Abstract:
A method includes performing a chemical-mechanical planarization (CMP) on an article, providing a polishing fluid capable of transferring charges to the article, and detecting a current generated in response to the charges transferred to the article. An apparatus that is capable of performing the method and a system that includes the apparatus are also disclosed.

Description:
CROSS-REFERENCE TO A RELATED APPLICATION 
     This application is a continuation application of U.S. patent application Ser. No. 14/097,400, filed Dec. 5, 2013, which is incorporated herein by reference in its entirety. 
    
    
     FIELD 
     The technology described in this disclosure relates generally to material processing and more particularly to planarization. 
     BACKGROUND 
     Semiconductor devices fabrication involves many processes, such as chemical-mechanical polishing/planarization (CMP) for planarizing surfaces of a wafer. The CMP process implements a combination of chemical and mechanical forces. For example, the CMP process involves both mechanical grinding and wet etching to generate a smooth surface on a wafer for subsequent processes (e.g., photolithography) in the fabrication of semiconductor devices. 
     SUMMARY 
     In accordance with one aspect of the teachings described herein, an apparatus configured to perform a chemical-mechanical planarization (CMP) on an article comprises a polishing head and a current detector. The polishing head is configured to hold the article. The current detector is configured to detect a current generated in response to charges transferred to the article. 
     In accordance with another aspect of the teachings described herein, a system configured to perform a chemical-mechanical planarization (CMP) on an article comprises a polishing head, a polishing fluid, and a current detector. The polishing head is configured to hold an article. The polishing fluid is capable of transferring charges to the article. The current detector is configured to detect a current generated in response to the charges transferred to the article. 
     In accordance with yet another aspect of the teachings described herein, a method comprises performing a chemical-mechanical planarization (CMP) on an article, providing a polishing fluid capable of transferring charges to the article, and detecting a current generated in response to the charges transferred to the article. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1( a ) - FIG. 1( b )  depict an example diagram showing a CMP system. 
         FIG. 2( a )  and  FIG. 2( b )  depict an example diagram showing a polishing fluid including light-absorption particles used in the CMP system as shown in  FIG. 1( a )  and  FIG. 1( b ) . 
         FIG. 3( a )  and  FIG. 3( b )  depict another example diagram showing a polishing fluid including light-absorption particles used in the CMP system as shown in  FIG. 1( a )  and  FIG. 1( b ) . 
         FIG. 4  depicts an example band diagram showing electrons being transferred from a light-absorption particle to a CMP stop layer. 
         FIG. 5  depicts an example flow chart for performing CMP on an article. 
     
    
    
     DETAILED DESCRIPTION 
     Fabrication of semiconductor devices usually includes a CMP process and an etching process. Oftentimes, a thin nitride layer is used as a CMP stop layer for the CMP process and/or an etching hard mask for the etching process that follows the CMP process. When the CMP stop layer (e.g., titanium nitride, titanium oxide) is exposed, the CMP process is usually supposed to stop. However, it is hard to accurately control the end point of the CMP process. The thin CMP stop layer may be completely removed during the CMP process and layers under the CMP stop layer cannot be protected during the subsequent etching process. 
       FIG. 1( a ) - FIG. 1( b )  depict an example diagram showing a CMP system. The CMP system  100  is configured to perform a CMP process with photo-current detection so that the CMP process stops with accuracy when a CMP stop layer (e.g., titanium nitride, titanium oxide) is exposed. 
     As shown in  FIG. 1( a )  and  FIG. 1( b ) , the CMP system  100  includes a polishing head  102 , a polishing pad  104 , a platen  106 , and a photo-current detector  108 . A polishing fluid (not shown) used for the CMP process includes light-absorption particles that are capable of transferring charges (e.g., electrons or holes) to a CMP stop layer included in a wafer  114  in response to an incident light  112  (e.g., from a light source  118 ). The photo-current detector  108  is configured to detect a photo-current generated as a result of the charge transfer. Upon the detection of the photo-current (e.g., the intensity of the photo-current exceeding a threshold), the CMP process is stopped. 
