Method for separating substrates

Abstract

A method for separating a substrate of a brittle-hard material is provided. The method includes the steps of introducing defects into the substrate at a spacing from one another along a separation line using at least one pulsed laser beam; selecting an average spacing between neighboring defects and a number of laser pulses for generating a respective defect such that a breaking stress (σ.sub.B) for separating the substrate along the separation line is smaller than a first reference stress (σ.sub.R1) of the substrate and such that an edge strength σ.sub.K of the separation edge obtained after separation is greater than a second reference stress (σ.sub.R2) of the substrate; and separating the substrate after introducing the defects by applying a stress along the separation line.

Claims

1. A method for separating a substrate made of brittle-hard material, comprising: introducing defects into the substrate at a spacing from one another along a separation line using at least one pulsed laser beam; setting a first reference stress (σ.sub.R1) according to a formula σ.sub.R1≤C.sub.R1.Math.α.Math.E.Math.(T.sub.g−100° C.) and a second reference stress (σ.sub.R2) according to a formula σ.sub.R2≥C.sub.R2.Math.α.Math.E.Math.(T.sub.g−100° C.), wherein C.sub.R1 and C.sub.R2 are reference stress coefficients with C.sub.R1=0.5/k and C.sub.R2=0.5.Math.k, wherein k is 1.5, wherein α is the coefficient of thermal expansion of the material of the substrate, wherein E is the Young's modulus of the material of the substrate, and wherein T.sub.g is the glass transition temperature of the material of the substrate; selecting an average spacing between neighboring defects that is at least 3 μm and a number of laser pulses for generating a respective defect such that a breaking stress (σ.sub.B) for separating the substrate along the separation line is smaller than the first reference stress of the substrate and such that an edge strength (σ.sub.K) of the separation edge obtained after separation is greater than the second reference stress of the substrate; and separating the substrate after introducing the defects by applying a stress along the separation line so as to provide resulting broken edges of the substrate along the separation line with an average roughness R.sub.a of less than 0.5 μm.

2. The method of claim 1, setting the first reference stress (σ.sub.R1) and the second reference stress (σ.sub.R2) identical to one another and to a maximum thermal stress (σ.sub.th) that depends on a material of the substrate.

3. The method of claim 2, further comprising determining the maximum thermal stress (σ.sub.th) according to a formula σ.sub.th=0.5.Math.α.Math.E.Math.(T.sub.g−100° C.), wherein α is the coefficient of thermal expansion of the material of the substrate, E is the Young's modulus of the material of the substrate, and T.sub.g is the glass transition temperature of the material of the substrate.

4. The method of claim 1, wherein, when the substrate is a chemically toughened substrate, the method comprises: setting the first reference stress (σ.sub.R1) and the second reference stress (σ.sub.R2) identical to one another and to an inner tensile stress (σ.sub.CT) that depends on properties of the chemically toughened substrate; and determining the inner tensile stress (σ.sub.CT) according to the formula σ.sub.CT=(σ.sub.CS.Math.d.sub.L)/(d−2d.sub.L), wherein σ.sub.cs denotes a surface compressive stress of the chemically toughened substrate, d.sub.L is a penetration depth of a preliminary stress, and d is a thickness of the chemically toughened substrate.

5. The method of claim 1, wherein, when the substrate is a thermally toughened substrate, the method comprises: setting the first reference stress (σ.sub.R1) and the second reference stress (σ.sub.R2) identical to one another and to an inner tensile stress (σ.sub.CT) that depends on properties of the thermally toughened substrate; and determining the inner tensile stress σ.sub.CT according to the formula σ.sub.CT=σ.sub.CS/2, wherein σ.sub.cs denotes a surface compressive stress of the thermally toughened substrate.

6. The method of claim 1, wherein the step of separating the substrate comprises moving a point of incidence of a laser radiation over the substrate along the separation line to cause the stress to be applied along the separation line.

7. The method of claim 1, wherein the step of selecting the average spacing between neighboring defects comprises selecting the spacing of at most 10 μm.

8. The method of claim 1, wherein the step of selecting the number of laser pulses comprises selecting from an interval [1, 20] or from an interval [2, 8].

