JP6005125B2 - Transparent material processing with ultrashort pulse laser - Google Patents

Description

  The present invention relates to ultrashort pulse laser processing of optically transparent materials, including material scribing, welding, and marking.

A Cutting and scribing Cutting of optically transparent materials is often done by mechanical methods. The usual method for processing thin flat materials probably uses a mechanical dicing saw. This is the standard method in the silicon wafer microelectronics industry. However, this method generates significant debris that must be managed to avoid particulate contamination, resulting in an increase in the overall cost of the process. In addition, thin wafers used in advanced microprocessor designs tend to shatter when cut with a dicing saw.

  To deal with these problems, state-of-the-art processing for material cutting such as “scribe and cleave” uses various lasers to scribe surface grooves into the material before breaking the material along this scribe. . For example, sub-picosecond laser pulses have been used to cut silicon and other semiconductor materials (H. Sawada, “Substrate cutting method,” U.S Patent No. 6,770,544). A focused astigmatic laser beam was also used to create a single surface groove for material cutting (JP Sercel, “System and method for cutting using a variable astigmatic focal beam spot,” US Patent Application No. 20040228004). This claims that an increase in processing speed is achieved by optimizing the astigmatism focusing geometry.

  In order to achieve accurate and high quality cleaving (cleavage), the groove must have a certain minimum depth, the value of which varies with the application (eg sapphire with a thickness of 100 μm is satisfactory) A groove with a depth of about 15 μm is required for proper cleaving). The groove depth decreases with increasing scanning speed, so the minimum depth limits the maximum scanning speed and thus the overall throughput. An alternative material cutting technique uses multiphoton absorption to form a single laser-modified line feature in the bulk of a transparent target material (F. Fukuyo et al., “Laser processing method and laser processing apparatus, "US Patent Application No. 20050173387). In the case of surface grooves, this sub-surface feature is equal to the limit on the processing speed of the material cutting, since there is a sub-surface minimum dimension feature that is necessary to cause accurate and high quality material cleaving.

  A notable application of “scribe and cleave” material cutting is wafer dicing for the separation of individual electronics and / or optoelectronic devices. For example, sapphire wafer dicing is used to unify blue light emitting diodes. Wafer singulation is done by laser ablation on the backside, minimizing contamination of the Uheha surface device (TP Glenn et al., “Method of singulation using laser cutting,” US Patent No. 6,399,463). . Assist gas is also used to assist the laser beam dicing the substrate (K. Imoto et al., “Method and apparatus for dicing a substrate,” U.S. Patent No. 5,916,460). In addition, the wafer is first diced using a laser to scribe the surface grooves and then use a mechanical saw blade to complete the cutting (NC Peng et al., “Wafer dicing device and method, "US Patent 6,737,606). Such applications are implemented in large volumes, and therefore processing speed is particularly important.

  One process uses two different types of lasers, one scribes the material and the other breaks the material (JJ Xuan et al., “Combined laser-scribing and laser-breaking for shaping of brittle substrates, "US Patent No. 6,744,009). A similar process uses a first laser beam to draw scribe lines on the surface and a second laser beam to split non-metallic material into separate pieces (D. Choo et al., “Method and apparatus for cutting a non-metallic substrate using a laser beam, "US Patent No. 6,653,210). Also, two different laser beams for scribing and cracking were used to cut glass plates (K. You, “Apparatus for cutting glass plate,” International Patent Application No. WO 2004083133). Finally, the laser beam is focused near the surface of the material, and the focal point is lowered through the material near the bottom surface while providing a relative lateral motion between the focused laser beam and the target material. (JJ Xuan et al., “Method for laser-scribing brittle substrates and apparatus therefore,” US Patent No. 6,787,732).

B Material Joining Joining two or more optically transparent materials, such as glass and plastics, is beneficial for various industrial applications. The manufacture of any type of device where optical transparency enables or adds functionality, or vice versa, provides added value (eg, aesthetics) would benefit from such a bonding process. One example is the hermetic sealing of components that require visual inspection (eg, communications and biomedical industries).

  In some applications, conventional bonding processes (eg, adhesives, mechanical bonding) are inadequate. For example, many adhesives have proven non-biocompatible with biomedical implant devices. For other devices, the adhesion is not strong at all for special applications (eg high pressure seals). For such demands, laser welding provides an ideal solution.

  In micro fluidic systems, the sealing of individual paths adjacent to each other with cap pieces covering the entire device is required. Due to the small contact area between different microfluidic paths, it is difficult to make a strong and firm sealing joint by other methods. Laser welding can accurately position the junction area between these microfluidic paths, resulting in a leak-proof seal.

Recent technology for laser welding of transparent materials is
(1) Use of a CO2 laser with a wavelength (-10 μm) that is linearly absorbed by many optically transparent materials, or (2) Specially designed to absorb laser radiation, thereby heating and melting the material Introduction of additives into the interface of transparent materials
including.

  Both of these methods are limited by their function and / or their implementation cost.

