3D printing device and 3D printing method for manufacturing a workpiece

Abstract

The present invention relates to a 3D printing device (100) for manufacturing a workpiece. The device (100) has: a first radiation source (1) for emitting a first radiation (2) to perform non-linear absorption polymerization, and a second radiation source (3) for emitting a second radiation (4) to perform optical coherence tomography, wherein the first radiation (2) can pass through a first beam path (5), and the second radiation (4) can pass through a second beam path (6), and wherein the first beam path (5) and the second beam path (6) are formed completely independently of each other. The present invention also relates to a 3D printing method (200, 400) for manufacturing a workpiece, another 3D printing device (300), a method for analyzing the quality of a starting material (12) for non-linear absorption polymerization, a method for checking the orientation of a substrate (8) to be printed by non-linear absorption polymerization, a method for determining the spatially resolved degree of conversion of non-linear absorption polymerization, a method for analyzing the structural clarity of a structure produced by non-linear absorption polymerization, and a three-dimensional reconstruction method for a workpiece manufactured by non-linear absorption polymerization.

Description

[0001] The present invention relates to 3D printing devices and a 3D printing method for manufacturing workpieces. The present invention further relates to a method for analyzing the quality of starting materials for non-linear absorption polymerization, a method for verifying the position and / or alignment of a substrate to be printed by non-linear absorption polymerization, a method for determining the spatially resolved degree of conversion of non-linear absorption polymerization, a method for analyzing the structural clarity of structures produced by non-linear absorption polymerization, and a three-dimensional reconstruction method for workpieces manufactured by non-linear absorption polymerization.

[0002] Non-linear absorption polymerization is an additive manufacturing method (3D printing method) in which a three-dimensional workpiece is constructed layer by layer by the controlled polymerization of starting materials with monomers and / or oligomers. The starting materials can also be referred to as photoresists.

[0003] Typically, the irradiation of photons emitted by a laser causes a photoinitiator to decompose into free radicals, which cause the free radical polymerization of monomers and / or oligomers to produce a polymer. Subsequently, the unpolymerized monomers and / or oligomers can be removed, for example, by washing them off, in order to obtain the desired workpiece made of polymer material. This washing step is also referred to as the development of the photoresist. By subsequent chemical processes, the polymer material can be modified or transformed if necessary.

[0004] The lasers used are typically femtosecond lasers, i.e., lasers with optical pulses having a duration in the femtosecond range. Contrary to photo-polymerization, which has been known for some time, non-linear absorption polymerization is capable of producing very small structural dimensions in the range of about 100 nm. The reason for this is that the formation of free radicals requires the non-linear absorption of multiple (e.g., two) photons, which is only possible at that location or in that reaction volume where the energy input is high enough (i.e., in the direct focus area of the laser). The power introduced here is proportional to (light intensity) N where N = 1 for the linear mechanism and N > 1 for the non-linear mechanism.

[0005] By appropriately controlling the laser focus in the lateral direction and along the depth axis, the site of the polymerization reaction can thus be accurately fixed, such that the desired three-dimensional workpiece can be formed step by step. The corresponding method is also referred to as direct laser writing (abbreviated as "DLW").

[0006] Two important subgroups of non-linear absorption polymerization are two-photon polymerization and two-step absorption polymerization. While two-photon polymerization is based on the simultaneous absorption of two photons, the absorption in two-step absorption polymerization takes place in two consecutive steps, as in HAHN, V., MESSER, T., BOJANOWSKI, N.M., CURTICEAN, E.R., WACKER, I., As described in "Two-step absorption instead of two-photon absorption in 3D nanoprinting. Nat. Photon" by R.R., BLASCO, E., and WEGENER, M., Nat. Photon. "Nature Photonics", 202115, 932–938. The basic idea of two-step absorption is to replace the virtual state in two-photon absorption with a real state (i.e., an electronic intermediate state that exists in the absence of a light field). Its lifetime is typically determined by non-radiative processes and can be several orders of magnitude longer than femtoseconds or picoseconds. Unless otherwise stated, the term "nonlinear absorption polymerization" is used here as an umbrella term for two-photon polymerization and two-step absorption polymerization.

[0007] Several challenges are associated with the practical use of nonlinear absorption polymerization, which are elaborated below.

[0008] Impurities in the starting material can affect the quality of the printed structure and thus the quality of the manufactured workpiece. For example, impurities may cause the formation of unfavorable light-scattering centers in the path of the radiation, such as in the path of a femtosecond (fs) laser beam, which will cause nonlinear absorption polymerization. Alternatively or additionally, impurities may cause local inhomogeneities within the voxel at the radiation focus (e.g., at the laser focus). This may in turn lead to an undesirably high surface roughness, a difference in shape relative to the desired 3D model, and / or cause local inhomogeneities, such as inhomogeneities in optical properties (e.g., refractive index), or mechanical properties of the printed material and thus the manufactured workpiece.

[0009] Possible impurities can be external particles (e.g., dust particles) or internal inhomogeneities that form within the starting material over time.

[0010] Therefore, a method is needed by which impurities in the starting material can be analyzed.

[0011] While in some applications, individual elements are printed onto a planar and horizontally aligned substrate, in other applications, for example, it is necessary to print structures onto:

[0012] - on a free-form surface,

[0013] - on sidewalls, such as at the edge of a substrate, as in JAYNE, R.K., KARAKAN, M. As described in "Direct laser writing for cardiac tissue engineering: a microfluidic heart on a chip with integrated transducers" by ZHANG, K., PIERCE, N., MICHAS, C., BISHOP, D. J., CHEN, C. S., EKINCI, K. L., WHITE, A. E., Lab Chip, 2021, 21(9), 1724-1737.

[0014] - At the tip of an optical fiber, as described in "Sub-micrometre accurate free-form optics by three-dimensional printing on single-mode fibres" by GISSIBL, T., THIELE, S., HERKOMMER, A., GIESSEN, H., Nat Commun, 2016, 7, 11763.

[0015] - In a semi-closed cavity or other areas that are difficult to access, for example, to functionalize microfluidic channels, as described in "3D microfluidics via cyclicolefin polymer-based in situ direct laser writing" by ALSHARHAN, A. T., ACEVEDO, R., WARREN, R., SOCHOL, R. D., Lab Chip, 2019, 19(17), 2799-2810.

[0016] - Together with and within already existing pieces produced by non-linear absorption polymerization or in some other way (LAMONT, A.C., RESTAINO, M.A., KIM, M.J., SOCHOL, R.D., "A facile multi-material direct laser writing strategy". Lab Chip, 2019, 19(14), 2340 - 2345), these pieces can be identified only via their refractive index or via volume information, for example, by printing similar pieces successively with alternating photoresists.

[0017] Combinations of the above requirements are also possible. In these cases, even when using alignment marks or if a single live camera is used in a 3D printer, the initial positioning and / or alignment of the substrate may be difficult.

[0018] Therefore, a method is needed by which the positioning and / or alignment of the substrate to be printed can be improved.

[0019] Many properties of the formed polymer are known to depend on the local degree of conversion (abbreviated as "DC"). The degree of conversion can be defined here as the mass percentage of starting materials (monomers and / or oligomers) that have covalently bonded to each other to form the product (polymer). The determination and control of the degree of conversion are crucial because this parameter affects the mechanical properties (e.g., elastic modulus), optical properties (e.g., refractive index), and / or thermal properties (e.g., thermal expansion) of the produced workpiece. Adjustment of the degree of conversion at the micron scale or even below that micron scale additionally enables the production of, for example, gradient refractive index optical devices (abbreviated as "GRIN") and mechanical parts with specific gradients of elastic modulus and stiffness.

[0020] The local degree of conversion depends on the locally applied power of the radiation source (i.e., e.g., laser power) and the local exposure time during the 3D printing process. Therefore, information about the degree of conversion can be inferred by measuring the local refractive index. If, for example, the degree of conversion of a test sample is analyzed ex-situ by ellipsometry, the optimization of the applied radiation power and exposure time can be very time-consuming.

[0021] Therefore, an improved method is needed by which the degree of conversion can be determined. In-situ analysis of the degree of conversion is desirable.

[0022] Furthermore, it would be helpful to be able to determine the degree of conversion within the printed photoresist in a spatially resolved manner because the degree of conversion may vary intentionally or unintentionally within the micron range or below the micron range.

[0023] After 3D printing, the diffusion of monomers into the interfacial layer between the unpolymerized starting material and the exposed starting material can immediately affect the structural clarity of the printed workpiece. The extent and time-dependence of this diffusion process depend in particular on the type of starting material used and the parameters of the non-linear absorption polymerization, such as the laser writing parameters. In the absence of in-situ monitoring methods, the consequences of monomer diffusion can only be analyzed after the excess starting material has been washed away (i.e., after photoresist development), which is particularly disadvantageous in the case of long printing applications.

[0024] Therefore, it would be desirable to have a method that can analyze the structural clarity in-situ, i.e., even while the printing operation is still running.

[0025] Impurities in the starting material, shrinkage of the polymerized starting material, proximity effects, and non-optimized laser writing parameters are examples of factors that can cause the actual morphology of the printed workpiece to differ from its target model (e.g., a CAD model (CAD = computer-aided design)). Proximity effects here refer to the effects caused by the presence of structures that have already been printed or polymerized in the immediate vicinity. These include a reduction in the minimum radiation dose required for polymerization, which can lead to distortion of fine structures due to a spatially less well-confined polymerization reaction, and a reduction in the radiation dose beyond which damage may be caused to already existing adjacent structures, as described, for example, in SAHA, S.K., DIVIN, C., CUADRA, J.A., PANAS, R.M., "Effect of Proximity of Features on the Damage Threshold during submicron Additive Manufacturing via Two-Photon Polymerization", J. Micro Nano-Manuf., September 2017, 5(3):031002.

[0026] For example, in “In Situ Volumetric Imaging and Analysis of FRESH 3D Bioprinted Constructs Using Optical Coherence Tomography” by TASHMAN, J.W., SHIWARSKI, D.J., RUESCH, A., LANNI, F., KAINERSTORFER, J., FEINBERG, A.W., bioRxiv, 2021, https: / / doi.org / 10.1101 / 2021.06.30.450389 and WO 2020 / 150251 A1, the actual morphological 3D reconstruction measured in situ by optical coherence tomography has been reported. However, the methods described in these two documents are based on defects within the printing area, which cause optical coherence tomography signals in an ideally uniform area. In the absence of these defects, it would be impossible to reconstruct a specific morphology after the printing operation.

[0027] Accordingly, what is desired is a method by which the above disadvantages can be avoided and 3D reconstruction can be performed even in the absence of defects.

[0028] The prior art discloses various methods for characterizing workpieces produced by two-photon polymerization and two-photon polymerization methods.

[0029] An overview of non-in-situ and in-situ metrology solutions capable of inspecting microstructures produced by two-photon polymerization can be found in “Two-photon polymerization metrology: Characterization methods of mechanisms and microstructures” by LAFRATTA, C.N., BALDACCHINI, T., Micromachines, 2017, 8(4), 101. The methods proposed are particularly used, for example, to analyze the structural dimensions of laser-written materials by non-in-situ scanning electron microscopy, to analyze their surface roughness by non-in-situ atomic force microscopy, and to analyze the differences in their shape relative to the target 3D model due to shrinkage and its degree of conversion during photopolymerization or photoresist development.

[0030] The standard method for extrapolating the degree of conversion is differential scanning calorimetry (abbreviated as "DSC"). However, it is not applicable to workpieces produced by non-linear absorption polymerization because it requires a minimum weight, cannot be performed in-situ and is at least associated with damage to the sample to be examined. Fourier transform infrared spectroscopy (abbreviated as "FTIR") can also be used to determine the degree of conversion, but in the case of non-linear absorption polymerization, it is subject to various limitations, especially with regard to poor spatial resolution.

