JP2023148049A - Laser processing method - Google Patents

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

The present invention relates to a laser processing method for processing a workpiece using a laser processing device.

A wafer on which multiple devices such as ICs and LSIs are formed on the front surface divided by division lines is ground on the back side using a grinding machine to form a desired thickness, and then individually separated using a dicing machine and a laser processing machine. It is divided into device chips and used in electrical equipment such as mobile phones and computers.

The grinding device generally includes a grinding means rotatably equipped with a chuck table that holds a wafer, and a grinding wheel having a grinding wheel disposed in an annular shape for grinding the wafer held on the chuck table. Thus, the wafer can be processed into a desired state (see, for example, Patent Document 1).

Patent No. 6366308

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, but some wafers are difficult to grind with a grinding machine. However, there is a problem in that productivity is poor.

Furthermore, wafers made of Si substrates are brittle, and it is necessary to grind them over time to prevent damage to the devices due to the stress caused by the grinding process, resulting in the same problem of poor productivity as described above.

The present invention has been made in view of the above facts, and its main technical problem is to provide a laser processing method for processing a wafer, which is difficult to grind, into a state where the surface can be ground relatively easily. There is a particular thing.

In order to solve the above-mentioned main technical problems, the present invention provides a chuck table having a holding surface for holding a plate-shaped workpiece, and a pulse having a wavelength that is absorbed by the workpiece held on the chuck table. A laser processing device that processes the surface of a workpiece using a laser processing device that includes a laser irradiation unit that irradiates a laser beam, and a feeding unit that processes and feeds the chuck table and the laser irradiation unit relative to each other. The processing method includes a correlation threshold generation step for generating a correlation threshold for forming a stratum corneum on the upper surface of a workpiece from a correlation between pulse energy, which is energy per pulse, and pulse interval, and a laser irradiation means. a step of setting laser processing conditions such that the pulse energy of the pulsed laser beam irradiated from the chuck table is greater than or equal to the correlation threshold, and the pulse interval is less than or equal to the correlation threshold; A laser processing method is provided, which includes a step of forming a stratum corneum layer on the upper surface of the workpiece by irradiating the workpiece with a pulsed laser beam based on the laser processing conditions.

The stratum corneum is exposed to the pulsed laser beam, causing large thermal stress to rapidly expand locally in the workpiece, and this large thermal stress is constrained to the low-temperature surroundings and exceeds the yield stress of the workpiece. Preferably, it is generated by a chain of compressive plastic strains occurring at .

The correlation threshold in the correlation threshold generation step is specified by an approximate formula, where the horizontal axis is the pulse interval [μs] and the vertical axis is the pulse energy [μJ],
Pulse energy=a{b-(pulse interval/[μs]-c) ² }
The coefficients a, b, and c are preferably set depending on the type of workpiece.

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 formula is
Pulse energy = 4 {1-(pulse interval/[μs]-0.9) ² }
The pulse interval is preferably set in a range of 0.02 μs or more and 0.8 μs or less.

The irradiation pitch of the pulsed laser beam is preferably set in the range of 10 to 200 nm. Further, the pulse width of the pulsed laser beam is preferably set in a range of 0.3 to 100 ps.

The laser processing method of the present invention includes a chuck table having a holding surface for holding a plate-shaped workpiece, and a laser irradiation means for irradiating the workpiece held on the chuck table with a pulsed laser beam having an absorbing wavelength. and a feeding means for processing and feeding the chuck table and the laser irradiation means relative to each other, a laser processing method for processing the surface of a workpiece using a laser processing device comprising: 1 A correlation threshold generation step for generating a correlation threshold for forming a stratum corneum on the upper surface of a workpiece from the correlation between pulse energy, which is energy per pulse, and pulse interval; a laser processing condition setting step of setting laser processing conditions such that the pulse energy is greater than or equal to the correlation threshold and the pulse interval is less than or equal to the correlation threshold; Since the process includes a stratum corneum forming step of forming a stratum corneum on the upper surface of the workpiece by irradiating a pulsed laser beam based on laser processing conditions, the stratum corneum formed on the plate-shaped workpiece The surface of a workpiece that is difficult to grind can be ground relatively easily, and productivity can be improved.