  A pulsed CO2 slab laser was used to weld Pyrex® to Pyrex® and to join polyimide and polyurethane to titanium and stainless steel (HJ Herfurth et al., “ Joining Challenges in the Packaging of BioMEMS, ”Proceedings of ICALEO 2004). Also, fused quartz and other difficult-to-melt materials were welded with a 10.2 μm CO2 laser (M.S. Piltch et al., “Laser welding of fused quartz,” U.S. Patent No. 6,576,863). The use of such a CO2 laser does not allow welding by focusing through a thick top layer material since the laser radiation is absorbed before reaching the interface. An additional disadvantage is that the large wavelength does not allow the beam to be focused into a small spot, thus limiting the benefit of forming small weld marks on the micron scale.

  Instead, an absorbent layer such as polyimide or acrylic that is transparent to the human eye is placed between the two materials to be welded (VA Kagan et al., “Advantages of Clearweld Technology for Polyamides,” Proceedings ICALEO 2002). Diode lasers with line focusing are used for welding (T. Klotzbuecher et al., “Microclear“ A Novel Method for Diode Laser Welding of Transparent Micro Structured Polymer Chips, ”Proceedings of ICALEO 2004). The laser is specifically designed to absorb the wavelength of the laser (RA Sallavanti et al., “Visibly transparent dyes for through-transmission laser welding,” US Patent No. 6,656, 315).

  One welding process for joining glass to glass or metal uses a laser beam to melt glass solder between the surfaces to be welded (M. Klockhaus et al., “Method for welding the surfaces of materials, "US Patent No. 6,501,044). Also, two fibers are welded together using an intermediate layer that is a linear absorber for the laser wavelength (MK Golldstein, “Photon welding optical fiber with ultra violet (UV) and visible source,” US Patent No. 6,663,297). Similarly, a fiber with a plastic jacket is laser welded to a plastic ferrule by inserting an absorbing interlayer (K.M. Pregitzer, “Method of attaching a fiber optic connector,” U.S. Patent No. 6,804,439).

  The use of an additional layer of absorbent material has major drawbacks. The most obvious is the cost of making or purchasing a suitable material for the process. A potential cost issue is the increased processing time associated with incorporating additional materials into the manufacturing process. In the case of biomedical implant devices, such costs are expected to rise dramatically as the required welding area becomes smaller and smaller. Another disadvantage of using an intermediate light absorbing layer is that it causes contamination in the area where this layer is sealed. For microfluidic systems, the light absorbing layer is in direct contact with the fluid through the channel.

  One method for welding a transparent material to an absorbent material is called through-transmission welding. In this method, a laser beam is focused on an absorbing material through a transmissive material, resulting in welding of the two materials (W.P. Barnes, “Low expansion laser welding arrangement,” U.S. Patent No. 4,424,435). This method was used to weld plastic by directing polychromatic radiation through the upper transparent layer and concentrating on the lower absorbing layer (RA Grimm, “Plastic joining method,” US Patent No. 5,840,147; RA Grimm, “Joining method,” US Patent No. 5,843,265). In another example of this method, a black mold material that is transparent to the laser wavelength is welded to the adjacent material or through the additional welding of an assist material that absorbs the laser wavelength (F. Reil , “Thermoplastic molding composition and its use for laser welding,” US Patent No. 6,759,458). Similarly, another method uses at least two diode lasers with a projection mask to weld two materials, at least one of which is a laser wavelength absorber (J. Chen et al., “Method and a device for heating at least two elements by means of laser beams of high energy density, "US Patent No. 6,417,481).

  In another laser welding method, the material is repeatedly heated until melting and welding occur, so the interface between the two materials is continuously scanned with a laser beam (J. Korte, “Method and apparatus for welding,” US Patent No. 6,444,946). ). In this patent, one material is transparent and the other material is opaque to the laser wavelength. Finally, one method uses ultraviolet light, laser, X-ray and synchrotron radiation to melt the two pieces of material and then contact them for welding (A. Neyer et al., “Method for linking two plastic work pieces without using foreign matter,” US Patent No. 6,838,156).

  Laser welding for hermetic sealing of organic light emitting diodes with at least one layer of organic material between two substrates has been disclosed ("Method of fabrication of hermitically sealed glass package", U.S. Patent Application Publication 20050199599). In the Japanese Journal of Applied Physics, Vol. 44, No. 22, 2005, Tamaki et al. Use a 1-kHz, 130-fs laser pulse to join transparent materials in “Welding of Transparent Materials Using Femtosecond Laser Pulses,” Are discussed. However, it is known that the interaction between the low repetition rate (kHz) ultrashort pulse and the material is clearly different from the high repetition rate ultrashort pulse (MHz) due to the electron-phonon coupling time constant and the storage effect. .

C Subsurface Marking Subsurface mark patterning on glass was modified by artists to create 2-D portraits and 3-D sculptures. These marks are designed to look good in a wide range of conditions that do not require external lighting.

  The energy that is focused closely below the surface of the optically transparent material creates visible radially propagating micro-cracks. Usually, a long pulse laser is used to make these marks. Some patents discuss changes in the size and density of these radial cracks to control the visibility of the next pattern (US Patent Nos. 6,333,486, 6,734,389, 6,509,548, 7,060,933). .

