METHOD AND DEVICE FOR THE LASER-BASED WORKING OF TWO-DIMENSIONAL, CRYSTALLINE SUBSTRATES, IN PARTICULAR SEMICONDUCTOR SUBSTRATES

Abstract

The present invention relates to a method for laser-based machining of a planar, crystalline substrate in order to separate the substrate into a plurality of parts, in which the laser beam of a laser is directed, for machining the substrate, onto the latter, in which, with an optical arrangement positioned in the beam path of the laser, a laser beam focal surface which is expanded, viewed both along the beam direction and viewed in precisely a first direction perpendicular to the beam direction, but is not expanded in a second direction which is both perpendicular to the first direction and to the beam direction, is formed from the laser beam radiated onto said arrangement on the beam output side of the optical arrangement, the substrate being positioned relative to the laser beam focal surface such that the laser beam focal surface in the interior of the substrate, along an expanded surface portion of the substrate material, produces an induced absorption by means of which crack formations in the substrate material induced along this expanded surface portion are effected.

Claims

1. Method for laser-based machining of a planar, crystalline substrate in order to separate the substrate into a plurality of parts, in which the laser beam (2a, 2f) of a laser (3) is directed, for machining the substrate (1), onto the latter, in which, with an optical arrangement (6) positioned in the beam path of the laser (3), a laser beam focal surface (2f) which is expanded, viewed both along the beam direction (z) and in precisely a first direction (y) perpendicular to the beam direction (z), but which is not expanded in a second direction (x) which is both perpendicular to the first direction (y) and to the beam direction (z), is formed, from the laser beam (2a) radiated onto said optical arrangement (6), on the beam output side of the optical arrangement (6), the substrate (1) being positioned relative to the laser beam focal surface (2f) such that the laser beam focal surface (2f) produces, in the interior of the substrate (1) along an expanded surface portion (2c) of the substrate material, an induced absorption by means of which induced crack formations in the substrate material along this expanded surface portion (2c) are effected.

2. Method according to the preceding claim, characterised in that the planar, crystalline substrate is or comprises the following: a semiconductor substrate, in particular a 4-6 or 3-5 semiconductor substrate, preferably a GaAs substrate, or an elementary semiconductor substrate, preferably an Si substrate, an insulator substrate, in particular an oxide, preferably Al.sub.2O.sub.3 (sapphire) or SiO.sub.2 (quartz), or a fluoride, preferably CaF.sub.2 or MgF.sub.2, or a chloride, preferably NaCl, or a nitride, preferably Si.sub.3N.sub.4 or BN, and/or a substrate comprising or consisting of at least one carbon-based material with crystalline or quasi-crystalline basic order, in particular comprising or consisting of carbon nanotubes.

3. Method according to one of the preceding claims, characterised in that the expansion of the laser beam surface (2f), subsequently termed length l, in beam direction (z) is configured to be larger by at least times, preferably by at least 20 times, preferably by at least 50 times, preferably by at least 100 times, preferably by at least 500 times, than the expansion D of the laser beam focal surface (2f) in the second direction (x), and/or in that the expansion of the laser beam focal surface (2f), subsequently termed width b, in the first direction (y) is configured to be larger by at least 5 times, preferably by at least 10 times, preferably by at least 50 times, preferably by least 100 times, than the expansion D of the laser beam focal surface (2f) in the second direction (x).

4. Method according to one of the preceding claims, characterised in that the substrate (1) is positioned relative to the laser beam focal surface (2f) such that, viewed in beam direction (z), the expanded surface portion (2c) of the induced absorption in the material, i.e. in the interior of the substrate (1), extends up to at least one of the two oppositely situated substrate surfaces (1a, 1b).

5. Method according to the preceding claim, characterised in that the substrate (1) is positioned relative to the laser beam focal surface (2f) such that, viewed in beam direction (z), the expanded surface portion (2c) of the induced absorption in the material, i.e. in the interior of the substrate (1), extends from one (1a) of the two oppositely situated substrate surfaces to the other (1b) of the two oppositely situated substrate surfaces, i.e. over the entire layer thickness d of the substrate (1), or in that the substrate (1) is positioned relative to the laser beam focal surface (2f) such that, viewed in beam direction (z), the expanded surface portion (2f) of the induced absorption in the material, i.e. in the interior of the substrate (1), starting from one (1a) of the two oppositely situated substrate surfaces, extends into the substrate (1), but not up to the other (1b) of the two oppositely situated substrate surfaces, i.e. not over the entire layer thickness d of the substrate (1), extends preferably over 80% to 98%, preferably over 85% to 95%, particularly preferably over 90%, of this layer thickness.

