Abstract:
A chemical mechanical polishing apparatus including a platen for holding a pad having a polishing surface, a subsystem for holding a substrate and the polishing surface together during a polishing step, and a temperature sensor oriented to measure a temperature of the polishing surface, wherein the subsystem accepts the temperature measured by the sensor and is programmed to vary a polishing process parameter in response to the measured temperature. In an aspect, a chemical mechanical polishing apparatus having a platen for holding a pad having a polishing surface, a fluid delivery system for transporting a fluid from a source to the polishing surface, and a temperature controller which during operation controls the temperature of the fluid transported by the delivery system.

Description:
TECHNICAL FIELD 
       [0001]    This invention relates to methods and apparatus for chemical mechanical polishing (CMP) of semiconductor substrates, and more particularly to temperature control during such chemical mechanical polishing. 
       BACKGROUND 
       [0002]    Integrated circuits are typically formed on substrates, such as silicon wafers, by the sequential deposition of various layers such as conductive, semiconductor or insulating layers. After a layer is deposited, a photoresist coating can be applied on top of the layer. A photolithographic apparatus, which operates by focusing a light image on the coating, can be used to remove portions of the coating, leaving the photoresist coating on areas where circuitry features are to be formed. The substrate can then be etched to remove the uncoated portions of the layer, leaving the desired circuitry features. 
         [0003]    As a series of layers are sequentially deposited and etched, the outer or uppermost surface of the substrate tends to become increasingly non-planar. This non-planar surface presents problems in the photolithographic steps of the integrated circuit fabrication process. For example, the ability to focus the light image on the photoresist layer using the photolithographic apparatus may be impaired if the maximum height difference between the peaks and valleys of the non-planar surface exceeds the depth of focus of the apparatus. Therefore, there is a need to periodically planarize the substrate surface. 
         [0004]    Chemical mechanical polishing (CMP) is one accepted method of planarization. Chemical mechanical polishing typically includes mechanically abrading the substrate in a slurry that contains a chemically reactive agent. During polishing, the substrate is typically held against a polishing pad by a carrier head. The polishing pad may rotate. The carrier head may also rotate and move the substrate relative to the polishing pad. As a result of the motion between the carrier head and the polishing pad, chemicals, which can include a chemical solution or chemical slurry, planarize the non-planar substrate surface by chemical mechanical polishing. 
         [0005]    The CMP process, designed to remove nonplanarity, nevertheless can lead to non-planar artifacts. For example, the fluid dynamics of the slurry, coupled with the mechanical aspects of the system can lead to turbulence variations across the polishing pad/substrate, proportional to the relative speed of rotation. These turbulence variations are believed to lead to erosion in the substrate which can result in deviations from planarity, contrary to the goal of the CMP. This erosion can be countered in part by also moving the substrate in relation to the CMP polishing pad, but such erosion is not entirely eliminated. Another defect or deviation in planarity which can arise from CMP is “dishing” or differential polishing and/or erosion which occurs between different material layers, typically material layers of different hardness. For example, when CMP breaks through an overlying hard layer, e.g. of oxide, an underlying layer of softer metal can be “dished.” Consequently, there is a need in the art to improve the ability of CMP to planarize a substrate and to reduce non-planar side-effects of CMP such as erosion and dishing. 
       SUMMARY 
       [0006]    Applicants have discovered that controlling temperature during CMP can lead to improved planarization, reduced erosion, and reduced dishing. In particular, Applicants have discovered that, for example, in CMP of copper using a slurry with ammonium persulphate (APS) oxidizer, dishing and erosion can depend on the temperature at the surface of a polishing pad and the temperature of the polishing slurry, where dishing is increased with decreasing temperature, whereas erosion is increased with increasing temperature. 
         [0007]    In general, in various aspects, the invention features a chemical mechanical polishing apparatus with a platen for holding a pad having a polishing surface, a subsystem for holding a substrate against the polishing surface during a polishing process, and a temperature sensor oriented to measure a temperature of the polishing surface. The subsystem accepts the temperature measured by the sensor and is programmed to vary a polishing process parameter in response to the measured temperature. 
