EP4323141A1

EP4323141A1 - Method for monitoring and control of pulsed laser micro-processing and apparatus for carrying out this method

Info

Publication number: EP4323141A1
Application number: EP21851962.7A
Authority: EP
Inventors: Jirí MARTAN; Denys MOSKAL; Milan Honner; Carlos BALLARDIN BELTRAMI; Vladislav LANG
Original assignee: Zapadoceska Univerzita v Plzni
Current assignee: Zapadoceska Univerzita v Plzni
Priority date: 2021-04-14
Filing date: 2021-12-23
Publication date: 2024-02-21
Also published as: CZ2021186A3; WO2022218451A1

Abstract

Subject-matter of the invention is the method for monitoring and control of pulsed laser micro-processing, wherein the laser pulses repeatedly impinge on the workpiece (10), which is heated by them and emits the thermal radiation (12), which is detected by the detection system (3), which creates the time course of the signal, and in the time course of the signal the changes are found corresponding to changes in the state, structure or properties of the material of the workpiece (10), caused by at least one laser pulse, and by using these changes at least one characteristic number is determined, which has a dimension of time or signal, and the value of the characteristic number is compared with a range of values predetermined for a correctly performed operation, and in case of an incorrect value of the characteristic number the parameters of the pulsed laser micro-processing are corrected, this method of monitoring and control of the pulsed laser micro-processing being repeated within the laser micro-processing, as well as the apparatus for carrying out the method for monitoring and control of pulsed laser micro-processing comprising a detection optical system (1), a detection system (3), a recording system (4), an evaluation system (5), a control system (6), a laser (7) and a laser optical system (8), and the recording system (4) comprising an analog-to-digital converter, the evaluation system (5) comprising a programmable gate array and the detection optical system (1) comprising a scanning head.

Description

Method for monitoring and control of pulsed laser micro-processing and apparatus for carrying out this method

Field of technology

The invention relates to a method for monitoring and control of pulsed laser micro-processing and to an apparatus for carrying out this method. It is a promising technology enabling fine and precise machining of material, creation of surface micro structure or nanostructure or micro structure and nanostructure at the same time or, conversely, polishing of the surface. However, this technology still has its limits and limitations and they are insufficient processing speed and the possibility of frequent defects, such as local surface degradation.

Prior art

Pulsed laser micro-processing is a promising technology that allows fine and precise machining of the material, creating a surface microstructure, nanostructure or both to affect the physical properties of the material, such as water repellence, light absorption, increased adhesion or surface polishing. However, this technology still has its limits and limitations and they are insufficient processing speed and the possibility of defects, such as local degradation, burning, excessive or insufficient machining and inhomogeneity of the structure in local places. These defects will be viewed as defects and the part must be repaired or discarded. Or it is necessary to process the whole part more slowly with weaker parameters, for example with lower average power or pulse repetition frequency, which leads to the first limitation. Defects on the part are usually discovered only after the overall processing during the inspection, for example by visual, 3D laser confocal microscope, measuring the dimensions of the micro-part, optical microscope or checking the functionality of the surface texture. Defects are therefore discovered only after the overall processing, when the part already has a high added value and its discarding means large financial and time losses.

There are already some devices and approaches for process control that allow capturing information during the process. However, they are not able to deal with process control and management during the process and for each laser pulse. These are, for example, cameras in the visible and infrared spectral regions, but which do not have a fast enough response to control the process. Alternatively, in the case of micro-drilling, sensing the emission of plasma by a photodiode is used to be able to terminate the process when the plasma disappears and the hole is already fully made. However, this device and method do not allow monitoring and control of the process during the micro-processing. Pulsed laser micro-processing uses pulses with a duration in nanoseconds, picoseconds and femtoseconds with a repetition frequency of pulses, usually from tens of kHz to tens of MHz. Once the pulse is released, it is not possible to control its action. However, it is possible to sense its response and control other pulses accordingly.

The present invention is directed to such a control. A large number of pulses, such as millions, strike the material during the laser processing. Tens, hundreds to thousands of pulses overlap in one place during the process. Residual heat then remains in the material, which is not sufficient to dissipate to the depth of the material and accumulates, which is reflected in an increase in the local surface temperature of the workpiece. If this temperature exceeds a certain threshold for a given material, the material begins to degrade. In other applications, on the other hand, heat accumulation is used to remelt the material and polish it. By using burst laser processing with a frequency of repeated pulses in the order of GHz, it is possible to achieve more efficient and faster surface treatment. However, this process involves thermal processes, even if there are picosecond or femtosecond pulses used. These thermal processes have not yet been possible to control during the burst, only after the overall processing during the inspection, as already mentioned. Finding suitable parameters of laser processes with GHz burst without control and management is difficult and time consuming and at the same time material processing is sensitive to material inhomogeneities and geometric shape of the part, such as corners and narrow grooves, where defects are formed all the more.

To increase the processing speed, multi-beam systems, such as parallel processing, or large-area laser beams, are beginning to be used. In these systems, it is not yet possible to control the process in individual laser beams, or in individual areas of a large beam, during the process. Also when using a scanning head to move the laser beam over the surface of the material, the measurement is always problematic at the point of action of the laser beam.

Summary of the invention

The essence of the method of monitoring and control of pulsed laser micro-processing, wherein the laser pulses repeatedly impinge on the workpiece (10), which is heated by them and emits the thermal radiation (12), which is detected by the detection system (3), which creates the time course of the signal, is that in the time course of the signal the changes are found corresponding to changes in the state, structure or properties of the material of the workpiece (10), caused by at least one laser pulse, and by using these changes at least one characteristic number is determined, which has a dimension of time or signal, and the value of the characteristic number is compared with a range of values predetermined for a correctly performed operation, and in case of an incorrect value of the characteristic number the parameters of the pulsed laser micro-processing are corrected, this method of monitoring and control of the pulsed laser micro-processing being repeated within the laser micro-processing.

