Abstract:
The present invention relates to an apparatus and method for polishing substrate surfaces. The method can include the steps of holding a substrate against a polishing surface and depositing slurry on the polishing surface. The method can further include the step of moving the holding device in a substantially curvilinear path relative to the polishing surface, or the step of moving the polishing surface in a substantially curvilinear path relative to the holding device. The apparatus comprises a polishing surface, a holding device for holding a substrate against the polishing surface, and a slurry supply system for depositing slurry on the polishing surface. The apparatus further includes a moving structure for moving the holding device in a substantially curvilinear path along the polishing surface, or a moving structure for moving the polishing surface in a substantially curvilinear path relative to the holding device. The substantially curvilinear path is preferably substantially a figure eight path.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     (Not Applicable) 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     (Not Applicable) 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to the field of semiconductor wafer fabrication, and more particularly to the field of chemical mechanical planarization (CMP) of thin films used in semiconductor wafer fabrication. 
     2. Description of the Related Art 
     The production of integrated circuits begins with the creation of high quality semiconductor wafers. A semiconductor wafer typically includes a substrate, such as a silicon or gallium arsenide wafer, on which a plurality of transistors have been formed. Transistors are chemically and physically formed in and on a substrate by patterning regions in the substrate and patterning layers on the substrate. The transistors are interconnected through the use of well known multilevel interconnects to form functional circuits. Typical multilevel interconnects are comprised of stacked thin films, commonly comprised of one or more of the following: titanium (Ti), titanium nitrite (TiN), tantalum (Ta), aluminum-copper (Al—Cu), aluminum-silicon (Al—Si), copper (Cu), and tungsten (W). 
     During the wafer fabrication process, the wafers may undergo multiple masking, etching, dielectric deposition, and conductor deposition processes. An extremely flat, or planarized, surface is generally needed on at least one side of the semiconductor wafer to ensure proper accuracy and performance of the microelectronic structures being created on the wafer surface. In general, a wafer can be polished to remove high topography, surface defects such as crystal lattice damage, scratches, roughness or embedded particles. As the size of integrated circuits continues to decrease and the density of microstructures on an integrated circuit continues to increase, the need for precise wafer surfaces becomes more important. Therefore, between each processing step, it is usually necessary to polish the surface of a wafer in order to obtain the most planarized surface possible. 
     CMP is routinely used to planarize the surface of the layers, or thin films, of the wafer during the various stages of device fabrication. CMP has emerged as the planarization method of choice because of its ability to planarize better than traditional planarization methods. During a CMP process, polishing planarizes surfaces to very precise tolerances, which is essential for maintaining the precise photolithographic depth of focus required for integrated circuit chip fabrication. In a typical CMP process, the wafer is held by a rotating carrier with the active wafer surface facing a rotating polishing table, called a platen. On top of the platen is a porous polyurethane polishing surface on which is poured a slurry. The slurry can be colloidal silica suspended in an aqueous solution. Slurries with different chemical compositions are used to polish metals and other films. During metal polishing, the slurry chemically reacts with the wafer&#39;s surface, forming a passive layer on a portion of the wafer&#39;s surface, while the mechanical force exerted by the pad and the colloidal silica particles abrades the wafer&#39;s surface, removing the passive layer. 
     A CMP slurry serves several functions. Most notably, it is the medium in which abrasive particles are dispersed. Additionally, it furnishes the chemical agents which promote the chemical process. To obtain optimum results from CMP processing, there must be a synergistic relationship between the chemical and mechanical processes. 
     For example, CMP slurries for polishing a metal layer commonly comprise a metal oxidizer and an abrasive agent. The oxidizer reacts with the metal to form a passive metal oxide layer. During the polishing process, the abrasive agent removes the passive oxide layer from elevated portions of the metal layer. Depressed portions of the metal layer surface are not subjected to mechanical abrasion and, therefore, the protected material underlying depressed portions of the passive oxide layer is not polished. This process continues until the elevated portions of the metal layer have been polished away, resulting in planarization. 
     The ideal polishing process can be described by Preston&#39;s equation: R=K p *P*V, where R is the removal rate, P is the applied pressure between the wafer and the polishing surface, V is the relative velocity between the wafer and the polishing surface, and K p  is a function of consumables such as polishing surface roughness, elasticity, and chemistry. The ideal CMP process has constant pressure between the polishing surface and the wafer, constant polishing surface roughness, elasticity, area, and abrasion effects, and constant velocity over the entire wafer surface. Having constant velocities at points which are distant from the center of the wafer is generally preferable to having fluctuating velocities because the removal rate is much easier to control when constant velocity conditions exist. For example, when points at a distance from the center of the wafer are exposed to alternating high and low velocities, the abrasive material may scratch the surface of the wafer and result in a non-planarized surface. Non-uniform removal of films from the surface of a wafer is a common problem encountered during CMP processing because there are numerous variables which can affect planarization. 
