Molding method for producing nanostructure and specific device for implementing method

Abstract

The invention relates to a thermoforming method, in particular a press forming method, for forming structures, in particular microstructures or nanostructures, in a surface (2) of a substrate (3) by means of a forming die (4). To this end, the surface (2) of the substrate (3) is locally heated by means of laser radiation (5) above a softening temperature, and then a negative structure of the press surface (6) of the forming press (4) is embossed into the deformable glass frit at a set pressure by means of a forming force (F). In order to maintain the heat input even during the forming process, in particular to ensure that the temperature is constant throughout the forming process and that the temperature distribution is uniform over the entire die surface (6), the forming die (4) and the die surface (6) are made of a material which is wavelength transparent to the laser radiation (5).

Description

Technical Field

[0001] The present invention relates to a thermoforming method, in particular a pressure forming method, for forming structures, in particular microstructures or nanostructures, in amorphous substrates, in particular glass substrates. The method is achieved by locally heating the surface of the substrate using applied laser radiation; positioning a forming die having a negatively structured die surface (the material of which is at least partially transparent to the wavelength of the laser radiation) relative to the substrate; and transmitting pressure between the surface and the forming die, thereby forming or deforming the surface and forming a structure in the surface corresponding to the negatively structured structure. The present invention also relates to an apparatus for carrying out the forming method. Background Art

[0002] According to US 2004 / 0046288 A1, structures can be directly embossed into the surface of a solid substrate. To do this, the substrate surface is softened or liquefied by radiation, and a forming die is pressed into the softened or liquefied surface to directly emboss the substrate. The forming die is transparent to radiation, allowing the radiation to pass through the transparent forming die and illuminate the substrate. This radiation can heat the surface layer of the substrate without significantly heating the forming die or the entire substrate. This embossing process can be repeated at other locations on the same substrate.

[0003] US2005 / 0146084 A1 describes a method for forming micro- and nanostructures in a heat-structurable layer using electromagnetic radiation, using a structured shaped pattern. The radiation is directed onto a radiation-absorbing plate positioned beneath the substrate, thereby indirectly heating the substrate. During this process, the radiation beam is shaped using a mask, resulting in localized melting of the substrate surface regardless of the size of the imprinted pattern.

[0004] EP 2 101 216 B1 relates to an embossing method using electromagnetic waves which are directed towards a heating layer through a mould which is transparent to electromagnetic radiation and which are absorbed by the heating layer.

[0005] Thus, the heating layer generates heat and softens the surface of the substrate, and the heat is transferred through contact with the substrate.

[0006] Nanoimprint lithography is a well-known technique for forming microstructures or nanostructures in substrates. To create nanostructures, a positive mold (particularly a monomer or polymer) and a stamper with the nanostructures are used. The positive mold is applied to the substrate, heated to above its glass transition temperature, and then pressed into the stamper. Furthermore, US 2007 / 0104813 A1 discloses a photolithographic method for imprinting flat components, such as integrated circuits.

[0007] In laser-assisted direct imprinting (LADI), a structured stamp is pressed onto a silicon substrate and then irradiated with excimer laser pulses. This causes the surface of the silicon substrate to melt and liquefy, transferring the desired pattern to the substrate.

[0008] Furthermore, according to WO 2007 / 012215 A1, microstructures can be generated by mask projection. The mask is used to form a predetermined intensity distribution of the laser beam, thereby allowing the laser beam to pass only through specific surface areas of the mask.

[0009] EP 2660074 A1 discloses an embossing method with microscopic resolution, in which the workpiece absorbs energy from an energy source only in the area to be embossed. Before, during and / or after the energy source applies energy, an embossing stamp is pressed into the heated area to be embossed of the workpiece.

[0010] JP 2005-111975 A relates to a method for forming fine non-uniform patterns by molding a pattern from a replica plate material into a resin material to produce a relief hologram, diffraction grating, or similar structure. To this end, a light-to-heat conversion layer is applied to the relief-forming material or the relief-patterned film. Upon contact, the light-to-heat conversion layer is heated, forming the fine non-uniform pattern.

[0011] US 2011 / 0215504 A1 discloses a method for transferring a non-uniform pattern onto an object. To this end, two molds, each formed with a concave-convex pattern, are aligned in a reference position with the central axis of a central rod carrying the object, and the molds are then pressed simultaneously onto different surfaces of the object.

