Patent Publication Number: US-2023150057-A1

Title: Laser cutting method for metal foil

Description:
REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of International Application No. PCT/JP2021/027216, filed on Jul. 20, 2021 which claims the benefit of priority of the prior Japanese Patent Application No. 2020-123947, filed on Jul. 20, 2020, the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present disclosure relates to a laser cutting method for metal foil. 
     Laser cutting by application of a laser beam is known as one of methods to cut processing targets made of metal materials. Laser cutting is a method of cutting a processing target by applying a laser beam onto the part of the processing target and melting the part with the energy of the laser beam (see, for example, Patwa, Rahul, et al. “High speed laser cutting of electrodes for advanced batteries.” International Congress on Applications of Lasers &amp; Electro Optics. 2010.). 
     SUMMARY OF THE INVENTION 
     When the processing target is a metal foil, because the metal foil is easily deformed or torn, it may be difficult to obtain the required quality, in the case where various parameters for laser cutting are set in the same manner as for thicker metal members. 
     In cases where metal foils are applied to battery electrodes, for example, higher quality laser cutting with less dross and spatter is required. 
     Therefore, it is desirable to obtain a new and further improved laser cutting method for metal foil that is capable of cutting metal foil serving as a processing target by laser. 
     In some embodiments, a laser cutting method for metal foil includes: cutting a processing target by laser by intermittently applying a pulse of laser beam to a surface of a metal foil serving as the processing target at a frequency of 1 [MHz] or less. 
     In the laser cutting method for metal foil, when the frequency is A [Hz] and a full width at half maximum of the pulse is B [s], a duty ratio Rd [%] expressed by following equation Rd=A×B×100 . . . (1) may be 0.1 or more and 80 or less. 
     The above and other objects, features, advantages and technical and industrial significance of this disclosure will be better understood by reading the following detailed description of presently preferred embodiments of the disclosure, when considered in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is an exemplary schematic diagram of a laser cutting device according to a first embodiment; 
         FIG.  2    is an exemplary schematic diagram of a laser device included in the laser cutting device according to the first embodiment; 
         FIG.  3    is a timing chart illustrating an example of the change over time of output of a laser beam applied from the laser cutting device according to the first embodiment; 
         FIG.  4    is an enlarged view of one pulse of the laser beam in  FIG.  3   ; 
         FIG.  5    is a photographed image in a plan view of an example of a cut edge and its vicinity of a metal foil of 100 [μm] thickness as a processing target cut with the laser cutting device according to the embodiment; 
         FIG.  6    is a photographed image in a side view of the cut edge of the metal foil in  FIG.  5   ; 
         FIG.  7    is a photographed image in a plan view of an example of the cut edge and its vicinity when the same metal foil as in  FIGS.  5  and  6    is cut by laser under different conditions from those in  FIGS.  5  and  6    as a reference example; 
         FIG.  8    is a photographed image in a side view of the cut edge of the metal foil in  FIG.  7   ; 
         FIG.  9    is a photographed image in a plan view of an example of the cut edge and its vicinity of metal foil of 10 [μm] thickness as a processing target cut with the laser cutting device according to the embodiment; 
         FIG.  10    is a photographed image in a side view of the cut edge of the metal foil in  FIG.  9   ; 
         FIG.  11    is a photographed image in a plan view of an example of the cut edge and its vicinity when the same metal foil as in  FIGS.  9  and  10    is cut by laser under different conditions from those in  FIGS.  9  and  10    as a reference example; 
         FIG.  12    is a photographed image in a side view of the cut edge of the metal foil in  FIG.  11   ; 
         FIG.  13    illustrates experimental results of laser cutting according to duty ratio and index for a plurality of samples of metal foil; and 
         FIG.  14    is an exemplary schematic diagram of a laser cutting device according to a second embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Exemplary embodiments of the disclosure will be disclosed below. Configurations of the embodiments illustrated below, as well as the actions and results (effects) provided by the configuration, are examples. The disclosure can also be achieved by configurations other than those disclosed in the following embodiments. In addition, according to the disclosure, it is possible to obtain at least one of various effects (including derivative effects) obtained by the configuration. 
