Abstract:
A method and apparatus provide polishing of a semiconductor wafer or other substrate. The apparatus includes multiple wafer carriers provided on the top surface of a table. A semiconductor wafer is seated face-up in the wafer carrier. Each wafer carrier is driven by an electric motor to rotate at a low speed. During operation, each wafer carrier is positioned at a work station where a specified task is performed. The table rotates when the task at each station is completed to move the wafers from station to station. Thus multiple tasks relating to polishing (e.g., buffing and drying) can be carried out in parallel. At one station, a polishing pad is positioned by a polishing pad carrier face-down to polish the surface of the semiconductor wafer. A motor drives the polishing pad to move in a high-speed circular motion.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to chemical-mechanical polishing (CMP) of semiconductor wafers. In particular, the present invention relates to an apparatus providing simultaneously polishing of multiple wafers. 
     2. Discussion of the Related Art 
     In integrated circuit manufacturing, CMP is widely used for planarizing the surface of the semiconductor wafer to allow multiple layers of conductors and dielectric to be formed. 
     In the prior art, CMP is achieved by pressing a semiconductor wafer against a polishing pad provided on a high-speed rotating table or a linear motion polishing belt. Typically, the semiconductor wafer is held by a wafer carrier, which also rotates. A slurry, typically including fine silicon oxide particles suspended in an alkaline solution, is provided as a chemically active abrasive. 
     In CMP using a rotational table, because each point on a semiconductor wafer experiences a polishing speed that depends, among other factors, the distance from the rotating table&#39;s axis of rotation and its own speed of rotation, uniformity of polishing across the wafer is very difficult to achieve. Furthermore, because of the complex motion, the wear and tear on the polishing pad at different points of the rotating table are also non-uniform, also contributing to non-uniform polishing results. 
     Linear polishing eliminates some of the contributing factors of non-uniformity. However, because the contact surface areas at different points of the polishing pads are different, polishing non-uniformity is still difficult to achieve. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method and an apparatus for accomplishing chemical-mechanical polishing of a wafer using a non-rotatory circular motion. This circular motion polishes every point on a wafer equally and also imparts equal wear and tear at every point of the polishing pad. Better uniformity than achievable in the prior art is therefore achieved. 
     According to one aspect of the present invention, an apparatus for polishing of a wafer includes (a) a wafer holder for holding the wafer so as to expose a surface of the wafer for polishing; (b) a polishing pad holder for holding a polishing pad against the exposed surface; and an actuator coupled to the polishing pad holder for driving the polishing pad in a non-rotatory circular motion so as to provide polishing action against the exposed surface of the wafer. In one embodiment, the apparatus multiple wafer holders are provided on a rotatable table, such that tasks related to chemical-mechanical polishing (CMP) of the wafer can be carried out simultaneously at stations around the rotatable table. In one embodiment of the present invention, the wafer holder includes a raised wall for containing a slurry used in polishing. 
     In one implementation, the actuator includes a motor which drives an off-center shaft to provide the non-rotatory circular motion. In that implementation, a linear bearing couples the polishing pad holder to the actuator, such that the off-center shaft, the linear bearing and the motor are enclosed in multiple connected chambers. In that arrangement, a pressurized air flow is provided to flow through the multiple connected chambers to effectuate cooling of the motor. 
     In one embodiment, the apparatus provides the polishing pad holder and the actuator in each of two polishing assemblies. In addition, a conditioning station is provided, so that when one of the polishing assemblies is positioned for polishing the wafer, the polishing pad in the other polishing assembly is positioned at the conditioning station for conditioning. A diamond plate on a rotatable platform is provided at the conditioning station. Conditioning is carried out by pressing the polishing pad against the rotating diamond plate. 
     In one implementation of the apparatus, an actuator is provided for rotating the wafer in the wafer holder during polishing. 
     According to another aspect of the present invention, the wafer holding includes an wafer edge extension ring which surrounds the wafer. The wafer edge extension ring has a surface flush with the surface of the wafer being polished, so that the edge of the wafer is “extended” to the outer edge of the extension ring. Since polishing is carried out over the surfaces of the wafer and the wafer edge extension ring, edge effects are substantially eliminated. 
