Laser processing apparatus and laser processing method

Abstract: In a laser processing apparatus according to the present invention, an aberration compensating device (9) is provided between a beam section forming device (2) and a half mirror (1e) for making a focal point of visible light which is reference light and a focal position of infrared light which is laser beam coincided with each other. In the aberration correcting device, an aberration correcting convex lens (9a) having long focal length and a space portion (9b) are provided in a rotatable lens holder (9c). When observing an object to be processed (6), the lens holder is rotated to bring the space portion into an optical path, and is rotated to bring a convex lens for correction into the optical path when emitting laser beam. The laser beam is converged until incidence on the objective lens by the convex lens for correction to correct the aberration. As the aberration compensating device, a combination of a concave lens (9f) and a convex lens (9e) having different refractive indexes and the same focal length may be employed. Otherwise, a concave lens (10) may be provided to make a beam waist of the laser beam coincide with the portion to be processed.

Description: The present invention relates to a laser processing apparatus and a laser processing method according to the preamble of claims 1 and 7. More particularly, the present invention relates to a laser processing apparatus and a laser processing method in which fine processings are performed with laser beam with a processed surface being observed with a microscope. Description of the Background Art Fig. 1 is a vertical section of a conventional laser processing apparatus, and Fig. 2 is a diagram for describing a processing method in the case where glass is provided above a surface of an object to be processed in the conventional laser processing apparatus. Referring to Fig. 1, the laser processing apparatus includes a microscope portion 1, a beam section forming device 2, an optical coupling device 3, a laser oscillator 4, a light source 5 and an observing portion 7. Microscope portion 1 constitutes a microscope of so-called infinite lens-barrel type, in which an objective lens 1b is attached to the lower end portion of lens-barrel 1a. A lens 1c for collimating light from a focal point outside the objective lens 1b and a lens 1d for converging the collimated light in turn are provided above objective lens 1b. Furthermore, in microscope portion 1, a first half mirror 1e is provided above lens 1d with an angle of approximately 45° with respect to the optical axis of these lenses, and an observing portion 7 is provided on the right side of half mirror 1e in the drawing. The upper end portion of lens-barrel 1a is opened, to which beam section forming device 2 is coupled. Beam section forming device 2, which is cylindrical-shaped similarly to lens-barrel 1a, includes a main body 2a provided on the upper end portion of lens-barrel 1a and a light shielding plate 2b provided in main body 2a vertical to the center axis of main body 2a. Light shielding plate 2b has an opening 2c at its central portion. Although not shown in the figure, in opening 2c, the opening shape can be arbitrarily changed. Optical coupling device 3 is coupled to the upper end portion of beam section forming device 2. Optical coupling device 3 includes a main body 3a, a second half mirror 3b, a collimator portion 3c and a correction plate 3d. Main body 3a is cylindrically shaped similarly to main body 2a of beam section forming device 2. Second half mirror 3b is provided inside main body 3a with an angle of 45° with respect to the central axis of the main body 3a. Collimator portion 3c is provided on the left side of half mirror 3b for collimating the light from the light source 5, and correction plate 3d is provided above half mirror 3b. Laser oscillator 4 is coupled to the upper end portion of main body 3a, and laser beam r1 is emitted from a laser device 4a therein. Furthermore, reference light r2 is emitted from an optical system 5, which is led to collimator portion 3c through an optical fiber 5a to be collimated. The laser beam r1 passes through correction plate 3d and second half mirror 3b, and reference light r2 is reflected at second half mirror 3b, both of which are led to a common optical path r. Light source 5 includes a tungsten halogen lamp and a film for changing white light produced by the tungsten halogen lamp into distinguishable monochromatic light therein. In laser processing with a laser processing apparatus configured as described above, an object to be processed 6 is provided in the vicinity of objective lens 1b of microscope portion 1 and a portion to be processed 6a in the object to be processed 6 is enlarged to be observed through observing portion 7. Next, a spot of reference light r2 is formed at the portion to be processed 6a. Variously changing the shape of opening 2c in beam section forming device 2 while observing the spot shape with the microscope, the spot shape of reference light r2 is made a shape for processing. Subsequently, laser beam r1 is emitted from laser device 4a of laser oscillator 4. The emitted laser light r1 is thus converged as a spot having substantially same shape as the spot of the reference light on the portion to be processed 6a by objective lens 1b, and desired processings are performed. In the conventional laser processing apparatus configured as described above, visible light is seen in the observation portion 7, and the reference light r2 for irradiating object to be processed 6 and indicating the spot shape is also visible light as a matter of course. On the other hand, as laser light r1 used for processings, YAG or CO₂ laser is employed, which are near infrared and far infrared rays, respectively. However, when objective lenses 1b, 1c have to be for use only for visible light, and when an object to be processed is a liquid crystal display, for example, a glass is provided above the electrode pattern as shown in Fig. 2, so that the light which passes through objective lenses 1b, 1c only for use for visible light forms an image at position P2 as shown by the broken line in Fig. 1 even if focusing on an electrode pattern as an object to be processed 6. This is because the refractive index of the laser beam r1 having long wavelength is made small. Accordingly, a precise processed shape cannot be formed