[0012] A disadvantage of existing methods is that in the actual shape replication process, the energy input uniformity of the laser radiation is often insufficient, especially due to the uneven temperature distribution formed, which may lead to surface quality defects in the produced nanostructured area, especially shape deviations. Summary of the Invention

[0013] The object of the present invention is to provide a method and a device by means of which the quality of the microstructures or nanostructures produced thereby can be significantly improved and reproducible precision can be achieved.

[0014] According to the invention, the first object is achieved by a method having the features of claim 1. Further developments of the invention can be gathered from the dependent claims.

[0015] Therefore, according to the present invention, a forming method, in particular a pressure forming method, for forming structures (in particular microstructures or nanostructures) in a substrate is provided. The method comprises: locally heating the surface of the substrate by means of applied laser radiation; positioning a forming die having a negatively structured die surface (the material of which is at least partially transparent to the wavelength of the laser radiation) relative to the substrate; transmitting a deforming force between the surface and the forming die for forming the surface, thereby forming a structure corresponding to the negative structure in the surface; and simultaneously exposing the surface of the substrate to the laser radiation in such a way that the laser radiation passes through the forming die and is directed onto the surface during the forming process, thereby heating the surface of the substrate above its softening temperature; wherein the intensity distribution of the laser radiation is adapted to the negative structure of the forming die and / or the structured surface of the forming die. In many cases, a uniform intensity distribution on the die surface or the structured surface of the forming die, or on the surface of the substrate to be formed, has proven to be advantageous.

[0016] The core insight of the present invention is that the quality of the structures produced during the forming process is primarily determined by the radiation intensity and the resulting temperature distribution. This temperature distribution depends not only on the applied laser radiation (e.g., a CO2 laser with a wavelength between 9.4 and 10.6 μm, or a CO laser with a wavelength between 4.8 and 8.3 μm), but also, in particular, on the type and size of the structure to be produced. For example, it has been shown that a uniform beam distribution of the applied laser radiation does not necessarily reliably achieve uniform heat input and temperature distribution on the substrate surface. According to the present invention, the intensity distribution can be optimized based on empirical parameters combined with measured values ​​from in-situ detection.

[0017] In the present invention, the term "structure" is to be understood not only as a contoured or structured surface (eg with regular or irregular depressions or protrusions), but also as a planar surface, the surface quality of which can be significantly improved by the treatment according to the invention.

[0018] For example, studies have shown that the type and characteristics of the resulting structure (including aspect ratio) can lead to locally different solidification conditions on the molded surface, which can generate internal stresses. Therefore, the subsequent solidification process must be taken into account when inputting heat.

[0019] Preferred materials for the forming die are silicon (Si), gallium arsenide (GaAs), germanium (Ge), zinc selenide (ZnSe), zinc sulfide (ZnS), calcium fluoride (CaF2), barium fluoride (BaF2) and / or indium arsenide (InAs).

[0020] In practice, it has proven particularly advantageous to bring the forming die into contact with the substrate surface only after the substrate surface has been heated above a predetermined temperature threshold (eg softening temperature) and has reached a sufficient temperature.

[0021] A particularly advantageous embodiment of the present invention is to maintain a constant temperature and / or radiation intensity during the thermoforming of the surface. For example, by dynamically adjusting the intensity or intensity distribution of the applied laser radiation based on continuous temperature monitoring, or by setting a predetermined intensity profile over time, it is possible to avoid an uneven forming process when the forming die is pressed into the softened surface of the substrate. In the prior art, such an uneven forming process can disadvantageously lead to local pressure differences and undesirable stresses during solidification of the formed substrate surface.

[0022] Practice has also proven particularly advantageous when the cross-section of the laser radiation is adapted to the surface of the forming die by means of particularly variable beam shaping, so that the cross-sectional area of ​​the applied laser radiation corresponds to the surface of the forming die (or the surface of the substrate to be formed). The intensity distribution within the cross-section of the shaped beam can be uniform or constant, or non-uniform, in order to adapt to the desired locally increased or decreased energy input to the substrate surface.

