JP2006068789A - Laser induced processing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a laser induced processing method that makes laser beam machining possible in a minute machining shape below a diffraction limit without being restricted by the type of material of a workpiece. <P>SOLUTION: A workpiece 7 is immersed and arranged in a solution 5 in which particles 6 are dispersed. A high intensity ultrashort pulsed laser beam 2 emitted from a high intensity ultrashort pulsed laser 1 is converged so as to be focused on a position on the near side of the workpiece 7, the solution 5 is pulse-irradiated. The output intensity of the high intensity ultrashort pulsed laser 1 is controlled in the manner evenly balancing the self-focusing generated by the non-linear optical effect of the solution 5 as induced by the laser beam 2 and the diffraction of the laser beam 2. As a result, the laser beam 2 is propagated in the solution 5 toward the workpiece 7 micro-linearly, and the particles 6 suspended in the solution 5 in the micro-linear region are made to collide with the workpiece 7 to perform the machining. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a laser-induced machining method capable of performing fine-shaped machining on workpieces of various materials.

A laser oscillator with a high peak output can be obtained by setting the oscillation pulse width to a small value. In the 1960s, a laser having an oscillation pulse width of nanosecond was already developed, and from the 1990s, a titanium sapphire laser was used. Lasers with a pulse width of femtosecond (fsec = 10 ^-15 sec) and picosecond (psec = 10 ^-12 sec) using a saturable absorber mirror have been developed, marketed, and used in various technical fields. . In recent years, laser oscillators that oscillate by being excited by a laser medium in a picosecond or femtosecond oscillation pulse width region using a semiconductor laser are commercially available.

  Fine processing using a laser having a high peak output with an ultrashort pulse as described above has a feature that a good processed surface can be obtained because there is little thermal damage to the workpiece, especially thin film processing and resin Suitable for fine processing. In contrast, in conventional microfabrication using a laser with an oscillation pulse width of nanoseconds and ablation processing using ultraviolet light from an excimer laser, the laser oscillation pulse width is large. Damage cannot be denied.

On the other hand, the limit of microfabrication depends on the diameter of the beam that condenses the laser beam ideally to its diffraction limit. In general, when a laser beam is condensed up to its diffraction limit in the TEM ₀₀ mode (fundamental mode) with good condensing property, the beam diameter d is λ, the focal length of the condensing lens is f, and the condensing lens If the beam diameter of is w,
d = 1.22 × λ × (f / w) or d = (1.22 × λ) / NA
It is expressed by the following formula.

  As is clear from the above equation, in order to obtain a small focused beam diameter d, a short wavelength laser is used to set the laser wavelength λ small, or a condensing lens with a large numerical aperture NA and a small focal length. It is conceivable to use a large beam.

  However, when an extremely large beam is used, the minimum focused beam diameter may not be obtained due to the aberration of the focusing lens, and the focal depth (range of focus distance) is NA. Therefore, a method of processing while adjusting the distance between the condenser lens and the workpiece is required. Further, the above expression is an expression for simply calculating the focused beam diameter of the laser beam, and when the workpiece is actually irradiated with the laser beam, not only the energy of the laser beam but also the workpiece The actual focused beam diameter varies depending on the absorption characteristics depending on the wavelength of the material, the processing shape, and the surface state. However, in general, there is no method for condensing light beyond the diffraction limit, and thus there has conventionally been no fine processing means that greatly exceeds the diffraction limit.

  However, in recent years, laser-induced channeling has been proposed as a processing method capable of obtaining a focused beam diameter of laser light exceeding the diffraction limit and performing fine processing (see Patent Document 1). In this laser-induced channeling process, high-intensity ultrashort pulse laser light is input to the work piece, and the Kerr effect causes a spatial change of the equiphase surface of the work piece, so that self-focusing of the high-intensity ultra-short pulse laser light occurs. In addition, optical breakdown occurs due to the high-intensity ultrashort pulse laser beam that is self-focused by the Kerr effect, and plasma is generated in the work piece to cause the high-intensity ultrashort pulse laser beam to self-diverge, Self-divergence and self-convergence of high-intensity ultrashort pulsed laser light are canceled by controlling the input intensity of the intense ultrashort-pulsed laser light to the workpiece by self-divergence of the high-intensity ultrashort pulsed laser light. Is balanced so that high-intensity ultra-short pulse laser light is linearly processed in the direction of propagation inside the solid, and the transparent material is processed. And performs processing only inside without the surface of damaging.

