DE102020213776A1 - Method of cleaving a crystal - Google Patents

Abstract

The invention relates to a method for cleaving a crystal (2), comprising: irradiating a pulsed laser beam onto the crystal (2) to modify, in particular to remove, material from the crystal (2). When the pulsed laser beam is radiated in, the material of the crystal (2) is modified to a penetration depth which corresponds to at least 30%, preferably at least 40%, of a thickness of the crystal (2), the crystal (2) being radiated along by the pulsed laser beam a predetermined breaking edge is split.

Description

The present invention relates to a method for cleaving a crystal, comprising: irradiating a pulsed laser beam onto the crystal to modify, in particular to remove, material from the crystal.

The modification of the material of the crystal can consist, for example, in the introduction of imperfections and/or defects into the material of the crystal, which are produced by the interaction of the material of the crystal with the pulsed laser beam. The imperfections or defects can be, for example, changes in density, chemical changes and/or local stresses that promote splitting of the crystal. The modification of the material of the crystal can also consist in material of the crystal typically being removed by laser ablation.

The crystal may be an optical crystal, i.e. a crystal that can be used for optical applications. The optical crystal can in particular be a non-linear optical crystal. However, the crystal can also be a semiconductor crystal, e.g. a silicon crystal, or another type of crystal.

Integrated optics based on microstructures and waveguide structures in optical crystals are an important prerequisite for modern (quantum) optical devices and switches. In order to ensure the efficiency of the resulting optical structures, it is necessary to minimize the losses during coupling, in particular during direct coupling, into the individual structures. A key factor is the nature of the end facets of waveguide structures, which typically must have a roughness on the order of less than 10 nm. These end facets are usually produced after the actual waveguide has been structured by controlled separation of the end areas.

The singulation methods typically used, such as sawing or polishing, are very time-consuming and expensive, with throughput limited either by the process itself, such as diamond sawing, or by the number of steps, such as scribing and mechanical cleaving.

In the article "Ultrabroadband Nonlinear Optics in Nanophotonic Periodically Poled Lithium Niobate Waveguides", M. Jankowski et al., arXiv:1909.08806, 19 September 2019, a conventional laser saw is used to produce optical quality end facets on periodically poled lithium niobate, with of the laser pulses with pulse energies on the order of µJ are focused into the sample to create a periodic array of damage sites that serve as nucleation sites for crack propagation. In such a process, also known as “stealth dicing”, the sample or the optical crystal is subsequently split by a carrier membrane on which the sample is arranged being stretched in the radial direction.

In the article "Smooth Surfaces with High Accuracies", S. Mahdi et al., Laser Technik Journal 9, No. 3 (2012): 36-39, a method is described in which a silicon-on-insulator chip with a femtosecond - Laser is scribed, forming nicks in the chip. After scribing, the chip is split along the center of the kerf using mechanical tools.

In the U.S. 9,636,783 B2 describes a method and a device for laser dicing, in which a semiconductor wafer and a substrate wafer are connected to one another by bonding or lamination. The laser parameters of a laser beam that cuts a gap in the semiconductor wafer are selected in such a way that an ablation threshold value of the semiconductor wafer is not exceeded, so that the semiconductor wafer is removed in a targeted manner.

In the US8969761B2 describes a method for cutting a wafer-like object, in which the object is irradiated with laser light to cut the object along a plurality of crossing cutting lines to form a plurality of modified points at a plurality of laser light-converging points along the cutting lines form. The object is cut into parts along the cut lines by propagating cracks in the object from the modified locations, extending to the front and back of the object. When the object is cut, a membrane on which the object is supported is deformed to apply tension to the object, thereby causing the object to be cut into pieces.

In the WO 2011/163149 A2 describes a method for cutting a semiconductor wafer having a plurality of integrated circuits. The method includes forming a mask over the semiconductor wafer. The mask consists of a layer that covers and protects the integrated circuits. The mask is patterned with a femtosecond-based laser scribing process to provide a patterned mask with gaps. The structuring lays down areas of the semiconductor wa fers between the integrated circuits. The semiconductor wafer is etched through the gaps in the patterned mask to singulate the integrated circuits.

object of the invention

The object of the invention is to improve a method for cleaving a crystal.

subject of the invention

This object is achieved by a method of the type mentioned at the outset, in which, when the pulsed laser beam is irradiated, the material of the crystal is modified, in particular removed, to a penetration depth which corresponds to at least 30%, preferably at least 40%, of a thickness of the crystal, wherein by irradiating (and during irradiating) the pulsed laser beam, the crystal is split along a predetermined breaking edge. The penetration depth into the crystal defines the area in which material of the crystal is modified, for example by material of the crystal being removed by laser ablation.

In the method according to the invention, the material of the crystal is modified when the laser beam is irradiated, so that it can be split along the breaking edge. For example, when the pulsed laser beam is radiated in, deep, crater-shaped predetermined breaking points can be introduced into the material of the crystal by laser ablation. It has been shown that a direct self-cleavage process of the crystal can be achieved in this way, i.e. the crystal is split along the fracture edge by an energy input that is introduced when the laser beam is irradiated. The direct self-cleavage distinguishes the method described here from the methods described above, in which the (optical) crystal is first laser-assisted scratched, but the (optical) crystal is only split by a subsequent mechanical splitting process.

