Patent Publication Number: US-2023150086-A1

Title: Polishing apparatus

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a polishing apparatus for polishing a wafer held on a holding surface of a chuck table. 
     Description of the Related Art 
     A wafer having a plurality of devices such as integrated circuits (ICs) and large-scale integration circuits (LSIs) formed on a front surface thereof in a state of being partitioned by a plurality of intersecting streets is thinned by grinding of a back side thereof by a grinding apparatus, is then subjected to polishing by a polishing apparatus for removing grinding marks, is thereafter divided into individual device chips by a dicing apparatus or a laser processing apparatus, and the device chips thus divided are utilized for electric equipment such as a mobile phone and a personal computer. 
     The polishing apparatus for polishing the back surface of the wafer generally includes a chuck table that holds the wafer, and a polishing unit that has, in a rotatable manner, a polishing pad for polishing the wafer held by the chuck table, and is capable of polishing the ground surface of the wafer to process the surface into a desired smooth state (see, for example, Japanese Patent Laid-Open No. Hei 08-099265). 
     SUMMARY OF THE INVENTION 
     Incidentally, when the grinding marks generated by processing by the grinding apparatus are removed by the polishing apparatus and a laser beam is then applied to the wafer in order to divide the wafer into individual device chips, inappropriate processing by the laser beam may cause insufficient division of the wafer, resulting in possible damage to the device chips. 
     As a result of the present inventors&#39; extensive and intensive study of the cause of the above problem, it becomes clear that, since the wafer becomes ununiform in thickness after the grinding marks are removed by the polishing apparatus, and at a time of applying the laser beam to the wafer to divide the wafer into the individual device chips, the focal point of the laser beam would not be positioned at a suitable position, so that processing by the laser beam may not be carried out suitably. 
     Accordingly, it is an object of the present invention to provide a polishing apparatus which is capable of solving the problem that insufficient division of the wafer attributable to inappropriate processing by a laser beam leads to damages to device chips. 
     In accordance with an aspect of the present invention, there is provided a polishing apparatus including a chuck table that has a holding surface and is rotatable, a polishing unit that rotatably supports a polishing pad for polishing a wafer held on the holding surface of the chuck table and that has a spindle formed with a through-hole extending from one end of the spindle to the other end of the spindle in an axis of the spindle, a vertical feeding mechanism that relatively puts the polishing unit into processing feeding in a vertical direction to the holding surface of the chuck table, a parallel feeding mechanism that relatively puts the polishing unit into processing feeding in a direction parallel to the holding surface of the chuck table, a thickness measuring unit that is disposed at one end of the through-hole of the spindle and that measures a thickness of the wafer, and a controller. The controller obtains information regarding the thickness of the wafer measured by the thickness measuring unit at a position defined by a variation in a distance between a rotational center of the rotating wafer held on the chuck table fed by the parallel feeding mechanism and a center of the other end of the through-hole facing the wafer and a rotational angle of the chuck table, and forms mapping data regarding the thickness of the wafer. 
     Preferably, the thickness measuring unit includes a light source that emits light in such a wavelength region as to be transmitted through the wafer, a condenser that applies the light emitted by the light source to the wafer held on the chuck table, a light branching section that is disposed in an optical path connecting the light source and the condenser and that branches return light reflected from the wafer held on the chuck table from the optical path, a diffraction grating that spectrally disperses the return light branched by the light branching section by wavelength, and an image sensor that detects intensity of the light spectrally dispersed by wavelength by the diffraction grating and that generates a spectral interference waveform. 
     Preferably, the controller includes a thickness calculation section that calculates the spectral interference waveform generated by the image sensor and that outputs thickness information and a determination section that determines, based on the mapping data, that the the thickness of the wafer is passed in a case where variation of the thickness of the wafer is less than an allowable value and that the thickness of the wafer is failed in a case where the variation of the thickness of the wafer is the allowable value or more. 