To find changes in the time course of the signal, signal derivation is preferably used and includes finding a local minimum, a local maximum or a zero value of the derived time course of the signal.

The characteristic number may be the heat accumulation signal, which is determined as the average of the signal in the short time, for example in time of 100 ns, before the time of fast rise of the signal.

The characteristic number may be the time of the melting threshold signal, which is determined as the difference between the time of exceeding the threshold value higher than the melting signal and the fast rise time of the signal, wherein for the case of determining the time of the melting threshold signal for the burst laser processing, with a repetition rate of pulses in the burst ranging from 20 MHz to 10 THz, the signal is smoothed to remove individual pulses in the burst before evaluating the threshold value.

The characteristic number may be the time of the solidification threshold signal, which is determined as the difference between the fall time below a threshold value lower than the solidification signal and the maximum signal time, wherein in the case of determining the time of the solidification threshold signal for the burst laser processing, with a repetition frequency of pulses in the burst ranging from 20 MHz to 10 THz, the course of the signal at the end of the batch after the last pulse is used.

The characteristic numbers may be the maximum and minimum values of the signal difference in the second part of the laser pulse burst, with a pulse repetition frequency in the burst ranging from 20 MHz to 10 THz, the signal difference being determined as the difference between the actual signal value and the signal average shortly before the fast increase in signal at the beginning of the burst.

The monitoring and control according to the invention can be repeated after each laser pulse or after each burst of laser pulses with a pulse repetition frequency in the burst in the range from 20 MHz to 10 THz.

The monitoring and control can be performed only in the next processed layer from the measured data in this or even previous layers and from the measured data at this point or even adjacent points.

Correction of pulsed laser micro-processing parameters can be done by changing the pulse energy, or by changing the pulse repetition frequency, or by changing the pulse duration or pulse burst length, or by changing the number of pulses in the burst, or by changing the laser spot size, or by changing polarization of laser light, or by changing the speed of movement of the workpiece or by changing the speed of movement of the laser beam.

Before or after finding changes, the time course of the signal can be converted to the time course of the temperature.

Thermal radiation can be collected by the detection system using a detection optical system from an area smaller than the laser beam or laser multi-beam assembly and the relative position of the centre of the measured thermal radiation area and the centre of the laser beam changes repeatedly during laser processing to measure the response in different parts of the large laser beam or laser multi-beam assembly and the control system, in the event of an incorrect characteristic number value, corrects the pulsed laser micro-processing parameters separately for each part of the large laser beam or individual parts of the laser multi-beam assembly.

The range of values for a properly performed operation is determined by experimental calibration, or computer simulation, or machine learning from previous laser processing of the material.

The essence of the apparatus for performing the method of monitoring and control of pulsed laser micro-processing according to the invention is that it comprises detection optical system, a detection system, a recording system, an evaluation system, a control system, a laser and a laser optical system, and the recording system comprises an analog-to-digital converter, the evaluation system comprises a programmable gate array and the detection optical system comprises a scanning head.

The advantage of using said apparatus and method is the possibility of on-line monitoring and control of laser micro-processing after each laser pulse or burst of pulses, which has not been possible before. The result is a higher speed of material processing without loss of processing quality, which leads to time savings and consequently to financial savings.

The advantages of using the mentioned method and apparatus for evaluation of heat accumulation are prevention of degradation, burning or excessive machining of the workpiece in local places, indication of places with possible insufficient machining and enabling their repair and overall reduction of scrap in laser micromachining and saving machining time, and so financial gain because it is not necessary to machine the whole part with weaker parameters, such as lower average power, for a long time due to potentially problematic places.

The advantage of the solution according to the invention is the possibility to monitor and control the laser micro-processing even with a rapid movement of the laser beam over the surface of the part, for example 10 m/s. It is also possible to program the relative position of the laser and measuring beam not only to the same location, but also at some distance from each other, for example a measuring beam in front of or behind the laser beam. Another advantage is the possibility of dynamic adjustment of the position of the measuring and laser beam, for example when moving both beams, which can be used for multi-beam or large-area laser surface treatment, where the measuring system will alternately evaluate the processing quality of different laser beams or groups of the beam and the beam guidance and routing system will dynamically adjust the energy density of the laser pulses at a given location, i.e. the measured beam, a group of beams or a part of a larger beam.

The advantages of using said method and apparatus for surface remelting evaluation are prevention of degradation, burning or insufficient remelting of the processed part in local places, indication of places with possible poor quality processing and enabling their repair and overall higher speed of material processing without loss of quality during laser remelting.

The advantages of using the mentioned method and apparatus for evaluating the quality of nanostructure formation are avoidance of degradation, burning, remelting, excessive material removal or insufficient formation of nanostructures on the processed part in local places, indication of places with possible poor quality processing and enabling their repair and overall reduction of laser waste during formation of nanostructures.

The advantages of using the method and apparatus for quality control and management of laser burst ablation are avoidance of degradation, burning or excessive or insufficient machining of the workpiece in local places, indication of places with possible insufficient machining and enabling their repair and overall higher material processing speed without loss of quality in laser micromachining and surface texturing, and thus a financial gain, because it is not necessary to machine the whole part with weaker parameters, such as lower average power, for a longer period of time due to potentially problematic places. Laser burst ablation allows more efficient removal of material than individual pulses, and in conjunction with the method and apparatus will allow its full use without the risk of quality reduction by too strong thermal effects.

