Method for producing a microstructured component having a layer structure

Abstract

In a method for producing a microstructured component which has a substrate (140) and at least one microstructured functional layer system (130) applied to the substrate, a workpiece is provided which has the substrate, a functional layer system applied to the substrate, and a cover layer system (110) applied to the functional layer system. Laser radiation is radiated through the cover layer system from a side facing away from the substrate onto a buffer layer (112), adjoining the cover layer, of the functional layer system in such a way that the laser radiation weakens or destroys a connection between the buffer layer and the cover layer system within at least one locally delimited target area in a boundary region between the buffer layer and the cover layer system. As a result, material of the cover layer system (110) is removed in the target area in order to expose the buffer layer (112) in the target area. The boundary region is irradiated by means of direct laser interference patterning (DLIP) by virtue of a laser beam emitted by a primary laser radiation source being split into at least two partial beams (TS1, TS2) and the partial beams being guided such that at least two mutually coherent partial beams pass through the cover layer system and a spatial intensity pattern is produced in a superimposition region (IVOL) of the coherent laser beams, said spatial intensity pattern having radiation microzones (MZ) with constructive interference and relatively high laser radiation power density adjacent to regions of destructive interference and low laser radiation power density in relation to the radiation microzones, wherein damaged microzones arranged with lateral spacings are produced in the boundary region by the laser radiation in the radiation microzones.

Description

[0001] Method for producing a microstructured component with a layered structure

[0002] SCOPE OF APPLICATION AND STATE OF THE ART

[0003] The invention relates to a method for producing a microstructured component comprising a substrate and at least one microstructured functional layer system applied to the substrate. The invention further relates to the use of a device for direct laser beam interference structuring for carrying out process steps of the method.

[0004] In the fabrication of microstructured components, such as microelectronic components or microelectromechanical systems (MEMS), the challenge often lies in structuring a stack of layers consisting of numerous layers and thereby exposing specific areas of deeper layers. Etching processes are frequently used in semiconductor technology for the fabrication of microelectronic circuits or microsystems components. In addition to wet chemical etching processes, dry etching processes such as plasma etching, reactive ionic deep etching, plasma-assisted etching, and ionic thinning are also employed.

[0005] The specialist article: “Piezoelectric Micromachined Ultrasonic Transducers (PMUTs): Performance Metrics, Advancements, and Applications” by Y. Birjis et al. in: Sensors 2022, 22, 9151 This describes a process for manufacturing piezoelectric micromachined ultrasonic transducers and other sensors. This process utilizes, among other things, lithographic processes in combination with wet and dry front-side and back-side etching steps.

[0006] Such processes are sometimes quite complex in terms of equipment, and keeping the workpiece clean can also be complex.

[0007] TASK AND SOLUTION

[0008] Against this background, one object of the invention is to provide a method for producing microstructured components that allows, during the production of microstructured components, a multi-layered stack of layers between two defined layers to be efficiently and gently separated and one or more upper layers to be removed in order to locally expose certain functional layers, e.g. electrodes for electrical contacting.

[0009] To solve this problem, the invention provides a method with the features of claim 1. Furthermore, claim 14 provides the use of a device for direct laser beam interference structuring (DLIP) for carrying out the method. Advantageous embodiments are specified in the dependent claims. The wording of all claims is made clear by reference to the content of the description.

[0010] According to one formulation, a method for producing a microstructured component is provided, comprising a substrate and at least one microstructured functional layer system applied to the substrate.

[0011] In this process, a workpiece is provided that comprises the substrate, a functional layer system applied to or arranged on the substrate, and a top layer system applied to or arranged on the functional layer system. In at least one process step, the workpiece undergoes laser processing using laser radiation in a laser processing station under the control of a control unit.

[0012] During the manufacturing of the workpiece, a functional layer system is formed on the substrate, comprising at least one functional layer. Preferably, the functional layer system has a plurality of functional layers. On a side facing away from the substrate, the functional layer system has a buffer layer located away from the substrate, the properties of which are explained below. A top layer system is formed on the functional layer system. This can consist of a single layer or material layer; more often, the top layer system comprises several different layers or material layers. The individual layers can be produced in the desired sequence by any suitable layer formation process, e.g., by material deposition processes such as CVD or PVD, and / or by processes in which complete layers or partial layers undergo a transformation, e.g., by oxidation or another chemical reaction.

[0013] These steps produce a workpiece that serves as an intermediate product and is further processed or refined in subsequent process steps. The production of the workpiece or intermediate product and its further processing can be carried out at different locations and / or by different institutions (e.g., individuals, departments, companies).

[0014] During the process, in a laser processing step or operation, laser radiation is directed from a side facing away from the substrate through the topcoat system onto the buffer layer adjacent to the topcoat system in such a way that the laser radiation weakens or destroys the connection between the buffer layer and the topcoat system within at least a limited target area in a boundary region between the two, either locally or over a larger area. This allows the topcoat material to be removed cleanly and with virtually no residue from the target area.

[0015] The term "target area" refers to a spatially limited local area within the entire surface covered by the surface layer system. A target area can also be described as a zone, section, region, or segment. The area of ​​a target area is usually only a fraction of the area of ​​the boundary between the surface layer system and the functional layer system, e.g., less than 20% or less than 10% of this area. Typical dimensions, e.g., diameter or edge lengths, can be on the order of 100 pm or more, possibly even in the millimeter range (e.g., from 1 mm to 10 mm or more).

[0016] During and / or after laser irradiation, material from the cover layer system is removed from the target area to expose the buffer layer in the target area.

[0017] The uppermost layer of the functional layer system that absorbs the radiation is called the buffer layer. This can be a separate layer with a sufficiently high absorption of the laser radiation compared to the overlying overlying overlying layer system to cause the overlying overlying layer system to detach or slough off. However, the buffer layer can also be an uppermost sublayer of the functional layer system, for example, the top layer of a layered electrode (e.g., the top 5% of an electrode layer). The absorption coefficient of the buffer layer for the laser radiation used should be high compared to the absorption coefficient of the overlying layers. This does not preclude partial transparency of the buffer layer, however, as long as any transmitted laser radiation does not cause damage to the functional layer system.The buffer layer thus forms the boundary between the portion of the surface layer system to be removed or already removed, and the remaining functional layer system, which is to be exposed in the target area. The buffer layer can remain on the component or be removed along with it. Partial removal of the buffer layer is also possible. It is important that the transparent surface layer system is completely removed.

