BACKGROUND

Semiconductor materials are grown, for example, in large cylinders of crystalline material, referred to as ingots, while industrial processes often require wafer material with differences in thickness and surface quality. Wafering of brittle semiconductor materials is frequently carried out with diamond-based or slurry-based wire sawing processes. These sawing processes lead not only to cutting-gap loss of potentially useful material, but also to surface roughness and, below the surface, to damage to the crystal. These aspects of wafering by sawing processes necessitate polishing and grinding steps in the wafer manufacturing process, leading to additional damage and process costs.

To address these problems in the conventional wafering and thinning of semiconductors, so-called kerf-less technologies have been developed which promise a reduction in the cutting-gap losses, if not, indeed, the elimination of such losses, and also of damage below the surface and of grinding steps. So-called spalling processes, especially those situated externally, use stresses—often temperature-induced—to separate crystalline materials along crystal planes with well-defined thickness. Spalling may take place using nickel-chromium alloys, silver-aluminum paste, epoxy resin, aluminum, and nickel. Kerf-less wafering technologies have the potential for drastic reduction in damage within semiconductor fabrication processes. Stress-based separation methods, such as spalling (or splitting), use externally applied stresses in order to separate crystalline materials along their crystal planes with well-defined thickness. After the spalling, however, substrates exhibit what are called Wallner lines, which originate from crack propagation within the crystal.

Spalling was made possible using differences in the coefficients of thermal expansion between a brittle material and a polymer adhering on the surface of the material. Cooling of the conjoined materials below the glass transition temperature of the polymer induces stresses which lead to the material separating along a cracking plane. An advantage of this specific method in contrast to other modes of spalling is that the process of cooling does not give rise to any increased diffusion of unwanted chemical components through the material, in contrast to high-temperature methods which are also used for spalling.

Spalling processes, however, tend to be limited in their control over the wafer thickness attained, and tuning of the vertical locus of crack propagation is complicated. With spalling, furthermore, very prominent patterns of Wallner lines develop on the surface. This pattern is made up of streaklike grooves and elevations which originate from crack propagation within the semiconductor material and enable diversion of the cracking dynamic in the substrate. The crack typically begins at a certain point on the margin, and then propagates rapidly from the edge of the substrate. Wallner lines on conventional spalling surfaces greatly increase the resulting surface roughness, often to a point necessitating additional polishing or grinding steps before further processing and the production of circuits on the substrate.

The article “Large Area 4H SiC Products for power Electronic Devices”, Material Science Forum, ISSN: 1662-9752, Vol. 858, pp 11-14, describes dislocation densities in solid-state SiC bodies.

The article “Exploration of Bulk and Epitaxy Defects in 4H-SiC Using Large Scale Optical Characterization”, Material Science Forum, ISSN: 1662-9752, Vol. 897, pp 226-229 likewise describes dislocation densities in 4H-SiC.

The applicant's publication WO2016/083610 discloses laser-assisted separation of solid-state layers from donor substrates, more particularly examples of generation of modifications.

The publication DE102016201780A1 discloses a method wherein laser beams are used to generate cracks for the purpose of detaching a wafer in an ingot. The detachment takes place in dependence on an exposure to ultrasound. This process is deleterious because, owing to its operating parameters, it is very determinate and, moreover, necessitates a high level of rework, with the rework generating high costs and losses of material.

SUMMARY

The object of the present invention is to provide an improved method for modifying donor substrates and/or solid-state bodies, and an improved method for dividing modified donor substrates and/or solid-state bodies.

The aforesaid object is achieved in accordance with the invention by a method for generating microcracks in a donor substrate. This method of the invention preferably comprises at least the steps specified hereinafter: providing the donor substrate. The donor substrate may be referred to alternatively here as solid-state body or boule or thick wafer or ingot. The donor substrate preferably comprises or consists of a crystalline material. More preferably the donor substrate comprises or consists at least predominantly of or consists entirely of a single crystal. The donor substrate comprises crystal lattice planes, where at least one extent direction of the crystal lattice planes is oriented with an inclination relative to a planar major surface. Consequently, at least the majority and preferably at least 95% of the crystal lattice planes, and with particular preference all the crystal lattice planes, are oriented with an inclination relative to a planar major surface, at least in one extent direction of the crystal lattice planes. The major surface forms one boundary of the donor substrate in the longitudinal direction of the donor substrate. The major surface is formed in this case by the donor substrate. Generating modifications in the interior of the specific donor substrate, to form a detachment region in the interior of the donor substrate, by means of a laser facility. The modifications are more preferably generated by means of laser beams of the laser facility, where the laser beams with particular preference penetrate via the major surface into the donor substrate. The detachment region is preferably planar or substantially planar in configuration. The donor substrate is bounded, on the side opposite the major surface, by a further boundary surface. The detachment region is preferably generated or formed closer to the major surface than to the other boundary surface. The major surface and the further boundary surface are preferably oriented substantially or exactly parallel to one another.

This solution is advantageous because the modifications can be introduced into the donor substrate with great precision and in a short time.

The modifications in the context of the present invention may with particular preference be regarded as, and referred to as, local alterations in material.

Further preferred embodiments of the present invention are subjects of the subsequent parts of the description.

According to a further preferred embodiment of the present invention, the modifications are generated along a line, more particularly dotted line/s or line/s generated from dots, or two or more lines, more particularly parallel lines, to form modification lines, or where the modifications are generated to form one modification line or two or more modification lines, more particularly parallel modification lines, more particularly dotted line/s or line/s generated from dots. According to a further preferred embodiment of the present invention, the individual modifications generate compressive stresses in the donor substrate, whereby the donor substrate, by virtue of the modification line or the modification lines, undergoes cracking at least predominantly in the extent direction of the crystal lattice planes that is preferably inclined relative to the major surface.

