Method for removing brittle material from a workpiece

Abstract

Method for removing brittle material (2) from a workpiece using pulsed laser radiation, which has a radiation axis (6) in the beam direction and which is moved perpendicular to the radiation axis (6) in a feed direction and in which the removal by means of at least one laser beam with a predefined width b L forms a depression (1), wherein the side surfaces of this cut-off depression (1) are referred to as flanks (3) and wherein the base surface of this cut-off depression (1) is referred to as cut-off base (7) and assumes a width b which is at least 1 / 3 of the predefined width b L of the laser beam and does not become smaller even with increasing ablation depth d and wherein both the base of the erosion (7) and the flanks (3) of the erosion depression (1) in the material (2) run at a spatially varying flank angle w, where the flank angle w is the local angle between the surface normal n F is defined on the flank (3) of the ablation depression (1) and the surface normal n0 on an unablated surface (5) of the material (2), wherein during the ablation process the flank angle w, independent of a spatial position along the flank (3), a critical angle w specific for the material (2) and a projected intensity I of the laser radiation max , where the critical angle w max is defined as the angle at which no further ablation takes place, and where the intensity I projected onto the surface of the ablation depression (1) is the product of the cosine of the flank angle w and the intensity I of the laser radiation, characterized by that an elliptical beam shape of the cross-section of the laser radiation is set in a plane perpendicular to the radiation axis (6), wherein, in the focus of the laser radiation, the major semi-axis of the ellipse is aligned in the feed direction and the minor semi-axis is perpendicular to it, and the beam radius w 01 in the direction of the minor semi-axis of the laser radiation, at the entrance of the ablation depression (1), where the unablated surface (5) of the workpiece transitions into the ablation depression (1), is set to a value such that the flank angle w reaches approximately 90 degrees and the achievable ablation depth d is at least 5 times greater than for a larger beam radius, where the flank angle w is smaller than the material-specific critical angle w max is, and that the beam radius w 02 the major semi-axis of the laser radiation is adjusted to such a value that the Rayleigh length z R02is at least as large as half the removal depth d / 2.

Description

[0001] The present invention relates to a method for removing brittle, hard material from a workpiece by means of pulsed laser radiation according to the preamble of claim 1.

[0002] Such a process uses pulsed laser radiation with a radiation axis in the beam direction. The radiation axis is moved in a feed direction perpendicular to the radiation axis. The ablation is carried out by pulsed laser radiation with a laser beam of predefined width b. Lan ablation depression. Typically, the speed of movement of the radiation axis in the feed direction is lower than the speed at which material is removed in the ablation direction. The ablation depression takes on a geometric shape that is longer in the feed direction than its extension perpendicular to the feed and beam directions. The ablation depression has a base area up to which ablation has taken place in the beam direction. The depth of the base area in the beam direction can reach the entire thickness of the material and the process is then referred to by the person skilled in the art as a through-hole or through-drilling because the material is continuously removed with regard to its thickness in the beam direction, or is referred to as a cut because the material is continuously removed in the feed direction and the material is separated into at least two parts. Regardless of whether after the end of the process orIf an ablation depression, a hole, or a cut was created during irradiation, an ablation depression with a base and side surfaces is created during these processes. The base surface is referred to as the ablation base. The side surfaces of this ablation depression are referred to as flanks. The flanks of the ablation depression in the material run at a spatially variable flank angle w, where the flank angle w is defined as the local angle between the surface normal n. Fon the flank of the ablation depression and the surface normal n0 on the non-ablated surface of the material. Typically, at the beginning of the ablation, a wide and shallow ablation depression is created with a small flank angle w in the center of the ablation depression and a large flank angle w at the flank, which is the edge of the ablation depression. Typically, with increasing number of pulses or increasing irradiation time, the flank angle w at the flank increases, independent of spatial positioning along the flank, to a value for the product of the cosine of the flank angle w with the intensity I of the laser radiation and a material-specific critical angle w max The critical angle w max is defined as the angle at which no more ablation occurs with increasing number of pulses or increasing irradiation time.

