Wafer processing method

Abstract

To provide a wafer processing method in which there is no problem such as a dividing line is meandered, warping of a wafer occurs, a device peels off from a protective sheet, or chipping of a part of the device occurs when dividing into individual devices.SOLUTION: A wafer processing method that divides a wafer formed on the surface of a plurality of devices into individual device chips by dividing lines includes at least a protective sheet placement step of placing a protective sheet on the surface of the wafer, and a modified layer forming step of forming a modified layer in which a focused point of a laser beam with a wavelength that is transparent to the wafer is positioned inside the dividing line and irradiated to create a modified layer that is to serve as the starting point for dividing, and in the modified layer forming step, the protective sheet provided on the surface of the wafer is heated to be flexible and allowed to expand due to the formation of the modified layer.SELECTED DRAWING: Figure 3

Description

Technical Field

[0006] , ,

[0001] The present invention relates to a method for processing a wafer in which a plurality of devices are partitioned by a dicing line and formed on the surface, and the wafer is divided into individual device chips.

Background Art

[0002] A wafer in which a plurality of devices such as ICs and LSIs are partitioned by a dicing line and formed on the surface is divided into individual device chips by a dicing device or a laser processing device, and is used in electric devices such as mobile phones and personal computers.

[0003] A laser processing device is generally composed of a chuck table for holding a wafer and a laser beam irradiation means for irradiating a laser beam on a dicing line of the wafer held on the chuck table, and can perform laser processing along the dicing line formed on the wafer.

[0004] When dividing into individual device chips using a laser processing device, for example, a protective sheet is disposed on the surface of the wafer, the protective sheet side is held on the chuck table, and the condensing point of a laser beam having a wavelength that is transmissive to the wafer is positioned inside corresponding to the dicing line and irradiated from the back side of the wafer to form a modified layer inside along the dicing line (see, for example, Patent Document 1).

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] When a modified layer is formed along the interior of a wafer corresponding to the planned division line using the laser processing apparatus described in Patent Document 1 above, the modified layer expands, causing minute deformation in the wafer. However, wafers attached to and held by a protective sheet have their position restricted by the protective sheet's action and cannot follow the changes in the wafer caused by the expansion of the modified layer. This results in problems such as the planned division line becoming meandering, the wafer becoming warped, and even the device peeling off from the protective sheet or chipping occurring in parts of the device.

[0007] Protective sheets attached to the surface of a wafer come in various types, including those with an adhesive layer covering the entire surface, those with an adhesive layer around the outer edge but not in the device area, and those without an adhesive layer that are attached to the wafer surface by heat and pressure bonding. However, the aforementioned problems can occur to a greater or lesser extent in any of these types. In particular, in wafers with bumps (electrode protrusions) on the device electrodes, these bumps can dig into the adhesive surface of the protective sheet, preventing it from following the changes in the wafer caused by the expansion of the modified layer, which makes the above problems more likely to occur.

[0008] The present invention has been made in view of the above facts, and its main technical problem is to provide a wafer processing method that, even when a modified layer is formed inside the wafer corresponding to the planned division line by irradiating it with a laser beam, does not cause problems such as the planned division line becoming meandering, the wafer becoming warped, the device peeling off from the protective sheet, or chipping occurring in part of the device when it is divided into individual devices. [Means for solving the problem]

[0009] To solve the above-mentioned main technical problems, the present invention provides a wafer processing method for dividing a wafer, which has multiple devices formed on its surface by division lines, into individual device chips, comprising a protective sheet placement step of placing a protective sheet on the surface of the wafer, The protective sheet holds the front side of the wafer and allows access from the back side of the wafer.A wafer processing method is provided, comprising at least a modified layer formation step of positioning the focal point of a laser beam with a wavelength that is transparent to the wafer within the wafer corresponding to the planned division line and irradiating it to form a modified layer that will serve as the starting point for division along the planned division line, wherein in the modified layer formation step, a protective sheet disposed on the surface of the wafer is heated to make it flexible and to allow for expansion caused by the formation of the modified layer.

