Laser processing method and laser processing apparatus

Abstract

To provide a laser processing method for removing laser-induced surface periodic structures formed on a workpiece. [Solution] The laser processing method includes the step of removing the laser-induced surface periodic structure of an object to be processed, by irradiating the laser-induced surface periodic structure of the object with the laser-induced surface periodic structure with a laser.

Description

[Technical Field]

[0001] This invention relates to a laser processing method and a laser processing apparatus. [Background technology]

[0002] Traditionally, laser processing has been used for micro-machining such as cutting and drilling of various materials, including metal sheets such as iron plates, stainless steel plates, and aluminum plates.

[0003] For example, Patent Document 1 discloses a method for manufacturing a sample stage for an electron microscope, in which a diamond film is formed on a metal substrate by plasma CVD, and then the diamond film is processed into a desired shape using a laser processing method. Molybdenum, tungsten, and silicon are used as the metal substrate. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 9-115471 [Overview of the project] [Problems that the invention aims to solve]

[0005] Incidentally, molybdenum and tungsten are industrially important rare metals, but because they have high hardness and high melting points, they are typical examples of difficult-to-process materials. It is desirable that laser processing can be applied to the micro-machining of workpieces containing such difficult-to-process materials.

[0006] However, our research has shown that when laser processing workpieces such as molybdenum plates, depending on the laser irradiation conditions, ripple-like processing marks called laser-induced surface periodic structures (LIPSS) can form on the surface of the workpiece. Since such surface periodic structures can degrade the appearance, it is desirable that they be removed.

[0007] The object of the present invention is to provide a laser processing method and a laser processing apparatus for removing laser-induced surface periodic structures formed on a workpiece. [Means for solving the problem]

[0008] The present invention relates to the following laser processing method and laser processing apparatus. [1] A laser processing method comprising the step of removing a laser-induced surface periodic structure from a workpiece having a laser-induced surface periodic structure by irradiating the laser-induced surface periodic structure with a laser. [2] The laser processing method according to [1], wherein the laser is a pulsed laser with a wavelength of 600 nm or less. [3] The laser processing method according to [2], wherein the pulse width of the pulsed laser is 10 picoseconds or less. [4] The light intensity per unit area at the irradiation spot of the pulsed laser is 0.2 J / cm² 2 More than 0.3J / cm 2 The laser processing method described in either [2] or [3] below. [5] A laser processing method according to any one of [1] to [4], wherein the laser is scanned while the laser is repeatedly irradiated onto the laser-induced surface periodic structure. [6] The laser processing method according to any one of [1] to [5], further comprising the step of forming a hole by irradiating the workpiece with a laser, wherein in the removal step, the laser is irradiated onto the laser-induced surface periodic structure formed around the hole in the workpiece in the hole-forming step. [7] The laser processing method according to [6], wherein in the step of forming a hole and the step of removing a hole, the laser is a pulsed laser, and the wavelength and pulse width of the laser in the removal step are the same as the wavelength and pulse width of the laser in the step of forming a hole. [8] The laser processing method according to [7], wherein the light intensity per unit area at the laser irradiation spot in the removal step is smaller than the light intensity per unit area at the laser irradiation spot in the hole forming step. [9] The laser processing method according to any one of [6] to [8], wherein the laser is linearly polarized, and in the removal step, the laser is scanned over the laser-induced surface periodic structure while rotating the polarization plane of the laser.

[10] The laser processing method according to any one of [1] to [9], wherein the workpiece includes a difficult-to-process metallic material.

[11] The laser processing method according to

[10] , wherein the difficult-to-process metal material is molybdenum.

[12] A laser processing apparatus comprising: a stage for placing a workpiece having a laser-induced surface periodic structure; a laser light source for emitting a laser; a light guide for guiding the laser from the laser light source to the workpiece placed on the stage; and a control unit for controlling the laser light source, the stage, and the light guide so as to form a hole in the workpiece by irradiating it with a laser, and then remove the laser-induced surface periodic structure formed around the hole in the workpiece by irradiating it with a laser.

[13] The laser processing apparatus according to

[12] , wherein the wavelength and pulse width of the laser used to remove the laser-induced surface periodic structure are the same as the wavelength and pulse width of the laser used to form the hole, and the control unit controls the laser light source, the light guide unit and the stage such that the light intensity per unit area of ​​the laser irradiation spot used to remove the laser-induced surface periodic structure is less than the light intensity per unit area of ​​the laser irradiation spot used to form the hole.