  Mark visibility is controlled by crack density around the central laser spot rather than by mark size (US Patent No. 6,417,485, “Method and laser system controlling breakdown proves development and space structure of laser radiation for production of high quality laser-induced damage image ”).

  US Patent No. 6,426,480 (“Method and laser system for production of high quality single-layer laser-induced damage portraits inside transparent material”) uses a single layer of smooth marks whose brightness (brightness) is controlled by spot density. use.

  Increasing pulse duration of writing laser light increases mark brightness (US Patent No. 6,720,521, “A method for generating an area of laser-induced damage inside a transparent material by controlling a special structure of a laser irradiation” ).

SUMMARY OF THE INVENTION Ultrashort laser pulses can energize very well defined regions in a bulk transparent material through nonlinear absorption. Matching laser properties and processing conditions can result in a series of features, refractive index changes that allow optical guiding, melting and subsequent bonding of internal interfaces, or formation of optical flaws that scatter light. Can do.

  The high repetition rate of the laser and the sufficient pulse and pulse overlap result in additional interaction between the next pulse in the same region as the material modification created by the previous laser irradiation. The light diffracts around the previously appearing modification, and through structural interference, another spot known as the “Arago spot” or “Poisson spot” in the “shadow” of the previous modification make. The spot size and intensity increase with distance and intensity asymptotically approaching the input laser intensity.

  One object of the present invention is to enable clean cleaving of the transparent material at a higher speed than in the prior art. This can be done with only a single pass beam across the material to create both a surface groove into the material and one or more laser modified regions into the material (or only multiple subsurface laser modified features). This is achieved using short laser pulses. Since multiple scribe features are created at the same time and are located both on and inside the bulk material, or only inside the bulk material, the success of the next cleaving need not depend on the depth of the surface grooves.

  During the scribing process of the scribe material, cracks begin with surface scribe features and propagate down the material. If the surface groove is too narrow, it results in a low quality split section and poor cleaving accuracy. However, if there is an additional scribe feature in the bulk material, the cracks are guided in a given direction through the material, giving a higher leveling accuracy and split section quality than would be expected with only a narrow surface scribe. Bring.

  When a sufficient portion of the bulk material is modified below the surface, the fissure can begin at the sub-surface modified region and propagate to the adjacent modified region of the bulk material without the need for surface scribe lines. Reducing the size of the surface grooves or eliminating them completely reduces the debris from processing that contaminates the processing environment or that requires extensive post-processing cleaning.

  Another object of the present invention is to focus ultrashort laser pulses below the surface to form subsurface flaw patterns in the transparent material. Slightly changing the processing conditions associated with scribing makes it possible to create subsurface optical scratches that scatter light. By controlling the nature and arrangement of these scratches, these patterns are made clearly visible when illuminated from the side and difficult to see when there is no illumination. This feature of sub-surface marking can be used for car indicator signs or lights, warning signs or lights, or simple value added to glass (eg, artistic) and others. This technique differs from known laser marking techniques that are designed so that scratches formed in the material are always clearly visible.

  In one embodiment of the invention, the pattern of optical flaws is formed at different depths of the transparent material. Having marks at different depths prevents the “shadow” effect that one mark blocks the illumination light from shining on the next mark. This structure reduces the amount of scattering from non-direct adjacent illumination sources and increases the on-off contrast. These scratches are separation points or stretch lines.

The small size and smooth profile of these scratches makes them invisible when there is no lighting. Also, the substrate will be stronger and not susceptible to crack propagation due to thermal or mechanical stress, especially for thin transparent materials. Also, the small size allows more jumpy writing positions per unit thickness and increases the pattern resolution for a given thickness of transparent material.

  There is a trade-off between the visibility of the mark when illuminated and the invisibility of the mark when not illuminated. This trade-off is adjusted by controlling the light source intensity of the illumination light, the mark size and smoothness, and the mark spacing. Control parameters for the size of the mark include the pulse duration, fluence, repetition rate, and wavelength of the laser, and the depth and rate of movement of the collection point within the material. It is important that these parameters need to be adjusted for transparent materials with different optical, thermal and mechanical properties.

  The desired pattern is made up of a collection of discrete pixels where each pixel is a collection of parallel lines. Utilizing pixels makes it possible to create a large overall image with fewer lines, with greater contrast visibility.

  The sub-surface pattern is illuminated with a suitably focused light source. Condensation is important for efficiently illuminating the pattern and minimizes leakage light. If the distance between the light source and the pattern is relatively short, this illumination light is supplied directly from the light source. If the distance is long, total internal reflection between the top and bottom surfaces of the transparent material is used for light guidance.

  Another option is to create an optical waveguide in the transparent material for supplying light. The advantage of the optical waveguide supply is that the path between the light source and the pattern need not be straight and / or short. In the case of an optical waveguide, the output end of the waveguide is suitably designed to illuminate a predetermined pattern.

