Patent Publication Number: US-2021175113-A1

Title: Plate-shaped workpiece holding tool

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a plate-shaped workpiece holding tool for holding a plate-shaped workpiece such as a wafer under suction. 
     Description of the Related Art 
     Wafers having a plurality of devices such as integrated circuits (ICs) and large-scale integrated (LSI) circuits formed in respective areas on a face side that are demarcated by a plurality of projected dicing lines are divided into individual device chips by a dicing apparatus or a laser processing apparatus, and the divided device chips will be each used in electric equipment such as a mobile phone or a personal computer. 
     Wafers on which devices are to be formed are produced by slicing an ingot such as a silicon ingot or a silicon carbide (SiC) ingot with a wire saw or a laser processing apparatus. Wafers sliced from an ingot have face and reverse sides ground and polished to a mirror finish (see, for example, JP 2000-94221A and JP 2016-111143A). 
     SUMMARY OF THE INVENTION 
     However, even the sliced wafers sliced from an ingot still have surface irregularities and undulations. When such a wafer is delivered by a holding tool having a holding surface that holds the wafer under suction thereon, gaps are likely to be formed between the wafer and the holding surface, introducing ambient air therethrough between the wafer and the holding surface. As a result, the wafer tends to drop off from the holding surface of the holding tool. 
     It is therefore an object of the present invention to provide a plate-shaped workpiece holding tool that is capable of reliably holding under suction a plate-shaped workpiece such as a wafer having surface irregularities and undulations. 
     In accordance with an aspect of the present invention, there is provided a plate-shaped workpiece holding tool for holding a plate-shaped workpiece under suction, including a holding base having a joint port coupled to a suction source and a holding surface for holding the plate-shaped workpiece thereon, a first O-ring disposed on the holding surface, a second O-ring disposed on the holding surface radially inwardly of the first O-ring, a suction port that is open in the holding surface between the first O-ring and the second O-ring and held in fluid communication with the joint port, and a liquid supply mechanism for forming a liquid seal between the holding surface and the plate-shaped workpiece radially outwardly of the first O-ring. 
     According to the present invention, even in a case where spaces are created between the first and second O-rings and an upper surface of the plate-shaped workpiece, such as a wafer, since the liquid seal is present radially outwardly of the first O-ring, ambient air is prevented from flowing in through the spaces between the first and second O-rings and the plate-shaped workpiece. Therefore, the workpiece can reliably be held under suction on the holding base even if the plate-shaped workpiece has surface irregularities and undulations. 
     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 an appended claim with reference to the attached drawings showing a preferred embodiment of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a plate-shaped workpiece holding tool according to an embodiment of the present invention; 
         FIG. 2  is a cross-sectional view of the plate-shaped workpiece holding tool illustrated in  FIG. 1 ; 
         FIG. 3  is a perspective view of a holding base of the plate-shaped workpiece holding tool illustrated in  FIG. 1 , the holding base being turned upside down to have its lower surface facing upwardly; 
         FIG. 4  is a cross-sectional view illustrating a manner in which the plate-shaped workpiece holding tool illustrated in  FIG. 1  is holding a plate-shaped workpiece; and 
         FIG. 5  is a perspective view of a grinding apparatus that incorporates the plate-shaped workpiece holding tool illustrated in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A plate-shaped workpiece holding tool according to a preferred embodiment of the present invention will hereinafter be described below with reference to the drawings. As illustrated in  FIGS. 1 and 2 , the plate-shaped workpiece holding tool, denoted by  2 , includes a holding base  4  having a joint port coupled to a suction source and a holding surface for holding a plate-shaped workpiece thereon, and a moving mechanism  6  for moving the holding base  4 . As illustrated in  FIG. 2 , the holding base  4  includes a main body  8  shaped as a circular plate, a hollow cylindrical portion  10  extending upwardly from a central area of an upper surface of the main body  8 , and a circular retainer  12  fixed to an upper end of the hollow cylindrical portion  10  and larger in diameter than the hollow cylindrical portion  10 . The hollow cylindrical joint port, denoted by  14 , is attached to the hollow cylindrical portion  10  and extends radially outwardly therefrom. The joint port  14  is coupled to the suction source, denoted by  18 , through a fluid channel  16 . 
     As illustrated in  FIG. 2 , the suction source  18  according to the present embodiment includes an ejector  20  shaped as a hollow cylinder as a whole, an air supply source  24  coupled to the ejector  20  through a fluid channel  22 , for supplying air to the ejector  20 , and a valve  26  for selectively opening and closing the fluid channel  22 . The ejector  20  includes a smaller-diameter portion  20   a  disposed in an axially intermediate portion thereof and smaller in inside diameter than the rest of the ejector  20 . The smaller-diameter portion  20   a  is connected to the fluid channel  16  extending to the joint port  14  of the holding base  4 . The ejector  20  has an axial end connected to the fluid channel  22  that extends to the air supply source  24 . 