     The embodiments illustrated below have similar configurations. Therefore, according to the configurations of the embodiments, similar actions and effects based on the similar configurations can be obtained. In the following description, those similar configurations will be provided with similar symbols, and overlapping explanations may be omitted. 
     In each of the drawings, an X direction is represented by an arrow X, a Y direction by an arrow Y, and a Z direction by an arrow Z. The x-, y-, and z-directions intersect and are orthogonal to each other. The Z direction is the normal direction of a surface Wa (machined surface) of a processing target W. Although the X direction is illustrated as a sweep direction SD in some drawings, the sweep direction SD is not limited to the X direction, as long as it intersects the Z direction. 
     First Embodiment 
     Configuration of Laser Cutting Device 
       FIG.  1    is a schematic diagram of a laser cutting device  100  according to a first embodiment. The laser cutting device  100  includes a laser device  110 , an optical head  120 , an optical fiber  130  connecting the laser device  110  and the optical head  120 , and a controller  140 . 
     A processing target W of the laser cutting device  100  is made of, for example, a conductive metal material. The metal material is, for example, a copper-based material, such as copper and a copper alloy, and/or an aluminum-based material, such as aluminum and an aluminum alloy. In such cases, the processing target W can also be referred to as a metal conductor. 
     The processing target W is, for example, a metal foil  10  with a thickness of  500  [pm] or less, but is not limited thereto. 
     The metal foil  10  may also be an electrode, for example, of a battery, such as a lithium-ion battery. In this case, the metal foil  10  may be coated with an active material, such as manganese dioxide or lithium. The laser cutting device  100  may continuously cut both a part coated with the active material and a part not coated with the active material in a single sweep. The active material can also be referred to as a film, a coating film, a coating material, a surface layer, or a surface layer material. The metal foil  10  as the processing target W of the laser cutting device  100  is not limited to an electrode of a battery. For example, the metal foil  10  may be coated with a different material other than the active material, it may have a surface layer, such as a plating layer, it may have a film or a surface layer formed over the entire surface. It may have parts where a film or a surface layer is formed and parts where no film or surface layer is formed, or it may have no film or surface layer. 
     The laser device  110  includes a laser oscillator and is configured to output a single-mode laser beam of several kW power, as an example. The laser device  110  will be described below. 
     The optical fiber  130  guides the laser beam output from the laser device  110  to the optical head  120 . When the laser device  110  outputs a single-mode laser beam, the optical fiber  130  is configured to propagate the single-mode laser beam. In this case, the M 2  beam quality of the single-mode laser beam is set to 1.2 or less. When the laser device  110  outputs a multi-mode laser beam, the optical fiber  130  is configured to propagate the multimode laser beam. 
     The optical head  120  is an optical device for applying the laser beam input from the laser device  110  to the processing target W. The optical head  120  includes a collimating lens  121  and a condensing lens  122 . The collimating lens  121  and the condensing lens  122  can also be referred to as optical components. The optical head  120  may include optical components other than the collimating lens  121  and the condensing lens  122 . 
     In the present embodiment, the optical head  120  is configured to be able to change its position relative to the processing target W to sweep the laser beam L while applying the laser beam L on the processing target W. Relative movement of the optical head  120  and the processing target W can be achieved by movement of the optical head  120 , movement of the processing target W, or movement of both the optical head  120  and the processing target W. 
     The collimating lens  121  collimates the input laser beam. The collimated laser beam becomes a collimated light. The condensing lens  122  converges the laser beam as a collimated light, and applies it as a laser beam L (output light) onto the processing target W. 
