Wafer processing method

Abstract

To provide a machining method for a wafer with which the wafer can be satisfactorily divided into individual device chips even in a case where a metal film is formed on a rear face of the wafer.SOLUTION: A machining method includes: a modified layer forming step of irradiating a laser beam LB with a wavelength having transmissibility with respect to the wafer 10 and positioning a focal point of the laser beam LB inside a predetermined dividing line 14 and executing irradiation to form a modified layer 100 to be a start point of division; and a division step of imparting ultrasonic waves W through a layer of water L to a metal film 16 coated a rear face 10b of the wafer 10 to plastically deform the metal film 16, and dividing the wafer 10 into individual device chips 12 along the modified layer 100.SELECTED DRAWING: Figure 5

Description

Technical Field

[0001] The present invention relates to a method for processing a wafer that divides a wafer having a front surface formed by partitioning a plurality of devices by a planned division line and a back surface coated with a metal film into individual device chips.

Background Art

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

[0003] In addition, in a wafer having a metal film (for example, Ti, Ni, Au, or a composite thereof) serving as an electrode coated on the back surface, a laser beam is irradiated on the back surface corresponding to the planned division line to remove the metal film, and then the planned division line is divided by a dicing device or a laser processing device (for example, see Patent Document 1).

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] In the technique described in Patent Document 1 above, since the metal film in the region corresponding to the planned division line is melted and removed by irradiation with a laser beam, particles of the melted metal film scatter and adhere to the back surface of the device chip, resulting in a problem of deteriorating the quality of the device chip.

[0006] Furthermore, when a wafer is formed from a SiC substrate, it is difficult to divide it using a cutting blade. Therefore, attempts have been made to divide the metal film simultaneously by positioning the focal point of a laser beam with a wavelength that is transparent to the SiC substrate inside the line to be divided and irradiating it to form a modified layer that serves as the starting point for division, and then applying an external force to divide it into individual device chips. However, when a metal film as described above is formed, especially when the thickness of the metal film is thick, there is a problem in that the metal film cannot be divided well together with the SiC substrate that constitutes the wafer.

[0007] 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 can divide a wafer into individual device chips well, even when a metal film is formed on the back surface of the wafer. [Means for solving the problem]

[0008] To solve the main technical problems described above, the present invention provides a wafer processing method for dividing a wafer having a surface formed by dividing lines and a back surface covered with a metal film into individual device chips, comprising: a modified layer formation step of irradiating the wafer with a laser beam of a wavelength that is transparent to the wafer to position the focal point of the laser beam inside the dividing lines and irradiating to form a modified layer that will serve as the starting point for division; and a division step of applying ultrasound to the metal film covered on the back surface of the wafer via a layer of water to plastically deform the metal film and dividing the wafer into individual device chips along the modified layer.

[0009] In the splitting process, when applying ultrasound to the metal film, it is preferable to expose the region corresponding to the planned splitting line and mask the metal film in other regions. The wafer is preferably formed from a SiC substrate. [Effects of the Invention]

[0010] The wafer processing method of the present invention is a wafer processing method for dividing a wafer having a surface formed by dividing lines for multiple devices and a back surface covered with a metal film into individual device chips, and comprises a modified layer formation step in which a laser beam of a wavelength that is transparent to the wafer is irradiated to position the focal point of the laser beam inside the dividing lines and irradiated to form a modified layer that will serve as the starting point for division, and a division step in which ultrasound is applied to the metal film covered on the back surface of the wafer via a layer of water to plastically deform the metal film and divide the wafer into individual device chips along the modified layer. As a result, the substrate constituting the wafer together with the metal film covered on the back surface of the wafer can be divided well along the dividing lines without degrading the quality of the devices. [Brief explanation of the drawing]

[0011] [Figure 1] (a) Perspective view of the wafer to be processed, (b) Side view of the wafer described in (a). [Figure 2] This is an overall perspective view of the laser processing equipment. [Figure 3] (a) A perspective view showing an embodiment of the modified layer formation process, and (b) A partially enlarged cross-sectional view of the modified layer formation process shown in (a). [Figure 4] This is a perspective view showing how protective tape is applied to wafer 10. [Figure 5] (a) A conceptual diagram showing an embodiment of the division process, and (b) A conceptual diagram showing another embodiment of the division process. [Figure 6] This is a perspective view showing an embodiment of a relocation process in which a wafer is held to a frame via adhesive tape. [Figure 7] This is a perspective view showing an embodiment of the pickup process. [Modes for carrying out the invention]

[0012] 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.

