JP2006110587A - Laser interference machining method and device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce a machining width and a machining depth by cutting interference light short in irradiation time. <P>SOLUTION: In the laser interference machining method where the surface or inside of a base material is machined or reformed by interference light obtained by superimposing two pulse beams P<SB>1</SB>, P<SB>2</SB>split by a beam splitter, the light path of the pulse beams P<SB>1</SB>, P<SB>2</SB>is provided with a mask-lens system for shaping the cross-sectional shape of the beams, and the interference region width DE of both the pulse beams P<SB>1</SB>, P<SB>2</SB>is made narrower than the interference-realizable region width AB by ideal plane waves, thus the interference light whose irradiation time is short is removed, and the machining width and machining depth by interference light are made uniform. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a laser interference processing method and apparatus for processing and modifying a workpiece using an ultrashort pulse laser.

It is known that the femtosecond laser has a very short pulse of 1 to 999 × 10 ⁻¹⁵ seconds and is hardly affected by heat diffusion in the ablation processing by absorption of laser energy. Taking advantage of this feature, non-patent document 1 and the like have reported processing on the order of 100 nm using a femtosecond laser. In addition, since the intensity of the pulse reaches 1 TW (10 ¹² W) or more, multi-photon absorption in which one electron absorbs multiple photons at a time when a material is irradiated with a femtosecond laser that has been condensed to increase the light intensity. It is known that this phenomenon occurs. If this phenomenon is used, it is possible to cause laser absorption only inside transparent materials such as glass, and voids can be formed inside these materials. It is also known that multiphoton absorption occurs even when irradiated with a relatively weak femtosecond laser that does not lead to void formation, causing a change in refractive index and etching rate.

  Examples of applications using these phenomena include the production of photonic crystals in which voids are periodically arranged inside the glass (see Non-Patent Document 2), and the production of optical waveguides that apply changes in the refractive index of glass (Non-Patent Documents). 3), and the production of a liquid flow channel using an etching rate change (see Non-Patent Document 4) is known. In addition, the application of these two features to the processing of living bodies such as the cornea and the analysis by multiphoton absorption has been actively studied.

  Furthermore, the femtosecond laser is a laser having a very high coherence, and when two femtosecond lasers are overlapped by an appropriate optical system, a light intensity fringe (interference fringe) is generated due to interference. Examples of applying the coherence of the femtosecond laser are disclosed in Patent Document 1 and Patent Document 2.

  Patent Document 1 discloses that a femtosecond laser pulse divided by a beam splitter is spatially and temporally overlapped to cause interference, irradiate the surface of the substrate, and the shape change or refractive index of the substrate surface due to ablation. The interference fringes are recorded on the surface of the substrate as a change in the above. It is also stated that grooves with the same pitch as the interference fringes can be processed on the glass surface by irradiating the glass surface with interference femtosecond laser of pulse energy that does not lead to ablation, and then etching with 1% -HF. ing. As described above, when the interference of the femtosecond laser is used, a periodic structure of several tens to several hundreds or a periodic refractive index change can be performed to several tens to several hundreds μm without a complicated process such as photolithography. This area can be easily processed.

Moreover, in patent document 2, the structure which has a period of two directions is obtained by irradiating the base material twice by changing the incident direction of the beam with the femtosecond laser pulse interfered with the apparatus similar to patent document 1. It is stated that it can be produced on the surface of a substrate. The content of the invention disclosed in Patent Document 1 or Patent Document 2 is described in detail in Non-Patent Document 5 as an academic paper.
Optics & Laser Technology34 (2005) 575 AppliedSurface Science 197-198 (2002) 705-709 Proceedings of the 42nd Laser Thermal Processing Seminar (1997.11) p.105 March 1, 2001 / Vol. 26, No.5 / Optics Letters 54th Annual Meeting of the Laser Processing Society (2001.11) p.16 JP 2001-236002 A JP 2003-57422 A

  However, when a conventional femtosecond laser interferometer is used, the depth, width, or refractive index change rate of the structure formed by the interference is widely distributed in the region irradiated with the pulse. It was.

  FIG. 14 and FIG. 15 are diagrams showing the processing shape (processing pattern) when the surface of the substrate 201 is ablated using conventional femtosecond laser interference. FIG. 14 shows a case where only one place is processed without moving the laser irradiation region, and FIG. 15 shows a case where a wide area is processed by arranging the processing patterns shown in FIG. In the fully processed region α in the center of the laser irradiation region in FIG. 14, a periodic structure by ablation is clearly formed. However, in the region β on both sides off the center of the laser irradiation region, the contrast of the periodic structure is not clear, and the boundary between the processed region and the unprocessed region is very ambiguous.

