Abstract:
A method of transporting a semiconductor wafer having a ring-shaped stiffening portion can include the steps of pressing the semiconductor wafer from the back surface side to the front surface side thereof on a place different from a place at which the semiconductor wafer is to be held, the step of pressing the semiconductor wafer being conducted before holding the semiconductor wafer having the ring-shaped stiffening portion. The method can include releasing the attachment by suction of the front surface of the semiconductor wafer by supplying a positive pressure onto the chuck table, releasing pressing the semiconductor wafer from the back surface side to the front surface side thereof on the place different from the place at which the semiconductor wafer is to be held and picking up the semiconductor wafer having the ring-shaped stiffening portion from the chuck table while holding the semiconductor wafer.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Embodiments of the invention relate to methods and apparatuses for manufacturing a semiconductor device, and, in particular, to methods and apparatuses for manufacturing thin semiconductor devices such as power semiconductor devices used in power conversion devices. 
     2. Description of the Related Art 
     In power semiconductor devices represented by IGBTs (insulated gate bipolar transistors), methods are being developed to reduce the thickness of semiconductor substrates or wafers for producing semiconductor devices to achieve high performance of the devices. In addition, wafers with enlarged diameters are being developed to increase the number of power semiconductor elements that can be formed of a sheet of wafer, thereby reducing the costs of the semiconductor devices. 
     The reduction in wafer thickness, however, in certain circumstances, may cause a break at the peripheral edge of the wafer resulting in chipping of the wafer. Because thin wafers can show lowered mechanical strength, a problem can also arise that the wafer is liable to generate cracks, breakage, and deflection. These problems are noticeable in wafers with a large diameter, in particular. Thin and large diameter wafers showing large deflection have difficulties in transporting the wafer between manufacturing steps and in positioning the wafer on manufacturing devices. 
     The manufacturing process of vertical power semiconductor devices typically includes the steps of ion implantation, heat treatment (annealing), metallic film deposition, etc., also on the back surface of the wafer. Hence, the above-mentioned problems can cause difficulty in carrying out these steps on the back surface of the wafer. 
     In order to solve or minimize such problems, a wafer has been proposed that has a ring-shaped stiffening portion at the periphery of the back surface side of the wafer to reinforce the wafer having a reduced thickness. The wafer having a ring-shaped stiffening portion has the periphery of the back surface side thicker than the central portion of the wafer. Use of the wafer having a ring-shaped stiffening portion substantially reduces warp and deflection of the wafer and enhances strength of the wafer in handling the wafer in the transport step, preventing the wafer from generation of breakage and chipping. 
       FIGS. 12A through 12D  show shapes of a wafer having a ring-shaped stiffening portion.  FIG. 12A  shows a wafer with an orientation flat  110  having a ring-shaped stiffening portion  111  at the periphery of the wafer. 
       FIG. 12B  shows a wafer with a notch  120  having a ring-shaped stiffening portion  121  at the periphery of the wafer. 
     The inner portion of both the wafers inside the ring-shaped stiffening portion is a region for forming semiconductor device elements. 
     Japanese Unexamined Patent Application Publication No. 2007-173487 (also referred to herein as “Patent Document 1”), for example, discloses a method for fabricating a wafer having the ring-shaped stiffening portion. The method uses a grinding apparatus provided with a grinding member having a diameter smaller than that of the wafer and grinds the central portion of the wafer thin leaving the peripheral portion of the back surface side of the wafer to form a rib. 
       FIGS. 13A and 13B  are simplified sectional views showing a grinding step in the process of fabricating a wafer having the ring-shaped stiffening portion. 
     The following describes a step of fabricating a wafer having the ring-shaped stiffening portion with reference to  FIGS. 13A and 13B . The wafer  20  is first set in a cassette (not depicted) of a grinding apparatus. The wafer  20 , after positioning by a transport robot or the like, is then transported above a chuck table  10  and put on the surface of an attachment plate  12  of the chuck table  10 . The chuck table  10  is connected to a vacuum system (not depicted) that supplies a negative pressure through the attachment plate  12  to attract and hold the wafer  20  as shown in  FIG. 13A . The attachment plate  12  is made of porous ceramics, for example. 
     The grinding apparatus is provided with a grinding member  133  having a diameter smaller than that of the wafer. The grinding member  133  has a grindstone on the contact surface with the wafer. The grinding member  133  rotates on its own axis and the axis itself turns around on the wafer to grind the central portion of the wafer. 
     The grinding apparatus grinds only a central portion of the wafer leaving the peripheral portion with just the thickness as of the original wafer inputted to the grinding apparatus. Thus, a wafer  22  is fabricated having the central portion ground to a thin desired thickness as shown in  FIG. 13B .  FIG. 12C  shows a cross-section of a thin fabricated wafer, which has a ring-shaped stiffening portion  122  (a rib structure) at the periphery of the wafer. 
       FIGS. 14A ,  14 B, and  14 C are sectional views of essential parts in another example of grinding step in the process for fabricating a wafer having the ring-shaped stiffening portion. 
     The grinding apparatus of  FIGS. 14A ,  14 B, and  14 C is provided with two grinding members  131  and  132  containing abrasive grains of different grain sizes, whereas the grinding apparatus of  FIGS. 13A and 13B  has a single grinding member  133 . 