         [0008]    Various implementations may include one or more or the following. The subsystem may hold the substrate against the polishing surface with a controlled pressure, and the polishing process parameter may be the controlled pressure. A carrier head may hold the substrate. A pressure controller may control the pressure with which the subsystem holds the substrate against the polishing surface. A processor may be electrically connected to the pressure controller. The pressure controller may control the pressure by regulating a flow of compressed fluid to the carrier head. A relative velocity between the substrate and the polishing surface may be the polishing process parameter. A chemical solution delivery system may deliver a chemical solution with a concentration to the polishing surface, and the polishing process parameter may be the concentration. 
         [0009]    In some aspects, a chemical mechanical polishing apparatus has a platen for holding a pad having a polishing surface, a fluid delivery system for transporting a fluid from a source to the polishing surface, and a temperature controller which during operation controls the temperature of the fluid transported by the delivery system. 
         [0010]    Several implementations may include one or more of the following. A heating/cooling element may adjust the temperature of the fluid. The apparatus may have a processor for controlling the temperature of the fluid. The source from which the fluid is transported may be a water tank. 
         [0011]    In various aspects, a method for polishing a surface of a substrate includes polishing the surface of the substrate with a polishing surface during a polishing process characterized by a plurality of process parameters, repeatedly monitoring a temperature of the polishing surface during the polishing process, and controlling one of the plurality of process parameters in response to the monitored temperature so as to achieve a target value for the monitored temperature. 
         [0012]    Some implementations may include one or more of the following. One of the plurality of process parameters may be a controlled pressure with which the substrate is held against the polishing surface. The pressure may be increased if the monitored temperature is below the target temperature, and the pressure may be decreased if the monitored temperature is above the target temperature. One of the plurality of process parameters may include a relative velocity between the polishing surface and the surface of the substrate. A chemical solution with a concentration may be delivered to the polishing surface, and one of the plurality of process parameters may be the concentration. 
         [0013]    In various aspects of the invention, a method for polishing a surface of a substrate includes transporting a fluid to a polishing surface and controlling the temperature of the fluid. 
         [0014]    A potential advantage of the chemical mechanical polishing apparatus described herein is that it can significantly reduce temperature variations during a polishing operation and from one polishing operation to the next. This, in turn, can improve the repeatability of the polishing process. 
         [0015]    The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
     
    
     
       DESCRIPTION OF DRAWINGS 
         [0016]      FIG. 1  is a block diagram of the main components of a chemical mechanical polishing system as described herein; 
           [0017]      FIG. 2  is a block diagram of a control system for controlling the carrier head in a polishing apparatus, such as the polishing apparatus of  FIG. 1 ; and 
           [0018]      FIG. 3  is a block diagram of the main components of a chemical mechanical polishing system constructed according to various implementations of the present invention. 
       
    
    
       [0019]    Like reference symbols in the various drawings indicate like elements. 
       DETAILED DESCRIPTION 
       [0020]    The invention described herein generally relates to methods and apparatus for chemical mechanical polishing of substrates in order to planarize such substrates. Applicants have discovered that the planarization efficacy of CMP processing relates to the temperature of the process and temperature variation during the process. In particular, it is believed that CMP side effects such as erosion and dishing are related to temperature and temperature variation during the CMP process. In particular, Applicants have discovered, for example in CMP of copper using a slurry with ammonium persulphate (APS) oxidizer, that dishing and erosion can depend on the temperature at the surface of a polishing pad and the temperature of the polishing slurry, where dishing is increased with decreasing temperature, and erosion is increased with increasing temperature. Accordingly, the apparatus and methods described below are directed towards controlling the average temperature and reducing temperature variation during CMP planarization of substrates, particularly towards a target temperature that improves planarization. The described methods and apparatus lead to improved planarization efficacy during CMP of substrates, with reduced side-effects such as erosion and dishing. 
