Methods for removing material from a surface

Abstract

A method for removing material from a surface, wherein material is removed from the surface by means of at least one laser system (100), wherein the surface is subjected to material removal a) in a first ablation mode with at least one low-frequency laser pulse train (140) of the at least one laser system (100), and b) in a second ablation mode with high-frequency pulse packets (170), wherein a single-pulse repetition rate of individual laser pulses (160) in the high-frequency pulse packets (170) is of at least 0.5 GHz to at most 100 GHz, characterized in that the surface is processed i. until a disturbance geometry occurs in the first ablation mode, and ii. after the occurrence of the disturbance geometry in the second ablation mode, wherein iii. steps i) and ii) are carried out repeatedly in at least one surface region.

Description

The invention relates to a method for removing material from a surface. In such a process, material is removed from a surface using at least one laser system. Increasing the amount of material removed per unit time by scaling the power is often limited by thermal surface effects. US 2010 / 0246611 A1 describes the use of a series of laser pulse bundles for the micromachining of target structures. EP 3639966 A1, a subsequently published prior art document, describes a method for cutting a semiconductor material by irradiating it with laser energy. In particular, residual heat in the material, due to a strong temperature gradient, leads to a melting dynamic that freezes rapidly due to the onset of thermal conduction. If surface irregularities are to be avoided, the achievable fluence, and thus the usable pulse energy, is limited.Especially at high fluences and / or high single-pulse repetition rates of individual pulses from the laser system, cone-like protrusions (CLPs) form, creating a foam-like structure and thus reducing the surface quality of the treated surface. This is particularly problematic in connection with newer laser systems that are actually capable of providing higher pulse powers and fluences, which, however, cannot be utilized due to the formation of these irregular geometries. Therefore, potential for achieving high surface quality is wasted in terms of both power and throughput. The invention is based on the objective of creating a method for removing material from a surface without the aforementioned disadvantages occurring. The problem is solved by creating the present technical teaching, in particular the technical teaching of the independent claims as well as the embodiments disclosed in the dependent claims and the description. The problem is solved in particular by creating a method for removing material from a surface, wherein material is removed from the surface by means of at least one laser system, wherein the surface is subjected to material removal in a first ablation mode with at least one low-frequency laser pulse train of the at least one laser system, and in a second ablation mode with high-frequency pulse packets, wherein the single-pulse repetition rate of individual laser pulses in the high-frequency pulse packets is at least 0.5 GHz up to a maximum of 100 GHz. In the second ablation mode, it is particularly possible – depending on the material being processed – to remove a larger amount of material per unit of time, thus increasing the throughput, and wherein any existing interference geometry can be smoothed, either alternatively or additionally.Thus, the restriction regarding fluence or pulse power for the first ablation mode is eliminated, allowing for the selection of individual pulses with higher fluence or pulse power for the low-frequency laser pulse train, which also contributes to an increase in the efficiency and throughput of the process. Any resulting geometries due to this increase in fluence can easily be removed in the second ablation mode. At the same time, the first ablation mode advantageously avoids burr formation at an ablation edge of the surface, particularly in the area where the laser beam enters the processed material. Depending on the material being processed, it is also possible that a higher ablation rate is achieved in the first ablation mode than in the second, but a higher surface quality is achieved in the second mode than in the first.This allows the respective advantages of the different removal modes to be combined to their advantage. A laser pulse train, whether high-frequency or low-frequency, is generally understood to be a sequence of individual laser pulses, also called single pulses, that follow one another at a specific single-pulse repetition rate. Such a laser pulse train can be a single pulse train or a pulse packet. A single-pulse train is defined as a sequence of individual laser pulses that follow one another at a specific single-pulse repetition rate, whereby the individual pulses are not grouped into defined pulse packets. If several single-pulse trains are generated, they will, at most randomly, have the same time interval, but preferably different time intervals from each other. Thus, the single-pulse trains are not generated at a specific pulse packet repetition rate. In contrast, a pulse packet is a group of at least two individual pulses that follow one another at a specific individual pulse repetition rate, also known as the micropulse repetition rate, with several successive pulse packets following one another at a specific pulse packet repetition rate, also known as the macropulse repetition