Abstract:
Microelectronic substrate assemblies are planarized using methods, planarizing solutions and planarizing machines according to various embodiments of the present invention. A substrate is assembly pressed against a planarizing surface of a fixed-abrasive polishing pad, covering an operative portion of the planarizing surface with a non-abrasive planarizing solution, and moving the substrate assembly and/or the polishing pad with respect to the other. The fixed-abrasive polishing pad includes a body having a suspension medium and abrasive particles fixedly attached to the suspension medium at the planarizing surface. The substrate assembly is a stop-on-feature device including a substrate, a polish-stop layer formed over the substrate to conform to a topography of features on the substrate, and a cover layer formed over the polish-stop layer. The planarizing solution includes a mechanical selectivity agent that increases the mechanical removal rate of the cover layer and/or reduces the mechanical removal rate of the polish-stop layer compared to planarizing solutions without the mechanical selectivity agent.

Description:
TECHNICAL FIELD 
     The present invention relates to methods, planarizing solutions and apparatuses for planarizing microelectronic substrate assemblies in mechanical and/or chemical-mechanical planarization. 
     BACKGROUND OF THE INVENTION 
     Mechanical and chemical-mechanical planarizing processes (collectively “CMP”) are used in the manufacturing of electronic devices for forming a flat surface on semiconductor wafers, field emission displays, and many other microelectronic substrate assemblies. CMP processes generally remove material from a substrate assembly to create a highly planar surface at a precise elevation in the layers of material on the substrate assembly. 
     FIG. 1 schematically illustrates a rotary CMP machine  10  for planarizing a microelectronic substrate assembly  12 . The rotary machine  10  has a platen  20 , a wafer carrier assembly  30  above the platen  20 , and a polishing pad  40  between the platen  20  and the carrier assembly  30 . The carrier assembly  30  generally includes a head  32  to pick up, hold and release the substrate assembly  12  at different stages of the planarizing process. The carrier assembly  30  can also include a backing pad  34  to support the back side of the substrate assembly  12 . The head  32  may be a weighted, free-floating unit (not shown), or the carrier assembly  30  can further include an actuator  36  attached to the head  32  to impart axial and/or rotational motion (indicated by arrows C and D, respectively). 
     The polishing pad  40  can be a non-abrasive polymeric pad (e.g., polyurethane), or it may be a fixed-abrasive polishing pad in which abrasive particles are fixedly dispersed in a resin or another type of suspension medium. A planarizing fluid  44  covers the polishing pad  40  during planarization of the substrate assembly  12 . The planarizing fluid  44  may be a conventional CMP slurry with abrasive particles that etch and/or oxidize the surface of the substrate assembly  12 , or the planarizing fluid  44  may be a “clean” non-abrasive planarizing solution without abrasive particles. Abrasive planarizing solutions can also include surfactants to help suspend the abrasive particles in the solution. For example, Brij  58 , ZONYL and other surfactants have been commercially used in only slurries with abrasive particles. In most CMP applications, abrasive slurries with abrasive particles are used on non-abrasive polishing pads, and non-abrasive cleaning solutions without abrasive particles are used on fixed-abrasive polishing pads. 
     To planarize the substrate assembly  12  with the CMP machine  10 , the carrier assembly  30  presses the substrate assembly  12  face-downward against a planarizing surface  42  of the polishing pad  40 . At least one of the platen  20  or the head  32  moves relative to the other to move the substrate assembly  12  across the planarizing surface  42  in the presence of the planarizing solution  44 . As the face of the substrate assembly  12  moves across the planarizing surface  42 , the polishing pad  40  and/or the planarizing solution  44  continually remove material from the face of the substrate assembly  12 . 
     CMP processes should consistently and accurately produce a uniform, planar surface on substrate assemblies to enable circuit and device patterns to be formed with photolithography techniques. As the density of integrated circuits increases, it is often necessary to accurately focus the critical dimensions of the photo-patterns to within a tolerance of approximately 0.1 μm. Focusing photo-patterns to such small tolerances, however, is difficult when the planarized surfaces of substrate assemblies are not uniformly planar. Thus, to be effective, CMP processes should create highly uniform, planar surfaces on substrate assemblies. 
