LASER BASED HOLE FORMATION AND ETCHING OF TRANSPARENT MATERIALS

Technical Field

[1] The present disclosure relates generally to laser processing of transparent materials such as glass, and more particularly to laser cutting using Bessel-like beam configurations.

Background

[2] Laser processing of material, specifically the controlled interaction of laser light with material, is well established in various fields of applications such as laser cutting and laser welding, be it, for example, in industrial as well as medical applications. The interaction depends on the laser light parameters such as wavelength, focus zone, laser power etc. as well as the material properties such as absorption at the respective wavelength, band gap of the material etc. In combination, those parameters and properties define the interaction that takes place and in particular the field strength that is provided at a specific position within the material.

[3] Gaussian beams are often used for modifying transparent materials with various laser processing methods such as disclosed in WO 2015/108991 A2. Gaussian beam based methods have their limitations due to the short dimension of the limited focal spot size, resulting, for example, in limited process speed for a given aspect-ratio.

[4] In "Bessel and annular beams for materials processing" by M. Duocastella, Laser

Photonics Rev. 6, No. 5, 607-621 (2012), Bessel beams are disclosed for modifying materials. Furthermore, the use of Bessel beams for laser processing is disclosed, for example, in "High aspect ratio nanochannel machining using single shot femtosecond Bessel beams" by M. K. Bhuyan eta al., Applied Physics Letters 97, 081102-1 (2010) and

"Femtosecond non-diffracting Bessel beams and controlled nanoscale ablation" by M. K. Bhuyan et al., IEEE (2011). Furthermore, the use of a pulsed Bessel-like laser beam for laser processing is disclosed, for example, in WO 2014/079570 Al, where damage zones are generated along a pre-cut line at a distance not too close to the previous laser damage zone as that may affect the present beam propagation.

[5] Bessel beams in combination with chemical etching were used in methods for rapid laser drilling of holes in glass as disclosed in WO 2015/100056 Al. Furthermore, the generation of birefringent waveguiding microchannels is disclosed in "Fabrication of microchannels in fused silica using femtosecond Bessel beams" by D.A. Yashunin,

JOURNAL OF APPLIED PHYSICS 118, 093106 (2015).

[6] Processes of laser trepanning a passage through a workpiece are disclosed in

US 5,223,692 B as well as in "Laser Trepanning for Industrial Applications", by

D. Ashkenasi, Physics Procedia 12 (2011) 323-331.

[7] The herein disclosed concepts are generally directed at using pulsed Bessel/Bessel- like beams (generally Bessel-type beams) for laser processing a glass sample with the objective of creating micron-sized holes with different shapes. In particular, the concepts are directed at generating micron-sized holes in glass from plain bulk by Bessel-like beams and at glass etching with Bessel-like beams for generating in particular square/rectangular apertures in glass. Herein, micron-sized holes refer to an opening width in the range from, for example, about 30 μιη to more than 1 mm.

[8] Thus, the present disclosure is directed, at least in part, to improving or overcoming one or more aspects of prior systems.

Summary of the Disclosure

[9] In a first aspect, the present disclosure is directed to a method for forming a through- hole in a material using a pulsed Bessel-like laser beam. The material is essentially transparent with respect to single photon absorption of the pulsed Bessel-like laser beam when propagating through the material. The method comprises scanning a through-hole forming pattern with the pulsed Bessel-like laser beam, thereby creating a multi-photon process in the regime of optical breakdown photoionization as the underlying process for forming single laser specific pulse damage regions across the material. Thereby, the through- hole forming pattern comprises a path along which the pulsed Bessel-like laser beam is scanned along the surface of the material and the path comprises trajectory sections along which sequences of single laser pulse damage regions are formed and a plurality of the trajectory sections form pairwise neighboring trajectory sections, the single laser pulse damage regions of successive laser pulses, which follow immediately one another, are displaced with respect to each other by a pulse machining step elected for providing an overlap of single laser pulse damage regions of the respective successive laser pulses, and the single laser pulse damage regions of neighboring trajectory sections, which extend immediately next to each other, are displaced with respect to each other by a trajectory machining step selected for providing an overlap of single laser pulse damage regions of the respective laser pulses of the neighboring trajectory sections.

[10] In another aspect, the present disclosure is directed to a method for forming a

through-hole in a material using a pulsed Bessel-like laser beam. The material is essentially transparent with respect to single photon absorption of the pulsed Bessel-like laser beam when propagating through the material. The method comprises scanning a through-hole forming pattern with the pulsed Bessel-like laser beam, thereby creating a multi-photon process in the regime of optical breakdown photoionization as the underlying process for forming single laser specific pulse damage regions across the material. Thereby, the through- hole forming pattern comprises a path along which the pulsed Bessel-like laser beam is scanned along the surface of material and the path comprises a plurality of concentric circular and/or elliptical trajectories for forming sequences of single laser specific pulse damage regions. For the concentric circular and/or elliptical trajectories, the single laser pulse damage regions of successive laser pulses following immediately one another are displaced with respect to each other by a machining step selected for providing an overlap of single laser pulse damage regions of respective successive laser pulses.

[11] In another aspect, the present disclosure is directed to a method for reshaping a preexisting through-hole in a material by sidewall-etching with a pulsed Bessel-like laser beam into a desired shape. The material is essentially transparent with respect to single photon absorption of the pulsed Bessel-like laser beam when propagating through the material. The method comprises the steps of providing an etching pattern, which defines a path along which the pulsed Bessel-like laser beam is scanned, wherein the shape of the pattern is associated with the desired shape, and the path comprises a plurality of trajectory sections for forming sequences of single laser specific pulse etching regions. The single laser pulse etching regions of successive laser pulses following immediately one another along one of the trajectories are displaced with respect to each other by a pulse machining step selected for providing an overlap of single laser pulse etching regions of respective successive laser pulses, and the plurality of trajectory sections comprise an initial trajectory section, a target trajectory section defining the desired shape, and a plurality of trajectory sections extending as a sequence in-between the initial trajectory section and the target trajectory section;

positioning the etching pattern with respect to the pre-existing through-hole such that the initial trajectory section extends within the aperture given by the pre-existing through-hole such that the initial trajectory section is positioned at an initial distance from the air-material interface defined by the sidewall of the pre-existing through-hole and a non-diffracting zone of the pulsed Bessel-like laser beam extends through the pre-existing through-hole, and scanning the pulsed Bessel-like laser beam along etching pattern with, thereby creating a multi-photon process in the regime of optical breakdown photoionization as the underlying process for ablating material from the sidewall along the non-diffracting zone.

[12] In some embodiments, the material has a plate-like shape and the scanning can be performed in direction of the extension of the plate such that neighboring elongate single laser pulse damage regions are displaced with respect to each other along the trajectory in the range from a minimum distance of 5 % up to a maximum distance of 70 % (in some embodiments from 5 % up to a maximum distance of 95 %) of a beam waist at full width half maximum of a core of the pulsed Bessel-like laser beam as present within a single laser pulse damage region. Optionally, for a beam waist at full width half maximum of the core of about 1.5 μιη, the pulse machining step may be in the range from about 0.1 μιη to about 1 μιη such as about 0.3 or 0.5 μιη. In some embodiments, the scanning can be performed in direction of the extension of the plate such that neighboring elongate single laser pulse damage regions of neighboring trajectory sections are displaced with respect to each other in the range from a minimum distance of 5 % up to a maximum distance of 70 % (in some embodiments from 5 % up to a maximum distance of 95 %) of a beam waist at full width half maximum of a core of the pulsed Bessel-like laser beam as present within a single laser pulse damage region. Optionally, for a beam waist at full width half maximum of the core of about 1.5 μιη, the trajectory machining step may be in the range from about 0.1 μιη to about 1 μιη such as about 0.3 or 0.5 μιη.

[13] In some embodiments, for two concentric circular and/or elliptical trajectories, the single laser pulse damage regions of laser pulses associated with trajectories immediately neighboring one another in a radial direction may be displaced with respect to each other by a radius increment associated with the respective trajectories for providing an overlap of the respective single laser pulse damage regions. For example, they may be displaced with respect to each other across the trajectories in the range from a minimum distance of 5 % up to a maximum distance of 70 % (in some embodiments from 5 % up to a maximum distance of 95 %) of a beam waist at full width half maximum of a core of the pulsed Bessel-like laser beam as present within a single laser pulse damage region. Optionally, for a beam waist at full width half maximum of a core of about 1.5 μιη, the radius increment may be in the range from about 0.1 μιη to about 1 μιη such as about 0.3 or 0.5 μιη.