In the laser induced channeling process, it is possible to perform a fine process on a transparent material, and the minimum beam diameter d is
d = (1.22 × λ) / route (8 × N0 × N2)
It is expressed by the following formula. However, in the above formula, the refractive index N is N = N0 + N2 and N2 = Δ + 1, where l is the laser integrity and Δ is a nonlinear constant. Therefore, as apparent from the above formula, the beam diameter of the laser light depends on the laser integrity l.

  Conventionally, as a means of finely processing a transparent material such as glass, which is difficult to use a direct laser etching method due to low light absorption, the transparent material can be simply applied by laser irradiation. In addition, a laser micromachining method for a transparent material designed to allow precise micromachining (see Patent Document 2), and a laser processing method designed to perform micromachining with high processing surface accuracy for a transparent material (see Patent Document 3) ) Has been proposed.

  In the laser microfabrication method of Patent Document 2 described above, as shown in FIG. 2, a fluid substance 23 having a strong absorption for the laser wavelength of the laser beam is applied to the back surface (the left side surface of the drawing) of the workpiece 21 made of a transparent material. ) Is incident on the surface of the workpiece 21 (the right side surface in the figure), and is irradiated to the contact surface between the fluid material 23 and the workpiece 21 through the workpiece 21. To do. As a result, there is no change on the surface on the workpiece 21 where the laser beam 22 is incident, but only the portion irradiated with the laser beam 22 is selected on the back surface of the workpiece 21 in contact with the fluid substance 23. Etching is performed. Here, if the laser beam 22 is irradiated through a mask pattern, for example, a fine structure having a line width of several micrometers can be formed, and the formed portion is not chemically degraded or damaged.

On the other hand, in the laser processing method of Patent Document 3, as shown in FIG. 3, a solution 33 in which fine particles 32 are dispersed in a solvent that hardly absorbs laser light 34 is applied to the surface of a substrate-like workpiece 31 (FIG. The laser beam 34 is contacted to the upper surface of the substrate 31 by means of coating or the like and spatially selected by an optical system or a mask, and the workpiece 31 is irradiated from the surface side so that the energy of the laser beam 34 is absorbed by the fine particles 32. The workpiece 31 is processed by collision of the fine particles 32 excited by the laser beam 34 with the workpiece 31 or electronic transition, and minute removal is performed. In this laser processing method, by using the energy transfer from the fine particles 32, it is possible to process only the pole surface of the workpiece 31, so that the absorption of the laser light usually enters the inside of the workpiece. Unlike direct machining of a workpiece by the laser ablation method, laser machining with a high machining surface system is possible.
Japanese Patent Laid-Open No. 11-207479 Japanese Patent No. 3012926 JP 2002-178171 A

  However, in the high-intensity ultrashort pulse laser processing method of Patent Document 1 described above, the processing object is limited to a transparent material such as glass even if the laser beam can be condensed to a minute beam diameter. However, there is a problem that the surface of the workpiece cannot be finely processed.

  As a means for solving this problem, conventionally, a method has also been proposed in which only the channeling is caused to the extent that laser-induced channeling is not performed on the transparent material, and the workpiece is placed on the end of the transparent material. However, in this case, in order to control the laser output so that channeling does not occur in the transparent material below the processing threshold of the workpiece, an optimal combination of materials is required, and there are also types of workpieces that can be processed. In addition, there is a problem that the laser beam emitted from the edge of the transparent material is controlled by the refractive index in the air, and the balance between self-diffusion and self-convergence of the laser beam is easily lost. In order to obtain the beam diameter, it is necessary to control the laser output and the focusing optical system, and very difficult control is required.

  On the other hand, both of the processing methods of Patent Documents 2 and 3 perform processing when the fluid material 23 or the fine particles 32 having high absorption with respect to the laser wavelength collide with the workpieces 21 and 31. Since the fine particles 32 are excited in the portion having the highest laser intensity, the processing range of the workpieces 21 and 31 depends on the beam diameter of the laser beam. Therefore, even in these processing methods, it is difficult to process a region extremely smaller than the diffraction limit. That is, it can be said that each of the processing methods of Patent Documents 2 and 3 is suitable for processing a relatively large region smoothly, as far as possible.

  Therefore, the present invention has been made in view of the above-described conventional problems, and can perform laser processing of a minute processing shape below the diffraction limit without being restricted by the material of the workpiece. Is intended to provide.

