JP2005021964A - Laser beam ablation processing method and device therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a laser beam ablation processing method capable of processing a high-aspect-ratio hole or groove with a fine hole opening or a fine line width, or capable of cutting a work with a fine line width, and to provide device therefor. <P>SOLUTION: An X-Y table 2, on which a work to be processed 13 is mounted, is located inside a pressure reducing chamber 11. A laser beam light source 14 which transmits pulses is provided in the outside of the pressure reducing chamber 11 and a laser light beam which is irradiated from the laser beam light source 14 is introduced into the inside of the pressure reducing chamber 11 through the window of the pressure reducing chamber 11 and is collimated by a magnification/collimate optical system 15. Then the laser light beam is formed to Bessel-beam by an axicon lens 16 and is irradiated onto the work to be processed 13. If required, gas contained in a gas cylinder 17 can be blown through a nozzle 18 as an assist gas to a laser beam irradiated region of the work to be processed 13. Again if desired, air(gas) inside the pressure reducing chamber 11 is evacuated. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an ablation processing method using a laser beam and an apparatus therefor, and more particularly, to a laser ablation processing method and an apparatus for processing with a high aspect ratio fine hole or fine line width.

Since ancient times, drilling has been performed by mechanical processing such as drilling. However, in mechanical machining, it is difficult to machine a fine hole diameter such that the diameter is less than 100 μm. In recent years, along with downsizing and high integration of products and parts, fine diameter through holes and blind holes (non-through holes), fine width groove processing, and fine width cutting processing are required. As a processing method that satisfies these requirements, there is ablation processing using a laser light pulse. This is because a laser light pulse with a pulse width of about 100 ns or less is focused on a minute spot with a convex lens or the like, and the power density is increased to about 100 MW / cm ² or more by a peak value, and the workpiece surface portion irradiated with this Is a processing method of removing by ablation (transpiration). The size (diameter) of a minute spot formed by condensing light such as a convex lens varies depending on the wavelength of the laser light, the focal length of the convex lens, and the like. When the wavelength of the laser beam is λ, the focal length of the convex lens is f, the laser beam has a top hat intensity distribution, and its beam diameter is a, the spot diameter φc in an ideal condensed state is approximate. Is expressed by the following equation (1).
φc = 2.44 λ f / a (1)
In addition, the focal depth zf of the spot in an ideal light collection state is approximately expressed by equation (2).
zf = 2 λ f ² / a ² (2)
For example, if a laser beam having a beam diameter of about 4 mm and a wavelength of 532 nm is condensed using a convex lens having a focal length of about 60 mm, the spot diameter is about 20 μm. However, since the focal depth of this spot is only about 250 μm at most, it is necessary to precisely adjust the position of the workpiece and the convex lens for condensing during processing.

  The surface of the workpiece is removed by ablation by irradiation with a laser beam pulse having a high peak power density focused on a minute spot, and a shallow hole is formed. When laser light pulses are repeatedly generated and irradiated (laser light pulse train irradiation), the next laser light pulse enters the inside of the hole through the opening of the shallow hole, and the workpiece surface inside the hole is ablated. Removed by. The depth of the hole is not gradually increased by the laser light pulse that is continuously irradiated. When the laser light pulse reaches a certain depth, the hole is not deepened even if the laser light pulse to be irradiated is increased. Accordingly, it is difficult to increase the depth d of the hole while keeping the diameter φ of the hole fine, that is, to perform drilling with a large aspect ratio d / φ.

  As a solution to this problem, each time a certain number of laser light pulses are irradiated, the stage on which the workpiece is placed is raised, so that the laser beam condensing spot (focal point) is gradually deepened inside the workpiece. There has been proposed a technique of inserting and machining a hole with a high aspect ratio while avoiding saturation of the depth of the machined hole (see, for example, Patent Document 1).

  As the drilling process progresses, the laser beam focusing spot (focal point) is sequentially inserted deeply into the workpiece by sequentially moving the lens focusing laser beam to the workpiece. A method of processing a hole having a high aspect ratio while avoiding saturation of the depth of the processed hole has also been proposed (for example, see Patent Document 2).

