Linear reciprocating disposable belt polishing method and apparatus

Information

  • Patent Grant
  • 6500056
  • Patent Number
    6,500,056
  • Date Filed
    Friday, June 30, 2000
    24 years ago
  • Date Issued
    Tuesday, December 31, 2002
    21 years ago
Abstract
An apparatus for chemically mechanically planarizing a semiconductor wafer is disclosed having a continuous polishing strip with first side having a fixed abrasive surface and a second side opposite the first side. In one embodiment, a first drive roller holds a first end of the polishing strip, a second drive roller holds a second end of the polishing strip, and a pair of support rollers contacts the second side of the polishing strip on either end of a polishing strip support. A drive motor is operably connected to the first and second drive rollers for moving the polishing strip in a linear, bi-directional manner.
Description




FIELD OF THE INVENTION




The present invention relates to polishing and planarization of semi-conductor wafers. More particularly, the present invention relates to a method and apparatus for linearly reciprocating at least a portion of a continuous polishing member to polish a semiconductor wafer.




BACKGROUND




Semiconductor wafers are typically fabricated with multiple copies of a desired integrated circuit design that will later be separated and made into individual chips. A common technique for forming the circuitry on a semiconductor is photolithography. Part of the photolithography process requires that a special camera focus on the wafer to project an image of the circuit on the wafer. The ability of the camera to focus on the surface of the wafer is often adversely affected by inconsistencies or unevenness in the wafer surface. This sensitivity is accentuated with the current drive toward smaller, more highly integrated circuit designs. Semiconductor wafers are also commonly constructed in layers, where a portion of a circuit is created on a first level and conductive vias are made to connect up to the next level of the circuit. After each layer of the circuit is etched on the wafer, an oxide layer is put down allowing the vias to pass through but covering the rest of the previous circuit level. Each layer of the circuit can create or add unevenness to the wafer. This unevenness is preferably smoothed out before generating the next circuit layer.




Chemical mechanical planarization (CMP) techniques are used to planarize the raw wafer and each layer of material added thereafter. Available CMP systems, commonly called wafer polishers, often use a rotating wafer holder that brings the wafer into contact with a non-abrasive polishing pad moving in the plane of the wafer surface to be planarized. A polishing fluid, such as a chemical polishing agent or slurry containing microabrasives, is applied to the polishing pad to polish the wafer. The wafer holder then presses the wafer against the rotating polishing pad and is rotated to polish and planarize the wafer. Another type of polisher is a linear polishing mechanism that rotates a polishing pad mounted on an endless loop. This type of polisher also utilizes an abrasive slurry to chemically-mechanically planarize or polish semiconductor wafers. With the recent introduction of fixed abrasive polishing media that does not require an abrasive slurry in order to planarize or polish a semiconductor wafer, new wafer polishers are desirable that can take advantage of the fixed abrasive media.











BRIEF DESCRIPTION OF THE DRAWINGS





FIG. 1

is an elevational side view of a semiconductor wafer polishing device according to a preferred embodiment;





FIG. 2

is an elevational side view of the second embodiment of a preferred semiconductor wafer polishing device according to the present invention;





FIG. 2A

is a top sectional view of a drive roller used in the wafer polishing device of

FIG. 2

;





FIG. 3

is an elevational side view of a third embodiment of a semiconductor wafer polishing device;





FIG. 3A

is a top sectional view of a roller suitable for use in the wafer polishing device of

FIG. 3

; and





FIG. 4

is an elevational side view of a fourth embodiment of a semiconductor wafer polishing device.











DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS




In order to address the need for wafer polishers that are suitable for use with fixed abrasive polishing media, a wafer polisher is disclosed below that provides an apparatus and method for applying fixed abrasive polishing media to linear polishing techniques. A preferred embodiment of the wafer polisher


10


is illustrated in FIG.


1


. The polisher


10


includes a pair of belt support rollers


12


,


14


used to control vertical position of a polishing strip


16


.




Positioned between the first and second support rollers is a polishing strip support


18


. Preferably, the polishing strip is oscillated by a drive assembly made up of a central drive motor


20


connected to a pair of drive rollers


22


,


28


through a belt pulley system. The drive rollers may be driven by any of a number of known types of DC servo motors.




