Method for simultaneously separating a plurality of wafers from a semiconductor ingot

Abstract

Method for the simultaneous separation of a plurality of slices from a semiconductor rod, wherein the semiconductor rod is moved in a wire saw while supplying a lapping suspension through a wire frame spanned by a saw wire, wherein the saw wire is guided through grooves of at least two wire guide rollers having a jacket surface, characterized in that a groove profile of a worn wire guide roller is determined by means of a stylus method in order to determine the wear of the wire guide roller and subsequently the wire guide roller is provided with renewed grooves, wherein the expected wear is taken into account during the regrooving by locally adapting the grooving.

Description

The invention relates to a method for simultaneously separating a plurality of wafers from a semiconductor rod.Semiconductor wafers are produced in a multiplicity of successive process steps, wherein in the first step a single crystal (rod) made of monocrystalline semiconductor material is pulled by the Czochralski method or by means of crucible pulling or a polycrystalline block made of semiconductor material is cast, and in a further step the resulting circular cylindrical or block-shaped semiconductor rod made of semiconductor material is cut into individual semiconductor wafers by means of wire sawing.In wire sawing, a distinction is made in principle between wire lapping and wire grinding. In wire lapping, a saw wire containing no bonded grain is used. The abrasive particles necessary for machining the semiconductor rod are introduced into the sawing gap via a lapping suspension (slurry). The slurry typically consists of polyethylene glycol (PEG) and SiC grains.In wire grinding, on the other hand, a saw wire is used which contains grain bonded on its surface. In wire grinding, water can be used as the cooling lubricant in the simplest case.An apparatus and method for wire separation lapping is disclosed, for example, in GB 717,874.Hereinafter, the term wire sawing is used regardless of whether the saw wire does not contain (lapping) or bonded grain (grinding).The essential components of a wire saw include a machine frame, a feed device and a sawing tool which consists of a gate (wire gate) of parallel wire sections.As a rule, the wire netting is formed by a plurality of parallel wire sections which are clamped between at least two wire guide rollers, wherein the wire guide rollers are rotatably mounted and at least one of which is driven.A wire guide roller is a roller-shaped body made of steel or composite plastics, the functional surface of which usually forms the shape of a straight circular cylinder. The functional surface usually comprises a covering made of a wear-resistant tough plastic, usually polyurethane, in particular thermosetting polyurethane. The polyurethane layer is provided with grooves which receive and guide the wire.A groove comprises a groove base and groove flanks. The wire comes to rest in the groove base and the groove flank "catches" the wire at its inlet and centers it in the groove, so that it does not jump over into one of the adjacent grooves. Wire guide rollers with V-shaped grooves are known in the prior art, for example from JP 2006102917 A.Furthermore, wire guide rollers are known which have a curved groove base. DE 102007019566 A1 discloses a wire guide roller for use in wire saws for simultaneously separating a plurality of disks from a cylindrical workpiece which is provided with a coating having a thickness of at least 2 mm and at most 7.5 mm and which is made of a material having a Shore A hardness of at least 60 and at most 99, which further includes a plurality of grooves through which the saw wire is guided, the grooves each having a curved groove base having a radius of curvature which is given by 0.25-1.6 times a saw wire diameter D and an opening angle of 60-130°. This invention thus describes grooves whose groove bases can be both narrower and equal to or wider than the saw wire. The special groove shape ensures optimum guidance of the wire, which improves the cut quality. The hardness and thickness of the covering minimize the wear of the wire guide roller.The longitudinal axes of the wire guide rollers are generally oriented perpendicular to the saw wire in the wire netting.The production of wafers from semiconductor material places particularly high requirements on the precision of the separation process. For this purpose, it is important that the plurality of grooves on the wire guide roller run exactly parallel and the grooves and the saw wire lie in one line (alignment).Both as a result of the heating of the wire guide roller and as a result of its wear, an alignment error can occur, i.e. the groove of the wire guide roller and the wire lying in this groove no longer lie on a straight line, i.e. the angle formed by the projected line of the groove of the wire guide roller and the direction of the free wire changes. This can lead to damage, for example, a formation of grooves, to the surfaces of the separated semiconductor wafers.DE 