WO2012031828A1

WO2012031828A1 - Method for sawing a workpiece

Info

Publication number: WO2012031828A1
Application number: PCT/EP2011/063009
Authority: WO
Inventors: Martin Schoepf
Original assignee: Robert Bosch Gmbh
Priority date: 2010-09-10
Filing date: 2011-07-28
Publication date: 2012-03-15
Also published as: TW201244860A; DE102010040535A1

Abstract

The invention relates to a method for sawing a workpiece (10), in particular a silicone workpiece, in which a sawing tool (12) is moved along the workpiece (10), wherein the sawing tool (12) interacts with the abrasive grains (14) during the movement in such a manner that the abrasive grains (14) effect a chip-removing ablation of the workpiece (10), and wherein an electrical voltage is applied between the workpiece (10) and the sawing tool (12) for the voltage-based ablation of the workpiece (10). Using the method according the invention increases the ablation rate and decreases the process duration.

Description

description

title

Method for sawing a workpiece The present invention relates to a method for sawing a workpiece.

The invention particularly relates to a method for wire sawing a

Silicon workpiece for producing a silicon wafer.

State of the art

It is often desirable to saw workpieces, such as silicon workpieces, to achieve a desired size or structure. Here, the so-called wire sawing is often used. This is a mechanical separation process, especially for silicon, which is also referred to as separation lapping process with unbound cutting grain and undirected cutting edge. The cutting grain is usually in a carrier medium, which forms a suspension together with the cutting grain, which is also referred to as slurry. A usually thin wire with a diameter of 100μηι to 180μηι often serves as a tool. An unwinding reel winds the wire over

Wire guide rollers with a defined wire speed, until it is finally wound up again via a take-up reel. The wire dips into the suspension and draws the slurry adhering to the wire surface into the kerf of the workpiece. Slurry is additionally defined on the wire field via a nozzle. The cutting grains are thus pulled with the help of the wire and with a defined processing speed through the kerf and tear small particles from the solid. In this case, the workpiece dips into the wire field at the appropriate feed rate, whereby a cutting removal takes place. The wire usually cuts many close together thin wafers, which may have a thickness of between 200μηι and 300μηι depending on the industrial application. Such a method is known, for example, from CH 696389 A5.

In particular, in the production of silicon wafers are nowadays by wire sawing with a wire diameter of 120μηι

Cutting widths of about 150μηι achieved. The achieved feed is about 0.4 mm / min, which often makes parallelizing the method economically sensible and necessary. Today, up to 2000 wires simultaneously process, for example, a silicon cylinder called ingot.

Furthermore, a special form of wire sawing is known, which operates with bound cutting grains. In this case, the conventional wire is replaced with a wire-cut tool with a cutting grain coating. The

kinematic relationships are identical to those of classic wire saws. When wire sawing with bonded grain no slurry is needed, only one necessary for the grinding process coolant is often used here.

Disclosure of the invention

The present invention is a method for sawing a workpiece, in particular a silicon workpiece, wherein a sawing tool is displaced along the workpiece, wherein the sawing tool cooperates with the displacement of abrasive grains such that the abrasive grains cause a machining removal of the workpiece, and wherein between the workpiece and the sawing tool an electrical voltage to

voltage-based removal of the workpiece is created.

Characterized in that a sawing tool is displaced along the workpiece, wherein the sawing tool in the displacement cooperates with abrasive grains such that the abrasive grains cause a machining removal of the workpiece, the material of the workpiece is first removed by an abrasive method and the workpiece thus sawed. This mechanical sawing method is for the Sawing a workpiece even in small dimensions, such as those used for the production of power semiconductors, well suited.

The abrasive grains are firmly attached to the sawing tool, for example, and may be present as an abrasive grain coating, for example. The

Cooperation of the sawing tool with the abrasive grains can therefore be effected by attachment of the abrasive grains to the sawing tool.

In this case, a displacement means, in particular, that the sawing tool is moved along the intended cutting edge of the workpiece, wherein the abrasive grains rub against on the surface of the workpiece, in order to effect a machining removal of the workpiece. In this case, a cutting removal means in particular that the removed material is obtained in the form of chips or particles.