         [0021]    Referring to  FIG. 1 , a chemical mechanical polishing (CMP) apparatus  10  includes a flat platen  12  with an attached or applied polishing pad  14 . Platen  12  is mounted on the end of a drive shaft  18  of a motor  20 , which rotates platen  12  during a polishing operation. Platen  12  may be made of a thermally conductive material, e.g., aluminum, and can include within its interior an array of fluid circulation channels  22  through which a coolant or heating fluid can be circulated during use. A pump  24  collects fluid from a reserve tank  25  through a reservoir outlet tube  25   a.  Pump  24  supplies fluid to channels  22  via an inlet tube  26  and collects the fluid flowing out of circulation channels  22  through an outlet tube  28 . Pump  24  returns fluid to reserve tank  25  through reservoir inlet tube  25   b.  A heating/cooling element  30  encircling reserve tank  25  can heat or cool the fluid flowing through the circulation system, e.g., to a predetermined temperature, thereby controlling the temperature of platen  12  during the polishing operation. The heating/cooling element can include heating and cooling elements known to the art. For example, a heating element can include a resistive electrical heating element, an infrared heating element, a heat exchanging system which directs a heated fluid through an exchange jacket or coil at reserve tank  25 , and the like. A cooling element can include a heat exchanging system which directs a cooled fluid through an exchange exchange jacket or coil at the reserve tank  25 , a Peltier element, and the like. A heating or cooling element can be employed to heat or cool platen  12  and a substrate at platen  12 . For example, an infrared heating element can be employed to heat platen  12  and a substrate at platen  12 . The infrared heating element can be positioned over the platen to direct infrared heat onto the polishing pad. A temperature controller  32 , which includes a temperature sensor  33  for monitoring the temperature of the fluid, is electrically connected to heating/cooling element  30 . Based on the signal supplied by sensor  33 , controller  32  operates heating/cooling element  30 , for example, to bring the fluid to a predetermined temperature. 
         [0022]    Typically, polishing pad  14  is adhesively attached to platen  12 . Polishing pad  14  can be, for example, a traditional polishing pad, a fixed abrasive pad, or the like. An example of a traditional pad is an IC1000 pad (Rodel, Newark, Del.). Polishing pad  14  provides a polishing surface  34 . 
         [0023]    A carrier head  36  faces platen  12  and holds the substrate during the polishing operation. Carrier head  36  is typically mounted on the end of a drive shaft  38  of a second motor  40 , which can rotate head  36  during polishing and at the same time that platen  12  is also rotating. Various implementations may further include a translation motor that can move carrier head  36  laterally over the surface of polishing pad  14 , for example, while carrier head  36  is rotating. 
         [0024]    Carrier head  36  can include a support assembly, e.g., piston-like support assembly  42 , which can be surrounded by an annular retaining ring  43 . Support assembly  42  has a substrate-receiving surface, such as a flexible membrane, inside of the central open region within retaining ring  43 . A pressurizable chamber  44  behind support assembly  42  controls the position of the substrate-receiving surface of support assembly  42 . By adjusting the pressure within chamber  44 , the pressure with which the substrate is pressed against the polishing pad can be controlled. More specifically, an increase in the pressure within chamber  44  causes support assembly  42  to push the substrate against polishing pad  14  with greater force, and a decrease in the pressure within chamber  44  reduces that force. 
         [0025]    This document presents typical elements of the CMP apparatus that relate to the invention described herein. Additional details about the structure and operation of typical CMP are known, for example, U.S. Pat. No. 5,738,574, incorporated herein by reference in its entirety. 
         [0026]    In various implementations, a pressure controller  46  in cooperation with source of pressure, e.g., a compressed air source  48  (e.g. container of pressurized air or a air pump) can control the pressure in chamber  44 . Pressure controller  46  can include a pressure sensor  50  for sensing the pressure in chamber  44 . Pressure sensor  50  is depicted within pressure controller  46 , but may alternatively be located at any place from which the pressure within the chamber  44  could be effectively monitored. Pressure controller  46  operates a valve, e.g., electronically controllable valve  52 , to flow air into chamber  44  and to release air from chamber  44 , thereby controlling the pressure within chamber  44 . 
         [0027]    To perform the polishing operation, a supply delivery tube  54  delivers a polishing liquid  56  to the surface of polishing pad  14 . In various implementations, polishing pad  14  comprises an abrasive, and polishing liquid  56  is typically a mixture of water and chemicals that aid in the polishing process. In some implementations, the polishing pad does not contain an abrasive, and polishing liquid  56  may contain an abrasive in a chemical mixture. In several implementations, both polishing pad  14  and polishing liquid  56  can include an abrasive. 