According to a further preferred embodiment of the present invention, the modifications are generated per modification line in such a way that, in the case of more than M crystal lattice dislocations (per cm2) in the donor substrate, the crack propagates less than 50 μm per modification line, in the extent direction of the crystal lattice planes that is inclined relative to the major surface, from the next point of the modification line or of the generation, or the modifications are generated per modification line in such a way that, in the case of fewer than O crystal lattice dislocations (per cm2) in the donor substrate, the crack propagates more than 100 μm, more particularly more than 200 μm or more than 250 μm or more than 300 μm or more than 350 μm, per modification line in the extent direction of the crystal lattice planes that is inclined relative to the major surface, or the modifications are generated per modification line in such a way that in the case of more than O crystal lattice dislocations (per cm2) in the donor substrate and fewer than M crystal lattice mismatches (per cm2) in the donor substrate, the crack per modification line propagates between 50 μm and 100 μm, more particularly between 50 μm and 100 μm or 50 μm and 200 μm or 50 μm and 250 μm or 50 μm and 300 μm or 50 μm and 350 μm, in the extent direction of the crystal lattice planes that is inclined relative to the major surface. Here, with particular preference, O<M.

This solution is advantageous since it provides a treatment protocol for different qualities of material. Through this treatment protocol it is possible to divide donor substrates of different qualities reliably and with minor losses of material. The treatment protocol additionally or alternatively defines how longer cracks or longer subcritical cracks are to be provided in cases of increasing quality of material, i.e., of decreasing dislocation density. Conversely, where the quality of material is lower, i.e., where there is increasing dislocation density or crystal lattice dislocation density, shorter cracks are to be provided.

The cracks propagating around the modifications may also be referred here to as subcritical cracks. Subcritical cracks are cracks which are connected together only by a later main crack, and which, by connecting, divide the donor substrate, dividing it more particularly into a solid-state layer and the remaining solid-state body. Subcritical cracks may alternatively be referred to here also as microcracks.

The dislocations, or crystal lattice dislocations, can be visualized and counted by way of etching methods on a surface. The dislocation density should be understood preferably as the mean dislocation density over the face as a whole—the type of dislocations is more preferably “threading dislocation”, referring to “penetrative dislocations” which “penetrate through a face” or extend to the surface (they can be seen, for example, by carrying out partial etching there). What this describes, accordingly, is a classification according to direction of dislocation in relation to the surface. Additionally or alternatively, the dislocations may be described or defined by the dislocation type (edge and/or screw dislocation).

An important mechanism of action is that the microcracks or subcritical cracks generated by the laser modification produce stresses in the interior of the solid-state body. These stresses cause the main crack to advance advantageously on the basis of the preliminary damage. The primary driver of this effect is that the vertical component of crack propagation is suppressed by the compressive stresses that have been introduced. The size of the stresses introduced here may be very heavily dependent on the length of the microcracks. For example, two short microcracks have a much lower stress than a single microcrack with a length corresponding to the sum of the lengths of the two short microcracks. The effect this produces is that the internal stress of numerous short cracks that is introduced is less than the stress of a few long cracks, even if the overall length of crack is the same in both cases. This can likewise be observed, for example, when the complete solid-state face experiences preliminary damage. In accordance with the invention, therefore, in crystals with a few dislocations and, consequently, a very high strength, preferably more internal stresses are generated in the solid-state body. This may be achieved in particular by microcracks which make up for their lack of numbers by being longer.

The stress needed in order to cause a microcrack to grow decreases in line with the size of the microcrack.

According to one preferred embodiment of the present invention, O is 250 cm−2, more particularly 500 cm−2or 1000 cm−2or 2000 cm−2or 3000 cm−2or 4500 cm−2. Additionally or alternatively, M is preferably 5000 cm−2, more particularly 7500 cm−2or 10 000 cm−2or 12 500 cm−2. This embodiment is advantageous because, depending on the dislocations in a donor substrate material, especially a specific boule or ingot, more particularly of a defined batch or manufacture, a defined crack propagation results or is generated.

According to a further preferred embodiment of the present invention, additionally or alternatively, the crack propagates in the extent direction of the crystal lattice planes, that is inclined relative to the major surface, as a function of the size of the individual modifications. Preferably at least the majority of the modifications are generated with a volume of less than A, or the modifications of one modification line are generated with a mean modification volume of less than A, if the number of crystal lattice dislocations (per cm2) is greater than M, or at least the majority of the modifications are generated with a volume of greater than C, or the modifications of one modification line are generated with a mean modification volume of greater than C, if the number of crystal lattice dislocations (per cm2) is less than O, or at least the majority of the modifications are generated with a volume of between A and C, if the number of crystal lattice dislocations (per cm2) is between M and O. Here, with particular preference, A<C.

The size of a modification is preferably the volume of the modification. In the case of SiC, the blackening of the material, particularly the fractions of material that are altered by a physical transformation, represents the size or the volume of the modification.

This solution is advantageous since it provides a treatment protocol for different qualities of materials. Through this treatment protocol it is possible for donor substrates of different qualities to be divided reliably and with small losses of material. The treatment protocol here defines, additionally or alternatively, the provision of greater modification volumes as the quality of material goes up, i.e., as the dislocation density decreases. In the case of lower quality of material, conversely, i.e., increasing dislocation density, smaller modification volumes are to be provided.

According to one further preferred embodiment of the present invention, the volume A or the mean volume A is less than or equal to 100 μm3, more particularly less than or equal to 75 μm3or less than or equal to 50 μm3or less than or equal to 25 μm3, and/or the volume C is greater than or equal to 250 μm3, more particularly greater than or equal to 500 μm3or greater than or equal to 750 μm3or greater than or equal to 1000 μm3. This embodiment is advantageous because, depending on the dislocations in a donor substrate material, more particularly a specific boule or ingot, more particularly of a defined batch or manufacture, a defined modification volume or mean modification volume results or is generated.

In accordance with a further preferred embodiment of the present invention, additionally or alternatively, the crack propagates along the extent direction of the crystal lattice planes that is inclined relative to the major surface, in dependence on the laser energy provided for generating the respective modifications. Preferably at least the majority of the modifications are generated with a laser energy of less than D, or the modifications of one modification line are generated with a mean laser energy of less than D, if the number of crystal lattice dislocations (per cm2) is greater than M, or at least the majority of the modifications are generated with a laser energy of greater than E, or the modifications of one modification line are generated with a mean laser energy of greater than E, if the number of crystal lattice dislocations (per cm2) is less than O, or at least the majority of modifications are generated with a laser energy between D and E if the number of crystal lattice dislocations (per cm2) is between M and O. Here, with particular preference, D<E.