     In the grinding step of  FIGS. 14A ,  14 B, and  14 C, a semiconductor wafer  20  before grinding, which can be an eight inch wafer having a thickness of 725 μm, for example, after positioning by a transport robot (not depicted) or the like, is transported on a chuck table  10  and held by suction on the surface of an attachment plate  12 . In the first grinding step, as shown in  FIG. 14A , the central portion of the wafer  20  is ground using a grinding member  131  that is provided with a grindstone containing abrasive grains having a relatively large average grain size. The grinding step is conducted on the central portion of the wafer  20  down to a predetermined remaining thickness of 100 to 150 μm, for example, leaving the peripheral portion with a width of 1 to 5 mm, for example. After the central portion is ground to the desired thickness, the second grinding step is conducted, as shown in  FIG. 14B , using a grinding member  132  that is provided with a grindstone containing abrasive grains having a smaller average grain size than that of the grindstone provided on the grinding member  131 . The second grinding step grinds the back surface of the wafer processed by the first grinding step down to a predetermined thickness of 60 to 120 μm, for example, on the region with an inner circumference diameter smaller than that of the recessed inner circumference formed by the first grinding step. Thus, a wafer  23  having a ring-shaped stiffening portion is fabricated as shown in  FIG. 14C . Here, the second grinding step grinds the wafer to an inner diameter that is smaller than the recessed inner diameter formed in the first grinding step, because positioning accuracy of the grinding machine used in the second grinding step is taken into consideration Thus, the grinding member  132  used in the second grinding step does not become in contact with the side wall of the ring-shaped stiffening portion that is formed in the first grinding step. 
     The wafer is ground only on the central portion leaving the peripheral portion of the wafer. Thus, a wafer  23  as shown in  FIG. 14C  is fabricated that is machined to a desired thickness only on the central portion of the wafer, leaving the peripheral portion with the thickness as inputted into the grinding apparatus.  FIG. 12D  is a section view of the wafer having a ring-shaped stiffening portion (a rib structure)  123  at the periphery of the wafer. 
     The wafer having the ring-shaped stiffening portion formed at the peripheral region of the wafer is transferred to the next step for cleaning and drying. A transport device, not depicted in  FIGS. 13A ,  13 B,  14 A,  14 B, and  14 C, picks up the wafer from the attachment plate  12  of the chuck table  10  and transported to the predetermined destination. 
     For transporting a wafer having a ring-shaped-stiffening portion (a rib structure), Japanese Unexamined Patent Application Publication No. 2009-059763 (also referred to herein as “Patent Document 2”), for example, discloses a construction in which the upper surface of the ring-shaped portion is attracted by evacuation suction to transport the wafer. 
     In the process of grinding by a grinding apparatus to form the ring-shaped stiffening portion at the periphery of the wafer, the process is generally conducted in consecutive steps. 
     After a grinding step is finished, the wafer having the ring-shaped stiffening portion (a rib structure) is removed from the chuck table and transferred to the next step of cleaning and drying. At the same time, the attachment plate on the chuck table is cleaned with a blush or the like, and the next wafer to be ground is supplied and held by suction on the chuck table. 
       FIGS. 15A ,  15 B, and  15 C show conventional apparatus and process for transporting a wafer. A wafer  22  after completion of the grinding step and having a ring-shaped stiffening portion formed in the step is removed from the chuck table  10  and transferred to the next step by a transport device  80 . The transport device  80  is provided with an attracting member  81  at the lower end of a support member  82  and transmits a negative pressure from a vacuum system (not depicted) to the bottom surface of the attracting member  81  through a supply and exhaust system (not depicted) inside the support member  82 . The attracting member  81  attracts the thin central portion of the wafer  22  by suction as shown in  FIG. 15A . The wafer  22  is removed from the chuck table  10  by elevating the support member  82  with the attracting member  81  attracting the thin region of the wafer  22 . 
     While a wafer may be attracted only at the ring-shaped stiffening portion as disclosed in Patent Document 2, enough attraction so as to pick up and transport the wafer requires a flat and enough area of the upper surface of the ring-shaped stiffening portion to be attracted. The large area of the flat upper surface decreases the area of the central portion of the wafer, which is a device-forming region. 
     Attracting the wafer at the central portion thereof as shown in  FIG. 15A  ensures an area necessary for attracting the wafer and transportation without failure. 
     Alternatively, as shown in  FIG. 16A , the wafer after completion of the grinding step having a ring-shaped stiffening portion can be held at the outer peripheral end of the wafer by a holding member  92  provided at the end of an arm  91  of a transport device  90 . The arm  91  of the transport device  90  is movable so that the holding member  92  can approach and leave the ring-shaped stiffening portion of the wafer  22 . The holding member  92  moves from a position apart from the ring-shaped stiffening portion and holds the wafer  22  at the ring-shaped stiffening portion thereof. The arm  91  is then elevated while holding the ring-shaped stiffening portion of the wafer  22  with the holding member  92  to remove the wafer  22  from the chuck table  10 . This procedure allows the wafer  22  to be transferred to the next step without touching the thinned portion of the wafer  22 . 
     The process including the grinding step needs to be consecutive. The total time required for the consecutive process should be decreased, the consecutive process on the wafer including: transfer, placing, grinding, picking up, cleaning of the chuck table, and transfer to the next wafer. In the step of picking up the wafer to transfer to the next step, in particular, two problems must be solved simultaneously: to shorten the time for picking up the wafer and to prevent the wafer from any damage. 
