WAFER PROCESSING METHODS

Abstract

A wafer processing method includes a polyolefin film providing step of positioning a wafer in an inner opening of a ring frame and providing a polyolefin film on a back side of the wafer and on a back side of the ring frame, a bonding step of heating the polyolefin film while applying pressure to the polyolefin film to thereby bond the wafer and the ring frame via the polyolefin film by thermocompression bonding, a dividing step of applying a laser beam to the wafer to form dividing grooves in the wafer, thereby dividing the wafer into individual device chips, and a picking-up step of heating the polyolefin film in each region of the polyolefin film corresponding to each device chip and pushing up each device chip from the polyolefin film side to pick up each device chip from the polyolefin film.

Description

BACKGROUND OF THE INVENTIONTechnical FieldThe present invention relates to a wafer processing method for dividing a wafer along a plurality of division lines to obtain a plurality of individual device chips, the division lines being formed on the front side of the wafer, thereby defining a plurality of separate regions in which a plurality of devices are individually formed.DESCRIPTION OF THE RELATED ARTIn a manufacturing process for device chips to be used in electronic equipment such as mobile phones and PCs, first, a plurality of crossing division lines (streets) are arranged on the front side of a wafer formed of, for example, a semiconductor, to thereby define a plurality of separate regions on the front side of the wafer. Next, in each divided region, a device such as an integrated circuit (IC), a large scale integration (LSI), and a light emitting diode (LED) is formed. Thereafter, a ring frame having an inner opening is prepared by previously attaching an adhesive tape called a division tape to the ring frame (the back side of the ring frame) at its outer portion so as to close the inner opening of the ring frame. Thereafter, a central portion of the adhesive tape is attached to the back surface of the wafer so that the wafer is accommodated in the inner opening of the ring frame. In this way, the wafer, the adhesive tape, and the ring frame are bonded to each other to form a frame unit. Thereafter, the wafer contained in this frame unit is processed to be divided along each division line, thereby obtaining a plurality of individual device chips having the respective devices.For example, a laser processing apparatus is used to divide the wafer (see Japanese Patent Application Laid-Open JP H10-305 420 A). The laser processing apparatus includes a chuck table for holding the wafer via the adhesive tape and a laser processing unit for applying a laser beam to the wafer held on the chuck table, the laser beam having an absorption wavelength for the wafer. In dividing the wafer using this laser processing apparatus, the frame unit is placed on the chuck table, and the wafer is held on the upper surface of the chuck table via the adhesive tape. In this state, the chuck table and the laser processing unit are relatively moved in a direction parallel to the upper surface of the chuck table. At the same time, the laser beam is applied to the wafer from the laser processing unit. When the laser beam is applied to the wafer, laser ablation occurs to form a dividing groove in the wafer along each dividing line, thereby dividing the wafer along each dividing line.Thereafter, the frame unit is transferred from the laser processing apparatus to another apparatus for applying ultraviolet light to the adhesive tape, thereby reducing the adhesion of the adhesive tape. Thereafter, each device chip is picked up by the adhesive tape. As a processing apparatus capable of manufacturing the device chips with high efficiency, there is a laser processing apparatus capable of continuously performing the process of dividing the wafer and the process of applying ultraviolet light to the adhesive tape (see, for example, Japanese Patent JP 3 076 179 B2). Each device chip picked up by the adhesive tape is next attached to a predetermined wiring substrate or the like.Further prior art helpful to understand the present invention can be found in the following documents:WO 2014 / 157471 A1 relates to a wafer processing tape that has uniform extensibility and a property that enables device chips to be picked up.US 2004 / 0 089 515 A1 relates to a method and an apparatus for receiving a semiconductor chip, a method and an apparatus for removing a semiconductor chip from a dividing band and a method for forming a perforated dividing band.US 2014 / 0 004 685 A1 relates to laser and plasma etching wafer separations using UV curable adhesive tapes.US 2016 / 0 007 479 A1 relates to an apparatus for maintaining a chip spacing, in which the spacing between adjacent component chips is maintained.JP 2003-152056 A relates to a semiconductor element holder and a method for manufacturing the same.SUMMARY OF THE INVENTIONThe adhesive tape has a base layer formed of, for example, a polyvinyl chloride sheet and an adhesive layer formed on the base layer. In the laser processing apparatus, the laser beam is applied to the wafer under the conditions where each dividing groove can be reliably formed in the wafer to have a depth from the front side to the back side of the wafer, in order to reliably divide the wafer by laser ablation. Consequently, the adhesive layer of the adhesive tape attached to the back surface of the wafer is melted by the heat due to the application of the laser beam to the wafer at the position under or around each division groove formed in the wafer, and a part of the melted adhesive layer is attached to the back surface of each device chip obtained from the wafer. In this case, in the step of picking up each device chip from the adhesive tape, ultraviolet light is applied to the adhesive tape to reduce the adhesion of the adhesive tape. However, the melted portion of the adhesive layer attached to the back surface of each device chip attached to the adhesive tape still remains on the back surface of each device chip received by the adhesive tape. Consequently, the quality of each device chip is degraded.The present invention has been made in view of such problems, and it is therefore an object