EP3976335A1

EP3976335A1 - Method for separating a plurality of slices from workpieces during a number of separating processes by means of a wire saw, and semiconductor wafer made of monocrystalline silicon

Info

Publication number: EP3976335A1
Application number: EP20722431.2A
Authority: EP
Inventors: Georg Pietsch; Peter Wiesner
Original assignee: Siltronic AG
Current assignee: Siltronic AG
Priority date: 2019-05-27
Filing date: 2020-04-29
Publication date: 2022-04-06
Also published as: TW202042941A; WO2020239348A1; TWI760753B; TWI830046B; KR20220014877A; TW202144107A; US20220234250A1; CN113891790A; CN214026490U; CN111993614A; DE102019207719A1; CN111993614B; SG11202113089RA; JP2022538517A

Abstract

The invention relates to a method for separating a plurality of slices from workpieces during a number of separating processes by means of a wire saw, which comprises a wire gate formed of moving wire sections of a saw wire, which is clamped between two wire guide rollers, wherein each of the wire guide rollers is mounted between a fixed bearing and a floating bearing. The invention also relates to a semiconductor wafer formed of monocrystalline silicon, which is provided using the method. The method comprises delivering one of the workpieces in the presence of a working fluid during each of the separating processes along a delivery direction against the wire gate in the presence of hard materials, which act abrasively on the workpiece; controlling the temperature of the fixed bearing of the respective wire guide roller during the separating processes according to a temperature profile, which specifies a temperature according to a cutting depth; and a first changing of the temperature profile over the course of the separating processes from a first temperature profile with a constant temperature curve to a second temperature profile that is proportional to the difference between a first average shape profile and a shape profile of a reference slice, wherein the first average shape profile is determined by slices that have been separated according to the first temperature profile.

Description

PROCESS FOR SEPARATING A VARIETY OF DISCS FROM WORKPIECES DURING A NUMBER OF CUT-OFF PROCESSES USING A WIRE SAW AND A SEMI-CYCLING WASHER

SILICON

5

The invention relates to a method for separating a large number of

Slicing of workpieces during a number of severing operations using a wire saw, which creates a wire mesh made of moving wire sections of a

Includes saw wire, which is stretched between two wire guide rollers, each 10 of the wire guide rollers is mounted between a fixed bearing and a floating bearing.

The invention also relates to a semiconductor wafer made of monocrystalline silicon, which is made accessible by the method.

State of the art

15th

For many applications, thin, uniform slices of material are required. An example of wafers which are subject to particularly high requirements with regard to uniformity and plane parallelism of their respective front and rear sides are wafers made of semiconductor material, which are used as substrates for the production of microelectronic components. Particular importance to

Production of such discs has the wire sawing, in which one at the same time

Large number of discs can be separated from a workpiece, as it is particularly economical.

25 In wire sawing, the saw wire is spiraled around at least two

Wire guide rollers led around that on the side facing the workpiece to be cut and glued to a retaining bar, two adjacent ones

Wire guide rollers a wire mesh is spanned from mutually parallel sections of the saw wire. The wire guide rollers have the shape of

30 circular cylinders, the axes of these circular cylinders are parallel to each other

arranged, and the outer surfaces of the wire guide rollers have a coating made of a wear-resistant material, which is closed with a ring and in

Planes is provided perpendicular to the wire guide roller axis extending grooves that guide the saw wire. Rotate the wire guide rollers in the same direction around their Cylinder axes cause the wire sections of the wire mesh to move relative to the workpiece, and by bringing the workpiece and the wire mesh into contact in the presence of an abrasive, the wire sections thus cause material to be removed. As the workpiece is continuously fed in, the wire sections form separating gaps in the workpiece and work their way through the workpiece until they all come to rest in the holding bar. The workpiece is then cut into a multitude of uniform disks which, like the teeth of a comb, hang from the holding bar by means of the adhesive joint. Wire saws and methods of wire sawing are known, for example, from DE 10 2016 211 883 A1 or DE 10 2013 219 468 A1.

Wire sawing can be done using abrasive lapping or abrasive cutting. At the

Separating lapping is supplied to the space between the wire surface and the workpiece in the form of a slurry of hard materials (slurry). During lapping, the material is removed by means of a three-body interaction of saw wire, hard materials and workpiece. In cutting-off grinding, saw wire is used in the surface of which hard materials are firmly bound, and a working fluid is supplied that does not itself contain any abrasive substances and which acts as a cooling lubricant. During cut-off grinding, the material is removed by means of a two-body interaction of saw wire with bonded hard materials and the workpiece.

The saw wire is mostly piano wire made of, for example, hypereutectoid

pearlitic steel. The hard materials of the slurry consist, for example, of silicon carbide (SiC) in a viscous carrier liquid, for example glycol or oil. The bonded hard material consists, for example, of diamond, which is positively and non-positively connected to the wire surface by galvanic nickel or synthetic resin bonding or by rolling.

Smooth or structured saw wire is used for separating lapping

Cut-off grinding only smooth saw wire. A smooth saw wire has the shape of a circular cylinder of very great height (namely the length of the wire). A structured saw wire is a smooth wire that has been provided with a large number of bulges and indentations along its entire length in directions perpendicular to the longitudinal direction of the wire. W013053622 A1 describes an example of smooth saw wire for separating lapping, an example of structured saw wire for separating lapping US9610641 BB and an example for smooth saw wire with diamond coating for

Cut-off grinding is described in US 7 926 478 B2.