     In a typical CMP process, the wafer carrier and the platen rotate in the same direction, but with the two rotating axes offset by some distance. This arrangement results in relative linear motion between any position on the wafer and the polishing surface. Thus, removal caused by the polishing surface is related to the radial position of the wafer relative to the platen. The removal rate increases as the wafer moves radially, or linearly, outward relative to the platen. Removal rates tend to be higher at the edges of the wafer than at the center of the wafer. As a result, wafer surfaces tend to become higher at the center of the wafer as compared to the edges of the wafer. Reducing this center-to-edge variation results in a more planarized wafer surface. 
     Attempts have been made to reduce this center-to-edge variation by polishing in non-linear polishing patterns. One approach includes affixing a mechanical template having a non-linear opening to a polishing surface. A rotating motor moves a wafer carrier along the edges of the non-linear template, allowing the wafer carrier to traverse the surface of the polishing surface in a non-linear manner. This approach is significantly limited, however, because it requires attaching a device to the polishing surface. Such a configuration can significantly reduce the polishing surface lifespan by causing uneven wear of the polishing surface. The direct contact between the template and the polishing surface also reduces the lifespan of the polishing surface because the template can introduce particles and other defects into the polishing surface. Another approach involves the use of a non-linear carrier displacement mechanism for moving a wafer carrier across a polishing surface. A drawback to this configuration is that it does not provide a means for moving a wafer across a polishing surface along a substantially figure eight path. 
     SUMMARY OF THE INVENTION 
     The present invention relates to an improved apparatus and method for planarizing the surface of a substrate, such as a semiconductor wafer. In one embodiment of the invention, the apparatus for polishing substrate surfaces includes a polishing surface, a holding device for holding a substrate against the polishing surface, a slurry supply system for depositing slurry on the polishing surface, and structure for moving the holding device in a substantially figure eight path relative to the polishing surface. The moving structure can comprise a motor and an actuating arm connecting the motor to the holding device. 
     In another embodiment, the apparatus for polishing substrate surfaces includes a polishing surface, a holding device for holding a substrate against the polishing surface, a slurry supply system for depositing slurry on the polishing surface, and a structure for moving the holding device in a substantially curvilinear path relative to the polishing surface. In this embodiment, the moving structure can include a drive which is attached to the holding device, structure for rotating the drive, and at least one steering device for steering the drive in a substantially curvilinear path relative to the polishing surface. The substantially curvilinear path can be a substantially figure eight path. The steering device can be one or more cams. 
     Yet another embodiment of the invention comprises a polishing surface, a holding device for holding at least one substrate against the polishing surface, a slurry supply system for depositing slurry on the polishing surface, and a moving structure. The moving structure can include a drive which is attached to the holding device and rotates the holding device, and counter-rotating devices having structure for engaging the holding device. The counter-rotating devices alternately engage the holding device, thereby moving the holding device in a substantially curvilinear path relative to the polishing surface. The substantially curvilinear path can be a substantially figure eight path. 
     A method for polishing substrate surfaces according to the invention includes the steps of holding a substrate against a polishing surface with a holding device, depositing slurry on the polishing surface, and moving the holding device in a substantially figure eight path relative to the polishing surface with moving structure. The step of moving the holding device in a substantially figure eight path relative to the polishing surface can be performed with a motor and an actuating arm connecting the motor to the holding device. 
     Another method according to the invention includes the steps of holding a substrate against a polishing surface with a holding device, depositing slurry on the polishing surface, rotating the holding device with a drive attached to the holding device, and steering the drive in a substantially curvilinear path relative to the polishing surface with at least one steering device. The substantially curvilinear path can be a substantially figure eight path, and the steering device can be one or more cams. 
     Still another method according to the invention includes the steps of holding a substrate against a polishing surface with a holding device, depositing slurry on the polishing surface, rotating the holding device with a drive attached to the holding device, providing a plurality of counter-rotating devices having structure for engaging the holding device, and rotating the counter-rotating devices. The counter-rotating devices alternately engage the holding device, whereby the holding device moves in a substantially curvilinear path relative to the polishing surface. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     There are shown in the drawings embodiments which are presently preferred, it being understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown, wherein: 
     FIG. 1 is a side schematic view of a conventional CMP apparatus. 
     FIG. 2 is a top schematic view of the conventional apparatus of FIG.  1 . 
     FIG. 3 a  is a top schematic view of a curvilinear polishing system showing curvilinear polishing according to the invention. 
     FIG. 3 b  is a top schematic view of an alternative curvilinear polishing system showing curvilinear polishing according to the invention. 