In the case where the workpiece is transparent to the laser beam, another type of technique is disclosed. For example, Non-Patent Document 1 reports drilling with a diameter of several μm and a depth exceeding 1 mm by repeatedly irradiating quartz glass with femtosecond laser light pulses (pulse width: 100 to 200 fs) having a wavelength of 790 nm.
Further, in Patent Documents 3 and 4, the focused spot of the laser beam pulse is positioned on the workpiece surface (back side) opposite to the surface on which the laser beam is incident, and the back side surface is ablated to sequentially collect the laser beam. A technique for processing a hole having a high aspect ratio by drawing a spot (focus) into the workpiece is disclosed.
JP-A-5-208288 JP 2002-307180 A JP 2001-212680 A JP 2003-20258 A Varel et al. Applied Physics A, 65, pp. 367-373, 1997

Conventionally, when drilling, grooving, or cutting with a fine hole opening or fine line width is carried out by ablation using a laser light pulse, the position of the workpiece and the lens for condensing are precise. There was a need to adjust to.
The methods reported in Non-Patent Document 1 and the methods disclosed in Patent Documents 3 and 4 are all limited to those that are transparent to the laser beam irradiated on the workpiece.
Further, the methods disclosed in Patent Documents 1 and 2 require a stage driving mechanism on which a workpiece is placed and a condensing lens moving mechanism, and the method and apparatus are complicated and expensive.
It is an object of the present invention, regardless of whether the workpiece is transparent or opaque to the laser beam, and does not require precise alignment for focusing with the workpiece and requires a complicated apparatus. It is an object of the present invention to provide a method and apparatus for processing a high-aspect-ratio hole, groove, or cut with no fine hole opening or fine line width.

  In order to achieve the above object, according to the present invention, in the laser ablation processing method in which the workpiece surface is removed by irradiating the workpiece surface with a laser beam, drilling, groove formation or cutting is performed. There is provided a laser ablation processing method, wherein a laser beam applied to a workpiece is a Bessel beam.

  In order to achieve the above object, according to the present invention, there is provided a laser ablation processing apparatus that removes a surface of a workpiece to form a hole, form a groove, or cut by irradiating the surface of the workpiece with a laser beam. There is provided a laser ablation processing apparatus including a laser light source, conversion optical means for converting a laser beam into a Bessel beam, and a table on which a workpiece is placed.

According to the present invention, regardless of whether the workpiece is transparent or opaque to the laser beam, precise alignment for the workpiece and light collection is unnecessary, and a complicated apparatus is required. There is provided a method and apparatus for processing fine hole openings or fine line width, high aspect ratio holes, grooves or cuts. The reason will be described below.
A feature of the present invention resides in that a workpiece is irradiated with a Bessel beam as a laser light pulse. The Bessel beam is described in detail in Japanese Patent Application No. 2002-339520. The center spot diameter φb of the Bessel beam is approximately expressed by equation (3), where θb is the intersection angle between the beam and the optical axis (laser beam propagation axis) (see FIG. 1).
φb = 2.405 λ / (π sin θb) (3)
The condensing spot diameter in the case of normal convex lens condensing can be rewritten into equation (4) by setting tan θc = a / (2 f) in equation (1).
φc = 1.22 λ / tan θc (4)
Here, the angle θc is an angle at which the outermost light beam of the laser light beam refracted by the convex lens toward the focal point intersects the optical axis (propagation axis) (see FIG. 2).

When a laser beam having a wavelength λ = 532 nm is used, the crossing angle necessary for obtaining the same spot diameter is calculated and shown in FIG. In any of the spot diameters, the crossing angle of the Bessel beam is smaller than that in the case of normal convex lens focusing.
FIG. 2 shows a state where drilling is performed with a spot condensed by a convex lens. When the surface of the workpiece 1 is irradiated with the laser beam 2 collected by the convex lens, a hole is formed by ablation. The inside of this hole advances while the next laser light pulse repeats reflection, but the angle between the laser beam outermost light and the propagation axis 3: the crossing angle θc is large, so many reflections are required to advance the unit distance. become. Further, the reflection angle θr with respect to the side surface of the hole is small, and as understood by Fresnel reflection, a lot of energy is lost for each reflection. Therefore, in the case of the spot condensed by the convex lens, the energy reaching the hole bottom is only small.
FIG. 1 shows the state of drilling with a Bessel beam. When the surface of the workpiece 1 is irradiated with the laser beam 2 which is a Bessel beam, a hole is formed by ablation, and the next laser beam pulse is repeatedly reflected inside the hole. In this case, the crossing angle θb And the number of reflections required to travel a unit distance is small. Further, the reflection angle θr with respect to the side surface of the hole is large, and as is understood by Fresnel reflection, the energy lost for each reflection is small. Therefore, in the case of a Bessel beam spot, much of the energy of the laser beam that has passed through the aperture reaches the bottom of the hole. Therefore, when the Bessel beam having a small crossing angle is used, the laser beam reaches the deeper hole bottom while maintaining high energy, so that a deep hole can be processed.