[14] The laser processing apparatus according to

[12] or

[13] , further comprising an imaging unit for imaging a surface of the workpiece, wherein the control unit controls at least one of the light guiding unit and the stage so as to irradiate the laser-induced periodic surface structure imaged by the imaging unit with the laser. Effect of the Invention

[0009] According to the present invention, there can be provided a laser processing method and a laser processing apparatus for removing a laser-induced periodic surface structure formed on a workpiece. Brief Description of the Drawings

[0010] [Figure 1] FIG. 1A is a schematic explanatory diagram of a droplet-like deposit, FIG. 1B is a schematic explanatory diagram of LIPSS, and FIG. 1C is an SEM photograph showing an example of LIPSS formed on the surface of a workpiece. [Figure 2] FIG. 2 is a flowchart of a laser processing method according to an embodiment of the present invention. [Figure 3] FIGS. 3A to 3C are schematic diagrams of a hole forming step. [Figure 4] FIGS. 4A to 4C are schematic diagrams of a LIPSS removing step. [Figure 5] FIG. 5 is a schematic diagram showing a configuration of a laser processing apparatus according to an embodiment of the present invention. [Figure 6] FIG. 6 is an SEM photograph showing the results of Example 1. [Figure 7] FIG. 7 is an SEM photograph showing the results of Example 3. [Figure 8] FIG. 8 is an SEM photograph showing the results of Example 4. Mode for Carrying Out the Invention

[0011] As described above, depending on laser irradiation conditions and the like, LIPSS may be formed on the surface of the workpiece. Hereinafter, the findings by the present inventors will be specifically described.

[0012] Figure 1A is a schematic diagram illustrating drop-like deposits, Figure 1B is a schematic diagram illustrating LIPSS, and Figure 1C is an SEM image showing an example of LIPSS formed on the surface of a workpiece.

[0013] When a laser is irradiated onto the surface of a workpiece 10, such as a molybdenum plate, in a circular scanning motion to form fine holes 10a, tiny droplet-shaped deposits 11 (droplet-like deposits) sometimes formed near the opening edge of the holes 10a on the opposite side from the laser incidence side (laser emission side) (see Figure 1A). These deposits are thought to be molybdenum deposits coated with oxygen.

[0014] In response to this, the inventors found that reducing the intensity of the irradiating laser made it difficult for the droplet-like deposits 11 to form. However, it was newly discovered that when the laser intensity is reduced, a processing mark called LIPSS 12 is formed near the opening edge of the hole 10a on the laser incidence side of the workpiece 10 (see Figures 1B and 1C). These phenomena are thought to occur not only in molybdenum plates but also in various other workpieces.

[0015] In response to this, the inventors have found that LIPSS 12 formed on the workpiece 10 can be effectively removed by irradiating it with a laser.

[0016] In other words, a laser processing method according to one embodiment of the present invention includes a step of removing the laser-induced surface periodic structure of a workpiece having LIPSS by irradiating the laser with a laser.

[0017] Laser-induced periodic surface structure (LIPSS) refers to a submicron to micron-sized periodic structure formed around the processed area of ​​an object when it is subjected to laser processing. The mechanism by which LIPSS is formed is thought to involve interference between the incident laser and its surface scattered waves, and especially in the case of metallic materials, the excitation of surface plasmon polaritons. LIPSS is particularly noticeable when performing microfabrication using ultrashort pulse lasers. This is thought to be because ultrashort pulse lasers have high coherence (coherence), and laser interference contributes significantly to the observed effect.

[0018] Hereinafter, one embodiment of the present invention will be described in detail with reference to the drawings. However, the present invention is not limited to this embodiment.

[0019] 1. Laser processing method Figure 2 is a flowchart of a laser processing method according to one embodiment of the present invention. Figures 3A to 3C are schematic diagrams of the hole formation process S1, and Figures 4A to 4C are schematic diagrams of the LIPSS removal process S2.

[0020] As shown in Figure 2, a laser processing method according to one embodiment of the present invention includes a step of forming a hole in a workpiece by irradiating it with a laser (step S1), and a step of removing a laser-induced surface periodic structure (LIPSS) formed around the hole in the workpiece by irradiating it with a laser (step S2).

