Patent Publication Number: US-6220938-B1

Title: Grinding machines

Description:
This invention relates to grinding machines and to grinding wheels for use in such machines for grinding notches in the edges of discs such as wafers of silicon for use in the construction of semi-conductor devices, and to methods of grinding edge regions of such discs so as to form notches therein. Since the notches are of relatively small dimensions relative to the size of the wafers, grinding wheels used to form such notches are commonly referred to as grinding pins. 
     BACKGROUND TO THE INVENTION 
     A grinding machine for grinding discs is disclosed in WO97/48522 and incorporated herein by reference. WO97/48522 discloses use of a metal-bonded CBN or diamond wheel on a grinding machine to rough grind the edge of a disc, such as a semiconductor wafer, before use of a softer resin-bonded CBN wheel for finish grinding the disc edge and further describes an in situ technique for forming and re-forming a groove in the resin bonded CBN grinding wheel to grind the correct shape around the disc edge. 
     The machine also includes a small diameter grinding pin for grinding a notch of predetermined proportions around the edge of the ground disc. 
     The use of a resin-bonded CBN wheel for notch grinding has the disadvantage that such wheels are relatively soft compared with metal-bonded CBN or diamond wheels, and as such wear rapidly and need to be replaced frequently. Therefore such wheels have tended not to be used in such applications although the reduced damage to the workpiece resulting from the use of such wheels means that it is desirable if they could be used for notch grinding. 
     The present invention aims to provide a formable grinding pin for notch grinding which can be used for longer before it has to be replaced, and to a method of forming such a notch grinding pin and to an improved method of notch grinding. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention, a grinding pin for notch grinding comprises a cylindrical region of formable grinding material, in which is formed a groove having a profile which corresponds to that desired for an edge of a notch to be forced, and wherein the axial extent of the cylindrical region is such that further grooves may be formed subsequently therein as the first and then each of the other grooves becomes too worn to be capable of being reformed, and reused. 
     The invention thus provides a notch grinding pin, or wheel, whose axial width is such as to enable a plurality of grooves successively to be machined therearound for notch grinding. 
     The advantage of the invention is obtained if the formable pin is mounted in a spindle of a grinding machine and is initially formed, and re-formed as required, in situ. 
     Desirably the formable material comprises a resin-bonded material, or a vitreous-bonded material, such as grinding grit bonded by a resin or vitreous material. 
     Preferably the grinding machine is a CNC grinding machine. 
     Using a wide (cylindrical) pin and forming and re-forming the grooves in situ, allows more grinding operations to be performed before the pin has to be replaced. Thus after each groove formed around the cylindrical surface of the pin is no longer capable of being reformed to accurately grind notches, a further groove can be formed at an axially spaced location across the width of the pin, so increasing the useful life of the pin and reducing the number of times the more complex operation of replacing the pin (involving considerable machine down-time) is required. 
     The length of the cylindrical region is typically of the order of 10 mm, preferably 6 mm, and its overall diameter is typically 4 mm. 
     In a method of notch grinding in accordance with the invention, a first groove is formed around a cylindrical region of a formable grinding material using a groove-forming grinding wheel also mounted on the same grinding machine, and after grinding to form one or more notches, or as required, the notch-forming groove is reformed using the same, or another, forming wheel, until it is not possible to accurately re-form the groove, after which a subsequent groove is formed in a similar manner in the cylindrical surface adjacent the first, to permit the notch grinding process to continue. 
     In a preferred embodiment of the invention, the cylindrical region of the grinding pin may comprise the formable grinding material region, and a metal-bonded grinding material region, wherein the metal-bonded region is provided with a groove for rough grinding a notch, and the formable grinding material region is formed with a groove to permit finish grinding of a notch previously formed by the groove in the metal-bonded region of the pin, and the axial extent of the formable grinding material region is sufficient to permit additional grooves to be formed therein as each groove becomes unusable. 
