Laser processing device

Abstract

Laser processing device (1) comprising: a clamping table (36) operable to hold a workpiece (W); and a laser beam irradiation means (5) which can be operated to irradiate the workpiece (W) held by the clamping table (36) with a laser beam, wherein the laser beam irradiation means (5) comprises: a pulse laser oscillator (51) designed to oscillate a pulsed laser beam (LB) at a predetermined repetition frequency, a first (52a) and a second (52b) condenser designed to collect the pulsed laser beam oscillated by the pulse laser oscillator (51), and a beam splitter means (53) arranged between the pulse laser oscillator (51) and the first (52a) and the second (52b) condenser for splitting the pulsed laser beam oscillated by the pulse laser oscillator (51) and directing the resulting beams (LB1, LB2) alternately towards the first (52a) and the second (52b) condenser, wherein the beam splitter means (53) includes a photoelastic modulator (532) having a piezoelectric element (532a) and a synthetic quartz (532b) formed in one piece, and modulates the laser beam so that a polarization plane of the laser beam is alternately at 0 and 90 degrees by applying a high-frequency voltage having a frequency to the piezoelectric element (532a), which corresponds to the natural frequency of the synthetic quartz (532b), where the beam splitter means (53) from one side of the pulse laser oscillator (51) towards one side of the first (52a) and second (52b) condensers includes: a first λ / 2 plate, a photoelastic modulator (532), a second λ / 2 plate, and a polarization beam splitter (534), and wherein, if the frequency of the photoelastic modulator (532) is f PEM , the repetition frequency of the pulse laser oscillator (51) with f Laser and a natural number is denoted by m, the repetition frequency f Laser from the following formula: f Laser = 4 f PEM ( 2 m + 1 ) [ H z ]

Description

Background of the inventionField of the invention

[0001] The present invention relates to a laser processing apparatus for laser processing a workpiece, such as a semiconductor wafer. Description of the state of the art

[0002] In a semiconductor device manufacturing method, the surface of a roughly disc-shaped semiconductor wafer is divided into a plurality of regions by designated division lines formed in a grid pattern, and a device, such as an IC or LSI, is formed in each of the divided regions. Subsequently, a semiconductor wafer is cut along designated division lines, thereby dividing the wafer into regions, each having a device formed therein, and individual device chips are fabricated. An optical device wafer having light-receiving elements, such as photodiodes, or light-emitting elements, such as laser diodes, laminated on the surface of a sapphire substrate is similarly cut along designated division lines, thereby dividing the wafer into individual optical devices, such as photodiodes or laser diodes, for use in a wide range of electronic devices.

[0003] In order to divide a wafer, such as a semiconductor wafer or an optical device wafer, along designated division lines as described above, a method has been proposed that forms laser-machined grooves by irradiating a pulsed laser beam having a wavelength absorbed by the wafer along designated division lines and refracting the wafer along the designated division lines where the laser-machined grooves have been formed. A laser processing apparatus for performing such laser processing includes a chuck table, laser beam irradiation means, and processing feeding means. The chuck table holds a workpiece. The laser beam irradiation means laser-machines the workpiece held by the chuck table. The processing feeding means relatively moves the chuck table and the laser beam irradiation means in the processing feeding direction.The laser beam irradiation device also includes a laser beam oscillation device and a condenser. The laser beam oscillation device oscillates a laser beam. The condenser includes a condenser lens designed to collect the laser beam oscillated by the laser beam oscillation device and irradiate the beam onto the workpiece held by the clamping table.

[0004] Furthermore, a laser processing apparatus has been proposed that enables two types of laser processing by splitting a pulsed laser beam oscillated by a laser beam oscillating means into two paths by using a polarization beam splitter (e.g., Japanese Patent Laid-Open No. JP 2008-114239 A).

[0005] Further information helpful for understanding the present invention can be found in the following documents: JP 2003 - 211 278 A relates to a laser beam machining apparatus in which a machining speed is increased without dividing the power of a laser beam. WO 2012 / 099151 A1 relates to an optical scanning device and a scanning verification device. WO 2007 / 082321 A1 relates to a photoelastic modulator for modulating light guided along a beam path.