rate. The pulse packets thus have a constant time interval between them. The prefixes “GHz-” and “low frequency-” refer to the single pulse repetition rate of the individual pulses before the terms “laser pulse train”, “single pulse train”, “pulse packet”, and “pulse packet repetition rate”, and in particular indicate whether this is at least 0.5 GHz (“GHz-”) or less than 0.5 GHz (“low frequency-”). According to a preferred embodiment, the low-frequency laser pulse train is a low-frequency single-pulse train, specifically a low-frequency laser pulse train to which no pulse packet repetition rate is assigned. The high-frequency pulse packets are high-frequency laser pulse trains or GHz laser pulse trains to which a defined pulse packet repetition rate is assigned. The individual pulses of the low-frequency laser pulse train in the first ablation mode are preferably generated with a single-pulse repetition rate of less than 0.5 GHz, in particular from a few kHz up to a few 100 MHz, preferably from a few 100 kHz, up to a few MHz, preferably from a few kHz, preferably from a few MHz. The laser system is preferably an ultrashort pulse laser system configured to generate laser pulses with a time width on the femtosecond timescale or the picosecond timescale, in particular with a time width of a few hundred femtoseconds or a few ten picoseconds. A high-frequency pulse packet in the second ablation mode with a single-pulse repetition rate of the individual laser pulses in the high-frequency pulse packet of at least 0.5 GHz up to a maximum of 100 GHz is also referred to as a GHz burst. The high-frequency pulse packets have a pulse packet repetition rate on the kHz scale or on the MHz scale, in particular a pulse packet repetition rate of a few kHz to a few MHz, preferably from a few hundred kHz to a few MHz, preferably a few kHz, preferably a few MHz. The duration of a high-frequency pulse packet is preferably at least 10 ns to at most 10 µs. This means, in particular, that a high-frequency pulse packet preferably comprises at least 5 individual laser pulses, which is also preferred regardless of the duration specified here. Preferably, a high-frequency pulse packet comprises at least 10 individual laser pulses. These values ​​yield particularly good results due to thermal and / or melting effects. According to a further development of the invention, the surface is first processed in the first ablation mode and then in the second ablation mode. The advantages of the first ablation mode, such as the avoidance of burr formation and / or, depending on the material, a higher ablation rate, can be utilized until disadvantages of the first ablation mode become apparent, particularly in the form of the formation of a disruptive geometry. The second ablation mode can then be used to improve the surface quality, in particular to remove the disruptive geometry, and / or, depending on the material, to increase the ablation rate. According to a further development of the invention, the low-frequency laser pulse train in the first ablation mode is configured as a single low-frequency pulse train. As already explained, it is therefore preferably a low-frequency laser pulse train to which no pulse packet repetition rate is assigned. Alternatively, however, it is also possible that a sequence of low-frequency laser pulse trains configured as low-frequency pulse packets is used in the first ablation mode. In particular, depending on the material to be processed, it can be selected whether the low-frequency laser pulse train in the first ablation mode is configured as a single low-frequency pulse train or as a low-frequency pulse packet. According to a further development of the invention, the first ablation mode is selected for processing or forming an ablation edge. In this first ablation mode, burr formation at the ablation edge can be avoided in particular. The ablation edge is an edge or area where the laser radiation first comes into contact with the surface, i.e., where it enters the surface. Alternatively or additionally, the first ablation mode is preferably selected until a disturbance geometry, particularly a thermally induced disturbance geometry, appears on the machined surface. Such disturbance geometry can, for example, take the form of cone-like protrusions. Generally, a disturbance geometry is defined as a geometry on the surface that results in a surface roughness greater than 2 µm. Conversely, a surface roughness greater than 2 µm can also be considered a disturbance geometry. If such a disturbance geometry appears on the surface, it significantly reduces the ablation rate, and the material removal becomes increasingly inhomogeneous. It is then advantageous to smooth the surface using the second ablation mode, particularly to remove the disturbance geometry. The interference geometry occurs primarily due to specific melting dynamics in certain materials. In particular, the single-pulse repetition rate in the first ablation mode is not high enough to prevent the area irradiated by a laser pulse from cooling into the surrounding material of the processed surface via heat conduction. This results in the uncontrolled solidification of material molten by the single laser pulse. In the second ablation mode, the single-pulse repetition rate of the individual laser pulses in the high-frequency pulse packets is much higher. Consequently, such heat conduction-induced solidification effects do not occur, as the time between two laser pulses in a high-frequency pulse packet is insufficient