The advantages of using the mentioned method and apparatus for quality control of laser micro- processing in the next layer are prevention of repeated overheating of the processed part in local places and thus degradation and excessive machining, indication of places with possible insufficient machining and enabling their repair and thus overall higher processing speed of the material without loss of quality in laser micromachining and surface texturing and thus financial gain, because it is not necessary to machine the whole part with weaker parameters, such as lower average power, for a long time due to potentially problematic places. Brief description of the drawings

An exemplary embodiment of the invention is explained in more detail in the accompanying figures, in which

FIG. 1 shows a schematic embodiment of an apparatus for the monitoring and control of laser micro-processing,

FIG. 2 shows the heat accumulation signal shortly before the time of the next fast increase of the signal,

FIG. 3 shows the time of fast signal rise,

FIG. 4 shows an apparatus according to the invention in an embodiment which is suitable, for example, for laser texturing or the production of surface nanostructures,

FIG. 5 shows the course of the melting sign and / or the solidification sign in the measured signal and their characteristic values,

FIG. 6 shows satisfactory and unsatisfactory melting signal time domains,

FIG. 7 shows the evaluation of the solidification feature,

FIG. 8 shows the evaluation of the quality of the laser formation of nanostructures by the evaluation of the solidification feature,

FIG. 9 shows the evaluation of the melting time using the local minimum of the signal derivative,

FIG. 10 shows the evaluation of the solidification plateau length as the difference of the local maximum and local minimum signal derivation times,

FIG. 11 shows the evaluation of the quality of the burst laser ablation using the difference of the signals in the second part of the burst, determined as the difference between the actual value of the signal and the average of the signal from the short time before the start of the burst;

FIG. 12 shows a microscopic thermal image of the micro-processing with peaks at the locations of the high heat accumulation signal, and

FIG. 13 shows an image of the adjusted micro-processing parameters with valleys of lower set average laser power at locations of the high heat accumulation signal in the previous processing layer.

Examples

The monitoring and control of the pulsed laser micro-processing according to the described invention can be performed in the processing of various materials, in particular metals, but also composites, plastics, ceramics and semiconductors. The typical length of laser pulses in this micro- processing is from femtoseconds to nanoseconds and the repetition frequency of the laser (7) pulses is from kHz to GHz. Pulsed lasers are typically used as the source of laser pulses, which always operate in pulse mode. This means that they repeatedly emit laser pulses, i.e. flashes of laser radiation. However, continuous or quasi-continuous lasers operating in pulse mode can also be used. These then emit longer pulses in the order of microseconds.

Example 1

The monitoring and control of the pulsed laser micro-processing according to the described invention can be performed on the device according to FIG. 1. The laser beam 11 is guided from the laser 7 to the workpiece 10 by means of a fixed laser optical system 8 and is focused by an objective 9 which is usually part of the laser optical system 8. The workpiece 10 is moved and rotated to precisely direct the laser beam 11 to the intended place of the workpiece 10. The individual laser pulses impinge on the surface of the material of the workpiece 10 and heat, melt, reshape or ablate the material from the surface. The surface of the material of the workpiece 10 is heated by the residual heat and emits thermal radiation 12. The thermal radiation 12 is guided and focused on the fast detection system 3 by the detection optical system 1. The detection system 3 is located next to the laser optical system 8. Partially reflected laser beam 11 from the workpiece 10, passed through the detection optical system 1, is stopped or reflected back by the filter 2 eliminating the laser radiation, so that the detection system 3 is not damaged.

By thermal radiation 12 is meant here the electromagnetic radiation generated by radiation, i.e. emitted by the thermal motion of particles of material. The intensity and wavelength of the thermal radiation 12 depends significantly on the temperature of the material. For the laser processes under consideration, the thermal radiation 12 has a wavelength mainly in the region of infrared radiation, i.e. 760 nm to 1 mm, and visible light, i.e. 400 nm to 760 nm.

The detection system 3 detects the thermal radiation 12 and generates a signal in the form of an electrical voltage according to its intensity. The detection system 3 sends the measured signal to a fast recording system 4, for example an oscilloscope card, where the analog voltage signal is converted into a digital signal by an analog-to-digital converter. The recording system 4 sends a digital signal to the fast evaluation system 5, which comprises a programmable gate array with a recorded program, enabling the execution of several function blocks at once and / or the implementation of algorithms in a massively parallel manner. The evaluation system 5 converts the measured time course of the signal into one or several characteristic numbers and sends them to the control system 6. The signal processing may include the conversion of the signal to temperature. The control system 6 evaluates whether the number corresponds to the operation being performed and, if not, corrects the average laser power, the repetition frequency of laser pulses or bursts of laser pulses, the pulse energy, the pulse length or burst length, the number of pulses per burst, the laser spot size, for example by defocusing, the polarization of the laser light, or the speed of movement of the workpiece 10. The correction performed will affect next laser pulses incident on the material.

The detection optical system 1 typically comprises two off-axis paraboloidal mirrors, one or more optical lenses, a microscopic objective, or a combination thereof. The detection system 3 typically comprises one electromagnetic radiation detector, for example of mercury-cadmium-tellurite, indium-gallium-arsenite, indium- antimonite or silicon. However, it may contain more detectors, for example for spectral measurements and easier calibration for temperature conversion from a signal, or for evaluating the spatial temperature distribution in the case of a large laser beam or laser beam assembly. Or, to sense the thermal response of a fast-moving beam, the detectors can be placed in a row and the beam will sequentially traverse the locations measured by different detectors. To obtain a larger signal, it is suitable to use a detector cooled by liquid nitrogen, Stirling engine or thermoelectric cooling. A DC detector is suitable for easier calibration. The response rate of the detection system 3 is typically in nanoseconds but can be from picoseconds to microseconds. The laser-eliminating filter 2 consists, for example, of germanium or silicon or a thin-film optical filter transmitting longer or shorter wavelengths than the wavelength of the laser light. For a simpler arrangement, the filter 2 can be incorporated into the detection optical system 1, for example as a material or the surface of one of the lenses.

If the monitoring and control system is already fine-tuned and production is performed repeatedly and single-purpose in a large series, it is advantageous to use a fully analog data evaluation and control system, which is faster and cheaper. The recording system 4, the evaluation system 5 and possibly also the control system 6 then operate in analog mode without converting the signal into digital form.

