Abstract:
Microelectronic devices including a layer of germanium and selenium, optionally including up to 10 atomic percent silver, show promise for select applications. Manufacturing microelectronic devices containing such layers using conventional CMP processes presents some significant challenges. Embodiments of the invention provide methods of planarizing workpieces with Ge—Se layers, many of which can be carried out using conventional CMP equipment. Other embodiments of the invention provide chemical-mechanical polishing systems adapted to produce planarized workpieces with Ge—Se layers or, in at least one embodiment, other alternative layers. Various approaches suggested herein facilitate production of such microelectronic devices by appropriate control of the down force of the Ge—Se layer against the planarizing medium and/or one or more aspects of the planarizing medium, which aspects include pH, abrasive particle size, abrasive particle hardness, weight percent of abrasive.

Description:
TECHNICAL FIELD  
       [0001]     The present invention provides certain improvements in processing microelectronic devices. The invention has particular utility in connection with planarizing microelectronic devices with Ge—Se—Ag layers.  
       BACKGROUND  
       [0002]     Mechanical and chemical-mechanical planarizing processes (collectively “CMP”) remove material from the surface of semiconductor wafers, field emission displays or other microelectronic workpieces in the production of microelectronic devices and other products.  FIG. 1  schematically illustrates a CMP machine  10  with a platen  20 , a carrier assembly  30 , and a planarizing pad  40 . The CMP machine  10  may also have an under-pad  25  attached to an upper surface  22  of the platen  20  and the lower surface of the planarizing pad  40 . A drive assembly  26  rotates the platen  20  (indicated by arrow F), or it reciprocates the platen  20  back and forth (indicated by arrow G). Since the planarizing pad  40  is attached to the under-pad  25 , the planarizing pad  40  moves with the platen  20  during planarization.  
         [0003]     The carrier assembly  30  has a head  32  to which a microelectronic workpiece  12  may be attached, or the microelectronic workpiece  12  may be attached to a resilient pad  34  in the head  32 . The head  32  may be a free-floating wafer carrier, or an actuator assembly  36  may be coupled to the head  32  to impart axial and/or rotational motion to the workpiece  12  (indicated by arrows H and  1 , respectively).  
         [0004]     The planarizing pad  40  and a planarizing solution  44  on the pad  40  collectively define a planarizing medium that mechanically and/or chemically removes material from the surface of the workpiece  12 . The planarizing pad  40  can be a soft pad or a hard pad. The planarizing pad  40  can also be a fixed-abrasive planarizing pad in which abrasive particles are fixedly bonded to a suspension material. In fixed-abrasive applications, the planarizing solution  44  is typically a non-abrasive “clean solution” without abrasive particles. In other applications, the planarizing pad  40  can be a non-abrasive pad composed of a polymeric material (e.g., polyurethane), resin, felt or other suitable materials. The planarizing solutions  44  used with the non-abrasive planarizing pads are typically abrasive slurries with abrasive particles suspended in a liquid. The planarizing solution may be replenished from a planarizing solution supply  46 .  
         [0005]     If chemical-mechanical planarization (as opposed to plain mechanical planarization) is employed, the planarizing solution  44  will typically chemically interact with the surface of the workpiece  12  to speed up or otherwise optimize the removal of material from the surface of the workpiece. Increasingly, microelectronic device circuitry (i.e., trenches, vias, and the like) is being formed from copper. When planarizing a copper layer using CMP, the planarizing solution  44  is typically neutral to acidic and includes an oxidizer (e.g., hydrogen peroxide) to oxidize the copper and increase the copper removal rate. One particular slurry useful for polishing a copper layer is disclosed in International Publication Number WO 02/18099, the entirety of which is incorporated herein by reference.  
         [0006]     To planarize the workpiece  12  with the CMP machine  10 , the carrier assembly  30  presses the workpiece  12  face-downward against the polishing medium. More specifically, the carrier assembly  30  generally presses the workpiece  12  against the planarizing solution  44  on a planarizing surface  42  of the planarizing pad  40 , and the platen  20  and/or the carrier assembly  30  move to rub the workpiece  12  against the planarizing surface  42 . As the workpiece  12  rubs against the planarizing surface  42 , material is removed from the face of the workpiece  12 .  
