Patent Grant
8512099
This application claims priority to German application DE 10 2008 052 793.9 filed Oct. 22, 2008, and application DE 10 2008 061 038.0 filed Dec. 3, 2008, which are herein incorporated by reference.
1. Field of the Invention
The invention relates to a device for the double-sided processing of flat workpieces, comprising upper and lower working discs, at least one of the discs being driven in rotary fashion by means of a drive, and which between themselves form a working gap in which is arranged a carrier disc with a cutout for at least one workpiece to be processed, wherein the carrier disc has teeth on its circumference by means of which it rolls on an inner and an outer gear wheel or pin ring if at least one of the gear wheels or pin rings is set in rotation, wherein the gear wheels or pin rings each have a multiplicity of gear or pin arrangements which the teeth of the carrier discs engage during rolling.
2. Background Art
With devices of this type, flat workpieces, for example semiconductor wafers, can be subjected to material removal processing, for example honing, lapping, polishing or grinding. For this purpose, the workpieces are held in floating fashion in cutouts in carrier discs guided in rotary fashion in the working gap and are simultaneously processed on both sides. In this case, the workpieces describe a cycloid movement in the working gap. With such devices, flat workpieces can be processed on both sides in a highly precise manner.
The contact between the outer teeth of the carrier discs and the teeth of the gear wheels or the pins of the pin rings results in wear on the gears or pins. It is therefore known from DE 295 20 741 U1 to mount sleeves in rotatable fashion on the pins of pin rings, wherein the carrier discs come into engagement with the sleeves. In the case of an embodiment of this type, frictional stress no longer occurs between the carrier disc teeth and the pins. Rather, such contact takes place between the sleeve and the pin. However, since the sleeve bears on the pin over a much greater length, the surface loading and hence the possible abrasion are correspondingly lower. Furthermore, the sleeves can be replaced in a simple manner in the event of wear. By contrast, replacing the pins is comparatively complicated. Further configurations of such sleeves have been disclosed by DE 101 59 848 B1 and DE 102 18 483 B4. EP 0 787 562 B1 discloses sleeves composed of a plastic material.
In the case of the known devices, one problem is that the loading of the carrier discs on account of contact with the gears or pins and sleeves can lead to the teeth of the carrier disc being bent away upward or downward, which regularly leads to damage to the workpieces and also the working discs or their working layers. On account of the low strength, this is particularly critical in the case of plastic carrier discs that are otherwise desired. Moreover, premature wear of the carrier discs can occur in the case of the known devices. This is because the carrier disc partially leaves the working gap, in particular in the region of the gear wheels or pin rings, which can perform undesirable vertical movements owing to the lack of guidance there by the working gap. When this part of the carrier disc reenters the working gap, these movements lead to undesirable contact between the carrier disc surface and the edge of the working discs or their working layer, as a result of which intensified wearing of the carrier disc surface occurs.
The present invention also relates to a method for the simultaneous double-sided material removal processing of a plurality of semiconductor wafers, in which each semiconductor wafer lies freely mobile in a recess of one of a plurality of carrier discs set in rotation by means of an annular outer wheel or pin ring and an annular inner gear wheel or pin ring and thereby moves on a cycloid path curve, while the semiconductor wafers are processed so as to remove material between two rotating annular working discs, and the carrier discs and/or semiconductor wafers temporarily leave the working gap, delimited by the working discs, with a part of their surface during the processing.
For electronics, microelectronics and micro-electromechanics, semiconductor wafers with extreme requirements for global and local planarity, one-side referenced local planarity (nanotopology), roughness and cleanness are used as starting materials (substrates). Semiconductor wafers are wafers of semiconductor materials, in particular compound semiconductors such as gallium arsenide or elementary semiconductors such as silicon and germanium.
According to the prior art, semiconductor wafers are produced in a multiplicity of successive process steps. In general, the following production sequence is used:
Mechanical processing of the semiconductor wafers serves primarily for global planarization of the semiconductor wafer, and also for thickness calibration of the semiconductor wafers as well as removal of the crystalline-damaged surface layer and of processing traces (sawing cuts, incision marks) caused by the previous cutting process.
Methods known in the prior art for mechanical wafer preparation are single-sided grinding (SSG) with a cup grinding disc which contains a bound grinding agent, simultaneous grinding of both sides of the semiconductor wafer together between two cup grinding discs (“double-disc grinding”, DDG) and lapping both sides of a plurality of semiconductor wafers simultaneously between two annular working discs while supplying a slurry of free grinding agent (double-sided plane-parallel lapping, “lapping”).