         [0028]    A pipe  58  connects delivery tube  54  to a supply reservoir  60 . A heating/cooling element  62  encircles reservoir  60  and provides a way of heating and/or cooling the polishing liquid, e.g., to a desired constant temperature, before it is delivered to the polishing pad. A temperature controller  64 , which operates heating/cooling element  62 , uses a thermal sensor  65  to monitor the temperature of the slurry and adjusts the power delivered to heating/cooling element  62  to control the slurry temperature. 
         [0029]    An IR sensor  66  located at polishing surface  34  is oriented to sense the temperature of polishing surface  34  adjacent to carrier head  36 , for example, when carrier head  36  is in contact with polishing surface  34 . A programmed computer or special purpose processor  68  can monitor the output of IR sensor  66  and can control pump  24 , temperature controller  32 , pressure controller  46 , and temperature controller  64 , as described in greater detail below. 
         [0030]    The polishing system can also include a pad rinse system, such as a water delivery tube  100  that delivers deionized water  102  to the surface  34  of polishing pad  14 . A pipe  104  connects delivery tube  100  to deionized water tank  106 . A heating/cooling element  108  encircles tank  106  and provides a way of heating and/or cooling the water before it is delivered to polishing pad  14 . A temperature controller  110 , which operates heating/cooling element  108 , uses a thermal sensor  112  to monitor the temperature of the water and adjusts the power delivered to heating/cooling element  108  to achieve the desired water temperature. 
         [0031]    During polishing, carrier head  36  holds substrate  16  against polishing surface  34  while motor  20  rotates platen  12  and motor  40  rotates carrier head  36 . Supply delivery tube  54  delivers a mixture of water and a chemical to polishing surface  34 . After polishing, debris and excess slurry can be rinsed from the pad surface by water from the water delivery tube  100 . 
         [0032]    During the polishing process, which is partially chemical in nature, the polishing rate depends on the temperature at of substrate  16  and polishing surface  34 . More specifically, the polishing rate increases when the temperature increases and it decreases when the temperature decreases. Further, it is believed that undesirable side-effects such as erosion and dishing increase with temperature variation and/or temperature deviation, where dishing is increased with decreasing temperature, and erosion is increased with increasing temperature. To achieve a more uniform and repeatable polishing rate, and to reduce side effects such as erosion and dishing, temperature in CMP can be regulated, particularly towards a target temperature that improves planarization, in one or more ways as follows. 
         [0033]    First, the temperature at polishing surface  34  can be partly regulated by controlling the temperature of the fluid circulating through fluid circulating channels  22 . Because the platen is made of a thermally conductive material, the temperature of the fluid in the channels can directly and quickly influence the temperature of the polishing pad. Computer  68  can set a target temperature of temperature controller  32 , then adjusts the power delivered to heating/cooling element  30  to control the temperature of the fluid, e.g., holding it at the target temperature. Thus, the target temperature can be reached, and temperature variations can be reduced. 
         [0034]    The temperature at polishing surface  34  may also be regulated by controlling the temperature of liquid that is delivered to polishing surface  34 . Polishing pad  14  may have insulating properties. Therefore, even if the temperature of platen  12  is controlled as described above, it may not provide as much control of the temperature of polishing surface  34  as desired. Additional temperature control at polishing surface  34  may include delivering liquid at a controlled temperature to polishing surface  34 , such as polishing fluid  56 , delivered through liquid delivery tube  54 . Temperature controller  64  senses the temperature of the polishing fluid in tank  62 . Computer  68  can set a target temperature, and temperature controller  64  can then adjust the power delivered to heating/cooling element  62  to control the temperature of the fluid, e.g., to the target temperature. Thus, the target temperature can be reached, and temperature variations can be reduced. 
         [0035]    A second liquid delivered to surface  34  can be deionized water  102 , delivered through water delivery tube  100 . Temperature controller  110  can sense the temperature of the water in water tank  106 . Temperature controller  106  can adjust the power delivered to heating/cooling element  108  to control the temperature of the water, e.g., to a pre-set target temperature. Water delivery tube  100  delivers deionized water, e.g., at a target temperature, to polishing surface  34 , for example, for several seconds prior to the initiation of a polishing step. The polishing surface  34  can thereby be brought to the target temperature when the polishing step begins. This procedure can improve process repeatability. 
