How to divide a transparent workpiece

Abstract

The invention relates to a method for dividing a transparent workpiece (1) by forming a beam focus zone (3) in the volume of the workpiece with pulsed laser radiation (2), the intensity of which is above the threshold for nonlinear absorption, moving the beam focus zone (3) and the workpiece (1) relative to one another, thereby forming a two-dimensional weakening in the workpiece (1) extending along a predefined separation line (4), and then dividing the workpiece (1) along the separation line (4).The invention proposes to suppress the nonlinear propagation of the laser radiation (2) in the volume of the workpiece (1) outside the beam focus zone by selecting the duration of the energy input caused by the nonlinear absorption of the pulsed laser radiation and by spatially shaping the beam.

Description

[Technical field]

[0001] A method of dividing a transparent workpiece by using pulsed laser radiation to create a beam focus zone within a volume of the workpiece, where the intensity of the laser radiation is above a threshold for nonlinear absorption, moving the beam focus zone and the workpiece relative to one another, thereby forming a two-dimensional weakened portion in the workpiece extending along a predefined separation line, and then dividing the workpiece along the separation line. [Background technology]

[0002] The division of a wafer substrate into chips, i.e. the so-called dicing of a wafer, is important in the production of semiconductor devices which are becoming smaller and more complex. Traditional methods of dicing are based on the use of diamond saws for wafers thicker than 100 μm. For thinner wafers, laser-based methods are increasingly being used.

[0003] US Pat. No. 5,399,433 describes a method of using pulsed laser radiation to form a beam focus zone in the volume of a workpiece, i.e. a semiconductor substrate. In the beam focus zone, the intensity of the laser radiation locally exceeds the threshold for nonlinear absorption. Correspondingly, in the beam focus zone, multiphoton processes occur, for example in the form of multiphoton ionization or avalanche ionization, which leads to the formation of plasma. The plasma formation rate increases significantly above a threshold value that depends on the material of the workpiece and the parameters of the laser radiation. This is therefore also called "optical breakdown". The resulting modification of the workpiece, and thus the processing, is highly accurate, since small amounts of spatially localized energy are reproducibly introduced into the material. This good spatial localization is achieved by focusing the laser radiation using an optical probe with a high numerical aperture that is as free of aberrations as possible. The beam focus zone is formed in the form of a "spike-like" focal volume in the above-mentioned US Pat. No. 5,399,433, which extends in the direction of the laser beam axis. This beam focus zone is moved relative to the workpiece, which allows the creation of a two-dimensional weakened portion in the workpiece extending along a predefined separation line. Possible weakening mechanisms resulting from the induced modifications are the creation of voids and / or cracks, structural changes in the workpiece material, cracks respectively connected to the modified area, transient or permanent stresses, thermo-mechanical stresses, localized volume increase or decrease stresses, solidification cracking, etc. Finally, the actual separation of the workpiece occurs by applying a small mechanical force or stress, which causes the workpiece to break in the area of ​​the weakened portion, i.e. along the separation line.

[0004] Of great importance is a uniform modification of the material over and up to a given depth of the workpiece. This improves the splitting, minimizes manufacturing defects such as chipping or material distortion, and higher edge strength is achieved. In known methods, the modification in the workpiece is caused by laser pulses with wavelengths between 500 nm and 2000 nm and repetition rates between 10 kHz and 2 MHz, and pulse durations in the range of 100 to 15000 fs. In known methods, the shaping of the beam to form the beam focus zone is designed based on the undisturbed linear propagation of the laser radiation in the volume of the workpiece. However, at the fluences required to produce the weakened parts of the material that allow the subdivision, the propagation of the laser radiation in the workpiece is subject to nonlinear effects in the above-mentioned pulse duration range. At narrow pulse durations and high energy densities, the propagation of the laser radiation within the volume of the workpiece is strongly hindered by nonlinear effects (e.g. self-focusing that occurs already outside the beam focus zone as well as two-photon absorption), so that effective energy coupling into the desired area of ​​the beam focus zone is significantly hindered. A well-defined localization of the energy deposition and the resulting modification of the workpiece material cannot be achieved in the case of a high peak intensity of the radiation, for the reason that this peak intensity exists at short pulse durations.

