Method for processing a workpiece using laser radiation in the form of Lissajous figures

Abstract

A method for processing a workpiece using laser radiation directed onto the workpiece along a processing line in the form of a variable or static Lissajous figure by means of a glass-based MEMS scanner, wherein the MEMS scanner has at least one mirror element deflectable about at least one axis against a restoring force of a spring element, a laser radiation power of more than 20 W, a pulse duration between 100 fs and 200 ns, pulse repetition rates of more than 200 kHz, a mirror aperture between 6 mm and 10 mm, scan frequencies between 5 kHz and 20 kHz, an angle of incidence, which is an angle to the surface normal of the mirror element, of less than 5°, and a scan angle of the MEMS scanner of less than 2°, wherein the scan angle is an angle between a direction of emission of the laser radiation after reflection at the non-deflected mirror element and the direction of emission of the laser radiation when the mirror element is deflected.The mirror element has a substrate made of glass that is highly transmissive for the wavelength of the laser radiation used, which is coated with a highly reflective coating for the laser radiation. Any residual radiation that occurs is transmitted as a non-reflected portion of the laser radiation and penetrates the transmissive substrate material largely unhindered, thus reducing the heating of the mirror element and the other elements of the MEMS scanner.

Description

[0001] The invention relates to a method for processing a workpiece using laser radiation, which is directed onto the workpiece along a processing line in the form of a Lissajous figure, which is variable or static, particularly during processing.

[0002] In principle, the energy of laser radiation is suitable for a wide variety of processing methods, such as laser cutting of metallic and non-metallic materials of varying thicknesses. This is based on a laser beam that is guided, shaped, and focused. When this beam strikes the workpiece, the material heats up so intensely that it melts or vaporizes.

[0003] The laser offers several key advantages. Firstly, the contactless process does not induce any mechanical stress in the material. Secondly, the process inherently prevents the accumulation of dust, which could cause malfunctions.

[0004] Laser radiation can be used to process a wide variety of materials, including FR4 composites. The thermal energy input during the process is controlled to prevent burning or charring of the material.

[0005] WO 2020 / 251 782 A1 concerns a method for producing through-holes in workpieces, such as printed circuit boards, using laser radiation, as well as the splitting of laser pulses or the modulation of the power of laser pulses from a laser source. An acousto-optic deflector (AOD) serves to deflect the beam path and modify the radiation as it passes through the AOD.

[0006] US Patent 10,507,544 B2 relates to a laser processing system comprising a first positioning system with one or more AODs for generating a first relative movement of a beam axis along a beam trajectory with respect to a workpiece, a second positioning system for generating the second relative movement, and a laser source for emitting laser beam pulses directed at individually selected points. The laser beam pulses can also be directed at spatially identical, overlapping, or non-overlapping areas on the workpiece. According to one embodiment, the positioning systems can be controlled such that the shape or size of the laser beam points can be changed and multiple pulses emitted onto the workpiece form a pattern. The AOD can also be controlled, for example, to attenuate the incident laser beam.

[0007] US Patent 2010 / 0140237A1 describes a method for preventing inaccuracies and quality degradation in laser processing resulting from dynamic and thermal stresses on laser beam positioning and optical components. This method utilizes, for example, an acousto-optic deflector (AOD), an electro-optic deflector (EOD), and a galvanometer scanner to position the laser beam.

[0008] In practice, the losses in acousto-optic or electro-optic deflectors, which can reach up to 50% with ultrashort pulses, have a detrimental effect. In particular, the image quality deteriorates in transmissive deflectors due to refraction, so a reflective element with a highly reflective coating proves superior in this respect.

[0009] Laser scanners with highly reflective elements are already used as micro-electro-mechanical systems (MEMS), for example in the field of measurement technology, in fingerprint sensors, barcode scanners or LIDAR scanners.

[0010] The mirrors of the MEMS scanners known from the prior art, designed as microactuators, include, for example, a vibrating body with a mirror that is suspended in a chip frame by means of spring elements.

[0011] In a typical design, the scanner, also known as a "gimbal mounted scanner", has a mirror element that is movably arranged in a second, also movably suspended frame.

[0012] EP 1 419 411 B1 describes a projection device with a deflection mechanism for deflecting a light beam about a first and a second deflection axis in order to move the light beam across the field of view. The light beam is deflected about the first deflection axis at a first deflection frequency and about the second deflection axis at a second deflection frequency. A gimbal-mounted mirror is used as the deflection mechanism.

[0013] Single-axis and multi-axis suspensions are possible, which make it possible to move the movable element linearly (quasi-static or resonantly), rasterically (one axis quasi-static, one axis resonant), Lissajous-shaped (both axes resonant) or completely vectorially (both axes quasi-statically).