[0013] Figure 1(a) shows a perspective view of a wafer 10 processed by the wafer processing method of this embodiment. The wafer 10 is made of a SiC substrate, and on the surface 10a, a plurality of devices 12 are formed, demarcated by division lines 14. As shown in Figure 1(b), a metal film 16 made of a composite of Ti, Ni, and Au that constitutes electrodes when the wafer is divided into individual device chips is formed on the back surface 10b of the wafer 10. In this embodiment, the thickness of the metal film 16 is 1 μm each for Ti and Ni, and 3 μm for Au, so the total thickness is 5 μm, and the total thickness of the wafer 10 is 570 μm. Note that the wafer 10 described above is just one example, and the wafer processed by the wafer processing method of the present invention is not limited to this, and for example, the above metal film 16 may be formed on a Si wafer or a sapphire wafer. Furthermore, the metal film 16 is not limited to being composed of the composite of Ti, Ni, and Au described above, but may be composed of only one of them, or a selection of two of them, or even a film formed from another metal.

[0014] When the wafer 10 described above is divided into individual device chips using the wafer processing method of this embodiment, the modified layer formation step and the division step described below are performed. The division step is performed after the modified layer formation step, and in the embodiment described below, ultrasonic waves are applied to the metal film 16 coated on the back surface 10b of the wafer 10 via a layer of water to plastically deform the metal film. The wafer processing method of this embodiment will be described in more detail with reference to Figures 2 to 5.

[0015] In carrying out the wafer processing method of this embodiment, a modified layer formation step is performed to form a modified layer on the wafer 10 that serves as the starting point for splitting. Referring to Figure 2, a laser processing apparatus 1 suitable for carrying out the modified layer formation step of this embodiment will be described. When processing the wafer 10 with the laser processing apparatus 1, it is possible to perform the processing by placing the wafer 10 directly on the holding means 3 as shown in the figure, but protective tape may also be attached to the back surface 10b of the wafer 10.

[0016] The laser processing apparatus 1 is disposed on a base 2 and includes a laser beam irradiation means 7 for irradiating a wafer 10 with a laser beam, a holding means 3 for holding the wafer 10, an alignment means 6 for imaging the wafer 10 held by the holding means 3 and performing alignment, an X-axis moving means 4a for moving the holding means 3 in the X-axis direction, a Y-axis moving means 4b for moving the holding means 3 in the Y-axis direction, a frame 5 consisting of a vertical wall portion 5a erected on the side of the X-axis moving means 4a and Y-axis moving means 4b on the base 2 and a horizontal wall portion 5b extending horizontally from the upper end of the vertical wall portion 5a, and control means (not shown) for controlling each operating part.

[0017] The holding means 3 is a means for holding the wafer 10 with the XY plane, specified by the X and Y coordinates, as the holding surface. As shown in Figure 2, it includes a rectangular X-axis movable plate 31 mounted on a base 2 so as to be movable in the X-axis direction, a rectangular Y-axis movable plate 32 mounted on the X-axis movable plate 31 so as to be movable in the Y-axis direction, a cylindrical support column 33 fixed to the upper surface of the Y-axis movable plate 32, and a rectangular cover plate 34 fixed to the upper end of the support column 33. A chuck table 35 is provided on the cover plate 34, extending upward through an elongated hole formed on the cover plate 34. The chuck table 35 is configured to be rotatable by a rotational drive means (not shown) housed within the support column 33. A circular suction chuck 36, formed from a breathable porous material and with the XY plane, specified by the X and Y coordinates, as the holding surface, is provided on the upper surface of the chuck table 35. The suction chuck 36 is connected to a suction means (not shown) by a flow path through the support column 33.

