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

Description

[0001] The invention relates to 3D printing devices and a 3D printing method for producing a workpiece.

[0002] Nonlinear absorption polymerization is an additive manufacturing (3D printing) process in which a three-dimensional workpiece is built layer by layer through the targeted polymerization of a starting material with monomers and / or oligomers. The starting material can also be referred to as a photoresist.

[0003] Irradiation with photons, typically emitted by a laser, causes the decomposition of a photoinitiator into radicals, which cause radical polymerization of the monomers and / or oligomers to form a polymer. Unpolymerized monomers and / or oligomers can then be removed, e.g., washed out, to produce the desired polymer material workpiece. This washing step is also referred to as developing the photoresist. The polymer material can be modified or converted as needed using subsequent chemical processes.

[0004] Femtosecond lasers, i.e. lasers whose light pulses have a duration in the femtosecond range, are typically used as lasers. In contrast to photopolymerization, which has been known for some time, nonlinear absorption polymerization enables the creation of very small structural sizes in the range of approximately 100 nm. The reason for this is that for radical formation, several, e.g. two, photons must be absorbed non-linearly, which is only possible at the position or in the reaction volume where the energy input is sufficiently high, i.e. in the immediate focal area of ​​the laser. The input power is proportional to (optical intensity) N< , where N = 1 for linear mechanisms and N > 1 for nonlinear mechanisms.

[0005] By controlling the laser focus in the lateral direction and along the depth axis, the location of the polymerization reaction can be precisely defined, allowing the desired three-dimensional workpiece to be built step by step. This process is also known as direct laser writing (DLW).

[0006] Two major subgroups of nonlinear absorption polymerization are two-photon polymerization and two-step absorption polymerization. While two-photon polymerization relies on the simultaneous absorption of two photons, two-step absorption polymerization involves absorption in two consecutive steps, as described in HAHN, V., MESSER, T., BOJANOWSKI, NM, CURTICEAN, ER, WACKER, I., SCHRÖDER, RR, BLASCO, E., WEGENER, M. Two-step absorption instead of two-photon absorption in 3D nanoprinting. Nat. Photon., 2021 15, 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 without the light field. Its lifetime is usually determined by non-radiative processes and can be orders of magnitude longer than femtoseconds or picoseconds.Unless otherwise stated, the term nonlinear absorption polymerization is used here as a generic term for two-photon polymerization and two-step absorption polymerization.

[0007] There are several challenges associated with the practical application of nonlinear absorption polymerization, which are outlined below.

[0008] Impurities in the source material can impair the quality of the printed structures and thus of the finished workpiece. For example, impurities can lead to the formation of undesired light scattering centers in the path of the radiation intended to induce nonlinear absorption polymerization, e.g., in the path of a femtosecond (fs) laser beam. Alternatively or additionally, impurities can cause local inhomogeneities within the voxels at the focus of the radiation, e.g., at the laser focus. This, in turn, can lead to undesirably high surface roughness, shape deviations with respect to the desired 3D model, and / or to local inhomogeneities, e.g., in the optical properties, e.g., the refractive index, or the mechanical properties of the printed material and thus of the finished workpiece.

[0009] Possible contaminants can be both extrinsic particles, e.g., dust particles, and intrinsic inhomogeneities that form within the starting material over time.

[0010] There is therefore a need to find a method to analyze impurities in the starting material.

[0011] While some applications require printing a single element on a flat and horizontally aligned substrate, others require printing a structure, for example: onto a freeform surface, onto sidewalls, e.g. the edge of a substrate as described in JAYNE, RK, KARAKAN, M. Ç., ZHANG, K., PIERCE, N., MICHAS, C., BISHOP, DJ, CHEN, CS, EKINCI, KL, WHITE, AE Direct laser writing for cardiac tissue engineering: a microfluidic heart on a chip with integrated transducers. Lab Chip, 2021, 21(9), 1724-1737, onto the tip of an optical fiber, e.g. as described in GISSIBL, T., THIELE, S., HERKOMMER, A., GIESSEN, H. Sub-micrometre accurate free-form optics by three-dimensional printing on single-mode fibers. Nat Commun, 2016, 7, 11763, into semi-closed cavities or other hard-to-reach areas, e.g. B. to functionalize a microfluidic channel ALSHARHAN, AT, ACEVEDO, R., WARREN, R., SOCHOL, RD 3D microfluidics via cyclic olefin polymer-based in situ direct laser writing.Lab Chip, 2019, 19(17), 2799-2810), next to and in already existing parts manufactured by nonlinear absorption polymerization or by other means (LAMONT, AC, RESTAINO, MA, KIM, MJ, SOCHOL, RD A facile multi-material direct laser writing strategy. Lab Chip, 2019, 19(14), 2340-2345), which can only be identified by their refractive index or by volumetric information, e.g. sequential printing of similar parts with alternating photoresists.

[0012] A combination of the above requirements is also possible. In these cases, initial positioning and / or alignment of the substrate can be difficult, even when using alignment marks or a single real-time (live-view) camera in the 3D printer.

[0013] There is therefore a need to find a method that can improve the positioning and / or alignment of a substrate to be printed.

[0014] It is known that many properties of the resulting polymer depend on the local degree of conversion (abbreviated "DC"). The degree of conversion can be defined as the mass percentage of the starting material (monomers and / or oligomers) that have been covalently bonded to form the product (polymer). Determining and controlling the degree of conversion is crucial because this parameter affects the mechanical properties, such as the elastic modulus, the optical properties, such as the refractive index, and / or the thermal properties, such as thermal expansion, of the manufactured workpiece. Adjusting the degree of conversion on the micrometer scale or even below also enables the production of, for example, graded-index optics (abbreviated "GRIN") and mechanical parts with specific elastic modulus and stiffness gradients.

[0015] The local conversion efficiency depends on the locally applied power of the radiation source, e.g., the laser power, and the local exposure time during 3D printing. Therefore, information about the conversion efficiency can be derived by measuring the local refractive index. When analyzing the conversion efficiency of test samples ex situ, e.g., using spectroscopic ellipsometry, optimizing the applied radiation power and exposure time can be very time-consuming.

[0016] There is therefore a need to find an improved method for determining the degree of conversion. The possibility of in-situ analysis of the degree of conversion would be desirable.

[0017] Furthermore, it would be helpful to be able to determine the degree of conversion within the printed photoresist in a spatially resolved manner, since the degree of conversion can vary intentionally or unintentionally in the micrometer range or less.

[0018] Immediately after 3D printing, the diffusion of monomers into an interface between the unpolymerized and the exposed feedstock can impair the structural sharpness of the printed workpiece. The extent and time dependence of this diffusion process depend particularly on the type of feedstock used and the parameters of nonlinear absorption polymerization, such as the laser writing parameters. Without an in situ control method, the consequences of monomer diffusion can only be analyzed after excess feedstock has been washed out, i.e., after the photoresist has been developed, which is particularly disadvantageous for long print runs.

[0019] It would therefore be desirable to have a method that enables an in-situ analysis of the structural sharpness, i.e. an analysis while the printing process is still ongoing.

[0020] 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 a printed workpiece to deviate from its target model, e.g., a CAD model (CAD stands for computer-aided design). Proximity effects are effects caused by the presence of already printed or polymerized structures in the immediate vicinity. These include, among other things, a reduction in the minimum radiation dose required for polymerization, which can lead to blurring of fine structures due to a less spatially confined polymerization reaction, as well as a reduction in the radiation dose above which damage to existing neighboring structures can occur, as in SAHA, SK, DIVIN, C. CUADRA, JA, PANAS, RMEffect of Proximity of Features on the Damage Threshold during submicron Additive Manufacturing via Two-Photon Polymerization, J. Micro Nano-Manuf., Sep 2017, 5(3): 031002.

[0021] The 3D reconstruction of the actual morphology using in-situ optical coherence tomography measurements has been previously reported, e.g., in TASHMAN, JW, SHIWARSKI, DJ, RUESCH, A., LANNI, F., KAINERSTORFER, J., FEINBERG, AW 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 and WO 2020 / 150 251 A1. However, the approaches described in these two documents rely on imperfections within the printed regions, which lead to an optical coherence tomography signal in ideally homogeneous regions. Without these imperfections, the reconstruction of certain morphologies after the printing process would be impossible.

[0022] It would therefore be desirable to have a method that avoids the disadvantages mentioned above and enables 3D reconstruction even without the presence of imperfections.

[0023] A variety of methods for characterizing both workpieces produced by two-photon polymerization and two-photon polymerization processes are known from the state of the art.

[0024] An overview of ex situ and in situ metrology solutions enabling the inspection of microstructures generated by two-photon polymerization can be found in LAFRATTA, CN, BALDACCHINI, T. Two-photon polymerization metrology: Characterization methods of mechanisms and microstructures. Micromachines, 2017, 8(4), 101. The presented methods are particularly used to analyze the structural dimensions of laser-etched materials, e.g., by ex situ scanning electron microscopy, their surface roughness, e.g., by ex situ atomic force microscopy, their shape deviation from the desired 3D model due to shrinkage during photopolymerization or photoresist development, and their degree of conversion.

[0025] A standard method for extrapolating the degree of conversion is differential scanning calorimetry (abbreviated "DSC"). However, it is not suitable for workpieces produced by nonlinear absorption polymerization because it requires a minimum weight, cannot be performed in situ, and is usually associated with damage to the sample being examined. Fourier transform infrared spectroscopy (abbreviated "FTIR") can also be used to determine the degree of conversion, but it faces several limitations in nonlinear absorption polymerization, particularly with regard to poor spatial resolution.

[0026] From JIANG, LJ, ZHOU, YS, XIONG, W., GAO, Y., HUANG, Optics letters, 2014, 39(10), 3034-3037, in particularAbbildung 3 , und 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, insbesondere Abbildung 4 , the use of Raman (micro)spectroscopy to determine and image the degree of conversion on the micrometer scale is demonstrated. The degree of conversion can be derived from the integrated peak intensity change of the C=C bonds that are converted into C–C bonds during polymerization. Although Raman (micro)spectroscopy enables non-destructive in situ testing, its applicability is hampered by a low signal-to-noise ratio, which requires either high laser powers or long acquisition times. Acquisition times can be several hours for imaging printed parts measuring tens of micrometers, which is incompatible with real-time analysis during nonlinear absorption polymerization.