[0010] The process includes a grinding step after the protective sheet placement step and before the modified layer formation step, in which the back surface of the wafer is ground to a desired thickness. In this grinding step, the protective sheet placed on the surface of the wafer is cured at a lower temperature than during heating, thereby strengthening the wafer's holding power compared to the modified layer formation step. Furthermore, after the modified layer formation step, the process includes a splitting step in which the back surface of the wafer is ground to a desired thickness and split into individual device chips. In this splitting step, the protective sheet placed on the surface of the wafer is cured at a lower temperature than during heating, thereby strengthening the wafer's holding power compared to the modified layer formation step. In addition, it is preferable that the protective sheet in the protective sheet placement step is a thermocompression sheet. [Effects of the Invention]

[0011] The wafer processing method of the present invention is a wafer processing method for dividing a wafer, which has multiple devices formed on its surface by division lines, into individual device chips, comprising a protective sheet placement step of placing a protective sheet on the surface of the wafer, The protective sheet holds the front side of the wafer and allows access from the back side of the wafer.The method includes at least a modified layer formation step, in which a laser beam with a wavelength that is transparent to the wafer is focused at a point located inside the wafer corresponding to the planned division line and irradiated to form a modified layer that will serve as the starting point for division, along the planned division line. In this modified layer formation step, a protective sheet disposed on the surface of the wafer is heated to make it flexible and to allow for expansion caused by the formation of the modified layer. Therefore, even when a modified layer is formed inside the wafer corresponding to the planned division line by irradiating it with a laser beam, it is prevented that the planned division line will meander, the wafer will warp, or that devices will peel off or chip. [Brief explanation of the drawing]

[0012] [Figure 1] This is a perspective view of the wafer processed in this embodiment. [Figure 2] This is a perspective view showing how a protective sheet is heat-pressed onto the surface of a wafer. [Figure 3] (a) A perspective view showing how a wafer is held in the holding means of a laser processing apparatus, and (b) A perspective view showing how a modified layer formation process is performed on the wafer. [Figure 4] (a) A perspective view showing how a wafer is held in the holding means of the grinding device, and (b) A perspective view showing how the grinding process is performed on the wafer. [Figure 5] (a) A perspective view showing a wafer being subjected to a splitting process, and (b) A perspective view showing a wafer that has been split by the splitting process. [Modes for carrying out the invention]

[0013] Hereinafter, embodiments relating to a wafer processing method constructed according to the present invention will be described in detail with reference to the attached drawings.

[0014] Figure 1 shows a wafer 10 processed by the wafer processing method of this embodiment, and a protective sheet T attached to the surface 10a of the wafer 10. The wafer 10 is, for example, a silicon (Si) wafer, and multiple devices 12 are formed on the surface 10a, demarcated by division lines 14. Also, as shown in the enlarged view of a part of the wafer 10 on the right, multiple bumps 16 with a protruding shape are formed on multiple electrodes arranged on the devices 12 of the wafer 10.

[0015] In carrying out the wafer processing method of this embodiment, as shown in Figure 1, a protective sheet T is placed on the surface 10a of the wafer 10, and the wafer 10 and the protective sheet T are integrated (protective sheet placement step). The type of protective sheet T placed in the protective sheet placement step and the method of placing the protective sheet T on the wafer 10 are not particularly limited. For example, one can choose from a type of protective sheet with an adhesive layer covering the entire surface, a type of protective sheet with an adhesive layer on the outer periphery but no adhesive layer in the device area, or a type of protective sheet that does not have an adhesive layer and is attached to the surface of the wafer by thermal pressure bonding.

[0016] Referring to both Figure 1 and Figure 2, the case in which the protective sheet T disposed on the wafer 10 in the protective sheet disposal process is a thermosealable sheet will be described. The thermosealable sheet is, for example, a polyolefin-based sheet or a polyester-based sheet. If it is a polyolefin-based sheet, it is preferable to select it from a polyethylene sheet, a polypropylene sheet, or a polystyrene sheet. If the thermosealable sheet is a polyester-based sheet, it is preferable to select it from a polyethylene terephthalate sheet or a polyethylene naphthalate sheet. Each of the above thermosealable sheets is a sheet that softens and exhibits adhesive strength when heated. In this embodiment, the case in which the protective sheet T is a polyethylene sheet will be described.

