Laser processing apparatus and laser processing method

Abstract

To provide a laser beam machining apparatus capable of breaking growing debris and suppressing generation of molten debris while forming a groove in a work.SOLUTION: A laser beam machining apparatus includes a laser oscillating mechanism 38 for oscillating pulsed laser beams. The laser oscillating mechanism 38 includes: a group setting part 48 for establishing the number of pulsed laser beams to be applied to a work until a time after which molten debris produced by the pulsed laser beams applied to the work is solidified, and assigns the pulsed laser beams to a group, under conditions that, within a period of time which is shorter than a period of time in which the melted debris is produced, and within a period of time until which a plasma generated by the pulsed laser beam becomes extinct, a next pulsed laser beam is applied to the work to sustain the plasma uninterruptedly to break growing debris; and a time interval setting part 40 for setting the time until the heat generated by the irradiation of the pulsed laser beams of one group are cooled as the time interval between the groups, and for setting the time interval of the pulsed laser beams of one group, wherein the repetition frequency is set with one group as one unit.SELECTED DRAWING: Figure 2

Description

【Technical Field】 【0001】 The present invention relates to a laser processing apparatus including holding means for holding a workpiece, laser beam irradiation means for irradiating a laser beam of a wavelength having absorbability to the workpiece held by the holding means, and feeding means for relatively feeding the holding means and the laser beam irradiation means, and a laser processing method for processing a workpiece. 【Background Art】 【0002】 A wafer on which a plurality of devices such as ICs and LSIs are partitioned by a dicing line and formed on the surface is divided into individual device chips by a laser processing apparatus, and each of the divided device chips is used in electric devices such as mobile phones and personal computers. 【0003】 A laser processing apparatus includes holding means for holding a workpiece, laser beam irradiation means for irradiating a laser beam of a wavelength having absorbability to the workpiece held by the holding means, and feeding means for relatively feeding the holding means and the laser beam irradiation means, and can divide a wafer into individual device chips with high precision. 【0004】 However, there is a problem that molten debris generated by irradiation of a laser beam adheres to the device and deteriorates the quality of the device chip. Further, particularly in a wafer in which a copper wiring is laminated on a silicon substrate, there is a problem that debris in which copper and silicon are melted and mixed adheres to the device by irradiation of a laser beam and deteriorates the quality of the device chip. This debris is debris that is likely to occur in a workpiece containing a semiconductor material and a metal material, and may be called growth debris because it grows with the passage of time after irradiation of a laser beam. 【0005】 Therefore, the present applicant has developed a technique of irradiating a laser beam again in order to remove molten debris and growth debris generated on the outer periphery of a device chip (see, for example, Patent Document 1). 【Prior Art Documents】 [Patent Documents] 【0006】 [Patent Document 1] Japanese Patent Publication No. 2015-133437 [Overview of the project] [Problems that the invention aims to solve] 【0007】 However, in the technology disclosed in Patent Document 1 mentioned above, it is necessary to irradiate the workpiece with a laser beam to remove molten debris and growing debris after irradiating it with a laser beam to form grooves, so there is room for improvement in productivity. 【0008】 The object of the present invention is to provide a laser processing apparatus and a laser processing method that can simultaneously irradiate a workpiece with a laser beam to form grooves and irradiate it with a laser beam to destroy growth debris and suppress the generation of molten debris. [Means for solving the problem] 【0009】 According to the present invention, the following laser processing apparatus is provided that solves the above problems. That is, A laser processing apparatus comprising: a holding means for holding a workpiece; a laser beam irradiation means for irradiating a laser beam onto the workpiece held by the holding means; and a feeding means for relatively feeding the holding means and the laser beam irradiation means for processing, The laser beam irradiation means includes an oscillation mechanism that emits a pulsed laser beam, and a concentrator that focuses the pulsed laser beam emitted by the oscillation mechanism and irradiates the workpiece held by the holding means. The oscillation mechanism is, A group setting unit sets the number of pulsed laser beams to be irradiated until the molten debris solidifies, under the conditions that the irradiation time with the pulsed laser beam on the workpiece is shorter than the time it takes for molten debris to be generated, and that the next pulsed laser beam is irradiated within the time it takes for the plasma generated from the workpiece by the pulsed laser beam to disappear, thereby continuing the plasma without interruption and destroying the growing debris. The system includes a time interval setting unit that sets the time required for the heat generated by the irradiation of a group of pulsed laser beams to dissipate as the time interval between a group and an adjacent group, and sets the time interval between the pulsed laser beams constituting the group. A laser processing apparatus is provided that sets the repetition frequency with the group as a single unit. 