  As shown in FIG. 15, even when the processing patterns of FIG. 14 are arranged and a wide region is processed, the processing width and processing depth distributions in the processing regions α and β are not eliminated.

  The present invention has been made in view of the above-mentioned unsolved problems of the prior art, eliminates the distribution of the processing depth and processing width in laser interference processing, and uses the interference light of a femtosecond laser for higher accuracy. It is an object of the present invention to provide a laser interference processing method and apparatus capable of processing and modifying a highly reliable device and the like.

  In order to achieve the above object, the laser interference processing method of the present invention was obtained by splitting a coherent pulse laser beam into two pulse beams and overlapping them by overlapping at a predetermined angle. In a laser interference processing method of irradiating a workpiece with interference light and processing / modifying the workpiece, an ideal plane wave having a cross-section in the interference period direction of the interference light having the same pulse length as the two pulse beams Is smaller than the interference possible region obtained by superimposing them at the predetermined angle.

  By using the interference light only at the center of the region where interference is possible, unevenness in the processing depth and processing width due to the distribution of the irradiation time can be reduced, and groove processing with high shape accuracy can be performed.

As shown in FIG. 1, pulse beams P ₁ and P ₂ obtained by dividing femtosecond laser light into two are overlapped at an angle θ to cause interference in a limited region. As shown in FIG. 2, when processing or modifying the base material 1 by irradiating the base material 1 which is a workpiece as shown in FIG. 2, the cross-sectional dimensions of the interfering pulse beams P ₁ and P ₂ are set. By limiting, the cross section of the interference light by the pulse beams P ₁ and P ₂ is compared with the possible interference width AB in the interference period direction in which the ideal plane waves S ₁ and S ₂ having the same pulse length are caused to interfere at the same angle θ. By narrowing the interference area width DE, which is the above, interference light having a short irradiation time is eliminated, and the distribution of the processing width and processing depth is eliminated.

The present inventors have found that the distribution of the processing width and the like in the laser interference processing described above is caused by the distribution of the interference light irradiation time on the surface of the base material through detailed examination. Since the femtosecond laser has a very short pulse length of several tens to 300 μm, even when infinitely wide ideal plane waves S ₁ and S ₂ interfere with each other as shown in FIG. (Region) is limited to the diamond-shaped region in the figure. As shown in FIGS. 4A to 4C, since the rhombus region is incident on the base material 1 while maintaining its shape, points A and B located at the end of the rhombus and the center of the rhombus At point 0 located at, the time of exposure to the interference light is significantly different as shown in FIG. The processing depth and width and the degree of modification depend on the irradiation time of interference light, and the processing or modification proceeds sufficiently in a region where the irradiation time is long, but is insufficient in a region where the processing time is short. Therefore, a distribution occurs in the degree of processing and modification between the irradiation regions A and B.

The interference possible area width AB is expressed by the following equation when the pulse length of each of the pulse beams P ₁ and P ₂ is L:

によって算出される。また、干渉パターンのピッチｄは、以下の式

Is calculated by The pitch d of the interference pattern is expressed by the following equation:

によって算出される。

Is calculated by

Therefore, as shown in FIGS. 1 and 2, in the xy cross section, the width DE in the interference period direction of the interference region cut off at the incident surface of the pulse beams P ₁ and P ₂ is made smaller than the interference possible region width AB. In addition, by using interference only in the central part of the interference possible area, the occurrence of interference is suppressed between the areas A and D and the areas E and B where the irradiation time of the interference light is insufficient.

By irradiating the base material 1 with such interference light, it is possible to improve the distribution of processing and modification caused by uneven irradiation time. Further, the boundary between the processed or modified region and the non-processed region becomes clear, which is convenient when a large region is processed or modified by arranging these patterns. The restriction on the interference region width DE is for the direction parallel to the incident surface of the pulse beams P ₁ and P ₂ , and for the interference region in the direction perpendicular to the beam incident surface (z direction), It does not add any restrictions. Therefore, if the interference region is elongated in the z direction, a sufficient processing area and modified area can be secured.

FIG. 5 is a schematic diagram showing a laser interference processing apparatus according to an embodiment. A regenerative amplification system using a titanium sapphire crystal was used for femtosecond laser oscillation. The laser wavelength, pulse width, oscillation period, pulse energy, and 1 / e ² intensity beam diameter (φ (1 / e ² )) are 800 nm, 100 fs, 40 kHz, 3 mJ, and 8 mm, respectively. A femtosecond laser oscillator (not shown) is combined with an optical shutter synchronized with the oscillator and an ND filter so that an appropriate number of pulses having an appropriate pulse energy can be taken out.

