Abstract:
A comprehensive measurement of properties of a polishing pad of a CMP apparatus is used to create a database by which the CMP apparatus can be maintained and the polishing process can be precisely controlled. The measuring apparatus includes a measuring table and a control section. The measuring table has a flat top surface on which the polishing pad is placed, a camera, a sensor for sensing the relative location of the top surface of the polishing pad, a bracket to which the camera and the sensor are fixed, and an X-Y drive for moving the bracket in X- and Y-directions orthogonal to each other. The control section controls the operation of the camera, the sensor and the X-Y drive, and processes signals from the camera and sensor, so that a profile of the surface of the polishing pad can be discerned and an image of the surface of the polishing pad can be produced. The control section also assigns values to the sensed data and displays the values of the measured data, graphs and a surface image of the polishing pad. The measuring apparatus can also measure the hardness of the polishing pad and the transmittance through a transparent window of the polishing pad.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a method and apparatus for measuring various properties of a polishing pad. More specifically, the present invention relates to a method and apparatus for measuring various properties of a polishing pad used to carry out the chemical and mechanical polishing of semiconductor wafers.  
           [0003]    2. Description of the Related Art  
           [0004]    Recently, as the need arises for more highly integrated semiconductor devices, a multi-layered structure has been used to yield a higher number of semiconductor chips per wafer. The multi-layered structure has been realized through the use of a conductive wiring pattern electrically connecting a plurality of layers on a semiconductor wafer. A planarization process is required to form such a multi-layered structure.  
           [0005]    Chemical mechanical polishing (hereinafter referred as CMP) is a known planarization technique for forming the above-mentioned multi-layered structure. In CMP, the wafer is polished by the motion of a polishing pad relative to the wafer while the wafer is under pressure and a polishing solution or a slurry is provided between the wafer and the polishing pad.  
           [0006]    The polishing pad carries out two major functions. That is, the polishing pad simultaneously causes the wafer to mechanically abrade and facilitates a chemical reaction between the slurry and the wafer. To this end, the polishing pad causes the polishing solution or slurry to flow smoothly by means of many small pores and grooves open at the surface of the polishing pad. Also, the polishing pad removes reactants from the surface of the wafer by means of its foam cell walls. The small pores have diameters of about 30˜70 μm so that the slurry deposited on the surface of the wafer can be stored temporarily in the pores. Hence, the Material-Removal-Rate (hereinafter referred to as MRR), namely the rate of polishing as a function of the pressure between the wafer and the polishing pad, can be kept constant during the polishing process. Also, a Within-Wafer-Non-Uniformity (hereinafter referred to as WIWNU) of the MRR is kept to a minimum.  
           [0007]    However, during the CMP process, an elastic deformation occurs in the polishing pad due to a stress concentration resulting from a difference in density or size of device patterns on the wafer. Generally, the larger the stress concentration, the higher the MRR is. The uniformity of the MRR is also influenced by the hardness of the polishing pad. Generally, though, a hard polishing pad has a good local planarizing characteristic for inner portions of a chip, but generates a defect on a surface of the chip.  
           [0008]    The polishing pads used in CMP apparatus are generally classified into two types, namely a non-woven fabric type and a foamed cross-linked polymer type. The non-woven fabric type of polishing pad is manufactured by impregnating or coating a non-woven fabric (e.g., polyester felt) with a cross-linked polymer (e.g., polyurethane resin). On the other hand, the foamed cross-linked polymer type of polishing pad is manufactured, for the most part, by coating a non-woven fabric pad with foamed polyurethane.  
           [0009]    Both types of polishing pads provide a great number of pores open over the entire surface of the polishing pad. As mentioned earlier, the pores temporarily store the polishing solution and provide the polishing solution for the surface of the wafer. Such polishing pads of a CMP apparatus are mainly used for polishing a glass substrate or a mono-crystalline silicon wafer. In addition, new polishing pads are continually being developed to provide better performance during the CMP process. For example, the IC-1000 (manufactured by Rodel Inc. U.S.A) has been recently used in CMP apparatus without producing scratches on the surface of the wafer and offers the same performance as the IC-60 (also manufactured by Rodel Inc. U.S.A.).  
           [0010]    Now, again, the performance of a polishing pad is basically decided by the hardness, surface state and compressibility of the polishing pad as these characteristics relate to the material being polished.  
           [0011]    As concerns hardness, if the hardness of the polishing pad is not uniform over the entire polishing pad, the magnitude of a load applied to the polishing pad varies at different portions of the polishing pad. Thus, the thickness of the polishing pad becomes non-uniform during the polishing process. Consequently, the wafer is not planarized correctly. Hence, polishing pads should have a hardness tailored to the particular polishing operation.  
