LASER PROCESSING OF A MULTI-PHASE TRANSPARENT MATERIAL, AND MULTI-PHASE COMPOSITE MATERIAL

Abstract

A method provides for producing modifications in or on a transparent workpiece using a laser processing device. The laser processing device has a short pulse or ultrashort pulse laser that emits laser radiation having a wavelength in the transparency range of the workpiece and which has a beam-shaping optical unit for beam shaping for focusing the laser radiation. The transparent workpiece is composed of a material that has a plurality of phases, of which at least two phases have different dielectric constants, of which in turn the one phase is a phase embedded in the form of particles, which phase is substantially surrounded by the other phase, and wherein the product of the volume of the particles specified in cubic nanometers and the ratio of the absolute value of the difference of the two different dielectric constants to the dielectric constant of the surrounding phase is greater than 500.

Claims

1. A method for producing modifications in or on a transparent workpiece utilizing a laser processing device, wherein the laser processing device has a short pulse or ultrashort pulse laser that emits laser radiation having a wavelength in the transparency range of the workpiece and which has a beam-shaping optical unit for beam shaping for focusing the laser radiation; and wherein the transparent workpiece is composed of a material that has a plurality of phases, of which at least two phases have different dielectric constants, of which in turn the one phase is a phase embedded in the form of particles, which phase is substantially surrounded by the other phase, and wherein the product of the volume of the particles specified in cubic nanometers and the ratio of the absolute value of the difference of the two different dielectric constants to the dielectric constant of the surrounding phase is greater than 500.

2. The method as claimed in claim 1, wherein the product of the volume of the particles specified in cubic nanometers and the ratio of the absolute value of the difference of the two different dielectric constants to the dielectric constant of the surrounding phase is greater than 1000.

3. The method as claimed in claim 1, wherein the product of the volume of the particles specified in cubic nanometers and the ratio of the absolute value of the difference of the two different dielectric constants to the dielectric constant of the surrounding phase is greater than 2000.

4. The method as claimed in claim 1, wherein a series of adjacent linear modifications is generated in the transparent workpiece along a line by moving the laser radiation relative to the workpiece along said line and by emitting laser pulses in time succession, either as individual pulses or in the form of bursts, wherein each of said individual pulses or bursts is used to produce a respective one of said linear modifications.

5. The method as claimed in claim 4, wherein the length of the linear modification is adjusted via the pulse energy level.

6. The method as claimed in claim 4, wherein said linear modification is generated by beam shaping with formation of a line focus or by beam shaping with formation of a sufficiently high intensity for a modification along the optical axis.

7. The method as claimed in claim 1, wherein an optical unit is used for focusing the laser radiation, which spatially extends the focus in the propagation direction in such a way that the maximum intensity of the laser radiation is less than 150% of the threshold value of the intensity above which the material is modified.

8. The method as claimed in claim 7, wherein the maximum intensity of the laser radiation is 130% of the threshold value of the intensity above which the material is modified.

9. The method as claimed in claim 1, wherein the laser radiation is focused so that a ratio of the length range along which the intensity is at least 110% of the threshold value of the modification to the length range along which the intensity is at least 10% of the threshold value is at least 0.4.

10. The method as claimed in claim 9, wherein the ratio of the length range along which the intensity is at least 110% of the threshold value of the modification to the length range along which the intensity is at least 10% of the threshold value is at least 0.5.

11. The method as claimed in claim 9, wherein the ratio of the length range along which the intensity is at least 110% of the threshold value of the modification to the length range along which the intensity is at least 10% of the threshold value is at least 0.7.

12. The method as claimed in claim 4, wherein the linear modification is generated by a beam shaping with a rectilinear or curvilinear high intensity distribution.

13. The method as claimed in claim 4, which comprises providing a workpiece table and a displacement device for aligning the beam-shaping optical unit to the workpiece for producing said linear modification in the workpiece and thereafter for an incremental relative displacement between the beam-shaping optical unit and the workpiece table according to the line.