[0014] An advantageous operating mode is based on resonant operation of the MEMS scanner, as this allows for favorable amplification of the mirror oscillation amplitude while simultaneously utilizing low energy consumption. This applies to both single-axis and multi-axis MEMS scanners.

[0015] For example, DE 10 2011 104 556 A1 discloses a deflection device for a scanner with Lissajous scanning with a micromirror oscillating in two deflection axes and a control for the resonant operation of the micromirror.

[0016] The coating of the mirrors in the MEMS scanners used in practice does not achieve 100% reflectivity, so some residual radiation is absorbed, leading to unwanted heating of the mirror.

[0017] The thermal energy input induced by the laser can shift the resonance frequency of the MEMS mirror, resulting in phase and amplitude changes, whereby the heat input can concentrate unfavorably on the thin torsion springs.

[0018] The problem is exacerbated when a fast and a slow axis are combined, because the slow axis is usually implemented using a very thin and therefore soft spring suspension, which hinders heat dissipation.

[0019] From DE 10 2012 005 546 A1 a MEMS scanner is known in which a microchannel is provided for passing a cooling medium through a chip frame and the spring elements in order to improve heat dissipation from the mirror plate and to ensure a high tolerance to heat input.

[0020] In order to dissipate the heating of the surface of the optical element associated with the incident light, it is known in a mirror arrangement according to DE 10 2007 034 652 A1 to provide a medium for heat dissipation in a chamber and to cool the chamber walls by means of cooling channels.

[0021] However, such a cooling arrangement on the elements of the MEMS scanner is complex to manufacture and also leads to larger masses and dimensions.

[0022] Senger et al., “A 2D circular-scanning piezoelectric MEMS mirror for laser material processing”, Proc SPIE 11697 (2021), describe a two-dimensional piezoelectrically driven MEMS mirror with large mirror apertures and highly reflective coatings for wavelengths in the green and infrared spectral range, designed for circular scanning in ultrashort pulse material processing with high dynamics.

[0023] From DE 10 2016 111 531 A1 an optical scanner is known in which a first frame pivotable about a rotational axis with a mirror element is driven by a Lorentz force drive unit arranged in a frame recess and which is designed in such a way that a sensor has direct access to the back of the mirror element for precise position detection.

[0024] Sasaki et al., “Low cost and large deflection angle polymer MEMS mirror using glass substrate”, Optical MEMS and Nanophotonics 2010, present an electromagnetically driven polymer MEMS mirror on a low-cost glass substrate with polymer torsion beams and multilayer wiring, which can be manufactured without dry etching processes and achieves optical deflection angles of more than ±40° at low drive currents as well as high repeatability.

[0025] In Yamashita et al., “Integration of angular rate sensor on large deflection polymer-MEMS mirror”, Optical MEMS and Nanophotonics 2013, a large deflection polymer-MEMS mirror with SU-8 hinges is described, on whose movable mirror plate a coil sensor structure is monolithically integrated, which provides a motion signal for active amplitude control of the laser scan via electromagnetic induction.

[0026] Khan et al., “Fabrication method of micromachined quartz glass resonator using sacrificial supporting structures”, Sensors and Actuators A 305 (2020) 111922, finally reveal a manufacturing method for quartz glass resonators in which the resonator structure is bonded to a substrate via Au microposts and temporary sacrificial supports provide homogeneous bond strength, improved heat dissipation during plasma etching, and mechanical support for the fragile resonator, thus enabling a process-friendly release at moderate process temperatures.

[0027] The invention is based on the objective of achieving particularly efficient laser processing.

[0028] This problem is solved according to the invention by a method according to the features of claim 1. Further embodiment of the invention can be found in the dependent claims.

[0029] According to the invention, a method for processing a workpiece is provided in which, first, a beam shape of a Lissajous figure, which is either variable or static, is generated by means of at least one glass-based MEMS scanner through resonant vibration excitation of two mutually perpendicular axes of the supplied laser radiation. The beam shape of the laser radiation thus generated is then moved over the workpiece by means of at least one further scanner, in particular designed as a galvanometer scanner, in order to carry out the desired processing, for example, an ablation on the workpiece surface.

[0030] The Lissajous figures are generated by two independent, resonant axes of at least one MEMS scanner, and the shape of the Lissajous figures is determined by the distance or deviation of the vibration excitation of the resonant axes of the MEMS scanner.

