Patent Publication Number: US-6669540-B2

Title: Chuck means for flat workpieces, in particular semi-conductor wafers

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not Applicable. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
     Not Applicable. 
     BACKGROUND OF THE INVENTION 
     A known method for the planarization of semi-conductor wafers in the semi-conductor industry is the so-called CMP process. This is a chemical-mechnical machining by means of a fluid (slurry), with the chemically reactive portion of the slurry having the objective to convert the material into a polishable condition. The slurry includes abrasive means in the form of colloidal abrasive small particles. The planarization of a semiconductor disk by CMP is an important step for the manufacture of plane defect-free and smooth semi-conductor disks. Frequently, the CMP process is the last processing step in the manufacturing line and considerably influences the shape and the surface qualities of the semiconductor disk which as known is the blank material for the manufacture of electrical, electronic and microelectronic components (prime wafer manufacture). Also after the coating of semi-conductor wafers, for example with an oxide, tungsten or other metal layer, a CMP machining takes place in order to provide the desired quality of the surface. In case this is not achieved, problems occur with lithographic processes in the form of focus failures by steps of focal field of the UV stepper or in form of disturbances of the conductor paths. 
     During the transport and the machining of the wafers the wafers are held by a carrier. The objective of such carrier is to transfer a homogenous pressure field or different pressure profiles to the back side of the wafers. Conventionally, the carrier is retained and moved by an apparatus whereby the carrier is rotated about a vertical axis and linearly moved in vertical and horizontal direction. 
     From DE 197 55 975 A1 a carrier has become known which is attached to a spindle or shaft which can be vertically moved. The carrier has a chuck plate at the lower side which through a universal joint is coupled with a support portion above the chuck plate. The chuck plate includes a plurality of bores which extend to the lower side of the chuck plate and which are connectable with a vacuum and/or a fluid source. The chuck plate is vertically movably guided by the support portion and an annular closed diaphragm is arranged between the support portion and the chuck plate, the diaphragm defining a sealed closed inner space which selectively can be connected to atmosphere, vacuum or a pressure source, respectively. 
     The objective of the invention is to provide chuck means for flat workpieces, in particular for wafers which enable a particular effective processing of the surfaces of the workpieces. 
     BACKGROUND OF THE INVENTION 
     Similar to conventional chucks or carriers for semi-conductor wafers for the CMP polishing the invention provides a circular housing which is connected to a driving spindle or a shaft for rotation therewith. The housing includes a top wall and an annular side wall. The chuck plate is made of relatively rigid, however, elastically deformable material, for example of steel or plastic material and has ports at the lower side for the holding of wafers by vacuum. The chuck plate is floatingly arranged in the housing. It can freely move upwards and downwards and is laterally limited by a retainer ring which forms the side wall of the housing in the area of the chuck plate. A plurality of pressure chambers is provided above the chuck plate which have a desired shape and distribution. The pressure chambers can be concentrically arranged about the spindle axis and divided by radial walls. The pressure chambers are connected to a pressure manifold means which in turn is connected to a fluid source under pressure. By means of the pressure manifold means the pressure in the individual pressure chambers can be controlled. The pressure chambers have lower wall portions which are resilient, in particular flexible. The lower wall portions engage the upper side of the chuck plate and determine the pressure of the chuck plate onto the workpiece beyond the gravity force of the chuck plate if the workpiece lies on a polishing table. Thus, the polishing pressure of the chuck plate is solely determined by the pressure in the individual pressure chambers. 
     The transfer of the torque from the spindle to the chuck plate solely takes place though the frictional engagement of the lower chamber walls with the chuck plate. 
     The pressure force which is transferred from the pressure chamber to the chuck plate has the same value at any location in the pressure chamber. By the selection of the pressure in the pressure chambers the polishing result can be influenced. 
     The retainer ring at the housing can be splitted so that it can be easily released from the housing which allows the removal of the chuck plate. Frequently, a polishing cloth is adhered to the chuck plate. The cloth is a wear part and thus must be replaced from time to time. By the easy removal of the chuck plate, the replacement can be easily carried out at a remote location. 
     It is conceivable to have separate pressure conduits connected with the individual valves which are connected to the pressure chambers. The supply apparently has to take place through the spindle. Therefore, it is more simple and not particularly disadvantageous if only switching valves are associated with the pressure chambers which are connected to a common pressure control valve in order to generate selectively the pressure in desired pressure chambers. The pressure control valve can be located outside of the housing or the spindle, respectively, and connected to an axial passage within the spindle through a rotary duct. The passage in the spindle is connected to the individual switching valves in the housing. The switching valves are electrically controllable, with an external control device for the switching valves being connected to the switching valves through electrical conductors. The conductors could be connected to electrical lines within the spindle or the housing by sliding contact ring means associated with the spindle or the carrier. Also a contactless transmission can take place, e.g. through electromagnetic waves or infrared means. 
