JP2009066657A - Laser machining method and laser machining apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a laser machining method and a laser machining apparatus by an ultra-short pulse laser in a liquid where the occurrence of stain and damage is suppressed in the circumference of a machined part. <P>SOLUTION: An object 8 to be machined is arranged in water 13 within machining housing 11, and the fluence of laser beam 3 generated from an ultra-short pulse laser generator is regulated to 2 times the light emission threshold of the water 13 or below or to the light emission threshold or below at the face 8a of the object 8 to be machined by a power attenuator 4, a mask 6 and a focusing lens 7, and the object 8 to be machined is irradiated with the ultra-short pulse laser beam 3 whose fluence is regulated to be thereby machined. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a laser processing method and a laser processing apparatus.

  In recent years, ultra-short pulse laser processing has attracted attention as a process that does not substantially affect the workpiece. Ultrashort pulse laser processing is to process a material by making the peak power per pulse very large instead of the pulse width being as short as 10 ps or less, which is called ablation processing (for example, patents). Reference 1).

In addition, there is a laser processing technique for performing laser processing in water (for example, Patent Document 2). According to this laser processing technique, when carried out in water, the splash generated by laser processing is prevented from adhering to the workpiece, and when the workpiece is toxic, the toxic material is splashed. Can be prevented.
JP 2005-533658 A Japanese Patent Laid-Open No. 06-269975

  An object of the present invention is to provide a laser processing method and a laser processing apparatus using an ultrashort pulse laser in a liquid, in which generation of dirt and damage around the processing portion is suppressed.

  One embodiment of the present invention provides the following laser processing method and laser processing apparatus in order to achieve the above object.

[1] In a state in which a liquid that emits light by irradiating an ultrashort pulse laser beam is present on at least the surface of the workpiece, the ultrashort pulse laser beam is applied to the surface via the liquid. Irradiation is performed to process the workpiece, and the processing with the ultrashort pulse laser beam is not substantially due to the light emission of the liquid, but is based on the laser beam transmitted through the liquid. A laser processing method of irradiating the laser beam with a certain intensity.

[2] The work piece is arranged so that the liquid exists on at least the processing side surface of the work piece, and the light is condensed so that the fluence upon irradiation of the work piece is equal to or less than the light emission threshold value of the liquid. A laser processing method for processing the workpiece by irradiating the processed side surface of the workpiece with the ultrashort pulse laser beam through the liquid.

[3] The laser processing method according to [1] or [2], wherein the ultrashort pulse laser beam is a femtosecond laser beam.

[4] The laser processing method according to [1] or [2], wherein the liquid is water.

[5] A holding means for holding the workpiece so that the liquid exists on at least the processing-side surface of the workpiece and a condensing unit, and when the workpiece is irradiated by the condensing unit Irradiating means for irradiating the surface on the processing side of the workpiece with the ultrashort pulse laser light condensed so that the fluence of the liquid becomes less than twice the light emission threshold of the liquid; A laser processing apparatus comprising: a moving unit that moves the holding unit relative to the irradiation unit.

[6] A holding means for holding the workpiece so that the liquid exists on at least the processing side surface of the workpiece and a condensing unit, and when the workpiece is irradiated by the condensing unit Irradiating means for irradiating the surface on the processing side of the workpiece through the liquid with ultrashort pulse laser light condensed so that the fluence of the liquid becomes equal to or less than the light emission threshold of the liquid; And a moving means for moving the holding means relative to the laser processing apparatus.

[7] The laser processing apparatus according to [5] or [6], wherein the liquid is water, and the ultrashort pulse laser beam is a femtosecond laser.

[8] The laser processing apparatus according to [5], further including charging means for charging the workpiece to a predetermined potential.

[9] A counter electrode disposed in the liquid so as to face the processing side surface of the workpiece, and charging means for charging the workpiece and the counter electrode with potentials of different polarities. The laser processing apparatus according to [5], further provided.

[10] The laser processing apparatus according to [8] or [9], wherein the liquid is water containing an electrolyte.

  According to the laser processing method of the first aspect, it is possible to perform processing so that processing by light emission of liquid does not substantially occur.

  According to the laser processing method of the second aspect, the energy utilization efficiency of the ultrashort pulse laser beam is increased.