     With this configuration, the optical head  120  applies the laser beam L to the surface Wa of the processing target W in the opposite direction of the Z direction. The application direction of the laser beam L from the optical head  120  is the opposite direction of the Z direction. The optical head  120  can, for example, converge the laser beam L such that, for example, the beam diameter is 10 [μm] or more. 
     The controller  140  controls operations of the laser device  110  and operations of the optical head  120  or a drive mechanism of a stage supporting the processing target W. 
     Laser Device 
       FIG.  2    is a schematic diagram of an example of the laser device  110 . The laser device  110  can also be referred to as a light source device. The laser device  110  in  FIG.  2    is a CW laser capable of outputting a laser beam continuously. The laser device  110  can also be referred to as a laser oscillator. 
     The laser device  110  illustrated in  FIG.  2    is an optical fiber laser, and includes a plurality of semiconductor pumping light sources  1 , a plurality of optical fibers  2 , an optical multiplexer  3 , an optical fiber Bragg grating (FBG)  4 , an optical amplifying fiber  5 , an FBG  7 , an optical multiplexer  8 , a plurality of optical fibers  9 , a plurality of semiconductor pumping light sources  6 , and an output optical fiber  11 . The elements are connected by optical fibers as appropriate. The output optical fiber  11  is optically coupled to the optical fiber  130  illustrated in  FIG.  1    or is part (input end) of the optical fiber  130 . 
     Each of the semiconductor pumping light sources  1  serving as pumping light sources outputs pumping light to be supplied to the optical amplifying fiber  5 . The pumping light has a wavelength that can optically excite the optical amplifying fiber  5 , for example,  915  [nm]. Each of the optical fibers  2  propagates the pumping light output from each semiconductor pumping light source  1  and outputs the pumping light to the optical multiplexer  3 . 
     The optical multiplexer  3  is formed of a tapered fiber bundle (TFB) in the present embodiment. The optical multiplexer  3  multiplexes the pumping light input from each optical fiber  2  into the optical fiber of the signal light port and outputs the multiplexed pumping light to the optical amplifying fiber  5 . 
     The optical amplifying fiber  5  is an ytterbium doped fiber (YDF) in which ytterbium (Yb) ions serving as an amplifying substance are added to a core part made of quartz glass, and is a double clad optical fiber in which an inner cladding layer made of quartz glass and an outer cladding layer made of resin or the like are formed sequentially around the periphery of the core part. The core part of the optical amplifying fiber  5  has a NA of, for example, 0.08 and is configured to propagate light emitted by Yb ions, for example, at a wavelength of 1070 [nm], in a single mode. The absorption coefficient of the core part of the optical amplifying fiber  5  is 200 [dB/m] at a wavelength of 915 [nm], for example. The power conversion efficiency from the pumping light input to the core part to the laser oscillation light is, for example, 70%. 
     The FBG  4  serving as the rear-end reflection unit is connected between the optical fiber of the signal light port of the optical multiplexer  3  and the optical amplifying fiber  5 . The FBG  4  has a center wavelength of, for example, 1070 [nm], and has a reflectivity of approximately 100% in the wavelength bandwidth of the center wavelength and a width of approximately 2 [nm] around it, so that light with a wavelength of 915 [nm] is mostly transmitted. The FBG  7  serving as the output-side reflection unit is connected between the optical fiber of the signal optical port of the optical multiplexer  8  and the optical amplifying fiber  5 . The FBG  7  has a center wavelength of, for example, 1070 [nm], which is substantially the same as that of the FBG  4 , a reflectivity of approximately 10% to 30% at the center wavelength, a full width at half maximum in the reflected wavelength bandwidth of approximately 1 [nm], so that almost all light with a wavelength of 915 [nm] is transmitted. 
     The FBGs  4  and  7  are placed against the respective ends of the optical amplifying fiber  5 , and form an optical fiber resonator for light with a wavelength of 1070 [nm]. 