on the surface of object to be processed 6. In the case shown in Fig. 2, even if objective lenses 1b and 1c are capable of dual use for visible light and infrared light, even if focusing on the surface of object to be processed 6 with visible light, the focal point of the infrared light (laser beam) is at P3 because of glass plate 8 on the surface of object to be processed 6, with the result that a precise processed shape cannot be formed on the surface of object to be processed 6 similarly to the description made above. EP-A-0 119 883 discloses an apparatus for positioning of a laser beam by means of an optical adjustment system using visible light to fix and control the position of the laser beam. The shape of the focus is not addressed therein. WO 87/06865 uses reflective optics which is not sensitive to the refractive index. SUMMARY OF THE INVENTION Accordingly, it is a main object of the present invention to provide a laser processing apparatus and a laser processing method in which difference in the refractive index of objective lenses can be corrected so that a focal point of visible light coincides with a focal point of infrared light. This object is achieved by an apparatus according to claim 1 and a method according to claim 7. Briefly stated, in the present invention, reference light and laser beam for processings having a common optical path are guided to the optical path of an objective lens of a microscope for enlarging and observing a portion to be processed of an object to be processed and converged at the portion to be processed. The spot shape of the reference light at the portion to be processed is observed with a microscope before processing with laser beam. Cross sectional shapes of the reference light and the laser beam are formed to be the shape of the opening, and the processed shape by the laser beam at the portion to be processed is made a shape to be processed. The aberration is corrected so that in processings the laser beam for processings is focused on the position in the optical axis direction the same as that in observation. Accordingly, according to the present invention, the difference in the refractive index of an objective lens can be corrected so that a focal point of reference light which is visible light and a focal point of laser beam which is infrared light coincide with each other. In a more preferable embodiment, an aberration correcting lens for correcting the aberration is held rotatably by a lens holder, in which laser beam passes through the aberration correcting lens in processing and reference light passes through space in observation. Accordingly, according to a preferable embodiment of the present invention, the aberration at the time when laser beam passes through an objective lens can be corrected in processing by rotating the lens holder. Furthermore, according to a more preferable embodiment of the present invention, the aberration of an objective lens is corrected with a correction lens in which concave and convex lenses of different materials are combined so that the focal length in the visible light range is large and that for the infrared light is short. The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a vertical section of a conventional laser processing apparatus. Fig. 2 is a diagram showing an example in which a glass plate is interposed between a conventional laser processing apparatus and an object to be processed. Fig. 3 is a vertical section of one embodiment of the present invention. Fig. 4 is a diagram showing a lens for correction in another embodiment of the present invention. Fig. 5 is a vertical section showing still another embodiment of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 3 is a vertical section of one embodiment of the present invention, and Fig. 4 is a diagram illustrating one example of a lens for correction. The embodiment shown in Fig. 3 is similarly constituted to the laser processing apparatus shown in Fig. 1 described above except the following points. That is to say, an aberration compensating device 9 is provided between a beam section forming device 2 and a half mirror 1e for making a focal point of visible light and a focal point of infrared light coincide with each other. Aberration compensating device 9 includes a lens holder 9c provided rotatably around a central axis 9d parallel to the optical axis of an objective lens 1b, a convex lens 9a for correcting aberration with long focal length provided in the lens holder 9c, and a space portion 9b. When observing an object to be processed 6, lens holder 9c is rotated to bring space portion 9b into the optical path, and normal operation is performed. When emitting laser beam, lens holder 9c is rotated so that-convex lens 9a for correction comes into the optical path Thus, the laser beam which is infrared light that passed through convex lens 9a for aberration correction is converged until incidence on objective lenses 1b, 1c as shown by the broken line, and the focal point of the converged laser beam coincides with the focal point in observation after it passes through objective lenses 1b, 1c, resulting in obtaining precise processed shape. Needless to say, the same effect can be obtained, providing aberration compensating device 9 between lens 1d and first half mirror 1e. The convex lens 9a for aberration correction corrects difference in the refractive indexes in objective lenses 1b, 1c of visible light and infrared light, of which refractive index ratio is approximately 0.5% (when using YAG laser). In the embodiment described above, lens holder 9c must be rotated every time in observation and in laser light emission. Fig. 4 shows an embodiment in which the troublesome problem is solved. The lens for aberration correction shown in Fig. 4 is made of combination of a convex lens 9e and a concave lens 9f having different refractive indexes and the same focal length. When employing general optical glass as material of convex lens 9e, the refractive index at the wavelength of 546.1nm is 1.5187, and the refractive index with the wavelength of 1064nm is 1.50669. On the other hand, using zinc selenide as the concave lens 9, the refractive index with the wavelength of 546.1nm is 2.705 and the refractive index with the wavelength of 1064nm is 2.488. Appropriately combining such two kinds of convex lens 9e and concave lens 9f made of different materials, a compound lens in which the focal length