Abstract:
The present invention is directed to a method and apparatus for polishing a surface of a semiconductor wafer using a pad moveable in both forward and reverse directions. In both VLSI and ULSI applications, polishing the wafer surface to complete flatness is highly desirable. The forward and reverse movement of the polishing pad provides superior planarity and uniformity to the surface of the wafer. The wafer surface is pressed against the polishing pad as the pad moves in both forward and reverse directions while polishing the wafer surface. During polishing, the wafer is supported by a wafer housing having a novel wafer loading and unloading method.

Description:
This is a continuation of application Ser. No. 09/201,928 filed Dec. 1, 1998, now U.S. Pat. No. 6,103,628. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the field of chemical mechanical polishing. More particularly, the present invention relates to a method and apparatus for polishing a semiconductor wafer to a high degree of planarity and uniformity. This is achieved when the semiconductor wafer is polished with pads at high bi-directional linear or reciprocating speeds. 
     BACKGROUND OF THE INVENTION 
     Chemical mechanical polishing (CMP) of semiconductor wafers for VLSI and ULSI applications has important and broad application in the semiconductor industry. CMP is a semiconductor wafer flattening and polishing process that combines chemical removal of semiconductor layers such as insulators, metals, and photoresists with mechanical buffering of a wafer surface. CMP is generally used to flatten/polish wafers after crystal growing during the wafer fabrication process, and is a process that provides global planarization of the wafer surface. For example, during the wafer fabrication process, CMP is often used to flatten/polish the profiles that build up in multilevel metal interconnection schemes. Achieving the desired flatness of the wafer surface must take place without contaminating the desired surface. Also, the CMP process must avoid polishing away portions of the functioning circuit parts. 
     Conventional systems for the chemical mechanical polishing of semiconductor wafers will now be described. One conventional CMP process requires positioning a wafer on a holder rotating about a first axis and lowered onto a polishing pad rotating in the opposite direction about a second axis. The wafer holder presses the wafer against the polishing pad during the planarization process. A polishing agent or slurry is typically applied to the polishing pad to polish the wafer. In another conventional CMP process, a wafer holder positions and presses a wafer against a belt-shaped polishing pad while the pad is moved continuously in the same linear direction relative to the wafer. The so-called belt-shaped polishing pad is movable in one continuous path during this polishing process. These conventional polishing processes may further include a conditioning station positioned in the path of the polishing pad for conditioning the pad during polishing. Factors that need to be controlled to achieve the desired flatness and planarity include polishing time, pressure between the wafer and pad, speed of rotation, slurry particle size, slurry feed rate, the chemistry of the slurry, and pad material. 
     Although the CMP processes described above are widely used and accepted in the semiconductor industry, problems remain. For instance, there remains a problem of predicting and controlling the rate and uniformity at which the process will remove materials from the substrate. As a result, CMP is a labor intensive and expensive process because the thickness and uniformity of the layers on the substrate surface must be constantly monitored to prevent overpolishing or inconsistent polishing of the wafer surface. 
     Accordingly, an inexpensive and more consistent method and apparatus for polishing a semiconductor wafer are needed. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a method and apparatus that polishes a semiconductor wafer with uniform planarity. 
     It is another object of the present invention to provide a method and apparatus that polishes a semiconductor wafer with a pad having high bi-directional linear or reciprocating speeds. 
     It is yet another object of the present invention to provide a method and apparatus that reduces the size of the polishing station thereby reducing the space and cost of such station. 
     It is another object of the present invention to provide a method and apparatus that eliminates or reduces the need for pad conditioning. 
     It is yet another object of the present invention to provide a method and apparatus for efficiently loading and unloading a semiconductor wafer onto a wafer housing. 
