Patent Publication Number: US-2010130013-A1

Title: Slurry composition for gst phase change memory materials polishing

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims benefit of U.S. provisional patent application Ser. No. 61/117,525, filed Nov. 24, 2008, which is herein incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Embodiments of the present invention generally relate to polishing compositions and methods for polishing a substrate using the same. More particularly, embodiments of the invention relate to chemical-mechanical polishing compositions suitable for polishing substrates comprising phase change alloys. 
     2. Description of the Related Art 
     Typical solid state memory devices (dynamic random access memory (DRAM), static random access memory (SRAM), erasable programmable read only memory (EPROM), and electrically erasable programmable read only memory (EEPROM)) employ micro-electronic circuit elements for each memory bit in memory applications. Since one or more electronic circuit elements are required for each memory bit, these devices consume considerable chip space to store information, limiting chip density. For typical non-volatile memory elements (like EEPROM i.e. “flash” memory), floating gate field effect transistors are employed as the data storage device. These devices hold a charge on the gate of the field effect transistor to store each memory bit and have limited re-programmability. They are also slow to program. 
     PRAM (Phase Change Random Access Memory) devices (also known as Ovonic memory devices) use phase change materials that can be electrically switched between an insulating amorphous and conductive crystalline state for electronic memory application. Typical materials suited for these applications utilize various chalcogenide (Group VIB) and Group VB elements of the periodic table (e.g., Te, Po, and Sb) in combination with one or more of In, Ge, Ga, Sn, or Ag (sometimes referred to herein as a “phase change alloy”). Particularly useful phase change alloys are germanium (Ge)-antimony (Sb)-tellurium (Te) alloys (GST alloys), such as an alloy having the formula Ge 2  Sb 2  Te 5 . These materials can reversibly change physical states depending on heating/cooling rates, temperatures, and times. Other useful phase change material alloys include indium antimonite (InSb). The memory information in PRAM is preserved with minimal loss through the conductive properties of the different physical states. 
     Chemical-Mechanical Polishing (CMP) techniques can be utilized to manufacture memory devices employing phase change materials. However, current CMP slurry, rinse, etc. compositions do not provide sufficient planarity when utilized for polishing substrates having relatively soft phase change alloys, such as a GST alloy. In particular, the physical properties of many phase change alloys (e.g., GST or InSb) make them “soft” relative to other materials utilized in phase-change memory (PCM) chips. For example, typical CMP polishing slurries containing relatively high solid concentrations (&gt;about 3%) remove a phase change alloy (e.g., a GST alloy) through the mechanical action of the abrasive particles resulting in heavy scratching on the surface of the phase change alloy. When such high solids CMP compositions are used, phase change alloy residues often remain on the underlying dielectric film after polishing, since the CMP slurry is not able to remove all of the phase change alloy material. The phase change alloy residues cause further integration issues in subsequent steps of device manufacturing. Additionally, even removal of a multi-component alloy poses a challenge for conventional CMP techniques. 
     Thus, there is an ongoing need to develop new CMP compositions that provide reduced scratching and residue defects, while still providing acceptably rapid removal of phase change alloys compared to conventional CMP compositions. 
     SUMMARY OF THE INVENTION 
     One embodiment of the invention generally provides chemical-mechanical polishing (CMP) slurry for removing at least a phase change alloy from a substrate surface. The slurry includes colloidal particles with a particle size less than 60 nm and in an amount between 0.2% to about 10% by weight of the slurry, a pH adjustor, a chelating agent comprising at least one organic carboxylic acid, an oxidizing agent in an amount less than 1% by weight of the slurry, and polyacrylic acid. 
     Another embodiment of the invention also provides a rinse solution for passivation of a phase change alloy on a substrate surface used in conjunction with CMP polishing of the phase change alloy. The rinse solution includes deionized water and at least one component in the deionized water selected from the group comprising polyethylene glycol, polyacrylic amide, polyethylene imine, an azole containing compound, benzo-triazole, and combinations thereof. The rinse solution has a pH between 2 and 12. 