[14] In some embodiments, a laser-matter interaction zone associated with a beam core for each pulse of the pulsed Bessel-like laser beam may overlap with at least one interaction zones in each direction along the trajectory section, in particular along the trajectory section of concentric trajectories such as along the respective concentric circular and/or elliptical trajectory. In some embodiments, a laser-matter interaction zone defined by a beam core for each pulse of the pulsed Bessel-like laser beam may overlap with at least one interaction zone of another trajectory section, in particular in a radial direction across concentric trajectories such as the respective concentric circular and/or elliptical trajectories. In some embodiments, a laser-matter interaction zone defined by a beam core for each pulse of the pulsed Bessel-like laser beam may overlap with at least one interaction zone of the same trajectory section and/or with at least one interaction zone of another trajectory section by about 30 % up to a 95 % (in some embodiments from about 5 % to 95 %) of the cross section of a beam waist at full width half maximum of a core of the pulsed Bessel-like laser beam as present within a single laser pulse damage region.

[15] In some embodiments, the scanning along the trajectory sections, in particular along one of the concentric circular and/or elliptical trajectories, may be performed in a single pass scan such that a single laser pulse damage region originating from a selected laser pulse has only a single directly neighboring single laser pulse damage region that originates from a single laser pulse irradiated in time immediately before the selected laser pulse and one directly neighboring single laser pulse damage region that originates from a single laser pulse irradiated in time immediately after the selected laser pulse.

[16] In some embodiments, the through-hole forming pattern may comprise further an initial position for generating an initial single laser specific pulse damage region from which a first of the trajectory sections starts and which is positioned in an area, where, after the processing, remaining material is present or the shape of a side wall of the through-hole is irrelevant. In addition or alternatively, the through-hole forming pattern may comprise in particular a central position for generating a central single laser specific pulse damage region and around which the plurality of concentric circular and/or elliptical trajectories extend.

[17] In some embodiments, the method further comprises setting an optical beam path and a laser characteristic of the pulsed Bessel-like laser beam such that the elongate single laser pulse damage regions extend in the material at least through 90 % of a thickness of the material such as completely through the material. [18] In some embodiments, the method further comprises setting parameters of the optical beam path selected from the group of parameters comprising a conical half-angle of the Bessel-like laser beam and a beam apodization function of the Bessel-like laser beam, wherein, for example, the conical half-angle of the Bessel-like laser beam is settable in the range from 7° to 20°, for example set to 15° in dependence of the material thickness, and/or the beam apodization function is settable via the real apodization FWHM diameter (Dapod) measured at the entrance of an axicon lens in dependence of the conical half-angle Θ, the length of the single laser pulse damage region L, the demagnification 1/M of the optical system without the axicon lens, wherein M is larger 1, the refractive index n of the material, and a selectable parameter k, with 0.5 < k < 2, according to the equation:

Dapod = k*2*L/n*tg(9) *M.

[19] In some embodiments, the method further comprises setting parameters of the laser characteristic of the Bessel-like laser beam selected from the group of parameters comprising a pulse duration, a pulse energy wherein, for example, the pulse duration of a single pulse is settable in the sub-nanosecond range, for example, in the range from 100 ps to 100 fs or from 50 ps to 250 fs or from 15 ps to 10 ps, and the pulse energy is settable in the range from, for example, 1 μΐ to 40 μΐ or 1 μΐ to 80 μΐ per 100 μιη thickness such as such as from, for example, 60 μΐ to 160 μΐ or more per 100 μιη thickness, and/or setting the pulse duration such that the multi-photon process is accompanied by an electron avalanche photoionization.

[20] In some embodiments, the method for forming a through-hole further comprises the steps of: receiving information on a thickness of the material and a desired size of the through-hole; determining a minimum length of a required single laser pulse damage region that is required for creating a damage region to extend completely across the material;

determining the beam core associated to the single laser pulse damage region; setting the pulse machining step and the trajectory machining step for the trajectory sections, in particular the plurality of concentric trajectories such as concentric circular and/or elliptical trajectories, of the through-hole forming pattern in line with a desired overlap between the beam waists at full width half maximum of a core of the pulsed Bessel-like laser beam associated with neighboring single laser pulse damage region along and across the, in particular concentric circular and/or elliptical, trajectory sections.

[21] In some embodiments, the method for forming a through-hole further comprises the steps of: receiving information on a thickness of the material and a desired size of the through-hole; determining a minimum length of a required single laser pulse damage region that is required for creating a damage region to extend completely across the material; determining the beam core associated to the single laser pulse damage region; determining a minimum number of neighboring trajectory sections required for building up a single laser pulse damage region that extends through the material; and defining the through-hole forming pattern to comprise at least the minimum number of neighboring trajectory sections. Thereby, the through-hole forming pattern may comprise at least the minimum number of neighboring trajectory sections that extend around a non-irradiated area of the material.

[22] Generally, the through-hole forming pattern may comprise trajectory sections in a shape of, in particular, enlarging ellipses or circles, in a triangular, rectangular, quadratic, or polygonal shape, in a spiral-like shape or generally an arbitrary shape that allows processing trajectory sections that extend next to each other in a sequence.

[23] In some embodiments, the method for reshaping further comprises the steps of:

receiving information on a thickness of the material, determining a minimum length of a required single laser pulse etching region that is required for creating an etching region to extend completely across the material, determining the beam core associated to the single laser pulse etching region, setting the pulse machining step and/or the trajectory machining step (e.g. radius increments) for the plurality of trajectory sections of the etching pattern in line with a desired overlap along and across the trajectory sections.

[24] In some embodiments of the method for reshaping, the etching pattern may comprise sections of parallel linear trajectories for forming an essentially planar sidewall of a desired shape and/or sections of curved trajectories that are displaced in direction of the curvature and/or sections of arbitrary shaped trajectories that neighbor each other. Alternatively or additionally, the etching pattern may comprises a plurality of concentric circular and/or elliptical trajectories for achieving a circular or elliptical formed desired shape. The etching pattern may optionally comprise a central position for generating a central single laser specific pulse etching region around which the plurality of concentric circular and/or elliptical trajectories extend.

[25] In some embodiments of the method for reshaping, the material may have a plate-like shape and the scanning is then performed in direction of the extension of the plate such that neighboring non-diffracting zones are displaced with respect to each other in the range from a minimum distance of 5 % up to a maximum distance of 70 % (in some embodiments from 5 % up to a maximum distance of 95 %) or less of a beam waist at full width half maximum of a core of the pulsed Bessel-like laser beam. Optionally, for a beam waist at full width half maximum of the core of about 1.5 μιη, the pulse machining step is in the range from about 0.1 μιη to about 1 μιη such as about 0.3 or 0.5 μιη. [26] In some embodiments of the method for reshaping, for two trajectories or sections of trajectories, the single laser pulse etching regions of laser pulses associated with trajectories immediately neighboring one another may be displaced with respect to each other by a trajectory machining step associated with the respective trajectories for providing an overlap of the respective single laser pulse etching regions. For example, they may be displaced with respect to each other across the trajectories in the range from a minimum distance of 5 % up to a maximum distance of 70 % (in some embodiments from 5 % up to a maximum distance of 95 %) or less of a beam waist at full width half maximum of a core of the pulsed Bessel- like laser beam as present within a single laser pulse etching region. Optionally for a beam waist at full width half maximum of a core of about 1.5 μιη the increment is in the range from about 0.1 μιη to about 1 μιη such as about 0.3 or 0.5 μιη.

[27] In some embodiments of the method for reshaping, a laser-matter interaction zone associated with a beam core for each pulse of the pulsed Bessel-like laser beam may overlap with at least one interaction zone in movement direction along the respective trajectory section. In addition or alternatively, a laser-matter interaction zone defined by a beam core for each pulse of the pulsed Bessel-like laser beam may overlap with at least one interaction zone of another trajectory section, in particular in a direction across the respective trajectory section.

[28] In some embodiments of the method for reshaping, the scanning along one of the trajectory sections may be performed in a single pass scan such that a single laser pulse etching region originating from a selected laser pulse has only a single directly neighboring single laser pulse etching region that originates from a single laser pulse irradiated in time immediately before the selected laser pulse and/or one directly neighboring single laser pulse etching region that originates from a single laser pulse irradiated in time immediately after the selected laser pulse.

[29] Herein, the beam core associated to a laser beam and thus to a single laser pulse of the laser beam is given by the full width half maximum of the intensity distribution of the laser beam's inner (core) peak.

[30] Herein, a Bessel-like beam will be understood as a Bessel-Gauss beam that is a finite energy Bessel beam generated by means of a Gaussian laser beam with finite transfer size impinging e.g. onto an axicon.

