Patent Publication Number: US-6910943-B2

Title: Planarization apparatus and method

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to a planarization apparatus and method, and more particularly to a planarization apparatus and method for planarizing the reverse of a semiconductor wafer on which no chip is formed in a semiconductor wafer manufacturing process. 
     2. Description of Related Art 
     A planarization apparatus for grinding the reverse (one side) of a semiconductor wafer has chucks for holding the wafer by suction, a rough grinding wheel, a fine grinding wheel, a reverse cleaning unit, and the like. One chuck holds the obverse (the other side) of the wafer, and then the rough grinding wheel is pressed against the reverse of the wafer. The reverse of the wafer is roughly ground by rotating the chuck and the grinding wheel. The roughly-ground wafer is detached from the chuck and is held by another chuck for the fine grinding so that the wafer can be finely ground by the fine grinding wheel. The finely-ground wafer is transferred to the reverse cleaning unit so that the reverse of the wafer can be cleaned. That completes the grinding of the reverse of one wafer by the planarization apparatus. 
     The wafer whose reverse has already been ground is transferred from the planarization apparatus to an etching apparatus, which etches the wafer to remove a machining deteriorated layer formed at the reverse of the wafer. 
     If the wafer is ground into an extremely thin wafer close to a standardized article, the wafer is damaged (cracked or chipped) because of the machining deteriorated layer when the wafer is transferred from the planarization apparatus to the etching apparatus. 
     To address this problem, the conventional planarization apparatus grinds the wafer to such a thickness as not to damage the wafer during the transfer. With respect to the thickness of the wafer, the planarization apparatus roughly grinds the wafer with the thickness of 725 μm sliced from an ingot to the thickness of 250 μm, and finely grinds the wafer to the thickness of 200 μm. The wafer is machined to the standardized thickness of 50 μm at the etching step. 
     The conventional planarization apparatus, however, cannot grind the wafer close to the standardized thickness in order to prevent the wafer from being damaged during the transfer. For this reason, a machining allowance (150 μm in the above example) is large at the etching step. Thus, it takes a long time to etch the wafer, and the throughput cannot be improved. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing, it is an object of the present invention to provide a planarization apparatus and method which improve the throughput without damaging a workpiece. 
     The above object can be achieved by providing a planarization apparatus which comprises a holding means for holding a workpiece and a grinding means which has a grinding wheel for grinding the workpiece and grinds one side of the workpiece by the grinding wheel, the planarization apparatus comprising: a polishing means for polishing the one side of the workpiece, the polishing means comprising a rotating means for rotating a polishing head and a positioning mechanism for setting an interval between the polishing head and the workpiece. 
     The above object can be achieved by providing a planarization apparatus comprising: a holding means for holding a workpiece; a rough grinding means for roughly grinding the workpiece held by the holding means; a fine grinding means for finely grinding the workpiece roughly ground by the rough grinding means in the state wherein the holding means is holding the workpiece; a polishing means for polishing the workpiece finely ground by the fine grinding means in the state wherein the holding means is holding the workpiece; and a moving means for moving the holding means to a rough grinding position for the rough grinding means, a fine grinding position for the fine grinding means, and a polishing position for the polishing means. 
     The above object can be achieved by providing a planarization method using a planarization apparatus comprising: holding means for holding a workpiece; grinding means for grinding the workpiece held by the holding means; polishing means for polishing the workpiece ground by the grinding means; moving means for moving the holding means to a grinding position for the grinding means to grind the workpiece and to a polishing position for the polishing means to polish the workpiece; and wherein the polishing means polishes the workpiece by an amount more than an amount required for removing a machining deteriorated layer formed by the grinding, calculated by doubling a standard deviation, and less than a larger value between an amount required for correcting an unevenness of a thickness during the grinding and removing the machining deteriorated layer, calculated by sextupling a standard deviation, and 20 μm. 
     The above object can be achieved by providing a planarization method using a planarization apparatus comprising: holding means for holding a workpiece; rough grinding means for roughly grinding the workpiece held by the holding means; fine grinding means for finely grinding the workpiece roughly ground by the rough grinding means; polishing means for polishing the workpiece finely ground by the grinding means; moving means for moving the holding means to a rough grinding position for the rough grinding means, a fine grinding position for the fine grinding means and to a polishing position for the polishing means; wherein the fine grinding means finely grinds the workpiece by an amount more than an amount required for removing a machining deteriorated layer formed by the rough grinding, calculated by doubling a standard deviation, and less than a larger value between an amount required for correcting an unevenness of a thickness during the rough grinding and removing the machining deteriorated layer, calculated by sextupling a standard deviation, and 150 μm; and wherein the polishing means polishes the workpiece by an amount more than an amount required for removing a machining deteriorated layer formed by the fine grinding, calculated by doubling a standard deviation, and less than a larger value between an amount required for correcting an unevenness of a thickness during the fine grinding and removing the machining deteriorated layer and calculated by sextupling a standard deviation, and 20 μm. 
     According to the present invention, the planarization apparatus having the grinding means is provided with the polishing means, and this enables the grinding and polishing of the wafer in one apparatus. In the polishing method using the polishing means, the positioning mechanism positions the polishing head with respect to the workpiece, and the polishing head is pressed against the workpiece and is rotated by the rotating means. Consequently, the workpiece is polished. Since there is no necessity of transferring the workpiece from the planarization apparatus to the etching apparatus, the grinding means can grind the workpiece close to the standardized thickness. This reduces the time required for polishing the workpiece, and improves the throughput. The polishing removes the machining deteriorated layer formed by the grinding, and eliminates the necessity of etching in a post-treatment. This simplifies the entire structure of the workpiece manufacturing line, and reduces the size of the workpiece manufacturing line. 