  In order to achieve the above object, the laser-induced machining method of the invention according to claim 1 is a high-intensity emitted from a high-intensity ultrashort pulse laser, wherein the workpiece is immersed in a solution in which fine particles are dispersed. By condensing the ultrashort pulse laser beam so as to focus on the near side position of the workpiece and irradiating the solution with pulses, the laser beam is controlled by controlling the output intensity of the high intensity ultrashort pulse laser. The self-focusing generated by the nonlinear optical effect of the solution induced by the balance is balanced so that the diffraction of the laser beam is balanced, and the laser beam is propagated toward the workpiece in a fine line shape in the solution. Processing is performed by causing the fine particles floating in a solution in a micro linear region to collide with the workpiece.

  According to a second aspect of the present invention, in the laser induced processing method of the first aspect of the invention, the finely processed dimension and the processed surface roughness applied to the workpiece by changing the size of the fine particles dispersed in the solution are variably adjusted. The processing depth applied to the workpiece is variably adjusted by variably controlling the irradiation time of the laser beam.

  According to the first aspect of the present invention, the laser beam can be condensed to a minute beam diameter of the diffraction limit or less, and the fine particles floating in the solution in the minute linear region of the condensed laser beam are processed. The workpiece is limited to a transparent material as in the prior art, and unlike the one that only processes or modifies the interior of the solid, there is a restriction on the material of the workpiece. Without being received, it becomes possible to perform fine processing below the diffraction limit of laser light on the surface of the processing surface of the processing object. In addition, in the region where the channeling effect acts, the length of the laser beam condensed to a small constant beam diameter below the diffraction limit becomes long, so that an optical system having a large NA is used. However, there is an advantage that adjustment of the focal position of the laser beam by this optical system becomes unnecessary.

  According to the second aspect of the present invention, the required fine processing dimension can be obtained by changing the size of the fine particles dispersed in the solution while maintaining the beam diameter of the focused laser beam without changing the laser irradiation energy. In addition, it is possible to set so that the processed surface roughness of the required shape can be obtained without hindrance, and it is possible to set the required processing depth to be obtained by variably controlling the irradiation time of the laser beam.

  Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a schematic explanatory diagram of a configuration for realizing a laser induced machining method according to an embodiment of the present invention. In the figure, the container 4 is filled with a solution 5 in which fine particles 6 are dispersed, and a substrate-like workpiece 7 is immersed in the solution 5 and disposed on the bottom surface of the container 4. It is only necessary that the workpiece 7 be arranged so that its processing surface (the upper surface in the figure in this embodiment) is immersed in the solution 5.

  The high intensity ultrashort pulse laser 1 emits high intensity long and short pulse laser light 2 having an oscillation pulse width of picosecond or less and a high peak output. The laser beam 2 is collected by the condenser lens 3 and is set so as to form a focal point F at the near side position with respect to the workpiece 7 in the solution 5. As the high-intensity long / short pulse laser beam 2, one having a femtosecond oscillation pulse width is preferably used.

  In the present invention, the absorption characteristics of the laser beam 2 of the fine particles 6 and the workpiece 7 are not particularly important. Therefore, for example, a magnetic material, plastic, or glass can be suitably used as the fine particles 6, but in order to obtain fine processing uniformity, the fine particles 6 having a uniform diameter as much as possible are used. Is preferred. Moreover, as the solution, for example, pure water can be used.

  A part of the vicinity of the focal point F of the high-intensity long / short pulse laser beam 2 in the solution 5 is induced by the laser beam 2 and acts as a nonlinear optical material, so that the nonlinear optical effect (laser beam) is increased by the high energy density of the laser beam 2. When the electric field becomes stronger, an effect that the polarization of the substance does not respond linearly in proportion to the electric field of the laser light occurs), and an action of focusing by this nonlinear optical effect, that is, self-focusing occurs.

  Here, in the high-intensity ultrashort pulse laser 1, the action of the laser light 2 emitted from itself is balanced so that the action of spreading by the diffraction of the laser light 2 and the action of focusing by the nonlinear optical effect (self-focusing) are balanced. The output intensity is controlled. By balancing the diffraction and self-focusing of the laser beam 2 in this way, the laser beam 2 is condensed to a beam diameter below the diffraction limit, and the solution 5 is in the form of a fine line while maintaining the beam diameter. Propagate inside. That is, a channeling phenomenon occurs in the solution 5. When the channeling phenomenon does not occur, the laser beam 2 is once focused and then propagates through the solution 5 while spreading as shown by a two-dot chain line in FIG.