     These and other objects of the present invention are obtained by providing a method and apparatus that polishes a wafer with a pad having high bi-directional linear speeds. In summary, the present invention includes a polishing pad secured to a timing belt mechanism that allows the pad to move in a reciprocating manner, i.e. in both forward and reverse directions, at high speeds. The constant forward and reverse movement of the polishing pad as it polishes the wafer provides superior planarity and uniformity across the wafer surface. The wafer housing of the present invention can also be used to securely hold the wafer as it is being polished. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects and advantages of the present invention will become apparent and more readily appreciated from the following detailed description of the presently preferred exemplary embodiment of the invention taken in conjunction with the accompanying drawings, of which: 
     FIG. 1 illustrates a perspective view of a method and apparatus in accordance with the preferred embodiment of the present invention; 
     FIG. 2 illustrates a side view of a method and apparatus in accordance with the preferred embodiment of the present invention; 
     FIG. 3 illustrates a front view of a method and apparatus for attaching a polishing pad to timing belts in accordance with the preferred embodiment of the present invention; 
     FIG. 4 illustrates side views of a polishing pad moving around the timing belt rollers in accordance with the preferred embodiment of the present invention; 
     FIG. 5 illustrates a side view of a wafer housing adapted to load and unload a wafer onto a wafer housing in accordance with the preferred embodiment of the present invention; 
     FIG. 6 illustrates a side view of a wafer housing having protruding pins adapted to load/unload a wafer onto a wafer housing in accordance with the preferred embodiment of the present invention; 
     FIG. 7 illustrates a side view of a wafer loaded onto a wafer housing in accordance with the preferred embodiment of the present invention; and 
     FIG. 8 illustrates a bottom view of a wafer being loaded and unloaded onto a wafer housing by three pins in accordance with the preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The preferred embodiment of the present invention will now be described with reference to FIGS. 1-8, wherein like components are designated by like reference numerals throughout the various figures. The present invention is directed to a CMP method and apparatus that can operate at high bi-directional linear pad or reciprocating speeds and a reduced foot-print. The high bi-directional linear pad speeds optimize planarity efficiency while the reduced foot-print reduces the cost of the polishing station. Further, because the polishing pad is adapted to travel in bi-directional linear directions, this reduces the pad glazing effect, which is a common problem in conventional CMP polishers. Because the pad travels in bi-directional linear directions, the pad is substantially self conditioning. 
     FIG. 1 illustrates a perspective view and FIG. 2 illustrates a side view of an apparatus of a preferred embodiment of the present invention. The wafer polishing station  2  includes a bi-directional linear, or reverse linear, polisher  3  and a wafer housing  4 . The wafer housing  4  (which can rotate about its center axis and move side to side, as known) securely positions a wafer  18  so that a surface  17  may be polished. In accordance with the present invention, a novel method and apparatus of loading and unloading the wafer  18  onto the wafer housing  4  is described more fully later herein. 
     The reverse linear polisher  3  includes a polishing pad  6  for polishing the wafer surface  17 , a mechanism  8  for driving the polishing pad  6  in a bi-directional linear or reciprocating (forward and reverse) motion, and a support plate  10  for supporting the pad  6  as the pad  6  polishes the wafer surface  17 . A polishing agent or slurry containing a chemical that oxidizes and mechanically removes a wafer layer is flowed between the wafer  18  and the polishing pad  6 . The polishing agent or slurry such as colloidal silica or fumed silica is generally used. The polishing agent or slurry generally grows a thin layer of silicon dioxide or oxide on the wafer surface  17 , and the buffering action of the polishing pad  6  mechanically removes the oxide. As a result, high profiles on the wafer surface  17  are removed until an extremely flat surface is achieved. It should also be noted that the size of the particles from the polishing agent or slurry used to polish the wafer surface  17  is preferably at least two or three times larger than the feature size of the wafer surface  17 . For example, if the feature size of the wafer surface  17  is 1 micron, then the size of the particles should be at least 2 or 3 microns. 
     The underside of the polishing pad  6  is attached to a flexible but firm and flat material (not shown) for supporting the pad  6 . The polishing pad  6  is generally a stiff polyurethane material, although other suitable materials may be used that is capable of polishing wafer surface  17 . 