[0021] (pore formation process; process S1) In step S1, a hole 10a is formed in the workpiece 10 by irradiating it with a laser 20 (see Figures 3A-3C).

[0022] The workpiece 10 is not particularly limited as long as it can be laser processed. Examples of the main component material of the workpiece 10 include semiconductors such as silicon, metals such as copper, iron, aluminum, molybdenum, and tungsten, stainless steel, and alloys such as Invar. The main component refers to a component that is preferably made up of 50% by mass or more, more preferably 70% by mass or more, and even more preferably 90% by mass or more, of the total mass of the workpiece. These materials may consist of only one type or two or more types. In particular, the workpiece 10 preferably contains a metallic material, and more preferably contains a difficult-to-process metallic material. Examples of difficult-to-process metallic materials include rare metals such as molybdenum and tungsten, with molybdenum being preferred. The shape of the workpiece 10 is not particularly limited, but for example, it may be plate-shaped. When the workpiece 10 is plate-shaped, the thickness of the workpiece 10 can be, for example, 1 μm or more and 1000 μm or less, preferably 1 μm or more and 100 μm or less.

[0023] The irradiating laser 20 may be a pulsed laser or a continuous wave laser, but it is preferably a pulsed laser.

[0024] The wavelength and pulse width of the pulsed laser are not particularly limited as long as the desired processing can be performed on the workpiece. For example, the wavelength of the pulsed laser is preferably 600 nm or less, and more preferably 200 nm to 600 nm. The pulse width of the pulsed laser is preferably 10 picoseconds or less, and more preferably 0.1 picoseconds to 5 picoseconds. This makes it possible to process workpieces with high shape accuracy, even those containing difficult-to-process materials.

[0025] Furthermore, the pulsed laser may be unpolarized or polarized. In this embodiment, for example, a linearly polarized pulsed laser can be used.

[0026] In this embodiment, it is preferable to perform a so-called trepanning process, in which the laser 20 is irradiated onto the surface of the workpiece 10 while scanning it in a circular motion. Trepanning is a method of cutting out a shape by scanning the laser 20 around the surface of the workpiece 10, for example, in a circular motion (see Figure 3A). Trepanning is suitable, for example, when processing a hole with a diameter larger than the beam spot diameter. In trepanning, the processing time is longer compared to when the laser 20 is not scanned around the surface, but the processing quality tends to be better. This is because the processing is done slowly with a low-intensity laser beam. In Figure 3A, a hole 10a is formed by irradiating the surface of the workpiece 10 with the laser 20 while scanning it in a circular motion (see Figures 3A and 3B).

[0027] From the viewpoint of suppressing the generation of droplet-like structures, it is preferable to keep the light intensity per unit area at the irradiation spot of the laser 20 low. Specifically, if the laser 20 is a pulsed laser, the light intensity per unit area at the irradiation spot of the pulsed laser should be 0.1 J / cm². 2 More than 1J / cm 2 Preferably, it is 0.2 J / cm². 2 More than 0.4J / cm 2 The following is more preferable: The light intensity per unit area at the pulsed laser irradiation spot can be adjusted by the pulse energy of the pulsed laser and the irradiation spot diameter.

[0028] Hole 10a may be a straight hole or a tapered hole. In Figure 3B, hole 10a is a tapered hole. When hole 10a is a tapered hole, the taper angle θ of the tapered hole is not particularly limited, but it is preferably 10° or more and 70° or less, and more preferably 20° or more and 50° or less (see Figure 3B).

[0029] The plan view shape of the opening of hole 10a is not particularly limited and may be circular, elliptical, or polygonal, such as a quadrilateral. In Figures 3A to 3C, the plan view shape of the opening of hole 10a is circular. The diameter of hole 10a when viewed from above is not particularly limited, but it is preferably larger than the diameter of the laser 20 irradiation spot, and can be, for example, 1 μm or more and 1,000 μm or less.

[0030] As described above, by performing step S1, LIPSS 12 is formed around the hole 10a on the laser incident side surface of the workpiece 10 (see Figure 3C).

[0031] (LIPSS removal process; process S2) In step S2, the LIPSS 12 formed around the hole 10a of the workpiece 10 is removed by irradiating it with a laser 20 (see Figure 4A). In this embodiment, the LIPSS 12 is removed by scanning the laser 20 in a circular motion along the LIPSS 12 (see Figures 4A and 4B).