【0010】 Preferably, the oscillation mechanism comprises a plurality of laser diodes that oscillate pulsed laser beams, and in the group setting unit, a group is set by the pulsed laser beams oscillated by the plurality of laser diodes, and in the time interval setting unit, a pulse delay generator inputs signals to the plurality of laser diodes at desired time intervals, and also inputs signals so that the time it takes for the heat generated by the irradiation of the pulsed laser beams of the group to dissipate becomes the time interval between the group and adjacent groups. 【0011】 The oscillation mechanism comprises a plurality of oscillators that emit pulsed laser beams, and in the group setting unit, a group is set by the pulsed laser beams emitted by the plurality of oscillators, and in the time interval setting unit, a voltage is applied to the plurality of oscillators at a desired time interval using a delay voltage converter, and a voltage may also be applied so that the time it takes for the heat generated by the irradiation of the pulsed laser beams of the group to dissipate becomes the time interval between the group and adjacent groups. 【0012】 It is desirable to set the repetition frequency by thinning out a predetermined number of groups from multiple groups that oscillate per second. 【0013】 Further, according to the present invention, the following laser processing method for solving the above problems is provided. That is, "A laser processing method for processing a workpiece, comprising: a preparation step of preparing the laser processing apparatus according to any one of claims 1 to 4 and irradiating the workpiece with a laser beam having a wavelength having absorbency with respect to the workpiece; a groove forming step of forming a groove by irradiating the workpiece with the laser beam; a debris destruction suppression step of destroying the growth debris formed in the groove forming step and suppressing the generation of molten debris; and in order to simultaneously perform the debris destruction suppression step during the groove forming step, in the group setting unit, the number of the group of pulsed laser beams irradiated until the time when the molten debris solidifies is set on the condition that the next pulsed laser beam is irradiated within a time shorter than the time when the molten debris is generated by the irradiation of the pulsed laser beam to the workpiece and within the time until the plasma generated from the workpiece disappears by the irradiation of the pulsed laser beam, so as to continuously break the growth debris without interruption of the plasma; in the time interval setting unit, the time until the heat generated by the irradiation of the group of pulsed laser beams cools down is set as the time interval between the group and the adjacent group, and the time interval of the pulsed laser beams constituting the group is set, and the laser processing method" is provided. 【0014】 When molten debris is generated at time t1 after irradiating the workpiece with a pulsed laser beam and the molten debris solidifies at time t2, the time interval setting unit sets the time interval t3 of the group of pulsed laser beams such that t3 < t1 and t4 < t3 < t5, where t4 is the time when plasma is generated from the workpiece and t5 is the time when the plasma disappears, and the group setting unit preferably sets the number n of the group of pulsed laser beams as the integer part of (t2 / t3) + 1. 【Advantages of the Invention】 【0015】 According to the present invention, since the irradiation of a laser beam for forming a groove in a workpiece and the irradiation of a laser beam for destroying growth debris and suppressing the generation of molten debris can be performed simultaneously, productivity can be improved. 【Brief Description of the Drawings】 【0016】 [Figure 1] Perspective view of a laser processing apparatus configured according to the present invention. [Figure 2] Block diagram of the laser processing apparatus shown in FIG. 1. [Figure 3] Schematic diagram of the oscillation mechanism shown in FIG. 2. [Figure 4] Schematic diagram of another form of the oscillation mechanism. [Figure 5] Schematic diagram showing the generation time and extinction time of plasma generated by irradiation with a pulsed laser beam. [Figure 6] Schematic diagram of the pulsed laser beam irradiated onto the workpiece. 【Embodiments for Carrying Out the Invention】 【0017】 Hereinafter, embodiments of a laser processing apparatus and a laser processing method according to the present invention will be described with reference to the drawings. 【0018】 (Laser processing apparatus 2) First, the laser processing apparatus according to the present invention will be described. The laser processing apparatus shown as 2 in its entirety in FIG. 1 includes a holding means 4 for holding a workpiece such as a wafer, a laser beam irradiation means 6 for irradiating the workpiece held by the holding means 4 with a laser beam, and a feeding means 8 for relatively feeding the holding means 4 and the laser beam irradiation means 6 for processing. 