of the present invention to provide a wafer processing method that prevents adhesion of the adhesive layer to the back surface of each device chip obtained from a wafer, thereby suppressing a decrease in the quality of each device chip due to the adhesion of the adhesive layer.According to an aspect of the present invention, there is provided a wafer processing method for dividing a wafer along a plurality of division lines to obtain a plurality of individual device chips, the division lines being formed on a front side of the wafer so as to define a plurality of separate regions in which a plurality of devices are individually formed. The wafer processing method includes a ring frame preparation step of preparing a ring frame having an inner opening for accommodating the wafer, a polyolefin film provision step of positioning the wafer in the inner opening of the ring frame and providing a polyolefin film on a back side of the wafer and on a back side of the ring frame, wherein an adhesive layer is not disposed between the polyolefin film and the wafer, a bonding step of heating the polyolefin film during application of a pressure to the polyolefin film after performing the polyolefin film provision step, thereby bonding the wafer and the ring frame via the polyolefin film by thermo-compression bonding so as to form a frame unit in a state where the front side of the wafer and the front side of the ring frame are exposed upward, a dividing step of applying a laser beam to the wafer along each division line, the laser beam having an absorption wavelength for the wafer, after performing the bonding step, thereby forming a division groove in the wafer along each division line to divide the wafer into the individual device chips, and a receiving step of heating the polyolefin sheet in each region of the polyolefin sheet corresponding to a respective device chip and pressing up each device chip from the polyolefin sheet side to receive each device chip from the polyolefin sheet after performing the dividing step.Preferably, the bonding step includes a step of applying infrared light to the polyolefin sheet, thereby performing the the thermoplastically bonding.Preferably, the polyolefin sheet is larger than the ring frame, and the bonding step includes an additional step of cutting the polyolefin sheet after heating the polyolefin sheet, thereby removing a part of the polyolefin sheet outside the outer periphery of the ring frame.Preferably, the receiving step includes a step of widening the polyolefin sheet to thereby increase a distance between respective adjacent devices.Preferably, the polyolefin film is formed of a material selected from the group consisting of polyethylene, polypropylene and polystyrene.In the case where the polyolefin film is formed of polyethylene, the polyolefin film in the bonding step is preferably heated in the range of 120° C. to 140° C. In the case where the polyolefin film is formed from polypropylene, the polyolefin film in the bonding step is preferably heated in the range from 160° C. to 180° C. In the case where the polyolefin film is formed from polystyrene, the polyolefin film is preferably heated in the range from 220° C. to 240° C. in the bonding step.Preferably, the wafer is formed of a material selected from the group consisting of silicon, gallium nitride, gallium arsenide, and glass.In the wafer processing method according to a preferred embodiment of the present invention, the wafer and the ring frame are bonded using the polyolefin sheet having no adhesive layer instead of an adhesive tape having an adhesive layer, and thereby form the frame unit consisting of the wafer, the ring frame, and the polyolefin sheet bonded together. The bonding step of bonding the wafer and the ring frame via the polyolefin sheet is realized by thermoplastically bonding. After performing the bonding step, a laser beam having an absorption wavelength for the wafer is applied to the wafer to thereby form a dividing groove in the wafer along each dividing line by laser ablation, so that the wafer is divided along each dividing line to obtain individual device chips attached to the polyolefin sheet. Thereafter, in each region of the polyolefin sheet corresponding to a respective device chip, the polyolefin sheet is heated, then each device chip is pressed up from the side of the polyolefin sheet and then received by the polyolefin sheet. Each packaged device chip is next mounted on a predetermined mounting substrate or the like. Note that when the polyolefin sheet is heated upon accommodating each device chip, the adhesion of the polyolefin sheet is reduced and a load applied to each device chip can be reduced.In performing laser ablation on the wafer, heat due to application of the laser beam to the wafer is transferred to the polyolefin sheet at the position below or near each division line. However, since the polyolefin sheet does not have an adhesive layer, there is no problem that the adhesive layer can be melted to be attached to the back surface of each device chip. That is, the frame unit according to an aspect of the present invention can be formed using the polyolefin sheet having no adhesive layer, so that an adhesive tape having an adhesive layer is not needed. Consequently, it is possible to prevent the problem that the quality of each device chip is degraded by the adhesion of the adhesive layer to each device chip.Thus, the wafer processing method according to an aspect of the present invention can exhibit the effect that the adhesive layer does not adhere to the back surface of each device chip obtained from the wafer, whereby a decrease in the quality of each device chip due to the adhesion of the adhesive layer can be suppressed.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 the appended claims with reference to the appended drawings showing a preferred embodiment of the invention.BRIEF DESCRIPTION OF THE FIGURESFIG. 