Method for producing linearly arranged defects in a transparent workpiece, as well as method and apparatus for separating a workpiece

Abstract

Method for producing linearly arranged defects (14) in a transparent workpiece (2), comprising the following steps: a) Providing a laser processing device (1) with ultrashort laser (10) and focusing optics (11), wherein the wavelength of the laser radiation is in the transparency range of the workpiece (2); b) The workpiece (2) or the laser beam are moved relative to each other in order to create the perforation in the depth of the workpiece (2) along a line (20) of the series of damage points (14); c) Emission of laser pulses in two or more successive periods during the positioning of the focusing optics (11) at each damage site (14), wherein the energy of the laser pulses during each period is dimensioned to form an associated filament structure (4, 5, 6) in the transparent workpiece (2) and the successive laser pulse periods result in filament structures (4, 5, 6) extending transversely through the workpiece (2), wherein the focusing optics (11) generates a beam (12) with such a cross-section that is more extended in the direction of the linearly arranged damage sites (14) than transversely to this direction.

Description

[0001] The present invention relates to a method for producing linearly arranged damage points in a transparent workpiece. Tempered glass or glass ceramic is particularly suitable as the workpiece material. The invention also relates to a method and a device for separating a workpiece.

[0002] From WO 2012 / 006736 A2, it is known that the Kerr effect can cause the formation of filaments in glass, resulting in irreversible damage. The succession of such damage in glass enables the separation of transparent substrates. A filament is created by an ultrashort laser pulse. Due to the Kerr effect, the laser beam undergoes self-focusing inside the glass until the energy density at one point becomes so high that a plasma is ignited. This results in a plasma explosion, in which the glass suffers irreversible damage around this plasma formation point. From there, further radiation emanates, which is subject to self-focusing and culminates in a plasma explosion. This effect repeats itself several times, depending on the intensity. The energy decreases across the entire glass thickness, so the first plasma spots have the greatest energy and also cause the greatest damage.Furthermore, the plasma spots are round, which leads to outgoing defects occurring statistically distributed in all directions.

[0003] Glass with inherent stresses, such as chemically toughened glass, can spontaneously break. This significantly impacts the processing of larger glass sheets. The breakage causes the glass sheet to change its position, making further precise processing impossible.

[0004] Patent DE 102 13 044 B3 describes a method for cutting or drilling material. Here, too, the nonlinear optical effect caused by high-intensity, ultrashort laser pulses is exploited to create a filament by alternating focusing and defocusing the laser beam. A filament is thus a small-diameter channel created by a high-intensity laser light beam.

[0005] Furthermore, document DE 10 2006 042 280 A1 describes a method for processing transparent material with a laser. Ultrashort laser pulses are used to create both a surface groove on the substrate and one or more laser-modified regions within the bulk of the material. The fracture ultimately leading to separation occurs at the surface groove structure and propagates downward through the substrate material. If the surface groove is too shallow, the fracture tends to migrate. The creation of fracture edges with consistently high quality is not described.

[0006] DE 10 2007 028 042 B3 also describes a method for laser processing transparent materials, using pulsed laser radiation in the nanosecond range. According to this method, there is a radiation intensity range in which material changes occur without plasma glow.

[0007] The patent specification DE 10 2005 024 563 B3 discloses a method for separating glass and using a liquid suitable for this purpose, comprising the steps of creating a linear scratch on the glass, wherein the scratch is exposed to an aqueous liquid containing an organic ionic compound consisting of a cation with a positively charged nitrogen atom and a hydroxyl ion as an anion during or immediately after its creation, and separating the glass along the scratch line by applying a force.

[0008] The published patent application DE 199 63 939 A1 describes a method and a device for severing flat workpieces made of brittle material, wherein a laser beam with a linear beam profile with a trailing cooling spot is moved along a severing line with a predetermined contour, and in which a linear focal spot is generated on the workpiece by scanning the laser beam, and during each scanning movement path data of the severing line are provided such that the linear focal spot experiences a curvature corresponding to the curvature of the contour of the severing line, and its length is also adjusted depending on the curvature of the contour of the severing line by adjusting the scanning amplitude.

