Abstract:
A device for holding a wafer-shaped article comprises an annular chuck base body having a plurality of movable contact elements for securing a wafer-shaped article to the annular chuck base body, and a gear mechanism driving the contact elements in unison between a first position and a second position. The annular chuck base body comprises a housing formed from a material that is resistant to attack by strong inorganic acids. The annular chuck base body also comprises a reinforcing ring fitted within the housing and formed from a material whose coefficient of linear thermal expansion is substantially less than that of the housing material.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The invention relates to devices for holding wafer-shaped articles, such as semiconductor wafers. 
         [0002]    Semiconductor wafers are subjected to various surface treatment processes such as etching, cleaning, polishing and material deposition. To accommodate such processes, a single wafer may be supported in relation to one or more treatment fluid nozzles by a chuck associated with a rotatable carrier, as is described for example in U.S. Pat. Nos. 4,903,717 and 5,513,668. 
         [0003]    The patents referenced above operate on the Bernoulli principle, such that the wafer receives subjacent support from a gas cushion rather than by contact with the chuck. Such chucks nevertheless ordinarily include a circular series of pins that are positioned radially outwardly of a wafer positioned on the chuck. Those pins prevent lateral displacement of the wafer relative to the chuck. 
         [0004]    Other spin chucks operate under control of magnetic fields, by fashioning the chuck body as a magnetic rotor positioned coaxially within a surrounding annular magnetic stator, as described for example in U.S. Pat. No. 6,485,531. In such chucks the magnetic rotor supports the wafer. Commonly-owned co-pending application U.S. Pub. No. 2012/0018940 describes magnetic rotor designs that better withstand the often extreme temperatures to which such chucks are exposed in use. However, the present inventors have discovered that there remains a need for improved chuck designs that are less adversely influenced by higher temperature use conditions. 
       SUMMARY OF THE INVENTION 
       [0005]    Thus, in one aspect, the present invention relates to a device for holding a wafer-shaped article, comprising an annular chuck base body having a plurality of movable contact elements for securing a wafer-shaped article to the annular chuck base body, and a gear mechanism driving the contact elements in unison between a first position in which they contact a wafer-shaped article positioned on the device, and a second non-contact position. The annular chuck base body comprises a housing formed from a material that is resistant to attack by strong inorganic acids, and the annular chuck base body comprises a reinforcing ring fitted within the housing and formed from a material whose coefficient of linear thermal expansion is substantially less than that of the housing material. 
         [0006]    In preferred embodiments of the device according to the present invention, the housing is formed from a material whose coefficient of linear thermal expansion is at least twice that of the reinforcing ring material, preferably at least three times greater, and more preferably at least five times greater. 
         [0007]    In preferred embodiments of the device according to the present invention, the reinforcing ring is formed from steel, and wherein the housing is formed from a plastic selected from the group consisting of polytetrafluoroethylene (PTFE), perfluoroalkoxy (PFA), polyphenylenesulfide (PPS), polyetheretherketone (PEEK), polystyrene/polyethylstyrene (PS/PES), ethylene tetrafluoroethylene (ETFE), polyvinylidene fluoride (PVDF), homopolymer of chlorotrifluoroethylene (PCTFE), fluorinated ethylene propylene (FEP), ethylene chlorotrifluoroethylene (ECTFE) and polypropylene (PP). 
         [0008]    In preferred embodiments of the device according to the present invention, the gear mechanism and the reinforcing ring are formed from materials whose coefficients of linear thermal expansion differ from one another by not more than 50% and preferably by not more than 20%. 
         [0009]    In preferred embodiments of the device according to the present invention, the gear mechanism and the reinforcing ring are formed from a same material. 
         [0010]    In preferred embodiments of the device according to the present invention, the gear mechanism and the reinforcing ring are each formed from stainless steel. 
         [0011]    In preferred embodiments of the device according to the present invention, the gear mechanism and the reinforcing ring are formed from different materials, each of which is a metal. 
         [0012]    In preferred embodiments of the device according to the present invention, the gear mechanism is formed from aluminum and the reinforcing ring is formed from stainless steel. 
         [0013]    In preferred embodiments of the device according to the present invention, the contact elements are embodied in a circular series of pin assemblies that are conjointly movable from the second position to the first position, and the gear mechanism comprises a gear ring having gear teeth in continuous meshing engagement with gear teeth provided on a base portion of each of the pin assemblies. 
