Abstract:
An improved chemical-mechanical polishing method and apparatus is provided. A brush is employed to continually brush slurry particles from surface features, e.g., grooves, on a polishing pad. In this manner slurry is prevented from becoming compacted within the grooves as the slurry passes beneath and is subjected to compressive forces of a wafer polishing head. The invention may be practiced by use of a stationary brush operatively coupled to the polishing pad surface, or by an improved conditioning assembly having both a diamond surface for conditioning the polishing pad and a brush for cleaning the pad&#39;s surface features. The brush portion of the conditioning assembly may or may not rotate as it is scanned across the surface of the polishing pad.

Description:
This application is a continuation of U.S. patent application Ser. No. 09/021,765, filed Feb. 11, 1998, now U.S. Pat. No. 6,135,868 titled “GROOVE CLEANING DEVICE FOR CHEMICAL MECHANICAL POLISHING”. 
    
    
     The present invention relates to the field of semiconductor processing, and more particularly to a method and apparatus for polishing and/or planarizing semiconductor wafers and the thin films formed thereon. 
     BACKGROUND OF THE INVENTION 
     Modern semiconductor devices are typically multilayered, having numerous metalization layers separated by numerous insulating oxides and interconnected with vias or contact holes. For instance, an interconnect for a typical multi-layer device is formed by depositing and patterning a first metal layer over the device, depositing an intermediate oxide over the patterned first metal layer, photolithographically defining a contact hole in the oxide, and depositing a second metal layer over the oxide that fills the contact hole and contacts the patterned first metal layer. 
     Patterning the first metal layer produces metal steps or undulations between where the first metal is removed and where the first metal remains. Because the intermediate oxide layer is a conformal layer, the oxide layer tracks these undulations. Accordingly, if the second metal layer were also deposited directly over the intermediate oxide layer, the undulations from the first metal layer would undesirably appear in the second metal layer. 
     Undulations in the second metal layer complicate patterning of the second metal layer, especially in high resolution, fine line-width applications, because no single focal plane exists on the second metal layer. A non-planar second metal layer, therefore, undesirably increases the line-widths produceable in the second metal layer. Furthermore, if the second metal layer undulations are large (e.g., on the order of the thickness of the second metal layer), voids or open circuits may form in the second metal layer. These problems may propagate to subsequently deposited material layers. 
     To prevent step or undulation propagation, the intermediate oxide layer is preferably planarized, removing any steps or undulations formed therein, prior to deposition of the second metal layer. Planarization is typically performed mechanically by forcing the semiconductor wafer face down against a semi-porous polishing pad which is saturated with an abrasive compound (i.e., a slurry) and by rotating the polishing pad relative to the wafer. The rotary motion between the polishing pad and the wafer mechanically removes layers of the intermediate oxide and is continued until the oxide steps or undulations are removed. This process is generally referred to as a chemical mechanical polishing (CMP). 
     To facilitate material removal during the CMP process the polishing pad is provided with grooves that channel slurry to the polishing pad/wafer interface, and that provide a path for wafer material to be removed from the polished wafer surface. During polishing, however, the downward force of the wafer against the polishing pad compacts slurry particles within these grooves, reducing the supply of fresh slurry to the polishing pad/wafer interface, the removal rate of wafer material, and the overall polishing efficiency and throughput of the CMP process, as well as giving rise to defects in the form of wafer scratches as described below. Additionally, the downward force of the wafer against the polishing pad causes the semi-porous surface of the polishing pad to pack down, causing polishing rates to become low and unpredictable, and necessitating frequent polishing pad replacement. 
     To extend the useful life of a polishing pad, a pad conditioner that roughens or “conditions” the polishing pad surface is employed insitu, while the polishing pad polishes a wafer; or ex-situ, after wafer polishing is complete. A typical pad conditioner comprises a diamond surface that continually roughens the polishing pad surface by scribing additional “microgrooves” in the polishing pad surface. Continuous roughening of the polishing pad surface ensures adequate abrasion (e.g., due to slurry saturation of the roughened surface) at the polishing pad/wafer interface. (See, for example, U.S. Pat. No. 5,216,843 to Breivogel et al.). 
     While pad conditioners significantly increase a polishing pad&#39;s abrasive lifetime, they do not address the problem of slurry debris (e.g., compacted, dried slurry) within the slurry grooves. In fact, during the polishing/conditioning process, the compacted slurry material which fills the pad&#39;s original grooves maybe freed in large chunks that can scratch and produce defects in the polished wafer. Thus the polishing process itself can become a defect source. 