[0018] The separation of parts of the surface layer system from the functional layer system is achieved using a laser lift-off (LLO) process or a laser-induced backward transfer (LIBT) process. In these processes, the buffer layer located at the interface between the surface layer system and the functional layer system is destroyed or removed by laser irradiation. The irradiation is applied from the surface of the surface layer system facing away from the substrate and passes through it, with the laser beam focused on the buffer layer or the interface. In the LLO process, the surface layer system can then be separated from the layers remaining on the intermediate product by applying an external force. In an LIBT process, the surface layer system is directly detached or blasted off by the pressure of the vaporized or decomposed material or by thermally induced mechanical stresses and can be removed by suction.

[0019] According to the claimed invention, the boundary region is irradiated by means of direct laser beam interference structuring (DLIP). A laser beam emitted from a primary laser radiation source is split into at least two partial beams, and these partial beams are guided such that at least two coherent partial beams pass through the cover layer system. In a superposition region of the coherent laser beams, a spatial intensity pattern is created that exhibits radiation microzones with constructive interference and relatively high power density of the laser radiation alongside regions of destructive interference and, relative to the radiation microzones, low power density of the laser radiation. This results in the creation of damaged microzones in the boundary region, arranged with lateral spacing between them.

[0020] The boundary region is thus irradiated using a variant of direct laser interference patterning (DLIP). Direct laser interference patterning (DLIP) is a laser-based technology that utilizes the physical principle of interference of high-intensity coherent laser beams to fabricate functional periodic microstructures. A corresponding laser processing system features a laser-optical arrangement for laser structuring, in which a laser beam emitted from a primary laser radiation source is split into at least two partial beams. These beams are guided in such a way that at least two interfering partial beams converge on the area to be processed. A spatial intensity pattern is generated in the region of the superimposed coherent laser beams.In this process, the areas with constructive interference form the radiation microzones that cause the formation of microzones in the boundary material, while the areas of destructive interference (or extinction) lead to the unirradiated or only weakly irradiated intermediate regions between adjacent microzones. Unlike other processing methods, such as laser writing, the beam diameter of the partial beams does not need to be focused, or at least not to a minimum beam diameter. This allows a significantly larger area to be processed per laser pulse.

[0021] Based on the inventors' experience, commercially available laser processing systems can be used for direct laser beam interference structuring within the framework of the inventive methods. For example, a laser-optical arrangement according to EP 3 466 598 A1 can be used. The disclosure content of this document is made by reference to the content of the present description.

[0022] In the interface material, a fine pattern of relatively small, locally confined zones is created. In these zones, the power density of the laser radiation is sufficient to damage or weaken the interface material to such an extent that separation along the interface or detachment from the growth substrate becomes possible. The term "microzone" indicates that the lateral dimensions of the interface zones, which are damaged and / or destroyed due to the locally increased power density, are in the single-digit micrometer range or below in at least one lateral direction. The mean lateral distances between immediately adjacent microzones are also very small, in the single-digit or low double-digit micrometer range.The term "distance" refers to the minimum center-to-center distance between immediately adjacent microzones, i.e., the center-to-center distance in the direction in which the center-to-center distance takes on its smallest value.

[0023] Within a radiation microzone, the intensity or power density of the laser radiation is increased by the beam conditioning compared to the surrounding area to such an extent that the power density is sufficient to initiate the processes required to weaken the material at the interface and to create the weakened microzones there. In the regions between immediately adjacent radiation microzones, the power density is orders of magnitude lower, so that the laser radiation does not directly cause weakening or destruction within the interface there. However, adjacent microzones are so close together that weakening is nevertheless generated throughout the entire interface covered by microzones, starting from the areas of microzones directly damaged by laser radiation, and the components to be separated can be reliably separated.

[0024] When transferring the top layer stack or parts thereof into the extraction system, a separation of the components to be separated takes place along the boundary area that is locally weakened or destroyed in the microzones, with the separation also extending from the microzones through the intermediate areas adjacent to the microzones that are not directly weakened.

[0025] It has been shown that with such a fine distribution of very small microzones, sufficient weakening of the functional layer system at the interface can be achieved without affecting the adjacent layer materials to the extent that sometimes occurs with significantly larger weakening zones. Therefore, this type of laser processing is particularly gentle on the areas bordering the interface.

[0026] The advantage of such methods compared to ablation by melting and / or vaporizing the entire material to be removed is that the process is much more efficient, since only a small portion of the material to be removed needs to be decomposed, heated, or vaporized, thus requiring significantly less process energy (energy of the laser radiation). Because the coating material is preferably ablated and removed largely as a solid, i.e., "in one piece," the risk of contamination of the microelectronic component by redeposited material, melt, and particles is also significantly reduced. However, unlike LLO or LIFT processes (Laser-Induced Forward Transfer processes), which are known from the production of optoelectronic, LED-based components, the ablated portion of the coating system is not part of the product here, but is treated as waste and, for example, recycled.The material is conveyed into an extraction system, while the remaining functional layer stack is further processed to produce the microelectronic component.

[0027] In the laser-induced forward transfer (LIFT) process, the laser irradiation passes through the transparent substrate, and the detached layer system is accelerated forward in the beam direction (forward transfer). In the process described here, the laser irradiation passes through the transparent cover layer system, and the detached segment of the cover layer system is accelerated backward in the opposite direction to the beam direction (backward transfer). For this process, a laser wavelength is selected such that the cover layer material is essentially transparent to this laser radiation, while the buffer layer material is significantly more absorbent. Preferred laser wavelengths are typically in the range of 200 nm to 5000 nm. These wavelengths can, for example, be in the ultraviolet range between 300 nm and 400 nm.

[0028] The process is preferably carried out such that, when material from the coating system is removed from the target area, a coherent piece, or possibly several coherent pieces, of the coating system material are detached from the target area. The coating material is thus removed in the form of at least one solid. This systematically avoids contamination through re-deposition, such as that which can occur in laser ablation processes.

[0029] In some embodiments, after the removal of the cover layer material from the target area, a contact material is brought into contact with an exposed surface of the buffer layer. The exposed target areas are thereby at least partially refilled with another material. The cover layer material can then perform its original function in the areas that remain uncovered.