(a) An overall perspective view of the laser processing apparatus according to the present embodiment, and (b) a block diagram showing an optical system of a laser irradiation means provided in the laser processing apparatus described in (a). FIG. 2 is a perspective view of a wafer processed in this embodiment. FIG. 2 is a perspective view showing an embodiment of a laser processing experiment carried out in a correlation threshold generation step. (a) A perspective view of a wafer on which a stratum corneum has been generated by the laser processing experiment shown in FIG. 3, and (b) a partially enlarged sectional view of the wafer shown in (a). It is a correlation diagram created by a correlation threshold value generation process. 4 is a perspective view showing another embodiment of the laser processing experiment shown in FIG. 3. FIG. (a) A perspective view showing a mode in which a wafer is placed on a chuck table of a grinding device, and (b) a perspective view showing an embodiment of a grinding process. It is a block diagram which shows the optical system of another form of the laser irradiation means shown in FIG.1(b).

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of a laser processing method based on the present invention will be described in detail with reference to the accompanying drawings.

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

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 X-axis movable plate 21 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 rotary drive means (not shown). The chuck table 23 is a means for holding a workpiece using the XY plane specified by the X and Y coordinates as a holding surface 25, and the holding surface 25 is made of a member having air permeability. The holding surface 25 is connected to a suction means (not shown) by a flow path passing through the chuck table 23, and by activating the suction means, negative pressure is generated on the holding surface 25 and the plate-shaped workpiece is suctioned. It is possible to hold.

The feeding 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 the motor 42 into linear motion via a ball screw 43 and transmits the linear motion to the X-axis movable plate 21, and is disposed on the base 1a along the X-axis direction. The X-axis movable plate 21 is moved in the X-axis direction along the pair of guide rails 1d and 1d. The Y-axis moving means 44 converts the rotational motion of the motor 45 into linear motion via the ball screw 46 and transmits the linear motion to the Y-axis movable plate 22, and moves the rotational motion of the motor 45 along the Y-axis direction onto the X-axis movable plate 21. The Y-axis movable plate 22 is moved in the Y-axis direction along the pair of guide rails 24, 24 provided.

The laser processing device 1 includes a vertical wall portion 1b that stands on the side of the X-axis moving means 41 and the Y-axis moving means 44 on the base 1a, and the vertical wall portion 1b extends in the horizontal direction from the upper end. It has a horizontal wall portion 1c. An optical system constituting the laser irradiation means 3 and an imaging means 5 are housed inside the horizontal wall portion 1c. A condenser 31, which constitutes a part of the laser irradiation means 3 and irradiates the workpiece with a laser beam, is disposed on the lower surface side of the tip of the horizontal wall portion 1c. The imaging means 5 is a means for taking an image of the workpiece held by the holding means 2 and detecting a position to be irradiated with a laser beam, etc. They are arranged at adjacent positions in the axial direction.

FIG. 1(b) shows a block diagram schematically showing the optical system of the laser irradiation means 3 described above. The laser irradiation means 3 includes an oscillator 32 that oscillates a laser beam LB, a reflection mirror 33 that changes the optical path of the laser beam LB emitted by the oscillator 32 toward a condenser 31, and a wafer 10 that is a workpiece that irradiates the laser beam LB. It includes at least a condenser lens 34 disposed on a condenser 31 that condenses light. Further, the control means 100 is connected to the oscillator 32 of the laser irradiation means 3, and the laser processing conditions when the laser beam LB is oscillated from the oscillator 32 are adjusted by the control means 100.