[31] Other features and aspects of this disclosure will be apparent from the following description and the accompanying drawings. Brief Description of the Drawings

[32] The accompanying drawings, which are incorporated herein and constitute a part of the specification, illustrate exemplary embodiments of the disclosure and, together with the description, serve to explain the principles of the disclosure.

[33] In the drawings:

Fig. 1 is a schematic representation of a laser system for processing of materials by employing a Bessel-like laser beam;

Fig. 2 is a diagram illustrating the Bessel-like beam formation in the optical system of the laser system of Fig. 1;

Figs. 3A and 3B illustrate exemplary radial profiles of the fluence at the focus orthogonal to the direction of the laser propagation;

Fig. 4 illustrates an exemplary profile of the fluence in the direction of the laser propagation;

Fig. 5 is an exemplary illustration of a hole formation pattern of single pulse interaction zones along concentric circular trajectories;

Figs. 6A to 9B are optical microscope images of through-holes generated with a hole formation pattern like the one of Fig. 5;

Fig. 10 is an exemplary illustration of an etching pattern of single pulse interaction zones of concentric circular trajectories;

Figs. 11A to l lC are optical microscope images illustrating enlargement of a through-hole using an etching pattern like the one of Fig. 10;

Fig. 12 is an exemplary illustration of an etching pattern of single pulse interaction zones along linear trajectories;

Figs. 13A to 14 are optical microscope images illustrating etching performed with an etching pattern comprising linear trajectories;

Fig. 15 is an exemplary illustration of a pattern of locally overlapping single pulse interaction zones consisting of parallel linear trajectories;

Figs. 16A and 16B are optical microscope images illustrating the entrance opening and the exit opening of an etched rectangular through-hole generated with the pattern of Fig. 15;

Fig. 17 is an exemplary illustration of a pattern of locally overlapping single pulse interaction zones consisting of concentric square trajectories;

Figs. 18A and 18B and Figs. 19A and 19B are optical microscope images illustrating the entrance openings and the exit openings of etched quadratic through-holes generated with the pattern of Fig. 17 with side length of about 100 μηι and 50 μηι, respectively;

Figs. 20A and 20B are optical microscope images illustrating the entrance opening and the exit opening of an etched triangular through-hole;

Figs. 21A and 21B are optical microscope images illustrating the entrance opening and the exit opening of an etched triangular through-hole generated with reduced of single laser pulse interaction zones in comparison with the through-hole shown in Figs. 20A and 20B;

Figs. 22A and 22B illustrate patterns consisting of concentric circles as trajectory sections;

Figs. 23A to 23E are schematic drawings illustrating the interaction of a Bessel-type laser pulse for locally overlapping single laser pulse interaction zones; and

Figs. 24A to 24E are lateral optical microscope images illustrating the formation of a cylindrical through-hole with about 50 μιη opening diameter.

Detailed Description

[34] The following is a detailed description of exemplary embodiments of the present disclosure. The exemplary embodiments described therein and illustrated in the drawings are intended to teach the principles of the present disclosure, enabling those of ordinary skill in the art to implement and use the present disclosure in many different environments and for many different applications. Therefore, the exemplary embodiments are not intended to be, and should not be considered as, a limiting description of the scope of patent protection. Rather, the scope of patent protection shall be defined by the appended claims.

[35] The disclosure is based in part on the realization that the high aspect ratio of a Bessel beam focal line can be used to reduce the fabrication time of specific through-holes. In particular, it was realized that micron-sized through-holes can be generated with a writing technique based on a specific distribution of pulsed micro-Bessel beam interaction zones (extending essentially orthogonal to the sample surface).

[36] Specifically, it was noticed that the complete length of the Bessel beam focus line can be used for processing a material when there is a sufficient overlap between neighboring pulsed micro-Bessel beam interaction zones. While a first pulsed micro-Bessel beam interaction zone is usually within otherwise not yet processed material, any further pulsed micro-Bessel beam interaction zone will - as proposed - at least partly overlap with an area of the material that was already processed. In other words, any further pulsed micro-Bessel beam interaction zone falls partly into one or more pulsed micro-Bessel beam interaction zones. As long as that condition is met, the shape of the trajectory of the laser on the material is not limited.

[37] The inventors had, in other words, discovered that the fact, that the arrival of a laser pulse falls on a previous irradiated and, for example, at least partly ablated zone or zones (this was generally thought to have disadvantageous effect onto the beam propagation as stated in the above mentioned prior art), is not relevant for the hole formation process along the Z direction and to the contrary allows even laser processing using the complete length of the pulsed micro-Bessel beam interaction zone, resulting in high aspect ratio through holes with dimensions and geometries not yet accomplished at the respective wall geometry.

[38] It is noted that the herein disclosed processing is based on a single laser pulse (or a burst of sub-pulses forming a pulse) per pulsed micro-Bessel beam interaction zone such that the trajectory followed by the laser in general only processes an area once and achieves nevertheless complete through-hole drilling at a very high speed. This stands in contrast to ablation drilling that removes layer- wise material by repeatedly processing (previously processed) areas until a through -hole is formed.

[39] A particular distribution of pulsed micro-Bessel beam interaction zones comprises interaction zones distributed along, for example, concentric circles around a common, usually fixed, central point. The circles formed by the interaction zones progressively increase in radius after each single pass. In an exemplary parameter constellation for processing a glass plate with Bessel-beam irradiation, the pulse duration used was greater than 5 ps FWHM e.g. greater than 12 ps. It was realized that a well-defined separation of the interaction zones along the concentric circles was needed to ensure the building of the through-hole. This applies generally to all possible shapes of through holes. In general, the pulse overlap interval can be in the range from 30 % up to 95 % (in some embodiments from about 5% to 95%) of the pulsed beam core. For example, assuming a beam core (core- FWHM of the intensity distribution at the beam waist) of 1.5 μιη, a separation between successive pulse interaction zones may be in the range from 0.1 μιη to 1 μιη (in some embodiments to 1.4 μιη) for obtain a through hole. The magnitude of the separation may affect the quality of the through hole and the energy needed.

[40] In some embodiments, the distance between the interaction zones along the circle was specifically selected such that partially overlapping etching /ablation/damage zones in particular at the sample's surface were present. The concentric circle distribution allowed the generation of holes within glass materials such as glass substrates of 30 μιη, 100 μιη and 150 μηι thick glass substrates, with an aperture size of the order of few tens of micrometers (such as 30 μιη and 50 μιη respectively for the two cases). A minimum achievable size of the through-holes was identified to at least partially depend on the sample thickness and on the aspect ratio of the through-hole.

[41] Alternative examples of trajectory types comprise enlarging ellipses, circle-like

shapes, ellipse-like shapes, and spiral-like shapes, triangular, rectangular, quadratic, polygonal geometric shapes or generally arbitrary shapes that preferably allow processing trajectory sections that extend next to each other in a sequence. In general, one should be able to process the trajectory sections with the herein defined separation of interaction zones being maintained.

[42] Exemplary particular distributions of pulsed micro-Bessel beam interaction zones may further comprise interaction zones distributed along concentric square or rectangular trajectories around a common central point as will be exemplary discussed in connection with the figures. The trajectories for the interaction zones, for example, progressively increase in radius/size after each single pass of a trajectory section.

[43] Exemplary particular distributions of pulsed micro-Bessel beam interaction zones may further comprise ring-like trajectory sections of the above type such as circular, elliptic, square-like, or rectangular trajectory sections. The ring-like trajectory sections may surround an "untreated" central area.

[44] As further discussed in connection with the figures, another particular distribution of pulsed micro-Bessel beam interaction zones may comprise interaction zones distributed gridlike by a sequence of lines (linear or non-linear lines such as curved lines) as trajectory sections. Each trajectory section may be formed by the interaction zones of a line and the next neighboring trajectory section may be caused by progressively shifting the

line/trajectory section in a constant or varying direction after a single pass along that line/trajectory section is completed.

[45] Moreover, it was realized that by positioning the Bessel beam in the center of an existing through-hole within a glass substrate, e.g. within an, e.g. round, aperture generated with the above concentric circle distribution, further machining of the glass material can be performed using specific etching scan-trajectories. For example, parallel line scanning procedures allowed forming apertures of rectangular shapes.

[46] For the etching mechanism, it was realized that the Bessel beam allows starting to machine a desired surface from an air/glass interface. However, as for the hole generation based in the above described manner, the Bessel beam's interaction zones were set with a well-defined separation along the desired writing lines (etching scan-trajectory sections).