           [0012]    A polishing pad for polishing an insulating film should have a hard and rough surface to remove a reactant of the insulation film produced as the result of the chemical etching of the film by means of the slurry. In the case of polishing a metal such as aluminum, a ductile polishing pad is preferably used because the aluminum is prone to being damaged and contaminated due to its own ductility. When such a ductile polishing pad is used for polishing process, the RMM is about 3,000 Å/min and the selectivity of aluminum with respect to an oxide is about 40:1. On the other hand, a hard polishing pad is advantageous when polishing a hard metal such as tungsten. In the case of polishing tungsten, the RMM is about 2,000 Å/min and the selectivity with respect to an oxide is about 20:1. When polishing copper, a polishing pad having a medium hardness should be used. In this case, the RMM is about 4,000 Å/min and the selectivity thereof with respect to an oxide is about 100:1  
           [0013]    Generally, the lower the density of the polishing pad is, the higher the MRR is. Also, the larger the compressibility is, the higher the MRR is.  
           [0014]    As concerns compressibility, the extent to which a polishing pad will deform under compression has a direct effect on the evenness of the wafer and uniformity of a residual thin film of the wafer. With this in mind, a hard polishing pad, i.e., having a small compressive deformability, should be used when polishing a stepped surface of a wafer. For instance, an IC-1000 polishing pad, manufactured by Rodel Inc. U.S.A., is usually used in this case. Also, a second generation IC-series pad, namely the IC-1400 polishing pad, offers an extended lifetime and improvement in the uniformity of the surface of a wafer. The IC-1400 polishing pad has a structure of two layers—a surface layer and a lower layer. The surface layer is formed using the IC-1000 polishing pad, and the lower layer is made of an independent foamed material serving as a buffer layer to improve water permeability and a slight change of a compressive characteristic of the polishing pad, thereby increasing the uniformity of the surface of the wafer. Furthermore, a surface of the second generation polishing pad has concentric grooves formed therein.  
           [0015]    The Q-2000 polishing pad (also a trade name of Rodel Inc. U.S.A.) has been developed for the purpose of decreasing a dependency of the polishing pad on the conditioner to improve a local planarity of the wafer. This pad, too, has a structure consisting of two layers. The surface layer thereof comprises a foamed polymer sheet having a high degree of hardness, and in which grooves are formed to ensure a smooth and uniform flow of the slurry.  
           [0016]    As described above, the polishing pads are designed and selected for use based on the type and surface characteristics of the material to be polished. Because the polishing pads play a very important role in the CMP process, a strict management of the maintenance and deployment of the polishing pads is required in fabricating semiconductor devices.  
           [0017]    To this end, the polishing pads are periodically replaced according to product specifications provided by the manufacturer. The product specifications are conservative in their approach to preventing a polishing pad from damaging a wafer. The reliance on the product specifications to manage the replacement of the polishing pads increases the manufacturing cost of the semiconductor devices because the polishing pad is sometimes replaced even if the used polishing pad is still functional. Furthermore, the end user of a CMP apparatus generally buys the polishing pad with the CMP apparatus from the manufacturer, and operates the CMP apparatus according to specifications provided by the manufacturer. Then, after the useful life of the polishing pad has expired, the end user mounts a new polishing pad to the apparatus. However, because the characteristics of the replacement polishing pad may differ slightly from the characteristics described in the specifications, operating the CMP apparatus according to the specifications usually results in a processing error. That is, a wafer can be damaged by a polishing pad even when the replacing of the polishing pads are being scheduled according to the product specifications of the manufacturer.  
           [0018]    Meanwhile, the polishing pad has a transparent window by which the polishing of the wafer can be monitored. Accordingly, when to end the CMP process can be determined by monitoring the surface state of the wafer through the transparent window.  
           [0019]    Now, when foreign material becomes lodged in the pores of the polishing pad, such foreign material may microscopically scratch the surface of the wafer. Furthermore, the gaps between adjacent pores and/or the depths of the pores are altered and made irregular by the foreign material. Hence, the condition of the slurry is changed by these changes in the gaps and/or depths of the pores which, in turn, leads to changes in the efficacy of the CMP process. Another problem that sometimes occurs is that the light transmittance through the transparent window of the polishing pad changes. When this occurs, the end point of the polishing process cannot be detected accurately. Thus, the wafer may be polished excessively or insufficiently.  
           [0020]    As described above, the planarization of the wafer is influenced during the CMP process by various properties of the polishing pad; these properties include the surface profile, hardness, distribution and uniformity of the pores, and the degree of transparency of the window. Accordingly, such properties of the polishing pad need to be measured precisely if a wafer is to be sufficiently and uniformly polished.  
