EP2978558A2

EP2978558A2 - Method and device for removing brittle-hard material which is transparent to laser radiation, by means of laser radiation

Info

Publication number: EP2978558A2
Application number: EP14721752.5A
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: 2014-03-24
Publication date: 2016-02-03
Also published as: DE102013005135A1; WO2014154345A3; KR20150135381A; CN105283265A; WO2014154345A2

Abstract

The invention relates to a method for removing brittle-hard material having a spatial volume of a given thickness between a top side and an underside by means of laser radiation, and the material has a defined optical penetration depth which is greater than or equal to the thickness of said material. The laser radiation is incident at a defined angle of incidence with respect to the surface normal of the top side; said laser radiation is designated as primary laser radiation. The primary laser radiation is at least partly reflected at the underside of the material, such that secondary laser radiation is generated in the volume of the material as a result of the reflection. For the processing of the brittle-hard material, the latter is backed by a further material on the underside in a positively locking manner. The invention also relates to a corresponding device.

Description

"Method and device for removing brittle-hard, laser-transparent material by means of laser radiation"

The invention relates to a method and a device for removing, such as drilling, cutting or scribing, of brittle, transparent to laser radiation material by means of laser radiation, wherein the material has a volume of a given thickness between a top and a bottom of the material and the material a defined optical penetration depth which is greater than or equal to the thickness of the material, wherein the laser radiation incident at a defined angle of incidence between an axis of the laser radiation and the surface normal of the top and the laser radiation incident on the top of the material referred to as the primary laser radiation wherein the primary laser radiation at the bottom of the material is at least partially reflected, so that in the volume of the material by the reflection of a secondary laser radiation is generated, by absorption of a coherent superposition primarily an electron density is generated in the volume of the material having at least two levels of electron density with different effects on the material, these levels of electron density being associated with material specific thresholds of intensity of the primary laser radiation by reaching the first threshold value of the laser radiation Intensity of the primary laser radiation is characterized by a modification of the material properties and / or mechanical damage in the form of cracks and the achievement of the second threshold value is characterized by a mechanical removal of the material in the form of flaking and / or erosion.

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 with sensitive "Touchscreen" 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 the case of mechanical scribing, microcracks are introduced or even small parts, so-called chips, are broken out, so that grinding or etching is necessary as a post-processing process.

Requirements for such a method and such a device for

Removal of brittle-hard material, as described above, are achieving a large removal rate, without causing damage to the material, so that no additional stresses or additional cracks are introduced into the material by the removal and thus after processing, otherwise with Method according to the prior art may occur.

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. If the cracks of the second type emanating from the abraded surface reach the underside or the surface of the workpiece or material facing away from the laser radiation, they are often no longer distinguished from the cracks of the first kind-if carelessly analyzed.

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. That the focal length, which can be assigned to the local roughness or the small radii of curvature of the rough surface is substantially smaller than the focal length of the comparatively large radii of curvature of the leading edge.

JP 2005 230 863A describes a method and an apparatus for processing transparent material by means of laser radiation. When the laser radiation enters the material to be processed reflections should be avoided, including the entire workpiece to be immersed in a liquid.

WO 2006 045 130A1 describes a method and an arrangement for dividing glass panes. The material to be cut is positively with a reflection glass disc deposited to allow reflections or amplify. Due to the different surface roughnesses of both the underside of the glass to be cut and the solid plate used, an air gap is formed so that considerable reflections nevertheless occur.

The present invention has for its object to provide a method and an apparatus with which a back damage, such as cutting, scribing and drilling of brittle-hard material that is transparent to laser radiation, is avoided, which means damage, cracking and / or Chipping on the back of the material, so cracks of the first kind, as described above, can be avoided.

This object is achieved with a method according to claim 1 and an apparatus according to claim 6.

It is essential that, for the processing of the brittle-hard material, this is deposited in a form-fitting manner by another material on the underside. The underside is in this case that surface of the material or material to be processed which lies opposite the surface on which the laser beam is incident.