to dissipate a sufficient amount of heat from the target area of ​​the laser pulses into the surrounding material. Alternatively or additionally, the second removal mode is preferred for surface removal. This applies particularly to materials where the removal rate in the second mode is significantly higher than in the first. In this case, the first mode can be used for machining or forming a cut edge – while avoiding burr formation – and the second mode is chosen for surface removal to achieve a high removal rate. Alternatively or additionally, the second ablation mode is preferably chosen to smooth the interference geometry. In this way, it is particularly possible to select a higher fluence or pulse power in the first ablation mode, thereby simultaneously increasing the ablation efficiency and preferably also the ablation rate in the first ablation mode, and to advantageously remove the interference geometry that then occurs again by means of the second ablation mode, i.e., to smooth the surface. Particularly with materials where a higher removal rate can be achieved in the first removal mode than in the second, it is preferable to continue working in the first removal mode until the removal rate decreases due to the resulting interference geometry. The system can then switch to the second removal mode to smooth out the interference geometry, after which it can switch back to the first removal mode to maintain the highest possible removal rate. It becomes clear that the effects that occur, as well as the efficiency of the different ablation modes, depend heavily on the specific material being processed. According to a further development of the invention, the surface is pre-treated, at least in certain areas, in the first ablation mode and then fine-machined in the second ablation mode. The roughness achievable with the first ablation mode is preferably higher than that achievable in the second ablation mode; thus, the first ablation mode involves a coarser material removal than the second. In particular, roughing of the machined surface is preferably performed in the first ablation mode, while finishing is performed in the second ablation mode. According to the invention, the surface is processed in the first ablation mode until the appearance of the interference geometry, and in the second ablation mode after the appearance of the interference geometry, particularly after its first appearance. The aforementioned steps are repeated and performed in at least one surface area. Thus, processing in the potentially advantageous first ablation mode, particularly with a higher ablation rate depending on the material, is possible initially and, in particular, repeatedly, while any interference geometries that have formed in the meantime can be smoothed in the second ablation mode. Therefore, the at least one surface area can be processed very efficiently by repeatedly and iteratively applying the first and second ablation modes. According to a further development of the invention, the surface is treated in a third ablation mode after processing in the second ablation mode, in which the surface is subjected to low-frequency pulse packets. The single-pulse repetition rate of the individual laser pulses in the low-frequency pulse packets is at least 10 MHz to a maximum of 100 MHz, preferably at least 40 MHz to a maximum of 80 MHz, and preferably 50 MHz. It has been found that even higher surface quality can be achieved with low-frequency pulse packets than with high-frequency pulse packets. However, smoothing with low-frequency pulse packets is significantly slower than with high-frequency pulse packets.Therefore, pre-smoothing is preferably carried out using high-frequency pulse packets, especially in the sense of coarse finishing, followed by post-smoothing with low-frequency pulse packets to further increase the surface quality, which can also be referred to as fine finishing or finishing. Overall, the process preferably achieves roughing of the machined surface through the first removal mode, coarse finishing through the second removal mode, and fine finishing through the third removal mode. The low-frequency pulse packets preferably follow each other in time with a pulse packet repetition rate on the kHz scale, in particular of a few kHz or a few tens of kHz, or a few hundred kHz. If a plurality of low-frequency laser pulse trains designed as low-frequency pulse packets are used in the first ablation mode, the third ablation mode preferably corresponds to the first ablation mode in this respect. According to a further development of the invention, in the third ablation mode, the surface is treated with a combination of low-frequency pulse packets and a low-frequency laser pulse train configured as a low-frequency single-pulse train, preferably a plurality of low-frequency single-pulse trains. In particular, low-frequency single-pulse trains and low-frequency pulse packets can alternate. This advantageously enables a particularly high ablation quality combined with a very high surface finish. In particular, it is possible to process the surface first with the first ablation mode and then with the second ablation mode. This sequence of ablation modes can also be repeated, especially multiple times, depending on the material being processed. This is particularly common when processing with the first ablation mode allows for a higher ablation rate than the second ablation mode, but where the first ablation mode introduces irregular geometries that are then removed or smoothed