     Specifically, one or more small windows  110  in the polishing pad  104  allow the incident light  112  to pass through and fall on the wafer  114  that includes the CMP stop layer (e.g., titanium nitride, titanium oxide). Once the CMP stop layer is exposed during the CMP process, the light-absorption particles begin to transfer charges (e.g., electrons or holes) to the CMP stop layer in response to the incident light  112 . For example, the windows  110  are fabricated using one or more materials that are approximately transparent to the incident light  112 . 
     The CMP system  100  further includes a polish-head-rotation controller  120  and a computer  122 . For example, the polish-head-rotation controller  120  is configured to control the polishing head  102  to rotate and oscillate to bring the wafer  114  into contact with the polishing pad  104  that moves in the plane of the wafer surface to be planarized (e.g., together with the platen  106 ). The computer  122  is configured to control the light source  118  and/or the photo-current detector  108 . As an example, the computer  122  compares the detected intensity of the photo-current with a predetermined threshold, and causes the polish-head-rotation controller  120  to stop the polishing head  102  if the detected current intensity is larger than the predetermined threshold. In certain embodiments, the polishing pad  104  is made of stacks of soft and hard materials (e.g., porous polymeric materials). As an example, the photo-current detector  108  is connected to an electrode in contact with the polishing fluid, as shown in  FIG. 2( a )  and  FIG. 2( b ) . 
       FIG. 2( a )  and  FIG. 2( b )  depict an example diagram showing a polishing fluid including light-absorption particles used in the CMP system  100 . The polishing fluid  202  includes an abrasive and corrosive chemical slurry (e.g., a colloid). For example, the polishing fluid  202  includes one or more abrasive materials  204 , and a plurality of light-absorption particles  206  capable of generating charges (e.g., electrons or holes) in response to the incident light  112  and attaching to a CMP stop layer  214  in the wafer  114 . Further, the polishing fluid  202  includes one or more electrolyte particles  216  (e.g., I −  ions) for conducting a current through an electrode  218  which is connected to the photo-current detector  108 . The wafer  114  includes multiple layers on a substrate  210 . One or more material layers  212  (e.g., silicon oxide) are formed on the CMP stop layer  214 . For example, the CMP stop layer  214  includes titanium oxide or titanium nitride. 
     At the beginning of the CMP process, the CMP stop layer  214  is covered by the material layers  212 , and the light-absorption particles  206  are not attached to the CMP stop layer  214 . For example, the photo-current detector  108  detects no current or a current with low intensity (e.g., below a threshold) through the electrode  218 . As the CMP process continues, the material layers  212  formed on the CMP stop layer  214  are removed, and at least part of the CMP stop layer  214  is exposed. The light-absorption particles  206  begin to attach to the CMP stop layer  214 , as shown in  FIG. 2( b ) . Charges (e.g., electrons or holes) are transferred from the light-absorption particles  206  to the CMP stop layer  214 . The photo-current detector  108  detects a current or an intensity increase of the current, and the CMP process is stopped when the intensity of the current becomes larger than a threshold. 
     Each of the light-absorption particles  206  includes one or more surfactant particles  208  that can attach to the CMP stop layer  214 . For example, the light-absorption particles  206  include CdS, CdSe, CdTe, ZnO, ZnS, ZnSe, ZnTe, InAs, InN, InP, GaN, GaP, GaAs, AlP, or other suitable materials. The abrasive materials  204  include silica or other suitable materials. The surfactant particles  208  include organic molecules that contain one or more hydroxyl-based (e.g., —OH) functional groups, one or more carboxyl-based (e.g., —COOH) functional groups, one or more ammonium-ion-based (e.g., —NH + ) functional groups, one or more sulfonic-acid-based (e.g., —SO 3 H) functional groups, or other suitable functional groups. As an example, the electrolyte particles (e.g., I −  ions)  216  combine to form particles  220  (e.g., I 3   −  ions) as a result of the charge transfer between the light-absorption particles  206  and the CMP stop layer  214 . 