In FIG. 4A, a laser beam having a wavelength distribution of 532 nm and a beam diameter (1 / e ² ) of 4 mm and having a Gaussian intensity distribution is generated using a condensing means such as an axicon lens or a diffractive optical element. The light intensity distribution on the propagation axis of a Bessel beam having an angle θb of about 1.17 ° is shown. In this case, the spot diameter is about 20 μm.
Further, the light intensity on the propagation axis has a maximum value when the propagation distance from the light collecting means is about 50 mm and maintains a value of 80% of the maximum intensity (focus depth, about 30 mm to about 30 mm). (Up to 75 mm) extends to a length of 45 mm, and at least in this section, the light intensity distribution in the beam cross section shown in FIG. 4B is maintained. In a normal convex lens condensing, the Bessel beam has a much deeper depth of focus when the condensing spot diameter is about 20 μm and is about 250 μm. Eliminates the need for precise adjustment of the lens position.
Hereinafter, the present invention will be described in detail with reference to embodiments and examples, but the present invention is not limited thereto. It will be apparent to those skilled in the art that various modifications, improvements, combinations, and the like can be made without departing from the scope of the present invention.

FIG. 5 is a cross-sectional view showing a schematic configuration of the laser ablation processing apparatus of the present invention. Although not an essential component, the XY table 2 on which the workpiece 13 is placed is placed in the decompression chamber 11. A laser light source 14 that pulsates is disposed outside the decompression chamber 11, and a laser light beam emitted from the laser light source 14 is introduced into the decompression chamber 11 through a window of the decompression chamber 11, and an expansion / collimating optical system. After being collimated by 15, it is converted into a Bessel beam by the axicon lens 16 and irradiated onto the workpiece 13. An optical attenuator for adjusting the intensity of the laser light beam may be disposed between the laser light source 14 and the magnifying / collimating optical system 15 or between the magnifying / collimating optical system 15 and the axicon lens 16. If the beam diameter of the light beam emitted from the laser light source 14 is sufficiently large and the parallelism thereof is high, the magnification / collimating optical system 15 may be omitted. Alternatively, the magnifying / collimating optical system 15, the axicon lens 16, and the optical attenuator may be placed outside the decompression chamber 11.
The pulse width of the light emitted from the laser light source 14 is preferably 100 ns or less, and more preferably 30 ns or less. As the laser light source 14, a solid laser such as an Nd: YAG laser, an Nd: ruby laser, or a Ti: sapphire laser, or a second harmonic output thereof is used. A Kepler type or Galileo type beam expander or the like is used for the magnifying / collimating optical system 15, whereby the light beam emitted from the laser light source 14 is expanded into a parallel beam. This parallel light beam is converted into a Bessel beam by the axicon lens 16 which is a conical lens, but a diffractive optical element having a similar function may be used instead of the axicon lens 16. The workpiece 13 is moved in the horizontal plane by the XY table 2. For example, when grooving is added to the workpiece 13, the XY table 2 is controlled by a control device (not shown). The XY table 2 can have a function of moving up and down.

If desired, the gas in the gas cylinder 17 is blown through the nozzle 18 as an assist gas to the laser beam irradiation site of the workpiece 13. As the assist gas, He, Ar, O ₂ , N ₂ gas or the like is used. If desired, air (gas) in the decompression chamber 11 is exhausted by a pump (not shown). In drilling, grooving and cutting, it is effective to irradiate laser light pulses while spraying an assist gas to the area including the irradiated part on the surface of the workpiece because the processing speed increases and the surface smoothness improves. is there. Also, in drilling, grooving and cutting, irradiating the laser light pulse while reducing the area including the irradiated part on the surface of the work piece promotes discharge of the removed material from the hole or groove, so that the processing speed This is effective for improving the smoothness of the processed surface.