[0003] In the context of this process, brittle materials include all materials that have very low light absorption and are therefore transparent, such as glass. Such materials have a band gap (greater than 1 eV) for the energies between the valence band and the empty conduction band of a material, which is greater than the energy of a laser photon. Such materials change their optical properties when illuminated with intense laser radiation and then become absorbent and can be ablated. The so-called "wide-band-gap" materials include glass and sapphire, for example, as special cases of "band gap" materials. Materials with a moderate band gap (band gap approx.1 eV) are brittle materials, where it is essential that their optical properties (reflectance, absorption) change when they are illuminated with intense laser radiation, such as silicon and related semiconductor materials.

[0004] US 2006 0 255 022 A1 describes a method for processing a wafer by means of laser radiation, wherein an elliptical beam shape of the cross section of the laser radiation is set in a plane perpendicular to the radiation axis and, at the focus of the laser radiation, the major semi-axis of the ellipse is aligned in the feed direction and the minor semi-axis is aligned perpendicular thereto.

[0005] DE 10 2013 208 833 A1 describes a laser beam processing device for a wafer.

[0006] US 2013 0 256 286 A1 describes laser processing using an elongated beam cross-section and using ultrashort pulses and / or long wavelengths.

[0007] DE 10 2013 005 139 A1 describes a method for ablating brittle, hard material using laser radiation. During the ablation, a removal depression with a flank angle w of the flanks of the removal depression is formed in the material. The flank angle w is defined as the angle between the surface normal to the removal depression and the surface normal to the non-ablated surface of the material. During ablating, the flank angle w exceeds a material-specific limit angle wmax.In order to achieve a high removal rate and a large removal depth with simultaneous damage-free removal / cutting, the beam radius of the laser radiation at the entrance of the removal recess is set so small that the flank angle w reaches almost 90 degrees and the achievable removal depth is at least a factor of 2 greater than for a larger beam radius, where the flank angle is smaller than the material-specific critical angle wmax.

[0008] The invention is based on the object of creating a method using laser radiation with which a high removal rate and a large removal depth can be achieved with simultaneous damage-free removal / cutting, so that a large material depth is processed by the effect of the laser radiation during removal / cutting and no additional stresses or additional cracks are introduced into the brittle-hard material after processing.

[0009] This object is achieved by a method according to claim 1. Advantageous embodiments of this method are specified in the dependent claims.

[0010] According to the method, an elliptical beam shape of the cross-section of the laser radiation is set in a plane perpendicular to the radiation axis. This elliptical cross-section of the laser radiation refers to a plane in the material to be ablated, perpendicular to the radiation axis. Accordingly, the major semi-axis of the ellipse is aligned in the feed direction, and the minor semi-axis is perpendicular to it and perpendicular to the beam direction. The beam radius in the direction of the minor semi-axis of the laser radiation, with w 01referred to as, at the entrance of the removal recess, where the unchanged part of the surface of the workpiece merges into the removal recess, is set to a value such that the flank angle w reaches almost 90 degrees and the achievable removal depth is at least a factor of 5 greater than for a larger beam radius, where the flank angle w is smaller than a material-specific limit angle w max where no more ablation takes place. Furthermore, the beam radius of the major semi-axis of the laser radiation, w 02 is set to such a value that the Rayleigh length, with z R02 is at least as large as half the removal depth.

[0011] Insofar as this description refers to a method for removing brittle, hard material, it should be noted that structuring, cutting, and drilling are special cases of removal. During structuring, a removal depression is created in the material, with the removal depth being less than the material thickness. During drilling, the removal depression can progress in the direction of the radiation axis until the removal depth reaches the material thickness. During cutting, the radiation axis is moved in addition to drilling. The radiation axis can be set at an incline relative to the surface normal, designated n0, of the non-removed material. The movement then occurs perpendicular to the surface normal, designated n0, of the non-removed material, with the radiation axis being set parallel to the surface normal n0 or inclined to it, or the inclination is changed along the movement.

