Abstract:
A method and apparatus for chemical mechanical polishing a semiconductor wafer by providing a novel high pressure mixture of gas and liquid in a pulsation mode for eliminating residual slurry, by-products, and slurry abrasive particles on or in the polishing pad.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates generally to manufacturing semiconductor wafers and more particularly the invention pertains to the field of Chemical Mechanical Polish (CMP) used during the fabrication of semiconductor chips to planarize the wafers. 
     2. Background Art 
     Rapid progress in semiconductor device integration demands smaller and smaller wiring patterns or interconnections which connect active areas. As a result, the tolerances regarding the planeness or flatness of the semiconductor wafers used in these processes are becoming smaller and smaller. One customary way of flattening the surfaces of semiconductor wafers is to polish them with a polishing apparatus. 
     Such a polishing apparatus has a rotating wafer carrier assembly in contact with a polishing pad. The polishing pad is mounted on a rotating turntable which is driven by an external driving force. The polishing apparatus causes a polishing or rubbing movement between the surface of each thin semiconductor wafer and the polishing pad while dispersing a polishing slurry to obtain a chemical mechanical polish (CMP). CMP in planarization requires the wafer surface to be brought into contact with a rotating pad saturated with either a slurry of abrasive particles or a reactive solution, or both, that attacks the wafer surface. This is done while exerting force between the wafer and polishing pad. The abrasive particles can scratch the surface during the polish operation especially if they agglomerate or if abrasive material becomes embedded in the polishing pad. Also, after polishing is completed and the wafer is rinsed to remove the abrasive particles, some of the abrasive particles will remain on the wafer surface or pad by simply being imbedded into the surface or strongly attracted to the surface. 
     The scratches can cause problems in subsequent processing steps as for example in a Damascene process, intra-level where shorting could result. Leaving the particles on the wafer or pad can likewise lead to process problems, most notably photolithographic and etching defects in subsequently processed levels. 
     Prior art for particle removal involves either a mechanical clean, such as a brush clean, after the wafers have been removed from the polishing apparatus and dried, or a washing step with a surfactant (soap) performed prior to the wafer being dried as disclosed in U.S. Pat. No. 5,320,706, issued to Blackwell on Jun. 14, 1994 and assigned to Texas Instruments Incorporated. These processes are not usually 100% effective and do not address the scratching problem. Therefore, it appears that no prior art per se exists that addresses the scratching problem directly. 
     SUMMARY OF THE INVENTION 
     The present invention discloses a method and apparatus for removing slurry abrasive particles in a polishing process for planarization of a semiconductor wafer. 
     The present invention discloses a method and apparatus for reducing the size of the slurry abrasive particles or agglomerations of slurry abrasive particles in a polishing process for planarization of a semiconductor wafer. 
     The present invention disclose a method for reduction in size of agglomerations and removal of abrasive particles from the polishing pad during a polishing process for planarization of a wafer, comprising: applying a high pressure mixture of gas and liquids to the polishing pad to reduce and remove the agglomeration of abrasive particles. 
     The present invention is a method for removal of abrasive particles and conditioning the polishing pad during a polishing process for planarization of a wafer, comprising: applying a high pressure mixture of gas and liquid to the polishing pad to remove the abrasive particles and condition the pad. 
     The present invention discloses an apparatus comprising: a wafer; a polishing pad which planarizes the wafer; a slurry dispenser which provides slurry containing abrasive particles to the polishing pad; and a high pressure fluid dispenser which controllably provides a mixture of gas and fluid to the polishing pad. 
     In accordance with the above, it is an advantage of the present invention to solve the problem of slurry abrasive particle removal post-CMP and the problem of scratches on the post-CMP surface. 
     It is an advantage of the present invention to remove the abrasive particles from the pad. 
     It is an advantage of the present invention to rapidly remove and reduce the abrasive particle size while conditioning the pad. 
