Abstract
A method for laser processing of workpieces in liquid, the method including the following steps: providing a workpiece in a process chamber filled with a liquid; focusing pulsed laser radiation on a surface of the workpiece using a focusing unit; producing a relative movement between the focused laser radiation and the workpiece surface using a positioning unit; detecting a gas bubble in a predefined detection region using a detection unit; and conducting a first action to avoid or reduce interaction effects between the laser radiation and the detected gas bubble.
Claims
1. A method for laser processing of workpieces in liquid, the method comprising the following steps: providing a workpiece in a process chamber filled with a liquid; focusing a pulsed laser radiation onto a surface of the workpiece using a focusing unit; generating a relative movement between the focused laser radiation and the workpiece surface using a positioning unit; detecting a gas bubble in a predefined detection region using a detection unit; and conducting a first action to avoid or reduce interference effects during laser processing which are caused by the detected gas bubble.
2. The method according to claim 1, wherein that the detection of the gas bubble comprises the following steps: capturing a photographic image of the predefined detection region using 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, wherein the detection of the gas bubble comprises a scattered light measurement, the scattered light measurement comprising in particular the following steps: illuminating the detection region using an LED; picking up a detection signal using a photodiode, the photodiode being configured to receive the radiation emitted by the LED after propagating through the detection region; evaluating the detection signal using an evaluation unit.
4. The method according to claim 1, wherein the first action comprises a change of the flow rate in the process chamber.
5. The method according to claim 1, wherein the first action comprises a positioning of the preceding laser radiation such that the laser radiation is directed onto the detected gas bubble.
6. The method according to claim 5, wherein the laser radiation is directed in a defocused form onto the region of the workpiece surface in which the gas bubble has been detected.
7. The method according to claim 1, wherein the first action comprises a generation of ultrasonic waves in the vicinity of the detected gas bubble using an ultrasonic generator.
8. The method according to claim 1, wherein the first action comprises a change in the flow type, wherein the flow type can in particular be varied between laminar, turbulent and pulsating.
9. The method according to claim 1, further characterized by the following steps: determining a transit time for the detected gas bubble, in which the gas bubble is present in a region in which an interaction between the laser radiation and the gas bubble is to be 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 arranged outside the gas bubble.
10. A system for laser processing of workpieces in liquid, comprising: a laser beam source for generating pulsed laser radiation; a focusing unit for focusing the laser radiation onto the surface; a process chamber for receiving the workpiece; a positioning unit for adjusting the position of the laser radiation on the surface of the workpiece. a detection unit for detecting a gas bubble in a predefined detection region; and a control unit, the control unit being configured to conduct a first action to avoid or reduce interference effects during laser processing which are caused by the detected gas bubble.
11. The system according to claim 10, wherein that the detection unit comprises a camera unit for monitoring the detection region.
12. The system according to claim 10, wherein the detection unit comprises an LED and a photodiode, the photodiode being configured to receive the radiation emitted by the LED after its propagation through the detection region.
13. The system according to claim 10, further characterized by a flow generator configured to vary the flow rate and/or the flow direction and/or the flow type in the process chamber in dependence on whether a gas bubble was previously detected in the detection region or not.
14. The system according to claim 10, further characterized by an ultrasonic generator configured to generate ultrasonic waves in the detection region if a gas bubble has previously been detected in the detection region.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0054] In the following, the present invention is illustrated in more detail with reference to the Figures which show:
[0055] FIG. 1 a system for laser material processing in liquid according to prior art,
[0056] FIG. 2 the laser processing process in the ideal case and in the presence of a gas bubble in the processing region,
[0057] FIG. 3 shows a schematic illustration of an embodiment of the method according to the present invention,
[0058] FIG. 4 a schematic illustration of embodiments for the detection of gas bubbles according to the present invention, and
[0059] FIG. 5 a schematic illustration of embodiments of the action for avoiding interaction effects between the laser radiation and the detected gas bubble, according to the present invention.
DESCRIPTION OF THE INVENTION
[0060] FIG. 1 shows a system 10 for laser processing in liquid which is already known from prior art. Such a system 10 comprises a laser beam source 12 that generates a pulsed laser radiation 14. The orientation of the laser radiation 14 can be adjusted through a positioning unit 16. The laser radiation 14 is focused into the interior of a process chamber 20 via a focusing unit 18. A workpiece 22 to be processed is arranged in 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 precisely at the desired location and can evaporate. Here, the laser radiation enters the process chamber 20 via a transparent process window 24, the process chamber 20 other-wise not being permeable to light. The process chamber 20 is filled with a liquid 26. The liquid 26 used may be water, for example. The liquid serves to cool the workpiece during the processing process, for example.
[0061] FIG. 2 illustrates the problems arising in a laser processing process according to prior art. As shown in FIG. 2, laser processing in liquid causes the formation of gas bubbles which have an adverse effect on the laser processing process.
[0062] First, FIG. 2(a) shows the ideal case in which no gas bubbles are present in the process chamber 20. In this case, the focused laser radiation 14 can impinge unhin-dered on the surface of the workpiece 22 and heat the workpiece 22.
[0063] In contrast thereto, FIG. 2(b) shows the case in which an adherent gas bubble 28 has formed on the surface of the workpiece 22. Due to the difference in the refrac-tion index between the liquid 26 and the gas 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 can thus not be used for the processing process. Furthermore, a part of the incident laser radiation 14 is deflected. The deflected laser radiation 14b thus does not impinge on the surface of the workpiece 22 at the desired location. This has an adverse effect on the accuracy of the laser processing process.