[47] Regarding the laser processing with Bessel-like beams, Bessel-like beams such as zero-order Bessel beams, may feature an intense central spot, which persists in propagation direction essentially without apparent diffraction - in contrast to the focusing of standard Gaussian beam, which is usually strongly diverging after a tight focus. Accordingly, with single laser Bessel-like laser beam pulses, interaction zones over up to a millimeter and more may be achieved that result in a very narrow needle like laser damage zones. The etching /ablation/damage zones may comprise index of refraction modified regions and nano- channels. By positioning the etching /ablation/damage zones close to each other, one may create a material zone which may result in a formation of an unstable material state that then can dissolve to form a through-hole (as used, e.g., with the ring-like trajectory sections described below).

[48] In the following, an exemplary laser system is schematically explained in connection with Figs. 1 to 5 that provide Bessel-like laser pulses. Through-hole forming applications and etching applications are then disclosed in connection with Figs. 6 to Fig. 9B and Fig. 10 to Fig. 14, respectively. Further aspects of through-hole forming applications are then disclosed in connection with Figs. 15 to 24.

[49] Referring to Fig. 1, an exemplary laser processing system 1 for processing a

transparent sample 3 by employing a Bessel-like laser beam (generally referred to as a Bessel-type laser beam) comprises a laser system 5, an optical system 7, and a translation unit 9 indicated schematically by arrows. The micromachining set-up can be used for through-hole formation (herein also referred to as "drilling" or "laser based drilling" although removing material is performed based on writing a hole formation pattern based on damage zones into a glass substrate) or etching of specific hole shapes within glass samples by employing e.g. zero-order Bessel beams and starting from a pre-existing aperture.

[50] Laser system 5 is configured to deliver short laser pulses of a specifically adjustable temporal duration. An example of laser system 5 is a Ti:Sapphire laser that generates laser pulses at λ=800 nm with about 40 fs pulse duration (e.g. transform limited) pulses e.g. at a repetition rate of about 20 Hz. Generally, the pulse duration can be stretched by slightly detuning a respective laser compressor. In general, the pulse duration is selected in accordance with the sample's material and thickness as well as the geometry of the Bessel- like laser beam. For example, the pulse duration can be in the ps-range for sample thicknesses in the range up to several hundred μηι; in particular pulse durations of at least about 12 ps were used in some of the later-on discussed laser processing examples.

[51] Similarly, also the energy per pulse is selected in accordance with the sample's

material and thickness as well as the geometry of the Bessel-like laser beam. Examples, of energy per pulse used in the later-on discussed laser processing examples were in the range from 50 μΐ to 100 μΐ.

[52] Another example for laser system 5 is a Pharos laser providing laser pulses of a minimum pulse duration of 230 fs at a central wavelength of 1030 nm with a pulse repetition rate up to 600 kHz.

[53] Optical system 7 comprises a Bessel-like beam shaping optical system 11 and a de- magnifying telescope 13 comprising a lens with e.g. focal length 300 mm and a microscope objective lens of magnification 20 X and a numerical aperture of e.g. 0.4. The optical system is configured for creating a Bessel-like beam based on a Gaussian (input) beam provided by laser system 5 and for focusing the same onto sample 3.

[54] As an example of a Bessel beam, a zero-order Bessel-like beam can be generated by means of a Spatial Light Modulator (SLM) capable of imprinting the phase profile of an axicon (conical lens) onto an input Gaussian laser beam. The resulting beam is then de- magnified at the sample position by means of an optical telescopic system constituted by the lens and the microscope objective in Fig. 1.

[55] An exemplary set-up of the optical system is described in more detail in

"Experimental investigation of high aspect ratio tubular microstructuring of glass by means of picosecond Bessel vortices" by O. Jedrkiewicz et al., Appl. Phys. A 120, 385 (2015).

[56] Fig. 2 illustrates the generation of a zero-order Bessel beam from a standard input laser beam impinging onto a conical lens (axicon lens 15) being an optical element approach in Bessel beam generation in contrast to the above mentioned SLM. Imprinting the phase profile of an axicon on the beam with an axicon base angle a of 1° generates a Bessel-like beam 17 from a finite energy laser beam that features a non-diffracting zone NDZ depending on the Bessel beam the conical half-angle Θ.

[57] Using such optical systems, Bessel beam features at the micromachining position may have a conical half-angle Θ of about 15° with a core radius of about 1.5 μιη (see Fig. 3A) and a Bessel focal length L (non-diffracting length of non-diffracting zone NDZ) of about 280 μιη.

[58] Non-diffracting zone NDZ extends along Z-direction during laser processing through sample 3 as illustrated in Fig. 1. The herein disclosed aspects in particular relate to laser processing with Bessel beams having a Bessel focal length L of non-diffracting zone NDZ that is at least comparable to the thickness of the processed material, usually having a platelike shape. The laser processing (e.g. the scanning) is performed in direction of the extension of the plate. Thereby, neighboring elongate single laser pulse damage regions (in the case of hole-forming) or the laser pulse etching regions (in the case of re-shaping of apertures) are displaced with respect to each other by usually a maximum distance of 80% or less such as 70% or less of a beam waist of the Bessel-like laser beam at full width half maximum of its core.

[59] Figs. 3A and 3B illustrate exemplary transverse fluence profiles 19A, 19B within non-diffracting zone NDZ having a peak fluence in dependence of a position in the X-Y- plane. Fig. 3A shows a plot of a normalized fluence along X-direction, while Fig. 3B illustrates a 2D-plot in the X-Y-plane, i.e. orthogonal to Z direction across the beam diameter.

[60] Specifically, transverse fluence profile 19A is taken at a longitudinal central position

Zc as shown in Fig. 2 (e.g. close to the center of non-diffracting zone where the peak-fluence longitudinal profile shows its maximum). Transverse fluence profiles 19 A, 19B are indicated for the experimentally generated Bessel-like beam with e.g. conical half-angle 15°.

[61] Transverse fluence profiles 19A and 19B show several characteristic concentric

fringes across the beam diameter that are set by the respective beam apodisation function of the Bessel-like beam. The full width at half-maximum of a central core of the Bessel-like beam is at the position Zc about 1.5 μιη as indicated in Fig. 3A. In Fig. 3B, a white intensity peak can be seen indicating non-diffracting zone NDZ that is surrounded by concentric intensity peaks (being shown in Fig. 3A in the cross-section).

[62] The beam apodization function may be set via the real apodization FWHM diameter

Dapod measured e.g. at the entrance of axicon lens 15 in Fig. 2. It is set in dependence of the conical half-angle Θ, the length of the maximum single laser pulse damage region which is similar to length L of non-diffracting zone NDZ, the demagnification 1/M of the optical system 7 without axicon lens 15, wherein M is larger 1, the refractive index n of the material, and, for example, a selectable parameter k, with 0.5 < k < 2, according to the equation: Dapod = k*2*L/n*tg(9) *M.

[63] Fig. 4 illustrates an exemplary peak fluence distribution 21 (along the focal line e.g. in Fig. 1 along Z-direction) of the Bessel-like laser beam 17 with a conical half-angle of 15°. A corresponding zone of laser induced damage/ablation/etching should have a length at least equal to the glass sample thickness. Vertical line-pairs 23A, 23B, 23C indicate the top and bottom surfaces of respective samples for three different thicknesses. E.g. line-pair 23A relates to 30 μιη thick (e.g. BK7 or FS) glass, line-pair 23B relates to 100 μιη thick glass, and line-pair 23C relates to a 150 μιη thick glass. The relative position of the respective glass plates across the Bessel focal line can be achieved with translation unit 9. For all samples, it is assumed that, along the sample thickness, the peak fluence provided within Bessel-like beam 17 is above the threshold for etching/ablation/damaging.

[64] Referring to Fig. 5, the laser processing with laser processing system 1 is illustrated for through-hole drilling using a specific hole formation pattern 25 as a first example of a distribution of pulsed micro-Bessel beam interaction zones. In general, through-hole forming pattern 25 comprises a path 27 along which pulsed Bessel-like laser beam 17 is scanned along surface 3 A of sample 3. Path 27 comprises a plurality of concentric circular and/or elliptical trajectories 27A (as an example of trajectory sections) for forming sequences of single laser specific pulse damage regions. Through-hole forming pattern 25 may comprise further a central position for generating a central single laser specific pulse damage region around which the plurality of concentric circular and/or elliptical trajectories 27A extend.

[65] Fig. 5 is a top view of a front face 3A of sample 3 being machined by irradiating laser pulses of Bessel-like laser beam 17 onto that front face. Sample 3 is scanned - by moving the sample and/or the laser position - along predetermined (scanning) path 27.

Specifically, sample 3 is scanned along circular trajectories 27A starting from a center position (not required) with increasing radius after each closed circular trajectory 27A (the increase in radius being indicated by an arrow). For example, the increase in radius may be performed by increments of about 0.5 μιη.