[0017] As shown in FIG. 1, a protective sheet T made of a polyethylene sheet is placed on the surface 10a of the wafer 10 and conveyed to the thermocompression bonding apparatus 20 shown in FIG. 2. The thermocompression bonding apparatus 20 of the present embodiment includes a table 22 with a flat upper surface and a heating roller 24. The heating roller 24 includes a heating heater and a temperature sensor (not shown) inside, and the surface of the heating roller 24 can be controlled to a desired temperature. The thermocompression bonding roller 24 is supported by a support means (not shown), and the surface is coated with a fluororesin so that the protective sheet T is not卷入 the heating roller 24 even when the protective sheet T exhibits adhesive force.

[0018] When the wafer 10 is conveyed to the above-described thermocompression bonding apparatus 20, it is placed on the upper surface of the table 22 with the side where the protective sheet T is adhered facing upward and the back surface 10b side facing downward. Next, the above-described heating heater of the heating roller 24 is operated to heat the surface of the heating roller 24 to near the melting temperature (120°C to 140°C) of the polyethylene sheet constituting the protective sheet T, and it is positioned at the front end side of the protective sheet T on the wafer 10 and pressed from above. Next, the heating roller 24 is rotated in the direction indicated by the arrow R1 and moved in the direction indicated by the arrow R2 to thermocompression bond the protective sheet T to the surface 10a of the wafer 10 to integrate them. After the protective sheet disposition step is completed, the operation of the heating heater is stopped, and the temperature of the protective sheet T drops to the room temperature (for example, 23°C) where the thermocompression bonding apparatus 20 is disposed. In addition, when a protective sheet of a type in which an adhesive layer exists on the entire surface or a type in which an adhesive layer exists on the outer periphery and not in the device region is selected as the protective sheet T, since heating is not necessary, after placing it on the surface 10a of the wafer 10, the protective sheet disposition step is completed only by pressing from above.

[0019] As described above, once the protective sheet placement process is carried out and the wafer 10 and the protective sheet T are integrated, they are transported to the laser processing apparatus 30 (only a portion is shown) shown in Figure 3. The laser processing apparatus 30 includes a holding means 32 shown in Figure 3(a) and a laser beam irradiation means 34 (see Figure 3(b)) that irradiates the wafer 10 held by the holding means 32 with a laser beam LB. The holding means 32 includes an adsorption chuck 32a made of a breathable porous material and a frame 32b that surrounds and supports the adsorption chuck 32a. The laser beam irradiation means 34 is a means for irradiating the wafer 10 with a laser beam LB of a wavelength that is transparent to the wafer 10, and the laser beam LB is focused by a light concentrator 34a and irradiated onto the wafer 10 held by the holding means 32. A suction source (not shown) is connected to the frame 32b and generates negative pressure on the upper surface of the adsorption chuck 32a. Furthermore, a heating means 36 is provided in the holding means 32 of this embodiment. The heating means 36 is a heating heater built inside the suction chuck 32a or between the suction chuck 32a and the frame 32b, and is connected to a power supply (not shown) that can heat the upper surface of the suction chuck 32a to a desired temperature (for example, 35 to 50°C). The laser processing apparatus 30 includes an X-axis feeding means that feeds the holding means 32 and the laser beam irradiation means 34 relatively in the X-axis direction, a Y-axis feeding means that feeds the holding means 32 and the laser beam irradiation means 34 relatively in the Y-axis direction perpendicular to the X-axis direction, and a rotational driving means that rotates the holding means (none of which are shown).

[0020] The wafer 10 conveyed to the laser processing apparatus 30 is placed on the holding means 32 with the surface 10a, to which the protective sheet T is thermocompression bonded, facing downward, and is sucked and held. Next, the heating means 36 disposed on the holding means 32 is operated to heat the protective sheet T disposed on the surface 10a of the wafer 10. The temperature at which the protective sheet T is heated is a temperature at which the protective sheet T exhibits sufficient flexibility, for example, T = 50°C. Next, alignment is performed on the wafer 10 held by the holding means 32 using alignment means (not shown) disposed in the laser processing apparatus 30, and the position of the division planned line 14 formed on the surface 10a is detected. The alignment means is means for irradiating infrared rays and detecting the division planned line 14 formed on the surface 10a from the back surface 10b side of the wafer 10, and the information on the position of the detected division planned line 14 is stored in control means (not shown).