The fracture edge produced in the method described here has a characteristic structure in which deep defects, for example in the form of ablation craters, are formed along that side of the crystal onto which the pulsed laser beam is irradiated, while on one of the irradiation of the pulsed laser beam Due to the direct self-cleavage, a completely smooth fracture edge is formed on the opposite side of the crystal, which has a roughness in the order of a molecular diameter or in the atomic range (Angstrom). In the smooth portion of the breaking edge, which can extend e.g. over about half the thickness of the crystal, a crystal facet with optical quality is produced, which can be used in an optical crystal, e.g. However, the crystal cleaved along the fractured edge in the manner described above may also constitute a composite material substrate having an optical crystal bonded to a surface of the substrate. If the optical crystal has a significantly smaller thickness than the crystalline substrate, the optical crystal is typically also cleaved along the fracture line, producing an optical quality crystal facet on the optical crystal.

The method described here for the self-cleavage of the crystal does not require any pretreatment of the material of the crystal. In addition, few or no cleaning steps are required before and after the splitting of the crystal, so that it may be possible to dispense with carrying out the method in a clean room. The method described here can therefore be carried out inexpensively and, with suitable optimization, is very stable and reproducible. In addition, it is a dry separation process in which there is no contamination from cooling or lubricating liquids. In contrast to diamond sawing, it is not an abrasive process either, so that there is no material wear during production, there is no contamination and therefore no material wastage.

In one variant, the breaking edge runs along a crystal axis or along a crystal plane and/or in a plane perpendicular to a surface of the crystal onto which the pulsed laser beam is radiated. In order to obtain a fracture edge that is as smooth as possible, it is generally advantageous if the fracture edge runs along a crystal axis or a crystal plane of the split crystal. If the laser beam is radiated perpendicularly onto the surface of the crystal, the breaking edge usually runs in a plane perpendicular to the surface of the crystal. In this case, cleavage of the crystal along the plane perpendicular to the surface may also be possible if this plane does not coincide with a crystal plane.

In contrast to conventional cleavage methods, the method proposed here can also be used to produce break edges along planes that do not run along the natural direction of cleavage (ie typically along a crystal plane). In this way it is possible to produce end facets which are inclined or angled with respect to the direction of the thickness of the crystal. This enable applications such as Brewster windows and thus improved coupling properties. However, as described above, it is typically favorable if the fracture edge runs along the natural direction of cleavage, ie along a predetermined crystal plane.

In one variant, the material of the crystal is modified when the pulsed laser beam is irradiated to a penetration depth which corresponds to no more than 70%, preferably no more than 60%, of the thickness of the crystal. Although the height of the region where an end facet of a waveguide structure is formed on an optical crystal is usually no more than 5% of the thickness of the optical crystal, the penetration depth of the pulsed laser beam should typically not exceed the value specified above. In the event that the crystal onto which the laser beam is radiated forms a crystalline substrate of a material composite with an optical crystal, the penetration depth can also be selected to be greater if necessary.

In a further variant, the pulsed laser radiation is radiated onto a surface of the crystal on a predetermined, in particular rectilinear path curve along a plurality of spaced apart, preferably self-contained irradiation contours. The pulsed laser beam and the crystal are moved relative to one another to generate the predetermined, generally linear or rectilinear trajectory. In order to achieve the accuracy required for the cleavage, it is favorable if the crystal remains stationary and the pulsed laser beam is moved over the surface.

Material of the crystal is modified along the irradiation contours, with cracks propagating in the crystal that reach to that surface of the crystal which faces away from the irradiated surface. In this way, the crystal is split along a fracture line that extends in the volume of the crystal along the predetermined trajectory. The irradiation contours along the predetermined trajectory thus act in the manner of a perforation, along which the optical crystal is split. The pulsed laser beam is typically focused on the surface or, if necessary, on a volume area of the crystal close to the surface. The irradiation contours are preferably closed contours, but in principle it is also possible for a respective irradiation contour not to have a closed contour.

The surface on which the pulsed laser beam is irradiated may be a surface of an optical crystal, but it is also possible that the pulsed laser beam is irradiated on the surface of a substrate bonded to the optical crystal, such as This is the case, for example, with so-called LNol (lithium niobate-on insulator), in which a LiNbO ₃ crystal is bonded to a (crystalline) substrate made of an insulator or a semiconductor. In this case, too, it is necessary for the penetration depth of the pulsed laser beam into the material of the crystal that forms the substrate to be at least 30%, preferably at least 40%, of the thickness of the crystal during the material modification.

In a further variant, a respective irradiation contour has an extent transverse to the predetermined trajectory, which is greater than an extent along (i.e. parallel to) the predetermined trajectory. In general, it is favorable for the cleavage of the crystal when the aspect ratio of the extension 2 a of a respective irradiation contour transverse to the trajectory and the extension 2 b parallel to the trajectory applies: 1 ≤ ( 2 a / 2 b ). In the event that the trajectory is a curved trajectory, the parallel or perpendicular extension relates to the instantaneously irradiated point along the trajectory.

