Patent Publication Number: US-7720562-B2

Title: Polishing method and polishing apparatus

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a polishing method and a polishing apparatus which are useful for polishing and flattening a surface (surface to be polished) of a substrate, such as a semiconductor wafer. 
   2. Description of the Related Art 
   In the formation of integrated circuits on a surface of a substrate, such as a semiconductor wafer, it is a general practice to deposit an insulating film, a conductive film or a semiconductive film, etc. on the surface of the substrate, and form integrated circuit interconnects in the deposited film. For the formation of such interconnects, lithography of an integrated circuit pattern with light or electron beams is carried out. In order to form fine interconnects, it is necessary to make the width of a lithography pattern as narrow as possible, which requires a shallower focus depth. This necessitates flattening of a surface of a semiconductor wafer on which lithography is to be carried out. As a method for the flattening, polishing by a chemical mechanical polishing (CMP) apparatus is generally practiced. A multi-stage CMP process is known which comprises polishing a laminate of films formed in a surface of a substrate, such as a semiconductor wafer, in a plurality of process steps. 
   For example, a substrate W to be polished, as shown in  FIG. 6A , is prepared by forming trenches  302  in an insulating layer  300  and forming a barrier film  304  of, e.g., SiN on a surface of the insulating layer  300 , and then depositing an oxide film  306  on a surface of the barrier film  304  while filling the oxide film  306  into the trenches  302 . In an exemplary multi-step CMP process, a first polishing step of the surface of the substrate W is carried out to polish the oxide film  306  partway, and then a second polishing step is carried out to polish the remaining oxide film  306  and a predetermined amount (to a target value) of the barrier film  304 , as shown in  FIG. 6A . In this case, the first polishing step may be carried out by using a polishing liquid (slurry) which has a high polishing rate for the oxide film  306 , though low in its surface irregularities-eliminating property for the oxide film  306 , and then the second polishing step may be carried out by using a polishing liquid which has a high surface irregularities-eliminating property for the oxide film  306 , though low in the polishing rate for the oxide film  306 , so as to increase the polishing amount of the oxide film  306  in the first polishing step and to thereby shorten the overall polishing time. 
   As shown in  FIG. 6A , when the oxide film  306  is deposited on the insulating layer  300  in which the trenches  302  are formed, depressions are formed in those portions of the surface of the oxide film  306  which correspond to the trenches  302  provided in the insulating layer  300 . In order to increase the throughput, it is desirable to polish the oxide film (uppermost-layer film)  306  as much as possible in the first polishing step using a polishing liquid having a high polishing rate for the oxide film  306  and to minimize the polishing of the oxide film  306  in the second polishing step using a polishing liquid having a low polishing rate for the oxide film  306 . However, the first polishing step using a polishing liquid having a low surface irregularities-eliminating property cannot flatten the depressions in the surface of the oxide film  306 , and the depressions need to be eliminated by the second polishing step. It is therefore necessary to terminate the first polishing step when the oxide film (uppermost-layer film)  306  is partly left. 
   On the other hand, the overall polishing time becomes longer if the second polishing step is initiated when the oxide film (uppermost-layer film)  306  remains in excess. Further, since a polishing liquid used in the second polishing step generally has low polishing ability for the lower-layer barrier film  304 , the second polishing step needs to be carried out over a considerably long time. When the second polishing step is thus carried out over a long time, excessive polishing may occur in the surface of the oxide film  306  in the trenches  302 , forming a depression having a depth “d”, as shown in  FIG. 6B , which may cause dishing or erosion. Thus, the second polishing step should desirably be initiated with an optimal thickness of the oxide film  306  to be polished. 
   However, a thickness of a film to be polished, like the oxide film  306 , forming an uppermost surface layer of a substrate, such as a semiconductor wafer, generally varies among substrates. In addition, the polishing rate of an uppermost-layer film can decrease, e.g., due to deterioration of a consumable member of a polishing apparatus. It has therefore been generally difficult to make a thickness of an uppermost-layer film constant at the start of the second polishing step. 
   SUMMARY OF THE INVENTION 
   The present invention has been made in view of the above situation in the related art. It is therefore an object of the invention to provide a polishing method and a polishing apparatus which make it possible to initiate a second polishing step of a workpiece with an optimal thickness of an uppermost-layer film to be polished, without being influenced by possible variation in the initial thickness of the uppermost-layer film among workpieces. 
