Patent Publication Number: US-2018040513-A1

Title: Processing method for wafer

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a wafer processing method, applicable to the division of a wafer along projected dicing lines thereon. 
     Description of the Related Art 
     Electronic devices that are typified by mobile phones and personal computers include as indispensable components device chips that are provided with devices such as electronic circuits, etc. Device chips are manufactured by demarcating the surface of a wafer made of a semiconductor material such as silicon, gallium arsenide, or the like, for example, with projected dicing lines also known as streets, forming devices in the demarcated areas, and dividing the wafer along the projected dicing lines. 
     According to one known process (SD: Stealth Dicing) for dividing such a wafer, a transmittable laser beam is focused inside the wafer to form a modified layer (modified region) therein by way of multiphoton absorption (see, for example, Japanese Patent Laid-open No. 2002-192370). After modified layers have been formed along respective projected dicing lines on the wafer, a mechanical stress is applied to the wafer by a blade-shaped member or the like, for example, starting to divide the wafer from the modified layers into a plurality of device chips (see, for example, Japanese Patent Laid-open No. 2016-40810). 
     SUMMARY OF THE INVENTION 
     However, since the wafer referred to above is generally brittle, the process that applies a mechanical stress to the wafer is likely to chip off the edges of the device chips. Furthermore, inasmuch as it is necessary to apply the mechanical stress to the wafer all along the projected dicing lines, if the device chips are reduced in size, e.g., to a size of 1 mm in length×1 mm in width, then the time required to divide the wafer is increased. 
     It is therefore an object of the present invention to provide a wafer processing method in order to divide the wafer within a short period of time while preventing the wafer from being chipped off. 
     According to an aspect of the present invention, there is provided a wafer processing method of dividing along a plurality of projected dicing lines set on the wafer, including: a placing step of placing the wafer on a heating table with a tape interposed therebetween, the wafer having modified layers, from which to start to divide the wafer, formed therein at positions aligned with the projected dicing lines, the tape being applied to one surface of the wafer, and a dividing step of dividing the wafer on the heating table by heating with the heating table and thereafter cooling an exposed opposite surface in its entirety of the wafer with a cooing unit whereby the wafer starts being ruptured from the modified layers along the projected dicing lines due to a thermal shock caused by a temperature difference developed between the heated and cooled surfaces of the wafer. 
     According an aspect of the present invention, the cooling unit may eject a cooling fluid to the exposed opposite surface in its entirety of the wafer. Alternatively, the cooling unit may have a contact surface for contacting the exposed opposite surface in its entirety of the wafer, and cools the contact surface by the Peltier effect. 
     Since the wafer processing method according to the first-mentioned aspect of the present invention divides the wafer utilizing the thermal shock caused by the temperature difference developed between the heated and cooled surfaces of the wafer, it is not necessary to apply a mechanical force to the wafer to divide the same. Consequently, the wafer is prevented from being chipped off due to a mechanical force which would otherwise be applied. Furthermore, as the thermal shock acts on the wafer in its entirety the wafer can be divided along all the projected dicing lines in a short period of time. 
     The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attaching drawings showing preferred embodiments of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a perspective view schematically showing a structural example of a wafer; 
         FIG. 1B  is a perspective view schematically showing the wafer which is supported on an annular frame; 
         FIG. 2A  is a side elevational view, partly in cross section, schematically showing a modified layers forming step; 
         FIG. 2B  is a side elevational view, partly in cross section, schematically showing a placing step and a dividing step; 
         FIG. 2C  is a side elevational view, partly in cross section, schematically showing a wafer that has been ruptured; 
         FIG. 3A  is a side elevational view, partly in cross section, schematically showing a dividing step according to a modification; and 
         FIG. 3B  is a side elevational view, partly in cross section, schematically showing a wafer that has been ruptured according to the modification. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will be described below with reference to the accompanying drawings. A wafer processing method according to an embodiment of present invention includes a placing step (see  FIG. 2B ) and a dividing step (see  FIGS. 2B and 2C ), which will briefly be described below. In the placing step, a wafer having therein modified layers from which to start dividing the wafer is placed on a heating table for heating the wafer. In the dividing step, the heating table heats the wafer from its surface that is held in contact with the heating table, and then the other exposed surface of the wafer is cooled in its entirety thereby rupturing the wafer due to a thermal shock caused by the temperature difference between the heated and cooled surfaces. The wafer processing method according to the present embodiment will be described in detail below. 