     According to the present invention, the thickness of the wafer is measured by the thickness measuring unit, and mapping data regarding the thickness of the wafer is formed. Hence, variation of the thickness of the wafer after polishing is carried out can be grasped, so that it is possible to permit the conveyance to the subsequent step in which laser processing is carried out if the variation of the thickness of the wafer is less than an allowable value, and to instruct reprocessing if the variation of the thickness of the wafer is the allowable value or more. Therefore, it is possible to prevent dividing processing of the wafer with an ununiform thickness from being carried out, and only a wafer with a uniform thickness capable of being divided suitably is subjected to dividing processing. As a result, the wafer can reliably be divided into individual device chips, and the problem that the device chips may be damaged due to the presence of a region in which the division is insufficient is eliminated. 
     The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attached drawings showing a preferred embodiment of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is an overall perspective view of a polishing apparatus according to a preferred embodiment of the present invention; 
         FIG.  2    is a block diagram depicting an optical system of a thickness measuring unit of the polishing apparatus depicted in  FIG.  1   ; 
         FIG.  3    is a conceptual diagram of mapping data stored in a table; and 
         FIG.  4    is a flow chart to be carried out by a determination section. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A polishing apparatus according to a preferred embodiment of the present invention will be described in detail below with reference to the attached drawings.  FIG.  1    depicts a perspective view of a polishing apparatus  1  of the present embodiment. The polishing apparatus  1  includes a substantially rectangular parallelepiped apparatus housing  2 . The apparatus housing  2  includes: a polishing unit  3  that polishes a wafer W as a workpiece; a thickness measuring unit  4  that measures a thickness of the wafer W; a holding unit  5  that holds the wafer W; a first cassette  6  that is disposed on the front side in the figure that accommodates the wafer W before polishing; a second cassette  7  that is disposed on the other side in an X-axis direction in the figure relative to the first cassette  6  and that accommodates the wafer W after polishing; a temporary placing unit  8  that is disposed adjacent to the first cassette  6  in an Y-axis direction in the figure and centering the wafer W; a cleaning unit  9  that is disposed adjacent to the second cassette  7  in the Y-axis direction; a first conveying mechanism  10  that unloads the wafer W accommodated in the first cassette  6  to the temporary placing unit  8  and that conveys the wafer W cleaned by the cleaning unit  9  to the second cassette  7 ; a second conveying mechanism  11  that conveys the wafer W having been placed on the temporary placing unit  8  and centered, to a chuck table  52  of the holding unit  5  positioned at a loading/unloading position (on the front side in the figure); a third conveying mechanism  12  that conveys the wafer W after polishing from the chuck table  52  positioned at the loading/unloading position to the cleaning unit  9 ; and a controller  100 . Note that the wafer W accommodated in the first cassette  6  is formed with a grinding mark due to back side grinding in the grinding carried out before conveying into the polishing apparatus  1 , and the polishing apparatus  1  of the present embodiment removes the grinding mark by polishing. 
     The polishing unit  3  is mounted to a pair of guide rails  22  and  22  extending vertically on an inside surface of a support wall  21  erected on a rear end side of the apparatus housing  2  in a vertically movable manner. The polishing unit  3  includes: a unit housing  31 ; a wheel mount  33  disposed at a lower end of a spindle  32  rotatably supported by the unit housing  31 ; a polishing wheel  34  which is mounted to the wheel mount  33  and has a polishing pad  35  attached to a lower surface thereof; an electric motor  36  which is mounted to an upper end of the unit housing  31  and which rotates the wheel mount  33 ; and a moving base  38  that supports the unit housing  31  through a support member  37 . The moving base  38  is provided with guided grooves slidably fitted to the guide rails  22  and  22 , and the polishing unit  3  is supported in a vertically movable manner. In addition, the spindle  32  is formed in an axis thereof with a through-hole  14  (indicated by a broken line) extending to penetrate the spindle  32  from one end (upper end) of the spindle  32  to the other end (lower end) of the spindle  32 , and an opening (omitted in illustration) at a position corresponding to a lower end of the through-hole  14  is formed on a lower surface side of the polishing pad  35  disposed on the polishing wheel  34 . 