[0009] In summary, various methods are known that allow modifying regions within a material using ultrashort, pulsed laser radiation, thus creating a component of a separation process. However, the separation and breaking required, for example, for singulating substrates modified in this way, have not yet been sufficiently accessible to industrial processes. This problem is particularly evident with substrates comprising tempered glasses or glass-ceramics, as these are prone to uncontrolled breakage due to the prestressing when processed with ultrashort, pulsed laser radiation.

[0010] For industrial use, however, precise control is required not only for the creation of a parting line in or on the substrate, but also for the cutting or breaking process itself. This is particularly challenging in the case of tempered glass, as the material modifications caused by laser irradiation can lead to the uncontrolled appearance and propagation of cracks, making precise control of the cutting process very difficult.

[0011] The following is stated regarding the problem: 1) Cutting / drilling by filamentation: Due to the process, the filament develops inhomogeneously: Due to the high initial energy density, comparatively larger plasma volumes are ignited on the entry side of the filament-generating laser beam than in the subsequent plasma points located deeper in the workpiece. This means that the damage channel (= the formed filament structure) in the workpiece tapers. The resulting damage (microcracks) is therefore significantly more severe on the entry side of the laser beam than on the exit side. Directed strength tests (four-point bending, etc.) show a significant difference in edge strength even at a glass thickness of 0.7 mm. 2) Spatial geometry of the plasma formation sites: The plasma formation sites caused by self-focusing essentially have a spherically symmetric shape with spherically symmetric energy distribution, resulting in direction-independent, statistically distributed microcracks around the plasma volume. As a result, the cracks also extend into the subsequent fracture edge and have a strength-reducing effect. 3) Spontaneous fracture: During the filamentation of stressed brittle materials, uncontrolled spontaneous fracture of the workpiece occurs during the process, resulting in an increased scrap rate. Furthermore, the spontaneous fracture leads to a

[0012] Shift in the position of the workpiece, which makes automated processing difficult or even impossible.

[0013] The invention is primarily intended to improve the quality of the edges produced.

[0014] The invention is defined in the independent claims. Preferred embodiments and further developments of the invention can be found in the respective subclaims.

[0015] The invention improves the separation process for brittle materials.

[0016] 1) Multifilamentation: In contrast to the state of the art, the preparation for separation (i.e., splitting the workpiece) is not carried out solely by creating a single tapered filament structure, but by arranging several filament structures in a row. Each of these filament structures is relatively narrow and generates a significantly smaller number of microcracks perpendicular to the direction of the filament structures compared to a separation process with a single filament structure passing through the workpiece. The lower conicity of the individual filament structures (=filament + microcracks) produces an overall higher geometric accuracy of the machining channel, coupled with a higher edge strength of the fracture edge after separation of the workpiece.

[0017] The single-filament structures are created by a picosecond / femtosecond pulse, which is split and introduced into the workpiece at staggered times, starting at the exit side of the laser beam. Greater cutting depths can be achieved using multiple pulse sequences at the same laser power, with a corresponding reduction in cutting speed.

[0018] 2) The spatial geometric shape of the plasma can be influenced by special optics. This creates an elongated cross-section of the laser beam, such as a lancet, elliptical, or droplet. This results in a controllable preferred direction of the damage / cracks generated by the plasma explosions.

[0019] The cross-sections of the laser beam should be adjusted / changed with their longitudinal extension in the cutting direction of the workpiece when rounding or changing direction.

[0020] 3) The production of filamentation under a protective atmosphere is primarily a mechanical engineering concept, whereby the production plant is designed in such a way that a defined setting of the atmosphere surrounding the workpiece to be processed is achieved.

[0021] By adjusting the atmosphere in a targeted manner, spontaneous fracture of the workpiece can be inhibited or prevented.

[0022] The invention also relates to a method for separating and splitting a workpiece having the features of claim 7.

[0023] One embodiment of the invention relates to a method for separating a substrate by means of focused laser radiation, comprising the following steps: - the substrate is exposed to a protective gas atmosphere, - pulsed, ultrashort laser radiation is directed onto the substrate, whereby the substrate is transparent in the wavelength range of the laser radiation, - laser irradiation creates a deep, filament-like material modification in a predetermined volume of the substrate, - Breaking the substrate along the separation line defined by the material modification.

[0024] Pulsed, ultrashort laser radiation can induce two effects of nonlinear optics in a transparent material. These include the optical Kerr effect and the defocusing of the laser beam in a plasma bubble. Such effects are already known and will therefore only be briefly outlined.