[0018] The X-axis moving means 4a converts the rotational motion of the motor 42a into linear motion via the ball screw 42b and transmits it to the X-axis direction movable plate 31, and moves the X-axis direction movable plate 31 in the X-axis direction along a pair of guide rails 2A, 2A arranged along the X-axis direction on the base 2. The Y-axis moving means 4b converts the rotational motion of the motor 44a into linear motion via the ball screw 44b, transmits it to the Y-axis direction movable plate 32, and moves the Y-axis direction movable plate 32 in the Y-axis direction along a pair of guide rails 31a, 31a arranged along the Y-axis direction on the X-axis direction movable plate 31.

[0019] Inside the horizontal wall portion 5b of the frame body 5, an optical system (not shown) constituting the above-described laser beam irradiation means 7 and the alignment means 6 are accommodated. On the lower surface side of the tip of the horizontal wall portion 5b, a condenser 7a which constitutes a part of the laser beam irradiation means 7 and condenses and irradiates the laser beam LB onto the wafer 10 is arranged. The laser beam irradiation means 7 is a means for irradiating a laser beam LB having a wavelength (for example, 1342 nm) that is transmissive to the SiC substrate. The alignment means 6 is an imaging means for imaging the wafer 10 held by the holding means 3 and detecting the position and orientation of the division planned line 14 of the wafer 10, and is arranged at a position adjacent in the X-axis direction indicated by the arrow X in the figure with respect to the above-described condenser 7a.

[0020] The above-described control means is constituted by a computer, and includes a central processing unit (CPU) that performs arithmetic processing according to a control program, a read-only memory (ROM) that stores the control program and the like, a readable and writable random access memory (RAM) for temporarily storing the detected detection values, arithmetic results, etc., an input interface, and an output interface. The alignment means 6, the laser beam irradiation means 7, the X-axis moving means 4a, the Y-axis moving means 4b, etc. are connected to the control means, and the information detected from the image captured by the alignment means 6 is stored in an appropriate memory and displayed on a display means (not shown).

[0021] When performing the modified layer formation process, the wafer 10 is transported to the laser processing apparatus 1, and with the surface 10a on which the device 12 is formed facing upwards, it is placed on the suction chuck 36 of the chuck table 35 of the holding means 3 and held by suction. Next, the wafer 10 held on the chuck table 35 has its position of the planned division line 14 formed on the surface 10a detected using the alignment means 6 provided in the laser processing apparatus 1, and the wafer 10 is rotated together with the chuck table 35 by the rotation drive means to align the planned division line 14 in a predetermined direction with the X-axis direction. The information of the position of the detected planned division line 14 is stored in the control means described above.

[0022] Next, based on the position information detected by the alignment means 6 described above, the focuser 7a of the laser beam irradiation means 7 is positioned at the processing start position of the division line 14 in a predetermined direction. As shown in Figures 3(a) and (b), the focus point of the laser beam LB is positioned inside the wafer 10 corresponding to the division line 14 and irradiated, and the X-axis moving means 4a is activated to process and feed the wafer 10 in the X-axis direction to form a modified layer 100 (shown by a dashed line) along the predetermined division line 14 of the wafer 10. Once the modified layer 100 has been formed along the predetermined division line 14, the wafer 10 is indexed and fed by the interval of the division line 14 to position the unprocessed division line 14 directly below the focuser 7a. Then, in the same manner as described above, the focus point of the laser beam LB is positioned inside the division line 14 corresponding to the division line 14 and irradiated, and the wafer 10 is processed and fed in the X-axis direction to form a modified layer 100. Similarly, the wafer 10 is processed and fed in the X-axis and Y-axis directions to form a 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 perpendicular to the division lines 14 on which the modified layer 100 has already been formed with the X-axis direction. Then, the focal point of the laser beam LB is positioned and irradiated onto each of the remaining division lines 14 in the same manner as described above to form a modified layer 100 along the interior of all division lines 14 formed on the surface 10a of the wafer 10. This completes the modified layer formation process.