【0019】 (Holding means 4) As shown in Figure 1, the holding means 4 includes an X-axis movable plate 12 supported on the upper surface of the base 10 so as to be movable in the X-axis direction, a Y-axis movable plate 14 supported on the upper surface of the X-axis movable plate 12 so as to be movable in the Y-axis direction, a support column 16 fixed to the upper surface of the Y-axis movable plate 14, and a cover plate 18 attached to the upper end of the support column 16. An elongated hole 18a extending in the Y-axis direction is formed in the cover plate 18, and a chuck table 20 extending upward through the elongated hole 18a is rotatably mounted on the upper end of the support column 16. Multiple clamps 22 are arranged around the periphery of the chuck table 20 at intervals in the circumferential direction. 【0020】 The upper end of the chuck table 20 is provided with a porous, circular suction chuck 24 connected to a suction means (not shown). In the holding means 4, the suction means generates suction force on the upper surface of the suction chuck 24 to hold the workpiece in place. The chuck table 20 is also rotated vertically around its axis by a motor (not shown) built into the support column 16. 【0021】 The X-axis direction is indicated by arrow X in Figure 1, and the Y-axis direction is indicated by arrow Y in Figure 1, both being perpendicular to the X-axis direction. The XY plane defined by the X-axis and Y-axis directions is essentially horizontal. 【0022】 (Feeding means 8) The feeding means 8 in the illustrated embodiment includes an X-axis feeding means 26 for machining feeding of the chuck table 20 in the X-axis direction and a Y-axis feeding means 28 for indexing feeding of the chuck table 20 in the Y-axis direction. 【0023】 The X-axis feed mechanism 26 includes a ball screw 30 connected to the X-axis movable plate 12 and extending in the X-axis direction, and a motor 32 that rotates the ball screw 30. The X-axis feed mechanism 26 converts the rotational motion of the motor 32 into linear motion using the ball screw 30 and transmits it to the X-axis movable plate 12, moving the X-axis movable plate 12 in the X-axis direction along the guide rail 10a on the base 10. As a result, the chuck table 20 is fed for machining in the X-axis direction. 【0024】 The Y-axis feed mechanism 28 includes a ball screw 34 connected to the Y-axis movable plate 14 and extending in the Y-axis direction, and a motor 36 that rotates the ball screw 34. The Y-axis feed mechanism 28 converts the rotational motion of the motor 36 into linear motion using the ball screw 34 and transmits it to the Y-axis movable plate 14, moving the Y-axis movable plate 14 in the Y-axis direction along the guide rail 12a on the X-axis movable plate 12. As a result, the chuck table 20 is indexed and fed in the Y-axis direction. 【0025】 (Laser beam irradiation means 6) Referring to Figures 1 and 2, the laser beam irradiation means 6 includes an oscillation mechanism 38 (see Figure 2) that emits a pulsed laser beam, and a concentrator 40 that focuses the pulsed laser beam emitted by the oscillation mechanism 38 and irradiates the workpiece held by the holding means 4. 【0026】 As shown in Figure 1, the laser beam irradiation means 6 has a housing 42 that extends upward from the upper surface of the base 10 and then substantially horizontally. The oscillation mechanism 38 is housed inside the housing 42, and the light concentrator 40 is mounted on the lower tip of the housing 42. In addition, an imaging means 44 for imaging the workpiece held by the holding means 4 is attached to the lower tip of the housing 42. 【0027】 (Oscillation mechanism 38) As shown in Figure 2, the oscillation mechanism 38 includes an oscillator 46 that emits pulsed laser light with a wavelength absorbed by the workpiece, a group setting unit 48, a time interval setting unit 50, and an attenuator 54 that adjusts the output of the pulsed laser light emitted by the oscillator 46, and the repeat frequency is set with one group as one unit. 【0028】 (Oscillator 46) Figure 2 shows a single box representing an oscillator 46, but the number of oscillators 46 provided in the oscillation mechanism 38 may be one or more. If only one oscillator 46 is provided, for example, as shown in Figure 3, that single oscillator 46 may be equipped with multiple laser diodes LD. Also, as shown in Figure 4, the oscillation mechanism 38 may be equipped with multiple oscillators 46. The laser medium is not limited to semiconductors, and other known mediums (for example, gases) may be used. 【0029】 (Group setting section 48) The group setting unit 48 sets the number of pulsed laser beams to be irradiated until the molten debris solidifies, under the conditions that the irradiation time is shorter than the time it takes for molten debris to be generated by the irradiation of the workpiece with a pulsed laser beam, and that the next pulsed laser beam is irradiated within the time it takes for the plasma generated from the workpiece by the irradiation of the pulsed laser beam to disappear, thereby continuing the plasma without interruption and destroying the growing debris, and sets this number of pulsed laser beams to be irradiated until the time it takes for the molten debris to solidify, and sets this number as one group. 【0030】 The applicant has confirmed that molten debris generated when a workpiece is irradiated with a pulsed laser beam occurs approximately 100 ns after irradiation and solidifies approximately 500 ns after irradiation. Therefore, the above-mentioned "time shorter than the time it takes for molten debris to be generated by irradiation of a workpiece with a pulsed laser beam" refers, for example, to a time shorter than 100 ns from the irradiation of the pulsed laser beam. 