In conventional wire saws, each of the wire guide rollers is in the vicinity of one of its end faces with a bearing which is firmly connected to the machine frame and is referred to as a fixed bearing, and in the vicinity of the opposite one

End face with a bearing that is movable in the axial direction of the wire guide roller and is referred to as a floating bearing. This is necessary to avoid mechanical overdetermination of the structure, which leads to unpredictable deformations.

In particular at the moment of the first contact of the wire mesh with the workpiece, i.e. when sawing in, there is an abrupt mechanical and thermal load change. The arrangement of wire mesh and workpiece to one another changes, and the component of this change in the direction of

Wire guide roller axles mean that the separating gaps, the sides of which form the front and back of adjacent panes, deviate from their planes perpendicular to the wire guide roller axles, so the panes become wavy. Wavy washers are unsuitable for demanding applications.

Methods are known which aim to improve the plane parallelism of the main surfaces of the disks obtained by wire saws.

A method is known from US Pat. No. 5,377,568 in which the position of a reference surface located on the outside of the wire guide roller is measured parallel to and in the vicinity of the floating bearing face relative to the machine frame and, by means of temperature control of the wire guide roller interior, a thermal increase or decrease in length of the Wire guide roller is effected until the measured

Change in position of the reference surface is balanced again. The positions of the wire sections of the wire mesh shift when the

Wire guide roller in the axial direction at best proportional to its distance from the fixed bearing. However, the wire guide roller actually heats up

unevenly, as it is heated from the outside (unevenly) (thermal load change) and cooled from the inside, but the radial heat conduction in the wire guide roller due to its construction - not least due to the cooling labyrinth itself - not for each axis position is identical, so that the expansion of the wire guide roller along its axis takes place unevenly.

From JP 2003 145 406 A2 a method is known in which an eddy current sensor measures the position of a point on the outside of a wire guide roller and the temperature of the cooling water that controls the inside of the wire guide roller is changed in accordance with this position measurement. The method does not adequately capture changes in the arrangement from workpiece to wire mesh as a result of thermal or mechanical load changes.

A method for wire sawing is known from KR 101 340 199 B1 in which

Wire guide rollers are used, each of which is rotatably mounted on a hollow shaft, the hollow shaft being heated or cooled in several sections at different temperatures and thus being able to be expanded or contracted in sections in the axial direction. As a result, the length of the wire guide roller in the axial direction is changed non-linearly (non-uniformly), at least for a few sectors. However, the method does not take sufficient account of changes in the arrangement of the workpiece and wire mesh as a result of thermal or mechanical load changes.

From US 2012/0240915 A1 a method for wire sawing is known in which

Wire guide rollers are used, the interior of which and one of their bearings, which rotate the wire guide rollers, are temperature-controlled independently of one another by means of a cooling liquid. However, the method does not take into account that thermal and mechanical deformation of the structural elements of the wire saw are not constant and reproducible and that time-dependent deformations are not taken into account

Disturbances also act.

Finally, from WO 2013/079683 A1, a method for sawing wire is known in which the shapes of disks resulting for different temperatures of the wire guide roller bearings are first measured and each of these shapes is stored with the respective associated bearing temperature and then in the subsequent section the

Storage temperature is selected so that it corresponds to the selection of stored shapes that best match the desired target shape. This procedure does not take into account the extent and behavior of the thermal response of the wire saw change from cut to cut according to a drift and time-variable disturbance variables act according to a noise. The mechanical load change that occurs when wire sawing is also not taken into account.

In particular, wafers made of semiconductor material are often subjected to further processing steps after the wire sawing. Such processing steps can be the grinding of the front and back (sequential or simultaneous on both sides), the lapping of the front and back (simultaneously on both sides), the etching of the

Semiconductor wafer and the polishing of the front and back (mostly carried out as a sequential or simultaneous coarse polishing on both sides and as a fine polishing on one side). The unilateral or sequential bilateral processing methods have in common that one side of the semiconductor wafers is held in a clamping device, for example by means of a vacuum table (vacuum chuck), while the opposite side is processed.

The thickness of a semiconductor wafer is usually small compared to yours

Diameter. When clamping, a semiconductor wafer is therefore elastically deformed in such a way that the wafer is deformed by forces (load on the processing tool and clamping forces, for example as a result of the applied vacuum) and

Resetting deformation forces (tensioning of the disk) are balanced: The side of the semiconductor disk that is held clings to the clamping device. After material has been removed from the machined side and the semiconductor wafer has been released from the clamping device, the semiconductor wafer, which has become thinner due to the machining, relaxes into its original shape. In other words, subsequent processing steps generally do not improve the degree of plane parallelism of the front and back.

The object of the present invention is to overcome the

problems outlined by providing a method which takes better account of the change in the arrangement of workpiece to wire mesh as a result of thermal or mechanical load changes and discs with less

Ripple supplies.