[0012] The method according to the invention is advantageous over other known removal methods, in particular those such as wire EDM or laser methods with a rotationally symmetrical cross-section of the laser beam and a very narrow, circular cross-section of the removal recess, in that greater removal depths in the material can be achieved. With a circular cross-section of the removal recess, only a very small removal depth can be achieved, since the removed material redeposits on the flank of the removal recess, closing or narrowing it again. The removal depth achievable with the method according to the invention is significantly increased by creating an elliptical removal recess.The flank is enlarged at least in a direction perpendicular to a radiation axis of the laser radiation used, for which purpose an elliptical beam shape of the cross-section of the laser radiation is set in a plane perpendicular to the radiation axis.

[0013] A small diameter of the ablation depression results in reduced absorption at the edge of the ablation depression and a guidance of the laser radiation along the flank of the ablation depression toward the ablation base through multiple reflection or waveguiding. According to the invention, this advantageous effect also occurs during ablation / cutting if the diameter of the ablation depression is reduced only in a direction perpendicular to the radiation axis and total internal reflection does not occur. According to the invention, this effect also occurs at a cutting front if the ablation or cutting front has a partially open, waveguiding geometric shape and does not enclose the radiation in an approximately cylindrical shape, as is known from fiber guidance.Pulse shaping increases the effect of reflection at the flank, which is particularly important with an elliptical beam shape and a resulting elliptical shape of the ablation depression in order to maintain the effect of beam guidance in the ablation depression even when the ablation depression extends greatly in the feed direction or in the direction of the semi-major axis. The beam shaping according to the invention reflects the radiation in the direction of the semi-minor axis, i.e., perpendicular to the feed and beam direction at the flank, and guides it towards the ablation base. The ablation base is defined as a base area or floor of the ablation depression that merges into the flanks of the ablation depression. After calculation and experimental verification, it was observed that at a wavelength of 1 µm of the laser radiation, the threshold-like transition of the flank angle is achievable for a beam radius of less than 6 µm.The threshold-like transition for the flank angle w from a material-specific limit angle w. max , where the critical angle w max is defined as the angle at which no more material removal takes place, can be observed with values ​​for the flank angle of almost 90 degrees in that the material removal depth increases significantly by typically and at least a factor of 5.

[0014] The method according to the invention is particularly advantageous for cutting, since an elliptical beam shape of the laser radiation is used, wherein the major semi-axis of the ellipse must be arranged in the cutting direction and the minor semi-axis perpendicular to it, with the result that an undesirable widening or widening of the ablation depression is avoided.

[0015] Furthermore, temporal pulse shaping is an essential parameter for both cutting and drilling.

[0016] The temporal progression of the pulse power (pulse energy / pulse duration) must be adjusted so that the reflectance of the material that was transparent before the pulse (transparent means that the laser radiation experiences very low absorption in the material and therefore the material is transparent, as is the case with glass, for example) increases and the material ideally becomes reflective. It then behaves like a metal. Excessive absorption at the cutting flanks of the current cut / pass with the number n (n = 1, 2, 3 ....; natural number) as well as in the area of ​​the cutting edge from the n-1 previous cuts / passes is avoided, and strong absorption at the cutting front or in the base of the material being removed is achieved.Typically, the temporal progression of the pulse power in the process consists of a short, intense pulse / pulse sequence with a duration of typically 1–10 ps, ​​followed by a longer pulse / pulse sequence with a duration of 10–1000 ps. The optimal pulse shape is calculated using a material-specific optimization calculation; the underlying calculation model is known. During the short, intense pulses / pulse sequences with a duration of typically 1–10 ps, ​​the material at the ablation site becomes highly absorbent and is ablated. Typically, the material changes its physical properties and also its temperature within 1–10 ps due to the laser radiation. The exact numerical value is material-specific and depends on the composition of the material.During the subsequent longer pulses / pulse sequences with a duration of 10 - 1000 ps, ​​the physical properties and also the temperature of the material are prepared due to the laser radiation in such a way that the next intensive pulse / pulse sequence is reflected at the flanks and absorbed at the base of the ablation.