     In accordance with the preferred embodiment of the present invention, the driving or transmission mechanism  8  for driving the polishing pad  6  in a bi-directional linear motion will now be described. Although FIGS. 1-2 illustrate only one driving mechanism  8  from the frontside of the reverse linear polisher  3 , it is understood that on the backside of the reverse linear polisher  3 , a similar driving mechanism  8  is also present. Driving mechanism  8  includes three timing belts, two vertically suspending timing belts  14 ,  15  and one horizontally suspending timing belt  16 . The timing belts  14 ,  15 , and  16  may be formed of any suitable material such as stainless steel or high strength polymers having sufficient strength to withstand the load applied to the belts by the wafer  18 . One end of the vertically suspending timing belts  14 ,  15  is secured to rollers  20  while the other end is secured to rollers  22 . Likewise, each end of the horizontally suspending timing belt  16  is secured to rollers  20 . As illustrated in FIG. 1, it is noted that the horizontally suspending timing belt  16  is placed in a z-plane slightly outside the z-plane of the vertically suspending timing belts  14 ,  15 . 
     Rollers  20  link the two vertically suspending timing belts  14 ,  15  with the horizontally suspending timing belt  16  so that each belts rate of rotation depends on the rate of rotation of the other belts. The rollers  20  and  22  retain the timing belts  14 ,  15 , and  16  under proper tension so that the polishing pad  6  is sufficiently rigid to uniformly polish the wafer surface  17 . The tension of the timing belts may be increased or decreased as needed by adjusting the position of rollers  22  relative to roller  20 . 
     Although the present invention describes a driving mechanism having three timing belts secured on four rollers, it is understood that any suitable number of rollers and/or timing belts, or a driving mechanism that does not rely on rollers/belts, i.e. a seesaw mechanism, such that it provides the bi-directional linear or reciprocating motion, are intended to be within the scope and spirit of the present invention. 
     An important aspect of the present invention is that the polishing pad  6  and the corresponding support material is adapted to bend at an angle at corners  24 , which angle is preferably about 90°. Each end of the polishing pad  6  is attached to a point on the two vertically positioned timing belts  14 ,  15  by attachments  12 ,  13 . One end of the polishing pad  6  is secured to attachment  12 , and the other end is secured to attachment  13 . Attachments  12  and  13  are preferably a sleeve and rod, as more fully described later herein. Referring again to FIGS. 1 and 2, as one end of the polishing pad  6  travels vertically downward with the assistance of timing belt  14  and attachment  12 , the other end of the polishing pad  6  travels vertically upward with the assistance of timing belt  15  and attachment  13 . The mechanical alignment of the timing belts  14 ,  15 , and  16  with the rollers  20  and  22  allows such movement to occur. 
     In order to drive the timing belts  14 ,  15 , and  16  to a desired speed, a conventional motor (not shown) is used to rotate rollers  20  and/or  22 . The motor is connected to rollers  20  or  22  or to any suitable element connected to rollers  20  and/or  22 , and it provides the necessary torque to rotate rollers  20  and  22  to a desired rate of rotation. The motor directly/indirectly causes rollers  20  and  22  to rotate so that the timing belts  14 ,  15 , and  16  are driven at a desired speed in both forward and reverse directions. For instance, when attachment  13  reaches roller  22  during its downward motion, it will reverse the direction of the polishing pad  6  as attachment  13  now travels upward. Soon thereafter, the same attachment  13  now reaches roller  20  and again changes direction in a downward direction. The reciprocating movement of attachment  13  allows the polishing pad  6  to move in both forward and reverse directions. Preferably, the speed at which the polishing pad  6  is moved is within the range of approximately 100 to 600 feet per minute for optimum planarization of the wafer surface  17 . However, it should be understood that the speed of the polishing pad  6  may vary depending on many factors (size of wafer, type of pad, chemical composition of slurry, etc.). Further, the pad  6  may be moved in both bi-directional linear directions at a predetermined speed, which preferably averages between 100 to 600 feet per minute. 