6. Method according to one of the preceding claims, characterised in that the induced absorption is produced such that the crack formation in the structure of the substrate (1) is effected without ablation and without melting of material of the substrate (1).

7. Method according to one of the preceding claims, characterised in that the length l of the laser beam focal surface (2f) is between 0.2 mm and 10 mm, preferably between 0.5 mm and 2 mm, and/or in that the width b of the laser beam focal surface (2f) is between 0.02 mm and 2.5 mm, preferably between 0.05 mm and 0.2 mm, and/or in that the layer thickness d of the substrate (1), measured perpendicular to the two oppositely situated substrate surfaces (1a, 1b), is between 2 m and 3,000 m, preferably between 100 m and 500 m, and/or in that the ratio V1=1/d of the length l of the laser beam focal surface (2f) and the layer thickness d of the substrate (1) is between and 0.5, preferably between 5 and 2, and/or in that the expansion D of the laser beam focal surface (2f) in the second direction (x) is between 1 m and 50 m, preferably between 5 m and 25 m.

8. Method according to one of the preceding claims, characterised in that a pulsed laser is used as laser (3), and/or in that the pulse duration t of the laser (3) is chosen such that, within the interaction time with the material of the substrate (1), the heat diffusion in this material is negligible, preferably no heat diffusion is effected, for which purpose preferably , F as surface expansion of the laser beam focal surface (2f) and the heat diffusion constant of the material of the substrate (1) are adjusted according to <<F/ and/or preferably is chosen to be less than ns, preferably less than 100 ps, and/or in that the pulse repetition frequency of the laser (3) is between 10 kHz and 1,000 kHz, preferably 100 kHz, and/or in that the laser (3) is operated as a single pulse laser or as a burst pulse laser, and/or in that the average laser power, measured directly on the beam output side of the laser (3), is between 5 watts and 100 watts, preferably between 15 watts and 30 watts.

9. Method according to one of the preceding claims, characterised in that the wavelength of the laser (3) is chosen such that the material of the substrate (1) is transparent at this wavelength or is substantially transparent, there being understood by the latter that the intensity reduction of the laser beam, effected along the beam direction (z), in the material of the substrate (1) is, per millimetre of penetration depth, 10% or less, the laser, in particular for crystals which are transparent in the visible wavelength range as substrate (1), is preferably an Nd:YAG laser with a wavelength of 1,064 nm or a Y:YAG laser with a wavelength of 1,030 nm, or, in particular for semiconductor substrates (1) which are transparent in the infrared wavelength range, is preferably an Er:YAG laser with a wavelength between 1.5 m and 2.1 m.

10. Method according to one of the preceding claims, characterised in that the laser beam (2a, 2f) is moved, relative to the surface (1a) of the substrate (1), along a line (5), preferably along a line (5) parallel to the first direction (y), along which the substrate (1) is to be separated in order to obtain a plurality of parts, a large number (2c-1, 2c-2, . . . ) of expanded surface portions (2c) of induced absorption in the interior of the substrate (1) being produced along this line (5), preferably the ratio V2=A/b of the average spacing A of directly adjacent expanded surface portions (2c) of induced absorption, i.e. of expanded surface portions (2c) of induced absorption being produced directly successively, and of the width b of the laser beam focal surface (2f) in the first direction (y) is between 1.0 and 1.3, preferably between 1.0 and 1.1.

11. Method according to the preceding claim, characterised in that during and/or after production of the large number (2c-1, 2c-2, . . . ) of expanded surface portions (2c) of induced absorption in the interior of the substrate (1), mechanical forces are exerted on the substrate (1) and/or thermal stresses are introduced into the substrate (1), in particular the substrate is heated non-uniformly and cooled again in order to effect, respectively between directly adjacent (2c-1, 2c-2) expanded surface portions (2c) of induced absorption, a crack formation in order to separate the substrate into the plurality of parts.

12. Device for laser-based machining of a planar, crystalline substrate in order to separate the substrate into a plurality of parts, with which the laser beam (2a, 2f) of a laser (3), for machining the substrate (1), is directable onto the latter, characterised by an optical arrangement (6) positioned in the beam path of the laser (3), with which a laser beam focal surface (2f) which is expanded, viewed both along the beam direction (z) and in precisely a first direction (y) perpendicular to the beam direction (z), but which is not expanded in a second direction (x) which is both perpendicular to the first direction (y) and to the beam direction (z), can be formed, from the laser beam (2a) radiated onto said optical arrangement (6), on the beam output side of the optical arrangement (6), the substrate (1) being positionable or positioned relative to the laser beam focal surface (2f) such that the laser beam focal surface (2f) produces, in the interior of the substrate (1) along an expanded surface portion (2c) of the substrate material, an induced absorption, as a result of which crack formations in the substrate material induced along this expanded surface portion (2c) are effected.