[66] Exemplarily, beam cores 29 are indicated for a sequence of circular trajectories at the low right side of Fig. 5. For two concentric circular and/or elliptical trajectories 27 A, the single laser pulse damage regions of laser pulses associated with trajectories 27A

immediately neighboring one another in a radial direction are displaced with respect to each other by a (radius/trajectory) increment dr (herein also referred to as trajectory machining step) associated with the respective trajectories 27 A for providing an overlap of the respective single laser pulse damage regions of the trajectory sections. As mentioned, for a beam waist at full width half maximum of cores 29 of about 1.5 μιη, radius increment dr may be in the range from about 0.3 μιη to about 1 μιη such as about 0.5 μιη.

[67] The laser scanning is performed such that consecutive laser pulses irradiate different areas of front face 3A. For the concentric circular and/or elliptical trajectories 27A, the single laser pulse damage regions of successive laser pulses following immediately one another are displaced with respect to each other by a pulse machining step dt that is specifically selected for providing an overlap of single laser pulse damage regions of respective successive laser pulses. Exemplarily for each circular trajectory 27A, the distribution of respective beam cores 29 of Bessel-like laser beam 17 is indicated

schematically by small circles along a circle segment 31 at the top right of Fig. 5, which schematically represent those areas where sample 3 is irradiated by the core of Bessel-like laser beam 17.

[68] For providing an overlap of the respective single laser pulse damage regions, a

displacement with respect to each other across/along the trajectories may be limited to a maximum distance of 70 % (in some embodiments of 80 %) or less of a beam waist at full width half maximum of a core of the pulsed Bessel-like laser beam as present within a single laser pulse damage region. In general, in some embodiments, a laser-matter interaction zone associated with a beam core for each pulse of the pulsed Bessel-like laser beam overlaps with at least one interaction zone in each direction along the respective concentric circular and/or elliptical trajectory section. Similarly, in some embodiments, a laser-matter interaction zone defined by a beam core for each pulse of the pulsed Bessel-like laser beam overlaps with at least one interaction zone in a radial direction across the respective concentric circular and/or elliptical trajectory section.

[69] Assuming a sufficient peak fluence, within each beam core 29, a damage zone

extends along the propagation axis Z. Exemplary spacing conditions may use a distance such that the damage regions overlap. Accordingly, the distance depends on the respective setting of optical system 7. In the herein discloses examples, a distance (pulse machining step dt along the trajectories) of about 0.5 μιη was identified to allow hole- formation as illustrated in Fig. 5. In other exemplary hole- formation processes, a pulse machining step dt along the trajectories of about 1 μιη or 0.5 μιη was used, e.g., in connection with laser parameters such as a pulse duration of about 10 ps and a pulse energy of 110 μΐ.

[70] In some embodiments, the scanning is performed in a single pass scan. Single pass relates to the fact that each section along circular trajectories 27A is only visited (passed) once by the laser beam. In other word, the scanning is performed such that, during the single pass scanning, the pulsed Bessel-like laser beam essentially does not return to an earlier irradiated position. (It will be understood that the partial overlapping between successive pulses may be present for any scanning process but is obviously not considered to be a second pass, in particular as used in ablative hole drilling.) [71] Using hole formation pattern 25, essentially circular holes can be generated as illustrated in Fig. 6A (top side) and Fig. 6B (bottom side) reproducing images for 100 μιη thick BK7 glass with an energy per pulse of about 90 μΐ, a pulse duration of about 15 ps, a Bessel beam conical half-angle of about 15°, and a machining step dt of about 0.5 μιη. The resulting hole diameter D was determined to be about 70 μιη.

[72] The images of Fig. 7A (bottom side) and Fig. 7B (top side) illustrate the hole

formation in a 100 μιη thick fused silica glass performed with the same parameter setting resulting in a smaller hole diameter D of about or less than 50 μιη.

[73] The images of Fig. 8A (bottom side) and Fig. 8B (top side) illustrate the hole

formation in 30 μιη thick BK7 glass performed with the same parameter setting resulting in a smaller hole diameter D of about 50 μιη.

[74] In Figs. 6 A to 9B, bars 33 indicate a length of 50 μιη.

[75] The images of Fig. 9A (bottom side) and Fig. 9B (top side) illustrate the hole

formation in 30 μιη thick BK7 glass performed with the same parameter setting resulting in a smaller hole diameter D of about 25 μιη. In Figs. 9A to 9B, bars 35 indicate a length of 20

[76] The minimum achievable hole size (i.e. diameter D) is expected to depend on the sample thickness and the beam parameters of Bessel-like laser beam 17.

[77] It should be understood that the above disclosed drilling mechanism is achieved by a combination of phenomena that occur during the laser interaction such as a plasma generation along the focal line of the Bessel beam inducing thermo-mechanical stress mechanisms in the affected area and the resulting pass after pass (along the partially superimposed trajectories) into a progressive in-depth ablation of the substrate.

[78] While the above hole formation pattern 25 was based on concentric circular

trajectories 27A, similar creation of openings with different shape may be based on, for example, concentric trajectories of arbitrary shape. A hole shape may even include an edge portion whereby a limit of the edge angle depends on the dimension of the Bessel beam core used in the machining process.

[79] Accordingly, hole formation patterns such as circular or elliptical patterns or the above mentioned shapes formed by, e.g., line trajectory sections, or rectangular trajectory sections (squares, triangles), or arbitrary shapes) can be used.

[80] The drilling processing as disclosed in connection with Fig. 5 to Fig. 9B is based on a limited extension of the damage regions/zone along predetermined path 27. There initially remains a structural connection between the sections of the material on the sides of circular trajectory 27A. Remaining structural connections may generally also be present in the case that centrally damage regions extend from the frontside (through which the laser beam enters the material) to the backside (through which the laser beam exits the material).

[81] With increasing number of e.g. circular trajectories 27A, the structural stability of the laser processed circular area being subject to hole formation pattern 25 is lost (e.g. by increasing tension build up) and a through-hole forms which is large with respect to each individual damaging zone. Specifically, the size of the through-hole relates to the outer ones of circular trajectories 27A.

[82] The laser processed material may fall out independently from external forces or may be removed by vibrations, a pressure difference, a thermo-mechanical stress mechanisms, because cracks of fused and re-solidified material zones being thinner and more and more fragile after the partially overlapping laser passes. Thereby, also a potentially not-processed central region may be removed from the material, because it is no longer connected to the surrounding (see also Fig. 22B).

[83] In connection with Figs. 10 to 14, a laser etching process is described that allows treating air-material interfaces, and in particular openings within a material, with Bessel-like laser beam 17. For example, the through -holes as drilled in accordance with the methods discussed above may be treated/etched/reshaped with the hereinafter disclosed methods. However, in general any through-hole or sidewall of an optically transparent material may be subject to the treating/etching/reshaping.

[84] Referring to the drilling process described above, the peak fluence of the

treating/etching/reshaping may be similar in value. Nevertheless, while in the foregoing, it was primarily referred to single laser pulse damage regions, in connection with the etching, it is referred to single laser pulse etching regions.

[85] Fig. 10 illustrates an etching pattern 37 that is essentially used for forming circular hole enlargements of pre-existing through-holes. Etching pattern 37 corresponds essentially to drilling pattern 25 and is illustrated similar to Fig. 5 on a top view of a front face 3A of sample 3 in which a through-hole 39 of arbitrary shape is present. As will be understood, through-hole 39 needs to be large enough that at least some circular trajectories 27A can be performed within through-hole 39. Again, sample 3 is scanned - by moving the sample and/or the laser position - along a predetermined path 27 comprising exemplarily circular trajectories 27A starting from a center position (not required). As for the drilling, circular trajectories 27 A increase in radius after each circular trajectory 27 A has been scanned (the increase in radius being indicated along an arrow by cores 29 being displace by (radius/ trajectory) increment dr). For example, the increase in radius may be performed by trajectory increments dr of about 0.5 μιη or 1 μιη, generally dependent on the material parameter and the laser parameter.

[86] Etching pattern 37 defines path 27 along which pulsed Bessel-like laser beam 17 is scanned. For the re-shaping, the shape of etching pattern 37 is associated with the desired shape. Path 27 comprises the plurality of trajectories 27A for forming sequences of single laser specific pulse etching regions, wherein the single laser pulse etching regions of successive laser pulses following immediately one another along one of the trajectories (trajectory sections) are displaced with respect to each other by pulse machining step dt. Pulse machining step dt is selected for providing an overlap of single laser pulse damage regions of respective successive laser pulses. The plurality of trajectories comprise an initial trajectory (section) 46A (in Fig. 12 discussed below 47 A), a target trajectory (section) 46B (in Fig. 12 discussed below 47B) defining the desired shape, and a plurality of trajectories (trajectory sections) extending as a sequence in-between the initial trajectory 46A and the target trajectory 46B - in Fig. 10, circular trajectory sections that are concentrically arranged with respect to each other.