102 20 640 A1 describes a method for checking and optionally correcting the alignment of the saw wire with respect to the grooves of the wire guide rollers.As a result of the constant contact with the saw wire, both the coating of the wire guide rollers and the grooves cut into the coating are subject to wear and must be regularly renewed.For this purpose, according to the prior art, the wire guide roller must be removed from the wire saw and a new coating and or new grooves must be cut into the surface of the coating of the wire guide roller.German patent application DE 103 49 287 A1 discloses a wire guide roller for wire saws, characterized in that the wire guide roller has a base body and a removable coating carrier. The coating carrier can be sent without the base body for recoating and grooving.This entire coating and or milling process taking place outside the wire saw is time-consuming and leads to undesirably long downtime of the respective wire saw.It is therefore an object of the invention to provide an improved method for grooving the wire guide rollers of a wire saw for separating wafers from a semiconductor rod, in which method both the service lives of the wire guide rollers, i.e. the operating time, can be extended and the operation of the wire saw can thus be made more economical, and the quality of the wafers separated from a semiconductor rod can be improved.The object is achieved by a method for simultaneously separating a plurality of wafers from a semiconductor rod, wherein the semiconductor rod is moved in a wire saw while supplying a lapping suspension through a wire web spanned by saw wire, wherein the saw wire is guided through grooves of at least two wire guide rollers having a lateral surface, characterized in that a groove profile of a worn wire guide roller is determined by means of a tactile cut method in order to determine the wear of the wire guide roller and subsequently the wire guide roller is provided with renewed grooves, wherein the expected wear is taken into account during the rerilling by locally adjusting the grooving.During rerilling, the groove depth or the groove geometry or both should be adapted depending on wear.In some cases, it is necessary to provide the wire guide roller with a new outer surface before the rerilling.It is advantageous to remove worn grooves and unevennesses from the jacket of the wire guide roller before the rerilling.The rerilling can be carried out by means of a mechanical method or by means of an optical method or by means of a combination of a mechanical method and an optical method.In one embodiment, both the removal of worn grooves and asperities and the rerilling are performed using an optical method.It was important for the gelling of the invention to be able to determine the local wear of the wire guide rolls exactly.Instead of, as before, aiming at a uniform depth of the individual grooves over the entire wire guide roller, the invention provides deviations, for example at the ends of the wire guide roller (on the machine side and on the grip side).A wire guide roller is a generally cylindrical roller with a lateral surface and two side surfaces. The wire guide roller is axially rotatably mounted about a roller core. The individual grooves for the wire guide are cut or ground into the outer surface of the wire guide roller.In one embodiment, the jacket of the wire guide roller comprises a polyester-based or polyether-based polyurethane or a copolymer.As a rule, the axis of rotation of the wire guide roller is fastened on one side to the machine frame, the fixed bearing side or the machine side.The other end of the axis of rotation of the wire guide roller is referred to as the loose bearing side or grip side.The grooves along the axis of the wire guide roller may be equidistant or variably spaced from each other.Each groove consists of two groove flanks and the groove base, wherein the groove profile results from the respective depth of the groove base of a groove with respect to the circumferential surface of the wire guide roller.The depth of the grooves is preferably in a range of 50 to 1500 μm.The invention is based on the observation that the grooves introduced into the lateral surface, when a semiconductor rod is sawn by means of a wire saw, experience both a mechanically abrasive load and a certain plastic deformation as a result of the surface load of the lateral surface of the wire guide roller by the wire web.Both the machining of the individual grooves and the plastic deformation of the jacket surface of the wire guide roller lead to wear of the grooves cut into the jacket surface of the wire guide rollers, so that after a certain time the wire guide roller has to be rerilled or even the jacket surface has to be renewed if a renewed grooving of the jacket surface is no longer possible.A renewed grooving of the circumferential surface of the wire guide roller can for example no longer be possible if the thickness of the circumferential surface is no longer sufficient to be able to cut grooves of the desired depth into the circumferential surface.It has been found