Abstract:
Embodiments of the invention comprise a machine adapted for polishing work pieces such as large silicon wafers. A wafer polishing machine in accord with the invention comprises a rotatable platen in a table base, above which is mounted a lid having a head moving assembly with four synchronously rotatable head assemblies. A motor and linkage connected to the head moving assembly imparts reciprocating linear motion to the head assemblies in a selected direction in a plane parallel to an upper surface of the platen. Embodiments of the invention produce a complex relative motion between a surface of a wafer to be polished and the platen. The complex relative motion, resulting from a combination of motions including rotation of the platen, rotation of the head assemblies, and translation of the head moving assembly, improves a uniformity of polish and a rate of polishing compared to wafer polishing machines known in the art.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of the priority date of provisional patent application Ser. No. 60/962,035, filed on Jul. 25, 2007, which is incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to semiconductor equipment and in particular to a machine for polishing semiconductor wafers. 
     BACKGROUND 
     Semiconductor integrated circuits are typically made from thin wafers cut from silicon ingots known as boules. Cutting a wafer from a boule generally leaves the surfaces of the wafer in a rough condition, so wafers are polished on wafer polishing machines prior to starting semiconductor processing operations. The difficulty in achieving desired values of flatness and surface roughness increases as the diameter of the wafer to be processed increases and as the size of semiconductor structures (also known as “feature size”) to be fabricated on the wafer decreases. Wafer diameters have steadily been increasing and feature sizes decreasing at the same time that manufacturers have been pressured by market forces to increase manufacturing throughput and reduce manufacturing costs. 
     In the past, the relatively small size of wafers permitted a single wafer polishing machine having one or more head assemblies, each head assembly adapted to hold a plurality of wafers, to flatten and smooth many wafers simultaneously. Polishing machines use an abrasive, corrosive slurry to mechanically and chemically remove microscopic projections from the surface of a wafer. Machines for polishing bare wafers and machines for polishing by a chemical and mechanical process are known in the art. A wafer polishing machine has a horizontal rotating platen in a table base with a polishing pad attached to the top of the platen. A lid attached to the table base has at least one head assembly that is rotated during polishing. A wafer carrier attached to a head assembly holds one or more wafers to be polished. Pumps deliver slurry at a selected rate to the polishing pad and motors rotate the platen and head assemblies. Parts of the head assembly for carrying wafers have vertical travel relative to the surface of the polishing pad and may be raised or lowered to contact the polishing pad and to apply a selected amount of pressure to the surface of the wafers to be polished. 
     One or more wafers to be polished are attached to a wafer carrier and a wafer carrier is attached to each head assembly. Next, slurry is deposited on the polishing pad. The lid with wafers attached to the carriers on the head assemblies is lowered to enclose a polishing envelope and bring wafers closer to the polishing pad, and slurry is deposited on the polishing pad. Separate drive motors for the platen and head assemblies enables independent control of speed and direction of rotation. Polishing continues until the wafers achieve a desired value of wafer material removal, a desired value of surface quality, or a combination of both. 
     A quality and a rate of wafer polishing depend in part on a magnitude and direction of motion of the wafers relative to the polishing pad. The relative motion between the wafers and the polishing pad includes a component of rotational motion from the platen combined with a component of rotational motion of the head assembly to which the wafer is attached. In the case of a head assembly having a carrier holding a plurality of wafers, rotation of the head assembly results in wafer rotation relative to the platen and orbital motion of each wafer to and from the center axis of the platen. As technology progresses, the diameter of processed wafers also increases and the number of wafers that fit onto a carrier is correspondingly reduced. Furthermore, as wafer diameter increases, an edge of the wafer moves closer to the rotational center of a head assembly. The contribution to the rate of polishing by the rotation of the head assembly decreases for those parts of the wafer that are closest to the center of rotation of the head assembly. Some wafers are large enough that only one wafer may be placed in the central area of a carrier on a head assembly, in which case the component of radial, orbital motion from rotation of the head assembly is effectively lost in the central area of the wafer, and the quality of polishing is significantly degraded. 