     It is conceivable to provide pressure control valves for the individual pressure chambers. However, small proportional valves which would be necessary, are not available. 
     The pressure chambers could be formed by a folded membrane which has upper and lower portions which alternate in radial direction. The upper portions can be connected to a plate within the housing, preferably through annular clamping rings so that the membrane rotates upon rotation of the spindle and the housing. The lower portions of the membrane have a larger thickness and engage the upper side of the chuck plate and transfer the torque onto the chuck plate through friction force. As the membrane is of an elastic material which has a small inherent rigidity, angle failures between the driving spindle and the polishing table and the polishing station can be compensated. 
     In the invention, the chuck plate can be formed as plane disk which can be deformed by means of the pressure chambers in the desired manner. The guidance of the chuck plate can be achieved by a cylindrical circumference of the plate which is guided by the housing or the mentioned retainer ring. To this purpose an annular groove can be formed at the circumference of the chuck plate, and a radially inwardly facing annular extension of the retaining ring can engage the groove in order to limit the vertical movement of the chuck plate. 
     The construction according to the invention can provide for a simple removal of the chuck plate as already described above. The passages of the chuck plate are connected to vacuum or a fluid source, respectively, through a passage in the spindle. Therefore, for this case the invention provides for a coupling which includes a part connected to the chuck plate and a part connected to the housing which in the coupled condition provides for a fluid connection to the passages in the chuck plate. The coupling must be structured such that the chuck plate can be easily released from the other coupling part in the housing after the retainer ring has been removed. As to this, a quick closure thread locking means is provided, e.g. formed by a helical groove in the coupling part secured to the housing, a projection of the coupling part connected to the chuck plate engaging the groove. By a respective rotation of the chuck plate, the coupling parts can be separated or coupled. By means of this construction the polishing cloth which is normally attached to the chuck plate can be easily removed from the chuck plate and replaced by another one. 
     The coupling part within the housing has to move with the vertical movement of the chuck plate, e.g. by its deformation but also by the vertical movement in operation. Therefore, this coupling part is vertically movable and preferably biased towards the chuck plate by means of a spring. A flexible conduit in the housing connects the coupling part in the housing with an axial passage in the spindle. Preferably, the coupling part within the housing consists of two portions, namely a first portion which forms a plug socket connection with a socket-like coupling part of the chuck plate and a second portion wherein the first portion is rotatably accommodated, however, secured against axial movement. The second portion is connected to a flexible conduit portion. The first portion is biased by a spring towards the chuck plate and is prevented from rotation in a lower position. If the first portion, however, is upwardly displaced after the coupling of the parts has taken place, the first portion can freely rotate. This is necessary because of the frictional transfer of the torque from the membrane to the chuck plate and a relative rotation between membrane and chuck plate may occur. Upon such relative rotation, the second portion of the second coupling part must not be rotated. The first portion, however, can freely rotate in the second portion so that no damages of the parts can occur. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     In the following, an embodiment example of the invention is explained in more detail, wherein 
     FIG. 1 shows a cross section through a chuck according to the invention during a polishing operation. 
     FIG. 2 shows in an enlarged scale a marginal portion of the chuck of FIG.  1 . 
     FIG. 3 shows enlarged a central portion of the chuck of FIG.  1 . 
     FIG. 4 shows enlarged a cross section through a central portion of the chuck of FIG. 1 with removed chuck plate. 
     FIG. 5 is a block diagram showing the strain gauges connected to an external computer. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     While this invention may be embodied in many different forms the are described in detail herein a specific preferred embodiment of the invention. This description is an exemplification of the principles of the invention and is not intended to limit the invention to the particular embodiment illustrated. 
     In FIG. 1 a cylindrical chuck  10  can be seen having a top wall  12  and an annular side wall  14 . A vertical shaft or a spindle  16  is rotated according to arrow  18  and is borne by a sleeve-shaped bearing arrangement  20 . The rotary bearing is not described in detail. The bearing arrangement  20  can be vertically moved by a suitable device (not shown), and the spindle  16  can be rotated by a suitable driving motor (not shown). Through a central hollow trunnion  22  the spindle  16  is connected to the top wall for rotation therewith. An axial passage  24  extends within the spindle, the passage being connected to a first conducting portion  26  (not shown in detail). A further axial passage in spindle  16  not shown in FIG. 1 is connected with a further conducting portion  28   a.  Upon rotation of spindle  16 , the housing of the chuck is rotated. 