Detailed description of the invention The object is achieved by a method for severing a plurality of slices of workpieces during a number of severing operations by means of a wire saw, which comprises a wire mesh from moving wire sections

Includes saw wire, which is stretched between two wire guide rollers, each of the wire guide rollers is mounted between a fixed bearing and a floating bearing, the method comprising

the infeed of one of the workpieces in the presence of a cooling lubricant during each of the severing processes along a infeed direction against the wire mesh in the presence of hard materials which act abrasively on the workpiece;

the temperature control of the fixed bearing of the respective wire guide roller during the separation processes according to a temperature profile that is a temperature in

Specifies the dependency of a cutting depth;

a first change in the temperature profile in the course of the separation processes from a first temperature profile with a constant temperature profile to a second temperature profile that is proportional to the difference between a first average shape profile and a shape profile of a reference disk, the first

Average shape profile of panes is determined, which have been separated according to the first temperature profile.

Disks that are separated from a workpiece by the method according to the invention are almost unaffected by the axial movements of the

Wire guide rollers as a result of thermal expansion of the fixed bearings. As a result, the deviation in shape of such disks from a reference disk is minimized.

The fixed bearing can be tempered, for example, by means of resistance heating or by means of one or more Peltier cooling elements. Particularly preferred, however, is temperature control of the fixed bearing by guiding a liquid through the fixed bearing of the respective wire guide roller during the separation processes, the temperature of the liquid for each of the separation processes following a temperature profile that specifies the temperature of the liquid as a function of the depth of cut.

Representing the other configurations, the further description of the method is directed to this preferred configuration of the invention.

A further change of the temperature profile to a further temperature profile is preferably provided. The further temperature profile is proportional to Difference of a further average shape profile of previously separated panes and the shape profile of the reference pane, the previously separated panes originating from at least 1 to 5 separation processes that immediately preceded a current separation process.

Determining the first average shape profile and the rest

Average shape profile can be performed based on a slice-related selection of slices. In the case of a pane-related selection, certain panes of a separation process are used to determine the respective

Average shape profile used by averaging and others

locked out. For example, only such discs are used in the

Averaging takes into account that have a certain position in the workpiece, approximately only every 15th to 25th slice along the length of the workpiece. Another option for the pane-related selection is the exclusion of panes with the greatest and the smallest deviation of the shape profile from the average shape profile of all panes in the cutting process. Alternatively, disks can also be excluded from the averaging whose shape profile is from

Average shape profile of all disks in the separation process deviates by more than 1 to 2 sigma.

Instead, the further average shape profile can also be determined based on a section-related selection of slices. In the case of a cut-related selection, all slices are from at least one

Separation process used to determine a further average shape profile by averaging and all panes excluded from at least one other separation process.

In addition, the further average shape profile can be determined based on a slice-related and a cut-related selection. In this case, at least one of the preceding separation processes is selected and at least one of the preceding separation processes

excluded, and at the same time certain slices are selected from the selected separation processes and others are excluded and the slices selected in this way are used for averaging. Definitions helpful to an understanding of the present invention, as well as considerations and observations which have led to the invention, are covered in the following sections of this specification.

The surface of a disc consists of the front, back and edge. The center of the disk is its center of mass.

The “regression plane” of a disc is the plane to which the sum of the distances of all points on the front and back is minimal.

The “median area” of a disc is the set of center points of all the lines that connect pairs of points lying mirror-symmetrically to the compensation plane, one of which is on the front and one on the

Back is located.

There is an “area-related thickness error” of a pane if the lengths of these lines change with the location on the front and the rear.

A “surface-related form error” of a disk is present if the central surface deviates from the compensation plane.

"Reference disk" is a disk without area-related thickness errors and without area-related form errors. A disc with a certain thickness profile or a certain shape profile over the location on the front and back can also be selected as a reference disc, if a wedge-shaped or spherical disc, for example, is desired as the target of bar cutting by wire saws. A convex disk is advantageous, for example, if the convexity counteracts a change in shape by subsequent application of a tensioned layer to the front (e.g. epitaxial layer) or rear (e.g. protective oxide). “Infeed direction” is the direction in which the workpiece is infeed onto the wire mesh. The "area-related thickness profile" of a pane describes the thickness of a pane as a function of the location on the compensation plane.

The “center line” of a disk is the line in the central surface that extends through the center of the disk in the feed direction.

The “thickness profile” of a pane is the thickness of the pane as a function of its location on the center line.

“Depth of cut” is a location on the center line and describes the extent of the

Separation gap in the infeed direction during the separation process. The “shape profile” of a disc is the course of the center line relative to the course of the center line of a reference disc. The course of the center line is determined at measuring points along the cutting depth.

"Average shape profile" is a shape profile obtained by averaging the shape profiles of several panes, whereby each shape profile is weighted equally (arithmetic averaging) or the shape profile of certain panes is specially weighted (weighted averaging) due to their position in the workpiece.

"Shape deviation" means the deviation of a shape profile from one

Target shape profile, for example from the shape profile of a reference disk.

"Temperature profile" is the course of the temperature of a liquid as a function of the depth of cut, whereby the liquid is used during the separation process

Tempering of the fixed bearing is passed through the fixed bearing of the respective wire guide roller of the wire frame. If necessary, the temperature control of the fixed bearing causes an expansion or contraction of the fixed bearing, the axial component of which is the floating bearing so the axial position of the associated wire guide roller along the

The axis of rotation of the wire guide roller moves. This movement of the

Wire guide roller then counteracts the occurrence of a form deviation.