[0017] For cutting, as a form of material removal, it should be noted that the critical angle w maxfor the flank angle w of the removal recess is not exceeded when removing and cutting transparent materials. In comparison to drilling, cutting also creates a cutting gap, which is limited in the feed direction and laterally by the cutting front and, in particular, is not limited where the cutting gap occurs, contrary to the feed direction. In order to achieve a large cutting depth regardless of the cutting gap, the laser beam must be elliptically shaped, i.e., perpendicular to the cutting direction, a small focus radius w 01 and have a large focus radius w along / parallel to the cutting direction 02 (w 02 >>w 01 ). The large focus radius w 02in the feed direction is necessary because the radiation can propagate freely in the feed direction, which is the direction of the resulting kerf, and therefore, according to the invention, should have a small divergence angle. As a result of a large focus radius w 02 is the divergence angle D 02 small, which follows from the well-known relationship that the product of focus radius w 02 and divergence angle D 02 is constant during the propagation of laser radiation. A small divergence angle or a large Rayleigh length z R02 is necessary to avoid an expansion of the radiation over a short propagation distance.

[0018] With an expansion of the radiation over a short propagation distance, the intensity in the cutting depth decreases over an undesirably short propagation distance and the cutting depth is undesirably limited.

[0019] Preferably, the value for the beam radius w 01the minor semi-axis by determining the beam radius w 01 is gradually reduced until a threshold-like change of the flank angle w from values ​​w<=w max less than or equal to the material-specific critical angle w max too large values ​​of the flank angle w of almost 90 degrees.

[0020] A threshold-like change in the flank angle w is a change that causes a large change in the flank angle with only a small change in the focus radius.

[0021] The value for the beam radius w 01 is calculated before the process is carried out in a simulated removal of a brittle material according to the workpiece.

[0022] The value for the beam radius w 01can also be determined experimentally before performing the process on a brittle-hard material corresponding to the workpiece by reducing the beam radius until a threshold-like change in the flank angle w to large values ​​occurs. The change in the flank angle to large values ​​can also be observed at a greater removal depth to be achieved.

[0023] In order to achieve a large removal depth and low absorption on the flank, which is associated with a reduction in damage to the workpiece, it is also intended that the temporal course of the pulse power is adjusted so that the degree of reflection on the flank of the removal depression increases.

[0024] The pulse shape of the laser radiation, i.e., the temporal progression of the pulse power, is also crucial, as it significantly alters the optical properties of the brittle, hard material, and thus also the reflectivity. The laser radiation is reflected from the flanks and redirected to the base of the hole or the ablation site, thereby preventing damage to the material on the flanks and increasing the ablation rate at the ablation site.

[0025] It is also intended that the temporal progression of the pulse power according to the method consists of pulse sequences, each with a short, intense pulse with a duration of typically <10 ps, ​​during which ablation is taking place, followed by a longer pulse with a duration of 10 ps to 1000 ps, ​​during which ablation is not taking place. This measure ensures that a high degree of reflection is present on the flank of the ablation depression before and during the intense pulse ablates the material. The person skilled in the art knows from the literature that brittle, hard materials change their optical properties within the physically possible ranges (the degree of reflection can assume values ​​between 0 and 1) when they are illuminated with intense laser radiation. The degree of reflection, e.g.of glass, can be changed by irradiation from a few percent of the reflectance - as is typically the case for light of low intensity - up to large values ​​of reflectance of 80-100%, as is typically the case for metals.

