Abstract:
The object of the present invention is to provide a thickness control method for a double side polisher, accuracy of which is not affected by wearing of polishing pad and applicable to polishing of nonconductive work pieces. An eddy current sensor in a cavity of an upper polishing plate measures distance from the senor to the upper surface of carrier with holes for the work pieces being inserted respectively. The measured distance is successively monitored and polishing is stopped when the distance has become a predetermined value corresponding to target amount of material removal from the work piece.

Description:
[0001]     This application is based on application No. 2005-050297 filed in Japan, the contents of which are hereby incorporated by reference.  
       FIELD OF THE INVENTION  
       [0002]     The present invention relates to a double side polisher for a work piece and a thickness control method thereof.  
       BACKGROUND OF THE INVENTION  
       [0003]     A double side polisher is a machine that polishes surfaces of both sides of a work piece at the same time. The work pieces are inserted in holes of a carrier respectively and the carrier with the work pieces is placed between upper and lower polishing plates on which polishing pads are plastered. Then, a planetary motion is provided to the carrier and a rotary motion is provided to the upper and lower polishing plates, while supplying slurry in the gap of the polishing plates and applying a predetermined polishing pressure to the work pieces by the polishing plates.  
         [0004]     Although the amount of polishing of the work piece is usually monitored by means of polishing duration time, there is a case where it is necessary to detect the amount of polishing or material removal. Therefore, there are made some attempts to provide a thickness control device in a polishing apparatus.  
         [0005]     Conventionally, there is known a thickness control device using a probe as disclosed in Japanese Examined Patent Publication No. S64-4126. The thickness control device disclosed in the document has a construction wherein a stylus of the probe is directed upward and the upper end of the stylus is in contact with the measurement chip fixed to the upper polishing plate. As the upper polishing plate goes down, with the advance of polishing, the chip of the upper polishing plate pushes down the stylus of the probe and the displacement, namely the amount of polishing, is measured by the probe.  
         [0006]     Also there is known another thickness control device using an eddy current distance sensor as disclosed in Japanese Examined Patent Publication No. S63-9943. This device mounted on the upper polishing plate measures the change of distance from itself to the lower polishing plate by detecting the change of impedance of the sensor.  
         [0007]     Furthermore, in the thickness control device disclosed in Japanese Unexamined Patent Publication No. H10-202514, a reference aluminum plate is provided on the carrier, and distance L 1  to the upper surface of the reference plate and distance L 2  to the upper surface of an aluminum disk, or a work piece, are measured by the eddy current sensor and a difference between distances L 1  and L 2  is calculated to determine the thickness of the aluminum disk (work piece).  
       SUMMARY OF THE INVENTION  
       [0008]     In the first document, it is assumed that the amount of downward displacement of the upper polishing plate corresponds to the amount of material removal from the work piece. However, as the upper and lower polishing plates are worn while lapping operations are repeated, the displacement of the upper polishing plate becomes no longer correspond to the amount of material removal when many work pieces are polished. Thus accuracy of the thickness control falls gradually. Moreover, since the stylus is in contact with the chip, the contact end of the stylus is abraded by rotation of the chip and error in measurement may occur. Because of this, accuracy the thickness control is about ±4 to 5 μm and thus it is difficult to achieve an accuracy of ±3 μm or less.  
         [0009]     In contrast, in the thickness control device using the eddy current sensor disclosed in the second Document, the distance between the upper and lower polishing plates is detected by radiating magnetic field from the eddy current sensor to the lower polishing plate, which allows for measurements including the wear of the lower polishing plate, and also allows the measurement of the work piece to an accuracy of ±3 μm or less, since measurement error decreases compared with the thickness control device disclosed in the first document. However, accuracy achieved by the above thickness control device is not enough to satisfy the level recently required as the measurement is influenced by deformation of polishing pad caused by polishing pressure.  
         [0010]     The thickness control device disclosed in the third document is limited for electrical conductive materials and cannot be applied for polishing work pieces made of semiconductor, glass or crystal as they are not electrically conductive.  
         [0011]     Therefore, an object of the present invention is to provide a thickness control method and device that can also be used for a work piece that is not conductive and is effectively not affected by wearing of polishing pad, and is capable of measuring with high accuracy.  
         [0012]     The aforementioned problems can be solved by the following means. That is, the first aspect of the present invention is a thickness control method for a double side polisher having: a machine base; a lower polishing plate on the upper surface of which a polishing pad being attached rotatably supported on said machine base; a sun gear with external teeth rotatably supported on said machine base; an internal gear with internal teeth rotatably supported on said machine base; an electro-conductive carrier with external teeth for meshing with said external teeth of said sun gear and said internal teeth of said internal gear having holes for work pieces to be inserted therein; a rotatable upper polishing plate, on the lower surface of which a polishing pad being attached, for applying polishing pressure to said work pieces inserted in said holes; a drive system with a singularity of or a plurality of driving sources for rotating said upper and lower polishing plates, said sun gear and said internal gear around the same axis; a slurry supplying unit for supplying slurry to polishing area; and an eddy current sensor mounted in a cavity of said upper polishing plate for measuring distance to the upper surface of said carrier, said polisher stops polishing when distance to the upper surface of said carrier measured by said sensor has decreased by a predetermined value corresponding to a target amount of material removal from the work piece.  