The shape of any disk can always be made up of a combination

Thickness profile and shape profile are described. TTV (total thickness variation, GBIR) is a key figure that denotes the difference between the largest and the smallest value of the area-related thickness profile. Warp is a key figure describing the shape deviation, which denotes the sum of the greatest distances in each case that the compensation surface has to the central surface in the direction of the front of the pane and in the direction of the rear of the pane. Bow is another such characteristic number and denotes the distance between the compensation plane and the central surface in the center of the disc. Another variable that describes the form deviation is the waviness. It can be quantified as the waviness number Wav _red and is determined on the basis of a waviness profile which is derived from the shape profile. Within a measuring window of a predetermined length, the characteristic wavelength, the maximum of the distance is determined that the measuring points of the shape profile have to the compensation plane. The beginning of the measuring window is moved along the cutting depth from measuring point to measuring point of the shape profile and the

Determination of the maximum of the distance for each position of the measuring window repeated. The amount of maxima determined in this way, plotted against the positions of the respective associated measurement window, results in a profile of the waviness as a function of the cutting depth with respect to the characteristic wavelength, the

Waviness profile. The waviness number Wav _red is a measure of the reduced linear waviness and designates the maximum value of the waviness profile, with values of areas of a given length at the start and end of the cut

go unnoticed. In principle, the characteristic wavelength and the lengths of the areas that are not considered can be freely selected. The

The characteristic wavelength is preferably 2 mm to 50 mm and the predetermined lengths of the areas that are not taken into account are preferably 5 mm to 25 mm each. In connection with the yet to be described

The semiconductor wafer according to the invention is based on a characteristic wavelength of 10 mm and lengths of the areas that are not taken into account of 20 mm each.

The observations mentioned relate to the separating lapping of a straight circular cylindrical rod made of silicon into wafers with a diameter of 300 mm. However, they apply equally to workpieces with a different shape and to cut-off grinding. The surface of a straight circular cylinder comprises its circular base area (first end face), its top surface congruent to the base area (second end face opposite the first) and its

Lateral surface (set of points on the member with the maximum distance from the member axis). A straight circular cylinder has a rod axis that is perpendicular to the base and top surface and runs through the center points of the same. The distance between the base and top surface along this rod axis is called the height of the cylinder.

First, it was observed that the thickness profiles and shape profiles of disks with positions on the rod axis that are close to one another differ only slightly from one another. The thickness profiles of disks with positions on the rod axis that are further apart are similar, but the

Shape profiles of such disks differ greatly from one another. As a result, there cannot be a temperature profile which can be used to make the shape of all disks of a workpiece flat at the same time. By shifting the workpiece relative to the wire mesh depending on the cutting depth during During the separation process, only panes with an approximately flat shape can be obtained.

Second, it was observed that the shape profiles of disks with the same

Positions on the rod axis and obtained by immediately successive cutting processes usually differ only slightly from one another, while those of such disks with the same positions but obtained by

Separation processes, between which several intermediate separation processes are carried out, differ considerably from one another. Hence it cannot

Provide a temperature profile which, when applied and maintained, the shape of disks with the same rod position and resulting from successive cutting processes remains unchanged over many cutting processes.

Instead, the temperature profile may have to be changed at least slightly from separation process to separation process in order to include panes

to be able to obtain approximately flat shape over a multitude of separation processes.

Thirdly, it was observed that the change in the shape profiles of identically positioned panes, obtained through successive cutting processes, can be divided into a steady, predictable part and a discontinuous spontaneous part. A pre-calculated temperature profile will consequently only be able to take into account the steady, predictable portion of the change, and despite the application of the temperature profile, a change in shape will be determined which varies in type and extent from separation process to separation process and is not predictable.

Fourth, it was observed that the relative arrangement of workpiece and

Wire mesh especially at the moment of the incision, i.e. at the moment of the first contact of the workpiece with the wire mesh, but also over the whole

Disconnection process, is subject to a strong thermal and mechanical load change. In particular, it was found that when the saw wire is cut into the workpiece, a heat output of a few kW is applied to the workpiece

Wire guide rollers and their bearings is transferred and the wire guide rollers are subjected to a mechanical load change with a force in the range of 10 kN in the transverse direction during a separation process. Fifthly, it was observed that the mechanical load change leads to an increase in the friction in the bearings via which the wire guide rollers are connected to the

Machine frame are connected. On the one hand, the rolling friction of the rolling elements increases as a result of the increased axial load; on the other hand, it increases

Friction as a result of the axis of the bearing bushes being tilted relative to the axis that the wire guide roller has in the unloaded state. This tilting leads to flexing of the bearing bush in the sleeve connected to the machine frame into which the bearing bush is fitted. This flexing work leads to heating at the bearing bush / sleeve transition.

Consequently, the change in storage temperature and the associated

Expansion of the bearing, in particular in the axial direction, can be used to adjust the axial position of the wire guide rollers to reduce the heating and the associated axial position change to a desired level by means of a cooling acting in the vicinity of the outer circumference of the bearing sleeve.

Sixth, it was observed that the warming of the fixed bearing of the

Wire guide roller of a wire saw due to increased bearing friction or deformation (heating by flexing) to a shift in the position of

Wire guide roller leads in the axis position relative to the machine frame.

Seventhly, it was observed that wire sawing produces disks with undulations which are particularly pronounced in the infeed direction and that such undulations with lateral wavelengths in the range of around 10 mm are caused by wire sawing

subsequent processing steps can practically not be reduced. The waviness of a completely machined disc is therefore already largely determined in this respect by the wire saw.