[0027] These limitations can be overcome by coherent anti-Stokes Raman scattering (CARS) microscopy, where the laser beam used to fabricate the workpiece can also be used to generate the pump and Stokes beams for CARS imaging. The signal intensity 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 Prince, R., Fan, P., Lu, Y., Baldacchini, T., Potma, E.O. Visualizing TPP structures with coherent Raman scattering microscopy. 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 using a phase determination algorithm and then applying the approach for Raman (micro)spectroscopy described above.CARS has been mentioned as a potential in situ process monitoring method due to its acquisition rate on the order of milliseconds and its sample volume close to the size of the printed voxels, see BALDACCHINI, T., ZIMMERLEY, M., KUO, CH, POTMA, EO, ZADOYAN, R. Characterization of microstructures fabricated by two-photon polymerization using coherent anti-stokes Raman scattering microscopy, J. Phys. Chem. B, 2009, 113(38), 12663-12668 and WO 2011 / 136 919 A1.

[0028] Finally, optical coherence tomography and quantitative phase imaging were combined to visualize the time- and spatially resolved degree of curing in polymer droplets (DONG, B., PAN, B. Visualizing curing process inside polymers. Appl. Phys. Lett., 2020, 116(5), 054103). However, a prerequisite for this approach is that the backscattered light is collected from the bulk of the polymer droplets, which may require the introduction of additional light-scattering nanoparticles into the polymer medium.

[0029] Optical coherence tomography (OCT for short) is an imaging technique for generating and displaying high-resolution, three-dimensional images, for example, from the various depth layers of a sample. For this purpose, electromagnetic radiation, e.g., infrared radiation with a wavelength of approximately 800 to 1400 nm, which has a short coherence length, is split into two beams or bundles of rays in a beam splitter. One of the beams is directed onto the sample to be examined, and the other beam serves as a reference and traverses a reference path. The beam reflected from the sample is superimposed on the reference beam. From the resulting interference signal, an image can then be generated that shows the structures of the sample along the depth axis. Three-dimensional optical coherence tomography images, i.e.,OCT images can be generated from the two-dimensional depth signals.

[0030] For optical coherence tomography, infrared radiation with a wavelength of approximately 800 nm to 1400 nm or visible light with a wavelength between approximately 400 nm and 800 nm can be used. The use of infrared radiation with a longer wavelength, e.g., up to approximately 2 µm, would also be possible in principle. The use of visible light has the advantage of higher spatial resolution, but can lead to problems with undesirable absorption in the source material.

[0031] OCT has previously been proposed as an online monitoring system for general application in 3D printing processes due to its non-destructive applicability and high penetration depth into low-absorbing materials (US 10 649 439 B2). One application involved comparing the actual printed structure with the model's design parameters. This strategy has also been pursued in the field of bioprinting to achieve in situ process monitoring using OCT in conjunction 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, JW, SHIWARSKI, DJ, RUESCH, A., LANNI, F., KAINERSTORFER, J., FEINBERG, AW 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). Using measurement software, the deviations of the 3D OCT reconstruction from the original 3D model were clearly visible (see Figure S5 in TASHMAN, JW, SHIWARSKI, DJ, RUESCH, A., LANNI, F., KAINERSTORFER, J., FEINBERG, AW 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).

[0032] US 2016 / 0 023 403 A1 describes the use of OCT as an image processing unit to check the conformity of the successively applied layers in an additive manufacturing process and to correct possible deviations.

[0033] US 2019 / 0 163 067 A1 discloses the use of OCT as a possible metrological method for characterizing surface topography before printing, especially for non-planar substrates.

[0034] WO 2020 / 150251 A1, corresponding to the preamble of independent claims 1 and 8, discloses a special OCT implementation, polarization-sensitive OCT (PS-OCT), for monitoring the two-photon polymerization process. PS-OCT was chosen for this purpose because of its ability to provide information about the birefringence of the cured material, which is cited as a common problem in 3D printing of polymers. However, to determine the curing volume and degree under different curing conditions, an analysis using a high-speed camera and Raman spectroscopy was proposed. The proposed arrangement of the radiation sources for two-photon polymerization and PS-OCT envisages the shared use of optical elements. A dichroic mirror is provided as a beam splitter, which, however, requires the use of radiation sources with significantly different wavelengths.This limits the selection of other optical elements that can be used together, as these must be suitable for both wavelengths.

[0035] Against this background, it is the object of the invention to provide devices and methods which enable improved in-situ process monitoring of a nonlinear absorption polymerization process and the most accurate possible analysis of the starting materials, products and substrates used in such a process.

[0036] This problem is solved by the subject matter of the independent claims. The dependent claims relate to embodiments of the inventive solutions.

[0037] A first aspect of the invention relates to a 3D printing device for manufacturing a workpiece. The invention is not limited to a particular field of application, but preferably micro- and / or nanostructured workpieces can be manufactured.

[0038] The device comprises a first radiation source configured to emit first radiation for carrying out a nonlinear absorption polymerization, for example, a two-photon polymerization. In other words, the first radiation causes a nonlinear absorption polymerization of a starting material as described above. The first radiation source can be configured as a laser light source, for example, as an fs laser. The wavelength of the first radiation can, for example, partially or completely cover the spectral range between 760 and 800 nm and / or the spectral range between 1520 and 1600 nm. The wavelength of the first radiation can also be in the wavelength range between 1000 and 1100 nm, e.g., when using an ultrafast Yb-doped fiber laser as the first radiation source. The use of a CW laser, e.g., with a wavelength of 405 nm, is also possible.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.

[0039] In addition to the first radiation source, the device comprises a second radiation source configured to emit a second radiation source for performing optical coherence tomography. In other words, optical coherence tomography can be performed using the second radiation, for example, to monitor nonlinear absorption polymerization, particularly in situ.

[0040] Depending on specific conditions, e.g., depending on the wavelength required for the decomposition of the photoinitiator in the starting material during nonlinear absorption polymerization, the wavelength of the second radiation can, for example, be selected such that it is shorter than the wavelength of the first radiation. This can advantageously increase the axial resolution of optical coherence tomography, since the minimum distance between two resolvable points in the axial z-direction is proportional to the square of the wavelength, i.e., Δz ∝ λ 2< .

[0041] Alternatively, the wavelength of the second radiation can, for example, be selected such that it is greater than the wavelength of the first radiation. Preferably, the wavelength of the second radiation can be greater than 500 nm. This can advantageously prevent undesired absorption of the second radiation in the starting material in many cases, since typically used photoinitiators decay at a wavelength between 350 nm and 500 nm. The wavelength of the second radiation can, for example, be between 750 nm and 1400 nm, preferably between 750 nm and 950 nm, more preferably between 780 nm and 920 nm, or even between 900 nm and 1100 nm. More preferably, the second radiation source should not be a pulsed radiation source and consequently the second radiation should not be pulsed radiation in order to avoid unintentionally triggering nonlinear absorption polymerization.The second radiation source can be, 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 when performing optical coherence tomography.

[0042] The first radiation passes through a first beam path, and the second radiation passes through a second beam path. This means that the first radiation travels from the first radiation source via the first beam path to the position in the starting material where nonlinear absorption polymerization is to be induced. The second radiation, in contrast, travels from the second radiation source via the second beam path to the location to be analyzed using optical coherence tomography.

[0043] Optionally, the position in the starting material and the location to be analyzed by optical coherence tomography, i.e. the focal points of the first and second radiation, can correspond to each other or deliberately differ slightly from each other, for example to be able to carry out an analysis by means of optical coherence tomography immediately before, during or immediately after the nonlinear absorption polymerization.

[0044] Furthermore, it is provided that the first beam path and the second beam path are completely independent of one another, i.e., separate. Consequently, the first radiation can be guided along the first beam path until the first radiation impinges on a polymerization site of the starting material to be polymerized at a focal point of the first radiation. The second radiation can be guided along the second beam path until the second radiation impinges on an analysis site of a sample to be analyzed at a focal point of the second radiation. Joint use of optical elements for the first and second beam paths is not provided, i.e., the first and second beam paths run independently of one another or separately from one another from the first or second radiation source to the focal point of the first or second radiation.

[0045] This advantageously enables the simultaneous performance of nonlinear absorption polymerization and optical coherence tomography. Furthermore, the separate design of the two beam paths allows the wavelength of the second radiation for performing optical coherence tomography to be selected independently of the wavelength of the first radiation for performing nonlinear absorption polymerization, or vice versa. Furthermore, the focal points of the first and second radiation can be selected independently of one another, either the same or different. Consequently, nonlinear absorption polymerization and optical coherence tomography can be performed simultaneously at the same focal point or at different focal points. Different focal points can be used, for example, to perform an OCT analysis of the fully polymerized starting material.Due to the polymerization process taking a certain amount of time, a focus point of the second radiation that is offset from the focus point of the first radiation by, for example, several lateral writing lines would be helpful.

[0046] Furthermore, the independent design of the beam paths can facilitate practical implementation compared to an at least partially shared beam path, for example, with regard to the available installation space. Using the proposed 3D printing device, an analysis using optical coherence tomography can be performed in situ and in real time during nonlinear absorption polymerization.

[0047] In order to reduce unwanted back reflections from the substrate or cuvette surface and thus achieve a high signal-to-noise ratio in optical coherence tomography, the use of immersion objectives is generally preferred for OCT imaging, i.e. in the second beam path.