14. The method as claimed in claim 4, wherein the linear modifications are introduced in the workpiece in a predetermined direction, wherein said direction is controlled with respect to the local surface normal of the workpiece and at a distance to the workpiece.

15. The method as claimed in claim 4, wherein the linear modifications are each formed of a plurality of defects arranged along a channel, wherein the lengths of the defects increase with increasing distance from a surface of the workpiece.

16. The method as claimed in claim 1, wherein modifications are generated at the surface of the transparent workpiece by removing material from the surface by said laser radiation.

17. The method as claimed in claim 1, wherein modifications are produced inside the transparent workpiece by adjusting the parameters of the short pulse or ultrashort pulse laser in such a way that damages to the surface of the transparent workpiece are avoided.

18. The method as claimed in claim 1, wherein the embedded phase is defined by at least one particle.

19. The method as claimed in claim 1, wherein said material is a glass ceramic or a composite material.

20. The method as claimed in claim 1, wherein a material is used whose particles have a median of particle sizes of 6 nm or more.

21. The method as claimed in claim 1, wherein an optical element is used to deflect the laser light and to successively modify the point of incidence of the laser beam.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0080] The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become more apparent and the invention will be better understood by reference to the following description of embodiments of the invention taken in conjunction with the accompanying drawings, wherein:

[0081] FIG. 1 shows a laser processing device while processing a workpiece on a workpiece table;

[0082] FIG. 2 shows a linear modification formed by defects in a glass ceramic;

[0083] FIG. 3 shows lengths of linear modifications which have been produced in glass ceramic and in the corresponding green glass, for comparison reasons, using lenses of different focal lengths of the focusing optical unit, as a function of the number of pulses per burst;

[0084] FIG. 4 schematically shows two intensity profiles along the central axis of rays focused in the glass ceramic;

[0085] FIGS. 5a and 5b show scanning electron micrographs of linear modifications in a glass ceramic and in a borosilicate glass, respectively.

[0086] Corresponding reference characters indicate corresponding parts throughout the several views. The exemplifications set out herein illustrates embodiments of the invention and such exemplifications are not to be construed as limiting the scope of the invention in any manner.

DETAILED DESCRIPTION OF THE INVENTION

[0087] FIG. 1 illustrates a laser processing device 1 above a workpiece 2 which is supported on a workpiece table 3. Laser processing device 1 comprises an ultrashort pulse laser 10 and a focusing optical unit 11 in order to emit a focused beam of rays 12 with a focus 13 that is located approximately on the upper face of workpiece 2. On workpiece 2, a cutting line or fracture line 20 is indicated, along which the workpiece 2 will be separated or divided. Provision is made for displacing the focus 13 along this line 20, which can be accomplished by adjusting the table 3 in the two coordinate directions 21, 22. Very small adjustment increments are used.

[0088] The wavelength of the radiation of ultrashort pulse laser 10 is selected so that it is in a transparency range of workpiece 2. Ultrashort pulse laser 10 is operated in a so-called burst mode in which the actual pulse itself, referred to as a burst, is defined by a packet of pulses recurring at a repetition rate R of approximately 100 kHz. The energy of the laser pulses or of the pulse packets (bursts) is dimensioned so that with each burst a damage channel 14, referred to as a filament, is formed perpendicular to an upper face 2a of workpiece 2 in the interior thereof. By displacing the beam-shaping optical unit 11 along a predefinable displacement line 20, a series of linear modifications 14 is generated in the workpiece 2. With an advancement rate of 1 m/s, the starting points of damage channels 14 which are directed towards the bottom face 2b, will have a spacing of 10 m on the upper face 2a along displacement line 20 of the ultrashort pulse laser 10. The displacement line 20 of ultrashort pulse laser 10 relative to workpiece 2 defines a fracture face and is therefore referred to as a fracture line 20.

[0089] Such linear modifications 14 produced by the method of the invention and in the composite material of the invention are shown in FIG. 2. As can be seen in FIG. 2, the linear modifications 14 are formed by defects 50, 51, 52, and 53 with lengths of 108 m, 221 m, 357 m, and 1037 m, which have been produced or which are arranged along a damage channel. The defects shown in FIG. 2 have a spacing 60, 61, and 62 of 263 m, 195 m, and 34 m, respectively. Mean values from three measurements equivalent to FIG. 2 yielded defect lengths of 104 m, 237 m, 350 m, and 1020 m with a spacing of 290 m, 209 m, and 150 m.

