Wafer processing method

Abstract

To provide a wafer processing method capable of solving a problem in which a thermal compression bond sheet arranged in a wafer peels from an outer periphery of the thermal compression bond sheet when flattening an upper surface of the thermal compression bond sheet.SOLUTION: A method for processing a wafer in which a plurality of devices are partitioned by division schedule lines on a surface includes a thermal compression bond sheet arranging step, a thermal compression bonding step, a step forming step, and a flattening step. The thermal compression band sheet arranging step arranges a thermal compression bond sheet on the surface of the wafer. The thermal compression bonding step heats and presses the thermal compression bond sheet and subjecting the surface of the wafer to thermal compression bond. The step forming step forms a step part in an outer periphery of the thermal compression bond sheet by cutting the thermal compression bond sheet along the outer periphery of the wafer. The flattening step flattens the upper surface of the thermal compression bond sheet by cutting the surface with a cutting tool. In the flattening step, a tip of the cutting tool is positioned at a place which does not fall below the step part formed in the step forming step.SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] The present invention relates to a wafer processing method for processing a wafer having a plurality of devices formed on its surface and partitioned by planned dividing lines. [Background technology]

[0002] A wafer has a plurality of devices such as ICs and LSIs formed on its surface, separated by planned dividing lines. The back surface is ground to the desired thickness, and then the wafer is divided into individual device chips using a dicing machine and laser processing machine. These chips are then used in electrical devices such as mobile phones and personal computers.

[0003] The grinding device is configured to include a chuck table that holds the wafer and grinding means that has a rotatable grinding wheel that grinds the wafer held on the chuck table, and can finish the wafer with high precision.

[0004] In addition, the present applicant has proposed a technology in which a thermocompression sheet that does not contain adhesive is thermocompressed onto the surface of the wafer so that the device will not be scratched even when the front side of the wafer is held on the holding surface of the chuck table, thereby avoiding contamination of the wafer surface due to adhesive residue, the intrusion of grinding water, etc. (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 2019-186488 Summary of the Invention [Problem to be solved by the invention]

[0006] Although the technology described in Patent Document 1 can avoid contamination of the wafer surface due to glue residue, intrusion of grinding water, etc., the unevenness of the device also causes unevenness on the upper surface of the thermocompression sheet, which causes a problem that even if negative pressure is generated on the holding surface of the chuck table to hold the sheet by suction, the sheet cannot be properly held on the holding surface of the chuck table.To address this problem, it is possible to flatten the upper surface of the thermocompression sheet by grinding it with a cutting tool, but when a thermocompression sheet laid on the surface of the wafer is ground with a cutting tool, the thermocompression sheet peels off from its outer periphery, making it impossible to flatten the sheet.

[0007] The present invention has been made in consideration of the above facts, and its main technical objective is to provide a wafer processing method that solves the problem of a thermo-compression-bonded sheet peeling off from its outer periphery when the top surface of the thermo-compression-bonded sheet placed on the wafer is flattened by grinding it with a bit. [Means for solving the problem]

[0008] In order to solve the above-mentioned main technical problem, according to the present invention, there is provided a wafer processing method for processing a wafer having a plurality of devices formed on its surface, the wafer having a plurality of devices divided by planned dividing lines, the method comprising: a thermo-compression sheet disposing step for disposing a thermo-compression sheet on the surface of the wafer; a thermo-compression bonding step for heating and pressing the thermo-compression sheet to thermo-compression bond it to the surface of the wafer; a step forming step for cutting the thermo-compression sheet along the outer periphery of the wafer to form a step portion on the outer periphery of the thermo-compression sheet; and a flattening step for cutting the upper surface of the thermo-compression sheet with a cutting tool to flatten the upper surface, wherein in the flattening step, the cutting edge of the cutting tool is positioned at a position that does not fall below the step portion formed in the step forming step.

[0009] The thermocompression-bonded sheet is a polyolefin-based sheet, and is preferably selected from any one of a polyethylene sheet, a polypropylene sheet, and a polystyrene sheet. The heating temperature when heating the thermocompression-bonded sheet in the thermocompression bonding step is preferably 120 to 140°C for a polyethylene sheet, 160 to 180°C for a polypropylene sheet, and 220 to 240°C for a polystyrene sheet.