[0048] The numerical aperture (NA) of the objective lens in the second beam path can be selected differently depending on the specific application. For example, if an OCT analysis in a laterally and axially small volume area with high resolution is desired, the objective lens should preferably have a high numerical aperture (NA). However, if an OCT analysis of an extensive depth area is desired, the objective lens should preferably have a small numerical aperture (NA), for example, a numerical aperture (NA) < 0.5 or even NA < 0.25. While a small effective numerical aperture (NA) reduces the lateral image resolution, it increases the image size in the axial direction, which is important for the three-dimensional imaging of tall printed structures.

[0049] Optical elements are arranged in both the first and second beam paths; these can form images and are arranged along an optical axis. Examples of optical elements include lenses and mirrors. At the end of each beam path opposite the respective radiation source, there is an objective lens that focuses the first or second radiation onto a focal point. The focal point corresponds to the position or location at which the nonlinear absorption polymerization takes place or at which the analysis is carried out using optical coherence tomography. The focal point can be moved, for example by moving mirrors arranged in the beam path accordingly. The movement path of the focal point can also be referred to as a trajectory.

[0050] The lens in the first and second beam paths can be designed as an immersion lens. Optionally, the lens of the first beam path can be arranged such that it is immersed in the source material.

[0051] Furthermore, the 3D printing device can comprise, among other things: a positioning and holding unit for positioning and holding the starting material and, if applicable, the substrate, a control unit, set up and designed to control the first and second radiation source and to control optical elements arranged in the beam paths in order to be able to bring about a change in the focal point of the first radiation and the focal point of the second radiation, an analysis unit, set up and designed to generate OCT images based on the second radiation, a storage unit, set up and designed to store OCT images and / or to store CAD models of the workpiece to be manufactured, and / or an evaluation unit, set up and designed to create 3D OCT scans from a plurality of OCT images.

[0052] According to various embodiments, the first beam path and the second beam path can be designed such that the first radiation and the second radiation meet at an angle α, where 0°< α < 180 °, for example 0°< α ≤ 90 °, applies.

[0053] The angle α refers to the smaller of the two angles formed between the chief rays of the first and second radiation at an actual or fictitious point of intersection of the chief rays of the first and second radiation in the common plane of the chief rays of the first and second radiation. The point of intersection and the plane refer to the last straight section of the chief rays of the first and second radiation respectively before they strike the focal point. An actual point of intersection exists if the focal point of the chief ray of the first radiation coincides with the focal point of the chief ray of the second radiation or if both focal points are arranged in such a way that the chief rays of both radiations meet before the respective focal point is reached.However, if the focal points of the principal rays of both beams are arranged such that the principal ray of the first beam and the principal ray of the second beam do not intersect, a fictitious intersection point is formed by fictitious extension of the principal rays of the first and second beams, respectively. The term principal ray refers to a ray that runs from the object or image point through the center of the pupil.

[0054] For example, the angle α can be selected such that the first radiation and / or the second radiation do not traverse the substrate. This advantageously allows printing on substrates that are not transparent to the first and / or second radiation used.

[0055] Preferably, the angle α can be selected such that the amount of already polymerized starting material outside the observation volume through which the second radiation must pass is minimized. Further preferably, the angle α can be selected such that the second radiation does not pass through already polymerized starting material.

[0056] For example, α = 90° - ω / 2 can apply, where ω is the object-side aperture angle of the second beam path. This can avoid difficulties and misinterpretations in the evaluation of the OCT signals, which could otherwise be caused, for example, by the superposition of a portion of the second radiation that passes through already polymerized starting material with a portion of the second radiation that does not pass through already polymerized starting material.

[0057] For example, the angle α can be 90°. The perpendicular alignment of the first and second beams can simplify the alignment of the focal points of the first and second beams.

[0058] If the principal rays of the first and second radiations are not in the same plane, the principal rays of the first and second radiations may also be skewed to each other, i.e., the principal rays may be offset from each other in depth, but not intersect. In 2D projection, the above statements may apply accordingly to the angle α, i.e., for example, α = 90° - ω / 2.

[0059] The first beam path can preferably be designed and arranged such that the first radiation impinges on the substrate surface parallel to the normal to the substrate surface, i.e., essentially perpendicularly. Optionally, the device can have a positioning device for the substrate to be printed, enabling positioning of the substrate surface with respect to the first radiation. This can contribute to greater accuracy of the structures to be printed.

[0060] The second beam path can be configured and arranged with respect to the substrate surface to be printed such that the second radiation impinges on the substrate surface at an angle other than 90° to the normal of the substrate surface, i.e., not parallel to the substrate surface. Optionally, the device can have a positioning device for the substrate to be printed such that positioning of the substrate surface with respect to the second radiation is possible. This can prevent undesired total internal reflection, which would result in OCT signals that cannot be adequately evaluated, since there is usually no exact match between the refractive index of the substrate and the refractive index of the immersion liquid or the starting material.An angle of less than 90° to the normal of the substrate surface can also be advantageous if structures are to be analyzed that are built into vertical side walls with overhanging elements.

[0061] The starting material can be arranged in a container that is transparent to the radiation used, e.g. a transparent cuvette, in which the 3D printing process is carried out.

[0062] Alternatively, one or both objective lenses of the first and second beam paths can be immersed directly into the source material. This has the advantage of allowing a container that is opaque to the first and / or second beam.

[0063] According to further embodiments, the second beam path can be designed such that the second radiation impinges on a focal point through a substrate to be printed.

[0064] This design variant requires that the substrate is sufficiently transparent for the second radiation. The working distance of the objective in the second beam path, which can preferably be designed as an immersion objective, should preferably be selected such that OCT images are possible through the substrate, which can, for example, have a substrate thickness of several mm. A large working distance is also advantageous in this case because the working distance limits the maximum height of printed structures that can be imaged.

[0065] Since an optically smooth surface is required on the entrance surfaces of the objective in the second beam path to prevent aberrations during imaging, the design variant in which the second radiation is guided through the substrate has the advantage that the substrate itself already represents this optically smooth surface and no separate optically smooth surface, e.g. in the form of a cuvette with an optically smooth surface, is required.

[0066] For example, the first radiation and the second radiation can impinge on their focal point from opposite directions, for example, essentially parallel to each other. In other words, the angle α at which the first radiation and the second radiation impinge can be α = 180°. This has the advantage that the lenses of the first and second beam paths can be arranged far apart from each other, thus reducing the likelihood of a mechanical collision between the two lenses. Furthermore, the lens selection is less restricted due to the larger installation space.

[0067] Preferably, the first radiation and / or the second radiation can impinge perpendicularly on the substrate surface.

[0068] Alternatively or additionally, the first beam path can be configured such that the first radiation passes through the substrate to be printed onto a focal point ("through-substrate configuration"). This configuration expands the selection of source materials, since not all source materials can be used in the "dip-in" configuration, in which the objective lens is immersed in the source material, without damaging the objective lens. Apart from the chemical compatibility between the source material and the objective lens, in a "dip-in" configuration the refractive index of the source material should be matched to the objective lens used, which is not satisfactorily achievable in all cases. In contrast, the "through-substrate" configuration potentially allows for a wider variety of source materials with different refractive indices to be specifically printed.This is because, in this configuration, the first beam travels only a comparatively short distance in the source material, which minimizes aberrations even when the refractive index is not matched. However, in the "through-substrate" configuration, the maximum height of the 3D print is limited, as aberrations occur through the printed structures, and the working distance of the lens used limits the structure height to avoid collision between the substrate and the lens.

[0069] Another aspect of the invention relates to another 3D printing device for manufacturing a workpiece.

[0070] The further device comprises a first radiation source configured to emit first radiation for carrying out nonlinear absorption polymerization. In other words, the first radiation causes nonlinear absorption polymerization of a starting material as described above. The first radiation source can be configured as a laser light source, for example, as an 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.

[0071] In addition to the first radiation source, the further device comprises a second radiation source configured to emit a second radiation source for performing optical coherence tomography. In other words, optical coherence tomography can be performed using the second radiation, for example, to monitor nonlinear absorption polymerization, particularly in situ.

[0072] Depending on specific conditions, e.g., depending on the wavelength required for the decomposition of the photoinitiator in the starting material during nonlinear absorption polymerization, the wavelength of the second radiation can, for example, be selected such that it is shorter than the wavelength of the first radiation. This can advantageously increase the axial resolution of optical coherence tomography, since the minimum distance between two resolvable points in the axial z-direction is proportional to the square of the wavelength, i.e., Δz ∝ λ 2< .

[0073] Alternatively, the wavelength of the second radiation can, for example, be selected such that it is greater than the wavelength of the first radiation. Preferably, the wavelength of the second radiation can be greater than 500 nm. This can advantageously prevent undesired absorption of the second radiation in the starting material in many cases, since typically used photoinitiators decay at a wavelength between 350 nm and 500 nm. The wavelength of the second radiation can, for example, be between 750 nm and 1400 nm, preferably between 750 nm and 920 nm, more preferably between 780 nm and 920 nm, or even between 900 nm and 1100 nm. More preferably, the second radiation source should not be a pulsed radiation source and consequently the second radiation should not be pulsed radiation in order to avoid unintentionally triggering nonlinear absorption polymerization.The second radiation source can be, 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 when performing optical coherence tomography.

[0074] The first radiation source and the second radiation source are arranged such that an optical system can be used at least partially jointly by the first radiation and the second radiation. The term "optical system" refers to the entirety of optical elements, e.g., lenses, mirrors, etc., in their specific arrangement. If the optical system is used at least partially jointly by the first radiation and the second radiation, this means that the optical elements of the optical system can be passed through by both the first radiation and the second radiation. The first beam path of the first radiation and the second beam path of the second radiation can be identical or different from one another, for example, by selecting or becoming different angles of incidence.

[0075] Furthermore, the further device comprises an objective lens arranged in the optical system, which is designed to focus the first and second radiation onto a focal point. In other words, both the first radiation and the second radiation are focused onto the source material or the location to be analyzed using the same objective lens.