According to the invention it is further provided that between the workpiece and the sawing tool, an electrical voltage for stress-based removal of the workpiece is applied. The abrasive removal of the workpiece is therefore supported by a stress-based removal, whereby at least two removal processes are performed in parallel. This will be the

Abtragrate, so the amount of removed material of the workpiece, increased.

In addition, the feed rate, ie the speed of displacement of the sawing tool along the surface of the workpiece can be increased. In detail, feed rates of up to 1.5 mm / min can be achieved. Such feed rates are suitable for both abrasive and stress-based removal of the workpiece. The feed rate is high enough to work economically, and low enough to handle even the smallest workpieces with high accuracy.

Therefore, according to the invention, it is possible to reduce the process time to be estimated in a sawing method. A production of workpieces to be formed can therefore also in small batches without a disproportionately high and therefore uneconomical

Cost incurred. According to the invention, the sawing tool and the workpiece serve as electrodes. It can thus be seen that the sawing tool and the workpiece should each be formed of an electrically conductive material. This has the advantage that the inventive method can be performed on existing sawing machines without a great deal of structural measures. Only one voltage source and possibly suitable insulators are needed. The method according to the invention is in particular for sawing a

Silicon workpiece suitable. However, other, electrically conductive workpieces can be sawn by the method according to the invention,

in particular semiconductor workpieces such as gallium arsenide or

Indium phosphide.

In this case, a method for sawing a workpiece is understood in particular as a method for separating or scoring a workpiece. By a sawing process can thus be separated from the workpiece or one or more parts of the workpiece or notches

or structures are created in the workpiece.

In a preferred embodiment of the method according to the invention is between the sawing tool and the workpiece a

Liquid arranged. A liquid can here on the one hand cool the sawing tool as well as the workpiece, which in particular in a cutting

Abtrag is preferred. In addition, the liquid can achieve a lubricating effect that minimizes the effort required to move the sawing tool. As a result, the sawing tool is spared. Consequently, the

Liquid have the function of a cooling lubricant. suitable

Liquids include, for example, glycol or oil, such as a mineral or synthetic oil.

In addition, the liquid can serve as a carrier medium for abrasive grains, whereby a method according to the principle of the unbound grain, ie with cutting grains in the carrier medium, is possible. In this

Embodiment forms the carrier medium with the abrasive grains a Suspension or the slurry, wherein the abrasive grains are entrained in this case by the displacing sawing tool and thus cause a machining removal of the workpiece. The interaction of the sawing tool with the abrasive grains can be effected by entrainment 5 of the abrasive grains by displacing the sawing tool.

In the context of a further advantageous embodiment of the method according to the invention, a discharge arc for removing the workpiece is generated by the tension between the sawing tool and the workpiece. In l o of this embodiment is thus essentially on the operating principle of

Spark erosion (EDM) is used. The discharge arc

or arc causes the material of the workpiece melts and / or evaporated and thus dissolves from the structure. By a tension arc is in addition to the abrasive removal of the workpiece

15 therefore a thermal removal of the workpiece possible.

In particular, such a thermal removal of the workpiece allows not only a significantly increased removal rate, the sawing of workpieces with very complex shapes and contours. In this case, a high dimensional accuracy 20 is possible. The method according to the invention is thus also very hard

Metals possible, since mainly the melting or evaporation temperature of importance, but less the hardness sets a limit.

25 It is particularly advantageous if between the sawing tool and the

Workpiece a dielectric is arranged. The dielectric can be formed by the liquid. In this way, the formation of a suitable discharge arc can be ensured, which is not by an electrically conductive component between the sawing tool and the

30 workpiece is disturbed or prevented. In a particularly advantageous manner, the

Dielectric selected in particular from deionized water or an oil, such as a synthetic oil.

In the context of a further advantageous embodiment of the invention

35 method, the workpiece is polarized as an anode and the sawing tool as a cathode to electron flow between the workpiece and the To enable sawing tool. In this embodiment is therefore in the

Essentially the functional principle of Electrochemical Ablation (ECM) is used. In this case, the flowing current generated by the applied voltage causes ions of the material of the workpiece to detach from the workpiece, whereby the workpiece is removed. In addition to a clear

Increased removal rate compared to a mere abrasive removal of the workpiece is made possible by using an electrochemical removal of creating the most complicated spatial shapes. In addition, machining with the highest precision in the micrometer range is possible with such a method. Further, after sawing, surfaces having a highest quality are obtained.