         [0013]     The second aspect of the present invention is a double side polisher comprising: a machine base; a lower polishing plate on the upper surface of which a polishing pad being attached rotatably supported on said machine base; a sun gear with external teeth rotatably supported on said machine base; an internal gear with internal teeth rotatably supported on said machine base; an electro-conductive carrier with external teeth for meshing with said external teeth of said sun gear and said internal teeth of said internal gear having holes for work pieces to be inserted therein; a rotatable upper polishing plate, on the lower surface of which a polishing pad being attached, for applying polishing pressure to said work pieces inserted in said holes; a drive system with a singularity of or a plurality of driving sources for rotating said upper and lower polishing plates, said sun gear and said internal gear around the same axis; a slurry supplying unit for supplying slurry to polishing area; an eddy current sensor mounted in a cavity of said upper polishing plate for measuring distance to the upper surface of said carrier; an initial value storage unit to store initial value of the distance from said sensor to the upper surface of said carrier measured at the beginning of polishing by said sensor; a comparator unit for successively comparing said initial value stored in said initial value storage unit with the distance currently measured by said sensor; and a control unit for stopping polishing when the difference calculated by said comparator unit has become a predetermined value corresponding to target amount of material removal from the work piece.  
         [0014]     According to the method and the polisher of the present invention, the eddy current sensor in the cavity of the upper polishing plate measures distance from the senor to the upper surface of carrier with holes for the work pieces being inserted respectively. The measured distance is successively monitored and polishing is stopped when the distance has become a predetermined value corresponding to target amount of material removal from the work piece. Thus, the double side polisher according to the present invention can be used for polishing of such a work piece as a semiconductor wafer that is normally electrically non-conductive, without depending on electrical property thereof, and almost without being affected by wearing of the polishing pad. Therefore thickness control is achieved with high accuracy for long time.  
         [0015]     Other objects and advantages besides those discussed above shall be apparent to those skilled in the art from the description of a preferred embodiment of the invention which follows. In the description, reference is made to accompanying drawings, which form a part thereof, and which illustrate an example of the invention. Such example, however, is not exhaustive of various embodiments of the invention, and therefore reference is made to the claims which follow the description for determining the scope of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention and together with the description, serve to explain the principles of the invention.  
         [0017]      FIG. 1  is a vertical cross sectional view showing the substantial part of a double side polisher  1  according to the present invention;  
         [0018]      FIG. 2  is a plan view of  FIG. 1  as seen from A-A;  
         [0019]      FIG. 3  is an enlarged view of the substantial part in  FIG. 1 ;  
         [0020]      FIG. 4  is a comparative diagram illustrating a thickness control operation: (1) is a cross sectional view of the substantial part at the start of polishing and (2) is at the end of polishing; and  
         [0021]      FIG. 5  is a flowchart describing the operation of a double side polisher of this embodiment including a calibration processing. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0022]     Now, preferred embodiments of the present invention will be described in detail while referring to the accompanying drawings.  
         [0023]     An upper polishing plate  11 , a lower polishing plate  12 , a sun gear  13 , and an internal gear  14  are rotatably supported around the same axis line on a machine base  10 . The upper polishing plate  11 , lower polishing plate  12 , sun gear  13 , and internal gear  14  have integrally coupled drive gears: a first drive gear  11   d,  a second drive gear  12   d,  a third drive gear  13   d,  and a fourth drive gear  14   d  respectively, in order to transmit rotation power. To these gears, a rotation power is transmitted from a first motor M 1 , a second motor M 2 , a third motor M 3 , and a fourth motor M 4  respectively. Although a drive unit  102  shown here consists of four motors, it is possible to drive respective gears with a single motor by distributing its power by means of a gear train.  
         [0024]     A polishing pad made of nonwoven fabric or the like is attached or plastered on the lower flat surface of the upper polishing plate  11  and the upper flat surface of the lower polishing plate, and the polishing plates  11  and  12  are disposed so that the flat surfaces thereof face each other. In the gap between these surfaces is disposed a carrier  15 . The carrier  15  thinner than the work piece  16  has external teeth meshing with the sun gear  13  and the internal gear  14 .  