Against the background of these observations, it is proposed that, in the course of a number of severing processes, a sequence of

To provide separation processes that differ in that the

Temperature profile, which specifies the temperature of the liquid that is passed through the fixed bearing of the respective wire guide roller of the wire frame,

is different. The sequence of separation processes expediently begins after a change in the sawing system, i.e. after a change in at least one feature of the wire saw, the saw wire or the cooling lubricant. The sawing system has changed, for example, when a change from

Wire guide rollers took place or mechanical adjustments were made to the wire saw. The first separation processes of the sequence, the so-called initial cuts, preferably consist of 1 to 5 separation processes. These

Separation processes are carried out according to a first temperature profile

performed, which specifies a constant temperature profile during the engagement of the wire sections in the workpiece.

Shape profiles are determined from all slices of the initial cuts or from slices of a slice-related selection of the slices of the initial cuts. A first average shape profile is determined from the shape profiles by averaging, which can optionally be weighted. The first average shape profile is then compared with the shape profile of a reference disk by subtracting the shape profile of the reference disk from the first average shape profile. The form deviation determined in this way corresponds approximately to an expected form deviation, the panes of a subsequent separation process

would have on average if the subsequent separation process were carried out in accordance with the first temperature profile.

The form deviation determined therefore serves as a benchmark for a

Corrective action that is directed against the expected form deviation. The separation processes following the initial cuts are therefore not carried out using the first temperature profile, but rather using a second temperature profile that is proportional to the form deviation determined. If the detected shape deviation indicates, for example, that maintaining the first temperature profile would result in slices whose center line would be offset by a certain amount on average in an axial direction of the wire guide rollers at a certain cutting depth, the second temperature profile provides a temperature of the liquid at the corresponding cutting depth which has the consequence that the fixed bearing due to thermal expansion, the wire guide roller assigned to it by the same amount in the opposite direction

shifts. By tempering the respective fixed bearing in accordance with the second temperature profile, the otherwise expected form deviation counteracted. The separation processes of the sequence following the initial cuts are therefore carried out in accordance with the second temperature profile, and the temperature profile is thus changed for the first time. The number of second separation processes in the sequence is, if no further change in the temperature profile is provided, preferably 1 to 15 separation processes. In principle, however, all cutting processes that follow the first change in the temperature profile can also be carried out using the second temperature profile, at least until a change in the sawing system occurs.

However, the number of separation processes that the

Initial cuts follow and which are carried out using the second temperature profile, to a number of 1 to 5 separation processes and to limit all further separation processes at least until a change occurs

Sawing system using a further temperature profile. The further temperature profile is determined anew before each of the further separation processes.

Of all the panes, the 1 to 5 cutting processes immediately preceding the respective current cutting process of the further cutting processes or of panes of a wafer-related selection of these panes or one

Cut-related selection of these slices or a slice-related and cut-related selection of these slices, shape profiles are determined. A further average shape profile is determined from the shape profiles by averaging, which can optionally be weighted, before the current separation process. The further average shape profile is then combined with the shape profile of the

The reference disk is compared by subtracting the shape profile of the reference disk from the further average shape profile. On the basis of the established

Shape deviation, a further temperature profile is determined which is proportional to the shape deviation determined. The current cutting process is shown under

Application of the further temperature profile carried out. For each subsequent separation process, a further temperature profile is determined in an analogous manner. In other words, after the number of 1 to 5 disconnections that the

Following initial cuts, the temperature profile is changed with each subsequent cutting process. A semiconductor wafer which is produced by a method according to the invention and optionally has a polished front and rear side after subsequent processing steps is characterized by particularly low waviness. The invention therefore also relates to a semiconductor wafer made of single-crystal silicon, which is characterized by a waviness number Wav _red of not more than 7 pm, preferably not more than 3 pm, provided the diameter of the

Semiconductor wafer is 300 mm, or characterized by a waviness number Wav _red of not more than 4.5 pm, preferably not more than 2 pm, provided the diameter of the semiconductor wafer is 200 mm. The characteristic

The wavelength for determining Wav _red is 10 mm and the lengths of the areas not taken into account at the start of the cut (incision) and at

The end of the cut (cutout) is 20 mm each. An inventive

Semiconductor wafer already has the waviness number Wav _red in the stressed area in the sawn state, i.e. in the unpolished state.

The method according to the invention is fundamentally independent of the material of the workpiece. However, it is particularly suitable for cutting off wafers made of semiconductor material and is preferably used for cutting off wafers

single crystal silicon used. Accordingly, a workpiece has

preferably the shape of a straight circular cylinder with a diameter of at least 200 mm, preferably at least 300 mm. However, other shapes, such as a cuboid or a straight prism, are also possible. The method is also independent of the number of wire guide rollers

Wire saw. In addition to the two wire guide rollers, between which the wire mesh is stretched, one or more further wire guide rollers can be provided.

The disks are severed during a severing process by

Cut-off grinding by supplying a cooling lubricant to the wire sections, which is free of substances that have an abrasive effect on the workpiece, or by lapping cutting while supplying a cooling lubricant to the wire sections, which consists of a slurry of hard materials. In the case of cut-off grinding, the hard materials preferably consist of diamond and are fixed on the surface of the saw wire by galvanic bonding or by bonding with synthetic resin or by form-fitting bonding. In the case of the separating lobe, the hard materials preferably consist of silicon carbide and are preferably in glycol or oil slurried. The saw wire preferably has a diameter of 70 gm to 175 gm and is preferably made of hypereutectoid pearlitic steel. Furthermore, the saw wire can be provided along its longitudinal axis with a multiplicity of protuberances and indentations in directions perpendicular to the longitudinal axis.