【0031】 Furthermore, the plasma generated when a workpiece is irradiated with a pulsed laser beam generally appears about 10 ns after the workpiece is irradiated with the pulsed laser beam and disappears about 30 ns after the workpiece is irradiated with the pulsed laser beam. Therefore, an example of the "time until the plasma generated from the workpiece by the irradiation of the pulsed laser beam disappears" is the time from the irradiation of the pulsed laser beam to about 30 ns. 【0032】 From the above, the phrase "a time shorter than the time it takes for molten debris to be generated by the irradiation of the workpiece with a pulsed laser beam, and within the time until the plasma generated from the workpiece by the irradiation of the pulsed laser beam disappears" can be defined as a time shorter than 100 ns from the irradiation of the pulsed laser beam, and within the time until approximately 30 ns have elapsed from the irradiation of the pulsed laser beam. 【0033】 Next, referring to Figure 5, we will explain how to maintain the plasma without interruption. Following the example above, with the time of irradiation of the workpiece with the first pulsed laser beam LB1 (1st pulse) as the reference (0 s), plasma P1 related to the 1st pulse is generated after 10 ns, and plasma P1 disappears after 30 ns. 【0034】 In this case, for example, if the second pulsed laser beam LB2 (second pulse) is irradiated 15 ns after the first pulse (within the time it takes for plasma P1 from the first pulse to disappear, which is 30 ns), then plasma P2 from the second pulse will be generated 25 ns after the first pulse and will disappear 45 ns later. In other words, plasma P2 will be generated before plasma P1 disappears. Similarly, if the time interval between the second and third pulses is also 15 ns, then plasma P3 from the third pulsed laser beam LB3 (third pulse) will be generated before plasma P2 disappears. 【0035】 In this way, by irradiating the workpiece with a pulsed laser beam for a time shorter than the time it takes for molten debris to be generated by the irradiation of the workpiece, and within the time it takes for the plasma generated from the workpiece by the irradiation of the pulsed laser beam to disappear (in the above example, the time interval between pulsed laser beams is set to 15 ns), the plasma generated from the workpiece can be continued without interruption. As a result, the growing debris generated by the irradiation of the pulsed laser beam can be destroyed by the plasma. 【0036】 Furthermore, the phrase "a time shorter than the time it takes for molten debris to be generated by the irradiation of the workpiece with a pulsed laser beam, and within the time until the plasma generated from the workpiece by the irradiation of the pulsed laser beam disappears" is not limited to 15 ns after the irradiation of the pulsed laser beam. 【0037】 As described above, molten debris solidifies approximately 500 ns after the workpiece is irradiated with a pulsed laser beam. Therefore, in the group setting unit 48 of the illustrated embodiment, the number of pulsed laser beams to be irradiated within 500 ns after the first pulsed laser beam (1st pulse) is set to form one group, on the condition that the plasma generated by the irradiation of the workpiece with a pulsed laser beam is continuously supplied without interruption to destroy the growing debris. Incidentally, in the case where the pulse interval is 15 ns as in the example above, the number of pulsed laser beams in one group can be set to 34 (34 pulses). 【0038】 The group setting unit 48, having the functions described above, can set up a group using pulsed laser beams oscillated by multiple (for example, 34) laser diodes LD, as shown in the example in Figure 3. Alternatively, the group setting unit 48 may set up a group using pulsed laser beams oscillated by some of the laser diodes LD provided by the oscillator 46 (for example, 34 out of 40). 【0039】 Furthermore, as shown in the example in Figure 4, the group setting unit 48 can also set a group using pulsed laser beams emitted by multiple oscillators 46. Alternatively, the group setting unit 48 may set a group using pulsed laser beams emitted by some of the oscillators 46 provided in the oscillation mechanism 38. 【0040】 Furthermore, by setting the number of pulsed laser beams in a group as described above, when pulsed laser beams are irradiated onto a workpiece, the growth debris generated by the irradiation of the pulsed laser beams can be destroyed by plasma, and the generation of molten debris can be suppressed. 【0041】 In this specification, among the debris generated on a workpiece by irradiation with pulsed laser light, debris that grows over time (gradually becomes larger after generation) is referred to as growing debris, and debris that does not grow over time is referred to as molten debris. 【0042】 (Time interval setting unit 50) The time interval setting unit 50 sets the time required for the heat generated by the irradiation of one group of pulsed laser beams to dissipate as the time interval between one group and an adjacent group, and also sets the time interval between the pulsed laser beams that make up one group. 