     Accordingly a need exists for a CMP apparatus and method that both extends the useful life of a polishing pad and eliminates wafer scratches caused by compacted slurry material. 
     SUMMARY OF THE INVENTION 
     The present invention addresses the shortcomings of the prior art by providing a chemical mechanical polishing (CMP) device that employs a brush for continually cleaning slurry particles from grooves (i.e., surface features in which slurry debris may collect), such as machined grooves, perforations or naturally occurring features. It will be understood that the pads described and claimed herein are hard pads such as those that are formed by casting (e.g., cast polyurethane), and that grooved pads refer to hard pads having surface features in which slurry debris may collect. The brush preferably comprises nylon bristles or other wear resistant material that is chemically stable in a corrosive CMP environment. The brush may be coupled in a stationary manner, or may rotate or roll, etc., as it impacts the polishing pad surface. 
     In a preferred embodiment the brush is coupled to a pad conditioner, such as a diamond embedded disk, and is scanned with the pad conditioner across the polishing pad surface. The brush may rotate with the pad conditioner if desired, or may be mounted to an anti-rotation device so as to remain stationary while the pad conditioner rotates. When coupled to the pad conditioner device, the brush is preferably spring loaded so that when the brush is not in. contact with the polishing pad, a polishing pad contacting surface of the brush projects beyond a polishing pad contacting surface of the pad conditioner. Thus, as the brush bristles wear they maintain sufficient contact with the bottom of each slurry groove to brush slurry particles therefrom. 
     Accordingly because the present invention continuously removes particles from the polishing pad grooves, no slurry debris builds up therein, and the present invention virtually eliminates defects caused by chunks of slurry debris such as particles that compact within, and subsequently dislodge from polishing pad slurry grooves scratching the wafer surfaces. A higher quality polished film results, scrapped wafer costs are reduced and thus the overall cost per wafer unit processed is reduced. 
     Other objects, features and advantages of the present invention will become more fully apparent from the following detailed description of the preferred embodiments, the appended claims and the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic top plan view of an inventive chemical mechanical polishing device which employs a brush for reducing slurry related defects; 
     FIG. 2 is a schematic side view of the bristles of the brush of FIG. 1 during wafer polishing; 
     FIG. 3 is a side sectional view of a first embodiment of an inventive conditioning assembly, which may replace the separate brush and conditioning head of FIG. 1; 
     FIG. 4 is a side sectional view of a second embodiment of an inventive conditioning assembly, which may replace the separate brush and conditioning head of FIG. 1; and 
     FIGS. 5A and 5B are a side sectional view and a bottom plan view, respectively, of a third embodiment of an inventive conditioning assembly, which may replace the separate brush and conditioning head of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a schematic top plan view of an inventive chemical mechanical polishing device  11  which employs a brush  13   a  for reducing slurry related defects as further described below. The polishing device  11  comprises a rotatable platen  15  on which a grooved polishing pad  17  for polishing semiconductor wafers is mounted. The polishing pad  17  has at least one groove  19  and typically has a plurality of concentric circumferential grooves  19  which are disposed along the outer portion of the polishing pad  17 . 
     The polishing device  11  further comprises a pivot arm  21 , a holder or conditioning head  23  mounted to one end of the pivot arm  21 , a slurry source such as a slurry/rinse arm  25 , a pad conditioner  27   a,  such as a pad embedded with diamond crystals, mounted to the underside of the conditioning head  23 , and a wafer mounting head  29  operatively coupled to the platen  15  so as to press a wafer (not shown) against the grooves  19  of the polishing pad  17 . 
     In the preferred embodiment of FIG. 1, the brush  13   a  is mounted to the slurry/rinse arm  25  so as to stationarily contact the surface of the polishing pad  17 . The pivot arm  21  is operatively coupled to the platen  15 , and holds the conditioning head  23  against the polishing pad  17 , as further described below. 
     In operation, a wafer (not shown) is placed face down beneath the wafer mounting head  29 , and the wafer mounting head  29  presses the wafer firmly against the grooved portion of the polishing pad  17 . Slurry is introduced to the polishing pad  17  via the slurry/rinse arm  25 , and the platen  15  rotates as indicated by the arrow R 1 . The pivot arm  21  scans from side to side in an arcing motion as indicated by the arrow S 1  and the conditioning head  23  rotates as indicated by the arrow R 2 . 