         [0036]    Referring also to  FIG. 2 , the temperature of substrate  16  during a CMP process can also be controlled by controlling the pressure with which substrate  16  is pressed against polishing surface  34  during polishing. The pressure between substrate  16  and surface  34  in part determines the friction. Increasing the pressure results in a higher friction and thus a higher temperature; conversely, decreasing the pressure results in lower friction and thus a lower temperature. Thus, computer  68  can vary the pressure in order control the temperature of polishing surface  34 , for example, towards a target temperature or to reduce temperature variation. 
         [0037]    The pressure which substrate  16  exerts against polishing surface  34  during processing can be controlled in the following manner. Using IR sensor  66 , computer  68  can monitor the temperature of polishing surface  34 . Computer  68  can be programmed to compare the temperature at sensor  66  to a predetermined target temperature profile. If the measured temperature is above the target temperature profile, computer  68  causes pressure controller  46  to reduce the pressure applied to substrate  16 , e.g. by reducing the pressure in the chamber  44  in carrier head  36  (see  FIG. 1 ). If the measured temperature is below the target temperature profile, computer  68  can cause pressure controller  46  to increase the pressure applied to substrate  16  by increasing the pressure in chamber  44 . Thus, computer  68  can control the temperature, for example at a predetermined target value throughout the polishing process. This process can be as short as 1-2 minutes for a given substrate. 
         [0038]    Typically, during a polishing run the temperature of polishing surface  34  will increase until a stable temperature is reached. One approach for establishing the target temperature to be used by computer  68  is to monitor a “good” polishing run to examine temperature variation throughout the run as a function of time, and at a fixed pressure. This measured temperature can be selected as the target temperature for similar runs. That is, computer  68  simply controls the pressure applied to the substrate for each run so that the temperature of the polishing surface follows the measured curve of a good polishing run. Thus, computer  68  tends to ensure that the averaged polishing rate of each polishing run is repeatable, thereby providing consistent results. A “good polishing run” occurs when temperature control leads to effective planarization with an acceptable amount of dishing and/or erosion. 
         [0039]    The temperature of substrate  16  during a CMP process can also be controlled by controlling the relative velocity with which platen  12  and carrier head  36  rotate with respect to each other. The friction between substrate  16  and surface  34  is determined in part by the relative velocity between substrate  16  and surface  34 . The relationship between the relative velocity and friction can be calculated. Then, the relative velocity can be adjusted to decrease friction if the temperature of polishing surface  34  is too high, or to increase friction if the temperature of polishing surface  34  is too low. For example, computer  68  can vary rotational velocities generated by motor  20  and/or motor  40  in order to control the temperature of polishing surface  34 , e.g., towards a target temperature. 
         [0040]    The relative velocity between platen  12  and carrier head  36  can be controlled in the following manner. Using IR sensor  66 , computer  68  monitors the temperature of polishing surface  34 . Computer  68  can be programmed to compare the sensed temperature to a predetermined target temperature profile. If the measured temperature is above or below the target temperature profile, computer  68  can proportionately changes the rotational velocity of motor  20  and/or motor  40 . Thus, computer  68  controls the temperature, e.g., at a predetermined target value during the polishing process. 
         [0041]    Typically, during a polishing run the temperature of polishing surface  34  will increase until a stable temperature is reached. In various implementations, the target temperature used by computer  68  can be selected by monitoring a “good” polishing run to examine temperature variation throughout the run as a function of time, while at a fixed relative velocity of substrate  16  to polishing surface  34 . This measured temperature can be selected as the target temperature for similar runs. Thus, computer  68  can control the relative velocity between substrate  16  and polishing surface  34 , so that the temperature of the polishing surface follows the measured curve of a good polishing run. Thus, computer  68  tends to ensure that the averaged polishing rate of each polishing run is repeatable, and thus leads to consistent results. A “good polishing run” occurs when temperature control leads to effective planarization with reduced dishing and/or erosion. 