[0010] The thermocompression-bonded sheet is a polyester-based sheet, and is preferably selected from either a polyethylene terephthalate sheet or a polyethylene naphthalate sheet. The heating temperature for heating the thermocompression-bonded sheet in the thermocompression bonding step is preferably 250 to 270°C for a polyethylene terephthalate sheet, and 160 to 180°C for a polyethylene naphthalate sheet.

[0011] The above-mentioned wafer processing method preferably includes a grinding step of holding the upper surface of the flattened thermocompression sheet on a holding surface of a chuck table of a grinding device and grinding the back surface of the wafer to a desired thickness, and also preferably includes a processing step of holding the back surface of the wafer to which the flattened thermocompression sheet is bonded on a holding surface of a chuck table of a dicing device or a laser processing device and performing cutting or laser processing along the intended division lines from the thermocompression sheet side. [Effects of the Invention]

[0012] The wafer processing method of the present invention is a wafer processing method for processing a wafer having a plurality of devices formed on its surface, partitioned by planned division lines, and includes a thermo-compression sheet disposing step of disposing a thermo-compression sheet on the surface of the wafer, a thermo-compression bonding step of heating and pressing the thermo-compression sheet to thermo-compression bond it to the surface of the wafer, a step forming step of cutting the thermo-compression sheet along the outer periphery of the wafer to form a step portion on the outer periphery of the thermo-compression sheet, and a flattening step of cutting the upper surface of the thermo-compression sheet with a bit to flatten the upper surface.In the flattening step, the cutting edge of the bit is positioned at a position that is not below the step portion formed in the step forming step, so that the pressure force of the step portion on the wafer is relatively greater than the force that attempts to peel the thermo-compression sheet from the outer periphery where the step portion is formed by the cutting, thereby eliminating the problem of the thermo-compression sheet peeling off from the outer peripheral edge when the upper surface of the thermo-compression sheet is cut with a bit to be flattened. [Brief explanation of the drawings]

[0013] [Figure 1] FIG. 10 is a perspective view showing an embodiment of a thermocompression sheet disposing step. [Figure 2] FIG. 1 is a perspective view showing an embodiment of a thermocompression bonding step. [Figure 3] FIG. 10 is a perspective view showing a manner in which a wafer is placed on a chuck table of a cutting device. [Figure 4] 1A is a perspective view showing an embodiment of a step forming step, and FIG. 1B is an enlarged side view showing a part of the outer peripheral edge of a wafer on which a step portion is formed. [Figure 5] FIG. 1A is a perspective view showing a cutting device for carrying out a planarization step, FIG. 1B is a perspective view showing an embodiment of the planarization step, and FIG. 1C is a perspective view showing a planarized wafer. [Figure 6] 1A is a perspective view showing a mode in which a wafer is placed on a chuck table of a grinding device, and FIG. 1B is a perspective view showing an embodiment of a grinding step. [Figure 7] FIG. 10 is a perspective view showing a mode of cutting along the planned dividing lines of the wafer. DETAILED DESCRIPTION OF THE INVENTION

[0014] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, preferred embodiments of a wafer processing method according to the present invention will be described in detail with reference to the accompanying drawings.

[0015] 1 shows a wafer 10 to be processed by this embodiment, a thermocompression sheet 2 disposed on a surface 10a of the wafer 10, and a table 4 for holding the wafer 10. The wafer 10 has a plurality of devices 12 formed on the surface 10a, partitioned by planned division lines 14. The thermocompression sheet 2 is formed to approximately the same dimensions as the wafer 10, and is a sheet that exhibits adhesive strength when heated, and is selected from, for example, either a polyolefin-based sheet or a polyester-based sheet.

[0016] When the thermocompression sheet 2 is made of a polyolefin-based sheet, it is selected from, for example, a polyethylene sheet, a polypropylene sheet, or a polystyrene sheet. When the thermocompression sheet 2 is made of a polyester-based sheet, it is selected from, for example, a polyethylene terephthalate sheet or a polyethylene naphthalate sheet. In this embodiment, it is assumed that a polyethylene sheet is selected as the thermocompression sheet 2.

[0017] When carrying out the wafer processing method of this embodiment, first, a thermocompression sheet providing step is carried out in which the above-mentioned thermocompression sheet 2 is provided on the front surface 10a of the wafer 10. When carrying out the thermocompression sheet providing step, as shown in Fig. 1, a table 4 having a flat upper surface 4a is prepared, and the back surface 10b of the wafer 10 is placed on the upper surface 4a, and a thermocompression sheet is laid on the front surface 10a of the wafer 10 to form an integral structure (see the lower part of Fig. 1). When placing and holding the back surface 10b of the wafer 10 on the table 4, a breathable suction chuck may be provided on the upper surface 4a of the table 4 (see, for example, the chuck table 32 in Fig. 3) to suction-hold the back surface 10b of the wafer 10.