   In order to achieve the object, the present invention provides a polishing method for carrying out a plurality of steps of polishing on a workpiece having a plurality of films to be polished, comprising: measuring a thickness of a film, forming the uppermost layer of a workpiece, before polishing, and then carrying out a first polishing step to polish the uppermost-layer film partway and a subsequent second polishing step to polish the remaining uppermost-layer film and a next-layer film, the first and second polishing steps being carried out under preset polishing conditions; determining the polishing rates of the uppermost-layer film in the first and second polishing steps based on the measured thickness of the uppermost-layer film and on the processing time taken to polish the uppermost-layer film in the first and second polishing steps; and measuring a thickness of a film, forming the uppermost layer of a predetermined nth workpiece, before polishing and, based on the measured thickness and on said polishing rates of the uppermost-layer film, setting a processing time for the first polishing step of the nth workpiece or a next predetermined nth workpiece. 
   By thus carrying out a multi-step polishing process for a workpiece after measuring a thickness of an uppermost-layer film of the workpiece, determining the polishing rates of the uppermost-layer film in the first and second polishing steps based on the measured thickness of the uppermost-layer film and on the processing time taken to polish the uppermost-layer film in the first and second polishing steps, and determining a processing time for the first polishing step of a predetermined nth workpiece based on the pre-determined polishing rates and on the thickness of an uppermost-layer film of the nth workpiece before polishing, it becomes possible to equalize a thickness of the uppermost-layer film at the start of the second polishing step for every workpiece. 
   The predetermined nth workpiece may be the next unpolished workpiece. 
   By applying the feedback of the polishing time to the next unpolished workpiece, a polishing rate can be set which responds to a change in polishing performance, e.g., due to deterioration of a consumable member of the polishing apparatus. 
   Preferably, the processing time for the first polishing step of the nth workpiece is set so that the thickness of the uppermost-layer film becomes a predetermined thickness at the start of the second polishing step of the nth workpiece. 
   In a preferred aspect of the present invention, the first polishing step is carried out by moving a polishing table, having a polishing face, and a top ring, holding the workpiece and pressing it against the polishing face, relative to each other, and a change of polishing object from the uppermost-layer film to the next-layer film is detected by detecting the torque of a drive section for driving the polishing table or the top ring. 
   By detecting a change of polishing object from the uppermost-layer film to the next-layer film by detecting the torque of a drive section for driving the polishing table or the top ring, it becomes unnecessary to transport, e.g., an optical device for measurement of a film thickness between the first polishing step and the second polishing step, which requires cleaning and drying of the workpiece, thereby leading to an increased throughput. 
   The present invention also provides a polishing apparatus comprising: a polishing section for carrying out a first polishing step of a film forming the uppermost layer of a workpiece and a second polishing step of the remaining uppermost-layer film and a next-layer film; a measurement section for measuring a thickness of the uppermost-layer film of the workpiece before polishing; and a control section for determining the polishing rates of the uppermost-layer film in the first and second polishing steps based on the thickness of the uppermost-layer film before polishing, measured with the measurement section, and on the processing time taken to polish the uppermost-layer film in the first and second polishing steps, and setting a processing time for the first polishing step of a predetermined nth workpiece based on said polishing rates of the uppermost-layer film and on the thickness of a film forming the uppermost layer of the nth workpiece before polishing. 
   Preferably, the processing time for the first polishing step of the nth workpiece is set so that the thickness of the uppermost-layer film becomes a predetermined thickness at the start of the second step of polishing of the nth workpiece. 
   In a preferred aspect of the present invention, the polishing section includes a polishing table having a polishing face, and a top ring for holding the workpiece and pressing the workpiece against the polishing face, and the change of polishing object from the uppermost-layer film to the next-layer film is detected by detecting the torque of a drive section for driving the polishing table or the top ring. 
   The polishing section may have a first polishing table for carrying out the first polishing step, and a second polishing table for carrying out the second polishing step. 
   Alternatively, the polishing section may have a polishing table for successively carrying out the first and second polishing steps. 
   The present invention also provides a program for causing a computer to control a polishing apparatus, for carrying out a plurality of steps of polishing on a workpiece having a plurality of films to be polished, to perform operations of: determining the polishing rates of a film, forming the uppermost layer of a workpiece, in a first polishing step and a second polishing step based on the thickness of the uppermost-layer film before polishing and on the polishing time taken to polish the uppermost-layer film in the first and second polishing steps; and based on said polishing rates and on the thickness of a film, forming the uppermost layer of a predetermined nth workpiece, before polishing, setting a processing time for the first polishing step of the nth workpiece. 