in the second ablation mode. After smoothing with the second ablation mode, especially after a final application of the second ablation mode, the third ablation mode can then be used to further improve the surface quality. Low-frequency pulse packets alone can be used, as well as a combination of low-frequency pulse packets with low-frequency single pulse trains. According to a further development of the invention, it is provided that the surface, after processing in the third ablation mode, is subjected to a low-frequency single pulse, preferably a plurality of low-frequency single pulses. This can be carried out in particular to advantageously remove a burr at an ablation edge. According to a further development of the invention, the low-frequency laser pulse train is generated by a first laser system, while the high-frequency pulse packets are generated by a second laser system, distinct from the first. In this way, different laser systems can be used, each specifically configured for generating low-frequency laser pulse trains on the one hand and high-frequency pulse packets on the other. Suitable optical components can direct the laser radiation from both laser systems to the same target position, in particular to the surface to be treated, and at least one optical component can be used to switch between the laser systems.This can be, in a particularly simple way, a foldable, tiltable or rotatable mirror, but in a more complex way also an optical modulator, in particular an acousto-optic modulator, an electro-optic modulator or a micro-electro-mechanical system. The second laser system is preferably configured in particular to generate the high-frequency pulse packets of the second ablation mode with a single-pulse repetition rate of the individual laser pulses in the high-frequency pulse packets of at least 0.5 GHz up to a maximum of 100 GHz. In contrast, the low-frequency pulse packets of the third ablation mode are preferentially generated by the first laser system, which also generates the low-frequency laser pulse trains for the first ablation mode. Alternatively, according to a preferred embodiment of the invention, the low-frequency laser pulse train and the high-frequency pulse packets are generated by the same laser system, which is switched between low-frequency and high-frequency operation. This represents a particularly compact, space-saving, and easy-to-operate and quickly switchable design of a laser system for carrying out the method. The laser system preferably comprises, on the one hand, a single-pulse source and, on the other hand, a high-frequency laser pulse source, between which switching is possible with a suitable beam switching device. This switching device preferably comprises at least one active optical component, in particular an optical modulator, preferably an acousto-optic modulator, an electro-optic modulator, or a micro-electro-mechanical system. The high-frequency laser source can be designed as a high-frequency laser diode or as a repetition rate multiplier, whereby such a repetition rate multiplier can be powered on the one hand by an excitation laser that also generates the individual pulses for the low-frequency laser pulse train, while on the other hand such a repetition rate multiplier can also be powered by a laser diode that is not itself designed to generate high-frequency laser pulses. A laser system is particularly preferred in which an excitation laser both generates the low-frequency laser pulse train and feeds the repetition rate multiplier. The invention is explained in more detail below with reference to the drawings. Figure 1 shows a schematic representation of a laser system configured to carry out one embodiment of a method for removing material from a surface; Figure 2 shows a schematic representation of an embodiment of a mode-changing device and, in particular, a repetition rate multiplier for the laser system according to Figure 1; Figure 3 shows a schematic representation of one embodiment of the method in the form of a flowchart; Figure 4 shows a schematic representation of the different ablation modes according to one embodiment of the method; Figure 5 shows a schematic representation of one embodiment of the method for a first material; and Figure 6 shows a schematic representation of a known method and a further embodiment of the method preferred according to the invention for a second material. Fig. 1 shows a schematic representation of an embodiment of a laser system 100, which is set up to carry out an embodiment of a method for removing material from a surface, which is explained in more detail below. In this embodiment of the method, material is removed from a surface by means of the laser system 100, wherein the surface is subjected to at least one low-frequency laser pulse train, preferably a low-frequency single pulse train, of the laser system 100 in a first ablation mode and to high-frequency pulse packets in a second ablation mode, wherein a single-pulse repetition rate of individual laser pulses in the high-frequency pulse packets is of at least 0.5 GHz to at most 100 GHz. The duration of a high-frequency pulse packet is preferably at least 10 ns to at most 10 µs. Alternatively or additionally, a high-frequency pulse packet preferably comprises at least 5 individual laser pulses. Preferably, a high-frequency pulse packet comprises at least 10 individual laser pulses. The laser system 100 shown in Fig. 1 is configured to generate both the low-frequency laser pulse trains for the first ablation