DE 103 44 602 A1 and DE 10 2006 032 455 A1 disclose methods for simultaneously grinding both sides of a plurality of semiconductor wafers together with a movement process similar to that of lapping, but characterized by the use of a grinding agent which is bound firmly in working layers (“films”, “pads”) which are applied onto the working discs. Such a method is referred to as “fine grinding with lapping kinematics” or “planetary pad grinding” (PPG).
Working layers used in PPG, which are adhesively bonded onto the two working discs, are described for example in U.S. Pat. No. 6,007,407 A and U.S. Pat. No. 6,599,177 B2. During the processing, the semiconductor wafers are placed into thin guide cages, so-called carrier discs, which have corresponding openings for receiving the semiconductor wafers. The carrier discs have outer teeth, which engage in a rolling device comprising an inner and outer gear wheel and are moved thereby in the working gap formed between the upper and lower working discs.
The ability to carry out the PPG method is crucially determined by the properties of the carrier discs and their guiding during the rolling movement. The semiconductor wafers must temporarily leave the working gap with a part of their surface during the processing. This temporary projection of a part of the area of the workpieces from the working gap will be referred to as the “workpiece excursion”. The latter ensures that all regions of the tool are used uniformly and experience uniform wear which preserves their shape, and the desired plane-parallel shape is imparted to the semiconductor wafers without “balling” (thickness reduction toward the margin of the semiconductor wafer). The same applies similarly for lapping with a free lapping abrasive.
The methods of PPG grinding known in the prior art, for example as described in DE 103 44 602 A1 and DE 10 2006 032 455 A1, are however disadvantageous in this regard. With the methods known from the prior art, it is not possible to provide semiconductor wafers with sufficient planarity as far as the outermost marginal region, which are suitable for particularly demanding applications and future technology generations.
Specifically, it has been found that the carrier discs are susceptible to vertical dislocation from their central position until they disengage from the rolling device owing to strong bending. This is to be expected in particular when high or strongly alternating process forces act on the carrier discs as in the case of high removal rates, unfavorably selected process kinematics, or when using particularly fine abrasives in the grinding pad.
The dislocation of the carrier discs is promoted because they have only a small total thickness (at most slightly greater than the final thickness of the semiconductor wafers to be processed) and thus have only a limited strength against bending. Furthermore, the carrier disc is conventionally made of a steel core which is provided with a protective layer. Direct contact of the steel core and the abrasive preferably used in PPG, i.e. diamond, leads to wear of the microedges of the diamond grains owing to the high solubility of carbon in iron, and therefore rapid loss of the cutting acuity of the working layers being used.
Frequent sharpening associated with high wear of the working layers, with the concomitant unstable process management, would also compromise the properties of the semiconductor wafers thereby processed and therefore make the use of PPG methods not only uneconomical, but possibly even unviable for future technology generations.
As is known, the protective layers applied onto the steel core of the carrier disc experience wear. They should therefore have a usable thickness which is as large as possible, in order to allow economical lifetimes of the consumable constituted by the “carrier disc”. The protective layers are furthermore required in order to achieve low sliding friction between the working layers and the carrier discs. Suitable layers consist, for example, of polyurethane. The layer is conventionally soft and does not therefore contribute to the stiffness of the carrier disc. The remaining steel core is therefore much thinner than the target thickness of the semiconductor wafers after the processing by means of PPG.
If the target thickness of a semiconductor wafer with a diameter of 300 mm after processing by means of PPG is for example 825 μm and the total thickness of the carrier disc being used is 800 μm, then 500-600 μm of this 800 μm total thickness of the carrier disc is given over to the steel core which imparts stiffness, and 100-150 μm each to the anti-wear coating on the two sides.
If for comparison the target thickness of the semiconductor wafer after processing by means of lapping is likewise 825 μm, then the carrier disc used for the lapping consists entirely of stiffness-imparting steel and has a thickness of 800 μm.
Since the bending of a plate, for the same material and the same shape and design, is known to vary with the third power of its thickness, a carrier disc with a 500 μm thick steel core bends during PPG about four times as much as an 800 μm thick carrier disc during lapping.
For a carrier disc with a 600 μm thick steel core, the bending during PPG is still 2.4 times that of an 800 μm thick carrier disc during lapping.
