Patent Application
20180040513
The present invention relates to a wafer processing method, applicable to the division of a wafer along projected dicing lines thereon.
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).
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.
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
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.
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 4a for attracting under suction and holding the front surface 11a of the wafer 11 through the tape 21. The holding surface 4a is connected to a suction source (not shown) through a channel 4b 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 11a of the wafer 11 and the holding surface 4a 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 4b to the holding surface 4a to attract under suction and hold the wafer 11 on the holding table 4 with the wafer 11 having a back surface 11b 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.
In the placing step, as shown in
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 11a of the wafer 11 is heated to a predetermined temperature by the heater 14 described above. Conditions for heating the entire front surface 11a 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 11a of the wafer 11 is heated to 85° C. or higher by the heater 14. When the front surface 11a 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 11b of the wafer 11 is quickly cooled to develop a large temperature difference across the wafer 11, i.e., between the front surface 11a and the back surface 11b of the wafer 11. According to the present embodiment, as shown in
When the cooling liquid F is applied to the entire back surface 11b 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.
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 11a of the wafer 11 and the back surface 11b of the water 11 is exposed. However, the tape 21 may be applied to the back surface 11b of the wafer 11 and the front surface 11a of the water 11 may be exposed. In other words, the back surface 11b of the wafer 11 may be heated and the front surface 11a 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 11b of the wafer 11. However, the present invention is not limited to any process of developing a temperature difference across the wafer 11.
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) 32a which is cooled when the Peltier device 32 is supplied with electric power (voltage). The cooling surface 32a is of a size large enough to contact the entire back surface 11b 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 11a of the wafer 11 is heated in the same manner as with the dividing step according to the embodiment shown in
According to the modification, the cooling surface 32a of the Peltier device 32 is brought into contact with the back surface 11b of the wafer 11 after the front surface 11a thereof has been heated. However, the cooling surface 32a of the Peltier device 32 may be brought into contact with the back surface 11b of the wafer 11 before the front surface 11a thereof is heated. Alternatively, the cooling surface 32a of the Peltier device 32 that has already been cooled may be brought into contact with the back surface 11b of the wafer 11 after the front surface 11a thereof has been heated. According to the latter alternative, as the back surface 11b 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.