【0043】 It is known that for workpieces formed from semiconductor materials such as silicon, the heat generated in the workpiece by the pulsed laser beam dissipates approximately 5 μs after irradiation, causing the workpiece temperature to drop to almost the same level as before irradiation. Therefore, as shown in Figure 6, the time interval setting unit 50 sets the time interval between one group and an adjacent group to 5 μs or more. 【0044】 This allows the heat generated in the workpiece to dissipate between the irradiation of one group of pulsed laser beams and the irradiation of the next group of pulsed laser beams, thus preventing a decrease in device quality due to heat. 【0045】 Furthermore, the time interval setting unit 50 sets the time interval (pulse interval) of the pulsed laser beams that constitute a group. In the case shown in Figure 3, the time interval setting unit 50 inputs signals to multiple laser diodes LD at desired time intervals (15 ns in the illustrated example) using the pulse delay generator 56, and also inputs signals so that the time it takes for the heat generated by the irradiation of one group of pulsed laser beams to dissipate is equal to the time interval between one group and an adjacent group (5 μs in the above example). The pulse width of the signal input from the pulse delay generator 56 to each laser diode LD can be any value (for example, 10 ps). 【0046】 On the other hand, in the case shown in Figure 4, the time interval setting unit 50 applies a voltage to the multiple oscillators 46 with a desired time delay using a delay voltage converter 58, and applies a voltage such that the time interval between one group and an adjacent group is equal to the time it takes for the heat generated by the irradiation of one group of pulsed laser beams to dissipate. As a result of the voltage being applied from the delay voltage converter 58 to each oscillator 46, the pulse width of the pulsed laser beams oscillated by each oscillator 46 can be any value. 【0047】 Then, as described above, when signals are input to multiple laser diodes LD by the pulse delay generator 56, or when voltages are applied to multiple oscillators 46 by the delay voltage generator 58, pulsed laser beams are generated, as shown in Figure 6, where each group (for example, 34 pulses) has a predetermined time interval (5 μs in the illustrated example). In other words, the repetition frequency of the pulsed laser beams is set with each group (34 pulses) as a unit. When the time interval between groups is 5 μs as in the illustrated example, the repetition frequency is 200 kHz. 【0048】 Then, the pulsed laser beams, each having a predetermined time interval between them, are adjusted in output by the attenuator 54, focused by the light concentrator 40, and irradiated onto the workpiece. A light guide means, such as an optical fiber, is provided between the oscillator 46 and the attenuator 54, so that each group of pulsed laser beams is guided from the oscillator 46 to the attenuator 54. 【0049】 The time interval between groups (5 μs in the illustrated example) may be set by the time interval setting unit 50 as described above, or it may be set by a decimation unit 52 (see Figure 2) which decimates a predetermined number of groups from the multiple groups that the oscillator 46 oscillates in one second. The decimation unit 52 may be composed of an acoustic-optic element or an electro-optic element. 【0050】 In other words, in the example shown in Figure 3, one group is formed by pulsed laser beams emitted from multiple laser diodes LD, and in the example shown in Figure 4, one group is formed by pulsed laser beams emitted from multiple oscillators 46. Then, in both the example shown in Figure 3 and Figure 4, with one group as one unit, the decimation unit 52 decimates a predetermined number of groups from the multiple groups emitted per second by the multiple laser diodes LD or the multiple oscillators 46, thereby generating pulsed laser beams with a predetermined time interval between groups. 【0051】 (Workpiece) Figure 2 also shows a wafer W as a workpiece that can be processed by the laser processing apparatus 2. The disc-shaped wafer W can be formed from a suitable semiconductor material such as silicon. The surface Wa of the wafer W is divided into multiple rectangular regions by grid-like division lines L, and a device D such as an IC or LSI is formed in each of the multiple rectangular regions. Although not shown, the surface Wa of the wafer W is coated with a metal film such as copper. 【0052】 In the illustrated embodiment, the back surface Wb of the wafer W is attached to an adhesive tape T fixed to an annular frame F, but the front surface Wa of the wafer W may also be attached to the adhesive tape T. 【0053】 (Laser processing method) Next, the laser processing method according to the present invention will be described. 【0054】 (preparation process) In the illustrated embodiment, first, a preparation step is performed to prepare a laser processing apparatus (for example, the laser processing apparatus 2 described above) that irradiates a wafer W with a laser beam of an absorbent wavelength. 【0055】 (Groove formation process) After the preparation process is completed, a groove formation process is carried out in which grooves are formed by irradiating the wafer W with a laser beam. 【0056】 In the groove formation process, first, the wafer W is held in place by suction on the upper surface of the chuck table 20 with its surface Wa facing upwards. The annular frame F is then fixed with clamps 22. Next, the wafer W is imaged by the imaging means 44, and the planned division line L is aligned in the X-axis direction based on the image of the wafer W captured by the imaging means 44. The pulsed laser beam is then aimed at the planned division line L aligned in the X-axis direction, and the height of the pulsed laser beam's focal point is adjusted to the surface Wa of the wafer W. 