EP2978557A2

EP2978557A2 - Method for removing brittle-hard material by means of laser radiation

Info

Publication number: EP2978557A2
Application number: EP14718506.0A
Authority: EP
Inventors: Wolfgang Schulz; Urs EPPELT
Original assignee: Fraunhofer Gesellschaft zur Forderung der Angewandten Forschung eV; Rheinisch Westlische Technische Hochschuke RWTH
Current assignee: Fraunhofer Gesellschaft zur Forderung der Angewandten Forschung eV; Rheinisch Westlische Technische Hochschuke RWTH
Priority date: 2013-03-26
Filing date: 2014-03-21
Publication date: 2016-02-03
Also published as: WO2014154341A2; KR102193056B1; US20160052082A1; DE102013005136A1; KR20150135383A; CN105377500A; WO2014154341A3; CN105377500B

Abstract

The invention relates to a method for removing brittle-hard material by means of laser radiation, wherein a removal depression having a flank angle of the flanks of the removal depression forms in the material as a result of the removal, with an entrance edge, which is defined as a spatially extended region of the surface of the material where an unchanged and thus unremoved part of the surface of the material merges into the removal depression, at which spatial portions of the power of the laser radiation are refracted and focused into the volume of the unremoved material. The distribution of the laser radiation is set such that the entrance edge assumes a small spatial extent such that that portion of the power of the laser radiation which is included in the focusing effect of the entrance edge does not suffice to generate a threshold value P_damage for the electron density in the volume of the material, in order thus to avoid damage to the material.

Description

"Method for removing brittle-hard material by means of laser radiation"

description

The invention relates to a method for removing, such as cutting, scribing, drilling, brittle material by means of laser radiation, wherein the removal forms a Abtragsvertiefung with a flank angle w of the flanks of the Abtragsvertiefung in the material, wherein the flank angle w as the angle between the surface normal is defined on the flank of the excavation cavity and the surface normal on the non-abraded surface of the material, and with an entrance edge serving as a spatially extended area of the surface of the material where an unaltered and thus untranslated portion of the surface of the material enters Abtragsvertiefung passes, is defined, and are refracted at the spatial proportions of the laser radiation in the volume of the non-abraded material and focused.

Such methods are used, inter alia, in display technology, in which thin glass substrates, a brittle-hard material, must be processed. Especially the industrial display technology conquers an ever larger market volume and tends to ever lighter devices and thus also thinner glass panes for for example Smart Phones and Tablet Computer.

Thin glass substrates offer advantages for displays when the durability and mechanical stability of thicker glass can be achieved. These thin glass panes are used in almost all Fiat Panel Displays (FDP's).

Conventional methods for processing such thin glass panes are milling with a defined cutting edge, or they are based on mechanical effects of cracking (cracking and breaking) deliberately introduced into the material or the material. A variety of known process variants using laser radiation is also based on utilizing the mechanical effects of the principle of scribing and subsequent fracturing by replacing the scribe with the action of laser radiation and refracting the material / material after exposure to the laser radiation. Conventional mechanical processing (cutting, drilling) is considerably more difficult for thin glass plates than for large material thicknesses. In fact, mechanical cracks cause microcracks introduced or even small parts, called chips, broken out, so that a grinding or etching as post-processing process is necessary.

It has been found that the surfaces or flanks of the removal depression formed in the material have a diffractive and refractive effect on the introduced laser radiation. As a result, interference diffraction patterns are generated by radiation components of the laser radiation. As soon as these radiation components strike the surfaces of the excavation recess again, the surface is roughened there more intensively; the refractive effect of this roughness leads to the focusing of the laser radiation and cracks in the adjacent material can be caused. The leading edge in the area of the forming recess has a very large influence on the formation of the excavation recess and the resulting cracks. Damage in the form of cracks originates from this leading edge, for which the laser radiation appears to be the cause, which impinges on the leading edge.

