JP2005305470A - Ultraviolet ray-assisted ultra short pulsed laser beam machining apparatus and method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a laser beam machining apparatus and method that make unique precision microfabrication possible at high speed by efficiently applying a ultra short pulsed laser beam to a workpiece. <P>SOLUTION: The laser beam machining apparatus works by possessing a machining head which is equipped with an optical system for guiding and converging the ultra short pulsed laser beam from a femtosecond laser oscillator 15 onto a machining point 30 of the workpiece 31 and also possessing a ultraviolet light source 22 which is for emitting the light near the machining point. The laser beam machining method includes an energy absorbing process in which two processes occur synchronously, namely, a ultraviolet electron exciting/absorbing process which excites to a higher level the valence band electron of an object to be irradiated with the ultraviolet ray and an absorbing process for a multi-photon which emits the ultra short pulsed laser beam to the object. Accordingly, the high precision and high-speed microfabrication by laser is implemented. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an ultrashort pulse laser processing apparatus and an ultrashort pulse laser processing method. More specifically, the present invention relates to an apparatus and method for improving processing capability and processing efficiency using ultrashort pulse oscillation output light and ultraviolet rays.

  Conventionally, a laser beam is focused and irradiated on a workpiece, the irradiated point is removed by evaporation, and removal processing such as drilling, cutting, and trimming is performed on semiconductors, metals, ceramics, resins, and glass. Yes. As the wavelength of the laser light, beams oscillated from various lasers from the ultraviolet region to the infrared region are used. Effective processing is performed on materials with a high laser beam absorptivity to be used on the workpiece. Absorbent coating is applied to the surface of workpieces with low light absorptivity, and the irradiated laser beam It is well known to increase the absorption and increase the processing efficiency by increasing the absorption due to the irradiation laser energy. These processing methods for performing surface treatment need to be provided with a surface treatment step, and further have a drawback of contaminating the material with foreign substances.

  On the other hand, a highly reflective object or a transparent object having a small light absorptance with respect to the wavelength of the laser beam to be used is irradiated with a wavelength having a high absorptivity, for example, an ultraviolet laser beam, and at the same time, the object is separated. Attempts have been made to improve the processing performance by simultaneously irradiating laser beams having the above wavelengths. In Japanese Patent Laid-Open Nos. 2003-94191, 2001-347388, and 5-104276, laser beams having a plurality of wavelengths such as a wavelength of 1064 nm in the infrared region, a wavelength of 532 nm in the visible light region, and wavelengths of 355 nm and 266 nm in the ultraviolet region are provided. Irradiation can be performed simultaneously or with a slight shift in time, and processing by chemical action using ultraviolet rays as well as thermal action is possible.

  As a result, the laser processing apparatus is effective for processing glass, ceramics, etc., which are difficult to process with a single wavelength laser processing apparatus having a wavelength of 1064 nm. Further, Japanese Patent Application Laid-Open No. 2002-1564 and Japanese Patent Application Laid-Open No. 2002-316282 introduce a technique for preventing the generation of cracks generated at or near a welding point by irradiating a laser beam having a plurality of wavelengths near the irradiation point of a laser beam for welding. ing.

  As described above, a processing method using both the action of an ultraviolet laser and a laser having another wavelength has been tried. The processing methods using these techniques have the disadvantage that the generation of a work-affected layer is unavoidable, and cracks and the like occur depending on the material. It is considered that the ablation processing mechanism using only the ultraviolet laser beam is based on the mechanism shown in FIG. That is, the electron 5 at the level of the valence band 4 is raised to the vacuum level 1 at once with the photon energy (short wavelength ultraviolet light 7) of DUV or VUV in the ultraviolet region, thereby breaking the electronic bond between atoms. Process.