In the working gap, the maximum deviation from the plane setting of the carrier disc is limited by the difference between the carrier disc thickness and instantaneous thickness of the semiconductor wafers. This is typically at most 100 μm. Wherever the carrier disc projects inward and outward from the annular working gap and engages into the rolling device comprising the inner and outer pin ring, no measures are implemented in the prior art of PPG methods to limit the possible bending of the carrier disc. Owing to the workpiece excursion required, this unguided region is particularly large.
Bending of the carrier discs leads to the following disadvantages for the semiconductor wafers and for the carrier discs, and therefore to an unstable and critical overall process:
JP 11254303 A2 discloses a device for guiding the carrier discs, which consists of two upper and lower spacers which converge conically or in a wedge shape and which are arranged on the inner margin of an outer gear wheel of the lapping machine. The deformation of thin carrier discs is intended to be able to be prevented by means of such a device. However, the modification described therein for the lapping machine, which moreover is directed at the processing of glass substrates, has substantial disadvantages and is unsuitable for carrying out methods of lapping and PPG grinding with workpiece excursion.
Both when lapping with free cutting abrasive in a slurry and for PPG grinding with abrasive bound firmly in grinding pads, the working layers (cast-metal lapping plates or grinding pad) experience constant wear. The height of the lapping plate or grinding pad decreases continuously and the position of the plane, in which the carrier discs move in the working gap formed between the lapping plates or grinding pads, is displaced progressively.
With increasing wear of the working layer and displacement of the movement plane of the carrier discs, the forcible guiding device disclosed in JP 11254303 A2 constrains the toothed outer region of the carrier discs to roll increasingly in a different plane. This means that the wedge-shaped guide blocks, screwed firmly to the outer toothed wheel, would additionally bend the carrier disc with increasing wear of the working disc, which is disadvantageous. Another disadvantage is that the guide blocks need to be unscrewed before it is possible to change the carrier disc, which is necessary from time to time. This represents additional outlay.
In PPG grinding methods, carrier discs are conventionally used with a coating, which is necessary in order to avoid direct contact between the stiffness-imparting core of the carrier disc and the abrasive of the grinding pad (for example diamond). Owing to the design, the spacers described in JP 11254303 A2 engage far into the carrier disc and in each case sweep over the coating of the carrier disc in its marginal region. Owing to the vertical constraining forces which occur during the guiding of the carrier disc, the coating of the carrier discs is therefore exposed to particularly high wear in the guided region, when a device according to JP 11254303 A2 is employed. Another disadvantage of using the solution proposed in JP 11254303 A2 for PPG methods is therefore that the guide ring is engaged far into the carrier disc and can thus damage the carrier disc coating (for example polyurethane).
No satisfactory solution to the problem of carrier disc bending in the region of the workpiece excursion is therefore known from the prior art.
It is an object of the present invention to provide a suitable device with which the wear and the risk of damage of the carrier discs and of the workpieces (e.g. semiconductor wafers) can be minimized, and furthermore a method for the simultaneous double-sided material removal processing of a plurality of semiconductor wafers, which prevents the carrier disc from bending out of the movement plane in the region of the workpiece excursion.
These and other objects are achieved, firstly, by means of a device for the double-sided processing of flat workpieces (1), comprising an upper working disc (4b) and a lower working disc (4a), wherein at least one of the working discs (4a, 4b) can be driven in rotary fashion by means of a drive, and wherein the working discs (4a, 4b), form between themselves a working gap (64), in which is arranged at least one carrier disc (5) with at least one cutout (25) for at least one workpiece (1) to be processed, wherein the at least one carrier disc (5) has teeth (10) on its circumference, by means of which teeth it rolls on an inner and an outer gear wheel or pin ring (7a, 7b) if at least one of the gear wheels or pin rings (7a, 7b) is set in rotation, wherein the gear wheels or pin rings (7a, 7b) each have a multiplicity of gear or pin arrangements with which the teeth (10) of the carrier discs (5) enter into engagement during rolling, wherein at least one of the pin arrangements has at least one guide (48) which delimits a movement of the margin of the at least one carrier disc (5) in at least one axial direction, wherein one guide (48) is formed by at least one shoulder (50) extending around the circumference of the pin arrangement between a first, larger diameter and a second, smaller diameter of the pin arrangement and a further guide (48) is formed by the side surfaces (56, 58) of at least one groove (15) extending around the circumference of the pin arrangement.