The first drive roller


22


holds a supply of unused polishing strip material that is wound, in a continuous strip, around a portion of the circumference of the first idler roller


24


, looped around the first belt support roller


12


, passed over the support platen


18


, and around the second support roller


14


. The polishing strip continues from the second support roller


14


around a portion of the circumference of the second idler roller


26


and is held at a second end by a take-up roller


28


. The take-up and feed rollers are preferably actively driven by the drive motor


20


through a pulley system. As shown in

FIG. 1

, the pulley system may include a plurality of belts


30


,


32


interconnecting the drive motor


20


to the first and second drive rollers


22


,


28


. In other embodiments, chains, gears or other methods of transferring movement between the motor and rollers may be used. Tension on the polishing strip


16


is maintained by the first and second drive rollers


22


,


28


. Preferably, the tension is maintained on these rollers using slip clutches


36


,


38


mounted on the first and second drive rollers


22


,


28


.




The preferred embodiment, distance measuring devices


52


,


53


constantly monitor the diameter of the drive rollers


22


,


28


to sense the change in diameter based on taking up or feeding out polishing strip material during operation. The distance measuring devices


52


,


53


monitor a distance d


1


, d


2


between the distance measuring device


52


,


53


and the respective drive roller


28


,


22


. The distance data is then feed to a CPU-based controller configured to calculate the appropriate torque that is necessary at each of the slip clutches. The torque information is provided to the proper slip clutch, for example in the form of a voltage. Using the voltage signal from the controller


51


, the slip clutches


36


,


38


maintain a torque proportionate to the change in torque moment arm resulting from drive roller diameter changes due to taking up or feeding out polishing strip material. By slipping at the required torque value, the slip clutches thus maintain the pre-established tension on the belt at all times. In one embodiment, the distance measuring device may be a laser-type, or other optical format, distance measuring device and the particle slip clutches may be magnetic. The controller


51


may have any one of a number of commonly available CPUs and memory for maintaining logic suitable for calculating torque values necessary to maintain a desired tension based on the measured diameter changes, and subsequently generate the appropriate voltage with, for example, standard digital-to-analog converter circuitry.




The drive motor


20


is preferably a bi-directional drive motor adjustable to linearly reciprocate a length of the polishing strip through the polishing area. The polishing area is defined by the area of polishing strip positioned between the support


18


and the wafer (not shown) held by a wafer carrier


40


that is pressed against the strip


16


by a spindle assembly


42


. In a preferred embodiment the length of polishing strip driven through the polishing area is adjustable from any desired incremental length to substantially the entire length of the strip. The number of oscillations of the polishing strip through the polishing area, per wafer treated, is selectable. While the polisher


10


may be adjusted to move the polishing member at various frequencies, the frequency of oscillation is preferably within the range of 0-25 Hertz.




The polishing strip


16


preferably has a width greater than the width of the wafer to be polished. Preferably the polishing strip is a consumable that may be constructed of any of a number of fixed abrasive materials suitable for use in planarization and/or polishing of semiconductor wafers. For example, the structured abrasive belts available under part numbers 3M 307EA or 3M 237AA from 3M Corporation of St. Paul, Minn. are suitable for this purpose. The polishing strip support


18


may be a platen producing a fluid bearing such as the platen used with the TERES™ polisher available from Lam Research Corporation of Fremont, Calif., or the wafer support assembly disclosed in U.S. Pat. No. 5,558,568, the entire disclosure of which is incorporated herein by reference. The slip clutches may be any of a number of available types of magnetic particle adjustable torque slip clutches. The support rollers may be hollow or solid cylinders preferably having a width greater than the width of the polishing strip. The support and idler rollers may be actively driven or passively rotatable by the polishing strip as it passes over the rollers. As described above, the slip clutches


36


,


38


on the first and second drive rollers preferably maintain a constant belt tension and allow for rotational speed changes as polishing strip accumulates onto or feeds off of the rollers.