     According to a further aspect, the invention also relates to a machine having a work spindle, a grinding spindle having a small diameter notch-forming grinding pin as aforesaid mounted thereon and engageable with an edge region of a workpiece carried by the work spindle, and a forming wheel mounted on and rotatable by a spindle which when engaged with the notch grinding pin, will form a notch-forming groove therearound as required. 
     The notch grinding pin is preferably formed from formable grinding material but may also comprise a first region of metal bonded grinding material also formed with a notch grinding groove, to allow rough grinding of the metal to be performed first, and thereafter the formable region to be used to finish grind the notch. 
     The forming wheel may be mounted on the same spindle as the workpiece, and the notch forming pin is moved as required to engage the forming wheel or the workpiece edge. 
     It has been found that formable notch grinding pins, formed on the machine (in situ), produce a better finish in, and less sub-surface damage around, the notch. 
     Preferably the grinding material is resin-bonded diamond, or resin bonded CBN, or vitreous bonded material. 
     The invention also lies in disc-shaped workpieces with at least one notch around their edge having an internal edge profile formed at least in part using a formable grinding pin having a grinding groove formed therein by in situ forming on the machine by a forming wheel mounted for rotation on the workspindle of the machine. 
    
    
     The invention will now be described by way of example, with reference to the accompanying drawings, in which: 
     FIG. 1 is a perspective view, not to scale, from the side on which a user normally stands of a grinding machine incorporating a formable grinding pin carried on a spindle; 
     FIG. 2 is a side view, not to scale, from the side on which a user normally stands of the machine shown in FIG. 1, and illustrating a sub-assembly; 
     FIG. 3 is an end view of the machine shown in FIGS. 1 and 2; 
     FIG. 4 is a side view of the sub-assembly end of the machine to an enlarged scale and partly in section; 
     FIG. 5 is a perspective view of the formable pin; 
     FIG. 6 is a perspective view, not to scale, of the formable pin with a first groove for notch grinding; and 
     FIG. 7 is a perspective view, not to scale, of a combined formable material and metal-bonded material grinding pin. 
    
    
     DESCRIPTION 
     FIGS. 1,  2  and  3  illustrate pictorially part of an overall machine station for notch grinding circular discs (wafers) of silicon or similar material. 
     The machine shown in FIGS. 1 to  3  comprises a control cabinet  10  from which extends a machine bed  12  which carries a floating platform  14  carried on three vibration absorbing feet, one of which can be seen in FIG. 1 at  16  and the second part  18  is mounted centrally before the base region  22  and is shown in dotted outline in FIG. 1, and the third can be seen at  20  in FIG.  3 . 
     The platform  14  includes an integral support structure or base  22  which carries a workhead  24  which is slidable axially along a slideway  26  mounted on an upper surface of the base  22  and which includes a spindle drive motor  28  and vacuum chuck  30  for carrying wafers to be ground. 
     Edge grinding is achieved by means of a grinding wheel  32  containing a number of annular grooves such as  34  for engaging the edge of a wafer workpiece designated in FIG. 2 at  36 . 
     The grinding wheel spindle (not shown) carried in bearing assembly  38  is rotated by an electric motor  40 . 
     Items  38  and  40  are carried on a support generally designated  42  which is mounted close to the centre line of the platform  14  to one side of a rigid strengthening plate  44  which is bolted through flanges to the platform  14  along its base and is secured at its upper end by bolts through another flange  46  to the machine base  22 . The function of the plate  44  is to increase the rigidity of the platform  14  relative to the base  22  and resist transverse vibrations which might otherwise be introduced. 
     Equidistant from and on the other side of the plate  44  is a second support  48  which carries a slideway  50  on which is mounted a second spindle drive  52  which carries a notch grinding spindle  57  having a notch grinding pin  53  at one end, and associated spindle motor  55 . 
     Axial movement of the spindle drive  52  is provided by a drive unit  54  (see FIG.  2 ). The spindle drive  52  can also be used to grind the internal diameter of an annular disc. 
     The workhead edge grinding and notch grinding spindles are mounted in air bearings and the workhead spindle typically has a speed range of 2 to 1000 revs per minute, the edge grinding spindle typically has a speed range up to 6000 revolutions per minute and the speed of the notch grinding spindle  53  is typically up to 70,000 revolutions per minute. 