     The grooves  19  channel the slurry (not shown) between the wafer and the polishing pad  17 . The semi-porous surface of the polishing pad  17  becomes saturated with slurry which, with the downward force of the wafer mounting head  29  and the rotation of the platen  15 , abrades and planarizes the surface of the wafer. The diamond crystals (not shown) embedded in the rotating conditioner  27   a  continually roughen the surface of the polishing pad  17  to ensure consistent polishing rates. 
     As the slurry filled grooves travel beneath the wafer mounting head  29 , the downward force of the wafer mounting head  29  and the wafer thereunder, in addition to other factors such as the pH, the temperature, and the act of polishing itself, tend to compact and/or dry the slurry particles within the grooves  19 , forming hard chunks which may dislodge and scratch the wafer as previously described. However, unlike conventional polishing devices, the inventive polishing device  11  employs the brush  13   a  which continually sweeps slurry particles from the grooves  19 , reducing the probability that slurry particles will remain in the grooves  19  long enough to form larger masses capable of dislodging and scratching the wafer being polished, as further described below with reference to FIG.  2 . 
     FIG. 2 is a schematic side view of the bristles  31 , specifically bristles  31   a-c,  of the brush  13   a  (FIG. 1) as the bristles  31  pass over a groove  19 . The configuration of the bristles  31  depends upon the dimensions of groove  19 ; i.e., the bristles  31  are longer than the depth d of the groove  19 , and are narrower than the width w of the groove  19 , so that the bristles  31  easily reach the bottom of the groove  19  as shown in FIG. 2 by bristle  31   b.    
     The bristles  31  are preferably made of a wear resistant material that is chemically stable in a corrosive environment, such as nylon, polypropylene, etc., and that is sufficiently stiff so as to transfer momentum to a slurry particle (not shown) positioned within the groove  19 . For example, as a bristle  31  passes through the groove  19 , it straightens from the position shown by bristle  31   c,  to the position shown by bristle  31   b,  ejecting any slurry particles within the groove  19  from the groove  19 , and then re-bends as the bristle strikes the front edge E of the groove  19 . 
     Although the brush  13   a  remains stationary, the platen  15  rotates therebeneath, causing the grooves  19  to move in an arcing path relative to the brush  13   a.  The arcing path of the grooves  19  causes a plurality of the bristles  31  to move through the grooves  19 . In this manner the brush  13   a  prevents slurry particles from building up within the grooves  19  and from being compressed by the repeated downward force applied to the slurry particle as the grooves  19  pass under the wafer mounting head  29 . As a result fewer compacted slurry chunks form and fewer defects arise during polishing. Even greater slurry clearing is achieved with the embodiments of FIGS. 3 and 4 which couple a brush to the conditioner  27 , as described below. 
     FIG. 3 is a side sectional view of a first embodiment of an inventive conditioning assembly  33   a,  which may replace the separate brush  13   a  and conditioning head  23  of FIG.  1 . The conditioning assembly  33   a  comprises the holder or conditioning head  23 , a conditioner  27   b  which assumes a ring shape and which is coupled to the conditioning head  23 , and a brush  13   b  which is preferably disk shaped and positioned within the ring shaped conditioner  27   b.    
     Like the conditioner  27   b,  the brush  13   b  is coupled to the conditioning head  23 , and may be coupled so as to rotate with the conditioning head  23  and the conditioner  27   b,  or may be stationarily coupled to the conditioning head  23  via an anti-rotation element  35 , as shown in FIG.  3 . The anti-rotation element  35  may comprise one or more bearings or other similar mechanisms as will be readily apparent to those of ordinary skill in the art. 
     The brush  13   b  is coupled to the anti-rotation element  35  via a spring loaded mechanism  37   a,  e.g., one or more springs, which causes a pad contacting surface  39  of the brush  13   b  to project beyond a pad contacting surface  41  of the conditioner  27   b  when no outside force is applied to the brush  13   b  (i.e., when the spring loaded brush  13   b  is in an unenergized state). Thus, as the pad contacting surface  39  of the brush  13   b  wears, the spring loaded mechanism  37   a  continues to force the pad contacting surface  39  of the brush  13   b  against the polishing pad  17 , maintaining sufficient contact between the bristles  31  and the bottom of the grooves  19  for proper slurry removal. Because the brush  13   b  scans across the polishing pad  17  with the conditioner  27   b,  the bristles  31  of the brush  13   b  have increased momentum relative to the grooves  19 , facilitating slurry removal from the grooves  19  as described previously with reference to FIG.  2 . To further increase momentum between the brush and the grooves, the anti-rotation element  35  may be omitted. 
