Laser processing method

Abstract

To provide a laser processing method for processing a wafer, for which cutting processing is difficult, into a state where grinding processing of a surface can be relatively easily performed.SOLUTION: The present invention relates to a laser processing method for applying processing to a surface of a workpiece using a laser processing device. The laser processing method includes: a correlation threshold generation step of generating a correlation threshold for forming a cuticle layer on a top face of the workpiece from a correlation between pulse energy, which is energy per pulse, and a pulse interval; a laser processing condition setting step of setting a laser processing condition in such a manner that the pulse energy of pulse laser beams radiated from laser radiation means is equal to or more than the correlation threshold and the pulse interval is equal to or less than the correlation threshold; and a cuticle layer forming step of forming the cuticle layer on the top face of the workpiece by irradiating the workpiece, which is held by a chuck table, with pulse laser beams based on the laser processing conditions.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a laser processing method for processing a workpiece using a laser processing device. [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 by a grinding machine to form the wafer to the desired thickness, and then the wafer is divided into individual device chips by a dicing machine and a laser processing machine. These chips are then used in electrical devices such as mobile phones and personal computers.

[0003] The grinding device is generally composed of a chuck table that holds the wafer and a grinding means that rotatably mounts a grinding wheel with a ring-shaped arrangement of grinding stones that grind the wafer held on the chuck table, and can process the wafer into a desired state (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 6366308 Summary of the Invention [Problem to be solved by the invention]

[0005] The above-mentioned wafers are composed of substrates such as Si substrates, SiC substrates, GaN substrates, and sapphire substrates, and various device chips are produced according to the characteristics of each substrate. However, there are wafers that are difficult to grind using a grinding device, which results in a problem of low productivity.

[0006] Furthermore, wafers made of Si substrates are brittle, and require time-consuming grinding to prevent damage to the devices due to stress caused by the grinding process, which also results in the same problem of poor productivity as above.

[0007] The present invention has been made in view of the above-mentioned facts, and its main technical object is to provide a laser processing method for processing a wafer that is difficult to grind into a state in which grinding of the surface becomes relatively easy. [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 laser processing method for processing the surface of a workpiece using a laser processing device comprising: a chuck table having a holding surface for holding a plate-shaped workpiece; laser irradiation means for irradiating a pulsed laser beam of a wavelength absorbable by the workpiece held on the chuck table; and feeding means for relatively feeding the chuck table and the laser irradiation means for processing, the laser processing method comprising: a correlation threshold generation step for generating a correlation threshold for forming a stratum corneum on the upper surface of the workpiece from the correlation between pulse energy, which is the energy per pulse, and a pulse interval; a laser processing condition setting step for setting laser processing conditions so that the pulse energy of the pulsed laser beam irradiated from the laser irradiation means is equal to or greater than the correlation threshold and the pulse interval is equal to or less than the correlation threshold; and a stratum corneum formation step for irradiating the workpiece held on the chuck table with a pulsed laser beam based on the laser processing conditions to form a stratum corneum on the upper surface of the workpiece.

[0009] The stratum corneum is preferably produced by a chain reaction of compressive plastic strain that occurs when a large thermal stress that causes rapid expansion occurs locally in the workpiece irradiated with the pulsed laser beam, and this large thermal stress is restrained by the low-temperature surrounding area, exceeding the yield stress of the workpiece.

[0010] The correlation threshold in the correlation threshold generating step is specified by an approximation formula, and the approximation formula is expressed as follows, where the horizontal axis represents the pulse interval [μs] and the vertical axis represents the pulse energy [μJ]: Pulse energy = a{b - (pulse interval / [μs] - c) 2} It is preferable that the coefficients a, b, and c are set depending on the type of workpiece.

[0011] Furthermore, when the workpiece is silicon (Si), the coefficients a, b, and c are as follows: a=4, b=1, c=0.9 and the approximation is Pulse energy = 4 {1 - (pulse interval / [μs] - 0.9) 2} The pulse interval is preferably set in the range of 0.02 μs to 0.8 μs.