This solution is advantageous because it provides a treatment protocol for different qualities of materials. Through this treatment protocol it is possible for donor substrates of different qualities to be divided reliably and with small losses of material. The treatment protocol additionally or alternatively defines the provision of a greater laser energy, to be used in particular for the modification, in the case of increasing quality of material, i.e., decreasing dislocation density. Conversely, in the case of lower quality of material, i.e., increasing dislocation density, a lower laser energy is to be provided, and more particularly used for generating the modification.

According to a further preferred embodiment of the present invention, the laser energy per modification D or the mean laser energy per modification D is less than or equal to 10 μJ, more particularly less than or equal to 5 μJ or less than or equal to 3 μJ or less than or equal to 2 μJ, and/or the laser energy E or the mean laser energy E is greater than or equal to 30 μJ, more particularly greater than or equal to 50 μJ or greater than or equal to 100 μJ or greater than or equal to 200 μJ. This embodiment is advantageous because, depending on the dislocations in a donor substrate material, more particularly a specific boule or ingot, more particularly of a defined batch or manufacture, a defined laser energy or mean laser energy results or is generated.

According to a further preferred embodiment of the present invention, additionally or alternatively, a multiplicity of modification lines parallel to one another are generated, where the distance between immediately adjacent modification lines, at least in the case of the majority of the modification lines, is less than J in the case of more than M crystal lattice dislocations (per cm2) or is greater than K in the case of fewer than O crystal lattice dislocations (per cm2), or is between J and K in the case of fewer than M crystal lattice dislocations (per cm2) and more than O crystal lattice dislocations (per cm2). Here, with particular preference, J<K.

This solution is advantageous since it provides a treatment protocol for different qualities of material. Through this treatment protocol, donor substrates of different qualities can be divided reliably and with small losses of material. The treatment protocol here additionally or alternatively defines the provision of larger distances between each pair of immediately adjacent modification lines in the case of increasing quality of material, i.e., decreasing dislocation density. Conversely, in the case of lower quality of material, i.e., increasing dislocation density, smaller distances are to be provided between respective pairs of immediately adjacent modification lines.

According to a further preferred embodiment of the present invention, the distance J is less than or equal to 50 μm, more particularly less than or equal to 40 μm or less than or equal to 25 μm or less than or equal to 10 μm, and/or the distance K is greater than or equal to 80 μm, more particularly greater than or equal to 100 μm or greater than or equal to 150 μm or greater than or equal to 250 μm. This embodiment is advantageous because, depending on the dislocations in a donor substrate material, more particularly a specific boule or ingot, more particularly of a defined batch or manufacture, a defined distance between respective pairs of immediately adjacent modification lines results or is generated.

A further important mechanism of action is that the laser modifications hinder the solid-state body from cracking open. This effect may arise because the modifications are preferably situated exactly where the solid-state body or donor substrate is intended to slip along the microcracks. The running dislocations of the fracture may indeed propagate very well along these microcracks, but at some point inevitably encounter the modification which caused the microcracks to develop. Within the modification, obviously, the crystal structure is destroyed, and so the fracture is unable to slip further along the existing fracture plane. Instead, the fracture has to go around the modification. This may happen, for example, by the fracture running on along a surface of the modification, in the vicinity of the interface between the material with phase transformation and the “normal” crystal. In this case, the situation arises that the material of the phase transformation becomes associated primarily with one particular side of the fracture face; more particularly, the material of the phase transformation is located primarily on the boule side and less on the wafer side. Going around the laser modification requires extra energy, and it is therefore advantageous to generate the laser modifications themselves at further distance from one another, particularly if the material possesses a high strength as a result of few dislocations. This may occur, for example, by the line distance of the modifications being adapted to the quality of crystal. More particularly, a greater distance of the modifications in the case of lower dislocation densities.

Another possibility is that of forming fewer modifications within a line, in other words increasing the distance between two modifications along a working section. Advantageous here are distances of >10 μm and/or >15 μm, and/or >20 μm and/or >30 μm, but more particularly preferably <500 μm or preferably <300 μm.

According to another preferred embodiment of the present invention, the modifications are generated on a generation plane, more particularly on at least one generation plane or on exactly one generation plane, and the crystal lattice planes of the donor substrate are preferably oriented with an inclination relative to the generation plane, more particularly at an angle of between 0.1° and 10°, more particularly between 1° and 6°, more particularly between 2° and 4°, or at an angle of 2° or 4°, where preferably a crystal lattice plane normal is inclined in a first direction relative to a major-surface normal. The modification lines here are preferably oriented with an inclination relative to an intersecting line which arises at the intersection between the generation plane and the crystal lattice plane. The angle in this case is preferably unequal to 0° and unequal to 90° and unequal to 180° and unequal to 270° and unequal to 360°. This embodiment is advantageous because preferably each modification intersects a different collective of crystal lattice planes. The effect of this is that the probability increases, or is very high, that the cracks propagate only with a predefined length.

According to a further preferred embodiment of the present invention, additionally or alternatively, the modification lines are oriented with an inclination relative to the intersecting line arising at the interface between the generation plane and the crystal lattice plane, at an angle smaller than the angle P or at an angle greater than the angle Q or at an angle between P and Q. The modification lines here are generated preferably at an angle smaller than the angle P if the number of crystal lattice dislocations (per cm2) is greater than M, or the modification lines are generated at an angle greater than the angle Q if the number of crystal lattice dislocations (per cm2) is less than O, or the modification lines are generated at an angle between the angles P and Q if the number of crystal lattice dislocations (per cm2) is between M and O.

This solution is advantageous since it provides a treatment protocol for different qualities of material. Through this treatment protocol, donor substrates of different qualities can be divided reliably and with low losses of material. This treatment protocol additionally or alternatively defines the generation of the modification lines preferably at an angle greater than the angle Q relative to the intersecting line, in the case of increasing quality of material, i.e., decreasing dislocation density. Conversely, in the case of lower quality of material, i.e., increasing dislocation density, the modification lines are generated preferably at an angle smaller than the angle P relative to the intersecting line.

