of the complete wire web or only of relevant wire segments e.g. those that actually cut the workpiece 2. This information can be obtained with method 400.

[0095] For accurate measurements a reference table may be made linking the actual deflection of the wire segments 3', 3", 3"' to what is measured by the sensor 8. This method for doing so containing the steps of:

- 301 : forming a wire web on the wire saw;

- 302: if desired attaching the workpiece 2 to the feed table 7, normally using a fixture attachment 6 and sacrificial plate 5;

- 303: moving the feed table possibly with workpiece towards the wire web;

- 304: moving the sensor over the wire web and measuring the deflection of the wires;

- 305: if at least a wire segment is deflected, storing the value measured by the sensor 8 at least for one wire segment, preferably for all relevant wire segments;

- 306: Moving the table 7 further over a predetermined distance to a measuring position;

- 307: Once the table has reached its measuring position, at least for one wire segment,

preferably for all relevant wire segments storing the value measured by the sensor 8.

Steps 306 and 307 are repeated for additional positions in the reference table.

[0096] In order to know how long the workpiece 2 being cut is and or how many workpieces 2 are attached to the feed table 7, a method 400 containing the following steps may be used:

- 401 : forming a wire web on the wire saw;

- 402: attaching the workpiece(s) 2 to the feed table 7, normally using a fixture attachment 6 and sacrificial plate 5;

- 403: moving the feed table towards the wire web;

- 404: pressing the workpiece into the wire web;

- 405: determining what wires are depressed. [0097] The thus determined information may be used to predict the number of wafers that will result from the workpiece(s), control the reciprocating movement of the sensor 8, determine the desired web speed, movement of the feed table 7, cutting load on the wire web 3, wear of the wire, control the measurements made etc.

[0098] The cutting of the wafers may be controlled by taking "images" of the wire web, meaning that the data on the wire segments is at least logically represented as an image, normally a 1 D image. On those images, known image processing techniques may be applied. [0099] After the cutting the cut work piece may be retracted form the wire web. The inventive sensor may be used for monitoring the condition of the wire web during this action as well. As shown in figure 13, the method 50 may contain the steps of:

- 501 : mounting a workpiece to the feed table;

- 502: forming the wire web;

- 503: moving the workpiece though the wire web while the web is rotating;

- 504: retracting the at least partially cut workpiece through the wire web, possibly while it is rotating;

- 505: monitoring the wire web with the inventive system while the workpiece is retracted. [00100] In all the above methods, steps may be omitted if not needed and their order may be altered if desired. Combining methods may also be done for optimization.

[00101] The invention is not restricted to the embodiments shown. Single or multiple combinations thereof are also possible.

List of reference marks

1. Wire saw

2. Workpiece

3. Wire web

3a, 3b Adjacent wires

3", 3", 3'" Wire segments

4. Wire guide roller

5. Sacrificial plate or beam

6. Fixture attachment

7. Feed table

8. Deflection measuring means

8'. Second deflection measuring means

9. Moving means

10. Housing

11. Outlet with connector plug

12 Sensor controller unit

13. Wire web monitoring system

14. Measuring range

14' Second measuring range

15. 15' Electrical connection to sensor

16 Crest of wire guide roller

17 Memory

d Distance to measurement area

M Direction of movement of the feed table

S Direction of movement of the sensor

S' Direction of movement of the second sensor

Claims

1. Wire saw (1), particularly a multi-wire saw, for cutting a workpiece (2) comprising:

- wire guide rollers (4) for forming a wire web (3) containing adjacent wire segments (3', 3", 3"') from at least one cutting wire (3), for cutting the workpiece (2);

- a feed table (7) for mounting the workpiece

(2) and moving it towards the wire web (3);

- a controller (12) for controlling the movement of the wire guide rollers and the feed table (7); and

- a wire web monitoring system (13) for monitoring wire segments (3', 3", 3"') of the wire web (3), containing:

deflection measuring means (8) for measuring the distance to or presence of at least one wire segment (3\ 3",

3"'),

characterized in that

moving means (9) are provided for moving the deflection measuring means (8) along the wire web, preferably over the wire web (3) in a direction mainly parallel to the wire web (3). 2. Wire saw (1) according to claim 1 , characterized in that the deflection measuring means (8) is located close to the wire web (3), preferably if the wire web (3) extends freely between the wire guide rollers (4), not further than 5mm away from the wire web (3) in a direction of

measurement, even more preferred not further than 4mm or even less than 3mm away.

