METHODS AND LASER PROCESSING MACHINES FOR THE SURFACE STRUCTURING OF LASER-TRANSPARENT WORKPIECES

Abstract

The disclosure provides methods and systems for producing surface structures on a laser-transparent workpiece, e.g. a glass or plastic workpiece, wherein one or more USP laser pulses are focused into the laser-transparent workpiece through the workpiece surface (11), to melt a modification in the workpiece interior by heating a focus volume, wherein the pulse parameters of the at least one USP laser pulse and the depth of the laser focus in the workpiece are chosen in such a way that the topmost part of the melted modification nearly touches the workpiece surface and the workpiece surface bulges outward to form a convex surface structure by means of thermal material expansion of the melted modification.

Claims

1. A method for producing surface structures on a laser-transparent workpiece, the method comprising: irradiating the laser-transparent workpiece with a pulsed laser beam in the form of ultra-short pulse (USP) laser pulses, wherein at least one USP laser pulse is focused into the laser-transparent workpiece through a workpiece surface, thereby melting a modification to form a melted modification in an interior of the workpiece by heating a focus volume in the interior of the workpiece; wherein pulse parameters of the at least one USP laser pulse and depth of laser focus in the workpiece are selected so that a topmost part of the melted modification nearly touches the workpiece surface and the workpiece surface bulges outward to form a convex surface structure by thermal material expansion of the melted modification.

2. The method of claim 1, wherein a plurality of USP laser pulses are focused into the laser-transparent workpiece through the workpiece surface, thereby melting the modification in the workpiece interior by heating the focus volume step-by-step; and wherein pulse parameters of the plurality of laser pulses and the depth of the laser focus in the workpiece are selected so that a topmost part of the melted modification nearly touches the workpiece surface and the workpiece surface bulges outward to form the convex surface structure by thermal material expansion of the melted modification.

3. The method of claim 2, wherein the plurality of USP laser pulses have a constant pulse spacing.

4. The method of claim 3, wherein the constant pulse spacing is not more than 100 ns.

5. The method of claim 4, wherein the constant pulse spacing is not more than 50 ns.

6. The method of claim 5, wherein the constant pulse spacing is not more than 20 ns.

7. The method of claim 2, wherein the USP laser pulses in the plurality of USP laser pulses are focused into the workpiece in the form of a plurality of laser bursts, where the USP laser pulses forming each of the laser bursts in the plurality of laser bursts have a pulse spacing that is less than a burst spacing between consecutive laser bursts in the plurality of laser bursts.

8. The method of claim 7, wherein each laser burst in the plurality of laser bursts includes no more than 10 USP laser pulses, and the pulse spacing is not more than 100 ns.

9. The method of claim 7, wherein the burst spacing corresponds to a burst repetition rate between 10 kHz and 10 MHz.

10. The method of claim 9, wherein the burst repetition rate is between 100 kHz and 1 MHz.

11. The method of claim 10, wherein the burst repetition rate is approximately 200 kHz.

12. The method of claim 1, wherein the at least one USP laser pulse has a pulse energy of between 0.1 J and 100 J.

13. The method of claim 1, wherein a beam cross section of the laser beam focused into the workpiece is shaped according to a desired cross section of the surface structures.

14. The method of claim 1, wherein the laser beam has a point-shaped or Gaussian laser focus or a linear laser focus running at right angles with respect to a beam axis, thereby melting a modification which is drop-shaped in a longitudinal cross section, with a spherical top side in the workpiece interior.

15. The method of claim 1, further comprising: producing linear surface structures by moving the laser beam over the workpiece and thus moving the laser focus through the workpiece interior.

16. The method of claim 1, further comprising: producing identical surface structures at different locations along the workpiece surface with identical predetermined pulse parameters.

17. The method of claim 1, wherein the at least one USP laser pulse is a plurality of USP laser pulses, and the method further comprises: measuring the workpiece surface between pulses in the plurality of USP laser pulses; and switching off or moving the laser beam as soon as a bulge height corresponding to a desired surface structure is measured.

18. The method of claim 1, wherein the at least one USP laser pulse has a pulse duration of less than 50 ps.

19. A laser processing machine for producing surface structures on a laser-transparent workpiece, comprising: a laser for producing a pulsed laser beam in the form of ultra-short pulse (USP) laser pulses; a focusing structure for delivering energy of the laser beam onto the workpiece; and a machine controller coupled to the laser and the focusing structure and configured to operate the laser and the focusing unit to melt a modification by heating a focus volume in an interior of the workpiece, where a topmost part of the modification nearly touches a workpiece surface.

20. The laser processing machine of claim 19, further comprising one or more a beam-shaping elements arranged in a beam path of the pulsed laser beam.

21. The laser processing machine of claim 20, wherein the one or more beam-shaping elements include one or more cylindrical lenses, spatial light modulators, or diffractive optical elements.

22. The laser processing machine of claim 19, further comprising a sensor system, connected to the machine controller, for measuring the workpiece surface.

23. The laser processing machine of claim 19, wherein the focusing structure includes a scanner for deflecting the laser beam over the workpiece.

24. The laser processing machine of claim 19, further comprising: a workpiece table for holding the workpiece; and an actuator coupled to the workpiece table to provide relative movement between the workpiece table and the laser.