Example 2

The apparatus and method of the present invention can be used in the context of Example 2 for quality control of laser micromachining and surface texturing. The evaluation system 5 evaluates the heat accumulation signal 13, as shown in FIG. 2, as the average of the signal values shortly before the next time of fast rise of the signal 14 caused by the next laser pulse. For a pulse repetition frequency of 100 kHz, 100 ns can be averaged over time. For a frequency of 1 MHz, from a shorter time, e.g. 10 ns, can be averaged. The time of fast rise of the signal 14 is determined as shown in FIG. 3 as the time when the difference of the signals 15 is several times higher, for example three times, than the magnitude of the noise 18 in the previous short time. The difference of the signals 15 is the difference between the actual value of the signal 16 and the average of the signal in the short time 17. The magnitude of the noise 18 is determined as the difference between the maximum and minimum value of the signal in this short time. This short time can be, for example, tens of nanoseconds.

The evaluation system 5 sends to the control system 6, as shown in FIG. 1, the value of the heat accumulation signal 13. The control system 6 compares this value with the set maximum permissible value of the heat accumulation signal for the operation being performed. If the sent value of the heat accumulation signal 13 is higher than the maximum permissible value, the control system 6 reduces the average laser power or pulse energy, or reduces the pulse repetition frequency, or increases the laser spot size, or increases the movement speed of the workpiece 10. The control system 6 records each such a change together with the current position of the laser beam 11 on the workpiece 10 for inspection and possible finishing in further layers. If in subsequent pulses the value of the heat accumulation signal 13 falls significantly below the maximum permissible value, for example by more than 5% of its value, the control system returns the power, energy, frequency or speed settings back to the original value and records this step together with the current laser beam 11 position on the workpiece 10.

Inspection of the surface shape at the recorded locations is performed by a standard method, such as a 3D laser confocal microscope, after one or more layers, or at the end of machining, and in the case of insufficient machining, corrective laser micromachining is performed at these locations. By layer is meant herein a layer of material to be removed in one iteration of the process. If the material is to be removed to a certain depth during laser micromachining or surface texturing, the depth is divided into layers and the material is removed successively by repeating the laser process. The dimensions of the hole and the method of placing the laser pulses do not need to be the same in different layers.

Example 3

The monitoring and control of the pulsed laser micro-processing according to the described invention can be performed on the apparatus according to FIG. 4, which is suitable, for example, for laser texturing, the production of surface nanostructures - laser-induced periodic surface structures - and micro-polishing.

Here, the laser optical system 8 comprises a scanning head which moves the laser beam 11, and the workpiece 10 does not move during processing. The scanning head is an optical system typically comprising two precisely controlled and rapidly tilting mirrors mounted on galvanometers or a rapidly rotating polygonal mirror, or both. The detection optical system 1 comprises a second scanning head which allows the detection system to sense the emitted thermal radiation 12 from the moving point of action of the laser beam 11 on the workpiece 10. It is suitable for both the scanning head in the laser optical system 8 and the scanning head in the detection optical system 1 to be of the same type, i.e. galvanometric, polygonal or hybrid. Alternatively, the radiated thermal radiation 12 can be guided back through the first scanning head and split from the laser beam 11 behind it and focused on the detection system 3.

In the case of combined processing of large areas, both the laser beam 11 and the workpiece 10 can move. When using the described apparatus, the relative positions of the place from which the thermal radiation 12 is collected and the place of action of the laser beam 11 are important. These places will typically move together at the same time, either having the centers exactly in the same place, or in a specific distance from each other. In the case of a large laser beam or laser multi- beam assembly, this relative distance and position may change periodically at certain times, for example after several laser pulses, to monitor and control the laser processing at different locations of the large laser beam 11 or assembly of laser beams 11.

Example 4

The apparatus and method according to the described invention can be used for monitoring and control of the quality of surface remelting, for example for laser micro-polishing of the surface or remelting of thin layers. By thin layer is meant a layer of different material with a thickness of tens of nanometers to units of micrometers applied to the base material.

The evaluation system 5 evaluates the existence of the melting sign 19 and / or the solidification sign 20 in the measured time course of the signal and their characteristic values, as can be seen from FIG. 5. A process with insufficient average laser power or pulse energy is shown by the heating curve 21 without signs of melting and solidification. The material was only heated and then cooled, or the amount of melt was so small that it was not detected. In laser polishing with ultrashort pulses, the time course of the signal can contain a solidification sign only. In laser remelting of thin films with longer pulses, the time course of the signal can contain a melting sign only. Characteristic numbers for monitoring and control of the quality of the laser remelting may be, for example, the melting time 22, the melting signal 24, the solidification plateau length 23 or the solidification signal 26.

A method of evaluating the surface remelting quality by evaluating the melting sign 19 is shown in FIG. 6. By means of experimental calibration, machine learning from previous laser remelting of material or by computer simulation of remelting variants, the range of corresponding threshold times 34 is determined for values of the time of the threshold signal of melting 28, which for given melting parameters, especially material of the workpiece 10 and its thickness, energy and pulse duration of the laser 7, the size of the laser spot, the frequency of repetition of the pulses and the speed of movement of the laser beam 11 along the workpiece 10, correspond to a well-executed remelting. This range is shown in FIG. 6 marked as "OK" and corresponds to a time of good quality remelting 37. The time of the threshold signal of melting 28 is calculated as the difference between the time when the signal value exceeds the UPA threshold and the time of fast rise of the signal 14. The UPA threshold is selected according to the material being processed as a slightly higher signal value than the signal UTA for melting of the material. When evaluating a signal with significant noise, before evaluating the time of the threshold signal of melting 28, the time course of the signal is smoothed, for example by means of a moving average.

If the time of the threshold signal of melting 28 is lower, we reach the range of lower threshold times 36, which indicates poor quality remelting, where the material may be damaged by high temperature, too oxidized or removed. If the time of the threshold signal of melting 28 is higher, we get to the range of higher threshold times 35, so that the monitoring also results in unsatisfactory remelting, where the remelting may not be homogeneous and completely performed. Unsatisfactory ranges of the time of the threshold signal of melting 28 are shown in FIG. 6 and are marked as "NOK". These ranges represent times of poor quality remelting 38.