         [0007]     CMP processes should consistently and accurately produce a uniformly planar surface on the workpiece to enable precise fabrication of circuits and photo-patterns. During the construction of transistors, contacts, interconnects and other features, many workpieces develop large “step heights” that create highly topographic surfaces. Such highly topographical surfaces can impair the accuracy of subsequent photolithographic procedures and other processes that are necessary for forming sub-micron features. For example, it is difficult to accurately focus photo patterns to meet tolerances approaching 0.1 micron on topographic surfaces because sub-micron photolithographic equipment generally has a very limited depth of field. Thus, CMP processes are often used to transform a topographical surface into a highly uniform, planar surface at various stages of manufacturing microelectronic devices on a workpiece.  
         [0008]     Chalcogenide materials can be used as electrically writable and erasable phase change materials, i.e., they can be electrically switched between generally amorphous and generally crystalline states with different resistive properties, or between different resistive states while in crystalline form. Such electrically writable and erasable materials are useful in a number of applications, including non-volatile or “state-changeable” memory devices such as EEPROMs and FLASH memory devices. Chalcogenide alloys have also garnered much attention as possible elements of optical memory devices. Certain aspects of manufacturing devices including chalcogenide materials are disclosed in U.S. Pat. No. 5,789,277 (Zahorik et al.), the entirety of which is incorporated herein by reference. Germanium-tellurium (Ge—Te) and germanium-tellurium-antimony (Ge—Te—Sb) are, perhaps, the most common chalcogenide-metal alloys in current EEPROM and Flash applications. Increasingly, though, memory device manufacturers are investigating other chalcogenides as possible candidates for both electrically writable and erasable materials and optical memory applications.  
       SUMMARY  
       [0009]     Various embodiments of the present dimension provide methods of planarizing workpieces with germanium-selenium layers or chemical-mechanical polishing systems adapted to produce planarized workpieces, e.g., workpieces with germanium-selenium layers. One exemplary embodiment provides a method of planarizing a microelectronic workpiece which includes a substrate having a surface, an outer layer defining a polished face of the workpiece, and an intermediate layer disposed between the substrate surface and the outer layer. This outer layer may comprise (Ge x Se y ) a Ag b , wherein x is about 20-80, y is about 20-80, a is about 90-100, and b is about 0-10. In accordance with this method, a planarizing solution is delivered to a planarizing surface of the planarizing pad. The planarizing solution and the planarizing pad together comprise a planarizing medium. The planarizing solution has a pH of at least 7; in one embodiment, the pH is greater than 7. The planarizing medium includes abrasive articles having a mean particle size of no greater than about 100 nm. Material of the outer layer is removed by pressing the unpolished face of the workpiece against the polishing medium with a down force of no greater than about 4 psi. One useful application employs a down force of no greater than about 2 psi.  
         [0010]     In one more specific adaptation of this embodiment, the planarizing solution includes a fluid component having a pH of about 8-11.5. The planarizing medium includes an abrasive comprising abrasive particles having a mean particle size of about 30-100 nm and a Vickers hardness number of less than about 1500. Material of the outer layer is removed by pressing the unpolished face of the workpiece against the polishing medium with a down force of at least about 0.1 psi and no more than about 2 psi.  
         [0011]     One alternative embodiment provides a method of planarizing a microelectronic workpiece which includes a substrate and an outer layer comprising germanium and selenium. The outer layer optionally includes silver, as well. The microelectronic workpiece is loaded into a carrier assembly of a CMP machine. A planarizing solution is delivered to a planarizing pad. The planarizing solution and planarizing pad comprise a planarizing medium. The planarizing solution includes a fluid component having a pH of about 8-11.5 and a solid component comprising abrasive particles. Material of the outer layer is removed from the workpiece by rubbing the outer layer against the planarizing medium with a controlled force. In one embodiment, the material of the outer layer is removed by rubbing without substantial delamination of the outer layer. In another embodiment, the controlled force is about 0.1-2 psi.  
