Method for finding the optimal focus position for laser ablation and cutting with minimal cutting width and good edge quality

Abstract

Method for controlling the distance (14) between the workpiece (9) and the processing optics (8) during cutting, drilling, and ablation of various materials with minimal side effects on non-irradiated areas, wherein the workpiece (9) is irradiated by a pulsed laser (7) with an intensity that generates plasma emission, wherein the strength of the plasma emission is detected, stored, and evaluated in such a way that the distance (14) of the processing optics to the workpiece surface is varied (15) until a local minimum (3) is reached near the maximum intensity of the plasma emission (target position) and sharp cuts and ablation channels are produced, characterized in that the detection of the strength of the plasma emission is carried out by means of a detection system (10) consisting of filters and photodetectors, wherein a spectrometer can also be used instead of a filter.

Description

Task

[0001] The invention relates to a method for controlling material removal and precision cutting of thin-film systems with good edge quality using a laser beam directed at a workpiece. The distance between the focusing optics and the workpiece is selectively varied during laser processing, and the plasma emission is simultaneously measured. The optimal distance for processing is determined by evaluating the plasma emission. State of the art

[0002] From DE 102 48 458 B4 and other patents, a method and a device suitable for carrying out this method are known. The focus position is determined by detecting the radiation emanating from the interaction zone between the laser beam and the workpiece. The method provides that the signal corresponding to the radiation is recorded as a function of the displacement position of the focusing optics. The focus position is thus determined from the change in radiation intensity as a function of the distance between the lens and the workpiece. The inventors base this on the fact that the maximum radiation intensity, or the setting accuracy of the focus position, increases where the highest signal value can be reliably determined from the entire signal profile. The reflected plasma emission is detected by a series of photodetectors in a processing head; that is, no special detection optics are required.The problem with the solution described in DE 102 48 458 B4 is that this approach of determining the highest signal value in terms of radiation intensity does not always identify the optimal focus position for good processing. Instead of a maximum radiation intensity, a plateau region often occurs, as the inventors themselves describe in DE 102 48 458 B4, which makes it very difficult for the user to reliably determine the focus position. Furthermore, the maximum radiation intensity correlates more with the amount of material removed from the workpiece by laser radiation, but this does not mean that this maximum removal rate represents an optimal focus position for precise laser processing.

[0003] DE 102 44 548 B4 discloses a method for determining the position of the focus of a pulsed laser beam used for laser material processing relative to the workpiece surface. During the processing process, a process characteristic is measured, and the focus position is determined by comparing the measured values ​​with function values ​​of this process characteristic, which were previously measured as a function of the focus position. The process characteristic measured is the power of an acoustic emission, wherein the measurement of the acoustic emission power is performed at a frequency that corresponds to one or more times the repetition rate of the laser.

[0004] US Patent 6,355,908 B1 discloses a method for adjusting the focus position of a laser beam based on the intensity of plasma emission. The described method considers only ultrashort laser pulses (ps and fs). A plasma is generated in the air above the workpiece with an energy lower than the material's ablation threshold. No material is removed. The plasma emission is detected by photodetectors mounted laterally at a specific angle. The optimal focus position is found when the plasma emission intensity is at its maximum. Inventive solution

[0005] The solution according to the invention relates to a method for controlling the distance between the workpiece and the processing optics when cutting, drilling and removing various materials, according to claim 1, and to a device for controlling the production of sharp insulation grooves on CIS, CIGS and other thin-film solar cells, according to claim 7.

[0006] Precise material removal and cutting with a laser requires not only a one-time setting, but also control of the focus position to reliably provide the ideal working distance between the laser processing optics and the workpiece to be processed for the most precise laser micromachining possible.

[0007] The focus position, or the lens-to-surface distance, is a parameter that strongly influences the quality of laser processing results. The optimal distance between the focusing optics and the workpiece surface, which produces the highest possible aspect ratio (depth to width of the laser processing track), maximally narrow and deep channels, and minimal thermal edge effects, is defined as optimal.

[0008] The invention described in this patent relates to finding the optimal focus position by analyzing the plasma emission generated during processing. Plasma emission is defined here as the radiation emitted by the laser-induced plasma. This radiation is optically detected and converted into an electrical signal by a detector or an array of detectors, e.g., photodiodes or photomultipliers, and one or no spectrometer or one, no, or several filters. The features of the detected plasma emission, e.g., individual emission lines, emission bands, and thermal radiation, are analyzed. The limitation of the wavelength range to be analyzed is based on the occurrence of these features. For example, in the case of analyzing CIS or CIGS solar cells, the spectral range of 300–500 nm is used, since the characteristic features of all the layers involved can be observed in this range.However, this is not the only area that can be used.