  Two patterns of the same area are clearly distinguished separately and are controllably illuminated by two different light sources. The axis of the illumination light source for each pattern is orthogonal (perpendicular) to the marks that make up the pattern. In this manner, maximum scattering (and maximum visibility) from individual illumination sources is selected for the indicated pattern only.

  Another object of the present invention is to use a high repetition rate femtosecond pulsed laser to allow the joining of two transparent materials without a supplemental bonding agent. Focusing the ultrafast laser beam with a high repetition rate on the contact area between the two transparent material pieces causes bonding by local heating. The repetition rate required for sufficient heat accumulation depends on many different process variables including pulse energy, beam focusing geometry, and individual material properties to be welded. Theoretical analysis of the conditions affecting the femtosecond laser spot bonding process emphasizes the determination of optimal focusing conditions for the process (M. Sarkar et al., “Theoretical Analysis of Focusing and Intensity Mechanisms for a Spot Bonding Process Using Femtosecond Laser, "IMECE2003-41906; 2003 ASME International Mechanical Engineering Congress, November 2003, Washington, DC, USA).

  Non-linear absorption of laser radiation (caused by ultra-short pulse duration) and heat-to-pulse accumulation in the material (caused by high repetition rate) make welding transparent materials simple and flexible And is achieved with unmatched effectiveness in other ways that exist. The non-linear absorption process results in a concentration of absorbed energy near the weld interface, minimizing damage and minimizing optical distortion to the rest of the material. A precise weld line is possible when dense channels need to be separated.

In addition, embodiments of the present invention direct a focused region of the ultrashort pulse beam at a high repetition rate near the interface of the materials to be joined, thereby allowing a laser of two materials transparent to the wavelength of the laser radiation. Allows joining. The repetition rate of the laser pulse is between about 10 kHz and 50 MHz, and the laser pulse duration is between about 50 fs and 500 ps. The laser pulse energy and beam focusing optics are selected to produce an energy fluence (energy per unit area) greater than about 0.01 J / cm ² in the welded area.

The optimum range of fluence depends on the individual material being welded. In the case of transparent polymer materials (polycarbonate, PMMA (polymethylmethacrylate), etc.), the required fluence is smaller than that of glass. This is due to the widely different physical properties of the material. For example, the melting temperature of PMMA is ˜150 degrees Celsius and that of fused quartz is ˜1585 degrees Celsius. Therefore, a sufficiently large fluence is required to melt the fused quartz. Other important material properties include specific heat and thermal conductivity. The range of fluence for welding polymer materials is between about 0.01 and 10.0 J / cm ² .

  In general, welding requires that the two surfaces to be joined have no vertical gap between them. One object of the present invention is to form a raised ridge at the interface between two pieces to be welded, which bridges any gap between them. By focusing the high repetition rate fs-second pulse slightly below the surface, heating, melting, and pressure can cause local bulge of the glass surface. These ridges are several tens of nm to several μm high. If the irradiated energy is not sufficient to cause a raised ridge to join the connecting piece, a second pass of the laser at a slightly higher focus position welds the ridge to the connecting piece. If the single ridge is not high enough to bridge the gap, a second ridge is created above the connecting surface.

  Furthermore, welding of materials with varying linear absorption is achieved with the present invention. Even though the present invention uses a nonlinear absorption phenomenon as the first means to couple energy to the material, materials that exhibit a small amount of linear absorption of the emitted laser pulse are also welded using the methods disclosed herein. Since linear absorption is relevant to the present invention, an important aspect of linear absorption is that for high linear absorption, the thickness of the material on which the beam is focused is reduced. Furthermore, higher linear absorption reduces the degree of localization of the weld feature.

  The spatial distribution of the laser fluence affects the welding quality. Although a typical laser process involves focusing a Gaussian laser beam to produce a smaller Gaussian laser beam, an ideal beam shaping method can improve the quality and / or efficiency of a particular welding process. Used to improve. For example, the conversion of a typical Gaussian fluence distribution into a spatially uniform fluence distribution (known as a “flat top” or “top hat” intensity distribution) results in a more uniform weld feature.

  The ultrashort nature of the pulse allows bonding to the transparent material through a non-linear absorption process of laser energy, but this process generally does not result in heating of the material and as such alone does not allow laser welding. It is an additional aspect of high pulse repetition rate combined with a specific range of other processing conditions. The high repetition rate allows the heat to accumulate in the material so that the material is melted and then cooled and joined.

  The elimination of supplemental bonding agents reduces processing time and costs, eliminates contamination within the device due to excess bonding agents, and maintains accurate dimensional tolerances. The junction points and lines can be brought very close to other features without causing any interference. Also, due to the concentrated fluence at the collection volume and the non-linear absorption process, a very limited thermal strain of the material in the vicinity of the weld area is possible.