[87] As for the hole formation pattern, for each circular trajectory section 27A, the

distribution of respective beam cores 29 of Bessel-like laser beam 17 is indicated

schematically by small circles along a circle segment 31 at the top right of Fig. 10. However, in this case, circle segment 31 does not relate to areas where sample 3 is present but instead is scanned within the aperture of pre-existing hole 39. Accordingly, the core of Bessel-like laser beam 17 can extend essentially undisturbed along the sidewalls of through-hole 39, thereby homogeneously interacting with similar intensity distributions (see Figs. 3A and 3B) with the material of sample 3.

[88] Etching pattern 37 is positioned with respect to the pre-existing through-hole 39 such that the initial trajectory 46A extends within the aperture given by the pre-existing through- hole 39 and initial trajectory 46A is positioned at an initial distance IN from the air-material interface defined by a sidewall of the pre-existing through-hole 39. Respective non- diffracting zone NDZ of pulsed Bessel-like laser beam 17 extends through the pre-existing through-hole 39.

[89] Assuming a sufficient peak fluence within each beam core 29, material is ablated with a homogeneous ablation/etching condition along the propagation axis Z up to a pre-set radial distance from the beam center line. Scanning along etching pattern 37 the pulsed Bessel-like laser beam creates a multi-photon process in the regime of optical breakdown photoionization as the underlying process for ablating material from the sidewall along the non-diffracting zone NDZ, assuming material of sample 3 is within the single laser specific pulse etching region.

[90] Using similar spacing conditions (pulse machining step dt and trajectory increments dr) as for the drilling, e.g. using a distance such that the damage regions sufficiently overlap, Bessel-like beam 17 etches the sidewall smoothly. As will be understood, the implemented distance between neighboring beam cores 29 may depend on the respective setting of optical system 7 and laser system 5, the material of sample 3, and the desired smoothness of an etched surface. In the herein discloses examples, a distance (pulse machining step dt) of about 0.5 μιη was used as schematically illustrated in Fig. 10 for a beam waist at full width half maximum of the core of the pulsed Bessel-like laser beam of e.g. about 1.5 μιη.

[91] During the etching process, e.g. while moving Bessel-like laser beam along the

circular trajectory sections within through-hole 39, those material areas closest will be ablated first, thus that a through-hole with an essentially circular cross-section is generated.

[92] In some embodiments, the scanning is performed in a single pass scan. Single pass relates again to the fact that each section along circular trajectories 27A is only visited (passed) once by the laser beam. In other word, the scanning is performed such that, during the single pass scanning, the pulsed Bessel-like laser beam does not return to an earlier irradiated position. In some embodiments, each circle may be traced two or more times, if single pass ablation is not sufficient to remove the material.

[93] Using etching pattern 37, circular holes can be etched as illustrated in Fig. 11 A (preexisting through-hole, top side), Fig. 11B (bottom side) and Fig. 11C (top side).

[94] Specifically, Fig. 11A shows a pre-formed hole 41 with significant damage around the aperture having a minimum width of about 25 μιη and a maximum width of about 40 μιη. Bar 43A illustrates 50 μιη.

[95] After having performed etching pattern 37 on a 150 μιη thick BK7 glass with

parameters such as an energy per pulse of about 90 μΐ, a pulse duration of about 15 ps, a Bessel beam conical half-angle of about 15°, and a pulse/trajectory machining step of about 0.5 μιη, the aperture was enlarged to about 100 μιη with a smooth and clean sidewall to form a Bessel-like laser beam etched though-hole 45. Bar 43B illustrates the length of 100 μιη. The associated beam waist at full width half maximum of the core of the pulsed Bessel-like laser beam was about 1.5 μιη. [96] In general, any (inner) surface shape of an aperture can be etched by providing the respective scanning trajectory. The etching pattern may comprise, for example, sections of parallel linear trajectories for forming an essentially planar (inner) sidewall of a desired shape and/or sections of curved trajectories that are displaced in direction of the curvature. In addition or alternatively, the etching pattern may comprise a plurality of concentric circular and/or elliptical trajectories for achieving a circular or elliptical formed desired shape.

Optionally, the etching pattern may comprise a central position for generating a central single laser specific pulse etching region around which the plurality of concentric circular and/or elliptical trajectories extend.

[97] For two trajectories or sections of trajectories, the single laser pulse etching regions of laser pulses associated with trajectories immediately neighboring one another are displaced with respect to each other by a trajectory increment associated with the respective trajectories for providing an overlap of the respective single laser pulse etching regions, such as by being displaced with respect to each other across the trajectories by a maximum distance of 80 % or less of a beam waist at full width half maximum of a core of the pulsed Bessel-like laser beam as present within a single laser pulse damage region, and optionally for a beam waist at full width half maximum of a core of about 1.5 μιη the trajectory increment is in the range from about 0.1 μιη to about 1 μιη such as about 0.5 μιη.

[98] A laser-matter interaction zone associated with a beam core for each pulse of the pulsed Bessel-like laser beam overlaps with at least one interaction zones in movement direction along the respective trajectory and/or with at least one interaction zone in a direction across the respective trajectories.

[99] For etching, the material may have a plate-like shape and the scanning may be

performed in direction of the extension of the plate such that neighboring non-diffracting zones are displaced with respect to each other by a maximum distance of 70 % (in some embodiments of 80 %) or less of a beam waist at full width half maximum of a core of the pulsed Bessel-like laser beam, and optionally for a beam waist at full width half maximum of the core of about 1.5 μιη the machining step is in the range from about 0.1 μιη to about 1 μιη such as about 0.3 or 0.5 μιη.

[100] While Fig. 10 illustrates circular trajectories, Fig. 12 illustrates a linear trajectory

47 A which is moved from the air- side into the material of sample 3. The direction is indicated by an arrow 49. Thereby, removing (etching) the material is performed along a straight line. To ensure a proper etching process, linear trajectory 47A is set at a distance IN from the border of sample 3. A scan be seen, the border of sample 3 as well as linear trajectory 47 A extend along X-direction in Fig. 12, while arrow 49 extends along Y- direction. By dashed lines 51, the resulting etched region 53 is indicated with respect to the remaining surface 3 A of sample 3 assuming a target trajectory 47B.

[101] While in Fig. 12, the etching process is schematically indicated for a linearly

extending sidewall of sample 3, respective linear trajectories can similarly be applied to curved surfaces as will be shown in the specific formation of through-hole configurations shown in Figs. 13A, 13B, 13C, and 14.

[102] As illustrated in Fig. 13A, the etching starts this time from a preformed through-hole

55 that was drilled in a 150 μιη thick BK7 glass by focusing multiple shots of a Gaussian beam onto the sample. The damaged region around the hole looks much more condensed than the initial glass material. The respective pulse energy of 66 μΐ was not sufficient to ablate the whole depth of the hole smoothly.

[103] For etching, higher energies such as 80 μΐ were used to remove material at the

interface air-cylindrical wall. Starting from preformed through-hole 55, images of Fig. 13B (top side) and Fig. 13C (bottom side) illustrate how through -hole 55 was given a desired shape. Exemplarily, the material of the sidewalls of through-hole 55 was etched by scanning Bessel-like laser beam 17 along an angled by 90°-trajectory 57. Thereby, a corner structure 59 was formed starting from through-hole 55. Bar 61 illustrates 50 μιη.

[104] Fig. 14 illustrates how a key-hole like shaped aperture 63 can be formed starting from a through-hole, which was initially enlarged using a circular scanning pattern of Fig. 10. Then, linear trajectories 65 of a length of about 100 μιη were driven at opposite sides of the through -hole into the material in the direction of arrows 67. Bar 69 illustrates 100 μιη.

[105] For the respective beam parameters, as a minimum size for a homogeneous lateral surface etching about 30 μιη were identified for BK7 glass with a thickness of 150 μιη. Moreover, as an optimum pulse/trajectory machining step of 0.5 μιη was identified for the 15° conical half-angle and the BK7 and fused silica glasses examined (assuming the beam waist at full width half maximum of the core of the pulsed Bessel-like laser beam to be in the range of 1 μιη to 1.5 μιη).