As described above, the fracture line along which the crystal is to be split is usually a fracture plane and the specified trajectory on the crystal is typically a straight line. Due to angular tolerances during the adjustment or during the manufacture of the crystal, the specified trajectory can deviate from the breaking edge or from the crystal plane along which the crystal is to be split. However, in order to reliably split the crystal, the pulsed laser beam should hit the specified breaking edge of the crystal at each irradiation contour along the predetermined trajectory, i.e. the breaking edge should intersect each irradiation contour along the trajectory.

wobei α die (maximale) Winkelabweichung einer Kristallachse bei der Ausrichtung des optischen Kristalls relativ zu der Bahnkurve bezeichnet, die sich aus dem Toleranzwinkel des Waferflats des Wafers des Kristalls und dem Toleranzwinkel bei der Justierung des Kristalls bzw. des Kristallstreifens zusammensetzt. Es versteht sich, dass in obiger Gleichung ein zusätzlicher Sicherheitsfaktor berücksichtigt werden kann.If the fracture edge, along which the crystal is to be split, is a fracture plane with length B along the specified trajectory (if the trajectory is correctly aligned with the crystal) and the extension of the irradiation contour is 2 a across the trajectory, then the (Half) extension of the irradiation contour transverse to the trajectory given by: $$\text{a=Btan}\left(\mathrm{a}\right),$$

where α denotes the (maximum) angular deviation of a crystal axis when aligning the optical crystal relative to the trajectory, which results from the tolerance angle of the wafer flat of the crystal wafer and the tolerance angle when adjusting the crystal or the crystal strip together people. It is understood that an additional safety factor can be taken into account in the above equation.

In a further development of this variant, the extension of the irradiation contour is between 40 μm and 200 μm transversely to the specified trajectory. This dimension of the extent of the irradiation contour transverse to the trajectory curve has proven to be favorable for the self-splitting of the crystal.

In a further development of the above variant, the extent of the irradiation contour along the specified trajectory is between 4 μm and 30 μm. An extension of the irradiation contour along the specified trajectory in the given order of magnitude favors the self-cleavage of the crystal through the material modification or the laser ablation.

In a further development, a geometry of the irradiation contour is selected from the group comprising: ellipses, polygons, in particular rhombuses or rectangles, and lines. The irradiation contours mentioned first are closed contours. A line-shaped irradiation contour can also be a closed contour, i.e. the start and end point of the contour coincide, but it is also possible for the line-shaped contour to be an open contour in which the start point and end point do not coincide. If the irradiation contour is an ellipse, the long semi-axis typically runs transversely to the specified trajectory and the short semi-axis runs along the specified trajectory. Also in the case of polygons, e.g. in the case of rectangles, but also in the case of a line-shaped irradiation contour, the extent transverse to the specified trajectory is generally greater than the extent along the specified trajectory. A line-shaped irradiation contour is understood to mean an irradiation contour whose length is at least 10 times greater than its width.

The pulsed laser beam that is radiated onto the surface typically has a beam diameter that lies between the diffraction limit and approx. 50 µm. The pulsed laser beam generally has a round beam cross-section, but it is also possible to shape the pulsed laser beam in a suitable manner with the aid of beam-shaping optics, so that it has a predetermined shape that deviates from a round geometry.

In a further development, the distance between adjacent irradiation contours is between 50 μm and 400 μm. As a rule, the distance between adjacent irradiation contours is constant, i.e. it does not vary along the trajectory. In principle, however, the distance between adjacent irradiation contours can also vary along the trajectory. By choosing a suitable distance between the irradiation contours, the self-cleavage of the crystal is favored or made possible.

In a further variant, a jump speed between adjacent irradiation contours is between 100 mm/s and 4000 mm/s. The jumping speed between adjacent irradiation contours is usually greater than the marking speed (see below) when the laser beam moves along the irradiation contour.

In a development, a marking speed during the movement of the pulsed laser beam along the irradiation contour is between 10 mm/s and 200 mm/s. During the movement of the pulsed laser beam along the irradiation contour, the beam cross sections of successive laser pulses typically overlap, so that a continuous irradiation contour is generated. As stated above, the marking speed is usually lower than the jump speed between adjacent irradiation contours. If necessary, the irradiation contour can also be traversed several times with the pulsed laser beam.

The splitting of the crystal with the aid of the method according to the invention is significantly faster than conventional methods for separating or splitting crystals, e.g. for the production of end facets of waveguide structures. Due to the fast splitting speed, the process is scalable, i.e. the crystal can be singulated or split not only at chip level, but also at wafer level, without significantly increasing the time required. The feed rate resulting from the jumping speed, the marking speed and the length of the respective irradiation contour, with which the pulsed laser beam is moved along the trajectory curve, corresponds approximately to the gap propagation speed in the material of the crystal.

In a development, the crystal is in the form of a strip and the predetermined trajectory extends transversely to a longitudinal direction of the crystal in the form of a strip (or the crystal strip). In this case, the crystal strip is split along its width, ie transversely to the longitudinal direction. The crystal strip has a defined length and width and can be produced from a wafer using a conventional singulation method. For example, the wafer can be divided into a plurality of crystal strips by laser perforation or by laser ablation with subsequent mechanical breaking. As above was described, but it is also possible that the method according to the invention is used for dividing the wafer into the crystal strips, in which self-cleavage of the crystal takes place.

