Method for laser processing a workpiece in a liquid - Patents.com

Abstract

The present invention relates to a method for laser processing a workpiece in a liquid, the method comprising: providing a workpiece (22) in a process chamber (20) filled with the liquid; focusing pulsed laser radiation (14) on a surface of the workpiece (22) using a focusing unit (18); generating relative movement between the focused laser radiation and the surface (22a) of the workpiece using a positioning unit (16); detecting a bubble (28) in a predetermined detection area using a detection unit (30); and performing a first action to avoid or reduce an interaction effect between the laser radiation (14) and the detected bubble (28).

Description

[Technical field]

[0001] The present invention relates to a method and corresponding system for laser processing a workpiece in a liquid. [Background technology]

[0002] The advantages of processing workpieces using laser radiation are generally known. The use of laser radiation in particular allows high precision and high processing speeds in material processing. In material processing with lasers, the strong local heating of the processed workpiece due to the use of focused laser radiation can be considered a problem. Thermal effects obtained outside the processing zone are generally undesirable.

[0003] To improve the laser processing process, in some applications the laser processing process is performed in a liquid, where the workpiece to be processed is placed in a process chamber filled with liquid so that the workpiece is cooled by the liquid throughout the process.

[0004] Furthermore, in such process chambers, the liquid within the chamber may be permanently exchanged during the treatment process. Generally, a pump is used to generate flow within the process chamber and to constantly exchange the liquid within the process chamber. Summary of the Invention [Problem to be solved by the invention]

[0005] A disadvantage of laser processing a workpiece in a liquid is considered to be the formation of gas bubbles in the process chamber. The reasons for the formation of gas bubbles are manifold. In particular, gas bubbles can be formed by the interaction of the laser radiation with the workpiece or by the circulation of the liquid.

[0006] For example, attached bubbles (also called persistent bubbles) can be caused by the laser process and formed on the workpiece surface in the area of ​​the processed surface. Over time, they can detach from the workpiece surface and travel through the process chamber as free bubbles. Furthermore, due to exchanges of liquids in the system, they can travel several times through the system, thereby repeatedly disrupting the laser processing process.

[0007] On the other hand, bubbles may form in the system that are generated independently of the interaction between the laser radiation and the workpiece. For example, free bubbles may be generated during system startup if the process chamber or hose is initially filled with air. When filling the process chamber with a liquid that is transported from a reservoir into the chamber, bubbles often form in the process chamber. Here, air is flushed several times in the form of bubbles from the hose into the liquid reservoir and then pumped back into the hose chamber system. The bubbles often reach the liquid surface in the reservoir only after a few minutes, and only then do they no longer interfere with the laser processing process.

[0008] As explained at the beginning, two different types of bubbles are usually formed in the process chamber: on the one hand, adherent or persistent bubbles that generally adhere to the workpiece surface, and on the other hand, free bubbles that can travel through the process chamber or even through the entire system (process chamber, hoses, reservoirs). Essentially, it is true that adherent bubbles can become free bubbles over time and vice versa.

[0009] Regardless of the specific type of bubble, it always applies that any bubble that is within the effective range of the laser radiation will cause undesirable interactions with the laser radiation. The reason for this is the difference in refractive index between air and liquid, which causes undesirable reflection and diffraction of the laser radiation used at the interface between the liquid and the bubble. [Means for solving the problem]

[0010] In order to solve the above problems, the present invention proposes a method for laser processing a workpiece in a liquid, the method comprising the steps of: Providing a workpiece in a process chamber filled with a liquid; focusing the pulsed laser radiation onto a surface of the workpiece using a focusing unit; generating relative motion between the focused laser radiation and a workpiece surface using a positioning unit; Detecting an air bubble within a predetermined detection area using a detection unit; Taking a first action to avoid or reduce interaction effects between the laser radiation and the detected air bubble.