that the grooves located on the fixed bearing side are often worn out to a greater extent than those in the center of the wire guide roller, so that the wire guide roller only has to be rerilled on account of the grooves worn on the fixed bearing side.Moreover, the wear of the individual grooves is also dependent on the respective position of a wire guide roller in the wire saw.If the wire saw comprises, for example, four wire guide rollers, of which the two upper wire guide rollers span the wire web and two lower wire guide rollers, the wear of individual grooves on the fixed bearing side of the upper wire guide rollers can be different than the wear of individual grooves on the fixed bearing side of the lower rollers, for example. The same naturally applies to the movable bearing side.The invention ensures that the individual grooves are cut into the outer surface of the wire guide roller with different geometries or depths according to their machine-specific position-dependent wear behavior, so that the service life of the respective wire guide roller is extended in comparison to a wire guide roller with the same groove geometry.In one embodiment, the wire guide rolls are measured. Suitable methods for measuring grooves of a wire guide roller are tactile cutting methods and optical methods with parallel light.In the tactile cut method, in its simplest embodiment, a small tip is drawn horizontally over the wire guide roller. As the tip follows the groove profile, a cantilever moves vertically. The vertical position is recorded as a groove profile.This measuring method makes it possible to measure the geometry of all grooves along the longitudinal axis of a wire guide roller by means of a tactile cut. The evaluation of the probe cut yields the distance of the grooves from one another and the respective groove depth.This makes it possible to compare the groove profiles of newly grooved wire guide rolls, e.g. the wire guide rolls obtained from the manufacturer, with the groove profiles of wire guide rolls used. This allows the different wear of the wire guide rollers to be determined.The invention now provides for wire guide rollers to be grooved in such a way that they counteract the variable wear during wire sawing, in that the wire guide rollers have a correspondingly adapted groove profile at the locations at which the wear is higher. Specifically, the groove base is shallower, i.e. less deep, at these points.New wire guide rollers include grooves that have at least the same depth of cut over the entire length of the wire guide roller. The term depth of cut is understood to mean the vertical distance of the groove base from the circumferential surface of the wire guide roller.Detailed DescriptionIt is advantageous to measure new unused wire guide rolls in a first step, i.e. to determine a groove profile.Subsequently, the wire guide rollers are installed in the wire saw, the wire netting is clamped and a plurality of semiconductor rods are successively separated into a plurality of wafers. During each trip, i.e. the complete sawing of a semiconductor rod into a plurality of disks, the individual grooves wear out both by a mechanically abrasive load of the grooves by the saw wire and by a plastic deformation of the outer surface of the wire guide roller.The deterioration of the disc quality is an indicator of the wear of the grooves of the wire guide rollers. If this deterioration of the disk quality occurs, the worn wire guide rollers are removed from the wire saw and remeasured in a second step, i.e. a second groove profile is determined.Due to the different wear of individual grooves along the length of the wire guide roller, the groove base course is no longer uniform. In particular, the inlet grooves are worn out to a greater extent, the groove base is deeper than the other grooves.Finally, the at least two wire guide rollers that span the wire web are grooved in such a way that grooves that have higher wear as a function of position than other grooves are cut into the outer surface of the wire guide roller with a lower groove depth.The smaller cutting depth of these grooves compensates for the greater wear, i.e. the greater cutting of the saw wire into the groove base, of these grooves in comparison with the other grooves and increases the service life of the wire guide roller.For grooving the wire guide rolls, grinding, turning or removal tools are used. A removal tool can act either mechanically or optically on the lateral surface or the lateral surface of the wire guide roller. If the jacket of the wire guide roller is no longer suitable for a grooving process, because, for example, in the case of a wire guide roller consisting of at least two materials, the outer coating forming the jacket is too thin for renewed grooving, the outer coating, the jacket, is replaced. The wire guide rolls are then grooved.The minimum thickness D min of the outer coating of the wire guide roller for a last reduction process is, for example, dependent on the wire tension, the groove spacing, the groove depth and is in the range of approximately 0.5 to 1 mm.If the