The control means 100 is composed of 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, etc., and a temporary storage system for storing detected values, calculation results, etc. It is provided with a readable and writable random access memory (RAM) for storing data in the memory, an input interface, and an output interface (details are omitted from illustration). The control means 100 is connected to the above-described laser irradiation means 3 as well as a feeding means 4, an imaging means 5, a display means 6, and the like.

The laser processing apparatus 1 suitable for carrying out the laser processing method of this embodiment has the above-mentioned configuration in general. Regarding the laser processing method of this embodiment carried out using the laser processing apparatus 1, This will be explained below.

The plate-shaped workpiece to be processed by the laser processing method of this embodiment has a thickness of 700 μm, for example, as shown in FIG. This is a silicon (Si) wafer 10 of. The laser processing described below is processing for bringing the back surface 10b of the wafer 10 into a desired state suitable for the grinding processing described later.

When carrying out the laser processing method of this embodiment, first of all, from the correlation between the pulse energy Pe, which is the energy per pulse, and the pulse interval Pw, the correlation between the formation of a stratum corneum, which will be described later, on the upper surface of the workpiece is determined. A correlation threshold generation step is performed to generate a relationship threshold.

When carrying out the correlation threshold generation step, a silicon (Si) wafer 10 is prepared as shown in FIG. Next, a protective tape T is attached to the front surface 10a of the wafer 10, the wafer 10 is turned over, and the back surface 10b of the wafer 10 is placed upward and the protective tape T is directed downward, and the chuck table 23 of the laser processing apparatus 1 is placed on the wafer 10. It is placed on the holding surface 25 of and held by suction. For convenience of explanation, FIG. 2 shows an example in which the wafer 10 on which the devices 12 are formed on the surface 10a is used when performing the correlation threshold generation step. The device 12 and the dividing line 14 are not formed because the processing experiment generates a correlation threshold value that distinguishes between regions where the stratum corneum is formed and regions where the stratum corneum is not formed on the upper surface of the workpiece. It is preferable to use a so-called dummy wafer having the same dimensions and materials as the wafer 10 except for this.

Once the above-described wafer 10 is prepared, the wafer 10 is positioned directly below the imaging means 5, and an image of the wafer 10 is taken and displayed on the display means 6 as appropriate, and the external shape of the wafer 10 is detected. Next, based on the information about 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 to be described later, as shown in FIG. The wafer 10 is processed and sent to perform laser processing on the back surface 10b of the wafer 10. Once the laser processing has been performed in this way, the wafer 10 is indexed and fed by a predetermined interval (for example, 15 μm) in the Y-axis direction, and the adjacent unprocessed area in the Y-axis direction is placed directly under the condenser 31. position. Then, in the same manner as described above, the condensing point of the pulsed laser beam LB is positioned and irradiated on the back surface 10b of the wafer 10, and the wafer 10 is processed and fed in the X-axis direction. By repeating such processing, the above-described laser processing is performed on the entire back surface 10b of the wafer 10, as shown in FIG. 4(a).

In the above correlation threshold generation step, by appropriately changing the laser processing conditions when performing the above-described laser processing, the pulse energy Pe [μJ], which is the energy per pulse, and the pulse interval Pw [μs] are Depending on the correlation, a laser processing experiment is carried out to find a threshold value for determining whether a good stratum corneum layer 18 is generated on the back surface 10b of the wafer 10, as shown in FIG. 4(b).

When the back surface 10b of the wafer 10 is irradiated with the pulsed laser beam LB, the stratum corneum 18 generates a large thermal stress that tends to rapidly expand locally on the wafer 10, and the large thermal stress is restrained in the low-temperature surroundings and the wafer 10 This layer is generated by a chain of compressive plastic strain caused by exceeding the yield stress of silicon (Si) that constitutes the silicon. As is clear from the enlarged image of a part of the internal configuration of the wafer 10 shown in FIG. 4(b), an interface 16 along the XY plane is generated inside the wafer 10 due to the chain of compressive plastic strain. The layer that peels off toward the upper surface with the interface 16 as a boundary becomes the stratum corneum 18 of this embodiment. The laser processing experiment in this embodiment is carried out by changing each parameter within the range of laser processing conditions shown below, for example.