     In one implementation of the apparatus, the wafer is supported by a housing seated on a rotatable platform. The housing is provided an inlet into a recessed portion of the housing underneath the wafer holder, and a flexible seal is provided over the recessed portion of the housing in contact with a surface of the wafer holder. Under this arrangement, a chamber is formed under the surface of the wafer holder. A gas can then be introduced into the chamber through the inlet to allow a pressure to be applied against the wafer holder. The polishing pressure can then be controlled by adjusting the pressure on the flexible seal. 
     The present invention is better understood upon consideration of detailed description below and the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of CMP apparatus  100 , according to one embodiment of the present invention. 
     FIG. 2 is a top view of CMP apparatus  100 . 
     FIG. 3 illustrates the motion of one point on a polishing pad relative to housing  18  of polishing motor  17 . 
     FIG. 4 is a cross-section view of polishing assembly  400 , including polishing pad carrier  21  and polishing motor  17 . 
     FIG. 5 is a cross-section view of wafer carrier assembly  500 . 
     FIG. 6 is a cross-section view of conditioning assembly  600 . 
     FIG. 7 is a top view of CMP apparatus  700 , according to a second embodiment of the present invention. 
     FIG. 8 shows, instead of bellows  24 , a special bearing assembly  820  provides support for the non-rotatory circular motion of polishing pad holder  809  (hence the motion of polishing pad  806 ). 
     FIG. 9 is a cross section view of bearing assembly  820 . 
     FIGS. 10 a-   10   d  show, respectively, the positions of upper plate  801  and lower plate  802  at four different positions during operation. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1 and 2 show a perspective view and a top view of a chemical mechanical polishing (CMP) apparatus  100 , respectively, according to one embodiment of the present invention. As shown in FIGS. 1 and 2, in CMP apparatus  100 , a table  8  transports semiconductor wafers in wafer carrier assemblies for processing at stations  1 - 6 , located 60 degrees apart relative to table  8 &#39;s axis of rotation. (The number of stations shown in FIG. 2 are provided merely for illustrative purpose; as many stations as practicable and necessary can be provided). An example of a wafer carrier assembly is wafer carrier assembly  500  shown in FIG.  5 . 
     Each of stations  1 - 6  performs a designated task on one semiconductor wafer under process. At any given time, all wafer stations are active, so that multiple wafers are simultaneously processed on table  8 . To complete processing, a semiconductor wafer to be processed is loaded at station  1  and is successively rotated in six steps through each of wafer stations  2 - 6 , finally returning to station  1  for unloading. 
     At station  1 , a semiconductor wafer is loaded and unloaded from one side by robot assembly  9 . A robotic arm in robot assembly  9  picks up from a wafer carrier on table  8  a semiconductor wafer that has completed processing, and deposits the semiconductor wafer into wafer cassette  10   a.  Then, the same or another robotic arm in robot assembly  9  picks up a semiconductor wafer to be processed from wafer cassette  10   b  and places the wafer on to the wafer carrier just unloaded. 
     At station  2 , a slurry-filling step is performed. In the slurry-filling step, a specified amount of slurry is introduced into the wafer carrier to “flood” the semiconductor wafer surface, in preparation for the CMP step at station  3 . 
     At station  3 , a CMP step is performed by a polishing pad in circular motion. The polishing pad is held by a polishing assembly (e.g., polishing  400   a ). An example of a polishing assembly  400  is shown in FIG.  4 . While polishing is carried out by the polishing pad at station  3 , another polishing pad held in another polishing assembly (e.g., polishing assembly  400   b ) is being conditioned by conditioning diamond plate  12  at conditioning station  11 . Conditioning of a polishing pad after each CMP step can improve polishing uniformity. In this embodiment, polishing assemblies  400   a  and  400   b  are rotated by a shaft  50  alternately into station  3  and conditioning station  11 . Polishing assemblies  400   a  and  400   b  are coupled respectively to linear mechanisms  7   a  and  7   b.  Linear mechanisms  7   a  and  7   b  each include linear bearings which allow polishing assemblies  400   a  and  400   b  to be vertically positioned for CMP. At station  3 , the wafer carrier on table  8  rotates at a low speed relative to the speed of the polishing pad&#39;s circular motion. The operations of polishing assemblies  400   a  and  400   b  are discussed in further detail below. 