[0030] In other applications, one or more target areas of the cover layer material can be removed from the underlying layers without refilling them. For example, the cover layer system can be an optically effective multilayer system that can have a specific optical function (such as reflection reduction or reflectivity enhancement) over the majority of its surface, while other properties can be locally generated in areas with exposed target regions. For example, transparent areas can be created in a dielectric mirror.

[0031] The buffer layer can essentially consist of an electrically conductive material, in particular a metallic material.

[0032] In some embodiments, the functional layer system comprises at least one piezo layer made of a piezoelectric material. The piezo layer can be sandwiched between electrically conductive layers, with these layers serving as electrodes. This allows the construction of piezoelectric transducers. When the DLIP method is carried out by superimposing two coherent laser beams, a line pattern is generated. Preferably, three or four laser beams are made to interfere, resulting in essentially point-like microzones in the irradiated area. These microzones then exhibit spacing from each other in three or more different lateral directions.

[0033] Preferably, many microzones, e.g. more than 100 or more than 500 or more than 1000 or more than 2000, are created simultaneously in one processing step, so that despite the small dimensions of the microzones and the distances between them, even larger areas of the boundary region can be weakened in a relatively short time and thus prepared for separation.

[0034] Using the DLIP technique, it is possible to process the laser radiation provided by the primary laser source without the need for a beam splitter mask with many apertures, creating a fine distribution of microzones. This allows the power provided by the primary laser source to be used with particularly high efficiency for attenuation at the interface. By eliminating the need for a beam splitter mask, losses that occur in conventional systems with a beam splitter mask due to this type of geometric beam splitting can be avoided. In other words, virtually no laser power is wasted; instead, a high proportion of the laser energy provided by the primary laser source can be used to attenuate the layer system at the interface.The energy of the laser radiation is largely conserved; it is only redistributed locally. Therefore, there is no additional process-related attenuation of the laser radiation beyond losses inherent in real components, such as reflection or scattering at interfaces.

[0035] Due to the high efficiency in utilizing the laser power provided by the primary laser radiation source, the method does not require the use of a high-pulse laser as the primary laser radiation source. In preferred embodiments, a suitable solid-state laser is therefore used as the primary laser radiation source, generating laser radiation from the ultraviolet region of the electromagnetic spectrum (UV solid-state laser), particularly at wavelengths below 400 nm. Alternatively, depending on the layer material (absorption coefficient) of the top layer of the functional layer system, wavelengths in the range between 400 nm and 5 pm can also be used. Commercially available short-pulse or ultrashort-pulse lasers can be used, e.g., with pulse durations in the range of a few tens of fs to a few hundred nanoseconds.Diode-pumped IR lasers with Nd:YAG, Yb:YAG, Nd:YVO4, or Nd:YLF as the laser medium can be used as the primary laser radiation source. Their radiation is converted to wavelengths in the UV range via frequency conversion, e.g., frequency tripling, thus providing wavelengths around 350 nm. Frequency quadrupling can generate laser wavelengths in the lower UV range around 266 nm. These are also suitable for the process but are not preferred due to the comparatively shorter lifetime of the crystals required for frequency quadrupling and the higher wear. Wavelengths around 350 nm are therefore currently preferred, among other reasons because the photon energy is sufficiently high for the LLO and LIBT applications considered here.

[0036] The optimal dimensions of the microzones and their spacing can vary from application to application. Based on the inventors' experience, very good results are achieved in most cases by ensuring that the lateral extent of the microzones in at least one lateral direction is in the range of approximately 0.5 pm to 3 pm. Alternatively or additionally, the mean lateral distance between immediately adjacent microzones can be in the range of approximately 1 pm to approximately 15 pm.

[0037] Generally, it is advantageous to create point-like microzones. The term "point-like" here means that the individual microzones are essentially round. This specifically means that diameter variations in different diametrical directions are at most 20%, and particularly at most 10%, of the largest diameter.

[0038] Alternatively, the microzones can also have other shapes, e.g., oval, approximately square, approximately rectangular, or linear. This makes it possible to better adapt the shape of the microzones to the application.

[0039] Occasionally, only two superimposed partial beams are used, resulting in an interference pattern with linear radiation microzones through two-beam interference. The use of linear microzones allows for largely uniform irradiation of the entire sample, provided a suitable pulse spacing is set. The lateral extent in the width direction can be in the aforementioned range (approximately 0.5 pm to approximately 3 pm), while the length can be significantly larger, e.g., several hundred pm or approximately 1 mm or more. In the DLIP method, under certain boundary conditions (e.g., low-interference coupling into the surface layer system), the microzones can exhibit a periodic micro-grid arrangement inherent to the system, which is beneficial for generating conditions that are as uniformly distributed as possible across the entire irradiated boundary area.

[0040] Regarding the spatial distribution of microzones that can be generated in the boundary region, the method offers numerous possibilities. It may suffice to generate all microzones of an irradiated area essentially according to a uniform grid. If necessary, combinations of two or more different grid arrangements can also be generated sequentially. In one embodiment, microzones are generated in a surface area of ​​the boundary layer in at least two successive processing steps. In a first processing step, a first micro-grid arrangement of microzones is generated, and in a subsequent second processing step, second microzones are generated in intermediate regions between the first microzones, distributed according to a second micro-grid arrangement.

[0041] In some cases, it is also possible that a more or less statistical distribution of microzones is generated in the boundary area, whose lateral distances and lateral sizes vary statistically in certain size ranges, especially within the size ranges already mentioned here.

[0042] The present application also relates to the use of a direct laser beam interference structuring (DLIP) system for irradiating an interface between a cover layer system and a functional layer system to weaken or destroy a connection between the cover layer stack and the functional layer stack in at least one local, spatially limited target area in a laser lift-off process or a LIBT process.

[0043] According to the inventors, the use of suitable variants of direct laser beam interference structuring (DLIP) within the framework of laser lift-off or LIBT processes offers advantages not only with regard to targeted weakening in the boundary region, but can also contribute elsewhere to reducing the overall costs of manufacturing processes that use laser lift-off operations and / or LIBT operations.

[0044] Many process variants possible within the scope of the invention can utilize coating systems with relatively rough surfaces. This, in turn, can lead to problems with the coupling of the laser radiation into the coating system using conventional laser processing methods. To avoid similar problems, DE 10 2017 205 635 A1, for example, proposed bringing a liquid layer of a liquid transparent to the laser radiation into contact with the (rough) back surface of a wax substrate before the laser beam is applied, and then shining the laser beam through this liquid layer. This significantly improved the efficiency of laser beam coupling, even with rough back surfaces.

