GB2149330A - Machining of workpieces consisting of brittle/friable materials - Google Patents

Abstract

The plane-side-transverse grinding process is described which is particularly advantageous for the machining of disc-shaped workpieces 3 of very hard, brittle/friable crystalline materials having a Vickers hardness of more than 7000 N/mm<2>. Using this process such materials may be ground to a thickness of between 60 and 250 mu m. The invention is particularly advantageous when applied to the grinding of workpieces made from monocrystalline or polycrystalline silicon. <IMAGE>

Description

SPECIFICATION Machining of workpieces consisting of brittle/friable materials The invention relates to the machining of workpieces consisting of very hard brittle/friable materials.

The machining of metals and non-metallic materials by grinding is being used increasingly in manufacturing technology, with two objectives, namely to achieve maximum accuracy, surface quality and least possible damage, on the one hand, and improve the machining performance, on the other hand. Hitherto, these objectives have been mutually exclusive since according to the state of the art the construction of machinery and particularly the specification of the grinding tools have to be designed specially in order to achieve one or other optimum.

At present, there are two known grinding processes, namely "circumferential grinding" and 'side grinding'. "Circumferential grinding" is further developed than "side grinding" and to some extent competes with turning, milling and planing in terms of the machining performance and accuracy. The two different grinding processes according to the prior art have been described in detail in the technical literature, and the performance limits are specified in scientific studies, specialist literature and product information and reflect the latest position achieved. Both the abovementioned grinding processes have their own partly overlapping fields of use in which they operate best. Thus, side grinding is superior for very hard, brittle and sensitive materials and very flat planar surfaces.Non-metallic crystalline materials which are used in large quantities in the form of large-area, very thin discs, for example as a substrate for electronic components, occupy a special position in machining. An example is the machining of very thin workpieces in disc form consisting of silicon, germanium, sapphires, garnet, spinel and Ill-V compound, which are used increasingly on a worldwide scale as a substrate for the manufacture of electronic components.

Lapping, which is used exclusively at the beginning of surface machining, is being replaced more and more by grinding with diamond-covered grinding discs. This process was perfected for production by the present applicants.

The aim of this invention is to achieve a better machining performance than in the prior art, with a gentle removal of material, when machining workpieces consisting of extremely brittle/friable, crystalline and often extremely hard materials, whilst the changes to the uppermost layers of material which are unavoidable in any cutting operation and the resulting damage, i.e. the depth of destruction of the crystalline layer near the surface, should be less than with the machining processes currently known and used and at the same time an improvement in surface quality should be achieved, in order to reduce the number of successive operations such as roughing, planing and fine grinding required for such materials.

The damage and visible traces of machining on the ground surface increase the tendency to fracture and alter the electrical values of the workpieces, particularly the substrate discs, to a considerable degree and have a very damaging effect on subsequent treatment and diffusion processes. The slighter the damage, the thinner the workpieces may be ground. Similarly, gentle grinding simplifies and shortens the subsequent operations since the removal of the unavoidable damage by etching is a less laborious process.

Extremely brittle/friable, monocrystalline or polycrystalline materials which are machined according to the invention have a minimum Vickers hardness of from 7000 N/mm2 up to a maximum of 25000 N/mm2. The best known examples of such materials are AIII-BV compounds with a Vickers hardness of up to 8500. Germanium with a Vickers hardness of up to 7500, silicon with a Vickers hardness of up to 11500, spine with a Vickers hardness of up to 14000 and sapphire and galliumgadolinium-garnet (GGG) with a Vickers hardness of from 19000 to 21500. These materials are preferably used in electronics.

Moreover, super-hard sintered materials such as silicon carbide SiC with a Vickers hardness from 15000 to 25500, silicon nitrite Si3N4 with a Vickers hardness of from 1 5000-20000 (or 35000 in the case of the mono crystal) and boron carbide B4C with a Vickers hardness of from 22500 to 31000 may also be advantageously machined. These materials are used in mechanical engineering, motor construction and plant construction.

Another preferred group of materials includes cutting ceramics with a Vickers hardness of from 17000 to 28500 and corundum Al2O3 with a Vickers hardness of 21500. These materials are used as non-metallic cutting materials.

The aim is achieved according to the invention by the use of the plane-side-transverse grinding method for machining disc-shaped workpieces of very hard brittle/friable crystalline materials with a Vickers hardness of more than 7000 N/mm2 to a thickness of between 60 and 250 ym. The invention is particularly advantageous in its application to the grinding of discs made from monocrystalline silicon.

Polycrystalline silicon is also very suitable.

The plane-side-transverse grinding process, by contrast with the grinding processes used hitherto for machining materials of this kind, comprises only two components of movement, namely the rotation of the grinding disc and the approach of the grinding disc at right angles to the workpiece surface which is to be ground. The movement of advance which was regarded as a dominant process parameter in the grinding processes used hitherto for machining materials of this kind no longer occurs.

The grinding process in plane-sidetransverse grinding consists of the immersion or dipping of the rotating grinding disc into the surface of the workpiece, thus suggesting the term "dip grinding" for this machining.