[0031] JIANG, L.J., ZHOU, Y.S., XIONG, W., GAO, Y., HUANG, X., JIANG, L., BALDACCHINI, T., SILVAIN, J.-F., LU, Y.F., "Two-photon polymerization: investigation of chemical and mechanical properties of resins using Raman microspectroscopy", Optics letters, 2014, 39(10), 3034-3037, especially Figure 3 , and RYS, J., STEENHUSEN, S., SCHUMACHER, C., CRONAUER, C., DARAIO, C., "Locally addressable material properties in 3D micro-architectures". Extreme Mechanics Letters, 2019, 28, 31-36, especially Figure 4, addresses the use of (micro) Raman spectroscopy for determining and imaging the degree of conversion at the micron scale. The degree of conversion can here be derived from the integral change in the peak intensity of C=C bonds, which are converted to C-C bonds during polymerization. Even though (micro) Raman spectroscopy enables non-destructive in-situ inspection, its usability is hampered by the need for a high laser power or long detection times due to a small signal-to-noise ratio. For imaging of prints with dimensions of several tens of microns, the detection time can be several hours, which is irreconcilable with real-time analysis during the non-linear absorption polymerization process.

[0032] These limitations can be avoided by using microscopy based on Coherent Anti-Stokes Raman Scattering (abbreviated as "CARS"), where the laser beam used to produce the workpiece can also be used to generate the pump beam and Stokes beam for CARS imaging. The signal intensity here depends on the concentration of C-H bonds, which in turn is related to the density of the polymer produced; see Figure 3 .3.8 in "Visualizing TPP structures with coherent Raman scattering microscopy" by PRINCE, R., FAN, P., LU, Y., BALDACCHINI, T., POTMA, E.O. "In Three-Dimensional Microfabrication Using Two-Photon Polymerization", 2020, pp. 229-249, William Andrew Publishing. However, the degree of conversion can only be determined indirectly by first retrieving the Raman spectrum with a phase determination algorithm and then applying the method of the above (micro) Raman spectroscopy. Since its detection rate is on the order of nanoseconds to microseconds and its sample volume is close to the size of the printed voxel, CARS has been mentioned as a possible in-situ process monitoring method; see "Characterization of microstructures fabricated by two-photon polymerization using coherent anti-stokes Raman scattering microscopy" by BALDACCHINI, T., ZIMMERLEY, M., KUO, C.H., POTMA, E.O., ZADOYAN, R., J. Phys. Chem. B, 2009, 113(38), 12663-12668 and WO 2011 / 136919 A1.

[0033] Finally, optical coherence tomography and quantitative phase imaging have been combined to visualize the time-resolved and spatially resolved degree of conversion on polymer microdroplets (DONG, B., PAN, B., “Visualizing curing process inside polymers”, Appl. Phys. Lett. 2020, 116(5), 054103). However, a prerequisite for this method is to collect backscattered light from the volume of the polymer microdroplets, which may require introducing additional light-scattering nanoparticles into the polymer medium.

[0034] Optical coherence tomography (abbreviated as “OCT”) is an imaging method for creating and displaying high-resolution three-dimensional images, for example, from different depth layers of a sample. For this purpose, electromagnetic radiation (for example, infrared radiation having a wavelength of about 800 nm to 1400 nm and having a low coherence length based on time) is split into two light beams or light ray bundles in a beam splitter, where one of the light ray bundles is directed onto the sample to be examined, and the other light ray bundle is used as a reference and passes through a reference path. The light ray bundle reflected by the sample is superimposed on the reference light ray bundle. Then, an image can be created from the resulting interference signal, which shows the structure of the sample along the depth axis, where a three-dimensional optical coherence tomography image, i.e., an OCT image, can be created from the two-dimensional depth signal by lateral scanning on the sample to be analyzed.

[0035] For optical coherence tomography, for example, infrared radiation having a wavelength of about 800 nm to 1400 nm or visible light having a wavelength of about 400 nm to 800 nm can be used. In principle, infrared radiation with a larger wavelength (for example, up to about 2 μm) can also be used. Using visible light has the advantage of higher spatial resolution, but using visible light may cause problems regarding unfavorable absorption in the starting materials.

[0036] OCT has been proposed as a general in-line monitoring system in 3D printing methods and is due to its non-destructive availability and its high penetration depth into low-absorbing materials (US10649439 B2). One use involves comparing the actual printed structure with the design parameters of the model. This strategy is also followed in the field of bioprinting in order to enable in-situ process monitoring by combining OCT with a 3D extrusion bioprinter (YANG, S., WANG, L., CHEN, Q., XU, M., "In situ process monitoring and automated multi-parameter evaluation using optical coherence tomography during extrusion-based bioprinting", Additive Manufacturing 47, 102251, 2021; TASHMAN, J.W., SHIWARSKI, D.J., RUESCH, A., LANNI, F., KAINERSTORFER, J., FEINBERG, A.W., "In Situ Volumetric Imaging and Analysis of FRESH 3D Bioprinted Constructs Using Optical Coherence Tomography", bioRxiv, 2021, https: / / doi.org / 10.1101 / 2021.06.30.450389). With the aid of measurement software, the differences in the 3D OCT reconstructions from the original 3D model become clearly visible (see Figure S5 in the following document: TASHMAN, J.W., SHIWARSKI, D.J., RUESCH, A., LANNI, F., KAINERSTORFER, J., FEINBERG, A.W., "In Situ Volumetric Imaging and Analysis of FRESH 3D Bioprinted Constructs Using Optical Coherence Tomography", bioRxiv, 2021, https: / / doi.org / 10.1101 / 2021.06.30.450389).

[0037] US2016 / 0023403 A1 describes the use of OCT as an image processing unit in order to monitor the conformity of successively applied layers in an additive manufacturing method and to correct possible differences.

[0038] US2019 / 0163067 A1 discloses the use of OCT as a possible metrology method, especially in the case of non-planar substrates, for characterizing surface morphology before printing.

[0039] WO 2020 / 150251 A1 discloses a specific embodiment of polarization-sensitive OCT (PS-OCT) for monitoring two-photon polymerization processes. PS-OCT is chosen for this purpose because it can provide information about the birefringence of the cured material, which is mentioned as a common problem in 3D printing of polymers. In contrast, for determining the cured volume and degree of cure under different curing conditions, analysis by high-speed camera and Raman spectroscopy is proposed. The envisaged arrangement of radiation sources for two-photon polymerization and PS-OCT contemplates the combined use of optical elements. The envisaged beam splitter is a dichroic mirror, but this requires the use of radiation sources with significantly different wavelengths. This limits the choice of further co-usable optical elements, as these optical elements must be correspondingly suitable for the two wavelengths.

[0040] In this context, the object of the present invention is to specify devices and methods that enable improved in-situ process monitoring of non-linear absorption polymerization methods and very accurate analysis of starting materials, products, and substrates used in such methods.

[0041] This object is achieved by the subject matter of the independent claims. The dependent claims relate to configurations of the solution according to the invention.

[0042] A first aspect of the invention relates to a 3D printing device for manufacturing workpieces. The invention is not limited to a specific field of use, but preferably can manufacture microstructured and / or nanostructured workpieces.

[0043] The device has a first radiation source which is designed to emit a first radiation for performing non-linear absorption polymerization, such as two-photon polymerization. In other words, the first radiation causes non-linear absorption polymerization of the starting material, as described by the introduced means. The first radiation source can take the form of a laser source, such as a fs laser. The wavelength of the first radiation can, for example, partially or completely cover the spectral range between 760 nm and 800 nm and / or the spectral range between 1520 nm and 1600 nm. The wavelength of the first radiation can also be within the wavelength range between 1000 nm and 1100 nm, for example in the case of using an ultrafast Yb-doped fiber laser as the first radiation source. A CW laser can also be used, for example with a wavelength of 405 nm. Depending on the target wavelength (i.e., the wavelength required for non-linear absorption polymerization) and the desired pulse width, other spectral ranges are possible.

[0044] In addition to the first radiation source, the device also has a second radiation source which is designed to emit a second radiation for performing optical coherence tomography. In other words, the second radiation can be used to perform optical coherence tomography in order to, for example, especially in-situ monitor non-linear absorption polymerization.

[0045] Depending on the circumstances, for example depending on the wavelength required for the decomposition of the photoinitiator in the starting material in non-linear absorption polymerization, the wavelength of the second radiation can be selected, for example such that the wavelength is less than the wavelength of the first radiation. In this way, the axial resolution of the optical coherence tomography can advantageously be increased, since the minimum separation of two points resolvable in the axial z direction is proportional to the square of the wavelength (i.e., Δz ∝ λ2).

[0046] Alternatively, the wavelength of the second radiation can be selected such that, for example, the wavelength is greater than the wavelength of the first radiation. The wavelength of the second radiation can preferably be greater than 500 nm. Thereby, in many cases, it is advantageously possible to avoid the second radiation being unfavorably absorbed in the starting material, since the photoinitiators typically used decompose at wavelengths between 350 nm and 500 nm. The wavelength of the second radiation can be, for example, between 750 nm and 1400 nm, preferably between 750 nm and 950 nm, further preferably between 780 nm and 920 nm, or alternatively between 900 nm and 1100 nm. Further preferably, the second radiation source should not be a pulsed radiation source and thus the second radiation should not be pulsed in order to avoid accidental triggering of non-linear absorption polymerization. The second radiation source can be designed as, for example, a tunable laser or a broadband radiation source, such as a superluminescent diode. The high spectral bandwidth of these radiation sources enables high axial resolution to be achieved when performing optical coherence tomography.

[0047] The first radiation can pass through a first beam path, and the second radiation can pass through a second beam path. In other words, the first radiation reaching the position in the starting material where non-linear absorption polymerization will occur from the first radiation source along the first beam path. In contrast, the second radiation reaches the site to be analyzed by optical coherence tomography along the second beam path from the second radiation source.

[0048] Optionally, the position in the starting material and the site to be analyzed by optical coherence tomography (i.e., the foci of the first and second radiations) can correspond to each other or be slightly different from each other in a controlled manner, e.g., so as to enable optical coherence tomography analysis before, during, or immediately after non-linear absorption polymerization.

[0049] Furthermore, it is provided that the first and second beam paths are formed completely independently of each other, i.e., separately. Thus, the first radiation can be guided along the first beam path until the first radiation irradiates the polymerization site of the starting material to be polymerized at the focus of the first radiation. The second radiation can be guided along the second beam path until the second radiation irradiates the analysis site of the sample to be analyzed at the focus of the second radiation. It is not envisaged to jointly utilize optical elements for the first and second beam paths, i.e., the first and second beam paths extend from the first or second radiation source independently of each other and separately to the foci of the first or second radiation.

[0050] This advantageously enables non-linear absorption polymerization and optical coherence tomography to be performed in parallel (i.e., simultaneously). Additionally, the separate formation of the two beam paths enables the wavelength of the second radiation used for performing optical coherence tomography to be selected independently of the wavelength of the first radiation used for performing non-linear absorption polymerization, and vice versa. Additionally, the foci of the first and second radiations can be selected identically or differently independently of each other. Thus, non-linear absorption polymerization and optical coherence tomography can be performed simultaneously at the same focus or at different foci. For example, different foci can be utilized so as to enable OCT analysis of the polymerized starting material. Since the polymerization process lasts for a certain period of time, it would be advantageous to offset the focus of the second radiation relative to the focus of the first radiation by, for example, a number of lateral writing lines for this purpose.

[0051] Furthermore, the independent formation of the beam paths can enable a more straightforward practical implementation compared to the case where the beam paths are at least partially jointly utilized, e.g., with respect to the available construction space. With the proposed 3D printing device, during the non-linear absorption polymerization process, analysis can be performed in-situ and in real-time by optical coherence tomography.