[0005] Longer pulse durations (e.g., >1 ns) and thus lower peak intensities can modify the material, but typically require higher energies and suffer greater damage due to the increased volume to which heat is applied as diffusion effects come into play. The results of the separation process are therefore less satisfactory in terms of the quality of the break line. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] International Publication No. 2016 / 059449 Summary of the Invention [Problem to be solved by the invention]

[0007] Against this background, the problem underlying the present invention is to provide an improved method for dividing transparent workpieces, which would preferably avoid the above-mentioned disadvantages of known methods. [Means for solving the problem]

[0008] The invention achieves this objective starting from the method described at the beginning by suppressing the nonlinear propagation of the laser radiation within the volume of the workpiece outside the beam focusing zone by selecting the duration of the energy input caused by nonlinear absorption of the pulsed laser radiation in the beam focusing zone and / or by spatially shaping the beam.

[0009] The gist of the invention is to take into account nonlinear propagation characteristics in order to introduce weakenings in the workpiece. In order to suppress, i.e. significantly reduce, the nonlinear propagation of the laser radiation outside the beam focus zone, optimal possible process parameters are defined in terms of the duration of the energy input and / or the shaping of the beam, which allows for improved control of the energy deposition. In this respect, finally, the local energy density in the volume of the workpiece is controlled by the duration of the energy input and the energy coupling in the beam focus zone is improved. Damage in the surrounding volume, i.e. outside the beam focus zone, is minimized. The invention achieves that the smallest possible interaction is achieved outside, in particular before the beam focus zone in the propagation direction of the laser radiation, in order to minimize disturbing nonlinear effects (e.g. self-focusing and nonlinear absorption) or other propagation disturbances. Furthermore, it is ensured that during the duration of the nonlinear absorption in the beam focusing zone, the zone of modification or high electron density occurring first in the beam direction does not block more than 50%, preferably more than 20%, particularly preferably more than 10% of the incident energy to the part of the beam focusing zone reached later in the beam direction.

[0010] By minimizing nonlinear propagation, the beam focusing in the beam focus zone allows the appropriate fluence to be achieved in a deliberate and defined manner. As a result, the desired modification occurs only in this region. The combination of beam shaping in time and in space results in a uniform and tailored energy deposition over the entire specific beam focus zone. As a result, the splitting process is facilitated. Chipping or material stress is minimized. The quality of the break line edges is improved compared to the prior art.

[0011] In other words, by deliberately selecting the process parameters (amount and duration of energy input, spatial beam shaping) according to the present invention, the following can be achieved: a) the intensity threshold for nonlinear absorption is exceeded in the beam focusing zone; b) the power or intensity threshold for undesirable nonlinear effects outside the beam focus zone is below; c) The energy required for the desired modification is introduced in a controlled and localized manner, which produces the desired surface weakening with the desired geometry. This is achieved.

[0012] Ideally, the wavelength of the laser radiation should be selected such that the linear absorption of the laser radiation in the material of the workpiece is less than 20% per centimeter of length in the direction of the laser beam, better still less than 10%, particularly preferably less than 5%. Furthermore, the wavelength of the laser radiation should be selected according to the condition that the nonlinear refractive index in the volume of the workpiece is low at that wavelength, so that nonlinear effects do not prevent sufficient energy deposition in the beam focusing zone. At the same time, the wavelength should be within a range in which good focusing is guaranteed.

[0013] According to the invention, it is important to appropriately select the duration of the energy input due to nonlinearly induced absorption of the pulsed laser radiation in the beam focus zone, i.e. at a defined position of the workpiece along the separation line. The duration of the energy input can be specified, for example, by the pulse duration of the pulsed laser radiation. An upper limit of the duration is obtained based on the tolerable size of the thermal damage zone due to thermal diffusion. Furthermore, an upper limit of the duration is given by the maximum tolerable energy absorbed in the beam focus zone. The longer the duration, the higher the energy input, if the intensity is above the threshold for nonlinear absorption. With too much energy and / or with energy introduced for too long a period of time, the location restriction of the weakening to the beam focus zone is impaired. With a view to avoiding nonlinear propagation of the laser radiation outside the beam focus zone, a lower limit of the energy input duration is important. In particular, the pulse duration should be greater than a critical value, which is, for example, the quotient of the pulse energy and the material-specific critical power, above which nonlinear propagation, in particular self-focusing, occurs in the volume of the workpiece. This ensures that the energy deposition is not unduly disturbed by non-linear effects and therefore ensures a sufficiently high localized energy deposition in the beam focus zone.