     FIG. 4 is a side sectional view of a second embodiment of an inventive conditioning assembly  33   b,  which may replace the separate brush  13   a  and conditioning head  23  of FIG.  1 . The conditioning assembly  33   b  comprises the conditioning head  23 , a brush  13 c which assumes a ring shape and which is coupled to the conditioning head  23  via a spring loaded mechanism  37   b,  and a conditioner  27   c  which is preferably disk shaped and positioned within the ring shaped brush  13   c.  Because, as shown in FIG. 4, the brush  13   c  is coupled directly to the conditioning head  23 , the brush  13   c,  the conditioner  27   c,  and the conditioning head  23  rotate as a unit. In this manner the bristles  31  of the brush  13   c  have considerably increased momentum relative to the grooves  19 , facilitating slurry removal from the grooves  19  as described previously with reference to FIG.  2 . Alternatively the brush  13   c  may be coupled to the conditioning head  23  via an anti-rotation element such as the anti-rotation element  35  of FIG.  3 . 
     FIGS. 5A and 5B are a side sectional view and a bottom plan view, respectively, of a third embodiment of an inventive conditioning assembly  33   c,  which may replace the separate brush  13   a  and conditioning head  23  of FIG.  1 . As shown in FIGS. 5A and 5B, the conditioning assembly  33   c  comprises the holder or conditioning head  23 , a conditioner  27   d  which assumes a ring shape and which is coupled to the conditioning head  23  via a position controller, such as pneumatic pistons  43   a,    43   b;  and a brush  13   d  which is preferably disk shaped and positioned within the ring shaped conditioner  27   d.    
     Like the conditioner  27   d,  the brush  13   d  is coupled to the conditioning head  23  via a position controller, such as pneumatic pistons  43   c,    43   d,  and may be coupled so as to rotate with the conditioning head  23  and the conditioner  27   d,  or may be stationarily coupled to the conditioning head  23  via an anti-rotation element (not shown) such as that described with reference to FIGS. 3 and 4. 
     The position controllers (e.g., pneumatic pistons  43   a-d ) allow the distance of both the conditioner  27   d  and the brush  13   d  above the polishing pad to be independently controlled. Thus, the position controllers not only can adjust to accommodate bristle wear, but also allow selective use of the conditioner  27   d  and/or the brush  13   d.  For example, only the conditioner  27   d  may be used during wafer polishing and both the brush  13   d  and the conditioner  27   d  may be used subsequently when the polishing pad  17  is cleaned using a high pressure spray of de-ionized water. Such selective use is advantageous in many applications, for instance, when worn bristle particles may damage the wafer being polished. 
     The present invention prevents large compacted slurry particles from forming within the grooves of a polishing pad, and prevents wafer scratches and defects that would otherwise occur as chunks of compacted slurry dislodge from the grooves and are forced across the wafer surface during polishing. Furthermore, because the slurry grooves are continuously cleared, slurry is more effectively channeled through the slurry grooves resulting in more efficient polishing rates. 
     The foregoing description discloses only the preferred embodiments of the invention, modifications of the above disclosed apparatus and method which fall within the scope of the invention will be readily apparent to those of ordinary skill in the art. For instance, while nylon or polypropylene bristles are presently preferred, other wear resistant and corrosive resistant bristle materials may be employed. Additional momentum may be provided to the bristles by mounting the bristles on a rotating, roller-type brush if so desired. Also, bristle free pliable rollers can be similarly used during polishing to wipe slurry from polishing pad grooves. 
     The present invention may be used with any polishing pad conditioners, including but not limited to those that have diamonds embedded in a metal (e.g., nickel) or polymer matrix, and those that have individual diamond crystals “embedded” in a screw-type holder. It will be understood that as used herein the term “diamond” includes any material abrasive enough to resurface a hard polishing pad, such as a cast polishing pad, without depositing debris on the polishing pad surface. 
     Finally, although the conditioning assemblies disclosed herein comprise concentric brushes and conditioners that may rotate together, the inventive conditioning assembly may comprise brushes and conditioners that are coupled adjacent each other, that are non-circular and/or that rotate in opposite directions. 
     Accordingly, while the present invention has been disclosed in connection with the preferred embodiments thereof, it should be understood that other embodiments may fall within the spirit and scope of the invention, as defined by the following claims.