[0021] Next, based on the position information of the division planned line 14 stored in the control means, the holding means 32 is rotated by the above-described rotational drive means to align the division planned line 14 in a predetermined direction with the X-axis direction of the laser processing apparatus 30. The condenser 34a of the laser beam irradiation means 34 is positioned directly above the processing start position of the division planned line 14 in the predetermined direction, and the condensing point of the laser beam LB is positioned inside the wafer 10 corresponding to the division planned line 14 and irradiated. As shown in FIG. 3(b), while applying heat energy Q to the protective sheet T and heating it, the wafer 10 is processed and fed in the X-axis direction together with the holding means 32 to form a modified layer 100 serving as a starting point for division along the division planned line 14 of the wafer 10. In the above-described embodiment, the alignment has been described as being performed after heating the holding means 32, but it may be performed before heating the holding means 32.

[0022] Once the modified layer 100 has been formed along the predetermined division line 14, the wafer 10 is indexed and fed in the Y-axis direction by the interval of the division line 14 to position adjacent unprocessed division lines 14 in the Y-axis direction directly below the concentrator 34a. Then, in the same manner as described above, the focal point of the laser beam LB is positioned inside the wafer 10 corresponding to the division line 14 and irradiated, and the wafer 10 is processed and fed in the X-axis direction to form the modified layer 100. Similarly, the wafer 10 is processed and fed in the X-axis and Y-axis directions to form the modified layer 100 along all division lines 14 along the X-axis direction. Next, the wafer 10 is rotated 90 degrees to align the unprocessed division lines 14 in the direction perpendicular to the division lines 14 on which the modified layer 100 has already been formed, in the X-axis direction. Then, the focal point of the laser beam LB is positioned and irradiated in the interior corresponding to each of the remaining division lines 14 in the same manner as described above, thereby forming a modified layer 100 along all the division lines 14 formed on the surface 10a of the wafer 10 (modified layer formation step). In the above embodiment, an example was shown in which a heating means 36 provided on the holding means 32 is used as a means to heat the protective sheet T provided on the wafer 10 and make it flexible, but the means to heat the protective sheet T and make it flexible is not limited to this. For example, since the area in the laser processing apparatus 30 where the holding means 3 is provided is usually formed in a sealed space, the protective sheet T may be heated by heating the space in which the holding means 3 that holds the wafer 10 is provided.

[0023] As a result of the modification layer formation process described above, multiple modification layers 100 are formed, causing the division line 14 on which the modification layers 100 are formed to expand, and the shape of the wafer 10 to change. In this embodiment, as described above, the protective sheet T is heated to a flexible state when the modification layer formation process is carried out. This allows for changes in the shape of the wafer 10 due to the expansion of the modification layers 100, preventing the division line 14 from meandering, the wafer from warping, or the device from delaminating or chipping.

[0024] The present invention includes a grinding step of grinding the back surface 10b of the wafer 10 to a desired thickness before carrying out the modified layer formation step described above, and a splitting step of grinding the back surface 10b of the wafer 10 to a desired thickness and splitting it into individual device chips after carrying out the modified layer formation step described above. The grinding step and the splitting step will be described below.

[0025] After the protective sheet placement process described above, and before the modified layer formation process, a grinding process is performed to grind the back surface 10b of the wafer 10 to a desired thickness. More specifically, the wafer 10, which has undergone the protective sheet placement process and is integrated with the protective sheet T, is transported to the grinding apparatus 40 (only a portion is shown) shown in Figure 4. As shown in Figures 4(a) and (b), the grinding apparatus 40 comprises a chuck table 42 and a grinding means 44 for grinding the back surface 10b of the wafer 10 held by suction on the chuck table 42. The chuck table 42 comprises an adsorption chuck 42a made of a porous material that allows for airflow, and a frame 42b that surrounds and holds the adsorption chuck 42a. A suction source (not shown) is connected to the frame 42b, and by operating the suction source, negative pressure is generated on the upper surface of the adsorption chuck 42a. The grinding means 44 includes a rotating spindle 44a that is rotated by a rotational drive mechanism (not shown), a wheel mount 44b attached to the lower end of the rotating spindle 44a, a grinding wheel 44c attached to the lower surface of the wheel mount 44b, and a plurality of grinding wheels 44d arranged in an annular shape on the lower surface of the grinding wheel.