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

Among the laser processing conditions described above, the wavelengths of 266 nm, 355 nm, and 532 nm are wavelengths that are absorptive to silicon (Si), and the wavelength of 1064 nm is a wavelength that is transmissive to silicon (Si). Furthermore, as described above, the pulse interval Pw is a value uniquely determined by determining the repetition frequency, and by determining the repetition frequency and average output, the pulse interval Pw is a value that is uniquely determined by determining the repetition frequency, and by determining the repetition frequency and the average output, The energy, pulse energy Pe, is determined. The procedure of the laser processing experiment described above will be explained below.

<Laser processing experiment procedure>
a) Determine the wavelength (532 nm in this embodiment) of irradiation from the oscillator 32 of the laser irradiation means 3.
b) Change the average output as appropriate in the range of 1 to 50W.
c) Change the repetition frequency appropriately in the range of 0.1 to 50 MHz.
d) Change the pulse width appropriately in the range of 0.3 to 1000 ps.
e) Perform laser processing by irradiating with pulsed laser beam LB.

According to the above-described procedure, laser processing is performed while changing each value (average output, repetition frequency, pulse width), and whether or not the above-mentioned stratum corneum 18 is well formed is confirmed using an appropriate electron microscope. It was confirmed whether the stratum corneum 18 could be easily peeled off from the back surface 10b of the wafer 10.

As a result of performing laser processing by changing the laser processing conditions according to the procedure of the laser processing experiment described above and checking whether the above-mentioned stratum corneum 18 was well formed each time, the following threshold values 1 to 10 were confirmed. It was confirmed as a correlation threshold.

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

In addition, in the laser processing experiment described above, the present inventors performed laser processing under the following laser processing conditions exceeding the threshold value of 10, but the stratum corneum 18 was not generated well. It was confirmed that this is the limit value at which the stratum corneum 18 is not produced well.
Pulse interval Pw: 0.9μs (repetition frequency: 1.11MHz)
Pulse energy Pe: 3.8μJ (average output: 4.22W)

The above threshold values 1 to 10 are plotted as actual values on a correlation diagram with the horizontal axis as the pulse interval Pw [μs] and the vertical axis as the pulse energy Pe [μJ], as shown in Figure 5, and connected by a straight line. (For reference, the above limit values are indicated by x). Thresholds 1 to 10 are seen as correlation thresholds that separate the region (A) in which the stratum corneum 18 is well formed and the region (B) in which the stratum corneum 18 is not well formed in the above laser processing experiment. This is the actual measured value. That is, according to the laser processing experiment described above, 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 value and the pulse interval Pw is equal to or less than the correlation threshold value. It is confirmed that if 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 larger than the correlation threshold, a good stratum corneum 18 is not formed, and the control means 100 The correlation threshold value is stored in .

In addition, in the laser processing experiment described above, even if other wavelengths of 266 nm and 355 nm, which are absorbent to the silicon wafer, are selected as the wavelength of the pulsed laser beam LB to be irradiated, it is possible to obtain the same correlation threshold. On the other hand, when the above laser processing experiment was carried out by selecting the wavelength of the pulsed laser beam LB to be irradiated with a wavelength of 1064 nm, which is transparent to the silicon wafer, a good stratum corneum was not obtained in any region. Not formed.

As described above, a correlation threshold value that allows 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. By storing the correlation threshold in the control means 100, the correlation threshold generation step is completed.