mode and the high-frequency pulse packets for the second ablation mode, so that the low-frequency laser pulse trains and the high-frequency pulse packets are generated by the same laser system 100. The laser system 100 is switchable between low-frequency and high-frequency operation and is switched between low-frequency operation – for the first ablation mode – and high-frequency operation – for the second ablation mode – as part of the process. Alternatively, it is possible for the low-frequency laser pulse trains to be generated by a first laser system, with the high-frequency pulse packets being generated by a second laser system, different from the first. The laser system 100 comprises an excitation laser 10 configured for generating laser pulses, in particular with a single-pulse repetition rate of less than 0.5 GHz, specifically a few kHz, a few hundred kHz, or even a few MHz, specifically a few hundred MHz. The laser system 100 also comprises a pulse stretcher 20 for pulse extension, a mode-changing device 30 for switching the laser system 100 between low-frequency and high-frequency operation, a first preamplifier 50, a first pulse selection device 60 for selecting individual pulses or pulse packets, a second preamplifier 70, and a main amplifier 80. Furthermore, the laser system 100 comprises a second pulse selection device 90 with which individual laser pulses or pulse packets can be selected.Furthermore, it has a pulse compressor 110 to compress the individual pulses or pulse packets that have been stretched in time by the pulse stretcher 20, after they have been amplified. It is possible that the laser system 100 also has a nonlinear optical component 120 which is set up for frequency conversion of the laser light of the excitation laser 10, in particular for frequency multiplication, for example frequency doubling, such as for generating a second harmonic of the excitation wavelength. Furthermore, the laser system 100 preferably has a control device 130 which is configured to control the mode-changing device 30 in order to switch from low-frequency operation to high-frequency operation - and back. The first pulse selection device 60 can be configured in particular as an optical modulator, preferably as an acousto-optic modulator or as an electro-optic modulator, or as a micro-electro-mechanical system. Similarly, the further, second pulse selection device 90 can alternatively or additionally be configured as an optical modulator, in particular as an acousto-optic modulator or electro-optic modulator, or as a micro-electro-mechanical system. Not all components shown here need to be present; additional components not shown here may also be included. Furthermore, the components shown here do not necessarily have to be arranged in the order presented; other arrangements of the components relative to each other are possible. Fig. 2 shows a schematic representation of an embodiment of the mode-changing device 30. This device has a beam splitter 31, which is configured to split a laser beam or laser pulse temporally and / or spatially into two beam paths, here a high-frequency laser pulse source 40 on the one hand and a single-pulse path 32 on the other. On the output side, the mode-changing device 30 has a beam selector 33, which is configured to direct laser light or laser pulses from the two beam paths onto a common path, in particular to a common target position. The high-frequency laser pulse source 40 is configured as a repetition rate multiplier 47, to which laser pulses from excitation laser 10 can be fed. The repetition rate multiplier 47 is configured to multiply the single-pulse repetition rate of the individual pulses of the excitation laser 10. The beam splitter 31 is configured here to divide the laser light from the excitation laser 10 temporally and / or spatially between the repetition rate multiplier 47 on the one hand and the single-pulse section 32 on the other. The beam selector 33 is configured here to supply, in particular, the amplifier 80, preferably the first preamplifier 50, with at least one high-frequency laser pulse train from the repetition rate multiplier 47 in high-frequency operation, and with laser pulses from the single-pulse section 32 in low-frequency operation. High-frequency pulse packets are preferably generated from the high-frequency laser pulse train by periodically controlling the first pulse selection device 60 and / or the second pulse selection device 90. Low-frequency pulse packets are preferably generated from the laser pulses of the single-pulse section 32 by periodically controlling the first pulse selection device 60 and / or the second pulse selection device 90. The beam splitter 31 is preferably configured as a passive beam splitter, preferably with a constant splitting ratio. Alternatively, it can also be configured as an active beam switching device, in particular as an acousto-optic modulator, as an electro-optic modulator, or as a micro-electro-mechanical system. Finally, it is also possible that the beam distributor 31 is designed as a combination of a passive beam splitter and an active beam switching device. The beam selector 33 is preferably configured as a beam switch, i.e., as an actively switchable component, in particular as an active beam switching device, especially as an acousto-optic modulator, as an electro-optic modulator, or as a micro-electro-mechanical system. Alternatively, the beam selector 33 is also possible as a combination of a passive beam combiner with at least one active beam modulation