       FIG. 1A  is a perspective view schematically showing a structural example of a wafer  11 . As shown in  FIG. 1A , the wafer  11  is in the shape of a disk made of a semiconductor material such as silicon (Si), gallium arsenide (GaAs), or the like. The wafer  11  has a front surface  11   a  separated into a central device region and an outer peripheral extra region surrounding the central device region. The central device region is demarcated into a plurality areas by a plurality of projected dicing lines  13 , also known as streets, arranged in a grid pattern, with a device  15  such as an integrated circuit (IC), a large-scale integration (LSI), or the like formed in each of the demarcated areas. The wafer  11  is not limited to any particular material, shape, and structure. A substrate made of ceramics, resin, or metal, for example, may be used as the wafer  11 . 
       FIG. 1B  is a perspective view schematically showing the wafer  11  which is supported on an annular frame  23 . As shown in  FIG. 1B , a tape  21  that is larger in diameter than the wafer  11  is applied to the front surface  11   a  of the wafer  11 . The tape  21  has an outer peripheral portion to which the annular frame  23  is secured. The wafer  11  is thus supported by the frame  23  through the tape  21 . 
     After the wafer  11  has been supported by the frame  23 , modified layers from which to start to divide the wafer  11  are formed within the wafer  11 .  FIG. 2A  is a side elevational view, partly in cross section, schematically showing a modified layers forming step of forming modified layers within the wafer  11 . The modified layers forming step is carried out using a laser processing apparatus  2  shown in  FIG. 2A , for example. 
     The laser processing apparatus  2  is provided with a disk-shaped holding table  4  for attracting under suction and holding the wafer  11 . The holding table  4  is coupled to a rotary actuator (not shown) such as a motor or the like, and is rotatable about a rotational axis extending substantially parallel to vertical directions. The holding table  4  is horizontally movable by a moving mechanism (not shown) disposed below the holding table  4 . The holding table  4  has an upper surface serving as a holding surface  4   a  for attracting under suction and holding the front surface  11   a  of the wafer  11  through the tape  21 . The holding surface  4   a  is connected to a suction source (not shown) through a channel  4   b  defined in the holding table  4 . A plurality of clamps  6  for securing a frame  23  that supports the wafer  11  are disposed around the holding table  4 . The laser processing apparatus  2  includes a laser processing unit  8  disposed above the holding table  4 . The laser processing unit  8  focuses a laser beam L that has been pulse-oscillated by a laser oscillator (not shown) inside the wafer  11  that is attracted under suction and held on the holder table  4 . The laser oscillator is arranged to oscillate the laser beam L at a wavelength that transmits the wafer  11 , i.e., a wavelength that is hard to be absorbed by the wafer  11 . 
     In the modified layers forming step, the wafer  11  is placed on the holding table  4  with the tape  21  interposed therebetween so that the tape  21  applied to the front surface  11   a  of the wafer  11  and the holding surface  4   a  of the holding table  4  face each other. The frame  23  is secured in position by the clamps  6 . Then, a negative pressure produced by the suction source is applied through the channel  4   b  to the holding surface  4   a  to attract under suction and hold the wafer  11  on the holding table  4  with the wafer  11  having a back surface  11   b  exposed upwardly. Then, the holding table  4  is moved and rotated to position the laser processing unit  8  above one of the projected dicing lines  13  to be processed in alignment therewith. Thereafter, the laser processing unit  8  applies the laser beam L to the wafer  11  while at the same time the holding table  4  is moved in a direction parallel to the projected dicing line  13  to be processed. The applied laser beam L causes multiphoton absorption in the vicinity of a focal point where the laser beam L is focused inside the wafer  11 , thereby forming modified layers  17  within the wafer  11  along the projected dicing line  13  to be processed. Various conditions including the wavelength, power density, and repetition frequency of the laser beam L, and the speed at which the holding table  4  moves are set in ranges for forming the modified layers  17  suitable for the division of the wafer  11 . The above processing sequence is repeated to form modified layers  17  along all the projected dicing lines  13 , i.e., at positions aligned with all the projected dicing lines  13 , whereupon the modified layers forming step is ended. 