[0026] A key effect of the method according to the invention is achieved by the type of pulse shape used for the pulsed laser radiation. Therefore, the short, intense pulse is reduced in duration and thus increased in intensity until the material on the flank strongly reflects the radiation, so that virtually no damage occurs. For a selected laser system, the time-averaged power is limited so that the product of intensity during the pulse and pulse duration remains constant, or the intensity increases when the pulse duration is reduced.

[0027] The pulse shape to be used for the laser radiation used in the process for ablating a brittle, hard workpiece is determined experimentally by observing the ablation depth per pulse and thus the ablation rate at the base of the ablation and achieving an increase in this depth. The temporal progression of the pulse power during a pulse sequence is determined using a material-specific optimization calculation. This material-specific optimization calculation is used to computationally determine the pulse shape according to the process. A calculation model is used with which the optical properties, such as reflectance and absorbance, can be calculated for any temporal progression of the pulse power during a pulse sequence. The calculation model also includes the characteristic values ​​for the properties of the material.

[0028] The temporal course of the pulse power is iteratively improved by repeated application of the calculation model until the calculation result yields an optimally large reflectance at the flank of the ablation depression.

[0029] The calculation model calculates the dielectric function of the material as a function of the material parameters and the pulse power during the pulse sequence. For this purpose, the propagation of the radiation in the ablation depression is calculated taking into account the laser-induced dielectric function, taking into account that the dielectric function has different values ​​along the flank.

[0030] Further details and features of the invention will become apparent from the following description of exemplary embodiments with reference to the drawings. Fig. 1 schematically shows an ablation depression produced by laser radiation, which serves to define geometrical dimensions as they are also used and explained in the above description, Fig. 2 a graph of the projected intensity, the absorbed intensity and the threshold intensity for an ablation as in Fig. 1 is shown, Fig. 3 schematically shows the removal depth of the Fig. 1 with marking of the various crack formations / damages of the second and third type, Fig. 4 a simulated erosion depression showing the propagation of the developing cracks of the second and third types, Fig. 5 a schematic sketch to explain the formation of a removal depression with rough removal flanks, and Fig. 6 the diffraction pattern created by diffraction of the incident laser radiation at the ablation flanks.

[0031] In the presentation of the Fig. 1 schematically shows an ablation depression 1 which is formed in a thin glass material 2 with a thickness d by means of at least one pulsed laser beam with an intensity I which leads to ablation and which has a predefined width b L and is incident in the direction of the dotted line 6.

[0032] This removal recess 1 has side surfaces 3, which are also referred to as removal flanks or simply flanks, which extend from a leading edge 4 on the surface 5 of the material 2. The transition of the surface 5 into the removal recess 1 is marked by the points A and A' (leading edge 4).

[0033] The leading edge 4 is a spatially extended region of the surface 5 of the workpiece 2, where an unchanged part of the workpiece's surface transitions into the part of the surface in which material removal has taken place, creating a removal depression. The leading edge 4 is an edge region of the removal depression 1, which lies at A or A' in the cross-section of the removal depression 1.

[0034] The leading edge 4 can also be referred to as the edge of the removal recess 1 and is accordingly a surface of the workpiece or edge created by the removal of material. The transition from the non-removed surface of the workpiece into the removal recess occurs at the edge or edge of the removal recess.

[0035] The flanks 3 of the removal depression 1 run at a spatially variable flank angle w, where the flank angle w is defined as the local angle between the surface normal nF on the flank 3 of the removal depression 1 and the surface normal n0 on the non-removed surface 5 of the material 2.

[0036] At the beginning of the removal process, a wide and shallow removal depression 1 with a small flank angle forms in the center of the removal depression 1. This part of the removed surface, i.e. the base area of ​​the removal depression 1, is referred to as the removal base 7. The division of the surface of the removal depression 1 into flanks 3 and removal base 7 is carried out by the very different value for their flank angle w. The transition between the removal flank 3 and the removal base 7 is marked by the points B and B'. Typically, the base area 7 of the removal depression 1 takes on a width b (between the points B and B'), which becomes smaller with increasing removal depth, which, starting from the surface 5, is designated by d, until a so-called "removal stop" occurs. The depth d of the ablation depression 1 cannot be further increased by a longer irradiation time or pulse duration or number of pulses.