[0018] After the thermocompression sheet disposing step is performed as described above, the thermocompression bonding step is performed, in which the thermocompression sheet 2 is heated and pressed to thermocompression bond it to the front surface 10a of the wafer 10, as described below. When performing the thermocompression bonding step of this embodiment, as shown in FIG. 2, a heating roller 22 (only a portion of which is shown) is positioned above the table 4. The heating roller 22 is rotatably held in the direction indicated by arrow R1 and is movable in the direction indicated by arrow R2, which is parallel to the upper surface 4a of the table 4. The surface of the heating roller 22 is coated with a fluororesin to prevent adhesion even when the thermocompression sheet 2 becomes adhesive when heated. An electric heater and a temperature sensor (not shown) are built into the heating roller 22, and the surface temperature of the heating roller 22 can be adjusted to a desired temperature by a separately provided control device.

[0019] Once the heating roller 22 is positioned on the table 4, as shown in FIG. 2, the heating roller 22 is rotated in the direction indicated by arrow R1 and moved in the direction indicated by arrow R2 while pressing the surface 2a of the thermocompression-bonding sheet 2. In this embodiment, the heating temperature when the thermocompression-bonding sheet 2 is heated by the heating roller 22 is set in the range of 120°C to 140°C. This heating temperature is near the melting point of the polyethylene sheet constituting the thermocompression-bonding sheet 2, and is a temperature at which the thermocompression-bonding sheet 2 does not melt excessively but softens and becomes adhesive. In this manner, the thermocompression-bonding sheet 2 is thermocompression-bonded to the surface 10a of the wafer 10 and integrated therewith. To prevent air from remaining in the gap between the surface 10a of the wafer 10 and the thermocompression-bonding sheet 2, the space in which the table 4 and the thermocompression-bonding roller 22 are disposed may be closed and depressurized to a vacuum state. This completes the thermocompression-bonding process.

[0020] Next, a step forming process is carried out in which the thermocompression bonding sheet 2 is cut along the outer periphery of the wafer 10 to form a step portion. More specifically, the wafer 10 that has been subjected to the above-described thermocompression bonding process is transported to a cutting device 30 (only a portion of which is shown) shown in FIG.

[0021] As shown in Fig. 3, the cutting device 30 is provided with a chuck table 32. The chuck table 32 includes an annular frame 32a and a suction chuck 32b that is surrounded and supported by the frame 32a and forms a holding surface. The suction chuck 32b is made of a breathable material and is connected to a suction source (not shown). When the suction source is activated, a negative pressure is generated on the upper surface of the suction chuck 32b. The chuck table 32 is rotatably supported by a rotation drive source (not shown).

[0022] The wafer 10 transported to the cutting device 30 is placed on the chuck table 32 with the back surface 10b side facing downward and is held by suction. Next, as shown in FIG. 4(a), the cutting means 32 is positioned above the wafer 10. The cutting means 32 includes a blade housing 34, a rotary shaft 35 rotatably supported by the blade housing 34, and a cutting blade 36 disposed at the tip of the rotary shaft 35. The cutting blade 36 is not particularly limited, but may be, for example, a circular saw having a saw blade formed as a cutting blade for processing the outer periphery of a cemented carbide substrate.

[0023] The cutting blade 36 is positioned on the outer periphery 2b of the thermocompression sheet corresponding to the outer periphery of the wafer 10, and while the chuck table 32 is rotated in the direction indicated by R3, the cutting blade 36 is rotated in the direction indicated by R4 at high speed while cutting to form a step portion 2c along the outer periphery 2b. As shown in FIG. 4(b), the step portion 2c has a width of, for example, 0.5 to 1.0 mm, and the height (step) of the upper surface 2a relative to the step portion 2c is, for example, 50 to 100 μm, while the thickness of the thermocompression sheet 2 is, for example, 300 to 500 μm. Note that the width and step of the step portion 2c shown in FIG. 4(b) are not shown to scale for ease of explanation. The step formation process is completed by forming the step portion 2c all around the outer periphery 2b of the thermocompression sheet 2.