It is particularly preferred that an electrolyte is arranged between the sawing tool and the workpiece. The electrolyte may also be formed by the liquid. In this way, a sufficient flow of current between the sawing tool and the workpiece can be ensured, wherein the ions dissolved from the material of the workpiece migrate into the electrolyte. It is particularly preferred if the electrolyte is a

Conductivity in a range of> 5mS / cm to <75 mS / cm.

Particularly preferred electrolytes which can be used in the invention are in particular selected from an aqueous one

Sodium chloride solution or an aqueous sodium nitrate solution.

In the event that a joint thermal and electrochemical removal is performed in addition to the abrasive removal of the workpiece, essentially a so-called ECDM method takes place. In this case, a substance, in particular a liquid, should be arranged between the workpiece and the sawing tool, which has an electrical conductivity of <10 mS / cm. In this way, the conductivity may be sufficiently high to provide at least a small electrical current between the

Workpiece and the sawing tool allows that allows an electrochemical removal of the workpiece. In addition, the conductivity is low enough to allow the formation of a discharge arc. Within the scope of a further advantageous embodiment of the present invention

Invention, the sawing tool with distance to the workpiece along displaced this, the distance is determined in particular by the arrangement and / or thickness of the abrasive grains. In the context of the invention, a distance between the tool and the saw-cutting piece is understood to mean that the outside of the sawing tool is not in direct contact with the workpiece. This is to be preferred in particular in the case of voltage-based or electrical removal methods, because these are in particular provided with a distance between the tool and the tool

Saw workpiece work as desired. The distance can be determined by the arrangement and / or thickness of the abrasive grains. In the case that the abrasive grains are attached to the sawing tool, the thickness of the

Abrasive grains directly the distance between the sawing tool and the workpiece. In the embodiment in which the abrasive grains are in one

Carrier medium are located and are entrained by the shifting sawing tool, the distance must of course be chosen so that allow the entrained Abrasivkörner a machining removal of the workpiece by an action of the sawing tool. Therefore, the distance is also determined in this case by the arrangement, or thickness of the abrasive grains. In the context of a further advantageous embodiment of the invention

Method, the sawing tool is selected from a copper, brass, graphite, tungsten or steel wire. In this way, the sawing tool provides a sufficiently high stability, tensile strength and is electrically conductive, so that both an abrasive, as well as a voltage-based

or electrical removal of the workpiece is possible.

It is also advantageous if a voltage of> 10V is used. In this case, the applied voltage is particularly preferably to be selected as a function of the type of stress-based removal of the workpiece. In particular, in an EDM and an ECDM method, voltages in one

Ranges from> 100V to <500V, whereas in an ECM method voltages in the range of> 10V to <80V are particularly suitable.

The subject of the present invention is furthermore an apparatus for sawing a workpiece according to the independent claim. Further advantages and advantageous embodiments of the subject invention are illustrated by the drawings and explained in the following description. It should be noted that the

Drawings are descriptive only and are not intended to limit the invention in any way. Show it

Fig. 1 is a schematic representation of an embodiment of the

inventive method;

Fig. 2 is a schematic representation of another embodiment of the method according to the invention.

FIG. 1 schematically shows the mode of operation of a device according to the invention

Process for sawing a workpiece 10 shown. The method according to the invention can be used, in particular, to carry out a wire sawing on a silicon workpiece, for example to produce a silicon wafer.

There is shown a workpiece 10. To saw the workpiece 10, along the workpiece 10, a sawing tool 12 is displaced or guided along this. The sawing tool 12 may in particular be a metal wire, such as a copper, brass, graphite, tungsten or steel wire. On the one hand, these metals are well suited for wire sawing. On the other hand, these wires are electrically conductive as metals, which in particular for a

stress-based removal of the workpiece 10 is important. The sawing tool 12 or the metal wire can have a thickness in a range of> 100μηι to <180μηι.