         [0025]     The upper polishing plate  11  and the first drive gear  11   d  are arranged so that they can engage at lower position or disengage at upper position of the plates  11 . It is possible to lift only the upper polishing plate  11  by an appropriate lifting means provided on a suspending member  21  and a beam  101 . The carrier  15  is inserted from space created when the upper polishing plate  11  is lifted. At this time, the external teeth of the carrier  15  are engaged with the external teeth of the sun gear  13  and the internal teeth of the internal gear  14 . The carrier  15  has a number of work piece holding holes in which flat work pieces  16  such as semiconductor wafers are mounted or inserted. Into the gap between the upper polishing plate  11  and the lower polishing plate  12 , slurry is supplied from a slurry supply unit (not shown).  
         [0026]     The upper polishing plate  11  has a cavity that opens downward and the eddy current sensor  22  is inserted therein. The eddy current sensor  22  is directed downward and measures a distance from the reference position of the sensor  22  to an upper surface  151  of the carrier  15 . The carrier, at least the upper surface thereof, is made of electrically conductive material. Since the position of the distance sensor  22  is known, it is possible to detect a distance to the carrier  15  relative to the upper polishing plate  11 . The detection principle of the eddy current sensor itself is not described here, since it is described in the second document (Japanese Examined Patent Publication S63-9943. The control unit  30  monitors the amount of polishing based on the output of the eddy current sensor, and stops the drive unit  102  when polishing has progressed to a preset desired amount of polishing.  
         [0027]     When polishing, the carrier  15  is placed on the lower polishing plate  12  to which the polishing pad  17  is attached; the external teeth of the carrier  15  are engaged with the sun gear  13  and internal gear  14 . The work pieces  16  are inserted in the work piece holding holes of the carrier  15 , and the upper polishing plate  11  is lowered. Then, slurry is supplied from a slurry supply unit into the gap between the upper and lower polishing plates  11 ,  12 . The plates  11 ,  12 , sun gear  13 , and internal gear  14  are driven to rotate. The carrier  15  revolves around the sun gear  13  and rotates around the axis of itself as the carrier is meshed with the sun gear  13  and the internal gear  14 . The upper and lower surfaces of the work pieces  16  are polished with the polishing pads of the plates  11 ,  12  by the planetary motion of the carrier  15 , the rotation and polishing pressure of the plates  11 ,  12 , and the slurry supplied. This polishing operation is a fairly standard operation and not special one.  
         [0028]     The operation of the thickness control device for a double side polisher of this embodiment is described below using  FIG. 5  and with reference to  FIGS. 1 through 4 .  
         [0029]     When the thickness control operation starts (step S 00 ), the operator is prompted to input an amount s 0  of polishing, and then the amount is inputted (step S 01 ). Here the amount “s 0 ” is a target value obtained by subtracting the desired thickness of the work piece from the present thickness of the work piece measured previously. Then the upper polishing plate  11  lowers, the drive unit  102  and slurry supply unit start to run (S 02 ). Then, the distance “d” from the eddy current sensor  22  to the upper surface of the carrier  15  is measured by the sensor  22  (S 03 ). The measured value “d” is stored as an initial value “d 0 ” (S 04 ).  
         [0030]     The distance “d” is measured again by the sensor  22  (S 05 ), and the difference between the initial value “d 0 ” and the current measured value “d” is calculated. This difference “d 0 −d” represents the total amount of polishing up to now, and is substituted for the current amount of polishing “s” (S 06 ).  
         [0031]     At the step S 07 , the value “s 0 ” and “s” are compared. There if the value “s” is equal to the value “s 0 ”, then the control advances to the next step S 08 , else returns to the step S 05 . Thereby the steps S 05  to S 07  are repeated until determination “yes” is realized at the step S 07 . As the work pieces are continued to be polished throughout the repetitions of the steps, the work pieces have the target thickness “s 0 ” at last.  
         [0032]     Describing this using  FIG. 4 , when polishing starts from the state shown at (1) of  FIG. 4  and progresses to reach the state shown at (2), the upper polishing plate  11  moves down. This downward movement causes the eddy current sensor  22  fixed in the cavity of the upper polishing plate  11  to move down by the same amount “s”. As the carrier  15  is kept in contact with the lower polishing plate  12  by gravitation, there decreases correspondingly the distance from the eddy current sensor  22  to the carrier surface  151 . That is, change of the distance represents the amount of polishing.  
         [0033]     Since the desired amount of polishing has been achieved, the drive unit  102  and slurry supply unit are stopped (S 08 ), the upper polishing plate  11  is raised, and thus the polishing is finished. New work pieces are inserted after the finished work pieces are removed.  
         [0034]     According to the thickness control method and the double side polisher of the present invention, the distance to the carrier surface is measured. Therefore the control is achieved without being affected by wear or deformation of the lower polishing pad and thereby accurate polishing can be performed. As the principle of the method is not dependent on electrical conductivity of the work piece, the method can be applied to polishing for work piece of non-electro-conductive material, such as a semiconductor wafer.  
         [0035]     Although only preferred embodiment is specifically illustrated and described herein, it will be appreciated that many modifications and variations of the present invention are possible in light of the above teachings and within the purview of the appended claims without departing from the spirit and intended scope of the invention.