  On the other hand, a laser processing method using laser-induced breakdown with an ultrashort pulse laser of 10 femtoseconds to 10 picoseconds generated from an ultrashort pulse oscillator is described in, for example, US Pat. No. RE37585. This document describes a technique for processing a transparent body or the like by irradiating an ultrashort pulse beam where the ablation threshold is not proportional to the square root value of the pulse width. The mechanism of this processing method is considered to be ablation processing by multiphoton absorption as shown in FIG. In this process, multiphoton absorption by a plurality of photons 8-1, 8-2, 8-3, 8-4, etc. having a quantum smaller than the energy difference from the band gap 3 or the valence band to the vacuum level. According to the process, the electrons 5 at the valence band level are raised to the vacuum level 1 at once, thereby breaking and processing the electronic bonds between the atoms.

At this time, since the probability of multiphoton absorption is much smaller than that of one-photon absorption, the photon energy density (number density of photons) must be increased exponentially. In principle, the occurrence probability of multiphoton absorption decreases exponentially as the number of participating photons increases, such as three-photon absorption rather than two-photon absorption. Therefore, as the photon absorption process increases, the photon energy density required for processing, and hence the peak value of the laser output power, is increasingly increased. For this reason, since the laser oscillation output energy of the ultrashort pulse must oscillate with the ultrashort pulse sufficiently to ensure a high output, a large-scale device configuration is required and the price is high. Therefore, the processing cost is high, and it is a processing method that is difficult to use industrially from a practical aspect.
JP 2003-94191 A JP 2001-347388 A JP-A-5-104276 JP 2002-1564 A JP 2002-316282 A US Reissue Patent No. 37585

  The problem to be solved is to improve the laser processing capability by ultrashort pulse laser irradiation.

  The present invention includes a processing head including an optical system that guides and collects an ultrashort pulse laser beam oscillated from a laser oscillator to a processing point of a workpiece, and an ultraviolet light source for irradiating near the processing point. A laser processing apparatus, an ultraviolet electron excitation absorption process for exciting valence band electrons of an object irradiated with ultraviolet light to a higher level, and an absorption process of multiphotons for irradiating an object with ultrashort pulse laser light. A high-precision and high-speed fine laser processing method is provided using a method including an energy absorption process that occurs synchronously.

  According to one preferred embodiment of the present invention, the ultrashort pulse laser beam laser oscillator of the laser processing apparatus is a mode-locked oscillation solid-state laser. According to another preferred embodiment of the present invention, the ultrashort pulse laser light of the laser processing apparatus is chirped pulse amplification and pulse compression mode-locked pulse output light. In the laser processing apparatus according to the present invention, the ultraviolet light source may be an ultraviolet laser or non-coherent ultraviolet light emitted from a laser oscillator, and the workpiece is any of a semiconductor, ceramics, glass, and transparent crystal. be able to. Furthermore, it is preferable that the ultrashort pulse laser is irradiated with the ultrashort pulse laser in the irradiation timing of the ultrashort pulse laser light and the ultraviolet light at least during a period when the influence of the ultraviolet light irradiation on the workpiece surface appears. The optical element that guides the laser light is preferably an optical element in which the pulse width of the ultrashort pulse compensates for the wavelength dispersion of the light guide and keeps the expansion of the pulse width small at the focal point.

  According to another preferred embodiment of the present invention, in the laser processing method, the excitation level of the ultraviolet electron excitation absorption process is a conduction band or forbidden band trap. The level of electrons that absorb multiphotons in the multiphoton absorption process is preferably an electron that has been excited by ultraviolet irradiation to a trap level in a conduction band or a forbidden band.

  As an effect of the present invention, compared with conventional processing methods, processing using ultraviolet and femtosecond ultrashort pulses makes use of the features of femtosecond lasers that do not generate heat, and furthermore, difficult-to-process materials such as semiconductor wafers and It is possible to realize processing with higher processing efficiency than conventional materials such as superconducting materials and ceramics. Since most of the conventional laser processing methods rely on processing by thermal action, a large cutting allowance was necessary in consideration of thermal strain in the processing portion. However, in the processing method of the present invention, a thermally deteriorated layer is generated. It can be prevented.