[0090] This exemplary embodiment, which is limited to the generation of linear modifications, shows that the defect lengths do not have to be consistent. Later, this can be of great advantage in order to be able to properly divide the workpiece along the alignment line of the filaments.

[0091] Accordingly, in a further embodiment of the method of the invention it is generally contemplated, without being limited to the specific exemplary embodiment explained above, that linear modifications 14 which are aligned along a line 20 in a transparent workpiece 2, are each formed of a plurality of defects that are arranged along a channel, and that the lengths the defects increase with increasing distance from a surface of the workpiece. Thus, in case of a tensile or bending load, as it is applied to the alignment line of the linear modifications for intentionally breaking the workpiece, there will be shorter defects near the surface, that is where the greater bending stress occurs. In the volume, towards the center of the workpiece, the tensile stress will be smaller in the case of a bending load. However, the defects are longer and the workpiece is weakened here to a greater extent. Thus, when the workpiece is subjected to a bending load, the critical bending stress is distributed more evenly along the filament structure, so that the fracture behavior is improved.

[0092] In the following, the investigation of a corresponding pair of glass ceramic and green glass will be described, which are referred to as glass ceramic A and green glass A. Green glass A has a glass composition that contains Li.sub.2O (lithium oxide), Al.sub.2O.sub.3 (aluminum oxide), SiO.sub.2 (silicon dioxide) and a total of about four weight percent of TiO.sub.2 (titanium dioxide) and ZrO.sub.2 (zirconium dioxide). The transformation temperature of this composition is 670 C.

[0093] On basis of this green glass A, the glass ceramic A has been produced by ceramization, during which finely dispersed individual crystals are being formed in the glassy material. For this purpose, the green glass A was processed in an electrically heated ceramization furnace. The resulting transformation process of the material can be subdivided into nucleation and crystal growth. At the beginning of each crystallization is a crystallization seed at which the crystal can begin to grow.

[0094] Green glass A contains impurities in the form of the added titanium dioxide and zirconium dioxide, which have a high melting temperature (1855 C. and 2715 C., respectively), and precipitate when heated so as to be effective as crystallization seeds. They represent heterogeneous seeds during the ceramization process, whereby a high seed density and a small crystal size is achieved. Due to crystal growth, a high-quartz solid solution (HQ.sub.ss) grows on the crystallization seeds of orthorhombically arranged ZrTiO.sub.4. It is based on the LAS system which takes its name from the crystal building blocks of lithium oxide, aluminum oxide, and silicon dioxide. Quartz (SiO.sub.2) transforms into so-called high-quartz at 573 C. However, due to the incorporation of other atoms, the HQ.sub.ss is stable when this temperature is undershot.

[0095] An ultrashort pulse laser was employed for the investigations, using a wavelength of 1064 nm, an average power of 12 W (at 1064 nm, 100 kHz, 1 pulse per burst), a repetition rate of 100 kHz, a burst frequency of 50 MHz, a pulse duration of about 10 ps (at 1064 nm and 100 kHz), and a Gaussian beam as the beam profile.

[0096] The energy that is introduced and absorbed by a laser pulse leads to a stronger heating in the glass ceramic A, due to the lower heat capacity compared to green glass, and the developed heat is also dissipated more efficiently than in the green glass A. However, in very short time regimes in the nanosecond range, no significant heat dissipation is to be expected, wherefore the assumption of a higher temperature in glass ceramic A compared to green glass A is justified.

[0097] In the burst mode, the second pulse of the burst will therefore impinge at an already preheated area. The burst frequency of 50 MHz results in an interval of only 20 ns between the individual pulses, which satisfies the condition mentioned in the previous paragraph. Although the effects of the burst mode in the USP laser blast technology have not yet been definitively clarified, yet there is a stronger presumption on the heat deposition which influences the behavior of the further pulses. The produced defects are formed more efficiently with increasing number a of pulses.