[0025] The Kerr effect describes a change in the optical properties of a transparent material (transparent in the wavelength range of the laser radiation) depending on the applied or occurring electric field strength. Laser radiation induces an electric field in the transparent material, the strength of which depends on the light intensity of the laser radiation. The electric field causes a change in the optical properties of the irradiated material, including an increase in the refractive index. This, in turn, leads to the self-focusing of the laser radiation.

[0026] Due to self-focusing and the resulting reduction in the radiation cross-sectional area, the areal power density, and thus the radiation intensity, increases significantly and can reach very high values. This further amplifies the electric field, leading to multiphoton ionization. Ionization means charge separation in molecules or atoms and plasma formation at the focusing points. In any case, damage occurs in the substrate material at the focusing points, which is subsequently referred to as material modification and manifests as plasma bubbles.

[0027] In the area of ​​the locally occurring plasma bubble, the laser beam is defocused, followed by another focusing of the laser beam. This can create a kind of string of pearls in the substrate material, consisting of several consecutively arranged focusing and defocusing areas, known as filaments.

[0028] By focusing and defocusing, the laser beam can be propagated into the substrate material, creating a kind of perforation in the material's depth. This effect is well known for processing transparent materials such as glass and is used, for example, to create a kind of perforated line in the substrate as a predetermined breaking point or dividing line by moving the substrate relative to the laser beam.

[0029] A higher laser power can achieve a correspondingly deeper perforation, which makes separation in the perforation area easier.

[0030] However, the use of deeper perforations in conjunction with tempered glass, a material that already has increased residual stress in its initial state, is problematic. Due to the increased residual stress, spontaneous crack formation and crack propagation can occur in the perforated areas, as described in WO 2012 / 006736 A2. Particularly at higher laser powers, this can lead to substrate fracture during the process. Industrial application is therefore significantly more difficult because the separation process cannot be reliably controlled and managed.

[0031] The inventors discovered that spontaneous crack formation can be reduced or even completely prevented if a specific atmosphere prevails on the workpiece during laser irradiation of the substrate. Crack formation is significantly reduced or delayed if a protective gas atmosphere that is low in OH ions or free of OH ions prevails during laser irradiation.

[0032] By applying a nitrogen atmosphere during laser irradiation, the time until spontaneous fracture cracks appeared in the area of ​​the perforation sites could be sufficiently extended.

[0033] In this way, it is possible to terminate the laser irradiation process of the substrate before spontaneous cracks occur in the substrate and thus before the substrate fractures. Accordingly, the substrate is exposed to pulsed, ultrashort laser radiation in an atmosphere low in OH ions or free of OH ions. For this purpose, a gas-tight chamber in which the substrate is held during laser irradiation can be used, for example.

[0034] Particularly good results have been achieved with a protective gas atmosphere whose water content is less than 0.2 vol%, preferably less than 0.1 vol%.

[0035] Laser irradiation can thus create a deep, filament-like material modification similar to a perforation in a predetermined volume of the substrate. The predetermined volume of the substrate refers to the defined separation area along which the perforation is to be created, thus forming the subsequent separation point or the separation line on the surface of the substrate.

[0036] Particularly advantageously, the substrate or the laser beam can be moved relative to each other to enable perforation deep into the substrate along the dividing line. Typically, the substrate can be moved, for example, via an XY-axis adjustment arrangement, which enables two-dimensional movement of the substrate at a constant distance from the laser beam. This arrangement can also be combined with a movable Z-axis arrangement to adjust the distance between the substrate and the laser beam.

[0037] The substrate material exhibits optical transparency at least in the wavelength range of the laser radiation. The optical transparency in this range is at least 80% / cm, preferably at least 85% / cm, and most preferably at least 90% / cm. This allows the laser radiation to penetrate the material.

[0038] The substrate can comprise materials selected from the group consisting of glass, sapphire, or diamond. Surprisingly, the laser irradiation according to the invention can also be used for tempered glass, as is known, for example, for display applications, or for flat glass. Equally surprisingly, glass-ceramic materials can also be processed.