           [0021]    U.S. Pat. No. 5,934,974, issued to Tzeng Huey-Ming, discloses a technique of measuring the degree of wear of a polishing pad by using a non-contact laser sensor. According to the patent, a device is mounted on a CMP apparatus to measure the thickness of the polishing pad during the polishing process without interrupting the process. Although the device can be easily adapted for use with a belt type of CMP apparatus, it is difficult to incorporate the device into a rotary type of CMP apparatus.  
           [0022]    U.S. Pat. No. 5,974,679, issued to Birang, et al, discloses a technique of measuring a surface profile of the polishing pad by bringing a sensor into contact with a surface of the polishing pad. In this system, the sensor is part of a measuring apparatus mounted on a rotary plate of the CMP apparatus for measuring the thickness of the polishing pad. According to the Birang et al. patent, the measuring apparatus can measure the surface profile of the polishing pad only when the CMP apparatus stops because, as mentioned above, the measuring apparatus is mounted on the rotary plate of the CMP apparatus.  
           [0023]    Japanese Patent Laid-open Publication No. 8-61949 discloses a technique of measuring the profile of a polishing pad by using a laser sensor and simultaneously measuring the profile of the rotary plate using an excess current sensor.  
           [0024]    In all of the conventional techniques described above, a measuring device mounted on the CMP apparatus itself is used to scan the polishing pad in the radial direction to detect a profile of the polishing pad in one direction.  
           [0025]    However, a typical CMP apparatus does not have enough space to accommodate such measuring devices. Also, the mounting structure for the measuring devices complicates the overall structure of the CMP apparatus. Furthermore, the conventional measuring devices can only measure the surface profile or thickness of the polishing pad. That is, the conventional measuring devices can not conduct a comprehensive measurement of the properties of the polishing pad.  
         SUMMARY OF THE INVENTION  
         [0026]    Accordingly, an object of present invention is to overcome the aforementioned problems and limitations of the prior art.  
           [0027]    More specifically, an object of the present invention is to provide a method of and apparatus for measuring and discerning properties of a polishing pad, such as the profile, surface state, hardness and transmittance through the transparent window thereof, before and/or after the polishing pad is used in a CMP apparatus.  
           [0028]    A further object of the present invention is to provide a method of and apparatus for measuring and discerning properties of a polishing pad to produce data by which the CMP process can be maintained and managed efficiently.  
           [0029]    A still further object of the present invention is to provide a method of and apparatus for discerning the wear of a used polishing pad to provide data useful for estimating the optimal operating parameters of the CMP apparatus.  
           [0030]    The apparatus for measuring properties of a polishing pad according to the present invention includes a measuring table and a control section. The measuring table has a flat table top on which the polishing pad is placed. A camera for taking a picture of the top surface of the polishing pad, and a proximity sensor for measuring profile of the top surface of the polishing pad, are fixed to a bracket. Drive means move the bracket along the directions of X and Y axes in a plane above the polishing pad. The control section includes a control system for controlling the movement of the camera and the sensor along directions of the X and Y axes, as well as the operation of the camera and the sensor. Accordingly, the profile of the surface of the polishing pad can be discerned, and an image of the surface of the polishing pad can be produced. The control system also includes a display having a screen on which measured values, graphs and images can all be displayed  
           [0031]    The table top is an upper plate that is precision-made to have a high degree of surface flatness so that it will affect the measurements of the surface profile of the polishing pad as little as possible. On the other hand, the upper plate has a plurality of holes formed at a central portion thereof. A vacuum pump communicates with the holes to produce suction by which the polishing pad is fixed to the upper plate. Therefore, vibrations and the like will not disturb the polishing pad when the polishing pad is being measured.  
           [0032]    The sensor for sensing the profile of the polishing pad is preferably a non-contacting laser sensor.  
           [0033]    The apparatus for measuring properties of the polishing pad may further include a hardness-measuring sensor for measuring the hardness of the polishing pad and a transmittance sensor for measuring the transmittance through a transparent window in the polishing pad. These sensors may also be mounted by the bracket to the drive means. Preferably, however, the transmittance sensor is fixed to the upper plate so as to overhang the edge of the upper plate.  
           [0034]    The transmittance sensor may comprise a light-receiving element that is disposed coplanar with the top surface of the upper plate, and a light-emitting element that is disposed a predetermined distance above the light-receiving element so that the polishing pad may be inserted between the light-receiving element and the light-emitting element.  