         [0042]    Referring to  FIG. 3 , the temperature of substrate  16  during a CMP process can be controlled by controlling the composition of polishing liquid  56 . Polishing liquid  56  is delivered to polishing surface  34  by supply/rinse tube  54 . Pipes  70  and  72  connect tube  54  to chemical solution reservoir  74  and water tank  76 , respectively. Valves  78  and  80  control flow of liquid from pipes  70  and  72  to tube  54 , respectively. Computer  68  can control valves  78  and  80 . The temperature of substrate  16  can depend in part on the rate of reaction of polishing liquid  56  with a surface of substrate  16 . The rate of reaction of polishing liquid  56  with a surface of substrate  16  can be directly proportional to the polishing rate. Increasing the concentration of chemical solution can increase the rate of reaction, and hence can increase the polishing rate. Decreasing the concentration of chemical solution can decrease the rate of reaction, and hence can decrease the polishing rate. 
         [0043]    The composition of polishing liquid  56  can be controlled in the following manner. Using IR sensor  66 , computer  68  can monitor the temperature of polishing surface  34 . Computer  68  can be programmed to compare the sensed temperature to a predetermined target temperature profile. If the measured temperature is above the target temperature profile, computer  68  can adjust valve  78  to decrease the flow of chemical solution from chemical solution reservoir  74 . Alternatively, computer  68  can adjust valve  80  to increase the flow of water from water tank  76 . This adjustment or adjustments can decrease the concentration of the chemical solution on polishing surface  34 , thus decreasing the polishing rate. On the other hand, if the measured temperature is below the target temperature profile, computer  68  can adjust valve  78  to increase the flow of chemical solution from chemical solution reservoir  74 . Alternatively, computer  68  can adjust valve  80  to decrease the flow of water from water tank  76 . This adjustment or adjustments can increase the concentration of the chemical solution on polishing surface  34 , thus increasing the polishing rate. 
         [0044]    Typically, during a polishing run the temperature of polishing surface  34  will increase until a stable temperature is reached. In various implementations, the target temperature used by computer  68  can be established by monitoring a “good” polishing run to examine temperature variation throughout the run as a function of time, and with a fixed concentration of chemical solution in water. This measured temperature can be selected as the target temperature for similar runs. Thus, computer  68  can control the concentration of the chemical solution in water, so that the temperature of the polishing surface follows the measured curve of a good polishing run. Computer  68  thus tends to ensure that the averaged polishing rate of each polishing run repeatable, leading to consistent results. A “good polishing run” occurs when temperature control leads to effective planarization with reduced dishing and/or erosion. If the measured temperature varies from the target temperature by more than a threshold amount, one or more of the polishing parameters, e.g., the pressure on the substrate, pressure on the retaining ring and/or slurry flow rate, can be adjusted to bring the temperature back toward the target temperature. The target temperature can be a constant through the polishing process. Moreover, the actual polishing rate can be allowed to drift during polishing, i.e., the feedback loop for the polishing parameters is based on keeping the temperature constant rather than keeping the polishing rate constant. 
         [0045]    Other embodiments are within the following claims. For example, in systems in which coolant can be delivered to the platen to regulate the temperature of the polishing surface, the platen can be made of any appropriate thermally conducting material, besides aluminum as described above. In addition, instead of measuring the temperature of the polishing surface with an IR monitor, other known techniques for measuring the temperature of the polishing surface can be employed, e.g. a thermocouple installed in the platen or embedded in the polishing pad. Also, other ways of controlling the pressure between the substrate and the polishing pad may be employed. For example, rather than applying pressure to the backside of the substrate, the entire carrier head can be moved vertically by an actuator (e.g., a pneumatic actuator, electromagnetic actuator, or the like) to control the pressure on the substrate. Furthermore, the temperature of the polishing liquid or water delivered to the polishing surface can be controlled by heating or cooling elements placed at locations in the delivery systems other than the locations described. In addition, liquid may be delivered to the polishing surfaces through multiple delivery tubes, with an independent temperature controller controlling the temperature of the liquid in each tube. 
         [0046]    A multi-step metal polishing process, e.g., copper polishing, can include a first polishing step in which bulk polishing of the copper layer is performed at a first platen with a first polishing pad without temperature control but using an in-situ monitor to halt the polishing step, and a second polishing step in which the barrier layer is exposed and/or removed and using the temperature control procedure discussed above. 
         [0047]    A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention.