Curvilinear chemical mechanical planarization device and method

Information

  • Patent Grant
  • 6537135
  • Patent Number
    6,537,135
  • Date Filed
    Monday, December 13, 1999
    25 years ago
  • Date Issued
    Tuesday, March 25, 2003
    21 years ago
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's surface, forming a passive layer on a portion of the wafer's surface, while the mechanical force exerted by the pad and the colloidal silica particles abrades the wafer'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'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.



Claims
  • 1. A method for polishing substrate surfaces, said method comprising 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, wherein the drive is 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, wherein the counter-rotating devices alternately engage the holding device, whereby the holding device moves in a substantially curvilinear path relative to the polishing surface.
  • 2. The method according to claim 1, further comprising the step of rotating the counter-rotating devices, wherein the counter-rotating devices alternately engage the holding device, whereby the holding device moves in a substantially figure eight path relative to the polishing surface.
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5549511 Cronin et al. Aug 1996 A
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5980363 Meikle et al. Nov 1999 A
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6007411 Feeney Dec 1999 A
6015337 Hiyama et al. Jan 2000 A
6290578 Bowman et al. Sep 2001 B1