   The predetermined nth workpiece may be the next unpolished workpiece. 
   Preferably, the processing time for the first polishing step of the nth workpiece is set so that the thickness of the uppermost-layer film becomes a predetermined thickness at the start of the second polishing step of the nth workpiece. 
   According to the polishing method and the polishing apparatus of the present invention, in carrying out a multi-step polishing process on workpieces, such as substrates, each having multi-layer films to be polished, a thickness of the uppermost-layer film at the start of the second polishing step can be equalized for all the workpieces even when there is variation in the initial thickness of the uppermost-layer film among the workpieces. Furthermore, the present invention makes it possible to formulate a polishing recipe taking into account wear of a polishing member, thereby preventing dishing or erosion in a surface of a film after polishing. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a plan view of the overall layout of a polishing apparatus according to an embodiment of the present invention; 
       FIG. 2  is a plan view of the overall layout of a polishing apparatus according to another embodiment of the present invention; 
       FIG. 3  is a schematic enlarged view of a polishing section of the polishing apparatus of  FIG. 1 ; 
       FIG. 4  is a control block diagram of the polishing apparatus of  FIG. 1 ; 
       FIG. 5A  is a diagram illustrating a two-step polishing process according to the present invention, and  FIG. 5B  is a graph showing the relationship between polishing time and the current value (torque) of a top ring motor in the second step of the two-step polishing process; and 
       FIG. 6A  is a diagram illustrating a conventional two-step polishing process, and  FIG. 6B  is a diagram illustrating the surface state of a substrate as observed when the second step of the conventional two-step polishing process is carried out over a long time. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Preferred embodiments of the present invention will now be described with reference to the drawings. The following description illustrates the case of using a substrate, such as a semiconductor wafer, as a workpiece, and polishing and flattening a surface (surface to be polished) of a substrate. 
     FIG. 1  shows a plan view of the overall layout of a polishing apparatus according to an embodiment of the present invention. As shown in  FIG. 1 , in the polishing apparatus, unpolished substrates (workpieces), such as semiconductor wafers, stocked in a cassette  204  are taken one by one by a transport robot  202 , which moves on traveling rails  200 , out of the cassette  204 , and placed on a substrate stage  206 . The unpolished substrate on the substrate stage  206  is transferred by a transport robot  208  onto a rotary transporter  210 , while a polished substrate is transferred by the transport robot  208  from the rotary transporter  210  onto the substrate stage  206 . The polished substrate on the substrate stage  206  is returned by the transport robot  202  into the cassette  204 . The unpolished substrate on the rotary transporter  210  is held by a top ring  1 , which is described below, and moved to a position on a polishing table  100  to carry out polishing of the substrate. The polishing apparatus is thus systematized so that a plurality of substrates can be polished successively. 
   The polishing apparatus includes cleaning machines  212 ,  214  for cleaning and drying a substrate after polishing, a polishing table  216  for carrying out a second polishing step of a substrate surface, dressers  218 ,  220  for carrying out dressing of the polishing tables  100 ,  216 , and a water tub  222  for cleaning the dresser  218 . The polishing apparatus is designed to be capable of carrying out two or more steps of polishing with one polishing table  100  by switching a plurality of polishing liquids and a plurality of polishing conditions (polishing recipes). 
     FIG. 2  shows a plan view of the overall layout of a polishing apparatus according to another embodiment of the present invention. As shown in  FIG. 2 , the polishing apparatus is provided with three loading/unloading stages  600  each for placing a cassette. A traveling mechanism  601  is provided along the loading/unloading units  600 . A first transport robot  602  having two hands is provided on the traveling mechanism  601 . Adjacent to the traveling mechanism  601  is disposed the below-described ITM  224 . Hands of the first transport robot  602  are accessible to each of the cassettes on the loading/unloading units  600  and the ITM  224 . 