1 is a schematic perspective view of a wafer; FIG. 2 is a schematic perspective view illustrating a manner of positioning the wafer and a ring frame on a holding surface of a chuck table; FIG. 3 is a schematic perspective view illustrating a polyolefin film providing step; FIG. 4 is a schematic perspective view illustrating a joining step; FIG. 5 is a schematic perspective view illustrating a modification of the joining step; FIG. 6 is a schematic perspective view illustrating another modification of the joining step; FIG. 7A is a schematic perspective view illustrating a manner of cutting the polyolefin sheet after performing the joining step; FIG. 7B is a schematic perspective view of a frame unit formed by performing the step illustrated in FIG. 7A ; FIG. 8 is a schematic perspective view illustrating a dividing step; FIG. 9 is a schematic perspective view illustrating a manner of loading the frame unit into a receptacle after performing the dividing step; FIG. 10A is a schematic sectional view illustrating a standby state in which a frame unit is fixed to a frame support table disposed at an initial position in a capturing step using the capturing apparatus illustrated in FIG. 9 ; and FIG. 10B is a schematic sectional view illustrating a working state in which the frame support table holding the frame unit with the polyolefin sheet is lowered to expand the polyolefin sheet in the accommodating step.DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTA preferred embodiment of the present invention will now be described with reference to the accompanying drawings. First, a wafer to be processed by a wafer processing method according to this preferred embodiment will be described. FIG. 1 is a schematic perspective view of a wafer 1. the wafer 1 is a substantially disk-shaped substrate formed of a material such as silicon (Si), silicon carbide (SiC), gallium nitride (GaN), and gallium arsenide (GaAs). The wafer 1 may be formed of any other semiconductor material. Further, the wafer 1 may be formed of a material such as sapphire, glass, and quartz. Examples of the glass include alkali glass, non-alkali glass, soda-lime glass, lead glass, borosilicate glass, and quartz glass. The wafer 1 has a front side 1 aand a back side 1 b. A plurality of intersecting division lines 3 are formed on the front surface 1 aof the wafer 1 to thereby each define a plurality of separate regions in which a plurality of devices 5 such as ICs, LSIs, and LEDs are formed. The intersecting division lines 3 are composed of a plurality of parallel division lines 3 extending in a first direction and a plurality of parallel division lines 3 extending in a second direction perpendicular to the first direction. In the processing method for the wafer 1 according to this preferred embodiment, laser ablation is performed to form a plurality of intersecting division grooves along the respective intersecting division lines 3 in the wafer 1, thereby dividing the wafer 1 into a plurality of individual device chips each including the device 5.Laser ablation is performed using a laser processing apparatus 12 (see FIG. 8 ). Before loading the wafer 1 into the laser processing apparatus 12, the wafer 1 is bonded with a polyolefin sheet 9 (see FIG. 3 ) and a ring frame 7 (see FIG. 2 ) to thereby form a frame unit 11 (see FIG. 8 ). Thus, the wafer 1 in the form of the frame unit 11 is loaded into the laser processing apparatus 12 and then processed by the laser processing apparatus 12 to obtain the individual device chips, with each device chip held on the polyolefin sheet 9. Thereafter, the polyolefin film 9 is expanded to thereby increase the spacing between all the adjacent device chips. Thereafter, each device chip is picked up using a pickup device. The ring frame 7 is formed of a rigid material such as metal and has a circular inner opening 7 athat has a diameter larger than that of the wafer 1. The ring frame 7 has a front side 7 band a rear side 7 c. In forming the frame unit, the wafer 1 is accommodated in the inner opening 7 aof the ring frame 7 and positioned in such a manner that the center of the wafer 1 substantially coincides with the center of the inner opening 7 a.The polyolefin film 9 is a flexible (expandable) plastic film and has a planar front side and a planar rear side. The polyolefin sheet 9 is a circular sheet having a diameter larger than the outer diameter of the ring frame 7. The polyolefin film 9 is a film of a polymer (polyolefin) synthesized by polymerizing an alkene as a monomer. Examples of the polyolefin film 9 include a polyethylene film, a polypropylene film, and a polystyrene film. The polyolefin film 9 is transparent or translucent to visible light. As a variation, the polyolefin film 9 may be opaque. Since the polyolefin sheet 9 does not have an adhesive property, it cannot be adhered to the wafer 1 and the ring frame 7 at room temperature. However, the polyolefin film 9 is a thermoplastic film, so that when the polyolefin film 9 is heated to a temperature near its melting point under a predetermined pressure in a state where the polyolefin film 9 is in contact with the wafer 1 and the ring frame 7, the polyolefin film 9 is melted and thereby bonded to the wafer 1 and the ring frame 7. That is, the polyolefin sheet 9 can be bonded to the wafer 1 and the ring frame 7 by applying heat and pressure in the state where the polyolefin sheet 9 is in contact with the wafer 1 and the ring frame 7 to the polyolefin sheet 9. Thus, in the processing method for the wafer 1 according to this preferred embodiment, both the wafer 1, the ring frame 7, and the polyolefin sheet 9 are joined by thermoplastically bonding as mentioned above, thereby forming the frame unit.The steps of the processing method for the wafer 1 according to this preferred embodiment will now be described. Before joining the wafer 1, the polyolefin sheet 9, and the ring frame 7, a polyolefin sheet providing step is performed using a chuck table 2 having a holding surface 2 aillustrated in FIG. 2. FIG. 2 is a schematic perspective view illustrating a manner of positioning the wafer 1 and the ring frame 7 on the holding surface 2 aof the chuck table 2. That is, the polyolefin film supplying step is performed on the holding surface 2 aof the chuck table 2 as illustrated in FIG. 2. The chuck table 2 has a circular porous member having a diameter larger than the outer diameter of the ring frame 7. the porous member forms a central upper portion of the chuck table 2. the porous member has an upper surface serving as the holding surface 2 aof the chuck table 2. A suction pipe (not shown) is formed in the chuck table 2, and one end