     During conditioning, diamond plates  12   a  rotates at a low speed similar to that of the wafer carrier. Cleaning arm  16   a  includes a cleaning mechanism for cleaning diamond plate  12   a  periodically to ensure uniformity of conditioning. Polishing pads are changed at station  14 . Depositing a used polishing pad and picking up a new polishing pad are performed by vacuum action in the polishing pad assembly. 
     At station  4 , the slurry is washed out of the wafer carrier by water. At station  5 , a fine polishing and/or cleaning step (“buffing”) is carried out. In this embodiment, the operation of the buffing step is similar to that of the polishing step in station  3 . The polishing pads for buffing are also conditioned at conditioning station  12   b  in a manner similar to that described above for conditioning station  11 . Cleaning arm  16   b  is shown in FIG. 2 to be performing the periodic cleaning operation on diamond plates  12   b.  Polishing pads for the buffing operation are changed at station  15  in a manner similar to that described above for station  14 . 
     At station  6 , the semiconductor wafer is rinsed and dried. 
     FIG. 4 is a cross-section view of polishing assembly  400 , which forms an actuator including housing  51 , polishing pad carrier  21  and polishing motor  17 . Polishing pad carrier  21  is coupled by linear bearing  20  to housing  51 . Polishing pad carrier  21  holds polishing pad  22 . The surface area of polishing pad  22  approximates the surface area of the wafer carrier assembly open to the polishing pad. 
     Motor  17  drives off-center shaft  23  to impart a circular motion to polishing pad carrier  21  during operation. This circular motion is not rotational about the axis of off-center shaft  23 . FIG. 3 shows the locus of motion of any point on the polishing pad. Unlike the prior art rotating table or linear polishing approaches, under this arrangement, every point in both the polishing pad and the semiconductor wafer surface experience substantially identical polishing action. Thus, the present invention provides more uniform polishing than the prior art approaches. 
     Polishing pad carrier  21 , bellows  24  and linear bearing support plate  19  provides a sealed environment enclosing linear bearing  20  and off-center shaft  23 . Bellows  24  prevents any rotational motion about off-center shaft  23 . Alternatively, as shown in FIG. 8, instead of bellows  24 , a special bearing assembly  820  provides support for the non-rotatory circular motion of polishing pad holder  809  (hence the motion of polishing pad  806 ). Bearing assembly  820  includes an upper plate  801 , a lower plate  802  and a plurality of ball bearings, represented in this embodiment by ball bearings  803   a-   803   d.  The rotation of upper plate  801  about off-axis  807  (relative to axis  808  at the center of polishing pad holder  809 ) provides the non-rotatory circular motion. 
     FIG. 9 is a cross section view of bearing assembly  820 . FIG. 10 a-   10   d  show, respectively, the positions of upper plate  801  and lower plate  802  at four different positions during operation. FIGS. 10 a-   10   d  show four different positions (approximately 90 degrees apart) along the polishing pad path traveled by polishing pad  806  over wafer carrier  805 . Upper and lower plates  801  and  802  are each a plate with a number of circular grooves with substantially semicircular cross sections. The circular grooves of upper and lower plates are positioned such that, at any given time, as shown in each of FIGS. 10 a-   10   d,  each circular groove overlaps a corresponding circular groove to form a spherical cavity where a ball bearing (e.g., any of ball bearings  803   a-   803   d ) is accommodated. As upper plate  801  rotates about axis  807 , each ball bearing travels along both the circular grooves of upper and lower plates  801  and  802 . As a result, lower plate  802  carries polishing pad  806  in the non-rotatory circular motion. 
     A cooling air flow through polishing assembly  400  is provided to polishing assembly  400 . Air enters into polishing assembly  400  through inlet  18 , through chambers  18   a,    18   b,  and  18   c  (in order) and exits through outlet  18   d.  Typically, the cooling air flow cools the surface of the polishing assembly sufficiently to provide a moisture condensation which prevents the slurry from drying up in the wafer carrier and on the outside of bellows  24 . The pressure in chambers  18   a,    18   b  and  18   c  are sufficiently low so as to lessen the force asserted by loaded linear bearing  20  against the semiconductor wafer surface. This lessened pressure allows better control of polishing rate and, consequently, better control of polish uniformity. 