     The polishing apparatus  1  illustrated in the figure includes a vertical feeding mechanism  39  that lifts up and down the moving base  38  of the polishing unit  3  along the guide rails  22  and  22 . The vertical feeding mechanism  39  includes a pulse motor  39   a,  a male screw rod  39   b  disposed vertically in parallel to the guide rails  22  and  22 , supported by the support wall  21  in a rotatable manner, and driven to rotate by the pulse motor  39   a,  and a female screw block (not illustrated) mounted to the moving base  38  and screw engaged with the male screw rod  39   b,  the male screw rod  39   b  being driven for forward rotation and reverse rotation by the pulse motor  39   a,  whereby the polishing unit  3  is moved in a Z-axis direction (vertical direction). Further, on the front side of the apparatus housing  2  on which the first conveying mechanism  10  is disposed, an operation panel  13  for instructing polishing to the controller  100  and designating polishing conditions is disposed, and a display unit  16  for displaying processing information and the like is connected to the controller  100 . 
     The holding unit  5  includes a cover table  51 , and a chuck table  52  disposed at a center of the cover table  51  in an upwardly projecting manner. The chuck table  52  includes a holding surface on which the wafer W is held, and the holding surface is configured by a gas-permeable member. The chuck table  52  is rotatably supported, and is rotated by a rotational drive source not illustrated. In addition, a parallel feeding mechanism (omitted in illustration) that puts the chuck table  52  into processing feeding in a Y-axis direction indicated by an arrow Y and in parallel to a horizontal surface is disposed inside the apparatus housing  2 , and moves the chuck table  52  between a loading/unloading position on the front side in the Y-axis direction and a polishing position directly below the polishing unit  3 . 
     On the front side of the loading/unloading position of the chuck table  52 , a cleaning water supply nozzle  23  for supplying cleaning water to the back surface of the wafer W after polishing is disposed. In addition, a slurry supply nozzle  24  is disposed in the vicinity of the polishing position, and, at a time of polishing, a liquid slurry containing free abrasive grains is jetted from the slurry supply nozzle  24 , to supply the slurry onto the wafer W which is to be polished by the polishing pad  35 . Note that the material of the polishing pad  35  is not particularly limited, and a material well-compatible with the slurry and elastically deformable, for example, a sheet of foamed polyurethane or felt (nonwoven fabric) can be used to form the polishing pad. 
     The thickness measuring unit  4  is disposed so as to face the opening  32   a  on one end side of the through-hole  14  formed in the spindle  32 . The thickness measuring unit  4  is connected to the controller  100 , measures the thickness of the wafer W at a predetermined position of the wafer W, and forms mapping data regarding the thickness of the wafer W described later. 
       FIG.  2    depicts a block diagram of an optical system constituting the thickness measuring unit  4 , and the thickness measuring unit  4  will be described more specifically with reference to  FIG.  2   . 
     As depicted in  FIG.  2   , the optical system constituting the thickness measuring unit  4  includes: a light source  41  that emits light LB 1  of such a predetermined wavelength region as to be transmitted through the wafer W held on the chuck table  52 ; a first optical path  41   a  formed of an optical fiber; a light branching section  42  that guides the light LB 1  from the light source  41  into a second optical path formed of an optical fiber on the chuck table  52  side and that branches a return light LB 2  obtained when the light LB 1  emitted from the light source  41  is reflected by the wafer W held on the chuck table  52  and goes reversely into an optical path on the side of a collimation lens  45  depicted in the figure; a collimation lens  43  to which the light LB 1  guided into the second optical path  41 b is applied; a condenser lens  44  that concentrates the light LB 1  applied from the collimation lens  43 ; a diffraction grating  46  that spectrally disperses the return light LB 2  branched from the light branching section  42  and made to be parallel light by the collimation lens  45 , by wavelength; and an image sensor  47  that receives the return light LB 2  spectrally dispersed by the diffraction grating  46 , detects light intensity by wavelength, and forms spectral interference waveforms. The spectral interference waveforms formed by the image sensor  47  are sent to the controller  100 . 