[0012] The irradiation pitch of the pulsed laser beam is preferably set in the range of 10 to 200 nm, and the pulse width of the pulsed laser beam is preferably set in the range of 0.3 to 100 ps. [Effects of the Invention]

[0013] The laser processing method of the present invention is a laser processing method for processing the surface of a workpiece using a laser processing device comprising: a chuck table having a holding surface for holding a plate-shaped workpiece; laser irradiation means for irradiating a pulsed laser beam of a wavelength absorbable by the workpiece held on the chuck table; and feeding means for relatively feeding the chuck table and the laser irradiation means for processing. The method comprises: a correlation threshold generating step for generating a correlation threshold for forming a stratum corneum on the upper surface of the workpiece from the correlation between pulse energy, which is the energy per pulse, and the pulse interval; a laser processing condition setting step for setting laser processing conditions such that the pulse energy of the pulsed laser beam irradiated from the laser irradiation means is equal to or greater than the correlation threshold and the pulse interval is equal to or less than the correlation threshold; and a stratum corneum forming step for irradiating the workpiece held on the chuck table with a pulsed laser beam based on the laser processing conditions to form a stratum corneum on the upper surface of the workpiece. Therefore, the stratum corneum formed on the plate-shaped workpiece makes it possible to relatively easily grind the surface of workpieces that are difficult to grind, thereby improving productivity. [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 2A is an overall perspective view of the laser processing apparatus of the present embodiment, and FIG. 2B is a block diagram showing an optical system of a laser irradiation means disposed in the laser processing apparatus shown in FIG. 2A. [Figure 2] FIG. 2 is a perspective view of a wafer to be processed in the present embodiment. [Figure 3] FIG. 10 is a perspective view illustrating an embodiment of a laser processing experiment performed in the correlation threshold generation process. [Figure 4] FIG. 4(a) is a perspective view of a wafer on which a stratum corneum was formed by the laser processing experiment shown in FIG. 3, and FIG. 4(b) is an enlarged cross-sectional view of a portion of the wafer shown in FIG. [Figure 5] FIG. 10 is a correlation diagram created by a correlation threshold generation step. [Figure 6] FIG. 4 is a perspective view showing another embodiment of the laser processing experiment shown in FIG. 3. [Figure 7] FIG. 2A is a perspective view showing a mode in which a wafer is placed on a chuck table of a grinding device, and FIG. 2B is a perspective view showing an embodiment of grinding processing. [Figure 8] FIG. 2 is a block diagram showing another form of optical system of the laser irradiation means shown in FIG. 1(b). DETAILED DESCRIPTION OF THE INVENTION

[0015] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an embodiment of a laser processing method configured based on the present invention will be described in detail with reference to the accompanying drawings.

[0016] 1(a) shows a laser processing apparatus 1 suitable for carrying out the laser processing method of this embodiment. The laser processing apparatus 1 is disposed on a base 1a and comprises a holding means 2 having a chuck table 23 for holding a workpiece, a laser irradiation means 3 for irradiating the workpiece held on the chuck table 23 with a pulsed laser beam, and a feeding means 4 for feeding the chuck table 23 and the laser irradiation means 3 relative to each other for processing.

[0017] The holding means 2 includes a rectangular X-axis movable plate 21 mounted on the base 1a so as to be movable in the X-axis direction, and a rectangular Y-axis movable plate 22 mounted on the X-axis movable plate 21 so as to be movable in the Y-axis direction. The chuck table 23 is rotatably disposed on the upper surface of the Y-axis movable plate 22 by a rotation drive means (not shown). The chuck table 23 is a means for holding a workpiece, with the XY plane defined by the X and Y coordinates as the holding surface 25. The holding surface 25 is made of a breathable material. The holding surface 25 is connected to a suction means (not shown) by a flow path passing through the chuck table 23. By operating the suction means, a negative pressure is generated on the holding surface 25, allowing the plate-shaped workpiece to be suction-held.

[0018] The feed means 4 includes an X-axis moving means 41 that moves the chuck table 23 in the X-axis direction, and a Y-axis moving means 44 that moves the chuck table 23 in the Y-axis direction. The X-axis moving means 41 converts the rotational motion of a motor 42 into linear motion via a ball screw 43 and transmits the linear motion to the X-axis movable plate 21, moving the X-axis movable plate 21 in the X-axis direction along a pair of guide rails 1d, 1d arranged on the base 1a along the X-axis direction. The Y-axis moving means 44 converts the rotational motion of a motor 45 into linear motion via a ball screw 46 and transmits the linear motion to the Y-axis movable plate 22, moving the Y-axis movable plate 22 in the Y-axis direction along a pair of guide rails 24, 24 arranged on the X-axis movable plate 21 along the Y-axis direction.