[0052] To reduce adverse back reflections from the surface of the substrate or the cuvette surface and thus to obtain a high signal-to-noise ratio in optical coherence tomography, i.e., on the second beam path, it is generally preferred to use an immersion objective for OCT imaging.

[0053] The numerical aperture NA of the objective on the second beam path can be selected differently according to specific applications. If, for example, OCT analysis in a small volume region with high resolution in the lateral and axial directions is desired, the objective can preferably have a high numerical aperture NA. In contrast, if OCT analysis in a region of the depth direction range is desired, the objective can preferably have a small numerical aperture NA, e.g., numerical aperture NA < 0.5 or even NA < 0.25. In the case of a small effective numerical aperture NA, the lateral image resolution decreases, but the image size in the axial direction increases, which is important for three-dimensional imaging of high-print structures.

[0054] Optical elements are provided on both the first beam path and the second beam path, and these optical elements can be imaging and are arranged along the optical axis. Examples of optical elements include lenses and mirrors. The objective is provided at the end of each beam path opposite the corresponding radiation source, and the objective focuses the first radiation or the second radiation onto the focal point. The focal point here corresponds to the location or the site where nonlinear absorption polymerization occurs or where analysis is performed by optical coherence tomography. For example, the focal point can be moved by a corresponding movable mirror provided on the beam path. The movement path of the focal point can also be referred to as a trajectory.

[0055] The objectives on the first beam path and the second beam path can take the form of immersion objectives. Optionally, the objective on the first beam path can be arranged or has been arranged such that the objective is immersed in the starting material.

[0056] In addition, the 3D printing device can in particular have the following:

[0057] - A positioning and holding unit for positioning and holding the starting material and optionally the substrate,

[0058] - A control unit that is set and designed to control the first radiation source and the second radiation source and to control the optical elements provided on the beam path so as to be able to cause changes in the focal point of the first radiation and the focal point of the second radiation,

[0059] - An analysis unit that is set and designed to generate an OCT image based on the second radiation,

[0060] - A storage unit that is set and designed to store the OCT image and / or to store the CAD model of the workpiece to be manufactured, and / or

[0061] - An evaluation unit, which is set up and designed to create a 3D OCT scan from a plurality of OCT images.

[0062] In different implementation variants, the first beam path and the second beam path can be formed such that the first radiation and the second radiation impinge on one another at an angle α, where 0° < α < 180°, for example 0° < α ≤ 90°.

[0063] The angle α here represents the smaller of the two angles formed between the principal beams of the first radiation and the second radiation at the actual or theoretical intersection point of the principal beams of the first radiation and the second radiation in the common plane of the principal beams of the first radiation and the second radiation. The intersection point and the plane here represent the respective last linear portions of the principal beams of the first radiation or the second radiation before they irradiate the focus. If the focus of the principal beam of the first radiation coincides with the focus of the principal beam of the second radiation, there is an actual intersection point, or the two foci are arranged such that the principal beams of the two radiations meet before reaching the respective foci. In contrast, if the foci of the principal beams of the two radiations are arranged such that the principal beam of the first radiation and the principal beam of the second radiation do not intersect, a theoretical intersection point is formed by the theoretical extension of the principal beams of the first radiation and the second radiation. The term "principal beam" refers to the beam that extends from an object or image point through the center of the pupil.

[0064] For example, the angle α can be selected or has been selected such that the first radiation and / or the second radiation does not pass through the substrate. Thus, it is also advantageously possible to print a substrate that is opaque to the first radiation and / or the second radiation used.

[0065] The angle α can preferably be selected or has been selected to minimize the amount of already polymerized starting material outside the observation volume through which the second radiation has to pass. Further preferably, the angle α can be selected or has been selected such that the second radiation does not extend through the already polymerized starting material.

[0066] For example, it can be the case that α = 90° - ω / 2, where ω is the object-side opening angle of the second beam path. In this way, difficulties and misinterpretations that might otherwise occur during the evaluation of the OCT signal can be avoided, for example, by superimposing a part of the second radiation that has passed through the already polymerized starting material with a part of the second radiation that has not passed through the already polymerized starting material.

[0067] For example, the angle α can be 90°. The perpendicular alignment of the first radiation and the second radiation with respect to one another can simplify the matching of the foci of the first radiation and the second radiation with respect to one another.

[0068] If the main beams of the first radiation and the second radiation are not in a common plane, the main beams of the first radiation and the second radiation can also be deflected to be aligned with each other, meaning that the main beams can be arranged offset from each other in the depth direction but do not intersect. In a 2D projection, the above observation can correspondingly be related to the angle α, i.e., the situation can be, for example, α = 90° - ω / 2.

[0069] The first beam path can preferably be formed and arranged such that the first radiation irradiates the substrate surface parallel to the normal of the substrate surface (i.e., substantially at a right angle). For this purpose, optionally, the device can have positioning means for the substrate to be printed in order to enable the substrate surface to be positioned relative to the first radiation. This can contribute to a higher accuracy of the structure to be printed.

[0070] With respect to the substrate surface to be printed, the second beam path can be designed and arranged such that the second radiation irradiates the substrate surface at an angle other than 90° to the normal of the substrate surface (i.e., not parallel to the substrate surface). For this purpose, optionally, the device can have positioning means for the substrate to be printed in order to enable the substrate surface to be positioned relative to the second radiation. In this way, adverse total internal reflections due to insufficient OCT signal evaluation can be avoided, since there is usually no exact correlation between the refractive index of the substrate and the refractive index of the immersion liquid or starting material. If a structure incorporated into a vertical sidewall with overhanging elements is to be analyzed, an angle of less than 90° to the substrate surface normal can be advantageous.

[0071] The starting material can be provided in a vessel transparent to the radiation used, such as a transparent cuvette, in which the 3D printing operation is carried out.

[0072] Alternatively, one or both objective lenses of the first beam path and the second beam path can also be directly immersed or have been directly immersed in the starting material. This has the advantage that a vessel opaque to the first radiation and / or the second radiation can be selected.

[0073] In a further implementation variant, the second beam path can be formed such that the second radiation irradiates the focus through the substrate to be printed.

[0074] The requirement for this implementation variant is that the substrate is sufficiently transparent to the second radiation. The working distance of the objective lens on the second beam path (which can preferably be in the form of an immersion objective lens) should preferably be selected such that the OCT image can pass through the substrate, which can have a substrate thickness of, for example, a few millimeters. In this case, a high working distance is also advantageous, since the working distance limits the maximum height of the printed structure that can be imaged.

[0075] Since an optically smooth surface is required at the entrance surface of the objective lens on the second beam path in order to avoid aberrations during imaging, the implementation variant in which the second radiation is guided through the substrate has the advantage that the substrate itself already forms this optically smooth surface and no separate optically smooth surface, such as in the form of a cuvette with an optically smooth surface, is required.

[0076] For example, the first radiation and the second radiation can irradiate their foci from opposite directions (e.g., substantially parallel to each other). In other words, the angle α at which the first radiation and the second radiation meet can be α = 180°. This has the advantage that the objective lenses of the first beam path and the second beam path can be arranged farther apart from each other, and thus the possibility of mechanical collision of the two objective lenses is reduced. In addition, due to the larger construction space, the choice of objective lenses is less restricted.

[0077] The first radiation and / or the second radiation can preferably irradiate the substrate surface at a right angle.

[0078] Alternatively or additionally, the first beam path can also be configured such that the first radiation irradiates the focus through the substrate to be printed ("through-substrate configuration"). This configuration increases the choice of starting materials because not all starting materials can be used in the "immersion" configuration in which the objective lens is immersed in the starting material without damaging the objective lens. In addition to the chemical compatibility between the starting material and the objective lens, in the case of the "immersion" configuration, the refractive index of the starting material should match the objective lens used, which cannot be satisfactorily achieved in all cases. In contrast, in the "through-substrate" configuration, potentially more types of starting materials with different refractive indices can be specifically printed. This is because in this configuration, the first radiation only covers a relatively short distance in the starting material, which minimizes aberrations even if the refractive indices do not match. However, in the case of the "through-substrate" configuration, the maximum height of 3D printing is limited, firstly due to aberrations occurring due to the printed structure, and secondly, the working distance of the objective lens used limits the structure height in order to avoid collision between the substrate and the objective lens.

[0079] Another aspect of the present invention relates to another 3D printing device for manufacturing a workpiece.

[0080] This other device has a first radiation source that is designed to emit a first radiation to perform nonlinear absorption polymerization. In other words, the first radiation causes nonlinear absorption polymerization of the starting material, as described by the introduced method. The first radiation source can be in the form of a laser source, such as a fs laser. The wavelength of the first radiation can, for example, cover the spectral range between 760 and 800 nm or the spectral range between 1520 and 1600 nm. Depending on the target wavelength (i.e., the wavelength required for nonlinear absorption polymerization) and the desired pulse width, other spectral ranges are also possible.

[0081] In addition to the first radiation source, the other device further has a second radiation source which is designed to emit a second radiation for performing optical coherence tomography. In other words, the second radiation can be used to perform optical coherence tomography so as to, for example, in particular, monitor non-linear absorption polymerization in situ.

[0082] Depending on the specific circumstances, for example, depending on the wavelength required for the decomposition of the photoinitiator in the starting material in non-linear absorption polymerization, the wavelength of the second radiation can be selected such that, for example, the wavelength is less than the wavelength of the first radiation. In this way, the axial resolution of the optical coherence tomography can be advantageously increased because the minimum separation between two points resolvable in the axial z-direction is proportional to the square of the wavelength (i.e., Δz ∝ λ2).

[0083] Alternatively, the wavelength of the second radiation can be selected such that, for example, the wavelength is greater than the wavelength of the first radiation. The wavelength of the second radiation can preferably be greater than 500 nm. Thereby, in many cases, it can be advantageously avoided that the second radiation is unfavorably absorbed in the starting material because the photoinitiators typically used decompose at wavelengths between 350 nm and 500 nm. The wavelength of the second radiation can be, for example, between 750 nm and 1400 nm, preferably between 750 nm and 920 nm, more preferably between 780 nm and 920 nm, or alternatively between 900 nm and 1100 nm. Further preferably, the second radiation source should not be a pulsed radiation source and thus the second radiation should not be pulsed radiation so as to avoid an accidental triggering of the non-linear absorption polymerization. The second radiation source can be designed as, for example, a tunable laser or a broadband radiation source, such as a superluminescent diode. The high spectral bandwidth of these radiation sources enables a high axial resolution to be achieved when performing optical coherence tomography.

[0084] The first radiation source and the second radiation source are arranged such that the optical system can be at least partially jointly utilized by the first radiation and the second radiation. The term "optical system" here refers to the entirety of the optical elements (such as lenses, mirrors, etc.) in their specific arrangement. If the first radiation and the second radiation at least partially jointly utilize the optical system, this means that both the first radiation and the second radiation can pass through these optical elements of the optical system. Here, the first beam path of the first radiation and the second beam path of the second radiation can be the same or formed differently from each other because, for example, the incident angles are selected differently or have been selected differently.

[0085] In addition, the other device has an objective lens which is arranged in the optical system and is designed to focus the first radiation and the second radiation onto a focus. In other words, both the first radiation and the second radiation are focused onto the starting material or the site to be analyzed using the same objective lens.

[0086] The optical system can be designed and arranged such that exclusively the first radiation or exclusively the second radiation can be focused onto a focal point by the objective lens. Alternatively, the optical system can be designed and arranged such that both the first radiation and the second radiation can be focused onto a focal point by the objective lens. The focal point here can be a common focal point, or different focal points can be or have been defined for the first radiation and the second radiation. In the case of different focal points, laterally different positions and / or axially different positions can be defined or have been defined. This can be possible, for example, via an inclination of the optical axes of the beam paths of the first radiation and the second radiation, for example, by means of a tiltable mirror or a scanning mirror or by means of a beam splitter on the beam path of the second radiation. In this way, for example, as already mentioned, OCT analysis of the polymerized starting material can be carried out.