     It is an advantage of the present invention to rapidly reduce the size of agglomerations of abrasive particles. 
     It is an advantage of the present invention to eliminate residual slurry and slurry abrasives and particulates from the polishing pad. It is an advantage of the present invention to break up or reduce the size of abrasive particle agglomerations. 
     It is an advantage of the present invention to eliminate slurry abrasive particle residues and reduce if not eliminate the sources of scratches. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a top plan view of a high pressure apparatus constructed according to the present invention located over a polishing pad; 
     FIG. 2 is a longitudinal sectional view of the high pressure apparatus and polishing pad taken substantially along the plane described by the lines  2 — 2  of FIG. 1; 
     FIG. 3 is a view of a portion of the high pressure apparatus as shown in FIG. 2 on an enlarged scale. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to the drawings, and initially to FIGS. 1-3, a polishing pad, indicated generally at  10 , is designed to polish a surface of a semiconductor wafer (indicated at  11 ) during the wafer fabrication process. The polishing process and the operation of the device of this invention will be described as they related to the Chemical Mechanical Planarization (CMP) process, as described in U.S. Pat. No. 4,793,895 which is assigned to the assignee of the present invention. It will be understood, however, that the apparatus and process with this invention can be used with other processes which remove different materials e.g., oxides, polysilicon and aluminum, from the surface of the semiconductor wafer. 
     The pad  10  is substantially circular in top plan view and is supported on a rotatable platen  12 . The pad  10  may be composed of number of elongated fibers  14 , normally extending substantially perpendicular to the upper surface of the pad for contacting and polishing the surface of the semiconductor wafer. The pad  10  is backed with an impervious surface  13  such as polyethylene terephthalate (i.e., MYLAR). A chemical, appropriate to the material being polished plus a suitable abrasive, is continuously applied, in the form of a slurry, to the pad to remove the unwanted material from the wafer during the polishing process. An effector (not shown) may be applied to the pad to assist in conditioning the surface of the pad. The effector may have abrasive material such as diamond material embedded in its surface to assist in the conditioning. This material may break off and become embedded in the pad. 
     During the semiconductor wafer fabrication process, the fibers  14  tend to become flattened during the polishing process Moreover, polishing by-products, abrasive material from the effector, and spent polishing material tend to become imbedded in the fibers of the pad. Additionally, a “glazed” effect becomes apparent on the surface of the polishing pad. The flattened fibers and the imbedded polishing by-products and spent polishing material degrades the polishing efficiency of the pad. Further, the abrasive material from the effector may cause severe scratching which would damage the wafer. 
     According to the present invention, a high pressure assembly, indicated generally at  16 , is supported in spaced relationship to the surface of the polishing pad. The high pressure assembly  16  includes an elongated pipe  18  coupled by a flexible tube  20  to a source of a liquid and gas mixture. The mixture is created from a source of pressurized liquid  30  and compressed gas  31 . 
     The pressurized liquid, such as water, is fed through a control valve  32  and check valve  34  to a flow meter controller  36  prior to being mixed with the compressed gas. The compressed gas, such as air, is fed through a control valve  33  and check valve  35  to a flow meter controller  37  prior to being mixed with the pressurized liquid. 
     The elongated pipe  18  is formed from rigid plastic or other appropriate material and preferably extends radially inward toward the rotational axis of the polishing pad. One end of the pipe  18  includes an inlet port  25  connected to the flexible tube  20 , while the other end is closed by an end cap  26 . The pipe  18  of the high pressure assembly can be pivotally connected on a swivel block (not shown) to the support structure of the platen. Accordingly, the pipe  18  can be moved away from the platen during replacement of the polishing pad. 