of the suction pipe is connected to the porous member. Further, a vacuum source 2 b(see FIG. 3 ) is connected to the other end of the suction pipe. The intake passage is provided with a selector switch 2 c(see FIG. 3 ) for switching between an ON state and an OFF state. When the ON state is established by the selector switch 2c, a vacuum generated by the vacuum source 2b is applied to a workpiece placed on the holding surface 2a of the chuck table 2, thereby holding the workpiece on the holding table 2 under suction.In the polyolefin film providing step, first, the wafer 1 and the ring frame 7 are placed on the holding surface 2 aof the chuck table 2 as illustrated in FIG. 2. At this time, the front side 1 aof the wafer 1 is oriented downward, and the front side 7 bof the ring frame 7 is also oriented downward. In this state, the wafer is positioned in the inner opening 7a of the ring frame 7. Thereafter, as shown in FIG. 3, the polyolefin sheet 9 is provided on the back surface 1 b(upper surface) of the wafer 1 and the back surface 7 c(upper surface) of the ring frame 7. FIG. 3 is a schematic perspective view illustrating a manner of providing the polyolefin sheet 9 to the wafer 1 and the ring frame 7. That is, as illustrated in FIG. 3, the polyolefin sheet 9 is provided so as to completely cover the wafer 1 and the ring frame 7. In the polyolefin film providing step, the diameter of the polyolefin film 9 is set larger than the diameter of the holding surface 2 aof the chuck table 2, If the diameter of the polyolefin film 9 is not larger than the diameter of the holding surface 2 a, such a problem may occur that, when the vacuum is applied from the vacuum source 2 bto the holding surface 2 aof the chuck table 2 in a joining step to be performed later, the vacuum may be lost from any gap between the polyolefin film 9 and the holding surface 2 abecause the holding surface 2 ais not completely covered with the polyolefin film 9, so that a pressure may not be appropriately applied to the polyolefin film 9.Next, in the processing method for the wafer 1 according to this preferred embodiment, a joining step is performed in such a manner that the polyolefin sheet 9 is heated to join the wafer 1 and the ring frame 7 by thermoplastically bonding via the polyolefin sheet 9. FIG. 4 is a schematic perspective view illustrating the joining step according to this preferred embodiment. As illustrated in FIG. 4, the transparent or visible light translucent polyolefin sheet 9 is provided so as to completely cover the wafer 1, the ring frame 7, and the holding surface 2 aof the chuck table 2, all of which are illustrated by broken lines in FIG. 4. In the joining step, the selector switch 2 cis operated to establish the ON state in which the vacuum source 2 bis in communication with the porous member of the chuck table 2, i.e., the holding surface 2 aof the chuck table 2, so that a vacuum generated by the vacuum source 2 bis applied to the polyolefin sheet 9 provided on the chuck table 2. Accordingly, the polyolefin sheet 9 is brought into close contact with the wafer 1 and the ring frame 7 by the atmospheric pressure applied to the upper surface of the polyolefin sheet 9.Thereafter, the polyolefin sheet 9 is heated in a state where the polyolefin sheet 9 is sucked from the vacuum source 2 b, and thereby performs thermoplastically joining. In this preferred embodiment shown in Fig. 4, the heating of the polyolefin sheet 9 is effected, for example, by a heat gun 4 provided above the chuck table 2. The heat gun 4 includes a heating means such as a heating wire and an air blowing mechanism such as a fan. Accordingly, the heat gun 9 can heat ambient air and blow the heated air. In a state where the vacuum is applied from the vacuum source 2 bto the polyolefin sheet 9, the heat gun 4 is operated to supply hot air 4 ato the upper surface of the polyolefin sheet 9. Accordingly, when the polyolefin sheet 9 is heated to a predetermined temperature, the polyolefin sheet 9 is bonded to the wafer 1 and the ring frame 7 by thermo-compression bonding.Another method may be used for heating the polyolefin sheet 9. For example, each member on the polyolefin sheet 9 heated to a predetermined temperature may be pressed against the wafer 1 and the ring frame 7. FIG. 5 is a schematic perspective view illustrating such a modification of the joining step. As illustrated in FIG. 5, the transparent or visible light translucent polyolefin sheet 9 is provided so as to completely cover the wafer 1, the ring frame 7, and the holding surface 2 aof the chuck table 2, all of which are illustrated by broken lines in FIG. 5. In this modification shown in Fig. 5, a heat roller 6 having a heat source is used. Specifically, first, the vacuum generated by the vacuum source 2 bis applied to the polyolefin sheet 9, so that the polyolefin sheet 9 is brought into close contact with the wafer 1 and the ring frame 7 by the atmospheric pressure applied to the upper surface of the polyolefin sheet 9.Thereafter, the heat roller 6 is heated to a predetermined temperature and next placed at an end located on the outer periphery of the holding surface 2 a, as shown in FIG. 5, on the holding surface 2 aof the chuck table 2. Thereafter, the heat roller 6 is rotated about its axis to roll on the chuck table 2 via the polyolefin sheet 9 from the above one end to another end diametrically opposite to the above one end. As a result, the polyolefin sheet 9 is bonded to the wafer 1 and the ring frame 7 by thermo-compression bonding. In the case where a force for pressing the polyolefin sheet 9 is applied from the heat roller 6, the thermoplastically bonding is effected at a pressure higher than the atmospheric pressure. Preferably, the cylindrical surface of the heat roller 6 is coated with a fluoroplastic. Further, the heat roller 6 may be replaced with an iron-like pressing member having a flat base plate and a heat source. In this case, the pressing member is heated to a predetermined temperature to thereby provide a hot plate which is next pressed onto the polyolefin sheet 9 held on the chuck table 2.Another method for heating the polyolefin sheet 9 may be used in the following manner. FIG. 6 is a schematic perspective view illustrating such another modification of the joining step. As