     In the highly competitive semiconductor industry, it is also desirable to maximize the throughput of CMP processing by producing a planar surface on a substrate assembly as quickly as possible. The throughout of CMP processing is a function of several factors, one of which is the ability to accurately stop CMP processing at a desired endpoint. In a typical CMP process, the desired endpoint is reached when the surface of the substrate assembly is planar and/or when enough material has been removed from the substrate assembly to form discrete components on the substrate assembly (e.g., shallow trench isolation areas, contacts, damascene lines, etc.). Accurately stopping CMP processing at a desired endpoint is important for maintaining a high throughput because the substrate assembly may need to be re-polished if it is “under-planarized,” or too much material can be removed from the substrate assembly if it is “over-polished.” For example, over-polishing can cause “dishing” in shallow-trench isolation structures or completely destroy a section of the substrate assembly. Thus, it is highly desirable to stop CMP processing at a desired endpoint within the substrate assembly. 
     One technique to endpoint CMP processing and to form a planar surface on a substrate assembly is to provide stop-on-feature (SOF) wafer structure having a polish-stop layer and a cover layer on the polish-stop layer. The polish-stop layer follows the contour of trenches or components on the wafer such that the polish-stop layer has high areas at the desired endpoint of the CMP process. The cover layer fills depressions in the polish-stop layer and covers the high areas on the polish-stop layer. The cover layer is generally softer than the polish-stop layer so that the polish-stop layer has a much lower polishing rate than the cover layer. For example, the polish-stop layer can be composed of silicon nitride or carbon, and the cover layer can be composed of a metal, silicon dioxide or polysilicon. 
     During planarization of an SOF wafer, certain high areas of the polish-stop layer are typically exposed before other high areas, such that some of the high areas remain covered by the cover layer. The harder polish-stop layer inhibits mechanical removal of material from the exposed high areas while the abrasive particles still aggressively remove material from the thicker portions of the softer cover layer. The polish-stop layer theoretically endpoints the CMP process when all of the high areas of the polish-stop layer are exposed. 
     One drawback of CMP processing with an SOF wafer is that the difference in polishing rate between the polish-stop layer and the cover layer may not be adequate to stop planarization at the polish-stop layer. For example, the first areas of the polish-stop layer that are exposed during planarization may be completely removed before the cover layer is removed from the other high areas of the polish-stop layer at the same elevation. This drawback is particularly problematic when using fixed-abrasive polishing pads because the fixed-abrasive particles aggressively remove material from the substrate assembly. Consequently, the difference in hardness between the polish-stop layer and the cover layer may not be adequate to effectively shut down planarization at the high areas of the polish-stop layer in fixed-abrasive CMP applications. 
     SUMMARY OF THE INVENTION 
     The present invention is directed toward planarizing solutions, planarizing machines and methods for planarizing semiconductor wafers, field emission displays and other microelectronic substrate assemblies. One method in accordance with the invention includes pressing a substrate assembly against a planarizing surface of a fixed-abrasive polishing pad, covering an operative portion of the planarizing surface with a non-abrasive planarizing solution, and moving the substrate assembly and/or the polishing pad with respect to the other. The fixed-abrasive polishing pad generally includes a body having a suspension medium and abrasive particles fixedly attached to the suspension medium at the planarizing surface. The substrate assembly is a stop-on-feature device including a substrate, a polish-stop layer formed over the substrate to conform to a topography of features on the substrate, and a cover layer formed over the polish-stop layer. 
     The planarizing solution is a non-abrasive solution without abrasive particles. The planarizing solution generally includes water and a mechanical selectivity agent that increases the mechanical removal rate of the cover layer and/or reduces the mechanical removal rate of the polish-stop layer. The mechanical selectivity agent can accordingly increase the polishing rate of the cover layer and/or reduce the polishing rate of the polish-stop layer to increase the polishing rate ratio between the cover layer and the polish-stop layer. By increasing the polishing rate ratio, it is less likely that exposed high areas of the polish-stop layer will be completely removed from the substrate assembly before the cover layer is removed from other high areas of the polish-stop layer. Accordingly, methods for fixed-abrasive CMP processing in accordance with the invention are expected to enhance the planarity of the finished surfaces and the accuracy of endpointing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic cross-sectional view of a planarizing machine in accordance with the prior art. 
