Laser machining device and laser machining method

Abstract

To provide a laser machining device that is able to reduce deposition and adhesion of debris to a workpiece even when a femtosecond pulse laser is used.SOLUTION: The laser machining device includes: a light source configured to emit a pulsed laser beam; a dividing unit configured to divide the pulsed laser beam into a first pulsed laser beam and a second pulsed laser beam; and a light guide unit configured to guide the first pulsed laser beam and the second pulsed laser beam to a to-be-machined object such that a spot of the first pulsed laser beam and a spot of the second pulsed laser beam overlap each other and a size of the spot of the first pulsed laser beam is smaller than a size of the spot of the second pulsed laser beam.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] In laser micromachining such as laser dicing and laser scribing (hereinafter also referred to as "laser processing"), the generation of processing waste called debris and the accumulation and / or adhesion of the debris around the processed area have always been problems. For this reason, various technologies have been developed to remove the debris generated during laser processing (see, for example, Patent Document 1).

[0003] Patent Document 1 discloses a laser processing method using an excimer laser, in which laser light is incident on not only the core but also the cladding at one end face of an optical fiber, and the laser light emitted from the other end face of the optical fiber is irradiated onto a workpiece. By guiding the light using not only the core but also the cladding of the optical fiber, it is possible to irradiate a processing region of the workpiece with laser light at a first intensity, and simultaneously irradiate a cleaning region surrounding the processing region with laser light at a second intensity that is weaker than the first intensity. The processing region is ablated with excimer laser light of the first intensity, and debris is removed from the cleaning region with excimer laser light of the second intensity. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 8-1357 Summary of the Invention [Problem to be solved by the invention]

[0005] Generally, the pulse width of a pulsed excimer laser is in the nanosecond range, so the laser processing described in Patent Document 1 is considered to be nanosecond pulse laser processing. This is supported by the fact that the laser processing described in Patent Document 1 involves ablation processing of the object to be processed.

[0006] Meanwhile, in recent years, femtosecond pulse laser processing, which has a pulse width in the femtosecond range of less than 1 picosecond, has been used. As will be described later, nanosecond pulse laser processing and femtosecond pulse laser processing have significantly different processing mechanisms, which results in significantly different chemical compositions, sizes, and charges of the debris generated. For example, nanosecond pulse laser processing mainly generates large debris of micron size or larger, while femtosecond pulse laser processing generates small debris of submicron size. Such small debris has a large specific surface area, making it difficult to remove once it adheres to the surface of the workpiece.

[0007] An object of the present invention is to provide a laser processing apparatus and a laser processing method that can reduce the accumulation and adhesion of debris on an object to be processed even when a femtosecond pulse laser is used. [Means for solving the problem]

[0008] The present invention relates to the following laser processing apparatus and laser processing method.

[0009] [1] A laser processing device comprising: a light source for emitting a pulsed laser beam; a splitting unit for splitting the pulsed laser beam into a first pulsed laser beam and a second pulsed laser beam; and a light guiding unit for guiding the first pulsed laser beam and the second pulsed laser beam to an object to be processed so that a spot of the first pulsed laser beam and a spot of the second pulsed laser beam overlap each other and the size of the spot of the first pulsed laser beam is smaller than the size of the spot of the second pulsed laser beam. [2] The laser processing apparatus according to [1], wherein the light guide unit has a beam size changing unit for changing the beam size of at least one of the first pulse laser beam and the second pulse laser beam so that the beam size of the first pulse laser beam is smaller than the beam size of the second pulse laser beam. [3] The laser processing device according to [1] or [2], wherein the light guide section has a combining section for combining the first pulse laser beam and the second pulse laser beam. [4] The laser processing device according to any one of [1] to [3], wherein the light intensity per unit area of ​​the spot of the first pulse laser beam is higher than the light intensity per unit area of ​​the spot of the second pulse laser beam. [5] The laser processing device according to any one of [1] to [4], wherein the pulse width of the pulse laser beam is less than 1 picosecond, and the wavelength of the pulse laser beam is 2000 nm or less. [6] The laser processing apparatus according to [3], wherein the splitting unit includes a first polarizing beam splitter for splitting the pulse laser beam into the first pulse laser beam and the second pulse laser beam, and the combining unit includes a second polarizing beam splitter for combining the first pulse laser beam and the second pulse laser beam. [7] The laser processing apparatus according to [2], wherein the beam size changing unit has a plano-concave lens arranged on the optical path of the second pulse laser beam and movable in the optical axis direction. [8] A laser processing method comprising: a step of splitting a pulse laser beam into a first pulse laser beam and a second pulse laser beam; and a step of irradiating an object to be processed with the first pulse laser beam and the second pulse laser beam so that a spot of the first pulse laser beam and a spot of the second pulse laser beam overlap and the size of the spot of the first pulse laser beam is smaller than the size of the spot of the second pulse laser beam. [9] The laser processing method according to [8], wherein in the step of irradiating the first pulse laser beam and the second pulse laser beam onto the workpiece, the beam size of at least one of the first pulse laser beam and the second pulse laser beam is changed so that the beam size of the first pulse laser beam is smaller than the beam size of the second pulse laser beam.