     The chuck table  10  has the cleaning water dropped on the wafer in the grinding step and the cleaning water used in the cleaning process of the chuck table  10  remained on the attachment plate  12  ( FIG. 15A ) or a combined attachment plate of  13  and  14  ( FIG. 16B ). The attachment plate  12  ( FIG. 15A ) or a combined attachment plate of  13  and  14  ( FIG. 16B ) is referred to as the attachment plate  12  or  13  and  14 ′ in the following. 
     If the cleaning water is remained between the wafer  22  and the attachment plate  12  or  13  and  14 , the wafer  22  is adhered to the attachment plate  12  or  13  and  14  due to the surface tension of the cleaning water. In the grinding step for forming the ring-shaped stiffening portion, a negative pressure is supplied onto the attachment plate  12  or  13  and  14  from a vacuum system (not depicted) through a supply and exhaust path  11 . After the grinding step, the negative pressure is released from the attachment plate  12  or  13  and  14  on the chuck table  10 . However, after the release of the negative pressure, the presence of the cleaning water still inhibits the progress of air leakage through the porous attachment plate  12  or  13  and  14  of the chuck table  10  and causes the wafer  22  remaining adhered onto the attachment plate  12  or  13  and  14 . If the wafer  22  were left on the attachment plate  12  or  13  and  14  for a long period of time for example ten minutes, after the release of the negative pressure, the air leakage through the attachment plate  12  or  13  and  14  would be advanced and the wafer  2  would become easy to be removed. This means, however, leaves the problem to reduce the time required for picking up the wafer unsolved and lowers the efficiency of the transport operation. 
     If the wafer  22  adhered to the attachment plate  12  or  13  and  14  were forced to be separated, the thin wafer  22  vulnerable to break would cause chipping and cracking. Thus, the problem to pick up the wafer with no damage is left unsolved. 
     The time duration for picking up the wafer  22  can be shortened by supplying, after release of the negative pressure, water, air, or a mixture thereof (indicated by the reference symbols  70  and  71  in  FIGS. 15B ,  15 C and  16 B) to the attachment plate  12  or  13  and  14  on the chuck table  10  through the supply and exhaust path  11 . A positive pressure is given to the surface of the attachment plate  12  or  13  and  14  on the chuck table  10  by blowing up the water, air, or the mixture thereof ( 70 ,  71 ) through the attachment plate. The wafer after releasing adhesion to and separation from the attachment plate on the chuck table  10  is transferred to the next step by the transport device ( 80  in  FIG. 15A ,  90  in  FIGS. 16A and 16B ). 
     The blowing up of the water, air, or a mixture thereof  70 ,  71  shown in  FIGS. 15B ,  15 C, and  FIG. 16B  causes deformation in the wafer  22 . Stress concentration is liable to be developed especially at places damaged by the grindstone in the grinding step and places with varied curvature. Such places are indicated by the symbol  223  in  FIG. 13B  and the symbol  233  in  FIG. 14C . 
     The deformation of the wafer  22  is very likely to occur at the part B indicated in  FIG. 15C  where the wafer  22  is pushed by the attracting member  81  of the transport device  80  and at the part B indicated in  FIG. 16B  where the wafer  22  varies in thickness thereof. 
     The wafer  22  undergoes occurrence of cracks on the ground surface thereof due to such deformation that makes the wafer locally float apart as indicated by the arrow A in  FIG. 15C  and  FIG. 16B . Thus, there is a need in the art for an improved method and apparatus for manufacturing semiconductor devices. 
     SUMMARY OF THE INVENTION 
     Embodiments of the invention are directed to these and other needs, and to provide for a method and apparatus for manufacturing a semiconductor device that allows a wafer to be picked up safely and securely after the wafer is attracted on a chuck table and ground on the surface thereof. 
     In certain embodiments, a front surface of a semiconductor wafer is attracted to an attachment plate of a chuck table and a back surface of the semiconductor wafer is ground in a inside region to a recessed configuration leaving a ring-shaped stiffening portion at a periphery of the wafer. In a procedure of transporting the semiconductor wafer having the ring-shaped stiffening portion from the chuck table, before holding the wafer having the ring-shaped stiffening portion, the semiconductor wafer is pushed at a place different from a place to be held from the back surface side to the front surface side. A positive pressure is supplied to the chuck table to release adhesion of the front surface of the semiconductor wafer. After releasing the pressure against the semiconductor wafer at the different place from the place to be held from the back surface side to the front surface side, the semiconductor wafer having the ring-shaped stiffening portion is picked up from the chuck table while being held. 