         [0012]     A different embodiment of the invention provides a chemical-mechanical polishing system. This system may include a carrier assembly including an actuator, a planarizing medium, a workpiece, and a controller operatively coupled to the actuator. The planarizing medium, which may comprise a planarizing solution and a planarizing pad, includes an abrasive comprising abrasive particles having a mean particle size of about 30-100 nm and a Vickers hardness number of less than about 1500. The planarizing solution may comprise a fluid component having a pH of at least about 7, and preferably at least about 8, e.g., about 8-11.5. The workpiece, which is loaded into the carrier assembly, has an outer layer comprising germanium and selenium which is oriented toward the planarizing medium. The controller controls the actuator to control a down force of the outer layer against the polishing medium to a range of about 0.1-2 psi.  
         [0013]     Still another embodiment provides an alternative chemical-mechanical polishing system that is well suited for use with a Ge—Se layer, but may be used to planarize other films, too. This system includes a carrier assembly including an actuator. The carrier assembly is adapted to hold the workpiece with an outer layer facing downwardly. A planarizing medium comprises a planarizing solution and a planarizing pad. The planarizing medium includes an abrasive comprising abrasive particles having a mean particle size of about 30-100 nm and a Vickers hardness number of less than about 1500. The planarizing solution comprises a fluid component having a pH of about 7-11.5, e.g., about 8-11.5. A controller, which may be operatively coupled to the actuator, controls the actuator to control a down force of the outer layer of the workpiece against the polishing medium to a range of about 0.1-2 psi. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]      FIG. 1  is a schematic cross-sectional view of a planarizing machine in accordance with the prior art.  
         [0015]      FIG. 2  is a schematic cross-sectional view of a portion of a microelectronic workpiece.  
         [0016]      FIG. 3  is a schematic illustration of a stage in the planarization of the microelectronic workpiece shown in  FIG. 2 .  
         [0017]      FIG. 4  is a schematic cross-sectional view of the microelectronic workpiece shown in  FIG. 2  after planarization. 
     
    
     DETAILED DESCRIPTION  
       [0018]     Various embodiments of the present invention provide methods and apparatus for processing microelectronic devices with Ge—Se—Ag layers. The following description provides specific details of certain embodiments of the invention illustrated in the drawings to provide a thorough understanding of those embodiments. It should be recognized, however, that the present invention can be reflected in additional embodiments and the invention may be practiced without some of the details in the following description.  
         [0019]     The owner of the present invention is investigating thin films or structures comprising germanium and selenium, or germanium, selenium, and silver (collectively referred to below as Ge—Se—Ag chalcogenides or Ge—Se—Ag films, even though some of these films may not include any silver) in forming functional elements of select microelectronic devices, e.g., in non-volatile memory applications. Materials currently contemplated for such applications have the general formula (Ge x Se y ) a Ag b , wherein x is about 20-80, y is about 20-80, a is about 90-100, and b is about 0-10. It should be appreciated, however, that in some embodiments the values of x, y, a, and b are not limited to these ranges.  
         [0020]      FIG. 2  schematically illustrates one possible structure of a microelectronic workpiece  50  employing a Ge—Se—Ag film. In this embodiment, the microelectronic workpiece  50  includes a substrate  52  having at least one recess  54  formed therein. The structure can be helpful in forming specific Ge—Se—Ag structures, e.g., conductive lines or vias of electronic circuitry, or electrically isolated lines, rings, or serpentine structures.  
         [0021]     A Ge—Se—Ag film may comprise an outer layer  70  of the workpiece  50 . This outer layer  70  may substantially fill the recess  54  and extend over the entire outer surface  56  of the substrate  52 , or at least a working portion of the substrate  52 . The Ge—Se—Ag chalcogenide may be deposited in any suitable fashion, e.g., using electrolytic deposition, chemical vapor deposition (CVD), physical vapor deposition (PVD), or atomic layer deposition (ALD). At this stage, the outer layer  70  is contiguous across the outer surface  56  of the substrate. If specific shapes or islands of Ge—Se—Ag chalcogenide are to be formed, an overburden of the outer layer  70  extending outwardly beyond the upper edge of the recess  54  will be removed, leaving the Ge—Se—Ag chalcogenide only in the filled recesses  54 . In other embodiments, it may be desirable to form a thin, monolithic, contiguous Ge—Se—Ag film. However, the outer, unpolished face  72  of the outer layer  70  may have an irregular, topographic surface. As noted above, such topographic surfaces may be unsuitable for further processing steps, e.g., photolithographic procedures, and it may be advisable to planarize this surface before such further processing.  