13. Device according to the preceding claim, characterised in that the optical arrangement (6) for forming the laser beam focal surface (2f) which is expanded, viewed both along the beam direction (z) and viewed in precisely the first direction (y) perpendicular to the beam direction (z), but which is not expanded in the second direction (x), comprises an optical element (9), preferably a conical prism or axicon, with a non-spherical free surface which is formed to form the laser beam focal surface (2f) with a defined length l, i.e. with a defined expansion viewed in the beam direction (z), and also on the beam output side of this optical element (9) with the non-spherical free surface and also at a spacing z1 therefrom, a diaphragm (8) which cuts the expansion of the laser beam (2a) in the second direction (x), i.e. which is orientated with the preferential direction in the first direction (y), preferably a slit diaphragm (8) which is orientated in the first direction (y).

14. Device according to the preceding claim, characterised in that between the optical element (9) with the non-spherical free surface and the diaphragm (8), an optical element (12) collimating the laser beam (2a), in particular a plano-convex collimation lens (12), is positioned and orientated such that the laser radiation emanating from the optical element (9) with the non-spherical free surface is projected parallel onto the diaphragm (8).

15. Device according to one of the three preceding claims, characterised in that the optical arrangement (6) for forming the laser beam focal surface (2f) which is expanded, viewed both along the beam direction (z) and viewed in precisely the first direction (y) perpendicular to the beam direction (z), but which is not expanded in the second direction (x), comprises an optical element (9), preferably a conical prism or axicon, with a non-spherical free surface which is formed to form the laser beam focal surface (2f) with a defined length l, i.e. with a defined expansion viewed in beam direction (z), and also on the beam output side of this optical element (9) with the non-spherical free surface and also at a spacing z2 therefrom, an optical element (7) which focuses the laser beam (2a) in the first direction (y) but not in the second direction (x), preferably a cylindrical lens (7) orientated parallel to the second direction (x).

16. Device according to one of the three preceding claims, characterised in that the optical arrangement (6), on the beam output side of the diaphragm (8) or of the optical element (7) which focuses the laser beam (2a) in the first direction (y) but not in the second direction (x), comprises an optical element (11) which focuses the laser beam (2a) at least in the first direction (y), preferably a collimation lens which focuses the laser beam (2a) in the first (y) and in the second (x) direction, for particular preference a plano-convex collimation lens (11).

17. Device according to one of the five preceding claims, characterised in that the optical arrangement (6) for forming the laser beam focal surface (2f) which is expanded, viewed both along the beam direction (z) and viewed in precisely the first direction (y) perpendicular to the beam direction (z), but which is not expanded in the second direction (x), has an optical element (13), preferably has a double wedge (13), with which beam bundles (s1, s2) are deflectable from the two half spaces (y1, y2), which half spaces (y1, y2) are situated, viewed in the first direction (y), oppositely relative to the optical axis (6z) of the optical arrangement (6), respectively parallel and towards the optical axis (6z), and also comprises, on the beam output side of this optical element (13), an optical element which focuses the laser beam (2a) at least in the first direction (y), preferably a collimation lens which focuses the laser beam (2a) in the first (y) and in the second (x) direction, particularly preferably a plano-convex collimation lens (11).

18. Use of a method or of a device according to one of the preceding claims for separating a semiconductor substrate, in particular a 4-6 or 3-5 semiconductor substrate, preferably a GaAs substrate, or an elementary semiconductor substrate, preferably an Si substrate, separating an insulator substrate, in particular an oxide, preferably of Al.sub.2O.sub.3 (sapphire) or of SiO.sub.2 (quartz), or a fluoride, preferably of CaF.sub.2 or of MgF.sub.2, or a chloride, preferably of NaCl, or a nitride, preferably of Si.sub.3N.sub.4 or of BN, or separating a substrate comprising or consisting of at least one carbon-based material with crystalline or quasi-crystalline basic order, in particular comprising or consisting of carbon nanotubes.

Description

[0049] Subsequently, the present invention is now described with reference to some of the embodiments based on what was described above. There are thereby shown:

[0050] FIG. 1 the principle of the production according to the invention of a laser beam focal surface with which the machining according to the invention of a substrate material which is transparent at the laser wavelength (here: silicon substrate) can be effected because of the induced absorption in the region of the laser beam focal surface,

[0051] FIG. 2 the positioning according to the invention of the laser beam focal surface in the substrate in detail,

[0052] FIG. 3 different possibilities for machining the substrate by different positioning of the laser beam focal surface relative to the substrate,

[0053] FIG. 4 a first optical arrangement which can be used according to the invention,

[0054] FIG. 5 a second optical arrangement which can be used according to the invention,

[0055] FIG. 6 a third optical arrangement which can be used according to the invention,

[0056] FIG. 7 the separation, according to the invention, of a substrate along narrow channels 1k between functional regions 1-1, 1-2 . . . of the substrate surface.