[0024] Next, a planarization process is performed in which the upper surface 2a of the thermocompression-bonded sheet 2 is cut with a cutting tool to flatten the upper surface 2a. To perform this planarization process, the wafer 10, on which a step 2c has been formed on the outer periphery of the thermocompression-bonded sheet 2 by the step-forming process described above, is transferred to a cutting device 40 (only a portion of which is shown) shown in FIG. 5(a). As shown in the figure, the cutting device 40 includes a chuck table 41 and a cutting unit 42. The cutting unit 42 includes a spindle housing (not shown), a rotating spindle 43 rotatably mounted in the spindle housing, and a drive source (not shown) for rotating the rotating spindle 43. A disk-shaped cutting tool mounting member 44 is mounted on the lower end of the rotating spindle 43. A cutting tool 46 extending downward is mounted on the outer periphery of the cutting tool mounting member 44. In the illustrated embodiment, the tool bit 46 is formed into a rod shape from tool steel such as a cemented carbide alloy, and a cutting edge 46a formed from diamond or the like is disposed at the lower tip of the tool bit 46. The tool bit 46 attached to the tool bit attachment member 44 is rotated together with the tool bit attachment member 44 in the direction indicated by arrow R5 by the rotation of the rotating spindle 43.

[0025] The illustrated cutting device 40 is equipped with a cutting feed mechanism that moves the cutting unit 42 vertically and a processing feed mechanism that moves the chuck table 41 in the direction indicated by arrow R6 (both not shown). The wafer 10 transported to the cutting device 40 is placed on the chuck table 42 and held by suction. As shown in FIG. 5(b), the cutting edge 46a of the cutting tool 46 is positioned at a height lower than the upper surface 2a of the thermocompression-bonded sheet 2 (e.g., 30 μm) but not lower than the step 2c formed in the step formation process. This height position allows the cutting tool 46 to remove irregularities formed on the upper surface 2a of the thermocompression-bonded sheet 2 thermocompression-bonded to the front surface 10a of the wafer 10. Next, the drive source of the cutting unit 42 is activated to rotate the rotary spindle 43 in the direction indicated by arrow R5, and the processing feed mechanism is activated to move the chuck table 42 in the direction indicated by arrow R6. As a result, the upper surface 2a of the thermocompression sheet 2 is cut and flattened, excluding the area of ​​the step portion 2c, as shown in Fig. 5(c). For convenience of explanation, the cutting path 2d of the cutting tool 46 is shown by a dashed line in Fig. 5(c), but in reality, the upper surface 2a of the thermocompression sheet 2 becomes a perfectly flat surface.

[0026] According to the above-described embodiment, the cutting edge 46a of the cutting tool 46 is positioned at a height that is not lower than the step portion 2c formed in the step formation process to cut the upper surface 2a of the thermo-compression sheet 2. Therefore, the pressure-bonding force of the step portion 2c against the wafer 10 is relatively greater than the force that attempts to peel the thermo-compression sheet 2 from the outer peripheral end portion 2e (see Figure 5(b)) where the step portion 2c is formed. Therefore, even if the upper surface 2a of the thermo-compression sheet 2 is cut and flattened with the cutting tool 46, the problem of the outer peripheral end portion 2e of the thermo-compression sheet 2 peeling off does not occur.

[0027] In this embodiment, after the above-described planarization process is performed on the thermocompression sheet 2 of the wafer 10, the wafer 10 is transferred to a chuck table 52 of a grinding device 50 (only a portion of which is shown) shown in FIG. 6(a). The chuck table 52 includes an annular frame 52a and a breathable suction chuck 52b that is surrounded by the frame 52a and forms a holding surface. The planarized thermocompression sheet 2 of the wafer 10 transferred to the grinding device 50 is placed face down on the suction chuck 52b of the chuck table 52, and is held by suction by operating a suction source (not shown). Next, a grinding means 53 shown in FIG. 6(b) is positioned above the back surface 10b of the wafer 10. The grinding means 53 includes a rotating spindle 54 rotated by a rotation drive mechanism (not shown), a wheel mount 55 attached to the lower end of the rotating spindle 54, and a grinding wheel 56 attached to the wheel mount 55 and having a plurality of grinding stones 57 arranged in an annular shape on its lower surface. The grinding means 53 is raised and lowered in the vertical direction by a grinding feed mechanism (not shown).