The above measures are particularly suitable for the processing of so-called brittle-hard, thin material, such as glass, with ultra-short pulsed laser radiation, which is referred to as "wide-band-gap" material. "Wide-band-gap" material is through characterized a band gap between valence and conduction band which is greater than 1 eV. Ultra-short pulsed laser radiation means laser radiation with pulse durations in the range of less than 500 ps (picoseconds). The absorption of a material transparent to laser radiation with a defined optical penetration depth greater than or equal to the thickness of the material has two physically different ranges with respect to the intensity of the laser radiation, which are termed "linear absorption" and "non-linear absorption be designated. Of particular importance is a threshold behavior for the intensity of the laser radiation present locally in the material for the achievement of a non-linear absorption in the material, which is transparent for a small intensity or has a small value for the absorption of laser radiation and for larger Upper values - above the at least two thresholds for modification / cracks and flaking / erosion - become strongly absorbing the intensity of the laser radiation.

Furthermore, it has been found that the temporal pulse shape strongly influences the achievement of the threshold values and thus the absorption in the material. Finding a suitable temporal pulse shape is the subject of research and not state of the art. For the special case of a Gaussian temporal pulse shape and for pulse durations of 10 ps (pulse duration greater than 10 ps (intensity based), the achievement of the thresholds is characterized by the value for the intensity of the laser radiation, and for a pulse duration smaller than 10 ps (fluence based) The thresholds are characterized by the value for the fluence of the laser radiation (surface-related energy input) It has been found that with the measures according to the invention, the material to be worked, the brittle-hard material or the material is deposited in a form-fitting manner by a further material, ie on the On the other hand, such backside damage can be observed when the brittle - hard material on the back side is surrounded by air, which is opposite the side over which the laser radiation is radiated into the material to be processed Results can be explained by the fact that by a suitable further material, the reflection on the back of the material does not occur. A material is suitable for avoiding backside damage if the values of optical properties for index of refraction and absorption index agree with values for the optical properties of the brittle-hard material to be removed. By comparison, e.g. in the case of air as the backing material, a reflection of laser radiation. By absorbing a coherent superposition of primary, directly incident laser radiation and secondary laser radiation reflected at the back, an electron density is created in the volume of the material.

It may be advantageous for the optical properties of the further material to be adjusted by a spatial structuring of the interface between the brittle-hard material and the further material. Such structuring can take place, for example, by virtue of the fact that a grid-shaped, periodic structure on the Rear of the material is generated so that no reflected and diffracted only from the material radiation occurs. In this case, the backside material is to be regarded as the area in which the amplitude of the patterning is present, and the backside material is to be regarded as the periodic change of material and environment (eg air). While structuring the back side of the material to be processed does not affect the optical properties in the volume of the material to be processed, the degree of reflection - a property of the surface - depends not only on the optical properties of the materials but also on their bulk the nature of the surface itself (including its geometric shape); this dependence of the degree of reflection on the shape of the boundary surface both prove the Fresnel law and the diffraction theory.

As a further material both a liquid and a solid plate can preferably be used.

If a liquid is used, which is particularly preferred, the brittle-hard material can be inserted into the corresponding liquid bath for processing so that the underside is in contact with the liquid. As a liquid, in particular immersion oils, as z. B. in immersion microscopy use. If a solid plate is used as another material, a sheet of ductile or thermosetting ductile material should be used, the type of material depending on the result of the manufacturing process of that material, for example curing, for the resulting refractive index.

If a liquid is used, the composition of the liquid should be adjusted so that the values of the optical properties for refractive index and absorption index match those for the optical properties of the brittle-hard material to be removed so precisely that the first (damage) or the second ( Erosion) threshold for the intensity should not be exceeded, depending on whether damage or erosion should be avoided. At the same time, as a further material, a solid plate may have the above-mentioned structuring of that surface which bears against the underside of the brittle-hard material to be processed. In any case, attention should be paid to the necessary form-fit between the material and the solid plate.