[0019] The laser processing apparatus 1 has a vertical wall 1b erected on a base 1a beside the X-axis moving means 41 and the Y-axis moving means 44, and the vertical wall 1b has a horizontal wall 1c extending horizontally from its upper end. The horizontal wall 1c houses an optical system constituting the laser irradiation means 3 and an imaging means 5. A condenser 31 constituting part of the laser irradiation means 3 and irradiating the workpiece with a laser beam is disposed on the underside of the tip of the horizontal wall 1c. The imaging means 5 is a means for imaging the workpiece held by the holding means 2 and detecting the position to be irradiated with the laser beam, and is disposed adjacent to the condenser 31 in the X-axis direction indicated by the arrow X in the figure.

[0020] 1(b) shows a block diagram illustrating an outline of the optical system of the laser irradiation means 3. The laser irradiation means 3 includes at least an oscillator 32 that oscillates a laser beam LB, a reflecting mirror 33 that redirects the optical path of the laser beam LB oscillated by the oscillator 32 toward a condenser 31, and a condenser lens 34 disposed on the condenser 31 that condenses the laser beam LB on the wafer 10, which is the workpiece. A control means 100 is connected to the oscillator 32 of the laser irradiation means 3, and the control means 100 adjusts the laser processing conditions when the laser beam LB is oscillated from the oscillator 32.

[0021] The control means 100 is configured by a computer and includes a central processing unit (CPU) that performs calculations according to a control program, a read-only memory (ROM) that stores the control program, etc., a readable and writable random access memory (RAM) that temporarily stores detected values, calculation results, etc., an input interface, and an output interface (details not shown in the drawings). In addition to the laser irradiation means 3 described above, the control means 100 is connected to a feed means 4, an imaging means 5, a display means 6, etc.

[0022] The laser processing apparatus 1 suitable for carrying out the laser processing method of this embodiment has roughly the configuration described above, and the laser processing method of this embodiment carried out using the laser processing apparatus 1 will be described below.

[0023] A plate-shaped workpiece to be processed by the laser processing method of this embodiment is, for example, a 700 μm thick silicon (Si) wafer 10 having a surface 10a on which a plurality of devices 12 are formed and partitioned by planned division lines 14, as shown in Fig. 2. The laser processing described below is a processing for bringing the back surface 10b of the wafer 10 into a desired state suitable for the grinding process described below.

[0024] When carrying out the laser processing method of this embodiment, first, a correlation threshold generation step is carried out to generate a correlation threshold at which a stratum corneum, described below, is formed on the upper surface of the workpiece from the correlation between the pulse energy Pe, which is the energy per pulse, and the pulse interval Pw.

[0025] To perform the correlation threshold generating step, a silicon (Si) wafer 10 is prepared as shown in Fig. 2. Next, a protective tape T is attached to the front surface 10a of the wafer 10, and the wafer 10 is inverted. The wafer 10 is placed on the holding surface 25 of the chuck table 23 of the laser processing apparatus 1 with the back surface 10b of the wafer 10 facing upward and the protective tape T facing downward, and is held by suction. Note that for convenience of explanation, Fig. 2 shows an example in which the correlation threshold generating step is also performed using a wafer 10 having a device 12 formed on its front surface 10a. However, since the correlation threshold generating step generates a correlation threshold that distinguishes between regions where a stratum corneum is formed and regions where it is not formed on the top surface of a workpiece through laser processing experiments, it is preferable to use a so-called dummy wafer that has the same dimensions and material as the wafer 10, except that the device 12 and planned division lines 14 are not formed.

[0026] Once the wafer 10 is prepared as described above, it is positioned directly below the imaging means 5, and an image of the wafer 10 is captured and displayed appropriately on the display means 6, while the outer shape, etc. of the wafer 10 is detected. Next, based on the information of the wafer 10 detected by the imaging means 5, the condenser 31 of the laser irradiation means 3 is positioned at an appropriate processing start position, for example, at the end in the Y-axis direction. Next, based on the laser processing conditions described below, as shown in FIG. 3(a), the focal point of the pulsed laser beam LB is positioned on the back surface 10b of the wafer 10 and irradiated, and the wafer 10 is processed and fed in the X-axis direction together with the chuck table 23 to perform laser processing on the back surface 10b of the wafer 10. After laser processing has been performed in this manner, the wafer 10 is indexed and fed in the Y-axis direction by a predetermined interval (for example, 15 μm) until an adjacent unprocessed area in the Y-axis direction is positioned directly below the condenser 31. Then, in the same manner as described above, the focal point of the pulsed laser beam LB is positioned on the back surface 10b of the wafer 10 and irradiated, and the wafer 10 is fed for processing in the X-axis direction. By repeating this processing, the above-mentioned laser processing is performed on the entire back surface 10b of the wafer 10, as shown in FIG. 4(a).