[0009] Experience shows that simply controlling the focus based on maximum plasma emission is not always suitable. Since emission intensity and the amount of material removed are proportional, these methods seek the focus position that generates the maximum amount of material heated by the plasma, i.e., the highest volume of material removed. Ideal machining quality is not always achieved at the machining parameters that produce maximum material removal. It has been observed that under these conditions, thermal effects significantly alter the machining results, leading to lower quality. This manifests itself in cracking, remelting, burr formation, or debris redistribution at the machining edges.

[0010] Surprisingly, it has been shown that the optimal focus position (lens-to-surface distance) of the laser beam directed at a workpiece can be found at a local minimum of plasma emission (target position). It has been determined that the focus position generating a local minimum of intensity produces the highest possible aspect ratio (depth to width of the laser processing track). Various known sensor arrangements and sensors, e.g., photodiodes and spectrometers, can be used to detect the plasma emission. Subsequently, the emission intensity is evaluated. In the present invention, the presence of this local minimum is crucial for precise laser processing and means that, at the optimal focus position, the laser processing does not exhibit the highest value of material removed, but rather the narrowest and deepest ablation tracks, i.e., with a high aspect ratio.Additionally, optimal focus position means being able to achieve a controlled ablation depth with this focus position. This type of trace is desirable to achieve high-quality processing results for various laser micromachining applications, such as the structuring of CIS, CIGS, or organic thin-film solar cells or other layers on glass substrates, metal foils, or flexible polymer films, as well as other multilayer systems; the production of MEMS and microfluidic components; silicon wafer cutting; the cutting of precious materials; the processing of flexible displays; etc.

[0011] These applications require narrow and deep channels with precise material processing to enhance the performance of the final product. Therefore, an automated system is needed to ensure adherence to the ideal narrow removal channels with a high aspect ratio.

[0012] The plasma emission detected during laser processing can also be used to guarantee selective ablation. This is possible because the characteristic emission lines of each material allow for differentiation. By monitoring suitable spectral bands or specific wavelengths, the ablated material can be identified. Changes in emission intensity along characteristic emission lines during laser processing indicate that different materials are being ablated.

[0013] By ensuring that only the desired material is removed, this process can significantly improve the quality of automated machining. This is particularly important for machining multi-layer systems. In this application, removing the selected top layer(s) without damaging the rest of the structure is crucial for the system's performance.

[0014] The method according to the invention enables the combination of detection of optimal focus conditions with the control of selective ablation. This method is ideal for various laser processing applications of multilayer systems such as solar cells, low-k dielectrics, flat panel displays, in the controlled removal of contaminant layers, etc.

[0015] The device for implementing the method according to the invention consists, analogously to the prior art, such as DE 102 48 458 B4, of the following parts: - a laser source, - a focusing optic, - a detection system consisting of a detector, filters and, if necessary, a detection optic. - an evaluation unit for analyzing, storing and comparing plasma emissions, - a unit for tracking the focus position.

[0016] Photodiodes or spectrometers can be used as detectors in the detection system. Filters, e.g., notch filters, are needed to suppress the reflected laser light or to select a desired spectral band.

[0017] The inventive method for finding the optimal focus position consists of the following steps: - Initially, the focusing optics are placed away from the sample (the focal length of the focusing optics is significantly above the sample surface). - A first lens-to-surface distance is used for laser processing, and at the same time the emission intensity is recorded and evaluated. - The intensity of a characteristic emission line or spectral band is stored and compared with previous intensity values ​​to find the local intensity minimum. - If the evaluated intensity is not the local minimum, the lens-to-surface distance is changed (in the first pass, this means moving the focusing optics towards the sample). - When the local minimum is found, the change in the lens-to-surface distance is stopped, as the optimal focus position (target position) of the system has been found. This process, or parts thereof, are repeated to ensure continuous tracking or to stop the processing. Naturally, the lens-to-surface distance can be varied alternately towards and away from the workpiece surface as the intensity increases, in order to prevent collisions. This distance can also be continuously adjusted or maintained within a predefined tolerance range.

[0018] A significant advantage over competing methods is that only one track is needed to find the optimal lens-to-surface distance, meaning that short time and little material are removed.