     In yet another embodiment of the invention, a method for chemical-mechanical polishing (CMP) of a phase change alloy on a substrate surface is provided. The method includes positioning a substrate comprising a phase change alloy material on a platen containing a polishing pad in a polishing slurry, polishing the substrate on the platen to remove a portion of the phase change alloy, and rinsing the substrate on the platen with a rinse solution. The polishing slurry includes colloidal particles with a particle size less than 60 nm and in an amount between 0.2% to about 10% by weight of the slurry, a pH adjustor, a chelating agent comprising at least one carboxylic acid, an oxidizing agent in an amount less than 1% by weight of the slurry, and polyacrylic acid. The rinse solution used in the method includes deionized water and at least one component in the deionized water selected from the group comprising polyethylene glycol, polyacrylic amide, polyethylene imine, an azole containing compound, benzo-triazole, and combinations thereof. The rinse solution has a pH between 2 and 12. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
         FIG. 1  is a cross sectional view of a chalcogenide semiconductor having a phase change alloy. 
         FIG. 2  shows a chemical mechanical polishing apparatus that may be used to polish a phase change alloy containing substrate. 
         FIG. 3  is a partial sectional view of one embodiment of a polishing station that includes a fluid delivery arm assembly. 
         FIGS. 4A-4C  are schematic cross-sectional views illustrating a polishing process performed on a phase change alloy containing substrate according to one embodiment of the invention. 
         FIG. 5  is a flow diagram of one embodiment of a method for chemical mechanical polishing a phase change alloy containing substrate. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the invention relate to chemical mechanical planarization or chemical mechanical polishing (CMP) of phase change alloys. Phase change memory devices may employ in their memory cells a phase change layer (a chalcogenide semiconductor thin film or the like) whose electrical resistance changes depending on its state. Chalcogenide semiconductors are amorphous semiconductors including chalcogen elements. 
     Chalcogen elements include S (Sulfur), Se (Selenium), and Te (Tellurium) in group VI in the periodic table. Chalcogenide semiconductors are used in generally two fields, optical disks and electric memories. Chalcogenide semiconductors used in the field of electric memories include Ge 2 Sb 2 Te 5  (hereinafter referred to as “GST”) which is a compound of Ge (Germanium), Te (Tellurium), and Sb (Antimony). 
     An example of a phase-change memory (PCM) cell  10  is illustrated in the cross-sectional view of  FIG. 1  although embodiments of PCM cells are not limited to such a structure. A dielectric layer  12 , for example silicon oxide, is grown over a bottom electrode  14 . A vertical structure is etched through the dielectric layer  12 . A via  16  in the lower portion is filled with a metal to contact the bottom electrode  14 . A wider plug  18  at the top of the dielectric layer  12  and contacting and overhanging the via  16  is filled with a phase change alloy, such as the metal chalcogenide germanium antimony telluride (GST). A top electrode  20  is deposited over the GST plug  18 . 
     As shown in  FIG. 1 , a chalcogenide semiconductor can take two stable states, i.e., amorphous state  23  and crystalline state  24 . In operation, a short electrical pulse is applied through the electrodes  14 ,  20  to the GST plug  18  to cause a phase-change region  22  to melt. The remainder of the GST plug  18  is preferably always in the conductive crystalline state  24 . Depending on whether the melting pulse is short or long, the phase-change region  22  either quickly cools and quenches to a high-resistance amorphous state  23  or slowly cools to a low-resistance crystalline state  24 . The state of the PCM cell  10  can be read by measuring its resistance between the electrodes  14 ,  20  across the GST plug. 
     The amorphous state exhibits a higher electrical resistance corresponding to a digital value “1” and the crystalline state exhibits a lower electrical resistance corresponding to a digital value “0”. This allows the chalcogenide semiconductor to store digital information. The amount of current flowing through the chalcogenide semiconductor or a voltage drop across the chalcogenide semiconductor is detected to determine whether the information stored in the chalcogenide semiconductor is “1” or “0”. 