     The suction source  18  operates as follows: When the air supply source  24  is actuated with the fluid channel  22  being opened by opening the valve  26 , the air supply source  24  supplies high-pressure air through the fluid channel  22  to the ejector  20 , developing a negative pressure in the small-diameter portion  20   a  to draw in air from the fluid channel  16 . The suction source  18  may include a suction pump. However, since the suction source  18  may draw in a liquid from a liquid supply mechanism  34  to be described later in some cases, the suction source  18  should preferably be of a configuration including the above-described ejector  20  that discharges the high-pressure air and the liquid from the other axial end of the ejector  20 . 
     As illustrated in  FIGS. 2 and 3 , the plate-shaped workpiece holding tool  2  further includes a first O-ring  28  disposed on a lower surface of the main body  8  that acts as the holding surface, a second O-ring  30  disposed on the lower surface of the main body  8  and positioned radially inwardly of the first O-ring  28 , a plurality of suction holes  32  that are open in the lower surface of the main body  8  radially between the first O-ring  28  and the second O-ring  30  and held in fluid communication with the joint port  14  via fluid channels defined in the main body  8  and the hollow cylindrical portion  10 , and the liquid supply mechanism, denoted by  34 , for forming a liquid seal between a portion of the holding surface that extends radially outwardly of the first O-ring  28  and a plate-shaped workpiece held on the holding surface of the holding base  4 . 
     Each of the first O-ring  28  and the second O-ring  30  is suitably made of an elastically deformable material such as synthetic rubber. The diameter of the second O-ring  30  is smaller than the diameter of the first O-ring  28 . As illustrated in  FIG. 2 , the lower surface of the main body  8  has a first annular groove  36  defined therein and a second annular groove  38  defined therein radially inwardly of the first annular groove  36  and smaller in diameter than the first annular groove  36 . The first O-ring  28  is fitted in the first annular groove  36 , and the second O-ring  30  is fitted in the second annular groove  38 . 
     As illustrated in  FIGS. 2 and 3 , the plurality of suction holes  32 , specifically four suction holes  32  according to the present embodiment, that are open in the lower surface of the main body  8  are positioned at spaced intervals in circumferential directions of the main body  8  between the first O-ring  28  and the second O-ring  30 . As can be understood from  FIG. 2 , the suction holes  32  are held in fluid communication with the fluid channels defined in the main body  8  and the hollow cylindrical portion  10  and also with the joint port  14 . 
     When the suction source  18  is actuated, it produces and transmits suction forces via the fluid channel  16 , the joint port  14 , and the fluid channels defined in the main body  8  and the hollow cylindrical portion  10  to the suction holes  32 , holding under suction a plate-shaped workpiece such as a wafer on the lower surface of the main body  8 . Thus, the lower surface of the main body  8  of the holding base  4  acts as the holding surface for holding the plate-shaped workpiece under suction thereon. When the plate-shaped workpiece is held under suction on the lower surface of the main body  8 , the first O-ring  28  and the second O-ring  30  have respective lower ends held in contact with an upper surface of the plate-shaped workpiece, creating gaps between the lower surface of the main body  8  and the upper surface of the plate-shaped workpiece, as illustrated in  FIG. 4 . 
     As illustrated in  FIGS. 2 and 3 , the liquid supply mechanism  34  includes an annular member  40  fixed to an outer circumferential surface of the main body  8  of the holding base  4 . The annular member  40  has a plurality of ejection holes  40   a  defined therein that are open in a lower surface thereof at circumferentially spaced intervals and a supply port  40   b  (see  FIGS. 1 and 2 ) held in fluid communication with the ejection holes  40   a . As illustrated in  FIG. 2 , the liquid supply mechanism  34  further includes a liquid supply source  44  coupled to the supply port  40   b  via a fluid channel  42  and a valve  46  for selectively opening and closing the fluid channel  42 . 
     As illustrated in  FIG. 4 , when the fluid channel  42  is opened by the valve  46  while the plate-shaped workpiece is being held under suction on the lower surface, i.e., the holding surface, of the main body  8  of the holding base  4 , the liquid supply source  44  supplies a liquid, such as water, through the fluid channel  42  to the supply port  40   b , from which the liquid is ejected out of the ejection holes  40   a , forming an annular liquid seal S between the lower surface, i.e., the holding surface, of the main body  8  of the holding base  4  and the upper surface of the plate-shaped workpiece such as a wafer W radially outwardly of the first O-ring  28 . With the plate-shaped workpiece holding tool  2  according to the present embodiment, the main body  8  of the holding base  4  and the annular member  40  of the liquid supply mechanism  34  are configured as separate members. However, the main body  8  and the annular member  40  may be configured as a single common member. 
     The moving mechanism  6  will be described below with reference to  FIGS. 1 and 2 . As illustrated in  FIG. 1 , the moving mechanism  6  includes an arm  48  extending essentially horizontally from an end thereof that is coupled to the holding base  4 , a rotational shaft  50  fixed to the other end of the arm  48  and extending vertically, an electric motor, not shown, for rotating the rotational shaft  50  about its vertical central axis, and lifting and lowering means, not shown, such as an electrically operated cylinder for lifting and lowering the rotational shaft  50 . 