device, preferably with one active beam modulation device in each of the beam paths, here in the single-pulse path 32 on the one hand and the repetition rate multiplier 47 on the other. The beam selector 33 can also be configured as a passive beam combiner, in particular if the beam distributor 31 is configured as an active beam switching device. An active beam control device is configured, in particular, to selectively allow light from the respective beam path to pass through to the beam selector 33, or to block the light path to the beam selector 33, especially by attenuating the light in the respective beam path or diverting it away from the beam selector 33, for example into a beam dump (beam trap). The active beam control device can be designed as an optical modulator, in particular as an acousto-optic modulator, as an electro-optic modulator, or as a micro-electro-mechanical system, or also as a hinged, tiltable, or rotatable mirror. Both the single-pulse section 32 and the repetition rate multiplier 47 are preferably designed as fiber optic components or have fiber optic components. The single-pulse section 32 preferably has a single-pulse delay section 34 to compensate for dispersion. The repetition rate multiplier 47 has an input multiplier beam splitter 41, which divides the incoming laser pulses into a delay path 42 on the one hand and a through path 43 on the other. The delay path 42 has a longer optical path than the through path 43, so that the laser pulse passing through the delay path 42 is delayed relative to the laser pulse passing through the through path 43. The repetition rate multiplier 47 also has a plurality of combination elements 44, each of which combines a beam combiner and a beam splitter, wherein the laser radiation from the through-path 43 on the one hand and the delay path 42 on the other hand is first combined and then split again into a subsequent delay path 42 and a subsequent through-path 43.This can, in principle, be repeated any number of times, with successive delay sections 42 preferably having a length that doubles or – as in the embodiment shown here – halves, depending on the design of the repetition rate multiplier 47, so that as a result either the laser pulse trains passing through the repetition rate multiplier 47 are multiplied, or – as in the embodiment shown here – the single-pulse repetition rate is multiplied, namely by a factor of 2 for each delay section traversed. On the output side, the repetition rate multiplier 47 has a multiplier beam combiner 45, which combines the laser radiation from a last passage section 43 with the laser radiation from a last delay section 42 and forwards it as a high-frequency laser pulse train to the beam combiner 33. Fig. 3 shows a schematic representation of an embodiment of the method in the form of a flowchart. The method starts in a first step S1. In a second step S2, the surface to be treated is subjected to at least one low-frequency laser pulse train of the laser system 100 in the first ablation mode, in particular to form or process an ablation edge without burr formation, and / or to ensure rapid, efficient ablation until a disturbance geometry occurs. In a third step, S3, the surface is then subjected to high-frequency pulse packets in the second ablation mode, with the individual pulse repetition rate of the laser pulses in the high-frequency pulse packets ranging from at least 0.5 GHz to a maximum of 100 GHz. This allows for particularly fast and / or efficient surface ablation, depending on the material being processed; and / or it can smooth out any irregularities created in the second step, S2. The second step S2 and the third step S3 can be repeated iteratively, alternating between them, to process the surface. This is particularly advantageous when, with the material being processed, ablation using a low-frequency laser pulse train or multiple low-frequency laser pulse trains in the first ablation mode is faster and / or more efficient than ablation using high-frequency pulse packets in the second ablation mode. In such cases, processing can continue in the first ablation mode until a disturbance geometry appears, at which point the disturbance geometry can be removed using the second ablation mode and the surface smoothed. The process can then be switched back to the first ablation mode. In particular, it is possible that in the first removal mode a roughing or pre-machining of the surface is carried out, resulting in a first, greater roughness of the surface, followed by a fine-machining, in particular finishing, process in the second removal mode, resulting in a second, lower roughness on the surface. In a fourth step S4, the surface is preferably treated in a third ablation mode after processing in the second ablation mode. In this third ablation mode, it is subjected to low-frequency pulse packets with a single-pulse repetition rate of individual laser pulses in the low-frequency pulse packets of at least 10 MHz and at most 100 MHz, particularly 50 MHz. In this third ablation mode, the surface can be smoothed even further, i.e., a third, even smaller roughness is achieved, which is smaller than the second roughness. Thus, a coarse finishing process is achieved in the second ablation mode, while a final smoothing or fine finishing process, also referred to as fine finishing, is achieved in the third ablation mode. It is possible that in the third ablation mode, the surface is subjected to a combination of low-frequency pulse packets and at least one low-frequency