GB2256822A

GB2256822A - Rounding off the edges of semiconductor discs

Info

Publication number: GB2256822A
Application number: GB9212891A
Authority: GB
Inventors: Hubert Hinzen
Original assignee: Gmn Georg Mueller Nuernberg AG
Current assignee: Gmn Georg Mueller Nuernberg AG
Priority date: 1991-06-18
Filing date: 1992-06-17
Publication date: 1992-12-23
Also published as: ITMI921435A1; ITMI921435A0; KR930001332A; IT1258972B; JPH05177524A; FR2677912A1; GB9212891D0; DE4120003C2; DE4120003A1

Description

Apparatus and method for rounding off the edges of semiconductor discs

This invention relates to apparatus and a method for rounding off the edges of work pieces such as semiconductor discs.

The processing of flat, disc-shaped workpieces is a standard problem in the field of semiconductor and photovoltaic manufacture, whilst the discshaped condition of the semifinished product is an intermediate stage which is virtually unavoidable according to the present state of the art. The discs measure up to 200 mm. in diameter and are generally less than 1 mm thick. Since the material initially takes the form of a cylinder, the discs have to be produced by cutting. Nowadays, it is preferred to use saws with an inner hole for this manufacturing step, but so-called "frame saws" or other methods of cutting with or without an abrasive grain may also be used.

Quite apart from the special method of cutting the -material and the dimensions of the workpiece, there is the problem, with current quality control requirements, of rounding off the edges of the disc-shaped workpiece thus obtained before it is further processed. This intermediate processing step is required for various reasons. First of all, it is essential to prevent the disc-shaped workpieces from scratching one another, in subsequent manufacturing processes, with the edges left behind by the cutting operation. In addition, fractures and cracks may occur at the corners, particularly as the materials used are brittle. A rbunded edge, by contrast, substantially reduces this risk. Moreover, the rounding of the edges preven.ts tiny fragments of 1k material from interfering with subsequent manufacturing steps as undesirable foreign particles, which is of exceptional importance in the manufacture of semiconductors, in particular, with the ever more stringent demands on the purity of the processes.

There are already machines in use for this machining operation, which are occasionally known as edge grinders. Since the materials in question are mainly very hard and the amount of material removed by machining is very small, grinding is the preferred process. The disc shaped workpiece is clamped with a flat side against a flat surface (e.g. by means of a vacuum) or clamped between two flat surfaces. The workpiece thus secured is machined on its outer surface using one or more suitably shaped grinding tools. By comparatively slow inherent rotation of the workpiece about its axis of symmetry, each point of its circumference is brought into engagement with the grinding tool rotating at high speed.

However, this commonly used process has a series of drawbacks which both affect the economics of the process and also raise considerable problems in connection with the requirements which will be imposed in future.

Since a shaped tool is used, a specific tool can only be used for a single contour shape. Any switch to a different shape of rounded edge and thus, generally, any switch to a different thickness of workpiece will necessarily require a change of tool. Apart from the time taken to reequip the machine, the need to store an extensive range of expensive tools is to the detriment of the economy of the process.

The standards demanded of the surface of the rounded edge are becoming ever higher, in view of the environmental requirements, which will become more stringent in future. Attempts have been made to meet these standards by carrying out a plurality of grinding steps one after the other, using finer and finer abrasive grain. However, the particle size of the abrasive grain has a lower limit. Below a certain limiting particle size, grinding is no longer possible in practice, because of the smaller and smaller volume of the machining chamber, which would cause the grinding disc to clog up. Currently, attempts are made to overcome this problem by carrying out, after the grinding operation, a polishing step which is problematic when working with geometries of this order.

The profiled grinding discs used are not easy to adjust, not least because of the minuscule dimensions of the contour. The tool therefore has to be renewed relatively frequently, incurring considerable expense.

A major disadvantage of profiled grinding discs is the fact that they wear out more or less unevenly. This is due to the fact that, depending on the geometric configuration of the grinding disc profile, the thrust component and the normal to the surface generally form different angles at different points on the contour. At those points where the two vectors coincide, comparatively great removal of material can be expected on the grinding disc, whereas at those points where these two vectors are at greater angles to one another, the removal of material at the grinding disc will be less. Thus, as the length of use of the grinding disc increases, its original profile will be lost. This-not only means that the accuracy of the process will increasingly deteriorate but also that the expensive coating of the grinding disc will not be used to the full.

According to one aspect of the present invention, there is provided a method for rounding off the edges of semiconductor discs by a grinding action, wherein the grinding action is carried out by an unprofiled grinding disc, a desired edge profile on a workpiece being obtained by subjecting the rotating workpiece and the rotating grinding disc to an appropriate sequence of movements relative to each other.