US 2006/0091126 A1 describes a method and a laser system for processing substrates of silicon, gallium arsenide, indium phosphide or a monocrystalline sapphire to produce microstructure patterns therein, using ultraviolet lasers. Here, two laser beams are superimposed to produce finely structured Abtragsvertiefungen with shallow depth. According to this method, only a small depth of the removal recess-a structuring-is produced, so that an optical effect of a removal recess is negligibly small. In addition, the material surface should be removed so that a fine structuring with as many right-angled, sharp edges in a small space is created.

US 2011/0240616 A1 describes a method of singling brittle electronic substrates into small cubes using a laser. As FIGS. 4 and 5 show, the singulation is carried out in two sub-steps. In a first step, an initial cut is performed with low power and with a small removal rate and a small heat-affected zone near the edge of the hole. Although this leads to the reduction of debris, but not to avoid damage by focusing laser radiation in the Material during the second step, in which a second, deep bore is created with an extended edge with undesirable focus at the location of the bottom of the hole of the first, flat cut (initial cut).

The invention has for its object to provide a method by which the damage described above, which emanate in particular from the leading edge or the cause of laser radiation, which impinges on the leading edge, avoided or at least largely prevented.

This object is achieved by a method having the features of claim 1.

Essential for the method according to the invention is that the distribution of the laser radiation is adjusted so that the leading edge assumes a small spatial extent such that the proportion of the power of the laser radiation, which is detected by the foaming effect of the leading edge, is insufficient to to create in the volume of the material a threshold pdamage for the electron density and thus avoid damage to the material.

Thus, the power of the laser radiation, which is detected by the focusing effect of the leading edge, is adjusted so that the intensity in the material, which is achieved by the focusing of the leading edge, does not reach a threshold Pdamage for the damage of the material. According to the method of the invention, the power is not simply reduced and a low-damage first bore is produced, but it is cut in one step with great power and only the portion of the power that leads to the damage is reduced.

The inventive measure exploits the knowledge that the effect of focusing at the leading edge into the volume is a relevant effect that must be avoided.

By this measure, the outgoing from the leading edge damage is substantially reduced or avoided, since this reduces the intensity of the laser radiation and thereby a spatially localized and thus excessive burden of the brittle-hard material is avoided.

As mentioned above, cracks occur in the machining of thin glass plates. Such cracks are also in the processing of To observe glass plates with laser radiation. The inventors have found that these cracks manifest themselves in at least three different manifestations:

Cracks of the first kind: Damage / cracking / chipping occurs on the back side of the material. Cracks of the first kind occur even when there is no damage on the front - from where the laser radiation is incident - and no abrasion has taken place.

Cracks of the second kind: Cracks or damages - also called spikes - originate from the leading edge, which represents the transition from the unchanged part of the surface of the workpiece into the lateral removal flanks of the forming removal recess.

The cracks or damages of the second kind run over a large depth into the volume of the material compared to cracks of the third kind. This from the

Outgoing material modifications / damages can also be visible or arise in the volume (they are also called "filaments", Kerr effect and self-focusing are the physical causes) or even the back or the laser radiation facing away from the surface

Reach workpiece.

Third type cracks: The formation of fine, not so deep penetrating cracks occurs in addition to the cracks of the second kind or damages of the second kind - along the worn surface (cut edge); they are not limited to the area near the leading edge and occur where the laser radiation in the Abtragsvertiefung on the abraded surface (Abtragsflanken), ie the Abtragsflanken, occurs. They spread from the worn surface into the material. The third type of cracks penetrate less deeply into the material compared to the first type of cracks. The rough surface of the Abtragsvertiefung has in comparison to the leading edge to a roughness with smaller radii of curvature. The focusing effect of the rough surface of the Abtragsvertiefung is much stronger than the focusing effect of the leading edge. These third type cracks are thus avoided or at least significantly reduced over conventional methods by the method of this invention.

Preferably, in the method according to the invention, the Poynting vector P is set inclined by the proportion of the laser radiation incident on the non-abraded surface of the material in the region of the ablation recess in the direction of the leading edge and the angle of incidence WE of the laser radiation is chosen so that it does not become smaller as zero (w _E > = 0 degrees).