     Specifically, after the chalcogenide semiconductor is supplied with heat at a temperature near its melting point, it switches into the amorphous state when the chalcogenide semiconductor is quickly cooled. After the chalcogenide semiconductor is supplied with heat at a crystallizing temperature lower than the melting point for a long period of time, it switches into the crystalline state when the chalcogenide semiconductor is cooled. For example, after the GST is supplied with heat at a temperature near the melting point (about 610° C.) for a short period of time (1 through 10 ns), it switches into the amorphous state when the GST is quickly cooled for about 1 ns. After the GST is supplied with heat at a crystallizing temperature (about 450° C.) for a long period of time (30 through 50 ns), it switches into the crystalline state when the GST is cooled. 
     Switching currents may be reduced by a variation of the structure of  FIG. 1  in which a smaller volume of GST is deposited near the bottom of the via  16  and the metal fills the rest of the via and the phase-change region  22 . 
       FIG. 2  shows a chemical mechanical polishing apparatus that may be used to polish a phase change alloy containing substrate. While the particular apparatus in which the embodiments described herein can be practiced is not limited, it is particularly beneficial to practice the embodiments in a REFLEXION® CMP system, REFLEXION® LK CMP system, and a MIRRA MESA® system sold by Applied Materials, Inc., Santa Clara, Calif. Additionally, CMP systems available from other manufacturers may also benefit from embodiments described herein. Although a CMP system is depicted for using embodiments of the invention, an electrochemical mechanical polishing system (eCMP) may also be suited to use embodiments of the invention. 
       FIG. 2  shows a chemical mechanical polishing apparatus  220  that can polish one or more substrates  210  such as wafers. Polishing apparatus  220  includes a series of polishing stations  222  and a transfer station  223 . Transfer station  223  transfers the substrates between carrier head assemblies  270  and a loading apparatus (not shown). 
     Each polishing station  222  includes a rotatable platen assembly  224  on which is placed a polishing pad assembly  230 . The first and second stations  222  can include a two-layer polishing pad with a hard durable outer surface or a fixed-abrasive pad with embedded abrasive particles. The final polishing station  222  can include a relatively soft pad. Each polishing station  222  can also include a pad conditioner apparatus  228  to maintain the condition of the polishing pad assembly  230  so that it will effectively polish substrates  210 . 
     A rotatable multi-head carousel  260  supports four carrier head assemblies  270 . The carousel  260  is rotated by a central post  262  about a carousel axis  264  by a carousel motor assembly (not shown) to orbit the carrier head assembly  270  and the substrates  210  attached thereto between polishing stations  222  and transfer station  223 . Three of the carrier head assemblies  270  receive and hold substrates  210 , and polish them by pressing them against the polishing pad assemblies  230 . Meanwhile, one of the carrier head assemblies  270  receives a substrate  210  from and delivers a substrate  210  to the transfer station  223 . 
     Each carrier head assembly  270  is connected by a carrier drive shaft  274  to a carrier head rotation motor  276  (shown by the removal of one quarter of cover  268  so that each carrier head can independently rotate about its own axis). In addition, each carrier head assembly  270  independently laterally oscillates in a radial slot  272  formed in carousel support plate  266 . 
     Slurry  238  is supplied to the surface of the polishing pad assembly  230  by a slurry supply port or combined slurry/rinse arm assembly  239 . The slurry  238  includes colloidal particles, a pH adjustor, a chelating agent comprising at least one organic carboxylic acid, an oxidizing agent in an amount less than 1% by weight of the slurry, and polyacrylic acid. In one embodiment, the slurry contains organic carboxylic acid in an amount of 0.1% to 3.0% by weight of the slurry, an oxidizing agent in an amount of 0.1% to 3.0% by weight of the slurry, and polyacrylic acid in an amount between 50 ppm and 5000 ppm. The slurry may also be adjusted to have a pH level between about 2 and about 9, such as between a pH of about 7 and about 9, between a pH of about 3 and about 6, or between a pH of about 2 and about 7. The oxidizing agent may comprise at least one of the following: hydrogen peroxide, organic peroxide, potassium iodate, and ammonium persulfate. The pH adjustor may comprise KOH or NH 4 OH added to the slurry in an amount sufficient to adjust the pH to be within any of the above ranges. 