[0011] The method of the invention makes it possible to eliminate or at least significantly reduce interference effects that may be caused by bubbles in the process chamber, which allows to increase the processing speed while at the same time improving the quality of the processed workpieces. The method of the invention in particular allows for an improvement of the cutting edge during the laser cutting process, which improvement is achieved by the reduction of interference effects.

[0012] The liquid used may be, for example, water. The focusing unit used may in particular be configured as a focusing lens, a focusing mirror or a lens system. The relative movement may be generated either by deflection of the radiation or by positioning of the workpiece. In this respect, the positioning unit may be implemented in the form of a pivotable mirror (also called a scanner mirror), a rotatable mirror (for example a polygon scanner) or a positioning table. The formation of bubbles can for example be monitored in a detection area encompassing the entire process chamber. Here, the detection area may comprise a continuous area, but also areas separated from one another. For example, it may be provided that the detection area comprises on the one hand the interior of the process chamber and on the other hand monitors a hose through which not only liquid but also bubbles may be introduced into the chamber. In this case, the detection area may be static or dynamic. A static detection area may be advantageous, for example, when the entire interior of the process chamber is monitored. However, in some embodiments, it may be explicitly desired that the entire process chamber is not monitored, but only that part of the process chamber in which the laser radiation is currently active. This may be desirable for relatively large process chambers. For example, if the occurrence of bubbles is far away from the laser emission and they do not interfere with the laser process, removal of such bubbles is not ultimately necessary, in which case the detection area may be dynamically set as a function of the current laser beam position.

[0013] In the first action, one or more steps are performed to avoid or at least reduce interference effects during the laser processing of the workpiece caused by the detected air bubble. The steps can in particular serve to remove the detected air bubble from the process area. In this context, the addition of "first" should not be understood as a limitation or a reference to a further action. Rather, this addition serves for easier reference in the context of the present application. The first action is performed each time an air bubble is detected in a given detection area. According to one embodiment of the present invention, the first action may be performed until there is no more air bubble in the detection area. The first action is performed in response to the detection of the air bubble, in particular depending on the detected air bubble (e.g., the position and size of the air bubble).

[0014] Although the method of the present invention is described above with a specific order of method steps for ease of understanding, it will be apparent to one of ordinary skill in the art that the present invention does not require the ultimate performance of the individual steps in a specific order to achieve the technical effects.

[0015] According to one embodiment of the method of the present invention, detecting an air bubble comprises: taking a photographic image of the predetermined detection area using a camera unit and generating a corresponding image file; In particular, evaluating the image file includes using a pattern recognition algorithm.

[0016] According to this embodiment, an automatic detection of the occurrence of air bubbles can be performed and, in case of detection, actions can be automatically taken to remove the air bubbles. Furthermore, depending on the position and the size or type of the detected air bubbles (e.g. free or attached bubbles), the actions to be performed may be automatically selected. The use of pattern recognition algorithms allows to further increase the degree of automation of the method according to the invention.

[0017] Further, according to the invention, detecting bubbles comprises scattered light measurement, the scattered light measurement being in particular Illuminating a detection area with an LED; acquiring a detection signal using a photodiode, the photodiode being configured to acquire radiation emitted by the LED that has propagated through a detection region; Evaluating the detection signal with an evaluation unit.

[0018] Here, the LED and photodiode may, for example, be positioned on two opposing sides of a process chamber, and light emitted by the LED can enter the chamber through a first process window, while scattered light can be directed out of the chamber through a second process window and then captured by the photodiode.

[0019] As an alternative to LEDs, other light sources such as halogen lamps or laser light sources can also be used. However, the use of LEDs is advantageous in that they are easy to control and available at low cost. The monitored detection area may for example include the entire interior of the process chamber or, alternatively, only a part of the process chamber. Also, individual sections of the supply or discharge hoses in which the formation of air bubbles may be expected may be monitored. The evaluation unit may in particular include a calculation unit. For example, the evaluation unit may include a PC, a laptop or a microcontroller. In the evaluation, the detection signal generated by the photodiode may in particular be compared to a reference signal previously obtained during a calibration process. For example, during calibration, a reference signal may have been obtained in an application scenario in which no air bubbles were present in the detection area. If the detection signal deviates significantly from the reference signal, it can be concluded therefrom that an air bubble is present between the LED and the photodiode.