[0076] 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 means of the 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 means of the lens. The focal point can be a common focal point, or different focal points can be or become predetermined for the first and second radiation. With different focal points, different lateral positions and / or different depth positions can be or become predetermined. This would be possible, for example, by inclining the optical axes of the beam paths of the first and second radiation, realized, for example, by a tiltable mirror or scanning mirror or by a beam splitter in the beam path of the second radiation.As already mentioned, this allows, for example, an OCT analysis of the fully polymerized starting material to be carried out.

[0077] Accordingly, either nonlinear absorption polymerization and / or optical coherence tomography analysis can be performed at the focal point at a specific time. The focal point can be moved, for example, by moving mirrors arranged in the beam path. The lens can be designed as an immersion lens or it can be arranged so that the lens is immersed in the source material.

[0078] The further device has a coupling element for coupling the first radiation and / or the second radiation into the optical system.

[0079] A coupling element is an optically active component with the aid of which the first and / or second radiation enters the optical system for the first time after being emitted by the first or second radiation source, or which feeds the first and / or second radiation to the optical system. The coupling element is therefore not a further optically active element within the optical system that interacts with the first or second radiation after it has already entered the optical system or has been coupled into the optical system. The coupling element is selected from a group consisting of a mechanically adjustable mirror, a galvo mirror, a polarizing beam splitter, an intensity splitter, and a partially coated mirror. Consequently, the coupling element is not designed as a dichroic mirror.Several of the coupling elements mentioned can also be combined with each other.

[0080] In addition to coupling the first and / or second radiation into the optical system, the coupling element can also act as an output element for the second radiation. This means that after the OCT analysis has been performed, i.e., after interaction with the sample to be examined and corresponding reflection in the sample volume, the second radiation can be output from the optical system again using the coupling element, e.g., by reflection toward an OCT analysis unit.

[0081] Both a mechanically adjustable mirror and a galvo mirror allow either the first radiation or the second radiation to be coupled into the optical system alternately by moving it between two positions. A surface reflecting the first or second radiation is aligned accordingly. With a mechanically adjustable mirror, alignment is achieved entirely mechanically, while with a galvo mirror, alignment is achieved using a galvanometer drive.

[0082] Both a mechanically adjustable mirror and a galvo mirror have the advantage that only one of the two beams is coupled into the optical system at a time, thus avoiding interactions between the two beams or interference with the OCT analysis caused by the first beam. Furthermore, the number and duration of the OCT analyses can be flexibly adapted to the geometry of the component to be printed and / or the quality requirements. For simple geometries, it may not be absolutely necessary to analyze each printed layer using optical coherence tomography; instead, the time interval between two OCT analyses can be increased. This can have a positive effect on the time required to manufacture the component. Furthermore, the computational effort can be reduced because less OCT analysis data is acquired.Accordingly, lower computing power in an OCT analysis unit and / or a smaller memory size in a storage unit may be sufficient. Sequential execution of nonlinear absorption polymerization and OCT analysis can also be advantageous in applications where the polymerization process takes some time, so that the focal point of the first radiation and thus also of the second radiation is already far away by the time polymerization is complete due to the high writing speed. In such a case, nonlinear absorption polymerization can be deliberately interrupted for an OCT analysis, or an OCT analysis can be performed after the writing process has been completed, so that the printed starting material can be analyzed using optical coherence tomography after its complete polymerization.

[0083] A polarizing beam splitter, an intensity splitter, and a partially coated mirror enable simultaneous coupling of the first and second beams into the optical system. Consequently, OCT analysis and nonlinear absorption polymerization can be performed simultaneously. This has the advantage that errors occurring during polymerization, e.g., due to proximity effects, can be detected early, and appropriate measures, such as stopping the faulty printing process, can be initiated.

[0084] The principle of a polarizing beam splitter is based on the use of differently linearly polarized radiation for the first and second beams. For example, while radiation with a first polarization state is transmitted, radiation with a second polarization state is reflected.

[0085] An intensity splitter is a beam splitter with a homogeneous, optically effective coating with a fixed transmission / reflection ratio of, for example, 80% transmission / 20% reflection, based on the wavelength of the first or second radiation. Consequently, for example, 80% of the first radiation can be transmitted while 20% of the first radiation is reflected. Similarly, 20% of the second radiation can be transmitted while 80% of the second radiation is reflected.

[0086] In contrast, a partially coated mirror does not have a homogeneous optically effective coating, but rather a coating with varying optical effectiveness in certain areas. For example, a central area of ​​the partially coated mirror can have a coating with a reflectance of 100%, based on the wavelength of the first and second radiation, while the reflectance in the edge area is almost 0%, meaning the coating is almost 100% transmissive. Beam splitting can be achieved, for example, by the second radiation having a beam with a smaller diameter than the beam of the first radiation, so that the second radiation only hits the partially coated mirror in the central area and is reflected there almost 100%. The first radiation, on the other hand, also hits the partially coated mirror in the edge areas and can be transmitted accordingly.In other words, the coating can block the first radiation by central shading with little impact on the spot shape and completely reflect the second radiation.

[0087] The use of a polarizing beam splitter, an intensity splitter, or a partially coated mirror has the advantage that the coupling element is rigid and mechanical wear is therefore not expected. Furthermore, OCT analysis and nonlinear absorption polymerization can be performed simultaneously.

[0088] In contrast to a dichroic mirror as a coupling element, all of the above-mentioned variants of the coupling element have the advantage that closely spaced wavelengths can be used for the first radiation and the second radiation. This, in turn, facilitates the selection of the remaining optical elements of the optical system, which must be suitable for both wavelengths.

[0089] By means of the proposed 3D printing device, an analysis by means of optical coherence tomography can advantageously be carried out in-situ and in real time during a nonlinear absorption polymerization.

[0090] The optical system may contain additional optical elements that may be image-forming and arranged along an optical axis. Examples of optical elements include lenses and mirrors.

[0091] Furthermore, the 3D printing device can comprise, among other things: a positioning and holding unit for positioning and holding the starting material and, if applicable, the substrate, a control unit, set up and designed to control the first and second radiation source, for controlling optical elements arranged in the optical system in order to be able to bring about a change in the focal point of the first radiation and the focal point of the second radiation, and for controlling a coupling element designed as a mechanically adjustable mirror or galvo mirror in order to switch between the first and second radiation, an analysis unit, set up and designed to generate OCT images based on the second radiation, a storage unit, set up and designed to store OCT images and / or to store CAD models of the workpiece to be manufactured, and / or an evaluation unit, set up and designed to create 3D OCT scans from a plurality of OCT images.

[0092] According to various embodiments, the device can comprise an analysis unit configured and designed to generate optical coherence tomography images based on the second radiation. The analysis unit comprises multiple measurement channels such that multiple analysis locations can be analyzed simultaneously using optical coherence tomography.

[0093] Each measuring channel has a radiation source or a radiation source position and a focal point. Radiation source position means that the radiation from a physical radiation source is spatially distributed such that radiation is provided to each measuring channel at a specific position. For example, the device can have a device, e.g., a photonic chip, that is designed to distribute the second radiation from the second radiation source across multiple measuring channels, e.g., six measuring channels. Using the various measuring channels, the second radiation can be provided at different lateral and / or axial positions and directed to different analysis locations. Each measuring channel can therefore generate a different focal point. The respective measuring channel serves as the detection channel for the returning radiation after interaction with the sample to be analyzed.

[0094] Providing multiple measurement channels has the advantage that multiple analysis locations, such as sample areas, can be analyzed simultaneously. For example, one analysis location with unpolymerized starting material, one analysis location with partially polymerized starting material, and one analysis location with fully polymerized starting material. This allows information about the chronological sequence of processes to be obtained. Furthermore, the analysis time can be shortened. Furthermore, the multiple measurement channels can enable the simultaneous acquisition of different depth ranges in OCT analyses with a high numerical aperture and thus limited A-scan depth.

[0095] According to further embodiments, the second radiation source can emit a temporally constant, i.e. non-pulsed, wave as second radiation.

[0096] This advantageously prevents unwanted polymerization of the starting material during analysis using optical coherence tomography. For example, sufficiently weak continuous-wave light sources that do not excite the photoinitiators in the starting material through single-photon absorption can be used as a second radiation source, such as a combination of several superluminescent light-emitting diodes. These also advantageously achieve a wide spectral bandwidth and thus an axial resolution of only a few µm.

[0097] According to further embodiments, the device can comprise a container for arranging a starting material to be polymerized, wherein the container is at least partially transparent to the first radiation and / or second radiation.

[0098] Transparent means that the first radiation and / or second radiation can be at least partially transmitted through a wall of the container and can thus reach the interior space enclosed by the container, in which the starting material to be polymerized can be arranged. For example, it can be provided that more than 70%, more than 80%, more than 90%, or more than 95% of the first radiation and / or second radiation is transmitted through the wall of the container. The container can be designed, for example, as a cuvette.

[0099] The at least partially transparent container allows for the polymerization and / or analysis of a starting material arranged in the container through the wall of the container in a simple and convenient manner. This can contribute to a simplification of the device's design, since the first or second radiation does not necessarily have to reach the starting material through an opening in the container.

[0100] Optionally, the container can be designed to be tightly sealed or sealable. This offers the possibility of polymerizing and / or analyzing starting materials that are sensitive to environmental influences, e.g., oxygen, moisture, etc.

[0101] According to further embodiments, an objective of the device can be designed and arranged in such a way that it can be immersed in a starting material to be polymerized.

[0102] This has the advantage that a container can be chosen for the arrangement of the starting material which is not transparent to the first and / or second radiation.

[0103] Another aspect of the invention relates to a 3D printing method for manufacturing a workpiece using one of the devices described above. The method provides for the nonlinear absorption polymerization and optical coherence tomography to be performed alternately or simultaneously, i.e., at the same time.

[0104] The above explanations of the 3D printing devices also serve 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.

[0105] Further, unclaimed, aspects of the disclosure relate to various methods in which optical coherence tomography is used in conjunction with nonlinear absorption polymerization. The methods can be carried out using the 3D printing devices described above. However, it should be noted that the methods explained below can also be carried out independently of the 3D printing devices described above, for example, with 3D printing devices that otherwise combine nonlinear absorption polymerization and optical coherence tomography, or devices that implement nonlinear absorption polymerization and optical coherence tomography independently of one another.