jacket of the wire guide roller is still suitable for a grooving process, the worn grooves and / or unevennesses are removed from the jacket of the wire guide roller by optical and / or mechanical methods. The jacket or jacket surface can be prepared both inside the wire saw, i.e. when the wire guide roller is installed, and outside the wire saw, i.e. when the wire guide roller is removed.Suitable optical methods for preparing the jacket are, for example, lasers which remove the worn grooves by melting or evaporating material.Suitable mechanical methods for preparing the jacket are, for example, grinding or machining methods in which the worn grooves are removed by material removal.A combination of mechanical and optical methods is also preferred, in order to realize, for example, a rough machining and a fine machining of the jacket and the jacket surface of the wire guide roller.The prepared jacket of the wire guide roller has, after the removal of the worn grooves, a jacket surface suitable for the subsequent grooving. The requirements for the shape, position, geometry and surface nature of the outer surface of the wire guide roller result from the subsequent processes (grooving and wire sawing) and can be set.Finally, the jacket of the wire guide roller is rerilled according to the determined position-dependent wear, i.e. a plurality of depressions are introduced into the jacket, which depressions run circularly around the entire jacket surface of the wire guide roller at a defined distance parallel to one another and ensure the clamping of the wire web at a parallel wire distance.In a likewise preferred embodiment of the method according to the invention, the two steps of preparing the lateral surface or the lateral surface and grooving are combined in one working step.In this case, both the preparation and the grooving of the cladding are preferably carried out using an optical method in that the pulse number and or the power of a laser are varied accordingly.Preferably, the wire saw itself or the wire guide roller has a marking which automatically establishes the zero point for the positioning of the first groove and thus the position of the outermost wire section of the wire web on the prepared or new outer coating by means of optical or electrical measurement. For the absolute and relative positioning of the cutting and shaping tool, linear axes with position measuring systems are suitable, for example.The first groove is positioned on the two wire guide rollers that are situated opposite one another in the horizontal plane in such a way that the saw wire that is later guided in the groove runs orthogonally to the respective axes of rotation of the wire guide rollers. This also ensures that the wire netting is cut into the semiconductor rod perpendicularly with respect to the longitudinal axis of the semiconductor rod to be cut.The grooves for the subsequent wire guiding are preferably introduced mechanically or optically by means of a laser into the jacket of the wire guiding rollers. In the following, both in mechanical and in optical methods, the term "brilliant tool" is used synonymous with the respective device (optical or mechanical) for preparing and or for introducing or cutting the grooves.For the purposes of introducing or cutting in the grooves, the reducing tool is positioned and fed on a precise feed and positioning axis, for example a ball screw with displacement measuring system. While this wire guide roller rotates about its longitudinal axis at a defined speed, the straightening tool mechanically or optically cuts a groove into the circumferential surface of the wire guide roller. In optical cutting, for example with a laser beam, the groove can be cut at various angles of tangential to 90° to the surface of the wire guide roller, as required.With the aid of a control program which takes into account the position-dependent wear of the individual grooves, the movements of the removal tool and the milling tool are controlled in such a way that further grooves are cut with the desired spacings, the desired depth and the desired groove shape in the circumferential surface of the first wire guide roller. The term groove shape comprises both the radius r of the groove base and the angle formed by the two groove flanks.The cutting and feed rate of the mechanical or optical brillianter tool and the speed of rotation of the wire guide roller during grooving depend on the cutting tool used, the groove geometry and the material of the outer coating.After the first wire guide roller is rerilled, the at least second wire guide roller isrilled according to the method described above.Likewise preferred is the simultaneous wear-predicting grooving (including the preceding preparation of the jacket or the jacket surface) of at least two wire guide rollers installed in a wire saw.The wear-predictive grooving of the wire guide rollers according to the invention brings about a respectively optimized groove depth of the individual grooves, so that the different wear of individual grooves, for example a greater wear in the edge region of the wire guide