[0020] The detection unit may comprise an ultrasonic detector, a radar sensor, a capacitive sensor, and / or an electromagnetic sensor. The detection unit may also comprise an oxygen sensor unit. Measuring the oxygen concentration in the process chamber provides information about the probability that gas bubbles are present in the liquid, since the oxygen concentration of the desorbed gas bubbles or microbubbles differs significantly from the oxygen concentration of the liquid.

[0021] According to another embodiment of the invention, the first action may comprise a change in the flow rate of the liquid in the process chamber. In particular, upon detection of an air bubble, the flow rate in the chamber may be increased sharply to allow the detected air bubble in the detection area to be removed. Also, the flow direction may be reversed to achieve a more efficient removal of the detected air bubble. Here, the reversal of the flow direction may be performed several times in succession, preferably repeated until the detected air bubble adhering to the surface is detached from this surface. The flow in the chamber may be generated in particular by using a pump that pumps the liquid into or out of the chamber. Alternatively, the flow may be generated by a movement of the chamber.

[0022] According to the invention, the first operation may also include positioning the laser radiation so that it is directed at the detected bubble. In particular, for attached bubbles, separation of the bubble from the surface to which it is attached can be achieved. For example, if a bubble is attached to the workpiece surface and the laser radiation is directed at the bubble, the interaction of the laser radiation with the workpiece surface causes a shock wave that contributes to the detachment of the bubble from the workpiece surface. Thus, the same laser beam is effectively used to process the material and to separate the attached bubble. In this embodiment of the invention, the laser beam is used as a combined tool to process the workpiece and to separate the bubble, and therefore no additional components are required to separate the bubble.

[0023] According to another embodiment of the invention, the laser radiation may be directed in a defocused manner onto the area of ​​the workpiece surface where the adhering bubbles are detected. This allows the generation of shock waves to take place over a larger area of ​​the workpiece surface, which allows a more uniform and efficient separation of the bubbles from the workpiece surface. The beam diameter on the workpiece surface can be set to be 50%, 80% or 100% of the bubble diameter. The defocusing of the laser radiation can in particular be performed by changing the position of the focusing unit or by changing the position of the workpiece.

[0024] According to an embodiment of the present invention, the first action may be the generation of ultrasonic waves in the vicinity of the detected gas bubble by using an ultrasonic generator. For this purpose, in particular, an ultrasonic generator may be placed in the immediate vicinity of the workpiece or in the immediate vicinity of a hose or line used to generate a flow in the process chamber. The ultrasonic generator may in particular comprise an electrically controlled piezoelectric element.

[0025] Furthermore, according to the invention, the first action can include a change in the type of flow, which can be changed in particular between laminar, turbulent and pulsatile flows. First investigations have shown that a particularly efficient detachment of attached bubbles can be achieved by changing the type of flow.

[0026] Further, according to the present invention, determining a transit time of the detected bubble when the bubble is present within a region where interaction between the laser radiation and the bubble is expected; deactivating the laser radiation for the determined transit time; Positioning the laser radiation such that it is located outside the bubble can be provided.