     Colloidal particles used with embodiments of this invention may be any particles suitable for use as an abrasive. The colloidal particles have a particle size less than 60 nm and are in an amount between 0.1% to about 10% by weight of the slurry. The colloidal particles may be between 10-30 nm in size. In another embodiment, the colloidal particles may be between about 5-85 nm in size. The colloidal particles that may be used include, but are not limited to, silica, alumina, modified silica with alumina, surface coated alumina-silica, or surface modified silica with organic groups. For example, colloidal silica may be positively activated, such as with an alumina modification or a silica/alumina composite. In the embodiment where the colloidal particles are silica with alumina or surface coated alumina, the pH may be between 3 and 7, such as between 5 and 7. 
     The organic carboxylic acid may include, but is not limited to citric acid, tartaric acid, succinic acid, oxalic acid, amino acids, salts thereof, or combinations thereof. For example, suitable salts for the chelating agent may include ammonium citrate, potassium citrate, ammonium succinate, potassium succinate, ammonium oxalate, potassium oxalate, potassium tartrate, or combinations thereof. The salts may have multi-basic states, for example, citrates have mono-, di- and tri-basic states. Other suitable chelating agents may include acetic acid, adipic acid, butyric acid, capric acid, caproic acid, caprylic acid, glutaric acid, glycolic acid, formaic acid, fumaric acid, lactic acid, lauric acid, malic acid, maleic acid, malonic acid, myristic acid, palmitic acid, phthalic acid, propionic acid, pyruvic acid, stearic acid, valeric acid, derivatives thereof, salts thereof or combinations thereof. Suitable chelating agents may be free of an amine or amide functional groups. The organic carboxylic acid is added to the slurry in an amount between 0.1% and 3.0% by weight of the slurry. 
     The polishing slurry may also include an inorganic acid for providing a suitable pH. Suitable acids include, for example, phosphoric acids, sulfuric acid, nitric acid, perchloric acid, or combinations thereof. In one embodiment, the slurry may contain H 3 PO 4  in an amount between 50 ppm to 5000 ppm. The acid may also buffer the composition to maintain a desired pH level for processing a substrate. For example, the slurry may have a desired pH level between 3 and 7, such as between 3 and 6 or between 5 and 7. 
     Examples of suitable acids include compounds having a phosphate group (PO 4   3− ), such as, phosphoric acid, copper phosphate, potassium phosphates (K x H (3-x) PO 4 ) (x=1, 2 or 3), such as potassium dihydrogen phosphate (KH 2 PO 4 ), dipotassium hydrogen phosphate (K 2 HPO 4 ), ammonium phosphates ((NH 4 ) x H (3-x) PO 4 ) (x=1, 2 or 3), such as ammonium dihydrogen phosphate ((NH 4 )H 2 PO 4 ), diammonium hydrogen phosphate ((NH 4 ) 2 HPO 4 ), compounds having a nitrite group (NO 3   1− ), such as, nitric acid or copper nitrate, compounds having a boric group (BO 3   3− ), such as, orthoboric acid (H 3 BO 3 ) and compounds having a sulfate group (SO 4   2− ), such as sulfuric acid (H 2 SO 4 ), ammonium hydrogen sulfate ((NH 4 )HSO 4 ), ammonium sulfate, potassium sulfate, copper sulfate, derivatives thereof or combinations thereof. 
     A clear window  236  is included in the polishing pad assembly  230  and is positioned such that it passes beneath substrate  210  during a portion of the platen&#39;s rotation, regardless of the translational position of the carrier head. The clear window  236  may be used for metrology devices, for example, an eddy current sensor and a laser may be placed below the clear window  236 . In certain the window  236  and related sensing methods may be used for an endpoint detection process. 