     On the workhead spindle to the rear of the chuck  30  are mounted forming wheels best seen in FIG. 2 at  56  and  58 . Indexing the workhead  24  in the direction of the arrow  60  in FIG. 2 allows the workpiece disc  36  to be engaged by one of the slots such as  34  in the grinding wheel  32  and further movement in the direction of the arrow  60  allows the disc  36  to clear the end face  62  of the grinding wheel assembly and to allow the forming wheels  56  or  58  to engage in the appropriate grooves in the grinding wheel  32 . 
     Lateral movement of the grinding wheel or notch grinder as required is achieved by tilting the support structures  42  and  48  as appropriate relative to the platform  14 . To this end both of the structures  42  and  48  are pivotally attached to the platform  14  near the centre line thereof and two stops  64  and  66  respectively (see FIG. 3) prevent excessive outward movement. 
     The pivoting is provided by means of flexures (as will be described) which allow for pivoting movement about two parallel axes close to the centre line of the platform  14  so that structure  42  can describe a small arc such as denoted by arrow  68  and structure  48  can describe an arc as denoted by reference numeral  70 . 
     Drive means for achieving the pivoting movement will be described with reference to later figures. 
     Attached to the base  22  is a clear polycarbonate rectilinear housing  72  through which the grinding wheel spindle protrudes. A large, generally oval opening  74  in the face of the housing  72  allows a similarly shaped closure  76  mounted on the workhead  24  to enter and seal off the opening  74  upon appropriate forward movement of the workhead  24  in the direction of the arrow  60  as aforesaid. 
     An inflatable ring seal  78  around the closure  76  (or alternatively around the internal lip of the opening  74 ) provides for a fluid tight seal between the closure  76  and the opening  74 . 
     The housing  72  is slidable relative to the base  22  and bellows seals  80  and  82  are provided between the spindle drives  38  and  52  so that after the seal has been made between the closure  76  and the opening  74 , the housing  72  will in fact move axially with the workhead assembly  24 . Sufficient clearance is provided to the rear of the bellows to allow the housing  72  to move in a continuing sense in the direction of the arrow  60  to allow for the grooves in the grinding wheel to be formed. Movement in the opposite sense is also accommodated by the bellows  80  and  82  so that the closed housing  72  can also follow the workhead  24  as it moves in an opposite sense to that of arrow  60  to allow for the edge of the disc  36  to be engaged by one of the grinding grooves such as  34 . 
     Coolant fluid is sprayed onto the workpiece through nozzles  84  and  86  and similar nozzles are provided for spraying similar fluid onto the forming wheels when required. An interlock is provided to prevent coolant fluid being jetted unless the housing  72  is closed and sealed by the closure  76 . 
     After a grinding operation has been completed and after a final wash with fluid, the housing  72  can be opened by deflating the edge seal  78  and withdrawing the workhead  24  in a direction opposite to that of arrow  60  to the position shown in FIG.  2 . The finished workpiece  36  can then be demounted and a fresh workpiece installed. 
     Wheel forming/dressing 
     Wheel forming can be performed initially before any workpiece has been mounted, in which case the housing  72  is closed by appropriate movement of the workhead  24  and closure  76  without first mounting a workpiece such as  36  on the chuck  30 . Wheel forming is performed by appropriate axial movement of the workhead  24  and lateral movement of support  42 , so that each of the grooves, such as groove  34 , is engaged by the appropriate forming wheel such as  56  or  58 . Coolant fluid is provided during the wheel forming operation. 
     After initial wheel forming, the assembly may be separated by breaking the seal  78  as before mentioned. After mounting a workpiece  36 , the assembly can be closed again and grinding undertaken as before described. 
     Typically re-forming of the groove is performed during machine downtime after one workpiece has been removed and before a subsequent workpiece has been installed, but in a development of the machine in which edge profile checking of the workpiece  36  is performed in situ on the workhead, it may be advantageous to allow for re-forming with the workpiece in place. 