     In some embodiments, a wafer can be safely and surely picked up and transported smoothly to the predetermined destination preventing the wafer from chipping and cracking. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIGS. 1A and 1B  show a first embodiment according to the present invention; 
         FIGS. 2A and 2B  show the first embodiment according to the present invention in the states following those in  FIGS. 1A and 1B ; 
         FIGS. 3A and 3B  show the first embodiment according to the present invention in the states following those in  FIGS. 2A and 2B ; 
         FIGS. 4A ,  4 B, and  4 C are timing charts in the first embodiment according to the present invention; 
         FIGS. 5A and 5B  show an essential part of a transport device in the first embodiment according to the present invention, in which  FIG. 5A  is a sectional view and  FIG. 5B  is a perspective view; 
         FIGS. 6A and 6B  show modifications of the transport device in the first embodiment according to the present invention; 
         FIGS. 7A and 7B  show a second embodiment according to the present invention; 
         FIGS. 8A and 8B  show the second embodiment according to the present invention in the states following those in  FIGS. 7A and 7B ; 
         FIGS. 9A and 9B  show the second embodiment according to the present invention in the states following those in  FIGS. 8A and 8B ; 
         FIGS. 10A ,  10 B, and  10 C are timing charts in the second embodiment according to the present invention; 
         FIGS. 11A and 11B  show modifications of the transport device in the second embodiment according to the present invention; 
         FIGS. 12A ,  12 B,  12 C, and  12 D show wafers having a ring-shaped stiffening portion; 
         FIGS. 13A and 13B  show a manufacturing process of a wafer having a ring-shaped stiffening portion; 
         FIGS. 14A ,  14 B, and  14 C show a manufacturing process of a wafer having a ring-shaped stiffening portion; 
         FIGS. 15A ,  15 B, and  15 C show a conventional transport device; and 
         FIGS. 16A and 16B  show a conventional transport device. 
     
    
    
     DETAILED DESCRIPTION 
     Now, some aspects of embodiments according to the present invention will be described in detail in the following with reference to the accompanying drawings. 
     &lt;First Embodiment&gt; 
     A wafer having a ring-shaped stiffening portion at the periphery of the wafer as shown in  FIGS. 12A through 12D  can be formed by a process as shown in  FIGS. 13A ,  13 B, and  FIGS. 14A ,  14 B, and  14 C. Descriptions on the  FIGS. 12A through 12D  and  FIGS. 13A ,  13 B, and  FIGS. 14A ,  14 B, and  14 C have been made in the foregoing and are not repeated here. 
       FIGS. 1A ,  1 B,  FIGS. 2A ,  2 B,  FIGS. 3A ,  3 B,  FIGS. 4A ,  4 B,  4 C, and  FIGS. 5A ,  5 B illustrates the first embodiment according to the present invention. Of the figures,  FIGS. 4A ,  4 B, and  4 C are timing charts showing pressures on some parts in the embodiment. 
       FIG. 1A  shows a wafer  22  attached by suction on the surface of an attachment plate  12  of a chuck table  10 . The attachment plate  12  is made of a porous material and has a surface machined to be flat for attaching the wafer  22 . The chuck table  10  is connected to a vacuum system (not depicted) through a supply and exhaust path  11 . A negative pressure (indicated by the symbol  60 ) supplied by the vacuum system is transmitted to the surface of the attachment plate  12  and attracts the wafer  22  through vacuum suction.  FIG. 1A  shows the wafer  22  after completion of a grinding step as illustrated in  FIGS. 12A through 12D ,  FIGS. 13A ,  13 B, and  FIGS. 14A ,  14 B, and  14 C. 
     The following describes a procedure of transferring the wafer  22  to the next step according to the sequence of the procedure. 
       FIG. 1A  shows a transport device  40  that is standing by above the wafer  22  after completion of the grinding step. The transport device  40  comprises an attracting mechanism  30  (indicated in  FIG. 2B ), which is a pick up device, and a pressing device including a pressing pad  43 , a pressing pad support arm  42 , and a pressing pad sliding member  41 . 
     The pressing pad  43 , which is a stiffening portion pressing mechanism, presses the ring-shaped stiffening portion of the wafer  22 . The pressing pad supporting arm  42  supports the pressing pad  43  and is fixed to a pressing pad sliding member  41  such that the pressing pad  43  is allowed to be vertically moved independently of the attracting mechanism  30 . 
     The pressing pad  43  is made of an elastic material in this embodiment example. A material for the pressing pad  43 , although not limited to an elastic material, preferably exhibits such elastic deformability that follows the configuration of the ring-shaped stiffening portion of the wafer  22  and such rigidity that withstands blowing up of air or the like through the attachment plate  12 , the blowing up being described afterwards. The favorable materials include foamed resin such as EVA resin and silicone rubber. The pressing pad  43  has a shape of a ring with a width wider than that of the ring-shaped stiffening portion of the wafer  22  and projects out toward both the inside and outside so as to cover whole the width of the stiffening portion. The pressing pad  43  has preferably such a width that projects out toward both inside and outside from the position of the ring-shaped stiffening portion by a dimension larger than 1 mm. The dimension of the pressing pad  43  covers the ring-shaped stiffening portion without failure irrespective of some positional deviation of the pressing pad  43 . The inside portion of the pressing pad  43  covers at least an inside wall of the recessed portion of the wafer  22 . In addition, in the case the wafer  22 , like the one illustrated in  FIG. 1A , has been ground through two stages as illustrated in  FIGS. 14A ,  14 B, and  14 C, the pressing pad  43  preferably covers the step part generated in the second grinding step. 
     An attracting member  31  shown in  FIG. 1A  of the attracting mechanism  30  indicated in  FIG. 2B  is fixed to a support member  32  (indicated in  FIG. 2B ) and vertically movable independently of the pressing pad  43 . The support member  32  includes an exhausting path (not depicted) communicating with a lower surface of the attracting member  31 . The exhausting path is connected to a vacuum system (not depicted) and supplies negative pressure to attract and hold the wafer  22  on the surface of the attracting member  31 . 