[0057] FIGS. 1 and 2 illustrate the basic procedure of the machining method according to the invention. A laser beam 2a emitted by the laser 3, not shown, is radiated onto the optical arrangement 6 (of this only the plano-convex collimation lens 11, which focuses the beam bundles of the laser beam 2a onto the substrate 1, is shown in FIG. 1ccf. also the embodiments described in the subsequent FIGS. 4 to 6).

[0058] FIG. 1a shows (in plan view on the substrate plane or the x-y plane perpendicular to the direction of incident radiation z) what would happen without beam formation essential according to the invention by the subsequently also described elements 8, 7 and 13 of the optical arrangements according to the invention: from the radiated laser beam 2a, merely an expanded laser beam focal line would be produced, on the beam output side, over a defined expansion region along the beam direction (length direction l or incident radiation direction z). This is denoted here with the reference number 2b. The laser beam focal line 2b (the diameter of which can be defined in the substrate plane x-y, for example by the full width at half maximum intensity value in the beam cross-section) is surrounded, viewed perpendicular to the beam direction z or radially from the centre of the laser beam focal line 2b to the outside, by a region of reducing beam intensity which is subsequently termed halo region. The halo region, the radial expansion of which perpendicular to the beam direction z is provided here with the reference a, can be defined for example as that region in which the intensity in the laser beam 2a drops to one hundredth (or even e.g. to a thousandth) of the maximum intensity in the laser beam focal line 2b (or in the centre of the same). As is described more precisely with reference to FIG. 7, the residual intensity which is still present in the outer edge regions of the halo can lead, on functional surface regions of the substrate to be isolated, to undesired damage or destruction. One of the aims according to the invention is therefore the formation of a beam cross-section or of a halo shape in such a way that the mentioned damage or destruction can be prevented.

[0059] The beam formation effected for this purpose via elements 8, 7 and 13 which are essential to the invention (cf. FIGS. 4 to 6) is evident in FIGS. 1b and 1c: according to the invention, instead of a one-dimensional focal line 2b, a laser beam focal surface 2f which extends over a surface region is produced, which laser beam focal surface is expanded, viewed both along the beam direction z and viewed in precisely a first direction y, perpendicular to the beam direction z, however is not expanded in a second direction x which is both perpendicular to the first direction y and to the beam direction z (x, y, z=Cartesian coordinate system). Overlapping, at least in portions, this laser beam focal surface 2f of the laser radiation 2a, the planar substrate 1 to be machined is positioned in the beam path after the optical arrangement 6. The reference number 1a denotes the surface of the planar substrate orientated towards the optical arrangement 6 or the laser 3, the reference number 1b denotes the rear-side surface of the substrate 1 which is at a spacing and normally parallel hereto. The substrate thickness (perpendicular to surfaces 1a and 1b, i.e. measured to the substrate plane) is denoted with the reference d; cf. FIG. 2.

[0060] By means of the beam formation according to the invention, which is described subsequently in more detail, the previously circular halo region (FIG. 1a), viewed in cross-section in the substrate plane x-y, is reduced greatly in expansion in the second direction x (relative to the expansion perpendicular thereto and perpendicular to the beam direction z, i.e. relative to the expansion in the first direction y). In the centre of the halo region, instead of a rotationally-symmetrical focal line, a flattened focal surface is produced. The expansion of the halo region defined in the case of FIG. 1a in the x-direction is denoted with a.sub.x, the expansion of the halo region in the y-direction with a.sub.y. Advantageously, a.sub.x is smaller than a.sub.y by at least the factor 10, preferably by at least the factor 50, preferably by at least the factor 100.