[10] The laser processing method according to [8] or [9], wherein the light intensity per unit area at the spot of the first pulse laser beam is higher than the light intensity per unit area at the spot of the second pulse laser beam.

[11] The laser processing method according to any one of [1] to

[10] , wherein the pulse width of the pulse laser beam is less than 1 picosecond, and the wavelength of the pulse laser beam is 2000 nm or less. [Effects of the Invention]

[0010] According to the present invention, even when a femtosecond pulse laser is used, it is possible to reduce the accumulation and adhesion of debris on a workpiece. Furthermore, according to the present invention, the light intensity per unit area, spot size, position, etc. of the first pulse laser beam spot and the second pulse laser beam spot can be adjusted separately, so that laser processing and debris removal can be performed under optimal conditions for each. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a schematic diagram showing the configuration of a laser processing apparatus according to an embodiment of the present invention. [Figure 2] FIG. 2 is a flowchart of a laser processing method according to an embodiment of the present invention. [Figure 3] 3A to 3C are photographs showing the results of Example 1. [Figure 4] 4A to 4C are photographs showing the results of Example 2. DETAILED DESCRIPTION OF THE INVENTION

[0012] An embodiment of the present invention will be described in detail below with reference to the drawings, although the present invention is not limited thereto.

[0013] [Laser processing equipment configuration] FIG. 1 is a schematic diagram showing the configuration of a laser processing apparatus 100 according to an embodiment of the present invention.

[0014] 1, the laser processing apparatus 100 has a light source 110, a splitting unit 120, and a light guiding unit 130. The laser processing apparatus 100 irradiates a pulsed laser beam 210 emitted from the light source 110 onto a workpiece 300 to process the workpiece 300. At this time, the splitting unit 120 and the light guiding unit 130 adjust the light intensity distribution of the pulsed laser beam 210, thereby suppressing the accumulation and adhesion (adsorption) of debris to the workpiece 300. Note that the term "adhesion" used in this specification with respect to debris also includes the concept of "adsorption," which is often used in academic fields.

[0015] There are no particular limitations on the workpiece 300 as long as it can be laser-processed. Examples of the workpiece 300 include semiconductors such as silicon, metals such as copper, and alloys such as invar.

[0016] The light source 110 emits a pulsed laser beam 210. The pulse width and wavelength of the pulsed laser beam 210 are not particularly limited as long as they can perform the desired processing on the workpiece 300. For example, the pulse width of the pulsed laser beam 210 is preferably less than 1 picosecond, and more preferably in the range of 50 to 500 femtoseconds. In other words, the pulsed laser beam 210 is preferably a femtosecond pulsed laser. Furthermore, the wavelength of the pulsed laser beam 210 is preferably 2000 nm or less, and more preferably in the range of 300 to 1500 nm. In the example shown in FIG. 1, the light source 110 emits a linearly polarized pulsed laser beam 210.

[0017] The splitting unit 120 splits the pulsed laser beam 210 emitted from the light source 110 into a first pulsed laser beam 220 for processing and a second pulsed laser beam 230 for cleaning. The configuration of the splitting unit 120 is not particularly limited as long as it can achieve the above function. In the example shown in FIG. 1 , the splitting unit 120 includes a ½λ wave plate 121 that adjusts the polarization direction of the pulsed laser beam 210, and a first polarizing beam splitter 122 that splits the pulsed laser beam 210, whose polarization direction has been adjusted by the ½λ wave plate 121, into the first pulsed laser beam 220 and the second pulsed laser beam 230. The light intensity ratio between the first pulsed laser beam 220 and the second pulsed laser beam 230 can be adjusted by rotating the ½λ wave plate 121. Here, the 1 / 2λ wave plate 121 is rotated so that the light intensity per unit area at the spot 221 of the first pulse laser beam 220 for processing is higher than the light intensity per unit area at the spot 231 of the second pulse laser beam 230 for cleaning.