         [0022]     In the structure illustrated in  FIG. 2 , the microelectronic workpiece  50  includes an intermediate layer  60  disposed between the outer surface  56  of the substrate  52  and the outer layer  70 . In the illustrated embodiment, the intermediate layer  60  extends over the entire outer surface  56  of the substrate  52 , including the interior surface of the recess  54 . In other embodiments, the intermediate layer  60  may be applied before the recess  54  is formed and the interior surface of the recess  54  will not include the intermediate layer  60 . Such an intermediate layer  60  can be used for a variety of purposes. For example, the intermediate layer  60  may function as a diffusion barrier, limiting diffusion of the material of the outer layer  70  into the substrate  52  and vice versa. In other embodiments, the intermediate layer  60  may function merely as a polish stop, as discussed in more detail below. The composition and thickness of the intermediate layer  60  will depend on a number of factors, including the function of the intermediate layer (e.g., as a diffusion barrier or as a polish stop). Suitable materials for the intermediate layer  60  include tungsten, tantalum, platinum, silica, and silicon nitride. An intermediate layer comprising silicon nitride has functioned effectively as a polish stop layer, as described below.  
         [0023]     CMP is commonly employed to planarize microelectronic workpieces having layers of copper or aluminum at commercially useful throughputs. As noted above, copper CMP operations typically employ neutral to acidic solutions. For example, International Publication Number WO 02/18099 (incorporated by reference above) suggests a copper planarizing slurry having a pH of about 2-6.  
         [0024]     Ge—Se—Ag films, however, cannot be effectively and safely manufactured under such conditions. The presence of selenium in the Ge—Se—Ag films presents some chemical handling difficulties, in part because selenium hydride is highly toxic. Planarizing Ge—Se—Ag films can generate selenium hydride as a toxic fume. If selenium is allowed to oxidize, the oxide is more readily converted to the hydride, increasing the risk of producing a toxic gas during planarization. When manufacturing limited quantities of microelectronic devices employing Ge—Se chalcogenides for research purposes, these films can be planarized in a highly ventilated area, such as a hood. This is not practical in mass production on a commercial scale using conventional CMP apparatus and techniques. Although a basic solution can help limit the generation of toxic byproducts, it has been discovered that Ge—Se—Ag films can also dissolve in strongly basic solutions, which may make it difficult to control the polishing rate.  
         [0025]     Silver also tends to be relatively mobile in Ge—Se—Ag films. Under CMP conditions commonly employed in planarizing copper films, for example, silver may tend to come out of solution and migrate to the surface during planarization and agglomerate on the surface. This leads to films with non-homogenous compositions and surface characteristics that vary from one location to another across the workpiece.  
         [0026]     Another concern in planarizing Ge—Se—Ag films is that such films may delaminate from the workpiece during a planarizing cycle. In some applications, Ge—Se—Ag films may be applied over a layer of tungsten, tantalum, platinum, silica, or silicon nitride. Unfortunately, adhesion of these Ge—Se—Ag films to such underlayers is relatively poor. Conventional CMP processing techniques and conditions could cause Ge—Se—Ag films to delaminate from the underlying substrate, leading to faulty electrical connections and defective products.  
         [0027]     It also can be difficult to predict and carefully control the polish rate of Ge—Se—Ag films, i.e., the rate at which the films are removed. This is due, at least in part, to significant variations of the polish rate with relatively small changes in the composition of the Ge—Se—Ag films. In some commercial CMP operations, the workpiece is planarized for a fixed period of time. If the polishing rate of a Ge—Se—Ag film varies from one workpiece to the next, planarizing for a fixed period of time could lead to appreciable variations in the thickness of the film removed during the planarization.  
         [0028]     Some embodiments of the invention address some or all of these difficulties to provide a practical, commercially viable process for planarizing Ge—Se—Ag films. In particular, smooth, planarized surfaces can be produced without substantial delamination or silver agglomeration by appropriately controlling aspects of the pH, controlling abrasive material in the planarizing medium, and/or limiting the down force against the Ge—Se—Ag film.  