  According to the laser processing method of the third aspect, compared to the case where the present invention is not used, the influence of heat on the workpiece is small, and fine processing is possible for a wide range of processing objects.

  According to the laser processing method of the fourth aspect, since water is used as the liquid, it is inexpensive and highly stable.

  According to the laser processing apparatus of the fifth aspect, it is possible to perform processing with an ultrashort pulse laser in a liquid while suppressing generation of dirt and damage around the processing portion as compared with the case where the present invention is not used. it can.

  According to the laser processing apparatus of the sixth aspect, the utilization efficiency of the energy of the ultrashort pulse laser beam is increased.

  According to the laser processing apparatus of the seventh aspect, since water is used as the liquid, it is inexpensive and highly stable, and since the femtosecond laser is used as the ultrashort pulse laser beam, the present invention is not used. Compared to the case, the influence of heat on the workpiece is small, and fine machining is possible for a wide range of workpieces.

  According to the laser processing apparatus of the eighth aspect, the processing scrap is processed by the electrostatic repulsive force between the processing scrap (ion) from the workpiece and the workpiece by processing with the ultrashort pulse laser beam. It can suppress adhering to an object.

  According to the laser processing apparatus of the ninth aspect, it is possible to further suppress the processing scraps from adhering to the workpiece as compared with the configuration in which the counter electrode is not used.

  According to the laser processing apparatus of the tenth aspect, water can be kept pure, and it is possible to stably suppress the processing dust from adhering to the workpiece.

[First Embodiment]
FIG. 1 is a diagram showing a schematic configuration of a laser processing apparatus according to the first embodiment of the present invention. In the figure, X, Y, and Z indicate an X-axis direction, a Y-axis direction, and a Z-axis direction (optical axis direction) orthogonal to each other.

  The laser processing apparatus 1 includes an ultrashort pulse laser generator 2 that generates an ultrashort pulse laser beam 3, a power attenuator 4, a shutter 5, a mask 6, and a light concentrator disposed on the optical path of the laser beam 3. A holding member 9 to which the lens 7 and the workpiece 8 are attached, a moving stage 10 for moving the holding member 9, a processing housing 11 installed on the moving stage 10, a controller for controlling the shutter 5 and the moving stage 10. 12. The workpiece 8 and the holding member 9 are disposed in water (liquid) 13 such as distilled water accommodated in the processing housing 11. The ultrashort pulse laser generator 2, the power attenuator 4, the shutter 5, the mask 6, and the condenser lens 7 constitute irradiation means for irradiating the workpiece 8 with the laser light 3.

  A pipe 15 is provided outside the processing housing 11 to connect a discharge port 15a formed below and an injection port 15b formed above, and a filter 16 and a pump 17 are interposed in the pipe 15. ing. The filter 16 removes processing waste and the like contained in the water 13 discharged from the processing housing 11. The pump 17 circulates the water 13 in the processing housing 11 from the discharge port 15a via the injection port 15b.

  The ultrashort pulse laser generator 2 generates an ultrashort pulse laser beam 3 having a pulse width of 10 ps or less at a predetermined repetition frequency. In the present embodiment, the output of the femtosecond laser oscillator is amplified by a regenerative amplifier to obtain a linearly polarized laser beam 3 having a pulse width of 100 fs, a center wavelength of 800 nm, an average power of 800 mW, an energy of 800 mJ / pulse per pulse, and a repetition frequency of 1 kHz. A femtosecond laser generator that emits light is used.

  The power attenuator 4 attenuates the average power 800 mW of the laser light 3 generated from the ultrashort pulse laser generator 2 to 0.5 mW, for example.

  In the condenser lens 7, the focal position in the optical axis direction (Z-axis direction) is adjusted by a moving mechanism or moving stage 10 (not shown) so that the laser beam 3 is condensed on the surface 8 a of the workpiece 8 or in the vicinity thereof. The In addition to the condensing lens 7, a concave mirror or the like may be used as the condensing element by changing the direction of the workpiece 8.

  The workpiece 8 can be a semiconductor substrate made of silicon, GaAs, or the like, or a metal such as stainless steel (SUS).

  The holding member 9 is fixed to the processing housing 11 and is made of a resin such as polymethyl methacrylate (PMMA), a metal such as aluminum or SUS. In addition, when forming a through-hole in the to-be-processed object 8, opening large enough for the laser beam 3 to pass through is formed.