     As illustrated in  FIG. 2 , the arm  48  has a vertically extending through opening  52  defined in the end thereof, and the hollow cylindrical portion  10  of the holding base  4  is inserted in the through opening  52 . The arm  48  also has a recess  54  defined in an upper portion of the end of the arm  48  circumferentially around an upper end of the through opening  52 , the recess  54  being larger in diameter than the through opening  52 . The retainer  12  of the holding base  4  is fitted in the recess  54 , preventing the holding base  4  from being dislodged from the arm  48 . 
     As can be understood from  FIG. 2 , a helical spring  56  is disposed between a lower surface of the end of the arm  48  and an upper surface of the main body  8  of the holding base  4 . When the lower surface of the holding base  4  is positioned on the upper surface of the plate-shaped workpiece, the holding base  4  is lifted, compressing the helical spring  56 . The helical spring  56  as it is compressed dampens shocks applied from the lower surface of the holding base  4  to the plate-shaped workpiece and allows the lower surface of the holding base  4  and the attracted surface of the plate-shaped workpiece such as the wafer W to fit each other better. 
     A process of delivering the plate-shaped workpiece, as the wafer W shaped as a circular plate, using the plate-shaped workpiece holding tool  2  as described above will be described below with reference to  FIG. 4 . 
     According to the process of delivering the wafer W using the plate-shaped workpiece holding tool  2 , the rotational shaft  50  is appropriately rotated about its central axis by the electric motor and appropriately lifted or lowered by the lifting and lowering means to position the lower surface, i.e., the holding surface, of the main body  8  of the holding base  4  above the wafer W that is placed on an appropriate table, not shown. Then, the rotational shaft  50  is lowered by the lifting and lowering means to bring the first and second O-rings  28  and  30  of the holding base  4  into intimate contact with an upper surface of the wafer W. 
     Then, while the fluid channel  22  is being opened by the valve  26 , the air supply source  24  is actuated to enable the ejector  20  to develop suction forces in the suction holes  32  in the holding base  4 , holding the upper surface of the wafer W under suction on the lower surface of the main body  8  of the holding base  4 . Then, as illustrated in  FIG. 4 , the fluid channel  42  is opened by the valve  46  to eject the liquid such as water from the liquid supply source  44  out of the ejection holes  40   a , forming the liquid seal S between the lower surface of the main body  8  and the upper surface of the wafer W radially outwardly of the first O-ring  28 . 
     Thereafter, the rotational shaft  50  is rotated about its central axis by the electric motor and appropriately lifted or lowered by the lifting and lowering means to deliver the wafer W held on the holding base  4  to bring the lower surface of the wafer W into contact with an upper surface of an appropriate table, not shown, on which the wafer W is to be placed. Then, the suction source  18  is inactivated to release the wafer W from the holding base  4 , whereupon the wafer W is placed onto the upper surface of the table. In this manner, the wafer W is delivered by the plate-shaped workpiece holding tool  2 . 
     With the plate-shaped workpiece holding tool  2  according to the present embodiment, even in a case where spaces are created between the lower ends of the first and second O-rings  28  and  30  and the upper surface of the wafer W, since the liquid seal S is present radially outwardly of the first O-ring  28 , ambient air is prevented from flowing in through the spaces between the first and second O-rings  28  and  30  and the wafer W. Therefore, the wafer W can reliably be held under suction on the holding base  4  even if the wafer W has surface irregularities and undulations. 
     The plate-shaped workpiece holding tool  2  may be mounted on a grinding apparatus  60  illustrated in  FIG. 5 . The grinding apparatus  60  includes a first cassette rest area  64   a  for placing thereon a first cassettes  62   a  housing a plurality of wafers W to be ground, a second cassette rest area  64   b  for placing thereon a second cassettes  62   b  housing the plurality of wafers W that have been ground, unloading and loading means  66  for unloading a wafer W to be ground from the first cassette  62   a  and loading a wafer W that has been ground into the second cassette  62   b , temporary rest means  68  for temporarily placing thereon the wafer W to be ground that has been unloaded from the first cassette  62   a  by the unloading and loading means  66 , first delivery means for delivering the wafer W to be ground that has been temporarily placed on the temporary rest means  68  to a chuck table  70 , grinding means  72  for grinding an upper surface of the wafer W held on the chuck table  70 , and second delivery means for delivering the wafer W that has been ground from the chuck table  70  to cleaning means  74 . Each of the first delivery means and the second delivery means referred to above may include the plate-shaped workpiece holding tool  2  described above. 
     In the grinding apparatus  60  illustrated in  FIG. 5 , the rotational shaft  50  of the moving mechanism  6  of the plate-shaped workpiece holding tool  2  is rotatably mounted on a base  76  of the grinding apparatus  60 . The wafer W that is delivered by the first delivery means may have surface irregularities and undulations because the wafer W is yet to be ground by the grinding means  72 . Since the plate-shaped workpiece holding tool  2  is capable of reliably holding under suction even the wafer W with surface irregularities and undulations, as described above, the wafer W is prevented from dropping off the plate-shaped workpiece holding tool  2  while being delivered thereby. 
     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 claim and all changes and modifications as fall within the equivalence of the scope of the claim are therefore to be embraced by the invention.