[0064] FIG. 2(c) shows another scenario, in which a free gas bubble 28 is present in the processing region, which interacts with the focused laser radiation 14. The gas bubble 28 has the effect that the incident laser radiation 14 is defocused and that, con-sequently, a defocused laser radiation 14c impinges on the surface of the workpiece 22. This regularly results in the radiation intensity (defined as power per area) is in-sufficient to evaporate the material at the surface of the workpiece 22.
[0065] The above illustrated examples show that gas bubbles formed in the process chamber 20 contribute to a significant interference with the laser processing process. In particular, the gas bubbles cause a reduced process speed, a reduced efficiency, in-stabilities, and deviations from the desired processing result.
[0066] FIG. 3 shows an embodiment of method 100 according to the invention. To illustrate the embodiment, reference is made hereinafter to a first step, a second step, etc. However, this terminology explicitly determines no order ultimately necessary in the framework of the present invention, but rather serves to differentiate between the individual method steps. In a first step 110, a workpiece is provided in a process chamber filled with liquid. A pulsed laser radiation is focused on the surface of the workpiece in a second step 120. Here, a focusing unit is used. 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, to which end 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 it may be configured as a positioning table designed to vary the position of the workpiece. In a fourth step 140, a predefined detection region is checked for the presence of a gas bubble. For this purpose, a detection unit is used which may in particular comprise a camera. When a gas bubble is detected, a first action is conducted in a fifth step 150 in order to avoid or reduce interaction effects between the laser radiation and the detected gas bubble. In other words, the first action serves to remove the detected gas bubble from the detection region or the processing region, respectively.
[0067] FIG. 4 shows embodiments of the present invention that regard the detection of the gas bubble.
[0068] FIG. 4(a) shows an embodiment in which a detection unit 30 is provided that is configured as a camera unit. The camera unit captures a detection region in which gas bubbles are considered as 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. When a gas bubble is detected in the generated image file, it may be provided that a corresponding action is conducted to remove the gas bubble and to reduce the interactions. As shown in FIG. 4(a), the camera can be arranged radially with respect to the laser radiation 14. As an alternative, the camera unit may also be positioned axially with respect to the laser radiation. For this purpose, a beam splitter can be used, for example. Further, it may be provided that the detection unit comprises two camera units, each arranged radially with respect to the laser radiation and offset from each other by 90?. By using two camera units, the three-dimensional position of the gas bubble 28 can be determined exactly.
[0069] FIG. 4(b) shows a further embodiment, in which the detection unit 30 comprises an LED 30a and a photodiode 30b. Die 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 via a transparent process window 24. If no gas bubble is present in the detection region, the light emitted by the LED 30a can exit directly through the opposite process window 24 and be captured by the photodiode 30b. However, if a gas bubble 28 is present in the detection region, the light emitted by the LED 30a is scattered at the gas bubble 28, so that the photodiode 30b produces a correspondingly modified signal. By comparing the output signal of the photodiode 30b with previously captured reference signals, it can be concluded on whether a gas bubble is present in the monitored detection region.
[0070] FIG. 5 shows different embodiments of the present invention, which provide different actions for avoiding interaction effects between the laser radiation 14 and the gas bubble 28. In FIG. 5(a), the embodiment illustrated comprises an additional ultrasonic generator 32 arranged at the lower side of the process chamber 20. If the detection unit 30 detects the presence of a gas bubble 28 in the detection region, the ultrasonic generator 32 can be activated by a control unit (not shown in this Figure). Thereby, the gas bubble 28 can be detached from the surface of the workpiece 22. For example, it may be provided that the ultrasonic generator 32 is activated only, when an adherent gas bubble 28 is activated. As already illustrated in FIGS. 2(b) and (c), adherent and free gas bubbles differ significantly in shape and can therefore be optically differentiated from each other.
[0071] 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, the pumps men-tioned not being illustrated in FIG. 5(b). The flow generator 34 is configured to generate a flow in the event of a detected gas bubble 28, by which the gas bubble is transported away from the processing region. It may also be provided that the ultrasonic generator 32 and the flow generator 34 are combined with each other. As such, it is possible, for example, to activate the ultrasonic generator 32 in case an adherent gas bubble 28 was detected, whereas the flow generator 34 is activated in case a free gas bubble was detected. Further, it may be advantageously provided that in the case of a detection of an adherent gas bubble, first the ultrasonic generator 32 is used to detach the gas bubble 28 from the surface of the workpiece 22, whereas the flow generator 34 is activated subsequently to transport the free gas bubble away from the detection region or the processing region, respectively.
LIST OF REFERENCE NUMERALS
[0072] 10 system for laser processing [0073] 12 laser radiation source [0074] 14 laser radiation [0075] 14a reflected laser radiation [0076] 14b deflected laser radiation [0077] 14c defocused laser radiation [0078] 16 positioning unit [0079] 18 focusing unit [0080] 20 process chamber [0081] 22 workpiece [0082] 22a workpiece surface [0083] 24 process window [0084] 26 liquid [0085] 28 gas bubble [0086] 30 detection unit [0087] 30a LED [0088] 30b photodiode [0089] 32 ultrasonic generator [0090] 34 flow generator [0091] 34a liquid inlet [0092] 34b liquid outlet [0093] 100 method for laser processing [0094] 110 first method step [0095] 120 second method step [0096] 130 third method step [0097] 140 fourth method step [0098] 150 fifth method step