【0057】 Next, while the chuck table 20 is fed in the X-axis direction, a pulsed laser beam LB with a wavelength absorbed by the wafer W is irradiated onto the wafer W from the focuser 40, and ablation is performed along the planned division line L. This forms a groove G (see Figure 2) that cuts along the planned division line L. 【0058】 Next, the chuck table 20 is indexed and moved in the Y-axis direction relative to the light concentrator 40 by the amount of the spacing in the Y-axis direction of the division line L. Then, by alternately repeating the irradiation of the pulsed laser beam and indexing and moving the table, grooves G are formed on all of the division line L that are aligned in the X-axis direction. 【0059】 Furthermore, by rotating the chuck table 20 by 90 degrees and repeatedly alternating between pulse laser beam irradiation and indexing feed, grooves G are formed on all of the division lines L that are perpendicular to the division lines L where grooves G were previously formed. By performing the groove formation process in this manner, grooves G are formed in a grid pattern along the grid-like division lines L. This makes it possible to divide the wafer W into individual device chips. 【0060】 (Debris destruction suppression process) During the groove formation process, a debris destruction suppression process is simultaneously carried out to destroy the growing debris formed during the groove formation process and to suppress the generation of molten debris. 【0061】 Therefore, in the debris destruction suppression process, the group setting unit 48 sets the number of pulsed laser beams to be irradiated in one group until the time it takes for the molten debris to solidify, under the conditions that the next pulsed laser beam is irradiated within a time shorter than the time it takes for molten debris to be generated by irradiating the workpiece with pulsed laser beams, and within the time it takes for the plasma generated from the workpiece by the irradiation of pulsed laser beams to disappear, thereby continuing the plasma without interruption and destroying the growing debris. The time interval setting unit 50 sets the time it takes for the heat generated by the irradiation of one group of pulsed laser beams to dissipate as the time interval between one group and an adjacent group, and sets the time interval between the pulsed laser beams that make up one group. 【0062】 (Pulse interval and number of pulses) The time interval (pulse interval) of the pulsed laser beam that constitutes a group and the number of pulsed laser beams in a group (pulse number) will be described. When molten debris is generated at time t1 after irradiating the workpiece with the pulsed laser beam and solidifies at time t2, the time interval setting unit 50 sets the time interval (pulse interval) t3 of the pulsed laser beam in a group such that t3 < t1, and also satisfies t4 < t3 < t5, where t4 is the time when plasma is generated from the workpiece and t5 is the time when the plasma disappears. The group setting unit 48 sets the number (pulse number) n of the pulsed laser beams in a group to n = the integer part of (t2 / t3)+1. 【0063】 As described above, it has been confirmed that the molten debris generated when irradiating the workpiece with the pulsed laser beam occurs after about 100 ns have elapsed since irradiating the workpiece with the pulsed laser beam and solidifies after about 500 ns have elapsed. In addition, the plasma generated when irradiating the workpiece with the pulsed laser beam generally occurs after about 10 ns have elapsed since irradiating the workpiece with the pulsed laser beam and disappears after about 30 ns have elapsed. 【0064】 Therefore, taking the time when the first pulsed laser beam (the first pulse) is irradiated to the workpiece as the reference (0 s), The time t1 when molten debris is generated = 100 ns The time t2 when molten debris solidifies = 500 ns The time t4 when plasma is generated = 10 ns The time t5 when plasma disappears = 30 ns Let it be so. In this case, the time interval setting unit 50 sets the time interval (pulse interval) t3 of the pulsed laser beam in a group to, for example, 15 ns. If t3 = 15 ns, t3 < t1 (100 ns) t4 (10 ns) < t3 < t5 (30 ns) Both will be satisfied. 【0065】 When the time interval setting unit 50 sets the pulse interval t3 to 15ns, the group setting unit 48 sets the number of pulsed laser beams (number of pulses) in one group as n, n = (t2 / t3) integer part + 1 = (500ns / 15ns) integer part + 1 = (33.333...) integer part + 1 =33+1 =34 Set to this. 【0066】 Thus, the group setting unit 48 sets the number of pulsed laser beams (n=34) to be irradiated in one group from the time the first pulsed laser beam (1st pulse) is irradiated until the time the molten debris solidifies (t2=500ns), under the condition that the next pulsed laser beam is irradiated within a time shorter than the time (t1=100ns) in which molten debris is generated by the irradiation of the workpiece by the pulsed laser beam (t5=30ns) in which the plasma generated from the workpiece by the irradiation of the pulsed laser beam disappears, thereby continuing the plasma without interruption and destroying the growing debris. This allows the growth debris generated by the irradiation of the pulsed laser beam to be destroyed by the plasma when the workpiece is irradiated with a pulsed laser beam, and also suppresses the generation of molten debris. 