Furthermore, it may be advantageous to the Poyntingvektor P of the proportion of

Laser radiation that falls in the Abtragsvertiefung in the Abtragsvertiefung perpendicular to the normal vector nF on the flank of Abtragsvertiefung set and the angle of incidence w _{E of} the laser radiation with w _E = 90 degrees to choose.

An advantageous embodiment of the method is given when the spatial distribution of the laser radiation at the entrance into the Abtragsveriefung

is set rectangular (rectangle). In this way, it is in fact achieved that the region of the leading edge assumes a small extent and thus the proportion of the laser radiation, which is detected by the region of the leading edge and focused into the material, becomes small.

Also, the spatial distribution of the laser radiation at the entrance to the Abtragsvertiefung perpendicular to the direction of incidence of the laser radiation can be set Gaussian, and the Gaussian distribution is at a distance from the beam axis, where the intensity in the volume of the material reaches a threshold Pdamage for the damage of the material , rectangular cut off; for greater distances from the beam axis, the intensity of the laser radiation is set to zero, also referred to as Gaussian rectangle. The laser beam axis is defined by the average value of the Poyntingvektoren averaged over the cross section of the laser beam. The direction of the laser radiation varies over the cross section of the laser beam and is defined by the local direction of the Poynting vector. Typically, the Poyntingvektoren are inclined above the focus of the laser beam on the laser beam axis and directed below the focus of the laser beam axis away. A Gaussian-rectangular distribution of intensity in the laser beam is defined as one Gaussian distribution, which from a defined distance from the laser beam axis - such as through a diaphragm - no more intensity. Mathematically, a Gaussian rectangle is the multiplication of a Gaussian distribution by one

Rectangular distribution, which has the maximum value of 1. By rectangular distribution is meant a 2D rectangular distribution that has been rotated about the laser beam axis.

In the specified method for removing brittle-hard material by means of laser radiation is formed by the removal of a Abtragsvertiefung in the material whose surfaces, also referred to as flanks, bending and acting on the introduced laser radiation and thereby radiation components of this laser radiation interference diffraction pattern within the Abtragsvertiefung produce. As soon as these radiation components strike the surfaces of the removal recess again and penetrate into the material volume, they cause a spatially variable removal along the surfaces and as a result roughen the surface and induce cracks in the material volume.

In a further embodiment of the method, a wavelength mixture of at least two wavelengths is now used as the laser radiation for the ablation, wherein the at least two wavelengths are selected such that interference diffraction patterns due to the diffraction and refraction both along the surfaces of the Abtragsvertiefung and in the material volume in comparison to laser radiation with only one of the wavelengths such that a contrast K in the spatial structure of the intensity distribution is reduced, the contrast K according to Michelson is defined as K = (lmax-lmin) / (lmax + lmin), where Imax and Imin indicates the maximum and minimum intensity of the spatial structure of the intensity distribution. Michelson's contrast K is a measure of the periodic pattern of diffraction maxima and diffraction minima.

By these measures, the intensity contrast is reduced in the area of the surface of the flanks of the Abtragsvertiefung and thereby avoided a spatially localized and thus excessive burden of the material, and as a result of the fact that for the processing of brittle material laser radiation with two different wavelengths, which are superimposed , is used. A superimposition of laser radiation with different wavelengths produces namely for each wavelength a spatially shifted in the Abtragsveriefung diffraction pattern. By choosing the appropriate wavelengths of the used

Radiation components, the achievements and the focal radii of the (at least two) wavelengths to be superimposed, the diffraction maxima of the laser radiation having the first wavelength can fall to the locations where the diffraction minima of the laser radiation lie at the second wavelength. As a result of this superimposition, the contrast of the superimposed diffraction structure becomes significantly smaller, with the result that a high removal rate and, if any, low stresses and / or cracks after removal are achieved.

To achieve the smallest contrast, the wavelengths of the

superimposed radiation components as well as belonging to the wavelengths

Achievements and the associated focus radii of the radiation components.