         [0029]     As noted above,  FIG. 1  illustrates a conventional CMP machine. Embodiments of the invention can be carried out on such a conventional CMP machine. In such embodiments, a microelectronic workpiece having a Ge—Se—Ag film, e.g., the microelectronic workpiece  50  of  FIG. 2 , is loaded in the carrier assembly  30  of the CMP machine  10 , e.g., by attaching the workpiece  50  to the resilient pad  34  of the head  32 . The actuator assembly  36  then lowers the head  32  to juxtapose the outer surface  72  of the outer layer  70  ( FIG. 2 ) with the planarizing medium, i.e., the planarizing pad  40  and the liquid planarizing solution  44 .  
         [0030]     The actuator assembly  36  may also rotate the workpiece  50  and pushes the workpiece  50  against the planarizing medium with a controlled, predetermined down force during the course of the planarizing operation. The actuator assembly may include a controller  37 , e.g., a programmed processor, which controls operation of the actuator assembly, including the rotational velocity of the workpiece  50  with respect to the planarizing medium and the down force. Rotating the workpiece  50  at a relative velocity of about 10-100 in. per second should be appropriate, though other rotational speeds may also work. In one embodiment, the down force is relatively low to reduce the likelihood of delaminating the Ge—Se—Ag film from the underlying intermediate layer  60  or substrate surface  56 . For certain applications, a down force of no more than about 2 psi is appropriate. In other applications, the down force should be about 0.1-1.9 psi.  
         [0031]     The planarizing solution  44  can be delivered from the planarizing solution supply to the planarizing surface  42  of the planarizing pad during the course of planarizing the workpiece. To avoid generating toxic byproducts from the selenium in the Ge—Se—Ag film, the planarizing solution may be basic. Depending on the stoichiometry of the film and other factors, the planarizing solution may range from a mild base to a fairly strong base. For most applications, it is expected that a pH from about 7 to about 11.5 will work well. For example, a pH level of about 8 to about 11.5 is expected to be useful in many applications. Lower pH levels than this will increase the risk of creating toxic byproducts from the selenium. On the other hand, as the pH level increases above this range, it has been found that Ge—Se—Ag chalcogenides become increasingly soluble in the planarizing solution, adversely affecting the integrity of the Ge—Se—Ag film. For films having more silver (e.g., 8-10 atomic % or greater), a planarizing solution with a pH over 11.5 may be employed. For Ge—Se—Ag films with relatively little or no silver (e.g., 0-2 atomic %), planarizing solutions advantageously may have a pH in a lower portion of the stated range, e.g., about 7-10 or about 8-10, though a higher pH may also suffice. In one embodiment, the planarizing solution comprises a base (e.g., potassium hydroxide or tetramethyl ammonium hydroxide) and deionized water in proportions selected to yield the desired pH. While an acidic buffer may be employed to help stabilize pH, planarizing solutions in some embodiments are substantially acid-fee to reduce the chance of generating toxic byproducts from the selenium in the Ge—Se—Ag chalcogenide material.  
         [0032]     In other embodiments, the planarizing solution includes ammonia. It is believed that the presence of the ammonia will reduce the rate of selenium hydride production. In one specific formulation, the planarizing solution comprises a combination of potassium hydroxide and ammonia.  
         [0033]     As noted above, the planarizing medium of the CMP machine typically includes an abrasive. Fixed-abrasive CMP machines employ a planarizing pad  40  that has abrasive particles imbedded therein. Such fixed-abrasive pads may be a conventional round rotary pad, a web-format pad that can be moved periodically to present a “fresh” abrasive area, or a belt, which is essentially a web-format pad arranged as a continuous loop. In other embodiments, a non-abrasive planarizing pad (e.g., a urethane having a Shore D hardness of about 40-60, commercially available from Rodel as model WWP 3000) is employed and the abrasive particles are included in the planarizing solution  44 .  