According to a further preferred embodiment of the present invention, the angle P is less than or equal to 10°, more particularly less than or equal to 8° or less than or equal to 6° or less than or equal to 4°, and/or the angle Q is greater than or equal to 10°, more particularly greater than or equal to 15° or greater than or equal to 20° or greater than or equal to 25°. This embodiment is advantageous because, depending on the dislocations in a donor substrate material, more particularly a specific boule or ingot, more particularly of a defined batch or manufacture, a defined angle between the modification lines and the intersecting line results or is generated.

According to a further preferred embodiment of the present invention, the donor substrate comprises or consists of SiC or the donor substrate comprises or consists of sapphire (Al2O3). This solution is advantageous since both materials are very hard and therefore low-loss separation methods on such materials are the most efficient, owing to the hardness of material.

The modifications which belong to the same linear architecture or writing line or points line or modification line and are generated in succession are preferably generated at a distance from one another which is defined by the function (d−x)/d<−0.31, more particularly <−0.4, where preferably x>d.

At the level of the generation plane or modification plane, or preferably parallel thereto, the subcritical cracks can propagate and so form a detachment plane. The detachment plane may therefore be nearer than the generation plane to the first surface, or the detachment plane may be further than the generation plane from the first surface. Preferably the entirety or the majority of the modifications are generated preferably on one plane, specifically the generation plane.

“In the region” of the modification may be understood here to mean that the solid-state body or the donor substrate undergoes cracking in the modified or transformed-material fraction and in the solid-state fraction adjacent thereto. It is, however, likewise possible for the modified region not to undergo cracking, and for the crack instead to crack in the longitudinal direction of the solid-state body, above or below the modification/s. If the solid-state body cracks above or below the modification/s, then the distance between the crack (more particularly the subcritical crack) and the generation plane is preferably less than 20 μm, more particularly less than 15 μm or less than 10 μm or less than 5 μm or less than 4 μm or less than 3 μm or less than 2 μm or less than 1 μm.

This solution is advantageous because the stated condition (d−x)/d<−0.31 defines the focal points of the successively generated modifications of the same linear architecture as being at a distance from one another such that the transformation of material generated beforehand produces little or no effects, more particularly little or no increase in absorption, in the case of the following transformation of material. This is advantageous because the modifications consequently can be generated with great precision, thereby allowing more effective control over the tendency of subcritical cracks or microcracks to propagate more strongly.

According to a further preferred embodiment of the present invention, the laser radiation is of defined polarization. The direction of polarization of the laser radiation here is oriented preferably at a defined angle, more particularly a fixed angle of 0° or 90°, or in a defined angle range, more particularly −20° to 20° or −10° to 10° or −5° to 5° or −1° to 1° or 70° to 110° or 80° to 100° or 85° to 95° or 89° to 91°, relative to the crystal axis of the solid-state body. Alternatively, the direction of longitudinal extent of the modifications generated by means of the laser beams in the interior of the solid-state body may be oriented at a defined angle, more particularly a fixed angle of 0° or 90°, or in a defined angle range, more particularly −20° to 20° or −10° to 10° or −5° to 5° or −1° to 1° or 70° to 110° or 80° to 100° or 85° to 95° or 89° to 91°, relative to an intersecting line, more particularly virtual intersecting plane, that arises at the intersection between the generation plane and the crystal lattice plane.

In the case of polarization of the laser parallel to the major flat, it has been observed, on laser machining into the C side of the SiC substrate, that a laser energy greater by around 50% is required in order to form comparable damage patterns, relative to a laser polarization perpendicular to the major flat in this configuration. If circularly polarized light is used, then the laser power transmitted in a polarizer, in contrast to pass polarization, is reduced by ⅓. This means that for circularly polarized light, the laser energy utilized relative to the linear ideal polarization must probably be increased by an amount of up to 50%. This difference, however, may also be smaller, in particular owing to the nonlinearity of the process and to the active cross sections for multiphoton effects in SiC for circularly polarized light. A value between the threshold energies for ideal laser polarization and laser polarization rotated by 90° in relation thereto is also possible, accordingly, since both polarizations occur transiently in the course of the rotating laser polarization. Multiphoton effects, however, usually have better active cross sections for linearly polarized light, and so even greater energies have to be expended for perfectly circularly polarized light.

According to a further preferred embodiment of the present invention, modifications which belong to the same linear architecture and are generated in succession are generated at a distance from one another which is defined by the function (d−x)/d<0, more particularly <−0.3 or <−0.31 or <−0.4 or <−0.45 or <−0.5 or <−0.55 or <−0.6 or <−0.65 or <−0.7 or <−0.75, where here, preferably x>d. The statement <−0.31 here means less than −0.31, the numbers in question here being numbers greater in terms of amount, such as −0.5, for example. In contrast, in terms of amount, −0.1 is smaller than the amount of −0.31, and therefore −0.1 would not be encompassed.

According to a further preferred embodiment of the present invention, the successively generated modifications of one linear architecture are separated from one another by unmodified solid-state material. In other words, the phase change brought about by laser modification preferably does not take place between individual modifications. The individual modifications of one linear architecture are therefore preferably separated from one another spatially to an extent such that a material transformation that has taken place beforehand has no effect on the absorption in the case of a modification generated immediately thereafter for the same linear architecture. This therefore results in a points pattern, or the linear architecture is formed by a points pattern.

According to a further preferred embodiment of the present invention, a first part of the modifications generated in the solid-state body are generated during a first relative movement of the solid-state body relative to an optics of the laser, and a second part are generated during a second relative movement of the solid-state body relative to the optics of the laser. The first relative movement here is preferably a straight-line movement in a first direction, and the second relative movement is preferably a straight-line movement in a second direction, with the traversal pathways preferably being parallel to one another. With preference the entire travel pathway forms a meander shape, or a travel movement produced by an X-Y table.

At least the setting of one laser parameter, particularly the polarization, according to a further preferred embodiment, during the first relative movement deviates from the setting during the second relative movement, with the first relative movement corresponding to a linear movement in a first direction and the second relative movement corresponding to a linear movement in a second direction, where the first direction and the second direction are oriented with parallel offset relative to one another.