In another variant, the crystal is an optical crystal selected from the group comprising: lithium niobate, LiNbO ₃ , lithium tantalate, LiTaO ₃ , or potassium titanyl phosphate, KTP. It goes without saying that the materials mentioned can also be doped, for example LiNbO ₃ can be MgO:LiNbO ₃ . The optical crystal can also have a refractive index structure for planar waveguiding or for ribbed waveguiding and can be in the form of PE (proton-exchanged) LiNbO ₃ or MgO:LiNbO ₃ , for example.

In an alternative variant, the crystal forms a substrate, preferably in the form of a semiconductor crystal, in particular a silicon crystal, which forms a material composite with an optical crystal is split along the breaking edge. As described above, the optical crystal to be cleaved by the method can also be bonded onto a crystalline substrate, i.e. the optical crystal can form a material composite with another material. In this case, the optical crystal typically has a thickness that is significantly (usually at least an order of magnitude) smaller than the thickness of the substrate. Therefore, when the substrate is cleaved along the fracture line, the optical crystal is also cleaved along the same fracture line. The irradiation of the pulsed laser beam onto the side of the substrate facing away from the optical crystal splits the optical crystal in the smooth partial area of the breaking edge and therefore has optical quality. The substrate can, for example, have a thickness of the order of about 500 μm, the thickness of the optical crystal is usually of the order of about 5 μm-10 μm.

The optical crystal can be connected to the substrate to form the composite material in different ways. For example, for bonding, a pulsed laser beam may be irradiated on a surface between the optical crystal and the substrate to form a fusion zone, thereby integrally bonding the optical crystal to the substrate. An intermediate layer may be formed between the optical crystal and the substrate, onto which the pulsed laser beam is irradiated to form the fused zone, as shown in FIG DE102020211282.7 of the applicant is described, which is incorporated by reference in its entirety into the content of this application. The—possibly amorphous—intermediate layer typically has a thickness of a few micrometers, so that the intermediate layer is also split along the fracture edge when the crystalline substrate is split.

In a further variant, at least one waveguide structure is formed on the optical crystal on a side facing away from the incident laser beam, with the breaking edge forming an end facet of the waveguide structure when the optical crystal is split. As described above, the optical crystal can already be structured with a waveguide. In this case, when the optical crystal is cleaved, an optical quality end facet is obtained. In this case, the end facet is formed in the smooth area of the fracture edge produced by the self-cleavage, which is not affected by the material modification or which is not removed during the laser ablation.

The optical crystal having a waveguide structure is typically a crystal strip formed from a wafer by dicing as described above. In the case of such a prestructured optical crystal, any internal stresses that may occur can influence the quality of the end facets that are produced. The suitability of the end facets produced with the aid of the method described here for wave guidance or for the coupling of radiation was proven by transmission measurements etc. It goes without saying that the optical crystal having the waveguide structure can also form a material composite with the crystalline substrate described above.

The waveguide structure is typically formed in the optical crystal by removing material around or laterally along the waveguide structure. The thickness of the optical crystal in this case is therefore measured from the top (planar) surface of the waveguide structure to the (planar) surface on the opposite side of the optical crystal.

It goes without saying that the method described here can also be used to cleave a non-prestructured optical crystal in order to produce chips with optical quality facets which can be used in any further manner.

It is favorable if the irradiation of the pulsed laser radiation is carried out in a protective gas atmosphere. The protective gas atmosphere or the gas used to generate the protective gas atmosphere can be nitrogen or argon, for example. For carrying out the procedure are suitable Atmospheric and flow conditions favourable. The correct form and attachment of the gas nozzles and the suction must be ensured here.

The correct mounting of the crystal, for example in the form of a crystal strip, also plays a role in carrying out the method and for removing the samples or the individual chips after the splitting, in order to increase the quality and the reproducibility of the method. The following options can be used here: Vacuum holders with a large suction surface, in particular using components made of porous ceramics, as well as complex clamping devices (e.g. vice variants) made of metal and/or plastics, for example with serrated and/or toothed structures. In particular, an entire wafer can be singulated (instantaneously) with the aid of a suitable holder.

For the successful implementation of the method, it is necessary to choose the parameters of the pulsed laser beam appropriately.

In one variant, the pulsed laser beam is irradiated with a wavelength of between 200 nm and 10 μm, preferably between 200 nm and 1100 nm. The modification of the crystal material typically requires that the crystal material absorbs the pulsed laser beam, i.e. the crystal material should not be transparent to the wavelength of the laser beam, or the crystal material should have increased absorption for the wavelength of the laser beam. This is usually the case at wavelengths of less than 550 nm or below the band edge of the crystal, with lithium niobate, for example, at wavelengths of less than approx. 400 nm, with silicon at wavelengths of less than e.g. 1100 nm.

For the modification or ablation of material, it is necessary for the pulsed laser beam to have a pulse energy that is above the modification or ablation threshold. For lithium niobate as the material of the optical crystal, the pulse energy of the pulsed laser beam is typically between 0.1 µJ and 10 µJ.