According to a second aspect of the present invention, there is provided a method for rounding off the edges of a semi-conductor disc by a grinding action, wherein the grinding action is carried out by a grinding disc, whose grinding surface is guided along a path relative to the semiconductor disc so as to define a desired profile on the semiconductor disc edge, the guide path being established by a predetermined sequence of movements of the grinding disc and/or the semiconductor disc relative to one another.

The invention also extends to apparatus for carrying out the above methods.

In the preferred embodiments of the present invention, the disadvantages of the prior art are overcome, since the desired contour on the workpiece is produced not by a suitably shaped grinding disc but by a tool, which does not need to be profiled, being guided along a suitable track relative to the workpiece, this,relative movement producing the desired contour of the workpiece.

Preferably the grinding disc consists of a disc-shaped base member surrounded by a grinding portion which can be used both on a peripheral face and on an end face. The peripheral face can provide a circumferential grinding disc action, whilst the end face (which faces the axial direction) can provide a cup or crown type 1 grinding disc.

A preferred embodiment of the present invention will now be described by way of example and with reference to the accompanying drawings, wherein:Figures 1 to 6 illustrate in side elevation the principle steps of a manufacturing method according to the preferred embodiment of the present invention; and Figure 7 shows in perspective view the components of an apparatus which is to be used to carry out the method depicted in Figures 1 to 6.

Figure 1 shows both the semiconductor disc or wafer 1, which is to be machined, and also the grinding disc 2 in the starting position before machining. The drawing shows only the top half of the periphery of the workpiece 1 in section; the lower half is symmetrical therewith and has therefore been omitted from the Figure. The semiconductor disc 1 is clamped, in its inner area, on a ceramic clamping plate by means of a vacuum or is clamped between two flat surfaces and rotates about its axis of symmetry. The grinding disc 2, of which only the left-hand half is shown here, is arranged at a specific angle thereto and rotates at the speed of the operation. The grinding disc 2 itself consists of a discshaped base member 3 and an annular grinding member 4 mounted on the circumference thereof, which takes on a specific wear contour 5, over its period of service, as will be shown hereinafter. The annular grinding member 4 projects axially beyond the disc-shaped base member 3, so that its end face 6 can also be used for grinding operations.

Figure 2 shows the start of the grinding operation. The grinding disc 2 is moved axially towards the - 6 semiconductor disc 1, whilst maintaining its inherent rotation. The circular end face 6 of the disc 2 comes into engagement with the wafer 1, so that material is machined off. The axial movement of the grinding disc 2 is stopped at a certain point and reversed after a sparking-out phase. In this way, a substantially rectilinear, conical bevel is produced at the periphery of the flat surface of the wafer 1.

Figure 3 shows the conical'bevel 6 formed by the first part of the grinding operation. After axial retraction, the grinding disc 2 is moved sideways by a defined amount, this movement being directed away from the wafer 1. Figure 3 shows the arrangement at the end of this movement.

As can be seen from Figure 4, the grinding disc 2 is then moved axially towards the wafer 1 again. D ' uring this movement, an additional volume of material is machined off the wafer 1. This movement is stopped, at the latest, when the lower point 8 of the outer surface of the grinding disc reaches the conical bevel 7 produced previously.

After this, the grinding disc 2 together with the spindle and drive unit (not shown here) is pivoted about the point 9. Point 9 is geometrically determined by the enveloping outer surface of the grinding ring 4 and the central plane 10 of the wafer 1. This pivoting_movement is stopped when the axis of rotation of the grinding disc 2 has reached the horizontal, as shown in Figure 5. During this pivoting movement, the point 8 of the grinding disc 2 describes a circular path about the fixed point 9. This geometrically generated arc is the sole determinant of the circular contour section produced on the wafer 1. As a result of the geometrically produced movement of the grinding method, 7 the consequent wear on the grinding disc causes the point 8 to be less remote or, at most, equally remote from the point 9 than any other point on the outer contour 5 of the grinding coating 4 produced by wear.

The shape of the wear contour 5 is shown only roughly in Figures 1 to 5. In actual operation it takes the form of a rather irregular curve which depends to a great extent on the particular conditions of use. However, the exact shape of this wear contour is totally irrelevant to the dimensional accuracy of the arcuate section, since the point out alone determines the contour produced on the workpiece, as has already been pointed out.

The increasing wear on the grinding disc will cause the point 8 to migrate upwards, but because of the geometric situation it will always remain on the outer cylindrical surface enveloping the grinding disc. To this extent, this wear on the grinding disc can very easily be compensated by a suitably axially compensated position of the grinding disc.