[0006] The following document concerns basic optical properties of photoelastic modulators: WANG, Baoliang; LIST, Jennifer: Basic Optical Properties Of The Photoelastic Modulator Part I: Useful Aperture and Acceptance Angle. In: Proceedings of SPIE, Vol. 5888 (1), 2005, pp. 58881I-58881I-8. - ISSN 0277-786X Summary of the invention

[0007] However, splitting a pulsed laser beam into two paths by using a polarization beam splitter results in halving the power density for each pulsed laser beam, preventing proper processing due to insufficient output.

[0008] To solve this problem, a method has been proposed that splits a pulsed laser beam at each pulse synchronously with the repetition frequency of the pulsed laser beam using an electro-optic modulator (EOM) or an acousto-optic modulator (AOM). However, both the EOM and AOM suffer from a 15 to 30% reduction in output due to low transmittance. Moreover, the EOM and AOM transmit a laser beam with a relatively high density. This is because it is necessary to reduce the beam diameter due to a small effective diameter of a few millimeters for receiving a laser beam. This leads to a deviation of the focal point due to thermal lensing and causes significant damage, resulting in a short lifespan of the condenser.

[0009] In view of the foregoing, an object of the present invention is to provide a laser machining apparatus that enables machining without reducing the power density of each pulsed laser beam even when the pulsed laser beam is split into two paths.

[0010] According to one aspect of the present invention, a laser machining apparatus is provided including a chuck table and laser beam irradiation means. The chuck table holds a workpiece. The laser beam irradiation means irradiates a laser beam onto the workpiece held by the chuck table. The laser beam irradiation means includes a pulse laser oscillator, first and second condensers, and beam splitter means. The pulse laser oscillator oscillates a pulsed laser beam at a predetermined repetition frequency. The first and second condensers collect the pulsed laser beam oscillated by the pulse laser oscillator. The beam splitter means is disposed between the pulse laser oscillator and the first and second condensers, splits the pulsed laser beam oscillated by the pulse laser oscillator, and alternately guides the resulting beams toward the first and second condensers.The beam splitter includes a photoelastic modulator comprising a piezoelectric element and a synthetic quartz crystal formed in one piece. The photoelastic modulator modulates the laser beam so that a polarization plane of the laser beam is alternately at 0 and 90 degrees by applying a high-frequency voltage to the piezoelectric element at a frequency that matches the natural frequency of the synthetic quartz crystal.

[0011] According to a first embodiment, the beam splitter means includes, from one side of the pulse laser oscillator toward one side of the first and second condensers, a first λ / 2 plate, a photoelastic modulator, a second λ / 2 plate, and a polarization beam splitter. When the frequency of the photoelastic modulator is f PEM , the repetition frequency of the pulse laser oscillator with f Laserand a natural number is denoted by m, the repetition frequency f Laser from the following formula: fLaser=4fPEM(2m+1)[Hz]

[0012] Preferably, the natural frequency of the synthetic quartz is 50 kHz. If f PEM = 50 kHz and m = 0, 1, 2, 3, 4 and so on, the repetition frequency f Laser of the pulse laser oscillator as f Laser = 200 kHz, 40 kHz, 8 kHz, 1.6 kHz, 320 Hz and 64 Hz.

[0013] According to a second embodiment, the beam splitter means includes, from the pulse laser oscillator side toward the first and second condenser side, a λ / 4 plate, a photoelastic modulator, a λ / 2 plate, and a polarization beam splitter. When the frequency of the photoelastic modulator is set to f PEM , the repetition frequency of the pulse laser oscillator with f Laserand a natural number is denoted by m, the repetition frequency f Laser from the following formula: fLaser=2fPEM(2m+1)[Hz]

[0014] Preferably, the natural frequency of the synthetic quartz is 50 kHz. If f PEM = 50 kHz and m = 0, 1, 2, 3, 4 and so on, the repetition frequency of the pulse laser oscillator is f Laser = 100 kHz, 20 kHz, 4 kHz, 800 Hz, 160 Hz and 32 Hz.