     An annular element  28  extends around trunnion  22  and is held by retaining means  30  which at  32  are threaded to the bearing arrangement  20 . A second annular element  32  is mounted on the upper side of top wall  12  and rotates therewith. The elements  28 ,  32  represent a sliding ring or commutator arrangement for the transmission of electrical signals. A cable  36  which is connected to a control device not shown is connected to the annular element  28  which forms the stator of the sliding ring arrangement, and a cable connection  38  is connected to the annular element  32  which forms the rotor and is introduced into the interior of the housing as can be seen at  40 . The function of cable  40  will be described later. 
     A flange  42  is attached to the lower side of top wall  12 , the flange  42  having a downwardly extending cylindrical hollow extension  44 . A plate  46  is screwed to the extension  44 , and a further plate  48  is connected to plate  46  by means of threaded fasteners. A retainer ring  50  is attached to the circumference of the circular plate  48  by threaded fasteners  52 . The retainer ring  50  at the lower portion has a radially inwardly extending annular extension  54 . The retainer ring  50  is splitted and can be easily removed after removal of screws  52 . 
     A folded diaphragm  56  is connected to the lower side of plate  48 , the diaphragm lying on a chuck plate  58 . A polishing cloth is attached to the lower side of the chuck plate. At  60 , a wafer is indicated which is polished by a polishing table diagrammatically shown at  62 . The polishing table for example is supported for rotation about an axis  64  and driven (not shown in detail). 
     The last mentioned parts can be seen more clearly in the FIGS. 2 to  4 . The integral diaphragm  56  has upper annular portions  66  and lower annular portions  68  which alternate in radial direction. The annular portions  66 ,  68  are interconnected by folds  70 . The upper portions  66  are pressed against the lower side of plate  48  by clamping rings  72  and screws. The lower annular portions  68  are relatively thick and engage the upper side of chuck plate  58 . The complete diaphragm  56  is annular and made of relatively flexible material. The attachment and sealing of the diaphragm  56  at the radially inner side takes place by a flange sleeve  74  to plate  48  (see FIGS.  3  and  4 ). Four annular pressure chambers  76  are defined by plate  48  and the described portions of the diaphragm  56 . The pressure chambers  76  are concentrically relative to each other and to the axis of spindle  16 . Each pressure chamber  76  is connected to a switching valve  78  through conduits  80  (see also FIG.  1 ). In FIG. 1 it can be seen that the four switching valves  78  are formed as a block which is arranged in housing  10  and connected with a conduit  82 . This conduit is connected with the conduit  28  not shown in detail, the latter being in communication with an axial passage in spindle  16 . The second axial passage is connected to an external pressure control valve through a rotary duct. This is indicated in FIG. 1 by block  84 . By means of the pressure control valve, in conduit  82  a predetermined pressure can be generated. By a respective control of the valves  78  the desired pressure chamber  76  can be selected wherein a predetermined pressure is to be generated. It is understood that all pressure chambers  76  can be provided with a predetermined pressure. The control of valves  78  takes place through cable  40  which is connected with the external cable  36  through the described sliding arrangement  28 ,  30 , the external cable  36  being led to the electrical control device not shown. 
     The circular chuck plate  58  has an annular groove  86  at its circumference, the radial annular extension  54  of the retainer ring  50  engaging the annular groove. Thus, the chuck plate  58  can be vertically moved within limits. 
     Star-like arranged passages  88  are provided in chuck plate  58 , the ends of the passages being closed as shown at  90 . The passages  88  are connected with bores  92  extending parallel to the axis of the chuck plate. The passages in the chuck plate terminate in radial openings  96  in the central bore of the chuck plate  58 . As already mentioned, a polishing cloth is attached to the lower side of chuck plate  58  which, however, is permeable for gas. Alternatively, the polishing cloth may be provided with bores aligned with the bores  92 . In case a vacuum is generated in passages  88  a holding force can be exerted upon wafer  60 . This is used for the transport of the wafer. In order to transfer the vacuum into passages  88  a fluid coupling is provided. The fluid coupling comprises a cup-shaped first portion  94  which is threaded into a central bore of the chuck plate  58 . In the upper enlarged portion of the coupling part  94  a sealing ring  98  is located. The upper end of the coupling part  94  is formed by diametrically opposed claws  100  which cooperate with helical groove portions  102  of a coupling portion  104 . This coupling portion  104  in conjunction with a further coupling portion  106  forms the second coupling part. 
     A retaining element  108  which is centrally arranged within housing  10  and is fixedly attached to extension  44  supports a coil spring  110  which with the lower end coacts with a radial flange  112  of portion  104  in order to bias the portion  104  downwardly as can be seen in FIGS. 3 and 4. The spring  110  surrounds a hollow portion  106  into which the upper cylindrical portion of portion  104  is inserted. At  114 , a rotary connection between portions  104  and  106  is formed which prevents an axial relative displacement of the theses parts. In the lower position of portion  104 , flange  112  is positioned within a recess  116  of plate  46  and thus secured against rotation. However, if flange  112  is above recess  116 , it can freely rotate (see FIG.  3 ). 