     To facilitate control of the polishing apparatus  220  and processes performed thereon, a controller  290  comprising a central processing unit (CPU)  292 , memory  294 , and support circuits  296 , is connected to the polishing apparatus  220 . The CPU  292  may be one of any form of computer processor that can be used in an industrial setting for controlling various drives and pressures. The memory  294  is connected to the CPU  292 . The memory  294 , or computer-readable medium, may be one or more of readily available memory such as random access memory (RAM), read only memory (ROM), floppy disk, hard disk, or any other form of digital storage, local or remote. The support circuits  296  are connected to the CPU  292  for supporting the processor in a conventional manner. These circuits include cache, power supplies, clock circuits, input/output circuitry, subsystems, and the like. 
     The polishing station  222  includes a combined slurry/rinse arm assembly  239 . During polishing, the arm  330  is operable to dispense slurry  238  containing a liquid and a pH adjuster. Alternatively, the polishing station includes a slurry port operable to dispense slurry onto polishing pad assembly  230 . 
     With reference to  FIGS. 2 and 3 , the polishing station  222  includes a carrier head assembly  270  operable to hold the substrate  210  against the polishing pad assembly  230 . The carrier head assembly  270  is suspended from a support structure, for example, the carousel  260 , and is connected by a carrier drive shaft  274  to a carrier head rotation motor  276  so that the carrier head can rotate about an axis  318 . In addition, the carrier head assembly  270  can oscillate laterally in a radial slot  272  formed in the support structure. In operation, the platen assembly  224  is rotated about its central axis  317 , and the carrier head assembly  270  is rotated about its central axis  318  and translated laterally across an upper surface  232  (see  FIG. 3 ) of the polishing pad assembly  230 . 
       FIG. 3  is a partial sectional view of one of the polishing stations  222  that includes the combined slurry/rinse arm assembly  239 . The polishing station  222  includes the carrier head assembly  270  and a platen assembly  224 . The carrier head assembly  270  generally retains the substrate  210  against a polishing pad assembly  230  disposed on the platen assembly  224 . At least one of a carrier head assembly  270  or platen assembly  224  is rotated or otherwise moved to provide relative motion between the substrate  210  and the polishing pad assembly  230 . In the embodiment depicted in  FIG. 3 , the carrier head assembly  270  is coupled to an actuator or motor  316  that provides at least rotational motion to the substrate  210 . The motor  316  may also oscillate the carrier head assembly  270 , such that the substrate  210  is moved laterally back and forth across the surface of the polishing pad assembly  230 . 
     The polishing pad assembly  230  may comprise a conventional material such as a foamed polymer disposed on the platen assembly  224  as a pad. In one embodiment, the conventional polishing material is foamed polyurethane. In one embodiment, the pad is an IC1010 polyurethane pad, available from Rodel Inc., of Newark, Del. IC1010 polyurethane pads typically have a thickness of about 2.05 mm and a compressibility of about 2%. Other pads that can be used include IC1000 pads with and without an additional compressible bottom layer underneath the IC1000 pad, IC1010 pads with an additional compressible bottom layer underneath the IC1010 pad, and polishing pads available from other manufacturers. The compositions described herein are placed on the pad to contribute to the chemical mechanical polishing of substrate. 
     In one embodiment, the carrier head assembly  270  includes a retaining ring  310  circumscribing a substrate receiving pocket  312 . A bladder  314  is disposed in the substrate receiving pocket  312  and may be evacuated to chuck the wafer to the carrier head assembly  270  and pressurized to control the downward force of the substrate  210  when pressed against the polishing pad assembly  230 . In one embodiment, the carrier head may be a multi-zone carrier head. One suitable carrier head assembly  270  is a TITAN HEAD™ carrier head available from Applied Materials, Inc., located in Santa Clara, Calif. 
     In  FIG. 2 , the platen assembly  224  is supported on a base  356  by bearings  358  that facilitate rotation of the platen assembly  224 . A motor  360  is coupled to the platen assembly  224  and rotates the platen assembly  224  such that the polishing pad assembly  230  is moved relative to the carrier head assembly  270 . 