         [0014]    In preferred embodiments of the device according to the present invention, the base body carries a plurality of rollers rotatable about axes parallel to the axis of rotation of the base body, each of the rollers being positioned radially outwardly of a corresponding one of the plurality of pin assemblies, with a respective section of gear teeth of the ring gear being positioned therebetween. 
         [0015]    In preferred embodiments of the device according to the present invention, each of the rollers comprises an elastically deformable contact material bearing on a surface of the ring gear. 
         [0016]    In preferred embodiments of the device according to the present invention, the pin assemblies each comprise a pin projecting from a respective pivotal base along an axis parallel to and offset from a pivot axis of the pivotal base. 
         [0017]    In preferred embodiments of the device according to the present invention, the gear ring teeth are formed on a radially inward edge thereof. 
         [0018]    In preferred embodiments of the device according to the present invention, the reinforcing ring comprises arcuate guide slots for guiding the ring gear in rotation relative to the annular chuck base body over a predetermined angular range. 
         [0019]    In preferred embodiments of the device according to the present invention, the reinforcing ring comprises a multiplicity of threaded bores for receiving correction weights for balancing the annular chuck base body. 
         [0020]    In preferred embodiments of the device according to the present invention, the annular chuck base is a magnetic rotor of a magnetic bearing and drive unit, the device further comprises a chamber in which the magnetic rotor is positioned, and the magnetic rotor and drive unit further comprises a stator positioned outside of the chamber and facing the magnetic rotor across a wall of the chamber. 
         [0021]    In preferred embodiments of the device according to the present invention, the contact elements depend downwardly from the annular chuck base body, such that a wafer-shaped article is suspended from the annular chuck base body when mounted in the device. 
         [0022]    In preferred embodiments of the device according to the present invention, the housing of the annular chuck base body comprises a main section having openings to receive the contact elements, an upper outer section housing magnets, and a lower outer section in which the reinforcing ring is fitted. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    Other objects, features and advantages of the invention will become more apparent after reading the following detailed description of preferred embodiments of the invention, given with reference to the accompanying drawings, in which: 
           [0024]      FIG. 1  is a perspective view, partly in section, illustrating a device according to one embodiment of the invention; 
           [0025]      FIG. 2  is a perspective view, also partly in section, of the detail II designated in  FIG. 1 ; 
           [0026]      FIG. 3  is a perspective view, also partly in section, taken along the line III-III of  FIG. 1 ; 
           [0027]      FIG. 4  is a perspective view of the device of  FIG. 1  with the main and upper outer housing components removed so as to reveal the internal mechanisms of the device; 
           [0028]      FIG. 5  is a perspective view of the reinforcing ring of the  FIG. 1  embodiment fitted in the lower outer housing component; and 
           [0029]      FIG. 6  is an exploded view of the components shown in  FIG. 5 . 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0030]    The device  100  of  FIG. 1  comprises a chamber, an annular chuck  20  positioned within the chamber for gripping and rotating a wafer W, and a stator  80 . The chamber comprises a cylindrical wall  60 , a bottom plate  65  and a preferably transparent top plate  67 . One or more upper dispensing tubes, indicated schematically at  63 , are led through the top plate  67  and one or more lower dispensing tubes (not shown) are led through the bottom plate  65 . 
         [0031]    Stator  80  is mounted to a stator base plate  85  and is concentric with the cylindrical wall  60 . The stator base plate  85  can be moved axially along the axis of the cylindrical wall  60 , e.g. with pneumatic lifting devices. Alternatively, the stator base plate  85  may be mounted in a stationary manner and the bottom plate  65  with cylindrical wall  60  mounted thereon can be moved axially relative to the stationary stator  80 , e.g. with pneumatic lifting devices. The stator base plate  85  and the stator  80  mounted thereto have central openings, whose diameter is greater than the outer diameter of the cylindrical wall  60 . The top plate  67  can also be moved axially to open the chamber, either manually or by a suitable mechanism (not shown). In its closed position the top plate is sealed against the cylindrical wall  60 . 
         [0032]    The stator  80  comprises several coils (not shown) for changing the radial and optionally also the axial orientation of the chuck  20 , as well as for driving the chuck in rotation. Such an arrangement is further described in U.S. Pat. No. 6,485,531. The diameter of the annular chuck  20  is less than the inner diameter of the cylindrical wall  60  so that it can freely levitate and rotate within the cylindrical wall  60 . The annular chuck  20  comprises a chuck base body with an annular groove circumferentially surrounding the outside of the inner chuck base body, with the annular groove receiving the gear ring  30 . The gear ring  30  is preferably made of aluminum or stainless steel. The gear ring  30  drives a circular series of pin assemblies  27 , of which there are six in this embodiment, and only three of which are visible in  FIG. 1 . 