9. The method of claim 1, wherein, when the substrate is made of a material with a coefficient of thermal expansion in an interval [3.Math.10.sup.−6K.sup.−1, 4.Math.10.sup.−6K.sup.−1], a Young's modulus in an interval [69 kN/mm.sup.2, 76 kN/mm.sup.2], and/or a glass transition temperature in an interval [700° C., 800° C.], the step of selecting the average spacing and the number of laser pulses comprises selecting the average spacing from an interval [6 μm, 8 μm] and the number of laser pulses from an interval [7, 9].

10. The method of claim 1, wherein, when the substrate is made of a material with a coefficient of thermal expansion in an interval [7.Math.10.sup.−6K.sup.−1, 8.Math.10.sup.−6K.sup.−1], a Young's modulus in an interval [69 kN/mm.sup.2, 76 kN/mm.sup.2], and/or a glass transition temperature in an interval [500° C., 600° C.], the step of selecting the average spacing and the number of laser pulses comprises selecting the average spacing from an interval [6 μm, 8 μm] and the number of laser pulses from an interval [1, 3].

11. The method of claim 1, wherein, when the substrate is made of a material with a coefficient of thermal expansion in an interval [3.Math.10.sup.−6K.sup.−1, 4.Math.10.sup.−6K.sup.−1], a Young's modulus in an interval [60 kN/mm.sup.2, 68 kN/mm.sup.2], and/or a glass transition temperature in an interval [500° C., 600° C.], the step of selecting the average spacing and the number of laser pulses comprises selecting the average spacing from an interval [4 μm, 8 μm] and the number of laser pulses from an interval [7, 9].

12. The method of claim 1, wherein, when the substrate is made of a material with a coefficient of thermal expansion in an interval [3.Math.10.sup.−6K.sup.−1, 4.Math.10.sup.−6K.sup.−1], a Young's modulus in an interval [60 kN/mm.sup.2, 68 kN/mm.sup.2], and/or a glass transition temperature in an interval [500° C., 600° C.], the step of selecting the average spacing and the number of laser pulses comprises selecting the average spacing from an interval [6 μm, 8 μm] and the number of laser pulses from an interval [3, 5].

13. The method of claim 1, wherein the step of selecting the average spacing between neighboring defects comprises selecting the spacing to at most 8 μm.

14. The method of claim 1, wherein the step of selecting the average spacing between neighboring defects comprises selecting the spacing from at least 5 μm to at most 8 μm.

15. The method of claim 1, wherein the step of selecting the average spacing between neighboring defects comprises selecting the spacing from at least 7 μm and at most 8 μm.

16. A method for separating an untoughened substrate made of brittle-hard material, comprising: introducing defects into the untoughened substrate at a spacing from one another along a separation line using a pulsed laser beam; selecting an average spacing between neighboring defects and a number of laser pulses for generating the defects such that a breaking stress (σ.sub.B) for separating the untoughened substrate along the separation line is smaller than a first reference stress (σ.sub.R1) of the untoughened substrate and such that an edge strength (σ.sub.K) of the separation edge obtained after separation is greater than a second reference stress (σ.sub.R2) of the untoughened substrate; setting the first reference stress (σ.sub.R1) and the second reference stress (σ.sub.R2) identical to one another and to a maximum thermal stress (σ.sub.th), wherein the maximum thermal stress (σ.sub.th) depends on a material of the untoughened substrate and is determined according to a formula σ.sub.th=0.5.Math.α.Math.E.Math.(T.sub.g−100° C.), wherein α is a coefficient of thermal expansion of the material, E is a Young's modulus of the material, and T.sub.g is a glass transition temperature of the material; and separating the untoughened substrate after introducing the defects by applying a stress along the separation line.

17. A method for separating a substrate made of brittle-hard material, comprising: introducing defects into the substrate at a spacing from one another along a separation line using a pulsed laser beam, wherein the substrate is chemically toughened; selecting an average spacing between neighboring defects and a number of laser pulses for generating the defects such that a breaking stress (σ.sub.B) for separating the substrate along the separation line is smaller than a first reference stress (σ.sub.R1) of the substrate and such that an edge strength (σ.sub.K) of the separation edge obtained after separation is greater than a second reference stress (σ.sub.R2) of the substrate; setting the first reference stress (σ.sub.R1) and the second reference stress (σ.sub.R2) identical to one another and to an inner tensile stress (σ.sub.CT), wherein the inner tensile stress (σ.sub.CT) depends on properties of the substrate and is determined according to the formula σ.sub.CT=(σ.sub.CS.Math.d.sub.L)/(d−2d.sub.L), wherein σ.sub.CS denotes a surface compressive stress of the substrate, d.sub.L is a penetration depth of a preliminary stress, and d is a thickness of the substrate; and separating the substrate after introducing the defects by applying a stress along the separation line.