[0031] For laser processing, the mirror of the secondary scanner is tilted by the associated actuators in such a way that the processing laser beam, onto which the Lissajous figure has been previously imprinted, is directed onto the surface of the workpiece. In this way, the beam parameters of the processing laser beam and the intensity distribution of the laser beam spot on the workpiece can be optimally designed or adapted for the respective laser processing method, and the focal point of the processing laser beam can be dynamically moved at frequencies between 5 kHz and 20 kHz. Simultaneously, the laser beam spot is shaped as a processing area bounded by the perimeter of the Lissajous figure.

[0032] The scan frequency refers to the frequency at which the scanner generates the Lissajous figures. Preferably, a frequency close to the resonant frequency of the respective oscillating system is used. For example, in a MEMS scanner with two tilt axes, a frequency is selected that lies between the resonant frequencies of the axes. An advantageous frequency range is 5 to 20 kHz.

[0033] The angle of incidence refers to the angle relative to the surface normal of the mirror element. A small angle of incidence means that the radiation strikes the mirror element almost perpendicularly. A small angle of incidence is advantageous because it results in a round laser beam becoming only slightly elliptical.

[0034] The scan angle is the angle between the direction of emission of the laser radiation after reflection from the mirror element of the scanner mirror when the mirror element is not deflected and the direction of emission of the laser radiation when the mirror element is deflected. It is preferably less than 2°, as larger angles can lead to an elliptical distortion of the laser beam.

[0035] At least one additional scanner preferably has a galvanometer scanner with two mirrors as a deflection device, each of which can be dynamically moved individually by means of controllable galvanometers. This allows for virtually any intensity distribution of the beam spot and beam parameter products of the processing laser beam to be provided by means of dynamic beam shaping; in particular, Lissajous figures can be written on the workpiece with the laser beam.

[0036] Surprisingly, the glass-based MEMS scanner, thanks to its highly reflective (HR) coatings, is significantly superior to acousto-optic deflectors as transmitting deflection elements. Furthermore, it has also been shown that glass is considerably more suitable than silicon. In particular, impairments to image quality can be avoided because the highly reflective coatings are better adapted to glass than to silicon, and glass, as the transmitting base material, does not heat up.

[0037] Furthermore, it has been shown that, contrary to the prejudice in the professional world, the mechanical properties of glass function mechanically better than those of silicon, despite its supposedly "brittle" properties.

[0038] The recurring or regular beam deflection generated by the MEMS scanner along closed, circumferential lines in the form of the Lissajous figure, which is deflected onto the workpiece by the second scanner, results in high ablation rates with at the same time not excessive local thermal energy input due to the regular beam path.

[0039] Furthermore, by adjusting the resonant frequencies of the two axes of the MEMS scanner relative to each other, the shape of the Lissajous figure can be changed during operation and thus adapted to the machining conditions of the workpiece.

[0040] In addition to a circular shape, virtually any Lissajous figure can be generated. Considering a settling time of only a few periods, the shape of the Lissajous figure and the orientation of its principal axes can be adjusted as needed during operation to meet the specific machining requirements. This allows the effective machining area, defined as the circumferential contour of the Lissajous figure, to be continuously adapted to the machining progress, specifically to the type, size, and orientation of the contour being machined.

[0041] A scanner not according to the invention can comprise a mirror element comprising a substrate with a material that is highly transmissive for the wavelength of the radiation used, in particular glass, or consisting exclusively of such a material which is provided with a coating that is highly reflective for the radiation. In a surprisingly simple manner, the use of a transmissive substrate material thus avoids unwanted heating. In particular, glass or comparable materials can be used as the transmissive substrate material, which is provided with a highly reflective coating and is used as a mirror in the scanner. This means that the residual radiation is transmitted as the unreflected portion of the radiation and thus penetrates the transmissive substrate material largely unimpeded. The heating of the mirror element and the other elements of the scanner is thereby effectively reduced.

[0042] The substrate can have a reflective coating only partially. Such a partially reflective coating is applied selectively and limited to those areas that are necessary or advantageous for the scanner's function and the desired deflection ranges. If the positioning, particularly the angular position of the mirror element, deviates from the range suitable for the application, the radiation, for example, laser radiation, does not strike the reflective coating but directly the substrate material or another coating that is non-reflective or only very slightly reflective, and penetrates the substrate unimpeded. This makes the radiation usable not only for other applications that require different beam deflection in these areas, but also enables, for the first time, the connection of multiple scanners in series or parallel.Furthermore, a significant improvement in operational safety is achieved for the first time when using potentially hazardous radiation, as beam deflection outside predefined areas is prevented by ensuring that the radiation cannot strike the reflective coating outside the assigned adjustment ranges of the mirror elements. This fundamentally eliminates beam deflection outside a predetermined deflection range, even in the event of a control malfunction.