The following is a list of references used in the specification:
The shoulder of the guide can particularly be perpendicular to a longitudinal axis of the pin arrangement or the sleeve respectively. The shoulder can also be formed by a sloped surface. The groove can also be perpendicular to the longitudinal axes of the pin arrangement or the sleeve respectively. The groove can have a square-shaped cross-section. The edges of the carrier discs are thereby guided by the side surfaces of the groove and therefore delimited in their movement on both sides in axial direction. The combination of shoulder and groove increases the flexibility of the use of the device as carriers with very different thicknesses can be used, wherein one type of carrier is guided in the groove and another, possibly much thicker type of carrier is guided by the shoulder.
The gear or pin arrangements according to the invention can have different diameters over their outer circumference in a longitudinal (or axial) direction, and can each have a substantially cylindrical form. They have on their outer surface, for example extending around their circumference, a guide which delimits the axial movement of the carrier disc margins, with the result that the latter are held substantially in the carrier disc plane. The gear arrangements can have corresponding guides. The guide can delimit the movement of the margins of the carrier discs in one or both axial directions, that is to say for example vertically upward and/or downward.
Furthermore, the delimitation can completely prevent the movement in the at least one axial direction or still permit a slight movement. According to the invention, therefore, the undesirable vertical movement of the carrier discs, in particular outside the working gap, is largely avoided by means of the guide. The risk of damage to the carrier discs and the workpieces to be processed is minimized.
The workpieces that are simultaneously processed on both sides by means of the device can be semiconductor wafers, for example. Material removal processing, for example grinding, lapping, polishing or honing, can be effected by means of the device according to the invention, and for this purpose, the working discs can have suitable working layers.
According to the invention, in particular, a plurality of carrier discs can be provided. The latter can in turn have a plurality of cutouts for a plurality of workpieces. The workpieces held in the carrier discs move along a cycloid path in the working gap.
Each pin arrangement can have a guide according to the invention. However, it is also conceivable to provide at least some of the pin arrangements with a guide, but not all. The gear or pin arrangements can be embodied in one part or in multiple parts. In principle, it is conceivable for the pin arrangements to consist in each case only of one pin, on whose outer surface itself the guide is formed. However, it is also conceivable for the pin arrangements to consist of a plurality of parts. In this case, therefore, the expression gear or pin arrangement encompasses not only the pins or gears themselves, but also for example separate component parts which, however, are connected thereto. Likewise, the feature that at least one gear or pin arrangement has a guide also encompasses for example the provision of the guide between adjacent pins or gears, whether or not the guide is connected to the pins or gears.
According to a further embodiment the at least one groove is formed at the larger diameter portion of the pin arrangement. Furthermore the smaller diameter of the pin arrangement can end in a free end of the pin arrangement starting from the shoulder without any diameter enlargement.
It is furthermore preferred if the pin arrangements of at least one of the pin rings are formed in each case from a pin and a sleeve mounted in rotatable fashion on the pin, wherein at least one of the sleeves, in particular all of the sleeves, for example, has the guide on the outer circumference thereof.
The sleeves, which can be embodied in one part or in multiple parts, can be arranged on the pin directly in rotary fashion or can be arranged on the pin for example by means of an inner casing serving as a sliding bearing.
The guide can be incorporated into the sleeve itself. However, it is also conceivable, of course, to arrange on the outer surface of the sleeve a further device, for example a ring or the like, which then forms the guide. Through the use of sleeves, the wear of the carrier discs and of the pin rings can be reduced in a manner known per se. At the same time, the wear and the risk of damage of the carrier discs are minimized further by the guide formed on at least one of the sleeves. The sleeves can be provided in particular on the outer and the inner pin ring or just one of the pin rings. They can furthermore consist for example of a steel material (e.g. a hardened steel material, in particular high-grade steel material). Such a material is particularly resistant to wear. However, it is also conceivable to produce the sleeves from a different material, for example a plastic material. Metal abrasion is avoided by choosing a plastic.
According to one configuration, at least one of the guides can have at least one radially extending guide surface. The guide surface extends in a radial plane, that is to say in particular a horizontal plane. The carrier disc then bears on the radial guide surface during processing and the movement of its margin is thus delimited in at least one axial direction.