[0045] In one embodiment of the invention, the (rough or sensitive) upper surface of the transparent coating system is brought into contact with a liquid that is transparent to the laser beam, so that the laser radiation passes through the transparent liquid and the transparent coating system. The liquid can be matched to the refractive index of the upper layer of the coating system to compensate for the roughness of the layer as effectively as possible. In the simplest case, however, water can also be used as the liquid. The liquid film also cools the area near the processing, thus enabling particularly gentle processing for the remaining coating area and the functional coating system.

[0046] Investigations by the inventors have now shown that such complex and costly additional measures in preparation for laser processing can be avoided if suitable variants of direct laser beam interference structuring are used to irradiate the interface. If the roughness of the coupling surface is not too great, essentially only the nature of the microzone distribution changes. If the top surface of the cover layer stack, used as the coupling surface, is coated with an immersion layer, regular lateral distributions of microzones with well-defined sizes can be generated. In contrast, if coupling occurs via an unpolished (relatively rough) coupling surface, more statistically distributed microzones with a broader size and spacing distribution are produced.

[0047] In this respect, this method variant for generating microzones appears to be relatively insensitive to problems induced by surface roughness. According to a further development, a top layer stack can therefore be used which has a surface roughness on the upper side with a mean roughness value Ra of more than 0.1 pm, wherein the mean roughness value is preferably in the range of 0.5 pm to 1 pm.

[0048] A system for the fabrication of microelectronic components is also disclosed, comprising a substrate and at least one functional layer system and at least one cover layer system. The system can be used, among other things, for LLO and LIBT processes, wherein a device for direct laser beam interference structuring (DLIP) can be used to irradiate an interface between the functional layer system and a cover layer system in order to weaken or destroy a connection between the functional layer system and the cover layer system in at least one local, spatially limited target area during a laser lift-off or LIBT process.

[0049] BRIEF DESCRIPTION OF THE DRAWINGS

[0050] Further advantages and aspects of the invention will become apparent from the claims and from the description of exemplary embodiments of the invention, which are explained below with reference to the figures.

[0051] Fig. 1 shows a schematic section through a workpiece in the form of a layered composite with an exemplary layer structure during laser processing using direct laser beam interference structuring;

[0052] Fig. 2 shows an example of a laser processing station set up to use direct laser beam interference structuring to locally weaken a layer within a layer structure as part of a laser lift-off process and to prepare it for separation;

[0053] Figs. 3A and 3B show microscopic images of an overview (3A) and a detail (3B) of a regular pattern of microzones generated by the use of DLIP in the boundary region of an irradiated workpiece when irradiated through a top surface of a cover layer stack which has a comparatively low roughness (R a <10 nm), through;

[0054] Fig. 4 shows an example of a regular micro-grid arrangement of microzones that were created in two successive processing steps with different settings of a DLIP device;

[0055] Fig. 5 shows an example of the irregular distribution of microzones after DLIP structuring through a rough top surface of a cover layer stack;

[0056] Figs. 6A and 6B show a top view (Fig. 6A) and a sectional view (Fig. 6B) of an embodiment of a microstructured component in the form of a piezoelectric transducer with an electrically controllable piezo layer arranged between electrode layers.

[0057] DETAILED DESCRIPTION OF THE EXECUTION EXAMPLES

[0058] The following describes various aspects of the inventive methods and systems for the production of microstructured components using a laser lift-off process and / or a LIBT process, based on exemplary embodiments.

[0059] Fig. 1 shows a schematic section through a workpiece 100 in the form of a layered composite with an exemplary layer structure. The workpiece 100 is an intermediate product produced during a manufacturing process, which is subjected, among other things, to laser beam processing. Fig. 2 shows an example of a laser processing station 200 provided for this purpose.

[0060] The workpiece 100 shown in Figs. 1 and 2 has a substrate 140 in the form of a flat glass plate. A functional layer system 130 is formed on the front side of the substrate, comprising several superimposed functional layers, e.g., one or more semiconductor layers, piezoelectric, magnetic and / or ferroelectric layers, 134, 136, as well as a metal layer as an electrode 132. The functional layer system is also referred to here as a functional layer stack. The layers can be structured so that the desired electronic function can be realized.

[0061] A thin buffer layer 120 is formed on the upper surface of the metal layer facing away from the substrate. This buffer layer can be a separate layer from the metal layer or a thin sublayer of the metal layer. The functional layer system or stack, or at least the buffer layer 120, exhibits a comparatively high absorption coefficient at the laser wavelength used, making the LLO or LIBT process feasible. When using ultrashort pulse lasers, a sufficient absorption coefficient can also be achieved via multiphoton absorption.

[0062] Thin transparent layers (e.g. made of SiO2, Si3N4) are then produced on the functional layer stack for electrical insulation and protection of the microelectronic functional units, which together form the cover layer system 110.

[0063] Generally, a cover layer system often consists of a multitude of transparent individual layers, which are required to implement complex electrical, optical, and / or mechanical functions. However, the cover layer system can also consist of just a single layer of cover layer material that is transparent to laser radiation.

[0064] For surface layer systems, e.g., in the form of optical coatings, layers made of oxides such as SiO2, Ta2O5, Ti2O3, Al2O3, HfO2, ZrO2, and Y2O3 are suitable. Alternatively or additionally to oxide layers, fluoride layers can also be present, e.g., with fluorides such as CaF2 and MgF2.

[0065] If conductive layers are required, materials such as TCO (transparent conducting oxide), e.g., indium tin oxide (ITO), indium zinc oxide (IZO) and / or indium gallium zinc oxide (IGZO) can be used for one or more layers of the cover layer system.

[0066] In this context, "transparent to laser radiation" means that the absorption coefficient of these layers is so low that the layers are not damaged, or only minimally damaged, during laser processing (i.e., not decomposed, melted, or vaporized), and the intensity of the laser radiation at the boundary is still sufficiently high to carry out the LLO or LIBT process. The layers typically have a thickness of less than one to a few millimeters each; the total thickness of the layer system with its various layers can, for example, be less than 10 millimeters.

[0067] Prior to further processing, the top layer system 110 can be structured, for example by laser processing, in order to define sharp boundaries of the areas to be removed from the top layer system.