     To achieve high quality polishing for large wafers, for example wafers having a diameter of 300 millimeters (12 inches), some polishing machines have only one head assembly above the platen. However, having only one head assembly per platen significantly reduces a rate of production compared to machines adapted to polish many wafers simultaneously. Adding more machines to make up the production rate difference per machine requires a higher capital investment in equipment and more factory floor space. 
     What is needed is a polishing machine having high throughput and a complex relative motion between a surface of a wafer to be polished and a polishing pad on a platen, for all parts of the surface of a large wafer. 
     SUMMARY 
     Embodiments of the present invention comprise a wafer polishing machine adapted for polishing large wafers efficiently and economically. In one embodiment, a wafer polishing machine in accord with the invention comprises a rotating platen and polishing pad in a table base, above which is mounted a lid having a head moving assembly with four rotating head assemblies. During operation, the head moving assembly collectively moves the head assemblies in reciprocating linear motion in a plane parallel to an upper surface of the platen while the platen and head assemblies are rotating. Embodiments of the invention produce a complex relative motion between a surface of a wafer to be polished and the polishing pad. The complex relative motion, resulting from a combination of motions including rotation of the platen, rotation of the head assemblies, and translation of the head moving assembly including four head assemblies, improves a quality and a throughput of polishing and prolongs a service life of the polishing pad compared to wafer polishing machines known in the art. 
     The above summary of the present invention is not intended to represent each disclosed embodiment, or every aspect, of the present invention. Other aspects and example embodiments are provided in the figures and the detailed description that follow. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present invention may be more completely understood in consideration of the following detailed description and accompanying drawings, in which: 
         FIG. 1  is a simplified pictorial view of a wafer polishing machine in accord with an embodiment of the invention; 
         FIG. 2  is a pictorial view of the embodiment of  FIG. 1 , in which the lid is raised from the table base and the platen and head assemblies are visible; 
         FIG. 3  is a simplified top view of an embodiment of the invention, showing a lid on top of a table base and a head moving assembly comprising four head assemblies in a square pattern sliding across the platen in an aperture formed in the lid; 
         FIG. 4  is a simplified top view of the same embodiment as  FIG. 3 , showing the head moving assembly moving in a direction opposite to the direction shown in  FIG. 3 ; and 
         FIG. 5  is a simplified view of a drive box, a part of the head moving assembly used to cause the head assemblies to rotate together at a same rate of rotation. 
         FIG. 6  is a top view of the embodiment of  FIG. 3  showing a reference position and a reference angle for motion of the head moving assembly. 
     
    
    
     DESCRIPTION 
     A wafer polishing machine adapted for polishing large wafers in accord with an embodiment of the invention is shown in  FIG. 1  and  FIG. 2 . The wafer polishing machine  100  comprises a table base  104  with a lid  102 , shown closed in  FIG. 1  and with the lid  102  raised in  FIG. 2 . Various electrical cables, slurry hoses, seals, switches, valves, and other support equipment have been omitted from the figures to facilitate a clearer view of the locations and functions of components discussed herein. 
     In  FIG. 1 , a top cover  110  encloses a head moving assembly (see  FIG. 3  and  FIG. 4 ) that partially protrudes through an opening formed in the lid  102 . A head assembly drive motor  106  in  FIG. 1  imparts rotation to four head assemblies  204  visible on the underside of the lid  102  in  FIG. 2 . The head assembly drive motor  106  and four head assemblies  204  are parts of the head moving assembly. A head moving assembly drive motor  108  attached to a fixed part of the lid  102  imparts a reciprocating linear motion to the head moving assembly. A round platen  202  is mounted into the table base  104  and rotates during polishing. A polishing pad (not shown) is placed on the upper surface of the platen  202  to facilitate polishing of a work piece. Work pieces like semiconductor wafers are placed on wafer carriers  205  and attached to the ends of the four head assemblies  204  visible in  FIG. 2 . 