The femtosecond laser pulse 2 which is a pulse laser beam whose energy and the number of pulses are controlled in this way is split into two pulse beams 2 a and 2 b having the same energy by the beam splitter 3. The divided pulse beams 2a and 2b were Gaussian beams with approximately φ (1 / e ² ) = 8 mm, and the light intensity at the center was ² mJ / cm ² . The divided pulse beams 2a and 2b are turned by mirrors 5a and 5b, respectively, and then pass through mask-lens systems 6a and 6b, which are beam shaping means, and overlap and interfere with each other on the surface of the substrate 1. _1, a P _2. The optical axes of the two pulse beams 2 a and 2 b are adjusted by the mirrors 5 a and 5 b, and their optical paths overlap on the surface of the substrate 1.

  Since the femtosecond laser has a pulse length of only 30 μm, in order to overlap the femtosecond pulses, it is necessary to match the optical path length from the beam splitter to the interference region on the order of several μm.

  Therefore, in order to adjust the optical path length of several μm, the optical path length adjuster 7 is installed in one of the optical paths of the divided pulse beams 2a and 2b. Specifically, in order to correct the optical path length difference caused by whether or not the beam passes through the beam splitter 3, a quartz plate having an appropriate thickness is inserted. In the present embodiment, the angle θ at which the divided pulse beams 2a and 2b overlap is 45 °. Further, a glass flat plate was used for the substrate 1.

FIG. 6 shows details of the mask-lens systems 6a and 6b. A point c in the optical path of the pulse beams P ₁ and P _{2 that} interfere with each other is the center of the interference region, and the surface of the substrate 1 that is a glass plate. Is consistent with Each mask-lens system 6a, 6b includes a mask 8 that is an optical mask and two cylindrical lenses 9 and 10 having different focal lengths. The focal lengths of the cylindrical lenses 9 and 10 are f = 48 mm and f = 10 mm, respectively. In the two cylindrical lenses 9 and 10, the axis of one cylindrical lens 9 is parallel to the xy plane that is the incident surface of the pulse beams P ₁ and P ₂ , and the axis of the other cylindrical lens 10 is perpendicular to each other. It is arranged to be.

The opening of the mask 8 is a 3 mm × 3 mm square, and is arranged so that the optical axis passes through the center thereof and one side of the square is parallel to the incident surfaces of the pulse beams P ₁ and P ₂ .

  The distances between the mask 8, the cylindrical lenses 9, 10 and the substrate 1 are as shown in FIG. 6. The positions of the cylindrical lenses 9, 10 are determined by the point c on the surface of the substrate 1 being the mask 8. Are finely adjusted so as to be the imaging conjugate point of the point b on the back surface of each. The reduction magnification of the mask image at the point c is different between the y direction parallel to the laser incident surface and the z direction perpendicular to the laser incident surface, and is 1/100 times and 1/20 times, respectively. Therefore, the 3 mm × 3 mm beam that has passed through the mask 8 becomes a 30 μm × 150 μm rectangle at point c.

FIG. 7 shows the spatial intensity distribution (beam profile) of the laser in the y direction and the z direction at points a, b, and c in FIG. The pulse in which the Gaussian beam skirt is cut by the mask 8 at the point b is reduced at different ratios in the y direction and the z direction, and forms a rectangular image at the point c. The peak light intensity of the pulse at this time is 4 J / cm ² .

Thus, the two pulse beams 2a and 2b divided by the beam splitter 3 are imaged as light beams P ₁ and P ₂ having a rectangular cross section of 30 μm × 150 μm at the point c. That is, when the shutter (not shown) is opened and the pulse laser beam is incident, the rectangular pulse beams P ₁ and P ₂ are overlapped, and interference occurs at the point c.

The substrate 1 which is a glass flat plate is installed so that the surface thereof coincides with the point c, and the surface is ablated every time interference occurs. FIG. 8 shows the spatial intensity distribution of the interference light formed by overlapping the _two pulse beams P ₁ and P _{2 at} the point c, and the formed periodic structure. The pitch d of the periodic structure as an interference pattern is shown in FIG. Was about 1.0 μm.

As can be seen from FIG. 8, since the pulse beams P ₁ and P ₂ are incident only on the central part of the interference possible area, the width of the interference possible area AB is about 78 μm, whereas the area in the central part thereof. Interference occurs only at DE (width of about 33 μm). As described above, by making the interference region width DE by the pulse beams P ₁ and P ₂ narrower than the interference possible region width AB, interference occurs in the regions (AD and EB) where the irradiation time of the interference light is short. To prevent that. Further, the width of the beam profile in the y direction is restricted as described above, but there is no such restriction in the z direction. In this embodiment, the processing area of 33 μm × 150 μm is secured on the surface of the substrate 1 by setting the width of the beam profile in the z direction to 150 μm.