   The polishing apparatus shown in  FIG. 2  is provided with four polishing sections  604 ,  605 ,  606  and  607 . These polishing sections  604 ,  605 ,  606 , and  607  are disposed along the longitudinal direction of the apparatus. Each polishing section includes a polishing table  608  with a polishing surface, a top ring  609  for holding a substrate, such as a semiconductor wafer, and polishing a substrate by pressing it against the polishing table  608 , a polishing liquid supply nozzle  610  for supplying a polishing liquid or a dressing liquid (e.g., water) onto the polishing table  608 , a dresser  611  for carrying out dressing of the polishing table  608 , and an atomizer  622  for spraying a mixed fluid of a liquid (e.g., pure water) and a gas (e.g., nitrogen) in the form of a mist from one or more than one nozzle to the polishing surface. 
   Adjacent to the polishing sections  604 ,  605  is disposed a first linear transporter  612  for transporting a substrate along the longitudinal direction. An turn over device  613  for turning over a substrate received from the first transport robot  602  is disposed above the first linear transporter  612  on the loading/unloading stage  600  side. Adjacent to the polishing sections  606 ,  607  is also disposed a second linear transporter  614  for transporting a substrate along the longitudinal direction. 
   The polishing apparatus includes a second transport robot  615 , a turn over device  616  for turning over a substrate received from the second transport robot  615 , four cleaning machines  617 ,  618 ,  619  and  620  for cleaning a polished substrate, and a transfer unit  621  for transferring a substrate between the inverter  616  and the cleaning machines  617 ,  618 ,  619  and  620 . The second transport robot  615 , the inverter  616  and the cleaning machines  617 ,  618 ,  619  and  620  are disposed in series along the longitudinal direction. 
   In the operation of such a polishing apparatus, a substrate in the cassette is carried in each of the polishing sections  604 ,  605 ,  606  and  607  via the turn over device  613 , the first linear transporter  612 , and the second linear transporter  614 . A polished substrate is carried in each of the cleaning machines  617 ,  618 ,  619  and  620 , via the second transport robot  615  and the turn over device  616 , where the substrate is cleaned. The substrate after cleaning is returned to the cassette by the first transport robot  602 . 
   Although four polishing tables are provided in this embodiment so that each set of two polishing tables carries out two-step polishing of a substrate, it is also possible to use four tables to carry out four-step polishing of a substrate. 
   Each of these polishing apparatuses is provided with an ITM (in-line thickness monitor)  224  as a measurement section for measuring a surface state, such as a thickness of a film to be polished, of a substrate before polishing or after post-polishing cleaning/drying. In particular, the ITM (measurement section)  224  is disposed at a location on a line extending from the traveling rails  200 , as shown in  FIG. 1 , and measures a thickness of an insulating film such as an oxide film, or the polishing state of a conductive film such as a copper film or a barrier layer, in a surface of a substrate, such as a semiconductor wafer, using an optical means which emits light toward the substrate surface and receives an optical signal of the reflected light, before the transport robot  202  places the substrate after polishing into the cassette  204  or after the transport robot  202  takes the substrate before polishing out of the cassette  204 . 
   Each of these polishing apparatuses is designed to be capable of detecting the removal of a conducive film from a substrate surface except a necessary region, such as an interconnect region, or the removal of an insulating film by monitoring sensor signals or measured values of such films during and/or after polishing of the substrate, determining polishing conditions for the respective steps of a multi-step polishing process and the endpoint of the polishing process, and repeating an appropriate polishing process. The ITM  224  is capable of measuring the surface state of a substrate over an entire surface (surface to be polished), so that the results of polishing at a particular portion of the substrate and the results of polishing over the entire substrate surface can be checked. 
   The polishing section of the polishing apparatus holds a substrate such as a semiconductor wafer and a polishing object, and presses the substrate against a polishing surface on a polishing table, thereby polishing and flattening the surface of the substrate.  FIG. 3  shows in detail a polishing section of the polishing apparatus shown in  FIG. 1 . As shown in  FIG. 3 , below the top ring  1  is disposed a polishing table  100  with a polishing pad (polishing cloth)  101  attached to the upper surface. Above the polishing table  100  is disposed a polishing liquid supply nozzle  102  which supplies a polishing liquid (slurry) Q onto the polishing pad  101  on the polishing table  100 . The top ring  1  is movable also to a position right above the polishing table  216  with a polishing pad (polishing cloth)  217  attached to the upper surface. The polishing table  216  is designed to make a so-called scroll movement. In the interior of the polishing table  216  is provided a polishing liquid supply section (not shown) for supplying a polishing liquid onto the polishing pad  217 . The polishing section is thus constructed. 