     The workpiece may be polished in a constant pressure processing or in a constant cutting depth processing. 
     According to the present invention, the grinding means grinds the workpiece held by the holding means, and then, the polishing means polishes the workpiece after the moving means moves the holding means to the polishing position. More specifically, the workpiece held by the holding means is ground and polished, and it enables the accurate machining without damaging the workpiece. On the other hand, an apparatus, which transfers the workpiece from a holding means for grinding means to a holding means for a polishing means has such a problem that the workpiece may be damaged by an external force. Moreover, the accuracy of the holding face of the holding means changes every time the workpiece is transferred, and the accuracy affects the machining accuracy of the workpiece. Thus, the workpiece cannot be machined accurately. The present invention solves this problem. 
     According to the present invention, the planarization apparatus has cleaning means for cleaning a polishing pad and/or dressing means for dressing a surface of the polishing pad. If the polishing pad becomes dirty or loaded, the planarization apparatus cleans and dresses the polishing pad. 
     According to the present invention, the grinding means or the polishing means machines a machining deteriorated layer formed at one side of the workpiece ground in previous machining or machines the machining deteriorated layer and an unevenness of the thickness of the workpiece. It is therefore possible to acquire an accurate workpiece. 
     According to the present invention, the holding means is provided in a plural number, and the moving means sequentially moves the holding means from the grinding position to the polishing position so that the grinding and the polishing can be performed at the same time. This increases the availability compared with the case where one holding means grinds and polishes the workpiece. 
     According to the present invention, the holding means connects to a spindle and is moved by the moving means. This eliminates the necessity of separating the spindle from the holding means and connecting to the holding means to a spindle at the next moved position every time the holding means is moved. 
     According to the present invention, the holding means is detachably connected to a spindle, and when the holding means is moved, the holding means is separated from the spindle and only the holding means is moved by the moving means. This reduces the load on the moving means. Moreover, it is only necessary to provide the spindles suitable for each machining, and this reduces the manufacturing cost for the apparatus. 
     According to the present invention, the holding means is holding means for holding the workpiece by suction, holding means for freeze-holding the workpiece through ice film, or electrostatic holding means for holding the workpiece with static electricity. Thus, the workpiece can be held securely. 
     According to the present invention, the planarization apparatus further comprises a partitioning member for partitioning off a polishing position for the polishing means to polish the workpiece. Therefore, the grinding fluid and the grinding layer never reach the polishing position, and the polishing fluid used by the polishing means never reaches the grinding position. This prevents a trouble resulting from the mixture of the grinding and polishing fluids and chips. Particularly if the polishing means performs a chemical-mechanical polishing, the polishing fluid includes a chemical-mechanical agent. If the grinding fluid is mixed in this polishing fluid, the concentration of the chemical-mechanical agent is lowered and the machining time becomes longer. The use of the partition solves this problem. 
     According to the present invention, a planarization apparatus comprises rough grinding means, fine grinding means and polishing means so that the workpiece can be roughly ground, finely ground and polished automatically. Moreover the planarization apparatus has moving means for moving the holding means to a rough grinding position for the rough grinding means, a fine grinding position for the fine grinding means, and a polishing position for the polishing means. Consequently, the apparatus can work without lowering the rate of operation thereof even if single or plural rough grinding means, fine grinding means and polishing means are combined. 
     According to the present invention, the planarization apparatus with the rough grinding means, the fine grinding means and the polishing means is provided with etching means for etching the workpiece. Thus, one planarization apparatus can perform the machining sequence from the rough grinding to the etching. In this case, the etching means may etch the workpiece having been finely ground before the polishing, or may etch the workpiece having been polished. More specifically, the light etching aiming at cleaning the ground workpiece may be performed before the polishing. The etching may be performed after the polishing in order to eliminate impurities, heavy metal or dot defects harmful to element characteristics in the chips or to perform gettering. 
     According to the present invention, the planarization method uses the planarization apparatus comprises grinding means and polishing means. The polishing means polishes the workpiece by an amount more than an amount required for removing a machining deteriorated layer formed by the grinding, calculated by doubling a standard deviation, and less than a larger value between an amount required for correcting an unevenness of a thickness during the grinding and removing the machining deteriorated layer, calculated by sextupling a standard deviation, and 20 μm. This enables the desired machining without decreasing the availability of the apparatus. 
     According to the present invention, the planarization method uses the planarization apparatus comprises rough grinding means and fine grinding means as grinding means. The fine grinding means finely grinds by an amount more than an amount required for removing a machining deteriorated layer formed by the rough grinding, calculated by doubling a standard deviation, and less than a larger value between an amount required for correcting an unevenness of a thickness during the rough grinding and removing the machining deteriorated layer, calculated by sextupling a standard deviation, and 150 μm. The polishing means polishes the workpiece by an amount more than an amount required for removing a machining deteriorated layer formed by the fine grinding, calculated by doubling a standard deviation, and less than a larger value between an amount required for correcting an unevenness of a thickness during the fine grinding and removing the machining deteriorated layer, calculated by sextupling a standard deviation, and 20 μm. 
