METHOD FOR LASER PROCESSING OF WORKPIECES IN LIQUID

Description

[0001] The present invention relates to a method for laser processing of workpieces in liquid.

[0002] CN 112 589 261 A describes a device for laser processing of workpieces. The processing device comprises a processing unit, a monitoring unit, and a control unit. The processing unit includes an ultrashort pulse laser beam source, an optical element, a positioning table, and a glass container.

[0003] In ZHOU JIA ET AL: "Study on the mechanism of ultrasonic-assisted water confined laser micromachining of silicon", OPTICS AND LASERS IN ENGINEERING, ELSEVIER, AMSTERDAM, NL, vol. 132, 27 April 2020 (2020-04-27), XP086163262, ISSN: 0143-8166, DOI: 10.1016 / J.OPTLASENG.2020.106 118, a device for laser processing of silicon in a water environment is described. The bubbles formed in the water during laser processing are monitored by a high-speed camera.

[0004] Publication CN 113 146 078 A, published after the priority date of the present application, describes a device and a method for observing and reducing gas bubbles that form during laser processing in a water environment. The device described in this publication comprises an ultrasonic vibration unit arranged in a process chamber, which is used to generate ultrasonic vibrations to prevent the formation of air bubbles during the processing of a workpiece.

[0005] In publication JP 2022 067857 A, filed after the priority date of the present application, a further device for laser material processing is described.

[0006] The advantages of processing workpieces using laser radiation are generally well-known. In particular, the use of laser radiation allows for high precision in material processing and high processing speeds. One challenge in laser material processing is that the workpiece heats up locally due to the use of focused laser radiation. The resulting thermal effects outside the processing zone are generally undesirable.

[0007] To improve the laser processing process, some applications perform the laser processing in a liquid. The workpiece to be processed is placed in a process chamber filled with the liquid, so that the workpiece is cooled by the liquid throughout the entire process.

[0008] Furthermore, some of these process chambers are designed to continuously exchange the fluid within the chamber during the machining process. This is typically achieved using pumps that create a flow within the process chamber and constantly replace the fluid.

[0009] One disadvantage of laser processing workpieces in liquid is that gas bubbles can form within the process chamber. The cause of these gas bubbles can be varied. In particular, they can be caused by the interaction of the laser radiation with the workpiece or by the circulation of the liquid.

[0010] For example, persistent gas bubbles (also known as persistent bubbles) can form during the laser process and accumulate on the workpiece surface in the treated area. Over time, these bubbles can detach from the workpiece surface and move through the process chamber as free gas bubbles. They can also circulate through the system multiple times due to fluid exchange, thereby repeatedly disrupting the laser processing.

[0011] On the other hand, gas bubbles can also form within the system independently of the interaction between the laser radiation and the workpiece. For example, free gas bubbles can form in the process chamber during system startup if the chamber or hoses are initially filled with air. Gas bubbles also frequently form within the chamber when it is filled with liquid pumped from a reservoir. During this process, air from the hoses is repeatedly forced into the liquid reservoir as gas bubbles and then pumped back into the hose-chamber system. These gas bubbles often only reach the liquid surface in the reservoir after several minutes, at which point they cease to interfere with the laser processing.

[0012] As explained in the introduction, two different types of gas bubbles typically form in the process chamber: firstly, adhering or persistent gas bubbles, which generally adhere to the workpiece surface, and secondly, free gas bubbles, which can move through the process chamber or even the entire system (process chamber, hoses, reservoir). Adhering gas bubbles can transform into free gas bubbles over time, and vice versa.

[0013] Regardless of the specific type of gas bubble, it is always the case that those gas bubbles located within the effective range of the laser radiation cause an undesirable interaction with the laser radiation. This is due to the different refractive indices of air and liquid, which leads to unwanted reflection and refraction of the laser radiation at the interface between the liquid and the gas bubble.