is long with visible light and it is short with infrared light can be obtained. Using such a compound lens formed of convex lens 9e and concave lens 9f as lens 9a for correction shown in Fig. 3, since the focal length with visible light is long, change of optical path by lens 9a for correction is small and the change is large with infrared light. That is, in the embodiment shown in Fig. 3, the lens holder 9c does not have to be rotated every time when observing and when emitting laser light, and the focal points can be well coincided with each other in any case. Fig. 5 is a diagram showing yet another embodiment of the present invention. In the embodiment shown in Fig. 5, a beam waist portion is coincided with a processed portion to enable processings with minimized processing dimensions. In order to decrease the processing dimensions to several µm or smaller, it is necessary to make the beam waist portion of the laser beam coincided with the processed portion. Accordingly, in the embodiment shown in Fig. 5, a concave lens 10 having focal length L is attached to a lens holder 9. Providing concave lens 10 in the optical path, the light incident upon concave lens 10 passes through concave lens 10 and then passes on the optical path expressed with a normal image forming optical system, so that the beam waist coincides with a processed portion on the surface of object to be processed 6. At this time, with an opening of main body portion 2a of beam section forming device 2 completely opened so that all the laser beam r1 from laser oscillator 4 can pass therethrough, the light can be effectively utilized. When the power of laser oscillator 4 is too large, a filter for attenuation (not shown) may be inserted between optical coupling device 3 and laser oscillator 4. As described above, according to embodiments of the present invention, a lens for correction is provided between objective lens 1b of a microscope and a beam section forming device 2, where laser beam passes through the lens for correction in processing, and a focal point of visible light and a focal point of infrared light can be coincided with each other. Furthermore, in the embodiment shown in Fig. 5, the beam waist of laser beam is located at a portion to be processed to enable processings possibly minimized. Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the scope of the present invention being limited only by the terms of the appended claims.

Claims: Laser processing apparatus for performing processing by irradiating laser beam to an object to be processed (6), comprising: a microscope (1) including an objective lens (1b) at its tip end for enlarging and observing a portion to be processed of said object to be processed; a laser beam source (4) for irradiating laser light to said object to be processed through said objective lens; a reference light source (5) for irradiating reference light for observing the portion to be processed of said object to be processed with said microscope; optical coupling means (3) for guiding the laser light and the reference light to an optical axis of the objective lens in said microscope; forming means (2) provided between said optical coupling means and said objective lens and having an opening corresponding to a processed shape of said object to be processed, characterized by chromatic aberration correcting means (9) provided between said objective lens (1b) and said forming means (2) such that, when processing, said laser beam is focussed at a position in the optical axis direction which coincides with the position at which said reference light is focussed when observing. The laser processing apparatus according to claim 1, wherein said correcting means comprises, aberration correcting lens (9a) for correcting aberration, and lens holder (9c) for rotatably holding said aberration correcting lens so that said laser beam passes through said aberration correcting lens when processing and said reference light passes through a space when observing. The laser processing apparatus according to claim 1, wherein said correcting means includes a correction lens (9e, 9f) in which convex and concave lenses of materials different from each other are combined so that focal length in visible light range is large and is short for infrared light. The laser processing apparatus according to claim 1, wherein said correcting means includes a concave lens (10) for making a beam waist of said laser beam coincide with said portion to be processed. The laser processing apparatus according to claim 1, wherein said microscope includes, a lens-barrel (1a) at which tip portion said objective lens is attached, a first half mirror (1e) provided in said lens-barrel and on the optical axis of said objective lens, and observing means (7) for observing an image of said object to be processed reflected by said first half mirror. The laser processing apparatus according to claim 1, wherein said optical coupling means comprises, a second half mirror (3b) provided on an optical axis of said laser light source and said objective lens, and collimator means (3c) provided perpendicular to an optical axis of said second half mirror for collimating the reference light emitted from said reference light source. Laser processing method for processing an object to be processed (6) by irradiating laser beam, comprising the steps of: guiding reference light and laser light for processing having a common optical path (r) to an optical path of an objective lens (1b) of a microscope (1) and converging them at a portion to be processed of said object to be processed; observing a spot shape of said reference light at the portion to be processed with said microscope before processing with said laser beam and forming a section shape of said reference light and said laser beam to a shape to be processed at the portion of said object to be processed, characterized by correcting chromatic aberration of the laser beam so that, when processing, said laser beam is focussed at a position in the optical axis direction which coincides with the position at which said reference light is focussed when observing, processing said object to be processed with the corrected laser beam.

Inventors: KOBAYASHI TOMOSHIGE, NAKAZEKI TSUGITO, KOBAYASHI, TOMOSHIGE, NAKAZEKI, TSUGITO
Applicants: NTN TOYO BEARING CO LTD, NTN CORPORATION
Application year: 1991
Main classification: B23K26
IPC codes: G02B21, B23K26, G02B27