     FIG. 5 is a cross-sectional view of wafer carrier assembly  500 . As shown in FIG. 5, wafer carrier assembly  500  includes an actuator for supporting and imparting motion to wafer carrier  54 . Wafer carrier  54  has a circular cavity for accommodating plate  27 , wafer edge extension ring  28  and guide ring  29 , and raised wall  30  for containing the slurry flooding the semiconductor surface during CMP. Plate  27  has a flat surface over which is coated a friction film for supporting a semiconductor wafer. Plate  27  has a diameter substantially the same as that of the semiconductor wafer, indicated by reference numeral  26 . Wafer edge extension ring  28 , which extends the surface area open to the opposing polishing pad, surrounds plate  27 . Guide ring  29  surrounds and positions wafer edge extension ring  28 . Plate  27 , wafer edge extension ring  28  and guide ring  29  are supported by housing  32  which, through flexible seal  31 , transmits a pressure against the semiconductor wafer. The pressure is provided by pressured air applied from the chamber  53  below seal  31 . Plate  27  and wafer edge extension ring  28  can be removed independently. Guide ring  29 , the surface of wafer  26  and wafer edge extension ring  28  form the surface of the wafer carrier assembly open to the polishing pad (not shown). Wafer edge extension ring  28  is designed to be flush with the semiconductor wafer surface to receive the same polishing action as the semiconductor wafer. Essentially, the edge of the semiconductor wafer is now positioned well inside the outer edge of the polishing surface, which is “extended” to the outer edge of the wafer edge extension ring  28 . Consequently, “edge effects” at the wafer edge characteristic in CMP are substantially minimized. Guide ring  29  guides the motion of the polishing pad in plane parallel to the wafer surface. 
     Housing  32  sits on base plate  39  of table  35 , which can be rotatably driven by shaft  36 . As explained above, during CMP, table  35  rotates at a low speed (relative to the speed of the polishing pad&#39;s circular motion). Seal ring  33  prevents the slurry from flowing into the shaft area. 
     Friction resulting from the polishing action tends to coerce the semiconductor wafer to follow the circular motion of the polishing pad. However, because of the speed of the polishing pad, the semiconductor wafer cannot keep up with the motion of the polishing pad. However, the circular motion tends to lessen the force exerted the wafer against the wafer edge extension ring, relative to the force that would be exerted by either the planetary motion of a rotating table or a linear polishing belt. Consequently, deformity of the edge of the semiconductor wafer is reduced, with corresponding improvement of polishing uniformity along the edge of the semiconductor wafer. This beneficial effect is expected even for 300 mm semiconductor wafers. 
     FIG. 6 is a cross-section view of conditioning assembly  600 . As shown in FIG. 6, conditioning assembly  600  includes a diamond plate (indicated by reference numeral  41 ) seated in a housing  55 . Housing  55  has raised wall  42  for containing a conditioning fluid used in conditional a polishing pad. Drain hole  40  is provided for draining conditioning assembly  600  with conditioning fluid. Table 43 is rotatably driven by shaft  40 . 
     FIG. 7 is a top view of CMP apparatus  700 , in accordance with a second embodiment of the present invention. As shown in FIG. 7, CMP apparatus  700  includes 5stations  701  to  705 . A wafer to be polished is loaded by a robotic assembly at station  701  onto a wafer holder on rotatable table  713  at the beginning of processing, and unloaded from rotatable table  713  by the robotic assembly at the end of processing. Rotatable  713  rotates in 90 degree steps to carry the wafer through stations  702  through  705  to complete the polishing process. In CMP appartus  700 , stations  702  and  704  are provided to perform one or more steps of rinsing, cleaning, conditioning of polishing pads, and changing polishing pads. Polishing and buffing are performed at stations  703  and  705  by polishing heads  711  and  714 , respectively. 
     The detailed description above is provided to illustrate the specific embodiments of the present invention and is not intended to be limiting. Numerous variations and modifications within the scope of the present invention are possible. The present invention is set forth in the following claims.