     FIG. 3 c  is a top schematic view of a curvilinear polishing system showing curvilinear polishing of a plurality of wafers with a plurality of wafer carriers according to the invention. 
     FIG. 4 is a top schematic view of a curvilinear polishing system with steering devices according to the invention. 
     FIG. 5 is a side schematic view of the rotating devices of FIG.  4 . 
     FIG. 6 is a top schematic view of a curvilinear polishing system with counter-rotating devices according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIGS. 1 and 2, a semiconductor wafer  10  is shown pressed against a polishing surface  12 , which is preferably a polishing pad. The wafer  10  is pressed against the polishing surface  12  by a wafer carrier  16 . In a conventional CMP device, the wafer  10  is held face-down by the wafer carrier  16 . A thin synthetic film (not shown) can rest on the back side of the wafer  10 . The synthetic film can have small holes through which back pressure may be applied during polishing. The back pressure can be used to prevent wafer bowing during polishing and to improve polishing uniformity. 
     The wafer carrier  16  is often composed of a material which can damage the wafer  10  if it directly contacts the wafer  10 . Therefore, the wafer carrier  16  can be pressed against a wafer holder  26  which helps protect the wafer  10  by separating the wafer carrier  16  from the wafer  10 . The wafer carrier  16  can also be rotated by a wafer carrier spindle  18 , causing the wafer  10  to rotate as it contacts the polishing surface  12 . 
     According to conventional CMP processes, the wafer  10  is pressed against the polishing surface  12 , and a slurry supply system  20  applies slurry  24  to the polishing surface  12 . During the CMP process, a platen spindle  22  rotates the platen  14 , independent of the rotation of the wafer  10  and the wafer carrier  16 . The rotation of the platen  14  and the wafer carrier  16  causes the wafer  10  to move through the slurry  24  in a rotary fashion. As slurry  24  flows over the surface of the wafer  10 , the suspended particles in the slurry  24  and the polishing surface  12  mechanically abrade the surface and the liquid in the slurry  24  chemically etches the abraded area. In this manner, a substantial amount of material from the high spots on the wafer  10  is removed, while a negligible amount of material from the low spots on the wafer  10  is removed, resulting in a flattened, or planarized, wafer  10 . 
     FIG. 3 a  shows curvilinear polishing according to the present invention. A wafer carrier  32  presses the surface of a wafer (not shown) against a polishing surface  30 . Preferably, the wafer carrier  32  can move axially and laterally relative to the polishing surface  30 . As the wafer carrier  32  moves the wafer across the polishing surface  30 , the wafer carrier  32  can be rotated by a drive  34 . The drive  34  is preferably a flexible rod or a connector that is rotated by a motor (not shown). The drive  34  can rotate the wafer carrier  32  in any suitable manner. 
     The wafer carrier  32  can be rotated by the drive  34  while the wafer carrier  32  moves curvilinearly across the polishing surface  30 . Curvilinear paths followed by the wafer carrier  32  preferably extend across the diameter of the polishing surface  30 . In a particularly preferred arrangement, the curvilinear path traveled by the wafer carrier  32  as it moves relative to the polishing surface  30  substantially takes the shape of one or more figure eight paths. An advantage of figure eight paths is that such paths expose the wafer to multiple directions of polishing. Accordingly, although a wafer traversing a figure eight path across the polishing surface  30  may be scratched by the polishing surface  30  as it moves along a first portion of the figure eight path, such abrasions can be removed as the wafer traverses a second portion of the figure eight path. Similarly, wafer surface imperfections not removed as the wafer moves through the first portion of the figure eight path can be removed as the wafer traverses the second portion of the figure eight path. 
     The substantially figure eight paths may be of any suitable size. For example, the substantially figure eight paths can be large enough to extend across the diameter of the polishing surface  30 . Substantially figure eight paths large enough to extend across the diameter of the polishing surface  30  can allow even wear of the polishing surface. 
     An actuating arm  36  can connect a motor  38  to the drive  34 . The motor  38  can move the arm  36 , and thus the attached wafer carrier  32 , curvilinearly across the polishing surface  30 . The motor  38  can be programmed to move the arm  36  in any desirable curvilinear direction, including a substantially figure eight path. 
     In FIG. 3 a,  the wafer carrier  32  is shown traversing a substantially figure eight path near the center of rotation of the polishing surface  30 . As shown in FIG. 3 b,  however, each substantially figure eight path can begin and end at any point along the polishing surface  30 . Additionally, as shown in FIG. 3 c,  the apparatus according to the invention may utilize multiple wafer carriers  32 . Each wafer carrier  32  can independently traverse the polishing surface  30  by following one or more substantially figure eight paths. Each wafer carrier  32  can be moved along the substantially figure eight paths by an arm  36  which is connected to a motor  38 . 