Using the polisher


10


of

FIG. 1

, a semiconductor wafer may be polished and/or planarized by lowering the wafer against the strip of fixed abrasive with the spindle assembly and wafer carrier. The strip may be set in motion prior to or shortly after the wafer contacts the strip. In a first embodiment, the drive motor


20


rotationally reciprocates such that the drive rollers


22


,


28


move the polishing strip back and forth at a desired oscillation rate. In an alternative embodiment, the drive motor


20


may be adjusted to oscillate such that substantially the entire length of the polishing strip is passed across the platen


18


each oscillation back and forth. In either instance, the wafer holder


40


and spindle assembly


42


preferably rotate the wafer while pressing the wafer against the linearly moving polishing strip.




In one embodiment, the polisher


10


may be operated to linearly oscillate a selected length of the polishing strip against the surface of a wafer and incrementally introduce new portions of the polishing strip by operating the drive rollers to steadily move the polishing strip more in one direction than the other with each oscillation. Alternatively, the polisher may be operated to treat each wafer with a different set amount of the polishing strip. In other embodiments, the polisher may use the same set amount of polishing strip for each of a group of wafers before moving a different portion of polishing strip into the polishing area for treatment of another group of wafers. Although not required, each of the embodiments described herein may utilize a non-abrasive liquid during polishing, such as deionized water, to facilitate the polishing process. The non-abrasive liquid may be applied via nozzles


43


(See

FIG. 1

) to the region of the polishing strip intended for contact with a wafer. In another embodiment, a pad conditioner


54


may be used to prepare the polishing strip for use. For example, if a protective coating, such as a polymer film, need to be removed from the polishing strip, the pad conditioner may be used to engage the appropriate portion of the polishing member to remove the protective coating. Any of a number of commercially available polishing pad conditioners may be used, including rotary disks and cylindrical rollers. The pad conditioner may be withdrawn from contact with the polishing strip after removal of any protective film.




Referring to

FIG. 2

, a second embodiment of the present invention is disclosed. The wafer polisher


110


of

FIG. 2

also includes a take-up roller and a feed roller,


112


,


114


. Each of the take-up and feed rollers preferably include a clutch, such as commonly available variable torque, magnetic particle clutches with internal roller motor


1




16


. A respective one of a pair of drive rollers


118


,


120


is mounted on a belt tracking device


122


and is positioned adjacent each of the take-up and feed rollers. Preferably, the drive rollers are covered with a high friction surface


124


, such as hypolon and also include internal drive motors.

FIG. 2A

illustrates the belt tracking device


122


in more detail. In one embodiment, the belt tracking device may use an optical detector to determine if the polishing strip


128


is moving laterally along the width of the drive roller and/or to determine the velocity of the strip. The polishing strip


128


may have a plurality of reference indicators


129


, such as marks or holes, that the belt tracking device


122


may use to monitor polishing strip motion and position. Pivot arms


125


may be manipulated to tilt the drive rollers


118


,


120


about pivot points


126


to compensate for the lateral strip movement.




A programmable reciprocating linear actuator equipped with a roller carriage


130


and having a pair of carriage mounted idler rollers


132


is positioned adjacent the drive rollers


118


,


120


. The programmable actuator


140


and roller carriage


130


is operably movable in a linear direction parallel to the longitudinal direction of the polishing strip


128


. As with the embodiment of

FIG. 1

, a pair of belt support rollers


134


,


136


are positioned on the side of a support platen


138


to maintain the height of the strip passing through the polishing area and avoid access wear of the strip against the support


138


. The polisher


110


applies a linear reciprocating motion to the polishing strip through linear motion of the programmable reciprocating linear actuator and roller carriage along the linear shaft


131


.




In order to maintain a constant tension on the polishing strip, the slip clutch in each of the take-up and feed rollers


112


,


114


is adjusted by a controller


151


based on diameter measurements made with distance measuring devices


152


,


153


. Suitable controllers


151


, distance measuring devices


152


,


153


and slip clutches are described with respect to the embodiment of FIG.


1


. Also, as descried in the embodiment of

FIG. 1

, a pad conditioner


154


may be used to remove any protective film on the polishing strip prior to planarizing semiconductor wafers.