The laser radiation according to one further preferred embodiment of the present invention is linearly polarized or elliptically polarized or circularly polarized. This embodiment is advantageous because a defined polarization of the laser radiation allows modifications to be generated that enable very short crack propagation—more particularly shorter than 100 μm—of the subcritical cracks.

According to another preferred embodiment of the present invention, the method likewise preferably comprises the step of altering a beam property of the laser beams before penetration of the solid-state body, where the beam property is the intensity distribution in the focus, where the alteration or adaptation of the beam property is produced by at least one or exactly one spatial light modulator and/or by at least or exactly one DOE, with the spatial light modulator and/or the DOE being disposed in the beam path of the laser radiation between the solid-state body and the radiation source.

Arranged in the path of the laser radiation, according to another preferred embodiment of the present invention, before the penetration of the laser radiation into the donor substrate or solid-state body, is a diffractive optical element (DOE). The DOE divides up the laser radiation over a plurality of light paths in order to generate a plurality of focusses. The DOE preferably produces, over a length of 200 μm, an image field curvature which is less than or equal to 50 μm, more particularly less than or equal to 30 μm or less than or equal to 10 μm or less than or equal to 5 μm or less than or equal to 3 μm, with the DOE generating at least 2 and preferably at least or exactly 3 or at least or exactly 4 or at least or exactly 5 or at least or exactly or up to 10 or at least or exactly or up to 20 or at least or exactly or up to 50 or up to 100 focusses at the same time for altering the physical properties of the donor substrate. This embodiment is advantageous because a significant acceleration of the process can be achieved.

In the context of the present invention, therefore, it has been recognized that high powers are divided up with diffractive optical elements (DOEs) over a plurality of foci in the focal plane. DOEs exhibit interference phenomena even before the focal plane; it has been recognized that interferences at the surface may, in front of the focal plane, generate local intensity maxima which can lead to damage to the surface and may result in reduced transmissivity for laser radiation for depth working. It has been recognized, moreover, that some materials (e.g., SiC) have local differences in refractive index and in other physical properties (e.g., absorption, transmission, scattering), as a result, for example, of the doping of material (frequent occurrence: doping spot). It has been recognized, moreover, that, in dependence on the surface roughness of the material at the surface of laser incoupling, the wavefront of the laser in the depth of the material may be significantly impaired, causing the focus to have reduced intensity (lower multiphoton transition probability), which would in turn lead to higher intensities, with problems as stated above.

Irradiating the laser beams onto or into the solid-state body or donor substrate below the Brewster angle is complicated and may be challenging, since the different beam components travel pathways of differing length in the higher-index medium. The focus, accordingly, must be adapted by higher energy and/or by beam shaping. Beam shaping here is accomplished preferably, for example, by way of one or more diffractive optical elements (DOEs), which compensates this difference dependently by way of the laser beam profile. The Brewster angle is relatively large, which with a high numerical aperture imposes requirements on the optics and its dimensions and also working distance. Nevertheless, this solution is advantageous, since reduced reflections at the surface also contribute to reduced surface damage, since the light intensity is coupled into the material more effectively. In the sense of this invention, laser beams can be irradiated at the Brewster angle or substantially at the Brewster angle in all other embodiments disclosed in this specification, as well. With regard to Brewster angle incoupling, reference is hereby made to the document “Optical Properties of Spin-Coated TiO2 Antireflection Films on Textured Single-Crystalline Silicon Substrates” (Hindawi Publishing Corporation International Journal of Photoenergy, Volume 2015, Article ID 147836, 8 pages, http://dx.doi.org/10.1155/2015/147836). This document is incorporated by reference in its entirety to become subject matter of the present patent application. The document stated and incorporated above discloses, in particular, calculations of the ideal irradiation angle for various materials and therefore refractive indices. The energy of the laser or of the laser-applying facility is adapted not so much as a function of the material, but, instead, of the possible transmission under a certain angle. If, therefore, the ideal transmission is 93%, for example, then these losses must be taken into account relative to experiments with perpendicular irradiation and losses in that case, for example of 17%, and the laser power must be adapted accordingly.

An example: 83% transmission perpendicularly relative to 93% at an angle means that, to achieve the same energy at depth, only 89% of the laser power used in the case of perpendicular irradiation is required (0.83/0.93=0.89). In the sense of the invention, therefore, the part of the oblique irradiation preferably serves to reduce light loss caused by surface reflection, and to bring more light into the depth. One possible downstream problem to which this may give rise in certain scenarios is that the focus in the depth may acquire a “skewed” profile, with the consequence that the intensities achieved—the key variable for multiphoton working—are again lower, and hence possibly lower even than in the case of perpendicular irradiation, where all of the beam components travel the same optical pathway in the material. This may then take place preferably by a diffractive optical element or by two or more diffractive elements or by a continuous wedge or two or more continuous wedges—and/or other optical elements—in the beam path, which compensate these additional pathways and/or the influence on the individual beams—especially different spherical aberrations over the beam profile. These DOEs can be calculated numerically using suitable software solutions (e.g., Virtuallab from Lighttrans, Jena) and then fabricated or provided.

The modifications, according to a further preferred embodiment of the present invention, are generated preferably by means of a multiphoton excitation, more particularly a two-photon excitation.

The method according to the present invention may comprise one or more, or all, of the following steps:

Moving the solid-state body relative to a laser-applying facility, successively generating a multiplicity of laser beams by means of the laser-applying facility, for generating in each case at least one modification, where the laser-applying facility is adjusted, in particular continuously, as a function of at least one parameter, more particularly of a multiplicity of parameters, for the purpose of the defined focusing of the laser beams and/or for the purpose of adapting the laser energy.

The LASER beam preferably penetrates into the donor substrate via a planar surface of the donor substrate. The LASER beam is preferably inclined relative to the surface, more particularly planar surface, of the donor substrate or solid-state body in such a way that it penetrates into the donor substrate at an angle unequal to 0° or 180° relative to the longitudinal axis of the donor substrate. Preferably the LASER beam is focused for the purpose of generating the modification in the donor substrate.