In a further variant, the pulsed laser beam is irradiated with a pulse duration of between 10 fs and 8 ns, preferably between 100 fs and 10 ps. For the modification of material or for laser ablation, pulse durations in the ultra-short pulse range between 100 fs and 10 ps have proven particularly useful.

In a further variant, the pulsed laser radiation is radiated in at a repetition rate between 1 kHz and 10 MHz, in particular between 500 kHz and 2000 kHz. The irradiation of the pulsed laser beam with these repetition rates favors the self-cleavage of the crystal.

In principle, the irradiation of the pulsed laser beam can take place continuously or in burst mode. As described above, the laser beam is typically only irradiated onto the crystal in the irradiation contours that are spaced apart from each other, i.e. the pulsed laser beam is not irradiated in the volume areas that lie between two adjacent irradiation contours and in volume areas that lie within (closed) irradiation contours .

Further advantages of the invention result from the description and the drawing. Likewise, the features mentioned above and those listed below can each be used individually or together in any combination. The embodiments shown and described are not to be understood as an exhaustive list, but rather have an exemplary character for the description of the invention.

• 1a-d schematische Darstellungen eines Wafers bei der Vereinzelung, eines bei der Vereinzelung erzeugten Kristallstreifens, des Einschreibens von Wellenleiterstrukturen in den Kristallstreifen, sowie der Selbstspaltung des Kristallstreifens,
• 2 eine Draufsicht auf den Kristallstreifen von 1 d mit einer geradlinigen Bahnkurve, entlang derer ein gepulster Laserstrahl an voneinander beabstandeten, ellipsenförmigen Einstrahlungskonturen auf den Kristallstreifen eingestrahlt wird,
• 3a,b Darstellungen einer Bruchkante des Kristallstreifens von 2 mit bzw. ohne eine Wellenleiterstruktur,
• 4 eine Detaildarstellung einer Endfacette der Wellenleiterstruktur des optischen Kristalls von 3a, sowie
• 5 eine schematische Schnittdarstellung eines Materialverbunds aus einem Kristall-Substrat und einem optischen Kristall.

Show it:

• 1a-d Schematic representations of a wafer during separation, a crystal strip produced during separation, the writing of waveguide structures in the crystal strip, and the self-cleavage of the crystal strip,
• 2 a plan view of the crystal stripe of FIG 1d with a rectilinear trajectory along which a pulsed laser beam is radiated onto the crystal strip at spaced, elliptical irradiation contours,
• 3a,b Representations of a broken edge of the crystal strip of 2 with or without a waveguide structure,
• 4 FIG. 12 is a detailed view of an end facet of the optical crystal waveguide structure of FIG 3a , such as
• 5 a schematic sectional view of a material composite of a crystal substrate and an optical crystal.

In the following description of the drawings, identical reference symbols are used for identical or functionally identical components.

1a shows a wafer 1, which in the example shown is formed from an optical crystal in the form of lithium niobate. The wafer 1 is divided into a plurality of strip-shaped optical crystals by means of a conventional dicing method tallen 2 divided, which are referred to below as crystal strips 2 for simplicity. The singulation takes place in two steps: The first step comprises a laser perforation using a pulsed laser beam, which is aligned in the Z-direction on the wafer 1 and which moves along the Y-direction of an XYZ coordinate system in a straight-line trajectory over the wafer 1 will like this in 1a is shown. 1b 1 shows a crystal stripe 2 formed by subsequent mechanical breaking of the wafer 1 material along the perforation lines (in the Y-direction) produced by the pulsed laser beam 3 in the wafer 1 material. The wafer 1 can be broken, for example, by tongue-shaped mechanical tools. As in 1b can be seen, crystal strips 2 are produced during the separation, which have a defined length and width.

in a 1c In the optional method step shown, a plurality of waveguide structures 4 are written into the crystal strip 2 , which extend in the longitudinal direction (Y-direction) of the crystal strip 2 . For the production of the waveguide structures 4, material from the 1c shown front 5a of the crystal strip 2 removed. The waveguide structures 4 are formed between strip-shaped areas of the wafer 1 in which material is removed from the wafer 1. FIG.

between the in 1c and the inside 1d The method step shown is the crystal strip 2 is rotated by 180 ° about the Y-axis. In the 1d The top view shown thus shows the rear side 5b of the crystal strip 2. The in 1d illustrated waveguide structures 4, which are located on the front side 5a of the crystal strip 2, are in 1d can be seen through the material of the crystal strip 2.

in the in 1d The method step shown is the crystal strip 2 irradiated with the pulsed laser beam 3 on its rear side 5b. The pulsed laser beam 3 is in this case moved along a plurality of parallel, rectilinear trajectories 6 in the positive X direction, ie transversely to the longitudinal direction Y of the crystal strip 2, and as in FIG 1a radiated onto the crystal strip 2 perpendicularly, ie aligned in the Z-direction. The irradiation of the pulsed laser beam 3 causes the crystal strip 2 to self-cleave along the rectilinear trajectories 6. The cleavage of the crystal strip 2 forms a plurality of chips 7 which each run between two adjacent trajectories 6.