Utilizing the polisher


110


of

FIGS. 2 and 2A

, a method of polishing a semiconductor wafer is described below. Preferably, a first supply of the polishing strip


128


is positioned in the polishing area (i.e. the area of the polishing strip over, or adjacent to, the support platen


138


) and the take-up and feed rollers lock in position using the magnetic particle clutches. Once the take-up and feed rollers have been locked in their positions, the programmable reciprocating roller carriage is linearly reciprocated along the shaft to provide a linear motion of the strip against the wafer. As described above with respect to

FIG. 1

, a spindle drive assembly


144


and wafer carrier


146


cooperate to press the wafer


148


against the strip and rotate the wafer. Tension and friction are used to prevent slippage of the polishing strip on the oscillating carriage rollers


132


. In an alternative embodiment, a clamping device may be used at each carriage roller


132


to hold the polishing strip and ensure that only a discrete portion of the polishing strip is used for any given series of oscillations.




A third embodiment of the present invention is best shown in FIG.


3


. In this embodiment, the feed


212


and take-up


214


rollers of the polisher


210


oscillate under the control of a synchronized closed-loop servo controller


216


that maintains a desired belt tension and adjusts roller velocity based on optically, or other type of, measured movement of the polishing strip. Each roller preferably includes an internal roller motor


213


,


215


. A pair of idle rollers


218


are positioned on either side of the polishing strip support


220


to maintain a fixed elevation of the polishing strip with respect to the polishing plane. The polishing strip support


220


may be the same type of platen assembly as described above. Standard preprogrammed algorithms or an index mark sensing system may be used to control the speed of rotation of the take-up and feed rollers to account for diameter variations as the consumable polishing strip material transfers from the feed roller


212


to the take-up roller


214


. Tension is preferably maintained through adjusting motor current for each roller motor with. The take-up and feed rollers may be hollow or solid cylinders used grip the extreme ends of the polishing strip and allow the polishing strip to roll of unroll as polishing proceeds. Alternatively, as shown in

FIG. 3A

, the take-up or feed roller


250


,


252


may be constructed in the shape of a spool with flanges


254


so as to assist with alignment of the polishing strip on each roller.




To aid in tracking and monitoring, the edges of the polishing strip


222


may be smooth, textured, or patterned. The edges may contain holes or other physical features that serve a functional purpose, such as aiding in alignment and tracking of the belt in use or such as aiding in triggering or counting. The edges of the polishing strip and any related features may be formed during molding or may be created in a secondary manufacturing operation such as cutting, drilling, lathing or punching. An optical sensor


224


may be connected to the servo controller


220


to sense polishing strip movement and provide feedback information usable to adjust the velocity of the polishing strip or alignment on the rollers


212


,


214


. The polishing strip


222


may also have holes cut in it to expose a portion of the wafer W held by the wafer carrier


226


and spindle assembly


228


during polishing. Operation of the embodiment of

FIG. 3

may proceed as described with respect to the embodiment of FIG.


1


. Additionally, distance measuring devices may monitor roller diameter of the feed and take-up rollers


212


,


214


, and a pad conditioner may be used, as described in the embodiment of FIG.


1


.




A fourth embodiment of the wafer polisher


310


is disclosed in FIG.


4


. In this embodiment, a belt clamping mechanism


313


is attached to each of a pair of drive rollers


316


positioned adjacent opposite sides of a polishing strip support


318


. The clamp attachment points


320


on each of the drive rollers


316


are preferably positioned past the top of each drive roller


316


in a direction away from the wafer polishing area defined by the region of polishing strip


322


over the polishing strip support


318


. The clamping mechanism


313


may include a clamping member


311


, such as a bar extending the width of the roller, that is movable into and out of engagement with the clamp attachment point


320


by a clamp driver


321


. The clamp attachment point may be a recessed region having a shape complementary to that of the clamping member on each of the rollers


316


. The clamp driver


321


may be any of a number of devices, such as pneumatic or hydraulic pistons and cylinders, an electrically driven motor or drive screw, or other known mechanisms.




A take-up roller


312


and a feed roller


314


are positioned adjacent a respective one of the drive rollers


316


. The take-up and feed rollers are preferably actively driven and controllable to maintain a desired slack region


328


of the polishing member


322


so that the take-up and feed rollers may remain substantially stationary while the drive rollers


316


move to polish a wafer W held on a wafer holder


330


. This reduces the possibility of stressing the polishing member and reduces the amount of roller mass that must be oscillated during polishing.