single pulse. In particular, low-frequency pulse packets and low-frequency single pulses can alternate. In a fifth step S5, the surface is preferably treated with at least one low-frequency single pulse after processing in the third ablation mode. This can be carried out in particular to advantageously remove a burr at an ablation edge. The procedure ends in a sixth step, S6. Fig. 4 shows a schematic representation of different ablation modes according to one embodiment of the method. The optical output power P of the laser system 100 is plotted against time t in a diagram. At A, the first ablation mode is shown schematically, where a low-frequency laser pulse train 140, preferably configured as a low-frequency single-pulse train 145, is generated with single pulses 150. For clarity, only one of the single pulses 150 is labelled with the corresponding reference numeral. The single pulses 150 follow one another at a single-pulse repetition rate of a few kHz, a few hundred kHz, or even a few MHz, and in particular, a few hundred MHz. Figure B schematically illustrates the second ablation mode. Specifically, it depicts a high-frequency pulse packet 170 comprising a plurality of individual laser pulses 160, which follow one another within the high-frequency pulse packet 170 with a single-pulse repetition rate of at least 0.5 GHz and at most 100 GHz. In the second ablation mode, a plurality of such high-frequency pulse packets 170 are generated, which follow one another temporally with a pulse packet repetition rate on the kHz or MHz scale, in particular of a few kHz, a few hundred kHz, a few MHz, or a few hundred MHz. Finally, the third ablation mode is shown in Figure C, where two low-frequency pulse packets 190 are shown as examples, wherein the individual pulse repetition rate of individual laser pulses 160 in the low-frequency pulse packets 190 is at least 10 MHz to at most 100 MHz, in particular 50 MHz. The individual low-frequency pulse packets preferably follow one another with a pulse packet repetition rate on the kHz scale, in particular of a few kHz, a few tens of kHz, or a few hundred kHz. Fig. 5 shows a schematic representation of an embodiment of the method for a first material, in particular for the volume removal of a silicon surface. Two vertically oriented diagrams plot two quantities against a given removal depth T: on the left, the removal rate V̇, and on the right, the roughness R of the treated surface. The vertical axis of the achieved removal depth T also represents a time axis, since the removal depth increases over time, although not necessarily linearly. Between the two diagrams, a sequence of surface processing steps is shown. The surface is first processed at step I in the first ablation mode with at least one low-frequency laser pulse train, preferably configured as a single-pulse low-frequency laser pulse train. It can be seen that the removal rate V̇ decreases with the achieved removal depth T, while the roughness R simultaneously increases.Above a certain threshold for the ablation rate V̇ or the achieved roughness R, the system switches to the second ablation mode, and the surface is subjected to high-frequency pulse packets (II). The single-pulse repetition rate of individual laser pulses within these high-frequency pulse packets ranges from at least 0.5 GHz to a maximum of 100 GHz. The ablation rate V̇ then increases again to a maximum, after which the ablation rate V̇, and thus the ablation efficiency, remains constant, resulting in a homogeneous ablation. Simultaneously, the roughness R decreases with increasing ablation depth T. The increase in roughness R in the first ablation mode (I) is primarily due to the build-up of a disturbance geometry, which is subsequently ablated in the second ablation mode (II), thus smoothing the surface.It is also important that the removal rate V̇ in the first removal mode at I decreases because the roughness R increases, as this is where the disturbance geometry develops, leading to an increasingly inhomogeneous removal and a decreasing removal efficiency. For silicon, as shown in Fig. 5, the removal rate V̇ in the second removal mode is higher than in the first removal mode. In the case of silicon, the process benefits significantly from a temperature-dependent increase in absorption due to the short pulse intervals in the high-frequency pulse packets of the second ablation mode. Therefore, considerably more volume per unit of time can be removed from silicon in the second ablation mode at the same average power than in the first ablation mode. In III, the surface is finally subjected to low-frequency pulse packets in the third ablation mode, whereby the ablation rate V̇ is lower than in the second ablation mode, but the surface is smoothed even more, so that the roughness R decreases further. Finally, at stage IV, the surface is again subjected to at least one low-frequency single pulse, particularly to remove burrs at a cutting edge. Fig. 6 shows a schematic representation of a known method (a), and a further embodiment of the method preferred according to the invention (b), each for a second material, here in particular stainless steel. The designations of the individual areas and sizes in the diagrams are chosen analogously to Fig. 5. As previously explained, figure a) illustrates a known embodiment of a method for removing material from a stainless steel surface. In this embodiment, material is removed at an optimal operating point of fluence, with maximum removal efficiency, using at least one