     In the embodiment example shown in  FIG. 1A , the contact surface with the wafer  22  of the attracting member  31  of the attracting mechanism  30  is in a common plane with the contact surface with the wafer  22  of the pressing pad  43 . Then, the attracting member  31  and the pressing pad  43  are lowered as shown in  FIG. 1B  until the attracting member  31  comes in contact with the thin portion of the wafer  22  and the pressing pad  43  is pushed against the ring-shaped stiffening portion of the wafer  22  and elastically deformed. The attracting member  31  and the pressing pad  43  in the above description are simultaneously lowered. However, another procedure is also possible in which one of the two members is lowered in advance and the other follows. 
       FIG. 4A  shows the pressure exerted on the wafer  22  placed on the attachment plate  12  of the chuck table  10 ;  FIG. 4B  shows the pressure exerted on the wafer  22  from the attracting member  31 ; and  FIG. 4C  shows the pressure exerted on the wafer  22  by the pressing pad  43 . 
     The state illustrated in  FIG. 1B  is the one at the time t 1  in  FIGS. 4A ,  4 B, and  4 C. A negative pressure has been supplied to the surface of the attachment plate  12  of the chuck table  10  as shown in  FIG. 4A . The attracting member  31  has been lowered from the position in  FIG. 1A  and reached the position of the wafer  22  but does not yet supply negative pressure for attracting the wafer  22  as shown in  FIG. 4B . The pressing pad  43 , after reaching the ring-shaped stiffening portion of the wafer  22 , is further pushed down with elastic deformation of the pressing pad itself until the time t 1  at which the pressure on the ring-shaped stiffening portion attains a predetermined value. 
     Then, the procedure advances to the state shown in  FIG. 2A . In the state of  FIG. 2A , the supply of negative pressure through the supply and exhaust path  11  has been stopped, and supply of positive pressure (indicated by the symbol  70 ) starts on the surface of the attachment plate  12  of the chuck table  10  from a positive pressure supplying system (not depicted) in order to facilitate removal of the wafer  22  from the chuck table  10 . The positive pressure is given by supplying water, air, or a mixture of water and air through the supply and exhaust path  11  and attains a predetermined value at the time t 2 . 
     Since the ring-shaped stiffening portion including the vicinity thereof is pushed toward the attachment plate  12  of the chuck table  10  by the pressing pad  43 , blowing out of water, air, or a mixture of water and air through the attachment plate  12  is suppressed to prevent the peripheral region from locally floating up like shown in  FIG. 15C . Since the local floating up does not occur, no stress concentration takes place at the places of remaining damages generated in the grinding step or the places of varying curvature. Thus, the wafer  22  does not suffer from any breakage such as cracks upon application of the positive pressure on the surface of the attachment plate  12 . 
     The application of positive pressure  70  is stopped at the time t 3  after continued application for a predetermined period of time. 
     Then, the procedure advances to the state shown in  FIG. 2B . After stopping the application of positive pressure  70 , the pressing pad sliding member  41  alone is elevated at the time t 4 . The support member  32  as well as the attracting member  31  does not move. The pressure exerted by the pressing pad  43  decreases in the process of elevating the pressing pad sliding member  41  and eventually becomes null when the pressing pad  43  departs from the ring-shaped stiffening portion of the wafer  22 . 
     Subsequently at the time t 5 , the attracting member  31  is supplied with a negative pressure through the exhausting path (not depicted) in the support member  32  to attract the thin portion of the wafer  22 . 
     At this moment of t 5 , as explained previously with reference to  FIG. 2A , adhesion between the wafer  22  and the attachment plate  12  has been released by application of positive pressure  70  in the period from t 2  to t 3 . Therefore, after attracting the wafer  22  by the attracting member  31 , the wafer  22  can easily be picked up from the chuck table  10  by elevating the attracting member  31  as shown in  FIG. 3A . The attracting member  31  is elevated by elevating the support member  32  to which the attracting member  31  is fixed. 
     Subsequently, the transport device  40  is moved in the horizontal direction with the attracting member  31  attracting the wafer  22  as shown in  FIG. 3B . 
     The embodiment described above allows the adhesion of the wafer  22  on the chuck table  10  to be quickly released. The time duration from t 2  to t 3  in  FIGS. 4A ,  4 B, and  4 C is about three seconds, for example, whereas it took conventionally about ten minutes for the adhesion to be released naturally (without adding any action for fast release) in order to avoid any damage on the wafer. Thus, substantial reduction of time has been achieved. The wafer is picked up without any damage and transferred to the next step. 
       FIGS. 5A and 5B  show essential part of the transport device  40 , in which  FIG. 5A  is a sectional view and  FIG. 5B  is a perspective view. The attracting member  31  is fixed to the support member  32 . The support member  32  includes an exhaust path for supplying negative pressure at the surface of the attracting member  31  from a vacuum system (not depicted). The pressing pad sliding member  41  is disposed around the support member  32  so as to vertically move independently of the support member  32 . The pressing pad sliding member  41  is driven vertically by the power from a driving system (not depicted).  FIGS. 5A and 5B  show the state in which the pressing pad sliding member  41  has been elevated to such a position that the contact plane of the pressing pad  43  to the wafer  22  is higher than the attracting surface of the attracting member  31 . The support member  32  and the pressing pad sliding member  41  are constructed coaxially, and the transport device  40  moves as a monolithic body combining the attracting member  31  and the pressing pad  43  in the vertical movement above the wafer  22  and the chuck table  10  and in the horizontal movement from a position above the chuck table  10  to a horizontally different position. 