Furthermore, the at least one pin arrangement or sleeve can have a plurality of grooves which are spaced apart axially with respect to one another, extend around the circumference of the pin arrangement or sleeve and the side surfaces of which in each case delimit a movement of the margin of the at least one carrier disc in an axial direction. The grooves can in turn extend perpendicular to the longitudinal axis of the pin arrangement or sleeve.
The grooves can furthermore have different widths. In this case, the groove width can be adapted to the thickness of the carrier discs that are to be guided in each case. In this way, by means of a suitable height adaptation of the pin arrangements, carrier discs of different thicknesses can be guided with the same pin arrangements or sleeves. This increases the flexibility of the device.
It goes without saying that the radial guide surfaces, shoulders and/or grooves according to the invention can be combined with one another in any desired manner. Thus, by way of example, the pin arrangements or sleeves can each have at least one such shoulder and/or at least one such guide surface and/or one or a plurality of such grooves. The scope of use of the device is thereby extended. In particular, workpieces having considerably different thicknesses can then also be guided with the same pin arrangements.
According to a further configuration, at least one groove can have a width which is greater than the thickness of the at least one carrier disc to be guided by 0.1 mm to 0.5 mm. This provides a small amount of play for the carrier disc in the groove opening which reduces the wear.
According to a further configuration, the at least one guide surface or the at least one shoulder or the at least one groove can have at least one circumferential bevel. Such a bevel leads to a facilitated entry of the carrier discs into the guide, for example the groove, and thus to a reduced wear. The risk of damage to the carrier discs and the workpieces is thereby reduced. The bevels can be formed at the edge of the shoulder or at one or both edges of the groove opening and in a manner extending around the circumference of the pin arrangement or sleeve. It has proved to be particularly suitable in practice for the bevel to have an opening angle of 10° to 45° relative to the guide surface or relative to the shoulder or relative to the groove.
As an alternative to the provision of bevels, for the same purpose it may also be provided that the at least one guide surface or the at least one shoulder or the at least one groove has at least one rounded edge. In turn, it is correspondingly possible in the case of grooves, of course, for both edges of the groove opening to be rounded.
Owing to the fact that for the carrier discs the risk of damage is minimized according to the invention, it is advantageously possible, in accordance with a further configuration, to produce said carrier discs from a nonmetallic material, in particular a plastic. In the prior art, such nonmetallic carrier discs are virtually impossible owing to the risk of damage to the carrier discs.
According to a further configuration, the gear wheels or pin rings can be mounted by means of a height-adjustable mount, wherein a lifting device is provided for the mount. The height of the gear wheels or pin rings and thus of their gear or pin arrangements can thus be varied. If the gears or pins or sleeves have for example a plurality of guides spaced apart in an axial direction, e.g. grooves and/or shoulders having different thicknesses, by means of the height setting it is possible to set the gear wheels or pin rings to the corresponding carrier discs having different thicknesses.
Objects of the invention are further achieved by a first method according to the invention for the simultaneous double-sided material removal processing of a plurality of semiconductor wafers, in which each semiconductor wafer (1) lies freely mobile in a recess of one of a plurality of carrier discs (5) set in rotation by means of an annular outer drive wheel (7a) and an annular inner drive wheel (7b) and thereby moved on a cycloid path curve, while the semiconductor wafers (1) are processed so as to remove material between two rotating annular working discs (4a) and (4b), and the carrier discs (5) and/or semiconductor wafers (1) temporarily leave the working gap, delimited by the working discs (4a) and (4b), with a part of their surface (6) during the processing, wherein the carrier discs (5) are guided in a movement plane which essentially extends coplanar with a midplane of the working gap by the carrier discs being guided in that movement plane, during the excursion of a part of the area of the carrier discs and/or semiconductor wafers from the working gap, in grooves (15) of a plurality of grooved sleeves (12) mounted on a pin (11) and fitted on at least one of the two gear wheels (7a) or (7b).