low-frequency single pulse. The pulse energy is limited by the optimal fluence at this operating point. This is particularly disadvantageous with regard to the use of modern laser systems, whose power cannot be adequately utilized, resulting in a low removal rate and thus, in particular, an in need of improvement in the process speed. Increasing the single-pulse repetition rate, the fluence, and / or the average power is not possible if the formation of irregular geometries, especially cone-like protrusions, is to be avoided. In section b), a further embodiment of the method preferred according to the invention is shown for stainless steel. Here, the operating point is deviated towards a higher fluence, which enables higher material removal rates despite lower efficiency. However, this necessitates the formation of interference geometries, which can be removed in the second operating mode of the method preferred according to the invention. Unlike silicon, the material removal rate V̇ in stainless steel is higher in the first removal mode, at least before the formation of the interference geometry, than in the second removal mode. Therefore, the first removal mode is applied at stage I until an interference geometry with a specific limiting roughness R is built up on the surface, or the material removal rate V̇ has dropped to a certain threshold. Subsequently, the interference geometry is smoothed at stage II in the second removal mode, after which the process switches back to the first removal mode, so that stages I and II alternate. On stainless steel - as well as on many other metals - there are strong shielding effects on the timescale chosen for the high-frequency pulse packets due to the short pulse intervals, which is why one does not gain efficiency in the second ablation mode, but can only generate advantages through the smoothing effects. Finally, a finer smoothing or finishing process follows at III in the third removal mode, and finally a further processing at IV again with at least one low-frequency single pulse pass, in particular to remove burr formation in the area of ​​a removal edge.

Claims

A method for removing material from a surface, wherein material is removed from the surface by means of at least one laser system (100), wherein the surface is subjected to material removal a) in a first ablation mode with at least one low-frequency laser pulse train (140) of the at least one laser system (100), and b) in a second ablation mode with high-frequency pulse packets (170), wherein a single-pulse repetition rate of individual laser pulses (160) in the high-frequency pulse packets (170) is of at least 0.5 GHz to at most 100 GHz, characterized in that the surface is processed i. until a disturbance geometry occurs in the first ablation mode, and ii. after the occurrence of the disturbance geometry in the second ablation mode, wherein iii. steps i) and ii) are carried out repeatedly in at least one surface region. Method according to claim 1, characterized in that the surface is first processed in the first ablation mode and then in the second ablation mode. Method according to one of the preceding claims, characterized in that the low-frequency laser pulse train (140) is designed as a low-frequency single-pulse train (145). Method according to one of the preceding claims, characterized in that the first removal mode a) is additionally selected for the processing or formation of a removal edge, and / or that the second removal mode b) is selected for surface removal. Method according to one of the preceding claims, characterized in that the surface is pre-processed at least in certain areas in the first ablation mode and is fine-machined in the second ablation mode. Method according to one of the preceding claims, characterized in that the surface is processed in a third ablation mode after processing in the second ablation mode, in which the surface is subjected to low-frequency pulse packets (190), wherein a single-pulse repetition rate of individual laser pulses (160) in the low-frequency pulse packets (190) is of at least 10 MHz to at most 100 MHz, in particular 50 MHz. Method according to claim 6, characterized in that the surface in the third ablation mode is subjected to a combination of the low-frequency pulse packets (190) and the at least one low-frequency single pulse train (145). Method according to one of claims 6 or 7, characterized in that the surface is subjected to at least one low-frequency single pulse train (145) after processing in the third ablation mode. Method according to one of the preceding claims, characterized in that the low-frequency laser pulse train (140) is generated by a first laser system (100), wherein the high-frequency pulse packets (170) are generated by a second laser system different from the first laser system (100). Method according to one of claims 1 to 8, characterized in that the low-frequency laser pulse train (140) and the high-frequency pulse packets (170) are generated by the same laser system (100), wherein the laser system (100) is switched between low-frequency operation and high-frequency operation. Method according to one of the preceding claims, characterized in that the time length of a high-frequency pulse packet (170) in the second ablation mode is at least 10 ns to at most 10 µs, and / or that a high-frequency pulse packet (170) of the high-frequency pulse packets (170) in the second ablation mode comprises at least 5 individual laser pulses (160).

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