[0032] The laser 20 irradiating LIPSS12 may be the same as the laser 20 irradiating in process S1. That is, the laser 20 irradiating LIPSS12 is preferably a pulsed laser, and more preferably a linearly polarized pulsed laser. Furthermore, the wavelength and pulse width of the pulsed laser irradiating LIPSS12 may be the same as the wavelength and pulse width of the pulsed laser irradiating in process S1. In this embodiment, it is preferable that the wavelength and pulse width of the pulsed laser in process S2 are the same as the wavelength and pulse width of the pulsed laser in process S1. This allows the same laser light source to be used in process S1 and process S2, thereby increasing processing efficiency.

[0033] The light intensity per unit area at the irradiation spot of the laser 20 in step S2 is not particularly limited, but is preferably smaller than the light intensity per unit area at the irradiation spot of the laser 20 in step S1. This is because LIPSS 12 can be removed more easily while reducing damage to the workpiece 10 (for example, an increase in surface roughness). Specifically, when the laser 20 is a pulsed laser, the light intensity per unit area at the irradiation spot of the laser 20 is 0.2 J / cm 2 or more and 0.3 J / cm 2 or less, more preferably 0.2 J / cm 2 or more and 0.25 J / cm 2 or less. When the light intensity falls within the above range, LIPSS 12 can be removed with higher efficiency while suppressing an increase in surface roughness and the like.

[0034] When the laser 20 is linearly polarized light, it is preferable to continuously or intermittently rotate the polarization plane of the laser while irradiating LIPSS 12 with the laser 20. That is, it is preferable to scan the laser over LIPSS 12 while rotating the polarization plane of the laser 20. Rotating the polarization plane continuously or intermittently both means changing the rotation angle of the polarization plane over time. For example, while rotating a λ / 2 wave plate, LIPSS 12 can be irradiated with the laser 20 that has passed through the λ / 2 wave plate (see FIG. 5 described later). Thereby, LIPSS 12 can be removed more uniformly. That is, since LIPSS can be formed depending on the polarization state of laser light, it often has an anisotropic striped structure. In order to erase LIPSS having such an anisotropic striped structure to obtain an isotropic and smooth shape, it is desirable to continuously change the polarization state (specifically, the polarization direction) of the laser to obtain isotropically polarized light as a time average.

[0035] Furthermore, from the viewpoint of removing LIPSS 12 more effectively and making the surface of the workpiece 10 smoother, it is preferable to repeatedly irradiate the LIPSS 12 with the laser 20 while scanning with the laser 20. In this embodiment, it is preferable to make the laser 20 circle multiple times along the LIPSS 12 formed around the hole 10a. The number of circle passes depends on the light intensity per unit area at the irradiation spot of the laser 20, but for example, two or more passes are preferable, and three to 100 passes are more preferable.

[0036] (effect) According to the laser processing method of the above embodiment, LIPSS formed on the surface of the workpiece can be effectively removed. In particular, by performing step S2 while reducing the light intensity per unit area at the laser irradiation spot in step S1, it is possible to suppress the formation of droplet-like deposits during laser processing while also removing LIPSS.

[0037] (Mechanism of LIPSS removal) LIPSS is presumed to be removed by the following mechanism. However, the mechanism of LIPSS removal is not limited to this.

[0038] When a laser is shone onto the workpiece 10, light absorption occurs, and the temperature of the irradiated area of ​​the workpiece rises. This temperature rise causes partial deformation and melting of the workpiece, which naturally involves atomic movement. Therefore, it is thought that LIPSS gradually disappears with repeated laser irradiation.

[0039] Furthermore, if the workpiece 10 is a molybdenum material, the work function of molybdenum is 4.2 eV (equivalent to a wavelength of 295 nm), and when one or two photons of ultraviolet light are absorbed, electrons are emitted due to the photoelectric effect. When the photoelectric effect occurs at high density, the chemical bonds of the components constituting LIPSS weaken, meaning that the electrons responsible for the chemical bonds are emitted due to the photoelectric effect, and it is thought that LIPSS becomes more easily destroyed by this photoelectric effect. Furthermore, an oxide film is formed on the surface of the molybdenum material as a passive layer, and its band gap is 2.5 eV or greater (equivalent to a wavelength of 495 nm or less). Therefore, by using a pulsed laser with a wavelength of 500 nm or less, LIPSS can be more sufficiently photoexcited, and thus LIPSS can be more effectively eliminated.