     According to the present invention, the planarization apparatus has a sensor for measuring the thickness of the workpiece prior to machining or during machining, and the planarization apparatus controls the amount of material to be ground or polished in accordance with a measured value. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The nature of this invention, as well as other objects and advantages thereof, will be explained in the following with reference to the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures and wherein: 
         FIG. 1  is a perspective view showing a semiconductor planarization apparatus according to an embodiment of the present invention; 
         FIG. 2  is a plan view showing the planarization apparatus in  FIG. 1 ; 
         FIG. 3  is a sectional view showing the structure of a polishing stage in the planarization apparatus in  FIG. 1 ; 
         FIG. 4  is a perspective view showing a partition of the planarization apparatus in  FIG. 1 ; 
         FIG. 5  is a plan view showing the partition in  FIG. 4 ; 
         FIG. 6  is a sectional view of the partition taken along line  6 — 6  in  FIG. 5 ; 
         FIG. 7  is an explanation drawing showing the state wherein a chuck and a spindle are separated by a fluid joint; 
         FIG. 8  is an explanation drawing showing the state wherein the chuck and the spindle are connected through the fluid joint; 
         FIG. 9  is a view showing the structure of a freezing chuck unit; 
         FIG. 10  is a side view showing a wafer thickness gage; 
         FIG. 11  is a flow chart showing a process for controlling the thickness of the wafer in the planarization apparatus; 
         FIG. 12  is a table showing machining speeds, amounts of material to be machined and machining times in a rough grinding, a fine grinding and a polishing; 
         FIG. 13  is a plan view showing the first embodiment of a planarization apparatus provided with an etching unit; 
         FIG. 14  a sectional view showing the structure of the etching unit in  FIG. 13 ; 
         FIG. 15  is a plan view showing the second embodiment of a planarization apparatus provided with an etching unit; and 
         FIG. 16  is a sectional view showing the structure of the etching unit in FIG.  15 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     This invention will be described in further detail by way of example with reference to the accompanying drawings. 
       FIG. 1  is a perspective view showing a semiconductor wafer planarization apparatus, and  FIG. 2  is a plan view thereof. 
     As shown in  FIG. 1 , a body  12  of a planarization apparatus  10  has a cassette housing stage  14 , an alignment stage  16 , a rough grinding stage  18 , a fine grinding stage  20 , a polishing stage  22 , a polishing pad cleaning stage  23 , a polishing pad dressing stage  27  and a wafer cleaning stage  24 . The rough grinding stage  18 , the fine grinding stage  20  and the polishing stage  22  are partitioned by a partition  25  indicated by alternate long and two short dashes lines in  FIG. 2  in order to prevent machining liquids used at the stages  18 ,  20 ,  22  from splashing to the adjacent stages. 
     The partition  25  is fixed on an index table  34  as shown in  FIGS. 4 and 5  and is cross-shaped in such a manner as to partition four chucks (equivalent to holding means)  32 ,  36 ,  38 ,  40  disposed on the index table  34 . The polishing stage  22  is covered with a casing  102  having a top board  100  so that the polishing stage  22  can be separate from the other stages. As shown in  FIG. 6 , a brush  104  is attached to the side of the casing  102  along the partition  25 . When the chuck  40  is positioned at a machining position, the brush  104  comes into contact with a top  25 A and a side  25 B of the partition  25 . Thus, the casing  102 , the partition  25  and the brush  104  keep the polishing stage  22  almost airtight. This prevents a grinding fluid used at the fine grinding stage  20  and chips from entering the polishing stage  22 , and prevents a polishing fluid used at the polishing stage  22  from splashing. It is therefore possible to prevent a malfunction resulting from the mixture of both fluids. The polishing stage  22  of this embodiment performs a chemical-mechanical polishing, and the polishing fluid includes a chemical polishing agent. If the grinding fluid is mixed in this polishing fluid, the concentration of the chemical polishing agent is lowered and this increases the machining time. The use of the partition  25  solves this problem. 
     As shown in  FIGS. 4 and 5 , the rough grinding stage  18  is enclosed by the side of the body  12 , a top board  106  and the partition  25 . Likewise, the fine grinding stage  20  is enclosed by the side of the body  12 , a top board  108  and the partition  25 . The top boards  100 ,  106 ,  108  have holes  101 ,  107 ,  109 , through which heads of the stages are inserted. In  FIG. 5 , reference numeral  110  denotes a brush for separating the rough grinding stage  18  from the outside, and the brush  110  is in contact with the top and side of the partition  25 . 
     Two cassettes  26  are detachably mounted at the cassette housing stage  14  in  FIGS. 1 and 2 , and the cassettes  26  contain a number of wafers whose reverses have not yet been ground. A hand  31  of a transfer robot  30  holds the wafers  28  individually, and sequentially transfers the wafers  28  to the alignment stage  16 . The transfer robot  30  may be suspended from a beam (not illustrated) standing on the body  12  through a lift, and may be arranged at the top  12 A of the body  12 . Suspending the transfer robot  30  decreases an interval between the cassette housing stage  14  and the alignment stage  16 , and this reduces the size of the planarization apparatus  10 . The transfer robot  30  is a well-known multi-joint robot, and this will not be described here. 
     The alignment stage  16  positions the wafer  28 , transferred from the cassettes  26 , at a predetermined position. The wafer  28  positioned by the alignment stage  16  is held by the hand  31  of the transfer robot  30  again, and is transferred toward the empty chuck  32 . The wafer  28  is held on the suction face of the chuck  32 . 