     As the light source  41 , for example, a halogen lamp that emits light having a wavelength of 400 to 1,200 nm can be adopted. The light branching section  42  can be selected, for example, from a polarization holding fiber coupler, a polarization holding fiber circulator, a single mode fiber coupler, a single mode fiber coupler circulator, and the like. In addition, the light source  41  is not limited to the abovementioned halogen lamp, but is selected according to the material of the wafer W to be polished, and is appropriately selected from a known light source that emits light of such a wavelength as to be transmitted through the wafer W. Besides, the light LB 1  guided by the condenser lens  44  is concentrated by the condenser lens  44  and is guided to the through-hole  14  inside the spindle  32 , and a focal point P is positioned at a position corresponding to a center of the other end (lower end) of the through-hole  14 , on the front surface of the wafer W held on the chuck table  52 . 
     The controller  100  includes a computer which includes a central processing unit (CPU) that performs arithmetic processing according to a control program, a read only memory (ROM) that stores the control program and the like, a readable and writable random access memory (RAM) that temporarily stores detected values, results of the arithmetic processing, and the like, an input interface, and an output interface (illustration of the details is omitted). The controller  100  is equipped with a thickness calculation section  110  and a determination section  120 . The thickness calculation section  110  performs Fourier transformation on the spectral interference waveforms generated by the image sensor  47  that receives the return light LB 2 , obtained when the light LB 1  emitted from the light source  41  is reflected by the front surface and the back surface of the wafer W held on the chuck table  52  and the reflected light as the return light LB 2  is guided through the condenser lens  44 , the collimation lens  43 , the light branching section  42 , and the diffraction grating  46 , and outputs thickness information of the wafer W. The determination section  120  determines whether or not variation of the thickness of the wafer W is within an allowable range, based on mapping data (described in detail later) formed by the thickness calculation section 
     The polishing apparatus  1  of the present embodiment has generally the above-described configuration, and the functions and actions of the polishing apparatus  1  will be described more specifically below. 
     In carrying out polishing of the wafer W by use of the polishing apparatus  1  of the present embodiment, the first conveying mechanism  10  and the second conveying mechanism  11  are operated to convey the wafer W accommodated in the first cassette  6  through the temporary placing unit  8  to the chuck table  52 , and the wafer W is placed on the chuck table  52  with the front surface side thereof directed downward and the back surface side thereof directed upward. When the wafer W is placed on the chuck table  52 , suction means omitted in illustration is operated to generate a suction negative pressure on the holding surface of the chuck table  52 , to thereby suction hold the wafer W. An appropriate protective film may be attached to the front surface side of the wafer W. 
     When the abovementioned wafer holding step is carried out, the chuck table  52  is moved to the polishing position directly under the polishing unit  3  by the abovementioned parallel feeding mechanism, and polishing is conducted in which the wafer W is polished while supplying the slurry from the slurry supply nozzle  24  to the back surface of the wafer W. More specifically, first, the chuck table  52  is rotated in a direction indicated by an arrow R 1  in  FIG.  1    by rotation drive means not illustrated, the abovementioned electric motor  36  is operated to rotate the spindle  32  of the polishing unit  3  in a direction indicated by an arrow R 2 , and the abovementioned vertical feeding mechanism  39  is operated to lower the polishing unit  3  in a direction indicated by an arrow R 3 . The polishing pad  35  is put into contact with the back surface of the wafer W, and is pressed against the wafer W with a predetermined pressing load while supplying the slurry from the slurry supply nozzle  24 , thereby carrying out polishing for a predetermined time, to remove the grinding mark due to the grinding previously carried out from the back surface of the wafer W and smoothen the back surface of the wafer W. 
     When the polishing is conducted, the chuck table  52  is moved to the loading/unloading position on the front side of  FIG.  1    by the abovementioned parallel feeding mechanism, and cleaning water is jetted from the cleaning water supply nozzle  23  to the back surface of the rotating wafer W, to clean the back surface of the wafer W. 