[0027] In the correlation threshold generation process, the laser processing conditions are appropriately changed when carrying out the above-mentioned laser processing, and a laser processing experiment is carried out to find the threshold for whether or not a good stratum corneum 18 is generated on the back surface 10b of the wafer 10, as shown in Figure 4(b), depending on the correlation between the pulse energy Pe [μJ], which is the energy per pulse, and the pulse interval Pw [μs].

[0028] The stratum corneum 18 is a layer formed by a chain reaction of compressive plastic strain that occurs when a pulsed laser beam LB is irradiated onto the back surface 10b of the wafer 10, generating large thermal stresses that cause rapid local expansion of the wafer 10. This large thermal stress exceeds the yield stress of the silicon (Si) that constitutes the wafer 10, which is constrained by the low-temperature surroundings. As is clear from the enlarged image of a portion of the internal structure of the wafer 10 shown in FIG. 4(b), this chain reaction of compressive plastic strain generates an interface 16 along the XY plane within the wafer 10, and the layer that peels off toward the upper surface across the interface 16 becomes the stratum corneum 18 of this embodiment. Laser processing experiments in this embodiment are carried out by varying each parameter within the range of laser processing conditions shown below, for example.

[0029] <Laser processing conditions> Target wafer: Silicon (Si) wafer Wavelength: 266nm, 355nm, 532nm, 1064nm Average power: 1~50W Repetition frequency: 0.1 to 50 MHz Pulse width: 0.3 to 1000 ps *Pulse energy Pe: average power / repetition frequency [μJ] *Pulse interval Pw: 1s / repetition frequency [μs]

[0030] Among the above laser processing conditions, wavelengths of 266 nm, 355 nm, and 532 nm are wavelengths that are absorbed by silicon (Si), and wavelength 1064 nm is a wavelength that is transparent to silicon (Si). Furthermore, as mentioned above, the pulse interval Pw is a value that is uniquely determined by determining the repetition frequency, and the pulse energy Pe, which is the energy per pulse of the pulsed laser beam LB, is determined by determining the repetition frequency and average output. The procedure for the above laser processing experiment is explained below.

[0031] <Laser processing experiment procedure> a) The wavelength (532 nm in this embodiment) to be emitted from the oscillator 32 of the laser irradiation means 3 is determined. a) The average output is changed appropriately within the range of 1 to 50W. c) The repetition frequency is changed appropriately within the range of 0.1 to 50 MHz. d) The pulse width is changed appropriately within the range of 0.3 to 1000 ps. E) Laser processing is performed by irradiating a pulsed laser beam LB.

[0032] Using the above procedure, laser processing was performed while changing each value (average output, repetition frequency, pulse width), and it was confirmed using an appropriate electron microscope whether the above-mentioned stratum corneum 18 was formed well, and whether the stratum corneum 18 could be easily peeled off from the back surface 10b side of the wafer 10.

[0033] Laser processing was performed by changing the laser processing conditions according to the procedure of the laser processing experiment described above, and each time it was confirmed whether the stratum corneum 18 was formed well or not.As a result, the following thresholds 1 to 10 were confirmed as correlation thresholds.

[0034] 1) Threshold 1 Pulse interval Pw: 0.02 μs (repetition frequency: 50 MHz) Pulse energy Pe: 0.25 μJ (average power: 12.5 W) 2) Threshold 2 Pulse interval Pw: 0.04 μs (repetition frequency: 25 MHz) Pulse energy Pe: 0.5 μJ (average power: 12.5 W) 3) Threshold 3 Pulse interval Pw: 0.1 μs (repetition frequency: 10 MHz) Pulse energy Pe: 1μJ (average power: 10W) 4) Threshold 4 Pulse interval Pw: 0.2 μs (repetition frequency: 5 MHz) Pulse energy Pe: 1.8μJ (average power: 9W) 5) Threshold 5 Pulse interval Pw: 0.25 μs (repetition frequency: 4 MHz) Pulse energy Pe: 2.2 μJ (average power: 8.8 W) 6) Threshold 6 Pulse interval Pw: 0.4 μs (repetition frequency: 2.5 MHz) Pulse energy Pe: 3μJ (average power: 7.5W) 7) Threshold 7 Pulse interval Pw: 0.5 μs (repetition frequency: 2 MHz) Pulse energy Pe: 3.3 μJ (average power: 6.6 W) 8) Threshold 8 Pulse interval Pw: 0.6 μs (repetition frequency: 1.66 MHz) Pulse energy Pe: 3.5 μJ (average power: 5.8 W) 9) Threshold 9 Pulse interval Pw: 0.7 μs (repetition frequency: 1.42 MHz) Pulse energy Pe: 3.6 μJ (average power: 5.1 W) 10) Threshold 10 Pulse interval Pw: 0.8 μs (repetition frequency: 1.25 MHz) Pulse energy Pe: 3.8 μJ (average power: 4.75 W)