[0098] Therefore, a series of tests was carried out, in which linear modifications were introduced and both materials were each processed with different numbers of pulses a per burst, according to a preferred embodiment of the invention. Four lenses of different focal lengths were used, in order to obtain a broader variety of data. According to the theory outlined above, the ratio of the total lengths of the defects or linear modifications in glass ceramic A to the corresponding total length in green glass A should increase with an increasing number of pulses per burst, since the effect of heat deposition becomes more significant with increasing number of partial pulses.

[0099] However, the energy distribution to the individual partial pulses of the burst must be taken into account. With only one partial pulse, the pulse energy is 120 J and the peak pulse power is 12 MW. In case of the two-pulse burst mode, on the other hand, the first pulse has about 77 J and the second 56 J. Therefore, if the number of pulses is high, the pulse energies and thus also the pulse peak powers and intensities may be too low to cause a modification in the material.

[0100] The repetition rate, the advancement rate in y-direction (1 m/s), and the power level (100%) were kept constant during the test series. FIG. 3 shows the results.

[0101] No damage to the material occurred only with two constellations: green glass A, 10 mm focal length, and four or eight pulses per burst, respectively. Here, the pulse energy of the first partial pulse of the burst was not sufficient to produce a modification of the material. For other optics, this absence was not observed, since the beam shape generated by such other optics is probably more suitably.

[0102] The results show a strong dependence of the total length on the number a of partial pulses. With one exception (green glass A, focal length equal to 80 mm, and a equal to two), the total length decreases with increasing number of partial pulses, as shown in FIG. 3, in which the measurement points were connected in order to illustrate the monotonically decreasing characteristic of the measured data. This is presumably due to the low pulse energy of the first pulse of the available laser, which decreases for increasing partial pulse numbers a. The assumption that the energy of the first pulse is responsible for the length and the further partial pulses only amplify the type of damage has thus been confirmed at least in the first point.

[0103] In the following, the investigation of a material that is used for producing optical filters will furthermore be described. This is the material RG610, and this material was investigated as green glass (referred to as green glass RG610 below), and also as so-called filter glass produced by thermal treatment of the green glass and including at least 2 different glassy or glassy-crystalline phases.

[0104] In the state of the filter glass that exhibits two phases, the refractive index is 1.52. The yellowish green glass RG610 changes color by the heat treatment, whereby the filter glass RG610 appears deep red later. The reason for this are the resulting mixed phases (amorphous or crystalline) which contain cadmium, sulfur, selenium, and zinc. Since there is no refractive index available for these mixed phases, it was derived from the refractive indices at the D-line of 588 nm of similar crystals: zinc selenide, zinc sulfide, cadmium selenide, cadmium sulfide. Thus, a refractive index of the crystal of 2.53 is resulting. Since the crystalline phase only accounts for a small volume fraction (about 12%), the refractive index of the residual glass phase can be approximated by that of the green glass RG610 of 1.52. This results in a ratio of the dielectric constants (ratio of the squared refractive indices) of 2.770. The size of the crystals (diameter) is about 12 nm.

[0105] By applying the ultrashort pulse laser technology to RG610, the discrepancy between green glass and glass ceramic could be clearly demonstrated: The defects in the glass ceramic RG610 are more than twice as long as with the green glass RG610 and reach a length of up to 5 mm in glass of 6.6 mm thickness.

TABLE-US-00001 TABLE 1 Particle Ratio of di- Interfacial Material diameter/nm electric constants effect IE Filter glass RG610 12 2.770 3059 Glass ceramic A 40 1.107 6848 Pre-seeded material A 4 2.374 88

[0106] Table 1 shows the factors of the interfacial effect IE for RG610 (as filter glass), and glass ceramic A (residual glass-phase and high-quartz solid solution interface), and pre-seeded material A (residual glass-phase and seed interface), which were calculated using the above stated formula.

[0107] Although the calculated value for glass ceramic A is more than twice as large as the value for the filter glass RG610 A, both values are of the same order of magnitude. This is different for the value of the pre-seeded material A.