During the self-cleavage of the crystal strip 2, the pulsed laser beam 3 is used to produce a material modification, for example material removal by laser ablation, which extends from the rear side 5b of the crystal strip 2 to a penetration depth T of more than 30%, to be more precise is more than 40% of the thickness D of the crystal strip 2, as shown in 3a,b can be seen. 3a,b each show a breaking edge 8 along which the crystal strip 2 was split. The thickness D of the crystal strip 2 is usually on the order of about 1 mm, for example between 500 μm and 700 μm. A larger or smaller thickness D is also possible. The fracture edge 8 runs along a fracture plane (XZ plane) of the crystal strip 2 at the level of one of the 1d illustrated linear trajectories 6.

How based on 3a,b can be seen, the material of the crystal strip 2 is modified from the rear side 5b to the penetration depth T. In the example shown, the material is removed in several adjacent crater-shaped volume areas. Starting from the penetration depth T to the front side 5a of the crystal strip 2, ie over the distance D - T, the crystal strip 2 is split by the energy input during the material modification in the form of laser ablation, without any external mechanical action being required for this purpose. A sub-area 9 of the breaking edge 8 produced during the self-cleavage, which is not within the modification area, is therefore smooth and has optical quality: In the sub-area 9, in particular in the vicinity of the front side 5a of the crystal strip 2, the breaking edge 8 has a roughness of the order of less than 10 nm.

Due to the self-cleavage in the in 3a shown breaking edge 8 of the crystal strip 2, an end facet 10 of the waveguide structure 4 is formed, which has optical quality (cf. also the detailed illustration of the breaking edge 8 in 4 ). Radiation can be coupled into the end facet 10 of the waveguide structure 4 with a low loss rate.

Even if the height H of the end facet 10 of the waveguide structure 4 is comparatively small and is generally less than 5% of the thickness D of the crystal strip 2, it is favorable if the crystal strip 2 only penetrates up to a penetration depth when the pulsed laser beam 3 is irradiated T is removed, which typically does not correspond to more than 70% of the thickness D, typically not more than 60% of the thickness D of the crystal strip 2 .

As in 3b can be seen, the self-cleavage can also be carried out on a crystal strip 2, in which the 1b illustrated optional step of inscribing waveguide structures 4 was not performed. Also in this case, in which the front 5a neighboring portion 9 of the breaking edge 8 produces a facet with optical quality that can be used further as desired.

To generate the in 3a,b shown removal of material at the breaking edge 8, are formed in the crater-shaped structures in the material of the crystal strip 2, the pulsed laser beam 3 as in 1d shown moved along a respective straight path curve 6. The pulsed laser beam 3 is irradiated onto the rear side 5b of the crystal strip 2 in a plurality of irradiation contours 11 spaced apart from one another, as is shown in FIG 2 can be seen.

At the in 2 In the example shown, the irradiation contours 11 are elliptical, with the long semi-axis a being oriented transversely to the rectilinear trajectory 6 (in the Y direction) and the short semi-axis b being oriented along the rectilinear trajectory 6 (in the X direction). A respective elliptical irradiation contour 11 thus has a (maximum) extension of 2a (2 x the long semi-axis a) transversely to the rectilinear trajectory 6, which is greater than a (maximum) extension 2 b (2 x the short semi-axis b) along the straight trajectory curve 6.

For generating the self-cleavage of the crystal strip 2 along the given trajectory 6, it has proven to be advantageous if the maximum extent 2a of the elliptical irradiation contour 11 transverse to the trajectory 6 is between 40 μm and 200 μm. Correspondingly, a maximum extension 2b of the irradiation contour 11 along the specified trajectory 6 of between 4 μm and 30 μm has proven favorable for the self-cleavage.

The irradiation contours 11 can also have a geometry other than an elliptical geometry. For example, the irradiation contours 11 can form polygons, e.g. rhombuses or rectangles, or be linear. In this case, too, it is favorable if the irradiation contours 11 have a greater maximum extent transversely to the straight path curve 6 (in the Y direction) than along the straight path curve 6 (in the X direction).

At the in 2 In the example shown, a distance d between adjacent irradiation contours 11 is between 50 μm and 400 μm. Such a distance d between the irradiation contours 11 or generally the provision of a lateral offset between adjacent irradiation contours 11 has proven favorable for the self-splitting of the crystal strip 2 . On the other hand, an overlap between adjacent irradiation contours 11 when the pulsed laser beam 3 irradiates the rear side 5b of the crystal strip 2 has proven to be rather unfavorable for the splitting.