According to a further preferred embodiment of the present invention, the laser radiation is generated with pulse lengths of less than 5 ns or less than 2 ns, more particularly of less than 1 ns or of less than 700 ps or of less than 500 ps or of less than 400 ps or of less than 300 ps or of less than 200 ps or of less than 150 ps or of less than 100 ps or of less than 50 ps or of less than 10 ps.

Alterations of the physical property, or modifications, are generated preferably in each case with laser pulses which are shorter than 5 ns, more particularly shorter than 2 ns or 1 ns. With particular preference the temporal duration of the individual laser pulses is between 50 ps and 4000 ps or between 50 ps and 2000 ps or between 50 ps and 1000 ps, more particularly between 50 ps and 900 ps or between 50 ps and 700 ps or between 50 ps and 500 ps or between 50 ps and 300 ps or between 300 ps and 900 ps or between 500 ps and 900 ps or between 700 ps and 900 ps or between 300 ps and 500 ps or between 500 ps and 700 ps or between 300 ps and 700 ps or shorter than 900 ps or shorter than 700 ps or shorter than 500 ps or shorter than 300 ps or shorter than 100 ps or shorter than 50 ps.

The laser radiation, according to a further preferred embodiment of the present invention, is generated with pulse energies, where the pulse energies are between 100 nJ and 1 mJ or 500 nJ and 100 μJ or 1 μJ and 50 μJ. The pulse energy per individual shot is preferably 0.1-50 μJ after the objective or after the last optical preparation means and before the penetration of the laser beam into the solid-state body. Where, for example, a DOE was intended to generate a plurality of focuses, the laser radiation assigned to each individual focus, after the objective or after the last optical preparation means and before the penetration of the laser radiation into the solid-state body, has a pulse energy of 0.1-50 μJ.

According to a further preferred embodiment of the present invention, for the purpose of defined thermal conditioning and/or for generating the modification and/or for altering, more particularly for locally altering, a physical property of the donor substrate, the LASER radiation is introduced into the solid-state body with a pulse density of between 0.1 nJ/μm2and 10 000 nJ/μm2, preferably between 1 nJ/μm2and 1000 nJ/μm2, and more preferably between 3 nJ/μm2and 200 nJ/μm2.

According to a further preferred embodiment of the present invention, initiating modifications for initiating subcritical cracks are generated, where at least one process parameter for generating the initiating modifications is different from at least one process parameter for generating the basic modifications; preferably, two or more process parameters are different from one another. Additionally or alternatively, the initiating modifications may be generated in a direction which is at a distance from or inclined relative to the running direction of the line along which the basic modifications are generated.

The subcritical cracks, generated in particular by initiating modifications and/or by modifications which define the detachment range or the detachment plane, and/or by modifications which form a linear architecture, propagate in accordance with the invention preferably less than 5 mm, more particularly less than 3 mm or less than 1 mm or less than 0.5 mm or less than 0.25 mm or less than 0.1 mm. An inclined orientation here may correspond, for example, to an angle of between 0° and 90°, preferably an angle of between 85° and 90°, and more preferably an angle of 90°.

This is a threshold process, triggered when a critical intensity (i.e. power/area) is exceeded. In other words, short pulses require less energy/pulse; higher numerical aperture concentrates the energy to a smaller point, therefore also requiring lower energy to achieve the threshold intensity.

The method of the invention preferably likewise comprises one or more of the following steps: providing the donor substrate and/or providing a donor substrate (or solid-state body) which comprises crystal lattice planes that are inclined relative to a planar major surface. The major surface of the donor substrate here preferably has one boundary in the longitudinal direction of the donor substrate, where a crystal lattice plane normal is inclined in a first direction relative to a major-surface normal. Providing at least one laser. Introducing laser radiation of the laser into the interior of the solid-state body, preferably by way of the major surface, to alter the physical properties of the solid-state body in the region of at least one laser focus. The laser focus is formed preferably by laser beams emitted by the laser. Altering the physical property forms a linear architecture through alteration of the site of penetration of the laser radiation into the donor substrate. The alterations in the physical property are generated preferably on a generation plane, which preferably extends parallel to the major surface. The linear architecture preferably extends in a straight or curved line, at least in sections. The crystal lattice planes of the donor substrate are preferably oriented with an inclination relative to the generation plane. The linear architecture, more particularly at least the section extending in a straight line or the section extending in a curved line, is inclined relative to an intersecting line which arises at the intersection between the generation plane and the crystal lattice plane, and consequently the altered physical property causes cracking of the donor substrate in the form of subcritical cracks. The step of separating the solid-state layer is preferably accomplished by introducing an external force into the donor substrate in order to connect the subcritical cracks, or as much material on the generation plane is altered by means of the laser radiation as is required to detach the solid-state layer from the donor substrate, with connection of the subcritical cracks. The major surface is regarded/defined here preferably as a surface of ideal planarity.

This method is advantageous because crack growth is confined perpendicularly to the writing direction, owing to the fact that the linear architecture is inclined relative to an intersecting line that arises at the intersection between the generation plane and the crystal lattice plane. The modifications per writing line are therefore not generated in the same crystal lattice planes. Consequently, for example, the first 1-5% of the modifications per writing line may intersect only a fraction, more particularly less than 75% or less than 50% or less than 25% or less than 10%, or no, crystal lattice planes, and the last 1-5% of the modifications may intersect the same writing line. The writing line here is preferably longer than 1 cm or longer than 10 cm or longer than 20 cm or up to 20 cm long or up to 30 cm long or up to 40 cm long or up to 50 cm long. Per writing line, therefore, significantly fewer modifications are generated in the same crystal lattice planes, thereby limiting crack propagation along these crystal lattice planes. Inclined should be understood here as nonparallel or nonsuperimposing, and may therefore be present, for example, from an angle of just 0.05°; even at very small angles, in particular below 1°, crystal lattice planes different from one another over the length of extent of the linear architecture, more particularly slip planes, are locally cut or modified or altered by the modification or modifications.