[106] Referring again to Fig. 4, a peak fluence above a threshold value is needed for optical damaging, essentially it is assumed that this threshold value corresponds to the threshold of optical break down. Specifically, a fluence above that threshold value may cause optical break down and thereby modify the internal structure of the material, e.g. form a damage region or even ablation in which material is destroyed. It is assumed that although ablation is one type of laser induced damage other types such as modifications of the index of refraction, changes of the density or even hardness exist and may have the same effect to induce a symmetry for a cleaving process such that high quality cut faces originate that, for example, primarily extend within one plane, e.g. the or next to the plane of the laser induced damage regions.

[107] Herein, a laser induced damage zone may be identified as the zone of the sample over which structures as a result of the laser interaction are observed after cleaving. In general, the length of the laser induced damage zone may be defined as the length of the sample section that shows damage in one plane (if the sample is scanned along X-direction, then in the XZ plane). A laser induced etching zone may be identified as the zone of the sample over which material as a result of the laser interaction is ablated/etched away.

[108] In general, the interaction of a single pulse of, for example, such a Bessel-like beam with a material being essentially transparent with respect to single photon absorption of the laser beam when propagating through the material may be based on multi-photon ionization. Multi-photon ionization may be accompanied by electron avalanche photoionization and result in a single laser pulse damage region. The length of the single laser pulse damage region may be within a range of several 100 μιη up to 1 mm and more in direction of the laser beam propagation and the width may be within a range below about 2 μιη in radial direction. The extension of the single laser pulse damage region depends on the field strength within the focus of, for example, the core beam and, thus, depends on the optical beam path within optical system 7 and the laser characteristic of the pulsed Bessel-like laser beam such as the laser pulse energy and the laser pulse duration provided by laser system 5.

[109] Transparency with respect to single photon absorption corresponds to the fact that single photon absorption is not the underlying ionization process as, for example, the band gap is larger than the photon energy. Ionization based on multi-photon ionization is generally characterized by an ionization threshold such that also the formation of a damage region is well defined in space. For example, the material may be transparent in the near infrared and/or visible spectral range

[110] When performing the hole formation and etching of the material, the position of the sample with respect to the laser beam may be maintained in such a way that the condensed beam zone (the volume in the space where the fluence (J/cm2) of the laser beam is above ½ of the maximum fluence to e.g. ensure ablation) is positioned across the complete sample thickness. [111] Figs. 15 to 24 further illustrate through-hole drilling performed with neighboring trajectory sections and illustrate in particular the aspect of the interaction zones overlapping along a trajectory section and across neighboring trajectory sections.

[112] Fig. 15 is an exemplary illustration of a pattern 71 of locally overlapping single pulse interaction zones 73 (exemplarily illustrated by the beam cores) applied along parallel linear trajectory sections 71A, 71B, .... Pattern 71 of Fig. 15 can be used for an asymmetric processing of a material, which starts at a first single pulse interaction zones 73A along the first linear trajectory section 71 A extending in a first direction Y. A distance dY between successive pulse interaction zones 73, i.e., the pulse machining step within trajectory section 71 A, is dY = 0.5 μιη. After a predetermined length and number of applied laser pulses, a second linear trajectory section 7 IB is defined that is displaced from the first trajectory section 71 A at a distance dX in a second direction X. The distance dX between successive trajectory sections, i.e., the trajectory machining step between neighboring trajectory sections 71A, 71B, is, for example, dX = 0.5 μιη. Thus, the displacements of the single pulse interaction zones 73 are identical in both directions X and Y in this example.

[113] In the example of Fig. 15, the trajectory comprises parallel trajectory sections

processed in the same direction such that the trajectory is not closed. In alternative embodiments, neighboring trajectory sections are processed in opposing directions, thereby forming a closed meander-like trajectory.

[114] Figs. 16A and 16B are optical microscope images that focus on the entrance opening

(Fig. 16A: top surface) and on the exit opening (Fig. 16B: bottom surface) of an etched rectangular through-hole 75 that was generated with the asymmetric processing pattern 71 of Fig. 15 in a sample 77. The sample 77 has a thickness of about 150 μιη. Starting from an origin area O, the linear trajectory sections 71 A, 7 IB are set up to cover a processing area in the order of 200 μιη x 100 μιη. The trajectory comprises parallel trajectory sections processed in the same direction as indicated by dotted arrows 78. Bar 69 in Fig. 16B illustrates 100 μιη.

[115] As illustrated in Fig. 15, the lines were written along the Y-axis and repeated for different positions along X with a distance between successive pulses and lines equal to 0.5 μιη (identical pulse and trajectory machining steps). This writing scheme creates a stress level inside the machined volume of sample 77. It was noticed that for the present parameters of the laser beam and the sample 77, a hole was formed, i.e., the material dropped out, when the processed area exceeded the dimension of 40 μιη x 40 μιη. [116] One recognizes in Fig. 16B that in the corner, where the process started, i.e., at the origin area O, and at the side of the first trajectory sections, the material of the sample 77 was not completely removed and some material remained close to/at the bottom surface. This can be seen, for example, at the dark triangular portion in the image shown in Fig. 16B in the corner of the origin area O, where part of the glass is still present on the lateral walls. In contrast, the hole formation extended completely through the sample 77 at the opposite side of through-hole 75, where the laser pulses ended and formed a smooth inner surface of through-hole 75. Do to the asymmetry of the physical processes in the region of the origin area O and that last processing line, the processing based on this type of pattern is referred to as asymmetric processing of a material. In conclusion, this asymmetric processing may not allow maintaining a desired homogeneous structure along all (side) walls throughout the sample's thickness. A potential explanation for the specific structure is illustrated in connection with Figs. 23 A to 23E.

[117] Fig. 17 is an exemplary illustration of a pattern 79 of locally overlapping single pulse interaction zones 73 that are formed along "concentric" square-type trajectory sections. Pattern 79 is an example of symmetrical processing as all (side) walls of the finally formed through-hole will be subject to similar physical processes and conditions and, thus, will be similarly processed. Specifically, enlarging square-shaped trajectory sections 79A, 79B, 79C... are processed step-wise from the most inner square up to the last square defining the final size of a through-hole with quadratic cross-section. Exemplarily a starting point S is illustrated for the first (most inner) trajectory section 79A together with arrows 80A illustrating the direction, in which the laser beam is moved along the trajectory section 79A. A distance dP between successive pulse interaction zones 73, i.e., the pulse machining step within trajectory sections 79A, 79B, 79C,..., is exemplarily indicated as dP = 0.5 μιη.

[118] Once a square-shaped trajectory section 79A, 79B, 79C... is completed, the next neighboring square-shaped trajectory being enlarged in its side length has a distance in "radial" direction (indicated by an arrow 80B) between successive trajectory sections, i.e., the trajectory machining step between neighboring trajectory sections 79A, 79B, 79C,... is set, for example, dr = 0.5 μιη.

[119] Figs. 18A and 18B are optical microscope images that focus on the entrance opening

(Fig. 18A: top surface) and on the exit opening (Fig. 18B: bottom surface) of a square- shaped through-hole 81 that was generated with the symmetric processing pattern 79 of Fig. 17 in the sample 77 having a thickness of about 150 μιη. Specifically, the applied pattern 79 included 80 trajectory sections up to a side length a of about 100 μιη. Bar 69 in Fig. 18A illustrates 100 μηι. It is noted that the images of the top surface and the bottom surface both show a smooth surface as the processing conditions along the final border of through-hole 81 are identical, i.e., associated with the "symmetric" processing.

[120] A through-hole 8Γ processed in the sample 77 with a pattern 79 having a maximum side length a of about 50 μιη is shown in Figs. 19A (top surface) and 19B (bottom surface). Bar 33 in Fig. 19A illustrates 50 μιη.

[121] It was realized that the geometrical shape is not a limitation of the processing as long as the condition of sufficient overlap between beam waists of the pulsed Bessel-like laser beam is ensured for laser pulses applied next to each other. Regarding corners in through- holes, the size of the beam waist limits the minimum angle that can be shaped with a desired quality. As stated above, the Bessel zone of the laser beam should be preferably greater or at least equal to about the thickness of the sample in order to drill through the sample on a single scan basis. The precision of the surface of the inner wall is then given by the respective Bessel core size (the smaller the smoother assuming a respective overlap).

[122] As an example, a triangular through-hole was processed with a pattern similar to Fig.

17 but with triangular shaped trajectory sections enclosing an angle of 60°.

[123] Figs. 20A and 20B are respective optical microscope images illustrating the entrance opening (Fig. 20A: top surface) and the exit opening (Fig. 20B: bottom surface) of an etched triangular through-hole 83 generated with overlapping single pulse interaction zones. As can be seen with respect to bar 33 (indicating a length of 50 μιη), the side length of the most outer triangular trajectory section was about 70 μιη.