     Each of the semiconductor pumping light sources  6  serving as pumping light sources outputs pumping light to be supplied to the optical amplifying fiber  5 . The pumping light has a wavelength that can optically excite the optical amplifying fiber  5 , for example, a wavelength of 915 [nm]. The optical fibers  9  propagate the pumping light output from the respective semiconductor pumping light sources  6  and output the light to the optical multiplexer  8 . 
     The optical multiplexer  8  is formed of a TFB in the present embodiment, like the optical multiplexer  3 . The optical multiplexer  8  multiplexes the pumping light input from each of the optical fibers  9  into the optical fiber of the signal light port and outputs the multiplexed pumping light to the optical amplifying fiber  5 . 
     In the optical amplifying fiber  5 , Yb ions in the core part are optically excited by the pumping light and emit light in the bandwidth including the wavelength of 1070 [nm]. The light emitted at a wavelength of 1070 [nm] is laser-oscillated by optical amplification action of the optical amplifying fiber  5  and action of the optical resonator formed of the FBGs  4  and  7 . 
     The output optical fiber  11  is located on the opposite side of the FBG  7  and is connected to the optical fiber of the signal light port of the optical multiplexer  8 . The oscillated laser beam (laser oscillated light) is output from the output optical fiber  11 . The laser device  110  operates as a pulsed laser outputting pulses of laser beam under the control of the controller  140  as described below. However, the laser device  110  may be a pulsed laser generating pulses using other methods or a laser device using an optical amplification method and different from an optical fiber laser. 
     Laser Cutting Method 
     In laser cutting using the laser cutting device  100 , the processing target W is first set so that the laser beam L is applied. Then, the laser beam L and the processing target W move relative to each other while the laser beam L is applied to the processing target W. This causes the laser beam L to move (sweep) in the sweep direction SD on the surface Wa while being applied onto the surface Wa. The part irradiated with the laser beam L melts and is cut. 
     Intermittent Application 
       FIG.  3    is a timing chart illustrating the change over time of the power of the laser beam L.  FIG.  4    is an enlarged view of a portion (one pulse) of  FIG.  3   . 
     In laser cutting, if a strong laser beam L strikes the metal foil  10 , the cut edges may bend or flip. However, if the output of the laser beam L is reduced, laser cutting takes more time. Therefore, after diligent research, the inventors have found that when the processing target W is a metal foil  10 , better results can be obtained in a shorter processing time by intermittently (intermittently) applying the laser beam L at a predetermined frequency to the surface Wa, as illustrated in  FIGS.  3  and  4   . 
     Through experimental studies, the inventors have found that the frequency of the pulse of the laser beam L (1/A, A: pulse period [s], see  FIGS.  3  and  4   ) is suitably to be 6 [KHz] or more and 1 [MHz] or less. 
     Through experimental studies, the inventors have also found a suitable range for the duty ratio Rd of pulses of the laser beam L. Here, the controller  140  controls the laser device  110  to switch between on and off, i.e., an output state (operating state) and a stop state (non-output state, non-operating state), at a high frequency as described above. Specifically, for example, the controller  140  outputs a control signal such that power in a rectangular waveform in time is supplied to the semiconductor pumping light sources  1  and  6 . In this case, the variation over time of the output of the laser beam L by the laser device  110  does not have a perfect rectangular waveform, but has a dull waveform, as illustrated in  FIGS.  3  and  4   , as an example. Therefore, the inventors have defined the duty ratio Rd [%] of the pulses of the laser beam L output from the laser device  110  as in the following equation (1). 
         Rd=A×B× 100   (1)
 
     Here, B [s] (see  FIG.  4   ) is the full width at half maximum of the pulse, which is the width (time width) of the value that is half value ( 0 . 5  Pmax) of the maximum value Pmax [W] of the pulse, and if there are a plurality of times with 0.5 Pmax within one pulse, it is the time width between the two farthest times. The inventors&#39; experimental studies have revealed that a duty ratio Rd [%] of 0.1 to 80 is suitable, and a duty ratio of 0.2 to 40 is even more suitable. 