         [0034]     Some abrasives used in conventional CMP operations may be too hard for polishing Ge—Se—Ag films. For example, use of high-purity alumina, which has a Vickers hardness number of 1500 or more, increases the rate at which the outer layer  70  is removed, which could increase throughput. It has been found, however, that such hard abrasives can promote delamination of the Ge—Se—Ag film from the underlying substrate  52  or intermediate layer  60 , e.g., by separating the outer layer  70  from the intermediate layer at the interface  62  therebetween. In certain embodiments of the invention, therefore, the abrasive particles are formed of a material having a Vickers hardness of less than about 1500. Silica particles (which typically have Vickers hardness numbers on the order of 1100 or less) have been found to work well and it is anticipated that ceria particles or mixtures of silica and ceria particles may also suffice.  
         [0035]     It has also been found that abrasives with larger particle sizes can increase the likelihood of delamination and agglomeration of silver at the surface of the planarized Ge—Se—Ag film. Abrasive particles having a mean particle size of less than about 100 nm are expected to suffice. In one specific application, the mean particle size is about 30-100 nm. In another embodiment, a mean particle size of about 30-50 nm is employed. In one specific system found to work well, silica particles having a mean particle size of about 50 nm were employed. The size distribution of the abrasive particles may be relatively narrow to avoid too many oversized particles.  
         [0036]     As noted above, the planarizing solution  44  may provide the abrasive particles used in the planarizing medium. Such a planarizing solution will have a fluid fraction, which may have a pH of about 7 to about 11.5, and a solid fraction comprising the abrasive particles. For example, a pH level of about 8 to about 11.5 is expected to be useful in many applications. Planarizing solutions in which the solid fraction is about 1-30 weight percent are expected to work well. While higher weight percentages of abrasive particles may be functional, this could increase the chances of delamination and/or silver agglomeration.  
         [0037]      FIG. 3  schematically illustrates a stage in the process of planarizing the microelectronic workpiece  50  shown in  FIG. 2 . At this stage, the outer layer  70  of the partially polished workpiece  150  has a polished face  74  which is juxtaposed with the planarizing face  42  of the planarizing pad  40 . The planarizing solution  44  is disposed between the polished face  74  of the outer layer and the planarizing face  42  of the planarizing pad  40 . The intermediate layer  60  remains beneath the polished face  74  of the outer layer  70  at this stage in the planarizing process.  
         [0038]      FIG. 4  schematically illustrates a final planarized microelectronic workpiece  250 . At this stage, the entire overburden of the Ge—Se—Ag outer layer  70  is removed, exposing a portion of the intermediate layer  60 . In the illustrated embodiment, a portion of the intermediate layer  60  in the recess  54  of the substrate  52  remains covered by the remainder of the outer layer  70 . In one embodiment of the invention, the intermediate layer  60  serves as a polish stop, helping automatically detect the end point of the planarizing process. In such an embodiment, the intermediate layer has a removal rate under the planarizing conditions of the CMP machine  10  that is slower than the removal rate of the Ge—Se—Ag film  70 . This can be accomplished by employing a material that is less subject to chemical attack by the basic planarizing solution  44  than is the Ge—Se—Ag film  70  and/or which is harder than the Ge—Se—Ag film. In one particular application, the intermediate layer  60  may comprise silicon nitride, which is both harder and more resistant to chemical attack than the adjacent Ge—Se—Ag film  70 . Once the overburden of the outer layer  70  is removed and the polishing medium begins to act on the intermediate layer  60 , the friction between the polishing medium and the workpiece  50  will change. As a consequence, the force necessary to drive the polishing pad at a constant speed will change. This will cause a change in power to the drive assembly  26 , which can be detected as an indication that the polishing is complete. If the actuator assembly  36  includes a controller  37 , as noted above, the controller  37  could be programmed to detect this change in friction and either set a flag signifying that polishing is complete or stop rubbing of the workpiece  50  against the polishing medium.  
         [0039]     Alternatively, the end point of the polishing process may be determined optically. Light reflected by the Ge—Se—Ag film  70  may differ in color and/or intensity from the light reflected by the intermediate layer  60  from the same light source. A controller  37  may be coupled to an optical sensor in a known manner to detect the change in reflectance and either set an endpoint flag or stop polishing the workpiece  50 .  
         [0040]     From the foregoing, it will be appreciated that specific embodiments of the invention have been described herein for purposes of illustration, but that various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.