     Notching 
     If a workpiece is to be notched, the support  42  is moved laterally to disengage the wheel from the workpiece and support  48  is moved laterally instead so as to engage the edge of the workpiece  36  by the notching pin  53 . After notching, the support  48  is moved in an opposite sense so as to disengage the pin from the workpiece. 
     Polishing 
     In an alternative arrangement, a polishing wheel may be mounted on the wheel spindle as well as the grooved grinding wheel, and by axially shifting the workpiece spindle, so the polishing wheel can be brought into engagement with the edge of the workpiece  36 . 
     A drive for shifting the workhead  24  along the slideway  26  is provided at  88 . 
     As shown in FIG. 3, a drainpipe  90  conveys fluid from the housing  72  to a storage tank  92  and a pump (not shown) is provided to recirculate the fluid from the tank. A filter may be provided in the tank or in the line between the tank and the pump. 
     The control housing  10  includes a television display  94  and keyboard  96  and a hand-held control unit  98  is connected via a flying lead  100  to a connection plug  102 . An operator can remove the unit  98  and walk to the machine with the unit  98  in his hand, and by pressing appropriate buttons instigate or arrest operation of the machine. The housing  10  houses a computer based control system for supplying control signals and power to the drives on the machine and for receiving signals from transducers, switching and other position/operation/touch etc signal generating sensors on the machine. 
     The slideway  26  on which the workhead slides, is preloaded, and the workhead is driven by server motors and fitted with a high resolution position coder to provide smooth motion during axis move interpolation. 
     Grinding infeed is achieved as previously described by tilting the structures  42  or  48  as required to bring the grinding element carried thereon into engagement with the edge of the workpiece  36 . Although the movement is not truly linear, but arcuate, this can be accommodated in the control signals generated by the control system housed within the housing  10 . 
     Whilst the jets such as  84  and  86  can be used to supply cutting fluid during grinding, they or other jets may be used to direct jets of cleaning fluid at the overhanging lip of the wafer whilst it is still being rotated but after grinding. This prevents grinding swarf from running down the back face of the wafer as it is removed from the chuck. 
     Grinding process 
     Typically edges are ground in a two-stage process using a plunge grind roughing operation and a second plunge grind finish cycle which includes a rapid advance of the grinding wheel until a touch sensor detects contact with the workpiece wafer. The grinding wheel axis position at touchdown is used to monitor wheel wear and to ensure that the material removed per finish cut cycle is kept constant. Grinding wheelforms are maintained by using metal-bonded diamond forming wheels permanently mounted on the workhead chuck. The reforming process can be fully automatic and can be programmed to occur every nth wafer, or whenever the ground edge profile becomes unacceptable (as determined by optical inspection of the disc edge profile) or when the touchdown point indicates excessive wheel wear. 
     Damping 
     In order to reduce unwanted vibration and resulting grinding damage to the minimum, the structural components making up the grinding machine are filled at least partially with polymer concrete, particularly sections of the base  22  and the bed  12  and if desired also the platform  14 . 
     Subassembly flexure mounting 
     FIG. 4 illustrates how the two structures  42  and  48  are mounted for hinging movement to permit wheel infeed. As shown in FIG. 4, the inboard edges of the two structures  42  and  48  are connected to the platform  14  by means of flexures (sometimes referred to as strip-hinges) one of which is shown at  104 . A second pair of flexures are provided towards the other end of the structures  42  and  48  nearer to the machine base  22 . 
     Whilst the flexures  102  and  104  permit tilting of the structures about one axis, they do not readily permit any other movement of the structures  42  and  48  relative to the platform  14  about any other axis. Consequently the coupling of the structures  42  and  48  to the platform  14  is very stiff in all directions except about the hinge axis of the flexures. 
     Cam drive 
     FIG. 4 is a side view of the end of the machine, albeit to a slightly reduced scale. As with the other views, it is shown partly cut-away so as to reveal the cam drive mechanism generally designated  114  which acts on the structure  48 . A captive washer  112  is shown at the side of the drive mechanism. 