Note that in the present invention, as described above, instead of performing the correlation threshold generation step of generating the correlation threshold based on the actual measurement value, the correlation threshold may be generated using an approximate expression. The present inventors calculated the correlation based on the actual measured value using the approximate formula (1) shown below within the range of laser processing experiments that showed good results in the laser processing experiments when generating the above correlation threshold value. We found that the relationship threshold can be approximated.

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

The above laser processing experiment was conducted on a silicon (Si) wafer 10, and when applying the above approximation formula (1) to the silicon (Si) wafer 10, at least the pulse By setting the coefficients a, b, and c to a=4, b=1, and c=0.9 within the range of the interval Pw from 0.02 μs to 0.8 μs, a good stratum corneum 18 can be achieved. It was found that it can be used as a generated correlation threshold. That is, the approximation formula (1) is expressed by the following approximation formula (2), and by using the following approximation formula (2), laser processing conditions that can produce a good stratum corneum 18 can be easily set. becomes possible.
Pe=4{1-(Pw/[μs]-0.9) ² } ...(2)

As described above, once the correlation threshold generation step of generating the correlation threshold based on the actual measurement value or the approximate formula is performed, the wafer 10 as the workpiece is prepared (see FIG. 2), and the control Laser processing condition setting in which laser processing conditions are set by the means 100 so that the pulse energy Pe of the pulsed laser beam LB irradiated from the laser irradiation means 3 is greater than or equal to the correlation threshold, and the pulse interval Pw is less than or equal to the correlation threshold. Implement the process. That is, in the correlation diagram shown in FIG. 5, 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 the region (A). In addition, 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, and by increasing the pulse energy Pe, the stratum corneum 18 can be formed. Since the thickness can be adjusted to a larger value, it is preferable to set the pulse energy Pe and pulse interval Pw of the pulsed laser beam LB in consideration of energy efficiency, the thickness of the stratum corneum 18 to be formed, and the like. Furthermore, if the value of the pulse energy Pe of the pulsed laser beam LB is too large, leakage light may reach the device 12 and cause damage, so it is set within a range that does not cause damage to the device 12.

In the laser processing experiment described above, 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. Therefore, in the laser processing condition setting step described above, it is preferable to set the pulse width within the above range. In addition, in the above laser processing experiment, it has been confirmed that a good stratum corneum 18 is formed when the irradiation pitch of the pulsed laser beam LB applied to the wafer 10 is 10 to 200 nm. It is preferable to adjust the machining feed rate in the X-axis direction appropriately according to the repetition frequency so as to achieve the above-mentioned irradiation pitch range.

As described above, once the laser processing condition setting step is performed, a protective tape T is attached to the surface 10a of the wafer 10, and the wafer 10 is held under suction on the chuck table 23 of the laser processing apparatus 1, and the laser processing Based on the conditions set in the condition setting step, the entire back surface 10b of the wafer 10 is irradiated with a pulsed laser beam LB to perform a stratum corneum forming step. As a result, the above-described stratum corneum 18 is formed 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 carried out by the same processing procedure as that described based on FIGS. 3 and 4 above, and detailed explanation will be omitted.

As described above, a laser processing condition setting step is carried out based on the correlation threshold value obtained in the correlation threshold value generation step, and a stratum corneum forming step is carried out according to the laser processing conditions set by the laser processing condition setting step. Thus, a good stratum corneum layer 18 is formed on the back surface 10b of the wafer 10.

In addition, in FIGS. 3 and 4 described above, when forming the stratum corneum 18, the chuck table 23 is processed and fed in the X-axis direction to irradiate the pulsed laser beam LB in a straight line, but the present invention For example, as shown in FIG. 6, the outer peripheral end of the back surface 10b of the wafer 10 is irradiated with a pulsed laser beam LB based on the laser processing conditions set in the laser processing condition setting step described above, and the chuck table is 23 in the direction shown by the arrow R1, and move the chuck table 23 in the direction of the arrow X-axis to move the irradiation position of the pulsed laser beam LB to the center O of the wafer 10. On the other hand, the pulsed laser beam LB may be applied in a spiral manner. Even when the pulsed laser beam LB is irradiated in this manner, a good stratum corneum layer 18 can be formed on the back surface 10b of the wafer 10, as in the above-described embodiment.