  The moving stage 10 includes a mechanism that moves the processing housing 11 in the X-axis direction, the Y-axis direction, and the Z-axis direction orthogonal to each other.

The processing housing 11 is formed from transparent glass or transparent resin. An AR (Anti Reflection) coat 14a as an antireflection film that suppresses reflection of the laser light 3 may be formed on the side on which the laser light 3 is incident. The AR coat 14 suppresses reflection by light interference, and is formed by alternately laminating thin films having different refractive indexes (for example, TiO ₂ and SiO ₂ ), for example, and the wavelength region of the laser light 3 ( The reflectance at a wavelength of 800 nm is preferably as small as 0.2% or less, for example.

  As the liquid stored in the processing housing 11, in addition to the water 13, an organic solvent such as alcohol, carbon tetrachloride, or the like can be used. In addition, a solution in which an acid, an alkali, a surfactant, or the like is dissolved in water can be used.

  The distance between the inner surface of the processing housing 11 and the surface 8a of the workpiece 8 is preferably short, preferably 3 mm or less, and more preferably 1 mm or less.

In the laser processing apparatus 1 configured as described above, the fluence on the workpiece 8 of the laser beam 3 irradiated to the workpiece 8 is equal to or higher than the processing threshold, and when the water is irradiated, It is adjusted so that the processing on the workpiece 8 is not substantially due to the light emission of the water 13. Specifically, the magnification and the focal length of the condenser lens 7 are selected so that the emission threshold fluence of the water 13 is less than or equal to or less than twice the emission threshold of the water 13 in this example, or the mask of the mask 6 is selected. The diameter of the opening 6a (mask opening diameter) is selected. The value of 2 will be described later. “Fluence” refers to the energy density (J / cm ² ) obtained by dividing the energy per pulse of laser light by the irradiation area. “Processing threshold” refers to the minimum fluence at which the workpiece 8 can be processed. “Luminescence threshold” refers to the minimum fluence at which a liquid emits light when irradiated with laser light. In addition, the specific numerical value of the light emission threshold value of the water 13 and the processing threshold value of the workpiece will be described later.

<Determination of light emission threshold>
FIG. 2 shows a system for measuring the water emission threshold. This system uses a mask 6 having a mask opening diameter = 1 mm and a condenser lens 7 having a focal length f = 40 mm in the processing apparatus 1 shown in FIG. The processing housing 11 containing distilled water is placed on the moving stage 10, and a CCD camera 18 having sensitivity from the visible light to the near-infrared region is arranged in a direction perpendicular to the incident beam, and the inside of the processing housing 11 The presence or absence of luminescence of the distilled water was observed. In the figure, 3a indicates a light emission position. When ultrashort pulse laser light is condensed by a condensing element such as a condensing lens and irradiates water, a nonlinear optical effect occurs, and the refractive index of water changes and exceeds a certain threshold (emission threshold). Emits a broader spectrum than incident light. Note that if a certain threshold is exceeded even in air, light is emitted with a broader spectrum than incident light, but the light emission threshold in air is much higher than the light emission threshold in water. Luminescence at is not a problem.

When the fluence was not less than the emission threshold of distilled water, light was emitted in white with the beam condensing position as the center. The incident energy was changed by the power attenuator 4 and the presence or absence of light emission was measured by the CCD camera 18 to determine the light emission threshold value. The threshold for light emission in water was 2.0 μJ / pulse, which was 1.2 J / cm ² in terms of fluence.

Next, when the emission threshold of distilled water was measured in the same manner using the mask 6 having a mask opening diameter = 4 mm and the condenser lens 7 having a focal length f = 10 mm, the emission threshold was 0.18 μJ / pulse. Yes, in terms of fluence, it was 3.8 J / cm ² . These results are shown in Table 1.

  FIG. 3 shows a system for measuring the luminescence threshold due to processing when the workpiece is placed in distilled water. As shown in the figure, a silicon substrate was attached as a workpiece 8 to a holding member 9 and installed in a processing housing 11 containing distilled water. The moving stage 10 adjusts the surface 8a of the workpiece 8 so as to coincide with the condensing position of the laser beam 3, the laser beam 3 is emitted from the ultrashort pulse laser generator 2, and the incident energy is input by the power attenuator 4. The presence or absence of light emission was observed with the CCD camera 18 while changing the above. Luminescence was observed when the fluence was higher than the processing threshold. In the figure, 3a indicates a light emission position. The light emission position 3 a was the surface 8 a of the workpiece 8. Moreover, in order to confirm that it was not light emission from water, the surface 8a of the to-be-processed object 8 was observed with the microscope after completion | finish of the said measurement, and it confirmed that the processing hole was formed. It was confirmed that a processed hole was always formed when there was luminescence.