When displacing the sawing tool 12 along the surface of the

Workpiece 10 act on the workpiece 10 cutting grains or abrasive grains 14 a. By the action of the abrasive grains 14 on the

Workpiece 10 is sawn mechanically or abraded.

Accordingly, the abrasive grains 14 should be designed such that they can effect a machining removal of the workpiece 10 or of material of the workpiece 10 when acting on the workpiece 10. The Abrasive grains 14 are for this purpose in particular particles which consist of silicon carbide or comprise this at least to a large extent.

Silicon carbide, a material very similar to diamond, thus has a very high hardness, which can range from 9.6 (Mohs) and 2600 (Vickers, Knoop). Such a high hardness is to be preferred, so that the

Abrasive grain 14 is not damaged even during a sawing or wears too high a degree. In addition, silicon carbide is also at temperatures above 800 ° C against oxygen to a high degree

oxidation resistant, and is also electrically non-conductive, which is particularly in a combination of the machining with a stress-based

Erosion is useful. Other preferred materials that may include the abrasive grains 14 include diamond or cubic boron nitride (CBN).

In the embodiment of Figure 1, the abrasive grains 14 are fixedly attached to the sawing tool 12, one speaks here of a sawing with bonded grain. As a result, a sawing process automatically takes place when the sawing tool 12 is displaced along the surface of the workpiece 10, since the abrasive grains 14 thus act on the workpiece 10. In this case, the sawing tool 12 is displaced at a distance from the workpiece 10 along this, wherein the distance is determined in particular by the thickness of the abrasive grains 14. It can by the sawing tool 12, a pressure in the direction of

Workpiece 10 are exercised, which is illustrated by the arrow 16. The sawing tool 12 or its surface can always exactly keep the distance to the workpiece 10, which corresponds essentially to the thickness or the diameter of the abrasive grains 14, wherein a penetration depth of the abrasive grains 14 into the workpiece 10 is to be considered. Of course, in this embodiment, the distance between the sawing tool 12 and the workpiece 10 can be adjusted by the attachment of the abrasive grains 14 on the sawing tool 12 of the

Diameter of the abrasive grains 14 is selected, respectively, a plurality of layers of abrasive grains on the sawing tool 12 is arranged.

By displacing the sawing tool 12 along the surface of the

Workpiece 10 and thereby preferably by exerting a pressure of the sawing tool 12 in the direction of the arrow 16 or a pressure of the workpiece 10 in the opposite direction can thereby a notch 18, which illustrates the sawing process. In this case, it is preferable to provide a substance 20, such as a liquid, between the sawing tool 12 and the workpiece 10, which can serve as cooling lubricant and thus simplify the sawing process.

In order to support the mechanical abrasive removal process, it is provided according to the invention that a voltage is applied between the sawing tool 12 and the workpiece 10, as is illustrated by the voltage source 22 in FIG. In this way, the purely mechanical removal of the workpiece 10 can be supported by a stress-based or electrical removal and the removal rate and the process duration can be improved.

For example, a discharge arc for removing the workpiece 10 can be generated by the tension between the sawing tool 12 and the workpiece 10. By the discharge arc respectively

Voltage arc melts or evaporates the material of the workpiece 10, whereby a thermal support of the mechanical Sägevorgangs is possible.

In this method, a distance between the sawing tool 12 and the workpiece 10 is preferably to be selected so that a gap in a

Magnitude of> 0.004mm to <0.5mm is formed. As already stated, this gap size can be adjusted by the selection and arrangement of the abrasive grains 14. A time-consuming setting and monitoring of the gap size or the distance is not necessary. Particularly preferably, a gap is formed in a range of approximately> 5 μm to <100 μm in the case of an EDM and an ECDM method. In an ECM method, the gap preferably has a size in a range of about> 5μηι to <1000 μηι. The exact gap size is preferably selected as a function of the applied voltage.

In particular, the abrasive grains 14 are formed of an electrically insulating material, such as silicon carbide, and are therefore on the

tension-based ablation process is not involved. Rather, they serve exclusively for the mechanical removal of the workpiece 10. Beyond In this case, the substance 20 is preferably also a dielectric. This can then be selected in particular from deionized water or an oil. Both substances serve as a good insulator to one

Suitably not hinder stress curves, and also have suitable properties for a cooling lubricant.