     The chuck  32  is disposed on the index table  34 , and the chucks  36 ,  38 ,  40  having the same function are disposed at intervals of 90° along the circumference of a rotary shaft  35  indicated by broken lines in FIG.  2 . The rotary shaft  35  connects to a spindle (not illustrated) of a motor (equivalent to a moving means)  37  indicated by broken lines in FIG.  2 . The chuck  36  is positioned at the rough grinding stage  18 , which roughly grinds the held wafer  28 . The chuck  38  is located at the fine grinding stage  20 , which finishes the held wafer  28  (fine-grinding and spark-out grinding). The chuck  40  is located at the polishing stage  22 , which polishes the held wafer  28  to eliminate a machining deteriorated layer caused by the grindings and the unevenness of the thickness of the wafer  28 . 
     The bottoms of the chucks  32 ,  36 ,  38 ,  40  connect to spindles  94  of rotary motors  92  as shown in  FIG. 3 , and the motors  92  rotate the chucks  32 ,  36 ,  38 ,  40 . The motors  92  are supported on the index table  34  through support members  93 . Thus, the motor  37  moves the chucks  32 ,  36 ,  38 ,  40  in the state wherein the spindles  94  of the motors  92  connect to the chucks  32 ,  36 ,  38 ,  40 . This eliminates the necessity for separating the spindles  94  from the chucks  32 ,  36 ,  38 ,  40  and connecting the chucks  32 ,  36 ,  38 ,  40  to the spindles  94  of the motors  92  located at the next positions every time the motor  37  moves the chucks  32 ,  36 ,  38 ,  40 . 
     In  FIG. 3 , the spindles  94  of the motors  92  connect to the chucks  32 ,  36 , 38 ,  40 , but this invention should not be restricted to this. As shown in  FIG. 7 and 8 , the spindles  94  may be detachably connected to the chucks  32 ,  36 ,  38 ,  40  through connecting members  112 . In this case, the connecting members  112  are separated from the chucks  32 ,  36 ,  38 ,  40  and the motor  37  moves only the chucks  32 ,  36 ,  38 ,  40  every time the chucks  32 ,  36 ,  38 ,  40  are moved. This reduces the load on the motor  37  and reduces the cost of the apparatus since it is necessary to provide only spindles and motors suitable for the rough grinding, the fine grinding and the polishing. 
     In  FIG. 7 , the chucks  32 ,  36 ,  38 ,  40  are placed on steps  116  of openings  114  formed in the index table  34 . Pistons  120  of cylinders  118  connect to the bottoms of the motors  92 . If the pistons  120  are expanded as shown in  FIG. 8 , the connecting members  112  are moved through the openings  114  and are fitted in concave parts  122  formed at the bottoms of the chucks  32 ,  36 ,  38 ,  40 . The continuous expansion of the pistons  120  moves the chucks  32 ,  36 ,  38 ,  40  from the index table toward grinding positions where grinding wheels  46 ,  54  grind the wafers  28 . 
     The suction face of the chucks  32 ,  36 ,  38 ,  40  of this embodiment are made of porous material  124  composed of a sintered body such as ceramics. If the connecting members  112  are connected to the concave parts  122 , fluid joints connect to the concave parts  122 . Thus, a suction force of suction pumps (not illustrated) connected to the concave parts  112  acts on the porous material  124  through air passages  126 , and this causes the wafer  28  to be securely held on the surface of the porous material  124 . When the connecting members  112  are separated from the concave parts  122 , a check valve (not illustrated) maintains the suction force. 
     In this embodiment, the chucks  32 ,  36 ,  38 ,  40  are used to hold the wafers  28  by suction, but a freezing chuck unit  128  in  FIG. 9  may be used instead of the chucks  32 ,  36 ,  38 , and  40 . 
     The freezing chuck unit  128  comprises a chuck plate  130 , a controller  132  and a cooling water supply unit  134 . The controller  132  applies a voltage to the chuck plate  130 , and the resulting Peltier effect freezes and holds the wafer  28  on the chuck plate  130  through ice film. The chuck plate  130  forms a closed circuit by connecting two kinds of metals (e.g., Cu and Bi) and passing an electric current through a contact thereof, thereby freezes and holds the wafer  28  on a thermo-element (Cu plate). The cooling water supply unit  134  supplies cooling water to a thermo-element (Bi plate) in order to cool the heat generated at the thermo-element (Bi plate). An electrostatic chuck unit, which holds the wafer by static electricity, may be used instead of the freezing chuck unit  128 . 
     The suction face of the chuck  32  located at the chucking position in  FIG. 2  is cleaned by a cleaning unit  42  (see  FIG. 2 ) before the chuck  32  receives the wafer  28 . The cleaning unit  42  is slidably provided on the rail  44 . To clean the suction face, the cleaning unit  42  is moved along the rail  44  and is positioned above the chuck  32 . The cleaning unit  42  has a removal member  43 , which comes into contact with the suction face of the chuck  32  to remove the sludge, etc. from the suction face. If the suction face of the chuck  32  is made of porous material composed of a sintered body such as ceramics, the removal member  43  is made of the porous material. 
     A pair of gages  136 ,  138  in  FIG. 10  measures the thickness of the wafer  28  held by the chuck  32 . The gages  136 ,  138  have contacts  140 ,  142 , respectively. The contact  140  contacts the top (the reverse) of the wafer  28 , and the contact  142  contacts the top of the chuck  32 . The gages  136 ,  138  can determine the thickness of the wafer  28  as a difference between readings of an in-process gage with the top of the chuck  32  being a reference point. 