[0018] The first pulsed laser beam 220 and the second pulsed laser beam 230 have the same pulse width and wavelength because they are both derived from the pulsed laser beam 210. The first pulsed laser beam 220 and the second pulsed laser beam 230 also have the same pulse timing.

[0019] The light guiding unit 130 guides the first pulse laser beam 220 and the second pulse laser beam 230 split by the splitting unit 120 to the work-piece 300. At this time, the light guiding unit 130 controls the first pulse laser beam 220 and the second pulse laser beam 230 so that the spot 221 of the first pulse laser beam 220 and the spot 231 of the second pulse laser beam 230 overlap on the surface of the work-piece 300 and the size of the spot 221 of the first pulse laser beam 220 is smaller than the size of the spot 231 of the second pulse laser beam 230. The spot 221 of the first pulse laser beam 220 corresponds to the area to be processed, and the spot 231 of the second pulse laser beam 230 corresponds to the cleaning area from which debris is to be removed.

[0020] In this specification, the phrase "the spot 221 of the first pulse laser beam 220 and the spot 231 of the second pulse laser beam 230 overlap" means that at least a part of the spot 221 of the first pulse laser beam 220 overlaps with at least a part of the spot 231 of the second pulse laser beam 230. Therefore, the entire spot 221 of the first pulse laser beam 220 does not have to be located within the spot 231 of the second pulse laser beam 230.

[0021] As described above, the light guiding unit 130 controls the first pulse laser beam 220 and the second pulse laser beam 230 so that the spot 221 of the first pulse laser beam 220 and the spot 231 of the second pulse laser beam 230 overlap on the surface of the work-piece 300. To achieve this, the light guiding unit 130 preferably includes a combining unit 131 for combining the first pulse laser beam 220 and the second pulse laser beam 230. In this case, the first pulse laser beam 220 and the second pulse laser beam 230 may be overlapped so that the optical axes of the first pulse laser beam 220 and the second pulse laser beam 230 coincide (become coaxial), or the first pulse laser beam 220 and the second pulse laser beam 230 may be overlapped so that the optical axes of the first pulse laser beam 220 and the second pulse laser beam 230 are misaligned (are not coaxial). The position of the spot 231 (corresponding to the cleaning portion) of the second pulse laser beam 230 relative to the spot 221 (corresponding to the processed portion) of the first pulse laser beam 220 can be adjusted as appropriate depending on the location of debris accumulation, etc.

[0022] The configuration of the multiplexing unit 131 is not particularly limited as long as it can achieve the above-mentioned functions. In the example shown in FIG. 1 , the multiplexing unit 131 is a second polarizing beam splitter 133. It is also possible to use a half mirror, a diffractive optical element (DOE), a refractive optical element (ROE), or the like as the splitting unit 120 and the multiplexing unit 131 instead of a polarizing beam splitter. However, when a half mirror, a diffractive optical element, or a refractive optical element is used, the light intensity ratio between the first pulse laser beam 220 and the second pulse laser beam 230 is fixed. However, when a polarizing beam splitter is used, the beam size and light intensity ratio between the first pulse laser beam 220 and the second pulse laser beam 230 can be changed. In this respect, it is preferable to use a polarizing beam splitter as the splitting unit 120 and the multiplexing unit 131.

[0023] Furthermore, the light guiding unit 130 controls the first pulsed laser beam 220 and the second pulsed laser beam 230 so that the size of the spot 221 of the first pulsed laser beam 220 is smaller than the size of the spot 231 of the second pulsed laser beam 230. To achieve this, the light guiding unit 130 preferably includes a beam size changing unit 132 that changes the beam size of at least one of the first pulsed laser beam 220 and the second pulsed laser beam 230 so that the beam size of the first pulsed laser beam 220 is smaller than the beam size of the second pulsed laser beam 230. For example, the beam size changing unit 132 may reduce the beam size of the first pulsed laser beam 220 or increase the beam size of the second pulsed laser beam 230. The configuration of the beam size changing unit 132 is not particularly limited as long as it can achieve the above function. In the example shown in FIG. 1 , the beam size changing unit 132 is a plano-concave lens 134 that is arranged on the optical path of the second pulsed laser beam 230 and is movable in the optical axis direction.

[0024] 1, the light guiding unit 130 includes a second polarizing beam splitter 133 (combining unit 131), a plano-concave lens 134 (beam size changing unit 132), one or more mirrors 135 for guiding the second pulsed laser beam 230 to the second polarizing beam splitter 133 through the plano-concave lens 134, and a focusing optical system 136 for guiding the first pulsed laser beam 220 and the second pulsed laser beam 230 to the work-piece 300.