[0040] 2. Laser processing equipment Figure 5 is a schematic diagram showing the configuration of a laser processing apparatus 100 according to one embodiment of the present invention.

[0041] As shown in Figure 5, the laser processing apparatus 100 includes a stage 110, a laser light source 120, a light guide unit 130, an imaging unit 140, and a control unit 150.

[0042] Stage 110 is a stage for placing the workpiece 10. Stage 110 may be fixed in position or may be configured to be movable in the XYZ directions. If Stage 110 is configured to be movable in the XYZ directions, Stage 110 may be configured to be movable under the control of Control Unit 150.

[0043] The laser light source 120 emits a laser. The laser light source 120 can continuously or pulsely oscillate a laser of a predetermined wavelength. In this embodiment, a light source that oscillates a pulsed laser of the above-mentioned wavelength and pulse width is preferred, and a light source that oscillates a linearly polarized pulsed laser having the above-mentioned wavelength and pulse width is more preferred. The laser light source 120 may be configured to adjust its output and repetition frequency under the control of the control unit 150.

[0044] The light guide unit 130 guides the laser emitted from the laser light source 120 to the workpiece 10 placed on the stage 110. The light guide unit 130 includes a λ / 2 wave plate 131, a beam size changing unit 132, a galvanometer scanner 133, an fθ lens 134, and two mirrors 135a and 135b.

[0045] The λ / 2 wave plate 131 adjusts the polarization direction of the laser emitted from the laser light source 120. Although a λ / 2 wave plate 131 is used in Figure 5, the system is not limited to this, and any device that can adjust the polarization direction according to the purpose may be used. Furthermore, it is preferable that the λ / 2 wave plate 131 is configured to be rotatable. In Figure 5, the λ / 2 wave plate 131 is fixed to a rotating stage 131a that rotates around its axis (rotates in the plane direction). The rotating stage 131a may be configured to allow adjustment of the rotation speed and rotation direction of the λ / 2 wave plate 131 under the control of the control unit 150.

[0046] The beam size changing unit 132 adjusts the beam size of the laser. In Figure 5, the beam size changing unit 132 is a beam expander (lens pair).

[0047] The galvanoscanner 133 adjusts the laser irradiation position or scanning pattern. The galvanoscanner 133 may be configured to adjust the laser irradiation position or scanning pattern under the control of the control unit 150. This allows the laser to be irradiated onto the surface of the workpiece 10 in any shape while scanning.

[0048] The fθ lens 134 adjusts the focal position of the laser. The fθ lens 134 may be configured to allow adjustment of the focal position under the control of the control unit 150.

[0049] Mirrors 135a and 135b adjust the optical path of the laser. In Figure 5, mirror 135a adjusts the optical path so that the laser emitted from the laser source 120 is incident on the λ / 2 wave plate 131. Mirror 135b adjusts the optical path so that the laser, after passing through the beam size changing section 132, is incident on the galvanometer scanner 133.

[0050] The imaging unit 140 captures images of the surface of the workpiece 10. The imaging unit 140 only needs to be capable of capturing images of the focal position, the processing process, and the surface condition of the workpiece 10. Such an imaging unit 140 can be a general-purpose camera, such as a CCD camera. The image information captured by the imaging unit 140 is output to the control unit 150.

[0051] The control unit 150 is a computer or the like connected to the stage 110, laser light source 120, light guide unit 130, and imaging unit 120, etc. The control unit 150 can adjust the laser output and repetition frequency by controlling, for example, the laser light source 120. The control unit 150 can also adjust the laser irradiation position or scanning pattern by controlling, for example, the galvanometer scanner 133 of the light guide unit 130 and at least one of the stage 110. Furthermore, the control unit 150 can adjust the laser focal position by controlling the fθ lens 134. The control unit 150 may also perform these controls based on, for example, image information captured by the imaging unit 140.

[0052] (operation) The operation of the laser processing device 100 will be explained below with reference to Figure 5. The control unit 150 controls the laser light source 120 and at least one of the stage 110 and the light guide unit 130. As a result, steps S1 and S2 described above are performed in succession.