       FIG. 3( a )  and  FIG. 3( b )  depict another example diagram showing a polishing fluid including light-absorption particles used in the CMP system  100 . The polishing fluid  302  includes one or more abrasive materials  304 , and a plurality of light-absorption particles  306  capable of generating charges (e.g., electrons or holes) in response to the incident light  112  and attaching to a CMP stop layer  314  (e.g., titanium oxide, titanium nitride) included in the wafer  114 . Further, the polishing fluid  302  includes one or more electrolyte particles  316  (e.g., I −  ions) for conducting a current through an electrode  318  which is connected to the photo-current detector  108 . In some embodiments, one or more material layers  312  (e.g., silicon oxide) are formed on the CMP stop layer  314 . 
     During the CMP process, the material layers  312  formed on the CMP stop layer  314  are removed, and at least part of the CMP stop layer  314  is exposed. The light-absorption particles  306  begin to attach to the CMP stop layer  314 , as shown in  FIG. 3( b ) . Charges (e.g., electrons or holes) are transferred from the light-absorption particles  306  to the CMP stop layer  314 . The photo-current detector  108  detects a current or an intensity increase of the current. In some embodiments, when most of the light-absorption particles  306  attach to the surface of the CMP stop layer  314 , the intensity of the detected current increases significantly, and the CMP process is stopped when the intensity of the detected current exceeds a predetermined threshold. 
     For example, the light-absorption particles  306  include certain dye materials, such as EBFP, Azunite, GFPuv, and T-sapphire. In another example, the light-absorption particles  306  include certain fluorescence conducting polymer materials, such as MEHPPV and P3HT. The light-absorption particles  306  include organic molecules that contain one or more hydroxyl-based (e.g., —OH) functional groups, one or more carboxyl-based (e.g., —COOH) functional groups, one or more ammonium-ion-based (e.g., —NH + ) functional groups, one or more sulfonic-acid-based (e.g., —SO 3 H) functional groups, or other suitable functional groups. The abrasive materials  304  include silica or other suitable materials. As an example, the electrolyte particles (e.g., I −  ions)  316  combine to form particles  320  (e.g., I 3   −  ions) as a result of the charge transfer between the light-absorption particles  306  and the CMP stop layer  314 . 
       FIG. 4  depicts an example band diagram showing electrons being transferred from a light-absorption particle to a CMP stop layer. As shown in  FIG. 4 , a light-absorption particle (e.g., the light absorption particles  206  or  306 ) contained in a polishing fluid (e.g., the polishing fluid  202  or  302 ) is in contact with a CMP stop layer (e.g., the CMP stop layer  214  or  314 ). In response to an incident light  402 , one or more electrons  404  of the light-absorption particle are excited from a first energy level  408  to a second energy level  410 , leaving behind one or more holes  406  at the first energy level  408 . The second energy level  410  is higher than an energy level  412  corresponding to a conduction band of the CMP stop layer. The one or more electrons  404  flow from the light-absorption particle to the CMP stop layer, and the one or more holes  406  flow from the light-absorption particle to an electrode (e.g., the electrode  218  or  318 ) through one or more electrolyte particles (e.g., I −  ions or I 3   −  ions) included in the polishing fluid for photo-current detection. 
     In some embodiments, the energy of the incident light is larger than a difference between the first energy level  408  and the second energy level  410  which corresponds to a band gap of the light-absorption particle (e.g., Eg). For example, the first energy level  408  is at about −6.0 eV, and the second energy level  410  is at about −4.0 eV. The energy level  412  of the CMP stop layer is at about −4.5 eV. As an example, certain electrolyte particles (e.g., I −  ions) combine to form other electrolyte particles (e.g., I 3   −  ions) as a result of the charge transfer between the light-absorption particle and the CMP stop layer. 