Description

DESCRIPTION OF THE DRAWINGS

(1) Reference will now be made to the accompanying figures. In the figures, the same reference numerals designate the same or equivalent elements, wherein:

(2) FIG. 1 shows test results of breaking stress σ.sub.B and edge strength σ.sub.K, in MPa, for SCHOTT AF32® glass having defects along a separation line, for different spacings A between neighboring defects and for different numbers L of laser pulses for generating a respective defect;

(3) FIG. 2 shows test results of breaking stress σ.sub.B and edge strength σ.sub.K, in MPa, for SCHOTT D263® glass having defects along a separation line, for different spacings A between neighboring defects and for different numbers L of laser pulses for generating a respective defect;

(4) FIG. 3 shows test results of breaking stress σ.sub.B and edge strength σ.sub.K, in MPa, for SCHOTT BOROFLOAT® 33 glass having defects along a separation line, for different spacings A between neighboring defects and for different numbers L of laser pulses for generating a respective defect;

(5) FIG. 4 shows test results of the edge roughness, in MPa, for SCHOTT BOROFLOAT® 33 glass having defects along a separation line, for different spacings A between neighboring defects and for different numbers L of laser pulses for generating a respective defect;

(6) FIG. 5 shows a workpiece with two separation lines crossing each other;

(7) FIG. 6 shows a workpiece with two sets of crossing separation lines;

(8) FIG. 7 shows an element separated out of a workpiece;

(9) FIG. 8 shows a variation of the exemplary embodiment of FIG. 6, with a web for holding together the strip-wise separated workpiece;

(10) FIGS. 9 and 10 show variations of the exemplary embodiment of FIG. 6, with a frame for holding together the strip-wise separated workpiece;

(11) FIG. 11 shows a workpiece on a carrier substrate; and

(12) FIG. 12 shows an embodiment of a workpiece subdivided into a plurality of portions.

DETAILED DESCRIPTION

(13) Referring to FIGS. 1 to 3, the results of a DOE series of tests are shown for three different glasses, namely SCHOTT AF32® (FIG. 1), SCHOTT D263® (FIG. 2), and SCHOTT BOROFLOAT® 33 (FIG. 3).

(14) This represents a selection of examples. More generally, the invention can be used for various substrates, in particular made of glass, glass ceramics, and/or silicon with filamentation, in particular even for materials having a low coefficient of thermal expansion.

(15) Filamentation was performed for each of a plurality of samples of these glasses, i.e. spaced apart defects were introduced into the volume of the respective sample along a separation line by laser pulses of a laser.

(16) For the samples made of SCHOTT AF32® or SCHOTT D263®, substrate thicknesses of approximately 100 μm were in particular selected, and for the samples made of SCHOTT BOROFLOAT® 33, substrate thicknesses of approximately 1 mm were in particular selected.

(17) After filamentation, the breaking strength σ.sub.B of the glass with respect to the filamented separation line was tested in each case. In other words, the samples were separated along the separation line, and the breakage stress required for separation was measured and logged. Depending on the thickness of the material, either the 4-point bending strength test method according to DIN EN 843-1 (thicker glasses) was performed, or the breaking strength was determined according to DE 10 2014 110 855 A1 (thinner glasses) with a step roller. DE 10 2014 110 855 A1 describes a method and a device for determining the breaking strength of the edges of thin ribbons of brittle material.

(18) In a next step, the separation edge obtained after separation was tested for its strength in each of the samples. In order to determine the respective edge strength OK, the samples were again tested either by the method according to DIN EN 843-1 (thicker glasses) or according to DE 10 2014 110 855 A1 (thinner glasses), depending on the thickness of the material, and the measured results were logged.

(19) The breaking stresses σ.sub.B and edge strengths σ.sub.K obtained in this way are plotted on the ordinate axis, in units of MPa, in FIGS. 1 to 3. The breaking stresses σ.sub.B are indicated by B and the edge strengths σ.sub.K are accordingly indicated by K at the bottom of the diagram.