[124] As illustrated in Figs. 15 and 17, the single laser pulse damage regions of successive laser pulses (which follow immediately one another when processing along a trajectory section) are displaced with respect to each other by a pulse machining step dY, dP that is selected for providing an overlap of single laser pulse damage regions of the respective successive laser pulses. Similar, the single laser pulse damage regions of neighboring trajectory sections, which extend immediately next to each other, are displaced with respect to each other by a trajectory machining step dX, dr selected for providing an overlap of single laser pulse damage regions of the respective laser pulses of the neighboring trajectory sections.

[125] Those displacements are set such that that neighboring elongate single laser pulse damage regions are displaced with respect to each other along the trajectory in the range from a minimum distance of 5 % up to a maximum distance of 70 % (such as in the range from a minimum distance of 10 % or 20 % up to a maximum distance of 90 % or 70 %) of a beam waist at full width half maximum of a core of the pulsed Bessel-like laser beam as present within a single laser pulse damage region. The displacement size may change the required energy and machining time but if the successive pulses overlap in the disclosed range, hole formation is possible, although the hole quality may vary. In the exemplary processed samples shown in Figs. 16A and 16B as well as 18A to 20B, a beam waist at full width half maximum of the core was about 1.5 μιη. In exemplary processings, the pulse machining step was set in the range from about 0.1 μιη to about 1 μιη such as at the indicated 0.5 μιη.

[126] As shown, one can generate holes with different shapes using different geometrical writing schemes. For the triangular and quadratic shapes, a concentric symmetry of the trajectory sections was used similar to the concentric circular trajectory sections of Fig. 5.

[127] If one increases the spatial distance between successive trajectories, one may still generate a hole but irregularities may occur, for example, at the bottom surface.

[128] For such a triangular pattern, Figs. 21A and 21B are respective optical microscope images illustrating the entrance opening (Fig. 21A: top surface) and the exit opening (Fig. 21B: bottom surface) of an etched triangular through-hole 83'. Besides the larger aperture dimension, the pulse and trajectory machining steps were increase by a factor of, for example, 10. While a through-hole is still generated, irregularities on the bottom surface can be seen in Fig. 21B that are cause by the reduced overlap between single pulse interaction zones.

[129] Figs. 22 A and 22B illustrate a modified approach for creating larger through-holes with less laser processing, exemplarily illustrated for patterns consisting of concentric circles of locally overlapping single pulse interaction zones.

[130] Fig. 22A illustrates that a minimum size, in this case diameter, is due to a minimum number of trajectory sections that are positioned side-a-side, to achieve a real through-hole through a sample. For the exemplary laser and sample parameters (pulse duration 10 ps, pulse energy 130 μΐ, 150 μιη sample thickness), the minimum diameter of the trough holes having an essentially smooth inner wall surface was about 50 μιη. In Fig. 22A, exemplarily four neighboring trajectory sections 85 were indicated, where the center was a single interaction zone 85' that is surrounded by three circles of interaction zones. For example, the beam waist of the laser beam may be about 1.5 μιη for an assumed overlap of 1.25 μιη of neighboring beam waists. A respective minimum number of trajectories, which would ensure through hole formation, was about 55 trajectories for the 150 μιη sample thickness (pulse duration 10 ps and energy 130 μΓ).

[131] Referring to Fig. 22B for bigger apertures, starting from, for example, a circular non- machined zone 87, one can apply a number of circular trajectories 85 that surround the non- machined zone 87. The number of circular trajectories 85 is determined to be at least a minimum number of neighboring trajectory sections that is required for building up a single laser pulse damage region that extends through the material as described below.

[132] Given the Bessel beam geometry needed to fulfill the previous condition, it is

possible to determine the minimum number of circular trajectories 85 (or in general neighboring trajectories) needed to drill a through-hole. The pattern of Fig. 22B removes the material both in a machined annular zone associated with the circular trajectories 85 and in the non-machined zone 87, i.e., the central disk. Thus, a given hole size can be generated faster as less laser pulses are needed.

[133] The minimum aperture size and the aspect ratio of a manufacturable aperture depend on laser beam parameter and material parameter, specifically, the Bessel beam geometry and the interaction with the specific material. It depends, for example, on the core size and the conical half-angle (defining the Bessel beam geometry and, thus, the extent of a single pulse damaged volume) and on the sample thickness and material type.

[134] Figs. 23A to 23E are schematic drawings illustrating the interaction of Bessel-type laser beam pulses with a transparent glass plate 89 and, in particular, the effect of locally overlapping single pulse interaction zones.

[135] Specifically, Fig. 23A illustrates schematically an interaction zone 91 of a single laser pulse within the glass plate. Within the interaction zone 91, the laser pulse generally damages the glass due to the high intensities generated along the Bessel-like beam 17 (see also Fig. 2). The damage comprises a modified material area 92 produced by the single shot and extending along the propagation direction in Z-direction through the glass plate 89. The damage further comprises, at the two surfaces 89A, 89B of the glass plate 89, material-free areas 93 A, 93B from which matter is removed by ablation (material etching by laser interaction). In the modified material area 92 inside the bulk, the material is still present but modified or partially damaged. In summary, the damage associated to the interaction zone 91 comprises superficial ablation (with material expulsion) from the top and bottom of the glass plate (i.e., the material-free areas 93A) and a damaged zone inside the glass plate 89 (i.e., the modified material area 92). For example, in the modified material area 92, a densification and/or cracks of the glass may be caused by the high intensities. [136] Referring to Fig. 23B, using the damage induced by a plurality of laser pulses of a pulsed Bessel-like laser beam, a hole may be formed in a fast and efficient way, when laser pulses partially overlap. In Fig. 23B, Bessel-like beams 17A, 17B overlap within a superposition zone 95. The size of the superposition zone 95 needs to be of a specific extent such that the damage created in a first interaction zone 91 A affects the extend of the damage created in a second interaction zone 91B, next to the first interaction zone 91A in scanning direction (pulse machining step dY) or within a neighboring trajectory (trajectory machining step dX).

[137] Fig. 23C illustrates the damage created within the second interaction zone 91B.

Specifically, in view of the previously damaged material, there takes place a larger etching to form deeper etched material-free areas 93 A', 93B' next to the etched material-free areas 93A, 93B on the surfaces 89A, 89B, respectively. In between etched material-free areas 93 A', 93B', a modified material area is formed. That modified material area and the material-free areas 93 A', 93B' are now affecting the laser interaction of the next laser pulse, resulting in even deeper etched material-free areas 93A", 93B" (see Fig. 23D, showing the progressive deepening of the ablation zone).

[138] It will be acknowledged that based on that pulse- wise (step-wise) extension into the glass plate 89, after a specific number of pulses, which are respectively displaced in a manner that enables that effect on later interaction zones, the glass plate will show a through- hole. Clearly to obtain a through-hole along the whole sample thickness, the number of required trajectory sections depends on the thickness.

[139] It is noted that the processing scheme used for the triangular aperture of Figs. 21A and 22B did not work properly because of a too large separation between successive trajectories. In other words, the superposition zone was too small to sufficiently affect the follow-up damage formation.

[140] While Fig. 23D shows a symmetric ablation from the front side and the bottom side of the glass plate 89, ablation may differ in extend under other configurations as illustrated in Fig. 23E. This may be caused by the focus position and/or absorption within the sample, for example.

[141] Fig. 23E further illustrates that after the required minimum number of trajectories (in

Fig. 15 in X-direction), a sidewall 97 will be formed that interacts homogeneously with the complete length of the Bessel beams 17'.

[142] The lateral optical microscope images of Figs. 24A to 24E illustrate this aspect of the through-hole formation using overlapping single pulse interaction zones as described above. Specifically, Fig. 24 shows the effect of the drilling process using an increasing number of circular trajectory sections. While for small numbers of circular trajectory sections, the wall still is largely tilted and forms a funnel on the top side, the through-hole becomes already at 30 to 40 circular trajectory sections cylindrical in shape, thus resulting in minimum diameters of cylindrical shaped holes of about 50 μιη.

[143] For completeness, it is noted that the hole formation process can be based on a spiral pattern that provides a sequence of trajectory section side-a-side, similar to concentric circular trajectory sections. However, it will be understood that there may form a step in the side wall, where the spiral ends. To avoid the step structure in the wall, the spiral pattern may be modified such as transition into a last circular trajectory section or group of last circular trajectory sections to provide a homogeneous aperture.

[144] Although the preferred embodiments of this invention have been described herein, improvements and modifications may be incorporated without departing from the scope of the following claims.