[0061] As FIG. 2a shows, the substrate 1 here is orientated perpendicular to the beam longitudinal axis and hence to the focal surface 2f produced in the space by the optical arrangement of 6 behind the same (the substrate is perpendicular to the drawing plane) and, viewed along the beam direction z, is positioned relative to the focal surface 2f such that the focal surface 2f, viewed in beam direction z, begins in front of the surface 1a of the substrate and ends in front of the surface 1b of the substrate, i.e. still inside the substrate. The laser beam focal surface 2f which is expanded in both spatial directions z and y hence produces (in the case of suitable laser intensity in the region of the laser beam focal surface 2f which is ensured by focusing of the laser beam 2 on a portion of length l and of width b, i.e. by a surface focus of the surface 1.Math.b) in the overlapping region of the focal surface 2f with the substrate 1, i.e. in the material of the substrate which is covered by the focal surface 2f, a surface portion 2c which is expanded viewed along the beam longitudinal direction z and over the width direction y via which an induced absorption in the material of the substrate is produced, which induces crack formations in the material of the substrate along the portion 2c. The crack formations are thereby effected not only locally but over the entire surface of the expanded portion 2c of the induced absorption. The length of this portion 2c (i.e. ultimately the length of the overlapping of the laser beam focal surface 2b with the substrate 1 in z-direction) is provided here with the reference L. The width of the portion 2c corresponds to the width b of the focal surface 2f. The average expansion of the portion of the induced absorption (or of the regions in the material of the substrate 1 which are subjected to crack formation) in the direction perpendicular to the surface expansion, i.e. viewed in x-direction, is denoted here with the reference D. This average expansion D corresponds to the average expansion of the laser beam focal surface 2f in x-direction.

[0062] As FIG. 2a shows, substrate material which is transparent at the wavelength of the laser beam 2a is hence heated according to the invention by induced absorption in the region of the focal surface 2f.

[0063] FIG. 2b illustrates that the heated material ultimately expands so that a correspondingly induced stress leads to microcrack formation according to the invention, the stress being greatest at the surface 1a.

[0064] Subsequently, concrete optical arrangements 6 which can be used for the production of the focal surface 2f are described. All of the arrangements are thereby based on what has previously been described so that respectively identical references are used for components or features which are identical or correspond in their function. Subsequently, respectively only the differences are therefore described.

[0065] Since the separation surface leading ultimately to separation is or should, according to the invention, be of high quality (with respect to breaking resistance, geometric precision, roughness and avoidance of re-machining requirements), the individual (more precisely: produced by individual laser pulses) focal surfaces to be positioned along the separation line 5 (cf. FIG. 7) on the surface of the substrate are produced as described with the subsequent optical arrangements (the optical arrangement is subsequently also termed alternatively laser lens system). The roughness is thereby produced in particular from the expansion D of the focal surface in x-direction. In order to be able to achieve, at a given wavelength of the laser 3 (interaction with the material of the substrate 1), a small expansion D of for example 0.5 m to 2 m, in general specific requirements should be placed upon the numerical aperture of the laser lens system 6. These requirements are fulfilled by the subsequently described laser lens systems 6.

[0066] FIG. 3 shows, for all subsequently described optical arrangements 6, that the laser beam focal surface 2f can be positioned differently by suitable positioning and/or orientation of the optical arrangement 6 relative to the substrate 1 and also by suitable choice of the parameters of the optical arrangement 6; as the first line of FIG. 3 illustrates, the length l of the focal surface 2f can be adjusted such that it exceeds the substrate thickness d (here by the factor 2). If the substrate is hence placed, viewed in the beam direction z, centrally relative to the focal surface 2f, then an expanded portion of induced absorption 2c is produced over the entire substrate thickness d.

[0067] In the case shown in FIG. 3b, second line, a focal surface 2f of length l which corresponds approximately to the extension of the substrate d is produced. Since the substrate 1 is positioned relative to the surface 2f such that the surface 2f begins in a line in front of, i.e. outside the substrate, the length L of the expanded portion of induced absorption 2c (which extends here from the surface of the substrate up to a defined substrate depth, however not as far as the rear-side surface 1b) is smaller here than the length l of the focal surface 2f. The third line in FIG. 3b shows the case in which the substrate 1 is positioned, viewed along the beam direction z, partially before the beginning of the focal surface 2f so that, here also for the length l of the focal surface, 1>L applies (L=expansion of the portion of induced absorption 2c in the substrate 1). The focal surface hence begins in the interior of the substrate and extends over the rear-side surface 1b until outside the substrate. The fourth line in FIG. 3b finally shows the case in which the produced focal surface length l is smaller than the substrate thickness d so thatwith central positioning of the substrate relative to the focal surface, viewed in direction of incident radiationthe focal surface here begins close to the surface 1a in the interior of the substrate and ends close to the surface 1b in the interior of the substrate (l=0.75.Math.d).

[0068] According to the invention, it is thereby particularly advantageous to produce the focal surface positioning such that at least one of the surfaces 1a, 1b is covered by the focal surface, the portion of induced absorption 2c begins hence at at least one surface. In this way, almost ideal cuts can be achieved by avoiding ablation, burr- and particle formation on the surface.

