Patent ID: 12194564

DESCRIPTION OF THE INVENTION

FIG.1shows a system10for laser processing in liquid which is already known from prior art. Such a system10comprises a laser beam source12that generates a pulsed laser radiation14. The orientation of the laser radiation14can be adjusted through a positioning unit16. The laser radiation14is focused into the interior of a process chamber20via a focusing unit18. A workpiece22to be processed is arranged in the process chamber20. The focused laser radiation14is directed onto a surface22aof the workpiece22, so that the workpiece22is heated precisely at the desired location and can evaporate. Here, the laser radiation enters the process chamber20via a transparent process window24, the process chamber20other-wise not being permeable to light. The process chamber20is filled with a liquid26. The liquid26used may be water, for example. The liquid serves to cool the workpiece during the processing process, for example.

FIG.2illustrates the problems arising in a laser processing process according to prior art. As shown inFIG.2, laser processing in liquid causes the formation of gas bubbles which have an adverse effect on the laser processing process.

First,FIG.2(a)shows the ideal case in which no gas bubbles are present in the process chamber20. In this case, the focused laser radiation14can impinge unhindered on the surface of the workpiece22and heat the workpiece22.

In contrast thereto,FIG.2(b)shows the case in which an adherent gas bubble28has formed on the surface of the workpiece22. Due to the difference in the refraction index between the liquid26and the gas bubble28, a part of the incident laser radiation14is reflected. The reflected laser radiation14adoes not impinge on the surface of the workpiece22and can thus not be used for the processing process. Furthermore, a part of the incident laser radiation14is deflected. The deflected laser radiation14bthus does not impinge on the surface of the workpiece22at the desired location. This has an adverse effect on the accuracy of the laser processing process.

FIG.2(c)shows another scenario, in which a free gas bubble28is present in the processing region, which interacts with the focused laser radiation14. The gas bubble28has the effect that the incident laser radiation14is defocused and that, consequently, a defocused laser radiation14cimpinges on the surface of the workpiece22. This regularly results in the radiation intensity (defined as power per area) is in-sufficient to evaporate the material at the surface of the workpiece22.

The above illustrated examples show that gas bubbles formed in the process chamber20contribute 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.

FIG.3shows an embodiment of method100according 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 step110, 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 step120. 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 step130, 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 step140, 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 step150in 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.

FIG.4shows embodiments of the present invention that regard the detection of the gas bubble.

FIG.4(a)shows an embodiment in which a detection unit30is 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 chamber20. 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 inFIG.4(a), the camera can be arranged radially with respect to the laser radiation14. 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 bubble28can be determined exactly.

FIG.4(b)shows a further embodiment, in which the detection unit30comprises an LED30aand a photodiode30b. Die LED30aand the photodiode30bare arranged on two opposite sides of the process chamber20. The light emitted by the LED30aenters the interior of the process chamber20via a transparent process window24. If no gas bubble is present in the detection region, the light emitted by the LED30acan exit directly through the opposite process window24and be captured by the photodiode30b. However, if a gas bubble28is present in the detection region, the light emitted by the LED30ais scattered at the gas bubble28, so that the photodiode30bproduces a correspondingly modified signal. By comparing the output signal of the photodiode30bwith previously captured reference signals, it can be concluded on whether a gas bubble is present in the monitored detection region.

FIG.5shows different embodiments of the present invention, which provide different actions for avoiding interaction effects between the laser radiation14and the gas bubble28. InFIG.5(a), the embodiment illustrated comprises an additional ultrasonic generator32arranged at the lower side of the process chamber20. If the detection unit30detects the presence of a gas bubble28in the detection region, the ultrasonic generator32can be activated by a control unit (not shown in this Figure). Thereby, the gas bubble28can be detached from the surface of the workpiece22. For example, it may be provided that the ultrasonic generator32is activated only, when an adherent gas bubble28is activated. As already illustrated inFIGS.2(b) and (c), adherent and free gas bubbles differ significantly in shape and can therefore be optically differentiated from each other.

Furthermore,FIG.5(b)shows another embodiment of the invention, in which a flow generator34is provided. In this embodiment, the flow generator34comprises a liquid inlet34aand a liquid outlet34b, as well as a pressure pump and a suction pump connected to the liquid inlet34aand the liquid outlet34b, the pumps mentioned not being illustrated inFIG.5(b). The flow generator34is configured to generate a flow in the event of a detected gas bubble28, by which the gas bubble is transported away from the processing region. It may also be provided that the ultrasonic generator32and the flow generator34are combined with each other. As such, it is possible, for example, to activate the ultrasonic generator32in case an adherent gas bubble28was detected, whereas the flow generator34is 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 generator32is used to detach the gas bubble28from the surface of the workpiece22, whereas the flow generator34is activated subsequently to transport the free gas bubble away from the detection region or the processing region, respectively.

LIST OF REFERENCE NUMERALS

10system for laser processing12laser radiation source14laser radiation14areflected laser radiation14bdeflected laser radiation14cdefocused laser radiation16positioning unit18focusing unit20process chamber22workpiece22aworkpiece surface24process window26liquid28gas bubble30detection unit30aLED30bphotodiode32ultrasonic generator34flow generator34aliquid inlet34bliquid outlet100method for laser processing110first method step120second method step130third method step140fourth method step150fifth method step