A similar view can be taken of the first part of the grinding operation: the annular end face 6 of the grinding coating 4 shown in Figure 1 will also be subject to wear and will thus gradually retreat. However, this movement can also be compensated by axial direction of the grinding disc 2.

The movements illustrated in Figures 1 to 5 relate to the machining of the upper half of the wafer. However, since the workpiece also has to be rounded off underneath, a second apparatus arranged in mirror symmetry therewith is needed to produce the complete workpiece profile as shown in Figure 6.

1 It will be apparent from the foregoing that the contour finally produced on the wafer consists essentially of three sections: adjoining the upper and lower flat surfaces of the wafer are two conical bevels which are in turn connected by an arc made up of two mirrorsymmetrical halves.

The angle of inclination of the conical bevel and the radius of the arc can be freely selected by a suitable setting of the machine parameters. At one extreme, the conical bevel may even merge tangentially into the arc. on the other hand, the angle of inclination of the conical bevel may merge into a horizontal, as a result of which the bevel itself would finally disappear altogether. Thus, by suitable setting of the machine, all possible conical bevels and all conceivable arcs can be combined with one another. Opposing this theoretically infinite variety are the actual practical requirements of the semiconductor industry, the main thrust of which is to avoid edges if at all possible. Consequently, the machine parameters are set so that there is a virtually tangential transition between the conical bevel and the arcuate portion. It will always be desirable to have a flat edge between the conical bevel and the flat surface of the wafer, since a certain thickness tolerance of the wafer must be expected. In the case of thicker wafers, the conical bevel applied at a constant angle will then be somewhat longer, whereas in thinner wafers it is correspondingly shorter., The thickness tolerance expected of the wafers will then determine the angle of inclination of the conical bevel.

Figure 7 shows by way of example, components of a preferred apparatus by means of which the method described above can be carried out. Since the apparatus for machining the upper half of the contour and the lower half of the contour are arranged mirror- 1 9 symmetrically to one another, the representation in Figure 7 is limited to the apparatus for the upper half of the contour. The Figure is purely diagrammatical and has omitted details of construction.

The Figure shows the wafer 1, applied by suction to a ceramic plate. This apparatus for holding the wafer 1 is adjustable by means of a lifting mechanism 11 so that the wafer 1 can be raised or lowered thereby. The wafer itself is rotatable by means of the rotation means 21 and is driven by the motor 22 via a toothed belt. A rotary guide 24 serves to introduce the vacuum needed for clamping the wafer 1 into the rotation means 21.

In the starting position, the grinding disc with the grinding surface 4 is arranged in a coaxial position. The grinding disc carrier 3 with the grinding surface 4 is mounted on a grinding spindle 12, in the form of a motor spindle in the example shown here. The grinding spindle 12 in turn is mounted on a spindle holder 13. The spindle holder 13 is movable by means of the carriage 14 in the axial direction of the grinding spindle 12 relative to the intermediate member 15. The intermediate member 15 is in turn pivotable about the journal 17 by means of the bearing 16. During this pivotal-movement, however, the grinding axis and the rotation axis of the workpiece always remain in a common plane. The position of the rotation axis of the pivotal movement is such that it is precisely at a tangent to the outer surface of the grinding surface 4. The pivotal movement is initiated by a motor 25 via a high stepping-down gear 26. The bearing 16 is securely attached to the displacement unit 18. The displacement unit 18 is movable relative to the machine frame 20 by means of a carriage 19.

- With this assembly of essential machine components the following movements are possible: The wafer 1 is rotatable, so that every point on its periphery is brought into engagement with the grinding surface 4. The wafer 1 is vertically adjustable, so that in the case of a wafer of varying thickness, its central plane can be adjusted relative to the grinding apparatus. The surface 4 of the grinding disc is set in rotation for the-grinding work. The surface 4 of the grinding disc is axially movable via the carriage 14. This movement determines the radius of rounding for the arcuate portion of the wafer contour. In addition, the wear on the grinding disc is thus compensated relative to the position of point 8 and also the end face 6. During the machining of the conical bevel the same movement component serves as an advance component. The rotary movement of the bearing 16 and journal 17 serves to pivot the entire grinding apparatus. This completes the relative movement to produce the arcuate portion of the ground profile. Furthermore, it determines the inclined position for machining the conical bevel.

The movement component of the carriage 19 determines the centre point of the arcuate portion of the edge profile of the wafer. At the same time, it is also adapted to different wafer diameters.