  For example, when a chip is cut out from a semiconductor wafer by a dicing process, a wasteful heat-affected layer is not generated, and the performance of the semiconductor device on the chip can be ensured even if the dicing area is minimized. This makes it possible to reduce costs. Therefore, a processing method using both an ultraviolet laser and a femtosecond laser is indispensable as a processing technology for semiconductors, MEMS, and the like because fine processing can be performed without a heat-affected layer when the chip size of a semiconductor device is reduced.

  The processing method that uses both UV laser and femtosecond laser improves the processing characteristics and processing performance of femtosecond lasers that do not generate heat. Conventional technologies such as difficult-to-process materials such as semiconductor wafers, superconducting materials, and ceramics Then, it becomes an innovative technology that can realize high-speed processing in the processing field where the application of fine processing is not easy. For the above reasons, an ultra-precise laser processing machine combining an ultraviolet laser and a femtosecond laser becomes extremely important as a processing machine responsible for the next generation technology.

According to one preferred embodiment of the invention described in claim 1 and claim 8 of the present invention, an optical system for guiding and condensing an ultrashort pulse laser beam oscillated from a laser oscillator to a processing point of a workpiece. A laser processing apparatus that performs processing with an ultraviolet light source for irradiating in the vicinity of the processing point, and an ultraviolet electron that excites valence band electrons of an object irradiated with ultraviolet light to a higher level High-precision and high-speed fine laser processing is realized using a method that includes an energy absorption process that synchronously generates an excitation absorption process and an absorption process of multiphotons that irradiate an object with ultrashort pulse laser light. is there. This will be described below with reference to FIGS.
Examples of the principle method of the present invention are shown in FIGS. 3 to 6, and explanatory diagrams of specific examples are shown in FIGS.

  FIG. 3 shows the first stage ultraviolet excitation process in the first example of the processing method according to the present invention, taking as an example processing for a semiconductor. The electrons 5 in the valence band 4 are excited to the conduction band by the excitation process 10 by the photon absorption 9 in the ultraviolet region. As a result, an insulator such as a ceramic has a conductive metallic property, but the processing does not proceed and the tissue is softened. By simultaneously irradiating the surface of this workpiece with an ultrashort pulse laser beam, excited electrons 5 in the conduction band 2 shown in FIG. 4 are excited to the vacuum level 1 in the multiphoton absorption process by the ultrashort pulse. The bond is broken and processing is performed.

  The processing point and its vicinity by this processing mechanism are softened and deteriorated by irradiation with ultraviolet rays, so cracking due to impact is prevented even when irradiated with ultrashort pulses, and it is ablation processing not due to thermal processing In the vicinity of the processing point, there is no occurrence of damage such as cracks, and precise processing is performed.

  As shown in FIG. 4, if the multiphoton absorption 10-1 and 10-2 are assisted by the ultraviolet light 10, the number of photons of the multiphoton absorption of the femtosecond laser is reduced by the amount excited by the ultraviolet light, thereby the femtosecond laser. The laser intensity can be reduced. On the contrary, if the strength is the same, the processing speed can be increased and the throughput can be increased.

  FIG. 5 is an explanatory diagram of a second embodiment in which an ultrashort pulse is irradiated after ultraviolet irradiation. In the first stage, ultraviolet irradiation is performed to excite the valence band to the conduction band. In this method, after being softened by ultraviolet irradiation, femtosecond laser irradiation with an ultrashort pulse is performed. While electrons excited in the conduction band stay in the conduction band for a certain period of time after ultraviolet irradiation. Ultra short pulse irradiation is performed to perform multiphoton absorption by ultra short pulses. Then, as in the implementation method of FIG. 4, the number of multiphotons absorbed by the ultrashort pulse femtosecond laser can be reduced, the power intensity of femtosecond laser irradiation can be reduced, and the throughput can be increased. .

  In the third embodiment shown in FIG. 6, the electrons trapped after the excitation 9 in the trap level 12 formed in the forbidden band are subjected to ultrashort pulses to the vacuum level by the multiphoton absorption 13-1 and 13-2. The case where the femtosecond laser is absorbed and processed is shown. As in the case of FIGS. 5 and 6, there is a possibility that the processing proceeds while the material is softened.