FIG. 2 is a diagram of a system used in a method for scribing a transparent material according to one embodiment of the present invention, showing a system configuration. FIG. 2 is a diagram of a system used in a method for scribing transparent material according to one embodiment of the present invention, showing a detailed perspective view of scribing and subsequent cleaving. FIG. 4 is a diagram of a surface and bulk scribe feature produced by a focused Gaussian beam for one embodiment of the present invention. 1 is a diagram of a system that uses an axicon lens to generate multiple subsurface scribe lines for one embodiment of the present invention. FIG. FIG. 4 is an intensity contour plot of a focused Gaussian astigmatism beam used in one embodiment of the present invention. It is a figure of the diffractive optical element (DOE) used for one Example of this invention. FIG. 2 is a diagram of a system used in a method for welding transparent material according to one embodiment of the present invention, schematically showing the system. FIG. 2 is a diagram of a system used in a method for welding transparent material according to one embodiment of the present invention, showing an enlarged perspective view showing details of beam focusing in nearby materials. FIG. 4 is a diagram of a welding process in which a raised ridge is used to fill a gap between two pieces. (A) shows the gap, (b) shows the ridge formed by condensing the laser beam slightly below the surface of the lower piece, and (c) is joined to the ridge where the laser condensing is raised. Figure 2 shows a weld formed when moved up to the interface between the upper piece to be made. A sub-surface marking is shown, where an arrow mark has been used as an example of a possible marking for the present invention. A sub-surface marking is shown, where an arrow mark has been used as an example of a possible marking for the present invention. A sub-surface marking is shown, where an arrow mark has been used as an example of a possible marking for the present invention. It is an optical microscope photograph which shows the experimental result of one Example of this invention. 2 is a continuous image showing fused quartz welding for one embodiment of the present invention. (A) shows the fused quartz before breaking the weld apart, (b) shows the bottom of the fused quartz after breaking the weld apart, and (c) shows the fused quartz after breaking the weld apart. Top view is shown. 3 is a photograph of a glass mark sample made in connection with the present invention. 3 is a photograph of a glass mark sample made in connection with the present invention. 3 is a photograph of a glass mark sample made in connection with the present invention. A photograph of a prior art decoration document and its individual marks formed by laser marking using a long pulse laser.

  The invention will be more clearly understood from the following description in conjunction with the accompanying drawings.

1. Ultrashort Pulse Laser Scribing FIG. 1 shows one embodiment of the present invention which is a method of scribing a transparent material for subsequent cleaving. This embodiment includes a laser system (1) that generates a beam (2) of ultrashort laser pulses, an optical system (6) that generates the required laser beam intensity distribution, and a scribe that is transparent to the wavelength of the laser pulses. The target material (7) to be used is used. Further, a Z-axis stage (8) is used for beam focusing position (depth) control, and an automatic XY axis stage assembly (9) causes the sample piece (7) to be relative to the focused laser beam. Required to move perpendicularly to each other. Instead, the scanning mirror (3), (4), (5) is used to move the laser beam (2) relative to the stationary target material.

  A laser beam (2) is directed through an optical system (6) that converts the laser beam (2) to create a predetermined three-dimensional intensity distribution. Individual regions of the converted laser beam cause ablation and / or modification of the target material through nonlinear absorption. Material ablation generally means evaporation of the material by intense laser irradiation. Material modification broadly means changes in the physical and / or chemical structure of the irradiated material and affects the propagation of cracks in the material. Laser modification generally requires lower light intensity than laser ablation for certain materials.

  The converted beam is directed to the target transparent material (7) to cause ablation / modification of the material (7) in the material (7) and / or on the surface at multiple determined positions. Ablated and / or modified regions are generally located in the material (7) along the optical propagation axis and separated by a predetermined distance within the material (7). The converted beam and the target material (7) are moved relative to each other, resulting in the simultaneous generation of multiple laser scribe features in the material (7). Multiple scribing allows the material to be cleaved by applying an appropriate force (see FIG. 1 (b)).

FIG. 2 shows another embodiment of the present invention, in which a laser beam having a Gaussian spatial intensity distribution (10
) Is collected to create sufficient intensity for non-linear absorption and subsequent ablation or modification of the target material (7). The strong collection region is located at a selected location in the bulk material below the surface of the material. Furthermore, by using an appropriate condensing optical system and laser pulse energy, a region of sufficient strength to cause material ablation is generated simultaneously at or near the surface of the material (11).

The important plane is not only in the bulk material (where the focused beam waist is located) but also at another point on the optical propagation axis before the beam waist (12) (both in the bulk and on the surface of the material) ) The pulse energy and focusing geometry are chosen so that there is sufficient intensity to cause ablation or modification simultaneously. When the laser pulse encounters the target material (11), their high intensity regions (near the center of the radial Gaussian intensity distribution) are nonlinearly absorbed by the material and ablation or modification occurs. However, certain areas outside the laser beam are too weak to be absorbed by the material and continue to propagate to the beam waist located further in the bulk material. The beam diameter at the beam waist position is small enough to regenerate enough intensity to cause nonlinear absorption and subsequent laser modification in the bulk material.

  The region immediately below the surface ablation is modified by subsequent pulse diffraction and structural interference after the initial surface feature was created (Arago spot). The relatively high repetition rate laser light source makes this process even more efficient with a reasonable movement speed.