[0023] The laser processing conditions used when carrying out the modified layer formation process described above are set, for example, as follows. Wavelength: 1342nm Average output: 1.0W Repetition frequency: 90kHz Machining feed rate: 700 mm / second

[0024] Once the modified layer formation process has been carried out as described above, a splitting process is performed to divide the wafer 10 into individual device chips along the modified layer 100.

[0025] In carrying out the splitting process, first, as shown in Figure 4, a protective tape T1 formed to the dimensions corresponding to the wafer 10 is attached to the surface 10a of the wafer 10 on which the modified layer 100 is formed, and the wafer is then inverted so that the back surface 10b of the wafer 10 faces upward (see the lower part of Figure 4).

[0026] Next, for example, a water tank 20 containing water L as shown in Figure 5(a) and an ultrasonic imparting means 21 are prepared (for the sake of explanation, only the water tank 20 is shown in cross-section). The ultrasonic imparting means 21 is, for example, equipped with an ultrasonic horn 22 having a piezoelectric element transducer inside, and emits ultrasonic waves W from its tip 23. The oscillation frequency of the ultrasonic waves W emitted from the ultrasonic imparting means 21 in this embodiment is, for example, 20 kHz, and the output of the ultrasonic horn 22 is 17 W.

[0027] Once the water tank 20 and ultrasonic application means 21 described above are prepared, the wafer 10 is submerged in the water L of the water tank 20. At this time, the back surface 10b of the wafer 10, i.e., the surface on which the metal film 16 is formed, is facing upwards, and the protective tape T1 side is facing downwards, and the wafer 10 is placed on the bottom wall 20a of the water tank 20, forming a layer of water L on the metal film 16. Next, the tip 23 of the ultrasonic horn 22 described above is submerged in the water L, and ultrasonic waves W are applied to the metal film 16. At this time, the tip 23 of the ultrasonic horn 22 and the back surface 10b of the wafer 10 are brought close together, for example, to a distance of 1 to 3 mm, and the ultrasonic horn 22 is moved horizontally (in the X-axis and Y-axis directions in the figure) above the wafer 10 using a moving means not shown, so that ultrasonic waves W are applied evenly and uniformly to the metal film 16 on the back surface 10b of the wafer 10. The ultrasonic waves W applied at this time cause cavitation to occur in the water layer L, and cavitation peening is performed on the metal film 16. The impact of this cavitation peening causes plastic deformation in the metal film 16, making it work-hardened and brittle, and the wafer 10 is divided into individual device chips along the modified layer 100. With this, the division process is completed, and the wafer processing method of this embodiment is completed. In this division process, the wafer 10 is divided into individual device chips, but since the wafer 10 is held together by the protective tape T1, the wafer 10 is prevented from falling apart.

[0028] According to the wafer processing method of this embodiment, a modified layer 100 that serves as the starting point for division is formed inside the division line 14. Subsequently, by applying ultrasonic waves W to the metal film 16 coated on the back surface 10b of the wafer 10 via a layer of water, the substrate constituting the wafer 10 can be divided along the division line 14 together with the metal film 16 coated on the back surface 10b of the wafer 10.

[0029] The method of carrying out the above-described splitting process is not limited to the method described based on Figure 5(a). For example, it is also possible to carry it out in another method as shown in Figure 5(b). In the method shown in Figure 5(b), a water tank 20 similar to the one described based on Figure 5(a) is prepared, and an ultrasonic transducer unit 24 is placed on the lower side of the bottom wall 20a of the water tank 20 as an ultrasonic imparting means. The ultrasonic transducer unit 24 is a unit that imparts ultrasonic vibration to the entire bottom wall 20a of the water tank 20, and imparts ultrasonic waves W to the water L from the bottom wall 20a of the water tank 20. When carrying out the splitting process with this configuration, the wafer 10 is held by suction using a wafer holding means 26 that can suction-hold the side of the wafer 10 to which the protective tape T1 is attached, and the back surface 10b side of the wafer 10 on which the metal film 16 is formed is turned downward and submerged in the water L of the water tank 20, bringing the back surface 10b close to the bottom wall 20a (for example, at a distance of 1 to 3 mm). Next, the ultrasonic transducer unit 24 is activated to apply ultrasonic waves W to the metal film 16 of the wafer 10 through the water layer L, thereby generating cavitation. With this configuration, as explained with reference to Figure 5(a), the metal film 16 is plastically deformed, and the wafer 10 can be divided into individual device chips along the modified layer 100 described above.