  FIG. 9 shows an example in which the processing shapes shown in FIG.

(Comparative example)
For comparison, a comparative example processed by the same method as shown in FIGS. 3 and 4 will be described. Conventionally, there was no idea of controlling the beam profile (cross-sectional shape) of two interfering pulse beams in order to improve the distribution of processing and modification, so that a substrate 101 such as glass as shown in FIG. In order to obtain sufficient energy for processing, only the pulse beams 102a and 102b are collected by lenses 106a and 106b such as convex lenses and objective lenses. The lenses 106a and 106b are convex lenses of f = 1000 mm, and the glass plate base material 101 is disposed at the focal position of the convex lenses. As shown in FIG. 11, a Gaussian beam with φ (1 / e ² ) = 8 mm at point a in FIG. 10 is condensed into a Gaussian beam with φ (1 / e ² ) = 127 μm at point c, which is the focal position. The

  As shown in FIG. 12 (a), Gaussian beam interference occurs in the entire interference area width AB. Therefore, interference occurs even in areas close to point A and point B where the interference light irradiation time is short. As shown in (b) and (c) of the figure, a large distribution occurs in the processing depth and width. FIG. 13 shows a pattern in which the processing patterns of FIG.

  Comparing FIG. 8 and FIG. 12 or FIG. 9 and FIG. 13, it can be seen that the working example has a smaller processing depth distribution than the comparative example. In the comparative example, when the periodic structure was manufactured on the glass flat plate, the depth distribution was 0.15 to 2 μm, but when processed according to the above example, the depth distribution was 1.2 to 2 μm. There was a significant improvement in the uniformity of the processing depth.

In the above embodiment, the ablation processing of the glass surface was taken as an example, but the present invention is not limited to the ablation processing. If the peak light intensity of the pulse is lowered to 800 mJ / cm ² or less, the change in the refractive index will be described. It can be applied as a method for periodically generating. Further, the base material to be processed is not limited to glass, and may be metal, plastic, organic substance, or other substance.

It is a figure explaining the principle of the laser interference processing method by one Example. It is a figure explaining the irradiation time of the interference light in the laser interference processing method by the said Example. It is a figure explaining the principle of the laser interference processing method by a comparative example. It is a figure explaining the irradiation time of the interference light in the laser interference processing method by the said comparative example. It is a figure explaining the laser interference processing apparatus by one Example. It is a figure which shows the structure of the principal part of the apparatus of FIG. 7 is a graph showing the intensity distribution of a pulse beam at points a, b, and c in FIG. 6. It is a figure which shows the example of a process by the apparatus of FIG. It is a figure which shows the case where a wide area | region is processed with the apparatus of FIG. It is a figure explaining the laser interference processing apparatus by a comparative example. 11 is a graph showing the intensity distribution of a pulse beam at points a and c of the apparatus of FIG. It is a figure which shows the example of a process by the apparatus of FIG. It is a figure which shows the case where a large area | region is processed with the apparatus of FIG. It is a figure which shows the example of a process processed with the laser interference processing method by a prior art example. It is a figure which shows the case where a wide area | region is processed by the prior art example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Base material 3 Beam splitter 5a, 5b Mirror 6a, 6b Mask-lens system 7 Optical path length adjuster

Claims (4)

  The workpiece laser is processed by irradiating the workpiece with the interference light obtained by splitting the coherent pulsed laser beam into two pulse beams and superposing them at a predetermined angle to cause interference. In the laser interference processing method to be modified, the cross-section in the interference period direction of the interference light is smaller than the interference possible region obtained by superimposing ideal plane waves having the same pulse length as the two pulse beams at the predetermined angle. A laser interference processing method characterized by the above.   An oscillator that oscillates laser pulse light having a coherence with a pulse width of 1 femtosecond <τ <999 femtoseconds, a beam splitter that divides the laser pulse light into two pulse beams, and the two pulse beams are predetermined. Interference obtained by superimposing an ideal plane wave having the same pulse length as the two pulse beams on the optical system that generates interference light by superimposing at an angle of A laser interference processing apparatus comprising beam shaping means for shaping the two pulse beams so as to be smaller than a possible region.   The beam shaping means has an optical mask and a lens means arranged in an optical path until each pulse beam reaches the interference area, and the back surface of the optical mask and the center of the interference area are in relation to the lens means. 3. The laser interference processing apparatus according to claim 2, wherein the laser interference processing apparatus is in an image forming relationship.   4. The laser interference processing apparatus according to claim 3, wherein the lens means comprises a plurality of cylindrical lenses.

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