[0087] Correspondingly, at the focal point, nonlinear absorption polymerization and / or optical coherence tomography analysis can be achieved at a specific junction point. For example, the focal point can be moved by means of a corresponding movable mirror arranged on the beam path. The objective lens can be designed as an immersion objective lens, or can be arranged or has been arranged such that the objective lens is immersed in the starting material.

[0088] In order to couple the first radiation and / or the second radiation into the optical system, the further device has an input coupling element. The input coupling element refers to an optically active component by means of which the first radiation and / or the second radiation first enters the optical system after being emitted by the first radiation source or the second radiation source, or the optically active component supplies the first radiation and / or the second radiation to the optical system. Thus, the input coupling element is not another optically active element that interacts with the first radiation or the second radiation after the first radiation or the second radiation has already entered or been coupled into the optical system. The input coupling element is selected from the group consisting of: a mechanically adjustable mirror, a current mirror, a polarization beam splitter, an intensity separator, and a partially coated mirror. Thus, the input coupling element does not take the form of a dichroic mirror. Two or more of the said input coupling elements can also be combined with each other.

[0089] In addition to the function of coupling the first radiation and / or the second radiation into the optical system, the input coupling element can also additionally serve as an output coupling element for the second radiation. This means that after the OCT analysis, i.e., after interacting with the sample to be examined and after corresponding reflections in the sample volume, the second radiation can be coupled out of the optical system again through the input coupling element, for example, by reflection in the direction of the OCT analysis unit.

[0090] By moving between two positions, both the mechanically adjustable mirror and the galvanometer mirror enable the first or second radiation to be alternately coupled into the optical system, since the surfaces reflecting the first or second radiation are correspondingly aligned. The alignment in the case of the mechanically adjustable mirror is achieved only by mechanical means, while the alignment in the case of the galvanometer mirror is achieved by a galvanometer drive.

[0091] An advantage that both the mechanically adjustable mirror and the galvanometer mirror have is that, in each case, only one of the two radiations is coupled into the optical system, and the interaction of the two radiations with each other or the interruption of the OCT analysis caused by the first radiation can be avoided. Additionally, the number and duration of the OCT analyses can be flexibly matched to the geometry and / or quality requirements of the part to be printed. For example, in the case of a simple geometry, it is not absolutely necessary to analyze each printed layer by optical coherence tomography, but a larger time interval can be chosen between two OCT analyses. This can have a positive impact on the time requirements for producing the part. Additionally, since fewer OCT analysis data are obtained, the computational requirements can be reduced. Therefore, a lower computing power in the OCT analysis unit and / or a lower storage size in the storage unit may be sufficient. The sequential execution of non-linear absorption polymerization and OCT analysis may also be advantageous in applications where the polymerization process takes some time, such that due to the high writing rate, the focus of the first radiation and thus also the focus of the second radiation have moved away from the time of complete polymerization. In this case, the non-linear absorption polymerization can be specifically targeted at the interruption of the OCT analysis, or the OCT analysis can be performed after the end of the writing process, such that the starting material for printing can be analyzed by optical coherence tomography after the starting material for printing has completely polymerized.

[0092] The polarization beam splitter, intensity separator, and partially coated mirror enable the first and second radiations to be simultaneously coupled into the optical system. Therefore, the OCT analysis and non-linear absorption polymerization can be performed simultaneously. This has the advantage that faults occurring during the polymerization process (e.g., due to proximity effects) are identified early, and corresponding measures, such as stopping the incorrect printing operation, can be initiated.

[0093] The mode of operation of the polarization beam splitter is based on using different linearly polarized radiations for the first and second radiations. For example, when the radiation with the first polarization state is transmitted, the radiation with the second polarization state is reflected.

[0094] The intensity separator refers to a beam splitter with a uniformly optically active coating that has a fixed transmission / reflection ratio, e.g., 80% transmittance / 20% reflectance, based on the wavelength of the first or second radiation. Thus, for example, 80% of the first radiation can be transmitted, while 20% of the first radiation is reflected. Correspondingly, 20% of the second radiation can be transmitted, while 80% of the second radiation is reflected.

[0095] In contrast, a partially coated mirror does not have a uniform optically active coating, but rather has different optically active coatings in different regions. For example, the central region of a partially coated mirror may have a coating with a reflectivity of 100% based on the wavelengths of the first and second radiations, while the reflectivity in the edge region is almost 0%, which means that the coating is almost 100% transmissive. Beam splitting can be achieved because, for example, the second radiation has a beam diameter smaller than that of the beam of the first radiation, such that the second radiation only irradiates the central region of the partially coated mirror, in which the second radiation is reflected to an extent of almost 100%. In contrast, the first radiation also irradiates the edge region of the partially coated mirror and can be correspondingly transmitted. In other words, the coating can block the first radiation and completely internally reflect the second radiation through a central occlusion that has a small impact on the spot shape.

[0096] The advantages of using a polarization beam splitter, an intensity separator, or a partially coated mirror are that the input coupling element is rigid and thus no mechanical wear is expected. Additionally, OCT analysis and nonlinear absorption polymerization can be performed simultaneously.

[0097] Compared to a dichroic mirror as an input coupling element, all variants of the described input coupling element have the advantage that the first and second radiations can have wavelengths that are close to each other. This in turn facilitates the selection of other optical elements of the optical system that must be suitable for both wavelengths.

[0098] With the proposed 3D printing device, during the nonlinear absorption polymerization process, analysis can advantageously be performed in situ and in real time by optical coherence tomography.

[0099] Additional optical elements can be provided in the optical system, and these optical elements can be imaging and are arranged along the optical axis. Examples of optical elements include lenses and mirrors.

[0100] Additionally, the 3D printing device can in particular have the following:

[0101] - A positioning and holding unit for positioning and holding the starting material and optionally the substrate,

[0102] - A control unit that is set up and designed to control the first radiation source and the second radiation source, control the optical elements provided in the optical system in order to be able to cause a change in the focus of the first radiation and the focus of the second radiation, and control the input coupling element in the form of a mechanically adjustable mirror or a current mirror in order to switch between the first radiation and the second radiation,

[0103] - An analysis unit, which is set up and designed to generate an OCT image based on the second radiation,

[0104] - A storage unit, which is set up and designed to store the OCT image and / or to store the CAD model of the workpiece to be manufactured, and / or

[0105] - An evaluation unit, which is set up and designed to create a 3D OCT scan from a plurality of OCT images.

[0106] In different execution variants, the device can have an analysis unit, which is set up and designed to generate an optical coherence tomography image based on the second radiation. The analysis unit here has a plurality of measurement channels, such that a plurality of analysis sites can be analyzed simultaneously by optical coherence tomography.

[0107] Here, each measurement channel in each case has a radiation source or a radiation source position and a focus. The "radiation source position" means that the radiation from the physical radiation source is spatially separated, such that each measurement channel is supplied with radiation at a specific position. For example, the device can have a device, such as a photon chip, which is designed to divide the second radiation from the second radiation source among a plurality of measurement channels (for example six measurement channels). Through these different measurement channels, the second radiation can be set or has been set at different lateral and / or axial positions and can be steered or has been steered to different analysis sites. Thus, each measurement channel can in each case form a different focus. The corresponding measurement channel serves as a detection channel for the return radiation after interaction with the sample to be analyzed.

[0108] The advantage of providing a plurality of measurement channels is that a plurality of analysis sites (for example sample regions) can be analyzed simultaneously, such as analysis sites where the starting material has not yet polymerized, analysis sites where the starting material is partially polymerized, and analysis sites where the starting material is fully polymerized. In this way, information about the process sequence can be obtained in terms of time. Additionally, the analysis duration can be shortened. A plurality of measurement channels in OCT analysis with a high numerical aperture and thus a limited A-scan depth enable simultaneous imaging of different depth regions.

[0109] In a further execution variant, the second radiation source can emit a wave that is constant over time (i.e., not a pulsed wave) as the second radiation.

[0110] In this way, it is advantageously possible to avoid the adverse polymerization of the starting material during the optical coherence tomography analysis. The second radiation sources used can be, for example, sufficiently weak continuous-wave light sources, which do not excite the photoinitiator in the starting material by single-photon absorption but by a combination of a plurality of superluminescent light-emitting diodes, which also advantageously achieve a wide spectral bandwidth and thus an axial resolution of only a few μm.

[0111] In a further implementation variant, the device can have a vessel for arranging the starting materials to be polymerized, wherein the vessel is designed to be at least partially transparent to the first radiation and / or the second radiation.

[0112] "Transparent" means that the first radiation and / or the second radiation can at least partially pass through the wall of the vessel, and thus can reach the interior enclosed by the vessel, and the starting materials to be polymerized can be arranged in this interior. For example, it can be provided that more than 70% or more than 80% or more than 90% or more than 95% of the first radiation and / or the second radiation passes through the wall of the vessel. For example, the vessel can take the form of a cuvette.

[0113] This at least partially transparent vessel enables the starting materials arranged in the vessel to be polymerized and / or analyzed through the wall of the vessel in a simple and convenient manner. This can contribute to simplifying the structural construction of the device, because the first radiation or the second radiation does not necessarily have to pass through an opening in the vessel to reach the starting materials.

[0114] The vessel can optionally be tightly sealed or be designed to be tightly sealable. This provides the option of polymerizing and / or analyzing starting materials that are sensitive to environmental influences (such as oxygen, moisture, etc.).

[0115] In a further implementation variant, the objective lens of the device can be formed and arranged such that the objective lens can be immersed in the starting materials to be polymerized.

[0116] This has the advantage that, for the arrangement of the starting materials, a vessel that is opaque to the first radiation and / or the second radiation can be selected.

[0117] Another aspect of the invention relates to a 3D printing method for manufacturing a workpiece by means of one of the devices described above. The method provides for non-linear absorption polymerization and optical coherence tomography to be carried out alternately or simultaneously (i.e., at the same time).

[0118] The observations made above for the 3D printing device are also used to describe the method according to the invention. The advantages of such a 3D printing device are correspondingly associated with the method according to the invention.

[0119] A further aspect of the invention relates to different methods in which optical coherence tomography is combined with non-linear absorption polymerization. These methods can be carried out by means of the 3D printing device described above. However, it should be noted that the methods described below can also be carried out independently of the 3D printing device described above, for example by means of a 3D printing device that combines non-linear absorption polymerization and optical coherence tomography with each other in some other way, or by means of a device that implements non-linear absorption polymerization and optical coherence tomography independently of each other.

[0120] The first method involves analyzing the quality of the starting material for non-linear absorption polymerization by optical coherence tomography, wherein the quality parameters of the starting material are determined by optical coherence tomography. This method can be carried out by one of the 3D printing devices described above. The optical coherence tomography can preferably be implemented three-dimensionally.

[0121] Optionally, the parameters of the non-linear absorption polymerization can be fixed with reference to the determined quality parameters. For this purpose, illustrative parameters include the (laser) power of the radiation (i.e., for example, the first radiation source or the first radiation) for non-linear absorption polymerization, the trajectory (i.e., the movement path along which the focus of the radiation for non-linear absorption polymerization moves across the starting material), and the writing time (e.g., the speed profile for executing the trajectory). The parameters fixed in such a way can then be used for non-linear absorption polymerization.

[0122] For example, at the location where contamination in the starting material has been detected, parameters other than the standard parameters for non-linear absorption polymerization can be used, such as a higher laser power, a longer writing time, etc. The trajectory can be changed so as to be able to write completely or in an improved manner at the contaminated location in the starting material.

[0123] Thus, in the case where the starting material is contaminated, the quality of the workpiece to be manufactured by non-linear absorption polymerization can be improved.