           [0035]    The drive means includes a Y-axis linear drive mechanism and an X-axis linear drive mechanism. The Y-axis linear drive mechanism comprises a Y-axis carrier extending along a first side of the upper plate, a Y-axis guide rail extending parallel to the Y-axis carrier along a second side of the top surface of the upper plate, a Y-axis slider supported by the Y-axis carrier so as to be movable longitudinally therealong, and a Y-axis guide rail slider supported by the Y-axis guide rail so as to be movable longitudinally therealong. The X-axis linear drive mechanism comprises an X-axis carrier disposed above the top surface of the upper plate and having opposite ends respectively mounted to the Y-axis slider and the Y-axis guide rail slider; and an X-axis slider supported by the X-axis carrier so as to be movable longitudinally therealong. The bracket to which the sensors are mounted is fixed to the X-axis slider so as to move therewith.  
           [0036]    A respective feed screw or linear motor is connected each of the X-axis slider and Y-axis slider for moving the same along the X-axis and Y-axis carriers.  
           [0037]    In a method of measuring properties of a polishing pad of a CMP apparatus according to the present invention, the polishing pad is placed on the table top and fixed thereto using a vacuum. The polishing pad is then scanned in the directions of the X and Y axes to locate the center of the polishing pad. Once the center of the polishing pad is located, the hardness thereof is measured and a value of the hardness is displayed on the screen. Next, a profile of the polishing pad is discerned by moving the proximity sensor over the polishing pad in the direction of the Y axis, and the profile is displayed in the form of a graph on the screen. In addition, the surface of the polishing pad is scanned with the camera while pictures of the surface of the polishing pad are taken. These pictures are then used to display an image of the surface of the polishing pad on the screen.  
           [0038]    In addition, the transparent window of the polishing pad may be aligned with the transmittance sensor so that the transmittance through the transparent window is measured. Likewise, the measured value of the transmittance through the transparent window is displayed on the screen. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0039]    The above and other objects and advantages of the present invention will become more apparent from the following detailed description thereof made in conjunction with the accompanying drawings, of which:  
         [0040]    [0040]FIG. 1 is a schematic diagram of a rotary CMP apparatus;  
         [0041]    [0041]FIG. 2 is a plan view of a polishing pad of the rotary CMP apparatus shown in FIG. 1;  
         [0042]    [0042]FIG. 3 is a perspective view of measuring apparatus for measuring properties of a polishing pad according to the present invention;  
         [0043]    [0043]FIG. 4 is a perspective view of a measuring table of the measuring apparatus according to the present invention;  
         [0044]    [0044]FIG. 5 is a perspective view of a sensor array fixed to an X-axis slider of the measuring table;  
         [0045]    [0045]FIG. 6 is a perspective view of a transmittance sensor of the measuring table for measuring the transmittance of a transparent window of the polishing pad;  
         [0046]    [0046]FIG. 7 is a block diagram of a control section of the measuring apparatus according to the present invention;  
         [0047]    [0047]FIG. 8 is a flowchart of the overall operation of the measuring apparatus according to the present invention;  
         [0048]    [0048]FIGS. 9 and 10 are flowchart of a subroutine in which the measuring apparatus detects a center of the polishing pad according to the present invention;  
         [0049]    FIGS.  11  to  14  are front views of a display screen of the control section of the measuring apparatus according to the present invention; and  
         [0050]    [0050]FIG. 15 is a graph illustrating a surface profile of the polishing pad measured by means of a laser sensor according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0051]    Hereinafter, the present invention will be described in detail with reference to the accompanying drawings.  
         [0052]    Referring to FIG. 1, a CMP apparatus includes a rotary platen  10  and a polishing pad  12  attached to the top surface of the rotary platen  10 . As the polishing process progresses, the pores of the polishing pad are clogged with residual material generated by the polishing process. If left unchecked, the essential function of the pores, e.g., of storing polishing slurry, would be degraded. To counteract this problem, a conditioner head  14  is supported at the periphery of the polishing pad  12  in contact with the polishing pad  12 . The conditioner head  14  comprises a nickel plate having diamond particles electro-deposited on a surface thereof. The conditioner head  14  thus produces fine cuts in the surface of the polishing pad  12  to expose new pores that can then store the slurry.  
         [0053]    A rotary head  16  is supported at the periphery of the polishing pad  12 , symmetrically to the conditioner head  14  with respect to the center of the polishing pad. The rotary head  16  produces a vacuum by which a wafer  18  is adhered to the bottom surface thereof. The rotary head  16  moves the wafer  18  adhered thereto downward to press the wafer  18  against the polishing pad  12 . The rotary head  16  also rotates eccentrically relative to the polishing pad  12  so that the wafer  18  makes contact with the polishing pad  12  over an area wider than that of just the wafer  18 . The slurry constituting a polishing solution is supplied through a nozzle  20  situated above the polishing pad  12 .  