[0025] As described above, the beam size changing unit 132 is not limited to the plano-concave lens 134. For example, a variable-focus lens, a transmission type diffractive optical element, or a transmission type phase modulation element (e.g., an LCOS-SLM) may be used instead of using the plano-concave lens 134. Furthermore, instead of using the plano-concave lens 134, the mirror 135 may be changed to a convex mirror, a deformable mirror, a reflection type diffractive optical element, or a reflection type phase modulation element (e.g., an LCOS-SLM).

[0026] The light intensity per unit area at the spot 221 of the first pulsed laser beam 220 for processing is higher than the light intensity per unit area at the spot 231 of the second pulsed laser beam 230 for cleaning. Here, "the light intensity per unit area at the spot 221 of the first pulsed laser beam 220" means the light intensity per unit area provided by the first pulsed laser beam 220 alone, and "the light intensity per unit area at the spot 231 of the second pulsed laser beam 230" means the light intensity per unit area provided by the second pulsed laser beam 230 alone. The method for adjusting the light intensities of the first pulsed laser beam 220 and the second pulsed laser beam 230 in this manner is not particularly limited. For example, the first pulsed laser beam 220 and the second pulsed laser beam 230 may be split by the splitting unit 120 so as to achieve a predetermined light intensity ratio, or the light intensities of the first pulsed laser beam 220 and / or the second pulsed laser beam 230 split by the splitting unit 120 may be individually adjusted. As described above, in the example shown in FIG. 1, the light intensity of first pulsed laser beam 220 is adjusted by ½λ wave plate 121 to be higher than the light intensity of second pulsed laser beam 230.

[0027] The light intensity per unit area of ​​the spot 221 of the first pulse laser beam 220 and the light intensity per unit area of ​​the spot 231 of the second pulse laser beam 230 can be set appropriately depending on the work-piece 300. For example, when the work-piece 300 is silicon, the light intensity per unit area of ​​the spot 221 of the first pulse laser beam 220 is 4×10 5 ~8×10 5 μJ / cm 2 The light intensity per unit area of ​​the spot 231 of the second pulse laser beam 230 is about 1×10 5 ~3×10 5 μJ / cm 2 When the workpiece 300 is copper, the light intensity per unit area of ​​the spot 221 of the first pulse laser beam 220 may be about 8 to 40 J / cm. 2The light intensity per unit area of ​​the spot 231 of the second pulse laser beam 230 is about 0.2 to 0.6 J / cm 2 When the workpiece 300 is made of invar, the light intensity per unit area of ​​the spot 221 of the first pulse laser beam 220 may be about 2 to 20 J / cm. 2 The light intensity per unit area of ​​the spot 231 of the second pulse laser beam 230 is about 0.1 to 0.4 J / cm 2 It may be to some extent.

[0028] [Laser processing method] FIG. 2 is a flowchart of a laser processing method according to an embodiment of the present invention.

[0029] As shown in FIG. 2, a laser processing method according to one embodiment of the present invention includes a step of splitting a pulse laser beam into a first pulse laser beam and a second pulse laser beam (step S10), and a step of irradiating an object to be processed with the first pulse laser beam and the second pulse laser beam (step S20).

[0030] The laser processing method according to this embodiment can be carried out using, for example, the laser processing apparatus 100 according to the embodiment of the present invention (see FIG. 1). The laser processing method according to this embodiment will be described below in connection with the operation of the laser processing apparatus 100.

[0031] (Dividing process; process S10) In step S10, the pulse laser beam is split into a first pulse laser beam for processing and a second pulse laser beam for cleaning.

[0032] The pulse width and wavelength of the pulse laser beam are not particularly limited as long as they can perform the desired processing on the workpiece. For example, the pulse width of the pulse laser beam is preferably less than 1 picosecond, and more preferably within the range of 50 to 500 femtoseconds. In other words, the pulse laser beam is preferably a femtosecond pulse laser. Furthermore, the wavelength of the pulse laser beam is preferably 2000 nm or less, and more preferably within the range of 300 to 1500 nm.