[0124] Alternatively or additionally, the optical coherence tomography parameters can also be fixed with reference to the determined quality parameters. For this purpose, illustrative parameters are the position of the focus of the radiation for optical coherence tomography and the fixing of an appropriate trajectory of the focus of the radiation for optical coherence tomography. The parameters fixed in such a way can then be used for optical coherence tomography.

[0125] For example, the focus of the radiation for optical coherence tomography and thus the analysis site can be fixed such that a more detailed analysis can be achieved by comparing with the uncontaminated location at the location where contamination in the starting material has been detected.

[0126] This can enable an improved analysis of the workpiece to be manufactured, such that a faulty workpiece can be identified early, for example, as early as during its manufacturing process, and countermeasures can be taken if necessary.

[0127] Another method involves verifying the position and / or alignment of the substrate to be printed by non-linear absorption polymerization, wherein the characteristic features of the substrate to be printed are identified by optical coherence tomography. This method can be carried out by one of the 3D printing devices described above. The optical coherence tomography can preferably be implemented three-dimensionally.

[0128] The specific features of the substrate to be printed refer to objects that can be clearly located on the surface or within the substrate, such as labels, boundaries, or contaminants, meaning that the position of the specific features relative to the substrate is known, and based on this, the position and / or alignment of the substrate can be determined. The term "position" describes here the absolute positioning of the substrate relative to a reference point (e.g., in a coordinate system). The term "alignment" describes the rotational position of the substrate relative to a reference point (e.g., in a coordinate system); in other words, the alignment determines, for example, which edge of a cubic substrate is where.

[0129] Such specific features can be identified by optical coherence tomography to assist in the printing alignment process and enable very precise positioning and / or alignment of the area to be printed relative to the focus of the radiation for nonlinear absorption polymerization. In this way, the quality of the workpiece to be manufactured can be improved, for example because the difference between the manufactured workpiece and the corresponding original can be reduced.

[0130] Another method involves determining the spatially resolved degree of conversion of nonlinear absorption polymerization by optical coherence tomography. This method can be performed by one of the 3D printing devices described above. Optical coherence tomography can preferably be implemented three-dimensionally. This advantageously enables a three-dimensionally resolved determination of the local degree of conversion.

[0131] Optionally, the parameters of the nonlinear absorption polymerization can be fixed with reference to the determined degree of conversion. For this purpose, illustrative parameters also include here the (laser) power, trajectory, and writing time of the radiation for nonlinear absorption polymerization.

[0132] For example, in the case of insufficient degree of conversion, parameters other than the standard parameters of the nonlinear absorption polymerization can be used, such as a higher laser power, a longer writing time, etc. The trajectory can be changed so as to cause, for example, the irradiation to restart or be extended at positions where the degree of conversion is too low. When writing subsequent layers during the layer-by-layer construction of the workpiece to be manufactured, the trajectory can also be fixed by (initially) omitting positions where the polymerization has not fully occurred (i.e., the degree of conversion is too low), for example to avoid an unfavorable polymerization process. Then the parameters fixed in such a way can be used for the nonlinear absorption polymerization.

[0133] Alternatively or additionally, the optical coherence tomography parameters can also be fixed with reference to the determined degree of conversion. For this purpose, illustrative parameters are the position of the focus of the radiation for optical coherence tomography, and the fixing of an appropriate trajectory of the focus of the radiation for optical coherence tomography. The parameters fixed in such a way can then be used for optical coherence tomography.

[0134] For example, the focus of the radiation for optical coherence tomography and thus the analysis site can be fixed such that, by comparison with positions having a sufficient degree of conversion, a more detailed analysis can be achieved at positions where the degree of conversion has been found to be too low.

[0135] This can enable an improved analysis of the workpiece to be manufactured such that faulty workpieces are identified early on, for example already during their manufacturing process, and countermeasures can be taken if necessary.

[0136] Thus, the degree of conversion can be influenced in a locally resolved manner and, together with it, the conversion-dependent properties of the manufactured workpiece.

[0137] Another method involves analyzing the structural clarity of structures produced by non-linear absorption polymerization. For this purpose, the refractive index distribution determined in a spatially resolved manner by optical coherence tomography (optionally in-situ) is used to determine the structural clarity. This method can be carried out by one of the 3D printing devices described above. The optical coherence tomography can preferably be implemented three-dimensionally.

[0138] Optionally, the parameters of the non-linear absorption polymerization can be fixed with reference to the determined structural clarity. For this purpose, exemplary parameters also include here the (laser) power, the track, and the writing time of the radiation for non-linear absorption polymerization. For example, in the case of insufficient structural clarity, parameters other than the standard parameters of non-linear absorption polymerization can be used, such as a higher laser power, a longer writing time, etc. The track can be changed in order to cause, for example, a restart or an extension of the irradiation at positions where the structural clarity is too low. The parameters fixed in such a way can then be used for non-linear absorption polymerization.

[0139] Alternatively or additionally, the optical coherence tomography parameters can also be fixed with reference to the determined structural clarity. For this purpose, exemplary parameters are the position of the focus of the radiation for optical coherence tomography and the fixation of a suitable track of the focus of the radiation for optical coherence tomography. The parameters fixed in such a way can then be used for optical coherence tomography.

[0140] For example, the focus of the radiation for optical coherence tomography and thus the analysis site can be fixed such that, by comparison with positions having sufficient structural clarity, a more detailed analysis can be achieved at positions where the structural clarity has been found to be too low.

[0141] This can enable an improved analysis of the workpiece to be manufactured such that faulty workpieces are identified early on, for example already during their manufacturing process, and countermeasures can be taken if necessary.

[0142] Thus, it is possible to locally resolve the structural clarity and, together therewith, the structure-clarity-dependent properties of the manufactured workpiece.

[0143] For example, it is possible to determine the in-situ degree of conversion and / or the structural clarity during the non-linear absorption polymerization. Then, depending on the determined degree of conversion and / or the determined structural clarity, for example taking into account geometric deformations and / or a degree of conversion different from the target function, the parameters of the non-linear absorption polymerization, such as the laser power and the writing time, can be controlled. Regions with insufficient polymerization can be irradiated again, for example by changing the trajectory of the focus of the radiation used for the non-linear absorption polymerization, in order to, for example, further increase the degree of conversion or change the refractive index and / or the elastic modulus. A closed-loop control circuit can be formed based on the OCT data.

[0144] Alternatively or additionally, it is also possible to control the optical coherence tomography parameters depending on, for example, the degree of conversion and / or the determined structural clarity determined according to the principles described above.

[0145] Another method relates to the three-dimensional reconstruction of a workpiece manufactured by non-linear absorption polymerization, wherein, during the manufacture of the workpiece, the order of printing achieved by non-linear absorption polymerization and the order of analysis achieved by optical coherence tomography are alternated. This method can be carried out by one of the 3D printing devices described above. The optical coherence tomography can preferably be carried out three-dimensionally.

[0146] Optionally, the number and / or the junctions of the order of analysis achieved by optical coherence tomography can be fixed depending on the expected morphology of the manufactured workpiece, the starting material of the manufactured workpiece and / or the parameters of the non-linear absorption polymerization. These non-linear absorption polymerization parameters can here be selected from the group comprising: the laser power, the trajectory and the writing time of the radiation used for the non-linear absorption polymerization. In this way, first, the printed workpiece can be accurately reconstructed and, second, the time requirements can be kept to a minimum.

[0147] The invention is explained in detail below by means of the figures and the accompanying description. In the figures:

[0148] Figure 1 Schematic illustration of a 3D printing device in a first working example;

[0149] Figure 2 Schematic illustration of a 3D printing device in a third working example;

[0150] Figure 3 Schematic illustration of a 3D printing device in a fourth working example;

[0151] Figure 4a 、 Figure 4b Illustrative flow chart of a 3D printing method for manufacturing a workpiece;

[0152] Figure 5 Images for illustrating illustrative methods for analyzing the quality of starting materials for non-linear absorption polymerization;

[0153] Figure 6a 、 Figure 6b Images for illustrating illustrative methods for determining the degree of conversion in non-linear absorption polymerization;

[0154] Figure 7a 、 Figure 7b Images for illustrating illustrative methods for analyzing the structural clarity of structures produced by non-linear absorption polymerization;

[0155] Figure 8a 、 Figure 8b Images for illustrating illustrative methods for three-dimensional reconstruction of workpieces manufactured by non-linear absorption polymerization; and

[0156] Figure 9 Images for illustrating illustrative methods for verifying the alignment of substrates to be printed by non-linear absorption polymerization.

[0157] In the examples described below, reference is made to the accompanying drawings, which form a part of these examples and in which specific embodiments in which the invention can be implemented are shown by way of illustration. It should be understood that other embodiments can be used and structural or logical changes can be made without departing from the scope of protection of the invention. It will be apparent that, unless otherwise specifically stated, the features of the different illustrative embodiments described herein can be combined with each other. Accordingly, the following detailed description should not be considered restrictive, and the scope of protection of the invention is defined by the appended claims. Where appropriate, the same or similar elements are given the same reference numerals in the drawings.

[0158] When used in a series of two or more elements, the expression “and / or” as used herein means that any of the listed elements can be used alone, or any combination of two or more of the listed elements can be used. For example, if a composition is described as containing components A, B, and / or C, the composition can contain: A alone; B alone; C alone; A and B together; A and C together; B and C together; or A, B, and C together.

[0159] It should be clearly pointed out that, for simplicity and for easier understanding of the components specified and claimed, these components are generally named as single components. Depending on the individual case, multiple corresponding components in series or in parallel can also be used to achieve the same function. Such a multi-component arrangement should also be clearly covered by the claims and the specification.

[0160] Figure 1Schematic diagram of a first working example of a 3D printing device 100 for manufacturing workpieces. The circular area is shown in an enlarged form.

[0161] The device 100 has a first radiation source 1 that emits a first radiation 2. By means of the first radiation 2, non-linear absorption polymerization, such as two-photon polymerization, can be carried out. For this purpose, the first radiation 1 is guided along a first beam path 5 until it irradiates a liquid starting material 12 to be polymerized at a focus 7, which is arranged in a vessel 13 in the form of a transparent cuvette. In the working example, the first radiation source 1 is designed as a fs laser radiation source, meaning that the first radiation 2 is fs laser radiation.

[0162] There are several optical elements for forming the first beam path 5, where, for the sake of clarity, Figure 1 only the mirror 17 is shown. Additionally, a first objective lens 15 is arranged on the first beam path 5, and the focus 7 of the first radiation 2 can be fixed by means of this first objective lens. The first objective lens 15 is designed as an immersion objective lens, i.e., an immersion liquid 14 is arranged between the front lens of the first objective lens 15 and the vessel 13 containing the starting material 12. In this way, the resolution ability can be increased, and smaller structures can be printed more precisely. The first objective lens 15 in the working example has a numerical aperture NA = 1.4 and a magnification ratio of 40x.

[0163]

[0161] The device 100 has a first radiation source 1 that emits a first radiation 2. By means of the first radiation 2, non-linear absorption polymerization, such as two-photon polymerization, can be carried out. For this purpose, the first radiation 1 is guided along a first beam path 5 until it irradiates a liquid starting material 12 to be polymerized at a focus 7, which is arranged in a vessel 13 in the form of a transparent cuvette. In the working example, the first radiation source 1 is designed as a fs laser radiation source, meaning that the first radiation 2 is fs laser radiation.

[0162] There are several optical elements for forming the first beam path 5, where, for the sake of clarity, Figure 1 only the mirror 17 is shown. Additionally, a first objective lens 15 is arranged on the first beam path 5, and the focus 7 of the first radiation 2 can be fixed by means of this first objective lens. The first objective lens 15 is designed as an immersion objective lens, i.e., an immersion liquid 14 is arranged between the front lens of the first objective lens 15 and the vessel 13 containing the starting material 12. In this way, the resolution ability can be increased, and smaller structures can be printed more precisely. The first objective lens 15 in the working example has a numerical aperture NA = 1.4 and a magnification ratio of 40x.