When controlling the process, the evaluation system 5 sends to the control system 6, as can be seen in FIG. 1, the value of the time of the threshold signal of melting 28. The control system 6 compares this value with the range of the corresponding threshold times 34 for the operation being performed. If the sent value of the time of the threshold signal of melting 28 is lower than the minimum allowable value, the control system 6 reduces the average laser power or pulse energy, or increases the size of the laser spot. If the sent value of the time of the threshold signal of melting 28 is higher than the maximum allowable value, the control system 6 increases the average laser power or pulse energy, or decreases the size of the laser spot. Each such change is recorded by the control system 6 together with the current position of the laser beam 11 on the workpiece 10 for inspection and possible finishing.

A method of evaluating the surface remelting quality by evaluating the solidification sign 20 is shown in FIG. 7. By means of experimental calibration, machine learning or by computer simulation of remelting variants, the range of corresponding threshold times 34 is determined for values of time of the threshold signal of solidification 33, which for given melting parameters, especially material of the workpiece 10, or the thin layer on the surface and its thickness, energy and pulse duration of the laser 7, the size of the laser spot, the pulse repetition frequency and the speed of movement of the laser beam 11 along the workpiece 10, correspond to a well-executed remelting. This range shown in FIG. 7 is marked as "OK" and corresponds to a time of good quality remelting 37. The time of the threshold signal of solidification 33 is calculated as the difference between the time when the signal value falls below the UPU threshold and the time of the maximum signal 27. The UPU threshold is selected according to the material being processed as a slightly lower signal value than the signal UTU for the solidification of the material, but higher than the heat accumulation signal 13 for the process. When evaluating a signal with significant noise, before evaluating the time of the threshold signal of solidification 33, the time course of the signal is smoothed, for example by means of a moving average.

If the time of the threshold signal of solidification 33 is higher, we get to the range of higher threshold times 35, which indicates poor quality remelting, where the material may be damaged by high temperature, too oxidized or removed. If the time of the threshold signal of solidification 33 is lower, we get to the range of lower threshold times 36, so that the control also results in unsatisfactory remelting, where remelting may not be homogeneous and completely performed. Unsatisfactory ranges of the time of the threshold signal of solidification 33 are shown in FIG. 7 marked as "NOK". These ranges represent times of poor quality remelting 38.

When controlling the process, the evaluation system 5 sends to the control system 6, as shown in FIG. 1, the value of the time of the threshold signal of solidification 33. The control system 6 compares this value with the range of corresponding threshold times 34 for the operation being performed. If the sent value of the time of the threshold signal of solidification 33 is higher than the maximum allowable value, the control system 6 will reduce the average laser power or pulse energy, or otherwise reduce the thermal load on the material. If the sent value of the time of the threshold signal of solidification 33 is lower than the minimum allowable value, the control system 6 increases the average laser power or pulse energy, or otherwise increases the thermal load of the material. Each such change is recorded by the control system 6 together with the current position of the laser beam 11 on the workpiece 10 for inspection and possible finishing.

The evaluation of the surface remelting quality by evaluating the melting sign 19 or the solidification sign 20 can also be performed by signal derivation. An example for evaluating the melting sign 19 by signal derivation is shown in FIG. 9. To evaluate the melting sign 19, the measured signal is derived in the range from the time of fast rise of the signal 14 to the time of the maximum signal 27. For every two points of the measured signal, the derivative of the signal is equal to the ratio of the difference between signal values U and difference between time values t at these two points. When evaluating a signal with significant noise, the time course of the signal is smoothed before and after the derivation, for example by means of a moving average. The melting time 22 is calculated as the difference between the time of local minimum of derivative of the signal 29 and the time of fast rise of the signal 14. A method of evaluating the surface remelting quality by evaluating the melting sign 19 using a local signal derivation minimum is shown in FIG. 9. By means of experimental calibration, machine learning or by computer simulation of remelting variants, the range of corresponding melting times 30 is determined for values of melting time 22, which for given melting parameters, especially material of the workpiece 10, or thin layer on surface and its thickness, pulse energy and pulse duration of the laser 7, the size of the laser spot, the pulse repetition frequency and the speed of movement of the laser beam 11 along the workpiece 10, correspond to a well-executed remelting. This range is shown in FIG. 9 marked as "OK" and corresponds to the time of good quality remelting 37.

If the melting time 22 is higher, we get into the range of higher melting times 31, which indicates a poorly performed remelting, where the remelting may not be homogeneous and completely performed. If the melting time 22 is lower, we get to the range of lower melting times 32, so that the inspection also results in unsatisfactory remelting, where the material may be damaged by high temperature, too oxidized or removed. Unsatisfactory ranges of the melting time 22 are shown in FIG. 9 marked as "NOK". These areas represent times of poor quality remelting 38.

In process control, the evaluation system 5 sends a value of the melting time 22 to the control system 6. The control system 6 compares this value with the range of corresponding melting times 30 for the operation being performed. If the sent value of the melting time 22 is higher than the maximum allowable value, the control system 6 increases the average laser power or pulse energy, or reduces the size of the laser spot. If the sent value of the melting time 22 is lower than the minimum allowable value, the control system 6 reduces the average laser power or pulse energy, or increases the size of the laser spot. Each such change is recorded by the control system 6 together with the current position of the laser beam 11 on the workpiece 10 for inspection and possible finishing.