[0028] Once the wafer 10 is held on the chuck table 52, the chuck table 52 is rotated in the direction indicated by arrow R7 at, for example, 300 rpm, while the rotating spindle 54 of the grinding means 53 is rotated in the direction indicated by arrow R8 in FIG. 6(b) at, for example, 6000 rpm. Next, while grinding water is supplied onto the back surface 10b of the wafer 10 by a grinding water supply means (not shown), the grinding feed mechanism is operated to lower the grinding means 53 in the direction indicated by arrow R9, bringing the grinding wheel 57 into contact with the back surface 10b of the wafer 10 and grinding at a grinding feed rate of, for example, 1 μm / sec. The thickness of the wafer 10 is measured by a contact or non-contact measuring means (not shown), and grinding is continued until the desired thickness is reached, completing the grinding process. Once the grinding process is completed and the grinding means 53 is stopped, cleaning, drying, and other processes are performed.

[0029] As described above, in this embodiment, the upper surface 2a of the thermocompression sheet 2 is properly flattened, so that even when the grinding process is performed, the wafer 10 is properly held on the chuck table 52, and the back surface 10b of the wafer 10 is well ground.

[0030] Furthermore, after the grinding step described above is performed, the wafer 10 may be transported to a processing device 60 (only a part of which is shown) shown in Fig. 7, where the processing step described below is performed. When the wafer 10 is transported to the processing device 60, as shown in Fig. 7, the wafer 10 is held by an annular frame F via a protective tape T.

[0031] The processing apparatus 60 of this embodiment is a dicing apparatus that cuts the wafer 10 into individual device chips and includes a chuck table (not shown) that suction-holds the wafer 10 and cutting means 62 that cuts the wafer 10 held by suction on the chuck table. The chuck table is rotatable and includes a moving means (not shown) that feeds the chuck table in the direction indicated by the arrow X in the figure. The cutting means 62 includes a rotating shaft 64 that is rotatably held in the Y-axis direction indicated by the arrow Y in the figure, and an annular cutting blade 66 held at the tip of the rotating shaft 64. The cutting means 62 also includes a Y-axis moving means (not shown) that indexes and feeds the cutting blade 66 in the Y-axis direction. The rotating shaft 64 is driven to rotate by a spindle motor (not shown).

[0032] To perform the cutting process, first, the wafer 10 is placed and held with the back surface 10b facing downward on the holding surface (not shown) of a chuck table. Then, the division lines 14 formed on the front surface 10a to which the thermocompression sheet 2 is pressed are detected, aligned in the X-axis direction, and aligned with the cutting blade 66. Because the thermocompression sheet 2 is a transparent sheet, the division lines 14 can be detected using an appropriate imaging device. Next, the cutting blade 66, rotated at high speed in the direction indicated by arrow R10, is positioned on the division lines 14 aligned in the X-axis direction and cuts into the division lines 14 from the thermocompression sheet 2 side, while the chuck table is moved in the X-axis direction to form cut grooves 100 that divide the wafer 10 along the division lines 14. Next, the cutting blade 66 of the cutting means 62 is indexed and moved to a position on a division line 14 adjacent in the Y-axis direction to the division line 14 where the cut groove 100 has been formed but where no cut groove 100 has been formed, and the cut groove 100 is formed in the same manner as described above. By repeating these steps, the cut grooves 100 are formed along all the division lines 14 along the X-axis direction.

[0033] Next, the chuck table is rotated 90 degrees, and the direction perpendicular to the direction in which the cut grooves 100 were previously formed is aligned with the X-axis direction. The above-mentioned cutting process is performed on all of the division lines 14 that have now been aligned with the X-axis direction, and cut grooves 100 are formed along all of the division lines 14 formed on the wafer 10. In this manner, the cutting process is performed to complete the processing step of dividing the wafer 10 into device chips for each device 12 along the division lines 14. Once the above-mentioned processing step is completed, the thermocompression sheet 2 is cooled or heated, for example, to make it easier to peel from the wafer 10, and the individually divided devices 12 are then picked up from the wafer 10.