A characteristic of the Bessel beam is that a center peak diameter of high power density and about 100 μm or less can be easily obtained, and the center peak diameter is maintained over a relatively long propagation distance. A condensing spot diameter larger than this can be easily obtained by ordinary convex lens condensing, and as can be seen from FIG. 3, the crossing angle between the Bessel beam at the large spot diameter and the propagation axis of the convex lens condensing is still Bessel. Although the beam is smaller, both values are sufficiently small, and sufficient processing can be performed by the conventional method. The short diameter, groove width, or cutting width of the hole in which the characteristics of the present invention are particularly exhibited is 100 μm or less. In many cases, the drilling process is circular drilling with high roundness, but there are also cases of elliptical or irregular hole shapes.
The short diameter in the drilling process is the circle or ellipse of the maximum area inscribed in the hole shape, the diameter in the case of a circle, and the short axis (shortest diameter) in the case of an ellipse. Although it depends on the value of the power density increased by condensing the laser beam to be irradiated, in general, drilling can be easily performed by the conventional technique at a depth similar to the short diameter of the hole. The ratio d / φ of the short diameter φ of the hole and the depth d of the hole, or the ratio t / w of the groove width or cutting width w and the groove depth or cutting thickness t, in which the features of the present invention are particularly exhibited. That is, the aspect ratio is 2 or more. In the case of drilling, the depth of the hole can be adjusted by appropriately selecting the number of laser light pulses to irradiate and the energy per pulse, and not only through holes but also blind holes of the desired depth can be adjusted. It is possible to process.
In the case of groove and cutting, the depth of the groove can be adjusted by appropriately selecting the energy per pulse of the laser light pulse to be irradiated and the moving speed of the irradiation position, and not only the cutting but also the desired depth It is possible to machine the groove.

The second harmonic output of a pulsed Nd: YAG laser with a repetition rate of 10 Hz, pulse width of about 10 ns, wavelength of 532 nm, light intensity distribution in the beam cross section is near Gaussian, and beam diameter (1 / e ² ) is about 7 mm. A laser light pulse train having a substantially parallel luminous flux was used as a light source.
The optical axis of the synthetic quartz glass axicon lens (conical shape, base circle radius 30 mm, half apex angle 68 °) and the axis of the laser beam pulse train light beam are made to substantially coincide with each other so that the laser light pulse train light beam is generally applied to the bottom surface of the axicon lens. A Bessel beam was generated that was incident perpendicularly and had a crossing angle θb of about 11.2 °, a center peak diameter of about 2 μm, and a depth of focus of about 17 mm to about 24.5 mm from the bottom of the axicon lens cone. A SUS304 foil having a thickness of about 20 μm was fixed to the XY table, and the surface of the SUS304 foil was set at a distance of about 20 mm from the conical bottom surface of the axicon lens. The surface of the SUS304 foil was adjusted so that the Bessel beam was incident substantially perpendicularly, and a laser beam pulse as a Bessel beam was irradiated at an energy of 1.25 mJ per pulse at a repetition rate of 10 Hz.
The short diameter of the hole is about 4 μm, the hole is a through hole reaching the back surface of the SUS304 foil, and the aspect ratio d / φ is about 5. FIG. 6 shows the result of measuring the cross-sectional shape of the through hole using a laser microscope. However, since the shape inside the microscopic hole cannot be evaluated with a laser microscope, it is hatched.

Using the same light source as in Example 1, the half apex angle of the axicon lens was changed to 87 °, the crossing angle θb was about 1.38 °, the center peak diameter was about 17 μm, and the focal depth was about 42 mm from the bottom surface of the axicon lens. A Bessel beam spanning about 110 mm from
An SUS304 plate with a thickness of about 300 μm is fixed to the XY table, and the surface of the SUS304 plate is adjusted and installed so that the Bessel beam is incident on the surface of the SUS304 plate at a distance of about 74 mm from the conical bottom surface of the axicon lens. did. A laser beam pulse that is a Bessel beam at a repetition rate of 10 Hz was irradiated with about 12000 pulses at an energy of 11 mJ per pulse. The short diameter of the opening of the hole is about 20 μm, the hole is a through hole reaching the back surface of the SUS304 plate, and the aspect ratio d / φ is about 15.