In a preferred embodiment of the method, a wavelength mixture of the at least two wavelengths is selected such that spatial positions of interference maxima of one wavelength (n) fall into interference minima of the other wavelength (n), whereby the ablation edge is not reached roughened and so the focusing effect of the rough Abtragskante is not formed, and so the threshold for the removal Pdamage, occur at the damage / cracks, is not achieved.

Furthermore, radiation components of the laser radiation may be used in addition to the at least two radiation components having wavelengths which are integer multiples or divisors of the at least two wavelengths, which may be referred to as fundamental wavelengths.

Each wavelength can be provided by a separate laser. This has the advantage that the focus radii and the power components of the different wavelengths of the laser radiation can be adjusted. If the laser source allows modulation of the wavelength, the different wavelengths can be provided by a laser source or a laser device. If the laser source emits several wavelengths, as is the case, for example, with diode lasers, the different wavelengths can be provided by a laser source or a laser device whose wavelength is modulated.

Further details and features of the invention will become apparent from the following description of exemplary embodiments with reference to the drawing. In the

Drawing shows

Figure 1 schematically shows a Abtragsveriefung with marking the

various cracking / damage of the second kind and third kind,

Figure 2 is a simulated Abtragsveriefung that the spread of

forming second-type and third-type tears,

FIG. 3 shows a schematic sketch in order to explain the formation of a removal depression with rough removal flanks,

Figure 4 shows the diffraction pattern, by diffraction of the incident

Laser radiation is generated at the ablation edges,

5 shows the principle of the formation of a crack of the second kind (FIGS. A, b) and the principle of the method according to the invention in order to avoid or at least suppress these cracks (FIGS. C, d), FIG.

Figure 6 shows a simulated Abtragsvertiefung, with a TopHat-shaped

Distribution of the intensity of the laser radiation was generated

Figure 7 is a view corresponding to Figure 6, but with a spatial

Distribution of the intensity of the laser radiation, which consists of a TopHat distribution and a Gaussian distribution,

Figure 8 is a view corresponding to Figure 6 with a narrow, spatial

Gaussian distribution of the intensity of the laser radiation using laser radiation with a beam radius <4 pm,

9 shows a view corresponding to FIG. 6 with a spatial TopHat distribution of the laser radiation at the entrance to the removal recess, FIG.

10 shows an image sequence a to e, to the formation of cracks of the third kind

explain, and FIG. 11 shows in an enlarged simulation representation the contrast of the spatial distribution of the intensity in the removal depression, corresponding to image a of FIG. 5.

In the illustration of Figure 1, a V-shaped Abtragsvertiefung 1 is shown schematically, which is formed in a thin glass material 2 with a thickness x. This removal recess 1 has Abtragsflanken 3, emanating from an inlet edge 4 on the surface 5 of the material.

For the various terms used herein, the following definitions apply:

Threshold p _a t> iation is the electron density threshold at which ablation / ablation begins,

Threshold Pdamage is the threshold of electron density in which

Use damage / cracks,

Pulse parameter is a set of parameters used to characterize the spatial, temporal and spectral properties of incident laser radiation. The pulse parameter contains at least the values for

- pulse duration,

- maximum value of the intensity in the pulse,

- temporal pulse shape; this is the temporal distribution of the intensity of laser radiation in a single pulse or in a sequence (multiple pulse, pulse burst) of pulses,

- spatial distribution of intensity and

- Spectral distribution of intensity (wavelength mixing).

The leading edge is a spatially extended area of the surface of the workpiece where an unchanged portion of the surface of the workpiece merges into the portion of the surface in which material has been removed and a

Abtragsveriefung has arisen.

The edge of the excavation recess is a material produced by the removal of material

Surface. Rear side or underside of the workpiece is the surface of the workpiece facing away from the laser radiation.

The three different types of damage / tear explained above are:

Cracks of the first kind about backside damage,

Cracks of the second kind about entry edge damage,

Cracks of the third kind about damage originating from the surface of the excavation recess, that is to say from the flanks of the excavation recess.