[0026] Once the wafer 10 is transported to the grinding apparatus 40, as shown in Figure 4(a), the wafer 10 is placed on the chuck table 42 with its back surface 10b facing upwards and the protective sheet T facing downwards, and then held in place by suction. Next, as shown in Figure 4(b), the rotating spindle 44a of the grinding means 44 is rotated at, for example, 3000 rpm in the direction indicated by arrow R3, while the chuck table 42 is rotated at, for example, 300 rpm in the direction indicated by arrow R4. Then, grinding water is supplied onto the back surface 10b of the wafer 10 by a grinding water supply means (not shown), and the grinding wheel 44d is brought into contact with the back surface 10b of the wafer 10. The grinding wheel 44c is then lowered in the direction indicated by arrow R5 at, for example, a grinding feed rate of 0.1 μm / second to perform grinding and feeding. In this process, grinding can be carried out while measuring the thickness of the wafer 10 using a measuring gauge (not shown), and the back surface 10b of the wafer 10 is ground by a predetermined amount to bring the wafer 10 to a predetermined thickness. Once the wafer 10 has reached the predetermined thickness, the grinding means 40 is stopped, and after cleaning, drying, etc., the grinding process of grinding the back surface 10b of the wafer 10 is completed.

[0027] Here, when performing the grinding process described above, the protective sheet T disposed on the surface 10a of the wafer 10 is hardened at a temperature lower than that during the heating process performed when the modified layer formation process is carried out, thereby strengthening the wafer's holding force compared to the modified layer formation process. The temperature lower than that during heating described above is, for example, a state in which the temperature of the protective sheet T is 20 to 25°C. The means by which the wafer 10 is hardened at a temperature lower than that during the heating process performed when the modified layer formation process is carried out, thereby strengthening the holding force of the wafer 10, are not particularly limited. As such means, for example, a circulation path can be formed in the chuck table 42 to circulate cooling water cooled to about 20°C, thereby maintaining the upper surface of the suction chuck 42a at 20°C at all times, and thus the protective sheet T = 20°C can be employed. Alternatively, the temperature of the space where the chuck table 42 of the grinding device 40 is located may be adjusted to 20°C, thereby cooling the protective sheet T attached to the surface 10a of the wafer 10 to maintain a temperature of 20°C.

[0028] According to the embodiment described above, when performing the modified layer formation process, the protective sheet T disposed on the surface 10a of the wafer 10 is heated to make it flexible, allowing for expansion caused by the formation of the modified layer 100. However, in the grinding process performed before this, the protective sheet T is hardened at a lower temperature than when heated during the modified layer formation process, thereby strengthening the holding force of the wafer 10 compared to the modified layer formation process. This prevents the wafer 10 from moving on the protective sheet T or peeling off the protective sheet T during the grinding process, and also allows for changes in the shape of the wafer 10 due to the expansion of the modified layer 100 during the modified layer formation process, preventing the planned division line 14 from meandering, the wafer from warping, or the device from peeling or chipping.

[0029] After performing the modified layer formation process described above, the back surface 10b of the wafer 10 is ground to the desired thickness, and a splitting process is performed to divide it into individual device chips starting from the modified layer 100. This splitting process will be explained with reference to Figure 5.

[0030] The splitting process, which is carried out after the modified layer formation process, can use the grinding apparatus 40 used in the grinding process described above. After the modified layer formation process described above has been carried out and the modified layer 100 that will serve as the starting point for splitting has been formed inside the wafer 10 along the planned splitting line 14, the wafer 10 is transported to the grinding apparatus 40 shown in Figure 5 (which is the same as the grinding apparatus 40 shown in Figure 4, and only a part of it is shown, and a detailed explanation is omitted), and is placed on the chuck table 42 with the protective sheet T side facing downwards and held in place by suction.

[0031] Next, as shown in Figure 5(a), the rotating spindle 44a of the grinding means 44 is rotated at, for example, 3000 rpm in the direction indicated by arrow R3, while the chuck table 42 is rotated at, for example, 300 rpm in the direction indicated by arrow R4. Then, grinding water is supplied onto the back surface 10b of the wafer 10 by a grinding water supply means (not shown), and the grinding wheel 44d is brought into contact with the back surface 10b of the wafer 10. The grinding wheel 44c is then lowered in the direction indicated by arrow R5 at, for example, a grinding feed rate of 0.1 μm / second to perform grinding and feeding. At this time, the thickness of the wafer 10 can be measured using a measuring gauge (not shown) as the grinding progresses, and the back surface 10b of the wafer 10 is ground by a predetermined amount to achieve a predetermined finished thickness of the wafer 10. By performing this grinding, an external force is applied to the wafer 10, and as shown in Figure 5(b), the wafer 10 is divided into individual device chips 12' starting from the modified layer 100 (dividing process). Then, the grinding means 40 is stopped, and after cleaning, drying, etc., the wafer is transported to a device that performs the appropriate next process, for example, a pickup device that performs the pickup process.