[0037] The vector arrow n0 indicates the surface normal, which is perpendicular to the non-removed surface 5 of the material 2, and the vector arrow n F indicates the surface normal on the flank 3 of the removal depression 1. The angle w between the surface normal n0 on the non-removed surface 5 and the surface normal n F on the flank 3 of the removal recess 1 is called flank angle w. The removal flanks 3 of the removal recess 1 run over the distance AB or A'B' in the Fig. 1 schematically shown cross section.

[0038] During the ablation process, the flank angle w, regardless of a spatial position along the flank, exceeds a limit angle w which is defined as the product of the cosine of the flank angle w with the intensity I of the laser radiation and a material-specific limit angle w max . The critical angle w maxis defined as the angle at which no more material is removed.

[0039] For the laser radiation incident with its beam axis along the dot-dash line 6, an elliptical beam shape of the cross section of the laser radiation is set in a plane perpendicular to the radiation axis (line 6).

[0040] In the focal plane of the laser radiation - in Fig. 1, the focus is preferably located in a plane perpendicular to the radiation axis 6 and on the surface 5 of the non-removed material - the major semi-axis of the ellipse lies in the feed direction, which is perpendicular to the plane of the drawing, and the minor semi-axis is perpendicular to it (in the direction of the double arrow b L ) aligned; accordingly, the beam radius w 01in the direction of the minor semi-axis of the laser radiation, at the entrance of the ablation depression where the unchanged part of the surface of the workpiece merges into the ablation depression, and is set to such a value that the flank angle w reaches almost 90 degrees and the achievable ablation depth is at least a factor of 5 greater than for a larger beam radius, where the flank angle w is smaller than a material-specific limit angle w max where no more material is removed. The beam radius w 02 the major semi-axis of the laser radiation is set to such a value that the Rayleigh length z R02 is at least as large as half the removal depth.

[0041] The creation of an extensive ablation base 7, which is almost perpendicular to the direction of the radiation axis 6 and has a very small flank angle w of <10 degrees, is an essential aspect of this invention. The width b of the ablation base 7 is achieved by a small beam radius (less than 6 µm for a 1 µm laser wavelength) and is almost equal to the width b L of the laser radiation. The ablation depression 1 takes for small beam radii w 01 the shape of a cylinder with an elliptical base, like a waveguide.

[0042] In contrast, with a very narrow, circular cross-section of a removal recess, only a very small removal depth d is achieved, since the removed material accumulates again on the flank and closes the removal recess again.

[0043] By means of the method according to the invention, the achievable removal depth d is significantly increased by producing an elliptical removal depression instead of a circular removal depression by enlarging the flank at least in one direction - the feed direction - perpendicular to the radiation axis.

[0044] In the graphic of the Fig. 2, the projected intensity is represented by a curve 8 (solid line), the absorbed intensity by a curve 9 (broken line) and the threshold intensity is represented by the line 10 (dash-dotted line) for the ablation as shown in Fig. 2 is shown.

[0045] Typically, a critical angle w max for the flank angle w or the pitch of the cutting edge / removal edge must not be exceeded. The limit angle w maxhas a value specific to the projected intensity and material of typically 70 angular degrees and can be determined experimentally.

[0046] As the Fig. 2 shows, the above-mentioned critical angle w max by the intersection of curve 9 for the intensity absorbed in the material and curve 10 for the threshold intensity for removal, as shown in the graph. If the absorbed intensity is smaller than the threshold intensity for removal, then no further removal can occur: a so-called drilling stop occurs when the flanks of the removal depression meet at depth d. The drilling stop occurs at a small depth when the maximum value w max for the flank angle w of the removal recess is small and consequently the flanks of the removal recess already meet at a smaller depth d.