[0163] Furthermore, the device 100 has a second radiation source 3 that emits a second radiation 4. By means of the second radiation 4, optical coherence tomography analysis can be carried out. For this purpose, the second radiation 4 is guided along a second beam path 6 until the second radiation irradiates a sample to be analyzed, such as the liquid starting material 12 to be polymerized, at the focus 7. In the working example, the second radiation source 3 is in the form of a tunable laser or a superluminescent diode. The second beam path 6 is formed independently of the first beam path 5.

[0164] There are several optical elements for forming the second beam path 6, and these optical elements are not shown in detail for the sake of clarity. Figure 1 Additionally, a second objective lens 16 is arranged on the second beam path 6, and the focus 7 of the second radiation 4 can be fixed by means of this second objective lens. The focus 7 of the first radiation 2 can correspond to the focus 7 of the second radiation 4, as Figure 1 [[ID=ID=18]]shown. Alternatively, these two foci 7 can be different from each other. For example, based on the movement of the focus 7 of the first radiation 2, the focus 7 of the second radiation 4 can be located immediately in front of or immediately behind the focus 7 of the first radiation 2. This enables analysis by OCT immediately before or after non-linear absorption polymerization.

[0165] The second objective lens 16 is likewise designed as an immersion objective lens, i.e., the immersion liquid 14 is arranged between the front lens of the second objective lens 16 and the vessel 13 containing the starting material 12. In this way, by preventing adverse back reflections at the surface of the substrate 8 or the vessel 13, an improved signal-to-noise ratio can be achieved in optical coherence tomography. Regarding the numerical aperture NA of the second objective lens 16, NA < 0.25.

[0166] Optionally, the first beam path 5 and / or the second beam path 6 can be designed to be fiber-guided.

[0167] In the first working example, the first beam path 5 and the second beam path 6 are designed such that the first radiation 2 and the second radiation 4 irradiate the focus 7 substantially perpendicular to each other, which means that for the angle α, α = 90° holds. Neither the first radiation 2 nor the second radiation 4 passes through the substrate 8 on the way to the focus 7, so an opaque substrate 8 can also be used for the first radiation 2 and the second radiation 4. As is evident in Figure 1 the diameter of the first beam path 5 is significantly larger than the diameter of the second beam path 6.

[0168] The device 100 additionally has an analysis unit 23, which is accommodated in a housing shared with the second radiation source 3, and with this analysis unit, OCT analysis can be carried out at the focus 7 using the second radiation 4. In other words, the analysis unit 23 generates an OCT image based on the second radiation 4. Optionally, the analysis unit 23 can have a number of measurement channels such that several analysis sites can be analyzed simultaneously.

[0169] Furthermore, the device 100 can have additional units (not shown), such as a control unit, a storage unit, an evaluation unit, a positioning and holding unit.

[0170] In the second working example (not shown), for the angle α, α = 80° holds. For the object-side opening angle of the second objective lens 16 being ω = 20°, this angle α can be found as follows: α = 90° - ω / 2.

[0171] Figure 2 A third working example of a 3D printing device 100 for manufacturing a workpiece is shown in schematic form. Compared with the first and second working examples, the second radiation 4 passes through the substrate 8 to irradiate the focus 7. This requires the substrate 8 to have a material that is transparent to the second radiation 4. The angle α between the first radiation 2 and the second radiation 4 is α = 180°.

[0172] Another difference from the first working example is that the starting material 12 is not arranged in the vessel 13; rather, the first objective lens 15 is directly immersed in the starting material 12. Thus, the first objective lens 15 is also not in the form of an immersion objective lens. For the rest, reference is made to the description related to Figure 1 which.

[0173] Figure 3 Shows a fourth working example of a 3D printing device 300 for manufacturing workpieces.

[0174] The device 300 has a first radiation source 1 that emits a first radiation 2. By means of the first radiation 2, non-linear absorption polymerization can be carried out. For this purpose, the first radiation 1 is guided along an optical system 9 until at the focus 7 the first radiation irradiates the liquid starting material 12 to be polymerized. In the working example, the first radiation source 1 is designed as a fs laser radiation source, meaning that the first radiation 2 is fs laser radiation. Optionally, the first radiation 2 can be fiber-guided.

[0175] The optical system 9 includes a number of optical elements, among which Figure 3 A number of lenses 19 are shown. Additionally, the optical system 9 includes an objective lens 18 by means of which the focus 7 of the first radiation 2 can be fixed. By means of a galvanometer mirror 20 in the x direction and a galvanometer mirror 21 in the y direction, the surface of the sample can be scanned, for example, according to a definable CAD model of the workpiece to be manufactured. The objective lens 18 is directly immersed in the starting material 12. The objective lens 18 in the working example has a numerical aperture NA = 1.4 and a magnification ratio of 40x.

[0176] Additionally, the device 300 has a second radiation source 3 that emits a second radiation 4. By means of the second radiation 4, optical coherence tomography analysis can be carried out. For this purpose, the second radiation 4 is likewise guided in the optical system 9 until at the focus 7 the second radiation irradiates the sample to be analyzed, for example, the liquid starting material 12 to be polymerized. The second radiation source 3 is designed as a tunable laser or a superluminescent diode in this working example, meaning that the second radiation 4 is laser radiation. The second radiation 4 can be transmitted through a single-mode optical fiber and coupled into the optical system 9 via an optical fiber coupler 22.

[0177] The device 300 has an input coupling element 10 which, in this working example, takes the form of a polarization beam splitter, but alternatively can also take the form of a mechanically adjustable mirror, a galvanometer mirror, an intensity separator or a partially coated mirror. By means of the input coupling element 10 in the form of a polarization beam splitter, the first radiation 2 and the second radiation 4 can be simultaneously coupled into the optical system 9 in order to be able to carry out non-linear absorption polymerization and optical coherence tomography analysis simultaneously.

[0178] By means of the optical fiber coupler 22, the diameter of the light beam of the second radiation 4 can be fixed such that when the second radiation irradiates the input coupling element 10, the diameter of the light beam of the second radiation is smaller than the diameter of the light beam of the first radiation 2. By means of the controlled adjustment of the diameter of the light beam of the second radiation 4, the aperture of the objective lens 18 can be deliberately underexposed, i.e., the effective numerical aperture NA of the objective lens 18 for the second radiation 4 can be deliberately reduced.

[0179] The device 300 additionally has an analysis unit 23, which is accommodated in a housing shared with the second radiation source 3, and with which OCT analysis can be performed at the focus 7 using the second radiation 4. In other words, the analysis unit 23 generates an OCT image based on the second radiation 4. Optionally, the analysis unit 23 can have a number of measurement channels such that a number of analysis sites can be analyzed simultaneously. For this purpose, the second radiation 4 can be split between the individual measurement channels by means of a photon chip.

[0180] Furthermore, the device 300 can have additional units (not shown), such as a control unit, a storage unit, an evaluation unit, a positioning and holding unit.

[0181] Reference Figure 4a and 4b , the illustrative 3D printing methods 200, 400 for manufacturing workpieces are described in detail below. Figure 4a A flow chart of the 3D printing method 200 is shown, in which non-linear absorption polymerization S3 and optical coherence tomography S4 are performed simultaneously. The method 200 can be performed, for example, by one of the 3D printing devices referred to above Figure 1 or Figure 2 described.

[0182] After the start of the method 200, in method step S1, a substrate is provided onto which the material will be applied by non-linear absorption polymerization, and which, together with the applied material, forms the manufactured workpiece after the end of the method 200.

[0183] In method step S2, the substrate is positioned in the starting material. The starting material used can be, for example, the IP product of manufacturer Nanoscribe GmbH. Alternatives to this starting material include the following products: IP-S, IP-Q, IP-Visio, IP-n162, IP-L from manufacturer Nanoscribe GmbH; upphoto, updraft, upbrix, upsol, upopto from manufacturer UpNano GmbH.

[0184] The starting material typically has a liquid consistency and can be present in a vessel, such as a cuvette, a dish, etc. The vessel is designed here to be transparent to the radiation that is used to perform the non-linear absorption polymerization and to perform optical coherence tomography. To position the substrate, it is at least partially immersed in the starting material such that the starting material is present at least at the interface between the substrate and the polymer to be formed.

[0185] In method step S3, non-linear absorption polymerization of the starting material is carried out. For a more detailed illustration of this method step S3, reference is made to the above description of non-linear absorption polymerization.

[0186] In method step S4, optical coherence tomography is carried out at least partially (i.e., simultaneously) in time with method step S3. By means of optical coherence tomography, in-situ analysis can be carried out during the 3D printing process. For example, the degree of conversion of the non-linear absorption polymerization can be determined in a spatially resolved manner, as described below with reference to Figure 6a and Figure 6b stated. Alternatively or additionally, the structural clarity of the structure produced by non-linear absorption polymerization can be analyzed, as described below with reference to Figure 7a and Figure 7b stated. Alternatively or additionally, three-dimensional reconstruction of the manufactured workpiece can be carried out, as described below with reference to Figure 8, for example.

[0187] Before the start of the non-linear absorption polymerization, optical coherence tomography can be used to align the substrate to be printed by identifying specific points on the substrate to be printed. Alternatively or additionally, the quality of the starting material can be analyzed by means of optical coherence tomography, as described below with reference to Figure 5 stated.

[0188] Once all the desired structures have been produced by non-linear absorption polymerization, in method step S5 the excess starting material is washed off with a suitable solvent in order to obtain the manufactured workpiece. This concludes method 200.

[0189] Figure 4b The flowchart of another illustrative 3D printing method 400 for manufacturing a workpiece is shown, in which non-linear absorption polymerization S3 and optical coherence tomography S4 are carried out alternately with each other. Method 400 can be carried out, for example, by the 3D printing device described above with reference to Figure 3 where the input coupling element 10 takes the form of a mechanically adjustable mirror or a current mirror.

[0190] After the start of method 400, as described above with reference to Figure 4aPerform method steps S1 and S2 as in method 200 of the description. However, compared to method 200, the subsequent method steps S3 and S4 are not carried out in parallel over time, but rather alternately with each other. First, for example, in step S3, non-linear absorption polymerization can be carried out in order to produce a part of the desired structure from the starting material. At a suitable junction, the non-linear absorption polymerization is stopped and the method continues to method step S4, where analysis is carried out by optical coherence tomography. In this regard, reference is made to the description related to method step S4 of Figure 4a above.

[0191] Once the optical coherence tomography analysis is complete, the method returns to method step S3 and non-linear absorption polymerization continues. Method steps S3 and S4 alternate until all of the desired structure has been produced. Subsequently, in method step S5, the excess starting material is washed away in order to obtain the manufactured workpiece. This concludes method 400.

[0192] As an alternative, before the first execution of method step S3, method step S4, i.e., optical coherence tomography, can also be carried out first in order to, for example, analyze the starting material or check substrate alignment.

[0193] Refer to Figure 4a and Figure 4b The two 3D printing methods 200, 400 described above can also be combined with each other, i.e., method steps S3 and S4 can also be carried out partially simultaneously and partially alternately with each other.

[0194] All of the methods described below that include optical coherence tomography analysis are carried out using an in-house built 3D printing device. Here, optical coherence tomography (fiber-coupled superluminescent diode, model: Exalos EXC250002-00) is implemented in the Fourier range using a radiation source with a central wavelength of 845 nm and a nominal full width at half maximum (FWHM) of 135 nm, obtaining a measured axial resolution of 2.7 μm and a lateral resolution of 2.2 μm, as well as a maximum sensitivity of 105 dB. The scan rate or A-scan frequency for each B-scan (a 2D image formed by several A-scans, i.e., individual depth signals or individual scans) is 25 kHz.