[0019] The variation of the lens-to-surface distance can be performed with a predetermined step size to improve the accuracy of the positioning.

[0020] The following steps must be completed to monitor selective removal: Laser processing and plasma scanning are performed simultaneously. - The plasma emission is analyzed to observe characteristic features of the emission spectra, i.e. emission lines and background radiation. The presence of features of the layer to be removed and the absence of features of the layer to be preserved determine the correct working area before the transition between the layers. The transition has occurred when a feature of a layer to be preserved appears. - When the transition is reached, the process is stopped or other suitable measures are taken to avoid damage to the layer being preserved.

[0021] The method is very advantageous for micro-machining, e.g. cutting and marking, and can be used with all wavelengths, i.e. with IR, VIS and UV laser wavelengths as well as with ns to fs pulse durations.

[0022] This method can be used to achieve ideal results with laser processing in automated processes. As with competing methods, the correct execution of this process depends on using significantly higher laser energy than the ablation threshold of the material being processed.

[0023] The process can be used for a wide variety of materials, e.g., metals, ceramics, technical glass, semiconductors, and thin-film solar cells. This list is not exhaustive.

[0024] The method according to the invention can be implemented in a focusing head or in an independent monitoring system. Depending on the system specifications, the plasma emission can be detected either opposite to the propagation direction of the laser processing radiation or only above a certain angle of inclination.

[0025] The same system can be used to detect contamination on a workpiece during processing and remove it immediately, or to stop the processing for inspection.

[0026] The invention is described with the help of the following figures: Fig. Figure 1 shows the basic structure of the device. Fig. Figure 2 is a graphical representation of the operating principle of the procedure. It is also shown in Fig. 2 the plasma emission intensity is represented as a control signal. Fig. Figure 3 is a flowchart of the procedure to find the optimal focus position. Fig. Figure 4 is a diagram of the processing results of multi-layer systems. Fig. Figure 4a shows the desired cutting depth. Fig. 4b shows over-editing and Fig. 4c Sub-processing. Fig. Figure 5 is a flowchart of the procedure to limit the processing to specific layer thicknesses or materials.

[0027] In Fig. Figure 1 shows a basic setup of the processing system. The laser beam (7) is focused onto the workpiece (9) by the focusing optics (8), with or without beam shaping. The process can be implemented in various configurations of the laser processing setup. The plasma emission generated by the interaction of the laser beam with the sample is collected by means of a detection system (10) consisting of filters and photodetectors. Instead of a filter, a spectrometer or another device can also be used. A portion of the plasma emission is transmitted through the focusing optics and directed to the detection system by means of a partially reflective optical element (11), e.g., a dichroic mirror or a simple glass plate. The plasma emission is converted into an electrical signal in the detection system (10) and stored and evaluated in an evaluation unit (12).The evaluation unit is responsible for analyzing the emission intensity to identify the local minimum. The signal sent from the evaluation unit (12) to the control unit (13) is used to adjust the position of the focusing optics.

[0028] In Fig. Figure 2 shows the intensity of the plasma emission generated during laser structuring. The method is based on shifting the focused beam along the Z-axis during laser processing. A track is ablated starting with an initial, significantly excessive lens-to-surface distance (1). During processing, the plasma emission is recorded, and its intensity is analyzed and evaluated. The emission intensity is used as a control signal. Since the initial intensity value is not a local minimum, the lens-to-surface distance is varied to initially reduce the relative distance between the focusing optics and the sample. As a result, the plasma emission intensity is then increased (2). Maximum intensity is observed when the beam is focused just above the surface (3).Further variation of the lens-to-surface distance towards the sample produces a decrease in plasma intensity (4). A local minimum in emission intensity is reached for the optimal focus position, i.e., the narrowest and deepest ablation crater (5). If the lens-to-surface distance is shifted further in the same direction, the intensity increases again (6). The lens-to-surface distance can be varied in small increments to improve the precise adjustment of the optimal focus position.

[0029] In Fig. Section 3 illustrates the process using a flowchart. First, the workpiece must be placed on the processing table. Then, the plasma emission is recorded during processing. Independent intensity values ​​are stored for each step of the lens-to-surface distance variation. The plasma intensity is analyzed to identify the local minimum. If the focus is not in the optimal position, the lens-to-surface distance is shifted in controlled steps during processing towards the presumed optimal distance—that is, initially towards the surface, and subsequently towards the distance previously recorded with minimum emission. Other strategies (coarse scanning followed by fine scanning, continuous small variation during processing) are also possible. Once the local minimum is reached, the shifting process is temporarily stopped.