Furthermore, these objects are also achieved by a second method according to the invention for the simultaneous double-sided material removal processing of a plurality of semiconductor wafers, in which each semiconductor wafer (1) lies freely mobile in a recess of one of a plurality of carrier discs (5) set in rotation by means of an annular outer drive wheel (7a) and an annular inner drive wheel (7b) and thereby moved on a cycloid path curve, while the semiconductor wafers (1) are processed so as to remove material between two rotating annular working discs (4a) and (4b) which comprise working layers (3a) and (3b), and the carrier discs (5) and/or semiconductor wafers (1) temporarily leave the working gap, delimited by the working discs (4a) and (4b), with a part of their surface (6) during the processing, wherein the carrier discs (5) are guided in a movement plane which essentially extends coplanar with a midplane of the working gap by the two working discs (4a) and (4b) respectively comprising an annular region (18a) and (18b), which contains no working layer (3a) and (3b) and ensures guiding of the carrier disc (5) during the excursion of the carrier discs (5) and/or semiconductor wafers (1) from the working gap.
The first and second methods preferably involve double-sided grinding of the semiconductor wafer, each working disc comprising a working layer of abrasive material (in particular PPG methods). In the first method according to the invention, double-sided lapping of the semiconductor wafers, while supplying a suspension which contains an abrasive material, is likewise preferred.
Lastly, the first and second methods according to the invention may also involve double-sided polishing, while supplying a dispersion which contains silica sol, in which case each working disc comprises a polishing pad as a working layer. In fact, no workpiece excursion takes places in double-sided polishing. Nevertheless, the carrier discs emerge from the working gap even in DSP so that guiding the carrier discs according to the first method according to the invention is also advantageous for DSP.
The invention will be described in detail below with the aid of
A double-sided processing machine 42 with planetary kinematics is illustrated in the example in
For processing purposes the workpieces to be processed are inserted into the cutouts 25 in the carrier discs 5 (not illustrated). As a result of the pivoting of the pivoting arm 43, the two working discs 4a, 4b are aligned coaxially with respect to one another. They then form between themselves a working gap, in which are arranged the carrier discs 5 with the workpieces held by the latter. With at least one rotating upper or lower working disc 4a, 4b, afterward the upper working disc 4b, for example, is pressed onto the workpieces by means of a highly precise loading system. From the upper and lower working discs 4a, 4b, in each case a press-on force thus then acts on the workpieces to be processed and the latter are simultaneously processed on both sides. The construction and the function of such a double-sided processing machine are known per se to the person skilled in the art.
The sleeve 12 illustrated in
In the case of the exemplary embodiment of a sleeve 12 according to the invention as illustrated in
The exemplary embodiment shown in
The sleeves 12 illustrated in
The sleeve 12 illustrated in
It is pointed out that even though a machine having pin rings (7a, 7b) is described in the exemplary embodiment illustrated, the pin arrangements correspondingly having a guide for the carrier discs, according to the invention a machine having a gear wheel arrangement, that is to say an inner and outer gear wheel instead of the inner and outer pin ring, could likewise be provided, in which case the gear arrangements can then have corresponding guides.
The lower working disc 4a is represented with the applied lower working layer, consisting of a working layer carrier 2a and a working layer 3a, and the rolling device formed by the inner pin ring (7b) and outer pin ring (7a) for the workpiece guide cage (“carrier discs”, 5) with inserted workpieces 1 (semiconductor wafers). 11 and 12 denote the pin and pin sleeve of the pin ring.
Owing to wear, the working layer experiences a thickness reduction during the processing. This takes place inside the annular surface over which the semiconductor wafers sweep in the course of the processing. When the annular surface lies inside the annular working layer, a “trough-shaped” thickness profile is set up radially over the working layer. This leads to enhanced material removal at the edge of the semiconductor wafer (“edge roll-off”), which is undesired. Yet when the working layer lies fully inside the swept annular surface, the semiconductor wafers experience a workpiece excursion and an edge roll-off does not occur.
A workpiece excursion is known, for example, from DE 102 007 013 058 A1. Owing to a workpiece excursion, the carrier disc also projects over a sizeable length without guiding from the working gap formed by the upper and lower working discs.