[0069] The optical arrangements shown in FIGS. 4 and 5 are based on the basic idea of using firstly a lens system (element 9) with a non-spherical free surface, in order to form the focal surface 2f, which free surface is shaped such that a focal surface of a defined length l is formed. For this purpose, aspheres can be used as optical elements 9 of the optical arrangement 6. For example in FIGS. 4 and 5, a so-called conical prism which is also often termed axicon is used. An axicon is a special, conically ground lens which forms a point source on a line along the optical axis (or also transforms a laser beam annularly). The construction of such an axicon is basically known to the person skilled in the art; the cone angle here is for example 10. The axicon denoted here with the reference number 9 is orientated with its cone tip towards the direction of incident radiation (here: z-direction) and is centred on the beam centre. In the beam path after the free surface lens system, a further lens system is inserted (element 7 or 8) which reduces the expansion of the beam bundle of the laser radiation 2a in the second direction x, hence constricts the beam bundle in x-direction.

[0070] FIG. 4 shows a first example of a device according to the invention together with an optical arrangement 6 for forming a laser beam focal surface 2f which is expanded in the y-z plane (optical axis 6z of the arrangement 6 or of the device and direction of incident radiation in z-direction). In the beam path of the laser 3 (not shown), the laser beam of which is denoted with 2a, firstly an optical element with a non-spherical free surface which is formed to form a laser beam focal line expanded in the z-direction, viewed along the direction of incident radiation, is positioned. This optical element here is an axicon 9 with 5 cone angle which is positioned perpendicular to the beam direction and centred on the laser beam 2a. The cone tip of the axicon 9 thereby points towards the direction of incident radiation. In the beam direction at the spacing z1 from the axicon 9, a collimating optical element, here a plano-convex collimation lens 12, the planar surface of which points in beam direction z, is disposed such that the laser radiation incident on the plano-convex collimation lens 12 is collimated, i.e. orientated parallel. The spacing z1 of the plano-convex collimation lens 12 from the axicon 9 is chosen here with approx. 300 mm such that the laser beam bundle formed by the axicon 9 impinges annularly on the externally situated regions of the lens 12. Hence due to the lens 12 being at a radial spacing from the optical axis 6z of the device, an annular beam bundle 2r which extends parallel to the optical axis 6z is produced.

[0071] On the beam output side of the lens 12, a one-dimensional slit diaphragm 8 is positioned at the spacing z1 with z1>z1 (here: z1=1.3z1). The slit diaphragm 8 is orientated with its preferential direction (i.e. slit direction) in the first direction, i.e. the y-direction. The slit diaphragm (subsequently also termed alternatively slit diaphragm) 8 is thereby positioned such that the optical axis 6z, viewed in the second direction x, extends centrally between the two slit edges. The slit width is chosen such that it corresponds with the inner diameter 2i of the annular beam bundle 2r on the output side of the lens 12: as FIG. 4a shows, viewed along a straight line extending in x-direction and also through the optical axis 6z, the relevant components of the annular beam bundle 2r are occluded hence by the material of the edges of the slit diaphragm 8 which is not transparent for laser radiation of the used wavelength. Because of the one-dimensionality of the slit diaphragm 8, the relevant beam components in the beam bundle 2r reach the space however without being impeded on the output side of the slit diaphragm 8 (cf. FIG. 4b), viewed along a straight line extending in y-direction and through the optical axis 6z.

[0072] On the beam output side of the diaphragm 8 and at a spacing from the latter, a further plano-convex collimation lens 11, which serves here as focusing lens, is positioned centred about the optical axis 6z: said focusing lens focuses all of the beam components, not occluded by the diaphragm 8, of the previously annular beam bundle 2r into the first y and into the second x direction towards the planar substrate 1 which is disposed on the beam output side of this lens 11, perpendicular to the optical axis 6z, i.e. in the x-y plane. The lens 11 (the planar side of which is orientated towards the substrate 1) hence focuses the beam components, not occluded by the diaphragm 8, of the previously annular beam bundle 2r at a defined spacing from the lens 11 onto a two-dimensional laser beam focal surface 2f with a defined expansion in the z-direction (due to the effect of the axicon 9) and also with a defined expansion in the y-direction (due to the effect of the diaphragm 8); see in this respect the beam formation illustrated in FIG. 1b. The effective focal width of the lens 11 here is 25 mm so that the laser beam focal surface 2f is produced for instance at a spacing of 20 mm from the lens 11 (the substrate 1 is positioned there).

[0073] The optical properties of the optical arrangement 6 which comprises the rotationally-symmetrical elements 9, 12 and 11, positioned on the optical axis 6z, and also the diaphragm 8 (in particular the geometric forming of elements 9, 12, 8 and 11 and the positioning thereof relative to each other along the main beam axis 6z) can thereby be chosen such that the expansion 1 of the laser beam focal surface 2f in z-direction is twice as large as the thickness d of the substrate in z-direction. If the substrate is then positioned centred relative to the focal surface 2f (cf. FIG. 3, uppermost line), then formation of the expanded surface portion 2c of induced absorption is effected over the entire substrate thickness. The expansion 1 of the focal surface 2f in z-direction can be adjusted via the beam diameter on the axicon 9. The numerical aperture over the focal surface 2f can be adjusted via the spacing z1 of the axicon 9 from the lens 12 and also via the cone angle of the axicon 9. In this way, the entire laser energy can be concentrated in the focal surface 2f.