     In another embodiment of the invention, the wafer can be held substantially stationary against the polishing surface  30 , while the polishing surface  30  moves in a substantially curvilinear manner. In this embodiment, any suitable motor (not shown) can be used to move the polishing surface  30  in a substantially curvilinear manner. The substantially curvilinear motion is preferably a substantially figure eight motion. 
     There are many other ways to impart curvilinear motion according to the invention. FIGS. 4 and 5 show an embodiment in which one or more steering devices  46 ,  48  steer a wafer carrier  42  across a polishing surface  40  in a curvilinear manner. The steering devices  46 ,  48  may be any mechanism suitable for steering the wafer carrier  42 , but preferably are cams. Each steering device  46 ,  48  can be attached to a motor (not shown) by an actuating arm  47 ,  49 . The motors and actuating arms  47 ,  49  rotate each steering device  46 ,  48  about its respective axis. The wafer carrier  42  has a drive  44  which rotates the wafer carrier  42  about its axis as it traverses the polishing surface  40 . The drive  44  is preferably a flexible rod or a connector that is rotated by a motor (not shown). The wafer carrier  42  is shown pressing the wafer holder  52  against the back surface of the wafer  54 , and the wafer holder  52  is shown pressing the surface of the wafer  54  being polished against the polishing surface  40 . 
     For this embodiment, the movement of the wafer carrier  42  in a substantially curvilinear or a substantially figure eight motion can be caused by two independent motions. For example, this movement can be caused by the wafer carrier  42  moving linearly across the polishing surface  40  as indicated by arrows  41  and  43 , while steering devices  46 ,  48  steer the drive  44  in a curvilinear manner by alternately pressing against the drive  44 . Contact between a steering device  46 ,  48  and the drive  44  communicates motion to the drive  44 , which permits the drive  44  to push the wafer carrier  42  relative to the polishing surface  40 . The motion communicated to the drive  44  can be dictated by the geometry of the edges of the steering devices  46 ,  48  or the geometry of one or more grooves cut in the edges of the steering devices  46 ,  48 . The steering devices  46 ,  48  can be configured to move the drive  44 , and thus the attached wafer carrier  42 , along any desirable curvilinear path along the polishing surface  40 . As previously indicated, however, it is preferable for this curvilinear path to substantially take the shape of a figure eight. 
     Another embodiment of the invention is shown in FIG.  6 . This embodiment includes a plurality of counter-rotating devices  66 ,  68  which move a wafer carrier  62  in a curvilinear path relative to a polishing surface  60 . Preferably, the curvilinear path is one or more substantially figure eight paths. Each counter-rotating device  66 ,  68  can be rotated about its axis by a drive  70 ,  72 . The wafer carrier  62  also has a drive  64  which rotates the wafer carrier  62  about its axis as it traverses the polishing surface  60 . Any suitable motor can provide the rotation of the drives  64 ,  70 ,  72 . 
     Each counter-rotating device  66 ,  68  has one or more extension arms  74  extending radially outward relative to its center. Preferably, the extension arms  74  have a main portion  76  and a contact portion  78 . Each main portion  76  can be attached to a contact portion  78  in any suitable manner. Preferably, the main portion  76  is attached to the contact portion  78  by a pin, so that the contact portion  78  can pivot relative to the main portion  76 . The contact portion  78  carries the wafer carrier  62  as the counter-rotating devices  66 ,  68  move the wafer carrier  62  relative to the polishing surface  60 . 
     During operation, the counter-rotating devices  66 ,  68  alternate moving the wafer carrier  62  relative to the polishing surface  60 . Accordingly, each counter-rotating device  66 ,  68  can receive the wafer carrier  62  in one of its extension arms  74 , complete approximately one revolution, and then transfer the wafer carrier to the other of the counter-rotating devices  66 ,  68 . The contact portion  78  of the extension arm  74  holding the wafer carrier  62  can pivot at least slightly towards or away from the main portion  76  of the extension arm  74  as the wafer carrier  62  is transferred from one counter-rotating device  66 ,  68  to another counter-rotating device  66 ,  68 . The counter-rotating devices  66 ,  68  allow the wafer carrier  62  to traverse the polishing surface  60  along a curvilinear path. Preferably, the curvilinear path is a substantially figure eight path. 
     It is understood that the embodiments of the present invention are described in the context of devices and methods for polishing semiconductor wafers, although those skilled in the art will recognize that the disclosed devices and methods are readily adaptable for other applications, including polishing of substrates other than semiconductor wafers. It should also be understood that the examples and embodiments described herein are for illustrative purposes only and that various modifications or changes in light thereof will be suggested to persons skilled in the art and are to be included within the spirit and purview of this application. The invention can take other specific forms without departing from the spirit or essential attributes thereof.