DE3844520A1 - Method and apparatus for splitting up semiconductor ingots into semiconductor wafers with at least one plane surface - Google Patents

Abstract

The invention relates to a method of producing wafers by cutting off monocrystalline or polycrystalline semiconductor ingots by means of a wire-cable saw, in which method the end face of the semiconductor ingot is in each case provided with a plane surface by means of a machining step before the cutting-off of each wafer to produce a plane reference surface for the further processing of this wafer. A plane end face is advantageously produced by grinding. Furthermore, the invention relates to an apparatus for carrying out the above method. It consists of a combination of a wire-cable saw with a grinding device which permits grinding in the work set-up of the wire-cable saw. The grinding device expediently forms a unit with the wire-cable saw. In this arrangement, the grinding of the end face of the ingot and the cutting-off of a further wafer can be carried out in succession by means of a wire-cable saw. The grinding of the end face of the ingot and the cutting-off of a further wafer are advantageously carried out in such a way that they are timed to overlap and while utilising the same feed movement.

Description

The invention relates to the cutting of hard, brittle non-metallic Materials with a Vickers hardness of up to HV 15,000 N / mm², which are due to their special properties make extreme demands on the machining process put.

The semi-finished material obtained from the melt is in cylindrical form, so-called "bars" before. Further processing of the material requires a slice-wise division of these bars into circular slices, so-called Discs or wafers. This separation process is due to the following requirements marked:

• a) Because the material because of its elaborate extraction in ultra pure Texture is very expensive, the machining losses be minimized. The cutting width should be well below the wafer thickness of about 1 mm.
• b) The divided wafers should have surfaces that are as plane-parallel as possible.

The principle of splitting is also available for the first requirement Ropes on which in its basic form has been known since ancient times is. When stones are cut into cuboid blocks, a rope is placed underneath Force applied over the workpiece, the abrasive with the Cooling lubricant is added loosely. Although the removal rate of such Sawing is very humble, even today they are still in quarries used.  

A significant improvement in productivity was achieved in that the abrasive was firmly attached to the rope. This can either be in Form of discrete elements (the rope surrounded and tied to it Sleeves with abrasive outer surface) or by direct application the rope happen.

Because the precision of the cut and the removal rate with increasing rope tension become larger, there are increased demands on the tensile strength of the Ropes, so the traditional hemp rope is none in today's industrial practice Role plays more.

The cutting of very hard materials is the current state of the art Technology based on the principle of rope separation. The special However, the problem lies in the requirement for the smallest possible cutting width justified. Allow wires with a diameter of 1 mm and more it also when using conventional drive mechanisms that required Transfer tension and cutting forces to the rope.

However, the cutting width required here requires the wire diameter to reduce to a few tenths of a millimeter. However, this can only be done succeed when the tensile strength of the rope is optimally picking up the clamping and cutting forces are exhausted. Here comes the interpretation the device that applies the driving forces to the rope, very special Significance to: It must be designed in such a way that the material strength limited as much as possible Cutting force can be transmitted.

The inevitably arises the difficulty that with those in the wire existing tensile and empty strand forces, on the one hand, slipping through on average must be enforced, because only then can there be a separation process, but that on the other hand with the same tensile and empty strand forces on the drive side slipping must be prevented in any case, because otherwise the drive mechanism itself would be cut. This problem is caused by the strongly fluctuating coefficient of friction between wire and workpiece or between wire and drive mechanism.  

The above requirements can only be met if if the wrap on the drive side of the wire is sufficient is great. However, a wrap must be provided cannot be accommodated on one roll, but on several rolls must be distributed. The distribution of forces over several roles can can only be used to advantage if the drive effects of individual roles with their respective engine precisely matched will. A moment branching that all roles from a common Motor powered, makes little sense here, because in this case would the wear indispensable due to expansion slip, especially in the partial load range audibly focus on the first role, which has adverse effects on the maintenance intervals and thus on the downtimes of the machine would have. In addition, the mechanical coupling of the drive rollers would be not so easily with each other according to the current state of the art possible: a positive branching gear (for example a gear) comes across at the high wire speeds and the associated speeds very soon to lubrication There are limits and any kind of frictional power split unsuitable solely because of the inevitable slip that occurs inevitably continues at the contact points of the drive roller-diamond wire and causes increased wear there.