[0035] In the above-mentioned laser processing experiment, the inventors also performed laser processing under the following laser processing conditions that exceeded the threshold value of 10, but the stratum corneum 18 was not formed well, and therefore it was confirmed that the following laser processing conditions are the limit values ​​at which the stratum corneum 18 is not formed well. Pulse interval Pw: 0.9 μs (repetition frequency: 1.11 MHz) Pulse energy Pe: 3.8 μJ (average power: 4.22 W)

[0036] The thresholds 1 to 10 are plotted as measured values ​​on a correlation diagram, as shown in FIG. 5 , with the horizontal axis representing the pulse interval Pw [μs] and the vertical axis representing the pulse energy Pe [μJ], and are connected by straight lines (for reference, the limit values ​​are indicated by ×). The thresholds 1 to 10 are measured values ​​determined through the laser processing experiment described above as correlation thresholds that separate the region (A) where the stratum corneum 18 is well formed from the region (B) where the stratum corneum 18 is poorly formed. That is, the laser processing experiment described above confirmed that a good stratum corneum 18 is formed when the laser processing conditions are set in the region (A) where the pulse energy Pe is equal to or greater than the correlation threshold and the pulse interval Pw is equal to or less than the correlation threshold, whereas a good stratum corneum 18 is not formed when the laser processing conditions are set in the region (B) where the pulse energy Pe is less than the correlation threshold and the pulse interval Pw is greater than the correlation threshold. The correlation thresholds are then stored in the control unit 100.

[0037] In the above-mentioned laser processing experiment, similar correlation thresholds were obtained even when other wavelengths, 266 nm and 355 nm, which are absorbent for silicon wafers were selected as the wavelength of the irradiated pulsed laser beam LB, whereas when the above-mentioned laser processing experiment was performed by selecting the wavelength of 1064 nm, which is transparent for silicon wafers, as the wavelength of the irradiated pulsed laser beam LB, a good stratum corneum was not formed in any region.

[0038] In this way, a correlation threshold that allows for the formation of a stratum corneum on the upper surface of the workpiece is generated from the correlation between the pulse energy Pe, which is the energy per pulse, and the pulse interval Pw, and the correlation threshold is stored in the control means 100, thereby completing the correlation threshold generation process.

[0039] In the present invention, instead of performing the correlation threshold generating step of generating a correlation threshold based on actual measurement values ​​as described above, the correlation threshold may be generated using an approximation formula. The inventors have found that, within the scope of the laser processing experiment that showed good results in the laser processing experiment when generating the correlation threshold described above, the correlation threshold based on the actual measurement values ​​can be approximated by the following approximation formula (1).

[0040] Approximation formula (1) is specified as follows, where the horizontal axis represents the pulse interval Pw [μs] and the vertical axis represents the pulse energy Pe [μJ]. Pe=a{b-(Pw / [μs]-c) 2} ···(1) *The pulse interval Pw is made dimensionless by dividing it by the unit [μs]. *Coefficients a, b, and c are set depending on the type of workpiece (e.g., Si, SiC, GaN, sapphire, etc.).

[0041] The above-mentioned laser processing experiment was conducted on a silicon (Si) wafer 10, and it was found that when the above approximate formula (1) is applied to the silicon (Si) wafer 10, the coefficients a, b, and c can be set to a = 4, b = 1, and c = 0.9 at least within the range of a pulse interval Pw of 0.02 μs or more and 0.8 μs or less, and the coefficients can be used as correlation thresholds for generating a good stratum corneum 18. That is, the approximate formula (1) is expressed by the following approximate formula (2), and by using the following approximate formula (2), it is possible to easily set laser processing conditions that can generate a good stratum corneum 18. Pe=4{1-(Pw / [μs]-0.9) 2} ···(2)