     When the wafer W has been cleaned by the cleaning water supply nozzle  23 , the chuck table  52  is again moved toward the polishing position directly under the polishing unit  3 . Incidentally, the thickness calculation section  110  has a table  112  (see  FIG.  3   ) stored therein, and in the table  112 , a plurality of predetermined positions are defined by a variation in a distance (Y 0 −Ym) between a rotational center O of the rotating wafer W held on the chuck table  52  fed by the abovementioned parallel feeding mechanism and a center of the other end of the through-hole  14  facing the wafer W and by a variation in the rotational angle (0° to 360°) of the chuck table  52  with a direction connecting the rotational center  0  and a notch Wn being set as a reference (0°), based on the shape data of the wafer W displayed on the display unit  16 . Then, the chuck table  52  is put to parallel movement in the Y-axis direction by the parallel feeding mechanism, and at predetermined positions defined by variations of the rotational center O of the wafer W rotating at an appropriate speed and a position corresponding to the center on the side of the other end (lower end) of the through-hole  14  of the spindle  32  facing the wafer W and by a variation in the rotational angle of the chuck table, the thickness of the wafer W is calculated. Then, thickness information corresponding to all the predetermined positions defined in the table  112  depicted in  FIG.  3    is calculated and stored, to complete the mapping data. 
     In the present embodiment, in the direction in which the notch Wn of the wafer W displayed on the display unit  16  of  FIG.  2   , measurement of the thickness of the wafer W is started from the inside (the position indicted by Y 0  in the figure) of an outer edge region where the notch Wn is formed. When the thickness of the wafer W has been measured at the measurement start point (angle 0°, Y 0 ), while rotating the wafer W, the thickness of the wafer W is calculated, for example, by intervals of 36° by the thickness calculation section  110 , and, by rotating through  360 °, thickness information at each point (92, 92, 93, 93, 92, . . . 92 μm) is stored at a predetermined position in the table  112  depicted in  FIG.  3   . Note that the thickness at 0° and the thickness at 360° are obtained by measuring the thickness of the wafer W at the same position, and therefore, measuring of the thickness at 360° may be omitted. Next, the chuck table  52  is put to parallel movement by the abovementioned parallel feeding mechanism, thereby to bring the position to be applied by the light LB 1  by the thickness measuring unit  4  closer toward the rotational center O, by a distance obtained by dividing the distance from the abovementioned measurement start point to the rotational center O by a predetermined interval, and 360° rotation is made while measuring the thickness of the wafer W by intervals of 36° as described above, thereby to measure the thickness of the wafer W. 
     By repeating this, thickness information regarding the whole region of the wafer W is recorded in the table  112  depicted in  FIG.  3   , thereby to complete the mapping data. Note that the position defined by Ym in the present embodiment is a position coinciding with the rotational center O of the wafer W, and the position to be applied by the light LB 1  by the thickness measuring unit  4  coincides with the rotational center O of the wafer W, so that the thickness of the wafer W does not vary even when the thickness of the wafer W is measured a plurality of times during 360° rotation. By these operations, the controller  100  acquires information regarding the thickness of the wafer W at all the positions defined in the table  112  by the thickness measuring unit  4 , resulting in mapping data as depicted in  FIG.  3   . Note that, in the above-described embodiment, the parallel feeding mechanism has been operated stepwise by intervals of 360° rotation of the wafer W to bring the position to be applied by the light LB 1  closer toward the rotational center O, but the present invention is not limited to this; the thickness information may be calculated, what is generally called in a spiral manner, by measuring the thickness of the wafer W, bringing the position to be applied by the light LB 1  closer toward the rotational center O bit by bit by intervals of 36° rotation. 
     The determination section  120  of the controller  100  of the present embodiment determines, based on the mapping data stored in the table  112 , that the thickness of the wafer W is acceptable (passed) when the variation of the thickness of the wafer W is less than an allowable value, and that the thickness of the wafer W is inacceptable (failed) when the variation of the thickness of the wafer W is the allowable value or more.  FIG.  4    depicts a flow chart  122  which the determination section  120  carries out. 