     FIG. 3 illustrates a front view and FIG. 4 illustrates a side view of a method and apparatus for attaching the polishing pad  6  to the timing belts  14 ,  15  in accordance with the preferred embodiment of the present invention. As described earlier herein, the underside of the polishing pad  6  is attached to the flexible but firm and flat material, which is non-stretchable. At each end of the material, and thus the ends of the polishing pad  6 , a rod  40  is attached. The rod  40  extends horizontally from the pad  6  as shown in FIG. 3. A sleeve  42 , i.e. a cylinder or a slit, is also attached to each of the vertically suspending timing belts  14 ,  15 , and a portion  44  of the sleeve  42  extends horizontally to join the rod  40 , as again illustrated in FIG.  3 . When the rod  40  and the sleeve  42  are joined, this allows the polishing pad  6  to travel bi-directional with high linear speeds without the problem of having the polishing pad  6  being wrapped around the rollers  20 ,  22 . FIG. 4 further illustrates a side view of the polishing pad  6  as it rotates around the rollers  20 ,  22 . 
     As described earlier, the polishing pad  6  bends at an angle, preferably about 90° at the two corners  24 . This approach is beneficial for various reasons. In accordance with the present invention, the length of the polishing pad  6  on the horizontal plane needed to polish the wafer surface  17  needs to be only slightly longer than the wafer  18  diameter. Optimally, the entire length of polishing pad should be only slightly longer than three times the wafer  18  diameter. This allows the most efficient and economical use of the entire polishing pad  6 . During polishing, slurry or other agent may be applied to the portions of the polishing pad  6  that are not in contact with the wafer surface  17 . The slurry or other agent can be applied to the polishing pad preferably at locations near corners  24 . The configuration of the polishing pad  6  described above also decreases the size of a support plate  10  needed to support the pad  6 . Furthermore, though the bi-directional linear movement provides for a substantially self conditioning pad, a conditioning member can also be disposed on or about this same location. 
     The novel approach described above has many other advantages and benefits. For example, the CMP device of the present invention takes up less space than most traditional CMP devices because about two-thirds of the polishing pad  6  can be in a vertical position. The bi-directional linear movement of the CMP device further increases the pad usage efficiency because the reciprocating movement of the pad  6  provides a self-conditioning function, since the pad  6  is moving in different, preferably opposite, directions. 
     In accordance with the present invention, only one wafer is generally polished during a single time. As described above, the polishing pad  6  moves bi-directional with high linear speeds so as to uniformly polish the wafer surface  17 . Because high pad speeds are needed to polish the wafer surface  17 , the momentum, and thus inertia created is very high. Thus, as the polishing pad  6  reverses direction, sufficient energy is needed to keep the pad moving at desired speeds. If the total area (length and width) of the polishing pad  6  is minimized, the energy needed to keep the pad moving at desired speeds is decreased accordingly. Thus, by limiting the length of the polishing pad  6 , a conventional motor can handle the necessary energy needed to keep the pad moving at desired speeds in both forward and reverse directions. The entire length of the polishing pad  6  should be slightly longer than two-diameter lengths of the wafer  18 , and preferably three-diameter lengths of the wafer  18 . The reason for this is so that the polishing pad  6  may be conditioned and slurry may be applied to both sides of the pad opposite where the wafer  18  is positioned, in close proximity to corners  24 . 
     Although the present invention is adapted to polish a single wafer at one time, one skilled in the art may modify the preferred embodiment of the invention in order to polish multiple wafers at one time. Slurry (not shown) can be applied to the surface of the polishing pad  6  in conventional manners and the pad  6  can further be conditioned in conventional manners. 