[0027] When determining the transit time, it is possible to use, in particular, a camera-based method, in which the speed at which the free bubble moves in the detection area is determined. The time and duration (= transit time) during which the detected bubble is in the area where an interaction between the laser radiation and the bubble is expected can then be calculated. Finally, the laser radiation is deactivated during the determined transit time or moved to another position where no interaction occurs between the laser radiation and the bubble. For deactivation of the laser radiation, it is possible, for example, to switch off the laser beam source or, alternatively, to use a beam absorber. Alternatively, beam modulators, in particular acousto-optical modulators (also called ACMs) or electro-optical modulators (also called EOMs), can be used for particularly fast deflection of the radiation. As soon as the detected bubble leaves the impact area, the laser radiation can be activated again. Alternatively, the trajectory of the laser beam may be predefined for the laser processing process and, upon detection of a bubble, it may be calculated that an interaction between the laser beam and the free bubble is imminent. In this case, the laser radiation can leave the preset trajectory and thereby "skip" the impact area, so that this area of ​​the workpiece is processed immediately after the bubble leaves the impact area. In this way, the interaction between the laser radiation and the bubble is significantly reduced, significantly improving the process quality and speed.

[0028] In order to achieve the above object, according to another embodiment of the present invention, a system for laser processing a workpiece in a liquid is proposed, the system comprising: a laser beam source for generating pulsed laser radiation; a focusing unit for focusing the laser radiation onto a surface of the workpiece; a process chamber for receiving a workpiece; a positioning unit for adjusting the position of the laser radiation on the surface of the workpiece; a detection unit for detecting an air bubble within a predetermined detection area; and a control unit configured to perform a first operation to avoid or reduce interference effects between the laser radiation and the detected air bubbles.

[0029] The system according to the invention allows to significantly increase the processing speed and processing quality by significantly avoiding or significantly reducing undesirable interaction effects between the focused laser radiation and the bubbles. The laser beam source may in particular be a laser beam source designed to generate ultrashort pulses with a pulse duration of about 100 fs to 100 ps. The control unit may in particular be designed to achieve a first action for avoiding or reducing interference effects during laser processing caused by interaction between the laser radiation and the detected bubbles.

[0030] According to one embodiment of the system of the present invention, the detection unit may comprise a camera unit for monitoring the detection area.

[0031] Furthermore, in the system of the present invention, the detection unit may comprise an LED and a photodiode, the photodiode being configured to acquire radiation emitted by the LED propagated through the detection region, where the LED and the photodiode may be located on two opposite sides of the process chamber, where the light of the LED may be introduced into the process chamber through a first process window, while the light is led out of the chamber through a second process window so that it may be detected by the photodiode.

[0032] According to another embodiment of the system of the present invention, the system preferably further comprises a flow generator configured to adjust the flow rate, the direction of flow, and / or the type of flow in the process chamber depending on whether a bubble has been previously detected in the detection region. In this way, if a bubble is detected, the flow in the process chamber can be adjusted to remove the bubble from the processing region. In this way, the bubble removal method described above can be performed. The flow generator can in particular comprise one or more pressure pumps and suction pumps.

[0033] Furthermore, according to an embodiment of the system of the present invention, the system may comprise an ultrasonic generator configured to generate ultrasonic waves in the detection region if an air bubble has previously been detected in the detection region, whereby it is possible to achieve in particular an efficient detachment of the adhering air bubble. [Brief description of the drawings]

[0034] In the following, the invention is illustrated in more detail with reference to the drawings.

[0035] [Figure 1] 1 is a system for laser material processing in liquid according to the prior art; [Diagram 2] Laser processing in the ideal case and when air bubbles are present in the processing area. [Diagram 3] 1 shows a schematic diagram of one embodiment of the method according to the present invention. [Figure 4] FIG. 2 is a schematic diagram of an embodiment for the detection of air bubbles according to the present invention; [Diagram 5] 4 is a schematic diagram of an embodiment of an operation for avoiding interaction effects between laser radiation and detected bubbles according to the present invention. FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0036] FIG. 1 shows a system 10 for laser processing in liquids, already known from the prior art. Such a system 10 comprises a laser beam source 12, which generates pulsed laser radiation 14. The direction of the laser radiation 14 can be adjusted via a positioning unit 16. The laser radiation 14 is focused via a focusing unit 18 into a process chamber 20, in which a workpiece 22 to be processed is arranged. The focused laser radiation 14 is directed onto a surface 22a of the workpiece 22, so that the workpiece 22 can be heated and evaporated precisely at the desired location. Here, the laser radiation enters the process chamber 20 via a transparent process window 24, which is otherwise not transparent to light. The process chamber 20 is filled with a liquid 26. Water can be used as the liquid 26, for example. The liquid serves, for example, to cool the workpiece during the processing process.