[0032] Here, when carrying out the above splitting process, the protective sheet T disposed on the surface 10a of the wafer 10 is cured at a temperature lower than that during the heating process carried out when the modified layer formation process is carried out, thereby strengthening the wafer's holding force compared to the modified layer formation process. The temperature lower than that during heating is, for example, a state in which the temperature of the protective sheet T is 20-25°C, similar to the grinding process described above. As a means of strengthening the holding force of the wafer 10 by curing the protective sheet T at a temperature lower than that during the heating process carried out when the modified layer formation process is to form a circulation path in the chuck table 42 through which cooling water cooled to about 20°C is circulated, thereby maintaining the upper surface of the suction chuck 42a at a constant temperature of 20°C and setting the protective sheet T = 20°C. Alternatively, the temperature of the space where the chuck table 42 of the grinding device 40 is located may be adjusted to 20°C, thereby cooling the protective sheet T attached to the surface 10a of the wafer 10 to maintain a temperature of 20°C.

[0033] According to the above-described embodiment, when the modified layer formation process is carried out, changes in the shape of the wafer 10 due to the expansion of the modified layer 100 are permitted, preventing the planned division line 14 from meandering, the wafer from warping, or the devices from peeling off or chipping. Furthermore, in the division process carried out after the modified layer formation process, the protective sheet T is cured at a lower temperature than when heated during the modified layer formation process, thereby strengthening the holding force of the wafer 10 compared to the modified layer formation process. This prevents the wafer 10 from moving on the protective sheet T or peeling off the protective sheet T during the division process. [Explanation of symbols]

[0034] 10: Wafer 10a: surface 10b: Back side 12: Devices 12': Device chip 14: Planned division line 20:Thermocompression bonding equipment 22: Table 24: Heating roller 30: Laser processing equipment 32: Holding means 32a: Suction chuck 32b:Frame body 34: Laser beam irradiation means 34a: Light concentrator 36: Heating means 40: Grinding equipment 42: Chuck Table 42a: Suction chuck 42b:Frame body 44: Grinding methods 44a: Rotating spindle 44b: Wheel mount 44c: Grinding Wheel 44d: Grinding wheel 100: Modified layer T: Protective sheet

Claims

1. A wafer processing method for dividing a wafer, which has multiple devices partitioned by dividing lines and formed on its surface, into individual device chips, A protective sheet placement process in which a protective sheet is placed on the surface of the wafer, A modified layer formation step is performed by holding the surface side of the wafer through the protective sheet and irradiating the back side of the wafer with a laser beam of a wavelength that is transparent to the wafer, positioning the focal point of the laser beam inside the wafer corresponding to the planned division line, thereby forming a modified layer inside along the planned division line that will serve as the starting point for division. It has at least the following features: A wafer processing method comprising a modified layer formation step, wherein a protective sheet disposed on the surface of the wafer is heated to make it flexible, thereby allowing expansion caused by the formation of the modified layer.

2. After the protective sheet placement step and before the modified layer formation step, the process includes a grinding step in which the back surface of the wafer is ground to a desired thickness. The wafer processing method according to claim 1, wherein in the grinding step, a protective sheet disposed on the surface of the wafer is hardened at a temperature lower than that at the time of heating, thereby strengthening the wafer's holding power compared to the modified layer formation step.

3. After the modified layer formation step, the back surface of the wafer is ground to a desired thickness, and the process includes a division step to divide it into individual device chips. The wafer processing method according to claim 1, wherein in the splitting step, a protective sheet disposed on the surface of the wafer is cured at a temperature lower than that at the time of heating, thereby strengthening the wafer's holding power compared to the modified layer formation step.

4. The wafer processing method according to any one of claims 1 to 3, wherein in the protective sheet placement step, the protective sheet is a heat-sealable sheet.

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