Furthermore, it is preferred to move the saw wire during a cutting process in a continuous sequence of pairs of direction reversals, with a pair of direction reversals each moving the saw wire in a first longitudinal direction of the wire by a first length and a second subsequent movement of the saw wire in a second Wire longitudinal direction comprises a second length, the second wire longitudinal direction of the first wire longitudinal direction

is opposite and the first length is greater than the second length.

Preferably, the saw wire when moving the first length is the

Wire mesh with a first tensile force in the longitudinal direction of the wire from a first

Wire supply is supplied and, when moving by the second length, is supplied with a second tensile force in the longitudinal direction of the wire from a second wire supply, the second tensile force being less than the first tensile force.

Details of the invention are explained below with reference to drawings.

Brief description of the figures

Fig. 1 shows a perspective view of features that are typical of a wire saw.

Fig. 2 shows a sectional view through a wire guide roller and its

Storage.

3 show the shape profile and the waviness profile (upper diagram) of a disk which was not produced according to the invention, and the temperature profile (lower diagram) which was used during the separation process not according to the invention.

4 show the shape profile and the waviness profile (upper diagram) of a disk which was produced according to the invention, and the temperature profile (lower diagram) Diagram) which was used during the separation process carried out according to the invention.

List of reference symbols used

1 wire guide roller

2 wire gates

3 saw wire

4 workpiece

5 fixed bearings

6 floating bearings

7 machine frames

8 topping

9 channel

10 control unit

11 Direction of movement of the floating bearing

12 shaped profile

13 Waviness profile

14 Temperature profile

15 Temperature profile

16 shaped profile

17 Waviness profile

18 Temperature profile

19 Temperature profile

20 incision area

21 Cutout area

22 incision area

23 Cutout Area

24 maximum in the inner sub-area of 13

Detailed description of exemplary embodiments according to the invention Fig. 1 shows features that are typical of a wire saw. This includes at least two wire guide rollers 1, a wire mesh 2 made of wire sections

Clamp the saw wire 3. To cut off panes, a workpiece 4 is fed against the wire mesh 2 in the feed direction symbolized by an arrow.

As shown in FIG. 2, the wire guide roller 1 is mounted between a fixed bearing 5 and a floating bearing 6. Fixed bearing 5 and floating bearing 6 are on one

Machine frame 7 supported. The wire guide roller 1 carries a covering 8 which is provided with grooves in which the saw wire 3 runs. The fixed bearing 5 comprises a channel 9 through which a liquid for controlling the temperature of the fixed bearing 5 is passed. If the temperature of the liquid is increased, the thermal expansion of the fixed bearing 5 causes an axial displacement of the wire guide roller 1 in the direction of the floating bearing 6, and the floating bearing 6 moves in the direction indicated by a double arrow 11 of the axis of the wire guide roller opposite the

Machine frame 7. If the temperature of the liquid is reduced, a

Shifting the wire guide roller 1 and the floating bearing 6 causes in the opposite direction. According to the invention, the temperature of the liquid in

Dependence of the cutting depth given by a temperature profile and that

Temperature profile changed at least once during a number of disconnections. A control unit 10, which is in connection with a heat exchanger and a pump, ensures that the through the fixed bearing 5 is directed

Liquid when a certain depth of cut is reached by the respective

Temperature profile has the required temperature.

Example and comparative example

The invention is based on one not according to the invention

Comparative example (Fig. 3) and an example according to the invention (Fig. 4)

clarified.

In the upper half, FIG. 3 shows the shaped profile 12 of a semiconductor wafer made of monocrystalline silicon with a diameter of 300 mm and separated by means of wire separating lobes over the cutting depth (D.O.C = Depth Of Cut). The separation process was carried out using a steel wire with a diameter of 175 μm within about 13 hours

Use of silicon carbide (SiC) with an average grain size of about 13 μm (FEPA F-500), slurried in a carrier liquid made from dipropylene glycol.

During the separation process, the temperatures for cooling the fixed bearings were kept constant at values that had been determined from previous separation processes to be optimal for obtaining semiconductor wafers that were as flat as possible. The lower diagram of FIG. 3 shows this as a function of the cutting depth

Temperature profile 14 of the cooling water temperature of the left (TL = temperature on the left; solid line) and the corresponding temperature profile 15 of the cooling water temperature of the right (TR = temperature right; dashed) fixed bearing of the two wire guide rollers that span the wire mesh.

The distance between two horizontal grid lines in the diagram below is 1 ° C. The temperature was actually kept very constant with Solst deviations of less than 0.1 ° C. The shape profile 12 of the semiconductor wafer obtained in this comparative example (S = shape, that is to say shape (profile); solid line) is, however, very uneven. In particular, the semiconductor wafer has a strong deformation in the incision region 20, that is to say within the first 10% of the

Depth of cut, which is referred to as the incision wave, and a strong deformation in the cutout area 21, that is, within the last approximately 10% of the depth of cut, which is called the cutout wave. That derived from the shaped profile 12

Waviness profile 13 (W = waviness, dashed line), which is the amount of the difference in the deformation of the semiconductor wafer within a along the

Depth of cut sliding measurement window depicts, has strong deflections in the incision area 20 and in the cutout area 21.