     FIG. 2 is a schematic cross-sectional view of a planarizing machine with a planarizing solution for stop-on-feature planarization of a substrate assembly on a fixed-abrasive polishing pad in accordance with one embodiment of the invention. 
     FIG. 3 is a partial cross-sectional view of a substrate assembly being planarized by a method in accordance with an embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is directed towards methods, planarizing solutions and planarizing machines for mechanical and/or chemical-mechanical planarization of semiconductor wafers, field emission displays and other microelectronic substrate assemblies. Many specific details of certain embodiments of the invention are set forth in the following description and in FIGS. 2 and 3 to provide a thorough understanding of such embodiments. One skilled in the art however, will understand that this invention may have additional embodiments, or that this invention may be practiced without several of the details described in the following description. 
     FIG. 2 is a schematic cross-sectional view of a planarizing machine  100  for planarizing an SOF substrate assembly  112  on a fixed-abrasive polishing pad  140  using a non-abrasive planarizing solution  160  in accordance with an embodiment of the invention. The planarizing machine  100  includes a table  102  and a wafer carrier assembly  104 . The table  102  can be a fixed support surface of a web-format planarizing machine, or the table  102  can be a rotary platen of a rotary planarizing machine. The wafer carrier  104  includes a carrier head  106  to hold the substrate assembly  112  and a drive system  108  to move the head  106  with respect to the table  102 . Suitable web-format planarizing machines having a table and wafer carrier assembly are manufactured by Obsidian, Incorporated of Fremont, California, and suitable rotary planarizing machines are manufactured by Westech Systems, Inc. and Strasbaugh Corporation. Additionally, web-format and rotary planarizing machines are disclosed in U.S. Pat. Nos. 5,456,627; 5,486,131; 5,421,769; 5,234,867; 5,232,875; 5,069,002; 5,036,015, all of which are herein incorporated by reference. 
     The fixed-abrasive polishing pad  140  is placed on the surface of the support table  102 . The fixed-abrasive pad  140  is typically adhered to a rotary platen in rotary applications, but slidably attached to a stationary table in web-format applications. The fixed-abrasive polishing pad  140  can include a body  141  having a suspension medium  142  and a plurality of abrasive particles  143  fixedly attached to the suspension medium  142  at a planarizing surface  144 . The suspension medium  142  is typically a resin, and the abrasive particles  143  can be alumina, titania, silica, ceria, tantalum oxide or other abrasive particles. The polishing pad  140  can also include a backing film  146  attached to the back side of the body  141 . The planarizing surface  144  can have a texture, such as truncated pyramids or mounds. Suitable fixed-abrasive pads are manufactured by 3M Corporation of St. Paul, Minn. Additionally, fixed-abrasive pads are disclosed in U.S. Pat. No. 5,645,471, and patent application Nos. 09/201,576 and 09/164,916, all of which are also herein incorporated by reference. 
     The stop-on-feature wafer  112  has a substrate  113  with a plurality of trenches  114  or holes, a conformal polish-stop layer  116  having lands  117  over portions of the substrate  113  outside of the trenches  114 , and a cover layer  118  formed on the polish-stop layer  116 . The polish-stop layer  116  has lower polishing rate than the cover layer  118 . The polish-stop layer  116  accordingly inhibits further planarization as the lands  117  are exposed during planarization. In applications for forming shallow trench isolation structures in the trenches  114 , the polish-stop layer  116  can be composed of silicon nitride and the cover layer can be composed of silicon dioxide, borophosphate silicon glass, or tetraethylorthosilicate glass. In other applications for forming damascene lines in the trenches  114 , the polish-stop layer  116  can be composed of silicon nitride, silicon dioxide, a low K material, or other hard materials, and the cover layer  118  can be composed of polysilicon, aluminum, tungsten, copper, gold or other metals. For the purposes of this invention, a low K material is a material having a low dielectric constant, such as bisbenzocyclobutene (BCB), alkyl-silsesquioxanes, parylene N, fluorinated parylene or fluorinated silicon dioxide. 