[0014] Advantageously, the duration (e.g. pulse duration) and the amount (e.g. pulse energy) of the energy input in a pulse event should be selected under the condition that the damage, i.e. the desired modification in the volume of the workpiece in the beam focus zone, is caused by a single laser pulse or a laser pulse burst consisting of a series of a predefined number of laser pulses. A suitable single pulse can be, for example, a laser pulse with a Gaussian time profile of a specific pulse duration. A burst contains a predefined number of laser pulses with a short time interval (pulse repetition frequency in the GHz or THz range). Such a single burst is also considered as a pulse event if the time interval between the bursts is at least 100 times greater than the duration of the individual bursts. The modification in a specific location of the workpiece is preferably caused completely during a single such "pulse event" (single pulse or single burst). The energy input in the context of the present invention therefore relates to the energy input during a single pulse event due to nonlinearly caused absorption in the beam focus zone. The duration of the energy input therefore results from the pulse duration or burst duration. It should be noted that the laser pulses do not necessarily have to be Gaussian, "flat-top" or have any other common shape. Any pulse shape is conceivable. What is important is the effective duration of the energy input. In the method according to the invention, the effective duration is preferably 20-500 ps.

[0015] The two-dimensional weakened portion is then repeatedly formed by incrementally moving the workpiece relative to the beam focus zone along the separation line with each pulse event. Where possible, the weakened portion is formed during a single motion event along the separation line. This allows high process speeds to be achieved and ensures that the workpiece is broken with high quality parted edges along the separation line.

[0016] Preferably, the shortest possible duration of the energy input is identified, which has a probability of at least 80%, preferably at least 90%, more preferably at least 95%, that the desired modification occurs. The duration of the energy input, when the weakening is introduced, is then selected to be greater than or equal to this identified shortest possible value. This value may depend on a number of factors (material, size and geometry of the beam focus zone, wavelength, pulse shape, etc.). For example, the selected duration of the energy input may exceed the identified shortest possible value by a factor of 10, preferably 5, particularly preferably 2. Ideally, the factor is in the range of 1.1 to 5. This provides a good control of the energy input for the desired formation of the weakening.

[0017] More preferably, the beam focusing zone has an elongated shape perpendicular to the surface of the workpiece. As in WO 2016 / 059449 cited above, the beam focusing zone is preferably elongated in the beam direction and therefore extends over a large part of the entire thickness of the workpiece to ensure a suitable weakening. For example, the length of the beam focusing zone in the beam direction may be at least 10 times, preferably at least 50 times, particularly preferably at least 100 times, greater than the extension of the beam focusing zone perpendicular to the desired two-dimensional weakening. The laser beam may have a primarily Gaussian profile or any other realizable input beam shape. Particularly suitable are Gauss-Bessel beams or other beam shapes, which may be described as substantially non-diffracting beams. The tailored spatial intensity distribution in the beam focusing zone is expediently achieved by suitable optical elements, such as beam shaping optics or focusing optics in combination with adaptive beam shaping elements, such as spatial light modulators (SLMs) or piezo mirrors. It can be improved by depth-dependent aberration correction. The goal of spatial beam shaping is to achieve as unobstructed an energy introduction as possible to the desired beam focal zone, extending from the surface of the workpiece (beam incidence plane) to the depth within the workpiece required for the separation process.

[0018] At the same time, the extent of the beam focus zone transverse to the beam axis is preferably greater in a direction parallel to the plane of the weakening than in a direction perpendicular to the plane of the weakening, so that during repeated introduction of modifications based on relative motion between the beam focus zone and the workpiece, the portion of the laser radiation that is each affected (blocked) by a previously introduced modification is minimized.