[0015] In the laser processing apparatus according to the present invention, the beam splitter means configured to split a pulsed laser beam and alternately direct the resulting beams toward the first and second condensers includes a photoelastic modulator comprising a piezoelectric element and a synthetic quartz crystal formed integrally. The photoelastic modulator modulates the laser beam so that the polarization plane is alternately at 0 and 90 degrees by applying a high-frequency voltage to the piezoelectric element at a frequency consistent with the natural frequency of the synthetic quartz crystal. This ensures that a pulsed laser beam is alternately modulated at each pulse in synchronization with the repetition frequency of the pulsed laser beam oscillated by the pulsed laser oscillator, thereby preventing the energy density of each pulse from being reduced by half.This makes it possible to process a workpiece simultaneously and appropriately using pulsed laser beams which are split by the beam splitter means and directed alternately towards the first and second condensers.

[0016] Furthermore, the synthetic quartz used in the photoelastic modulator has a high transmittance, so the photoelastic modulator's output does not decrease by 15 to 30%, as is the case with electro-optic modulators (EOM) and acousto-optic modulators (AOM). Furthermore, the effective diameter for receiving a laser beam is large, or approximately 16 mm, enabling the transmission of a laser beam with a relatively low density. This prevents thermal lensing, thus eliminating the problems of focal point drift and short condenser life.

[0017] The above and other objects, features and advantages of the present invention and the mode for carrying them out will become more apparent and the invention itself will be best understood by studying the following description and the appended claims with reference to the accompanying drawings which show some preferred embodiments of the invention. Short description of the drawings Fig. 1 is a perspective view of a laser processing apparatus constructed in accordance with the present invention; Fig. 2 is a block diagram of a laser beam irradiation means used in the Fig. 1 shown laser processing device; and Fig. 3 is a block diagram illustrating another embodiment of the laser beam irradiation means used in the Fig. 1 shown laser processing device. Detailed description of the preferred embodiments

[0018] Preferred embodiments of a wafer processing method and a laser processing apparatus according to the present invention will be described in detail below with reference to the accompanying drawings. Fig. Figure 1 illustrates a perspective view of a laser processing apparatus 1 constructed according to the present invention. Fig. The laser processing apparatus 1 illustrated in FIG. 1 includes a stationary base 2, a chucking table mechanism 3, and a laser beam irradiation unit 4 disposed on the stationary base 2 and serving as a laser beam irradiation means. The chucking table mechanism 3 holds a workpiece disposed on the stationary base 2 in a movable manner in the processing feed direction (X-axis direction) indicated by an arrow X.

[0019] The chuck table mechanism 3 includes a pair of guide rails 31, first and second slide blocks 32 and 33, a holding table 35, and a chuck table 36. The guide rails 31 are arranged on the stationary base 2 so as to be parallel to each other along the X-axis direction. The first slide block 32 is arranged on the guide rails 31 in a manner movable in the X-axis direction. The second slide block 33 is arranged on the first slide block 32 in a manner movable in the indexing feed direction (Y-axis direction) perpendicular to the X-axis direction. The holding table 35 is held on the second slide block 33 by a cylindrical member 34. The chuck table 36 serves as a chuck table. The chuck table 36 includes a suction chuck 361 formed with a porous material. A workpiece, such as a metal part, is clamped therein. B. a circular semiconductor wafer, is held on the holding surface, iethe upper surface of the suction chuck 361 by a suction means not shown. The chuck table 36 constructed as described above is rotated by a pulse motor not shown, which is arranged inside the cylindrical member 34. Note that a clamp 362 is arranged on the chuck table 36 for fixing an annular frame that holds the workpiece, such as a semiconductor wafer, through a protective tape.