By carrying out the laser processing method including the above-described correlation threshold generation step, laser processing condition setting step, and stratum corneum formation step, the grinding process is performed by the stratum corneum 18 formed on the back surface 10b of the wafer 10. It becomes possible to grind the surface of difficult wafers with relative ease, improving productivity.

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 10 is transported to a grinding device 80 (only a portion of which is shown) shown in FIG. 7 to perform a grinding process. As shown in FIG. 7A, the grinding device 80 includes a suction chuck 81b and a chuck table 81 including a frame 81a surrounding 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, negative pressure is generated on the surface of the suction chuck 81b.

The grinding device 80 also includes a grinding means 82 for grinding and thinning the workpiece suctioned and held by the chuck table 81, as shown in FIG. 7(b). The grinding means 82 includes 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 plurality of grinding wheels 86 attached to the lower surface of the wheel mount 84. and a grinding wheel 85 arranged in an annular manner.

Once the wafer 10 with the above-mentioned stratum corneum 18 formed on the back surface 10b is placed on the chuck table 81 with the back surface 10b facing upward and held under suction, a drive motor (not shown) is activated to grind the wafer 10. The rotating spindle 83 of 82 is rotated at, for example, 6000 rpm in the direction shown by arrow R2 in FIG. 7(b), and the chuck table 81 is rotated at, for example, 300 rpm in the direction shown by arrow R3. Next, while supplying grinding water onto the back surface 10b of the wafer 10 by a grinding water supply means (not shown), the grinding wheel 86 is brought into contact with the back surface 10b of the wafer 10, and the grinding wheel 85 is moved at a grinding feed rate of, for example, 1 μm/sec. feed the grinding downward. At this time, the 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 by a predetermined amount and the wafer 10 has a desired thickness, the grinding means 82 is stopped, and through cleaning, drying steps, etc., the grinding process is completed to thin the wafer 10 and make the back surface 10b a flat surface, as shown in FIG. 7(c).

In the grinding process in 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 where it is easily peeled off with the interface 16 as a boundary. Therefore, when carrying out the grinding process, the above-mentioned stratum corneum 18 is destroyed by the grinding means 82 and detached as a large solid from the back surface 10b of the wafer 10, making it possible to shorten the time involved in the grinding process. productivity is improved. Furthermore, since the stratum corneum 18 is destroyed by the grinding means 82 and detached as large particles, the grinding water used for grinding can be easily purified and reused, so grinding has a low environmental impact. It becomes possible to realize processing.

The present invention is not limited to the embodiments described above. For example, instead of the laser irradiation means 3 provided 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 irradiation means 7 includes an oscillator 72 that oscillates a pulsed laser beam LB, an X-axis galvano scanner 74 that oscillates the pulsed laser beam LB emitted by the oscillator 72 in the X-axis direction, and an X-axis galvano scanner 74 that oscillates the pulsed laser beam LB in the Y-axis direction. a moving Y-axis galvano scanner 75; a condenser 71 including an fθ lens 76 that irradiates a desired position of the wafer 10 held on the chuck table 23 with a pulsed laser beam LB oscillated in the X-axis direction and the Y-axis direction; , is equipped with.

The X-axis galvano scanner 74 and the Y-axis galvano scanner 75 have a known configuration including a mirror (not shown) and an angle adjustment actuator that adjusts the reflection angle of the mirror. The fθ lens 76 irradiates the back surface 10b of the wafer 10 perpendicularly with a pulsed laser beam LB that is oscillated in the X-axis direction and the Y-axis direction by the action of the X-axis galvano scanner 74 and the Y-axis galvano scanner 75. ing. The oscillator 72, the X-axis galvano scanner 74, and the Y-axis galvano scanner 75 are controlled by a control means 100'. Note that the X-axis galvano scanner 74 and the Y-axis galvano scanner 75 in this embodiment function as feeding means for relatively processing and feeding the chuck table and laser irradiation means in the present invention.