<Determination of processing threshold>
The mask opening diameter was set to 4 mm, the focal length of the condenser lens 7 was set to 10 mm, the incident energy was changed by the power attenuator 4 and the presence or absence of the processing hole was observed with a microscope, and the processing threshold was determined. The processing threshold of the GaAs substrate in water was 6.1 μJ / pulse, which was 0.12 J / cm ² in terms of fluence. The processing threshold value of the Si substrate determined by the same method was 13 μJ / pulse, which was 0.26 J / cm ² in terms of fluence. These results are shown in Table 2.

Table 1 above, as apparent from the experimental results shown in Table 2, the fluence can be processed GaAs is in the range of 0.12~3.8J / cm ^2, in the range of 0.26~3.8J / cm ² It can be seen that Si can be processed and energy loss due to underwater light emission is suppressed. That is, the processing threshold is determined according to the material of the workpiece 8, and the light emission threshold is determined by the liquid in which the workpiece 8 is disposed.

<Maximum fluence>
FIG. 4 is a diagram showing a result of measuring the depth of the hole by changing the fluence and performing the hole processing. As a result of this experiment, a low fluence region (F <10 J / cm ² ) in which the depth of the processed hole 8b shown in FIG. 5 is proportional to the fluence F and a high fluence region in which the depth of the hole 8b is saturated with respect to the fluence ( F> 10 J / cm ² ). The boundary between the two regions was F = 8.6 J / cm ² . On the other hand, the light emission threshold value of water under these processing conditions was 3.8 J / cm ² .

FIG. 5 is a diagram showing the state of the surface of the workpiece when processed in the low fluence region and the high fluence region. In the high fluence region, dirt 8c was generated around the machining hole 8b due to adhesion of machining waste. The fluence in the high fluence region exceeds twice the emission threshold of water (7.6 J / cm ² ), and the incident light energy is dissipated due to underwater emission and not only effectively used for processing but also scattered. It is thought that the emitted light caused damage around the hole.

  Therefore, in this condition, if the fluence is less than twice the light emission threshold fluence (the inflection point in the graph), the processing by the light emission of the liquid does not substantially occur, and the surface of the workpiece has passed the liquid. It can be processed substantially by a short pulse laser beam.

(Operation of laser processing equipment)
Next, the operation of the laser processing apparatus 1 will be described.

  The laser light 3 generated from the ultrashort pulse laser generator 2 is adjusted in power by the power attenuator 4, passes through the shutter 5 controlled to be opened by the controller 12, and passes through the mask opening 6 a of the mask 6. Then, the light is condensed on the surface 8 a of the workpiece 8 by the condenser lens 7, and the workpiece 8 is irradiated with the laser beam 3. On the other hand, the moving stage 10 is controlled to move the workpiece 8 at, for example, 10 μm / s in the X-axis direction. Thereby, for example, a groove having a width of 1 to 10 μm and a depth of 1 to 10 μm is formed on the workpiece 8 along the X-axis direction.

[Second Embodiment]
FIG. 6 is a diagram showing a laser processing apparatus according to the second embodiment of the present invention. In the figure, X, Y, and Z indicate an X-axis direction, a Y-axis direction, and a Z-axis direction (optical axis direction) orthogonal to each other.

  The present embodiment is different from the first embodiment mainly in the processing housing 11, and the rest is configured in the same manner as in the first embodiment. That is, the laser processing apparatus 1 includes an ultrashort pulse laser generator 2 that generates an ultrashort pulse laser beam 3, and a power attenuator 4, a shutter 5, a mask 6, and the like disposed on the optical path of the laser beam 3. A holding member 9 to which a mirror 20, a condenser lens 7 and a workpiece 8 are attached, a moving stage 10 for moving the holding member 9, a processing housing 11 installed on the moving stage 10, a shutter 5 and a moving stage 10 and a controller 12 for controlling 10. The workpiece 8 and the holding member 9 are disposed in water (liquid) 13 such as distilled water accommodated in the processing housing 11. Note that the ultrashort pulse laser generator 2, the power attenuator 4, the shutter 5, the mask 6, the mirror 20, and the condenser lens 7 constitute irradiation means for irradiating the workpiece 8 with the laser light 3.