     Rotating the index table  34  by 90° in the direction of an arrow A in  FIGS. 1 and 2  positions the measured wafer  28  on the rough grinding stage  18 . A cup-shaped grinding wheel  46  roughly grinds the reverse of the wafer  28 . As shown in  FIG. 1 , the cup-shaped grinding wheel  46  connects to an output shaft (not illustrated) of a motor  48 , and is attached to a grinding wheel feed unit  52  through a support casing  50  of the motor  48 . The grinding wheel feed unit  52  moves up and down the cup-shaped grinding wheel  46  as well as the motor  48 , and the downward movement causes the cup-shaped grinding wheel  46  to be pressed against the reverse of the wafer  28 . Thus, the reverse of the wafer  28  is roughly ground. The downward movement amount of the cup-shaped grinding wheel  46 , i.e. the amount of material removed by the cup-shaped grinding wheel  46  is determined according to a previously-registered reference position of the cup-shaped grinding wheel  46  and the thickness of the wafer  28  detected by the gages  136 ,  138 . 
     The thickness of the wafer  28  whose reverse has been roughly ground by the rough grinding stage  18  is measured by thickness gages with the same structure in  FIG. 10  after the cup-shaped grinding wheel  46  moves away from the wafer  28 . Rotating the index table  34  by 90° in the direction of the arrow A positions the measured wafer  28  on the fine grinding stage  20 , and the cup-shaped grinding wheel  54  finely grinds and spark-out grinds the wafer  28 . The structure of the fine grinding stage  20  will not be explained here since it has the same structure as the rough grinding stage  18 . In this embodiment, there are two grinding stages, but it is possible to provide only one grinding stage. The thickness gages may measure the thickness of the wafer  28  in line. 
     The thickness of the wafer  28  whose reverse has been finely ground by the fine grinding stage  20  is measured by thickness gages with the same structure in  FIG. 10  after the cup-shaped grinding wheel  54  moves away from the wafer  28 . Rotating the index table by 90° in the direction of the arrow A positions the measured wafer  28  at the polishing stage  22 . The wafer  28  is polished by a polishing pad  56  of the polishing stage  22  in FIG.  3  and slurry supplied from the polishing pad  56 . Consequently, the machining deteriorated layer is removed from the reverse of the wafer  28 . The thickness gages may measure the thickness of the wafer  28  in line. 
     A description will be given of the control on the thickness of the wafer by the planarization apparatus  10  with reference to FIG.  11 . First, the initial thickness of the wafer is measured before the rough grinding (S 100 ), and the amount of material to be machined in the rough grinding is determined according to the measured thickness. Then, the wafer  28  is roughly ground at the rough grinding stage  18  (S 110 ). The thickness of the roughly-ground wafer is measured (S 120 ), and the amount of material to be machined in the fine grinding is determined according to the measured thickness. The wafer  28  is finely ground at the fine grinding stage  20  (S 130 ). Then, the thickness of the finely-ground wafer is measured, and the polishing time is determined according to the measured thickness, the polishing conditions and the final thickness (S 140 ). The wafer  28  is polished at the polishing stage  22  (S 150 ). The planarization apparatus  10  controls the thickness of the wafer  28  in this manner. 
     The amount of material to be machined at the fine grinding stage  20  is preferably more than the amount required for removing the machining deteriorated layer generated by the rough grinding, calculated by doubling a standard deviation. It is preferably less than the larger value between the amount required for correcting the unevenness of the thickness during the rough grinding and removing the machining deteriorated layer, calculated by sextupling the standard deviation, and 150 μm. This enables the removal of the machining deteriorated layer generated by the rough grinding without decreasing the availability. 
     If the amount required for removing the machining deteriorated layer formed by the rough grinding is calculated by multiplying the standard deviation by less than two, the machining deteriorated layer cannot always be removed completely. On the other hand, if the amount required for removing the unevenness of the thickness and the machining deteriorated layer generated during the rough grinding is set at a value in excess of the larger value between a value found by multiplying a standard deviation by six, and 150 μm; the machining time becomes longer and the rate of operation is lowered. 
     The amount of material to be machined at the polishing stage  22  is preferably more than the amount required for removing the machining deteriorated layer generated by the fine grinding, calculated by doubling a standard deviation. It is preferably less than the larger value between the amount required for correcting the unevenness of the thickness during the fine grinding and removing the machining deteriorated layer, calculated by sextupling the standard deviation, and 20 μm. This enables the removal of the machining deteriorated layer generated by the fine grinding without lowering the rate of operation. 
     If the amount of material for removing the machining deteriorated layer formed by the fine grinding is calculated by multiplying the standard deviation by less than two, the machining deteriorated layer cannot always be removed without fail. On the other hand, if the amount required for correcting the unevenness of the thickness during the fine grinding and removing the machining deteriorated layer is set at a value that exceeds the larger value found by multiplying a standard deviation by six, and 20 μm; the machining time becomes longer and the rate of operation is lowered. 
       FIG. 12  is a table showing an example of the machining. If the wafer with a diameter of 200 mm and an initial thickness of 725 μm is to be machined to the thickness of 50 μm; the rough grinding speed, the fine grinding speed and the polishing speed are set at 225 (μm/min), 65 (μm/min) and 6 (μm/min), respectively, and the amounts of material to be machined in the fine grinding, the rough grinding and the polishing are set at 510 μm, 150 μm and 14.9 μm, respectively. In this case, the rough grinding time, the fine grinding time and the polishing time are substantially equal (2.27-2.48 min), so that the wafer  28  with the thickness of 725 μm can be machined to the thickness of 50 μm without lowering the rate of operation. In this case, the standard deviation of the unevenness in the thickness during the fine grinding is 2.25 μm, and six times the standard deviation is 13.5 μm. The mean of the depth of the machining deteriorated layer during the fine grinding is 0.7 μm, the standard deviation of the depth of the machining deteriorated layer is 0.11 μm, and six times the standard deviation is 0.66 μm. The maximum depth of the machining deteriorated layer is 1.36 μm. Therefore, the amount of material for eliminating the unevenness of the thickness at the fine grinding and removing the machining deteriorated layer can be set at 14.9 μm. 