[0023] Another particularly advantageous embodiment of the present invention provides for generating a predetermined, particularly uniform, laser radiation intensity distribution on the surface of the forming die or the substrate surface to be formed by means of an optical element that is adjustable and / or movable, particularly during the substrate forming process. For this purpose, the optical element is preferably arranged in a housing serving as a carrier for the forming die and protected from environmental influences. During the forming process, the optical element moves along with the forming die, so that its distance from the substrate surface remains constant, resulting in particularly good results. The optical element is preferably equipped with a beam expander to vary the cross-section of the applied beam.

[0024] Another particularly advantageous embodiment of the present invention provides that the cross-sectional area of ​​the laser radiation (in particular, the cross-sectional area of ​​the intensity distribution relevant or sufficient for heating the substrate) is smaller than the die surface of the forming die. Thus, the laser radiation is scanned across the die surface for heating, exposing continuously varying localized areas of the surface to be heated or formed to the laser radiation. This allows the temperature to be adjusted by varying the heat input in different localized areas of the substrate surface based on a controlled feed motion. Of course, the energy input is not limited to a single laser beam; rather, multiple laser beams can be independently moved along the localized areas of the substrate surface to be heated, and these beams can also be superimposed for rapid energy input.

[0025] Practice has also proven particularly advantageous in limiting the heating of the substrate to the surface of the forming die or the negative structure. By shaping the beam to adapt the energy input to the surface to be formed, heating of adjacent areas can be minimized. Monitoring of the heating occurring in adjacent surface areas of the substrate is preferably provided. To prevent unintended exposure to adjacent surface areas, for example, a light shield can be provided that surrounds the forming die and simultaneously serves as a support for the forming die. Active cooling of adjacent surface areas, for example using a cooling medium or a conformable cooling element, can also be provided.

[0026] Furthermore, it has proven particularly practical to use a step-and-repeat method for forming, i.e., to process different partial surfaces of the substrate in sequence. This allows for the processing of adjacent or neighboring partial surfaces. Preferably, adjacent partial surfaces are not processed in consecutive steps, thus creating cooling times between the processing of adjacent surfaces. This minimizes the distance between adjacent partial surfaces to be formed, thus ensuring optimal surface utilization.

[0027] In another advantageous embodiment of the invention, the surface of the substrate and / or the die surface of the forming die is also heated by radiation directed through the substrate from the side facing away from the forming die. This allows for targeted heating from opposite sides, thus avoiding any obstructions caused by any supporting structures. Furthermore, this allows heating of the surface areas directly adjacent to the surface to be formed, thereby reducing or eliminating any resulting stresses.

[0028] Practice has shown that the temperature of the substrate surface is preferably measured contactlessly, in particular using a pyrometer, wherein a wavelength different from that of the laser radiation is detected. This allows for continuous temperature monitoring.

[0029] In practice, the control of the method is preferably based on the temperature, the pressing stroke or feed stroke and / or the force curve during the pressing process (preferably force-controlled). When the substrate is heated and softened, the forming die and the substrate are brought closer to each other under the action of force. This distance, which is called the "pressing stroke", is measured. Based on the detected pressing stroke and / or the force measured by the force sensor in the press, the force can be controlled, for example to maintain a constant force until a specific pressing stroke is reached. The temperature is controlled by the laser power, taking into account the heat conduction in the forming die. Ideally, the temperature remains constant during the embossing process, so that the temperature can be measured and the laser power can be controlled accordingly.

[0030] The power of the laser radiation is preferably controlled based on the measured temperature, with the target temperature being compared to the actual temperature. However, continuous control (especially uninterrupted monitoring) is not absolutely necessary. Instead, for example, for repetitive die shapes, the intensity distribution (power) and / or beam shape of the laser radiation can be continuously monitored and adapted only once, and then, for subsequent processing of the same shape, this setting can be repeated without continuous measurement. Preferably, the temperature distribution is characterized and then controlled by local measurements during repeated, at least essentially identical, process cycles.

[0031] According to the present invention, the second objective (i.e., providing an apparatus for implementing the method) is achieved by providing a device for detecting the laser power intensity distribution and / or the temperature distribution of the forming die (particularly the die surface and / or the substrate surface). Thus, according to the present invention, the laser beam (particularly the intensity distribution and beam shaping) can be controlled based on the detected measured values, thereby controlling the forming process and the degree of forming according to the structure to be formed. In this way, the quality of the substrate's formed surface can be significantly improved.