  The same light source, axicon lens, and SUS304 foil having a thickness of about 20 μm were used as in Example 1. The SUS304 foil was fixed to the XY table, and the surface of the SUS304 foil was adjusted and installed so that the Bessel beam was incident on the surface of the SUS304 foil at a distance of about 20 mm from the conical bottom surface of the axicon lens. While the XY table is moved linearly at a speed of about 60 μm per minute in a direction perpendicular to the incident direction of the laser light pulse for 5 minutes, a laser light pulse that is a Bessel beam with a repetition of 10 Hz is 1 per pulse. Continued irradiation repeatedly at an energy of .25 mJ. The SUS foil had a width of about 4 μm and a length of about 300 μm, was cut to the back surface, and the aspect ratio t / w was about 5.

Sectional drawing which shows the state of the propagation of the laser beam pulse inside a hole in the fine hole drilling process by this invention. Sectional drawing which shows the state of the propagation of the laser beam pulse inside a hole in the fine hole drilling process by the spot condensed with the normal convex lens. The chart which shows the crossing angle required in order to obtain the target spot diameter. Light intensity distribution (a) on the propagation axis and light intensity in the beam cross section of a Bessel beam having a crossing angle θb of about 1.17 ° generated from a Gaussian beam having a wavelength of 532 nm and a beam diameter (1 / e ² ) of 4 mm The graph which shows distribution (b). The conceptual diagram which shows one Embodiment of the laser ablation processing apparatus of this invention. The graph which shows the cross-sectional shape measurement result using the laser microscope of the through hole of 20-micrometer-thick SUS304 foil.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Workpiece 2 Laser beam 3 Propagation axis (theta) b, (theta) c Crossing angle (theta) r Reflection angle 11 Decompression chamber 12 XY table 13 Workpiece 14 Laser light source 15 Enlarging / collimating optical system 16 Axicon lens 17 Gas cylinder 18 Nozzle

Claims (17)

In the laser ablation processing method in which the surface of the workpiece is removed by irradiating the surface of the workpiece with drilling, groove formation or cutting, the laser beam irradiated on the workpiece is a Bessel beam A laser ablation processing method characterized by that. The laser ablation processing method according to claim 1, wherein the laser light source is a laser that performs pulse oscillation, and the workpiece is irradiated with a laser light pulse or a pulse train. The laser ablation processing method according to claim 2, wherein the laser light source is a laser that performs pulse oscillation with a pulse width of 100 ns or less. 4. The laser ablation processing according to claim 1, wherein a short diameter φ of a hole formed in the workpiece or a width or a cutting width w of a groove to be formed is 100 μm or less. 5. Method. The ratio d / φ of the short diameter φ of the hole to be drilled on the surface of the workpiece and the depth d of the hole, or the ratio t / 5. The laser ablation processing method according to claim 1, wherein w is 2 or more. 6. The laser ablation processing method according to claim 1, wherein the processing is performed while spraying an assist gas on a region including a laser beam irradiation site on the workpiece. The laser ablation processing method according to claim 6, wherein the assist gas is any one of He, Ar, O ₂ , and N ₂ gas. The laser ablation processing method according to any one of claims 1 to 7, wherein processing is performed while decompressing a region including a laser beam irradiation site on the workpiece. A laser ablation processing device that removes the surface of a workpiece, drills, forms grooves, or cuts by irradiating the surface of the workpiece with a laser beam, and converts the laser light source and the laser beam into a Bessel beam. A laser ablation processing apparatus comprising optical means and a table on which a workpiece is placed. The laser ablation processing apparatus according to claim 9, wherein the conversion optical means is an axicon lens or a diffractive optical element. 11. The laser ablation processing apparatus according to claim 9, wherein an enlargement / collimation optical means for expanding and collimating a beam is disposed between the laser light source and the conversion optical means. The laser ablation processing apparatus according to claim 11, wherein the magnifying / collimating optical means is a beam expander. The laser ablation processing apparatus according to any one of claims 9 to 12, wherein an optical attenuator for adjusting the intensity of a laser beam is disposed between the laser light source and the conversion optical means. The laser ablation processing apparatus according to any one of claims 9 to 13, wherein the table is movable in an XY direction or an XYZ direction. The laser ablation processing apparatus according to any one of claims 9 to 14, further comprising a nozzle for spraying an assist gas on a region including a laser beam irradiation site on the workpiece. The laser ablation processing apparatus according to claim 9, wherein the table is disposed in a decompression chamber that can be evacuated by a pump. The laser ablation processing apparatus according to any one of claims 9 to 16, wherein the laser light source uses an Nd: YAG laser or a second harmonic output thereof.

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