A method of evaluating the surface remelting quality by evaluating the solidification sign 20 using a local minimum and a maximum of the derivative of the signal is shown in FIG. 10. In this case, the measured signal is derived in the range from of time of the maximum signal 27 to the time of the next fast rise of the signal 14. The solidification plateau length 23 is evaluated as the difference between the local minimum of derivative of the signal 29 and the local maximum of derivative of the signal 48. Using experimental calibration, machine learning or by means of computer simulation of remelting variants the range of corresponding solidification times 49 is determined for values of solidification plateau length 23, which for given melting parameters, especially material of the workpiece 10, or thin layer on surface and its thickness, pulse energy and pulse duration of the laser 7, the size of the laser spot, the pulse repetition frequency and the speed of movement of the laser beam 11 along the workpiece 10, correspond to a well-executed remelting. This range is shown in FIG. 10 marked as "OK" and corresponds to the time of good quality remelting 37.

If the solidification plateau length 23 is higher, we get into the range of higher solidification times 50, which indicates poor quality remelting, where the material may be damaged by high temperature, too oxidized or removed. If the solidification plateau length 23 is lower, we get to the range of lower solidification times 51, so that the result of the inspection is also unsatisfactory remelting, where the remelting may not be homogeneous and completely performed. Unsatisfactory ranges of solidification plateau length 23 are shown in FIG. 10 marked as "NOK". These areas represent times of poor quality remelting 38.

When controlling the process, the evaluation system 5 sends to the control system 6 a value of the solidification plateau length 23. The control system 6 compares this value with the range of the corresponding solidification times 49 for the operation currently being performed. If the sent value of the solidification plateau length 23 is higher than the maximum allowable value, the control system 6 reduces the average laser power or pulse energy, or increases the size of the laser spot. If the sent value of the solidification plateau length 23 is lower than the minimum allowable value, the control system 6 increases the average laser power or pulse energy, or reduces the size of the laser spot. Each such change is recorded by the control system 6 together with the current position of the laser beam 11 on the workpiece 10 for inspection and possible finishing.

The methods of quality evaluation and process control of the surface remelting by evaluating the melting sign 19 or solidification sign 20, as described above, can also be used in burst laser processing with a hurts pulse repetition frequency in the GHz range of tens MHz to THz, e.g. during laser micro-polishing. The evaluation of the melting sign 19 is then performed at the beginning of the burst, for example within the first fifty pulses, and the evaluation of the solidification sign 20 at the end of the burst, i.e. after the last pulse. To evaluate the melting sign 19, it is only necessary to perform signal smoothing, for example by moving average, to remove the peaks of the individual pulses in the signal. During control, it is then also possible to change the energy and number of pulses in the burst, the length of the burst, the frequency of repetition of pulses in the burst and the frequency of repetition of bursts of pulses. In some cases, for example in the context of very fine melting, the solidification signs 20 will appear within the individual pulses, i.e. their peaks, already during the burst of pulses with a frequency of repetition of pulses in the burst in the GHz range. It is then possible to evaluate the quality already during the burst of pulses by means of the solidification sign 20 on some or all of the pulses in the burst. The surface remelting at the recorded locations is checked by a standard method, such as a microscope, and in the case of poor quality remelting, a corrective laser treatment is performed at these locations.

Example 5

The apparatus and method according to the described invention can be used for quality control of nanostructures, such as laser-induced periodic surface structures. The evaluation system 5 evaluates the existence of the solidification sign 20 in the measured signal and its characteristic values, which is shown in FIG. 5.

A method of evaluating the quality of laser formation of nanostructures by evaluating the solidification sign 20 is shown in FIG. 8. By means of experimental calibration, machine learning or by computer simulation of process variants the range of corresponding threshold times 34 is determined for values of the time of the threshold signal of solidification 33, which for given parameters of nanostructure formation, especially material of the workpiece 10, pulse energy and pulse duration of the laser 7, the size of the laser spot, the pulse repetition frequency and the speed of movement of the laser beam 11 along the workpiece 10, correspond to a well-formed nanostructure. This range is shown in FIG. 8 marked as "OK" and corresponds to the time of good quality nanostructure 39. The time of the threshold signal of solidification 33 is calculated as the difference between the time when the signal value falls below the UPU threshold value and the time of the maximum signal 27. The UPU threshold value is selected according to the processed material as a slightly lower signal value than the solidification signal of the UTU material, but higher than the heat accumulation signal 13 for the process.

If the time of the threshold signal of solidification 33 is higher, we get into the range of higher threshold times 35, which indicates a poorly formed nanostructure, where the material may be remelted, damaged by high temperature, too oxidized or removed. If the time of the threshold signal of solidification 33 is lower, we get to the range of lower threshold times 36, so that the inspection also results in an unsatisfactory nanostructure, where the nanostructure may not be homogeneous and completely formed. Unsatisfactory time ranges of the time of the threshold signal of solidification 33 are shown in FIG. 8 marked as "NOK". These areas represent times of poor quality nanostructure 40.

When controlling the process, the evaluation system 5 sends to the control system 6 according to FIG. 1 the value of the time of the threshold signal of solidification 33. The control system 6 compares this value with the range of the corresponding threshold times 34 for the operation being performed. If the sent value of the time of the threshold signal of solidification 33 is higher than the maximum allowable value, the control system 6 will reduce the average laser power or pulse energy, or otherwise reduce the thermal load on the material. If the sent value of the time of the threshold signal of solidification 33 is lower than the minimum allowable value, the control system 6 increases the average laser power or pulse energy, or otherwise increases the thermal load of the material. Each such change is recorded by the control system 6 together with the current position of the laser beam 11 on the workpiece 10 for inspection and possible finishing.

The inspection of the formed surface nanostructure at the recorded locations is performed by a standard method, for example a microscope, and in the case of poor quality nanostructure, a corrective laser treatment is performed at these locations.

Example 6

The apparatus and method of the present invention can be used for monitoring and control of the quality of laser burst ablation performed using pulse bursts with a pulse repetition rate in the GHz range, i.e. tens of MHz to THz, such as laser micromachining and surface texturing. The evaluation system 5 evaluates the difference of the signals 15, as shown in FIG. 3 and FIG. 11, as the difference between the actual value of the signal 16 and the average of the signal in the short time 17 before the time of fast rise of the signal 14. The difference of the signals 15 is evaluated in the time range ti to t2. The time from the time of fast rise of the signal 14 to the time ti is typically several tens of nanoseconds and contains several tens of laser pulses. The time between ti and t2 is usually from tens of nanoseconds to units of microseconds.