[0014] To address the problem described above, the invention defined in claim 1 proposes a method for laser processing of workpieces in liquid, wherein the method comprises the following steps: Providing a workpiece in a process chamber filled with a liquid; focusing a pulsed laser beam onto a surface of the workpiece using a focusing unit; generating a relative movement between the focused laser beam and the workpiece surface using a positioning unit; detecting a gas bubble in a predefined detection area using a detection unit; and performing an initial action to avoid or reduce interaction effects between the laser beam and the detected gas bubble.

[0015] The method according to the invention makes it possible to eliminate or at least significantly reduce any disruptive effects caused by gas bubbles in the process chamber. This allows the process speed to be increased while simultaneously improving the quality of the processed workpieces. In particular, the method according to the invention allows for an improvement in the cut edges during the laser cutting process, with the improvement being achieved through the reduction of disruptive effects.

[0016] The fluid used can be, for example, water. The focusing unit can be a focusing lens, a focusing mirror, or a lens system. The relative motion can be generated either by deflecting the radiation or by positioning the workpiece. The positioning unit can be a swiveling mirror (also called a scanner mirror), a rotatable mirror (e.g., a polygon scanner), or a positioning stage. The formation of any gas bubbles can be monitored within a detection area that, for example, encompasses the entire process chamber. The detection area can be a single, continuous area or separate, distinct sections.

[0017] For example, the detection area may encompass both the interior of the process chamber and the hoses through which liquid, gas, or gas bubbles can be introduced into the chamber. The detection area can be static or dynamic. A static detection area can be advantageous, for instance, when the entire interior of the process chamber needs to be monitored. However, in some embodiments, it may be explicitly desired not to monitor the entire process chamber, but only a portion where the laser radiation is currently active. This might be desirable, for example, in relatively large process chambers, as any gas bubbles could be located far from the laser radiation, thus preventing them from affecting the laser process and making their removal unnecessary.In this case, it may be possible to dynamically adjust the detection range depending on the current laser beam position.

[0018] In the first action, one or more steps are performed to avoid or at least reduce disruptive effects during the laser processing of the workpiece caused by the detected gas bubble. These steps can, in particular, serve to eliminate the detected gas bubbles from the process area. The term "first" is not to be understood as a limitation or as an indication of further actions. Rather, this addition serves for easier reference within the scope of this application. The first action is always performed when a gas bubble is detected in the predefined detection area. According to one embodiment of the present invention, the first action can be performed until no gas bubble remains in the detection area. The first action is performed in response to the detection of the gas bubble and can, in particular, depend on the type of bubble detected (e.g.,Location and size of the blister) are determined.

[0019] Although the inventive method is described above with a specific sequence of process steps for easier understanding, it is evident to the person skilled in the art that it is not necessary to carry out the individual steps in a specific order in order to achieve the technical effect of the present invention.

[0020] According to one embodiment of the method according to the invention, the detection of the gas bubble may comprise the following steps: Creating a photographic image of the predefined detection area using a camera unit and generating a corresponding image file; and evaluating the image file, the evaluation of which includes in particular the use of a pattern recognition algorithm.

[0021] According to this embodiment, automated detection of any gas bubbles can be performed, and in the event of a detected gas bubble, an action to remove the bubble can be carried out automatically. Furthermore, it can be provided that the action to be performed is selected automatically depending on the location or type of the detected bubble (for example, free or adherent bubble). The use of a pattern recognition algorithm also allows the degree of automation of the method according to the invention to be increased.

[0022] Furthermore, according to the present invention, the detection of the gas bubble may include a scattered light measurement, wherein the scattered light measurement in particular comprises the following steps: Illumination of the detection area using an LED; recording of a detection signal using a photodiode, wherein the photodiode is designed to record the radiation emitted by the LED after propagation through the detection area; evaluation of the detection signal using an evaluation unit.

[0023] The LED and the photodiode can, for example, be arranged on two opposite sides of the process chamber, whereby the light emitted by the LED can enter the chamber through a first process window, while the scattered light can be led out of the chamber through a second process window and subsequently absorbed by the photodiode.