Figure 7 shows the arrangement of the essential machine components merely by way of example. A virtually unlimited variety of other combinations is possible, all of which are suitable for carrying out the process described above. For example, the linear guide 19 may be replaced by a rotary movement about a point which is fixedly connected to the machine frame 20. The axial movement of the wafer 1 may also be carried out by a cross-wise carriage in conjunction with the carriage 19. The carriages 14 and 19 may take the form of roller guides or sliding guides. Instead of pivoting the tool it is also possible to pivot the workpiece. In this case, the pivoting movement about the journal 17 is replaced by a corresponding pivoting of the wafer 1.

An advantage of the preferred embodiments of the invention over the prior art described hereinbefore is the fact that the tool itself does not need to be profiled, since the contour achieved on the workpiece is obtained not by a profiled grinding disc but by a succession of relative movements between the tool and the workpiece which is freely selectable within certain limits.

Additionally, the same tool can be used for grinding when the contour to be ground is changed or when the wafer thickness is changed. There is no longer any need to change the tool; only the parameters of the movement need be changed. This does away with the expense of reztooling and storing an assortment of profiled grinding discs.

Another advantage of preferred embodiments of the present invention is the fact that it is possible to obtain a substantially finer surface than has hitherto been possible. Since the tool no longer needs io be profiled but only has to be sharpened, this grinding process can be operated using an electrolytic truing tool. Whereas, with conventional grinding discs, it is only possible to grind satisfactorily to a particle size down to 15 gm without the grinding disc becoming clogged, the use of metallically bound grinding discs in conjunction with electrolytic planning or truing tools makes it possible to use particle sizes down to about 1 gm. As a result, roughness values on the workpiece are achieved which were hitherto possible only by carrying out a laborious and complex polishing operation.

A further advantage is that virtually the entire volume of the grinding coating actually participates in the grinding process, since there are no profiling operations and the problem of uneven wear of the disc which is characteristic of the prior art does not arise. Consequently, the quantity of abrasive agent used is utilised to the full. This leads to a large increase in the-service life of the tool and drastically reduces the costs of abrasive agent.

Claims

1. A method for rounding off the edges of semiconductor discs by a grinding action, wherein the grinding action is carried out by an unprofiled grinding disc, a desired edge profile on a workpiece being obtained by subjecting the rotating workpiece and the rotating grinding disc to an appropriate sequence of movements relative to each other.

2. A method as claimed in claim 1, wherein the grinding disc consists of a disc-shaped base member surrounded by a grinding portion which can be used both on a peripheral face and on an end face.

3. A method as claimed in claim 1 or 2, wherein the desired edge profile on the semiconductor disc is produced by the end face of the grinding disc generating a conical bevelled portion adjacent to the flat surface of the semiconductor disc, and the peripheral face of the grinding disc generating an adjacent arcuate rounded portion. -

4. A method as claimed in claim 1, 2 or 3, wherein the grinding method is carried out with electrolytic truing.

5. A method for rounding off the edges of a semiconductor disc by a grinding action, wherein the grinding action is carried out by a grinding disc, whose grinding surface is guided along a path relative to the semiconductor disc so as to define a desired profile on the semiconductor disc edge, the guide path being established by a predetermined sequence of movements of the grinding disc and/or the semiconductor disc relative to one another.

1 1

6. Apparatus for carrying out the method as claimed in any of claims 1 to 5, comprising a vacuum suction plate on which the semiconductor disc to be machined is held by suction, the periphery of the semiconductor disc projecting beyond the edge of the vacuum clamping plate, a mounting unit for rotating the vacuum clamping plate together with the semiconductor disc clamped thereon, a mechanism for providing axial positioning of the semiconductor disc, a grinding spindle with a grinding disc mounted thereon which can grind both on its periphery and on its end face, the grinding spindle being capable of being moved axially relative to an intermediate member by means of a linear guide, a pivoting device by means of which the rotation axis of the unit comprising the grinding spindle and grinding disc and the rotation axis of the unit comprising the vacuum clamping plate and semiconductor disc can be tilted or pivoted, but whereby both axes of rotation always remain in a common plane, and a guide by means of which the rotation axes of tool and workpiece, located in a common plane, can be moved towards each other and away from each other.

7. Apparatus as claimed in claim 6, wherein said guide for the relative movement of tool and workpiece relative to one another comprises a linear guide.

8. Apparatus as claimed in claim 6, wherein said guide for the relative movement of tool and workpiece relative to one another comprises rotary units.

9. Apparatus as claimed in claim 6, 7 or 8, wherein the grinding disc is fitted with an electrolytic truing tool.

10. A method substantially as hereinbefore described with reference to Figures 1 to 7 of the accompanying drawings.

t 1 - 15

11. Apparatus substantially as hereinbefore described with reference to Figures 1 to 7 of the accompanying drawings.