The bending of the carrier disc in the prior art and also the guiding of the carrier discs outside the working gap by means of a support ring are illustrated schematically below (
In PPG methods, the bending is further promoted by the carrier discs consisting only of a thin stiffness-imparting core material 30, for example of steel, which is coated on both sides with an anti-wear coating 29 that does not contribute to the stiffness (
In the prior art without carrier disc guiding in excursion (
In the prior art, suitable carrier discs usually have “plastic insert” which lines the reception opening. An example is shown by
As a suitable countermeasure,
The upper carrier disc guide 13b may be guided around the pin sleeves 12, as shown in
Further variants consist in fitting the upper carrier disc guide 13b on the machine frame 8 (
In the latter case (
The pins 11, or pin sleeves 12, of the inner and outer pin rings 7a and 7b of the PPG grinding device transmit all the forces required for rolling and movement of the carrier disc 5 in the working gap. High compressive forces therefore occur between the (rotatable) pin sleeve 12 and the flank of the outer teeth of the carrier disc 5, and also friction forces in the case of pin rings 7a/7b with stiff forces (non-rolling running). The pins/pin sleeves 11/12 and tooth flanks must therefore have a high material strength. The material of the core of the carrier disc 5, which imparts the stiffness necessary for the carrier disc 5 and therefore usually consists of (hardened) steel, another (hardened) metal or a (fiber-reinforced) composite of high-strength plastic, satisfies this strength condition anyway. Similar materials with high strength and low wear are preferred for the pins 11 and pin sleeves 12. The pins 11 and pin sleeves 12 are therefore preferably made of steel or another (hardened) metal, particularly preferably of hard metal (cemented carbides, tungsten carbide etc.). For critical applications, in which contamination of the workpieces by metal abrasion must be avoided, it is also preferable to use sleeves 12 made of a high-strength composite plastic, in particular glass- or carbon fiber-reinforced PEEK (polyether ether ketone) or other thermo- or duromer composite plastics, as well as those made of materials with high abrasion strength and/or low sliding friction, for example fiber-reinforced polyamide (“nylon”), aramid (PAI, PEI), polyacetal (POM), polyphenyl (PPS), polysulfone (PSU).
The embodiment of pin rings in which the pins 11 carry rotatable sleeves 12 that can follow by corotation the relative movement between the gear wheel 7a/7b and the outer teeth of the carrier disc 5, which occurs during the rolling of the carrier disc 5, is particularly preferred. So that the sleeves 12 can rotate particularly easily and with low wear on the pins 11, the sleeve 12 may also be configured in multiple parts and consist externally of the particularly suitable high-strength material described, which enters into engagement with the carrier disc 5, and internally of a material with a low sliding friction coefficient (for example polypropylene PP, polyethylene PE, polyamide [polyamide 6, polyamide 12, polyamide 6], polyethylene terephthalate PET, polytetrafluoroethylene PTFE (“Teflon”), polyvinylidene difluoride PVDF etc.). The inner sliding layer may be configured in the form of an inner coating, or inner sleeves or rings which are pressed or adhesively bonded in.
Vertically, the sleeves 12 are preferably guided loosely by screwed “caps” of the pins 11 or by a ring connected to the entire outer pin ring 7a/7b, so that they cannot slip off the pins and are guided on them with a more or less large play in the vertical direction, more or less uniformly in a plane.
The carrier discs 5 preferably consist of hardened material (for example hardened steel), and the engagement surface of the outer teeth with the sleeves 12 of the pin ring 7a/7b is very small. The pin sleeves 12 therefore experience increased wear. The wear is particularly high in the outer pin ring 7a, since high torques are transmitted there (greater lever).
Multiply grooved sleeves 12 are preferably used, since a different channel 15 can be used after wear without having to replace the entire sleeve.
In the PPG grinding method, carrier discs are used which are provided with a coating that prevents contact of the (metallic) core of the carrier disc with the abrasive of the working layer. During processing of the workpieces with the PPG grinding method, the carrier discs in the working gap slide over the working layer (grinding pad). Shear and friction forces then occur on the coating of the carrier discs. At contour edges of the coating, these forces are particularly high and particularly detrimental peel forces occur. In order to avoid detachment of the coating or increased wear, in particular on the contour edges of the carrier disc coating, the coating—particularly in the case of only partial surface coating likewise according to the invention—is configured so that the length of the contour edges is as short as possible and the profile of the contour edges has curvatures which are as small as possible. Preferably, therefore, in particular an e.g. annular region along the profile of the outer teeth of the carrier discs is left uncoated. For example, the coating is configured circularly and extended onto the outer teeth only as far as the base circle of the outer teeth. Particularly preferably, the diameter of such a circular coating is even somewhat smaller than the base circle diameter of the outer teeth. (On the other hand, the region left free by the coating must not be so large that parts of the exposed metallic carrier disc core come in contact with the diamond of the grinding pad owing to bending of the carrier disc. The preferred width of the annular region, exposed in addition to the teeth left uncoated inside the base circle of the outer teeth, is therefore 0-5 mm.)