[0108] The green glass and the glass ceramic A is a lithium aluminosilicate glass of the composition: [0109] 60-73.0 wt % of SiO.sub.2; [0110] 15-25.0 wt % of Al.sub.2O.sub.3; [0111] 2.2-5.0 wt % of Li.sub.2O; [0112] 0-5.0 wt % of CaO+SrO+BaO; [0113] 0-5.0 wt % of TiO.sub.2; [0114] 0-5.0 wt % of ZrO.sub.2; [0115] 0-4.0 wt % of ZnO; [0116] 0-3.0 wt % of Sb.sub.2O.sub.3; [0117] 0-3.0 wt % of MgO; [0118] 0-3.0 wt % of SnO.sub.2; [0119] 0-9.0 wt % of P.sub.2O.sub.5; [0120] 0-1.5 wt % of As.sub.2O.sub.3; [0121] 0-1.2 wt % of Na.sub.2O+K.sub.2O, with respective amounts within the following specified ranges: [0122] 0-1.0 wt % of Na.sub.2O; [0123] 0-0.5 wt % of K.sub.2O; and [0124] 0-1.0 wt % of coloring oxides.

[0125] For two lenses it has been shown above that with a parameter adjustment it is possible to achieve the same filament structure length in glass ceramic A as in green glass A. In this case, it is also possible to achieve the same defect category as in green glass A. Accordingly, the energy of the first partial pulse of the burst is decisive for the defect length, whereas it has been found that the further pulses only increase the defect category. With the same parameters, the defects in glass ceramic A will in fact be longer than in green glass A, but with a lower defect category. With an increased number of partial pulses per burst, the defects become shorter and the defect category becomes higher. It was found that with eight partial pulses, the same results can be achieved in glass ceramic A as with two partial pulses in green glass A.

[0126] Due to the interfacial effect IE in glass ceramic A, damage is already caused at lower intensities, so that the defects are longer than in green glass A. However, since not so much energy can be deposited per area, the defects are not so pronounced. Green glass A, on the other hand, requires higher intensities, so that the damage is more pronounced here. This can be compensated with an increase in the number of partial pulses per burst, as mentioned above.

[0127] The usually higher number of refocusing cycles for green glass can also be explained with this model. Since higher intensities of the laser are necessary in case of green glass to ignite the plasma, the state of equilibrium which determines the defect length is more unstable and collapses more rapidly. However, if the pulse has still sufficient energy afterwards, it can refocus.

[0128] For producing the material modification, a power threshold value is reached or exceeded. Due to the described effect of field strength superelevation, this threshold value will usually be significantly smaller for glass ceramics than the value for the corresponding raw glass (see the table, all values in W/m.sup.2):

TABLE-US-00002 Material 8712 8724 state raw glass 1.0*1E16 1.5*1E16 ceramized 0.4*1E16 0.6*1E16

[0129] In the case of not optically optimized setups, e.g. when the laser radiation is focused using a lens with spherical aberration, a significantly higher intensity and power occurs in the material at the rear end of the focusing area (in beam direction) than in the vicinity of the imaging optics, so that the threshold values are substantially only exceeded in the vicinity of the intensity peaks. With adapted optics, however, the same pulse energy (equal areas under the curves) is distributed more homogeneously in the focus area, ideally as homogeneously as possible above the glass ceramic threshold, so that as a result of the field strength amplification effect described above the material modification can be produced over a significantly longer region than in glass.

[0130] This effect is illustrated below with reference to FIG. 4.

[0131] FIG. 4 schematically shows two curves J1, J2 of the power per volume along the central axis of the propagation direction of laser beams focused in the glass ceramic. The values x along the abscissa indicate the distance to the lens. The desired linear modifications are produced above certain power per volume thresholds. In FIG. 4, a threshold value (P/V).sub.min,glass is indicated for producing the modification in glass, and a lower threshold value (P/V).sub.min,glass for producing corresponding modifications in glass ceramics.