There are two thresholds Pdamage, Pabiation for the electron density p in

Material, each having a damage pdamage or an ablation pabiation of the

Cause material, defined. For each material can this different

Pdamage thresholds, Pabiation for the electron density p, where Pdamage <Pabiation is assigned to two sets of values for the parameters of the laser radiation.

A light-refractive property, for example focusing property, of the leading edge is of particular importance to the invention. Namely, the leading edge may have a geometric shape and an extent which may cause two undesirable effects, which however may be avoided or substantially reduced by the method according to the invention. On the one hand can occur due to the geometric shape undesirable focusing of the incident laser radiation in the material and on the other can, by the

Expansion, the power of the incident laser radiation, which is detected by the leading edge and then focused in the material, undesirably take a value, so that the intensity in the focus of the leading edge

Generates electron density p, which exceeds the threshold value P _d amage of the electron density for a damage of the material / material and does not reach the threshold value pabiation of the electron density for a removal.

In the damage of the material / material, the three different types of cracks occur, which have already been explained above.

Cracks of the first kind are those which occur even when no damage or abrasion has taken place from the front side (where the laser radiation is incident). Cracks of the second kind and third kind, which are illustrated with reference to Figures 1 and 2.

In FIGS. 1 and 2, the cracks of the second type are identified by the reference numeral 22 and the cracks of the third type by the reference numeral 33.

Reach the 22 emanating from the worn surface cracks

Underside or the laser radiation facing away from the surface of the workpiece, then they can often no longer be distinguished from the cracks of the first kind, which means damage to the underside of the workpiece without the top of the workpiece is already removed. Cracks or damage of the third kind begin at the rough excavation pit, i. at the abraded surface, and where the abraded surface has a deviation from flatness.

This deviation of the removal depression from the flatness arises from the fact that the incident laser radiation at the entrance of the removal depression and in its course is diffracted into the depth of the workpiece (removal front, cutting edge) and has a diffraction structure as shown in FIGS. 3 and 4 ,

This diffraction structure is a spatial modulation of the intensity and generates the deviation from a flat erosion front. The resulting diffraction structure for the intensity of the radiation in the Abtragsvertiefung leads to increases in the intensity of the Abtragsfront and thus to a deviation of Abtragfront of a smooth or flat Abtragsfront.

According to the invention, in order to avoid the occurrence of cracks of the second kind in the form of damage / cracking emanating from the leading edge of the material to be processed, the power of the laser radiation, which is detected by the focusing effect of the leading edge, is adjusted so that the Intensity in the material, which is achieved by the focusing of the leading edge, does not reach a threshold value for the damage of the material.

FIG. 5 shows an image sequence in which the illustrations a) and b) represent the principle of the formation of a crack of the second kind, while the image sequence with the figures c) and d) serve to illustrate the measures according to the invention. in order to avoid or substantially reduce the occurrence of such second-type cracks.

In FIGS. 5a) and 5b), the respective entry edges of a removal depression are indicated by a region 40. This leading edge thus comprises a spatially extended region 40, in which the laser radiation is focused. In Figures 5c) and 5d), the spatially extended region 40 is associated with its location as a transition region from the non-abraded surface into the flank of the ablation well.

FIG. 5c shows that the Poyntingvektor P of the proportion of the laser radiation, which falls in the region of the leading edge in the Abtragsveriefung, perpendicular to

Normal vector nF is set on the flank of Abtragsveriefung and that the angle of incidence WE of the laser radiation w _E = 90 degrees.

In the material of the workpiece, a region of damage or the beginning of a filament is formed, which is designated by the reference numeral 41.

The arrows 42 indicate the Poynting vectors P (with direction and magnitude) whose time-averaged amount is also referred to as intensity.