       FIGS. 5A and 5B  shows a pressing pad support plate  44 . Although the construction can be used in which the pressing pad  43  is directly supported by the pressing pad support arm  42  as shown in  FIGS. 1A through 3B , another construction can also be employed in which the pressing pad  43  is fixed to the pressing pad support plate  44  that is fixed to the pressing pad support arm  42  as shown in  FIGS. 5A and 5B . The pressing pad support plate  44  has a configuration that does not obstruct vertical movement of the pressing pad  43  due to the attracting member  31  and is an annular disk, for example, as shown in  FIG. 5B . The use of the pressing pad support plate  44  for supporting the pressing pad  43  securely holds the pressing pad  43  made of an elastic material such as a foamed resin and applies uniform pressure on the wafer  22 . 
       FIGS. 6A and 6B  shows a modification in the first embodiment according to the invention. 
     In the device of  FIG. 6A , the pressing pad  43  made of an elastic material provided in the transport device  40  shown in  FIG. 1A  is replaced by a pressing member  45  made of a material less deformable elastically. The pressing member  45  can be made of an engineering plastic such as polycarbonate, polyamide, etc. 
     The pressing member  45 , being made of an elastically less deformable material, does not deform following the shape of the ring-shaped stiffening portion. However, the ring-shaped stiffening portion, being pushed toward the chuck table by the pressing member  45 , does not locally float up by blowing up of water, air, or a mixture of water and air through the attachment plate  12  like in a configuration shown in  FIG. 15C . 
     The pressing member  45 , having certain rigidity by itself, can be readily attached to the pressing pad support arm  42 . 
     In the device of  FIG. 6B , the pressing pad  43  made of an elastic material provided in the transport device  40  shown in  FIG. 1A  is replaced by a hollow pressing tube  47 . The pressing tube  47  is a ring-shaped tube made of an elastic material such as rubber containing air or the like in the hollow thereof. Like the pressing pad  43  shown in  FIG. 1B , the pressing tube  47  elastically deforms upon pressing against the ring-shaped stiffening portion following the shape of the stiffening portion as shown in  FIG. 6B . 
     The pressing tube  47  is attached, with air or the like filling the hollow thereof, to the pressing pad support arm  42 . Alternatively, the pressing tube  47  can be attached to, in place of the pressing pad support arm  42 , a pressing tube support arm  46  provided with a supply and exhaust path to the pressing tube  47 . 
     The ring-shaped stiffening portion, being pushed toward the chuck table by the pressing tube  47 , does not locally float up by blowing up of water, air, or a mixture of water and air through the attachment plate  12  like in a configuration shown in  FIG. 15C . 
     The use of the pressing tube support arm  46  controls the pressure in the tube, thereby adjusting degree of deformation of the tube following the configuration of the stiffening portion and the pushing force on the stiffening portion. 
     &lt;Second Embodiment&gt; 
     The second embodiment according to the present invention will be described in the following with reference to  FIGS. 7A ,  7 B,  FIGS. 8A ,  8 B,  FIGS. 9A ,  9 B, and  FIGS. 10A ,  10 B, and  10 C. 
       FIGS. 7A ,  7 B,  FIGS. 8A ,  8 B,  FIGS. 9A ,  9 B, and  FIGS. 10A ,  10 B, and  10 C illustrate the second aspect of embodiment according to the present invention. Of these figures,  FIGS. 10A ,  10 B, and  10 C are timing charts showing pressure at some parts in the device of the embodiment. 
       FIG. 7A  shows a wafer  22  attached by suction on the surfaces of attachment plates  13  and  14  of a chuck table  10 . The attachment plates  13  and  14  are made of a porous material and have a surface machined to be flat for attaching the wafer  22 . The attachment plate  13  to attach the outer peripheral portion of the wafer  22  is made of a relatively high density porous material and has a structure that effectively transmits a negative pressure from a supply and exhaust path  11  to the attracted surface of the wafer  22 . Because of the relatively high density porous material, the attachment plate  13  performs just minimum leakage of atmosphere to the supply and exhaust path  11  for vacuum suction even though the outer peripheral part of the attachment plate  13 , the outer peripheral part being not covered with and not in contact with the wafer  22 , is exposed to the atmosphere. 
     The attachment plate  14  to attach by suction the central portion of the wafer  22  is made of a relatively low density porous material and has a structure that effectively transmits a negative pressure from a supply and exhaust path  11  to the attracted surface of the wafer  22 . 
     The chuck table  10  is connected to a vacuum system (not depicted) through the supply and exhaust path  11 . A negative pressure (indicated by the symbol  60 ) supplied by the vacuum system is transmitted to the surface of the attachment plates  13  and  14  and attracts the wafer  22  through vacuum suction.  FIG. 7A  shows the wafer  22  after completion of a grinding step as illustrated in  FIGS. 12A through 12D ,  FIGS. 13A ,  13 B, and  FIGS. 14A ,  14 B, and  14 C. 
     The following describes a procedure of transferring the wafer  22  to the next step according to the sequence of the procedure. 