[0037] A problem that occurs in prior art laser machining processes is shown in Figure 2. As shown in Figure 2, laser machining in liquids can cause the formation of bubbles that can adversely affect the laser machining process.

[0038] 2(a) illustrates an ideal case in which no gas bubbles are present within the process chamber 20. In this case, the focused laser radiation 14 can impinge unimpeded on the surface of the workpiece 22 and heat the workpiece 22.

[0039] In contrast, Fig. 2(b) shows a case where an attached bubble 28 is formed on the surface of the workpiece 22. Due to the difference in refractive index between the liquid 26 and the bubble 28, a part of the incident laser radiation 14 is reflected. The reflected laser radiation 14a does not impinge on the surface of the workpiece 22 and cannot be used for processing. Furthermore, a part of the incident laser radiation 14 is deflected. Therefore, the deflected laser radiation 14b does not impinge on the surface of the workpiece 22 at the desired position. This has a negative effect on the accuracy of the laser processing.

[0040] 2(c) illustrates another scenario in which a free gas bubble 28 is present in the processing region and interacts with the focused laser radiation 14. The gas bubble 28 results in the incident laser radiation 14 being defocused, resulting in defocused laser radiation 14c impinging on the surface of the workpiece 22. This results in insufficient radiation intensity (defined as power per area) to vaporize material at the surface of the workpiece 22.

[0041] The above examples demonstrate that bubbles formed within the process chamber 20 can contribute to significant interference with the laser machining process. In particular, bubbles can cause slower processing speeds, reduced efficiency, instability, and deviations from the desired processing results.

[0042] FIG. 3 shows an embodiment of the method 100 according to the invention. To describe the embodiment, reference is made in the following to a "first step", a "second step", etc. However, this terminology does not explicitly determine the final necessary order in the framework of the invention, but rather serves to distinguish the individual method steps. In a first step 110, a workpiece is provided in a process chamber filled with liquid. Pulsed laser radiation is focused on the surface of the workpiece in a second step 120. A focusing unit is used here. For the processing of the workpiece, a relative movement between the focused laser radiation and the workpiece surface is generated in a third step 130, for which a positioning unit is used. The positioning unit may for example be configured as a scanner mirror designed to set the position of the focused laser radiation on the surface of the workpiece or as a positioning table designed to vary the position of the workpiece. In a fourth step 140, at a predetermined detection area, the presence of air bubbles is checked. For this purpose, a detection unit is used, which may in particular include a camera. If an air bubble is detected, a first action is taken in a fifth step 150 to avoid or reduce interaction effects between the laser radiation and the detected air bubble. In other words, the first action serves to remove the detected air bubble from the detection or processing area, respectively.

[0043] FIG. 4 illustrates an embodiment of the invention relating to air bubble detection.

[0044] FIG. 4(a) shows an embodiment in which a detection unit 30 configured as a camera unit is provided. The camera unit images the detection area in which the bubble is believed to be interfering. In particular, the camera unit can monitor the interior of the process chamber 20. The camera unit generates an image file that is subsequently evaluated. If an air bubble is detected in the generated image file, corresponding actions can be taken to remove the air bubble and reduce the interaction. As shown in FIG. 4(a), the camera can be arranged radially with respect to the laser radiation 14. Alternatively, the camera unit may also be positioned axially with respect to the laser radiation. For this purpose, for example, a beam splitter can be used. Furthermore, the detection unit may comprise two camera units arranged radially with respect to the laser radiation and offset by 90° from each other. By using two camera units, the three-dimensional position of the air bubble 28 can be precisely determined.