[0020] The first slide block 32 has, on its lower surface, a pair of guided grooves 321 that fit the pair of guide rails 31, and, on its upper surface, a pair of guide rails 322 formed in parallel along the Y-axis direction. Constructed as described above, the first slide block 32 can move along the pair of guide rails 31 in the X-axis direction because the guided grooves 321 fit the pair of guide rails 31. The chuck table mechanism 3 includes a machining feed means 37 configured to move the first slide block 32 along the pair of guide rails 31 in the X-axis direction. The machining feed means 37 includes an externally threaded rod 371 and a drive source such as a pulse motor 372. The externally threaded rod 371 is arranged parallel to and between the pair of guide rails 31. The pulse motor 372 rotates and drives the external threaded rod 371.One end of the male threaded rod 371 is freely rotatably supported by a bearing block 373 fixed to the stationary base 2, and the other end of the male threaded rod 371 is coupled to the output shaft of the pulse motor 372 for power transmission. Note that the male threaded rod 371 is screwed into a female threaded through hole formed in a female threaded block (not shown). The female threaded block is provided to protrude from the lower surface of the central portion of the first slide block 32. Therefore, when the male threaded rod 371 is rotated in the normal and reverse directions by the pulse motor 372, the first slide block 32 moves along the guide rails 31 in the X-axis direction.

[0021] The second slide block 33 has a pair of guided grooves 331 on its lower surface, which fit onto the pair of guide rails 322 provided on the upper surface of the first slide block 32. Since the guided grooves 331 fit onto the pair of guide rails 322, the second slide block 33 can move in the Y-axis direction. The chuck table mechanism 3 includes a pitch feeder 38 configured to move the second slide block 33 along the pair of guide rails 322 provided on the first slide block 32 in the Y-axis direction. The pitch feeder 38 includes an externally threaded rod 381 and a drive source such as a pulse motor 382. The externally threaded rod 381 is arranged parallel to and between the pair of guide rails 322. The pulse motor 382 rotates and drives the externally threaded rod 381.One end of the male threaded rod 381 is freely rotatably supported by a bearing block 383 fixed to the upper surface of the first slide block 32, and the other end of the male threaded rod 381 is coupled to the output shaft of the pulse motor 382 for power transmission. Note that the male threaded rod 381 is screwed into a female threaded through hole formed in a female threaded block (not shown). The female threaded block is provided to protrude from the lower surface of the central portion of the second slide block 33. Therefore, when the male threaded rod 381 is rotated in the normal and reverse directions by the pulse motor 382, ​​the second slide block 33 moves along the guide rails 322 in the Y-axis direction.

[0022] The laser beam irradiation unit 4 includes a holding member 41, a housing 42, a laser beam irradiation means 5, and an imaging means 6. The holding member 41 is arranged on the stationary base 2. The housing 42 is held by the holding member 41 and extends substantially horizontally. The laser beam irradiation means 5 is arranged on the housing 42. The imaging means 6 is arranged at the front end portion of the housing 42 for detecting an area to be laser processed. Note that, in the illustrated embodiment, the imaging means 6 includes, in addition to a normal imaging element (CCD) configured to capture an image using visible light, an infrared illuminator, an optical system, another imaging element (infrared CCD), and so on. The infrared illuminator irradiates infrared light onto a workpiece.The optics capture infrared light emitted by the infrared illumination device. The imaging element (infrared CCD) outputs an electrical signal proportional to the infrared light captured by the optics. The imaging device 6 transmits the captured image signal to a control device (not shown).

[0023] The laser beam irradiation means 5 is described with reference to Fig. 2. The Fig. The laser beam irradiation means 5 illustrated in FIG. 2 includes a pulse laser oscillator 51, first and second condensers 52a and 52b, and beam splitter means 53. The pulse laser oscillator 51 oscillates a pulsed laser beam at a predetermined repetition frequency. The first and second condensers 52a and 52b collect the pulsed laser beam oscillated by the pulse laser oscillator 51 and irradiate the pulsed laser beam onto a workpiece W held by the chuck table 36. The beam splitter means 53 is arranged between the pulse laser oscillator 51 and the first and second condensers 52a and 52b, and splits the pulsed laser beam oscillated by the pulse laser oscillator 51 and directs the resulting beams alternately toward the first and second condensers 52a and 52b.In the present embodiment, the pulse laser oscillator 51 oscillates pulsed laser beams LB at repetition frequencies of 200 kHz, 40 kHz, 8 kHz, 1.6 kHz, 320 Hz, and 64 Hz for the reason that will be described later.