[0030] Before performing the division process involving the application of ultrasound as described above, a mask may be formed on the back surface 10b of the wafer 10 coated with the metal film 16, exposing the region corresponding to the division line 14 and covering the other regions. When forming the mask, for example, a liquid resin formed from a volatile solvent that is poorly soluble in water is applied to the entire back surface 10b of the wafer 10 to form a layer, which is then cured and removed along the region corresponding to the division line 14. This forms a mask that exposes the metal film 16 of the wafer 10 along the region corresponding to the division line 14. By forming such a mask and performing the division process described above, it becomes possible to intensively plastically deform the substrate and metal film 16 of the wafer 10 in the region corresponding to the division line 14, thereby efficiently embrittle the metal film 16 along the region corresponding to the division line 14 and dividing it into individual device chips along the division line 14, while protecting the metal film 16 that forms electrodes in areas other than the region corresponding to the division line 14. When a mask is formed in this way, the mask formed on the back surface 10b of the wafer 10 is removed after performing the division process.

[0031] Furthermore, it is conceivable to prepare a water tank 20 and an ultrasonic application means 21 as described above based on Figure 5(a), and to increase the effect of the cavitation peening described above, to mix abrasive particles S, such as diamond or alumina, into the water L stored in the water tank 20. However, according to experiments by the inventors of the present invention, when such abrasive particles S are mixed into the water L and ultrasonic waves W are applied to the metal film 16 through the layer of water L as described based on Figure 5(a), in addition to the cavitation peening described above, the effect of shot peening, in which the abrasive particles S collide with the metal film 16, is excessively exhibited, and the device 12 of the wafer 10 is destroyed along with the metal film 16. Therefore, when the above-mentioned abrasive grains S are mixed in and ultrasonic waves W are applied to perform the splitting process, it is necessary to carefully adjust the processing conditions, such as the output of the ultrasonic horn 22 and the distance between the tip 23 of the ultrasonic horn 22 and the wafer 10, compared to the embodiment of the splitting process that does not use the above-mentioned abrasive grains S. In addition, it is necessary to consider forming a mask in areas other than the area of ​​the planned splitting line 14 on the back surface 10b of the wafer 10.

[0032] If the splitting process is carried out as described above, a pickup process described later may be carried out as necessary. When carrying out the pickup process, first, the relocation process shown in Figure 6 is carried out. In this relocation process, an annular frame F having an opening Fa capable of accommodating the wafer 10 as shown in the figure is prepared, the wafer 10 is positioned with the side to which the protective tape T1 is attached facing upwards in the center of the opening Fa of the frame F, and the wafer 10 is integrally formed with the frame F using an adhesive tape T2 with excellent elasticity, and the protective tape T1 is peeled off from the surface 10a of the wafer 10. As a result, the wafer 10 is transferred from the protective tape T1 to the adhesive tape T2 shown in the figure. As a result of the splitting process described above, the wafer 10 is divided into individual device chips 12' by forming a split groove 110 with the modified layer 100 described above as the starting point for splitting, as shown in the figure.

[0033] Once the above relocation process is completed, the wafer is transported to the pickup device 60 (shown in part only) shown in Figure 7. The pickup device 60 includes an expansion means 62 and a pickup means 64. The expansion means 62 includes a cylindrical expansion drum 62a, a plurality of air cylinders 62b adjacent to the expansion drum 62a and extending upward at circumferential intervals, an annular holding member 62c connected to the upper end of each air cylinder 62b, and a plurality of clamps 62d arranged at circumferential intervals on the outer edge of the holding member 62c. The inner diameter of the expansion drum 62a is larger than the diameter of the wafer 10, and the outer diameter of the expansion drum 62a is smaller than the inner diameter of the frame F. The holding member 62c corresponds to the diameter of the frame F, and the frame F is placed on the flat upper surface of the holding member 62c.