[0033] In the laser processing apparatus 100 shown in Fig. 1, the splitting unit 120 splits the linearly polarized pulsed laser beam 210 emitted from the light source 110 into a first pulsed laser beam 220 and a second pulsed laser beam 230. As described above, the pulsed laser beam 210 has its polarization direction adjusted by the ½λ wave plate 121, and is then split into the first pulsed laser beam 220 (e.g., P-polarized) and the second pulsed laser beam 230 (e.g., S-polarized) by the first polarizing beam splitter 122. At this time, the angle of the ½λ wave plate 121 is adjusted in advance so that the light intensity per unit area at the spot 221 of the first pulsed laser beam 220 for processing is higher than the light intensity per unit area at the spot 231 of the second pulsed laser beam 230 for cleaning.

[0034] (Irradiation process; process S20) In step S20, the workpiece is irradiated with the first pulse laser beam and the second pulse laser beam such that the spot of the first pulse laser beam and the spot of the second pulse laser beam overlap and the spot size of the first pulse laser beam is smaller than the spot size of the second pulse laser beam.

[0035] In this step, it is preferable to change the beam size of at least one of the first pulse laser beam and the second pulse laser beam so that the beam size of the first pulse laser beam is smaller than the beam size of the second pulse laser beam.

[0036] In the laser processing apparatus 100 shown in FIG. 1, the first pulsed laser beam 220 for processing is directly guided to the second polarizing beam splitter 133 (combining section 131) in the light guiding section 130. On the other hand, the second pulsed laser beam 230 for cleaning passes through a plano-concave lens 134 (beam size changing section 132) in the light guiding section 130 and is then guided to the second polarizing beam splitter 133 (combining section 131). The size of a spot 231 of the second pulsed laser beam 230 on the surface of the workpiece 300 (the size of the cleaning area) can be adjusted by moving this plano-concave lens 134 in a direction along the optical axis. In the second polarizing beam splitter 133 (combining section 131), the first pulsed laser beam 220 and the second pulsed laser beam 230 are coaxially combined.

[0037] The combined first pulse laser beam 220 and second pulse laser beam 230 are focused and irradiated onto the surface of the work-piece 300 through the focusing optical system 136. At this time, the first pulse laser beam 220 and the second pulse laser beam 230 or a stage on which the work-piece 300 is placed may be moved to scan the first pulse laser beam 220 and the second pulse laser beam 230 along the processing portion of the work-piece 300. In the examples described below, the first pulse laser beam 220 and the second pulse laser beam 230 were scanned along the processing portion of the work-piece 300 using a galvanometer mirror.

[0038] As described above, the angle of the ½λ wave plate 121 is adjusted so that the light intensity per unit area of ​​the spot 221 of the first pulsed laser beam 220 for processing is higher than the light intensity per unit area of ​​the spot 231 of the second pulsed laser beam 230 for cleaning. Furthermore, the position of the plano-concave lens 134 is adjusted so that the size of the spot 231 of the second pulsed laser beam 230 is appropriately larger than the size of the spot 221 of the first pulsed laser beam 220. As a result, as shown in FIG. 1 , the small spot 221 of the first pulsed laser beam 220 and the large spot 231 of the second pulsed laser beam 230 are concentrically positioned on the surface of the work-piece 300. Furthermore, the light intensity per unit area of ​​the spot 221 of the first pulsed laser beam 220 is higher than the light intensity per unit area of ​​the spot 231 of the second pulsed laser beam 230.

[0039] By irradiating the workpiece 300 with the first pulse laser beam 220 and the second pulse laser beam 230 in this manner, the processing portion of the workpiece 300 is processed by the first pulse laser beam 220, and at the same time, debris generated by the processing is removed by the second pulse laser beam 230. As a result, it is possible to reduce the accumulation and adhesion of debris around the processing portion.

[0040] It should be noted that since the first pulsed laser beam 220 and the second pulsed laser beam 230 are both derived from the common pulsed laser beam 210, there is no need to adjust the irradiation timing of the first pulsed laser beam 220 and the second pulsed laser beam 230.

[0041] [Debris removal mechanism] In the laser processing apparatus and laser processing method according to the present invention, it is believed that debris is removed by the following mechanism: However, the mechanism of debris removal is not limited to this.

[0042] When laser processing is performed, decomposition products are ejected from the area irradiated with the laser light. Among the ejected products, gaseous substances are dispersed into the surrounding area. On the other hand, molecular clusters and microparticle-like decomposition fragments of a certain size (more than a few tens of nanometers) are called debris, and do not disperse but instead fall and accumulate around the irradiated area (processed area). The chemical composition of this debris is not necessarily the same as that of the object being processed. For example, if the object being processed is a silicon wafer (single crystal silicon), the debris will include not only silicon clusters and silicon microcrystals, but also silicon oxide. Silicon oxide is formed when silicon atoms or silicon clusters in the ejected product combine with oxygen in the atmosphere.