【0067】 (Time interval between groups) In the time interval setting unit 50, in addition to setting the time interval of the pulsed laser beams constituting one group (15 ns in the above example), the time required for the heat generated by the irradiation of one group of pulsed laser beams to dissipate is set as the time interval between one group and an adjacent group. This is because if pulsed laser beams are continuously irradiated onto the wafer W, heat will accumulate on the wafer W, raising concerns about adverse effects on the device due to heat (deterioration of device quality). 【0068】 As described above, in the case of workpieces formed from semiconductor materials such as silicon, it is known that after about 5 μs of pulsed laser irradiation, the heat generated in the workpiece by the pulsed laser irradiation dissipates, and the temperature of the workpiece drops to almost the same level as before the pulsed laser irradiation. For this reason, as shown in Figure 6, the time interval setting unit 50 sets the time interval between one group and an adjacent group to 5 μs or more. 【0069】 This allows the heat generated on the wafer W to dissipate between the irradiation of one group of pulsed laser beams and the irradiation of the next group of pulsed laser beams, thus preventing adverse effects on the device caused by heat. 【0070】 Furthermore, it is preferable that the number of pulsed laser beams constituting a group is set by the group setting unit 48 so that the heat generated by the irradiation of one group of pulsed laser beams is below a temperature that does not adversely affect the device. 【0071】 As described above, the group setting unit 48 sets the number of pulsed laser beams in one group (n=34), and the time interval setting unit 50 sets the time interval between groups (5μs) and the time interval between pulsed laser beams constituting one group (t3=15ns). Then, the time interval setting unit 50 uses the pulse delay generator 56 to input signals to multiple laser diodes LD at 15ns intervals, and also inputs signals so that the time interval between groups becomes 5μs (see Figure 3). 【0072】 Alternatively, the time interval setting unit 50 may apply voltage to multiple oscillators 46 at 15 ns intervals using a delay voltage converter 58, and apply voltage so that the time interval between groups is 5 μs (see Figure 4). 【0073】 Furthermore, the time interval between groups may be set to 5 μs by thinning out a predetermined number of groups from multiple groups that oscillate per second using the thinning unit 52. 【0074】 In other words, signals may be input to multiple laser diodes LD at 15 ns intervals via a pulse delay generator 56, or voltages may be applied to multiple oscillators 46 at 15 ns intervals via a delay voltage converter 58, while a predetermined number of groups are thinned out by a thinning unit 52 from multiple groups that oscillate per second, so that the time interval between groups becomes 5 μs. When the time interval between groups is 5 μs, the repetition frequency becomes 200 kHz. 【0075】 In this way, by simultaneously performing the groove formation process and the debris destruction suppression process, a pulsed laser beam is generated as shown in Figure 6, with n=34 pulses per group, a pulse interval t3=15ns, and a time interval of 5μs between groups. The output of this pulsed laser beam is adjusted as appropriate by the attenuator 54, then focused by the concentrator 40, and irradiated along the planned division line L of the wafer W. This allows for the formation of grooves G while destroying the growing debris generated by the irradiation of the pulsed laser beam with plasma and suppressing the generation of molten debris. 【0076】 The groove formation process and the debris destruction suppression process can be carried out, for example, under the following processing conditions. Wavelength of pulsed laser beam: 355nm Average output: 60W Repetition frequency per group: 200kHz One group consists of 34 pulsed lasers. Power density per group: 60 J / cm² 2 Pulse width of one group: 495 ns = t3 × (34 - 1) = 15 ns × 33 Spot size per group: 10 μm in the X-axis direction, 50 μm in the Y-axis direction. Overlap rate between groups: 50% Feed rate: 1 m / s Time t1 for the formation of molten debris: Occurs 100 ns after laser irradiation. Solidification time t2 of molten debris: Solidification occurs 500 ns after laser irradiation. Time interval t3 between pulses: 15 ns (see Figures 3 and 4) Plasma generation time t4: Generated 10 ns after laser irradiation (see Figure 5). Plasma annihilation time t5: Annihilation 30 ns after laser irradiation (see Figure 5). Power density per pulse: 1.7 J / cm² 2 Pulse width per pulse: 10 ps Pulse overlap rate: 99.8% 【0077】 As described above, in the illustrated embodiment, it is possible to simultaneously irradiate the workpiece with a laser beam to form grooves and irradiate it with a laser beam to destroy growing debris and suppress the generation of molten debris. Therefore, since it is not necessary to irradiate the workpiece again with a laser beam to remove debris after forming the laser-processed grooves, productivity can be improved. Furthermore, since the heat generated in the workpiece cools down between the irradiation of one group of pulsed laser beams and the irradiation of the next group of pulsed laser beams, it is possible to prevent deterioration of device quality due to heat. [Explanation of symbols] 【0078】 2: Laser processing equipment 4: Holding means 6: Laser beam irradiation means 8: Feeding method 38: Oscillation mechanism 40: Light concentrator 46: Oscillator 48: Group Setting Section 50: Time interval setting section 56: Pulse delay generator 58: Delay voltage converter LD: Laser Diode