[0106] A first method relates to the analysis of the quality of a starting material from nonlinear absorption polymerization using optical coherence tomography, wherein a quality parameter of the starting material is determined using optical coherence tomography. The method can be carried out using one of the 3D printing devices described above. Preferably, the optical coherence tomography can be carried out three-dimensionally.

[0107] Optionally, parameters for nonlinear absorption polymerization can be defined taking into account the determined quality parameter. Examples of these parameters include the (laser) power of the radiation used for nonlinear absorption polymerization, e.g., the first radiation source or first radiation; the trajectory, i.e., the path of motion along which the focal point of the radiation used for nonlinear absorption polymerization is guided over the starting material; and the writing duration, e.g., the velocity profile with which the trajectory is completed. The parameters defined in this way can then be used for nonlinear absorption polymerization.

[0108] For example, at positions where an impurity has been detected in the starting material, parameters deviating from standard parameters can be used for nonlinear absorption polymerization, e.g., higher laser power, longer writing time, etc. The trajectory can be modified to be able to write at all or to improve the writing at positions of an impurity in the starting material.

[0109] This can improve the quality of the workpiece to be manufactured using nonlinear absorption polymerization when the starting material is contaminated.

[0110] Alternatively or additionally, parameters for optical coherence tomography can also be defined taking into account the determined quality parameter. Examples of such parameters include the positions of the focal points of the radiation used for optical coherence tomography and the definition of a corresponding trajectory for the focal point of the radiation used for optical coherence tomography. The parameters defined in this way can then be used for optical coherence tomography.

[0111] For example, the focal points of the radiation used for optical coherence tomography and thus the analysis locations can be defined in such a way that a more detailed analysis can be carried out at positions where contamination was detected in the source material compared to positions without contamination.

[0112] This can enable improved analysis of the workpiece to be manufactured, so that defective workpieces can be detected early, for example during production, and countermeasures can be taken if necessary.

[0113] Another method relates to checking the position and / or orientation of a substrate to be printed using nonlinear absorption polymerization, wherein characteristic features of the substrate to be printed are identified using optical coherence tomography. The method can be carried out using one of the 3D printing devices described above. Preferably, the optical coherence tomography can be carried out three-dimensionally.

[0114] Characteristic features of the substrate to be printed are objects, e.g., markings, boundaries, contaminants, on the surface or inside the substrate, which can be clearly located, i.e., whose position relative to the substrate is known, and based on which the position and / or orientation of the substrate can be determined. The term "position" describes the absolute positioning of the substrate relative to a reference point, e.g., in a coordinate system. The term "orientation" describes the rotational position of the substrate relative to a reference point, e.g., in a coordinate system. e.g., the orientation determines, for example, which edge of a cube-shaped substrate is located where.

[0115] Identifying such characteristic features using optical coherence tomography can be used to support the print alignment process and enable the most precise positioning and / or alignment of the area to be printed with respect to the focal point of the radiation used for nonlinear absorption polymerization. This can improve the quality of the workpieces to be manufactured, for example, by reducing deviations between the finished workpiece and the corresponding template.

[0116] Another method relates to the determination of a spatially resolved degree of conversion of a nonlinear absorption polymerization using optical coherence tomography. The method can be carried out using one of the 3D printing devices described above. Preferably, the optical coherence tomography can be performed three-dimensionally. This advantageously enables a three-dimensionally resolved determination of the local degree of conversion.

[0117] Optionally, parameters for nonlinear absorption polymerization can be specified taking into account the determined degree of conversion. Examples of such parameters include the (laser) power of the radiation used for nonlinear absorption polymerization, the trajectory, and the writing time.

[0118] For example, if the degree of conversion is insufficient, parameters deviating from the standard parameters can be used for nonlinear absorption polymerization, e.g., higher laser power, longer writing time, etc. The trajectory can be modified, for example, to cause repeated or extended irradiation at positions where the degree of conversion is too low. It is also possible to define the trajectory in such a way that positions where polymerization has not yet fully taken place, i.e., where the degree of conversion is too low, are (initially) skipped when writing the subsequent layer during the layer-by-layer construction of the workpiece to be manufactured, for example, to avoid undesired polymerization processes. The parameters defined in this way can then be used for nonlinear absorption polymerization.

[0119] Alternatively or additionally, optical coherence tomography parameters can also be defined taking into account the determined degree of conversion. Examples of such parameters include the positions of the focal points of the radiation used for optical coherence tomography and the definition of a corresponding trajectory for the focal point of the radiation used for optical coherence tomography. The parameters defined in this way can then be used for optical coherence tomography.

[0120] For example, the focal points of the radiation used for optical coherence tomography and thus the analysis locations can be defined in such a way that a more detailed analysis can be carried out at positions where an insufficient degree of conversion was found compared to positions with a sufficient degree of conversion.

[0121] This can enable an improved analysis of the workpiece to be manufactured, such that defective workpieces can be recognized at an early stage, for example already during their manufacture, and appropriate countermeasures can be taken if necessary.

[0122] Thereby, it is possible to locally influence the degree of conversion and, along with it, the properties of the manufactured workpiece that depend on the degree of conversion.

[0123] Another method relates to the analysis of the structural sharpness of a structure generated by non-linear absorption polymerization. For this purpose, the structural sharpness is determined based on a refractive index profile determined with spatial resolution by means of optical coherence tomography, optionally in-situ. The method can be carried out by means of one of the 3D printing devices explained above. Preferably, the optical coherence tomography can be three-dimensional.

[0124] Optionally, parameters for nonlinear absorption polymerization can be defined taking into account the determined structural sharpness. Examples of parameters include the (laser) power of the radiation used for nonlinear absorption polymerization, the trajectory, and the writing time. For example, if structural sharpness is insufficient, parameters deviating from standard parameters can be used for nonlinear absorption polymerization, e.g., a higher laser power, a longer writing time, etc. The trajectory can be modified, for example, to cause repeated or extended irradiation at positions with insufficient structural sharpness. The parameters defined in this way can then be used for nonlinear absorption polymerization.

[0125] Alternatively or additionally, optical coherence tomography parameters can also be defined taking into account the determined structural sharpness. Examples of such parameters include the positions of the focal points of the radiation used for optical coherence tomography and the definition of a corresponding trajectory for the focal point of the radiation used for optical coherence tomography. The parameters defined in this way can then be used for optical coherence tomography.

[0126] For example, the focal points of the radiation used for optical coherence tomography and thus the analysis locations can be defined in such a way that a more detailed analysis can be carried out at positions where insufficient structural sharpness was found compared to positions with sufficient structural sharpness.

[0127] This can enable improved analysis of the workpiece to be manufactured, so that defective workpieces can be detected at an early stage, for example during production, and countermeasures can be taken if necessary.

[0128] This makes it possible to influence the structural sharpness and, together with this, the properties of the manufactured workpiece that depend on the structural sharpness in a locally resolved manner.

[0129] For example, the degree of conversion and / or structural sharpness can be determined in situ during nonlinear absorption polymerization. Parameters of the nonlinear absorption polymerization, e.g., laser power, writing time, can then be controlled depending on the determined degree of conversion and / or structural sharpness, e.g., taking into account geometric deformations and / or a degree of conversion deviating from a target function. Insufficiently polymerized areas can be re-irradiated, e.g., by changing the trajectory of the focal point of the radiation used for nonlinear absorption polymerization, in order to, for example, further increase the degree of conversion or change the refractive index and / or the Young's modulus. A closed control loop can be created based on the OCT data.

[0130] Alternatively or additionally, parameters of optical coherence tomography can also be controlled depending on the determined degree of conversion and / or the determined structural sharpness, for example according to the principles explained above.

[0131] Another method relates to the three-dimensional reconstruction of a workpiece manufactured by nonlinear absorption polymerization. During the production of the workpiece, sequences in which printing is carried out by nonlinear absorption polymerization alternate with sequences in which analysis is carried out by optical coherence tomography. The method can be carried out using one of the 3D printing devices described above. Preferably, the optical coherence tomography can be carried out three-dimensionally.

[0132] Optionally, the number and / or timing of the sequences in which an analysis using optical coherence tomography is performed can be determined depending on the expected morphology of the manufactured workpiece, the starting material of the manufactured workpiece, and / or the parameters of the nonlinear absorption polymerization. The parameters of the nonlinear absorption polymerization can be selected from a group comprising: laser power of the radiation used for the nonlinear absorption polymerization, trajectory, and write duration. This allows the printed workpiece to be accurately reconstructed while minimizing the required time.

[0133] The invention is explained in more detail below with reference to the figures and the accompanying description. They show: Fig. 1 shows a schematic representation of a 3D printing device according to a first embodiment; Fig. 2 shows a schematic representation of a 3D printing device according to a third embodiment; Fig. 3 shows a schematic representation of a 3D printing device according to a fourth embodiment; Fig. 4a, b shows exemplary flow charts for 3D printing methods for producing a workpiece; Fig. 5 shows an illustration to explain an exemplary method for analyzing the quality of a starting material for non-linear absorption polymerization; Fig. 6a, b shows illustrations to explain an exemplary method for determining a degree of conversion in non-linear absorption polymerization; Fig. 7a, b shows illustrations to explain an exemplary method for analyzing the structural sharpness of structures produced by non-linear absorption polymerization;8a, b: Illustrations explaining an exemplary method for the three-dimensional reconstruction of a workpiece manufactured by means of nonlinear absorption polymerization; and Fig. 9: an illustration explaining an exemplary method for checking the alignment of a substrate to be printed by means of nonlinear absorption polymerization.

[0134] In the examples explained below, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. It is to be understood that other embodiments may be utilized and structural or logical changes may be made without departing from the scope of the present invention as claimed. It is to be understood that the features of the various exemplary embodiments described herein may be combined with one another unless specifically stated otherwise. The following detailed description, therefore, is not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims. In the figures, identical or similar elements are designated by identical reference numerals where appropriate.