[0043] Such debris not only accumulates on the surface of the workpiece near the processing location, but also adheres to it. There are two main possible mechanisms for adsorption: physical adsorption due to Coulomb force, and chemical adsorption, in which a chemical bond is formed between the debris and the surface of the workpiece. The mechanism by which debris is removed by irradiating it with a pulsed laser beam (cleaning laser) is thought to be as follows.

[0044] 1) The debris and the workpiece absorb the cleaning laser and are heated by a photothermal process, causing instantaneous thermal expansion and scattering. This mechanism of debris removal is effective for debris adsorbed by both physical and chemical adsorption mechanisms.

[0045] 2) The debris and the workpiece absorb the high-frequency cleaning laser, generating acoustic waves on the workpiece surface. These acoustic waves agitate the debris, promoting its detachment from the workpiece surface. This mechanism of debris removal is particularly effective for debris adsorbed by physical adsorption.

[0046] 3) If the chemical composition of the debris is different from that of the workpiece, the absorption coefficient of the debris to the cleaning laser may be greater than the absorption coefficient of the workpiece to the cleaning laser. In this case, the debris may be instantly photodecomposed and vaporized, and removed. This mechanism of debris removal is effective for debris adsorbed by both physical and chemical adsorption mechanisms.

[0047] 4) When the cleaning laser directly excites the chemical bond between the debris and the workpiece, this chemical bond is cleaved. The debris released from this chemical bond is then removed by the effects of 1) to 3) above. This mechanism of debris removal is particularly effective for debris adsorbed by chemical adsorption.

[0048] 5) When the cleaning laser photoionizes the debris and the workpiece, Coulomb repulsion occurs between the positively charged debris and the workpiece, promoting the detachment of the debris from the workpiece surface. This mechanism of debris removal is effective for debris adsorbed by both physical and chemical adsorption mechanisms.

[0049] What is important here is that when the cleaning laser is a nanosecond pulse laser, unless the workpiece and debris absorb the laser light, the above mechanisms 1) to 5) do not work effectively and the debris is not removed. On the other hand, when the cleaning laser is a femtosecond pulse laser, multiphoton absorption occurs easily, so all of the above mechanisms 1) to 5) work effectively regardless of the type of workpiece. In particular, mechanisms 4) and 5) only work when the laser is a femtosecond pulse laser. Therefore, using a femtosecond pulse laser as a cleaning laser, as in the present invention, is extremely effective in removing debris.

[0050] [Relationship between pulse width and processing mechanism] As mentioned above, the laser processing method of Patent Document 1 is thought to use a nanosecond pulse laser. When processing an object by irradiating it with a nanosecond pulse laser, the occurrence of damage on the surface of the object that becomes the starting point of processing is called "ablation." There are two mechanisms for this ablation: the "photothermal mechanism" and the "photochemical mechanism."

[0051] (Photothermal Ablation) When the workpiece absorbs the laser light, it becomes electronically excited, and a high density of electronically excited states of the material that makes up the workpiece is generated at the laser light irradiated area. This electronically excited state quickly converts the excitation energy into vibrational energy (phonons) of the material (this is called internal conversion in the case of molecules), and a high density of vibrationally excited states is generated. This vibrational energy is released as heat at the irradiated area (this is called vibrational relaxation). As a result, the temperature at the irradiated area rises rapidly, and the material that makes up the workpiece is thermally decomposed. The temperature, pressure, and volume then rise rapidly, and the irradiated area ejects and removes decomposed materials, causing damage to the surface. This is the mechanism of photothermal ablation.

[0052] (Photochemical Ablation) When the workpiece absorbs laser light, it becomes electronically excited, generating a high density of electronically excited states in the material that makes up the workpiece at the irradiated area of ​​the laser light. If the energy potential curve of the electronically excited state intersects with the dissociative potential, there is a certain probability that it will switch to the dissociative potential, causing bond dissociation in the material (called predissociation). It is also possible for the material to be directly photoexcited from its ground state to this dissociative potential, in which case the dissociation is called direct dissociation. As a result, this bond dissociation is induced at a high density, causing the material to decompose, resulting in a sudden increase in pressure and volume, ejecting and removing decomposed materials from the irradiated area and causing damage to the surface. This is the mechanism of photochemical ablation.

[0053] When nanosecond pulsed lasers are used, photothermal ablation dominates. As the wavelength of the laser light decreases from the visible to the ultraviolet range, the contribution of photochemical ablation increases. However, pure photochemical ablation does not occur when using nanosecond pulsed lasers.