[0024] In the present embodiment, the beam splitter 53 includes, from the pulse laser oscillator 51 side toward the first and second condensers 52a and 52b side, a first λ / 2 plate 531, a photoelastic modulator 532, a second λ / 2 plate 533, and a polarization beam splitter 534. The first λ / 2 plate 531 rotates the polarization plane of the pulsed laser beam LB oscillated by the pulse laser oscillator 51, thereby adjusting the polarization plane so that the pulsed laser beam LB is incident on the photoelastic modulator 532 at an appropriate angle. Note that the first λ / 2 plate 531 is rotated by an actuator 531a such as a pulse motor.

[0025] In the present embodiment, the photoelastic modulator 532 includes a piezoelectric element 532a and a synthetic quartz 532b formed integrally. The photoelastic modulator 532 modulates the pulsed laser beam oscillated by the pulse laser oscillator 51 so that the polarization plane is alternately at 0 and 90 degrees by applying a high-frequency voltage to the piezoelectric element 532a at a frequency that matches the natural frequency of the synthetic quartz 532b. In the photoelastic modulator 532 of the present embodiment, the natural frequency of the synthetic quartz 532b is 50 kHz. As a result, a high-frequency voltage of 50 kHz is applied to the piezoelectric element 532a from a high-frequency power supply 532c.Therefore, the photoelastic modulator 532 alternately modulates the pulsed laser beam LB at each pulse in synchronism with the repetition frequency of the pulsed laser beam LB oscillated by the pulse laser oscillator 51 (which alternately modulates the pulsed laser beam LB into S and P waves relative to the polarization beam splitter 534).

[0026] The second λ / 2 plate 533 rotates the polarization plane of the pulsed laser beam LB, which has been modulated by the photoelastic modulator 532 so that the polarization plane is alternately at 0 and 90 degrees, thereby adjusting the polarization plane so that the pulsed laser beam LB is incident on the polarization beam splitter 534 at an appropriate angle. Note that the second λ / 2 plate 533 is rotated by an actuator 533a, such as a pulse motor.

[0027] The polarization beam splitter 534 splits the pulsed laser beam LB incident through the second λ / 2 plate 533 into pulsed laser beams LB1 and LB2, and directs the pulsed laser beam LB1 toward the first condenser 52a and the pulsed laser beam LB2 toward the second condenser 52b. The pulsed laser beam LB1 consists of S waves, and the pulsed laser beam LB2 consists of P waves.

[0028] The first condenser 52a includes a condenser lens 521a. The condenser lens 521a collects the pulsed laser beam LB1, which is split by the polarization beam splitter 534 and consists of S waves, and radiates the pulsed laser beam LB1 onto the workpiece W held by the chuck table 36. On the other hand, the second condenser 52b includes a direction-changing mirror 522b and a condenser lens 521b. The direction changing mirror 522b changes the direction of the pulsed laser beam LB2, which is split by the polarization beam splitter 534 and consists of P waves, and directs the pulsed laser beam LB2 toward the holding surface of the chuck table 36. The condenser lens 521b collects the pulsed laser beam LB2, the direction of which has been changed by the direction changing mirror 522b, and irradiates the pulsed laser beam LB2 onto the workpiece W held by the chuck table 36.Note that in the illustrated embodiment, the second condenser 52b is arranged at a distance from the first condenser 52a in the Y-axis direction, and the second condenser 52b can be adjusted by a moving means 523 for moving in the Y-axis direction. The first and second condensers 52a and 52b, constructed as described above, are mounted on the top of the housing 42, as shown in FIG. Fig. 1. The laser beam irradiation means 5 is constructed as described above, and the adjustment of the repetition frequency of the pulse laser oscillator 51 will be described. When the frequency of the photoelastic modulator 532 is set to f PEM , the repetition frequency of the pulse laser oscillator 51 with f Laser and a natural number is denoted by m, the repetition frequency can be determined by the formula 1 shown below: fLaser=4fPEM(2m+1)[Hz]

[0029] The natural frequency of the synthetic quartz 532b, which forms the photoelastic modulator 532, is 50 kHz. If f PEM = 50 kHz and m = 0, 1, 2, 3, 4 and so on, the repetition frequency f Laser the pulse laser oscillator 51 as f Laser = 200 kHz, 40 kHz, 8 kHz, 1.6 kHz, 320 Hz and 64 Hz.