FIGS. C) and d) of FIG. 5 show, in addition to the polyhedral vectors P (reference numeral 42), the normal vectors ns on the non-ablated surface and the normal vectors nF on the abraded surface (cutting edge, edge of the ablation recess). Finally, in the figure d) of Figure 5, the angle of incidence WE of Poyntingvektors P is indicated on the non-abraded surface. As can be seen from Figure 5, the angle of incidence w _{E is} defined as the angle between the Poyntingvektor P of the laser radiation and the normal vector n _{s of} the surface on which the laser beam is incident. The laser beam either falls on the flank of the removal depression with the surface normal n _F (FIG. 5 c) or falls on the non-ablated surface of the surface normal n _s (FIG. 5 d). The angle of incidence w _E is equal to the flank angle w when the polyhedral vector is parallel to the surface normal ns on the non-abraded surface of the material (see eg Figure 5c). According to the invention, the laser radiation is now adjusted so as to avoid that two spatial portions of the radiation from the leading edge are so refracted and focused in the non-abraded material and are superimposed in the material that the threshold Pdamage for the damage is exceeded and thus the Threshold for the ablation Pabiation is not reached. As a result, no crack / damage of the second kind arises.

The extent of the surroundings of the leading edge is defined by the fact that the laser radiation incident into the focusing part of the leading edge contains sufficient power so that at least the damage threshold of the material or material can be achieved in the focus of this power. Consequently, in order to avoid damages in the material, which are caused by laser radiation, which is refracted and focused at the leading edge to the Abtragsvertiefung in the material, to avoid two variables to be considered and correctly set, namely on the one hand, the geometric shape of the leading edge and the other the direction of the incident laser radiation and thus the angle w of the Poynting vector to the normal vector ns on the non-ablated part of the surface.

As mentioned above, the geometric shape of the leading edge leads to a refraction of the laser radiation and in an unfavorable case to the focusing of the incident laser radiation, as shown schematically in Figures a) and b) of Figure 5. The geometric shape of the leading edge points

ideally a sharp edge, which has no spatial extension; consequently, the geometric shape of the leading edge is ideally one without

Curvature (ideally an edge with a radius of curvature r taking the value r = 0). In order to achieve an edge with the radius of curvature close to r = 0 (with the criterion from the following paragraph), an inventive measure is to set a Gaussian-rectangular distribution of the incident intensity.

According to the method according to the invention, the geometric shape of the leading edge is to be adjusted so that the power of the laser radiation, which is focused by the leading edge or by the focusing effect of the leading edge, is detected is so small that the achievable by the focus intensity does not reach the threshold value Pdamage for the damage of the material of the workpiece.

The second quantity to be considered is the direction of the incident laser radiation, that is, the direction of the polarization vector P of the laser radiation on the non-abraded surface of the material of the workpiece. Ideally, the direction of the incident laser radiation outside the Abtragsvertiefung, ie on the non-abraded part of the workpiece surface, parallel to the normal vector ns on the non-abraded surface and within the Abtragsvertiefung perpendicular to the normal vector n _F on the edge of the Abtragsveriefung.

According to the method of the invention, the direction of the incident laser radiation, hence the direction of the Poynting vector P of the laser radiation, is inclined on the non-abraded surface of the material of the workpiece towards the ablation recess by an angle w to the normal vector ns, that is, it forms an angle of incidence w> = 0 on the non-ablated surface to the normal vector ns (see Figure d) of Figure 5) and is ideally within the Abtragsvertiefung perpendicular to the normal vector n _F on the edge of the Abtragsveriefung.

FIGS. 6 to 9 now show the results of various measures

shown that can be applied to the geometric shape of the

To influence the leading edge.