illustrated in FIG. 6, the transparent or visible light translucent polyolefin sheet 9 is provided so as to completely cover the wafer 1, the ring frame 7, and the holding surface 2 aof the chuck table 2, all of which are illustrated by broken lines in FIG. 6. In this modification shown in Fig. 6, an infrared lamp 8 is provided above the chuck table 2 to heat the polyolefin sheet 9. The infrared lamp 8 can apply infrared light 8a having an absorption wavelength at least for the material of the polyolefin sheet 9. In the modification shown in FIG. 6, the vacuum generated by the vacuum source 2 bis also first applied to the polyolefin sheet 9, so that the polyolefin sheet 9 is brought into close contact with the wafer 1 and the ring frame 7 by the atmospheric pressure applied to the upper surface of the polyolefin sheet 9. Thereafter, the infrared lamp 8 is operated to apply the infrared light 8 ato the polyolefin sheet 9, and thereby heats the polyolefin sheet 9.When the polyolefin sheet 9 is heated to a temperature near its melting point by performing any of the above methods, the polyolefin sheet 9 is bonded to the wafer 1 and the ring frame 7 by thermoplastically bonding. After bonding the polyolefin sheet 9, the selector switch 2c is operated to establish the OFF state in which the connection between the porous member of the chuck table 2 and the vacuum source 2b is disconnected. Accordingly, the suction holding by the chuck table 2 is finished.Thereafter, the polyolefin sheet 9 is circularly cut along the outer periphery of the ring frame 7 to remove an undesirable peripheral portion of the polyolefin sheet 9. FIG. 7A is a schematic perspective view illustrating a manner of cutting the polyolefin sheet 9. As shown in Fig. 7A, a disk-shaped (ring-shaped) cutter 10 is used to cut the polyolefin sheet 9. The cutter 10 has a central through hole 10a into which a rotating shaft 10b is fitted. Accordingly, the cutter 10 is rotatable about the axis of the rotating shaft 10b. First, the cutter 10 is positioned above the ring frame 7. At this time, the rotating shaft 10 bis arranged to extend in the radial direction of the chuck table 2. Thereafter, the cutter 10 is lowered until the outer periphery (cutting edge) of the cutter 10 comes into contact with the polyolefin sheet 9 placed on the ring frame 7. That is, the polyolefin sheet 9 is caught between the cutter 10 and the ring frame 7, so that the polyolefin sheet 9 is cut by the cutter 10 to form a cut trace 9 a. Further, the cutter 10 is rotatably guided on the polyolefin sheet 9 along a circular line disposed between the inner periphery of the ring frame 7 (i.e., outside of the inner opening 7 aof the ring frame 7) and the outer periphery of the ring frame 7, thereby forming the cut track 9 acircularly along the above circular line. As a result, a predetermined central portion of the polyolefin sheet 9 is surrounded by the circular cut trace 9a. Thereafter, a remaining outer portion of the polyolefin sheet 9 outside the circular cut trace 9a is removed. That is, an undesired outer portion of the polyolefin sheet 9 including an outermost outer portion outside the outer periphery of the ring frame 7 can be removed.The cutter 10 may be replaced with an ultrasonic cutter for cutting the polyolefin sheet 9. Further, a vibration source for vibrating the cutter 10 at a frequency in an ultrasonic band may be connected to the cutter 10. Further, when the polyolefin sheet 9 is cut, the polyolefin sheet 9 may be cooled to be cured to facilitate the cutting operation. By cutting the polyolefin sheet 9 as mentioned above, a frame unit 11 shown in FIG. 7B is formed in which the frame unit 11 is composed of the wafer 1, the ring frame 7, and the polyolefin sheet 9 bonded together. That is, the wafer and the ring frame 7 are bonded to each other via the polyolefin sheet 9 to form the frame unit 11 as illustrated in FIG. 7B. FIG. 7B is a schematic perspective view of the frame unit 11 in a state where the front side 1 aof the wafer and the front side 7 bof the ring frame 7 are exposed upward.In performing the the thermoplastically bonding as mentioned above, the polyolefin sheet 9 is preferably heated to a temperature equal to or below the melting point of the polyolefin sheet 9. When the heating temperature is higher than the melting point of the polyolefin sheet 9, there is a possibility that the polyolefin sheet 9 is melted to such an extent that the shape of the polyolefin sheet 9 cannot be maintained. Further, the polyolefin sheet 9 is preferably heated to a temperature equal to or higher than the softening point of the polyolefin sheet 9. When the heating temperature is lower than the softening point of the polyolefin sheet 9, the thermoplastically bonding cannot be carried out properly. Accordingly, the polyolefin sheet 9 is preferably heated to a temperature equal to or higher than the softening point of the polyolefin sheet 9 and equal to or lower than the melting point of the polyolefin sheet 9. Further, there is a case where the softening point of the polyolefin sheet 9 may be unknown. In order to deal with such a case, when the thermoplastically joining is performed, the polyolefin film 9 is preferably heated to a temperature equal to or higher than a predetermined temperature and equal to or lower than the melting point of the polyolefin film 9, the predetermined temperature being lower than the melting point of the polyolefin film 9 by 20° C.In the case where the polyolefin film 9 is a polyethylene film, the heating temperature in the bonding step is preferably set in the range of 120° C. to 140° C. Further, in the case where the polyolefin film 9 is a polypropylene film, the heating temperature in the joining step is preferably set in the range of 160° C. to 180° C. Further, in the case where the polyolefin sheet 9 is a polystyrene sheet, the heating temperature in the joining step is preferably set in the range of 220° C. to 240° C.The heating temperature is defined herein as the temperature of the polyolefin sheet 9 to be heated in performing the joining step. As the heat sources including the heat gun 4, the heat roller 6, and the infrared lamp 8 mentioned above, some kinds of heat sources capable of setting an output temperature have been put into practical