This leads to a key second advantage of the present invention, namely that the writing direction need not necessarily be implemented in such a way that the further cracks generated are required to be superimposed on the latterly generated cracks. It is now also possible for the writing direction to be the opposite. Because of the possible shortness of the cracks as a result of the method of the invention, indeed, there is no shading by the cracks accomplished latterly. This makes it possible, in spite of the opposed writing direction, to realize, for example, linear spacings of less than 100 μm, more particularly of less than 75 μm or of less than 50 μm or of less than 30 μm or of less than 20 μm or of less than 10 μm or of less than 5 μm or of less than 2 μm.

Alteration in the physical property may be understood here preferably to refer to the generation of a materials modification or the generation of a crystal lattice defect, more particularly the production of a locally limited phase change.

According to a first preferred embodiment of the present invention, the linear architecture or writing line is inclined relative to the intersecting line in an angle range between 0.05° and 87°, more particularly in an angle range between 3° or 5° and 60° and preferably between 10° and 50°, more particularly between 10° and 30°, such as, for example, between 12° and 20° or between 13° and 15°, or between 20° and 50°, more particularly between 25° and 40° or between 30° and 45° or between 28° and 35°. This solution is advantageous since the inclination is such that a sufficiently large number of different crystal lattice planes are a constituent of each further modification of the same linear architecture or writing line.

According to a further preferred embodiment of the present invention, the amount of material of the donor substrate that is altered, forming a linear architecture or two or more linear architectures, is such that the ends of the individual crystal lattice planes, exposed as a result of the separation of solid-state layers, and the materials alterations produce moiré patterns; for this purpose, a multiplicity of materials alteration regions are generated that extend linearly, and preferably in a straight line, and that are oriented parallel to one another.

A linear architecture is regarded here preferably as an amount of points that forms a straight or curved line. The distances between the centers of the individual points are preferably less than 250 μm, more particularly less than 150 μm or less than 50 μm or less than 15 μm or less than 10 μm or less than 5 μm or less than 2 μm, from one another.

Preferably a plurality of linear architectures are generated on the same generation plane; preferably, at least two or more of the linear architectures are arranged at the same distance from one another. The linear architectures may preferably have the form of arcs, more particularly circular arcs, or of straight lines.

According to a further preferred embodiment of the present invention, the method of the invention may comprise the step of moving the solid-state body relative to the laser, with the laser being set for defined focusing of the laser radiation and/or for adaptation of the laser energy, preferably continuously, in dependence on at least one parameter and preferably on a multiplicity of parameters, more particularly at least two parameters, with one parameter preferably being the degree of doping of the solid-state body at a predetermined site or in a predetermined region, especially in the interior, of the solid-state body, more particularly at a distance from the solid-state body surface.

According to a further preferred embodiment, an additional or alternative parameter is the degree of doping of the solid-state material that is determined preferably by the analysis of backscattered light (preferably Raman scattering), where the backscattered light has a different wavelength or a different wavelength range from light irradiated in a defined way in order to trigger the backscattering; a Raman instrument is preferably a constituent of the apparatus, and the degree of doping is determined preferably by means of the Raman instrument; one or more or all of these parameters are captured, in particular simultaneously, preferably by means of a common detection head. Raman spectroscopy is preferably likewise employed in the case of glasses, sapphire, aluminum oxide ceramic. The Raman method is advantageous since it measures in the depth of the material, but only from one side, does not require high transmission, and, by means of a fit to the Raman spectrum, yields the charge carrier density/doping, which can be correlated with the laser parameters.

An additional or alternative parameter, according to another preferred embodiment of the present invention, is the degree of doping of the solid-state body at a predetermined site or in a predetermined region, in particular in the interior, of the solid-state body, more particularly at a distance from the solid-state body surface. The degree of doping is preferably linked with site information in such a way that a treatment card is produced or site-resolved treatment instructions are provided, which site-dependently mandates or mandate the laser parameters, especially laser focus and/or laser energy, and/or other machine parameters, more particularly the speed of advance.

The degree of doping, according to a further preferred embodiment of the present invention, is determined by the analysis of backscattered light with an inelastic scattering (Raman scattering), where the backscattered light has a different wavelength or a different wavelength range from light irradiated in a defined way in order to trigger the backscattering, with the backscattered light being backscattered from the predefined location or from the predetermined region.

This embodiment is advantageous because in the laser process, particularly on SiC (though also other materials), the operation must take place in a manner adapted to the site (e.g., different laser energy, etc.). In accordance with the invention it has been recognized that in the case of SiC, for example, the doping is particularly critical for this, since it changes the transparency of the material to the working wavelength and necessitates higher laser energies.

According to a further preferred embodiment of the present invention, the degree of doping is determined by means of an ellipsometric measurement (e.g., Muller matrix ellipsometry with back-side reflection). The ellipsometric measurement is based preferably on an optical transmission of the material.

According to another preferred embodiment of the present invention, the degree of doping is determined by means of a purely optically calibrated transmission measurement, the calibration being effected by means of Hall measurement and 4-point measurement. This method is likewise able to ascertain the doping/number of free charge carriers in the material, which then allows a determination of the laser energy required for the operation.

The degree of doping, according to a further preferred embodiment of the present invention, is determined by means of an eddy current measurement, with determination and evaluation, preferably, of conductivity differences in the solid-state material.

With eddy current measurements or when using eddy current sensors or in the eddy current measuring technique, preference is given to utilizing a transmitting and receiving coil in order to detect local conductivity differences. In the emitting coil, a radiofrequency, electromagnetic, primary alternating field is generated. In the conductive material, eddy currents (currents with local flow) are then induced, which give rise in turn to a secondary electromagnetic alternating field with opposite direction. The superimposition of these fields can be measured, separated and evaluated. Hence it is possible to measure various quality features (layer thickness, layer resistance, homogeneity of material) principally of thin conductive layers, though also of bulk material. In transmission arrangement (test body between emitting coil and receiving coil), optimum resolutions are achieved; however, the arrangement of both coils on one side of the sample, for reflection measurements, is also possible. Through adapted design of the coils and selection of frequency it is possible to utilize different depths of penetration and sensitivities.

In principle, therefore, there are a host of measurement methods which allow the doping to be measured, in principle. Important here is a rapid, contactless, nondestructive method.