     The unevenness of the thickness and the machining deteriorated layer cannot always be removed without fail within the machining time that is calculated in the above-mentioned manner. 
     To solve this problem, in this embodiment, the amount of material 150 μm in the fine grinding is compared with the amount required for eliminating the unevenness of the thickness and the machining deteriorated layer in the rough grinding, calculated by doubling a standard deviation. If the former is larger, the amount of material to be machined is set at 150 μm. If the latter is larger, the amount of material is set at the latter amount. Consequently, the unevenness of the thickness and the machining deteriorated layer can be eliminated without lowering the rate of operation and without failure during the fine grinding. 
     Moreover, the amount of material to be machined in the polishing is compared with the amount required for eliminating the unevenness of the thickness and the machining deteriorated layer in the fine grinding. If the former is larger, the amount of material to be machined is set at 20 μm. If the latter is larger than 20 μm, the amount of material to be machined is set at the latter value. It is therefore possible to eliminate the unevenness of the thickness and the machining deteriorated layer without lowering the rate of operation and without failure during the polishing. 
       FIG. 3  shows the structure of the polishing stage  22 . 
     The polishing pad  56  of the polishing stage  22  in  FIG. 3  is attached to a polishing head  61  connected to an output shaft  60  of a motor (equivalent to a rotating means)  58 . Guide blocks  62  of a direct-acting guide are provided at the side of the motor  58 . The guide blocks  62  are inserted into a guide rail  66  formed at the side of a support plate  64  in such a manner as to freely move vertically. Thus, the polishing pad  56  and the motor  58  are attached to the support plate  64  in such a manner as to freely move in the vertical direction. 
     The support plate  64  is provided at an end of a horizontally-arranged long arm  68 . A base end of the arm  68  connects to an output shaft  74  of a motor  72  in a casing  70 . Running the motor  72  rotates the arm  68  about the output shaft  74 . Consequently, the polishing pad  56  can be moved within a range between the polishing position indicated by a solid line in  FIG. 1 , the polishing pad cleaning position for the polishing pad cleaning stage  23  and the dressing position for the polishing pad dressing stage  27 . When the polishing pad  56  is moved to the polishing pad cleaning position, the polishing pad cleaning stage  23  removes the polishing chips and the like from the surface of the polishing pad  56 . The polishing pad  56  is made of, for example, polyurethane foam, and the polishing pad cleaning stage  23  has a removal member such as a brush for removing the polishing chips. When the polishing pad  56  is cleaned, the removal member and the polishing pad  56  are rotated by the motor  58  (see FIG.  3 ). The polishing pad dressing stage  27  is made of, for example, of polyurethane foam as is the case with the polishing pad  56 . 
     Guide blocks  76  of a direct-acting guide are provided at the side of the casing  70 , and the guide blocks  76  are inserted into a guide rail  80  formed at the side of a screw feed unit housing  78  in such a manner as to freely move vertically. A nut member  82  projects from the side of the casing  70 . The nut member  82  is inserted into the housing  78  through an opening  79  formed in the housing  78 , and is screwed down on a thread rod  81  of a screw feed unit (equivalent to a positioning mechanism). An output shaft  84  of a motor  82  connects to the top end of the thread rod  81 . Running the motor  82  and the rotating the thread rod  81  vertically move the casing  70  due to the feeding of the screw feed unit and the straight movement of the guide blocks  76  and the rail  80 . Consequently, the polishing pad  56  is moved vertically to set a desired interval between the polishing head  61  and the wafer  28 . 
     A piston  88  of an air cylinder apparatus (equivalent to a pressurizing mechanism)  86  connects to the top of the motor  58  through a hole  69  of the arm  68 . The air cylinder apparatus  86  connects to a regulator  90  that controls the inner pressure P of the cylinder. Therefore, the pressing force of the polishing pad  56  against the wafer  28  can be controlled by controlling the inner pressure P with the regulator  90 . 
     After the rotation of the arm  68  moves the polishing pad  56  away from the wafer  28 , the wafer  28  having been polished by the polishing stage  22 , is held by a hand  97  of a robot  96  in FIG.  2  and is transferred to the wafer cleaning stage  24 . The robot  96  is not illustrated in FIG.  1 . The machining deteriorated layer has already been eliminated from the polished wafer  28  to prevent the wafer  28  from being damaged. Therefore, the wafer  28  is not damaged during the transfer by the robot  96  and the cleaning by the wafer cleaning stage  24 . 
     In this embodiment, the polishing pad  56  is used to polish the wafer  28 , but this invention should not be restricted to this. For example, the wafer  28  may be polished by a polishing wheel or electrophoresis of abrasive grains. In this case, it is preferable to perform a constant cutting depth polishing. 
     A stage having a rinsing function and a spin drying function is employed as the wafer cleaning stage  24 . The wafer  28  cleaned and dried by the wafer cleaning stage  24  is held by the hand  31  of the robot  31 , and is placed on a predetermined shelf of the cassettes  26 . That completes the wafer machining process in the planarization apparatus  10  of this embodiment. 