         [0054]    In addition, the polishing pad  12  and the wafer  18  are rotated in directions opposite to each other while the polishing solution is supplied between the polishing pad  12  and the wafer  18  so as to polish the wafer  18 .  
         [0055]    The pores that are formed in a surface of the polishing pad  12  have diameters of about 30˜70 μm so as to be capable of accommodating the slurry, of rendering the MRR (Material Removal Rate) constant and of minimizing the WIWNU (Within Wafer Non-Uniformity).  
         [0056]    [0056]FIG. 2 is a plan view of the polishing pad  12 . The polishing pad  12  is in the form of a disc and has a transparent window  12   a  at a location where the wafer is brought into contact with the rotary head  16 . The polishing pad  12  also has a plurality of concentric circular grooves  12   b . Laser light is transmitted to the surface of the wafer  18  through the transparent window  12   a , and is then reflected by the wafer  18 . The reflected light is transmitted back through the window  12   a  and is then received in a light sensor (as illustrated back in FIG. 1). The concentric grooves  12   b  provide space for storing and supplying the slurry. Accordingly, the shape and a distribution of the grooves  12   b  as well as those of the pores are very important factors affecting the polishing characteristics of the polishing pad  12 . In particular, these factors dictate the uniformity of contact between the slurry and the surface of the wafer  18 .  
         [0057]    [0057]FIG. 3 shows a measuring apparatus for measuring and discerning properties of the polishing pad  12 , namely, the surface profile, surface state, hardness and transmittance of a transparent window thereof. As will be described in further detail later on, the properties of the polishing pad,  12  before and after use in the CMP apparatus, can be filed in a database. The measuring apparatus of the present invention basically includes a measuring table  100  and a control section  200 .  
         [0058]    The control section  200  is in the form of a cabinet. A monitor  202  is disposed at the top of the control section  200  and a first tray  204  for supporting a keyboard  208  and a second tray  206  for supporting a mouse  210  are mounted at an intermediate portion of the control section  200 . A disc driver  212  for driving a data disc on which data is recorded, for example, a floppy disc, an optical disc or the like, is located at the center of a lower portion of the control section  200 . The control section  200  also includes a computer processor, an interface circuit board and a power supply.  
         [0059]    The measuring table  100  basically comprises an XY-carrier  110  to which a plurality of sensors are mounted, an additional transmittance sensor  120 , and a vacuum pump  130  disposed at the bottom of the measuring table  100 . Referring also now to FIG. 4, the XY-carrier  110  is installed on an upper plate  102  of the measuring table  100 . The transmittance sensor  120  is fixed to the front of the upper plate  102 . The upper plate  102  of the measuring table  100  is made by a precision machine-process so as to possess a high degree of surface flatness. A pad-accommodating section on which the polishing pad  12  is placed is formed at the center of the top surface of the upper plate  102 , and a plurality of vacuum holes  104  are formed in the pad-accommodating section. The vacuum pump  130  is connected to the plurality of vacuum holes  104  to produce a vacuum by which the polishing pad  12  is adhered to the upper plate  102  of the measuring table.  
         [0060]    The XY-carrier  110  comprises X-axis and Y-axis linear drive mechanisms. The Y-axis linear drive mechanism includes a Y-axis carrier  111 , a Y-axis slider  112 , a Y-axis guide rail  114 , and a Y-axis guide rail slider  115 . The X-axis linear drive mechanism includes an X-axis carrier  116  and an X-axis slider  117 . The Y-axis carrier  111  extends along the left edge of the upper plate  102  of the measuring table  100  (in the direction of a Y axis). The Y-axis guide rail  114  extends along the right edge of the upper plate  102  of the measuring table parallel to the Y-axis carrier  111 . The Y-axis carrier  111  carries the Y-axis slider  112  in the direction of the Y axis. The Y-axis guide slider  115  is guided for movement along the Y-axis guide rail in the direction of the Y axis.  
         [0061]    One end of the X-axis carrier  116  is fixed to the Y-axis slider  112  and the other end of the X-axis carrier is fixed to the Y-axis guide slider  115 . As a result, the X-axis carrier  116  can traverse the measuring table  100 .  