[0030] The beam splitter means 53 of the laser beam irradiation means 5 constructed as described above modulates the pulsed laser beam oscillated by the pulse laser oscillator 51 so that the polarization plane is alternately at 0 and 90 degrees by applying a high-frequency voltage having a frequency that matches the natural frequency of the synthetic quartz 532b to the piezo element 532a of the photoelastic modulator 532, which includes the piezo element 532a and the synthetic quartz 532b formed integrally. This ensures that a pulsed laser beam is alternately modulated at each pulse in synchronism with the repetition frequency of the pulsed laser beam oscillated by the pulse laser oscillator 51 (the pulsed laser beam is alternately modulated into S and P waves relative to the polarization beam splitter 534), thereby preventing the energy density of each pulse from being reduced by half.This makes it possible to simultaneously and appropriately machine a workpiece using the pulsed laser beams LB1 and LB2, wherein the pulsed laser beam LB1 consists of S waves and the pulsed laser beam LB2 consists of P waves, and the laser beams are split by the polarization beam splitter 534 of the beam splitting means 53 and alternately directed toward the first and second condensers 52a and 52b.

[0031] Furthermore, the synthetic quartz 532b constituting the photoelastic modulator 532 has a high transmittance, and therefore, the output of the photoelastic modulator 532 does not decrease by 15 to 30% as in the electro-optic modulator (EOM) and an acousto-optic modulator (AOM). Furthermore, the effective diameter for receiving a laser beam is large, or approximately 16 mm, enabling the transmission of a laser beam with a relatively low density. This prevents thermal lensing, thus eliminating the problems of focal point deviation and short condenser life.

[0032] In the following, another embodiment of the laser beam irradiation means will be described with reference to Fig. 3. The Fig. The laser beam irradiation means 50 illustrated in Figure 3 includes a λ / 4 plate 535 instead of the first λ / 2 plate 531 constituting the beam splitter means 53 of the laser beam irradiation means 5. The λ / 4 plate 535 is rotated by an actuator 535a such as a pulse motor. Other components are essentially the same. Therefore, the same components are denoted by the same reference numerals, and their descriptions are omitted.

[0033] The λ / 4 plate 535, which is the beam splitter means 530 of the Fig. 3, changes the linearly polarized pulsed laser beam LB, which has been oscillated by the pulse laser oscillator 51, to circular polarization. The pulsed laser beam LB, which has been changed to circular polarization as described above, is guided to the synthetic quartz 532b of the photoelastic modulator 532, as in the beam splitter 53 of the laser beam irradiation means 5. The pulsed laser beam LB is split, causing the resulting beams to be alternately directed toward the first and second condensers 52a and 52b, as in the beam splitter 53 of the laser beam irradiation means 5.

[0034] The following describes the setting of the repetition frequency of the pulse laser oscillator 51, which irradiates the laser beam 50 in the Fig. 3. If the frequency of the photoelastic modulator 532 is set to f PEM' the repetition frequency of the pulse laser oscillator 51 with f Laser and a natural number is denoted by m, the repetition frequency f Laser of the pulse laser oscillator 51 can be determined by formula 2 shown below: fLaser=2fPEM(2m+1)[Hz]

[0035] The natural frequency of the synthetic quartz 532b, which forms the photoelastic modulator 532, is 50 kHz. If f PEM = 50 kHz and m = 0, 1, 2, 3, 4 and so on, the repetition frequency f Laser the pulse laser oscillator 51 as f Laser = 100 kHz, 20 kHz, 4 kHz, 800 Hz, 160 Hz and 32 Hz.

[0036] Therefore, the pulse laser oscillator 51, which oscillates the laser beam irradiation means 50 in the Fig. 3, the pulsed laser beams LB with repetition frequencies of 100 kHz, 20 kHz, 4 kHz, 800 Hz, 160 Hz and 32 Hz. The laser beam irradiation means 50 constructed as described above offers the same advantageous effects as the one shown in Fig. 2 illustrated laser beam irradiation means 5.

[0037] The present invention is not limited to the details of the preferred embodiments described above. The scope of the invention is defined by the appended claims, and all changes and modifications that are within the equivalence of the scope of the claims are therefore encompassed by the invention.