     The combined slurry/rinse arm assembly or fluid delivery arm assembly  239  is utilized to deliver slurry from a slurry supply  328  to a top or working surface of the polishing pad assembly  230 . In the embodiment depicted in  FIG. 3 , the fluid delivery arm assembly  239  includes an arm  330  extending from a stanchion  332 . A motor  334  is provided to control the rotation of the arm  330  about a center line of the stanchion  332 . An adjustment mechanism  336  may be provided to control the elevation of a distal end  338  of the arm  330  relative to the working surface of the polishing pad assembly  230 . The adjustment mechanism  336  may be an actuator coupled to at least one of the arm  330  or the stanchion  332  for controlling the elevation of the distal end  338  of the arm  330  relative to the platen assembly  224 . 
     The fluid delivery arm assembly  239  may include a plurality of rinse outlet ports  370  arranged to uniformly deliver a spray and/or stream of rinsing fluid to the surface of the polishing pad assembly  230 . The ports  370  are coupled by a tube  374  routed through the fluid delivery arm assembly  239  to a rinsing fluid supply  372 . In one embodiment, the fluid delivery arm may have between 12 and 15 ports. The rinsing fluid supply  372  provides a rinsing fluid to the polishing pad assembly  230  before, during, and/or after polishing the phase change alloy containing substrate and/or after the substrate  210  is removed to clean the polishing pad assembly  230 . The polishing pad assembly  230  may also be cleaned using fluid from the ports  370  after conditioning the pad using a conditioning element, such as a diamond disk or brush (not shown). 
     The rinsing fluid may be used for passivation of a phase change alloy on a substrate surface used in conjunction with CMP polishing of the phase change alloy. The rinse solution includes deionized water and at least one component in the deionized water. The component is selected from the group consisting of polyethylene imine, polyethylene glycol, polyacrylic amide, alcohol ethoxylates polyacrylic acid, an azole containing compound, benzo-triazole, and combinations thereof. Examples of organic compounds having azole groups include benzotriazole (BTA), mercaptobenzotriazole, 5-methyl-1-benzotriazole (TTA), tolyltriazole (TTA), 1,2,4 triazole, benzoylimidazole (BIA), benzimidazole, derivatives thereof or combinations thereof. 
     The rinse solution may be formed by mixing 10 to 14 liters of deionized water with 300 milliliters of a 1%-10% component solution i.e. the component solution has 1% to 10% of component by weight. The rinse solution has a pH between 2 and 12. In one embodiment, the pH range is between a pH of about 2 and about 7.5. 
     The nozzle assembly  348  is disposed at the distal end of the arm  330 . The nozzle assembly  248  is coupled to the slurry supply  328  by a tube  342  routed through the fluid delivery arm assembly  239 . The nozzle assembly  348  includes a nozzle  340  that may be selectively adjusted relative to the arm, such that the fluid exiting the nozzle  340  may be selectively directed to a specific area of the polishing pad assembly  230 . 
     In one embodiment, the nozzle  340  is configured to generate a spray of slurry. In another embodiment, the nozzle  340  is adapted to provide a stream of slurry. In another embodiment, the nozzle  340  is configured to provide a stream and/or spray of slurry  346  at a rate between about 200 to about 500 ml/second to the polishing surface. 
     One embodiment of the process will now be described in reference to  FIGS. 4A-4C , which are schematic cross-sectional views of a substrate being processed according to methods and compositions described herein. Referring to  FIG. 4A , a substrate generally includes a dielectric layer  410  formed on a substrate  400 . A plurality of apertures, such as vias, trenches, contacts, or holes, are patterned and etched into the dielectric layer  410 , such as a dense array of narrow feature definitions  420  and low density of wide feature definitions  430 . The apertures may be formed in the dielectric layer  410  by conventional photolithographic and etching techniques. 
       FIG. 4A  depicts a substrate  400  and a phase change alloy  460  with a passivation layer  490  formed thereon after using the rinse described above.  FIG. 4B  illustrates the contact of the substrate surface with a polishing article to remove a portion of a passivation layer  490  formed thereon and the underlying phase change alloy  460 .  FIG. 4C  illustrates the substrate after a portion of the phase change alloy  460  on the dielectric layer  410  has been removed by applying a CMP process using the slurry described above. Alternatively, and not shone, the phase change alloy  460  may be removed in multiple processing steps. 