[0054] In contrast, the mechanism of laser processing (damage induction) using femtosecond pulsed lasers is completely different. With femtosecond pulsed lasers, photons are concentrated in an extremely short femtosecond time span and irradiated onto the workpiece, facilitating multiphoton absorption, such as three- or four-photon absorption, which is difficult to induce with nanosecond pulsed lasers. As a result, the workpiece is easily excited to a higher electronically excited state (compared to that irradiated with nanosecond pulsed lasers). In such an electronically excited state, there are countless dissociation potentials, so the material that makes up the workpiece easily decomposes to the atomic level or even to extremely small molecular clusters, and is removed as gas. In other words, damage is induced without thermal disturbance. Photoionization also occurs in parallel. When high-density photoionization occurs due to femtosecond pulsed laser irradiation, electrons are emitted from the workpiece's surface, creating a high density of positive charges on the workpiece's surface, which then decomposes and removes the irradiated area through Coulomb explosion.

[0055] As described above, the mechanisms of laser processing using nanosecond pulsed lasers and femtosecond pulsed lasers are significantly different. As a result, the chemical composition, size, and charge of the debris that is decomposed and / or removed are significantly different. Generally, nanosecond pulsed laser processing mainly generates debris of micron size or larger, while femtosecond pulsed laser processing often generates a large amount of small debris of submicron size. Such small debris is highly dependent on surface area, and when it adheres to the surface of the workpiece, the adhesion force becomes strong, making it difficult to remove. Therefore, it is necessary to remove debris using the laser processing device and laser processing method according to the present invention.

[0056] [effect] As described above, the laser processing apparatus and laser processing method according to an embodiment of the present invention can reduce the accumulation and adhesion of debris on the workpiece, even when using a femtosecond pulse laser. Furthermore, the laser processing apparatus and laser processing method according to an embodiment of the present invention can adjust the light intensity per unit area, spot size, position, etc. of the first pulse laser beam spot and the second pulse laser beam spot separately, so that laser processing and debris removal can be performed under optimal conditions for each.

[0057] The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples. [Example]

[0058] [Example 1] Silicon wafers were processed using the laser processing apparatus shown in Figure 1. The pulsed laser beam had a wavelength of 513 nm, a pulse width of 263 fs, and an oscillation repetition rate of 50 kHz. Using a galvanometer scanner and an fθ lens, the surface of the silicon wafer was scanned with the first and / or second pulsed laser beam at a speed of 100 mm / s. The spot diameter of the first pulsed laser beam used for processing was 5.6 μm (calculated value), the pulse energy was 0.1 to 0.2 μJ, and the light intensity per unit area at the spot was 0.4 to 0.8 J / cm. 2 The spot diameter of the second pulsed laser beam for cleaning was ~80 μm (estimated value), the pulse energy was 10 μJ, and the light intensity per unit area at the spot was 0.2 J / cm. 2 The silicon wafer after being irradiated with the pulsed laser beam was observed using an industrial microscope (STM7, Olympus Corporation).

[0059] FIG. 3A is a photograph of a silicon wafer after being irradiated with only the first pulse laser beam for processing, FIG. 3B is a photograph of a silicon wafer after being irradiated with only the second pulse laser beam for cleaning, and FIG. 3C is a photograph of a silicon wafer after being irradiated simultaneously with the first pulse laser beam and the second pulse laser beam.

[0060] As shown in Figure 3A, when only the first pulsed laser beam was used, silicon scribing was possible, but debris accumulated around the etched groove. As shown in Figure 3B, when only the second pulsed laser beam was used, no change was observed on the silicon surface. On the other hand, as shown in Figure 3C, when both the first and second pulsed laser beams were used simultaneously, silicon scribing was possible, and the amount of debris was significantly reduced compared to when only the first pulsed laser beam was used (see Figure 3A).

[0061] [Example 2] The copper plate was processed using the laser processing apparatus shown in Figure 1. The pulsed laser beam had a wavelength of 513 nm, a pulse width of 263 fs, and an oscillation repetition rate of 20 kHz. Using a galvanometer scanner and an fθ lens, the surface of the copper plate was scanned with the first and / or second pulsed laser beam at a speed of 100 mm / s. The spot diameter of the first pulsed laser beam used for processing was 5.6 μm (calculated value), the pulse energy was 2 to 10 μJ, and the light intensity per unit area at the spot was 8 to 40 J / cm. 2 The spot diameter of the second pulsed laser beam for cleaning was ~80 μm (estimated value), the pulse energy was 10 μJ, and the light intensity per unit area at the spot was 0.4 J / cm. 2 The copper plate after being irradiated with the pulsed laser beam was observed using an industrial microscope (STM7, Olympus Corporation).