[0042] After the correlation threshold generating step of generating a correlation threshold based on actual measurements or an approximation formula as described above has been performed, a wafer 10, which is the workpiece, is prepared (see FIG. 2 ). The control means 100 then performs a laser processing condition setting step of setting laser processing conditions such that the pulse energy Pe of the pulsed laser beam LB irradiated from the laser irradiation means 3 is equal to or greater than the correlation threshold and the pulse interval Pw is equal to or less than the correlation threshold. That is, the laser processing conditions are set so that the pulse energy Pe and pulse interval Pw of the pulsed laser beam LB are set within region (A) in the correlation diagram shown in FIG. 5 . By increasing the repetition frequency and decreasing the pulse interval Pw, it is possible to form the stratum corneum 18 with a smaller pulse energy Pe. Furthermore, by increasing the pulse energy Pe, it is possible to adjust the thickness of the stratum corneum 18 to a larger value. Therefore, it is advisable to set the pulse energy Pe and pulse interval Pw of the pulsed laser beam LB taking into consideration factors such as energy efficiency and the desired thickness of the stratum corneum 18. Furthermore, if the pulse energy Pe of the pulsed laser beam LB is too large, there is a risk that leaked light may reach the device 12 and cause damage, so the pulse energy Pe is set within a range in which the device 12 will not be damaged.

[0043] In the above-mentioned laser processing experiment, it was confirmed that the thickness of the stratum corneum 18 and the thickness of the interface 16 were stably formed by setting the pulse width of the pulsed laser beam LB to 0.3 to 100 ps, ​​so it is preferable to set the pulse width within the above range in the above-mentioned laser processing condition setting step. Furthermore, in the above-mentioned laser processing experiment, it was also confirmed that a good stratum corneum 18 was formed when the irradiation pitch of the pulsed laser beam LB irradiated onto the wafer 10 was 10 to 200 nm, so it is preferable to adjust the processing feed rate in the X-axis direction appropriately depending on the repetition frequency of the pulsed laser beam LB so that the irradiation pitch falls within the above-mentioned range.

[0044] After the laser processing condition setting step described above has been performed, a protective tape T is applied to the front surface 10a of the wafer 10, which is then suction-held on the chuck table 23 of the laser processing apparatus 1. A pulsed laser beam LB is then applied to the entire back surface 10b of the wafer 10 based on the conditions set in the laser processing condition setting step, thereby performing a stratum corneum forming step. This results in the formation of the stratum corneum 18 on the back surface 10b of the wafer 10. The thickness of the stratum corneum 18 is, for example, 200 μm. This stratum corneum forming step is performed using the same processing procedures as those described above with reference to FIGS. 3 and 4, and a detailed description thereof will be omitted.

[0045] As described above, the laser processing condition setting process is carried out based on the correlation threshold obtained by the correlation threshold generation process, and the stratum corneum formation process is carried out using the laser processing conditions set by the laser processing condition setting process, thereby forming a good stratum corneum 18 on the back surface 10b of the wafer 10.

[0046] 3 and 4, when forming the stratum corneum 18, the chuck table 23 is moved in the X-axis direction to irradiate the pulsed laser beam LB linearly, but the present invention is not limited to this. For example, as shown in Fig. 6, based on the laser processing conditions set in the laser processing condition setting step, the pulsed laser beam LB may be irradiated onto the outer peripheral edge of the back surface 10b of the wafer 10, and the chuck table 23 may be rotated in the direction indicated by arrow R1 and moved in the X-axis direction to move the irradiation position of the pulsed laser beam LB to the center O of the wafer 10, thereby irradiating the back surface 10b of the wafer 10 with the pulsed laser beam LB in a spiral manner. Even when the pulsed laser beam LB is irradiated in this manner, a good stratum corneum 18 can be formed on the back surface 10b of the wafer 10, as in the above embodiment.

[0047] By carrying out the laser processing method including the correlation threshold generation process, the laser processing condition setting process, and the stratum corneum formation process described above, the stratum corneum 18 formed on the back surface 10b of the wafer 10 makes it possible to relatively easily grind the surface of wafers that are difficult to grind, thereby improving productivity.

[0048] Once the stratum corneum 18 has been formed on the back surface 10b of the wafer 10 by the above-described laser processing method, the wafer is transferred to a grinding device 80 (only a portion of which is shown) shown in Figure 7, where grinding processing is performed. As shown in Figure 7(a), the grinding device 80 is equipped with a chuck table 81 including a suction chuck 81b and a frame 81a that surrounds the suction chuck 81b. A suction means (not shown) is connected to the frame 81a of the chuck table 81, and by operating the suction means, a negative pressure is generated on the surface of the suction chuck 81b.