[0074] Instead of the plano-convex lenses 11, 12 shown in FIG. 4 (and also in FIGS. 5 and 6, see subsequently), also focusing meniscus lenses or other more highly corrected focusing lenses (aspheres, multilenses) can be used.

[0075] FIG. 5 shows a further example of a device according to the invention which is formed basically as the one shown in FIG. 4. Therefore, only the differences are subsequently described (the optical arrangement 6 comprises here the rotationally-symmetrical optical elements 9 and 11 centred on the axis 6z and also a cylindrical lens 7).

[0076] In the beam path 2a, the plano-convex, focusing cylindrical lens 7 is positioned on the beam output side of the axicon 9 instead of the lens 12 in FIG. 4, at a spacing of z2, viewed along the optical axis 6z. The spacing z2 of the lens 7 from the axicon can thereby be chosen as the spacing z1 in FIG. 4. The planar side of the cylindrical lens 7 which is positioned in the x-y plane lies on the side orientated away from the axicon 9. The preferential direction, i.e. the direction of the cylindrical axis of the cylindrical lens 7, is orientated parallel to the x-direction, and the cylindrical lens 7 is disposed centred, viewed with respect to the optical axis 6z. The spacing z2 and the expansions of the lens 7 are chosen such that the beam bundle, which is produced by the axicon 9 and diverges annularly on the input side of the cylindrical lens 7, impinges on the outer edge regions of the cylindrical lens 7. The shaping and positioning of the elements 9, 7 is thereby effected such that, viewed along a straight line extending in x-direction and through the optical axis 6z, the relevant beam components, impinging on the lens 7, of the annularly diverging beam bundle are not deflected (cf. FIG. 5a), whereas, viewed along a straight line extending in y-direction and through the optical axis 6z, the relevant beam components, impinging on the lens 7, of this beam bundle are collimated by the cylindrical lens 7, i.e. directed parallel (FIG. 5b).

[0077] At a defined spacing z2 behind the cylindrical lens 7, the focusing plano-convex collimation lens 11 is positioned in the beam path, centred about the optical axis 6z, as in the embodiment of FIG. 4. The spacing z2 is thereby chosen such that, viewed along the straight line extending in x-direction and through the optical axis 6z, the relevant beam components 2x pass by the lens 11 at the side without deflection, whereas, viewed along the straight line extending in y-direction and through the optical axis 6z, beam components 2y are intercepted completely by the lens 11, are deflected and the relevant ones are focused 2f on the substrate 1 positioned on the beam output side of the lens 11.

[0078] Also due to the combination of the rotationally-symmetrical axicon 9 with the cylindrical lens 7 and also the subsequent focusing by the rotationally-symmetrical plano-convex collimation lens 11, the beam formation from FIG. 1b can hence be produced. The expansion of the laser beam focal surface 2f in y-direction and in z-direction is thereby adjusted as follows: [0079] displacement spacing of workpiece 1 relative to focusing lens 11 [0080] changing the focal length of the focusing lens 11 [0081] illumination of axicon 9.

[0082] A further example of a device according to the invention for producing an expanded focal surface 2f is shown in FIG. 6.

[0083] In the beam path 2a of the laser 3 (not shown), firstly a non-rotationally-symmetrical optical element 13 provided with a preferential direction (here: x-direction) is positioned. This is configured as a planar element on the beam output side which deflects on the beam input side and is centred on the optical axis 6z. The planar side therefore points towards the substrate 1. The deflecting side situated opposite the planar side (i.e. pointing towards the laser 3) is configured as a pointed-roof-shaped double wedge, the central backbone of which extends along the x-direction and though the optical axis 6z. The element 13 is subsequently termed also double wedge for simplification.

[0084] As FIG. 6a shows, the relevant beam components of the beam bundle 2a, viewed along a straight line extending in x-direction and also through the optical axis 6z, is merely transmitted through the double wedge 13 but not deflected. Perpendicular thereto, i.e. viewed along a straight line extending in y-direction and through the optical axis 6z, the relevant partial beam bundles, situated on both sides of the optical axis 6z, are deflected by the double wedge structure towards each other, per se respectively parallel and also viewed as a whole (FIG. 6b): all of the beam components s1 of the beam bundle 2a radiated onto the double wedge 13, which are situated in the half space y1 above a plane extending parallel to the x-z plane and through the optical axis 6z, are hence deflected parallel and towards the oppositely situated half space y2 (extending below said plane parallel to the x-z plane and through the optical axis 6z). Conversely, all of the beam components s2 of the beam bundle 2a which are incident on the wedge 13 below said plane are deflected out of the half space y2 parallel and towards the half space y1.