[0034] As shown in Figure 7, the multiple air cylinders 62b raise and lower the holding member 62c between a reference position where the upper surface of the holding member 62c is at approximately the same height as the upper end of the expansion drum 62a, and an expanded position where the upper surface of the holding member 62c is below the upper end of the expansion drum 62a. In Figure 7, for illustrative purposes, the wafer 10 held by the adhesive tape T2 is shown to be raised and lowered (shown by solid and dashed lines), but in reality, it is the holding member 62c that is raised and lowered.

[0035] The pickup means 64 shown in Figure 7 is configured to be movable in the horizontal and vertical directions. A suction means (not shown) is also connected to the pickup means 64, and the individually divided device chips 12' are attracted to the lower surface of the tip of the pickup means 62. When performing the pickup process, first, the surface 10a of the wafer 10 is turned upward, and the frame F is placed on the upper surface of the holding member 62c positioned at the reference position. Next, the frame F is fixed with a plurality of clamps 62d. Then, by lowering the holding member 62c to the expanded position, an external force is applied to the wafer 10 attached to the adhesive tape T2, which is shown by the dashed line in Figure 7, causing it to expand radially. As a result, the adhesive tape T2 expands, and the spacing between the device chips 12', which are individually divided by the cut grooves 110, is expanded.

[0036] As shown in the figure, if the spacing between adjacent device chips 12' is increased, the pickup means 64 is positioned above the device chip 12' to be picked up and lowered, and the upper surface of the device chip 12' is attracted to the lower tip of the pickup means 64. Next, the pickup means 64 is raised, and the device chip 12' is peeled off the adhesive tape T2 and picked up. The picked-up device chips 12' are transported to a tray or the like (not shown), or to a predetermined loading position in the processing equipment for the next process. This pickup operation is then performed sequentially for all the device chips 12', and the pickup process is completed.

[0037] In the above-described embodiment, after the wafer 10 is subjected to the modified layer formation process, a protective tape T1 is attached to the surface 10a of the wafer 10 and the splitting process is carried out. However, the present invention is not limited to this. For example, a protective tape T1 that transmits the laser beam LB described above may be selected and attached to the surface 10a of the wafer 10 before the modified layer formation process is carried out, and the modified layer formation process may be carried out by irradiating the laser beam LB through the protective tape T1. By doing so, damage to the wafer 10 starting from the modified layer 100 is suppressed compared to the case where the protective tape T1 is attached to the wafer 10 after the modified layer formation process is carried out. [Explanation of symbols]

[0038] 1: Laser processing equipment 2: Base 3: Holding means 35: Chuck Table 36: Suction Chuck 4a:X-axis movement means 4b: Y-axis movement means 5:Frame body 6: Alignment means 7: Laser beam irradiation means 7a: Light concentrator 10: Wafer 10a: surface 10b: Back side 12: Devices 12': Device chip 14: Planned division line 16: Metal film 20: Aquarium 20a: Bottom wall 21: Ultrasonic imparting means 22: Ultrasonic horn 23: Tip 24: Ultrasonic transducer unit 26: Wafer holding means 60: Pickup device 62: Expansion means 62a: Expansion Drum 62b: Air cylinder 62c: Retaining member 62d: Clamp 64: Pickup method 100: Modified layer 110: Cutting groove

Claims

1. A wafer processing method for dividing a wafer having a surface formed by dividing multiple devices by dividing lines and a back surface covered with a metal film into individual device chips, A modified layer formation step involves irradiating the wafer with a laser beam of a wavelength that is transparent to it, positioning the focal point of the laser beam within the line to be divided, and irradiating it to form a modified layer that will serve as the starting point for division. A wafer processing method comprising: a splitting step of applying ultrasonic waves to a metal film coated on the back surface of a wafer via a layer of water to plastically deform the metal film, and dividing the wafer into individual device chips along the modified layer.

2. The wafer processing method according to claim 1, wherein, in the splitting step, when ultrasonic waves are applied to the metal film, a region corresponding to the planned splitting line is exposed and the metal film in other regions is masked.

3. The wafer is formed from a SiC substrate, according to the wafer processing method described in claim 1.

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