[0039] The perforation created in the substrate, extending deep into the substrate, runs along a path along the dividing line where the substrate is to be separated. The dividing line can be straight, uneven, or curved. For example, the dividing line can also have very small radii, enabling material separation similar to a drill hole.

[0040] The perforation can extend vertically from the substrate surface into the depth. However, it can also be created at a specific angle relative to the surface, for example, to create angled separation edges on the substrate. In this case, the laser beam is directed not vertically but at a predetermined angle to the substrate surface.

[0041] The material is then separated along the perforated separation line. This requires that the material modification caused by the perforation, which is equivalent to material damage, reaches a certain extent in order to enable separation with a certain quality of the separation edge in terms of surface finish and the strength of the separated substrate.

[0042] It has been shown that good separability at the separation line can be achieved if the perforation extends to a depth of at least 40% of the material thickness of the substrate, preferably at least 50%, and most preferably at least 55%. The filaments should be spaced apart at a distance in a range of approximately 200 µm to 800 µm. A filament can have a cross-section in a range of approximately 15 µm to 250 µm.

[0043] In this way, sufficient pre-damage to the substrate material can be created along the separation line, resulting in good separability. By separating the material along the separation line, separation edges are formed, and the quality of these resulting separation edges is surprisingly good.

[0044] The separation edges typically exhibit a structure with adjacent, parallel filaments in the perforation area, and a more shell-like fracture structure in the underlying separation edge area. Roughness values ​​of the separation edge in the range of Ra < 100 µm were achieved.

[0045] The edge strength of the resulting cut-off edges is also of high quality. This was determined using a 4-point bending test. An average strength of at least 120 MPa was achieved for tempered glass.

[0046] The laser source is selected according to the wavelength range in which the substrate exhibits transparency. The wavelength range of the emitted radiation falls within the transparency range of the substrate.

[0047] The laser beam can be spatially focused with a Gaussian intensity distribution to achieve a sufficiently high intensity. The first focal point is located within the substrate, i.e., within the substrate volume. If the laser pulse hits this point within the substrate volume, plasma formation can occur there, leading to material modification. Due to the subsequent defocusing and further focusing effects, the filament-like perforation, comprising multiple focusing points, can be created within the substrate volume.

[0048] The focal points in transparent glass, for example, often have a spherical, approximately symmetrical shape. However, using special optics, it has been possible to create focal points in tempered glass that have a non-spherical spatial configuration. For example, focal points with elliptical, lancet, or drop-shaped shapes have been created. Such focal point structures facilitate crack formation, particularly by improving the crack progression from one focal point to the next, which also allows for better quality of the separating edge.

[0049] A suitable laser source according to the present invention operates with a repetition rate which is between 10 kHz and 120 kHz, preferably between 30 kHz and 110 kHz and most preferably between 35 kHz and 105 kHz.

[0050] The suitable pulse duration of a laser pulse is in the range of less than 100 picoseconds, preferably less than 10 picoseconds, and most preferably less than 1 picosecond. The power of the laser source is particularly advantageous in the range of approximately 7 to 12 watts.

[0051] Using such laser radiation, very good results could be achieved for the creation of deep perforated dividing lines on tempered glass.

[0052] Increasing the laser power can lead to a greater tendency for spontaneous cracking, especially in toughened glass, whereas in non-toughened glass a denser perforation can be achieved, which in turn improves separability.

[0053] The invention further relates to a device for separating a substrate having the features of claim 9.

[0054] An embodiment of the device for separating a substrate, in particular tempered glass or glass ceramic, by means of focused laser irradiation, comprises: - a gas-tight chamber to accommodate the substrate, - a pulsed ultrashort laser light source, - a device for moving the substrate and / or the laser light source relative to each other, wherein - the substrate is exposed to a protective gas atmosphere, - pulsed, ultrashort laser radiation is directed onto the substrate, whereby the substrate is transparent in the wavelength range of the laser radiation, - the laser irradiation produces a deep, filament-like material modification in a predetermined volume of the substrate, - a separation occurs along the dividing line defined by the material modification.

[0055] The invention further provides an article made of tempered glass or glass ceramic which has been processed on at least one side using a method according to the invention.

[0056] In a further development of the method for separating a substrate, in particular tempered glass or glass ceramic, using focused laser irradiation, after the substrate has been perforated along the future separation point, the substrate is exposed to a second atmosphere. This second atmosphere differs from the protective gas atmosphere, which therefore represents the first atmosphere, in its OH ion content. Thus, the OH ion content of the second atmosphere is higher than that of the first atmosphere.