     Although the polish-stop layer  116  alone inhibits planarization as the lands  117  are exposed, the fixed-abrasive polishing pad  140  may not selectively remove the cover layer  118  before removing too much of the exposed portions of the polish-stop layer  116 . More particularly, the difference in hardness between the polish-stop layer  116  and the cover layer  118  alone may not produce a sufficiently high polishing rate ratio between the cover layer  118  and the polish-stop layer  116 . It may also be detrimental to use a planarizing solution that chemically etches the cover layer at a faster rate than the polish-stop layer  116  with a selective etchant because such etching will produce depressions in the portions of the cover layer  118  remaining in the trenches  114 . Accordingly, to increase the polishing rate ratio between the cover layer  118  and the polish-stop layer  116 , several embodiments of this invention use a non-abrasive planarizing solution  160  that increases the mechanical removal rate of the cover layer  118  and/or reduces the mechanical removal rate of the polish-stop layer  116 . 
     The non-abrasive planarizing solution  160  includes a mechanical selectivity agent that chemically interacts with the materials at the substrate surface to increase the polishing rate ratio between the cover layer  118  and the polish-stop layer  116 . The planarizing solution  160  does not contain abrasive particles. The selectivity agent can chemically interact with the cover layer  118  to enhance the mechanical removal of the cover layer  118  and/or the selectivity agent can chemically interact with the cover layer  106  to impede the mechanical removal of the cover layer  106 . The selectivity agent, for example, can be a chemical that adsorbs more to the cover layer  118  than to the polish-stop layer  116  so that the abrasive particles  143  in the polishing pad  140  can more readily remove material from the cover layer  118  than the polish-stop layer  116 . Adsorption of the selectivity agent to the surface of the cover layer  118 , for example, can increase the hydrolysis rate of the cover layer  118  or reduce the hydrolysis rate of the polish-stop layer  116  so that the cover layer  118  is much easier to mechanically remove than the polish-stop layer  116 . Additionally, because mechanical removal dominates chemical removal in fixed-abrasive CMP, the increase in the difference of the mechanical removal rate between the cover layer  118  and the polish-stop layer  116  can significantly increase the mechanical selectivity of removing the cover layer  118  without removing significant portions of the polish-stop layer  116 . 
     In one specific application for forming shallow-trench-isolation structures on the substrate assembly  112 , the polish-stop layer  116  is a thin, conformal silicon nitride layer and the cover layer  118  is a silicon dioxide layer. The planarizing solution  160  for this embodiment can include water and ammonia or potassium hydroxide, and the selectivity agent in the planarizing solution  160  can be a non-ionic surfactant. One suitable non-ionic surfactant is polyoxyethylene (20) cetyl ether, such as Brij  58  available from Aldrich Chemical Co. of Milwaukee, Wis. The planarizing solution  160 , for example, can include 50-1000 ppm Brij  58  in a water/ammonia solution containing 85%-99.5% water and 0.5%-15% ammonia. This particular application is expected to produce a 10:1 polishing rate ratio between the silicon dioxide cover layer and the silicon nitride polish-stop layer using the fixed-abrasive polishing pad  140 . The addition of Brij  58  to the planarizing solution produces the significant increase in the polishing rate ratio by decreasing the hydrolysis of the silicon nitride polish-stop layer  116  relative to the silicon dioxide layer to make the silicon nitride polish-stop layer  116  even more resistant to mechanical removal. Other suitable non-ionic surfactants include polyoxyethylene ethers and polyoxyethylene alcohols, such as (a) polyoxyethylene (4) lauryl ether (Brij 30) and (b) polyoxyethylene (10) isooctyl phenol ether (Triton X-100). 