[0135] As used herein, the term "and / or," when used in a series of two or more elements, means that any of the listed elements may be used alone, or any combination of two or more of the listed elements may be used. For example, if a composition is described containing components A, B, and / or C, the composition may contain A alone; B alone; C alone; A and B in combination; A and C in combination; B and C in combination; or A, B, and C in combination.

[0136] It is expressly pointed out that for the sake of simplicity and to facilitate understanding of the specified and claimed components, these are generally referred to as individual components. Depending on the specific circumstances, several corresponding components may be used in series or in parallel for the same function. Such multi-part arrangements are also expressly intended to be encompassed by the claims and the description.

[0137] In Fig. 1 A first embodiment of a 3D printing device 100 for manufacturing a workpiece is schematically shown. The circled area is shown enlarged.

[0138] The device 100 has a first radiation source 1 that emits a first radiation 2. A nonlinear absorption polymerization, e.g., a two-photon polymerization, can be carried out using the first radiation 2. For this purpose, the first radiation 1 is guided along a first beam path 5 until it impinges at the focal point 7 on the liquid starting material 12 to be polymerized, which is arranged in a container 13 designed as a transparent cuvette. In the exemplary embodiment, the first radiation source 1 is designed as an fs laser radiation source, i.e., the first radiation 2 is fs laser radiation.

[0139] To form the first beam path 5, several optical elements are present, of which Fig. 1 For reasons of clarity, only the mirror 17 is shown. Furthermore, a first objective 15 is arranged in the first beam path 5, with which the focal point 7 of the first radiation 2 can be determined. The first objective 15 is designed as an immersion objective, i.e. an immersion liquid 14 is arranged between the front lens of the first objective 15 and the container 13 with the starting material 12. This can improve the resolution and smaller structures can be printed with greater precision. In the exemplary embodiment, the first objective 15 has a numerical aperture NA = 1.4 and a magnification number of 40x.

[0140] Furthermore, the device 100 has a second radiation source 3 that emits a second radiation 4. Using the second radiation 4, an analysis can be performed by means of optical coherence tomography. For this purpose, the second radiation 4 is guided along a second beam path 6 until it impinges on the sample to be analyzed, e.g., the liquid starting material 12 to be polymerized, at the focal point 7. In the exemplary embodiment, the second radiation source 3 is designed as a tunable laser or as a superluminescent diode. The second beam path 6 is designed independently of the first beam path 5.

[0141] To form the second beam path 6, several optical elements are present, which are arranged in Fig. 1 are not shown in detail for reasons of clarity. Furthermore, a second lens 16 is arranged in the second beam path 6, with which the focus point 7 of the second radiation 4 can be determined. The focus point 7 of the first radiation 2 can be aligned with the focus point 7 of the second radiation 4, as in Fig. 1 shown. Alternatively, the two focal points 7 can differ from one another. For example, the focal point 7 of the second radiation 4 can be located directly in front of or directly behind the focal point 7 of the first radiation 2, relative to a movement of the focal point 7 of the first radiation 2. This enables analysis by OCT immediately before or after the nonlinear absorption polymerization.

[0142] The second objective 16 is also designed as an immersion objective, i.e., an immersion liquid 14 is arranged between the front lens of the second objective 16 and the container 13 containing the starting material 12. This allows an improved signal-to-noise ratio to be achieved in optical coherence tomography by avoiding unwanted back reflections at the surface of the substrate 8 or the container 13. The numerical aperture NA of the second objective 16 is NA < 0.25.

[0143] Optionally, the first beam path 5 and / or the second beam path 6 can be fiber-guided.

[0144] In the first embodiment, the first beam path 5 and the second beam path 6 are designed such that the first radiation 2 and the second radiation 4 impinge on the focal point 7 essentially perpendicular to each other, ie for the angle α, α = 90° Neither the first radiation 2 nor the second radiation 4 pass through the substrate 8 on their way to the focal point 7, so that non-transparent substrates 8 can also be used for the first radiation 2 and the second radiation 4. As in Fig. 1 As can be seen, the diameter of the first beam path 5 is significantly larger than the diameter of the second beam path 6.

[0145] The device 100 also has an analysis unit 23, which is housed in a common housing with the second radiation source 3 and with which an OCT analysis can be performed at the focal point 7 based on the second radiation 4. In other words, the analysis unit 23 generates OCT images based on the second radiation 4. Optionally, the analysis unit 23 can have multiple measurement channels so that multiple analysis locations can be analyzed simultaneously.

[0146] In addition, the device 100 may comprise further units (not shown), such as a control unit, a storage unit, an evaluation unit, a positioning and holding unit.

[0147] In a second embodiment (not shown), the angle α is α = 80°. This angle α results for an object-side aperture angle of the second lens 16 of ω = 20° according to α = 90° - ω / 2.

[0148] Fig. 2 shows a schematic representation of a third embodiment of a 3D printing device 100 for manufacturing a workpiece. In contrast to the first and second embodiments, the second radiation 4 impinges on the focal point 7 through the substrate 8. This requires a substrate 8 whose material is transparent to the second radiation 4. The angle α between the first radiation 2 and the second radiation 4 is α = 180°.

[0149] A further difference from the first embodiment is that the starting material 12 is not arranged in a container 13, but the first objective 15 is immersed directly into the starting material 12. The first objective 15 is therefore not designed as an immersion objective. For the rest, reference is made to the explanations for Fig. 1 referred to.

[0150] Fig. 3 shows a fourth embodiment of a 3D printing device 300 for manufacturing a workpiece.

[0151] The device 300 has a first radiation source 1 that emits a first radiation 2. Nonlinear absorption polymerization can be carried out using the first radiation 2. For this purpose, the first radiation 1 is guided along the optical system 9 until it impinges on the liquid starting material 12 to be polymerized at the focal point 7. In the exemplary embodiment, the first radiation source 1 is designed as an fs laser radiation source, i.e., the first radiation 2 is fs laser radiation. Optionally, the first radiation 2 can be fiber-guided.

[0152] The optical system 9 comprises several optical elements, of which Fig. 3 several lenses 19 are shown. Furthermore, the optical system 9 comprises an objective lens 18, with which the focal point 7 of the first radiation 2 can be determined. The surface of the sample can be scanned using the galvo mirror 20 for the x-direction and the galvo mirror 21 for the y-direction, for example, according to a predeterminable CAD model of the workpiece to be manufactured. The objective lens 18 is immersed directly into the starting material 12. In the exemplary embodiment, the objective lens 18 has a numerical aperture NA = 1.4 and a scale number of 40x.

[0153] Furthermore, the device 300 has a second radiation source 3 which emits a second radiation 4. By means of the second radiation 4, an analysis can be carried out by means of optical coherence tomography. For this purpose, the second radiation 4 is likewise guided in the optical system 9 until it strikes the sample to be analyzed, e.g., the liquid starting material 12 to be polymerized, at the focal point 7. In the exemplary embodiment, the second radiation source 3 is designed as a tunable laser or as a superluminescence diode, i.e., the second radiation 4 is laser radiation. The second radiation 4 can be transported by means of an optical single-mode fiber and coupled into the optical system 9 via the fiber coupler 22.

[0154] The device 300 has a coupling element 10, which in the exemplary embodiment is designed as a polarizing beam splitter, but can alternatively also be designed as a mechanically adjustable mirror, galvo mirror, intensity splitter, or partially coated mirror. By means of the coupling element 10 designed as a polarizing beam splitter, the first radiation 2 and the second radiation 4 can be simultaneously coupled into the optical system 9 in order to simultaneously perform nonlinear absorption polymerization and an analysis using optical coherence tomography.

[0155] By means of the fiber coupler 22, the diameter of the beam of the second radiation 4 can be set such that, upon impingement on the coupling element 10, it is smaller than the diameter of the beam of the first radiation 2. By specifically adjusting the diameter of the beam of the second radiation 4, the aperture of the objective 18 can be specifically under-illuminated, i.e., the effective numerical aperture NA of the objective 18 for the second radiation 4 can be specifically reduced.

[0156] The device 300 also has an analysis unit 23, which is housed in a common housing with the second radiation source 3 and can be used to perform an OCT analysis at the focal point 7 based on the second radiation 4. In other words, the analysis unit 23 generates OCT images based on the second radiation 4. Optionally, the analysis unit 23 can have multiple measurement channels so that multiple analysis locations can be analyzed simultaneously. For this purpose, the second radiation 4 can be divided among the individual measurement channels using a photonic chip.

[0157] In addition, the device 300 may comprise further units (not shown), such as a control unit, a storage unit, an evaluation unit, a positioning and holding unit.

[0158] With reference to the Fig. 4a and 4bIn the following, exemplary 3D printing processes 200, 400 for the production of a workpiece are explained in more detail. Fig. 4a shows a flow chart of a 3D printing method 200 in which the nonlinear absorption polymerization S3 and the optical coherence tomography S4 are carried out simultaneously. The method 200 can be carried out, for example, by means of one of the methods described above with reference to Fig. 1 or Fig. 2 explained 3D printing devices.

[0159] After the start of the method 200, a substrate is provided in method step S1, onto which material is to be applied by means of non-linear absorption polymerization and which, together with the applied material, forms the finished workpiece after completion of the method 200.

[0160] In process step S2, the substrate is positioned in the starting material. The starting material can be, for example, IP Dip ® from Nanoscribe GmbH. Alternatives to this starting material include the following products: IP-S, IP-Q, IP-Visio, IP-n162, and IP-L from Nanoscribe GmbH; OrmoComb ® from Micro resist technology Gesellschaft für chemische Materialien besonderenr Photoresistsysteme mbH; and upphoto, updraft, upbrix, upsol, and upopto from UpNano GmbH.

[0161] The starting material typically has a liquid consistency and can be contained in a container, e.g., a cuvette, a dish, etc. The container is transparent to the radiation used to conduct nonlinear absorption polymerization and optical coherence tomography. To position the substrate, it is at least partially immersed in the starting material, ensuring that starting material is present at least at the interface between the substrate and the polymer to be formed.