         [0062]    In addition, the X-axis linear drive mechanism and the Y-axis linear drive mechanism comprise feed screws  116   b  and  111   b  that are rotated by stepper motors  116   a  and  111   a , respectively (briefly refer to FIG. 7). The X-axis slider  117  and the Y-axis slider  112  are threaded to the feed screws  116   b  and  111   b , respectively, so as to move in the direction of the axis X and the Y axis when the feed screws  116   b  and  11   b  are rotated by the stepper motors  116   a  and  111   a . Alternatively, the X-axis linear drive mechanism and the Y-axis linear drive mechanism may comprise the stators of linear motors, and the X-axis slider  117  and the Y-axis slider  112  may respectively be integrated with the respective movers of the linear motors. In any case, the X-axis carrier  116  is moved in the direction of the Y axis, and the X-axis slider  117  is moved in the direction of the X axis. In this way, the X-axis slider  117  can be scanned across the pad-accommodating portion of the upper plate  102  in an XY-plane defined by the X and Y axes of the measuring table  100 .  
         [0063]    Referring to FIG. 5, a bracket  118  is mounted to the X-axis slider  117 , and a camera  140 , a laser sensor  150  and a hardness-measuring sensor  160  are fixed to the X-axis slider  117  by bracket  118 .  
         [0064]    Referring to FIG. 6, the transmittance sensor  120  for measuring the transmittance through the transparent window  12   a  of the polishing pad  12  includes a light-receiving element  122  and a light-emitting element  124 . The light-receiving element  122  is mounted to one end of a first holding arm  121 , the other end of which is fixed to the top portion of the measuring table  100 . The light-emitting element  124  is attached to an end portion of a second holding arm  123  disposed above and in parallel with the first holding arm  121  as spaced a predetermined distance therefrom. The first and second holding arms  121  and  123  have lengths that are sufficient to extend from an edge of the polishing pad  12  to the center of the transparent window  12   a  of the polishing pad  12 . The light-receiving element  122  and the light-emitting element  124  are positioned such that an extension of the top surface of the upper plate  102  of the measuring table  100  lies between the light-receiving sensor  122  and the light-emitting sensor  124 . In particular, the light-receiving sensor  122  lies coplanar with the top surface of the upper plate  102 .  
         [0065]    Accordingly, when the transmittance through the transparent window  12   a  is to be measured, the polishing pad can be supported on the top surface of the upper plate  102  with the transparent window  12   a  disposed in the path of light transmitted to the light-receiving element  122  from the light-emitting element  124  (FIG. 7).  
         [0066]    The camera  140  is, for example, a CCD (charge coupled device) camera having a high resolution and a high magnification. Such a camera is suitable for observing the surface of the polishing pad  12  using the naked eye. The camera  140  can also search for foreign matter in the grooves or the pores of the polishing pad  12 .  
         [0067]    The laser sensor  150  is a common sensor widely used for measuring the interference of laser light. More specifically, the laser sensor  150  directs a laser onto the surface of the polishing pad  12  and detects the interference pattern of light reflected by the surface of the polishing pad  12 . Hence, the relative position of the surface of the polishing pad can be detected, whereby the surface profile of the polishing pad  12  can be discerned.  
         [0068]    The hardness-measuring sensor  160  is a common sensor widely used for measuring hardness.  
         [0069]    Referring now especially to FIG. 7, the control section  200  comprises a control system that may include a personal computer, the sensors and camera connected to the personal computer through an interface board. In this case, each of the sensors and the camera can communicate with the interface board through an RS 232  serial bus or a USB (universal serial bus).  
         [0070]    Specifically, the control section  200  includes a microcomputer  218 , a memory  216  including a DRAM, a SRAM and an EPROM, a keyboard  208 , a mouse  210 , a hard disc driver  216 , a floppy disc driver  212 , a CD-ROM driver  214  and a monitor  202 . The microcomputer  218  is connected through a system bus to a hardness-measuring sensor interface  220 , a laser sensor interface  222 , a camera interface  224 , a motor operating portion  226  and a transmittance sensor interface  228 .  
         [0071]    As shown in FIG. 7, the profile, the surface state and the hardness of the polishing pad  12  are measured while the polishing pad  12  is adhered by a vacuum to a central portion (position A in FIG. 7) of the upper plate  102  of the measuring table  100 . On the other hand, the transmittance through the transparent window  12   a  is measured while the polishing pad  12  is positioned on the measuring table  100  with the transmittance sensor  120  aligned with the transparent window  12   a  (position B in FIG. 7).  
         [0072]    The operation of the measuring apparatus will now be described in more detail with reference to FIGS.  8 - 14 .  