Fig. 4 shows in the upper diagram the shape profile 16 and the waviness profile 17 derived therefrom of a semiconductor wafer separated by a method according to the invention and in the lower diagram the temperature profiles 18 and 19 of the left and right fixed bearing of the wire gate spanning

Wire guide rollers. For the production of the semiconductor wafer with the properties according to FIG. 4, five separation processes were initially carried out using a constant temperature profile corresponding to the lower half of FIG. 3

carried out and the shape profiles of the semiconductor wafer obtained from each separation process randomly averaged (every 15th semiconductor wafer, from the beginning of the rod to the end of the rod), whereby the shape profiles of the semiconductor wafers closest to the end faces of the rod were not considered (wafer-related Selection), and then the resulting pane-related average shape profiles of each cutting process are averaged over the five cutting processes

(cutting-related selection).

By multiplying the resulting slice and cut related

Average shape profile with a previously experimentally determined,

machine-specific constant (in ° C / pm) that determines the sensitivity of the

Specifies shape profile change (in pm) per change in temperature of the fixed bearing (in ° C), a first non-constant temperature profile was obtained for the cutting depth-dependent temperature control of the fixed bearing, and a further separation process was thus carried out. This resulted in semiconductor wafers with a wafer-related

Average shape profile, which was already significantly flatter than the slice and cut-related average shape profile of the first five cutting processes using the constant temperature profile. Since the manipulated variable, namely the first non-constant temperature profile, for this separation process by recourse

(Regression) on the constant temperature profile was obtained, the

Application of this temperature profile can also be referred to as regressive control.

The separation process that produced the semiconductor wafer, the shape profile of which is shown in the upper diagram of FIG. 4, was finally carried out using a

Temperature profile carried out, which was calculated from the deviation of the pane-related average shape profile of the previous separation process from the shape profile of a reference pane. This further temperature profile is shown in the lower diagram of FIG. 4. The temperature profile shows im

Incision area 22 within the first 10% of the cutting depth significantly increased and in the cutout area 23 within the last approximately 10% of the cutting depth significantly reduced temperatures, with the result that the incision wave in the incision area 20 and the cutout wave in the cutout area 21 according to the upper diagram of Fig 3 cannot be observed.

Since the manipulated variable, namely the further temperature profile, differs from that of the previous cutting process only by the difference (increment) of the change from the pane-related average shape profile of the preceding and that of the preceding cutting process, the application of the further temperature profile can also be used as incremental parts

Regulation are referred to.

The machine-specific constant used to calculate the temperature profiles indicates by how many micrometers the shape profile changes when the fixed bearing temperature is increased or decreased by one degree Celsius, and is dependent on the effectiveness of the cooling, e.g. the flow temperature, the cooling capacity of the heat exchanger, which supplies the cooling water and is determined by the flow rate (cross section) of the cooling water. Since all of these variables are subject to fluctuations and are also specific to the wire saw, the machine-specific constant can only be determined very imprecisely.

The sign of the machine-specific constant results from which of the two sides of the semiconductor wafer is defined as the front and which as the rear. In the present examples, the rod made of semiconductor material was always with the seeding end (that end face of the rod of two end faces, the position of which was closer to a single-crystal seed crystal during the manufacture of the rod) in the direction of the wire guide roller fixed bearing and with the second

Front face oriented in the direction of the floating bearing, and as the front of the

Semiconductor wafer defines the area of the semiconductor wafer pointing towards the end of the vaccine and, as the rear side of the semiconductor wafer, the area of the semiconductor wafer pointing away from the end of the vaccine. In accordance with the representations in FIG. 3 and FIG. 4, the front side of the semiconductor wafer faces upwards and its rear side faces downwards. In this arrangement the sign for converting the

Average shape profile in the temperature profile negative. With a reverse orientation of the rod in the wire saw, the machine-specific constant would be positive.

The particular effectiveness of the incremental control according to the invention in particular is that the machine-specific constant does not have to be precisely known, since an incremental control has the fundamental property of converging towards the target value (the shape profile of the reference disk), provided that the proportionality factor, namely the machine-specific constant is not selected too high. In the latter case, the regulation would oscillate and not, as desired, converge. So there will be a mere estimate for the Constant always obtained in the course of a few separating processes semiconductor wafers with very flat shape profiles, provided that this estimated value is assumed to be too small in terms of amount.

In particular, the same estimated value for the machine-specific constant can therefore be assumed for different wire saws, preferably a constant with an amount in the range from 0.2 to 5 pm / ° C. The sign of the machine-specific constant results, as described, from the definition of the directions in which the front and back of the semiconductor wafers point in relation to the rod built into the wire saw. Differences between wire saws with different actual constants then only result in the speed of convergence, but not in the achievable amount

Plane parallelism of the semiconductor wafers. Their residual unevenness is only from the unforeseeable ones that occur from separation process to separation process

Fluctuations of the respective separation process (noise quantities) determined.