Claims

[Claim 1] A laser processing apparatus comprising: a holding means for holding a workpiece; a laser beam irradiation means for irradiating a laser beam onto the workpiece held by the holding means; and a feeding means for relative feeding of the holding means and the laser beam irradiation means, The laser beam irradiation means includes an oscillation mechanism that emits a pulsed laser beam, and a concentrator that focuses the pulsed laser beam emitted by the oscillation mechanism and irradiates the workpiece held by the holding means. The oscillation mechanism is, A group setting unit sets the number of pulsed laser beams to be irradiated until the molten debris solidifies, under the conditions that the irradiation time with the pulsed laser beam on the workpiece is shorter than the time it takes for molten debris to be generated, and that the next pulsed laser beam is irradiated within the time it takes for the plasma generated from the workpiece by the pulsed laser beam to disappear, thereby continuing the plasma without interruption and destroying the growing debris. The system includes a time interval setting unit that sets the time required for the heat generated by the irradiation of a group of pulsed laser beams to dissipate as the time interval between a group and an adjacent group, and sets the time interval between the pulsed laser beams constituting the group. A laser processing apparatus that sets the repetition frequency with the group as a single unit. [Claim 2] The oscillation mechanism comprises multiple laser diodes that emit pulsed laser beams, In the group setting unit, one group is set by the pulsed laser beams emitted by the plurality of laser diodes. The laser processing apparatus according to claim 1, wherein the time interval setting unit inputs signals to the plurality of laser diodes at desired time intervals using a pulse delay generator, and inputs signals so that the time it takes for the heat generated by the irradiation of one group of pulsed laser beams to dissipate becomes the time interval between one group and an adjacent group. [Claim 3] The oscillation mechanism comprises multiple oscillators that emit pulsed laser beams, In the group setting unit, one group is set by the pulsed laser beams emitted by the plurality of oscillators. The laser processing apparatus according to claim 1, wherein the time interval setting unit applies voltage to the plurality of oscillators at desired time intervals using a delay voltage converter, and applies voltage so that the time it takes for the heat generated by the irradiation of one group of pulsed laser beams to dissipate becomes the time interval between one group and an adjacent group. [Claim 4] The laser processing apparatus according to claim 1, wherein the repetition frequency is set by thinning out a predetermined number of groups from a plurality of groups that oscillate per second. [Claim 5] A laser processing method for processing a workpiece, A preparation step for preparing a laser processing apparatus according to any one of claims 1 to 4, which irradiates a workpiece with a laser beam of a wavelength that has absorption properties, A groove forming process in which grooves are formed by irradiating a workpiece with a laser beam, The groove formation process includes a debris destruction suppression step that destroys the growing debris formed in the groove formation process and suppresses the generation of molten debris, In order to carry out the debris destruction suppression process simultaneously with the groove formation process, In the group setting unit, the number of pulsed laser beams for a group to be irradiated before the time it takes for the molten debris to solidify is set, provided that the next pulsed laser beam is irradiated within a time shorter than the time it takes for molten debris to be generated by irradiating the workpiece with a pulsed laser beam, and within the time it takes for the plasma generated from the workpiece by the irradiation of the pulsed laser beam to disappear, thereby continuing the plasma without interruption and destroying the growing debris. A laser processing method in which the time interval setting unit sets the time until the heat generated by the irradiation of one group of pulsed laser beams dissipates as the time interval between one group and an adjacent group, and sets the time interval between the pulsed laser beams constituting one group. [Claim 6] When a pulsed laser beam is irradiated onto a workpiece, molten debris is generated at time t1, and the molten debris solidifies at time t2, The time interval setting unit sets the time interval t3 of the group of pulsed laser beams to satisfy t3 < t1 and the interval t4 < t3 < t5 between the time t4 when plasma is generated from the workpiece and the time t5 when the plasma disappears. The laser processing method according to claim 5, wherein the group setting unit sets the number n of pulsed laser beams in one group to the integer part of n = (t2 / t3) + 1.

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