         [0073]    Referring to FIG. 8, at first, the control system of the control section  200  is initialized (step S 1 ). In this step, the motors  111   a ,  116   a  are first initialized and then the sensors  120 , 140 ,  150  and  160  are initialized using the initialization screen shown in FIG. 11. Referring to FIG. 11, when the motors are initialized, the current X-axis and Y-axis positions of the motors are input, and “X-axis motor homing”, “Y-axis motor homing”, “motor initialization” and “manual motor movement” actions are performed. Also, when a sensor is initialized, a current sensor reading is input and then a “sensor initialization” action is performed.  
         [0074]    Furthermore, the environment is set up by inputting reference values before the properties of the polishing pad are measured. In order to set up the environment, a pad data reference value, a hardness reference value and a transmittance reference value of the polishing pad are inputted using the environment setup screen shown in FIG. 12, and then a “setup reference value” action is performed.  
         [0075]    When the system initialization is completed, a project start screen is displayed on the monitor  202 , as shown in FIG. 13 (step S 2 ). The project start screen includes an image display window for displaying images taken by the CCD camera  140 , a data display window for displaying the pad data and a graph display window for displaying a graph of the surface profile of the pad.  
         [0076]    The image display window includes an image display region, a “CCD” button for selecting the CCD camera, a “previous” button for selecting a previous image, a “delete” button for deleting a current image and a “next” button for selecting a next image.  
         [0077]    The data display window has a display portion for displaying information pertaining to the polishing pad such as its serial number, pad data, reference value of the hardness, measured value of the hardness, error of the hardness, reference value of the transmittance through the window, measured value of the transmittance and error of the transmittance. Furthermore, the data display window has a key pad portion including various buttons such as a “serial number” button, a “reading pad” button, a “measuring hardness” button, a “measuring transmittance” button, a “transmittance reference value” button, a “hardness reference value” button, a “save” button, a “preview” button and a “print” button.  
         [0078]    Each measuring mode can be selected using the project start screen (steps S 3 -S 6 ).  
         [0079]    Firstly, when the “measuring transmittance” button is pressed after the transparent window  12   a  of the polishing pad  12  is aligned with the transmittance sensor  120  (step S 3 ), the transmittance sensor  120  measures the transmittance through the transparent window  12   a  (step S 7 ). A transmittance signal generated by the transmittance sensor is transferred through the transmittance sensor interface  228  to the microcomputer  218 . The transmittance signal is converted into the measured value by the microcomputer  218  and compared with the transmittance reference value to calculate the error of the transmittance. Then, the error and the measured value of the transmittance are displayed on the screen of the data display window.  
         [0080]    Next, the polishing pad  12  is placed at the center of the measuring table  100 , and then is fixed to the measuring table  100  using the vacuum produced by vacuum pump  130 . After the polishing pad  12  is fixed to the measuring table  100 , the XY-carrier  110  is moved in the directions of the X and Y axes to scan the surface of the pad  12  with the camera  140  and the sensors  150 , 160  in a plane defined by the X and Y axes. At this time, signals produced by the camera  140  and the sensors  150 ,  160  are processed to discern the surface profile and state of the polishing pad  12  and to measure the hardness of the polishing pad  12  (S 8 -S 10 ). In particular, when the hardness mode is selected (step S 4 ), the hardness of the polishing pad  12  is measured (step S 8 ). When the pad-reading mode is selected (step S 5 ), the surface profile of the polishing pad  12  is read (step S 9 ). When the CCD mode is selected (step S 6 ), a picture of the polishing pad  12  is taken (step S 10 ).  
         [0081]    Referring to FIG. 9, the microcomputer  218  performs the homing of the X-axis motor  116   a  and the Y-axis  111  a motor to search out the center of the polishing pad  12  placed on the measuring table  100 . To detect the position of the center of the polishing pad  12 , the X-axis motor  116   a  is operated to move the X-axis slider  117  along the X-axis carrier  116  to a center point along the axis X (step S 12 ). When the movement to is completed (step S 13 ), the Y-axis motor  111   a  is operated to move the X-axis carrier  117  along the Y-axis carrier  111  to the point where the axis Y starts on the polishing pad  12  (step S 14 ). The point where the axis Y starts is checked (step S 15 ) and then, the Y-axis motor  111   a  accurately moves the X-axis carrier  116  in the direction of the Y axis (step S 16 ). The starting point of the axis Y is detected during the accurate movement of the X-axis carrier  117  in step S 16  (step S 17 ).  