To date, there is still no satisfactory solution to this whole problem area been found. For this reason, wire rope saws are where it is high precision is still an issue, for example in the semiconductor sector have not gone beyond the experimental stage and have so far been able to not yet prevail in industrial practice.

The solution to the problem according to the invention is that of those involved in the drive Rollers to be equipped individually with one motor each and by electrical or mechanical coordination of their speed-torque characteristics with each other to divide the total force between the individual drive units that

• 1. the individual drive rollers with the same or at least approximately same safety against slippage are involved in the drive, whereby the transferable force is increased to a maximum and  
• 2. the wear forced by expansion slip in any case distributed almost evenly on all roles.

An example may clarify this fact:
A drive mechanism consisting of 3 rollers with given constructive boundary conditions can transmit a force of almost 8 N without adapting the individual drives. If the individual drives are adapted, this force can be increased to almost 10 N.

The wear distribution when adapting transfers regardless of which actually transmitted power always 34.0% for the first, 38.9% for the second and 27.2% for the third role.

In the case of unmatched individual drives, the first role takes over at full load 37.7%, the second roller 43.2% and the third roller 19.1% of wear.

If the load drops from 70% of its maximum value, the first 46.6% of the roll and 53.4% of the wear from the second roll, while the 3rd roll has no more wear at all. If the transmitted force is less than a quarter of the maximum value, then the wear concentrates exclusively on the first roll.

This circumstance gains special importance by the fact that wire saws are operated essentially only in the partial load range and only for occasional use occurring larger loads and the associated risk of slipping must be dimensioned correspondingly larger.

An embodiment of the invention is below with reference to the drawings described as an example. It shows

Fig. 1 shows the basic arrangement of the essential components of such a machine

Fig. 2 shows the distribution of the total force between the individual rollers required with regard to the rope friction

Fig. 3 shows the implementation of this required force distribution by adapting and utilizing the torque-speed characteristics of the drive motors.

Fig. 1 shows the most important elements of such a machine. The saw wire 1 is in engagement with the workpiece 2 and is driven by several, in this case 3 rollers, which are each coupled to a motor, not shown here. The rollers 4 serve only as deflection or guide rollers and have no influence on the tensile force of the wire. The three drive rollers are arranged close together with their parallel axes of rotation as corner points of an equilateral triangle in order to achieve the largest possible wrap angle.

On average, the process forces create a frictional force R , which is noticeable as the difference between tensile strand force S ₁ and empty strand force S ₄. In the sense of a constant preload, the empty span force S ₄ is kept at a constant value by the weight and spring mechanism. The three drive rollers are driven by their motors in such a direction that the tensile strand force is generated in strand 1 and the empty strand force in strand 4 in accordance with the force requirement of the cut.

Depending on the drive involvement of the individual rollers, the intermediate levels S ₂ and S ₃ located between the maximum tensile strand force S ₁ and the minimum empty strand force S ₄ are formed.

Proper functioning of the machine requires safe transmission of frictional force on the three drive rollers. This situation is illustrated in FIG. 2:
The drive mechanism as a whole is shown in the 1st quadrant. The leetrum force S ₄ is - as described above - kept constant for all operating states. If the wire is not engaged in the workpiece, all strand forces, including S ₁, are as large as S ₄, the operating state is on the bisector. If a frictional force is introduced into the wire due to the engagement of the wire in the workpiece, a force of the same magnitude occurs as peripheral force U _tot in the drive mechanism, which increases the tensile strand force S ₁ compared to the empty strand force S ₄. This force difference caused by the cutting process U _tot = S ₁- S ₄ may on one hand be as large as permitted by the coefficient of friction and the wrap of all the drive rollers; The first quadrant of FIG. 2 shows the limit case of the slip limit which is critical for the power transmission (straight line with the greatest slope).