     A simplified top view of an embodiment of the polishing machine  100  of  FIG. 1  and  FIG. 2  is shown in  FIG. 3  and  FIG. 4 . In  FIG. 3  and  FIG. 4 , the lid  102  is shown atop the table base  104 . The platen  202 , marked with a hidden line, is shown beneath the lid  102  in the table base  104 . An example of a platen rotation direction  306  is marked with an arrow drawn with a dashed line. The platen  202  may optionally be rotated in a direction opposite to the platen rotation direction  306  shown. The platen may be rotated at a selected rate of rotation in the selected direction of rotation. 
     The lid  102  is formed with a rectangular opening  301  in which a head moving assembly  302  slides back and forth above the platen  202 . The head moving assembly  302  comprises four head assemblies  204 . In some embodiments, the four head assemblies are attached to the head moving assembly in a square pattern, as shown in  FIG. 3 ,  FIG. 4 , and  FIG. 5 . An example of a first head moving assembly translation direction  308  is shown in  FIG. 3 . An example of a second head moving assembly translation direction  402 , representing a direction opposite to the direction shown in  FIG. 3 , is shown in  FIG. 4 . The head moving assembly  302  may be constrained to move on a linear path by slides, rails, channels, the sides of the aperture in the lid  102 , or equivalent linear guiding means. Motion is imparted to the head moving assembly  302  by the head moving assembly drive motor  108  shown in  FIG. 1 . A mechanical linkage (not illustrated) connected to the head moving assembly drive motor  108  and to the head moving assembly  302  converts a continuously rotating output from the drive motor to a reciprocating linear motion of the head moving assembly. In some embodiments, the linkage converts the motor&#39;s rotary output to an approximately sinusoidal linear motion. Linkages for converting rotary to linear motion, for example rotary to sinusoidal linear motion, are well known in the art and will not be described further here. The head moving assembly may be moved at a selected rate of translation in each of the directions of translation. 
     An example of a head assembly direction of rotation  304  is shown by an arrow drawn with a solid line in  FIG. 3  and  FIG. 4 . All four head assemblies  204  rotate in a same selected direction and at a same selected rate of rotation. In other embodiments, the head assembly direction of rotation  304  may be opposite to the direction shown in  FIG. 3  and  FIG. 4 . A means of causing all four head assemblies  204  to rotate at a same rate and in a same direction is shown in  FIG. 5 . In  FIG. 5 , a drive box  502  comprises mechanical support and components for driving the four head assemblies  204 . A drive motor pulley  504  is rotationally coupled to the head assembly drive motor  106  of  FIG. 1 , either by direct attachment to the motor drive shaft or by additional gears, belts, or pulleys. A head assembly pulley  506  is attached to a shaft for each head assembly  204 . Rotating the head assembly pulley  506  causes the head assembly  204  connected to the pulley to rotate. A power coupling means  508  engages the drive motor pulley  504  and the head assembly pulleys  506  as shown in  FIG. 5  such that a rotation of the drive motor pulley  504  causes a corresponding rotation of the head assembly pulleys  506  and correspondingly rotates the head assemblies  204 . In some embodiments, the power coupling means  508  is a double-sided timing drive belt having teeth and in other embodiments it can be a drive chain. 
     In the embodiment of  FIG. 3  and  FIG. 4 , the head moving assembly  302  is shown moving in a first translation direction  308  and a second translation direction  402 . The first translation direction  308  and the second translation direction  402  are collinear and in opposite directions. A direction of translation of the head moving assembly  304  is selected such that a tangent to a circular rotation path that is concentric with the platen&#39;s center of rotation is at an angle of 45 degrees to the direction of translation when the head moving assembly is in a reference position. The reference position referred to herein is defined as a middle or nominal position of the head moving assembly  304 . With the head moving assembly in the reference position, all four heads simultaneously have a tangent at 45 degrees to the direction of translation, as shown in  FIG. 6 . 