[0045] FIG. 4(b) shows a further embodiment 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 the transparent process window 24. If no air bubbles are present in the detection region, the light emitted by the LED 30a can directly exit through the opposite process window 24 and be captured by the photodiode 30b. However, if an air bubble 28 is present in the detection region, the light emitted by the LED 30a will be scattered at the air bubble 28, so that the photodiode 30b will generate a correspondingly modified signal. By comparing the output signal of the photodiode 30b with a previously captured reference signal, it can be concluded whether an air bubble is present in the monitored detection region.

[0046] FIG. 5 shows different embodiments of the invention, which provide different actions to avoid interaction effects between the laser radiation 14 and the gas bubble 28. In FIG. 5(a), the illustrated embodiment comprises an additional ultrasonic generator 32 arranged on the underside of the process chamber 20. If the detection unit 30 detects the presence of a gas bubble 28 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, the ultrasonic generator 32 may be activated only when an attached gas bubble 28 is activated. As already shown in FIG. 2(b) and (c), attached and free gas bubbles have significantly different shapes and can therefore be optically distinguished from one another.

[0047] Furthermore, FIG. 5(b) shows another embodiment of the invention in which a flow generator 34 is provided. In this embodiment, the flow generator 34 comprises 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, which pumps are not shown in FIG. 5(b). The flow generator 34 is configured to generate a flow when an air bubble 28 is detected, whereby the air bubble is moved away from the processing area. The ultrasonic generator 32 and the flow generator 34 may also be combined. Thus, for example, the ultrasonic generator 32 can be activated when an attached air bubble 28 is detected, while the flow generator 34 can be activated when a free air bubble is detected. Furthermore, it may be advantageously provided that in the case of detection of an attached air bubble, first the ultrasonic generator 32 is used to separate the air bubble 28 from the surface of the workpiece 22, while the flow generator 34 is subsequently activated to move the free air bubble away from the detection area or the processing area, respectively. [Explanation of symbols]

[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: Work piece 22a: Surface of the workpiece 24: Process Window 26:Liquid 28: Air bubbles 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 method step 120: Second method step 130: Third method step 140: Fourth method step 150: Fifth method step

Claims

1. 1. A method for laser processing a workpiece in a liquid, comprising: Providing a workpiece in a process chamber filled with a liquid; focusing pulsed laser radiation onto a surface of the workpiece using a focusing unit; generating relative movement between the focused laser radiation and a surface of the workpiece using a positioning unit; Detecting an air bubble within a predetermined detection area using a detection unit; performing a first operation to avoid or reduce interference effects during laser processing caused by the detected bubbles; The first operation includes: determining a transit time, which is the time the bubble is in a region where an interaction between the laser radiation and the bubble is expected; deactivating the laser radiation during the determined transit time or positioning the laser radiation such that the laser radiation is located outside the bubble.

2. Detecting the air bubble taking a photographic image of the predetermined detection area using a camera unit and generating a corresponding image file; 2. A method according to claim 1, characterized in that it comprises evaluating said image file by an evaluation which in particular involves the use of a pattern recognition algorithm.

3. Detecting the air bubbles includes measuring scattered light, the measuring scattered light comprising: Illuminating the detection area with an LED; acquiring a detection signal using a photodiode, the photodiode being configured to acquire radiation emitted by the LED that has propagated through the detection area; 3. The method according to claim 1, further comprising evaluating the detection signal by means of an evaluation unit.

4. 3. The method of claim 1, wherein the first action comprises varying a flow rate of the liquid in the process chamber.

5. 3. The method of claim 1, wherein the first action comprises generating ultrasonic waves in the vicinity of the detected gas bubble using an ultrasonic generator.

6. 3. The method according to claim 1 or 2, characterized in that the first action comprises changing the type of flow of the liquid, the type of flow being in particular capable of being changed between laminar, turbulent and pulsatile flow.

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