[0019] As mentioned above, the present invention aims to achieve the smallest possible interaction outside, in particular in front of the beam focusing zone in the direction of propagation of the laser radiation, in order to minimize disturbing nonlinear effects. Furthermore, it is desirable to ensure that the zone of reformation or high electron density that occurs first during a pulse event in the beam focusing zone blocks only as small a portion of the incident energy as possible to the part of the beam focusing zone that is reached later. A suitable beam shaping contributes to this. The beam shaping is advantageously carried out in such a way that the angle formed by the beam axis with the beam components (consisting of individual beams or bundles of individual beams) of the laser radiation that focus in the volume of the workpiece near the surface of the workpiece is equal to or smaller than the angle formed by the beam axis with the beam components that focus further away from the surface of the workpiece in the volume of the workpiece. This should in any case apply to the majority of the beam components that focus in the beam focusing zone, and in individual cases deviations of small portions of the beam components from this geometry can be tolerated as long as nonlinear effects are sufficiently suppressed.

[0020] The method according to the invention is particularly suitable for dicing semiconductor wafers into chips. The proposed temporal and spatial beam shaping prevents or at least minimizes nonlinear propagation of the laser radiation, which is detrimental to the dicing process. As a result, the propagation of the laser radiation in the material of the substrate is not hindered and an elongated modification zone can be introduced with each pulse event. By repeating the procedure with the relative movement of the laser beam and the substrate, a two-dimensional weakening is formed along a predefined separation line. This weakening serves as a predefined breaking point during the subsequent breaking due to the applied tensile stress. Thus, even thin semiconductor substrates on the one hand and thick semiconductor substrates on the other hand can be effectively separated at minimally thermally stressed or differently weakened breaking points. Due to the small extension of the modification zone transverse to the weakening plane, it is possible to introduce modifications closely adjacent to one another in the feed direction without the beam propagation being significantly hindered by the modifications introduced as described above. The contour accuracy of the separation line is particularly high. Production errors and rejects are minimized. At the same time, high process speeds can be achieved.

[0021] The method according to the invention is also suitable for dividing flat glass products or ceramic and crystalline workpieces.

[0022] Further features, details and advantages of the invention are apparent from the following description of exemplary embodiments, based on the claims and the drawings. [Brief description of the drawings]

[0023] [Figure 1] 1 is a schematic diagram of a method according to the present invention for cleaving or dividing a transparent workpiece. [Diagram 2] FIG. 2 is a schematic diagram illustrating spatial beam shaping according to the present invention. [Diagram 3] FIG. 13 illustrates the introduction of repeated modified zones with different spacing. [Figure 4] 1 is a flow chart of a process for optimizing method parameters. [Diagram 5] 1 is a flow chart of a process for optimizing pulse duration. [Figure 6] 1 is a flow chart of a process for optimizing pulse energy. [Figure 7] FIG. 13 illustrates the interrelationship of process parameters for the repeated introduction of modified zones. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] FIG. 1 shows the main steps of a method according to the invention for cleaving or dividing a workpiece 1, for example a wafer in semiconductor manufacturing. A laser beam 2 in the form of pulsed laser radiation is emitted from above onto the workpiece. The laser beam 2 is shaped (for example by focusing optics in combination with a spatial light modulator (not shown)) in such a way that a beam focus zone 3 elongated in the beam direction is created in the volume of the workpiece 1. In the beam focus zone 3, the intensity of the laser radiation exceeds the threshold for nonlinear absorption, which leads to a correspondingly spatially limited modification of the material of the workpiece 1. The beam focus zone 3 is gradually moved (arrow direction) relative to the workpiece. During the process, a number of modification zones 5 are formed in the volume of the workpiece 1, which are located next to each other along a separation line 4, which together form a weakened plane. The workpiece 1 is then broken into two parts 1b, 1c along the separation line 4 by applying a small mechanical force. The separation line does not have to be straight as shown in FIG. 1. Separation of the workpiece parts 1a, 1b along a curved separation line is also conceivable.

[0025] The gist of the invention is to take into account the nonlinear propagation properties of the laser radiation 2 in order to introduce a modified zone 5 in the workpiece 1. The propagation of the laser radiation 2 into the volume of the workpiece 1 is on the one hand restricted to the desired region of the beam focus zone 3, but on the other hand is so strongly hindered by nonlinear effects (e.g. self-focusing and two-photon absorption already occurring outside the beam focus zone) that an effective energy coupling that fills the beam focus zone 3 as completely as possible is considerably hindered. In order to suppress, i.e. to reduce, such a nonlinear propagation of the laser radiation 2 outside the beam focus zone 3, the invention defines possible process parameters that are optimal in terms of the duration of the energy input and the shaping of the beam, thus allowing a wide range of control of the energy deposition. Damage in the surrounding volume, i.e. outside the beam focus zone 3, is reduced to a minimum.