The dimensions of the grinding surface are preferably such that the entire surface of the workpiece to be ground is covered; however, in the case of very extended workpieces, repeated partial dip-grinding is possible, the workpiece being moved along by the width of the grinding surface relative to the axis of the grinding disc after each dip-grinding operation.

Plane-side-transverse grinding, which has hitherto been used only as a relatively rough coarse machining operation with low demands on surface quality, surprisingly results in extraordinary, gentle and good machining characteristics in the case of hard and extremely brittle materials which are prone to destruction of the crystal lattice at the machined surface, in the optimum process according to the invention. Thus, the machining performance is many times greater than the values known at present, even though the surface quality achieved and the harmful changes to the outermost layer of material coming into contact with the grinding disc are comparable with the present machining processes developed especially for these purposes.

Extremely brittle/friable monocrystalline silicon can thus be ground to a final thickness of about 80 item, whilst with the known grinding methods it is generally only possible to carry out reliable machining on workpieces with a thickness 160 ym. Measurements confirm that detrimental changes in the crystal lattice of the material at the grinding surface are comparatively smaller than with the grinding processes used hitherto.

An important advantage of the invention is that the front side of the silicon disc, which has been treated during grinding of the back and is generally coated with relatively soft materials, and which constitutes the contact side with the clamping point of the workpiece during the grinding operation, is much less in danger of being indented and crushed and consequently rejects are avoided.

The most favourable grinding processes used in practice, in which the diamond-coated side grinding disc moves with a very slow advance and low depth of machining over the surface of the workpiece, presuppose a finegrained grinding disc with an elliptically shaped rounding of the cutting edge in order to attain the desired parameters of quality.

These optimal cutting conditions cannot be reliably reproduced in mass-production machines, thus resulting in considerable fluctuations in quality. In the grinding process according to the invention, the only point to watch is that the grinding surface is flat.

Machining performances are achieved with a relatively fine-grained grinding disc which were hitherto achieved only with very coarse roughing discs, but the smoother surface peculiar to the finer grinding disc is retained on the workpiece and is even improved and moreover the surface of the workpiece coming into contact with the grinding disc is less altered or destroyed in the outermost layer.

The smoothness and peak-to-valley height can be improved substantially if, after the end of the preparation of the "dip grinding" the workpiece is drawn out of the area of engagement parallel to the surface of the grinding disc. This evens out the traces of the tips of the grain projecting somewhat further from the surface of the grinding disc. This additional operation can be compared with socalled "sparking out" in greatly simplified form.

The use of the "plane-side-transverse grinding" process according to the invention for machining workpieces consisting of brittle/friable materials of great hardness provides a new grinding technology having numerous advantages.

Since there is no movement of advance during machining, it is possible to construct a simpler and very stable machine. Known automatic single-or multi-station grinding machines operate with rotary attachments as the workpiece carriers and advance mechanism, in which different speeds of advance increas ing towards the outside will necessarily occur.

This disadvantage, which has a negative effect on the material stressing and the surface quality during engagement of the abrasive grain, no longer arises, since, as described above, a final "sparking out" may optionally be carried out as a linear movement, for example of the grinding headstock, in order to obtain a very fine and uniform surface. Conventional rotary attachment-type automatic grinding machines with a plurality of stations have the same speed of advance at every grinding station, irrespective of whether preli minary, fine or very fine grinding is being carried out. In the plane-side-transverse grind ing method used according to the invention in which there is no movement of advance dur ing the grinding process, the "sparking out movement" can be adapted individually and optimally with little mechanical expenditure, and generally a linear sparking out movement is required only at the last grinding station.

Since the workpiece carrier, which may be in the form of a cyclically operating rotary attachment, for example, is stationary during grinding, very precise loading and unloading of the workpiece is achieved, which is also simple to handle.

Another advantage is that during this period it is possible to obtain total spatial separation between the grinding, loading and unloading areas, which is of great importance in the case of high precision workpieces, since the workpiece clamping point can be cleaned and kept clean until the next substrate disc is put on. Any small particles of dust between the workpiece support surface and the substrate disc press on the surface and unavoidably lead to the local cracks known as crows feet" on account of their appearance, in the case of very thin substrate discs.

This invention brings the exceptional advantage of greatly increasing the machining performance whilst keeping the stress on the workpiece and tool relatively low.

This makes it possible to grind brittle materials to very thin thicknesses of between 60 and 250 ym, preferably between 80 and 200 clam, since advantageously the crystal lattice of the material is only slightly changed. Another advantage is that during the grinding operation the workpiece carrier bench is fixedly blocked on the machine frame and is therefore very secure against vibration.

Another advantage of the present invention is the low specific tool costs which result from the long service life of the grinding disc and a major contribution to this is the fact that the maximum possible number of grinding members participate in the cutting process, thereby reducing the strain on the abrasive grain and facilitating the removal of chips, whilst the constant low-vibration cutting force acting on the individual grains can be represented as a spatial vector in terms of the grinding surface.