[0030] In Fig. Figure 4a shows the desired machining depth for the selective removal of multi-layer systems. The top layer was completely removed without damaging the underlying layer.

[0031] In Fig. Figure 4b shows the result of over-processing a sample. The lower layer is damaged and selective material removal is not achieved.

[0032] In Fig. Figure 4c shows the result of undercutting a sample. The desired removal depth is not achieved, and therefore the expected result is not obtained. The lower layer cannot be accessed from the working side because one of the upper layers has left a protective layer on it. If isolation between areas is to be created, this is prevented by the residual thickness of the layer to be removed.

[0033] In Fig. Section 5 illustrates the selective ablation process using a flowchart. First, the workpiece must be placed on the processing table. Then, the plasma emission is recorded during processing. The spectrally resolved emission intensity of the plasma is analyzed. The decrease in plasma emission characteristics from the layer being ablated and the increase in plasma emission characteristics from the layer to be created lead to the identification of a transition between the materials. The processing process is repeated until the desired transition is observed. Finally, the laser processing is stopped to prevent damage to the deeper layers of the multilayer system. A similar procedure can be used to enable the controlled removal of contaminants from surfaces to be processed. In this case, the traverse speed is additionally controlled.The travel path of the focusing optics on the workpiece surface is controlled by the occurrence of undesired features in the plasma emission.

[0034] The in Fig. 3 and in Fig. The 5 methods shown can be used together or separately.

[0035] The presented technical solutions for the process steps serve only for explanation and are interchangeable with equivalent solutions according to the prior art without affecting the process according to the invention. Reference symbol list 1 Initial state with first lens-to-surface distance 2 lens-to-surface distance, which produces an increase in emission intensity 3. Lens-to-surface distance that produces a maximum intensity value 4. Lens-to-surface distance, which produces a reduction in emission intensity 5. Optimal focus position, i.e., the lens-to-surface distance that produces the local intensity minimum. 6. Lens-to-surface distance, which produces a further increase in emission intensity 7 Laser beam 8 Focusing optics, possibly with beam shaper e.g. for a tophat profile 9 workpiece 10 Data collection system 11 partially reflective optical element 12 evaluation units 13 Control unit 14 Lens-to-surface distance 15. Change in lens-to-surface distance

Claims

[1] Method for controlling the distance (14) between the workpiece (9) and the processing optics (8) during cutting, drilling and ablation of various materials with minimal side effects on non-irradiated areas, wherein the workpiece (9) is irradiated by a pulsed laser (7) with an intensity that generates plasma emission, wherein the strength of the plasma emission is detected, stored and evaluated in such a way that the distance (14) of the processing optics to the workpiece surface is varied (15) until a local minimum (3) is reached near the maximum intensity of the plasma emission (target position) and sharp cuts and ablation channels are produced, characterized by , that the detection of the strength of the plasma emission is carried out by means of a detection system (10) consisting of filters and photodetectors, whereby a spectrometer can also be used instead of a filter. [2] Method according to claim 1, characterized by, that additionally an evaluation of specific spectral lines of the plasma emission is used to monitor the processed material and, in case of change, to switch off the laser (7) or to initiate other suitable steps. [3] Method according to claims 1 and 2, characterized by , that the occurrence of specific spectral lines is used to remove layers in solar cells and other layer systems in a controlled manner. [4] Method according to claims 1 and 2, characterized by , that the appearance of specific spectral lines is used to remove impurities and unwanted layers from surfaces in a controlled manner, without damaging the surface layer to be protected. [5] Method according to claim 1, characterized by , that a permanent readjustment to maintain the local minimum is carried out by small variation of the distance around the target position and evaluation of the intensity change with corresponding adjustment of the target position. [6] Method according to claim 1, characterized by , that the laser (7) emits pulses with a time length in the range of ns to fs. [7] Device for controlling the production of sharp isolation trenches on CIS, CIGS and other thin-film solar cells, - wherein the device comprises a spectrally resolving detection system (10), an evaluation unit (12), a control unit (13) and a focusing optic (8), and - wherein a plasma emission generated by laser pulses is measured with the spectrally resolving detection system (10) at one or more element-specific wavelengths and the distance between the workpiece and a processing optic (14) is adjusted according to the method of claim 1.

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