A first parameter here may be the mean refractive index of the material of the donor substrate, or the refractive index of the material of the donor substrate in the region of the donor substrate through which laser radiation must pass in order to generate a defined alteration of material, and

a second or alternative first parameter here may be the depth of working in the region of the donor substrate through which laser radiation must pass in order to generate a defined alteration of material. The first parameter is determined preferably by means of a refractive index determination means, more particularly by means of spectral reflection, and/or the second parameter is determined preferably by means of a topography determination means, more particularly by means of a confocal-chromatic distance sensor.

According to a further preferred embodiment of the present invention, a first parameter is the mean refractive index of the material of the solid-state body, or is the refractive index of the material of the solid-state body in the region of the solid-state body through which laser beams must pass in order to generate a defined modification, or is the transmission of the solid-state body at defined sites of the solid-state body, and preferably for a defined solid-state body depth. A second or alternative first parameter, according to a further preferred embodiment of the present invention, is the working depth in the region of the solid-state body through which laser beams must pass in order to generate a defined modification. The first parameter, according to one further preferred embodiment of the present invention, is determined by means of a refractive index determination means, more particularly by means of spectral reflection, and/or the second parameter is determined by means of a topography determination means, more particularly by means of a confocal-chromatic distance sensor.

According to a further preferred embodiment of the present invention, a first parameter is the advance direction in which a linear architecture is formed as a sequence of modifications in the generation plane, more particularly an outward and/or return journey. Accordingly, a first parameter may represent the laser parameters in the case of an outward journey, and a second parameter may represent the laser parameters in the case of a return journey, especially in the case of meanderlike working by means of an X-Y table.

Data on the parameters, particularly on the first parameter and on the second parameter, are provided, according to a further preferred embodiment of the present invention, in a data storage facility, and are supplied, at least before the generation of the alteration in material, to a control facility, where the control facility adjusts the laser in dependence on the respective site of the material alteration to be generated; the control facility, for adjusting the laser, preferably likewise processes distance data relating to a distance parameter, where the distance parameter reproduces the distance of the respective location at which laser radiation for generating the alteration in material is introduced into the donor substrate at the time of the alteration of material, relative to the laser, the distance data being captured by means of a sensor facility.

Data on the parameters, particularly on the first parameter and on the second parameter, are provided, according to a further preferred embodiment of the present invention, in a data storage facility, and are supplied, at least before the generation of the modifications, to a control facility, where the control facility adjusts the laser-applying facility in dependence on the particular site of the modification to be generated.

The object stated above is likewise achieved by a method for separating at least one solid-state slice from a donor substrate. This method preferably comprises at least one of the preceding methods for generating microcracks in a donor substrate, and at least the following step:

Generating as many modifications in the same plane as are required to cause the cracks generated by the modifications to detach the solid-state slice from the donor substrate, or introducing an external force into the donor substrate to connect the cracks. This solution is advantageous because a

According to a further preferred embodiment of the present invention, to initiate the external force, an acceptor layer is arranged on an exposed surface of the donor substrate, more particularly on the solid-state layer to be separated, where the acceptor layer comprises a polymer material, more particularly PDMS, and the acceptor layer is exposed thermally to generate, more particularly to generate mechanically, stresses in the solid-state body, where the thermal exposure represents a cooling of the acceptor layer to a temperature below the ambient temperature, more particularly to a temperature below 0° or to a temperature below −10° or to a temperature between −20° and −200° C., where the cooling takes place such that the polymer material of the acceptor layer fulfils a glass transition and where, as a result of the stresses, the main crack in the solid-state body propagates along the detachment region, more particularly so as to connect the cracks, more particularly subcritical cracks, that are generated by the modification lines.

Additionally or alternatively, the solid-state body, for the purpose of initiating the external force, may be treated with sound, more particularly ultrasound, and/or, for initiating the external force, the circumferential surface of the solid-state body, especially at the level of the detachment plane, may be treated thermally and/or ablatively.

According to a further preferred embodiment of the present invention, the initiation of the external force takes place at an ambient temperature and/or a core temperature of the donor substrate of less than 10° C., or of less than or equal to 1° C. or of less than or equal to 0° C. or of less than or equal to −1° C. or of less than or equal to −10° C. or of less than or equal to −50° C. or of less than or equal to −100° C. or of less than or equal to −125° C. or of less than or equal to −150° C. or of less than or equal to −175° C. This solution is advantageous because the lattice planes slide off better at low temperatures. This is a result preferably of the fact that there are fewer disruptions to sliding from movements in the crystal.

According to a further preferred embodiment of the present invention, the mean propagation velocity of the main crack in a radial direction of the donor substrate is less than 5 cm/second or less than 3 cm/second or less than 1 cm/second, more particularly less than 0.5 cm/second and preferably less than 0.1 cm/second. The mean propagation velocity of the main crack in a radial direction of the donor substrate is preferably between 0.5 cm/second and 0.1 cm/second, more particularly between 0.4 cm/second and 0.15 cm/second or between 0.35 cm/second and 0.2 cm/second. This embodiment is advantageous because a slow crack propagation velocity allows very precise crack guidance to be brought about.

Starting from a trigger site, the crack initially propagates more rapidly in the circumferential direction of the donor substrate than in a radial direction of the donor substrate. Preferably the crack or the main crack propagates completely in the circumferential direction, and up to complete propagation in the circumferential direction the crack propagates, in particular on average or at maximum, less than 5 cm, more particularly less than 3 cm or less than 2 cm or less than 1 cm or less than 0.5 cm or less than 0.2 mm, in a radial direction of the donor substrate.

The article “Large Area 4H SiC Products for power Electronic Devices”, Material Science Forum, ISSN: 1662-9752, Vol. 858, pp 11-14 and the article “Exploration of Bulk and Epitaxy Defects in 4H-SiC Using Large Scale Optical Characterization”, Material Science Forum, ISSN: 1662-9752, Vol. 897, pp 226-229 are incorporated by reference in their entirety to become part of the subject matter of the present invention.

The use of the word “substantially”, preferably in all cases where this word is used in the context of the present invention, defines a deviation in the range of 1%-30%, more particularly of 1%-20%, more particularly of 1%-10%, more particularly of 1%-5%, more particularly of 1%-2%, from the stipulation which would be the case without the use of this word.