  Under these focusing and pulse energy conditions, the relative movement of the material (11) with respect to the laser beam (10), together, allows multiple laser modification regions (ie, high precision cleaving of the material). , Resulting in the formation of surface grooves (13) and one or more bulk modified regions (14), or two or more bulk modified regions).

  FIG. 3 shows another embodiment of the present invention, where an axicon lens (20) is used to generate multiple internal scribe lines (21). When illuminated with a laser beam (22), the axicon lens (20) produces a beam known as the zero order Bessel beam. This name comes from the fact that the mathematical description of the optical intensity distribution in the plane perpendicular to the propagation axis is defined by a 0th order Bessel function whose position in the radial direction from the beam center is an independent variable. High intensity capable of propagating with the same small size for much larger distances (ie, greater than the Rayleigh range of a normal focused beam) than for similarly sized spots produced by a normal focusing method This beam has the rare property of having a central beam spot. The central intensity field is surrounded by a plurality of concentric light rings that decrease with increasing radius. Due to the inward radial component of the propagation vector, these rings of light continue to reproduce a small central beam spot (25) as the Bessel beam propagates. Thus, a small high intensity central beam spot (23) is generated, whose small diameter maintains a small diameter over the entire thickness of the target material (24). A Bessel beam is also commonly referred to as a non-diffracted beam because of its tight beam collection over a wide area.

  Since the outer ring reproduces a strong center spot (23), if the center spot (23) is strong enough to cause ablation at the material surface (26), the ring (has a larger diameter than the ablated area). Refocuses on the center of the beam after a short distance and reproduces a strong central spot where ablation or material modification can occur again. With appropriate optical system design and sufficient pulse energy, this ablation and subsequent beam reproduction process can be repeated throughout the entire bulk of transparent material (24). Other optical elements such as distributed index lenses and diffractive optical elements are also used for Bessel beam generation.

  In additional embodiments of the present invention, alternative beam intensity conversion techniques well known to those skilled in the art are employed in the optical system of the present invention to tailor the beam intensity to produce multiple scribe lines in the target material. Is done. One such method utilizes astigmatism beam focusing to create two different high intensity regions that are separated by a predetermined distance. FIG. 4 shows an intensity distribution plot of the collected astigmatism Gaussian beam, and the focal planes of the X and Y axes in the figure are 20 μm apart. Note the presence of two different high intensity regions (distinguishable by constant intensity contours). When directed to the target material, these two regions are used to create a multiple scribe feature.

  Another method for generating multiple scribe features in transparent materials employs diffractive optical elements (DOEs) designed to generate multiple regions of high light intensity at different locations along the beam propagation axis. FIG. 5 shows how such a DOE can function. When these multiple high-strength regions are directed to the target material, it creates a multiple scribe feature for material cleaving.

  For the various beam collection and / or intensity mapping methods used to generate multiple scribe ablation features, additional optical elements are introduced to generate an elliptical component for the overall beam shape. . By aligning the elliptical beam so that the long axis is parallel to the beam scanning direction, a higher scanning speed is achieved. The elliptical beam shape allows for sufficient pulse-to-pulse overlap for machining of smooth and continuous scribe features (dot scribe features where the spatially separated pulses are ablated on the opposite side) Thus, a high scanning speed is achieved. Increasing pulse overlap and high scan speed are achieved with a large circular beam spot, but this often results in undesirably wide scribe features.

2. Ultrashort Pulse Laser Welding Another embodiment of the present invention relates to a process for laser welding of transparent materials. As shown in FIG. 6, this embodiment uses a laser system (50) that generates a high repetition rate ultrashort laser pulse (51) beam, a condensing element (55) with sufficient condensing power (eg, Lens, microscope objective lens) and at least two materials (56) and (57) joined together. At least one of the two materials is transparent to the wavelength of the laser. In addition, a beam focusing and positioning stage (58) is used to adjust the focusing position of the laser beam (51), and an automatic stage assembly (59) focuses the sample pieces (56) and (57) into the focusing laser. Usually required to move relative to the beam.

  In this example, the two materials to be laser welded “top piece” (56) and “bottom piece” (57)) create a slight or no gap interface between their faces. They are placed in contact with each other. A lens (55) is placed in the path of the laser beam to create a collection area for high intensity laser radiation. The two transparent materials (56) and (57) are positioned relative to the focused laser beam so that the beam focusing area extends to the interface between the top piece (56) and the bottom piece (57). Is done. With sufficient laser intensity, material interface welding occurs. By moving the transparent materials (56) and (57) with respect to the beam focusing area and at the same time keeping the interface of the materials (56) and (57) very close to the beam focusing area, the predetermined length of laser welding is reduced. can get. In a very specific application of this embodiment, the focused laser beam moves through the top (transparent) piece (56) and the focusing area is near the interface between the top piece (56) and the bottom piece (57). The materials (56) and (57) are arranged so that the two materials are welded together.