         [0033]    Each pin assembly  27  is rotatable relative to the chuck base body  21  via the gear ring  30 , about axes parallel to the axis of rotation of the chuck. The pin assemblies terminate downwardly in contact elements positioned so as to contact a wafer W on its peripheral edge. As the pins assemblies  27  also support the weight of the wafer W, the contact portions thereof may either be cylindrical in shape or have recessed portions on their radially inwardly facing sides contacting the wafer edge, to prevent axial displacement of the wafer W relative to the pin assemblies  27  when the wafer is being gripped. 
         [0034]    As shown in  FIG. 2 , a plurality of rotor magnets  83 , which are permanent magnets, are evenly arranged around the chuck base body  21 . These rotor magnets  83 , which are preferably at least  24  in number, are part of the drive and positioning unit, namely, part of the rotor (elements of the active bearing), which is mounted to the chuck base body  21 . 
         [0035]    The plurality of rotor magnets  83  and the gear ring  30  carrying the permanent magnets  33  are housed within chuck base body  21 , outer lower chuck cover  23 , and the rotor magnet cover  25 . Such rotor magnet cover  25  can be a stainless steal jacket. The covers  23  and  25  are annular and concentric with the chuck base body  21 . Two permanent magnets  33  are mounted to the gear ring  30 . 
         [0036]    When the pins are to be opened e.g. to release a wafer the following procedure is conducted: the stator  80  is moved upwardly relative to chamber wall  60  (either by raising the stator or lower the chamber wall, or both) so that the chuck  20  is lifted such that the cylindrical wall  60  is no longer in the gap between the locking magnets  55  and the chuck  20 . Thereafter the pneumatic cylinders  50  move the locking magnets  55  in close proximity to the chuck  20  and the chuck is turned so that the permanent magnets  33  and therewith the gear ring  30  is locked by the locking magnets. Now the chuck is turned while the gear ring stands still and thus the pins  28  open. Alternatively the chuck base body might stand still while the pneumatic cylinders are moved so that the locking magnets tangentially turn (along the circumference of the chuck), whereby the gear ring is turned. 
         [0037]    Turning now to  FIG. 3 , the structure of pin assembly  27  and its associated contact element  28  is shown in greater detail. Also shown in  FIG. 3  are the chuck base body  21  in which the pin assembly is fitted, the magnet holder  25  containing the rotor magnets  83 , the ring gear  30  in continuous meshing engagement with the gear teeth of the pin assembly  21 , a reinforcing ring  70  that is fitted with the outer lower chuck cover  23 , and a roller  71  whose function will be described below. 
         [0038]    The housing components  21 ,  23 ,  25  making up the chuck  20  are each preferably formed of an engineering plastic that is highly resistant to the strong mineral acids commonly used during processing of semiconductor wafers. For example, these components are preferably made from one or more materials selected from the group consisting of polytetrafluoroethylene (PTFE), perfluoroalkoxy (PFA), polyphenylenesulfide (PPS), polyetheretherketone (PEEK), polystyrene/polyethylstyrene (PS/PES), ethylene tetrafluoroethylene (ETFE), polyvinylidene fluoride (PVDF), homopolymer of chlorotrifluoroethylene (PCTFE), fluorinated ethylene propylene (FEP), ethylene chlorotrifluoroethylene (ECTFE) and polypropylene (PP). Among these materials, PVDF (polyvinylidene fluoride) and ECTFE (ethylene-chlorotrifluoroethylene) are particularly preferred. 
         [0039]    On the other hand, the gear ring  30  and reinforcing ring  70  are made from a material that has a significantly greater resistance to thermal expansion than do the plastic housing components  21 ,  23 ,  25 . In particular, ring gear  30  and reinforcing ring  70  are preferably each made of stainless steel. Ring gear  30  and reinforcing ring  70  may also be made of different materials; however, in that case, it is preferred that their respective coefficients of linear thermal expansion differ from one another by less than 50%, and preferably less than 20%. 
         [0040]    It is preferred that at least the reinforcing ring  30  be made of a material whose coefficient of linear thermal expansion is not more than half that of the housing components, preferably not more than one third, and more preferably not more than one fifth. 
         [0041]    In the present disclosure, coefficients of linear thermal expansion are expressed in units of m/mK. Thus, for example, stainless steel has a reported linear temperature expansion coefficient of 17.3 (10 −6  m/mK), whereas stainless steel has a reported linear temperature expansion coefficient of 17.3 (10 −6  m/mK), whereas polyvinylidene fluoride (PVDF) has a reported linear temperature expansion more than seven times greater, 127.8 (10 −6  m/mK). 