[0162] In process step S3, a nonlinear absorption polymerization of the starting material is performed. For a more detailed explanation of this process step S3, please refer to the above description of nonlinear absorption polymerization.

[0163] At least partially overlapping in time, i.e., simultaneously with process step S3, optical coherence tomography is performed in process step S4. Optical coherence tomography can be used to perform in-situ analysis during 3D printing. For example, it is possible to determine the degree of conversion of nonlinear absorption polymerization with spatial resolution, e.g., as described below with reference to Fig. 6a und Fig. 6b Alternatively or additionally, the structural sharpness of the structures produced by nonlinear absorption polymerization can be analyzed, e.g., as described below with reference to Fig. 7a, und Fig. 7b Alternatively or additionally, the finished workpiece can be reconstructed three-dimensionally, e.g., as described below with reference to Fig. 8 described.

[0164] Before starting nonlinear absorption polymerization, optical coherence tomography can be used to align the substrate to be printed by identifying characteristic points of the substrate to be printed. Alternatively or additionally, the quality of the starting material can be analyzed using optical coherence tomography, as described below with reference to Fig. 5 described.

[0165] After all desired structures have been produced by nonlinear absorption polymerization, excess starting material is washed out with a suitable solvent in process step S5 to obtain the finished workpiece. This concludes process 200.

[0166] In Fig. 4b is a flowchart of another exemplary 3D printing method 400 for manufacturing a workpiece, in which the nonlinear absorption polymerization S3 and the optical coherence tomography S4 are carried out alternately. The method 400 can be carried out, for example, by means of the method described above with reference to Fig. 3 explained 3D printing device, wherein the coupling element 10 is designed as a mechanically adjustable mirror or as a galvo mirror.

[0167] After the start of the method 400, the method steps S1 and S2 are carried out as described above with reference to Fig. 4a explained method 200 is carried out. In contrast to method 200, however, the subsequent method steps S3 and S4 are not carried out in parallel, but alternately. For example, a non-linear absorption polymerization can first be carried out in step S3 in order to produce some of the desired structures from the starting material. At a suitable time, the non-linear absorption polymerization is interrupted and the method continues to method step S4, in which an analysis is carried out by means of optical coherence tomography. For this purpose, reference is made to the above explanations of method step S4 with reference to Fig. 4a referred to.

[0168] Once the optical coherence tomography analysis is complete, the process returns to process step S3, and the nonlinear absorption polymerization continues. Process steps S3 and S4 alternate until all desired structures have been produced. Excess starting material is then washed out in process step S5, yielding the finished workpiece. Process 400 is thus completed.

[0169] Alternatively, before the first execution of process step S3, process step S4, i.e. optical coherence tomography, can also be carried out first, for example to analyze the starting material or to check the substrate alignment.

[0170] The two, with reference to Fig. 4a and Fig. 4b The 3D printing processes 200, 400 explained can also be combined with each other, ie the process steps S3 and S4 can also be carried out partly simultaneously and partly alternately.

[0171] All procedures described below, which involve analysis using optical coherence tomography, were performed using a self-built 3D-printed device. Optical coherence tomography was performed in the Fourier domain using a radiation source with a central wavelength of 845 nm and a nominal full width at half maximum (FWHM) of 135 nm (fiber-coupled superluminescent diode, model: Exalos EXC250002-00), which achieved 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 scanning speed or A-scan frequency was 25 kHz for each B-scan (2D image formed from multiple A-scans, i.e., individual depth signals or individual scans).

[0172] With reference to Fig. 5 A method for analyzing the quality of a starting material for nonlinear absorption polymerization using optical coherence tomography is explained in more detail below. In this method, a quality parameter of the starting material is determined using optical coherence tomography.

[0173] To analyze the quality of the starting material, volumetric images are recorded and evaluated using optical coherence tomography (3D-OCT scans) before nonlinear absorption polymerization begins. During the evaluation, one or more quality parameters are determined. These quality parameters can provide information, for example, about whether the 3D-OCT scans and thus the examined starting material show impurities, the proportion of impurities, the size and / or size distribution of the impurities, how the impurities are distributed within the starting material, etc. Furthermore, the 3D coordinates of the impurities can also be determined. The evaluation can be performed computer-assisted and optionally using artificial intelligence methods. Based on this information, the suitability of the starting material for the planned 3D printing process can then be assessed.This assessment can also be carried out computer-assisted and optionally using artificial intelligence methods.

[0174] Fig. 5 shows simplified 3D OCT scans of an aged source material (left) and a fresh source material (right) as black and white line drawings. Both source materials were applied to a glass substrate to create the 3D OCT scans.

[0175] In the case of the aged starting material (left image), the presence of numerous microscale, intrinsic inhomogeneities can be observed as impurities, while the 3D OCT scan of the fresh starting material (right image), simplified as a black and white line drawing, shows hardly any impurities. Depending on the requirements placed on the starting material, it can then be assessed whether the aged starting material can still be used or whether it should be discarded.

[0176] If the starting material analyzed as described above is used for 3D printing using nonlinear absorption polymerization, quality parameters determined during the quality analysis, such as the proportion and 3D coordinates of impurities, can be used to define and optimize the nonlinear absorption polymerization parameters, such as the so-called laser writing parameters. This can influence the morphology and properties of the manufactured workpieces. Workpieces with improved quality can be obtained.

[0177] Furthermore, it is possible to re-inspect the volumes containing impurities analyzed before nonlinear absorption polymerization after nonlinear absorption polymerization to evaluate the quality of the workpiece in the vicinity of the previously detected impurities. This allows insights into the influence of the presence of impurities on the quality of the finished workpiece to be derived. In other words, a quality analysis can be performed before, during, and / or after nonlinear absorption polymerization.

[0178] With reference to Fig. 6a und Fig. 6b A method for determining the degree of conversion in a nonlinear absorption polymerization using optical coherence tomography is explained in more detail below.

[0179] 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 the refractive index. The local refractive index is easily determined if the refractive index profile is assumed to consist of discontinuities. In this case, if one refractive index is known, the other refractive index can be determined from the Fresnel reflection coefficients. At the interface with a glass substrate, for example, the refractive index of the glass substrate is known, so the refractive index of the adjacent polymer or monomer can be determined.

[0180] The spatially resolved determination of the degree of conversion can therefore be carried out in-situ using optical coherence tomography by detecting the optical coherence tomography signal resulting from the reflection at the interface between polymer and substrate and first determining the refractive index in a spatially resolved manner. Fig. 6a, b show the influence of the power of the laser used for nonlinear absorption polymerization on the refractive index of the produced polymer and thus the degree of conversion for a low printing speed of 10 mm / s ( Fig. 6a ) and a high printing speed of 30 mm / s ( Fig. 6b The refractive index was determined in situ immediately after laser irradiation using optical coherence tomography. The error bars represent the standard deviation of six measurements performed on six different samples with the same laser power and the same printing speed.

[0181] A comparison of the Fig. 6a und Fig. 6b shows that at higher printing speeds, higher laser power is required to achieve the same refractive index. Furthermore, it is evident that at a speed of 10 mm / s and above a laser power of approximately 17.5 mW, the refractive index increases only insignificantly, and therefore the maximum conversion efficiency appears to have been reached. At a speed of 30 mm / s, this is only the case at a laser power of approximately 22.5 mW. The maximum achievable refractive index is approximately identical for both speeds, so that the maximum conversion efficiency can, in principle, be achieved for both combinations of speed and laser power.

[0182] The spatially resolved degree of conversion can be used to determine the parameters of nonlinear absorption polymerization. For example, if 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 to achieve a higher degree of conversion. Since the degree of conversion can be determined in situ, the desired parameters of nonlinear absorption polymerization can be determined more quickly. This advantageously allows for the production of higher-quality workpieces in a shorter time.

[0183] With reference to Fig. 7a und Fig. 7b A method for analyzing the structural sharpness of structures produced by nonlinear absorption polymerization is explained in more detail below.

[0184] To determine and assess structural sharpness, volumetric images are recorded and evaluated using optical coherence tomography (3D OCT scans). During the evaluation, the abruptness of the refractive index changes at characteristic points is determined. If the refractive index changes abruptly, this indicates high structural sharpness. If the refractive index changes only gradually, structural sharpness is low. The determination and assessment of structural sharpness can be performed computer-assisted and optionally using artificial intelligence methods based on the spatially resolved refractive index curve determined using optical coherence tomography.

[0185] The optical coherence tomography signal depends on the refractive index profile between the unpolymerized and polymerized starting materials and is therefore influenced by the presence of an interface between the unpolymerized and polymerized starting materials. Optical coherence tomography scans at selected positions along the interface and at regular time intervals can be used to monitor structural sharpness during the 3D printing process.

[0186] Fig. 7a und Fig. 7b show exemplary measurement data for workpieces printed with different laser powers and immersed in unpolymerized starting material. Shown is the time dependence of the OCT (optical coherence tomography) signal recorded at the developing interface between the unpolymerized and the polymerized starting material. The OCT signal is plotted in the logarithmic unit decibels (dB). Fig. 7a shows measurement data of a sample in which an already polymerized starting material was immersed in liquid starting material after its development. Fig. 7b shows an in-situ measurement in which a polymerized starting material was left in the surrounding unpolymerized starting material during optical coherence tomography. In Fig. 7a und Fig. 7b Measurement data for three different samples are shown, in which the nonlinear absorption polymerization was carried out with different laser powers - in Fig. 7a 25 mW, 27 mW and 29 mW, in Fig. 7b 21 mW, 23 mW, and 25 mW. The starting material used in each case was IP Dip ®< from Nanoscribe GmbH. Fig. 7a und Fig. 7b show how the OCT signal decreases over time depending on the power of the writing laser. For the developed or polymerized structures ( Abb. 7a ) applies: With increasing laser power, 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 monomer due to diffusion processes. Abb. 7b No dependence of the OCT signal on laser power is evident. In this case, diffusion likely has a more complex and stochastic character.