On the other hand, avoidance of the material-damaging slip is only guaranteed if the frictional engagement of each individual roller is not exceeded. To clarify this state of affairs, the 3 individual roles are similarly shown in FIG. 2 in 3 further quadrants. The slip limit guide beam in these three quadrants is of course much flatter than that in the first quadrant, since on the basis of a critical coefficient of friction of the same size on all rolls, the total wrap angle of all 3 rolls in the first quadrant, but only proportionately in the other 3 quadrants assigned wrap angle is taken into account.

From this representation it can be seen that the circumferential force applied to the individual roll may only be as great as is the result of the respective empty strand force. The applied to the individual rollers circumferential forces U ₁, U ₂ and U ₃ finally add up to the total circumferential force U _tot . For optimal operation, the division of U _tot into the individual components U ₁, U ₂ and U ₃ must therefore always have a very specific proportion, regardless of the bias S ₄. In order to be able to utilize a larger total circumferential force U _ges , the preload S ₄ must be increased. The limit is reached, however, where the maximum strand force S ₁ that occurs exceeds the tensile strength of the wire.

In terms of the uniform wear of all three rollers and the same performance all 3 drive motors would be a distribution of the total circumferential force in 3 individual components of equal size desirable. However, this can be done only close to reaching.

For this reason, the three rollers are also arranged asymmetrically ( FIG. 1) in order to be able to exert somewhat more circumferential force on the third roller, which has been disadvantaged from the start, by increasing the wrap angle. As a result of this arrangement, the wrap angle of the first roller becomes smaller, which is also useful for aligning the circumferential forces with one another.

The main task is therefore the peripheral forces on the three individual roles individually by the motor driving them. This process should preferably be done with simple means, ie without measuring and control engineering effort can be accomplished.  

The solution to the problem of power split is shown in this example by adjusting the electromechanical properties of the individual motors to the respective power transmission requirements sets.

The principle is explained in Fig. 3:
The motor characteristics are plotted in the 1st quadrant, which must show a shunt behavior, since otherwise the drive would assume an excessively high speed in the no-load state. The speed of each motor, multiplied by the associated roll radius, gives the circumferential speed of the rolls, which is equivalent to the wire speed (quadrant 4).

The torque of the motor divided by the corresponding roller radius leads to the force arising on the circumference (2nd quadrant). Both connections are linear and if scaled accordingly, the same straight line can be created use in both the 2nd and 4th quadrants.

Since all three drive rollers are coupled via the common wire, the peripheral speeds for a certain operating state be the same for all three roles. Because the individual rolls have the same diameter all 3 motors inevitably run at the same speed.

To further clarify the division of the circumferential force, the third quadrant can be used: The circumferential forces U ₁, U ₂ and U ₃, which can be found as individual components on the abscissa, can be summed up as U _tot = U ₁ + U ₂ + U ₃ plotting the ordinate. The optimal circumferential force distribution determined according to FIG. 2 can then be represented as a bundle of straight lines in the third quadrant of FIG. 3.

The proportionality between U ₁, U ₂ and U ₃ should maintain a certain value regardless of the total load U _tot . The individual circumferential forces result in corresponding moments over the roller radius. However, these generally different moments must be effective at the same speed. This results in different characteristics for the three motors in the 1st quadrant: The same speed is required when idling, depending on the load, the individual motor takes on as much torque as is intended for it according to the proportionality shown in the 3rd quadrant.

The second of the requirements mentioned at the beginning is of particular importance characterized in that the cutting tool under the influence of Process forces and the wear-related uneven cutting ability of the tool is deflected, resulting in an imprecise workpiece geometry leads.

The dividing surfaces are neither flat nor parallel, but twisted in themselves, which is simply called "bow" or "warp".

As FIG. 4 shows, this error can no longer be remedied even by subsequent processing steps.