[0057] The inventors discovered that an increased proportion of OH ions can promote the separation or fracture of the workpiece along the perforated parting line. By applying an atmosphere enriched with OH ions, such as wet steam, crack formation can be promoted and thus controlled. In this way, the fracture process step for creating the material separation can also be specifically influenced, allowing for easy industrial application. In particular, the occurrence of spontaneous fracture cracks can be prevented.

[0058] Particularly good experiences have been made with a second atmosphere which has an OH ion content of at least 1.4 vol%, preferably at least 2 vol%.

[0059] Therefore, the chamber for accommodating the substrate can be particularly advantageously designed to be gas-tight, allowing a first, OH-ion-poor atmosphere to be easily established. Furthermore, this chamber can also be particularly advantageously designed to expose the substrate to an OH-ion-enriched atmosphere. Likewise, the device can also comprise two separate chambers, each designed for exposure to the first or second atmosphere.

[0060] Further details of the invention emerge from the description of the illustrated embodiments and the appended claims.

[0061] Examples of implementation are described using the drawings. The drawings show: Fig. 1: A laser processing device during the processing of a workpiece on a workpiece table, Fig. 2: an enlarged detail from Fig. 1, Fig. 3: an elliptical radiation cross-section of the laser processing device, Fig. 4: lancet-shaped radiation cross sections and Fig. 5: drop-shaped radiation cross sections.

[0062] Fig. Figure 1 shows a laser processing device 1 above a workpiece 2 resting on a workpiece table 3. The laser processing device has an ultrashort laser 10 and focusing optics 11 for emitting a focused beam 12, the focal point 13 of which is located approximately on the top side of the workpiece 2. A cutting or breaking line 20 is indicated on the workpiece 2, along which the workpiece is to be separated or split. It is intended that the focus 13 can be moved along this line 20, which can be accomplished by adjusting the table in the two coordinate directions 21, 22. Very small adjustment steps are used.

[0063] The ultrashort laser 10 is capable of emitting laser pulses in two or more consecutive periods. The wavelength of the radiation is selected to lie within a transparency range of the workpiece 2. The energy of the laser pulses is dimensioned such that a linear damage site 14 is formed transversely to the surface of the workpiece 2. By moving the focusing optics 11 along the intended fracture line 20, a series of damage sites 14 are created in the workpiece 2, which, in a sense, predetermine the intended fracture surface. The invention relates to the production of these linearly arranged damage sites 14.

[0064] Fig. Figure 2 schematically shows a damage site 14 extending transversely through the workpiece 2. In the illustrated embodiment, the damage site 14 comprises three filament structures 4, 5, and 6 aligned in a line. Each of these filament structures is generated by an ultrashort laser pulse. In the case of the filament structure 4, this leads to a plasma site 41 through self-focusing of the laser beam 12, at which the workpiece material transitions into the plasma phase. This is accompanied by outgoing radiation 42, which leads to another plasma site 43 through self-focusing. The process continues until the energy of the laser pulse is exhausted. At the plasma formation points 41, 43, 45, a plasma explosion takes place due to thermal expansion, which leads to crack formation mainly along a forming gap into the interior of the workpiece, which is desired, but also transversely to this channel, as indicated by cracks 46.These transverse cracks 46 are undesirable and should be kept as small as possible in the invention.

[0065] For this purpose, the damage site 14 is created in several stages. This is achieved by emitting the laser pulses in two or more consecutive periods. The energy of the laser pulses during each period is selected to be so small that only small and few plasma explosion sites occur during one emission period. This greatly reduces the formation of damaging lateral cracks 46. Gaps and cleavage cracks form along the filament structure 4, which precede the subsequent fracture in the workpiece.

[0066] In a second laser pulse period, filament structure 5 is created in a similar manner to filament structure 4. Plasma bubbles 51, 53, 55 and defocusing / focusing points 52, 54 form; similarly, plasma bubbles 61, 63 and defocusing / focusing points 62 form in the third laser pulse period. The greater depth of penetration is achieved due to the previously formed gaps and cracks in the direction of the damage site, which provide a guide channel for the second and subsequent laser pulse periods. The number of consecutive laser pulse periods depends on the thickness of workpiece 2.