[0024] As an alternative to LEDs, other light sources can also be used, such as a halogen lamp or a laser beam source. However, using an LED has the advantage that LEDs are easy to control and inexpensive. The monitored detection area can, for example, encompass the entire interior of the process chamber or, alternatively, only a portion of it. It is also possible to monitor individual sections of the inlet or outlet hoses where gas bubbles are expected to form. The evaluation unit can include a processing unit, such as a PC, a laptop, or a microcontroller. During evaluation, the detection signal generated by the photodiode can be compared with reference signals previously recorded during a calibration process.For example, during calibration, a reference signal may have been recorded in an application scenario where no gas bubble was present in the detection area. If the detection signal shows a significant deviation from the reference signal, it can be concluded that a gas bubble is present between the LED and the photodiode.

[0025] Furthermore, the detection unit may include an ultrasonic sensor, a radar sensor, a capacitive sensor, and / or an electromagnetic sensor. It may also include an oxygen sensor unit. Measuring the oxygen concentration within the process chamber provides information about the probability of gas bubbles being present in the liquid, since the oxygen concentration of dissolved gas bubbles or microbubbles differs significantly from the oxygen concentration of liquids.

[0026] According to a further embodiment of the invention, it can be provided that the first action involves a change in the flow velocity of the

[0027] The process chamber comprises liquid. In particular, it can be provided that the flow velocity within the chamber is abruptly increased upon detection of a gas bubble, so that the gas bubble detected in the detection area can be removed. It can also be provided that the flow direction is reversed to achieve more efficient removal of the detected gas bubble. The reversal of the flow direction can be performed several times in succession and preferably repeated until a detected gas bubble adhering to a surface is detached from that surface. The flow within the chamber can be generated, in particular, by using pumps that convey liquid into or out of the chamber. Alternatively, the flow can be generated by movement of the chamber.

[0028] Not part of the present invention is the fact that the first action comprises positioning the laser radiation such that the laser radiation is directed at the detected gas bubble. Particularly in the case of adhering gas bubbles, this can achieve the detachment of the gas bubble from the surface to which it adheres. If, for example, the gas bubble adheres to the workpiece surface and the laser radiation is directed at the gas bubble, shock waves are generated by the interaction of the laser radiation with the workpiece surface, which contribute to the detachment of the gas bubble from the workpiece surface. This advantageously allows one and the same laser beam to be used for both material processing and for detaching the adhering gas bubbles.In this embodiment, the laser beam is used as a combined tool for processing the workpiece and for releasing the gas bubbles, so that no additional components are required for releasing the gas bubbles.

[0029] According to a further embodiment, which is not part of the present invention, the laser radiation can be directed in a defocused form onto the area of ​​the workpiece surface where an adhering gas bubble has been detected. This allows the shock wave to be generated over a larger area of ​​the workpiece surface, thereby enabling a more uniform and efficient detachment of the gas bubble from the workpiece surface. The beam diameter on the workpiece surface can be set to be 50%, 80%, or 100% of the diameter of the gas bubble. The defocusing of the laser radiation can be achieved, in particular, by varying the position of the focusing unit or by varying the position of the workpiece.

[0030] Furthermore, according to one embodiment of the invention, the first action may include the generation of ultrasonic waves in the vicinity of the detected gas bubble using an ultrasonic generator. For this purpose, it may be provided, in particular, that an ultrasonic generator is arranged in the immediate vicinity of the workpiece or in the immediate vicinity of the hoses or lines used to generate the flow in the process chamber. The ultrasonic generator may, in particular, comprise a piezoelectric element that is electrically controlled.

[0031] Furthermore, according to the present invention, the first action can include a change in the flow mode, wherein the flow mode can be varied, in particular, between laminar, turbulent, and pulsating. Initial investigations have shown that varying the flow mode can achieve a particularly efficient release of adhering gas bubbles.