  FIG. 7 shows a configuration example of an apparatus for carrying out the present invention. A horizontal laser beam 16 oscillated from a femtosecond laser 15 which is an ultrashort pulse laser is converted into a parallel beam 20 having a diameter enlarged by lenses 17 and 18 constituting a collimating optical system, and reflected by a femtosecond mirror 19. Then, it is deflected in the vertical direction, passes through the dichroic mirror 27, enters the condenser lens 29, and is irradiated onto the workpiece 31. A laser beam 23 emitted in the horizontal direction from the ultraviolet laser oscillator 22 is converted into a beam 26 having a diameter enlarged by collimating optical system lenses 24 and 25, reflected by a dichroic mirror 27, and coaxially with the femtosecond laser beam 21. A converging lens 29 irradiates the condensing point 30 of the workpiece 31 with the converging lens 29.

  The workpiece 31 is installed on an x, y, z drive stage 32 and used for setting a processing point. The femtosecond laser beam 16 and the ultraviolet laser 22 are synchronously emitted with an oscillation beam, and when they reach the surface of the workpiece, the timing is such that the beams arrive at the same time, or the excited state by the irradiation of the ultraviolet laser beam. Control is performed so that the femtosecond laser is irradiated while the influence continues. The fact that the oscillation wavelength of the femtosecond laser is not particularly limited is apparent from the fact that the gist of the present invention is processing by ablation of a substance accompanying absorption of multiphotons from an electronic state in which the surface of the workpiece is excited by ultraviolet rays. It is.

  Since the ultraviolet laser excites the workpiece surface to increase the number of electrons in the valence band, it does not have to be a light source having a coherent quality like laser light, and has sufficient power for excitation. Any light source may be used. The wavelength of the ultraviolet light source may be shorter than the wavelength having energy corresponding to the band gap of the insulator. The collimating optical systems 17 and 18, the femtosecond mirror 19, and the dichroic mirror 27 have chromatic dispersion characteristics so that the pulse width is not broadened at the condensing point 30 due to the equivalent of the ultrashort pulse 16 or chromatic dispersion caused by reflection. It is necessary to select. For example, if the dispersion of the lens or dichroic mirror shows positive dispersion, the dispersion characteristic of the femtosecond reflection mirror is set to a negative dispersion characteristic, and the entire optical path is designed to be zero dispersion.

  FIG. 8 is a structural example of an apparatus showing another embodiment of the present invention. Parts having the same functions as those in FIG. In this configuration, the ultraviolet light source 22 is arranged so as not to be coaxial with the ultrashort pulse femtosecond laser beam, and is irradiated from an oblique direction. An arrangement is used in which the ultraviolet beam 23 is reflected by a mirror 41 and irradiated with a condenser lens 42 while aiming at a processing point 30 of the workpiece. Since the effect of the present invention can be obtained by exciting the surface of the processing point with the ultraviolet beam, it is not always necessary to irradiate it with the ultrashort pulse beam.

  As an application example of the present invention, precision processing without thermal influence can be realized at high speed by irradiation with an ultrashort pulse laser and ultraviolet light. Therefore, processing by irradiation with only a femtosecond laser has a low processing cost due to low speed. Even in application fields that have become too practical, high speeds can be achieved by the present invention, so precision microfabrication of nanotechnology, micromachines, semiconductors, superconducting ceramics, functional ceramics, crystalline materials, dielectrics, and other hard and brittle materials Can be applied to. It can be applied to various fields such as laser processing, laser medical use, information processing, measurement, bioscience, physics and chemistry, display, etc., and has a great effect in promoting practical application to the industrial field as a highly reliable laser system.