(20) Furthermore, the laser filamentation was performed with different parameters, i.e. different perforated samples were produced. The spacing between neighboring defects was varied, as well as the numbers of laser pulses for producing a respective defect. These parameters were set for a sample and were kept constant during the filamentation of a particular sample in each case.

(21) Accordingly, FIGS. 1 to 3 show the breaking stresses σ.sub.K and edge strengths K for the different parameters of laser filamentation, which are plotted on the abscissa axis. Here, the spacing between neighboring defects, in micrometers, is denoted by A, and the number of laser pulses for generating a respective defect is denoted by L.

(22) Based on the experimental results presented, it can be verified that it is particularly favorable for a cleaving step following the filamentation, to set the filamentation process parameters A and L with regard to the breaking stress σ.sub.K for separation along the separation line and with regard to the edge strength σ.sub.K of the separation edge obtained after separation.

(23) What can be verified below, in particular, is the finding that it has proved to be particularly favorable to set the breaking stress σ.sub.K for separating the glass along the separation line so as to be smaller than a first reference stress σ.sub.R1 that depends on the respective glass, and the edge strength σ.sub.K after separation so as to be greater than a second reference stress σ.sub.R2 that depends on the respective glass. For example, it has been found to be particularly advantageous if σ.sub.R1=σ.sub.R2=σ.sub.th holds, wherein σ.sub.th is the maximum thermal stress which can be assumed as σ.sub.th=0.5.Math.α.Math.E.Math.(T.sub.g−100° C.), for example.

(24) In FIGS. 1 to 3, the maximum thermal stress 0.5.Math.α.Math.E.Math.(T.sub.g−100° C.) so determined for the respective glass has been calculated and is plotted as a horizontal line.

(25) It turns out that for all three glasses, i.e. SCHOTT AF32® (FIG. 1), SCHOTT D263® (FIG. 2), and SCHOTT BOROFLOAT® 33 (FIG. 3), this value is in the order of magnitude of the measured breaking stresses σ.sub.B and edge strengths σ.sub.K, and that therefore the possibility arises to select the process parameters A and L in an optimized manner with regard to this value.

(26) From the test results shown in FIG. 1, which relate to SCHOTT AF32® glass, it can be seen that in particular a defect spacing of 7 μm in combination with a number of 8 laser pulses is particularly suitable, in particular with respect to the value 0.5.Math.α.Math.E.Math.(T.sub.g−100° C.), in order to minimize the breaking stress σ.sub.B and on the other hand to maximize the edge strengths σ.sub.K.

(27) For SCHOTT glass D263®, it can be seen from FIG. 2 that a defect spacing of 7 μm in combination with a number of 2 laser pulses constitute particularly suitable parameters.

(28) For SCHOTT glass BOROFLOAT® 33, it can be seen from FIG. 3 that either the combination of a number of 4 pulses and a defect spacing of 7 μm or the combination of a number of 8 pulses and a defect spacing of 5 μm or 7 μm are particularly suitable parameters.

(29) So, by optimizing the filaments in terms of breaking stress, the subsequent separation process (cleaving step) by thermally induced stresses can be performed in an optimized manner.

(30) The tests in particular show that, surprisingly, perforations at a larger spacing bring about advantages for separability, across the materials.

(31) This includes higher process reliability: even with marginal process windows, i.e. processes with parameter ranges that could previously not be performed reliably according to the prior art, the substrates can be reliably separated with the method according to the invention.

(32) Moreover, the laser can be operated at reduced power, and/or the displacement rate (advance rate) of the laser can be adjusted, in particular increased. For example, for materials with a perforation spacing of 5 μm and process setting with an advance rate of 40 mm/s, it was possible to reduce the laser power from 110 W to 75 W.

(33) In one application example, a sample made of SCHOTT BOROFLOAT® 33 of 1 mm thickness, with a CTE of 3.3.Math.10.sup.−6K.sup.−1, was filamented with a defect spacing of 7 μm and 6 laser pulses for producing a respective defect, at 300 kHz and with an advance rate of 2100 mm/s.

(34) Another advantage arises with regard to the shaping of the separation edge. Due to lower stress during the cleaving step, more complex geometries can be produced, for example smaller corner radii.

(35) Furthermore, the edge quality can be improved: chipping at the edge or microcracks are avoided or are not perceptible, neither visually nor microscopically. This has a positive effect in particular on edge strength.