     FIG. 4 also shows the two flexure mountings at the base of the unit  48 , the outboard one being designated  104  and the inboard one being designated  118 . 
     As previously mentioned each of the cam drive arrangements is carried within a rigid housing  110  and the latter is more clearly visible in FIG. 4 as is the horizontal leg  120  by which it is bolted to a protruding plate from the end of the base  22 . 
     Also visible in FIG. 4 is the motor  55  for driving the chuck  122  from which the notch grinding spindle  124  protrudes. The motor  55  is carried within a housing  52  previously described in respect of FIG. 1, and the housing  52  slides along a slideway  50  as previously described. 
     FIG. 4 shows the bellows seal  82  attaching the housing  52  sealingly to the opening in the casing  72  through which the motor  55  and spindle carrying pin  53  protrude. 
     The formable grinding pin, or wheel,  53  shown in FIG. 1 is now discussed on more detail in relation to FIGS. 5,  6  and  7 . 
     The formable cylindrical grinding pin  53  shown in FIG. 5 is carried by a smaller diameter cylindrical metal core  130 ,  132  for fitting to the spindle (not shown) of a grinding machine. The cylindrical grinding pin  53  is of approximately 4 mm diameter by 6 mm axial extent in which a first groove  140  is formed in situ by a forming wheel (not shown). The cylindrical nature of the pin  53  allows a series of adjacent grooves such as  142 ,  144  to be machined into the grinding material region as each groove becomes worn and ceases to be operational as shown in FIG.  6 . 
     The pin is formed of resin-bonded diamond, resin-bonded grinding grit or vitreous-bonded material. 
     In use, the operational groove in the resin-bonded diamond wheel  53  is brought into contact with the edge of a stationary semi-conductor disc (not shown) to grind a notch in the disc edge. To this end, the pin  53  is rotated at speeds of the order of 30,000 rpm or more. 
     After several notch grinding operations (and re-forming to the extent permitted by material and depth considerations) the groove in use (eg  140 ) wears out, and another groove (eg  142 ), is formed into the resin so that notch grinding can continue without the need to demount and replace the pin  53 . Typically a total of three or four grooves can be formed along the pin  53  before it has to be replaced. Whilst use of a cylindrical pin permits several grooves to be cut, increasing the axial length to allow even more grooves to be cut, increases the risk of whip, and the risk of errors occurring in the grinding of the notch. 
     The use of an elongated resin-bonded or vitreous-bonded pin thus allows for a succession of different grooves to be formed in the pin as each groove in turn wears out, and this reduces the number of times the pin  53  must be replaced. Each groove can be reformed a few times before it becomes too deep, and/or break-through occurs into the core material  130 . 
     Although not shown, the groove-forming grinding wheel may to advantage be mounted on the workspindle carrying the workpiece in which a notch is to be ground. 
     When the pin is being used on a CNC grinding machine, the latter can be programmed automatically to calculate the diameter of the root of the groove in the notch grinding wheel and compensate by interpolation to produce the desired form of notch during the grinding process. 
     A further embodiment of the invention is shown in FIG. 3, where a composite grinding pin  150  is provided. Here the resin-bonded diamond section  152  is integral with or simply abuts an axially adjacent metal-bonded diamond section  154 . In use, a groove  156  in the metal bonded section  154  is used to rough grind the bulk of each notch, and finish grinding occurs during a second pass, using the current groove formed in the resin-bonded diamond section  152 . 
     The metal-bonded section is preferably designed to outlive the expected useful life of the resin-bonded section so that replacement or reforming of the rough grinding groove  156  is not necessary. Should reforming be necessary, a tougher metal-bonded forming wheel or more preferably a diamond forming wheel will be required to reform the rough-grinding section groove  156 . Preferably any such reforming of the groove in the metal-bonded section is also done in situ in the machine, using an appropriately mounted forming wheel, which is preferably mounted on the workspindle. 
     A CNC grinding machine such as described in WO97/48522 may be used to mount the notch grinding pin and the groove forming wheels.