[0195] Refer to Figure 5 for a method of analyzing the starting material for non-linear absorption polymerization by optical coherence tomography. In this method, the quality parameters of the starting material are determined by optical coherence tomography.

[0196] For the quality analysis of the starting material, before starting the non-linear absorption polymerization, volume images are recorded by optical coherence tomography (3D OCT scan) and evaluated. During the evaluation, one or more quality parameters are determined. These quality parameters can give information, for example, about the 3D OCT scan and thus whether the starting material under examination shows impurities, the size of the impurity fraction, the size and / or size distribution of the impurities, the distribution of the impurities within the starting material, etc. In addition, the 3D coordinates of the impurities can also be determined. The evaluation can be carried out in a computer-aided manner and optionally by means of artificial intelligence methods. Using this information, the suitability of the starting material for the planned 3D printing method can subsequently be evaluated. This evaluation can also be carried out in a computer-aided manner and optionally by means of artificial intelligence methods.

[0197] Figure 5 The 3D OCT scans of the aged starting material (left) and the fresh starting material (right) are shown, simplified in the form of a black-and-white line drawing. Both starting materials have been applied to a glass substrate to produce the 3D OCT scan.

[0198] In the case of the aged starting material (left image), the presence of many micron-scale internal inhomogeneities can be observed as contaminants, while the 3D OCT scan of the fresh starting material (right image), simplified in the form of a black-and-white line drawing, shows almost no contaminants. Depending on the requirements placed on the starting material, it can subsequently also be evaluated whether the aged starting material is still usable or should be discarded.

[0199] If the starting material analyzed as described above is used for 3D printing by non-linear absorption polymerization, the quality parameters determined in the quality analysis (such as the proportion and 3D coordinates of the impurities) can be used for the fixation and optimization of the non-linear absorption polymerization parameters (such as the so-called laser writing parameters). In this way, the morphology and properties of the manufactured workpiece may be affected. Workpieces with improved quality can be obtained.

[0200] It is also possible to re-examine the volume with the contaminants analyzed before the non-linear absorption polymerization after the non-linear absorption polymerization in order to evaluate the quality of the workpiece in the vicinity of the pre-detected impurities. In this way, findings about the influence of the presence of the contaminants on the quality of the manufactured workpiece can be inferred. In other words, quality analysis can be carried out before, during and / or after the non-linear absorption polymerization.

[0201] Reference Figure 6a and Figure 6b , a method for determining the degree of conversion in non-linear absorption polymerization by optical coherence tomography is described in detail below.

[0202] The basic idea is to determine the degree of conversion based on the refractive index. This is possible because polymerization increases the density of the material and thus causes an increase in the refractive index. If it is assumed that the refractive index profile consists of abrupt jumps, the local refractive index can be easily determined. In this case, if the refractive index is known, another refractive index can be determined based on the Fresnel reflection coefficient. For example, at the interface with a glass substrate, the refractive index of the glass substrate is known, and thus the refractive index of the adjacent polymer or monomer can be determined.

[0203] Therefore, the degree of conversion can be determined in-situ in a spatially resolved manner by optical coherence tomography, by detecting the optical coherence tomography signal generated by reflection at the interface between the polymer and the substrate, and first using this optical coherence tomography signal to determine the refractive index in a spatially resolved manner. Figure 6a 、 Figure 6b shows the influence of the power of the laser used for non-linear absorption polymerization on the refractive index of the resulting polymer, and thus on the degree of conversion for a low printing speed of 10 mm / s ( Figure 6a ) and a high printing speed of 30 mm / s ( Figure 6b ). In each case, the refractive index was determined in-situ by optical coherence tomography immediately after laser irradiation. The indicated error bars represent the standard deviation of six measurements on six different samples at the same printing speed and with the same laser power.

[0204] Figure 6a and Figure 6b The comparison shows that at higher printing speeds, a higher laser power is required to obtain the same refractive index. It is further evident that at a speed of 10 mm / s, the refractive index only increases insignificantly when the laser power exceeds and is higher than approximately 17.5 mW, so it seems that the maximum degree of conversion has been reached. At a speed of 30 mm / s, this is only the case at a laser power of approximately 22.5 mW. For both speeds, the maximum achievable refractive index is approximately the same, such that in principle the maximum degree of conversion can be achieved for a combination of speed and laser power.

[0205] The degree of conversion determined in a spatially resolved manner can be included in the fixation of non-linear absorption polymerization parameters. For example, when the degree of conversion is too low, the radiation power can be increased and / or the exposure time can be extended by reducing the printing speed in order to achieve a higher degree of conversion. Since the degree of conversion can be determined in-situ, the desired non-linear absorption polymerization parameters can be found more quickly. Therefore, it is advantageously possible to produce workpieces of higher quality in a shorter period of time.

[0206] Reference Figure 7a and Figure 7b details the method for analyzing the structural clarity of structures produced by non-linear absorption polymerization below.

[0207] To determine and evaluate structural clarity, volumetric images are recorded by optical coherence tomography (3D OCT scan) and evaluated. During the evaluation, the abruptness of the refractive index change at specific points is evaluated. If there is an abrupt change in the refractive index, a higher structural clarity can be concluded. In contrast, if the refractive index only changes gradually, the structural clarity is lower. The spatially resolved refractive index gradient determined by optical coherence tomography can be referred to for computer-aided and optionally artificial intelligence-based evaluation and determination of structural clarity.

[0208] The optical coherence tomography signal depends on the refractive index distribution between the unpolymerized starting material and the polymerized starting material and is thus affected by the presence of the interface between the unpolymerized starting material and the polymerized starting material. By using optical coherence tomography at selected positions along the interface and at regular time intervals, structural clarity can be monitored during 3D printing operations.

[0209] Figure 7a and Figure 7b Illustrative measurement data of workpieces that have been printed with different laser powers and immersed in the unpolymerized starting material are shown. Shown is the dependence of the OCT signal (optical coherence tomography signal) recorded at the interface formed between the unpolymerized starting material and the polymerized starting material on time. The OCT signal is plotted in logarithmic units of decibels (dB). Figure 7a Measurement data of a sample are shown, in which the polymerized starting material has been immersed in the liquid starting material after development. Figure 7b In-situ measurements are shown, in which the polymerized starting material has remained in the surrounding unpolymerized starting material during optical coherence tomography. Figure 7a and Figure 7b Measurement data of three different samples in each case are shown, for which nonlinear absorption polymerization was achieved at different laser powers, at Figure 7a 25 mW, 27 mW, and 29 mW in Figure 7b and 21 mW, 23 mW, and 25 mW in The starting material used in each case is the IP Figure 7a and Figure 7b Show how the OCT signal decreases over time according to the power of the writing laser. The following rules apply to the developed or polymerized structure ( Figure 7a ): As the laser power increases, there is a tendency for the exponential decay constant to decrease (flattening of the curve). The inventors attribute this signal behavior to the blurring of the refractive index transition between the polymer matrix and the monomer associated with the diffusion process. In Figure 7bAmong them, the dependence of the OCT signal on the laser power is not obvious. In this case, diffusion may be more complex and random.

[0210] Referring to FIG. 8, a method for three-dimensional reconstruction of the radiation exposure curve of the starting material during the non-linear absorption polymerization by optical coherence tomography is described in detail below.

[0211] As already explained by the introduction, the known morphological reconstruction methods after 3D printing operations assume defects in the printed workpiece that cause refractive index inhomogeneities. Figure 8a The left image of shows the 3D OCT scan reconstruction of a workpiece fabricated by non-linear absorption polymerization. The cross-hatching introduced during the fabrication process (the model of which is shown in Figure 8a (the right image)) results in refractive index inhomogeneities within the fabricated workpiece, such that the OCT scan performed after 3D printing can be fully utilized for reconstruction.

[0212] In contrast, for example, if the radiation used for optical coherence tomography is incident on the analysis site from the top (i.e., parallel to the sidewalls), the vertical sidewalls between regions with a constant refractive index in each section do not generate an optical coherence tomography signal. Therefore, they can only be reconstructed to a limited extent (if at all). Figure 8b The right image of shows the 3D OCT scan reconstruction of a workpiece fabricated by non-linear absorption polymerization. The unidirectional hatching introduced during the fabrication process (the model of which is shown in Figure 8b the left image of ) results in a uniform refractive index within the fabricated workpiece. Therefore, the vertical sidewalls cannot be detected by OCT scan and thus can only be incompletely reconstructed using the OCT scan performed after 3D printing.

[0213] The improved method for 3D reconstruction proposed here provides for in-situ OCT scanning, where the sequence of printing by non-linear absorption polymerization is alternated with the sequence of analysis by optical coherence tomography. Here, corresponding OCT signals are generated at the interfaces between materials with different refractive indices (e.g., at the interface between starting materials with a low degree of polymerization and a higher degree of polymerization), provided that the refractive index difference is large enough to backscatter light or radiation. The position of the interface changes as the 3D printing progresses, after which optical coherence tomography can be performed. A number of individual OCT scans can ultimately be used to reconstruct the entire morphology of the fabricated workpiece.

[0214] This has the advantage that even when the fabricated (i.e., finished) workpiece contains surfaces that do not generate any distinguishable OCT signals, a comprehensive 3D reconstruction can still be performed.

[0215] The frequency of the OCT scan can be or can have been fixed, for example depending on the printing time elapsed, i.e. the duration for which non-linear absorption polymerization has taken place. This has the advantage of simple metrological implementation.

[0216] Optionally, the frequency of the OCT scan can be optimized, i.e., made to match a specific printing scenario, for example depending on the expected morphology of the workpiece based on the attached CAD model, starting material, radiation power, etc. First, as many printed layers as possible should be imaged in order to reconstruct the printed workpiece with maximum accuracy. Second, the frequency of the OCT scan should be adjusted such that complete post-polymerization of the layers being examined is possible and adverse delays in 3D printing are avoided.

[0217] One strategy can be to adjust the frequency of the OCT scan according to the morphology to be imaged. For example, large volumes can be imaged at once, e.g. for reconstructing the morphology of inclined sidewalls relative to the incident radiation for performing optical coherence tomography. For the reconstruction of elements (such as sidewalls) that cannot be imaged with a single OCT capture, i.e., elements aligned in the direction of the incident radiation for performing optical coherence tomography, the frequency of the OCT scan can be increased.

[0218] The frequency of the OCT scan can be fixed, for example, using the analysis of the corresponding 3D (CAD) model before starting 3D printing and, if necessary, implemented in the control system for the 3D printing device.

[0219] Reference Figure 9 , a method for verifying the position and / or alignment of the substrate 8 to be printed by non-linear absorption polymerization is described in detail below. Figure 9 A transparent substrate 9 is shown, in which four cross-shaped distinctive marks 11 are introduced. The position and alignment of the distinctive marks 11 relative to the substrate 8 are known, so the position and alignment of the substrate 8 can be determined by the positioning of the distinctive features 11.

[0220] For this purpose, one or more of the distinctive features 11 of the substrate 8 to be printed are identified by optical coherence tomography (i.e., using two-dimensional or preferably three-dimensional OCT images).

[0221] Verification of the position and / or alignment by optical coherence tomography can be carried out especially in the cases described below.

[0222] When the area of the substrate to be printed cannot be imaged by the real-time camera of the 3D printing device, e.g. when it is at an inclined sidewall or when it is hidden, optical coherence tomography can be used to image the area of the substrate to be printed and determine the corresponding coordinates, and if appropriate, also the coordinates related to the section hiding the area of the substrate to be printed. This assumes that the absorption coefficient of the material of the hidden section does not impede the detection of the optical coherence tomography signal.

[0223] Optical coherence tomography can also be used to locate unique features 11 that cannot be imaged with a real-time camera, either because of the alignment of the substrate 8 or the morphology of the already printed sections, or because the unique features 11 are intentionally embedded within the substrate 8. For example, unique features 11 in the form of alignment marks have been directly written into the volume of a glass substrate using an fs laser, or alignment marks have been written by nonlinear absorption polymerization and then hidden with another photoresist to create a refractive index contrast below the surface.