     The dielectric layer  410  may comprise one or more dielectric materials conventionally employed in the manufacture of semiconductor devices. For example, dielectric materials may include materials such as silicon dioxide, phosphorus-doped silicon glass (PSG), boron-phosphorus-doped silicon glass (BPSG), and silicon dioxide derived from tetraethyl orthosilicate (TEOS) or silane by plasma enhanced chemical vapor deposition (PECVD). The dielectric layer may also comprise low dielectric constant materials, including fluoro-silicon glass (FSG), polymers, such as polyamides, carbon-containing silicon oxides, such as BLACK DIAMOND® dielectric material, silicon carbide materials, which may be doped with nitrogen and/or oxygen, including BLOk® dielectric materials, available from Applied Materials, Inc. of Santa Clara, Calif. The dielectric layer may also include SiN. 
     A phase change alloy  460  is disposed on the dielectric layer  410  and in the vias, trenches, contacts, or holes. The phase change alloy  460  may comprise chalcogen elements such as GST. The CMP process may begin by positioning the substrate in a polishing apparatus and exposing the substrate to a rinse solution  495  that can form a passivation layer  490  on the phase change alloy  460 . The passivation layer  490  may be formed by the rinse solution described herein. 
       FIG. 4B  illustrates chemical mechanical polishing during processing. During processing, the substrate surface and a polishing pad assembly  230  are contacted with one another and moved in relative motion to one another, such as in a relative orbital motion, to remove portions of the passivation layer  490  formed on the exposed phase change alloy  460 , which may additionally remove a portion of the underlying phase change alloy  460 . A view of the final planarized substrate surface containing the phase change alloy  460 , as depicted in  FIG. 4C , is formed by using the polishing slurry and rinse described above and according to the methods disclosed herein. 
     The substrate surface and polishing pad assembly  230  are contacted at pressure less than about 2.5 pounds per square inch (lb/in 2  or psi). Removal of the passivation layer  490  and some phase change alloy  460  may be performed with a process having a pressure of about 2 psi or less, for example, from about 0.3 psi to about 2.2 psi. In one aspect of the process, the substrate surface and polishing article are contacted at a pressure of about 1 psi or less. In one embodiment, the CMP process may have a pressure between about 0.5 psi to 1.5 psi. 
     In one embodiment the platen is rotated at a velocity from about 20 rpm (rotations per minute) to about 120 rpm, and the polishing head is rotated at a velocity from about 20 rpm to about 120 rpm and also moved linearly at a velocity from about 0.3 cm/s (centimeters per second) to about 3 cm/s in a direction radial to the platen. The preferred ranges for a 300 mm diameter substrate are a platen rotational velocity from about 40 rpm to about 100 rpm and a polishing head rotational velocity from about 40 rpm to about 100 rpm and a linear (e.g., radial) velocity of about 2 cm/s. A removal rate of phase change alloy of between about 100 nm/min to about 145 nm/min can be achieved by the processes described herein. A down force pressure of 0.5 psi to 1.0 psi may also be used to vary the removal rate. 
     Optionally, a rinse solution may be applied to the substrate after the polishing process to remove particulate matter and spent reagents from the polishing process as well as help minimize metal residue deposition on the polishing articles and defects formed on a substrate surface. The rinse solution includes deionized water and at least one component in the deionized water. The component is selected from the group consisting of polyethylene imine, polyethylene glycol, polyacrylic amide, alcohol ethoxylates, polyacrylic acid, an azole containing compound, benzo-triazole, and combinations thereof. The rinse solution has a pH between 2 and 12. For example, the rinse solution may be formed by mixing 10 to 14 liters of deionized water with 300 milliliters of a 1-10% component solution. 