   According to the polishing apparatus shown in  FIG. 1 , a first polishing step of a substrate is carried out with the polishing table  100  and a second polishing step of the substrate is carried out with the polishing table  216  while the substrate is kept held by the top ring  1 . A polishing liquid (slurry) Q, which has a high polishing rate for a film forming the uppermost layer of the substrate, such as an oxide film  306  (see  FIG. 5 ), though low in its surface irregularities-eliminating property for film, is supplied to the polishing pad  101  of the polishing table  100 . On the other hand, a polishing liquid (slurry) Q, which has a high surface irregularities-eliminating ability for the uppermost-layer film, such as the oxide film  306 , though low in the polishing rate for the film, is supplied to the polishing pad  217  of the polishing table  216 . 
   Various commercially-available polishing pads can be used as the polishing pads  101 ,  217 . Examples include SUBA 800,IC-1000 and IC-1000/SUBA 400 (two-layer cloth), manufactured by Rodel, Inc., and Surfin xxx-5 and Surfin 000, manufactured by Fujimi Incorporated. SUBA 800, Surfin xxx-5 and Surfin 000 are non-woven fabrics each comprising fibers fixed with a polyurethane resin, and IC-1000 is a rigid foamed polyurethane (single layer). The foamed polyurethane is porous, and has numerous fine recesses or holes in the surface. The polishing pads  101 ,  217  basically are consumable members, and gradually wear out as they polish a surface of a substrate. In an actual polishing process, polishing pads  101 ,  217  are replaced with new ones when the polishing pads  101 ,  217  have come to a predetermined thickness or the polishing rates have become lower. 
   As shown in  FIG. 1 , the top ring  1  is connected via a universal joint  10  to a top ring drive shaft  11 , and the top ring drive shaft  11  is coupled to a top ring air cylinder  111  secured to a top ring head  110 . The top ring drive shaft  11  moves vertically by the top ring air cylinder  111 , thereby moving the entire top ring  1  up and down and pressing a retainer ring  3 , fixed to the lower end of a top ring body  2 , against the polishing table  100  or  216 . The top ring air cylinder  111  is connected to a compressed air source  120 . The pressure of fluid, such as pressurized air, supplied to the top ring air cylinder  111  can be regulated, whereby the pressure of a substrate, held by the top ring  1 , on the polishing pad  101  or  217  can be adjusted. 
   The top ring drive shaft  11  is coupled via a key (not shown) to a rotating cylinder  112  which is provided with a timing pulley  113  at its outer surface. A top ring motor  114  as a rotational. drive section, which is provided with a timing pulley  116 , is secured to a top ring head  110 . The timing pulley  113  is connected to the timing pulley  116  via a timing belt  115 . Thus, by rotationally driving the top ring motor  114 , the rotating cylinder  112  and the top ring drive shaft  11  rotate by the timing pulley  116 , the timing belt  115  and the timing pulley  113 , whereby the top ring  1  rotates. The top ring head  110  is supported by a top ring head shaft  117  secured to a frame (not shown). 
   The top ring motor  114  is provided with a torque measurement section  122  for measuring the torque of the motor  114 . For example, when an insulating film on a substrate is removed and a metal film, formed under the insulating film, becomes exposed to a polishing surface during polishing of the substrate surface, the torque of the top ring motor  114  changes due to a change in the frictional force between the substrate surface and the polishing surface. The removal of the insulating film can be determined by detecting the change in the torque with the torque measurement section  122 . Though the torque measurement section  122  measures the electric current of the top ring motor  114  in this embodiment, it may be one that actually measures the torque of the top ring motor  114 . Though, in this embodiment, the torque measurement section  122  is provided in the top ring motor  114 , it is also possible to provide a torque measurement section in a polishing table motor for rotating the polishing table  216 . 
   As shown in  FIG. 3 , signals from the ITM  224  and the torque measurement section  122  are inputted into a control section  400 . As shown in  FIG. 4 , based on input from an input section  401 , e.g., comprised of a man-machine interface, such as an operation panel, and input from a host computer  402  that performs various data processings, the control section  400  controls the polishing apparatus to polish a substrate W at a target polishing rate (polishing amount) so as to obtain a target profile, such as an intended surface configuration. 