       FIG. 7A  shows a transport device  50  that is standing by above the wafer  22  after completion of the grinding step. The transport device  50  comprises: a holding member  52  which is a holding mechanism for holding the ring-shaped stiffening portion of the wafer  22  at the outermost peripheral portion thereof; a support arm  51  for supporting the holding member  52 ; a pressing pad  54  which is a inner region pressing mechanism for pressing the thin portion of the wafer  22  and vertically moves independently of the holding member  52 ; and a support member  53  for supporting the pressing pad  54 . The pressing pad  54  presses the thin central portion (inner region) of the wafer  22 . The pressing pad  54  in this embodiment example is made of an elastic material. A material for the pressing pad  54 , although not limited to an elastic material, preferably exhibits such elastic deformability that follows the configuration of the ring-shaped stiffening portion of the wafer  22  and such rigidity that withstands blowing up of air or the like through the attachment plates  13  and  14 , the blowing up being described afterwards. The favorable materials include formed resin such as EVA resin and silicone rubber. 
     The pressing pad  54  has a shape of a disk covering the thin recessed portion and the inside portion of the ring-shaped stiffening portion. The pressing pad  54  has a dimension that is a little larger than that of the thin recessed portion and able to cover the inside portion of the ring-shaped stiffening portion. The diameter of the pressing pad  54  is preferably about 1 mm larger than that of the inner circumferential edge of the ring-shaped stiffening portion. The dimension of the pressing pad  54  covers the stepped part at the inner circumferential end of the ring-shaped stiffening portion without failure irrespective of some positional deviation of the pressing pad  54 . The pressing pad  54  preferably covers the outer side wall of the thin recessed portion (or the inner circumferential side wall of the ring-shaped stiffening portion). However, the pressing pad  54  that presses at least the thin recessed portion can suppress the local floating up as shown in  FIG. 16B . 
     In addition, in the case the wafer  22 , like the one illustrated in  FIG. 7A , has been ground through two stages as illustrated in  FIGS. 14A ,  14 B, and  14 C, the pressing pad  54  preferably covers the step part generated in the second grinding step. 
     The holding member  52  stands by at the position outer than the outer periphery of the wafer  22  as shown in  FIG. 7A  and is vertically movable independently of the pressing pad  54  fixed to the support member  53 . The pressing pad  54  stands by above the wafer  22  in the example shown in  FIG. 7A . 
     Then, the holding member  52  and the pressing pad  54  are lowered as shown in  FIG. 7B  until the pressing pad  54  comes in contact with the thin recessed portion of the wafer  22  and presses the wafer  22  with elastic deformation of the pressing pad  54 . The holding member  52  and the pressing pad  54  in the above description are simultaneously lowered. However, another procedure is possible as well in which one of the two members is lowered in advance and the other follows. 
       FIG. 10A  shows the pressure exerted on the wafer  22  put on the surface of the attachment plate of the chuck table  10 .  FIG. 10B  shows the force exerted by the holding member  52  to hold the wafer  22  at the outer peripheral part thereof (which is the ring-shaped stiffening portion).  FIG. 10C  shows the pressure exerted on the wafer  22  by the pressing pad  54 . 
     The state illustrated in  FIG. 7B  is the one at the time t 1  in  FIGS. 10A ,  10 B, and  10 C. A negative pressure is supplied to the surface of the attachment plates  13  and  14  of the chuck table  10  as shown in  FIG. 10A . The holding member  52  has been lowered from the position in  FIG. 7A  and arrived at the same level as of the wafer  22  but has not yet moved to the position of holding the wafer  22 . The pressing pad  54 , after reaching the thinned recessed portion of the wafer  22 , is further pushed down with elastic deformation of the pressing pad itself until the time t 1  at which the pressing pad  54  presses the thin recessed portion and a part of the ring-shaped stiffening portion with a predetermined pressure. 
     Then, the procedure advances to the state shown in  FIG. 8A . In the state of  FIG. 8A , the holding member  52  approaches the wafer  22  from the outside of the wafer and holds the ring-shaped stiffening portion. The holding member  52  in this example has a construction to hold both the upper and lower surfaces of the ring-shaped stiffening portion as shown in  FIG. 8A . Holding the upper surface of the ring-shaped stiffening portion suppresses floating up of the outer peripheral portion of the wafer  22  due to blowing up of water, air, and the like. Holding the lower surface of the ring-shaped stiffening portion facilitates picking up of the wafer  22  in a later step. 
     Then, the supply of the negative pressure through the supply and exhaust path  11  is stopped, and supply of positive pressure (indicated by the symbol  70 ) starts on the surface of the attachment plates  13  and  14  of the chuck table  10  at the time t 2  indicated in  FIGS. 10A ,  10 B, and  10 C in order to facilitate removal of the wafer  22  from the chuck table  10 . The positive pressure is given by supplying water, air, or a mixture of water and air through the supply and exhaust path  11 . 
     In this state, the pressing pad  54  presses the thin recessed portion, the boundary region between the thin recessed portion and the ring-shaped stiffening portion, and a part of the ring-shaped stiffening portion of the wafer  22  toward the chuck table  10 . Therefore, blowing out of water, air, or a mixture of water and air through the attachment plates  13  and  14  is suppressed to avoid the local floating up as shown in  FIG. 16B . Since the local floating up does not occur, no stress concentration takes place at the places of remaining damages generated in the grinding step or the places of varying curvature. Thus, the wafer  22  does not suffer from any breakage such as cracks upon application of the positive pressure on the surface of the attachment plates  13  and  14 . After applying the positive pressure  70  for a predetermined period of time, the application of the positive pressure  70  is stopped at the time t 3  indicated in  FIGS. 10A ,  10 B, and  10 C. 