     Through experimental studies, the inventors have also found a suitable range for the waveform of the pulses. Here, the inventors have defined the pulse ratio Rp as in the following equation (2). 
         Rp=F/B    (2)
 
     Here, F is a 1/e 2  width of the pulse and B is the full width at half maximum of the pulse. The 1/e 2  width of a pulse is defined as the width of the pulse being 1/e 2  (≈0.135) relative to the maximum value Pmax of the pulse, and if there are a plurality of values that is 1/e 2  within a pulse, it is the time width between the two farthest times. The pulse ratio Rp is an index indicating the spread of pulses, and a higher value indicates a wider pulse spread. The inventors&#39; experimental studies have revealed that a pulse ratio Rp of 1 to 7 is suitable. It has been also found that the M 2  beam quality of the laser beam L is suitably 1.2 or less and the energy applied by the pulse is suitably 0.1 [mJ] or more. 
     Furthermore, through experimental studies, the inventors have found a suitable range for the index I expressed in equation (3) below. 
         I=A×B×C×D×E    (3)
 
     Here, C [m/s] is the sweep rate of the laser beam L, D [m] is the spot diameter of the laser beam L on the surface Wa of the processing target W, and E[W] is the output of the laser beam L. The spot diameter D (beam diameter) is defined as the diameter of an area of power that includes the peak of power at that spot and is 1/e 2  or more of the peak power. In the case where the spot is not circular, the spot diameter can be defined as the length of the area in a direction perpendicular to the sweep direction SD where the power is 1/e 2  or more of the peak power. The power distribution at the spot is not limited to a Gaussian shape. The inventors&#39; experimental studies have revealed that an index I of 1.0×10 −7  or more and 1.0×10 −1  or less is suitable. 
     Experimental Results 
       FIGS.  5  and  6    illustrate the experimental results in the case where the metal foil  10  serving as the processing target W is an aluminum foil of thickness 100 [μm] as an electrode with the surface coated with active material, the duty ratio Rd is 6.1, and the index I is 3.84×10 −3 .  FIG.  5    is a plan view of a cut edge  10   a  and its vicinity, and  FIG.  6    is a side view of the cut edge  10   a . As is clear from  FIGS.  5  and  6   , under these conditions, a good cut edge  10   a  (processed state) is obtained, with little or no bending, flipping, dross, spatter, etc. 
       FIGS.  7  and  8    illustrate the experimental results as a reference example in the case where the application conditions of the laser beam L are changed for the same processing target W as in  FIGS.  5  and  6   . In  FIGS.  7  and  8   , the duty ratio Rd is 100 and the index I is 3.84×10 −3 . A duty ratio Rd of 100 means that the laser beam L is continuously applied, not intermittent application of a pulsed laser beam L.  FIG.  7    is a plan view of a cut edge  10   a  and its vicinity, and  FIG.  8    is a side view of the cut edge  10   a . As is clear by comparing  FIGS.  7  and  8    with  FIGS.  5  and  6   , deformation of the cut edge  10   a  is greater under these conditions, with more flipping and bending. 
       FIGS.  9  and  10    illustrate the experimental results in the case where the metal foil  10  serving as the processing target W is a copper foil of thickness 10 [μm], the duty ratio Rd is  6 . 1 , and the index I is 3.84×10 −3 .  FIG.  9    is a plan view of a cut edge  10   a  and its vicinity, and  FIG.  10    is a side view of the cut edge  10   a . As is clear from  FIGS.  9  and  10   , under these conditions, a good cut edge  10   a  (processed state) is obtained, with little or no bending, flipping, dross, spatter, etc. 
       FIGS.  11  and  12    illustrate the experimental results as a reference example in the case where the application conditions of the laser beam L are changed for the same processing target W as in  FIGS.  9  and  10   . In  FIGS.  11  and  12   , the duty ratio Rd is 100 and the index I is 6.3×10 −2 . Since the duty ratio Rd is 100, the laser beam L is continuously applied in this case as well.  FIG.  11    is a plan view of a cut edge  10   a  and its vicinity, and  FIG.  12    is a side view of the cut edge  10   a . As is clear from  FIGS.  11  and  12   , under these conditions, dross  10   b  is generated and the cut edge  10   a  heavily flips or bents. 