The separated blank 1 has two uneven boundary surfaces, and the "warp" can be up to a few hundredths of a millimeter. If this thin workpiece is now expediently clamped onto a flat surface for further processing, the blank surface in contact with this flat clamping surface is also forced into a flat position using the elastic deformability of the blank ( 2 ). The opposite surface can be converted into a flat state in this position by appropriate processing methods, so that two plane-parallel surfaces are created in this position ( 3 ). However, if the workpiece is relaxed again, the side of the round plate facing the flat clamping point resumes its original shape due to its elasticity ( 4 ). All other subsequent processing steps cannot remedy this faulty situation. You get parallel, but not flat surfaces.

The problem of absolutely plane-parallel surfaces can, as in OS-DE 36 13 132 described by an integration of separation and Leveling process to be solved.

As explained in FIG. 5, the uneven end face of the ingot ( 1 ) is leveled by machining, whereby in addition to the grinding that is preferably used, milling, turning, electrolytic and erosive removal are also possible. The subsequent cutting process with a wire saw leaves an uneven boundary surface ( 3 ) both on the ingot and on one side of the cut blank.

However, since the separated blank has an absolutely flat reference surface, can be tensioned on this without distortion, so that then the opposite surface can also be machined plane-parallel to it can. If the blank is then removed from the clamping point again, so it no longer rejects. Before another disconnect the front of the ingot leveled again.

With this method, it is irrelevant whether the removal or removal process refer to a surface perpendicular to the bar axis or a - mostly minor - to this vertical position Assume misalignment.

A device that meets the two requirements set out at the beginning Exploitation of what has been explained above must therefore be a combination of wire rope saw and material removal machine, for the sake of To save time, a constellation is to be striven for, which is the removal or the Do not flatten the face of the ingot and detach the round blank one after the other, but overlapping in time.

FIGS. 6-10 show the main components of a device to the process described above.

In FIG. 6, the initial position of the cutting operation is shown: the to Separating bars 2 having no planar end face in the general case, extends into the annular grinding wheel 11 in a rotating grinding cup, wherein the end face of the billet must be completely immersed in the ring 11.

If the ingot 2 is moved perpendicularly to the axis of rotation of the grinding wheel 11 in the manner shown in FIG. 7, this grinding wheel removes material at the end of the ingot, so that regardless of the original geometry, the end face of the ingot becomes completely flat.

In the further course of this feed movement, the ingot 2 comes into engagement with the saw wire 12 ( FIG. 8). The saw wire 12 is arranged relative to the grinding wheel 11 in such a way that it separates a wafer from the ingot 2 , the thickness being such that the finished size still has to be increased by an allowance determined by experience.

As the feed movement continues ( FIG. 9), the grinding wheel 11 disengages while the separation process is still ongoing. The sawing wire 12 also migrates slightly in the axial direction of the ingot under the influence of the process forces, so that the step leaves unevenness both on the ingot and on the wafer surface just cut.

After the separation process ( FIG. 10), the wafer 13 is transported out of the processing zone, which is particularly simple in the case of the wire saw, because the tool, in this case the wire, can no longer exert disruptive forces on the workpiece.

One side of the wafer 13 has been given an absolutely flat surface by grinding, so that the wafer can then preferably be clamped on this flat surface, preferably with a vacuum, on a flat surface, so that the opposite surface can be processed with the aim to achieve a plane that is also plane and parallel to the first surface. This latter processing step is also preferably accomplished by grinding.

Claims (6)

1. A method for producing round blanks by separating monocrystalline or polycrystalline semiconductor ingots using a wire saw, characterized in that the end face of the semiconductor ingot before separating each round blank to produce a planar reference surface for the further processing of this round blank in each case by means of a machining step is provided with a flat surface. 2. The method according to claim 1, characterized in that a plane End face is generated by grinding. 3. Device for performing the method according to claim 1 or 2, characterized by a combination of a wire saw with a grinding device, which grinding processing in the Workpiece clamping of the wire rope saw allowed. 4. The device according to claim 1, characterized in that the Grinding device forms a unit with the wire saw. 5. The device according to claim 2, characterized in that the end face Grinding the ingot and severing another Circular blank is carried out in succession using a wire saw. 6. The device according to claim 2, characterized in that the end face Grinding the ingot and severing another Round blank overlapping in time and using the same feed movement is carried out.