(36) By way of example, FIG. 4 shows the edge roughness for SCHOTT BOROFLOAT® 33 of 1 mm thickness for various process parameters. The number of laser pulses L is denoted by burst in this figure. It can be seen that with the same number of laser pulses, the edge roughness decreases with increasing spacing of the defects.

(37) In the following tables, properties of the aforementioned Schott glasses AF32®, D263®, and BOROFLOAT® 33 are listed in detail.

(38) TABLE-US-00016 TABLE 1 Properties of SCHOTT AF32 ®: Technical Data: Dimensions: round and square custom size wafer formats, e.g. 6″, 8″, or 12″ Surface roughness <1 nm RMS Thicknesses 0.03 mm up to 1.1 mm Standard Thicknesses 0.3 mm, 0.4 mm Luminous transmittance τ.sub.n065 (d = 0.5 mm) 91.9% Coefficient of mean linear thermal expansion α 3.2 .Math. 10.sup.−4 K.sup.−1 (20° C.; 300° C.) (static measurement) Transformation temperature T.sub.g 717° C. Dielectric constant ∈.sub.r at 1 MHz 5.1 Refractive index n.sub.D 1.5099 Density ρ (annealed at 40° C./h) 2.43 g/cm.sup.3

(39) The Young's modulus of SCHOTT AF32® is estimated to be 74.8 kN/mm.sup.2.

(40) TABLE-US-00017 TABLE 2 Properties of SCHOTT glass D263 ®: Technical Data Dimensions 440 mm × 360 mm, other size on request Surface roughness <1 nm RMS Thicknesses 0.03 mm up to 1.1 mm Standard thicknesses and packaging units 0.21 mm 100 pcs 0.30 mm 100 pcs 0.40 mm 50 pcs 0.55 mm 50 pcs Luminous transmittance τ.sub.n065 (d = 0.5 mm) 91.7% Coefficient of mean linear thermal expansion α 7.2 .Math. 10.sup.−4 K.sup.−1 (20° C.; 300° C.) (static measurement) Transformation temperature T.sub.g 557° C. Dielectric constant ∈.sub.r at 1 MHz 6.7 Refractive index n.sub.D 1.5230 Density ρ (annealed at 40° C./h) 2.51 g/cm.sup.3

(41) The Young's modulus of SCHOTT D263® is estimated to be 72.9 kN/mm.sup.2.

(42) TABLE-US-00018 TABLE 3 Properties of SCHOTT BOROFLOAT ® 33: Coefficient of linear thermal α.sub.(20/300° C.) 3.25 × 10.sup.−6 K.sup.−1 expansion (C.T.E.) (to ISO 7991) Specific heat capacity c.sub.(20/100° C.) 0.83 KJ × (kg × K).sup.−1 Thermal conductivity λ.sub.(90° C.) 1.2 W × (m × K).sup.−1 Viscosity η Working point 10.sup.4  dPa .Math. s 1270° C.  Softening Point 10.sup.7.6 dPa .Math. s 820° C. Annealing Point 10.sup.13  dPa .Math. s 560° C. Strain Point .sup. 10.sup.14.5 dPa .Math. s 518° C. Transformation temperature 525° C. (T.sub.g)

(43) The Young's modulus of SCHOTT BOROFLOAT® 33 is estimated to be 64 kN/mm.sup.2.

(44) The problem that a crack does not follow a predetermined separation line during the severing and runs away, or that the crack does not start or that it stops is aggravated when crossing separation lines are provided. When separating substrates that have separation lines arranged at angles to one another and consist of filaments, by thermally induced stress, the problem often arising at the crossing points of the separation lines is that the cracking runs in potentially all directions. If the separation lines are used as a preparation for breaking in the context of a multi-step process (e.g. with intermediate steps of washing, coating, etc.) for being separated or singulated into multiple portions later, the pre-processed substrate often has initial cracks at the crossing points of the separation lines. In this case, the separation lines that are to be separated later (for example after further process steps such as washing and coating), are also subject to initial crack formation. This increases the risk that the separation line will break in a non-controlled manner during the further processing.

(45) Even in this case, due to the precisely set breaking stress, the invention is generally particularly suitable for enabling separation along an intended line, even if the latter is crossed by a further separation line of spaced-apart defects. According to one embodiment of the invention it is therefore contemplated that the workpiece according to the invention has at least two crossing separation lines of spaced-apart defects, or that at least two crossing separation lines with spaced-apart defects are generated in the substrate.