[0187] With reference to Fig. 8 In the following, a method for the three-dimensional reconstruction of a radiation exposure profile of a starting material during nonlinear absorption polymerization using optical coherence tomography is explained in more detail.

[0188] As already explained in the introduction, known methods for reconstructing morphologies after completion of the 3D printing process require imperfections in the printed workpiece that lead to inhomogeneities in the refractive index. Fig. 8a The left figure shows a 3D OCT scan reconstruction of a workpiece manufactured using nonlinear absorption polymerization. The cross-hatching introduced during manufacturing, the model of which is shown in Fig. 8a , shown in the right figure, leads to an inhomogeneous refractive index within the manufactured workpiece, so that the reconstruction can be carried out entirely using OCT scans performed after 3D printing.

[0189] In contrast, vertical sidewalls between regions with a constant refractive index in some sections do not lead to an optical coherence tomography signal if the radiation used for optical coherence tomography hits the analysis site from above, i.e., parallel to the sidewall. Therefore, they cannot be reconstructed, or only to a limited extent. Fig. 8b The right figure shows a 3D OCT scan reconstruction of a workpiece manufactured using nonlinear absorption polymerization. The unidirectional hatching introduced during manufacturing, the model of which is shown in Fig. 8b , shown in the left figure, results in a homogeneous refractive index within the finished workpiece. The vertical sidewalls cannot therefore be captured using OCT scans, so reconstruction can only be performed incompletely using OCT scans performed after 3D printing.

[0190] The improved 3D reconstruction method proposed here involves performing OCT scans in situ, alternating between sequences of printing using nonlinear absorption polymerization and sequences of analysis using optical coherence tomography. The respective OCT signal is generated at the interface between materials with different refractive indices, e.g., at the interface between starting materials with a low and a higher degree of polymerization, provided the refractive index difference is large enough to backscatter the light or radiation. The change in the position of the interface as the 3D printing progresses can be tracked using optical coherence tomography. The entire morphology of the finished workpiece can ultimately be reconstructed from several individual OCT scans.

[0191] This has the advantage that a comprehensive 3D reconstruction is also possible if the manufactured, i.e. finished, workpiece contains surfaces that do not generate a detectable OCT signal.

[0192] The frequency of OCT scans can be fixed, for example, depending on the elapsed printing time, i.e., the duration of nonlinear absorption polymerization. This has the advantage of simple metrological implementation.

[0193] Optionally, the frequency of OCT scans can be optimized, i.e., adapted to the specific printing situation, for example, depending on the expected morphology of the workpiece according to the associated CAD model, the starting material, the radiation power, etc. On the one hand, as many printed layers as possible should be imaged to reconstruct the printed workpiece as accurately as possible. On the other hand, the frequency of OCT scans should be adjusted to allow complete post-polymerization of the tested layers and avoid adverse delays in 3D printing.

[0194] One strategy may be to adjust the frequency of OCT scans depending on the morphology to be imaged. For example, a large volume can be imaged at once, e.g., to reconstruct morphologies of sidewalls tilted with respect to the incident radiation for optical coherence tomography. To reconstruct elements, such as sidewalls, that cannot be imaged with a single OCT acquisition—that is, elements oriented in the direction of the incident radiation for optical coherence tomography—the frequency of OCT scans can be increased.

[0195] The frequency of OCT scans can be determined, for example, based on an analysis of the corresponding 3D (CAD) model before starting 3D printing and, if necessary, implemented in a control system for controlling the 3D printing device.

[0196] With reference to Fig. 9 A method for checking the position and / or alignment of a substrate 8 to be printed by means of non-linear absorption polymerization is explained in more detail below. Fig. 9 shows a transparent substrate 9 into which four cross-shaped characteristic markings 11 are introduced. The position and orientation of the characteristic markings 11 with respect to the substrate 8 is known, so that by locating the characteristic features 11, the position and orientation of the substrate 8 can be determined.

[0197] For this purpose, one or more of the characteristic features 11 of the substrate 8 to be printed are identified by means of optical coherence tomography, ie using two- or preferably three-dimensional OCT images.

[0198] Verification of position and / or alignment using optical coherence tomography can be carried out, among other things, in the cases described below.

[0199] If the substrate area to be printed cannot be imaged with a real-time camera of the 3D printing device, e.g., if it is located on a sloped sidewall or if it is obscured, optical coherence tomography can be used to image the substrate area to be printed and determine the corresponding coordinates, possibly also in relation to the section obscuring the substrate area to be printed. This assumes that the absorption coefficient of the material of the obscuring section does not prevent the acquisition of the optical coherence tomography signal.

[0200] Optical coherence tomography can also be used to locate characteristic features 11 that cannot be imaged with a real-time camera, either due to the orientation of the substrate 8, the morphology of the already printed section, or because the characteristic features 11 are intentionally embedded inside the substrate 8, e.g., characteristic features 11 in the form of alignment marks written directly into the volume of a glass substrate with a fs laser, or alignment marks written with nonlinear absorption polymerization and subsequently covered with another photoresist to create subsurface refractive index contrast.

[0201] Since optical coherence tomography is very sensitive to refractive index contrasts and enables subsurface imaging of materials with low to moderate absorption and scattering, i.e., as long as the backscattered signal matches the sensitivity of the OCT system, optical coherence tomography can be used to verify substrate position and / or substrate orientation within an array of elements with identical topography that differ only in their refractive index, as in multi-material printing, or in their internal structures, e.g., elements with different porosity.

[0202] After the substrate position and / or substrate orientation has been verified, it can be determined whether the found substrate position and / or substrate orientation corresponds to the desired substrate position and / or substrate orientation. If this is not the case, the position and / or orientation of substrate 8 can be changed accordingly and, if necessary, verified again using optical coherence tomography. Bezugszeichenliste

[0203] 1First radiation source 2First radiation 3Second radiation source 4Second radiation 5First beam path 6Second beam path 7Focus point 8Substrate 9Optical system 10Coupling element 11Characteristic feature 12Source material 13Container 14Immersion liquid 15First objective 16Second objective 17Mirror 18Objective 19Lens 20Galvo mirror x-direction 21Galvo mirror y-direction 22Fiber coupler 23Analysis unit 1003D printing device 2003D printing process 3003D printing device 4003D printing process 3D three-dimensional CAD computer-aided design CW continuous wave NA numerical aperture fs femtosecond OC optical coherence tomography S1 Providing a substrate S2 Positioning the substrate in the starting material S3 Performing nonlinear absorption polymerization S4 Performing optical coherence tomography S5 Washing out excess starting material αAngle at which the first radiation and the second radiation meet ωAperture angle of the second beam path

Claims

1. 3D printing apparatus (100) for manufacture of a workpiece, the apparatus (100) having: - a first radiation source (1) designed to emit a first radiation (2) for performance of nonlinear absorption polymerization, and - a second radiation source (3) designed to emit a second radiation (4) for performance of optical coherence tomography, wherein a first beam path (5) is traversable by the first radiation (2) and a second beam path (6) is traversable by the second radiation (4), characterized in that the first beam path (5) and the second beam path (6) are formed entirely independently of one another.

2. Apparatus (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 one another at an angle α, where 0° < α < 180°, for example 0° < α ≤ 90°.

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

4. Apparatus (100) according to any of the preceding claims, wherein the first beam path (5) is formed such that the first radiation (2) hits the substrate surface parallel to a normal of a substrate surface of the substrate (8) to be printed.

5. Apparatus (100) according to any of the preceding claims, wherein the second beam path (6) is formed such that the second radiation (4) hits the substrate surface at a non-90° angle to the normal of the substrate surface of the substrate (8) to be printed.

6. Apparatus (100) according to Claim 5, wherein the second beam path (6) is formed such that the second radiation (4) hits the substrate surface at an angle of less than 90° to the normal of the substrate surface of the substrate (8) to be printed.

7. Apparatus (100) according to any of the preceding claims, wherein the second beam path (6) is formed such that the second radiation (4) hits a focal point (7) through a substrate (8) to be printed.

8. 3D printing apparatus (300) for manufacture of a workpiece, the apparatus (300) having: - a first radiation source (1) designed to emit a first radiation (2) for performance of nonlinear absorption polymerization, - a second radiation source (3) designed to emit a second radiation (4) for performance of optical coherence tomography, wherein the first radiation source (1) and the second radiation source (3) are arranged such that an optical system (9) is at least partly utilizable collectively by the first radiation (2) and the second radiation (4), - an objective lens (18) disposed in the optical system (9) and designed to focus the first and second radiation to a focal point, and - an input coupling element (10) designed to couple the first radiation (2) and / or the second radiation (4) into the optical system (9), characterized in that the input coupling element (10) is selected from a group consisting of a mechanically adjustable mirror, a galvanomirror, a polarizing beam divider, an intensity divider and a partly coated mirror.

9. Apparatus (100, 300) according to any of the preceding claims, having: - an analysis unit (23), set up and designed to generate optical coherence tomography images based on the second radiation (4), wherein the analysis unit (23) has multiple measurement channels such that multiple analysis sites can be analyzed simultaneously by optical coherence tomography.

10. Apparatus (100, 300) according to any of the preceding claims, wherein the second radiation source (3) emits a wave which is constant over time as the second radiation.

11. Apparatus (100, 300) according to any of the preceding claims, having: - a vessel (13) for positioning of a starting material (12) to be polymerized, wherein the vessel (13) is designed to be at least partly transparent to the first radiation (2) and / or second radiation (4).

12. Apparatus (100, 300) according to any of the preceding claims, wherein an objective lens (18) of the apparatus (100, 300) is designed and arranged such that it can be immersed into a starting material (12) to be polymerized.

13. 3D printing method (200, 400) for manufacture of a workpiece by means of an apparatus (100, 300) according to any of the preceding claims, wherein nonlinear absorption polymerization (S3) and optical coherence tomography (S4) are conducted in mutual alternation or simultaneously.