[0032] It is particularly advantageous if the holding device has a support body transparent to the wavelength of the laser radiation, to which the forming die is releasably fastened; during the embossing process, the support body serves to increase the rigidity and prevent the die surface (whose material thickness is 10 μm to 1 mm) from bending.

[0033] In another, equally suitable embodiment of the present invention, the support body comprises a support structure, in particular with regular through-holes. For example, a metal grid or honeycomb structure made of steel or aluminum is opaque to the laser radiation, so that small areas of the substrate surface are shielded. Heat is supplied to these areas via heat conduction, so that a uniform temperature distribution can be easily achieved in practice.

[0034] The forming die can be releasably fixed at the periphery and / or at the edges, allowing for easy replacement when necessary. A particularly advantageous refinement of the invention provides that the support body has a plurality of recesses, and the forming die is fixed to the support body by applying a negative pressure relative to the ambient pressure. This allows the entire surface of the forming die to be used as the forming surface, without the available surface being restricted by retaining devices at the edges. In this case, the forming die can have almost any shape and size, eliminating the need for complex adaptations of the support body or retaining devices.

[0035] In another, likewise particularly advantageous embodiment of the invention, the forming die is provided with an antireflection coating on the back side facing away from the die surface in order to minimize the losses that occur when coupling in the laser radiation.

[0036] Furthermore, the die surface of the forming die can be provided at least partially (especially in the area of ​​the negative structure) with a coating which reduces the adhesion of the glass to the die and thus enables a fast and defect-free separation of the forming surface from the forming die.

[0037] The device preferably has a housing with a forming die holder, which includes a deflection element for the laser beam. This ensures, on the one hand, optimal protection of the forming die, and, on the other hand, protects the substrate surface area to be formed from external influences. In particular, this reliably prevents contamination of the die surface with foreign particles, which could impair the quality of the produced nanostructures.

[0038] Practice has shown that when selecting the molding die material, it is preferred that the thermal expansion coefficient of the substrate be higher than that of the molding die; in this way, the substrate shrinks more significantly when cooling, thereby simplifying the demoulding process. DETAILED DESCRIPTION

[0039] The present invention can be implemented in various ways. To further illustrate its basic principles, one embodiment is shown in the accompanying drawings and described below.

[0040] This schematic diagram shows a device 1 for thermoforming a surface 2 of a substrate 3 by means of a forming die 4 in order to cost-effectively form structures, particularly nanostructures, in the substrate 3. To this end, the surface 2 of the substrate 3 is locally heated to above its softening temperature by means of laser radiation 5. Subsequently, the negative structure of the die surface 6 of the forming die 4 is impressed into a deformable glass material using a forming force F at a set pressure. To maintain heat input during this forming process, and in particular to ensure a constant temperature throughout the entire forming process and a uniform temperature distribution across the die surface 6, the forming die 4 and the die surface 6 are made of a material that is transparent to the wavelength of the laser radiation 5. As a result, the laser radiation 5 can pass through the forming die 4 and strike the surface 2 during the entire forming process. The surface 2 of the substrate 3 is heated to above its softening temperature by absorption of the laser radiation, with the intensity distribution of the laser radiation 5 being adapted to the negative structure of the forming die 4. The device 1 comprises a housing 7, which houses an environmentally protected optical element 8. This optical element deflects the applied laser radiation 5 so that its intensity distribution is adapted to the surface of the forming die 4 or the surface 2 of the substrate 3 to be formed, and, if necessary, to its cross-sectional area. On the side of the housing 7 facing the substrate 3, a support body 9, transparent to the wavelength of the laser radiation, is provided. This support body serves to transmit the applied forming force F to the forming die 4 over a surface, in particular to prevent bending of the die surface 6 during the forming process. The device for positioning the forming die 4 relative to the substrate 3 is a flat worktable 10 that is movable along two spatial axes and temperature-controlled (in particular, heatable). The substrate 3 is fixed to this worktable by applying a negative pressure p and can be preheated to a temperature below the softening temperature of the substrate 3. Once the thus formed substrate 3 has cooled below its softening temperature, the forming die 4, provided with a release coating, can be easily separated from the surface 2.