   A description will now be made of a polishing method according to the present invention, executed by the control section  400  of the polishing apparatus. In this embodiment, a substrate W to be polished, as shown in  FIG. 5A , is prepared by forming interconnect trenches  302  in an insulating layer  300  and forming a barrier film  304  of, e.g., SiN on a surface of the insulating layer  300 , and then depositing an oxide film  306  on a surface of the barrier film  304  while filling the oxide film  306  into the trenches  302 . A first polishing step of the substrate W is carried out to polish the oxide film  306  partway, and then a second polishing step is carried out to polish the remaining oxide film  306  and a predetermined amount (to a target value) of the barrier film  304 , as shown in  FIG. 5A . 
   According to the polishing apparatus shown in  FIG. 1 , substrates W housed in the cassette  204  are transported one by one by the transport robots  202 ,  208  to the rotary transporter  210  and held by the top ring  1 . The substrate W held by the top ring  1  is subjected to a first polishing step with the polishing table  100 , and then to a second polishing step with the polishing table  216 . The substrate W after polishing is cleaned and dried by the cleaning machines  212 ,  214 , and then retuned to the cassette  204 . 
   In advance of the polishing process, the substrate W before polishing is transported by the transport robot  202  to the ITM  224  to measure the thickness of the oxide film  306  forming the uppermost layer of the substrate W. The thickness of the oxide film  306 , which has been formed in the pre-polishing process, generally varies among substrates. In order to formulate a polishing recipe that compensates for the variation, the thickness of the oxide film (uppermost-layer film)  306 , a polishing object, is measured before polishing. When a pre-polishing processing apparatus and the polishing apparatus can share information by a network or the like, information on a thickness of the uppermost-layer oxide film  306  of a substrate, if measured in the pre-polishing processing apparatus after the processing of the substrate, can be shared with the polishing apparatus. Thus, in this case, the pre-polishing measurement of a film thickness in the polishing apparatus is unnecessary. 
   The substrate W after the measurement of the thickness of the oxide film  306  is transported by the transport robot  208  to the rotary transporter  210 , where the substrate W is held by the top ring  1 . Using the polishing table  100 , the first polishing step of the substrate W is carried out under preset polishing conditions. In particular, while rotating the polishing table  100  and the top ring  1 , the substrate W held by the top ring  1  is pressed against the polishing pad  101  of the polishing table  100  at a predetermined pressure and, at the same time, a polishing liquid Q is supplied from the polishing liquid supply nozzle  102  to the polishing pad  101  of the polishing table  100 . By carrying out the first polishing step using a polishing liquid (slurry) Q having a high polishing rate for the oxide film  306  but having a low surface irregularities-eliminating property for the oxide film  306 , the polishing amount of the oxide film  306  is increased and the overall polishing time is shortened. 
   The first polishing step is terminated with the oxide film  306  slightly left on the barrier film  304  and not completely removed, as shown in  FIG. 5A . The termination of the first polishing step is, for example, by time control. Thus, the first polishing step is terminated after carrying out the polishing for a predetermined time based on polishing time data in the polishing recipe. 
   Next, the substrate W after the first polishing step, which is kept held by the top ring  1 , is moved to right above the polishing table  216 . Using the polishing table  216 , the second polishing step of the substrate W is carried out under preset polishing conditions. In particular, while rotating the polishing table  216  and the top ring  1 , the substrate W held by the top ring  1  is pressed against the polishing pad  217  of the polishing table  216  at a predetermined pressure and, at the same time, a polishing liquid is supplied through the polishing liquid supply section formed in the polishing table  216  to the polishing pad  217  of the polishing table  216 . In the second polishing step, the oxide film  306  on the barrier film  304  is completely polished, and the barrier film  304  is polished to a polishing target (in a predetermined amount), as shown in  FIG. 5A . 
   By carrying out the second polishing step using a polishing liquid (slurry) having a lower polishing rate for the oxide film  306  than that of the first polishing step but having a higher surface irregularities-eliminating property for the oxide film  306  than that of the first polishing step, the oxide film  306  remaining on the barrier film  304  can be completely removed while flattening the surface of the oxide film  306 . The termination of the second polishing step is, for example, by time control. Thus, a polishing time after the change of polishing object from the oxide film  306  to the barrier film  304  in the second polishing step is set in the polishing recipe, and the end point of polishing is determined based on the set time. The change of polishing object from the oxide film  306  to the barrier film  304  is detected by signals from the torque measurement section  122 . In particular, when polishing in the second step shifts from the oxide film  306  having a thickness B to the barrier film  304  having a thickness C as shown in  FIG. 5A , the current value (torque) of the top ring motor  114  gradually increases and, after complete removal of the oxide film  306 , the current value (torque) of the top ring motor  114  gradually decreases. The time point “t 1 ” when the current value (torque) has reached the maximum is therefore regarded as the time of the change of polishing object from the oxide film  306  to the barrier film  304 , and a time period from the time point t 1 , “t 1 -t 2 ”, is set to control time at the end of polishing. 