Energy diagram of conventional ablation mechanism using ultraviolet rays. The energy diagram of the mechanism of the conventional multiphoton absorption ablation processing (femtosecond processing). The energy diagram at the time of softening phenomenon of ceramics etc. by ultraviolet irradiation. The energy diagram of the present invention for ultraviolet excitation and multiphoton absorption by simultaneous absorption of ultraviolet and ultrashort pulse laser focused beam. The energy diagram of this invention which irradiates the ultra-short pulse laser focused beam to the workpiece in the excitation state by the ultraviolet rays irradiated previously, and performs the process by multiphoton absorption. FIG. 3 is an energy diagram of the present invention in which an ultra-short pulse laser focused beam is irradiated to a workpiece in an excited state trapped in a forbidden band level impurity level irradiated with ultraviolet rays in advance and processed by multiphoton absorption. The processing apparatus of the present invention for irradiating a workpiece with an ultrashort pulse laser beam and ultraviolet rays in a coaxial arrangement. The processing apparatus of the present invention for irradiating a workpiece with an ultrashort pulse laser beam and ultraviolet rays in a non-coaxial arrangement.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Vacuum level 2 ... Conduction band 3 ... Band gap 4 ... Valence band 5 ... Electron 6 ... Ultraviolet excitation path 8-1, 8-2, 8-3, 8-4 .. Ablation by multiphoton absorption,
9 ・ ・ ・ ・ ・ Photon (UV)
10-1, 10-2 ... Electron excitation to the vacuum level of the conduction band by ultrashort pulses (multiphoton absorption)
11-1, 11-2... Electron excitation to the vacuum level of the conduction band by ultrashort pulses 12... Electron 13-1 trapped in the impurity level of the forbidden band level, 13-2... Electron excitation of the conduction band to the vacuum level by an ultrashort pulse 15... Ultrashort (femtosecond) laser oscillator 16. 18 .. Collimating optical system 19... Femtosecond mirror 22... Ultraviolet laser oscillators 24 and 25.
27 ... Dichroic mirror 29 ... Condensing lens 30 ... Condensing point (processing point)
31 ... Workpiece 32 ... x, y, z axis drive stage

Claims (10)

  Laser processing including a processing head having an optical system for guiding and condensing ultrashort pulse laser light oscillated from a laser oscillator to a processing point of a workpiece, and an ultraviolet light source for irradiating the vicinity of the processing point apparatus.   The laser processing apparatus according to claim 1, wherein the ultrashort pulse laser beam laser oscillator is a mode-lock oscillation solid-state laser.   The laser processing apparatus according to claim 1, wherein the ultrashort pulse laser light is chirped pulse amplification and pulse compression mode-locked pulse output light.   4. The laser processing apparatus according to claim 1, wherein the ultraviolet light source is an ultraviolet laser emitted from a laser oscillator or an incoherent ultraviolet ray.   The laser processing apparatus according to claim 1, wherein the workpiece is any one of a semiconductor, ceramics, glass, and transparent crystal.   6. The laser processing according to claim 1, wherein, at the irradiation timing of the ultrashort pulse laser beam and the ultraviolet light, the ultrashort pulse laser is irradiated at least during a period in which the influence of the ultraviolet light irradiation on the surface of the workpiece appears. apparatus.   The optical element that guides the ultrashort pulse laser beam uses an optical element that keeps the expansion of the pulse width small by compensating the wavelength dispersion of the light guide through the pulse width of the ultrashort pulse at the condensing point. The laser processing apparatus according to any one of 1 to 6.   Energy absorption process in which the ultraviolet electron excitation absorption process that excites the valence band electrons of the object irradiated with ultraviolet light to a higher level and the absorption process of multiphotons that irradiate the object with ultrashort pulse laser light occur synchronously A laser processing method including:   The laser processing method according to claim 8, wherein an excitation level in the ultraviolet electron excitation absorption process is a conduction band or a forbidden band trap.   10. The laser processing method according to claim 8, wherein the electrons of a level that absorbs the multiphotons in the absorption process of the multiphotons are electrons excited by ultraviolet irradiation to a trap level in a conduction band or a forbidden band.

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