[0043] Preferably, the radiation striking the area excluded from the reflective coating is transmitted through the substrate and, further along the beam path, strikes, for example, another scanner. Accordingly, the beam path can also be controlled such that several scanners are arranged one after the other in the beam path, and the scanner illuminated by the radiation can be selected as needed.

[0044] A sensor and / or measuring system allows for a quick check of, for example, the radiation intensity of the supplied radiation, even during normal operation. Particularly advantageous is the ability to perform the measurement without interrupting the scanner's operation.

[0045] In such cases, where further use of the radiation is not intended, it has already proven particularly useful to assign a deflection element and / or a radiation trap to at least one area of ​​the surface, so that the energy is dissipated not within the scanner, but outside and without any effect on it.

[0046] Furthermore, a variant is also feasible in which the substrate has multiple coatings, allowing, for example, different types of radiation to be deflected differently. Naturally, not only can the coatings differ in the various areas, but also the properties of the substrate itself. In particular, this makes it possible to create scanners with different properties for radiation of different wavelengths.

[0047] If the reflective coating is applied to the substrate as a dielectric coating, very low absorption of the coating and, for example, a reflection of more than 95% at an angle of incidence of + / - 5° can be achieved.

[0048] The scanner may feature active and / or passive damping of its mechanical structures to avoid unwanted influences that could lead to errors.

[0049] In practice, a variant of the scanner with two independent, resonant axes could be considered.

[0050] Also not pertaining to the invention is a mirror element for a scanner, wherein the mirror element comprises a substrate with a material, in particular glass, that is highly transmissive at the wavelength of the radiation used and is provided with a coating that is highly reflective for the radiation. This prevents the non-reflected portion of the radiation from causing undesirable heating of the substrate, as in the prior art, and allows it to pass through the substrate largely unchanged. Furthermore, the transmitted radiation is thus available for other applications. Optionally, the entire surface of the substrate can be coated, or only certain areas, so that the radiation incident on the uncoated areas is transmitted accordingly and can be used for other purposes.

Claims

[1] Method for processing a workpiece using laser radiation directed along a processing line in the form of a variable or static Lissajous figure onto the workpiece by means of a glass-based MEMS scanner, wherein the MEMS scanner has at least one mirror element which can be deflected about at least one axis against a restoring force of a spring element, a power of the laser radiation more than 20 W, a pulse duration between 100 fs and 200 ns, pulse repetition rates more than 200 kHz, a mirror aperture between 6 mm and 10 mm, scan frequencies between 5 kHz and 20 kHz, an angle of incidence which is an angle to the surface normal of the mirror element, less than 5° and a scan angle of the MEMS scanner less than 2°, wherein the scan angle is an angle between a direction of emission of the laser radiation after reflection at the non-deflected mirror element and the direction of emission of the laser radiation when the mirror element is deflected,The mirror element has a substrate made of glass that is highly transmissive for the wavelength of the laser radiation used, which is coated with a highly reflective coating for the laser radiation. Any residual radiation that occurs is transmitted as a non-reflected portion of the laser radiation and penetrates the transmissive substrate material largely unhindered, thus reducing the heating of the mirror element and the other elements of the MEMS scanner. [2] Method according to claim 1, characterized by , that the Lissajous figure is generated by means of the glass-based MEMS scanner and is moved over the workpiece by means of at least one further scanner, wherein the Lissajous figure is generated by two independent, resonant axes of the MEMS scanner and the shape of the Lissajous figure is determined by the distance or the deviation of the excitation frequencies of the axes of the MEMS scanner. [3] Method according to claim 1 or 2, characterized by , that at least two Lissajous figures are generated by the MEMS scanner and at least one other scanner, as well as by a superimposed linear movement of the MEMS scanner and at least one other scanner. [4] Method according to any one of the preceding claims, characterized by that at least two of the movements of the MEMS scanner, the further scanner and / or the superimposed linear movement occur independently of each other. [5] Method according to any one of the preceding claims, characterized by that the movements of the MEMS scanner and the other scanner are not synchronized. [6] Method according to any one of the preceding claims, characterized by that the movements of the MEMS scanner and the other scanner take place in a common plane. [7] Method according to any one of the preceding claims, characterized by, that the superimposed motion is generated by one degree of freedom of the MEMS scanner in combination with two degrees of freedom of the other scanner. [8] Method according to any one of the preceding claims, characterized by that the deflection of the laser radiation is synchronized by means of at least one MEMS scanner and at least one other scanner. [9] Method according to any one of the preceding claims, characterized by that the laser radiation is deflected by a combination of at least one MEMS scanner and at least one galvo scanner.