[0224] Since optical coherence tomography responds very sensitively to refractive index contrasts and enables imaging below the surface of materials with low to medium absorption and scattering, i.e., provided that the backscattered signal corresponds to the sensitivity of the OCT system, optical coherence tomography can be used to verify the substrate position and / or substrate alignment within the arrangement of elements having the same morphology, where the elements differ only in their refractive index (as in the case of multi-material printing), or in their internal structure, e.g., the elements have different porosities.

[0225] Once the substrate position and / or substrate alignment has been verified, it can be determined whether the found substrate position and / or substrate alignment corresponds to the desired substrate position and / or substrate alignment. If this is not the case, the position and / or alignment of the substrate 8 can be changed accordingly and, if appropriate, verified again by optical coherence tomography.

[0226] In addition to the appended claims, the invention can be defined by the following clauses:

[0227] 14. A method for analyzing the quality of a starting material (12) for nonlinear absorption polymerization, wherein the quality parameters of the starting material (12) are determined by optical coherence tomography.

[0228] 15. The method according to clause 14, wherein the method is carried out by a 3D printing device (100, 300) as described in any one of claims 1 to 12.

[0229] 16. The method according to clause 14 or 15, wherein the optical coherence tomography is implemented in three dimensions.

[0230] 17. The method according to any one of clauses 14 to 16, wherein the parameters of the nonlinear absorption polymerization and / or the parameters of the optical coherence tomography are fixed with reference to the determined quality parameters.

[0231] 18. The method according to clause 17, wherein the parameters of the nonlinear absorption polymerization are selected from the group consisting of: laser power, trajectory, and writing time of the radiation for nonlinear absorption polymerization.

[0232] 19. The method as described in clause 17 or 18, wherein the parameters of the optical coherence tomography include the position of the focus of the radiation for the optical coherence tomography and / or the trajectory of the focus of the radiation for the optical coherence tomography.

[0233] 20. A method for verifying the position and / or alignment of a substrate (8) to be printed by non - linear absorption polymerization, wherein specific features (11) of the substrate (8) to be printed are identified by optical coherence tomography.

[0234] 21. The method as described in clause 20, wherein the method is carried out by a 3D printing device (100, 300) as described in any one of claims 1 to 12.

[0235] 22. The method as described in clause 20 or 21, wherein the focus of the radiation for the non - linear absorption polymerization matches the verified position and / or alignment of the substrate (8) to be printed.

[0236] 23. A method for determining the spatially resolved degree of conversion of non - linear absorption polymerization, wherein the degree of conversion is determined by optical coherence tomography.

[0237] 24. The method as described in clause 23, wherein the method is carried out by a 3D printing device (100, 300) as described in any one of claims 1 to 12.

[0238] 25. The method as described in clause 23 or 24, wherein the optical coherence tomography is implemented in three dimensions.

[0239] 26. The method as described in any one of clauses 23 to 25, wherein the parameters of the non - linear absorption polymerization and / or the parameters of the optical coherence tomography are fixed with reference to the determined degree of conversion.

[0240] 27. The method as described in clause 26, wherein the parameters of the non - linear absorption polymerization are selected from the group comprising: the laser power, trajectory, and writing time of the radiation for non - linear absorption polymerization.

[0241] 28. The method as described in clause 26 or 27, wherein the parameters of the optical coherence tomography include the position of the focus of the radiation for the optical coherence tomography and / or the trajectory of the focus of the radiation for the optical coherence tomography.

[0242] 29. A method for analyzing the structural clarity of a structure generated by non - linear absorption polymerization, wherein the structural clarity is determined based on the refractive index gradient determined by optical coherence tomography.

[0243] 30. The method as described in clause 29, wherein the method is carried out by means of a 3D printing device (100, 300) as described in any one of claims 1 to 12.

[0244] 31. The method as described in clause 29 or 30, wherein the optical coherence tomography is implemented in three dimensions.

[0245] 32. The method as described in any one of clauses 29 to 31, wherein the parameters of the non-linear absorption polymerization and / or the parameters of the optical coherence tomography are fixed with reference to the determined structural clarity.

[0246] 33. The method as described in clause 32, wherein the parameters of the non-linear absorption polymerization are selected from the group consisting of: laser power, trajectory and writing time of the radiation for non-linear absorption polymerization.

[0247] 34. The method as described in clause 32 or 33, wherein the parameters of the optical coherence tomography include the position of the focus of the radiation for the optical coherence tomography and / or the trajectory of the focus of the radiation for the optical coherence tomography.

[0248] 35. The method as described in any one of clauses 23 to 34, wherein the degree of conversion and / or the structural clarity are determined in-situ during the non-linear absorption polymerization process, and the parameters of the non-linear absorption polymerization and / or the parameters of the optical coherence tomography are controlled based on the determined degree of conversion and / or the determined structural clarity.

[0249] 36. A method for three-dimensional reconstruction of a workpiece manufactured by non-linear absorption polymerization, wherein during the manufacturing process of the workpiece, the order of printing achieved by the non-linear absorption polymerization alternates with the order of analysis achieved by the optical coherence tomography.

[0250] 37. The method as described in clause 36, wherein the method is carried out by means of a 3D printing device (100, 300) as described in any one of claims 1 to 12.

[0251] 38. The method as described in clause 36 or 37, wherein the number and / or the junction points of the orders of analysis achieved by the optical coherence tomography are fixed based on the expected morphology of the manufactured workpiece, the starting material of the manufactured workpiece and / or the parameters of the non-linear absorption polymerization.

[0252] List of Reference Numerals

[0253] 1. First radiation source

[0254] 2. First radiation

[0255] 3. Second radiation source

[0256] 4. Second radiation

[0257] 5. First light beam path

[0258] 6. Second light beam path

[0259] 7. Focus

[0260] 8. Substrate

[0261] 9. Optical system

[0262] 10. Input coupling element

[0263] 11. Characteristic feature

[0264] 12. Starting material

[0265] 13. Vessel

[0266] 14. Immersion liquid

[0267] 15. First objective lens

[0268] 16. Second objective lens

[0269] 17. Mirror

[0270] 18. Objective lens

[0271] 19. Lens

[0272] 20. Current mirror, x - direction

[0273] 21. Current mirror, y - direction

[0274] 22. Optical fiber coupler

[0275] 23. Analysis unit

[0276] 100. 3D printing device ?

[0277] 200. 3D printing method

[0278] 300. 3D printing device

[0279] 400. 3D printing method

[0280] 3D Three - dimensional

[0281] CAD Computer - aided design

[0282] CW Continuous wave

[0283] NA Numerical aperture

[0284] fs Femtosecond

[0285] Note: There seems to be a question mark in the translation of "100. 3D printing device " in the original text. It might be a formatting or content error in the original. If this is a known issue, it should be corrected in the source for more accurate translation.Optical Coherence Tomography (OCT)

[0286] S1 Provide a substrate

[0287] S2 Position the substrate in the starting material

[0288] S3 Perform non - linear absorption polymerization

[0289] S4 Perform optical coherence tomography

[0290] S5 Wash away the excess starting material

[0291] α The angle at which the first and second radiations meet

[0292] ω The opening angle of the second beam path

Claims

1. A 3D printing device (100) for manufacturing a workpiece, the device (100) having: - a first radiation source (1) designed to emit a first radiation (2) to perform non-linear absorption polymerization, and - a second radiation source (3) designed to emit a second radiation (4) to perform optical coherence tomography, Among them, The first radiation (2) passes through a first beam path (5), and the second radiation (4) passes through a second beam path (6), and wherein the first beam path (5) and the second beam path (6) are formed completely independently of each other.

2. The device (100) according to claim 1, wherein, The first beam path (5) and the second beam path (6) are formed such that the first radiation (2) and the second radiation (4) are incident on each other at an angle α, where 0° < α < 180°.

3. The device (100) according to claim 1, wherein, The first beam path (5) and the second beam path (6) are formed such that the first radiation (2) and the second radiation (4) are incident on each other at an angle α, where 0° < α ≤ 90°.

4. The device (100) according to claim 2 or 3, wherein, α = 90° - ω / 2, where ω is the object-side opening angle of the second beam path (6).

5. The device (100) according to any one of claims 1 to 3, wherein, The first beam path (5) is formed such that the first radiation (2) irradiates the substrate surface parallel to the normal of the substrate surface of the substrate (8) to be printed.

6. The device (100) according to any one of claims 1 to 3, wherein, The second beam path (6) is formed such that the second radiation (4) irradiates the substrate surface at an angle other than 90° with respect to the normal of the substrate surface of the substrate (8) to be printed.

7. The device (100) according to claim 6, wherein, The second beam path (6) is formed such that the second radiation (4) irradiates the substrate surface at an angle less than 90° with respect to the normal of the substrate surface of the substrate (8) to be printed.

8. The device (100) according to any one of claims 1 to 3, wherein The second beam path (6) is formed such that the second radiation (4) passes through the substrate (8) to be printed and irradiates the focal point (7).

9. The device (100) according to any one of claims 1 to 3, having: - an analysis unit (23) which is arranged and designed to generate an optical coherence tomography image based on the second radiation (4), wherein, The analysis unit (23) has a plurality of measurement channels such that a plurality of analysis sites can be analyzed simultaneously by optical coherence tomography.

10. The device (100) according to any one of claims 1 to 3, wherein, The second radiation source (3) emits a wave that is constant over time as the second radiation.

11. The device (100) according to any one of claims 1 to 3, having: - A vessel (13) for positioning starting materials (12) to be aggregated, wherein, The vessel (13) is designed to be at least partially transparent to the first radiation (2) and / or the second radiation (4).

12. The apparatus (100) according to any one of claims 1 to 3, wherein, The objective lens (18) of the device (100) is designed and arranged such that the objective lens can be immersed in the starting material (12) to be polymerized.

13. A 3D printing device (300) for manufacturing a workpiece, the device (300) having: - a first radiation source (1) designed to emit a first radiation (2) to perform non-linear absorption polymerization, - a second radiation source (3) designed to emit a second radiation (4) to perform optical coherence tomography, Among them, The first radiation source (1) and the second radiation source (3) are arranged such that the optical system (9) can be at least partially jointly utilized by the first radiation (2) and the second radiation (4), - An objective lens (18) that is provided in the optical system (9) and is designed to focus the first radiation and the second radiation to a focal point; and - An input coupling element (10) that is designed to couple the first radiation (2) and / or the second radiation (4) into the optical system (9), where the input coupling element (10) is selected from the group consisting of: a mechanically adjustable mirror, a current mirror, a polarization beam splitter, an intensity separator, and a partially coated mirror.

14. The device (300) according to claim 13, having: - an analysis unit (23) which is arranged and designed to generate an optical coherence tomography image based on the second radiation (4), wherein The analysis unit (23) has a plurality of measurement channels such that a plurality of analysis sites can be analyzed simultaneously by optical coherence tomography.

15. The device (300) according to claim 13 or 14, wherein, The second radiation source (3) emits a wave that is constant over time as the second radiation.

16. The device (300) according to claim 13 or 14, having: - A vessel (13) for positioning starting materials (12) to be aggregated, wherein, The vessel (13) is designed to be at least partially transparent to the first radiation (2) and / or the second radiation (4).

17. The device (300) according to claim 13 or 14, wherein, The objective lens (18) of the device (300) is designed and arranged such that the objective lens can be immersed in the starting material (12) to be polymerized.

18. A 3D printing method (200, 400) for manufacturing a workpiece by means of a device (100, 300) as described in any one of the preceding claims, wherein, Nonlinear absorption polymerization (S3) and optical coherence tomography (S4) are alternated with each other or performed simultaneously.

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