In the preferred embodiment of the present invention for guiding the carrier discs in the region of the workpiece excursion in the form of grooved pin sleeves or pins, the guiding channels come into contact with the carrier disc only along the tooth flanks of the outer teeth. In particular, the grooved pins or pin sleeves therefore never come in contact with the coating of the carrier discs, so that this is spared and not exposed to any additional wear. It is particularly preferable to coat the carrier discs with a thermosetting polyurethane elastomer with a layer hardness of from 50 to 90 Shore A, and particularly preferably from 60 to 70 Shore A.
Partial coating of the working layer carrier with a working layer, however, is difficult in terms of manufacturing technology.
The embodiment in
Outer rings 18a and 18b which are mounted outside the outer edge of the working layer (
The outer and inner rings preferably have the same ring width, since the extent of the workpiece excursion “outward” and “inward” is usually identical.
The inner diameter of the outer rings is equal to or greater than the outer diameter of the working layer carriers 2a/2b with a working layer 3a/3b, and the outer diameter of the inner rings is equal to or less than the inner diameter of the working layer carriers 2a/2b with a working layer 3a/3b.
Most preferably, the outer margins of the outer rings and the inner margins of the inner rings project beyond the outer and inner edges 4a (bottom) and 4b (top), respectively, and as close as possible to the sleeves 12 on the outer (7a) and inner pin ring 7b (not shown), so that the carrier discs are guided over a region which is as large as possible and only very small maximum bending of the carrier disc is achieved (16 in
The present invention also relates to a semiconductor wafer.
Semiconductor wafers, which are produced by PPG according to the prior art, have a range of undesired properties which makes them unsuitable for demanding applications.
Thus, the migration 21 described in
Furthermore, semiconductor wafers processed by PPG methods not according to the invention often have an anisotropic distribution of the processing marks (grinding cuts) due to the PPG processing. 37 denotes grinding marks which have been imparted with a preferential direction along the grinding tool movement in the excursion region of the semiconductor wafer (
A semiconductor wafer processed by methods according to the invention, in which the carrier disc was guided in the movement plane without constraint, does not show these defects (
The method according to the invention therefore also provides a semiconductor wafer which has particularly good properties in respect of isotropy, rotational symmetry, planarity and constant thickness, and is therefore suitable for particularly demanding applications.
A Peter Wolters AC-1500P3 polishing machine was used for the examples. The technical features of such a device are described in DE 10007389 A1. Grinding pads with abrasives firmly bound therein were used. Such grinding pads are disclosed in U.S. Pat. No. 6,007,407 A and U.S. Pat. No. 5,958,794 A.
Monocrystalline silicon wafers with a diameter of 300 mm were provided as workpieces to be processed, which had an initial thickness of 915 μm. In the PPG grinding, material removal of 90 μl m took place so that the final thickness of the silicon wafers after processing was about 825 μm.
The carrier discs used had a steel core with a thickness of 600 μl m and were coated on both sides with a PU anti-wear layer having a thickness of 100 μl m on each side.
The process pressure selected for the working discs was 100-300 daN to simulate different loading situations, and on average gave removal rates of 10-20 μm/min.
Deionized water (DI water) was used as a cooling lubricant with flow rates of between 3 and 20 l/min, adapted to the respectively resulting removal rates and the different heat inputs resulting from this during the processing.
In the first example, a corresponding process was carried out without any carrier disc guiding. Even during the first run, damage of the silicon wafer at the margin occurred owing to the insert being torn out of the carrier disc and a grinding block or parts thereof being torn off.
In a second example, a process was carried out with a removed grinding pad section taken off. No damage of the silicon wafer occurred here, although there was slight roughening of the semiconductor wafers in the outer marginal region. The geometry of the silicon wafers was acceptable.
In a second example, a process was carried out with carrier disc guiding by grooves in sleeves on the outer pin ring. The silicon wafers showed good geometry, a homogeneous micrograph up to the wafer margin, and no damage of the semiconductor wafer edge. Four runs were possible without damage/roughening of the running discs, the insert, the coating and without attack or tearing of the outermost grinding blocks.
While embodiments of the invention have been illustrated and described, it is not intended that these embodiments illustrate and describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention. It should be noted that the term “pin” in the claims corresponds to a single part pin as well as pins composed of a plurality of parts, for example, but not by limitation, a pin with a grooved or shouldered sleeve surrounding it.
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