[0053] In step S1, the control unit 150 activates the laser light source 120 and adjusts the galvanometer scanner 133 and fθ lens 134 of the light guide unit 130. This adjusts the scanning pattern and focal position of the pulsed laser. The control unit 150 also adjusts the laser light source 120 and fθ lens 134. This adjusts the light intensity per unit area at the irradiation spot. Then, the pulsed laser is repeatedly scanned around the surface of the workpiece 10 to form a hole 10a.

[0054] Specifically, a pulsed laser is emitted from the laser light source 120. The emitted pulsed laser is incident on the λ / 2 wave plate 131. The polarization direction of the pulsed laser incident on the λ / 2 wave plate 131 is adjusted by the λ / 2 wave plate 131. At this time, the polarization plane of the laser may be rotated intermittently or continuously by rotating the λ / 2 wave plate 131 with a rotating stage 131a under the control of the control unit 150. After the pulsed laser has passed through the λ / 2 wave plate 131, the beam diameter is adjusted by the beam size changing unit 132 and then incident on the galvanometer scanner 133. The galvanometer scanner 133 scans the pulsed laser on the surface of the workpiece 10 into a predetermined shape, and the focal position is adjusted by the fθ lens 134. Then, the pulsed laser is repeatedly scanned in a circular motion on the surface of the workpiece 10 to form a hole 10a.

[0055] The control unit 150 may acquire the focal position and processing status through the imaging unit 140, and adjust the laser's focal position and scanning position by controlling the fθ lens 134 and galvanometer scanner 133 based on the acquired information.

[0056] In step S2, the control unit 150 controls the laser light source 120 and the light guide unit 130 in the same manner as in step S1 described above, to irradiate the LIPSS 12 formed around the hole 10a of the workpiece 10 with a pulsed laser.

[0057] For example, the control unit 150 adjusts the laser light source 120 and the fθ lens 134 of the light guide unit 130. It is preferable to make the light intensity per unit area of ​​the pulsed laser irradiation spot smaller than the light intensity per unit area of ​​the pulsed laser irradiation spot in step S1. It is also preferable to irradiate the LIPSS with a pulsed laser having the same wavelength and pulse width as the pulsed laser irradiated in step S1. Furthermore, it is preferable to repeatedly irradiate the LIPSS 12 with the pulsed laser by repeatedly performing a circular scan of the laser.

[0058] The irradiation position and number of pulses (number of passes) of the pulsed laser may be set based on separately stored data, or adjusted based on image information captured by the imaging unit 140. For example, the control unit 150 may control at least one of the light guide unit 130 and the stage 110 based on image information captured by the imaging unit 140 to adjust the irradiation position so that the laser is irradiated onto the LIPSS of the workpiece 10. Alternatively, the control unit 150 may control at least one of the light guide unit 130 and the stage 110 based on image information captured by the imaging unit 140 to adjust the number of passes that scan the LIPSS 12.

[0059] In this embodiment, the control unit 150 adjusts the laser scanning pattern and focal position by adjusting the galvanometer scanner 133 and the fθ lens 134, but is not limited to this. For example, the control unit 150 may adjust the laser scanning pattern, etc. by adjusting the position of the stage 110. Alternatively, the control unit 150 may adjust the laser scanning pattern, etc. by adjusting both the light guide unit 130 and the stage 110.

[0060] Furthermore, in this embodiment, the control unit 150 adjusts the light intensity per unit area of ​​the irradiation spot by adjusting the fθ lens 134 of the laser light source 120 and the light guide unit 130, but is not limited to this. For example, the control unit 150 may adjust the light intensity using only the fθ lens 134, or it may adjust the light intensity by placing a light-reducing member such as an ND filter in the optical path, or it may adjust the light intensity by moving the stage 110 in the Z-axis direction.

[0061] 3. Variant In the above embodiment, an example was shown in which the LIPSS formed in step S1 is removed in step S2, but the embodiment is not limited to this. For example, a workpiece on which LIPSS has been formed may be obtained from another source, and step S2 may be performed on that workpiece. [Examples]

[0062] The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples.