  Unlike other welding processes, the process of the present invention initially welds using nonlinear absorption rather than linear absorption. For this reason, this welding process has unique properties. Nonlinear absorption is very intensity dependent so that the process is limited to the focus of the laser beam. Thus, absorption can be made to occur only around the focal point at a deep location in the transparent material. Typical nonlinear absorption with ultrashort pulses results in plasma formation and very little (if any) heat buildup, so ablation with ultrafast lasers can create a very small heat affected zone (HAZ). Bring. However, if the intensity is high enough to cause non-linear absorption but low enough not to cause ablation, some heat is accumulated. When the laser repetition rate is increased sufficiently, heat is sufficiently accumulated in the material to cause melting.

The laser system (50) emits a substantially collimated laser beam (51) of pulses having a pulse duration in the range of about 200-500 fs and a wavelength of about 1045 nm with a pulse repetition rate between 100 kHz and 5 MHz. The first beam deflection mirror (52) directs the laser beam to the power conditioning assembly (53). This assembly is used to adjust the pulse energy used in the welding process. Specific methods for performing such attenuation are well known to those skilled in the art. The second beam deflection mirror (54) directs the beam to the beam focusing objective lens (55). The beam focusing objective (55) has a maximum value at approximately a distance (F) from the beam focusing objective (55) to achieve a suitable fluence (energy / unit area) for processing. Focus laser pulses. The beam condensing positioning stage (58) moves the beam condensing objective lens (55) so that the maximum fluence region is located at the interface between the target materials (56) and (57). The XY stage assembly (59) targets the focused beam to provide a focused beam so that an array of linear or circular weld features can be generated at the interface of the target material (56) and (57). The materials (56) and (57) are moved.

  FIG. 7 shows another embodiment of the present invention where welding is required between two pieces separated by a small gap (60). Initially, the laser beam (51) is focused below the surface of the bottom piece (57). With proper control of the pulse energy and focusing conditions, the sample is transferred in response to the beam focus (or because the beam moves in response to the target), thus forming a raised ridge (61). This raised ridge (61) bridges the gap between the top and bottom targets. The second pass of the laser with that beam focus rises to a height near the interface between the top of the raised ridge (61) and the top piece (56), forming a weld (62).

3. Visible / Invisible Laser Mark The same system shown in FIG. 1a is used to form a sub-surface mark in a transparent material, where the applied laser beam is focused below the surface of the transparent material substrate. FIG. 8 is a schematic top view of an arrow pattern (63) drawn on a transparent material (64) such as glass. The light source (65) injects light into an optical waveguide (66) that supplies light to the arrow marks to illuminate the pattern. The output numerical aperture of the optical waveguide should be appropriately designed to fully illuminate a given source. Multiple optical waveguides are used to illuminate different areas of the pattern. Controlling the timing of different illumination sources can create different decorations and cues. Instead, rather than using an optical waveguide, the pattern is illuminated directly with a suitably focused light source.

  FIG. 9 (a) shows an enlarged schematic top view of the arrow mark (63) made entirely of parallel lines orthogonal to the illumination light. These parallel lines are generated by tightly focusing the laser light within the target substrate to create a region of material modification. FIG. 9B shows a schematic side view of the arrow mark (63). Arrows consist of groups of marks with different depths. These marks scatter the light supplied in the optical waveguide (66) towards the observer (67). The brightness is controlled by the intensity of the illumination light, the size of the individual marks, and the density of the marks.

  FIG. 10 shows a schematic diagram of a concept where the pattern consists of “pixels” (68), where each pixel is formed at a different depth formed by tightly focusing laser light to modify the substrate material. A group of parallel lines (69).

Experimental results Ultra-short pulse laser scribing As shown in FIG. 11, a pair of scribe lines (surface groove (70) and sub-surface scribing are formed on a 100 μm-thick sapphire wafer using a 20 × aspherical focusing objective lens (focal length 8 mm). (71)) was processed simultaneously with a single pass of the laser beam. The split surface shows good quality. The scanning speed was 40 mm / s (non-optimized).

  Using the same laser pulse energy and repetition rate, for only surface scribe lines under the same processing conditions (ambient atmospheric environment, etc.), the maximum scribe speed that yields a good grade of material is ~ 20 mm / S.

2. Ultrashort Pulse Laser Welding When multiple laser pulses are absorbed in individual regions of the material to be welded, heating, melting and mixing of the materials occur, and upon cooling, the individual materials are fused together. The number of pulses required to weld the materials together depends on the physical properties of the material as well as process variables (laser energy, pulse repetition rate, etc., focusing geometry, etc.). For example, materials with a combination of high thermal conductivity and high melting point require higher pulse repetition rates and lower travel speeds in order to allow sufficient heat accumulation in the irradiated area for welding to occur.

A. Welding of polycarbonate Experiments with a high repetition rate, femtosecond pulsed laser operating at a pulse repetition rate of 200 kHz and having a wavelength of 1045 nm resulted in laser bonding of two optically transparent materials. In particular, a laser pulse of ˜2 μJ was focused on the bottom surface interface with the top surface of the same size transparent polycarbonate through the top surface of the transparent polycarbonate of 1/4 inch thickness with a lens having a focal length of 100 mm. The polycarbonate piece was linearly moved in a plane perpendicular to the laser propagation direction while maintaining the position of the beam converging region near the material interface. The two pieces were fused together at the laser irradiation interface and sufficient force was required to pull them apart from one.