(46) In the case of crossing separation lines it is advantageous to provide the workpiece with different separation lines that can be separated by different stress levels.

(47) More generally, without being limited to specific exemplary embodiments, a workpiece is provided according to a further aspect of the invention, in particular a glass product, a glass ceramic product, and/or a silicon product, which is pre-damaged along at least two crossing separation lines, so that the workpiece can be separated by applying a breaking stress along the separation lines, and wherein the breaking stresses for severing along the separation or modification lines differ by at least 3 MPa, preferably by at least 5 MPa, more preferably by at least 10 MPa.

(48) It is particularly advantageous in this case, if the main structure of those separation lines that are to be separated first have a significantly lower stress than the separation lines which are to be separated later.

(49) More generally, it is preferred that the workpiece again has defects that are spaced apart along the separation lines, and that the breaking stress σ.sub.B for separating the workpiece along the separation line is smaller than a first reference stress σ.sub.R1 that depends on the respective workpiece, and that the edge strength σ.sub.K of the separation edge obtained after the separation is greater than a second reference stress σ.sub.R2 that depends on the respective workpiece. Accordingly, the breaking stress of the other separation line will then be higher or lower by at least 3 MPa. Most preferably, this second breaking stress is higher.

(50) The different breaking stresses may preferably be set by changing the laser parameters such that all of the features described herein for the separation lines produced by the ultra-short pulsed laser also apply to this embodiment with crossing separation lines. However, it is also conceivable to produce defects along the crossing separation lines in other ways. So, this embodiment of the invention is therefore not necessarily limited to the introduction of filamentary defects or to the laser processing.

(51) FIG. 5 shows a workpiece or substrate 1 which has two separation lines 3, 5, as described above. Preferably, the substrate 1 generally has a sheet-like shape and accordingly has two opposite generally parallel faces 10, 11, without being limited to the illustrated example. With the described method, filamentary defects 7 were introduced into the substrate 1 using an ultrashort pulsed laser. For this purpose, the laser light is irradiated onto one of the faces 10, 11, so that the filamentary defects 7 extend between the two faces 10, 11, following the beam direction. In the illustrated example, the separation lines 3, 5 are crossing at right angles, as is preferred to separate rectangular portions.

(52) As can be seen from FIG. 5, the spacings a.sub.1, a.sub.2 of the filamentary defects 7 along the separation lines 3, 5 are different. In the illustrated example, the spacing a1 of the filamentary defects along separation line 3 is smaller than the spacing a.sub.2 of the filamentary defects along separation line 5. Thus, the breaking stress σ.sub.B1 for severing along the separation line 3 is different from the breaking stress σ.sub.R2 for severing along the separation line 5. According to the observations discussed above, with the smaller spacing a.sub.1, the breaking stress for separation line 3 might be either higher or lower. Typically, a minimum of breaking stress is found for a spacing of the linear filaments in the range from 5 to 7 μm. More generally, without being limited to the illustrated example, a workpiece or substrate 1 is provided in which the spacing of the filamentary defects differ, wherein the spacing in one of the separation lines 3 is smaller than in the other one of the crossing separation lines.

(53) According to a further embodiment, the workpiece or substrate 1 is distinguished by having at least two different separation lines, as shown in FIG. 5, and the breaking stress σ.sub.B1 for separating the workpiece 1 along a separation line 3 is smaller than a first reference stress σ.sub.R1 that depends on the respective workpiece, and the edge strength σ.sub.K of the separation edge obtained after the separation is greater than a second reference stress σ.sub.R2 that depends on the respective workpiece, and the second separation line 5 has a significantly higher breaking stress of at least σ.sub.B1+5 MPa, preferably σ.sub.B1+10 MPa, most preferably σ.sub.B1+15 MPa. Preferably, the second separation line is even formed such that the breaking stress associated therewith is greater than σ.sub.R1.