[0063] [Example 1] (Investigation of the number of irradiations) (1) Process S1 A molybdenum plate (10 μm thick) was processed using the laser processing apparatus shown in Figure 5. Specifically, a molybdenum plate was placed on the stage of the laser processing apparatus described above. Then, using a galvanoscanner and an fθ lens, the following pulsed laser was scanned around the surface of the molybdenum plate in a circular motion with a predetermined diameter under the following irradiation conditions. This formed a tapered hole (diameter 30 μm on the incident side, diameter 20 μm on the exit side, taper angle 26 degrees). The focal point of the pulsed laser was set on the surface of the molybdenum plate.

[0064] (Pulsed laser) Wavelength: 355nm Pulse width: 3.0 picoseconds Oscillation repetition frequency: 1.0kHz Output (Power): 0.84mW Pulse energy: 0.84 J

[0065] (Irradiation conditions for pulsed laser) Diameter of the irradiation spot on the surface of the molybdenum plate: 17 μm Light intensity per unit area at the illumination spot: 0.39 J / cm² 2 Laser scanning speed: 3 mm / second

[0066] When the surface of the molybdenum plate on the laser incidence side after laser irradiation was observed using a scanning electron microscope, it was confirmed that LIPSS had formed along the opening edge of the hole (see Figure 6, without LIPSS removal process). No droplet-like deposits were observed near the opening edge of the hole on the laser emission side of the molybdenum plate.

[0067] (2) Process S2 Next, using the laser processing apparatus described above, a pulsed laser was irradiated onto the LIPSS formed on the surface of the molybdenum plate using a galvanoscanner and an fθ lens from the same laser light source as above, under the following irradiation conditions, and a circular scan was performed. One rotation around the outer circumference of the hole was defined as one irradiation cycle (one circular scanning pass), and the number of irradiation cycles was repeated. The focal position of the pulsed laser was set on the surface of the molybdenum plate, as described above.

[0068] (Pulsed laser) Wavelength: 355nm Pulse width: 3.0 picoseconds Oscillation repetition frequency: 1.0kHz Output (Power): 0.50mW Pulse energy: 0.50 J Polarization: Linear polarization, during processing, the polarization plane is constantly rotating (the direction of rotation is the same as the direction of circumferential scanning).

[0069] (Irradiation conditions for pulsed laser) Diameter of the irradiation spot on the surface of the molybdenum plate: 16.6 μm Light intensity per unit area at the illumination spot: 0.23 J / cm² 2 Laser scanning speed: 3 mm / second Laser spot spacing: 3 μm Laser spot overlap rate: 77% Laser circumference scanning diameter: Adjust as appropriate according to the taper diameter.

[0070] (3) Discussion Figure 6 shows SEM images of the results of Example 1. In the figure, the lower SEM image is a magnified image of the area enclosed by the dotted line in the upper SEM image. As shown in Figure 6, it can be seen that LIPSS disappears as the number of laser passes for LIPSS reduction increases from 1 to 3. With 5 and 7 passes, LIPSS disappears even further, and the surface of the molybdenum plate becomes smoother. As you can see, with each subsequent irradiation, the smoothness gradually increases, indicating that more LIPSS is being removed.

[0071] [Example 2] (Investigation of pulse width) In step S2, the laser was irradiated while performing a circular scanning motion under the same conditions as in Example 1, except that the laser pulse width was changed to 8 picoseconds, 3 picoseconds, and 250 femtoseconds, and the number of irradiations was set to 1.

[0072] We confirmed that LIPSS disappears at pulse widths of 8 picoseconds, 3 picoseconds, and 250 femtoseconds. However, there was no difference in the degree of LIPSS reduction among these three pulse widths.

[0073] [Example 3] (Investigation of light intensity per unit area in the irradiation spot 1) In step S2, the light intensity per unit area at the irradiation spot is set to 0.27 J / cm². 2 The same tests as in Example 1 were conducted, except for the change made to [specific component].

[0074] Figure 7 is an SEM image showing the results of Example 3. For comparison, Example 1 (0.23 J / cm²) is also shown. 2 The SEM image of the subject is also shown. As shown in Figure 7, 0.27 J / cm 2 However, Example 1 (0.23 J / cm²) 2 It can be seen that LIPSS can be almost completely erased, similar to ).

[0075] [Example 4] (Investigation of light intensity per unit area at the irradiation spot 2) In step S2, the light intensity per unit area at the irradiation spot is set to 0.32 J / cm². 2 , 0.51 J / cm² 2 1.02 J / cm² 2 1.53 J / cm² 2 The same test as in Example 1 was performed, except that the parameters were changed and the number of irradiations was set to one.