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

In the laser processing apparatus 1 equipped with the laser irradiation means 7 described above, the laser processing condition setting step is also carried out based on the correlation threshold value generated by the correlation threshold value generation step of the present embodiment, and the laser processing condition setting step is performed based on the laser processing condition. A stratum corneum forming step is performed on the back surface 10b of the wafer 10 using the laser irradiation means 7. As a result, a stratum corneum layer 18 similar to that described with reference to FIG. 4 is formed on the back surface 10b of the wafer 10. Then, the back surface 10b of the wafer 10 on which the stratum corneum layer 18 has been formed by the laser irradiation means 7 is ground by the grinding device 80 described based on FIG. , good grinding can be performed.

In the above-described embodiment, the above-described correlation threshold generation step, laser processing condition setting step, and stratum corneum formation step were performed using the silicon (Si) wafer 10 as the workpiece. The process is not limited to a plate-shaped workpiece made of other materials, such as a SiC substrate, a GaN substrate, a sapphire substrate, etc., and can be performed in the same manner. This makes it possible to relatively easily grind the surface of a workpiece that is difficult to grind, thereby improving productivity.

1: Laser processing device 1a: Base 1b: Vertical wall 1c: Horizontal wall 1d: Guide rail 2: Holding means 21: X-axis movable plate 22: Y-axis movable plate 23: Chuck table 24: Guide rail 25 : Holding surface 3: Laser irradiation means 31: Concentrator 32: Oscillator 33: Reflection mirror 34: Condensing lens 4: Feeding means 41: X-axis moving means 42: Motor 43: Ball screw 44: Y-axis moving means 45: Motor 46: Ball screw 5: Imaging means 6: Display means 7: Laser irradiation means 71: Concentrator 72: Oscillator 74: X-axis galvano scanner 75: Y-axis galvano scanner 76: fθ lens 10: Wafer 10a: Surface 10b: Back surface 12: Device 14: Planned dividing line 16: Interface 18: Stratum corneum 80: Grinding device 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 (6)

A chuck table having a holding surface for holding a plate-shaped workpiece, a laser irradiation means for irradiating the workpiece held on the chuck table with a pulsed laser beam having an absorbing wavelength, the chuck table and the laser. A laser processing method for processing the surface of a workpiece using a laser processing device including a feeding means for processing and feeding the irradiation means relatively,
a correlation threshold generation step of 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 of setting laser processing conditions such that the pulse energy of the pulsed laser beam irradiated from the laser irradiation means is greater than or equal to the correlation threshold, and the pulse interval is less than or equal to the correlation threshold;
A laser processing method comprising: forming a stratum corneum layer on the upper surface of the workpiece by irradiating the workpiece held on the chuck table with a pulsed laser beam based on the laser processing conditions. The stratum corneum is exposed to the pulsed laser beam, causing large thermal stress to rapidly expand locally in the workpiece, and this large thermal stress is constrained to the low-temperature surroundings and exceeds the yield stress of the workpiece. 2. The laser processing method according to claim 1, wherein the laser processing method is produced by a chain of compressive plastic strain caused by. The correlation threshold in the correlation threshold generation step is specified by an approximate formula, where the horizontal axis is the pulse interval [μs] and the vertical axis is 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 the workpiece. 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 formula 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 a range of 0.02 μs or more and 0.8 μs or less. The laser processing method according to claim 1, wherein the irradiation pitch of the pulsed laser beam is set in a range of 10 to 200 nm. The laser processing method according to claim 1, wherein the pulse width of the pulsed laser beam is set in a range of 0.3 to 100 ps.

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