The waviness number Wav _red is determined on the basis of the shaped profile of a disk, as explained below using the example of the shaped profiles 12 in FIG. 3 and 16 in FIG. 4. From such a shape profile, the amount of the difference between the maximum and minimum of the shape profile within the measurement window is determined within a measurement window with a characteristic wavelength of 10 mm in the direction of the depth of cut (DOC). The position of the start of the measuring window is gradually determined along the cutting depth for each measuring point of the shape profile and the amount of the difference is determined for each of these positions. The amounts of differences obtained in this way are plotted as a function of the cutting depth, with the position of the start of the measuring window indicating the respective cutting depth. In this way, a waviness profile is obtained which, for example, the curves 13 in FIG. 3 and 17 in FIG. 4 embody. The waviness number Wav _red is determined from the waviness profile in that at the start and end of the cut the values of the amounts of the differences are disregarded within a length of 20 mm and the maximum of the remaining values of the amounts of the differences is determined as the waviness number Wav _red .

Correspondingly, starting from the shape S of the shaped profile 12 in FIG. 3, the waviness number Wav _{red of} the semiconductor wafer not produced according to the invention about 12 pm, corresponding to the maximum 24 of the waviness W of the waviness profile 13 and taking into account a grid spacing of the ordinate of 4 pm.

Starting from the shaped profile 16 in FIG. 4, the waviness number Wav _{red is} the

Semiconductor wafer produced according to the invention about 3 μm, corresponding to the maximum of the waviness W of the waviness profile 17 and taking into account a grid spacing of the ordinate of 4 μm.

The above description of exemplary embodiments is to be understood as exemplary. The disclosure made in this way enables the person skilled in the art, on the one hand, to understand the present invention and the advantages associated therewith, and, on the other hand, also includes those which are obvious in the understanding of the person skilled in the art

Changes and modifications to the structures and procedures described. Therefore, it is intended that all such changes and modifications, and equivalents, be covered by the scope of the claims.

Claims

1. A method for cutting a plurality of wafers from workpieces during a number of cutting operations by means of a wire saw, which is a

Wire mesh comprising moving wire sections of a saw wire, which is stretched between two wire guide rollers, each of the wire guide rollers is mounted between a fixed bearing and a floating bearing, the method comprising the infeed of one of the workpieces in the presence of a working fluid during each of the severing processes along an infeed direction against the wire mesh in the presence of hard materials that have an abrasive effect on the workpiece;

Specifies the dependency of a cutting depth;

2. The method of claim 1, comprising

a further change of the temperature profile to a further temperature profile which is proportional to the difference between a further average shape profile of previously separated panes and the shape profile of the reference pane, with the previously separated panes originating from at least 1 to 5 separation processes that immediately preceded a current separation process.

3. The method of claim 1 or claim 2 including using the first temperature profile during a first one of the severing operations occurring after a change in at least one feature of the wire saw, saw wire, or working fluid.

4. The method according to any one of claims 1 to 3, comprising determining the first average shape profile and the further average shape profile based on a slice-related selection of slices.

5. The method according to any one of claims 1 to 3, comprising determining the further average shape profile based on a section-related selection of slices.

6. The method according to any one of claims 1 to 3, comprising determining the further average shape profile based on a slice-related and a section-related selection of slices.

7. The method according to any one of claims 1 to 6, comprising determining the first average shape profile and the further average shape profile based on a weighted averaging of the shape profile of panes.

8. The method according to any one of claims 1 to 7, wherein the saw wire is a hypereutectoid pearlitic steel wire.

9. The method according to any one of claims 1 to 8, wherein the saw wire has a diameter of 70 pm to 175 pm.

10. The method according to claim 8 or claim 9, wherein the saw wire is provided along a wire longitudinal axis with a plurality of protuberances and indentations in directions perpendicular to the wire longitudinal axis.

11. The method according to any one of claims 1 to 10, comprising supplying a cooling lubricant as working fluid to the wire sections during the cutting process, wherein the hard materials consist of diamond and are fixed on the surface of the saw wire by galvanic bonding, by synthetic resin bonding or by form-fitting bonding, and the cooling lubricant is free from substances that have an abrasive effect on the workpiece.

12. The method according to any one of claims 1 to 10, comprising supplying the working fluid in the form of a slurry of the hard materials in glycol or oil to the wire sections during the separation processes, wherein the hard materials consist of silicon carbide.

13. The method according to any one of claims 1 to 12, comprising moving the saw wire in a continuous sequence of pairs of reversals of direction, wherein a pair of reversals of direction each a first movement of the

Saw wire in a first longitudinal wire direction by a first length and a second subsequent movement of the saw wire in a second longitudinal wire direction by a second length, the second longitudinal wire direction is opposite to the first longitudinal wire direction, and the first length is greater than the second length.

14. The method according to claim 13, wherein the saw wire is fed to the wire mesh with a first tensile force in the longitudinal direction of the wire from a first wire supply when moving in the length of the wire, and when moving the second length with a second tensile force in the longitudinal direction of the wire from a second wire supply is supplied, and wherein the second tensile force is less than the first tensile force.

15. The method according to any one of claims 1 to 14, wherein the workpiece consists of a semiconductor material.

16. The method according to any one of claims 1 to 15, wherein the workpiece has the shape of a straight prism.

17. The method according to any one of claims 1 to 16, wherein the workpiece has the shape of a right circular cylinder.

18. Semiconductor wafer made of single-crystal silicon, characterized by a waviness number Wav _red of not more than 7 pm and a diameter of 300 mm, or by a waviness number Wav _red of not more than 4.5 pm and one

Diameter of 200 mm.

19. A semiconductor wafer according to claim 18, characterized by a

Wav _{red number} of not more than 3 pm and a diameter of 300 mm, or by a wav _{red number} of not more than 2 pm and one

Diameter of 200 mm.