[0032] According to the present invention, the following steps are also provided: Determining a transit time for the detected gas bubble, within which the gas bubble is located in an area where an interaction between the laser radiation and the gas bubble is expected; and deactivating the laser radiation for the duration of the determined transit time; or positioning the laser radiation such that the laser radiation is located outside the gas bubble.

[0033] A camera-based method can be used to determine the transit time, in which the speed at which a free gas bubble moves within the detection area is determined. Subsequently, it can be calculated when and for how long the detected gas bubble remains in the area where an interaction between the laser radiation and the gas bubble is expected (transit time). Finally, the laser radiation is deactivated for the determined transit time or moved to a position where no interaction between the laser radiation and the gas bubble occurs. For example, the laser beam source can be switched off or, alternatively, a beam absorber can be used to deactivate the laser radiation.Alternatively, a beam modulator, in particular an acousto-optic modulator (also known as an AOM) or an electro-optic modulator (also known as an EOM), can be used to redirect the radiation very quickly. Once the detected gas bubble has left the collision zone, the laser radiation can be reactivated. Alternatively, a laser beam trajectory can be predefined for a laser processing operation, where the gas bubble detection process calculates that an interaction between the laser radiation and a free gas bubble is imminent. In this case, the laser radiation can deviate from the predefined trajectory and thereby "skip" the collision zone, allowing this area of ​​the workpiece to be processed at a later time, once the gas bubble has left the collision zone.In this way, interactions between the laser radiation and the gas bubble are significantly reduced, and the process quality and speed are considerably increased.

[0034] The present invention is illustrated below with reference to the figures, which show the following: Fig. 1 a system for laser material processing in liquid according to the prior art, Fig. 2 the laser processing process in the ideal case and in the presence of a gas bubble in the processing area, Fig. 3 a schematic representation of an embodiment of the method according to the invention, Fig. 4 a schematic representation of embodiments for the detection of the gas bubbles according to the invention, and Fig. 5 a schematic representation of embodiments for the action according to the invention for avoiding interaction effects between the laser radiation and the detected gas bubble.

[0035] In the Fig. 1 A system 10 for laser processing in liquids, known from the prior art, is shown. Such a system 10 has a laser beam source 12 that generates pulsed laser radiation 14. The orientation of the laser radiation 14 can be adjusted via a positioning unit 16. The laser radiation 14 is focused into the interior of a process chamber 20 by a focusing unit 18. A workpiece 22 to be processed is arranged inside the process chamber 20. The focused laser radiation 14 is directed onto a surface 22a of the workpiece 22, so that the workpiece 22 is heated selectively at the desired location and can vaporize. The laser radiation enters the process chamber 20 through a transparent process window 24, the process chamber 20 being otherwise opaque. The process chamber 20 is filled with a liquid 26. The liquid 26 can be, for example, water.The liquid is used, for example, to cool the workpiece during the machining process.

[0036] In the Fig. 2 The problems arising during a laser processing process according to the state of the art are illustrated. As in the Fig. 2 As shown, laser processing in liquid leads to the formation of gas bubbles, which negatively affect the laser processing process.

[0037] In the Fig. 2 (a) The ideal case is initially depicted, in which no gas bubbles are present within the process chamber 20. In this case, the focused laser radiation 14 can strike the surface of the workpiece 22 undisturbed and heat the workpiece 22.

[0038] In contrast, in the Fig. 2 (b) The case shown involves the formation of an adhering gas bubble 28 on the surface of the workpiece 22. Due to the refractive index difference between the liquid 26 and the gas bubble 28, a portion of the incident laser radiation 14 is reflected. The reflected laser radiation 14a does not strike the surface of the workpiece 22 and therefore cannot be used for the processing. Furthermore, a portion of the incident laser radiation 14 is deflected. The deflected laser radiation 14b thus does not strike the surface of the workpiece 22 at the desired location. This impairs the accuracy of the laser processing.