[0068] The top surface 114 of the top layer system, or its free surface, can have different surface roughnesses depending on the coating process, e.g., a medium surface roughness (R). a ) in the range of 1 pm or slightly below. In the example case, the top surface has a roughness of R a approximately 0.3 nm.

[0069] Fig. 2 schematically shows a laser processing station 200, configured as part of a system for manufacturing microelectronic components for a laser lift-off (LLO) or laser laser bonding (LIBT). The system includes a workpiece carrier 210 for holding the workpiece 100. The workpiece is placed on the workpiece carrier with the top surface 114 of the coating system 110 facing upwards. The laser processing station 200 includes a workpiece motion system 280, which is configured to position a workpiece to be processed in a desired processing position of the laser processing station in response to motion signals from the control unit 290. In the configuration of Fig.2. The workpiece motion system 280 comprises the workpiece carrier 210, which can be moved very precisely to a desired position parallel to the (horizontal) xy-plane of the system coordinate system and in the vertical direction (parallel to the z-direction), as well as rotated about a vertical axis of rotation (PHI axis). For this purpose, precisely controllable electric direct drives are provided in this example.

[0070] The laser processing station serves to direct laser radiation from the top surface 114 of the cover layer system 110 through the cover layer system in such a way that the laser radiation is guided into a planar boundary region between the cover layer system 110 and the functional layer system 130 (see Fig. 1) and a connection between the cover layer system 110 and the functional layer system 130 is weakened or destroyed in the boundary region. The boundary region can, for example, lie in or contain the buffer layer 120.

[0071] In the immediate vicinity of the processing position, a compressed gas nozzle 320 and a suction nozzle 330 are arranged in such a way as to support the removal and extraction of detached material from the top layer system.

[0072] A unique feature is that the interface is irradiated using a variant of direct laser interference patterning (DLIP). DLIP is a laser-based technology that utilizes the physical principle of interference of high-intensity coherent laser beams to create functional periodic microstructures on surfaces. Unlike conventional applications, DLIP is used here to weaken a thin layer (the interface) located within the workpiece at a distance from its free surfaces.

[0073] The laser processing station features a laser-optical arrangement 300 for laser structuring. A raw laser beam LSR emitted by a primary laser radiation source 310 is split into three coherent partial beams TS1, TS2, TS3 by means of three physical beam splitters 312-1, 312-2, 312-3. The primary laser radiation source 310 is an ultrashort pulse laser with a solid-state laser medium, emitting in the ultraviolet range at a wavelength of approximately 355 nm. The three partial beams are guided by a 45° deflecting mirror 314 and a focusing optic 315 such that they interfere with each other in a three-dimensionally extended superposition region (interference volume IVOL, see Fig. 1). The focusing optic 315 can be formed by a single lens or by a lens system with multiple lenses. The three partial rays are symmetrical about the center of the focusing optics at the vertices of an equilateral triangle.whose optical axis is arranged 314.

[0074] Fig. 1 schematically illustrates the situation with two coherent partial beams TS1 and TS2. The partial beams propagate in directions of propagation that are oriented at an acute angle W to the optical axis AX of the focusing optics 315. They are azimuthally offset from each other by 120°. The angle W is continuously adjustable.

[0075] The setup of the laser-optical arrangement 300 is only an example. Commercially available DLI P systems exist that can be used for the purpose presented here. Documents EP 3 466 598 B1 and EP 3 735 332 B1 show some examples of systems that could be used in principle.

[0076] The interfering partial beams penetrate the transparent cover layer system 110 largely without absorption. The laser radiation is only strongly absorbed upon impact with the buffer layer 120, where it alters the material. In a superposition region of the coherent laser beams (partial beams TS1, TS2, TS3), a spatial intensity pattern is generated. This results in locally confined, relatively high laser power densities in the areas of constructive interference. The small volume regions where the laser power density is sufficient to weaken the material of the interface are referred to here as radiation microzones. Where these extend into the interface material, microzones MZ are formed in the interface material, in which the interface material is vaporized and / or otherwise destroyed or weakened. The microscopically small microzones MZ are spaced apart from each other in a regular micro-grid arrangement.In areas of destructive interference, there remain unirradiated or only weakly irradiated intermediate areas, e.g., between neighboring microzones.

[0077] Although the power density only directly weakens the boundary material (here, for example, a metal) in the area of ​​the radiation microzones and causes the microzones there, a weakening effect is nevertheless achieved across the entire surface. This is because the material in the intermediate regions, e.g., between the treatment points (microzones), is also detached by the pressure resulting from the decomposition or vaporization of the boundary material and / or by the thermally induced mechanical stresses in the boundary material. To illustrate the changes in the irradiated boundary region, Figures 3A and 3B show microscopic images of the structures in the boundary region of an irradiated workpiece during a series of tests.Figure 3A essentially shows the entire, approximately circular area (enclosed by the dashed line LP) that was irradiated by a single laser pulse, i.e., in principle, the intersection between the planar boundary region and the interference volume. Figure 3B shows an enlarged section of the central region of the circular processing area irradiated by a single pulse.

[0078] It is clearly evident that a regular structure of more or less circular microzones (MZ) was generated within the interference volume, following a rectangular grid pattern. A single pulse simultaneously generated several hundred microzones (MZ) evenly distributed across the approximately circularly irradiated area. According to the intensity distribution within the interfering partial beams, the microzones (MZ) are more pronounced in the central area than at the radial edge of the illuminated area (LP) when laser beams with a Gaussian distribution are used. The mean diameter (DMZ) of the individual circular microzones was approximately 1.9 pm, corresponding to a microzone area of ​​approximately 2.8 to 2.9 pm. 2The smallest distance AB between a microzone and an immediately adjacent microzone, measured between the centers of the respective microzones, was approximately 5.1 pm. The microzones are uniformly distributed within an area irradiated by a pulse; in the overlapping areas with the neighboring pulse, superimpositions of offset grid arrangements can occur.

[0079] The distance AB between the microzones (i.e., the interference period) is adjustable in the chosen arrangement by varying the angle of incidence W between the interfering partial beams and the optical axis in the processing zone, e.g., in the range between approximately 1 pm and approximately 15 pm.