Thus, thanks to the fact that there is no need for any advance movement, the breaking out of abrasive grain from the binding is retarded.

The grinding discs required for the process used according to the invention are simple and easy to produce. Generally, abrasive materials, preferably diamond particles as the cutting material, will be secured in the form of discs, sheets or strips on a base member. The spaces left free between the individual abrasive elements intensify the cooling effect and make it easier for the chips to be removed.

The spacing between the individual abrasive elements or abrasive segments is smaller than the surface which is to be machined. This prevents a workpiece from getting between two segments and then, when the grinding disc rotates, a conventional side grinding operation occurring in which only the edge of the grinding segment is operative. However, it is also possible to use, to good advantage, grinding discs with a continuous surface the dimensions of which are matched to the size of the workpiece.

In the process used according to the invention, finer grains may be used economically, resulting in finer surfaces when grinding nonmetallic materials.

The accompanying drawings show some advantageous embodiments of the present invention in more detail, which will be described hereinafter by way of example only.

In the drawings: Figure 1 shows a substrate disc of typical dimensions; Figure 2 shows an embodiment of an apparatus for applying the plane-side-transverse grinding process according to the invention; Figure 3 is a diagrammatic view of the size ratios between the grinding surface of the grinding disc and the workpiece in an apparatus as shown in Fig. 1; Figure 4 shows diagrammatic view 4a to 4e of examples of advantageous embodiments of the grinding surfaces of grinding discs for the process used according to the invention.

The substrate disc made of silicon shown in Figure 1 has dimensions which are the prerequisite for the manufacture of high-performance electronic components.

The embodiment of an apparatus shown in Figure 2 consists of a substantially boxshaped machine frame 1 on whose horizontal upper surface is mounted a cyclically rotatable or horizontally movable bench 2 which can be clamped in position during the grinding process for receiving and further transporting the workpieces 3. The workpiece receiving means has a support 4 projecting over it, on which are fixed the drive motor 8 and the grinding spindle 5 which carries the grinding disc 6 with the grinding surface 7 facing the workpieces. The support 4 is fixed to the frame 1 with accurate rigid guide elements 9 so as to be movable vertically up and down.

This vertical up and down movement may be initiated, for example, by an electrically driven feed spindle 10. This feed spindle 10 moves the grinding disc 6 according to the present invention down to the end dimension of the workpiece in "dip grinding" and rushes the entire support back upwards. During this return movement of the support, the finished machined workpiece is moved out of the region of the grinding disc by cyclical rotation or horizontal movement of the bench 2, and at the same time a new workpiece to be machined is brought under the grinding disc and positioned in the working position.

This simultaneous unloading and loading is carried out in harmony with the return movement of the support so that there is no collision between the new workpiece and the grinding disc as long as the transporting movement has not ended and the workpiece carrier has been clamped firmly in place again.

Figure 3 shows a grinding disc for the process used according to the invention. The grinding disc 6 shown has a grinding surface 7 facing the workpiece 3, which covers the entire surface of the workpiece 3 which is fixedly positioned during the grinding operation and is not subjected to any advancing movement. This ensures that all the abrasive grains of the grinding surface 7 act to machine the workpiece during the grinding operation.

Figure 4 diagrammatically shows various possible embodiments of abrasive surfaces which may be used for this invention.

Figure 4a shows a grinding surface formed of pellets 43 of abrasive material. Figures 4b and 4c show grinding discs the grinding surfaces of which consist of square or trapezoidal grinding surface elements 44, 45.

In Figures 4d and 4e the grinding surface elements are rod-shaped 46 or arc-shaped 47.

In all the embodiments 4a to 4e, circular workpieces 41 are shown, and it is clear that the grinding surfaces cover the entire surface area of the workpieces. In each of these embodiments of grinding discs, care is taken to ensure that during the grinding operation all the abrasive grains of the grinding surface segments facing the workpiece are effective in removing material. This forms a clear contrast with conventional side grinding in which only the abrasive grains of the edge of the grinding disc act to remove material.

Another advantage of the invention is the fact that the grinding load and the area of engagement of the grinding disc on the workpiece are constant throughout the grinding period. This ensures a very uniform grinding picture and a very stable grinding operation.

The plane-side-transverse grinding process according to the invention is described, for example, in draft DIN standard 8589, part 11, page 10.

Claims (4)

1. Use of the plane-side-transverse grinding process for machining disc-shaped workpieces of very hard, brittle/friable, crystalline materials with a Vickers hardness of more than 7000 N/mm2 to a thickness of between 60 and 250 item.

2. Use of the plane-side-transverse grinding process as claimed in claim 1 for machining discshaped workpieces consisting of very hard brittle/friable crystalline materials, preferably to a thickness of between 80 and 120 ym.

3. Use of the plane-side-transverse grinding process as claimed in claim 1 or claim 2 wherein the material of the workpiece is monocrystalline or polycrystalline silicon.

4. Use of the plane-side-transverse grinding process substantially as herein described with reference to the accompanying drawings.