B. Welding of fused silica A fused quartz plate having a thickness of 200 μm was welded to a fused quartz plate having a thickness of 1 mm using a 40 × aspherical lens and a laser repetition rate of 5 MHz. The 1 / e ² beam diameter of the laser is ˜3.6 mm and the focal length of the aspheric lens is 4.5 mm, resulting in an operating NA (numerical aperture) of 0.37. FIG. 12 shows a fused quartz weld feature where images were taken both before and after splitting the two quartz plates. The first image (a) shows the raw weld structure showing the area of the smoothly melted glass, and the next images (b) and (c) show the two glass surfaces after the weld has been peeled off. , Shows the peeled glass surface.

Although speeds of 5 mm / s and higher are possible, the welding speed is 0.1 to 1.0 mm / s and the maximum speed increases with increasing pulse repetition rate. The nominal fluence range for this process is 5-15 J / cm ² and the nominal pulse duration range is 10-100 fs. In these fluence and pulse duration ranges, the nominal pulse repetition rate range is 1-50 MHz. For stringent process optimizations, these ranges for fluence, pulse duration, and repetition rate are expanded to 1-100 J / cm ² , 1 fs-500 ps, and 100 kHz-100 MHz, respectively. A high repetition rate is necessary for the accumulation of sufficient heat to begin melting the fused quartz.

  If higher energy pulses are available at the same repetition rate, loose collection can create a large collection volume with the required fluence. The size and shape of this weld collection volume is adapted based on the area to be welded.

3. Visible / Invisible Laser Marks FIG. 13 shows a glass sample with arrow marks illuminated with a green light source from the side. You can see the arrow pattern clearly. The schematic diagrams of FIGS. 8 and 9 show the details of the arrow pattern, where lines perpendicular to the illumination source and of different depths were generated with closely focused laser light.

  FIG. 14 shows the same glass sample with the illumination light source off. Obviously, the arrow pattern is not visible.

  FIG. 15 shows micrographs of individual pixelels used to clearly outline the arrow marks of FIG. FIG. 16A shows a photograph of the decorative pattern inside the glass, and FIG. 16B shows a microscopic image of each mark.

  The mark in FIG. 16 (b) is very rough with a size of about 200 μm and has a clear crack radiating from the center. The pixel of FIG. 15 is made up of a series of parallel lines, each line approximately 10 μm wide and 250 μm long. The line spacing is 50 μm. The difference in size and smoothness between FIGS. 15 and 16 (b) requires side illumination for the arrows in FIGS. 13 and 14 to be visible, but the engraving in FIG. 16 is clearly in the best lighting conditions. It shows why it looks. The size and smoothness of the feature produced is controlled by the pulse energy, pulse duration and wavelength of the laser and the moving speed of the beam in the target. The optimal parameters depend on the individual target material. The visibility of the pixel in FIG. 15 is controlled by controlling the width and length of each line in the pixel, the smoothness as well as the line density within the pixel.

  Therefore, while controlling the parameters of the laser as described, the roughness of the line is controlled, but one way to generate a visible pattern of laser-modified features under the surface of the transparent material is precisely focused. Begin by first forming multiple lines at different depths in the material using an ultrafast pulsed laser. The line is illuminated using light that propagates to the line or is generally directed orthogonally. A pattern formed by this method is clearly visible to the naked eye when illuminated from an orthogonal direction, and is not visible to the naked eye without illumination, that is, under normal surrounding light conditions as shown in FIG. Illumination is guided by directing the focused light source onto the line or directing light onto the line through an optical waveguide with an output numerical aperture selected to fully illuminate the pattern.

  One different from the line, for example a line of different pixels, is at an angle that clearly outlines the other, and multiple light sources are arranged so that they point the light approximately orthogonal to the subset of lines, Illuminated separately or simultaneously.

  Thus, the present invention provides a transparent material having a pattern of sub-surfaces formed with a laser, e.g. an ultrafast pulsed laser, where the marking is formed with lines of different depth in the material, the lines being It is well visible to the naked eye when illuminated with a light source oriented in a direction perpendicular to the line.

Claims (2)

  A method for scribing a transparent material, wherein the material is irradiated with a focused beam of an ultrashort laser pulse having a laser wavelength at which the material becomes transparent, at two or more positions in the depth direction. Modifying the material, wherein the intensity of the focused beam becomes non-linear absorption in a region defined by a three-dimensional intensity distribution including a beam waist, and at least a part of the irradiation is a propagation axis of the input beam A diffractive optical element that produces multiple regions of high light intensity at different depth locations along the optical axis, wherein the diffractive optical element receives the input beam and is spaced apart in the depth direction. A method that produces a continuous region. Surface grooves are formed in the material, at least one modified region within the bulk of the material is formed, the method according to claim 1.

Images