FIG. 6 shows the simulated formation of an ablation recess which is achieved with incident laser radiation having a top hat-shaped, spatial distribution (ie transversely to the direction of incidence) of the intensity of the incident laser radiation. By this measure, the area of the leading edge is greatly reduced or no longer present and the remaining damage has a much smaller penetration depth into the material starting from the leading edge than in a Gaussian spatial distribution of laser radiation, which is commonly used. FIG. 7 now shows a simulated representation corresponding to FIG. 6, but in which the laser radiation has a spatial distribution of the intensity of the incident laser radiation, which is composed of a TopHat distribution for large distances from the laser beam axis and a Gaussian distribution near the laser beam axis , It can be clearly seen that even here the proportion of laser radiation due to the TopHat distribution in the upper region of the Abtragsvertiefung results in approximately parallel Abtragsflanken, but with a round Abtragsgrund, which is a consequence of the proportions of the laser radiation due to the Gaussian distribution. The result of this simulation is also a slightly greater penetration depth into the material than the case where the spatial distribution of the intensity of the incident laser radiation is only TopHat-shaped.

In the simulation, as shown in Figure 8, laser radiation was used with a narrow beam radius (<4μηι) and a Gaussian distribution. In the region of the leading edge, the crack-forming effect of the laser radiation focused from the region of the leading edge, that is to say cracks of the second type or impact damage, no longer exists.

Only the cracks of the third kind, i. Damage emanating from the surface of the Abtragsvertiefung, that means emanating from the flanks of Abtragsvertiefung, still occur. The cracks of the third kind are still present, but much smaller pronounced and erosion or the drilling speed assumes larger values. The achievement of small flank angles has been proven experimentally.

Figure 9 shows a simulation in which the laser radiation is pulsed and the

Wavelength of the laser radiation changes from pulse to pulse alternately from 500nm to 1000nm. The geometric shape of the advantageously forming large curvature of the region of the leading edge causes a reduction of the focused intensity from the region of the leading edge into the volume and thus falls below the damage threshold and avoiding this cause for the cracking. In one embodiment of the method, a wavelength mixing of at least two wavelengths is used as the laser radiation for the removal. In this case, the at least two wavelengths are selected such that interference patterns due to the diffraction and refraction both in the volume of material and in the volume of Abtragsvertiefung compared to laser radiation with only one of the wavelengths adjust such that a contrast K in the spatial structure of the intensity distribution is reduced, so that a spatially localized and thus excessive burden of the material is avoided. The contrast K is defined by Michelson as K = (lmax-lmin) / (lmax + lmin), where I indicates the intensity.

Thus, the contrast between intensity maxima and intensity minima is reduced, which is otherwise responsible for the diffraction of the laser radiation at the surface or the flanks of the Abtragsvertiefung and due to the ability of the laser radiation to interference.

As a result of the superimposition of laser radiation according to the invention with at least two different wavelengths, a diffraction pattern spatially displaced in the removal depression is generated for each wavelength. The at least two wavelengths, also in conjunction with the adjustment of the powers and focus radii of the corresponding laser radiation, can be chosen so that the diffraction maxima of the laser radiation having the first wavelength fall to the locations where the diffraction minima of the laser radiation coincide with the second Wavelength lie. As a result of this overlay, the contrast of the superimposed diffraction pattern becomes much smaller.

FIG. 10 illustrates once again in the image sequence of the images a to e the formation of cracks of the third kind, as can be seen in the last image e of the image sequence, after 8 pulses of laser radiation.

Figure a shows the causal distribution of intensity in the Abtragsveriefung, image b that in brittle-hard material. Image c represents the free electron density, image d the surface of the excavation cavity and image e the resulting distribution of

Modifications / damage / cracks after eight pulses of laser radiation. It can be seen from the images of FIG. 10 that the spatial structure of the intensity distribution in the removal depression (FIG. 1 a) continues in an undesirably pronounced spatial structure of the intensity of the laser radiation in the brittle-hard material (FIG. As a result, the geometric shape of the

Surface of the Abtragsvertiefung (image d), the generated density of free electrons (image c) and the modifications / damage (image e) spatially structured and it form unwanted cracks of the third kind.

The spatial extent of the counts is 40pm in both directions to illustrate the proportions.

As contrast, as used here, the deviation of the intensity of the laser radiation from a spatially weakly variable distribution is referred to, as would be present in an undisturbed propagating laser radiation in the Abtragsveriefung (Figure a of Figure 10).