[0076] Figure 8 shows SEM images of the results of Example 4. In the figure, the lower SEM image is a magnified image of the area enclosed by the dotted line in the upper SEM image. As shown in Figure 8, the light intensity per unit area is 0.32 J / cm². 2 In this case, Example 1 (0.23 J / cm²) 2 Although the LIPSS is reduced to the same extent as in the case of a single pass, it can be seen that the surface around the aperture becomes slightly rougher. And the light intensity per unit area is 0.51 J / cm². 2 1.02 J / cm² 2 1.53 J / cm² 2 As the intensity increases, LIPSS disappears more, but the surface roughness around the aperture increases. From these observations, the light intensity per unit area at the irradiation spot is 0.32 J / cm². 2 Preferably 0.3 J / cm 2 The following are suitable. [Industrial applicability]

[0077] The laser processing method and laser processing apparatus according to the present invention are useful for manufacturing parts such as support plates for electron microscopes. [Explanation of Symbols]

[0078] 10. Object to be processed 10a hole 11 Drip-like deposits 12 LIPSS 20 lasers 100 Laser Processing Equipment 110 stages 120 light source 130 Light guide section 131 λ / 2 wave plate 131a Rotating stage 132 Beam size change section 133 Galvanometer Scanner 134 fθ lens 135a, 135b Miller 140 Imaging Unit 150 Control Unit

Claims

1. The process includes a step of removing the laser-induced surface periodic structure of a workpiece having a laser-induced surface periodic structure by irradiating the laser-induced surface periodic structure with a laser, Laser processing method.

2. The laser is a pulsed laser with a wavelength of 600 nm or less. The laser processing method according to claim 1.

3. The pulse width of the aforementioned pulsed laser is 10 picoseconds or less. The laser processing method according to claim 2.

4. The light intensity per unit area at the irradiation spot of the pulsed laser is 0.2 J / cm². ２ 0.3J / cm or more ２ The following is: The laser processing method according to claim 2.

5. The laser is scanned while repeatedly irradiating the laser-induced surface periodic structure with the laser. The laser processing method according to claim 1 or 2.

6. The process further includes the step of forming a hole in the workpiece by irradiating it with a laser, In the removal step, a laser is irradiated onto the laser-induced surface periodic structure formed around the hole in the workpiece during the hole-forming step. The laser processing method according to claim 1.

7. In the steps of forming the hole and removing the hole, the laser is a pulsed laser. The wavelength and pulse width of the laser in the removal step are the same as the wavelength and pulse width of the laser in the hole forming step. The laser processing method according to claim 6.

8. The light intensity per unit area at the laser irradiation spot in the removal step is smaller than the light intensity per unit area at the laser irradiation spot in the hole forming step. The laser processing method according to claim 7.

9. The aforementioned laser is linearly polarized, In the removal step, the laser is scanned over the laser-induced surface periodic structure while rotating the polarization plane of the laser. The laser processing method according to any one of claims 6 to 8.

10. The workpiece includes a difficult-to-process metal material. The laser processing method according to claim 1.

11. The aforementioned difficult-to-process metallic material is molybdenum. The laser processing method according to claim 10.

12. A stage for placing a workpiece having a laser-induced surface periodic structure, A laser light source for emitting a laser, A light guide unit for guiding the laser from the laser light source to the workpiece placed on the stage, A control unit for controlling the laser light source, the stage, and at least one of the light guide unit, so as to form a hole in the workpiece by irradiating it with a laser, and then remove the laser-induced surface periodic structure formed around the hole in the workpiece by irradiating it with a laser, Having, Laser processing equipment.

13. The wavelength and pulse width of the laser used to remove the laser-induced surface periodic structure are the same as the wavelength and pulse width of the laser used to form the holes. The control unit controls the laser light source, the light guide unit, and the stage such that the light intensity per unit area of ​​the laser irradiation spot when removing the laser-induced surface periodic structure is less than the light intensity per unit area of ​​the laser irradiation spot when forming the hole. The laser processing apparatus according to claim 12.

14. The system further includes an imaging unit for imaging the surface of the workpiece, The control unit controls at least one of the light guide unit and the stage so as to irradiate the laser-induced surface periodic structure captured by the imaging unit with the laser. The laser processing apparatus according to claim 12 or 13.

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