As in 1d is shown, the laser beam 3 is moved along the positive X-direction during the movement over the crystal strip 2, specifically with a (mean) feed speed which corresponds approximately to the gap propagation speed in the material of the crystal strip 2. The dwell time of the pulsed laser beam 3 on a respective irradiation contour 11 is typically longer than the time between the traversing of two adjacent irradiation contours 11. The pulsed laser beam 3 covers the distance d between two adjacent irradiation contours 11 at a jump speed vs, which is typically between 100 mm/s and 4000 mm/s. A marking speed v _M when traversing a respective irradiation contour 11 with the pulsed laser beam 3 is typically of the order of magnitude between 10 mm/s and 200 mm/s.

wobei a die große Halbachse der ellipsenförmigen Einstrahlungskontur 11 und B die Breite des Kristallstreifens 2 bezeichnen.As a rule it is favorable if the respective breaking edge 8 runs along a crystal plane of the crystal strip 2 . The X-direction and the Y-direction along which the wafer 1 at in 1a shown is aligned, can not run exactly along the crystal axes due to production, but have a predetermined tolerance angle to the crystal axes. Adjustment tolerances can also occur when aligning the crystal strip 2 for the cleavage in a holding device provided for this purpose, for example in the form of a vice or the like. In order to ensure that the pulsed laser beam 3, when moving along the straight trajectory 6, strikes a crystal axis 12 that is aligned at a tolerance angle α or a (maximum) angular deviation from the X-direction, the following condition must be met: $$\text{a=Btan}\left(\mathrm{a}\right),$$

where a denotes the semimajor axis of the elliptical irradiation contour 11 and B denotes the width of the crystal strip 2 .

In principle, it is possible by means of the self-cleavage method described here to cleave the crystal strip 2 along a fracture plane or fracture edge 8 that can be oriented in practically any way. In particular, it is not absolutely necessary for the breaking edge 8 to run in a plane perpendicular to the rear side 5b of the crystal strip 2 or in the thickness direction (Z-direction). Rather, the breaking edge 8 can also be aligned at an angle to the thickness direction (Z-direction). This is particularly favorable in order to form end facets 10 on the waveguide structures 4, which are aligned, for example, at Brewster's angle to the incident radiation. To create laterally inclined breaking edges 8 the pulsed laser beam 3 is generally not radiated perpendicularly onto the rear side 5b of the crystal strip 2, as described above, but at an angle other than 90°.

For the ablation of material of the crystal strip 2 it is necessary that the pulsed laser beam 3 is absorbed in the material of the crystal strip. It is therefore favorable if the material of the crystal strip 2 is not transparent to the wavelength λ of the laser beam 3. In principle, the wavelength λ of the laser beam 3 can lie in a value range between approximately 200 nm and approximately 10 μm. With typical optical crystals 2 such as lithium niobate, lithium tantalate or potassium titanyl phosphate, it is favorable if the wavelength λ of the laser beam 3 is less than 550 nm, in particular less than 400 nm, and more than 200 nm. In the example shown, the pulse energy of the pulsed laser beam 3 is greater than the ablation threshold of the material of the crystal strip 2. In the example shown, in which the material of the crystal strip 2 is lithium niobate, the (mean) pulse energy should be between approx µJ and 10 µJ.

Pulse durations τ of the laser beam 3 that are between 10 fs and 8 ns, in particular between 100 fs and 10 ps (ultra short pulses) have proven to be favorable for the self-cleavage or for the material modification. The ablation of the material of the crystal strip 2 is also promoted if the pulsed laser beam 3 is irradiated with a repetition rate between 1 kHz and 10 MHz, in particular between 500 kHz and 2000 kHz.

Although the self-cleavage process has been described above in connection with strip-shaped optical crystals or crystal strips 2, it is also possible to use an optical crystal in the form of the in 1a to split the wafer 1 shown in this way. In this case, the associated with 1a described step of separating the wafer 1 into a plurality of crystal strips 2 is carried out using the self-cleavage method described above. For example, vacuum holders with a large suction surface, eg with components made of porous ceramics, or complex clamping devices made of metal and plastics, eg with serrated and toothed structures, can be used to hold the wafer 1 or the crystal strips 2 during the splitting.

The irradiation of the pulsed laser beam 3 is generally carried out in a protective gas atmosphere. The protective gas atmosphere or the gas or gases used to generate the protective gas atmosphere can be, for example, nitrogen or argon. Suitable atmospheric and flow conditions are favorable for carrying out the process.

It goes without saying that the self-cleavage method described here can also be carried out on optical crystals 2 which are doped or which have a refractive index structure for planar waveguides or for rib waveguides, eg PE (proton-exchanged) LiNbO ₃ .

The optical crystal 2 can also be part of a composite material, for example the composite material can be LNol. In this case, the pulsed laser beam 3 is not irradiated on the back side 5b of the optical crystal 2 but on the back side 5b of a crystalline substrate 13. The optical crystal 2 is bonded to the substrate 13 at a front side 5a of the substrate 13 . In the example shown, an intermediate layer 14 is formed between the substrate 13 and the optical crystal 2, which is lithium niobate. The intermediate layer 14, which can consist of SiO ₂ , for example, favors the production of a material connection of the optical crystal 2 to the substrate 13 when the connection is produced by irradiating a pulsed laser beam, as is the case in FIG DE102020211282.7 is described by the applicant. It goes without saying that the composite material does not necessarily have to have an intermediate layer 14, ie the optical crystal 2 can, if necessary, be connected directly to the crystalline substrate 13.