use. However, even if such a heat source is used to heat the polyolefin sheet 9, the temperature of the polyolefin sheet 9 does not reach the above-set discharge temperature in some cases. In order to deal with such a case, the output temperature of the heat source may be set to a temperature higher than the melting point of the polyolefin sheet 9 to heat the polyolefin sheet 9 to a predetermined temperature.After performing the above-mentioned joining step, a dividing step is performed in such a manner that the wafer 1 in the state of the frame unit 11 is processed by laser ablation to form a plurality of intersecting division grooves along the plurality of intersecting division lines 3 in the wafer 1, thereby dividing the wafer 1 into individual device chips. The dividing step is performed using a laser processing apparatus 12 illustrated in FIG. 8 in this preferred embodiment. FIG. 8 is a schematic perspective view illustrating the dividing step. As illustrated in FIG. 8, the laser processing apparatus 12 includes a laser processing unit 14 for performing laser ablation on the wafer 1 and a chuck table (not illustrated) for holding the wafer 1. The laser processing unit 14 includes a laser oscillator (not shown) for generating a laser beam 16 having an absorption wavelength for the wafer 1 (having a wavelength absorbable for the wafer 1). The chuck table has an upper surface as a holding surface for holding the wafer 1. The chuck table is movable in a direction parallel to its upper surface, that is, movable in a feeding direction. The laser beam 16 generated by the laser oscillator in the laser processing unit 14 is applied to the wafer 1 held on the chuck table. The laser processing unit 14 further includes a processing head 14 awith a mechanism for focusing the laser beam 16 at a predetermined vertical position in the wafer 1.In performing laser ablation on the wafer 1, the frame unit 11 is placed on the chuck table in the state where the front surface 1 aof the wafer 1 is exposed upward. Accordingly, the wafer 1 is held on the chuck table via the polyolefin sheet 9. Thereafter, the chuck table is rotated to cause the division lines 3 extending in the first direction on the front side 1 aof the wafer 1 to be parallel to a feeding direction in the laser processing apparatus 12. Further, the chuck table and the laser processing unit 14 are relatively moved to set a relative position, thereby positioning the processing head 14 adirectly above an extension of a predetermined one of the division lines 3 extending in the first direction. Thereafter, the laser beam 16 is applied to the wafer from the processing head 14a. At the same time, the chuck table and the laser processing unit 14 are relatively moved in the feeding direction parallel to the upper surface of the chuck table. Consequently, the laser beam 16 is applied to the wafer 1 along the predetermined dicing line 3, thereby performing laser ablation along the predetermined dicing line 3. Accordingly, a dividing groove 3a is formed in the wafer 1 along the predetermined dividing line 3 by the laser beam 16. The dividing groove 3a has a depth from the front side 1a to the back side 1b of the wafer 1. In this dividing step, the laser beam 16 may be applied under the following machining conditions. The following machining conditions are only exemplary.Wavelength: 355 nmRepetition frequency: 50 kHzAverage power: 5 WFeed rate: 200 mm / sAfter forming the dividing groove 3 aalong the predetermined dividing line 3, the chuck table and the laser processing unit 14 are moved in an index direction perpendicular to the feeding direction to similarly perform laser ablation along the next dividing line 3 extending in the first direction. Thereafter, laser ablation along all the other division lines 3 extending in the first direction is similarly performed. Thus, a plurality of similar dividing grooves 3 aare formed along all the dividing lines 3 extending in the first direction. Thereafter, the chuck table is rotated 90 degrees about its vertical axis perpendicular to its holding surface to similarly perform laser ablation along all division lines 3 extending in the second direction perpendicular to the first direction. Thus, a plurality of similar dividing grooves 3 aare similarly formed along all the dividing lines 3 extending in the second direction. In this manner, the plurality of intersecting division grooves 3 aare formed at a depth from the front side 1 ato the back side 1 bof the wafer in the wafer 1 along the respective intersecting division lines 3, so that the wafer 1 is divided by these division lines 3 ato obtain the individual device chips.When the laser processing unit 14 is operated to perform laser ablation on the wafer 1, processing dust is generated from the wafer 1 at the position where the laser beam 16 is applied. This machining dust spreads around this laser deposition position to adhere to the front surface 1a of the wafer 1. Although the front surface 1 aof the wafer 1 is cleaned after performing laser ablation on the wafer 1 using a cleaning unit described below, it is not easy to completely remove this machining dust attached to the front surface 1 aof the wafer 1. If the machining dust remains on each device chip obtained from the wafer 1, the quality of each device chip is lowered. To address this problem, a water-soluble liquid resin may be previously applied to the front surface 1 aof the wafer 1, and this water-soluble liquid resin serves as a protective film for protecting the front surface 1 aof the wafer 1. In the case where the liquid resin is previously applied to the front surface 1 aof the wafer 1, the machining dust spreading when performing laser ablation adheres to the upper surface of the applied liquid resin. That is, the processing dust is prevented from adhering directly to the front surface 1 aof the wafer 1. After performing the laser ablation, the wafer 1 is cleaned using the cleaning unit. At this time, the processing dust may be removed together with the applied liquid resin when cleaning the wafer 1.That is, the laser processing apparatus 12 may include such a cleaning unit (not illustrated). In this case, the wafer 1 processed by the laser processing unit 14 as mentioned above is transferred to the cleaning unit and then