Wire saw (1) according to claim 1 or 2, characterized in that the deflection measuring means (8)

is movable along a line parallel to the crest of a wire guide roller (4), the distance (d) between the crest and the centre of the measurement range of the deflection measuring means (8) being smaller than 6 cm, preferably smaller than 4 cm or even more preferred between 30 and 40 mm, the distance (d) being measured parallel to the wire web (3) when it extends freely between the wire guide rollers (4).

4. Wire saw (1) according to any of the preceding claims, characterized in that the deflection

measuring means (8) and the moving means (9) for moving the deflection measuring means (8) together are inside a housing (10), preferably a waterproof housing.

5. Wire saw (1) according to any of the preceding claims, characterized in that the deflection

measuring means (8) has a measuring range (14) wider than the distance between two adjacent wire segments (3', 3", 3"'), preferably wider than 10 times this distance or even more preferred wider than 20 times this distance.

6. Wire saw (1) according to any of the preceding claims, characterized in that the wire web

monitoring system (13) further comprises a memory (17) for storing measurement data from the deflection measuring means (8).

7. Wire saw (1) according to any of the preceding claims, characterized in that the deflection measuring means (8) is adapted to make a reciprocating movement, preferably with a scan frequency smaller than 5/min, even more preferred smaller than 2/min or even smaller than 1/min.

8. Wire saw (1) according to any of the preceding claims, characterized in that the deflection

measuring means (8) is adapted to make a measurement at least twice while moving from one wire segment to the next (pitch), preferably at least 3 times and even more preferred at least 4 times.

9. Wire saw (1) according to any of the preceding claims, characterized in that the wire web

monitoring system (13) comprises at least two deflection measuring means (8, 8') that preferably are associated with one moving means (9).

10. Wire saw (1) according to any of the preceding claims, characterized in the control unit being capable of any of setting an alarm, controlling the movement of the feed table , controlling the movement of the wire guide rollers and any combination thereof.

1 1. Method for operating a wire saw according to any of the previous claims, containing the steps of:

- Forming the wire web (101);

- Preferably mounting a workpiece to the feed table (102);

- Rotating the wire web or keeping it still;

- Moving the feed table towards the wire web (103);

- Pushing the feed table or the workpiece attached thereto against the wire web; and

- Monitoring the wire web while the feed table or workpiece touches it (104).

12. Method (200, 500) for using a wire saw according to any of the claims 1 to 9, containing the steps of: - Mounting a workpiece to the feed table (102);

- Forming and rotating the wire web (101 , 202);

- Moving the workpiece though the wire web (203);

- monitoring the wire web while the workpiece is moved (104, 505).

13. Method (500) for using a wire saw according to any of the claims 1 to 9, containing the steps of:

- Mounting a workpiece to the feed table;

- Forming the wire web;

- Moving the workpiece though the wire web while the web is rotating;

- retracting the cut workpiece though the wire web;

- monitoring the wire web while the workpiece is retracted.

14. Method according to claim 11 , further comprising at least one of the following steps:

- Determining the position of the feed table in which it or workpiece attached to it first touches the wire web (200);

- moving the feed table over a predetermined distance and determining the amount the web is pressed down (306, 307);

- determining the difference between the position of the feed table in which the first wire is touched and the last wire is touched;

- determining what wire segments are pressed down and what wire segments are not (405);

- determining the difference between the angles of at least two wires;

- determining the value the deflection sensor is measuring as the web is first pressed down (105);

- retracting the cut workpiece from between the wire segments (504);

- monitoring the wire segments with the wire web monitoring system (104);

- monitoring the speed of a wire segment in the direction perpendicular to the wire web;

- Recognizing when all relevant wire segments have returned to one plane;

- determining the absolute position wires relative to the wire saw;

- or any combination thereof.

15. Wire web monitoring system (13) to be used in any of the above wire saws (1) or methods (100, 200).