[0132] Intensity profile J1 results when a lens with spherical aberration is used for focusing, for example.

[0133] Intensity profile J2 is an optimized profile as it can be achieved using an axicon, for example. With the focusing, an elongated intensity maximum is achieved. With the same pulse energy, the peak intensity will then be correspondingly lower than in the case of intensity profile J1.

[0134] The length of the produced linear modifications now depends on the length of the region in which the pulse power per volume exceeds the respective threshold value. Intensity profile J1 still allows for a linear modification of a length Lglass in glass. However, with intensity profile J2, no modification can be achieved, since the threshold value (P/V).sub.min,glass is no longer exceeded with the comparatively lower maximum intensity. In glass ceramics, on the other hand, this is not only possible with the lower threshold value (P/V).sub.min,glass ceramic, but moreover the achievable length L.sub.glass ceramic is longer. Because the focal range in which the threshold value is exceeded is elongated in the direction of the beam, a correspondingly elongated linear modification is achieved. Accordingly, the length of the linear modification can be maximized by a preferably homogenous distribution of the laser pulse energy and power per unit volume above the minimum energy and minimum power per unit volume along the focal line.

[0135] In order to achieve a longest possible length of the modification, an optical systems may therefore be selected for focusing the laser beam depending on the material of workpiece 2 and at a given pulse energy, which generates an elongated focus with a reduced maximum intensity compared to a spherical lens, and so that the threshold value for producing the material modification will not be undershot. In other words, an optical unit is therefore used for focusing the laser radiation, which spatially extends the focus in the propagation direction, so that the maximum intensity of the laser radiation is less than 150%, preferably less than 130% of the intensity threshold value above which the material is modified.

[0136] A suitably intensity distribution is in particular achieved when the beam is focused using the optical unit so that the ratio of the length range along which the intensity is at least 110% of the threshold value of the modification to the length range along which the intensity is at least 10% of the threshold value is at least 0.4, preferably at least 0.5, more preferably at least 0.7. In this way, a particularly suitable distribution of the intensity is achieved so that a very elongated region in the material is irradiated with intensities above the threshold value.

[0137] From FIG. 4 it is moreover apparent that the length of the modification can be easily adjusted via the pulse power. With increasing power, the length section is extended along which the threshold value for producing the modification is exceeded.

[0138] FIG. 5a shows an SEM image of a plurality of adjacently arranged (from left to right in the figure) linear modifications 14 (and extending from top to bottom in the figure) which are produced by the method according to the invention and in the composite material according to the invention.

[0139] The nine visible linear modifications 14 are spaced apart from each other by about 7 micrometers and were produced by laser pulses in the form of bursts and with a biconvex lens (16 mm) using the following process parameters: 6 bursts, burst frequency: 50 MHz, total burst energy: about 500 J, decreasing burst shape, pulse energy of the first pulse about 170 J, 12 mm tube beam 1/e.sup.2, wavelength: 1064 nm.

[0140] It can be seen in FIG. 5a that the linear modifications are substantially wider than for example in a borosilicate glass which is shown in FIG. 5b, for comparison reasons, and which has filamentary channels with diameters in the sub-micrometer range.

[0141] A linear modification or a defect of a linear modification can accordingly have a width, in particular an average width 70, which is greater than 1 m, preferably greater than 2 m. Width herein means a dimension perpendicular to the extension of the linear modification.

[0142] Such a relatively wide linear modification or defect of a linear modification may in particular exist in the form of a melting zone. Accordingly, material of the composite material may melt around the impact region of the laser and may undergo a phase transformation, for example.

[0143] In addition to the relatively wide impact zones, it can also be seen from the SEM image of FIG. 5a that the linear modifications are accompanied by bubble formation in certain regions along their extension direction (along the laser beam axis). The bubbles may in particular be voids, for example at the phase boundaries, due to the field strength superelevation which was already described above. It can also happen that an additional filament channel is formed along the linear modifications.

[0144] Accordingly, a linear modification may include at least partially open areas 80, in particular pore-like or bubble-shaped areas.

[0145] While this invention has been described with respect to at least one embodiment, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.