     In another application of the present invention for forming conductive damascene lines in the trenches  114  or contacts in holes through a dielectric layer, the polish-stop layer  116  is a thin layer of silicon dioxide or silicon nitride, and the cover layer  118  is a layer of a conductive metal, such as aluminum, tungsten, copper or gold. The planarizing solution for forming W conductive damascene lines or contacts can include water, hydrogen peroxide and/or ammonium persulfate, and the selectivity agent in the planarizing solution can be a polyoxyethylene non-ionic surfactant. 
     The methods and planarizing solutions described above with reference to FIG. 2 are expected to increase the difference in the polishing ratio between the cover layer  118  and the polish-stop layer  116 . In the particular example of using a non-abrasive planarizing solution  160  with Brij  58  to planarize a silicon nitride polish-stop layer  116  and a silicon dioxide cover layer  118 , the polishing rate ratio is increased to 10:1 from 4:1 without the Brij  58 . The planarizing solution  160  with the mechanical selectivity agent is thus expected to reduce tie removal of material from the substrate assembly  112  beyond the lands  117  of the polish-stop layer  116 . 
     FIG. 3 is a schematic cross-sectional view illustrating the advantages of planarizing the substrate assembly  112  with the planarizing solution  160  at a relatively late stage of a method in accordance with an embodiment of the invention. In this embodiment, the substrate assembly  112  has been planarized so that a first land  117   a  is exposed and a second land  117   b  is still covered by the cover layer  118 . Because the mechanical selectivity agent in the planarizing solution  160  produces a 10:1 polishing rate ratio between a silicon nitride polish-stop layer  116  and a silicon dioxide cover layer  118 , even the first land  117   a  of the polish-stop layer  116  is not likely to be completely removed before enough material is removed from the cover layer  118  to expose the second land  117   b  and isolate the portions of the cover layer  118  in the trenches  114 . Therefore, planarizing solutions  160  with mechanical selectivity agents and the methods for planarizing semiconductor wafers using such planarizing solutions are expected to produce more accurate endpointing in CMP processing. 
     The non-abrasive planarizing solution  160  with the selectivity agent is also expected to produce highly planar surfaces without significant dishing in the trenches  114 . The global planarity of the substrate assembly  112  is expected to be highly planar because the mechanical selectivity agent in the planarizing solution  160  effectively inhibits further planarization beyond the polish-stop layer  116 . Thus, by accurately forming the lands  117  of the polish-stop layer  116  at a desired endpoint elevation, the substrate assembly  112  should be globally planar. Additionally, because the mechanical selectivity agent does not etch the cover layer  118  and instead changes the ability of the abrasive particles  143  in the polishing pad  140  to remove the cover layer  118  with respect to the polish-stop layer  116 , the dominant mechanical removal characteristics of the fixed-abrasive pad  140  continue to control the removal of material from the substrate assembly  112 . Consequently, the mechanically selective planarizing solution  160  does not produce dishing in the remaining portions of the cover layer  118  in the trenches  114 . Therefore, the planarizing solution  160  with the mechanical selectivity agent is expected to provide a highly planar surface on the substrate assembly  112 . 
     The non-abrasive solution  160  with the selectivity agent is also not readily apparent to a person of ordinary skill in the art. Although slurries with abrasive particles often include surfactants, such as Brij  58 , the purpose of adding surfactants to abrasive slurries is to maintain the suspension of the abrasive particles in the slurry. It follows, therefore, that because the non-abrasive solution  160  does not have abrasive particles, there is no immediate reason to add surfactants to non-abrasive solutions for the same reason that surfactants are added to abrasive solutions. Thus, adding the selectivity agent to the non-abrasive solution  160  is not readily apparent to a person skilled in the art based upon abrasive solutions with surfactants. 
     From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. For example, the mechanical selectivity agent can be materials other than surfactants that enhance the mechanical removal of the cover layer relative to the polish-stop layer without dissolving or otherwise etching material from the cover layer. Accordingly, the invention is not limited except as by the appended claims.