[0026] FIG. 2 shows the spatial beam shaping according to the invention diagrammatically by a cross section through the workpiece 1. In the left diagram of FIG. 2, two beam components 6, 7 of the laser radiation 2 (FIG. 1) incident from above are focused in a beam focus zone 3 far below the surface (beam incidence surface) of the workpiece. The beam components 6, 7 form an acute angle with the beam axis 8, resulting in an elongated beam focus zone 3. The beam components 6, 7 propagating through the volume of the workpiece 1 at a predefined angle to the beam axis 8 ensure that the overlap of the beam components 6, 7 occurs exclusively within the beam focus zone 3. Outside the beam focus zone, the fluence of the laser radiation in the volume of the workpiece remains low so that as few nonlinear effects as possible occur. In the central diagram of FIG. 2, further beam components 9, 10, 11, 12 of the laser radiation 2 (FIG. 1) are added, which "fill" the area between the lowest beam focus zones 3 with further beam focus zones 3', 3''. To prevent nonlinear propagation of the laser radiation 2 outside the beam focus zones 3, 3', 3'' the angle between the beam components 9, 10, 11, 12 of the laser radiation 2, which are focused closer to the workpiece surface in the volume of the workpiece 1, and the beam axis 8 is the same (as in FIG. 2) or has a smaller angle between the beam components 6, 7, which are focused further away from the workpiece surface in the volume of the workpiece 1, and the beam axis 8. This eliminates the possibility of overlapping the different beam components 6, 7, 9, 10, 11, 12 outside the beam focus zones 3, 3', 3'', thus creating a fluence that allows nonlinear effects. In the example shown on the right hand side of FIG. 2, the different beam components 6, 7, 9 and 10, 11, 12 respectively merge with each other in two wider beam components 13, 14, thereby forming a single elongated beam focus zone 3.The beam shaping ensures that during one pulse event (single laser pulse or pulse burst) in the beam focus zone 3, the alteration (or zone of high electron density) occurring first (i.e., far up in FIG. 2) blocks only a small portion of the incident energy to the later arriving (i.e., far below) portion of the beam focus zone 3. This can generally be achieved by the narrowest possible angular range of the beam components 6, 7, 9, 10, 11, 12, 13, 14 of the incident laser radiation with respect to the beam axis 8.

[0027] FIG. 3 shows, by means of a cross-section (top) and a plan view (bottom) of the workpiece 1, the gradual introduction of a number of modification zones 5 by converging beam components 13, 14 of the laser radiation 2. Here, as described above, the workpiece 1 is moved relative to the beam focusing zone 3 (towards the right in FIG. 3). In the left image, a number of modification zones 5 are introduced closely adjacent to each other. Here, a part of the laser radiation 2 is blocked by the previously already introduced modification zone 5 in each case. The corresponding blocking angle segment is shown in the bottom image of FIG. 3. It can be seen that this angle segment is larger in the case of closely spaced modification zones 5 (left image) than in the case of more widely spaced modification zones 5 (middle image). On the right side, the modification zones are again closely adjacent to each other. However, in this case the beam is shaped in such a way that the extent of the beam focus zone 3 and the correspondingly appearing modified zone 5 is greater transversely to the beam axis 8 than in the direction parallel to the weakening plane (i.e. along the parting line 4) and perpendicular to the weakening plane. As a result, during the repeated introduction of modified zones 5, the portion of the laser radiation 2 blocked by each previously introduced modified zone 5, respectively as described above, is reduced. It can be seen that in the lower right diagram the blocking angle segment is smaller than in the lower left diagram. Thus, the smaller extent of the modified zones 5 transversely to the parting line 4 allows for closely spaced modified zones, so that an overall larger proportion of the area may be weakened and thus the quality of the break can be improved.