     As stated above, the planarization apparatus  10  according to this embodiment roughly grinds, finely grinds and polishes the wafer  28  since the body  12  is provided with the polishing stage  22  as well as the rough grinding stage  18  and the fine grinding stage  20 . 
     This eliminates the necessity of transferring the wafer  28  from the planarization apparatus  10  to an etching apparatus. Thus, the rough grinding stage  18  and the fine grinding stage  20  can grind the wafer  28  to the thickness close to the standardized thickness. The conventional planarization apparatus grinds the wafer in such a manner as to remain the etching allowance of 150 μm in order to prevent the wafer from being damaged during the transfer, whereas the planarization apparatus  10  of this embodiment may grind the wafer with the polishing allowance of, for example, 3 μm. 
     This substantially reduces the polishing time, and increases the throughput. Polishing the wafer  28  removes the machining deteriorated layer formed by the grindings, and this eliminates the necessity for etching the wafer  28  in the post-treatment. This simplifies the entire structure of the wafer manufacturing line. Moreover, the planarization apparatus  10  has the polishing pad cleaning stage  23  for cleaning the polishing pad  56  of the polishing stage  22  and the polishing pad dressing stage  27  for dressing the polishing pad  56 . Therefore, the polishing pad  56  can be cleaned and dressed in the same apparatus  10 . Consequently, the polishing pad  56  can be handled easily. There is provided a sensing means for sensing a loading in the polishing pad  56  (such a loading as to affect the machining). If the apparatus is automatically controlled so that the polishing pad dressing stage  27  dresses the polishing pad  56  when the sensing means senses the loading, the planarization apparatus  10  can be fully automated. An example of the sensing means is a means for sensing the rotational torque of the motor  58  for the polishing pad  56 . The polishing pad  56  is cleaned when the rotational torque exceeds a reference value. 
     The planarization apparatus  10  can roughly grind, finely grind and polish the wafer  28 , which is held by the same chuck  32  ( 36 ,  38 ,  40 ), by rotating the index table  34 . This prevents the wafer from being damaged due to the transfer of the wafer  28 , and enables the accurate machining of the wafer  28 . On the other hand, if the wafer  28  is transferred from one chuck to another at each stage, the wafer may be damaged during the transfer. Moreover, the accuracy of the suction face of the chuck changes every time the wafer is transferred, and the changes in the accuracy affect the wafer machining accuracy. This makes impossible the accurate machining. Thus, the planarization apparatus  10  of this embodiment solves the problem of the conventional apparatus. 
     Furthermore, the polishing stage  22  removes the machining deteriorated layer from the wafer  28  and eliminates the unevenness in the thickness of the wafer  28  by extending the machining time. 
     In this embodiment, the wafer is used as a workpiece, but this invention should not be restricted to this. The planarization apparatus of this embodiment may also be applied to any kinds of workpieces that require the polishing after the grinding. 
       FIG. 13  is a plan view showing the first embodiment of a planarization apparatus  152  provided with an etching unit  150 . Parts similar to those described with reference to  FIG. 10  are denoted by the same reference numerals, and they will not be described. 
     The planarization apparatus  152  in  FIG. 13  etches the wafer  28  polished by the polishing stage  22 . The robot  97  holds the polished wafer  28 , which is positioned on the chuck  32  by rotating the index table  34  by 90° clockwise. Then, the robot  97  transfers the wafer  28  to the cleaning stage  24 , which cleans the wafer  28 . The cleaned wafer  28  is transferred to the etching unit  150 . Wafers that are not etched while being held by the chuck may be damaged during the transfer by the robot  97 . However, since the machining deteriorated layer is removed at the polishing stage  22  prior to the etching process, the wafer  28  is never damaged during the transfer by the robot  97 . 
     The etching unit  150  is a spin etching unit, which is comprised mainly of a chuck  154  for holding the wafer  28  by suction, a motor  156  and a spindle  158  for rotating the chuck  154 , a nozzle  162  for supplying an etching liquid  160 , and an etching tank  164 . In the etching unit  150 , the wafer  28  is held by a porous material  155  of the chuck  154 , and the etching liquid  160  is supplied to the center of the top of the wafer  28  through the nozzle  162  while the motor  156  is rotating the wafer  28  at a predetermined rotation speed. The radially-diffused etching liquid  160  etches the wafer  28 . The nozzle  162  connects to an etching liquid tank  168  through a pump  166 , and running the pump  166  supplies the etching liquid  160  from the etching liquid tank  168  through the nozzle  162 . 
     The spindle  158  is inserted into a hole  172  formed at the center of a bottom  170 , and the bottom  170  is inclined downward toward the outer peripheral in order to prevent the etching liquid  160  scatted from the wafer  28  from leaking through the hole  172 . A drain pipe  174  connects to the outer periphery of the bottom  170 , and the etching liquid is discharged through the drainpipe  174 . 
     In the planarization apparatus  152  of this embodiment, the polishing stage  22  has a wafer cleaning unit  176  as shown in  FIG. 13 , and a spin cleaner  178  is provided near the cleaning stage  24 . The wafer cleaning unit  176  is capable of running on a pair of rails  180 , and is moved to a position above the wafer  28  held on the chuck in order to clean the wafer  28  before or after the polishing. On the other hand, the spin cleaner  178  cleans the wafer  28  before and after the etching. The etched wafer  28  having been transferred to the spin cleaner  178  is spin cleaned, and the robot  97  holds the wafer  28  again and places it on a predetermined shelf of the cassettes  26 . 