The motors


324


driving the drive rollers


316


, preferably synchronized DC servo motors controlled by a standard servo controller


326


such as described with respect to

FIG. 3

, are controlled so that a tension is maintained on the portion of the polishing strip extending between the attachment points and so that the attachment points do not pass below the polishing plane as the polishing member is oscillated against a wafer. The positioning of the attachment points allows oscillation with motion control and avoids the problem of an attachment point


320


passing below the polishing plane during operation. The take-up and feed rollers


312


,


314


are preferably only driven between polishing steps to draw a new portion of the polishing strip across the polishing region when the clamps


313


are released and the wafer holder is not pressing and turning a wafer W against the polishing strip. Although shown as connected to the drive rollers by belts


332


, the motors may be direct drive motors, internal or external, connected to the axis of rotation of each drive roller


316


. The take-up and feed rollers are preferably connected to motors


334


selectively operable to rotate the take-up and feed rollers and move a different portion of the polishing strip over the drive rollers.




It is intended that the foregoing detailed description be regarded as illustrative rather than limiting, and that it be understood that the following claims, including all equivalents, are intended to define the scope of this invention.



Claims
  • 1. An apparatus for chemically mechanically planarizing a semiconductor wafer, the apparatus comprising:a continuous polishing strip comprising a first side and a second side opposite the first side, wherein the first side comprises a fixed abrasive surface; a pair of polishing strip support rollers positioned adjacent opposite ends of a polishing strip support, wherein the pair of polishing strip rollers are in contact with the second side of the polishing strip and the polishing strip support is configured to support a section of the polishing strip during a semiconductor wafer polishing process; a first drive roller holding a first end of the polishing strip; a second drive roller holding a second end of the polishing strip, wherein at least one of the first and second drive rollers comprises an torque adjustment mechanism configured to maintain a tension on the polishing strip; a drive motor operably connected with the first and second drive rollers and configured to move the polishing strip in a linear, bi-directional motion, wherein both of the first and second drive rollers are operably connected with the drive motor by belts; a first passively rotatable idler roller positioned between the first drive roller and a first one of the pair of polishing strip support rollers; and a second passively rotatable idler roller positioned between the second drive roller and a second one of the pair of polishing strip support rollers.
  • 2. The apparatus of claim 1, wherein the torque adjustment mechanism comprises a slip clutch.
  • 3. The apparatus of claim 1, wherein each of the first and second drive rollers further comprise a slip clutch.
  • 4. The apparatus of claim 1, wherein the polishing strip support comprises a fluid bearing platen disposed beneath the second side of the polishing strip.
  • 5. An apparatus for chemically mechanically planarizing a semiconductor wafer, the apparatus comprising:a continuous polishing strip comprising a first side and a second side opposite the first side, wherein the first side comprises a fixed abrasive surface; a pair of polishing strip support rollers positioned adjacent opposite ends of a polishing strip support, wherein the pair of polishing strip rollers are in contact with the second side of the polishing strip and the polishing strip support is configured to support a section of the polishing strip during a semiconductor wafer polishing process; a first drive roller holding a first end of the polishing strip; a second drive roller holding a second end of the polishing strip, wherein at least one of the first and second drive rollers comprises an torque adjustment mechanism configured to maintain a tension on the polishing strip; a drive motor operably connected with the first and second drive rollers and configured to move the polishing strip in a linear, bi-directional motion; a first passively rotatable idler roller positioned between the first drive roller and a first one of the pair of polishing strip support rollers; a second passively rotatable idler roller positioned between the second drive roller and a second one of the pair of polishing strip support rollers; and a feedback circuit for adjusting the torque adjustment mechanism during a polishing process, the feedback circuit comprising a drive roller diameter sensing device in electrical communication with a controller, wherein the controller is in communication with the torque adjustment mechanism and is configured to provide a signal to the torque adjustment mechanism based on a sensed drive roller diameter, whereby a torque may be maintained on the polishing strip regardless of an amount of polishing strip on a drive roller.
  • 6. The apparatus of claim 5, wherein the torque adjustment mechanism comprises a slip clutch.
  • 7. The apparatus of claim 5, wherein each of the first and second drive rollers further comprise a slip clutch.
  • 8. The apparatus of claim 5, wherein the polishing strip support comprises a fluid bearing platen disposed beneath the second side of the polishing strip.
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Entry
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