[0080] The overall pulse size, i.e., the size of the area LP that can be irradiated with a single pulse, is also adjustable by varying the size of the interfering partial beams in the processing zone. This can be achieved, for example, by adjusting the raw beam diameter or varying the focus or magnification of the optics used. This variation of the overall pulse size also changes the intensity within the areas of constructive interference, i.e., the single-spot energy, which can also be adjusted by selecting the laser pulse energy.

[0081] The size of the individual microzones MZ, also referred to as single-spot size, is continuously adjustable in the selected arrangement from approximately 0.5 pm to approximately 3 pm. The diameter I, the size of the individual spots, changes depending on the laser energy used and the angle of incidence W between the interfering partial beams and the optical axis in the processing zone.

[0082] If the edge drop in laser intensity towards the edge of a partial beam is perceived as disruptive due to a Gaussian-like profile, some embodiments allow for the masking of edge regions with lower intensity. Such a mask can be designed as a direct mask 319 with a small distance to the sample surface 114. Alternatively, it is also possible to map the shape of a mask 311 onto the sample using a mask projection method. The geometry of the contact surfaces to be exposed can be precisely adjusted using such masks.

[0083] It is also possible to homogenize the raw beam before the mask and beam splitting (e.g. by a diffractive optical element / DOE).

[0084] Generally, it is advantageous to avoid multiple pulses during the transition to the next pulse to prevent an uneven distribution of attenuation at the boundary, which is difficult to control. However, the lateral offset from pulse to pulse can also be set with a defined overlap so that individual points are hit multiple times (by several pulses), thus extending the interference pattern over large areas. This superposition allows positions at the edge of the processing zone, which were initially processed with insufficient fluence, to be processed with sufficient fluence overall.

[0085] In this example, a raw laser beam (LSR) was used. Homogenization of the primary laser beam can be provided if required.

[0086] Within this process, different surface patterns of microzones can be generated. For example, individual pulses can be generated at intervals without overlap, and then other grids with different laser power and / or spot size and / or spot spacing can be generated in the spaces between them, for example to avoid excessive energy input in the transition area between adjacent irradiation zones.

[0087] It is also possible to modify the structure during processing, for example, spirally from the outside in. For instance, a denser grid can be created in the outer area than in the inner area. Dot grids with varying single-spot dimensions and spacings are also possible, as are grids with alternating large and small effects. Fig. 4 shows an example in which microzones were created in a surface area of ​​the boundary layer in at least two successive processing steps. In a first processing step, a first micro-grid arrangement of microzones MZ1 with diameters D1 and spacings AB1 was created. Then, in a subsequent second processing step, with a modified average laser power, second microzones MZ2 with smaller diameters D2 were created in the intermediate areas between the first microzones MZ1, distributed according to a second micro-grid arrangement with spacings AB2.

[0088] Another embodiment was designed to achieve gentle, successive processing while simultaneously implementing complete processing of the entire surface.

[0089] The use of a direct laser interference structuring (DLIP) method or device for irradiating an interface between a surface layer system and a functional layer system, for the purpose of weakening or disrupting the bond between these two systems in a laser lift-off or LIBT process, offers further advantages beyond those already mentioned, particularly regarding the quality of the finished product and the associated costs. These advantages demonstrate the suitability of this method for structuring planar areas within a workpiece, i.e., areas not located on the workpiece's free surfaces.

[0090] The advantages arise, among other things, from the fact that the quality of the laser radiation coupling into the workpiece material does not have to be as high as with conventional laser processing. As mentioned in the introduction, in some conventional processes the top surface of the coating system, which serves as the entry point for the laser radiation, must be polished before laser processing to avoid disruptive scattering. Alternatively, the coupling surface can be smoothed using an immersion technique.

[0091] In contrast, when applying the DLIP approach, comparatively rough surfaces of cover layer systems can be used as the coupling surface for the laser radiation without prior modification of the coupling surface. This will be demonstrated below using experimental data. Using DLIP offers an additional advantage regarding a large tolerance of the permissible deviation of the Z-position during processing: The three or four individual beams are focused by the focusing optics and superimposed in a larger area above the focal plane. In this area (also called the interference volume), where the beams overlap, the interference pattern is created, which does not change in the Z direction; that is, the XY position of the individual interference points is independent of Z. The processing position is located above the focal position and, in the example shown, has a diameter of approximately 250 pm.The tolerance of the Z-position is approximately four times this diameter. Therefore, in this example, a potential wafer bow (i.e., a possible wafer deformation) of up to approximately 1 mm can be tolerated without Z-tracking. The laser radiation intensity changes only relatively little in this range. The high depth of field of the DLIP process can thus be used to implement the process more cost-effectively without compromising the quality of the final product.

[0092] Conventional laser lift-off methods required care to ensure that scattering effects did not excessively impair the coupling of the laser radiation into the growth substrate. Therefore, the coupling surface was smoothed by polishing or the use of an immersion layer. In contrast, laser lift-off using direct laser interference structuring (DLIP) is significantly less sensitive to scattering effects due to the roughness of the coupling surface. This will be illustrated with reference to Fig. 5. Fig. 5 shows a microscopic image of DLIP structuring through a surface with a mean surface roughness of approximately R. a=1 m. In this case, the processing zone at the boundary no longer exhibits the defined, uniform pattern seen when coupled through a polished back surface, but instead displays a stochastic structuring. The microzones MZ, which originated in the area of ​​particularly high radiation intensity, exhibit a certain size distribution within and below the single-digit micrometer range, and the lateral distances to immediately adjacent microzones are no longer uniform, but vary statistically over a certain distance range. This stochastic structuring is attributed to the formation of speckles through scattering of the radiation at the rough coupling surface and a largely stochastic superposition in the processing plane. Here, too, a weakening of the boundary layer occurs throughout the entire irradiated area.of the interface in such a way that the components can subsequently be separated from each other along the interface. This variant is particularly suitable when a large-area laser lift-off is required, for example, to remove an entire layer system from a wafer. There are also applications where the individual functional components are already present on the semi-finished product for the production of microelectronic components and where the top layer system must be locally removed at defined positions (e.g., at surfaces for the electrical contacts of the components). For such applications, the use of masks is advantageous in order to precisely define the size of the areas to be removed.

[0093] Figures 6A and 6B illustrate some steps of an exemplary method for fabricating a layered piezoelectric transducer. Figure 6B shows a vertical section through the component, which is shown in top view in Figure 6A. For clarity, the same reference numerals are used for identical or similar features as in Figure 1.