[0034] In the above-described embodiment, the processing apparatus 60 is a dicing apparatus. However, the present invention is not limited to this, and the processing apparatus may be a laser processing apparatus that irradiates the wafer 10 with a laser beam to perform laser processing. When the processing step is performed using a laser processing apparatus, for example, a laser beam having a wavelength that is absorbed by the wafer 10 may be irradiated from the thermocompression sheet 2 side along the intended division lines 14 to perform ablation processing and divide the wafer 10 into individual device chips. As in the above-described embodiment, by performing the processing step of dividing the wafer 10 into device chips without peeling the thermocompression sheet 2 from the wafer 10, cutting chips generated when cutting the wafer 10 and debris generated by laser processing are prevented from adhering to the surfaces of the divided device chips, contributing to improved quality of the device chips.

[0035] In the above-described embodiment, the thermocompression sheet 2 is a polyethylene sheet, but the present invention is not limited thereto. For example, the thermocompression sheet 2 can be selected from other polyolefin-based sheets, such as a polypropylene sheet or a polystyrene sheet. When a polypropylene sheet is selected as the thermocompression sheet 2, the heating temperature during the thermocompression bonding step is preferably 160 to 180°C, and when a polystyrene sheet is selected, the heating temperature is preferably 220 to 240°C. The thermocompression sheet 2 can also be selected from polyester-based sheets, such as a polyethylene terephthalate sheet or a polyethylene naphthalate sheet. When polyethylene terephthalate is selected as the thermocompression sheet 2, the heating temperature during the thermocompression bonding step is preferably 250 to 270°C, and when polyethylene naphthalate is selected, the heating temperature is preferably 160 to 180°C. [Explanation of symbols]

[0036] 2: Heat-sealed sheet 2a:Top surface 2b: Outer circumference 2c: Step 2d: Cutting marks 2e: Outer edge 4: Table 4a:Top surface 10: Wafer 10a: surface 10b: Back side 12: Device 14: Planned division line 22: Heating roller 30: Cutting equipment 32: Chuck table 34: Blade housing 35: Rotation axis 36: Cutting blade 40: Cutting equipment 41: Chuck table 42: Cutting unit 43: Rotating spindle 44: Tool holder 46: Byte 46a: cutting edge 50: Grinding equipment 52: Chuck table 53: Grinding means 54: Rotating spindle 55: Wheel mount 56: Grinding wheel 57: Grinding wheel 60: Processing equipment (dicing equipment) 62: Cutting means 66: Cutting blade 100: Cutting groove F: Frame T: Protective tape

Claims

1. A wafer processing method for processing a wafer having a plurality of devices formed on a surface thereof by dividing the wafer into sections by planned division lines, comprising the steps of: a thermocompression sheet disposing step of disposing a thermocompression sheet on the front surface of the wafer; a thermocompression bonding step of heating and pressing a thermocompression bonding sheet to the surface of the wafer; a step forming step of cutting the thermocompression bonded sheet along the outer periphery of the wafer to form a step portion on the outer periphery of the thermocompression bonded sheet; a flattening step of flattening the upper surface of the thermocompression sheet by cutting the upper surface with a cutting tool; Including, In the planarization step, the cutting edge of the tool is positioned at a position that does not fall below the step portion formed in the step formation step.

2. 2. The wafer processing method according to claim 1, wherein the thermocompression sheet is a polyolefin sheet selected from the group consisting of a polyethylene sheet, a polypropylene sheet, and a polystyrene sheet.

3. 3. The wafer processing method according to claim 2, wherein the heating temperature when heating the thermocompression-bonded sheet in the thermocompression bonding step is 120 to 140°C for a polyethylene sheet, 160 to 180°C for a polypropylene sheet, or 220 to 240°C for a polystyrene sheet.

4. 2. The wafer processing method according to claim 1, wherein the thermocompression sheet is a polyester sheet selected from the group consisting of a polyethylene terephthalate sheet and a polyethylene naphthalate sheet.

5. 5. The wafer processing method according to claim 4, wherein the heating temperature when heating the thermocompression bonding sheet in the thermocompression bonding step is 250 to 270°C in the case of a polyethylene terephthalate sheet, and 160 to 180°C in the case of a polyethylene naphthalate sheet.

6. 6. A wafer processing method according to claim 1, further comprising a grinding step of holding the top surface of the flattened thermocompression sheet on the holding surface of a chuck table of a grinding device and grinding the back surface of the wafer to a desired thickness.

7. 7. A wafer processing method according to claim 1, further comprising a processing step of holding the back side of the wafer to which the flattened thermocompression sheet is bonded on the holding surface of a chuck table of a dicing device or a laser processing device, and performing cutting or laser processing along the intended division line from the thermocompression sheet side.

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