     In carrying out the flow chart  122 , first, in step S 1 , all the data of the mapping data described above is referred to, and a maximum Max of the thickness information of the wafer W is read. Note that the maximum Max of the thickness information of the wafer W in the mapping data stored in the table  112  in the present embodiment is 93 μm. Next, proceeding to step S 2  is made, and a minimum Min of the thickness information of the wafer W in the mapping data is read. The minimum Min of the thickness information of the wafer W in the mapping data in the present embodiment is 85 μm. Next, proceeding to step S 3  is made, and the difference D=Max −Min is calculated (in the present embodiment, D=8 μm). Here, the controller  100  stores the allowable value T of the variation of the thickness of the wafer W, the determination section  120  reads the allowable value T (in the present embodiment, 5 μm) and in step S 4 , determines whether or not the difference D is less than the allowable value T. In the present embodiment, since it is determined that the difference D (8 μm) is more than the allowable value T (5 μm) (No), the determination result in step S 6  is “failed.” Note that, if the difference D is less than the allowable value T (5 μm), the determination result in step S 5  is “passed,” and the flow chart  122  comes to an end (END). The determination results of steps S 5  and S 6  are appropriately displayed on the display unit  16 , and are stored in the controller  100 . 
     According to the above-described embodiment, by forming the mapping data, the variation of the thickness of the wafer W can be grasped, and the polishing carried out in the polishing apparatus  1  can be evaluated. In addition, if the variation of the thickness of the wafer W is less than the allowable value, conveyance of the wafer W to the subsequent step in which laser processing is to be carried out is permitted, whereas, if the variation of the thickness of the wafer W is the allowable value or more, reprocessing is instructed, and if reprocessing is not possible, discarding of the wafer W or the like is instructed. As a result, carrying out dividing processing of a wafer with a nonuniform thickness can be prevented, and only a wafer having a uniform thickness which can be divided suitably is subjected to dividing processing. As a result, the wafer can be divided into individual device chips reliably, and the problem that device chips may be damaged due to the presence of a region in which the division is insufficient is eliminated. 
     Note that the present invention is not limited to the above-described embodiment. For example, in the above-described embodiment, in measuring the thickness at a predetermined position of the wafer W, the thickness calculation section  110  performs waveform analysis such as Fourier transformation on the spectral interference waveform generated by the image sensor  47  to thereby output thickness information, but the present invention is not limited to this. For example, a thickness measuring unit can be provided in which the spectral interference waveform acquired based on the return light of light applied through the through-hole  14  and a reference waveform preliminarily recorded in a reference waveform recording section are checked with each other, to determine the thickness from the reference waveform that has been coincident with the spectral interference waveform (see, for example, Japanese Patent Laid-Open No. 2020-176999). Further, a white light source, a first pinhole mask through which light of a part of a waveform of diffracted light obtained by spectrally dispersing white light by acoustooptic deflecting means is allowed to pass, a chromatic aberration lens that concentrates the light having passed through the first pinhole mask to apply the concentrated light to a workpiece, a beam splitter which deflects the reflected light of the light applied to the workpiece, a second pinhole mask through which the reflected light deflected by the beam splitter is allowed to pass, a light receiving element that outputs a light reception signal of the reflected light having passed through the second pinhole mask, and a thickness measuring unit that determines a height position of the workpiece based on a control signal for the acoustooptic deflecting means and the light reception signal from the light receiving element (see, for example, Japanese Patent Laid-open No. 2008-209299) may be prepared, and they may be disposed on the other end side of the abovementioned spindle  32 . 
     In addition, in the above-described embodiment, at a time of measuring the thickness of the wafer W by the thickness measuring unit  4 , the rotational angle of the wafer W held on the chuck table  52  and rotated has been set at intervals of 36°, but the present invention is not limited to this; for example, the rotational angle may be set by intervals of another angle, for example, by intervals of 5°, 10°, or the like. 
     Further, in the above-described embodiment, measurement of the thickness of the wafer W has been carried out after polishing of the wafer W is finished, but the present invention is not limited to this. For example, the measurement of the thickness of the wafer W may be carried out during polishing of the wafer W by the abovementioned polishing apparatus. In that case, the slurry is not supplied from the abovementioned slurry supply nozzle  24 , and the polishing pad may preliminarily be filled with abrasive, and dry polishing is preferably carried out. 
     The present invention is not limited to the details of the above described preferred embodiment. The scope of the invention is defined by the appended claims and all changes and modifications as fall within the equivalence of the scope of the claims are therefore to be embraced by the invention.