According to the invention the optical interface is minimized in its effect on the reflection of laser radiation, so that exceeding the first threshold for the intensity is avoided and thus damage to the bottom (back) is avoided. It should be noted that in the physically and technically unattainable limiting case of perfectly matched optical refractive indices between the material to be processed and the material to be deposited, the optical effect of this interface could be eliminated. A perfect or almost perfect adjustment of the refractive indices is given in particular if the first and the second threshold of the intensity of the laser radiation are not exceeded, wherein the first threshold value is assigned to the occurrence of a damage and the second threshold value is assigned to the occurrence of an ablation. A lack of (back) damage on the underside characterizes the inventive composition of the liquid composition. A slight mismatch will then result in the first threshold being exceeded causing damage (e.g., cracks). A coarser mismatch then leads to erosion on the bottom (back) in the form of flaking.

According to the invention, the reflections on / from the material backside are to be avoided.

If a fluid is used to deposit the material to be processed, care must be taken to wet only the back of the material with a liquid.

Further details and features of the invention will become apparent from the following description of an embodiment with reference to the drawing. In the drawing shows

FIG. 1 schematically shows the structure of an arrangement according to the invention, FIG. 2 shows a further arrangement according to the invention,

Figure 3 is a schematic representation to the principle of the emergence of a

Explain the backside damage of a thin, brittle material when machining by means of laser radiation, and

Figure 4 shows two schematic illustrations, wherein the left image A shows the back of a processed glass without the deposit by another material, while the right image B shows the back with deposit by another material.

Figures 1 and 2 show two arrangements with which a brittle, laser-transparent material, designated by the reference numeral 1, can be processed with laser radiation in order to achieve the effects according to the invention, in particular cracks of the first kind on the back side of the material avoided or prevented as a result of processing.

According to the invention, according to the arrangement of FIG. 1, the brittle-hard material 1 to be processed is deposited in a form-fitting manner on its underside 2 by a further material 3, which is, for example, a solid plate. In order to remove or drill the brittle material by means of laser radiation, thereby to scratch or cut the surface, the laser radiation is radiated from the upper side 4 onto the thin, plate-shaped material. It is therefore essential that the material 1 on the bottom 2 - where the laser radiation impinges after passing through the material thickness - which is opposite to the top 4, on which the primary laser radiation is opposite, flat and thus positively locked.

The arrangement according to the invention, as shown in Figure 2, differs from that of Figure 1 in that the brittle-hard material is not deposited on its underside 2 by a solid plate, but immersed in a liquid bath 5, so that the underside 2 is covered by the liquid. The liquid 5 is filled for example in a tub 6; the material 1 is carried on the bottom 7 of the tub 6 by two supports 8. In Figure 3, the principle of the formation of a back damage is shown, which is just avoided with the arrangements and measures of Figures 1 and 2. The transparent material 1 and its bottom 2 are with the same

Reference numeral as indicated in Figures 1 and 2. The surrounding the bottom 2 of the material 1 medium, in the present case, for example, air, is designated by the reference numeral 9. An incident laser beam 6 is for the better

Clarity with a large angle of incidence 10 shown. This simulation shows a periodic increase in intensity near the backside 2, indicated by region 11, which causes the damage / cracks / chipping.

FIG. 3 simulates a case in which the brittle-hard material 1 has a thickness d and a primary laser radiation 12 is incident thereon. The material 1 has an optical penetration depth for the wavelength of the primary laser radiation 12 which is greater than or equal to the thickness d of the brittle-hard material 1 (d _op t> = d).

The region 11 of the periodic increase in the radiation intensity arises from the fact that in the volume of the brittle-hard material 1, a secondary radiation 13 is generated by reflection of primary radiation 12 at the bottom. By absorbing laser radiation, an electron density in the brittle-hard material 1 increases in at least two stages, wherein these stages are assigned threshold values of an intensity of the laser radiation. The achievement of the first threshold value is characterized by a modification of the material properties and the achievement of the second threshold value is characterized by a mechanical damage, the mechanical damage occurring in the form of cracks / chipping or erosion.