     After the modified layers forming step, the wafer  11  is divided by the wafer processing method according to the present embodiment. Specifically, the placing step is carried out to place the wafer  11  including the modified layers  17  from which to start to divide the wafer  11 , on a heating table.  FIG. 2B  is a side elevational view, partly in cross section, schematically showing the placing step and the dividing step. 
     In the placing step, as shown in  FIG. 2B , the wafer  11  is placed on an upper surface  12   a  of a heating table  12 , which is of a disk shape greater in diameter than the wafer  11 , with the tape  21  interposed between the wafer  11  and the upper surface  12   a . As a result, the back surface  11   b  of the wafer  11  is exposed upwardly. A heater  14  for heating the wafer  11  is disposed in the upper surface  12   a  of the heating table  12 . The heater  14  is capable of heating the entire front surface  11   a  of the wafer  11 . 
     The placing step is followed by the dividing step wherein the wafer  11  is ruptured by a thermal shock. In the dividing step, the entire front surface  11   a  of the wafer  11  is heated to a predetermined temperature by the heater  14  described above. Conditions for heating the entire front surface  11   a  of the wafer  11  are arbitrary. According to the present embodiment, however, the temperature of the heater  14  is set to 95° C., and the front surface  11   a  of the wafer  11  is heated to 85° C. or higher by the heater  14 . When the front surface  11   a  of the wafer  11  is heated to 85° C. or higher, it is easy to establish a temperature difference across the wafer  11  that is necessary to cause a thermal shock. According to the present embodiment, the heater  14  is energized after the placing step has been completed. However, the heater  14  may be energized before the placing step is completed, i.e., before the placing step is carried out or while the placing step is being carried. In this case, since the wafer  14  starts being heated immediately after it has been placed on the heating table  12  in the placing step, the time required to divide the wafer  11  is reduced for an increased throughput. 
     After the wafer  11  has been heated, the entire exposed back surface  11   b  of the wafer  11  is quickly cooled to develop a large temperature difference across the wafer  11 , i.e., between the front surface  11   a  and the back surface  11   b  of the wafer  11 . According to the present embodiment, as shown in  FIG. 2B , the temperature difference is developed by applying a cooling fluid F to the entire back surface  11   b  of the wafer  11 . Specifically, an ejection nozzle (cooling unit)  22  is disposed above the heating table  12 , and a cooling fluid F is ejected from the ejection nozzle  22  to the back surface  11   b  of the wafer  11 . The cooling fluid F may be a gas such as air or the like that has been sufficiently cooled or a liquid such as water, a solution, or the like. If a liquid is used as the fluid F, then the liquid may be cooled to a temperature not low enough to freeze the liquid, e.g., a temperature higher than its freezing point by a temperature in the range from 0.1° C. to 10° C. Alternatively, a low-temperature volatile liquid that is capable of removing heat from the wafer  11  upon its vaporization may be used as the fluid F. In this case, the low-temperature volatile liquid allows the necessary temperature difference to be developed easily as it can quickly cool the back surface  11   b  of the wafer  11 . The necessary temperature difference refers to a temperature difference for bringing out a thermal shock in excess of the rupture stress of the wafer  11 . The temperature difference is determined depending on the material and thickness of the wafer  11  and the state of the modified layers  17  in the wafer  11 , for example. Conditions such as the type and flow rate of the fluid F are set in ranges for developing the necessary temperature difference. 
     When the cooling liquid F is applied to the entire back surface  11   b  of the wafer  11 , developing a sufficient temperature difference across the wafer  11 , the wafer  11  starts being ruptured from the modified layers  17  due to a thermal shock.  FIG. 2C  is a side elevational view, partly in cross section, schematically showing the wafer  11  that has been ruptured. When the wafer  11  is divided into a plurality of device chips  19  along the projected dicing lines  13 , the dividing step is finished. 