Description

DESCRIPTION OF DRAWINGS

[0025] Further advantages and advantageous embodiments of the subject matter of the invention are evident from the description, the claims and the drawing. Likewise, the features mentioned above and those presented below can be used in each case by themselves or as a plurality in any desired combinations. The embodiments shown and described should not be understood to be an exhaustive enumeration, but rather are of exemplary character for outlining the invention. In the figures:

[0026] FIG. 1 is a schematic diagram that shows an example of a laser processing machine for producing surface structures on a laser-transparent workpiece using a pulsed laser beam.

[0027] FIG. 2 is a longitudinal cross-section through an example of a workpiece with a plurality of surface structures produced along the advance direction of the pulsed laser beam.

DETAILED DESCRIPTION

[0028] FIG. 1 shows an example of a laser processing machine 1 as described herein, which can be used to produce surface structures 10 on a laser-transparent workpiece 2 made of glass (e.g., fused silica) using a pulsed laser beam 3. By way of example, only glass is discussed in the following. The processes presented are, however, also conceivable for other laser-transparent materials, such as plastics.

[0029] The laser processing machine 1 includes a USP laser 4 for producing the laser beam 3 in the form of USP laser pulses 5 with pulse durations of less than 10 ps, e.g., in the femtosecond range; a laser processing head 6, which is height-adjustable in the Z direction, with an objective 7 of high numerical aperture (NA>0.1), from which the laser beam 3 exits in a manner focused toward the workpiece 2; a workpiece table 8, which is adjustable in the X-Y direction, on which the workpiece 2 lies; and a machine controller 9, which controls the laser parameters of the USP laser 4, the Z position of the laser processing head 6, and the X-Y movement of the workpiece table 8.

[0030] For producing a surface structure 10, a plurality of USP laser pulses 5 are focused into the workpiece 2 through the workpiece surface 11, in order to melt a drop-shaped modification 12, which is convex toward the workpiece surface 11, with a spherical top side in the workpiece interior by heating the focus volume step-by-step. In this way, the pulse parameters (given by pulse energy, number of pulses, pulse duration, temporal pulse spacing, wavelength, focusing) of the plurality of USP laser pulses 5 and the depth of the laser focus in the workpiece 2 are chosen in such a way that the topmost part of the melted modification 12 nearly touches the workpiece surface 11 (FIG. 2). Then, the workpiece surface 11 bulges outward to form a surface structure 10 in the shape of a sphere-like segment by means of thermal material expansion of the melted modification 12. In this way, the Z position of the laser focus determines the magnitude of the diameter of the sphere-like surface structure 10 on the workpiece surface 11.

[0031] In various embodiments, the plurality of USP laser pulses 5 can, as detail A in FIG. 1 shows, have a constant pulse spacing or a constant repetition rate or, as detail B in FIG. 1 shows, be grouped in a plurality of laser bursts 13, wherein the USP laser pulses 5 forming a respective laser burst 13 have a ns pulse spacing, which is therefore considerably less than the ms burst spacing between two laser bursts 13. By using laser bursts 13, it is possible to stretch the melted modification 12 in the Z direction and, as a result, to draw the surface structure 10 somewhat further out from the workpiece surface 11 in comparison to the constant pulse spacings shown in detail A. The burst repetition rate may be selected to be sufficiently high enough to enable heat accumulation in the workpiece 2. For this purpose, for example, a pulse energy of 10 J may be provided for a burst repetition rate of 100 kHz, while a pulse energy of 1 J may be provided for a burst repetition rate of 1 MHz. In general, more average power produces a larger melt volume in the workpiece 2.

[0032] Instead of the objective 7 of high numerical aperture (NA>0.1), other beam-shaping units can also be arranged in the beam path of the laser beam 3 for spatial pulse- and beam-shaping of the USP laser pulses 5, e.g., cylindrical lenses, diffractive optical elements or an SLM modulator, in order to produce other modifications 12 in the material and thus other surface structures 10.

[0033] In embodiments for producing a linear surface structure 10, the laser beam 3 can be continuously moved over the workpiece 2 in the advance direction v and thus the laser focus can continuously move through the workpiece interior. In other embodiments, as shown in FIG. 2, identical surface structures 10 can be produced at different locations respectively with the same, fixedly predetermined pulse parameters by sequential movement of the laser beam 3 in the advance direction v. When producing a surface structure 10, the workpiece surface 11 can be measured between the plurality of USP laser pulses 5 using a sensor system 14, e.g., attached to the laser processing head 6; with input from the sensor system, the machine controller 9 can then switch off or move the laser beam 3 further as soon as a bulge height h corresponding to the desired surface structure 10 is achieved. The surface structures 10 can thus be produced in a controlled manner, either a sequential approach or by automatic process monitoring.

[0034] In some contexts, alongside the thermal expansion of the material, other processes which would lead to a bulge can may occur. For example, the material around the region of the intended bulge could be modified so that internal stresses are produced, which cause the bulge.

[0035] Experiments carried out with the following parameters led to surface structures 10 with suitable quality:

[0036] average laser power 8 W

[0037] focusing: NA 0.2

[0038] focal length: 11 mm

[0039] laser spot diameter on the workpiece: approx. 4 m

[0040] pulse duration 500 fs

[0041] laser burst with 4 laser pulses with pulse spacing of 20 ns [0042] burst repetition rate 200 kHz [0043] pulse energy: 10 J