[0094] The microstructured electromechanical component 610 has a substrate 140 in the form of a plane-parallel plate, which can be made of silicon, for example. Several layers of a functional layer system 130 were successively deposited on the flat front surface of the substrate. The functional layer system comprises a first electrode layer 132 made of an electrically conductive material, for example, a metal. Immediately adjacent to this is a piezo layer 134, i.e., a layer made of a piezoelectric material, for example, lead zirconate titanate (PZT). While the first electrode layer was, for example, vapor-deposited, the piezo layer can be produced, for example, using a sol-gel process. Immediately adjacent to the piezo layer is a second electrode layer 136, which can be made of the same material as the first electrode layer.Depending on the application, the second electrode layer can be laterally structured or, as in the example case, applied continuously.

[0095] A coating system 110 was applied to the substrate-facing upper surface of the second electrode layer. In this example, this coating system serves, among other things, to protect the underlying layers of the functional layer system and to provide electrical insulation for the underlying electrode layer. The coating system can be, for example, a layer of Si3N4 or a coating system with multiple layers of materials transparent to laser radiation. Alternatively, it can also be a layer of silicon dioxide (SiO2). The coating system can comprise various oxide and / or nitride layers. In this example, the coating system primarily serves for electrical insulation. In other applications, the coating system can also have an optical function, for example, similar to a dielectric multilayer coating for reflection reduction (antireflective coating) or with another optical function.

[0096] In this example, the second electrode layer 136 applied to the piezo layer is to be electrically contacted in specific, locally limited areas according to a particular spatial pattern (see Fig. 6A). For this purpose, the cover layer material is to be completely removed from the otherwise closed cover layer 110 within locally limited, in this example square, target areas 160, so that the underlying electrode layer is then exposed in the target areas.

[0097] To remove the material of the cover layer system 110 in the target areas 160 as completely and gently as possible, the interface between the cover layer system and the upper second electrode is irradiated there by means of direct laser interference structuring (DLIP) as described in connection with Fig. 1. For this purpose, laser radiation, for which the material of the cover layer system is largely transparent, but the second electrode layer is strongly absorbing, is directed as described from the side facing away from the substrate and passes through the cover layer system onto the interface or a buffer layer, which here is a thin sublayer of the electrode layer 136 located away from the substrate and bordering the cover layer system. Microzones MZ of high laser power density are formed there according to a regular pattern.

[0098] In this example, the lateral extent of the microzones (MZ) in at least one lateral direction is approximately 0.5 pm to 3 pm. Mean lateral distances between immediately adjacent microzones (MZ) can range from approximately 1 pm to approximately 15 pm. A target area typically contains many microzones, for example, at least ten, at least 100, or 1000 or more.

[0099] This results in coherent, cuboid pieces 110' of the top layer material being gently detached from the underlying material within the target areas and blasted outwards (see Fig. 6B).

[0100] In this process, all target areas 160 can be irradiated simultaneously by directing the laser radiation through a beam splitter mask (e.g., mask 311 in Fig. 2) that has corresponding square apertures or openings defining the spatial distribution of the target areas. The parts of the top layer system 110, preferably detached as solids or in the form of macroscopic flakes, can be conveyed to the waste via a suction system (here with a blow nozzle 320 and a suction nozzle 330) after detachment from the functional layer system. Thus, preferably, a complete, contiguous part of the top layer system is transferred from the functional layer system to the waste. The transfer is selective, such that only selected, locally confined parts or segments of the layer stack of the top layer system are detached.

[0101] This approach to exposing defined target areas of the electrode layer using DLIP is significantly gentler on the irradiated material than full-surface laser irradiation, which can occasionally lead to cracking in the metal layer. Compared to lithographic structuring processes combined with etching, many process steps can be eliminated. Furthermore, the use of chemicals and water, etc., is unnecessary. Compared to laser ablation, this variant of the LIFT process offers significant advantages in terms of material preservation and cleanliness, as there is no risk of re-deposition of vaporized material and minimal particle contamination. Additionally, laser energy and processing time can be saved.

[0102] For the technical implementation, the following options exist as alternatives or in addition to those described so far. The aggregation and superposition of the partial beams can be achieved using a focusing element, such as a lens, objective, focusing mirror, or axicon. Other methods for generating the interference pattern can also be used, for example, with a grating, such as a reflection grating. Lasers with pulse durations in the picosecond or nanosecond range can also be used.

[0103] By modifying the LIBT process, it can be advantageously used for the efficient opening of inner layers of complex layer stacks. This allows, for example, the creation of openings for electrode contact after the deposition of insulating and / or passivating layers or protective layers on complete device structures / wafers or large-area substrates. This eliminates, for instance, the need for complex steps such as lithographic masking and subsequent chemical etching of these layers / cover layer systems.

[0104] The use of DLIP devices and processes in laser lift-off (LLO) or LIBT offers several advantages. Among other things, the alternating processing in this micro-scale process requires less laser power per irradiated area than homogeneous irradiation. Therefore, cost-effective solid-state lasers with comparatively low pulse energies but high pulse repetition frequencies in the MHz range can be used. High laser pulse repetition frequencies enable high throughput in the application of this process. Furthermore, the alternating processing in this process places less mechanical stress on the parts of the layer system adjacent to the interface, so increased production yield can be expected.

[0105] Another advantage of the method is that the generated pattern of microzones across the entire laser beam diameter is stored as an interference pattern, and hundreds to thousands of microzones (weakening zones in micro or nano dimensions) can be generated in the boundary region within a few nano- to femtoseconds using only one laser pulse.

[0106] The beam diameter does not need to be focused; depending on the required pulse energy, it can even be widened to generate microscopically small structures with a significantly larger processing area per laser pulse. The focusing optics are not used to focus each individual partial beam to its minimum diameter at the boundary, but rather to redirect the parallel partial beams entering the focusing optics towards the targeted processing area and adjust the beam diameter through partial focusing so that the laser fluence required for the process is present in the buffer layer.

[0107] Compared to laser writing, a very high depth of field can be achieved because DLIP does not rely on precise focusing of the laser beam, but instead generates a spatial "interference volume" with three-dimensionally extended radiation microzones, within which the boundary region is structured with the corresponding interference pattern. This makes the process less sensitive to variations in the workpiece's position in the z-direction, allowing the use of more cost-effective systems for z-direction positioning.