[0028] FIG. 4 shows an example of a procedure for optimizing process parameters according to the invention. First, the wavelength of the laser radiation is identified based on the state of the workpiece 1 (material, thickness). Ideally, the wavelength of the laser radiation should be selected such that the linear absorption of the laser radiation in the material of the workpiece is less than 20%, better still less than 10%, particularly preferably less than 5% over a length of one centimeter in the direction of the laser beam. Furthermore, the wavelength of the laser radiation should be selected under the condition that the nonlinear refractive index in the volume of the workpiece is as low as possible at this wavelength. For example, a longer wavelength reduces the two-photon absorption outside the beam focusing zone. At the same time, the wavelength should be in a range in which good focusing is guaranteed, and from this point of view, a shorter wavelength is preferred. Based on mutually different optimization criteria, a suitable wavelength is identified. In the next step, the beam shaping is identified according to the above-mentioned criteria, also taking into account the thickness and material (refractive index) of the workpiece. An iterative optimization of the pulse duration and pulse energy is then performed, again with the proviso that non-linear effects in the propagation of the laser radiation 2 through the volume of the workpiece 1 outside the beam focusing zone 3 are avoided or at least reduced. Further details on this are given below with reference to Figures 5 and 6.

[0029] In order to achieve the best possible control of the energy deposition, the shortest possible duration of the energy input caused by the nonlinear absorption of the pulsed laser radiation is identified, i.e. in this case the shortest possible pulse duration for the achievement of the desired modification in the workpiece 1. This pulse duration depends on the parameters already specified beforehand, namely the material, the geometry of the beam focus zone 3, i.e. the beam shape and the wavelength. The optimization steps shown in FIG. 5 are used to find the optimal pulse duration. Possible starting points for the iterative optimization may arise on the one hand from the energy density required to modify the material of the workpiece 1 and on the other hand from the respective critical power at which self-focusing of the laser radiation 2 propagating in the volume of the workpiece 1 occurs. The pulse duration must at least be selected long enough so that the peak power of the laser radiation achieved is below the parameter of the material-specific limit for self-focusing. In the sequence shown in FIG. 5 and FIG. 6, the demonstration of successful modification with at least 95% probability is carried out, for example, according to ISO 21254 ("Lasers and laser-related equipment - Test methods for laser-induced damage threshold"). The pulse duration and pulse energy of the pulsed laser radiation 2 are determined by optimization in such a way that the modification by a laser pulse burst consisting of a single laser pulse or a predefined train of laser pulses occurs reliably (at least with a probability of 95%). At the same time, the shortest necessary pulse duration is selected, for which the modification still reliably occurs. If improved results and / or increased process stability can be achieved after the optimization of the pulse duration according to FIG. 5, the pulse duration can be adjusted upwards as required accordingly. The upper limit is further determined by the thermal damage range during the laser irradiation. According to FIG. 6, the fluence in the desired modification range (probability >95%) is then adjusted by increasing or decreasing the pulse energy according to a similar procedure.The optimization steps illustrated in FIGS. 5 and 6 ensure that optimal pulse duration and pulse energy are used for the selected beam shaping, i.e. for the desired geometry of the modification zone 5.

[0030] In a practical embodiment of the invention, a silicon wafer with a thickness of 525 μm is irradiated with pulsed laser radiation at a wavelength of 1960 nm. A significant decrease in the nonlinear propagation and therefore the first occurrence of modification can be observed from a pulse duration of 20 ps. The determination of the probability of modification by a single laser pulse shows that a probability of >95% is achieved from a pulse duration of 25 ps. In this case, the pulse energy is 15 μJ. Thus, a modification zone with a diameter of 5 μm and a length in the beam direction of 350 μm can be formed by spatial pulse shaping. The modification zones are aligned at intervals of 10 μm by moving the focus of the laser radiation, i.e. the beam focusing zone, relative to the wafer. The wafer can then be broken at right angles with small mechanical forces. A precise break line is formed. The heat affected zone along the break line is small. The surface roughness is less than 5 μm.