[0039] In the Fig. 2 (c) Another scenario is shown in which a free gas bubble 28 is present in the processing area, interacting with the focused laser radiation 14. The gas bubble 28 causes the incoming laser radiation 14 to be defocused, resulting in defocused laser radiation 14c striking the surface of the workpiece 22. This regularly leads to the radiation intensity (defined as power per unit area) no longer being sufficient to vaporize the material on the surface of the workpiece 22.

[0040] The examples shown above demonstrate that gas bubbles forming in process chamber 20 significantly disrupt the laser processing process. Specifically, the gas bubbles cause reduced process speed, reduced efficiency, instabilities, and deviations from the desired processing result.

[0041] In the Fig. 3 Figure 100 illustrates an embodiment of the method according to the invention. For the sake of clarity, reference is made below to a "first step," "second step," etc. However, this terminology does not expressly define a sequence that is mandatory within the scope of the present invention, but rather serves to distinguish the individual process steps. In a first step 110, a workpiece is placed in a process chamber filled with liquid. In a second step 120, pulsed laser radiation is focused onto a surface of the workpiece. A focusing unit is used for this purpose. In a third step 130, relative movement between the focused laser radiation and the workpiece surface is generated for processing the workpiece, using a positioning unit.The positioning unit can be, for example, a scanner mirror designed to adjust the position of the focused laser beam on the workpiece surface, or a positioning table designed to vary the workpiece's position. In a predefined detection area, a fourth step (140) checks for the presence of a gas bubble. This is done using a detection unit, which may include a camera. If a gas bubble is detected, a fifth step (150) involves an initial action to prevent or reduce interaction effects between the laser beam and the detected gas bubble. In other words, the first action serves to remove the detected gas bubble from the detection area or the processing area.

[0042] In the Fig. 4 Exemplary embodiments of the present invention are shown, relating to the detection of the gas bubble.

[0043] In the Fig. 4 (a) An embodiment is shown in which a detection unit 30 is provided, which is designed as a camera unit. The camera unit records a detection area in which gas bubbles are considered disruptive. In particular, the camera unit can monitor the interior of the process chamber 20. The camera unit generates an image file, which is then evaluated. If a gas bubble is detected in the generated image file, it can be provided that a corresponding action is carried out to remove the gas bubble and to reduce the interactions. The camera unit can, as shown in the Fig. 4 (a) The camera unit is shown to be arranged radially to the laser radiation 14. Alternatively, it can also be positioned axially to the laser radiation.

[0044] For this purpose, a beam splitter can be used, for example. Furthermore, the detection unit can be designed to have two camera units, each arranged radially to the laser beam and offset from each other by 90°. By using two camera units, the three-dimensional position of the gas bubble 28 can be determined precisely.

[0045] In the Fig. 4 (b) Another embodiment is shown in which the detection unit 30 comprises an LED 30a and a photodiode 30b. The LED 30a and the photodiode 30b are arranged on two opposite sides of the process chamber 20. The light emitted by the LED 30a enters the interior of the process chamber 20 through a transparent process window 24. If no gas bubble is present in the detection area, the light emitted by the LED 30a can exit directly through the opposite process window 24 and be detected by the photodiode 30b. If, however, a gas bubble 28 is present in the detection area, the light emitted by the LED 30a is scattered by the gas bubble 28, causing the photodiode 30b to generate a correspondingly altered signal. By comparing the output signal of the photodiode 30b with previously recorded reference signals, it can be determined whether a gas bubble is located in the monitored detection area.

[0046] In the Fig. 5 Various embodiments of the present invention are shown, in which different actions are provided to avoid interaction effects between the laser radiation 14 and the gas bubble 28. In the Fig. 5 (a) The illustrated embodiment features an additional ultrasonic generator 32, which is arranged on the underside of the process chamber 20. If the detection unit 30 detects that a gas bubble 28 is present in the detection area, the ultrasonic generator 32 can be activated by a control unit (not shown in this figure). This allows the gas bubble 28 to be detached from the surface of the workpiece 22. For example, it can be provided that the ultrasonic generator 32 is only activated when an adhering gas bubble 28 is detected. As already described in the Fign. 2 (b) und (c) As shown, adhering and free gas bubbles differ significantly in their shape and can therefore be visually distinguished from one another.