[0108] Embodiments of the methods described here can be used, for example, in the fabrication of microelectronic and / or microelectromechanical components based on semiconductor layers, piezoelectric, magnetic, and / or ferroelectric layers. A functional layer system can, for example, comprise a piezoelectric layer system for generating ultrasound transmitters / receivers, position sensors or actuators, and / or medical sensors. A microstructured component can, for example, be designed as a CMUT (capacitive micromachined ultrasound transducer) or PMIIT (piezoelectric micromachined ultrasound transducer). Embodiments can also be used in the fabrication of directly applied and / or bonded actuators for adjusting / positioning substrates, for example, for use in high-precision positioning systems.It is also suitable for beam stabilization and / or for use in adaptive optics for manipulating / shaping laser beam shapes and / or wavefronts, e.g., for forming tophats, donut profiles, etc., but also for correcting absorption-induced "distorted / deformed" wavefronts (e.g., in telescopes). Applications in imaging, for haptic components, for generating mechanical adjustment units (e.g., for lasers or optical components), or for sound field generation are possible.

Claims

Patent claims 1. A method for producing a microstructured component comprising a substrate and at least one microstructured functional layer system applied to the substrate, comprising the following steps: Providing a workpiece comprising the substrate, a functional layer system applied to the substrate, and a top layer system applied to the functional layer system; Directing laser radiation from a side facing away from the substrate through the cover layer system onto a buffer layer of the functional layer system adjacent to the cover layer in such a way that the laser radiation weakens or destroys a connection between the buffer layer and the cover layer system within at least a locally limited target area in a boundary region between the buffer layer and the cover layer system; Detachment of material from the cover layer system in the target area to expose the buffer layer in the target area, wherein the boundary region is irradiated by means of direct laser beam interference structuring (DLIP) by splitting a laser beam emitted from a primary laser radiation source into at least two partial beams and guiding the partial beams such that at least two coherent partial beams pass through the cover layer system and a spatial intensity pattern is created in a superposition region of the coherent laser beams, which exhibits radiation microzones with constructive interference and relatively high power density of the laser radiation next to regions of destructive interference and, relative to the radiation microzones, low power density of the laser radiation, wherein damaged microzones are generated in the boundary region by the laser radiation of the radiation microzones, which are arranged with lateral distances to each other.

2. Method according to claim 1, characterized in that when material of the cover layer system is removed from the target area, a continuous piece of cover layer material is removed from the target area.

3. Method according to claim 1 or 2, characterized in that, after the removal of cover layer material from the target area, a contact material is brought into contact with an exposed surface of the buffer layer.

4. Method according to one of the preceding claims, characterized in that the functional layer system comprises at least one piezo layer made of a piezoelectric material, wherein preferably the piezo layer is adjacent to the buffer layer, and / or that the buffer layer consists essentially of an electrically conductive material.

5. A method according to any of the preceding claims, characterized in that the coating system is designed as an optically effective coating system and comprises at least one layer of a material selected from the group consisting of: an oxide, in particular SiO2, Ta2O5, Ti2O3, Al2O3, HfO2, ZrO2, Y2O3; a fluoride, in particular CaF2, MgF2; and / or that the coating system comprises at least one electrically conductive layer, in particular of a TCO (transparent conducting oxide), preferably indium tin oxide (ITO), indium zinc oxide (IZO) and / or indium gallium zinc oxide (IGZO), and / or that the coating system comprises at least one layer of Si2O3 or Si3N4.

6. Method according to one of the preceding claims, characterized in that a lateral extent of the microzones in at least one lateral direction is in the range of 0.5 pm to 3 pm and / or that mean lateral distances of the microzones are in the range of 1 pm to 15 pm and / or that in one processing step more than 100, in particular more than 1000 microzones are produced simultaneously.

7. Method according to one of the preceding claims, characterized in that the laser beam emitted by the primary laser radiation source is divided into three or four partial beams.

8. Method according to one of the preceding claims, characterized in that essentially point-shaped or oval or approximately square or approximately rectangular microzones are produced or that linear microzones are produced.

9. Method according to one of the preceding claims, characterized in that microzones are generated in a surface area of ​​the boundary layer in at least two successive processing steps, wherein in a first processing step a first grid of first microzones is generated and in a subsequent second processing step second microzones are generated in intermediate areas between first microzones.

10. Method according to one of the preceding claims, characterized in that a top layer system is used which has a surface roughness on the upper side with a mean roughness value R a exhibiting a roughness greater than 0.1 pm, wherein the mean roughness value is preferably in the range of 0.5 pm to 1 pm.

11. Method according to one of the preceding claims, characterized in that pulsed laser radiation from a solid-state laser is used to irradiate the boundary region.

12. Method according to one of the preceding claims, characterized in that the laser radiation is directed by the use of a beam splitter mask (311, 319)) such that radiation microzones of high power density of the laser radiation are generated in the boundary region such that damaged microzones are generated in the boundary region by the laser radiation of the radiation microzones, which are arranged with mean lateral distances in the micrometer range to each other, wherein the area filled with radiation microzones is limited by the contour of the beam splitter mask or the beam splitter mask shown.

13. Method according to one of the preceding claims, characterized by: applying a liquid transparent to laser radiation to the top surface of the transparent cover layer system; Radiation of laser radiation through the transparent liquid and the transparent cover layer system in such a way that the laser radiation weakens or destroys a connection between the functional layer system and the transparent cover layer system in a boundary area between the functional layer system and the transparent cover layer system in locally limited areas or over a large area; Removal of the transparent top layer system or parts thereof and preferably removal via the liquid film.

14. Use of a device for direct laser beam interference structuring (DLIP) for irradiating an interface between a cover layer system and a functional layer system to weaken or destroy a connection between the cover layer system and the functional layer system within at least a locally limited target area in a method according to one of the preceding claims, wherein a laser beam emitted from a primary laser radiation source is split into at least two partial beams, preferably three or four partial beams, and the partial beams are guided such that at least two coherent partial beams pass through the cover layer system and a spatial intensity pattern is created in a superposition region of the coherent laser beams.The radiation microzones with constructive interference and relatively high power density of the laser radiation are located next to areas of destructive interference and, relative to the radiation microzones, have lower power density. exhibits laser radiation, wherein in the boundary area damaged microzones are generated by the laser radiation of the radiation microzones, which are arranged with lateral distances to each other.

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