The crystalline substrate 13 is the one in 5 The example shown is a semiconductor, more precisely monocrystalline silicon. The substrate 13 is split along a breaking edge as described above in connection with the optical crystal 2 by modifying the material of the crystal substrate 13 to a penetration depth T which is at least 30%, preferably at least 40%, of a thickness D of the crystal -Substrate 13 corresponds. When the crystal substrate 13 self-cleaves, the optical crystal 2 and the intermediate layer 14 are also cleaved. In this case, the optical crystal 2 is located in a partial area of the breaking edge, which is formed by the self-cleavage and in which the breaking edge has an optical quality.

The thickness D _M of the optical crystal 2 is at the in 5 shown example is significantly less than the thickness D of the substrate 13. As a rule: D _M <0.1 D. In the example shown, the optical crystal 5 has a thickness D _M between approximately 5 μm and approximately 10 μm. The thickness D of the substrate 13 is of the order of approx. 500 μm. The intermediate layer 14 has a thickness of the order of about 2 μm, which is negligible for the cleavage. That The method for cleaving the crystal substrate 13 is carried out analogously to the method described above in connection with the cleaving of the optical crystal 2. Only the wavelength λ of the laser beam 3 is adapted to the silicon material of the substrate 13 and is approximately 1030 nm in the example shown.

QUOTES INCLUDED IN DESCRIPTION

This list of documents cited by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent Literature Cited

• US 9636783 B2 [0008]
• US8969761B2 [0009]
• WO 2011/163149 A2 [0010]
• DE 102020211282 [0036, 0074]

Claims (18)

Method for splitting a crystal (2, 13), comprising: irradiating a pulsed laser beam (3) onto the crystal (2, 13) to modify, in particular to remove, material of the crystal (2, 13), characterized in that when Radiating in the pulsed laser beam (3), the material of the crystal (2, 13) is modified to a penetration depth (T) which corresponds to at least 30%, preferably at least 40%, of a thickness (D) of the crystal (2), with the Radiation of the pulsed laser beam (3) the crystal (2, 13) along a predetermined breaking edge (8) is split. procedure after claim 1 , in which the breaking edge (8) runs along a crystal axis (12) and/or in a plane (XZ) perpendicular to a surface (5b) of the crystal (2, 13) onto which the pulsed laser beam (3) is irradiated. procedure after claim 1 or 2 , in which the material of the crystal (2) is modified when the pulsed laser beam (3) is irradiated to a penetration depth (T) which is no more than 70%, preferably no more than 60%, of the thickness (D) of the crystal (2 , 13). Method according to one of the preceding claims, in which the pulsed laser beam (3) radiates onto a surface (5b) of the crystal (2 , 13) is irradiated. procedure after claim 4 , in which a respective irradiation contour (11) has an extent (2a) transversely to the predetermined trajectory (6) which is greater than an extent (2b) along the predetermined trajectory (6). procedure after claim 4 or 5 , in which the extension (2a) of the irradiation contour (11) is transverse to the predetermined trajectory (6) between 40 microns and 200 microns. Procedure according to one of Claims 4 until 6 , in which the extension (2 b) of the irradiation contour (11) along the specified trajectory (6) is between 4 μm and 30 μm. Procedure according to one of Claims 4 until 7 , in which a geometry of the irradiation contour (11) is selected from the group comprising: ellipses, polygons, in particular rhombuses or rectangles, and lines. Procedure according to one of Claims 4 until 8th , in which a distance (d) between adjacent irradiation contours (11) is between 50 μm and 400 μm. Procedure according to one of Claims 4 until 9 , in which a jumping speed (v _s ) between adjacent irradiation contours (11) is between 100 mm/s and 4000 mm/s. Procedure according to one of Claims 4 until 10 , in which a marking speed (v _M ) along the irradiation contours (11) is between 10 mm/s and 200 mm/s. Procedure according to one of Claims 4 until 11 , in which the crystal (2, 13) is strip-shaped and the predetermined trajectory (6) extends transversely to a longitudinal direction (Y) of the strip-shaped crystal (2). Method according to one of the preceding claims, in which the crystal is an optical crystal (2), which is preferably selected from the group comprising: lithium niobate, lithium tantalate or potassium titanyl phosphate. Procedure according to one of Claims 1 until 12 , in which the crystal forms a substrate (13), preferably in the form of a semiconductor crystal, in particular in the form of a silicon crystal, which forms a material composite with an optical crystal (2), wherein when the pulsed laser beam (2) is irradiated on a the side (5b) of the substrate (13) facing away from the optical crystal (2), the optical crystal (2) is also split along the breaking edge (8). procedure after Claim 13 or 14 , in which at least one waveguide structure (4) is formed on the optical crystal (2) on a side (5a) facing away from the irradiated laser beam (3), the breaking edge (8) having an end facet (10 ) of the waveguide structure (4) forms. Method according to one of the preceding claims, in which the pulsed laser beam (3) is irradiated with a wavelength (λ) which is between 200 nm and approx. 10 µm, preferably between 200 nm and 1100 nm. Method according to one of the preceding claims, in which the pulsed laser beam (3) is irradiated with a pulse duration (τ) of between 10 fs and 8 ns, preferably between 100 fs and 10 ps. Method according to one of the preceding claims, in which the pulsed laser beam (3) is irradiated with a repetition rate between 1 kHz and 10 MHz, in particular between 500 kHz and 2000 kHz.

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