By means of experimental calibration, machine learning or by means of computer simulation of process variants, the range of corresponding signals 41 is determined, as can be seen from FIG. 11 for the values of the difference of the signals 15 between the times ti and t2, which for the given parameters of laser burst ablation, especially the material of the workpiece 10, pulse duration of laser 7, pulse energy of laser 7 for individual pulses in burst, repetition frequency of pulses and bursts, duration of bursts of pulses, the number of pulses in the burst, the size of the laser spot and the speed of movement of the laser beam 11 along the workpiece 10, correspond to a well-executed machining. This range is shown in FIG. 11 marked as "OK" and corresponds to a signal of good quality burst ablation 44.

If the difference of the signals 15 is at least at one point between times ti and t2 higher than the maximum allowable value, we get to the range of higher signals 42, which indicates poor quality ablation, where the material may be remelted, damaged by high temperature, oxidized or too removed. If the difference of the signals 15 is at least at one point between the times ti and t2 lower than the lowest allowable value, we get to the range of the lower signals 43, so that the inspection also results in unsatisfactory ablation, where machining may not be homogeneous and completely performed. Unsatisfactory ranges of difference of the signals 15 are shown in FIG. 11 marked as "NOK". These areas represent signals of poor quality burst ablation 45.

When controlling the process, the evaluation system 5 sends to the control system 6, as shown in FIG. 1, the minimum and maximum values of the difference of the signals 15 between times ti and t2. The control system 6 compares these values with the range of the corresponding signals 41 for the operation being performed. If the sent maximum value of the difference of the signals 15 is higher than the maximum allowable value, the control system 6 reduces the average laser power or pulse energy, or otherwise reduces the thermal load of the material. If the sent minimum value of the difference of the signals 15 is lower than the minimum allowable value, the control system 6 increases the average laser power or energy in the pulse, or otherwise increases the thermal load of the material. Each change made is recorded by the control system 6 together with the current position of the laser beam 11 on the workpiece 10 for inspection and possible finishing. If both sent values, i.e. minimum and maximum, are outside the range of corresponding signals 41, this means that the burst ablation process is too unstable. The control system 6 does not make any change, it only records this information together with the current position of the laser beam 11 on the workpiece 10. If there are too many unstable places, there may be a material defect or process parameters need to be changed: pulse energy, pulse distribution in during the burst, the repetition frequency of the pulses in the burst, the number of pulses in the burst, the speed of movement of the laser beam 11 along the workpiece 10.

The inspection of the shape of the surface at the recorded locations is performed by a standard method, such as a 3D laser confocal microscope after one or more layers, or at the end of the machining, and in case of insufficient machining, corrective laser machining is performed at these locations.

Example 7

The apparatus and method of the present invention can be used for monitoring and control of the quality of laser micro-processing in the next layer by collecting data from the previous layer or layers. This is particularly advantageous for micromachining and texturing, where the process consists of several to many layers to achieve the desired depth or shape.

The evaluation system 5 converts the measured time course of the signal into one or several characteristic numbers and sends these numbers to the control system 6. Part of the signal processing may be the conversion of the signal to temperature. The control system 6 records the characteristic numbers together with the actual position of the laser beam 11 on the workpiece 10. It thus forms in memory a microscopic thermal image 46 of the micro-processing in the form of a 2D or 3D surface from one layer of micro-processing, as shown in FIG. 12, where U is the characteristic number of the signal. After finishing the layer, or already in progress in parts, the control system 6 evaluates the differences in individual places and prepares the parameters of the laser process for the next layer. These are represented by an image of the modified parameters 47, as shown in FIG. 13, where P is the average laser power, which will mean lower energy in the pulse at places where the thermal signal was higher. The resulting parameters at a certain point can be calculated on the basis of the values of the characteristic numbers at this point only, or even on the basis of the values at the surrounding points, even from the previous several layers of micro- processing. By surrounding points are meant both points in one line to which the laser beam on the workpiece is moved, i.e. the places of previous and subsequent laser pulses, and also points in adjacent lines, i.e. places of laser pulses incident in the vicinity on adjacent lines.

Characteristic numbers for laser micromachining and texturing can be, for example, the heat accumulation signal 13, the maximum signal 25, the time of the threshold signal of solidification 33 and the minimum and maximum values of the difference of the signals 15 between times ti and t2 in the burst ablation. The correct setting of parameter adjustments is determined by experimental calibration, machine learning or by computer simulation of micro-processing variants.

Industrial applicability

The invention can be used in all methods of monitoring and control of pulsed laser micro- processing and for the construction of equipment for carrying out this method. It is a very promising technology enabling fine and precise machining of the material, creation of surface microstructure or nanostructure or micro structure and nanostructure at the same time or, conversely, polishing of the surface.