[0031] To create a continuous weakening in the material of the workpiece 1, the speed of the relative motion of the laser beam 2 and the workpiece 1 can be reduced to such an extent that there is an overlap of the modified zones 5 within the volume of the workpiece 1. The graph of FIG. 7 shows the relationship between the feed rate α, the modification probability P and the input pulse energy E in X1 indicates the area where the modification is introduced in a continuous form (overlapping each other). Each selected energy E inIn regime X2, the pulse energy is too low and thus the probability of modification is too low. Sufficient weakening of the workpiece 1 to allow fracture is not achieved. If the feed rate is increased, then, at the same repetition rate of the pulse events, the distance between the introduced modified zones 5 increases, resulting in the formation of separated, i.e. non-overlapping modified zones 5 (regime X3). In regime X4, the pulse energy is too low and thus the probability of modification is too low.

Claims

1. 1. A method for dividing a transparent workpiece (1) by forming a beam focus zone (3) in a volume of the workpiece with pulsed laser radiation (2), in which the intensity of the laser radiation (2) is above a threshold for nonlinear absorption, moving the beam focus zone (3) and the workpiece (1) relative to one another, thereby forming a two-dimensional weakening in the workpiece (1) extending along a predefined separation line (4), and then dividing the workpiece (1) along the separation line (4), A method comprising: suppressing nonlinear propagation of the laser radiation (2) within the volume of the workpiece (1) outside the beam focusing zone (3) by selecting the duration of energy input and / or spatially shaping the beam resulting from nonlinear absorption of the pulsed laser radiation in the beam focusing zone.

2. 2. The method according to claim 1, characterized in that the wavelength of the laser radiation (2) is selected in accordance with the proviso that the linear absorption of the laser radiation (2) at said wavelength is less than 20% per centimeter, preferably less than 10%, particularly preferably less than 5% per centimeter.

3. 3. The method according to claim 2, characterized in that the wavelength of the laser radiation (2) is selected according to the condition that the nonlinear refractive index in the volume of the workpiece (1) is as low as possible at said wavelength, in particular so low that the nonlinear propagation does not prevent a sufficient energy input into the beam focusing zone to form the weakened portion.

4. 4. The method according to claim 1, wherein the pulse duration of the pulsed laser radiation is greater than a critical value, which is the quotient of the pulse energy and a material-specific critical power, above which nonlinear propagation, in particular self-focusing, occurs within the volume of the workpiece (1).

5. 5. The method according to claim 1, characterized in that the pulse duration and the pulse energy of the pulsed laser radiation (2) are selected in accordance with the condition that a modification by nonlinear absorption of the laser radiation (2) occurs in the volume of the workpiece in the beam focusing zone (3) by a single laser pulse or a laser pulse burst consisting of a train of a predefined number of laser pulses.

6. 6. The method according to claim 1, further comprising determining a shortest possible duration of energy input for which the modification occurs with a probability of at least 80%, preferably at least 90%, particularly preferably at least 95%, and selecting the duration of energy input such that it is longer than or equal to the determined shortest possible value, preferably 1 to 20 times longer, particularly preferably 1.1 to 5 times longer.

7. 7. The method according to claim 1, wherein the beam focusing zone (3) has an elongated shape along a beam axis (8) directed substantially perpendicular to the surface of the workpiece, and the length of the beam focusing zone in the direction of the beam is at least 10 times, preferably at least 50 times, particularly preferably 100 times, greater than the extension of the beam focusing zone perpendicular to the beam axis.

8. 8. The method according to claim 7, characterized in that the extent of the beam focusing zone (3) in a direction transverse to the beam axis is preferably more than 1.2 times, particularly preferably more than 2 times, greater in a direction parallel to the weakening plane than in a direction perpendicular to the weakening plane.

9. 9. The method according to claim 1, wherein the shaping of the beam is performed such that an angle formed by the beam components (6, 7, 9, 10, 11, 12) of the laser radiation, which converge in the volume of the workpiece near the surface of the workpiece, and the beam axis (8) is less than or equal to an angle formed by the beam components (6, 7, 9, 10, 11, 12) which converge in the volume of the workpiece further away from the surface of the workpiece, and the beam axis (8).

10. 10. The method according to claim 1, wherein the material of the workpiece is silicon, the pulse duration of the pulsed laser radiation is in the range of 20-500 ps, ​​and the wavelength of the laser radiation is in the range of 1300-2500 nm.

11. 11. Use of the method according to any one of claims 1 to 10 for dividing a semiconductor wafer into chips.

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