According to the invention, by positioning a second material on the

Bottom 2 of the brittle-hard material 1, as shown in Figures 1 and 2, the optical properties of the interface between the brittle-hard material and the second material adjusted so that the intensity of radiation in the volume and in the vicinity of the bottom 2 of the brittle-hard material due to interference between the incident primary radiation 12 and the reflected one Secondary radiation 13 does not reach the threshold value of the intensity for reaching the second stage and so damage to the bottom 2 is avoided. This further material on the underside 2 of the brittle-hard material 1 may be a solid body 3 (FIG. 1) or a liquid 5 (FIG. 2).

The above effects on the underside of the brittle-hard material 1 can also be enhanced by setting the optical properties of the interface between the brittle-hard material 1 and the further material by periodic structuring so that no reflections (diffraction orders) in the volume the first material 1 are directed.

FIG. 4 shows a comparison in the two schematic images in order to clarify the effects of the invention. While the left image A shows the back side of a processed glass plate without being deposited by another material, the right image B shows the back side where processing has been performed under the same conditions with deposition by another material. Damage is indicated schematically in the form of lines or line segments. It becomes clear that the glass plate according to image B shows far less damage than the glass plate of image A.

Claims

P atentanmeldung and device for removing brittle, transparent to laser radiation material by means of laser radiation "claims

A method of removing brittle-hard, laser-transparent material by means of laser radiation, wherein the material has a volume of a given thickness between a top and a bottom surface of the material and the material has a defined optical penetration depth greater than or equal to the thickness of the material is, wherein the laser radiation incident at a defined angle of incidence between an axis of the laser radiation and the surface normal of the top and the laser radiation incident on the top of the material is referred to as the primary laser radiation, wherein the primary laser radiation at the bottom of the material at least partially reflected is so that in the volume of the material by the reflection of a secondary laser radiation is generated, wherein by absorbing a coherent superposition of primary and secondary laser radiation, an electron density is generated in the volume of the material, having at least two levels of electron density with different effects for the material, wherein these levels of electron density are associated with material specific thresholds of intensity of the primary laser radiation by achieving the first threshold intensity of the primary laser radiation by a modification of the material properties and / or mechanical properties Damage in the form of cracks is characterized and reaching the second threshold by a mechanical removal of the material in the form of flaking and / or a Abtrag is characterized, characterized in that for the processing of the brittle-hard material is deposited in a form-fitting manner by another material on the bottom.

2. The method according to claim 1, characterized in that the optical properties of the further material are adjusted by a spatial structuring of the interface between the brittle-hard material and the further material.

3. The method according to claim t or 2, characterized in that as a further material, a liquid is used.

4. The method according to claim 3, characterized in that the composition of the liquid is adjusted so that the values of the optical properties for refractive index and absorption index with the values for the optical properties of the abradable brittle-hard material match such that the first threshold and the second Threshold of the intensity of the laser radiation are not exceeded, wherein the first threshold associated with the occurrence of damage and the second threshold associated with the occurrence of a removal.

5. The method according to claim 1 or 2, characterized in that as a further material, a solid plate is used.

6. An apparatus for removing brittle-hard, transparent to laser radiation material by means of laser radiation, wherein the material has a volume of a given thickness between a top and a bottom of the material and the material has a defined optical penetration, which is greater than or equal to the thickness of the material is, wherein the laser radiation incident at a defined angle of incidence between an axis of the laser radiation and the surface normal of the top and the laser radiation incident on the top of the material is referred to as the primary laser radiation, wherein the primary laser radiation at the bottom of the material at least partially reflected so that in the volume of the material By absorbing a coherent superposition of primary and secondary laser radiation, an electron density is generated in the volume of the material having at least two levels of electron density with different effects for the material, these levels of electron density being material-specific Thresholds are associated with an intensity of the primary laser radiation by the achievement of the first threshold of the intensity of the primary laser radiation is characterized by a modification of the material properties and / or mechanical damage in the form of cracks and reaching the second threshold by a mechanical removal of the material in Form is characterized by flaking and / or erosion, characterized in that for the processing of the brittle-hard material of this form-fitting another material on the bottom is deposited.