[0047] A small diameter of the ablation depression 1 results in reduced absorption at the edge of the ablation depression 1 and guidance of the laser radiation along the advancing front of the ablation depression through multiple reflection or waveguiding. According to the invention, this advantageous effect also occurs during ablation / cutting if only the diameter of the ablation depression is reduced and total internal reflection does not occur. According to the invention, this effect also occurs at a cutting front if the ablation or cutting front has a partially open waveguiding geometric shape and does not enclose the radiation in an approximately cylindrical shape, as is known from fiber guidance.

[0048] As used in this description, the back of the workpiece or material is the surface of the workpiece or material facing away from the laser radiation.

[0049] A property of the leading edge 4 that refracts the light, for example, focuses it, is of particular importance for the invention. The leading edge 4 can have a geometric shape and an extension that can cause two undesirable effects, which can, however, be avoided or significantly reduced by the method according to the invention. Firstly, the geometric shape can cause an undesirable focusing of the incident laser radiation into the material, and secondly, the extension can cause the power of the incident laser radiation, which is captured by the leading edge 4 and then focused into the material, to assume an undesirable value such that the intensity in the focus of the leading edge 4 generates an electron density ρ that exceeds a threshold value ρ damage of the electron density for damage to the material at which damage / cracks begin and exceeds a threshold value ρ ablationthe electron density for removal is not reached.

[0050] Three different types of cracks occur when the material is damaged. These three different types of damage / cracks are: Type I cracks, which are back-side damage; Type II cracks, which are leading-edge damage; and Type III cracks, which are damage originating from the surface of the erosion depression, i.e., from the flanks of the erosion depression.

[0051] First-type cracks are those that occur even when no damage or removal has yet occurred on the front side - where the laser radiation hits.

[0052] Second and third type cracks are classified based on the Fig. 3 and Fig. 4; these cracks are precisely avoided by the method according to the invention.

[0053] Cracks or damage of the second type – also called spikes – originate from the leading edge 4. The cracks or damage of the second type extend into the volume of the material over a greater depth than the cracks of the third type. These material modifications / damages originating from the leading edge 4 can also become visible or arise in the volume (they are then also called "filaments"; the Kerr effect and self-focusing are the physical cause) or even reach the back or the surface of the material facing away from the laser radiation. In particular, the method according to the invention reduces the extent of the leading edge 4 and thus reduces the power of the laser radiation, which is the cause of the cracks of the second type.

[0054] Third-type cracks, which are also avoided or significantly suppressed by the method according to the invention, are cracks that do not penetrate as deeply and occur in addition to second-type cracks or second-type damage—along the ablated surface (cutting edge). They are not limited to the area near the leading edge 4 and occur where the laser radiation in the ablation recess 1 impinges on the ablated surface (ablation flanks 3). They propagate from the ablated surface into the material. Third-type cracks penetrate less deeply into the material than first-type cracks. The rough ablation recess 1 has a roughness with smaller radii of curvature compared to the leading edge 4. The focusing effect of the rough ablation recess is significantly stronger than the focusing effect of the leading edge 4.

[0055] In the Fig. 3 and Fig. 4, the cracks / damages in the glass 2 appear as shaded areas; the second type of cracks are identified by reference numeral 22 and the third type of cracks by reference numeral 33.

[0056] If the cracks 22 originating from the ablated surface reach the underside or the surface of the material facing away from the laser radiation, they can often no longer be distinguished from type I cracks, i.e., damage to the underside of the workpiece without the top surface having been ablated. Type III cracks or damage 33 begin at the rough ablation depression 1, i.e., at the ablated surface, and where the ablated surface exhibits a deviation from flatness.

[0057] This deviation of the ablation depression 1 from flatness is caused by the fact that the incident laser radiation is diffracted at the entrance of the ablation depression and in its course into the depth of the workpiece (ablation front, cutting edge) and has a diffraction structure as shown in the Fig. 5 and Fig. 6 is shown.