     In the wafer processing method according to the present embodiment, as described above, as much as the wafer  11  is divided utilizing a thermal shock caused by the temperature difference between the heated and cooled surfaces of the wafer  11 , it is not necessary to apply a mechanical force to the wafer  11  to divide the same. Consequently, the wafer  11  is prevented from being chipped off due to a mechanical force which would otherwise be applied. Furthermore, as the thermal shock acts on the wafer  11  in its entirety the wafer  11  can be divided along all the projected dicing lines  13  in a short period of time. 
     The present invention is not limited to the above illustrated embodiment, but many changes and modifications may be made therein. In the illustrated embodiment, the tape  21  is applied to the front surface  11   a  of the wafer  11  and the back surface  11   b  of the water  11  is exposed. However, the tape  21  may be applied to the back surface  11   b  of the wafer  11  and the front surface  11   a  of the water  11  may be exposed. In other words, the back surface  11   b  of the wafer  11  may be heated and the front surface  11   a  of the water  11  may be cooled. 
     In the wafer processing method according to the present embodiment, moreover, the temperature difference is developed across the wafer  11  by applying the cooling fluid F to the entire back surface  11   b  of the wafer  11 . However, the present invention is not limited to any process of developing a temperature difference across the wafer  11 .  FIG. 3A  is a side elevational view, partly in cross section, schematically showing a dividing step according to a modification, and  FIG. 3B  is a side elevational view, partly in cross section, schematically showing a wafer that has been ruptured according to the modification. 
     In the dividing step according to the modification, a temperature difference is developed across a wafer  11  using a Peltier device (cooling unit)  32 . The Peltier device  32  is made of two different metals joined to each other, for example, and has a cooling surface (contact surface)  32   a  which is cooled when the Peltier device  32  is supplied with electric power (voltage). The cooling surface  32   a  is of a size large enough to contact the entire back surface  11   b  of the wafer  11 . Electric wires  34  for supplying electric power (voltage) are connected to the Peltier device  32 . 
     In the dividing step according to the modification, the entire front surface  11   a  of the wafer  11  is heated in the same manner as with the dividing step according to the embodiment shown in  FIG. 2B . After the wafer  11  has been heated, the cooling surface  32   a  of the Peltier device  32  described above is brought into contact with the back surface  11   b  of the wafer  11  with a gel  36  of high thermal conductivity interposed therebetween. However, the gel  36  may be dispensed with. Thereafter, electric power (voltage) is supplied through the electric wires  34  to the Peltier device  32 , cooling the cooling surface  32   a  of the Peltier device  32 . The entire back surface  11   b  of the wafer  11  is now quickly cooled, developing a large temperature difference across the wafer  11 , i.e., between the front surface  11   a  and the back surface  11   b  of the wafer  11 . When the sufficient temperature difference is developed across the wafer  11 , the wafer  11  starts being ruptured from the modified layers  17  due to a thermal shock. When the wafer  11  is divided into a plurality of device chips  19  along the projected dicing lines  13 , the dividing step is finished. 
     According to the modification, the cooling surface  32   a  of the Peltier device  32  is brought into contact with the back surface  11   b  of the wafer  11  after the front surface  11   a  thereof has been heated. However, the cooling surface  32   a  of the Peltier device  32  may be brought into contact with the back surface  11   b  of the wafer  11  before the front surface  11   a  thereof is heated. Alternatively, the cooling surface  32   a  of the Peltier device  32  that has already been cooled may be brought into contact with the back surface  11   b  of the wafer  11  after the front surface  11   a  thereof has been heated. According to the latter alternative, as the back surface  11   b  of the wafer  11  is cooled more quickly, it is easy to develop the necessary temperature difference across the wafer  11 . 
     The present invention is not limited to the details of the above described preferred embodiments. The scope of the invention is defined by the appended claims and all changes and modifications as fall within the equivalence of the scope of the claims are therefore to be embraced by the invention.