       FIG. 6  shows a table base  102 , a lid  102 , and a head moving assembly  302  comprising four head assemblies  204 , as in  FIG. 3  and  FIG. 4 . A platen pad  610  on top of the platen  602  is represented by a phantom line.  FIG. 6  further illustrates a reference position for the head moving assembly  302  and a direction of translation for the head moving assembly. A displacement of the head moving assembly  302  from the reference position illustrated in  FIG. 6 , also referred to as a middle position of the head moving assembly, corresponds to a magnitude of translation of the head moving assembly, a maximum value for which is determined by the size of the opening  301  in the lid  102 . A platen circular rotation path  604 , indicated with a phantom line, is shown concentric with the center of rotation  606  of the platen  202  and intersecting all four centers  608  of the head assemblies  204 , thereby defining a reference position of the head assemblies and head moving assembly. Lines  602 A,  602 B,  602 C, and  602 D, each tangent to the platen circular rotation path  604  and each passing through a head assembly center or rotation  608 , represent a direction of wafer center motion from platen  202  rotation relative to the platen  202 . A direction of translation represented by a line  402  passing through the centers of rotation  608  of the head assemblies  204 , or alternately an opposite direction of translation represented by lines  308 , is selected such that a line representing the linear translation path for all four head assemblies is at an angle of 45 degrees to the rotational part of the wafer center motion relative to the platen. For example, tangent line  602 A is one of four lines tangent to the platen circular rotation path  604 . A translation direction is selected such that an angle of 45 degrees is formed between a line  402  representing the linear translation path of the head moving assembly  302  and the tangent line  602 A. Similarly, 45 degree angles are formed between line pairs ( 402 ,  602 C), ( 308 ,  602 B), and ( 308 ,  602 D). The 45 degree angle described herein is to be formed for all four heads simultaneously when the head moving assembly is in the reference position illustrated in  FIG. 6 . 
     Embodiments with four head assemblies in the head moving assembly have high throughput and provide high quality wafer polishing. Wafer polishing machines with one or two head assemblies process fewer wafers per unit time than embodiments of the invention. Wafer polishing machines with three head assemblies will not have the symmetries apparent from an examination of the four-head configuration of  FIG. 3 ,  FIG. 4 , and  FIG. 6 , leading to differences in polishing rates compared to embodiments of the invention, and three head assemblies will not simultaneously meet the preferred 45 degree direction of translation described herein and in  FIG. 6 . Wafer polishing machines with more than four head assemblies in the head moving assembly will not simultaneously meet the preferred 45 degree direction of translation as defined in  FIG. 6  and will not provide uniform optimal polishing conditions for the polishing process. 
     A method of polishing a plurality of wafers on a polishing machine in accord with an embodiment of the invention comprises mounting wafers to be polished to wafer carriers  205  and installing the wafer carriers  205  on the head assemblies  204  as shown in  FIG. 2 . The platen  202  with a polishing pad attached is rotated in a selected direction  306  as in  FIG. 3  and  FIG. 4 . The head assemblies  204  with carriers  205  holding wafers are rotated at a selected rate and in a selected direction as in  FIG. 3  and  FIG. 4 . The head moving assembly  302 , also referred to as a drive box, is moved back and forth relative to the platen  202  within the opening  301  in a first translation direction  402  and a second translation direction  308 . Slurry is supplied to the polishing pad, the carriers  205  are lowered until the wafers contact the rotating platen  202 , and a separation distance between the wafers in the carriers  205  on the head assemblies  204  and the polishing pad on the platen  202  is adjusted to apply a selected amount of pressure between the wafers and the polishing pad. Pressure and motion continue until a selected quality of polish is achieved or until a selected amount of material is removed from the wafers. One skilled in the art will recognize that the steps above may optionally be performed in many different alternative sequences. 
     The present disclosure is to be taken as illustrative rather than as limiting the scope, nature, or spirit of the subject matter claimed below. Numerous modifications and variations will become apparent to those skilled in the art after studying the disclosure, including use of equivalent functional and/or structural substitutes for elements described herein, use of equivalent functional couplings for couplings described herein, or use of equivalent functional steps for steps described herein. Such insubstantial variations are to be considered within the scope of what is contemplated here. Moreover, if plural examples are given for specific means, or steps, and extrapolation between or beyond such given examples is obvious in view of the present disclosure, then the disclosure is to be deemed as effectively disclosing and thus covering at least such extrapolations. 
     Unless expressly stated otherwise herein, ordinary terms have their corresponding ordinary meanings within the respective contexts of their presentations, and ordinary terms of art have their corresponding regular meanings.