cleaned by the cleaning unit. For example, the cleaning unit includes a cleaning table having a holding surface for holding the frame unit 11, and a cleaning water nozzle configured to be horizontally reciprocated above the frame unit 11 held on the holding surface of the cleaning table. The cleaning water nozzle acts to supply cleaning water such as clean water to the wafer 1. The cleaning table is rotatable about its axis perpendicular to its holding surface. In operation, the cleaning table is rotated about its axis and at the same time the cleaning water is supplied from the cleaning water nozzle to the wafer 1. During this supply of the cleaning water, the cleaning water nozzle is horizontally reciprocated along a path passing through the position just above the center of the holding surface of the cleaning table. Accordingly, the entire surface of the front surface 1 aof the wafer 1 can be cleaned by the cleaning water.After performing the dividing step or the cleaning step, a picking step is performed to pick up each device chip from the polyolefin sheet 9. The capturing step is performed using a capturing device 18 illustrated in FIG. 9. FIG. 9 is a schematic perspective view illustrating a manner of loading the frame unit 11 into the receptacle 18. As shown in FIG. 9, the take-up device 18 includes a cylindrical drum 20 and a frame holding unit 22 including a frame support table 26 provided around the cylindrical drum 20. The cylindrical barrel 20 has an inner diameter larger than the diameter of the wafer 1 and an outer diameter smaller than the inner diameter of the ring frame 7 (the diameter of the inner opening 7a). The frame support table 26 of the frame holding unit 22 is an annular table having a circular inner opening larger in diameter than the drum 20. that is, the frame support table 26 has an inner diameter larger than the outer diameter of the drum 20. First, the height of the upper surface of the frame support table 26 is set equal to the height of the upper end of the drum 20 (see FIG. 10A ). Further, the upper end portion of the drum 20 is surrounded by the inner periphery of the ring frame 7 at this initial stage.A plurality of clamps 24 are provided on the outer periphery of the frame support table 26. Each clamp 24 acts to hold the ring frame 7 supported on the frame support table 26. That is, when the ring frame 7 of the frame unit 11 is placed on the frame support table 26 via the polyolefin sheet 9 and then held by each clamp 24, the frame unit 11 can be fixed to the frame support table 26. The frame support table 26 is supported by a plurality of rods 28 extending in a vertical direction. That is, each rod 28 is connected at its upper end to the lower surface of the frame support table 26. An air cylinder 30 for vertically moving each rod 28 is connected to the lower end of each rod 28. Specifically, the lower end of each rod 28 is connected to a piston (not shown) movably accommodated in the air cylinder 30. Each air cylinder 30 is supported on a disk-shaped base 32. That is, the lower end of each air cylinder 30 is connected to the upper surface of the disk-shaped base 32. Accordingly, when each air cylinder 30 is operated in the initial stage, the frame support table 26 is lowered with respect to the fixed-in-place drum 20.Further, a push-up mechanism 34 for pushing up each component chip carried on the polyolefin sheet 9 is provided inside the drum 20. The push-up mechanism 34 has, at its upper end, a heater 34 athat has a heat source such as a Peltier element or a heating wire. That is, each device chip is configured to be pushed up by the push-up mechanism 34 disposed below the polyolefin sheet 9 by blowing the air 34 aup via the polyolefin sheet 9. Further, a gripper 36 (see FIG. 10B ) capable of holding each device chip under suction is provided above the drum 20. Both the push-up mechanism 34 and the gripper 36 are movable in a horizontal direction parallel to the upper surface of the frame support table 26. The gripper 36 is connected to a vacuum source 36a (see Fig. 10B) via a selector switch 36b (see Fig. 10B).In the accommodating step, first, each air cylinder 30 in the accommodating device 18 is operated to adjust the height of the frame support table 26 so that the height of the upper end of the drum 20 corresponds to the height of the upper surface of the frame support table 26. Thereafter, the frame unit 11 transferred from the laser processing apparatus 12 is placed on the drum 20 and the frame support table 26 in the pick-up device 18 in a state where the front side 1 aof the wafer 1 of the frame unit 11 is directed upward. Thereafter, each clamp 24 is operated to fix the ring frame 7 of the frame unit 11 to the upper surface of the frame supporting table 26. FIG. 10A is a schematic sectional view illustrating a standby state in which the frame unit 11 is fixed to the frame support table 26 set at the initial position. At this time, the plurality of division grooves 3 ahave already been formed in the wafer 1 in the dividing step, so that the wafer 1 has already been divided into a plurality of individual device chips 1 c(see FIG. 10B ).Thereafter, each air cylinder 30 is operated to lower the frame supporting table 26 of the frame holding unit 22 with respect to the drum 20. As a result, the polyolefin sheet 9 fixed to the frame holding unit 22 by each clamp 24 is expanded radially outward as shown in FIG. 10B. FIG. 10B is a schematic sectional view illustrating a processing state in which the frame support table 26 holding the ring frame 7 with the polyolefin sheet 9 is lowered to expand the polyolefin sheet 9. When the polyolefin sheet 9 is expanded radially outward as mentioned above, the distance between adjacent device chips 1c supported on the polyolefin sheet 9 is increased as shown in Fig. 10B. Accordingly, contact between the adjacent device chips 1 cmay be suppressed, and each device chip 1 cmay be easily accommodated. Thereafter, a target device chip 1c is selected and the push-up mechanism 34 is next moved to a position directly below this target device chip 1c as shown in FIG. 10B. Moreover, as illustrated in FIG. 10B, the gripper 36 is also moved to a position directly above this target device chip 1 c. Thereafter, the heater 34a is operated to raise