The invention can be used in production and research laser machines and devices for laser micromachining, e.g. small parts, local removal of materials into precise shapes, drilling micro- holes, further for laser texturing of surfaces to create functional surfaces containing microstructures and nanostructures, e.g. hydrophobic, light absorption, increased adhesion, antibacterial, antiviral, tribological reducing or increasing friction, and also for laser micro- polishing, for example casting molds, 3D printed parts, optical lenses and mirrors, molding tools, medical instruments. List of reference numerals

1 - detection optical system

2 - filter

3 - detection system

4 - recording system

5 - evaluation system

6 - control system

7 - laser

8 - laser optical system

9 - objective

10 - workpiece 11 - laser beam

12 - thermal radiation

13 - heat accumulation signal

14 - time of fast rise of the signal

15 - difference of the signals

16 - actual value of the signal

17 - average of the signal in the short time

18 - magnitude of the noise

19 - melting sign

20 - solidification sign

21 - heating curve

22 - melting time

23 - solidification plateau length

24 - melting signal

25 - maximum signal

26 - solidification signal

27 - time of the maximum signal

28 - time of the threshold signal of melting

29 - minimum of derivative of the signal

30 - range of corresponding melting times

31 - range of higher melting times

32 - range of lower melting times

33 - time of the threshold signal of solidification

34 - range of corresponding threshold times 35 - range of higher threshold times

36 - range of lower threshold times

37 - time of good quality remelting

38 - time of poor quality remelting 39 - time of good quality nanostructure

40 - time of poor quality nanostructure

41 - range of corresponding signals

42 - range of higher signals

43 - range of the lower signals 44 - signal of good quality burst ablation

45 - signal of poor quality burst ablation

46 - microscopic thermal image

47 - image of the modified parameters

48 - maximum of derivative of the signal 49 - range of corresponding solidification times

50 - range of higher solidification times

51 - range of lower solidification times

Claims

1. A method for monitoring and control of pulsed laser micro-processing, wherein the laser pulses repeatedly impinge on the workpiece (10), which is heated by them and emits the thermal radiation (12), which is detected by the detection system (3), which creates the time course of the signal, characterized in that in the time course of the signal the changes are found corresponding to changes in the state, structure or properties of the material of the workpiece (10), caused by at least one laser pulse, and by using these changes at least one characteristic number is determined, which has a dimension of time or signal, and the value of the characteristic number is compared with a range of values predetermined for a correctly performed operation, and in case of an incorrect value of the characteristic number the parameters of the pulsed laser micro-processing are corrected, this method of monitoring and control of the pulsed laser micro-processing being repeated within the laser micro-processing.

2. The method for monitoring and control of pulsed laser micro-processing, according to claim 1, characterized in that signal derivation is used to find changes in the time course of the signal and comprises finding a local minimum, a local maximum or a zero value of the derivative of the time course of the signal.

3. The method for monitoring and control of pulsed laser micro-processing, according to claim 1, characterized in that the characteristic number is the heat accumulation signal (13), which is determined as the average of the signal in the short time (17), for example in time of 100 ns, before the time of fast rise of the signal (14).

4. The method for monitoring and control of pulsed laser micro-processing, according to claim 1, characterized in that the characteristic number is the time of the threshold signal of melting (28), which is determined as the difference between the time of exceeding the threshold value, which is higher than the melting signal (24), and the time of fast rise of the signal (14), wherein for determining the time of the threshold signal of melting (28) for burst laser processing, with a burst pulse repetition frequency ranging from 20 MHz to 10 THz, the signal is smoothed to remove individual pulses in the burst.

5. The method for monitoring and control of pulsed laser micro-processing, according to claim 1, characterized in that the characteristic number is the time of the threshold signal of solidification (33), which is determined as the difference between the fall time below the threshold value, which is lower than the solidification signal (26), and the time of the maximum signal (27), wherein for the case of determination of the time of the threshold signal of solidification (33) for burst laser processing, with a repetition frequency of pulses in the burst ranging from 20 MHz to 10 THz, the time course of the signal at the end of the burst after the last pulse is used.

6. The method for monitoring and control of pulsed laser micro-processing, according to claim 1, characterized in that the characteristic numbers are the maximum and minimum values of the difference of the signals (15) in the second part of the laser pulse burst, with a pulse repetition frequency in the burst ranging from 20 MHz to 10 THz, the difference of the signals (15) being determined as the difference between the actual value of the signal (16) and the average of the signal in the short time (17) before the time of fast rise of the signal (14) at the beginning of the burst.

7. The method for monitoring and control of pulsed laser micro-processing, according to claim 1, characterized in that the monitoring and control is repeated after each laser pulse or after each burst of laser pulses with a pulse repetition frequency in the burst in the range from 20 MHz to 10 THz.

8. The method for monitoring and control of pulsed laser micro-processing, according to claim 1, characterized in that the monitoring and control is performed only in the next processed layer using the measured data in this or even previous layers and from the measured data at this point or even adjacent points.

9. The method for monitoring and control of pulsed laser micro-processing, according to claim 1, characterized in that the correction of pulsed laser micro-processing parameters is performed by changing the pulse energy, or by changing the pulse frequency, or by changing the pulse duration or pulse burst duration, or by changing the number of pulses in the burst, or by changing the laser spot size, or by changing the polarization of the laser light, or by changing the speed of movement of the workpiece or by changing the speed of movement of the laser beam.

10. The method for monitoring and control of pulsed laser micro-processing, according to claim 1, characterized in that before or after the changes are found, the time course of the signal is converted to the time course of the temperature.

11. The method for monitoring and control of pulsed laser micro-processing, according to claim 1, characterized in that the thermal radiation (12) is collected by the detection system (3) by means of a detection optical system (1) from an area smaller than the laser beam (11) or assembly of laser beams and at the same time the relative position of the center of the measured area of the thermal radiation (12) and the center of the laser beam (11) is changed repeatedly during laser processing to measure the response in different parts of the large laser beam (11) or the assembly of multiple laser beams and the control system (6) corrects the parameters of the pulsed laser micro-processing separately for individual parts of the laser beam (11) or individual parts of the assembly of multiple laser beams.

12. The method for monitoring and control of pulsed laser micro-processing, according to claim 1, characterized in that the range of values for a properly performed operation is determined by experimental calibration or computer simulation or machine learning from previous laser treatments of the material.

13. Apparatus for carrying out the method for monitoring and control of pulsed laser micro- processing, comprising a detection optical system (1), a detection system (3), a recording system (4), an evaluation system (5), a control system (6), a laser (7) and a laser optical system (8), characterized in that the recording system (4) comprises an analog-to-digital converter, the evaluation system (5) comprises a programmable gate array and the detection optical system (1) comprises a scanning head.