  Further, the processing housing 11 is provided with an inlet 21 above one end side, a discharge pipe 22 is provided above the other end side, and a discharge tank 23 is connected to the discharge side of the discharge pipe 22. Has been. When water 13 is supplied from the inlet 21 to the processing housing 11, the water 13 passes through the processing housing 11 and is discharged to the discharge tank 23 via the discharge pipe 22. At this time, the water 13 discharges the processing waste generated by the irradiation of the laser light 3 to the discharge tank 23 through the discharge pipe 22.

  FIG. 7 is a cross-sectional view showing another example of a method for holding a workpiece. By providing a transparent plate 30 made of transparent glass or transparent resin at a predetermined distance on the surface 8 a of the workpiece 8, water is placed between the surface 8 a of the workpiece 8 and the back surface of the transparent plate 30. The flow path may be formed. By injecting water from the upper injection port 31 and discharging from the lower discharge port 32, the processing waste by the laser beam can be discharged together with water, as shown in FIG. No processing waste remains on the surface 8a.

[Third Embodiment]
FIG. 8 is a diagram showing a laser processing apparatus according to the third embodiment of the present invention. In the figure, X, Y, and Z indicate an X-axis direction, a Y-axis direction, and a Z-axis direction (optical axis direction) orthogonal to each other.

  In the present embodiment, the workpiece 8 is charged to a positive or negative potential so that the machining waste (ions) is less likely to adhere to the workpiece 8.

  In the laser processing apparatus 1 of the present embodiment, the counter electrode 40 is disposed in the water in the processing housing 11 so as to face the workpiece 8 in the first embodiment, and the workpiece 8 and the counter electrode 40 are arranged. Is connected to a DC power supply 41 for applying a DC voltage, and the rest is configured in the same manner as in the first embodiment.

  A negative voltage is applied from the DC power source 41 to the counter electrode 40 to charge the counter electrode 40 to a negative potential, and a positive voltage is applied to the workpiece 8 to charge the workpiece 8 to a positive potential. deep. Similarly to the first embodiment, when the workpiece 8 is ablated by laser light, the processing waste generated from the workpiece 8 reacts with the water 13 and decomposes, and metal ions (cations) are generated. An electrostatic repulsive force is generated between the workpiece 8 that is generated and charged to a positive potential, and metal ions are prevented from adhering to the workpiece 8. Some of the metal ions adhere to the counter electrode 40, and other metal ions are discharged to the discharge tank 23 through the discharge pipe 22 together with the water 13.

  In the third embodiment, an electrolyte such as ammonia water may be included in the water. Further, the counter electrode 40 may be provided at a location away from the workpiece 8 in the liquid.

(Operation of laser processing equipment)
In this embodiment, the output of a femtosecond laser oscillator is directly used as the ultrashort pulse laser generator 2, and laser light having a repetition frequency of 80 MHz, a pulse width of 100 fs, a center wavelength of 800 nm, and an average power of 2.0 W is power attenuated. The power is attenuated to an average power of 800 mW by the device 4. The fluence when the laser beam 3 is condensed using the condensing lens 7 having a focal length f = 4 mm and irradiated onto the workpiece 8 is 0.3 J / cm ² . The workpiece 8 is a GaAs substrate, the output voltage of the DC power supply 41 is set to 18 V, and a voltage of 18 V is applied between the workpiece 8 and the counter electrode 40 to charge the workpiece 8 to a positive potential. Let me. The pump 17 is operated to flow a constant flow of water into the processing housing 11. Simultaneously with opening the shutter 5, the moving stage 10 is controlled to move the workpiece 8 in the X-axis direction at 1 mm / s. As a result, a groove having a width of 10 μm and a depth of 5 μm is formed on the workpiece 8 along the X-axis direction. The surface of the workpiece 8 after processing has less adhesion of processing waste than when the DC power source is not used. Depending on the type of the workpiece 8, the workpiece 8 may be charged to a negative potential. For example, when n-type silicon is used as the workpiece 8, it is effective to charge the workpiece 8 to a negative potential.