(54) In a further embodiment, the substrate is provided with a plurality of separation lines for subsequent separation, and separation lines running in the same direction are generated with the same laser process parameters and therefore have the same breaking stress, and breaking stresses only differ between separation lines running in different directions. In other words, more than two crossing separation lines are provided, and the number of separation lines can be divided into at least two sets of juxtaposed separation lines, and the average breaking force of the separation lines of one set differs from the average breaking force of another set by the aforementioned value of at least 3 MPa. An example of such a workpiece or substrate 1 is shown in FIG. 6. A plurality of separation lines 3 run parallel to one another and so are juxtaposed along the face 10. These separation lines 3 are crossed by separation lines 5 which likewise run parallel to one another. As in the example shown in FIG. 5, the separation lines 3, 5 cross each other at right angles. The set of parallel separation lines 3 forms a first set 30 of separation lines, the separation lines 5 running perpendicular thereto form another set 50. The sets 30, 50 are distinguished by comprising separation lines with filamentary defects 7 that have the same spacing to one another within a set, but a different spacing compared to a separation line of the other set. Therefore, the breaking stresses of the separation lines 3 of set 30 have a value σ.sub.B1 which differs from the value σ.sub.B2 of the separation lines 5 of the other set 50.

(55) In principle, there are several ways to modify or set the breaking stress along a separation line. For example, the number of burst pulses used to form the filament can be varied (in the range from 1 to 100 pulses, preferably in the range from 2 to 20 pulses). Also, the pulse energy of the single pulse or burst pulse can also be varied in the range from 100 μJ to 1 mJ for single pulses, or from 400 μJ to 4 mJ for burst pulses. Furthermore, as shown in the figures, the spacing of the filaments 7 in the individual separation lines 3, 5 can be varied to assume different values in a range from 1 μm to 25 μm, most preferably in a range from 2 μm to 20 μm, so that different values are resulting for the spacings a.sub.1 and a.sub.2.

(56) For example, for an alkali boroaluminosilicate glass of 0.5 mm thickness, a system of separation lines with different breaking stresses can be produced by reducing the refractive power from 25 MPa to about 15 MPa by altering the burst energy from 300 μJ to 400 μJ (for a USP laser with a repetition rate of 100 kHz and pulse duration of 10 ps). Furthermore, an increase in the pitch spacing results in a decrease in the breaking stress that is required for the separation process by about 5 MPa: with a pitch of 5 μm between the introduced modifications, a breaking stress of about 45 MPa is resulting for a glass as described above, whereas the breaking stress will only be 15 MPa in the case of a spacing of 10 μm.

(57) Once the separation lines have been introduced, the substrate 1 can be separated by a two-step singulation process, first by breaking along the first direction, then by breaking along the second direction. In this case, the adjacent edges of the portions will exhibit periodic patterns with a different spacing (as the filament channels open into two cylindrical halves) which are accessible to common topological measurement techniques such as tactile or optical profilometry or electron microscopy. Such an element 2 that has been separated from the workpiece 1 is shown in FIG. 7.

(58) As can be seen from the schematic view, a sheet-like glass element or glass ceramic element 2 is provided, which has two opposite faces 10, 11 and edge surfaces 13, 14, 15, 16, wherein at least two of the edge surfaces 13, 14, 15, 16 exhibit filamentary defects 7 that extend side by side along the edge surface, with a periodic spacing and in the direction from one of the faces 10, 11 to the other one of the faces, and the period of the spacing of the filamentary defects is different for at least two of the edge surfaces 13, 14, 15, 16.

(59) As in the illustrated example, a generally quadrangular shape is preferred, so that the element 2 will have two pairs of opposite edge surfaces 13, 15 and 14, 16. In this case, the period of the filamentary defects 7 is preferably the same for a pair of opposite edges. All information disclosed herein with regard to the method of the invention and the workpiece that can be produced thereby apply accordingly with regard to the period or spacing between the filamentary defects, the thickness of the element 2 and the material thereof.

(60) Without restricting generality, the separation may be performed either by a mechanical breaking process or by a laser-based thermal separation process (using a CO.sub.2 laser) or by other methods.

(61) In a further embodiment, the individual strips (formed by opposite edges of the same breaking stress) remain connected to one another through a common web that can be used as a handling aid for subsequent process steps such as washing, coating, and later singulation. An exemplary embodiment of such a workpiece 1 is shown in FIG. 8. As can be seen from the exemplary figure, the separation lines 5 of one of the sets, 50, terminate at a distance from one of the edges of the workpiece 1, so that a web 18 is formed between the ends of the separation lines 5 and the edge 20 of the workpiece, to which the separable elements 2 remain attached even when the workpiece 1 is severed along the separation lines 5 that terminate at a distance from the edge 20.