     Then, the procedure advances to the state shown in  FIG. 8B . After stopping the application of the positive pressure  70 , the pressing pad  54  alone is elevated at the time t 4  indicated in  FIGS. 10A ,  10 B, and  10 C. The holding member  52  is not moved. The pressure exerted by the pressing pad  54  decreases in the process of elevating the pressing pad  54  and ultimately becomes null when the pressing pad  54  departs from the wafer  22 . 
     In this process, the adhesion of the wafer  22  onto the attachment plates  13  and  14  has been released by the application of the positive pressure  70  on the attachment plates  13  and  14  during the period from the time t 2  to the time t 3 . The outer peripheral portion (or the ring-shaped stiffening portion) of the wafer  22  is held by the holding member  52 . Consequently, in the next step, the wafer can be readily picked up from the chuck table  10  by elevating the holding member  52  as shown in  FIG. 9A . The holding member  52  is elevated together with the support member  53  that fixes the pressing pad  54 . Accordingly, the pressing pad  54  does not become in contact with the wafer  22 . 
     Subsequently, the transport device  50  starts to move horizontally with the holding member  52  holding the outer peripheral portion (or the ring-shaped stiffening portion) of the wafer  22  as shown in  FIG. 9B . 
     The embodiment described above allows the adhesion of the wafer  22  onto the chuck table  10  to be quickly released. The time duration from t 2  to t 3  in  FIGS. 10A ,  10 B, and  10 C is about three seconds, for example, whereas it took conventionally about ten seconds for the adhesion to be released naturally (without adding any action for fast release) in order to avoid any damage on the wafer. Thus, substantial reduction of time has been achieved. The wafer is picked up without any damage and transferred to the next step. 
     Although not shown in the figures, the pressing pad  54  illustrated in  FIG. 7A  can be fixed to a pressing pad support plate that is additionally provided like in the construction illustrated in  FIGS. 5A and 5B . The pressing pad support plate has a configuration that does not interfere with vertical movement of the pressing pad  54  due to the holding member  52 . The use of the pressing pad support plate for supporting the pressing pad  54  securely holds the pressing pad  54  made of an elastic material such as a foamed resin and applies uniform pressure on the wafer  22 . 
       FIGS. 11A and 11B  shows a modification in the second embodiment according to the invention. 
     In the device of  FIG. 11A , the pressing pad  54  made of an elastic material provided in the transport device  50  shown in  FIG. 7A  is replaced by a pressing member  55  made of a material less deformable elastically. The pressing member  55  can be made of an engineering plastic such as polycarbonate, polyamide, etc. 
     The pressing member  55  is made in contact solely with the thin recessed portion of the wafer  22 . This is because the pressing member  55 , being made of an elastically less deformable material, does not deform following the shape of the ring-shaped stiffening portion. However, the thin recessed portion of the wafer  22 , being pushed toward the chuck table  10  by the pressing member  55 , does not locally float up by blowing up of water, air, or a mixture of water and air through the attachment plate  14  like in a configuration shown in  FIG. 16B . 
     The pressing member  55 , having certain rigidity by itself, can be readily attached to the support member  53 . 
     In the device of  FIG. 11B , the pressing pad  54  made of an elastic material provided in the transport device  50  shown in  FIG. 7A  is replaced by a hollow pressing balloon  57 . The pressing balloon  57  is a balloon made of an elastic material such as rubber containing air or the like in the balloon. Like the pressing pad  54  shown in  FIG. 7B , the pressing balloon  57  elastically deforms upon pressing against the ring-shaped stiffening portion following the shape of the stiffening portion as shown in  FIG. 11B . 
     The pressing balloon  57  is attached, with air or the like filling the interior thereof, to the support member  53 . Alternatively, the pressing balloon  57  can be attached to, in place of the pressing member  53 , a pressing balloon support member  56  provided with a supply and exhaust path to the pressing balloon  57 . 
     The thin recessed portion and the inside of the ring-shaped stiffening portion of the wafer  22 , being pushed toward the chuck table by the pressing balloon  57 , does not locally float up due to blowing up of water, air, or a mixture of water and air through the attachment plates  13  and  14  like in a configuration shown in  FIG. 16B . 
     The use of the pressing balloon support member  56  controls the pressure in the balloon, thereby adjusting degree of deformation of the balloon following the configuration of the stiffening portion and the pushing force on the stiffening portion. 
     Examples of specific embodiments are illustrated in the accompanying drawings. While the invention is described in conjunction with these specific embodiments, it will be understood that it is not intended to limit the invention to the described embodiments. On the contrary, it is intended to cover alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims. In the above description, specific details are set forth in order to provide a thorough understanding of embodiments of the invention. Embodiments of the invention may be practiced without some or all of these specific details. Further, portions of different embodiments and/or drawings can be combined, as would be understood by one of skill in the art. 
     This application is based on, and claims priority to, Japanese Patent Application No. 2011-054332, filed on Mar. 11, 2011. The disclosure of the priority application, in its entirety, including the drawings, claims, and the specification thereof, is incorporated herein by reference.