[0041] Reference Signs List

[0042] 1 Device 2 surface 3 substrate 4 Molding die 5 laser radiation 6 Die surface 7 case 8 element 9 Support 10 Workbench F Forming force P negative pressure

Claims

1. A forming method, in particular a pressure forming method, for forming a structure, in particular a microstructure or nanostructure, in a substrate (3) made of an amorphous material, comprising: locally heating the surface (2) of the substrate (3) by means of the applied laser radiation (5); A forming die (4) having a die surface (6) with a negative structure (the material of which is at least partially transparent to the wavelength of the laser radiation (5)) is positioned relative to the substrate (3); a pressure is transmitted between the surface (2) and the forming die (4), by which the surface (2) is formed and a structure corresponding to the negative structure is formed in this surface (2); at the same time, the surface (2) of the substrate (3) is exposed to the laser radiation (5), specifically in such a way that during the forming process the laser radiation (5) passes through the forming die (4) and is directed towards the surface (2), thereby heating the surface (2) of the substrate (3) to above the softening temperature; it is characterized in that the intensity distribution and / or the beam shape of the laser radiation (5) are adapted to the forming die (4) (in particular the die surface (6)), the structured surface of the forming die (4) and / or the surface (2) to be formed of the substrate (3).

2. The molding method according to claim 1, characterized in that During the profiling of the surface (2), the temperature and / or the radiation intensity of the laser radiation (5) are kept constant and / or are varied, adapted or controlled over time, in particular according to a predefined cycle.

3. The molding method according to claim 1 or 2, characterized in that: By means of a particularly variable beam shaping, the cross-sectional area of ​​the laser radiation (5) is adapted to the surface of the forming die (4).

4. The forming method according to at least one of the preceding claims, characterized in that A predetermined, in particular uniform, intensity distribution of the laser radiation (5) is generated on the surface of the forming die (4) by means of an in particular adjustable and / or movable optical element (8).

5. The forming method according to at least one of the preceding claims, characterized in that The cross-sectional area of ​​the laser radiation (5) (in particular the cross-sectional area of ​​the intensity distribution associated with heating the substrate (3)) is smaller than the die surface (6) of the forming die (4), and energy input is achieved by scanning the die surface (6) with the laser radiation (5).

6. Forming method according to at least one of the preceding claims, characterized in that The heating of the substrate (3) is limited to the surface or negative structure of the forming die (4).

7. Forming method according to at least one of the preceding claims, characterized in that The molding is carried out using a step-by-step repetitive method.

8. Forming method according to at least one of the preceding claims, characterized in that The die surface (6) of the forming die (4) is also heated by radiation from the side facing away from the forming die (4) through the substrate (3).

9. Forming method according to at least one of the preceding claims, characterized in that The temperature of the surface (2) of the substrate (3) is measured by non-contact means, in particular using a pyrometer.

10. A device (1) for forming structures, in particular microstructures or nanostructures, in a substrate (3) by means of laser radiation (5), comprising a forming die (4), the die surface (6) of which consists of a material at least partially transparent to the wavelength of the laser radiation (5), for carrying out the forming method according to at least one of the preceding claims, characterized in that The device is provided with a device for detecting the power intensity distribution of the laser radiation (5) and / or the temperature distribution of the forming die (4), in particular the die surface (6) and / or the substrate (3) surface (2).

11. The device (1) according to claim 10, characterized in that The device (1) has a holding device with a support (9) transparent to the wavelength of the laser radiation (5) and to which the forming die (4) is releasably fastened.

12. The device (1) according to claim 11, characterized in that The support body (9) has a support structure, in particular with regular through holes.

13. The device (1) according to claim 11 or 12, characterized in that The support body (9) has a plurality of recessed portions, and the forming die (4) is fixed on the support body (9) by applying relative negative pressure.

14. Device (1) according to at least one of claims 10 to 13, characterized in that The forming die (4) is provided with an anti-reflective coating on the back side facing away from the die surface (6).

15. Device (1) according to at least one of claims 10 to 14, characterized in that The die surface (6) of the forming die (4) is at least partially (especially in the negative structure area) provided with a coating, which can reduce the adhesion of the substrate (3) on the die surface (6).

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