[0062] FIG. 4A is a photograph of the copper plate after being irradiated only with the first pulsed laser beam for processing, FIG. 4B is a photograph of the copper plate after being irradiated only with the second pulsed laser beam for cleaning, and FIG. 4C is a photograph of the copper plate after being irradiated simultaneously with the first pulsed laser beam and the second pulsed laser beam.

[0063] As shown in Figure 4A, when only the first pulsed laser beam was used, copper scribing was possible, but debris accumulated around the etched groove. As shown in Figure 4B, when only the second pulsed laser beam was used, almost no change was observed on the copper surface. On the other hand, as shown in Figure 4C, when both the first and second pulsed laser beams were used simultaneously, copper scribing was possible, and the amount of debris was significantly reduced compared to when only the first pulsed laser beam was used (see Figure 4A). [Industrial Applicability]

[0064] The laser processing apparatus and the laser processing method according to the present invention are useful, for example, in the manufacture of semiconductor elements. [Explanation of symbols]

[0065] 100 Laser processing equipment 110 Light source 120 Division 121 1 / 2λ wave plate 122 First polarizing beam splitter 130 Light guide section 131 Multiplexing section 132 Beam size change unit 133 Second Polarizing Beam Splitter 134 Plano-concave lens 135 Mirror 136 Condensing Optical System 210 Pulsed Laser Beam 220 First pulse laser beam 221 First pulse laser beam spot 230 Second Pulse Laser Beam 231 Second pulse laser beam spot 300 Processing Objects

Claims

1. a light source for emitting a pulsed laser beam; a splitter for splitting the pulsed laser beam into a first pulsed laser beam and a second pulsed laser beam; a light guiding unit for guiding the first pulse laser beam and the second pulse laser beam to a workpiece such that a spot of the first pulse laser beam and a spot of the second pulse laser beam overlap each other and the size of the spot of the first pulse laser beam is smaller than the size of the spot of the second pulse laser beam; A laser processing device having the above structure.

2. 2. The laser processing apparatus according to claim 1, wherein the light guiding unit has a beam size changing unit for changing the beam size of at least one of the first pulse laser beam and the second pulse laser beam so that the beam size of the first pulse laser beam is smaller than the beam size of the second pulse laser beam.

3. 2. The laser processing apparatus according to claim 1, wherein the light guide section has a combining section for combining the first pulse laser beam and the second pulse laser beam.

4. 2. The laser processing device according to claim 1, wherein a light intensity per unit area of ​​a spot of the first pulse laser beam is higher than a light intensity per unit area of ​​a spot of the second pulse laser beam.

5. the pulsed laser beam has a pulse width of less than 1 picosecond; The wavelength of the pulsed laser beam is 2000 nm or less. The laser processing device according to claim 1.

6. the splitting unit includes a first polarizing beam splitter for splitting the pulsed laser beam into the first pulsed laser beam and the second pulsed laser beam, the combining unit includes a second polarizing beam splitter for combining the first pulse laser beam and the second pulse laser beam; The laser processing device according to claim 3.

7. 3. The laser processing device according to claim 2, wherein the beam size changing unit has a plano-concave lens disposed on an optical path of the second pulse laser beam and movable in the direction of an optical axis.

8. splitting the pulsed laser beam into a first pulsed laser beam and a second pulsed laser beam; irradiating the workpiece with the first pulse laser beam and the second pulse laser beam so that a spot of the first pulse laser beam and a spot of the second pulse laser beam overlap each other and a size of the spot of the first pulse laser beam is smaller than a size of the spot of the second pulse laser beam; A laser processing method comprising:

9. 9. The laser processing method according to claim 8, wherein in the step of irradiating the first pulse laser beam and the second pulse laser beam onto the workpiece, a beam size of at least one of the first pulse laser beam and the second pulse laser beam is changed so that the beam size of the first pulse laser beam is smaller than the beam size of the second pulse laser beam.

10. 9. The laser processing method according to claim 8, wherein a light intensity per unit area of ​​a spot of the first pulse laser beam is higher than a light intensity per unit area of ​​a spot of the second pulse laser beam.

11. the pulsed laser beam has a pulse width of less than 1 picosecond; The wavelength of the pulsed laser beam is 2000 nm or less. The laser processing method according to claim 8.

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