         [0042]    As shown in  FIG. 4 , gear ring  30  comprises six regions of inwardly facing teeth  31 . The inwardly facing teeth  31  in turn drive the teeth of a pin assembly  27 . A pin  28  is mounted to or formed integrally with each pin assembly  27 , at a position that is eccentric with respect to the axis of rotation of the pin assembly  27 . Consequently, the pins  28  are displaced radially of the chuck when the pin shafts  27  are turned by the gear ring  30 . As the pins and the gear ring  30  are both carried by the chuck base body  21 , the pins shafts  27  are rotated by the gear ring  30  only when the gear ring  30  rotates relative to the chuck base body. 
         [0043]    In order to mount the gear ring  30  into the annular groove of the chuck base body  21  the gear ring  30  consists of two separate segments, which are fixed together when inserted into the annular groove. The chuck base body  21  and the gear ring  30  are connected via one or more helical springs  73  so that the gear ring  30  urges the pins  28  into their radially innermost positions, corresponding to the gripping of a wafer W. 
         [0044]    When assembling the chuck  20  the pin shafts  27  are inserted from above into their respective seats so that the pin shafts tightly seal against the chuck base body  21  as shown in  FIG. 3 . Additionally there is a dynamic sealing acting between pin shaft and base body. Each pin shaft  27  is fixed in position with a screw  24 . Additionally, each pin shaft may be pressed into its seat by a helical spring between the pin shaft and the screw. 
         [0045]    In  FIG. 4 , behind each set of teeth  31  there is an enclosed cut-out  32  that defines a narrower region of ring gear  30  that is thereby significantly weakened, as described in commonly-owned co-pending application U.S. Pub. No. 2012/0018940. The material of the ring gear  30  has sufficient elasticity that the narrower region will deflect radially outwardly upon thermal expansion of chuck base body  21  and associated displacement of pin assembly  27 , yet will return to its initial position as the chuck base body  21  cools and contracts. 
         [0046]    However, the present inventors have discovered that, during processes conducted at elevated temperatures, such as 80° C., a chuck with a base body of ECTFE and an aluminum or stainless steel gear ring, made as described in commonly-owned co-pending application U.S. Pub. No. 2012/0018940, would expand such that there would result a reduction of pin clamping forces, and in some cases, of chuck open/close malfunctions. In a worst case scenario this could result in a wafer being irreparably damaged. 
         [0047]    The reinforcing ring  70  prevents this undesired expansion of the chuck, while also improving the design of the device in other ways. In particular, the reinforcing ring  70  significantly increases the stiffness of the entire chuck assembly and reduces the expansion of the chuck main body  21 . 
         [0048]    Additionally, as shown in  FIG. 4 , the base body  21  carries a series of rollers  71 . These rollers  71  serve as counterpart to the pin assemblies  27  so that the ring gear  30 , which drives the pins, is prevented from being excessively deformed, especially in the regions where the ring gear  30  is thinned by the cut-outs  32 . 
         [0049]    However, it was also observed that when such a chuck was used at temperatures in excess of 100° C., expansion of the ring gear  30  toward the rollers  71  caused the rollers  71  to be contacted with sufficient pressure that they would not rotate. Therefore, rollers  71  are preferably equipped with a relatively soft material with which to contact the ring gear  30 , such as rubber. This arrangement permits the ring gear  30  to expand without seizing of the rollers  71 . 
         [0050]    As shown in  FIGS. 5 and 6 , the reinforcing ring is provided with twelve through bores  73  that align with bores on the outer lower chuck cover  23  such that the reinforcing ring  70  may be fixed in position relative to the outer lower chuck cover  23  (as well as chuck base body  21  and magnet cover  25 ). The reinforcing ring  70  also includes a pair of opposed arcuate openings or recesses  72 , which receive a downwardly projecting roller (not shown) on the ring gear  30 , so as to guide the ring gear across its limited angular range of motion relative to the chuck, and against the action of springs  73 . 
         [0051]    The reinforcing ring furthermore includes a multiplicity of additional bores  74 . Selected ones of these additional bores  74  may be used to accommodate one or more correction weights, to aid in spin-balancing the rotary chuck. 
         [0052]    While the present invention has been described in connection with various preferred embodiments thereof, it is to be understood that those embodiments are provided merely to illustrate the invention, and should not be used as a pretext to limit the scope of protection conferred by the true scope and spirit of the appended claims.