[0049] 7(b), the grinding device 80 is equipped with grinding means 82 for grinding and thinning the workpiece held by suction on the chuck table 81. The grinding means 82 is equipped with a rotating spindle 83 rotated by a rotation drive mechanism (not shown), a wheel mount 84 attached to the lower end of the rotating spindle 83, and a grinding wheel 85 attached to the lower surface of the wheel mount 84 and having a plurality of grinding stones 86 arranged in an annular shape on the lower surface side.

[0050] The wafer 10 having the above-described stratum corneum 18 formed on its back surface 10b is placed on the chuck table 81 with the back surface 10b facing upward and held by suction. Then, a drive motor (not shown) is operated to rotate the rotating spindle 83 of the grinding means 82 in the direction indicated by the arrow R2 in Fig. 7(b) at, for example, 6000 rpm, and the chuck table 81 in the direction indicated by the arrow R3 at, for example, 300 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 stone 86 is brought into contact with the back surface 10b of the wafer 10, and the grinding wheel 85 is fed downward for grinding at a grinding feed rate of, for example, 1 µm / sec. At this time, grinding can be carried out while measuring the thickness of the wafer 10 using a contact-type measuring gauge (not shown). Once the back surface 10b of the wafer 10 has been ground a predetermined amount to a desired thickness, the grinding means 82 is stopped, and after cleaning, drying, etc., the grinding process is completed, in which the wafer 10 is thinned and the back surface 10b is made flat, as shown in Figure 7(c).

[0051] In the grinding process of this embodiment, as described above, the stratum corneum 18 is formed on the back surface 10b of the wafer 10, and is in a state in which it is easily peeled off at the interface 16. Therefore, when performing the grinding process, the stratum corneum 18 is broken down by the grinding means 82 and removed as large pieces from the back surface 10b of the wafer 10, which makes it possible to shorten the time required for the grinding process and improve productivity. Furthermore, because the stratum corneum 18 is broken down by the grinding means 82 and removed as large pieces, the grinding water used in the grinding process can be easily purified and reused, making it possible to realize a grinding process with a low environmental impact.

[0052] The present invention is not limited to the above-described embodiment. For example, instead of the laser irradiation means 3 arranged in the laser processing apparatus 1 shown in Fig. 1, another form of laser irradiation means 7 shown in Fig. 8 may be provided. The illustrated laser beam application means 7 includes an oscillator 72 that oscillates a pulsed laser beam LB, an X-axis galvanometer scanner 74 that oscillates the pulsed laser beam LB oscillated by the oscillator 72 in the X-axis direction, a Y-axis galvanometer scanner 75 that oscillates the pulsed laser beam LB in the Y-axis direction, and a condenser 71 including an fθ lens 76 that irradiates a desired position on the wafer 10 held on the chuck table 23 with the pulsed laser beam LB oscillated in the X-axis and Y-axis directions.

[0053] The X-axis galvanometer scanner 74 and the Y-axis galvanometer scanner 75 are configured with a known configuration including mirrors (not shown) and angle adjustment actuators that adjust the reflection angles of the mirrors. The fθ lens 76 irradiates the pulsed laser beam LB, which is oscillated in the X-axis and Y-axis directions by the action of the X-axis galvanometer scanner 74 and the Y-axis galvanometer scanner 75, perpendicularly to the back surface 10b of the wafer 10. The oscillator 72, the X-axis galvanometer scanner 74, and the Y-axis galvanometer scanner 75 are controlled by a control means 100'. Note that the X-axis galvanometer scanner 74 and the Y-axis galvanometer scanner 75 in this embodiment function as a feed means for relatively feeding the chuck table and the laser irradiation means in the present invention.

[0054] The pulsed laser beam LB oscillated by the oscillator 72 of the laser irradiation means 7 is oscillated in the X-axis and Y-axis directions by the X-axis galvanometer scanner 74 and the Y-axis galvanometer scanner 75, and is guided to the fθ lens 76, and then irradiated via the fθ lens 76 onto a desired position on the back surface 10b of the wafer 10 held on the chuck table 23.