     One metal foil  10 , such as aluminum foil or copper foil, may have a coated region covered with a film, such as active material, and an exposed region that is not covered with the film. The inventors have confirmed through experimental studies that even in such a case, by properly setting the duty ratio Rd and index I according to specifications, such as the material and thickness of the metal foil  10  and the film, for example, a good processed state can be obtained in both the coated region and the exposed region, in continuous laser cutting of the coated and exposed regions by a single sweep of the laser beam L across the coated and exposed regions. Such a method makes it possible to perform laser cutting more quickly on the metal foil  10  including coated and exposed regions, thereby reducing the time required to manufacture components and products containing the metal foil  10 . 
     In the manufacturing process of the metal foil  10 , the sweep rate may change, for example, when the cutting processing is started with replacement of the product lot, when the cutting processing is finished to replace the product lot, or when other problems occur. Changes in sweep rate may adversely affect the processing quality. Through experimental studies, the inventors have confirmed that even when the sweep rate varies in continuous laser cutting, a good processed state can be obtained by appropriately setting or changing, for example, the duty ratio Rd and/or the index I according to the sweep rate. Such a method can suppress the decline in processing quality due to changes in sweep rate, making it easier to ensure the required processing quality and increasing the processing yield. 
       FIG.  13    is a graph illustrating some of the experimental results in the case where the horizontal axis is the duty ratio Rd and the vertical axis is the index I. From  FIG.  13   , it can be understood that suitable laser cutting can be performed when each of the duty ratio Rd and the index I is within a predetermined range. 
     The inventors&#39; diligent research has also revealed that the spot diameter (beam diameter) D is preferably 100 [μm] or less, more preferably 50 [μm] or less, and even more preferably 30 [μm] or less. This is because, if the spot diameter D is too large, thermal energy is given to the outside of the cutting area (kerf), i.e., the processing target W, and quality defects, such as thermal oxidation, may occur. 
     Furthermore, as illustrated in  FIGS.  3  and  4   , when the pulse has a first peak P 1  and a second peak P 2  after the first peak P 1 , it has been found that the ratio Ro (=Pp 2 /Pp 1 , hereinafter referred to as output ratio Ro) of the output Pp 2  of the second peak P 2  to the output Pp 1  of the first peak P 1  is preferably 0.1 or more and 0.5 or less. For example, when a laser beam serving as an infrared light is applied to a material with relatively high reflectivity to the laser beam, such as a copper-based material and an aluminum-based material, it may be difficult to provide the energy necessary for cutting, depending on the pulse having only one peak. If the output of the second peak P 2  is higher than the output of the first peak P 1 , the thermal effect on the surroundings of the cutting area may become too large. In this respect, when the output ratio Ro is set to 0.1 or more and 0.5 or less, as described above, even for materials with relatively high reflectivity to the laser beam, first, the material is melted with the first peak P 1 , and then the second peak P 2 , which has a lower output than the first peak P 1 , ensures the amount of energy necessary and appropriate for cutting. This allows for more reliable cutting and also suppresses thermal effects on the surroundings of the cutting area, resulting in a good cut edge  10   a  with little or no bending, flipping, dross, spatter, etc. From this viewpoint, it is preferable that, before the area melted with the first peak P 1  solidifies, the area is irradiated with the second peak P 2 , which has a lower output than the first peak P 1 . The time interval between the first peak P 1  and the second peak P 2  can be 5.0 [psec] or less, for example. 
     It has been also found that a plurality of applications may be performed as in (1) or (2) below. 