This contrast in the spatial distribution of the intensity in the removal depression, which is to be reduced, is shown again in the enlarged FIG. This contrast in the spatial structure of the intensity distribution in the removal recess is reduced in accordance with an embodiment of the invention by superimposing laser radiation with at least two different wavelengths.

According to the invention, the power of the laser radiation is not reduced to avoid the damage, but the geometric shape of the leading edge is set according to the invention by adjusting the spatial distribution of the power so that the focusing effect of the leading edge is reduced. According to the invention, therefore, at high power, and as a result thereof at a desirably high removal rate, the proportion of the power is reduced, which is detected by the leading edge and is focused and thus leads to undesired damage.

According to the inventive method can be cut with great power in one step and still a small extension of the leading edge can be generated. As a result of the small extent of the leading edge, only a minor portion of the power is focused into the material, thus avoiding damage.

Claims

Patent application “Method for removing brittle-hard material using laser radiation” patent claims

Method for removing brittle-hard material using laser radiation, the removal forming a removal recess with a flank angle w of the flanks of the removal recess in the material, the flank angle w being the angle between the surface normal on the flank of the removal recess and the surface normal on the not removed surface of the material is defined, and with a leading edge, which is defined as a spatially extended area of the surface of the material, where an unchanged and therefore not removed part of the surface of the material merges into the removal recess, and at which spatial components of the performance the laser radiation is refracted and focused into the volume of the material that has not been removed, characterized in that the distribution of the laser radiation is adjusted so that the leading edge assumes a small spatial extent so that the proportion of the power of the laser radiation that depends on the focusing effect of the The leading edge is detected, is not sufficient to generate a threshold Pdamage for the electron density in the volume of the material and thus avoid damage to the material.

Method according to claim 1, characterized in that the Poynting vector P is set inclined towards the leading edge by the proportion of the laser radiation that is incident on the non-ablated surface of the material in the area of the ablation recess and that the angle of incidence w _E of the laser radiation is not smaller than zero (WE>=0 degrees).

Method according to claim 1 or 2, characterized in that the Poynting vector P of the portion of the laser radiation that falls into the removal recess in the area of the leading edge is perpendicular to the normal vector nF is set on the flank of the removal recess and that the angle of incidence w _E of the laser radiation is WE=90 degrees.

4. Method according to one of claims 1 to 3, characterized in that the spatial distribution of the laser radiation at the entrance to the

Ablation depression is set rectangular as seen perpendicular to the direction of the laser beam axis.

5. Method according to one of claims 1 to 3, characterized in that the spatial distribution of the laser radiation at the entrance to the

Ablation depression is set Gaussian-shaped and the Gaussian-shaped distribution at a distance from the beam axis where the intensity in the material reaches a threshold value Pdamage for damage to the material,

is cut off in a rectangular shape, and for larger distances from the

Beam axis the intensity is zero.

6. The method according to claim 1, characterized in that a wavelength mixture of at least two wavelengths is used as laser radiation for the ablation and the at least two wavelengths are selected so that interference diffraction patterns due to the diffraction and refraction occur both along the surfaces of the ablation recess and also in the material volume compared to laser radiation with only one of the wavelengths in such a way that a contrast K in the spatial structure of the intensity distribution is reduced, the contrast K being defined according to Michelson as K = (lmax-lmin)/(lmax+lmin) , where Imax and Imin indicate the maximum and minimum intensity of the spatial structure of the intensity distribution.

7. The method according to claim 6, characterized in that the wavelength mixture of the at least two wavelengths is selected such that spatial positions of interference maxima of one wavelength (s) fall into interference minima of the other wavelength (s).

8. The method according to claim 6 or 7, characterized in that additional wavelengths to the at least two wavelengths are selected so that they are integer multiples or divisors of the at least two wavelengths.

9. Method according to one of claims 6 to 8, characterized in that each wavelength is provided by a separate laser.

10. The method according to any one of claims 6 to 9, characterized in that the different wavelengths are provided by a laser source whose wavelength is modulated in time.