[0055] In the laser processing apparatus 1 equipped with the above-described laser irradiation means 7, the laser processing condition setting step is also performed based on the correlation threshold generated by the correlation threshold generation step of this embodiment, and the stratum corneum formation step is performed on the back surface 10b of the wafer 10 using the above-described laser irradiation means 7 based on the laser processing conditions. As a result, a stratum corneum 18 equivalent to that described with reference to Fig. 4 is formed on the back surface 10b of the wafer 10. Then, by grinding the back surface 10b of the wafer 10 on which the stratum corneum 18 has been formed by the laser irradiation means 7 using the grinding device 80 described with reference to Fig. 7, good grinding can be performed, as in the previously described embodiment.

[0056] In the above-described embodiment, the correlation threshold generating step, the laser processing condition setting step, and the stratum corneum forming step were carried out using a silicon (Si) wafer 10 as the workpiece, but the present invention is not limited to this, and these steps can also be carried out on plate-shaped workpieces made of other materials, such as SiC substrates, GaN substrates, sapphire substrates, etc., and the stratum corneum formed on the workpiece makes it relatively easy to grind the surface of workpieces that are difficult to grind, thereby improving productivity. [Explanation of symbols]

[0057] 1: Laser processing equipment 1a: Base 1b: Vertical wall 1c:Horizontal wall 1d: Guide rail 2: Holding means 21:X-axis movable plate 22: Y-axis direction movable plate 23: Chuck table 24: Guide rail 25: Holding surface 3: Laser irradiation means 31: Concentrator 32: Oscillator 33: Reflective mirror 34: Condenser lens 4:Feeding means 41:X-axis movement means 42: Motor 43: Ball screw 44: Y-axis movement means 45: Motor 46: Ball screw 5: Imaging means 6:Display means 7: Laser irradiation means 71: Concentrator 72: Oscillator 74: X-axis galvanometer scanner 75: Y-axis galvanometer scanner 76: fθ lens 10: Wafer 10a: surface 10b: Back side 12: Device 14: Planned division line 16: Interface 18: stratum corneum 80: Grinding equipment 81: Chuck table 82: Grinding means 83: Rotating spindle 84: Wheel mount 85: Grinding wheel 86: Grinding wheel 100, 100': control means Pe: Pulse energy Pw: Pulse width

Claims

1. A laser processing method for processing a surface of a workpiece using a laser processing device including: a chuck table having a holding surface for holding a plate-shaped workpiece; laser irradiation means for irradiating a pulsed laser beam having a wavelength absorbed by the workpiece held on the chuck table; and feeding means for relatively feeding the chuck table and the laser irradiation means for processing, a correlation threshold generating step of generating a correlation threshold for forming a stratum corneum on the upper surface of the workpiece based on the correlation between pulse energy, which is energy per pulse, and the pulse interval; a laser processing condition setting step of setting laser processing conditions so that the pulse energy of the pulsed laser beam irradiated from the laser irradiation means is equal to or greater than the correlation threshold value and the pulse interval is equal to or less than the correlation threshold value; a stratum corneum forming step of irradiating the workpiece held on the chuck table with a pulsed laser beam based on the laser processing conditions to form a stratum corneum on the upper surface of the workpiece.

2. The laser processing method according to claim 1, wherein the stratum corneum is produced by a chain reaction of compressive plastic strains that occur when a large thermal stress that tries to rapidly expand the local area of ​​the workpiece irradiated with the pulsed laser beam is generated, and the large thermal stress is restrained by the low-temperature surrounding area and exceeds the yield stress of the workpiece.

3. The correlation threshold in the correlation threshold generating step is specified by an approximate expression, and the approximate expression is expressed as follows, where the horizontal axis represents the pulse interval [μs] and the vertical axis represents the pulse energy [μJ]: Pulse energy = a {b - (pulse interval / [μs] - c) ２ } 2. The laser processing method according to claim 1, wherein the coefficients a, b, and c are set depending on the type of workpiece.

4. When the workpiece is silicon (Si), the coefficients a, b, and c are: a=4, b=1, c=0.9 and the approximation is Pulse energy = 4 {1 - (pulse interval / [μs] - 0.9) ２ } 4. The laser processing method according to claim 3, wherein the pulse interval is set in the range of 0.02 μs to 0.8 μs.

5. 2. The laser processing method according to claim 1, wherein the irradiation pitch of the pulsed laser beam is set in the range of 10 to 200 nm.

6. 2. The laser processing method according to claim 1, wherein the pulse width of the pulsed laser beam is set in the range of 0.3 to 100 ps.

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