rollers, is counteracted in a targeted manner and the service life of the wire guide rollers in the wire saw is thus increased.Since the shape of the worn grooves can also be determined by the sensing method used, the geometry of the groove can also be adapted to the respective wear during the renewed grooving. If, for example, the flank of one groove is worn out to a greater extent than in the case of another groove, the method according to the invention allows the angles of the flanks to be adapted in accordance with the respective wear during renewed grooving.The features described with respect to the above-mentioned embodiments of the invention can be realized either separately or in combination as embodiments of the invention. Furthermore, they can describe advantageous embodiments that are independently protectable.FIGS. FiguresList of Reference Numerals Used1 Wire guide roller having an outer lateral surface containing guide grooves 2a flanks of a groove 2b groove base 2c surface of the wire guide roller 3a saw wire 3 wire netting, consisting of a plurality of wire sections 4 formed from the saw wire 3a and arranged parallel to one another slurry 5 semiconductor rodFIG. 1 shows a wire saw according to the prior art, comprising two wire guide rollers 1, between which a saw wire 3 ais clamped to form a wire netting 3 consisting of parallel wire sections, which saw netting is sawn into a semiconductor rod 5 fastened to a semiconductor rod receptacle (not shown). For reasons of clarity, the wire saw is greatly simplified and is shown, for example, without deflection rollers. FIG. 2 shows a cross-section of a single guide groove of a wire guide roller 1. The groove comprises two groove flanks 2 aand a curved groove base 2 b. The groove flanks form an angle a. During wire sawing, the saw wire 3 ais located in the groove and guided through it. The lapping suspension 4 is also shown. The perpendicular distance between the groove base 2 band the surface of the wire guide roller 2 ccorresponds to the groove depth and is, for example, 170 μm.Examples:Example 1 - Determination of Groove ProfilesUnused Wire Guide RollerThe depth of the individual guide grooves is the same over the entire length of the wire guide roller, i.e. the jacket of the wire guide roller has a uniform groove base course.Worn Wire Guide RollerDue to the different wear of individual grooves along the length of the wire guide roller, the groove base course is no longer uniform. In particular, the inlet grooves are worn out to a greater extent, wherein the groove base of the inlet grooves is deeper than in the case of the other grooves.Example 2 - Measurements and RerillingAfter a defined number of passes, i.e. a defined number of sawn semiconductor rods, the wear of the wire guide rollers was measured. It was found that the machine-side grooves were cut into the jacket to a depth of up to 0.2 mm than the movable-bearing-side grooves.The wear of the grooves decreased continuously from the machine side toward the idle bearing side. Accordingly, when this wire guide roller was again grooved, the grooves on the machine side were cut into the jacket to a depth of up to 0.2 mm than in the case of the loose bearing side.The grooves on the movable bearing side of the wire guide roller are cut into the jacket to a lesser extent than on the machine side, so that the higher wear of the grooves on the machine side assumed in the example is compensated for by the lower groove depth of the newly grooved wire guide roller and both the service life of the wire guide roller and a better quality of the sawn wafers are achieved.

Claims

Method for simultaneously separating a plurality of wafers from a semiconductor rod, wherein the semiconductor rod is moved in a wire saw while supplying a lapping suspension through a wire gate spanned by saw wire, wherein the saw wire is guided through grooves of at least two wire guide rollers having a lateral surface, characterized in that a groove profile of a worn wire guide roller is determined by means of a tactile cut method in order to determine the wear of the wire guide roller and subsequently the wire guide roller is provided with renewed grooves, wherein the expected wear is taken into account during the rerilling by locally adapting the grooving.Method according to claim 1, wherein during rerilling the groove depth or the groove geometry are adapted depending on wear.Method according to claim 1 or according to claim 2, wherein the wire guide roller is provided with a new circumferential surface before the rerilling.Method according to one of Claims 1 to 3, worn grooves and unevennesses being removed from the jacket of the wire guide roller before the rerilling.The method according to any one of claims 1 to 4, wherein the rerilling is performed by a mechanical method or by an optical method or by a combination of a mechanical method and an optical method.Method according to one of Claims 1 to 5, wherein the removal of worn grooves and unevennesses and the rerilling are carried out by means of an optical method.

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