Alternatively or in addition to a thermal removal of the workpiece 10, according to the invention, an electrochemical or anodic removal of the workpiece 10 can take place. For this purpose, the workpiece 10 can be polarized as an anode and the sawing tool 12 as a cathode. In addition, in order to allow a sufficient flow of current between the workpiece 10 and the sawing tool 12, as the substance 20, an electrolyte can be selected. The electrolyte can be selected in particular from an aqueous sodium chloride solution or an aqueous sodium nitrate solution.

Furthermore, the electrolyte preferably has an electrical conductivity in a range of> 5mS / cm to <75mS / cm.

In this embodiment, a current flowing through the applied voltage between the workpiece 10 and the sawing tool 12, the one

electrochemical removal of the workpiece 10 allows. In detail go at the anode, so the workpiece 10, ions in solution and migrate in particular through the electrolyte to the sawing tool 14th

In this case, the thermal and the electrochemical removal method can be used equally in combination in addition to the abrasive method. In this case, therefore, the increase in the removal rate by a parallel use of a mechanical, thermal and a

Electrochemical removal possible. In a hybrid process, ie a combination of thermal and electrochemical removal processes, the anodic dissolution of the material of the workpiece preferably acts during the ignition delay of the thermal process.

FIG. 2 shows a further embodiment of the method according to the invention. The method according to FIG. 2 essentially corresponds to FIG

Process of Figure 1, which is why corresponding components with the same

Reference numerals are provided. The difference of the method according to FIG. 2 While the abrasive grains 14 in FIG. 1 are fastened to the sawing tool 12, they lie loosely in the substance 20, in this case a carrier medium, according to FIG. This method is therefore also referred to as sawing with loose grain. In this embodiment, the abrasive grains 14 are entrained by the moving sawing tool and thus act on the workpiece 10, to effect a chipping.

Also in this case, a precise distance control is not required, since the distance between the sawing tool and the workpiece is determined by the thickness, or the diameter and the arrangement of the abrasive grains 14.

Claims

claims

1. A method for sawing a workpiece (10), in particular a

Silicon workpiece in which a sawing tool (12) along the workpiece (10) is displaced, wherein the sawing tool (12) in the displacement with abrasive grains (14) cooperates such that the abrasive grains (14) cause a machining removal of the workpiece (10) , And wherein between the workpiece (10) and the sawing tool (12) has an electrical

Voltage for stress-based removal of the workpiece (10) is applied.

2. The method according to claim 1, characterized in that between the sawing tool (12) and the workpiece (10) a liquid is arranged.

3. The method according to claim 1 or 2, characterized in that a discharge arc for removing the workpiece (10) is generated by the voltage between the sawing tool (12) and the workpiece (10).

4. The method according to claim 3, characterized in that between the sawing tool (12) and the workpiece (10) a dielectric is arranged, which is in particular selected from deionized water or an oil.

5. The method according to any one of claims 1 to 4, characterized in that the workpiece (10) as the anode and the sawing tool (12) is polarized as a cathode to allow an electron flow between the workpiece (10) and the sawing tool (12) ,

6. The method according to claim 5, characterized in that between the sawing tool (12) and the workpiece (10) an electrolyte is arranged, which is in particular selected from an aqueous sodium chloride solution or an aqueous sodium nitrate solution.

7. The method according to claim 5, characterized in that between the

5 sawing tool (12) and the workpiece (10) a substance is arranged, which has an electrical conductivity of <10mS / cm.

8. The method according to any one of claims 1 to 7, characterized in that the sawing tool (12) is displaced at a distance from the workpiece (10) along this l o, wherein the distance in particular by the arrangement

and / or thickness of the abrasive grains (14) is determined.

9. The method according to any one of claims 1 to 8, characterized in that the sawing tool (12) is selected from a copper, brass,

15 graphite, tungsten or steel wire.

10. The method according to any one of claims 1 to 9, characterized in that a voltage of> 10V is used.

20 1 1. Device for sawing a workpiece (10), in particular one

Silicon workpiece, comprising a sawing tool (12), characterized

characterized in that the device comprises means for carrying out the

Method according to one of claims 1 to 10.

25