[0067] To promote the formation of gaps in the direction of the fracture surface 20, it is advisable to select cross-sectional shapes of the laser beam that extend in the direction of the desired fracture. Such cross-sectional shapes are shown in the Fig. 3, Fig. 4 and Fig. 5. The elliptical cross-sectional shape can be obtained from an originally circular cross-sectional shape of the laser beam by combining cylindrical lenses. The lancet shape of the Fig. 4 and the drop shape of the Fig. 5 of the beam cross-section can be obtained using special lenses. These shapes promote cracks and gaps in the direction of the desired fracture line 20.

[0068] If, as shown, the fracture line is to have a curved shape, the laser beams must be tracked so that the larger cross-sectional area points in the desired fracture direction.

[0069] When machining brittle materials, there is a risk of spontaneous fracture of the workpiece before all damage along the intended fracture line 20 has been completed. To reduce or completely avoid this risk, machining is carried out in a "neutral" atmosphere, such as nitrogen. This produces workpieces prepared for separation or cleavage. The final separation or cleavage is then performed by applying mechanical stress to the workpiece and under steam or another atmosphere containing a hydroxyl group (OH).

[0070] It should be noted that the spacing between the linearly arranged damage points 14 should be chosen more closely, the finer the fracture pattern desired in the separation plane. The spacing is of the same order of magnitude as the diameter of the damage points.

Claims

[1] Method for producing line-shaped damage points (14) in a transparent workpiece (2), comprising the following steps: a) providing a laser processing device (1) with an ultrashort laser (10) and focusing optics (11), wherein the wavelength of the laser radiation lies in the transparency range of the workpiece (2); b) the workpiece (2) or the laser beam are moved relative to each other in order to produce the perforation in the depth of the workpiece (2) along a line (20) of the row of damage points (14); c) Emitting laser pulses in two or more successive periods during the positioning of the focusing optics (11) at each damage point (14), wherein the energy of the laser pulses during each period is dimensioned to form an associated filament structure (4, 5, 6) in the transparent workpiece (2) and the successive laser pulse periods result in filament structures (4, 5, 6) extending transversely through the workpiece (2), wherein the focusing optics (11) generates a beam (12) with a cross-section which is more extensive in the direction of the line-shaped damage points (14) than transversely to this direction. [2] Method according to claim 1, wherein each filament structure (4, 5, 6) has a plurality of focusing and defocusing points extending transversely to the plate-shaped workpiece (2) in the manner of a string of pearls. [3] Method according to claim 1 or 2, wherein the steps of the relative displacement of the focusing optics (11) to the workpiece (2) are in the size range of the lateral dimension of the filament structures (4, 5, 6) along the line-like lined-up damage sites (14). [4] Method according to one of claims 1 to 3, wherein the number of successive laser pulse periods at a damage site (14) is dimensioned according to the local thickness of the workpiece (2). [5] Method according to one of claims 1 to 4, wherein the focusing optics (11) is adjustable with respect to the larger beam cross-sectional extent and the direction of the larger cross-sectional extent is adjusted to the course of the line-shaped damage points (14). [6] Method according to one of claims 1 to 5, wherein the workpiece (2) is kept in a neutral atmosphere during the production of the line-shaped damage points (14) in order to avoid premature fracture along the line-shaped damage points (14). [7] Method for separating and splitting workpieces, in which the method for producing line-shaped damaged areas (14) according to one of claims 1 to 6 is preceded and separated as a preparatory step. [8] Method according to claim 7, wherein gas containing OH ions is supplied to the damaged areas (14) to promote the separation or splitting of the workpiece (2). [9] Device for separating a workpiece, in particular toughened glass or glass ceramic, by means of focused laser irradiation for carrying out a method according to one of claims 7 or 8, comprising: - a workpiece space for holding the workpiece (2), - Feeding into the workpiece space, - a pulsed ultrashort laser light source (10) for producing a deep, filament-like material modification in a predetermined volume of the workpiece (2) by laser irradiation, - a device for moving the workpiece (2) and / or the laser light source (10) relative to each other, - a wet steam supply into the workpiece space, and - a cutting device for cutting the workpiece along the cutting line defined by the material modification.

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