     Next, with reference to FIG. 5, a wafer housing  4  in accordance with the preferred embodiment of the present invention will now be described. Wafer housing  4  includes a nonconductive, preferably circular, head assembly  28  with a cavity  29  that is preferably a few millimeters deep at its center and having a resting pad  30  thereof. The wafer  18  is loaded into the cavity  29 , backside first, against the resting pad  30 . A conventional type of securing mechanism  31  (i.e. vacuum) is used to ensure that the wafer  18  is securely positioned with respect to the wafer head assembly  28  while the wafer  18  is being polished. The resting pad  30  may also be of a type that secures the wafer  18  by suctioning the backside of wafer  18  when the resting pad  30  is wet. 
     As described above, the reverse linear polisher  3  may polish the wafer  18  during various stages of the wafer fabrication process. Accordingly, a method for loading the wafer  18  into the cavity  29  so that an additional loading mechanism is not needed will now be described with reference to FIG.  6 . First, the wafer housing  4  is aligned to load the wafer  18  into the cavity  29 . The head assembly  28  includes a pin housing  32  adapted to move up and down with respect to the cavity  29  using a motor or pneumatic control (not shown). During loading of the wafer  18 , the pin housing  32  extends down from an original position, which is illustrated by the dashed lines, below the surface  17  of the wafer  18 . At least three pins  34  are then automatically caused to protrude out of the pin housing  32  using a conventional retraction device under motor control so that the wafer  18  can be picked up and loaded into the cavity  29  of the head assembly  28 . With the pins  34  protruding out, the pin housing  32  automatically retracts back to its original position, and thus the wafer  18  is loaded into cavity  29 . When the head assembly  28  and the resting pad  30  secures the position of the wafer  18 , as described above, the pins  34  automatically retract back into the pin housing  32  and the pin housing  32  retracts back to its original position so that the wafer  18  may be polished, as illustrated in FIG.  7 . 
     Referring back to FIGS. 1 and 2, after the wafer  18  is securely loaded onto the wafer housing  4 , the wafer housing  4  is automatically lowered until the wafer surface  17  is in contact with the polishing pad  6 . The polishing pad  6  polishes the wafer surface  17  in accordance with the method described herein; the wafer  18  is then ready to be unloaded from the wafer housing  4 . 
     With reference to FIG. 6, the wafer  18  is unloaded from the wafer housing  4  using essentially a reverse order of the loading steps. After polishing the wafer  18 , the wafer housing  4  is raised from the polishing pad  6 , and the pin housing  32  extends down from its original position, which is illustrated by the dashed lines, below the surface  17  of the wafer  18 . The pins  34  are then automatically caused to protrude out so that the wafer  18  may be supported when unloaded from the cavity  29 . With the pins  34  protruding, the vacuum is reversed with opposite air flow, thus dropping the wafer  18  away from head assembly  28  and onto the pins  34  (i.e., wafer  18  is positioned from the resting pad  30  onto the pins  34 ). From this position, the wafer can then be transported to the next fabrication processing station. 
     FIG. 8 illustrates a bottom view of the wafer  18  surface being loaded and unloaded into the cavity  29  by the pins  34 . Although FIG. 8 illustrates three protruding pins  34 , it should be understood that more than three pins, or an alternative support mechanism, may be used in accordance with the present invention. 
     Referring again to FIGS. 1-2, the support plate  10  for supporting the polishing pad  6  will now be described. The polishing pad  6  is held against the wafer surface  17  with the support of the support plate  10 , which may be coated with a magnetic film. The backside of the support material to which the polishing pad  6  is attached may also be coated with a magnetic film, thus causing the polishing pad  6  to levitate off the support plate  10  while it moves at a desired speed. It should be understood that other conventional methods could be used to levitate the polishing pad  6  off the support plate  10  while it polishes wafer surface  17 , such as air, lubricant, and/or other suitable liquids. 
     It is to be understood that in the foregoing discussion and appended claims, the terms “wafer surface” and “surface of the wafer” include, but are not limited to, the surface of the wafer prior to processing and the surface of any layer formed on the wafer, including oxidized metals, oxides, spin-on glass, ceramics, etc. 
     Although various preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and/or substitutions are possible without departing from the scope and spirit of the present invention as disclosed in the claims.