       FIG. 5  depicts an example flow chart for performing CMP on an article. At  502 , the CMP process begins on an article (e.g., a wafer) including a CMP stop layer. A polishing fluid that is used for the CMP process includes a plurality of light-absorption particles capable of generating charges (e.g., electrons or holes) in response to an incident light and attaching to the CMP stop layer included in the article. Once the light-absorption particles attach to the CMP stop layer, the charges (e.g., electrons or holes) transfer from the light-absorption particles to the CMP stop layer. At  504 , a photo-current resulting from the charge transfer from the light-absorption particles to the CMP stop layer is detected. At  506 , a determination whether the intensity of the detected photo-current is increasing (e.g., becoming larger than a threshold) is made. If the intensity of the photo-current is increasing (e.g., becoming larger than a threshold), it indicates that at least a large part of the CMP stop layer is exposed. The CMP process ends to avoid removing the CMP stop layer, at  508 . Otherwise, the CMP process continues. 
     For example, the light-absorption particles include CdS, CdSe, CdTe, ZnO, ZnS, ZnSe, ZnTe, InAs, InN, InP, GaN, GaP, GaAs, AlP, EBFP, Azunite, GFPuv, T-sapphire, MEHPPV, P3HT, or other suitable materials. In some embodiments, the light-absorption particles include surfactant particles capable of attaching to the stop layer. As an example, the surfactant particles include organic molecules that contain one or more hydroxyl-based functional groups, one or more carboxyl-based functional groups, one or more ammonium-ion-based functional groups, one or more sulfonic-acid-based functional groups, or other suitable functional groups. 
     This written description uses examples to disclose embodiments of the disclosure, include the best mode, and also to enable a person of ordinary skill in the art to make and use various embodiments of the disclosure. The patentable scope of the disclosure may include other examples that occur to those of ordinary skill in the art. One of ordinary skill in the relevant art will recognize that the various embodiments may be practiced without one or more of the specific details, or with other replacement and/or additional methods, materials, or components. Further, persons of ordinary skill in the art will recognize various equivalent combinations and substitutions for various components shown in the figures. 
     Well-known structures, materials, or operations may not be shown or described in detail to avoid obscuring aspects of various embodiments of the disclosure. Various embodiments shown in the figures are illustrative example representations and are not necessarily drawn to scale. Particular features, structures, materials, or characteristics may be combined in any suitable manner in one or more embodiments. The present disclosure may repeat reference numerals and/or letters in the various examples, and this repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed. Various additional layers and/or structures may be included and/or described features may be omitted in other embodiments. For example, a particular layer described herein may include multiple components which are not necessarily connected physically or electrically. Various operations may be described as multiple discrete operations in turn, in a manner that is most helpful in understanding the disclosure. However, the order of description should not be construed as to imply that these operations are necessarily order dependent. In particular, these operations need not be performed in the order of presentation. Operations described herein may be performed in a different order, in series or in parallel, than the described embodiments. Various additional operations may be performed and/or described. Operations may be omitted in additional embodiments. 
     This written description and the following claims may include terms, such as top, on, under, etc. that are used for descriptive purposes only and are not to be construed as limiting. The embodiments of a device or article described herein can be manufactured, used, or shipped in a number of positions and orientations. For example, the term “on” as used herein (including in the claims) may not necessarily indicate that a first layer/structure “on” a second layer/structure is directly on or over and in immediate contact with the second layer/structure unless such is specifically stated; there may be one or more third layers/structures between the first layer/structure and the second layer/structure. The term “under” as used herein (including in the claims) may not indicate that a first layer/structure “under” a second layer/structure is directly under and in immediate contact with the second layer/structure unless such is specifically stated; there may be one or more third layers/structures between the first layer/structure and the second layer/structure. The term “substrate” used herein (including in the claims) may refer to any construction comprising one or more semiconductive materials, including, but not limited to, bulk semiconductive materials such as a semiconductive wafer (either alone or in assemblies comprising other materials thereon), and semiconductive material layers (either alone or in assemblies comprising other materials).