(62) According to a further embodiment, the workpiece 1 prepared for separation has a peripheral frame from which entire strips or individual portions can be broken out during singulation. This embodiment with a frame 22 is shown in FIG. 9. In this case, the handling frame may be preserved as a whole during the singulation process, or else it may be disintegrated step by step. As with the web 18 of the embodiment according to FIG. 8, the separation lines of one of the sets terminate at a distance from the edge 20 of the workpiece 1. In the case of a frame 22, this then applies to both sets 30, 50.

(63) Other variants are also conceivable, in which the filamented separation lines partially protrude into the handling frame or into the handling strip, for example. An example of this is shown in FIG. 10. In this example, one side of the frame 22 is defined by the distance of the ends of the separation lines 5 from the edge 20, while the other sides of the frame are bounded by the longitudinal ends of the separation lines 3, 5.

(64) FIG. 11 shows an example which allows to handle a workpiece 1 according to the invention, for example for industrial further processing, while avoiding premature separation along the separation lines 3, 5. In this example, the workpiece 1 is secured on a carrier substrate 24 with one of its faces 11. The carrier substrate 24 may for instance be a glass wafer or a silicon wafer or a polymer such as a plastic sheet.

(65) In the embodiment of FIG. 6, the spacing of the filamentary defects 7 and the breaking stress differ for different sets, the respective sets being defined by juxtaposed separation lines that do not cross each other. This does not preclude providing one or more of the juxtaposed separation lines with a different and in particular lower breaking stress. For example, according to a further embodiment, the substrate or workpiece 1 is subdivided, by a suitable choice of the breaking stresses, into a plurality of larger portions that may in turn be separated further. This subdivision need not be an actual separation but is to be understood in particular as a subdivision of the workpiece along the separation lines between the portions. In the example shown in FIG. 12, the workpiece 1 is subdivided into two portions 8, 9 along a separation line 6.

(66) For example, only every n-th parallel separation line has the same breaking stress, and the separation lines therebetween have a different breaking stress value. In this case, any combination of parallel and perpendicular separation lines is possible. A respective variation of breaking stresses is possible, as mentioned above, by varying the pulse energy, number of bursts, or spacing of the introduced modifications. Conceivable would be a subdivision in which the separation line at the boundary between the portions has a higher breaking stress than neighboring separation lines. However, in order to allow for a separation first into the portions 8, 9 and then further into individual elements 2, it is particularly preferred to set a lower breaking stress for the separation line(s) 6 than for the neighboring separation lines within the portions 8, 9. According to this embodiment of the invention, it is therefore contemplated that the workpiece 1 has a plurality of juxtaposed separation lines 5, 6, wherein at least one of the separation lines 6 extends between two adjacent separation lines 5, and wherein the separation line 6 extending between the two adjacent separation lines 5 has a lower breaking stress than the adjacent separation lines 5. Preferably, two sets of separation lines 5 exhibiting a higher breaking stress extend on both sides of the separation line 6 exhibiting the lower breaking stress, as in the example shown. The difference in the breaking stresses may generally be selected as in the embodiments with crossing separation lines, i.e. so as to preferably amount to at least 3 MPa, in particular at least 5 MPa.

(67) All workpieces described herein may, in principle, be coated substrates as well. Optionally, the coating may also be applied prior to the laser-assisted introduction of the separation lines. According to one embodiment of the invention, an organic functional coating for protein or DNA analyzes may be applied as a coating. Thus, the workpiece 1 or an element 2 separable from the workpiece 1 can be used as a DNA or protein microarray. Suitable coatings for this purpose include aminosilanes, epoxysilanes, aldehyde silanes, hydrogels, streptavidin, and certain polymers. The coating may then be provided with a microarray of oligonucleotides, cDNA/PCR, bacterial artificial chromosomes (BACs), peptides, proteins, antibodies, glycans, or cell samples or tissue samples. In this way, the samples can be prepared together, and after having been separated they can then be examined and/or shipped separately.

LIST OF REFERENCE NUMERALS

(68) 1 Workpiece 2 Element 3, 5, 6 Separation line 7 Filamentary defect 8, 9 Portions of 1 10, 11 Faces of 1 30, 50 Sets of separation lines 13, 14, 15, 16 Edge surface 18 Web 20 Edge of 1 22 Frame 24 Carrier substrate