     The portion  106  in the upper area has two conduit fittings  118 ,  120  on opposing sides which are connected to the interior of the hollow portion  106 . The portion  104  has a central throughbore  122  which terminates in a conical portion  124  which as can be seen in FIG. 3 can be inserted into the interior of coupling part  94 . By this, a connection to the radial passages  88  is established. The fittings  118 ,  120  are connected to conduit portions  124  which are connected to a fluid coupling  126  which in turn is in communication with conduit  26 . The latter as described is in communication with the axial passage  24  of the spindle. In this way, a vacuum can be generated in bores  92  or a fluid can be supplied to the bores if desired. 
     At the lower end of portion  104  an annular rib  126  is provided which sealingly engages the sealing ring  98  of coupling part  94 . 
     When a wafer  60  is retained by vacuum at the lower side of chuck plate  58 , the carrier  10  can lower onto the polishing table  62 . The lowering takes place such that the chuck plate  58  may freely float so that the annular extension  54  forms a space with respect to the walls of groove  86 . Thus, the chuck plate  58  only engages portions  68  of diaphragm  56 . By means of the valves  84  and  78  in selected pressure chambers  76  a predetermined pressure in a predetermined distribution is established whereby a pressure on the chuck plate  58  is exerted and a partial deformation of the chuck plate  58  can take place if the pressure in chambers  76  is not equal. Thus, the polishing pressure on wafer  60  is solely generated by the pressure in the pressure chambers  76 . The gravity force of chuck plate  58  and the force of spring  110  are constant forces which add to the pressure generated by the pressure chambers. 
     In FIG. 2 an annular extension can be seen at  130  having an inner diameter which is slightly larger than the outer diameter of wafer  60 . By this, the wafer is radially secured if the polishing process takes place. 
     If the chuck plate  58  is to be removed, the retainer ring  50  is released as already described. A plate-like arrangement  132  not described in detail is placed below chuck plate  58  which may support the chuck plate  58  with the polishing cloth. The chuck plate then is rotated approximately about 90° in one rotary direction whereby the claws  100  are turned out of grooves  102  (FIG.  3 ). Then, the chuck plate  58  can be freely lowered. 
     Upon the mounting of chuck plate  58  the chuck plate is moved upwardly against diaphragm  56 , with the portion  124  being inserted into coupling part  94  and rib  126  engaging sealing ring  98 . The arrangement  132  is slightly movable and thus facilitates the centering of coupling part  94  and portion  124  upon assembly of chuck plate  58 . The claws  100  of coupling part  94  are aligned with the grooves  102 . Thereafter, the chuck plate  58  is rotated in opposite direction whereby the parts  104  and  94  are pressed against each other and are clamped. The chuck plate  58  is then moved upwardly towards diaphragm  56  until engagement therewith. The coupling portions  104  and  106  are commonly moved upwardly as indicated in FIG.  3 . By this, a fluid connection is established with conduit  26 . The coupling part  104  can freely rotate within coupling portion  106  in housing  10 . Thus, a relative rotation of coupling portion  104  and coupling portion  106  is possible. This may occur through the frictional torque transmission from diaphragm  56  to chuck plate  58 . Thus, a safety clutch is formed by this construction. The conduits  124  are flexible and allow the described vertical movement of coupling portion  106 . 
     An annular wall portion  134  which for example is made of plastic forms a shroud and protects the interior of housing  10 . The shroud  134  must not provide a support function. 
     As mentioned, the pressure distribution on the deformable chuck plate  58  takes place by the pressure in the pressure chambers  76 . The chuck plate  58  is relatively thick and for example is made of steel. The achievable deformations are relatively small, however, completely sufficient to obtain the desired pressure distribution upon the polishing process. 
     The pressure in the pressure chambers  76  is controlled by the adjustment of the pressure control valve  84  and the control of the switching valves  78 . The switching valves are connected through pressure chambers  76  through axial parallel bores, as for example shown at  136 . 
     Referring now to FIG. 5, strain gauges  200  are attached to the upper side of the chuck plate, said strain gauges connected to an external computer  202  through transfer means  201  associated with the spindle. 
     The above Examples and disclosure are intended to be illustrative and not exhaustive. These examples and description will suggest many variations and alternatives to one of ordinary skill in this art. All these alternatives and variations are intended to be included within the scope of the attached claims. Those familiar with the art may recognize other equivalents to the specific embodiments described herein which equivalents are also intended to be encompassed by the claims attached hereto.