     In one embodiment of the invention, the slurry has a particle size of about 50 nm. The slurry is modified by adding polyacrylic amide or other polymers from 0.1%-0.5% by weight. The modified slurry may be diluted with deionized water in a ratio of 1:0 to 1:9. In this embodiment, the removal rate may be from 400 Å/min to 2000 Å/min with a down force from 0.5 psi to 2 psi. In another embodiment of the invention, the slurry may be a commercially available slurry that is modified according to the above conditions. One example of a commercially available slurry from Cabot Microelectronics located in Aurora, Ill. is iCue® EP-C7092. 
     After polishing, some stains may remain on the polishing pad, which may be byproducts of the GST alloy on the pad. To prevent unwanted particulate transfer from pad to other substrates, a pad cleaning solution may be used. The pad cleaning solution is phosphoric acid based with about 0.2%-2.0% of phosphoric acid and about 0.2%-10% hydrogen peroxide. Other organic acids may also be added such as citric acid at 0.1%-2.0%. Using a pad cleaning solution as described, the stains may be removed from the pad in about one minute. 
     The pad cleaning solution may be applied between every wafer polishing or between multiple wafers polishing. In one embodiment, the pad cleaning solution is delivered onto the pad while the pad slowly rotates at 20 rpm or less, for example at about 5 rpm. The pad is soaked for about 30 seconds and then conditioned for about 20 seconds. After which, the pad is rinsed for 5 seconds with deionized water at a fast rotation of the platen or pad at 80 rpm or more, for example at about 100 rpm. 
     Referring to  FIG. 5 , a flow chart of one embodiment of the polishing method  500  is described herein. A substrate is positioned on a platen containing a polishing pad (box  502 ). The substrate has a phase change alloy material disposed thereon, such as GST. A polishing slurry is delivered to the polishing pad (box  504 ) where the polishing slurry comprises colloidal particles with a particle size less than 60 nm and in an amount between 0.2% and 10% by weight of the slurry, a pH adjustor, a chelating agent comprising at least one carboxylic acid, an oxidizing agent in an amount less than 1% by weight of the slurry, and polyacrylic acid. 
     The slurry may also include H 3 PO 4  in an amount between 50 ppm to 5000 ppm. In one embodiment, the slurry includes colloidal particles such as silica, alumina, modified silica with alumina, surface coated alumina-silica, or surface modified silica with organic groups. In embodiments that have surface modified alumina or modified silica, the pH may be between 6 and 7. In another embodiment, the polyacrylic acid is in an amount between 50 ppm and 5000 ppm. The carboxylic acid is chosen from the group consisting of citric acid, oxalic acid, tartaric acid, and succinic acid, or combinations thereof. The polishing slurry may include any of the embodiments described herein. 
     The substrate on the platen is then polished to remove a portion of the phase change alloy (box  506 ). A rinse solution is used to rinse the substrate on the platen (box  508 ). The rinse duration may be between 5 and 30 seconds. The rinse solution includes deionized water and at least one component in the deionized water selected form the group comprising polyethylene glycol, polyacrylic amide, polyethylene imine, an azole containing compound, benzo-triazole, and combinations thereof. In some embodiments, two or more components may be used in the rinse solution. The rinse solution has a pH between 2 and 12 and 300 milliliters of the component may be mixed in 10 to 14 liters of deionized water. Rinsing the substrate may be performed before, during, or after polishing the substrate. 
     In another embodiment, a pad cleaning solution may be used to clean any stains remaining on the polishing pad after polishing the phase change alloy (box  510 ). The cleaning solution is delivered onto the pad at a slow pad rotation followed by soaking the pad for 30 seconds with the pad cleaning solution. Next, the pad is conditioned for 30 seconds, and rinsed with deionized water for about 5 seconds at a fast rotation of the platen. Cleaning the polishing pad may be between about 30 seconds and about 90 seconds. The pad cleaning may include 0.2-2% phosphoric acid and 0.2%-5% hydrogen peroxide, such as a 1% hydrogen peroxide solution. Further, 0.1%-2.0% citric acid may also be added to the cleaning solution. 
     It has been observed that substrate planarized by the processes described herein have exhibited reduced topographical defects, such as dishing, reduced residues, improved planarity, and improved substrate finish. According to embodiments of the invention, a 50% decrease in defects was observed. 
     While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.