  The laser beam 2 having a high peak output focused to a small beam diameter below the diffraction limit due to the occurrence of the channeling phenomenon passes through the solution 5 in the form of a minute line while maintaining the minute beam diameter. 7, and the fine particles 6 that float in the solution 5 existing in the propagation path of the micro linear laser beam 2 collide with the work piece 7, so that the work surface on the surface of the work piece 7 Extremely fine processing is applied. At this time, the stage 8 on which the container 4 is installed is controlled to move in the horizontal direction. Thereby, a fine required pattern is formed and processed on the surface to be processed, which is the surface of the workpiece 7 that is relatively moved in the direction orthogonal to the optical axis of the laser beam 2.

  In the high-intensity ultrashort pulse laser processing method of Patent Document 1 described above, the workpiece is limited to a transparent material, and only the inside of the solid is processed or modified. In the laser-induced processing method of the above embodiment, Since the fine particles 6 floating in the solution 5 collide with the workpiece 7 in the minute linear region of the laser beam condensed to a minute beam diameter below the diffraction limit, the workpiece 7 is processed. Without being restricted by the material, it is possible to perform fine processing below the diffraction limit of the laser beam 2 on the surface of the processing surface of the workpiece 7. In addition, in the region where the channeling effect acts, the length (depth) of the laser light 2 condensed to a small constant beam diameter below the diffraction limit becomes longer (deeper), so that the condenser lens 3 is included. Even when an optical system having a large NA is used, there is an advantage that adjustment of the focal position of the laser beam 2 by this optical system becomes unnecessary.

  In practical use, it is preferable that the solution 5 is circulated using a pump or the like so that the distribution of the fine particles 6 is always uniform, whereby the workpiece 7 is always subjected to uniform fine processing. be able to. In particular, in the above embodiment, bubbles may be generated in the solution 5 due to the high intensity energy of the laser light 2, and when the bubbles are generated, there is a possibility that the workpiece 7 is not sufficiently irradiated with the laser light 2. For this reason, it is desirable to employ means for circulating the solution 5 as described above.

  By the way, in many existing ablation processes, there is a case where fine processing is performed at a high energy part distributed in the beam part by lowering the laser irradiation energy density. However, in the laser induced machining method of the above-described embodiment, when the fine machining dimensions such as the machining width and the machining diameter, the machining surface roughness, or the machining depth are adjusted and set to the required values or desired states, If a means for changing the laser irradiation energy is adopted, the balance between self-focusing and diffraction due to the nonlinear optical effect is lost, and the location of the channeling phenomenon, the length of the channeling effect, and even the beam diameter of the laser beam 2 change. There is a risk of malfunction.

  Therefore, in the above-described embodiment, while maintaining the beam diameter of the focused laser beam 2 as it is, the required fine processing dimensions, processing surface roughness, and processing depth can be obtained by employing the following means. I am trying so. That is, the fine processing dimension and the processing surface roughness are set so that the required value and the required shape can be obtained without hindrance by changing the size (diameter) of the fine particles 6 dispersed in the solution 5, and the processing depth is The required value can be obtained by variably controlling the irradiation time of the laser beam 2. The irradiation time of the laser beam 2 is controlled by varying the number of irradiations, that is, the number of irradiation pulses.

  The laser induced machining method according to the present invention is not limited to only transparent materials, and it is possible to machine fine shapes below the diffraction limit of laser light on the surface of the workpiece surface of workpieces of various materials. Can be applied.

BRIEF DESCRIPTION OF THE DRAWINGS Schematic explanatory drawing of the structure for embodying the laser induction processing method which concerns on one embodiment of this invention. The schematic sectional drawing which shows the principle of the conventional laser microfabrication method. The schematic perspective view which shows the structure which actualized the conventional laser processing method.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 High intensity | strength ultrashort pulse laser 2 High intensity | strength ultrashort pulse laser beam 5 Solution 6 Fine particle 7 Workpiece

Claims (2)

Place the work piece soaked in a solution in which fine particles are dispersed,
The high-intensity ultrashort pulse laser beam emitted from the high-intensity ultrashort pulse laser is focused so as to focus on the near side position of the workpiece, and pulsed to the solution,
By controlling the output intensity of the high-intensity ultrashort pulse laser, the self-focusing generated by the nonlinear optical effect of the solution induced by the laser beam and the diffraction of the laser beam are balanced to balance the laser beam. A laser which propagates toward the workpiece in the form of a fine line in the solution and makes the fine particles suspended in the solution in the fine line region collide with the work piece to perform the processing. Induction machining method. By changing the size of the fine particles dispersed in the solution, the fine processing dimensions and surface roughness applied to the workpiece are variably adjusted, and the laser beam irradiation time is variably controlled to change the processing depth applied to the workpiece. The laser induced machining method according to claim 1, which is adjusted.

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