[Other embodiments]
The present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the spirit of the present invention.

  For example, in the above embodiments, the workpiece side is moved, but the irradiation means side may be moved. In the above embodiment, the moving stage 10 has moved the processing housing 11, but the holding member 9 may be moved with respect to the processing housing 11. Further, the moving stage 10 may be disposed in the processing housing 11. Further, the incident energy to the workpiece 8 may be adjusted by the power attenuator 4 while monitoring the light emission in the processing housing 11 with a CCD camera.

FIG. 1 is a diagram showing a schematic configuration of a laser processing apparatus according to the first embodiment of the present invention. FIG. 2 is a diagram showing a system for measuring the light emission threshold of water. FIG. 3 is a diagram showing a system for measuring a light emission threshold when a workpiece is placed in distilled water. FIG. 4 is a diagram showing a result of measuring the depth of the hole by changing the fluence and performing the hole processing. FIG. 5 is a diagram showing the state of the surface of the workpiece when processed in the low fluence region and the high fluence region. FIG. 6 is a diagram showing a laser processing apparatus according to the second embodiment of the present invention. FIG. 7 is a cross-sectional view showing another example of a method for holding a workpiece. FIG. 8 is a diagram showing a laser processing apparatus according to the third embodiment of the present invention.

Explanation of symbols

1 laser processing equipment, 2 ultrashort pulse laser generator, 3 laser light, 3a emission position,
4 power attenuator, 5 shutter, 6 mask, 6a mask aperture, 7 condenser lens,
8 Workpiece, 8a surface, 8b hole, 8c dirt, 9 holding member, 10 moving stage, 11 processing housing, 12 controller, 13 water, 14 AR coating, 15 piping, 15a outlet, 15b inlet, 16 filter, 17 pump, 18 CCD camera, 20 mirror, 21 inlet, 22 outlet pipe, 23 outlet tank, 30 translucent plate, 31 inlet, 32 outlet, 40 counter electrode, 41 DC power supply

Claims (10)

In a state where the liquid that emits light is irradiated by irradiating at least the processing side surface of the workpiece with the ultrashort pulse laser beam, the ultrashort pulse laser beam is applied to the processing side surface through the liquid. Irradiating to process the workpiece,
A laser processing method of irradiating the laser beam with an intensity such that the processing with the ultrashort pulse laser beam is not substantially due to light emission of the liquid but due to laser light transmitted through the liquid. Arranging the workpiece so that there is a liquid on at least the surface of the workpiece to be processed,
The surface of the workpiece side of the workpiece is irradiated with the ultrashort pulse laser beam condensed so that the fluence upon irradiation of the workpiece is equal to or less than the emission threshold of the liquid. A laser processing method for processing the workpiece.   The laser processing method according to claim 1, wherein the ultrashort pulse laser beam is a femtosecond laser beam.   The laser processing method according to claim 1, wherein the liquid is water. Holding means for holding the workpiece so that liquid exists on at least the surface of the workpiece on the processing side;
An ultrashort pulse laser beam condensed by the light condensing unit so that a fluence upon irradiation of the workpiece is not more than twice a light emission threshold of the liquid; Irradiating means for irradiating the processing side surface of the workpiece through
A laser processing apparatus comprising: a moving unit that moves the holding unit relative to the irradiation unit. Holding means for holding the workpiece so that liquid exists on at least the surface of the workpiece on the processing side;
An ultrashort pulse laser beam condensed by the light condensing unit so that a fluence at the time of irradiation to the workpiece is equal to or lower than a light emission threshold of the liquid. Irradiating means for irradiating the surface on the processing side of the workpiece;
A laser processing apparatus comprising: a moving unit that moves the holding unit relative to the irradiation unit.   The laser processing apparatus according to claim 5, wherein the liquid is water, and the ultrashort pulse laser beam is a femtosecond laser.   The laser processing apparatus according to claim 5, further comprising charging means for charging the workpiece to a predetermined potential. A counter electrode disposed in the liquid so as to face the surface of the workpiece on the processing side;
6. The laser processing apparatus according to claim 5, further comprising charging means for charging the workpiece and the counter electrode with different polar potentials.   The laser processing apparatus according to claim 8, wherein the liquid is water containing an electrolyte.

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