[0058] This diffraction pattern is a spatial modulation of the intensity and creates a deviation from a flat ablation front. The resulting diffraction pattern for the radiation intensity in ablation depression 1 leads to intensity enhancements at the ablation front and thus to a deviation of the ablation front from a smooth or flat ablation front.

Claims

[1] Method for removing brittle-hard material (2) from a workpiece by means of pulsed laser radiation, which has a radiation axis (6) in the beam direction and which is moved perpendicular to the radiation axis (6) in a feed direction and in which the removal by means of at least one laser beam with a predefined width b L forms an abrasion depression (1), wherein the side surfaces of this removal depression (1) are referred to as flanks (3) and wherein the base surface of this removal depression (1) is referred to as removal base (7) and assumes a width b which is at least 1 / 3 of the predefined width b L of the laser beam and does not decrease with increasing ablation depth d and wherein both the removal base (7) and the flanks (3) of the removal recess (1) in the material (2) run at a spatially variable flank angle w, wherein the flank angle w is defined as the local angle between the surface normal n F on the flank (3) of the removal depression (1) and the surface normal n0 on a non-removed surface (5) of the material (2), wherein during the ablation process the flank angle w, independent of a spatial position along the flank (3), has a critical angle w specific for the material (2) and a projected intensity I of the laser radiation max , where the critical angle w max is defined as the angle at which no more ablation takes place, and where the intensity I projected onto the surface of the ablation depression (1) is the product of the cosine of the flank angle w and the intensity I of the laser radiation, characterized by , that an elliptical beam shape of the cross-section of the laser radiation is set in a plane perpendicular to the radiation axis (6), wherein, in the focus of the laser radiation, the major semi-axis of the ellipse is aligned in the feed direction and the minor semi-axis is aligned perpendicular thereto and the beam radius w 01 in the direction of the minor semi-axis of the laser radiation, at the entrance of the ablation recess (1), where the non-removed surface (5) of the workpiece merges into the ablation recess (1), is set to such a value that the flank angle w reaches almost 90 degrees and the achievable ablation depth d is at least a factor of 5 greater than for a larger beam radius, where the flank angle w is smaller than the material-specific critical angle w max and that the beam radius w 02 the major semi-axis of the laser radiation is set to such a value that the Rayleigh length z R02is at least as large as half the removal depth d / 2. [2] Method according to claim 1, characterized by that the value for the beam radius w 01 the small semi-axis in a simulated removal on a brittle-hard material (2) is determined according to the workpiece by the beam radius w 01 is gradually reduced until a threshold-like change of the flank angle w from values w<=w max less than or equal to the material-specific critical angle w max too large values of the flank angle w of almost 90 degrees. [3] Method according to claim 1, characterized by that the value for the beam radius w 01 the small semi-axis in an experimental removal on a brittle-hard material (2) is determined according to the workpiece by the beam radius w 01 is gradually reduced until a threshold-like change of the flank angle w from values w<=w maxless than or equal to the material-specific critical angle w max too large values of the flank angle w of almost 90 degrees. [4] Method according to one of claims 1 to 3, characterized by that a temporal course of the pulse power is set so that the degree of reflection at the flank (3) of the ablation depression (1) increases. [5] Method according to claim 4, characterized by that the temporal course of the pulse power according to the method is composed of pulse sequences each with a short, intense pulse with a duration of typically <10 ps, during which ablation takes place, and a subsequent longer pulse with a longer duration of typically 10 to 1000 ps, during which no ablation takes place. [6] Method according to claim 5, characterized bythat the short, intensive pulse is reduced in duration and increased in intensity until the material (2) on the flank (3) strongly reflects the radiation, so that almost no damage to the flank (3) of the workpiece occurs. [7] Method according to one of claims 4 to 6, characterized by that the temporal course of the pulse power according to the process is determined via a material-specific optimization calculation.

Images