[0085] Viewed in beam direction at a spacing from the wedge 13 (behind the intersection point of the two beam components s1 and s2), a cylindrical lens 7 is positioned, as in the example of FIG. 5: viewed in x-direction, no deflection of the beam components s1 and s2 is undertaken by said cylindrical lens and, viewed in y-direction, the two beam components s1 and s2 are deflected by the latter towards the axis 6z and collimated (both beam components s1 and s2 extend on the beam output side of the lens 7 parallel to the axis 6z).

[0086] Viewed in beam direction, the plano-convex collimation lens 11 is positioned behind the cylindrical lens 7 i.e. at a defined spacing z3 on the beam output side of the double wedge 13 (as in the examples from FIGS. 4 and 5). Said lens is likewise configured as described in the previous examples and is disposed centred on the axis 6z. The spacing z3 is chosen such that the two beam bundles s1 and s2 which diverge, viewed in the direction of incident radiation (z-direction) parallel away from each other and directed parallel after the lens 7, impinge, viewed in y-direction, on externally situated edge regions of the collimation lens 11. The lens 11 hence focuses the two beam bundles s1 and s2 on a laser beam focal surface 2f expanded in the y-z plane, in which, as described in the other embodiments, the substrate 1 to be machined is positioned. Also with an optical arrangement 6 comprising the elements 11, 7 and 13, the beam formation according to FIG. 1b can hence be produced.

[0087] FIG. 7 illustrates, in plan view on the substrate plane (x-y plane), a possible machining according to the invention of a semiconductor substrate 1 already provided with functional structures 1-1, 1-2, 1-3 and 1-4. The functional structures 1-1, . . . disposed here in quadrant form must not be subjected to any laser irradiation during isolation (the halo region H of the laser beam shown in FIGS. 1a and 1b and indicated with the references a or a.sub.x, a.sub.y must hence also be prevented in particular from covering the mentioned functional regions during isolation). Laser radiation must be effected hence exclusively on the channel-shaped structures 1k which extend between the functional regions and have no functional structures at all and therefore may be laser-irradiated.

[0088] As FIG. 7 indicates, the operation can take place with a laser beam focal surface 2f produced according to FIG. 1b and adapted to the width of the channels 1: the feed direction of the laser beam 2a over the substrate 1 is set precisely parallel to a channel longitudinal axis (here: of the vertically extending channel 1k orientated in y-direction). At the same time, the laser beam focal surface 2f is produced perpendicular to the substrate plane (x-y plane) and parallel to the feed direction. Hence, for example in the case shown in FIG. 4 (the deflection lens system which is known per se to the person skilled in the art, and for example producible on the basis of galvanometer scanners is not shown in the embodiments of FIGS. 4 to 7), a feed of the laser beam 2a is effected along a line 5 (which corresponds to a desired separation line, along which the substrate is intended to be separated) such that a large number 2c-1, 2c-2, . . . of expanded surface portions 2c of induced absorption (cf. FIG. 3) is produced. Each portion of induced absorption 2c-1, . . . thereby corresponds to a defect zone produced by a single laser pulse of the pulsed laser in the substrate material along the separation line 5. Since the individual defect zones 2c-1, . . . are produced in the centre of the channel 1k and these or the laser beam focal surfaces 2f extend respectively parallel to the edges of the functional structures 1-1, . . . , it can be ensured by suitable choice of the optical parameters that the halo zone H surrounding the focal surface 2f in x-direction has a substantially smaller diameter than in y-direction. The expansion of the halo zone H in x-direction is thereby chosen such that it is smaller than the channel width of the channel 1k.

[0089] The repetition rate of the laser pulses is coordinated to the feed speed of the laser such that the average spacing A of immediately adjacent expanded surface portions 2c of induced absorption, i.e. produced by temporally directly successive laser pulses, is slightly (e.g. by the factor larger than the width b of the laser beam focal surface 2f in feed direction or y-direction. Hence, without intensity overlapping, introduction of a large number of defect structures 2c which are placed immediately in a row along the channel axis 1k or the desired separation line 5 is effected and hence efficient separation of the substrate 1 along such channels 1k. The substrate residues which still remain between two adjacent defect structures 2c and are detectable here as gaps can readily effect crack formations due to the effect of mechanical forces and/or thermal stresses in order to separate finally from each other the substrate fragments produced on both sides of the separation line 5.