[0047] Furthermore, in the Fig. 5 (b) Another embodiment of the invention is shown, in which a flow generator 34 is provided. In this embodiment, the flow generator 34 has a liquid inlet 34a and a liquid outlet 34b, as well as a pressure pump and a suction pump connected to the liquid inlet 34a and the liquid outlet 34b, wherein the pumps are located in the Fig. 5 (b) The flow generator 34 is designed to generate a flow, in the event of a detected gas bubble 28, by which the gas bubble is transported out of the processing area. It is also possible to combine the ultrasonic generator 32 and the flow generator 34. This allows, for example, the ultrasonic generator 32 to be activated if an adhering gas bubble 28 is detected, while the flow generator 34 is activated if a free gas bubble is detected. It is also advantageous to provide that, in the event of the detection of an adhering gas bubble, the ultrasonic generator 32 is first used to detach the gas bubble 28 from the surface of the workpiece 22, while the flow generator 34 is subsequently activated to transport the free gas bubble out of the detection area or the processing area. BEZUGSZEICHENLISTE

[0048] 10 Laser processing system 12 Laser beam source 14 Laser radiation 14a Reflected laser radiation 14b Deflected laser radiation 14c Defocused laser radiation 16 Positioning unit 18 Focusing unit 20 Process chamber 22 Workpiece 22a Workpiece surface 24 Process window 26 Liquid 28 Gas bubble 30 Detection unit 30a LED 30b Photodiode 32 Ultrasonic generator 34 Flow generator 34a Liquid inlet 34b Liquid outlet 100 Laser processing method 110 First process step 120 Second process step 130 Third process step 140 Fourth process step 150 Fifth process step

Claims

1. A method for laser processing of workpieces in liquid, the method comprising the following steps: - providing (110) a workpiece (22) in a process chamber (20) filled with a liquid; - focusing (120) a pulsed laser radiation (14) onto a surface of the workpiece (22) using a focusing unit (18); - producing (130) a relative movement between the focused laser radiation and the workpiece surface (22a) using a positioning unit (16); - detecting (140) a gas bubble (28) in a predefined detection region using a detection unit (30); and - carrying out a first action (150) to avoid or reduce interference effects during laser processing, which are caused by the detected gas bubble (28); - the method being characterized by the following steps: - determining a transit time for the detected gas bubble (28), in which the gas bubble (28) is present in a region in which an interaction between the laser radiation (14) and the gas bubble (28) is to be expected; and - deactivating the laser radiation (14) for the duration of the determined transit time; or - positioning the laser radiation (14) such that the laser radiation (14) is arranged outside the gas bubble (28).

2. The method according to claim 1, characterized in that the detection (140) of the gas bubble comprises the following steps: - capturing a photographic image of the predefined detection region with the use of a camera unit and generating a corresponding image file; and - evaluating the image file, the evaluation particularly comprising the use of a pattern recognition algorithm.

3. The method according to claim 1 or 2, characterized in that the detection (140) of the gas bubble (28) comprises a scattered light measurement, the scattered light measurement comprising in particular the following steps: - illuminating the detection region with the use of an LED (30a); - capturing a detection signal with the use of a photodiode (30b), the photodiode (30b) being configured to pick up the radiation emitted by the LED (30a) after its propagation through the detection region; - evaluating the detection signal with the use of an evaluation unit.

4. The method according to any one of the preceding claims, characterized in that the first action (150) comprises a change of the flow velocity in the process chamber (20).

5. The method according to any one of the preceding claims, characterized in that the first action (150) comprises a generation of ultrasound waves in the vicinity of the detected gas bubble (28) with the use of an ultrasound generator (32).

6. The method according to any one of the preceding claims, characterized in that the first action (150) comprises a change in the flow type, where the flow type can in particular be varied between laminar, turbulent, and pulsating.