     (1) The processing target W is cut by performing a plurality of sweeps on the same path under conditions where cutting of the processing target W is not completed in a single sweep. In the case where the surface of the processing target W is coated, for example, with brittle and easily breakable materials, such as ceramics and polymers, or active materials (hereinafter referred to as “coating materials”), the coating materials may be adversely affected, such as cracking or evaporation, if excessive energy is applied. In this respect, as described above, by performing the laser beam sweep a plurality of times under the condition where the cutting of the processing target W is not completed in a single sweep, it is possible to suppress adverse effects such as cracking or evaporation of the coating material caused by excessive energy input. 
     As for (1), when the thickness of the processing target W is 100 [μm] or more, it has been found that, by performing the sweep twice under the condition where the index I is 1.0×10 −3  or more and 1.0×10 −2  or less, it is possible to cut the processing target W while suppressing adverse effects on the coating material. 
     (2) First, the coating material is removed by laser beam application in the cutting area (area to be cut) and its vicinity, and then the processing target W is cut by performing the laser beam sweep of the present embodiment described above. In this case, laser cutting with higher energy efficiency and higher quality can be performed after eliminating the effects of the coating material. The laser beam may be swept along the same path in the process of removing the coating material and in the process of performing laser cutting. The laser beam applied in the process of removing the coating material may be pulsed or continuous wave. 
     As explained above, in the present embodiment, for example, the surface Wa of the metal foil  10  serving as the processing target W is intermittently irradiated with pulses of laser beam L at a frequency of 1 [MHz] or less to cut the processing target W by laser. 
     In the present embodiment, for example, the duty ratio Rd [%] of the pulses of the laser beam L may be 0.1 or more and 80 or less, or 0.2 or more and 40 or less. 
     In the present embodiment, for example, the pulse ratio Rp may be 1 or more and 7 or less, the M 2  beam quality of the laser beam L may be 1.2 or less, and the energy applied by the pulse may be 0.1 [mJ] or more. 
     In the present embodiment, for example, the index I may be 1.0×10 −7  or more and 1.0×10 −1  or less. 
     Such a method of cutting metal foil by laser, for example, enables higher quality cutting to be performed with less energy and shorter required time. 
     Second Embodiment 
       FIG.  14    is a schematic diagram of a laser cutting device  100 A according to a second embodiment. In the present embodiment, an optical head  120  includes a galvano scanner  126  between the collimating lens  121  and the condensing lens  122 . The galvano scanner  126  includes two mirrors  126   a . The change in attitude of these two mirrors  126   a  changes the application direction and application position of the laser beam L. In other words, the laser cutting device  100 A can move the application position of the laser beam L and sweep the laser beam L without moving the optical head  120 . The controller  140  can control the operation of each of motors  126   b  corresponding to respective mirrors  126   a  such that the angle (attitude) of the mirror  126   a  changes. The present embodiment also provides the same actions and effects as those of the first embodiment described above. 
     The embodiments of the disclosure illustrated above are examples and are not intended to limit the scope of the disclosure. The above embodiments can be implemented in various other forms, and various omissions, substitutions, combinations, and changes can be made without departing from the gist of the disclosure. In addition, each configuration, shape, and other specifications (structure, type, direction, model, size, length, width, thickness, height, number, arrangement, position, material, etc.) can be changed as needed. 
     When the laser beam is swept against the processing target, the sweep may be performed by known wobbling, weaving, or output modulation to adjust the surface area of the molten pool. 
     The processing target can also be a metal surface with a thin layer of another metal, such as a plated metal sheet. 
     The laser device (laser oscillator) is not limited to a CW laser, but may be, for example, a pulsed laser that can achieve intermittent application at a predetermined high frequency and a duty ratio of a predetermined range. 
     The disclosure can be used for laser cutting methods for metal foil. 
     According to the disclosure, a new and further improved laser cutting method for metal foil can be obtained, which enables laser cutting of metal foil as a processing target. 
     Although the disclosure has been described with respect to specific embodiments for a complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art that fairly fall within the basic teaching herein set forth.