Claims

[1] Laser processing device (1) comprising: a clamping table (36) operable to hold a workpiece (W); and a laser beam irradiation means (5) which can be operated to irradiate the workpiece (W) held by the clamping table (36) with a laser beam, wherein the laser beam irradiation means (5) comprises: a pulse laser oscillator (51) designed to oscillate a pulsed laser beam (LB) at a predetermined repetition frequency, a first (52a) and a second (52b) condenser designed to collect the pulsed laser beam oscillated by the pulse laser oscillator (51), and a beam splitter means (53) arranged between the pulse laser oscillator (51) and the first (52a) and the second (52b) condenser for splitting the pulsed laser beam oscillated by the pulse laser oscillator (51) and directing the resulting beams (LB1, LB2) alternately towards the first (52a) and the second (52b) condenser, wherein the beam splitter means (53) includes a photoelastic modulator (532) having a piezoelectric element (532a) and a synthetic quartz (532b) formed in one piece, and modulates the laser beam so that a polarization plane of the laser beam is alternately at 0 and 90 degrees by applying a high-frequency voltage having a frequency to the piezoelectric element (532a), which corresponds to the natural frequency of the synthetic quartz (532b), where the beam splitter means (53) from one side of the pulse laser oscillator (51) towards one side of the first (52a) and second (52b) condensers includes: a first λ / 2 plate, a photoelastic modulator (532), a second λ / 2 plate, and a polarization beam splitter (534), and wherein, if the frequency of the photoelastic modulator (532) is f PEM , the repetition frequency of the pulse laser oscillator (51) with f Laser and a natural number is denoted by m, the repetition frequency f Laser from the following formula: fLaser=4fPEM(2m+1)[Hz] [2] Laser processing device (1) according to claim 1, wherein the natural frequency of a synthetic quartz (532b) is 50 kHz and at which, if f PEM= 50 kHz and m = 0, 1, 2, 3, 4 and so on, the repetition frequency of the pulse laser oscillator (51) is defined as f Laser = 200 kHz, 40 kHz, 8 kHz, 1.6 kHz, 320 Hz and 64 Hz. [3] Laser processing device (1) comprising: a clamping table (36) operable to hold a workpiece (W); and a laser beam irradiation means (5) which can be operated to irradiate the workpiece (W) held by the clamping table (36) with a laser beam, wherein the laser beam irradiation means (5) comprises: a pulse laser oscillator (51) designed to oscillate a pulsed laser beam (LB) at a predetermined repetition frequency, a first (52a) and a second (52b) condenser designed to collect the pulsed laser beam oscillated by the pulse laser oscillator (51), and a beam splitter means (53) arranged between the pulse laser oscillator (51) and the first (52a) and the second (52b) condenser for splitting the pulsed laser beam oscillated by the pulse laser oscillator (51) and directing the resulting beams (LB1, LB2) alternately towards the first (52a) and second (52b) condenser, wherein the beam splitter means (53) includes a photoelastic modulator (532) comprising a piezo element (532a) and a synthetic quartz (532b) formed in one piece, and modulates the laser beam so that a polarization plane of the laser beam is alternately at 0 and 90 degrees by applying to the piezo element (532a) a high-frequency voltage having a frequency that matches the natural frequency of the synthetic quartz (532b), where the beam splitter means (53) from one side of the pulse laser oscillator (51) towards one side of the first (52a) and second (52b) condensers includes: a λ / 4 plate, a photoelastic modulator (532), a λ / 2 plate, and a polarization beam splitter (534), and wherein, if the frequency of the photoelastic modulator (532) is f PEM , the repetition frequency of the pulse laser oscillator (51) with f Laser and a natural number is denoted by m, the repetition frequency f Laser from the following formula: fLaser=2fPEM(2m+1)[Hz] [4] Laser processing device (1) according to claim 3, wherein the natural frequency of a synthetic quartz (532b) is 50 kHz and at which, if f PEM = 50 kHz and m = 0, 1, 2, 3, 4 and so on, the repetition frequency of the pulse laser oscillator (51) is defined as fLaser = 100 kHz, 20 kHz, 4 kHz, 800 Hz, 160 Hz and 32 Hz.

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