a temperature and brought into contact with a portion of the polyolefin sheet 9 corresponding to the target device chip 1c to heat the portion. Further, the push-up mechanism 34 is operated to push up the target device chip 1c via the polyolefin sheet 9. Further, the selector switch 36b is operated to cause the gripper 36 to communicate with the vacuum source 36a. As a result, the target device chip 1 cis held by the gripper 36 under suction and thereby picked up by the polyolefin sheet 9. Such a pick operation is similarly performed for all the other device chips 1c. Thereafter, each packaged device chip 1 cis attached to a predetermined wiring substrate or the like for actual use.Note that, when the portion of the polyolefin sheet 9 is heated by the heater 34 a, for example, the corresponding portion is heated at a temperature near a melting point of the polyolefin sheet 9. Since the polyolefin sheet 9 reduces adhesion during a period in which the polyolefin sheet 9 has a temperature near its melting point, a load applied when each device chip is peeled from the polyolefin sheet 9 is reduced.In the case of forming the frame unit 11 using an adhesive tape, heat generated by the application of the laser beam 16 in the dividing step is transferred to the adhesive tape, so that the adhesive layer of the adhesive tape is melted and attached to the back surface of each device chip. Accordingly, in this case, there is a problem that the adhesion of the adhesive layer to each device chip causes a quality reduction. In comparison, in the wafer processing method according to this preferred embodiment, the frame unit 11 can be formed using the polyolefin sheet 9 having no adhesive layer, the polyolefin sheet 9 being attached to the wafer 1 and the ring frame 7 by thermoplastically bonding. That is, an adhesive tape having an adhesive layer is not required. Consequently, it is possible to avoid a degradation in quality of each device chip due to the adhesion of the adhesive layer to the back surface of each device chip.The present invention is not limited to the above preferred embodiment, but various modifications may be made within the scope of the present invention. For example, while in the above preferred embodiment, the polyolefin film 9 is selected from a polyethylene film, a polypropylene film and a polystyrene film, this is illustrative only. That is, the polyolefin sheet usable in the present invention may be formed of any other materials (polyolefins) such as a copolymer of polypropylene and ethylene and an olefin elastomer.

Claims

A wafer processing method for dividing a wafer (1) along a plurality of division lines (3) to obtain a plurality of individual device chips (1c), the division lines (3) being formed on the front side (1a) of the wafer (1) so as to define a plurality of separate regions in which a plurality of devices (5) are individually formed, the wafer processing method comprising: a ring frame preparing step of preparing a ring frame (7) having an inner opening (7a) for accommodating the wafer (1); a polyolefin film providing step of positioning the wafer (1) in the inner opening (7a) of the ring frame (7) and providing a polyolefin film (9) on a back side (1b) of the wafer (1) and on a back side (7c) of the ring frame (7), wherein no adhesive layer is provided between the polyolefin film (9) and the wafer (1); a joining step of heating the polyolefin sheet (9) while applying pressure to the polyolefin sheet (9) after performing the polyolefin sheet providing step, thereby joining the wafer (1) and the ring frame (7) via the polyolefin sheet (9) by thermo-compression bonding so as to form a frame unit (11) in a state where the front side (1a) of the wafer (1) and the front side (7b) of the ring frame (7) are exposed upward; a dividing step of applying a laser beam to the wafer (1) along each dividing line (3), the laser beam having an absorption wavelength for the wafer (1), after performing the bonding step, thereby forming a dividing groove in the wafer (1) along each dividing line (3) to divide the wafer (1) into the individual device chips (1c); and a receiving step of heating the polyolefin sheet (9) in each region of the polyolefin sheet (9) corresponding to a respective device chip (1c), and pushing up each device chip (1c) from the polyolefin sheet (9) side to receive each device chip (1c) from the polyolefin sheet (9) after performing the dividing step.The wafer processing method according to claim 1, wherein the bonding step includes a step of applying infrared light to the polyolefin sheet (9), thereby performing the the thermoplastically bonding.The wafer processing method according to claim 1 or 2, wherein the polyolefin sheet (9) is larger than the ring frame (7), and wherein the bonding step includes an additional step of cutting the polyolefin sheet (9) after heating the polyolefin sheet (9), thereby removing a part of the polyolefin sheet (9) outside an outer periphery of the ring frame (7).The wafer processing method according to any one of the preceding claims, wherein the accommodating step includes a step of expanding the polyolefin sheet (9) to thereby increase a distance between respective adjacent device chips (1c).The wafer processing method according to any one of the preceding claims, wherein the polyolefin film (9) is formed of a material selected from the group consisting of polyethylene, polypropylene and polystyrene.The wafer processing method according to claim 5, wherein the polyolefin film (9) is formed of polyethylene, and the polyolefin film (9) is heated in the range of 120°C to 140°C in the bonding step.The wafer processing method according to claim 5, wherein the polyolefin sheet (9) is formed of polypropylene, and the polyolefin sheet (9) is heated in the range of 160°C to 180°C in the bonding step.The wafer processing method according to claim 5, wherein the polyolefin sheet (9) is formed of polystyrene, and the polyolefin sheet (9) is heated in the range of 220°C to 240°C in the bonding step.The wafer processing method according to any one of the preceding claims, wherein the wafer (1) is formed of a material selected from the group consisting of silicon, gallium nitride, gallium arsenide, and glass.

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