         [0082]    Referring to FIG. 10, once the starting point of the axis Y is detected (step S 17  in FIG. 9), the Y-axis motor  111   a  is operated to accurately move the X-axis carrier  117  along the Y-axis carrier  111  to a point where the Y axis ends on the polishing pad  12  (step S 18 ). The end point is checked (step S 19 ), and then the Y-axis motor  111   a  accurately moves the X-axis carrier  116  in the direction of the Y axis (step S 20 ). The end point along the Y axis is detected during the accurate movement of the X-axis carrier  116  in step S 20  (step S 21 ). When the end point along the Y axis is detected at step S 21 , a center point along the Y axis is calculated based on the starting point and the ending point along the Y axis. Then the X-axis carrier  116  is moved along the Y-axis carrier  111  to the center point along the Y axis (step S 22 ). When the movement of the X-axis carrier  116  to the center point along the Y axis is completed (step S 23 ), the center of the polishing pad  12  has been found.  
         [0083]    When the movement of the X-axis slider  117  to the center of polishing pad  12  is completed, the “measuring hardness” button is clicked (step S 4 ) so that the hardness-measuring sensor  160  measures the hardness of the polishing pad  12  at its central portion (step S 8 ). A hardness signal generated by the hardness-measuring sensor  160  is transferred through the hardness-measuring sensor interface  220  to the microcomputer  118 . The hardness signal is converted into a measured value by means of the microcomputer  218  and compared with the reference value of the hardness to calculate the error of the hardness. Then, the error and the measured value of the hardness are displayed on the screen of the data display window.  
         [0084]    When the “reading pad” button is pressed on the project start screen (step S 5 ), the laser sensor  150  measures the surface profile of the polishing pad  12  (step S 9 ). The laser sensor  150  measures the surface profile of the polishing pad while accurately moving from the center of the polishing pad  12  to the outer peripheral edge of the polishing pad. Accordingly, a profile signal generated by the laser sensor  150  is transferred to the microcomputer  118 . The microcomputer  118  converts the profile signal into digital data and displays the digital data as a graph on the project start screen shown in FIG. 13.  
         [0085]    [0085]FIG. 15 is an exemplary graph of a surface profile of the polishing pad measured by the laser sensor  150  according to the present invention. A plurality of peaks  300  regularly arranged at a low portion of the graph in FIG. 15 correspond to the grooves formed in the surface of the polishing pad. According to the present invention, the measuring of the profile of the polishing pad serves as a two-dimensional examination by which the distance between adjacent grooves and the depths of the grooves can be determined. Accordingly, a polishing pad whose grooves have an irregular depth and spacing can be detected, thereby insuring that the only polishing pads that are used are those that will supply the slurry uniformly over the surface of the wafer.  
         [0086]    When the “CCD” button is clicked on the project start screen (step S 6 ), a surface image of the polishing pad is displayed (step S 10 ). Therefore, an operator can observe an enlarged image of the surface of the polishing pad as scanned by the laser sensor  150 , can capture important ones of these images and can save the images in a database.  
         [0087]    The measured data of the polishing pad according to the above-mentioned procedure and screen images of the polishing pad are filed in the database according to the serial number of the polishing pad and are thus saved on the hard drive. As shown in FIG. 14, the measured data and screen images of the polishing pad filed in the database can be searched on the basis of measured data or serial number of the polishing pad.  
         [0088]    As described above, the measuring apparatus according to the present invention can measure the profile, the hardness, the transmittance of the transparent window and the surface state of the polishing pad, wherein the properties of each polishing pad can be exactly observed. Accordingly, when the polishing pad is mounted in the CMP apparatus, the polishing process can be set up based on the actual characteristics of the polishing pad. As a result, the polishing process can be carried out under conditions that minimize the number of polishing defects in the wafer.  
         [0089]    Furthermore, the presence of foreign matter in the pores and grooves of the polishing pad is easily discerned from the displayed image of the surface of the polishing pad. Therefore, measures can be taken to prevent micro-scratches from being produced in the wafer. In addition, the measured hardness of the polishing pad can be used to ensure that the polishing pad is suitable for the particular material layer of the wafer to be polished.  
         [0090]    Also, because the transmittance through the transparent window of the polishing pad can be measured, an excessive or insufficient polishing of the wafer can be prevented.  
         [0091]    Furthermore, operating characteristics of the CMP apparatus can be estimated from the wear exhibited by its polishing pad as discerned using the present invention. Therefore, a schedule for replacing the polishing pads can be accurately determined. Moreover, using such information, the CMP apparatus can be set-up to optimize the efficacy of the polishing pads and prolong the useful life thereof. Accordingly, the present invention contributes to reducing the manufacturing cost of the semiconductor devices.  
         [0092]    Finally, although the present invention has been described in connection with the preferred embodiments thereof, the present invention is not so limited. Rather, various changes and modifications can be made to the preferred embodiments by one skilled in the art within the spirit and scope of the present invention as hereinafter claimed.