     As illustrated in FIGS. 2 and 3, a plurality of perforations, about 22 in number, as indicated at  28 , are formed in the wall of the pipe  18 . Each of the perforations  28  has a diameter D. The perforations are preferably equally spaced at d intervals along the length of the pipe  18 . The distance d between the edges of the perforations and the diameter of the perforations is preferably equal. It has been determined that for the above-described process the diameter D and distance d should be approximately ⅛″. Alternatively, it is also within the scope of the invention to provide a thin slot extending along the length of the pipe in lieu of the perforations. 
     The compressed gas supply  31  is conventional in design and provides compressed air (or other appropriate gas) at a predetermined pressure that is adjusted by the control valve  33  and the rate of flow by flow meter  37 . 
     Likewise, the pressurized liquid  30  is conventional in design and provides high pressure liquid such as water (or other appropriate liquid) at a predetermined pressure that is adjusted by control valve  32  and the rate of flow by flow meter  36 . The mixture of gas and fluid is supplied through the flexible tube  20  to the inlet port  25  of the pipe  18 . The mixture fills the pipe and is forced through the perforations  28  in a substantially uniform stream. The perforations  28  direct the flow downwardly against the surface of the polishing pad at an angle Ø, which angle is approximately 45 to 90 degrees and preferably about 90° in the direction of rotation of the platen. 
     As illustrated most clearly in FIG. 1, as the platen  12  is rotated, the chemical slurry is applied through an applicator, as indicated generally in phantom at  29 , to the fibers of the polishing pad. The fibers move across the surface of the wafer as the polishing pad rotates. The action of the chemical slurry and the pad removes the material from the wafer being polished. However, the polishing by-products, along with other unwanted abrasive materials becomes imbedded in the fibers of the pad. Additionally, the fibers on the pad may become flattened during contact with the wafer surface. The downwardly directed stream of the liquid and gas mixture as a pulsation of fluid under high pressure is applied against the surface of the polishing pad  10 . The high pressure pulsation of the gas—liquid mixture tends to scrub or loosen the polishing by-products, abrasive material from the affector, and spent polishing material from the pad. The gas—liquid mixture also washes the pad and raises the flattened fibers to their normal orientation substantially perpendicular to the surface of the polishing pad. Moreover, the “eglazing” of the fibers during polishing is prevented by this blowing away of the material and the polishing by-products. As the platen continued to rotate, the raised fibers are again coated with the chemical slurry and contact and polish the surface of the semiconductor wafer. Although the high pressure system may be used to thoroughly clean and condition the pad during polishing, it has been found most effective if used as a pre-polishing or post-polishing step in the CMP process. 
     Tests have been conducted using the high pressure assembly supported over a polishing pad where 8″ semiconductor wafers were being polished. The pad was rotated on the platen at a rate of 35 rpm. The perforations in the pipe provide the gas—liquid mixture under pressure in about a 1:1 ratio in order to create a pulsating effect. The gas is supplied with a flow from 10 to 300 CFM and preferably set at about 250 CFM. The flow of the liquid is adjusted to supply liquid at 1 to 4 GPM. The system uses matched pressures of about 40 PSI (approximately in a 1:1 ratio), gas and liquid to create the pulsation of gas—liquid stream at inlet  25 . It has been determined that the high pressure assembly removed the spent polishing materials and by-products including the abrasive effector material from the pad and raises the flattened pad fibers. It has further been determined that the high pressure assembly thoroughly cleans the pad from all unwanted materials and may be effectively used to reduce unwanted scratches occurring on the wafers. 
     Accordingly, the present invention provides a high pressure apparatus which is simple and convenient to use and which efficiently removes spent polishing materials and by-products from the surface of a polishing pad by the pulsation action of the mixture of gas and liquid. 
     Although the invention has been shown and described with respect to a certain preferred embodiment, it is obvious that equivalent alterations and modifications will occur to others skilled in the art upon their reading and understanding of the specification. For example, fluids such as water, solvents, the polishing fluid or the like can be substituted for the air discussed above. The present invention includes all such equivalent alterations and modifications, and is limited only the scope of the following claims.