   After the second polishing step, is the substrate W is transported transported by the transport robot  208  from the rotary transporter  210  to the cleaning machine  214  and then to the cleaning machine  212 , where the substrate is cleaned and dried. Thereafter, the substrate W is transported by the transport robot  202  to the ITM  224  to measure the thickness of the barrier film  304  after polishing and the number of surface portions where dishing or erosion has occurred. The substrate W after the measurement is returned by the transport robot  202  to the cassette  204 . 
   Data on the thickness of the oxide film (uppermost-layer film)  306  before polishing, the processing (polishing) time of the first polishing step, the processing (polishing) time (t 1 ) for the oxide film  306  in the second polishing step, the processing (polishing) time (t 1 -t 2 ) for the barrier film (next-layer film)  304  in the second polishing step and the thickness of the barrier film  304  after the second polishing step, with respect to the substrate W, is stored in a database in the control section  400 . Based on these data and on the ratio of the polishing rate for the oxide film  306  between the polishing liquid used in the first polishing step and the polishing liquid used in the second polishing step, the control section  400  calculates the polishing rates for the oxide film  306  in the first and second polishing steps. 
   After the calculation of the polishing rates, the next substrate W taken out of the cassette  204  is transported to the ITM  224  before polishing to measure the thickness of the oxide film  306 . Based on the measured film thickness and on the above polishing rates calculated by the control section  400 , a polishing time for the first polishing step is newly set. A first polishing step of the substrate W held by the top ring  1  with the polishing table  100  is carried out over the newly-set time and terminated. The polishing amount A of the oxide film  306  in the first polishing step is thus corrected, and the second polishing step can be started with the intended thickness of the oxide film  306 . 
   The above-described setting of polishing conditions or formulation of polishing recipe makes it possible to ensure the intended thickness of a film forming the uppermost layer of a substrate W, such as the oxide film  306 , at the start of the second polishing step without being influenced by the initial thickness of the uppermost-layer film. In addition, feedback of data on the last substrate makes it possible to formulate a polishing recipe taking account of a decrease in the polishing rate, e.g., due to deterioration of a consumable member used in the polishing process. 
   Furthermore, the two-step polishing process according to the present invention eliminates the need for film thickness measurement between the first and second polishing steps, thus increasing the throughput. 
   In this embodiment, the two sets of two tables  100 ,  216  are operated in parallel. In the parallel operation, the database or the control section  400  stores data on polishing or formulates a polishing recipe independently for each set of tables  100 ,  216 . It is also possible to carry out the first and second polishing steps with one polishing table  100  by supplying two different types of polishing liquids Q to the polishing pad  101  of the polishing table  100 . Also in this case, the database or the control section  400  stores data on polishing or formulates a polishing recipe independently for each of the two polishing tables  100  of  FIG. 1 . The same is true for each of the four polishing tables  608  of  FIG. 2 . 
   In this embodiment, the next substrate is in a standby state until the polishing rate of the oxide film  306  of the substrate W is calculated. On the other hand, in a so-called series operation in which the first polishing step and the second polishing step are carried out using different top rings and polishing tables, during shift from the first polishing step of a substrate using a first top ring and a first polishing table to the second polishing step using a second top ring and a second polishing table, the next substrate is transported to the first top ring. In such an operation, the polishing recipe for the next substrate is based on the polishing rate of the last-but-one substrate. Thus, during shift from the first polishing step of an nth substrate to the second polishing step, the next of (n+1)th substrate is subjected to the first polishing step. The polishing recipe for the (n+1)th substrate is based on the polishing rate of the (n−1)th substrate which has already finished polishing. 
   The previous description of embodiments is provided to enable a person skilled in the art to make and use the present invention. Moreover, various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles and specific examples defined herein may be applied to other embodiments. Therefore, the present invention is not intended to be limited to the embodiments described herein but is to be accorded the widest scope as defined by limitation of the claims and equivalents.