       FIG. 15  is a plan view showing the second embodiment of a planarization apparatus  192  provided with an etching unit  190 . Parts similar to those of the planarization apparatus  10  in FIG.  2  and the planarization apparatus  152  in  FIG. 13  are denoted by the same reference numerals, and they will not be described. 
     The planarization apparatus  192  in  FIG. 15  etches the wafer  28  finely ground by the fine grinding stage  20 , and more particularly etches the wafer  28  before polishing. The finely-ground wafer  28  is transferred to the etching unit  190  by rotating the index table  34  clockwise by 90°. The etching unit  190  etches the wafer  28  held on the chuck  40 . The robot  97  holds the etched wafer  28  on the chuck  32 . Then, the robot  97  transfers the wafer  28  to the cleaning stage  24 . The wafer  24  is cleaned at the cleaning stage  24 , and is then transferred to the polishing stage  22 . The planarization apparatus  192  etches the wafer  28  held on the chuck to remove the machining deteriorated layer. Thus, the wafer  28  is never damaged during the transfer by the robot  97 . 
     The etching unit  190  is a spin etching unit, which is comprised mainly of the chuck  40  for holding the wafer  28  by suction, a motor  194  and a spindle  196  for rotating the chuck  40 , a nozzle  200  for supplying an etching liquid  198 , and an etching tank  202 . 
     The chuck  40  is placed on a step  206  of an opening  204  formed in the index table  34 , and a piston  210  of a cylinder  208  connects to the bottom of the motor  194 . When the piston  210  is contracted, the piston  210  is located at a position away from the chuck  40 , and when the piston  210  is expanded as shown in  FIG. 16 , the spindle  196  passes through the opening  204  and a connecting member  212  provided at the top of the spindle  196  is fitted in a concave part (not illustrated) formed at the bottom of the chuck  40  so that the spindle  196  can be connected to the chuck  40 . The continuous expansion of the piston  210  rises the chuck  40  from the index table  34 , and positions the chuck  40  in the etching tank  202  through a hole  216  formed at the bottom  214  of the etching tank  202 . 
     In the etching unit  190 , the wafer  28  is held by the porous material  41  of the chuck  40 , and the etching liquid  198  is supplied to the center of the top of the wafer  28 , that is rotated at a predetermined rotation speed by the motor  194 , through the nozzle  200  and the radially-diffused etching liquid  198  etches the wafer  28 . The nozzle  200  connects to an etching liquid tank  218  through a pump  217 , and running the pump  217  supplies the etching liquid  198  from the etching liquid tank  218  through the nozzle  200 . 
     The hole  216  is formed at the center of the bottom  214  of the etching tank  202 , and the bottom  214  is inclined downward toward the outer periphery in order to prevent the etching liquid  198  scattered from the wafer  28  from leaking through the hole  216 . A drain pipe  220  connects to the outer periphery of the bottom  214 , and the etching liquid is discharged through the drain pipe  220 . 
     The polishing stage  22  of the planarization apparatus  192  has a chuck  222  for holding the etched wafer  28  transferred by the robot  97  as shown in FIG.  15 . The wafer  28  is held on the chuck  222 , and is polished in the state wherein the polishing pad  56  is pressed against the top of the wafer  28  while a motor (not illustrated) for rotating the chuck  222  is rotating the wafer  28 . The robot  97  transfers the polished wafer  28  to the spin cleaner  178 , which spin cleans the wafer  28 . Then, the robot  97  holds the wafer  28  again and places it on a predetermined shelf of the cassettes  26 . 
     As set forth hereinabove, the planarization apparatus according to the present invention has both the workpiece grinding means and the workpiece polishing means, so that the workpiece can be ground and polished by one planarization apparatus. This increases the throughput without damaging the workpiece. 
     According to the present invention, the workpiece is ground and polished in the state of being held by the same holding means, and this enables the accurate machining without damaging the workpiece. 
     According to the present invention, the planarization apparatus is provided with the cleaning means for cleaning the polishing pad of the polishing means and/or the dressing means. Thus, the same apparatus cleans and dresses the polishing pad when the polishing pad becomes stained or loaded. 
     According to the present invention, the polishing means or the etching means removes the machining deteriorated layer from one side of the workpiece ground by the grinding means and eliminates the unevenness in the thickness of the workpiece. This enables the accurate machining. 
     In the planarization method according to the present invention, the fine grinding means finely grinds the workpiece by an amount more than an amount required for removing a machining deteriorated layer formed by the rough grinding, calculated by doubling a standard deviation, and less than a larger value between an amount required for correcting an unevenness of a thickness during the rough grinding and removing the machining deteriorated layer, calculated by sextupling a standard deviation, and 150 μm. This enables the removal of the machining deteriorated layer formed by the rough grinding without lowering the rate of operation. The polishing means polishes the workpiece by an amount more than an amount required for removing a machining deteriorated layer formed by the fine grinding, calculated by doubling a standard deviation, and less than a larger value between an amount required for correcting an unevenness of a thickness during the fine grinding and removing the machining deteriorated layer, calculated by sextupling a standard deviation, and 20 μm. This enables the removal of the machining deteriorated layer formed by the fine grinding without lowering the rate of operation. 
     It should be understood, however, that there is no intention to limit the invention to the specific forms disclosed, but on the contrary, the invention is to cover all modifications, alternate constructions and equivalents falling within the spirit and scope of the invention as expressed in the appended claims.